New Materials in Civil Engineering 0128189614, 9780128189610

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New Materials in Civil Engineering

New Materials in Civil Engineering Edited by

Pijush Samui Department of Civil Engineering, NIT Patna, Patna, Bihar, India

Dookie Kim Department of Civil and Environmental Engineering, Structural System Laboratory, Kongju National University, Cheonan, Chungnam, Republic of Korea

Nagesh R. Iyer FNAE Dean & Visiting Professor, Indian Institute of Technology Dharwad, Dharwad, India

Sandeep Chaudhary Discipline of Civil Engineering, Indian Institute of Technology Indore, Indore, India

Butterworth-Heinemann is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-818961-0 For Information on all Butterworth-Heinemann publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Editorial Project Manager: Ana Claudia Garcia Production Project Manager: Nirmala Arumugam Cover Designer: Mark Rogers Typeset by MPS Limited, Chennai, India

Dedication

Dedication in memory of my father

Prof. G.R. Ranganatha Iyer (June 26, 1922 October 30, 2019) Chief Engineer & Joint Secretary (Retd), Govt. of Gujarat Professor of Civil Engineering, L.D. College of Engineering, Gujarat University & Technical Advisor, World Bank He has been a fountainhead of knowledge, inspiration, philosophy, and an excellent disciple of spiritual learning; a noble soul radiating love and warmth; he has left behind a rich harvest of memories to cherish, honor, and emulate. He was a magnanimous presence, an endearing soul who spread happiness and love with his brilliant smile and words of encouragement. We pray for his soul and salute him for his foot prints on the sands of time! We will remember him in every moment in every walk of our lives, as we continue to be inspired by him, forever. People of his kind never die!

Contents

List of Contributors 1

2

3

An overview of cementitious construction materials Nagesh R. Iyer 1.1 Cement and concrete 1.2 High-performance concrete 1.3 Geopolymer concrete 1.4 Fiber-reinforced concrete 1.5 Fiber-reinforced concrete polymer composites 1.6 Lightweight concrete 1.7 Ultrahigh-strength concrete 1.8 Biomimetics and bacterial concrete Acknowledgments References Computational intelligence for modeling of pavement surface characteristics Behrouz Mataei, Fereidoon Moghadas Nejad, Hamzeh Zakeri and Amir H. Gandomi 2.1 Introduction 2.2 Computational intelligence methods 2.3 Conclusion References Further reading Computational intelligence for modeling of asphalt pavement surface distress Sajad Ranjbar, Fereidoon Moghadas Nejad, Hamzeh Zakeri and Amir H. Gandomi 3.1 Introduction 3.2 CI methods 3.3 Methodology and application 3.4 Application of CI frameworks in PMS 3.5 Conclusion References

xvii 1 1 10 13 16 23 25 40 50 61 61

65

65 67 75 76 77

79

79 80 84 97 102 104

viii

4

5

6

7

Contents

Expanded polystyrene geofoam S.N. Moghaddas Tafreshi, S.M. Amin Ghotbi Siabil and A.R. Dawson 4.1 Introduction 4.2 EPS properties 4.3 EPS in embankments 4.4 EPS in bridge abutments and retaining structures 4.5 EPS in utility protection 4.6 EPS in other uses 4.7 Conclusions References Recycling of industrial wastes for value-added applications in clay-based ceramic products: a global review (2015 19) M. Contreras, M.J. Ga´zquez, M. Romero and J.P. Bolı´var 5.1 Introduction 5.2 Industrial waste materials as aggregate in clay ceramics 5.3 Review of studies into the incorporation of waste materials in brick making 5.4 Discussion References Emerging advancement of fiber-reinforced polymer composites in structural applications Kishore Kumar Mahato, Krishna Dutta and Bankim Chandra Ray 6.1 Introduction 6.2 Assessment of fiber-reinforced polymer composites by mechanical, chemical, and thermal behaviors 6.3 Evaluation of special structural properties 6.4 Environmental durability of fiber-reinforced polymer composites in civil structures 6.5 Conclusions and future perspectives Acknowledgment References Fiber-reinforced concrete and ultrahigh-performance fiber-reinforced concrete materials Francesco Micelli, Angela Renni, Abdou George Kandalaft and Sandro Moro 7.1 Fiber-reinforced concrete 7.2 Ultrahigh-performance concrete ultrahigh-performance fiber-reinforced concrete References

117 117 120 131 138 144 149 150 151

155 155 158 164 208 209

221 221 224 233 241 261 262 262

273

273 294 310

Contents

8

9

10

11

The superplasticizer effect on the rheological and mechanical properties of self-compacting concrete Mouhcine Ben Aicha 8.1 Introduction 8.2 Chemical structure of superplasticizers 8.3 Action mechanisms of superplasticizers 8.4 Superplasticizer effect on cement paste 8.5 Superplasticizer effects on concrete rheology 8.6 Superplasticizer effect on concrete compressive strength 8.7 Conclusion References Trends and perspectives in the use of timber and derived products in building fac¸ades Anna Sandak, Marcin Brzezicki and Jakub Sandak 9.1 Introduction 9.2 Biobased fac¸ade materials 9.3 Trends and perspectives 9.4 Conclusions Acknowledgment References Dynamic response of laminated composite plates fitted with piezoelectric actuators S.K. Sahu, A. Gupta and E.V. Prasad 10.1 Introduction 10.2 Formulation 10.3 Linear static analysis of cross-ply laminated plates 10.4 Dynamic and transient analyses 10.5 Nonlinear vibration analysis of composite plates embedded with piezoelectric materials 10.6 Conclusion References Functional nanomaterials and their applications toward smart and green buildings Kwok Wei Shah, Ghasan Fahim Huseien and Teng Xiong 11.1 Introduction 11.2 Sustainability of traditional ordinary Portland cement-based concrete 11.3 Self-healing concrete 11.4 Nanomaterials 11.5 Nanomaterial-based self-healing concrete 11.6 Sustainability of nanomaterial-based self-healing concrete

ix

315 315 315 318 321 324 326 327 328

333 333 335 348 369 370 370

375 375 378 383 383 384 392 392

395 395 396 398 410 413 419

x

Contents

11.7

Advantages and disadvantages of nanomaterials for self-healing concrete 11.8 Economy of nanomaterial-based self-healing concretes 11.9 Environmental suitability and safety features of nanomaterial-based concretes 11.10 Conclusions References 12

13

Production of sustainable concrete composites comprising waste metalized plastic fibers and palm oil fuel ash Hossein Mohammadhosseini, Mahmood Md. Tahir, Rayed Alyousef and Hisham Alabduljabbar 12.1 Introduction 12.2 Waste metalized plastic fibers 12.3 Concrete incorporating waste metalized plastic fibers 12.4 Applications 12.5 Conclusions References Alkali-activated concrete systems: a state of art R. Manjunath and Mattur C. Narasimhan 13.1 Introduction 13.2 Geopolymers and alkali-activated cementitious systems 13.3 Requirements for alkali activation of ground granulated blast furnace slag 13.4 Alkali-activated slag systems 13.5 Effect of dosage and modulus of activator solutions 13.6 Workability and strength characteristics of geopolymers and alkali-activated composites 13.7 Alkali-activated composites with alternative binders 13.8 Alkali-activated composites with different activators 13.9 Alkali-activated composites with alternative aggregates 13.10 Durability studies on alkali-activated composites 13.11 Elevated-temperature performance of alkali-activated composites 13.12 Behaviour of alkali-activated composites incorporated with fibers 13.13 Behaviour of rebar-reinforced structural elements made from alkali-activated concrete mixes 13.14 Summary of alkali-activated composite systems 13.15 Future trends for AA composites—research needs References

420 420 421 422 423

435

435 437 439 454 454 455 459 459 460 463 463 464 465 469 471 472 473 475 477 479 480 482 482

Contents

14

15

16

17

Porous concrete pavement containing nanosilica from black rice husk ash Ramadhansyah Putra Jaya 14.1 Introduction 14.2 Literature review 14.3 Materials 14.4 Experimental plan 14.5 Results and discussions 14.6 Conclusions Acknowledgment References Porous alkali-activated materials Priyadharshini Perumal, Tero Luukkonen, Harisankar Sreenivasan, Paivo Kinnunen and Mirja Illikainen 15.1 Introduction 15.2 Porous alkali-activated materials 15.3 Characterization of porosity in alkali-activated materials 15.4 Properties of porous alkali-activated materials 15.5 Functional properties and applications 15.6 Conclusions Acknowledgments References Lightweight cement-based materials Teresa M. Pique, Federico Giurich, Christian M. Martı´n, Florencia Spinazzola and Diego G. Manzanal 16.1 Introduction 16.2 Lightweight/low-strength aggregates 16.3 Lightweight/high-strength aggregates 16.4 Extenders 16.5 Outlook and future trends References Development of alkali-activated binders from sodium silicate powder produced from industrial wastes Parthiban Kathirvel 17.1 Introduction 17.2 Alternative for Portland cement 17.3 Alkaline activators 17.4 Waste glass 17.5 Silica fume 17.6 Rice husk ash

xi

493 493 496 499 501 512 523 523 523 529

529 530 541 546 549 554 555 555 565

565 566 575 580 586 587

591 591 592 593 594 597 597

xii

Contents

17.7 Sugarcane bagasse ash 17.8 Other materials 17.9 Cost analysis 17.10 Summary and conclusions References 18

19

20

Innovative cement-based materials for environmental protection and restoration Hosam M. Saleh and Samir B. Eskander 18.1 Introduction 18.2 Innovative cement-based material 18.3 Conclusions References Comparative effects of using recycled CFRP and GFRP fibers on fresh- and hardened-state properties of self-compacting concretes: a review M. Mastali, Z. Abdollahnejad, A. Dalvand, A. Sattarifard and Mirja Illikainen 19.1 Introduction 19.2 Experimental plan 19.3 Results and discussion 19.4 Analysis 19.5 Conclusions References Corrosion inhibitors for increasing the service life of structures B. Bhuvaneshwari, A. Selvaraj and Nagesh R. Iyer 20.1 Introduction 20.2 What is corrosion? 20.3 Severity of corrosion 20.4 Concrete corrosion inhibitors 20.5 Limitation of inhibitors 20.6 Mechanism of inhibition 20.7 Techniques to assess inhibitor performances 20.8 Concrete corrosion assessing techniques 20.9 Surface characterization of the metals/rebars after corrosion 20.10 Corrosion product analysis techniques 20.11 Durability studies of concrete with admixtures 20.12 Conclusion Acknowledgments References

602 605 606 609 609

613 613 617 636 638

643

643 645 647 650 652 654 657 657 658 660 661 663 664 665 666 668 668 670 673 673 673

Contents

21

22

23

24

Use of fly ash for the development of sustainable construction materials Sanchit Gupta and Sandeep Chaudhary 21.1 Introduction 21.2 Sustainable development of fly ash utilization 21.3 Characterization of fly ash 21.4 Fly ash applications 21.5 Developments in industrial fly ash applications 21.6 Conclusions References An innovative and smart road construction material: thermochromic asphalt binders Henglong Zhang, Zihao Chen, Chongzheng Zhu and Chuanwen Wei 22.1 Introduction 22.2 Three-component organic reversible thermochromic materials 22.3 The performance characterization of thermochromic asphalt binders 22.4 The adjustment of bituminous pavement temperature 22.5 Recommendations for future research and applications References Resin and steel-reinforced resin used as injection materials in bolted connections Haohui Xin, Martin Nijgh and Milan Veljkovic 23.1 Introduction 23.2 Computational homogenization 23.3 Experiments 23.4 Numerical simulation of resin 23.5 Numerical simulation of steel-reinforced resin 23.6 Conclusions References Swelling behavior of expansive soils stabilized with expanded polystyrene geofoam inclusion S. Selvakumar and B. Soundara 24.1 Effect of geobeads inclusion 24.2 Effect of the geofoam granules column 24.3 Conclusions Acknowledgments References

xiii

677 677 678 679 681 682 686 687

691

691 693 699 713 714 715

717 717 721 722 733 734 741 743

745 745 755 774 774 775

xiv

25

26

27

28

Contents

New generation of cement-based composites for civil engineering Danna Wang, Wei Zhang and Baoguo Han 25.1 Introduction 25.2 Smart and multifunctional cement-based composites 25.3 Nanocement-based composites 25.4 Conclusions Acknowledgments References Potential use of recycled aggregate as a self-healing concrete carrier Chao Liu and Zhenyuan Lv 26.1 Introduction 26.2 Self-healing concrete materials 26.3 Method and results 26.4 Effect of recycled aggregate in self-healing concrete 26.5 Outlook References Self-healing concrete Xu Huang and Sakdirat Kaewunruen 27.1 Introduction 27.2 Materials and methods 27.3 Results 27.4 Discussion 27.5 Conclusion Acknowledgments Author contributions Conflicts of interest References Equations for prediction of rubberized concrete compressive strength: a literature review ˇ c´ Marijana Hadzima-Nyarko and Ivana Milicevi 28.1 Introduction 28.2 Literature review 28.3 Database description 28.4 Expressions for compressive strength in the literature 28.5 Expressions for compressive strength of concrete 28.6 Comparison of existing expressions 28.7 Conclusion Acknowledgment References

777 777 778 784 789 790 790

797 797 802 805 817 820 821 825 825 828 835 851 853 854 854 854 854

857 857 858 859 864 865 869 872 872 872

Contents

29

30

31

Influence of cobinders on durability and mechanical properties of alkali-activated magnesium aluminosilicate binders from soapstone Z. Abdollahnejad, M. Mastali, F. Rahim, Tero Luukkonen, Paivo Kinnunen and Mirja Illikainen 29.1 Introduction 29.2 Experimental plan 29.3 Results and discussion 29.4 Conclusions Acknowledgment References Fly ash utilization in concrete tiles and paver blocks S.K. Sahu, S. Kamalakkannan and P.K. Pati 30.1 Introduction 30.2 Experimental procedure 30.3 Results and discussion 30.4 Conclusion References Problems in short-fiber composites and analysis of chopped fiber-reinforced materials Vahid Monfared 31.1 Introduction 31.2 Analytical methods 31.3 Numerical methods 31.4 Experimental methods 31.5 Constitutive and fundamental researches 31.6 Solved problems References

Index

xv

877

877 878 884 892 894 894 897 897 900 905 915 916

919 919 933 954 973 990 992 1035 1045

List of Contributors

Z. Abdollahnejad Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland; Civil & Environmental Engineering Department, University of Connecticut, CT, United States Hisham Alabduljabbar Department of Civil Engineering, College of Engineering, Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia Rayed Alyousef Department of Civil Engineering, College of Engineering, Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia Mouhcine Ben Aicha National School of Architecture, Rabat, Morocco B. Bhuvaneshwari Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, India J.P. Bolı´var Department of Applied Physics, Faculty of Experimental Sciences, University of Huelva, Natural Resources, Health and Environment Research Center (RENSMA), Huelva, Spain Marcin Brzezicki Wroclaw University of Science and Technology, Faculty of Architecture, Wroclaw, Poland Sandeep Chaudhary Discipline of Civil Engineering, Indian Institute of Technology Indore, Indore, India Zihao Chen Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, P.R. China M. Contreras Department of Applied Physics, Faculty of Experimental Sciences, University of Huelva, Natural Resources, Health and Environment Research Center (RENSMA), Huelva, Spain A. Dalvand Department of Engineering, Lorestan University, Khorramabad, Iran A.R. Dawson Nottingham Transportation Engineering Centre, University of Nottingham, Nottingham, United Kingdom

xviii

List of Contributors

Krishna Dutta Composite Materials Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India Samir B. Eskander Radioisotope Department, Nuclear Research Center, Atomic Energy Authority, Giza, Egypt Amir H. Gandomi Faculty of Engineering & Information Technology, University of Technology Sydney, NSW, Australia M.J. Ga´zquez Department of Applied Physics, University of Cadiz, University Marine Research Institute (INMAR), Ca´diz, Spain S.M. Amin Ghotbi Siabil Department of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran Federico Giurich Polymers for the Oil and Construction Industry, Engineering Faculty, Universidad de Buenos Aires, Buenos Aires University, Argentina A. Gupta Department of Civil Engineering, OP Jindal University, Raigarh, Chhattisgarh, India Sanchit Gupta Discipline of Civil Engineering, Indian Institute of Technology Indore, Indore, India Marijana Hadzima-Nyarko Josip Juraj Strossmayer Univеrsity of Osijek, Faculty of Civil Engineеring and Architecturе Osijek, Osijek, Croatia Baoguo Han School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China Xu Huang Department of Civil Engineering, School of Engineering, University of Birmingham, Birmingham, United Kingdom Ghasan Fahim Huseien Department of Building, School of Design and Environment, National University of Singapore, Singapore Mirja Illikainen Civil & Environmental Engineering Department, University of Connecticut, CT, United States; Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland Nagesh R. Iyer Fellow, Indian National Academy of Engineering, Dean & Visiting Professor, Indian Institute of Technology, Dharwad, India Ramadhansyah Putra Jaya Department of Civil Engineering, College of Engineering, University of Malaysia Pahang, Kuantan, Malaysia

List of Contributors

xix

Sakdirat Kaewunruen Laboratory for Track Engineering and Operations for Future Uncertainties (TOFU Lab), School of Engineering, University of Birmingham, Birmingham, United Kingdom S. Kamalakkannan Department of Civil Engineering, NIT Rourkela, Odisha, India Abdou George Kandalaft BASF Construction Chemicals, Italy Parthiban Kathirvel School of Civil Engineering, SASTRA Deemed University, Thanjavur, India Paivo Kinnunen Fibre and Particle Engineering Research Unit, University of Oulu, Oulu, Finland; Civil & Environmental Engineering Department, University of Connecticut, CT, United States Chao Liu Xi’an University of Architecture and Technology, Xi’an, China Tero Luukkonen Fibre and Particle Engineering Research Unit, University of Oulu, Oulu, Finland; Civil & Environmental Engineering Department, University of Connecticut, CT, United States Zhenyuan Lv Xi’an University of Architecture and Technology, Xi’an, China Kishore Kumar Mahato School of Mechanical Engineering, Vellore Institute of Technology, Vellore, India R. Manjunath Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, India Diego G. Manzanal National Univerity of Patagonia, Comodoro Rivadavia, Argentina; ETS of Roads, Canals and Ports, Polytechnique University of Madrid, Madrid, Spain Christian M. Martı´n Polymers for the Oil and Construction Industry, Engineering Faculty, Universidad de Buenos Aires, Buenos Aires University, Argentina M. Mastali Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland; Civil & Environmental Engineering Department, University of Connecticut, CT, United States Behrouz Mataei Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran Francesco Micelli University of Salento, Lecce, Italy

xx

List of Contributors

Ivana Miliˇcevi´c Josip Juraj Strossmayer Univеrsity of Osijek, Faculty of Civil Engineеring and Architecturе Osijek, Osijek, Croatia S.N. Moghaddas Tafreshi Department of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran Hossein Mohammadhosseini Institute for Smart Infrastructure and Innovative Construction (ISIIC), School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Skudai, Johor, Malaysia Vahid Monfared Department of Mechanical Engineering, Zanjan Branch, Islamic Azad University, Zanjan, Iran Sandro Moro BASF Construction Chemicals, Italy Mattur C. Narasimhan Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, India Fereidoon Moghadas Nejad Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran Martin Nijgh Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands P.K. Pati Department of Civil Engineering, NIT Rourkela, Odisha, India Priyadharshini Perumal Fibre and Particle Engineering Research Unit, University of Oulu, Oulu, Finland Teresa M. Pique Polymers for the Oil and Construction Industry, Engineering Faculty, Universidad de Buenos Aires, Buenos Aires University, Argentina E.V. Prasad Department of Civil Engineering, OP Jindal University, Raigarh, Chhattisgarh, India F. Rahim Civil & Environmental Engineering Department, University of Connecticut, CT, United States Sajad Ranjbar Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran Bankim Chandra Ray Composite Materials Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India

List of Contributors

xxi

Angela Renni Roughan & O’Donovan Consulting Engineers, Dublin, Ireland M. Romero Department of Construction, Eduardo Construction Science (IETcc-CSIC), Madrid, Spain

Torroja

Institute

for

S.K. Sahu Department of Civil Engineering, NIT Rourkela, Odisha, India Hosam M. Saleh Radioisotope Department, Nuclear Research Center, Atomic Energy Authority, Giza, Egypt Anna Sandak InnoRenew CoE, Izola, Slovenia; University of Primorska, Faculty of Mathematics, Natural Sciences and Information Technologies, Koper, Slovenia Jakub Sandak InnoRenew CoE, Izola, Slovenia; University of Primorska, Andrej Maruˇsiˇc Institute, Koper, Slovenia A. Sattarifard Faculty of Civil Engineering, Semnan University, Semnan, Iran S. Selvakumar Department of Civil Engineering, Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology, Chennai, India A. Selvaraj CBM College, Bharathiar University, Coimbatore, India Kwok Wei Shah Department of Building, School of Design and Environment, National University of Singapore, Singapore B. Soundara Department of Civil Engineering, Bannari Amman Institute of Technology, Sathyamangalam, Erode, India Florencia Spinazzola Polymers for the Oil and Construction Industry, Engineering Faculty, Universidad de Buenos Aires, Buenos Aires University, Argentina Harisankar Sreenivasan Fibre and Particle Engineering Research Unit, University of Oulu, Oulu, Finland Mahmood Md. Tahir Institute for Smart Infrastructure and Innovative Construction (ISIIC), School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Skudai, Johor, Malaysia Milan Veljkovic Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands Danna Wang School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China

xxii

List of Contributors

Chuanwen Wei Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, P.R. China Haohui Xin Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands Teng Xiong Department of Building, School of Design and Environment, National University of Singapore, Singapore Hamzeh Zakeri Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran Henglong Zhang Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, P.R. China Wei Zhang School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China Chongzheng Zhu Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, P.R. China

An overview of cementitious construction materials

1

Nagesh R. Iyer Fellow, Indian National Academy of Engineering, Dean & Visiting Professor, Indian Institute of Technology Dharwad, Dharwad, India

1.1

Cement and concrete

1.1.1 Introduction The purpose of this chapter is to introduce different engineering materials of construction that have potential to be employed [1
3]. Considering various engineering attributes such as durability, sustainability, enhanced performance, reduction of use of natural resources, and low embodied energy, and the way forward, the reader is introduced to the new materials, however no attempt is made to provide a treatise of each material. The concepts introduced also give insight into the challenges and scope for innovation that exist. The extraction, production, transportation, utilization, and recycling of construction materials have impacts on the environment, sustainability, and built environment. Generally, investment and rate of growth of infrastructure act as one of the key indicators of economic growth and prosperity of any country. There are reports of structures having suffered severe degradation. Investigations have revealed that most of the distress, damage, or degradation are due to the combined effects of aggressive environments, and increased live loads or altered function from the original/intended design. Civil engineers face challenges of restoring the original design life, and preserving and maintaining retrofitted structures [1] through technological interventions. After water, concrete is the most commonly used building material in the world. Concrete has been through different stages of development; the earliest was conventional normal-strength concrete (NSC). Cement, water, fine aggregates, and coarse aggregates are the four key ingredients to developing the concrete mix matrix. For faster and leaner RCC construction of civil engineering infrastructure use of concrete with very high compressive strength is the preferred solution today. Civil infrastructure referred to here is concerned with urban infrastructure, development of smart cities, high-rise buildings, and long-span bridges, etc. In the next stages of development, (1) high-strength concrete (HSC), (2) high-performance concrete (HPC), and (3) ultraHSC (UHSC) have been successfully developed and deployed. It may be noted that HSC has compressive strength over 50 MPa, whereas HPC and UHSC have exhibited compressive strengths of over 100 MPa and high tensile strength (more than 10% of the compressive strength). There are also reports of achieving over 140 MPa compressive strength. The range of applications of such UHSCs is far and wide. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00001-6 © 2020 Elsevier Inc. All rights reserved.

2

New Materials in Civil Engineering

To substantiate, successful uses have been reported in the construction of strategic sectors (for example, blast shelters, impact-resistant structures, nuclear structures, etc.), high-rise buildings, structures/infrastructures in coastal areas for corrosion resistance, pavements, etc. In cases where one wants to achieve higher axial compressive strength, the water
cement ratio is reduced. This is done by adding a water-reducing agent or superplasticizer (SP). In contrast to NSC, two other ingredients, namely, admixtures and additives, are added to the mix. Silica fume (SF), fly ash (FA), and blast furnace slag are preferred as admixtures. These are waste materials and are industrial by-products. Therefore, HPC is considered as a green HPC (GHPC). Considerable studies were reported on the behavior and applications of HPC toward the end of the 20th century. UHSC thus has a clear advantage and is preferred in a wide range of engineering applications such as, to improve strength, deformability, and toughness of UHSC, short steel fibers are introduced during mixing to restrain cracks. Introduction of steel fibers in the matrix improves the toughness and deformation of UHSC, thereby avoiding high brittleness [2,3]. Ordinary Portland cement (OPC) is an energy-intensive material and is also associated with high CO2 emissions. Stricter regulations and proactive actions of major cement manufacturers have reduced the emission levels and experiments on alternate fuels to replace coal are on-going. The best way to reduce the carbon footprint is to use supplementary cementitious materials (SCMs) like FA, ground slag, rice husk ash, metakaoline, etc. The use of SCMs not only reduces the OPC content in concrete, but also enhances the durability, which is one of the foundations of sustainability. The use of blended cements in which the right kind of SCM of right quality is used would obviate all apprehensions about using SCMs. Emphasis should also be on optimizing the use of cement by applying mix proportioning. In India, the majority of “normal” concrete of grades M20 and M25 is still manufactured by volume batching. Use of appropriate construction chemicals judiciously to reduce the water
cement ratio and thus obtain a dense and durable concrete is imperative. Concrete that is made well, transported carefully, cast properly, and cured sufficiently would last a very long time, making concrete an extremely sustainable material. The absence of large-scale mechanization, absence of weigh-batching facilities, improper shuttering, inadequate compaction, and virtually no curing are the bane of concrete construction. There is a need for extensive training of masons and construction workers to inculcate in them this level of professionalism.

1.1.2 Proportioning Concrete is a composite heterogeneous material substantially influenced by the constituent materials and the proportional distribution in the mix. Well-developed mix design methods form the nucleus of concrete technology for sustainable development. A good concrete mix should lead to strong and durable concrete as the endproduct. This can be attained by a well-established procedure of mixing of the ingredients. Much depends upon how the voids are filled and packed in a concrete mix, otherwise it will result in rough, honeycombed, and porous concrete.

An overview of cementitious construction materials

3

A mix is easy to handle and can produce a better and smoother finish when it has an excess amount of cement paste. However, at the same time the mix would shrink more and turn out to be uneconomical. Thus, optimum workability of concrete through a “magic” mix becomes the key. The most desired attributes in the mix after it is hardened and is prepared with acceptable quality of paste are durability and strength. The water
cement ratio used to produce the mix determines the strength and quality of the paste. The water
cement ratio is the weight of the water used for a given weight of the cement. This optimum and “magic” mix of cement, aggregate, and water is generally in the range of 10
15:60
75:15
20, respectively. The numbers mentioned are percentages by volume. The air that is trapped could be anywhere between 5% and 8%. This is the reason for vibrating the mix, so that there are virtually no voids in the mix after it hardens. One can thus obtain high-quality concrete by lowering the water
cement ratio.

1.1.3 Other ingredients The type and size of the aggregate used to prepare the mix depends on the geometry and final application or the intended purpose [4]. In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete. Almost any natural water without impurities that is almost pH neutral or not acidic qualifies to be used for concrete, otherwise this can disturb adversely the setting time and concrete strength. Poor concrete can show symptoms of efflorescence, staining, volume instability, and reduced durability. Furthermore, it can also lead to corrosion in rebars. A number of studies and investigations have helped in framing guidelines and specifications to define the limits or amount of chlorides, sulfates, alkalis, and solids permitted. For thin building sections, small coarse aggregates should be used. For large structures such as large dams, aggregates up to 150 mm (6 inches) in diameter have been used. A continuous gradation of particle sizes is desirable for efficient use of the paste.

1.1.4 Hydration The hardening process begins once the appropriate proportion of cement, aggregates, and water is used to produce a mix. Obviously, because of the presence of water, this starts a chemical reaction called hydration. There is a formation of a node on each cement particle surface. This node starts growing and expanding and becomes connected with either nodes from other cement particles or adheres to adjacent aggregates. This process leads to progressive stiffening, hardening, and strength development of the mix. After the concrete is mixed thoroughly and is workable, it is placed in molds or within the formwork that is already created to get the desirable form/shape, etc. At this time, the concrete is consolidated and compacted using vibrator(s) to remove voids, air to avoid forming honeycombs. Technologies, instruments, and tools are used to achieve a smooth surface and desirable outcome with precision. Curing ensures progressive gain of strength and

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New Materials in Civil Engineering

stiffness through continued hydration. There are a number of methods of curing that are well developed today. Concrete continues to get stronger as it ages.

1.1.5 Cement “Cement,” in general, can be defined as any material which has the property to bind together different materials through different reactions. Commonly the reactions that are involved are chemical reactions which are aided by the presence of water. A great deal of literature is available in the form of textbooks, standards, technical papers, etc. which discuss in details cement, types of cements, and the various reactions that take place during the strength-gaining process. The intention of this section is to give a very brief overview of the types of cement generally available in the market and major properties of such cements.

1.1.5.1 Types of cement OPC: OPC is by far the most common cement used in India. Depending upon the 28 days strength of the cement mortar cubes, as per IS 4031-1988, OPC is classified into three grades, namely 33, 43, and 53 grades. It is expected that for a particular grade of cement the test results of the mortar cubes do not fall below the specified value. Rapid hardening cement (IS 8041-1990): Rapid hardening cement starts gaining strength and develops strength at the age of 3 days that OPC achieves in 7 days. Higher fineness of grinding and higher C3S and lower percentage of C2S increase the rate of development of strength. Extra rapid hardening cement: When calcium chloride (up to 2%) is intergraded with rapid hardening Portland cement (PC), extra rapid hardening cement is produced. Although the strength of extra rapid hardening cement is about one-fourth higher than that of rapid hardening cement at 1 or 2 days and 10%
20% higher at 7 days, it is almost the same at 90 days. Sulfate resisting cement (IS 12330-1988): During OPC production when tricalcium aluminate (C3A) is added restricting it to the lowest permissible value, it results in sulfate resisting cement. It also has low C4AF content. Use of this type of cement is more beneficial for structural elements in contact with soils and ground water, where there is significant presence of sulfates, seawater, or exposure to the sea coast. Portland slag cement (PSC) (IS 455-1989): PSC is produced by intimate interground mixing in suitable proportions of PC clinker, gypsum, and granulated blast furnace slag with permitted additives. Except for slowness in hydration during the first 28 days, other attributes of this cement are similar to OPC. Therefore it can be employed for mass concreting. It has very low diffusivity to chloride ions and therefore has better resistance to corrosion of steel reinforcements. Quick setting cement: At the time of clinker grinding, reducing gypsum content produces quick setting cement. This cement can reduce the pumping time, making it more cost-effective. Super sulfated cement (IS 6909-1990): This is a hydraulic cement produced by intergrinding or intimate blending mixture of granulated blast furnace slag, calcium sulfate,

An overview of cementitious construction materials

5

and a small amount of PC or PC clinker or any other lime in the proportions of 80
85:10
15:5, respectively. IS:6909-1990 (reaffirmed 2016) provides more details. Low heat cement (IS 12600-1989): This type of cement has low heat of hydration and displays a slow rate of gain of strength. However, the ultimate strength is the same as that of OPC. The cement is produced by intimately mixing together calcareous and argillaceous and/or other silica-, alumina-, or iron oxide-bearing materials burnt at clinkering temperature and grinding them. Hydrophobic cement (IS 8043-1991): Hydrophobic cement is obtained by intimately mixing together calcareous and argillaceous and-or other silica-, alumina- or iron oxide-bearing materials burnt at clinkering temperature and grinding them with natural or chemical gypsum with a small amount (say 0.1%
0.5%) of hydrophobic agent, forming a film which is water-repellant around each cement grain. The film is broken out when the mixing together of cement and aggregate breaks the film. This exposes the cement particles for normal hydration. The film-forming waterrepellant material is expected to improve workability and also protect from deterioration due to moisture during storage and transportation. Masonry cement (IS 3466: 1988): Masonry cement is made by intimate grinding and mixing of PC clinker and gypsum with pozzolanic or inert materials and in suitable proportions air entraining plasticizer resulting normally in fineness better than OPC. It finds use mainly for masonry construction. Expansive cement: In this type of cement, there is a significant increase in volume (instead of shrinking) vis-a`-vis PC paste when mixed with water. The key element is the presence of sulfoaluminate clinker mixed with PC and stabilizer in the proportions of 10:100:15, respectively. This process not only improves the density but also the integrity of concrete. Oil-well cement (IS 8229-1986): Oil-well cement is used by the petroleum industry for cementing gas and oil wells at high temperature and pressure. There are eight classes (A to H) defined by IS:8229 that are manufactured. Each class essentially contains hydraulic calcium silicates. As per the IS code, no material other than one or more forms of calcium sulfate are interground with clinker or blended with ground clinker during production. The common agents, which are known as retarding agents, are starch, cellulose products, or acids to prevent quick setting. Rediset cement: Cement which yield high strengths in about 3
6 hours, without showing any retrogression is rediset cement. It has similar 1- or 3-day strength as OPC. High alumina cement (IS 6452: 1989): As per the IS specifications, high alumina cement is obtained by either fusing or sintering aluminous and calcareous materials and grinding the resulting clinker. Only water can be added during the grinding process. One of the key features of high alumina cement concrete is its very high rate of strength development. In 1 day it can gain about 20% of the ultimate strength.

1.1.6 Cement composition A complicated process known as hydration is caused due to chemical reaction during mixing of cement and water in suitable proportions [2]. This gives strength to PC. Lime, silica, alumina, and iron oxide are the chief raw materials used to

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New Materials in Civil Engineering

produce cement. The four major oxides of cement, CaO, SiO2, Al2O3, and Fe2O3, in decreasing order, interact in the kiln at high temperature, forming complex compounds. The relative proportions of these oxide compositions influence various attributes/properties of cement, including the rate of cooling and fineness of grinding. Table 1.4 shows the oxide composition range of OPC. Oxides in smaller quantities that are important for cement behavior include SO3, MgO, Na2O, and K2O. PC is manufactured by crushing, milling, and proportioning the following materials: G

G

G

G

G

lime or calcium oxide, CaO: from limestone, chalk, shells, shale, or calcareous rock; silica, SiO2: from sand, old bottles, clay, or argillaceous rock; alumina, Al2O3: from bauxite, recycled aluminum, clay; iron, Fe2O3: from clay, iron ore, scrap iron, and FA; gypsum, CaSO4.2H2O: found together with limestone.

During the process of cement manufacture, the following methods are generally used to determine the oxide composition of cement: 1. 2. 3. 4. 5. 6. 7.

chemical analysis; X-ray diffraction (XRD); optical microscopy; scanning electron microscopy (SEM) with energy dispersive X-ray analysis; electron microprobe analysis; selective dissolution; thermal analysis.

The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes: a dry process or a wet process. The materials are then fed through a kiln at 1428 C (2600 F) to produce grayish-black pellets known as clinker. The alumina and iron act as fluxing agents, which lower the melting point of silica from 1650 C (3000 F) to 1428 C (2600 F). After this stage, the clinker is cooled, pulverized, and gypsum added to regulate the setting time. It is then ground extremely fine to produce cement. Because of the complex chemical nature of cement, a notational form as given below is used to denote the chemical compounds. CaO 5 C; SiO2 5 S; Al2O3 5 A; Fe2O3 5 F; H2O 5 H; MgO 5 M; Na2O 5 N; SO3 5 S; CO2 5 C

The following compounds contribute to the properties of cement. Tricalcium aluminate, C3A: This generates considerable heat during the initial hydration stages but has little strength contribution. The presence of gypsum delays the hydration rate of C3A. A lower quantity of cement makes it sulfate resistant. Tricalcium silicate, C3S: This compound hydrates and hardens rapidly. It is largely responsible for PC’s initial set and early strength gain. Dicalcium silicate, C2S: C2S hydrates and hardens slowly. It is largely responsible for strength gain after 1 week.

An overview of cementitious construction materials

7

Ferrite, C4AF: This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln [from 1650 C (3000 F) to 1428 C (2600 F)]. It hydrates rapidly, but does not contribute much to the strength of the cement paste. By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments. The thermodynamics of cement chemistry have long been studied and were first applied by Le Chatelier in 1905. The use of thermodynamic methods in cement hydration was often doubted, as the water
cement system was considered to be too complex. Thermodynamic modeling of the interactions between solid and liquid phases in cements using geochemical speciation codes can be the basis for the interpretation of many of the observed experimental results. Thus, there are many phases possible through different combinations within the CaO
SiO2
Al2O3
Fe2O3 system. This permits extrapolation of the same to longer time scales by varying different parameters within the system. The phases that get formed in the CaO
SiO2 2 Al2O3 2 Fe2O3 system as a result of combinations of number of components such as two, three, or four can be described as binary, ternary, and quaternary phase diagrams [5
7]. A typical phase diagram for this system is shown in Fig. 1.1 [5], with an expanded view of the lime-rich part of the system. For example, C3S and C2S compounds are formed as a binary system due to CaO and SiO2 phase relation and, similarly, C3A and C12A7 are formed due to CaO and Al2O3, again as a binary phase relation.

1.1.7 Aggregates Aggregates are the constituents which give the strength and mass to the concrete [2
4]. Generally, aggregates are classified as coarse or fine aggregates. Usually SiO2 Liquid

Liquid

Two liquids

Temperature oC

Liquid

Cristobalite

Liquid Two Liquids

Trioyhite Mullite

Cristobalite - liquid Tridymite - liquid

Anorthite

Tridymite Genlenite Tridymite

Corundum

Lime

Quartz

CaO

A12O3

Weight %Sio2 The CaO-SiO2 system

The CaO-A12O3SiO2 system

Figure 1.1 The system CaO
Al2O3
SiO2. Source: Reproduced with permission from ,https://www.cementequipment.org/home/ cement-chemistry-home/everything-you-need-to-know-about-cement-chemistry-from-ancienttimes-to-2019/. (under chemical clinker formation).

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New Materials in Civil Engineering

aggregates less than 4.75 mm in size are described as fine aggregates and those greater than 4.75 mm are coarse aggregates. Indian standard IS 383:1970 gives the specifications for aggregates from natural sources that are used in making concrete. Grading of aggregates is important in order to obtain good concrete. Based on the particle size distribution, the aggregates can be classified as uniformly graded, well graded, and gap/poorly graded. It is always recommended that well-graded aggregates be used, since concrete produced with well-graded aggregates will have minimum voids. The fine aggregates that are being used should have very low slit content and preferably be free from organic materials.

1.1.8 Fine aggregates Sand Natural river sand is the most commonly used fine aggregate in the construction industry. It is also the most suitable material as of today. However, the exponential increase in the demand for sand has led to a supply
demand gap. More excessive exploitation of river beds leads to many environmental issues. Hence there is a growing need to find alternates to river sand.

1.1.8.1 Alternate fine aggregates Sea sand This is available in abundance and, with the long shore line available in India, it is a viable alternative [8]. The major problem that one faces when sea sand is used is that it usually contains chloride in excess of the permissible level. Generally, use of sea sand is not recommended. Hence, sea sand if used for construction, especially in reinforced concrete, should be screened for its chloride content. Apart from this, the particle size of sea sand may also lead to problems, hence one has to take care that the sand being used is well graded. Coarse ash/bottom ash Coarse ash/bottom ash forms a major part of thermal power plant waste. There have been some studies on replacing sand with coarse ash in cement mortar used for plastering works. It has also been used to replace sand during the making of hollow, paver blocks, etc. Blast furnace/copper slag Blast furnace and copper slag are waste products produced during the extraction of iron and copper, respectively, from their ores. They have pozzolanic properties and hence have been used as a cement replacement for a number of years. However, due to the increasing demand for, and unavailability of, sand, it is also being thought of as an alternative filler material. Studies have found that replacement of sand with slag results in performance in terms of workability, refractory properties, and resistance to alkali
silica reactions. Manufactured sand This is sand that is obtained by crushing stones to the required shape and size. The major factor involved here is the cost of production, however, with the increasing scarcity of natural sand and places where other alternate materials are not available, manufactured sand is emerging as a good alternative. In general, when an alternate material is used in making concrete, it has to be ensured that the characteristics of the materials are thoroughly studied and

An overview of cementitious construction materials

9

suitable modifications are made to the mix design so that the performance of the concrete is ensured.

1.1.8.2 Coarse aggregates Coarse aggregates form the major volume of the concrete mass. In India, crushed granite is mostly used as coarse aggregates, however other stones such as limestone, basalt, and most igneous rocks are suitable to be used as coarse aggregate. It is important to note that most of the strength contribution in a concrete mix is from coarse aggregate, hence the stones used as coarse aggregates should be sufficiently strong and inert to environmental factors. However, in some places even broken bricks are used in making concrete. Special caution needs to be exercised while using such materials as coarse aggregate. Use of stone, such as shale pumice, which have very high water absorption capacity, should be avoided, as this can lead to excessive cracking in the concrete. The other ingredients of concrete are discussed separately in other chapters in this book.

1.1.9 Reinforcing bars Concrete is strong in compression, but weak in tension. Normally, we use concrete in applications in which the primary stresses will be compressive. However, this is rarely the case, therefore we use steel reinforcement, because steel is equally strong in both tension and compression. However, it is much more expensive than concrete so we do not use it as the only building material, but use it in the form of reinforcing bars, also called rebars. We use the amount of rebars as per design that will be enough to take on any tensile force that the concrete is subjected to before the concrete would fail. The reinforcing steel or rebar is used in different forms or compositions, such as deformed bars and TMT bars. A deformed bar, a common steel bar, is used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression. It may be noted that there can be several material candidates to be used as a reinforcement material to take the tensile force that may be used in concrete. However, steel is the most preferred as the coefficients of thermal expansion of concrete and steel are similar. This phenomenon produces minimal stress in the composite matrix due to differential expansion. However, the use of rebars also brings with it other challenges, such as corrosion. In concrete structures, corrosion is a large concern. The effect of corrosion on structures can significantly deteriorate the physical integrity and progressively lead to the destruction of property and loss of life. Corrosion of reinforcing steel is a spontaneous irreversible electrochemical process [9] which is accelerated by the presence of electrolytes, especially salt corrosion. Again, the chlorides initiate corrosion and oxygen fuels the reaction. The concrete containing cement paste provides an alkaline environment around the rebar steel and helps to form a protective, tenacious, and passive oxide film. The pH of the pore solution as well as the

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New Materials in Civil Engineering

migration of aggressive species toward steel reinforcement plays a major role in concrete corrosion. The use of inhibitors in protecting steel reinforcement from corrosion is essential, not only for new structures to be constructed, but also for existing structures by means of repair. Researchers are now paying more attention toward the synthesis of novel and efficient inhibitors and their proper usage in construction applications [9].

1.1.9.1 Types of rebars There are various types of reinforcing bars used in construction, such as [2]: G

G

G

G

G

G

plain and ribbed (hot rolled) mild steel bars—the ribs improve the mechanical bond; cold twisted deformed (CTD) bars—ribbed low-carbon steel bars, twisted to increase the yield strength by work hardening. The resistance to corrosion decreases due to the residual stresses caused by the work hardening; thermomechanically treated (TMT) bars—bars with a hard high-strength surface and a ductile core; corrosion-resistant TMT bars—bars with small quantities of copper and chromium, and a higher than usual percentage of phosphorus; galvanized bars, epoxy-coated bars; stainless steel bars.

1.1.9.1.1 High yielding strength deformed bars These include grades Fe415, Fe500, and Fe550 (the number indicates the yield stress). Grade Fe250 mild steel is also available but is used only as a secondary reinforcement. One is advised to refer to the corresponding table provided in IS 1786-1985 for the (1) chemical composition of reinforcements and (2) mechanical properties of reinforcements. The chemical composition is described by the constituents such as carbon, sulfur, phosphorous, and mix of sulfur and phosphorous for all grades. The mechanical properties are characterized by (1) 0.2% proof stress or yield stress, (2) percentage minimum elongation on gauge length 5.65OA, where A is the cross-sectional area of the test piece, and (3) minimum tensile strength for all grades, namely, Fe 415, Fe 500, and Fe 550. Further, the code (IS:1786, reaffirmed in 2008) prescribes that (1) for Fe 415, the minimum tensile strength should be 10% more than the actual 0.2% proof stress but not less than 485 MPa, (2) for Fe 500, the minimum tensile strength should be 8% more than the actual 0.2% proof stress but not less than 545 MPa, and (3) for Fe 550, the minimum tensile strength should be 6% more than the actual 0.2% proof stress but not less than 585 MPa.

1.2

High-performance concrete

1.2.1 Introduction The classification of HSC is straightforward, since it can be based on compressive strength [2,3,10
12]. This is not the case for high-durability concrete (HDC) as strength can be a poor indicator of resistance to deterioration, particularly chemical,

An overview of cementitious construction materials

11

such as chloride attack. It is shown by considering the theoretical role of each material that high performance can be approached as a whole technology, that is, from the paste to the aggregate has to be taken into account. As the strength and durability requirement increases, so the demands on all of the material components working together become more critical and pose ever greater demands on concrete technology. Consideration is also given to the cost comparison of using HSC in place of conventional-strength concrete in structures. The durability of concrete has been a major concern for the past two decades, when it was found that the structures built during the rapid expansion of infrastructure in the 1960s and 1970s were deteriorating significantly. Indeed, the legacy of this is that more fiscal resources are now being spent on repair and rehabilitation than on new construction. Thus, there has been a great deal of interest in the use of both reactive and unreactive additional materials such as the pozzolanic binders and rock flour to produce HDC. HDC can be defined as a concrete with enhanced resistance to degradation, but not necessarily high strength. Frequently, higher strengths necessary to achieve high durability are not possible, as it is difficult to give a precise requirement for the lifespan of a structure. There are two distinct, but interrelated routes, to achieving HDC, these are to reduce the continuity and spaces in the capillary pore system, until in an ideal situation, no fluid movement can occur. In practice, this is unlikely to be possible and all concrete will have at least some interconnected pores, or else provide chemically active sites to immobilize and retard passing aggressive ions (this is mainly for chloride-bearing environments). This is an important factor for structures exposed to chloride-bearing environments, since fluid movement is always likely to occur. Chloride binding is a complex and not fully understood phenomenon. In HDC, high levels of pozzolanic binders are used, increasingly in poly blends to provide both chloride binding and reduced capillary pore size and their interconnection.

1.2.2 Characterization and design philosophy Two complimentary but different indexes are usually used to describe high performance, namely high strength and high durability. Depending upon the compressive strength, post-set heat treatment and application of pressure before and during setting may be necessary. The key characteristics of HPC can be summarized as: G

G

G

G

low water
binder ratio; large quantity of fine mineral powder (e.g., SF); aggregates containing fine sand; high dose of SPs.

It is the use of mineral admixtures (MAs) acting as fine fillers in the production of HPC that separates it from conventional concrete (CC). Pozzolanic materials like FA and SF are used as MAs. Due to the presence of fine fillers in the mix, HPC has a strong, denser, and hardened microstructure [2]. The low porosity and the stronger transition zone of HPC result in its superior durability and strength characteristics. However, quality control measures on the MAs are required basically to get HPC

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New Materials in Civil Engineering

with low w/c ratios. Moreover, the pozzolanic action of MAs depends upon their amorphous content as well as SiO2 content; the higher these contents the better will be their hardened (hydrated) state. In fresh HPC mixes, the particle sizes and their distribution in MAs play a major role. Finer and more spherical-shaped particles of MAs are preferred in HPC. Thus, the tests for characterization of MAs are very important requirements for HPC mix design. Characterization of HPC is similar to that of conventional cement concrete, however, in view of low w/c ratios and generally high cement contents of HPCs, the following characterizations are particularly important: G

G

G

G

G

G

G

G

G

G

G

peak temperature reached in fresh concrete; rate of retention of workability; effect of methods/sequence of mixture of ingredients; sensitivity to charge in small variations in dosages of admixtures; ambient temperature/humidity conditions; method of measurement of workability (compaction faction for stiff mixes, flow table for highly workable mixes and slump cone for medium workable mixes, K-slump, Kelly ball); time of start of curing (to avoid self-desiccation problems); curing method (curing compound, water ponding/spraying, etc.); air content (entrained/entrapped air contents); amount and type of vibration/compaction; and mode of transportation of fresh HPC concrete mix.

Most of the above-stated characterization tests of HPC mixes are performance oriented and, therefore, have to be conducted for each set of field conditions. Hence, the laboratory in which the HPC mix is begin developed must have facilities to simulate the field conditions so that the HPC mix can be appropriately developed and the laboratory result can be more reliably used in the field. The conventional cement concrete also requires generally similar considerations. However, the HPC has to perform well from many considerations other than strength alone, in contrast to conventional cement concrete whose performance is mostly measured in terms of strength only and therefore more stringent quality control measures are required at every stage of the production of HPC. HPC is usually designed to suit a particular application. Therefore characterization of hardened HPC mixes should be with reference to its end-usage. The following characterization studies which are required to be done on hardened HPCs are given as general guidelines. G

G

G

G

G

G

G

strength properties at different ages such as 1, 3, 7, 14, 28, 56, and 90 day. HPC usually contains pozzolanic admixtures and hence, the strength development beyond 28 days would be substantial. In literature, both 56- and 90-day tests are reported (AASHTO T-22, ASTM C39, IS: 516); permeability to water (ISTST, AUTOCLAM, BS 1881 Part 5); volume changes due to moisture movements (IS:4031); creep, shrinkage, and long-term properties; stress
strain relationship (IS:516); electrical resistance; pH and free lime content of hardened concrete;

An overview of cementitious construction materials

G

G

G

G

G

G

G

G

G

G

G

G

13

bond with steel reinforcement (IS:2770); nature of transition zone between cement matrix and aggregate in hardened HPC; air-void analysis and microstructure; resistance to attack by sulfates and other aggressive agents; resistance to abrasion, erosion, scaling, cavitation, etc. (IS:1234, ASTM C672); ductility of RC structural elements; permeability to chlorides (AASHTO T 277-831, ASTM C 1202); permeability to CO2/resistance to carbonation; permeability to air/oxygen; electrochemical potential of steel-reinforced HPC; corrosion current in steel-reinforced HPC subjected to accelerated corrosion cycles; freeze
thaw test (ASTM C 666, AASHTO T 161).

It may be noted that the above characterization tests on hardened HPC can be also performed on CC. In fact, it is essential to perform them both on CC and HPC simultaneously so that the superior characteristics of HPC are brought out clearly, for any particular set of ingredients. Some of the test methods available for CC are mentioned in the above list. However, in view of the special nature of HPC and also its high potential for use in important structures requiring a high degree of durability combined with long service life, it is necessary to formulate standard test methods so that the characteristics of HPCs developed all over the world can be compared more meaningfully. This would also help in creating a database from which the standard Codes of Practice can be prepared for use by field engineers. Apart from the above, specific tests to study the performance of HPC with the actual type and nature of structure need to be planned. Some examples in this context are nuclear power plant structures, off-shore structures, marine structures, irrigation and hydraulic structures, highway applications, airport pavements, overlays in factory floorings, and repair of chemically deteriorated or corrosion-damaged RC structures.

1.3

Geopolymer concrete

1.3.1 Introduction Geopolymer cements, eco-cements, and sulfoaluminate cements are considered as three alternative cements holding high potential in recent years [2,13]. Geopolymer cement concretes (GPCCs) are the most preferred among the new binder systems. Geopolymer is a generic and broad term. It comprises nine classes of materials representing a chain of inorganic molecules. However, Class F material consisting of aluminosilicate materials qualifies for civil engineering applications as it has the potential to replace partially at least OPC. However, its utility for structural and nonstructural elements and its durability characteristics need to be established from extensive R&D studies [2]. The program on waste to wealth undertaken internationally to use the large amount of industrial wastes and by-products by cleverly attempting to replace

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New Materials in Civil Engineering

partially or substitute the ingredients of concrete mix mainly, cement and aggregates have been the subject of research and applications. Some of these wastes include FA, ground granulated blast furnace (GGBS), alkaline sludges like red mud, and other materials. The wastes used are not necessarily pozzolanic. Considering these aspects, deployment of GPC can provide significant environmental benefits. Over OPC, the setting process in GPC is much faster and does not affect the hydration process. The polymerization takes place under alkaline conditions on silicon
aluminum minerals. This creates a three-dimensional polymeric chain and ring structure. The ratio of Si to Al determines the final structure of the geopolymer. This mix gains strength over different timescales. However, one disadvantage is that one needs over 30 C temperature scales for curing. This results in a reduction of the extent of amorphous order within the binder. Aside from their application as high-performance cements, GPCs find a range of niche applications such as in automobile car parts, waste immobilization, thermal boards, roof tiles, tooling materials, and decorated ceramics. GPCs result in a microstructure that is more heat resistant, fire resistant, and that has superior thermal expansion, cracking, and swelling properties compared to PC. They exhibit a smooth surface and can be molded easily. Several studies indicate that for geopolymerization, natural Al
Si minerals are most suitable. Due to the complexity of the reaction mechanisms involved, it is as yet difficult to identify and assess the suitability of the specific mineral. So far, FA and slags such as GGBS which are the by-products, have shown very encouraging results for use as geopolymers in the studies conducted. Between FA and slag, FA exhibits high reactivity—one of the reasons for this being that FA is finer than slag.

1.3.2 Development of structural grade geopolymer cement concretes There are no standard mix design approaches available for GPCs. As mentioned earlier, the water
cement ratio influences the strength of cement concrete. Studies have been conducted for the formulation of the GPC mixtures on a trial-and-error basis through liquid to binder (l/b) ratio and suitable composition of GPC solids (GPS). This is done till it meets the workability and strength requirements through a good cohesive mix. Recommended requirements for such mix are slump of 75
100 mm and 28-day compressive strength of 20
45 MPa [14
16]. The mixes were designed such that the test specimens cast were demoldable after 24 hours of wet gunny curing and the required strength could be realized after 28 days. Table 1.1 shows the typical mix composition of the geopolymer concrete. The mechanical properties of the GPCC mixes, including the stress
strain characteristics, were evaluated. Table 1.2 shows the strength characteristics of the mixes. The elastic modulus of high-volume GGBS-based GPCCs was slightly less than that of conventional OPCCs but the high-volume FA-based GPCCs showed considerably lower elastic modulus compared to OPCCs. The strain at peak stress ranged

Table 1.1 Typical mix composition for GPCC [2]. Mix ID

FAB-1 FAB-2 FAB-3 GGB-1 GGB-2 GGB-3 CC1 CC2 CC3

Binder

75% F 25% G 75% F 25%G 75% F 25% G 0% F 100% G 25% F 75% G 50% F 50% G OPC OPC OPC

Mix proportion (B:S:CA)

Molar ratios

l/b

Na2O/ GPS%

SiO2/ GPS%

H2O/ Na2O

SiO2/ Na2O

SiO2/ Al2O3

Na2O/ (Al2O3 1 SiO2)

1:1.64:2.82

7.77

2.49

4.24

0.33

0.70

11.38

4.28

1:1.43:2.6

10.34

3.18

4.58

0.26

0.70

12.47

8.06

1:1.10:1.83

9.61

3.64]

4.43

0.22

0.55

10.18

6.58

1:184:2.82

11.96

5.36

4.30

0.15

0.70

9.18

3.45

1:1.78:2.82

9.42

3.78

4.16

0.21

0.70

9.18

3.45

1:1.64:2.62

6.80

2.72

3.97

0.29

0.70

9.18

3.45

1:2.35:2.95 1:1.95:2.58 1:1.49:2.15





B, Binder; CA, coarse aggregate; F, FA; G, GGBS; l/b, liquid/binder; S, sand.

0.55 0.48 0.40

35 41 52

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New Materials in Civil Engineering

Table 1.2 Strength characteristics of the mixes [2]. Mix ID

Binder

FAB-1

75% F, 25% G 75% F, 25%G 75% F, 25% G 0% F, 100% G 25% F, 75% G 50% F, 50%G OPC OPC OPC

FAB-2 FAB-3 GGB-1 GGB-2 GGB-3 CC1 CC2 CC3

σ

cu, MPa

σ ft, MPa

Ec, GPa

σ ft, MPa (IS-456)

Ec, GPa (IS-456)

Ec, GPa (ACI-318)

17

2.35

11.2

2.07

14.79

14.7

49

4.65

20.8

4.47

31.92

25.0

52

4.81

22.4

4.63

33.07

25.8

63

5.53

28.3

5.18

37.00

28.4

57

4.84

26.5

4.89

34.91

27.0

52

4.86

22.7

4.63

33.07

25.8

35 41 52

4.03 4.32 4.85

3.62 4.01 4.63

25.86 28.61 33.07

24.9 26.9 30.3

σcu, Compressive strength; σspt, split tensile strength; σft, flexural tensile strength; Ec, elastic modulus.

from 3216 to 4516 μm/m for GPCCs, which is higher than that for CCs (around 2700 microstrains). The strain at failure ranged up to 6000 μm/m.

1.3.3 Geopolymer cement concrete building blocks and paver blocks With the scarcity in availability of fired clay bricks, concrete building blocks and pavers are the most widely used concrete components other than structural concrete [17]. Therefore the use of eco-friendly GPCCs in lieu of OPCCs for the production of building blocks is an attractive proposal. Table 1.3 shows the engineering properties of some of the paver blocks with indigenous materials, the GPCC-based building blocks and pavers are feasible on a large scale and using the same tools and plants as OPCC elements, and these blocks meet the relevant performance requirements. This technology was released by CSIR-SERC to AEON Construction Products Ltd., Chennai, in 2008
09 [18].

1.4

Fiber-reinforced concrete

1.4.1 Introduction It is well realized that concrete is essentially considered quasi-brittle or nearly brittle. This brittleness can be significantly reduced by adding fibers to the concrete mix. Historically, different materials were introduced as fibers in the mix such as

Table 1.3 Engineering properties of GPCC building/paver blocks. ID

GB1 GB2 GB3 GB4 GB5 FB1 FB2 FB3 FB4 FB5 LWG LWF LWC

Average value of

Suitable application

σ cu (MPa)

σspt (MPa)

SD (MPa)

18.2 36.4 57.2 58.0 53.8 22.6 18.3 26.3 28.8 27.2 23.2 20.7 19.9

4.85 6.33 8.15 6.14 5.44 3.77 3.66 4.14 5.14 4.76 -

2.2 4.3 4.9 4.9 4.3 2.8 3.3 4.1 4.6 4.4 4.6 4.4 2.0

Building block Paver block

Building block

σcu, Compressive strength; σspt, split tensile strength; SD, standard deviation. a IS: 1185 -Part I(C & M 8). b IS:1185-Part 2 (C & M 9), 1 IS:15658 (C & M 19).

Grade designation as per IS code

Grade Aa M-30 1 M-50 1 M-50 1 M-50 1 Grade Aa Grade Aa Grade Aa Grade Aa Grade Aa Grade Ab Grade Ab Grade Ab

Average value of Water absorption (%)

SD (%)

3.3 2.4 1.2 0.7 1.4 4.3 4.9 4.0 3.7 3.1 5.3 5.8 4.3

1.0 0.47 0.29 0.23 0.8 1.0 2.5 1.4 1.6 1.9 1.5 1.6 1.1

18

New Materials in Civil Engineering

steel, natural glass, polypropylene, asbestos, carbon, and polymeric fiber (kevlar, aramid). Fibers with suitable aspect ratio of length to diameter and volume fraction are introduced during the mixing process. Another advantage of the use of fibers is that they act as a secondary reinforcement, thereby arresting cracking due to shrinkage and improving the energy absorption capacity of the concrete. This makes the product durable. Road pavements, industrial flooring, machine foundations, etc. are some of the typical applications of fiber-reinforced concrete [2].

1.4.2 Steel fiber-reinforced concrete Typically steel fibers are added to the concrete matrix in the range of 0.25%
2% by volume of concrete [2]. It has been noticed that if the fiber content is increased beyond this range both the strength and workability of the concrete reduce drastically. Generally, a fiber dosage of 0.5%
1% is found to perform very satisfactorily. Again this depends on the type of fibers used and their sizes/dimensions. Fibers are available in a variety of shapes; straight, crimped, hooked, etc., and the length of the fibers can vary from 19 to 60 mm, fibers of length less than 19 mm are also available and are generally referred to as microfibers. They are normally not used in general concreting, however they find good application in UHSCs such as reactive powder concrete (RPC). The shapes of fibers used are (1) straight, (2) crimped, (3) hooked, (4) deformed, and (5) glued. The geometry of fibers used can be circular, square, or rectangular. Some of the important features of SFRC are: G

G

G

G

G

G

G

the weight density of concrete increases with the increase in the steel fiber content; slump will decrease at a higher percentage of steel fiber and lower SF; the workability of concrete improves when SF percentage increases; the compressive strength increases significantly due to the addition of SF compared with normal concrete; the split tensile strength increases significantly due to the addition of steel fibers; the flexural strength increases significantly due to the addition of steel fibers; as the percentage of steel fibers increases, the percentage of tensile strength and flexural strength properties increase more than the compressive strength.

In addition to the above, properties of FRC can be enhanced due to: 1. 2. 3. 4. 5.

aspect ratio; volume fraction; fiber profile; fiber efficiency factor; and strength of matrix.

Thus, one can note that use of fiber can result in changes to the basic stress
strain characteristics. Further, it also turns out that while the slope of the stress
strain curve in the linear range will be similar or close to that of NSC, the slope in the downward portion, that is, after reaching the fully elastic linear portion, is significantly different. This can be seen from Fig. 1.2A where hooked fibers are used and Fig. 1.2B where straight fibers are used. In both cases, the results are shown for

An overview of cementitious construction materials

19

Control Axial strain (A) hooked steel fibers

Smooth steel fibers Compressive stress

Compressive stress

Smooth steel fibers

Control Axial strain (B) straight steel fibers

Figure 1.2 Stress
strain response of FRC under compression [14]: (A) hooked steel fibers; (B) straight steel fibers.

different volume fractions of the fiber content against the control concrete [14]. Obviously, a higher fiber content in the mix makes it more ductile and improves the toughness as there is an increase in strain at the peak stress. This results in a strength improvement from 0% to 15% [14]. Direct tensile testing as carried out for metals, is not possible to be adopted for quasi-brittle materials like concrete and FRC. Presently, there are no guidelines or established standards for such tests. In view of this, a number of test schemes have been attempted to conduct direct and indirect tensile tests. Similar to tensile tests, flexural toughness is another parameter SFRC needs to be assessed. However, in the case of flexural toughness there are established standards and guidelines: ASTM C-1018, JCI-SF4, JSCE-S4, and ACI 544 are some that can be referred to. The third parameter to assess the performance and behavior of SFRC is the impact resistance. This determines the utility and range of applications of SFRC. Again, testing procedures for impact resistance of SFRC members need to be established, though considerable research has been reported. The tests involve investigation into crushing, shear failure, and tensile fracturing. Charpy-type impact test, weighted pendulum, drop-weight test, rotating impact test, blast impact test, projectile impact test, and instrumented impact test are some of the tests used to investigate impact resistance. The addition of fibers improves the impact resistance of the plain concrete. However, as already mentioned, the amount and the type of fibers used will determine the extent of improvement. From the various studies conducted, it is noted that [2,4,19]: G

G

G

G

G

fibers near the surface corroded causing brown stains but the strength and toughness characteristics of SFRC were not affected; the SFRC shows better performance in beam column joints with enhancement of strength by about 20%; SFRC as shear reinforcement substitute has no effect on the shear capacity of joints; the joints with SFRC provided better confinement of the concrete, showing less structural damage of the joint both under static and cyclic loading; in the case of exterior beam
column joints, addition of 1% steel fibers in the joint portion spacing of stirrups at the beam
column joint can be increased by twice the normal rate;

20

G

G

New Materials in Civil Engineering

addition of fibers, even in a small quantity, considerably improves the impact resistance of concrete; with an adequate fiber volume, the failure mode under repeated impact loading is transformed from brittle failure to multiple cracking, concrete crushing, and disintegration.

1.4.3 Fiber-reinforced concrete with nonmetallic fibers Nonmetallic fibers can be either natural or synthetic [2]. Coconut husk, sisal, sugar cane bagasse, bamboo, akara, plantain, and musamba are some of the natural fibers used in cement paste, mortar, and concrete. Glass, polypropylene, carbon, polymeric fiber, hybrid fiber-reinforced concrete, etc. are some of the examples of synthetic fibers. A number of studies have been conducted to assess the performance of the use of such fibers in concrete in terms of compressive strength, flexural strength, etc. The results have been encouraging but, as of now, the applicability of each of these is limited to specific applications. Durability, sustainability, and reliable composition are the key parameters to assess the performance to evolve suitable test procedures, validation methods, and guidelines.

1.4.4 Applications of steel fiber-reinforced concrete SFRC is generally used in repairs of abrasion, cavitation, or impact damage in various components of structures and in new construction of some products. For new constructions, these are presented in the following. Precast products: There are a good number of manhole covers every kilometer of road. These are manhole covers with frames that are needed to cover chambers and there can be anywhere between 5
20 every kilometer. Cast iron manhole covers have been preferred in the past for such applications. However, cast iron covers and frames/rings are susceptible to pilferage and work out costlier. They can crack or break as the material is brittle. As a reliable and cost-effective substitute, SFRC manhole covers with frames (Fig. 1.3) are employed for such applications. These covers and frames are about 60% cheaper than cast iron ones. Having established through a number of studies [2,4,19] for its ductility and high impact resistance, SFRC is perfectly suitable for such applications. CSIR-Structural Engineering Research Centre, Chennai (CSIR-SERC) [2] (https://www.serc.res.in) transferred the technology on manhole covers and frames to more than 40 agencies in the country. Based on the intensity of vehicular traffic, these are produced as heavy-, medium-, and light-duty. Similarly, SFRC has good potential for use in other precast concrete products such as lost forms, dolosses, and wall panels. Another CSIR laboratory in India, CSIR-Central Building Research Institute (CBRI), Roorkee, developed and transferred technologies to produce different building components, such as, precast doubly curved roofing tiles (1000 3 1000 3 20 mm and 700 3 700 3 20 mm), precast lintels (120 3 230 3 75 mm), and precast planks (1200 3 400 3 25 or 50 mm) using steel as well as vegetable fibers. In the early 1980s, another product, namely, corrugated roofing sheets, made out of coconut fiberreinforced concrete was used in a major leprosy settlement in a village near Titilagarh

An overview of cementitious construction materials

21

Figure 1.3 SFRC manhole covers and frames.

in Orissa, India. There is no report of any damage till today, and after several years this is evidence of its reliable and sustainable performance. In the neighboring state of Andhra Pradesh, similar FRC roofing is also now being used in a number of villages. SFRC for pavements and industrial floors: Cement concrete cannot provide adequate wear resistance and quality/strength of concrete against impact, abrasion, etc. needed for industrial floors. Further, pavements and industrial paved floors are often in aggressive environments. In the changing scenario of industries where material-handling equipment/machines/forklift trucks are deployed in high numbers with frequent use, and use of robots in production, SFRC meets these stringent requirements in full. SFRC is able to generate a finish that is very flat and provides a smooth surface. SFRC brings with it naturally several advantages and provides an ideal solution as a replacement for plain concrete for the applications mentioned in above, as listed here: 1. With the higher flexural strength of SFRC, it is observed that one can obtain a reduction in the thickness of concrete floors of up to 30%, while at the same time increasing the spacing of contraction joints by up to 50%; 2. Higher tensile strength minimizes shrinkage and warping cracks that might occur due to thermal stresses; 3. Scaling in concrete is arrested due to higher abrasion resistance; 4. Because of its precrack and postcrack load-carrying capacities, it provides better resistance to the development and propagation of cracks originating from underlying pavement. This delayed propagation of cracks provides a two- to three-fold increase in the life of the overlay. The above features and advantages make SFRC perfect for providing overlays for pavements and industrial floors. There are also reported uses of SFRC in heavy vehicle factories, boiler plants, and thermal power plants, where very heavy machinery and tools are moved on tracked vehicles.

22

New Materials in Civil Engineering

One of the other important applications of SFRC is shotcrete, popularly known as “steel fiber-reinforced shotcrete” (SFRS). SFRS is mortar or concrete containing discontinuous discrete fibers that is pneumatically projected at high velocity on to a surface. This process of shotcreting enhances its mechanical properties/attributes significantly. Some of these are (1) flexural strength, (2) shear strength, (3) durability, (4) ductility, (5) high fatigue and impact resistance, and (6) toughness. The advantages of use of SFRS are: (1) increased load-bearing capacity, (2) homogeneously reinforced shotcrete layer, (3) increased crack control, (4) less energy absorption, and (5) easy and simple to use. As stated here, increased crack control allows, even after cracking, the material to continue to carry a higher load. This permits failure to take place only after considerable deformation. The properties and advantages mentioned in the above facilitate efficient design of members, resulting in thinner or lighter sections. Substituting mesh reinforcement by steel fibers, SFRS can offer considerable time savings and makes it less labor intensive. Thus, there is no overconsumption of concrete as compared to traditional reinforcement. A reinforced shotcrete lining can be applied immediately after excavation for immediate safety.

1.4.5 Slurry infiltrated fibrous concrete Slurry infiltrated fibrous concrete (SIFCON) is a special type of fiber-reinforced concrete and relatively recent material [2]. In FRC, 1%
2% by volume fibers are used, whereas in SIFCON between 6% and 15% fibers are used. The constituents (Fig. 1.4) used to produce SIFCON are (1) cement paste or flowing cement mortar, (2) sand (it is preferable to sieve it through a 1.18-mm sieve), (3) FA or slag or SF, (4) SP, and (5) water. Because of the high fiber content, cement slurry needs to be infiltrated into a bed of preplaced fibers. SFRC can be successfully employed to

Figure 1.4 Constituents of SIFCON.

An overview of cementitious construction materials

23

Figure 1.5 Stress
strain plot in compression for 8% fibers.

provide high impact resistance and high ductility where standard modes of reinforcement are not effective, such as precast concrete products, refractory applications, pavements and overlays and bridge decks, strategic applications, and structures subjected to blast and dynamic loading. Investigations [2] considering two mix proportions of volume and cement to sand ratio (1:1 and 1:1.5) and two w/c ratios (0.40 and 0.35) revealed that a mix proportion of 1:1 with a water
cement ratio of 0.35 and polycarboxilic-based SP and viscositymodifying agent (VMA) exhibited better performance in terms of compressive and split tensile strengths. Similarly, it was noted that with addition of 8% fibers, the compressive strength achieved was in the range of 70
80 MPa and the split tensile strength was found to be between 15 and 18 MPa. The aspect ratio of straight and crimped fibers used in the study was 66, whereas that for the hooked fiber was 48. Fig. 1.5 is a typical stress
strain plot for the various types of fibers at 8% fiber volume.

1.5

Fiber-reinforced concrete polymer composites

1.5.1 Fiber-reinforced polymer composite laminates Fiber-reinforced polymer (FRP) composite materials are produced from three main fiber types, namely, carbon, glass, and aramid [2,7,19,20]. Each fiber has different engineering properties and, therefore, selection must be made to suit the requirements of a particular application. Carbon fibers have great strength. Stresses at

24

New Materials in Civil Engineering

failure can be in excess of 3000 N/mm2. However, they are very expensive. Conversely, glass fibers are relatively inexpensive but have less strength and greater elasticity. Aramid fibers, renowned for their high impact resistance, tread the middle ground and their true potential is yet to be exploited in structural engineering applications. The composite materials currently utilized for repair and structural applications are produced in the form of laminate or wrap. Laminates consist of groups of unidirectional fibers, referred to as rovings, which are pultruded through a bath of resin into a dye before being baked in an oven. The resulting product is usually between 1 and 2 mm thick with a width of less than 150 mm and can be coiled for transportation. Laminates and wraps are produced from either one or a combination of the fiber types. In the plane of the material, it can be woven with the fibers oriented in almost any direction and with different percentages allocated to the wrap and weft. The resulting fabric is then fully impregnated with resin. Figs. 1.6 and 1.7 show the carbon fiber mat and typical execution procedure for CFRP mat used for repair and strengthening of RC structural elements. Despite its higher cost than steel, the ultralightweight and durability of FRP makes it one of the most preferred material/sheets because of its reliability in the use of distressed infrastructure. Some of the features/advantages of this material are: G

G

durable; better fatigue life;

Carbon fiber mat

Figure 1.6 Carbon fiber mat roll.

An overview of cementitious construction materials

25

Protective coating 2nd resin coat Carbon fiber 1st resin coat Epoxy putty filler Primer Concrete substrate

Figure 1.7 Typical sketch showing repair and retrofitting steps.

G

G

G

G

G

G

corrosion resistance; resistance to chemical attack; high strength-to-weight ratios; easy to handle on site; sufficiently pliable to fit almost any shape and size of structure; reduced labor cost.

1.6

Lightweight concrete

1.6.1 Introduction Lightweight concrete contains an expanding agent that increases the volume of the mix due to which the concrete becomes nailable and has low density or lower dead weight. These qualities allow faster construction, with lower handling costs. This suits high-rise buildings and large infrastructures, as the total dead load on foundations is substantially reduced. Because of its relatively low thermal conductivity, it facilitates maintaining comfortable conditions in buildings. Lightweight concretes allows use of industrial wastes such as clinker, FA, and blast furnace slag in large quantities. Finally, the traditional concrete materials, such as sand and coarse aggregates, are becoming very scarce and thus lightweight aggregate concretes could contribute immensely to consuming less of these depleting natural resources [2]. Lightweight concrete has the ability to hold its large voids without the formation of cement films when placed on a wall. However, sufficient water
cement ratio is essential to produce adequate cohesion between the cement and water to maintain

26

New Materials in Civil Engineering

good strength of concrete. Otherwise, too much water can cause cement to run off aggregate to form laitance layers, leading to weakening of concrete strength [15,16].

1.6.1.1 Types of lightweight concrete Concretes can be produced with different densities of concretes varying from 300 to 2000 kg/m3 and can be produced with corresponding (1) compressive strength from 1 to 60 MPa and (2) thermal conductivities of 0.2
1.0 W/mk. Concrete can be made lighter by adding air in its composition. This can be done in the following three ways [2]: 1. By introducing very fine bubbles of gas in a cement paste or mortar mix to form a cellular structure containing approximately 30%
50% voids (aerated concrete and foamed concrete); 2. By replacing either wholly or partially natural gravel or crushed aggregates in a conventional mix with aggregates containing a large portion of voids (lightweight aggregate concrete); 3. By omitting the finer fraction from normal weight aggregate grading to create air-filled voids called no-fines concrete.

1.6.2 Foam concrete/cellular concrete Foam or foamed concrete is also known as cellular concrete [2,15,16]. This is a versatile material principally comprising a cement-based mortar or paste mixed with at least 20% by volume of air in the form of preformed foam [2,15,16]. The material generally contains no coarse aggregates. As the name suggests, it requires a foaming agent. Hydrolyzed protein-based concentrated liquid that does not chemically react with cement is used as a foaming agent in producing the foam. The foam serves as a temporary wrapping material for the air bubbles till the cement mortar develops its own final set and strength. The mix can be designed with high volumes of industrial waste materials and recycled aggregates. In foamed concrete both strength and density are normally specified as the constituents and the proportions are flexibly used. This gives a wide band of densities and strengths for varied applications. Foam concretes with dry densities varying from 360 to 1550 kg/m3 and air contents from 28% to 78% showing strengths ranging from 1 to about 30 MPa have been reported [2]. Foamed concrete has been established as an accepted building material. It finds application in many areas due to its relatively low cost, light weight, ease of production, easy placement, simple compaction, etc. Some of the features offered by foam concrete are (1) flowability, (2) self-compactability and self-leveling nature, (3) ultra-low density, (4) excellent thermal and sound insulation properties, (5) dimensional stability, (6) economical, and (7) eco-friendly.

1.6.2.1 Applications of foamed concrete Internationally, the material has been used in low-cost housing in the Middle East, where its good thermal insulation, ease of placing, and relatively nondemanding

An overview of cementitious construction materials

27

technical input are beneficial. It is used in a variety of applications such as void filling, floor construction, bridge decks, roofing insulation, road sub-base, sewer infill, swimming pool infill, raising the levels of flooring, underfloor infilling, train platform infilling/reprofiling, floor and roof screeds, wall casting, complete house casting, sound barrier walls, subsurface for sport arenas, aircraft arresting beds, road crash barriers, floating barges, jetty platforms and floating homes, trench reinstatement, storm drain infilling, bridge strengthening, culvert abandonment filling, culvert or bridge approaches, subway abandonment filling, large-diameter shafts, tunnel abandonment, bridge abutments, slope protection, basement infill vaults, pipeline infill, tank infill, and fuel tank infill [2].

1.6.2.2 Material constituents Foamed concrete is a blend of cement, sand, water, and prefoamed foam with the vast majority of foamed concrete containing no large aggregates but only fine sand (Fig. 1.8) [4]. The extremely lightweight foamed concrete contains only cement, water, and foam. The raw materials used for the production of foam concrete are binding agent, aggregates, foaming agent, and water. The OPC is used with contents varying from 300 to 600 kg/m3. In addition to OPC, rapid hardening PC, high alumina cements can be used to reduce the setting times and improve early strengths. Partial cement replacements with FA, GGBS, and other fine materials can be made. SF can be added to improve the compressive strength of concrete. However, the compatibility of these admixtures with foaming agents should be ascertained. GGBS gives the foamed concrete a cohesive, almost sticky, consistency. The use of FA tends to make the mix fluidier. The key requirement here is to have stable foam. Only fine sands with particle sizes up to 5 mm are used, as coarse aggregate tends to settle in the lightweight mortar mix and causes collapse of the foam during mixing. Very low-density sand with a fineness modulus of approximately 1.5 are preferred, including FA, lime, calcium carbonate, crushed concrete granite dust, expanded

Figure 1.8 Materials used for foam concrete.

28

New Materials in Civil Engineering

polystyrene granules, sintered FA aggregate fines, rubber crumbs, recycled glass, and foundry sand. Lightweight aggregates such as sintered FA aggregate and vermiculite can also be used to produce foamed concrete. The preformed foam is a mixture of foaming agent, water, and air, with a density of 75 kg/m3. The addition of preformed foam lowers the density of the mix, increasing the yield. The higher the quantity of foam added, the lighter the resultant material. Two types of foam, wet foam and dry foam, are used in the production of foamed concrete. Wet foam is produced by spraying foaming agent solution and water over a fine mesh. The foam produced in this case is similar in appearance to bubble bath foam, with a bubble size ranging from 2 to 5 mm. However, the foam that is added must remain stable without collapsing during pumping, placement, and curing. This factor becomes prominent when the quantity of foam is greater than 50% of the base mix (that is, for a density of approximately 1100 kg/m3). Foamed concrete below this density needs to be manufactured and used with care. The water
cement ratio typically ranges from 0.4 to 0.8, depending on the mix proportions and consistency requirements. When extremely fine materials are used in large quantities the water demand increases, lowering the strength of foamed concrete. Chemical admixtures such as SPs, VMAs, and accelerators can be used in foamed concrete, however their effect on the stability of the foam should be ensured. Addition of fibers such as polypropylene and polyester fibers can be used to limit both plastic and drying shrinkage strains. The constituents of the base mix can react with certain foaming chemicals resulting in destabilization of the mix.

1.6.2.3 Mix proportioning of foamed concrete There is currently no standard or accepted method for designing a foamed concrete mix. However, foamed concrete is specified by its strength and density. Similar to normal concretes, the higher the air content in the mix, the weaker the resultant material. That is, the lower the density of the concrete, the less its strength. In addition to the water
cement ratio, the volume of voids is an important factor that decides the strength of concrete. Strength will also be controlled by cement and fine aggregate content. Unlike in normal concretes, strength is achieved purely by cementing action, rather than by consolidation and mechanical interlocking of aggregate particles. To design foamed concrete, through selection/assignment of casting density, sand/cement and FA/cement ratios, the water requirement is determined. Using these ratios and the relative densities of the materials, the mass of the cement and the volume of foam that should be added to obtain the required density is determined. Using the following equation, the foam quantity in the mix is calculated by adding the mix quantities (per m3) to the target plastic density value [9,10]: Fm 5 Bm 3 Fd ½1=Td 2 1=Bd

An overview of cementitious construction materials

29

where Fm 5 Mass of foam, kg (this may be converted in volume using foam density, typically 40
60 kg/m3); Bm 5 base mix mass, kg; Fd 5 foam density, kg/m3; Td 5 target density, kg/m3; Bd 5 base mix density, kg/m3 (this varies greatly depending on the aggregate type used).

The sum of the material weights equal to the required casting density will produce 1 m3 of foamed concrete and the sum of the volume of all the constituent materials should be 1 m3 or 1000 L. The flow table test can be used to determine the water demand of cement or a mixture of cement and FA. The mix should not absorb water from the foam. No visual breakdown of the foam should take place. The higher the water content in the mix the less the density.

1.6.2.4 Strength ranges The British Cement Association reported on work on a range of mixes with dry densities varying from 360 to 1550 kg/m3 and air contents from 28% to 78.5%, with strengths ranging from 1 to about 10 MPa. Concretes with densities at the upper limit can produce roughly strengths in excess of 15 MPa.

1.6.2.5 Characteristics of foamed concrete The characteristics of foamed concrete are generally constant across a range of mix designs such as: G

G

G

G

G

G

G

G

G

G

high strength to weight ratio; low coefficient of permeability; low water absorption; good freeze and thaw resistance; high modulus of elasticity (compared to soils); a rigid well-bonded body; low shrinkage; thermal insulating properties; shock-absorbing qualities; not susceptible to breakdown due to hydrocarbons, bacteria, or fungi.

1.6.2.6 Experimental investigations The following section details the investigations carried out to develop foamed concrete having a density of approximately 1000 kg/m3 for structural applications. A pilot study undertaken to develop foamed concrete panels to be used as infill for precast roof and floor systems is also described in detail.

30

New Materials in Civil Engineering

1.6.2.6.1 Ingredients The ingredients used in the study to develop foamed concrete are as follows: OPC (53 grade), sand (river), FA (class F), and foam produced using foaming agent (Rheocell 30).

1.6.2.6.2 Production of foam A compact portable foam generator called a foam gun was used to produce a constant supply of foam during casting. The equipment consists of a 0.5 HP pump and motor which ensured supply of water at a pressure of approximately 1.5
2 kg/cm2. A pressure gauge at the inlet pipe indicated the water pressure. Sufficient quantity of foaming agent was ensured in the receptacle before start of the operation. The water flowing picks up the required foaming agent to produce form foam of a density of 40 kg/m3 coming out of the pipe of larger diameter. The average bubble size was about 3
5 mm.

1.6.2.6.3 Details of mix The mix proportions were arrived at for a target density of 1000 kg/m3. To achieve a cost-effective foamed concrete, use of a minimum cement content was ensured. The proportions of the ingredients were as follows: cement: FA:sand 5 1:1:1 by weight and a w/c 5 0.50. The mass of cement, FA, sand, water, and foam per m3 were computed by equating the mass of ingredients to the wet density. From the specific gravities of the individual ingredients the volumetric quantities were arrived at. The volume of foam that is required to produce a foamed concrete density of 1000 kg/m3 was also arrived at. The proportions arrived at for 1 m3 (1000 L) of concrete are as given below. For assumed wet density of foamed concrete 5 1000 kg/m3 Cement 5 400 kg Foam 5 500 L (0.5 m3) Foam: base mix 5 50:50 by volume

Sand 5 400 kg Water 5 200 kg FA 5 can replace up to 60%a (refer below)

a

Partial replacement of cement/sand with FA (class F) exhibits pozzolanic properties. It was observed that FA can replace PC up to 60% with an improvement in strength compared with conventional foamed concrete. Also, the addition of FA leads to better cohesion of the fresh mixture and lowers early plastic shrinkage deformation. The consistency of the wet mix is very important before the addition of foam. Too dry a consistency results in breakage of foam due to absorption of water from the foam, while too wet a consistency results in segregation of the mix.

1.6.2.6.4 Mixing, casting, and placing procedures The ingredients were weighed. Tilting type mixer was used. First the sand, cement, and FA were dry mixed for a few minutes to obtain a homogeneous mixture. Water was added and thoroughly mixed. The foam was next added to the wet slurry mortar. The mix had good consistency after addition of foam. The higher consistency was of great advantage. It flows by itself into the molds and cavities, thereby doing away with the necessity of compaction or vibration. The flow of the concrete was measured using a marsh cone. The high stability of foam prevented reduction of mix volume during mixing, pouring, placing, and the hardening process. Normal water curing was adopted on demolding the specimens after 24 hours. After mixing was completed the wet density of the foamed concrete was measured. The dry density (oven dried) of foamed concrete is normally less than its wet density,

An overview of cementitious construction materials

31

depending on the w/c ratio, density, and also the cellular structure obtained. The target value of wet density should be much higher than the required dry density. Precast roofs, floors, and walls of single- and multistoried buildings offer the highest scope for affecting the economy both in terms of cost and consumption of materials. An economical floor and wall system needs to be economical both in terms of initial cost and recurring maintenance. Ideally it shall be comfortable, durable, and maintenance-free. The major advantages of replacing conventional in situ concrete construction of roofs and floors by precast concrete construction are: G

G

G

possibility to adopt efficient structural forms; better quality of workmanship due to adoption of systematic production techniques involving skilled labor and constant supervision; and savings in building materials by replacing scarce materials with locally available lower cost, substitute materials.

Further, mass housing programs in earthquake-prone zones are highly advantageous if the roof and floor systems are of lightweight precast concrete elements, which can reduce the inertia forces, thus making it seismic resistant.

1.6.3 No-fines concrete No-fines concrete is another variation of lightweight concrete composed of cement and fine aggregate. The main characteristic of this type of lightweight concrete is that it maintains in the whole mass uniformly distributed large voids. It is used for load-bearing walls, nonload-bearing walls, infilled walls for framed structures, fillings, and roof screeds. The mix commonly used is 1:8 by volume with a water
cement ratio of 0.40 and densities range between 800
1400 kg/m3. The cement slurry is first prepared and added to the conventional coarse aggregates in a saturated condition so that each particle of coarse aggregate gets coated with a layer of cement paste and the aggregates are bonded to each other to leave interstitial voids. The voids are interconnected and produce a porous open-textured concrete. The density of concrete depends on the type and grading of the aggregates. For lower densities single-sized aggregates can be used. Aggregate sizes generally used are 10
20 mm but sizes can range from 7 to 75 mm. No particles less than 5 mm should be used, similarly no flaky or elongated particles are used. The aggregates should be clean to allow good cohesion with the cement paste. To obtain densities ranging from 1200 to 1900 kg/m3 gravel or crushed aggregates without sharp edges can be used. With lightweight aggregates, densities ranging from 800 to 1400 kg/m3 can be obtained. All types of cements can be used for making no-fines concrete. However, curing of concrete must be given particular attention, as longer curing periods may be required for blended cements.

1.6.4 Lightweight aggregate concrete Lightweight aggregate concrete is made with lightweight aggregates, either natural or manufactured, comprising gravel or crushed stone. Therefore it has substantially lower bulk density than concrete. Further, as many types of aggregates can be used,

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New Materials in Civil Engineering

one can design concretes of required densities and strengths. This can be from lowdensity concretes, structural lightweight concretes, to moderate-strength lightweight concretes. These are discussed in the following.

1.6.4.1 Low-density concretes and associated aggregates Low-density concretes are generally used for insulation as they have high thermal insulation values. They have a density of 800 kg/m3 or lower. Being of low density, they have a low compressive strength between 0.7 and 7.0 MPa. Vermiculite and perlite are commonly used aggregates. They are known to have bulk density in the range from 96 to 192 kg/m3. During a rise in temperature, with vermiculite, being a micaeous mineral, layers of combined water in the mica’s laminar structure are converted to steam. Due to this, successive layers are peeled off, leading to material disintegration. At the same time, perlite, being a volcanic glass, contains combined water making the internally generated steam expand violently.

1.6.4.1.1 Structural lightweight concrete and associated aggregates Structural lightweight concretes are produced using aggregates such as expanded slags [2]; sintering grate expanded shale, clay, or FA; and rotary kiln expanded shale, clay, or slate. These aggregates produce concretes that have densities ranging from 1360 to 1920 kg/m3 and minimum compressive strengths of 17.0 MPa. Although the insulating efficiency is lower than that of low-density concretes, it is still higher than that of normal-weight concretes. Sintering can produce either crushed or pelletized aggregates. Crushed aggregates contain organic matter that can serve as fuel. It can also be mixed with fuel such as finely ground coal or coke. The premoistened raw materials are burnt. This leads to production or release of gases causing expansion. This forms clinker which is then cooled, crushed, and screened to the required gradation. However, there is no control of the shape and therefore this produces clinkers that can be sharp and angular with a porous surface texture. In order to get pelletized aggregates, first mixing of clay, pulverized shale, or FA with water and fuel is done. This mix is then pelletized or extruded and burnt. In this case, there is good control of the shape that provides generally spherical or cylindrical aggregate particles.

1.6.4.1.2 Moderate-strength lightweight concrete and associated aggregates Moderate-strength lightweight concretes are made from pumice which is spongy lava, or scoria aggregate which is volcanic cinder [2]. As a result of a rise in temperature, steam or gas escapes from the lava while pumice is hot, creating tube-like, interconnected void pores. Scoria has a pore structure and possesses isolated voids. These concretes have a density and strength approximately midway between those of low-density and structural concretes. They are also called fill concrete. As per ACI 213R-87, typical ranges of densities of concretes made with various lightweight aggregates are illustrated in Fig. 1.9.

An overview of cementitious construction materials

Low density concrete

33

Moderate strength concrete

Structural concrete Expanded slag Sintering grate expanded shale, clay or flyash Rotary kiln expanded shale, clay and slate

Scoria Pumice Perlite Vermiculite kg /m3

pcf

400 20

600 40

800

1000

1200

1400

60 80 28-Day air dry unit weight

160

1800

100

kg /m3 120 pcf

Figure 1.9 Broad distribution of lightweight aggregates (ACI 213R-87).

1.6.5 Textile-reinforced concrete/FabCrete Upgrading reinforced concrete structural components with cement-based bonding agents and nonmetallic high-performance fibers gives a more compatible repair or strengthening system. In this context, textile/fabric-reinforced concrete (TRC), which is also called FabCrete [21], overlays are applied on the tension face of the elements to be strengthened using a hand lay-up method, to increase the flexural resistance of reinforced concrete beams. The strengthening technique using TRC consists of a fine-grained, HSC as the matrix and bonding agent and the reinforcing textile either of carbon, polypara-phenylene benzobisoxazole (PBO), or AR-glass [22,23] as reinforcement. The TRC strengthening method maximizes the use of the favorable properties of the fiber material in textile, such as high strength-to-weight ratio, corrosion resistance, and flexibility. It also helps to overcome the constraints caused by organic resin binders while using high temperatures and damp surfaces. Several researchers have investigated the increase of flexural resistance of RC beams while strengthening with TRC overlays [22,24]. However, there are varying results reported with respect to the efficiency of TRC systems in such applications, which depends on the performance of binder, textile, and composite as a whole. It is found that the ultimate behavior is also limited by the bonding performance between the matrix and the concrete substrate [24]. Some of these studies have confirmed that using a hand lay-up method for TRC overlays limits the maximum strengthening effect achievable in a TRC system. Both delamination and slippage lead to underutilization of the full potential of textile and neither the ultimate tensile strength nor rupture is attained for the textile in the TRC system. In most of the textiles, the waviness due to the production method leads to a dependence on the

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New Materials in Civil Engineering

placement of textile layers, which also delays the activation of textile during composite performance. In this context, Zhou et al. [25] reported considerable enhancement in the flexural capacity of RC beams strengthened with precast and prestressed carbon fabric-reinforced cementitious plate compared to nonprestressed TRC strengthening. With the realization of such possible improvements for TRC, more investigations are needed to realize the utilization of textiles toward achieving the desired reinforcing targets than conventionally used TRC strengthening methods.

1.6.5.1 Textile-reinforced concrete characteristics TRC [21] consists of a fine-grained cementitious binder and alkali-resistant glass textiles. The significance of prestressing the textile for its better utilization is demonstrated by conducting a tensile test. Based on the advantages realized by prestressing the textile, Refs. [21,26
29] illustrate the suitability of the proposed method for achieving enhanced performance of RC beams while strengthening the same with TRC. A fine-grained cementitious binder, consisting of PC (578 kg/m3), FA (206 kg/ 3 m ), SF (41 kg/m3), quartz sand (589 kg/m3), quartz powder (QP) (354 kg/m3), water (330 kg/m3), and polycarboxylate-based SP was used in TRC. The flows measured using minislump apparatus have an initial value of more than 150% and 80% after 1 hour. The cube compressive strength of the mix is 44.5 MPa ( 6 4.2%). The glass textile, which is used as reinforcement is alkali-resistant mesh type reinforcement, with a mesh size of 25 3 25 mm. Uniaxial tensile characterization was carried out on the textile specimens of 500 mm length and 60 mm width. It is observed that the maximum load-carrying capacity of textile per unit width is around 45 kN/m and a slack was observed in the initial textile response (see textile alone response in Fig. 1.10). Further details about textile characterization can be seen elsewhere [21]. Studies indicated that a certain amount of tensile force is needed to straighten the yarns while casting TRC for achieving better composite action. In the studies reported [21,26
30], a prestressing/mechanical stretching was provided to textiles while casting TRC. Accordingly, to determine the textile contribution in TRC, rectangular specimens of 500 (length) 3 60 (width) 3 8 mm (thickness) with the mechanically stretched textiles were cast and tested. The details about the mechanical stretching methodology and test are reported by Gopinath [21]. The comparison of results with that of TRC with nonprestressed textiles is illustrated in Fig. 1.10, where TRC specimens had three and four layers of textiles placed in the mold without applying any mechanical force during casting of the specimen and, in others, a mechanical force was applied to the textile layers using a specially designed apparatus during casting. Based on the load versus displacement behavior, the nominal stress for the textile was obtained following the procedures mentioned in ACI 549 [31] by dividing the load by the textile reinforcement crosssection area of 33.58 mm2/m. The stress versus strain of three- and four-layer reinforced prestressed and nonprestressed textiles in TRC is shown in Fig. 1.10. In

An overview of cementitious construction materials

35 3a prestressed 4a prestressed 3a non prestressed 4a non prestressed Textile alone

1600 1400

Stress (MPa)

1200 1000 800 600 400 200 0 0

0.5

1

1.5

Strain (%)

Figure 1.10 Typical stress
strain for textiles.

addition, the strain was obtained by dividing the displacement of LVDT with a gauge length of 350 mm. Responses have also been superimposed with various textile behaviors in TRC obtained from a uniaxial test (see Fig. 1.10). When TRC is cast without giving any mechanical stretching (nonprestressed) to the textile, it is noticed that the slope of multiple cracking and stabilized state is parallel to the textile slope, as shown in Fig. 1.10. However, the peak strain needs to be obtained, since the textiles in TRC are not elongating till the failure strain in the textile is reached. When textiles are prestressed/mechanically stretched, the slope of textile behavior in the multiple cracking state is parallel to the slope of the textile. However, once the TRC moves to a stabilized state, the slopes of bare textile and that of textile in TRC are not parallel. It is noticed that there is a particular stain (0.8%), where the stress in bare textile coincided with that of the stress experienced by textile in TRC. This indicates that, until this point, the full potential of the textile is used and beyond which only the elongating ability of the textile is predominantly utilized. Beyond 0.8% strain, the stiffness of textile in TRC is lower compared to that of bare textile, indicating that there is a deficit of stiffness caused by the premature failure of a certain portion of filaments and by premature debonding of core filaments. This was further confirmed using X-ray CT analysis, which is explained in the following section. From the investigations conducted [21,26
29], it is reported that the tensile strength of bare textile is higher compared to that of textile in TRC in both prestressed and nonprestressed cases. This is because textile yarns and also their placement are highly nonhomogeneous and therefore they produce partially discontinuous stress distribution in a yarn in TRC combined. This illustrates the low ductility of single filaments. The TRC with prestressed textile is found to experience

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New Materials in Civil Engineering

more stress (about 60%) compared to TRC with nonprestressed textile. This indicates that prestressing can enhance the composite performance and lead to better utilization of textiles in TRC. The ultimate stress of bare textile is around 1400 MPa. Textiles when prestressed show ultimate stress around 900 MPa can be experienced, whereas in the case of nonprestressed TRC, the maximum stress experienced is only 400 MPa, indicating the underutilization of textiles. To exploit the advantages offered by prestressed TRC, this concept can be upscaled for flexural strengthening of RC beams with TRC.

1.6.5.2 Debonding characteristics of textiles in textile-reinforced concrete Microcracking and crack distribution are two main internal parameters that result in pseudo-ductility of TRC [31], which has relevance in structural strengthening with TRC. The TRC strengthened beam displays a failure mechanism governed by interlaminar shear, at the same time exhibiting pseudo-ductility [32]. The loss of strengthening action is mainly due to fiber debonding from the matrix interface [33]. If more layers of textiles are used in TRC, it is essential to ensure that all meshes are fully impregnated with the matrix system. This will ensure that not only the matrix shear strength is sufficient to transfer the shearing load between meshes, but also ensures that the bond strength between the substrate and TRC system is sufficient to transfer design forces. In the studies reported [21,26
30], samples cut from the TRC panel of 500 3 60 3 8 mm size that was untested and tested under uniaxial tension, a surface cut parallel to the weft direction in the specimen with two layers of textile was used for the investigations. Observations on an untested TRC specimen showed good bonding between the matrix and the textile yarns. In Fig. 1.11, textile with two yarns per line shows the space between the closely spaced yarns (of the same line) filled with binder. This reflects the efficiency of binder penetration and bonding between textile and matrix. Further, microscopical observations on a sample cut from a tested specimen under tensile load (see Fig. 1.12), indicated that the matrix retains its integrity under load. Generally, the failure modes [31] for TRC-strengthened section include (1) crushing of the concrete in compression before yielding of the reinforcing steel, (2) shear/tension delamination of the concrete cover or cover delamination, (3) debonding of the TRC from the concrete substrate (TRC debonding), and (4) interlaminar debonding. Among these, interlaminar debonding depends on the casting method or method of production of TRC and positioning of textile. Since prestressing of textile is a novel practice, X-ray CT analysis was carried out on the TRC specimen, which was cast for conducting a uniaxial tensile test to internally evaluate the binder penetration and the fiber slippage possibilities by increasing the number of layers to three, four, and six. Both tested and untested specimens used for a uniaxial tensile test were considered for the investigation. X-ray CT is a method of digitally visualizing a physical object to reveal its interior details. This can be used to reconstruct full three-dimensional images by

An overview of cementitious construction materials

37

Closely spaced warp yarns of a line Cementitious binder (white color) Yarns in warp direction (layer 1 of textile)

Weft yarn (layer 2 of textile)

Figure 1.11 Image of surface cut parallel to the weft yarns of glass textile in untested specimen.

1 mm

Warp yarns

Core filaments

Sleeve filaments

Figure 1.12 Image of surface cut parallel to the weft yarns of glass textile in tested specimen.

collecting two-dimensional slices. Specimens with varying number of layers (three, four, and six) of alkali-resistant glass textile were used to understand the interior details of TRC. Samples of size 60 mm length were cut from the TRC panel of 500 3 60 3 8 mm size that were untested and tested. The dimensions were chosen in such a way that at least two meshes were incorporated along the length of the specimen. Tomography scans on TRC samples were performed using a General Electric/Phoenix VtomexS CT system. A typical visualization of the X-ray CT image across thickness and along height is shown in Fig. 1.13. In X-ray CT, the denser the material, the more whitish the image will be; TRC glass is relatively dense and shows up more white. Further, the successive slices of textile stacked

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New Materials in Civil Engineering

Figure 1.13 X-ray CT 3D-image of a TRC specimen.

one over the other demonstrated that the stacked layers have gaps for the matrix penetration. Specimens with three, four, and six layers of textile (3ST, 4ST, and 6ST) were further used to understand the influence of a number of layers in TRC. Only typical images for the intersections between layers and binder where most of the damage was visualized in Fig. 1.13 are shown in Fig. 1.14. The debonding of the textile layer from the matrix is greater as the number of layers increases to six. Examining the specimens through the thickness (see Fig. 1.13) also indicates that as the number of layers increases (for 6ST), debonding is seen, compared to 3ST and 4ST. The tomography images in Figs. 1.14A
C show that the crack width reduces as the number of layers increases and the number of cracks per meter length also increases, indicating better distribution of forces between textile and matrix in the TRC system. Since a ductile behavior is anticipated for the TRC strengthened system in the present study, increasing the number of layers will lead to fiber slippage, leading to more ductile types of failure. Especially the fiber slippage is visible for the number of layers beyond four. These types of possibilities for incorporating a greater number of textile layers in TRC can be considered for flexibility or customization possibility for designing a TRC system to achieve a ductile failure while strengthening RC beams. Further prestressing of textile allows for more uniformity in binder penetration, leading to better bonding characteristics of textile in a TRC system. During casting of TRC, it is quite possible that the yarns in the textiles are bent and this induces initial internal stresses. This can be avoided by prestressing the textiles, where textiles are stretched and kept during TRC casting, which again leads to better efficiency of the TRC system. Placing a number of textile layers together also increases the resistance to pullout since load is easily distributed to the adjoining fibers which may be in contact.

An overview of cementitious construction materials

(A)

3ST

(D)

3ST

Cracks

39

(B)

4ST

(C)

6ST

(E)

4ST

(F)

6ST

Voids and debonding

Figure 1.14 Comparison of X-ray CT images TRC specimens with three, four, and six layers of textile.

Based on the realization of possible added advantages for the TRC system, by providing prestressing textiles, laboratory-level investigations were carried out to develop a method for strengthening of RC beams by placing a greater number of layers of textile in a prestressed way.

1.6.5.3 Summary The advantage of prestressing the textiles in TRC can be clearly observed by comparing the textile performance under uniaxial tension with that of TRC with nonprestressed textiles. The results, if correlated with X-ray CT images, confirm the debonding characteristics of textile in TRC with prestressed textiles by increasing the number of layers of textile. It is reported that TRC with prestressed textile is found to experience more stress (about 60%) compared to TRC with nonprestressed textile, leading to better utilization of textiles in TRC. Investigations carried out for flexural strengthening of uncracked and cracked RC beams strengthened with TRC by applying prestressing to the textiles show that TRC strengthened beams showed an enhancement in load-carrying capacity and ductility over unstrengthened RC beams [21,26
30]. In addition, the strengthened beams exhibit a residual loadcarrying capacity close to that of the ultimate load of unstrengthened beam and capability to elongate up to the extent of maximum elongation of textile. The energy absorption for strengthened beams was significantly higher compared to that of unstrengthened beams. With respect to crack pattern, the TRC strengthening layer showed multiple cracking behavior and the number and width of cracks were decreased in the main beam when it was strengthened. The proposed strengthening

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New Materials in Civil Engineering

methodology also leads to better control over the TRC thickness, ability to place more layers of textiles in a particular thickness, greater uniformity, better bonding of textiles in binder system, and avoidance of special adhesives for repair and retrofitting of structural members.

1.7

Ultrahigh-strength concrete

1.7.1 Introduction The UHSC concept assumes that concrete with minimum defects such as microcracks and internal voids will be able to realize higher load-carrying capacity and offer better durability. The process used in the introduction of reinforcement in UHSC is such that it minimizes the differences between cement and aggregate. This enables creation of a more homogeneous cement-based material and therefore this concrete product possesses properties that are close to those of the metals and allows for the design of new products and structures using concrete with ease. UHSC has the following advantages/features [34]: 1. 2. 3. 4.

Elimination of coarse aggregates leading to better homogeneity; Postheating enhances the microstructure; Ductility enhancement due to small steel fibers; Higher concrete density is achieved through optimization of the granular mixture, and application of pressure before and during setting.

Very high compressive strength is obtained in UHSC because of the above factors. Measures relating to production (e.g., the application of pressure and heat-curing) are some of the variations one can use to enhance the performance of UHSC. Credited to Richard and Cheyrezy [34], RPCs are a new kind of UHSC. Due to the presence of short steel fibers, in addition to the other similar constituents to HSC, RPC exhibits high split tensile strength as well as high ductility, besides enhanced mechanical and physical properties [2,34]. The mechanical properties that can be achieved by UHSC include a compressive strength in the range between 200 and 800 MPa, fracture energy in the range between 1200 and 40,000 J/m2, and ultimate tensile strain of the order of 1%. This is generally achieved by a microstructural engineering approach, that considers (1) elimination of the coarse aggregates, (2) reducing the water-to-cementitious material ratio, (3) lowering the CaO to SiO2 ratio by introducing the silica components, and (4) incorporation of steel microfibers. It is noted that when SF is added in RPC matrix, it develops an interfacial-toughening effect upon fiber slip [2]. This is found to significantly improve the steel fiber
matrix bond characteristics. It is further reported [35] that when mortar is mixed with brass-coated steel fibers both physical and mechanical properties are influenced by autoclaving under saturated vapor at 180 C. Thereby, both flexural and compressive strengths have been found to improve. On curing cycles, Massida et al. [35] reported that when specimens were precured at ambient temperature for 3 days and later under high-pressure steam

An overview of cementitious construction materials

41

curing for 3 hours, these specimens displayed flexural strength of 30 MPa and compressive strength of 200 MPa. It was further reported [36] that RPC has excellent freeze
thaw resistance with no apparent damage at up to 600 cycles as per the ASTM C 666 test procedure. UHSC can be produced with low water
cement ratio, cement dosage in the higher range of 800
1000 kg/m3. However, this can lead to undesirable heat of hydration and can cause shrinkage problems. This will also increase production cost. Use of MAs can be one of the solutions [2] in such cases.

1.7.2 Mechanism of production of ultrahigh-strength concrete Advanced cementitious composites (ACCs), such as UHSC/RPC, macro defect-free (MDF) cement and high-volume hybrid fiber-reinforced composites (HVHFRC) are a new generation of concrete composites employing MAs, hard, fine fillers, fibers, and modified processing and curing techniques [11,37]. Conventional cement concrete is a heterogeneous material in which the strength is mainly limited by the strength of the binder and the transition zone at the binder
aggregate interface. As a result, the performances obtained are not commensurate with the optimum achievable. In ACCs, homogeneity is improved by using a powder concrete in which the particle size distribution is reduced by nearly two orders of magnitude by replacing the coarse aggregate as well as coarse portion of fine aggregates by fine, hard fillers, such as ground quartz. The incorporation of fibers and MAs, better packing of components (particle sizes ranging from a maximum of 600 down to less than 0.1 μm) and improved processing and curing techniques enhance the matrix strength and improve the aggregate interface bond. The compressive strength (around 200 MPa) and Young
s modulus are increased by a factor of three. The flexural strength can be increased to 50 MPa or more depending on the fiber content. It is important to note that all such composites defined by their component materials with minimum of defects such as microcracks and pore spaces, only can provide better ultimate load-carrying capacity and possess enhanced durability properties.

1.7.3 Criteria for material selection Very dense packing of the matrix is achieved by optimizing the granular packing of dry fine powders [11,34,37,38]. The constituents [34] are (1) fine quartz sand, (2) cement, (3) crushed quartz, and (4) SF. Sand: Attributes of sand are defined with respect to (1) mineral composition, (2) mean particle size, (3) granularity, (4) particle shape, and (5) mixture ratio by mass. Quartz, being a very hard material, is preferred in the mineral composition and is readily available at low cost. Fine sand is obtained by screening crushed sand, where the grains are highly angular, or natural quartz sand, where the grains are more spherical. Both types of sand can be used for RPC/UHSC. Water demand is lower for natural sand. Cement: It is preferable to have cements with low tricalcium aluminate (C3A). High silica modulus improves the rheological and mechanical properties. However, it may be noted that this type of cement suffers from a very slow setting rate,

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New Materials in Civil Engineering

preventing its use for certain applications. Thus, in spite of higher water demand, conventional quick-setting high-performance cement offers similar mechanical performance. It may be noted that cement selection should be done along with that of the SP. SP: Polyacrylate-based agents are the most efficient SPs. However, the retarding characteristics of the agent can be a disadvantage. The conventional SPs on the other hand are preferred for their compatibility even though they are poorer in comparison. As in the case of UHSC/RPC one needs to maintain a low water
cement ratio, and 1.6% of cement content should be satisfactory. SF: SF does not have aggregates and is amorphous. The SF is used in UHSC/RPC for filling the voids in the matrix/mix, acting a as lubricant to improve rheological characteristics because of its spherical shape and production of secondary hydrates by pozzolanic reaction with the lime resulting from primary hydration. Particle aggregation and size, impurities, etc. form part of the characterization of SF. Studies indicate that a SF to cement ratio of 0.25 is optimum. Closer to this ratio can lead to entire consumption of lime and total hydration of cement. However, since cement hydration is incomplete in RPC, a greater quantity of SF will be available than that required by the pozzolanic reaction. Crushed quartz: As far as UHSC/RPC is concerned, crushed QP forms the key ingredient in the mix. It is observed that a mean particle size of between 5 and 25 μm yields maximum reactivity during heat treatment. The mean particle size of 10 μm is the same as that of cement used for RPC, which makes it granular. The ratio (by weight) corresponds to the stoichiometric optimum for conversion of amorphous hydrates into tobermorite characterized by a C/S molar ratio of 5/6 5 0.83. This is achieved with a silica
cement ratio of 0.62. This ratio is obtained by adding SF and crushed quartz as a complement. Table 1.4 describes the different ingredients of UHSC/RPC and their selection parameters [33]. Fibers [2]: The behavior of the UHSC/RPC matrix is purely linear and elastic, with a fracture energy of about 30/Jm2. Steel fibers 13 mm long with a diameter of 0.15 mm with 1.5%
3% of volume fraction are recommended. The addition of fibers improves ductility. For UHSC/RPC heat-treated at 250 C and above with shorter fibers (less than 3 mm) of irregular form, enhanced mechanical performance (compressive and tensile strength) can be obtained. In this case, the fracture energy obtained is sharply reduced with simultaneous compressive strength increase.

1.7.4 Curing Concrete specimens can be moist cured in three different conditions: 1. Standard curing: Curing at room temperature (always at 20 C) for 28 days; 2. Steam curing: Curing at 90 C after a preliminary curing at 20 C for 6 hours; 3. High-pressure steam curing (autoclave process): Curing at 160 C and 1.7 MPa pressure after a preliminary curing at 20 C for 24 hours.

Table 1.4 Selection of parameters for UHSC components. Component

Selection criteria

Function parameters

Particle size

Types

Sand

Good hardness, readily available at low cost C3S: 60% C2S: 22% C3A: 3.8% C4AF: 7.8% Fineness

Acts as aggregate, gives strength

150
600 μm

Natural, crushed

Binding material, produces primary hydrates

1
100 μm

OPC, medium fineness

Hard filler, shows maximum reactivity during heat-treating Very fine filler, fills the voids, enhances rheology, produces secondary hydrates Improves ductility

5
25 μm

Crystalline

0.1
1 μm

Procured from ferrosilicon industry (highly refined) Straight

Cement

Quartz powder Silica fume

Much lower quantity of impurities

Steel fibers

Good aspect ratio

Superplasticizer

High range water reduction, less retarding characteristic

Reduce W/B ratio

L:13
25 mm Φ: 0.15
0.2 mm


Polyacrylate based

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New Materials in Civil Engineering

1.7.5 Benefits of ultrahigh-strength concrete G

G

G

G

G

G

UHSC comparable with structural steel; In order to improve the strength deformability and toughness of a UHSC member, a number of short steel fibers are embedded to restrain cracks in the matrix; Embedding steel fibers in the matrix enhances the toughness and deformation of UHSC and overcomes the disadvantage of high brittleness; Offers higher shear capacity resulting in a reduction of dead load and adaptable structural member shape; Except direct primary tensile stresses, can resist other stresses, therefore there is no need for supplemental shear and other auxiliary reinforcing steel; Offers better seismic, blast, shock, and impact resistance and therefore is preferred for a variety of related applications such as blast shelters, nuclear structures, skyscrapers, corrosion-proof structures and pavements.

1.7.6 Characterization of materials and development of mix The materials used for the development of HSC, HSC1, and UHSC are shown in Fig. 1.15 and their corresponding mechanical properties are given below [11,34,36
40].

Figure 1.15 Constituents of HSC, HSC1, and UHSC.

An overview of cementitious construction materials

45

Cement: The physical properties of OPC Grade-53 tested as per IS: 4031 are: Particle size range: 31
7.5 μm Initial setting time: 110 min

Specific gravity: 3.15 Normal consistency: 28% Final setting time: 260 min

Compressive strength: 3-days strength: 29 MPa 7-days strength: 34 MPa 2-days strength: 57 MPa

SF: The physical properties of SF are given below: Specific gravity: 2.2 Particle size range: 0.2
25 μm Percentage of passing: 92%(45 μm sieve in wet sieve analysis)

QP: The physical properties of QP are given below: Specific gravity: 2.61 Particle size range: 2.3
75 μm Percentage of passing: 52% (45 μm sieve in wet sieve analysis)

Quartz sand: The physical properties of quartz sand used in the investigation are given below: Specific gravity: 1.2

Particle size range: 400
800 μm

Steel fibers: The physical properties of steel fiber are given below: Specific gravity: 7.8 Diameter: 0.18 mm

Length: 13 mm Yield stress: 1500 MPa

Standard sand: The physical properties of sand used in the investigation are given below: Specific gravity: 2.65

Particle size range: 0.5
0.09 μm (Grade 3 of IS:650)

Coarse aggregate: The physical properties of coarse aggregate used in the investigation are given below: Specific gravity: 2.8

Particle size range: 4.75
20 mm

SP: Polycarboxylate ether-based SP is used. The properties of this are given below: Appearance: light yellow colored liquid Volumetric mass at 20 C: 1.06 kg/L

pH: 6.5 Normal consistency: 28%

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New Materials in Civil Engineering

1.7.7 Mix proportion There is considerable scope to arrive at mix design procedure/methodology on UHSC to obtain the desired strength. As there are no established guidelines, a number of trials had to be conducted [36
38] to arrive at a design methodology for the design of UHSC to get the desired strength. After a number of trials the final mix design was arrived at. The mix proportions and ratio obtained are given in Tables 1.5 and 1.6.

1.7.8 Equipment used The following pieces of equipment (Fig. 1.16) are normally used for casting, compacting, curing, and testing of HSC, HSC1, and UHSC beams: G

G

G

G

G

Pan type mixing machine of 150L capacity (EIRICH) or Hobart mixer; Table vibrator; Autoclave for hot water curing at 90 C; Oven for steam curing at 200 C; Compressive strength testing machine.

1.7.9 Specimen preparation The following sequence is followed to prepare the test specimen [36]: 1. In the mixer machine, well-mixed dry binder powder is slowly poured into the bowl while the mixer is rotating at a slow speed; 2. The speed of the mixer is increased and the mixing process is continued for about 2
3 minutes; Table 1.5 Mix proportions for HSC, HSC1, and UHSC. Mix proportion

UHSC

Water/cement ratio Silica fume, kg/m3 Quartz powder, kg/ m3 Coarse aggregate, kg/m3 Steel Fiber, kg/m3

Mix proportion

UHSC

3

0.23 209.73 335.57

Cement, kg/m Quartz sand, kg/m3 Fine aggregate, kg/m3

838.93 922.82





Water, kg/m3

192.95

158.50

Superplasticizer (% weight of cement content in mix)

3.5 %

Table 1.6 Mix ratio of HSC, HSC1, and UHSC. Mix ratio

Cement

Fine aggregate

Coarse aggregate

Silica fume

Quartz sand

Quartz powder

Steel fiber

Water

% Sp

UHSC

1







0.25

1.1

0.4

2%

0.23

3.5

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47

Figure 1.16 Equipment used. 3. The determined quantity of water is then added; 4. Additional mixing is performed at this speed until a uniform mixture is achieved; 5. In the case of HSC1 and UHSC, after mixing all the ingredients as per Fig. 1.15 (with water and SP), fibers are added. Fig. 1.17 shows the flowable consistency of the HSC1 mix; 6. Fresh mixture is poured using a steel scoop into the molds; 7. Compaction is done by placing the filled molds on a table vibrator for about 2 minutes; 8. After 24 hours, the specimens are demolded.

In the case of HSC, normal curing is done till testing. In the case of HSC1 and UHSC, specimens are immersed in potable water for 2 days subsequent to demolding. The curing is done at room temperature. After 2 days, the specimens are placed in an autoclave at 90 C for 2 days. Following the autoclaving for 2 days, the specimens are placed in an oven and maintained at 200 C for 1 day. The curing methodology employed for UHSC is shown in Fig. 1.18.

1.7.10 Mechanical properties Various mechanical properties, such as compressive strength and spilt tensile strength of HSC, HSC1, and UHSC mix at 28 days are shown in Table 1.7.

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New Materials in Civil Engineering

Figure 1.17 Slump.

Figure 1.18 Curing methodology.

Table 1.7 Mechanical properties of HSC, HSC1, and UHSC. S. no.

Mix ID

Compressive strength (MPa)

Split tensile strength (MPa)

Modulus of elasticity (MPa)

1

UHSC

122.52

20.65

42,987

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49

From selected mixtures of RPC, prismatic specimens of size 40 mm 3 40 mm 3 160 mm, were prepared (both with and without steel fibers and with both normal and hot water curing)[34,36
38]. Three specimens each were tested for all mixtures after 28 days of curing in a 400-kN capacity UTM. Two-point loading was used for testing and the flexural strength was calculated using the equation Flexural Strength ðMPaÞ 5 Pl=bd2

(1.1)

where P is the cumulative point load, l is the effective span, and b and d are the breadth and depth of cross section of the specimen. Prismatic specimens of size 40 3 40 3 160 mm were tested under two-point loading. It is noted that plain RPC had a flexural strength of 13
22 MPa and by the introduction of fibers and hot water curing, it reached 23
32 MPa. However, with the incorporation of fibers, there was considerable enhancement in the flexural strength of these mixtures. G

G

G

For determining the water absorption, three water-saturated, surface dry cube specimens were weighed and the mass of the cubes was noted as the wet mass (Mw). The specimens were then placed in an oven at 105 C after the required number of days of curing, and dried to a nearly constant mass (Md). The percentage of water absorption was calculated as:

Water absorption ð%Þ 5 ðMw 2 MdÞ 3 100=Md

(1.2)

The water absorption of RPC is found to be in the range of 1.2%
1.63% G

G

G

G

The water permeability test is conducted using the Germann water permeability apparatus. The sealed pressure chamber of this apparatus was clamped to the surface of the concrete cube specimens of size 150 mm. Clean water was filled into the chamber and a water pressure of 500 kPa was applied to the concrete surface. The pressure was kept constant operating a micrometer gauge with attached pin such that the volume of a portion of the pin that penetrates into the pressure chamber due to the screw operation is equal to the quantity of water penetrating into concrete.

The difference in the gauge readings (d) over 10 minutes was recorded to calculate the water permeability in mm/s for the given water pressure of 5 bar using Water Permeability ðKÞ ðmm=secÞ 5 v=I

(1.3)

where, v 5 Q/At, and i 5 h/tg, Q 5 a 3 d and h 5 p/γw, where v is the velocity, Q is the discharge, a is the micrometer pin area (78.6 mm2), A is the water pressure

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New Materials in Civil Engineering

surface area (3018 mm2), d is the difference in gauge reading for time t seconds, and h is the pressure selected, depth of water penetration assumed to be equal to the gasket thickness tg. The water permeability of concrete as measured by the Germann water permeability test mainly depends on its surface conditions after the final stage of setting. The permeability values are in the range of 2.55 3 10211 to 5.36 3 10211 m/s. In general, the water permeability values reported here are on the higher side compared to expected values, which indicates that there is a need for further improvement. It should also be noted that the values reported here are at the age of 28 days and further improvement in water permeability could be expected at later ages.

1.7.11 Rapid chloride permeability test (as per ASTM C 1202) G

G

G

G

G

G

Cylindrical specimens of thickness 50 mm were cut from the central portion of cylinders of length 200 and diameter 100 mm (three specimens from each cylinder). The curved surface of the specimens was coated with an enamel paint to prevent any surface leakage of the solutions used for the experiment. Rapid chloride permeability test (RCPT) cells were connected on either side of specimens after applying silicone gel for water tightness. A constant voltage of 60 volts was applied between the cells, one of which was filled with a solution of 3% NaCl (positive) and the other with 0.3 N NaOH (negative). The current flow through each specimen was measured by a digital ammeter every 30 minutes for 6 hours. The charge passed through each specimen in coulombs (product of current in Amperes and time in seconds) and the cumulative charge passed for 6 hours was calculated using the formula:

Q5

X

i t;

where Q is the charge in coulombs, i is the current magnitude in amperes, and t is the time in seconds. Results of RCPT were conducted after 28 days of curing. RCPT values are in the range of 180
220 coulombs, which is considered to be very low as per ASTM C 1202, given the range (less than 1000 coulombs). It should also be noted that the values reported here are at the age of 28 days and further improvement in RCPT could be expected at later ages.

1.8

Biomimetics and bacterial concrete

1.8.1 Introduction Water is a highly exhausted material by livestock, flora, and fauna as it is the basic element of life, and at the same time water is one of the major ingredients in concrete. Therefore usage of huge quantities of water in concrete technology is still an

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irrevocable problem among researchers worldwide. As mentioned at the start of this chapter, after water, cement is the most consumed material. Further, when water is mixed with cement the mix reacts and hydration occurs to provide the desirable strength. However, presence of excess water in the matrix is not only responsible for making the concrete weaker, it is also detrimental to the health of the concrete as it adds porosity in the microstructure of the matrix. Heterogeneity, low tensile strength, and porosity of the concrete mix pose challenges to improve its durability and sustainability. Cracks and microcracks are common during casting or construction, in addition to the lower tensile strength and brittleness. These can severely affect the service life of concrete structures. Therefore, concrete is much more sensitive and vulnerable than is the common perception. On one hand, rational design and construction procedures for concrete structures along with modernization, automation, and many more sophistications can produce safe and durable structures, while on the other hand formation of cracks in a critical location can lead to the collapse of a structure. Many procedures have been evolved, implemented, and further refined to improve the performance of concrete structures by researchers for decades. Introduction of biomimetics concept/approach in the procedures is one of the modern areas of research for improving the mechanical and durability properties of heterogeneous concrete. The interdisciplinary and transdisciplinary approaches in the biomimetics concept can lead researchers to develop novel aspects of remedial/durable methods. In this chapter, transdisciplinary works on the application of a biomimetics concept by using various types of bacteria on improving the performance of concrete structures are described in brief [2]. Biomimetic approaches can offer a novel and untried alternative to the development of high-performing, eco- and environmental-friendly and durable engineering materials with low-embodied energy. Biomimetics is not about copying or mimicking nature, but about learning from nature. Though many applications of biomimetics can be observed in industries concerned with aerospace, AI, textiles, computing, and sensor technologies, its application is presently seen only in a limited manner in the civil construction sector, particularly for construction materials and design. For example, self-cleaning paints that have the “Lotus effect.” This surface has a unique ability to avoid wetting as it has the presence of microscale protuberances covered with waxy nanocrystals on their surface [2,41,42]. Superhydrophobic property and microscale protuberances of lotus leaves with waxy nanocrystals reduce the gap between leaves and allows smooth flow of droplets that take dust and other contaminants with them. This process, which is very important for water plants, keeps the surface clean, which is also known as the “self-cleaning effect.” This phenomenon occurs in the “Lotus leaf.” This idea from nature led to the development of a new self-cleaning paint. Several structures created around the world, such as honeycomb structure, biomes, stone veneer cladding products, roof structures resembling branching trees, and a combination of cylindrical membranes supported by cables and struts are testimony to this [41,42]. Such bio-inspired concepts and evolving theories have facilitated several engineering applications including infrastructure development. Since up-to-date applications of Tensairity include bridge

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and roof structures, the static response of such beams under bending loads has been numerically and experimentally studied by Luchsinger and Crettol. Current areas involving biomimetics include energy and resource efficiency, added functionality in materials and structures, robotics, lightweight structures, architecture and design, etc. [41].

1.8.2 Background As mentioned earlier, biomimetics can offer several advantages, such as sustainability, environmental friendly, cost effectiveness, higher levels of performance, preservation of natural resources by considering substitutes to natural materials, minimization of waste, and energy savings [2,41,43]. Minimization of waste, cost, and energy is in fact the biological principle. Biomimetic approaches have provided an inspiration to many problems, as mentioned above. Studies revealed that the bacteria Sporosarcina pasteurii is abundantly available in natural soil deposits. This precipitates into calcite mineral, which is an excellent binding agent to create sandstone from sand. This is thus a natural biological process we have witnessed. The challenge posed is: can we mimic this in concrete to obtain better bonding property to get a better pore-filling effect? Along with many useful properties, some of the weaknesses of concrete have been mentioned earlier. A number of attempts and considerable efforts have been made to augment and enhance the properties of concrete through innovative interventions. Modification has been made from time to time to overcome such difficulties of concrete but those processes are not easy or good enough [2]. There are advanced technologies and tools available today to protect/retrofit/repair distress in concrete members and surfaces. As is well known, cracking of concrete is natural. Besides synthetic materials and coatings, organic coatings are also used. Those products with organic coatings consist of volatile organic compounds. However, during the production of these compounds and application of coatings, there is a substantial air polluting effect. This has led to the development of products and coating materials that are basically inorganic. As a powerful alternative, biomimetics emerges as a potentially deserving candidate which can substantially contribute to improving strength and durability. One solution is to employ a microorganism that can be biodisposed inside the concrete. The other solution that has generated considerable interest is biomineralization [41]. Biomineralization is achieved through biological synthesis of minerals. The composite materials comprise both mineral and organic components. As they are formed in controlled conditions, they have distinguishing properties based on the phase properties like shape, size, crystalline nature, elemental composition (isotopic and trace), etc.; hence, they are called true minerals. It is known that species of Bacillus group are able to precipitate on their cell constituents and in their microenvironment by conversion of urea into ammonia and carbon dioxide. Urea undergoes bacterial degradation, which increases pH. This facilitates microbial disposition of carbon dioxide resulting in the formation of calcium carbonate [41,42].

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1.8.2.1 Case studies The number of studies conducted over concrete specimens [2] indicate a considerable increase in resistance toward carbonation, chloride penetration, and freezing and thawing when treated with biodisposition. This is equivalent to coating when cementitious materials are employed for biodisposition since carbonate precipitation was mainly a surface phenomenon due to the limited penetration of the bacteria in the porous matrix [2]. The high compressive strength of concrete cubes, whose cracks were retrofitted by calcite precipitation induced by polyurethane, immobilized Bacillus pasteurii cells and they believed that the calcite might remain as a form of precipitation, not as a bonding material within the materials. Pioneering results of innovative techniques based on microbial mineral precipitation have led to a number of studies on the use of bacteria in concrete. The endospore-forming soil microorganism B. pasteurii was used for calcite precipitation by producing urease enzyme. In the same paper an attempt was made to study the two types of specimen, one with microorganisms and the other with microbial mixtures. Studies conducted on mortar cubes with low concentration of B. pasteurii showed improvement in the compressive strength. However, with an increase in cell concentration and curing time, the strength decrease. This indicated biomass affecting the mortar integrity [41,43]. During further studies, when bacteria and sand were filled with cracks [44], it showed an increase in compressive strength and stiffness vis-a`-vis those without cells. It was concluded by the authors that the B. pasteurii was effective in crack remediation but not in strength enhancement [44]. Researchers [45] have also tried using the bacterium S. pasteurii with lactose mother liquor (LML) as an alternative nutrient source. The study reported an increase in the compressive strength, as shown in Fig. 1.19. It was further reported that for calcite precipitation, LML can be an alternative medium to yeast which is costly. Further research work by Achal et al. [46] was to increase the production of calcite in a high pH environment. An attempt to create a mutant of S. pasteurii resulted in creating phenotypic mutants of S. pasteurii namely MTCC 1761. UV radiation was used on the bacteria to test their ability for enhanced urease activity. This is a chemical process by which the microorganism consumes urea and breaks it down to form ammonia and then to calcite. Among the mutants created, a mutant named Bp M-3 [46] was found to be more efficient compared to other mutants and the wild-type strain. It enhanced the highest urease activity and calcite production. One of the key findings of this study is the effectiveness of the suggested biomineralization process for calcite production. This can pave the way to find an effective sealing agent for filling the pores or cracks in structural concretes. The improved strain of S. pasteurii by mutagenesis also showed survival at very high pH values. The enhanced urease activity of Bp M-3 was compared with other mutant strains and a wild strain which is shown in Fig. 1.20. Different microorganisms have been used [47] to increase the compressive strength of cement mortar and for remediation of cracks in concrete. It was shown

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3 days 30

7 days

Compressive strenght (MPa)

28 days

20

10

0 Control

LML Media

NB

YE

Figure 1.19 Compressive strength achieved using different growth media [45]

Figure 1.20 Comparison of urease activity for different strains [46].

that the addition of specific microorganisms to cement
sand mortar or to concrete, deposit inorganic substances inside the pores of the matrices. This can be used as a filling material to remediate cracks within the structures. One other suggested alternative is the calcium carbonate precipitation that is induced bacterially for use in the repair of cracks. This is environment friendly. Further, to enhance the compressive strength, an anaerobic hot spring bacterium into the concrete or mortar can be added. The bacterium suggested is closely related to Shewanella species and can

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enhance the compressive strength up to 30%. The anaerobic microorganism (of about 105 cells/mL of water) concentration is added in the mortar after 28 days of curing. It is observed that the filler material within the pores of the cement
sand mortar increased the compressive strength of the mortar by about 25%. This can be seen from Table 1.8. However, they did not find any improvement in the cement mortar by using another bacterial microorganism, namely E. coli. Using SEM, the growth of anaerobic bacteria inside the concrete was observed [47]. Figs. 1.21(A) and (B) show the SEM observations of concrete mortar with and without microorganisms, respectively. The identification of an appropriate microorganism was also required for the study of biologically imitated constructional studies under biomimetics. The biologically induced cement-based material thus also exhibited better durability and crack-repairing performance compared to normal concrete materials. Material scientists have been working on phenomena occurring in the natural environment to create a remarkable variety of intricate inorganic structures get created. Interdisciplinary research is performed at understanding biological processes accomplishing synthesis of inorganic materials under mild ambient conditions. The idea is to extend the understanding or knowledge created to materials chemistry through an interface that can be used in the fabrication of advanced hybrid structures. Some of the examples of biomineralization (fabrication of inorganic materials by organisms) include magnetite and silver nanoparticles. These particles are synthesized by bacteria and cadium sulfide, whereas nanoparticles are synthesized by yeast cells; the silica frustules of diatoms; and the silica spicules of the sponge Euplectella. Proteins and/or biological macromolecules contribute to transforming microorganisms to minerals. This basic concept is exploited as these also control the nucleation and growth of the inorganic structure. During biomineralization and assemblage, an array of inorganic components create systematic hierarchical structures. This gives rise to several possibilities and approaches of development that can mimic the recognition and nucleation capabilities found in biomolecules for inorganic material synthesis. The understanding developed so far of the biomineralization has enabled significant inroads to be made into the follow-up of biochemical processes both at nano and micro scales.

1.8.3 Durability studies on bioconcrete Bacterially induced carbonate precipitation was first proposed as an environmentally friendly method to protect a construction material when it was able to induce a compatible carbonate precipitate on limestone [42,48]. And unlike the lime
water treatment, the carbonate cement (as a result of the precipitation) appeared to be highly coherent. Then the concept of biomineralization was applied to increase the durability of a wide variety of construction materials such as calcareous stone, granite, mortar, and concrete. For simulating the deteriorative conditions for water absorption, tests were conducted as described below.

Table 1.8 Strength improvement for different cell concentration [47]. Cell concentration/ml of water

Nil 10 102 103 104 105 106 107

Average mortar compressive strength (MPa) 3 days

7 days

14 days

28 days

Strength 6 SD

Strength 6 SD

Strength 6 SD

Strength 6 SD

8.67 6 0.28 8.68 6 0.44 8.76 6 0.47 8.80 6 0.69 8.89 6 0.87 9.34 6 0.81 9.20 6 0.28 8.86 6 1.01

12.60 6 0.47 12.74 6 0.89 12.87 6 0.46 12.98 6 0.81 13.40 6 0.53 14.70 6 0.74 13.80 6 0.58 13.00 6 0.23

16.00 6 0.81 16.21 6 0.22 16.44 6 0.38 16.87 6 0.64 17.10 6 0.37 19.50 6 0.42 17.50 6 0.81 17.00 6 0.45

23.13 6 0.23 24.21 6 0.43 25.00 6 0.88 25.40 6 0.84 25.44 6 0.97 28.98 6 0.86 26.52 6 0.27 25.69 6 0.74

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Figure 1.21 SEM micrographs of mortar [47].

1.8.3.1 Water absorption tests In order to ascertain the efficiency of the concrete, Achal et al. [46] determined the increase in resistance toward water penetration, a sorptivity test performed on the basis of the RILEM 25 PEM (II-6). They obtained the sorptivity coefficient, k [cm  s21/2] by using the following expression pffi Q=A 5 k t where Q 5 amount of water absorbed (cm3); A 5 cross section of the specimen that was in contact with water (cm2); and t 5 time (s). From the Q/A plot against the square root of time, k is calculated from the slope of the linear relation between the former.

1.8.4 Techniques used in microbiology 1.8.4.1 Growing the strain in alkalophilic conditions The strains B. pasteurii ATCC 11859 and Pseudomonas aeruginosa ATCC 27853 were procured from CSIR-IMTECH Chandigarh [2]. B. pasteurii wild type was isolated from an old concrete buildings at CSIR-SERC. These were all cultured to check their morphology on nutrient agar. The composition of nutrient agar (NA) is: Peptic digest of animal tissue: 5000 Beef extract: 1500 Agar: 15,000 each in g/L

Sodium chloride: 5000 Yeast extract: 1500

The final pH of the medium was found to be (at 25 C) 7.4 6 0.2. The culturing was done by spreading the stock culture of the bacteria onto the plates and allowing it to be incubated for 24 hours at 37 C. Both Pasteurii strains were fine but the P.

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aeruginosa did not grow at all, even after serially culturing it. Next, a urea CaCl2 medium was prepared as per Refs. [36,37,39]. The composition (g/l) of the urea was as follows: Agar . 5 NB 3 g Urea: 20 g

NH4Cl: 10 g NaHCO3: 2.12 g (equivalent to 25.2 mM)

The CaCl2 concentration taken to be 25 g, which would equate to 25 mM/L. After autoclaving, CaCl2 was used to make the pH of the media reach 8, 8.5, and 9. The volumes of CaCl2 added were 10, 25, and 40 mL approximately. These were poured out into the plates in duplicate set and then the cultures were spread. These cultures then showed growth after the first 24 hours. They were observed every day to note the changes of growth in bacteria. The basic morphology of the bacterial colony on the urea agar was found out to be (1) round, (2) irregular edged, (3) dull white, (4) raised, (5) opaque, (6) protruding out, (7) moist, and (8) 1 mm in diameter. Note that the colony morphology was carried out on NA only. The growth on NA alone is to described as the colony morphology. Some images of culture can be viewed after 72 hours. Fig. 1.22 is of the strain B. pasteurii at 80 3 magnification.

Figure 1.22 Bacillus pasteurii (80 3 ).

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1.8.5 Culturing of cells for use in bioconcrete From the cell-counting experiments, one can determine the cell concentration/mL for any given OD for the culture of a strain grown in certain conditions. The bought and wild strains of B. pasteurii were cultured overnight in LB (Luria Bertani) broth for 16 hours and then the cells were centrifuged at 8000 rpm for 20 minutes. Then these pellets were washed and suspended in the PBS at a concentration of 50 mM and pH 7.5. Then the OD (absorbance) was taken at 600 nm in the spectrophotometer. Distilled water was taken as the blank. From the OD taken, the cell concentrations were taken by referring to the results of the serial dilution and plating experiment. This is simple unitary calculation. The following were the count standards obtained at 600 nm. 0:158
1:6 3 107 for wild culture=ml 0:102
2 3 107 for ATCC 11859=ml Hence, for any optical density in terms of absorbance grown in LB, one can obtain the concentration.

1.8.6 Effect of bacteria on compressive strength The cubes incorporated with bacteria showed a significant increase in compressive strength as compared to the cubes without bacteria. Both the wild strain and ATCC 11859 showed an increase in compressive strength with respect to the control at 3 days. The wild strain showed an increase in compressive strength up to 14 days. The 107 concentration showed a 68% increase in compressive strength with respect to the control, whereas the 105 concentration showed an increase of 41% with respect to the control. All four concentrations of the incorporated wild strain have been plotted with respect to the control as shown in Fig. 1.23 and the details are presented in Table 1.9. The two concentrations of ATCC 11859 also showed an increase in compressive strength as of the 3rd day. The strengths obtained are as shown in Table 1.10.

1.8.7 Summary It can be seen that there is ample scope for the development of a durable bioconcrete using microorganism-mediated calcium precipitation. It can also be developed further in order to act as a good cover or sealant, especially in making structures more watertight. The precipitation of calcite within the pores of the concrete has also resulted in increased particle packing and the extracellular polymeric substances (EPS) secreted by the bacteria help in increased binding of the calcite to the cementitious particles in the pores. The concrete also is sustainable in the terms of durability and CO2 emissions. However, there is a need for validation of the methodology using a greater number of corrosion tests and with more samples. There is also a clear need to define the properties of the microorganism to be used in the development of such a bioconcrete, so that the best strains could be used in practice when implementing this methodology of microbe-induced calcite precipitation.

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New Materials in Civil Engineering ↑ C o m p r e s s i v e

45 40 35 30 25

Control Wild 7 Wild 5 Wild 6 Wild 4

20 S t r e n g t h ( M P a )

15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 28 29 30 DAY →

Figure 1.23 Compressive strength comparison of wild strain incorporating mortar and plain mortar.

Table 1.9 Compressive strengths S-2. Day

Control

5th day 7th day 14th day 21st day 28th day

17.93 22.77 25.67 36.64

Wild 7

Wild 6

Wild 5

Wild 4

30.59 34.9 38.32

25.54 31.41

24.9 25.65 32.8

12.72 33

Wild 7, Cell concentration 5 107 colony-forming units (CFU)/mL; Wild 6, cell concentration 5 106 CFU/mL; Wild 5, cell concentration 5 105 CFU/mL; Wild 4, cell concentration 5 104 CFU/mL.

Table 1.10 Compressive strengths S-1. Day

Control

5th day 7th day 14th day 21st day 28th day

17.93 22.77 25.67 36.64

AATCC 7 31.3

ATCC 6

ATCC 5

ATCC 4

28.65

ATCC 7, Cell concentration 5 107 colony-forming units (CFU)/mL; ATCC 6, cell concentration 5 106 CFU/mL; ATCC 5, cell concentration 5 105 CFU/mL; ATCC 4, cell concentration 5 104 CFU/mL.

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Further study is also required in order to deduce the best composition of culture media that could be used for the effectiveness of a certain strain. Also, the microbial interactions with the concrete environment need to be better understood, especially with regard to the pH of the concrete.

Acknowledgments The author thanks many of his colleagues, co-workers, and students for their contributions during the entire period of this study. Some of those who have contributed to enhancement of knowledge in pursuit of better understanding of the studies undertaken include B. Bhuvaneshwari, Bharath Kumar, G. S. Palani, A. Ramachandra Murthy, Smitha Gopinath, S. Maheswaran, P. S. Ambily, T.S. Krishnamoorthy, Prabhat Prem, K. Sarayu, and V. Ramesh Kumar.

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224. [17] ,http://rpsonline.com.sg/proceedings/9789810911393/html/Golden-Legacy.pdf. [18] ,https://www.cpi-worldwide.com/in/journals/artikel/32038/geopolymer-concrete-forprecast-application2. [19] H.V.S. Ganga Rao, N. Taly, P.V. Vijay, Reinforced Concrete Design with FRP Composites, CRC Press, 2006. [20] V. Bhashya, S.S. Kumar, G. Ramesh, B.H. Bharatkumar, T.S. Krishnamoorthy, N.R. Iyer, Long term studies on FRP strengthened concrete specimens, Ind. J. Eng. Mater. Sci. 22 (2015) 465
472. [21] S. Gopinath, Development of fabric reinforced concrete for flexural strengthening of RC beams, PhD Thesis, Department of Civil Engineering, IIT Madras, India, 2017. [22] T.C. Triantafillou, C.G. Papanicolaou, P. Zissimopoulos, T. Laourdekis, Concrete confinement with textile-reinforced mortar jackets, ACI Struct. J 103 (1) (2006) 28
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15. Available from: https://doi.org/10.1177/1369433217735988. First Published Online October 27, 2017. [26] T.C. Triantafillou, Textile-reinforced mortars (TRM) versus fibre-reinforced polymers (FRP) as strengthening and seismic retrofitting materials for reinforced concrete and masonry structures, in: International Conference on Advanced Composites in Construction (ACIC07), University of Bath, 2007. [27] A.D. D’Ambrisi, L. Feo, F. Focacci, Experimental analysis on bond between PBOFRCM strengthening materials and concrete, Compos. Part B Eng 44 (1) (2013) 524
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[28] S. Gopinath, N.R. Iyer, An apparatus for cast-in-place strengthening of structural members using textile reinforced concrete and method thereof, Indian Patent Application 2339DEL2015. [29] S. Gopinath, R. Gettu, N.R. Iyer, Influence of prestressing the textile on the tensile behaviour of textile reinforced concrete, Mater. Struct. 51 (64) (2018). [30] S. Gopinath, N.R. Iyer, R. Gettu, G.S. Palani, A.R. Murthy, Confinement effect of glass fabrics bonded with cementitious and organic binders, Procedia Eng. 14 (2011) 535
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2323.

Computational intelligence for modeling of pavement surface characteristics

2

Behrouz Mataei1, Fereidoon Moghadas Nejad1, Hamzeh Zakeri1 and Amir H. Gandomi2 1 Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran, 2Faculty of Engineering & Information Technology, University of Technology, Sydney, NSW, Australia

2.1

Introduction

There are many factors that degrade the quality of the images. In practice, corruption by Gaussian noise, speckle noise, salt and pepper noise, and Poisson noise are the cause of most common degradations [1,2]. The source of these degradations can be the camera image capturing quality, lighting condition, environmental factors, etc. Such degradations can have a significant impact on the image quality and affect the accuracy of computer-based methods. Additionally, due to the poor quality of images, feature extraction, analysis, recognition, and quantitative measurements become difficult and unreliable. Thus, image enhancement is one of the primary requirements for any application. Image enhancement basically improves the interpretability or perception of information in images for viewers and provides appropriate input for automated image processing techniques [3]. It is well known that image enhancement is a problem-oriented procedure. The goal of the image enhancement is to improve the visual appearance of the image to retain the important image features as much as possible for future analysis, detection, segmentation, and recognition [4]. During this process, one or more attributes of the image are modified. For a given task it is specific to choose attributes and the way they are modified. Moreover, observer-specific factors, such as the human visual system and the observer’s experience, will introduce a great deal of subjectivity into the choice of image enhancement methods. Denoising and compressing are two actions to preprocess and enhance the image quality. Denoising is removing noise of the images while retaining the important signal features as much as possible. Image compression means reducing the volume of data needed for representing a given image. Computer images are extremely data-intensive, hence requiring a great deal of memory for storage. The compression techniques reduce the size of a file in order to facilitate the efficient transfer of its storage. In this regard, it is of high importance to compress images without losing their valuable features for future analysis. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00002-8 © 2020 Elsevier Inc. All rights reserved.

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To improve the process of denoising, researchers have tried to develop more efficient thresholding functions, because they are a more effective and simplified way of implementation in transforms domain [1]. Despite the large number of methods proposed for image enhancement [5
13], they all lack general standard criteria for image quality measures, which can be used as optimization targets for image enhancement algorithms. The enhancement methods can be generally classified into two categories: (1) spatial domain methods, and (2) frequency domain methods. Initial efforts for image enhancement methods started with the ideas based on statistical filtering in spatial domain [14]. In spatial domain techniques, the image pixels are directly dealt with and the pixel values are manipulated to achieve desired enhancement. In frequency domain methods, the image is first transferred into the frequency domain by computing the Fourier transform of the image. All the enhancement operations are performed on the transformed image and then the inverse Fourier transform is performed to get the processed image. These enhancement processes are performed in order to modify the image brightness, contrast, or the distribution of the gray levels. As a consequence, the pixel value (intensities) of the output image will be modified according to the transformation function applied to the input values. These methods are operated on transforms of the image, such as the Fourier, wavelet, and cosine transforms. The basic advantages of transform-based image enhancement techniques are the low complexity involved in computations, and the facilitated viewing and manipulating of the frequency composition of the image, without direct reliance on the spatial information [15]. The analysis of the existing transform-based image enhancement techniques [6,8
10] shows that they face some common problems, including [16]: 1. Introduction of certain artifacts called “objectionable blocking effects” [10]. 2. Such methods cannot simultaneously enhance all parts of the image with the desired accuracy. 3. It is difficult to select optimal processing parameters, and there is no efficient measure that can serve as a building criterion for image enhancement.

Finding a solution to this problem is very important especially when the image enhancement procedure is used as a preprocessing step for other image processing techniques such as detection, recognition, and segmentation. Image enhancement is applied in many fields such as pavement texture assessment. There are several approaches for measuring texture characteristics of pavement surface. In this chapter, we present a study on computational intelligence methods for the enhancement of the pavement surface images that prepare images for use in texture characteristics evaluation systems. Fig. 2.1 introduces an example of such systems. It is necessary to use a proper method to enhance and prepare the image for a given analysis to find proper indices for texture extraction. The main contribution of this study is to explore the computational intelligence methods for denoising and compressing pavement surface images. In this chapter, we used wavelet, curvelet,

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Figure 2.1 Flowchart of a pavement texture evaluation system.

ridgelet, shearlet, and contourlet transform methods to enhance the pavement images and arrange a study on the performance of these transforms with the image processing parameters. The results of this study can be a guidance to finding an appropriate technique for enhancing images taken to evaluate the texture of the pavement surface. In the following sections, a brief review of computational intelligence transforms is provided.

2.2

Computational intelligence methods

Computational intelligence analysis [17] allows for the preservation of an image according to certain levels of resolution. The computational intelligence analysis allows for the zooming in and out on the underlying texture structure. Therefore, the texture extraction is not affected by the size of the pixel neighborhood. This multiresolution quality is among the useful features of transforms in many applications from image compression to image denoising and edge detection [18]. Since their development in the early 1980s, several wavelet families have emerged. The curvelet, ridgelet, and other such transforms are all extensions of the wavelet transform. The following section contains a brief review of the multiresolution transforms.

2.2.1 Wavelet transform Wavelet transforms are a multiresolution representation of signals and images that decompose signals and images into multiscale details [19
21]. The basic functions used in wavelet transforms, which are locally supported, are nonzero only over part of the domain represented. A sharp transition in the image is preserved and depicted extremely well in wavelet transforms [21]. This special treatment of edges by wavelet transform is very attractive in image filtering. Wavelet has several transformers with different characteristics such as daubechies, coiflet, and symlet.

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Image resolution enhancement in the wavelet domain is a relatively new research topic with recently many new algorithms proposed for it [22
25]. The continuous wavelet transform (CWT) and discrete wavelet transform (DWT) [26] are the main wavelet transforms used in image processing. The continuous wavelet transform (CWT) in R2 of f ðtÞ can be written as:
 CWTf ðs; uÞ 5 f ðtÞ; ψs;u

(2.1)

where u is the translation parameter, indicating which region we concern and s is the scaling parameter greater than zero because negative scaling is undefined. The discrete wavelet transform (DWT) in R2 of f ðnÞ can be written as: 1 X DWT ϕ ½j0 ; k 5 pffiffiffiffiffi f ½nϕj0 ;k ½n M n

(2.2)

where j is the dilation parameter or the visibility in frequency and k is the position parameter. Also, f ½n, ϕj0 ;k ½n; and ϕj;k ½n are discrete functions defined in ½0; M 2 1 totally M points. Table 2.1 illustrates a comparison of the performance results of wavelet transform with different filters (haar, daubechies, coiflets, symlets, discrete Meyer, biorthogonal, and reverse biorthogonal). Comparing different filters, the de Meyer family outperformed other filters for most performance measures. The haar, daubechies, biorthogonal, and reverse biorthogonal filters had similar performance and symlet filter performed slightly higher in PSNR, SNR, UQI, and SSIM. The coiflet with a little difference performed higher than symlet filter. It should be noted that there is no difference between MSE and MAE in using each filter. Hence, it can be concluded that changing filters does not make any difference in calculated errors. The symlet, reverse biorthogonal, and haar were less time consuming than de Meyer filter but since the difference is really small (0.03
0.59 seconds) it can be ignored. The de Meyer family has the highest SSIM. It has also the highest UQI, PSNR, and SNR, respectively, with a 3%, 16%, and 27% superiority over coiflet.

Table 2.1 Performance of different filter families in wavelet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

Haar Daubechies Coiflets Biorthogonal R biorthogonal Symlets Discrete Meyer

3.44 3.56 3.55 3.97 2.88 3.12 3.47

18.57 18.57 21.25 18.57 18.57 20.68 24.72

9.54 9.54 12.13 9.54 9.54 11.94 15.46

6.2 6.2 6.2 6.2 6.2 6.2 6.2

0.23 0.23 0.23 0.23 0.23 0.23 0.23

0.89 0.89 0.94 0.89 0.89 0.93 0.97

0.9985 0.9985 0.9990 0.9985 0.9985 0.9990 0.9995

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2.2.2 Ridgelet transform Ridgelet transform was developed over several years in order to break an inherent limit plaguing wavelet denoising of images. To overcome the limitations of wavelets in higher dimensions, Candes and Donoho pioneered a new system of representations named ridgelets which dealt effectively with line singularities in two dimensions [27,28]. The main idea behind this system is to map a line singularity into a point singularity using the Radon transform [29,30]. Then, the wavelet transform can be used to effectively handle the point singularity in the Radon domain. Thus, ridgelet transform allows representing edges and other singularities along lines in a more efficient way for a given accuracy of reconstruction, in terms of compactness of the representation, than traditional transformations, such as the wavelet transform [31]. Given an integrable bivariate function f(x), its continuous ridgelet transform (CRT) in R2 is defined by [27,28]: ð CRT f ða; b; θÞ 5

R2

ψa;b;θ ðxÞf ðxÞdx

(2.3)

where the ridgelets ψa; b; θðxÞ in two dimensions are defined from a wavelet-type function in 1-D ψðxÞ as: ψa; b; θðxÞ 5 a21=2 ψððx1 cosθ 1 x2 sinθ 2 bÞ=aÞ:

(2.4)

where a, b, θ are parameters of ψa; b; θðxÞ. Discrete ridgelet transform, which also is called finite ridgelet transform, can be written as: FRIT f ½k; m 5

X


 ωðkÞ m ½l f ; ϕk;l

(2.5)

lEZp

where ϕk;l 5p21=2 δLk;l kEZplEZp ZP 5 0; 1; . . . ; p 2 1 ; ZP 5 0; 1; . . . ; p

where p is a prime number, δs denotes the charactristics function for a set s in Zp2 , k is the slope or direction of line, and l is its intercept. Table 2.2 presents a comparison of the performance of ridgelet transform in image denoising and compressing using seven filters. As shown in this table, all families show the equal MSE, MAE, UQI, and SSIM. Besides, haar, daubechies, biorthogonal, and reverse biorthogonal had a similar result and showed higher performance in comparison with other filters in PSNR and SNR parameters. Applying symlet and coiflet filters in ridgelet technique showed a lower performance, but the worst results were obtained for the de Meyer filter. Time differences in all filters are short about 0.7 seconds and daubechies has the minimum time with 2.7 seconds.

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Table 2.2 Performance of different filter families in ridgelet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

Haar Daubechies Coiflets Biorthogonal R biorthogonal Symlets Discrete Meyer

2.9 2.7 3.4 2.8 2.8 2.9 3.1

99.27 99.27 98.58 99.27 99.27 98.37 96.18

90.43 90.43 88.85 90.43 90.43 88.51 86.93

6.19 6.19 6.19 6.19 6.19 6.19 6.19

0.23 0.23 0.23 0.23 0.23 0.23 0.23

1 1 1 1 1 1 1

1 1 1 1 1 1 1

Table 2.3 Performance of four subsets of daubechies filters in ridgelet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

Daubechies 1 Daubechies 2 Daubechies 10 Daubechies 15

2.7 4 3 4.1

99.27 99.19 95.60 96.42

90.43 90.13 87.67 88.29

6.19 6.19 6.19 6.19

0.23 0.23 0.23 0.23

1 1 1 1

1 1 1 1

Hence, considering all parameters, daubechies is overall an appropriate family in ridgelet domain. Four subsets of daubechies family were compared on the basis of the better performance of this filter in ridgelet technique (Table 2.3). The results show no significant difference in MSE, MAE, UQI, and SSIM results. Here, the “db1” has the highest PSNR and SNR, with about 2% superiority over the mean of other filters. The least time consuming also belongs to “db1.” Thus, according to this comparison, “db1” is the best filter for ridgelet transform.

2.2.3 Curvelet transform Curvelet transform is the directional transform that overcomes the limitation of the wavelet transform. The curvelet transform, like the wavelet transform, is a multiscale transform with frame elements indexed by scale and location parameters and the curvelet pyramid contains elements with a very high degree of directional specificity. The orientation selectivity behavior and anisotropic nature of the curvelet transform allow to represent suitably the objects with curves and handle other 2D singularities better than wavelets, which makes it a more proficient transformation for image compression application. The first generation curvelet transforms originally developed in the continuous domain was through multiscale filtering, followed by a block ridgelet transform on each band pass image. Due to its computational complexity and high redundancy,

Computational intelligence for modeling of pavement surface characteristics

71

this technique is seldom used for image compression but find application in image denoising, image fusion, etc. Curvelet transform combines the merits of ridgelet transform (which is good at expressing straight line characteristics) and wavelet transform (which is suitable for point characteristics). This transform is applied efficiently to image intensification, image fusion, and image denoising [32]. The continuous curvelet transform (CCT) of f ER2 is given as: D E ð CCT f ðj; l; kÞ:¼ ϕj;l;k ; f 5 f ðxÞϕj;l;k ðxÞdx;

(2.6)

R2

ψj;l;k :jEð0; 1Þ; lER2 ; kEð0; 2πÞ Fast discrete curvelet transform (FDCT) is mathematically expressed as: FDCT ðj; l; kÞ 5

X

f^½n1 ; n2 2 n1 tanθl U~ j ½n1 ; n2 ei2πðk1 n1 =L1;j 1k2 n2 =L2;J Þ

(2.7)

n1 ;n2 EPj

Pj 5 ðn1 ; n2 Þ:n1;0 # n1 , n1;0 1 L1;j ; n2;0 # n2 , n2;0 1 L2;j where θl is the rotation angle and U~ j ½n1 ; n2  is supported on the rectangle of length L1;j and width L2;j . Table 2.4 shows the performance of curvelet transform in image denoising and compression using seven filter families. Like the wavelet transform, haar, daubechies, biorthogonal, and reverse biorthogonal families had similar performance. Moreover, symlet and coiflet filters indicated a close competition with a little superiority for coiflet in PSNR, SNR, and UQI. MSE and MAE are equal in all filters. The de Meyer filter provided the best results in comparison with other filters and had better performance in four parameters (PSNR, SNR, UQI, and SSIM). It has the highest UQI, PSNR, and SNR, respectively, with a 0.4%, 9%, and 13% superiority over coiflet. However, in terms of time, the best performance belongs to haar with about 30 seconds less than de Meyer family. Hence, it can be concluded that in a particular task in which time is a determinant, haar is a proper choice, but if higher quality is demanded de Meyer is the best. Table 2.4 Performance of different filter families in curvelet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

Haar Daubechies Coiflets Biorthogonal R biorthogonal Symlets Discrete Meyer

91 111 132 113 95 104 119

26.44 26.44 30.45 26.44 26.44 30.05 33.32

17.60 17.60 21.73 17.60 17.60 21.03 24.67

6.2 6.2 6.2 6.2 6.2 6.2 6.2

0.23 0.23 0.23 0.23 0.23 0.23 0.23

0.977 0.977 0.991 0.977 0.977 0.989 0.995

0.9995 0.9995 0.9998 0.9995 0.9995 0.9998 0.9999

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2.2.4 Shearlet transform A representation scheme called the shearlet representation has been introduced recently [33,34]. This new representation system, which yields nearly optimal approximation properties [35], is based on a simple and rigorous mathematical framework that not only provides a more flexible theoretical tool for the geometric representation of multidimensional data, but is also more natural for implementation. As a result, the shearlet approach can be associated with a multiresolution analysis (MRA), leading to a unified treatment of both the continuous and discrete worlds [34]. However, all known constructions of shearlets so far are band-limited functions which have an unbounded support in the space domain. In fact, in order to capture the local features of a given image efficiently, representation elements need to be compactly supported in the space domain. Furthermore, this property often leads to a more convenient framework for practically relevant discrete implementation. The continuous shearlet transform can be written as the following function:
 CSTða; s; tÞ 5 f ; ψast ; aAR1 ; sAR; tAR2

(2.8)

where ψast is the affine function which is defined as follows: 1

22   21 ψastðxÞ 5 jdetMas j ψ M ð x 2 t Þ as

a 2 aα s Mas 5 ;0,α,1 0 aα

(2.9)

We now define discrete shearlet transform (DST) as: DST ðf Þ 5


  f ; Tn ϕ00 nEZ 2 ; S0 ðf Þ; S~0 ðf

(2.10)

where

j

j Λ0 5 ðj; kÞ:j 5 0; 1; . . . ; J 2 1; 22 2 # k # 2 2

8 1 0 > > > > C B > > C B > > C B     > 0 > C B 6 7 > P S ð f Þ 5 w f 0 sjk j Cð j; kÞEΛ0 > B 6 7 > > C B 6 7 j > > A @ J2j;J24 5 > > < 2 1 0 > > > > C B > > C B >     > B > ~0 ðf Þ 5 Bw~ 0 6 7 P~ sjk fj C > S Cð j; kÞEΛ0 > 6 7 > C B > 6j7 > A @ > > J2j;J24 5 > > : 2

(2.11)

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Table 2.5 Performance of different filter families in shearlet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

9
7 maxflat pyr pyrexc

9.7 11.1 7.6 7.2

302.96 298.11 307.56 308.17

294.51 288.96 297.92 298.16

6.2 6.2 6.2 6.2

0.23 0.23 0.23 0.23

1 1 1 1

1 1 1 1

Table 2.5 shows a comparison of using shearlet transform for image denoising and compressing with four filters for the Laplacian Pyramid/ATrous decomposition including 9
7, maxflat, pyr, and pyrexc. The MSE, MAE, UQI and SSIM values are equal for all filters. The pyrexc has the highest PSNR and SNR with a 2% superiority over the mean of other filters and the minimum time consumption (2 seconds less than mean of time of other filters). Therefore, it is appropriate for using in shearlet technique.

2.2.5 Contourlet transform Do and Vetterli [36
38] developed the contourlet representation as a directional transform based on an efficient 2D nonseparable filter bank that can deal effectively with images having smooth contours. In Ref. [39], it was shown that despite the redundancy of the contourlet transform, by using this transform in an image coding system, one can obtain better visual results for texture and contour parts of images in comparison to a wavelet transform. Contourlets not only possess the main features of wavelets (namely, multiresolution and time-frequency localization), but also show a high degree of directionality and anisotropy. The main difference between contourlets and other multiscale directional systems is that contourlets allow for a different and flexible number of directions at every scale while achieving nearly critical sampling. In addition, contourlet transform employs iterated filter banks, which makes it computationally efficient. In the contourlet transform, the image is first decomposed into subbands by the Laplacian pyramid (LP) and then each detail image is analyzed by the directional filter banks (DFB). Under certain conditions, the low-pass filter G in the LP uniquely defines an orthogonal scaling function ϕðtÞEL2 ðR2 Þ via the two-scale equation: ϕ ðt Þ 5 2

X

g½nϕð2t 2 nÞ

(2.12)

nEz2

ϕj;n 5 22j ϕ



t 2 2j n jEZ; nEZ 2 2j

The DFB is applied to the multiresolution subspace Vj :

(2.13)

74

New Materials in Civil Engineering lÞ θðj;k;n ðt Þ 5

X

ðlÞ gðlÞ k ½m 2 Sk nϕj;m ðt

 (2.14)

mEz2 lÞ ðlÞ is an orthonormal basis of a directional subspace Vj;k for The family θðj;k;n 2 l nEZ each k 5 0; . . . ; 2 2 1. Table 2.6 illustrates a comparison of different filter families used in denoising and compressing images with contourlet transform as filters for the Laplacian pyramid (contourlet uses two kinds of filters, one for the Laplacian pyramid and one as a directional filter). Note that MSE, MAE, UQI, and SSIM are equal in all filters. Besides, coiflet and symlet filters had an unacceptable performance in PSNR and SNR. Although, haar, daubechies, biorthogonal, and reverse biorthogonal filters all had a similar performance, because of less time needed by daubechies filter (3.26 seconds difference with the mean time of other filters) it was chosen as the appropriate filter for contourlet transform. The de Meyer family is not applicable in contourlet transform. To choose the best filter for the Laplacian pyramid, a comparison is established between four subsets of daubechies family (Table 2.7). The MSE, MAE, UQI, and SSIM are equal for all filters while “db1” has the highest PSNR and SNR, with a 26% superiority over the mean of other filters. Since the least time consuming also is for “db1,” it was selected for contourlet transform technique.

Table 2.6 Performance of different filter families in contourlet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

Haar Daubechies Coiflets Biorthogonal R biorthogonal Symlets Discrete Meyer

16.7 14.5 16.4 19.2 18.4 18.1


295.52 295.52 238.14 295.52 295.52 240.97


285.35 285.35 228.18 285.35 285.35 231.31


6.2 6.2 6.2 6.2 6.2 6.2


0.23 0.23 0.23 0.23 0.23 0.23


1 1 1 1 1 1


1 1 1 1 1 1


Directional filter is assumed “pkva.”

Table 2.7 Performance of four subsets of daubechies filters in contourlet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

Daubechies 1 Daubechies 2 Daubechies 10 Daubechies 15

14.5 16.3 20.6

295.52 240.37 222.46

285.35 232.37 214.21

6.2 6.2 6.2

0.23 0.23 0.23

1 1 1

1 1 1

27.7

239.42

230.54

6.2

0.23

1

1

Directional filter is assumed “pkva.”

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Table 2.8 Performance of different directional filters in contourlet transform. Filter

Time (s)

Psnr (db)

SNR (db)

MSE

MAE

UQI

SSIM

5
3 9
7

15 23 6.2 20 14.5

295.39 275.21 0.23 275.21 295.51

284.75 265.33 1 265.33 285.35

6.2

0.23

1

1

1 6.2 6.2

0.23 0.23

1 1

1 1

cd pkva

Filter for the Laplacian pyramid is “db1.”

Figure 2.2 (A) The original image and (B) the image enhanced by a computational intelligence method after thresholding.

To choose the directional filter another comparison was arranged between four filters (Table 2.8). As “pkva” has the best performance (higher PSNR and SNR and lower TIME), it is the best directional filter for contourlet transform. Fig. 2.2 shows an image denoised and compressed with a computational intelligence method.

2.3

Conclusion

This research presents a study on computational intelligence methods that are applicable in enhancing and analyzing of the pavement surface images. The advantage of these methods is their efficiency in texture extraction. The first step of any image processing work is preprocessing and enhancement of the images. Thus it is important to find the best method for denoising and enhancing images in any case. In this work a comparison was established between wavelet, curvelet, ridgelet, contourlet, and shearlet transforms. Seven assessment parameters were used to investigate the performance of these transforms. The performance of the computational intelligence methods was compared for different filters. The result of this assessment showed

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that ridgelet transform is appropriate for projects in which time is the most important parameter; but in projects in which the quality of the denoised and compressed image is the primary target, shearlet transform is the best method for image enhancement. The results of this work can be a guide in future work for evaluating characteristics of pavement surface such as drainage quality and skid resistance.

References [1] G. Bhutada, R. Anand, S. Saxena, Edge preserved image enhancement using adaptive fusion of images denoised by wavelet and curvelet transform, Digit. Signal Process. 21 (1) (2011) 118
130. [2] A. Jaiswal, J. Upadhyay, A. Somkuwar, Image denoising and quality measurements by using filtering and wavelet based techniques, AEU-Int. J. Electron. Commun. 68 (8) (2014) 699
705. [3] R. Maini, H. Aggarwal, A comprehensive review of image enhancement techniques. arXiv preprint arXiv:1003.4053, 2010. [4] P. Hedaoo, S.S. Godbole, Wavelet thresholding approach for image denoising, Int. J. Netw. Secur. & Its Appl. (IJNSA) 3 (4) (2011) 16
21. [5] A. Rosenfeld, A.C. Kak, Digital Picture Processing., vol. 1, Elsevier, 2014. [6] A.K. Jain, Fundamentals of Digital Image Processing, Prentice-Hall, Inc, 1989. [7] T.-L. Ji, M.K. Sundareshan, H. Roehrig, Adaptive image contrast enhancement based on human visual properties, Med. Imaging, IEEE Trans. on 13 (4) (1994) 573
586. [8] S. Agaian, Advances and problems of the fast orthogonal transforms for signal-images processing applications (part 1), Pattern Recognition, Classification, Forecasting. Yearbook, The Russian Academy of Sciences, 3, Nauka, Moscow, 1990, pp. 146
215. [9] S. Agaian, Advances and problems of fast orthogonal transform for signal/image processing applications, Part 1 (1991) 146
215. [10] S. Aghagolzadeh, O.K. Ersoy, Transform image enhancement, Optical Eng. 31 (3) (1992) 614
626. [11] D.C. Wang, A.H. Vagnucci, C. Li, Digital image enhancement: a survey, Comput. Gr. Image Process. 24 (3) (1983) 363
381. [12] W.M. Morrow, et al., Region-based contrast enhancement of mammograms, Med. Imaging, IEEE Trans. on 11 (3) (1992) 392
406. [13] A. Beghdadi, A. Le Negrate, Contrast enhancement technique based on local detection of edges, Comput. Gr. Image Process. 46 (2) (1989) 162
174. [14] R.C. Gonzalez, R.E. Woods, Digital Image Processing, Prentice Hall, Upper Saddle River, NJ, 2002. [15] A.M. Grigoryan, S.S. Agaian, Transform-based image enhancement algorithms with performance measure, Adv. Imaging Electron. Phys. 130 (2004) 165
242. [16] S.S. Agaian, K. Panetta, A.M. Grigoryan, Transform-based image enhancement algorithms with performance measure, IEEE Trans. Image Process. 10 (3) (2001) 367
382. [17] J.-L. Starck, M. Elad, D. Donoho, Redundant multiscale transforms and their application for morphological component separation, Adv. Imaging Electron. Phys. 132 (82) (2004) 287
348. [18] J. Li, A Wavelet Approach to Edge Detection, Sam Houston State University, 2003. [19] I. Daubechies, Orthonormal bases of compactly supported wavelets, Commun. Pur. Appl. Math. 41 (7) (1988) 909
996.

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[20] S.G. Mallat, A theory for multiresolution signal decomposition: the wavelet representation, Pattern Anal. Mach. Intelligence, IEEE Trans. on 11 (7) (1989) 674
693. [21] S.G. Mallat, S. Zhong, Complete Signal Representation With Multiscale Edges, New York University, Courant Institute of Mathematical Sciences, Computer Science Division, 1989. [22] Y. Piao, L.-H. Shin, H. Park, Image resolution enhancement using inter-subband correlation in wavelet domain, in: Image Processing, 2007. ICIP 2007. IEEE International Conference on. 2007, IEEE. [23] H. Demirel, G. Anbarjafari, Satellite image resolution enhancement using complex wavelet transform, Geosci. Remote. Sens. Letters, IEEE 7 (1) (2010) 123
126. [24] C.B. Atkins, C.A. Bouman, J.P. Allebach, Optimal image scaling using pixel classification, in: Image Processing, 2001. Proceedings. 2001 International Conference on. 2001. IEEE. [25] W.K. Carey, D.B. Chuang, S.S. Hemami, Regularity-preserving image interpolation, Image Processing, IEEE Trans. on 8 (9) (1999) 1293
1297. [26] S. Mallat, A Wavelet Tour of Signal Processing, (Wavelet Analysis & Its Applications), second ed., Academic Press, 1999. [27] E.J. Candes, Ridgelets: Theory and Applications, Stanford University, 1998. [28] E.J. Cande`s, D.L. Donoho, Ridgelets: a key to higher-dimensional intermittency? Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 357 (1760) (1999) 2495
2509. [29] S.R. Deans, The Radon Transform and Some of Its Applications, Courier Corporation, 2007. [30] E.D. Bolker, The finite Radon transform, Contemp. Math. 63 (1987) 27
50. [31] P. Campisi, A. Neri, G. Scarano, Model based rotation-invariant texture classification, in: Image Processing. 2002. Proceedings. 2002 International Conference on. 2002, IEEE. [32] E.J. Candes, D.L. Donoho, Curvelets: A Surprisingly Effective Nonadaptive Representation for Objects With Edges, DTIC Document, 2000. [33] K. Guo, et al., Wavelets with composite dilations and their MRA properties, Appl. Comput. Harmon. Anal. 20 (2) (2006) 202
236. [34] D. Labate, et al., Sparse multidimensional representation using shearlets, Optics & Photonics 2005, International Society for Optics and Photonics, 2005. [35] K. Guo, D. Labate, Optimally sparse multidimensional representation using shearlets, SIAM J. Math. Anal. 39 (1) (2007) 298
318. [36] M. Do, M. Vetterli, Contourlets, in: J. Stoeckler, G.V. Welland (Eds.), Beyond Wavelet, Academic Press, New York, 2003. [37] M. Do, M. Vetterli, Contourlet: a computational framework for directional multiresolution image representation, in: IEEE Trans. Image Proc., 2003. [38] M.N. Do, D.M.I., Representations (Ph.D. dissertation), Swiss Federal Institute of Technology, 2001. [39] R. Eslami, H. Radha, On low bit-rate coding using the contourlet transform, in: Signals, Systems and Computers, 2004. Conference Record of the Thirty-Seventh Asilomar Conference on, 2003, IEEE.

Further reading R. Gonzalez, R. Woods, Digital Image Processing, Pearson Prentice Hall, Upper Saddle River, NJ, 2008. W.K. Pratt, Digital Image Processing, hird ed., Wiley, 2009.

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Sajad Ranjbar1, Fereidoon Moghadas Nejad1, Hamzeh Zakeri1 and Amir H. Gandomi2 1 Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran, 2Faculty of Engineering & Information Technology, University of Technology Sydney, NSW, Australia

3.1

Introduction

The significant importance of transportation infrastructures in human life and economic development of countries lead to performing widespread efforts to develop, manage, and maintain these infrastructures. A large part of these efforts is conducted on roads as one of the most important subsets of transportation infrastructures. Pavement conditions have a great impact on the safety and efficiency of roads and are affected by various factors such as traffic characteristics, climatic effects, design parameter, construction process, maintenance activities. The pavement conditions usually deteriorate during the lifetime and it is necessary to perform a management process known as pavement management system (PMS). In the procedure of PMS, at first, the pavement inventory is defined, then the current condition of the pavement is determined by detection and analysis of pavement distresses and calculation of pavement indices. Pavement inspection information is the fuel of the PMS engine. In the next step, it is necessary to predict the future condition of pavement based on deterioration function and inspection information (pavement indices, distresses). Pavement condition prediction is the engine of PMS. After predicting the future condition of the pavement, a decision-making process from several M&R strategies is performed with regard to several factors such as budget constraint, the importance of pavement sections, technical needs [1]. An efficient PMS have several objectives, such as achieving an optimum state in the enhancement level of serviceability, budget activities, and scheduling maintenance for entire networks and projects [2]. It means that the outputs of PMS lead to performing the best maintenance and rehabilitation (M&R) alternative in the best time, and with optimum cost and benefits for entire networks and projects. Depending on the breadth of roads network, a variety of pavement distresses, type, and various M&R strategies, the analysis of collected data, prediction process, and decision-making process by the traditional method (expert staff) are very timeconsuming procedures, and practically impossible. Thus, using the proper expert system based on computer science is unavoidable. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00003-X © 2020 Elsevier Inc. All rights reserved.

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Artificial intelligence (AI) is a branch of computer science that attempts to create an intelligent machine that works like a human [2,3]. AI systems provide several important abilities for a computer-based system such as problem-solving, learning, reasoning, and planning. Also, computational intelligence (CI) provides natureinspired techniques that create robust tools for solving complex problems for various challenges that traditional techniques would be ineffective to address [4,5]. In the methodology of CI techniques, input data is processed to explore representational patterns and rules for generating a reliable response in various applications such as object detection, decision making, optimization [6]. Generally, the techniques based on CI can be divided into five main frameworks including artificial neural network (ANN), fuzzy logic (FL), evolutionary computation (EC), swarm intelligence (SI), and hybrid method (HM). In recent years, the growth of computing power leads to the extensive use of CI methods in various engineering applications such as automatic control system, decision-making system, speech recognition, handwriting recognition, audio processing, robotics, computer vision, self-driving car, optimization problem, and expert systems [7
12]. According to the abilities of CI techniques, they can be used in different parts of PMS and can create great contributions in various challenges such as pavement inventory definition, condition assessment (distress detection, classification, and quantification of distresses), prediction (budget, pavement condition), prioritization, and optimization in M&R operations selection. This chapter presents a comprehensive view of CI frameworks and their applications in PMS procedure. The chapter is organized as follows: Section 2 presents the summarized information on CI methods. The methodology of the new and effective techniques from the CI frameworks is presented in Section 3 with several examples of their application in PMS. Section 4 tries to provide a comprehensive view of the application of CI methods in various subsystems of PMS. Finally, Section 5 presents the conclusion of the chapter.

3.2

CI methods

The primary basis of CI can be categorized into various forms. In this chapter, artificial neural network (ANN), fuzzy logic (FL), evolutionary computation (EC), swarm intelligence (SI), and hybrid methods (HM) have been introduced as the most important foundation of CI. It should be noted that other methods and algorithms such as probabilistic methods, artificial immune systems can be considered as CI methods. In this section, summarized information has been provided on the ANN, FL, EC, SI, and HM.

3.2.1 Artificial neural network ANN is a framework that has been built based on a simulation of the biological nervous system to imitate the human brain performance in learning [13,14]. In the

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structure of ANN, a functional relationship (usually non-linear function) between inputs and affecting parameters is being considered, and historical data is being used in determining the best functional coefficient [15,16]. It means that the structure of the model and unknown parameters of the models are being determined based on data in the ANN process. ANNs are trained by experiences (input data) and can create a model of a system where there is no clear relationship between its inputs and outputs [4]. According to this ability, ANN is one of the most flexible and powerful techniques in machine learning science and has a wide application in performing the data mining tasks such as classification, prediction, estimation, clustering [4,17]. Also, ANN is a powerful framework for some applications such as feature extraction [9,18
20], object detection [19,21
23], image classification [21,24], computer vision [19,25], speech recognition [19,26], text and handwriting recognition [21,27], etc. There are various ANN architectures including feed-forward neural network (FFNN) such as multilayer perceptron network (MLP) [4,13,28], and Radial Basis Function (RBF) [4,13,29], recurrent neural network (RNN) such as Hopfield’s recurrent network [28,30
32], self-organizing neural network (SONN) [28,33,34], deep learning (DL) [35
37], convolutional neural network (CNN) [37,38], etc. Also, there are three learning strategies for using ANNs in problems including supervised, unsupervised, and semi-supervised learning. G

G

G

Supervised learning works based on known training dataset with labeled data, and it means that the network is created according to inputs and desired outputs or target values. These learning strategies are applicable for some data mining tasks such as classification, estimation, and prediction [16,36,39
41]. Unsupervised learning uses original data without utilizing label information and the network is built just according to inputs. These learning strategies are usable for data mining tasks such as clustering and affinity grouping. The objective of unsupervised learning is to discover “interesting structure” in the data [16,36,39
41]. Semi-supervised learning is used when class labels have been missed, and the training data has labeled as well as unlabeled data [28,42,43].

3.2.2 Fuzzy logic A considerable part of concepts and information in human reasoning, analyzing, and programming have inherent uncertainty, and they are almost always inexact. On the other hand, the classical logical systems such as Boolean logic are not appropriate systems to solve problems in an environment of uncertainty and imprecision [2,44]. FL is one of the CI methods that provide the ability to model the vagueness and imprecision in the procedure of solving problems [13]. FL and fuzzy sets are two mine concepts in the structure of fuzzy systems. Each input element in a fuzzy set has a degree of belonging to the set that can be specified numerically by the membership function (MF) [13,39,45]. In fact, fuzzy logic presents a generalization of classical logic [46]. The MF is a curve that can be constructed by any method: exact, heuristic and metaheuristic, and can be different shapes based on different types of fuzzy sets such as bell, sigmoid, triangular,

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trapezoidal, Gaussian, S, or p function in the domain [2,47,48]. The selection of proper MF is a difficult task. The procedure of implementing the fuzzy systems consists of four main stages including fuzzification, fuzzy rule base, fuzzy inference engine, and defuzzification [13,45]. In the fuzzification stage, each piece of the input data is converted to degrees of belonging by membership functions. The fuzzy rule base contains rules as a set of IF-THEN statements including all possible fuzzy relations between the inputs and outputs. In a fuzzy inference engine, all the fuzzy rules are being considered to formulate the mapping from a set of inputs to their corresponding outputs. Then, the fuzzy outputs are converted to a number through the defuzzification process [13,45,47]. The fuzzy systems are powerful systems for approximate reasoning and nonlinear complex system modeling in ambiguous, vague, and not crisp applications [2], and have a wide application in automatic control systems [45,49], performing some data mining task [50] (such as clustering and classification [51]), decision making systems [10,52], modeling and forecasting tasks [8,10], image processing systems [47], expert systems [47,51]. There are two popular types of fuzzy systems known as fuzzy type I and fuzzy type II. The main difference between two types of fuzzy systems is that the type II fuzzy sets can model such uncertainties because their membership functions are themselves fuzzy, while the membership functions in type I fuzzy sets are crisp [2,53]. Accordingly, type II fuzzy sets have better performance in more uncertain environments [51]. Moreover, some researches have been performed to develop a higher level of fuzzy sets such as type III, IV, and type n [54,55].

3.2.3 Evolutionary computation EC is one of the common CI frameworks and has been inspired by the process of evolution in nature. In this type of CI, an iterative procedure is being used to imitate the biological evolution to perform optimization tasks [56]. In recent years, the significant progress in computation power leads to transforming EC into the popular and robust framework in optimization problems [57]. The distinctive features of EC methods compared with other classical optimization methods are that the EC methods rrely on a population of solutions instead of searching using a single solution; it means that the search is being conducted in parallel from a population of solutions [57]. Therefore, they are flexible in solving various types of optimization problems with a high level of complexity. The structure of EC methods consists of several main concepts including encoding, generating population, fitness evaluation, selection, crossover, mutation, and stopping criteria. In the encoding process, a proper encoding scheme (such as binary, numbers, and strings) is being selected to transform the variables into a suitable representation in conducting operations. For example, genetic algorithms use binary encoding. Generating population is the start of evolutionary algorithms, often performed with a randomly initialized population. By applying a fitness function, a ranking and evaluation of individuals are being done. Then, the best

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individuals are selected as parent individuals and cross-over with each other to generate new individuals that are known as offspring individuals. Randomly selected offspring individuals are subjected to specified mutations, and again, the fitness evaluation and ranking of new individuals are being done. This iterative process is stopped when the stopping criteria are met. The stopping criteria are checked after each cycle, and the number of generations (iteration of the process) usually is considered as the stopping criteria [57,58]. EC can be implemented by several evolutionary algorithms that have wide application in optimization problems including genetic algorithms (GA) [13,59], differential evolution (DE) [13,59], evolutionary programming (EP) [59], Harmony search algorithm (HSA) [7,13]

3.2.4 Swarm intelligence SI is a branch of CI that has been created by simulating the collective intelligent behavior of insect or animal groups such as flocks of birds, colonies of ants, schools of fish, swarms of bees [13,60]. Also, SI is known as bio-inspired computation method, developed based on studying the collective behavior of swarms in nature through the complex interaction of individuals with no supervision [60,61]. It should be noted that SI can be created by the collective behavior of artificial agents such as robots in foraging robots [62,63]. As with EC framework, SI is a very popular and influential tool for optimization tasks; the algorithms based on SI have high flexibility for use in almost all areas of sciences, and have high efficiency in solving the complex problems that for which classical methods have no efficient solution [64]. In the process of SI methods a fixed-size population of individuals is being used for search across generations, and in each generation, the findings of individuals are being evaluated to adjust the search strategy in the next generation without any selection operations on individuals; it is the main difference between EC and SI methods [58]. Self-organization strategies and independent work of each individual for solving the problems are two important characteristics of the SI methods. Self-organizing strategy leads to creating a system of individuals that react to local stimulation individually, and may act together to perform a global task; and independent working state leads to avoid centralized supervision [60,64]. These features make it possible to simulate the collective behavior of insect or animal groups in nature. In recent years, a large number of swarm algorithms have been developed based on simulating the behavior of various animals including particle swarm optimization (PSO) [13,65], ant colony optimization (ACO) [13,64,66], firefly algorithm (FA) [13,67], bat algorithm (BA) [68,69], krill herd algorithm (KHA) [13,70,71], cuckoo search algorithm (CSA) [13,72], honey bee algorithm (HBA) [13,64], flower pollination algorithm (FPA) [64], lion optimization algorithm (LOA) [73], cat algorithm (CA) [74] It should be noted that selecting the proper algorithm is a key stage in the procedure of problem solution. The algorithms based on EC and SI framework are often categorized in metaheuristic algorithms and have a wide application in engineering problems [7,72,75
83].

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3.2.5 Hybrid method The combination CI methods can be applied to achieve a more efficient system in different applications. There are six possible dual combinations for developing a hybrid method including ANN-FL [15,84], ANN-EC [85], ANN-SI [86], FL-EC [87,88], FL-SI [86], and EC-SI [83,89]. Moreover, some researches were performed to combine more than two CI frameworks [86] or combine several techniques from the same CI framework [90] to create an agent for problem-solving. According to presented information on CI frameworks, a comprehensive view of CI techniques and their main applications are shown in Fig. 3.1.

3.3

Methodology and application

According to presented information in Section 2, the CI frameworks (ANN, FL, EC, SI, and HM) are being applied with three main objectives including (1) learning from historical data and using in future tasks, (2) providing the ability of problem-solving in uncertain and ambiguous environment, and (3) optimizing the performance of systems. In this section, the methodology of some of the newest and more efficient techniques in recent years for achieving the mentioned goals has been essential. Therefore, the methodologies of DL and CNN techniques have been explained as

Figure 3.1 A comprehensive view of CI techniques.

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two prevalent tools for learning and modeling. Also, type-II fuzzy logic systems (T2FL) has been explained as an efficient tool for solving problems with uncertainty, and the methodology of emperor penguin algorithm (EPA) has been presented as the powerful new tool for optimization applications. Also, the application of these techniques has been presented in this section.

3.3.1 Deep learning Deep learning is a machine learning method based on neural networks that applied multiple layers of processing information enabling computers to automaticly extract features from data with multiple levels of abstraction; in each transition of layers, representation at one layer transforms into representation at a higher abstract level in the next layer [9,12,35,91,92]. The distinctive property of deep learning is that layers of the feature are learned automatically from data [12,19]. Deep learning can be used for supervised, unsupervised, and semi-supervised learning and has a widespread application in computer vision, analysis of big data, speech recognition, object detection, handwriting recognition, image classification, audio processing, robotics, self-driving car, drug discovery [11,12,91,93]. A deep network is a hierarchical model where each layer uses a linear transformation after a non-linearity to the previous layer [94]. In a simple example, it can be assumed that input data be XAℝN 3 D , where each row of X is a D-dimensional data point such as a grayscale image with D pixels, and N is the number of training examples [94]. W k was defined as a matrix for indicating a linear transformation applied to the output of the layer k 2 1 to achieve a dk -dimensional representation at layer k: [94] Xk21 W k AℝN 3 dk

(3.1)

In the next step, a non-linear activation function (ψk ) is applied to each entry of Xk21 W k to generate the kth layer of a neural network as:
 Xk 5 ψk Xk21 W k

(3.2)

The non-linear activation function (ψk ) can be a different function, such as a hyperbolic tangent (tanhðxÞ) or a sigmoid (ð11e2x Þ21 ) Accordingly, the output of the network is given by:





     Φ X; W 1 ; W 2 ; . . . ; W k 5 ψk ψk21 . . . ψ2 ψ1 XW 1 W 2 . . . W k21 W k (3.3) Where Φ is an N 3 C matrix, in which C is the dimension of the output of the network, equal to the number of classes in the case  of a  classification problem. k Learning the parameters of the deep network W k k51 from N training examples is performed as an optimization problem. In a classification setting, each row of X

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indicates a data point in ℝD and each row of Y indicates the membership of each data point to one out of C classes. In a regression setting, the rows of Y denote the dependent variables for the rows of X. Consequently, learning the network weights (W) is formulated as: [94] YAℝN 3 C

;

XAℝN 3 D




 Min ‘ Y; Φ X; W 1 ; W 2 ; . . .; W k 1 λΘ W 1 ; W 2 ; . . .; W k

(3.4)

In this formula, a loss function ‘(Y, Φ) measures the compliance between the pre-determined classes of the classification problem as true output (Y), and the predicted classes as the output of the network (Φ) from Eq. (3.3). Also, Θ is a regularization function considered to prevent overfitting and λ is a balancing parameter which has positive value [94]. For more details on the mathematical basis of deep learning see Refs. [94
96].

3.3.2 Convolutional neural network (CNN) CNN is a special form of deep neural networks that was designed to process data that have multiple arrays and grid-like topology [97
99]. CNNs can be used on 3D (video), 2D (image), and 1D (text or audio) input data to perform one of the mentioned deep learning applications [11,38]. In recent years, a large number of researches were conducted in pavement management systems to automatic feature extraction and distress detection by applying CNN on the pavement image as input data [19,99
105]. The structure of CNNs consisted of three main substructures, which include: convolutional layers, pooling layers, fully connected layers. Convolutional layers are made from several feature maps, and each unit of feature maps is made from convolving a small region in input data which is called the local receptive field. As can be seen in Fig. 3.2, a new feature map is created by sliding a local receptive field over the input. The convolution can be used in various kinds of data such as image, text. For example, in the image, an area of pixels is convolved, and in the text, a group of characters or words are convolved. Unlike the standard neural network, each neuron in the layers is not connected to all of the nodes (neurons) in the previous layer but is just connected to nodes in a special region known as the local receptive field [11,19,37,97
99,106,107]. Pooling layers are commonly used immediately after convolutional layers. These layers were generated to simplify the information and reduce the scale of feature maps. In other words, pooling layers make a condensed feature map from each feature map in convolutional layers. In some references, these layers are called the subsampling layer. Pooling operation can be performed in various types such as geometric average, harmonic average, maximum pooling [108]. Max-pooling and average-pooling are two of the most prevalent processes for pooling that have been presented in Fig. 3.3. The pooling layers are necessary to reduce the computational time and overfitting issues in the CNN [11,19,37,97
99,106,107,109].

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Figure 3.2 Convolution process to create feature maps.

Figure 3.3 An example of (A) max-pooling and (B) average-pooling operation.

Fully connected layers are the final layers in the CNN structure that can be one or more layers and placed after a sequence of convolution and pooling layers. This part of CNN comprises the composite and aggregates of the most important information from all procedures of CNN. Consequently, these layers provide the feature vector for the input data, which can be used for some machine learning tasks such as classification, prediction [11,19,37,97
99,106,107]. The last layer of fully connected layers is known as softmax classifier and determines the probability of each class label over N classes [11,37,97]. Designing the CNN structure is a big challenge because there are many hyperparameters that have significant influence on the efficiency of CNNs such as depth (which includes the number of convolutional, pooling, and fully-connected layers), the number of filters, stride (step-size that the local receptive field must be moved), pooling types, locations, and sizes, and the number of units in fully-connected layers [97,98]. Finding the proper hyperparameters combination needs expert

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knowledge and is often performed as a trial and error process. Recently, this challenge has been raised as an optimization problem [97,110
113]. An example of the CNN structure based on its applications in computer vision and image classification for pavement distress detection and classification is shown in Fig. 3.4. Generally, there are two methods for applying CNN models that include: training from scratch and performing transfer learning by use of pre-trained models. If the first method (training from scratch) was applied for training a CNN model, it would be necessary to define the number of layers and filters and use massive amounts of data which are a time-consuming procedure. Also, the CNN structure design to achieve proper results is a big challenge because there are many hyperparameters that have an influence on the efficiency of CNNs, such as depth (which includes the number of convolutional, pooling, and fully-connected layers), the number of filters, stride (step-size that the local receptive field must be moved), pooling locations and sizes, and the number of units in fully-connected layers [97,98]. Furthermore, finding the proper hyperparameters combination needs expert knowledge and is often performed as a trial and error process. Recently, this challenge has been raised as an optimization problem [97,110
113]. In the other method (transfer learning), one of the pre-trained CNN models is being used that was trained on the source domain (big image data set). This means that in transfer learning, the ability of pre-trained models to learn the predictive function helps to train the new target domain (new image dataset) instead of training from scratch [18,19,114
116]. Transfer learning is a much faster and easier method for applying deep learning, and in this method, it is not necessary to understand the structure and combinations of network layers. The application of transfer learning in pavement distresses detection and classification is displayed in Fig. 3.5. As can be seen in Fig. 3.5, it is possible to create a pavement distress detector and classifier model by using transfer learning techniques, and a proper dataset of pavement distresses image. Pre-trained models are CNNs that trained by a huge number of the image with the aim of detection and classification data (images) in a large number of classes. AlexNet [117,118], GoogleNet [119,120], SqueezNet [121], ResNet [122], DenseNet-201 [123], Inception-v3 [124], and VGG [125] are some of the more prevalent pre-trained models used in transfer learning technique.

3.3.3 Type 2 fuzzy logic systems Type 2 fuzzy system (T2FS) is one of the most popular and efficient techniques based on FL as a CI framework. Fuzzy systems provide an efficient tool for solving problems with an uncertainty that is an inherent feature of information in different branches of science. Despite the wide application of type 1 fuzzy system (T1FS) in various disciplines such as pattern recognition, control system, and engineering problem, it cannot provide an effective solution in problems with more uncertainty. Accordingly, T2FS was developed with more efficiency at a high level of uncertainty.

Figure 3.4 CNN structure for pavement distress detection and classification.

Figure 3.5 Transfer learning application in PMS.

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In fact T2FSs are considered as a blurred membership function. The blurring is used to model the uncertainty of crisp T1FSs. A T2FS can be formulated as follow: [47] ð

μA5 ðxÞ 5 A5 x xAX

ð

Ð xAX

xAX

fx ðuÞ u

X Jx

; XJx D½0; 1

(3.5)

where fx ðuÞ the blurred membership function and Jx is the original membership. The footprint of uncertainty (FOU) is a region between the blurred membership function. The FOU of A 5 can be expressed by as: [47] 5 5 5   FOU A 5 , ’ xAX Jx 5 ðx; uÞ:uAJx D½0; 1 μA~ 5 FOU A and by μA~ 5 FOU A

(3.6) FOU constructed form upper membership function (UMF) and lower membership function (LMF). In General, FOU (A 5 ) can be expressed as: [47] 5 h 5  5 i FOU A 5 , FOU A ; FOU A

’ xAX

(3.7)

General type 2 fuzzy system (GT2FS) and interval type 2 fuzzy system (IT2FS) are two popular types of T2FS. The GT2FS considers all possible combinations of secondary membership values and has the ability to model uncertainty, based on the (FOU) which, compared with its third dimension, is more precise than other types, and shows a higher degree of stability in the simulations. On the other hand, GT2FSs have high complexity related to structure and computational time, which makes them unsuitable methods for real-world applications. IT2FS is an efficient method to reduce the computational complexity. IT2FSs are a unique form of GT2FS that secondary membership values for the entire members of the primary domain are 1. This property reduces computational efforts required for analysis of GT2 FSs while significantly enhancing fuzziness handling capabilities of T1FSs [2,47,51]. A view of two kinds of T2FS is presented in Fig. 3.6. As mentioned in Section 2.2, fuzzy systems generally consist of four main steps including fuzzification, fuzzy rule base, fuzzy inference engine, and defuzzification but, as can be seen in Fig. 3.7, T2FSs require an additional step for representing a single value as the representative of the uncertainty. This step is called typereduction, which converts a T2FS into its type 1. Type-reduction is a necessary step in different computational intelligence fields, including fuzzy systems, clustering, and classification [51]. The MF is one of the most essential parts in structure of fuzzy systems. As mentioned in Section 2.2, Membership functions for fuzzy sets can have various shapes and be constructed by any heuristic or metaheuristic method in the domain. The selection of proper MF is a difficult task, and several pieces of research are conducted with the aim of automatic MF generator. Two most important constraints

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Figure 3.6 A view of T2FSs: (A) IT2FS and (B) GT2FS [2,51].

Figure 3.7 Main steps in the structure of T2FSs.

must be considered for selecting a membership function. First, a membership function must be restricted to between [0 1], and the next μAðxÞ must be unique [2,53]. In T2FSs, the MFs are themselves fuzzy, and four possible fuzzy membership functions (type I, II, III, and polar) are presented in Fig. 3.8 [2,47]. The new generation of fuzzy membership functions (type III and polar) are efficient to use for several applications in the control and classification domains. In the field of PMS, this new generation of MF provides a powerful link among several tools such as multi-resolution methods (wavelet and beyond the wavelet), image processing, ANN and expert system. As an example of T2FS in the PMS, it can be insinuated to the application of these techniques in pavement crack detection. When radon transform (an effective tool in pavement cracking detection) is applied to a wavelet modulus, distress (crack) is transformed into a peak in the radon domain. In this method, it is necessary to determine the thresholds for distress detection. According to uncertainties in 3D radon transform, it may have different thresholds. T2FS can create a flexible threshold for automatic thresholding and build a high-

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Figure 3.8 Possible fuzzy membership functions.

accuracy relationship between the patterns of the peaks and the properties of the crack using automatic fuzzy threshold selection [2,47,126]. Two examples of the application of T2FS in pavement distress detection are presented in Fig. 3.9 based on the three-dimensional domain of FOU for 3D fuzzy sets and three-dimensional polar domain of FOU for 3D fuzzy sets.

3.3.4 Emperor penguin algorithm As previously mentioned, Nature has inspired scientists to develop new algorithms for solving difficult problems. In this part of the chapter, another swarm intelligence-based algorithm has been presented called emperor penguin algorithm (EPA), based on the spiral-like behavior of the emperor penguins, in response to climate change and seasonal variation processes. These penguins can maintain their body temperature in a large colony by timed movements toward the center of a large group. This new optimization algorithm was inspired by emperor penguin’ huddling motions and each emperor penguin independently. Emperor penguins and their huddling behavior are amazing, and they are one of the most adaptive animals on the planet, both in terms of their breeding habits and their ability to adjust to cold climate conditions. Most animal species begin their breeding cycle in the spring and summer; however, emperor penguins have developed a cycle that requires them to begin their nesting when temperatures can be as low as 245 C. Their efficient metabolism as well as standing in a compact huddle can be considered as a behavioral adaptation or a form of swarm intelligence. Hibernation behavior of emperor penguins and huddling in a dense group enables them to keep warmth with the highest temperature in the center and lowest on the outside, so as to minimize the overall heat loss. The penguins on the outside face into the huddle herd and slowly move inwards, thus forming a churning and rotational action and not leaving one the outer layers exposed to the extreme cold too long. From a colony-scale and algorithm point of view, penguins may have a spirallike movement towards the center so as to achieve a maximum spread of any

Figure 3.9 The application of T2FS in pavement crack detection: (A) three-dimensional domain of FOU for 3D fuzzy sets, (B) threedimensional polar domain of FOU for 3D fuzzy sets [2,47].

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temperature over the whole colony and thus minimize the effect on each individual. This step can be considered as a diversification step. After reaching the center which can be considered as intensification, a spiral-like movement to the outside continues in a logarithmic spiral. Intensification and diversification play a crucial and indeed central role in metaheuristic algorithms. The intensification phase searches around promoting solutions and selects better candidates or solutions. Intensification guarantees that the algorithm converges to the good solution and the diversification phase ensures that the algorithm explores the search space more efficiently. For developing the EPA, it is necessary to idealize the characteristics of a colony of penguins. Accordingly, several idealized rules were followed: 1. All penguins use spiral-like motion to sense distance, and they can control their metabolism by activities such as shivering, walking, and swimming. 2. Penguins converge to the center randomly with a logarithmic velocity ðvi Þ at position ðxi Þ, varying ðrÞ, and arc length ðli Þ to reach the heat center. They can automatically track the spiral route depending on the proximity heat and swarm behavior. 3. Although the spiral function can vary in many ways, it is assumed that the movement pattern varies from a large (positive) l0 to a minimum lmin D0. 4. Attractiveness is proportional to their heater neighbor, thus for any two emperor penguins, the warmer one plays the role of the center of colony, and the colder one will move towards the warmer one. 5. The attractiveness is proportional to the heat of the emperor penguin’s body, and they decrease as their distance (Arc-length) increases.

The mathematical expression of spiral movement in an N-dimensional space can be modeled as: 

dp dn

i 5 f fPn ; Sn11 ; randi g

(3.8)

where pn is the position of an emperor penguin; sn11 is the spiral motion as an attractiveness function, and rand i is the function that generates randomization near or inside the updated position in iteration (i). Initially, each emperor penguin should have random values of heat and position with random sni arc, and this can be achieved by the randomization function. The distance between any two emperor penguins i and j, determined as a polar distance with respect to selected spiral function. The arc length of the polar curve is given by: Sni 5

ð ðθ2 Þn qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2
2
rij n 1 dr=dθ dθ

(3.9)

ðθ 1 Þn

The intensification and diversification of an emperor penguin i that is attracted to another more attractive (heater) emperor penguin j is determined by: Sni11 5 S0i ½ð12e2τ r Þm  1 R

(3.10)

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Where S0i is the initial length of the arc in iteration i, and the first term is due to the intensification while the third term (R) is randomization parameter which is uniformly distributed in ½0; 1 and help to provide diversification. Also, τ and m are consistent parameters as the controller of the speed rate to converge. As can be seen, the parameter r plays a crucial role and will determine the total performance of the algorithm. This parameter represents the spiral term that can be generated by a variety of functions such as Archimedes’ Spiral, Circle Involute, Conical Spiral, Cornu Spiral, Cotes’ Spiral, Daisy, Epispiral, Fermat’s Spiral, Helix, Hyperbolic Spiral, Logarithmic Spiral, Mice Problem. According to Sni11 the new position is then estimated based on the new ri11 and n θi11 as follow:

 xi11 5 ri11 jUcos θni11 ; yi11 5 ri11 jUsin θni11

(3.11)

Where i and j are ID number of emperor penguins in the swarm. The heat or warmness of an emperor penguin can be associated with the objective function. For a maximization problem, the warm point and the intensity of heat can simply be proportional to the value of the objective function. The minimum length of the spiral arc and the maximum number of iterations are two stop criteria in the EPA process. The spiral-like movement and update the position of any individual emperor penguins have been showed in Fig. 3.10.

Figure 3.10 The process of the EPA.

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In fact, EPA can find the global optima as well as all the local optima simultaneously by a novel diversification and intensification method, and this may be extended to the original idea of the firefly algorithm. EPA is very flexible because of its powerful movement and searching in a variety of spiral-like functions. This algorithm is self-adaptive and can simultaneously handle diversification and intensification by a spiral-like motion of emperor penguins. The randomization function helps to diversification phase and has a very key role in converging to the local and global optima. As an example of the applications of EPA in PMS, it can be referred to as crack detection, classification, and quantification. The pavement images (or their transforms like radon, wavelet) thresholding, is the first step toward success in highquality distress detection and classification. The numbers of the peak are corresponding to the number of cracks. So the numbers can be used as one of the parameters to determine the type of cracks as a single crack or multiple cracks. Thus, finding local and global maximum is significant. The position of the peaks can be easily used to classify the orientations and location of the cracks. Area of the peaks (density of emperor penguins) can be used to quantify the width and severity of a crack, and the value of a peak (maximum heat in a colony) can be used to quantify the length and extent of the crack. There is another parameter of the peak (average heat of emperor penguin colony), which is volume covered by peak, that can be used to serve as a general index for crack quantification. The application of EPA in the detection of peaks in radon domain for pavement cracking diagnosing has been shown in Fig. 3.11.

3.4

Application of CI frameworks in PMS

As can be seen in Fig. 3.12, PMS consists of several subsystems including inventory definition, pavement condition assessment, pavement condition prediction, maintenance and rehabilitation (M&R) operations analysis [1,2,127]. In this section, an explanation of PMS subsystems has been presented, and the applications of CI frameworks in each subsystem of PMS have been investigated.

3.4.1 Inventory definition In this substructure of PMS, network, branches, and sections are being defined. A network is a logical grouping of pavements for managing the M&R activities and is broken into several smaller parts known as branches. Each branch is a part of a network that has a special application and can be divided into several sections according to some distinctive factors including pavement structure, condition, construction history, size [1]. Definition of pavement networks, branches, and sections is an essential step in PMS procedure and is a key factor that affects time and cost. Accordingly, some researches were conducted with the aim of evaluating the pavement network

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Figure 3.11 The application of EPA algorithm for pavement cracking diagnosing.

Figure 3.12 Subsystems of PMS.

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reliability based on fuzzy set technique to describe the condition state of pavement management segments, pavement links, pavement routes, and eventually the whole network, and the results lead to more effective decisions on pavement network maintenance with a limited budget [128,129]. The arrangement of inspection units is another key factor that is defined before the inspection process. Investigation of large numbers of sections and inspection units is time-consuming, needs high computation efforts, and leads to a costly inspection process. On the other hand, the low number of inspection units may lead to unreliable results. Accordingly, optimization CI frameworks (EC and SI) such as HM with a combination of GA and PSO approaches can be used to create an optimal arrangement of surveyed pavement inspection units for cost reduction, minimization of inspection errors and accuracy improvement of pavement network analysis [83,89,130].

3.4.2 Condition assessment Pavement condition assessment consists of several steps such as pavement inspection, evaluating the collected data for distress detection, classification, and quantification, calculating the pavement indices that presented the condition of pavement. As shown in Fig. 3.13, pavement inspection and collecting information on the pavement condition (specifically on pavement distresses) can be done in two general ways: (1) visually by human experts and (2) automatic systems by applying various technologies such as image-based systems (IBS) [104,115,131
135], 3D inspector system (3DIS) such as laser [136
138], Kinect [139
141], and photographic stereo [142,143], ground penetration radar (GPR) [106,144
146], ultrasonic sensor (US) [147
149], accelerometer sensor (AS) [150
155], hybrid systems (HS) [146,156,157]. Departments of road maintenance, repair, and transportations have become more interested in using automatic systems for pavement assessment because of the defects of visual inspection such as high labor costs, time-consuming, unreliable results, unsafe working conditions for laborers, and disturbing the traffic flow [158
162].

Figure 3.13 Pavement condition assessment steps.

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As previously mentioned, the analysis of pavement distresses is a significant component in PMS and creates the input of PMS process. Generally, pavement distresses can be categorized into cracking distress (CD) and non-cracking distress (NCD). CD is the most prevalent pavement distresses, and cracks have a high impact on reducing the design-life of pavement. Also, NCD caused a significant decline in serviceability and safety of roads. As can be seen in Fig. 3.14, the cracks have several types including fatigue crack, transverse crack, longitudinal crack; and NCD can be divided into surface distresses (bleeding, polished aggregate, raveling) and deformation distresses (rutting, shoving, swelling). The collected data from pavement defects cannot be used alone as the input of the PMS process. It is necessary to use proper software and models to analyze the collected data with aim of distress detection, classification, and quantification and extracting the related indexes that represented the pavement conditions such as pavement condition index (PCI), present serviceability index (PSI), distress manifestation index (DMI), international roughness index (IRI), etc. These indices are applied in the next steps of PMS procedure. The main steps of the condition assessment process have been presented in Fig. 3.13. AI systems based on CI frameworks (such as ANN, FL, EC, SI, and HM) have a wide application in the data analysis process. Detection, classification, and quantification of the distress are the most crucial aims in pavement condition assessment. For these aims, a large number of researches were performed based on classical ANN [14,17,163
168], FL systems [126,169
172], EC system such as genetic algorithm [173], SI system such as differential flower pollination algorithm [174], HM with combination of ANN-FL [2,162,175,176], ANN-EC [177], and ANN-SI [178]. In these years, due to the ability of the new forms of ANN such as DL and CNN to automatic feature extraction and object detection, these techniques have become powerful and popular tools for pavement distress detection, classification, and quantification [19,99,100,102,104,114,141,179
185].

3.4.3 Condition prediction Pavement condition prediction is the most critical subsystem of PMS that analyzed information (such as pavement indices, traffic information, climatic data) is being used as input data and modeling the pavement performance and deterioration as output results. Also, the collected data in the inspection process can be used to create predictive models for pavement distresses. The main aim of this step of PMS is to predict the future condition of the pavement networks, branches, and sections. The accuracy of pavement condition prediction results is paramount because the choosing M&R strategies and analysis of different budget are being performed based on condition prediction results. Accordingly, a weak prediction process for the future pavement condition may cause to select inappropriate strategies and inefficient use of resources [1,186]. The ability of learning, efficiency in the uncertain and ambiguous problem, and flexibility to solve problems with a high level of complexity are some properties of CI frameworks that have been transforme into a suitable method to pavement

Figure 3.14 Pavement distresses classification.

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condition prediction. Accordingly, several researches are performed with the aim of predicting the pavement performance, deterioration, and distresses based on ANN [187
194], FL systems [186,195
197], EC technique such as genetic programming [198], HM with the combination of ANN-FL [199,200], ANN-EC [201].

3.4.4 M&R operations analysis After prediction of the future condition of the pavement, it is necessary to determine M&R alternatives for improving the pavement condition with minimum cost and maximum performance level. Technical needs, budget constraints, and environmental aspects are some of the essential factors that have an effect on the selection of optimal M&R strategy [202]. Optimizing the pavement M&R strategy selection problem is very difficult to solve optimally using traditional optimization techniques. On the other hand, CI techniques provided effective solutions for optimizing problems with high complexity. Accordingly, some researches are conducted to optimize pavement M&R strategy selection based on EC methods such as genetic algorithm [203
209], SI techniques such as PSO [127,205,210
213], and ANN [214,215]. Also, the prioritization of M&R activities is another important concept in PMS. Some researchers try to apply CI methods such as ANN [216], FL [217
219], and SI [220] techniques to assign priorities to the pavement section based on several important factors such as budget constraints, pavement condition.

3.5

Conclusion

According to the high importance of roads in transportation infrastructure, it is necessary to develop and manage the road networks. The pavement is the main part of road infrastructures that has great influence on the safety and serviceability of the roads. Evaluating the quality and serviceability of pavements is performed in a system known as PMS and consists of several important tasks such as inventory definition, pavement inspection, pavement condition assessment, pavement performance prediction, M&R operation selection, work planning. Due to the extension of pavement networks and various types of tasks in PMS procedure, the automation and using expert systems in the PMS process are transformed into an essential need. The complexity of problems and uncertain environment in PMS task lead to failure of use of classical techniques and encourage the researchers to apply AI system by use of CI techniques. In this chapter, the CI techniques were categorized into five frameworks, including ANN, FL, EC, SI, HM, and the methodology of the newest and effective techniques was explained. Also, a comprehensive view of the application of CI methods in the different part of PMS was presented. A qualitative comparison between the applications of CI techniques in four main subsystems of PMS has been presented in Fig. 3.15. In this figure, black stars

Figure 3.15 The qualitative comparison of CI frameworks applications.

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represent the direct application of CI methods, and gray stars represent the application of CI methods in combination with other methods as HM. As can be seen in Fig. 3.15, the lowest application of CI methods has occurred in inventory definition as a subsystem of PMS. In recent years, ANN, EC, and SI techniques are not used directly in this part of the PMS procedure. According to the high importance of inventory definition for achieving proper outputs in PMS, this subsystem needs more attention. On the other hand, the most use of CI methods has occurred in pavement condition assessment. The classical ANN has wide application in pavement distress analysis, and today the new branches of ANN (DL and CNN) are the popular and robust tools in pavement condition assessment. EC and SI techniques have little application in this part of PMS despite the excellent ability in optimization problems. These techniques can be used to optimize the learning process in DL and CNN models. In pavement condition prediction, FL techniques are the most prevalent CI method, and optimization tools (such as EC and SI) have the lowest application. Conversely, the optimization techniques have the most use in the M&R operation analysis process, while the ANN and FL frameworks have little use and HM systems are not used in this part of PMS. As shown in Fig. 3.15, ANN methods with the ability to training, feature extraction, and object detection used more in pavement condition assessment for determination and classification of pavement distresses. FL systems are robust techniques to solve the uncertain and ambiguous problem, and they have more application in pavement condition prediction and pavement condition assessment. Also, optimization tools in CI frameworks such as EC and SI have more used in the analysis of M&R operation for optimizing and prioritization of M&R selection problem, and the most use of HM has occurred in pavement condition assessment task. In this chapter, a comprehensive view of CI frameworks and their application in PMS was presented, and it can be concluded that despite the wide attempt to apply expert systems in PMS, the CI frameworks need to be used more in the different part of PMS to create a more effective and optimal system.

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49.

Expanded polystyrene geofoam

4

S.N. Moghaddas Tafreshi1, S.M. Amin Ghotbi Siabil1 and A.R. Dawson2 1 Department of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran, 2 Nottingham Transportation Engineering Centre, University of Nottingham, Nottingham, United Kingdom

4.1

Introduction

Geofoam is the generic name given to lightweight blocks of expanded polystyrene (EPS) or extruded polystyrene (XPS). The main purpose of EPS geofoam is to provide lightweight material fill used in highway or railway construction and bridge abutments. Additionally, other areas of application for EPS geofoam include pipe and culvert protection, thermal breaks (insulation), wave barrier systems, etc. A close view of EPS geofoam is shown in Fig. 4.1. The material structure in EPS geofoam consists of abundant open-ended air bubbles that are attached, bonded, or fused together, and sometimes bundled together; it is manufactured from the extrusion process of raw polystyrene beads. The individual tubes can have simple random geometric shapes in their cross-section (circle,

Figure 4.1 Close view of EPS geofoam under uniaxial compression test.

New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00004-1 © 2020 Elsevier Inc. All rights reserved.

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Beads

Air Steam Expanding

Steam Storage

Molding

Curing

Figure 4.2 Production procedure of EPS geofoam [1].

ellipse, hexagon, octagon, etc.), which on average vary from less than 1 mm to a few millimeters in size. When bundled together, the composite resembles a honeycomb in its overall cross-section assembly. Production of expanded polystyrene is comprised of two processes, preexpansion, and molding. In the pre-expansion stage, the polystyrene beads are placed in a container and heated to a temperature between 80oC and 110oC by steam. The result of this process (also called “pre-puff”) is %polystyrene %beads expanded to approximately 50 times larger than their original size. The new polystyrene spheres are then cooled so as to enable their stabilization process during the next several hours. Subsequently, the pre-puffs are placed in molds and heated by steam for further expansion or possible softening. At this stage, EPS blocks are formed and, in the following, they are released from the mold and left for several days to “season.” This process must be done to allow the blowing agent used in the manufacturing process to diffuse out of the geofoam structure. This allows dimensional changes and swelling related to the completion of the cooling process. The complete procedure is represented in Fig. 4.2 [5]. In this section, a brief history of EPS geofoam is first presented and then, some of the practical applications and principles of design with EPS geofoam are introduced.

4.1.1 History The first application of EPS geofoam was in embankments around Flom Bridge in Oslo, Norway, in 1972 for reducing settlements. Preceding the installation of EPS geofoam, 20
30 cm of settlement occurred annually in this area, severely damaging the existing road pavements [1]. With the successful outcome of the Oslo geofoam project, a series of international conferences was started to extend the knowledge, disseminate research, share new applications, and discuss case histories. The first conference of these series was held in Oslo at 1985. Since then, there have been many successful applications of EPS geofoam in civil engineering projects around the world, mostly due to the advantages it possesses as a lightweight fill. For example, over 1,300,000 m3 of geofoam was used in 2000 projects in Japan between 1985 and 1987 [5]. These applications included

Expanded polystyrene geofoam

119

runway fill embankment in Japanese airports, demonstrating geofoam’s capability for sustaining heavy and repeated pressure applications. The first use of geofoam in the United Stated took place in 1989 on Highway 160 between Durango and Mancos, where a landslide occurred due to heavy rainfall destroying part of the highway as a consequence. To prevent similar events, geofoam was utilized to stabilize highway side slopes along suspect areas. This method reduced the total cost of the project by 84% compared to conventional renovation techniques [2]. From 1997 to 2001, the largest geofoam project took place in the United States on Interstate 15 in Salt Lake City, Utah. Approximately 100,000 m3 geofoam was used to minimize relocation and remolding operations within the project with a total saving of around $450,000 [3]. Likewise, geofoam was applied in bridge abutments and approaches to increase its base stability [2]. With the achievements of EPS geofoam use for the I-15 project, Utah Transit officials incorporated geofoam embankments for their light rail (i.e., TRAX) and commuter rail lines (i.e., FrontRunner) [4]. Since 2009, a large amount of EPS geofoam is used in the Highway projects of Montreal, Canada. Thus, it is clear that the use of EPS geofoam in civil engineering projects is continuously increasing and it is becoming required for professionals to increase their knowledge regarding this material.

4.1.2 Design manuals As mentioned earlier, the most typical applications of EPS geofoam in the civil engineering industry include road construction and widening, bridge abutments, culverts and pipes, and retaining structures; these will be discussed further in the following sections. Based on these applications, several design manuals have been prepared for various areas of EPS geofoam application. The Swedish standard [6] and the Norwegian standard [7] are two of the pioneers which have attempted to provide minimum requirements for safe and reliable performance of pavement foundations supported on EPS geofoam blocks. Although they appear to involve basic design parameters (e.g., thickness of soil layer), they merely discuss the performance of EPS geofoam per se, or as a part of pavement foundations. Following those efforts, Stark et al. [8] summarized the research on EPS geofoam embankments into a conclusive design procedure, incorporating a systematic approach; called Guideline and Recommended Standard for Geofoam Applications in Highway Embankments. This guideline is the most complete reference for designing EPS embankments and considers internal stability, external stability, seismic stability, hydrostatic uplift, etc. This document [8] comes with a supplementary guideline prepared by Stark et al. [9], called Geofoam Applications in the Design and Construction of Highway Embankments. This reference provides comprehensive details regarding the response of EPS geofoam material, concepts of the design methodology, external and internal stability concepts, design examples, construction practices, quality control (QC)/quality assurance (QA) issues, and a few case histories. This guideline can be addressed as the most detailed reference presenting design concepts and procedures for designing EPS geofoam embankments (at least based on the available research up to 2004). At the end of this guideline, some of

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the future (of course, according to the research available at that time) research topics are also listed, among which using soil reinforcement methods is also addressed. The most recent reference that includes chapters on EPS geofoam is Ground Modification Methods Reference Manual of FHWA by Schaefer et al. [10], which mostly presents a summary and guide to previous resources. They show that geofoam is suitable for a wide range of geologic conditions, although the costs might be up to threefold that of conventional fill materials (without considering installation costs) [10]. Furthermore, Use of ultra-lightweight geofoam to reduce stresses in highway culvert extension by Sun et al. [11] provides insight into the area of utility protection. Regarding slope stability design, Guidelines for geofoam applications in slope stability projects by Arellano et al. [12] is available. It is worth mentioning that research on EPS geofoam is still ongoing and these guidelines may be updated in the future.

4.2

EPS properties

The engineering properties of EPS geofoam can be categorized into three main groups: physical properties, static properties, and dynamic/cyclic properties. Standard procedures for characterizations exist; some of these are currently available as listed in Table 4.1. In the following sections, the most important characteristics of EPS geofoam are described.

4.2.1 EPS density Because the density of EPS material is very low (as little as 0.01 of commonly used fill materials, see Table 4.2), it is an exceptional fill material. The engineering properties of expanded polystyrene, including Young’s modulus and creep behavior, are directly associated with its density. Therefore it is essential to use the appropriate EPS Table 4.1 Standards related to EPS geofoam and its application. Standard no.

Description

ASTM D 7180-05 [13]

Standard guide for use of expanded polystyrene (EPS) geofoam in geotechnical projects Standard specification for rigid cellular polystyrene geofoam Standard specification for rigid, cellular polystyrene thermal insulation Standard test method for apparent density of rigid cellular plastics Standard test method for compressive properties of rigid cellular plastics

ASTM D 6817-17 [15] ASTM C 578-19 [16] ASTM D 1622-08 [14] ASTM D 1621-00 [17]

Expanded polystyrene geofoam

121

Table 4.2 Density range for typical lightweight fills [5]. Lightweight fill type

Range of density (kg/m3)

Geofoam (EPS) Foamed concrete Wood fiber Shredded tires Expanded shale and clay Fly-ash Boiler slag Air-cooled slag

12
35 335
770 550
960 600
900 600
1040 1120
1440 1000
1750 1100
1500

Table 4.3 EPS geofoam densities [5]. ASTM C 578 type

Density (kg/m3)

ASTM D 6817 type

I II VIII IX XI

15 22 18 29 12

EPS15 EPS22 EPS19 EPS29 EPS12

density in specific applications. For instance, the optimum value of EPS density in highway embankment construction is typically in the range between 16 and 32 kg/m3, although densities of up to 100 kg/m3 are sometimes used where high strength, exceptional load spreading, and/or low compressibility are required [5]. EPS geofoam is manufactured in typical densities around the world according to the local standards [5]. According to ASTM C 578-19 and ASTM D 6817-17, some of these typical values are provided in Table 4.3.

4.2.2 Typical stress
strain behavior A typical stress
strain curve of EPS geofoam under static loading is displayed in Fig. 4.3. The figure generally consists of four distinguishing parts including: zone 1, an initial linear elastic response; zone 2, yielding; zone 3, linear 1 work hardening; and zone 4, nonlinear 1 work hardening [8]. To define the initial elastic limit and compressive strength of EPS geofoam, the stress at 1% and 5% (or more commonly 10%) strain is taken into account [19].

4.2.3 Young’s modulus and Poisson’s ratio Young’s modulus and Poisson’s ratio of expanded polystyrene are typically obtained from uniaxial compression tests on 50 mm samples of EPS geofoam according to ASTM D 1621-00, EN 826, and ISO 844. As mentioned earlier, these

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4 3.6

Applied stress (MPa)

3.2 2.8 2.4 2

Zone 3

Zone 4

1.6 Zones 1 and 2 1.2 0.8 0.4 0 0

20

40

60

80

100

Axial strain (%)

Figure 4.3 Typical stress
strain curve of EPS geofoam.

parameters are directly related to the density of EPS geofoam. However, due to the absence of a standard test procedure, a widely accepted relationship between EPS density and its elastic parameters does not exist in the current literature [8]. Supposing that the EPS is of sufficient quality, an average of the linear empirical equations between EPS density and initial Young’s modulus of EPS (Eti) can be presented as Eq. (4.1) [6]: Eti 5 450 ρ 2 3000

(4.1)

where Eti has units of kPa and ρ 5 EPS density in kg/m3. To estimate Poisson’s ratio of EPS geofoam, Eq. (4.2) is suggested by EDO-EPS of Japan: υ 5 0:0056ρ 1 0:0024

(4.2)

where υ 5 Poisson’s ratio of EPS ρ 5 density of EPS (kg/m3). However, Poisson’s ratio values are highly variable with strain level as, once cells in the foam start to break, the polymer walls contract and the EPS volume decreases and can even result in an apparent negative value!

4.2.4 Compressive strength The compressive strength of EPS is a significantly important parameter due to the predominant mode of loading (i.e., compression) in geotechnical applications. As

Expanded polystyrene geofoam

123

Table 4.4 EPS geofoam densities [5]. Physical property

EPS12

EPS15

EPS19

EPS22

EPS29

EPS39

Compressive resistance at 1% strain (kPa) Compressive resistance at 5% strain (kPa) Compressive resistance at 10% strain (kPa) Flexural strength (kPa)

15

25

40

50

75

103

35

55

90

115

170

241

40

70

110

135

200

276

69

172

207

240

345

414

previously mentioned, the compressive stress sustained at 10% strain is taken as the compressive strength of EPS by ASTM standards (see Table 4.4 for typical values). At this stress level, the EPS geofoam crushes one-dimensionally into solid particles and a general rupture failure does not take place. Note that there is nothing fundamental about a strain level of 10% (or 5% for that matter) in Table 4.4, except that it is located after the initial yielding region of the EPS. Research indicates that the compressive strength of EPS does not correlate directly to the creep behavior. Therefore to prevent long-term consequences of permanent loading, it is suggested that the designers keep the applied pressures in the elastic range, which is defined as 1% of compressive strain in a rapid loading test [5]. The compressive strength of EPS geofoam grows linearly with increasing EPS density, hence the following suggested correlation (Eq. 4.3) [17]: σc10 5 8:82ρ 2 61:7

(4.3)

where σc10 5 compressive strength (defined at 10% compressive strain) in kPa and ρ 5 EPS density in kg/m3.

4.2.5 Shear strength The shear strength of EPS geofoam is categorized into two types: internal and external shear strengths. The internal shear resistance is related to the resistance against sliding along planes that may develop inside the material, while the external shear strength is associated with the resistance of the interface between EPS blocks, or the sliding resistance occurring at the contact surface of EPS geofoam and other materials. Research has shown that the internal shear strength of EPS geofoam is directly related to the density of the EPS geofoam. As the density is increased, the shear strength is increased. To obtain internal shear resistance, the rapid loading method on the EPS geofoam specimen is adopted until reaching the maximum shear stress. The shear strength at the EPS/EPS contact surface could be approximated using the traditional Mohr
Coulomb formulation shown in Eq. (4.4). It is obvious that the shear strength depends on the vertical pressure at the interface of the

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New Materials in Civil Engineering

blocks, originated from the surcharge loads and weight of the EPS blocks. If additional shear strength is needed, special connectors might be utilized to limit relative lateral movement of the blocks [5]. τ e 5 σn tan ϕ

(4.4)

where τ e5external shear strength at the EPS/EPS contact surface (kPa); σn 5 vertical pressure produced at the EPS/EPS interface (kPa); ϕ 5 interface friction angle between EPS blocks (degrees).

The test procedure introduced by ASTM D5321-19 is recommended for determination of the interface friction angle, which is typically between 27 degrees and 32 degrees. The above equation can also be used for calculating the external shear strength at the interface between EPS geofoam and other materials. The friction angle is different in this case, however, a designer can assume 30 degrees for the frictional angle between sand and EPS geofoam. In one study, the friction angles of EPS/geomembrane and EPS/geotextile interfaces were taken to be 55 degrees and 25 degrees, respectively [5].

4.2.6 Behavior under cyclic/dynamic loading Cyclic loading is defined as the loads that are applied, removed, and reapplied, for example on a pavement, in a relatively rapid and repetitive form. Current research shows that when the amplitude of applied pressure is kept under the elastic stress limit (σe), there is: G

G

No residual strain after the removal of the applied stress; No reduction in the Young’s modulus during cyclic loading.

The initial Young’s modulus, E0, can be obtained from cyclic uniaxial tests (Fig. 4.4). Trandafir and Erickson [20] used a loading frequency of 1.5 Hz and various geofoam densities (15
25 kg/m3), measuring the E0 value (in kPa) at a cyclic axial strain amplitude, εac, of 0.01%. Then, using regression techniques they obtained the relationship given in Eq. (4.5) and illustrated in Fig. 4.5. E0 5 59:93ρ3 2 1622:8ρ 1 15602

(4.5)

where ρ is the geofoam density expressed in kg/m3. However, by increasing the applied pressure beyond the elastic limit, residual deformation and modulus degradation will occur. Degradation in elastic modulus is well demonstrated with the progressive flattening of the loading
unloading curves (see Fig. 4.5). This figure is the result of cyclic loading on a 200 mm cubic specimen with a density of 30 kg/m3. The loading amplitude was in the postyield range, that is, the applied stress was beyond the elastic stress limit. As can be seen in Fig. 4.5, the average of tangential Young’s modulus decreases with an increase in

Expanded polystyrene geofoam

125

Initial young’s modulus, E0 (kPa)

14000 E0 = 59.93 2 − 16.22.8 + 15602 R2 = 0.9678 11000

8000

E0 = 4724 ± 571 kPa for EPS15 E0 = 6469 ± 734 kPa for EPS19 E0 = 12312 ± 391 kPa for EPS25

5000

2000 14

14

14

14

14

14

14

EPS density,  (kg/m3)

Figure 4.4 Variation of the initial Young’s modulus with geofoam density [8].

Figure 4.5 Typical stress
strain curve of EPS geofoam subjected to cyclic loading [6].

load cycles and is smaller than the initial tangent Young’s modulus. When increasing the strains to very large values (substantially beyond those tolerable in practice), Young’s modulus increases [8].

4.2.7 Dynamic characterization An accepted theory for linking shear modulus, G, and Young’s modulus, E, of EPS geofoam has not been developed yet. However, a simplified empirical expression to estimate average properties of cellular foams, based on the theory of elasticity, has

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New Materials in Civil Engineering

been expressed as [21]: G5α

E 2ð1 1 υÞ

(4.6)

where υ is Poisson’s ratio, as before. Based on triaxial test results, values of α vary between 1.5 and 2.2, which include most EPS geofoam applications. However, more research is required to prove these values. Based on Fig. 4.6, with a growth in shear strain, shear modulus reduces and damping ratios increase. For shear strains between 1024% and 1021%, the shear modulus and damping ratio remain almost constant and then vary significantly with a further increase in the shear strain [21]. 14

15

ρ=24 kg/m3

12

8 6 4

λ(%)

G (Mpa)

10 σ3=0 kPa σ3=30 kPa σ3=60 kPa

10

ρ=24 kg/m3 σ3=0 kPa σ3=30 kPa σ3=60 kPa

5

2 0

0 1. E-04 1. E-03 1. E-02 1. E-01 1. E+00 1. E+01

1. E-04 1. E-03 1. E-02 1. E-01 1. E+00 1. E+01

γ(%)

γ(%) 15

18 ρ=30 kg/m3

16

ρ=30 kg/m3

10 6 4

10 λ(%)

G (Mpa)

12 σ3=0 kPa σ3=30 kPa σ3=60 kPa

σ3=0 kPa σ3=30 kPa σ3=60 kPa

5

2 0

0 1. E-04 1. E-03 1. E-02 1. E-01 1. E+00 1. E+01

1. E-04 1. E-03 1. E-02 1. E-01 1. E+00 1. E+01

20 18 16 14 12 10 8 4 2 0

γ(%) 15 ρ=32 kg/m3

ρ=32 kg/m3 10

σ3=0 kPa σ3=30 kPa σ3=60 kPa

λ(%)

G (Mpa)

γ(%)

σ3=0 kPa σ3=30 kPa σ3=60 kPa

5

0 1. E-04 1. E-03 1. E-02 1. E-01 1. E+00 1. E+01 γ(%)

1. E-04 1. E-03 1. E-02 1. E-01 1. E+00 1. E+01 γ(%)

Figure 4.6 Shear modulus and damping ratio curve for EPS from suggested equations and test results [9].

Expanded polystyrene geofoam

127

As mentioned in previous sections, shear modulus and damping ratio are closely related to the EPS density (ρ) and the applied confining stress (σ3). Similar to the model derived for clay material, the following equations are obtained to estimate dynamic parameters of EPS geofoam [21]: G 5 ðGmin 2 Gmax Þ H ðγ Þ 1 Gmax

(4.7)

λ 5 ðλmax 2 λmin Þ H ðγ Þ 1 λmin

(4.8)

"


2B #A γ=γ r H ðγ Þ 5
2B 11 γ=γ r

(4.9)

where, γ r is a reference strain at 50% of the maximum shear modulus. A and B are dimensionless parameters dependent of EPS density, estimated from a multilinear fitting method for 24 kg/m3 # ρ # 32 kg/m3 and 0 kPa # σ3 # 60 kPa. These parameters are estimated using the following formulas [21]: A 5 2 0:99 B 5 0:26

 σ 2 3

100

 σ 2 3

100

γ r ð%Þ 5 0:40

1 0:65

2 0:7

σ  3

100

σ  3

100

2 0:40

1 0:40

 ρ 2 10

 ρ 2 10

1 0:22ðρÞ 2 1:92

2 0:22ðρÞ 1 3:61

 ρ 2 σ  3 1 0:26 2 0:27ðρÞ 1 5:09 100 100

(4.10)

(4.11)

(4.12)

Using the same method, the following expressions are suggested for computation of Gmax (MPa) and λmin (%) values (maximum shear modulus and minimum damping ratio, respectively) based on EPS density and confining stress [21]: Gmax 5 9:62

 ρ 2 σ  3 2 2:78 2 4:66ðρÞ 1 67:03 100 100

λmin 5 2 1:26

 ρ 2 10

1 0:36

σ  3

100

1 0:69ðρÞ 2 8:64

(4.13)

(4.14)

These equations are used to estimate shear modulus and damping ratio curves based on Eqs. (4.8) and (4.9). When there are no experimental data to hand, deviator stress
strain curves can be predicted from the available literature [21].

4.2.8 Creep and time-dependent behavior A permanent dead load applied on to EPS geofoam blocks can trigger creep behavior in an overlying pavement structure after the construction phase. The initial

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phase of time-dependent creep behavior could start with application of a permanent dead load on the pavement structure, closing the gaps between EPS geofoam blocks. The magnitude of such creep deformation is directly dependent on the magnitude of the applied pressure on the pavement. Additionally, it has been shown that once the permanent (continuous loading) exceeds the 2% compressive strain limit, creep deformation of EPS geofoam material will start. Designers attempt to avoid such creep by keeping the applied pressures within the linear portion of the stress
strain curve of EPS geofoam [5]. Creep behavior of EPS geofoam is determined from testing samples of EPS geofoam. Since smaller samples would overestimate the actual strain, the minimum recommended dimension of the specimens is 300 mm to derive an equation for predicting the creep settlement [22]. To use 2 or 3-day creep test data for extracting the equation, it is necessary to compare the results of these tests with longer creep test results. Srirajan et al. [22] report that 2 m height with full size block and compressive strength of 100 kPa approximately yielded 1.1% strain over 3 years, with 64% of the total strain occurring within the first 2 days. An empirical Eq. (4.5) for total strain (ε) was obtained from test results performed on 300 mm cubic of EPS20 material. Creep strains would be insignificant for the applied stresses below 25% of the EPS geofoam compressive strength (ε , 1%). For operational pressures between 25% and 50% of strength: [22] ε 5 ð3α 1 0:1Þ 3 ½ð 20:0004D 1 β Þ 3 LnðtÞ 1 γ 

(4.15)

where ε 5 total strain, percent; σ 5 applied load, kPa; D 5 density of geofoam, kg/m3 (12 kg/m3
35 kg/m3); t 5 time, minutes; α, β, γ are parameters as defined below: α 5 σ/(7.5D
41.3) β 5 0.230α
0.045 γ 5
1.95α 1 0.985

According to this equation, when EPS20 geofoam is subjected to 50% compressive strength, a total strain of 2% would be generated over 50 years. In other words, if the pavement system could withstand 2% long-term creep deformation, a maximum permanent pressure of 50% of compressive strength would be allowed [22]. Finally, the following issues are notable regarding creep behavior of EPS geofoam: G

G

G

EPS sample size directly affects its creep performance; smaller samples overestimate creep strains due to stress concentration at sample edges and noticeable seating effects; For the same applied pressure, denser EPS geofoam demonstrates less creep. The amount of reduction in creep is dependent on the applied stress level. The difference is negligible for the applied pressures around 30% of the compressive strength, while at pressures equivalent to 50% of EPS compressive strength, the creep can reduce by 25% with an increase in EPS density from 10 to 30 kg/m3; Poisson’s ratio is reduced with an increase in creep strains;

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Figure 4.7 Accumulated plastic strain at different cyclic deviator stress amplitudes (Δσdc) under different loading frequencies (f) [8]. G

Using creep tests on large geofoam samples under stress levels up to 50% of compressive strength could be indicative of the creep behavior under working loads for geofoam design.

Additionally, Effects of loading frequency on the plastic yielding of EPS19 under a static deviator stress level of 50 kPa and cyclic deviator stresses of 25 and 35 kPa are shown in Fig. 4.7. For both selected cyclic deviator stress amplitudes, plastic strain accumulated under a specific number of cycles is greater for lower cyclic loading frequencies compared to higher loading frequencies [20].

4.2.9 Other important issues Besides the engineering properties discussed above, there are several issues that must be considered during design and application of EPS geofoam. These include UV protection, fire and solvent risk, environmental hazard, recycling, etc. and are addressed in the following.

4.2.9.1 UV protection Although EPS geofoam does not deteriorate as do other geosynthetic products when subjected to ultraviolet light, it is still recommended to protect it from direct sunlight to prevent its surface from becoming yellow after a few weeks. Depending on the geometry of the EPS geofoam on the site, a suitable type of covering system should be selected. Often a covering will serve several purposes, such as excluding light, solvents, and water [9].

4.2.9.2 Solvent risk If EPS is touched with hydrocarbonate solvents (gasoline or diesel oil), it will be dissolved. Therefore a big concern regarding the use of EPS geofoam is potential accidents accompanied by fuel leakage downwards into the ground, thence dissolving EPS geofoam. After any fuel spill, the uppermost layer of the foundation must

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be removed. Therefore a fuel-resistant geomembrane is the proper solution to protect EPS geofoam from such dangers, this should not be placed immediately below the surface construction but at a depth where equipment removing a contaminated covering will not damage it [9].

4.2.9.3 Fire risk EPS is a plastic material prone to flammability and burning in fire. Full-scale fire tests in Japan showed that by using 500 mm soil cover on EPS geofoam, melting of EPS is avoided even after a 1-hour fire of kerosene on a sloped embankment (although thinner soil layers were not investigated). EPS should always be covered, either by soil or by a fire-protective material. Adding fire-retardant agents to EPS during production also helps prevent fires from starting, and limits their spread, thereby reducing this concern [9].

4.2.9.4 Environmental impact Using EPS geofoam is beneficial to the environment as it reduces construction time and requires lower fuel consumption compared to when heavier soil material is transferred. However, it has to be kept in mind that EPS geofoam is a plastic that is obtained from oil and its utilization should be limited to cases where other methods are not efficient.

4.2.9.5 Recycling To recycle EPS geofoam, it can be crushed and reused in a variety of applications including lightweight concrete, plastic boards, durable exterior terrace floors, drainage grains, etc. In practice, however, very little is recycled, partly due to the lack of a significant market for the material. Of the 377,580 tons of polystyrene produced in the state of California in 1 year in the early 2000s, only 0.8% was recycled [23]. More recently, anecdotal evidence suggests that this has changed little since then. Therefore the “green” credentials of the material referred to above can only be assured where long-lifetime use of geofoam is envisaged.

4.2.9.6 Insulation and permafrost regions EPS geofoam is a thermal isolative material. Although it may be a secondary function, when EPS geofoam is used in permafrost regions, it can reduce heat transfer to deeper ground levels and prevent melting of the frozen soil, maintaining the bearing capacity, for example of airport pavement foundations [24].

4.2.9.7 Fixing while placing The inherent cohesion between EPS blocks might be insufficient in some circumstances. When using EPS geofoam on sloping ground, it might be necessary to keep EPS blocks in place by using barbed plates or other mechanical devices. Additionally, these connectors help EPS blocks fix in their place during seismic events. Of course,

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these connectors increase construction costs and their use should be limited to circumstances where sliding or flexing apart are important concerns [9]. In the next section, application of EPS geofoam in different areas, including relevant case histories, is discussed, and some of the basic design concepts are introduced.

4.3

EPS in embankments

4.3.1 Introduction Nowadays, the need for new roads and highways is growing and many of these infrastructures are built over weak ground, which is incapable of tolerating additional loads without excessive settlements. Among various construction techniques and materials available to address these cases, a proficient method would be to use innovative lightweight material (e.g., EPS geofoam) to reduce surcharge and to bridge over sensitive existing utilities. This accelerates the project speed at the same time. Thus the loading on the underlying soils and nearby structures decreases, as EPS geofoam is much lighter than ordinary soil material (1%
2% of soil density). The compressive strength of EPS is sufficiently high to withstand secondary and interstate highway traffic loads. Furthermore, EPS geofoam is easily handled and does not require special machinery, which speeds up construction procedures. Unlike common fill materials that come with complicated QA/QC testing during construction, EPS geofoam production is an engineered procedure and is delivered on site tested QA/QC already [25]. As shown in Fig. 4.8, a typical EPS geofoam road section from top to bottom includes: placing a layer of compacted sand at the base of the road section to adjust to the desired level and to permit a free-draining construction surface. Below this, EPS geofoam blocks are placed in a staggered manner to prevent vertical joints from aligning in subsequent vertical locations. Moreover, a layer of geotextile could be employed to separate EPS blocks from the overlying soil layer. This separation layer improves the performance of the pavement with two functions. One is a reinforcing mechanism and the other is increasing the durability of EPS geofoam by avoiding soil particles from damaging the EPS surface during and after construction. Depending on the situation, geocell filled with soil, geomembrane with

Figure 4.8 Graphic representation of EPS geofoam used for road construction.

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hydrocarbon resistivity, geogrid, pozzolanic alleviated materials, reinforced concrete slab, or soil cement could be used instead of a geotextile. Geomembranes/ geotextiles also help to ensure that, if there are any unwanted block movements, the covering sand cannot fall into joints between blocks causing them to slowly, and permanently, wedge apart. For example, when only a fuel spill is of concern, a hydrocarbon-resistant geomembrane would be the proper selection. If additional protection against traffic overstressing is required, a reinforced concrete load distribution slab (RCLDS) can be employed to avoid overstressing and hydrocarbon attack simultaneously. Another benefit of RCLDS is its ability to support other anchored structural elements such as impact barriers, tilt-up panel walls, power poles, and light [25]. In the next section, a selection of projects, including application of EPS geofoam, are introduced.

4.3.2 Case histories and performance 4.3.2.1 Road embankment: Port Mann/Highway 1 Improvement Project, Vancouver to Langley, BC Port Mann/Highway 1 (PMH1) Improvement Project is a 5-year design-build, with a $2.4-billion budget for upgrading the Trans-Canada Highway 1 and relevant transportation infrastructure between Vancouver and Langley in British Columbia, Canada. The project was scheduled to be completed in late 2014. A 37 km portion of the Trans-Canada Highway corridor from the McGill Interchange in Vancouver to 208th Street in Langley was included in the project. The designing procedure included the structural design of 45 bridges, 52 retaining walls, and 25 expanded polystyrene (EPS) embankments [26]. The project corridor passes over various ground conditions, encompassing unique geotechnical features of the Metro Vancouver area including sedimentary rock, ice age glacial deposits, and highly variable marine and deltaic sediments. Therefore localized basins of soft, compressible, or liquefiable soils were present at the location of many interchanges. In the original construction, wood waste was used extensively as a lightweight fill but this is no longer available due to environmental concerns. Therefore in order to deliver the long-term performance requirements of the project, EPS was considered the most suitable alternative to building conventional fill embankments [26]. Some of the marine deposits are very sensitive to disturbance. Some of the deltaic deposits include soft compressible sediments comprising loose to compact sand, overbank and organic silt, and peat overlying till-like soils. Due to the shallow groundwater levels, the soil sensitivity, and the large thicknesses, removal and replacement of these soils was not feasible while improvement methods based on preloading and surcharge would have significantly increased the construction duration. Where soft clays exist below a surface crust, careful treatment would have enabled them to tolerate the 3
4 m required embankment, but not to the desired,

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tight, schedule. For these reasons, EPS geofoam was used to meet the project requirements [26]. As there is a seismic risk, the design requirements included provision for postshock immediate, albeit limited, access, repairable damage, and no collapse. These requirements applied to bridges, retaining walls, as well as the embankments. In the Cape Horn area, the potential of liquefaction was identified up to a depth of 25 m, which might result in large post-seismic settlement and lateral ground spreading, damaging the new infrastructure. Therefore several methods including stone columns, compaction piles, wick drains, and jet grouting were needed to improve the ground condition. These measures were combined with lightweight EPS fill to deliver a safer and less expensive development of the highway infrastructure by reducing the inertial forces due to seismic activity. Two views of the construction procedure are shown in Figs. 4.9 and 4.10 [26].

4.3.2.2 Manchester railway embankment In the north-west of England, on the western edge of Manchester, Bridge 193 over a historic, but now empty, channel of the River Irwell, had long-term maintenance

Figure 4.9 EPS embankment under construction [14].

Figure 4.10 Placement of blocks at Kensington Avenue EPS embankment [14].

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problems. It was decided to replace it with a new embankment to be built under the old bridge so that the railway could remain open during the works. Geotechnical investigations showed a relatively shallow water table with a variety of complications (including a thick layer of weak soil, layers of soapwork waste, dredging deposits, arsenic, and other dangerous substances). Furthermore, with work room of less than 10 m under the bridge, most of the rehabilitation methods (including: traditional embankments on piled foundations and rehabilitation of the current bridge) were not suitable. EPS geofoam was found to be a good solution for the following reasons: much less disturbance was caused to the ground and less headroom was needed compared to a piled foundations and other solutions [5]. In particular, the contaminated ground could be left safely in place, without being disturbed and without being significantly stressed. Either might have caused contaminants to spread while excavation would have been extremely costly due to the need to take the contaminated soil to a licensed hazardous waste dump.

4.3.2.3 Watford Junction replacement station platform At Watford Junction station in the UK, on one of the major rail lines north from London, a timber-supported platform needed replacement. The existing platform was supported on concrete sleepers placed more-or-less directly on the ground surface. Electrical/telecommunication cables were fixed to the timber supports. Cutting the cables and rewiring would have necessitated temporary closure of the rail line due to the need for the replacement wires to be tested to check that none of the signal interlocking had been compromised. To install foundations for the replacement platform compliant with modern structural requirements would also have necessitated closure of the rail line. For these reasons brick-built, precast concrete, steel framed, or glass-reinforced-plastic framed solutions were all rejected in favor of EPS blocks with precast concrete capping to provide the surface. The EPS was faced in a fire-resistant covering. The cables were simply moved sideways into cutouts made in the EPS. The existing catenary and lighting posts were retained and the EPS cut to go around them, avoiding any need to provide new earthing connections. In this way the platform was replaced in 48 hours over the Christmas holiday period when the rail network was nonoperational.

4.3.3 Practical issues A complete discussion is available in Stark et al. (2004b), therefore only some of the important issues are briefly addressed here.

4.3.3.1 Layout of the blocks To obtain appropriate vertical interlocking between EPS blocks, the following guidelines should be considered: G

The thinnest dimension of the block should be oriented in the vertical dimension, tightly placed near adjacent blocks.

Expanded polystyrene geofoam

G

G

135

At least two layers of EPS geofoam blocks should be used. For better interlocking, vertical joints should not be aligned close to each other. It is recommended that the blocks in each layer be placed with their length perpendicular to the preceding layer.

4.3.3.2 Longitudinal geometry Two issues should be considered regarding the longitudinal geometry of the embankment: G

G

The finished surface of the EPS geofoam bed must be parallel to the pavement surface, therefore any sloping should be established by regulating the foundation soil slope. Any transition between EPS geofoam and soil must be gradual to reduce the possibility of differential settlement. The blocks at subsequent layers have to be placed such that they form steps at their end, where EPS geofoam and soil reach together.

4.3.3.3 Site preparation The following items are recommended during site preparation: G

G

G

Even if the design is performed against hydrostatic pressure, it is recommended that any free-standing water be drained from the site and a proper drainage system be installed to avoid the water level rising. Except where the designer has taken into account the effect of thawing in permafrost regions, the EPS should not be placed directly on frozen ground. Any vegetation and debris should be removed from the ground surface. To avoid EPS damage from direct contact with coarse granular soil on the ground, it is recommended to place a 12
25 mm thick layer of sand. This cover will also help leveling or sloping of the ground surface.

4.3.4 Design procedure and notes The design procedure of an EPS geofoam embankment contains three phases: 1. Global stability analysis: the overall pavement system including EPS geofoam, overlying soil, and subgrade is analyzed and checked under serviceability and ultimate limit including settlements, bearing capacity, collapse, and slope stability, and various loading conditions including gravity, seismic, wind, hydrostatic, etc. As EPS geofoam is relatively expensive, the design should be optimized for the volume of EPS geofoam used. 2. Internal stability: the main action is to check whether the selected EPS can sustain the applied pressure without excessive settlement, creep (related service limit), or failure. 3. Design of the pavement system: based on the two described phases, the proper pavement system is designed to prevent failure and satisfy different factors including rut depths and cracks. Proper support or anchorage system for road utilities (guardrails, median dividers, lighting, etc.) also must be considered.

Various components of the pavement have interactions with each other that affect the mentioned design phases in different ways. For example, increasing the overlying soil layer would decrease the safety factor against slope stability, while

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increasing the safety factor against uplift. Therefore the design procedure for these embankments is an iterative effort to achieve an optimized plan. Some of the important elements of this design procedure are explained further in the following.

4.3.4.1 Buoyancy and seismic loading As EPS geofoam is exceptionally lightweight and has a closed cell structure, it is prone to buoyancy in the presence of groundwater and the possible effects of water rise should be reflected during the design phase. It should be noted that this buoyancy is not decreased by any water absorption in the EPS blocks after a while. To handle this issue, the mount of dead load on the EPS must be sufficient to compensate for the potential uplift forces. Seismic loads can adversely affect the internal or external stability of the structures. Thus in all geotechnical applications, seismic loading must be considered in the design to prevent common problems such as excessive settlements or liquefaction. Fortunately, the considerations used for seismic design with ESP geofoam are very similar to those used in design with other construction materials. It has been proved that the seismic hazard mainly depends on the location and the thickness and properties of the soil layer on the bedrock, rather than the EPS geofoam material in use [8].

4.3.4.2 Flexural strength and bearing capacity A key parameter in designing embankments with EPS geofoam is the bearing capacity. In the absence of a proper design, the EPS geofoam can fail in bearing and result in possible extreme vertical settlements in the pavement or damage to the nearby infrastructure. The common equation for calculating ultimate bearing capacity is (in the variant presented by [8]): qult 5 cNc 1 γDf Nq 1 γBW Nγ

(4.15)

where c 5 Mohr
Coulomb shear strength parameter termed cohesion, kN/m2; Nγ, Nc, Nq 5 Terzaghi’s bearing capacity factors; c 5 Unit weight of soil, kN/m3; BW 5 Bottom width of embankment, m; and Df 5 Depth of embedment, m.

Eq. (4.15) can be shortened into Eq. (4.16); as the EPS geofoam blocks are commonly positioned on a soft bed of saturated cohesive soil. In this situation, the cohesion (c) can be associated with the undrained shear strength (su) as the short-term condition will almost invariably be less stable than the long-term one. This simplification also requires that the length of the embankment is considerably larger than the width of the embankment. qult 5 su Nc

(4.16)

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where Nc 5 Terzaghi’s bearing capacity factor 5 5 (1 1 0.2Bw/L) [8]; qult 5 Ultimate bearing capacity of the soil (kPa); su 5 Undrained shear strength (kPa).

If the EPS embankment is a continuous footing (Bw/L!0), Eq. (4.16) can be further simplified to the following equation: qult 5 su 3 5

4.3.4.3 Settlement The settlement that must be noticed during design arises as a consequence of instant settlement, primary consolidation of the fill material, secondary consolidation of the fill material, and the long-term creep of the fill material. However, the settlement associated with the lateral deformation of subgrade soil is not usually taken into account. Such settlement is small compared to other previously mentioned modes of settlement, as the factor of safety for external instability is usually taken to be greater than 1.4. If the embankment is to be constructed over underground utilities, lateral creep deformations must be taken into account [8].

4.3.4.4 Pavement composition considerations In addition, to the basic parts of an EPS embankment (i.e., soil, EPS geofoam, and separation geotextile), a load distribution slab (LDS) or reinforced soil layer (e.g., Ref. [27]) can be utilized over the EPS embankment to keep stresses and deformations within an acceptable range and therefore reduce the cost of the pavement or rail structure. Such a system is specifically applicable to high-volume traffic highways or where heavy vehicles/trains are allowed to pass. Regarding the LDS method, several risks and drawbacks must be considered: cost, which typically accounts for 20%
30% of the project cost; possible sliding during earthquake loading; the ponding of water on the slab inside the pavement system; and the increased risk of differential icing and solar heating. To minimize such consequences, it is recommended that a minimum of two layers of EPS blocks should be used to prevent blocks from moving during service, a situation that could result in failure. Furthermore, the overall minimum depth of the EPS fill mass should be 1.2 m, so that the risk of differential icing is minimized. By using soil reinforcement methods instead of LDS, many of these problems are minimized [8].

4.3.4.5 Further details There are a few more issues that have to be noted. For example, the approach for slope stability concerns is the same as common ones, however, it is recommended to refer to Guidelines for geofoam applications in slope stability projects by Arellano et al. [12].

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Figure 4.11 Schematic view of the testing apparatus (not to scale) and test parameters (units in mm) [16].

For pavement systems, Ghotbi Siabil et al. [27] performed a series of large-scale cyclic plate load tests using the setup shown in Fig. 4.11 and reported the following findings: 1. For design under cyclic loading conditions, the elastic moduli of EPS obtained from static testing of small cubic samples (e.g., 200 mm) can be doubled. 2. When the underlying EPS geofoam is stiffer, the compaction ability of the overlying soil layer increases. The greater the thickness of the overlying soil layer, the better compaction is achieved. 3. The resilient modulus of a pavement foundation including EPS 30 placed on EPS20 and 400 mm compacted soil thickness varied from 10 to 27 MPa under a cyclic applied pressure from 800 to 275 kPa. 4. Thickness of the overlying soil layer is an important factor in the performance of EPS geofoam-supported embankments. For example, 200 mm soil thickness will certainly fail under a few load cycles from a heavy truck. 5. Using 300 mm soil thickness and an applied pressure of 550 kPa, the minimum thickness of the upper EPS layer with larger density should be at least 200 mm. This is to prevent rupture in the thin layer of stiffer EPS placed above softer EPS geofoam. 6. Rut depths can be decreased with the help of a denser EPS block, however, this can increase the cost of the project.

4.4

EPS in bridge abutments and retaining structures

Application of EPS geofoam in construction of bridge abutment fills has several benefits. It distributes traffic-induced stresses over a wide area of foundation soil

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and minimizes the dead load stresses on that soil. As a result, the differential settlement between the bridge/approach fill, construction costs, and maintenance costs is reduced. Because of the much lighter weight of EPS compared to soil, the lateral forces of the backfill applied to the abutment walls, foundations, and other retaining structures are also considerably decreased; which means a great saving for the construction of these structures as they no longer need to withstand significant static/dynamic lateral forces. In the reconstruction project of York Bridge in Washington State, the presence of compressible peat and clays soil on the west bank of the Sammamish River led the designers to adopt EPS geofoam to construct the west approach of the bridge. With this method, the potential settlement at the bridge approach and the need for relocation of existing lifelines was almost eliminated [25]. As mentioned for bridge support structures, EPS geofoam can address a similar benefit to retaining structures by the same lateral pressure reduction mechanism. As the density of EPS geofoam is much lower than soil, the horizontal pressure from the active soil zone acting on the wall or retaining structure decreases significantly compared to the case of backfilling with soil, thus permitting a substantially less robust structure. In addition, to static loads, the lower mass of EPS backfill leads to the generation of lower earthquake forces on the structure in seismic zones. For sites with shallow groundwater and loose soils, proper drainage systems must be installed to avoid the development of hydrostatic stress and buoyancy forces on EPS geofoam. A schematic view of EPS geofoam used behind retaining walls is shown in Fig. 4.12.

4.4.1 Cases histories and performance 4.4.1.1 Case I A shopping mall was planned to be built in soft deposits in Syracuse, New York. The foundation of the building had to be placed at a depth of 2.7 m. To reduce the fill pressure on the basement retaining wall, the building perimeter was filled with 28,000 m3 of EPS geofoam. Earth pressure cells were installed at depths of 0.7 m and 1.6 m to record the pressure applied on the retaining wall. It was observed that the vertical stress was reduced from 42 to 4 kPa and the upper pressure cell showed negligible pressure [28].

4.4.1.2 Case II A new bridge had to be constructed to replace the Norman Kill in Albany, New York. EPS geofoam was used to reduce the earth pressure on the bridge abutments. EPS geofoam helped in a considerable reduction in the lateral pressure applied to the bridge abutment. A numerical modeling using FLAC approved the results of this observation [28].

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4.4.2 Basic design concepts In general, earth-retaining structures are mainly categorized as nonyielding and yielding: [29] G

G

Nonyielding walls are defined as the walls which are fixed against both rotational and horizontal movements under service load. Examples include basement walls, common bridge abutments or other free-standing retaining structures. Such structure are designed for “at rest” condition; Yielding walls can displace or rotate under service load and trigger active earth pressure within the soil
wall system.

With this basis, there are two main mechanisms in reducing the lateral pressure on a retaining wall by EPS geofoam: G

G

Due to the exceptionally low density of EPS geofoam and its negligible Poisson’s ratio, the lateral pressure on the wall decreases, considering the fact that EPS geofoam is selfsupporting (low-strain function). EPS is a highly compressible material which can deform in a controlled manner under pressure (controlled yielding). Such function can mobilize active earth pressure within the soil mass behind the wall and consequently reduce the lateral pressure (high-strain function).

A typical cross-section of an EPS geofoam backfilled wall for yielding or nonyielding walls is presented in Fig. 4.12. Although the dotted line is the failure surface for regular backfill soil under active state, the real failure surface for EPS geofoam backfill occurs along the dashed line. This has two advantages: (1) the applied pressure applied from EPS geofoam and the overlying soil is insignificant and (2) if an adequately small value of θ is selected, the pressure transmitted (Coulomb for static or Mononobe
Okabe for seismic) from soil to the wall through EPS mass would be practically zero [29]. Based on these concepts, the general steps of design are explained in the next section.

4.4.2.1 Design steps When EPS geofoam is incorporated into a part of the bridge approach, a geosynthetic drainage layer (not granular soil) has to be installed along the back of the

Figure 4.12 Typical view and Basic mechanism involved in the function of an EPS geofoam backfill (redrawn from [18]).

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Figure 4.13 Typical view and Loading of the bridge approach (redrawn from Stark et al., 2004b).

retaining structure to minimize water pressure. The overall steps for design of abutments and retaining walls are (Stark et al., 2004b): Selection of initial wall proportions and dimensions; Establish acting pressure against the wall; Calculation of the resultant of forces on the wall and its footing; Checking stability and safety factors including: normal component location of forces, sufficiency bearing capacity, and sliding safety factor; 5. Review and modify wall dimensions and repeat steps 2
4 to satisfy all requirements; then check: settlements and safety factor of deep-seated failure; 6. If irrational dimensions were obtained for the wall, a piled foundation or shaft might be considered; 7. Finally, the economic aspects of the design should be evaluated. 1. 2. 3. 4.

For bridge approaches consisting of EPS geofoam backfill, the following two pressure sources should be considered at step 2 based on Fig. 4.13: G

G

Dead load of the overlying pavement system and EPS blocks (Wp) that directly act at the back of the abutment; The active earth pressure transferred from the soil at the back of EPS geofoam fill (PA).

More details of the calculation of seismic and gravity loads can be found in Stark (2004).

4.4.2.2 Further details To simulate and examine this mechanism and obtain design charts, Ertugrul and Trandafir [30] constructed flexible retaining wall models from St-37 steel plates of 700 mm height and 980 mm length. Their structures were rigidly welded to a steel base of 980 mm length, 500 mm width, and 8 mm thickness. To consider various levels of flexural rigidity which activate different lateral soil pressures on the walls,

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New Materials in Civil Engineering ti tw D-1 D1-D2 Wall displacements P1-P4 Wall pressures P5-P6 Base pressures

Granular backfill

P1 200 P2 200 P3 200 P4

Stem

350

D-2 Base

D-2

Base 350

P5

P6

Compacted base layer

Figure 4.14 Cross-sectional view of the test installations and sensors [19].

the thickness of the wall stem was selected as 2, 4, 5, and 8 mm. A view of the physical test setup in cross-section is presented in Fig. 4.14 [30]. The main parameter for studying the response of the cantilever wall-backfill system is called “relative flexibility” (dw) and is defined as: dw 5

GH 3 Dw

(4.17)

in which G is the backfill shear modulus, H is the wall height, and Dw is the flexural rigidity of the wall which is defined by: Dw 5

E t3
ww  12 1 2 υ2w

(4.18)

Ew denotes the wall’s Young’s modulus, tw and ν w are the wall thickness and Poisson’s ratio of the materials, respectively. Combining Eqs. (4.17) and (4.18), the relative flexibility (dw) of the wall is expressed as:  
G H 3 dw 5 12 1 2 υ2w Ew tw

(4.19)

With the above definition, relative flexibilities were calculated as 128, 524, 1024, and 8197 for different wall thicknesses and Young’s modulus of the retained earth. Two different types of geofoam (EPS and XPS) and two different thickness ratios (ti/H 5 0.07 and ti/H 5 0.14) were selected for inclusion in the test series. Control tests (walls without geofoam buffer) were also performed to use as the benchmark for the performance of the lightweight-filled walls [30]. As shown in Fig. 4.15, lateral earth pressures were normalized with respect to the vertical earth pressure at the base of the wall [(σv)base] for the rigid (with the horizontal supports kept in place to prevent movements, representing dw 5 0 case)

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σh /(σv)base (A)

0.00

0.20

0.40

0 Coulomb active No buffer EPS (t/H=0.07)

0.25

EPS (t/H=0.14) H/z

0.5

0.75

1

σh /(σv)base (B)

0

0.00

0.15

0.30 Coulomb active

0.25

No buffer EPS (t/H=0.07)

H/z

0.5

EPS (t/H=0.14)

0.75 1

Figure 4.15 Lateral pressure distribution for (A) a rigid wall (dw 5 0), and (B) a flexible wall (dw 5 524) [19].

and flexible (horizontal supports were released, dw 5 524) wall models. In this figure, the vertical axis represents the normalized elevation (z, measured positive from wall top, downwards) with the horizontal axis showing the lateral earth pressure normalized against overburden pressure [(σv)base]. Depending on the type and thickness of geofoam inclusion, the pressure distribution is significantly reduced behind the retaining wall system. For the rigid wall systems, using a geofoam inclusion of least stiffness (lowest Ei/ti ratio) could assist in delivering a 50% reduction in the lateral earth pressure distribution on the nonyielding wall model. However, the results show that the effect of inclusion is less noticable on flexible wall models due to pressure redistribution related to their flexural movement [30]. The effects of geofoam properties and relative wall flexibility (dw) on the horizontal wall displacements (ρh) are demonstrated in Fig. 4.16. The most influential factor on wall displacement was found to be the relative wall flexibility. When

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2.0

1.6

No buffer EPS-15 t/H=0.07 EPS-15 t/H=0.14 XPS-22 t/H=0.07

1.2

XPS-22 t/H=0.14

0.8

0.4

0.0 1

10

100

1000

dw

Figure 4.16 Effect of wall flexibility and geofoam characteristics on wall displacements (ρh) [19].

using the compressible (EPS/XPS) inclusion, the wall displacement could be reduced to one-fourth of the wall displacement with no geofoam. Since base sliding and rotation were constrained during physical tests, these measured displacements are considered purely flexural [30].

4.5

EPS in utility protection

Development plans sometimes require placement of an infrastructure over present underground utility lines and structures that were not originally designed to sustain such an overstressing. In these conditions and instead of strengthening or reinforcing, a set of EPS geofoam blocks could be placed over or adjacent to the existing utilities to prevent any increase in the applied load. Recent methods for the protection of buried pipelines across roadways and railways using EPS geofoam include: (1) lightweight embankments, (2) imperfect trenches, (3) slot-trench cover systems, and (4) post and beam cover systems. A combination of these methods might also be used. Two of the most commonly used methods are briefly described in the following sections [31].

4.5.1 Case histories and performance 4.5.1.1 Case I A concrete pipe with 1.95 m diameter had to be buried in a 14 m high embankment with rock-fill. With this geometry, the recorded pressure above the pipe remained

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at about 70 kPa during a 12-year period (as shown in Fig. 4.18), which is about 25% of the estimated pressure with regular fill. The registered value of deformation also stabilizes at 140 mm over 12 years, which is 28% of the initial thickness of EPS geofoam [32].

4.5.1.2 Case II Another installation included a 1.7 m diameter pipe culvert with 15 m rock-fill overburden. As above, the recorded pressure on the top of the pipe stabilized at about 75 kPa, which is equivalent to 25% of the pressure when the regular infilling method is used.

4.5.2 Practical issues Soil arching is the essential mechanism affecting the magnitude and distribution of earth pressure on buried pipes. In the case of protection by EPS geofoam, the effectiveness of this mechanism depends on the pipe installation depth and the relative stiffness of the pipe/trench with the surrounding soil and compressible inclusion. In the “imperfect trench” method (also called the induced trench or imperfect ditch method), the EPS geofoam is placed above the semirigid pipe or culvert (see Fig. 4.17). Positive arching is activated since the softer EPS zone compresses more than the adjacent soil. With mobilization of the shear strength in soil above the culvert, the vertical pressure applied on the pipe would be less than the weight of the overlying soil [33].

Figure 4.17 Placing EPS geofoam above a concrete pile in the imperfect trench system with pressure plate instrumentation evident in the foreground, Eidanger, Norway, 1988. Photo courtesy of the Norwegian Public Roads Administration; S.F. Bartlett, B.N. Lingwall, J. Vaslestad, Methods of protecting buried pipelines and culverts in transportation infrastructure using EPS geofoam. Geotext. Geomembr. 43 (5) (2015) 450
461.

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Figure 4.18 Recommended arrangement of a pipe and EPS in the imperfect trench method (Handbook 16 of [23]) [22].

The recommended installation for an imperfect trench by NPRA [34] is shown in Fig. 4.18. This comprises 20 kg/m3 EPS blocks able to carry 100 kPa at 5% strain. A suggested value of 1.5D for block width and 0.5 m for its thickness are found to be appropriate for a pipe with diameter of D. The block should be placed 0.2D from the top of the pipe with an overlying soil thickness of 5 m or greater. It is also recommended that the granular backfill should be compacted to 97% of the standard Proctor value. Additionally, a D/3 uncompacted zone should be utilized at the base of the pipe [33]. According to NPRA [34] research, granular fill has a better effect in improving the performance of an imperfect trench; the percent of pressure measured above the pipe compared to a trench was about 24% with granular backfill and 45% with cohesive backfill. However, the applied pressure on the section of the culvert without the induced trench installation was measured as 124% of the calculated overburden pressure. During the tests and as the deformation continued, the strain increased in the geofoam block in a complicated, nonlinear manner, with most of the strain concentrated near the geofoam
top-of-pipe interface, resulting in failure near the top of the pipe [33]. When brittle pipe systems are used, the traditional method would be improper for pipe protection. For example, failure and rupture of a pipe due to ground settlement is evident in Fig. 4.19 (left). Therefore another method suitable for pipe protection in this situation is EPS geofoam in a post and beam system (see Fig. 4.19, middle and right). In this method, post and beam elements are used to form positive

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Figure 4.19 Imperfect trench method shown on the right compared to non-geofoam trench installation on the left [22].

Figure 4.20 Graphical representation of a post and beam testing system [24].

soil arching. With this arrangement, the settlement of the foundation would almost avoid any additional pressure from soil on the pipe. To obtain a better insight regarding the post and beam method, Abdollahi et al. [35] performed a large-scale laboratory plate load test and obtained the following results regarding this method (see Fig. 4.20 for the test setup): 1. The thickness of the EPS beam has a key role in pavement deformations. The use of a thicker geofoam block for the EPS beam considerably decreases surface settlements. When the beam thickness was 200 mm, excessive deformations were encountered.

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2. Increasing the thickness of the soil cover leads to less pavement deformations and reduces the maximum stress on the beam. Using a soil thickness less than 200 mm results in failure even in the reinforced system. 3. The density of the EPS geofoam is critical to the post-beam system. Denser EPS has a larger Young’s modulus and higher compressive strength, which helps it to sustain larger applied pressures. 4. The span length between EPS posts is also very important. By decreasing the span length from 500 to 300 mm, the beam deflection can be reduced by up to 40%. 5. Using a single layer of geogrid or other soil reinforcement methods could considerably improve the performance of the post-beam system, especially when larger pressures are applied.

4.5.3 Design considerations To evaluate the vertical pressure on an imperfect ditch culvert, the following equation can be used: [36] σv 5 NA γHðkN=m2 Þ

(4.25)

where γ 5 unit weight of the soil (kN/m3); H 5 height of cover (m); and NA 5 arching factor.

NA 5

1 2 e2A A

(4.26)

where A 5 2Sv

H and B 5 width of the culvertðmÞ B

Sv is the friction number which can be determined from the following equation of Janbu for friction on piles: Sv 5 jr jtanρKA

(4.27)

where tan ρ is the mobilized soil friction equal to f tan ϕ; f 5 degree of mobilization, (ranges from 0 to 1), tan ϕ 5 soil friction, and KA 5 active earth pressure coefficient: 1 KA 5 hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffii2 11tan2 ρ 1tanρ 12 jr j

(4.28)

Expanded polystyrene geofoam


;  pA 1 ctanϕ tanδ tanδ  #1 5 r5
; pA 1 ctanϕ tanρ tanρ

149

(4.29)

where c 5 cohesion, pA 5 active earth pressure, and δ 5 inclination of front wall.

4.6

EPS in other uses

The major areas of using EPS geofoam in civil engineering were discussed in the previous section. In this section, the use of EPS geofoam in two other civil engineering area, wave attenuation and EPS core panel system is introduced.

4.6.1 Wave attenuation and blast protection Centrifuge tests (see the schematic view in Fig. 4.21) were used to study the effect of EPS geofoam in the protection of buried structures from impact loading. Different densities and arrangements of EPS geofoam with respect to the underground structure were investigated. The following results are obtained in agreement with other studies [37]: G

G

G

EPS geofoam is an appropriate material for attenuating blast wave effects on the underground structures. Fig. 4.22 shows a Fourier spectrum analysis of acceleration on a tunnel wall with and without EPS geofoam. The location of EPS geofoam is very important to its performance; the optimum performance is obtained when an EPS block is closer to the structure than to the explosion zone. When EPS geofoam is located horizontally between the buried structure and the blast loading source on the ground surface, the pressure on the structure walls increases due to the arching effect induced by EPS geofoam.

Figure 4.21 A typical graphical view of an EPS geofoam barrier placed above the underground structure [26].

Amplitude (m.s/s2)

150

New Materials in Civil Engineering 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Without barrier Barrier near the structure

0

50

100

150

200

Frequency (Hz)

Figure 4.22 Fourier spectrum of the acceleration time histories on the wall [26].

G

G

The applied load mainly consists of two phases, namely, impulse and continuous loads. Using geofoam, the strains due to the impulse loading phase were reduced significantly. With the presence of an EPS geofoam barrier, the first natural frequency of the system was reduced.

4.6.2 EPS core panel system A novel, safe, and cost-effective method for building construction is the EPS core panel system. These panels are available in both load-bearing and nonload-bearing systems. The main structure of such panels consists of 3D wire space frames welded together with EPS geofoam used in the core. The panel is installed in the place and covered with shotcrete layers on both sides [38]. These panel have some benefits including: G

G

G

G

G

G

Construction cost reduction; Faster construction; Reduction in transportation costs; No need for heavy equipment; Thermal inclusion; Proper strength and durability.

4.7

Conclusions

The main objective of this chapter has been to deliver a primary understanding regarding the basic properties of EPS geofoam, basic addressing of the design method with EPS geofoam, and brief case histories in the chief areas of application. For any practical design of structures with EPS geofoam, the designer must refer to the introduced guidelines and research material or any relevant resource. EPS geofoam is used in a great variety of civil engineering applications, especially geotechnical engineering. However, further research and development will be required to improve existing guidelines and to deliver more cost-effective solutions.

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References [1] Elragi, Ahmed Fouad. Selected Engineering Properties and Applications of EPS Geofoam
Introduction Softoria Group. 2006. Web. 18 Nov. 2010. [2] Geofoam Research Center Syracuse University Syracuse, 2000. Web. 18 Nov. 2010. [3] Meier, Terry. Lighter Loads: Geofoam Shortens Construction Schedules by Reducing the Weight of Embankment Fill and Settlement Time HubDot. HubDot, 1 Apr. 2010. Web. 18 Nov. 2010. http://archive.is/NNoe [4] Bartlett, Steven. "Use of EPS Geofoam in Transportation Systems" (http://www.civil. utah.edu/Bbartlett/Geofoam/Slideshow%20TRAX%20Salt%20Lake%20City.pdf). www.civil.utah.edu. EPS Geofoam Consortium. [5] A. Mohajerani, M. Ashdown, L. Abdihashi, M. Nazem, Expanded polystyrene geofoam in pavement construction, Constr. Build. Mater. 157 (2017) 438
448. [6] R. Gandahl, Polystyrene Foam as a Frost Protection Measure on National Roads in Sweden, Transportation Research Record No. 1146, Transportation Research Board, Washington, D.C., 1987. [7] Expanded Polystyrene Used in Road Embankments - Design, Construction and Quality Assurance (1992). Form 482E. Oslo: Public Roads Administration, Road Research Laboratory. [8] T.D. Stark, D. Arellano, J.S. Horvath, D. Leshchinsky, Guideline and Recommended Standard for Geofoam Applications in Highway Embankments, National Cooperative Highway Research Program (NCHRP) Report 529, Transportation Research Board of the National Academies, 2004. 71pp. ,http://onlinepubs.trb.org/onlinepubs/nchrp/ nchrp_rpt_529.pdf.. [9] T.D. Stark, D. Arellano, J.S. Horvath, D. Leshchinsky, Geofoam applications in the design and construction of highway embankments, NCHRP Web Doc. 65 (2004) 792. [10] V.R. Schaefer, R.R. Berg, J.G. Collin, B.R. Christopher, J.A. DiMaggio, G.M. Filz, et al., Ground Modification Methods Reference Manual, vol. II, Federal Highway Administration, Washington, DC, 2017. [11] L. Sun, T.C. Hopkins, T.L. Beckham, Use of Ultra-Lightweight Geofoam to Reduce Stresses in Highway Culvert Extensions, Publication KTC-05-34/SPR-297-05-1I. Kentucky Transportation Center, University of Kentucky, Frankfort, KN, United States, 2005. [12] D. Arellano, T.D. Stark, J.S. Horvath, D. Leshchinsky, Guidelines for geofoam applications in slope stability projects. Preliminary Draft Final Rep., NCHRP Project No. 2411 (02), 2011. [13] American Society for Testing and Materials, Stand ard Guide for Use of Expanded Polystyrene (EPS) Geofoam in Geotechnical Projects. ASTM Int’l D7180-05, 2005. [14] American Society for Testing and Materials, Standard Test Method for Apparent Density of Rigid Cellular Plastics. ASTM Int’l D1622-08, 2008. [15] American Society for Testing and Materials, Standard Specification for Rigid Cellular Polystyrene Geofoam. ASTM Int’l D6817-17, 2017. [16] American Society for Testing and Materials, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. ASTM Int’l C578-19, 2019. [17] American Society for Testing and Materials, Standard Test Method for Compressive Properties of Rigid Cellular Plastics. ASTM Int’l D1621-00, 2000. [18] A. O’Brien, Design and construction of the UK’s first polystyrene embankment for railway use, in: 3rd International EPS Conference, Salt Lake City, UT, 2001.

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[19] J.S. Horvath, Expanded polystyrene (EPS) geofoam: an introduction to material behavior, Geotext. Geomembr. 13 (4) (1994) 263
280. [20] A.C. Trandafir, B.A. Erickson, Stiffness degradation and yielding of EPS geofoam under cyclic loading, J. Mater. Civ. Eng. 24 (1) (2011) 119
124. [21] A. Ossa, M.P. Romo, Dynamic characterization of EPS geofoam, Geotext. Geomembr. 29 (1) (2011) 40
50. [22] S. Srirajan, D. Negussey, N. Anasthas, Creep behavior of EPS geofoam, in: Proceedings of EPS Geofoam 3rd International Conference, 2001, pp. 10
12. [23] CIWMB, Use and Disposal of Polystyrene in California: A Report to the California Legislature, California Integrated Waste Management Board, December, 2004. [24] D.C. Esch, Road and airfield design for permafrost conditions, in: T.S. Vinson, J.W. Rooney, W.H. Haas (Eds.), Roads and Airfields in Cold Regions: A State of the Practice Report, ASCE, 1996. Section 5, 330pp. [25] T. Stark, S. Bartlett, D. Arellano, Expanded Polystyrene (EPS) Geofoam Applications & Technical Data, EPS Industry Alliance, 2012. [26] M. Buksowicz, S. Culpan, Use of EPS as a Lightweight Fill Material on the Port Mann/Highway 1 Improvement Project, Vancouver to Langley, BC, in: Transportation 2014: Past, Present, Future-2014 Conference and Exhibition of the Transportation Association of Canada//Transport 2014: Du passe´ vers l’avenir-2014 Congre`s et Exposition de’Association des transports du Canada, 2014. [27] S.M.A. Ghotbi Siabil, S.N. Moghaddas Tafreshi, A.R. Dawson, M. Parvizi Omran, Behavior of expanded polystyrene (EPS) blocks under cyclic pavement foundation loading, Geosynth. Int. 26 (1) (2019) 1
25. [28] A.J. Lutenegger, M. Ciufetti, Full-scale pilot study to reduce lateral stresses in retaining structures using geofoam. Final report, Project no. RSCH010-983 Vermont DOT, University of Massachusetts, Amherst, MA, 2009. [29] J.S. Horvath, Extended Veletsos-Younan model for geofoam compressible inclusions behind rigid, non-yielding earth-retaining structures, in: Geotechnical Earthquake Engineering and Soil Dynamics IV, 2008, pp. 1
10. [30] O.L. Ertugrul, A.C. Trandafir, Lateral earth pressures on flexible cantilever retaining walls with deformable geofoam inclusions, Eng. Geol. 158 (2013) 23
33. [31] S. Bartlett, E.C. Lawton, C.B. Farnsworth, M.P. Newman, Design and Evaluation of Expanded Polystyrene Geofoam Embankments for the I-15 Reconstruction Project, University of Utah. Department. of Civil and Environmental Engineering, Salt Lake City, UT, 2012 (No. UT-12.19). [32] Jan Vaslestad, N., Sayd, M. S., Johansen, T. H., & Louise Wiman, N. (2011). Load Reduction and Arching on Buried Rigid Culverts using EPS Geofoam. Design Method and Instrumented Field Tests, Norwegian Pubic Roads Administration. [33] S.F. Bartlett, B.N. Lingwall, J. Vaslestad, Methods of protecting buried pipelines and culverts in transportation infrastructure using EPS geofoam, Geotext. Geomembr. 43 (5) (2015) 450
461. [34] NPRA, Ha˚ndbok 016: Geoteknikk i vegbygging (in Norwegian). Norwegian Public Roads Administration, 2010. [35] M. Abdollahi, S.N. Moghaddas Tafreshi, B. Leshchinsky, Experimental-numerical assessment of geogrid-EPS systems for protecting buried utilities, Geosynth. Int. 26 (4) (2019) 333
353.

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[36] J. Vaslestad, M.S. Sayd, Load reduction on buried rigid culverts, instrumented case histories and numerical modeling, 5th International Conference on Geofoam Blocks in Construction Applications, Springer, Cham, 2019, pp. 115
128. [37] M.H. Baziar, H. Shahnazari, M. Kazemi, Mitigation of surface impact loading effects on the underground structures with geofoam barrier: centrifuge modeling, Tunn. Undergr. Space Technol. 80 (2018) 128
142. [38] CSIR-CBRI, Manual for Expanded Polystyrene (EPS) Core Panel System and Its field Application, Central Building Research Institute, Roorkee, India. 2017.

Recycling of industrial wastes for value-added applications in clay-based ceramic products: a global review (2015
19)

5

M. Contreras1, M.J. Ga´zquez2, M. Romero3 and J.P. Bolı´var1 1 Department of Applied Physics, Faculty of Experimental Sciences, University of Huelva, Natural Resources, Health and Environment Research Center (RENSMA), Huelva, Spain, 2 Department of Applied Physics, University of Cadiz, University Marine Research Institute (INMAR), Ca´diz, Spain, 3Department of Construction, Eduardo Torroja Institute for Construction Science (IETcc-CSIC), Madrid, Spain

5.1

Introduction

Industry is the economic activity that uses and transforms natural resources (raw materials and energy sources) to produce semifinished products that are used to manufacture other goods or ready-made processed materials. In modern societies, industrial activities play a fundamental role in favoring economic development and supporting our living standards. However, industrial activities also have destructive impacts on human health and our environment, basically caused by the occupation of space, the use of natural resources, and the generation of wastes and pollutants. In this sense, the inexorable growth of wastes generated as a result of industrial activity has become one of the most difficult problems to deal with in recent years. Inadequate management of these wastes can cause important alterations to the biosphere, due both to the danger of some of them and to the increasing inability to manage them, as well as contributing indirectly to the depletion of natural resources. Consequently, it is essential for the application of integrated waste management systems that allow giving an acceptable solution to the problem from an environmental point of view. Therefore it is indispensable to reduce the amount of waste generated and, when the generation of waste is inevitable, it is necessary to promote waste as a resource, and increase the levels of reuse and recycling, establishing disposal as the least desirable alternative. In recent decades, regulatory regimes have been established to control and reduce the effects of industrial activity and develop a sustainable industrial production model, balancing economic growth and technological development with environmental protection and human health. It was in this spirit that, in September 2015, world leaders adopted 17 global objectives (Sustainable Development Goals) to eradicate poverty, protect the planet, and ensure prosperity for all as part of a New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00005-3 © 2020 Elsevier Inc. All rights reserved.

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new sustainable development agenda [1]. Among them, some of the targets of Goal 12 “Responsible consumption and production” are: by 2020, achieve the environmentally sound management of chemicals and all wastes throughout their life cycle and significantly reduce their release to air, water, and soil; and by 2030, substantially reduce waste generation through prevention, reduction, recycling, and reuse. In 2016, the amount of waste generated in European OECD member countries by various sectors of economic activity (agriculture, mining and quarrying, manufacturing industry, energy production, water purification and distribution, construction, etc.) was 2.4 3 108 tons. Fig. 5.1 shows the waste generated by economic activities and households in the EU-28 countries. Construction contributed 36.4% of the total in 2016 and was followed by mining and quarrying (25.3%), manufacturing (10.3%), waste and water services (10.0%), and households (8.5%); the remaining 9.5% was waste generated from other economic activities, mainly services (4.6%) and energy (3.1%). Regarding waste disposal, Fig. 5.2 shows the development of waste treatment in the EU-28. In 2016, slightly more than half (53.3%) of waste was treated in recovery operations: recycling (37.8% of the total treated waste), backfilling (9.9%), or energy recovery (5.6%). The remaining 46.7% was either incinerated without energy recovery (1.0%) or disposed of otherwise, mainly by landfilling (45.7%). Therefore, currently, a very important amount of waste is still destined for landfills. To comply with international directives regarding waste management and the environment, it is necessary to make an effort to find a way to waste recovering as raw materials in other production processes. The biggest drawback of the use of waste as a resource is its heterogeneity. Generally, its physical-chemical characteristics vary throughout different campaigns and sometimes they are also dependent on the season. Consequently, its use as a raw material could only be possible in the production process of heterogeneous materials. In this sense, in recent decades,

Figure 5.1 Waste generation by economic activities and households, EU-28, 2016. Source: Eurostat.

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157

Figure 5.2 Development of waste treatment, EU-28, 2004
16. Source: Eurostat.

clay-based ceramic or traditional ceramics have been distinguished as suitable materials for this purpose. The raw materials used in traditional ceramic manufacture are very heterogeneous and, according to their role in the ceramic body, they are classified as: G

G

G

G

Skeleton raw materials. These constitute the base material. The finest fractions are made up of clay minerals (kaolinite, montmorillonite, and illite), micas, chlorite, and quartz. In the thicker fractions are variable proportions of quartz, micas, calcite, dolomite, feldspar, and plaster as major mineral phases. It is also possible to find reduced amounts of soluble salts and organic matter. The clay fraction favors a shaping operation as it allows plasticity to the green body and also provides sufficient mechanical strength to resist the stresses before firing. Fluxes. These form eutectic mixtures with the other components and reduce the firing temperature of the ceramic body. In this group are sodium carbonates, sulfates, and borates, as well as potassium and sodium feldspars. Degreasing raw materials. These reduce the plasticity of the green ceramic body, avoid retractions and reduce the drying time, increasing the mechanical resistance of the fired product. The most commonly used are quartz, feldspar, chamotte (crushed ceramic material), and, in some cases, micas and calcite grains. Colorants. Natural and artificial additives are usually used.

During firing, raw materials react and new crystalline phases develop. Consequently, fired clay-based ceramics show a very heterogeneous microstructure, which consists of quartz grains, crystalline phases, a silica-rich amorphous phase, and porosity. Therefore, given their heterogeneity, both in composition and microstructure, clay-based ceramic materials are suitable candidates for the recovery of wastes of different nature, even in high proportions. Depending on its physicochemical characteristics, the waste will play a specific role (skeleton, flux, degreaser, or additive) in the ceramic body. The incorporation of waste in traditional ceramics has raised great expectations in the scientific community; therefore many scientific works on the use of wastes in

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brick manufacture have been published in recent decades, which have been included in a number of reviews. Dondi et al. [2,3] provided an overview of the status of knowledge about the recycling of industrial and urban wastes in brick production based on studies of literature from 1977 to 1996. In this review, wastes were divided into four main categories based on their principal effect (fuel wastes, fly-ash, fluxing wastes, and plasticity-reducing and plastifying wastes). They concluded that the recycling of industrial and urban wastes in clay raw materials is theoretically useful for waste producers and brick manufacturers. However, often the reuse of wastes is not economically advantageous, due to the high transport cost, which has a very heavy influence on the global production cost. In later years, the incorporation of waste into the body of bricks was subject to different revisions [4
7]. Thus, Raut et al. [8] reviewed various methodologies for the design and development of waste-created bricks and highlighted that wastes enhance brick performance in terms of lighter density, lower thermal conductivity, and higher compressive strength, which gives an economical option to the design of green buildings. The different methods reported for producing bricks from waste materials can be divided into three general categories: firing, cementing, and geopolymerization [9]. The firing and cementing retain the drawbacks of high-energy consumption and large carbon footprint inherent in conventional brick production methods. The method for producing bricks from waste materials through geopolymerization seems to be the trend to follow in terms of energy and environmental concerns. However, as Monteiro and Fontes-Vieira [10] have underlined, geopolymerization is restricted to wastes containing solid aluminosilicate materials and many wastes, such as water treatment plant sludge, biomass, petroleum residues, blast furnace (BF) dust, and paper mill sludge that are not appropriated for geopolymerization. On the other hand, Bories et al. [11] reviewed the incorporation of wastes as pore-forming agents in porous brick manufacture. They also presented the impact of pore-forming agents of the physical, mechanical, and thermal properties of clay bricks. As can be seen, so far the different bibliographical reviews (2015
2019) on the recovery of wastes in clay-based ceramics are limited exclusively to their incorporation into bricks. However, other ceramic building materials have also been investigated as suitable candidates for industrial waste valuation. In this sense, this chapter presents an overview of works published in the literature over the last 5 years on the reuse of industrial wastes in clay-based ceramics, such as bricks, stoneware, and porcelain stoneware tiles and lightweight aggregates.

5.2

Industrial waste materials as aggregate in clay ceramics

5.2.1 Ashes Energy valorization in a power plant produces significant quantities of ash, mainly through coal energy production, and municipal solid waste (MSW) and biomass incineration. Three mainly phases are found in the ash, inorganic matter

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(amorphous or crystalline/mineral phases), organic matter not combusted (char, organic minerals, etc.), and fluid matter associated with both inorganic and organic matter. About 70%
85% of the mass incinerated is released mainly as CO2 into the atmosphere, 10%
25% is bottom ash (BA) formed from dry-ash discharge boilers or slag from melting discharge boilers, and the final 5% is fly ash (FA) from the gas cleaning systems. Coal energy production generates as its main waste, fly (CFA) and bottom (CBA) ashes, being generally formed by spherical particles and with a grain size smaller than 100 μm. It is composed mostly by silicon dioxide (SiO2), which is present in two forms: amorphous (rounded and smooth) and crystalline (sharp and pointed). Other major components are aluminum oxide (Al2O3), calcium oxide (CaO), and iron oxide (Fe2O3). Nevertheless, the composition is very heterogeneous, depending on the origin, and is mainly formed by glassy particles such as quartz, mullite, and various iron oxides. The worldwide production of CFA and CBA coming from the use of coal for energy production is about 600 million tons/ year [12]. MSW is the abridgment of domestic, commercial, and construction residues managed by municipalities, approximately 2 billion tons are generated worldwide annually [13]. The energy valorization of MSW is extended around the world, which in 2016 generated over 1900 million tons [14]. MSW components are very heterogeneous (food, cardboards, paper, glass, metals, plastics, etc.) therefore, the ash characteristics depend on the composition of the MSW, the combustion system, and the FA treatment technology [15]. Biomass energy valorization, estimated at about 9 billion tons annually, is another significant source of ash. Biomass ash results from the complete or incomplete combustion (or occasionally low-temperature oxidation) of organic matter. The chemical composition of the BA is highly variable (O . Ca . K . Si . Mg . Al . Fe . P . Na . S . Mn . Ti, plus some Cl, C, H, N, and trace elements) and depends on the biomass matter and the thermal process used [16]. The use of ashes from power plants was traditionally for manufacturing cement and concrete, while the nonvalorized fraction is stored in large stacks (or landfills) near to the plants [17,18]. In the last three decades many different applications for the fly and BAs have been developed in agriculture, to reduce the acidity of soils, as fertilizer, or as an alkaline agent to remove contaminants in liquid industrial effluents, for example [19]. In addition, this waste is appropriated for ceramic production due to the mineral content, and the low particle granulometry leads us to investigate applications in the production of ceramics, which is of great interest.

5.2.2 Artificial gypsum Gypsum is a mineral formed by calcium sulfate and two molecules of water according to the following formula CaSO4  2H2O, and is usually called dihydrate. This molecule can exist in other hydrous or nonhydrous forms: hemihydrate (CaSO4  0.5H2O) and anhydrate (CaSO4). Gypsum-based products are known as one of the most important environmentally friendly materials.

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Phosphogypsum (PG) is a waste from the manufacture of phosphoric acid after dissolving phosphate ore [mainly formed by fluorapatite, Ca5(PO4)3F] [20]. Ca5 ðPO4 Þ3 F 1 5H2 SO4 1 10H2 O ! 3H3 PO4 1 5CaSO4  2H2 O 1 HF

(5.1)

About 4.5
5 tons of PG are generated per ton of phosphoric acid production, and worldwide PG generation is estimated to be around 100
280 Mt/year [21, 22]. PG is mainly composed of gypsum, but also contains a high level of impurities such as phosphates, fluorides, naturally occurring radionuclides, heavy metals, and other trace elements. Average activity concentrations of PG around the world are 600
1200 Bq/kg for 226Ra, and around 500
1400 Bq/kg for 210Pb and 210Po [22], showing wide variability. All of this adds up to a negative environmental impact and many restrictions on PG applications. Usually the main problem associated with the use of this waste is the radiological content and its environmental implications [23], depending on the origin of the phosphate rock. Borogypsum (BG), which consists mainly of gypsum crystals, boron oxide (B2O3), and some impurities, is formed during the production of boric acid. The boron minerals most widely used by industry worldwide are colemanite, kernite, tincal, and ulexite, with colemanite being the most important reserve [24]. For industrial applications, boric acid is generally produced by dissolving colemanite (50.81% B2O3) in a sulfuric acid solution, according to the following reaction: 2CaB3 O4 ðOHÞ3 :H2 O 1 2H2 SO4 1 6H2 O ! 2CaSO4 :2H2 O 1 6H3 BO3 It is estimated that 2
2.5 tons of BG are generated per ton of colemanite used [25]. Currently, large amounts of BG waste are discharged by the boric acid industry into the environment, where about 62% of the world’s boron ores are found [26].

5.2.3 Metal slags and metallurgy waste Mining and metallurgical waste processes generate considerable volumes of wastes. Extractive processes lead to large amounts of wastes, most of which are discarded to sterile dumps or tailings/piles. Moreover, a high quantity of metallurgical waste is produced during the smelting process of the ore to remove the metal, during the alloy manufacturing process and during the production of metallic components for use in consumer or engineering products. Thus, the steel industry generates from 2 to 4 tons of scrap and 0.7 to 1.7 tons of slag per ton of steel produced. Similarly, 0.9
2.2 tons of slag is produced for each ton of Fe-Si-Mn alloy manufactured. If we consider that the worldwide steel production in 2017 was 1689 3 106 tons, it can be evaluated that 4560
9627 3 106 tons was the amount of waste generated solely by the steel sector. Therefore it is mandatory to find applications for these wastes in other sectors of the economy to diminish the effects of mining, and metallurgical and metal manufacturing wastes. This chapter mainly includes the use of tailings and metallurgical slags in clay-based ceramic manufacture.

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5.2.4 Sludge Sludge can be defined as semiliquid residue from industrial processes and the treatment of sewage and wastewater. The incorporation of industrial sludge into clayey masses can be an alternative to mitigate the high consumption of clays in the manufacture of red ceramics. Red mud (RM) (alkaline leaching residue) is the first waste obtained after digestion steps in the Bayer process where alumina from bauxite (aluminum ore containing 30%
60% of hydrated aluminum oxide) by selective extraction of pure aluminum oxide dissolved in sodium hydroxide (caustic soda, NaOH) is obtained. Usually, each ton of alumina produces 1
1.5 tons of RM, with the annual worldwide production estimated at 120 million tons of RM and 3.9 Gt being the estimated amount stockpile [27], posing a real environmental problem. Mining mud. On the other hand, the wastes generated in mining operations and their adequate management have an essential importance to environmental sustainability, improving the efficiency of natural resources consumption. For example, in the borate industry where the ore beneficiation produces boron-bearing residues with a potential use in the manufacture of ceramic. Galvanic sludge wastes (GSWs). Galvanization is an important process that prevents corrosion in metals and alloys. These processes consume a large amount of water, resulting in large amounts of wastewater, which needs treatments, that generate a large amount of sludge. This sludge contains nonnegligible quantities of heavy metals, colloidal aluminum hydroxide, aluminum sulfate (used as a flocculating agent), sodium and calcium ions (generated in neutralizing solutions), and water. The production of GSW in developed and developing countries is higher than 1,000,000 tons/year [28]. The oil sludge (petroleum sludge), which is the result of various oil industry processes (drilling of wells, collecting pools, transport, storage tanks, and refining) is one of the most important solid wastes. It is a complex emulsion of various hydrocarbons, water, heavy metals, and solid particles. The hydrocarbons can be removed using different chemical processes, however, the solid objects and heavy metals remain as real problems after the manufacturing processes. Petroleum waste sludge is composed mainly of barium oxide, silicon oxide, and sulfur trioxide. The accumulation of such wastes (sludge) creates environmental hazards as the soil, water, and air pollution. A petroleum refinery with a production capability of 105,000 drums per day creates approximately 50 tons of oily sludge per year [29], which equates to 9 billion tons of oil sludge waste globally [30]. Wastewaters (sewage) sludge. The generation of sludge in the treatment and cleaning process of wastewaters coming from different industrial process is growing increasingly. Generally, water pollutants include pathogenic organisms, oxygendemanding wastes, plant nutrients, synthetic organic chemicals, inorganic chemicals, microplastics, sediments, radioactive substances, oil, and heat. In general, three types of wastewater can be distinguished: domestic (usually called sanitary sewage), industrial (water from manufacturing or chemical processes), and storm (water from precipitation and collected runoff).

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5.2.5 Ornamental rock waste Ornamental rocks have accompanied humanity throughout history, not just as a basic construction material, but also as a decorative accessory. They are natural hard rock of visibly crystalline texture formed essentially of SiO2, Al2O3, FeO2, and CaO. A range of groups of ornamental rocks exists, mainly, granites, marbles, slates, limestone, etc. The global ornamental rock production included 350 million tons of limestone in 2018 [31]. Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of mica and amphibole minerals. The chemical composition of granite is typically 70%
77% SiO2, 11%
13% Al2O3, 3%
5% K2O, 3%
5% CO2, 2%
3% total Fe, and less than 1% MgO and TiO2. The production of granite waste (GPW) around the world was estimated at about 20 millions of tons of granite in 2016 [32]. Limestone is a sedimentary rock that contains at least 50% CaCO3, in the form of the mineral calcite (CaCO3). It may also contain quartz, feldspar, clay minerals, pyrite, siderite, and other minerals. The chemical composition of limestone is 40%
50% CaO2, 1%
15% MgO, 1%
6% Al2O3, 1%
5% Fe2O3, and 1%
10% SiO2. Finally, within the family of metamorphic rocks are marble and slate. Marble is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. According to chemical composition, marble contains a significant amount of CaO2 (55%). It also presents smaller amounts of 1%
5% MgO, 1%
3% Al2O3, 1%
3% Fe2O3, and 1%
10% SiO2. The total production of marble was 17 million tons in 2017 [33]. Slate, on the other hand, is composed mainly of clay minerals or micas; it can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals. The chemical characterization shows 50%
70% SiO2, 15%
30% Al2O3, 3%
9% FeO, 3%
5% K2, and 1%
3% MgO and Na2O. In 2016, the world slate production was 2 million tons [34]. Ornamental rocks are extracted from the quarry as massive blocks, and in the processing plant, the rock block is cut into the desired shape and size. During the block-cutting step, around 0%
20% of the blocks are discarded. It is estimated that only 52.6% is finished product from the total extracted [35]. Therefore, millions of tons of ornamental waste are generated each year [36], which are deposited in tailings close to the factories, causing serious environmental problems.

5.2.6 Glass waste Glass is a noncrystalline, amorphous material with excellent characteristics and technological properties (hardness and brittleness, weather resistant, heat and chemical resistant, workability, etc.), which have turned glass into a material with widespread technological and decorative uses. With very few exceptions, most glasses are silicate based, with SiO2 being the main component. According to its chemical

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composition, glass can be classified as: soda
lime glass (71%
75% SiO2, 12%
16% Na2O, 10%
15% CaO); lead crystal and crystal glass (54%
65% SiO2, 25%
30% PbO, 13%
15% Na2O or K2O); borosilicate glass (70%
80% SiO2, 7%
15% B2O3, 4%
8% Na2O or K2O, 2%
7% Al2O3), and special glass (variant composition depending on the required properties of the products). The first three of these categories accounts for over 95% of all glass produced [37]. The most usual products fabricated in the overall glass industry are flat glass, glass containers, fiberglass, and special products such as lenses, optic fibers, mirrors, glassware, and TV tubes. In 2018, EU-28 glass production reached a volume of 36.5 million tons [38], whereas worldwide, around 195 million tons of glass are currently produced annually, of which 44% is for flat glass and 46% for container glass [39]. Consequently, a large amount of glass waste is generated annually worldwide. As an indication, 16.3 million tons of glass waste from packaging were produced in 2016 by the member states of the European Union according to EUROSTAT [40]. Glass wastes from the manufacture of glass and glass products (except glass powder containing heavy metals, i.e., from cathode ray tubes), as well as those from municipal wastes (household, waste and similar commercial, industrial, and institutional wastes) are classified as nondangerous by the European Waste Catalogue [41]. Regarding their management, 26% of the volume of glass generated is recycled, 13% is destined to combustion with energy recovery, whereas the remaining 61% ends up in landfill. For this reason, numerous investigations have been carried out with the purpose of using glass waste as a raw material in secondary markets, among which the production of ceramic-based products is included [42].

5.2.7 Organic waste Organic waste is any material that comes from either a plant or an animal, and can be decomposed by microorganisms (biodegradable) into carbon dioxide, methane, and simple organic molecules, or consists of the remains, residues, or waste products of any organism. Moreover, an organic compound is any of a large class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen. Therefore, organic industrial waste includes a wide range of organic materials obtained from industrial and commercial operation. Large and increasing volumes of industrial organic waste are generated annually worldwide. Paper mill sludge, the large amounts of waste produced in paper manufacturing plants (11 million tons in Europe in 2018), require the introduction of recycling and/or alternative recovery solutions to minimize the amounts of generated waste. MSW is the abridgment of the waste generated from domestic, commercial, and construction activities by households that is collected and treated by municipalities. The worldwide MSW production is expected to increase to approximately 2.2 billion tons/year by 2025 (D. Hoornweg, P. Bhada-Tata, What a Waste: A Global Review of Solid Waste Management. Urban Development Series, World Bank,

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2012. https://openknowledge.worldbank.org/handle/10986/17388 License: CC BY 3.0 IGO. http://hdl.handle.net/10986/17388). Biomass is defined as the biodegradable fraction of products, waste, and residues of biological activities from agriculture (including vegetal and animal substances), forestry and related industries, including fishing and aquaculture, as well as the biodegradable fraction of industrial and municipal waste. Therefore the ceramic manufacturing industry is open to innovation, developing sustainable and low-cost products with possible improvements to their thermal and mechanical properties.

5.3

Review of studies into the incorporation of waste materials in brick making

5.3.1 Ashes in clay-based ceramic applications Many researches have been developed for the utilization of fly and BAs for clayceramic manufacturing (Table 5.1).

5.3.1.1 Bricks A wide variety of these works are focused on the valorization of ashes from biomass incineration, including olive pomace (OP) ash [43], pine-olive pruning ash [45], wood ash from biomass [44,46], rice husk and sugarcane bagasse ashes [44,47
50], MSW incineration ash [51], and coal energy production (coal FA [6,54], coal fly and BAs [55]).The recycling technologies mainly include acid leaching, extraction [52], ion exchange, precipitation, and bioassay methods [67]. Kazmi et al. [47
49], Hwang and Huynh [50], and Eliche-Quesada [44] evaluated the effect of the addition (5%
30% by clay weight) of sugarcane ash (SBA) or rice husk ash (RHA) in clay bricks manufacturing. It was observed that these two types of clay bricks exhibited lower compressive strength and modulus of rupture (MOR), but satisfied the requirements of building bricks. Furthermore, an increase in apparent porosity with a decrease in thermal conductivity was also observed with increasing SBA or RHA content in burnt clay bricks, which resulted in lower unit weight leading to lighter and more economical structures. Based on these studies, it can be concluded that the utilization of SBA and RHA (up to 15% by clay weight) in the manufacturing of burnt clay bricks is not only helpful in landfill reduction but also leads toward the development of sustainable and thermally efficient construction material. The utilization of biomass as a fuel is very attractive from the ecological point of view. Such a method of energy extraction is ecologically clean (zero CO2 emission) and biomass is renewable and a theoretically inexhaustible local energy source. However, during the utilization of secondary raw materials of biomass for energy production, a higher amount of ash accumulates. Eliche-Quesada and LeiteCosta [43] used OP ash (OPA) to replace different amounts (10
50 wt.%) of clay

Table 5.1 Summary of prominent reviewed research for use of ash wastes in clay-based materials. Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

TCLP

Highlights

[AP (%)]

[FS (MPa)]

18.5
31.0

10.2
33.9

X

kLS; kBD; mAP; mWA

[33.3
39.0]

[N.S.]

19.5
27.7

11.8
51.1

[34.1
43.0]

[N.S.]

22.1
32.9

1.5
38.1

[30.3
45.2]

[N.S.]

kMetal leachability

13.5
29.3

3.7
14.9

kLS; kBD; mWA

[N.S.]

[N.S.]

18.3
26.4

4.1
7.2

[37.3
44.0]

[0.7
1.5]

16.9
20.9

36.5
53.7

[30.9
36.2]

[N.S.]

Bricks Olive pomace ash Eliche-Quesada and Leite-Costa [43] Pine-olive pruning ash Eliche-Quesada et al. [44] Wood ash Eliche-Quesada et al. [45] Wood ash Kizinieviˇc and Kizinieviˇc [46] Rice husk and sugarcane bagasse ashes Kazmi et al. [47
49] Rice husk ash Eliche-Quesada et al. [44]

10, 20, 30, 40, 50

10, 20, 30

10, 20, 30

5, 10, 20, 40, 60

54.5 MPa; 30 3 10 3 60 mm3 950 C for 4 h

1.28
1.64

54.5 MPa; 30 3 10 3 60 mm3 900 C, 1000 C for 4 h

1.55
1.68

54.5 MPa; 30 3 10 3 60 mm3 900 C, 1000 C for 4 h

1.34
1.70

70 3 70 3 70 mm3

1.40
1.75

950 C, 1000 C for 4 h 5, 10, 15

228 3 114 3 76 mm3

N.S.

Kiln, 800 C for 36 h 10, 20, 30

54.5 MPa; 30 3 10 3 60 mm3 900 C, 1000 C for 4 h

1.69
1.83

X

X

X

X

kThermal cond.; kMetal leachability kLS; kBD; mWA; kThermal conduct. kMetal leachability, kFiring temperature kBD; mWA; mAP; mCS

mAP; mWA; kMetal leachability kEfflorescence; mFreeze resistance kBD; mWA; mAP kMetal leachability

(Continued)

Table 5.1 (Continued) Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References] Rice husk ash Hwang and Huynh [50] MSW incineration ash

10, 20, 30, 40 55

Taurino et al. [51] MSW incineration ash

5, 10, 15, 20, 25

Deng et al. [52] Coal fly ash

5, 10, 15, 20, 25

Eliche-Quesada et al. [54] Coal fly and bottom ashes Sutcu et al. [55]

10, 20, 30, 40, 50

3, 10, 15, 20, 30

CS (MPa)

[AP (%)]

[FS (MPa)]

TCLP

Highlights

mLS; kBD; mWA

35 MPa

1.6
1.89

9.9
17.8

20.9
28.1

220 3 105 3 60 mm3 40 MPa; 50 3 5 3 4 mm3

1.72
2.00

[N.S.] 6
14

[5.7
6.5] 32
66

[27
36]

[N.S.]

N.S.

N.S.

[N.S.]

[2.6
3.7]

15
25

8
19

[N.S.]

[3
6]

18
27

10.8
50.6

[33.3
43.2]

[N.S.]

11.2
21.2

8.0
35.7

[22.7
35.2]

[N.S.]

kMetal leachability; kThermal cond.

4.3
4.6

N.S.

mBD; mFS; mWA

[N.S.]

[37.7
38.4]

microwave 800 C, 900 C, 1000 C 5 min 40 3 5 3 5 mm3 1040 C, 1060 C, 1080 C, 1100 C, 1120 C Kiln, 800 C for 20 days

8.5
11.2

N.S.

Abbas et al. [53] Coal fly ash

WA (%)

10 MPa; 3 3 10 3 60 mm3 1000 C for 4 h

1.65
1.78

20 MPa; 12 3 40 3 80 mm3 950 C, 1050 C for 2 h

1.66
2.03

50 MPa; 115 3 25.4 3 2.54 mm3 1190 C for 2 h

2.16
2.28

X

mBD; mLS; kWA; kAP; mCS kEnergy consumption process mLS; kBD; mWA; mAP kMetal leachability; mBiosecurity mWA; mAP

X

kArea affected by efflorescence mLS; kBD; mWA; mAP kMetal leachability

X

kBD; mWA; mAP; mCS

Stoneware tiles Sugarcane bagasse ash Schettino and Holanda [56]

2.18, 2.28

Coal fly ash Kockal [57] MSW incineration ash Andreola et al. [58]

5, 10, 15

1130 C, 1190 C, 1210 C

N.S.

60

40 MPa 1190 C and 1210 C for 1h

N.S.

12.5, 60

80 3 20 3 6 mm3 Kiln, 1140 C
1260 C 1100 C
1400 C

2.12
2.36

45 MPa; 150 3 10 3 10 mm3 1000 C
1500 C 20 MPa; 100 3 100 3 4 mm3 1200 C for 2 h

10, 20, 30, 40

1150 C

10, 20, 30, 50 44

,0.1
6.0 [N.S.] 2.6
4.7 [N.S.]

N.S. [33.0
39.1] N.S. [42.1
47.7]

kLS; mFS; kWA

1.38
2.44

,0.1
0.2 [ , 0.1
0.5] ,0.1
34.1 [N.S.] N.S.

N.S. [48
67] N.S. [N.S.] N.S.

mBD; kAP; mCS

1.92
2.55

[2.1
42.4] 0
13.3

[23.6
70.8] N.S.

mWA; mWA

[N.S.]

[26.7
54.5]

1.97
2.32

0.8
55.1

0.3
8.9

kBD; kWA; mCS

Microwave

0.89
1.16

[N.S.] 11.0
18.0

[N.S.] 0.5
1.0

kWA; mCS; kBD

Extruded Microwave 700 C, 1000 C 60 3 30 3 11 mm3

2.15

[32.0
69.0] 12.8
23.5 [N.S.]

[N.S.] 5.5
21.1 [N.S.]

kBD; kWA mResistance

1.22
1.89

N.S.

N.S.

[N.S.]

[N.S.]

X

kMetal leachability mBiosecurity

Porcelain stoneware tiles Rice straw ashes Guzma´n et al. [59] Bovine bone ash Gouvˆea et al. [60] Coal fly ash Wang et al. [61] Coal fly ash

1, 2, 5 30, 40, 50, 60, 70, 80 15, 35, 50

Luo et al. [62,63]

N.S.

kBD; mLS; mWA; mAP mBD; mWA

Clay-expanded aggregates MSW incineration bottom ash Cao et al. [64] Coal fly ash Franus et al. [65] Sugarcane bagasse ash Pitolli Lyra et al. [66] Bagasse from beer Moreno-Maroto et al. [73]

5, 10, 25, 33, 40

1000 C, 1050 C, 1100 C, 1150 C, 1200 C

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S., not specified; WA, water absorption.

X

k BD kMetal leachability

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New Materials in Civil Engineering

in brick manufacturing (fired at 950 C). The properties of waste bricks were compared to conventional products following standard procedures in order to determine the maximum waste percentage. The amount of OPA is limited to 20 wt.%, obtaining bricks with superior engineering properties when 10 wt.% of waste is added. Adding a higher amount of OPA (30
50 wt.%) resulted in bricks with water absorption and compressive strength values on the edge of meeting those established by standards. Moreover, fired bricks fulfill standards requirements for clay masonry units, offering, at the same time, better thermal insulation of buildings due to a reduction in thermal conductivity of 17%, compared to control bricks (only clay). Moreover, Eliche-Quesada et al. [45] investigated the incorporation up to 30 wt.% of fly or bottom biomass ashes from combustion of biomass (olive pruning and pine pruning) into clay for the production of fired bricks (900 C and 1000 C), demonstrating that the optimum amounts of bottom or FA were 20 wt.% and the optimum sintering temperature was 1000 C. Regarding the results, the fired bricks displayed high water absorption; the mechanical properties were suitable, the thermal conductivity decreased by 21% with respect to the standard bricks and they did not present environmental problems according to the leaching study. A comprehensive study on evaluating the role of wood ash (WBA) in stoneware tiles was carried out by Eliche-Quesada et al. [45], and Kizinieviˇc and Kizinieviˇc [46], who analyzed the feasibility of using wood BA (WBA) from boards to replace different amounts (0
60 wt.%) of clay in brick manufacture fired at temperatures of 900 C, 950 C and 1000 C. The results showed small variations due to firing temperature. Firing at 1000 C achieved greater densification and thus lower water absorption and higher compressive strength, firing at 900 C produced higher porosity, which reduced the compressive strength. Moreover, WBA addition influences the properties of ceramic bodies, that is, it reduces drying and burning shrinkage, density, compressive strength, thermal conductivity, and increases water absorption and porosity. Finally, bricks containing 20 wt.% WBA showed properties similar to the control bricks containing only clay and improved thermal conductivity, and those containing 30 wt.% WBA fulfilled standard requirements for clay masonry units. On the other hand, Deng et al. [52] investigated the incorporation (0
25 wt.% with a difference of 5%) of MSW incineration (MSWFA) FA, silica-based aggregate, rich in Ca, Al, and Fe, into clay bricks (mold compressed at 20 MPa/fired at 1040 C
1120 C). The best physical properties were achieved with a sintering temperature higher than 1100 C. The amount of MSWFA has significant effects on the final properties of bricks. For the maximum 25 wt.% incorporation and 1120 C firing, significant changes are observed in the water absorption (11.2%), linear shrinkage (1%), and bending strength (3.4 MPa). TCLP leaching tests (heavy metals) and bioassays (E. coli, S. aureus, and MG63) reveal that leaching concentrations were lower than their limits and have favorable biosecurity. Furthermore, municipal solid waste incineration bottom ash (MSWBA) was studied by Taurino et al. [51]. This research evaluated the production of ceramic bricks with 55 wt.% of MSWBA by an innovative microwave-assisted sintering process (an isothermal

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169

holding of 5 minutes at 800 C
900 C
1000 C) compared with the conventional process (solid molded/sintered at 1000 C). According to the results, the microwave heating led to an increase in the density of bricks caused by a high crystallization due to new crystal phases that were precipitated (Na-rich crystal, nepheline, quartz, and anorthite) and an improvement in the mechanical properties of the bricks obtained in extremely short heat treatment of 5 minutes at 900 C. The authors concluded that the microwave process could have practical implications as a means of the reduction in the energy demand and achieving cost savings in brick production in a very short thermal cycle and at relatively low temperatures. Another example of this type of research is the work developed by Abbas et al. [53], who investigated the valorization of FA from a coal power plant (CFA) by the incorporation of different percentage of FA (0
25 wt.%) into clay bricks (pressmolded/burning in kiln at 800 C). According to the results, lighter bricks can be produced by incorporating CFA in clay bricks, the weight was reduced (18%) and the compressive strength decreased (45%). However, bricks with an ash content of up to 25 wt.% CFA showed compressive strength of approximately 10 MPa and flexural strength of 3 MPa, which satisfies the minimum strength requirement of bricks according to the Pakistan Building Code. Similar results were shown in the review published by Zhang et al. [6]. Furthermore, Eliche-Quesada et al. [54] evaluated the incorporation of CFA (0
50 wt.%) in fired clay bricks. The bricks were molded at 10 MPa and fired at 1000 C. Similar physical properties were achieved with the incorporation of up to 20 wt.% of CFA in fired clay bricks compared with the reference brick (only clay). A higher amount (30
50 wt.%) decreased the mechanical properties (between 25% and 50%) due to an increase in open porosity. Furthermore, Gupta et al. [67] assessed the negative effects of CFA bricks on the environment. In the current study, the leaching potential of heavy metals (test TCLP) from CFA and bricks manufactured using CFA were evaluated. The leaching procedure reveals that CFA contains detectable concentrations of heavy metals (especially Al . Fe . Mn . Zn) but it is nonhazardous in nature. Moreover, the leaching test proved that CFA bricks passed the toxic leachability tests showing lower values in CFA bricks compared to bricks containing only clay due to their pozzolanic nature and metal encapsulation in the ceramic body. This report concluded the suitability in the terms of leaching behavior. Finally, Sutcu et al. [55] studied the addition up to 30 wt.% of FA, and BA from a CFA into clay to produce bricks (950 C and 1050 C). The optimum temperature was 1050 C (high resistance, lower apparent porosity, and water absorption) compared to 950 C (lower bulk density [BD] and thermal conductivity). The incorporation of FA led to an increase in the apparent porosity and water absorption, but a decrease in the BD and thermal conductivity. On the other hand, BA addition does not have a significant influence on the brick properties. The obtained brick incorporating about 5 wt.% FA, 5 and 10 wt.% BA fired at 1050 C presented better technical properties than the reference brick (containing only clay). The leaching results show significantly lower values than those of the limit concentrations due to the heavy metals being immobilized in the ceramic structures after firing.

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5.3.1.2 Stoneware tiles Some methods have been developed to recycle bottom and FA from incinerators which is obtained during the combustion from biomass (sugarcane bagasse ash [56], rice straw ash (RSA) [68], lignite fly and BAs [69], MSW incinerator ash [58,70], and coal FA [57]) in stoneware tile manufacturing. Schettino and Holanda [56], demonstrated the viability of manufacturing ceramic floor tiles containing up to 2.5 wt.% SBA waste, prepared by uniaxial pressing and sintered at 1190 C. SBA addition creates only slight differences with the linear shrinkage; the apparent density and water absorption tends to increase; and, the flexural strength presented only slight increases with SBA. According to these results, SBA is rich in quartz particles, and has potential to be used as an alternative raw material for the production of ceramic floor tiles replaced with up to 2.5% of SBA in the production of floor tile type BIIa (3.0 , WA , 6.0 and FS . 22 MPa) [71]. According to Guzma´n et al. [68], RSA, generated on the combustion of rice straw (a waste product of rice harvesting), is a new alternative ceramic raw material that can be used as a replacement for the fluxing (mainly feldspar) and inert (quartz) materials. Since RSA contains a high amount of silica (over 80%), and alkaline (10.53%) and alkaline earth (2.80%) metals, there is potential to utilize it in the manufacture of triaxial ceramic, through the alkali-silica reaction. Moreover, two sintered methods, two-step sintering (TSS) and conventional, were compared using lignite combustion class-C (highly calcareous) FA and residual carbon BA in the study of Karayannis et al. [69]. Compared to the commercial sintering method, ashes obtained employing the TSS process contain more pronounced crystallinity and finer microstructures, however they also exhibited an interconnected porosity. The promoting effect of obtaining effectively densified ceramics by the TSS process is a new promising approach. The potential use of coal FA derived from two power plants at different locations was assessed by Kockal [57]; here, clay was replaced by FA at 5%, 10%, and 15% by weight of the mix. The results of variance analysis (ANOVA) revealed that temperature and FA addition were the factors most associated with the output parameters. The regression models ranged from 0.89 to 0.98. Moreover, the FA type had no significant effect on the responses. An increase in shrinkage and strength, and moreover, a decrease in water absorption, were observed with 5 wt.% FA incorporation sintered at 1130 C. On the other hand, higher addition of FA ( . 10 wt.%) produced high water absorption, and low shrinkage and strength, while sintering at 1210 C. On the other hand, in the Sappa et al. study [70], an application of the life cycle assessment (LCA) procedure to the industrial manufacture of clay ceramic tiles using MSWBA and conventional materials, was developed. Similar mineralogical and rheological compositions were obtained between ashes and traditional materials. LCA results indicate large environmental and energy benefits incorporating MSWBA in ceramic bodies, high metal recovery, and lower use of clay, even leaching tests verified the environmental compatibility. Andreola et al. [58] characterized the leachability and toxicology of stoneware tiles produced with 60 wt.%

Recycling of industrial wastes for valuedded applications

171

MSWBA. Regarding the results, this ceramic tile is classified as a nonhazardous material because of TCLP and ecotoxicological tests denoting low metal leachability and high biosecurity.

5.3.1.3 Porcelain stoneware tiles Another means for valorization of RSA [59,72], bovine bone ash (BBA) [60], and coal ash [61
63], is the production of porcelain tiles. A laboratory investigation by Guzma´n et al. [72] studied RSA as a substitute for the nonplastic raw materials (feldspar and feldspathic sand), for porcelain tile manufacturing. The research was carried out with two sets of samples, one prepared using only clay, feldspar, and feldspathic sand and the other using RSA, where waste was added at 12.5 and 60 wt.% using the dry route and fired at 1140 C
1260 C. The addition of 12.5 wt.% RSA did not produce remarkable changes in the physical properties and mechanical behavior. Another study by Guzma´n et al. [59] enhanced the optimum mix proportion of clay (40 wt.%), feldspar (20
50 wt.%), feldspathic sand (5
20 wt.%), and RSA (0
25 wt.%). A study on similar ground was conducted by Gouvˆea et al. [60] where up to 5 wt.% BBA was used to manufacture porcelain tiles (triaxial pressed/sintered ranging from 1100 C to 1400 C). The study found that the addition of 2 wt.% BBA reduced the sintering temperature by 50 C compared to control (0 wt.% BBA) due to the promotion of liquid-phase sintering and the best tensile strength was obtained. In general, it was observed that the increase in high-alumina CFA content leads to a reduction in the inversion temperature of densification, which is normally attributed to the high presence of ferruginous and calcareous minerals. The optimum sintering temperature was 1300 C in ceramic tiles containing 70 wt.% CFA with the highest flexural strength of 67 MPa, largest BD of 2.43 g/cm3, and lowest apparent porosity of 0.13% [61]. In this respect, Luo et al. [62] investigated the use of alkali-activated CFA for the production of ceramic tiles compared to untreated CFA. The results revealed better sintering properties (low firing temperature and a wide sintering range) and noted an increment in the strength of green and sintered tiles, which was more significant when using alkali-activated CFA, due to hydrogen bonding and fluxing and mullite skeleton effects. In terms of physical properties, tiles manufactured with only treated CFA sintered at 1100 C obtain a rupture modulus of 50.1 MPa, BD of 2.5 g/cm3, and water absorption of 0%. In addition, Luo et al. [63] demonstrated that there was little effect on the properties of ceramic products when FCA was incorporated with different sizes and their alkali activation. The researchers concluded that ash particles can be divided into three classes based on the chemical composition and size, large particles with a high acidic oxide content, large quartz particles, and small particles with a high alkaline oxide content, which act as clay, quartz, and feldspar, respectively, in ceramic bodies. According to the alkali activation, a relatively higher plasticity was achieved due to the crystal skeleton and the fluxing agent roles.

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New Materials in Civil Engineering

5.3.1.4 Clay-expanded aggregates Recycling of different ashes like coal FA [65], MSW BA [64], and biomass FA [66,73] from a power plant in clay-expanded aggregates has also been achieved. The role of MSWBA was studied by Cao et al. [64]. Here, MSWBA (major oxides SiO2, Al2O3, and CaO) and their combination with clay were used in different proportions for producing lightweight aggregates (LWAs). The effect of MSWBA (25
40 wt.% SiO2) had a noticeable impact on LWA properties with reduced water absorption from 50% to 14% and improved compressive strength and particle density from 0.5 to 6.18 MPa and from 755 to 1268 kg/m3, respectively. However, when used higher than 20 wt.% Al2O3 and CaO, dramatic changes occurred with an increase in the porosity and a decrease in the resistance of samples. On the other hand, a recent study by Franus et al. [65] investigated the influence on the properties of LWA by different coal FAs (F class FA and C-rich FA), clay addition, and microwave heating time. The test results showed that the incorporation of high-carbon FA increased water absorption (12%
18%) and decreased particle density (1.92
2.35 g/cm3), apparent particle density (0.63
0.95 g/cm3), and compressive strength in comparison to class F FA. The study concluded that use of longer microwave heating time improved the mechanical characteristics. Furthermore, Pitolli Lyra et al. [66] developed LWA by modifying red clay with SBA ash addition. The mixes were prepared by the addition of 44 wt.% of SBA that was extruded and sintered between 700 C and 1000 C using two sintering methods (conventional sintering in an electric oven and microwave sintering). The results indicate the addition of SBA reduced drastically the density of the samples, in particular those sintered in a microwave oven. Additionally, sintering in a microwave oven exhibited better compressive strength result and lower water absorption values compared with the electronic oven, due to the microstructure having a more homogeneous porosity with less connectivity. SBA can be considered as an alternative lightweight material that has a low environmental impact during its production process, due to the greater energy efficiency of the microwave oven and the reuse of waste. Use of ash along with other generated waste can also lead to improved LWA properties and result in better waste management practice. This behavior was demonstrated by Moreno-Maroto et al. [73] in a laboratory investigation, where FA enriched to 60 wt.% with K2O from a biomass plant (FBA), bagasse from beer (BB) production, and GSW were added to the clay expanded aggregate composition. Mixtures of clay incorporating FBA (25, 29.2, and 33.3 wt.%), another with FBA and BB together (31.77-5, 31-7, and 30-10, FBA-BB wt.%), and finally incorporating all the waste were prepared. Regarding the results, lightweight sintered at 1000 C and the high FBA content (33.3 wt.%) yielded the best technological properties: oven-dry density of 1.22 g/cm3, water absorption of 39.8%, and open porosity of 48.6%. The addition of FS increased the density and colored the specimen into an intense red with the incorporation of greater than 25 wt.%. On the other hand, BG addition decreases the density, but the incorporation with FS has not been successful because the color was lost due to the reduction of the red iron oxides.

Recycling of industrial wastes for valuedded applications

173

According to the results, the production of colored LWA is a possible alternative for these wastes.

5.3.2 Artificial gypsum in clay-based ceramic applications In this section, we will see the use of two types of rich gypsum waste (GW) coming from different industrial processes in the ceramic manufacture (Table 5.2).

5.3.2.1 Bricks BG from boric acid production [74] was valorized into clay-bricks. Abi [74] evaluated the feasibility of utilizing BG in the manufacture of brick. A proportion of 2.5%
15% by weight of BG was added to the brick production and the results showed a significant reduction (up to 16.47%) in the BD of the produced bricks with an increase in BG content. In addition, the water absorption test revealed clear increments (from 19.1% to 22.2%) with regards to BG content and the compressive strength of the produced bricks increased from 28 up to 30 MPa (at 10 wt.% BG), decreasing to 24 MPa with increasing BG content at 15%.

5.3.2.2 Stoneware tiles Some authors have been also used PG [76] and GW [75] in ceramic tiles. Radulovi´c et al. [75] assessed the feasibility of adding GW to clay-based materials. This research evaluated the viability of recycling GW, established the optimal regime for processing and reapplication of the GW and, furthermore, optimized the characteristics of the final product. The investigation revealed the viability of substitute GW for clay in stoneware tile manufacture, with economic and environmental benefits. Thus, Contreras et al. [76] reported the use of PG as an additive in ceramic manufacturing following the firing route. Ceramic samples were produced by adding different concentrations of PG (5, 7.5, and 10 wt.%) to natural clay, and sintering the final tiles at different temperatures (950 C, 1050 C, and 1150 C) in order to check the technological and mechanical properties of the ceramics obtained. In relation to the technological properties, this manuscript showed that the water absorption and apparent porosity decreased with the addition of PG for all temperatures. In addition, bending strength values were similar or even higher in the new ceramic bodies than in the materials taken as reference, obtaining the best resistance results for samples containing 5 wt.% of PG firing at 1050 C and 1150 C. In addition, the environmental impact generated for the new ceramics obtained was analyzed. The radioactive characterization of the ceramic bodies and raw materials should be the first step in evaluating the radiological impact associated with the use of PG due to the high content in 226Ra and 210Po with activities concentrated around 1000 and 700 Bq/kg, respectively. Taking into account the content of radionuclides in the waste used, European Union Radiation Protection No. 112 [77] defines an external risk index (I), also called an activity concentration

Table 5.2 Summary of prominent reviewed research for use of gypsum waste in clay-based materials. Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

8
21

8
45

[N.S]

[N.S.]

TCLP

Highlights




kBD; mAP; mWAmCS

X

kBD; mFS; kWAkLSkAP kEnvironmental impact Gypsum waste might provide a viable Substitute for clay in the ceramics

Bricks Borogypsum

2.5, 5, 7.5, 10, 15

Abi [74]

Cylindrical sample

1.70
1.95

800 C
1100 C at 200 C/h

Bioecotoxy Metal leachability

Stoneware tiles Phosphogypsum Contreras et al. [77] Gypsum waste Radulovi´c et al, [75]

5, 7.5, 10

N.S.

19 MPa; 60 3 20 3 5 mm3 950 C, 1050 C and 1150 C for 1 h N.S.

1.75
2.34

N.S.

2.99
19.9

N.S.

[6.8
34.8]

[4.6
34.5]

N.S.

N.S.

[N.S.]

[27.2]

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S, not specified; WA, water absorption.

Recycling of industrial wastes for valuedded applications

175

index, to ensure that external gamma dose rates inside a room from building materials does not exceed 1 mSv/year where NORM wastes are used. This index should not exceed the value I # 1 for materials used in bulk amounts, for example, concrete, or I # 6 for superficial materials and those with restricted use, for example, tiles, boards, etc. In this study, all samples showed a risk index below 6. In a second step, the TCLP (Toxicity Characteristic Leaching Procedure, USEPA) leaching test was carried out in order to evaluate the mobility of radionuclides and metals, reporting a negligible environmental impact.

5.3.2.3 Porcelain stoneware tiles No research has been assessed due to the increase in water absorption and apparent porosity with the gypsum addition, because of the different gases released during the firing process. 5.3.2.4 Clay expanded aggregates Similarly, in the period covered by this study no application of clay-expanded aggregates has been found using gypsum as a traditional ceramic raw material.

5.3.3 Mineral slags and metallurgy waste in clay-based ceramic applications Although the greater effort in this chapter has been focused on waste reuse through incorporation into brick pastes, some research studies have also been carried out aimed at preparing other clay-based ceramic products, such as expanded clay or ceramic tiles (Table 5.3).

5.3.3.1 Bricks In the period studied, the greatest effort to reuse mining and metallurgical wastes was aimed at the incorporation into clay brick pastes of different wastes, such as, calamine hydrometallurgical tailings [78], steel industry waste [79,80,88], slag from alloy industry [81], BF slag [82], aluminum filter dust (AFD) [83], and Waelz slag [84]. In this sense, Taha et al. [78] investigated the effect of adding the waste generated by a calamine (Zn4Si2O7(OH)2  H2O) hydrometallurgical processing plant to brick production. The waste, mostly composed of gypsum (CaSO4.2H2O), quartz (SiO2), and calcite (CaCO3), was mixed in different percentages (10
50 wt.%) with natural shale for brick manufacture. The results indicated that the lower proportion of SiO2 and alumina in calamine waste compared to natural shale has a significant effect on the physical properties of fired bricks. Thus, the incorporation of calamine waste results in higher water absorption and open porosity, which leads to reduced flexural strength and apparent density. However, the authors indicated that the adverse effect of the incorporation of calamine on technological properties could be corrected by the addition of glass waste. In this manner, fired light bricks

Table 5.3 Summary of prominent reviewed research for use of metal slags and metallurgy waste in clay-based materials. Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

TCLP

Highlights

X

kBD; mWA; mAP

Bricks Calamine hydrometallurgical tailings Taha et al. [78] Steel industry waste Quaranta et al. [79] Steel industry wastes

10, 20, 30, 40, 50 10, 20, 30, 40, 50 5, 10, 15

Karayannis [89] Steel industry waste Zong et al. [80]

30

Slag from alloy industry

25, 50, 75

Ferreira et al. [81] Blast furnace slag Esmeray and Atıs [82]

5, 10, 15

Aluminum filter dust

5, 10, 15, 20, 25

Bonet-Martı´nez et al. [83] Waelz slag Mun˜oz et al. [84]

20, 70

6 MPa; 100 3 20 3 12 mm3 1020 C for 5 h Extrusion

1.6
2.1

9
26

N.S.

N.S.

[22
42] N.S

[3.5
16] N.S.

Extrusion; 80 3 43.5 3 18 mm3

 1.6

[N.S.]  21

[N.S.] [  8.5]

[  34]

[N.S.]

0.1
6.0 [N.S.]

120
350 [N.S.]

19.5
19.9

0.63
2.10

[N.S.]

[N.S.]

2.2
3.38

2.3
18.5 [N.S.]

8.6
15.2 [17.9
44.7]

2.4
2.5

18
22

20
55

[N.S.]

[N.S.]

N.S. [N.S.]

N.S. [N.S.]

850 C, 950 C, and 1050 C for 15 min 10 MPa 1150 C, 1160 C, and 1170 C for 1 h 2.5 MPa; 5 3 10cm2

Curing for 28 and 60 days 1.5 3 4 mm2 (cylinder) 900 C and 1050 C for 2h 10 MPa; 60 3 30 3 10 mm3 950 C for 1 h N.S.

N.S.

N.S.

kMetal leachability kPlasticity index

Not opposite effects on technological properties of sintered bricks

k WA; mCS

X

Not opposite effects on technological properties; kMetal leachability kLS; mWA

X

kBD; m WA; mCS; kMetal leachability mSurface area: kThermal conductivity kImpact on climate change kImpact freshwater ecotoxicity

Stoneware tiles Bayer process waste

31.32, 29.67, 32.61

Gonc¸alves et al. [85] Waste foundry sand

15

Lin et al. [86]

20.68 MPa; 50 3 15 3 15 mm3 1000 C
1400 C for 1h 35MPa; 120 3 60 3 10 mm3 1000 C, 1050 C, 1100 C, and 1150 C

N.S.

N.S.

[6.25
46.5]

[4
65]

N.S.

[N.S.]

[N.S.]

14
22

[34
58]

[N.S.]

[N.S.]

[0
30]

[70
80]

kBD; mFS

[N.S.]

[N.S.]

kThermal expansion

[55
65]

N.S.

mBD; kAP

[N.S.]

[N.S.]

mThermal conductivity

mLS; mWA; mFS

Porcelain stoneware tiles Blast furnace slag and basic oxygen furnace slag Pal et al. [87]

30

40 MPa; 65 3 15 3 4 mm3 1150 C, 1200 C, 1250 C, and 1280 C

1.85
2.3

Pelletization; 50 r/min

1.05
1.25

Clay-expanded aggregates Waste foundry sand Xiang et al. [91]

50, 60, 70, 80

3% polyvinyl alcohol

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S., not specified; WA, water absorption.

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New Materials in Civil Engineering

with appropriate physical and mechanical properties were fabricated from mixtures containing up to 30 wt.% calamine waste and 10 wt.% glass waste. Quaranta et al. [80] evaluated different steel discards (converter steel slag, white powders, BF sludge, and postmortem aluminosilicate refractories) as feedstock aggregate to clay. Converter steel slag is comprised of quartz, calcite, calcium hydroxide, periclase, wustite, and calcium silicate; the predominant crystalline phases in white powders are calcite, dolomite, and calcium hydroxide; BF sludge contains hematite and quartz as major phases, as well as magnetite, wustite, and iron in minor amounts. Meanwhile, aluminosilicate refractory wastes contains mullite and aluminum oxide as well as cristobalite, quartz, and titanium oxide. The authors studied the effect of waste addition (10
50 wt.%) on the plastic behavior of clay bodies. The results showed that by increasing the proportion of each waste in the body, the plasticity index reduces. They stated that the plastic behavior of steel wastes was not appropriate for processing ceramic bodies by extrusion and that the manufacture of materials using uniaxial pressure should be more suitable. However, recent studies contradict these results. Thus, Karayannis [88] reported on the development by plastic extrusion and firing of clay-based bricks including steel industry electric arc furnace dust (EAFD), and employed pilot-plant simulation procedures of industrial brick manufacturing. EAFD is comprised of enstatite and the results pointed out that efficient extrusion of brick samples incorporating up to 15 wt.% EAFD into the clay mixture was viable, without noteworthy deviations in both their mechanical performance and thermal conductivity. Zong et al. [80] evaluated the influence of steel-making slag (SS) particle size and sintering temperature on the sintering process and end properties of red clay ceramic bricks. The results showed that the water absorption rate of the specimens diminished with decreasing slag particle size and that the compressive strength of the sample was higher at a moderate SS particle size (,0.075 mm) and sintering temperature (1150 C). The authors endorsed this behavior with the fact that the smaller SS particles were more reactive, generating calcium feldspar and pyroxene phases, which have been revealed to enhance the mechanical properties of fired bricks. Moreover, they observed a logarithmic relationship between the SS particle size and activation energy of sintering, which indicated that small SS particles reduced the sintering activation energy and decreased the sintering temperature. Furthermore, Ferreira et al. [82] studied the inclusion of Fe-Si-Mn slag, from ferroalloys manufacture, as partial replacement (25, 50, and 75 wt.%) of lime in the fabrication of nonfired clay-lime bricks through a process consisting of a curing stage for 28 and 60 days in a humidity chamber. The prepared brick showed water absorption values near 20% and compressive strength in the 0.63
2.10 MPa region. The authors highlighted that the incorporation of Fe-Si-Mn slag resulted in a 25% reduction in the cost of processing. A similar research by Esmeray and Atis [83] substituted clay with different rates of BF slag (0, 5, 10, and 15 wt.%) fired at two different temperatures (900 C and 1050 C). The addition of this waste produces no significant impacts on the physicomechanical performance of fired bricks. This study also determined that the increase in curing temperature has a positive effect, as described in the literature.

Recycling of industrial wastes for valuedded applications

179

Bonet-Martı´nez et al. [83] carried out an investigation aimed at evaluating the use of AFD, a waste from the aluminum secondary industry, as a alternative for clay in the manufacture of fired bricks. AFD is constituted principally of aluminum oxide (60
70 wt.%), CaO (8 wt.%), sodium chloride (15 wt.%), and potassium chloride (5
10 wt.%). The authors considered the partial substitution of a clay mixture (40% black, 30% red, and 30% yellow clay) by different percentages of AFD (0
25 wt.%). After firing at 950 C, the results indicated that the addition of up to 20 wt.% AFD resulted in bricks with physical features similar to pure clay-based bricks and improved compressive strength and thermal conductivity. These sustainable bricks also met the directives for heavy metals leached to the environment. However, despite the benefits of manufacturing bricks using waste materials, their commercial production is still irrelevant, which in part is attributed to the lack of information in the industry and the public in general regarding the environmental benefits of these materials. For this reason, Mun˜oz et al. [85] carried out an investigation for additional information about the environmental consequences of including Waelz slag into fired bricks. The study was carried out using an LCA methodology and a cradle-to-grave approach. The results showed that incorporating Waelz slag into ceramic bricks has a lower impact on climate change and decreases the impact on freshwater ecotoxicity and fossil depletion. These benefits were attributed to impact savings due to avoiding the landfilling of slag and lowered fuel requirements during manufacturing. However, due to the higher SO2 and HF emissions produced in the firing of slag containing bricks, these advantages are counterbalanced by greater impacts on human toxicity and terrestrial acidification. The results indicated very restricted environmental benefits in this practice even taking into account different end-of-life scenarios.

5.3.3.2 Stoneware tiles Bayer process waste [85] and foundry sand waste [86] have been evaluated as ceramic tile raw materials. Gonc¸alves et al. [85] analyzed the microstructure and behavior during sintering of tile pastes containing clays (smectite [SM] and kaolin) and alumina waste produced by electrostatic precipitation in the Bayer process, with the aim of correlating the newly formed phases with the end properties of the ceramic products. They concluded that the chemical composition, firing temperature, and Al/Si ratio were the main factors having an influence on the formation of crystalline phases. Moreover, the addition of alumina waste in ceramic pastes contributed to the formation of mullite and cordierite phases, which enhanced the mechanical properties of the ceramic materials. Similarly, Lin et al. [89] studied the effects of adding waste foundry sand (WFS) on the properties of sintered ceramic tiles. Clay in tile composition was replaced with 0 or 15 wt.% WFS. They stated that WFS includes impurities such as Fe, Ca, and Mg, which supports the formation of a glassy phase and promotes the sintering of tiles.

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New Materials in Civil Engineering

5.3.3.3 Porcelain stoneware tiles On the other hand, Pal et al. [88] prepared porcelain stoneware tiles from conventional ceramic minerals and slag from metallurgical industries. Thus, they studied a standard body (45 wt.% kaolin, 40 wt.% feldspar, and 15 wt.% quartz) and two other pastes with BF and basic oxygen furnace slag substituting 30% of the feldspar. The authors found that the slag caused early vitrification compared to the standard body. They also observed that the thermal expansion of slag-containing bodies was lower compared to a standard porcelain body due to the development of lowexpansive anorthite crystals. Consequently, a BF slag-containing vitrified body presented greater bending strength ( . 80 MPa) due to the development of prestress originated by the difference in thermal expansion between glassy matrix, quartz, and anorthite grains.

5.3.3.4 Clay-expanded aggregates Xiang et al. [90] formulated expanded clay aggregates adding (50
90 wt.%) WFS with the addition of plastic clay, alumina powder, and bran. They found that lowcost aggregates from a waste foundry prepared by sintering exhibited thermal conductivity with comparable values to those of commonly used clay and mullite heatinsulating bricks with similar Al2O3 contents.

5.3.4 Sludge in clay-based ceramic applications In this chapter, several types of sludge from different industrial processes and certain wastewater treatments are analyzed in order to obtain a variety of ceramic products (Table 5.4).

5.3.4.1 Bricks The literature has valorized different sludges, including the use of sludge from the wastewater treatment process [83], galvanic sludge [91
93]), electroplating sludge [94,95,110], and RM from an alumina processing plant [96]. Esmeari and Atis [83] reported similar results, in a study on brick production from sewage sludge (SWS). Different mixtures with diverse rates (5, 10, 15 wt.% of SWS) were produced (900 C and 1050 C). They attributed the worsening of the technological properties of the bricks to the disordered and rough structure of the SWS, which had negative effects on the brick characteristics, with the aim of correcting these adverse impacts of SWS on the physical-mechanical performance of fired bricks. However, the pressure values and SEM images of the SWS may be suitable for use of the SWS in the construction sector. In recent times, some investigations have been carried out using GSW from electroplating plants in the manufacture of clay bricks. Pe´rez-Villarejo et al. [91] investigated the feasibility and effect of the incorporation of GSW in industrial clay to produce clay bricks. The authors used a mixture of three types of clay (white, red, and black) in the same proportion, mixing with GSW up to 5 wt.%. The bricks

Table 5.4 Summary of prominent reviewed research for use of sludge in clay-based materials. Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

TCLP

Highlights

[AP (%)]

[FS (MPa)]

15.5-16.1

65
95

X

kBD; kWA; mCS

[N.S.]

[N.S.]

kMetal leachability

12.5
17

7.1
21.3

kBD; mWA; mAP

[11.5
15]

[N.S.]

3.1
6.6

8.4
25.2

kBD; mWA; mAP

[2.8
5.5]

[N.S.]

2.9
4.2

18
24

kFrost resistance; k Thermal conduct. mLS; mWA

[N.S.]

[N.S.]

kMetal leachability

Bricks Galvanic sludge Pe´rez-Villarejo et al. [91] Galvanic sludge Sukharnikova et al. [92] Galvanic sludge Vorob’eva et al. [93] Electroplating sludge Zhang et al. [111] Electroplating sludge Mao et al. [94] Electroplating sludge Dai et al. [95] Red mud alumina processing plant

1, 2, 3, 4, and 5

54.5 MPa; 60 3 30 3 10 mm3

1.35
1.46

950 C for 1 h 5, 10, 15

15 MPa; 50 3 50 mm2

1.75
2.06

1050 C for 0.5 h 2.5, 5, 7.5, 10

2, 4, 6, 8, 10

15 MPa; 50 3 50 mm2 1050 C, 1100 C, 1150 C, 1200 C for 0.5 h 40 MPa; 50 3 35 3 10 mm3

1.59
2.05

N.S.

950 C for 3 h

X

40 MPa; 50 3 35 3 10 mm3

N.S.

5
9

13
22

mWA

10

950 C for 1, 2, 3, 4, 5, and 6 h 40 MPa; 50 3 35 3 10 mm3

2.21
2.65

[N.S.] 0.6
10.5

[N.S.] 18
45

10, 20, 30, 40, 50, 60

950 C and 1050 C for 3 h 4 3 4 3 4 cm3; 5 3 10 3 20 cm3

mSurface area Na2SiO3 additions enhance the physicalmechanical properties

1.8
2.2

[N.S.] 14
26

[N.S.] 8
32

2, 4, 6, 8, 10

mLS; mWA

(Continued)

Table 5.4 (Continued) Waste

Addition (wt.%)

BD (g/cm3)

Production method

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

[N.S.]

[N.S.]

2.2
3.38

2.3
18.5 [N.S.]

8.6
15.2 [17.9
44.7]

kLS; mWA

1.45
1.71

5.5
5.7

N.S.

mBD; kFS; mWA

[N.S.]

[185.6
168.3]

[References] 

Tran et al. [96] Sewage sludge Esmeray and Atıs [82]

5, 10, 15





900 C, 950 C, and 1000 C for 1h 1.5 3 4 mm2 (cylinder) 900 C and 1050 C for 2 h

TCLP

Highlights

Stoneware tiles Red mud from Bayer process Wang et al. [97] Red mud from Bayer process Xu et al. [98] Red boron mud

Zanelli et al. [99] Sewage sludge Ferreira et al. [100] Sewage sludge

Amin et al. [101]

65.8, 67.2, 68.6, and 70

25 MPa; 70 3 6 3 6 mm3 1150 C
1200 C for 2 h

40

40 MPa; 37 3 6.5 3 6.5 mm3

1.7-2.3

0.12-9.92

N.S.

mFS; kWA;kAP;kBD

2, 5, and 10

940 C
1160 C for 2 h 40 MPa; disks (50 mm diameter 3 5 mm thickness) 1120 C
1240 C

2.25
2.46

[2.5-22.5] 0.5-10

[30.57
60.96] N.S.

No radiological impact mBD; kWA

[N.S.]

[N.S.]

1.7-1.8

10-13.5 [N.S.]

N.S. [11.7
12.6]

kLS; kBD; kWA mEfflorescence

5
22.5

5
22.5

N.S.

mLS; mBD; mWA; mAP; kFS

[10
32]

[5-30]

2 and 4

5, 10, 15, 20, 25, 30, and 35

50 3 50 3 35 mm3; 900 C, 940 C, 980 C, and 1020 C 30 MPa; 110.4 3 55.4 3 8 mm3; 1150 C for 15 min

Sewage sludge Cremades et al. [102] Sewage sludge Tang et al. [103] Ceramic sludge

0, 20, 30, 40, and 70

Rectangular cross-section, 70 3 10 mm2; 980 C for 3 h

N.S.

19.8
37

N.S.

[N.S.]

[3.33-5.68]

X

mLS; mWA; kFS kMetal leachability

N.S.

650 C
1350 C for 3 h

N.S.

N.S. [N.S.]

N.S. [N.S.]

10, 20, 30, 40, and 50

30 MPa; 111 3 55 3 7 mm3

1.8-1.9 WT

14
16 WT

N.S.

kBD; mWA for wall tiles

1160  C wall (WT); 1180 C floor (FT)

2.0-2.2FT

2
10 FT

[N.S.]

kBD; mWA for floor tiles

1150 C, 1165 C, and 1175 C for 22 min




[N.S.]

[N.S.]

[N.S.]

[N.S.]

1.06-43.8 [N.S.] 5.85
10.1 [N.S.]

[N.S.] [N.S.] 17.07 [N.S.]

1.1-40.1

7.1-20.5

[N.S.]

[N.S.]

Amin et al. [104]

X

kMetal leachability

Porcelain stoneware tiles Sewage sludge

20, 30, 40, 50, 60, 70, and 80

Nandi et al. [105]

X

mMetal leachability (barium)

Clay-expanded aggregates Sewage sludge Lee et al. [146] Sewage sludge Mingwei et al. [106,107] Washing aggregate sludge Gonza´lezCorrochano et al. [108]

75

1000 C
1150 C for 15 min

0.35
0.67

50

1100 C for 30 min

0.84
0.89

15, 30, and 45

1000 C
1150 C

N.S.

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S., not specified; WA, water absorption.

mBD; kWA; mLS mWA; mBD

X

kWA; kWA kMetal leachability

184

New Materials in Civil Engineering

were then manufactured using conventional processes, firing at 950 C for 1 hour. The BD of the fired clay was 1.760 g/cm3, while the addition of GSW (1
5 wt.%) produced a decrease of up to 18% in BD. The water absorption capacity (WAC) of the samples after addition of GSW, even 4
5 wt.% in relation to the original clay (16.4 wt.%), showed only a slight variation, at around 1%. The open porosity of reference sample showed the highest value (28.86 vol.%), being minimum (21.98 vol.%) when the addition of GSW was 4 wt.%. The compressive strength [109] values increased linearly with the addition of GSW up to 2
3 wt.%, where a value of ca. 86 MPa was reached, around 30% higher than clay. Finally, the environmental study carried out on the fired samples using the TCLP test showed that the environmental impact was negligible, being an efficient inertization method of the metals present in the GSW. Sukharnikova et al. [92] conducted a similar study. Different mixtures of GSW (5, 10, and 15 wt.%) were added into a clay matrix to produce ceramic bricks pressed at 15 MPa and fired at 1050 C. According to the results, the BD was reduced (range: 1.75
2.06 g/cm3), the water absorption (range: 12.5%
17%), and the apparent porosity (11.5%
15%) increased with the GSW addition compared to control (only clay). Moreover, the resistance showed acceptable values, and the compressive strength ranged from 7.1 to 21.3 MPa. Finally, Vorob’eva et al. [93] produced a facing ceramic based on low-plastic clay. Mixtures containing titanium dioxide (10 wt.%), boric acid (5 wt.%), and GSW (2.5, 5, 7.5, 10 wt.%) were pressed at 15 MPa and sintered at 1050, 1100, 1150, 1200 C. The influence of GSW confirms the possibility of synthesizing environmentally safe ceramic obtained as a result of the glazing (self-glazing) effect on the surface of articles and on the surface of the ceramic particles over the entire volume of the ceramic. Zhang et al. [110] examined the possibility of using electroplating sludge as an alternative raw material in the manufacture of fired clay bricks. Electroplating sludge is a solid waste originating in electroplating production or surface treatment plants, which has been extensively classified as hazardous waste by most countries and regions due to the significant quantity of heavy metals (Cr, Zn, Cu, Ni, etc.). Prolonged leaching analyses highlighted that heavy metals might be converted into stable crystalline phases during the heating process, and the highest addition of electroplating sludge was below 8 wt.%. However, the results revealed that compressive strength failed from 23.5 to 15.5 MPa, and water absorption increased from 2.7% to 3.46% with the addition of electroplating sludge up to 10 wt.%. Given these results, Mao et al. [94] studied the effects of adding electroplating sludge on the microstructure, porosity, and crystalline phase of bricks. They pointed out that the texture of bricks became rough and porous with the incorporation of electroplating sludge, while surface area, pore volume, and pore size were greatly increased. As a result, compressive strength and water absorption declined. Dai et al. [95] suggested an alternative by adding Na2SiO3. They highlighted that the addition of Na2SiO3 reduced water absorption and enhanced significantly compressive strength. Moreover, apparent porosity, surface area, pore volume, and pore size decreased appreciably, mainly at 1050 C. The mineralogical analysis indicated that the Na2SiO3 reacted with quartz and formed albite, which played an important role

Recycling of industrial wastes for valuedded applications

185

in reducing porosity and improving mechanical behavior. Additionally, Na2SiO3 diminished the leachability of heavy metals, mostly due to the reduction in water absorption, surface area, pore volume, and pore size. Extending the firing time could also reduce water absorption and improve compressive strength in the presence of Na2SiO3. On the other hand, Tran et al. [96] reported on the manufacture of sintered bricks from clay and RM from the Bayer process of alumina extraction from bauxite. The main composition of RM consisted of iron and aluminum oxide minerals, calcium aluminum silicates, sodium, and a redundant alkaline amount. The authors prepared bricks including a high proportion of RM (10
60 wt.%) and estimated the influence of RM content, sintering temperature, and holding time on physicomechanical properties. They stated that bricks enclosing 50 wt.% of RM and sintered at 1000 C for 1 hour exhibited excellent physicomechanical properties, such as compressive strength, water adsorption, BD, and sintering shrinkage of 23.3 MPa, 18.3%, 2.22 g/ cm3, and 19.8%, respectively. The alkaline release of brick was trivial and the pH of the solution after soaking a brick for 6 days was approximately neutral (7.8). Moreover, its radioactive level was small (0.28 μSv/h) and harmless for human health and environment. The authors pointed out that the quantity of Al2O3 (18.28 wt.%) in RM is higher than that of SiO2 (4.39 wt.%), which was positive for developing mullite phase (3Al2O3.2SiO2) in sintered brick, with a resulting improvement in the technological properties.

5.3.4.2 Stoneware tiles Some works have incorporated sludge from ceramic [104], RM from Bayer process [97,98], red boron mud [99], and SW [100
103]. Wang et al. [97] used RM with the addition of kaolin in the manufacture of new ceramic floor tiles with high strength and lightweight using of ammonium molybdate (NH4)6Mo7O24 in order to promote the growth of mullite crystals. Several mixtures of RM, kaolin, and ammonium molybdate were carried out adding 6 wt.% distilled water. The mixtures were pressed at 25 MPa and dried at room temperature and sintered until 1150 C
1200 C for 2 hours, before being cooled to room temperature. The results indicated the ceramic floor tile with 68.2 wt.% RM, 28.2 wt.% kaolin, and 6 wt.% (NH4)6Mo7O24 sintered at 1180 C showed the best properties, with a flexural strength of 185.6 MPa, BD of 1.45 g/cm3, and water absorption of 5.5%. The authors stated that the alkali metal oxides in RM could be used as sintering aids to lower the sintering temperature. Xu et al. [98] analyzed the microstructure and properties of ceramic tiles manufactured with several samples of RMs and other materials as quartz, talc, flint clay, and shale in order to control the content in SiO2 and Al2O3. Raw materials were mixed and wet-milled. The mixture was dried at 100 C for 24 hours, then granulated by adding 8 wt.% water. The new mixtures were pressed into rectangular bars and disks at 40 MPa. The results showed that the optimal firing temperature was 1040 C, obtaining 4.93% water absorption and 60.96 MPa bending strength with an addition of RM of 40 wt.%, and 5, 30, 14, and 11 wt.% of quartz, talc, flint clay, and shale, respectively. These parameters are in

186

New Materials in Civil Engineering

accordance with international standards. In addition, and taken into account that RM is a NORM (Naturally Occurring Radioactive Materials), having a typical 232 Th activity concentration of 0.1
3 Bq/g IAEA-TECDOC-1712 [111], the radiological implication must be performed and a radiological study was carried out. The results indicated that all the internal and external exposure indexes are within the required values for building according to national standard GB6566-2010. A study resubmitted by Zanelli et al. [99] agreed about the technological feasibility of utilizing borate residues, although with different amounts depending on the end use. This research studied the recycling of residual boron muds into ceramic tiles, specifically in stoneware tile, increasing waste amounts 2
5
10 wt.% carrying out a comparison with a reference material. The batches were designed by two different strategies: adding boron waste in replacement of ball clay (“Argentineanstyle bodies A0, A2, A5 and A10”) and in substitution of Na-feldspar (“Italian-style bodies B0, B2, B5 and B10”), considering two key indicators: (1) the temperature of maximum densification (maximum value of BD) and (2) the maturing temperature (corresponding to 3% of water absorption, prescribed as the threshold for BI stoneware tiles by standard ISO 13006). In the Argentinean-style bodies, the firing shrinkage increased with the amount of boron waste, improving the sintering kinetics of the body with 10 wt.% waste (A10). In general, the BD increased with boron waste additions in both series. This section looks more closely at the problem related to the sludge generated in the treatment of industrial waters and wastewaters and the incorporation of this sludge in ceramic manufacture, analyzing the technical properties of these new mixtures. Ferreira et al. [100] studied, at a laboratory scale, the effect of incorporating sludge from a poultry slaughterhouse wastewater treatment system in ceramic mass as a clay substitute for tile production in Brazil. The ceramic mass/poultry dry sludge ratios analyzed were 100/0 (as reference), 98/2, and 96/4. Properties such as BD, linear drying shrinkage (LS), MOR, and the formation of efflorescence were measured. These properties did not show significant differences between the samples, showing values for BD of 1.9, 1.8, and 1.7 g/cm3 for LS 6.8%, 7.9%, and 7.6% for MOR 77%, 72%, and 73% for reference and samples with 2 and 4 wt.% of sludge, respectively. In the case of efflorescence, the sample containing 4 wt.% of poultry sludge showed high formation, recommending the use of 2 wt.% waste sludge. Amin et al. [101] tested the use of SW from a municipal water plant in the production of ceramic floor tiles. Several percentages of sludge, from 5 up to 35 wt. %, were added to a standard floor tile mix. Linear firing shrinkage [112], water absorption and apparent porosity [113], and breaking strength (BS) and MOR [114] were calculated. Generally, the shrinkage increased with the firing temperature, while it decreased when the concentration of sludge exceeded 15 wt.%. Water absorption and apparent porosity are closely related and both were strongly affected by the addition of sludge, reaching 40% and 50%, respectively, when the addition of sludge was 35 wt.%. The mechanical strength takes into account two parameters, the BS and the MOR. Both parameters decreased when the sludge was added. Finally, the authors showed that it was possible to obtain tiles according to ISO standards with the addition of sludge of 7 wt.% fired at 1150 C with water

Recycling of industrial wastes for valuedded applications

187

absorption less than 10%. Furthermore, Cremades et al. [102] analyzed the use of recycling sludge from drinking water treatment as ceramic material for the manufacture of tiles. Clay was mixed with different percentages of sludge (0, 20, 30, 40, and 70 wt.%) and fired up to 980 C. The technical properties and environmental implications of these mixtures were measured. In general, the addition of sludge in ceramic pastes increases the coefficient of linear expansion, total porosity, and water absorption, and it is possible to use samples with additions of up to 40 wt.% in regions that are not exposed to frequent frost cycles. The results of flexural strength indicate that the higher the sludge percentage in ceramic pastes, the lower the flexural strength, recommending a maximum addition of sludge of between 15 and 20 wt.%. The resulting ceramic material does not pose any environmental hazard, far surpassing the NEN 7345 [115] leaching test by adding up to 70 wt.% of the sludge. In addition, the ceramic tile can be used not only to introduce some wastes, but also to stabilize some heavy metals. Thus, Tang et al. [103] evaluated the use of residues from SW incineration rich in aluminum, iron, and silicon for ceramic products with potential for zinc stabilization, by using a molar ratio of Zn: Al 5 1:2. The results indicated that Zn was incorporated into a ZnAlxFe2
xO4 spinel solid solution, improving the incorporation at high temperature with the optimal range from 950 C to 1250 C. A leaching test using liquids at different pHs (initial values of 2.9, 4.9, 6.0, and 13) was carried out. In the case of Zn, an increase in the pH in the extracting liquid decreased the concentration of zinc. On the other hand, Amin et al. [104] studied the incorporation of ceramic sludge waste in the preparation of dry pressed stoneware tiles for production of wall and floor tiles. The sludge was dried and ground, being added in percentages of 5 up to 50 wt.% for standard wall and floor tiles mixes and then, the samples were fired at 1180 C for floor tiles and 1160 C for wall tiles. Mechanical properties as BS and the MOR are the most important properties in using the tiles and the International Standard [116] gives a minimum value for each separately. In this case, water absorption of wall tiles is .10% and corresponds to a minimum BS of 200 N and MOR of 15 MPa (thickness ,7.5 mm). In this case, 650 N for BS and around 20 MPa for MOR were obtained with an addition of up 10 wt.% of sludge. For floor tiles, the range of water absorption was 3%
6%, which corresponded with a BS of 600 N and MOR of 22 MPa. Values of 700 N for BS and 30 MPa for MOR were obtained with an addition of sludge up to 20 wt.%.

5.3.4.3 Porcelain stoneware tiles It is important to note that the ceramic sludge from a ceramics manufacturing wastewater treatment plant can be also incorporated for production of porcelain stoneware tiles. In this sense, Nandi et al. [105] analyzed the use of ceramic sludge mixed with recycled glass in order to obtain engobes for the production of ceramic tiles. Seven frit mixtures were prepared with different compositions of recycled glass from 12 to 48 wt.%, (increasing in steps of six), ceramic sludge from 20 to 80 wt.% (increasing in steps of 10), calcite, dolomite, ulexite, and saltpeter varying from 2 to 8 wt.%. The samples were melted at 1460 C (1 hour) and dry-crushed.

188

New Materials in Civil Engineering

The frits to produce engobes are selected depending on the results of the coefficient of thermal expansion (CTE) by dilatometry and glaze fluidity flow test (as a measure of the viscosity). These tests showed that the use of both wastes could be feasible in the manufacture of ceramic engobes. Thus an engobe’s formulation containing 26 wt.% of frits (with 18 wt.% of recycled glass and 70 wt.% of ceramic sludge), 9 wt.% of kaolin, 24.7 wt.% of clay, 15 wt.% of feldspar, 14 wt.% of zirconia, 10 wt.% of talc, and 1 wt.% of bentonite was selected for an industrial test, showing a lower porosity in relation to the commercial engobe taken as a reference. Thus, the environmental impact study showed the effectiveness of the inertization process of the ceramics sludge in the ceramics frits.

5.3.4.4 Clay-expanded aggregates On the other hand, SWS is widely used in the manufacture of lightweight aggregates by mixing with saline clay (SC) [146]), river sediment [106,107], and washing aggregate sludge (WAS) [108]. Usually, the main chemical compositions of SW are SiO2, Al2O3, and Fe2O3, which represent more than 60% of the total weight. LWA is artificially manufactured by a thermal process using natural clay, shale, and slate, or industrial waste materials. Lee et al. [146] studied the effects of a two-step heating process on the properties of LWA prepared with SW and SC, using SM as an additive for easy production of aggregates. The preheating temperature was between 400 C and 600 C while the sintering temperature was 1000 C
1150 C. All samples analyzed (11 specimens) were prepared with a mixing ratio 75:20:5 (SS:SC:SM), varying the preheating temperature (400 C
600 C) and the sintering temperature (1000 C
1150 C). The preheating processes considered in this study were 400 C, 500 C, and 600 C and 30, 40, and 50 minutes, respectively, while sintering temperatures (1000 C, 1050 C, 1100 C, and 1150 C) were held for 5, 10, 15, and 20 minutes. LWA preheated at 400 C and sintered at 1150 C exhibited the highest crushing strength (11.1 MPa), decreasing with increasing preheating temperature, while water absorption remained lower than 2.5% in all cases. The densities were in the range of 0.66
0.72 g/cm3 for BD and 1.20
1.35 g/cm3 for apparent density. In the case of sintering temperature, the authors showed that the influence in water absorption was as high 43.8% and 33.0%, at 1000 C and 1050 C, significantly reducing to 14.3% and 1.06% for 1100 C and 1150 C, respectively. Moreover, Mingwei et al. [106,107] analyzed first the effect of the ratio of components and second the effect of SiO2 and Al2O3 on the characteristics of LWA made from SW and river sediment, due to the control of the components in the manufacture of LWA being important. Initially, they investigated the effect of the mass ratio of basic (Fe2O3:CaO:MgO; FCM) and acidic (SiO2:Al2O3; SA) oxides on the characteristics of LWA where (Fe2O3 1 CaO 1 MgO)/(SiO2 1 Al2O3) is defined as the mass ratio (K). The best results obtained were when K was in the range of 0.15
0.3 and for that reason K 5 2 was fixed. The mass ratio of SW:river sediment was1:1. The mixed material was pelletized and dried at room temperature. The LWA samples were further dried at 110 C in a blast roaster for 24 hour. Samples

Recycling of industrial wastes for valuedded applications

189

were heated (8 C/min), soaked respectively at 200, 600, and 800 C (10 minutes) and at test temperatures of 1100 C (30 minutes), cooling naturally to room temperature. Simulated ratios of SA (or FCM) were prepared by adding acid (SiO2, Al2O3) or base (Fe2O3, CaO, and MgO) to the raw materials. The maximum bulk, grain densities, and compressive strength (882 and 1756 kg/ m3 and 17.7 MPa) and the minimum water absorption (6.32%) were obtained at SA 5 2:1. The maximum bulk, grain densities, and compressive strength (821 and 1832 kg/m3, and 17.02 MPa, respectively) and water absorption (5.59%) were obtained at FCM 5 3:2:1. In a second study the authors deepened the effect of variation in the SA ratio, maintaining the relation SW:river sediment at 1:1. Initially, the original mass ratio of SiO2:Al2O3 was found to be 13:5, it being necessary to add different doses of oxides (SiO2, Al2O3) in order to study different SA ratios. The results showed that LWAs with the highest compressive strength, lowest porosity and water absorption, and best solidification of heavy metals were obtained with an SiO2 content of between 30 and 45 wt.% and an Al2O3 content of between 11 and 19 wt.%, respectively. In addition, heavy metals were encapsulated within the aluminosilicate matrix structure when LWA pellets were sintered at 1100 C. Finally, Gonza´lez-Corrochano et al. [108] tested the mixture of WAS from a gravel pit and SW from a wastewater treatment plant for LWA production. The materials were mixed, milled, and made into granules, preheated for 2 and 5 minutes, and sintered in a rotary kiln at temperatures between 1175 C and 1275 C for different dwell times ranging from 1 to 30 minutes. Their water absorption values were between 23.54% and 38.36% and compressive strength values between 1.23 and 3.03 MPa. These properties were affected by the heating temperature, prefiring, and firing dwell times.

5.3.5 Ornamental rock waste in clay-based ceramic applications In this section, some applications incorporating ornamental rock waste are summarized (Table 5.5).

5.3.5.1 Bricks The most recent researches have reported on the incorporation in ceramic brick of ornamental rock waste residue from different sources, such as natural stone [117], granite [118], marble [119
121], and trachyte [122]. Lougon et al. [117] considered the use of natural stone waste (NSPW) as a raw material in the manufacture of blocks and bricks, as a partial substitute for less plastic clay. Mixtures containing two types of clays (kaolinite and illite) and NSPW (0, 14, and 28.5 wt.%) were prepared (molded by uniaxial pressure/sintered at 850 C, 900 C, and 950 C) similar to those used in the red ceramics industries. The experimental results indicated higher NSPW incorporation produce higher density and lower plasticity due to the glass-phase formation and either filling pores obtaining similar compressive strength, lower porosity, and water absorption compared to reference material (only clay). Moreover, all the mixing studies comply with the

Table 5.5 Summary of prominent reviewed research for use of ornamental waste in clay-based materials. Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

TCLP

Highlights

Bricks Natural stone waste

14, 28.5

850 C, 900 C, and 950 C for 2 h

1.89
2.35

10.1
10.9

N.S.

mLS; mWA

Lougon et al. [117] Granite waste

10, 20, 30

18 MPa; 124.3 3 11 3 25.4 mm3 850 C
1100 C for 3 h 50 MPa; 60 3 30 3 10 mm3

N.S.

[N.S.] 9.2
25.7

[N.S.] N.S.

kPlasticity mLS; mWA; mFS

1.70
1.76

[N.S.] 17.9
20.2

[1.2
14.1] 48.1
53.8

[33.9
36.8]

[N.S.]

Vieira et al. [118] Marble powder waste Cobo-ceacero et al. [119] Marble powder waste Sutcu et al. [120] Marble powder waste Munir et al. [121] Trachyte waste Coletti et al. [122]

2.5, 5, 7.5, 10

950 C for 1 h 5, 10, 15, 20, 25, 30, 35 5, 10, 15, 20, 25

40 MPa

1.61
2.07

12.5
26.2

5.7
33.9

950 C for 2 h 228 3 114 3 76 mm3

N.S.

[26.3
42.5] 16.5
24.5

[N.S.] 4.2
8.2

[33.5
42.3]

[N.S.]

800 C for 36 h

kLS; mBD; mAP; mWA; mCS

X

kLS; kBD; mWA; mAP kThermal conductivity mAP; mWA

5, 10, 15

5 3 12 3 20 mm3 900 C, 1000 C, and 1100 C for 5 h

1.67
1.82

23.5
18.0 [35.4
37.9]

N.S. [N.S.]

kMetal leachability; kThermal cond. kLS; kBD; kWA kThermal conductivity

10, 20, 30, 40, 50

800 C, 850 C, and 900 C for 1 h

1.89
2.35

N.S.

N.S.

mLS; kBD; mFS

[N.S.]

[7.3
12.2]

kPlasticity

Stoneware tiles Rock powder waste Vijayaragavan et al. [123]

Ornamental stone waste Vieira et al. [124] Ornamental stone waste Xavier et al. [125] Ornamental stone waste Sant’Ana et al. [126] Ornamental stone waste Gadioli et al. [127] Granite sawdust

10, 20, 30

950 C for 4 h

1.95
2.68

19.5
27.7

N.S.

5, 10

25 MPa; 110 3 25 3 10 mm3

1.34
1.70

[34.1
43.0] 20.0
21.9

[11.8
51.1] N.S.

[33.3
37.2]

[2.6
4.6]

19.5
21.9

N.S.

[33.3
37.2]

[N.S.]

10, 20, 30, 40, 50

N.S.

950 C for 3 h

N.S.

N.S.

N.S.

10, 20, 30, 40, 50

50 3 50 3 15 mm3

1.63
1.79

[N.S.] 3.1
8.3

[N.S.] 31.1
52.7

[N.S.]

[1.2
3.6]

12.5
21.2

16.1
49.2

[33.3
37.2]

[4.6
14.2]

10, 20, 30

Amaral et al. [129] Limestone waste Allegretta et al. [130] Soapstone waste Souza et al. [131] Marble waste Ye¸silay et al. [132]

1.81
1.93

950 C

Sultana et al. [128] Granite sawdust

750 C, 850 C, and 950 C for 3 h 114 3 25 3 1 mm3

5, 15, 25

5, 15

10, 20, 27

850 C, 900 C, 950 C, 1000 C, 1150 C for 1 h 34 MPa; 115 3 25 3 10 mm3

1.90
1.96

900 C, 950 C, 1000 C, 1050 C for 3 h 25 MPa; 70 3 10 3 15 mm3 500 C, 750 C, 1000 C for 1h 14 and 28 MPa

N.S.

N.S. [N.S.]

N.S. [N.S.]

1.48
1.67

22.1
28.3

9.2
83.4

850 C and 1000 C 200 3 20 3 15 mm3 1160 C for 7 h

N.S.

[N.S.] 3.9
11.2 [N.S.]

[N.S.] N.S. [N.S.]

X

mLS; mBD; mAP; mWA; mFS kThermal conductivity mBD; mFS; kWA; kAP kMetal leachability

X

kBD; mLS; kAP; kWA kMetal leachability

X

kMetal leachability

kLS; mWA

mBD; kWA; kAP; kLS

kFiring times mNonstoichiometric phases kLS; mCS; kWA; mBD kLS; kWA mWhiteness; mThermal expansion

(Continued)

Table 5.5 (Continued) Waste

Addition (wt.%)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

TCLP

Highlights

Porcelain stoneware tiles Granitic rock wastes and basalts Pazniak et al. [133] Quartzite waste

2.5, 5, 7,5, 10

Kiln, 1180 C
1210 C for 51 min

N.S.

0.1
1.1

N.S.

mLS; kWA

10, 15, 20, 25

Slip casting process; 6 3 2 3 0.5 mm3 1200 C

2.3
2.4

[N.S.] 0.6
0.9

[N.S.] N.S.

mWA; kLS

[N.S.]

[27.5
46.4]

mHardness

Pellets

0.63
1.82

0.8
55.1

0.3
8.9

kBD; mWA

[N.S.]

[N.S.]

0.1
25.7 [10.5
50.7]

0.6
12.7

kThermal conduct.; kAcoustic cond. kBD; mWA

Medeiros et al. [134]

Clay-expanded aggregates Granite waste

10, 20, 30, 40, 50

1000 C
1200 C for 15 min

Soltan et al. [135] Granite and marble

Moreno-Maroto et al. [137] Granite and marble Moreno-Maroto et al. [138]

2.5, 5, 7.5, 10

pellets

2.5, 5, 7,5, 10

1100 C, 1125 C, 1150 C for 4
16 min Pellets 1100 C, 1125 C, 1150 C for 4
16 min

0.81
1.41

[N.S.] 0.91
1.41

0.1
24.2 [19.8
55.9]

2.7
14.1 [N.S.]

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S., not specified; WA, water absorption.

kBD; mWA; mCS

Recycling of industrial wastes for valuedded applications

193

Brazilian technical standard, water absorption between 8% and 22%, and flexural strength higher than 1.5 MPa (for bricks used horizontally) or 3.0 MPa (for bricks used vertically). Consequently, this application is ideal when lower water absorption is required. With regard to granite cutting powder waste, Vieira et al. [118] evaluated the effect of GPW incorporation (0, 10, 20, and 30 wt.%) in clay brick fabrication (uniaxial pressed at 18 MPa/fired from 850 C to 1100 C). The application of granite up to 10 wt.% partially generated an increase in the mechanical strength of the bricks fired at 900 C or higher. On the other hand, the mechanical strength was reduced with the inclusion of higher than 10 wt.% GPW in relation to a control (without waste incorporated). Several studies introduced the possibility of recycling marble powder (MPW) for brick ceramic manufacturing. Cobo-Ceacero et al. [119] applied the principles of the circular economy in ceramic bricks produced with MPW (0, 2.5, 5, 7.5, and 10 wt.%) sintered at 950 C in a muffle. Based on the physical results, the marble waste incorporation produced mainly lighter color and low density. Therefore, lower open porosity and mechanical strength, together with higher water absorption, were produced. MPW was found to have excellent recycling potential to obtain eco-friendly lightweight ceramic bricks. Similar results were obtained by Sutcu et al. [120]. Clay bricks were produced by a semidry pressing process, adding up to 35 wt.% MPW fired at 950 C and 1050 C. The addition up to 30 wt.% increased porosity (40%), and decreased the compressive strengths (8.2 MPa), BD, and thermal conductivity (from 0.97 to 0.40 W/m/K). However, their resistance was in accordance with the standards. In accordance, Munir et al. [121] prepared bricks (0%
25% by weight of clay). Although the bricks still required deep technological evaluations, MPW could be used as a substitutable raw material for up to 15 wt.% for producing environment-friendly lightweight bricks. Clay-based bricks containing trachyte waste were examined by Coletti et al. [122], giving promising results. Mixtures with 5, 10, and 15 wt.% of trachyte were incorporated into the clay (hand-molding/sintered at 900 C, 1000 C, and 1100 C). The influence of trachyte as a fluxing agent favors partially the densification of the ceramic matrix, showing similar or even greater compressive strength than control bricks, especially with higher sintering temperatures. The trachyte addition revealed that water absorption and open porosity decreases; nevertheless, the thermal conductivity remained constant.

5.3.5.2 Stoneware tiles Ornamental rock is a nonplastic material, which acts as a degreaser and fluxing agent, which promotes the reduction of drying shrinkage and increases mechanical properties. The incorporation of undefined ornamental rock powder waste into ceramic specimens [123
127], granite sawdust [128,129,131], limestone waste [130], and even marble waste [132], is becoming common practice. Vijayaragavan et al. [123], Vieira et al. [124], Xavier et al. [125], Sant’Ana et al. [126], and Gadioli et al. [127] described the usability of mixtures of NSPW

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New Materials in Civil Engineering

waste as an additive raw material in ceramic product to determine the quality of the ceramic samples made from 5 to 50 wt.% of NSPW. The ceramic specimens were oven dried and sintered between 750 C
950 C. Moreover, the specimens were analyzed for water absorption, linear shrinkage, and flexural strength. All the experimental data show that the addition of the rock residue does not affect the physical and mechanical properties. Moreover, according to the leachability test (TCLP) the specimens leached a low level of heavy metals and did not cause environmental problems. Since GS waste is rich in silica it has also received research attention. Sultana et al. [128] incorporated different amounts of GS waste (10
50 wt.%) into red clay to produce roof tiles fired at 850 C
1100 C. The results concluded that incorporating up to 40 wt.% granite sawdust is promising for the manufacture of roof tiles because of the improved mechanical properties (31.97 MPa) and reduced water absorption when (6.5%) fired at 900 C compared to reference material. The GS acts as a fluxing agent and reduces the sintering temperature. Moreover, Amaral et al. [129] formulated clayey bodies with up to 30 wt.% of GS waste to produce ceramic paver. The results revealed a clayey body plasticity by coarse granulometry, and higher dry BD because of a higher packing. The use of ornamental rock waste is viable for some types of ceramic pavers due to the results of water absorption and compressive strength obtained, with a firing temperature of 1050 C, is in accordance with legislation. Allegretta et al. [130] investigated the mineralogical and microstructural changes that occur in a kaolinitic clay incorporating different limestone waste contents (0, 5, 15, and 25 wt.%) fired at 500 C, 750 C, and 1000 C. The results revealed that short firing times and sand-sized limestone temper promoted the formation of nonstoichiometric phases at the clay/limestone boundary, ruled by the lateral variation of CaO activity. Souza et al. [131] studied the influence of soapstone waste in the technological properties of red ceramics (pressed at 14 and 28 MPa/fired at 850 C and 1000 C). The best physical properties (linear firing shrinkage, water absorption, and resistance to simple compression) were achieved by a replacement of 15 wt.% soapstone waste into clay fired at 1000 C and pressed at 28 MPa. Ye¸silay et al. [132] used a technology for ceramic artwork tile manufacturing from MPW (up to 27 wt.%) for clay development. According to the firing color, the whiteness increased and redness decreased, and the water absorption (6.6%
11.2%) and thermal expansion coefficient (ranged from 53 to 63) increased. These properties illustrated the feasibility of ceramic artwork manufacturing.

5.3.5.3 Porcelain stoneware tiles The several options available to treat ornamental rock wastes also involve porcelain stoneware tile production using granitic rock wastes [133] and quartzite waste [134]. In this respect, Pazniak et al. [133] carried out a study into the fluxing potential of GPW and basalts and their effect on the properties and microstructure of porcelain tiles. The test results showed that the addition of up to 7.5 wt.% basalt increased compressive strength and decreased shrinkage properties, and even

Recycling of industrial wastes for valuedded applications

195

resulted in water absorption values near zero, because of the formation of a lowviscosity glassy phase sintered at 1150 C
1160 C. On the other hand, de Medeiros et al. [134] investigated the incorporation up to 25 wt.% of quartzite waste into ceramic porcelain (slip casting process/fired at 1200 C) as a substitute for feldspar. The study results showed improved mechanical resistance (35 MPa), water absorption (lower than 0.5%), and shrinkage characteristics, and it was concluded that quartzite waste could be an acceptable option for porcelain tile production.

5.3.5.4 Clay-expanded aggregates Finally, some applications have been developed for producing LWA, a very low density granular material with enormous applications in construction, agriculture, or civil and environmental engineering, to recycle ornamental rock waste obtained during the cutting process [135], and mixed with other inorganic wastes [136,137], in recent years. Soltan et al. [135] collected LWA-produced sintering green pellets at temperatures ranging from 1100 C to 1200 C and analyzed the viability of incorporating up to 50 wt.% GPW. Regarding the results, the granite involves a decrease in BD (1.2 g/cm3) and water absorption increment, due to large pores produced because of the liquid phase pushing the gaseous phase upward after firing at 1200 C. This research revealed that LWA, including GPW, could be used in lightweight concrete as thermal and acoustic insulators. Other researchers not only studied the incorporation of ornamental rock waste, but also produced expanded aggregates incorporating other waste. In the work of Moreno-Maroto et al. [136], two types of waste were used: (1) GPW and MPW (90 wt.%) and (2) sepiolite (10 wt.%). This mineral matrix added polyethylenehexene thermoplastics (0
10 wt.%), shaped into pellets and fired at 1100 C, 1125 C, and 1150 C. As was expected, higher temperatures involved greater sintering provoking improvements in the physical and mechanical properties. The study concludes that LWAs produced without thermoplastics were appropriated in hydroponics and/or water filtration systems, showing even better results than commercial LWA (Arlita G3). Similarly, another study by Moreno-Maroto et al. [137] investigated the influence of carbon fiber waste (0, 2.5, 5, and 10 wt.%) in the previous mineral matrix (90 wt.% granite and marble waste and 10 wt.% sepiolite rejection). The results indicated that these LWA produced with carbon fiber improved the lightness and mechanical properties because of promoted bloating and the formation of an internal structure.

5.3.6 Glass waste in clay-based ceramic applications Glass is a nonplastic raw material, which acts as a degreaser in the course of the shaping process, with a consequent reduction of drying shrinkage. Moreover, glass is a fluxing agent, forming a viscous liquid during sintering, which promotes densification on firing, with a consequent increase in the mechanical properties (Table 5.6).

Table 5.6 Summary of prominent reviewed research for use of glass waste in clay-based materials. Waste

Addition (wt. %)

Production method

BD (g/cm3)

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

2.74
7.64

20
32

[1.6
10.69]

[N.S.]

N.S.

18.5
28.7 [N.S.] 3
8

1.8
6.9 [N.S.] 30
70

X

kWA; mCS

1.6
2.1

[N.S.] 9
26

[N.S.] N.S.

X

kMetal leachability kBD; mWA; mFS; kAP

1.08
1.47

[22
42] 18.5
42.4 [39.3
56.7]

[3.5
16] 6.8
14.4 [N.S.]

kMetal leachability mLS; mBD; kWA; mCS

1.24
2.43

1.40
9.48

26.84
37.18

mLS; mBD; kWA; mCS

[N.S.]

[N.S.]

1.62
1.67

14.7
16.4

5.77
10.04

mBD; kAP; kWA; mCS

1.61
1.84

[N.S.] 12.5
25.8 [20.1
3.95]

[N.S.] 3.34
14.7 [N.S.]

mBD; kWA; mCS

, 10 [N.S.] 0.7
31.8

21.4
23.5 [N.S.] 9.5
37.4

[References]

TCLP

Highlights

X

mBD; kWA; mCS; kAP

Bricks Glass bottles

5, 10, 15, 20, 25, 30

Mao et al. [138] Glass bottles Ponce Pen˜a et al. [139] Glass bottles

20, 25, 30 5, 10

Rahman et al. [140] Glass bottles

5, 10, 15

Taha et al. [78] Window glass Kizinieviˇc et al. [141]

10, 20, 40

Window glass

20, 35, 50

Akhtar et al. [142] Glass fiber waste

5, 10

Guzlena et al. [143] Liquid crystal displays Tang [144]

10, 20, 30

Cathode ray tube waste Lee et al. [145] Solar panel glass waste

1
10 30

40 MPa; 50 3 35 3 10 mm3 950 C for 3 h

N.S.

190 3 100 3 50 mm3 1000 C for 12 h 1 MPa; 3 3 3 cm2 (cylinder) 1150 C for 2 h 6 MPa; 100 3 20 3 12 mm3 1020 C for 5 h 50 3 50 3 50 mm3 900 C and 1000 C for 1h 5 tons; 50 3 50 3 50 mm3 650 C, 850 C, and 1050 C for 3 h Extrusion; 12 3 3.4 3 7 cm3 1000 C for 1 h 50 3 50 3 50 mm3 900 C, 950 C, 1000 C, and 1050 C 1000 C for 48 h

N.S.

Extrusion; 120 3 28 3 17 mm3

N.S. 1.44
2.11

kSurface area; kMetal leachability kWA; mCS

X

kWA; mCS kMetal leachability N.S.

Jimenez-Millan et al. [147] Fluorescent glass Cordeiro-Morais et al. [147] Sludge from glass polishing

10, 20, 30

5, 10, 15, 20, 25

[N.S.]

[N.S.]

N.M

12
24

7
20

mBD; mLS; kWA; mCS

1.02
1.00

[N.S.] 17
21

[N.S.] 10
13

kWA; kAP; mCS; mFS

[ 40
44 ]

[1.09
2.07]

17
20

9
12.5

[38
42]

[N.S.]

1.38
1.86

14
30

N.S.

mThermal cond.; kmetal leachability mWA; mFS

1.24
2.43

[N.S.] 1.40
9.48

[2
11] 26.84
37.18

mLS; mBD; kWA; mCS

[N.S.]

[N.S.]

1.70
1.76

14.8
18.7 [N.S.]

15
19 [19.3
24.6]

mBD; kWA; kAP; mCS

N.S.

13.4
13.9 [N.S.]

11.2
21.3 [N.S.]

kWA; kAP; mCS

0.93
18.94 [1.6
32.7]

N.S [N.S.]

kWA; mhardness mAbrasion resistance

850 C for 36 h

Kazmi et al. [148] Sludge from glass polishing

975 C, 1025 C, and 1075 C for 3 h 18 MPa; 114.5 3 2.54 3 10 mm3 850 C
1100 C for 2 h 228 3 114 3 76 mm3

5, 10, 15, 20, 25

228 3 114 3 76 mm3

1.35
1.38

850 C for 36 h

Kazmi et al. [49] Scraps glass

10, 15

Mymrin et al. [149] Borosilicate glass

20, 35, 50

Akhtar et al. [142] Waste glass Phonphuak et al. [150]

5, 10

Waste glass Lissy et al. [151]

15, 25, 30

10 MPa; 60 3 20 3 10 mm3 700 C
1100 C for 4 h 5 tons; 50 3 50 3 50 mm3 650 C, 850 C, and 1050 C for 3 h 160 3 65 3 40 mm3 900 C, 950 C, and 1000 C for 1 h 80 3 40 3 40 mm3 600 C for 48 h

X

kEfflorescence; m freeze resistance mBD; kWA; kAP; mCS

Stoneware tiles Solar panel glass Lin et al. [86]

10, 20, 30, 40

5 MPa; 40 3 45 3 6 mm3 800 C
1100 C for 2 h

(Continued)

Table 5.6 (Continued) Waste

Addition (wt. %)

Production method

BD (g/cm3)

[References]

WA (%)

CS (MPa)

[AP (%)]

[FS (MPa)]

TCLP

Highlights

Porcelain stoneware tiles Glass bottles

10, 20, 30, 40

Njindam et al. [152] Glass bottles

50

Chitwaree et al. [153] Glass bottles 1 borosilicate lab. glassware Lassinantti Gualtieri [156] Liquid crystal displays Kim et al. [154] Window glass waste

41

10, 20, 30, 40

25, 50

Delvasto-Arjona et al. [155]

8 tons; 80 3 40 3 10 mm3 1000 C
1200 C for 2 h 20 MPa; 120 3 20 3 5 mm3 1230 C for 30 min 30 3 60 cm2

1.97
2.32

0.4
13

N.S.

mBD; kWA; mFS

N.S.

[N.S.] 0.09

[7
39] N.S.

kWA; mFS

2.15

[N.S.] 0.15

[54.87] N.S.

mAbrasion resistance N.S.

50 MPa 1100 C and 1150 C for 1h 50 MPa; 117 3 27 3 4 mm3 1250 C

2.0
2.4

[N.S.] ,0.5
8 [N.S.]

[42] N.S [N.S.]

2.23
2.31

0.8
2.7

N.S.

[6.8
7.7]

[26.0
33.0]

1
14

20
200

[N.S.]

[N.S.]

kBD; kWA kThermal expansion coefficient kLS; kBD; mAP; mWA

Clay-exposed expanded aggregates Glass bottles Oliveira et al. [157]

5, 10, 20

35 MPa; 20 3 20 mm2 (cylinder) 870 C
1100 C for 20 min

2.10
2.25

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S., not specified; WA, water absorption.

kWA; mCS

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199

5.3.6.1 Bricks As stated by data reported in the literature, glass waste is a promising additive in brick manufacture. In this sense, most recent researches have reported on the incorporation of glass residues from different sources, such as bottle glass [78,138
140], window glass [141,142], fiber glass [143], TFT-LCD glass waste [144], scrap glass [149], cathode ray tube panel glass [145], solar panel glass [147], borosilicate glassware [142], fluorescent lamp glass[147], or glass waste sludge (GS) [49,148]. In addition, some investigations dealt with glass waste in the manufacture of bricks without specifying their origin [150,151]. Akhtar et al. [142] analyzed the influence of adding different types of glass waste, namely soda
lime glass (window and sheet glass), colored glass (from Thai aluminum color glass) and borosilicate glass (laboratory glassware) on physical and technological characteristics of bricks. They concluded that compressive strength depends on the fluxing agent. Thus, 35 wt.% soda
lime glass as a fluxing agent exhibits the best compressive strength. Soda
lime glass includes an optimal amount of Na2O and CaO with a higher quantity of SiO2, which leads to a decrease in the sintering temperature. Kizinieviˇc et al. [141] used water treatment sludge and milled window glass waste for the manufacture of ceramic products. Compressive strength of ceramic body without glass waste was in the 8.5
9.1 MPa interval, while the highest compressive strength of 14.4 MPa was revealed for a ceramic body prepared with 40 wt.% glass waste. Recently, Jimenez-Millan et al. [147] assessed the utilization of solar panel glass waste in the production of sepiolite-based clay bricks. The addition of glass waste decreased the plasticity of the sepiolite and fired samples presented higher shrinkage values. The study concluded that the increase in shrinkage and the associated densification of the samples led to higher mechanical strength of samples fabricated with glass waste (95, 226, and 374 kg/cm2); therefore, these ceramics can be considered as high resistance materials. Moreover, Guzlena et al. [143] evaluated the effect of the addition of fiberglass waste (FGW) on the properties of fired clay bricks. Different percentages by weight of FGW, 5 and 10, were extruded and sintered at 1000 C for 1 hour. The addition of FGW waste (size range: 2
20 μm) showed a good influence on the mechanical characteristics of clay bricks such as compressive strength and density increases by increasing the FGW content in the mixture. FGW works as a reinforcement for the clay matrix. However, in future it is planned to use submicron reinforcing materials to gain greater improvement in crack reduction of clay materials. Other investigations have been carried out with the purpose of using technological glass waste as a raw material in secondary markets, among which the production of ceramic-based products is included. Cordeiro-Morais et al. [147] included different amounts of fluorescent lamp glass wastes into a clayey body for conventional red ceramics. Above 900 C, the strength noticeably increased with both the firing temperature and the percentage of glass waste. In particular, at 1050 C and 1100 C, the incorporation of 30 wt.% of glass waste led to material with strength values suitable for civil construction structural blocks. Tang [144] evaluated the

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New Materials in Civil Engineering

addition of 10, 20, and 30 wt.% of liquid crystal displays into clay-bricks sintered at 1000 C. The results revealed an increase in the ceramic matrix, improving the compressive strength and reducing the water absorption. A similar study was conducted by Lee et al. [145], which assessed cathode ray tube glass as a substitute for clay. Mixtures with 1
10 wt.% of waste were sintered at 900 C
1050 C. As the main conclusion, water absorption was reduced and, consequently the resistance increased. The TCLP test result exposed a lower metal leachability, in accordance with the limits established. Nevertheless, without a doubt, an extraordinary improvement in the technological properties of ceramic bricks is achieved when glass waste, which contains extremely fine glass particles, is introduced into a brick paste composition. Mao et al. [138] proposed the addition of glass bottle mixture (GBM) as a novel strategy for enhancing the immobilization of heavy metals when electroplating sludge is used in the manufacture of clay bricks. The leaching concentration of Cu and Zn declined substantially with GBM addition up to 30 wt.%. It was pointed out that GBM addition promoted brick densification and, hence, the porosity volume and surface area were reduced. During firing, glass waste melted and developed into a liquid phase, which might incorporate heavy metals, preventing their effective release. Moreover, the addition of GBM results in the modification of mineralogical composition after firing. GBM promotes a decrease in quartz content, while spinel develops. The chemical formula of the spinel structure is usually AB2O4 (A is a divalent metal, and B is a trivalent metal). The formation of spinel supports heavy metal immobilization as it exhibits high chemical resistance to acid solutions. Ponce Pen˜a et al. [139] reported on the effect of GBM in clay mixtures for the brick manufacturing process. The results indicated that decreasing the glass particle size enhanced significantly the brick properties of water absorption and compressive strength. The authors stated that a smaller GBM particle size facilitates glass melting, reducing porosity, and enhancing brick properties. GBM has been also studied to counteract the negative effect on the mechanical properties observed when other residues are introduced into brick bodies. Thus, Rahman et al. [140] investigated the effect of GBM addition in sludge
clay brick manufacture on brick quality to increase the sludge addition proportion in brick specimens. Previous research remarked on the possibility of textile sludge replacing clay as a raw material in brick production. However, increasing the addition of sludge led to worsened brick quality and hindered realistic applications of sludge-based bricks with reference to compressive strength standards. However, the addition of GBM to the textile sludge
clay mixture for brick manufacture enhances the compressive strength and water absorption parameters of textile sludge-based bricks, reaching values of .70 N/mm2 and ,4 wt.%, respectively, in bricks made using 30 wt.% textile sludge, 60 wt.% clay, and 10 wt.% GBM. Similarly, Taha et al. [78] found that the addition of GBM improved the mechanical properties of fired bricks containing calamine [Zn4Si2O7(OH)2  H2O] mine tailings. However, when more than 15 wt.% GW was used, a white skin constituted of sodium sulfate emerged at the fired brick exposed surface. Thus, fired light bricks with suitable physical and mechanical properties could be obtained from mixtures enclosing up to 30 wt.% calamine and

Recycling of industrial wastes for valuedded applications

201

10 wt.% of GBM. Reduction of crystalline quartz with glass waste addition has been also reported by Phonphuak et al. [150]. α-SiO2 was the main crystalline phase in clay bricks formulated without glass addition but when the glass waste amount was increased to 5 and 10 wt.%, anorthite [Ca(Al2SiO8)] was developed with a decrease in global crystalline quartz. A similar studied was conducted by Lissy et al. [151] adding 15, 25, and 30 wt.% of GBM, achieving good sintering and better technological properties. Another waste material related to glass is GS, which is generated during the cutting and polishing processes of glass on an industrial scale. Kazmi et al. [49,148] explored the potential of using GS as a secondary material in clay brick manufacturing using various dosages (0%, 5%, 10%, 15%, 20%, and 25% by clay weight). They found that the density of bricks decreased with increasing GS content, which was attributed to the lower density of GS (998 kg/m3) in comparison with that of clay (1118 kg/m3). Moreover, the results indicated that a 50 wt.% increase in flexural strength was observed for brick specimens incorporating 25 wt. % replacement of GS for clay. The authors examined the effect of GS addition on the initial rate of absorption (IRA), which is considered to be an indirect evaluation of the bond strength of bricks with mortar IRA values for brick specimens incorporating GS being lower compared with control bricks, enhancing the bond between strength. GS addition also had a beneficial effect on the durability of bricks. Thus, the area affected by efflorescence for brick specimens decreased with the addition of WGS. Generally, a higher free CaO content is correlated with the occurrence of efflorescence and, in GS, the CaO content is lower than that of the clay and, consequently, leads to reduced efflorescence. On the other hand, the results indicated that a higher percentage of GS in bricks can lead to improved freeze
thaw resistance. However, in this case the addition of GS has a detrimental result on immobilization of heavy metals. Brick specimens incorporating 25 wt.% GS presented higher values of leaching toxicity as compared with control brick specimens. The authors attributed this result to the lower amount of iron oxide in GS as compared to clay since the sorption character of iron oxide incorporates heavy metals and reduces the leaching toxicity. However, values of leaching toxicity for all brick specimens were observed to be well below the specified limits. Mymrin et al. [149] produced bricks incorporating scrap glass (10
15 wt.%) and red clay sintered between 700 C and 1100 C. The results meeting all requirements were studied. The flexural strength tests of ceramics reached good values of around 11.7 MPa and water absorption above 12.6%.

5.3.6.2 Stoneware tiles Some authors have investigated the addition of solar panel glass waste as an additive in stoneware tiles [86]. This research established that it can act as a substitute raw material for the manufacture of ceramic tiles. The authors indicated the possibility of manufacturing suitable ceramic tiles by sintering at 1050 C of a paste containing an appropriate combination of raw materials. Solar panel waste glass encouraged melting of quartz, developing a more abundant and less viscous liquid

202

New Materials in Civil Engineering

phase, which assists sintering. In this way, they highlighted that the inclusion of 30
40 wt.% solar panel waste glass in stoneware leads to ceramic tiles with appropriate technological performance.

5.3.6.3 Porcelain stoneware tiles Several researchers have also reported on the feasibility of using glass waste as raw material in porcelain stoneware tile manufacture, such as, glass bottles [152,153], glass bottles mixed with borosilicate laboratory glassware [156], window glass waste [155], and liquid crystal display waste [154], where glass can replace both quartz sand as an inert component and feldspar as a fluxing agent. In this sense, Njindam et al. [152] studied the viability of fabricating porcelain stoneware wall and floor tiles from binary formulations of clay. From the results, the optimum glass waste content of 30 wt.% and a firing glass waste temperature of 1150 C were recommended to produce porcelain stoneware tiles. As just mentioned, the incorporation of glass into a ceramic paste leads to an important decrease in firing temperature, which in turn enables energy savings and a reduction in CO2 emissions. Chitwaree et al. [153] contrasted the energy consumption during the sintering process of conventional porcelain stoneware tiles and those where glass waste and pottery stone were used as substitutes for clay, feldspar, and quartz. The sintering temperatures of the commercial and modified tiles were 1230 C and 1050 C, respectively. The energy consumption for a conventional sample was 13,303.84 kJ/kg of product, while the modified sample consumed about 9191.24 kJ/ kg of product (about 30 wt.% lower). Regarding the CO2 emitted during the sintering process, it was about 0.1132 kg CO2eq/kg product and 0.0782 kg CO2eq/kg product for conventional and modified tiles, respectively. On the other hand, Lassinantti Gualtieri et al. [156] developed low-temperature stoneware tiles by adjusting a traditional porcelain stoneware formulation. They stated a significantly lower firing temperature (1080 C instead of 1215 C) when a mixture containing 34 wt.% borosilicate glass and 7 wt.% soda
lime
silica glass was added to a conventional porcelain stoneware composition. Pilot-scale experiments confirmed the fulfillment in existing powder processing routes. The mineralogical composition of the novel tile was 60 wt.% amorphous phase, 15 wt.% residual quartz, and about 24 wt.% plagioclase, both newly formed and residual. The mechanical properties in terms of flexure strength and abrasion resistance were in line with the requirements of tiles classified as porcelain tiles (group BIa). Kim et al. [154] examined the gradual substitution of feldspar by glass waste from LCD panels in a porcelain stoneware body. The authors highlighted that, for the feldspar, the sintering process progressed by diffusion via slightly viscous flow of the partially melted feldspar, while diffusion via full viscous flow of the glass prevailed for LCD waste. Moreover, the character and percentage of crystalline phases (quartz, mullite, andesite, and glassy phase) in bodies sintered at 1100 C did not show a noteworthy modification in relation to those present in conventional porcelain stoneware samples. This result was attributed to both the Al2O3 content and viscosity at sintering temperature of LCD glass waste.

Recycling of industrial wastes for valuedded applications

203

Finally, Delvasto-Arjona et al. [155] investigated the substitution of sodiumpotassium feldspar in a porcelain stoneware composition by window glass waste. The proportion of feldspar in a standard formulation was replaced in 25 and 50 wt. % by glass powder. The results revealed that the incorporation of glass waste resulted in a decrease in the optimal firing temperature, although the optimum vitrification interval is somewhat narrower, indicating the requirement for more accurate temperature control during sintering. However, according to the criteria of the IS0 13006 standard, the tiles with residual glass presented with water absorption and flexural strength values that did not admit their classification as porcelain tiles but rather as semistoneware or stoneware. Nevertheless, other researchers have successfully obtained porcelain stoneware with the addition of glass wastes.

5.3.6.4 Clay-expanded aggregates Finally, Alves de Oliveira et al. [157] in a recent study described the production of synthetic aggregate of calcined clay with the incorporation of soda
lime glass waste. When 20 wt.% of glass waste was included in the composition, a volume increase was observed in samples fired at 1000 C, due to the higher liquid phase formation that sealed the void spaces and delayed the release of trapped gases. In this case, glass waste acted as a density reducer, which is fundamental for the production of LWA.

5.3.7 Organic waste in clay-based ceramic applications Organic waste is a lightweight material, the addition of which causes a decrease in BD. Moreover, one of the methods of weight reduction and at the same time hardness-improved performance is to introduce voids by incorporating organic particles into the clay mixture (Table 5.7).

5.3.7.1 Bricks As a result, over the last decade, a lot of research has been carried out to incorporate various types of organic waste into the production of bricks. This report focuses on specific types of industrial organic waste generated, paper mill waste (PMS) (paper mill sludge ([169,158
160,170]), paper mill sludge compost) [160], biomass (e.g., wheat straw, sunflower seed cake and olive stone flour) ([163,164]; Aouba et al., 2017), vine shoot (VS) [162], and degraded MSW [160,161]. Cusido´ et al. [169], Vieira et al. [158], Goel and Kalamdhad [159], and Kizinieviˇc et al. [170], reported the use of PMS, a nonhazardous industrial waste mainly consisting of cellulose and calcite, as an additive in clay brick manufacturing in various mix ratios (up to 20 wt.% PMS), and fired at different low temperatures (ranging from 750 C to 1000 C). Physical properties such as bulk porosity, water absorption, and BD were measured. Mechanical and thermal performances have also been characterized. Based on their results, the PMS content in the clay mixture increased the porosity and water absorption, consequently decreasing the

Table 5.7 Summary of prominent reviewed research for use of organic waste in clay-based materials. Waste

Addition (wt.%)

Production method

Density (g/cm3)

WA (%)

CS (MPa)

TCLP

Highlights

[AP (%)]

[FS (MPa)]

N.S.

11.5
26.6

24
49

X

mWA; mAP; mCS

N.S.

[N.S.] 22.1
23.5

[N.S.] 1.8
3.4

X

[N.S.] 14.3
31.8 [20.1
32.0]

[N.S.] 2.5
1.2 [N.S.]

1.31
1.86

11.2
26.3 [23.4
46.0]

4.0
22.0 [N.S.]

N.S.

N.S.

N.S.

[N.S.]

[N.S.]

13.4
25.4

3.4
28.1

[17.1
26.3]

[N.S.]

Bricks Paper mill sludge Cusido´ et al. [169] Paper mill sludge Vieira et al. [158] Paper mill sludge Goel and Kalamdhad [159]

5, 10, 15, 20, 25 10

5, 10, 15, 20

Extruded 10 bar; 120 3 50 3 50 mm3 980 C for 3 h 190 3 190 3 90 mm3 Kiln, 750 C for 12 h 61 3 29 3 19 mm3 850 C and 950 C for 8h 50 3 50 3 50 mm3 900 C and 1000 C for 1 h

1.03
1.25

Paper mill sludge Kizinieviˇc et al. [170]

5, 10, 15, 20

Paper mill sludge compost; MSW Goel et al. [160]

5, 10, 15, 20

61 3 29 3 19 mm3

MSW

5, 10, 15, 20

Kiln, 850 C and 900 C for 8 h 61 3 29 3 19 mm3

5, 11, 17

Kiln, 850 C and 900 C for 8 h 250 kPa

1.12
1.48

22.5
36.0

1.6
9.0

4, 8

Kiln, 900 C 175 3 79 3 17 mm3

1.46
1.77

[33.2
40.5] 17.8
30.0

[N.S.] N.S.

[31.6
43.5]

[5.3
11.9]

Goel and Kalamdhad [161] Vine shoots waste Velasco et al. [162] Wheat straw, sunflower seed cake, and olive stone flour Bories et al. [163]

920 C for 1 h

1.20
1.51

X

kMetal leachability kLS; mWA; mAP; 5 CS kMetal leachability mBD; kLS; mAP kMetal leachability

X

kLS; mAP; mWA mFrost resistance; kMetal leachability 5 CS

X

kLS; mAP; kBD; mWA kMetal leachability kThermal conductivity mWA; mAP; kLS kLS; kBD; mWA; mAP kThermal conductivity

Wheat straw, sunflower seed cake, and olive stone flour Aouba et al. [164]

1
8

170 3 75 3 17 mm3

1.57
1.85

920 C for 1 h

15.2
28.6

18.1
36.2

[27.9
43.5]

[7.1
14.2]

kLS; mWA; mAP; kBD mPlasticity; kThermal conductivity

13.9
30.0

N.S.

kBD; mAP; mWA

[N.S.]

[1.9
11.8]

mFS

22.9
30.0

N.S.

[28.1
44.0]

[10.8
18.9]

kBD; kLS; mWA; mAP kThermal conductivity

Stoneware tiles Pulp paper waste

12.5, 15, 17.5, 20

Extruded 10 MPa; 60 3 20 3 10 mm3 700 C
1100 C for 4h 80 MPa; 100 3 50 3 10 mm3 1150 C

1.44
1.69

2, 4, 6, 8, 10, 12.5, 15

Kiln, 900 C, 950 C, 1000 C for 1 h

1.11
1.76

18.0
51.2

N.S.

kBD; mAP; mWA

5, 10, 15

900 C and 1000 C for 1 h

1.12
1.69

[31.6
56.8] 11.7
60.4

[N.S.] N.S.

mRedness kBD; mWA; mAP

[22.2
60.3]

[N.S.]

1.45
1.61

20.4
25.7 [32.9
37.4]

N.S. [N.S.]

kElectrical conductivity mWA; mAP; kBD kMetal leachability

1.70

40.5
42.7

N.S.

[49.8
56.7]

[N.S.]

Mymrin et al. [149] Coffee waste

10, 20, 30

Manni et al. [165]

1.46
1.64

Clay-expanded aggregates Brewery waste

Farı´as et al. [166] Brewery, meat-bone meal, and corn cob Farı´as et al. [167] Bagasse from beer production Moreno-Maroto et al. [73]

5, 7, 10

Olive pomace

2.5

Moreno-Maroto et al. [168]

60 3 30 3 11 mm3 1000 C
1200 C for 1h Extruded and pelletized Kiln, 1050 C for 4 m

AP, Apparent porosity; BD, bulk density; CS, compressive strength; FS, flexural strength; LS, linear shrinkage; N.S., Not specified; WA, water absorption.

X

kBD; mWA; mAP

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New Materials in Civil Engineering

BD due to the open pore structure in the clay body that was developed as a result of burnt cellulose fibers and calcite decarbonization. Therefore PMS incorporation improves properties regarding thermal and acoustic insulation, but in turn, it decreases the mechanical strength. However, this fragility of the material is compensated for by increased ductility. Moreover, the addition of 10 and 15 wt.% of PMS at firing temperatures of 900 C and 1000 C, respectively, met the technical and environmental standards. Recycling of PMS by composting is becoming an acceptable practice for converting these chemically complex materials into useful soil amendments, while eliminating any negative environmental impacts. Goel et al. [160] statistically studied the effects of paper mill sludge compost (PMSC) and firing temperature on the final properties of fired bricks. Different PMSC ratios were added and fired at two different temperatures (850 C and 900 C) in line with the commercial kiln practice. The performance characteristics were analyzed using ANOVA, signal to noise (S/ N) ratio, and random forest regression analysis. The results showed that the firing temperature is the least important parameter affecting a brick’s compressive strength, water absorption, or linear shrinkage. Furthermore, the addition up to 15 wt.% of PMSC can still achieve compressive strength comparable to normal clay bricks. Goel and Kalamdhad [161] demonstrated the feasibility of incorporating degraded MSW as one of the constituents for the production of fired bricks. The raw materials, degraded MSW and two different soils, that is, laterite soil and alluvial soil, were mixed together in different proportions ranging from 5 to 20 wt.%. Specimens of these mixtures were then fired at 850 C and 900 C, respectively. Various properties such as BD, linear shrinkage, and loss on ignition, water absorption, compressive strength, and modulus of elasticity were studied and compared. An optimum constituent mix of 20 wt.% degraded MSW with (laterite or alluvial) soil fired at temperature of 900 C was found to be most appropriate for brick production. The ultimate uptake of this study is an 8% net saving in the energy consumption of external fuel by mixing 20 wt.% degraded MSW. Velasco et al. [162], studied the addition (ranged from 2 to 17 wt.%) of kindling from VS, which is a widely produced waste in vineyards, in the production of fired (at 950 C) clay bricks in order to achieve better thermal insulation of buildings. Therefore the influence of kindling addition on the thermal and mechanical properties of the fired clay bricks containing up to 17 wt.% VS has been investigated. It could be concluded that the amount of kindling VS that can be added is around 11 wt.%, whereby the bricks’ mechanical and physical properties abide by settled regulations for structural clay bricks, in accordance with current regulations. The added VS has improved brick conductivity properties by reducing it by up to 62% compared to bricks made without any waste. This means an improvement of up to 34% for the equivalent thermal transmittance of a typical single-leaf wall assembly. Bories et al. [163] and Aouba et al. [164] evaluated three different agricultural solid wastes (wheat straw, sunflower seed cake, and olive stone flour) using different particle size and incorporation rate (up to 8 wt.%) on the properties on fired clay bricks at 920 C. The results showed an increase in the total porosity and water

Recycling of industrial wastes for valuedded applications

207

absorption, and so a decreasing BD for mixtures containing these residues, implying lower thermal conductivity (up to 23%). There is a significant positive correlation between the increasing amount of organic matter and porosity. Moreover, different grain sizes create higher total pore volume owing to the difference in grain size distribution. The compressive strength was substantially reduced with the waste incorporation and higher particle size. The incorporation of 4 wt.% of sunflower seed cake, with the lowest grinding, leads to the best compromise between mechanical and thermal results compared to the reference brick (100 wt.% clay).

5.3.7.2 Stoneware tiles Moreover, some recent researches have reported on the incorporation of organic waste as additive in stoneware tile manufacture, like paper mill sludge [149] and coffee waste (CW) [165]. Mymrin et al. [149] produced red ceramics using PMS and scrap glass. Mixtures containing 12.5
20 wt.% PMS, 10
15 wt.% SG, and red clay were shaped and sintered between 700 C and 1100 C. The results meeting all the requirements were studied. The flexural strength tests of obtained ceramics reached 11.7 MPa, water absorption was 12.6%, bulk densities varied from 1.79 to 1.86 g/cm3, and shrinkage was 1.4%. CW, a residue generated from coffee shops, was valorized by Manni et al. [165] on ceramic tile manufactured incorporating 10, 20, and 30 wt.% CW and firing at 1150 C. The study reported that a 30 wt.% replacement ratio produced porous (42.81%) lightweight ceramics (water absorption 5 29.25%/class BIII) with similar flexural strength (10.75 MPa) and high insulating properties (0.39 W/m/k).

5.3.7.3 Porcelain stoneware tiles On the other hand, due to the increase in water absorption with organic waste incorporation, because of the CO2 gas released during the firing process, it is not possible to use it as a porcelain additive.

5.3.7.4 Clay-expanded aggregates The growing concern on sustainability and environmental issues has led many researchers around the world to focus their efforts on the investigation of wastes as possible raw materials for the production of LWA production, such as food production waste (i.e., brewery industry waste [166], BB production [73], sludge from wastewater treatment plants from meat-bone meal and corn cob [167], and OP [168]). Farı´as et al. [166], described the utilization of three brewery wastes (BB, diatomaceous earth, and brewery sludge), in the production of expanded clay-based aggregates. Different amounts of waste (0
15 wt.%) were combined with clay, formerly, the dried samples were subjected to a firing process in a kiln (temperatures of 900 C, 950 C, and 1000 C) that sintered the grains and changed their density and porosity. The results showed that the incorporation of these wastes decreased

208

New Materials in Civil Engineering

the BD and increased water absorption and porosity, a phenomenon more evident in the BB. Furthermore, these properties are not affected by temperature. According to the LCA, waste from the brewing industry in the combustion process releases CO2 emissions that can be recaptured and regrown by photosynthesis. In addition, the calorific value, largely in the case of bagasse by burning, generates an exothermic reaction, which lowers power consumption in the sintering process, therefore having economic savings. These new expanded aggregates have significant insulating properties and are suitable for use on green roofs. On the other hand, Farias et al. [167] elaborated LWA using different percentages (0, 5, 10, and 15 wt.%) of three food industries waste (sludge from a wastewater treatment plant from the brewery industry, meat-bone meal, and corn cob), which were mixed with three types of clays (white, black, and red) in two different clay-based mixtures treated at two different temperatures (900 C and 1000 C). The results indicate the potential for manufacturing high-quality LWA for agronomic purposes, using relatively simple processing and low sintering temperature that contribute to the reduction of greenhouse gases to the atmosphere and energy consumption of the furnace. Moreno-Maroto et al. [73] examined the possibility of incorporating BB and FBA in a clay-expanded aggregate composition. Mixtures of clay incorporating together FBA (31.7, 31, and 30 wt.%) and BB (5, 7, and 10 wt.%) were blended with water, shaped into prismatic specimens, oven-dried, muffle sintered at 1000 C
1200 C, and finally crushed into LWA. The result showed the formation of pores with the addition of BB but it did not illustrate noticeable improvements in lightness, porosity, or water absorption with respect to the reference material (without any additive). This investigation proves the usefulness of recycling FBA and BB to obtain light weight, which makes it interesting for agriculture, gardening, or hydroponics. Other research by Moreno-Maroto et al. [168] recycled OP, a waste generated through olive tree management and cultivation (widely used in the Mediterranean agricultural sector), as a bloating and/or fluxing agent in LWA manufacturing. Different clays were used with the addition of 2.5 wt.% OP. Mixtures were moistened and blended, then, after 1 day, the mixture was extruded and pelletized. Finally, the product was fired at 900 C in a tubular rotary kiln for 4 minutes. Based on the results, the aggregates containing OP presented higher porosity and water absorption, lower density, and acceptable mechanical strength.

5.4

Discussion

In the extensive literature reviewed steady interest in the valorization of different industrial wastes in clay-based ceramic manufacturing has been revealed. The different industrial wastes used in the production of clay ceramics are generated in massive amounts worldwide, such as: (1) ash from combustion, (2) artificial gypsum, (3) metal slag and metallurgical waste, (4) sludge, (5) ornamental rock sawdust, (6) glass waste, and (7) organic waste from food production and agricultural management.

Recycling of industrial wastes for valuedded applications

209

A wide variety of the most common methods studied during the period 2015
19 on the production of stoneware floor and roof tiles, porcelain stoneware tile, bricks, and lightweight (clay expanded) aggregates, incorporating various waste materials as partial replacement of the raw materials has been extensively presented. The results are described in the form of tables describing the ceramic mix composition, conditions of molding, firing, and main physical properties, such as resistance (compressive and flexural strength), water absorption, BD, apparent porosity, and shrinkage on sintering. As an environmental result, the potentially leachable contaminants inherent in the different wastes were evaluated mainly through the leaching behavior of the materials, but they are also based on LCA and gas emission analyses. Moreover, the ecotoxicological potential of ceramics was assessed. The analysis and evaluation of the vast amount of experimental research concluded that utilizing industrial waste materials to produce eco-friendly clay-ceramic materials improved the products’ physical and mechanical attributes. However, there are many conditioning factors related to the nature of the compatibility between the waste and the natural raw material; the kind of products where the waste will be introduced (tiles, bricks, etc.); and the characteristics of the finished product (sintered/porous). The leaching and ecotoxicological tests carried out in accordance with various specifications, which were reviewed in parallel with the corresponding regulations, have established the viability of these products from an environmental point of view, due to having low metal leachability, high biosecurity, and most importantly, no significant adverse environmental impacts. Finally, the use of recycling industrial waste can absorb great amounts of materials, including hazardous by-products, that would otherwise be disposed of in landfill and high amounts of waste by-products can be reused, even if the waste incorporation is done in small amounts, as high production rates will translate into significant consumption of wastes.

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696.

Emerging advancement of fiber-reinforced polymer composites in structural applications

6

Kishore Kumar Mahato1, Krishna Dutta2 and Bankim Chandra Ray2 1 School of Mechanical Engineering, Vellore Institute of Technology, Vellore, India, 2 Composite Materials Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India

6.1

Introduction

6.1.1 Definition and development of composite materials Technological development in today’s world is greatly dependent on advancements in the field of materials. This needs continuous research and development of newer approaches toward specific engineering applications. Frequently, the modern technological era necessitates materials with an amalgamation of properties that are rarely available with conventional metals and metal alloys, as well as ceramics. For instance, hard and strong materials are relatively dense, with low ductility and impact properties. Conversely, many materials with appreciable formability possess lower strength. Hence, the range of beneficial properties in a single material could be tailor made by the development of composite materials. A composite material is a kind of two/multiphase material that exhibits the combined properties of both/all its components. Composites, perhaps the most significant material in any application, are nowadays widely accepted as they provide high strength and stiffness combined with low density, when compared with bulk materials; furthermore, they allow for a considerable weight reduction in the finished part. The assessment of composite materials in critical applications is nothing new. At the start of the 1960s, diversified applications of composites were started in civil constructions, aerospace, and energy sectors. Nevertheless, nature is the best example of using composite structures in several of its creations, for example, wood, palm leaf, bones, etc. The composite materials may be classified according to the reinforcement material used as particle-reinforced or fiber-reinforced composites. Fig. 6.1 illustrates the classification of composite materials in accordance with the type of reinforcement. Technologically, the fiber-reinforced composites can be considered as the most important variant of composites as they provide high strength and/or stiffness with high specific modulus. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00006-5 © 2020 Elsevier Inc. All rights reserved.

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Large particle Particulate Small particle Reinforcement type Continuous Fiber Dis continuous

Aligned

Randomly oriented

Figure 6.1 Classification of composite materials in accordance with the type of reinforcement.

6.1.2 Technological superiorities of fiber-reinforced polymer composites Since the exceptional innovation of macromolecules called polymers by the Nobel prize winner German chemist Hermann Staudinger in 1953 [1], the human civilization has become increasingly dependent on the use of polymer-based materials. The excellent range of mechanical properties with their light weight, as well as many other noteworthy benefits, have made polymer-based materials a top choice in many areas. Further enhancement of their properties was possible by introducing polymer-based composites where different reinforcing materials were added to the polymer matrix. Fiber-reinforced polymeric (FRP) composite materials are nowadays globally one of the main counterparts of the metallic materials used in various structural and construction fields. During the 1980s, early research work on sustainability of FRP composites for civil engineering structural applications was performed by Professor Urs Meier at EMPA (Swiss Federal Laboratories for Materials Testing and Research) in Switzerland. Since then, numerous researchers throughout the world have explored the potential applications of FRP composites in civil structures. In the last few decades, fairly significant numbers of researchers have contributed toward the development of FRP composites with improved properties. These composite materials attain superiority due to their low density with high specific strength and stiffness, and have led to the use of such materials in many critical and supercritical applications including automobile, aerospace, marine, bridges, cryogenic tanks, pressure vessels, and so on [2]. Moreover, lower fabrication cost coupled with low maintenance and ease of handling make these materials useful in acute applications, replacing metallic materials. The overall properties of such polymeric matrix composites (PMCs) are governed by the fiber
polymer interface and/or interphase. The identity of the interface is determined by a three-dimensional area between the bulk matrix and bulk fiber [3]. Furthermore, the morphological properties of the fiber and the diffusivity of elements in each component also have prominent effects [4]. Laminated

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composites again attract attention toward their excellent in-plane mechanical properties, which are directly subjective to the properties of the reinforcing fiber. In contrast to in-plane behavior, the out-of-plane properties are limited by the response of the polymer phase [5,6] and the corresponding interface/interphase generated. One of the main concerns in laminated FRP composites is the delamination of fiber and matrix phase due to the interfacial debonding. Superior interfacial properties can be imposed by proper alteration of the polymer matrix and/or interface/interphase with the incorporation of different nanofillers. Inclusion of metal oxides (TiO2, Al2O3, ZnO, etc.), inorganic fillers (SiO2, SiC etc.), and carbonaceous-based fillers (CNT, SWCNT, MWCNT, and graphene) certainly improves the mechanical properties of the PMCs [7
15]. Modification of the matrix trough interface allows drastic improvement of the toughness of the resultant material. The stability of laminated FRP composites however, is subjected to the residual stresses at the interfacial region and the proper transfer of stress from matrix to fiber through the interface. The overall improved fracture toughness by matrix shear yielding and the crack pinning mechanism is attributed to the reinforcement of polymer with micron-sized particles like rigid glass spheres [16]. Also, in recent times extensive research has been carried out into the environmental durability of different nanofiller-embedded FRP composites.

6.1.3 Applications of fiber-reinforced polymers in structural fields There is an increasing demand for glass FRP (GFRP) composites worldwide, for several applications in civil engineering, specifically as reinforcement in concrete structures. The key advantages of FRP composite materials in such applications are their noncorrosive properties in association with their high tensile strength and light weight [17,18]. Furthermore, these materials are nonconductive and nonmagnetic. Lightweight concrete reduces the structure’s total mass and hence, the proportional seismic loads; it further causes significant cost savings in the construction by retaining the structural integrity and the earthquake resistance of reinforced concrete (RC) structures [19]. The fatigue response however, may be questionable when the structures are subjected to high tensile stresses [20]. The material durability can be improved and controlled through a suitable choice of composite constituents [17,21]. Substitution of the conventional steel reinforcing bar in the precast sections of underground tunnel lining by FRP reinforcement was reported by Caratelli et al. [22]. Use of FRP reduces the maintenance cost and durability of such structures. It appears that the use of this noncorrosive reinforcement in tunnel segments contributes to reducing the requirement for concrete cover that is usually a weak point for such structures, as it can crash due to the thrusts produced from a tunnel-boring machine. FRP composites are also being used in making bridge decks, replacing traditional steel decks. Pont y Ddraig (Dragon Bridge), Rhyl, United Kingdom, is a lifting bridge, which was built with lightweight glass and carbon FRP (CFRP) composites on the bridge deck and soffit (Fig. 6.2). Use of FRP enabled weight

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Figure 6.2 Rhyl Pont y Ddraig lifting bridge. Source: From Composite materials: enter the dragon, ,https://www.themanufacturer.com/ articles/composite-materials-enter-the-dragon/..

reduction of the bridge, as it is an important consideration for such lifting bridges to reduce energy consumption [23]. Wolf Trap Pedestrian Bridge, shown in Fig. 6.3, is another example of a lightweight bridge in Vienna, Austria, constructed using 55 prefabricated FRP bridge deck panels connected to a steel truss using mechanical fasteners and Z-clips. Fig. 6.3 illustrates Rhyl Pont y Ddraig lifting bridge that possesses a high strength-to-weight ratio for pedestrians and also enables it to support an emergency vehicle, if necessary [24]. Other than these, the followings are examples where FRP composites have been used in bridge structures [25,26]: G

G

G

G

G

G

G

G

G

G

INEEL Bridge, Idaho (1995); Magazine Ditch Bridge, Delaware (1997); Wickwire Run Bridge, West Virginia (1997); Washington Schoolhouse Road Bridge, Maryland (1998); Wilson’s Bridge, Pennsylvania (1998); Bennet’s Bridge, New York (1998); Woodington Run Bridge, Ohio (1999); Greensbranch Bridge, Delaware (1999); Schroon River Truss Bridge, New York (2000); Kings Stormwater Canyon Bridge, California (2000)

6.2

Assessment of fiber-reinforced polymer composites by mechanical, chemical, and thermal behaviors

6.2.1 Macro characterization The basic mechanical properties of fiber-reinforced composites are evaluated by various tests such as tensile, fatigue, flexural, interlaminar shear stress (ILSS), and

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Figure 6.3 Wolf trap pedestrian bridge in Vienna. Source: From FRP transportation infrastructure, ,https://www.compositeadvantage.com/ gallery/pedestrian-deck-wolf-trap-virginia/..

creep. The above-mentioned mechanical properties are highly essential in the design and development of fiber-reinforced composites.

6.2.1.1 Tensile test Basically, the tensile test is one of the important tests used to calculate tensile behavior, such as tensile strength, tensile strain, tensile modulus, and Poisson’s ratio. In the case of fibrous-based polymeric composites, a flat rectangular sample can be used to perform the test in accordance with the American Society for Testing and Materials (ASTM) D 3039 [27]. This test technique is designed to yield tensile property data for material specifications, research and development, structural design and analysis, and quality assurance. A minimum of five specimens should be tested per test condition unless valid results can be gained through the use of fewer specimens, such as in the case of a designed experiment. There are many material configurations, such as fabric-based materials, multidirectional laminates, or randomly reinforced sheet-molding compounds, which can be successfully tested without tabs. However, tabs are highly recommended when testing unidirectional materials (or strongly unidirectional dominated laminates) to failure in the fiber direction. Tabs may also be necessary when testing unidirectional specimens in the matrix direction to avoid gripping damage. The most consistently used bonded tab material has been continuous E-glass FRP matrix materials (woven or unwoven) in a [0/90] laminate configuration. The tab material is commonly

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applied at 45 degrees to the force direction to provide a soft interface. Adhesive bonding of composite may be used at the end tabs to prevent failure within the tab area. The specimen is fixed in the tensile fixture within the grip and a constant standard crosshead velocity, that is, 2 mm/min, is fixed in the software to perform the test. Strain gauges are deployed to measure the longitudinal and transverse strains.

6.2.1.2 Fatigue test The standard test method for tension
tension fatigue of PMC materials is designated by ASTM D3479/D3479M-12 [28]. The test sample geometry, preparation, dimensions, and tabbing are the same as those described in ASTM D3039/D3039M. This test technique defines the fatigue behavior of PMC materials subjected to tensile cyclic loading. The FRP composite material forms are restricted to continuousfiber or discontinuous-fiber reinforced composites for which the elastic behaviors are specifically orthotropic with respect to the direction of test. It is restricted to unnotched test samples subjected to constant amplitude uniaxial in-plane loading, where the loading is clear in relation of a test control factor. The prime test result is the fatigue life of the test sample under a specific loading and environmental parameter. Repetition of tests may be done to obtain a distribution of fatigue life for specific material types, environments, laminate stacking sequences, and loading conditions. The obtained fatigue life data may be statistical in nature; using this linearized stress life (S
N) or strain life (ε
N) curves need to be plotted. The investigations into fatigue damage in a PMC are needed to identify the existence of microscopic cracks, delamination, or fiber fractures. The sample’s residual strength or stiffness, or both, may vary due to these damage mechanisms. The loss in stiffness may be computed by stopping cyclic loading at certain cycle intervals to get the quasistatic axial stress
strain curve using modulus determination procedures found in Test Method D3039/D3039M. The loss in strength linked to fatigue loss may be accumulated by discontinuing cyclic loading to get the static strength using test method D3039/D3039M.

6.2.1.3 Flexural test The flexural testing method is relevant to FRP composite materials. It is a testing mechanism with a convenient loading rate, which is used in combination with a loading fixture. The theoretical calculation of the strength and modulus of laminated FRP composites in tensile and compressive loading is simple and well recognized. However, due to the complexity of stress distribution at the time of flexural loading, estimation of the mechanical properties of the materials becomes difficult. The standard test method for flexural properties of PMC is described in ASTM D7264/D7264M-15 [29]. This test technique determines the flexural properties (comprising strength, stiffness, and load/deflection behavior) of PMC materials. This test method was established for optimum use with continuous-fiber-reinforced polymer matrix composites and differs in several respects from other flexure techniques, including the use of a

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standard span-to-thickness ratio of 32:1 versus the 16:1 ratio. Flexural properties may be used for specification purposes, quality control, and design applications. It may also be used to estimate the flexural properties of structures. The load (P) and displacement (h) graph obtained from flexural testing can be converted into stress (σ) and strain (ε) plots by the following equations: σ5

3Pl 2bd2

ε5

6hd l2

where b, d, and l represent the width, thickness, and span length of the specimen, respectively. Generally, the strength of the material is taken as stress which resembles the maximum load carried by the specimen after the test. The modulus value has been calculated from the slope of the preliminary linear portion of the stress
strain plot.

6.2.1.4 Interlaminar shear stress or short beam shear test The three-point short beam shear test is carried out on a sample with a minute span, which stimulates failure by interlaminar shear [4]. Shear stress is usually involved in the beam encountered with the bending load, which is directly proportional to the magnitude of the employed load, independent of the span length. Therefore the support span in the short beam shear (SBS) sample is usually kept short to produce an excessive interlaminar shear failure ahead of bending failure. The short beam shear test is well documented by ASTM D 2344 [30], which describes a span length to sample thickness ratio of five for lower stiffer composite materials and four for higher stiffer composite materials. The test has a natural problem related to the nonlinear plastic deformation and the stress concentration incorporated by the loading roller nose of small diameter. This is schematically shown in Fig. 6.4, where the effects of stress concentration in a thin sample (Fig. 6.4B) are compared with those in a thick sample (Fig. 6.4C). In both cases, the samples have a similar span-to-depth ratio (SDR). Therefore the stress condition

Figure 6.4 (A) Schematic of the three-point bend test; and stress distribution of a (B) thin and (C) thick specimen. Source: Adapted from J.K. Kim, Y.W. Mai, Elsevier, Amsterdam, The Netherlands, 1998.

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is much more complex than the pure shear stress condition given by the simple beam theory. In relation to the classical beam theory, the distribution of shear stress along the thickness of the sample is a parabolic function, which is equivalent to the neutral axis where it is highest and then decreases to zero at the tensile and compressive profile. In real cases, the stress field is influenced by the stress concentration close to the loading roller nose, which totally demolishes the parabolic shear distribution used to evaluate the evident ILSS. Detailed studies into the short beam shear stress technique have been carried out by several investigators to identify the imperfection. The experiments reveal that by significantly increasing the diameters of the support and loading cylinders and increasing the span to length thickness ratio to about eight shows some good results. With these variations, the results show that the parabolic division of shear stress throughout the thickness of the sample estimated by simple beam theory could be extremely good in the areas between the loading roller and support cylinder, so that the sample fails in a shear mode. With these improvements, ASTM D 2344 could turn out to be technically appropriate with an accepted shear test method.

6.2.1.5 Creep test The standard test method for tensile creep rupture of FRP matrix composite bars is well demonstrated in ASTM D7337/D7337M-12 [31]. The initial test result is the million-hour creep rupture size of the sample. This test technique plans requirements for tensile creep rupture testing of FRP matrix composite bars frequently used as tensile components in prestressed, reinforced, or post-tensioned concrete. Creep behaviors of reinforced, prestressed, or post-tensioned concrete structures need to be considered from a design point of view. This method of exploring creep rupture of FRP bars is proposed for practice in laboratory tests in which the principal variable is the type or size of the FRP bars, magnitude of applied force, and period of force application. Unlike steel reinforcing bars or prestressing tendons exposed to substantial sustained stress, creep rupture of FRP bars may take place below the static tensile strength. Therefore it is very important to discover the creep rupture strength when determining suitable stress levels in FRP bars used as reinforcement or tendons in concrete members intended to resist sustained loads. Creep rupture strength fluctuates according to the size and type of FRP bars used. This standard calculates the creep rupture time of FRP bars under a particular set of controlled environmental situations and force ratios.

6.2.2 Micro characterization The evaluation of FRP composites at the micro and nano levels can be conducted through the following methods: differential scanning calorimeter (DSC), Fourier transformation infrared (FTIR) spectroscopy, atomic force microscopy (AFM),

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dynamic mechanical thermal analysis (DMTA), scanning electron microscopy, and transmission electron microscopy (TME).

6.2.2.1 Differential scanning calorimetry analysis DSC analysis is a thermoanalytical method used to measure melting temperature, glass transition temperature, heat of fusion, latent heat of melting, specific heat or heat capacity, crystalline phase transition temperature and energy, precipitation energy and temperature, and specific heat or heat capacity oxidation induction times. DSC analysis measures the quantity of energy absorbed or released by a specimen when it is heated or cooled, providing quantitative and qualitative data on endothermic (heat absorption) and exothermic (heat evolution) processes. The specimen is placed in a suitable sample pan and held in a constantan disc on a platform in the DSC analysis cell with a chromel wafer immediately underneath. A chromel-alumel thermocouple under the constantan disc measures the specimen temperature. An empty reference pan sits on a symmetric platform with its own underlying chromel wafer and chromel-alumel thermocouple. Heat flow is measured by comparing the difference in temperature across the specimen and the reference chromel wafers. Temperature-modulated differential scanning calorimetry (TMDSC) is an upgraded thermoanalytical technique over conventional methods which includes superimposition of modulation on the linear heating, cooling, or isothermal temperature program. The temperature modulation can be defined by a linear sawtooth pattern, waveform pattern, sinusoidal function, or other mathematical function. A discrete Fourier transformation of the modulated heat flow (raw signal) generates the dynamic heat flow and deconvoluted (average) response. The latter is denoted as total heat flow. It can be separated into its heat capacity and kinetic components, also known as reversing and nonreversing heat flow, respectively. Alternative probability of interpretation of the results is supported by calculation of complex heat capacity, which denotes the ratio of the modulated heat flow amplitude and modulated heating rate amplitude. TMDSC is also described as alternating DSC (ADSC). ADSC is recognized as temperature modulation during a constant heating rate in nonisothermal experiments, but the quasi-isothermal conditions mean that the temperature is altered in a sinusoidal fashion with an angular frequency ω (radian s21) and suitably at lower amplitude about a constant temperature. Pans of Al, Cu, Pt, Au, Al2O3, and graphite are used and need to be selected to avoid reactions with specimens and with regard to the temperature range of the measurement. Any of the following atmospheres may be used in the DSC chamber, that is, nitrogen, argon, air, oxygen, vacuum as low as 30 mTorr, and controlled mixed gases. DSC is used to identify the thermal properties of plastics, polymers, adhesives, pharmaceutical materials, waxes, sealants, metal alloys, foods, lubricants, oils, shape-memory alloys, fertilizers, catalysts, and intermetallic compound formations.

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6.2.2.2 Fourier transformation infrared spectroscopy FTIR spectroscopy is normally denoted as FTIR analysis or FTIR spectroscopy. This infrared spectroscopy technique is used to identify chemical functional groups existing in organic, polymeric, and in some cases, inorganic materials. The FTIR test relies on infrared light to scan samples and observe bond properties. This technique is used to assess the purity of some inorganic samples and is highly reliable for identifying polymer composition. Basically, the infrared spectrum is distributed in three regions. The common region, which is also known as the mid-infrared region, ranges from 4000 to 667 cm21, and results from vibrational plus rotational transitions. This region is mostly meant for organic chemists as the vibrations induced in organic molecules are absorbed in this region. The regions on either side of infrared are known as the close infrared (12,500
4000 cm21) and far infrared (667
50 cm21) regions. The close infrared regions reveal bands assignable to harmonic overtones of fundamental bands and combination bands, whereas the far infrared region deals with pure rotational motion of the molecules. The physical property that is evaluated in infrared spectroscopy is the capacity of some molecules to absorb infrared radiation. Atoms in molecules are not static, as one might think, but rather they vibrate about their equilibrium positions. The frequency of these vibrations depends on the mass of the atom and the length and strength of the bonds. Molecular vibration is stimulated by bonds absorbing radiation of the same frequency as their natural vibrational frequency (usually in the infrared region). The different chemical bonds in this excited state absorb the light energy at frequencies unique to the various bonds. This activity is denoted as a spectrum. The spectrum can be stated as % transmittance (%T) or % absorbance (%A) with respect to wavenumber (cm21). The wavenumber of the peak tells what kinds of bonds exist there and %T indicates the signal strength. Low signal strength directly affects the resolution of the peaks, creating the sample size and preparation key to acquire a quality spectrum [32]. The region of the infrared spectrum from 4000 to 1400 cm21 reveals absorption bands that fall under the functional group regions. These bands are useful diagnostically, but more usually they supplement the region below 1400 cm21. The region from 1400 to 900 cm21 is complex because it contains, in addition to fundamental stretching and bending vibrations, many bands resulting from the sum or difference of their vibration frequencies. Specific vibrational assignments in this region are very difficult to perform. Thus, this part of the spectrum is characteristic of a compound and is called the fingerprint region. Similar molecules may show very similar spectra in the functional group region, but exhibit an apparent difference in the fingerprint region. A ratio of specific peak heights can sometimes be used to quantify proportions in simple mixtures, degree of oxidation or decomposition, purity, and so on. The FTIR supports identifying chemical bonds as well as the chemical composition of materials.

6.2.2.3 Atomic force microscopy AFM is one of the most useful and powerful microscopy technologies for studying specimens at the nanoscale. It is versatile because an atomic force microscope

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cannot take only images in three-dimensional topography, but also provides several types of surface measurements to meet the needs of scientists and engineers. It is powerful because it can produce images at atomic resolution in the angstrom scale, with minimal sample preparation. AFM has become a versatile tool for describing the topography and properties of solid materials since its first use. Besides topography information, the phase lag of the cantilever oscillation, relative to the signal sent to the cantilever’s piezo driver, is simultaneously monitored, giving information about the local mechanical properties, such as adhesion and viscoelasticity. Phase imaging is a powerful tool that provides nanometer-scale information that is often not revealed by other microscopy techniques. The AFM comprises a cantilever with a sharp tip, called a probe, at its end that is used to scan the specimen surface. The cantilever is normally made up of silicon or silicon nitride with a tip radius of curvature of the order of nanometers. When the tip is carried into the proximity of a specimen surface, forces between the tip and the specimen lead to a deflection of the cantilever according to Hooke’s law. There are two primary modes of operation for an AFM, that is, contact mode and noncontact mode, depending on whether the cantilever vibrates during the operation. In the contact mode, the cantilever drags across the specimen surface and uses the deflection of the cantilever to measure the contours of the surface. In the noncontact mode, the tip vibrates marginally above its resonance frequency and does not contact the surface of the specimen. Any long-range forces, such as van der Waals forces, decrease the resonant frequency of the cantilever.

6.2.2.4 Dynamic mechanical thermal analysis The DMTA is an instrument that helps to evaluate the viscoelastic response of the material to a wide range of temperatures [33
35]. The DMTA instrument applies a dynamic load to the sample, and the response of the material is recorded in the form of dynamic displacement. For a perfectly elastic solid, the applied stress and resulting strain remain in phase, whereas there is a phase difference in the case of polymeric or viscoelastic materials. The data acquired from DMTA as storage modulus (E0 ) represent the elastic modulus of the material and loss modulus (E00 ) replicates the viscous modulus. The damping behavior of the material is calculated from the tan δ factor (i.e., the ratio between E00 and E0 ). The values such as E0 , E00 , and tan δ are calculated using the following equations: E0 5

σo cosδ εo

(6.1)

E00 5

σo sinδ εo

(6.2)

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tanδ 5

E00 E0

(6.3)

where σo and εo represent the peak stress and peak strain, respectively, and δ is the phase difference between the stress and strain.

6.2.2.5 Scanning electron microscopy Fractography has become identical with an examination in which both the mechanisms of cracking are identified and the influence of the environment and/or the internal structures of the component on the mechanics of fracture are determined. The investigation of the fracture surface of polymer-based composites is generally carried out successively using low-power optical microscopy, high-power optical microscopy, and either scanning electron microscopy (SEM) applied directly to the surface or TME of replicas taken from the surface of specimen. The use of replicas has declined in recent years due to the development and better availability of SEM. SEM, due to its very large depth of fields, is ideally suited to the study of fractures in polymers and can focus on even the most fibrous fracture surfaces. It allows correlation of the detailed structures observed at high magnification with the coarser optically visible features of the fracture.

6.2.2.6 Transmission electron microscopy TEM is a microscopy method whereby a beam of electrons is transmitted through an ultrathin specimen, interacting with the specimen as it passes through. When electrons are accelerated up to high-energy levels (a few hundred keV) and focused on the material, they can scatter or backscatter elastically or inelastically, or produce many interactions, which are the source of different signals such as X-rays, Auger electrons, or light. Some or all of these are used in TEM. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. The TEM mechanism is like a slide projector. A projector shines a beam of light which transmits through the slide. The patterns painted on the slide only allow certain parts of the light beam to pass through. Thus the transmitted beam replicates the patterns on the slide, forming an enlarged image of the slide when falling on the screen. In the TEM, only thin samples, which allow a fraction of the incident electron beam to pass through the specimen, can be studied. When an accelerated beam of electrons impinges upon a specimen a wide variation of interactions takes place, as shown in Fig. 6.5. The interactions that occur during the collision of the electron beam and the sample include directly transmitted electrons, secondary electrons, backscattered electrons, coherent elastic scattered electrons, incoherent inelastic electrons, incoherent elastic forward-scattered electrons, characteristic and continuum X-rays,

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Incident beam Backscattered electrons

233

Secondary electrons Characteristic X-rays

Auger electrons

Visible, IR, UV lights

X-rays Elastically scattered electrons

Direct beam Direct beam

Figure 6.5 Schematic of an incident electron beam through a sample.

Auger electrons, long-wavelength radiation in the visible, ultraviolet, and infrared regions of the spectrum, lattice vibrations (phonons), electron oscillations in metals (plasmons), electron
hole pair generation, and electrical current. In principle, all these products of primary beam interaction can be used to derive information on the nature of the specimen. To derive full benefit of the various interactions the electron microscopist needs a working knowledge of the electron
sample interactions. This knowledge should be at least broadly qualitative. In special cases as characterization of the characteristic x-rays and electron diffraction the level of understanding must be quantitative. The working principle of TEMs acts in the same way except that they shine a beam of electrons (like the light in a slide projector) through the sample (like a slide). However, in TEM, the transmission of the electron beam is highly dependent on the properties of the material being studied. Such properties include composition, size, density, etc. For example, porous material will allow more electrons to pass through, while dense material will allow less. As a result, a sample with nonuniform density can be observed by this method. Whatever part is transmitted is projected onto a phosphor screen for the user to see.

6.3

Evaluation of special structural properties

6.3.1 Vibrational properties In the building construction sector, the steel frame structure is one of the most important major structural systems across the world. With the introduction of lightweight walls, the cost of these steel structures is greatly decreased, and this has

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been reported by numerous researchers [36,37]. Traditional lightweight wall systems generally integrate external facing sheets sandwiching a series of wall studs. These wall studs are made from tubular steel sections or solid timber sections [38,39]. Pultruded GFRP (PGFRP) profiles have pronounced potential to serve as a better alternative as wall studs in civil constructions [40
43]. These profiles reveal necessary advantages comprising low thermal conductivity, light weight, and high resistance to corrosion and chemical attack [2,44
48]. Such advantages permit the design and construction of structural systems [49
53] with lower maintenance costs and carbon dioxide emissions [44]. For example, a built-up web-flange sandwich structure has been established for beam/slab applications in buildings [54,55]. This built-up unit involves a series of regular PGFRP box or I-profiles sandwiched between two PGFRP flat panels. Such a type of sandwich structure may be further integrated as a wall in the frame structures. Wu et al. [56] studied the connections of tubular GFRP wall studs to steel beams for building construction. They reported that a PGFRP stud was fixed firmly to the sleeve connector by one of three techniques: adhesive bond, ordinary bolt, and blind bolt. The connector was then attached to the steel beam through ordinary bolts. For comparison purposes conventional steel angles were also fabricated. Sequences of moment
rotation tests were carried out on these stud-to-beam connections. In addition, two stud lengths were fabricated in order to examine the connection behavior under moment dominant loading and shear force dominant loading conditions. Experimental results revealed the following behaviors including failure mode, shear
rotation response, moment
rotation response, joint rotational stiffness, and capacity. Among all the connections, the bonded sleeve connection was found to be more effective and was categorized as a rigid and partial strength connection. The failure methods of all the connections were explored after being dismounted from the test set-up. The complete progressive failure mechanism of each sample was studied with the help of video camera recording. Fig. 6.6 illustrates the final failure modes of the shear force dominant specimens and Fig. 6.7 shows that for the moment dominant specimens. The detailed failure form at the end of the GFRP stud is also shown in Figs. 6.6 and 6.7. For the shear samples in Fig. 6.6, all bolted connections (OB and BB in Fig. 6.6A
C) failed with damage seen at the end of the PGFRP studs, and most of the damage was concentrated at the top and bottom flanges of the GFRP part without any damage on the webs. This may be attributable to the top and bottom flanges of the GFRP stud which were well connected to the connector through bolts. To stabilize the external moment and shear force, the top and bottom flanges of the stud are subjected to the bearing forces from the connector (AG or SL), which may cause flange damage (particularly at the four corners of the tubular section) as stated by the authors of References [47,48]. Furthermore, the failure mode of the SL connection seems independent of the bolt type (i.e., OB or BB). Both the OB sample and the BB sample showed similar failure modes as presented in Fig. 6.6B and C. in contrast, no damage on the GFRP stud was seen when it was adhesively bonded to the SL connector (S-SL-AB in Fig. 6.6D). This may be attributable to the GFRP stud being incorporated with the SL connector through adhesive bonding. Therefore the moment and shear force

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Figure 6.6 Failure modes of shear specimens: (A) S-AG-OB; (B) S-SL-OB; (C) S-SL-BB; and (D) S-SL-AB. The numbers indicate damages to (1) web/flange separation; (2) shear out of top flange; (3) yielding of the end plate; and (4) bending and fracture of the threaded rod.

Figure 6.7 Failure modes of moment specimens: (A) M-AG-OB; (B) M-SL-OB; (C) M-SLBB; and (D) M-SL-AB. The numbers indicate damages to (1) web/flange separation; (2) shear out of top flange; (3) yielding of the end plate; and (4) bending of the threaded rod.

are transferred through the whole composite section (GFRP stud and steel tube) to the steel beam. This structural reliability of the bonded SL connection suitably allows the achievement of plasticity through the yielding of the steel end plate (see Fig. 6.6D) rather than the brittle failure of the GFRP stud as experienced by bolted SL connections. The large yield deformation of the steel end plate led to the bending and fracture of the first row of the threaded rod which is close to the top surface of the steel tube (see Fig. 6.6D). For the moment dominant samples in Fig. 6.7, the failure methods are identical to those of the corresponding shear samples with the same connection technique as shown in Fig. 6.6. For example, all bolted samples exhibited web-flange splits of the GFRP stud, while the bonded SL connection showed no stud damage but rather extensive yielding of the steel end plate. However, the difference in the failure methods between the shear and moment samples was the way in which their web/ flange separated.

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Zhang et al. [57] investigated the cyclic performance of bonded sleeve connections for joining tubular FRP beams and columns. Samples with various endplate thicknesses and numbers of bolts were studied under cyclic loading. The hysteretic moment-rotation responses of samples, including ultimate moment, rotation capacity, and rotational stiffness, and local strain reactions were experimentally acquired and comparatively examined. The cyclic performance of samples was also considered in terms of their ductility and energy dissipation capacity. From the failure modes observed, it was seen that the beam-column samples with bonded sleeve connection failed first through yielding of the steel endplate. After that, cohesive failure generated at the interface between the steel tube and GFRP column, especially in samples with a thicker endplate. These cohesive failures were then propagated, eventually caused web-flange junction cracking at the GFRP beam end. In contrast, yielding of the steel endplate was the dominant mode in samples with smaller endplate thickness, in which case the experiment was stopped due to the large deflection at the beam end caused by the yielding deformation of the endplate in the connection. Qi et al. [58] explored the flexural behaviors of novel composite sandwich beams that feature Paulownia or Southern pine wood core and GFRP face-skins reinforced with lattice-webs. In the nomenclature of samples, CON denotes control sample; PA and SP indicate Paulownia wood core and Southern pine wood core, respectively; 0, 1, 2, 3, and 4 signify the number of lattice-webs; and F and S denote flatwise and sidewise directions, respectively. “Flatwise” or “sidewise” signify the configuration of lattice-web reinforcement directions with horizontal lattice-webs or vertical latticewebs. The horizontal lattice-webs mean that the direction of the lattice-webs is perpendicular to the applied load. The vertical lattice-webs mean that the direction of the lattice-webs is parallel to the applied load. The control samples PA-CON and SPCON failed in a brittle manner. Tensile cracks were first detected at the bottom of the loading point. The maximum shear strain of the wood was above its crushing strain, these cracks then quickly prolonged upward and produced instant failure with the increase of the load, as revealed in Fig. 6.8A. Meanwhile, a large deflection and loud sound occurred. For composite sandwich samples PA-0 and SP-0, without lattice-webs, brittle failure did not occur since the tension face-sheet bridged the cracked core, avoiding brittle failure. At the starting of loading, microcracking occurred in addition to noises from slight flexural cracks of the wood fibers in the area of the loading state but no other noticeable phenomena could be seen on the external surface, representing an effect of stress concentration. Tensile cracks were generated in the core and transmitted rapidly when the load continued to increase. When the highest compressive strain of the external surface exceeded its compressive strain, upper local indentation occurred while compressive cracks on the lateral face-skin extended downward, followed by compressive failure of the side face-skins, as revealed in Fig. 6.8. This experimental phenomenon could be attributed to compressive failure of the lateral face-skins. Composite sandwich samples with one, two, three, and four webs failed in a ductile fashion owing to the existence of multilayered GFRP lattice-webs that may

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Figure 6.8 Failure modes of GFRP sandwich beams: (A) SP-CON-F/S, (B) SP-0-F/S, (C) PA-3-F, (D) PA-3-S, (E) SP-4-F, and (F) SP-4-S.

possibly increase the adhesive bonding, allowing full play of the characteristics of the glass fiber between the outermost skins and core. Cracks initiated in the core and transmitted in the lateral face-skins with the application of load. A substantial decrease in the load value and an increase in crack widths were seen when the depth of the cracks on the core extended to the level of the composite lattice-webs in the vicinity. This may be attributed to the fact that GFRP lattice webs disallowed the propagation of the cracking of the core to the core of adjacent webs and the cracks in the side face-skin. The composite sandwich beams in the flatwise directions mainly failed by crushing of the fiber composite skins at the loading point followed by splitting of the bottom glass fiber caused by large deformations at the bottom of the beams, as shown in Fig. 6.8C and E. As a higher load was subjected to the uppermost faceskin, pressed sunken failure ultimately followed in the samples using Paulownia core material, because it was not hard enough. The nonhorizontal lattice-webs, however, prohibited the cracks from propagating vertically in the side face-skin at the area of loading point. Fig. 6.8D and F indicated compressive wrinkling in the sidewise

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directions and constant application of load produced crack propagation ranging from the uppermost to the lower section, followed by compressive failure. Satasivam et al. [59] studied the shear-stiffness of novel blind bolts used as shear connectors in steel
FRP composite systems. An experimental study was executed to enumerate the shear-stiffness, or slip modulus, of the shear connection by loading steel
FRP joints in tension. The shear stiffness of the connector was then used in a new design technique to predict bending stiffness of the composite beam system with partial composite action at the steel
FRP interface. This planned design technique also considered the partial composite action between the upper and lower flat panels of the FRP slab caused by the weak in-plane shear stiffness of the transverse webs. Hizam et al. [60] investigated the behavior of a PGFRP truss system connected using a through-bolt with a mechanical insert. They performed experimental and analytical studies of a double-chorded composite truss system connected using stainless steel through-bolts with mechanical inserts. The composite trusses were accumulated by means of rectangular hollow sections of PGFRP, where adhesively bonded mechanical inserts were introduced at the vicinity of the joining areas. The trusses were tested under two load conditions: (1) under four-point and (2) threepoint bending. The load-vertical deflection behavior of the truss, joint behavior and internal forces distribution in the members were explored. Under load case 1, the structural PGFRP truss members continued physically undamaged as the maximum capacity of the equipment of 450 kN was reached, although a decline in stiffness was obvious prior to this. The maximum internal tension force of 77.18 kN was recorded at the mid-span of the bottom chord, while the maximum internal compression force was recorded at 170.98 kN for the external diagonal members. Based on the calculated factor of safety, all the structural members accomplished carrying the load safely, with the lowest factor of safety of 1.10 being accomplished by the external diagonal members. Under load condition 2, the top chords and joint were placed under greater stress and this mostly affected the overall load distribution paths. The maximum axial tension force of 30.21 kN was measured at the bottom chord member, while higher axial compression forces of 60.30 and 54.44 kN were recorded for the external diagonal member and the vertical member, respectively. Based on the load-deflection curve, at around 130
160 kN, compressive failure progressively occurred in the outermost part of the tubular cross-section, and beyond this point, the load-carrying capacity decreased gradually until the end of the load application.

6.3.2 Toughening mechanisms through implications of nanofillers Conventional FRP composites reveal an excellent specific strength and stiffness, low weight, impact resistance, and good chemical and environmental resistance. These features have made them one of the most imperative materials for many structural applications, including automotive, aeronautical, renewable energy,

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construction materials, and sports equipment [51,61
63]. Even though they exhibit excellent in-plane properties, the through-thickness behavior of FRP is weaker; specifically the interlaminar delamination resistance. Interlaminar delamination is a major challenge in the design of composite structures [64]. The key factor initiating delamination in laminates is the weak fiber/matrix interface and the brittle behavior of the resins. Numerous methods have been explored to enhance the interfacial adhesion, such as Z-pinning, stitching, and braiding, but these processes are complex and expensive, and also tend to reduce the in-plane laminate performances, as they damage the primary fibers [65,66]. Another substitute is to increase the surface area and reactivity of the fibers through surface modifications, plasma treatment, thermal modification, or chemical functionalization, but the increase in properties has usually been modest [10
13]. Over the last decade, there has been growing interest in the progress of FRP composite materials, in which a nanoparticle is incorporated into the conventional fiber composite material. Inclusion of metal oxide nanoparticles (TiO2, Al2O3, ZnO, etc.), inorganic fillers (SiO2, SiC, etc.), and carbonaceous-based fillers (CNT, SWCNT, MWCNT, and graphene) improves the mechanical properties of the PMC [7
15]; specifically nanofillers enhance the matrix-dominated properties such as the toughness of the FRP composites. Chandrasekaran et al. [67] investigated the effect of the addition of 0.5 wt.% of MWCNTs on the ILSS of carbon fiber/epoxy composites, using the compression shear test method. They observed a 41% enhancement in the ILSS value for the nonfunctionalized particles, while a 61% improvement was reported while the nanoparticles (NPs) were functionalized. By comparing the effect of the nanoparticle on the fracture toughness of the matrix and the overall performances of the FRP, the authors concluded that the enhancements were attributed to the chemical interlocking between the fibers and the NPs (with a stronger interlocking in the case of the functionalized NPs), rather than by improving the properties of the matrix itself. Rathore et al. [9] reported on an experimental investigation into the assessment of elevated temperature durability of glass fiber/epoxy (GE) composite with various levels of multiwalled carbon nanotubes (MWCNTs). Flexural testing at room temperature revealed that the addition of 0.1% MWCNTs yielded maximum strength (132.8% over control GE) and modulus (111.5% over control GE) among all the CNT-modified composite systems. Furthermore, MWCNT
GE composites resulted in accelerated degradation of mechanical performance with increasing temperature as compared to a GE composite. DMTA was carried out to study the viscoelastic behavior of all composites over a range of temperatures. Fractographic analysis illustrated various failure modes in all composites at various temperatures. The flexural stress versus strain plots for samples with various MWCNT loading at various in situ temperatures are shown in Fig. 6.9. The service temperature of these materials is limited in accordance with their glass transition temperature (Tg). Hence the maximum in situ testing temperature was chosen that was close to the Tg of the composites (nearly 110 C). The impact of temperature on GE composite with 0% (control) and 0.1% MWCNTs can be seen in Fig. 6.9A. The addition of 0.1% MWCNTs into the GE composite resulted in enhancement of both modulus

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and strength by 11.5% and 32.8%, respectively, when tested at room temperature, as shown in Fig. 6.9A. This huge increment in strength might be attributable to the efficient stress transfer from the soft polymer matrix to the stiff MWCNTs through the subtle CNT/polymer interface. The potential exploitation of CNTs in a composite can only be achieved if the majority of the load could be transferred from the matrix to the nanotube. Fig. 6.10 shows the dispersion state of MWCNTs in the matrix of 0.1% and 0.5% MWCNT
GE laminated composites. The MWCNTs are mostly isolated from each other and uniformly distributed throughout the matrix of 0.1% MWCNT
GE composite (Fig. 6.10A), whereas in 0.5% MWCNT
GE composites, local bunches of MWCNTs are found which form agglomerates as shown in Fig. 6.10B. The reduction of strength at higher MWCNT content can be attributed to the formation of these agglomerates, reducing the total CNT/epoxy interfacial area. The toughness increment in GE composite due to the addition of 0.1% MWCNTs may be attributed to the nanotube pull-out and crack bridging by nanotubes, as can be seen from Fig. 6.10C. In FRP composites, most of the transverse mechanical properties are governed by the matrix and fiber/matrix interface. After morphological analysis of the modified polymer matrix in each of the GE/MWCNT composites, the fiber/ matrix interface was analyzed for control GE and 0.1% MWCNT
GE composites. In GE composite (Fig. 6.10D), the bare and smooth fiber surface without any signature of polymeric phase indicates fiber/matrix debonding due to poor interfacial bonding. In contrast, a good fiber/matrix interfacial adhesion has been noticed in the case of 0.1% MWCNT
GE composite (Fig. 6.10E) and a coating of the polymeric phase can be seen on the surface of the fibers. Davis et al. [68] investigated the variation of mechanical properties of a fiberreinforced epoxy composite using functionalized CNTs. A comparison with

Figure 6.9 Flexural stress
strain curves for GE composites with various MWCNT contents at (A) room temperature (RT) (20 C), (B) 70 C, (C) 90 C and (D) 110 C.

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Figure 6.10 Dispersion of (A) 0.1%, (B) 0.5% MWCNTs in GE composite, and (C) CNT pull out and crack bridging by CNT in 0.1% MWCNT
GE composite after room temperature testing. Glass fiber bundle (D) GE and (E) 0.1% MWCNT
GE composite.

composite laminate material without CNT reinforcements indicated that there are modest enhancements of strength and stiffness; but a potentially significant increase is demonstrated for the long-term fatigue life of these functionalized CNTreinforced composite materials. They further reported that amine functionalized SWCNTs (a-SWCNTs) enhanced the fiber/fabric
matrix interfaces of a CFRP composite material, which exhibited enhancements in the tensile strength and stiffness and resistance to tension
tension fatigue damage. At the a-SWCNTs fiber/ fabric
matrix interfaces an “a-SWCNT reinforced region” of strengthen and stiffened matrix material was created resulting in overall improvements of these properties in the nanocomposite laminate in comparison to the neat material. The aSWCNTs deposited at the fiber/fabric
matrix interfaces inhibited longitudinal interfacial cracking and delamination during fatigue cycling and the carbon fiberreinforced epoxy nanocomposite laminate material showed greater durability under tension
tension cyclic loading.

6.4

Environmental durability of fiber-reinforced polymer composites in civil structures

FRP composite materials have been utilized predominantly in the construction of civil infrastructure [69
71]. In the last three decades there has been growing awareness for building civil infrastructure systems with FRP composites amongst civil engineers in aggressive environments for the enhancement of the mechanical and in-service properties [62,72
76]. In fact, this class of materials possesses immense potential for the FRP fabric, and plates are often used as reinforcement, rehabilitation, or retrofitting of existing structures, externally bonded or

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mechanically fastened to RC beams, slabs, and columns [77
82] for use in civil engineering. These FRP composites find an extensive range of applications in several sectors such as aerospace, automotive, sporting goods, marine, and oil and water pipelines due to their ease of handling and usage, better mechanical properties, and lower manufacturing cost [62,83,84]. The overall properties of PMCs are governed by the fiber/polymer interface/interphase. The identity of the interface is determined by a three-dimensional area between the bulk matrix and bulk fiber [3]. Further, the morphological properties of the fiber and the diffusivity of elements in each component also have prominent effects [85], therefore between individual fiber/matrix composites there is an exclusive interface region. During the fabrication, storage, and in-service period these materials are exposed to numerous environmental conditions and loading ranging from quasistatic to dynamic loading [86]. The environmental conditions can be temperatures [87] (low and subzero environment [88,89], high [90], thermal spike, thermal shock [91], freeze
thaw and cyclic variation of temperatures), humidity [92], seawater exposure, UV light exposure, alkaline environment, thermal cycling [93], also different types of loadings [94
97], and often a combination of different types of loadings and aforesaid environments [70,74,76,87,98]. These environments lead to vulnerable damage and degradation of PMCs. This section of the chapter includes some reports in the developments of civil infrastructure using FRP composites in different harsh and hostile environments. The material responses of FRP-based composites subjected to several environmental effects have been widely reported. However, reviews of their performance at structural levels such as for joints and connections are still incomplete and initiatives on applications and practices of FRP composites in civil constructions are not widely found in the literature. As FRP composites are comparatively less commonly used materials for structural construction in civil engineering, it becomes essential and necessary to review the results on the mechanical performance of their joints and connections and to introduce developments of new structures made from such materials in actual fields of applications, especially under aggressive environments. This section focuses on research into the mechanical performance of FRP composites at a structural level, mainly for joints, adhesion, and connections exposed to different environmental effects.

6.4.1 Temperature In most of the structural applications, FRPs impinge on high-energy and highvelocity dynamic loadings that may result in multiaxial dynamic states of stress. The response of these composite materials is very short at these kinds of loadings, which are generally highly transient. These materials possess poor resistance to fire, and are susceptible to high temperatures. The damage and degradation at the interface can arise due to differential thermal expansion of fiber and matrix at high temperature [99], and thereby a reduction in the overall strength of the composite. At higher temperatures, the formation of microcracks was observed due to the differential coefficient of thermal expansion of fiber and matrix [100] at the fiber/matrix

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interface. The fiber/matrix interface also becomes sensitive to destructive reactions in a high-temperature environment, which may lead to damage and degradation of both the fiber phase as well as the matrix phase in the composite [101]. Nowadays, in aerospace-related applications, fuel tanks made from conventional materials are being substituted by advanced PMC materials. The liquid hydrogen fuel used in spacecraft is at LN2 temperatures. The composite material becomes brittle when exposed to LN2 temperatures [102]. Shindo et al. studied the LN2 compressive properties of glass/epoxy laminates at RT, LN2 temperature (
196 C) [103]. Shindo et al. [104] investigated the tensile behavior of plain weave glass/ epoxy composites at LN2 conditions. They reported that the tensile properties are not affected by various gauge lengths. Hei-lam Ma et al. [105] evaluated the impact performance of glass/epoxy composites in an LN2 environment. Kim et al. [106] investigated the tensile response of graphite/epoxy composites at low temperatures. They reported that tensile stiffness significantly increases as the temperature decreases. They observed various modes of failure due to impact damage like fiber breakage and matrix cracking. At LN2 temperatures, the material becomes stiffer. In polymeric composite materials, the effect of crosshead speed has shown an enhancement of the mechanical properties when exposed to low temperatures [94,107].

6.4.1.1 Low and cryogenic temperatures Shindo et al. [108] performed cryogenic fatigue behavior of plain weave glass/ epoxy composite laminates under tension
tension cycling loading. They conducted the tests at 4 and 10 Hz frequencies, with a stress ratio of 0.1. The tests were carried out at room temperature, 77K, and 4K. The S
N curves showed the usual trend that as the maximum applied stress σmax increases, the fatigue life decreases. The 106 cycle fatigue limits at cryogenic temperatures were considerably lower than the knee stresses reported from monotonic tensile test data. Flat fracture surface like failure modes were obtained at room temperature. At cryogenic temperatures, however, delaminations appeared in the tested samples. The damage growth in the fatigue samples was mainly in the form of matrix microcracks on the sample surface and matrix failure in the fiber bundles oriented transverse to the load direction. Mahato et al. [109] studied the effect of liquid nitrogen (LN2) conditioning (for different intervals of time) on the loading rate sensitivity of the tensile response of GFRP composites. In order to assess this, tensile tests of the unconditioned and conditioned specimens were carried out at different crosshead speeds, namely, 1, 10, 100, 500, and 1000 mm/min. At 1 mm/min crosshead speed, an improvement of 3.33% and 7.3% ultimate tensile strength (UTS) value was observed in the case of 0.25 and 1 hour conditioned GFRP composites, respectively, as compared to unconditioned GFRP composites. Similarly, the specimens tested at 1000 mm/min show an improvement of 11.39% and 12.02% UTS for 0.25 and 1 hour LN2 conditioned GFRP composites, respectively, as compared to unconditioned GFRP composites. The effect of LN2 conditioning on crosshead speed sensitivity of modulus and strain at break was also reported.

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Fig. 6.11A and B represent the comparisons between the tensile stress
strain behaviors of unconditioned GFRP and LN2 conditioned (0.25, 1, 4, and 8 hours) GFRP composites tested at 1 and 1000 mm/min crosshead speeds, respectively. It is clearly noticeable from Fig. 6.11A that at 1 mm/min crosshead speed, UTS increases with LN2 conditioning time up to 1 hour as compared to unconditioned GFRP composites. Improvements in the UTS value of 3.33% and 7.3% were observed in the case of 0.25 and 1 hour LN2 conditioned GFRP composites, respectively. Sreenivasa and Joshi [110] also reported an improvement of 3% in tensile strength of LN2 tested GFRP composites as compared to unconditioned composite specimens. A similar trend was also observed in the case of GFRP composites tested at 1000 mm/min crosshead speed. The specimen tested at 1000 mm/min crosshead speed showed improvements in UTS values of 11.39% and 12.02% for 0.25 and 1 hour LN2 conditioned GFRP composites, respectively. This improvement in strength was attributed to the hardening of the polymer matrix due to almost complete absence of disentanglement [111]. Kumar et al. [112] reported the mechanical behavior of glass/epoxy composites at cryogenic temperature. Woven and chopped E-glass fibers of 50 weight percentages were reinforced with epoxy matrix to prepare the FRP laminated composites. Three-point bend tests were carried out to assess interlaminar fracture behavior at cryogenic and ambient conditions. The samples were tested at a range of 2
500 mm/min crosshead speed to evaluate the sensitivity of mechanical response during loading at these conditions. The mechanical performances of the laminated specimens at cryogenic conditions were compared with room temperature properties using SEM photographs. Differential scanning calorimetry (DSC) was carried out to study whether there was any change in the glass transition temperature. Glass/ epoxy composites were found to be loading rate sensitive. DSC analysis showed an increase in the glass transition temperature after cryogenic conditioning, which might be due to irreversibility of the chain mobility. Phenomenological behavior of these FRP composite materials may be attributed by polymer relaxation at low temperature, cryogenic hardening, matrix cracking, and misfit strain due to the differential thermal coefficient of the fiber and the matrix, and also by enhanced mechanical keying factor by compressive residual stresses generated at cryogenic temperatures.

Figure 6.11 Tensile stress versus tensile strain curve at RT (30 C), 0.25, 1, 4, and 8 h liquid nitrogen conditioning tested at (A) 1 mm/min and (B) 1000 mm/min crosshead speeds.

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Ma et al. [105] investigated the impact properties of GE composites in the cryogenic environment. They reported that GFRP samples were fabricated by vacuum infusion method and some were post-cured at 353K for 3 hours to confirm that a complete chemical reaction inside the samples was achieved. A low-velocity drop weight test was executed for the samples prepared and then stored at room temperature (295K), dry ice temperature (199K), and liquid nitrogen temperature (100K) environments. The apparent damage and the extent were visually examined and measured. Impact parameters such as impact load, deflection, and energy absorption of each damage type were also analyzed. Experimental results showed that GFRP composites at cryogenic conditions exhibited less apparent damage and were stiffer as compared with other cases. However, they demonstrated relatively poor energy absorbability in low-temperature conditions. Fig. 6.12 illustrates the depth of damage at both the testing temperature and curing conditions and shows the relationship between temperature and depth of damage of naturally cured and post-cured samples. It was observed that, in both cases, the depth of damage decreases with temperature. Also, it is observed that postcured samples exhibit better energy absorbability than naturally cured samples.

6.4.1.2 Elevated temperatures

Damage depth (mm)

Structural FRP-based components can be shaped in standard contours similar to those used for steel members such as I, L, box, channel, and tube sections for the construction sector. Experimental studies have been carried out of pultruded FRP materials cut from structural components in a few investigations, where the failure modes signifying load capacities for different loading situations (e.g., compression, shear, and tension) were found at various temperature levels [113
115]. This section stresses the influences of FRP composites at the structural level. Mechanical bolting, a usual approach for steel structural construction, is often used to connect FRP structures and members. Temperature effects on bolted FRP joints are still limited in comparison to those on FRP materials. Turvey et al. [116] studied two types of single-bolt FRP joints for their tensile behavior at 20 C, 60 C, and 80 C. The 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

With post-curing With post-curing 0

100

200 300 Temperature (K)

Figure 6.12 Depth of damage versus temperature.

400

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tests were planned to accomplish a bearing failure (type I) or a tension failure (type II) at room temperature, based on two different arrangements of the width (W) to bolt diameter (D) ratio and end-distance (E) to bolt diameter (D) ratio. All type I joints exhibited a steady bearing failure mode that occurred at room temperature. A decrease of 38.7% and 51% in the load-carrying capacity was found at 60 C and 80 C, respectively. In case of type II joints, the reduction in load-carrying capacity was identified with 48.5% and 56.4% decreases at 60 C and 80 C, respectively. This may be attributed to the transition of failure modes from tension to bearing in type II joints started at 60 C. Recently, Wu et al. [75] examined the pultruded FRP bolted joints exposed to tensile loading at different temperatures. Constant shear-out failure mode was recognized for the joints observed at different temperatures from room temperature to 220 C. The decrease in load-carrying capacity compared to that at room temperature were observed as 14%, 38%, 64%, 78%, and 85% at 60 C, 100 C, 140 C, 180 C, and 220 C, respectively. Up to 60 C, a decrease in load-carrying capacity was observed for a new type of blind bolted joint, while similar reductions were reported after 60 C. Anwar [117] studied the short- and long-term responses of an E-glass/polyester pultruded FRP material subjected to single pinbearing loads. The study comprised experiments on single-bolt tension connections under a double-lap shear configuration at normal and elevated service temperatures. Elevated temperature tests were conducted at 43.3 C and 60 C. About a 44% decrease in pin-bearing strength was observed when the temperature was increased to 60 C. Turvey and Wang [118] studied the multibolt FRP joints with a series of geometric configurations in terms of end distance (E), pitch distance (P), side distance (S), and bolt diameter (D) at room temperature and 60 C. The geometric structures were found to significantly affect the decrease in joint strength at the elevated temperature. The maximum degradation of the ultimate joint strength was found to be 36%, corresponding to a state of E/D of 4, P/D of 2, and S/D of 4. No sign of degradation was recognized in the joints connected with the geometry of E/D of 2, P/D of 4, and S/D of 2. Finally, an average decrease in ultimate joint strength was found as 17% for 12 groups of multibolt FRP joints with different geometric profiles. A more substantial decrease was observed in the damage loads of multibolt FRP joints when the temperature was increased from room temperature to 60 C. The decomposition and glass transition temperature of the composite material were examined through thermogravimetric and dynamic mechanical analysis. Like bolted connections, adhesive bonding for FRP structural components has produced solid attention as a joining method. Zhang et al. [119] investigated adhesively bonded double-lap joints (with a bond length of 50 mm) in tension, composed of pultruded FRP adherents and an epoxy adhesive at temperatures ranging from 235 C to 60 C. Almost no difference was observed for the joint ultimate loads at 235 C and room temperature. However, a clear decrease in joint ultimate load was found when the temperature was increased to 45 C, and a maximum decrease of 32% was recorded at 60 C in comparison to that at room temperature.

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The failure mechanism was also found to change from fiber split to adhesive failure when the temperature was increased, especially at temperatures above 50 C. The bond length affected the temperature dependence of the load-carrying capacity of adhesively bonded joints. The stiffness decrease at temperatures above 40 C was higher than the drop in ultimate load, illustrating that the elastic properties were more sensitive than the strength within this temperature range, as revealed in Fig. 6.13 (values at 23 C were used for normalization). The decrease in stiffness value leads to a significant increase in elongation at failure with increasing temperature. All the parameters changed significantly in the range between 40 C and 50 C, that is, about the Tg of the adhesive. Mahato et al. [74] studied the high-temperature tensile behavior at different crosshead speeds during loading of GFRP composites. They investigated the tensile behavior of GFRP composites at 50% and 70% volume fractions of reinforcement tested at room (25 C), 70 C, 90 C, and 110 C temperatures with 1, 10, 100, 500, and 1000 mm/min crosshead speeds to investigate the impact of high temperature on the mechanical properties and different dominating failure modes. The experimental results revealed that with an increase in crosshead speeds the tensile strength of the composites is increased. The effect of crosshead speeds and temperature with changing fiber volume fractions affected the GFRP composite. Crosshead speed sensitivity seems to be more unpredictable at high temperatures and at high speed. Furthermore, when the GFRP composite with higher fiber volume fraction resulted in unprecedented nature of fluctuation in the tensile stress-strain curve with various

4 Ultimate load Elongation to failure Specimen stiffness

Normalized values

3.5 3 2.5 2 1.5 1 0.5 0

-40

-20

0

20

40

60

Temperature (°C)

Figure 6.13 Normalized ultimate load, elongation, and specimen stiffness versus temperature.

80

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crosshead speeds. In case of 50% fiber volume fraction composites, the tensile strength decreased with an increase in temperature, as expected. However, for the composites with 70% fiber volume fraction the tensile strength tends to increase as the temperature increases. The possible reasons may be due to higher volume fraction of fibers, which generates more interfaces that ensure a high degree of decohesion between the fibers. Thus fiber breakage is likely to be the dominating failure behavior. Fig. 6.14 represents the effect of temperature on GFRP composites with 50% and 70% fractions of fibers at a crosshead speed of 1 mm/min. It is evident from Fig. 6.14A that with an increase in temperature the strength of the composite decreases. This may be due to the differential coefficient of thermal expansion between the fiber and the matrix. The composite containing a fiber volume fraction of 70% showed an increment in the tensile strength as the temperature increased (Fig. 6.14B). This is attributed to the low matrix content o in the composite and it is known that during longitudinal loading the strength mainly depends upon the fiber content and orientation. Li et al. [120] studied the fatigue behavior of GFRP bars exposed to elevated temperatures. The GFRP bars were tested with different cyclic loading times after exposing them to an elevated temperature. The results showed that the tensile strength and elastic modulus of GFRP bars decreased with the increase in temperature and fatigue cycling time, and the tensile strength of GFRP bars decreased by 19.5% when the temperature reached 250 C. Within the test temperature range, the tensile strength of GFRP bars decreased at most by 28%. The cyclic loading accelerated the degradation of GFRP bars after elevated temperature exposure. The coupling of elevated temperature and cycling enhanced the degradation effect of the cyclic loading on GFRP bars. The tensile strength of GFRP bars after elevated temperature exposure at 350 C under cyclic loading was reduced by 50.5% compared with that at room temperature, and by 36.3% compared with that after exposure at 350 C without cyclic loading. In addition, the elastic modulus of GFRP bars after elevated temperature exposure at 350 C under cyclic loading is reduced by 17.6% compared with that at room temperature and by 6.0% compared with that after exposure at 350 C without cyclic loading.

Figure 6.14 Stress versus strain curves at room temperature (RT), 70 C, 90 C, and 110 C at 1 mm/min loading rate; (A) reinforcement content (VF) 5 50%, (B) reinforcement content (VF) 5 70%.

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6.4.2 Humid environments 6.4.2.1 Hydrothermal response It has been reported in the literature that the existence of water to some extent decreases the elastic modulus and strength of polymer matrices or structural adhesives, due to the moisture ingression, which can lead to plasticization or chemical and physical breakdown of the interfacial adhesion forces within the molecular structure [121]. Since FRP composites are made up of a combination of polymer matrices, and fibers may show degradation of mechanical properties, such degradation has been experimentally studied for the last three decades. Liao et al. [122] investigated the pultruded glass
fiber-reinforced vinyl ester matrix composite exposed to environmental aging in order to explore their durability since such composites are of interest for infrastructural applications. The pultruded FRP composites were exposed to deionized water or salt (NaCl) solutions at different concentrations at either room temperature or 75 C for different durations. Both strength and modulus values were normally found to decrease with environmental aging. The flexural behavior (i.e., strength and modulus) were calculated for the as-received as well as aged samples. The fluids for aging were water, and 5% and 10% NaCl solutions. The aging environments were as follows: 1. 2. 3. 4.

Deionized water for up to 3900 hours at room temperature (25 C); 5% NaCl solution for up to 3980 hours at room temperature; 10% NaCl solution for up to 6570 hours at room temperature; and Deionized water for 2400 hours at 75 C.

Figs. 6.15 and 6.16 show the results from the flexural tests for 0 degree samples which were compared to the as-received samples, the flexural modulus of 0 degree samples aged in conditions (A
C) remained similar, as shown in Fig. 6.15. The effects of salt concentration on flexural modulus could not be differentiated for up to 6570 hours. The aging time for samples in condition (C) (10% salt solution) was 65% longer than those in condition (B) (5% salt solution). Since condition (C) might be considered the most severe, this indicated that significant modulus drop happened in this case, which was not expected for samples in conditions (A) and (B), for at least up to 6570 hours. No significant modulus drop was seen for 0 degree samples aged in condition (D) (75 C water for 2400 hours) either. Fig. 6.16 reveals the flexural strength after aging of the composites, which exhibited some damage and degradation. The mean flexural strength for the 0 degree samples aged in conditions (A), (B), and (C) indicated 4.8%, 12%, and 13% reductions in strength, respectively. It was evident that aging in salt solutions caused a significant decrease in the flexural strength value of the composites. In addition, the failure strains also showed a decreasing trend for the aged samples. The average failure strain for as-received and those aged in conditions (A), (B), and (C) was 2.14%, 1.90%, 1.97%, and 1.94%, respectively. Furthermore, a substantial decrease in flexural strength value was observed in the samples aged in condition (D), where the average reduction in the strength was 40%, as shown in Fig. 6.15. The failure strain was 1.34% as compared with as-received samples.

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20

Flexural modulus (GPa)

Water

5% NaCl 10% NaCl 75 °C water

20

10

5

0 As-received 3940 h in room temp water 4980 h in room temp 5% NaCl 6570 h in room temp 10% NaCl 2400 h in 75 °C water

Figure 6.15 Flexural modulus for 0 degree specimens of pultruded composite coupons before and after environmental aging.

Flexural strength (MPa)

400 350 300 250

Air Water

5% NaCl 10% NaCl 75 °C water

200 150 100 50 0

Figure 6.16 Flexural strength for the specimens of pultruded composite coupons before and after environmental aging.

Chu et al. [123] studied several material properties such as tensile strength, short beam shear strength, and glass transition temperature of pultruded E-glass vinyl ester composites exposed to deionized water saturation at various temperatures of

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23 C, 40 C, 60 C, and 80 C for up to 75 weeks. Tests were conducted after different time intervals of immersion and subsequently after a period of drying out to evaluate the regaining of performance owing to a decrease in the absorbed moisture content. It was observed that, although the initial effect of immersion was only matrix plasticization, higher periods of immersion and/or use of high temperatures gave rise to hydrolysis, microcracking, interfacial debonding, and even degradation of the fiber. These mechanisms of irreversible degradation substantially reduced the performance, but further, these decreases were increasingly irreversible. Investigations were also explored by Nkurunziza et al. [124] on pultruded glass
fiber-reinforced vinyl ester rebar under a combination of tensile loading and water immersion. The bars were exposed to two stages of constant tensile stress at 25% and 38% of tensile strength, while being surrounded by either alkaline solution (pH 12.8) or deionized water (pH 7.0). The results obtained through experiments exhibited that the performance of the GFRP bar under these extreme loading and environmental conditions was found to be better. The average residual tensile strength was reported to be 139% and 144% of the design tensile strength for bars exposed to deionized water at 25% and 38% stress level, respectively. In alkaline medium, this range was calculated as 126% and 97%. More importantly, no substantial alteration in the elastic modulus was reported under the stress levels and the different environmental parameters. Riebel et al. [125] studied the PGFRP composites under compression with exposure of 18 months to an alkaline solution (pH 13.4) at different temperatures (20 C, 40 C, and 60 C). A durability investigation was implemented on a PGFRP compression component of a hybrid GFRP/steel joint for concrete structures. The GFRP component was immersed in alkaline water solutions of different temperatures for a period of 18 months. Initially, the rate of moisture uptake was very fast, mostly through a wicking effect along the matrix cracks and fiber/matrix interfaces. This eventually led to a reduction of the matrix stiffness property attributed to swelling and causing a significant decrease in the compression strength. In the second stage, the rate of degradation was found to be slower in the chemical glass and matrix. Due to the less harsh environment in practice, the strength and stiffness decrease was found to be acceptable for in the structural field of applications. Jesthi and Nayak [126] studied the improvement of mechanical properties of hybrid composites through interply rearrangement of glass and carbon woven fabrics for marine applications. In this study, five varieties of composites were used, that is, plain glass ([G]s), plain carbon ([C]s), [G3C2]S, [G2C2G]S, and [GCG2C]S types of hybrid composites. The composites were aged for 90 days in seawater. The results showed that the tensile strength of seawater aged [GCG2C]S type hybrid composite was enhanced by 14% as compared to plain GFRP composite. The flexural strength and modulus of seawater aged [GCG2C]S type hybrid composite were increased by 43% and 64%, respectively, in comparison to plain GFRP composite. However, it was observed that the impact strength of seawater aged [GCG2C]S type hybrid composites was decreased by 44.5% as compared to plain GFRP composite. Fig. 6.17A and B illustrates the tensile strength versus strain of the composites in dry and seawater aged conditions, respectively. The results exhibited that in the

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Figure 6.17 Tensile stress versus strain curve of (A) dry and (B) seawater-aged composites.

dry condition, the hybrid composite [G3C2]S has the maximum tensile strength, that is, 329 MPa followed by [GCG2C]S type hybrid composite, that is 313 MPa. It was seen that there was no substantial improvement in the tensile strength with varying of the stacking sequence. Song [127] reported that the central carbon layers improve the tensile strength of hybrid composites. The marginal improvement in the tensile strength of the hybrid composites has good agreement with the earlier findings [128]. The flexural stress and modulus of dry and seawater-aged composites are shown in Fig. 6.18A and B, respectively. In dry conditions, the results revealed that the flexural strength decreased with the increase in glass layers in the outer region of the hybrid composites. Dong [129] observed that the flexural strength changed with the stacking sequence of carbon/glass fibers in the hybrid composites. Among the hybrid composites, [GCG2C]S type had the highest flexural strength and modulus, that is, 462 MPa and 27.8 GPa, respectively, which was increased by 46.1% and 49.5%, respectively, as compared with plain GFRPC. The hybrid composite of [GCG2C]S type contains alternate glass and carbon fiber layers. Since the glass fiber layers can withstand higher strain, it helps to bridge low strain capacity carbon fiber layers, which helped to avoid the brittle failure of the hybrid composite. This strengthening mechanism of the hybrid composite enhances the flexural strength and modulus. Zhang et al. [130] reported similar findings. However, the flexural strength of seawater-aged composites was reduced due to degradation of the polymer matrix [131]. The flexural strength of seawater-aged hybrid composites of [G3C2]S, [G2C2G]S, and [GCG2C]S types was reduced by 12.6%, 11.2%, and 10.1%, respectively. The flexural modulus of seawater-aged hybrid [GCG2C]S composites was reduced by 7.6%. In both dry and seawater-aged conditions, the plain carbon and glass composite have the highest and lowest flexural strengths, respectively. Turvey and Wang [116] conducted an experimental investigation into pultruded FRP single-bolt tension joints after water immersion at various temperatures, that is, room temperature, 60 C, and 80 C for a maximum of 91 days. After water immersion at room temperature for 13 weeks, the joints (type I) which had been designed to achieve a bearing failure mode at room temperature exhibited almost no loss of load-bearing capacity, whereas a loss of about 10% was recorded in the

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Dry condition Seawater aged

(A)

400 300 200 100 0 [G]10

Flexural modulus (GPa)

Flexural stress (MPa)

40 500

35

Dry condition Seawater aged

(B)

30 25 20 15 10 0

[G3C2]S [G2C2G]S[GCG2C]S [C]10

Composite

[G]10

[G3C2]S [G2C2G]S[GCG2C]S [C]10

Composite

Figure 6.18 Flexural stress (A) and modulus (B) versus composite type in dry and seawateraged conditions.

joints (type II) which had been designed with a tension failure mode at room temperature. A more substantial decrease in load-bearing capacity was observed in the joints after water immersion at higher temperatures. It was found that the residual load-bearing capacity of type I joints was only 22.5% and the residual load-bearing capacity of type II joints was only 30.6% of that at 80 C before water immersion. It was found that the joint strength also decreased significantly at elevated temperatures, that is, 60 C and 80 C prior to any water immersion. Turvey and Wang [118] designed pultruded FRP joints with two bolts in one column with a series of geometric parameters. Such joints were immersed in water for about 45 days at two different temperatures (room temperature and 60 C) and the residual load-bearing capacity was recorded. All the joints with different geometric parameters showed greater or lower reductions in load-bearing capacity after environmental conditioning. The most significant loss of 28% after water immersion at room temperature corresponded to the joints with E/D of 2, S/D of 2, and P/D of 2. A more severe loss of load-bearing capacity was found in the joints immersed to 60 C in water, with a maximum reduction of 56% for the scenario of E/D of 2, S/D of 4, and P/D of 4.

6.4.2.2 Hygrothermal behavior Zhang et al. [132] examined the fatigue behavior of adhesively bonded PGFRP double-lap joints under various environmental conditions. Tests were executed at 235 C, 23 C, and 40 C. The fourth set of fatigue data was collected from tests on preconditioned specimens in warm (40 C) water. The tests were conducted at 40 C and at 90% relative humidity (RH). In addition to the S
N curves, stiffness fluctuations, and crack initiation and propagation during fatigue were monitored. In adhesively bonded pultruded FRP joints, samples with a bond length of 50 mm were all preconditioned for 70 days in 40 C water; subsequently, they were exposed to fatigue loading (tensile stress ratio of 0.1 and 4 stress levels of about 44%, 53%, 63%, and 76% of the ultimate joint strength) simultaneously under various

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environmental parameters [132]. The various conditionings were (1) 235 C without humidity control, (2) room temperature (23 C) and 50% RH, (3) 40 C and 50% RH, and (4) 40 C and 90% RH. The joints loaded at a stress level of about 44% of the ultimate joint strength attained fatigue lives ranging from 484,288 to 1,909,376 cycles under 40 C and 50% RH, and ranging from 553,739 to 1,313,940 cycles under 40 C and 90% RH. At the same stress level, the fatigue life of joints examined under room temperature and 50% RH ranged from 1,030,600 to 2,215,704 cycles. At higher stress level, shorter fatigue life was recorded; this fact was not evident in the existence of an elevated temperature of 40 C and/or the presence of a higher humidity level (90% RH). The dominant failure mode was identified as fiber-tear failure that occurred in the mat layers of the GFRP laminates. In high humidity, the failure occurred in the adhesive/composite interface. Although the testing temperature was lower than the glass transition temperature of the adhesive, its effect on the fatigue life and fracture behavior of the observed joints was apparent and was intensified by the existence of humidity. The overall conclusion was that at higher temperatures and/or humidity levels the fatigue life of the composites is shortened. Xin et al. [133] investigated the moisture diffusion characteristic and hygrothermal aging properties in PGFRP composite laminates of bridge profiles exposed to water and artificial seawater at temperatures of 40 C, 60 C, and 80 C, respectively. They reported that with an increase in temperatures of 40 C
80 C, the different sets of composite specimens experienced a damage-dominated diffusion process after exposure to several hours of conditioning. In the initial stages moisture uptake was enhanced rapidly in the composites, with a maximum of about 25% of the initial weight. Chaichanawong et al. [134] studied the effect of moisture on the mechanical properties of GFRP composites. The moisture absorption of the GFRP composites was subjected to four phases: (1) the starting phase (1
7 days), (2) the second phase (8
24 days), (3) the third phase (25
35 days), and (4) the saturated phase (36
60 days). Ultimate tensile strength, yield strength, and flexural strength of all samples were significantly reduced at the starting phase of moisture absorption and then these mechanical properties were observed to be constant. The modulus of elasticity of all samples reduced at the starting phase and saturated phase. The ductility (% elongation) of all samples showed no significant change at the starting stage; however, it was considerably reduced in the third phase, and then became almost constant. Fig. 6.18 illustrates the relationship between the moisture absorption content and immersed time of glass fiber/polyamide composites. It is evident from Fig. 6.19 that the moisture absorption contents decreased when increasing the weight percentage of the glass fiber in the glass fiber/polyamide composites. This showed that the moisture is mostly adsorbed into polyamide due to the almost no moisture-absorption property of glass fiber. The absorbed moisture content of all samples increased with increasing immersion time and reached saturation at around 50 days after immersion. Fig. 6.20 indicates the effect of moisture on the yield strength and ultimate tensile strength. Yield strength and ultimate tensile strength of all samples

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8 I

Moisture content (wt.%)

7

II

III

IV

6 5 4 0 wt.% 10 wt.% 20 wt.% 30 wt.%

3 2 1 0

0

5

10

15

20 25 30 35 40 Immersion time (day)

45

50

55

60

Figure 6.19 Relationship between the moisture absorption content on the glass fiber/ polyamide composites and the immersion time.

Ultimate tensile strength (Mpa)

140 I

120

II

III

15

20 25 30 35 40 Immersion time (day)

IV

0 wt.% 10 wt.% 20 wt.% 30 wt.%

100 80 60 40 20 0

0

5

10

45

50

55

60

Figure 6.20 Effect of moisture content on the ultimate tensile strength.

considerably decreased at the starting phase of moisture absorption (phase I: days 1
7) and slightly decreased at phase II (days 8
24). The mechanical properties were almost constant after immersion for 24 days (phase III: days 25
35 and IV: days 36
60). The modulus of elasticity of all specimens decreased at the starting phase (phase I: days 1
7) and slightly decreased at phase II (days 8
24). It significantly decreased further at phase III (days 25
35). This was attributed to the lowering of the glass fiber-matrix (polyamide) interfacial shear strength after immersion in water [135]. The effect of the combination of water immersion and continuous bending on the degradation response of E-glass/vinylester composites was considered through mechanical testing (tension and short-beam-shear) and moisture uptake [136]. It was observed that moisture uptake and the coefficient of diffusion increased with levels of constant strain and the uptake characteristics could be predicted with a

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reasonable level of accuracy using a model incorporating the free volume approach. Both short-beam-shear strength and tensile strength decreased with increasing duration of immersion and level of constant strain. It was highlighted that the synergistic effects of constant bending and water immersion were accelerative and that further research was needed to ascertain the safe level of constant loading for use in civil infrastructure.

6.4.3 UV irradiation Despite the widespread study of the environmental durability of FRP composite systems, investigations into the effects of UV irradiation on the bond between steel, fiber, and polymer are still limited. Although the Earth’s atmosphere screens out most of the total solar radiation, 6% of the total solar radiant flux is UV radiation that is able to reach the Earth’s surface [137]. UV light is thought to have damaging effects on most polymers as it is related to wavelengths in the range of 290
400 nm and thus can detach the molecular bonds in most polymers. The degradation may range from simple surface discoloration to extensive loss of mechanical properties [69]. Degradation in the case of polymers typically originates from the outer surface and infiltrates progressively into the bulk of the material [137,138]. Therefore UV radiation aging is one of the critical environmental parameters for polymer-based materials, especially FRP composites. Karbhari et al. [138] studied UV effects on different types of FRP composites, mainly at the material level. Further experimental investigations were reported by Correia et al. [140] for UV effects on pultruded FRP composites (i.e., E-glass fiber as reinforcement and polyester as a matrix), including two conditions of UV exposure. In the first condition, a UV-accelerated weathering instrument was used to provide repeated cycles of light (with an irradiance of 0.77 W/m2/nm at 340 nm) and moisture up to 6346 hours (about 265 days). In the second conditioning, a xenon-arc light accelerated weathering instrument was used to supply constant irradiation of 0.5 W/m2/nm at 340 nm, and cycles of water spray [18 minutes, dry intervals (102 minutes)], up to 2000 h (about 84 days). No significant reduction in flexural strength was identified from the first condition. A small reduction in the average tensile strength was detected for the samples after condition of 2000 h (about 84 days) in both conditions. It was determined that UV generally caused shine loss and a change in color (surface yellowing), rather than a destruction of the mechanical properties. It seems that UV effects alone are limited to a thickness near the surface. Investigations into UV effects on the mechanical performance of FRP composites and structures are thus of less concern. Nguyen et al. [141] explored the effect of UV irradiation of 1.26 W/m2/nm at 340 nm for 371 hours (about 16 days) on the steel/ CFRP composites double-strap joints, on both sides of it. Significant enhancement of joint stiffness was detected and this was attributed to post-curing effects of the heat associated with the UV exposure. However, the joint strength after UV exposure exhibited a significant decrease in comparison to the initial strength, with the maximum value of 50% corresponding to the shortest bond length of 30 mm. Neither a conditioning

Emerging advancement of fiber-reinforced polymer composites in structural applications

CF Temperature alone CFRP1 Temperature alone CFRP3 Temperature alone

(A)

1.2

CF UV CFRP1 UV CFRP3 UV

Normalized stiffness degradation

Normalized strength degradation

1.2

1

0.8

0.6

0.4

0.2

CF Temperature alone CFRP1 Temperature alone CFRP3 Temperature alone

(B)

257

CF UV CFRP1 UV CFRP3 UV

1

0.8

0.6

0.4

0.2

0

0

0

124 248 372 Exposure time (h)

596

744

0

124

248 372 596 Exposure time (h)

744

Figure 6.21 (A) Strength and (B) stiffness retention of CF and CFRP specimens exposed to scenarios 1 and 2 after 0, 124 3 2, 248 3 2, and 372 3 2 h.

temperature of 40 C nor UV conditioning affected the elastic modulus or tensile strength of CF and CFRP samples, even with deviations in the thickness and number of carbon fiber layers. The holding of tensile strength and stiffness were normalized, as illustrated in Fig. 6.21. Although there were some differences in sample strength and stiffness, the differences were due to data scatter rather than the effect of UV and/or heating effect after 372 3 2 hours UV exposure. Unidirectional CFRP composites contain carbon fibers and epoxy resin. Although the resin might be damaged, the tensile strength and stiffness in the fiber direction were mainly directed by the strength and stiffness of the fibers, which were not affected by the environments identified in this investigation.

6.4.4 Thermal shock Temperature variations are among the most important environmental factors that may affect the durability of FRP composites for civil engineering applications [32,62,73,74]. The problems with thermal cycles are generated from the thermal deformations of the adherends and polymers, and the potential dissimilarity between the coefficients of thermal expansion (CTE) of the adhesive, polymers, adherends, and fibers, which may lead to the generation of interfacial residual stresses at the joining areas and eventually lead to microcracks at the interfaces or even premature debonding failure [142
144]. Sousa et al. [145] performed experimental and numerical investigations into the effects of thermal cycles on the mechanical response of single-lap bonded joints between pultruded glass-unsaturated polyester laminates. GFRP adherends were bonded with two commercial adhesives, epoxy (EP) and polyurethane (PUR), and were conditioned to a range of temperature variations of 25 C to 40 C for about 350 cycles in a dry state. Single-lap joint tests were then executed at predetermined

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times to assess the effect of thermal cycles in overall joint performance, in terms of load versus elongation response, bond strength, joint stiffness, and failure modes. Exposure to thermal cycles caused a considerable reduction in joints for adhesives in stiffness and strength value. The maximum decreases in stiffness and strength of 18% and 22%, respectively, for EP-GFRP joints and 19% and 11% for PUR-GFRP joints were observed. The degradation of performance was subjected by post-curing effects, and was more significant in the PUR adhesive. Before conditioning to thermal cycles, both varieties of samples showed similar failure mechanisms, which generally (80%
90% of cases) involved light fiber tears. Conditioning to thermal cycles did not affect the failure modes of the PUR-GFRP joints; however, EP-GFRP joints became further susceptible to adhesive failure after being exposed to thermal cycles. In general, the thermal cycling damages bonded joints between PGFRP adherends. The degradation was mainly due to detrimental effects on the constituent materials, that is, adhesives; however, the environments used in the exploration, seemed to be well-suited with the structural use of this type of joints in civil infrastructure. Mahato et al. [98] investigated the effect of short-term exposure of thermalshock conditioning on the mechanical properties of glass/epoxy (GE) composites. The specimens were conditioned at
60 C for 36 hours followed by further conditioning at 170 C for the same duration. In order to assess the effect of thermal shock on the mechanical properties, tensile tests of the conditioned and unconditioned specimens were done with various loading rates of 1, 10, 100, 500, and 1000 mm/min. The tensile strength and “strain to failure” were found to increase with an increase in the loading rates at room temperature; the thermal shockconditioned specimens exhibited even higher UTS and failure strain compared with the unconditioned specimens. It can be stated that different coefficients of thermal expansion during thermal-shock conditioning and a significant amount of preexisting residual stresses governed the stress distribution and ultimately the mechanical properties of glass/epoxy composite. Various dominating modes of failure in the composites were analyzed under scanning electron microscopy. Fig. 6.22 shows the stress
strain behavior of both control as well as thermal shock-conditioned GE composites. It can be seen from Fig. 6.22A that with an increase in the loading rate, that is, from 1 to 1000 mm/min, the maximum tensile stress value increased. At lower loading rates more time was available and thus small microcracks propagated to potential cracks which led to a substantial reduction in the tensile stress of the composite system. However, at the higher loading rates, the time availability to propagate these microcracks was much reduced. Hence, the load-carrying capacity of the GE composite was found to be greater. The failed glass/epoxy composites at different loading rates (1) without conditioning and (2) thermal shock-conditioned specimens are shown in Fig. 6.22. Fig. 6.22B reveals the stress
strain curve of TCS with various loading rates. Fig. 6.22B shows the same trend as that of the control GE composite. Compared with the RT specimens, the TCS exhibited a greater tensile stress value at each loading rate. This may be attributable to the difference in the coefficient of thermal expansion/contraction at the fiber/matrix interface, which altered the nature of

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Figure 6.22 A tensile stress versus tensile strain curve with 1, 10, 100, 500, and 1000 mm/ min loading rates tested at (A) room temperature (30 C) and (B) a thermal-shock conditioned specimen (TCS).

residual stresses in the composite. The fiber/matrix interfacial adhesion was firmly altered by the existence of residual stresses. Conditioning to low temperature caused differential shrinkage at the fiber/matrix interface, leading to improved interfacial adhesion, which ultimately enhanced the debonding resistance by a mechanical keying factor [91]. The evolution of the misfit strain emerged at the fiber/matrix interface due to variance in the coefficient of thermal expansion/contraction [89]. The TCS exhibited better fiber/matrix interfacial bonding by a mechanical keying factor imparting better strengthening of the GE composites. In the case of TCS, the load-carrying capacity of the composite was enhanced with an increase in the loading rate. This was attributed to uniform stress distribution in polymer matrix at lower loading rates. However, as the loading rate increased, the transfer of load from the polymer matrix to the fiber reinforcement was not smooth and steady, leading to higher strength of the composite. Fig. 6.23A
C show SEM micrographs of the broken specimens tested at a 1 mm/min loading rate depicting fiber imprints, textured microflow, and matrix microcracking morphology, respectively. The textured microflow morphology usually occurs at a lower loading rate imparting proper load distribution in the matrix region. The flow direction of the granular structures indicates the direction of fracture and is most prevalent in brittle matrix systems [146]. Fig. 6.23D
F show SEM micrographs at 1000 mm/min loading rates indicating fiber pullout, mirror, mist and hackle, and fiber fracture morphology, respectively. The mirror, mist, and hackle morphology usually occurs in brittle materials. The term mirror refers to the smooth surface of crack imitation. As the crack propagates, a smooth, matt region called mist develops, incorporating the start of scraps. Finally, as the crack attains its terminal velocity the hackle type of fractures occurs [146]. Furthermore, during tensile pulling, fiber fracture was one of the most important modes of failure in the glass/epoxy composites.

6.4.5 Freeze
thaw Lopez-Anido et al. [147] studied the freeze
thaw resistance of single-lap bonded joints between GFRP composites (glass-epoxy-based vinylester), created by

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(A)

(B)

Fiber Imprints

Matrix microcracking

Textured microflow (D)

Fiber pullout

(C)

(E)

(F)

Fiber fracture Mist Mirror

Hackle

Figure 6.23 Scanning electron micrographs of thermal-shock conditioned GE composite tested at (A) 1 mm/min, (B) 1 mm/min, (C) 1 mm/min, (D) 1000 mm/min, (E) 1000 mm/min, and (F) 1000 mm/min loading rates, respectively.

VARTM, and an underwater curing epoxy adhesive. The specimens were immersed in tap water at 38 C to allow the epoxy adhesive to cure in an underwater environment. After 2 weeks, the control samples were removed and tested in a dry state, while the samples subjected to thermal cycles were removed after 3 weeks. The freeze
thawing conditioning involved 20 cycles characterized by (1) 8 hours at 218  C and (2) 16 hours of tap water immersion at 38 C. After exposure to freezing and thawing, the bond strength of the samples showed a reduction of about 43%. This reduction in the bond strength was mainly attributed to enhanced moisture ingress in voids existing in the adhesive layer. At the time of freezing, the void content was affected due to water expansion, which produced cracks and damaged the epoxy adhesive. The degradation occurring at the joints was attributed to numerous factors, such as water immersion, thermal cycles, and freeze
thaw. Ray [148] investigated the freeze
thaw response of polymer composites under different loading rates and also at ambient and subambient temperatures. The investigation was carried out with three different weight fractions (i.e., 0.55, 0.6, and 0.65) of GFRP composites. The short beam shear specimens were suddenly exposed to 80 C for 2 hours and then a three-point bend test was conducted instantaneously at this temperature. Another batch of samples was allowed to thaw at ambient temperature for 1 hour and tested on the flexural mode at ambient temperature. The testing was carried out for a range of 0.5
500 mm/min crosshead speeds. The shear strengths at cryogenic temperature and thawing temperature were compared with the room temperature-tested data of the shear value of laminates. Li et al. [149] investigated the freeze
thaw conditioning of unidirectional glass, carbon, and basalt FRP (GFRPs, CFRPs, and BFRPs, respectively) epoxy wet layups from

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230 C to 30 C in dry air. Using optic fiber Bragg grating sensors, the variation of the internal strain and coefficient of thermal expansion (CTE) during the freeze
thaw cycles were calculated. With the CTE values, the stresses developed in the matrix of the FRPs were also monitored. It was noted that the CFRPs possess marginally higher stresses in comparison to the BFRP and GFRP. The freeze
thaw cycles exhibited an insignificant effect on the tensile behavior of both GFRP and BFRP composites, but showed a contrasting effect on CFRP, producing a decrease of 16% and 18% in the strength and modulus value, respectively, after conditioning 90 freeze
thaw cycles. The exposure of the bonding between the carbon fibers and epoxy to the freeze
thaw conditioning cycles was assigned to the damage and degradation of CFRP composites.

6.5

Conclusions and future perspectives

In this chapter, the disseminate literature has been presented and discussed focusing on the implications of understanding the emerging improvement of FRP composites in structural applications at different environmental exposures, with and without modified nanofiller-enhanced composites. A micron-level change with environmental conditions may lead to a significant change in properties and performances in FRPs. The chapter focuses on several aspects related to macro and micro characterization of FRP composites, evaluation of some special structural properties such as vibrational and toughening mechanisms, environmental durability (low and cryogenic temperatures, elevated temperatures, hydrothermal and hygrothermal behavior, UV irradiation, thermal shock, freeze
thaw) of FRP composites in civil structures and the implications of nanofiller incorporation in the structural behavior of FRP composites. The evaluation of nanofillers as reinforcement in FRPs has added a new aspect to the field of composites, as the nanofillers reveal exceptionally high strength and stiffness, high specific surface area, high thermal stability, and fatigue resistance. By the effective inference of all these properties into the FRP composite, they create durable and reliable structural materials. Nevertheless, research and proper investigation are not yet conclusive for these FRP-based materials at in-service environmental conditions. There is a dearth of investigation regarding the combined effect of detrimental environmental parameters (low and elevated temperatures, moisture, thermal spike, thermal shock, UV irradiation, and thermal cycling of temperatures) and alteration of static and dynamic loading, which leads to extreme complex situations and further more than one damaged micromechanisms reacting at the same time and resulting in failure of the composite. The durability of FRPs is a matter of concern as exposure to static or dynamic loading with environmental conditioning can damage and degrade the properties of the fiber/matrix, the fiber/matrix interface, or the entire composite. Therefore the evolution of FRP composites is the opening of a generation of advanced low-

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weight and indestructible structural materials. Urgency is required here for the development and evaluation of this novel material for advanced solutions and methods, otherwise the capabilities of FRP will remain uncertain.

Acknowledgment The authors take this opportunity to express their appreciation for financial and infrastructural support from the National Institute of Technology, Rourkela, India.

References [1] The Nobel Prize in Chemistry, 1953, NobelPrize.Org, ,https://www.nobelprize.org/ prizes/chemistry/1953/summary/., n.d. (accessed 24.05.19). [2] L.C. Hollaway, A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties, Constr. Build. Mater. 24 (2010) 2419
2445. Available from: https://doi.org/10.1016/j. conbuildmat.2010.04.062. [3] L.T. Drzal, M.J. Rich, M.F. Koenig, P.F. Lloyd, Adhesion of graphite fibres to epoxy matrices: II. The effect of fibre finish, J. Adhes. 16 (1983) 133
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Fiber-reinforced concrete and ultrahigh-performance fiberreinforced concrete materials

7

Francesco Micelli1, Angela Renni2, Abdou George Kandalaft3 and Sandro Moro3 1 University of Salento, Lecce, Italy, 2Roughan & O’Donovan Consulting Engineers, Dublin, Ireland, 3BASF Construction Chemicals, Italy

7.1

Fiber-reinforced concrete

7.1.1 General In recent decades the growing interest in high-performance materials in civil engineering applications has met new requirements in terms of safety, costs, and sustainability. Fiber-reinforced concrete (FRC) is one of these subjects that has become a very promising material in civil engineering applications for its advantages including: G

G

G

G

Toughness, in regards to compressive strength and flexural bending strength; Tensile strength; Durability; Energy-absorbing capacity.

The use of fibrous reinforcement in concrete is often considered from an economic point of view, as costs are lower than those of ordinary concrete for the elimination of the installation times of conventional reinforcement. Moreover, the use of FRC has its advantages as regards casting as the fibers are already incorporated into the concrete. This may be found, that is, for the construction of secondary precast concrete elements, used in civil and industrial buildings and infrastructures. This chapter aims to provide a complete overview of the use of FRC materials and their recent innovative applications in civil engineering. The first part of the chapter illustrates the state of the art of FRC, in order to spotlight the history of this material. The second part addresses the development of the material, constituents, structural codes, and new application trends. According to ACI11R, Cement and Concrete Terminology, FRC is defined as a concrete containing dispersed randomly oriented fibers. This makes FRC one of the materials suitable to be carefully studied concerning the orientation and functionality of fibers inside the mix. Recently, even the structural design software or finite element software have been taken into consideration for the FRC materials in their libraries. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00007-7 © 2020 Elsevier Inc. All rights reserved.

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In the 1960s, Romualdi, Batson, and Mandel interested many researchers around the world with their published works on FRC [1,2]. The study of FRC started from the concept linked to fracture mechanics, whereby the brittleness of Portland cement concrete could be improved by reinforcing the matrix with fibers. An improvement was observed in strength when the filament spacing was decreased. Initial tests were conducted on continuous filament and later, discrete fibers were used. In connection to the chemical nature of fibers, first steel and later polypropylene fibers were used in the experimental tests. The first fibers used in the research were characterized by higher dimensions (0.15
0.25 mm diameter), with respect to modern applications. The use of these fibers did not produce the expected effects, and no convenient results were obtained. In the mid-1980s new fiber types were created through new production techniques. All these aspects provided favorable effects for fiber applications [3,4]. The main aspect of using these composite materials is focused on the possibility of reducing conventional shear reinforcement in reinforced and prestressed structural elements, and even the reduction of diameter of the main reinforcement, not to mention crack control, corrosion resistance, joint distance increase, and sustainability of elements under creep loading. Therefore, FRC is becoming widely used in building engineering because of its astonishing and interesting mechanical properties [5,6]. This composite material could overcome concrete limitations such as high brittleness. In fact, fibers add a significant tensile strength in the cracked phase to concrete, thanks to the fiber-bridging mechanisms across the crack surface. For its improved cracking behavior, FRC proves its suitability to be used in many applications, including pavements, shotcrete tunnel linings, slabson-ground, and precast bridge decks. Pioneering research on deck slabs reinforced by low synthetic fibers without conventional steel bars has been reported by Mufti et al. [7
9]. In the work of Mufti, the failure of the investigated slabs was always due to bending. Afterwards, the arching action in concrete deck slabs of bridges was studied and this was recognized in some codes, where new approaches were introduced in the use of this innovative material in respect to safety limits [8]. The enthusiasm of the academic world for this innovative material has been opposed initially by the diffidence of the construction industry, because of the uncertainty about the durability of the material and quantification of the possible long-term benefits. This created a gap between the knowledge obtained in laboratories and the development of design guidelines that could allow practitioners to consider FRC in structural design. Since then, many concrete applications using short fiber reinforcement have been realized, such as slabs, steel decks, precast, and shotcretes, as shown in Fig. 7.1. All the methods were different in the method of defining the tensile capacity of the concrete provided by the fibers. A conservative approach was defined by ACI 544.6R, mainly for structural members such as beams, columns, walls, and slabs. According to the latter code, fibers could be used to reduce the reinforcing bars in structural members. In those applications where the presence of steel bars was not essential, such as slabs-on-ground and pavements, fibers may be used as the only reinforcement in specific cases. ACI Committee 544 explains the design concepts

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Figure 7.1 Applications of FRC: (A) industrial pavement; (B) tunnels; (C) bridge decks.

of fiber reinforcement, including constitutive laws, design for flexure, design for shear, and design for crack-width control, and also, applications and recommendation about mixing, placing, and finishing of elements [10]. Later, other guidelines for structural elements were introduced, such as RILEM TC162-TDF [11], and recommendations were produced by various countries including Sweden [12], Germany [13], Austria [14], Italy [15], and Spain [16]. Nowadays, fib Model Code 2010 is considered to be an exhaustive reference for future guidelines [17,18].

7.1.2 Constituent materials 7.1.2.1 Cement-based matrix FRC is a cement matrix inside which random fibers are dispersed randomly with respect to the dimensions of the element. Unlike traditional reinforcement with steel bars, the added short fibers usually do not increase the compressive strength of the concrete, however, thanks to their homogeneous diffusion, they are more effective in improving the postcracking behavior of the concrete element. Their purpose is therefore to increase the ductility and capacity to absorb energy, through the control of crack propagation [19
22]. The main components of the matrix in FRC do not vary compared to those of traditional cementitious materials, which are typically: Portland cement, water, aggregates of various sizes, and additives. The choice of quality of the components and their proportions in the mixture depend on the main requirements of strength grade, workability at the fresh state, porosity, and durability. Essentially, the matrix generally consists of Portland cement-based concrete when low amounts of fibers are added. In fact, the main technological challenge is to introduce high volumes of dispersed fibers within the matrix, without losing high workability. At higher dosages and depending on the fiber type and dimensions, it is necessary to consider proper adjustments to the mix design. This is possible by adding or increasing the dosage of superplasticizers and by considering a greater volume of fine aggregates, to maintain a constant water
cement ratio. When high amounts of fibers are added, the mix design has to be modified, for instance, by increasing the amount of fine aggregates and additives, which may lead to the path of self-compacting concrete (SCC). SCC is self-compacting concrete without the need for vibration. When cast, possible segregation is eliminated as shown in Fig. 7.2A; their internal structure is made of high quantities of fine aggregates and they represent the mixtures suitable at accommodating higher dosages of fibers and

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Figure 7.2 SCC (self-consolidating concrete) (A), cement-based matrix without additives (B), balling of fibers (C). Table 7.1 Example of mix design of conventional FRC (A) and SCCFRC (B). Materials (A) Cement type CEM I 52.5 R Sand 0/3 mm Sand 0/12 mm Gravel 8/15 mm Water Superplasticizer

350 kg/m3 120 kg/m3 800 kg/m3 670 kg/m3 150 L/m3 3.5 L/m3

Materials (B) Cement type CEM I 52.5 R Filler Sand 0/3 mm Sand 0/12 mm Gravel 8/15 mm Water Superplasticizer

400 kg/m3 50 kg/m3 825 kg/m3 190 kg/m3 735 kg/m3 160 L/m3 6 L/m3

good pumpability. The diffusion of fluidifying and superfluidifying chemical additives was introduced in the early 1970s and they are now widely used. They improve properties at the fresh state without compromising the properties at the hardened state, as shown in Fig. 7.2B, where the effects of an SCC concrete without additives are illustrated. The most commonly used additives are superplasticizers, in accordance with EN 934-2 (2009), which are capable of reducing the demand for water with equal spillage and increasing the fluidity with the same ratio of water/ binder. Nevertheless, it could have an effect on the early strength of the concrete structure. The superfluidifying agents are water-soluble polymers, whose main effect is deflocculating, that improve the dispersion of solid particles. The use of superfluidifying agents may be considered in order to avoid two possible problems: exudation and segregation of concrete. If the additive is used to reduce the water content, without changing the mix, it will be necessary to check that the viscosity of the fresh FRC does not become too high, causing slow flow. Moreover, in some cases, as has been found that when relatively longer steel fibers are used, “fiber balling” can occur, as noted in Fig. 7.2C.

Table 7.2 Steel fibers. Equivalent diameter (mm)

Tensile strength (N/mm2) R1

R2

(1)

0.15 # dt # 0.50 0.50 # dt # 0.80 0.80 # dt # 1.20 (1) For smooth fibers. (2) For hooked fiber.

(2)

R3

(1)

(2)

(1)

(2)

Rm

Rp0,2

Rm

Rp0,2

Rm

Rp0,2

Rm

Rp0,2

Rm

Rp0,2

Rm

Rp0,2

400 350 300

320 280 240

480 450 390

400 350 300

800 800 700

720 640 560

1080 1040 910

900 800 700

177 1550 1400

1360 1240 1120

2040 2015 1820

1700 1550 1400

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In Table 7.1, an example of FRC mix design is illustrated, whereas in Table 7.2 an example of SCCFRC concrete mix design is shown, designed with a greater amount of fine aggregates, to be considered as filler.

7.1.2.2 Fibers Fibers are added within the matrix with the aim of inhibiting cracking, controlling the brittle fracture process, and providing postcracking toughness. Fibers can contribute to improving the performance of concrete elements as follows (according to ACI 544.6R): 1. By resisting the tensile stresses; 2. By controlling crack development and consequently increasing the durability of concrete.

Fibers can be characterized according to a variety of aspects, such as material and geometric forms, including prismatic, irregular cross section, and collated (Fig. 7.3). Generally, fiber efficiency can be evaluated through different parameters: G

G

G

G

The aspect ratio, defined as the ratio between the length and diameter of the fiber, which influences the bond force transfer, and the workability of the matrix in the fresh state; The roughness of the surface and the finishing, which affects the quality of mechanical contact and friction behavior under tensile forces that act to promote slipping between fibers and the matrix at their interface (embossing or crimping); Tensile strength, elastic modulus, and breaking behavior that determine the quantity of energy that can be dissipated in the cracking phase; The volumetric percentage of fiber with respect to the total volume, which typically falls between 0.2% and 2%. This parameter greatly affects the diffusion of internal forces inside the cement matrix, which may lead to brittle or ductile behavior.

Crack control depends mainly on the fiber ratio, mechanical anchorage, and strength of fibers.

Material

-Steel -Synthetic (polypropilene, polyethylene.) -Mineral (carbon,...) -Natural

Chemical/physical

-Density -Surface roughness -Chemical stability -Fire resistance

Mechanical

-Tensile strength -Shear elastic modulus -Stiffness -Ductility

Fiber characteristics

-Lenght -Diameter Geometric

Figure 7.3 Fiber characteristics.

Cross section

Circular, square triangular, rectangular elliptical

Shape

Smooth deformed, rounded, hooked

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279

In modern FRC materials, polymeric fibers (polypropylene) are also commonly used in place of traditional steel fibers. This is justified by three advantages of polypropylene fibers over steel fibers: corrosion resistance, low cost, and good behavior against impact loads. Fibers can be fabricated and used according to different shapes: G

G

G

Prismatic: rounded or polygonal cross-section; Irregular cross section; Collated: monofilament or multifilaments.

Regarding fibers (not including circular or prismatic), their equivalent diameter is an essential parameter. It is the diameter of the circle with the same area of the average cross-sectional area of an effective fiber. There is a correspondence between the equivalent diameter and the flexural stiffness. Flexural stiffness increases as the equivalent diameter increases. American Concrete Institute (ACI) Committee 544 distinguishes four categories of FRC based on fiber material type: 1. 2. 3. 4.

SFRC (steel FRC); GRFC (glass FRC); SNFRC (synthetic FRC including carbon fibers); NFRC (natural FRC).

7.1.2.2.1 Steel fibers ASTM A820/A820M is the standard specification for steel fibers within concrete, and classifies fibers into five types based on the production process: type I (colddrawn wire); type II (cut sheet); type III (melt-extracted); type IV (mill cut); and type V (modified cold-drawn wire). According to ASTM A820/A820M, the average tensile strength of fiber should not be less than 345 MPa. There are three classes to distinguish the production process [23
26]: G

G

G

Derived from drawn wire (type A); Derived from sheet metal (type B); Derived from other processes (type C).

Moreover, there are two classes to define the form: G

G

Linear; Shaped (hooked, wavy, etc.).

And three classes for the material: G

G

G

Low-carbon steel (C , 0.2, type 1); Steel with high carbon content (C . 0.2, type 2); Stainless steel (type 3).

As for the mechanical characteristics, the fibers can be divided into three categories (R1, R2, and R3) summarized in Table 7.2 (Fig. 7.4).

7.1.2.2.2 Glass fibers The brittleness of common glass is due to the large number of defects in crystallization which act as microfractures and regions of stress concentration. In contrast,

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Smooth surface

Roughened surface

Round with end paddles

Round with end buttons

Round with hooked ends

Crimped

Figure 7.4 Various shapes of steel fibers.

Table 7.3 Glass fibers. Properties Diameter (μm) Density (kg/m3) Tensile strength (MPa) Elastic modulus (GPa) Rupture deformation (%)

10 2540
2780 2500
3500 70
73 3.6
4.8

glass fibers do not have these defects, and so the mechanical strength is much higher than in common glass. Different types of fibers are distinguished according to their characteristics and chemical susceptibility. Glass fibers are widely produced and used in the manufacture of structural composites used in aerospace, boats, and automotive fields. They are not commonly used for the fabrication of composites with a metallic or ceramic matrix. For these materials there is a technological problem, due to the high temperature of production, that does not allow for the use of glass fibers. In the field of civil engineering, glass fibers are used for the manufacturing of fibrocement artifacts and for external strengthening of existing buildings, in the form of glass fiber-reinforced polymers (GFRPs) (Fig. 7.5; Table 7.3). The main limit of glass fibers is their sensitivity to alkaline environments, which promote the production hydrolysis products inside the fibers. The so-called E-glass

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281

Figure 7.5 Glass fibers (Tongban).

Figure 7.6 Polymeric fibers.

fibers are greatly weakened by high-pH solutions, such as those found in concrete pores. Thus, correct use of glass fibers in civil engineering would suggest the use only of alkali-resistant glass fibers (AR-glass).

7.1.2.2.3 Polymeric fibers For polymeric fibers, ASTM D7508/D7508M is the standard specification, including microfibers and macrofibers. According to ASTM D7508/7508M, the average tensile strength of fiber should not be less than 345 MPa for macrofibers, whereas no limitations are found for microfibers in terms of tensile strength. Polymeric fibers have much lower stiffness and strength compared with steel fibers, and they are mainly used to reduce shrinkage cracking of fiber concrete structural elements. Other types of synthetic fibers such as glass, basalt, aramid, and carbon fibers have high tensile strength and stiffness, coupled with low specific gravity. Unless these fibers are stable in severe conditions, they are characterized by a small diameter which leads to the formation of fiber balling [27
29] (Fig. 7.6; Table 7.4).

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Table 7.4 Polymeric fibers. Properties

Acrylic

Nylon

Polyester

Polyethylene

Polypropylene

PVA

Diameter (μm) Density (kg/m3) Tensile strength (MPa) Elastic modulus (GPa) Rupture deformation (%)

12.7
104.1 1.16
1.18

22.86 1.14

19.81 1.36

25.4
1016 0.92
0.96

100
50,000 0.9

14
600 1.3

269
1000

965

276
1103

76
586

130
689

880
1600

13.7
19.3

5.2

17.2

5

3.4
4.8

25
40

7.5
50

20

12
150

3
80

15

6
10

Table 7.5 Carbon fibers. Properties

PAN

PITCH

Diameter (μm) Density (kg/m3) Tensile strength (MPa) Elastic modulus (GPa) Rupture deformation (%)

7.0
9.7 1950 2200 390 0.5

1600 600
750 30
32 2.0
2.4

7.1.2.2.4 Carbon fibers Carbon fibers, which appeared on the market in 1960, are produced by the modification of organic fibers (rayon, acrylic, etc.) or by residues from the distillation of oil or tar. The former is called carbon-PAN (polyacrylonitrile), the other carbon by pitch (PITCH). Carbon fibers are generated in the form of ribbons made of thousands of filaments, with average diameters between 7 and 15 μm, constituted by hexagonal graphite crystals arranged in planes with high atomic density. The methods for changing the carbon structure are based on the pyrolysis of organic precursors such as polyacrylonitrile fibers in the PAN method and the derivatives of petroleum (petroleum pitch) in the PITCH method. In both processes, first the extraneous chemical elements are eliminated and finally they undergo a graphitization treatment at 3000 C, which allows the complete formation of the crystalline structure. Table 7.5 shows the main mechanical characteristics of the two fiber categories (Fig. 7.7).

7.1.2.2.5 Natural fibers Natural fibers (Fig. 7.8) have been and are still being developed for the construction of low-cost structures and they can be produced by macerating and drying the roots,

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Figure 7.7 Carbon fibers.

Figure 7.8 Natural fibers.

as in jute and flax, from the removal of the superficial layer of fruits and seeds of plants, as in the coconut fibers, and finally from the mechanical treatment of the cellulose of bamboo and cane. All-natural fibers have a complex microstructure,

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Table 7.6 Natural fibers. Properties

Juta

Agave

Coconut

Sugar cane

Diameter (μm) Density (kg/m3) Tensile strength (MPa) Elastic modulus (GPa) Rupture deformation (%)

0.1
0.2 1800 250
350 26
32 1.5
1.9



280
750 13
26 3
5

0.1
0.4 50
350 120
200 19
26 10
25

0.2
0.4 50
300 170
290 15
29


made up of hundreds of cells in length between 2 and 5 mm, and with a diameter of less than 0.2 mm, joined together by lignin, a natural adhesive. Being made of organic material they are vulnerable to biological attack, influenced by humidity, and alkaline attack, which decomposes the lignin. Their main mechanical properties are described in Table 7.6.

7.1.3 Rheology and mechanical properties 7.1.3.1 Fresh state Rheological parameters define the fresh state characteristics of cementitious materials. The most commonly used model is the Bingham model: τ 5 τ 0 1 μ0 γ G

G

G

where τ 0 is the yield stress, describing the stress needed to initiate flow; μ0 is the plastic viscosity, which expresses the resistance of the material to flow; τ and γ are the shear stress and shear rate, respectively.

Rheological parameters that are the main factors to describe the properties of concrete mix at the fresh state, are workability, compatibility, flowability, and pumpability [21,30,31]. The workability of fresh concrete can be determined by using the slump test (Abram cone). The instrument is used to measure workability consisting of a truncated cone open at both ends which is placed on a nonabsorbent metal base and filled with concrete. By lifting the cone after filling it with the freshly mixed concrete, the mixture by the action of gravity force tends to spread until a balance is reached between internal resistance forces and external forces. Spreading is measured with respect to the original cone height. The test is conducted according to EN 12350-2. If concrete is of high flow, a slump test is conducted in the same way but by measuring the spread diameter. Other tests can be conducted in order to define rheology, such as using a Vebe` consisto-meter. This test simulates the material compaction during the casting phase. The fresh concrete mix is put inside a cone, similar to the Abrams cone. The cone is placed on a plate and subjected to vibration at a constant frequency and the duration of compaction is measured. This test considers the influence of the shape and aggregate volume,

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void contents, presence of superplasticizers, and superficial friction of fibers. The two tests define the same behavior at the fresh state for FRC: a decrease in workability is observed as fiber volume within the matrix increases. The addition of fibers decreases the slump and consequently the workability of the concrete. Other parameters affecting the latter of the FRC are fiber shape and aggregate dimensions. When fibers with an aspect ratio higher than 100 are used, the fiber balling phenomenon is more likely to occur. Different approaches can be followed in order to get rid of the low workability due to the introduction of fibers within the matrix: 1. Introducing fibers with a low aspect ratio to the mix, but on equal terms of postcracking ductility, fiber volume must be increased; 2. Increasing the amount of fine aggregates; 3. The use of fly ash and air-entraining additives, which helps in obtaining plastic or semifluid concrete, even with high fiber contents; 4. The use of superplasticizers leads to FRC characterized by good workability, if the aspect ratio is moderate; higher amounts of additives lead to higher flowability of the matrix and the consequent segregation, with the separation of fibers from the matrix; 5. The introduction of silica fume (SFs) and superplasticizers leads to better workability of the mix, because FRC concrete results are characterized by high viscosity at rest and good workability in movement, reducing segregation problems.

The tendency of concrete to segregate or the separation of its constituents, both ordinary and fibrous, is evaluated by a test which measures the resistance to segregation of SCC, according to the European Guidelines and the Standard UNI EN 206-9, with the test methodology of the UNI EN 12350-11 Standard. After mixing, the fresh concrete is rested for 15 minutes, then the concrete is poured into a sieve which has openings with a square mesh of 5 m. After 2 minutes the mass of material that has passed through the sieve is measured. Segregation is finally calculated as the percentage of the remainder of mass that has passed through the sieve, as shown in Fig. 7.9.

7.1.3.2 Hardened state The properties of fresh and hardened FRC depend on the characteristics of the single components [18,19]. The addition of fibers within the matrix can increase the resistance of the material to cracking that can occur for several reasons, such as shrinkage, thermal gradients, or external loads. The increase in the resistance to cracking leads to an increase in toughness and durability of the material. The presence of fibers can also increase the resistance of the material to impaction, abrasion, and fatigue [32]. Moreover, the introduction of fibers within the matrix assists in obtaining more ductile behavior in compression and shear capacity.

7.1.3.2.1 Compressive strength The contribution of fibers to the compressive behavior of concrete is not substantial in terms of maximum strength, but it influences the postpeak behavior demonstrated in Fig. 7.10. The material is much more ductile than plain concrete and can dissipate a greater amount of energy.

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Figure 7.9 Segregation test.

Compressice strength (mm)

40 With fibers 30

20

Without fibers

10

0.001

0.002

0.003

0.004

0.005

Axial deformation (mm)

Figure 7.10 Compressive strength of concrete with and without fibers.

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Fibers > 2% Load (kN)

P

P Multicracking

Fibers < 2% Fibers > > > > > > > > > 2 @x > > > > > >   > > 2 > > = < 1 @w @y fεNL g 5 2 > > > > > > > 0 10 1 > > > > > > @wA@@wA > > > @ > > > > : @x @y ;

(10.6)

(10.7)

10.2.2 Stress
strain relations The stress
strain relation of the k-th layer is expressed as 9k 2 8 σ1 > Q11 > > > > > > 6 Q12 = < σ2 > 6 τ 12 56 6 0 > > > 4 0 > > τ 23 > > > ; : 0 τ 13 Q11 5

Q12 Q22 0 0 0

0 0 Q66 0 0

0 0 0 Q44 0

9k 3k 8 0 ε1 > > > > > > > > 0 7 7 < ε2 = 7 0 7 γ 12 > > 0 5> γ > > > > ; : 23 > γ 13 Q55

E1 ν 12 E2 E2 ; Q12 5 ; Q22 5 ; 1 2 ν 12 ν 21 1 2 ν 12 ν 21 1 2 ν 12 ν 21 Q66 5 G12 ; Q44 5 G23 ; Q55 5 G13

(10.8)

(10.9)

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The off-axis elastic constant matrix is given by 

   Qij 5 ½T T Qij ½T 

(10.10)

where ½T  is the transformation matrix and is defined as 2 6 6 ½T  5 6 6 4

cos2 θ sin2 θ 22 cos θ sin θ 0 0

sin2 θ cos θ sin θ cos2 θ 2cos θ sin θ 2 cos θ sin θ cos2 θ 2 sin2 θ 0 0 0 0

3 0 0 0 0 7 7 0 0 7 7 sin θ cos θ 5 2cos θ sin θ

(10.11)

10.2.3 Constitutive relations The constitutive relations of an orthotropic lamina are 8 9k 2 < σx = Q11 σy 5 4 Q12 : ; τ xy Q16 

τ yz τ xz

k

5

Q44 Q45

Q12 Q22 Q26 Q45 Q55

9k 2 3k 8 Q16 < εx = 0 εy 140 Q26 5 :γ ; Q66 0 xy

k 

γ yz γ xz

k

1

d 14 d 15

d 24 d 25

3k 8 9 0 d 31 < 0 = 0 0 d 31 5 : ; φ;z 0 d 31 8 9 k < 0 = 0 0 0 :φ ; ;z

(10.12a)

(10.12b)

For the laminated piezoelectric plate, the electric field is dominant in the thickness direction only and is given by Ez 5 2 φ;z ; where φ is the potential field. The relation between applied voltage (Vk) and the electric field is Ez 5 Vhpk ; where hp is the thickness of the piezoelectric layer.

10.2.4 Energy expressions The strain energy due to initial stresses and strains is given by U0

5 5

U0

5

U0

5 5

ððð 1 fεgT fσgdv 2 ððð 1 fεgT ½Cfεgdv 2 ð 1 fd gT ½BT ½C½Bfd gdA 2 ð 11 ð 11  ð 1 ½BT ½C ½BjJ jdrds fd g fd gT 2 21 21 ð 1 fd gT ½kfd g 2

(10.13)

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Ð 11 Ð 11 where k 5 21 21 ½BT ½C ½BjJ jdrds and is defined as the element stiffness matrix. The kinetic energy of the plate is given by (    2 )# ð "  2 h @w h3 @θx 2 @θy 1 1 T5 dxdy 2 @t 24 @t @t

(10.14)

10.2.5 Solution process The governing equation for the nonlinear vibration of the composite plate integrated with the piezoelectric layer is given by

 1 1 € M δ 1 K 1 KET 1 N1 1 N2 δ 5 0 2 3

(10.15)

where M and K are global mass and stiffness matrices, respectively. KET is the initial stress stiffness matrix due to thermoelectrical loads, and N1 and N2 are nonlinear stiffness matrices. The solution to Eq. (10.15) is calculated as δ 5 δmax sin wt After substituting this into the above equation 2

3 1 1 2 ω2 Mδmax sin wt 1 4K 1 KET 1 N1 ðδÞ 1 N2 ðδ; δÞ5δmax sin wt 5 0 2 3 2 3 1 1 2 ω2 Mδ 1 4K 1 KET 1 N1 ðδÞ 1 N2 ðδ; δÞ5δ 5 0 2 3 (10.16) As Eq. (10.16) does not fulfill the governing equation at all the points, Eq. (10.16) is modified as,

 1 1 2ω Mδ 1 K 1 KET 1 N1 ðδÞ 1 N2 ðδ; δÞ δ 5 R 2 3 2

(10.17)

Applying the weighted residual method along the path ðT

4

R sin wt dt 5 0

(10.18)

0

Since

ÐT

4

0

sin2 wt dt 5 T8 ;

ÐT

4

0

sin3 wt dt 5

ÐT

T 4 3π ; 0

sin4 wt dt 5

3T 32

; we have

Dynamic response of laminated composite plates fitted with piezoelectric actuators

 4 1 N1 ðδmax Þ 1 N2 ðδmax ; δmax Þ δmax 5 0 2ω2 Mδmax 1 K 1 KET 1 3π 4

383

(10.19)

The preceding equation is a generalized eigenvalue system and is solved to determine the eigenvalues and eigenvectors. The solution to Eq. (10.19) is achieved via a direct iteration technique. The different stages involved in this process are: Step 1: The linear frequency and corresponding linear mode shape are determined using Eq. (10.19) by putting all the nonlinear terms as zero. Step 2: The mode shape is normalized suitably by scaling the eigenvector to make sure that the maximum displacement is equal to the required amplitude, c 5 Wmax =h. Step 3: Based on the normalized mode shape, the nonlinear terms in the stiffness matrix are determined. Step 4: The equations are then solved to evaluate the new eigenvalues and eigenvectors. Step 5: Steps 2
4 are repeated for a few rounds until the value of the frequency converges up to the required exactness.

10.3

Linear static analysis of cross-ply laminated plates

The correctness of the present formulation is checked by conducting static analysis of cross-ply laminated plates resting on simply supported edges and comparing with the results of Chandrashekhara and Tenneti [30], as shown in Fig. 10.2. The temperature distribution across the thickness is calculated as T 5 T2 z/h. The sideto-thickness ratio is 50.

10.4

Dynamic and transient analyses

Table 10.1 displays the comparison of nondimensional frequencies of composite plates subjected to thermal loading based on the present formulation and the results obtained from FSDT and the Ritz method of Ram and Sinha [31]. The accuracy of the current formulation is also tested by performing vibration analysis and comparing the natural frequencies of piezoelectric laminated plates with the results of Huang and Shen [17], as presented in Table 10.2. The dimensions and material properties are considered from Huang and Shen [17]. The transient response of composite plates integrated with piezoelectric material (P/0/90/90/0/P) based on the present formulation is shown in Fig. 10.3 and compared with the HSDT outcomes of Huang and Shen [17]. The material properties are considered from Huang and Shen [17]. The present formulation is also validated for nonlinear vibration criteria by comparing the present results with Huang and Shen [17] under electrical load for different amplitude ratios (Wmax/h) as depicted in Table 10.3. The current finite element method (FEM) outcomes are matching closely with the published one.

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Figure 10.2 Comparison of central deflection of a laminated composite plate resting on simply supported edges.

Table 10.1 Comparison of dimensionless frequencies of composite plate (0/90/90/0) resting on simply supported edges; a/b 5 1, a/h 5 100, T 5 325 K. Frequency

1 2 3 4

10.5

Present FEM

8.5824 19.5209 39.7136 45.8484

Ram and Sinha [31] FSDT

Ritz method

8.088 19.196 39.624 45.431

8.068 18.378 38.778 44.778

Nonlinear vibration analysis of composite plates embedded with piezoelectric materials

After verifying the consistency and exactness of the present formulation, the study is extended to nonlinear dynamic characteristics of laminated composite plates integrated with piezoelectric layers under thermal loading. The various types of boundary conditions used in the present investigation are: For both the cases w 5 0 at x 5 0; a and y 5 0; b:

Table 10.2 Comparison of dimensionless frequencies of composite plates integrated with piezoelectric material. Stacking sequence

(P/0/90/0/90/ P) (P/0/90/90/0/ P)

Voltage (V) Vlower 5 Vupper

2 50 0 50 2 50 0 50

Temperature 0 C

Temperature 100 C

Temperature 300 C

Present FEM

Huang and Shen [17]

Present FEM

Huang and Shen [17]

Present FEM

Huang and Shen [17]

10.8569 10.8098 10.7625 10.8696 10.8226 10.7753

10.664 10.617 10.568 10.861 10.841 10.768

10.3327 10.2832 10.2334 10.3461 10.2967 10.2470

10.133 10.082 10.032 10.343 10.293 10.244

9.1949 9.1393 9.0833 9.2103 9.1548 9.0989

8.975 8.918 8.862 9.217 9.163 9.106

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Figure 10.3 Comparison of the transient response of a piezoelectric laminated plate.

Table 10.3 Validation of frequency ratio (ωNL/ωL) of laminated plates [P/0/90/90/0/P] integrated with piezoelectric layers. Voltage (V)

Amplitude ratio (Wmax/h)

2 50

Present FEM Huang and Shen [17] Present FEM Huang and Shen [17] Present FEM Huang and Shen [17]

0 50

0.2

0.4

0.6

0.8

1.0

1.019 1.019 1.019 1.020 1.020 1.020

1.075 1.075 1.075 1.076 1.076 1.077

1.161 1.163 1.163 1.164 1.164 1.166

1.272 1.275 1.274 1.277 1.276 1.280

1.402 1.406 1.405 1.410 1.408 1.413

In addition to the above restraints, the following restraints are also considered: ImmovableðSS1Þu0 5 v0 5 0 at x 5 0; a and y 5 0; b: Partially movableðSS2Þv0 5 0 at x 5 0; a; u0 5 0 at y 5 0; b: The material constants considered in the present investigation are: Graphite/epoxy composite: E1 5 150 GPa; E2 5 9 GPa; G12 5 G13 5 7:1 GPa; G23 5 2:5 GPa; ν 12 5 0:3 and ρ 5 1580 kg=m3 PZT-5A is selected as the piezoelectric layer:

E 5 63 GPa; G 5 24:2 GPa; ν 12 5 0:3; ρ 5 7600 kg=m3 and d31 5 d32 5 2:54 3 10210 m=V: The square plate of dimensions a 5 b 5 24 mm and the total thickness of 1.2 mm is considered for the present study. The thickness of each orthotropic layer is the same, and the thickness of the piezoelectric layer is 0.1 mm.

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10.5.1 Influence of temperature rise on frequency ratio of crossply laminates embedded with a piezoelectric layer The impact of a temperature increase on the frequency ratio of the piezoelectric composite plate [P/0/90/90/0/P] with SS1 boundary condition is presented in Fig. 10.4. From the figure, it is clearly seen that the frequency ratio is increasing as the temperature increases. Because of the reduction in stiffness of the plate with temperature increment, the linear frequency is decreasing, and so the frequency ratio is increasing. The frequency ratio is increasing as the amplitude ratio increases. The same type of trend is observed in the case of Huang and Shen [17]. The influence of temperature on the frequency ratio of the hybrid plate [P/0/90/ 90/0/P] for the SS2 boundary condition is illustrated in Fig. 10.5. From this figure, for an amplitude ratio of 0.2%, 0.18% and 1.01% increments are seen in the frequency ratio with an increase in the temperature from 0 C to 300 C. The frequency ratio increases as the Wmax/h ratio increases. The variation of frequency ratio of the unsymmetric cross-ply hybrid plate with amplitude ratio for SS1 and SS2 boundary conditions is shown in Figs. 10.6 and 10.7, respectively. From the figures, it is understood that the frequency ratio

Figure 10.4 Variation of frequency ratio with temperature rise for SS1 boundary condition.

Figure 10.5 Variation of frequency ratio with temperature rise for the SS2 boundary condition.

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Figure 10.6 Variation of the frequency ratio of the hybrid plate [P/0/90/0/90/P] with a temperature rise for the SS1 boundary condition.

Figure 10.7 Variation of the frequency ratio of the hybrid plate [P/0/90/0/90/P] with a temperature rise for the SS2 boundary condition.

increases as the temperature increases from 0 C to 300 C. With the increase in the amplitude ratio, the frequency ratio increases. Also, a symmetric cross-ply hybrid plate shows higher frequency ratio compared to the unsymmetric cross-ply hybrid plate.

10.5.2 Effect of control voltage on the frequency ratio of crossply laminates embedded with a piezoelectric layer The influence of control voltage on frequency ratio is tabulated in Table 10.4. From the table, it is noted that for an amplitude ratio of 0.2, the frequency ratio is increasing in the range of 0.03% and 0.06% for the control voltages of 0 and 100 V, respectively, compared to 2100 V. With the increase in the Wmax/h ratio the frequency ratio is increasing. There is an insignificant increment in the frequency ratio as the control voltage increases. The variation in the frequency ratio with control voltages for the SS2 boundary condition is presented in Table 10.5. The frequency ratio of the hybrid plate is increasing in the range of 0.62% and 1.28% for the applied voltages of 0 and 100 V compared to 2100 V and for the Wmax/h ratio of 1.2. There is no significant variation in the frequency ratio.

Dynamic response of laminated composite plates fitted with piezoelectric actuators

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Table 10.4 Variation of the control voltage with frequency ratio (ωNL/ωL) of the hybrid plate [P/0/90/90/0/P] for the SS1 boundary condition. Amplitude ratio (Wmax/h)

0.2 0.4 0.6 0.8 1.0 1.2

Control voltage 2100 V

0V

100 V

1.0191 1.0742 1.1601 1.2703 1.399 1.5412

1.0194 1.0754 1.2745 1.1627 1.4049 1.549

1.0197 1.0767 1.1654 1.2788 1.411 1.5569

Table 10.5 Variation of the frequency ratio (ωNL/ωL) of the hybrid plate [P/0/90/90/0/P] with control voltage for the SS2 boundary condition. Amplitude ratio (Wmax/h)

0.2 0.4 0.6 0.8 1.0 1.2

Control voltage 2100 V

0V

100 V

1.0089 1.035 1.0769 1.1327 1.2002 1.2774

1.0092 1.0361 1.0793 1.1367 1.2062 1.2853

1.0095 1.0373 1.0819 1.141 1.2125 1.2938

10.5.3 Influence of temperature rise on frequency ratio of angleply laminates embedded with a piezoelectric layer The influence of temperature rise on frequency ratio of the angle-ply laminated plate [P/45/-45/-45/45/P] embedded with a piezoelectric layer with the SS1 boundary condition is illustrated in Fig. 10.8. The frequency ratio is increasing in the range of 2.04% and 7.18% for a temperature rise of 100 C and 300 C, respectively, when compared to 0 C for the Wmax/h ratio of 1.2. Also, with the increase of amplitude ratio, the frequency ratio is significantly varying. The angle-ply hybrid plate displays a higher frequency ratio as compared to the cross-ply hybrid plate. Fig. 10.9 illustrates the impact of temperature on the frequency ratio of the hybrid plate [P/45/-45/-45/45/P] with the SS2 boundary condition. From this figure, it is noted that the frequency ratio is increasing as the temperature increases from 0 C to 300 C and Wmax/h ratio. The impact of temperature on an unsymmetric angle-ply plate embedded with a piezoelectric layer for SS1 and SS2 boundary conditions is depicted in Figs. 10.10 and 10.11, respectively. From the figures, it is clearly seen that the frequency ratio is increasing as the temperature increases. Also, the frequency ratio varies significantly with the amplitude ratio.

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Figure 10.8 Influence of temperature rise on the frequency ratio of the angle-ply laminated plate with the SS1 boundary condition.

Figure 10.9 Variation of the frequency ratio of the angle-ply laminated plate with temperature rise for the SS2 boundary condition.

Figure 10.10 Variation of the frequency ratio of a angle-ply laminated hybrid plate [P/45/45/45/-45/P] with temperature for the SS1 boundary condition.

Figure 10.11 Variation of the frequency ratio of the angle-ply laminated hybrid plate [P/45/45/45/-45/P] with temperature for the SS2 boundary condition.

Dynamic response of laminated composite plates fitted with piezoelectric actuators

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10.5.4 Effect of control voltage on the frequency ratio of angleply laminates embedded with a piezoelectric layer The variation of the frequency ratio of the hybrid plate [P/45/-45/-45/45/P] with control voltage for the SS1 boundary condition is tabulated in Table 10.6. From this table it can be seen that the frequency ratio is increasing in the range of 0.3% and 0.7% for voltages of 0 and 100 V, respectively, as compared to 2100 V and for the Wmax/h ratio of 1.2. There is no significant variation in the frequency ratio with the increment in control voltages. The influence of control voltage on the frequency ratio of the hybrid plate is depicted in Table 10.7. As observed from this table, for an amplitude ratio of 1.2, there is an increment of 0.43% and 0.89% in the frequency ratio for the control voltages of 0 and 100 V, respectively, as compared to 2100 V. When the hybrid plate is subjected to negative voltage, the fundamental frequency increases because of the pinching action of the piezoelectric material, which causes stretching of the plate, thus enhancing stiffness.

Table 10.6 Variation of the frequency ratio (ωNL/ωL) of the hybrid plate [P/45/-45/-45/45/ P] with control voltage for the SS1 boundary condition. Amplitude ratio (Wmax/h)

0.2 0.4 0.6 0.8 1.0 1.2

Control voltage 2100 V

0V

100 V

1.013 1.051 1.1113 1.1903 1.2843 1.3901

1.0132 1.0517 1.1128 1.1928 1.2879 1.3949

1.0134 1.0524 1.1143 1.1953 1.2917 1.3999

Table 10.7 Variation of the frequency ratio (ωNL/ωL) of the hybrid plate [P/45/-45/-45/45/ P] with control voltage for the SS2 boundary condition. Amplitude ratio (Wmax/h)

0.2 0.4 0.6 0.8 1.0 1.2

Control voltage 2100 V

0V

100 V

1.0084 1.0332 1.0731 1.1264 1.1912 1.2649

1.0086 1.0339 1.0747 1.1292 1.1953 1.2704

1.0088 1.0347 1.0764 1.1321 1.1996 1.2762

392

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New Materials in Civil Engineering

Conclusion

In order to examine the influence of various parameters such as temperature, lamination sequence (i.e., cross-ply and angle-ply), and control voltage on vibration characteristics of composite plates embedded with piezoelectric materials, an accurate nonlinear analysis has been performed. According to MFSDT, the governing equations are derived. Through von Karman strains, the geometric nonlinearity is introduced. The Galerkin’s weighted residual technique is employed to derive the final governing equation and is solved using a direct iteration technique. Numerical results have been presented for laminated composite plates and piezoelectric laminated plates subjected to electrical and thermal loads. The results revealed that the angle-ply plate embedded with piezoelectric layers shows higher linear fundamental frequency compared with the cross-ply plate. Significant variation is noticed in the piezoelectric plates under thermal loads, whereas the electrical load has a minor effect on the frequency ratio. The plate with SS1 boundary conditions has a greater frequency ratio as compared with the SS2 boundary conditions.

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[29] K. Nguyen-Quang, T. Vo-Duy, H. Dang-Trung, T. Nguyen-Thoi, An isogeometric approach for dynamic response of laminated FG-CNT reinforced composite plates integrated with piezoelectric layers, Comp. Meth. Appl. Mech. Eng. 332 (2018) 25
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148.

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Kwok Wei Shah, Ghasan Fahim Huseien and Teng Xiong Department of Building, School of Design and Environment, National University of Singapore, Singapore

11.1

Introduction

Cement industries that produce the main constituent of concrete remain the foremost concern to the world in terms of contributors to climate change and harming sustainability. In addition to the United States, the fast-developing nations such as China, Indonesia, India, and Turkey are also causing major environmental pollution. According to Robert Hutchinson, above 4 billion metric tons of ordinary Portland cement (OPC) that are produced annually from numerous cement industries worldwide as the basic constituent of concrete are largely responsible for a major part of the CO2 footprint [1
4]. The identification of satisfactory and realistic alternatives is a challenging task. In the civil engineering sector, OPC is broadly used as an efficient binder in concrete and other construction materials. Meanwhile, OPC manufacturing is commonly accepted as the main provider of emitted greenhouse gases in the atmosphere [5
8]. International Energy Agency (IEA) reports suggest that this amounts to 6%
7% of total CO2 emissions [8
11]. By the year 2050, the worldwide demand for OPC is expected to be increased by nearly 200% [8]. Mitigation of CO2 emissions from OPC-related activities requires new types of sustainable, smart, and environment-friendly self-healing materials [12,13]. It is known that cement materials have very low resistance to aggressive environments and that is one of the most severe problems affecting the durability and service life of concrete structures in natural climates. The effect of an aggressive environment can manifest in the form of expansion and cracking of concrete. Sometimes, the cracking of concrete may cause serious structural problems [14,15]. In this sense, the continuous increase in the durability requirements of concrete structures has led to the development of smart concretes (using self-healing technology) where excellent durability is mandatory [16]. Generally, self-healing is beneficial for the materials’ durability. Particularly, it is advantageous when human interference is difficult, such as in construction purposes in the midst of harsh physical and chemical environments. Nanomaterials invariably reveal excellent functional attributes. Compared to ordinary materials, nanomaterials degrade faster due to the presence of numerous interfacial atoms. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00011-9 © 2020 Elsevier Inc. All rights reserved.

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Many functional nanostructures can be combined to fabricate diverse nanosystems, wherein some components can also be incorporated to offer self-healing actions. This strategy is simplistic compared to the design of a more robust nanosystem [17]. Of late, due to the rapid advancements in nanoscience and nanotechnology, the design and fabrication of self-healing materials has opened new frontiers, wherein materials with particle sizes below 500 nm are described as nanomaterials. Self-healing materials can recover from damage autonomously. In many circumstances, the self-healing action can also be prompted via temperature, such as external stimuli; systems with this capacity are called nonautonomic self-healing materials [18]. The chapter is organized into three main sections. Section 11.2 discusses the importance of self-healing concrete toward more sustainable and smarter growth in building sectors than existing traditional concretes in terms of enhanced environmental friendliness and pollution reduction. Section 11.3 highlights some strategies used for self-healing processes that disclosed repairing mechanisms with sophistication, elegance, and efficiency. Various mechanisms of self-healing, benefits, and drawbacks of every strategy are emphasized and compared. Section 11.4 deals with self-healing systems as a feasible solution for self-recovery mechanisms when active concretes are exposed to corrosion, deterioration, degradation, and cracking.

11.2

Sustainability of traditional ordinary Portland cement-based concrete

11.2.1 Cement-generated environmental problems Universally, OPC has been continually exploited as a concrete binder and for different building substances. It is known that production of OPC-based concretes needs a huge amount of fuel and raw ingredients that are acquired via resource mining and energy-exhaustive processing [9]. This, in turn, causes great quantities of greenhouse gas (essentially CO2 and NOX) emissions into the atmosphere. Per ton of OPC manufacturing, almost 1 ton of CO2 is created by consuming 2.5 tons of raw materials and fuel [19
22]. Some estimates have revealed that approximately 1.35 billion tons of CO2 are emitted annually from the OPC production industries alone, accounting for nearly 7% of the total greenhouse gases released to the environment [8,23]. Compared to CO2 levels in 2010, the CO2 emissions from OPC industries into the atmosphere will be a major environmental concern because the global demand for OPC is predicted to grow by almost 200% by 2050 [8]. Over the years, numerous strategies have been adopted to reduce OPC manufacturing, including the durability enhancement of concrete and elongation of service life-span. From this standpoint, self-healing concrete has emerged as one of the most promising solutions to mitigate OPC-generated environmental pollution.

11.2.2 Concrete durability Characteristically, the serviceability of construction materials has considerable economic significance, particularly with regard to modern infrastructures and

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components. For urbanization, concrete materials that are greatly exploited must meet the standard codes of practices requirements related to strength and durability [24]. For instance, poor planning, low capacity or overload, faulty material design and structures, wrong construction practices or unsatisfactory maintenance, and lack of engineering knowledge can often diminish the service life span of concrete under operation [25]. In the construction industries the rapid decline of concrete structures is a major setback necessitating additional improvement. Varieties of physical, chemical, thermal, and biological processes are responsible for the progressive deterioration of concrete structures during their service [7,26]. Several studies [27
29] have revealed that concrete performance is greatly affected by improper usage, and the physical and chemical conditions of the environment. It is verified that both external and internal factors involving physical, chemical, or mechanical actions are often responsible for the deterioration of concrete structures. Mechanical damage to concrete structures occurs for different reasons such as impact, abrasion, cracking, erosion, cavitation, or contraction. Chemical actions that cause a decline in concrete attributes are carbonation, reaction associated with alkali and silica, and alkali and carbonate, as well as efflorescence. Moreover, outside attacks by chemicals happen primarily due to CO2, Cl2, and SO4, as well as several other liquids and gases generated by industries. Physical causes of deterioration include the effects of high temperature or differences in the thermal expansion of aggregate and of the hardened cement paste. Another reason for deterioration is the occurrence of alternating freezing and thawing of concrete and the associated action of deicing salts. Physical and chemical processes of deterioration often act in a synergistic way, including the influence of seawater on concrete. Poor durability performance of OPC in aggressive acidic or sulfate (especially marine) environments is caused by the existence of calcium complexes. These calcium complexes are very easily dissolved in an acidic atmosphere, leading to enhanced porosity and thus fast deterioration [30]. In many places worldwide, OPC structures that have existed for many decades are facing rapid deterioration [31]. Definitely, the permanence of OPC is linked with the nature of concretes’ constituents, where CaO of 60%
65% and the hydration product of Ca(OH)2 at nearly 25% is responsible for rapid structural decay. Several studies have indicated that the occurrence of a fast reaction of Ca(OH)2 in acidic surroundings allows OPC to be deprived of water, leading to acid fusion and weakening of resistance against aggressive attack. The intense reaction of evolved CO2 with Ca(OH)2 contributes to rapid corrosion of concretes containing OPC [31]. The safety, service life, permanence, and life span of the mix design of concretes are considerably influenced by crack development and subsequent erosion. These distinguished drawbacks of OPC-based concretes have driven researchers to enhance the properties of conventional OPC by adding pozolanic materials, polymers, and nanomaterials so that it becomes more sustainable and endurable. The immediate consequence for affected concrete structures is the anticipated need for maintenance and execution of repairs [32]. Thus, there is renewed interest in the development of sustainable concrete to solve all these existing shortcomings involving harsh environmental conditioning and durability.

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11.2.3 Energy problems in cement industries Nowadays, cement is the primary workhorse in the construction sectors worldwide. It is processed at high temperature (  1450 C) in a rotary kiln by mixing limestone (LS) or chalks with clay. Next, the yielded hard nodules of clinker are crushed with a low amount of gypsum using a ball-milling technique. Firing of such cement constituents at higher temperature consumes substantial amounts of energy (burning of coal or petroleum coke). Rapid decay of land setting, generation of dust during transport, creation of noise throughout the quarrying and processing of raw materials are regarded as major environmental concerns in OPC production. According to previous publications [28,33], OPC manufacturing without considerable CO2 release is impossible. Actually, the CO2 emission occurs in two phases including fuel burning (to achieve very high temperatures in the kiln) and calcining (a chemical reaction that occurs during LS firing). Until now, even the most efficient cement production plant emits 60% or more CO2 from various unavoidable chemical pathways. The consumption of a considerable quantity of energy during crushing of cementitious raw materials in the clinker phase remains the foremost ecological problem in the OPC industries [34,35]. Developing autonomic self-healing cement-based materials must be sought as a practical solution to the existing problems.

11.3

Self-healing concrete

11.3.1 Sustainability of smart concrete Low carbon emission and energy-saving building materials incorporated with smart materials (in self-healing technology) is a well-known candidate energy technology in enhancing the energy efficiency and sustainability of buildings. The major aim of sustainable development is to make life on Earth sustainable for the future with absolute support or care so that the ecological balance is not disturbed [36]. Sustainability is founded on three basic elements: economic security, environmental safety, and societal benefits. Sustainable advancement must preserve these factors to protect biodiversity with a balanced ecosystem. In the current growing industrial era, engineers, scientists, policymakers, and architects are attempting to use a sustainable model resourcefully to reduce the negative impacts on our ecosystem. Therefore, within the perspective of building materials, the phrase sustainability is used synonymously with a robust or friendly and green environment [37,38]. From this viewpoint, self-healing materials have attracted increasing interest due to their potential to reduce the degradation, prolong the functional lifespan, and suppress the maintenance costs of materials [39,40]. However, the self-healing technology contributes directly to enhancing the environment and reducing pollution while increasing concrete life-span and reducing the demand for and consumption of OPC as well as saving energy and increasing the sustainability of concrete.

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11.3.2 Life cycle analysis of self-healing concrete Over the past decade, the self-healing technique has been extensively studied and introduced as an innovative technique for concrete crack self-repair. However, several innovative strategies to self-healing for cementitious materials have been proposed and developed. Nowadays, this is known as the life cycle assessment (LCA) methodology which has been standardized in ISO 14040
14044. Thus far, it has been used for assessing the environmental impact of all kinds of products and services, preferably from the cradle to the grave. The goal of this LCA was a quantification of the environmental impact reduction that could be achieved by using the proposed self-healing concrete instead of a more traditional concrete. Some direct benefits of concrete self-healing include a reduction in the rate of deterioration, extension of service life, and reduction of repair frequency and cost over the life cycle of the concrete infrastructure. These direct benefits may be expected to lead to enhanced environmental sustainability as fewer repairs implies a lower rate of material resource usage and a reduction in energy consumption and pollutant emission in material production and transport, as well as that associated with traffic alterations in transportation infrastructure during repair/reconstruction events [41]. Van Belleghem et al. [42] reported that the self-healing of cracks with encapsulated polyurethane precursor formed a partial barrier against immediate ingress of chlorides through the cracks. Application of self-healing concrete was able to reduce the chloride concentration in a cracked zone by 75% or more. The service life of selfhealing concrete in marine environments could reach 60
94 years as opposed to only 7 years for ordinary (cracked) concrete. However, LCA calculations indicated important environmental benefits (56%
75%) that were mainly induced by the achievable service life extension.

11.3.3 Mechanism of self-healing in cementitious materials In the human body, injured surfaces (skin) and tissues are self-recovered due to the assimilation of nutrients that creates new substitutes to repair the injured parts. Likewise, in the self-healing of cement-based materials, essential products (acting as nutrients) are supplied to heal the cracks in damaged concrete. Over the past few decades, intensive investigations have been performed to explore novel strategies for efficient self-healing with durability. Fig. 11.1 provides a schematic presentation involving the developed self-healing strategies for cementitious materials.

11.3.3.1 Hollow fibers Hollow fibers store some functional material components (acting as healing agents) inside the empty spaces that form a composite network matrix [43,44]. When a concrete structure is damaged and develops cracks under certain external stresses or stimuli these functional elements, so-called healing agents, flow out of these hollow spaces to repair the cracks instantly. Analogous to the arteries in a living system, self-healing processes use hollow fibers implanted with engineering materials

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Figure 11.1 Developed strategies for self-healing in cement-based materials.

varying lengths depending on the damaged structures being repaired [44,45], for instance, bulk polymers used in polymeric composites for self-healing purposes [46
49]. It has been shown that these hollow fibers or tubular structures are greatly effective for discharging the healing agents upon encapsulation [50,51]. To identify easily and rapidly the internal damage in composite structures, Pang and Bond proposed a method called damage visual enhancement [49]. It was demonstrated that fibers packed with engineered healing materials and tagged with fluorescent dye can easily monitor the repair progression. The idea of a biological self-healing mechanism such as bleeding can now be applied to repair the cracks of cementitious materials [52,53] as appropriate. Using this concept, the functional components (healing elements) can be packed inside the fragile fibrous vessels dispersed inside the concrete network. With sustained damage these fragile fibers are broken and functional repair materials are released to initiate the self-repair. Liquid methyl methacrylate (MMA; a methyl ester of methacrylic acid and a reactive resin) was embedded within hollow polypropylene fibers and then encapsulated inside the concrete [53], which could release MMA and reduced the concrete porosity. Some studies were conducted on the release of crack-holding cementitious glue from hollow pipettes of glass inside the concrete

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immediately following the flexural test. Such concrete structures packed with glue/ adhesive components were found to display about 20% greater load-carrying capacity than those without glue following the flexural testing. Several researchers [52,54,55] independently clarified the buoyant process involving self-healing. To get a basic insight into the self-healing processes, hollow fibers (pipettes) were positioned within a cementitious network matrix, where one terminal was connected to the self-healing mediator and the other end was fastened. In this study, concrete mixes were designed [54] and glass tubes with respective external and internal diameters of 2 mm and 0.8 mm were positioned within the specimens. Both dilute (27%) and nondilute alkali-silicate mixtures and two components combined with epoxy resin of low viscosity were utilized for the self-healing component. Subsequently, this complex solution was loaded until the crack mouth opening displacement (CMOD) reached 0.03
2 mm after the elimination of the load. Next, these cracked samples were again cured to examine the improvement in self-healing ability. Compared to normal specimens (without healing agent) specimens that contained the dilute and nondilute alkali-silica solution as the healing component correspondingly revealed a mean ratio of the strength recovery of 1.1 and 1.5. Conversely, specimens in the presence of epoxy resin displayed little improvement in the strength recovery ratio, which was much lower than the value that was obtained by direct mixing of resin and manual injection into the cracked regions (about threefold better recovery). This observation was majorly ascribed to the inadequate blending and stirring of the two constituents, which produced a low healing ratio due to less hardening of resin. It could also be due to the presence of some residual epoxy that remained inside the pipes because of the sealing at one end. According to Joseph et al. [55], the testing regime was analogous except for some insignificant distinctions. Bent plastic tubes with respective external and internal diameters of 4 mm and 3 mm were employed as the vehicle for a healing mediator (ethyl cyanoacrylate). It was concluded that supply of such a healing component externally could achieve self-healing successfully. After healing of the damaged area the stiffness after cracking, the peak load, and the ductility were enhanced considerably. Results during and after testing revealed that ethyl cyanoacrylate as an adhesive could penetrate a large area of the fractured surfaces due to capillary suction and gravitational effects.

11.3.3.2 Microencapsulation Over time, manmade materials for encapsulation have been developed from natural examples spanning from the macro- to nanoscale. Birds’ egg or seeds are the simplest illustration of natural encapsulation on a macroscopic scale and a cell within it is an example on a microscopic scale [56
58]. The growth of microencapsulation started with the synthesis of capsules comprised of dyes, which was then included in paper for copying and replacing carbon-paper [59]. Over the passage of time, several novel technologies have been developed in diverse areas. Microencapsulation is not considered as a disconnected product or part of it, but is rather defined as

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a development including micrometer-sized solid granules or liquid drops or gases within an inert shell, isolating and protecting them from unnecessary reactions in the outside surroundings [60,61]. The implementations of microencapsulation with self-healing products have been established [60,62]. Nishiwaki [63] reported the filling of microcapsules using epoxy resin as a healing material where the shell of urea
formaldehyde formalin in the size range of 20
70 μm was incorporated. These microcapsules plus acrylic-resin (a hardener in the form of a gelatin shell of size range 125
297 μm) were employed. It was shown that microcapsules incorporated with sodium silicate were effective for selfhealing [64]. In this work, concrete was first stacked to almost the breaking point before the load was removed and then cured for 1 week. Results revealed that concrete with 2% sodium silicate microencapsulation could recover as much as 26% of its actual strength compared to the reference sample (recovery of 10% only). It was concluded that by increasing the healing material’s amount the upper strength recovery ratio could be attained. Du et al. [65] inspected one-component microcapsules prepared with toluene-di-isocyanate (TDI) as the healing agent and paraffin as the shell. The results indicated that the TDI was successfully encapsulated in the paraffin shell and the mortars with the microcapsules showed more favorable selfhealing capacity.

11.3.3.3 Expansive agents and mineral admixtures Kishi et al. [66] reported the creation of cementitious materials such as Al2O3
Fe2O3-tri (AFt), Al2O3
Fe2O3-mono (AFm), and calcium carbonate (CaCO3) in cracked concrete and air voids of Ca(OH)2 crystals. It was assumed that such hydration yields could leach out and recrystallize in the water flowing through the fractures. According to this idea, Kishi and co-workers [66
68] evaluated selfhealing concrete performance with different healing agents. Expansive agents, geomaterials, and chemical mixtures together with their blends were included in the investigation of agents [68,69]. A comparison of the reference specimen was made with a specimen containing 10% cement substituted by expansive materials comprised of C4A3S, CaSO4, and CaO (lime). It was revealed that concrete beams containing expansive agents could almost heal an early crack of 0.22 mm after 1 month, in which the rehydration yield among cracks was identified. Nevertheless, for the ordinary concrete structure these cracks were partially healed during the same time period. Therefore, recrystallization of expansive agents in air voids for self-recovery was extra efficient compared with normal concrete [66]. Tanvir Qureshi et al. [29] reported that the expansive minerals improved the self-healing capacity of concrete mixes. The autogenous self-healing performance of any particular cement mix could be numerically predicted based on their hydration degree, which is an indicator of the cement mix state at any particular age. It was also demonstrated that a geo-material containing SiO2 (71.3%) and Al2O3 (15.4%) when added to the expansive material could form a geo-polymer via polymerization of aluminate and silicate complexes individually [67]. Subsequently, these were dissolved at high pH due to the existence of alkali metals. A thorough

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investigation revealed that the size of the geo-polymer gel particles was below 2 μm and the cracked interfacial phases of the original ruptured zone generated numerous hydro-garnet or AFt phases. Thus, the presence of an expansive agent played a decisive role in bridging those cracks. Furthermore, EDX spectra illustrated that the majority of the modified geo-polymer gel existed in a dense phase rather than the hydro-garnet. Further study into chemical additives [67] indicated that the inclusion of NaHCO3, Na2CO3, and Li2CO3 to standard concrete could enhance cementitious re-crystallization and precipitation of particles in concrete. It was concluded that, using the correct amounts of carbonates and expansive agents, the self-healing attributes of cracks could considerably be improved.

11.3.3.4 Bacteria as a self-healing agent In recent years, the utilization of biological techniques for repairing was proposed by introducing bacteria inside the concrete [70
74]. In the mid-1990s Gollapudi et al. [75] conducted a study and introduced an environment-friendly approach to repairing concrete cracks, where ureolytic bacteria were incorporated to accelerate the precipitation of CaCO3 in the microcrack areas in concrete. The microbes assisted the precipitation of CaCO3, which was characterized using several parameters such as dissolved inorganic carbon contents, material pH, calcium ion concentration, and the accessibility of nucleation sites (accessible localized areas of reaction). The bacterial cell walls acted as nucleation sites and bacterial metabolism controlled the other factors [71]. Van Tittelboom and coworkers [71] used bacteria in concrete to generate an enzyme called urease that could catalyze urea [CO(NH2)2] into ammonium ions (NH41) and carbonate radicals (CO22 3 ). In the chemical reactions, 1 mol of urea underwent intracellular hydrolyses to 1 mol of carbonate and 1 mol of ammonia following path I. Then, carbamate was hydrolyzed spontaneously to form one extra mole of ammonia and carbonic acid via path II. These products later formed 1 mol of bicarbonate (HCO23) and 2 moles of ammonium (NH14) and hydroxide (OH21) ions (paths III and IV). Paths IV and V were responsible for the enhancement of pH, drifting the bicarbonate equilibrium to form carbonate ions. COðNH2 Þ2 1 H2 O ! NH2 COOH 1 NH3 NH2 COOH 1 H2 O ! NH3 1 H2 CO3

Path I Path II

H2 CO3 1 H2 O ! HCO23 1 H11

Path III

2NH3 1 2H2 O ! 2NH14 1 2OH21

Path IV

14 HCO23 1 H11 1 2NH14 1 2OH21 ! CO22 1 2H2 O 3 1 2NH

Path V

The bacteria could accept cations (with Ca12 deposited on the surface of the cell wall) from the surroundings because of their negatively charged cell walls. Then,

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the Ca12 could react with the CO22 3 and lead to precipitate CaCO3 at the cell wall surfaces which offers an active nucleation site (paths VI and VII). Using this approach of localized precipitation of CaCO3 via bacteria, the cracked surfaces could be repaired. Ca12 1 Cell ! Cell-Ca12 Cell 2 Ca12 1 CO22 3 ! Cell-CaCO3

Path VI Path VII

11.3.3.5 Shape memory materials as a self-healer Several researchers [76
79] have revealed the usefulness of functional material incorporation in cementitious concrete structures to achieve efficient self-healing. These candidates include shape memory alloys (SMAs) or shape memory polymers. The fundamental idea is that when cracks are formed then such materials with short predefined memory shape could shrink in a controlled condition, thereby generating a contraction to optimally act as a crack closure agent. Chang and Read [78] identified a reversible phase change in gold
cadmium (Au-Cd) alloy, which was referred to as the first observation on shape memory effect [79]. Over the years, several SMAs have been realized with excellent thermomechanical and thermoelectrical features [80]. For instance, Nitinol reveals superelasticity together with a shape memory effect by returning back to its predetermined shape under heat treatment. In superelasticity it undergoes an exceedingly large inelastic deformation and recovers the shape soon after unloading [81]. Wires of SMAs was used by Song and Mo [82] to create intelligent reinforced concrete (IRC). The post-tension effects in IRC were achieved by utilizing stranded martensite wires of SMA. The alteration in the electrical resistance of SMA wires was monitored to obtain the strain distribution within the concrete. Any crack developments due to explosions or earthquakes could be identified, wherein electrical heating of the SMA wires could reveal contractions and reduce the cracks, thus handling macrocracks by selfrehabilitation. The concrete structure was intelligent enough to detect selfrehabilitation, and thus the name was coined. Sakai et al. [76] examined the self-restoration process in concrete beams by superelastic wires made of SMAs. The results showed that the mortar beam containing SMA wires could recover almost totally after sustaining a very large crack. Likewise, shape memory polymers (SMPs) have been developed by Jefferson et al. to incorporate into cementitious materials [78]. When a crack arises in the cementitious matrix because of early age shrinkage, thermal influences, and/or mechanical loading, then the shrinking process in the included SMA tendons can be stimulated by heating. This, in turn, can repair the cracks by developing measurable compressive stresses across the closed crack surfaces. Such a crack-closing mechanism can improve the natural self-healing processes and the durability performance of the structural elements in concrete. It was concluded that post-tensioned mortar beams via aligned shrunk polymer tendons could be effective for crack closing and weak

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prestressing. Results from several screening tests revealed that the most efficient tendons were polyethylene terephthalate (PET) Shrinktite with a shrinking potential of about 34 MPa in controlled situations with 90 C heating and subsequent cooling down to room temperature. The influence of heating together with extra curing could enhance the mortar’s strength by around 25%.

11.3.3.6 Coating Recent developments in materials technology provide extra functionalities leading to the term “intelligent material” and are characterized by the desired response to some external stimuli such as temperature, light, and humidity. Innovative materials for buildings (coatings) have been developed and tested successfully [83,84]. Amongst these materials we may find coatings for self-healing concrete with specific durability properties. The technique of a self-healing coating used with reinforcement concrete for steel bar self-healing reduced corrosion damage. Selfhealing coatings are a recent avenue of research that could provide a method to fight the deterioration of modern infrastructures. Conventional anticorrosive coatings have limited effectiveness if even a small portion of the coating is damaged [85]. Self-healing coatings, however, can continue to function due to their ability to heal after fracture [86]. In effect, this self-repair is anticipated to be able to greatly prolong the life of steel rebar structures. An initial investigation into application on steel rebar was conducted by Chen et al. [87]. This technology could be applied to rebar structures that are typically coated with epoxy coatings in the highly corrosive areas of the northeast Asia to combat rapid corrosion rates. In the past, many studies have been conducted [12,13,26] to determine the mechanism of protecting concrete roads, bridges, and other structures from the development of tiny cracks. However, no conclusive decision has been made and thus it remains a major issue. Cracks allow water, salt used for deicing, and air to enter concrete. During the winter season, frozen water inside the cracks expands and makes them larger, speeding up the concrete’s deterioration in the presence of road salt. Many investigations have been conducted into self-healing anticorrosive coatings for protection of metals but no work has been performed on self-healing protective coatings for concrete. Self-healing coatings that initiate self-healing through external cracking or damage have undergone substantial research. These coatings often have microcontainers mixed in that can rupture easily when acted upon. The containers hold healing agents that then seal the crack and prolong the functionality of the coating. Containers that hold these healing agents can vary from polyurethane microcapsules to microfilament tubes and often have little effect on the mechanical properties of the coating. Research in this area has provided good results and applications in realworld environments can be further investigated [86,88,89]. Many types of organic and inorganic materials are used for this purpose. Epoxy coatings are widely used to prevent corrosive agents, such as water and salts, from coming into contact with rebar. The development of polymer coatings that react to environmental stimuli, such as heat or pH changes, to initiate crack healing has also been investigated [90,91]. Coatings that react to heat have achieved success, with

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some retaining all of their mechanical properties after multiple healing cycles [92]. As polyelectrolyte nanocontainer coatings, changes in the pH of the solution have been able to set a response within seconds [93]. Full recovery of the mechanical properties of the coatings was also achieved, making it a promising option for research in the future [93]. Drying oils such as tung oil and linseed oil have received significant investigations regarding their healing properties and encapsulation [94
96]. When exposed to air, tung oil will polymerize into a tough, glossy, waterproof coating. These characteristics have made drying oils a valuable component in paints, varnishes, and printing inks. Tung oil encapsulation was first achieved by Samadzadeh et al. [97]. These urea-formaldehyde microcapsules displayed good adhesion to the epoxy matrix, compared to industry standards, by testing the microcapsules pull-off strength following ASTM D4541-09. Testing of damaged samples to measure the service life through immersion in sodium chloride solutions was conducted. The results were promising, as the tung oil microcapsules extended the service life up to ninefold that of epoxy coatings after damage.

11.3.3.7 Engineered cementitious composites In the early 1990s, the special class of concrete called ultraductile fiber reinforced cementitious composites was designed. Originally, it was known as engineered cementitious composite (ECC) [98], which has been constantly improved over the last two decades [16,98]. This material possesses high ductility (3%
7%), tight crack size (about 60 μm), and comparatively low fiber content (roughly 2% or less by volume) [99]. The metal-like feature is the most significant mechanical character of ECC. It can sustain elevated loads after initial cracking during auxiliary distortions. Li et al. [100] examined the self-repairing idea of Dry [52] related to bleeding which was used to discharge chemicals that could close the tensile cracks with subsequent air curing. This mechanism led to regaining the mechanical properties of the uncracked composites. It was acknowledged that the self-healing process was ineffective for usual concrete, cement, or concrete reinforced with fiber. This was due to the difficulty in controlling the size of the tensile cracks in these materials. Local breakages could multiply continuously inside the crack width under declining tensile load and exhaust quickly the accessible chemicals responsible for crack closure and rehealing. Therefore, effective self-recovery requires control of the tensile crack width, which must be restricted within tens of microns. Otherwise, very large glass pipes are needed to modify the mechanical characteristics of the composites. This argument was consistent with several other reports, which emphasized the role of crack width [101
103]. The tightly controlled crack size is one of the inherent characteristics that ECC can offer advantages for when applying the self-healing idea. Later, two different experiments were conducted [30] to examine the viability of the proposed bleeding notion. In the first experiment, the ECC materials with a single empty glass fiber free of any healing chemical were tested in situ at a loading stage under an FESEM. The idea was to confirm the sensing and actuation properties of ECC. The other experiment was performed to measure the flexural strength of ECC containing glass fibers packed with sealing material (ethyl cyanoacrylate). Both studies were

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performed under MTS load-frames to ascertain the rehealing effectiveness of the sealing mediator after sustaining damage (fracture) in the load-cycles. The sensing and actuation processes were authenticated using FESEM images and the influences of revitalization were authenticated via flexural stiffness regain. It was also acknowledged that several other issues needed further investigation before practical applications could be made. Li et al. [50,104,105] also examined the self-healing phenomena of cementitious materials in addition to the inclusion of external chemicals as healing glue in concrete. The concrete matrix and its interaction with the exposed surroundings were examined. The ECC specimens were precracked and exposed to various environments such as water permeation and submersion, cycles related to wetting and drying, and chloride ion attack. The results revealed that the mechanical and transport characteristics was principally recovered, particularly for ECC preloaded with tensile strain below 1%. Apart from the tiny cracks, the small water/binder ratio and the presence of a great quantity of FA in the mixture could support the self-recovery action through hydration and pozzolanic mechanisms. The ECC materials were locally developed from waste matter and/or byproducts [106]. Several mixtures were designed using powders of blast furnace slag (BFS) as well as LS and subsequently analyzed by measuring the tensile strain (2%
3%) and stiff cracks (with typical size ,60 μm). It was found that [107] the mixes designed by Zhou et al. [106] were dissimilar from the one cast by Li et al. [50] in terms of basic constituents (high proportion of BFS and LS instead of FA) and a very high water to binder ratio in the range of 0.45
0.60 against 0.23. This indicated that the quantity of unhydrated cementitious materials cured later than 28 days was much below that used by Li et al. [100]. It was shown that ECC specimens made from a great quantity of BFS and LP, and a reasonably high ratio of water to binder that could restore comparable self-repairing properties than ECC materials analyzed with a high proportion of FA and low water to binder ratio. This observation was ascribed to the presence of tight crack width, wherein the selfhealing of ECC was decided by the availability of unhydrated cement and other complementary products including BFS. Therefore, the low ratio of water to cementitious material ratio and high fraction of cementitious specimen were useful in supporting the self-recovery process. The significance of crack size was underscored in the context of constant hydration-dependent self-recovery because it consumed a very low amount of healing materials to close the cracks and it was easy for the healing specimens to develop from both faces of the cracks for joining. Commonly, the microcrack behavior and the cracking opportunities of ECC in the presence of microencapsulated modules were found to improve where healing materials were released. This assured the efficiency of the sensing and actuation processes through microencapsulation. The tight crack size in ECC is highly significant because it needs low quantities of healing products for crack filling and it is efficient for the healing products to develop from both fac¸ades to connecting these cracks. In short, ECC possesses a higher fraction of cementitious materials and a lower water to binder ratio than conventional cements, and thus it is a more effective self-healer. Table 11.1 lists the benefits and drawbacks of various self-healing concrete strategies developed in the past.

Table 11.1 Comparison of various self-healing concrete strategies Mechanism References

Strategy

Precursors

Products

Merits

Demerits

[48,49,53, 108,109]

Internal encapsulation and hollow fiber

Epoxy/polymer hardened

Resin hardened

[110]

External supply system

Epoxy/polymer hardened

Resin hardened

Complexity in casting. Negative influence on the mechanical characteristics of the cement matrix in the presence of a large number of hollow fibers. Probable intricacy in the release of healing materials Low recovery ratio, which was much lower than direct mixing of resin and manual injection into the cracks. Later one revealed three times better healing

[39,68]

Microcapsule

SiO2 Al2O3 TiO2 Fe2O3

C-S-H gel calcium nitrate

[111
113]

Expansive agent and mineral admixtures

CSH2, CH, C4A3S

C-(A)-S-H gels

Healing systems are discharged when needed. Intermediate quantity of healing materials. Efficient under multiple damages Healing materials are discharged when needed. Adaptable high quantity of healing material Healing materials are discharged when needed. Response for multiple damages simultaneously. Possible effectiveness under multiple damage events Superior efficiency in healing. Excellent compatibility of the produced healing products with the cementitious matrix

Complexity for capsule making and casting. Inadequate healing system. The bonding among capsules and matrix is uncertain. Negative influence on the mechanical characteristics of the cement matrix in the presence of several capsules Unwanted growth under ill-treatment. Formation of healing products is uncertain. Not very useful in the presence of several damages

[71,110,114,115]

Bacteria

CO2, Ca(OH)2

CaCO3

Good biological effectiveness, free of pollution and natural

[116
118]

Shape memory materials

Enhanced ettringite formation

C-S-H gel

[119]

Coating method

Polymer epoxy

Reduction in the porosity

[120,121]

Nanomaterials

SiO2, Al2O3 TiO2, Fe2O3

C-(A)-S-H gels

Macrocracks can be tackled. Strong recovery in the mechanical behavior. Effectiveness toward many damage events Easy maintenance. Cheap. Decrease in corrosion problems Strong recovery in the mechanical behavior. Effectiveness toward many damages. For internal and external application. Cheap and saves energy. Excellent healing efficacy

Need several prerequisites. Measures must be taken to shield bacteria in concrete. Mechanical features recovery and efficacy for many damages may be a concern Expensive. Induction of healing via heat treatment leads to uncertainties

Many prerequisites to be met. Efficiency toward numerous damages may be a setback for external healing High safety requirements

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New Materials in Civil Engineering

Nanomaterials

11.4.1 Production of nanomaterials It is well known that nanoparticles produce a superior effect on filler than micronsized materials. Guterrez [122] acknowledged that all materials can be converted into nanoparticles via crushing or chemical treatment. The production accuracy of nanoparticles is decided by the purity and chemical constituents of the parent materials. Two routes are followed for the large-scale production of nanomaterials, including the top-down [123] and bottom-up approaches [124]. These approaches are chosen based on the appropriateness, price, and knowledge of the nanoscale properties of the material under consideration [125]. The milling technique falls in the category of the top-down approach, wherein the selection of milling machine is favored due to its accessibility, inexpensiveness, and feasibility of easy modification without requiring any chemical reagents or complex electronic equipment. In the top-down approach, big structures (bulk) are transformed to small ones (nanodimensions) while keeping their physical or chemical behavior intact via atomic level control [126], which has been applied at an industrial level. Using the milling technique, a high volume of nanoparticles can be produced. Though the top-down approach is contemporary for nanofabrication, the consistency and superiority of the yields are often unpredictable. To overcome such drawbacks, the milling techniques (a top-down approach) can further be improved by increasing the number of balls, ball types, milling speed, and nature of the jar, where the quality of nanoparticles is improved [127]. The high-energy ball-milling technique has been widely used to fabricate diverse nanomaterials such as nanoparticles, nanograins, nanoalloys, nanocomposites, and nanoquasicrystals. John Benjamin introduced this technique to produce oxide particles inside a matrix of nickel superalloys (1970). Utilizing a milling technique the properties of alloy components effective for high thermal structure were altered and the mechanical strengths were improved. Factors including fractures, plastic distortions/deformations, and cold welds during the milling process affect the material conversion into the desired morphology. Milling not only crushes the material into much smaller fragments but also blends numerous particles or materials to transform them into new phases with different compositions. Usually, the final yields of the milling technique are in the form of flakes where refinements are performed based on the selected ball and milling standard. Most of the nanomaterials (nanosilica, nanoalumina, nanoclay) utilized in concrete can be obtained via the bottom-up approach, which is adopted for materials engineering at atomic or molecular levels via the self-assembling process—so-called molecular nanotechnology or molecularlevel processing. It is applied indirectly in nanomaterials and chemical production [32]. Varied morphology of nanoparticles obtained via the bottom-up strategy is often customized through a chemical synthesis technique. In comparison to the topdown strategy, the bottom-up strategy can generate nanomorphology with greater uniformity and reproducibility. In addition, using the bottom-up approach one can produce novel nanocrystals with perfect atomic or molecular ordering. Bottom-up

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approach-based production of nanomaterials is useful for achieving high electronic conductivity, optical absorption, and chemical reactivity [128]. Through the bottom-up strategy, one can get much smaller sizes and formations of consistent surface atoms with modified surface energies and morphologies. Usually, the bottom-up technique is exploited to prepare self-healing and self-cleaning nanomaterials with improved catalytic properties, sensing capacity, and novel pigment characteristics. Moreover, the main drawbacks of the bottom-up scheme are related to the high running cost, need for experts for chemical synthesis, and limitation to laboratory orientation only [129]. The nanoparticles produced using the bottom-up approach are excellent for advance applications including electronic components and biotechnology.

11.4.2 Nanomaterial-based concrete In recent years, the growth in nanotechnology and the accessibility of nanomaterials suited for construction use, including nanosilica, nanoalumina, polycarboxylates, and nanokaolin, has improved concrete properties remarkably [125,130,131]. Intensive researches have revealed that the mechanical properties such as compressive strength, splitting tensile and flexural strength of cement pastes [132,133], mortars [130,134], and concretes [135] can be improved by a tiny quantity of nanomaterials. Early strengths of pastes, mortars [130,134], and concrete [135] in the presence of nanomaterials were reported to be much higher than those formulated with conventional OPC. The development of such higher strengths was ascribed to the faster cement hydration process and pozzolanic reaction, reduction of pore density, and enhanced interfacial bonding amid hardened cement paste and constituents (aggregates). Nanomaterials were also exploited to reduce the porosity and enhance the durability properties of concrete [136,137]. With developing concrete technology, the nanomaterials were used in many applications (Fig. 11.2) and the self-healing technology was one of the most important applications to produce sustainable and smart concrete.

Figure 11.2 Applications of nanomaterials in concrete.

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11.4.3 Production of nanoconcrete Using materials with a particle size less than 500 nm in concrete production as an admixture or part cement replacement creates nanoconcrete. It was shown that the strength of normal concrete tended to be enhanced with the inclusion of nanoparticles. The bulk properties and packing model structure of concrete can remarkably be improved by the incorporation of nanoparticles. Nanoparticles act as excellent filling agents through the refinement of intersection zones in cementitious materials and production of high-density concrete. The manipulation or modification of these nanoparticles in the cement matrix can render new-fangled nanostructures [138
140]. General deficiencies in the microstructures of concretes including voids, microporosity, and corrosion originating from the reaction of alkaline silica can be discarded. The advancement of nanomaterials occurred due to their characteristics as new binding agents with particle sizes much smaller than traditional OPC. This property enhances the hydration gel product by imparting a neat and solid structure. In addition, using a blend of filler and extra chemical reaction in the hydration scheme, high-performing novel nanoconcrete with enhanced durability can be achieved. The application of nanotechnology in concrete is still in its infancy. Evergrowing demand for ultrahigh-performance concrete (UHPC) and recurring environmental pollution caused by OPC has forced engineers to exploit nanotechnology in construction materials. Classical blends of UHPC with incorporated silica fumes can achieve enhanced strength and high durability. However, limited accessibility and high pricing of nanomaterials not only slowed down the growth of UHPC technology but also made it less demanding compared to conventional high-strength concrete (HSC). To overcome these limitations, nanotechnology combined with production of UHPC emerged in its own right wherein an alternative to silica fume was developed. Exploiting the nanoproduction idea, a typical nanomaterial mimicking the attributes of silica fume was designed. Nanosilica is the newest material in nanotechnology-based processing that has been used as a substitute for silica fume [141]. Using this celebrated nanosilica component, several types of nanoparticles have been synthesized which are effective for concrete production [142]. Nanoalumina [143], titanium oxide nanoparticles [144], carbon nanotube, [145] and nanopolycarboxylates [146] are the emergent nanomaterials in the new nanoconcrete era. The production and possible applications of nanomaterials are discussed next.

11.4.4 Significance of nanomaterials as a self-healer The contribution of nanomaterials in enhancing the workability, strength, and durability of building materials can affect the hydration kinetics of cement. Furthermore, the addition of nanomaterials can improve the performance of cement appreciably. The high performance of nanomaterials attracted researchers to apply nanomaterials to create sustainable concrete by self-healing technology. A small amount of healing materials was packed inside the microcapsules. The strength of bonds amid the microcapsules and the cement network were required to be stronger

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than that of the former. This led the researchers to resolve these problems and find another way to produce sustainable concrete with increased performance. In order to solve these problems, the merits of nanomaterials are attractive the researchers to use these materials for sustainable concrete.

11.5

Nanomaterial-based self-healing concrete

The advancement of self-healing materials in concrete industries is an emerging trend. The generic term self-healing concrete is used for cement-based materials that repair themselves after the material or structure is damaged by diverse deterioration mechanisms. Nowadays, using nanomaterials for sustainable concrete also receives a lot of attention around the world. Compounding the two technologies (self-healing and nanomaterials) together contributed to enhancing the durability of concrete and successfully led to producing sustainable concrete [147,148]. Most nanomaterial applications in self-healing have been used to control the corrosion of steel bars in reinforcement concrete. Koleva [147] reported the ability to improve reinforced concrete performance by incorporating nanoscale materials with tailored properties, that is, core
shell polymer vesicles or micelles in the cement-based system. Some researches have been conducted into using nanomaterials for selfhealing concrete. Qian et al. [149] studied the curing situations in the presence of air, CO2, and water in the wet state as well as the dry state. Furthermore, the effects of nanoclay with water (used as an inner water-furnishing agent for hydration) on microcracks were determined. The results revealed that the healing extent could be considerably better with the inclusion of nanoclay and better cementitious material concentrations for mixes. It was stated that for all studied air-cured mixtures, the final crack did not occur at the new location, indicating good healing. Despite the interior water supply from nanoclay it revealed comparatively weak strengthening, making it hard to relocate the final crack sites from preexisting ones in other positions. This unproductive recovery with air curing was also reported by Sun [62]. It was shown that the self-healing behavior of ECC with superabsorbent polymer (SAP) capsules with water as the interior pool for extra hydration was effective. Meanwhile, various repairing products were recognized across the cracked fac¸ades. However, noticeable healed cracks were absent completely. It was acknowledged that the cracks could very likely undergo the self-recovery mechanism without much effectiveness. The significance of moisture accessibility was highlighted that acted as the reactant for additional hydration.

11.5.1 Nanosilica In sustainable concrete development, nanosilica (SiO2) is a gifted nanomaterial broadly utilized in ultrahigh-performance concrete (UHPC). In general, nanosilica was manufactured from micron-sized silica. Strong reactions produced by

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nanosilica in UHPC are comparable to silica fume or microsilica involving their strength, performance, and durability improvement [133,150,151]. Qing et al. [133] demonstrated that concrete with nanosilica can gain strength earlier than with silica-fume. It was shown that the incorporation of nanosilica in concrete could enhance its workability when the inclusion of superplasticizers was at its lowest amount. In addition, the nanosilica particle size could act as an ultrafiller inside the concrete, leading to dense and refined microvoids to impart smart microstructures [152]. Other benefits involving nanosilica include the better control of water to cement ratio, wherein the strength can be customized easily. Quercia et al. [153] reported the incorporation of nanosilica at a certain dosage improved the strength of concrete and performed as a cement substitution component. The results showed the ability to replace the cement with 20 to 30% without effect on strength performance of prepared concrete. In self-healing applications, the capacity of nanosilica to react with available Ca(OH)2 in concrete and formulate (CSH gel) was examined [154
157]. Using nanosilica, the self-healing concrete technology as additional mineral admixtures or in nanoencapsulation methods has been studied. However, the disadvantage of nanosilica is the cost and lack of availability in some countries where nanosilica needs to be imported for use in the concrete industries [32].

11.5.2 Nanoalumina In cement hydration, it is known that silica and alumina are the two main components involved in formulating C-(A)-S-H gels with calcium. The role of silica inside cement was to alter strength, whereas alumina controlled the setting time. Nanoalumina is produced from alumina, where the use of nanoalumina in concrete is seldom reported. The inclusion of nanoalumina in concrete, particularly highperformance concrete, can enormously affect concrete characteristics because it regulates the setting time of cement [158,159]. The presence of nanoalumina within cement can accelerate the early setting time for high-performance concrete, which in turn diminishes the segregation and flocculation. In high-performance concrete mixes, any disturbance in the cement creates inhomogeneity and affects the working performance. Nanoalumina in high-performance concrete functions as a dispersive mediator in cement particulates [160,161]. In addition, nanoalumina can refine the porosity of hydration-gel products as nanofiller because the cement proportion in high-performing concrete is high. Therefore, grain size distribution in such concretes is essential simultaneously with silica-mediated hydration. Without nanoalumina-mediated refinement, the hydration mechanism is weaker as the silica component cannot penetrate the interior structure of the gel. Incorporation of nanoalumina can create a pathway for easy injection of the silica or binding materials into the interior microstructures of the hydration gel to start the refinement [162,163]. All these advantages mentioned above make nanoalumina one of the most important materials for future use in producing smart and sustainable concrete by using self-healing and nanotechnology.

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11.5.3 Carbon nanotubes Carbon nanotubes (CNTs) are carbon allotropes having a cylinder-shaped nanostructure. CNTs can be produced with length to diameter (aspect) ratios of up to 132,000,000:1, considerably greater than any other material. They possess atypical properties, which are valuable for applications in diverse fields of materials science and nanotechnology [32,122]. Particularly, due to their unusual thermal conductivity, and mechanical and electrical characteristics, CNTs can be used as additives for diverse structural applications. CNTs are affiliated to the fullerene family, in which the long and hollow construction the walls created by one-atom-thick sheets of carbon, so-called graphene, makes them distinctive. Such sheets can be rolled in definite orientations, where the revolving angle and radius together determine the attributes of CNTs [145]. Nanotubes are divided into single-walled carbon NTs (SWCNTs) and multiwalled carbon NTs (MWCNTs). Individual CNTs are aligned in the form of ‘‘ropes” that hold them mutually by van der Waals forces or via pistacking [164,165]. These ropes make chemical bonds in the CNT structure, where the bonds of CNTs are similar to graphite. These bonds are much stronger than those formed in alkanes and diamonds, making CNTs exceptionally strong. Flexibility is one of the significant merits of CNTs in producing sustainable concrete. Using CNTs, the sustainable concrete design may be changed into a distinctive or rigid type. The flexible nature of CNTs is advantageous for enhancing sustainable concrete strength. Compared to other nanomaterials, CNTs are superior in terms of flexibility improvement and strength enhancement of sustainable concrete [145]. Predominantly, the dimensions of CNTs are much smaller than other nanomaterials. The major function of CNTs in sustainable concrete is to advance the stress and compressive strength [68]. CNTs can be use in self-healing concrete using the engineered cementitious composite (ECC) method which contributes to producing sustainable and smart concrete for the future construction industry. Nevertheless, the deficiency of CNT resources may reduce the attention toward potential concrete applications for this material.

11.5.4 Polycarboxylates In the past, polycarboxylate (PCE) nanomaterials were utilized in concrete [166]. PCE is a polymer-based compound obtained from methoxy-polyethylene glycol copolymer. Usually, the carboxylate group is comprised of water molecules, rendering a negative charge along the backbone of PCE. The polyethylene oxide group provides a nonuniform electron cloud distribution and chemical polarities to the secondary or side reactions. The number and length of the secondary or side groups can easily be changed. In the case that the secondary or side reactions possess numerous electrons, it lowers the large molar mass and alters the polymer density, resulting in reduced performance of cement suspensions [32,167]. For both chains to combine and be paired simultaneously, longer side groups and strong charge density from one to the other reaction end must form. Usually, polycarboxylate is used

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in concrete as a high-range water reducer (HRWR). The inclusion of PCE allows for controlling the concrete workability better at lower water to cement ratios. The characteristics of PCE in concrete rely on its amount, where an elevated amount may cause a false setting without any hydration incidence in the concrete [168]. Also, the addition of PCE at the required level can create self-compacting concrete (SCC), which enhances the workability of concrete. It also creates a flow concrete with enormous effects at small and elevated intensity regions. Another benefit of PCE inclusion in UHPC or concrete is related to its capacity to be applied in marine atmospheres. Pores or voids in UHPC can remarkably be reduced to get a more compact structure in the presence of PCE because it can eliminate air bubbles and improves concrete density. Furthermore, refinement of UHPC microstructure can avoid or reduce the permeability rate under marine conditions, lowering the attack from seawater containing sulfate and chloride ions. On top of this, the use of polycarboxylate is regarded as a relatively green strategy compared with the use of silica fume and other stabilizers in UHPC microstructures refinement. Birgisson et al. [169] used PCE in HSC instead of silica fume to improve the workability and performance of UHPC. Additionally, approximately 70% of constituent materials in UHPC, including silica fume, superplasticizer, and fiber, were reduced. Ultimately, it was shown that the incorporation of PCE within UHPC could improve the overall performance compared to pure UHPC or HSC. It was acknowledged that a PCE addition of 2.5% of cement weight in HSC could rapidly enhance the early age strength. Consequently, on the first day the HSC strength was enhanced from 40 to 80 MPa. Meanwhile, at 28 days, the obtained strength was about 70
100 MPa at low PCE level, which proved that PCE can act as a good substitute to improve concrete performance. In short, simple management with least parameters or protocols of PCE makes it a celebrated nanomaterial to be incorporated in UHPC.

11.5.5 Titanium oxide Titania or titanium oxide (TiO2) has been widely been used as a pigment in food coloring, paints, photocatalases, implants, solar cells, and many other applications. Normally, it is resourced from ilmenite, rutile, and anatase phases and exists in nature in mineral phases such as rutile, anatase, and brookite. At high pressure, TiO2 is transformed to monoclinic baddeleyite and orthorhombic structures, recently discovered at the Ries crater in Bavaria [164,170]. Ilmenite and rutile are the most abundant forms of titanium dioxide-containing ore worldwide (98%). Heating above temperatures of 600 C
800 C metastable anatase and brookite phases are achieved [171]. Incorporation of TiO2 into UHPC and other concretes revealed a remarkable influence on self-cleaning capability and contributed to green material implementation in engineering construction [170]. Jubilee Church in Rome (Italy) exploited this self-cleaning attribute of TiO2 in buildings, material paving, and product finishing [125]. TiO2 provides accelerated strength of concrete at an early age, wherein the concrete’s performance and abrasion resistance are both enhanced [172]. TiO2

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acting as a glassy layer (extreme porosity) or pigment in the exterior of the particles in UHPC and concrete modifies their microstructure and thus their performance. Moreover, such layers can react with hydration gel products in mixing by producing protective layers, thereby imparting self-cleaning capacity to the surface of concrete. This self-cleaning action of titania in the exterior and coated concrete surface makes them extremely durable with permeability. This enhanced activity of titania incorporated concrete is regarded as a fiber-reinforced system that mimics the glassy fiber effect. It is realized that refinement and tailoring of the hydration gel via such fibers can be useful for the production of concrete with improved strength and prolonged durability [173]. However, health and safety is a major concern related to the use of TiO2 because of its dusty nature, where a small dose can result in a notable environmental impact, especially to workers during packaging and manufacturing. Some reports have shown that TiO2 generates inflammation and causes cancer to factory workers [172]. Consequently, it must be handled with care, particularly during mixing and the TiO2 process.

11.5.6 Nanokaolin Nanokaolin is a by-product of kaolin also called kaolinite, and is a very important industrial clay mineral with the chemical formula Al2Si2O5(OH)4 [174,175]. It is a silicate mineral with many layers, where one tetrahedral layer is connected via oxygen atoms to another octahedral layer of alumina [176]. Kaolinite-enriched rocks are generally called kaolin or china clay [177]. This clay encloses a white mineral called dioctahedral phyllosilicate that is formed by chemical changes to aluminum silicate minerals including feldspar [178]. For ceramic applications, kaolin is heat treated and altered into Al2O3.2SiO2.2H2O. After treatment or endothermic dehydration, the crystalline phase of kaolin is transformed to amorphous structures [179], where the new phase is known as metakaolin. It contains amorphous silica and alumina with some hexagonal layering [180], which is very reactive pozzolan and reveals a comparable reaction to silica fume. Furthermore, by refining the microstructure of metakaolin both strength and durability can be improved, thereby allowing consistent water penetration. These properties make kaolin stronger and more cost effective than silica fume [32,181]. Nanokaolin is generated following either a top-down or bottom-up approach, where the final product is influenced by the processing conditions. Usually, the generation of nanokaolin is comprised of layered or stacked flakes. The kaolin particles are identical to nanokaolin, where the morphologies after the size conversion from microparticles to nanoparticles offer a wider surface area. In concrete, nanokaolin is treated to form an extra reactive or stable product called nanometakaolin. Being a newly used supplementary ingredient in concrete, the presence of nanometakaolin improves the concrete properties unexpectedly [32]. The positive effects of metakaolin in UHPC and other kinds of concretes have been demonstrated [137]. Morsy et al. [182] showed that nanometakaolin inclusion in concrete could improve the mortars’ compressive strength by about 8%
10%. Interestingly, the tensile and flexural strength enhancement of mortar containing nanometakaolin was discerned

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to be nearly 10%
15% compared to plain OPC [182,183]. Despite the advantages of nanometakaolin incorporation into mortars to improve the performance, some shortcomings in its application in UHPC were identified. The lack of raw kaolin in some countries makes nanometakaolin less used than silica fume. Thus, standard guidelines and commercial protocols must be developed for large-scale production of nanokaolin and nanometakaolin as a useful nanomaterial alternative to concrete for the construction industries.

11.5.7 Nanoclay Nanoparticles of layered mineral silicates are called nanoclay. Based on the chemical compositions and morphologies of these nanoparticles, nanoclays are categorized into many classes including montmorillonite, bentonite, kaolinite, hectorite, and halloysite. It is one of the most inexpensive materials with beneficial outcomes in polymers, and is prepared from deposits of montmorillonite mineral with platelet structures of average thickness 1 nm thick and width 70
150 nm. It possesses many good qualities and an is an outstanding base material for nanotechnology maneuvering. These notable attributes are the stability, interlayer spacing, elevated hydration, and swelling capacity, as well as strong chemical reactivity. Clays and their improved organic products can be analyzed via simple and modern instruments that can evaluate the chemical compositions. These tools include gravimetric analyzers, inductively coupled plasma (ICP) or X-ray fluorescence (XRF) spectroscopy, cation exchange capacity (CEC) using standard ammonium acetate technique, surface area determination, Fourier transform infrared spectroscopy (FTIR), powdered X-ray diffraction (PXRD), etc. [57,87
89]. These clays are also distinguished via cation exchange capacity which can differ broadly based on the source and nature of the clay. The clay purity can influence the nanocomposite behaviors. Therefore it is significant to obtain montmorillonite with minimal impurities such as crystalline silica (quartz), amorphous silica, calcite, and kaolin [184]. For clay purification, several techniques have been developed such as hydrocyclone, centrifugation, sedimentation technique, and chemical treatment [180]. Clays are regarded as economical and easily accessible materials. Despite their abundance worldwide, the guidelines and methods for transforming clay to nanomaterials are not well documented. It is thus important to explore the constructional benefits and disadvantages of nanoclay as potential materials, although it has been diversely employed in the polymeric system. Nonetheless, proof of improvement in the material hardness, thermal stability, barrier coating, and solvents together with the enhancement of the electronic and novel types of materials is required. In the construction process, nanoclay is used as an additive to improve the mechanical and binding concrete characteristics. Morsy et al. [182] acknowledged an enhancement of the compressive and tensile strength of mortar cement in the presence of nanoclay as an additive. Furthermore, the thermal properties of concretes can be enhanced via the inclusion of nanoclay as a cement additive in the paste [185,186]. Qian et al. [149] studied the inclusion of nanoclay with water (working as the inner water provider to promote hydration alongside the microcracks). It was shown that the recovery level

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could be considerably improved with the incorporation of nanoclay in the mixtures. The mechanism of healing depended on the reaction between calcium hydroxide and nanoclay to formulate CSH gel, which led to healing of the crack.

11.5.8 Nanoiron Copper, cobalt, and nickel are ferromagnetic materials with very limited applications due to their toxicity and susceptibility to oxidation. Unlike these, iron oxide nanoparticles are attractive owing to their superparamagnetic properties and their potential applications in many fields. They have many applications in the construction industry, but of particular interest are uses a coloring and an anticorrosion agent in construction materials and coatings. Iron oxide nanoparticles have very good UV-blocking capabilities, making these nanoparticles ideal for glass applications ranging from glass coatings to sunglasses. They also allow for better dispersion in paints and coatings, especially in high gloss and automotive applications [187,188]. The possibility and quick reaction between Ca(OH)2 and Fe2O3 nanoparticles led to high amounts of reaction products being formed and closing the cracks.

11.5.9 Nanosilver Nanosilver is well known for its antibacterial, antivirus, and antifungal efficacy, as well as for cellular metabolism in inhibiting cell growth. It can slow down the growth of bacterial and fungal infections that cause bad odors, itching, and sores. Using nanotechnology tools it is now possible to produce silver nanoparticles of accurate morphology (size and shape) with very uniform distribution. Surface coating of nanoparticles from diverse materials can enhance the surface area by several orders of magnitude compared with their bulk counterpart. In particular, silver is an attractive metal because of its extraordinary size- and shape-dependent optical characteristics, efficient plasmon excitation, and high electrical and thermal conductivity in bulk form. These unique features allow silver nanoparticles (Ag NPs) to have potential applications in catalysis, selective oxidation of styrene, antimicrobial coatings, optical sensing, printed electronics, and photonics. Ag NPs in the size range of 1
100 nm have several applications in the field of catalytic processes, photonic devices, electronics, optoelectronics, etc. due to their distinct physical, structural, morphological, chemical, electrical, optical, and magnetic attributes. In addition, Ag NPs have been used widely as antibacterial and antifungal agents in the biomedical field, in textile engineering, for water treatment, and several other consumer products [187,188].

11.6

Sustainability of nanomaterial-based self-healing concrete

Currently, nanotechnology is one of the most significant scientific and industrial breakthroughs of the 21st century. Nanomaterials offer great advantages toward

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concrete sustainability in construction fields such as energy storing, high performance, corrosion resistance, environmental remediation, and self-healing applications. Sustainability is a celebrated topic nowadays, as nanomaterials become incorporated into concrete products in increasing amounts it may help to develop an understanding of their interaction with the environment. Nanotechnology has the potential to dramatically change the strength, sustainability, and total properties of concrete. Hamers [189] reported the broad range of complex nanomaterials required to understand the molecular-level design rules. It is challenging to exploit the power of chemistry to guarantee that nanosystem-incorporated technologies can make better environment-friendly products.

11.7

Advantages and disadvantages of nanomaterials for self-healing concrete

In general, sustainable energy and the environment are the central priorities for researchers; triggering a massive capital investment into research to define new trajectories in construction material sustainability and consequent pollution abatement. Researchers have attempted to solve self-healing problems by using nanomaterials as the healing agent, contributing to achieving many advantages. Moreover, the deployment of nanomaterials in self-healing concrete is an emergent concept. High performance of nanomaterials affects positively the enhancement and development of self-healing concrete. The applied nanotechnology has led to the development of a molecular model for hydration products (C-S-H gels) of OPC [172]. As well as enhancing the strength and sustainability of self-healing concrete using nanomaterials, controlled self-healing would be better and the price of the material considerably below that of epoxy-based materials. The cost of self-healing concrete compared to conventional concrete remains high even when using nanomaterials. Thus, self-healing concrete is a probable product for several civil engineering structures where the concrete cost is much higher due to the better quality, for instance, in tunnel linings and marine structures where security is a major issue or in structures in which there is limited accessibility for repairing and maintaining. In such special circumstances, even if the self-healing agent-incorporated concretes cost more, the safety and future benefits are worth this increased cost.

11.8

Economy of nanomaterial-based self-healing concretes

Generally, concrete is the distinct construction material exploited worldwide. It has been documented that over 2.6 billion tons of OPC was manufactured in the year

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2007 [156] worldwide, amounting to above 17 billion tons. OPC was used in a variety of products including building basements, walls, footpaths, lamp posts, bridges, dams, tall towers, skyscrapers, etc., to name but a few. Usually, concrete goods are proposed to achieve long lifetime and are tolerant of local aggressive atmospheric conditions. Eventually, these concrete structures are usually demolished and recycled upon reaching the final stage of their service life. In addition, in the building sector it is easy to apply process innovations instead of modernizations of disruptive goods. Manufacturing combines products from a variety of areas and uses skills in a broad array of trades to create a single completed structure. An alteration in the built structure can be evaluated by the construction corporation itself. Moreover, a considerably novel product produced by a supplier needs to be understood and approved by the architects, engineers, and client before being applied by the skilled and trained on-site workers. Several factors must be accounted for in developing nanotechnology-related concrete. First, concrete and related products must be manufactured on a large scale. Even if the cost of expensive concrete structures is lower, it must be capable of handling massive material weights in a safe and environment-friendly way. Second, innovations are required to be methodically developed, with field testing to achieve the understanding and assurance of the construction sector. Lastly, concrete structures are difficult to destroy and need explosives or high-energy methods for breaking up. Thus, nanotechnology-based concrete production should be compatible with these conventional practices. With these restrictions, early nanomaterial implementations for self-healing purposes must render notable benefits in terms of extra functions with comparatively low quantities of nanomaterials. This low amount can be offered via standard construction practices and must not influence the materials’ performance. Innovative products (smart self-healing concrete) must be able to advance the delivery of traditional materials, including control of the released admixtures in order to penetrate the marketplace.

11.9

Environmental suitability and safety features of nanomaterial-based concretes

Commonly, gaseous CO2 is released from OPC concrete during the cement clinker’s decarbonation of lime and calcination reactions. Using nanomaterial-based self-healing technology, the emission of CO2 can be reduced greatly. Currently, the world’s use of concrete is immense [190]. Without concrete, wonderful structures like the Sydney Opera House, the Chrysler Building, or Taj Mahal would not have been built. Furthermore, skyscrapers in metropolis all over the world would not have reached such striking altitudes without the use of concrete. In every aspect, the durability of concrete has played a remarkable role in erecting these historic buildings centuries ago without modern technology and qualified engineers.

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Definitely, concrete in its own right is an integral part of everyday life. The manufacturing of smart and new concrete demands a great deal of money, where billions of tons of raw materials are wasted annually because of inefficient concrete production processes. Moreover, the production of OPC (primary concrete binder) adds over 5% of total released greenhouse gases annually worldwide. It creates a threat to our environment where global development is striving for sustainable and green building deployment [191]. Therefore the future aim is targeted at building cleaner, safer, efficient, reliable, and stronger smart materials as alternatives to conventional OPC-based concretes. In this spirit, the notion of nanomaterial-based smart concrete and self-healing technology has been created.

11.10

Conclusions

In recent times, the production of sustainable concrete by self-healing technology has become more common in the construction industries worldwide. An exponential increase in the use of OPC has caused severe environmental damages. The immense benefits and usefulness of self-healing concrete technology were demonstrated in terms of its sustainability, energy-saving traits, and environmental affability. The foremost challenges, current progress, and future trends of nanotechnologyenhanced self-healing concretes are emphasized. An all-inclusive overview of the appropriate literature on nanomaterial-based self-healing concrete has allowed us to draw the following conclusions: 1. Self-healing concretes are characterized by many significant traits such as less pollution, cheap, eco-friendly, and elevated durability performance in harsh environments. These properties make them effective sustainable materials in the construction industries. 2. The design of nanomaterial-based self-healing concretes with improved performance and endurance that are useful for several applications is a new avenue in nanoscience and nanotechnology. 3. Environmental pollution can considerably be reduced by implementing highstrength and durable cementitious composites fabricated using diverse nanoparticles, carbon nanotubes, and nanofibers. 4. In the domain of building and construction, the production of materials by the nanotechnology route is going to play a vital role toward sustainable development in the near future. 5. The use of nanomaterials in concrete is advantageous in terms of improved engineering properties of cementitious materials, especially for the generation of self-healing and sustainable concrete. 6. This comprehensive review is aimed at providing a taxonomy to navigate and underscore the research progress toward nanomaterial-based self-healing concrete technology.

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11.

Production of sustainable concrete composites comprising waste metalized plastic fibers and palm oil fuel ash

12

Hossein Mohammadhosseini1, Mahmood Md. Tahir1, Rayed Alyousef2 and Hisham Alabduljabbar2 1 Institute for Smart Infrastructure and Innovative Construction (ISIIC), School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Skudai, Johor, Malaysia, 2Department of Civil Engineering, College of Engineering, Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia

12.1

Introduction

12.1.1 General appraisal From the sustainability point of view, a reduction in the utilization of nonrenewable raw materials is a critical factor in sustainable construction. Sustainability helps the environment by minimizing waste materials and reducing the requirement for raw materials. Through industrialization and technological developments in various fields, huge amounts and different types of solid waste materials have been generated by the industrial, mining, agricultural, and domestic sectors. Therefore waste management has become one of the main environmental concerns around the world [1,2]. With the growing attentiveness to the environment, lack of landfill areas, and because of its high cost, the utilization of by-products and waste materials has become an attractive substitute to discarding waste. Recycling of nonbiodegradable wastes such as plastics is complicated [3]. Utilization of natural sources, the massive quantity of industrial waste produced, and environmental contamination need new and applicable solutions for sustainable development. Over the decades there has been a rising affirmation in the use of by-products and waste materials in the construction industry. The utilization of wastes not only helps in applying them in concrete composites and similar applications, it also decreases the cost of concrete production, and has many indirect advantages such as decreasing landfill area, saving energy, and protecting the environment from harmful impacts. Moreover, consumption of these waste materials may enhance the physicomechanical properties, durability performance, and microstructure of concrete composites, which are challenging to attain with the usage of only raw materials [4 6]. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00012-0 © 2020 Elsevier Inc. All rights reserved.

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A primary challenge facing construction industries is to execute project incompatibility with the environment by adopting the concept of sustainable growth. This includes the use of high-performance and eco-friendly materials manufactured at a reasonable quality and cost [7]. Current researches on many waste materials such as supplementary cementing materials (SCMs), plastics and textiles, aggregates, and a host of others have shown that the addition of such waste materials in concrete has the potential to enhance the physical, mechanical, and durability properties of concrete as well as reducing the cost of construction [8]. The challenges are more a consequence of the facts that Portland cement is not particularly eco-friendly and there is a lack of landfill space for waste materials. One could then decrease these challenges to the succeeding simple formulation: use as much concrete, but with as low OPC as possible, and waste materials as much as possible, which means substituting as much raw material as possible with waste and SCMs, particularly those that are by-products of industrial processes, and the use of waste instead of raw materials.

12.1.2 Background Concrete is an essential construction material, and its utilization is increasing globally. In addition to the regular applications, higher energy absorption capacity and ductility are most important in various fields such as industrial buildings, highway pavements, and bridge decks. Nevertheless, conventional concrete possesses very slight tensile strength, limited ductility, low resistance to cracking, and little energy absorption. Internal microcracks inherently exist in concrete specimens, and its low tensile strength is due to the propagation of such microcracks, ultimately leading to brittle fracture of the concrete. Therefore enhancing the toughness of concrete and decreasing the size and possibility of weaknesses would lead to better concrete performance [9]. Previously, efforts were made to enhance the ductility and tensile strength of concrete with the addition of a small fraction (0.5% 2%) of short fibers to the concrete mixture throughout the mixing process [10 12]. In such situations, fiberreinforced concrete (FRC) has been shown to perform its purposes adequately. FRC can be described as a composite material containing mixtures of cement and binders, coarse and fine aggregates, and short fibers that are dispersed in the concrete matrix. There are various types of fibers, whether polymeric or metallic, utilized in FRC for their benefits. Among others, the most common types of fiber used in concrete composites are glass fibers, steel fibers, polypropylene (PP), natural fibers, and fibers produced from waste. Fibers in general, and PP fibers in particular, have gained popularity recently for use in improving the properties of concrete [12]. In brittle materials like plain concrete without any fibers, microcracks develop even before applying a load, mainly owing to drying shrinkage or other causes of volume variation. While loading, the cracks propagate and open up, and due to the effect of stress concentration and formation of additional cracks in places of minor defects. The development of such microcracks along the concrete members is the main reason for the inelastic deformation in concrete [13,14]. It has been

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recognized that the adding of short fibers in a concrete mixture has potential in bridging the cracks, load transfer, and improving the microcrack dispersal system [15]. Moreover, the fibers can act as crack arresters and significantly enhance the properties of concrete not only under compression, tensile, and flexure [16], but also under impact blows [17] and plastic shrinkage cracking [18]. One of the fundamental solutions for attaining enhanced concrete properties in terms of strength, durability, and microstructures is the combined use of PP fiber and pozzolanic materials in concrete. Polymeric fiber is included in the mixture to reduce the brittleness of the matrix, thus reducing the susceptibility to cracking of concrete [19]. As most of the problems related to the durability properties such as permeability, chloride penetration, carbonation, and acid and sulfate attacks start from concrete cracking, a method that reduces the brittleness of concrete is necessary. Fiber-reinforced cementitious composites address the brittleness of concrete. This ductile material exhibits good ductility under mechanical loads and durability under severe environmental exposure [20]. Since the early 1990s, the identification and recognition of waste materials that could be used in concrete have grown massively [21]. Utilization of agricultural waste such as ashes can help in making the construction industries more sustainable and eco-friendly. Furthermore, the employment of pozzolanic materials in the production of concrete for their benefits is a common practice. POFA is one of the most recent inclusions to the pozzolanic ash category. POFA is generated from the incineration of palm kernel shells and palm oil husks as fuel in palm oil mills [14]. Malaysia is the second-largest producer of palm oil crops in the world. According to Alsubari et al. [22], in Malaysia alone, about 4 million tons of waste ash were produced in 2010, and this rate of manufacture is expected to increase, due to the growth in the plantation of palm trees. The discarded ash is now considered as a valuable pozzolanic material, as it has properties that can be used in the manufacture of durable concrete composites to enhance the durability and strength properties [23].

12.2

Waste metalized plastic fibers

Over the past 50 years, plastic production has increased massively globally, and various types and forms of plastics have come to be vital parts of our modern lifestyle. This has significantly contributed to the generation of wastes related to plastics. Polymeric-based plastics are extensively used in more or less all fields, mainly in food packaging, electronics and electrical, automotive, agriculture, and other industries [24,25]. According to Gu and Ozbakkaloglu [26], and Sharma and Bansal [1], the overall global manufacture of plastics reached 288 million tons in 2012. Approximately half of these products were for single-use consumers, which caused the generation of excessive amounts of different sorts of plastic wastes. The useful life of plastic products may vary from the short, medium, to long term [27]. Fig. 12.1 displays the waste management hierarchy. It can be seen that the most

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favored option is prevention. It can be attained with the use of less raw materials in the design and the production process, which, in turn, decreases the waste produced, in addition to the greenhouse gas emissions related to the production of plastics [28,29]. Based on Fig. 12.1, prevention is the best proposal for the environment, followed by reuse, recycling, energy from waste (by burning), and finally landfill disposal. Polymers are industrialized to meet the high demand for plastic products. The high demand for various types of plastic by packaging industries illustrates that the ideas of recycling, reduction, and reprocessing are yet to be attained by developing countries [30]. Growth in littering and discarding of different kinds of plastic, particularly in urban areas, demonstrates limitations in the disposal of postconsumer waste plastics. Therefore mismanagement of waste plastics leads to severe environmental concerns such as human health hazards, effects on animal life, water, air pollution, and soil impurities. However, most of these waste plastics have potential for recycling and reuse by chemical or thermal processes, but not all waste plastics are appropriate for this classification [3]. Waste metalized plastics (WMPs) used by food packaging industries are unfit for reprocessing and reuse. Currently, the main methods of disposal of this considerable amount of waste plastics are limited to incineration and landfill [31,32]. Therefore reliable and sustainable discarding substitutions to the existing methods have become essential. Metalized plastic films are made of a polymer which is coated with a thin sheet of metal, typically aluminum. These films offer the glossy metallic appearance of an aluminum foil at a lower cost and mass. Some of the common properties of used WMPs fibers are presented in Table 12.1. Metalized plastic films are extensively used for food packaging, particular applications such as insulation and electronics, and also for decorative purposes [33]. Aluminum is the most popular metal applied for coatings; however, other metals, for instance, nickel or chromium, are also

Figure 12.1 Waste management hierarchy.

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Table 12.1 Engineering properties of WMP fibers Resin

Plastic

Thickness

Size

Density

Tensile

Elongation

Reaction

category

type

(mm)

(W 3 L)

range

strength

(%)

with water

(mm)

(kg/m3)

(MPa)

2 20

0.915 0.945

600

8 10

Hydrophobic

Polypropylene

LDPE

0.07

used, as shown in Fig. 12.2. This coating will not disappear or fade with time, and it is considerably thinner than a metal foil could be made, which is about 0.5 µm. Polyethylene terephthalate (PET) and PP are the most common polymeric films used for metallization. The purpose of the film coating is to reduce the penetrability to liquids, air, and light. The advantages of using these waste metalized films include the capability to be heat sealed, higher toughness, lower density, and lower cost as compared to an aluminum foil. Therefore it offers metalized films some benefits over other types of waste plastics. In this research work WMP films used for food packaging products such as snack foods, coffee and candy were collected from postconsumer waste [32].

12.3

Concrete incorporating waste metalized plastic fibers

12.3.1 Fresh properties 12.3.1.1 Density The experimental results of the fresh density of the various mixes are exposed in Fig. 12.3. It has been observed that the fresh density was reduced with rising WMP fiber dosage. This was predictable owing to the lesser density of the WMP fibers, which is approximately about 915 kg/m3 associated with that of conventional concrete. The inclusion of POFA into the mixtures also resulted in in lower density than OPC-based mixtures. This phenomenon could be because of the lesser relative density of POFA than OPC. As illustrated in Fig. 12.3, the minimum fresh density was noted for the POFA mix with 1.25% fibers. The lower density of concrete containing WMP fibers and POFA could be an advantage for concrete which is used in lightweight structural and nonstructural applications [34].

12.3.1.2 Workability The workability of fresh concrete mixes containing WMP fibers and POFA in terms of slump test and VeBe time test were conducted. The results of the workability tests are displayed in Figs. 12.4(A,B). It can be observed that the workability of concrete mixes was considerably reduced by adding WMP fibers. From Fig. 12.4 (A), the slump of the plain concrete mix without any fibers and POFA was noted as

Figure 12.2 (A) Postconsumer waste metalized plastic films, (B) fabricated metalized plastic fibers, and (C) WMP fibers used in this study.

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Figure 12.3 Influence of WMP fibers on the fresh density of concrete mixes.

Figure 12.4 Influences of WMP fibers on (A) slump and (B) VeBe time of fresh concrete.

190 mm. With the inclusion of WMP fibers at 0.25%, 0.5%, 0.75%, 1%, and 1.25%, the slump values dropped to 120, 80, 65, 40, and 30 mm, respectively. Moreover, due to the higher surface area of POFA than OPC, the matrix absorbs a greater amount of water and thus makes the mixture stiffer resulting in lower workability [35]. From the results given in Fig. 12.4(B), it can be observed that in mixes with 20% POFA, the slump dropped to 170 mm, and the VeBe time raised to 16 s as compared to that of 190 mm and 15.3 s for OPC plain concrete. A similar tendency like that of OPC mixes was observed for POFA mixes reinforced with WMP fibers. The higher the fiber dosage, the lower the workability of concrete. There appears to be a good correlation between slump values and VeBe times. A power relation with a correlation coefficient of R2 5 0.948 for Eq. 12.1 is obtained

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Figure 12.5 The relationship among slump and VeBe time of concrete containing WMP fibers.

between VeBe time and slump value, as shown in Fig. 12.5. It can be seen that the higher the slump value, the lower was the VeBe time. This indicates that the WMP fibers influenced the workability of concrete that resulted in lower slump values and greater VeBe times. Presumably, the higher amount and large surface area of WMP fibers demanded extra binder paste and sand to wrap over the fibers, due to the strong bond among WMP fibers and other components in the concrete composite [36]. y 5 194:24 3 20:782

R2 5 0:9481

(12.1)

12.3.2 Hardened properties 12.3.2.1 Compressive strength The experimental results reveal that the cube compressive strength of all concrete mixtures was reduced by increasing the fiber dosage. Fig. 12.6 demonstrates the experimental results for the compressive strength of OPC and POFA-based concrete mixes incorporating WMP fibers at different curing periods. Comparing the 28 days compressive strength values of the plain concrete mixture, the inclusion of WMP fibers at dosages of 0.25%, 0.5%, 0.75%, 1%, and 1.25% reduced the cube compressive strength by 6%, 7%, 11%, 18%, and 21%, respectively. In concrete mixtures with 20% POFA, further decreases in compressive strength of 13% at 7 days and 10% at 28 days curing were observed related to that of the OPC-based concrete mixture. The obtained results of this study are in agreement with those findings by Bhogayata and Arora [32], who reported the decrease in compressive strength values by adding metalized plastic waste fiber. This reduction in the compressive strength could be attributed to the existence of air voids in the matrix, which are

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Figure 12.6 Variation in the compressive strength of concrete mixes reinforced with WMP fibers.

increased by adding fibers and therefore, the effects of air voids in reduction of strength were more effective compared to arresting further crack openings. Beyond 28 days, the compressive strengths of POFA-based concrete tended to increase with the curing age for all fiber volume fractions and gave higher compressive strength than OPC concrete at 91 days. This can be explained by the fact that the higher fineness of POFA develops pozzolanic properties and particle packing density. These characteristics tend to develop concrete strength as well as its density [23]. To attain a detailed understanding of the influence of POFA on the hydration process and strength improvement of the concrete matrix at the age of 91 days, the microstructural analysis in terms of scanning electron micrography (SEM) was applied. The SEM images for OPC- and POFA-based concrete mixes are revealed in Fig. 12.7. It can be observed that in both POFA- and OPC-based mixtures, the development of hydration products is significant. Fig. 12.7 shows that the C S H gels were more consistently formed and spared in the POFA-based mixture as compared to the OPC mixture. When OPC is mixed with water, it forms a hydrated binding cement paste (hcp) of C S H, and it liberates calcium hydroxide (CH). This reaction is quite rapid. Nevertheless, when a pozzolanic material such as POFA is present, its reactive silica (SiO2) constituent reacts with liberated CH in hcp (in the presence of water) to form secondary C S H gels. This reaction is relatively slow, causing a slow rate of heat liberation and slow strength improvement. This reveals that the pozzolanic reaction consumes lime instead of producing it [37].

12.3.2.2 Splitting tensile strength Fig. 12.8 displays the variation in the results of tensile strength vs. WMP fiber volume fractions. It can be seen that with the addition of WMP fibers and an increase in dosage, the tensile strength values of concrete mixtures were considerably higher

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Figure 12.7 SEM images of OPC- and POFA-based concrete specimens at 91 days.

than those of the control mix. When the splitting occurred and was sustained, the WMP fibers bridging the split parts of the specimens acted over the stress transfer from the matrix to the fibers and therefore slowly supported the full tensile stress. The resistance against the indirect tension improved the strain capability of the specimens and therefore resulted in higher splitting tensile strength of those specimens reinforced with short fibers than those of a plain concrete mixture. Fig. 12.8 further reveals that the incorporation of WMP fibers and POFA contributed to the improvement of tensile strength. At the age of 91 days, the splitting tensile strength of OPC-based concrete mixes increased by 12%, 19%, 17%, 13%, and 8% for fiber dosages of 0.25%, 0.5%, 0.75%, 1%, and 1.25%, respectively, associated with the control concrete mix. Over the same curing period, the inclusion of POFA to the fibrous concrete, for instance, enhanced the strength values by 13%, 22%, 18%, 15%, and 11% for the similar fiber contents, compared to the control mixture. The improvement in the tensile strength could be due to the greater contact surface area among WMP fibers and the binder paste caused by the pozzolanic hydration process, which is in line with the good pozzolanic nature of POFA at the ultimate ages [22]. Bhogayata and Arora [32] also found similar observations on the enhancement in the tensile strength of concrete by the adding metalized plastic waste fibers.

12.3.2.3 Flexural strength Fig. 12.9 demonstrates the obtained results of the flexural strength test. It can be observed that the flexural strength of concrete mixtures incorporating WMP fibers was significantly greater than that of control mix. The flexural strength values varying from 3.0 to 3.99 MPa, 3.84 to 5.13 MPa, and 4.90 to 5.95 MPa were recorded for all concrete mixtures with increasing fiber dosages from 0% to 1.25%, at curing periods of 7, 28, and 91 days, respectively. A comparable tendency like that of splitting tensile was also noted for the concrete flexural strength values. For example, for the POFA-based mixture comprising 0.5% WMP fibers at the age of 91

Figure 12.8 Effects of WMP fibers on the tensile strength of concrete composites.

Figure 12.9 Effects of WMP fibers on the flexural strength of concrete composites.

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days, a value of 5.95 MPa was recorded, which was the highest and 21% higher than that of the control mixture. The said enhancement was attributed to the higher pozzolanic action of POFA and formation of extra C S H gels, particularly at ultimate ages, which enhanced the strength values of concrete mixtures, as reported by Nagaratnam et al. [38]. The fibers interconnect the microcracks in the tension region of the concrete prisms, and therefore improve the flexural strength. WMP fibers act as crack arresters and stop crack propagation from extending; consequently, the fibers afford a higher energy absorption capacity for the crack zones contiguous with the tips of the cracks. Nevertheless, further rises in fiber content greater than 0.75% were caused by a reduction of flexural strength, but the obtained values are even higher than those of the control mixture. This reduction in flexural strength can be due to the lower workability and existence of pores at high fiber dosages and also uneven distribution of fibers [39]. This rise in porosity can be associated with the growth of microcracks in the specimen, and therefore poor fiber matrix bonding and lower strength.

12.3.2.4 Impact resistance In this chapter, the impact resistance of different concrete mixtures incorporating WMP fibers was examined through a number of blows necessary to obtain the initial crack (N1) and failure (N2) of the concrete disk. Also, the difference among the obtained number of blows at failure and first crack (N2 2 N1), as well as the percentage rise in the number of post first crack blows to ultimate failure (N2 2 N1/ N1) are presented in Table 12.2. Table 12.2 Variation in the impact resistance of concrete mixtures at the initial and ultimate cracks Impact resistance

Mix OPC

POFA

Vf (%)

First crack (N1)

Failure (N2)

0.00 0.25 0.50 0.75 1.00 1.25 0.00 0.25 0.50 0.75 1.00 1.25

17 29 48 65 87 105 19 37 61 83 99 128

20 41 74 82 108 131 23 49 85 99 124 156

Impact energy (kN mm) First crack

Failure

346.0 590.3 977.0 1323.0 1770.8 2137.2 386.7 753.1 1241.6 1689.4 2015.0 2605.3

407.1 834.5 1506.2 1669.0 2198.2 2666.4 468.1 997.3 1730.1 2015.0 2523.9 3175.2

(N2 2 N1/ N1) 3 100

N2 2 N1 3 12 26 17 21 26 4 12 24 16 25 28

17.6 41.4 54.2 26.2 24.1 24.8 21.1 32.4 39.3 19.3 25.3 21.9

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According to the obtained results, adding WMP fibers enhanced the initial and ultimate crack impact resistance of the concrete composites, and further development was noted for the impact resistance at the ultimate crack, as associated with the impact resistance at the initial crack. As given in Table 12.2, reinforcement of plain concrete mixtures with WMP fibers of 0.25%, 0.5%, 0.75%, 1%, and 1.25% improved the impact resistance of disk specimens by 71%, 182%, 282%, 412%, and 518%, respectively, at the initial crack. Furthermore, the impact resistance at the ultimate crack was raised by 105%, 270%, 310%, 440%, and 555% for the concrete mixtures containing the same fiber volume fractions, respectively. The addition of WMP fibers correspondingly increased the percentage rise in the number of post first-crack blows to failure (N2 2 N1/N1) value over the plain concrete disks without any fibers. This signifies that the WMP fibers significantly decreased the brittleness of the concrete specimens. The obtained values of the impact resistance at the initial and ultimate crack for POFA-based concrete composites show a significant improvement compared to those of OPC-based concrete mixtures. Table 12.2 further reveals that the combination of WMP fibers and POFA significantly increased the impact resistance of concrete. For instance, by the addition of WMP fibers at dosages of 0.25%, 0.5%, 0.75%, 1%, and 1.25%, the first crack impact resistance increased by 95%, 221%, 337%, 421%, and 574%, respectively, as compared to that of plain POFA-based mixture. Furthermore, the ultimate crack impact resistance was raised by 113%, 270%, 330%, 439%, and 578% for the concrete mixtures containing the same fiber volume fractions, respectively, which are considerably higher than those of OPCbased fibrous mixtures. As was mentioned earlier, owing to the pozzolanic behavior of POFA, it modified the microstructure of the concrete mixtures and resulted in higher strength and energy absorption, particularly at the ultimate ages [14]. Fig. 12.10 displays the crack patterns formed on the concrete disk specimens, which were tested under a repeated drop weight impact test. Rising WMP fiber dosages resulted in more uniform cracks, that is, a higher number of cracks on the surface of concrete disks. The development of multiple cracks on the top surface of disk specimens is owing to the bridging action delivered by WMP fibers which absorbed more energy and prevented the sudden failure of the samples [40]. Based on the obtained fractured surface of the concrete specimens, it is indicated that WMP fibers are consistently dispersed along the section of the concrete disks. The results obtained and observations made in the current study confirm the outcomes by Mastali et al. [36] for the utilization of waste glass fibers, and by Nili and Afroughsabet [17] for PP fibers.

12.3.2.5 Sorptivity and water absorption The measured values of the water absorption test are illustrated in Fig. 12.11. Based on the obtained results, the addition of WMP fibers leads to a reduction in water absorption of concrete specimens, as the minimum water absorption was measured as 3.64% for concrete reinforced with 0.25% WMP fibers and containing 20% POFA at the curing period of 90 days, which is 11.2% lower as compared to

Figure 12.10 Fracture surfaces of the reinforced specimens under a repeated drop weight impact test.

Figure 12.11 Effects of WMP fibers on water absorption of concrete mixes at the ages of (A) 28 days; (B) 90 days.

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those values recorded for control mix. This improvement could be due to the bridging action of WMP fibers by providing a grid structure and prevent the entry of water into the concrete specimens. However, the water absorption was increased for mixes containing fibers by more than 0.75%. The higher water absorption in specimens with higher fiber content could be due to the balling effects by fibers, which interrupted the compaction of concrete in molds, and therefore initiated the pores in the matrix. It is interesting to note that the anticracking nature of WMP fibers decreased the size and amount of cracks in the matrix and prevented the absorption of water into the concrete. The outcomes of the sorptivity test on concrete disk specimens are shown in Fig. 12.12. Concerning the recorded results, the addition of WMP fibers improved the permeability of the concrete specimens. The results indicated that the reinforcement of concrete with WMP fibers up to 0.5% significantly enhances the durability of concrete through a reduction in water absorption. The measured values for the sorptivity test show that the water absorption of POFA-based mixes was significantly lower than that of OPC mixes. It could be attributed to the development of additional C S H gels in the matrix owing to the pozzolanic nature of POFA. These extra gels fill up the voids and microspores in the concrete and, therefore, provide a dense microstructure and lower permeability of concrete, as stated by Karahan and Ati¸s [19].

12.3.2.6 Chloride diffusion In this study, the chloride diffusion depth was inspected by immersion of concrete samples in 5% chloride solution, and the results are illustrated in Fig. 12.13. The inclusion of WMP fibers in concrete with POFA provided a grid structure in the matrix, which has a significant effect on decreasing the chloride penetration into concrete as well as a reduction in the creation of cracks [41]. The outcomes of tests revealed that the depth of penetration in those mixes reinforced with WMP fibers at dosages of 0.25%, 0.5%, and 0.75% was noticeably reduced. The chloride penetration depth was recorded as 14.8 mm for OPC-based specimens reinforced with 0.5% fibers, which is about 24% lower than that of the recorded value of 19.5 mm for plain concrete without any fiber. In addition, the lower penetration depths were recorded for POFA-based mixes for the same conditions. At the age of 90 days, the penetration depth of 10.5 mm was recorded for POFA-based mix with 0.5% fibers, which was relatively lower than that of the measured value of 13.1 mm for concrete mix without any fiber. However, a further increase in fiber dosage increased the permeability of concrete and therefore increased the depth of penetration. By comparing the depth of chloride penetration for various mixes, it can be seen that the incorporation of POFA and fibers into concrete caused a breakdown of larger voids in the matrix and therefore filled the cavities through the formation of additional hydration products. This was due to the finer particle size of POFA as compared to OPC particles. in addition to the high pozzolanic activity of POFA. During the pozzolanic reaction of POFA, a significant amount of CH [Ca(OH)2] involved in the pozzolanic reaction reacted with active SiO2 of POFA, resulting in

Figure 12.12 Water absorption vs. square root of time: (A) OPC- and (B) POFA-based concrete mix reinforced with WMP fibers.

Figure 12.13 Effects of WMP fibers on the chloride penetration depth for (A) OPC and (B) POFA concrete.

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the formation of extra C S H gel [28,29,42]. This produces additional hydration, and therefore provides a dense microstructure in the concrete matrix and reduces the amount of porosity and, consequently, there is a lower depth of chloride penetration.

12.4

Applications

FRC is increasingly used on account of the benefits of higher tensile and flexural strengths, more significant energy absorption capacity as well as the better ductility performance. The consistent distribution of short fibers into the concrete mixture offers isotropic properties not common in conventional steel-reinforced concrete. Since its introduction into the market in the late 1960s, the use of FRC has increased steadily. The following are the most common applications for concrete reinforced with WMP fibers: G

G

G

G

G

Concrete reinforced with WMP fibers can be efficiently used as concrete pavements for highways and airport runways, bridge components, housing, and industrial floors, and other similar applications as its ductility is comparatively higher than that of plain concrete. Concrete reinforced with WMP fibers can be used as utility concrete components such as underground vaults and junction boxes, drains for industrial wastes, sewer channels and pipes, and power line transmission poles due to its lower water absorption and high resistance to chemical attacks compared to that of plain concrete. The existing sorts of waste PP plastic fibers can be used to fabricate marine construction materials that are economically competitive and environmentally superior to conventional marine construction products. Concrete containing WMP fibers has potential to be used as railroad ties, median barriers, and bridge panels in transportation-related works. It can also be used in the production of precast wall panels as sound isolation components.

12.5 G

G

G

G

Conclusions

The presence of WMP fibers in concrete decreased the unit weight of the mixture. The higher the fiber dosage, the lower the unit weight due to the lower density (915 kg/m3) of fibers compared to that of ordinary concrete. The inclusion and further rise in the WMP fiber dosage resulted in lower workability of concrete. Generally, higher fiber content leads to lower slump and higher VeBe time. At the early ages, cube compressive strength diminished slightly with the adding of WMP fibers and POFA. However, for POFA mixes, the compressive strength was higher than that of OPC mixes at 91 days. It is believed that at more extended curing periods, the pozzolanic activity of POFA has aided in enhancing the strength of concrete mixtures. Unlike diminution in compressive strength, remarkable enhancements in both tensile and flexural strengths of all concrete specimens were noted. All specimens containing WMP

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G

G

G

455

fibers obtained higher tensile and flexural strength values than those of plain concrete mixes. With the addition of 0.5% WMP fibers, the splitting tensile strength was enhanced by about 19% for OPC mix and 22% for the POFA mix at the age of 91 days. Likewise, the flexural strength improved by approximately 19% and 22% for the same conditions. The inclusion of WMP fibers in concrete specimens revealed better ductility performance and higher impact resistance due to the bridging action of fibers. The addition of WMP fibers and POFA enhanced the water absorption and sorptivity properties of concrete, which was due to the development of extra C S H gels by hydration of POFA and the bridging action of WMP fibers. A reduction in the depth of chloride penetration by approximately 54% was observed in concrete specimens reinforced with WMP fibers.

The outcomes of this chapter recommend that concrete reinforced with WMP fibers and palm oil fuel ash (POFA) have potential for manufacture and industrialization as they have adequate engineering properties and durability performances for both structural and nonstructural applications. Nevertheless, to ensure the feasibility of this new type of concrete composite as structural components, large-scale applications, together with the behavior in steel-reinforced concrete components, are suggested for upcoming studies.

References [1] R. Sharma, P.P. Bansal, Use of different forms of waste plastic in concrete a review, J. Clean. Prod. 112 (2016) 473 482. [2] A.A. Awal, H. Mohammadhosseini, Green concrete production incorporating waste carpet fiber and palm oil fuel ash, J. Clean. Prod. 137 (2016) 157 166. [3] R. Siddique, J. Khatib, I. Kaur, Use of recycled plastic in concrete: a review, Waste Manag. 28 (10) (2008) 1835 1852. [4] J. Kanadasan, H.A. Razak, Engineering and sustainability performance of selfcompacting palm oil mill incinerated waste concrete, J. Clean. Prod. 89 (2015) 78 86. [5] M.S. Meddah, M. Bencheikh, Properties of concrete reinforced with different kinds of industrial waste fibre materials, Constr. Build. Mater. 23 (10) (2009) 3196 3205. [6] M. Batayneh, I. Marie, I. Asi, Use of selected waste materials in concrete mixes, Waste Manag. 27 (12) (2007) 1870 1876. [7] N.H.A.S. Lim, H. Mohammadhosseini, M.M. Tahir, M. Samadi, A.R.M. Sam, Microstructure and strength properties of mortar containing waste ceramic nanoparticles, Arab. J. Sci. Eng. 43 (10) (2018) 5305 5313. [8] S. Chandra, Waste Materials Used in Concrete Manufacturing, Elsevier, 1996. [9] H. Mohammadhosseini, A.A. Awal, J.B.M. Yatim, The impact resistance and mechanical properties of concrete reinforced with waste polypropylene carpet fibres, Constr. Build. Mater. 143 (2017) 147 157. [10] J. Yahaghi, Z.C. Muda, S.B. Beddu, Impact resistance of oil palm shells concrete reinforced with polypropylene fibre, Constr. Build. Mater. 123 (2016) 394 403. [11] R.F. Zollo, Fiber-reinforced concrete: an overview after 30 years of development, Cem. Concr. Compos. 19 (2) (1997) 107 122. [12] A.M. Brandt, Fibre reinforced cement-based (FRC) composites after over 40 years of development in building and civil engineering, Compos. Struct. 86 (1-3) (2008) 3 9.

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[13] M. Hsie, C. Tu, P.S. Song, Mechanical properties of polypropylene hybrid fiberreinforced concrete, Mater. Sci. Eng.: A 494 (1-2) (2008) 153 157. [14] H. Mohammadhosseini, J.M. Yatim, Microstructure and residual properties of green concrete composites incorporating waste carpet fibers and palm oil fuel ash at elevated temperatures, J. Clean. Prod. 144 (2017) 8 21. [15] M.A.A. Aldahdooh, N.M. Bunnori, M.M. Johari, Influence of palm oil fuel ash on ultimate flexural and uniaxial tensile strength of green ultra-high performance fiber reinforced cementitious composites, Mater. Des. 1980-2015 (54) (2014) 694 701. [16] S.P. Yap, U.J. Alengaram, M.Z. Jumaat, Enhancement of mechanical properties in polypropylene and nylon fibre reinforced oil palm shell concrete, Mater. Des. 49 (2013) 1034 1041. [17] M. Nili, V. Afroughsabet, The effects of silica fume and polypropylene fibers on the impact resistance and mechanical properties of concrete, Constr. Build. Mater. 24 (6) (2010) 927 933. [18] P. Zhang, Q. Li, Z. Sun, Influence of silica fume and polypropylene fiber on fracture properties of concrete composite containing fly ash, J. Reinf. Plast. Compos. 30 (24) (2011) 1977 1988. [19] O. Karahan, C.D. Ati¸s, The durability properties of polypropylene fiber reinforced fly ash concrete, Mater. Des. 32 (2) (2011) 1044 1049. [20] K.H. Mo, U.J. Alengaram, M.Z. Jumaat, M.Y.J. Liu, Contribution of acrylic fibre addition and ground granulated blast furnace slag on the properties of lightweight concrete, Constr. Build. Mater. 95 (2015) 686 695. [21] C. Meyer, The greening of the concrete industry, Cem. Concr. Compos. 31 (8) (2009) 601 605. [22] B. Alsubari, P. Shafigh, M.Z. Jumaat, Utilization of high-volume treated palm oil fuel ash to produce sustainable self-compacting concrete, J. Clean. Prod. 137 (2016) 982 996. [23] E. Khankhaje, M.W. Hussin, J. Mirza, M. Rafieizonooz, M.R. Salim, H.C. Siong, et al., On blended cement and geopolymer concretes containing palm oil fuel ash, Mater. Des. 89 (2016) 385 398. [24] J.L. Ruiz-Herrero, D.V. Nieto, A. Lo´pez-Gil, A. Arranz, A. Ferna´ndez, A. Lorenzana, et al., Mechanical and thermal performance of concrete and mortar cellular materials containing plastic waste, Constr. Build. Mater. 104 (2016) 298 310. [25] S. Yang, X. Yue, X. Liu, Y. Tong, Properties of self-compacting lightweight concrete containing recycled plastic particles, Constr. Build. Mater. 84 (2015) 444 453. [26] L. Gu, T. Ozbakkaloglu, Use of recycled plastics in concrete: a critical review, Waste Manag. 51 (2016) 19 42. [27] Y. Wang, Fiber and textile waste Utilization, Waste Biomass Valoriz. 1 (1) (2010) 135 143. [28] H. Mohammadhosseini, M.M. Tahir, M.I. Sayyed, Strength and transport properties of concrete composites incorporating waste carpet fibres and palm oil fuel ash, J. Build. Eng. 20 (2018) 156 165. [29] H. Mohammadhosseini, M.M. Tahir, A.R.M. Sam, N.H.A.S. Lim, M. Samadi, Enhanced performance for aggressive environments of green concrete composites reinforced with waste carpet fibers and palm oil fuel ash, J. Clean. Prod. 185 (2018) 252 265. [30] M. Gesoglu, E. Gu¨neyisi, O. Hansu, S. Etli, M. Alhassan, Mechanical and fracture characteristics of self-compacting concretes containing different percentage of plastic waste powder, Constr. Build. Mater. 140 (2017) 562 569.

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[31] A.I. Al-Hadithi, N.N. Hilal, The possibility of enhancing some properties of selfcompacting concrete by adding waste plastic fibers, J. Build. Eng. 8 (2016) 20 28. [32] A.C. Bhogayata, N.K. Arora, Fresh and strength properties of concrete reinforced with metalized plastic waste fibers, Constr. Build. Mater. 146 (2017) 455 463. [33] A. Bhogayata, A. Nakum, Strength characteristics of concrete containing post consumer metalized plastic waste, Inter. J. Res. Eng. Tech. 4 (9) (2015) 430 434. [34] H. Mohammadhosseini, M.M. Tahir, Durability performance of concrete incorporating waste metalized plastic fibres and palm oil fuel ash, Constr. Build. Mater. 180 (2018) 92 102. [35] A.M. Zeyad, M.A.M. Johari, B.A. Tayeh, M.O. Yusuf, Pozzolanic reactivity of ultrafine palm oil fuel ash waste on strength and durability performances of high strength concrete, J. Clean. Prod. 144 (2017) 511 522. [36] M. Mastali, A. Dalvand, A. Sattarifard, The impact resistance and mechanical properties of the reinforced self-compacting concrete incorporating recycled CFRP fiber with different lengths and dosages, Compos. Part. B: Eng. 112 (2017) 74 92. [37] H. Mohammadhosseini, A.A. Awal, A.H. Ehsan, Influence of palm oil fuel ash on fresh and mechanical properties of self-compacting concrete, Sadhana 40 (6) (2015) 1989 1999. [38] B.H. Nagaratnam, M.E. Rahman, A.K. Mirasa, M.A. Mannan, S.O. Lame, Workability and heat of hydration of self-compacting concrete incorporating agro-industrial waste, J. Clean. Prod. 112 (2016) 882 894. [39] Y. Ding, D. Li, Y. Zhang, C. Azevedo, Experimental investigation on the composite effect of steel rebars and macro fibers on the impact behavior of high performance selfcompacting concrete, Constr. Build. Mater. 136 (2017) 495 505. [40] E. Kantar, T.Y. Yuen, V. Kobya, J.S. Kuang, Impact dynamics and energy dissipation capacity of fibre-reinforced self-compacting concrete plates, Constr. Build. Mater. 138 (2017) 383 397. [41] K. De Weerdt, D. Orsa´kova´, M.R. Geiker, The impact of sulphate and magnesium on chloride binding in Portland cement paste, Cem. Concr. Res. 65 (2014) 30 40. [42] C. Chandara, K.A.M. Azizli, Z.A. Ahmad, S.F.S. Hashim, E. Sakai, Analysis of mineralogical component of palm oil fuel ash with or without unburned carbon, Advanced Materials Research, Vol. 173, Trans Tech Publications, 2011, pp. 7 11.

Alkali-activated concrete systems: a state of art

13

R. Manjunath and Mattur C. Narasimhan Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, India

13.1

Introduction

In the modern world, issues associated with global warming and resulting climate changes are gaining increasing importance. Global warming is caused by the increased emissions of greenhouse gases, including methane and carbon dioxide, to the atmosphere and the consequent depletion of the protective ozone layer. Again, depletion of other nonrenewable natural resources has been an issue of concern. Concrete is one of the most extensively used commodities for the development of infrastructural facilities and the demand for it is increasing unabated. Production of increased quantities of Portland cement (PC) requires large amounts of natural resources, and it releases significant quantities of carbon dioxide to the atmosphere during its production process. Large amounts of crushed, natural rock-based aggregates are used as coarse aggregates, and river sand, generally mined from the banks and beds of rivers, is used as fine aggregates in the production of concrete (almost 70% 80% of its bulk). Their increased use is also causing greater stress to the environment. The crucial importance in developing concrete infrastructure for the growth of any community is thus leading to a search for alternate, greener, durable, less energy-intensive, and economical construction materials with lower carbon footprints. This, in turn, has led to the evolution of alkali-activated (AA) concrete mixes and is also an important current study area for researchers. One of the main drivers for the development of AA concrete-based materials is their environmental credentials. In the entire of AA concrete production process, when carried out at ambient temperature, there are very few steps, like the production of the alkali activators, wherein some carbon dioxide synthesis and emission are involved. Again, unlike concrete mixes with blended cements (fly-ash (FA) based or slag based) and/or site-produced concrete mixes, wherein any of the mineral admixtures like FA, ground granulated blast furnace slag (GGBFS), silica fume, etc., are used as partial binder materials, no PC is used in AA concrete mixes. Usually materials rich in alumina and/or reactive silica are used as the starting materials in the production of AA cement systems. These, in general, cannot react directly with water and they require an activator for their setting and hardening reactions. Alkaline New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00013-2 © 2020 Elsevier Inc. All rights reserved.

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compounds such as caustic alkalis, silicate salts, and nonsilicate salts of weak acids [1] are the most commonly used alkaline activators. AA concrete systems have the potential to use large volumes of industrial byproducts, such as FA and GGBFS, as source materials, which otherwise would lead to safe disposal problems, thus leading to more costs. It is estimated that the production of FA -based geopolymeric cement is responsible for the emission of 0.18 ton/ton of concrete produced, which is about 70% lower than the production of OPC, thus proving its eco-friendly nature. Furthermore, the production of heatcured, FA -based geopolymer concrete is 20% 30% cheaper than ordinary PCbased concrete [2]. Partial replacement of FA with GGBFS in the above mixes may further prove to be more effective by avoiding heat curing [3]. Current practices in the construction industry are highly unsustainable, due to the consumption of enormous amounts of natural aggregates in the form of both fine and coarse aggregates, which act as inert filler materials. About 70% of the volume of concrete is composed of aggregates and the availability of good aggregates in any location is proving to be increasingly scarce and costly. Hence the search is on for suitable alternate materials for possible use as aggregates in concrete mixes. Herein the researchers are again identifying the possibilities of using different types of waste materials such as glass powder, stone powder, (iron) slag sand, iron ore tailings, copper slag (CS), electric arc furnace slag, and waste plastics, as partial/full replacements for natural aggregates in concrete mixes. Steel is the basic material of construction for developing all manufacturing units, transportation vehicles, railway engines and coaches, etc. Large facilities like industrial buildings, warehouses, auditoria, space stadiums, and large-span bridges/flyovers are generally steel-intensive constructions. Steel is also used in the form of rebars in RCC constructions. Production of iron and steel is always associated with the production of large amounts of varieties of slags, economical disposal of which is again a challenge to the steel industry. Such materials, with some processing, can be advantageously used as partial/full replacements for natural aggregates in concrete mixes, leading to sustainability in both concrete and steel industries. For example, steel slag or Linz-Donawitz (LD) slag is produced during the conversion of raw iron into steel using the LD process and the by-product obtained after this process is called blast oxygen furnace (BOF) slag. This is processed and made available in both coarser and finer fractions, which can be used as aggregates in concrete systems.

13.2

Geopolymers and alkali-activated cementitious systems

13.2.1 Geopolymeric cementitious systems Fernandez-Jimenez and Palomo [4] studied the composition and microstructure of AA FA binder and found that the main reaction product of FA-based geopolymer is

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an alkaline silico-aluminate gel. (OH2) ions act as catalysts for the reaction during the activation process and the alkaline metal (Na1) acts as a structure-forming element. The structure of pre-zeolite gel contains Si and Al tetrahedrons randomly distributed along the polymeric chains that are cross-linked so as to provide cavities of sufficient size as to accommodate the charge-balancing hydrated sodium ions. According to Fernandez-Jimenez et al. [5], the dissolution process starts when the alkaline solution attacks FA particles. Reaction products will be generated both inside and outside the spherical shell surfaces until the FA particles are consumed almost completely. The precipitation of the reaction products occurs as the alkaline solution penetrates into the larger sphere and fills the interior space with the reaction products forming a dense matrix. Due to this massive precipitation of reaction products, some of the smaller particles are covered with the products providing a crust, which prevents contact with alkaline solution, resulting in unreacted particles, attached to the alkaline solution but which maintain their spherical shape, and reaction products which may co-exist in a single paste. Davidovits [6] found the geopolymers to be amorphous to semicrystalline, threedimensional silicon aluminate structures which can be further classified based on their Si/Al ratios mainly consisting of polysialate, polysialate-siloxo, and polysialate-disiloxo. An aluminosilicate source such as FA can be activated by using commercially available sodium hydroxide (SH) or potassium hydroxide, either singly, or in combination with sodium or potassium silicate solutions, respectively. In the case of geopolymer composites, water does not play a significant role in the chemical reaction, except for increasing the workability of the mixtures. FA, that is activated in the presence of the alkaline activator, is responsible for causing the process of polymerization, leading to the formation of a geopolymeric gel. FA-based geopolymer concrete mixes generally require heat or steam curing, at a temperature of between 60 C 70 C for about 24 hours, to initiate the process of geopolymerization. Heat curing in an electric oven is the most commonly used type of curing for FA -based geopolymer composites, as it is not possible to achieve minimum required strengths when cured at ambient temperature. The heat-curing arrangements, however, really become impracticable for use in actual site conditions except for their possible application in precast elements. Several research studies have been carried out to date to completely eliminate heat curing in FAbased systems by additionally using OPC or GGBFS in the binder system, where the specimens attain required mechanical properties when cured at ambient temperatures [7 9].

13.2.2 Alkali-activated cementitious systems The powdered blast-furnace slag was stabilized using caustic soda which was first demonstrated by Kuhl [10] in 1908, with reference to the setting behavior of mixtures of ground slag powder and caustic potash solution. Purdon [11] carried out the first extensive laboratory study on clinkerless cement consisting of slag and caustic soda produced using a base and an alkaline salt. Glukhovsky [12] was first to

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discover the possibility of producing binders using calcium-free aluminosilicates (clays) and solutions of an alkaline metal. He described the binders as soil cements and the corresponding concretes as soil silicates. In 1979, Davidovits produced binders using alkalis mixed with a burnt mixture of limestone, kaolinite, and dolomite and called them “geopolymers” due to their polymeric structure. Krivenko [13] classified the AA cementitious materials based on the composition of the hydrated products. The alkaline aluminosilicate systems were called geocements, emphasizing the similarity of their formation process materials to the geological process for the formation of natural zeolites and the alkaline earth systems where the hydration products are low, basic calcium silicate hydrates (C S H). Davidovits [14] proposed that alkaline liquids can be used with silicon and aluminium-rich source materials, possibly with a geological origin or from industrial by-products such as FA, GGBFS, and rice husk. PC also hardens in an alkaline environment and the same happens during the pozzolanic reactions, which mean that the designation “alkaline cement” is not very accurate; Davidovits described PC as AA calcium silicate. The important historic developments of AA binders are summarized briefly by Roy [15], with the use of slag as cement by Feret in 1939 to the development of alkaline cement by Krivenko in 1994. In the activation process of slag (GGBFS), as postulated by Wang et al. [16] the reaction begins with the attack of alkalis on slag particles, thus breaking the outer layer and then continues as polycondensation of reaction products. The initial reaction products are formed due to the process of dissolution and precipitation. At later stages, however, a solidstate mechanism is followed where the reaction occurs on the surface of formed particles dominated by slow diffusion of the ionic species into the unreacted core. In the initial stages of hydration, alkali cation (R1) behaves like a mere catalyst in the cation exchange with Ca21 ions for the reactions shown in the following equations ( [13,17]: Si 2 O2 1 R1 5 Si 2 O 2 R

(13.1)

Si 2 O 2 R 1 OH2 5 Si 2 O 2 R 2 OH2

(13.2)

Si 2 O 2 R 2 OH2 1 Ca21 5 Si 2 O 2 Ca 2 OH 1 R1

(13.3)

While the alkaline cations act as structure-creators, the anions in the solution play a significant role in activation, especially during the early stages, particularly with regard to paste-setting [18]. The final hydration products in the case of activation of slag are similar to the products of OPC hydration, that is, calcium silicate hydrates (C S H), but with a low Ca/Si ratio. However, the rate and intensity of activation of slag differ substantially as compared to that of hydration of OPC. It is reported that the alkalis are not freely available in the pore solution since they are bound to the reaction products, thereby negating the potential for alkali silica reactivity; however this depends on the concentration of alkali used [18].

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13.3

463

Requirements for alkali activation of ground granulated blast furnace slag

GGBFS is obtained by finely grinding the granulated blast furnace slag (BFS), which in turn is obtained by sudden quenching of molten slag removed from the blast furnaces of the iron and steel industry. It mainly consists of oxides of calcium (CaO), silica (SiO2), alumina (Al2O3), and magnesia (MgO), along with some other minor oxides in small quantities. GGBFS is probably the most widely investigated and most effective cement replacement material used in concrete manufacturing. McGannon [19] quantified the hydraulic activity of GGBFS in terms of the basicity coefficient (Kb) which is the ratio between the total content of basic constituents to total content of acidic constituents as given in Eq. (13.4) Kb 5

CaO 1 MgO 1 Fe2 O3 1 K2 O 1 Na2 O SiO2 1 Al2 O3

(13.4)

Wang et al. [16] and Bakharev et al. [20] further simplified the equation by excluding the minor components such as Fe2O3, K2O, and Na2O (generally less than 1%) in the computation of the basicity coefficient, Kb. Kb 5

CaO 1 MgO SiO2 1 Al2 O3

(13.5)

Based on the basicity coefficient (Kb), the GGBFS is classified into three groups: acidic (Kb , 0.9), neutral (Kb 5 0.9 1.1), and basic (Kb . 1.1). Neutral and alkaline slags are preferred as starting materials for activation in AA slag (AAS) binders. Chang [21] introduced a parameter called hydration modulus (HM), as given in Eq. (13.6), and suggested that it should be greater than 1.4 in order to ensure good hydration properties. HM 5

13.4

CaO 1 MgO 1 Al2 O3 SiO2

(13.6)

Alkali-activated slag systems

Finely grounded slag was the first industrial by-product to be activated by alkali due to its pozzolanic nature and GGBFS has proved to be the most suitable material for AA binders. These AASs have been commercially produced and are used in the construction industry [13]. Most of the applications of AAS mixes have taken place in the former Soviet Union, China, and some Scandinavian countries, where the huge impact on the environment in the form of depletion of natural resources, such as limestones and other minerals, has led to the search for alternative binders other

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than PC. Krivenko also found that alkaline cations play a catalytic role during the early stages of hydration of these slags, involving the interchange of Ca21 cations and, in the later stages, they themselves combine into the structure to form zeolitelike phases. Taylor [22] stated that the role of alkalis in AAS is similar to that in blended GGBS OPC cement systems, that is, to maintain the supply of OH2 anions in the system. The hydrated calcium silicate hydrate (C S H) gel is the most abundant product formed in the hardened slag-based geo-polymer pastes [20]. Brough and Atkinson [23] found that, while the inner product contains regions of hydrates of AAS mortars having C S H gel with a higher ratio of Ca/Si (0.9), mixed with higher amounts of magnesium hydrotalcite, the outer product regions have a lower Ca/Si ratio of 0.7 and lower magnesium hydrotalcite contents. Pan et al. [24] developed a composite cementitious material with alkali slag red mud and solid water glass with modulus 1.2. They concluded that the hardened paste mainly consisted of C S H gel, very fine in size, and extremely irregular in shape. According to Bellmann and Stark [25], as BFS contains less lime, calcium hydroxide is not formed during the hydration of the slag particles; instead, ettringite and C S H with a low calcium/silicate ratio are formed.

13.5

Effect of dosage and modulus of activator solutions

Activator modulus (Ms) is defined as the mass ratio of SiO2 to Na2O or (K2O) in an alkaline activator. The dosage or total mass of Na2O in the alkaline activator solution (AAS) basically includes the sum of mass of Na2O present in sodium silicate solution and that is present in the SH. Glukhovsky et al. [26] classified the alkaline activators into six groups according to their chemical composition, namely caustic alkalis, nonsilicate strong and weak acid salts, and aluminosilicates. The majority of researchers have found that activation with sodium silicate blended with SH or sodium silicate alone leads to higher strength. Wang et al. [16] reported that the nature of the activator influences the mechanical strength of AAS mortars and that use of sodium silicate powder leads to lower performance when compared to when taken in a liquid form. Further activator modulus plays a major role in the strength development according to the type of slag [16]. The fineness of slag, concentration and nature of activators, and curing temperatures play a major role in the mechanical properties of AA BFS mortars as reported by Fernandez-Jimenez et al. [27]. They also noticed that optimum Na2O concentration in the alkaline activators varied between 3% 5% by mass of slag. Bakharev et al. [1] found liquid sodium silicate (LSS) to be a better activator as compared to SH and sodium carbonate (SC) activators in terms of the strengths developed in geopolymer concretes. Bakharev et al. [20] investigated the effect of admixtures and the type of activator used on the workability of AAS concrete mixes. They observed that concretes developed using LSS as activator solution exhibited enhanced workability and provided better mechanical properties and also showed reduced shrinkage as compared to normal OPC-based concrete.

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Puertas et al. [28] investigated the strength behavior of activated FA-based cements blended with BFS with different proportions, using SH solution. They concluded that an increase in the slag content and molarity of NaOH solution resulted in an increase in the compressive strength. Again, the slag cements activated with sodium silicate solution with modulus varying between 0.6 1.5, and with appropriate dosage of Na2O, will provide higher compressive strength as compared to PCbased concretes [29]. With increasing contents of metakaolinite and sodium silicate solution, showed an increase in the setting time, compressive strength and fire resistance properties of geopolymer concretes containing blends of GGBFS and metakaolinite as the source materials [30]. Hardijito and Rangan [2] observed that, at a constant ratio of water glass to SH maintained at 2.5, an increase in molarity of SH solution increased the strength of geopolymer concrete mixes.

13.6

Workability and strength characteristics of geopolymers and alkali-activated composites

Addition to ordinary PC causes an increase in angular sized particles in FA -based geopolymer composites leading to a decrease in the workability of these mixtures [31]. On the other hand, the heat evolved during the process of hydration further accelerates the geopolymeric reactions, causing a decrease in the initial setting time characteristics of these composites. Several researchers have proved that an increase in the addition of OPC further accelerates the setting action of these composites. These reactions are responsible for the formation of C S H, along with the AA or geopolymer gel, namely N A S H or C S H gel [32 35]. In general, the workability of FA-based geopolymer composites is usually lower and cannot be easily achieved as compared to the normal OPC-based composites [36]. Inclusion of slag as an additive in the case of FA -based geopolymer mixtures further decreases the workability, due to the higher activation of slag grains in the presence of alkaline solution, and so also decreases the initial setting times. Both initial and final setting of FA -based geopolymer composites can be greatly reduced with an increase in the addition of GGBFS Addition of slag up to 25% can reduce the initial setting time from 300 minutes to as low as 45 minutes [37 41]. Collins and Sanjayan [42] showed that the partial replacement of slag with ultrafine FA significantly improves the workability and setting action of AASC mixes at early stages. A lignosulfonate admixture showed an increase in workability for LSS as well as for combined (NaOH 1 Na2CO3) activators on the AASC mixes. However, naphthalene formaldehyde superplasticizer showed an increase in workability at the initial stage and then quick setting occurred at the later stages [20]. Palacios et al. [43] observed that the dosages of the superplasticizers required to attain similar reduction in the yield stress were about 10 times lower than for ordinary PC when compared with SH AAS pastes. Cahit et al. [44] observed that the

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higher percentage of sodium present in the NaOH and sodium silicate-activated slag paste and mortars led to the development of higher workability and strengths, however the setting time was rapidly decreased. Jang et al. [45] investigated the fresh and hardened properties of AA FA/slag pastes with polycarboxylate-based and naphthalene-based superplasticizers. The polycarboxylate-based superplasticizer showed a retarding effect on AA FA/slag pastes and improved the workability more significantly than naphthalene-based superplasticizer. The rheological behavior in AAS pastes activated with NaOH alone or combined with Na2CO3 was similar to the rheology observed in OPC pastes, and fits the Bingham model. Conversely, the AAS pastes activated with water glass fit the Herschel Bulkley model and their rheology proved to depend on both the activator modulus and the dosage of sodium oxide [46]. The study concluded that initial setting time and compressive strength of the materials are highly dependent on the composition of the materials [47]. Criado et al. [48] showed the alkali activation of FA pastes with four different alkaline solutions having different soluble silica contents. Na2O dosages up to 8% by weight of binder were used to activate soluble silica contents, with sodium alumino silicate (NASH) gel as the primary product. Based on the microstructural studies, they concluded that the amount of gel formed has a decisive effect on the mechanical strength developing in the material. Anurag et al. [49] conducted a series of experiments on FA-based AA concrete by varying the concentration of NaOH and heat-curing time. The performance of mixes produced with different NaOH concentrations (8 M, 12 M, and 16 M) and periods of heat-curing at 60 C with duration of 24, 48, and 72 hours were observed. Compressive strength up to 46 MPa was obtained with curing at 60 C. Cengiz et al. [50] showed that an increase in sodium concentration of the activators results in an increase in all of the compressive, flexural and tensile strengths of AAS mortars and also suggested that there is an optimum ratio to obtain the highest compressive and tensile strengths. Ubolluk and Prinya [51] studied the leaching of SiO2 and Al2O3 by mixing FA with NaOH solution for different time intervals. They found that the solubility of FAbased geopolymer mortars depended on the concentration of NaOH and duration of mixing with NaOH. They also developed geopolymer mortars of 70 MPa strength with a mixture formulated with 10 M NaOH and the ratio of sodium silicate to SH maintained at 1. Deepak et al. [52] showed that the concentration of activator solution has a major influence on the compressive strength of the activated concretes made with FA, whereas the activator to binder ratio influences the compressive strength of activated GGBFS concretes the most. The highest compressive strength of about 64 MPa was obtained for only FAbased AA concrete using the combination of SH and sodium silicate as activator solutions and cured at 75 C for 8 hours followed by curing at 23 C for 28 days [53]. Manjunath et al. [54] developed geopolymer mortars with better strength at ambient curing; the strength increased with an increase in the GGBFS content; but it was found that the addition of a small quantity of silica fume had no significant effect on the strength development characteristics. Keun et al. [55] concluded that

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the calcium hydroxide-based AAS concrete mixes showed enhanced workability, delayed slump loss, and so also increased compressive strengths with W/B ratio as observed in OPC concrete. Hamdy et al. [56] showed the influence of using seawater on the properties of activated slag pastes mixed with SH and sodium silicate liquid (6 wt.% of slag). Their results revealed that the bulk density and compressive strength were increased by increasing the sodium silicate content in the presence of NaOH. Lee and Lee [57] investigated the setting and mechanical strength properties of AA FA/slag concrete manufactured at room temperature. Based on their test results, they concluded that with an increase in the amounts of slag and NAOH solution, there was a decrease in the setting times of the mixes. Maochieh and Ran [58] used AA FA/slag (AAFS) mortars with various ratios of FA to slag. They used constant sodium oxide (Na2O) concentrations of 4% and 6% of total weight of cementitious material, and modulus at 1. The total liquid/binder ratio was kept at a constant of 0.5. Except drying shrinkage, better engineering properties such as compressive and flexural strengths and lower water absorption were obtained in AAFS mortars as compared with OPC mortars. The strength and drying shrinkage of reactive MgO-modified AAS (MAAS) pastes were measured up to 90 days and it was concluded that MgO with high reactivity accelerated the early hydration of AAS, while MgO with medium reactivity had little effect on the characteristics [59]. The experimental results of Yang et al. [60] showed that the addition of nanoTiO2 to AAS enhances the mechanical strength, and decreases the shrinkage of AAS pastes (AASPs). Microstructural results therein also demonstrated that the addition of nano-TiO2 into the AASP results in more hydration products and denser structure, leading to a change in the pore structure, leading to a large reduction in the total porosity of AASP, possibly due to enhanced hydration of AASP. Susan [61] investigated the effects of activator dose on the properties of AAS/metakaolinbased concrete blends in fresh and hardened states. She concluded that there was no significant loss in the compressive strength observed, along with decreased permeability characteristics. GGBFS from different manufacturing locations were selected for CaOactivation by Yeonung et al. [62]. They studied the influence of the slag characteristics on strength development and the reaction products. Despite the seemingly similar characteristics of the slags, each slag developed significantly different strengths varying from 25 to 52 MPa at 28 days. The main reaction products obtained were C S H and calcium hydroxide. Ettringite was also found in the case of raw slag with calcium sulfates. Zuhau et al. [63] investigated the possible use of FA and a local high-magnesium nickel slag (HMNS) as source materials to manufacture geopolymer cement pastes under the room temperature conditions. The results show that, by using optimal quantities of alkali activator and HMNS, compressive strengths up to 60 MPa can be achieved in the resulting geopolymer cements, which are comparable to hardened PCs. Various studies have reported that the AAS concretes display similar or better mechanical properties as compared to OPC concretes.

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Puertas et al. [64] studied the behavior of the fresh and hardened properties of AAS concrete along with OPC-based reference concrete. The results concluded that the slump and rheological behavior vary between OPC-based and AAS concrete mixes. In OPC-based mixes and NaOH-activated AAS concretes, longer mixing times showed an adverse effect on rheology, with a slight improvement in the hardened properties. On activation with water glass, however, longer mixing times improved both the rheological behavior as well as the mechanical strength properties of AAS concrete mixes. Chaitanya and Gunneswara [65] presented a methodology for designing mix proportions of AAS concrete (AASC) for a targeted compressive strength, using particle packing theory. It was observed that the SH concentration and alkaline solution to binder ratio are the most important influential parameters affecting the workability and compressive strengths of AAS concrete mixes. Dali et al. [66] carried out an investigation on AASC mixes in order to provide a comprehensive review of the effect of mix design variables on the slump, strength, and chloride transport. It was concluded that AASC can be designed for different workability levels as well as for different strength grades of concrete. Hilal et al. [67] investigated the performance and microstructure of AASC mixes subjected to different 28-day curing regimens, namely air curing, intermittent water curing (7 days in water followed by 21 days in air), and continuous water curing. Three concrete mixes were prepared with fixed contents of slag, desert dune sand, and aggregates, and were activated by alkaline solutions consisting of sodium silicate and SH. The ratio of AAS to slag was varied between 0.45 and 0.55. It was observed that an alkaline solution:slag ratio of 0.50 showed the optimal performance. Intermittent water curing was found to be the most effective curing regimen, resulting in a reduction in porosity and sorptivity, and also led to increases in bulk electrical resistivity, modulus of elasticity, and compressive strengths of the mixes. In a detailed experiment investigation, Ning et al. [68] developed AAS FA concretes of three representative strength grades: 40, 60, and 80 MPa. While they maintained a low water/binder ratio, and low sodium oxide/binder ratio, they used SC as an admixture. While calcium alumino silicate hydrate (C A S H) gel was recognized as the main reaction product in these mixes, sodium alumino silicate hydrate gel (N A S H) was detected only in air-cured counterparts. A high slump of 200 mm was achieved in the mixes, with quite satisfactory initial setting times (1 3 hours) recorded. Similarly, the results of Ali et al. [69] showed that the increases in SH molarity and the ratio of SH to sodium silicate, enhanced all the mechanical properties of the AASC mixes. However, an increase in alkaline solution-to-slag ratio beyond 0.5 or a curing temperature greater than 90 C had an adverse effect on the strength performance of AAS concrete. A curing period of 2 days showed the best results in terms of the mechanical properties of AASC. Possible utilization of seawater and sea-sand in the production of large volumes of AAS concrete mixes was contemplated in building artificial islands by Shutong et al. (2019) [70]. The study mainly aimed at evaluating the mechanical properties of four different types of AASC mixes: mixes using freshwater and river sand

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(FRASC) (control), mixes using freshwater and sea sand (FSASC), mixes using seawater and river sand (SRASC) and mixes using seawater and sea sand (SSASC). While all four ASSC mixes showed similar strength performances, the AASC prepared with seawater or sea sand showed better resistance to chloride ion permeability, even though their drying shrinkage values were slightly higher. Analyses of results of microstructural studies carried out using SEM (scanning electron microscopy) and XRD (X-ray diffraction), also showed slight variations in the morphology and hydration products in the ASSC mixes, using both seawater and sea sand, as compared to other mixes. Virendra et al. [71] carried out research on an AA binder containing 85% GGBFS and 15% invented chemicals in powdered form as chemical activators. The results showed that the strengths generated by AA binder herein were very similar to those of a control OPC-based mix. Furthermore, the AA binder also exhibited better performance in the case of durability aspects. Zhenzhen et al. [72] explored the influence of alkaline activator (NaOH or NaOH/Na2CO3 solutions) on the performance of AAS mortars using pottery sand as fine aggregate. The setting time, fluidity, compressive strength, and drying shrinkage were measured for all the AAS mortars. The experimental results showed that both the Na2CO3 to NaOH ratio and Na2O content had significant effects on the fresh and hardened properties of AAS mortars. Higher Na2CO3 to NaOH ratio and lower Na2O content resulted in AAS mortars with longer initial and final setting times. Furthermore, higher Na2CO3 to NaOH ratios led to higher later-age compressive strengths of AAS mortars; the AAS mortars with a Na2O dosage of 6% showed the best performance.

13.7

Alkali-activated composites with alternative binders

Escalante Garcia et al. [73] investigated the effect of using coarser granulated BFS (specific surface area 290 m2/kg) as a partial replacement for 30% 70% of the PC. They observed that the strength of the concrete increased with an increase in the amount of slag content, along with the increase in the percentage of Na2O. The effect of admixing silica fume (SF) as a partial binder in an otherwise GGBFS-based AA cement paste was studied by Alaa and Mervat [74]. The replacement of slag with silica fume in the range of 0% 15%, in increments of 5% (by weight), was considered (i.e., 100% slag only to 85% slag 1 15% SF). Sodium silicate solution was used as an activator. It was observed that admixing with silica fume was beneficial in terms of higher strength development, with 5% being the optimum percentage of SF replacement. Aylin and Kemalettin [75] investigated the strength of AA BFS mortars (AAS) with small percentages of very finely ground pumice powder added to them. Alkaline activator with the combination of sodium silicate and SH with a modulus

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of 0.75 was used in the mixes. They concluded that compressive strengths in the range of 29 50 MPa were recorded for different ages (i.e., 7, 28, and 90 days). Small increases in the strengths above were recorded with the addition of 10% of fine pumice powder. Moruf et al. [76] studied the effect of blending ultrafine, palm-oil fuel ash with GGBFS for the development of high-strength AA concrete. They concluded that the use of finely divided, ground BFS (GBFS) only can contribute to higher compressive strengths of AA concretes with combined GGBFS ultrafine palm oil fuel ash (AAGU) binders. Wen and Tsung [77] used desulfurization slag powder (DSP), a waste from the iron and steel industry, and GGBFS as raw materials for producing mortars with no-Portland-cement-based binders. They concluded that molten iron desulfurization slag is a harmless industrial waste that can serve as an efficient alkali activator. Its high-alkali (CaO) contents provide sufficient conditions for triggering the hydration process of GGBFS, attaining the highest strength development for specimens (about 17 MPa) with GGBFS/DSP ratios of 8:2 and 7:3. Adriana et al. [78] used sugarcane bagasse ash (SBA) along with BFS as source materials for preparing AA mortars. Molar solutions of either NaOH or KOH along with sodium silicate solution, prepared such that the activating solutions had modular ratio (SiO2/Na2O) of 0.5, were used. Reasonably high compressive strengths, in the range of 16 MPa to 51 MPa, were recorded. Gao et al. [79] attempted to study the performance on AA, ternary systems. The binders used were GGBFS, FA, and limestone. They concluded that good workability can be achieved in AAS FA limestone blends by using high amounts of fly ash and limestone. It has been recorded that, for a constant limestone powder replacement, a higher compressive strength was observed in samples with a higher slag content, for a constant slag content, the compressive strength increases with increasing limestone powder content. Paulo et al. [80] studied the effect of silicate content on the performance of blended metakaolin/ BFS AA mortars. A reference mortar based on the alkalineactivated metakaolin-based mortar was compared with 60% MK:40% BFS mortars containing different SiO2/Na2O molar ratios in the activator. Based on the results obtained they concluded that the inclusion of BFS in MK-activated mortars showed the reduction of alkaline activation to maintain workability, with significant improvements in the mechanical strength. The blending of AAS concrete with ferronickel slag (FNS) as a partial binder was investigated by Ruilin et al. [81]. It was observed that a partial substitution at dosages below 40% of FNS did not lead to a significant influence on the initial and final setting times; however, blending FNS above 60% prolongs the setting times and also causes a substantial reduction of mechanical strength. Abdollahnejad et al. [82] researched on the possible utilization of fired and unfired ceramic wastes as an additional binder with partial replacement in AAS concrete. The results showed that replacing ground granulated blast-furnace slag with either type of ceramic waste reduced the compressive strength, mainly due to a reduction in the calcium content.

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13.8

471

Alkali-activated composites with different activators

Cengiz et al. [50] investigated the feasibility of using an AA ground Turkish slag to produce a mortar without PC. Three different activators were used, LSS, SH, and SC, at different sodium concentrations. Based on the results of the tests conducted, they found that LSS- and SH-activated slag pastes took more time to set as compared to a PC paste. LSS-, SH-, and SC-activated slag mortars developed maximum compressive strengths of 81(highest), 29 (lowest), and 36 MPa, respectively, and maximum flexural tensile strengths of 6.8 (maximum), 3.8 (minimum), and 5.3 MPa, respectively, by 28 days. Ben et al. [83] investigated the hydration of two slags with different Al2O3 contents, activated with SH and hydrous sodium metasilicate. They concluded that in all systems, C S H incorporating aluminum and a hydrotalcite-like phase with Mg/Al ratio equal to 2, were the main hydration products. The C S H gels present in NaOH-activated pastes were more crystalline and contain less water; a calcium silicate hydrate (C S H) and a sodium-rich, calcium sodium silicate hydrate (C N S H) with a similar Ca content were observed at longer hydration times. Keun et al. [55] strengthened the practical application of the Ca(OH)2-based, AAS (GGBFS) system. Ca(OH)2 of 7.5% was used as the main activator and either 1% Na2SiO3 or 2% Na2CO3 was added as an auxiliary activator. The test results showed higher calcium silicate hydrate (CSH) gels for water curing than for airdried curing and with the use of the single Ca(OH)2-based activator. Maochieh [84] investigated the AASC specimens both with and without phosphoric acid (H3PO4) cured in air, under saturated limewater and in a curing room at relative humidity of 80% RH and temperature of 60 C, respectively, and the performance of AASC mixes was comparable with reference concretes produced using ordinary PC concrete (OPC). It was reported that the strength of slag pastes activated with MgO CaO mixtures decreased with the increasing ratio of MgO to CaO in the early stage, while a much steeper strength gain was observed in pastes with MgO/CaO higher than19/1, that is, Mg by 9.5% and Cao by 0.5% after 28 days and longer [85]. Puertas and Torres [86] explore the feasibility of using urban and industrial glass waste as a potential alkaline activator for BFS (AAS). AAS pastes were prepared with three activators, namely water glass, an NaOH/Na2CO3 mix, and the solutions resulting from dissolving glass waste in NaOH/Na2CO3. They concluded that the strength and microstructural development in the pastes activated with glass waste were similar and comparable to the parameters observed in AAS pastes prepared with conventional activators. Wan et al. [87] studied the use of anhydrous sodium sulfate (Na2SO4) as an alkaline activator in a system in which PC of about 10% 20% and ground granulated blast-furnace slag of about 80% 90% were mixed and added to light-burnt dolomite (LBD) and the effect of hydration and the strength of the cement were also analyzed. The strength of the resulting mixes material improved after the 3rd, 7th,

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and 28th days and confirmed that the pore structure of the paste became dense after 28 days of aging.

13.9

Alkali-activated composites with alternative aggregates

Puertas et al. [88] investigated the alkali silica reaction in water glass activated slag (water glass AAS) and ordinary PC (OPC) mortars using three types of (siliceous and calcareous) aggregates. The tests were conducted as per the method recommended in ASTM C1260-94. Based on the results obtained, they concluded that water glass AAS mortars are stronger and more resistant to alkali aggregate reactions than OPC mortars. Semiha and Cuneyt [89] showed the utilization of ground waste PET bottle aggregate in AAS and slag/metakaolin blended mortars. In PET aggregate mixtures, slag aggregate was replaced with waste PET aggregate, in amounts of 20% 100% by volume. Sodium hydroxide (NaOH) pellets and LSS were used as activators. It was found that the compressive strengths of AAS mortars decreases with an increase in the amount of waste PET aggregate; however they suggested the potential for the use of waste PET as aggregate in the production of AAS mortars. Mithun and Narasimhan [90] proposed CS as an alternative to river sand for use as fine aggregate in AAS concrete (AASC) mixes. They verified the comparable performances of such mixes with CS aggregate as compared to a control OPCbased mix, in terms of their workability, strength, and durability parameters. Parthiban and Saravana [91] studied the influence of using recycled concrete aggregates (RCAs) derived from the demolished concrete waste on the mechanical properties of FA-based geopolymer concrete (GPC) under ambient curing conditions. Based on the test results, they concluded that workability of mixes containing increased amounts of RCA could be improved, using higher of dosages of an appropriate superplasticizer. Alkali activated concrete (AAC) mixes with replacement of RCA even up to 50% exhibited improved compressive strength and water absorption characteristics. Alaa et al. [92] investigated the possibility of using granulated blast-furnace slag (GBFS) as a partial or full replacement for natural silica sand in AAS mortars. The ratio of binder to fine aggregate was 1:2. The strength-behavior of the mortar mixtures after exposure to elevated temperatures in the range 200 C 800 C (at intervals of 200 C) for 2 hours was also investigated. The results indicated that the compressive strengths of the mortar specimens increased with increasing GBFS sand content and showed better performance after exposure to elevated temperatures as compared to controlled OPCC mix. Nitendra et al. [93] investigated the mechanical strength and fatigue performance of AAS FA-based concrete mixes (AASFC) incorporating steel slag as coarse aggregates. AASFC mixes were prepared with steel slag coarse aggregates partially replacing the natural silica coarse aggregates at different replacement levels (0%

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100% by volume, in increments of 25%). The incorporation of steel slag aggregates resulted, in general, in decreases in the mechanical strength parameters of AASFC mixes. The fatigue lives of AASFC mixes containing steel slag were again found to be lower than AASFC with natural coarse aggregates.

13.10

Durability studies on alkali-activated composites

Durability-related issues of any new class of concretes are always of research concern as their acceptance in the field of concrete construction depends on their assured performance in aggressive environments. Density of paste, permeability, and relative availability of pores—their sizes and interconnectivity—in the entire concrete microstructure have a predominant effect on the concrete durability aspects [94].

13.10.1 Chloride resistance of AAC mixes Chloride permeability and hence its relative corrosion of the steel rebars is a major cause of deterioration of reinforced concrete infrastructures [95]. The corrosion resistance of AASC mixes has been shown to be very similar to OPC-based concretes when immersed in 3.5% NaCl solution [96]. Deepak and Narayanan [97] studied the chloride transport resistance of alkali silicate-powder activated slag concretes. Two different Na2O to source material ratios (n) and two SiO2 to Na2O ratios of the activator (Ms) were used to produce concretes proportioned using two slag contents (300 and 400 kg/m3). Rapid chloride permeability (RCP) and nonsteady-state migration (NSSM) tests were used to evaluate the chloride transport behavior. In the tests, alkali silicate powder-activated concretes demonstrated comparable or better chloride transport resistance than OPC-based concretes. The chloride migration coefficients determined for AASC mixes, using NSSM tests, showed smaller values when compared to OPC-based concrete mixes [98]. It can be appreciated that chloride ion penetrability in a concrete mix mainly depends on the concrete microstructure [99], but the RCPT values of AASC mixes are affected by the activator modulus and its alkalinity. Higher activator modulus leads to better resistance to chloride penetration in that lower amounts of charge passed are recorded [100,101]. It has been shown that the corrosion resistance also depends on the type of activator used in the AASC mixes (i.e., KOH or NaOH). AASC mixes activated using calcium hydroxide showed a better resistance against chloride permeability. Calcium hydroxide available in the matrix reacts with chloride ions, leading to the formation of calcium hypochlorite, which reduces the free chlorides available in the pore solution [102]. There are investigations that are mainly focusing on the chloride permeability of AASC. While the test results therein have indicated a good resistance of these mixes against chloride ion penetration [103,104], there are however, a few difficulties in ascertaining the chloride permeability of AAC using the RCPT test. This is

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because of the fact that the pore solution chemistry of AAC may greatly affect the results of RCPT values more than the pore structure [105]. Several other researchers also sounded the same concern on using the RCPT test for measuring the chloride permeability of AAC mixes [84,106,107]. However, a recent review suggested that chloride permeability of AAC is comparable with the OPC-based materials [108]. Thomas et al. [109] reported the results of a detailed experimental study of chloride permeability of AA FA-based concrete, AAS (GGBFS) concrete, and also PCbased concrete mixes. Contrary to previous studies, the RCPT provided a good estimate of chloride permeability in both AAS and AA FA-based concrete mixes. RCPT results showed excellent correlation with diffusion coefficients determined from salt ponding tests. Resistivity measurements, however, exhibited a poor correlation with diffusion coefficients and overestimated the resistance to chloride ion penetration. Again AC and DC resistivity measurements showed significant disagreement between them for AA concrete mixes. Silica-fume admixed AASC showed decreased electrical conductivity and hence improved resistance against chloride penetration [110].

13.10.2 Acid resistance of AAC mixes AA binder showed mass losses of about 6% and 7%, when immersed in 5% concentrated hydrochloric acid and sulfuric acid solutions, respectively, for a period of 30 days. However, for the same acids and concentrations, OPC-based mixes recorded high mass-losses of about 78% 98% [111] Shi and Stegemen [112] investigated the corrosion of different hardened cementing materials such as PC, AA BFS cement (ASC), lime FA (LFA) blend, and high alumina cement (HAC), with gypsum and lime in nitric acid (pH 5 3) and acetic acid (pH 5 5). They observed that PC paste corroded faster than ASC and LFA pastes and HAC paste quickly dissolved in the acid solution. Bakharev et al. [113] studied the durability of AAS concrete containing a neutral slag with a basicity coefficient of 0.93. LSS and SH were mixed providing a modulus Ms of 0.75% and 4% sodium oxide in the alkaline mixture added to slag. Strength losses up to 47% and 33% were obtained for OPC and AAS concretes, respectively, on exposure to acetic acid solution (pH 5 4) for an extended period of 1 year. The poorer performance of OPC-based concrete is attributed to the presence of higher calcium contents present in OPC-based concrete, which form soluble compounds on reacting with acetic acid; on the other hand, the AASC mixes, having lower Ca/Si molar ratio, are more stable in acidic environments. In another investigation by Gourley and Johnson [114], higher mass losses up to about 25% were observed with OPC-based concrete designed for a service life of 50 years after 80 immersion cycles in a concentrated H2SO4 solution (pH 5 1). However, specimens of AAC mix required 1400 immersion cycles to produce the same amounts of mass losses, which accounted for a calculated service life of about 900 years. Similarly better acid-resistance was reported by Sathia et al. [115] for FA-based geopolymer concrete mixes prepared with increased FA contents with sodium silicate and SH as activators, as compared to normal OPC-based concrete.

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Chaitanya and Gunneswara [116] investigated the durability performance of AASC mixes prepared using GGBFS as the sole binder material. Durability tests like sorptivity tests and acid attack (5% of which was acid, up to 56 days) were carried out to in order to evaluate the performance of AASC subjected to aggressive environments. The sorptivity of AASC mixes was found to be more significantly affected by the activator concentration. Results from acid attack test and XRD indicated higher durability of AAS-based concrete mixes. Hammad et al. [117] evaluated the performance of low-calcium FA-based geopolymer (FA-GPm) and AAS-based mortars (AASm) subjected to an aggressive sewer environment. Specimens were extracted from field exposure, after 6 and 12 months. It was concluded that the overall matrix deterioration was much higher in FA-GPm, as a result of crystallization of thenardite, as compared to AASm after 12 months of exposure to the aggressive environment.

13.10.3 Sulfate resistance of AAC mixes The destruction of calcium silicate hydrate is the major deterioration mechanism in the case of sulfate attack on AASC specimens, gypsum being one of the major reaction products [118]. Studies by Heikal et al. [119] suggest a better durability performance of the specimens of sodium silicate-activated slag cement (strength loss of only about 23%) as compared to OPC-based concrete (strength loss up to 37%) mixes, when immersed in 5% magnesium sulfate solution for 180 days. Nitendra et al. [120] studied the durability performance of AA concrete mixes containing steel slag aggregate as partial-to-full replacement (0%, 50%, and 100% by volume) for natural quartz-based coarse aggregates. The test results confirmed that, while AA concrete mixes with natural aggregates exhibited better resistance to sulfuric acid and magnesium sulfate environments as compared to OPC-based concretes, such resistances decreased with increased replacement of natural aggregates with steel slag aggregates, possibly due to incomplete weathering of steel slag aggregates leading to expansive reactions at later stages.

13.11

Elevated-temperature performance of alkaliactivated composites

Uncontrolled fire incidents, especially those occurring in underground constructions, may result in severe risk to human life along with enormous losses of unmovable assets. Further severe damage is also caused to the structures themselves due to such fire accidents [121 123]. Concrete and steel are the two major components of almost all civil engineering structures. During fire accidents, severe damage in the form of spalling, degradation of mechanical strength, etc., can take place in the concrete components, although it is an incombustible material. The impulsive release of huge amounts of heat and aggressive fire gases are mainly responsible for these damages to concrete. The

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phenomenon of concrete spalling and reduction of concrete strength generally occur after being exposed to temperatures beyond 300 C [124,125]. The resistance to degradation of OPC concrete when exposed to elevated temperatures is dependent on the type of constituent materials. The extent of deterioration also depends on the mineralogy and porosity of aggregates used [126]. In OPC-based concrete (OPCC) mixes, cement paste is the least stable constituent at elevated temperatures. Both chemical and physical properties of the OPCC display deteriorations, on being exposed to elevated temperatures beyond 400 C or so, which are due to loss of the interlayer and chemically bound water, and hence the decomposition of calcium hydroxide (CH) and C S H. Such decompositions are responsible for causing an increase in volume by about 44%, which takes place during the process of cooling, due to rehydration of calcium oxide, leading to severe cracking, as suggested by investigators such as Sullivan and. Sharshar [127], Bazant and Kalpan [128], and Handoo et al. [129]. When the concrete specimens are exposed to fire, in OPC pastes, the conversion of calcium hydroxide to calcium oxide takes place in the temperature range of 450 C 500 C. This leads to an expansion taking place followed by a shrinkage, causing the degradation of calcium oxide and hence the strength degradation, leading to cracking and softening phenomena inside the concrete surface. In addition to the above, during heating over a temperature range of 480 C and 510 C, explosive spalling of concrete occurs leading to a large reduction in the load-carrying capacity of concrete structures [130]. When the cement paste is exposed to elevated temperatures, capillary and gel water is evaporated at a temperature ranging between 100 C 150 C. A further increase in the temperature to 150 C 250 C causes shrinkage cracks accompanied by a reduction in the tensile strength. A reduction in the compressive strength, along with evaporation of the chemically bounded water, occurs at a temperature varying between 250 C 300 C. Finally, degradation of calcium hydroxide and C S H gels occurs at a temperature ranging between 400 C 600 C [131]. Several studies have been carried out by many researchers reporting the improved mechanical properties of concrete exposed to elevated temperatures with partial replacement of OPC with FA, GGBFS, and other replacement materials [132 135]. For the blended concrete mixes with 80% OPC blended with GGBFS, and with a low water/binder ratio of 0.23, the compressive strengths were very much improved and so there was also a substantial reduction in the cracking characteristics of those mixes at elevated temperatures [136]. There is only very limited literature available on the elevated temperature performance of AA FA/slag concrete mixes. It is reported that residual strength characteristics of AAS exposed up to 1000 C were similar to OPC, regardless of the activator type used that decalcification of C S H led to strength decreases as in OPC-based concrete [30,137]. Studies carried out by researchers [125,138 141] have reported better stability of AAC mixes (FA/slag based) elevated temperatures (up to 1200 C), and hence great potential for their use in high-temperature applications, as compared to

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OPCC-based mixes. This may be due to the presence of a highly condensed binder gel, the low content of chemically bonded water in the AA gel products, and the absence of CH as a reaction product in AAS systems. Again, it was observed that, with the increase in sodium dosage, initial reference strengths increased, but the residual strengths, after exposure to higher temperatures, exponentially decreased. Natali et al. [142] investigated the high-temperature behavior of ambient cured AA concrete mixes using combinations of ladle slag and metakaolin or ladle slag and FA. The ladle slag/FA-based AAMs exhibited superior strength gains and better thermal stability than the ladle slag/metakaolin-based AAMs, when exposed to temperatures up to 1000 C, which may be due to the instability of C A S H phases formed in the latter group of samples. Park et al. [143] studied the physicochemical properties of binder gel in AA FA/ slag exposed to high temperatures. Test results showed that the strength increased until exposure to a temperature of 400 C, and thereafter started to decrease. The strength increases below 400 C were attributed to the additional binder gel which gets formed on exposure to such temperatures, decreasing the porosity. The dehydration of C A S H and the formation of N A S H simultaneously occurred, inducing the transformation of pore structure from microporous to mesoporous states. Danial et al. [144] studied the influence of alkali activator concentration and curing conditions (ambient, water, and hydrothermal curing) on the heat resistance properties of AAS mortars. The results revealed that hydrothermal curing has a positive effect on the compressive strength of AAS mortars, after exposure to high temperatures, as compared to ambient and water-curing conditions, regardless of the dosage of alkali activator. In terms of flexural strength, the hydrothermal-cured AAS mortars experienced strength gain after exposure to high temperatures and the increase was higher at a certain level of alkali activation. Kiachehr and Mohammad [145] investigated the tensile strength properties of AAS concrete specimens after exposure to elevated temperatures up to 800 C in a fossil fuel gas furnace. The specimens had been initially water-cured for varying periods of 7, 28, and 90 days. The results concluded that the residual tensile strengths of the AAS concrete mixes, after exposure to elevated temperatures, were noticeably higher than those of normal control OPC-based concrete mix.

13.12

Behaviour of alkali-activated composites incorporated with fibers

Susan et al. [146] investigated the mechanical and permeability properties of AAS concrete (AASC) mixes, at early ages, reinforced with steel fibers. They observed a slight reduction in compressive strengths on incorporations of fibers. However, split-tensile (3.75 4.64 MPa) and flexural strengths (6.40 8.86 MPa) were largely improved with increasing fiber volumes, at 28 days of curing. Natali et al. [147] observed that, by incorporating 1% of fibers (by weight of binder) embedded in the

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geopolymer matrix, it would be possible to have a 30% 70% increase in the flexural strengths, depending on the type of fiber used. They also observed that geopolymers exhibited significantly improved post-crack behavior and enhanced ductility of the material and hence better energy absorption capacity, with the addition of PVC or carbon fibers. Alaa [148] adopted different fibers in AAS system which were used to modify some properties of the system. They observed slight increases in the workability with the admixture and also decreased shrinkage, with the increase in steel fiber contents and fiber lengths. They also observed an increase in compressive and flexural strengths as well as toughness of the AAS system. The addition of polypropylene fibers has not yielded significant increases in compressive strengths, but a slight increase in tensile strength. Carbon fibers and basalt fibers also failed to improve the compressive strength of the systems. The effects of length and volume fraction of steel fibers on the mechanical properties and drying shrinkage behavior of steel fiber-reinforced AAS/silica fume (AASS) mortars was investigated by Serdar and Bulent [149]. Steel fibers were used with two different lengths (6 and 13 mm), and at four different volume fractions (0.5% 2.0% by total volume of mortar mixes). A PC (OPC)-based control concrete having 1.5% steel fibers (13 mm length) was used as the reference mix. Test results showed the mechanical performance of AASS mortars was better than OPC-based control mortar. They also observed a dramatic improvement of mechanical performance of AASS with the increment of fiber length from 6 to 13 mm, and also decreased drying shrinkage of AASS mortars was observed with the increase in fiber dosage. Se et al. [150] explored the rheological and mechanical properties of fiberreinforced AAS-based, clinkerless composites. Single fiber pull-out test and matrix fracture tests were conducted to characterize the micromechanical behavior of the composites. They concluded that low plastic viscosity, low yield stress, and high ductility can be attained by employing an AAS-based binder with a water to binder ratio of 40% and 1.3% (by volume of composites) of PVA fibers. A detailed study on the mechanical properties of AAS (AAS) pastes, reinforced with carbon fibers (CFs) and the effect of the concentration of alkaline activator (Na2O% and silica modulus) and fiber aspect ratio (using different length fibers with the same diameter) on such properties was completed by Vilaplana et al. [151]. They concluded that CF was capable of improving the mechanical strengths of AAS pastes. The addition of CF increased bending strengths up to fivefold and increased the compressive strength by up to 20%. Jeong et al. [152] made a detailed study of meso-level composite properties of an ultrahigh ductile polyethylene fiber-reinforced AAS-based composite. Four mixtures were formulated with 1.75% of polyethylene fibers (by volume of slag), with varying water-to-binder ratios. Enhanced tensile strain capacity (up to 7.50%) and tensile strength (up to 13.06 MPa) were also observed in these polyethylene fiberreinforced AAS-based composites. Again, the average ratio of tensile strength to compressive strength of the composites in the series of tests was a high 19.8%, which is nearly double that of normal concrete.

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13.13

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Behaviour of rebar-reinforced structural elements made from alkali-activated concrete mixes

Over the years, relatively limited studies have been carried out on the flexural characteristics of structural beams, with steel rebar reinforcement, made up of AA concrete mixes. Sumajouw and Rangan [153] conducted extensive studies on FA-based geopolymer concrete reinforced beams and columns. The behavior and failure modes of reinforced GPC columns and beams were similar to those observed in the case of reinforced PC concrete beams. Detailed test results of Chang [154] showed that geopolymer concrete (GPC) beams can achieve sufficient strength for structural designs, but both compressive strength and flexural tensile strength are affected by drying, any differential drying shrinkage leading to microcracking at the drying surfaces. Furthermore, the shear characteristics of the reinforced GPC beams were also similar to those of reinforced OPCC beams. Similar results on flexural and shear performance were found for only slag-based AA concrete beams also by Lee et al [155]. Narasimhan et al. [3] studied the flexural strength properties of reinforced FAand GGFBS-based geopolymer concrete beams. According to them, load deflection curves of GPC-RC beams are very similar to normal RC beams. They recorded appreciable first load and ultimate loads for the beams; the ratios of ultimate load to first crack load were in the range of 1.7 2.7. The flexural and shear performance of reinforced AAS concrete beams are very similar to reinforced concrete beams made of OPC. Dattatreya et al. [156] conducted tests on a set of under-reinforced geopolymer concrete beams, which were observed to behave quite similarly in terms of first load crack, width of crack, load deflection behavior, flexural stiffness, and ultimate load as compared to reinforced OPCC beams, under flexural loading. A slightly more brittle failure was observed during crushing of reinforced GPC when compared to conventional reinforced OPCC beams. Similar observations were made by Yost et al. [157] and Kumaravel and Thirugnanasambandam (2013) [158] during their experimental investigations into FA-based geopolymer concrete beams. Pradeep et al. [159] and Sanjay et al. [160] carried out investigations into the flexural behavior of heat-cured FA- and GGBS-based geopolymer concrete (90% FA 1 10% GGBS, by volume), with normal aggregates as well as recycled aggregates. SH (NaOH, 12 16 M) solution along with sodium silicate solution, taken in the ratio (1:2.5), was used as the activator solution. From their test results, they concluded that with an increase in tensile reinforcement, the first crack load and stiffness of the beam would also increase. The service load deflections were well within the limits prescribed under IS:456-2000 [161]. The experimental ultimate momentcarrying capacity was within (1.5% 1.7% more) the theoretical ultimate moment capacity computed for the beam. Kathirvel and Kaliyaperumal [162] also researched on the possible use of recycled concrete aggregates in FA-based geopolymer concrete mixes. They observed that an increase in the recycled concrete aggregate content in the

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representative AASC mixes showed a marked increase in the load-carrying capacity up to 22.5%, up to an aggregate replacement level of 75%. A further increase in the RCA content showed a detrimental effect. Again, the geopolymer concrete beams with higher RCA contents also showed improved ductility as well as deflection characteristics. The failure of beams, designed as under-reinforced beams, was mainly due to the flexural failure and no sign of shear cracks was observed. Thus, it is suggested also to use the current provisions in the design codes for OPC-based reinforced concrete elements for a conservative design of reinforced geopolymer concrete members [163]. Un et al. [164] experimented the long-term behavior of concrete beams constructed of geopolymer concrete (GPC) mixes. A self-weight and sustained load of 1 kPa was applied on top of the beams at the age of 14 days to simulate construction conditions. Predictions of beam deflections are performed using alternative methods such as rate of creep method (RCM), effective modulus method (EMM), and age-adjusted effective modulus (AEMM). Experimentally determined properties of the GPC mixes, such as elastic modulus, modulus of rupture, creep, and shrinkage were used in the predictions of long-term deflections. It was concluded that the AEMM could be used for satisfactory prediction of long-term deflections for GPC beams with minor parameter modifications. In order to facilitate fast-track constructions, it is imperative that the advancements in precast technology should be fully exploited in the context of any new concrete material. Large-scale adoptability of AA concretes in the precast industry is then to be verified and ascertained [165]. Possible use of geopolymer concretes/AAS concretes as fast-repair materials, if demonstrated, would prove useful, since undertaking small repairs is inevitable for maintaining any infrastructure facility [166]. In one scenario, where new exploits in the domains of nano-science and nanotechnology are affecting all types of human activities, the effect of incorporating nanosilica in geopolymer concrete mixes for use in flexural members was investigated [167]. The results showed a comparatively higher flexural performance for reinforced concrete beams made with GPC mixes cured at ambient temperature, as compared to GPC beams which were heat cured or were of OPCC beams without nanosilica. Similar load-deflection characteristics and the moment curvature relationship along with cracking behavior were observed for all the reinforced GPC beams as compared to reinforced OPCC beams. The first, full-scale alkali activated slag (AAS) concrete mix structural application in terms of a multistoried building with an area of 4500 m2 was constructed in China. It consists of all the structural components such as slab, beams, and columns with 550 m3 of AASC mix [168].

13.14

Summary of alkali-activated composite systems

Based on a detailed review of the available research, it can be observed that the mechanical and durability properties of concrete/mortar mixes with AA binders are

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affected by several factors and acceptable performance characteristics can be achieved as compared to ordinary PC concrete mixes, by proper designing of these mixes. AASC mixes are now distinctly differentiated from the geopolymer concrete (GPC) mixes based on the hydration products obtained. In their hardened states, while AASC mixes show the signatures of products of hydration like calcium silicate hydrates (C S H), calcium alumino silicate hydrates (C A S H), and sodium alumino silicate hydrates (N A S H), geopolymer concretes should have the signatures of sialates, sialate siloxo, and/or sialate disilaxo also. Furthermore, these AA composite mixes can be obtained by activation of a large set of alternative binder materials using, again, an alternative range of alkaline solutions. The major factors affecting the strength of AAC mixes are type, chemical composition and content of the binder, type of alkaline solution and percentage of alkali oxide (sodium or potassium) present in it, water-to-binder ratio, and curing temperature. SH and sodium silicate solutions, individually or in combination, present themselves as the most effective alkaline activators, both in terms of their performance as well as cost. However w/b ratio and activator dosages are to be properly controlled so as to obtain significantly improved strengths in AASC mixes. Many researchers have suggested the use of molarity of SH solution and the modulus (sodium silicate to SH ratio) for describing the chemistry of the activator solution in their studies on FA-based and FA/slag-based geopolymers. In general, while an increase in the modulus, Ms, of the activator solution increases the strength, it decreases the workability of the mixes. An optimum modulus of 1.0 1.25 is recommended in the mixes for better strength and durability properties. Not many chemical admixtures are commercially available, as of now, which can lead to more acceptable performances of the AAC concrete mixes. It is also possible to consider a host of alternative materials such as coarser GGBS, recycled aggregates (construction demolition wastes), steel slag, CS, waste bottle aggregates, and waste plastics, as fine/coarse aggregates in the production of AAC mixes, at appropriate, optimum replacement levels. The role of ratios of aggregate-to-binder and fine-to-coarse aggregates, also plays a very important role in the strength performance of AAC mixes. In general, specifications in terms of provisions of the standard code of practice provisions are not yet formulated for mix design of AAC mixes and hence mix proportions are being arrived at, as of now, most often, on a trial-and-error basis, to meet the specified requirements. AA FA/slag-based concrete mixes, with satisfactory properties, both in their fresh and hardened states, can be produced even with curing under ambient (room temperature) conditions. Furthermore, fiber-reinforced AAC mixes can be produced with an optimum quantity of appropriately selected fibers for better fracture performance of the mixes. AAC mixes using conventional aggregates, in general, exhibit relatively better performance, when subjected to a variety of chemically aggressive environments, as compared to their OPC-based counterparts. AAC concrete mixes also are found to exhibit a comparatively better performance, when subjected to elevated temperatures, as compared to normal OPC-based concrete mixes. AA concretes are also observed to perform well under alternate freezing and thawing. The results of

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limited studies on structural elements made of steel rebar-reinforced AAC mixes have indicated comparable, and at times better, performance under loads, as compared to OPC-based structural concrete elements. The embodied energy, carbon footprints, and cost in developing AAC mixes are comparatively low when compared to the normal ordinary Portland concrete mixes.

13.15 G

G

G

G

G

G

G

Future trends for AA composites—research needs

Detailed studies on the issues related to initial setting and hardening behavior of alkali activated (AA) concrete mixes so as to enable use of RMC technology, with appropriate choice of chemical admixtures. Detailed studies connected with rheological properties of the AA concrete mixes so that pumpable and self-compacting types of mixes can be developed. Studies toward developing AA concrete mixes which can be classified under highstrength and ultrahigh-strength concretes. (Compressive strengths in the order of say, 100 120 MPa and above, as in OPC-based reactive powder concrete mixes.) Studies on durability performance of high-strength, high-flowability AA concrete mixes with satisfactory setting characteristics. Research has been conducted on AAC mixes with a variety of industrial slags, individually as partial/full replacements for either river sand (fine aggregate) and crushed granite chips (as coarse aggregates); however, there is scope for detailed studies on the possible use of such slags simultaneously as both fine and coarse aggregates, in high-strength, high-performance AA concrete mixes. Evaluation of performance characteristics of high-performance AA concrete mixes when subjected to sustained elevated temperatures. Evaluation of performance of structural elements like RC columns and beams made of such high-performance AA concrete mixes, under axial loads and/or flexure.

References [1] T. Bakharev, J.G. Sanjayan, Y.B. Cheng, Alkali activation of Australian slag cements, Cem. Concr. Res. 29 (1999) 113 120. [2] D. Hardijito, B.V. Rangan, Development and properties of low calcium flyash-based geo-polymer concrete, Research Report GCI, Faculty of Engineering Curtin University of Technology, Perth, Australia, 2005. [3] M.C. Narasimhan, G. Nayak, B.T. Ajith, M.K. Rao, Development of alternative binders to Portland cement concrete using fly ash and blast furnace slag: some experiences, in: Proceedings of the International UKIERI Concrete Congress New Delhi, India, 2011, pp. 43 60. [4] A. Ferna´ndez-Jime´nez, A. Palomo, Characterization of fly ashes. Potential reactivity as alkaline cements, Fuel 82 (2003) 2259 2265. [5] A. Fernandez-Jime´nez, A. Palomo, M. Criado, Microstructure development of alkaliactivated fly ash cement: a descriptive model, Cem. Concr. Res. 35 (2005) 1204 1209. [6] J. Davidovits, Geopolymer, Green Chemistry and Sustainable Development Solutions, Institute Geopolymer, Saint-Quentin, France, 2005, pp. 9 15.

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[141] M. Guerrieri, J. Sanjayan, F. Collins, Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures, Mater. Struct. 43 (6) (2010) 765 773. [142] M.A. Natali, W.D.A. Rickard, M.C. Bignozzi, A.R. Van, High temperature behaviour of ambient cured alkali-activated materials based on ladle slag, Cem. Concr. Res. 43 (2013) 51 61. [143] S.M. Park, J.G. Jang, N.K. Lee, H.K. Lee, Physico-chemical properties of binder gel in alkali-activated fly ash/slag exposed to high temperatures, Cem. Concr. Res. 89 (2016) 72 79. [144] N. Danial, H.P. Amir, G. Hossein, The influence of curing conditions and alkaline activator concentration on elevated temperature behaviour of alkali-activated slag (AAS) mortars, Constr. Build. Mater. 190 (2018) 108 119. [145] B. Kiachehr, S. Mohammad, The effect of elevated temperature on the residual tensile strength and physical properties of the alkali-activated slag concrete, J. Build. Eng. 20 (2018) 442 454. [146] B. Susan, D.G. Ruby Silvio, R. Erich, Performance of an alkali-activated slag concrete reinforced with steel fibers, Constr. Build. Mater. 24 (2010) 208 214. [147] M.A. Natali, S. Manzia, M.C. Bignozzia, Novel fiber-reinforced composite materials based on sustainable geopolymer matrix, Procedia Eng. 21 (2011) 1124 1131. [148] M.R. Alaa, A comprehensive overview about the influence of different additives on the properties of alkali-activated slag—a guide for civil engineer, Constr. Build. Mater. 47 (2013) 29 55. [149] A. Serdar, B.S. Bulent, The effect of fiber properties on high performance alkaliactivated slag/silica fume mortars, Compos. Part B 45 (2013) 63 69. [150] J.C. Se, C. Jeong, K.S. Jin, Y.L. Bang, Rheological and mechanical properties of fiber-reinforced alkali-activated composite, Constr. Build. Mater. 96 (2015) 112 118. [151] J.L. Vilaplana, F.J. Baeza, O. Galao, E.G. Alcocel, E. Zornoza, P. Garce´s, Mechanical properties of alkali activated blast furnace slag pastes reinforced with carbon fibers, Constr. Build. Mater. 116 (2016) 63 71. [152] C. Jeong, B.Y. Lee, R. Ravi, C. Victor, Y. Lee, Ultra-high-ductile behaviour of a polyethylene fiber-reinforced alkali activated slag-based composite, Cem. Concr. Compos. 70 (2016) 153 158. [153] D.M.J. Suumajouw, B.V. Rangan, Low calcium based Geopolymer concrete: reinforced beams and columns, Research report GC3, Curtin University of Technology, Perth, Australia, 2006. [154] E.H. Chang, Shear and bond behaviour of reinforced fly ash-based geopolymer concrete beams, Doctoral Thesis, Curtin University of Technology, Australia, 2009, pp. 1 409. [155] K.M. Lee, S. Choi, J.F. Choo, Y.C. Choi, S.W. Yoo, Flexural and shear behaviour of reinforced alkali-activated slag concrete beams, Adv. Mater. Sci. Eng., Article ID5294290 (2017) 1 12. [156] J.K. Dattatreya, N.P. Rajamane, D. Sabitha, P.S. Ambily, M.C. Nataraja, Flexural behaviour of reinforced geopolymer concrete beams, Int. J. Civ. Struct. Eng. 2 (1) (2011) 138 159. [157] J.R. Yost, A. Radlinska, S. Ernst, M. Salera, N.J. Martignetti, Structural behaviour of alkali activated fly ash concrete. Part 2: structural testing and experimental findings, Mater. Struct. 46 (2013) 449 462. [158] S. Kumaravel, S. Thirugnanasambandam, Flexural behaviour of reinforced low- calcium fly ash based geopolymer concrete beams, Glob. J. Res. Eng. 13 (8) (2013).

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[159] J. Pradeep, R. Sanjay, M.U. Ashwath, Experimental study on flexural behavior of fly ash and GGBS based geopolymer concrete beams in bending, Int. J. Emerg. Trends Eng. Dev. 6 (2012) 419 428. [160] R. Sanjay, R.S. Singh, M.U. Aswath, An experimental study on flexural behavior of reinforced geopolymer concrete beams with recycled aggregate in bending, Int. J. Emerg. Trends Eng. Dev. 6 (2) (2012) 186 199. [161] IS: 456-2000, Reaffirmed 2005. Plain and Reinforced Concrete Code of Practice, Bureau of Indian Standards, New Delhi, 2005. [162] P. Kathirvel, S.R.M. Kaliyaperumal, Influence of recycled concrete aggregates on the flexural properties of reinforced alkali activated slag concrete, Constr. Build. Mater. 102 (1) (2016) 51 58 (2016). [163] P.K. Sarker, Structural behaviour and design of geopolymer concrete members, Civ. Eng. Dimens. 17 (3) (2015) 133 139. [164] C.H. Un, J.G. Sanjayan, R.N. San, J.S.J. van Deventer, Predictions of long-term deflection of geopolymer concrete beams, Constr. Build. Mater. 94 (2016) 10 19. [165] C.A. Jeyasehar, G. Saravanan, M. Salahuddin, S. Thirugnanasambandam, Development of fly ash based geopolymer precast concrete elements, Asian J. Civ. Eng. 14 (4) (2013) 605 615. [166] Y. Wanchai, Application of fly ash-based geopolymer for structural member and repair materials, Adv. Sci. Technol. 92 (2014) 74 83. [167] D. Adak, M. Sarkar, S. Mandal, Structural performance of nano-silica modified flyash based geopolymer concrete, Constr. Build. Mater. 135 (2017) 430 439. [168] K. Yang, C. Yang, J. Zhang, Q. Pan, L. Yu, Y. Bai, First structural use of site-cast, alkali-activated slag concrete in China, Struct. Build. 171 (10) (2018) 800 809.

Porous concrete pavement containing nanosilica from black rice husk ash

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Ramadhansyah Putra Jaya Department of Civil Engineering, College of Engineering, University of Malaysia Pahang, Kuantan, Malaysia

14.1

Introduction

Malaysia has a tropical rainforest climate and rainfall throughout the year. Due to its rapidly expanding built-up areas, more and more regions in Malaysia are being paved. This will result in increasing runoff and accumulation of water during heavy rainfall in the low-lying areas. Conventional pavement, whether asphalt or concrete, are designed with the purpose of preventing water from seeping through to the soil beneath. In the other words, if the stormwater runoff cannot be controlled, flooding will occur. Lately, during the heavy rainfall season, there have been several flooding incidents in Malaysia. Various methods have been used in order to mitigate flooding on roads, such as designing proper drainage systems at the road shoulders, designing proper super elevation on the roads to flow water out of pavement, and also by using permeable or porous pavement to allow flowing water to seep through it structure. During heavy rainfall, there is a lot of water splash and spray on roads due to vehicles hitting the accumulated water. The water splash and spray is well known as a cause of accidents during the rainfall season [1]. Recently, environmental issues have become a matter of global concern. Because of the environmental benefits, porous concrete has become increasingly used in a variety of infrastructures, pavements, and overlays subjected to heavy traffic load. Due to these extended applications, superior strength and durability constitute the main concerns associated with porous concrete [2]. Over the years, developed countries have begun to use porous concrete pavement as a solution for stormwater runoff on road pavement. In addition to its characteristic of allowing water through its structure, porous concrete pavement has the capacity to absorb noise generated from vehicles and other sources. Porous concrete pavement is well known as concrete with limited or no fine aggregates that have high porosity in their structure. This porosity tends to allow water through its structure and allows it to absorb surrounding noise. The concept of the concrete is indicative of the fact that the void in the concrete structure tends to decrease the strength of the concrete. According to the Specifier’s Guide for Pervious Concrete Pavement Design, the New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00014-4 © 2020 Elsevier Inc. All rights reserved.

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recommended structure’s void for porous concrete is 15% minimum and 25% maximum. This high percentage of voids tends to lower the strength of the concrete. At present, many waste materials have been processed and used in concrete mixtures in order to improve their performance, such as rice husk ash (RHA), palm oil fuel ash (POFA), fly ash (FA), bottom ash, slag, and many more [3
8]. Each of these materials contains silica or alumina capable of improving the performance in concrete. However, very limited information is available on concrete pavement containing black RHA (BRHA), specifically in relation to porous concrete pavement. In addition, no research has been conducted in order to evaluate the effects of porous concrete pavement incorporating nanomaterials from BRHA. Hence, BRHA was selected as a replacement material in porous concrete pavement mixtures. BRHA is an RHA containing a high percentage of carbon after the burning process. This material has a high silica content that was needed in the concrete reaction in order to improve performance. In line with the research concept to reuse the waste material, the use of BRHA in the porous concrete pavement mixture tends to decrease the production of waste material globally and provides benefits to the pavement industries.

14.1.1 Description of the problem The use of concrete pavement is well known for its durability and reduced maintenance compared to asphalt pavement over the long term. The durability of concrete is a major concern in the construction industry around the world. Different from conventional concrete pavement, porous concrete pavement has a high percentage of voids ranging from 15% to 25% [9]. The strength of porous concrete is significantly affected by the porosity of its internal structure [10]. One of the approaches to strengthen concrete strength is to minimize the number of voids in the concrete structure. Compared with conventional concrete pavement, porous concrete pavement is designed to have a high percentage of voids in it structure. At the same time, the required strength must be achieved to meet vehicle loads. In order to improve the porous concrete pavement strength, one of the elements in the porous concrete structure that needs to be considered is the cement paste binder. The cement paste binder plays an important role in the porous concrete pavement. Besides aggregates, the strength of the porous concrete pavement depends on the strength of the cement paste binder. The existence of micropores and microcracks in the cement paste binder tends to weaken the hardened cement paste structure. One of the reasons for micropores resulting in the hardened cement paste binder structure is the process of fresh concrete becoming hardened concrete. The water in the fresh cement paste binder dries out and leaves micropores in the structure. Recently, a great deal of research has been conducted in order to explore the advantages of using nanomaterials in these products. In the concrete industry, nanosilica is one of the nanomaterials studied in research. The advantage of using nanosilica is the capacity thereof to fill the gaps created by micropores. Studies have been conducted in order to explore the advantages of using nanomaterials in

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concrete mixtures and cement mortar. It was found that by using nanomaterials, the strength of the concrete was improved [11
13]. Various sources can be used to produce silica, such as BRHA, POFA, and FA. In this research, the nanosilica produced from BRHA was selected as a main component for the porous concrete pavement mixture. The BRHA was ground until reaching a nano size. This ground BRHA is known as nano-BRHA and was used for this research. In this study, the laboratory mill grinder was developed to produce the nano-BRHA. The lack of knowledge on the outcomes of developed laboratory mills grinder is an issue that needs to be solved. The lack of research conducted on porous concrete containing nanosilica produced from BRHA is the main reason this research needs to be conducted. The main reason porous concrete pavement has low strength is the high void percentage in its structure. Porous concrete has been used for many years, but there are still many outstanding issues related to its structural performance [14]. Although there is fundamental information, including the influence of the water
cement (W/C) ratio, void ratio, cement paste characteristic, volume ratio of coarse aggregate, size of coarse aggregate, and strength of porous concrete, the optimum condition to produce good porous concrete is remains to be established [15,16]. In addition, it has been observed that the optimum quantity of nanosilica to be used is contradictory and it is the individual researcher’s responsibility to decide the optimum quantity for their own material [17]. In addition, the nanoscale study of hydration products (CSH, calcium hydroxide, ettringite, monosulfate, unhydrated particles, and air voids) is aimed at overcoming durability issues as a crucial step in concrete sustainability [18]. Furthermore, to produce clear and good laboratory results, the method and procedure for preparing porous concrete specimens, such as compacting and curing specimens, must be accentuated. In order to improve the mechanical properties of porous concrete pavement and to explore the advantages of using nanosilica produced from BRHA in the porous concrete mixture, this research was conducted.

14.1.2 Significance of research Rice husk is a waste material that can be used in the concrete industry. The high silica content in rice husk can be extracted using certain methods. From the literature review, the inclusion or replacement of nanosilica in the cement content of a concrete mixture has high potential to improve the properties of hardened concrete. The lack of research conducted on the properties of porous concrete pavement containing nanosilica produced from rice husk is the main idea for the basis on which this research was conducted. Nanosilica produced from BRHA is known as nanoBRHA. The difference percentages of nano-BRHA are replaced in the cement content producing different performance results. Therefore it is important to investigate and find the optimal percentage of nano-BRHA replacements in the cement content of the porous concrete mixture. The outcomes of this research created the idea for future demand within the porous concrete pavement industry and among other researchers.

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Literature review

A typical pavement consists of up to five specific layers, as illustrated in Fig. 14.1. The subgrade, selected, and subbase layers typically consist of granular materials, while the base can either be a bound or unbound granular material. Bound granular base layers can be engineered using either cement or bitumen. The surfacing of the pavement typically consists of bituminous or cementitious materials. These can consist of surfacing seals (a layer of aggregate bound by a layer of bitumen), asphalt (a manufactured mixture of aggregate, fillers, and bitumen) or concrete (a manufactured mixture of aggregate, fillers, and cement). In addition, reference may be made to another type of pavement, namely the porous pavement. This type of pavement consists of few or no fillers to produce a porous structure. Porous concrete is also referred to as pervious concrete, no-fines concrete, and permeable concrete. Porous concrete is a special type of cementitious material composed of gap-graded aggregates, coated with a thin layer of cement paste, and bonded by the cement paste layers partially being in contact [19]. Porous concrete is a concrete with continuous voids, which are purposely incorporated into concrete. The finished surface is not tight and uniform, but is open and varied, to admit large quantities of storm water [9]. The ability of the porous concrete pavement to allow water to percolate into the underlying strata depends on its porosity, which is one of its most important pore structure features [20]. This type of concrete is a completely different category from conventional concrete and therefore its physical characteristics differ greatly from those of normal concrete [16]. The research into porous pavement materials has been conducted by developed countries, such as the United States and Japan since the 1980s [21]. Because of the multiple environmental benefits associated with controlling stormwater runoff,

Figure 14.1 Typical pavement structure.

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restoring groundwater supplies, and reducing water and soil pollution, they have been widely used in Japan, the United States, and Europe. Porous concrete has been utilized in road pavements due to its water-permeating, water-draining, and waterretaining capacities [22]. In addition, it is currently used in various applications that require noise absorption or thermal insulation [19,23,24]. Schaefer et al. [25] mentioned four reasons why a porous concrete overlay is suitable as a road pavement. First, the porous concrete pavement can absorb noise on the road. Second, it increases skid resistance between tires and the pavement. Third, it decreases water splash and spray on the road, and finally it improves friction as a surface wearing coarse. In addition to its advantages, porous concrete pavement also has a series of disadvantages. When placing the fresh concrete, the workability of fresh Portland cement porous concrete is usually decreased by the exclusion of fine aggregates [26]. Scholz and Grabowiecki [27] found in their review that porous concrete pavement systems are prone to clogging, usually within 3 years after installation. Different from the conventional concrete pavement, because of the high porosity percentage, the porous concrete pavement is characterized by low strength and durability [22,26].

14.2.1 Problems regarding the porous concrete structure Various studies on porous concrete have been conducted by other researchers, and usually, the voids percentage ranges between 15% and 30% [16,22,23,28,29]. There are several reasons why the hardened concrete strength tends to decrease. One such reason is the voids content in the hardened concrete itself. From the literature review, it is also found that, when the void percentage increases, the strength of the hardened concrete tends to decrease [22
24]. As reported by Chen et al. [2], there are four major factors that will influence porous concrete strength, namely concrete porosity, water-to-cement ratio (W/C), cement paste characteristics, and size and volume content of the coarse aggregate. In order to enhance the strength of the concrete, silica fume and superplasticizers can be used in the mixture’s design [30]. Therefore, to improve the engineering properties of porous concrete pavement, the mixture needs to be modified by adding additional material (superplasticizer, steel fiber) or by replacing some percentage of cement with other materials (nanomaterial, silica fume).

14.2.2 Black rice husk ash One of the sources of nanosilica is RHA. According to Carmona et al. [31], approximately 20% of the rice husk is inorganic compounds. They also found in their literature review that of this 20% inorganic compounds, 94% contained silica. RHA is an agricultural waste classified as “a highly active pozzolan,” because it contains a very high amount of amorphous silica and a large surface area. BRHA is defined as RHA that contains a high amount of carbon after the burning process. Another name for BRHA is rice husk char. Noorvand et al. [32] conducted a scanning electron microscopy analysis on BRHA and ground black rice husk for their study, and

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found that the particle mean sizes of BRHA and ground BRHA are 100 and 34.9 μm, respectively. On the other hand, the BRHA maintained its cellular structure with an angular, irregular, and porous particle shape and porous cellular surface. Ismail et al. [33] characterized RHA using X-ray diffraction (XRD) and found that cristobalite, tridymite, and quartz are the major phases present in the SiO2 obtained from rice husk. Sarangi et al. [34] confirmed in their study that the major peak in BRHA is silica. Ramadhansyah et al. [35] conducted a thermal analysis on RHA in their study, and found that there are four endothermic peaks. The first endothermic peak is located at 100 C
350 C and corresponds to polysaccharide depolymerization. The second endothermic peak is located at 350 C
600 C and results from the dehydration of sugar. The third endothermic peak was found at 650 C
850 C and corresponds to silica in amorphous form. Lastly, the forth endothermic peak was detected in the range of 900 C
1200 C and was attributed to the crystallization of tridymite and cristobalite.

14.2.3 Nanomaterials In recent years, the use of nanomaterials has received particular interested in various fields of application, in order to improve existing technologies and also produce materials with new functionalities [36,37]. Research and developments have established that the application of nanotechnology can improve the performance of conventional construction materials, such as concrete and steel [38]. Nanomaterials are used as replacements for cement parts. The nanoscale size of the materials can result in significantly enhanced properties from conventional grain-size materials of the same chemical composition. Previous research studies into mixing nanomaterials in cement indicate that the inclusion of nanomaterials modifies the fresh and hardened properties [39]. Nanoparticles have a high surface area to volume ratio and also provide high chemical reactivity [40].

14.2.4 Porous concrete pavement containing nanosilica According to Tennis et al. [41], porous concrete pavement contains 15%
25% voids. This void percentage tends to lower the strength. One of the ways to strengthen the concrete is by enhancing the strength of the cement binder. The existence of some pores and microcracks in the hardened cement paste will influence the cement paste strength greatly [30]. The advantage of using nanosilica is that it can act as a nanofiller to fill up the pores in the cement paste binder. PachecoTorgal et al. [40] mentioned in their research that nanomaterials allow for a dramatic increase in the mechanical strength of the cementation composites. The nanomaterials fill the voids of the CSH structure and lead to a denser hardened cement binder. Heikal et al. [11] concluded in their study that the composite cements containing nanosilica are characterized by optimum mechanical properties. In theory, a lower void percentage in the hardened cement binder resulted in greater strength. Indirectly, it improves the strength of the hardened cement binder and porous concrete pavement.

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14.2.5 Mixed design method of porous concrete pavement The porous concrete pavement differs from the conventional concrete pavement. The placement of porous concrete pavement requires unique procedures compared to the conventional concrete pavement. Because of that, it is important to consider the procedures used to prepare the test specimens in the laboratory [42]. There are several previous studies conducted by other researchers that compare test specimen preparation techniques for the porous concrete pavement. Mahboub et al. [43] evaluated two different methods, namely by rodding in accordance with Ref. [44] and a custom-built pneumatic press that applied a compaction effort of 70 kPa uniformly over 100 mm diameter cylinders. They found that the cylinders compacted using a pneumatic press had statistically similar properties of compressive strength, permeability, and porosity as the pavement cores. The rodded specimens had compressive strength values higher than the pavement cores and porosity values lower than the pavement cores. Rizvi et al. [45] conducted research in order to evaluate different compaction methods for porous concrete cylinders (size: 150 mm diameter by 300 mm height). They use rodding and a 2.5 kg standard Proctor hammer in their study. From the study, it was found that the specimens prepared in two layers with 10 blows of a 2.5 kg Proctor hammer per layer resulted in the closest properties to the field porous concrete. Putman and Neptune [42] evaluated different porous concrete test specimen preparation techniques in an effort to produce laboratory specimens with properties similar to the in-place porous concrete pavement. In their study, cylinders and slabs were cast using porous concrete from three different paving projects using different procedures. The study was based on the infiltration rate, density, and porosity. Eight different methods to prepare the specimens were evaluated. These methods included the use of a 15.9 mm diameter steel rod (standard tamping rod), a standard Proctor hammer (2.5 kg), and dropping the mold on a concrete surface from a height of 50 mm. The specimen types used in their study were 150 mm diameter 3 300 mm height cylinder, 300 3 300 3 150 mm thickness slab, 450 3 450 3 150 mm thickness slab, and 600 3 600 3 150 mm thickness slab. The result indicated that the standard Proctor hammer produced cylinders with the porosity and density closest to the porous concrete pavement in all three projects. While the cylinder porosity values were close to the pavement porosity, the density of the cylinders was generally greater than the pavement. However, the slab size of 600 3 600 3 150 mm in thickness had porosity and density values relatively close to the pavement, with the exception of the infiltration rate.

14.3

Materials

14.3.1 Ordinary Portland cement Cement is the main material used to bind aggregates together. It also contributes strength to the concrete itself. The cement used in this investigation is an ordinary

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Portland cement (OPC) (Tasek Cement), which was supplied by the Tasek Corporation Berhad in one batch for the entire experimental work. The cement was kept air tight to ensure consistency of its quality and properties.

14.3.2 Water Water is an important material to produce concrete. In order to produce goodquality concrete, the water must not contain any substances that might affect the chemical reaction between cement and water. Table 14.1 show the elements contained in water.

14.3.3 Black rice husk ash The BRHA specimens were collected from the rice mill factory at Johor Bahru (Malaysia). The rice husk was burned at the factory, as shown in Fig. 14.2. The burned RHA was then ground to obtain nano size. The details of the grinding process procedure are outlined in section below. Table 14.1 Chemical analysis of the tap water used in this research. Element

Dissolved solid

Calcium

Chloride

Magnesium

Sulfate

Bicarbonate

Potassium

mg/L pH

80

23

2

2.6

4

81

3.8

6.9

Figure 14.2 Black rice husk ash.

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Table 14.2 Coarse aggregate gradation used for this study. Aggregate size (mm)

Upper and lower limits (%)

Mid-gradation (%)

19.00 12.50 9.50 4.75 2.36

100 90
100 40
70 0
15 0
5

100 95 55 0 0

14.3.4 Coarse aggregates To prepare the specimens for the experimental works, the coarse aggregates were graded in the range of 12.5
4.75 mm nominal size. The gradation of the coarse aggregate used in this study is tabulated in Table 14.2 and shown in Fig. 14.3. From this figure, it can be seen that the sieve analysis conducted on the specimen of the coarse aggregate complied with the requirements of Ref. [46].

14.4

Experimental plan

14.4.1 Grinding procedure Initially, the raw BRHA was sieved using a sieve size of 150 μm. The purpose of sieving the BRHA is to ensure the homogeneity of the BRHA size before the grinding process. Then, the BRHA passing through the 150 μm sieve was taken and ground for four different grinding periods. Each BRHA ground for four different grinding periods was designated as shown in Table 14.3. The BRHA was ground by a laboratory mill grinder, as shown in Fig. 14.4. The drum, balls, and rods used in this study were made from steel. Each drum contained one size of steel rods and four different sizes of steel balls. The milling speed used in this procedure was 60 rev/min. During the grinding process, each drum contained 500 g BRHA. The same amount of BRHA was placed in each drum for the following grinding process to ensure the consistency of the ground BRHA property. Fig. 14.5 shows the nanosized ground black RHA.

14.4.2 Concrete mix design and proportion The workability of fresh porous concrete and compressive strength were chosen in order to conduct the concrete mix design process. When the concrete mix proportions were obtained, the trial mixes were conducted to check whether the target concrete properties were achieved. Should the mix proportion not achieve the targeted properties, repeated mixes were conducted. In order to mix the porous concrete mixture, all the materials were placed into the drum mixer in the following order: coarse aggregate and OPC. These materials were first mixed for

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Figure 14.3 Gradation of coarse aggregate [46].

Table 14.3 The grinding designation of nano-BRHA. Type of nano-BRHA

Period of grinding (h)

NS1 NS2 NS3 NS4

33 48 63 81

approximately 1 minute under dry conditions to achieve a homogeneous dry mixture. Water was then added into the drum mixer, and the mixing operation was performed continuously for another 3 minutes. Immediately after mixing the mixture, the workability test was conducted. Then, the mixture was cast into a steel cube mold (100 3 100 3 100 mm) in two layers, followed by the compaction process. After casting, all the molded specimens were covered by wet hessian for 24 hours in order to avoid any evaporation of moisture from the specimens. Finally, the molded cubes were demolded and cured for 7, 14, 28, 56, and 90 days. At each testing age, three cubes were tested for compressive strength. After several trial mixes were conducted, a total cement content of 450, 1115 kg/m3 of coarse aggregate, and 153 kg/m3 of water were found and used as the control mix proportion. Table 14.4 shows the mix proportion of porous concrete specimens.

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Figure 14.4 Grinding methods.

Figure 14.5 Nanosized ground black rice husk ash.

Table 14.4 Mix proportion of porous concrete used in this study. Mix

w/c

OPC

0.34

Target slump (mm) 0
15

Water (kg/ m3 )

Cement (kg/ m3)

153

450

Coarse aggregate (kg/m3) 1115

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14.4.3 Workability Workability is one of the physical parameters of concrete, which affects its strength and durability. Generally it is defined as the easiness with which concrete can be handled, placed, compacted, and finished with minimum segregation. The workability of the concrete was measured by a slump test according to Ref. [47].

14.4.4 Compaction process There are several procedures used for compacting porous concrete specimens. In order to minimize the error rate in this process, the established procedure was implemented in accordance with Ref. [48]. This standard method proposed to use a Proctor hammer and bearing plate as apparatus for compacting the porous concrete specimens. Fig. 14.6 illustrates the compaction process for the specimens. From this figure, it can be seen that the bearing plate is placed on top of the concrete before compacting using the Proctor hammer.

14.4.5 Curing condition In this study, the curing condition is one of the most important factors that need to be considered. It has a strong influence on the properties of the hardened porous concrete. Proper curing will increase durability, strength, and other properties. The standard for curing no-fines test cubes was followed according to the established British standard [48]. Immediately after making the cube specimens, the specimens were stored in a place free from vibration and under conditions that prevented moisture loss. Upon demolding the cube specimens for a period of 16
28 hours, each specimen was marked clearly with an identification code. Immediately after marking, the specimens were immersed in water until air bubbles ceased to rise. The specimens were drained and immediately placed in a polyethylene bag. Finally, the polyethylene bags were sealed and stored. Fig. 14.7 shows the curing condition for porous concrete as per Ref. [48].

14.4.6 Compressive strength test Compressive strength can be defined as the capacity of concrete to withstand loads before failure. Of the many tests applied to the concrete, the compressive strength test is the most important, as it gives an idea about the characteristics of the concrete. The MATEST compression strength machine was used for this test. Fig. 14.8 illustrates the machine used for this test. In this study, the compressive strength test of all the concrete mixes was performed on 100 3 100 3 100 mm3. The specimens were compressed using a compression machine with a loading rate of 3.5 kN/s. The reported compressive strength was the average of the three specimens tested. The test was conducted according to the British standard test method [49].

Figure 14.6 Compacting process.

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Figure 14.7 Curing condition using polyethylene bags.

Figure 14.8 Compression strength machine used in this investigation.

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14.4.7 Permeability test The permeability test was conducted with the purpose of measuring the water flow through the specimens. This test is conducted according to the specification described in Ref. [50]. Fig. 14.9 shows the falling head permeameter used in this study. To perform this test, the discharge time of the specimens was recorded once the water started to flow at a specific marked interval on the standpipe. The measurement of the discharge time was recorded four times for each specimen. This test also records the temperature of water each time the measurements are performed. The permeability coefficient, k (cm/s) was calculated using Eq. 14.1.   al h1 k 5 ln At h2

(14.1)

where, A (cross-section area of specimen, cm2), a (cross-section area of standpipe, cm2), l (height of specimen, cm), h1 (initial height of water above the specimen, cm), h2 (height of the water after time, cm), and t (time taken for the water to fall from h1 to h2, second).

Figure 14.9 Falling head permeameter equipment used in this study.

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14.4.8 Sound absorption test The sound absorption test was conducted according to the standard test method for the impedance and absorption of acoustical materials using a tube, two microphones, and a digital frequency analysis system [51]. This test method has been applied in order to measure the sound absorption coefficient of absorptive materials. The porous concrete specimens were placed in the impedance tube. Then, the planed waves are generated in the tube using a broadband signal from a noise source. The decomposition of the stationary sound wave pattern into forward and backward traveling components is achieved by measuring sound pressures simultaneously at two spaced locations in the tube’s side wall. The calculations of the normal incidence absorption coefficients for the acoustical material are performed by processing an array of complex data from the measured transfer function. Fig. 14.10 shows the apparatus used for this test.

14.4.9 X-ray fluorescence test The chemical and mineralogical compositions of OPC and BRHA were determined by X-ray fluorescence (XRF). XRF is a nondestructive elemental and chemical method of analysis of rocks, minerals, metals, sediments, and fluids. To prepare the specimen for this test, 10 g OPC/BRHA in dry powder form was compacted by pressing the specimen to make the circle pellet at a load of 20 tons for 10 seconds. The specimen was then tested according to Ref. [52]. Fig. 14.11 shows the BRUKER AXS S4 PIONEER XRF microscopy machine used in this study in order to determine the chemical composition of the specimens.

Figure 14.10 Impedance tube apparatus.

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Figure 14.11 XRF instrument.

14.4.10 X-ray diffraction test XRD is a rapid analytical technique primarily used for the phase identification of crystalline materials. In this study, the OPC, BRHA, and cement paste powder specimens were characterized by the RIGAKU Smartlab X-ray Deffractometer, as shown in Fig. 14.12. As described in Ref. [53], the specimens were scanned in steps of 2θ with a fixed counting time of 1 second. The range of the X-ray scan was from 2θ 5 10
90 degrees, using copper (Kα Cu) with a wavelength (λ) of 1.5406 nm as an X-ray source.

14.4.11 Transmission electron microscopy The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. The TEM uses electrons as a light source and their much lower wavelength makes it possible to obtain a resolution a thousand times better than with a light microscope. It is a technique in which a beam of electrons is transmitted through an ultrathin specimen. In order to characterize the specimens, a Hitachi HT7700 TEM was used to capture and measure the microstructure image. Fig. 14.13 illustrates the TEM machine used in this

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Figure 14.12 XRD machine used in this study.

Figure 14.13 TEM machine used in this study.

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study. To prepare the specimen, a small amount of BRHA was placed in a small glass bottle and suspended in the iso-propanol solvent. The bottle containing BRHA powder was placed in an ultrasonic machine to sonicate (15
30 seconds) to disperse the particles. After sonication, two drops of the specimen were put on the copper grid. The specimen was left to dry and placed in the TEM machine for viewing.

14.4.12 Field emission scanning electron microscopy Field emission scanning electron microscopy (FE-SEM) is an advanced technology used to capture the microstructure image of the materials. FE-SEM is typically performed in a high vacuum because gas molecules tend to disturb the electron beam and the emitted secondary and backscattered electrons used for imaging. In order to characterize the specimens in this study, Zeiss Crossbeam 340 was used to capture the microstructure image. The specimens (except powder) were cut into smaller sizes of about 5 3 5 3 5 mm and coated using aurum prior to the morphological observation. In addition to FE-SEM, energy-dispersive X-ray (EDX) was conducted on the same specimens for further analysis. EDX is an X-ray technique used to identify the elemental composition of materials. These systems are attachments to electron microscopy instruments where the imaging capability of the microscope identifies the specimens of interest. Fig. 14.14 shows the FE-SEM and EDX machine used in this study.

Figure 14.14 FESEM and EDX machine used in this study.

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Table 14.5 Mean particle size of OPC and nano-BRHA at different grinding times. Grinding (h) Description Mean particle size (nm)

14.5

0 OPC 1353

33 NS1 85

48 NS2 76

63 NS3 66

81 NS4 64

Results and discussions

14.5.1 Particle size of the ordinary Portland cement and nanoblack rice husk ash In this study, the mean particle size of the OPC and nano-BRHA was measured using the Zetasizer and TEM, respectively. From Table 14.5, it can be seen that the mean particle size of the OPC is 1353 nm. The mean particle size for the nanoBRHA is as follows: NS1, 85 nm; NS2, 76 nm; NS3, 66 nm; and NS4, 64 nm. Compared to NS1, the other nano-BRHA sizes decrease to 10.6%, 22.4%, and 24.7% for NS2, NS3, and NS4, respectively. This clearly shows that an increase in the grinding time will decrease the mean particle size of nanosilica. This is because the increase in grinding time will increase the grinding effort to the BRHA.

14.5.2 Particle morphology In order to understand the morphology properties of OPC, BRHA, and nanoBRHA, the particle morphology images were captured using the FE-SEM and TEM, respectively. The particle morphologies of OPC, BRHA, and nano-BRHA are illustrated in Fig. 14.15. This clearly shows that the particle morphology of OPC is mostly irregularly shaped (Fig. 14.15A). While the BRHA have an elongated shape and crocodile skin surface with approximately 180 μm width and 1
3 mm length (Fig. 14.15B), the nano-BRHA consist mainly of fine irregularshaped particles (Fig. 14.15C
F). This indicates that the grinding process affects the morphology of the BRHA. This discovery is similar to that of Abu Bakar et al. [54], who found that ground RHA had irregular-shaped particles. From the morphological assessment, it was observed that the increase in the grinding time decreases the mean particle size of the nano-BRHA.

14.5.3 Chemical composition In this study, the XRF determines the main oxide compounds of the OPC and BRHA, such as silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na2O), potassium oxide (K2O), and sulfur trioxide (SO3). Table 14.6 shows the chemical composition of OPC and BRHA. It was found that calcium oxide had the highest percentage of composition in OPC, namely 62.27%, followed by silica at 22.68%. The other elements had smaller percentages of 4.72%, 3.5%, 1.89%, 0.31%, and 4.29% for alumina, iron

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Figure 14.15 The morphologies of OPC, BRHA, and nano-BRHA. (A) OPC (Ordinary Portland Cement); (B) BRHA (Black Rice Husk Ash); (C) NS1 (Nanosilica 1); (D) NS2 (Nanosilica 2); (E) NS3 (Nanosilica 3); (F) NS4 (Nanosilica 4).

Table 14.6 Chemical composition of OPC and BRHA. Oxides

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

OPC BRHA

22.68 91.33

4.72 0.07

3.5 0.07

62.27 0.45

1.89 0.28


0.01

0.31 2.64

4.29 0.05

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oxide, magnesium oxide, potassium oxide and sulfur trioxide, respectively. From the results obtained, the chemical composition for OPC fulfilled the standard composition requirements as stated in the Standard Specification for Portland cement [55]. The BRHA contained a high percentage of silica of 91.33%. The other elements in BRHA are alumina, iron oxide, magnesium oxide, sodium oxide, potassium oxide, and sulfur trioxide, which contained 0.07%, 0.07%, 0.45%, 0.28%, 0.01%, 2.64%, and 0.05%, respectively. Different from OPC, the BRHA has a small percentage of calcium oxide and a high percentage of silica. According to Ref. [56], the BRHA used in this study satisfied the Class N requirement for natural pozzolan used in concrete, which indicates that the total amount of silicon dioxide, aluminum oxide, and iron oxide must be equal or greater than 70%.

14.5.4 Mineralogical and phase identification The result for XRD of OPC is shown in Fig. 14.16. This figure shows the phase mineralogy composition of the element in OPC. It can be observed that the main components in OPC are alite and belite. The locations for each peak are shown in Table 14.7. Most of the intensity peaks of the diffractogram are alite and belite. This result has confirmed the XRF result for OPC, indicating that the major constituents are calcium and silica. According to Poulsen et al. [57], the hydraulic properties of Portland cement are mainly related to alite and belite, which are impure forms of the calcium silicates, Ca3SiO5 (C3S) and Ca2SiO4 (C2S), respectively. Both of these silicates will react in the cement hydration process to strengthen its structure. Fig. 14.17 shows the XRD analysis result of BRHA. From this figure, it may be inferred that the highest intensity peak located at 21.84 degrees 2θ is silicon oxide

Figure 14.16 X-ray diffraction of ordinary Portland cement.

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Table 14.7 Elements in OPC from XRD analysis. 2θ (degrees)

Elements

29.38, 32.58, 38.82, 41.28, 51.72, 56.6, 60.02, 62.24 32.2 23 25.18 34.32, 36.64, 45.72

Hatrurite (alite) Calcium silicate (belite) Calcium sulfate Silicon oxide Calcium silicon oxide

Figure 14.17 X-ray diffraction of black rice husk ash.

(SiO2). This indicates that the major composition in the BRHA material is silica. This result confirmed the XRF result for BRHA in Table 14.6, where silica is the major element in the BRHA.

14.5.5 Concrete mix design and mix proportions In this evaluation of the optimum nano-BRHA grinding time, all mix proportions had the same sum of nano-BRHA replacement at 10% by weight of cement. A previous researcher found that by replacing approximately 10% of NS in the cement concrete, the compressive strength at 28 days had the best result [12,58
60]. In this study, the porous concrete pavement mixes were designed to achieve a concrete of grade 20 MPa at 28 days. The proportions of the porous concrete pavement mix are given in Table 14.8.

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Table 14.8 Mix proportions of porous concrete pavement containing nano-BRHA. Symbol

w/c

Slump (mm)

Water (kg/m3)

OPC NS1 NS2 NS3 NS4

0.34

15 12 10 11 10

153

Cement (kg/m3) 450 405

NanoBRHA (kg/ m3)

Coarse aggregate (kg/ m3)

0 45

1115

14.5.6 Workability In this study, the low slump value has been applied in order to create a high percentage of voids in the concrete specimens. The workability of the OPC specimen and nano-BRHA blended cement at different grinding times is shown in Table 14.8. This table shows that the partial replacement of cement with nano-BRHA in the porous concrete mixture influences the slump test results. The result of the slump test for the OPC specimen was 15 mm, while for the NS1, NS2, NS3, and NS4 specimens, the results were 12, 10, 11, and 10 mm, respectively. The slump results for each nano-BRHA decrease by 20%, 33%, 27%, and 33% compared to the OPC for NS1, NS2, NS3, and NS4, respectively. The slump results decrease probably because of the nano-BRHA consuming more water for mixing. From these results, the partial replacement of cement with nano-BRHA affects the workability of the porous concrete specimens. The slump test result for all specimens decreased after replacing a certain amount of cement with nano-BRHA. This is because the fine particles will absorb more water compared to coarse particles, thus decreasing the slump result.

14.5.7 Compressive strength Fig. 14.18 shows the result of compressive strength for the various nano-BRHA replacements. Generally, the compressive strength for all specimens increases with the increase in curing age. It can be seen that the highest compressive strength for each age is 10% for the replacement specimen, which is 20.88, 24.45, 27.80, 34.20, and 35.23 MPa for 7, 14, 28, 56, and 90 days, respectively. The 40% replacement of nano-BRHA is the lowest compressive strength, which is 14.64, 16.28, 22.92, 27.53, and 29.30 MPa for 7, 14, 28, 56, and 90 days, respectively. The compressive strength for each day of testing shows the same trend where the compressive strength starts to increase from 0% replacement to 10% replacement of the nanoBRHA. Then, the compressive strength starts to decrease from 10% to 20% replacement of the nano-BRHA. The same applies to 20%
30% and 30%
40% replacements. This finding is similar to Siddharth et al. [61], who reported that the strength starts to increase with 10% replacement and decrease when the replacement is beyond 10%. This phenomenon probably occurred due to workability and the lack

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Figure 14.18 Compressive strength at different nano-BRHA replacements.

of required water in the mixture. The use of NS in porous concrete had a greater compressive strength than the control specimens [62]. Nevertheless, Wang et al. [63] found in their study that too high or too low a content of NS was not beneficial to upgrading the cement strength. This is probably due to the amount of NS in the mix being higher than the amount required to combine with the liberated lime during the process of hydration. From Fig. 14.18, it can be seen that the 10% replacement of nano-BRHA may have the best property for compressive strength evaluation.

14.5.7.1 Relationship between compressive strength and density The relationship between the compressive strength and density of porous concrete pavement containing different nano-BRHA replacements is shown in Fig. 14.19. It can be seen that the density of the porous concrete pavement specimens does not have a significant relationship with the compressive strength. It appears that the Rsquare of the relationship is 0.2626. Both high- and low-density values have a high strength. This phenomenon contrasts with the typical theory which stipulates that when the density of concrete increases, the compressive strength of the concrete also increases, and vice versa. This is probably due to the reaction of the nanoBRHA replacement in the porous concrete mixture. A different percentage of nanoBRHA replacement gives a different reaction. Although the porous concrete pavement specimens have low density, if the amount of nano-BRHA contained in the porous concrete mixture is correct and enough to have a good reaction, it will result in good compressive strength.

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Figure 14.19 Relationship between the compressive strength and density.

Figure 14.20 Relationship between compressive strength and the age of specimens.

14.5.7.2 Relationship between compressive strength and curing age Fig. 14.20 shows the trend lines between the compressive strength and the age of the specimens for different percentages of nano-BRHA replacements. Table 14.9

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Table 14.9 Coefficients value of the correlation between strength and the age. Nano-BRHA replacement R2 value

Control (0%) 0.9046

10% 0.8834

20% 0.9495

30% 0.9308

40% 0.8718

shows the coefficient of determination (R2) value for the relationship between the compressive strength and the age of the specimens. All of the specimens show a good relationship between compressive strength and the ages of the specimens with R-square values of 0.9046, 0.8834, 0.9495, 0.9308, and 0.8718 for 0%, 10%, 20%, 30%, and 40%, respectively. The compressive strength of the specimens for each of the nano-BRHA percentage replacements increases with an increase in the specimens’ age. The continued hydration develops the strength of cement-based materials [64]. The increase in age gives more time for the hydration process in the specimens. Indirectly, the specimens become more hardened with respect to age. This is because the figure shows that the strength of the specimens increases with an increase in age.

14.5.7.3 Compressive strength activity index As prescribed in Ref. [65], the strength activity index is used to determine whether natural pozzolan results in an acceptable level of strength development when used with hydraulic cement in concrete. This calculation was adapted here in order to determine the different strength values when a different percentage of replacement is used. The strength activity index was calculated following Eq. 14.2. Strength activity index 5

A ð100Þ B

(14.2)

where A is the compressive strength of the test mixture cube (MPa) and B is the compressive strength of the control mix cube (MPa). The strength activity index of the compressive strength results for all of the nano-BRHA replacement is shown in Fig. 14.21. At any specimen age, the 10% of nano-BRHA replacement shows an increase in strength activity index. The strength activity index increased by 5.14%, 4.01%, 9.08%, 5.10%, and 3.94% for 7 days, 14 days, 28 days, 56 days, and 90 days, respectively. For the 20%, 30%, and 40% of nano-BRHA replacement a decrease in the strength activity index is observed at all ages. The lowest strength activity index at all ages belongs to specimens with 40% nano-BRHA replacement. The strength activity index decreased to 73.72%, 69.26%, 89.93%, 84.60%, and 86.45% for 7 days, 14 days, 28 days, 56 days- and 90 days, respectively. This may be due to the quantity of NS present in the mixes being higher than the amount required to combine with the liberated lime during the hydration process, consequently leading to excess silica leaching out, causing a deficiency in strength as it replaces part of the cementitious material [66]. Overall, for the strength activity index, all the specimens have a strength activity index of more than 75% for the 28 days age.

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Figure 14.21 Compressive strength activity index.

14.5.8 Permeability The permeability coefficient results of the porous concrete specimens for various nano-BRHA percentage replacements are shown in Fig. 14.22. It can be seen that the lowest permeability coefficient is for specimens with 0% nano-BRHA replacement and the highest permeability coefficient is for specimens with 40% nanoBRHA replacement. The permeability coefficient for specimens with 0%, 10%, 20%, 30%, and 40% nano-BRHA replacement is 0.10, 0.11, 0.19, 0.20, and 0.35 cm/s, respectively. The permeability coefficient of porous concrete specimens was found to increase with an increase in the percentage of nano-BRHA replacement. The permeability coefficient for all specimens increases by 10%, 90%, 100%, and 250%, respectively, from 0% to 40% nano-BRHA replacement. There is a large different between 30% and 40% nano-BRHA replacement of the permeability coefficient. This is probably due to the large difference in porosity of specimens with 30% and 40% nano-BRHA replacement. The increase in permeability is due to an increase in the interconnected voids in the specimens when the percentage of the nano-BRHA replacement increases. The increase in the water absorbed due to the increasing percentage of the nano-BRHA replacement results in reduced workability of the mixtures. This contributes to the increase in the interconnected voids and directly increases the permeability of the hardened specimens. In their study, Kia et al. [67] mentioned that the permeability is not only dependent on the total pore volume, but also on other characteristics, such as size distribution, shape, degree of connectivity, and tortuosity of the pores.

14.5.8.1 Relationship between permeability and compressive strength The relationship between the compressive strength and permeability of porous concrete specimens is shown in Fig. 14.23. The relationship was determined using

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Figure 14.22 Permeability coefficient of porous concrete pavement at varying nano-BRHA replacement.

Figure 14.23 Relationship between compressive strength and permeability.

values of 0.10, 0.11, 0.19, 0.20, and 0.35 cm/s for the permeability coefficient and 25.48, 27.80, 24.07, 23.06, and 22.92 MPa for compressive strength, respectively. The R2 value for the relationship is 0.7355 and it shows a negative correlation. It was found that the increasing permeability will decrease the compressive strength. The increase in permeability was caused by the increase in the number of interconnected voids in the specimens. It been noted that the weakness of compressive strength is represented by the voids in the compressive strength. Therefore the increasing permeability results in a decrease in compressive strength. The present

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findings seem to be consistent with other research studies which found that when the permeability increases the strength is reduced [58]. Furthermore, Cui et al. [68] found that the porous concrete strength has a negative correlation with its permeability, which is similar to the present findings.

14.5.9 Sound absorption Fig. 14.24 shows the sound absorption of porous concrete specimens with various nano-BRHA percentage replacements. The sound absorption coefficients for all specimens are 0.59, 0.58, 0.62, 0.63, and 0.79 for OPC (0%), 10%, 20%, 30%, and 40%, respectively. The figure shows that the highest sound absorption coefficient is 0.79, which is the specimen with 40% nano-BRHA replacement. The specimen with 10% nano-BRHA replacement has the lowest sound absorption coefficient of 0.58. The sound absorption coefficient starts to decrease by 0.01 from 0.59 (0%) to 0.58 (10%). Then, the coefficient starts to increase by 0.04 from 0.58 (10%) to 0.62 (20%). The coefficient continues to increase by 0.01 from 0.62 (20%) to 0.63 (30%). Finally, the coefficient increases by 0.16 from 0.63 (30%) to 0.79 (40%). In this study, the sound absorption increases with an increase in the nano-BRHA replacement. This is because, by referring to Fig. 14.24, which is the porosity result, the increase in the percentage of nano-BRHA replacement results in an increase in the porosity of the specimens. According to Park et al. [69], the sound absorption became higher as the specific surface area of voids increased. This means that when the porosity of the specimen increases, the sound absorption will increase too. That is, why the 40% nano-BRHA replacement has the highest sound absorption coefficient.

Figure 14.24 Sound absorption at varying nano-BRHA replacements.

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G

G

G

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Conclusions

Four different nano-sizes of BRHA were examined and it was found that BRHA ground for the optimum grinding time had a median nano-size of 66 nm. It was also found that the particle size of the BRHA decreased with an increase in grinding time. On the other hand, the morphology of the nano-BRHA changed with grinding. There appears to be an optimum grinding time of approximately 63 hours, during which time the compressive strength and strength activity index increase significantly. Based on permeability and sound absorption, when the percentage of the nano-BRHA replacement increased, the permeability and sound absorption of the specimens also increased. This is probably due to a lack of water used in the mixture.

Acknowledgment The support provided by the Malaysian Ministry of Higher Education and University of Malaysia Pahang in the form of a research grant (RDU/UMP) Project number RDU190339 for this study is highly appreciated.

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370. [41] P.D. Tennis, M.L. Leming, D.J. Akers, Pervious Concrete Pavements (No. PCA Serial No. 2828), Portland Cement Association, Skokie, IL, 2004. [42] B.J. Putman, A.I. Neptune, Comparison of test specimen preparation techniques for pervious concrete pavements, Constr. Build. Mater. 25 (8) (2011) 3480
3485. [43] K.C. Mahboub, J. Canler, R. Rathbone, T. Robl, B. Davis, Pervious concrete: compaction and aggregate gradation, ACI Mater. J. 106 (6) (2009) 523. [44] ASTM C192/C192M, Standard practice for making and curing concrete test specimens in the laboratory, Annual Book of ASTM Standards, American Society for Testing and Materials, 2013. [45] R. Rizvi, S.L. Tighe, V. Henderson, J. Norris, Laboratory sample preparation techniques for pervious concrete, in: Transportation Research Board 88th Annual Meeting (No. 09-1962), 2009. [46] ASTM C33/C33M, Standard specification for concrete aggregates, Annual Book of ASTM Standards, American Society for Testing and Materials, 2013. [47] BS EN 12350-2, Testing Fresh Concrete—Slump Test, British Standards Institution, 2000. [48] BS 1881-113, Testing Concrete—Part 113: Method for Making and Curing No-Fines Test Cubes, British Standards Institution, 2011.

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[49] BS EN 12390-3, Testing Hardened Concrete—Part 3: Compressive Strength of Test Specimens, British Standards Institution, 2009. [50] ASTM PS 129, Standard provisional test method for measurement of permeability of bituminous paving mixtures using a flexible wall permeameter, Annual Book of ASTM Standards, American Society for Testing and Materials, 2001. [51] ASTM E1050, Standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system, Annual Book of ASTM Standards, American Society for Testing and Materials, 2012. [52] BS EN ISO 12677, Chemical Analysis of Refractory Products by X-ray Fluorescence (XRF). Fused Cast-Bead Method, British Standards Institution, 2011. [53] BS EN 13925-1, Non-Destructive Testing—X-ray Diffraction From Polycrystalline and Amorphous Materials: General Priciples, British Standards Institution, 2003. [54] B.H. Abu Bakar, P.J. Ramadhansyah, M.J. Megat Azmi, Effect of rice husk ash fineness on the chemical and physical properties of concrete, Mag. Concr. Res 63 (5) (2011) 313
320. [55] ASTM C150/C150M, Standard specification for Portland cement, Annual Book of ASTM Standards, American Society for Testing and Materials, 2011. [56] ASTM C618, Standard specification for coal fly ash and raw or calcined natural Pozzolan for use in concrete, Annual Book of ASTM Standards, American Society for Testing and Materials, 2012. [57] S.L. Poulsen, V. Kocaba, G.L. Saout, H.J. Jakobsen, K.L. Scrivener, J. Skibsted, Improved quantification of alite and belite in anhydrous Portland cements by 29Si MAS NMR: effects of paramagnetic ions, Solid. State Nucl. Magn. Reson 36 (2009) 32
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691. [59] H. Li, H. Xiao, J. Yuan, J. Ou, Microstructure of cement mortar with nano-particles, Compos.: Part. B 35 (2) (2004) 185
189. [60] M.H. Zhang, R. Lastra, V.M. Malhotra, Rice-husk ash paste and concrete: some aspects of hydration and the microstructure of the interfacial zone between the aggregate and paste, Cem. Concr. Res. 26 (6) (1996) 963
977. [61] T. Siddharth, P. Jayeshkumar, M.V. Chetna, Effect of rice husk ash on properties of pervious concrete, Int. J. Adv. Eng. Res. Stud. 4 (2) (2015) 296
299. [62] V. Lo´pez-Carrasquillo, S. Hwang, Comparative assessment of pervious concrete mixtures containing fly ash and nanomaterials for compressive strength, physical durability, permeability, water quality performance and production cost, Constr. Build. Mater. 139 (2017) 148
158. [63] L. Wang, D. Zheng, S. Zhang, H. Cui, D. Li, Effect of nano-SiO2 on the hydration and microstructure of Portland cement, Nanomaterials 6 (12) (2016) 241. [64] A.A.A.E. Metwally, Compressive strength prediction of Portland cement concrete with age using a new model, HBRC J. 10 (2) (2014) 145
155. [65] ASTM C311/C311M, Standard test methods for sampling and testing fly ash or natural Pozzolans for use in Portland-cement concrete, Annual Book of ASTM Standards, American Society for Testing and Materials, 2013.

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[66] A. Naji Givi, A.R. Suraya, N.A.A. Farah, A.M.S. Mohamad, Assessment of the effects of rice husk ash particle size on strength, water permeability and workability of binary blended concrete, Constr. Build. Mater. 24 (2010) 2145
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1854.

Porous alkali-activated materials

15

Priyadharshini Perumal, Tero Luukkonen, Harisankar Sreenivasan, Paivo Kinnunen and Mirja Illikainen Fibre and Particle Engineering Research Unit, University of Oulu, Oulu, Finland

15.1

Introduction

Porous concrete is a type of lightweight concrete (density 400
1850 kg/m3) with cementitious binders entrapping air voids. Though the foam concrete technology was introduced in the early 1900s, its application in building materials has become more popular in the last few decades [1,2]. Foam concrete attracted attention owing to its easy workability, reduced binder and aggregate content, and importantly its thermal insulation efficiency [1,3]. Over the past few years, the foaming technology evolved in terms of equipment, foaming agents, and mix design to enable its usage as a structural concrete material. The importance of lightweight concrete was realized with the cost saving in reduction of dead load, foundation, transport, and labor. Additionally, it has functional properties as a thermal and acoustic insulation material, and, resistance to high temperature. Portland cement has been used mainly as the binder material in porous concrete [1]. With growing pressure for construction industries to move toward sustainable materials, there is a growing need for alternative building materials with low CO2 emissions [4,5]. Supporting this, the idea of introducing alkali-activation technology in porous concrete is relatively new and has its own advantages. Alkali activation was started in the 1900s with blast furnace slag as precursor [6]. Commercialization of the materials with the alkali-activated slag in Belgium happened in the 1950s with the construction of buildings which still stand as an example of the durability of the material (Purdon, 1940). Following this, in the 1970s, Davidovits studied alkali-activated metakaolin and coined the term geopolymer (Davidovits, 1991). Any aluminosilicate material can be used as a source for alkali activation. Fly ash, slag, and metakaolin were widely used as source materials [7
9] and sodium/potassium-based alkali solution were used for activation. Although considerable research was carried out in geopolymer technology and proven for its economic, environmental, and functional benefits, it is not commonly used for practical applications. Currently, porous alkali-activated materials (AAMs) are attracting the attention of the construction industry due to their performance benefits and unique properties, in addition to the emission reduction by the use of industrial wastes [10,11]. Moreover, porous AAM can be used in several environmental engineering New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00015-6 © 2020 Elsevier Inc. All rights reserved.

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applications as well. Porous AAM can be prepared by several different methods such as direct foaming, sacrificial filler method, and 3D printing [10]. In direct foaming, blowing agents are used to incorporate air or other gas bubbles into the AAM paste or mortar. In the sacrificial filler method, the filler material acts as a template for the cellular structure and can be dissolved or thermally decomposed after curing of AAMs. 3D printing is a recent advancement in the preparation of porous AAMs. Direct ink writing (DIW) with fresh AAM and powder-based techniques is used for printing. In 3D-printed materials, the pores are the cavities between filaments and thus the dimensions can be controlled accurately. Currently, there is only limited information relating to the properties of source material, preparation method, structure, mechanical, and functional properties of porous AAMs. The challenge is to relate the functional properties of the material to be utilized for its practical application, owing to the various different parameters as mentioned earlier. To partially address this need, in this chapter, source materials, alkali solutions, production techniques, properties, and potential applications of porous AAMs are summarized.

15.2

Porous alkali-activated materials

The synthesis of porous AAMs involves aluminosilicate precursors, alkali activators, and a method to introduce air voids. A variety of aluminosilicate precursors are used for the synthesis of porous AAMs (Fig. 15.1), the most commonly used ones being metakaolin and fly ash. The alkali activators used are mostly sodium hydroxide/sodium silicate and potassium hydroxide/potassium silicate. A detailed description of blowing agents and surfactants together with other production methods is provided in Section 15.2.2.

Figure 15.1 Aluminosilicate precursors used for making porous alkali-activated materials.

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531

A typical synthesis procedure for porous AAMs is shown in Fig. 15.2. Aluminosilicate precursor (such as metakaolin) and silica fume (blowing agent) are added to the premixed activator solution. After stirring foaming agents and surfactants, the mixture is poured into molds and placed in an oven. After curing, demolding is done to obtain the porous AAM. Table 15.1 summarizes the synthesis conditions of porous alkali-activated materials reported in literature.

15.2.1 Alkali activation conditions affecting the properties of porous alkali-activated materials This section deals with the influence of alkali activation conditions on the properties of porous AAMs. This includes the type of aluminosilicate precursor, alkali activator, and curing conditions. The influence of blowing agents/surfactants/stabilizers is discussed in Section 15.2.2.

15.2.1.1 Types of aluminosilicate precursor The selection of aluminosilicate precursors has a critical role in determining the performance of AAM. Zhang et al. [11] described the differences between two commonly used aluminosilicate precursors: metakaolin and fly ash [11]. Both contain reactive amorphous Al and Si. Metakaolin powder consists of plate-like particles with a specific surface area of 9
20 m2/g as measured by N2 adsorption. On the other hand, fly ash consists of fine, spherical, mostly glassy particles with a surface area of only 0.6
4.2 m2/g. Due to this morphological difference, metakaolin-

Figure 15.2 Schematic diagram for the production of porous AAMs via direct foaming [12].

Table 15.1 Synthesis conditions of porous alkali-activated materials. Aluminosilicate precursor

Alkali activator

Blowing agents and surfactants

Fly ash

NaOH/Na2SiO3

Al

Metakaolin Metakaolin, red mud

NaOH/Na2SiO3 NaOH/Na2SiO3

Al Al

Metakaolin, rice hush ash, volcanic ash Fluid catalytic cracking (FCC) residue Metakaolin, glass steel plant waste, aluminum scrap Metakaolin Metakaolin, fly ash

NaOH/Na2SiO3

Al

NaOH/Na2SiO3

Al, recycled Alfoil

NaOH/Na2SiO3

Al, AlN

KOH/K2SiO3 NaOH/Na2SiO3

Si powder Si powder

Metakaolin, clay Slag, silicon sludge

KOH/K2SiO3 NaOH/Na2SiO3

SiO2 fume Si sludge

Fly ash, sand

NaBO3

Metakaolin, biomass ash Soil

NaOH/Na2SiO3/ Ca(OH)2 KOH/K2SiO3 NaOH/Na2SiO3

Metakaolin

NaOH/Na2SiO3

Metakaolin, diatomite Fly ash Fly ash

NaOH/Na2SiO3 NaOH/Na2SiO3 NaOH/Na2SiO3

Al, virgin monofilament polypropylene fibers Si/protein H2O2/oelic acid Sika Lightcrete 02

H2O2 H2O2

Curing 

References 

22 C (2 h), 80 C (12 h) RT (S-14 d) 0 C (S-7 d), RT (1
28 d) RT

[7]

23 C
65 C (4 h
6 d) 70 C
87 C (6 h)

[15]

RT
80 C (24 h) RT (28 d), 60 C (24 h) 70 C (4 h) 70 C (S-24 h), RT (3 d) RT (28 d)

[17] [18]

60 C (S-24 h; 12 h) 60 C (S-24 h), 220 C (4 h) 70 C (S-24 h)

[22] [23]

40 C (24 h) 80 C (10 h) 70 C (S-24 h)

[25] [26] [27,28]

[8] [13] [14]

[16]

[19,20] [9] [21]

[24]

Metakaolin, fly ash

KOH/K2SiO3

Metakaolin

KOH/K2SiO3

Metakaolin

NaOH/Na2SiO3

Fly ash

NaOH/Na2SiO3

Metakaolin

NaOH/Na2SiO3

SiC, carbon fibers, rice starch, cellulose fibers Sodium dodecyl benzene sulfonate 1 triethanolamine Si/Epojet, Globasil AL20

Metakaolin, fly ash, rice husk ash

KOH/K2SiO3

Al

Fly ash, iron ore tailing

NaOH/Na2SiO3

H2O2

S, curing under closed/sealed condition.

(Triton X-100; Tween 80) polyacrylic acid Tween 80

80 C [(S-1 h), (4 h)]

[29]

40  C (B24 h), 75 C (S-24 h) 70 C (S-72 h)

[30,31]

70 C (S-24 h)

[33]

RT (24 h), 60 C (24 h) RT, 50  C [S(1
28 d)] RT (S-24 h)

[34]

[32]

[14] [36]

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based alkali-activated paste requires a higher liquid quantity than its fly ash-based counterpart. The second difference is that metakaolin-based AAMs show a higher reaction rate and faster strength development when compared to fly ash-based AAMs, especially at low temperatures. The third difference is the structural stability of metakaolin-based AAMs which can influence their potential applications. It has been observed that under prolonged curing conditions there is transformation of amorphous to zeolite-like crystalline structures in metakaolin-based AAMs, leading to a drop in compressive strength. In comparison, fly ash-based AAMs show less transformation and negligible strength loss.

15.2.1.2 Types of foaming agent Porosity is produced in geopolymer paste with foaming agents such as hydrogen peroxide (H2O2) or Al powder. Hydrogen peroxide decomposes into H2 gas, or Al powder forms H2 gas. This reaction is faster in a high pH environment and pressurizes the unfilled volume. The induced pressure is inversely proportional to the size of the pores formed as per the Young
Laplace equation [35]. Hence, the pore size and total porosity can be adjusted with the amount of foaming agent [37,38]. Pore size increased considerably by extending the curing time, which is presented in Fig. 15.3 [39]. A similar trend was also observed with the increase in curing temperature, as shown in Fig. 15.4 [40]. It was also observed that the extension of

Figure 15.3 Pore size with different curing ages [39]

Figure 15.4 Pore formation with different curing temperatures [40].

Porous alkali-activated materials

535

oxidation of aluminum as a foaming agent is regulated with alkali activators and the porosity can be controlled by adjusting the ratio of NaOH to Na2SiO3 [41].

15.2.1.3 Effects of setting time The nature of the alkali activation reaction can influence pore development in AAMs [42]. A faster setting reaction results in trapping and maintaining small voids within the matrix before they can join and grow in size. This means a faster setting reaction favors the development of smaller pore in AAMs when compared to large pores. The size of the gel particulates formed initially can also influence the final size of the voids [42]. If the surface of the initially formed bubbles is much smaller than their surrounding particulates, the bubbles coalesce to form larger bubbles until their surfaces are protected by stabilizing layers of particles. Hence, large gel size can result in large pore size distribution in the final AAMs.

15.2.1.4 Alkali dosage Alkali dosage is critical when chemical foaming agents are used in the production of porous AAMs. Hajimohammadi et al. [41] studied the influence of alkali activators (sodium silicate/sodium hydroxide) on aluminum chemical foaming and the porosity of resulting AAMs [41]. Sodium hydroxide is known to act as a catalyst for the aluminum foaming reaction. On the other hand, sodium silicate is known to act as an inhibitor for aluminum corrosion in alkaline conditions due to the formation of amorphous aluminosilicate film on the metal surface. A higher sodium hydroxide/sodium silicate ratio led to a higher aluminum foaming reaction and the development of higher porosities in AAMs. Hence, by controlling the sodium hydroxide/sodium silicate ratio, porosity in the AAMs can be tuned.

15.2.2 Production methods The production method affects significantly the pore size distribution of the obtained AAM, as illustrated roughly in Fig. 15.5. AAMs have intrinsic pores with a diameter of approximately 1
100 nm (i.e., micro- to macropores) of which most are in the mesopore range (2
50 nm) [43]. The production methods of porous AAMs aim to add large pores (sometimes referred to as ultramacropores) up to hundreds of micrometers in size, depending on the application. In general, the direct foaming method is capable of adding pores starting from approximately 1 µm diameter up to the cm scale. However, the pore sizes and shapes are randomly oriented. The sacrificial template and additive manufacturing methods are able to produce controlled porosity, but in general sizes smaller than 10 µm cannot be obtained.

15.2.2.1 Direct foaming In the direct foaming method, air or other gas is introduced to the fresh-state AAM paste, mortar, or concrete by mechanical or chemical means before curing [10]. As the material sets, the gas bubbles are trapped in the structure to form pores, which

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New Materials in Civil Engineering

Additive manufacturing

Sacrificial template

Direct foaming

Intrinsic porosity

1 nm

10 nm

100 nm

1 µm

10 µm

100 µm

1 cm

Pore sizes

Figure 15.5 Approximate presentation of the obtainable pore sizes with different production methods of porous AAMs (the intrinsic porosity refers to the pores present in AAMs naturally).

can be either closed or open, that is, isolated or interconnected, respectively. Therefore, setting time and rheology of AAM are important factors affecting the pore structure. In general, closed pores are obtained by the use of blowing agents, which are chemicals that release gas upon decomposition [44]. Open pores, on the other hand, can be obtained by a combination of mechanical mixing or blowing agents and surfactants [29]. Surfactants consist of nonpolar carbon chain and anionic, nonionic, or cationic polar functional groups and they stabilize the gas
liquid interface. The stabilization mechanisms include the prevention of spontaneous drainage, continuous Ostwald ripening, and bubble coalescence [10]. Typically, surfactants are directly mixed into the fresh-state AAM. However, alternatively, in the prefoaming method, surfactant can be premixed with water (using a foam generator) [45] or alkali-activator solution [46] to prepare preformed foam, which is then mixed into fresh-state AAM. In some studies, a precuring step at a slightly elevated temperature (  75 C) is used before the addition of the blowing agent and/or surfactant [47]. When porous materials are prepared by direct foaming, a significant volumetric expansion of the material occurs frequently during curing [48], which can be controlled by the optimization of the dose and type of blowing agent and surfactant. As an alternative to surfactants, fibers, particles, or starch also have been employed to prevent bubble coalescence [32,49].

15.2.2.1.1 Blowing agents Hydrogen peroxide is one of the most widely used blowing agents in the preparation of porous AAMs. It decomposes into water and oxygen at high pH according to the overall reaction (15.1) [50]. H2 O2 ðaqÞ ! H2 OðlÞ 1 1=2O2 ðgÞ

(15.1)

Porous alkali-activated materials

537

The advantages of hydrogen peroxide are the lack of any residues in the mixture and the fact that it does not interfere with the alkali-activation reactions [51]. However, hydrogen peroxide introduces extra water, which should be taken into account. The typical dosing range of hydrogen peroxide is 0.1
5 wt.% of the wet slurry [27,47] or 0.75
3 wt.% of the precursor [48,52]. In general, increasing the concentration of hydrogen peroxide increases pore sizes [48,53], whereas a moderate dose (0.1
0.3 wt.%) promotes the formation of micrometer scale porosity [27]. When hydrogen peroxide is added as a low concentration solution (e.g., 3% [47]), a less anisotropic pore structure is obtained [37,54]. The reaction rate constant for oxygen release from hydrogen peroxide at alkali-activation conditions was determined to be 5.5 3 1024 s21, irrespective of its concentration [53]. Aluminum powder (i.e., metallic or elemental aluminum) is also a commonly used blowing agent for AAMs. Aluminum reacts at high pH conditions according to the overall reaction (15.2) by releasing hydrogen gas [55]. 2AlðsÞ 1 6H2 OðlÞ ! 2AlðOHÞ3 ðsÞ 1 3H2 ðgÞ

(15.2)

Aluminum addition affects the alkali-activation reactions: the precipitating aluminum hydroxide blocks some of the reactive sites of precursors and delays the strength development; the connectivity of unreacted particles increases due to the increased amount of soluble alumina; and carbonation can decrease as a result of alkalinity consumption [56]. The typical dosing range for aluminum powder is 0.01
0.25 wt.% of wet slurry [27,57,58]. As the doses are relatively small, it can be difficult to disperse aluminum powder effectively and, as a result, pore formation can occur only locally resulting in a heterogeneous pore structure [53]. Furthermore, aluminum production is highly CO2 intensive and thus it is not an environmental-friendly reagent [59]. A high dose of aluminum causes decreases in the compressive strength, density, and homogeneity of pore structure due to a collapse of pores [27]. Elemental silicon can be used as a blowing agent to generate hydrogen gas according to the overall reaction (15.3) [40]. SiðsÞ 1 4H2 OðlÞ ! 2H2 ðgÞ 1 SiðOHÞ4 ðaqÞ

(15.3)

Silica-rich industrial by-products can be also utilized as they contain small amounts of elemental silicon: for example, silica fume [40,60] or silicon carbide (SiC) sludge [32,49,61,62]. Also, ferrosilicon (FeSi) has been tested as a blowing agent: the decomposition reaction of FeSi has been proposed to occur according to reaction (15.4) [49,63]. The benefit of using FeSi is a slower reaction compared to pure elemental silicon or aluminum [63]. FeSiðsÞ 1 2OH2 ðaqÞ 1 4H2 OðlÞ ! FeðOHÞ2 ðsÞ 1 SiO2 ðOHÞ22 2 ðaqÞ 1 3H2 ðgÞ (15.4) The dose of elemental silicon is typically less than 0.04 wt.% of wet slurry [64]. Silica fume is required in a significantly higher amount (due to the low, typically

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,1%, content of elemental silicon): often more than 20 wt.% (of precursors) [19,20,65] but in some cases even more than 50 wt.% of silica fume was required to obtain significant porosity [39]. SiC contains a higher amount of elemental silicon (  15%) and thus the needed doses are in the range of approximately 1
5 wt. % [32]. However, large doses of elemental silicon are detrimental to the alkaliactivation reactions due to the water consumption [see reaction (15.3)] and excessive heat release [17]. Pore size distribution of silica fume-foamed AAM could be controlled by the duration of curing at elevated temperature (70 C): when the duration is increased, larger pores are obtained [39]. Sodium perborate is also an alternative blowing agent, which forms first hydrogen peroxide and subsequently oxygen gas in highly alkaline conditions according to the overall reaction (15.5) [66]. Sodium perborate is presented as monohydrate (the most water-soluble form) in reaction (15.5), but it is also available as tetrahydrate, trihydrate, and anhydrous [66].   1 Na2 B2 O4 ðOHÞ4 ðsÞ 1 2H2 OðlÞ ! 2BðOHÞ2 4 1 O2 ðgÞ 1 2Na

(15.5)

Sodium perborate is used decreasingly in oxidizing and bleaching agents as a source of active oxygen [66]. However, it is listed as a substance of very high concern by the European Chemicals Agency due to its toxicity and suspected damage to fertility [67]. In foamed AAMs, sodium perborate has been tested with 1
3 wt. % (of binder) dose and it was reported to result in lower thermal conductivity than hydrogen peroxide [21]. Sodium hypochlorite is also an alternative and less-used foaming agent for AAMs. It releases oxygen upon decomposition according to the overall reaction (15.6) [68] NaOClðaqÞ ! 2NaClðaqÞ 1 1=2O2 ðgÞ

(15.6)

When sodium hypochlorite is added to AAM slurry, it takes a relatively long time ($1 hour) before gas generation occurs, which is beneficial from a practical point of view [68]. Bo¨ke et al. [68] mixed sodium hypochlorite (12% solution) with fly ash in a weight ratio of 0.5, added sodium hydroxide pellets (NaOH/fly ash 5 0.22 was the optimum in terms of porosity), and cured gradually increasing the temperature up to 90 C.

15.2.2.1.2 Surfactants Surfactants employed for the preparation of porous AAMs can be divided into nonionic, cationic, and anionic (also proteinic and amphiphilic are sometimes mentioned as separate groups) [10]. Examples of surfactant structures are shown in Fig. 15.6. Additionally, many studies have employed commercially available air-entrapping and foaming agents developed for lightweight or aerated concrete applications—these products are typically combinations of several surfactants and other components. Examples of such products include Sika AER5 (mainly anionic surfactants) [53], Sika Lightcrete 02 (anionic surfactants: fatty acid, amide, and

Porous alkali-activated materials

539 O

H

O

O O

O

w

n

Triton X-114 (n = 7–8) Triton X-100 (n = 9–10) Triton X-405 (n = 40)

O

OH O x

HO

OH O z

O

Tween 20 (w + x + y + z = 20)

O y

O

–

O O

OH x OH

O

S

Tween 80 (w + x + y + z = 20)

O HO

10

O

z

O

Na+

H

O O

HO x

O

Sodium dodecyl sulfate (SDS) (also known as sodium lauryl sulfate)

y

O

O

O

w

y

Pluronic L61 (x = 2, y = 30) Pluronic P85 (x = 26, y = 40) Pluronic F127 (x = 100, y = 66)

x

Br

–

N

+

Cetyl trimethylammonium bromide (CTAB)

Figure 15.6 Examples of some commonly employed nonionic, anionic, and cationic surfactants for the preparation of porous AAMs.

sodium salt of C14
C16 sulfonic acid) [28], BASF Microair 210 (alkyl ether sulfate) [57], or EABASSOC foaming agent (anionic and amphoteric surfactants and protein) [69]. Surfactant type and dosing amount affect the morphology and topology of the pore network. Typical dosing ranges for surfactants shown in Fig. 15.6 are 0.05
6 wt.% [46,47,53]. In one comparative study, nonionic surfactants (Pluronic L35, Tween 80, and Triton X-100) resulted in a mechanically weak material and nonhomogeneous pore structure; anionic surfactant (SDS) resulted in homogeneous pore sizes but still low mechanical strength; whereas cationic surfactant (CTAB) resulted in higher strength and homogeneous pore size distribution for metakaolin geopolymer foam [53]. However, sodium dodecyl benzenesulfonate (anionic surfactant) was reported to result in relatively high compressive strength (  2.7 MPa at 7 days) at the optimal dose of 4% [46]. CTAB can also increase the viscosity of the AAM paste due to strong interactions with precursor (e.g., metakaolin) [53] and increase specific surface area significantly [70]. When comparing nonionic surfactants, Tween 80 was observed to produce smaller cell size, higher open porosity, but lower total porosity in comparison to Triton X-100 in a foaming process involving mechanical mixing to provide gas bubbles [29]. In general, a longer carbon chain should result in a more stable air
water film due to the increased surface elasticity of the interface [71]. It should be noted that other factors also affect the pore structure in addition to the surfactant chemistry: for instance, when surfactants are used with mechanical mixing, a higher mixing speed induces smaller cell size [29]. Surfactants can be also formed in situ using the saponification process: triacylglycerides are hydrolyzed at high-pH conditions of alkali activation to release fatty acids, which act as anionic surfactants [72]. The triacylglycerides can be of vegetable or animal origin [54,73]. However, the required dose of the triacylglyceride source (oil or fat) is relatively high: a range of 20
25 wt.% (of slurry) has been

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studied [73,74]. The triacylglyceride selection affects the material properties. For instance, canola oil, sunflower oil, and olive oil were compared as sources of triacylglycerides and it was observed that the oil affected the mechanical strength of the obtained foam, whereas the total porosity was less affected: this was explained by the different compositions of fatty acids in different oils and varying effects on rheology [74].

15.2.2.2 Sacrificial filler and replica method The sacrificial filler method comprises the addition of template material into the AAM slurry and its removal from the hardened AAM by dissolution or thermal treatment to obtain a negative replica of the template pore structure [10]. Possible sacrificial filler materials include ice (referred to as ice-templating or freeze-casting) [75,76], polystyrene [77], and 3D-printed polylactic acid [78]. The replica method resembles the sacrificial filler method but a positive replica of the original template is obtained [10]. This can be achieved, for instance, by immersing a highly porous polyurethane template into AAM slurry, which absorbs the slurry and, after removing the excess slurry, the template is removed by thermal treatment [79]. It should be noted that if very high temperatures are used for removal of template, the obtained material is not AAM but conventional high-temperature ceramic [10].

15.2.2.3 3D printing 3D printing, or additive manufacturing, allows the preparation of porous AAMs with accurately controlled pore dimensions, shapes, and amounts [10]. It should be noted that 3D printing of AAMs is also studied intensively for construction purposes [80
87] but, in that application, porosity per se is not usually a desired property and thus these studies are excluded from this review. The DIW or robocasting of AAMs consists of computer-controlled extrusion of layered AAM paste (referred to as “ink” in this context) filaments by air pressure and/or Archimedes screw and the resulting structure is cured usually at slightly elevated temperature (  40 C
75 C) and controlled humidity [30,31,88]. One of the challenges in the 3D printing of AAMs is the ongoing polycondensation reactions that change the rheological behavior of the ink as a function of time [89]. Furthermore, the ink should retain its shape after extrusion [88]. These challenges can be managed by controlling precursor particle size, alkali-activator composition, printing speed, air pressure and screw rotation speed, ink temperature, or by adding microfiller or rheology-modifying agents (such as polyethylene glycol) [30,31,88]. Graphene oxide was also shown to be an excellent rheology modifier of 3D-printed honeycomb-like AAM structures and it also improved mechanical properties and, after sintering at 1000 C, electrical conductivity [90]. Another 3D printing method, a powder-based system in which alkali-activator solution is delivered on a bed of precursor and other solids, results in randomly oriented highly porous (up to slightly over 70%) structures [91
93].

Porous alkali-activated materials

15.3

541

Characterization of porosity in alkali-activated materials

Porous features of the AAMs greatly influence their properties and hence, play a crucial role in determining their commercial application. Of the various pore features, total pore volume, size distribution, morphology, and connectivity are of particular interest. Bai et al. [94] studied how total porosity influences the compression strength and thermal conductivity of porous AAMs prepared from metakaolin [94]. The results are shown in Fig. 15.7. It can be seen that there is an inverse relationship between compressive strength and total porosity as well as thermal conductivity and total porosity. It is reported that circular and homogeneously distributed pores contribute positively to the insulating features of concrete foams [37,60,75]. In comparison, the irregular shape of voids with large pore size distribution can form a complex network of interlinking air channels, thereby leading to significant sound wave dissipation property, which is important in sound absorption [60,75,95]. Considering the significant role of porous features in determining the commercial application of porous AAMs, it is necessary to undertake their detailed characterization. This section discusses the various methods used for characterizing the porous properties of AAMs. This includes open and total porosity determination, optical microscopy characterization, SEM characterization, microcomputed tomography (µCT) characterization, mercury intrusion porosimetry, and ultrasonic pulse velocity measurement.

15.3.1 Open and total porosity determination Open porosity can be calculated according to the following equation [96]:   Open porosity 5 ðMw 2 Md Þ=ðMw 2 Ms Þ 3 100

(15.7)

Figure 15.7 (A) Compression strength versus total porosity and (B) thermal conductivity versus total porosity for porous AAMs prepared from metakaolin [94].

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where Mw 5 mass of water-saturated specimen in air (g); Md 5 mass of oven-dried specimen (g); Ms 5 mass of specimen when suspended in water (g). For calculating the total porosity, bulk density (ρb) is estimated by dividing the mass of AAM cut into a prism by its geometric volume measured with a caliper [44]. The true density of material (ρt) can be obtained with the help of a helium pycnometer [94]. The total porosity can be calculated according to the following equation [74]:   Total porosity 5 1 2 ρb= ρt 3 100

(15.8)

Fig. 15.8 shows the comparison between total porosity and open porosity for porous AAMs produced from metakaolin and fly ash using 2 wt.% surfactant (Tween 80/Triton X-100) at two different mixing speeds: 1500 and 2000 rpm [44].

15.3.2 Optical microscopy characterization Optical microscopy can be used for characterizing microstructures of the porous AAMs [12,42,61,62,94]. The microscopic images from the cross-section of the samples obtained using an optical microscope can be used for estimating the pore size distribution through image analysis. Hajimohammadi et al. [42] performed optical microscopy studies of porous AAMs obtained from blast furnace slag [41]. The results are shown in Fig. 15.9. Note that samples are represented as GBFS X(Y), where X represents Si/Al molar ratio and Y indicates H2O/Na2O molar ratio. It can be seen that GBFS 3.6 (14) possesses the smallest size of pores with the majority of pores (more than 60%) smaller than 0.5 mm. The sample GBFS 3.6 (17) has relatively larger pores than GBFS 3.6 (14) with nearly 45% of the pores smaller than 0.5 mm, about 35% having size between 0.5 and 1.5 mm, and the remaining pores larger than 1.5 mm. Among all the samples, GBFS 4.5 (14) possesses the largest pore size distribution, with only about 30% of the pores smaller than 0.5 mm, about 45% pores having size between 0.5 and 2 mm, and a few larger pores also.

15.3.3 Scanning electron microscopy characterization Scanning electron microscopy (SEM) can be also used for studying the microstructure of porous AAMs [41,94,97,98]. In comparison with optical microscopy, SEM allows microstructural characterization at higher magnification, thereby allowing observation of finer features. Fig. 15.10 shows the SEM analysis of AAMs

Figure 15.8 Comparison between total porosity and open porosity [44].

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Figure 15.9 (A) Microscopic images of cross section and (B) pore size distribution analysis of porous AAMs prepared from blast furnace slag [42].

Figure 15.10 SEM analysis of AAMs synthesized using two different foaming agents: (A, B) aluminum powder; (C, D) hydrogen peroxide [97].

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synthesized from fly ash using two different foaming agents: aluminum powder and hydrogen peroxide [97]: (A) has 0.07 wt.% aluminum powder, while (B) has 0.13 wt.% aluminum powder; (C) has 0.50 wt.% hydrogen peroxide, while (D) has 1.00 wt.% hydrogen peroxide. It can be seen that pores in AAMs produced with aluminum powder are irregular in shape and less uniformly distributed. In comparison, pores of AAMs produced with hydrogen peroxide are spherical in shape and more uniformly distributed. SEM analysis also indicates that there is high pore percolation when the amount of foaming agent used is higher. This is due to the high gas pressure created when a higher dosage of foaming agent is used.

15.3.4 Microcomputed tomography characterization Microcomputed tomography (µCT) imaging can be used for studying the porosity, pore size distribution, and circularity of voids in porous AAMs [41]. A µCT machine can be used to capture CT scan images of porous material. The captured CT images can be merged using image analysis software (for instance, Fiji ImageJ) to build a 3D image of the porous material. Hajimohammadi et al. [41] used µCT to study the influence of alkali-activator ratio (sodium silicate/sodium hydroxide) on the aluminum chemical foaming and the porosity of resulting AAMs [41]. Results of the analysis are shown in Fig. 15.11. The activator ratio in the samples followed the order: X . Y . Z. It can be seen from Fig. 15.11A that porosity increased as the activator ratio decreased. This is due to the fact that sodium hydroxide acts as a catalyst for aluminum foaming reaction, while sodium silicate acts as an inhibitor for the same. Results of the morphology analysis are shown in Fig. 15.11B. When the circularity of voids is less than 20%, voids are mostly irregularly shaped. As circularity increases, voids become closer to a round shape. When circularity of voids is

Figure 15.11 (A) 3D images of µCT scanned AAMs, (B) quantifying the circularity of voids in AAMs, and (C) pore size distribution analysis of AAMs [41].

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80%
100%, voids are mostly oval and circular. It can be seen that sample X with the highest activator ratio has the highest circularity of voids (meaning that it has more circular pores). Sample Y has similar percentages of circular and irregular voids. Sample Z, with the lowest activator ratio, has the highest amount of irregular pores. Fig. 15.11C shows the pore size distribution of AAMs. It can be see that the extent of porosity is increasing from samples X to Z. Another interesting feature is that in all the samples, the majority of pores are less than 0.5 mm.

15.3.5 Mercury intrusion porosimetry Mercury intrusion porosimetry can be used for the characterization of porous AAMs. Masi et al. [27,28] used mercury intrusion porosimetry for characterizing porous AAMs produced from fly ash with varying amounts of surfactant [27,28]. The results of the analysis are shown in Fig. 15.12. The surfactant content varied from 1 to 5 wt.%. The samples are represented as X Surfactant, where X indicates the wt.% of surfactant. It can be seen that there is a clear increase in porosity of the sample foamed with 2 wt.% surfactant when compared with 1 wt.% surfactant. However, further addition of surfactant led to a reduction in porosity. The open porosity (OP) observed is according to the following order: OP2 . OP3  OP4  OP5 . OP1 (where OPy indicates open porosity of a sample with y wt.% surfactant). This observation indicates the need to optimize the quantity of surfactant in the preparation recipe.

15.3.6 Ultrasonic pulse velocity measurement Ultrasonic pulse velocity measurement can be used for studying the directional homogeneity of pore distribution. Hajimohammadi et al. [42] used ultrasonic pulse

Figure 15.12 Pore distributions of porous AAMs synthesized with different quantities of surfactant [27,28].

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Figure 15.13 (A) Ultrasonic pulse velocities in three different directions of cubic AAM samples, (B) pore homogeneity results [42].

velocity measurement for characterizing the porous AAMs prepared from blast furnace slag with mechanical foaming [42]. The results are shown in Fig. 15.13. Note that samples are represented as GBFS X(Y), where X represents the Si/Al molar ratio and Y indicates the H2O/Na2O molar ratio. This means that the impact of the H2O/Na2O ratio is considered in these two cases, while keeping similar Si/Al and Na/Al ratios. The pulse velocity is measured in three directions in the cubic sample (Fig. 15.13A). If there is a noticeable difference in the pulse velocity in different directions, this would mean that pores are not uniformly distributed in the sample. The pulse velocities from side to side are almost the same in both samples, while there is a noticeable difference in the pulse velocities from top to bottom in the samples [with a larger difference in GBFS 3.6(17)]. This indicates the extent of bubble size rearrangement from the top to bottom within the samples. The pore homogeneity results (Fig. 15.13B) indicate that GBFS 3.6(14) has 95.9% homogeneity, while GBFS 3.6(17) has 76.1% homogeneity. This is due to the fact that foams with higher water content are more susceptible to bubble size rearrangement (foam sorting) during mixing and pouring into molds. This leads to poor pore homogeneity in the sample.

15.4

Properties of porous alkali-activated materials

Table 15.2 gives an overall idea of the properties of porous AAMs studied with different source materials. There is not much information on the fresh and durability properties of porous AAMs. Most of the literature emphasis is on mechanical properties including density and compressive strength. Regarding the functional properties, porous AAMs are mainly studied for their thermal conductivity and high temperature resistance, which is favorable for AAMs.

15.4.1 Foam stability Foam consists of air-filled voids dispersed in a liquid phase (fresh paste/mortar). The material then hardens by forming a gel and solid phase after final setting.

Table 15.2 Summary of properties of porous alkali-activated materials from literature. References

Aluminosilicate precursor

Foaming agent

Density (kg/m3)

Compressive strength (MPa)

Thermal conductivity/ specific heat (W/m K)/ (kJ/kg K)

[99] Bell and Kriven (2009) [100] [39] [50] [61,62] [40] [27,28]


Metakaolin

H2O2/perborates H2O2/Al

100
800 1090
1380

0.5
2 49
77

.0.037/1.2

Metakaolin 1 fly ash Calcined clay Perlite Metakaolin Metakaolin Fly ash

Al Silica fume H2O2 SiC Silica fume H2O2/Al

600
1200 534 335
665

1
16

0.47
1.65 0.22
0.24 0.03
0.05

[33] [101] [21]

Fly ash Fly ash Fly ash

H2O2 Al H2O2/NaBO3

[37] [102] [103] [104] [74] [97]

Fly ash Fly ash Fly ash Fly ash 1 slag Metakaolin Fly ash

H2O2 Al Al Chemical foam H2O2 H2O2/Al

[105] [41] [42]

Metakaolin 1 fly ash Fly ash Slag

[106] [94] [106]

Fly ash Metakaolin Fly ash 1 slag

H2O2 Al Sodium dodecyl sulfate Al H2O2 H2O2

[38]

Metakaolin 1 fly ash

Al

400
850 910
1400/ 800
1420 300
1000 900 600
1200/ 800
1300 240
360 403
1309 671 900
1100 260
840 600
1000/ 600
700 600
1200 300
800 300
775 400
1600 370
740 650
690 590
800 430
850

0.3
0.8 0.9

0.12
0.33 2
21/3
21 0.35
7 5.5
10.9 0.5
4.5/3
4.5 0.6
1.4 0.9
4.35 6 7
12 0.38
25.96 2
9/2
4 1.23
8.9 0.42
1.59 50
85

0.25
0.39 0.075
0.2/0.05
0.15 0.075
0.092 0.145 0.15
0.49

0.1
0.43 0.22
0.24

0.5
24 0.3
11.6 1.25
2.35/1
1.9

0.11
0.17

0.5
4.4

0.078
0.17

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However, it is important to maintain the foam stability to design the pore volume and pore structure of the material, which affects the density of the final product. Stability of the foam depends on the reaction kinetics and hardening time. For instance, fly ash takes a long time to set owing to its chemical composition and reactivity, by which the bubbles collapse within the paste and the same effect is observed with metakaolin geopolymer paste when binder is replaced with sand. This indicates a proper combination of cementitious composition and reaction conditions should be employed to compensate between workability and setting issues [100]. The type and concentration of foaming agent and foam generation method influence the stability of the foam [1,108]. Novais et al. [38] indicated that incorporation of Al helps in quicker setting by reducing the time to reach the peak temperature of geopolymerization. Use of Al as foaming agents also helps in the rapid release of aluminate ions that helps in gel network formation and reduces the induction period [109]. Additionally, the water content is also an important factor at an initial stage that affects the stability of the bubbles in foam [42]. It was found that aggregating the liquid film around the bubbles improves the stabilizing effect and addition of a stabilizing agent like xanthan gum considerably improved the stability and properties of porous AAMs [27,28,106].

15.4.2 Mechanical properties Density plays a major role in defining the properties of the porous AAMs. Densities of porous materials are measured in fresh and dry states and the difference in these values should not exceed 120 kg/m3. Fresh density (which changes with time) is measured by weighing the sample in a container of known volume and helps in designing the actual volume of the mix and to control the dry density for mechanical and durability properties [1]. The reported high density of geopolymer foams in certain insulation applications [three times higher than its counterparts like mineral wool/extruded polystyrene (EPS)] was considered to be a drawback for its applications. It was suggested to use a higher curing temperature ( . 100 C) to remove the water from the geopolymer structure which decreases the apparent density [110]. This can also be achieved by different manufacturing processes such as blowing air under controlled pressure or by using source materials with lower density, such as expanded perlite [50]. As mentioned earlier, pore structure and volume affect the mechanical properties of the porous AAMs. It can be roughly stated that compressive strength increases exponentially with a decrease in total porosity when similar manufacturing processes and materials are maintained [94]. However, this cannot be the case in practical production of porous AAMs. There are many factors that influence the properties by variation in composition, degree of polymerization, curing conditions, alkaline activator content, and production methodology. However, it is observed from Table 15.2 that it is possible to produce a wide range of porous AAMs with desired strength and density. The strength is also directly proportional to the foaming agent dosage, that is, the pores formed. For lower foaming agent content, a network of small and closed

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pores hinders water removal during polymerization. This results in incomplete alkali activation, and therefore in lower strength [50]. The ration of Si/Al affects the compressive strength of AAMs and it was found that a ratio of three gave good strength. Also, the ratio of Si/Na affects the dissolution of the species needed for polymerization and a lower ratio results in an incomplete reaction. The unreacted particles act as sites of failure and result in strength reduction [33]. On the other hand, excessive sodium may lower the compressive strength and durability as any sodium in excess of Na/Al 5 1 will not be chemically bound to the structure.

15.4.3 Durability properties There are only a few studies available reporting on the durability properties of porous AAM. Resistance to a chemically aggressive environment was studied with fly ash-based porous AAMs with a density of 671 kg/m3. The degree of pore saturation does not exceed 0.6, indicating the closed nature of pores. However, the porous AAM deteriorated in an acid environment (pH 2) by two different modes. Al depletion by leaching from N
A
S
H gel and leaves behind stronger Si
O
Si skeleton or dissolution N
A
S
H gel happens in acid medium. Still, porous AAMs could retain more than 50% of the strength after 360 days of exposure [103]. Whereas, C
S
H gel from Portland cement would completely disintegrate in such a situation [111].

15.5

Functional properties and applications

15.5.1 Thermal conductivity Air voids in porous AAM acts as a barrier against heat flow and help in insulation. Thermal conductivity depends on the cellular structure, type, size, and volume, which is comparable to other insulation foams [50]. Pore volume controls the thermal conductivity of the material and most porous materials exhibit the lowest conductivity [40]. It was reported that with a high water content, the bubbles were protected from breaking and gel particles around the bubble prevented coarsening. However, the bubble rearrangement due to high water content in the parallel direction of the heat flow increased the thermal conductivity of the material, despite its lower density. Though more voids mean more air content and lower conductivity, this also depends on the orientation and position of the voids [42]. The addition of quartz aggregate increases the thermal conductivity of the alkali-activated porous materials by reducing the porosity [100]. There are no significant literature data on functional properties and their relationship with porous AAMs. More detailed investigations into these grounds will give confidence on practical application of this material in a wider range. It can be observed from Table 15.2 that porous AAMs possess acceptable strength to be used as a construction material. In fact, it has added privileges of stability in high temperature, is free from fibers, and has high strength. For

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instance, insulation with brick work is possible only by increasing the thickness of the wall but occupies a great deal of space. Alternatively, polyurethane or mineral wool can be used as insulation material with brick works but has fibers which are hazardous to health. Also, they do not match the property of bricks and allow water absorption that affects the humidity inside buildings. Additionally, porous AAMs are fire resistant, unlike the other alternatives [99]. It was also been proved that the cost of porous AAMs is comparable to that of expanded polystyrene, with simplicity in production as an added advantage [105]. Porous AAMs have problems of efflorescence due to leaching of Na1 ions as reported by some authors, but this is mostly an aesthetic issue which can be rectified [11].

15.5.2 Sound absorption Noise pollution in both indoor and outdoor environments is an increasing problem. For instance, it has been estimated that, in Europe alone, at least one million healthy life-years are lost due to traffic noise [112]. One option to reduce excessive and harmful noises is sound-absorbing materials, in which the acoustic energy is dissipated (into heat energy) via viscous flow, internal friction, and panel vibration [113]. Three main parameters affecting the effectiveness of sound absorption are pore sizes, degree of open porosity, and material thickness [113]. In the case of AAMs, these can be largely controlled by the production method (see Section 15.2), aggregate/filler selection, and by selection of the precursor and alkali activator, as discussed below. The efficiency of sound-absorbing materials is frequently characterized by the absorption coefficient (α), which is the proportion of the absorbed energy on the surface, according to the standards ISO 10534-2 or ASTM E1050. The human hearing range is usually 20
20,000 Hz, but the lowest intensity to reach hearing threshold is required between 2000 and 5000 Hz [114]. Another parameter used to describe acoustic properties is the sound transmission loss (STL), which is the decrease in sound intensity at different frequencies as it passes through a sample. Pervious AAM concretes or mortars have been studied as sound-absorbing materials. They are prepared by adding lower than optimum amount of binder into aggregate/filler mixture to result in empty voids. For instance, Perna et al. [115] found that kaolinite-based geopolymer mortar (20% binder and 80% aggregate/ filler) had maximum sound absorption (α 5 0.7
0.9) at 1200 and 2000 Hz when the sample thickness was 30 or 20 mm, respectively. Moreover, increasing the content of geopolymer binder (kaolinite and activator) to over 20% of the mixture decreased sound absorption [115]. Gandoman and Kokabi [116] used geopolymer (binder consisting of metakaolin and NaOH) concrete (aggregate/metakaolin 5 4.5) containing rubber waste (0%
14% of aggregate) and observed sound absorption maximums (highest α 5 0.35
0.40) at 1000 and 3000
4000 Hz frequencies. When the rubber content increased, the ability to absorb sound also increased [116]. Chang et al. [117] prepared pervious geopolymer concrete from blast furnace slag activated with sodium silicate and hydroxide containing electric arc furnace slag and gravel as aggregates. They observed maximum sound absorption (highest

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α  0.45) at 500 Hz and the sound absorption increased as the amount of binder decreased (i.e., the concrete was more pervious with open porosity up to 25%) [117]. Arenas et al. [118] prepared porous AAM concrete (class F fly ash activated with sodium silicate/hydroxide) for an outdoor traffic noise absorber by incorporating coarse demolition waste aggregate into mortar. Their materials exhibited two peaks: at approximately 1000 Hz (α 5 0.7
0.9) and at 3000 Hz (α 5 0.45
0.65), increased sample thickness caused better absorption of low frequencies, and the more porous demolition waste aggregates performed better than natural coarse aggregate [118]. Another group of AAM-based acoustic materials has been prepared using the foaming methods described in Section 15.2. Zhang et al. [104] prepared foamed AAM concrete from fly ash and slag activated with sodium silicate and hydroxide using the prefoaming method (generation of foam from water
surfactant mixture using foam generator). Their material was especially effective at absorbing low frequencies (40
150 Hz) with an α of approximately 0.7
1 [104]. When the slag content was increased, higher frequencies (800
1600 Hz) were absorbed more effectively (α up to  0.3) [104]. An increase of the foam dose from 5% to 10% decreased absorption at low frequencies but improved absorption at the medium range (600
1000 Hz) [104]. Papa et al. [60,75] prepared foamed AAMs from silica fume with or without metakaolin activated with Na/K hydroxide and silicate. Their materials exhibited two peaks in sound absorption: one at 500
2000 Hz and another at approximately 4500
5500 Hz [60]. The highest absorption (α 5 0.8
0.9) was obtained with material containing silica fume and metakaolin activated with potassium-based activator: the difference compared to a sodiumbased system was significant (in that case α 5 0.4
0.5), which might have been because of the larger pore sizes (especially 500
1700 µm diameter) of potassiumbased material [60]. Mastali et al. [69] prepared fiber-reinforced alkali-activated blast furnace slag foam concrete using lightweight aggregates made by alkali activation of by-product from sponge iron manufacturing (Petrit-T). It was observed that a foam dose of 35% resulted in material with high sound absorption (α 5 0.8
1) in the range 1600
2500 Hz and the density had a linear correlation with sound absorption coefficient [69]. Stolz, Boluk, and Bindiganavile [119] prepared fiber-reinforced fly ash-based prefoamed alkali-activated concrete. Their material had high sound absorption (α up to 0.7
0.85) at low frequencies (125
250 Hz), but also moderate sound absorption (α up to 0.5) at high frequencies (4000
6300 Hz). The optimum density in terms of noise reduction coefficient (the average of sound absorption coefficients at 250, 500, 1000, and 2000 Hz) was 1130 g/cm3 (lower or higher density decreased sound absorption) [119]. LunaGaliano et al. [65] used silica fume as a blowing agent for the preparation of fly ash-based sound-absorbing material: their material had α ,0.3 with maximum absorption occurring at approximately 400 and 2500 Hz. They observed that an increase in the silica fume content (up to 40%) and curing temperature (up to 70 C) improved the acoustic properties [65]. To summarize, porous AAMs appear as promising materials for the absorption of sound. However, the most efficient sound absorption range of AAM seems to

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vary from one material to another and is likely due to porosity differences induced by the production method and different precursors.

15.5.3 Fire resistance Fire resistance testing involves the rapid heating of the sample on one side and temperature is measured from the other side (cold side) using thermocouples [101]. The temperature in the cold side increases with time as the porous AAM conducts heat and reaches 100 C with evaporation of water. This helps to analyze the combustibility, release of gas, and structural stability of the material during fire. It was observed from the test that porous material reached a higher temperature in a shorter time compared to solid materials. This was explained as the geopolymer paste being replaced with pores, and it also having a lower water content and hence less time for dehydration. However, porous material reaches an equilibrium temperature after the dehydration plateau and that helps in improved thermal insulation of the material [101]. Porous structure could be maintained until 800 C. With increasing the firing temperature beyond 800 C, cell size reduces by linear shrinkage due to complete removal of water, and sintering and crystallization occurs [54]. This even improves the strength of the porous AAMs at high-temperature exposure though it results in high shrinkage of 30% at 1200 C [54,103,104]. It was concluded that fire resistance of porous material depends on the water content, thermal conductivity, and crack resistance. In industries involving high-temperature furnaces, it needs refractory lining for kilns, which is mostly made of ceramics or mineral wools which have fibers. This always poses a health risk due to the release of these fibers during installation, maintenance, and repair. Also, complicated shapes of equipment to be insulated make the installation process difficult. Porous AAMs can be foamed on such places with ease and without the risk of any fibers [99]. Although geopolymeric foam can withstand 1000 C, it softens between 700 C and 1000 C, and hence 700 C should be the maximum application temperature [50]. It was also observed that porous AAMs by exposure to high temperature (up to 1200 C) improved in strength [10], which is an excellent property for fireproofing.

15.5.4 Application in water and wastewater treatment Porous AAMs have been studied for the following water and wastewater treatment applications: adsorbents and ion exchangers, photocatalysts and catalyst supports, membranes, filtration media, pH buffer material, and substrate for bioreactors. All of these applications are briefly discussed in this section from the viewpoint of porous AAMs. As adsorbents/ion exchangers, AAMs have shown a preliminarily promising potential for the removal of a wide scale of different metal(loid)s, rare earth elements, ammonium, sulfate, and organic dyes [120,121]. The cationic pollutant removal occurs through the ion-exchange of the charge-balancing cations (usually Na1 or K1) but secondary minerals (such as hydrotalcite) or AAM modification

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can contribute to anion exchange/adsorption properties [122
124]. The adsorption and ion-exchange reactions use the intrinsic microporosity of AAMs. Most of the adsorption/ion-exchange studies have been conducted with powdered AAMs but in full-scale water treatment processes, filter set-ups are preferred instead of powder dosing. Thus, open-cell AAMs with hierarchical porosity, high enough permeability (i.e., low pressure drop), and good mass transfer properties are interesting for this application. For instance, spherical or granular AAMs (diameters in the millimeter scale) have been prepared for this purpose by the geopolymerization
granulation process [125], addition of alginate and calcium to fresh paste to promote crosslinking [126], or by the suspension and solidification method [127]. Another option is to prepare monolithic porous filters [26,128,129] or 3D printing [30,31] using the methods discussed in Section 15.2. Regarding catalysts in water and wastewater treatment, current studies have focused on the photocatalytic decomposition of organic dyes [130]. In this application, the catalyst contains semiconductor material, which, when exposed to radiation with energy higher than the semiconductor band cap, is able of generating highly oxidizing hydroxyl radicals. Beyond water treatment, other catalytic reactions also have been studied, such as ash photocatalytic degradation of volatile organic compounds [131], biodiesel production [132,133], oxidation of hydrocarbons and selective catalytic reduction of NOx by NH3 [134], ethanol to syngas by (autothermal) steam reforming and partial oxidation [135], Beckmann rearrangement of cyclohexanone oxime to caprolactam [136], and Friedel-Crafts alkylation [137]. The catalytically active metals can be introduced to AAMs by adding metal compounds to fresh-state paste or to the cured material via ion exchange either directly or by first converting the geopolymer into NH41 form [138]. The preparation of the AAMs for catalyst support follows the procedures described in Section 15.2. AAM membranes and filters could have many beneficial properties for ceramic materials, but lower cost as no sintering step is required. As high-pressure membranes, AAM materials could be used as micro- or ultrafiltration membranes (i.e., pore sizes are 20
100 nm) [139,140]. This application does not require any additional foaming or other production method but utilizes the intrinsic porosity of geopolymers. Furthermore, AAMs could also be used in lower pressure separation applications as filtration media in sand filters, permeable reactive barriers, or pointof-use water treatment filters, as an example. This application requires porosity in the range of the micrometer to millimeter scale. Porous AAMs have been used as pH buffer material. The pH adjustment of various wastewaters is important for the efficient operation of chemical and biological treatment processes. AAMs could be used as pH buffering materials due to free leachable alkalis in their pore solution [141]. High SiO2 and Na2O content of metakaolin geopolymer promotes leaching, whereas high Al2O3 decreases it [142]. The Na2O/Al2O3 ratio of 0.2
2.0 was used with geopolymer buffers [13,142
144]. Other factors affecting the leaching are geopolymer porosity (while geopolymer particle size had only a minor effect when pores were open) [143] and the liquid to solid ratio during geopolymer preparation [142,144]. Novais et al. [105] prepared

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porous geopolymer buffer material from metakaolin, biomass fly ash, and hydrogen peroxide as a foaming agent. They noticed that larger amounts of hydrogen peroxide increased the total leaching of OH2 [up to 0.0180 mol/(dm3 3 g)] and the initial pH via the more porous structure. In another work, they used sodium dodecyl sulfate as a foaming agent and obtained total hydroxyl ion leaching of 0.0317 mol/ (dm3 3 g) [145]. Another geopolymer pH buffering material was based on metakaolin and red mud activated with sodium silicate and hydroxide with aluminum as a foaming agent [13]. Prolonged leaching of OH2 was achieved with geopolymers having .35 and ,0.025 wt.% of red mud and foaming agent, respectively [13]. Red mud is alkaline (pH typically 10
12.5 [146]) and its addition increased open porosity up to approximately 35% [13]. Wastewater treatment processes using fixed biofilms use various types of floating carrier media to enable adhesion and biofilm development. For instance, polystyrene and lightweight expanded clay aggregate (LECA) have been used. Silva et al. [147] prepared biofilm carrier media (diameter 2
3 cm) by treating tungsten mine waste mud (calcined at 800 C, 2 hours) with sodium silicate and hydroxide. The obtained material exhibited suitable pH (should be lower than 8) and stability when submerged in water [147]. Therefore, a porous AAM-based biofilm carrier media could be a low-cost alternative for wastewater treatment.

15.6

Conclusions

In summary, alkali activation of industrial wastes and its utilization in civil engineering applications are developing steadily all over the world. Although its application is mainly focused on nonstructural purposes, the advantages of this material on the possibility of designing the pore structure, usage of low-cost materials, and eco-friendly technology makes them a valuable addition to the porous, lightweight material applications. In this chapter, porous AAMs have been reviewed for their materials, manufacturing methods, amount of porosity, and pore structure, which is also related to the mechanical and functional properties of the material. It is important to understand the effect of source material, its mineralogy and reactivity, to understand the properties of the product. This is inevitable to form a database by experimental investigations on different locally available sources for alkali activation, to have control and confidence of the efficiency of the material for practical applications. Different processing methods for producing porous AAMs were discussed, of which direct foaming appears to be the simplest and most widely used. However, the control of the nature of the pores and their characteristics are unpredictable when using this method. Functional properties of porous AAMs define their applications. For instance, thermal conductivity is reduced when pores are not interconnected, which has important implications in thermal insulation applications. On the other hand, connected pores allow high sound absorption and can be used as an acoustic panel. Such special applications benefit from known

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pore architectures. Sacrificial filler method leads to controlled production of porous material, nevertheless there are no much scientific data available to assess the effect of this method on the properties of different combinations. In addition, it needs detailed study to define the relationships between pore characteristics and functional properties, and to clarify many other issues involved in alkali treatment such as efflorescence. Shrinkage and other durability studies were not given wide importance in these materials, which can lead to potential problems in practical applications. Research is needed in these areas to increase confidence among industries for the use of porous AAMs. This chapter has summarized the applications of porous AAMs in thermal and sound insulation, high-temperature applications, and water treatment materials. This demonstrates the wide range of potential for this material and the possible future research areas to be concentrated to increase confidence in it.

Acknowledgments Tero Luukkonen acknowledges the financial support received from the Academy of Finland (grant #315103).

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628.

Lightweight cement-based materials

16

Teresa M. Pique1, Federico Giurich1, Christian M. Martı´n1, Florencia Spinazzola1 and Diego G. Manzanal2,,3 1 Polymers for the Oil and Construction Industry, Engineering Faculty, Universidad de Buenos Aires, Buenos Aires University, Argentina, 2National Univerity of Patagonia, Comodoro Rivadavia, Argentina, 3ETS of Roads, Canals and Ports, Polytechnique University of Madrid, Madrid, Spain

16.1

Introduction

Nowadays, different properties are expected for building materials. Durability and high resistance are the common requirements, but every day more applications demand reduced weight as a required property. As a means of reducing environmental impact, sustainable construction requires considering not only the type of materials used, but also the way they affect the building process and the long-term performance of the structure. Lightweight materials are known for their insulating properties, thus low-energy conditioning of housing is one of the main reasons for their use in construction. However, these materials usually have poor mechanical properties. For this reason, lightweight materials are mostly used in nonstructural applications, such as covering mortars for walls, flat roof insulation, or concrete sandwich wall panels. In these cases, different approaches to lower the density of cement-based materials can be considered, such as using lightweight/low-strength aggregates, like expanded polystyrene (EPS), or increasing the water-to-cement ratio. Despite these approaches that tend to reduce the mechanical properties, it is also possible to reduce the density of a material and maintain a satisfactory strength. For example, cement can be replaced with lightweight/high-strength aggregates such as hollow glass microspheres (HGMSs), which are widely used for oil well lightweight cement slurries. These slurries usually require low density, as the pressure they exert over the formation when pumping it, which directly depends on density, is a limiting variable for its application. Another way to obtain lightweight cementbased materials with acceptable resistance is to enhance the mechanical properties of the binder. This chapter reviews three techniques for lightening cement-based materials: using EPS, HGMSs, and additives known in the oil well industry as extenders. Both insulating and mechanical properties are specially studied.

New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00016-8 © 2020 Elsevier Inc. All rights reserved.

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16.2

New Materials in Civil Engineering

Lightweight/low- strength aggregates

EPS lightweight concrete is one example of cement-based material used in construction. Its production involves partially or fully replacing stone aggregates by EPS particles, depending on the application intended for the concrete. EPS is obtained by the process of expanding atactic polystyrene (PS), an amorphous thermoplastic polymer, whose density is 1050 kg/m3. Preexpanded atactic PS particles, made of atactic PS mixed with a low boiling point hydrocarbon that serves as a foaming agent, are heated so that the hydrocarbon expands, resulting in an expanded particle around 50 times its original size [1]. Consequently, EPS density is around 10 30 kg/m3. Compared to other lightweight aggregates, EPS stands out for being highly porous (95% 98% of its volume is void space) but at the same time hydrophobic [2]. The latter property makes EPS different from other lightweight aggregates, as it has almost zero absorption of the concrete mixing water [3]. This reduces the needed mixing water but also makes EPS adherence to cement paste more difficult [1], and makes it more prone to segregation as a consequence [4]. Using EPS as concrete aggregate has been the subject of various researches. In most cases, a partial replacement of stone aggregates by EPS particles has been studied. Table 16.1 sums up the relevant features of the analyzed researches.

16.2.1 Production: fresh state Mehta and Monteiro [20] define workability through two properties: consistency and cohesion. Whereas consistency describes the ability of a mixture to flow in the production and casting process, cohesion describes the stability of phases in the mixture, that is, the absence of segregation and bleeding. Both properties must be considered together to achieve a suitable mixture in fresh state, as they are usually affected in opposite ways: when one increases, the other decreases. In lightweight concretes, segregation due to floating of the lightweight aggregate may happen due to the large difference in densities between the matrix and the aggregate. The main reasons for this are a very low mix cohesion or excessive vibration during compaction [20]. In both cases, low internal friction between aggregates and paste is responsible for segregation: naturally low in the former, vibration-induced in the latter. Vibration makes the mixture less cohesive and allows it to flow toward empty spaces, reducing cavities. However, if vibration is too intense, segregation follows, even in rigid-like mixtures. EPS lightweight concrete is not exempt from the above-mentioned challenges. Different researches have pointed out aspects that need to be considered during the mix design process and during casting and compacting in order to avoid segregation. Park and Chisholm [11] reported that there is only a slight difference between the water content that makes a mix too cohesive to allow good mixing with EPS particles and the one that makes a mix too fluid to avoid segregation. Similarly, Cook [1] indicated that reducing the water content prevents segregation and stressed

Table 16.1 Using EPS as concrete aggregate, relevant aspects of analyzed researches. References

Type of cement

[1]

Mineral admixtures

Type of EPS

EPS range (%vol/ total vol)

N/D

Fine and coarse

Spherical beads

20 70a

[2]

ASTM type I

Fine

5 35a

2200 1700

48 17

[3]

ASTM type I

Spherical beads (preexpanded PS and EPS); recycled/ crushed; recycled/powder Spherical beads

16 67

1723 582

12.5 1

[5]

ASTM type I

15 60a

900 2100

[6]

ASTM type I

33 36

1350 1165

Fly ash C

Fine and coarse Fine and coarse

Silica fume

Fine and coarse

Recycled/ thermally modified Spherical beads

Density range (kg/m3 )

28-day compressive strength range (MPa)

Stone aggregates

11.9 5.6

Analyzed properties

Mechanical strength; fresh state; drying shrinkage Mechanical strength; fresh state; durability

Mechanical strength; durability Fresh state; thermal conductivity; drying shrinkage Mechanical strength; fresh state; thermal conductivity; drying shrinkage; durability

(Continued)

Table 16.1 (Continued) References

Type of cement

[7]

ASTM type II ASTM type I ASTM type I

[8] [9]

Mineral admixtures

Stone aggregates

Type of EPS

EPS range (%vol/ total vol)

Density range (kg/m3 )

28-day compressive strength range (MPa)

Analyzed properties

Spherical beads

38 77a

1192 464

8.53 0.11

Mechanical strength; fresh state Mechanical strength Mechanical strength; fresh state; durability Mechanical strength; drying shrinkage; durability Mechanical strength; fresh state; thermal conductivity; drying shrinkage Mechanical strength; thermal conductivity; durability Fresh state

Silica fume

Fine

Spherical beads

9 47

1900 1124

30 6.1

Fly ash C

Fine and coarse

Spherical beads

20 50

1858 1012

20 5.5

[10]

ASTM type I

Silica fume, rice husk ash

Fine and coarse

Spherical beads

11 41a

1900 949

35 2.9

[11]

ASTM type I

Fly ash C

Fine

Spherical beads

53 68a

1040 520

6.7 0.7

[12]

ABCP CPII

Fine

Recycled/crushed

55 65

1250 1110

15.6 8.4

[13]

ASTM type I

Fine and coarse

Spherical beads

7 21a

Silica fume, nanosilica

[14] [15] [16]

ASTM type I ASTM type I ASTM type IV

Fine and coarse Fine

Spherical beads

10 40

1200 2440

Spherical beads

36 71

1199 797

6.2 3.1

Recycled/crushed

20 80

1567 648

16.9 1.82

381 584

2.4 0.9

[17]

ASTM type I

Nanosilica

Spherical beads

34 36a

[18]

ASTM type I ASTM type I

Nanosilica

Spherical beads

26 44a

Silica fume

Fine and coarse

Spherical beads

7 21a

1712 2235

17 31.5

ASTM type I

Fly ash F

Fine and coarse

Spherical beads

0.25

2060 2092

28 35

[19]

[4]

a

Values have been estimated through the proposed dosages.

Thermal conductivity Mechanical strength Mechanical strength; thermal conductivity; durability Mechanical strength; fresh state; thermal conductivity Fresh state Mechanical strength; fresh state; durability Mechanical strength; fresh state

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New Materials in Civil Engineering

the importance of using an adherence-improving additive. Sri Ravindrarajah and Tuck [6] reported good cohesion in mixtures made with surface-treated EPS beads to improve adherence. Kan and Demirboga [7] highlighted that a relatively uniform lightweight concrete can be achieved if vibration is limited to prevent EPS particles from moving toward the top of the mold. Giurich and Pique´ [18] stated that the amount of vibration needed to achieve a nonsegregated, well-finished EPS concrete member is directly related to the mixture consistency: the higher the consistency, the less the vibration that is needed. The consistency of EPS lightweight concrete generally decreases with an increase in the EPS content [2,5,7,12,18]. This phenomenon is mainly attributed to a lower weight of the mix itself, as weight is the driving force that causes slump. As the mix weight is reduced, lower slump values are obtained. In the case of concrete made purely with EPS particles as aggregates, consistency is also reduced due to the fact that the higher the EPS content, the lower the free cement paste (not adhered to the surface of particles), which is responsible for making the mix flow as a whole [18]. However, in the case of self-compacting EPS concrete, Ranjbar and Mousavi [19] reported that replacing stone aggregates by EPS yields increasing flow values, due to the lower internal friction caused by EPS particles in the mix compared to that caused by stone aggregates. Madandoust et al. [13] showed that, for a given EPS content, reducing the waterto-cement ratio reduces consistency, due to a decrease in the cement paste viscosity. Garcı´a-Alcocel and Ferra´ndiz-Mas [2] reported that for the same EPS content and water-to-cement ratio, consistency of mixtures made with spherical EPS beads is higher than that of mixtures made with crushed recycled EPS. This is because the former has a lower surface area than the latter, thus reducing the surface area to which cement paste adheres and increasing the free cement paste responsible for the mixture flow. Many researchers have warned about the difficulty of casting and compacting EPS concrete due to its usually rigid-like and sticky consistency [5,6,11]. Kan and Demirboga [7] reported larger compaction efforts associated with a reduction in consistency when the EPS volume is increased. Giurich and Pique´ [18] added that vibration is not effective in reducing cavities in low-consistency, high-cohesive mixtures, only leading to an increase in segregation. Sri Ravindrarajah and Tuck [6] recommend the usage of plasticizers in mixtures with low water-to-cement ratio in order to reduce the number of cavities, in agreement with other authors [2,7]. Airentraining agents can also be used to improve the workability of EPS concrete; however, the arising reduction of mechanical strength must be considered. Madandoust et al. [13] proposed developing a self-compacting concrete as a global solution to the issues involving the workability of EPS concrete. The goal is to achieve a naturally nonsegregating mixture that is fluid enough to allow its casting without the need for vibration, which is another source of segregation. To prevent EPS from floating, they recommend increasing the viscosity of mixtures by reducing the water-to-cement ratio or by incorporating mineral admixtures with high specific surface area, such as silica fume or nanosilica.

Lightweight cement-based materials

571

Giurich and Pique´ [18] reported an increase in cohesion and a reduction in consistency when nanosilica was added to EPS lightweight concrete. For a given water content, increasing the fines means increasing the surface area to which water molecules adsorb, therefore reducing both the amount of free water and the distance between particles. Consequently, there is higher internal friction between particles, which leads to a higher viscosity and a lower consistency of the mixture. This is especially true when nano-sized particles such as nanosilica are added to concrete, even in small quantities, due to the fact that nanoparticles have a specific surface area three orders of magnitude higher than that of ordinary Portland cement. For this reason, an adequate amount of plasticizer is recommended along with the usage of nanosilica in EPS lightweight concrete, in order to achieve both satisfactory cohesion to avoid segregation and satisfactory consistency to allow proper casting [18]. Li et al. [4] also suggest the usage of viscosity-modifying agents to reduce segregation. These agents act in the same way as high specific surface area admixtures, promoting the adsorption of water molecules on their surface and therefore reducing the content of free water. However, they claim that although stability of the mixture is enhanced, consistency is reduced, which affects negatively the achievement of a self-compacting concrete. For this reason, an adequate balance between cohesion and consistency must be reached through an adequate proportion between a viscosity-modifying agent and a plasticizer. Both Li et al. [4] and Giurich and Pique´ [18] performed similar evaluations of segregation of EPS concrete by producing specimens and then cutting them into equal slices from top to bottom in order to measure the density of each slice. With increasing content of the viscosity-modifying agent in the case of Li et al. [4] and nanosilica in the case of Giurich and Pique´ [18], densities of slices of each specimen became more homogeneous, meaning that segregation was effectively reduced.

16.2.2 Hardened state properties All studies report a decreasing density of concrete as EPS volume increases, because EPS has a density of approximately 10-30 kg/m3. Fig. 16.1, made with 66

Figure 16.1 Density change with EPS volume change, results from researches in Table 16.1.

572

New Materials in Civil Engineering

results obtained from 13 different studies [2 4,6 12,15,16,19], shows that density is linearly related to EPS volume in a wide range of densities. When it comes to strength, EPS particles can be considered as a macropore in concrete, as EPS has a negligible elastic modulus compared to that of the matrix [1]. This means that given a deformation in an EPS lightweight concrete member, the resistance borne by a deformed EPS particle is almost zero due to its low modulus. Stone aggregates, usually having a higher modulus than that of the matrix, work the other way, bearing resistance to deformation and thus increasing the overall strength of concrete. Therefore the strength of EPS lightweight concrete will be provided exclusively by the resistance offered by the rigid components, that is, the cement paste and the stone aggregates when they are present. Fig. 16.2, made with 65 results from 12 studies [2,3,6 12,15,16,19], shows a reduction of compression strength as EPS volume increases. These compression strength values have also been graphed against density in Fig. 16.3, showing an increase in strength as density increases. The dispersion of dots observed in both figures can be explained by the different materials and dosages used in each study,

Figure 16.2 Compressive strength change with EPS volume change, results from researches in Table 16.1.

Figure 16.3 Compressive strength change with density change, results from researches in Table 16.1.

Lightweight cement-based materials

573

which play an important role in the qualities of the interfacial zone and the matrix. Therefore if measures are taken to improve those qualities, it is possible to partially counteract the strength loss product of EPS inclusion [13,17]. Reducing the overall porosity of cement paste results in an improvement of its mechanical properties. Capillary pores can be reduced by lowering the water-tocement ratio and can be refined with mineral admixtures. Intentionally air-entrained bubbles should be avoided, especially if they are not needed due to durability reasons. The water-to-cement ratio determines the distance between cement particles and therefore the degree of contact reached by hydration products, which has a key impact on strength [20]. Several authors recommend keeping the water-to-cement ratio as low as possible in EPS concrete so as to avoid further strength loss [1,6,10,17]. Pozzolanic mineral admixtures help transform part of the hydration products with low specific surface area, such as calcium hydroxide crystals, into high specific surface hydration products, thus increasing the contact between solids and the overall strength [20]. The effectiveness of pozzolanic admixtures depends on the amount of noncrystalline silica contained, which determines the extra amount of high specific surface products formed, and on the specific surface area, which determines the reaction velocity. For pozzolanic reactions to occur, an alkali-rich pore solution is required [21]. At early ages, the presence of sulfate ions in pore solution prevents these reactions from happening. Pozzolanic admixtures, however, prompt changes in the hydration kinetics of cement by showing filler effects. On one hand, they act as nucleation sites, promoting a faster precipitation of clinker hydration products; on the other hand, they provide extra space for those products, as the volume of pozzolanic admixtures remains unchanged while they show filler effects [21]. Once dissolved sulfate ions have reacted with aluminates to form hydration products, the alkalinity in the pore solution grows, and thus, pozzolanic admixtures start to react, leading to a reduction in the content of calcium hydrates and an increase in an additional phase of C S H. Chen and Liu [22] studied the influence of fly ash and silica fume on the strength of an expanded clay lightweight concrete. At a given constant binder content, replacements of cement by fly ash and silica fume were carried out. The former has 54.9% of SiO2 with a specific surface area of 4500 cm2/g, whereas the latter has 92.4% of SiO2 with a specific surface area of 18,000 cm2/g. For a 10% cement replacement, results for 28-day compression strength show a 5% increase for the fly ash mix and a 17% increase for the silica fume mix. Both admixtures enhance the mix strength; however, the higher increase in the silica fume mix is directly related to its higher content of noncrystalline silica. For the same cement replacement, results for 7-day compression strength show an 11% decrease for the fly ash mix and a 14% increase for the silica fume mix. This difference in outcome at an early age is related to the reaction velocity, which is faster in the silica fume mix as it has a higher surface area than fly ash. Sadrmomtazi et al. [10] found that replacing cement by 10% silica fume in EPS lightweight concretes led to 28-day strength increases ranging from 4% to 46% in concretes with 10% 30% volume EPS. Giurich and Pique´ [18] reported that the

574

New Materials in Civil Engineering

incorporation of nanosilica also led to a strength increase of EPS lightweight concrete, in agreement with other researches carried out on the incorporation of nanosilica to concrete [23]. They also remarked that segregation should be avoided by all means if a maximization of EPS lightweight concrete strength is desired. When segregation occurs, EPS tends to accumulate on the top layer of the cast element, creating a weak layer that reaches failure earlier than the bottom layer with low EPS content and therefore reducing the overall strength of the whole. Li et al. [4] studied the influence of segregation on strength by casting cubic specimens of a concrete with 0.25% volume of EPS and varying degrees of segregation. When compression tests were performed loading specimens on directions both perpendicular and parallel to the casting direction, they found a growing difference between both strengths as segregation increased. Giurich [24] analyzed the impact of segregation on strength using a finite element model representing the compression test of cylindrical specimens of EPS concrete with constant EPS volume. The model yielded results congruent to the experimental results: as segregation increased, the weak layer containing a high volume of EPS was larger and caused the specimen to fail at lower loads. Therefore producing a material that is homogeneous throughout the cast element will lead also to homogeneous and enhanced mechanical properties. Nanosilica not only benefits EPS concrete strength through the promotion of pozzolanic reactions and through a reduction in segregation, but also through a reduction in porosity that is caused by additives that show an air-entraining secondary effect [24]. Air-entrained bubbles affect negatively the strength of EPS lightweight concretes: Yu et al. [25] reported a 33% strength loss when an air-entraining agent is used in a recycled glass lightweight concrete; similarly, Garcı´a-Alcocel and Ferra´ndiz-Mas [2] reported a 30% strength loss when this additive was used. As opposed to strength, thermal conductivity benefits from an increase in both the EPS and air bubble contents. Many studies report a reduction in thermal conductivity as the EPS content increases, or equally, as density decreases [6,7,14,16,18]. The reason for this is that EPS has a thermal conductivity of 0.041 W/m K [14], which is much lower than that of stone aggregates typically used in normal-density concretes (1.6 3.7 W/m K). This reduction in EPS lightweight concrete can be further increased when air-entraining agents are used. Kaya and Kar [16] studied the influence of tragacanth resin in EPS lightweight concrete, a natural resin known for increasing the microporosity of concrete. They found that at the same EPS volume, concretes with this resin had lower thermal conductivities. Giurich [24] explained that differences in thermal conductivity values at the same concrete density are probably related to the volume of both EPS and air, with it being lower in concretes with a higher air content. This is due to the fact that air has a thermal conductivity of 0.024 W/m K, about half of that of EPS. In brief, EPS concrete is a versatile material whose properties are as much related to the content of EPS itself as they are to the interactions with the rest of the mix components; as long as a proper workability in fresh state is achieved, a wide variety of mixes with different densities, strengths, and thermal conductivities in the hardened state can be obtained.

Lightweight cement-based materials

16.3

575

Lightweight/high-strength aggregates

Another alternative for reducing the density of materials in civil engineering consists of the addition of HGMSs. HGMSs have very low densities that range from 100 to 800 kg/m3. The main difference between these and traditional lightweight additions or aggregates is their high crush strength. This property defines the greatest compressive strength that a brittle material can bear without rupture. Usual lowweight materials used to reduce the density of cement-based materials tend to have low crush strength, resulting in low compressive strength of the composite, whereas HGMSs behave differently due to their higher crush strength. HGMSs have a crush strength that goes from 20 to 186 MPa, meaning that they can be added in composite materials in order to obtain low densities without severely affecting their mechanical properties [26]. HGMSs have the appearance of a white powder in a macro scale. They are manufactured from soda-lime borosilicate glass, with SiO2 being its principal component (60% 87%) [27]. Particle size varies between 20 and 70 µm. As shown in Fig. 16.4, there is a dependency between this property and their crush strength. HGMS classes are defined by this last property. The relationship between crush strength, particle size, and density is also notable. As is expected, the crush strength decreases with increasing particle size and increases with density. In addition to their low density and high crush strength, HGMSs have good chemical resistance and are stable at temperatures up to 600 C, depending on the duration of exposure. Because of the properties presented, HGMSs can be used both as insulation agents and as an addition in cement pastes for cementing oil wells.

16.3.1 Hollow glass microspheres as insulation agents Many researchers have studied the influence of HGMSs as insulation agents when added to composite materials. HGMSs are not only used in cement-based materials but also in polymeric materials, which demonstrate the versatility of these additions. Moreover, they can be added in a wide range of percentages to obtain composite materials with the desired final properties. The main properties to be considered when analyzing their application are density, strength, thermal conductivity, and

Figure 16.4 Density and particle size of different classes of commercial HGMSs.

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heat resistance. The results obtained by different authors for polymeric and cementbased materials with addition of HGMSs are presented in Table 16.2. Given the variability in the parameters analyzed by the different researches presents in Table 16.2, in order to achieve a comparative analysis, results are expressed as percentual variations in Figs. 16.5 and 16.6. For all reported researches, it can be noticed that there is a significant reduction in the density of the composites when HGMSs are added (Fig. 16.5). Polymeric composites [29] showed lower density reduction with increasing content of HGMSs when compared to cement-based materials [30 32]. This difference could be attributed principally to the higher density of Portland cement. Furthermore, either for the shear or compressive strength, there exists a correlation between the decreased density and final strength when adding HGMSs, no matter the type of binder used. Generally, as for other low-strength addition or aggregates, a reduction in the density of the composite material means a reduction Table 16.2 Using HGMS as insulating agents. Relevant aspects of analyzed researches. HGMS

Density

Compressive strength

Thermal conductivity

(MPa)

(W/m K)

References

Binder

(mass ratio %)

(kg/m3)

[28]

Urethane acrylate

[29]

Phenolic resin

[30]

Portland cement

[31]

Portland cement Portland cement

0 2 5 10 77 83 91 100 111 125 143 0 (w/c 0.55) 3 (w/c 0.55) 6 (w/c 0.55) 9 (w/c 0.55) 15 (w/c 0.40) 30 (w/c 0.67) 25 (w/c 1.40) 30 (w/c 1.40) 35 (w/c 1.40) 40 (w/c 1.40) 45 (w/c 1.40) 50 (w/c 1.40) 55 (w/c 1.40)

1159 1056 921 752 400 390 370 340 330 310 300 983 930 906 836 993 660 675 615 540 465 390 345 305

[32]

14.5 13.7 10.5 9.4 9.0 7.1 6.8 1.8 2.2 2.0 1.7 25.9 13.9 2.8 2.4 1.8 1.3 0.8 0.5 0.3

0.230 0.210 0.185 0.160 0.103 0.101 0.088 0.086 0.085 0.079 0.073 0.251 0.260 0.203 0.240 0.160 0.1370 0.1220 0.1060 0.0940 0.0800 0.0730 0.0680

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Figure 16.5 Density variation of composite and cement-based materials with the addition of HGMSs. Results correspond to researches in Table 16.2.

Figure 16.6 Strength variation of polymeric composite and cement-based materials with the reduction of its density due to the addition of HGMSs. Results correspond to researches in Table 16.2.

in its final strength (Fig. 16.6). Regardless of the binding material, it can be appreciated that the behavior of all composites studied is practically identical, showing a linear correlation between density and strength. As was expected, thermal conductivity of both, polymer composites and cement-based materials, decreased with the density of the material (Fig. 16.7). From the above-presented results, there is a plausible application of HGMSs as an innovating insulation agent regardless of the composite binder. Composites with HGMSs have outstanding thermal conductivity behavior in addition to their low density. When studying the mechanical properties of these materials, it can be outlined that it is not severely affected, especially when compared to other lightweight additions.

16.3.2 Hollow glass microspheres for oil well cements HGMSs are used in oil well cements due to their low density and high crush strength, in order to substantially reduce the specific weight of cement slurries without compromising their final performance. In oil wells, cement slurry is placed

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Figure 16.7 Thermal conductivity change with density change. Results from researches in Table 16.2.

between the drilled formation and the casing. Its main objective is to provide zonal isolation in the well. Therefore a hydraulic seal must be generated between the casing and the cement, and between the cement and the formation, while avoiding the generation of channels within the cement slurry. HGMS are used, for example, when boreholes go through weak formations. These cannot withstand the high hydrostatic pressures exerted by normal weight cement slurries when pumped, as their fracture gradient is low. Therefore, lightweight cement slurries are needed, with this being one of their most common applications. The first reported field application of HGMSs in the oil production industry dates to 1980 when it was intended to overcome the low fracture gradient on an off-shore well in the Gulf of Mexico [33]. Two cementing jobs were accomplished using a slurry with the addition of 27% by weight of cement of HGMS. The first operation consisted of cementing a 183 m high annular zone of 61 and 86 cm casing and hole diameters, respectively. The second operation consisted of a 305 m high annular zone, 41 and 66 cm casing and hole diameters, respectively. This case showed that, despite the elevated cost of HGMSs, a substantial improvement in final properties can be obtained by their usage. Densities of 1115 kg/m3 and better mechanical properties than usual lightweight cement slurries were obtained. Given this result, many researches took place in the petroleum industry to study and apply cement slurries with different classes and percentages of HGMS. Some of these researches are presented in Table 16.3. It is interesting to note that incorporating HGMSs into oil well cement slurries has significant benefits when compared to incorporating commonly used extenders in the oil well industry to lighten the slurries. For example, when using foamed cements, the densities obtained may be as low as those obtained with the addition of HGMSs, but other properties are severely affected. In this aspect, for the same target densities, a foamed cement slurry will have lower compressive strength and greater porosity than a cement slurry with HGMSs [26]. It is important to highlight that, given the need for an impermeable cement sheath, durability and resistance are fundamental. The strengths obtained with the addition of HGMSs are generally high enough to meet target requirements.

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Table 16.3 Using HGMS in oil well cement slurries, relevant aspects of analyzed researches. HGMS crush resistance

Addition Slurry proportion density

References

(MPa)/(kg/m3)

(mass ratio %)

(g/cm3)

(MPa)

[34]

Not specified

[35]

Not specified

0 3 12 18 20 18

1.74 1.62 1.56 1.50 1.50 1.32

33.3 17.9 20.2 2.0 17.3 9.7

[36]

27.6/380

10.43 12.41 14.82 17.67 11.86 14.16 16.92 20.19 13.31 16.02 19.15 22.98 19.28

1.463 1.401 1.340 1.278 1.454 1.393 1.329 1.260 1.462 1.401 1.340 1.278 1.462

11.4 9.5 6.4 5.3 10.4 9.2 6.8 5.1 9.1 7.0 5.7 4.2 8.1

55.2/420

131.0/460

41.4/600

Compressive strength Other properties studied

Thickening time test and HGMS segregation Slurry density variation with pressure

Generally, in oil well applications, the proportion of HGMSs is selected in order to achieve a target cement slurry density. This is chosen in accordance with the maximum specific weight acceptable for the lowest fracture strength of the drilled formation. As can be noted in Table 16.3, this proportion differs from one class of HGMSs to another due to the difference in their specific weight. Regarding this last issue, Mata and Calubayan [36] analyzed the relation between density, cement replacement percentage, HGMS crush strength (or class) and compressive strength of the cement slurries. They kept constant the water-to-cement ratio and four target densities were fixed: 1260, 1320, 1380, and 1440 kg/m3. These densities were achieved by replacing cement in different percentages. The results of the compressive strength for each target density obtained with different classes of HGMSs are shown in Fig. 16.8.

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Figure 16.8 Compressive strength of targeted densities for lightweight cement slurries designed with HGMSs with different crush strengths. Source: Adapted from C. Mata, A. Calubayan, Use of hollow glass spheres in lightweight cements—selection criteria, in: Proceeding of Engineers Asia Pacific Oil and Gas Conference and Exhibition SPE-182399-MS, Perth, Australia, October 25 27, 2016.

It can be observed that the final strength for the target densities varies with the class of HGMS. This can be explained by the fact that different classes of HGMS have different densities. It is interesting to notice that the HGMS class with the highest compressive strength is not the one that yields the highest compressive strength for a targeted density of a lightweight oil well cement slurry. AlBahrani et al. [37] prepared a specially designed experimental procedure to analyze the performance of different classes of HGMS under oil well conditions. HGMSs were subjected to an aging process that simulates the high-pressure and high-temperature conditions in an oil well and the impact forces generated during the cement slurry mixing and pumping into the formation. No clear relation between HGMS crush strength and aging resistance was found in this case. Given these results, the selection criteria of HGMSs is key when designing a lightweight cement slurry since no clear laws govern their behavior. Despite this difficulty, researches show that there is a great range of applications for this material, especially when considering the low densities and relatively high compressive strengths obtained by its use in the oil well industry.

16.4

Extenders

In addition to the HGMSs, the oil well industry uses a wide range of density reducer additives or density reducer admixtures, also called extenders. Extenders are materials that reduce the density and/or quantity of cement per unit volume of cement paste or, in the case of the oil well industry, of cement slurry. The most usual solution to lighten oil well cement slurries is to increase the water-to-cement ratio, since the density of the water is a third of that of cement. Nevertheless, increasing the water content causes other problems such as fluid loss

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and lost circulation. Fluid loss is the leakage of the cement slurry liquid phase into the formation, resulting from high slurry segregation. Lost circulation is the total or partial loss of the cement slurry into the formation. Both problems are controlled by using extenders, since these additives can reduce segregation and increase the yield point of the cement slurry. In addition, using extenders also allows reducing of the quantity of cement per unit volume. Usually, extenders are divided into three groups according to the way they decrease the density of the cement slurry: low-density materials, gas, and water extenders. In the next section low-density materials and water extenders will be described. Gas extenders, used to obtain foamed cement, exceed the scope of this chapter.

16.4.1 Low-density materials Low-density materials consist of low-weight additions (in the oil well industry also called additives) that can be added by replacing a cement fraction. Usually, the water-to-cement ratio increases since the water-to-solids ratio is maintained constant. Since replacement solids have lower specific weight than cement, the density of slurries decreases. Examples of this group are HGMSs, expanded perlite (30 150 kg/m3), gilsonite (1070 kg/m3), and other pozzolanic materials. To obtain expanded perlite, a siliceous volcanic glass is heat-processed. This glass can expand by a factor of 4 20 when heated rapidly up to 760 C 980 C. As a result, a porous particle with entrained air is obtained. Unexpanded perlite has a bulk density of approximately 1100 kg/m3, but, when expanded it can reach a bulk density as low as 30 150 kg/m3. Expanded perlite also segregates from the cement slurry due to its low density, therefore it is usually incorporated with a thickening agent, such as bentonite, to prevent this. Expanded perlite is also used in the construction industry because it is an excellent thermal and sound insulator. Lanzo´n and Garcı´a Ruiz [38] studied its application in render mortars and found that by adding expanded perlite, fresh density is reduced, and workability is enhanced. In addition, water retention slightly increased at low dosages (which is important to control water evaporation), due to the water retained within perlite internal voids. Philippacopoulos and Berndt [39] measured thermal properties on cement samples lightened with perlite which had a specific gravity of 1.72 and compared them with properties of neat cement samples with a specific gravity of 1.98. They found that the thermal conductivity was slightly lower than that of neat cement samples, the coefficient of thermal expansion was higher, and the thermal diffusivity notoriously decreased. In addition, the thermoelastic response of these cement samples was studied by means of finite element models of an oil well. They found that cement samples modified with perlite presented the lowest thermal stresses. Anya [26] reported the usage of gilsonite as a low-density extender. Gilsonite is a natural asphaltic material. Its low density (1070 kg/m3) helps to obtain a lightweight cement slurry. Densities as low as 1440 kg/m3 can be achieved without the aid of high quantities of water. Although it is a lightweight material, gilsonite is nonporous: along with its impermeability, it prevents premature slurry dehydration [40]. When it is used at high concentrations, a viscosity modifier admixture is

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New Materials in Civil Engineering

needed to control particle segregation. It can be used in oil well cement slurries but should be avoided in wellbores with high temperatures, as gilsonite particles begin to fuse above 115 C. William et al. [40] studied gilsonite as a self-healing cement (SHC) additive in a lightweight cement slurry. The self-healing ability would enable the cement sheath to repair microcracks and prevent hydrocarbon migration through the cement-casing and cement-formation interfaces. SHC slurries act as regular cement slurries do, but in addition they can expand when exposed to hydrocarbons, thus sealing microcracks. William et al. [40] worked with cement slurries with a density of 1580 kg/m3 and different dosages of gilsonite. They found porosity decreased as the concentration of gilsonite increased, both in exposed and unexposed samples to CH4 environments. Nevertheless, the decrease was higher in exposed samples. Pozzolanic materials, in addition to supplementary cementitious materials, are the most used extenders of the cement slurries in the oil well cement industry. They have lower density than nonhydrated cement and can decrease the hydration heat and permeability of the slurry. They also enhance the resistance to alkali-silica reaction and reduce the susceptibility to acid attack by decreasing the calcium hydroxide content. As examples of pozzolanic materials, fly ash, silica fume, and diatomaceous earth (DE) are described below. Fly ash is the residue of the combustion of powdered mineral coal. It consists mostly of solid spheres with a particle size that varies from 1 to 100 µm. The bulk density of noncompacted fly ash ranges from 540 to 860 kg/m3, while with closepacked storage or vibration it can reach 1120 1500 kg/m3 [41]. Elmrabet et al. [42] added different dosages of fly ash to ordinary Portland cement. They observed a reduction in the water demand with the increase in fly ash content. They also obtained significant delays in initial and final set of the samples with fly ash. They attributed this result to the reaction of SiO2 present in fly ash. The same reason was given by the authors to explain both the slower early strength development and the lower measured hydration heat as the dosage of fly ash was increased. Another pozzolanic material is silica fume, also known as microsilica, which is a by-product of the manufacture of silicon or ferrosilicon alloy. The bulk density of the densified silica fume goes from 130 to 430 kg/m3. Silica fume has a spherical shape, as fly ash, and is extremely fine, with an average particle diameter of about 0.1 µm. It is used to develop high-strength concrete and high impermeability oil well cement slurries. Its addition reduces bleeding and enhances short-term strength development [41]. Between 5% and 10% by weight of cement of silica fume is generally used. Higher dosages are used to improve mechanical performance but undesired effects, such as mixing difficulty and increasing water demand, become more pronounced. These effects where studied by Burroughs et al. [43] and it was concluded that the higher the dosage of silica fume is, the thicker the cement slurry becomes. Furthermore, they observed that this effect was stronger when the specific area of silica fume increased, probably due to a thinner layer of water between particles. In addition, the different specific surface areas affected differently the rheology of the cement slurry. Wang et al. [44] subjected silica fume to a sonication treatment before adding it to the cement slurry and compared the results with a

Lightweight cement-based materials

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slurry with nonsonicated silica fume. They observed that, after sonication, the slurry became less thin, presumably because silica fume agglomerates were broken during sonication, leading to a better distribution of the particles and to a higher specific surface area. They also attributed these reasons to the higher compressive strength obtained. DE, also called diatomite, is a natural pozzolan. Its main components are opal and amorphous hydrated silica, containing more than 10% water. Its chemical composition varies depending on the extraction site. To be used as an extender, it is essential that the fineness of the material is similar to that of cement. It has a large surface area and high water demand [45]. Its average particle density can vary from 1200 to 2500 kg/m3 and its porosity value from 50% to 70%. Fragoulis et al. [46] studied diatomite rocks from two different extraction sites and found differences in their compositions. However, when adding both ground diatomite rocks to ordinary Portland cement, they found similar effects in their performance. In this work, the water demand of all cements with diatomite was higher than that of the neat cement, and the late compressive strength of most cements was improved with the addition of diatomite. Kastis et al. [47] studied the effects of different percentages (from 0% to 35%) of replacement of cement with diatomaceous rocks which contained mainly CaCO3 and amorphous silica. They found that samples with up to 10% diatomite developed the same compressive strength as those of neat cement, but a higher water demand was observed with increasing diatomite content.

16.4.2 Water extenders Water extenders allow adding more water to the cement slurry and, therefore, the unit volume of slurry per bag of cement increases (that is why they are called extenders) and its density decreases. When a high quantity of water is added to the slurry, the water-to-solids ratio increases, and the slurry becomes extremely fluid. The presence of a large quantity of water in a slurry generates excessive free water, solids settling, segregation, high permeability, slow strength development, and reduced compressive strength. To prevent these undesired effects from happening, extenders are needed. The most common of these additives is bentonite, which is used to prevent solids separation, to reduce free water and fluid loss, and to increase the slurry yield point. Bentonite is a clay composed mostly of sodium montmorillonite, that expands when in contact with water. This expansion increments the viscosity, gel strength, and the ability to keep solids suspended in the cement slurry [45]. Bentonite can absorb about 5.3% of water for every 1% by weight of cement added, so slurry stability and water retention can be maintained with high water-to-cement ratios. A cement slurry with bentonite can achieve a density as low as 1400 kg/m3; nevertheless, the low density impacts on the compressive strength [26]. Fig. 16.9 illustrates the effect of bentonite addition on cement compressive strength. A water extender as effective as bentonite is sodium metasilicate. This consists of a white soluble powder produced by fusing silica with sodium carbonate at high temperatures. Slurry densities of 1400 kg/m3 can be achieved with 4% sodium

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New Materials in Civil Engineering

Figure 16.9 Effect of bentonite on the compressive strength of the lightweight cement slurry. Source: Adapted from E. Nelson, D. Guillot, Well Cementing, Schlumberger, 2008 [48].

metasilicate [26]. The silicates react with the lime in the cement or with calcium chloride, forming a calcium silicate gel, which provides enough viscosity to allow larger water-to-solids ratios without excessive bleeding. This process is different to that exhibited by bentonite extenders that absorb water. Some disadvantages of sodium metasilicates are that they can only be used up to a temperature of 90 C and that it tends to reduce the effectiveness of other additives, in particular retarders and agents that prevent fluid loss [45]. Expanded vermiculite is also used as a water extender for cement slurries. It is a magnesium-hydrated silicate with aluminum and iron that expands 8 30 times its size when heated. It has a lamellar structure with air in between that gives the material low density and low thermal conductivity. It is also resistant to high temperatures and to exposure to acids [49]. Gomes da Silva Araujo Filho et al. [49] studied the addition of expanded vermiculite to lightweight oil well cement slurries with high water-to-cement ratios. The density of expanded vermiculite was 730 kg/m3 and the lightweight cement slurry achieved a density as low as 1500 kg/m3. To thicken the slurry, they added colloidal silicon that improved cohesion between vermiculite and cement particles. They obtained the highest compressive strength with the highest dosages of vermiculite. In addition, they observed vertical homogeneity and no free water, contrary to what is expected in high water-to-cement ratio slurries. In another work, Minaev et al. [50] observed smaller volumetric shrinkage in lightweight cement slurries containing vermiculite and stated that it was due to the water absorption of expanded vermiculite, since water replaces the air contained between its lamellas. They also reported that increasing dosages of vermiculite resulted in lower density and flowability, reduced thickening times, decreased flexural strength, and higher dehydration. They studied different concentrations of vermiculite with different water-to-solids ratios and showed that a cement slurry with a density of 1480 kg/m3 could be satisfactorily achieved with 12.5% vermiculite and 0.8 water-to-solids ratio. Water extenders can also be water-soluble polymers, which increase the viscosity of the slurry, allowing higher water-to-cement ratios. Among these are cellulose

Lightweight cement-based materials

585

ethers (CEs), which are cellulose derivatives. Cellulose is a biopolymer. It is the major constituent of wood and most natural fibers and can also be produced by tunicates (sea animals), some algae species and bacteria, amoebae, and fungi. Some of the cellulose derivatives most used in oil well cement slurries are hydroxyethyl cellulose, methylethylhydroxy cellulose, and carboxymethyl hydroxyethyl cellulose. Hydroxyethyl cellulose works as a fluid-loss control agent, that allows use of high water-to-cement ratios. Its ability to perform as such is achieved under diverse salinity conditions and under temperatures of up to 150 C. Carboxymethyl hydroxyethyl cellulose also works as a fluid-loss control agent, inducing, in addition, a setting delay on the cement slurry [51]. Cellulose derivatives have the capacity of thickening the cement slurry, which is convenient when a high water-to-cement ratio is used. Moreover, its water retention ability can prevent fluid loss and segregation, which are common problems in slurries with high water-to-cement ratios, and it can also reduce cement slurry filtration to porous substrates. In addition to cellulose derivatives, other cellulose products are used, such as micro- and nanocellulose fibers. Natural cellulose macrofibers consist of smaller entities with higher mechanical properties, which can be extracted under proper conditions. Nanocellulose from natural fibers can be extracted by chemical proceedings, like acid hydrolysis used to obtain cellulose nanocrystals (CNCs), or by mechanical treatments. Mechanical treatments make use of elevated shear forces that disintegrate the fibers to obtain cellulose nanofibers (CNFs) [52]. Several papers have been published about cellulose in the form of micro- and nanofibers, and their influence in cement slurry rheology was highlighted. It was observed that shear stress is increased with an increase in the content of CNF in cement matrix. Also, with an increment of the addition, yield stress (defined as the shear stress necessary to start the flow of the slurry) was increased and the slurry exhibited a larger shear thinning effect [53]. These fibers contribute not only to the gain of viscosity but also to the strength development, especially of the flexural modulus, as several research works have shown [53,54]. However, the use of natural fibers is limited in cement-based materials because cellulose fibers contain components that suffer a degradation in the alkaline ambient of the cement paste (lignin, hemicellulose, and amorphous cellulose), which causes fragilization of the fiber. Herein lies the growing interest in other sources of CNFs, such as bacterial nanocellulose (BNC). This is produced by species of bacteria (Komagataeibacter xylinus) through fermentation of low-molecular-weight glucose. BNC has a chemical composition identical to vegetal cellulose, but is purely crystalline and does not contain lignin or hemicellulose. Martin et al. [55] studied the effect of BNC in oil well cement slurries. With an addition of 0.1% BNC by weight of cement they obtained an increment in the yield stress of 2000%, which shows the strong influence of BNC on the viscosity of the slurry. They also obtained a drastic decrease in the free fluid percentage of the slurries with BNC, which is an important water retention capacity. Finally, they found an increment of 16% and 19% in compressive strength with the addition of 0.1% and 0.2% of BNC, respectively, compared to samples without any addition.

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New Materials in Civil Engineering

Outlook and future trends

In this chapter, different techniques for lightening cement-based material were studied and reviewed. It is remarkable the great amount of research available regarding this subject, especially when it comes to low-weight aggregates and extenders. The authors have narrowed down the research to three main techniques. These consisted of using EPS, a synthetic lightweight/low-strength aggregate, HGMSs, a synthetic lightweight/high-strength addition, and some of the extenders used to lighten oil well cement slurries. After this wide research it can be concluded that EPS used as concrete aggregate is effective for producing a lightweight material that shows insulating properties. Its incorporation to the mix, however, needs to be done in careful proportion with the other materials present in the mix so that a homogeneous concrete is achieved. Reducing the water-to-cement ratio or incorporating high specific-area admixtures or viscosity-modifying agents are effective ways to create a nonsegregating EPS concrete; however, along these measures, a verification of workability must be assessed in order to obtain a material that can actually be cast and compacted. Reaching an equilibrium between both acceptable cohesion and consistency will lead to a reduction in the compaction energy needed during casting of concrete, which in turn will lead to a reduction in segregation caused during this process. Deciding which property is to be enhanced will also determine the composition of the mix. Depending on the degree of strength required, a partial or full replacement of stone aggregates by EPS should be made. For a given EPS replacement, reducing the water-to-cement ratio, incorporating pozzolanic admixtures and lowering air content will lead to greater strength values. The opposite actions will lead to a concrete with reduced thermal conductivity, which is beneficial in terms of insulating properties. HGMSs are newer and mainly used in the oil well industry. Nevertheless, researches have demonstrated that there is a plausible application of HGMSs as an innovating insulation agent for different types of materials. Composites with HGMSs have outstanding thermal conductivity behavior in addition to their low density. When studying the mechanical properties of these materials, it can be outlined that it is not severely affected as when their densities are reduced using regular lightweight additions. When using HGMSs, the selection criteria is key in the composite design, since no clear laws govern their behavior and there are not many researches in this field. Despite this, given the low density and high crush strength of this material, it is expected that there will be an increase in its application for lightweight cement-based materials. Regarding extenders, their numbers are enormous and it is safe to say that the best for each application is the one that is closer to the construction site. It could be any pozzolanic material, natural expanded material, or additives that allows controlling the free fluids and segregation when increasing the water-to-cement ratio. Special attention should be taken with the strength loss, which varies with the material chosen as an extender, being extreme in some cases and acceptable in others.

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References [1] D.J. Cook, Lightweight concrete using expanded polysterene, Constr. Rev. (1972) 53. [2] E. Garcı´a-Alcocel, V. Ferra´ndiz-Mas, Caracterizacio´n fı´sica y meca´nica de morteros de cemento Po´rtland fabricados con adicio´n de partı´culas de poliestireno expandido (EPS), Mater. Constr. 62 (308) (2012) 547 566. [3] K.G. Babu, D.S. Babu, Performance of fly ash concretes containing lightweight EPS aggregates, Cem. Concr. Compos. 26 (1) (2004) 605 611. [4] C. Li, L. Miao, Q. You, S. Hu, H. Fang, Effects of viscosity modifying admixture (VMA) on workability and compressive strength of structural EPS concrete, Constr. Build. Mater. 175 (1) (2018) 342 350. [5] R. Demirboga, A. Kan, Thermal conductivity and shrinkage properties of modified waste polystyrene, Constr. Build. Mater. 35 (1) (2012) 730 734. [6] R. Sri Ravindrarajah, A.J. Tuck, Properties of hardened concrete containing treated expanded polysterene beads, Ceme. Concr.te Compos. 16 (1) (1994) 273 277. [7] A. Kan, R. Demirboga, Effect of cement and EPS beads ratios on compressive strength and density of lightweight concrete, Ind. J. Eng. Mater. 14 (1) (2007) 158 162. [8] B. Chen, N. Liu, Experimental study of the influence of EPS particle size on the mechanical properties of EPS lightweight concrete, Constr. Build. Mater. 68 (1) (2014) 227 232. [9] D.S. Babu, K.G. Babu, W. Tiong-Huan, Effect of polystyrene aggregate size on strength and moisture migration characteristics of lightweight concrete, Cem. Concr. Compos. 28 (6) (2006) 520 527. [10] A. Sadrmomtazi, J. Sobhani, M.A. Mirgozar, M. Najimi, Properties of multi-strength grade EPS concrete containing silica fume and rice husk ash, Constr. Build. Mater. 35 (1) (2012) 211 219. [11] S.G. Park, D.H. Chisholm, Polystyrene Aggregate Concrete, Building Research Association of New Zealand, Judgeford, 1999. [12] A. Schackow, C. Effting, M.V. Folgueras, S. Gu¨ths, G.A. Mendes, Mechanical and thermal properties of lightweight concretes with vermiculite and EPS using airentraining agent, Constr. Build. Mater. 57 (1) (2014) 190 197. [13] R. Madandoust, M.M. Ranjbar, S. Mousavi, An investigation on the fresh properties of self-compacted lightweight concrete containing expanded polystyrene, Constr. Build. Mater. 25 (1) (2011) 3721 3731. [14] Y. Xu, L. Jiang, J. Liu, Y. Zhang, J. Xu, G. He, Experimental study and modeling on effective thermal conductivity of EPS lightweight concrete, J. Therm. Sci. Technol. 11 (2) (2016) 1 13. [15] C. Cui, Q. Huang, D. Li, C. Quan, H. Li, Stress strain relationship in axial compression for EPS concrete, Constr. Build. Mater. 105 (1) (2016) 377 383. [16] A. Kaya, F. Kar, Properties of concrete containing waste expanded polystyrene and natural resin, Constr. Build. Mater. 105 (1) (2016) 572 578. [17] F. Giurich, T.M. Pique´, Consideraciones para el disen˜o de hormigones livianos con EPS, in: Proceedings of Congreso de Jo´venes Investigadores en Materiales. Buenos Aires, Argentina, 2017. [18] F. Giurich, T.M. Pique´, Estudio de la trabajabilidad del hormigo´n liviano con adicio´n de nanosı´lice, in: Proceedings of Congreso de la Asosiacio´n Argentina de Tecnologı´a del Hormigo´n. Olavarrı´a, Argentina, 2018. [19] M. Ranjbar, Y.S. Mousavi, Strength and durability assessment of self-compacted lightweight concrete containing expanded polystyrene, Mater. Struct. 48 (4) (2013) 1001 1011.

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[20] P.K. Mehta, P.J.M. Monteiro, Concrete Microstructure, Properties, and Materials, McGraw-Hill, 2006. [21] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (1) (2011) 1244 1256. [22] B. Chen, J. Liu, Experimental application of mineral admixtures in lightweight concrete with high strength and workability, Constr. Build. Mater. 22 (1) (2008) 1108 1113. [23] P. Aggarwal, R. Singh, Y. Aggarwal, Use of nano-silica in cement based materials—a review, Cogent Eng. 2 (2015) 1078018. [24] F. Giurich, Disen˜o de paneles livianos prefabricados con nanotecnologı´a (Graduate theses of Univesidad de Buenos Aires), Buenos Aires, Argentina, 2019. [25] Q.L. Yu, P. Spiesz, H.J.H. Brouwers, Ultra-lightweight concrete: conceptual design and performance evaluation, Cem. Concr. Compos. 61 (1) (2015) 18 28. [26] A. Anya, Lightweight and ultra-lightweight cements for well-cementing—a review, in: Proceedings of SPE Western Regional Meeting SPE-190079-MS, Garden Grove, CA, USA, April 22 26, 2018. [27] M. Lanzo´n Torres, P.A. Garcı´a Ruiz, Lightweight pozzolanic materials used in mortars: evaluation of their influence, Cem. Concr. Compos. 31 (1) (2009) 114 119. [28] L.C. Herrera-Ramı´rez, M. Cano, R. Guzman de Villoria, Low thermal and high electrical conductivity in hollow glass microspheres covered with carbon nanofiber-polymer composites, Compos. Sci. Technol. 151 (1) (2017) 211 218. [29] H. Yang, H. Jiang, D. Xie, C. Wan, H. Pan, S. Jiang, Mechanical, thermal and fire performance of an inorganic-organic insulation material composed of hollow glass microspheres and phenolic resin, J. Colloid Interf. Sci. 530 (2018) 163 170. [30] S. Shahiron, A. Eeydzah, M.N. Khairiyah, R.H. Nurul Izzati Raihan, S.B. Nur Amira, Potential of hollow glass microsphere as cement replacement for lightweight foam concrete on thermal insulation performance. MATEC Web Conf. 103 (01014), 2017. [31] D. Oreshkin, V. Semenov, T. Rozocskaya, Properties of light-weight extruded concrete with hollow glass microspheres, Procedia Eng. 153 (2016) 638 643. [32] J. Gong, Z. Duan, K. Sun, M. Xiao, Waterproof properties of thermal insulation mortar containing vitrified microsphere, Constr. Build. Mater. 123 (1) (2016) 274 280. [33] R.C. Smith, C.A. Powers, T.A. Dobkins, A new ultra-lightweight cement with super strength, J. Petrol. Eng. 32 (8) (1980) 1438 1444. [34] F.J. Mavares, A.D. Pertuz, Disen˜o de un sistema cementante para pozos utilizados en anclaje de plataformas de produccio´n en aguas profundas. caso: Rio de Janeiro, Brasil. Revista de la Facultad de Ingenierı´a U.C.V. 28 (1) (2013) 73 82. [35] M.N. Abdullah, D. Bedford, S.R. Wong, H.S. Yap, Prehydrating high-strength microspheres in lightweight cement slurry creates value for offshore Malaysian operator, in: Proceedings of SPE Asia Pacific Oil and Gas Conference and Exhibition SPE-165796MS. Jakarta, Indonesia, October 22 24, 2013. [36] C. Mata, A. Calubayan, Use of hollow glass spheres in lightweight cements—selection criteria, in: Proceeding of Engineers Asia Pacific Oil and Gas Conference and Exhibition SPE-182399-MS, Perth, Australia, October 25 27, 2016. [37] H.I. AlBahrani, V. Wagle, A.S. Al-Yami, An overview of experimental studies examining the reliability of hollow glass spheres as a density reduction agent in oil field applications, in: Proceedings of SPE Middle East Oil & Gas Show and Conference SPE183681-MS, Manama, Kingdom of Bahrain, March 6 9, 2017. [38] M. Lanzo´n, P.A. Garcı´a Ruiz, Lightweight cement mortars: advantages and inconveniences of expanded perlite and its influence on fresh and hardened state and durability, Constr. Build. Mater. 22 (8) (2008) 1798 1806.

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[39] A. Philippacopoulos, M. Berndt, Mechanical response and characterization of well cements. Society of petroleum engineers, in: Proceedings of SPE Annual Technical Conference and Exhibition, September 29 October 2, San Antonio, TX, 2002. [40] B. William, V. Daniel, M. Radonjic, 2019. Nature’s solution to wellbore gas-leakage: gilsonite, in: Proceeding of the 53rd U.S. Rock Mechanics/Geomechanics Symposium, June 23 26, New York, NY, 2019. [41] S. Kosmatka, B. Kerkhoff, W. Panarese, Design and Control of Concrete Mixtures, Portland Cement Association, 2002. [42] R. Elmrabet, A. El Harfi, M. El Youbi, Study of properties of fly ash cements, Mater. Today Proc. 13 (3) (2019) 850 856. [43] J. Burroughs, J. Weiss, J. Haddock, Influence of high volumes of silica fume on the rheological behavior of oil well cement pastes, Constr. Build. Mater. 203 (1) (2019) 401 407. [44] X. Wang, J. Huang, S. Dai, B. Ma, H. Tan, Q. Jiang, Effect of silica fume particle dispersion and distribution on the performance of cementicious materials: a theoretical analysis of optimal sonication treatment time, Constr. Build. Mater. 212 (1) (2019) 549 560. [45] M. Rojas, G. Quercia, K.A. Gorrı´n, K.C. Gorrin, A. Del Toro, N. Vera, Mecanismos de accio´n de los aditivos utilizados en cementacio´n de pozos, in: Conferencia sobre materiales cementantes para pozos petroleros, Caracas, Venezuela (USB/MT6511), 2004. [46] D. Fragoulis, M. Stamatakis, D. Papageorgiou, E. Chaniotakis, The physical and mechanical properties of composite cements manufactured with calcareous and clayey Greek diatomite mixtures, Cem. Concr. Compos. 27 (2) (2005) 205 209. [47] D. Kastis, G. Kakali, S. Tsivilis, M. Stamatakis, Properties and hydration of blended cements with calcareous diatomite, Cem. Concr. Res. 36 (10) (2006) 1821 1826. [48] E. Nelson, D. Guillot, Well Cementing, Schlumberger, 2008. [49] R. Gomes da Silva Araujo Filho, J.C. Oliveira Freitas, M.A. Freitas Meloa, R. Martins Braga, Lightweight oil well cement slurry modified with vermiculite and colloidal silicon, Constr. Build. Mater. 166 (1) (2018) 908 915. [50] K. Minaev, V. Gorbenko, O. Ulyanova, Lightweight cement slurries based on vermiculite, IOP Conf. Ser.: Earth Environ. Sci. 21 (1) (2014) 012034 012039. [51] A. Va´zquez, T. Pique, Biobased additives in oil well cement, in: S. Goyanes, N. D’Accorso (Eds.), Industrial Applications of Renewable Biomass Products, Springer, 2017, pp. 179 198. [52] C. Go´mez Hoyos, R. Zuluaga, P. Gan˜a´n, T.M. Pique, A. Vazquez, Cellulose nanofibrils extracted from fique fibers as bio-based cement additive, J. Clean. Prod. 235 (1) (2019) 1540 1548. [53] X. Sun, Q. Wu, J. Zhang, Y. Qing, Y. Wu, Rheology, curing temperature and mechanical performance of oil well cement: combined effect of cellulose nanofibers and graphene nano-platelets, Mater. Des. 114 (1) (2017) 92 101. [54] O. Onuaguluchi, D. Panesar, M. Sain, Properties of nanofibre reinforced cement composites, Constr. Build. Mater. 63 (1) (2014) 119 124. [55] C.M. Martin, I. Zapata Ferrero, P. Cerrutti, A. Va´zquez, D. Manzanal, T.M. Pique´, Oil well cement modified with bacterial nanocellulose, in: M. Taha (Ed.), International Congress on Polymers in Concrete (ICPIC 2018), Springer, Cham, 2018. ICPIC 2018.

Development of alkali-activated binders from sodium silicate powder produced from industrial wastes

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Parthiban Kathirvel School of Civil Engineering, SASTRA Deemed University, Thanjavur, India

17.1

Introduction

Concrete is probably the most prolific building material, and Portland cement (PC), a principal constituent of concrete, is the main part of building material generated globally. With an increase of over half a ton per person annually, cement production has grown rapidly. Since 1950, the production of cement has increased over 30-fold, and nearly fourfold since 1990, with considerably faster development than worldwide production of fossil energy over the past two decades. This increase is mainly due to the rapid growth in China where the production of cement has been raised by an aspect of above 11, with 75% of global production occurring in China since 1990. The increasing demand for cement creates ecological issues not only for the access to raw materials (limestone), but also increases CO2 emissions to the atmosphere, with increased demand for energy in the manufacturing of PC, as shown in Fig. 17.1. The manufacture of PC requires a huge amount of energy; and it transmits an immense amount of CO2 to the atmosphere due to the calcination reaction during its manufacturing process. Lawrence [2] stated that 0.53 tons of CO2 is discharged into the air in the production of 1 ton of PC due to the calcination of CaCO3, and this may be as much as 1 ton if carbon gasoline is used as the energy source. About 83% of the total energy is consumed by cement in the manufacturing of nonmetallic minerals, with CO2 emissions of 94%. In the manufacturing of PC, the decomposition of limestone emits CO2. The components of limestone may be in the form of calcium and magnesium carbonates. Their decomposition reactions to emit CO2 are given in Eqs. (17.1) and (17.2). CaCO3 ! CaO 1 CO2

(17.1)

MgCO3 ! MgO 1 CO2

(17.2)

New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00017-X © 2020 Elsevier Inc. All rights reserved.

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Figure 17.1 Global cement and fossil energy production and growth rate [1].

Using the molecular weight in Eq. (17.1), 0.44 kg of CO2 will be discharged to the atmosphere in the manufacture of 1 kg CaCO3. From Eq. (17.2), 0.52 kg of CO2 will be emitted in the production of 1 kg of MgCO3. In addition to these reactions, the energy required in the quarrying and crushing, grinding, fueling in kiln, and finishing also plays a significant contribution to the consumption of energy and CO2 emissions. The energy consumed during production includes coal, fuel, and electricity.

17.2

Alternative for Portland cement

In contrast, the manufacture of industrial by-products discharges a low amount of emissions compared with PC. Fly ash generates 80%90% [3] and slag produces 80% [4] lower greenhouse gas (GHG) discharges to the air compared with PC. Therefore, 100% substitution of PC with fly ash or slag would significantly reduce the impact on the environment. Consequently, there is a need to find an ecofriendly replacement to concrete in order to reduce GHG discharges. Geopolymers are a fairly innovative group of building materials [5] that does not need CSH gel but makes use of the polycondensation of Si and Al from source materials to attain a greater strength [6]. Alkali-activated binders (AABs) are typically produced with an aluminosilicate precursor and an activator which primarily consists of alkalis of Na or K and waterglass [7]. The process of geopolymerization forms an aluminosilicate framework, which consists of sialate (silicon-oxo-aluminate), the alkali being Na, K, or Ca, and polysialate covers all geopolymers including at least one (Na, K, Ca)(SiOAl) and (Na, K, Ca)-sialate unit [8]. The innovation of AABs illustrates the prospect of setting at an ambient temperature condition and not with an elevated temperature, thereby reducing the GHG discharges, as an eco-friendly novel substitute to PC [9]. Due to these alluring

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properties this innovation is gaining expanding consideration in various fields such as fire protection structures and low-density panels for insulation purposes. PC also typically demands a high amount of raw materials, for example, limestone, clay, and water, in the production methodology [10]. Despite what might be expected, low-cost materials can be utilized in the geopolymerization reaction, including waste materials that are not as of now reused in other areas, but which are, however, too valuable and critical to discard. Through geopolymerization it is conceivable to utilize a great deal of waste generated from hazardous and nonhazardous materials and to manufacture new items, while also limiting environmental issues [11]. Geopolymer innovation could change waste containing aluminosilicates discharged from industries into useful items because of its adaptability and ability to immobilize and balance out the losses inside the geopolymer framework [12]. On a basic level, any waste material containing a certain measure of silica and alumina can be used as a source material for geopolymerization [13].

17.3

Alkaline activators

A variety of silicates, carbonates, and alkali metal hydroxide-based activators has been utilized in the formulation of AAB over the past few decades. Based on the available literature, it has been observed that the development of the mechanical and structural properties of these binders is strongly dependent on the nature of the activators used. Hydroxides and silicates of sodium or potassium were employed as activators, with most of the past literatures addressing the extensive utilization of NaOH in combination with Na2SiO3 as an activator in the manufacture of AAB. However, in combination, the application of sodium silicate leads to a stable and densified microstructure resulting in the improved development of mechanical properties with reduced permeability. The production of Na2SiO3, however, involves calcination of sodium carbonate (Na2CO3) and quartz (SiO2) at a very high temperature in the order of 1200 C1500 C, leading to the decomposition of CO2 into the atmosphere, resulting in the emission from the fuel to necessitate the temperature condition with high embodied energy requirement. Quartz sand dissolution using NaOH in a reactor is an alternate method of producing sodium silicate, however it requires high energy in its production [14,15]. These environmental issues have endorsed the progress of different methods for producing activators with a vision toward improved sustainability. In this regard, a solid activator must be developed when premixed with a precursor results in a “one-part” AAB which requires only water for the required mix that is highly preferred from an industrial point of view. The progress of alternative methods of producing activators could lead to a reduction in greenhouse emissions, resulting in sustainable development when utilized in the production of AABs. SiO2 1 2NaOH ! Na2 SiO3

(17.3)

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The following sections focus on alkali-activator binders that incorporate sodium silicate derived from industrial wastes to complete the geopolymerization process.

17.4

Waste glass

Millions of tons of glass are generated annually as municipal waste, which are difficult to recycle due to their color, impurities, and composition. Waste glass has been identified as a source of silica, due to its high amorphous silica content in the order of 70%75%. Thanks to its higher availability and chemical composition, waste glass can be a superior alternative ingredient in the production of AABs. This can effectively reduce or entirely abolish the required quantity of commercial sodium silicate and sodium hydroxide. This method can greatly reduce the utilization of activator solutions that lead to both economical and environmental concerns. Thermochemical or fusion method of extracting sodium silicate involving heating a mixture of glass and sodium hydroxide powder at a very high temperature in the order of 500 C1300 C, resulting in a higher rate of conversion from glass to sodium silicate, has been reported by various authors [1618]. The resulting silicate powder requires reheating for at least 1 hour at 175 C due to its lower solubility at ambient pressure when it is not fully dissolved. While, the hydrothermal method of extracting sodium silicate involves heating of glass in an alkaline solution at a temperature range of 150 C250 C, where the rate of dissolution is influenced by the composition of glass and its size, type of alkali (Na or K), and temperature [1922]. This can be achieved by using pressure reactor vessels which can dissolve silica up to 72%; however, the pressure reactor vessel may be damaged due to the corrosive nature of the alkaline solution employed. This process is well suit to a temperature of less than 100 C [2327], but the rate of dissolution is very slow (26% SiO2 from 70% in glass). While this method was successfully utilized in the activation of slag, the produced silicates had lower molarity than the commercially available one, resulting in the failure to attain the maximum expected strength of the precursor [23,27]. Puertas and Torres-Carrasco [23] also observed that the compressive strength of AAB made of slag as a source material increases with an increase in the volume of glass and the resulting mixture has a comparable distribution of pore size with commercial sodium silicate. It was also concluded that the silicon produced by the dissolution of glass waste in NaOH/Na2CO3 can have indistinguishable impacts to the silicon in sodium silicate, thus these glass wastes can be utilized as an activator within the AAB framework. In addition, the developed activator was utilized in the production of geopolymer paste with slag as a precursor to study the strength and microstructural properties and a comparable development was observed in the performance of AAB made with conventional activators as shown in Fig. 17.2. Sodium silicate was obtained by a dry mix of glass powder with NaOH with a silica modulus of 1 and developed in a furnace at 150 C and 330 C for 2 hours and it was found that the activator produced at 150 C was more efficient than the one produced at 330 C, which resulted in a considerable reduction in the energy

Figure 17.2 BSEM/EDX images of slag pastes [25]: (A) activated with NaOH/Na2CO3; (B) activated with waterglass; and (C) activated with NaOH/Na2CO3 and 25 g of glass waste.

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requirement, thereby reducing the global carbon footprint in the production of powder activator [28]. The step-by-step procedure involved in the production of activator powder is shown in Fig. 17.3. The resulting activator proved to be capable of activating frequently used precursors such as fly ash and ground granulated blast furnace slag (GGBFS). The compressive strength of the mortars produced with fly ash and blend of fly ash and GGBFS to check the efficiency of the powder activator was developed and found to be around 80%. Zhang et al. [29] utilized waste glass as a partial mineral precursor in the development of AAB with a blended slag/fly ash system and observed that the reactivity of waste glass powder was higher than powder coal fly ash (PCFA) when activated under room temperature due to its high amorphous phases content (Fig. 17.4) and high reactive phases content.

Figure 17.3 Step-by-step procedure for extraction of activating powder [28].

Figure 17.4 XRD patterns of unreacted solid precursors [29].

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The activator solution was prepared with waste glass and RHA and compared in activating metakaolin-based AABs [30]. It was observed that a higher compressive strength and compact microstructure for geopolymer binders was made with activator produced from glass waste than with RHA which can be observed from the pore volume and compressive strength distribution shown in Fig. 17.5.

17.5

Silica fume

Silica fume, a very fine noncrystalline polymorph of silica (about 1/100th the size of an average cement particle) is a by-product of generating silicon metal or ferrosilicon alloy in electric arc furnaces. Silica fume contains, principally, amorphous silica in the order of 85% and above, with a specific surface area in the range of 13,00030,000 m2/kg, categorizing it as a very reactive pozzolan when utilized in concrete. Zivica [31] utilized silica fume activator in the production of mortar with PC, silica fume, and slag as binders. It was reported that the mixes prepared with silica fume activators produced higher compressive strength than that prepared with water and Na2O. It was also reported that the total porosity was about 29%54% less than the conventional mix as a result of the dense pore structure. Rodrı´guez et al. [32] investigated the mechanical and permeability properties of fly ash-based geopolymers prepared with commercial sodium silicate solution (SNa) and potassium silicate solution (SK), and the results were compared with the chemically modified nanosilica-based activators (L-300-Na and L-300-K). A slight reduction in the silica discharge from the nanosilica particles at an early age results in reduced water demand and porosity of the mixes than the mixes produced from a commercial silicate solution. The compressive strength of the mixes produced from sodium silicate has superior values to the mix with potassium silicate solution (Fig. 17.6) and, in both cases, it was reported that the mixes with commercial activators produced slightly higher compressive strength than the nanosilica-based activator. A comparative analysis was made by Bernal et al. [33] to investigate the performance of slag-based geopolymer mixes when exposed to elevated temperatures (50 C, 200 C, 400 C, 600 C, and 800 C) and it was observed that the geopolymers with RHA-based activators (RHAAs) showed superior performance at elevated temperatures than geopolymers made of silica fume-based activators (SFA) and commercial activators (WG), as shown in Fig. 17.7. It was also suggested that the application of RHA can greatly reduce the environmental issues as well as its inability to be a supplementary cementitious material in the production of blended PC products. Zivica and Rowsekova [3436] performed various investigations into the use of modified silica as an activator and recognized its enormous prospects in the production of AAB with slag as the precursor.

17.6

Rice husk ash

Rice is one of the chief food crops globally and its production creates an enormous amount of waste in the form of rice husk. Rice grains have a firm covering in the

Figure 17.5 Variation of pore volume and compressive strength distribution of alkali-activated binder [30]: (A) cumulative pore volume distribution; (B) compressive strength distribution.

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Figure 17.6 Compressive strength results of the alkali-activated fly ash mortar mixes [32].

Figure 17.7 Compressive strength results of slag-based GP mixes exposed to different temperature conditions [33].

form of the rice husk, which contains minerals that need to be removed prior to consumption. This derivative is typically utilized as a source of energy in the rice mill boiler, fertilizer in agricultural fields, or in power-generating plants. The combustion of agricultural waste leaves a residue in the form of rice husk ash of variable volume (13%29%), depending on the rice diversity, geographic locality, and environmental conditions where it is produced. This rice husk ash may contain an enormous amount of reactive silica in the order of 90% and above, with additional inorganic salts, making it suitable as a sustainable source of silicon for various

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purposes. With more than 80 million tons (Mt) of rice husk estimated as the output globally, China is the main contributor, and this can be used as a source of producing alternative activators for alkali-activator binder applications. Geopolymer binders were prepared with sodium silicate activator derived from waste glass (G1) and RHA (G2) of similar proportions and it was observed that the mixes made of sodium silicate from waste glass (G1) resulted in superior compressive strength at different curing ages than the activators made of RHA (G2) due to the lower cumulative pore volume and compact microstructure (Fig. 17.8). Rice husk ash has been mixed with a sodium hydroxide solution of 3 M concentration and heated to a temperature of 80 C for 3 hours to produce a sodium silicate solution. The resulting sodium silicate solution was included as an activator in the production of blended AAB to attain a compressive strength of 60 MPa using fly ash and ground granulated blast furnace slag as the source material [37]. Mejia et al. [38] produced sodium silicate solution with RHA and compared this with a commercial sodium silicate solution of similar silica modulus, resulting in the production of geopolymer paste to yield a 7-day compressive strength of 42 MPa. Sodium silicate of various silica modulus (SiO2/Na2O 5 0.31, 0.47, 0.62, 0.78, 0.93, 1.09 and 1.25) was prepared using white rice husk ash and the resulting activator was utilized in metakaolin-based geopolymer binder and it was observed that the compressive strength increases with the increasing molar ratio [39]. They also observed that the gel structure was formed with the silica modulus ratio between 0.93 and 1.25. Thuadaij et al. [40] investigated the extraction of silica from RHA and observed that the pure silica yield increases with an increase in the NaOH concentration, as can be seen in Fig. 17.9. Bernal et al. [41] investigated the possibility of valorizing RHA as a silica source for activating AAB for slag/metakaolin blends and observed comparable

Figure 17.8 Compressive strength results of geopolymers with sodium silicate produced by glass waste (G1) and RHA (G2) [30].

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Figure 17.9 Pure silica yield with NaOH concentration.

mechanical properties to the AAB produced with commercial activators. High slag/ metakaolin ratios resulted in higher 7-day compressive strength due to the presence of silica at a delayed rate as a result of the partial preliminary dissolution of RHA. Selvakumar et al. [42] extracted sodium silicate from RHA using an NaOH solution of various concentrations and the filtrate was washed with various acid solutions and it was concluded that more than 85% of silica could be extracted when washed with HCl rather than HNO3 or H2SO4 and it can found to increase with increasing NaOH concentration (Fig. 17.10). Herve et al. [43] produced AAB with commercial sodium waterglass (S1) and activators derived from pure rice husk ash (S2) and raw rice husk ash (S3), and the production mechanism is illustrated in Fig. 17.11. The resulting AABs were denoted as Geo1, Geo2, and Geo3, respectively, and it was observed that S3 had the highest amount of condensed silica as a result of 4% phosphate in S3. It was also observed that Geo3 shows a greater amount of unreacted metakaolin than Geo1 and Geo2, in the order of 35%, 20%, and 25%, respectively. The use of activator powders in the production of one-part AABs is more advantageous in terms of handling and storage than conventional geopolymers, although there was reduced strength compared with the conventional geopolymers due to the low degree of reaction as a result of crystalline by-products [44]. Yuvakkumar et al. [45] synthesized high-purity nanosilica powder from RHA using a simple precipitation technique by treating RHA with 2.5 N NaOH solution, which yields 99.9% pure silica. Irrespective of the extraction method and concentration, the complete extraction of sodium silicate can be obtained by washing the filtrate with hot water. An investigation was carried out [46] to extract silica gel from RHA by heating RHA-mixed NaOH solution of different concentrations in a microwave oven for 510 minutes and ending with neutralization. It was observed that the silica yield

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Figure 17.10 Silica yield (in %) under different acids washing [42].

Figure 17.11 Production mechanisms for geopolymers.

increased with an increase in the NaOH concentration and microwave heating, and the maximum yield was observed at 2 M NaOH concentration with a microwave heating temperature of 800 W for 10 minutes.

17.7

Sugarcane bagasse ash

The sugar extracted from sugarcane leaves behind sugarcane bagasse which, when powdered to ash, is called sugarcane bagasse ash (SCBA), which was acknowledged to be a pozzolanic material recently and has been utilized as a supplementary cementitious material. Nonetheless, due to its additional silica volume, SCBA might likewise be utilized as a pozzolan [47]. The annual production of sugarcane globally increased from 1910.88 in 20112013.72 Mt in 2016. SCBA was cleared of impurities using hydrochloric acid and washed with distilled water to remove metallic ions, and then dried at 60 C for 24 hours. The silica was extracted from SCBA by heating at 600 C, 800 C, and 1000 C for 2 and 4 hours

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Figure 17.12 Silica composition in SCBA at various temperatures and durations.

durations with and without acid washing, and the results are illustrated in Fig. 17.12. It has been observed that the acid-washed samples resulted in greater amounts of silica extraction than those without acid washing. The resulting sodium silicate extract is then mixed with 3 M NaOH solution and heated at 80 C for 4 hours [48]. Tchakoute et al. [49] investigated the properties of metakaolin-based geopolymers using sodium waterglass derived from SCBA and concluded that the compressive strength at the age of 28 days was comparable both in air curing and water curing. They also investigated the infrared spectra of the geopolymer mixes with 28 days curing at room temperature with and without soaking in water, and the results are illustrated in Fig. 17.13. It was observed that the intensity of the band was found to reduce for the samples soaked in water after 28 days of room temperature curing compared with the specimens without soaking, which could be related to minor damage to the polysiloxo units under alkaline conditions (Eq. 17.4). Si 2 O 2 Al 1 H2 O ! Si 2 OH 1 Al 2 OH

(17.4)

Norsurayaa et al. [50] investigated the utilization of sugarcane bagasse ash as a source of an alternative activator and observed that the silica present in the raw sample was increased from 53% to 88% after acid treatment. It was also observed, based on the FTIR, that the extracted sodium silicate and commercial sodium silicate had almost the same spectral patterns. The SEM images (Fig. 17.14) reveal a round structure with a well-defined formation of mesoporous silica, which shows identical morphology to commercial mesoporous silica and therefore it has been concluded that the sodium silicate extracted from sugarcane bagasse can be utilized to synthesize mesoporous material.

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Figure 17.13 Infrared spectra of geopolymer mixes [49].

Figure 17.14 SEM morphology of synthesized mesoporous silica [50].

Alves et al. [51] investigated the synthesis of biosilica from SCBA using various techniques and a schematic flow chart of this is illustrated in Fig. 17.15. It was observed, based on the test results, that the fusion technique was better suited than the hydrothermal technique in the production of biosilica with an aging temperature of 80 C for 1 hour during gel formation. This has been justified by the amorphous nature of the silica synthesized and the availability of 99% silica.

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Figure 17.15 Schematic flow chart of biosilica extraction from sugarcane bagasse ash [51].

Silica was synthesized from SCBA under different temperature conditions (600 C, 700 C, and 800 C) in order to remove the impurities present in ash. The resulting ash was subjected to oxygen feeding with further hydrochloric acid treatment to obtain a high amount of pure silica, as detailed in Fig. 17.16. It can be observed that the silica content increases from 19.42% (2 hours of heating at 600 C without oxygen feeding and acid treatment) to 89.04% (3 hours of heating at 800 C with oxygen feeding and acid treatment) [52].

17.8

Other materials

Vaibhav et al. [53] investigated the feasibility of utilizing (a) rice husk, (b) sugarcane bagasse, (c) bamboo leaves, and (d) groundnut shell in the production of high-

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Figure 17.16 Composition of silica in sugarcane bagasse ash at various treatment levels.

purity silica by sintering them at 900 C for 7 hours followed by treating with 1 M NaOH to form sodium silicate, which is then washed with 6 M H2SO4 to precipitate silica in the order of 52%78% (Fig. 17.17). They also suggested further treatment of groundnut waste due to magnesium-substituted silica. Amorphous silica extraction from corn cob ash (CCA) was investigated by Okoronkwo et al. [54] by dissolving CCA in alkali solution to produce waterglass. The silica xerogel was obtained by the addition of HCl to lower the pH to 7 and subsequent drying. This method of extraction also improves the amount of silica present in CCA from 47.66% to 97.13%. The ash derived from wheat straw, an agricultural waste, by microwave digestion and calcination has been utilized to extract silica gel [55]. The method adopted to extract silica gel was alkaline extraction and acid neutralization and it was concluded that wheat straw can be used to extract high-purity amorphous silica.

17.9

Cost analysis

A cost comparison has been made between concrete made from PC, AAB with commercial activators, and AAB with powder activator from waste glass for both normal-strength and high-strength mixes [28]. It was observed that the concrete produced with AABs with commercial activators is significantly costlier than the corresponding concrete produced with PC, and varies at 48% and 18% in the production of normal-strength and high-strength concrete, respectively, whereas alkaliactivated concrete with powder activator is 10% and 19% cheaper than the corresponding concrete produced with PC for developing normal-strength and highstrength concrete, as shown in Fig. 17.18A. Similarly, the cost of commercial

Figure 17.17 Methodology adopted to extract silica from waste materials [53].

Figure 17.18 Cost analysis comparison between a powder activator and a commercial activator: (A) production cost (%) in terms of PCC production; (B) total production cost.

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activator was 41% and 45% of the total production cost of concrete made with AAB and that with powder activator was found to be 14% and 16%, respectively (Fig. 17.18B).

17.10

Summary and conclusions

AABs are the next generation of cement concrete to reduce the use of PC, thereby reducing the emissions and energy requirements in its production. AAB relies on precursors (which are rich in Si and Al) and activators (mostly sodium-based hydroxides and silicates), which results in improved mechanical and durability properties. However, the cost, embodied energy, and environmental impacts connected with the manufacture of activators, in particular sodium silicate, which requires more than 1000 C for melting sodium carbonate and silica, results in a huge amount of CO2 emissions. Hence, an alternative method of extracting sodium silicate is vital to reduce the huge energy required in its production. Many industrial wastes, such as waste glass, silica fume, rice husk, and sugarcane bagasse have been utilized as a starting material for extracting sodium silicate in a conventional way. This chapter overviews the various methodologies adopted to extract sodium silicate from industrial waste for use in the production of AAB. This alternative method leads to reduction in the GHG emission and results in sustainable construction technology.

References [1] R.M. Andrew, Global CO2 emissions from cement production, Earth Syst. Sci. Data 10 (2018) 195217. [2] C.D. Lawrence, The Production of Low-Energy Cements Lea’s Chemistry of Cement and Concrete, fourth ed., Butterworth-Heinemann, Oxford, 2003, pp. 421470. [3] P. Duxson, J.L. Provis, G.C. Lukey, J.S.J. van Deventer, The role of inorganic polymer technology in the development of ‘green concrete’, Cem. Concr. Res. 37 (2007) 15901597. [4] D.M. Roy, G.M. Idorn, Hydration, structure, and properties of blast-furnace slag cements, mortars, cnd Concrete, J. Am. Concr. Inst. 79 (1982) 444457. [5] J. Davidovits, Geopolymer Chemistry and Applications, fourth ed., Institut Ge´opolyme`re, 2015. [6] F.N. Okoye, J. Durgaprasad, N.B. Singh, Effect of silica fume on the mechanical properties of fly ash based-geopolymer concrete, Ceram. Int. 42 (2015) 17. [7] P. Duxson, S.W. Mallicoat, G.C. Lukey, W.M. Kriven, J.S.J. van Deventer, The effect of alkali and Si/Al ratio on the development of mechanical properties of metakaolinbased geopolymers, Colloids Surf. A Physicochem. Eng. Asp. 292 (2007) 820. [8] A.M. Mustafa Al Bakri, H. Kamarudin, M. Bnhussain, I. Khairul Nizar, W.I.W. Mastura, Mechanism and chemical reaction of fly ash geopolymer cement—a review, J. Asian Sci. Res. 1 (2011) 247253. [9] F. Skvara, L. Kopecky, J. Nemecek, Z. Bittnar, Microstructure of geopolymer materials based on fly ash, Ceram. Silik. 50 (2006) 208215.

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[31] V. Zivica, Effectiveness of new silica fume alkali activator, Cem. Concr. Compos. 28 (2006) 2125. [32] E.D. Rodrı´guez, S.A. Bernal, J.L. Provis, J. Paya, J.M. Monzo, M.V. Borrachero, Effect of nanosilica-based activators on the performance of an alkali-activated fly ash binder, Cem. Concr. Compos. 35 (2013) 111. [33] S.A. Bernal, E.D. Rodrı´guez, R. Mejı´a de Gutie´rrez, J.L. Provis, Performance at high temperature of alkali-activated slag pastes produced with silica fume and rice husk ash based activators, Mater. Constr. 65 (2015) e049. [34] V. Zivica, High effective silica fume alkali activator, Bull. Mater. Sci. 27 (2004) 179182. [35] A. Bajza, I. Rousekova, V. Zivica, Silica fume-sodium hydroxide binding systems, Cem. Concr. Res. 28 (1998) 1318. [36] I. Rousekova, A. Bajza, V. Zivica, Silica fume-basic blast furnace slag systems activated by an alkali silica fume activator, Cem. Concr. Res. 27 (1997) 18251828. [37] K.T. Tong, R. Vinai, M.N. Soutsos, Use of Vietnamese rice husk ash for the production of sodium silicate as the activator for alkali-activated binders, J. Clean. Prod. 201 (2018) 272286. [38] J.M. Mejia, R. Mejia de Gutierrez, F. Puertas, Rice husk ash as a source of silica in alkali-activated fly ash and granulated blast furnace slag systems, Mater. Constr. 63 (2013) 361375. [39] H.K. Tchakoute, C.H. Ru¨scher, S. Kong, N. Ranjbar, Synthesis of sodium waterglass from white rice husk ash as an activator to produce metakaolin-based geopolymer cements, J. Build. Eng. 6 (2016) 252261. [40] N. Thuadaij, A. Nuntiya, Synthesis and characterization of nanosilica from rice husk ash prepared by precipitation method, CMU. J. Nat. Sci. Special Issue on Nanotechnol. 71 (2008). [41] S.A. Bernal, E.D. Rodriguez, R.M. de Gutierrez, J.L. Provis, S. Delvasto, Activation of metakaolin/slag blends using alkaline solutions based on chemically modified silica fume and rice husk ash, Waste Biomass Valor 3 (2012) 99108. [42] K.V. Selvakumar, A. Umesh, P. Ezhilkumar, S. Gayatri, P. Vinith, V. Vignesh, Extraction of silica from burnt paddy husk, Int. J. ChemTech Res 6 (2014) 44554459. [43] H.K. Tchakoute, C.H. Ruscher, S. Kong, E. Kamseu, C. Leonelli, Comparison of metakaolin-based geopolymer cements from commercial sodium waterglass and sodium waterglass from rice husk ash, J. Sol-Gel Sci. Technol. 78 (2016) 492506. [44] P. Sturm, G.J.G. Gluth, H.J.H. Brouwers, H.C. Kuhn, Synthesizing one-part geopolymers from rice husk ash, Constr. Build. Mater. 124 (2016) 961966. [45] R. Yuvakkumar, V. Elango, V. Rajendran, N. Kannan, High-purity nano silica powder from rice husk using a simple chemical method, J. Exp. Nanosci. 9 (2014) 272281. [46] S. Rungrodnimitchai, W. Phokhanusai, N. Sungkhaho, Preparation of silica gel from rice husk ash using microwave heating, J. Metal Mater. Miner. 19 (2009) 4550. [47] K. Ganesan, K. Rajagopal, K. Thangavel, Evaluation of bagasse ash as supplementary cementitious material, Cem. Concr. Compos. 29 (2007) 515524. [48] N. Sahiron, N. Rahmat, F. Hamzah, Characterization of sodium silicate derived from sugarcane bagasse ash, Malaysian J. Anal. Sci. 21 (2017) 512517. [49] H.K. Tchakoute, C.H. Ruscher, M. Hinsch, J.N.Y. Djobo, E. Kamseu, C. Leonelli, Utilization of sodium waterglass from sugar cane bagasse ash as a new alternative hardener for producing metakaolin-based geopolymer cement, Geochemistry 77 (2017) 257266.

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[50] S. Norsurayaa, H. Fazlenaa, R. Norhasyimia, Sugarcane bagasse as a renewable source of silica to synthesize Santa Barbara amorphous-15 (SBA-15), Procedia Eng. 148 (2016) 839846. [51] R.H. Alves, T.V. da Silva Reis, S. Rovani, D.A. Fungaro, Green synthesis and characterization of biosilica produced from sugarcane waste ash, Hindawi J. Chem. (2017). Article ID 6129035. [52] P. Worathanakul, W. Payubnop, A. Muangpet, Characterization for post-treatment effect of bagasse ash for silica extraction, world academy of science, engineering and technology, Int. J. Chem. Mol. Eng. 3 (2009) 360362. [53] V. Vaibhav, U. Vijayalakshmi, S. Mohana Roopan, Agricultural waste as a source for the production of silica nanoparticles, Spectrochim. Acta A Mol. Biomol. Spectrosc. 139 (2015) 515520. [54] E.A. Okoronkwo, P.E. Imoisili, S.O.O. Olusunle, Extraction and characterization of amorphous silica from corn cob ash by sol-gel method, Chem. Mater. Res. 3 (2013) 6872. [55] K.G. Patel, N.M. Misra, R.R. Shettigar, Preparation and characterization of silica gel from wheat straw, Int. J. Chem. Eng. Appl. 7 (2016) 344347.

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Hosam M. Saleh and Samir B. Eskander Radioisotope Department, Nuclear Research Center, Atomic Energy Authority, Giza, Egypt

18.1

Introduction

Concrete is a major material for building construction. It consists of a chemically inert, solid particulate material known as aggregate (usually consisting of different types of sand and gravel) that is bound to cement as a binder and water as a hydration medium. Concrete has been used for more than 2000 years. It is best known for its long-life and dependable nature. The Assyrians and Babylonians utilized clay as an alternative binder with cement. At the same time, lime and gypsum were applied in the same manner by ancient Egyptians. After some thousands of years, in 1756, the British engineer John Smeaton prepared the first modern concrete by incorporating gravels with milled bricks into cement as a binder. Another British inventor, Joseph Aspdin, invented Portland cement in 1824 as a widespread cement for concrete production. Moreover, he produced the first true artificial cement by incineration of both limestone and clay. The combustion process modified the chemical characterization of the original materials and, consequently, strong cement was obtained in preference to crushed limestone. The other important integral part of concrete composition with cement is the aggregates, including several additives such as crushed stones, sands, gravels, ashes, slags, burned shale, and clays. The fine-size granulates are usually introduced for the production of smooth surfaces and concrete slabs. Concrete with embedded steel is known as reinforced concrete. Reinforced concrete was invented in 1849 by Joseph Monier, and patented in 1867. He was a Parisian gardener who manufactured garden pots and pipes using cement reinforced with a network of iron mesh. Reinforced concrete combines the tensile strength or rebate resistance of the metal and the compressive strength of concrete to bear high loads. He showed this reinforced constituent at the Paris Exhibition of 1867. In addition to pots, pipes, and vases, he modified reinforced concrete for railway ties, floors, arches, and bridges. The expanded history of types, production, and use of cementitious materials is closely associated with the history of civilization. One of the first cementitious materials was unburnt clay. Over time, methods were discovered for producing gypsum and lime by burning the appropriate rocks, which were specified by high strength with low water resistance compared with cementing materials from unburnt clay. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00018-1 © 2020 Elsevier Inc. All rights reserved.

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Cement is a powdery mineral binder, that is mixed with water or aqueous solutions of some salts to form a paste that is able to set and harden, and that consequently over time converts into a hardened solid block due to physicochemical reactions. Cement as an inorganic binder is subjected to physical, chemical, and/or biological impacts. Osmotic pressure is an example of physical attack, while calcium hydroxide leaching illustrates the most vulnerable impact due to chemical attack. Oxidation due to microbial attack and affecting the integrity of the hardened cement product can be considered as an example of biological aggression. All of these impacts cause an increase in permeability, which in turn elevates the leachant flow inside the pores of the hard product, increasing the leach rates of the cement ingredients and diminishing its integrity. Different methods for increasing their strength and water resistance were reported by introducing various additives, keeping the principal material of cement as the basic constituent. The introduction of mineral and chemical additives increased the development from several to hundreds of types of cements. Cement-based materials (CBMs) are characterized by light weight, heat insulation, cost effectiveness, and high integrity, where conventional additives were replaced by modern efficient alternatives. Recently, natural and artificial ultralightweight ingredients, such as blast furnace slag (BFS), natural clay, silica fume and fly ash, spinning waste fibers, wood waste, ceramsite, pumice, recycled styrofoam granules, polyurethane foam, expanded polystyrene, and several nanomaterials have been introduced into the industry. Eco-friendly biocement composites from cement and plant origin waste materials are novel products of the 21st century as a solution to the elevated environmental threat and to reduce the uncertainty of the petroleum supply which was expected to downturn between 2010
20. Natural plant wastes have put forward the opportunity for developing countries to utilize their own resources in various national valueadded composite industries. Natural wastes can be used to produce new products or can be used as admixtures, so that natural resources are limited and used more efficiently; in addition, the environment is protected from these waste deposits. Green innovative CBM is the concept of using eco-friendly materials in concrete, to make the system more sustainable. They are widely available and cheap to produce, since natural waste fractions are used as a partial substitute for native cement, consequently the costs for their disposal are avoided. Moreover, the energy consumption in production of cementitious materials is lowered, and the durability of the end products is greater. Inorganic residual products such as stone dust, crushed concrete, and marble waste are used as green aggregates in concrete. Furthermore, by replacing cement with fly ash, and microsilica in larger amounts, to develop new green cements and binding materials, increases the use of alternative raw materials and alternative fuels by developing or improving cement-based products characterized with low energy consumption. Considerable researches also have been carried out on the use of various industrial by-products and microfillers in concrete. The main concern with using pozzolanic wastes was not only the cost effectiveness but also to improve the properties of concrete, especially its durability. In general, the application of green cement-based products can reduce CO2 emissions to the atmosphere and allow the reuse of the natural waste materials, helping to reduce ecosystem pollution.

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This chapter discusses the crucial role of the innovative cement materials in environment protection and restoration, especially in the field of radioactive waste management.

18.1.1 Portland cement Portland cement is obtained by heating limestone and clay or other silicate mixtures at high temperatures ( . 1500 C) in a rotating kiln. The resulting clinker, when cooled, is mixed with gypsum (calcium sulfate) and ground to a highly uniform fine powder. Anhydrous Portland cement consists mainly of lime (CaO), silica (SiO2), and alumina (Al2O3), in addition to small amounts of magnesia (MgO), ferric oxide (Fe2O3), sulfur trioxide (SO3), and other oxides that are added as impurities in the raw materials during its manufacture. When these oxides are blended together, they form the four basic components of Portland cement, namely: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. Table 18.1 describes the main oxide compositions, their chemical formulas, and abbreviations, in addition to listing the average of each in commercially available ordinary Portland cement (OPC) (wt.%) [1].

18.1.2 Cement hydration The addition of water to OPC powder commences immediately with the cement hydration reactions. These series of chemical reactions result in the subsequent setting and hardening of the cement paste. Needle-like crystals of calcium sulfoaluminate hydrate, namely ettringite, are formed within a few minutes. The ettringite subsequently transforms to monosulfate hydrate after a time. Two hours after the start of the cementation process, large prismatic crystals of calcium hydroxide (CH) and very small crystals of calcium silicate hydrates (C
S
H) begin to fill the empty pores previously occupied by water and the hydrated cement particles. Therefore the major components of the hydrated cement paste are: Calcium silicate hydrate: This is the most important hydration product and forms nearly 60% of the volume of solids. It is formed by a layer of sponge-like structures with a large high

Table 18.1 The main oxide compositions, their oxide formulas, and abbreviations in addition to the average of each in commercially available Portland cement (wt.%). Oxide constituent

Oxide composition and abbreviation

Average weight (%)

Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite Others

3CaO  SiO2 (C3S) 2CaO  SiO2 (C2S) 3CaO  Al2O3 (C3A) 4CaO  Al2O3  Fe2O3 (C4AF)

40
60 13
50 4
11 7
13 7
10

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Table 18.2 Estimated reaction rates for cement hydration reactions. Component

Estimated reaction rate

3CaO  SiO2 2CaO  SiO2 3CaO  Al2O3 4CaO  Al2O3  Fe2O3 CaSO4  2H2O (gypsum)

Fast relative to 2CaO  SiO2 Slow compared to 3CaO  SiO2 Fastest, but addition of gypsum retards the rate Relatively slow Retardant

surface area (B 500 m2/g). The end product strength is largely contributed by the C
S
H formation and is attributed mainly by their van der Waals physical adhesion forces. Calcium hydroxide: This is the second most abundant component with respect to the volume of solids and constitutes about 25%. It is formed of large plate-like crystals with a smaller surface area compared to C
S
H. It contributes to limited van der Waal binding forces and is relatively highly soluble compared to C
S
H, and renders the concrete reactive to acidic solutions. Calcium sulfoaluminate: This has a minor role in the cementitious structure properties, and forms almost B15% of the volume of solids. The chemical durability of the cementitious end product to sulfate attack is of concern, and this is attributed to the presence of the monosulfate hydrate (Table 18.2).

The approximate hydration reactions between water and the cement components can be represented for each of the Portland cement major ingredient as follows. These equations are not stoichiometrically balanced due to the variations in the products formed and their compositions. For C3 S: For C2 S: For C3 A: For C4 AF:

C3 S 1 6H2 O ! C3 S2 :3H2 O 1 3 CaðHOÞ2 C2 S 1 4H2 O ! C3 S2 :3H2 O 1 3 CaðHOÞ2 C3 A 1 6H2 O ! C3 A:6H2 O 4C4 AF 1 2CaðOHÞ2 1 10H2 O ! C3 A:6H2 O 1 C3 F:6H2 O

It is clear that both silicates, C3S and C2S, need almost the same mass of water for hydration. However, calcium hydroxide resulting from C3S hydration is more than twice that obtained from C2S hydration. It is worth stating that the reaction rate of C3A is quicker than that of calcium silicates [2,3].

18.1.3 Hydration of cement is assumed to takes place in five arbitrary phases 1. Initial hydration—The unreacted clinker grains disperse in water. Clinker begins to dissolve, and calcium and hydroxide ions begin to fill the water in the void space between the grain. At this stage, the reaction rate is phase formation controlled. 2. Dormant period—The solution quickly achieves saturation. Hydration products grow from the surface of each clinker granule. Initial crystallization begins, but the material is mostly colloidal and is referred to as a gel phase.

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3. Acceleration period—The hydration product gel layer completely encases each clinker grain. These hydration products are Ca3Si2O7.3H2O and Ca3Al2O3, and to a lesser extent Ca2Si2O4.3H2O. This layer blocks the hydroxide ions which must diffuse through it to the clinker surface to continue the growth of hydration products. The reaction begins to be diffusion controlled and crystallization increases. 4. Deceleration period—The thickening gel expands to fill the voids between clinker granules, and further slows down incoming ions. 5. Steady state period—The reaction is at a completely diffusion-controlled rate. Interlocking polysilicate crystals grow within the gel and provide strength and hardness. These crystals provide a “skeleton.” The reaction rate is governed almost completely by diffusion of water and ions to reaction sites. This process can continue for up to 15 years. When it is complete, the cement has attained its ultimate strength [4].

18.2

Innovative cement-based material

The cement industry is responsible for a significant proportion of carbon dioxide emissions, moreover it consumes many raw materials and energy. Therefore, CBMs can contribute to sustainable development of their production and applications. The main and most important issues concerning the innovation of a new CBM are energy efficiency, recycling materials, polluting gas emissions, and final product durability, all of which can be ended to give environmental gains [5]. In more detail, the main aims for developing an innovative CBM are: G

G

G

Environmental protection by reducing CO2 and other greenhouse gases (GHG) emissions. According to Ref. [6], for each ton of Portland cement clinker produced, approximately 1 ton of carbon dioxide and about 10 kg of nitrogen oxides escape to the atmosphere [6]. Innovating CBM will save products where it is based on wastes and by- products, targeting environment protection and restoration. Consequently, there is the prospect of innovative CBM reducing the consumed energy for cement production by substituting part of the plain cement in the end product manufacture or during its formulation.

Selected waste materials can be applied for producing innovative CBMs. They substitute the plain cement in the manufacturing process. The waste materials are differentiated into two main groups: preconsumer and postconsumer materials. The preconsumer ones are by-products generated from industrial processes. These include slag, silica fume, fly ash, rice-husk ash, and others. This category of byproducts is in the powder state, therefore, they are mainly utilized in cement production as a substitute for clinker. Moreover, the preconsumer materials can be generated also from extractive processes such as ornamental stone waste, and from boat manufacturing, such as glass reinforced plastic. On the other hand, the most common postconsumer materials originated from the recycling of domestic wastes including glass and plastic or from demolition waste, for example, bricks, concrete, and rubble. Municipal solid incinerated wastes, wooden ash, agriculture residues, paper sludge ash, and others are also used for production of CBMs. In the last few years, some achievements for using

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polluted soil valorization to produce cement-based aggregates have been made. The process is characterized as a solidification/stabilization (S/S) process by solidifying the soil by incorporating into a cement matrix and, consequently, stabilizing the inorganic contaminants retarding their release to the surrounding groundwater. The produced aggregates can have acceptable mechanical integrity.

18.2.1 Fly ash Fly ash is produced during coal combustion in electricity generation plants [7]. It contains pozzolan which make it useful in waste stabilization. It can also be used for blended cement production.

18.2.2 Stone crusher waste as fine aggregates Quarry rock dust is generated after blasting, crushing, and screening processes for coarse aggregate production. It has rough, sharp, and angular particles and its application results in a gain in strength that is attributed to better interlocking. However, utilization of quarry dust sometimes causes an increase in the fraction of cement needed to maintain workability. Concrete based on quarry rock dust performs an acceptable acid and sulfate resistance and reduced permeability of the end product relative to conventional concrete. The main disadvantage for applying quarry sand is the increase in water required for concrete mixture production.

18.2.3 Marble waste as filler material Marble waste is one of the most worldwide environmental problems. It mainly comprises very fine powders. These waste materials are utilized to improve some properties of fresh and hardened end-product characteristics of mortar and concrete.

18.2.4 Blended cements Blended cement has been used for many decades around the world. The production of blended cements involves interring one or more additives, for example, fly ash, granulated BFS, silica fume, volcanic ash, and other by-products to the clinker, in various proportions, at the grinding stage of cement production. The use of blended cements is a particularly attractive efficiency option as introducing these additives not only aids in reducing the amount of energy used, in addition to reducing GHG emissions in clinker production, but also directly corresponds to a reduction in carbon dioxide emissions in calcinations [8]. Although it is most common to make use of supplementary cementing materials (SCMs) in the replacement of cement in concrete mixtures, the most pronounced advantages of adding blended cement include expanded production capacity and close monitoring of the quality of the product [9].

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18.2.5 Recent chemically innovative engineered cement substitutes In recent years, researches have been undertaken aimed at a viable alternative to OPC and recognized replacements. Those alternatives are often products of a lowtemperature chemical process which bypasses the energy input needed by OPC. Moreover, these researches have tried to avoid utilizing waste materials which arise from processes that are themselves environmentally harmful. Some of this is produced by the chemical mixing of various waste materials to obtain a product that acts in a way similar to traditional cement. It can be produced using a variety of industrial locally available by-products such as food wastes. It is obtained through a chemical process using minimal heat compared to traditional OPC production which uses a massive amount of heat energy. The CO2 emissions associated with conventional OPC production can be more than 850 kg/ton of cement, accounting for up to 10% of the world’s carbon dioxide emissions. It is noted that this product is estimated to generate around 43 kg carbon dioxide per ton. Moreover, producers have plans to supply plants with an anaerobic digestion system fueled by food waste. This will result in overall CO2 emissions falling as low as 18 kg/ton. Another example of chemically engineered cement substitution uses magnesium silicate as a raw material, which is processed at relatively low temperatures (B 700 C), to produce magnesium carbonate (MgCO3). The most significant target is that, besides the low production temperatures rendering low energy content fuels viable (such as biomass), the chemical process of carbonation actually traps carbon dioxide from the surroundings. The manufacturers reported that the production process for 1 ton of product can absorb up to 100 kg more CO2 than it emits [7].

18.2.6 Application of ordinary Portland cement in environment protection Most environmentally threatening waste components can be incorporated into a water
cement system. The suspended pollutants can be immobilized into the final hardened cement. During the solidification/stabilization process, the cement formation binds and strengthens the mass, coating and fixing most contaminant molecules in the siliceous solids, and filling the pathways between pores. Due to the high alkaline pH of cement admixtures (pH  9
11), many multivalent cations are precipitate as insoluble carbonates or hydroxides. Thus, this process is highly efficient for immobilization of hazard waste components, for example, heavy metal waste streams and some categories of radioactive wastes. The main characteristics desired for solidifying/stabilizing matrix can be summarized as follows: simplicity in formulation and application, cost effective, extended shelf-life, physical stability, acceptable mechanical integrity, high chemical resistance against various environmental impacts, optimal radiation resistance, resistance to microbial degradation, acceptable packaging capability, noncorrosive to the contained container, and no or very low leaching rate, especially for hazardous heavy metals and radionuclides.

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According to the International Atomic Energy Agency (IAEA) [10], OPC is the traditional material to solidify most radioactive waste. Therefore it is the most widely used candidate inorganic-based binding system used for solidification/stabilization of hazardous, low- and intermediate-level radioactive wastes [11
13]. A study was carried out by Eskander et al. [14] utilizing OPC to solidify the bioproducts originating from the bioremediation process of solid cellulose-based radioactive waste simulated using a mushroom (Pleurotus pulmonarius). It is worth mentioning that the values of compressive strength obtained for the cement-waste form incorporating up to 5% dry bioproducts, and cured for 28 days or after 90 days, remained higher than that recommended by the Nuclear Commission Regulatory [15] (Fig. 18.1). Based on the data collected, it can be stated that cement can provide a highly durable final waste form based on OPC that ensures long-term stability of the solidified waste material and can act as a first barrier against the release of radiocontaminants from radioactive wastes to the surrounding environment [14]. The global nanotechnology market is expected to record a turnover of $90.5 billion in 2021, compared to $39.2 billion in 2016, with a product annual growth rate of 18.2% [16]. The application of nanotechnology for environmental purposes is one of the multiple uses for this broad technology and is based on the use of cement
nanomaterial composites in construction and solidification/stabilization of hazard wastes [17]. Duque-Redondo et al. [18] discussed the molecular dynamics simulations to investigate the adsorption and diffusivity of Cs into C
S
H gel nanopores.

Figure 18.1 Mechanical integrity of a cement-waste form containing various ratios of bioproduct and cured for different periods of time. Source: Reproduced from S.B. Eskander, S.M.A. El-aziz, H.M. Saleh, Cementation of bioproducts generated from biodegradation of radioactive cellulosic-based waste simulates by mushroom 2012. https://doi.org/10.5402/2012/329676.

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18.2.7 Advantages and disadvantages of cement or cementbased materials The main advantages of applying cement or CBMs in various fields are: G

G

G

G

G

Raw materials are normally available and the production technology is well experienced. Ease of workability and formulation. Compatible with many wastes when applied for environmental protection and restoration. Low cost. Acceptable mechanical integrity, chemical resistance, and physical characterizations.

On the other hand, the most pronounced disadvantages of the cement end products are: G

G

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Swelling and cracking in the case of water intrusion into some end products. Volume increases and high density for shipping and disposal. Some wastes can affect setting and hardening of the cement ingredients, leading to improper waste forms [19
21].

In general, the most commonly reported disadvantages of CBMs, as siliceous and calcareous binders, are that they are vulnerable to chemical, physical, and biological attack. Large quantities of chemicals and additives are needed to incorporate some categories of waste, for example, organic wastes. Treatment of these wastes before solidification in cement can overcome this disadvantage, for example, the wet oxidative degradation of organic liquid scintillator using hydrogen peroxide before cementation [22]. Siliceous and calcareous binders have doubtful long-term durability. When they are applied for long-lived waste, an extended monitoring period is needed. The long-term monitoring can include additional maintenance costs in case of cement matrix failing. Siliceous and calcareous binders may not bind a few amphoteric metals, like lead, properly. In a cement high-pH environment, these binders become more soluble and vulnerable to leaching [23]. Another example for pretreatment prior to immobilization in CBM is the following: iodine is fixed as a chemical form of IO3 due to its high affinity for the cementitious materials, namely the high-performance cement technique. The oxidized iodine is immobilized through kneading (mixing by pressing and squeezing) and solidification in a cementitious material with high affinity for anions. Because of the pretreatment in alkaline solutions, transition of iodine to the gas phase is rare, and the iodine recovery rate is extremely high, at over 99.96% (Fig. 18.2) [24]. One of the most important applications of the cement and/or innovative cement materials is in the field of hazardous and radioactive waste management. Radioactive waste is created during all processes of nuclear research or power reactor operation, and in the production and applications of radioisotopes in different fields, for example, research, industry, agriculture, and medicine. Waste is a material that is not needed and is not useful economically, even with further processing. It can be solid, liquid, or gas. It is generated from many daily human activities.

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Figure 18.2 Leaching model of the components from cement materials [24].

The main causes of waste pollution, and the increase in its volume, especially solid wastes, are over population, urbanization, and affluence (where in affluent countries, the per capita consumption is very high and citizens discard many things regularly). Moreover, technology has that changed the culture toward how we use things [25]. Direct dumping and burning of hazardous waste is an unacceptable issue from environment and health perspectives. A multibarrier system is a necessity for management of hazardous and radioactive wastes. That system should ensure safety and protection of humans and the environment. The hazardous and radioactive wastes have to be solidified and stabilized using a solid and durable matrix. The most widely used matrix is made from cement or CBMs. The final solid waste form, that is, the waste and matrix, is introduced into a container, which can be metal or concrete. This container or the primary package serves as the first engineered barrier to avoid the dispersion of hazard ingredients (Fig. 18.3). This package is disposed of in an insulated disposal site to prevent the leaching out of the hazardous contaminants and to preserve the surrounding ecosystem (the most important element is the groundwater) leaving a safe, tidy, and clean environment. The disposal site is considered to be the secondary barrier and can be a deep borehole old mine or an engineered vault (Fig. 18.4). Disposal in a deep geologic repository is the most common working plane for disposing of high-level radioactive wastes [27]. The three main processes in a waste management scheme, that is, solidification, placing in a container, and disposal, fundamentally utilize CBMs. According to the IAEA, immobilization of waste includes its conversion into a waste form by embedding, solidification, or encapsulation using an inert matrix. This is necessary for safe handling, transportation, interim storage, and final disposal. On the other hand, conditioning usually refers to a process that results in a waste package that has optimal durability for the upcoming processes, that is, handling, transportation, storage, and disposal. Conditioning can include the conversion of waste to a solid waste form and enclosure of waste in containers [26]. Based on the class and category of the sorted radioactive waste, the suitable specific solidifying matrix material is a candidate for the immobilization process [28].

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Figure 18.3 Waste packages in use: (A) drum, (B) metal box, (C) concrete cylinder, and (D) concrete container [26].

Figure 18.4 A multibarrier system, based on cement and/or cement-based material for management of hazardous and radioactive wastes.

The most important targets for the multibarrier concept are the safety of all living organisms and protection of the environment, keeping it clean and tidy for future generations. This issue includes all the processes in the radioactive waste management scheme, that is, immobilization, packing, transportation, storage, and final disposal. Therefore this concept is concentrated on two aims: to accommodate stabilization of the radioactive waste ingredients within the inert matrix and

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including packaging, and additionally to restrict the radiation exposure dose to the public during all management schemes, namely handling, collection, sorting, pretreatment, treatment, immobilization, conditioning, transportation, storage, and final disposal [29]. Solidification/stabilization (S/S) is a process that includes the mixing of a waste with a binder to reduce the release of hazardous ingredients by both physical and chemical means and to convert the hazardous waste into an environmentally acceptable waste form for land disposal. Moreover, it provides the waste form with optimal mechanical integrity to withstand handling and transport processes. The requirements for the final waste form are to acquire physical, chemical, and thermal stabilities of the contained radioactive materials. Moreover, the immobilized final waste forms should be characterized by a very low or nonleaching rate, and resist cracking, powdering, and all forms of degradation. Convenience of the process, low cost, and products of acceptable quality and higher level elimination than other techniques are some of the advantages of the cementation technique. On the other hand, the most common disadvantages of the technique are low mechanical durability of solidified waste products and elevated corrosion risk for long-term disposal in environmental conditions [30,31]. Cementitious materials have many applications in low- and intermediate-level radioactive waste disposal facilities. These include the application of cement as a waste form (i.e., solidification) as well as its utilization as a backfill and construction material for the storage vault. The formulations of these cements are expected to contain substantial amounts of OPC, therefore the long-term behavior of hydrated cements and their constituent phases in natural groundwaters is important for the performance assessment of the stated waste disposal systems and the potential release of radionuclides. Encapsulation in cement is the preferable technique in the United Kingdom for treatment, conditioning, and disposal of medium- and low-level radioactive wastes. It is typical for applied composite cement systems to incorporate BFS or pulverized fuel ash (PFA)for their several advantages over Portland cement, especially their low hydration heat. The use of these mineral additions from a final waste form can itself require a disposal route that is advantageous due to the minimizing of the amount of Portland cement used, which provides a reduction in cost and energy consumption [32]. Several OPC-based mixtures have been innovated to overcome the drawbacks of using OPC, especially in the radioactive waste management field and to reduce the incompatibility of OPC associated with the chemical composition of certain types of waste. Biochar as a natural product was used as an additive in cement for solidification of spent resins. Biochar has a relatively structured carbon matrix with a high degree of porosity and an extensive surface area. An experimental study was conducted research into the role of biochar as an additive material for CBM solidifying radioactive spent ion exchange resins. Biochar was obtained from the pyrolysis of oil palm empty fruit bunches at a medium temperature (250 C
450 C). It was ground and sieved before used. The data searched the leaching rates for several

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radionuclides, which include Cs-134, Mn-54, Co-60, Zn-65, and Eu-152. Released data indicate that only one of the formulations showed the leaching of Cs-134 from the solidified spent resins. In addition, there were no other radionuclides, such as Mn-54, Co-60, Zn-65, and Eu-152, detected in any leachates [33]. Cement matrices have been applied for many years in different areas for the immobilization of hazardous waste materials, especially categories of low- and intermediate-level radioactive wastes, aiming to ensure safe handling, transport, storage, and final disposal [34]. The pronounced high porosity of the native hard cement matrix facilitates water intrusion, leading to significantly elevated leach rates of the radionuclide contaminants to the surrounding ecosystem. To overcome this serious drawback, polymer
cement composites have been developed as candidate matrices for incorporating radioactive waste efficiently. Polymers as part of the composite have been applied, chiefly, for improving the mechanical properties and reducing the leachability of radioactive wastes and enhancing the waterproofing characteristics of mortars and cement matrices. The properties of polymer-modified mortar depend significantly on the polymer content or polymer to cement ratio, that is, the mass ratio of the amount of polymer solids in a polymer-based admixture to the amount of cement in a polymer-modified cement [35,36,20]. The most common ways to add polymers to CBMs are: (1) in bulk where the polymer is used as a solid additive; (2) dissolved in water; (3) as a repair/impregnation material; and (4) as fibers [37
39]. The nature of the polymer/cement interaction plays a crucial role in the most determinate properties of the final cement
polymer composites (CPCs) [40], in addition to their chemical resistance [41]. The addition of polyvinyl alcohol (PVA) polymer to silica fume-modified OPC generates a CPC with enhanced compressive strength and lower porosity compared to the native cement matrix. The increase in strength can be attributed to the interaction of PVA with OPC that leads to elaboration of somewhat new components that fill the cement pores and enhance the bond between the cement ingredients. This filling component can act, also, as an additional diffusion barrier for the chemisorbed radionuclides, and results in a significant enhancement in the immobilization of those hazardous contaminants in the modified polymer cement. Moreover, the pozzolanic reaction of the silica fumes increases the calcium silicate hydrate contents in the hardened composites [40]. Composite materials can be defined as a combination of two or more dissimilar materials, through chemical or mechanical methods. Although they have many advantages in terms of cost-effectiveness and performance in application, the endof-life options for composite materials can be limited. To enhance the cement waste form behavior in aqueous media, during storage or disposal, various additives have been reported [42]. In selecting such admixtures, two issues are created: the first is to reduce the water permeability and porosity of the cement material using commercially available inorganic or organic additives incorporation. The use of inorganic admixtures reduces the release rate of radiocontaminants by selective sorption of contained radio-ions. The second approach, addition of organic polymer, can result in decreasing the porosity and permeability, consequently lower the leaching out of radionuclides from the incorporated radioactive waste materials [43].

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Many published experimental data discussing the applications of OPC, polymer
cement composites, and other CBMs in the field of low- and medium-level active waste have been carried out at our laboratory (Inorganic and Applied Chemistry Unit, Radioisotopes Department, Egyptian Atomic Energy Authority) [12,13,36,44
52]. A slurry
mortar composite was formulated by hydrating the sand
cement mixture with a slurry of partially degraded spinney waste fiber which is an added-value product due to its acceptable mechanical, physical, and thermal stabilities. The advantages of the obtained composite included light weight, acceptable durability, and the capacity to provide certain performance characteristics under specific circumstances. The composite product can be shaped into pipes, flat panels, bricks, and decorative edging (Fig. 18.5A
D). Therefore the mortar composite could be a suitable candidate for utilization as a structural sheathing, flooring, roofing, and decoration material. Moreover, the stated advantages of the composite could make it a candidate as an inert matrix for solidification of low-radioactivity wastes [12]. Saleh and Eskander proposed a mortar composite which was formulated from an admixture of Portland cement and sand then mixed with a slurry of degraded spinney waste fibers as a hydrating agent. Spinney waste fibers are generated during the spinning of cotton raw materials [13]. This waste is mainly cellulose with a low

Figure 18.5 Micrographs of mortar
slurry composite blocks immersed in seawater (A,C,E), groundwater (B,D), or acetic acid (F,G,H).

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hemicellulose and lignin contents. These wastes were subjected to chemical partial degradation based on a wet oxidation process using hydrogen peroxide as an oxidant [44]. The nominated composite was prepared at the laboratory scale. At the end of the setting and curing period, the performance of the obtained composite was evaluated under immersion conditions imitating a flooding scenario that could happen at a radioactive dumping site. Mass changes, porosity, and compressive strength characteristics were evaluated after complete static immersion in three different leachants, namely, acetic acid, groundwater, and seawater for 1 year. Scanning electron microscopy (SEM) and X-ray diffraction were performed to trace and compute the changes that can take place for the proposed composite under flooding conditions. Fig. 18.6 shows a SEM micrograph for the candidate hard composite after immersion in seawater, groundwater, or acetic acid for increasing periods of time. G

G

G

G

G

G

G

Micrograph (A) shows some flat hexagonal (or pseudohexagonal) crystals due to Friedel’s salt in hard blocks immersed in seawater for 1 year. Monograph (B) demonstrates ettringite crystals in the hard composite dipped in groundwater for 1 year. Micrograph (C) discloses the differences in the pore diameter and dormancy for the composite immersed in seawater for 1 year. Micrograph (D) clarifies the pore microstructure computed for small figures for the blocks immersed in groundwater compared to those in seawater. Crystals were present within intergrain spaces. Micrograph (E) illustrates that the Friedel’s salt crystals are growing inside pores after 1 year of immersion in seawater. Micrographs (F and G) exhibit the immersion of the hard mortar composite in acetic acid, the most clear observation is that the sample immersed for 1 month [micrograph (F)], is less dense than that immersed for 1 year [micrograph (G)]. Micrograph (H) shows calcite crystals deposited within the pores after 1 year of immersion in acetic acid.

In addition to SEM, other performed measurements including thermal stability, mechanical integrity, mass loses, and X-ray diffraction, confirmed that the proposed mortar composite based on partial degraded slurry of spinning wastes can be applied as an innovative matrix for the solidification of radioactive wastes. The application of treated plant fibers to replace cement in its hard matrix represents a new concept aimed at strengthening the final cementitious product. The hardened end products can be utilized in the construction field. Therefore introducing wood fibers enhances the bending strength, the energy absorption of the cement paste, an elevation in the cumulative heat of hydration, and the degree of hydration in the cement
wood fiber composite. The elevation in the degree of hydration of the composite can be based on the steric stabilization, and consequently the dispersion of cement particles. The role of the incorporated wood fibers is to create channels for the movement of the water through the hydration products (i.e., high-density CSH) to nonhydrated cement grains which positively affect the hydration process [53]. One of those recent publications described the evaluation of polymer
cement composite based on recycled expanded polystyrene foam waste immobilizing radioactive

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Figure 18.6 Micrographs of mortar
slurry composite blocks immersed in seawater (A,C,E), groundwater (B,D), or acetic acid (F,G,H) [44].

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sulfate waste simulated under freeze
thaw conditions. The study was carried out in laboratory-scale experiments, searching the impact of freeze
thaw cycling between
4 C and 160 C on the properties of the final solid waste form, namely, mass changes, mechanical integrity, bulk density, and apparent porosity after 90 cycles. As the freeze
thaw treatment proceeds for 180 days, gypsium (G) was accumulated as products of the sodium sulfate and ettringite (E) reactions. Gypsite has dense tubular crystals inside the cured specimens in contrast to ettringite which fills the specimen pores (Fig. 18.7A and C). The production of the gypsum burden pressure on the microstructure and consequently producing more pores in the solid composite was accompanied with a detectable decrease in the compressive strength values of the treated blocks after 90 cycles (Fig. 18.7C). Even so, it should be stated that the compressive strength values for the candidate polymer
cement composite incorporating 15% sulfate waste concentration, relative to the dry mass of powder OPC, are still nearly fourfold higher than that recommend by the Nuclear Regulatory Commission (NRC) for transport and disposal requirements (Fig. 18.8) [54,55]. Our most recent publication concerning the formulation of the proposed cement
polymer composite, which comprises OPC and 7% (mass:mass) recycled postconsumer polystyrene foam, to immobilize 15% (mass:mass of CPC) sulfate waste concentrate was generated from pressurized water reactors (PWRs). In addition to the user-friendly and attractive cost-effectiveness advantages, even after complete continuous static submersion intervals of up to 60 weeks, the candidate matrix can fairly maintain its structural and mechanical integrities due to immersion in the three types of water, namely tap, ground, and sea, imitating a flooding incident. This issue involves two safety concepts for disposal sites: reduction in the amount of sulfate ion (SO42
) released from solidified waste forms, and should retard the back release of radionuclides to the surrounding environment even in an incident of water breakthrough at the disposal site. The immersion of the final waste form in the three leachants was accompanied by precipitation of detectable amount of salts in the pores of the waste form.

Figure 18.7 The scanning electron micrographs of the final waste forms subjected to increasing freeze
thaw cycling (CH), portlandite, (E) ettringite, and (G) gypsite: (A) treated for 30 cycles; (B) treated for 60 cycles; and (C) treated for 90 cycles. Source: Reproduced from T.A. Bayoumi, M.E. Tawfik, Immobilization of sulfate waste simulate in polymer—cement composite based on recycled expanded polystyrene foam waste: evaluation of the final waste form under freeze—thaw treatment, 2017. ,https://doi. org/10.1002/pc..

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Compressive strength (Mpa)

35 30 25 20 15 10 5 0 0

15

30

45

60

90

Freeze/thaw cycles

Figure 18.8 The change in compressive strength values of the cement
polymer composite immobilizing sulfate waste simulated as a function of freeze
thaw cycles. Source: Reproduced from T.A. Bayoumi, M.E. Tawfik, Immobilization of sulfate waste simulate in polymer—cement composite based on recycled expanded polystyrene foam waste: evaluation of the final waste form under freeze—thaw treatment, 2017. ,https://doi.org/10.1002/pc..

This can be attributed to internal and/or external sulfate attacks. The amounts of precipitated components depend on the sulfate content in each leachant. The immersion in seawater showed the largest precipitation, compared to ground and tap water (Fig. 18.9A
E). It is worth noting that further cement essential hydration products were regenerated over time, especially in seawater, where portlandite (CH) plate-like crystals were detected (Fig. 18.9E) [56].

18.2.8 Containers for radioactive waste based on innovative cement-based materials The packaging provides an easy and safe step to handle and store radioactive wastes or immobilized waste forms. The container is an essential item in the chain of the multibarrier system in a repository [57]. In addition, it can transform the wastes or immobilized waste into forms which can be transported and disposed of with no further treatment at a future date; the container can reduce the cost and enhance the technical feasibility of the management scheme. The container works, mainly, as a receptacle for the waste forms from the solidification process, and in some cases as a mixing vessel. Handling the waste forms is a means of dealing with radioactive chemically complicated hazardous systems. Therefore, when aiming at improving repository performance, it is preferable to develop the preformed nonradioactive cement containers, rather than working with radioactive complicated waste forms. The cement container is assumed to act as a second barrier, retarding the leaching out of radionuclides from packed waste forms or waste materials to the surrounding ecosystem [43].

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Figure 18.9 Scanning electron microscopes of cement
polymer composites (CPCs), which comprised OPC and 7% (mass:mass) recycled postconsumer polystyrene foam, to immobilize 15% (mass:mass of CPC) sulfate waste concentrate simulated during immersion in: (A) seawater for 270 days; (B) tap water for 420 days; (C) groundwater for 420 days; and (D, E) 420 days in seawater [56].

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A study carried out in Ref. [43], to improve the cement container as a nonradioactive barrier, aimed at enhancing its retention capability for the radioactive nuclides in borate waste simulates. Borate waste concentrate is generated from the primary cooling circuit of PWRs [58]. Experimental data were collected by introducing predetermined weights of spiked powder borate waste simulate into cement containers. The containers were closed with a cement cover and completely immersed in groundwater leachate. To reduce the leaching rate of radionuclides through the cement containers, additives supposed to affect the container confinement efficiency were utilized, for example, organic additives, inorganic additives, and chemical treatment. Fig. 18.10 presents a schematic diagram of the container used. Three local clays were used to evaluate their suitability as inorganic additives to produce innovative cement containers. The inorganic additives, namely, natural clay, bentonite, or kaolin, were mixed with cement paste which was then molded as a container. Polymethyl methacrylate (PMMA) and unsaturated styrenated polyester (PE) were used as organic additives for the container construction. Two techniques were applied for the addition of organic polymers based on the polymer viscosity. The low-density methyl methacrylate was introduced into an already hard cement container using an impregnation technique based on heat and suction. On the other hand, the high-density styrenated PE premix was mixed with plain cement paste before molding into a container form. Cobalt hexacyanoferrate solutions were prepared with increasing concentration and used as chemical treatment agents for the production of innovative cement containers. Three methods were carried out to add the cobalt hexacyanoferrate solutions: first, soaking; second, impregnating of the previously prepared hard cement containers; or, third, mixing the solution with the cement paste prior to cement setting and molding. The cumulative leach fractions for both cesium-137 and cobalt-60 radionuclides from the various prepared improved innovative cement containers were evaluated and compared with their release from solid immobilized plain cement waste forms or from a plain cement container.

Cover

7.5 cm

Cement-based material

3.5 cm

Figure 18.10 Schematic diagram of the cement container [43].

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The release of radiocesium and radiocobalt in groundwater when using plain cement container was lower than their release from plain cement waste forms, moreover, the leaching of both radionuclides was reduced significantly in applying any of the innovated cement containers compared to applying the plain cement containers for waste conditioning (Figs. 18.11 and 18.12).

Figure 18.11 Cumulative leach fraction of Cs-137 (A) and Co-60 (B) in groundwater versus square root of time in days from the plain cement waste form and plain cement container [43].

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Figure 18.12 Cumulative leach fraction of radiocobalt as a function of square root of time from innovative cement-based containers modified with (A) natural clay, (B) cobalt hexacyanoferrate, and (C) polymethyl methacrylate [43].

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Concrete containing increasing ratios of natural zeolite rock is utilized for casting the secondary engineering barriers in the disposal site, aiming as retarding the leaching back of radionuclides contaminants into the final protection barrier. The addition of natural zeolite is accompanied with decreases in both compressive and flexural strength values. On the other hand, thermal conductivity and specific heat capacity are enhanced by natural zeolite rock addition and record lower values relative to the plain concrete. Therefore this new product is a candidate for engineered vault construction [59].

18.2.9 Concrete based on innovative cement-based materials applied for disposal sites A system of multiple barriers that provides optimal assurance of isolation of the ecosystem is required for the disposal of radioactive wastes. The overall safety against release and spreading of hazardous radionuclides requires proper selection of the waste form, a suitable engineered barrier, back filling, and the properties of the geo-ecosystem of the disposal site. Fig. 18.13 presents a schematic diagram for the multibarrier disposal concept. Many innovative designs for disposal repositories should use extensively hydraulic cements and/or concretes, that is, cement materials or innovative cement materials were applied for the different barriers, namely, waste matrix, container, vaults, and backfill. Collier et al. [60] applied cementitious grouts based on class G oil well cement as an inert matrix for securing a low-temperature, sealing and support matrix (SSM) for deep borehole disposal (DBD) [60]. This matrix can seal containers incorporating high-level radioactive waste within the disposal zone located in the bottom B2 km of holes drilled into the basement rock of the continental crust. Partial substitution of the cement with silica flour is practiced to reduce the Ca/Si ratio, to ensure the formation

Figure 18.13 A schematic diagram for a multibarrier disposal concept.

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of the most durable cement hydrate phases. According to these authors, over the longer term, the seal around the waste packages had to be maintained after the boreholes were sealed at the surface, and this could increase the safety of the DBD. A multibarrier container simulating a disposal site (Fig. 18.14) was subjected to a groundwater flooding scenario, and was completely immersed for more than 10 years. The data recorded for interval radioanalytical measurements for the Cs-137 and Co-60 illustrated that the total activity measures were in the range of the local background measurements. This confirmed the barrier capability of the final cemented waste form, the confinement natural clay filling layer, the second cement barrier and the outer container cement wall to condition, solidify, and stabilize the radiocontaminants and retard their release to the surrounding area even after 10 years. Moreover, Monte Carlo modeling was applied to evaluate the radiation exposure due to the contained radionuclides for long-term storage [29]. The results obtained showed that the photon flux was diminished rapidly after passing the cement waste form and clay layer. This approved the important role of cementitious products as shielding materials against dangerous radiation during handling, transportation, and final disposal of radioactive wastes.

18.3

Conclusions

Evidently, cement and CBMs contribute, to a significant extent, to social progress, economic growth, and environmental protection. The cement industry and its applications will be used to aid in two of the most pressing requirements for human society, namely, the infrastructural needs for elevating urbanization and industrialization of the world, and more importantly, the protection and restoration of the human environment. Industrial by-products and various postconsumer wastes have been utilized in CBMs and concrete manufactures to produce “green products.” Old tires, spent plastics, agriculture remains, and wood fibers can also be applied. Recycling of these byproducts and wastes is now a well-organized processes. Recycling acts to minimize solid waste disposal, improve air quality, reduce solid waste accumulation, and, consequentially, results in a sustainable concrete and cement industry. The extra cost, if incurred, can be offset by utilizing less native OPC, applying less water for production, providing innovative chemical admixtures, a reduction in global GHG air emissions, and moreover producing durable, specific, and high-quality aggregates. Due to the large size of the concrete industry, it is, undoubtedly, considered the ideal field for the economic and safe investment of billions of tons of industrial and waste additive by-products due to their highly pozzolanic and other optimal cementitious properties. It is clear that large-scale plain cement substitution (60%
70%) in concrete and other cementitious products with these industrial by-products will be advantageous concerning energy efficiency, cost economy, durability, and overall ecological gains. Therefore, in the near future, the utilization of by-products supplementary to cementing materials will be a necessary and urgent requirement.

Innovative cement-based materials for environmental protection and restoration

Figure 18.14 Schematic diagram of the multibarrier container and its dimensions.

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An alternative approach to the traditional applications of innovative CBMs for construction is their applications in the field of radioactive waste management and more specifically in the incorporation, solidification/stabilization, and conditioning of low- and medium-level radioactive wastes, in addition to engineered vault preparation. The application of cement and concrete to immobilize radioactive waste is a complex process due to the wide-spectrum nature of inorganic cementing components available, in addition to the scope of service environments in which cement is applied and the different functions offered by cement. For example, Portland cement-based concretes and/or plain cement are widely used as structural materials in the construction of vaults and tunnels. Moreover, cement and concrete are applied to form backfills as impermeable barriers retarding radionuclide release and reduce intrusion of groundwater to the disposal site. Portland cement has been used in a wide range of areas to encapsulate radioactive wastes for storage, transport, and as a radiation shield. It has also been applied to form and stabilize vaults and silo structures. Compared to other inert matrices, cement is characterized by chemical immobilization potential, binding and reacting with many radionuclides. Cement minerals undergo internal aging and slow reaction with other materials in the near field, hence the immobilization and isolation potentials of plain cement can be changed. Therefore innovative CBMs have been developed to replace plain cement in some applications. The authors hope that the fruits of the recent innovations for local, regional, and global environments will be harvested in the very near future.

References [1] IAEA, Handling and Processing of Radioactive Waste from Nuclear Applications, Technical Reports Series, International Atomic Energy Agency, Vienna, 2001. [2] I. Engkvist, Y. Albinsson, E. Johansson, The Long-Term Stability of Cement-Leaching Tests, Swedish Nuclear Fuel and Waste Management Co, 1996. [3] A.M. Neville, J.B. Jeffrey, Concrete Technology. Longman Scientific & Technical, England, 1987. [4] F. Tomei, P. Temkar, Solidification Technologies for Restoration of Sites Contaminated With Hazardous Wastes, Army Environmental Policy Inst., Arlington, VA, 1998. [5] A. Morbi, S. Cangiano, E. Borgarello, Cement based materials for sustainable development, in: Second International Conference on Sustainable Construction Materials and Technologies, Coventry University and the University of Wisconsin Milwaukee Center for by-Products Utilization, 2010. [6] V.M. Malhotra, Role of Supplementary Cementing Materials and Superplasticisers in Reducing Greenhouse Gas Emissions, ICFRC, 2004. [7] J. Denison, C. Halligan, Building Materials and the Environment. Stephen Georg. Partners LLP, 2010. [8] E. Worrell, C. Galitsky, L. Price, Energy efficiency improvement and cost saving opportunities for cement making. LBNL-54036-Revision. Ernest Orlando Lawrence Berkeley Natl. Lab. University of California, March 2008.

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[9] T.R. Naik, G. Moriconi, Environmental-friendly durable concrete made with recycled materials for sustainable concrete construction, in: International Symposium on Sustainable Development of Cement, Concrete and Concrete Structures, Toronto, ON, Citeseer, October 2005, pp. 5
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56, 2002. [16] McWilliams, The Maturing Nanotechnology Market: Products and Applications, BCC Res. Wellesley, MA, 2016. [17] NPD, Introduction [WWW Document]. ,https://product.statnano.com/., 2019. [18] E. Duque-Redondo, Y. Kazuo, I. Lo´pez-Arbeloa, H. Manzano, Cs-137 immobilization in CSH gel nanopores, Phys. Chem. Chem. Phys. 20 (2018) 9289
9297. [19] S.B. Eskander, S.M.A. Aziz, H. El-didamony, M.I. Sayed, Immobilization of low and intermediate level of organic radioactive wastes in cement matrices, J. Hazard. Mater. 190 (2011) 969
979. Available from: https://doi.org/10.1016/j.jhazmat.2011.04.036. [20] S.B. Eskander, M.E. Tawfik, T.A. Bayoumi, Immobilization of borate waste simulate in cement-water extended polyester composite based on poly(ethylene terephthalate) waste: 2-frost resistance of the polymer modified cement composite, Polym. Plast. Technol. Eng. 45 (2006) 939
945. Available from: https://doi.org/10.1080/ 03602550600723415. [21] M. Brownstein, G. Duratek, Radioactive waste solidification, ASME RW Syst. Comm. Radwaste Short Course (1991) 1
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448. Available from: https://doi.org/10.1016/j.jhazmat.2005.11.053. [31] N. Yanikomer, S. Asal, S. Haciyakupoglu, S.A. Erenturk, New solidification materials in nuclear waste management, Intl. J. Engn. Tech. 2(2) (2016) 76
82. [32] NEA, R&D and Innovation Needs for Decommissioning Nuclear Facilities, NEA No. 71. ed. Nuclear Energy Agency (NEA), 2014. [33] Z. Laili, M.S. Yasir, M.A. Wahab, Solidification of radioactive waste resins using cement mixed with organic material, AIP Conference Proceedings, vol. 1659. No. 1. AIP Publishing LLC, 2015. [34] IAEA, Development of Specifications for Radioactive Waste Packages IAEATECDOC-1515, IAEA, VIENNA, 2006. [35] F. Glasser, Application of inorganic cements to the conditioning and immobilisation of radioactive wastes, Handbook of Advanced Radioactive Waste Conditioning Technologies, Elsevier, 2011, pp. 67
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polymer composite immobilizing borate waste simulates during flooding scenarios, J. Nucl. Mater. 420 (2012) 175
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1598. [38] M.E. Tawfik, S.B. Eskander, T.A. Bayoumi, Immobilization of borate waste simulate in cement-water extended polyester composite based on polyethylene terephthalate waste 1-mechanical properties of the final waste forms, Polym. Plast. Technol. Eng. 44 (2005) 1355
1368. [39] H. Toutanji, N. Delatte, S. Aggoun, R. Duval, A. Danson, Effect of supplementary cementitious materials on the compressive strength and durability of short-term cured concrete, Cem. Concr. Res. 34 (2004) 311
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level radioactive wastes, Ain Shams University of Cairo, 1997. [44] S.B. Eskander, H.M. Saleh, H.M. Fahmy, Incorporation of the spinning wastes in cement and mortars, J. Radiat. Res. Appl. Sci. 2 (2009) 119
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PET composite waste form containing radioactive borate waste simulates, J. Nucl. Mater. 411 (2011) 185
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541. Available from: https://doi.org/10.1016/j. jclepro.2018.08.303. [50] H.M. Saleh, S.M. El-sheikh, E.E. Elshereafy, A.K. Essa, Mechanical and physical characterization of cement reinforced by iron slag and titanate nanofibers to produce advanced containment for radioactive waste, Constr. Build. Mater. 200 (2019) 135
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530. [57] IAEA, Packaging of radioactive wastes for sea disposal, Report of a Technical Committee Meeting, Vienna, December 3
7, 1979. TECDOC-240, 1981. [58] T.A. Bayoumi, Treatment and Solidification of Medium Active Waste (MS thesis), Ain Shams University, Faculty of Science, Cairo, Egypt, 1990. [59] E. Vejmelkova, M. Cachova, L. Scheinherrova, P. Konvalinka, M. Keppert, P. Bezdicka, R. Cerny, Mechanical and thermal properties of concrete suitable for radioactive waste disposal sites, in: IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2018, p. 12061. [60] N.C. Collier, N.B. Milestone, K.P. Travis, A review of potential cementing systems for sealing and support matrices in deep borehole disposal of radioactive waste, Energies 12 (2019) 2393.

Comparative effects of using recycled CFRP and GFRP fibers on fresh- and hardened-state properties of self-compacting concretes: a review

19

M. Mastali1, Z. Abdollahnejad1, A. Dalvand2, A. Sattarifard3 and M. Illikainen1 1 Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland, 2Department of Engineering, Lorestan University, Khorramabad, Iran, 3 Faculty of Civil Engineering, Semnan University, Semnan, Iran

19.1

Introduction

Hardened-state properties of plain concrete, like compressive strength, tensile strength, flexural strength, impact resistance, absorption strength, toughness, and load-carrying capacity have high potential to be improved by the addition of different contents and types of fibers [1 3]. Moreover, reinforcement of the compositions has positive effects on controlling the negative effects of shrinkage, which enhances the durability characteristics [4 7]. The efficiency of fiber reinforcement in enhancing hardened-state properties and limiting the negative aspects of shrinkage is greatly governed by fiber type and dosage. The production of industrial fibers results in increasing the carbon footprint, cost, and consumption of raw materials. Therefore using recyclable waste materials in different products becomes favorable because it is more economical and environment-friendly. Significant studies have been executed in using recycled materials in concrete in different ways, such as reinforcement of concrete with recycled fibers or using waste materials as powder to replace ordinary Portland cement (OPC) [8 12]. Using fiber reinforced polymer (FRP) materials has greatly increased in industry, construction, and transportation due to its light weight, high stiffness, high tensile strength, good durability resistance, and high temperature resistance. The most commonly used fibers in FRP materials are glass, carbon, aramid, or basalt. Therefore the mechanical and physical properties of FRP materials are significantly affected by the fibers that are used.

New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00019-3 © 2020 Elsevier Inc. All rights reserved.

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Recently, many products have been manufactured by FRP materials in the construction industry, such as sheets for strengthening structural elements (like beams, columns, and slabs), bars to reinforce concrete due to good properties of FRP materials against corrosion, structural profiles, and sandwich panels [13]. The production process usually results in some waste FRP materials, which should be recycled through material recycling, chemical recycling, or thermal recycling. Otherwise, they are presumed to be buried in the ground or remain unusable. It is worth stating that landfilling of these materials is forbidden in many countries, therefore recycling and reusing these materials could be an effective and economic solution. Using carbon FRP (CFRP) leads to improved mechanical properties of the reinforced or repaired components. However, due to its high costs, materials reinforced with virgin CFRP are not as likely to be used in many different products. In addition, the recycling and reusing of CFRP materials are large problems due to its hardness and chemical stability. A device is used to cut large-scale FRP wastes into small FRP pieces in the material recycling method. In comparison to cutting of other waste materials, cutting larger scale FRP waste is a more difficult process due to the high mechanical properties of FRP materials. It is estimated that 5000 tons of “r-CFRP” are wasted annually, which are presumed to be largely buried or discarded. About 50% 70% of the residue materials of incinerated FRP materials are minerals and ashes, which still require to be landfilled. Except for some researches on a laboratory scale, none of the cementitious products which use recycled CFRP have been studied [14], and studies regarding reinforced concrete with recycled CFRP wastes are quite limited. In 2005, Keiji et al. reinforced plain concrete with crushed CFRP pieces, using three different fiber lengths of 3, 10, and 20 mm and three different fiber-to-cement ratios (5%, 7.5%, and 10%) [14]. Their results showed that the efficiency of using CFRP pieces was significantly altered by the length of the recycled fibers. Mastali et al. demonstrated that the findings for recycled CFRP fibers are also validated for reinforced concrete with recycled glass FRP (GFRP) fibers [15]. Similar to CFRP sheets, GFRP sheets are also used widely in the polymer and construction industries, however GFRP sheets have lower costs compared to CFRP sheets. Moreover, in general, CFRP fibers have greater mechanical characterizations than GFRP fibers. Therefore it can also be expected to have a significant amount of unusable content of this fiber, which highlights the importance of recycling this fiber type. Therefore, regarding the growth trends in producing CFRP and GFRP fibers and the importance of clarifying the effects of fiber lengths and dosages on fresh- and hardened-state properties of concrete, this chapter reports the comparative effects of using recycled CFRP and GFRP fibers in different fiber volume fractions (0.5%, 1%, 1.5%, and 2%) and with differing lengths (10, 20, and 30 mm) on the freshand hardened-state properties of fiber reinforced self-compacting concrete (SCC). To execute this comparative study, fresh-state properties were evaluated by workability indexes such as slump flow diameter, T500, and V-funnel. Moreover, hardened-state properties were addressed in terms of compressive and flexural strengths, and impact resistance.

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19.2

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Experimental plan

19.2.1 Materials and mix designs The mixtures were prepared by combining OPC, fly ash, sand, water, and superplasticizer. The chemical composition and physical properties of the used OPC and fly ash are listed in Table 19.1. Regarding the chemical analysis in Table 19.1 and the ASTM C 618 recommendation, the used fly ash could be categorized as F class [16]. Superplasticizer was used to adjust the workability of fresh-state properties, so that the agent used can decrease the amount of water used by 10% 15% at small dosage rates, while reductions of up to 30% can be achieved at high dosages. The size of sand particles varied in the range of 0.2 4.76 mm. The mix composition was designed based on obtaining a slump flow diameter equal to or greater than 600 mm (without addition of fiber) and compressive strength of 50 MPa for the plain SCC. According to the fiber lengths and dosages, 13 different mix compositions could be designed for each fiber type. The proposed mixtures are listed in Table 19.2. Both recycled GFRP and CFRP fibers were recovered from leftover unusable sheets. Using a shredding machine, the sheets were cut into small fibers of average lengths of 10, 20, and 30 mm. The tensile strength, elastic modulus, and density of the GFRP sheets used to produce the recycled glass fibers were 1103 MPa, 44.8 GPa, and 2080 kg/m3, respectively. Moreover, the tensile strength, elastic modulus, and density of the CFRP fibers were 3550 MPa, 235 GPa, and 1550 kg/m3, respectively. The length characterization of recycled fibers consisted of a statistical analysis to evaluate the variability of the fiber length after the shredding process. The Table 19.1 Chemical composition and physical properties of OPC and fly ash [15,17]. Composition SiO2 Al2O3 Fe2O3 MgO K2O Na2O CaO TiO2 SO3 C3S C2S C3A C4AF

OPC (%)

Fly ash (%)

21.10 4.37 3.88 1.56 0.52 0.39 63.33

72.10 24.70 1.20 0.18 0.50 0.10 0.10 1.40 # 0.10

51.00 22.70 5.10 11.90

Physical properties Specific gravity Specific surface (cm2/g)

3.11 3000

2.30 3430

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Table 19.2 The proportions of the materials used in the mix compositions (kg/m3) [15,17]. Designation of specimens

Reference (F0L0) F0.5L10 F1L10 F1.5L10 F2L10 F0.5L20 F1L20 F1.5L20 F2L20 F0.5L30 F1L30 F1.5L30 F2L30

OPC

457

Fly ash

457

Sand

457

SP

2.74

Water

347

Recycled fiber Volume of fractions (%)

Length (mm)

0

0

0.5 1 1.5 2 0.5 1 1.5 2 0.5 1 1.5 2

10 10 10 10 20 20 20 20 30 30 30 30

average fiber lengths of recycled fibers were calculated using 100 fibers. More details can be found elsewhere [18 21]. To compare the efficiency of recycled CFRP and GFRP fibers, similar cementitious compositions were reinforced. In the batching process, OPC, fly ash, and sand were stirred for 1 minute. Then, a combined superplasticizer and water were added and the composition was mixed for a further 4 minutes. Finally, fibers were gradually added to the fresh mixture, while mixing was continued for another 2 minutes to avoid unfavorable effects such as fiber balling. The fresh mortars were cast into cubic molds with dimensions of 100 3 100 3 100 mm3, prismatic beams with dimensions of 420 3 80 3 60 mm3, and cylindrical disk molds with dimensions of 150 3 65 mm2 to evaluate the compressive and flexural strengths, and impact resistance of specimens, respectively. The results for strength obtained were presented based on averaging three replicated specimens tested. Due to the additional uncertainties involved in the impact test including the surface roughness and loading position, four disks were tested under impact test.

19.2.2 Test procedure V-funnel and slump flow tests (diameter and time) were carried out based on EFNARC and ACI 237R [22,23] to assess fresh-state properties. Slump flow time and diameter are two common methods to characterize flowability on a horizontal surface [24]. In total, 75 cubic specimens with dimensions of 100 3 100 3 100 mm3 were cast, prepared, and tested based on the ASTM C39 recommendation to evaluate the

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compressive strength [25]. A testing machine with a load capacity of 1000 kN was used to apply the compressive load, with a loading rate of 0.30 MPa/s. Moreover, a three-point bending test was carried out to evaluate and clarify the efficiency of different recycled fibers on the flexural strength. Thus, 75 prismatic beams with dimensions of 60 3 80 3 420 mm3 were tested by imposing a load (with a load-carrying capacity of 50 kN) to the mid-span of beams under displacement control. This evaluation was implemented in accordance with the ASTM C78 recommendations [26]. A repeated drop weight impact test was adopted based on ACI Committee 544 recommendations [27]. One hundred disks with diameters of 150 mm and height of 65 mm were tested under the impact test. A steel hammer with a mass of 4.45 kg was dropped from a height of 457 mm on a steel ball with a diameter of 63.5 mm located on the central surface of the cylindrical specimens.

19.3

Results and discussion

19.3.1 Fresh-state properties Characterizations of reinforced SCC mixtures in fresh state are depicted in Fig. 19.1. Regarding the results in Fig. 19.1A, increasing fiber dosage and length consistently reduced the flow diameter, regardless of fiber type. It was revealed that recycled GFRP fibers had a higher impact on reducing workability compared to recycled CFRP fibers. This could be justified by lower water absorption and moisture absorption of CFRP fibers than GFRP fibers [28,29]. Additionally, it was observed in Fig. 19.1B and C that introducing fibers influences the viscosity and time of flowability. Due to the absorption of a higher content of water by GFRP fibers, the reinforced mix compositions with GFRP fibers indicated higher viscosity and, consequently, higher time of flowability than the reinforced mixtures with CFRP fibers. In general, differences in fresh-state properties due to the use of different fiber characterizations were ignorable (under 1%). Moreover, it was detected that the fiber dosage had a higher impact on fresh characterizations of mortars than fiber length.

19.3.2 Hardened-state properties Fig. 19.2 illustrates the results obtained for hardened-state properties of the plain and fiber reinforced concretes. Fig. 19.2A shows the impact of using recycled fibers on the compressive strength. The effect of reinforcement on compressive strength is not straightforward. The addition of fibers could increase the porosity, which reduces the compressive strength. In contrast, fibers provide a bridging action to transfer the tensile stress across the fracture surface and arrest further crack opening, which could improve the compressive strength. Therefore the introduction of fibers either increases or decreases the strength.

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Figure 19.1 (A) Slump flow diameter; (B) T500; (C) V-funnel.

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Figure 19.2 ( A) Compressive strength; (B) flexural strength; (C) first crack impact resistance; (D) ultimate crack impact resistance.

As shown in Fig. 19.2A, reinforcement increased the compressive strength due to the capacity of fibers to arrest further crack opening [30 35]. Regarding the results, recycled CFRP fibers had a slightly higher impact on increasing the compressive strength than recycled GFRP fibers. This could be explained by providing better bond properties at the interfacial transition zone (ITZ) between fiber/matrix for recycled CFRP fibers compared to recycled GFRP fibers. According to the results, the addition of fibers increased, in the maximum case, 50% compressive strength compared to the plain concrete. Regardless of the recycled fiber type, the maximum enhancement was recorded in the mix compositions containing 2% recycled fiber with a length of 20 mm. Similar to the results obtained for the compressive strength, introducing fibers improved the flexural strength, so that the maximum increase was noticed as more than 60% in the reinforced mixtures with 2% fiber and 20 mm fiber length compared to the plain mixture. This comes from the fiber bridging action. Similar to fresh-state properties, fiber dosage has a much greater impact than fiber length on mechanical characterizations. This result can be expressed by the number of bridging actions which were provided by the fibers. Fig. 19.2C and D depict the impact performance of fiber reinforced disks under impact loadings. Comparing the effects of fibers on improving mechanical characterizations and impact resistance demonstrated that recycled polymer fibers have much greater influence on the impact resistance of the reinforced disks than mechanical properties. Reinforcing the mixtures could improve the first and ultimate crack resistance up to four- and fivefold, respectively. Moreover, it was illustrated that recycled fibers have a greater influence on ultimate crack impact

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resistance than the first impact resistance. The differences between the obtained results for the mechanical properties of mixtures reinforced with recycled CFRP and GFRP fibers were not significant and were lower than 1%, while this difference for impact resistance of disks was lower than 20%. Therefore microstructural analysis was carried out to justify the differences in the achieved results. Fig. 19.3 shows the morphology of the embedded recycled fibers in the matrix. Regarding the scanning electron microscopy (SEM) images, recycled CFRP and GFRP fibers were debonded and ruptured, respectively. Debonding recycled CFRP fibers showed better mechanical and impact properties than recycled GFRP fibers. Moreover, recycled CFRP fibers were fully covered by the hydration products.

19.4

Analysis

Some equations were derived, as shown in Fig. 19.4, where the empirical equations derived based on the linear regression analysis compare and predict hardened properties to fiber content. As shown in Fig. 19.4, the correlation of the experimental results and equations could be determined by the coefficient of determination (R2). The maximum gain rates of the mechanical properties and impact resistance of the reinforced specimens were evaluated by the slopes of the developed equations. Regardless of the recycled fiber type in Fig. 19.4A and B, increasing the fiber length increased the rate of increase of the compressive strength with fiber content. On the other hand, the maximum gain rate of the compressive strength was detected

Figure 19.3 Fiber failure for embedded recycled: (A) CFRP fibers [17]; (B) GFRP fibers [15].

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Figure 19.4 Correlation between: (A) compressive strength and recycled CFRP fiber; (B) compressive strength and recycled GFRP fiber; (C) flexural strength and recycled CFRP fiber; (D) flexural strength and recycled GFRP fiber; (E) the first impact resistance and recycled CFRP fiber; (F) the first impact resistance and recycled GFRP fiber; (G) ultimate impact resistance and recycled CFRP fiber; (H) ultimate impact resistance and recycled GFRP fiber (Fc, Compressive strength; Fr, flexural strength; Vf, fiber content; FC, first crack impact resistance; UC, ultimate crack impact resistance).

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by the reinforced mixtures with 30 mm. Comparing the results revealed that recycled CFRP fibers had higher gain rates of compressive strength than recycled GFRP fibers. For the flexural strength, a higher gain rate of flexural strength was found for the reinforced specimens with recycled GFRP fibers than recycled CFRP fiber. Introduction of recycled CFRP fibers indicated higher rates of enhancement for the impact resistance compared to recycled GFRP fibers. Interestingly, it was noticed that the efficiency of recycled polymer fibers in improving ultimate crack impact resistance was higher than the first crack impact resistance. Additionally, according to the results, the maximum rate of gaining strength and impact resistance due to use of recycled polymer fibers was recorded in the mixtures containing a fiber length of 30 mm. Regardless of the recycled polymer fiber dosage, length, and type, the mechanical and impact properties were correlated in Fig. 19.5. It was noticed that there was good agreement between mechanical and flexural strengths, and the flexural strength was linearly increased by increasing the compressive strength (R2 $ 0.94). Moreover, the highest correlation was observed between the first and ultimate crack impact resistance (R2 $ 0.99).

19.5

Conclusions

This chapter has presented an extensive experimental campaign, including fresh and hardened properties. This study aimed to investigate the comparative effects of using different recycled CFRP and GFRP fiber lengths and dosages on the fresh and hardened properties of SCC. In the first stage of this study, the impacts of using different fiber types, lengths, and contents on fresh- and hardened-state properties were assayed. The collected experimental data were then used for an analysis, based on which following remarks can be highlighted: 1. Regardless of recycled polymer fiber types, increasing fiber length and content degraded the flowability characteristics and improved the mechanical and impact properties. 2. Due to the use of recycled CFRP and GFRP fibers, the differences in fresh-state properties and mechanical characterizations were ignorable, as these differences were significant for the impact resistance. 3. Fiber dosage has a much greater impact on fresh- and hardened-state properties than fiber length. 4. Microstructural analysis indicated recycled CFRP fibers predominantly failed by debonding, while recycled GFRP fibers were ruptured. 5. The maximum rate of enhancing the mechanical and impact properties was achieved in the reinforced specimens with fiber length of 30 mm. 6. Recycled CFRP fibers had higher gain rates of compressive strength than recycled GFRP fibers, while GFRP fibers had greater increasing rates of flexural strength than recycled CFRP fibers. 7. Compressive strength and impact resistance of the reinforced specimens were correlated linearly, including coefficient of determination (R2) higher than 87%.

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Figure 19.5 Correlation between: (A) flexural and compressive strengths; (B) compressive and the first crack impact resistance; (C) the first and ultimate crack impact resistance.

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References [1] Faiz U.A.S., Review of mechanical properties of short fibre reinforced geopolymer composites, Constr. Build. Mater. 43 (2013) 37 49. [2] Y. Mohammadi, S.P. Singh, S.K. Kaushik, Properties of steel fibrous concrete containing mixed fibres in fresh and hardened state, Constr. Build. Mater. 22 (2008) 956 965. [3] S. Iqbal, A. Ali, K. Holschemacher, T.A. Bier, Mechanical properties of steel fiber reinforced high strength lightweight self-compacting concrete (SHLSCC), Constr. Build. Mater. 98 (2015) 325 333. [4] F. Pacheco-Torgal, Z. Abdollahnejad, S. Miraldo, S. Baklouti, Y. Ding, An overview on the potential of geopolymers for concrete infrastructure rehabilitation, Constr. Build. Mater. 36 (2012) 1053 1058. [5] W.-C. Choi, H.-D. Yun, J.-W. Kang, S.-W. Kim, Development of recycled strainhardening cement-based composite (SHCC) for sustainable infrastructures”, Compos. Part B: Eng. 43 (2012) 627 635. [6] Z. Abdollahnejad, F. Pacheco-Torgal, T. Fe´lix, W. Tahri, J. Barroso Aguiar, Mix design, properties and cost analysis of fly ash-based geopolymer foam, Constr. Build. Mater. 80 (2015) 18 30. [7] T.M. Grabois, G. Chagas Cordeiro, R. Dias Toledo Filho, Fresh and hardened-state properties of self-compacting lightweight concrete reinforced with steel fibers, Constr. Build. Mater. 104 (2016) 284 292. [8] X. Jianzhuang, L. Li, L. Shen, C.S. Poon, Compressive behaviour of recycled aggregate concrete under impact loading, Cem. Concr. Res. 71 (2015) 46 55. [9] D. Foti, Preliminary analysis of concrete reinforced with waste bottles PET fibers, Constr. Build. Mater. 25 (2011) 1906 1915. [10] D. Foti, Use of recycled waste pet bottles fibers for the reinforcement of concrete, Compos. Struct. 96 (2013) 396 404. [11] N.M. Ghasemi, M.K. Sharbatdar, M. Mastali, Repairing reinforced concrete slabs using composite layers, Mater. Des. 58 (2014) 136 144. [12] Z. Abdollahnejad, M. Mastali, M. Mastali, A. Dalvand, A comparative study on the effects of recycled glass fiber on drying shrinkage rate and mechanical properties of the self-compacting concrete and fly ash/slag geopolymer concrete, J. Mater. Civ. Eng. (2017). Available from: https://doi.org/10.1061/(ASCE)MT.1943-5533.0001918. [13] M. Mastali, I.B. Valente, J. A.O. Barros, D. M.F. Gonc¸alves, Development of innovative hybrid sandwich panel slabs: experimental results, Compos. Struct. 133 (2015) 476 498. [14] O. Keiji, S. Tomoyuki, M. Makoto, Strength in concrete reinforced with recycled CFRP pieces, Compos. Part. A: Appl. Sci. Manuf. 36 (2005) 893 902. [15] M. Mastali, A. Dalvand, A.R. Sattarifard, Z. Abdollahnejad, Effects of using recycled glass fibers with different lengths and dosages on fresh and hardened properties of selfcompacting concrete (SCC), Mag. Concr. Res. 70 (22) (2018) 1175 1188. [16] ASTM C618-17a, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM International, West Conshohocken, PA, 2017. Available from: www.astm.org. [17] M. Mastali, A. Dalvand, A. Satarifard, The impact resistance and mechanical properties of the reinforced self-compacting concrete incorporating recycled CFRP fiber with different lengths and dosages, Compos. Part. B: Eng. 112 (2017) 74 92.

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[18] E. Martinelli, A. Caggiano, H. Xargay, An experimental study on the post-cracking behaviour of hybrid industrial/recycled steel fibre-reinforced concrete, Constr. Build. Mater. 94 (2015) 290 298. [19] A.M. Brandt, J. Olek, M.A. Glinicki, C.k.Y. Leung, Brittle matrix composites 10, in: Institute of Fundamental Technological Research Polish Academy of Sciences, 2012, pp. 346 347. [20] V.R. Patel, N.S. Darji, D.I.I. Pandya, Experimental study of cracking behaviour for SFRC beams without stirrups with varying A/D ratio, Int. J. Eng. Res. Dev. 1 (2012) 1 5. [21] BS EN 12390-1:2000, Testing hardened concrete. Shape, dimensions and other requirements for specimens and moulds. [22] EFNARC, Specifications and Guidelines for Self-Compacting Concrete, Englished. European Federation for Spec Constr Chem and Concr Syst, 2005. [23] ACI Committee 237, Self-Consolidating Concrete, ACI 237R-07, American Concrete Institute, FarmingtonHills, 2007. [24] S. Nagataki, H. Fujiwara, in: V.M. Malhotra (Ed.), Self-Compacting Property of Highly-Flowable Concrete, American Concrete Institute, 1995, pp. 301 314. SP 154. [25] ASTM C39/C39M-18, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, ASTM International, West Conshohocken, PA, 2018. ,www. astm.org.. [26] ASTM C78/C78M-18, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading), ASTM International, West Conshohocken, PA, 2018. ,www.astm.org.. [27] ACI Committee 544, Measurement of properties of fiber reinforced concrete, ACI 555 Mater. J. 85 (1988) 583 593. [28] T.P. Sathishkumar, S. Satheeshkumar, J. Naveen, Glass fiber-reinforced polymer composites—a review, J. Reinf. Plast. Compos. 33 (2014) 1258 1275. [29] K. Tanaka, T. Hanasaki, T. Katayama, Effect of water absorption on the mechanical properties of carbon fiber reinforced polyoxamide composites, Comput. Methods Exp. Meas 55 (2013) 297 305. [30] M. Mastali, Z. Abdollahnejad, F. Pacheco-Torgal, Carbon dioxide sequestration of fly ash alkaline-based mortars containing recycled aggregates and reinforced by hemp fibres, Constr. Build. Mater. 160 (2018) 48 56. [31] M. Mastali, A. Dalvand, A.R. Sattarifard, M. Illikainen, Development of eco-efficient and cost-effective self-consolidation concretes reinforced with hybrid industrial/ recycled steel fibers, Constr. Build. Mater. 166 (2018) 214 226. [32] Z. Abdollahnejad, M. Mastali, T. Luukkonen, P. Kinnunen, M. Illikainen, Fiberreinforced one-part alkali-activated slag/ceramic binders, Ceram. Int. (2018). [33] Z. Abdollahnejad, F. Pacheco-Torgal, J.B. Aguiar, Cost-efficient one-part alkaliactivated mortars with low global warming potential for floor heating systems applications, J. Eur. J. Environ. Civ. Eng. (2016). Available from: https://doi.org/10.1080/ 19648189.2015.1125392. [34] Z. Abdollahnejad, P. Hlavacek, F. Pacheco Torgal, J.B. Aguiar, Compressive strength and microstructure of hybrid alkaline cements, Mater. Res. Ibero-Am. J. Mater. 17 (4) (2014) 829 837. [35] Z. Abdollahnejad, F. Pacheco-Torgal, J.B. Aguiar, C. Jesus, Durability performance of fly ash based one-part geopolymer mortars, Key Eng. Mater. 634 (2014) 113 120.

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B. Bhuvaneshwari1, A. Selvaraj2 and Nagesh R. Iyer3 1 Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, India, 2CBM College, Bharathiar University, Coimbatore, India, 3Fellow, Indian National Academy of Engineering, Dean & Visiting Professor, Indian Institute of Technology, Dharwad, India

20.1

Introduction

As far as the global economy is concerned, the expected cost of corrosion is B2.5 trillion US dollars per year worldwide [1]. Therefore corrosion is considered as one of the main issues which needs strict attention for structures that are prone to its effects. Furthermore, it results in an economic burden and imposes a safety threat to the environment. Corrosion is a perennial problem in many industrial systems and offshore structures, including bridges, docks, and lighthouses. The economic growth of countries is greatly affected by its infrastructural and industrial facilities. For example, buildings, highway and railway bridges, strategic structures, communication towers, power plant structures, etc. come under the infrastructure category and are required to be functional over their life time [2]. In India, the proposed investment by government on infrastructure facilities is about 46,000 billion (10.7% GDP) about 3600 billion of which is meant to meet the housing demand [1]. Usually, these infrastructural and industrial facilities are designed to last over 100 years. Some of these structures remain functional long beyond their design life expectancy. It is well known that India has a number of heritage structures, such as temples, palaces, and buildings/monuments of historical importance, which constitute a significant portion of the national assets. Hence, it is important to ensure the enhanced functionality, serviceability, and life span of these important structures. Due to climatic conditions and other harmful factors, these structures deteriorate with time, resulting in aging of materials, excessive use, and overloading, with unfavorable environmental conditions leading to corrosion, inadequate maintenance, and deficiencies in inspection methods, requiring huge maintenance costs. Nearly 2.5 trillion US dollars per year are being spent to address corrosion-related maintenance[3]. One of the major threats to the durability of any type of concrete structure is concrete corrosion, which is currently one of the major research topics in civil and materials science engineering [4]. Rebar corrosion leads to damage and sudden failure of structures. The chloride content in the cement, water used in preparing concrete, and the environment are the major reasons for rebar corrosion [5]. Many research studies have addressed the problem of corrosion of reinforcing bars New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00020-X © 2020 Elsevier Inc. All rights reserved.

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embedded in concrete. This is necessary to discover the proper solution to enhance the corrosion inhibition of rebars, to increase the lifetime and strength of structures. Also, steel structures placed offshore are subjected to the stresses of a severely corrosive environment [6]. Recently Guerra et al. [6] published the experimental investigations on the atmospheric corrosion of low-carbon steel exposed to coastal zone and the results are based on 1 year evaluation of corrosion data. The surface morphological studies of corroded steel and nature of the corrosion products are confirmed by SEM and XRD methods, respectively. It was concluded that marine aerosols are the main responsible factor for inducing the corrosion of steel. Furthermore, exploring the costeffective environmentally friendly corrosion inhibitors is a main field of interest in civil/structural engineering aimed at sustainability [7].

20.2

What is corrosion?

Corrosion is a complex physicochemical reaction of the environment with materials which degrades the electrical and mechanical functionality of materials. Corrosion has a major impact in designing materials and usage of these materials. Metals corrode when they interact with the environment as they are conductive in nature. Corrosion is a destructive phenomenon that, besides its economic effects, is detrimental to the appearance of metals and in some cases can cause equipment/structural component failure. It occurs in almost all environments. Since corrosion is the destruction of metal or alloy by chemical or electrochemical change, it is apparently preceded by a wide variety of processes. Usually this destruction process is associated with the formation of tarnish or oxide films, when directly combined with gases or liquids in the environment. The mechanisms of corrosion attack have never been fully understood. Past experiences have shown several theories to be reasonable, although without complete answers for all types of corrosion. Usually the corrosion of metals is electrochemical in nature. In the presence of oxygen and/or water, the corrosion process of steel occurs based on the following mechanism [8,9]: Fe ! Fe21 1 2e2

(20.1)

Fe21 ! Fe31 1 e2

(20.2)

O2ðgÞ 1 2H2 O 1 4e2 ! 4OH2

(20.3)

4Fe21 1 O2ðgÞ 1 2H2 O ! 4FeOOH2 1 8H1

(20.4)

Also, possibilities of hydrogen reduction reaction (H1 1 e2 5 0.5 H2) and water reduction (2H2 O 1 2e2 5 H2 1 2OH2 ) occur simultaneously. It is difficult to stop corrosion completely, because of the influence of the thermodynamic process. However, corrosion can be controlled or minimized by to some extent by proper control methods [9].

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Metals corrode because they are used in environments where they are chemically unstable. Only the precious metals such as gold, silver, and platinum are found in nature in their metallic state. As far as the rebar corrosion is concerned, the rate of corrosion depends on several factors such as the type and surface configuration of rebars, type of cement employed in the mortar, dose of concrete, permeability of concrete, presence of cracks and fissures, humidity of concrete, and, most importantly, in the presence of contaminants and aggressive species such as O2 ; Cl2 ; CO2 ; SO2 ; S22 ; SO22 4 , etc., In general, aggressive actions of chloride ions and carbonation of concrete environment are the main factors which cause concrete corrosion. The carbonation reaction changes the pH of the concrete environment, thereby initiating corrosion when the pH shifts to the acidic region. In the case of chloride ion-induced corrosion, the surface of the rebar can be uniform or localized pits, however it damages the steel severely. The popular practice to combat rebar corrosion on bare metals is to add corrosion inhibitors and other additives in a significant way, thereby corrosion attack can be controlled or totally eliminated. The chemistry involved in determining the concrete steel corrosion crosses several disciplinary boundaries [10]. Fig. 20.1 indicates the corrosion induced by the chloride ions on the rebar inside concrete [11]. Hansson et al. [11] presented the effect of inhibitors on corrosion potential and corrosion current density with various possible mechanisms. The suitability of inhibitors for use in the concrete environment were discussed.

Figure 20.1 Chloride-induced rebar corrosion in concrete [11].

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Zhu and Zi [12] predicted the process of steel reinforcement corrosion and concrete damage using a two-dimensional comprehensive model. In this model, the input parameters are mainly based on ingress of chloride ions, electrochemical reaction, carbonation, and corrosion-induced mechanical damage. It is possible to predict the nonuniform distribution of the corrosion product, pressure expansiveness, and the corrosion-induced crack growth. The crack patterns occur on concrete due to corrosion; a model prediction is shown in Fig. 20.2. Comparison has been made between the experimentally observed crack patterns (Fig. 20.2A) and the model which predicted the crack patterns based on the numerical results obtained by their model (Fig. 20.2B). It was concluded that the numerical crack pattern obtained based on their model (Fig. 20.2C) shows significant differences between the uniform displacement-control model and the experimental results.

20.3

Severity of corrosion

The corrosion of metals takes several forms. First, an overall surface attack slowly reduces the thickness or weight of the metal. Second, instead of an overall surface attack, isolated areas may be affected, producing the familiar localized corrosion. Third, it also occurs along grain boundaries or other lines of weakness, because of a difference in resistance to corrosive destruction. Metals and alloys react with

Figure 20.2 Comparison of cracking patterns of concrete cover between the results of (A) experiment, (B) model used by authors, and (C) the uniform displacement-control model [12].

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corrosive media to form a stable compound, which may be called corrosion as a loss of metal occurs and the metal surface becomes corroded. Various methods are used to reduce the corrosion rate of metals and alloys, among which use of a corrosion inhibitor is very popular. Metals and their alloys tend to enter into chemical union with the components of a corrosive medium to form stable compounds similar to those found in nature. In concrete, the embedded steel was protected from corrosion by a microscopically thin oxide layer called passive film, γ-Fe2O3.H2, which was formed during the highly alkaline condition of the concrete pore solution environment [13]. The iron dissolution was suppressed by the as-formed protective film to negligibly low values, and this as-formed oxide is insoluble as well as being highly stable. It can be noted that even in well-constructed concrete, steel corrosion cannot be controlled once the chlorides are continuously accumulating at the depth of the steel. Once the corrosion initiation starts, it rapidly propagates to the next level. As a result, continuous corrosion of steel takes place, which creates more rust products having a volume that is, 38 times greater than the original metal [14]. Subsequently, cracking and spalling of the cover of concrete happens due to stress, which accelerates the corrosion. When metal loss occurs this way, the compound formed is referred to as the corrosion product. Uses of corrosion-resistant materials, treatment of steels, application of protective coatings, or control of the environment are some of the methods for combating corrosion [9,1526]. The selection of materials or methods of protection must be determined for each environmental condition and within prescribed economic limits. Past experience and laboratory testing can serve as a guide in this selection, but exposure under real conditions is necessary.

20.4

Concrete corrosion inhibitors

Materials which retard the corrosion kinetics reactions of any metals are called corrosion inhibitors. The corrosion inhibitor may be composed of organic, inorganic, or a mixture of organic and inorganic materials. Corrosion inhibitors act by adsorption of ions or molecules over the metal surfaces. They reduce the corrosion rate mainly by increasing or decreasing the anodic and/or cathodic reactions and decreasing the diffusion rate for reactants to the surface of the metal. Also, due to the electrical resistance of the metal surface, the corrosion rate will be controlled. There are different types of concrete corrosion inhibitors investigated, namely anodic, cathodic, and migrating or penetrating inhibitors [10,2731], as shown in Fig. 20.3. Based on their use in concretes, they have been divided into two groups: the first type is admixed inhibitors (e.g., calcium nitrite, calcium nitrate), which are admixed to the fresh concrete for new construction, and the second type is migrating organic-based corrosion inhibitors, which are incorporated as admixtures during new construction or applied as a surface impregnant on existing structures [28]. Some of the compounds are treated both as admixed and migrating types (e.g., amine-based unsaturated and saturated fatty acid and aliphatic carboxylic acid, etc.). Some inhibitors are referred to as mixed corrosion inhibitors, as they

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Figure 20.3 Types of corrosion inhibitors.

influence both anodic and cathodic processes of corrosion [32]. There are different types of inhibitors such as sodium nitrite, sodium benzoate, sodium nitrate, and amine-based organic compounds. Based on their action mechanism, they are classified into different categories, for example, anodic inhibitor which controls the anodic reaction of metal, cathodic inhibitor which controls the cathodic reaction of metal, and migrating or mixed corrosion inhibitor which controls both the anodic and/or cathodic reaction of metal and provides stable protective film formation over the steel surface. Recently polymer-based inhibitors have taken centerstage in the research into concrete-related corrosion. However, from the literature it is understood that not much work on polymer-based corrosion inhibitors has been reported for concrete/rebar corrosion [9,28]. In the case of chloride-induced corrosion, inhibitors act based on scenarios such as (1) chloride ingress rate; (2) the extent of chemically bound or physically trapped chloride inside the cover of concrete; (3) steel tolerance level on chloride attack; (4) rate of dissolved oxygen ingress on withstanding the cathodic half-cell reaction; (5) the concrete’s electrical resistance; and (6) the electrolyte chemical composition, which basically depends on the pore solution of cement paste [11]. For example, Ann et al. [33] used a calcium nitrate-based corrosion inhibitor for assessing the corrosion rate at all levels of chloride ion ingress in mortar and it was found that by

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increasing the dosage of corrosion inhibitor, the threshold level of chloride was raised, which ranges from 0.22% to 1.95% by weight of cement. As far as the nitrite-free specimen is concerned, the values ranged from 0.18% to 0.33%. It was 2 also observed from the experiment that the threshold mole ratio of [NO2 2 ]:[Cl ] ranged from 0.34 to 0.66 toward corrosion prevention of rebar. However, earlier 2 studies reported that the threshold ratio [NO2 2 ]:[Cl ] falls in the range of 0.51.0. Also, the effect of inhibitors on concrete strength was also studied by these authors. It was found that the addition of a corrosion inhibitor increased the compressive strength of the concrete at early ages, however, at 900 days, reduction of the concrete strength occurred. The results were presented according to the control concrete. A recent review on nitrate-based inhibitors used in concrete corrosion protection and its environmental concerns was carried out by Song et al. [34]. Furthermore, corrosion inhibitors, when mixed with other admixtures, show an associated effect which sometimes enhances or decreases the concrete-related parameters [3537]. In general, admixtures are chemicals that are added to concrete at some dosage during its making to give the concrete new properties either when fluid or plastic and/or in the set or cured condition [3840]. There are many types of admixtures available for imparting new properties to concrete such as superplasticizers, air entrainers, set retarders, nanooxides or nanotubes, and accelerators and inhibitors mainly to control steel rebar corrosion [36,39,4146]. Also, computational methods such as molecular dynamics and quantum mechanical analysis are used to assess the interaction mechanism of steel with the corrosion inhibitors. These methods are used for quantitative study of the relationship between inhibition efficiency with respect to the molecular reactivity. Based on the analysis of interaction energies, highest occupied molecular orbital and lowest unoccupied molecular orbital of the inhibitors are evaluated, which helps in choosing the best inhibitors for controlling or eradicating the metal corrosion [47,48].

20.5

Limitation of inhibitors

As far as the use of nitrite-based inhibitors is concerned, a high rate of corrosion may occur due to an improper dose. Another major concern is toxicity, which limits the usage of nitrite-based corrosion inhibitors for concrete applications. Nowadays, organic-based inhibitors are preferred over inorganic-based inhibitors due to their high efficiency and lower environmental impact. Hence, current research is focused on the synthesis of new organic-based inhibitors. Despite this, the influence of such inhibitors on setting time, hydration, workability, and compressive strength of mortar/concrete due to the chemical alteration of phases are not well understood. This knowledge gap needs to be filled by conducting research in these areas. Polymers play a vital role in inhibiting corrosion of aluminum, mild steel, copper, etc., due to their chelating ability and the high efficiencies of functional groups, even when used in small concentrations. Hence the polymer-based corrosion inhibitors are defined as “emerging corrosion inhibitors for compacting reinforced steel corrosion.”

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20.6

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Mechanism of inhibition

The retardation of the corrosion reaction is effected by the reduction of anodic reaction rate or cathodic reaction rate, or both. This action may occur by any of the following mechanisms: 1. Changes in the double layer; 2. Formation of a physical barrier; 3. Reduction of metal reactivity; and 4. Participation of the inhibitor in partial electrochemical reactions. Modern instrumental techniques and surface analysis methods can contribute to the understanding of the inhibition mechanism involved in this system. In particular, the recording of the electrochemical characteristics such as anodic and cathodic polarization curves, electrochemical impedance, etc., in the presence and absence of an inhibitor may contribute to the identification of the prevailing mechanism of inhibitor action. Different mechanisms of inhibitor actions are discussed below. 1. Changes in the electrical double layer

Corrosion inhibition is related to changes in the electrical double layer nature at the metal/solution interface. This change takes place when the ionized inhibiting species formed due to electrostatic adsorption and the changes in electrical double layer are referred to the appearance of an adsorption potential jump. Organic-based molecules adsorb at the metalsolution interface due to electrostatics interactions. There are two types of adsorption, columbic adsorption like physical adsorption, and specific adsorption. a. Columbic adsorption In columbic adsorption, there is no direct contact between ions or molecules with the metal. A layer of solvent molecules which is formed at the electrode surface divides the metallic electrode from the inhibitor molecules. b. Specific adsorption

In the case of specific adsorption, chemisorption dominates when organic compounds with polar functional groups are adsorbed over metal surface. Due to strong metal-inhibitor binding energy, strong interactions occur between the metal inhibitor and mainly π-electrons present in organic compounds play a vital role. 2. Formation of a physical barrier Inhibitors with large number of branching groups create multimolecular layers over the metal surface. The resulting barrier action is independent of the nature of adsorption forces between the inhibitor molecule and the metal surface. The corrosion inhibition occurs while the mass transport causes are hindered. From the nature of polarization curves, the concentration polarization and resistance polarization on the cathodic branches can be well understood. From the Evans diagrams [49], the nature of inhibition by inhibitors such as the anodic, cathodic, or mixed polarizations can be assessed. The effectiveness of an inhibitor increases with its ability to induce anodic polarization at relatively low current values. Inhibitors inducing anodic polarization on the metal surfaces are called anodic inhibitors. Accordingly, cathodic and mixed-type inhibitors are also

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classified based on their inhibition nature. If the corrosion current is proportional to the anodic reaction of the metal, the decrease in corrosion should be proportional to the anodic area of the metal being polarized. 3. Reduction of metal reactivity As far as this mechanism of action is concerned, the inhibitor adsorbs on the metal sites activated due to the partial electrochemical reactions. The rate of anodic and cathodic reactions or both are reduced or controlled because of the blockage of active sites, which induces corrosion. Hence the rate of corrosion is controlled by the active site blockage. Also, it can be seen that the polarization curves shift toward lower current densities without changing the nature of the Tafel slope. Moreover, this mechanism does not guarantee complete coverage of the metal surface due to inhibitor adsorption. Generally, interaction forces are important and higher efficiencies result when stronger bonds, such as chemisorption bonds, are established. 4. Participation of inhibitor in partial electrochemical reactions

Both anodic reaction of metal dissolution and cathodic reaction of hydrogen evolution proceed by steps with the formation of adsorbed intermediates on the metal surface. According to this mechanism, the adsorbed molecules would participate in the intermediate formation, promoting either a decrease or stimulation of electrode reaction depending on the stability of the adsorbed surface complex. As a consequence, a variation in the Tafel slope can be observed. For example, Bockrise suggested the following mechanism for iron dissolution: Step 1: Fe 1 OH2 #ðFeOHÞads 1 e2 Slow Step 2: ðFeOHÞads! FeOH1 1 e2 1 Step 3: FeOH FastFe21 1 OH2 where step (1) is in quasiequilibrium, step (2) is the rate-determining step, and step (3) is the fast reaction step. Donahue and Nobe [50] suggested a sort of chelation mechanism for organic inhibitors. According to them the following steps are probable: Step 4: ðFeOHÞads 1 nIn#ðFeðOHÞðInÞn Þads Step 5: ðFeðOHÞðInÞn Þads ! ðFeðOHÞðInÞn Þ1 1 e2

In step (4) the adsorbed intermediate interacts with “n” molecules of inhibitor to form a complex that can undergo charge transfer as in step (5), and then adsorb as a complex ion. Likewise, the value of the equilibrium constant of step (4) is expected to determine the extent of inhibition provided the rate of step (5) is much slower than that of step (2).

20.7

Techniques to assess inhibitor performances

20.7.1 Corrosion monitoring techniques for the evaluation of inhibitor efficiency and corrosion rate The rate of corrosion can be determined by (1) physicochemical and (2) electrochemical methods. Fig. 20.4 shows the corrosion monitoring techniques used currently for assessing the corrosion rate of metals/rebar. Among all the techniques,

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Corrosion monitoring techniques

Physicochemical methods 1. Weight loss method 2. Gasometric method 3. Hydrogen permeation method

Electrochemical methods

Direct current methods 1. Tafel extrapolation 2. Linear polarization 3. Coulostatic 4. Low amplitude 5. Cyclic voltammetry

Alternate current methods

1. Faradaic impedance method 2. Faradaic rectification 3. Faradaic distortion method

Figure 20.4 Classification of corrosion monitoring techniques.

generally electrochemical methods have the most advantages over other methods as they require only short measuring time, possibility of continuous monitoring, and are helpful in arriving at mechanistic studies. However, too much polarization of metal surfaces to several hundred millivolts leads to changes in the specific properties of the system. Direct (DC) or alternating current (AC) is applied and the resultant current (or voltage) is measured.

20.8

Concrete corrosion assessing techniques

Potential measurement, potential mapping, DC polarization, electrochemical impedance spectroscopy (EIS), and transient methods are mainly used for assessing the concrete corrosion rate [4]. The probability of corrosion in a steel concrete system based on its potential characteristics is shown in Table 20.1.

20.8.1 Potential measurement This method was used for routine inspection of reinforced concrete structures for the detection of corrosion. The technique is well known and is described in the American National Standards ANSI/ASTM C876. The interpretation of the potential readings is described by: G

The mechanism of passive film formed on the rebar and its failure mechanism are explained by many models. However, it is assumed that the mechanism mainly relies on forming chloride ion soluble complexes on iron, thereby localized acidification leads to the growth of pits. Also, the bound chlorides in the concrete are free to participate when the pH environment of the concrete drops.

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Table 20.1 Probability of corrosion based on the potential measured [4]. Ecorr (vs Cu/CuSO4)

Probability of corrosion

, 0.35 V . 0.20 V 0.20 to 0.35 V

.95% ,5%  50%

Potential mapping techniques help to detect rebar corrosion, and are recommended for preliminary study of the corroding system. Although this technique has proven advantages for on-site monitoring, the results need careful interpretation, especially where the oxygen supply is restricted.

20.8.2 DC polarization measurements Through this technique, the polarization resistance of the system can be measured. This method has been used since 1970. The accuracy of the technique is improved significantly when the measuring devices work based on the guard ring technique principle. The instruments used in this technique are portable and have operationfriendly monitoring systems, and these characteristics make this technique adequate for rapid on-site determination of the corrosion rate. The fundamentals of the technique are mainly based on the SternGeary equation [51]: Icorr 5 B=Rp ; where B 5 ßaßc/[2:3(ßa 1 ßc)] with: Rp denoting the polarization resistance and ßa and ßc, the anodic and cathodic Tafel constants, respectively. In concrete, however, the main difficulty in the use of this technique arises from the irregular distribution of the electrical signal applied via the counterelectrode of much smaller dimensions than the reinforced concrete structure under test. A uniform distribution over the whole metallic system fails as the electrical signal tends to vanish while increasing the distance from the location of the counterelectrode. To overcome this, one of the best analytical models, namely the “transmission line” model, was developed [4] which clearly differentiates the behavior of steel in the passive state (Rp  105106 Ω cm2, e.g., concrete in the absence of chlorides) and in an active state (Rp  103104 Ω cm2, e.g., concrete with 3% CaCl2). This model was adopted by many authors to estimate the rate of corrosion [52].

20.8.3 Electrochemical impedance spectroscopy This is a powerful tool to study the nature of a steel/concrete system. The information includes parameters such as surface film formations, bulk characteristics of concrete, mass-transfer phenomena, and interfacial mechanisms. However, it is a time-consuming reaction. In impedance data analysis, with the help of equivalent circuit fit for the steel in the simulated experimental condition, it is easier to make

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a conclusions on the interaction of steel/concrete system based on the laboratory studies conducted. Also, detailed knowledge of the steel/concrete system can be explained based on this technique. Some of the equivalent circuits which have been commonly used are shown in Fig. 20.5 [4].

20.8.4 Transient methods This method is based on the time domain process, which is very effective in the onsite monitoring of steel/concrete response. It is a very rapid and nondestructive technique and the information on steel condition and concrete resistivity can be assessed based on this technique. Sometimes this technique can cause an unexpected deviation from the expected exponential behavior of the system, which leads to difficulty in estimating the system parameter results with over- or underestimation of the corrosion rate.

20.9

Surface characterization of the metals/rebars after corrosion

There are three main surface characterization techniques, namely scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), which are used for assessing the depth of corrosion attack, formation of corrosion pits, surface layer formation by inhibitors and its nature, etc. [9,28,5355]. These techniques are also very useful for correlating the inhibitor efficiency against the rebar corrosion and corrosion mechanisms.

20.10

Corrosion product analysis techniques

To understand the corrosion product nature after corrosion or corrosion inhibition reaction, mainly X-ray diffraction (XRD), FT-IR, TGA, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) detection methods are used [5,32,5357]. The nature and formation of oxides formed during corrosion on the metal surface can be assessed by using these techniques, which would be supportive for arriving at the mechanisms and conclusions. Standards and methods used for assessing the corrosion performance of inhibitors and reinforced concrete include the following: G

G

G

ANSI/ASTM C876: The detection of corrosion by using potential measurements is one of the most typical procedures for the routine inspection of reinforced concrete structures. ASTM C1582 and ASTM C494 Type C: This standard is used to assess the requirement for a corrosion inhibitor. ACI committee report, ACI 222R-96: This report comprises the corrosion of metals in concrete and the recommended chloride limits in concrete.

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Figure 20.5 Equivalent circuit for simulation of impedance data. (A) Rc, concrete resistance; Rf, Cf, film resistance and capacitance; Zd, diffusional impedance; Rct, Cdl, charge transfer resistance and double layer capacitance. (B) Discretized transmission line model for rebars in reinforced concrete. R, Resistance of rebar/segment; Rj, resistance of concrete/ segment; Zj, rebar/concrete interfacial impedance segment. (C) Equivalent circuit for simulation of impedance data. R0, concrete resistance; R1, C1, resistance and capacitance of a lime-rich layer; R2, C2, charge transfer resistance and double layer capacitance; R3, C3, resistance and capacitance of an adsorbed intermediate. (D) Equivalent circuit for simulation of impedance data. R0, concrete resistance; R1, resistance of products formed on steel; C1 and Rd1, dispersion capacitance and resistance (frequency dependent), accounting for inhomogeneity of concrete surrounding steel; R2, resistance of steel interface; C2 and Rd2, dispersion capacitance and resistance, accounting for homogeneity of products on metal surface. (E) Equivalent circuit proposed for macrocell corrosion behavior in reinforced concrete. Rcc and Rca, resistance of concrete in cathodic and anodic zones; Rc and Ra, resistance of cathodic and anodic sites; Cc and Ca, capacitance of cathodic and anodic sites [4].

G

G

G

G

ASTM A767: This standard specification is for using hot-dipped zinc (galvanized) coatings (coatings on steel rebar for concrete reinforcement). ASTM G180: This is a standard test method for evaluating corrosion-inhibiting admixtures for steel in concrete through polarization resistance in cementitious slurries. ASTM G109: This test discusses the standard specimen required for electrochemical impedance testing and its analysis. Rapid chloride ion penetration test (RCPT): The rapid chloride penetration test is used mainly to assess the permeability of concrete mixed with corrosion inhibitors.

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Half-cell potential (mV): This type of measurement technique describes the conditions of reinforcement corrosion. AASHTO T 289: This is a standard test method for determining the pH of soil used during corrosion testing.

Apart from the above-stated methods, there are some other reports/standards that may be available based on the reinforced concrete environmental conditions and nature.

20.11

Durability studies of concrete with admixtures

Studies on the durability of the inhibitors are crucial to understanding the performances of the synthesized inhibitors in terms of long-term behavior. In this regard, several authors have adopted various standard test protocols to study the durability of corrosion inhibitors, which are briefly reviewed here. The influence of chemical admixtures on the hydration of cement has been investigated by several workers and this is also presented here. It is an accepted fact that Cl2 ions present in concrete influence the corrosion of rebars. It is also accepted that chloride levels between 0.4% and 1% (by weight of cement) produce a medium risk of corrosion and above 1% they cause a high risk. Monticelli et al., using the test with calcium nitrite, sodium monofluorophosphate, and ethanolamine-based treatment, observed a moderate reduction in the overall rates of corrosion after inhibitive treatments to those specimens in which levels of chloride contamination were fairly low—at a level of 0.6% Cl2 by weight of cement. It is important to point out that none of the inhibitors used by them functioned effectively in concrete with high chloride levels (2.4% Cl2 by weight of cement) [58]. Gaidis made a similar observation with an admixture-type corrosion inhibitor coupled with a commercial nitrite-based corrosion inhibitor. Experiments were performed by him using varying concentrations of the two inhibitors at different chloride contents. The measured pitting potentials indicate that the passive domain is left as soon as chlorides are present in some amount. However, it was not possible for him to draw conclusions about the critical chloride ratios based on these measurements. Furthermore, the experiments were performed only in solution and not under real conditions in concrete or mortar. For the nitrite-based inhibitors, the nitrite to chloride ratio for effective corrosion prevention has also been discovered. It was also found that the mixed corrosion inhibitors reduced the rate of corrosion in Cl2-contaminated concretes at a level of 1% Cl2 by weight of cement [10]. Brown et al. found that organic esters and amines exhibited a much lower level of diffused chloride than the control mixes. Their investigation also included two amino alcohols. In the case of the first amino alcohol, the electrical conductance for chloride penetrability and the measured diffused Cl2 content were both less than the control. It was found that diffused chloride in the inhibited specimens after 2 years was significantly lower than the control. This observation indicates that the noticed early retarding effects did not extend beyond the earliest phases of curing. In the case of specimens containing the second amino alcohol, it was noticed that at

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reinforcement depth, less diffused Cl2 content was present and the electrical conductance values were very similar to those for the control. Furthermore, the amino alcohol produced significant benefits in the reduction of concrete permeability and long-term strength development, although there was high variability in diffused Cl2 content compared to the control [59]. Solution chemistry during cement hydration in the presence of metal hydroxide waste has been studied by Asavapisit et al. using conduction calorimetry and differential thermal analysis. Pb was resolubilized from the waste into solution at higher concentrations during early hydration. Zn waste was rapidly resolubilized, and Zn was adsorbed onto solid surfaces causing severe inhibition of hydration. Cd was present in both the waste and the cement and it was found to promote the rapid formation of crystalline Ca(OH)2, and thereby it behaved very differently from Pband Zn-containing wastes [60]. The influence of mineral additives such as fly ash, slags, limestone, and lime sludge on the hydration of ordinary Portland cement (OPC) was studied by Sharma et al. The samples of OPC with 10 wt.% mineral additives were examined separately at 28 and 90 days of hydration by XRD and differential thermal analysis. The hydration products in the OPC1 slags and OPC1 fly ash systems were found to be almost identical to that of pure OPC, except for the low quality of Ca(OH)2 during the early period of hydration. The hydration of OPC with lime sludge was found to be better than that with limestone due to the better crystallinity and fineness of powdered sludge [61]. Andrew et al. studied the analysis of calcium silicate hydrate (CSH) gel and cement paste by small-angle neutron scattering (SANS). The information obtained from SANS was compared with that obtained from another neutron method. Application of the SANS method to cement paste was demonstrated by analyzing the effects of CaCl2 acceleration, and sucrose retardation as the resulting hydrated microstructure [62]. The initial stages of cement hydration have been investigated by Preece et al. It was found that after the initial mixing of cement, an induction period occurred during which its consistency remained constant and thickening occurred at the end of this period. They proposed a reaction-diffusion model for the hydration of tricalcium silicate, a principal constituent of cement, which was believed to be responsible for the initial development of its strength. The results of their model suggest that the driving mechanism during the initial stages of cement hydration is a combination of a delayed nucleation and a protective coating mechanism [63]. Cement hydration and microstructure formation in the presence of water-soluble polymers was studied by Knapern et al., by employing isothermal calorimetry, thermal analysis, FT-IR spectroscopy, and SEM techniques. Hardening of cement mortars modified with small amounts of water-soluble polymers implied both cement hydration and polymer film formation [64]. Thomas et al. studied the influence of nucleation seeding on tricalcium silicate and cement hydration mechanisms, using various additives and by employing a calorimetric technique. It was found that the CSH content, which was the main hydration product, increased both the early hydration rate as well as the total amount of hydration during the early nucleation and growth period [65].

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The mechanism of cement hydration was investigated by Jeffrey et al. Several remaining controversies in understanding this mechanism were identified, such as the nature and influence on kinetics of an early surface hydrate, the mechanistic origin of the start of the acceleration period, the manner in which microscopic growth processes lead to the characteristic morphologies of hydration products at longer length scales, and the role played by diffusion in the deceleration period were analyzed [66]. The modification of cement concrete by means of polymers in solution was studied by Knapen et al. It was found that the addition of aqueous polymer emulsions or redispersible polymer powders in the fresh concrete mix resulted in polymer modification of cement mortar and cement [64]. Stein investigated the influence of some nonionic organic additives, dissolved in the water used for mixing cement pastes on cement hydration by isothermal calorimetry. The additives, all poly-alcohols, were found to exert a retarding action on cement hydration. Glycerol was found to have a specific influence on the crystallization of ettringite and calcium hydroaluminum monosulfate, and thereby accelerate the consumption of sulfate [67]. Larbi et al. discussed the mechanism of interaction of polymers with cement hydration products. The pore solution squeezed out from the cement paste with the help of an appropriate pore solutions expression device and was immediately subjected for chemical analysis. The results revealed that an interaction occurred 2 between the polymers and Ca21, SO22 4 , and OH ions released by the cement during hydration. Furthermore, it reveals that dosages of 5% or more of polyvinylidine chloride are sufficient to release Cl2 ions to exceed the tolerable corrosion limits in reinforced concrete [58]. Burris and Kurtis investigated the interaction of the use of citric acid as a set retarder with two commercially available calcium sulfoaluminate cements (CSAs) and found their interaction during hydration, setting, phase development, and compressive strengths. The key findings concluded that the citric acid was acting as a good set retarder and helped for prolonging the cements initial setting time beyond 120 minutes. Although citric acid retarded the hydration reaction, it did not reduce the total hydration of the systems. Also, citric acid did not affect the strength of the system and, in fact, it increased the strength in some combinations used. Overall, it was concluded that citric acid can be a good set retarder for large-scale applications [68]. Li et al. discussed the effects of chemical admixtures on the workability and strength development of CSA subjected to different temperatures. XRD and TG studies were used to understand the interaction mechanism between the admixture and cement. An improvement in setting time and early strength of CSA was noted in the presence of lithium carbonate-aluminum sulfate (LC-AS) at 0 C and sodium gluconate-borax (SG-B) admixture at 40 C. Furthermore, in the LC-AS system an enhanced ettringite formation, however accelerated belite hydration was also noticed. However, in the case of the SG-B, the formation of ettringite was retarded [69]. There are several methods to assess the durability of the inhibitors at length scales or component-level studies toward enhancing the service life of structures.

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Also, standard test protocols or test methods are available for the durability analysis of corrosion inhibitors which can be used for fresh concrete as well as for repair and rehabilitation to extend the service life of concrete structures.

20.12

Conclusion

Based on the available reports, the influence of corrosion inhibitors for increasing the service life of concrete structures is reviewed in this chapter. The importance, novelty, and standard methods for assessing the inhibitors and their emerging role in the construction industry are briefly presented. Also, standard methods used for corrosion evaluation of structures and inhibitors are discussed. Furthermore, the durability of concrete with admixtures is briefly reported on.

Acknowledgments The authors acknowledge the Indian Institute of Technology Kanpur, CSIR—Structural Engineering Research Centre Chennai, and the Indian Institute of Technology Dharwad for the support and encouragement provided for the writing of this chapter.

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Use of fly ash for the development of sustainable construction materials

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Sanchit Gupta and Sandeep Chaudhary Discipline of Civil Engineering, Indian Institute of Technology Indore, Indore, India

21.1

Introduction

Fly ash (FA) or pulverized fuel ash (PFA) is a fuel combustion residue, that is transported along with flue gases and collected in precipitators. Offering no combustion energy, FA is regarded as a waste by the energy sector. Waste management hierarchy suggests prevention, reduction, reuse, recycle, and energy derivation in preference to disposal in this respective order for the handling of wastes. In the case of FA, reduction is not possible due to global dependency on fuel-derived energy, and, reuse or energy derivation is not possible on account of FA having negligible calorific value. The initial trend of waste management was in terms of disposal as ash ponds, leading to several environmental concerns. Growing concerns about FA disposal led to various studies on the reduction and recycling of ash. The collective result of these studies recognizes FA as an industrial by-product with recycling potential in the form of a cement substitute, filler material, etc. in the field of civil engineering. FA utilization in the industry is not new—dating back to the 1930s—however, utilization technologies and patterns have greatly changed [1]. Over time, new products, like alkali-activated binders, have been developed, while the use of existing products has been improvised, such as beneficiation of FA. It is imperative that one should be aware of the new developments in FA utilization, to enable use of the best available sustainable technology. Different industrial uses offer their own advantages. Considering a simple case of cement substitution, different characteristics of concrete improve at a low level of FA incorporation and reduce at a higher level of incorporation [2]. In application, one can use either low-level replacement to achieve maximum benefits from the concrete, or achieve maximum incorporation while keeping the properties of concrete within performable standards. In this regard, sustainable development has become a major driver for research and development among the global research community. Most industrial uses are selected in such a way that they promote sustainable development. Therefore, in order for one to appreciate the new technological developments and justify their use in the industry, one should be aware of the concept of sustainability with reference to FA, as discussed in the following section. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00021-1 © 2020 Elsevier Inc. All rights reserved.

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Sustainable development of fly ash utilization

An environmental concern arising from the disposal of FA, like heavy metal contamination of soil, has been to a great extent managed by high-volume utilization of FA using various recycling technologies in the field of civil engineering. Furthermore, the use of FA has reduced the requirements for other materials like clinker and clay, enabling sustainable development in the industry. As the thrust for sustainability is not restricted to any single industrial sector, recent technological developments like coprocessing of wastes as fuel, municipal waste-derived fuels, and change in coal type have greatly impacted the characteristics of FA available [3]. Bhuiyan et al. [3] reviewed the thermochemical characteristics of FA obtained from the burning of materials other than coal, such as coprocessing of wood. Studies indicate that FA obtained from biomass can contain alkali, chlorine, and other substances that can cause damage when incorporated in concrete [3]. Due to the close interdependency of industries, change in one industry leads to change in others, such as the promotion of sustainability in the energy sector which is influencing the composition of FA being utilized for construction. As per a technology roadmap for the Indian cement industry, published by the International Energy Agency [4], if available technologies of sustainability are followed across all industries, the supply of industrial by-products like FA and slag will drop significantly. In the near future, if alternate technologies are not developed, reduced availability of FA will mean higher requirement of materials like clinker, which will pose additional environmental concerns. This highlights the fact that the scope of sustainable development should not be restricted to better utilization of conventional FA available in the industry—it should also account for variations arising due to technological changes like a change in fuel use. The community is a part of sustainable development; therefore, studies on FA have also been greatly influenced by the needs of individual countries or regions, arising from regional variations in terms of available resources, nature of industries, governing legal policies, and the threat level to the environment. India, which initiated its FA Mission in 1994 [5], has reached a point where FA may become a limited resource [4]. In contrast, Vietnam, on identifying FA as an environmental concern, was promoting FA utilization within its industries as late as 2014 [6]. Even among the nations where FA is widely accepted as an industrial by-product one can find a difference in usage patterns and governing standards, based on factors like the socioeconomic thrust of community, type of industrial usage, and the characteristics of the available ash. Therefore sustainable development of FA cannot be the same for all regions—while some regions may be initiating their use of FA and may target reliable and economically beneficial strategies for easy acceptance within the industry, others may have limited availability of FA and will prioritize use of FA for products that are more beneficial. Fig. 21.1 presents an overview of different factors that can govern the studies on sustainability in a region. By understanding the various factors governing sustainable development, one can understand the technologies being developed and suitable strategies for FA

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Figure 21.1 Factors governing strategies for sustainable development.

utilization. However, only technological factors have been discussed in this Chapter, independent of community and environmental conditions, as the role of the community is region-specific and the environmental impact remains similar globally.

21.3

Characterization of fly ash

Jones et al. [1] showed that, with time, the understanding of FA has improved, resulting in better utilization of ash in the industry. Hemalatha and Ramaswamy [7] observed that high-volume utilization of FA is possible in the industry, by understanding the characteristics of FA. Although characterization of FA is important for improving its utilization, characterization becomes even more important when developments for sustainability in the energy sector, like the use of alternate fuels, influence the characteristics of the FA being generated. Several organizations and agencies have developed standards on the current understanding of FA. These standards use information like chemical composition, fineness, loss on ignition, and pozzolanic activity for characterizing FA.

21.3.1 Physical characterization Physical characteristics play an important role both as a filler material and pozzolanic reactions. Particle size distribution, bulk density, specific gravity, and moisture content have been considered important parameters for the use of FA as an inert filler material, like landfill and lightweight aggregate. This form of addition has been described as “inert addition” in standards [1]. Similarly, for utilizing the

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pozzolanic properties of FA other than the properties influencing its use as a filler material, fineness or specific surface area also become important. Finer particles provide for additional surface area, allowing for more areas for nucleation and resulting in better pozzolanic reactions [8]. Recent studies [9,10] have also shown that particle size distribution plays an important role in the mechanical and physical properties of ash-based products. Sevim and Demir [10] showed that optimum particle size distribution results in a compact ash-based product, leading to better mechanical characteristics. As by simple modifications in the grading of FA utilization can be improved significantly, understanding of physical characterization becomes important for improving the applications of FA.

21.3.2 Chemical characterization FA, being a chemically active material, is used for its binding properties, resulting from the chemical reaction with other reactive constituents, thus a suitable understanding of chemical constituents is needed. In general, silica from FA reacts with alkaline compounds like lime to cause cementing actions. Silica, reacting in the form of silicic acid, is used for characterizing pozzolanic materials. Often, high silica content is referred to as acidic and is preferred. However, over time this characterization has shifted from chemical analysis to mineralogical identification, due to the development of an understanding of reactive phases and their role in pozzolanic activity [11,12]. Identification of chemical constituents still remains an integral part of ash characterization. As silica reacts with an alkaline compound to provide cementing action, the presence of alkaline substances in FA can make it self-cementing. Composition of free lime and other reactive alkalis becomes important for justifying the use of FA as a pozzolanic or self-cementing material. In addition, chemical analysis shows the presence of trace elements, like chloride, lead, and cadmium, which may be harmful to ash-incorporated products or the environment [13]. This makes chemical characterization an important process for checking the suitability of new forms of FA.

21.3.3 Microstructural characterization Extending to the understanding of physical characteristics, researchers have identified that microstructural characteristics like particle shape and particle surface also influence the application of FA. Modern techniques like SEM and optical microscopy have revealed that particles of FA have different morphologies [11,14].

21.3.4 Mineral characterization Although mineral characterization can be done from extensive chemical analysis, modern techniques like XRD, FTIR, and Raman spectroscopy make mineral characterization faster and more efficient. These methods, as a collective approach, identify different forms of minerals present in any given sample of FA; and from the understanding of roles of minerals in pozzolanic reactivity, one can justify the use of any given ash [11,12]. Methods of mineral characterization, being fast and less

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prone to error, have become an integral part of the studies into the development of FA as a sustainable resource.

21.3.5 Characterization from the application perspective Characterization techniques discussed so far are only indicators of the performance of FA in its respective ash-based products. Industry often warrants quick approaches that justify the application of FA as a specific product. For the same reason, several tests and indices have been developed to justify the suitability of FAs [15]. Activity indices are one of the most commonly used tests, and involve the preparation of cement FA mix for detection of strength gain at different ages. Since testing with cement involves multiple chemical constituents and multiple chemical reactions may occur, the lime pozzolan strength test has been suggested to be a better assessment tool. Other tests like the Chapelle test and electrical conductivity tests are also employed for identifying the pozzolanic properties of different ashes [16].

21.4

Fly ash applications

Composite cement, in addition to all types of concrete, autoclaved aerated concrete, nonaerated concrete blocks, sand-lime brick, bricks and ceramics, lightweight aggregate, cement raw material, road construction, and grouting are typical examples of FA utilization in the field of civil engineering [17]. However, prior to understanding the application of FA, one should understand the reasons for utilizing FA in the construction industry.

21.4.1 Rationale for use of fly ash FA, due to its widely accepted use, is regarded as an industrial by-product rather than a waste product of the energy sector. This widespread use of FA can be justified by the two primary factors of environmental benefit and engineering benefit. This is illustrated in Fig. 21.2. While the environmental benefits are apparent and directly depend on the quantity of FA utilized, they do not govern the specific use of FA in the industry. FA being employed for its use as a filler material does not warrant it to be chemically active, as is the case for the pozzolanic reaction. In such a scenario, the utilization of pozzolanic ash will provide environmental advantages but will not be a case of optimum utilization if chemically inert FA is available. In light of this, engineering benefits have been considered more important and are the basis of the discussion in this chapter.

21.4.2 Use as fine particles FA, as a means of disposal, is often used as a fine filler material. In some cases, like high-volume FA for self-compacting concrete and landfills, this is done to act as a fine-sized substitute to other fine-sized particles and provides proper grading or

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Figure 21.2 Rationale for the utilization of fly ash.

compact packing. This type of usage of FA treats FA as an inert material. As a preferable approach, FA with little or no pozzolanic activity should be utilized in such cases. The additional advantage achieved by the incorporation of FA is the improved workability resulting from proper grading of the resulting mix in concrete. This has promoted the use of FA as a mineral admixture in the field. In more recent developments, FA has been utilized as a filler material as a substitute for aggregates, as discussed in a separate section. Examples of the application of FA as fine filler particles include self-compacting concrete, landfills, fly-ash bricks, and other similar high-volume FA-based products.

21.4.3 Use for chemically active minerals Due to the presence of active aluminosilicate, FA has been utilized for its potential as a pozzolanic material. Another major use of FA is its role as a raw material for the manufacture of OPC and similar types of cement, as a source of aluminosilicates. Recent developments have seen an improvement in the utilization of FA as a source of chemically active minerals, such as composite cement and geopolymers. As its chemical activity offers advantages of binding action among several others, the use of FA has increasingly been preferred for chemical activity over its use as a filler material. Recent studies have shown ash of other forms, like bottom ash, with low pozzolanic activity that have been increasingly studied for their filler effect as a substitute to FA. Examples of the application of FA for its use for chemically active minerals include PPC, blended cement, lime FA mortar, and geopolymers.

21.5

Developments in industrial fly ash applications

As discussed in previous sections, FA over the years has found several industrial applications, such as the raw material for Portland pozzolana cement (PPC), fines

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for replacement of cement in concrete, filler product in landfill and road embankments, and the raw material for construction products like FA bricks and in agriculture. With a better understanding of the material, FA has moved from its use as a waste filler material to a fine-sized aluminosilicate industrial by-product. Realizing the importance of FA as a resource, researchers now focus on studies with technical advantages in addition to the aspect of sustainability. Keeping in line with the scope of this book, only the recent FA-based products or industrial applications are discussed.

21.5.1 Fly ash blended cement A characteristic advantage of FA utilization is its pozzolanic reaction with lime formed during the hydration of cement. FA with active aluminosilicates reacts with lime-based chemically active compounds to form additional C S H or similar hydrated phase. Taking advantage of this blended cements have been formed using FA and lime-based slag or other similar compounds. The reaction involved in FA and slag is time-consuming and the blend may fail to deliver early-age strength. However, in the case of blended cement the initial strength is achieved by hydration of clinker, while delayed strength gain by FA and slag reduces the requirement of clinker in cement. Therefore, by utilizing industrial wastes as major constituents, blended cements are an eco-friendly approach as compared to its counterparts. Blended cement as an industry-manufactured product does not require technical expertise for their use in the field and is more reliable to use than FA as an admixture. Therefore blended cement as a more sustainable strategy has been a focus of several researches. Blended cement has been prepared using FA along with acid mine drainage treatment sludge [18], ground granulated blast furnace slag [19,20], limestone [21], activated paper sludge [22], and others in ternary as well as quaternary forms. Studies indicate that blended cement performs comparably to OPC and indicate the optimum proportion of the blend.

21.5.2 Artificial aggregates Studies on FA and its potential as an artificial aggregate are not new to the industry [23]. Primarily two forms of artificial aggregate using FA are reported in the literature, sintered FA lightweight aggregate and FA cenosphere aggregate, as coarse and fine aggregates, respectively [24]. FA-based lightweight aggregates exhibits high porosity, which, apart from reducing the dead weight of structure also provide excellent thermal insulation [25]. Several studies exist on sintered FA lightweight aggregates, while use of FA cenosphere as the lightweight filler has recently gained momentum.

21.5.2.1 Sintered fly ash lightweight aggregates FA particles are bonded together to form green pellets which are then converted into artificial aggregates by the process of sintering [26]. Artificial aggregates can

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also be produced by cold bonding or autoclaving of FA as an alternative to sintering, however, due to high FA utilization (90% 100%) as compared to other processes, sintering is discussed primarily [26]. Sintered FA lightweight aggregates, as reported in the literature [26], are spherical, having a specific gravity of 1.33 2.35 and water absorption as high as 33.9% indicating a porous structure, depending on the constituents used for preparing green pellets and temperature of sintering. The main advantage arising from high porosity is the internal curing of concrete from excess water, coupled with pozzolanic reactions resulting in a stronger interfacial transition zone ( ITZ) in sintered FA lightweight aggregate as compared to normal aggregate [26]. Despite stronger ITZ, concrete from artificial aggregates in general exhibited reduced mechanical properties with reference to the control sample [24]. However, as the failure is exhibited due to the lower strength of aggregate, it can be used in civil works like nonstructural walls where the strength requirement is not high. Incorporating a higher amount of artificial aggregate resulted in higher water absorption and higher volume of permeable voids in concrete [24]. Higher permeability of concrete resulted in increased chloride ion penetration, while improved ITZ improved corrosion resistance [26]. A reduction in acid resistance was also observed [27]. This limits the application of sintered FA aggregate in high-strength works and severe acid exposures. Interestingly, sintered FA lightweight aggregate exhibited lower compressive strength than normal aggregates at lower water binder ratios, but exhibited higher compressive strength at higher water binder ratio [26]. This can be attributed to the high porosity of sintered FA lightweight aggregate; at low water binder ratio high porosity will result in excess water and effectively lower compressive strength; meanwhile, at high water binder ratio excess water due to porosity is less than the excess mixing water available from the selection of high water binder ratio; reducing the influence on strength at higher water binder ratios. Additionally, hydrated products of cement pastes form inside pores, resulting in improved aggregate and cement matrix bonding, and higher compressive strength, which is called the hook effect [26] (Fig. 21.3). The combined influence of the hook effect and excess water availability makes sintered FA lightweight aggregate suitable for high water binder ratio applications like self-compacting concrete.

Figure 21.3 Representative diagram for the hook effect.

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Although sintered FA lightweight aggregate is used as a coarse aggregate, its use in the crushed form significantly increases its porosity, making it unsuitable for use as fine aggregate [26]. As a result, sintered FA lightweight aggregates are used with FA cenosphere as coarse aggregate and fine aggregate, respectively.

21.5.2.2 Fly ash cenosphere FA cenosphere is a form of FA with its particles resembling a hollow spheroid structure [24,28] (Fig. 21.4). Due to its small size, it is used for replacement of fine aggregates as lightweight aggregates. Cenospheres are microspheres within the size range 1 600 µm [29]. FA cenosphere behaves similarly to sintered FA lightweight aggregate and shows a reduction in mechanical properties [24]. Use of FA cenosphere results in higher water absorption and permeable pores [27]. FA cenosphere usually exhibits a great reduction in density of concrete, while retaining most of its mechanical strength; this results in higher mechanical strength per unit weight of concrete and makes FA cenosphere suitable for preparation of lightweight concrete [29]. Based on these principles, ultralightweight aggregate concrete, with a density as low as 1120 kg/m3 and compressive strength higher than 18.63 MPa, was prepared using FA cenosphere and aerogel [25].

21.5.2.3 Recent developments in artificial aggregates The thrust for sustainability has resulted in new types of FA, such as biomass FA. Moreno-Maroto et al. [30] have shown the formation of lightweight artificial aggregate using biomass FA by the process of sintering, providing solutions for its utilization. Meanwhile other works toward sustainability include showing the industrial application of the developed artificial aggregates. Satpathy et al. [24] and Patel et al. [27] have conducted studies on incorporating sintered FA aggregate as coarse aggregate and FA cenosphere as fine aggregate. Their works provide a solution for high-volume utilization of FA in the form of aggregates for manufacturing of lightweight concrete. However, recent developments in artificial aggregates are not limited to waste incorporation. Studies have been carried out to obtain additional benefits from using FA-based artificial aggregates like internal curing and admixture carriers, other than its general application as a lightweight filler. Ma et al. [31] studied the influence of internal curing for lightweight aggregates and observed better ITZ, refined pore structure, and reduced autogenous shrinkage.

Figure 21.4 Fly ash cenosphere particle representative diagram.

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This prompts utilization of artificial aggregates in concretes where shrinkage is a critical issue, rather than its mechanical properties. Chen et al. [32] utilized FA cenospheres as admixture carriers for concrete. Cenospheres being hollow can store desired admixtures and deliver them to the concrete as needed. Chen et al. [32] used chemical etching to produce perforated holes on the surface of the cenosphere and demonstrated its application as a carrier by transporting water. While the future applications of using cenosphere or similar as the carrier may be endless, it is yet to be introduced in the industry. Technologies to use FA as artificial aggregates are still being developed in the industry, while the induction of technology has already started.

21.5.3 Processed fly ash The discussion so far has covered the application of FA in industry. The influence of FA greatly depends not only on its incorporated proportion but also on the characteristics of FA. As discussed in the preceding section on characterization, even a simple change in characteristics can lead to high in corporation of advantages, and so several methods have been developed for modification of the characteristics of FA. Bicer [33] observed that a reduction in grain diameter of FA improved the strength characteristics of concrete. Similarly, mineralogical and chemical modifications have been carried out to improve the reactive phases of FA. In order to improve the characteristics of ash, thermal activation (change in chemical phase) and mechanical activation (reduction in grain size) changes have been made [16]. Treated FA has shown improved characteristics when incorporated in concrete [34]. Processing of FA also includes treatment for removal of any harmful substances and surface treatment. Fu et al. [35] and Krishnaraj and Ravichandran [14] showed that treatment changes the characteristics of FA, resulting in improved applications. Processed FA, owing to its improved characteristics, has found multiple applications in the industry. Not only conventional FA, but other previously considered unusable ashes, can be processed to improve their industrial applications. This highlights the importance of treatment or processing of FA prior to use, and by drawing maximum benefits sustainability of ash-based products can be improved through the use of processed FA.

21.6

Conclusions

Considering the importance and impact of FA in civil engineering, it is important to understand any new developments regarding FA. Over time FA has moved from a critical waste to an important industrial by-product in many countries. A better understanding and push toward sustainable development have shifted the focus of research from high-volume recycling to high-value utilization. One should be able to appreciate the role of new technological developments on the use of FA as a sustainable construction product and apply them in the industry. It is the hope of the

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authors that readers will be able to apply this knowledge and move from the phase of waste minimization to that of sustainable development in the case of FA, and in time apply the lessons of sustainable development, learned from the use FA, toward the development of other such industrial wastes.

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[27] S.K. Patel, R.K. Majhi, H.P. Satpathy, A.N. Nayak, Durability and microstructural properties of lightweight concrete manufactured with fly ash cenosphere and sintered fly ash aggregate, Constr. Build. Mater. 226 (2019) 579 590. Available from: https:// doi.org/10.1016/j.conbuildmat.2019.07.304. [28] U. Kleinhans, C. Wieland, F.J. Frandsen, H. Spliethoff, Ash formation and deposition in coal and biomass fired combustion systems: progress and challenges in the field of ash particle sticking and rebound behavior, Prog. Energy Combust. Sci. 68 (2018) 65 168. Available from: https://doi.org/10.1016/j.pecs.2018.02.001. [29] A. Hanif, Z. Lu, Z. Li, Utilization of fly ash cenosphere as lightweight filler in cementbased composites—a review, Constr. Build. Mater. 144 (2017) 373 384. Available from: https://doi.org/10.1016/j.conbuildmat.2017.03.188. [30] J.M. Moreno-Maroto, P.N. Camacho, T. Cotes-Palomino, C.M. Garcı´a, J. AlonsoAzca´rate, Manufacturing of lightweight aggregates from biomass fly ash, beer bagasse, Zn-rich industrial sludge and clay by slow firing, J. Environ. Manage. 246 (2019) 785 795. Available from: https://doi.org/10.1016/j.jenvman.2019.06.059. [31] X. Ma, J. Liu, C. Shi, A review on the use of LWA as an internal curing agent of high performance cement-based materials, Constr. Build. Mater. 218 (2019) 385 393. Available from: https://doi.org/10.1016/j.conbuildmat.2019.05.126. [32] P. Chen, J. Wang, F. Liu, X. Qian, Y. Xu, J. Li, Converting hollow fly ash into admixture carrier for concrete, Constr. Build. Mater. 159 (2018) 431 439. Available from: https://doi.org/10.1016/j.conbuildmat.2017.10.122. [33] A. Bicer, Effect of fly ash particle size on thermal and mechanical properties of fly ash-cement composites, Therm. Sci. Eng. Prog. 8 (2018) 78 82. Available from: https://doi.org/10.1016/j.tsep.2018.07.014. [34] Y. Hefni, Y.A.E. Zaher, M.A. Wahab, Influence of activation of fly ash on the mechanical properties of concrete, Constr. Build. Mater. 172 (2018) 728 734. Available from: https://doi.org/10.1016/j.conbuildmat.2018.04.021. [35] X. Fu, Q. Li, J. Zhai, G. Sheng, F. Li, The physical chemical characterization of mechanically-treated CFBC fly ash, Cem. Concr. Compos. 30 (3) (2008) 220 226. Available from: https://doi.org/10.1016/j.cemconcomp.2007.08.006.

An innovative and smart road construction material: thermochromic asphalt binder

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Henglong Zhang, Zihao Chen, Chongzheng Zhu and Chuanwen Wei Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, P.R. China

22.1

Introduction

Asphalt pavement is a paramount type of pavement and has been widely used around the world due to its excellent road performance, convenient rehabilitation measures, and comfortable driving conditions. As an adhesive material in pavement, asphalt binder is considered to have predominant effects and contributions to pavement performance. Asphalt is a viscoelastic material with rheological behaviors that are primarily dominated by temperature and time factors. Due to being susceptible to temperature, asphalt binder is prone to be soft at high temperatures while stiff at low temperatures, which results in rutting and cracking distresses of pavement, respectively. In addition, the inherent nature of the black exterior of asphalt makes it absorb a great deal of heat from solar radiation, which causes the surface temperature of pavement usually to be much higher than ambient air temperature in summer. This further exacerbates the permanent deformation of asphalt pavement in summer under vehicle loading influence. Asphalt is also an organic mixture composed of hydrocarbons and their derivatives, which makes it aged rapidly under influences of heat, oxygen, solar radiation, and water. After being seriously aged, asphalt becomes extremely stiff and brittle, leading to low-temperature and fatigue cracking of asphalt pavement. Therefore, the high surface temperature of pavement resulting from absorption of substantial solar energy by asphalt also exacerbates aging-related distresses of pavement. In addition, the urban heat island effect is a climatic phenomenon that increases the temperature of urban areas and brings about a series of consequent environmental issues. Among various contributing factors, the fact that asphalt pavement absorbs considerable heat from solar energy plays an important role. In terms of the above problems, there are two solutions to address them: one is that various modifiers are developed and added into asphalt binder to improve antiaging and high- and low-temperature performance of asphalt pavement itself, and the other is maintenance of temperature of asphalt pavement within a relatively reasonable range by innovative technologies. About the former method, there are many New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00022-3 © 2020 Elsevier Inc. All rights reserved.

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published studies on developing various asphalt modifiers [1 3]. For the latter method, there is a cool pavement which is coated by cool paving materials presenting fixed high reflectivity to solar radiation and high emissivity. Such cool pavements can reduce the surface temperature of asphalt pavement in summer, while the cooling effect on pavement also aggravates the distress of low-temperature cracking during cold weather, which compromises the general service life of such pavement. To compensate for the weakness of cool pavement, an innovative thermochromic asphalt binder, which implies adding thermochromic materials into conventional asphalt binder, has been increasingly investigated in recent years. Differing from cool paving materials, thermochromic materials are innovative substances which are able to dynamically adjust their appearance colors according to the ambient temperature and thus they also can dynamically adjust their reflectivity to solar radiation. Every thermochromic material has its own fixed transition temperature below which thermochromic materials are colored substances with lower reflectivity to solar radiation, while above this temperature they change to be colorless and exhibit higher solar energy reflection. Thermochromic materials have been effectively applied in the building construction field due to their dynamic features in optical and thermal properties. In the field of asphalt pavement construction, Hu et al. carried out pioneering work on the application of thermochromic materials into asphalt binders to control pavement surface temperatures within the appropriate range. They demonstrated that compared with pure asphalt binder, asphalt binders with thermochromic materials can largely reduce the pavement surface temperature in hot summer and increase it in winter [4 6]. Furthermore, they found that due to thermochromic material effects, asphalt binders with thermochromic materials revealed higher reflectivity within the near-infrared range above the transition temperature and higher heat capacity and lower thermal conductivity than pure asphalt binder [4 8]. Zhang et al. investigated the abilities of thermochromic asphalt binders to resist thermal and photo oxidation aging by indoor artificial simulations, such as the thin-film oven test (TFOT), pressure aging vessel (PAV) test, and ultraviolet (UV) radiation. The results revealed that aging resistances of thermochromic asphalt binders against three aging methods are better than those of pure asphalt binder [9,10]. Furthermore, Zhang et al. demonstrated that thermochromic asphalt binders exhibit better weathering aging resistance than pure asphalt binder [11]. Based on the above analyses, thermochromic asphalt binders are innovative and smart road construction materials which have promising prospects for addressing the problems mentioned above. Therefore, this chapter mainly introduces relevant knowledge and published research outcomes about thermochromic asphalt binders. The chapter first introduces thermochromic materials used in road construction concerning their components, structures, thermochromic mechanism, and thermal and optical properties. Then, the physical, rheological, and antiaging properties of thermochromic asphalt binders are illustrated in detail. Furthermore, the effects of thermochromic asphalt binders on keeping the surface temperature of pavement within a relatively reasonable range are evaluated. Finally, recommendations for future work into research and application of thermochromic asphalt binders are envisaged and proposed.

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22.2

693

Three-component organic reversible thermochromic materials

Reversible thermochromic materials are substances which are sensitive to temperature. Commonly, they display different appearance colors when the ambient temperature is above or below a certain critical value. More interestingly, they can reversibly change their appearance colors according to the ambient temperature. In accordance with their constituents, reversible thermochromic materials can be generally classified into three types—liquid crystals, inorganics, and organics. Among these, multicomponent organic reversible thermochromic compounds are the most attractive and prospective ones due to their wide range of transition temperatures and high sensitivities to temperature. Additionally, in order to improve the endurance and stability of this type of material, multicomponent thermochromic compounds are usually encapsulated by in situ polymerization, which produces socalled thermochromic microcapsule materials. Currently, multicomponent organic reversible thermochromic microcapsule materials have been extensively applied in many fields, such as the military, medical, building, and painting industries. For their applications in asphalt road construction field, three-component organic reversible thermochromic microcapsule materials are mainly investigated as asphalt binder modifiers.

22.2.1 Components and structures Generally, three-component organic reversible thermochromic microcapsule materials are comprised of two parts—core and shell materials. The core materials are referred to as three-component thermochromic compounds that contain three components of the color former, the color developer, and the solvent. The color former is usually a cyclic ester, which determines the color of the final product in its colored state. The color developer is usually a weak acid, which imparts the reversible color change to the thermochromic material and is responsible for the color intensity of the final product. The solvent is usually an alcohol or an ester whose melting point controls the transition temperature at which the color change occurs [12]. The shell materials of thermochromic microcapsules function as a barrier between the three-component thermochromic compounds and external surroundings to protect the thermochromic system from physical disruption or chemical erosion. The substance of urea-formaldehyde resin is widely used as a shell material of thermochromic microcapsules because microcapsules made from it possess excellent sealing and optical properties. A structural schematic diagram of a three-component organic reversible thermochromic microcapsule has been presented in Fig. 22.1. Moreover, Fig. 22.2 exemplifies the FTIR spectra of three kinds of thermochromic powders adopted by Zhang et al. who investigated their effects on aging behaviors of asphalt [9]. As presented in Fig. 22.2, the absorption bands centered around 3355 and 1509cm-1 correspond to hydroxyl (2OH) and benzene skeleton vibration derived from bisphenol a, a weak acid that functions as the color developer. The

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Figure 22.1 Structural schematic diagram of a three-component organic reversible thermochromic microcapsule.

Figure 22.2 The FTIR spectra of three types of thermochromic powders [9]. Black, blue, and red powders refer to three types of thermochromic powders.

characteristic peaks of methyl stearate that is used as solvent here lie in 2925, 2850, and 1740cm-1, indicating methyl (2CH3), methylene (2CH2), and carbonyl (C 5 O) group stretching vibration, separately. The peaks centered around 1552 and 813cm-1 are related to thiotriazinone stretching and melamine skeleton bending vibration, which are characteristics of melamine-formaldehyde resin, a common shell material of microcapsules.

22.2.2 Thermochromic mechanism Three-component organic reversible thermochromic microcapsule materials are substances which can dynamically change their appearance colors in accordance with the ambient temperature. Specifically, three-component organic reversible thermochromic microcapsules will lose their original colors and fade to be almost colorless when the ambient temperature is above a certain value, whereas they will return to

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Figure 22.3 Colors of thermochromic powders below and above the transition temperature: (left) below 31 C, (right) above 31 C [9].

their initial colors when the temperature declines below that value. The critical temperature at which color change occurs is called the transition temperature. Every three-component organic reversible thermochromic microcapsule has its own transition temperature. Fig. 22.3 illustrates when the ambient temperature is above their transition temperatures, three organic reversible thermochromic microcapsules whose transition temperatures are the same and equal to 31 C fade to be almost colorless [9]. The thermochromic mechanism of three-component organic reversible thermochromic microcapsule materials is that electron transfer and acquisition between the color former and the color developer due to varying temperature cause the change in molecular structure of the color former, which consequently makes the color former absorb or reflect different waves of light, mainly referring to visible light. From a macro perspective, it is shown that the thermochromic microcapsule changes its appearance color in accordance with the temperature. Specifically, the color former is an electron donor, while the color developer is an electron acceptor. When the ambient temperature is below the transition temperature, the structure of the electron donor will transfer from the lactone ring to quinone under the influence of the electron acceptor. When the ambient temperature rises above the transition temperature again, the structure of the electron donor will transfer from the quinone to lactone ring. In fact, the change in structure of the electron donor leads to the change in appearance color of the thermochromic microcapsule. In addition, the solvent determines the transition temperatures of thermochromic materials. Fig. 22.4 exhibits the molecular structures of electron donors contained by different thermochromic microcapsules and their structural changes with temperature [8].

22.2.3 Thermal and optical properties Three-component organic reversible thermochromic microcapsule materials display special thermal and optical properties due to their constituents, structures, and thermochromic reaction. Consequently, a full understanding of their thermal and optical properties contributes to further ascertaining their effect mechanisms on the properties of asphalt binders.

22.2.3.1 Spectrophotometry Hu et al. measured the spectral reflectance of three types of three-component organic reversible thermochromic microcapsules with a transition temperature of

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Figure 22.4 Molecular structures of electron donors contained by different thermochromic microcapsules and their structural changes with temperature: (A) black powder; (B) blue powder; (C) red powder [8].

31 C within the wavelength range of 300 1800 nm by UV-vis-IR spectrophotometry [4,8]. It can be seen from Fig. 22.5 that all thermochromic powders present high reflectivity in the spectrum of infrared range, whether the temperature is below or above their transition temperatures. In addition, compared with those at 25 C, thermochromic powders present significantly higher reflectivity in the visible as well as UV range at 35 C. This is mainly because the molecular structure of the constituent contained by the thermochromic microcapsule changes with the increment of temperature, which consequently makes the thermochromic microcapsule change its reflectivity in different waves of light. By spectrophotometry analysis, it can be further confirmed that the optical property of three-component organic reversible thermochromic microcapsules will change in accordance with temperature fluctuation (Fig. 22.5).

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Figure 22.5 Spectral reflectance of thermochromic powders at 25 C and 35 C [4,8]. Black, blue, and red refer to three types of thermochromic powders.

22.2.3.2 Thermogravimetric analysis In addition, Hu et al. further investigated the high-temperature stabilities of three types of three-component organic reversible thermochromic microcapsules with a transition temperature of 31 C by thermogravimetric analysis (TGA) technology [4]. As is shown in Fig. 22.6(a), the weights of all thermochromic powders start to decrease sharply once the temperature exceeds 200 C. Furthermore, based on differential TGA (DTG) curves which display the relationship between the mass loss rate and temperature, it can be also seen that the mass loss rates of all thermochromic powders begin to obviously increase after 200 C. The above experiment illustrates that the main pyrolysis procedures of these thermochromic microcapsules occur above 200 C, indicating their relatively good high-temperature stabilities. The reason for this is that the shell of the microcapsule has good thermal resistance, which can protect the whole microcapsule from serious heat influences. It also should be noted that pyrolysis temperatures of thermochromic microcapsules should be taken into consideration when thermochromic microcapsules are added into molten asphalt binder to prepare thermochromic asphalt binders.

22.2.3.3 Differential scanning calorimetry As previously mentioned, the transition temperatures of three-component organic reversible thermochromic microcapsules are dependent on the phase transition temperature of the solvent contained by the core materials of thermochromic microcapsules. Commonly, when the solvent melts as temperature increases, thermochromic microcapsules will fade to be colorless. Conversely, when the solvent freezes as temperature declines, the thermochromic microcapsules will return to their original colors. The thermochromic transition behaviors of thermochromic microcapsules can be characterized by differential scanning calorimetry (DSC). The phase

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Figure 22.6 TGA and DTG curves of thermochromic powders [4]. Black, blue, and red powders refer to three types of thermochromic powders.

Figure 22.7 DSC curves of thermochromic powders [6]. Black, blue, and red powders refer to three types of thermochromic powders.

transition temperature is determined as the maximum point on the endothermic or exothermic process, and the latent heat is evaluated through the integrated area of the peak profile. Hu et al. investigated phase transition temperatures and latent heat values of three types of three-component organic reversible thermochromic microcapsules with a transition temperature of 31 C [6]. The results are shown in Fig. 22.7. It can be seen that the melting and freezing temperatures are 27.28 C and 20.38 C, 34.41 C and 23.31 C, and 31.76 C and 22.21 C for black, blue, and red thermochromic powders, respectively. The melting and freezing latent values are

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33,660 and 35,680 J/kg, 25,040 and 29,370 J/kg, and 23,270 and 38,510 J/kg, respectively. The phase transition temperature is close to the transition temperature (31 C) reported by the manufacturer. This is because the experiment objects here are the whole thermochromic microcapsules rather than their solvents. Therefore, there are discrepancies between DSC test results and their real transition temperatures.

22.3

The performance characterization of thermochromic asphalt binders

22.3.1 Optical and thermal properties Modified by thermochromic microcapsule materials, thermochromic asphalt binders may show different optical and thermal properties from conventional pure asphalt binders. Hu et al. measured the spectral reflectance of three types of thermochromic asphalt binders which were respectively modified by black, blue, and red thermochromic powders whose transition temperatures are about 31 C [4,8]. It can be seen from Fig. 22.8 that all thermochromic asphalt binders are more reflective than conventional pure asphalt binder in the near-infrared range at both 25 C and 35 C. Hu et al. asserted that the increase in reflectance in the near-infrared region can be explained by the significantly high reflection of thermochromic powders [4,8], which is illustrated in Fig. 22.5. In the visible range, the difference of reflectance between thermochromic asphalt binders and conventional pure asphalt binder is negligible. In addition, Hu et al. investigated the high-temperature stabilities of these thermochromic asphalt binders by TGA technology [4]. Fig. 22.9 indicates that the main pyrolysis procedure occurs after 200 C for both thermochromic asphalt binders and conventional pure asphalt binder. Moreover, the weight losses of thermochromic asphalt binders are greater than that of conventional pure asphalt binder,

Figure 22.8 Spectral reflectance of various asphalt binders at 25 C (A) and 35 C (B) [4,8]. Black, blue, and red asphalt binders refer to three types of thermochromic asphalt binders.

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Figure 22.9 TGA and DTG curves of different asphalt binders [4]. Black, blue, and red asphalt binders refer to three types of thermochromic asphalt binders.

possibly because of decomposition of thermochromic microcapsules at high temperature. Table 22.1 presents the heat capacity and thermal conductivity results of various asphalt binders [5]. It can be seen that the heat capacity and thermal conductivity values of thermochromic asphalt binders are all larger than those of conventional pure asphalt binder, further indicating the effects of thermochromic powders on thermal properties of asphalt binder. To sum up, the special optical and thermal properties of thermochromic asphalt binders play a key role in the maintenance of a reasonable surface temperature of pavement.

22.3.2 Physical properties Softening point, penetration, and viscosity tests are commonly used to characterize the physical properties of asphalt binders. The softening point can reflect hightemperature stability to some extent, penetration can indicate the consistency, and viscosity can evaluate the flow resistance of asphalt binders. Zhang et al. investigated the effects of different thermochromic powders on the physical properties of base asphalt [9]. The results can be seen in Fig. 22.10. Compared with a blank sample, all asphalt binders with thermochromic powders exhibit higher softening point and viscosity while lower penetration values, indicating that the introduction of thermochromic powders could improve high-temperature stability, resistance to flow deformation, and consistency of base asphalt. Furthermore, Zhang et al. compared the effects of thermochromic powder on physical properties of base asphalt (70 pen grade) and styrene butadiene styrene copolymer (SBS) modified asphalt [13]. It can be seen from Fig. 22.11 that the thermochromic microcapsule increases the softening point and viscosity values of both base and SBS modified asphalt binders while it decreases their penetration. Zhang et al. explained that, before aging, thermochromic microcapsules are almost undamaged, and can function as fillers in

Table 22.1 Heat capacity and thermal conductivity results of various asphalt binders [5]. Black, blue, and red asphalt binders refer to three types of thermochromic asphalt binders. Heat capacity (J/kg
K)

Thermal conductivity (W/m
K)

Temperature ( C)

Calibration constant

Pure asphalt binder

Black asphalt binder

Blue asphalt binder

Red asphalt binder

Pure asphalt binder

Black asphalt binder

Blue asphalt binder

Red asphalt binder

15 25 35

0.019966 0.018217 0.017569

1392 1414 1412

1479 1563 1501

1447 1503 1484

1626 1724 1664

0.1498 0.1581 0.1749

0.1527 0.185 0.1801

0.1478 0.1581 0.1752

0.1657 0.2210 0.2282

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Figure 22.10 The effects of thermochromic powders on physical properties of base asphalt: (A) softening point, (B) penetration, (C) viscosity [9]. Black, blue, and red refer to three types of thermochromic powders; the number (4 or 6) refers to the contents of thermochromic powder; 4Blue refers to a thermochromic asphalt binder which is composed of base asphalt with 4% content of blue thermochromic powder.

the asphalt matrix to hinder movements of asphalt molecules and thus increase consistency and flow resistance of asphalt binders.

22.3.3 Rheological properties 22.3.3.1 Viscoelastic properties Asphalt exhibits an absolute elasticity at an extremely low temperature but becomes a fully viscous fluid at a relatively high temperature. Within the range of common service temperature, asphalt is a typical viscoelastic material whose rheological properties are dependent on two factors: temperature and time. As fundamental rheological parameters, complex shear modulus and phase angle indicate the binder total resistance to deformation and viscoelastic balance of behavior, separately. Similarly, Zhang and Du et al. investigated the effects of different thermochromic powders on viscoelastic properties of base asphalt (70#) and SBS copolymermodified asphalt by complex shear modulus and phase angle [9,13]. As shown in Fig. 22.12, it is clear that, in comparison with base asphalt, complex shear modulus values of asphalt binders with thermochromic powders are larger, while phase angle values are smaller, which illustrates that the addition of thermochromic powders

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Figure 22.11 The effects of thermochromic powder on physical properties of base asphalt (70#) and SBS-modified asphalt (SMA): (A) softening point, (B) penetration, (C) viscosity [13]. 70# refers to a base asphalt binder; 70# RT refers to a thermochromic asphalt binder which is composed of base asphalt and red thermochromic powder. SMA refers to SBS modified asphalt; SMART refers to a thermochromic asphalt binder which is composed of SBS modified asphalt and red thermochromic powder.

can increase the elasticity proportion in base asphalt and improve base asphalt total resistance to deformation. Zhang et al. maintained that the above phenomenon can be attributed to thermochromic microcapsules which function as fillers in asphalt matrix to constrain the movements of asphalt molecules and thus increase stiffness of asphalt binder [9]. It can be seen from Fig. 22.13 that complex modulus of SBS modified asphalt is increased due to the introduction of thermochromic powders. However, the different effects of thermochromic powder on the phase angle of base asphalt and SBS modified asphalt may be attributed to the special cross-link structure of SBS modifier [13].

22.3.3.2 Rutting performance Rutting is a common pavement distress which results from the cumulative permanent deformation of pavement under influences of high temperature and repeated vehicle loads. The high-temperature multiple stress creep recovery (MSCR) test is appropriate to evaluate the rutting performance of asphalt binder because it can

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Figure 22.12 The effects of thermochromic powders on viscoelastic properties of base asphalt: (A) complex modulus, (B) phase angle [9]. Black, blue, and red refer to three types of thermochromic powders; the number (4 or 6) refers to contents of thermochromic powder; 4Blue refers to a thermochromic asphalt binder which is composed of base asphalt and 4% content of blue thermochromic powder.

Figure 22.13 The effects of thermochromic powder on viscoelastic properties of base asphalt (70#) and SBS-modified asphalt (SMA): (A) complex modulus, (B) phase angle [13]. 70# refers to a base asphalt binder; 70# RT refers to a thermochromic asphalt binder which is composed of base asphalt and red thermochromic powder. SMA refers to SBS modified asphalt; SMART refers to a thermochromic asphalt binder which is composed of SBS modified asphalt and red thermochromic powder.

simulate actual loading conditions of pavement, which consists of 1 second of creep loading followed by 9 seconds of recovery over multiple stress levels of 0.1 and 3.2 kPa at 10 cycles for each stress level. Nonrecoverable creep compliance (Jnr) is the rutting potential index in the MSCR test, which is equal to the average nonrecovered strain for the 10 creep and recovery cycles divided by the corresponding applied stress in those cycles. In addition, average percent recovery (R) can also reflect the elasticity of asphalt binder, which is equal to the average ratio of recovered strain to maximum strain in every cycle under corresponding applied stress. The smaller nonrecoverable creep compliance (Jnr) combined with larger average

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Figure 22.14 MSCR test results of 70# and 70#RT: (A) nonrecoverable compliance, (B) average percent recovery [13]. 70# refers to a base asphalt binder; 70# RT refers to a thermochromic asphalt binder which is composed of base asphalt and red thermochromic powder.

Figure 22.15 MSCR test results of SMA and SMART: (A) nonrecoverable compliance, (B) average percent recovery [13]. SMA refers to SBS modified asphalt; SMART refers to a thermochromic asphalt binder which is composed of SBS modified asphalt and red thermochromic powder.

percent recovery (R) indicates better rutting resistance of asphalt binder. Du et al. investigated the effects of thermochromic powder on the rutting performance of base asphalt (70#) and SBS copolymer modified asphalt [13]. The results are shown in Figs. 22.14 and 22.15. It can be observed that the rutting resistances of PAV aged 70# and 70#RT are much better than their TFOT aged counterparts. In contrast, rutting resistances of PAV aged SMA and SMART are decreased compared with their TFOT aged samples. In addition, compared with 70#, 70#RT exhibits larger Jnr and smaller R when suffering TFOT or PAV aging conditions, which indicates thermochromic microcapsules weaken the hardening effect caused by aging and make aged base asphalt softer. However, there is almost no obvious discrepancy between SMA and SMART in Jnr and R values, indicating thermochromic microcapsules have few effects on rutting resistances of TFOT and PAV aged SBS modified asphalt binder.

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Du et al. considered that the combined effects of SBS modifier and thermochromic microcapsules on asphalt matrix cause behavioral discrepancies between SMA and 70# influenced by thermochromic microcapsules [13].

22.3.3.3 Fatigue performance The fatigue failure of asphalt binder leads to the occurrence of microfractures in the binder, which further induces other distresses such as moisture damage and fatigue crack. The linear amplitude sweep (LAS) test is usually used to assess fatigue resistance of asphalt binder by applying cyclic loading at increasing amplitudes. Based on damage characteristics of material, the fatigue life of asphalt binder can be calculated using predictive modeling techniques. Du et al. investigated the effects of thermochromic powder on fatigue performance of base asphalt (70#) and SBS copolymer modified asphalt [13]. It is observed from Fig. 22.16(a) that for four types of asphalt binders, their integrity parameters gradually decrease as damage intensity increases. At the same damage intensity, integrity levels of asphalt binders modified with thermochromic microcapsule are higher than their counterparts without thermochromic microcapsules. Based on viscoelastic continuum damage (VECD) analysis, fatigue lives of four types of asphalt binders at different strain levels are represented in Fig. 22.16(b). It is clear that thermochromic microcapsules can improve the fatigue lives of both base and SBS modified asphalt binders and improvement effect on base asphalt is more obvious than on SBS modified asphalt binder. Du et al. proposed that methyl stearate outflowing from damaged microcapsules may heal minimal structural damage caused by accelerated amplitude sweep and preclude further expansion of cracks and thus improve the fatigue lives of asphalt binders [13].

Figure 22.16 LAS test results of four types of asphalt binders: (A) damage characteristic curves, (B) predictive fatigue life at different strain levels [13]. 70# refers to a base asphalt binder; 70# RT refers to a thermochromic asphalt binder which is composed of base asphalt and red thermochromic powder. SMA refers to SBS modified asphalt; SMART refers to a thermochromic asphalt binder which is composed of SBS modified asphalt and red thermochromic powder.

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Figure 22.17 The effects of thermochromic powders on low-temperature performance of base asphalt: (A) stiffness, (B) m-value at different low temperatures [9]. Black, blue, and red refer to three types of thermochromic powders; the number (4 or 6) refers to contents of thermochromic powder; 4Blue refers to a thermochromic asphalt binder which is composed of base asphalt and 4% content of blue thermochromic powder.

22.3.3.4 Low-temperature performance It is known that asphalt is susceptible to temperature. The stiffness of asphalt binder increases while the stress relaxation ability decreases with a decrement of temperature, which easily induces temperature cracking of pavement when a sharp temperature drop occurs. The bending beam rheolometer (BBR) test is a method which is commonly used to evaluate low-temperature performance of asphalt binder. Zhang et al. explored the effects of thermochromic powders on lowtemperature performance of base asphalt [9]. The results are shown in Fig. 22.17. It can be seen that asphalt binders with thermochromic powders display smaller stiffness values and larger m-values than a blank sample, illustrating that the addition of thermochromic powders could improve the deformability capacity and stress relaxation capability of asphalt at low temperatures. The reason for this phenomenon may be that the methyl stearate outflows from damaged thermochromic microcapsules and functions as maltene to soften the colloidal structure of aged asphalt [9]. Thus, the low-temperature performance of asphalt binder can be improved by thermochromic microcapsules.

22.3.4 Antiaging properties 22.3.4.1 Aging resistance evaluation Aging of asphalt is one of the critical factors causing the deterioration of pavements [14]. Commonly, the aging process evolves with changes in the physical and chemical properties of asphalt, leading to stiffness increase, poor adhesion, and reduced coating properties [15,16]. The aging of asphalt can be categorized into two types: thermal oxidation and photo oxidation aging (the latter mainly refers to UV

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irradiation) [17,18]. Short-term thermal oxidation aging occurs when asphalt is exposed to heat and air in the process of asphalt mixture production and paving, which is primarily due to the oxidation and loss of volatile components at high temperatures. Long-term thermal oxidation aging proceeds during the service life of pavement as a result of continuous oxidation. In terms of UV irradiation aging, the structures of asphalt molecules change with the absorption of UV energy that induces the cleavage of chemical bonds and produces the oxidation components, finally causing an increment in the stiffness and embrittlement of asphalt binder. Generally, indoor artificial simulations, such as TFOT, PAV test, and UV radiation, are adopted to simulate short-term and long-term thermal oxidation and photo oxidation aging of asphalt, respectively. Furthermore, in order to fully simulate the real natural environment that asphalt binder suffers, weathering aging, namely putting asphalt binder in an outdoor environment, is also an important aging simulation method. Currently, the methodology for assessing the aging degree of asphalt binder is primarily through measurement of the changing amplitude of certain parameters before and after aging; these parameters are often physical, chemical, and rheological properties. The greater amplitude of these parameters means more serious aging of asphalt binder. Zhang et al. utilized indices including complex modulus aging index (CMAI), phase angle aging index (PAI), carbonyl index (CI), and sulfoxide index (SI) to evaluate the effects of thermochromic powders on aging resistance of asphalt binders. The specific definitions of these aging indices are presented in the following equations [9 11,19]. CAI 5

Aged complex modulus Unaged complex modulus

(22.1)

PAI 5

Aged phase angle Unaged phase angle

(22.2)

AC5O CI 5 P A

(22.3)

AS5O SI 5 P A

(22.4)

where AC5O is the integral area of the carbonyl group centered around 1700cm-1 and AS5O is the integral area of the sulfoxide group centered around 1030cm-1. P The sum of the area represents: A 5 A1700cm 21 1 A1600cm 21 1 A1460cm 21 21 21 21 1 A1376cm 1 A1030cm 1 A864cm 1 A814cm 21 1 A743cm 21 1 A724cm 21 . Zhang et al. investigated the effects of thermochromic microcapsules on TFOT, PAV, and UV aging resistances of base asphalt by aging indices of CAI and PAI

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Figure 22.18 Effects of thermochromic microcapsules on TFOT aging resistance of base asphalt: (A) CAI, (B) PAI [9]. Black, blue, and red refer to three types of thermochromic powders; the number (4 or 6) refers to contents of thermochromic powder; 4Blue refers to a thermochromic asphalt binder which is composed of base asphalt and 4% content of blue thermochromic powder.

Figure 22.19 Effects of thermochromic microcapsules on PAV aging resistance of base asphalt: (A) CAI, (B) PAI [9]. Black, blue, and red refer to three types of thermochromic powders; the number (4 or 6) refers to contents of thermochromic powder; 4Blue refers to a thermochromic asphalt binder which is composed of base asphalt and 4% content of blue thermochromic powder.

[9]. It is known that the smaller CAI and the larger PAI mean better aging resistance of asphalt binder. The results obtained are shown in Figs. 22.18 22.20. It can be seen that all asphalt binders containing thermochromic powders exhibit smaller CAI and larger PAI than the blank sample, no matter what aging method is used, illustrating that thermochromic powders can improve both thermal oxidation and photo oxidation aging resistances of base asphalt. In addition, it is well known that the oxidation of asphalt leads to the products of highly polar and strongly interacting oxygen-containing functional groups like carbonyl group (C 5 O) and sulfoxide group (S 5 O). Therefore, it is viable to evaluate the aging degree of asphalt binder

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Figure 22.20 Effects of thermochromic microcapsules on UV aging resistance of base asphalt: (A) CAI, (B) PAI [9]. Black, blue, and red refer to three types of thermochromic powders; the number (4 or 6) refers to contents of thermochromic powder; 4Blue refers to a thermochromic asphalt binder which is composed of base asphalt and 4% content of blue thermochromic powder.

Table 22.2 Carbonyl index (CI) of blank sample and 4% BTP binder after different aging modes [10]. Blank sample refers to a base asphalt binder; 4% BTP binder refers to a thermochromic asphalt binder which is composed of base asphalt and 4% content of red thermochromic powder. Blank sample

4% BTP binder

Sample

CI

WCI

CI

WCI

Unaged TFOT aging UV aging PAV aging

0.0120 0.0167 0.0201 0.0343

— 0.0047 0.0081 0.0223

0.0074 0.0083 0.0136 0.0261

— 0.0009 0.0062 0.0187

quantitatively by tracking the contents of these groups. The larger CI difference (ΔCI) and SI difference (ΔSI) between aged and unaged samples, the deeper the aging of binder is. Zhang et al. further demonstrated improving effects of thermochromic powders on aging resistance of base asphalt by Fourier transform infrared spectroscopy (FTIR) [10]. As shown in Table 22.2, it is clear that with deterioration of aging, ΔCI is gradually magnified for both the blank sample and asphalt binder with thermochromic powder. In addition, ΔCI of asphalt binder with thermochromic powder is smaller than that of the blank sample after all three aging methods, further demonstrating the superior antiaging properties (including short- and longterm thermal oxidation and photo oxidation aging) of thermochromic asphalt binder. Zhang et al. further investigated the weathering aging resistance of thermochromic asphalt binder [11]. It can be seen in Fig. 22.21 that weathering aging increases

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Figure 22.21 Carbonyl and sulfoxide indices with different weathering aging time for blank sample and thermochromic asphalt binder [11]. 70# refers to a base asphalt binder; TAB refers to a thermochromic asphalt binder which is composed of base asphalt and 6% content of red thermochromic powder.

Figure 22.22 Effect of red thermochromic microcapsule on aging susceptibility: (A) base asphalt, (B) SBSmodified asphalt [13]. 70# refers to a base asphalt binder; 70# RT refers to a thermochromic asphalt binder which is composed of base asphalt and red thermochromic powder. SMA refers to SBS modified asphalt; SMART refers to a thermochromic asphalt binder which is composed of SBS modified asphalt and red thermochromic powder.

the contents of both carbonyl and sulfoxide groups, implying that oxidation indeed occurs for the blank sample and thermochromic asphalt binder. In comparison with the blank sample, growth rates and amplitudes of carbonyl and sulfoxide index of thermochromic asphalt binder are much lower, further verifying the reduced aging susceptibility of thermochromic asphalt binder. In addition, Du et al. investigated the effects of thermochromic powder on aging resistances of base and SBS modified asphalt [13]. The results of their experiment

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Table 22.3 Carbonyl index (CI) of asphalt binders before and after different aging modes [13]. 70# refers to a base asphalt binder; 70# RT refers to a thermochromic asphalt binder which is composed of base asphalt and red thermochromic powder. SMA refers to SBS modified asphalt; SMART refers to a thermochromic asphalt binder which is composed of SBS modified asphalt and red thermochromic powder. 70#

70#RT

SMA

SMART

Sample

CI ( 3 1023)

WCI ( 3 1023)

CI ( 3 1023)

WCI ( 3 1023)

CI ( 3 1023)

WCI ( 3 1023)

CI ( 3 1023)

WCI ( 3 1023)

Unaged TFOT UV PAV

1.181 4.909 7.419 37.471

3.728 6.238 36.290

1.260 2.341 6.693 7.398

1.082 5.433 6.139

1.567 3.554 5.649 43.271

1.988 4.082 41.705

1.283 2.984 4.726 10.659

1.701 3.443 9.376

are presented in Fig. 22.22 and Table 22.3. It can be seen that, after TFOT, UV, and PAV aging, the CMAI and the increment of carbonyl group (WCI) values of base and SBS modified asphalt containing thermochromic powders are all much smaller than those of their counterparts. He concluded that like base asphalt binder, aging resistance of SBS modified asphalt binder was also improved due to the introduction of thermochromic microcapsules, while the influence of thermochromic microcapsules on antiaging behaviors of SBS modified asphalt binder was more complex in comparison with base asphalt due to the existence of SBS modifier [13].

22.3.4.2 Aging mechanism discussion Based on the above analyses, thermochromic microcapsules can indeed improve both the thermal oxidation and photo oxidation aging resistances of asphalt binders. Zhang et al. discussed the aging mechanisms of thermochromic asphalt binders. They contend that asphalt matrix stiffens with an increment of aging severity because volatilization of light components and association between resins and asphaltenes make asphalt colloid structure transform from sol to gel under heat, oxygen, and sunlight influences. However, asphalt binders with thermochromic microcapsules reveal higher reflectivity within the near-infrared range at high temperature than pure asphalt binder. This indicates that thermochromic microcapsules can help reduce asphalt binder solar absorption and thus prevent asphalt matrix being stiffer due to thermal aging. Furthermore, because of aging effects, shells of parts of thermochromic microcapsules are damaged, which results in outflow of core materials. As the largest part of core materials, methyl stearate with low density and melting point is a substance like fluid oil, which can compensate for light components volatilisation and improve the solubility of micelle. Additionally, methyl stearate is also a phase change material with relatively low phase temperature. Methyl stearate outflowing from microcapsules and melted

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with asphalt matrix can absorb much heat from asphalt matrix while keep its temperature almost constant, which can also reduce the thermal influence on the asphalt matrix [11].

22.4

The adjustment of bituminous pavement temperature

Thermochromic materials have been effectively applied in the building construction field due to their dynamic features in optical and thermal properties. In the field of asphalt pavement construction, Hu and Yu carried out pioneering work on evaluating the effects of thermochromic coating materials on the thermal responses of pavements [4]. The coatings were applied to the surface of hot mixture asphalt (HMA) specimens. Surface temperatures were recorded under the coating layer. The specimens were exposed to an open space on the top of a building with no tree shade during a typical hot summer day and a typical cold winter day. The difference between the surface temperatures of thermochromic asphalt coatings and conventional pure asphalt coating (WT 5 Tthermochromic 2 Tpure) with time was recorded under different weather conditions. It can be seen in Fig. 22.23 that, at high ambient temperatures, the surface temperatures of the thermochromic coatings are consistently lower than that of the traditional pure asphalt coating. The maximum

Figure 22.23 Difference between the surface temperatures of thermochromic asphalt coatings and conventional pure asphalt coating on a hot sunny day [4]. Black, blue, and red asphalt binders refer to three types of thermochromic asphalt binders.

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Figure 22.24 Difference between the surface temperatures of thermochromic asphalt coatings and conventional pure asphalt coating on a cold snowy day [4]. Black, blue, and red asphalt binders refer to three types of thermochromic asphalt binders.

reductions in surface temperature are 6.6 C, 2.7 C, and 4.9 C for the black, blue, and red thermochromic asphalt coatings, respectively. The effectiveness of surface temperature reductions between the samples was attributed to the increases in the solar reflectance of the asphalt coatings due to the introduction of thermochromic powders [4]. Fig. 22.8 shows that thermochromic asphalt binders are more reflective than conventional pure asphalt binder in the near-infrared range. Therefore, the thermochromic asphalt coatings can make bituminous pavement mitigate the absorption of heat more effectively than conventional pure asphalt coating. Fig. 22.24 illustrates the differences between the surface temperatures of asphalt coatings during a cold winter day. Compared to a conventional asphalt coating, thermochromic asphalt coatings keep the surface warmer under low-temperature conditions. The use of a thermochromic asphalt coating increases the maximum surface temperature by about 1 C. This means that unlike the cool pavements mentioned previously, thermochromic asphalt coatings do not have undesirable cooling effects on pavement in low-temperature conditions. Hu and Yu explained that the increase in surface temperature in winter is probably mainly due to the high heat capacity and low thermal conductivity of thermochromic asphalt coatings [4].

22.5

Recommendations for future research and applications

Based on the above analyses, thermochromic asphalt binders exhibit excellent antiaging performance and maintain the surface temperature of bituminous pavement

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within a reasonable range, which contributes to the alleviation of pavement distresses, extension of pavement durability, and improvement of the urban heat island problem. However, there are still some problems which need to be addressed in future research and applications. G

G

Currently, the effects of thermochromic materials on the performance of asphalt binders have been considerably investigated, while investigations into asphalt mixtures prepared with thermochromic materials are insufficient. Problems, such as the determination of optimum preparation conditions of asphalt mixtures prepared with thermochromic materials and the effects of thermochromic materials on performance of asphalt mixtures, need to be further addressed in the future. It is known that thermochromic powders are microcapsule materials. Core materials, namely three-component thermochromic compounds, are effective components which make thermochromic powders change their appearance colors in accordance with temperature. However, under aging influences, thermochromic microcapsules are damaged and core materials outflow from the microcapsule shell. On one hand, these outflowing core materials will be beneficial to softening aged asphalt matrix. On the other hand, the wastage of core materials will make thermochromic powders lose thermochromic function, which is not beneficial to control of the surface temperature of pavement. How to keep a balance between this pair of contradictory problems is an essential question.

References [1] C.Z. Zhu, H.L. Zhang, L.K. Huang, C.W. Wei, Long-term performance and microstructure of asphalt emulsion cold recycled mixture with different gradations, J. Clean. Prod. 215 (2019) 944 951. [2] D.M. Zhang, H.L. Zhang, C.J. Shi, Investigation of aging performance of SBS modified asphalt with various aging methods, Constr. Build. Mater. 145 (2017) 445 451. [3] H.L. Zhang, C.Z. Zhu, J.Y. Yu, B.Y. Tan, C.J. Shi, Effect of nano-zinc oxide on ultraviolet aging properties of bitumen with 60-80 penetration grade, Mater. Struct. 48 (10) (2015) 3249 3257. [4] J.Y. Hu, X. Yu, Innovative thermochromic asphalt coating: characterisation and thermal performance, Road. Mater. Pavement 17 (1) (2016) 187 202. [5] J.Y. Hu, X. Yu, Experimental study of sustainable asphalt binder: influence of thermochromic materials, Transp. Res. Rec. J. Transp Res. Board. 2372 (2013) 108 115. [6] J.Y. Hu, N. Wanasekara, X. Yu, Thermal properties of thermochromic asphalt binders by modulated differential scanning calorimetry, Transp. Res. Rec. J. Transp Res Board. 2444 (2014) 142 150. [7] J.Y. Hu, X. Yu, Reflectance spectra of thermochromic asphalt binder: characterization and optical mixing model, J. Mater. Civ. Eng. 28 (2) (2016). 04015121-1B0401512110. [8] J.Y. Hu, Q. Gao, X. Yu, Characterization of the optical and mechanical properties of innovative multifunctional thermochromic asphalt binders, J. Mater. Civ. Eng. 27 (5) (2015). 04014171-1B04014171-10. [9] H.L. Zhang, Z.H. Chen, L. Li, C.Z. Zhu, Evaluation of aging behaviors of asphalt with different thermochromic powders, Constr. Build. Mater. 155 (2017) 1198 1205.

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[10] H.L. Zhang, Z.H. Chen, G.Q. Xu, C.J. Shi, Physical, rheological and chemical characterization of aging behaviors of thermochromic asphalt binder, Fuel 211 (2018) 850 858. [11] Z.H. Chen, H.L. Zhang, C.J. Shi, C.W. Wei, Rheological performance investigation and sustainability evaluation of asphalt binder with thermochromic powders under solar radiation, Sol. Energy Mat. Sol. C. 191 (2019) 175 182. [12] T. Karlessi, M. Santamouris, K. Apostolakis, A. Synnefa, I. Livada, Development and testing of thermochromic coatings for buildings and urban structures, Sol. Energy 83 (4) (2009) 538 551. [13] P.F. Du, Z.H. Chen, H.L. Zhang, Rheological and aging behaviors of base and SBS modified asphalt with thermochromic microcapsule, Constr. Build. Mater. 200 (2019) 1 9. [14] Q. Qin, J.F. Schabron, R.B. Boysen, M.J. Farrar, Field aging effect on chemistry and rheology of asphalt binders and rheological predictions for field aging, Fuel 121 (2) (2014) 86 94. [15] M. Zaumanis, R.B. Mallick, Review of very high-content reclaimed asphalt use in plant-produced pavements: state of the art, Int. J. Pavement Eng. 16 (1) (2015) 39 55. [16] R. Karlsson, U. Isacsson, Material-related aspects of asphalt recycling: state-ofthe-art, J. Mater. Civ. Eng. 18 (1) (2006) 81 92. [17] F. Zhang, J.Y. Yu, J. Han, Effects of thermal oxidative ageing on dynamic viscosity, TG/DTG, DTA and FTIR of SBS- and SBS/sulfur-modified asphalts, Constr. Build. Mater. 25 (1) (2011) 129 137. [18] F. Durrieu, F. Farcas, V. Mouillet, The influence of UV aging of a styrene/butadiene/ styrene modified bitumen: comparison between laboratory and on site aging, Fuel 86 (s 10-11) (2007) 1446 1451. [19] H.L. Zhang, Z.H. Chen, G.Q. Xu, C.J. Shi, Evaluation of aging behaviors of asphalt binders through different rheological indices, Fuel 221 (2018) 78 88.

Resin and steel-reinforced resin used as injection materials in bolted connections

23

Haohui Xin, Martin Nijgh and Milan Veljkovic Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands

23.1

Introduction

The joints and connections play an important role in the construction of steel structures. It is common practice to use mechanical fasteners such as bolts to join the elements of a steel structure on site. Bolted connections in steel structures are generally fabricated with hole clearances for easy assembling. The clearances can lead to some slips occurring in the joints, making the bolted connection slip-critical [1] under static or dynamic loading. Conventional methods, including rivets, fitted bolts, or preloaded high-strength friction grip (HSFG) bolts, are generally used to improve the slip resistance. Injection bolts are regarded as a suitable alternative for a renovation of conventional connections of large-span structures [2
4]. As shown in Fig. 23.1, injection bolts are bolts in which the cavity produced by the clearance between the bolt and the wall of the hole is completely filled up with a two-component resin. Filling of the clearance is carried out through a small hole in the head of the bolt. After injection and complete curing, the connection is slip resistant [5]. Injection bolts also have the advantage that no sudden large displacements occur in the case of overloading compared with HSFG bolts [7]. The applications of injection bolts include not only repairing of old riveted structures and new construction [5,6,8]. Damaged riveted connections have been replaced with injection bolts in the Netherlands since the 1970s [8]. The choice for using resin-injected bolted connections as a replacement rather than using new rivets lies in the fact that riveting is no longer a very common connection method. As shown in Fig. 23.2, a successful application of injection bolts to repair a riveted structure was performed on the bridge “Schlossbru¨cke Oranienburg” [3] in Germany. In addition, injection bolts have been successfully applied for compact connections in a glass roof structure near Amsterdam Central Station [5]. The rotations introduced with normal bolted connections were too large. Injection bolts have been applied successfully in this application to limit rotations in the connections of the glass roof structure. In order to prevent connection slippage at the connection between the truss and ball bearing in Maeslant Storm Surge Barrier [6] in New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00023-5 © 2020 Elsevier Inc. All rights reserved.

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Figure 23.1 The injected bolts: (A) Photo [5]; (B) Schematic [6].

the Netherlands, resin-injected preloaded bolts have been used of metric sizes M56, M64, M72, and M80. In the ECCS recommendations for injection bolts [7], the design bearing stress is determined based on long-duration testing. The slip for the serviceability limit state verification is suggested to be smaller than 0.3 mm [7]. Since resins are susceptible to creep deformation if the bearing stress is too high, the bearing stress needs to be kept within certain limits. Recently, the injection material, typically an epoxy resin, was modified at TU Delft by adding steel shots (spherical particles) to mitigate the effects of resin compliance in the shear connection of reusable composite (steel
concrete) structures [6], as shown in Fig. 23.3. The steel shots serve as reinforcement to the epoxy resin matrix. The increase in compressive strength and the expected improvement of creep characteristics of the reinforced injected materials, especially in a bolt hole serving as nature confinement environment, are expected to improve the performance of connections exposed to monotonic and cyclic loading. In addition to experimental research, numerical simulations could play an important role in the qualification and certification of the short- and long-term behavior of

Figure 23.2 Corrosion of steel structure and subsequent repair with injection bolts [5].

Figure 23.3 Steel-reinforced resin [6].

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injection bolts. The material models of resin/steel-reinforced resin should be investigated before conducting finite element simulation on injection bolts. It is important to adopt a multiscale analysis to determine the mechanical properties of the steelreinforced resin. The numerical homogenization method [9], which accurately considers the geometry and spatial distribution of the phases, and also precisely estimates the propagation of damage to accurately predict the failure strength [10
12], is considered to be an effective modeling tool to analyze steel-reinforced resin. Computational homogenization methods of fine-scale models provide a pathway to use high-fidelity models (HFMs) to predict macroscopic mechanical responses of steel-reinforced resin. However, the high-fidelity numerical homogenization methods are reported to be computationally expensive [13
16]. The hierarchical strategy, where experimental results and HFM are employed to train a low-fidelity model (LFM) and to supplement the experimental database, is adopted to model the material behavior of steel-reinforced resin [13]. The performance of the steelreinforced resin is effectively predicted by an elaborate but computationally inexpensive LFM identified by a more fundamental but computationally taxing HFM, which was calibrated to the experimental results. Experimental compressive material tests on unconfined/confined resin and steelreinforced resin are evaluated in this chapter. The uniaxial model which combines damage mechanics and the Ramberg
Osgood relationship is proposed to describe the uniaxial compressive behavior of resin and steel-reinforced resin. First-order numerical homogenization is employed as a HFM, where a combined nonlinear isotropic/kinematic cyclic hardening model is employed to define the steel plasticity, the linear Drucker
Prager plastic criterion was used to simulate resin damage, and the cohesive surfaces reflecting the relationship between traction and displacement at the interface. The linear Drucker
Prager plastic model is used as a LFM.

23.2

Computational homogenization

The link between microscale and macroscale behavior is established based on the Hill
Mandel computational homogenization method. The macroscale Cauchy stress σij is obtained by averaging the microscale Cauchy stress, σ~ ij , in the unit cell domain, expressed as [9]: 1 σij 5 jΘj

ð Θ

σ~ ij dΘ

(23.1)

where σij is the macroscale Cauchy stress, σ~ ij is the microscale Cauchy stress, and Θ is the domain of the unit cell. The unit cell problem is solved for the leading order translation-free microscale displacement. The microscale displacement ufi ðx; yÞ is expressed in the form [9]: ufi ðx; yÞ 5 εcij yj 1 uði 1Þ ðx; yÞ

(23.2)

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where x is the macroscale position vector in the macroscale domain, y is the microscale position vector in the unit cell domain, εcij is the strain tensors in the macroscale domain, and uði 1Þ ðx; yÞ is the perturbation displacement of the microscale. The periodic boundary conditions [9] in the unit cell domain could be implemented by so-called “mixed boundary conditions” via constraint equations, is expressed by the following equations [9,17]: ð @ΘY

  ufi ðx; yÞ 2 εcik yk NjΘ dγ Y 5 0

   f  ui ðx; yÞ 2 εcik yk NjΘ # Tol

(23.3)

(23.4)

where NjΘ is the unit normal to the unit cell boundary @Θy .

23.3

Experiments

23.3.1 Material tests The epoxy resin used in the tests is RenGel SW 404 with hardener HY 2404 at ambient temperature. Reinforcing steel particles are chosen as steel shot S330 with a nominal diameter of 0.84 mm. Compression tests on both unconfined and confined conditions were carried out. As is shown in Figs. 23.4 and 23.5, the dimensions of the unconfined specimen are 26 mm 3 50 mm. The nominal dimensions of the confined specimen are 22 mm 3 22 mm, confined by S235 steel tube with dimensions of 30mm 3 50 mm 3 4 mm and loaded by a S355 steel cylinder with dimensions of 22mm 3 40 mm. Five specimens of each type, totaling 20 specimens, were prepared and tested in order to investigate the compressive behavior of resin and steel-reinforced resin. The load is applied with a displacement speed of 0.01 mm/s. Two GS-551 linear variable displacement transformers (LVDTs) were employed to measure the axial deformation of the specimens.

23.3.2 Experimental results 23.3.2.1 Unconfined specimens The compressive results of unconfined resin and steel-reinforced resin specimens are summarized in Tables 23.1
23.4. It is noted that the true ultimate strength of unconfined resin is 17.6% smaller than the nominal ultimate strength. Attention should be paid to large differences between nominal and true ultimate compressive strength of resin during finite element simulation. The stress
strain relationship of unconfined resin and steel-reinforced resin specimens is shown in Figs. 23.6 and 23.7. The stress
strain curve of unconfined resin generally consists of three stages: (1) the stress increases linearly with strain increasing; (2) yielding occurs, the stress

Figure 23.4 Schematic of unconfined/confined specimens: (A) unconfined; (B) confined.

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Figure 23.5 Experimental set-up. Table 23.1 Results of unconfined resin specimen from nominal stress/strain. Specimen

U-R-1 U-R-2 U-R-3 U-R-4 U-R-5 Average S.D.

Young’s modulus

Ultimate strength

Fracture initiation strain

Fracture strain

E (GPa)

σu (MPa)

εf0 (%)

εfu (%)

5.30 6.15 5.83 5.45 5.49 5.64 0.34

171.7 168.9 173.2 168.7 166.6 169.8 2.62

18.20 18.34 18.20 17.34 17.96 18.01 0.40

21.59 21.86 20.24 22.31 24.84 22.17 1.68

increases nonlinearly with increasing strain; and (3) fracture initiates when the resistance is reached, followed by a decrease in stress with an increase in strain. The stress
strain curve of unconfined steel-reinforced resin generally included two stages: (1) the stress increases almost linearly with increasing strain; and (2) damage occurs when the maximum strength is reached, followed by a decrease in stress with increasing strain. The failure modes of resin and steel-reinforced resin specimens are shown in Fig. 23.8. The longitudinal and diagonal cracks of resin were initiated with increasing load. The final failure of resin occurred after the long cracks propagated through the whole specimen and the specimen was split into two. The diagonal cracks of steel-reinforced specimens were initiated from the bottom of the specimens. The steel-reinforced specimen failed when the diagonal cracks propagated to the bottom of the specimen.

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Table 23.2 Results of unconfined resin specimen from true stress/strain. Specimen

U-R-1 U-R-2 U-R-3 U-R-4 U-R-5 Average S.D.

Young’s modulus

Ultimate strength

Fracture initiation strain

Fracture strain

E (GPa)

σu (MPa)

εf0 (%)

εfu (%)

5.20 6.02 5.72 5.33 5.38 5.53 0.33

141.69 139.23 142.43 141.47 138.72 140.71 1.63

19.70 19.43 19.78 18.64 19.71 19.45 0.47

24.66 24.72 24.34 24.51 26.58 24.96 0.92

Table 23.3 Results of unconfined steel-reinforced resin specimen from nominal stress/ strain. Specimen

U-SR-1 U-SR-2 U-SR-3 U-SR-4 U-SR-5 Average S.D.

Young’s modulus

Ultimate strength

Fracture initiation strain

Fracture strain

E (GPa)

σu (MPa)

εf0 (%)

εfu (%)

15.90 16.30 15.50 15.60 15.10 15.70 0.41

117.97 119.52 124.13 119.48 122.14 120.30 2.72

0.97 1.01 0.94 1.08 1.03 1.01 0.054

3.86 4.87 3.97 4.84 4.98 4.51 0.54

Table 23.4 Results of unconfined steel-reinforced resin specimen from true stress/strain. Specimen

U-SR-1 U-SR-2 U-SR-3 U-SR-4 U-SR-5 Average S.D.

Young’s modulus

Ultimate strength

Fracture initiation strain

Fracture strain

E (GPa)

σu (MPa)

εf0 (%)

εfu (%)

15.63 15.72 15.03 15.16 14.91 15.29 0.36

116.74 118.32 122.88 118.27 120.92 119.43 2.45

0.98 1.02 1.01 1.09 1.02 1.03 0.04

3.90 4.98 4.12 4.92 5.18 4.62 0.57

Figure 23.6 Stress
strain relationship of unconfined resin specimens: (a) Nominal stress versus nominal strain and (b) True stress versus true strain.

Figure 23.7 Stress
strain relationship of unconfined steel-reinforced resin specimens: (a) Nominal stress versus nominal strain and (b) True stress versus true strain.

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Figure 23.8 Typical failure mode for resin (A) and steel-reinforced resin (B).

23.3.2.2 Confined specimens The compressive elastic moduli of confined resin and steel-reinforced resin specimens are summarized in Table 23.5. It is noted that the confined specimen is not loaded to ultimate failure and therefore the ultimate strength of the confined specimen is not obtained. The stress
strain relationships of confined resin and steelreinforced resin are shown in Figs. 23.9 and 23.10. The stress
strain curve of confined specimens consists of two stages: (1) the stress increases linearly with strain increasing; and (2) yielding occurs, and the stress increases nonlinearly with increasing strain. The nonlinear branch of the stress
strain curve of confined specimen is due to: (1) the nonlinear behavior of the material itself, the yield surface of resin and steel-reinforced resin is hydrostatic pressure dependent; and (2) yielding

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Table 23.5 Elastic modulus of confined resin and steel-reinforced resin. Specimen

C-1 C-2 C-3 C-4 C-5 Average S.D.

Confined resin

Confined steel-reinforced resin

E (GPa)

E (GPa)

E (GPa)

E (GPa)

6.84 6.84 7.15 7.66 8.09 7.32 0.54

6.66 6.52 6.91 7.59 7.78 7.09 0.56

17.99 19.56 19.59 18.66 16.18 18.40 1.41

17.61 19.36 19.43 18.45 15.96 18.16 1.44

of the confining steel tube, this leads to the situation where the resin is subject to a loss of confinement. The deformation of the confined specimen is shown in Fig. 23.11. Obvious yielding is observed at the bottom half of the confined steel tube. The yielding of the steel tube is more pronounced in the case of the confined resin specimen compared to the confined steel-reinforced resin specimen, indicating that the Poisson’s ratio of resin is larger than that of the steel-reinforced resin.

23.3.2.3 Results and discussion The apparent Young’s modulus increased by 29.7% for the confined resin specimens and increased by 7.5% for the confined steel-reinforced resin specimens. An explanation for the different increases in elastic modulus is that the Poisson’s ratio of resin is larger than that of the steel-reinforced resin. The strength of confined specimens has obviously increased. The yield strength increased by 95.6% for confined resin specimens, and the yield strength increased by 189% for confined steelreinforced resin. It is assumed that the uniaxial compressive behavior is described by combining the damage mechanics and Ramberg
Osgood relationship [19], represented by: σ 5 ð1 2 DÞσR2O ðεÞ ε5

 R2O n σR2O σ 1K E E

(23.5) (23.6)

where D is the damage variable. The parameters of the Ramberg
Osgood relationship are fitted based on the experimental results before any damage occurred. The fitted material parameters are listed in Table 23.6. The comparisons of stress
strain relationship from the Ramberg
Osgood relationship and experimental results are shown in Figs. 23.6,

350

350

300

300

True stress (MPa)

(B) 400

Nominal stress (MPa)

(A) 400

250 200 150 100

C-R-1 C-R-2 C-R-4 C-R-5 Ramberg–Osgood

50 0 0.00

0.05

0.10

0.15

Norminal strain

C-R-3

250 200 150 100

C-R-1 C-R-2 C-R-4 C-R-5 Ramberg–Osgood

50

0.20

0.25

0 0.00

0.05

0.10

0.15

C-R-3

0.20

0.25

0.30

True strain

Figure 23.9 Stress
strain relationship of confined resin specimens: (a) Nominal stress versus nominal strain and (b) True stress versus true strain.

500

500

True stress (MPa)

(B) 600

Norminal stress (MPa)

(A) 600

400 300 200

C-SR-1 C-SR-2 C-SR-4 C-SR-5 Ramberg–Osgood

100 0 0.00

0.05

0.10

0.15

Norminal strain

C-SR-3

0.20

400 300 200 C-SR-1 C-SR-2 C-SR-4 C-SR-5 Ramberg–Osgood

100

0.25

0 0.00

0.05

0.10

0.15

0.20

C-SR-3

0.25

0.30

True strain

Figure 23.10 Stress
strain relationship of confined resin specimens: (a) Nominal stress versus nominal strain and (b) True stress versus true strain.

Figure 23.11 Deformation of confined specimens: (A) resin; (B) steel-reinforced resin.

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Table 23.6 Ramberg
Osgood relationship parameters of resin and steel-reinforced resin. Item Unconfined resin Unconfined steel-reinforced resin Confined resin Confined steel-reinforced resin

Nominal stress True stress Nominal stress True stress Nominal stress True stress Nominal stress True stress

K

n

R2

6.07 3 1011 1.62 3 1016 7.81 3 1015 4.43 3 1016 1.82 3 105 3.28 3 106 5.68 3 106 2.00 3 1011

8.27 10.62 8.83 9.15 4.55 5.27 4.99 7.59

0.98 0.95 0.99 0.94 0.90 0.85 0.97 0.89

23.7, 23.9, and23.10. A good agreement is observed when no damage occurred. It is assumed that the fracture initiation occurred when the load reached the peak value. The damage variable is defined as:

D5

8 >
: εf 2 εf u 0

ε , εf0 ε $ εf0

(23.7)

where εf0 is the plastic strain at fracture initiation, and εfu is the plastic strain at the failure. The fracture initiation strain εf0 is assumed to be the corresponding strain at the peak load, while the failure strain is obtained by extended the softening stage. The values of εf0 and εfu are listed in Tables 23.1
23.4, based on the experimental results. The comparisons between combined damage Ramberg
Osgood relationship and experimental results are shown in Figs. 23.6 and 23.7. A good agreement is observed.

23.4

Numerical simulation of resin

23.4.1 Unconfined resin simulation The unconfined resin compressive tests were simulated numerically using the commercial finite element software ABAQUS/Standard [18]. The finite element model is shown in Fig. 23.12. The linear Drucker
Prager model is employed to model the resin behavior. The friction angle β, the ratio of the yield stress in triaxial tension to the yield stress in triaxial compression K, and the dilation angle ψ are summarized in Table 23.7. The nominal stress
strain relationship of unconfined resin comparisons between finite element simulation and experimental results is shown in Fig. 23.13. A good agreement is observed, indicating that the material model could effectively model the uniaxial loading of unconfined resin.

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Figure 23.12 Finite element model of unconfined resin specimen. Table 23.7 Material parameters of the linear Drucker
Prager model. Material

Associated flow β

Resin Steel-reinforced resin

K 

12.16 49.80

0.92 0.78

Nondilatant flow ψ

β 

12.16 49.80



12.18 52.04

K

ψ

1.00 1.00

0 0

23.4.2 Confined resin simulation As shown in Fig. 23.14, a finite element model on confined resin tests was built to validate the efficiency of the linear Drucker
Prager model when predicting resin behavior with confinement. The nominal stress
strain relationship of confined resin comparisons between finite element simulation and experimental results is shown in Fig. 23.15. A good agreement is observed, indicating that the Drucker
Prager model could effectively model the confinement effects of resin. The deformation comparisons between FEM and experiments of confined resin are shown in Fig. 23.16.

23.5

Numerical simulation of steel-reinforced resin

23.5.1 Unconfined steel-reinforced resin Due to the limit of the manufacturing process of steel-reinforced resin, it is difficult to make dog-shaped tensile specimens to obtain tensile behavior experimentally.

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Figure 23.13 Stress
strain relationship comparisons between FEM and experiments of unconfined resin.

Figure 23.14 Finite element model of confined material tests.

The computational homogenization method provides an alternative way to obtain the tensile and shear behavior numerically after validating the multiscale model with compressive test results. The unit cell is shown in Fig. 23.17. The interface parameters are calibrated based on compressive test results. The normal interface stiffness is calibrated as 5.53 3 105 N/mm3, and the shear interface stiffness is calibrated as 2.01 3 105 N/mm3. The normal interface strength is calibrated as 40.8 MPa. The shear interface strength is calibrated to be 41.5 MPa. The normal

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Figure 23.15 Stress
strain relationship comparisons between FEM and experiments of confined resin.

critical fracture energies Gcn are determined as 0.04 kJ/mm, and the shear critical fracture energies Gcs and Gct are determined as 0.45 kJ/mm. The material parameter is assumed to be 1.8. The viscosity coefficient for the cohesive surface is assumed to be 0.001 second. Compressive stress
strain relationship comparisons between numerical homogenization and experiments of unconfined steel-reinforced resin are shown in Fig. 23.18. The macroscale stress is obtained based on Eq. (23.1), therefore the homogenization results are compared with the true stress
strain relationship. A good agreement is observed, indicating it is reliable to use the computational homogenization method to predict the tensile and shear behavior of steel-reinforced resin. The uniaxial stress
strain relationship and shear stress
strain relationship based on numerical homogenization method is shown in Fig. 23.19. The ultimate tensile strength of steel-reinforced resin is 39.8 MPa. The steel-reinforced resin material parameters of the linear Drucker
Prager model are summarized in Table 23.7. The finite element simulation of unconfined steel-reinforced resin is shown in Fig. 23.20. The nominal stress
strain relationship of unconfined steel-reinforced resin and a comparison between finite element simulation and experimental results is shown in Fig. 23.21. A good agreement is observed.

23.5.2 Confined steel-reinforced resin Similar to confined resin tests, a finite element model on confined steel-reinforced resin tests was built to validate the efficiency of the linear Drucker
Prager model

Figure 23.16 Deformation comparisons between FEM and experiments of confined resin.

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Figure 23.17 Unit cell of steel-reinforced resin.

Figure 23.18 Stress
strain relationship comparisons between numerical homogenization and experiments of unconfined steel-reinforced resin.

when predicting steel-reinforced resin behavior with confinement. The nominal stress
strain relationship of confined steel-reinforced resin and a comparison between finite element simulation and experimental results is shown in Fig. 23.22. The finite element simulation results from an “associated flow” model agreed well

Figure 23.19 Stress
strain relationship of steel-reinforced resin from numerical homogenization: (A) Uniaxial stress versus uniaxial strain; (B) Shear stress versus shear strain.

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Figure 23.20 Finite element model of unconfined steel-reinforced resin specimen.

Figure 23.21 Stress
strain relationship comparisons between FEM and experiments of unconfined steel-reinforced resin.

with the experimental results, but the finite element simulation results from “nondilatant flow” tend to be smaller than the experimental results in the hardening stages. The Drucker
Prager models with “associated flow” rules predict the confinement effects of steel-reinforced resin efficiently. Fig. 23.23 showed deformation

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Figure 23.22 Stress
strain relationship comparisons between FEM and experiments of confined steel-reinforced resin.

comparisons of confined steel-reinforced resin tests between FEM and experiments. A good agreement is observed.

23.6

Conclusions

Compressive material tests on unconfined/confined resin and steel-reinforced resin were experimentally evaluated in order to validate the numerical results. Finite element simulation and multiscale homogenization methods were successfully used in this study to effectively model the material properties of resin and steel-reinforced resin. 1. A combined damage mechanics and Ramberg
Osgood relationship is proposed in this chapter to describe the uniaxial compressive behavior of resin and steel-reinforced resin. Related material parameters were fitted based on experimental results. The proposed uniaxial compressive model could effectively describe the uniaxial hardening/softening behavior of resin and steel-reinforced resin during finite element simulation. 2. The friction angle, the ratio of the yield stress in triaxial tension to the yield stress in triaxial compression K, and the dilation angle ψ of the linear Drucker
Prager plastic model are obtained based on experiments and numerical homogenization to efficiently consider the confinement effects on resin and steel-reinforced resin. The confinement effects on resin and steel-reinforced resin could be effectively simulated by combining the above parameters and a uniaxial compressive model. Finite element simulations on unconfined/ confined resin and steel-reinforced resin material tests were conducted to validate the linear Drucker
Prager plastic model and material parameters proposed in this chapter. A good agreement is observed, indicating the model and parameters proposed in this chapter could be effectively used in the numerical simulation of injected bolted connections.

Figure 23.23 Deformation comparisons between FEM and experiments of confined steel-reinforced resin.

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References [1] B.D. Reid, Fastener Sealant Injection System, Google Patents, 1975. [2] A.M.N. Gresnigt, J.W.B.J. Stark, Design of bolted connections with injection bolts, Connections in Steel Structures III, Elsevier, 1996, pp. 77
87. [3] A.M. Gresnigt, G. Sedlacek, M. Paschen, Injection bolts to repair old bridges, Proceedings of Connections in Steel Structures IV, Citeseer, Roanoke, VA, 2000. [4] C.M. Steenhuis, J.W.B. Stark, A.M. Gresnigt, Cost-effective connections, Prog. Struct. Eng. Mater. 1 (1) (1997) 18
24. [5] A. Koper, Assessment of Epoxy Resins for Injected Bolted Shear Connections, Delft University and Technology, 2017. [6] M.P. Nijgh, New Materials for Injected Bolted Connections, Delft University of Technology, 2017. [7] ECCS, European Recommendations for Bolted Connections with Injection Bolts, ECCS Publication NO.79, Brussels, 1994. [8] H. Kolstein, et al., Behaviour of double shear connections with injection bolts, Steel Constr 10 (4) (2017) 287
294. [9] J. Fish, Practical Multiscaling, Wiley, 2013. [10] H. Xin, A. Mosallam, Y. Liu, C. Wang, et al., Analytical and experimental evaluation of flexural behavior of FRP pultruded composite profiles for bridge deck structural design, Constr. Build. Mater., 150, 2017, pp. 123
149. [11] H. Xin, Y. Liu, et al., Evaluation on material behaviors of pultruded glass fiber reinforced polymer (GFRP) laminates, Compos. Struct., 182, 2017, pp. 283
300. [12] H. Xin, A. Mosallam, Y. Liu, Y. Xiao, et al., Experimental and numerical investigation on in-plane compression and shear performance of a pultruded GFRP composite bridge deck, Compos. Struct. 180 (2017) 914
932. [13] H. Xin, A. Mosallam, Y. Liu, M. Veljkovic, J. He, Mechanical characterization of a unidirectional pultruded composite lamina using micromechanics and numerical homogenization, Construction and Building Materials 216 (2019) 101
118. [14] H. Xin, M. Nijgh., M. Veljkovic, M, Computational homogenization simulation on steel reinforced resin used in the injected bolted connections, Composite Structures 210 (2019) 942
957. [15] H. Xin, S. Sun, J. Fish, A surrogate modeling approach for additive-manufactured materials, Int. J. Multiscale Comput. Eng. 15 (6) (2017). [16] H. Xin, W. Sun, J. Fish, Discrete element simulations of powder-bed sintering-based additive manufacturing, Int. J. Mech. Sci. 149 (2018) 373
392. [17] J. Fish, R. Fan, Mathematical homogenization of nonperiodic heterogeneous media subjected to large deformation transient loading, June 2008, pp. 1044
1064. doi: 10.1002/ nme. [18] V. Abaqus, 6.14 Documentation, Dassault Systemes Simulia Corporation, 2014. [19] W. Ramberg, W.R. Osgood, Description of stress-strain curves by three parameters, 1943, Report Number: NACA-TN-902. https://ntrs.nasa.gov/search.jsp?R 5 19930081614.

Swelling behavior of expansive soils stabilized with expanded polystyrene geofoam inclusion

24

S. Selvakumar1 and B. Soundara2 1 Department of Civil Engineering, Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology, Chennai, India, 2Department of Civil Engineering, Bannari Amman Institute of Technology, Sathyamangalam, Erode, India

24.1

Effect of geobeads inclusion

A laboratory investigation program was conducted to study the efficacy of geobeads (GB) inclusion in arresting the swelling potential of expansive soils. The swelling behavior of soil without GB was studied and compared with a soil specimen with GB inclusion. The experimental investigation was conducted on a statically compacted soil specimen and tested on large one-dimensional consolidation equipment. Swell-compression tests also were performed on soil specimens with and without GB inclusion. The details of the laboratory tests are described in the subsequent sections.

24.1.1 Material characteristics 24.1.1.1 Expansive soil Commercially obtained sodium bentonite was used as an expansive soil in this study which poses a free swell index (FSI) value of 450%. Table 24.1 shows the physical properties of the expansive soil. According to USCS classification, the soil was classified as clay of high plasticity (i.e., CH) based on plasticity charateristics. In order to study the mineralogical constituents, X-ray diffraction (XRD) analysis was carried out on the soil sample. Fig. 24.1 shows the XRD plot of the soil sample, the diffraction data were collected from 10 degrees to 45 degrees (2θ degrees). The dotted vertical lines in the figure, identify the mineral names with d˚ ). The intensity peaks (in the spacing values in parentheses (in Angstrom units, A units of counts per second, Cps) indicates the presence of the smectite group/expansible phyllosilicate minerals group, such as montmorillonite (M) and vermiculite (V), responsible for volume changes. Some other mineral groups such as oxides/ hydroxides group like quartz (Q) and hematite (H), kaolins group like kaolinite (K), New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00024-7 © 2020 Elsevier Inc. All rights reserved.

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Table 24.1 Physical properties of the expansive soil. Property

Expansive soil

Specific gravity Liquid limit (%) Plastic limit (%) Shrinkage limit (%) Plasticity index (%) FSI (%) Clay (%) Silt (%) Sand (%) Maximum dry densitya (g/cm3) Optimum moisture contenta (%) USCSb classification Mineralogical compositionc (%)

2.80 276 33 8 243 450 96 4
1.38 38 CH Montmorillonite: 42
44, vermiculite: 24
26, quartz: 14
16, calcite: 8
10, feldspar: 2
4

a

Laboratory light compaction test. Unified soil classification system. c XRD spectrum. b

Figure 24.1 X-ray diffraction pattern for a soil sample.

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carbonate group like calcite (C), and some feldspar group are also present in the clay fraction [1].

24.1.1.2 Waste expanded polystyrene beads In this research, the investigation was carried out on the recyclability of waste EPS blocks in geotechnical application by using EPS beads as an additive in expansive soils. The cut pieces of waste EPS blocks were collected from manufacturing units to reuse the wastes. The collected EPS blocks were converted into beads by hand crushing. EPS beads of size approximately in the range of 2
6 mm (D10 of 2.3, D50 of 2.9 mm) were used in this study (Fig. 24.2). The specific gravity and unit weight of EPS beads were determined using a modified procedure similar to that of the standard used for fine aggregates [2
6]. The unit weight of EPS beads was determined from the net weight of beads that occupy a volume of a 1-litrer hydrometer jar without applying any compaction effort on the beads. The calculated unit weight of EPS beads based on the method was 0.15 kN/m3. For the determination of specific gravity, the opening of the hydrometer was covered with a piece of gauze fixed with an O-ring. Then de-aired water was added through the gauze until the total weight of the hydrometer remained constant. The absolute volume occupied by the EPS beads in the hydrometer was arrived at and thus the specific gravity of EPS beads was calculated as 0.03. Table 24.2 summarizes the physical properties of the EPS beads.

Figure 24.2 Particle size distribution curve for EPS beads.

Table 24.2 Physical properties of the EPS beads. Material

EPS beads

Properties Specific gravity, Gs

Dry unit weight (kN/ m3)

Effective particle size (D10: in mm)

Average particle size (D50: in mm)

Uniformity coefficient, Cu

Coefficient of curvature, Cc

0.03

0.15

2.3

2.9

1.4

0.9

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24.1.2 Experimental program 24.1.2.1 Large consolidation apparatus The large one-dimensional consolidation apparatus (LCA) was used to investigate the swelling behavior of expansive soils with and without GB inclusion. The LCA accommodates the California bearing ratio (CBR) mold (150 mm internal diameter and 175 mm height) covered with an outer tank provision for bottom drainage purposes. The equipment provision has been described in detail in Reference [3]. Some of the information is concisely repeated here for completeness. A photographic view of the LCA is shown in Fig. 24.3. The height of the compacted specimen was maintained at 100 mm for all tests. The remaining height of the mold accommodates adequate swelling as well as submerging the soil specimen. The top loading plate and base plate of the CBR mold have uniform pores for inundation of specimen with double-way drainage. The vertical displacement of the specimen was measured using a dial gauge with 0.01 mm sensitivity attached to the reaction frame in the LCA.

Figure 24.3 Photographic view of the large consolidation equipment.

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1. Dial gauge indicator, 2. reaction frame; 3. fixed frame; 4. CBR mold; 5. outer tank; 6. lever arm (1:10); 7. surcharge load.

24.1.2.2 Specimen preparation Laboratory light compaction tests were carried out on expansive soil with and without EPS beads to determine the optimum moisture content (OMC) and maximum dry density (MDD) in accordance with ASTM D698. The GB content (gc) was taken as 0.25%, 0.5%, 0.75%, and 1% by dry weight of the soil. The compaction test results are shown in Fig. 24.4. Minor variations were observed for OMC, while the MDD exhibited a marginal decreasing trend with increasing EPS beads content. This could be attributed due to the lower specific gravity of EPS beads and maximum specific dried mass compared to soil particles [7,8]. For swell-compression tests, all the samples were prepared by static compaction at the respective OMC and MDD of expansive soil (see Table 24.3). In the case of GB inclusion, the required amount of water corresponding to the desired OMC was added to each mixture and thoroughly mixed with GB content (gc 5 0.25%, 0.5%, 0.75%, and 1%) by hand. Care was taken while mixing the soil sample with beads mainly targeting the homogeneity of mixtures. Mixtures were then kept in a desiccator for 24 hours to maintain the moisture equilibrium. A specially made prefabricated metal spacer was used to achieve the static compaction of soil specimens. The inner surface of the CBR mold was smeared with silicone grease to avoid friction during compaction. Mixtures were gradually compressed in the mold by three layers to attain the maximum dry unit weight.

Figure 24.4 Compaction characteristics of soil with and without geobeads.

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Table 24.3 Mechanical properties of the prepared samples. Test setup

Test identifier

Optimum moisture content (OMC) (%)

Maximum dry density (MDD) (g/cm3)

Soil without geobeads content (soil alone) Soil mixed with 0.25% geobeads content Soil mixed with 0.5% geobeads content Soil mixed with 0.75% geobeads content Soil mixed with 1% geobeads content

SA

37.33

1.38

gc 5 0.25 %

37.15

1.32

gc 5 0.5 %

36.98

1.27

gc 5 0.75 %

36.94

1.25

gc 5 1 %

36.92

1.21

24.1.2.3 Test procedure Upon completion of specimen preparation, the series of swell-compression tests was performed on LCA in accordance with ASTM D4546. The test involved two stages: (1) swelling phase and (2) compression. In the first stage, the specimen was inundated with water to swell freely under a nominal seating pressure of 6.25 kPa. The incurred swelling strain was recorded at different time intervals to discover the equilibrium state. This resultant state is equivalent to the swelling potential of specimens (determined from Eq. 24.1). This could be attained after 18
22 days of inundation. During the compression stage, the swollen specimen was gradually loaded to counteract the build-up of swelling strain. The pressure required to return to the specimen’s initial thickness is referred to as the swelling pressure (SP). Percent swell ðPSÞ 5

ΔH 3 100 H

(24.1)

where, ΔH 5 ultimate changes in the soil sample with respect to time intervals and H 5 the actual thickness of the soil sample.

24.1.3 Results and discussions Time (logarithmic scale) versus swell strain plots for the soil alone (SA) and varying GB content of the soil samples are provided in Fig. 24.5. This figure clearly indicates that there is a significant reduction in the magnitude of swelling strain for GB inclusion when compared to the swelling potential of SA. With an increase in GB contents (gc), there is a gradual decrease in the swelling strain, indicating that gc was effective in arresting the swelling. This could be due to the replacement of swelling clay by compressible GB, which act as an inclusion or rather act as an obstruction of movement of water into the soil [9,10].

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Figure 24.5 Time versus percentage swell for varying geobeads contents.

Figure 24.6 Effect of geobeads content on PSr values.

Fig. 24.6 shows the relationship between the percent swell reduction factor (PSr) of the samples to the corresponding gc. From the plot, it is clear that percent swell (PS) is decreased with increasing gc. For increasing gc from 0.25% to 1%, there is a substantial reduction in the PS of around 10%
45%. PSr is defined as the ratio of maximum PS of soil with varying gc to the maximum PS of soil without gc (i.e., SA). The results of the

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measured PS and PSr values are given in Table 24.4. This clearly specifies that PSr for 0.25%, 0.5%, 0.75%, and 1% of gc is about 0.90, 0.81, 0.64, and 0.55, indicating a reduction in PS by 0.10, 0.19, 0.36, and 0.45 times, respectively, when compared to SA. Fig. 24.7 shows a graph of vertical stress (logarithmic scale) versus vertical strain for all the swell tests to evaluate the SP, which corresponds to the zero vertical strain. The values of SP are noted from the plot and listed in Table 24.4. The data shown in both Fig. 24.7 and Table 24.4 indicate that SP is reduces upon increasing the gc in the samples. As the maximum PS decreased with introduction of GB, obviously the SP of the sample is also decreased. For example, with an increase in gc from 0.25% to 1%, there is a noticeable reduction in the SP by 15%
61% when compared to SA. Fig. 24.8 shows the relationship between SP reduction factor (SPr) and the corresponding gc of the samples. SPr is defined as the ratio of SP for soil with varying gc Table 24.4 Summary of measured percent swell and swelling pressure from the swell test program. Test identifier

Percent swell (PS) (%)

Swelling pressure (SP) (kPa)

Percent swell reduction factor (PSr)

Swelling pressure reduction factor (SPr)

SA gc50.25% gc50.5% gc50.75% gc51%

14.69 13.23 11.87 9.44 8.09

530 452 328 273 210

1.00 0.90 0.81 0.64 0.55

1.00 0.85 0.62 0.52 0.40

Figure 24.7 Vertical stress versus vertical strain for varying geobeads content.

Figure 24.8 Effect of geobeads content on SPr values.

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to the SP of soil without gc. From the graph, it can be seen that the SPr values decreased with a gradual increase in gc of the samples. The results of the calculated SPr values are given in Table 24.4. This indicates that SPr for 0.25%, 0.5%, 0.75%, and 1% of gc is about 0.85, 0.62, 0.52, and 0.40, indicating a reduction in SP by 0.15, 0.38, 0.49, and 0.60 times, respectively, when compared to SA. Based on the experimental results from the present study, when compressible EPS beads are mixed randomly, they tend to act as an inclusion or obstruction to the flow of water into the soil. This is reflected in the reduction of magnitude of swelling strain in the expansive soils as discussed earlier.

24.2

Effect of the geofoam granules column

The natural expansive soil was obtained from Chennai, Tamilnadu, India, at a depth of about 1.5 m from ground level and used for the laboratory tests. The laboratory studies on the behavior of expansive soils with and without geofoam granules column (GGC) inclusion were carried out on a LCA. The apparatus setup is provided with double-way drainage, ceramic discs, porous loading plate, and the required surcharge load is applied in the ratio of 1:10 through a lever arm arrangement which is similar to that in a conventional 1D consolidation oedometer apparatus [11]. All the tests were conducted on remolded dry samples at two different placement conditions, that is, OMC and hygroscopic moisture content (HMC) at MDD obtained from dry density vs. water content curve through a standard Proctor compaction test in a laboratory. First, the swelling behavior of the compacted expansive soil specimen alone was studied for both HMC and OMC at MDD. Second, the influence of GGCs in the swelling behavior of the soil specimen was studied with four different diameters of GGC inclusion corresponding to the two different densities of geofoam granules in compacted soil specimens for two different moisture contents. The subsequent sections provide the details of the laboratory tests.

24.2.1 Material properties 24.2.1.1 Soil characteristics The natural soil received from the field was completely air-dried at room temperature and then ground to break up the aggregation thoroughly with the help of a pestle. The physical index properties of the soil sample were determined and the results are summarized in Table 24.5. Based on the FSI tests conducted on the soil sample, the soil can be classified as having a high degree of expansiveness [12]. According to the USCS classification system, the soil is classified as clay of high plasticity (CH). In order to understand the mineralogical constituents, X-ray diffraction (XRD) analyses were carried out for the soil sample. The air-dried soil sample was sieved through an ASTM standard sieve No. 200 (75 μm) and about 10 g of soil containing silt and clay fraction was thoroughly mixed with 1 L of sodium hexametaphosphate

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Table 24.5 Index properties of the expansive soil. Property

Expansive soil

Specific gravity Liquid limit (%) Plastic limit (%) Plasticity index (%) Shrinkage limit (%) Clay (%) Silt (%) Sand (%) Free swell index (%) USCS classificationa Optimum moisture content (%) Maximum dry unit weight (kN/m3) Mineralogical composition (%)

2.65 76 41 35 9 72.5 23.5 4.0 108 CH 17 17.65 Vermiculite, 46
48; montmorillonite, 33
34; quartz, 10
12; feldspar, 4
6

a

Unified soil classification system.

solution followed by the sedimentation process. After the sedimentation process, the soil suspension was air-dried and thoroughly crushed to collect the powdered particles. The collected suspension particles were then centrifuged to separate out the clay fraction (,2 μm) and used to carry out the XRD test [13,14]. The diffraction data were collected from 10 degrees to 45 degrees (2θ degrees) at a rate of 0.05 degrees 2θ/seconds. The results of the XRD plot are shown in Fig. 24.9. The dashed vertical lines in the figure mark the mineral names and d-spacing value in ˚ ). The intensity peaks indicate the presence of square brackets (in the unit of A smectite group/expansible phyllosilicates mineral groups such as montmorillonite (M) and vermiculite (V), responsible for volume changes. Some other mineral groups such as feldspars and oxides/hydroxides showing anorthite (A) and hematite (H) are also present in the clay fraction [1].

24.2.1.2 Waste expanded polystyrene geofoam EPS manufacturers are still finding the feasibility of using cutting-wastage of EPS blocks as a reusable material in manufacturing units. The cut pieces of waste EPS blocks of different sizes and densities were procured from a nonproprietary source free of cost as the recycling cost of cutting wastes is highly complex and uneconomical [15,16]. In this research, waste EPS from manufacturing units and molded packaging products from garbage were used in order to find a possible way to reuse and reduce these wastes. The different sizes and densities of manufacturer wastes and solid wastes of EPS geofoam blocks were used in these studies. The performance of recycled EPS beads as GGCs on the swelling behavior of deeper expansive soil was done through an extensive experimental program. The procured and

Figure 24.9 X-ray diffraction pattern of soil sample.

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collected waste EPS blocks were converted into granules by hand crushing, to a size ranging from approximately 2
6 mm and were used for GGCs formation in the compacted soil specimen.

24.2.2 Experimental setup 24.2.2.1 Large-scale one-dimentional consolidation apparatus The laboratory LCA was developed to perform column studies. The prefabricated equipment (Fig. 24.3) is allowed to accommodate the CBR mold of 150 mm internal diameter and 175 mm height (circular area of 17,671 mm2), covered with an outer tank provision for bottom drainage purposes. The CBR mold is specially made with a rigid body for compaction, and is zinc coated for noncorrosion and porous in the base plate for inundated soil samples. The thickness of the compacted soil is 100 mm and the remaining height of the CBR mold (75 mm) provides space for sufficient vertical swell and submerging of the soil specimen. The vertical load is applied on the loading plate which has uniform pores for facilitating the top drainage purposes. A ceramic porous stone is placed between the standard filter paper and the loading plate to minimize the seating displacements and also to avoid clogging of the soil grains into the pores of the loading plate and base of the mold. The developed LCA allows a maximum surcharge load of 600 kPa applied in the lever arm. The vertical displacement was measured using a standard dial gauge with an accuracy of 0.01 mm, placed on the top of the moving frame in the apparatus.

24.2.2.2 Specimen preparation It was decided to achieve a thickness of the soil specimen of 100 mm for all the tests so as to maintain sufficient height (about one-third the height of the mold) for free swelling of the soil specimen on imbibing water [17,18]. The soil sample passing through ASTM standard sieve no. 16 (1.18 mm) was used to prepare the compacted soil specimens for swell tests at two different placement conditions of OMC and HMC at MDD obtained from the compaction curve. Two types of placement conditions for the swell tests represent the swell and SP primarily dependent upon the initial moisture content and dry density [19]. All swell tests were conducted for soil samples at OMC (adding optimum water) and HMC (normally in the range of 5%
7%) at MDD. The required quantity of soil sample was thoroughly mixed and kept in a desiccator for 24 hours for moisture equilibrium. Before placing the soil sample into the mold, the inner surface of the mold was cleaned, dried, and lubricated with standard vacuum silicone grease to reduce friction. A thin steel plate of 5 mm thick was provided over the porous base of the mold to avoid clogging of pores by the soil. Then the well-prepared soil sample was placed on the mold layer by layer with a small amount of hand compaction by a steel tamping rod to fill the total mass of soil into the mold. After placing the soil into the mold, the top surface was covered with a thin steel plate for initial leveling of the soil with slight

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pressure. The prefabricated metal spacer of 148 mm diameter with a grooving mark is placed over the steel plate to achieve the sample thickness of 100 mm in the mold. The sample was statically compacted in a universal testing machine by gradually applying the load. During compaction the utmost care was taken so that the grooving mark on the spacer coincides with the mold height, as the height of the mold is 175 mm and the spacer grooving mark height from the bottom is 75 mm. When the grooving mark reached the mold height, the static compaction was stopped and hence the desired sample thickness was achieved. The top and bottom thin steel plates were removed from the soil with care and then replaced with standard filter papers and ceramic porous stones on the top and bottom faces of the sample. The mold was firmly fixed on the base plate and placed into the outer water tank and then assembled in the large 1D consolidation frame.

24.2.2.3 Column preparation In the case of tests with GGC inclusion, an open-ended steel tube with outer diameter (D) (25, 40, 50, and 75 mm) or L/D ratios (4, 2.5, 2, and 1.3) corresponding to the decided diameter of the GGC was placed centrally inside the CBR mold with the help of top and bottom steel guide plates (5 mm thick). The precalculated quantity of the prepared soil samples (OMC and HMC at MDD) was poured into the CBR mold around the steel tubes and roughly packed by hand compaction to accommodate the entire mass of soil sample into the mold. After filling the soil sample into the mold, the top surface was covered with a guide plate and then a metal spacer with a central hole and a grooving mark in the side was placed to achieve the desired diameter and thickness of the column. The prefabricated metal spacer has an inner diameter of 77 mm and an outer diameter of 148 mm for forming the maximum column diameter of 75 mm in the mold. For forming the other lower diameter columns an appropriate guide plate was used with the same metal spacer. Upon completion of static compaction, the top guide plate was removed carefully using a trowel and steel tubes were taken out by gradually rotating the steel tubes and pulling vertically with the help of a pipe wrench. The smooth outer surface of the steel tubes was lubricated with silicone grease to enable the free movement of the guide plate, reduce the friction, and minimize the disturbance of surrounding soil during pull-out of steel tubes vertically from the compacted specimen. After complete removal of the steel tubes, the formed circular hole was filled with the required quantity of recycled EPS geofoam granules (Fig. 24.10) of size ranging from 2 to 6 mm with the predecided densities of 20 and 15 kg/m3 ( 6 1 kg/ m3). For the purposes of compressible inclusion, an EPS geofoam granule density of 19 kg/m3 was considered adequate based on the works described in References [9,20
22]; wherein the size of EPS geofoam granules plays a major role in achieving the decided densities. As the bead size increases, correspondingly the density and specific gravity decrease. Additionally, the lowest density was found to wbe satisfactory with compressible inclusion, which is similar to that in the EPS geofoam blocks [23
26]. It was noticed from earlier literature that the size of EPS geofoam granules is the basic parameter to achieve the desired density. Determination of the

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Figure 24.10 Photographic view of formed GGC.

density of the EPS beads was performed using a modified method similar to that for fine aggregates [2,5,6]. The calculated average density of EPS beads without any compaction using this method was 15 kg/m3 and the same was adopted for the formation of GGC density of 15 kg/m3 (GGC15). The formation of GGC density of 20 kg/ m3 (GGC20) was achieved by little or moderate compaction effort (arrived at by a trial and error procedure) without affecting the individual EPS beads.

24.2.3 Experimental procedure After the preparation of soil specimens with and without GGCs inclusion, the mold was placed in the LCA to study the swelling behavior of expansive soils. The tests were conducted for two placement conditions and for different L/D ratios, where L/ D is defined as the ratio of length to the diameter of the GGCs in the compacted soil specimen. To achieve a different L/D ratio, the diameter of GGCs was varied while the length of the column remained constant. The test details with different L/ D ratios of GGC are provided in Table 24.6. Furthermore, the test setup is arranged to conduct the laboratory swell test, based on the procedure of a 1D swell-consolidation/swell-load method followed by many practicing geotechnical engineers around the world. In this method, generally simulation for reconstituted soil specimens to allow swell freely in loading and wetting conditions is followed. A similar procedure is outlined in ASTM D4546 [27] Method C. After assembling the specimen in the LCA under the seating load of 6.25 kPa, there is no marked deviation in the dial gauge readings. The specimen was then inundated with distilled water and the free swell of the specimen was frequently recorded from the dial gauge reading with the corresponding time until equilibrium was reached. The free swell/swell potential under any surcharge which is taken as the ratio of the increase in thickness

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Table 24.6 Experimental details of the swell test program. Test setup

Test identifier

L/D ratio

Soil without GGC inclusion (soil alone) 25 mm diameter and 20 kg/m3 density of GGC 40 mm diameter and 20 kg/m3 density of GGC 50 mm diameter and 20 kg/m3 density of GGC 75 mm diameter and 20 kg/m3 density of GGC 25 mm diameter and 15 kg/m3 density of GGC 40 mm diameter and 15 kg/m3 density of GGC 50 mm diameter and 15 kg/m3 density of GGC 75 mm diameter and 15 kg/m3 density of GGC

SA 25D-20 40D-20 50D-20 75D-20 25D-15 40D-15 50D-15 75D-15

0 4 2.5 2 1.3 4 2.5 2 1.3

of the specimen to the actual thickness of the soil specimen and is expressed in percentage of swell (PS) [28] was arrived at using Eq. 24.1. After the specimen reached equilibrium swell, the vertical load increment (12.5, 25, 50, 100, 200, and 300 kPa) was applied to the soil specimens after allowing sufficient time to measure the load-induced strains for each load increment (loading after wetting) and a similar procedure is outlined in ASTM D2435 [11]. A plot of change in thickness corresponding to the load increments was obtained and the pressure required to bring the specimen back to its original thickness was referred to as the SP.

24.2.4 Results and discussions 24.2.4.1 Effect of L/D ratio and diameter ratio First, the swell test was conducted for a soil specimen without GGC inclusion (SA) for the placement condition of OMC at MDD and the free swell movement was recorded by dial gauge readings for a period of around 12 days to attain equilibrium swell. After the completion of free swell, the vertical load increment was applied to prevent swelling and the dial readings were recorded. Second, the swell tests were conducted for the soil specimen with GGC inclusion of varying L/D ratio and two different densities. A similar procedure was adopted to record the data of both free swell and applied vertical pressure for nominal GGC inclusion. The results of the swell test with and without varying GGC inclusion after the completion of the experimental studies have been shown as a plot of time (logarithmic scale) versus PS in Fig. 24.11. From the plots, it is clear that the PS is reduced with decreasing L/D ratios or increasing the diameter of GGC as expected as GGC inclusion gave room to accommodate the lateral swelling and hence resulted in vertical swell reduction. The ratio of PS (PSr) decreases with an increase in the diameter ratio (DR) or a decrease in the L/D ratio of the GGC inclusion, irrespective of the density of GGC. PSr is defined as the ratio of maximum PS for clay with GGC inclusion to the maximum PS for clay without inclusion, while DR is defined as the ratio of the

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Figure 24.11 Time vs. percentage of swell for varying L/D ratio (OMC at MDD): (A) GGC20; (B) GGC15.

diameter of GGC inclusion to the original diameter of soil specimen. This is given by the following equations: PSr 5

PSGGC PSSA

(24.2)

DR 5

DGGC DSA

(24.3)

From the results of measured PSr values (Table 24.7), it is clear that the PS ratio for 25D, 40D, 50D, and 75D of GGC is about 0.82, 0.78, 0.76, and 0.60, indicating a reduction in swell by 0.18, 0.22, 0.24, and 0.40 times when GGC density is maintained at 20 kg/m3 when compared to SA. Similarly, for a GGC density of 15 kg/ m3, the swell decreased by 0.25, 0.29, 0.32, and 0.46 times when compared with SA for 25D, 40D, 50D, and 75D inclusion of GGC. For both densities of GGC, the

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Table 24.7 Summary of the measured PSr and SPr values from the swell test program. Placement condition

Mode

Test identifier

L/D ratio

DR

PSr

SPr

OMC at MDD

SA GGC20

SA 25D-20 40D-20 50D-20 75D-20 25D-15 40D-15 50D-15 75D-15

0.0 4.0 2.5 2.0 1.3 4.0 2.5 2.0 1.3

0.00 0.16 0.26 0.33 0.50 0.16 0.26 0.33 0.50

1.00 0.82 0.78 0.76 0.60 0.75 0.71 0.68 0.54

1.00 0.60 0.38 0.32 0.26 0.57 0.34 0.28 0.24

GGC15

Figure 24.12 Vertical stress vs. vertical strain for varying L/D ratio (OMC at MDD).

maximum reduction in swell is observed for 75D GGC (i.e., maximum DR of 0.5). This is due to the lower availability of soil for swelling and also the presence of GGC acting as an absorber of lateral swelling. Fig. 24.12 shows a graph of vertical stress (logarithmic scale) versus vertical strain for GGC20 and GGC15 with varying L/D ratio. The SP, which corresponds to the zero vertical strain, is noted from the plots and listed in Table 24.7. It is observed that the ratio of SP (SPr) is decreased with increasing diameter or decreasing L/D ratio of the GGCs inclusion, irrespective of GGC density. SPr is defined as the ratio of SP for clay with GGC inclusion to the SP for clay without inclusion (Eq. 24.4). The SPr for 25D, 40D, 50D, and 75D of GGC20 inclusion are observed to be 0.60, 0.38, 0.32, and 0.26 as compared to SA, that is, 0.40, 0.62, 0.68, and

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0.74 times reductions in SP. Similarly, the SP decreased by 0.43, 0.66, 0.72, and 0.76 times when compared to SA for 25D, 40D, 50D, and 75D of GGC15 inclusion. Figs. 24.13A and B show the relationship of L/D ratio with PSr and SPr for the

Figure 24.13 Effect of L/D ratios on (A) PSr and (B) SPr (OMC at MDD).

Figure 24.13 Continued.

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stabilized expansive soils. Similarly, the relationship of DR with PSr and SPr is shown in Figs. 24.14A and B. SPr 5

SPGGC SPSA

(24.4)

24.2.4.2 Effect of geofoam granules columns densities From time versus swell plots shown in Fig. 24.11, it is interesting to note that the plots of increasing diameter of GGC are shifting to the left of SA curve, which shows that the time taken for any amount of swell is less for specimens with GGC inclusion when compared to SA. Time ratio (Tr) is defined as the ratio of time taken for maximum PS with GGC inclusion to the time taken for maximum PS without inclusion (Eq. 24.5). Tr 5

TGGC TSA

(24.5)

Fig. 24.15 shows the relationship of Tr with density of GGC and diameter of the column. From the plot, it is clear that the time taken for maximum swell depends on the diameter and density of GGC. The time taken for maximum swell is decreased by 0.09
0.59 times compared to SA upon introducing various diameters of GGC with two different densities. The reduction in Tr for the density of GGC20

Figure 24.14 Effect of DR on (A) PSr and (B) SPr (OMC at MDD).

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New Materials in Civil Engineering

Figure 24.14 Continued.

for 25D, 40D, 50D, and 75D is 0.09, 0.15, 0.23, and 0.38 times with regard to SA, respectively. Similarly, for the density of GGC15 the reduction in Tr with regard to SA varied from 0.27, 0.46, 0.51, and 0.59 times for 25D, 40D, 50D, and 75D of GGC inclusion, respectively. The percentage reduction in time increases with increasing diameter of GGC irrespective of the density of GGC. This may be due to the fact that GGC inclusions were also acting as vertical drains and allowed the water to flow through the voids and hence accelerate the swelling process at a faster rate when compared to SA. Also, it is clear that the rate of swell is faster for GGC15 than GGC20, the variation being about 1.5 times when compared to the similar diameter GGC. This could be due to the presence of a relatively higher void volume for GGC15 when compared with GGC20, which allows quicker swelling. Fig. 24.16 shows the values of PSr and SPr for the nominal density of GGC20 and GGC15 for varying L/D ratio of GGC inclusion. From the plots, it is clear that the PSr and SPr are decreasing with decreasing L/D ratio of GGC irrespective of the density of the inclusion, while a larger L/D ratio resulted in a smaller reduction in swelling. PS and SP are significantly reduced at a smaller L/D ratio of 1.3 by around 50% and 25%, respectively, when compared to SA. Similarly, the percentage reductions of PS and SP for the medium L/D ratios of 2.0 and 2.5 are nearly equal and the values are 72% and 75%, and 30% and 36%, respectively. Thus, L/D ratios of 1.3 and 2.5 are taken as representative values for further studies. It is also observed from Fig. 24.16 that the degree of reduction of PSr and SPr is greater for GGC15 than GGC20. Table 24.8 summarizes that the PS reduction is 1.09, 1.10, 1.11, and 1.12 times higher for GGC15 when compared to GGC20 for L/D ratios

Figure 24.15 Effect of GGC densities on Tr (OMC at MDD).

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Figure 24.16 Effect of GGC densities on PSr and SPr (OMC at MDD). Table 24.8 Ratio of percent swell and swelling pressure for GGC densities (OMC at MDD). Test identifier

25D-20 25D-15 40D-20 40D-15 50D-20 50D-15 75D-20 75D-15

Maximum swell (%)

4.79 4.37 4.56 4.11 4.45 3.98 3.53 3.14

Swelling pressure (kPa)

165 158 105 93 87 76 72 65

GGCratio5GGC20/GGC15 Percent swell

Swelling pressure

1.09

1.04

1.10

1.12

1.11

1.14

1.12

1.10

of 4, 2.5, 2, and 1.3, respectively. There is uniform variation of 0.01 times PS reduction irrespective of the L/D ratio of GGC which indicates that the variation in density alone plays a role in swell reduction. Similarly, the SP reduction is 1.04, 1.12, 1.14, and 1.10 times higher for GGC15 when compared to GGC20 for L/D ratios of 4, 2.5, 2, and 1.43, respectively. Hence, it is clear that GGC15 performs better in swell and SP reduction when compared to GGC20 for all L/D ratios of GGC inclusion. This, in turn, could be defined as an increase in the density of GGC will result in a decrease in cushioning effect. Moreover, from the above discussion it is reasonable to consider GGC15 and L/D ratios of 2.5 and 1.3

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as representative parameters for finding the swelling behavior of expansive soil in another placement condition.

24.2.4.3 Effect of placement condition The amount of expansion in compacted clay increases with an increase in the dry density and with a decrease in the initial moisture content [29,30]. In this study, the effect of initial moisture content on swelling was observed by conducting the swell test on samples with and without GGC inclusion prepared with HMC at MDD. The swell tests were conducted for two different L/D ratios of 1.3 and 2.5 and for an optimum density GGC15 on the samples prepared as described in the previous sections, and the results are shown in Figs. 24.17 and 24.18. When compared with the OMC placement condition, the sample with HMC at MDD shows higher PS. It is increased by 52% for the soil sample when HMC placement condition is adopted over OMC condition keeping dry density (MDD) as constant. Similarly, the PS for GGC15 is increased by 59% for the HMC condition compared to OMC condition, irrespective of the column diameter. The effect of L/D ratio with GGC inclusion on PS is shown in Fig. 24.17 and it is observed that PS decreases with decreasing L/D ratio of GGC. The reduction of PSr is 0.17 and 0.37 for 2.5 and 1.43 L/D ratios of GGC15 when compared to SA, respectively. This is in agreement with the observations for OMC condition. Additionally, the rate of the swelling process is earlier for GGC inclusion since GGCs also act as vertical drains. It is evident from Fig. 24.17 that the time taken for achieving the maximum swell is around 18 days (25,410 minutes) for SA, 12 days (17,280 minutes) for 40D, and 8 days (11,250 minutes) for 75D.

Figure 24.17 Time vs. percent swell for GGC15 (HMC at MDD).

Figure 24.18 Vertical stress vs. vertical strain for GGC15 (HMC at MDD).

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After the equilibrium swell of the sample, the specimen was loaded with a vertical load increment until the initial thickness of the sample was reached. The strain was monitored for each load increment and is plotted in Fig. 24.18. From the plot, the SP is evaluated for samples with and without GGC15 inclusion for HMC placement condition. The SP of the sample with HMC initial moisture content is 329 kPa and for 40D and 75D GGC15 inclusions are 225 and 97 kPa, respectively. The reduction of SPr is 0.32 and 0.65 for 40D and 75D GGC15 when compared to SA. Figs. 24.19A and B show the effect of L/D ratio on PSr and SPr for both placement conditions (OMC and HMC) of the stabilized expansive soils with GGC15 inclusion. When compared with the OMC placement condition, the stabilized sample with HMC at MDD shows higher PSr. It is increased by 15% for the soil sample when HMC placement condition is adopted over OMC condition, irrespective of the column diameter. Similarly, the effect of DR on PSr and SPr for both placement conditions (OMC and HMC) of the stabilized expansive soils with GGC15 inclusion is shown in Figs. 24.20A and B.

Figure 24.19 Comparing the effect of L/D ratio on (A) PSr and (B) SPr for HMC and OMC.

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Figure 24.20 Comparing the effect of Dr on (A) PSr (B) SPr for HMC and OMC.

Fig. 24.21 shows the correlation between the SP and PS for representative L/D ratios of 1.3 and 2.5 for both placement conditions (OMC and HMC) of the stabilized expansive soils with GGC15 inclusion. The empirical relationships between SP and PS are found for both placement conditions and the correlation is reasonably high (R2 . 0.9). Using Eqs. (24.6) and (24.7), it is possible to estimate SP from PS for OMC and HMC placement conditions, respectively. SP 5 81:57PS 2 211:0

(24.6)

SP 5 51:24PS 2 291:6

(24.7)

24.2.5 Variation of moisture content on the specimens In order to check the penetrability of water over the specimen for varying test conditions in HMC at MDD, the water content of the specimen was tested by an oven

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Figure 24.21 Variation of percent swell and swelling pressure.

Table 24.9 Determined values of water content (%) over the depth of the specimens. Placement condition

Mode

Optimal L/D ratio

Water contenta (%) Specimen portion

(HMC at MDD)

a

Variation of water contentb

Upper

Middle

Lower

SA

0.0

30.45

28.68

30.12

1.61

GGC15

2.5 1.3

30.68 30.74

29.45 29.72

30.25 30.54

1.02 0.92

All tests provide top and bottom drainage. Difference between average water content of upper and lower to the water content at middle.

b

drying method. After completion of the swell test, the specimen was immediately removed from the mold with the help of a hydraulic extruder. Then the specimen was cut into three parts to represent the upper, middle, and lower portions of the sample. For the case of GGC inclusion, the surrounding soil was sliced into three parts and the water content was determined for each portion and the values are reported in Table 24.9. The variation of water content in the middle portion of the specimen is reduced due to introduction of vertical drains (i.e., GGC). From

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Table 24.9, it can be also observed that the variation of water content depends on the L/D ratio.

24.3

Conclusions

A series of swell-compression tests were carried out on large 1D consolidation equipment which can accommodate the CBR mold. An attempt has been made to ascertain the performance of EPS beads mixed with expansive soil (GB) and GGC inclusion in expansive soil. The effect of GB and GGC on percent swell and SP of expansive soil was studied through an extensive experimental program. Based on the observed results, the following conclusions can be drawn: G

G

G

G

G

G

G

G

The compressible recycled GB inclusion leads to a significant reduction in the swelling potential (i.e., percent swell and SP). Swelling potential was reduced at higher recycled GB content up to 1%, this could be considered as an acceptable choice. Reduction in swelling potential can be attributed due to replacement of swelling clay by compressible EPS beads which acts as an inclusion or obstruction to the flow of water into the soil. GGC inclusion accommodates the lateral swell and hence a reduction in the subsequent vertical swell was observed. Vertical swell of the expansive soil is decreased with increasing diameter of the column or decreasing L/D ratio of GGC inclusion. Obviously, if the vertical swell is decreased then the applied vertical pressure to prevent swelling is also decreased. This results in a gradual decrease in SP with increasing column diameter. GGC15 (15 kg/m3) can better reduce swelling and the rate of swell is faster compared to GGC20 (20 kg/m3). Hence, it can be recommended that the density corresponding to GGC15 is the optimal density for the reduction of swell and SP. When compared with the OMC placement condition, the sample with HMC at MDD showed higher swelling ability. PS and SP increased by 50% for the soil sample when the HMC placement condition is adopted over the OMC condition, keeping dry density (MDD) as a constant. For the HMC placement condition with the inclusion of GGC15, PS is increased by 2.4 times, irrespective of the column diameter and SP is increased by 2.4 and 1.5 times (for L/D 2.5 and 1.43, respectively) when compared to OMC placement condition.

The above conclusions are arrived at based on laboratory swell-consolidation tests conducted on large 1D consolidation equipment. Therefore these conclusions need verification through other expansive soils with full-scale model tests for better understanding. In addition, further studies are needed to ascertain the performance of GGC inclusion under cyclic swell
shrink aspects.

Acknowledgments The authors are grateful for the funding provided by Science and Engineering Research Board-Department of Science and Technology (SERB-DST), Government of India, New Delhi, India, under File No. ECR/2015/000019.

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Danna Wang, Wei Zhang and Baoguo Han School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China

25.1

Introduction

With the advancement of society and the complexity of the service environment, traditional cement-based composites can no longer meet the needs of current building materials, especially for engineering with special requirements and in rugged surroundings. The proposal of high-performance and smart/multifunctional cementbased composites has great potential to achieve the sustainable development of civil engineering structures [1 4]. High-performance cement-based composites require not only high compressive, flexural, and tensile strength, but also favorable workability and good durability. Smart materials means that their properties or structure, even their functions themselves, can respond and change with changes in the external environment [5]. The concept of smart concrete was originally proposed and then developed by Japanese researchers in the 1980s [6]. Smart/multifunctional cement-based composites refer to those possessing one or more intelligent functional properties, such as self-sensing, self-healing, self-heating, excellent electrical conductivity, and electromagnetic properties. High-performance and smart/multifunctional cement-based composites can be fabricated by incorporating admixtures and various fillers into cement-based composites. This new generation of cementbased composites can be applied to develop smart infrastructures and achieve automatic structural health monitoring, to enhance the security, durability, resilient and versatility of infrastructures. With the maturity and wide application of nanotechnology, increasing numbers of studies have explored its effects on civil engineering building materials. Its penetration into the field of civil engineering provides new impetus for developing highperformance and smart/multifunctional cement-based composites. The characteristics of nanomaterials play a crucial role in achieving the target performance of the new cement-based composites. First, nanomaterials themselves have favorable mechanical and electrical properties. Then, nanomaterials with particle size less than 100 nm can effectively improve the pore structure and reduce porosity, which makes the matrix denser, thereby improving the strength and durability of the cement-based composites. What is more, nanoparticles have good interfacial properties, strong adsorption, and chemical reactivity due to their small size effect, surface, and interface effects. The nanofillers commonly used in modified cement-based composites include carbon nanotubes (CNTs), graphite, nano-SiO2, nano-TiO2, etc. [7 16]. It has been New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00025-9 © 2020 Elsevier Inc. All rights reserved.

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Table 25.1 Increase rate of properties of cement-based composites with different nanofillers [17 22]. Types of cement-based composites

Compressive strength

Flexural strength

With multiwalled carbon nanotubes With multilayer graphenes With nano-SiO2 With nano-TiO2

78.8% (74 MPa)

64.6% (5.6 MPa)

With nano-ZrO2 With nano boron nitride

54% 48.7% 12.26% (12.2 MPa) 16.3% (16.18 MPa) 12.96% (11.61 MPa)

45.6% 87% (6.69 MPa) 36.6% (4.19 MPa) 15.7% (2.65 MPa)

Conductivity

References [19]

15.6%

[18] [20] [17]

20.3%

[21] [22]

confirmed that nanofiller-reinforced cement-based composites possess not only superior mechanical properties (including compressive strength [Cs] and flexural strength [Fs]), but also preeminent functional properties such as self-sensing ability and electrical conductivity. In addition, various types of nanofillers can endow cement-based composites with different functional properties. Meanwhile, the modification degree of cement-based composites varies with the content of nanofillers and curing conditions. Therefore this chapter summarizes the influences of some commonly used nanofillers on the properties of cement-based composites, and the increased rates of some properties are demonstrated in Table 25.1. In this chapter, the properties of smart and multifunctional cement-based composites (including self-sensing, self-healing, self-adjusting, self-heating, self-damping, wearing resisting, and electromagnetic wave-shielding/absorbing cement-based composites) as well as cement-based composites with different types of nanofillers including CNTs, graphene, nano-SiO2, nano-TiO2, nano-ZrO2, nano-boron nitride (nano-BN), and CNT/nano-carbon black (NCB) composite fillers are summarized. The strengthening/modifying mechanisms of nanofillers on cement-based composites are also summarized. In addition, the applications and further development of a new generation of cement-based composites are also discussed.

25.2

Smart and multifunctional cement-based composites

25.2.1 Self-sensing cement-based composites At present, some researchers have incorporated conductive fillers into cement-based composites to improve their conductivity and self-sensing abilities. The frequently employed conductive fillers are carbon fiber, carbon black, nickel powder, CNTs,

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stainless wire, and nano-graphite platelets (NGPs), which are all beneficial to forming a good conductive network in the composites. Han et al. observed that the piezoresistivity of carbon fiber and carbon black reinforced cement-based composites has excellent repeatability and stability under cyclic compressive loading in the elastic phase according to the electrical resistivity tests [7]. Experimental results showed that the maximal piezoresistive sensitivities can reach 1.35%/MPa and 227 in compressive stress and strain, respectively. The electrical conductivity and piezoresistivity of cement-based composites filled with three types of particle sizes of spiky spherical nickel powder (3 7, 2.6 3.3, and 2.2 2.8 µm) were studied by Han et al. [8]. It can be observed from the tests that the incorporation of nickel powder with high dosage and small size is effective in improving the electrically conductivity of cement-based composites. In addition, the piezoresistive sensitivity is influenced by the content and particle size of nickel powder according to the tests, which is directly correlated with the conductive network in cement-based composites. The maximal piezoresistive sensitivities of 6.0%/MPa can be achieved by adding 22 vol.% nickel powder with a diameter of 3 7 µm into cement-based composites. Han et al. also used cementbased composites filled with nickel particles as sensors, which were buried into concrete pavement for traffic detection [9]. The self-sensing cement-based composite pavement has been proven to be able to accurately detect vehicle flow, and it has some feasibility for traffic detection and monitoring. Han et al. studied the piezoresistive response of cement-based composites filled with 0.1% and 0.5% carboxyl multiwalled CNTs (MWCNTs), and both specimens exhibited stable and repeatable piezoresistive properties based on the analysis of AC voltage response and resistance response [10]. The response curves of AC voltage of 0.5% carboxyl MWCNT-filled cement-based composites are more repeatable and smoother compared with that of 0.1% carboxyl MWCNT-filled cement-based composites. Also, the cement-based composites with low-amplitude AC voltage have better piezoresistive properties, and the response curves are more regular. Dong et al. incorporated short-cut super-fine stainless wire (SSSW) with a length of 10 mm and diameter of 8/20 µm into cement-based composites to investigate their self-sensing property [11]. The electrical resistivity and its fractional change of cement-based composites with SSSW under cyclic compressive/monotonic compressive/flexural loading were tested and calculated. It can be seen from the results that for SSSW with a diameter of 8 µm, the piezoresistive sensitivity has better stability and reversibility with the increasing SSSW content under cyclic compressive loading. In contrast, the piezoresistive sensitivity is decreased as the SSSW content increases to a diameter of 20 µm, and the cement-based composites incorporating 1.5% SSSW with a diameter of 20 µm have almost no piezoresistive sensitivity. The addition of 1.5% SSSW with a diameter of 8 µm and 0.5% SSSW with a diameter of 20 µm provides an excellent piezoresistive sensitivity of cement-based composites, although the fractional changes of electrical resistivity are only 0.3% under cyclic compressive loading. Moreover, the strain sensitivity of cement-based composites containing SSSW

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with a diameter of 8 µm modified can separately reach 22.5, 94.9, and 43.6 under cyclic compression, monotonic compression, and flexure loading. Sun et al. explored the piezoresistivity of NGP-modified cement-based composites at different loading rates (0.1, 0.2, 0.3, and 0.4 mm/min) and different loading amplitudes of 2.5, 5, 10, and 20 MPa [12]. The experimental results presented that the NGP-doped cement-based composites have favorable piezoresistive sensitivity and repeatability under multiple static/dynamic cyclic loadings in the elastic range. The fractional change in electrical resistivity of 5 vol.% NGP-filled cement-based composites obtains a maximum value of 15.6% under compressive loading with an amplitude of 20 MPa. In addition, cement-based composites incorporating NGPs show maximal sensitivities of 0.78%/MPa and 156 in compressive stress and compressive strain, respectively. Piezoresistive sensitivity of cement-based composites with the above fillers under cyclic compressive loadings is illustrated in Table 25.2. These conductive filler-reinforced cement-based composites have high sensitivity and superior selfsensing ability, and some of these cement-based composites are even more sensitive than traditional metal strain gauges with a strain sensitivity of 2 3 [13]. In consequence, it has potential to use self-sensing cement-based composites as sensors for structural health monitoring.

25.2.2 Self-healing cement-based composites It has been indicated that incorporating nanofillers into cement-based composites is helpful to enhance their self-healing property. Wang et al. introduced nano-SiO2, nano-TiO2, and nano-ZrO2 into cement-based composites, and the results showed that the compressive and Fs of nanofiller-reinforced cement-based composites have some self-healing capability when reloaded [14]. The self-healing coefficient of cement-based composites filled with nano-SiO2/ nano-TiO2/nano-ZrO2 under compressive and flexural loadings is listed in Table 25.3. The self-healing coefficients of Cs and Fs in Table 25.3 are used to characterize the self-healing ability of cement-based composites in terms of compressive Table 25.2 Piezoresistive sensitivity of cement-based composites with different fillers under cyclic compressive loadings [7,8,11,12]. Types of cement-based composites

Stress sensitivity (%/MPa)

Strain sensitivity

References

With carbon fiber and carbon black With nickel powder With short-cut super-fine stainless wire With nano-graphite platelets

1.35

227

[7]

22.5

[8] [11]

156

[12]

6.0

0.78

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and Fs, respectively. The composites are thought to have satisfactory self-healing capability when the Cs and Fs values exceed 1. Larger Cs and Fs values indicate greater self-healing properties of cement-based composites [15]. As shown in Table 25.3, the self-healing ability of cement-based composites with water curing is better than that with air curing, except for the 3.0% nano-ZrO2 filled cement-based composites. The maximal self-healing coefficients Cs of 1.31 and Fs of 1.19 can be achieved by adding 3.0% nano-SiO2 to cement-based composites. The strengthening mechanisms of self-healing ability on cement-based composites filled with nanofillers can be summarized into three aspects [14]. First, the incorporation of nanofillers provides nucleation sites for hydration products and promotes the hydration of unhydrated cement, which is known as the nanonucleation effect [16,23]. Secondly, nanofillers are filled in the voids of the cement matrix to make the matrix more compact [17,24,25]. Also, the addition of nanofillers can enhance the 3D network structure of the cement-based composite matrix and disperse the distribution of microcracks [26]. Finally, nano-SiO2 and nano-TiO2 have a pozzolanic effect and can react with calcium hydroxide to generate calcium silicate hydrate (C S H) gels, thereby resulting in the enhancement of compactness and strength of the composite matrix [27,28].

25.2.3 Self-adjusting cement-based composites Phase change materials (PCMs) absorb or release a large amount of latent heat during the process of transforming physical properties (i.e., the phase transition process) [29]. Consequently, PCM has been introduced into cement-based composites to store thermal energy and reduce energy loss in constructions [30 32]. PCMreinforced cement-based composites have the function of self-adjusting the indoor temperature of constructions, so as to improve the indoor thermal comfort. Nonetheless, Han et al. pointed out that the incorporation of PCM leads to a decrease in the strength and thermal conductivity of cement-based composites [33]. In order to enhance the strength and thermal conductivity of construction materials, Table 25.3 Self-healing coefficient of cement-based composites under compressive and flexural loadings [14]. Types of cement-based composites

Without nanofillers With 3.0% nano-SiO2 With 3.0% nano-ZrO2 With 3.0% nano-TiO2

Self-healing coefficients of compressive strength

Self-healing coefficients of flexural strength

Water curing

Air curing

Water curing

Air curing

0.94 1.31 0.98 1.26

0.94 1.05 1.03 0.87

0.89 1.19 0.93 1.02

0.9 0.99 1.04 0.93

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they incorporated the aqueous microencapsulated PCM and CNTs into cementbased composites. The results indicated that the cement-based composites filled with PCM and CNTs have preferable thermal insulation performance. The maximum indoor temperature difference between the cement-based composites filled with PCM and CNTs and that without admixtures can be as high as 6.8 C. Furthermore, cement-based composites doped with PCM and CNTs have better mechanical properties compared with those doped with PCM alone. As a result, cement-based composites incorporating PCM and CNTs can be applied as selfadjusting, thermally comfortable, energy-efficient construction materials.

25.2.4 Self-heating cement-based composites In a cold environment or wintery weather, infrastructures such as urban roads, bridges, and airport runways are prone to freeze or be covered with snow, affecting their safety and the daily lives of users. The proposal of self-heating cement-based composites allows the infrastructure to heat itself, thereby melting the snow and ice on it. Conductive fillers are widely incorporated into cement-based composites, which improve the electrical conductivity and the electrical resistance self-heating cement-based composites are prepared. The most commonly used conductive fillers are carbon fibers, steel fibers, graphite, and nickel particles. [34 39]. However, the heating element requires that the electrical resistivity of cement-based composites should not be too low or too high. This is because, to achieve a certain power, low electrical resistivity will require a large current, while high electrical resistivity will require a high voltage [39]. Zhang et al. studied the electrical resistivity of different types (Type 123 and Type 287) and different contents of nickel particle-filled cement-based composites as well as the surface temperature of specimens under different input voltages (10, 15, and 20 V) [39]. The electrical resistivity of cement-based composites decreases as the content of both types of nickel particles increases. In addition, the cementbased composites doped with T287 nickel particles have a lower electrical resistivity with respect to that doped with the same content of T123 nickel particles, which indicates that nickel particles with small size are more advantageous for the conductivity of composites. Experiments showed that the surface temperature of the cement-based composites containing nickel particles rises from the environmental temperature of 216.0 C to 73.7 C within 500 seconds when the current is connected, and an ice cube with thickness of 3 mm melts completely within 478 seconds, while 2 cm-thick snow melts absolutely within 368 seconds. All these prove that the introduction of nickel particles is capable of strengthening the self-heating ability of cement-based composites. However, the heating system using self-heating cement-based composites containing nickel particles themselves as heating elements was costly. In future studies, it will be necessary to specifically observe the self-heating ability of cement-based composites and develop a comprehensive ice- and snow-melting system. Meanwhile, there is also a need to control the cost of the self-heating system.

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25.2.5 Self-damping cement-based composites Cement-based composites are brittle materials, which will suffer fatigue accumulation or damage under environmental extremes such as earthquakes or strong winds [2,40]. The self-damping cement-based composites can provide a damping effect for infrastructure buildings without additional energy dissipation devices, ensuring the safety of structures in severe environments [5]. The incorporation of polymers, fibers, silica fume, graphite, etc., has been shown to enhance the energy consumption capacity of cement-based composites, but their strength and stiffness are difficult to satisfy [41 51]. Ruan et al. found that the introduction of multilayer graphenes (MLGs) does not reduce the original mechanical strength of cementbased composites, and can improve its damping capacity [52,53]. Ruan et al. observed that the incorporation of 5.0% MLGs maximally increases the damping ratio of cement-based composites, and the increase rate is 45.73% in comparison with the composites without MLGs [53]. Meanwhile the damping ratio of cement-based composites containing 1.0% of MLGs is 16.22% higher than that of control composites. Moreover, the self-damping ability of MLG-doped cementbased composites is primarily due to the interlayer dislocation slip of MLGs, the viscous friction between MLGs and composite matrix, and the superior thermal conductivity of MLGs, all of which will consume energy. Obviously, the MLGreinforced cement-based composites possess excellent self-damping ability and can be used for infrastructure construction in earthquake-prone areas or environments prone to strong winds, for example, highways, bridges, and dams.

25.2.6 Wear-resisting cement-based composites Wear resistance is one of the indicators for characterizing the durability of cementbased composites. For cement-based composite pavements, wear resistance is particularly important. Previous studies have demonstrated that nanoparticles are effective fillers to improve the wear resistance of cement-based composites [54 56]. Wang et al. incorporated different contents (0%, 1.0%, and 3.0%) of nano-SiO2, nano-TiO2, and nano-ZrO2 into cement-based composites to investigate their wear resistance under room temperature curing and heat curing [57]. The results showed that the wear resistance of nanoparticle-reinforced cement-based composites under heat curing is preferable compared with that under room temperature curing, as heat curing can promote the hydration of cement [58,59]. In addition, cement-based composites filled with 1.0% nano-SiO2 under room temperature curing show the greatest increase of 39.68% in wear resistance, while the wear resistance of cementbased composites under heat curing obtains a maximal increase of 48.46% as 3.0% nano-ZrO2 is introduced. The mechanism of nanoparticles improving the wear resistance of cement-based composites can be explained by the fact that the nucleation effect of nanoparticles promotes cement hydration, reduces the orientation and size of calcium hydroxide crystals, makes the structure more compact, and improves the interface transition zone (ITZ) [60,61]. Cement-based composites with excellent wear resistance are of practical significance for airport runways and highways.

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25.2.7 Electromagnetic wave-shielding/absorbing cement-based composites Using electromagnetic wave-shielding/absorbing cement-based composites as building materials can reduce harmful radiation intensity and shield from redundant electromagnetic interference. Therefore the development of electromagnetic waveshielding/absorbing cement-based composites has great application value in the construction and military fields [62 65]. Graphite, carbon fibers, and steel fibers are generally added to cement-based composites to enhance their electromagnetic properties [66 68]. However, the mechanical properties of graphene-modified cement-based composites are notably decreased, although the electromagnetic properties are increased [69,70]. The cement-based composites containing MLGs have been confirmed to have excellent mechanical properties and functional characteristics, especially electromagnetic properties [18,71]. Wang et al. investigated the electromagnetic wave-shielding/absorbing properties of MLG-doped cement-based composites in the frequency of 2 18 GHz [71]. It could be observed from this that the maximal increase in electromagnetic shielding effectiveness of cement-based composites incorporated with 10.0% MLGs is 73.95% at 14 GHz, and the value reaches 10.35 dB. The reflectivity value of 10.0% MLG-filled cement-based composites with a thickness of 10 mm is up to 233 dB at a frequency of 8 GHz. Additionally, the corresponding bandwidths of 10 mmthick MLG-doped cement-based composites with reflectivity below 25 dB (with practical significance) are 2 GHz and 2.5 GHz. This indicates that electromagnetic wave-shielding/absorbing MLG-doped cement-based composites have practical value in engineering.

25.3

Nanocement-based composites

25.3.1 Cement-based composites with carbon nanotubes The addition of CNTs is beneficial to enhancing the mechanical properties of cement-based composites. The enhancement mechanisms can be summarized as the crack bridging and pull out effects, as well as the improvement of ITZ due to the wide distribution of MWCNTs [19,26,72,73]. Cui et al. studied the Cs and Fs of cement-based composites filled with 12 kinds of MWCNTs [19]. They found that the Cs and Fs of cement-based composites obtain a maximal increase of 47% and 55%, when 0.1% MWCNTs with short length of 0.5 2 µm and large diameter of 20 30 nm are incorporated. Hydroxylfunctionalized MWCNTs are preferable for providing higher mechanical properties of cement-based composites compared with carboxyl-functionalized MWCNTs. The compressive/Fs of 0.5% hydroxyl-functionalized MWCNT-reinforced cementbased composites are increased by 64.6%/76% compared with the control specimen without MWCNTs. Among the 12 types of MWCNTs, 0.5% nickel-coated MWCNTs gives the cement-based composites the maximum enhancement in Cs

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and Fs, and their maximal relative/absolute increases are 78.8%/74 and 64.6%/ 5.6 MPa, respectively. Ruan et al. introduced four kinds of MWCNTs into cement-based composites and tested their mechanical properties (including Fs, fracture energy, Cs, and toughness, as well as Fs Cs ratio) under water curing and heat curing [73]. The results presented that the mechanical properties of MWCNT-doped cement-based composites with heat curing are more favorable than with water curing. The Fs and fracture energy of cement-based composites with 0.5% of carboxyl-functionalized MWCNTs achieve a maximal relative/absolute increase of 31.0%/5.05 MPa and 36.2%/167.15 (J/m2), respectively. The Cs and toughness of 0.25% hydroxylfunctionalized MWCNT-filled cement-based composites are maximally improved by 30.4% and 44.6%. Cement-based composites incorporating with 0.25% helical MWCNTs exhibit the largest enhancement of 34.4% in Fs Cs ratio. In future research, it will be necessary to further promote the uniform dispersion of CNTs in cement-based composites. Moreover, the self-sensing ability, electrical conductivity, and durability of the cement-based composites with CNTs should be studied comprehensively to meet the requirements of high-performance and smart/ multifunctional cement-based composites.

25.3.2 Cement-based composites with graphene The incorporation of a small amount of graphene can not only improve the mechanical and electrical properties of cement-based composites, but also improve the functional properties such as electromagnetic properties [74 76]. However, the preparation of graphene is complicated and expensive. The performance of MLGs is similar to that of graphene and its prepared process is relatively simple and economical, thus it can be considered as a substitute for graphene. On the one hand, the small size, low density, and interlaminar structure of MLGs make them widely distributed in the composite matrix to form a reinforcing network. On the other hand, MLGs themselves have excellent physical properties such as high strength/ toughness and good electrical conductivity, chemical stability, and favorable compatibility with cement-based composites. As a result, the incorporation of MLGs has great potential for improving the properties of cement-based composites [71]. Research results obtained by Han et al. illustrated that the incorporation of MLGs increases the Cs and Fs of cement-based composites by 54% and 21% [45]. The reinforcing mechanisms can be interpreted as the enhanced network in cementbased composite matrix, the decrease of major cracks, and the superior compatibility between MLGs and composites owing to the addition of MLGs. Sun et al. reported that the incorporation of 2.0 vol.% MLGs maximally improves the Cs/elastic modulus of cement-based composites, and the increase rates are 54%/50% [18]. Five vol.% MLG-modified cement-based composites present the highest fractional change of 15.6% in electrical resistivity under a loading compressive stress of 20 MPa. Moreover, the piezoresistive sensitivity and stability of MLG-reinforced cement-based composites are excellent under both static cyclic loading and dynamic loading. This is believed to be related to the improvement of tunneling

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conduction during the compressive deformation of specimens. The piezoresistive sensitivity of 5.0 vol.% MLG-modified cement-based composites reaches 0.78%/ MPa in compressive stress. Cement-based composites containing MLGs possess good repeatability of thermal resistance, but the thermal-resistance sensitivity reduces with the increasing content of MLG. The effect is probably associated with the enhancement of thermal conductivity as the MLG content is increased. In addition, cement-based composites incorporated with MLGs have been reported to possess excellent self-damping and electromagnetic wave-shielding/ absorbing properties as described in Sections 25.2.5 and 25.2.7.

25.3.3 Cement-based composites with nano-SiO2 Numerous studies have reported that the presence of nano-SiO2 can improve the mechanical properties and durability of cement-based composites, which may be attributed to the small size, large specific surface area, and pozzolanic effect of nano-SiO2 [77 83]. Zhang et al. investigated the compressive and Fs of cement-based composites containing 0.0%, 0.5%, 1.0%, 1.5%, and 2.0% of nano-SiO2. The enhancement mechanisms of nano-SiO2 on compressive and Fs were explored through mechanical tests and the analysis of scanning electron microscope (SEM) images, thermogravimetry (TG), and X-ray powder diffraction (XRD) [20]. The introduction of nano-SiO2 is more effective in improving the Fs of the cement-based composites cured for 3 days than that cured for 28 days. With respect to a control specimen without nano-SiO2, the Fs of cement-based composites doped with 1.5% nano-SiO2 is improved by 45.6% after a curing period of 3 days. The maximum Fs of 8.7 MPa is obtained by cement-based composites doped with 2.0% nano-SiO2 after a 28 days curing period, of which the increase rate reaches 16.0% compared with the control specimen. Different from the influence of nano-SiO2 on Fs, the enhancement effects of the nano-SiO2 on the Cs of cement-based composites at 3 and 28 days are both significant. Two per cent nano-SiO2-reinforced cement-based composites achieve a maximal Cs of 61.4 MPa with an increase rate of 48.7%. According to the analysis of SEM images, TG, and XRD, the presence of nano-SiO2 leads to the formation of a more compact composite matrix, reducing the calcium hydroxide size and crystal orientation, and it has been shown to react with calcium hydroxide to form C S H gels. All of these possess a positive effect on the improvement of the mechanical properties of cement-based composites. Furthermore, the self-healing ability and wear-resisting properties of cementbased composites with nano-SiO2 are both satisfactory according to Sections 25.2.2 and 25.2.6.

25.3.4 Cement-based composites with nano-TiO2 Owing to the nucleation effect and filling effect, the addition of nano-TiO2 can enhance the mechanical properties, including Fs and fracture toughness, of cementbased composites [84 86]. Nevertheless, it has been proven that a high specific

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surface area and unsaturated bond will lead to serious agglomeration of nano-TiO2 in a composite matrix [87,88]. An effective method for preventing agglomeration is coating silicon on the surface of nano-TiO2 to form Ti O Si bonds, which can accumulate more negative charges on the surface, thereby dispersing the nano-TiO2 through electrostatic repulsion [89 92]. Han et al. reported that the incorporation of nano-SiO2-coated TiO2 is conducive to enhancing the mechanical behavior of cement-based composites [17]. The results presented that the maximal relative/absolute increase rate of 87%/6.69 MPa in Fs at 28 days can be reached by adding 5.0% of nano-SiO2-coated TiO2 to cement-based composites, and its value reaches 14.38 MPa. The Cs of 3.0% nano-SiO2-coated TiO2-reinforced cement-based composites cured for 28 days achieves the maximum of 111.75 MPa, which is 12.26%/12.2 MPa higher than the specimen without nanoSiO2-coated TiO2. Nano-SiO2-coated TiO2 with a higher content will absorb water in the mixture which is unfavorable for cement hydration, thus the reinforcing effect of 5.0% nano-SiO2-coated TiO2 on Cs is lower than that of 3.0% nano-SiO2coated TiO2. The improvement mechanisms of nano-SiO2-coated TiO2 on mechanical properties of cement-based composites are similar to those of nano-SiO2.

25.3.5 Cement-based composites with nano-ZrO2 Nano-ZrO2 possesses excellent strength, toughness, and favorable wear-resisting performance. Considering its small size characteristic and phase transition effect, nano-ZrO2 has been applied as fillers in the field of civil engineering to modify the properties of cement-based composites [93]. Han et al. introduced different contents of nano-ZrO2 into cement-based composites and tested their mechanical properties and electrical properties [21]. The flexural, compressive, and splitting tensile strength of cement-based composites filled with nano-ZrO2 after a 28-days curing period obtain a maximal relative/absolute increase of 36.6%/4.19, 16.3%/16.18, and 34.0%/1.08 MPa, respectively. Furthermore, the addition of 5.0% nano-ZrO2 maximally decreases the electrical resistivity of composites by 20.3%. Ruan et al. found that the flexural/compressive/splitting tensile strengths of 3.0% nano-ZrO2-modified cement-based composites with heat curing of 90 C for 2 days are all higher than those with water curing for 28 days, and the increase rates were 35.0%, 15.0%, and 17.0% [24]. Under uniaxial compression, the elastic modulus of cement-based composites doped with 3.0% nano-ZrO2 is reduced by 12.7% compared with that of an undoped control specimen. The increase in initial peak deflection/strength and fracture energy of cement-based composites containing nano-ZrO2 under four-point bending is 78.2%/5.6% and 85.7%, respectively. The addition of nano-ZrO2 improves the fracture peak load of cement-based composites by 25.4% under four-point shear fracture with a crack-depth ratio of 0.25, while it is 11.1% lower than the specimen without nano-ZrO2 with a crack-depth ratio of 0.5. It can be summarized from the research of Ruan et al. that the mechanical properties and toughness of cement-based composites doped with nano-ZrO2 have been greatly modified. The strengthening mechanisms of nano-ZrO2 are close to those of nano-

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SiO2 and nano-TiO2, except that nano-ZrO2 has no pozzolanic effect and cannot promote cement hydration.

25.3.6 Cement-based composites with nano-boron nitride Nano-BN, similar in structure to CNTs, has a series of excellent intrinsic properties, for example, satisfactory mechanical strength, fine heat resistance and corrosion resistance, and favorable thermal conductivity [94 98]. Some studies have used nano-BN as nanofillers in building materials for civil engineering applications. A few researches have presented that nano-BN has a significant enhancement effect on the mechanical strength of cement-based composites [22,99,100]. Zhang et al. explored the influence of different particle sizes (120 nm, 500 nm, and 11 µm) of nano-BN, different dosages of nano-BN, and different curing conditions on the mechanical properties and durability of cement-based composites [22]. Cement-based composites filled with 0.5% nano-BN with a particle size of 120 nm demonstrate an increase of 15.7%/2.65 MPa and 12.96%/11.61 MPa in flexural and Cs under standard curing for 28 days, respectively. Moreover, the wear resistance of this specimen is also improved by 55.56%/0.35 kg/m2 and 34.96%/0.17 kg/m2 under standard curing for 28 days and heat curing for 2 days, respectively. Heat curing is more beneficial to improve the Cs of cement-based composites doped with nano-BN, while standard curing is more effective in improving their wear resistance. According to the analysis of SEM images, TG, XRD, and nuclear magnetic resonance, the reinforcing mechanisms of nano-BN on the mechanical properties and durability are associated with a small size effect, nucleation effect, filling effect, and bridging effect.

25.3.7 Cement-based composites with carbon nanotube/nanocarbon black composite fillers Incorporating appropriate content of nano-carbon materials like CNTs and NCB contributes to the improvement of the mechanical and electrical properties of cement-based composites and endows them with functional properties such as selfsensing property [101 103]. In order to overcome the agglomeration of CNTs in the matrix and form the synergetic modification effect of composite fillers, the electrostatic self-assembly fillers composed of CNTs and NCB can be added to the cement-based composites to achieve effective modification [104,105]. The CNT/ NCB composite fillers can be evenly dispersed in the composite matrix without ultrasonic dispersing because of their grape bunch structure. Han et al. reported that the Fs and conductivity of CNT/NCB composite fillerreinforced cement-based composites are improved effectively, but the Cs drops slightly [104]. Furthermore, the piezoresistivity sensitivities of CNT/NCB composite filler-reinforced cement-based composites can be up to 2.69%/MPa and 704 in compressive stress and strain. In addition, the piezoresistivity is observed to be stable and reversible. Zhang et al. found that the introduction of CNT/NCB composite fillers

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with a high aspect ratio of CNT and large size of NCB is conducive to enhancing the functional properties of cement-based composites [105]. Cement-based composites containing 1.41 vol.% CNT/NCB composite fillers exhibit piezoresistivity sensitivity in compressive stress and strain of 3.12%/MPa and 521, respectively, and the fractional change of electrical resistivity is 13.4%. It is therefore concluded that the cement-based composites containing CNT/NCB composite fillers possess favorable mechanical properties and excellent piezoresistivity sensitivity.

25.4

Conclusions

This chapter introduced the research findings of smart and multifunctional cementbased composites and nanocement-based composites. Mechanical and functional properties of the cement-based composites modified by various types of fillers were described. To conclude, the piezoresistivity sensitivity of cement-based composites filled with carbon fiber and carbon black/SSSW/NGPs are more favorable, and these fillers all endow composites with self-sensing property. The addition of PCM and CNTs enhances the thermal insulation performance of composites, which is helpful in developing self-adjusting cement-based composites. The incorporation of nickel particles is beneficial to improve both the self-sensing and self-heating properties of cement-based composites. Furthermore, CNTs can not only give cement-based composites with self-sensing property, but also help increase their Cs, Fs, fracture energy, or toughness, as a result of crack bridging and pulling out effects. Cementbased composites containing MLGs possess excellent self-damping and electromagnetic wave-shielding/absorbing properties as well as favorable compressive and Fs. The introduction of nano-SiO2/nano-TiO2/nano-ZrO2/ nano-BN has different enhancement effects on the mechanical properties of cement-based composites, and enables the composites to have self-healing and wear-resistance abilities. The reinforcing effects are mainly related to the small size effect, nucleation effect, filling effect, and pozzolanic effect of nanoparticles. In addition, the cement-based composites with CNTs and NCB composite fillers are confirmed to have preferable selfsensing, electrical conductivity, and Fs. All of the above materials are beneficial to fabricate high-performance and smart/multifunctional cement-based composites. This new generation of smart/ multifunctional cement-based composites can be widely used in civil engineering fields, such as large infrastructures, buildings in complex service environments, and automatic structural health monitoring and repair. In future work, the preparation process of the new generation of cement-based composites needs to be perfected and the costs need to be controlled to achieve the maximum economic benefits. Finally, the in-depth development of the detection and characterization methods for the properties and functions of smart/multifunctional cement-based composites is considered to be an important direction for further research, which will be helpful for a comprehensive understanding of the properties of composites.

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Acknowledgments The authors express their thanks for the funding from the National Science Foundation of China (51978127 and 51578110), and the Fundamental Research Funds for the Central Universities in China (DUT18GJ203).

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Potential use of recycled aggregate as a self-healing concrete carrier

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Chao Liu and Zhenyuan Lv Xi’an University of Architecture and Technology, Xi’an, China

26.1

Introduction

Along with industrialization and the continuous advancement of urbanization, a large number of new urban complexes have emerged. At the same time, the existing cities are also constantly upgrading their landmark buildings and other outlying areas. This has led to a large demand for building materials resources [1
4]. In addition, the growing population density, as a result of the surge in residential, catering, medical services, and other public facilities, has greatly accelerated the construction waste generated by construction and demolition [5]. The global data statistics of the past decade have indicated that construction waste is produced that includes the arbitrary disposal of construction and demolition waste and illegal dumping, accompanied by a lack of specific control. As a result, the space for construction waste landfill and landfill is insufficient and the environment has further deteriorated [6]. Concrete is one of most commonly used building materials. Furthermore, natural aggregate, as a concrete component, is also heavily consumed. The process of rapid development has led to the gradual lack of availability of natural rough resources and further deterioration of the natural ecology. The contradiction between the need for development and the increasing scarcity of natural resources has become more acute over time [7]. In addition, damaged parts of the construction and demolition activities, along with manufacturing, are considered as demolition construction waste. Construction waste includes sand, gravel, asphalt, bricks, masonry, and recycled aggregates [8]. The recycled aggregate, which is taken from abandoned buildings, is a product formed by natural aggregate and cement-based hydration [9,10]. Recycled aggregate concrete (RAC) is prepared from recycled aggregates [11]; studies have shown that the performance of recycled concrete is affected by the properties of recycled coarse aggregates [12,13]. With an increase in the particle size of recycled aggregates, the cracks of recycled concrete have been further expanded and extended, compared with the cracks of natural aggregate concrete [14]. Among these, recycled aggregate is in the position of being wrapped by old mortar after undergoing garbage sorting. Therefore the inherent porous nature of the recycled attached mortar results in a more complex mechanical composition New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00026-0 © 2020 Elsevier Inc. All rights reserved.

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compared to natural aggregates [15]. The exploration of the performance of recycled aggregates has been ongoing for a long time. Ganesh et al. [16] prepared partial water and RAC. It was found that RAC has higher compressive strength and lower resistance than natural aggregate concrete; and the cost of preparing RAC is about 4.5% cheaper, without losing its mechanical properties. However, the inherent physical properties of recycled aggregates show lower slump and higher water absorption. Experiments by Gholamreza et al. [17] show that the creep shrinkage of RAC at different ages is significantly affected by the mixing ratio method. This phenomenon is mainly caused by the nature of the recycled aggregate attached to the mortar. The creep contraction prediction model established by Liu et al. [18] for 1200-day long-term load experiments has an influential relationship between the recycled aggregates with different substitution rates and the creep shrinkage of concrete. This experiment revealed that with the further extension of time and the replacement rate of recycled aggregates, the recycled concrete showed late stability energy close to that of natural concrete. Fonseca et al. [19] also conducted related experiments, taking compressive strength, split tensile strength, elastic modulus, and wear resistance as factors. It was found that in terms of mechanical properties, recycled concrete is affected by the same curing conditions as natural concrete. However, as the replacement rate of recycled aggregates increases, both the elastic modulus and the split tensile strength are lost. Recycled concrete with a higher substitution rate in the external environment is more susceptible to natural concrete. A study by Xiao et al. [20] shows that the quality of recycled concrete has a considerable influence on the probability distribution of strength. As the replacement rate of recycled aggregates increases, the elastic modulus of recycled concrete generally decreases; however, the strain at peak stress is greater than that of conventional concrete. The replacement ratio of the recycled aggregate has almost no effect on the bond strength between the test piece and the deformed steel bar. Experiments by Benito et al. [21] show that it is feasible to have a maximum replacement ratio of recycled aggregates of about 40%. At 90 days, for a test piece with a substitution rate of about 20% (better performance with respect to different substitution rates), the average compressive strength reduction value was less than 15%. This indicates that the overall effect of the recycled aggregate interface on the test piece has a more significant negative impact, with an increase in the amount of aggregate. Kou et al. [22] found that the compressive strength and splitting tensile strength of recycled concrete were lower than those of the corresponding natural aggregate concrete by observing the specimens cured for 5 years. However, from 28 days to 5 years, the compression and splitting tensile strength of recycled concrete increased. Manzi et al. [23] studied the short-term and long-term performance of concrete structures incorporating recycled aggregates. The results show that for different short-term and long-term performances, fine recycled coarse aggregates have some degree of influence on the mechanical properties of the structure. This is mainly due to the fact that the recycled aggregate added to the test piece can cause a very dense microstructure. Chakradhara et al. [24] compared recycled concrete specimens with 0%, 25%, 50%, and 100% substitution rates with natural aggregate concrete specimens. The experiment showed that

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the test piece, which was wet-cured for 7 days in air, showed better strength than the test piece which was completely cured under water for 28 days. The void volume and water absorption of the recycled aggregate were 2.61% and 1.82% higher than that of the natural aggregate concrete, respectively. And in the first 28 days, the strength growth rate of recycled concrete was lower than that of natural concrete. In terms of durability, recycled aggregates also exhibit a different and complex form of performance than natural aggregates. Kou and Poon [25] are committed to improving the durability of recycled coarse aggregate concrete. Experiments have shown that the use of recycled coarse aggregate in concrete can degrade the durability of concrete. Adding 25%
35% fly ash to the high substitution rate concrete will reduce the drying shrinkage of the test piece and weaken the creep of the concrete. However, the penetration resistance of concrete to chloride ions also decreases, and as the substitution rate increases, the carbonization depth increases accordingly. In particular, for recycled aggregates with larger capillary diameters and higher void ratios, the durability of the test pieces added with such recycled aggregates is more pronounced. Wai et al. [26] pointed out that recycled concrete has good ultrasonic pulse velocity values, low water absorption, and low intrinsic permeability. However, when the test piece was expanded in water curing, the recycled concrete reached the highest expansion rate at 80 days. Compared with the natural concrete at 56 days under the same conditions, the expansion increased by about 64.8%, and the expansion effect was more pronounced with an increase in the substitution rate. Thomas et al. [27] also found that high porosity is responsible for the poor durability of recycled concrete. Experiments show that for the same water
cement ratio, the density of the specimens with 20% recycled coarse aggregate is about 5% lower than that of the control specimens. The higher the inherent porosity of the recycled aggregate, the more the durability of the test piece is affected by it. Xiao et al. [20] showed that the carbonation resistance and chloride permeability of recycled concrete decreased with an increase in the replacement rate of recycled aggregate. The drying shrinkage of recycled concrete increases with the replacement ratio of recycled aggregate and the water
cement ratio. However, when a certain amount of fly ash and water-reducing agent are mixed in the test piece, the drying shrinkage rate of the recycled concrete is lowered. Compared with traditional concrete, the creep of recycled concrete is high; and the creep of recycled concrete increases with an increase in the replacement rate of recycled aggregate. However, the addition of slag and high-quality recycled aggregate can significantly reduce creep. Zong et al. [28] found that when recycled coarse aggregate was used in concrete specimens, the permeability, gas permeability, and chloride ion permeability of the specimens increased accordingly. In particular, recycled concrete containing clay brick waste exhibits a loose paste-like matrix, which is considered to be caused by an increase in permeability. The experimental results show that the concrete specimen with a 30% recycled aggregate replacement rate has better economical efficiency without affecting the durability of the specimen. Roz-Ud-Din and Parviz [29] attempted to add a mixture of milled waste glass and recycled aggregates to the concrete to improve the regenerative coarse aggregate limitations (higher water

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absorption and weaker adhesion to the mortar). The experiment illustrated that compared with the durability performance of the control concrete, the durability of the RAC containing the milled waste glass was partially improved. Kou and Poon [30] used a different substitution rate and fly ash content as a substitute for a 10-year cure. The results showed that, after 10 years, compared with the control group, the carbonization depth of the recycled concrete increased by 100% substitution rate; the strength of the 100% replacement rate of recycled concrete increased the most (more than 60%) within 28 days to 5 years; 5 years later, 100% replacement rate recycled concrete has the highest split tensile strength and strength gain (from 16.3% to 45.4%). The long-term durability of the test piece was significantly improved due to the pozzolanic reaction between fly ash and Ca(OH)2. However, the admixture improves durability while also reducing compressive strength [31]. A large number of experiments have shown that the improvement of the durability of recycled concrete is accompanied by the loss of one or more other properties, without modifying the recycled aggregate itself. With the deepening of research on the mechanical properties and durability of recycled aggregates, attempts to apply them to more aspects have continued. Mohammed et al. [32] added an auxiliary cementitious material to the recycled concrete to reduce the dependence on the cement as a binder. The experimental results show that the recycled concrete with 30% added materials has poor performance at 28 days; however, with the passage of time, the durability-related performance in the later stage is significantly enhanced. This recycled concrete can better reduce CO2 emissions. The study by Zhang et al. [3] also found that environmental improvements in recycled coarse aggregates, especially benign changes in land and reductions in greenhouse gas emissions, should be considered as priority concerns. Xiao et al. [33] found that the use of RAC as a structural material in high-rise buildings instead of natural aggregate concrete can reduce the carbon footprint under this particular engineering condition by about 2.175 3 105 kgCe. Mukesh et al. [34] added recycled aggregate to fly ash concrete. With the increase in the replacement rate of recycled aggregate, the carbonation resistance, chloride ion penetration resistance, and sulfate resistance of concrete are reduced; but at a suitable substitution rate, the long-term resistance of the specimen to carbonization can be enhanced. Hamid et al. [35] and Butler et al. [36] studied recycled aggregates and steel materials as a combination. It was found that the bond strength of recycled concrete is 9%
19% higher than that of natural concrete; recycled concrete has a lower slump, probably due to the shape of the regenerated aggregate interface and the rough surface. Meng et al. [37] attempted to use recycled aggregates as block fillers, as a natural aggregate substitute. The amount of material incorporated into the concrete block can be up to 100%; in some applications it should be limited to less than 30% to meet the standard requirements for concrete blocks. Experiments by Guo et al. [38] show that concrete blocks containing 75% recycled aggregate meet the strength, drying shrinkage, and freeze
thaw resistance requirements of concrete blocks specified by Chinese standards. In addition, a large number of trials and assessments focusing on the sustainable use of recycled aggregates and the recycling of construction waste has been studied [39
41]. Rawaz

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et al. [42] and Carlos et al. [43] evaluated and predicted the life cycle and fatigue properties of recycled aggregate products, respectively. Pedro et al. [44] and Shi et al. [45] also studied the use of recycled aggregates and aggregates in highperformance concrete. Li et al. [46] developed a class of planted concrete made from recycled aggregate. Through optimization, the porosity, water permeability, and 28-day compressive strength of the planted concrete can be increased to 40.9%, 2.88 cm/s, and 6.5 MPa, respectively, which can provide a suitable environment for plant growth. However, many studies have shown that recycled aggregates, no matter which aspect they are used for, cannot be cut off from the high porosity and microcracks in the transition zone caused by the properties of the old mortar. As an appendage of concrete hydration condensation shrinkage and related causes, the damage to concrete structures are mostly caused by the continuous extension of cracks [47]. Compared with natural aggregates, microcracks in the transition zone are more likely to occur due to the adhesion of recycled aggregates [48,49]. Effective control of cracks can improve the integrity of the concrete structure and effectively extend the life of the structure [50,51]. However, most of the existing artificial crack control methods use postrepair, which requires a lot of manpower and material resources, low economic benefits, and only repairs external cracks. The structural crack initiation is mostly caused by internal cracks, which in turn develop into a morphological representation that affects the structure. Existing research attempts to achieve this through passive repair, with the automatic repair of concrete during crack formation, that is, from the inside to the outside, inhibit the extension of cracks [52,53]. Therefore, effectively improving the passive repairability of concrete cracks is of great significance for improving the performance of recycled concrete. The additives currently used to achieve passive healing of concrete can be divided into three types of penetration: crystallization healing mechanisms, crystallization-related expansion, and pozzolanic reactions [54]. Of these, Maria et al. [55] used a polymer cylindrical capsule made of polyester (methyl methacrylate) (PMMA) for carrying a healing agent in self-healing. An experiment by Luthfi et al. [56] found that the thickness of the core shell of the capsule does not have a significant effect on the strength of the specimen, but has a great influence on the strength of the specimen. Therefore, there is a limit to the high level of process requirements in capsules [57]. Mohamed et al. [58] used expanded perlite to immobilize carrier components of bacterial spores. The experimental results show that self-repair not only requires sufficient healing compounds, but also requires a suitable carrier environment. Xu et al. [59] and Luo et al. [60] explored a new concept of self-healing, using microbes for self-healing to promote calcium mineral precipitation to fill concrete cracks. In fact, the relationship between microbes and concrete has long been a concern [61]. Jiang [62] believes that microbial repair methods do not create additional changes to the environment and additional resource occupancy, and are currently considered one of the cleanest passive repair methods. Among them, Bacillus bacillus, a bacterium capable of producing calcium minerals in a specific environment, has been studied for repairing various carriers [63]. Mayuri et al. [64] believe that recycled aggregates are more economical and environmentally friendly than the currently widely used carriers. However, the use

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of recycled aggregates as a Bacillus carrier to verify crack repair has rarely been reported. In order to clarify the possibility that recycled aggregates may be used, this chapter summarizes the application of recycled aggregates in concrete based on recent research advances in this field. For this purpose, the chapter consists of three main parts: the first part describes the nature of the recycled aggregate, the characteristics of Bacillus bacillus, and the repair mechanism. The second part summarizes the effects of the recycled coarse aggregate on the repair effect, such as the relevant properties of concrete crack repair morphology and repair speed. The last part deals with the possible contribution and impact of recycled aggregates on concrete repair. This chapter also attempts to study the benefits and limitations of recycled aggregates as self-healing carriers in concrete manufacturing in order to provide new research ideas for the future development of recycled aggregates.

26.2

Self-healing concrete materials

26.2.1 Bacillus bacillus The alkalophilic strain was Bacillus bacillus, which was purchased from the Shaanxi Institute of Microbiology. The purchased strain was inoculated and cultured using a liquid medium according to a conventional culture method, and the bacterial solution was diluted to an OD value of 1.2 [65]. The medium used was: 1 L of ultrapure water, 5 g of peptone, 3 g of beef extract, 0.42 g of NaHCO3, and 0.53 g of NaCO3. After inoculating Bacillus bacillus into liquid medium, it was placed in a constant temperature shaker and incubated at 30 C, 120 rpm for 24 hours. The repair principle of Bacillus bacillus is found in Eq. 26.1. The principle of this repair is to form calcium minerals on the spores by the consumption of nutrients by the strain in water and oxygen. Calcium lactate, the nutrients, and its free calcium ions continue to aggregate in the alkaline environment until the formation of CaCO3 crystals. Bacteria

Substrate 1 Calcium lactate 1 O2
! CaCO3 1 CO2 1 H2 O

(26.1)

26.2.2 Recycled aggregate The test materials used PC32.5R ordinary Portland cement, recycled coarse aggregate (bacteria carrier), natural fine aggregate, ordinary fine sand, and urban tap water. The recycled coarse aggregate was provided by Shaanxi Jianxin Technology Environmental Protection Co., Ltd., and a certain amount of recycled coarse aggregate was selected to test its bulk density, aggregate porosity, crushing index, water absorption rate, and water content to obtain recycled coarse aggregate. The physical properties are shown in Table 26.1.

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Table 26.1 Physical properties of recycled coarse aggregate. Bulk density (kg=m3 )

Porosity (%)

Aggregate

Apparent

Loose

Tight

Loose

Tight

Crushing

Water

Moisture

type

density

bulk

bulk

porosity

porosity

index

absorption

content

(kg=m3 )

density

density

(%)

rate (%)

(%)

2458

46.7

41.8

17.0

3.83

1.33

Recycled

46.7

41.8

coarse aggregate

Table 26.2 Composition statistics of recycled coarse aggregate. Original stones (%)

Secondary aggregate (%)

Mortar blocks (%)

Impurities (%)

29.5

51.2

16.0

3.3

Through observation, the appearance of the regenerated coarse aggregate was flat, the edges and corners were more prominent, and a layer of cement mortar was wrapped, so that the surface was rough and the void large. In addition, the outer surface of the recycled coarse aggregate is also covered with some other impurities. The components of the recycled coarse aggregate can be divided into undisturbed stones, secondary aggregates, mortar blocks, and impurities. The original stone refers to the coarse aggregate particles which do not contain the attached mortar on the surface of the coarse aggregate or have a small area of adhering mortar, and the surface is relatively smooth, angular, and distinct, and the appearance of the natural coarse aggregate is not much different. Secondary aggregate refers to aggregate particles with a small area attached to the surface of the aggregate. A mortar block is an aggregate of pure mortar or granules that is completely covered by mortar. Impurities generally include bricks, bricks, asphalt blocks, clods, and tiles. Three batches of recycled coarse aggregates weighing 15 kg were randomly weighed and classified. The screening method was manual screening. The original stones, secondary aggregates, mortar blocks, and impurities were separately screened and weighed to obtain the regeneration. The composition content of the coarse aggregate is shown in Table 26.2, and the composition of the recycled coarse aggregate is shown in Fig. 26.1. The gradation of the recycled coarse aggregate has a certain influence on other properties, such as strength. Therefore, after the division by the quarter method, the gradation composition of the recycled coarse aggregate can be obtained by using a shaker and artificial sieve, as shown in Table 26.3.

Figure 26.1 Recycled coarse aggregate components.

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Table 26.3 Recycled coarse aggregate grading composition. Particle size range (mm) Aggregate type

0
5

5
10

10
15

15
20

Recycled coarse aggregate

0.15%

13.15%

28.19%

58.51%

26.3

Method and results

26.3.1 Preparation and maintenance method The concrete mixing procedure is carried out in accordance with the GB/T 500812002 standard [66]. The fresh mixture was then cast into a cubic mold of 70.7 3 70.7 3 70.7 mm and a mold of 40.0 3 40.0 3 160.0 mm (Fig. 26.2). A total of 12 concrete specimens were poured into two groups: group I was a test piece group with Bacillus bacillus attached to the recycled aggregate as a carrier; group II was a carrier-free control group of Bacillus bacillus. Each group had three columns and three blocks. Fresh samples were shaken evenly and demolded after 24 hours. The samples were then cured at 85 6 5% RH and 22 6 2 C for 7 days. It should be noted that the particle size distribution of each test piece is varied. Group II was calculated and considered as the percentage of additional water added to the recycled aggregate at the time of mixing.

26.3.2 Prefabricated crack method The column specimens were placed under the pressure test machine of the Structural Laboratory of Xi’an University of Architecture and Technology, using a three-point method to distribute the pressure across the middle, by controlling the loading time and loading speed until significant cracks appeared in the loading span. In order to prevent the test piece from breaking during the loading process, the surface of the test piece should be cleaned before loading, and the noncompressed surface should be taped as a lateral prestress. The block test piece is formed by uniformly distributing the pressure on the pressure-receiving surface, and prefabricating the crack by other similar conditions and measures. The initial crack was immediately observed and recorded after the crack was pressed.

26.3.3 Self-healing characteristics 26.3.3.1 Apparent repair analysis Crack observation was performed under a 150-fold crack observer. The crack repair of the two sets of specimens was observed on the 7th and 14th days, respectively. Among these, the recycled aggregate was used for the width of the cracks A and B of the carrier test pieces RC1 and RC2, and the width of the crack C of the noncarrier test piece NC1 is as shown in Fig. 26.3. The width gauges at different cure times and results are shown in Table 26.4.

Figure 26.2 Schematic diagram of the specimen.

Potential use of recycled aggregate as a self-healing concrete carrier

Figure 26.3 Repair of cracks in RC and NC specimens: (A) repair of crack A in the RC1 specimen; (B) repair of crack B in the RC2 specimen; (C) repair of crack C in the NC1 specimen.

807

Table 26.4 Crack repair on the 7th and 14th days of the specimen. Healing 7 days crack width (mm)

Healing 14 days crack width (mm)

Specimen number

Initial crack

Minimum place

Maximum place

Minimum place

Maximum place

Mark repair rate (%)

RC1—column A RC2—column B RC3—column C RC4—cube A RC5—cube B RC6—cube C NC1—column A NC2—column B NC3—column C NC4—cube A NC5—cube B NC6—cube C

0.20 0.25 0.30 0.20 0.25 0.30 0.20 0.25 0.30 0.20 0.25 0.30

0.00 0.00 0.10 0.05 0.15 0.25 0.10 0.20 0.25 0.15 0.20 0.30

0.15 0.20 0.25 0.20 0.25 0.30 0.20 0.25 0.30 0.20 0.25 0.30

0.00 0.00 0.00 0.00 0.00 0.10 0.05 0.15 0.15 0.10 0.15 0.25

0.05 0.00 0.15 0.10 0.10 0.20 0.20 0.25 0.30 0.20 0.25 0.30

75 100 50 50 60 33 75 40 50 50 40 8

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The crack repair rate is used to characterize the crack repair effect, which is found in Eq. 26.2: w5

d0 2 dt 3 100% d0

(26.2)

where d0 is the initial width value; dt is the measured width value; w is the crack repair rate. Comparing Fig. 26.3 with Table 26.4, it can be seen that as the maintenance time is extended, the calcium carbonate crystal content increases in the fracture transverse direction of the crack, and the width reduces. Crack A was completely filled with calcium carbonate crystals after 14 days of repair; most of B was completely filled with calcium carbonate crystals; C was filled at local points, but most of the cracks were not completely filled. The average repair width da of the crack is measured by measuring six points on each test piece at the same time; that is, one observation maximum value and one observation minimum value are removed immediately; the remaining sum is averaged to obtain the width value, as illustrated in Eq. 26.3: 6 P

da 5

di 2 dmax 2 dmin

i50

4

(26.3)

where da is the average repair width of the crack; dmax is the maximum value of the observed repair; and dmin is the minimum value of the observed repair. The average repair of cracks in groups I and II is shown in Fig. 26.4. Using Table 26.4, comparing groups I and II, it can be seen that in the same environment of the same group, the repair of concrete columns is better than that of concrete blocks; the healing of group I with recycled aggregate as carrier is better than that of group II without carriers. The healing efficiency of the early group I was generally higher than that of group II. On the 14th day, the smallest crack in group II was generally not closed; while the crack in group I basically reached the state of almost collapse. Carrier-attached Bacillus bacillus showed better repairability than Bacillus bacillus in the vehicle-free control group [67,68]. Observation of experimental results: in the average repair trend (Fig. 26.4), the repair effect of the recycled aggregate group became apparent with the passage of time. The repair width shows a clear linear trend, and the slope indicates the rate at which the crack width was repaired per unit time increase. The average repair rate of RC1, RC2, and RC3 in group I was about 0.015 mm/ D in the first 7 days; the average repair rate was about 0.023 mm/D in days 7
14, and the repair rate was increased by 53.33%. The average repair rate of RC4, RC5, and RC6 was about 0.004 mm/D in the first 7 days; the average repair rate was about 0.011 mm/D in days 7
14, and the repair rate was increased by 175%. The column was 275% higher than the block repair rate in the first 7 days, and 109% higher in the last 7 days.

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New Materials in Civil Engineering

Figure 26.4 RC and NC average repair trend chart.

The average repair rate of NC1, NC2, and NC3 in group II was about 0.003 mm/ D in the first 7 days; the average repair rate was about 0.010 mm/D in days 7
14, and the repair rate was increased by 233.33%. The average repair rate of RC4, RC5, and RC6 was about 0.002 mm/D in the first 7 days; the average repair rate was about 0.004 mm/D in days 7
14, and the repair rate was increased by 100%. The column was 50% higher than the block repair rate in the first 7 days, and 150% higher in the last 7 days.

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Comparing the repair rates of columns and block of groups I and II, the former was 400% and 100% higher in the first 7 days, respectively, and 130% and 175% higher in the first to 14th days, respectively. The total amount of crack repair, based on the crack width of the marked point, extends to both ends of the mark until the mark width changes. It is assumed that the crack width does not change at the intersection of different marking points, and the repair amount is the sum of each point at the repair bridging (the length sum under the equal width). In the total repair trend chart (Fig. 26.5), it can be found that group I exhibits better early repair performance than group II, whether it is the column or the block. Group I had more obvious bacterial activity in the early stage, and with the increase of time, the column maintained relatively stable repair activity; the repair activity of the block slowly decreased. Group II had low bacterial activity in the early stage, but with the prolongation of time, the activity of the column repaired slowly increased; the activity of the repair of the block increased rapidly and then decreased. The total amount of repair in group I is much higher than that of group II. Among these, the optimal total repair ratio of the column (maximum ratio), group I was higher than group II by about 159%; for the weakest repair total of the column (minimum ratio), group I was higher than group II by about 218%. For the best total repair ratio of the block (maximum ratio), group I was higher than group II by about 145%; and for the weakest repair total of the block (minimum ratio), group I was higher than group II by about 156%. It should be noted that the total repair amount of RC1, RC2, and RC3 specimens is much smaller than that of other specimens in 35
56 days. This is because the cracks of the specimens completely healed in the time period, so that it was impossible to continue to confirm the additional repair amount after healing.

26.3.3.2 Microscopic repair analysis As the time of self-healing increases, the length of the crack becomes smaller. When the repair time reached 35 days, the transverse crack was completely repaired. Through SEM-EDS electron microscopy, the longitudinal section of the concrete crack will be repaired slowly after repairing the transverse surface, and the repair rate will gradually decrease until it stops. The calcium carbonate formed in the lateral direction initially gathers at various points, and then the different repair points are gradually connected, and finally the entire crack transverse surface is completely filled. After the entire crack was completely filled in this process, the calcium mineralization of Bacillus bacillus did not end. As the density of calcium carbonate increases, the surface of the crack and the outer edge of the crack are covered by calcium carbonate. By observing the test pieces of group I, it was found that, as shown in Fig. 26.6, the calcium carbonate formed in the longitudinal section forms a small irregular crystal cluster around the recycled aggregate. As shown in Fig. 26.7, the calcium carbonate crystals locally formed in the depth of the cross section are bonded to the

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New Materials in Civil Engineering

Figure 26.5 RC and NC total repair trend chart.

hydration product to exhibit a relatively uniform distribution. Near the outer edge of the crack, the calcium carbonate crystals are mostly in a ladder-like stacked state, and the crystal grain size is generally larger than the outer edge of the crack.

Figure 26.6 EDS diagram of the outer edge of a crack of the longitudinal section of the RC specimen.

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New Materials in Civil Engineering

Figure 26.7 Internal EDS diagram of a crack of the longitudinal section of the RC specimen.

The observation of the specimens of group II illustrated that, as shown in Fig. 26.8, the distribution of calcium carbonate crystals formed in the longitudinal section mostly showed nonuniformity, and it was in a tiled state at the bubble of the section and the outer edge of the crack. There is little or no calcium carbonate component left from the section, no calcium carbonate component is found around the hydration product, and the crystal grain size is generally small. Elemental analysis was performed on the Bacillus product, and the points were taken on the RC specimen and the NC specimen, respectively, and it was found that the product element component was indeed calcium carbonate. The analysis of groups I and II is shown in Fig. 26.9, Table 26.5, and Fig. 26.10, Table 26.6, respectively.

26.3.4 Self-repairing weakening principle After the crack is formed, as its depth increases, the water and oxygen decrease, and the distribution of calcium mineral concentration along the longitudinal section shows a decrease from the surface to the inside. This is because Bacillus bacillus can grow and multiply on the outer edge of the crack, and the mineralization reaction is more likely to occur. As moisture and nutrients (calcium lactate) enter, the matrix dissolves in the pore water and continues to flow internally. And mineralization is first carried out near the outer edge until the calcium carbonate formed in the fracture cross section is denser (effectively preventing the ingress of moisture or oxygen).

Figure 26.8 EDS diagram of the outer edge of a crack of the longitudinal section of the NC specimen.

Figure 26.9 Energy spectrum analysis of the RC crack section: (A) energy spectrum analysis of RC A point; (B) energy spectrum analysis of RC B point.

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Table 26.5 RC section element components. RC-A Standard sample

Total amount

RC-B

Element

Weight percentage

Atomic percentage

Element

Weight percentage

Atomic percentage

CK OK Ca K Si K Al K

11.67 53.58 32.67 1.18 0.90 100.00

18.65 64.27 15.64 0.80 0.64

CK OK Ca K Si K Al K

10.04 48.44 39.63 1.28 0.61 100.00

17.00 61.52 20.09 0.93 0.46

This also led to the inability to continue microbial mineralization. Therefore the calcium carbonate content is characterized by a large outer edge and a small internal content. Surface cracks are easily filled, and only a small amount of calcium carbonate is produced in the cracks, so that internal cracks cannot be filled.

26.4

Effect of recycled aggregate in self-healing concrete

In fact, as the repair time increased to 35 days (except for the cracks that have been repaired), the repair speed of most of the specimens dropped to a stable value and maintained a small fluctuation on the 56th day. In this case, it is assumed that as time passes, the crack repair rate of the component will not change much, and the repair rate will decrease rapidly after the crack is closed. The repairing effect of the specimen, with the recycled aggregate as the carrier, was better than that of a specimen without the carrier. The reason for this may be that during the process of mixing the cement base forming member, the air present in the void of the recycled aggregate adhered to the mortar and the added water, Bacillus bacillus had begun the calcium mineralization reaction, and CaCO3 started inside the test piece. When the curing is completed and the crack is pressed, the entry of external air and moisture causes the outer edge to start the calcium mineralization to the inside, which greatly accelerates the rate of formation of the CaCo3 crystals to the bridging crack. The distribution of Bacillus bacillus in the unsupported test piece was considered to be in a more uniform state. This led to the fact that Bacillus bacillus began to participate in the healing process when the cracks were produced, and the healing rate was slower than that of the regenerated aggregate carrier group, as shown in Fig. 26.11. At the same time, the uniform density distribution makes the CaCO3 formation form a tiled layer stack, the healing efficiency of the crack surface is greatly reduced, and the maximum crack width is

Figure 26.10 Energy spectrum analysis of the NC crack section: (A) energy spectrum analysis of NC A point; (B) energy spectrum analysis of NC B point.

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Table 26.6 NC section element components. NC-A Standard sample

Total amount

NC-B

Element

Weight percentage

Atomic percentage

Element

Weight percentage

Atomic percentage

CK OK Ca K

9.42 55.06 35.53

15.34 67.32 17.34

CK OK Ca K Si K

11.04 52.79 33.00 3.17 100.00

17.83 64.01 15.97 2.19

100.00

Figure 26.11 Schematic diagram of NC and RC crack repairs.

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New Materials in Civil Engineering

smaller than that of the recycled aggregate carrier group. It can be shown by SEM that CaCo3 formed by the recycled aggregate carrier group presents irregular stacked crystal blocks. This point-like aggregation speed and crystal grain size are larger than the tile layer stack, which has a good advantage in suppressing corrosion of steel by CO2 3 and other harmful elements. Recycled aggregates also have an advantage in terms of economy and environmental friendliness. Current self-healing carriers include a sealed epoxy resin liquid adhesive system invented by White et al. [69], which breaks the capsule by crack fracture, and the liquid resin flows on the surface of the crack due to surface tension, thereby repairing cracks; and ceramic materials such as MgO, Al2O3, etc., through thermal shock, long-range atomic rearrangement of the surface of the material to repair cracks; shape memory alloy (SMA) is added to the material to repair the damage when the structure is cracked. SMA generates compressive stress to restore the structure to its original state; the microbial self-repair method using expanded perlite, diatom mud, and other materials as carriers, and the use of alkalophilic microorganisms to produce calcium mineralization products (mainly CaCO3) to achieve repair. Microbial concrete with recycled aggregate as a carrier exhibits better economics than other repair systems; it also exhibits better environmental friendliness than other carrier microbial concretes. Recycled aggregates are derived from the characteristics of abandoned construction waste, making them highly sustainable and enabling the recycling of construction waste. This feature helps to reduce the extraction of natural resources which are irreplaceable by other materials, providing a new idea for green cleaning products.

26.5

Outlook

The porous characteristics of recycled aggregate attached to mortar is conducive to the attachment of Bacillus bacillus. Compared with the crack repair of a noncarrier specimen, the specimen with the recycled aggregate as a carrier exhibits the advantages of fast repair speed, high calcium mineralization product, large calcium carbonate crystal grain size, and compact structure. This is because the recycled aggregate greatly increases the density of the bacteria, and the air and water adhering to the mortar can be mineralized at the first crack generation. The oxygen and water required for the noncarrier bacteria need to pass through the outer edge of the crack to the internal listed process, and the hydration reaction of the cement will further affect the mineralization of the Bacillus, resulting in slow repair speed and a poor repair effect. The repair effect of the specimens of the two groups is better than that of the block. The reason for this may be that the column crack is located in the middle of the span, and the longitudinal section of the crack is smaller than the block, which shortens the time for water, oxygen, and nutrients to reach the Bacillus and mineralize; or the cracks in the column are mostly recycled aggregates. Because the adhesion of the recycled aggregate to the cement mortar is relatively poor, more aggregated Bacillus is in the crack for repair work. Another possibility is that the

Potential use of recycled aggregate as a self-healing concrete carrier

821

cracks in the column specimens are mostly generated in the surface mortar, and the cracks fail to effectively reach the Bacillus and recycled aggregate. As a result, the repair reaction is inefficient or fails. Recycled aggregate, as a repair carrier, has good application potential in concrete. It can be inferred that the viable factor of using recycled aggregate as a repair carrier is due to the porosity and weak alkalinity of the attached mortar. As long as the recycled aggregate has appropriate particle size and porosity, it can improve the crack repair performance of the concrete. In recent studies, the use of recycled aggregates as a microbial vector shows potential signs, and given the limited information, this deserves further study. On the other hand, due to the complexity of the transition zone of the recycled aggregate interface, the use of it as a replacement for repair carriers should be limited to low replacement levels. Therefore, while self-repairing concrete has a beneficial effect on the environment, more attention should be paid to the improvement of its strength and durability after repairing.

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549. [17] F. Gholamreza, A.R. Ghani, O.I. Burkan, et al., Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate, Cement Concr. Compos 33 (2011) 1026
1037. [18] C. Liu, Z.Y. Lv, C. Zhu, G.L. Bai, et al., Study on calculation method of long-term deformation of RAC beam based on creep adjustment coefficient, KSCE J. Civ. Eng. 1 (23) (2019). [19] N. Fonseca, J. Brito, L. Evangelista, The influence of curing conditions on the mechanical performance of concrete made with recycled concrete waste, Cem. Concr. Compos 33 (2011) 637
643. [20] J.Z. Xiao, W.J. Li, Y.H. Fan, et al., An overview of study on recycled aggregate concrete in China (1996
2011), Concr. Struct. Mater. 31 (2012) 364
383. [21] M. Benito, C. Antoni, D.O. Teodoro, et al., Influence of the amount of mixed recycled aggregates on the properties of concrete for non-structural use, Constr. Build. Mater. 27 (2012) 612
622. [22] S.C. Kou, C.S. Poon, A. Francisco, Comparisons of natural and recycled aggregate concretes prepared with the addition of different mineral admixtures, Cem. Concr. Compos. 33 (2011) 788
795. [23] S. Manzi, C. Mazzotti, M.C. Bignozzi, Short and long-term behavior of structural concrete with recycled concrete aggregate, Cement Concr. Compos. 37 (2013) 312
318. [24] R.M. Chakradhara, S.K. Bhattacharyya, S.V. Barai, Influence of field recycled coarse aggregate on properties of concrete, Mater. Struct. 44 (2011) 205
220. [25] S.C. Kou, C.S. Poon, Enhancing the durability properties of concrete prepared with coarse recycled aggregate, Concr. Struct. Mater. 35 (2012) 69
76. [26] H.K. Wai, R. Mahyuddin, J.K. Kenn, et al., Influence of the amount of recycled coarse aggregate in concrete design and durability properties, Constr. Build. Mater. 26 (2012) 565
573. [27] C. Thomas, J. Setie´n, J.A. Polanco, et al., Durability of recycled aggregate concrete, Concr. Struct. Mater. 40 (2013) 1054
1065. [28] L. Zong, Z.Y. Fei, S.P. Zhang, Permeability of recycled aggregate concrete containing fly ash and clay brick waste, J. Clean. Prod. 70 (2014) 175
182. [29] N. Roz-Ud-Din, S. Parviz, Strength and durability of recycled aggregate concrete containing milled glass as partial replacement for cement, Concr. Struct. Mater. 29 (2012) 368
377. [30] S.C. Kou, C.S. Poon, Long-term mechanical and durability properties of recycled aggregate concrete prepared with the incorporation of fly ash, Cem. Concr. Compos., 37, 2013, pp. 12
19. [31] S.C. Kou, C.S. Poon, Etxeberria M: influence of recycled aggregates on long term mechanical properties and pore size distribution of concrete, Cem. Concr. Compos. 33 (2011) 286
291. [32] F.A. Mohammed, U.A. Johnson, Z.J. Mohd, Assessment on engineering properties and CO2 emissions of recycled aggregate concrete incorporating waste products as supplements to Portland cement, J. Clean. Prod. 203 (2018) 822
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[33] J.Z. Xiao, C.H. Wang, T. Ding, et al., A recycled aggregate concrete high-rise building: structural performance and embodied carbon footprint, J. Clean. Prod. 199 (2018) 868
881. [34] L. Mukesh, S.M. Mohammed, O. Youssef, Use of recycled concrete aggregate in flyash concrete, Concr. Struct. Mater 27 (2012) 439
449. [35] R.C. Hamid, G. Mansour, K. Arash, et al., Experimental study on the flexural behaviour and ductility ratio of steel fibres coarse recycled aggregate concrete beams, Constr. Build. Mater. 186 (2018) 400
422. [36] L. Butler, J.S. West, S.L. Tighe, The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement, Cem. Concr. Resear, 41, 2011, pp. 1037
1049. [37] Y.Z. Meng, T.C. Ling, H.M. Kim, Recycling of wastes for value-added applications in concrete blocks: an overview, Resour. Conser. Recyc. 138 (2018) 298
312. [38] Z.G. Guo, A. Tu, C. Chen, et al., Mechanical properties, durability, and life-cycle assessment of concrete building blocks incorporating recycled concrete aggregates, J. Concr. Struct. Mater. 199 (2018) 136
149. [39] T. Nikola, M. Snezana, D. Tina, et al., Multicriteria optimization of natural and recycled aggregate concrete for structural use, J. Clean. Prod. 87 (2018) 766
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334. [41] R.V. Silva, J. Brito, R.K. Dhir, Properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production, Concr. Struct. Mater 65 (2014) 201
217. [42] K. Rawaz, D.S. Jose´, B. Jorge, Life cycle assessment of concrete made with high volume of recycled concrete aggregates and fly ash, Resour. Conser. Recycling. 139 (2018) 407
417. [43] T. Carlos, S. Israel, S. Jesu´s, et al., Evaluation of the fatigue behavior of recycled aggregate concrete, J. Clean. Prod. 65 (2014) 397
405. [44] D. Pedro, J. Brito, L. Evangelista, et al., Technical specification proposal for use of high-performance recycled concrete aggregates in high-performance concrete production, ASCE-J. Mater. Civ. Eng., 30, 2018, p. 04018324. [45] C.J. Shi, Y.K. Li, J.K. Zhang, et al., Performance enhancement of recycled concrete aggregate
A review, J. Clean. Prod. 112 (2016) 466
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64. [47] P.K. Mehta, Cracking stresses, Concr. Int 30 (4) (2018) 16. [48] W.G. Li, J.Z. Xiao, Z.H. Sun, Interfacial transition zones in recycled aggregate concrete with different mixing approaches, Constr. Build. Mater. 35 (2012) 1045
1055. [49] J.Z. Xiao, W.G. Li, Z.H. Sun, et al., Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation, Cem. Concr. Compos., 37, 2013, pp. 276
292. [50] Rattner, Cost-effective self-healing concrete. ,http://thefutureofthings.com/news/10104/costeffective-self-healingconcrete.html. (accessed January 2011). [51] W. Min, J. Bjo¨rn, G. Mette, A review: self-healing in cementitious materials and engineered cementitious composite as a self-healing material, Concr. Struct. Mater. 28 (2012) 571
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[52] Breugel, Is there a market for self-healing cement-based materials?, in: Proceedings of the first International Conference on Self-Healing Materials, Noordwijkaan Zee, The Netherlands, 2007. [53] A. Tae-Ho, K. Toshiharu, Crack self-healing behavior of cementitious composites incorporating various mineral admixtures, J. Adv. Concr. Techn. 8 (2) (2010) 171
186. [54] Jiang, W.T. Li, Z.Z. Yuan, et al., Self-healing of cracks in concrete with various crystalline mineral additives in underground environment. J. Wuhan Univer. Techn. ,https://doi.org/10.1007/s11595-014-1024-2., 2014 (in Chinese). [55] A. Maria, C. Sutima, G. Stijn, et al., Poly (methyl methacrylate) capsules as an alternative to the ‘’proof-of concept’’ glass capsules used in self-healing concrete, Cem. Concr. Compos. 89 (2018) 260
271. [56] M.M. Luthfi, X.Y. Zhuang, R. Timon, Computational modeling of fracture in encapsulation-based self-healing concrete using cohesive elements, Compos. Struct. 196 (2018) 63
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14. [60] J. Luo, X.B. Chen, C. Jada, et al., Interactions of fungi with concrete: significant importance for bio-based self-healing concrete, Constr. Build. Mater. 164 (2018) 275
285. [61] C.D. Parker, Species of sulphur bacteria associated with the corrosion of concrete, Nature. 4039 (159) (1947) 439. [62] Z.W. Jiang, Cement-Based Self-Healing Materials: Theory and Method, Tongji University Press, ShangHai, China, 2016. [63] C. Qian, R.Y. Li, M. Luo, et al., Distribution of calcium carbonate in the process of concrete self-healing, J. Wuhan Univer. Techn. Mater. Sci. Ed. (2016). Available from: https://doi.org/10.1007/s11595-016-1410-z (in Chinese). [64] W. Mayuri, M. Priyan, R.H. Crawford, Integrated assessment of the use of recycled concrete aggregate replacing natural aggregate in structural concrete, J. Clean. Prod. 174 (2018) 591
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244 (in Chinese). [66] GB/T 50081-2002, Standard for Test Method of Mechanical Properties on Ordinary Concrete, Ministry of Construction of the People’s Republic of China, China, 2003. [67] S. Mostafa, K.S. Ajit, K.S. Ali, et al., Mechanical properties of bio self-healing concrete containing immobilized bacteria with iron oxide nanoparticles, Appl. Microbio. Biotechn. 05 (102) (2018) 4489
4498. [68] Y.S. Lee, W.J. Park, Current challenges and future directions for bacterial self-healing concrete, Appl. Microbio. Biotechn 05 (102) (2018) 3059
3070. [69] S.R. White, N.R. Sottos, P.H. Geubelle, et al., Autonomic healing of polymer composites, Nature. 09 (415) (2001) 794
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Self-healing concrete

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Xu Huang1 and Sakdirat Kaewunruen2 1 Department of Civil Engineering, School of Engineering, University of Birmingham, Birmingham, United Kingdom, 2Laboratory for Track Engineering and Operations for Future Uncertainties (TOFU Lab), School of Engineering, University of Birmingham, Birmingham, United Kingdom

27.1

Introduction

27.1.1 Background Concrete is made by mixing cement, coarse and fine aggregate, water, and sometimes other substances. It is widely used for construction and has high compressive strength, high stiffness, and low cost [1]. The obvious disadvantage of concrete is its weak tensile strength that can easily induce cracks [2]. Cracks will expand gradually to increase permeability in concrete if there is no immediate repair [3]. This means that carbon dioxide, chloride ions, and water in the air can easily attach to steel bars and cause corrosion [4]. Thus, these cracks do not only result in a reduction of the strength of concrete, but can also shorten the durability of buildings [5]. A total of 45% of annual construction cost is spent on maintenance of existing buildings in the UK [6]. If cracks can be made to heal by themselves, then the maintenance cost of buildings can be dramatically reduced. Moreover, it is difficult to check cracks in places where access is poor. It is also very difficult to properly check the stability of buildings, especially for large-scale structures [7]. Furthermore, manual methods to repair cracks have limitations, such as using chemically and environmentally unfriendly construction materials [8,9]. In order to protect the environment, crumb rubber is used to replace sand. Discarded tires, which are out of service, are estimated to be the second largest waste material worldwide [10]. The major component of rubber, styrene, is bad for the environment. Moreover, it may irritate the eyes and upper respiratory tract mucosa when it is burned [11]. Furthermore, land which is a potential living site for animals may be filled with waste tires. It is urgent to deal with the negative influence from wasted tires. In this chapter, crumb rubber made from crushed waste tires substitutes a small part of the sand to test the mechanical properties of rubberized concrete. Self-healing concrete has received much attention in the past 40 years. There have been a number of research works into self-healing with bacterial additions [12]. However, there has been no study of the self-healing abilities of crumb rubber New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00027-2 © 2020 Elsevier Inc. All rights reserved.

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concrete with fiber. If rubberized concrete with fiber can satisfy the strength requirements and has good performance in terms of self-healing, it could be used to manufacture sleepers or bearers which are difficult to access for maintenance. Moreover, it is very helpful in keeping the environment healthy and removing the pollution from waste tires. Thus, it is meaningful to investigate the self-healing abilities of rubberized concrete with fiber. Additionally, in this chapter, a new measurement of self-healing abilities will be used for the first time, introduced from the crack inspection system.

27.1.2 Literature review 27.1.2.1 Crumb rubber concrete There are three types of rubber which researchers have tested so far. These are ground rubber, rubber chips, and crumb rubber. Comparing the three types of rubber, rubber chips and ground rubber reduce the compressive strength more than crumb rubber [13]. The final goal of this study is to establish real-life applications in railway industry whereas material strength is not significantly compromised. Thus, crumb rubber has been chosen as an additional material in concrete in order to reduce the effect of waste rubber while improve the dynamic attenuation properties of concrete. Different views have been put forward on the mechanical properties of rubberized concrete. There was a type of concrete with rubber that had lower compressive strength than plain concrete [14,15]. Nevertheless, Faraz et al. [16] obtained a different result with a compressive strength of 5% rubberized concrete that was higher than that of plain concrete. The reason for this special case is that rubber in concrete generated voids which may reduce the bonding strength between cement and rubber [17]. Splitting tensile strength and flexural strength will also decrease by adding rubber to concrete [18,19]. Thus, it is imperative to find other materials to offset the influence of adding rubber.

27.1.2.2 Fiber in concrete Fiber is the material which could potentially reduce the influence on mechanical properties when using rubber. Steel fiber is a fiber with good performance on ductility and fracture toughness [20]. However, steel fiber is easily corroded when exposed to a high sulfate and chloride environment. Thus, another kind of fiber called Duras EasyFinish fiber is used in concrete to avoid corrosion. Furthermore, fibers can act as cores for the precipitation of calcium carbonate (CaCO3) [21]. This mechanism can stimulate the generation of calcium carbonate to improve the self-healing performance. In the works described in this chapter, Duras EasyFinish fiber is added to concrete to the enhance mechanical properties and self-healing abilities. Adding different proportions of the fiber into concrete was carried out to identify the percentage that can perform the highest self-healing ability.

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27.1.2.3 Self-healing concrete 1. Background to self-healing concrete The mechanism of healing cracks is to produce calcium carbonate and then cracks can be filled with calcium carbonate. There are two methods to generate calcium carbonate during self-healing procedures. The first is unreacted cement particles are used to start hydration to form CaCO3. The second is that CaCO3 is formed after dissolution of Ca (OH)2 [22]. Eq. (27.1) shows different methods for forming calcium carbonate in different pH values of water [23]. 22 1 H2 O 1 CO2 3H2 CO3 3H1 1 HCO
2 3 32H 1 CO3 21 22 Ca 1 CO3 3CaCO3 pHwater . 8
1 Ca21 1 HCO2 7:5 , pHwater , 8 3 3CaCO3 1 H

(27.1)

2. Influence factors of self-healing Many factors which may influence self-healing abilities have been concluded by previous researchers. There are five main factors as follows: a. Moisture content: pilot specimens stored in water can heal by themselves more effectively. b. Crack width: cracks less than 0.3 mm in width can be healed completely [24]. Cracks which are wider than 0.3 mm may not be healed. Cracks of width 0.1 mm are completely healed after around 200 hours. Moreover, 0.2 and 0.3 mm width cracks are mostly healed within 30 days [25]. Cracks ranging in width from 0.15 to 0.3 mm significantly decrease in 7 days and are completely healed in 33 days [26]. c. Time for hydration: hydration for a longer time can yield a better self-healing performance [27]. d. Pressure loaded on cracks: loading proper pressure on cracks can stimulate better selfhealing ability. e. Water
cement ratio: a higher water
cement ratio includes more unreacted cement particles that can be used for further hydration to boost the generation of calcium carbonate.

Furthermore, the time of cracking is also important. Earlier cracking concrete has more unreacted cement particles, thus it can perform a high self-healing ability with ongoing hydration [28].

27.1.2.4 Assessment of self-healing concrete 1. Making cracks There are specimens with standardized cracks and natural cracks described in this chapter. For natural cracks, they were made using a compressive test machine and the width of cracks was controlled carefully by visual inspection learned from previous researchers [29,30]. However, it was almost impossible to make cracks on unreinforced specimens when only using a compressive machine according to tests conducted by the author. This is because the flexural strength of specimens is very low. Thus, specimens were completely broken into two pieces only using a compressive machine.

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Standardized cracks were usually made by inserting plates. A thin copper plate of 0.3 mm was inserted in the center of fresh concrete [31]. In this chapter, 0.25 mm thickness plastic films are used to make standardized cracks. 2. Methods of measuring self-healing abilities 2.1. Ultrasonic pulse velocity values Ultrasonic pulse velocity (UPV) values are usually used to detect the integrity of concrete and nondestructively measure the level of damage of concrete [24]. Moreover, researchers have proved that there is a connection between damage and a decrease in UPV results [32
34]. If cracks are healed, UPV results of specimens with cracks will decrease to the values of unbroken specimens. This can be recognized as the mechanism for measuring self-healing abilities by applying UPV tests [30]. In this chapter, another aided method is to visually inspect whether cracks can be healed by themselves. 2.2. Natural frequencies Natural frequencies are used to detect inside damage in construction industries, especially on road and bridge construction sites. Natural frequencies of concrete specimens change when the hardness of the specimen is varied [35]. This is why the natural frequency can be a potential property to identify self-healing abilities of specimens with different cracks, as used in this chapter. Detecting natural frequencies to identify self-healing abilities of concrete is first applied in this chapter.

27.2

Materials and methods

The methodology of this investigation can be divided into four main steps. The first step is to investigate the behaviors of concrete with crumb rubber and fiber, using engineering test standards, reviewing self-healing concrete theories, developing evaluation methods of self-healing effectiveness, and natural frequencies. The second step is to design experiments. The third is to test specimens. And the final step is to analyze and discuss the results.

27.2.1 Materials 27.2.1.1 Cement According to the British standard BS EN 197-1, ordinary Portland cement type 1 (CEM 1) with a strength of 42.5 MPa is used to make concrete.

27.2.1.2 Water The water in the laboratory was used.

27.2.1.3 Fine aggregate and coarse aggregate According to BS EN 12620:2002, sand particles smaller than 5 mm can be used as fine aggregate [36]. Gravel particles which are between 5 and 10 mm can be used as coarse aggregate. A vibrating sieve can screen out useable aggregates. Before

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making concrete, aggregate needs to be dried in an oven at 100 for an hour and moved for cooling to reach the indoor temperature. Furthermore, moisture content needs to be calculated during each test. 100 g wet sand and 1000 g wet gravel were weighed and burned with a 10-minute mix until there was no free water on the surface of aggregates. Afterwards aggregates were weighed again. Then, the moisture contents of aggregates were calculated, as shown in Table 27.1.

27.2.1.4 Crumb rubber The sizes of crumb rubber used in experiments described in this chapter are 180 and 400 µ (Fig. 27.1), which were obtained free of charge from Lehigh Technologies Incorporation. They were mixed at a ratio of 1:1. Moreover, Chen and Xiang [18,37] showed that 5% sand replaced by crumb rubber obtained a higher compressive strength than other proportions of crumb rubber. Thus, in this study, 5% sand was substituted by crumb rubber from mixes 2 to 6.

27.2.1.5 Fiber High-performance construction fiber called Duras EasyFinish from the ADFIL Construction Fibre Company, as shown in Fig. 27.2, was used in this study. The fiber can reduce embodied carbon dioxide and create more durable structures. The properties of fiber are shown in Table 27.2.

27.2.2 Methods 27.2.2.1 Design of concrete In this study, there are six mixes of concrete which are listed in Table 27.3. The water
cement ratio of concrete is 0.44 and slump values are from 60 to 180 mm. Mix 1 is the reference concrete (RFC) which does not contain crumb rubber or fiber. From mixes 2 to 6, all contain 5% crumb rubber of the mass of sand. Mixes 3, 4, 5, and 6 contain 0.1%, 0.15%, 0.2%, and 0.25% fiber of the mass of gravels, Table 27.1 Moisture contents of aggregates. No.

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6

Mixes

Reference concrete 5% 180 and 400 CRC 5% 180 and 400 CRC 1 0.1% fiber 5% 180 and 400 CRC 1 0.15% fiber 5% 180 and 400 CRC 1 0.2% fiber 5% 180 and 400 CRC 1 0.25% fiber

Moisture content (%) Sand

Gravel

5 5 4.6 8 5 10

1.5 1.5 1.2 1.4 1.5 1

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New Materials in Civil Engineering

Figure 27.1 (A) 180-µ crumb rubber; (B) 400-µ crumb rubber.

respectively. Concrete specimens are not reinforced, because reinforcement may interfere with the self-healing performance of the fiber.

27.2.2.2 Mixing concrete The making of concrete followed the steps in BS ISO 1920-3 [38]. Slump tests are shown in Fig. 27.3.

27.2.2.3 Casting concrete The concrete has been casted using a mixer. The process includes : (1) ensure the mixer is clean; (2) if it is dry, wipe it with a damp cloth2 (ensure moulds are clean

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Figure 27.2 Duras EasyFinish fiber.

Table 27.2 Properties of Duras EasyFinish fiber. Fiber length Fiber type Shape Absorption Specific gravity Electrical conductivity Softening point (melt point) Color Tensile strength E-modules Chloride content SO3 content

40 mm Macro monofilament Embossed elongated design None 0.92 kg/dm3 None 165 C Gray 470 Mpa 6000 Mpa None None

and lightly oiled; (3) measure the quantities of materials required (refer to design mix); (4) add fine aggregate into the mixer; (5) add 10 mm coarse aggregate into the mixer; (6) add 20 mm coarse aggregate into mixer; (7) start mixer and run for 30 seconds; (8) gradually add half of the water into the mixer and mix for 1 minute; (9) stop mixer and cover for 4 minutes; (10) add cement by spreading evenly over aggregate mixer for 1 minute; (11) gradually add rest of water (add admixture if used) and continue mixing for 2 minutes; (12) undertake consistency tests on a sample of the batch; (13) pre-prepared moulds can now be filled.The mixing process for standard and non-standard mixes (with variation of crumb rubbers) is almost identical, excepting addition of admixtures.

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Table 27.3 Concrete design. Design of concrete mixtures No Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6

Mixes Reference concrete 5% 180 and 400 CRC 5% 180 and 400 CRC 1 0.1% fiber 5% 180 and 400 CRC 1 0.15% fiber 5% 180 and 400 CRC 1 0.2% fiber 5% 180 and 400 CRC 1 0.25% fiber

Cement

Water

Gravel

Sand

Rubber

Fiber

530

233

986 986 983.621

630 598 598

32

982.431

598

3.569

981.242

598

4.758

980.052

598

5.948

2.379

Unit: kg/m3

27.2.2.4 Making cracks G

G

Standardized cracks For making standardized cracks, there are three steps. First, concrete is poured into molds. Second, plastic films of 0.25 thickness and 100 mm length are inserted into the middle of the concrete after plastic films are completely oiled. Moreover, the depths of the plastic films are 10, 20, and 30 mm, respectively. Finally, plastic films are pulled out after 24 hours. Thus, three types of standardized cracks are generated. The size of the first type of cracks, which is called A, is W0.25 3 D10 3 L100 mm. The size of the second type of cracks, which is called B, is W0.25 3 D20 3 L100 mm. The size of the third type of cracks, which is called C, is W0.25 3 D30 3 L100 mm. Fig. 27.4 shows the types of standardized cracks. Natural cracks For making natural cracks, a four-point bending machine is used. Twenty-four prisms (W100 3 H100 3 L500 mm) were taken from a curing tank at 28 days. Then, loads pressed by a four-point bending machine gradually increased at a rate of 100 N/s. Afterwards, loads were kept maintained cracks were visualized.

27.2.2.5 Concrete tests G

G

Compressive strength Based on BS EN 12390-3, three 100 mm3 samples were used for compressive strength tests at 7, 14, and 28 days, respectively [39]. The compressive machine was obtained from Avery-Dension Limited. The model of the machine was 7225 DT, made in Leeds, UK. During compressive tests, three aspects received attention. First, cubes needed to be dried naturally before testing. Second, all surfaces of cubes were cleaned. Finally, cubes were put in the center of the compression plate [40]. Splitting tensile strength Based on BS EN 12390-6, three cylinders ([100 3 L 200 mm) were tested with a fourpoint bending machine at 28 days [36]. The machine was obtained from Avery-Dension

Self-healing concrete

Figure 27.3 Slump tests.

Figure 27.4 Standardized cracks.

833

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G

G

New Materials in Civil Engineering

Limited. The model type was AD-T42, which was manufactured in Leeds, UK. Before testing, the loading pad of the machine and all surfaces of cylinders were cleaned. During testing, the rate of loading was 100 N/s. The loading pad needed to be lifted immediately when cylinders failed. Then, the maximum splitting tensile strength values shown on the machine were recorded. Flexural strength Based on BS EN 12390-5, three prisms (W100 3 H100 3 L500 mm) were tested using a bending machine at 28 days [41]. Specimens were cleaned and dried and then put in the center of the machine. Afterwards, continuous loading was essential at a rate of 100 N/s until the prisms failed. Self-healing evaluation

There are some methods for calculating self-healing rates which have been utilized by previous researchers, such as using scanning electron microscopy (SEM) [9]. By using SEM methods, it is possible to see the deposition sites. However, it is impossible to distinguish compositions of precipitation. The UPV test is an alternative measuring method for self-healing. Based on BS EN 12504-4, a UPV test is used to evaluate the speed of passing concrete [42]. Before UPV testing, prisms are brought from a curing tank and dried in an oven at 90 degrees for 30 minutes to reduce the negative effects from a high moisture content. Then, prisms are taken out for cooling to reach room temperature for eliminating errors caused by high temperature. The UPV equipment in Fig. 27.5 needs to be calibrated before use by referring to 25.1us with a reference concrete block whose traveling time is known and can

Figure 27.5 UPV tests.

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Table 27.4 Speed classifications of qualities of concrete. No

Pulse velocity cross probing (km/s) . 4.5 3.5
4.5 3.0
3.5 , 3.0

1 2 3 4

Concrete quality grading Excellent Good Medium Doubtful

then be zeroed. For every prism, it is essential to test three times from different dimensions to obtain more accurate data. Testing dimensions of each prism need to be recorded and tested in an orderly fashion every time. The results of the UPV test should remain steady and then can be recorded. For natural and standardized cracks, the UPV test starts from 28 days after casting and they are tested every 2 days. Eq. (27.2) can be used to convert passing time to speed: V

L T

(27.2)

where, T is the time taken by the pulse to transverse specimens (µs); L is the length of specimens (500 mm). Qualities of concrete classified by speed are listed in Table 27.4 [43]. According to UPV results presented in Tables 27.6, it shows that qualities of concrete in this study is good. G

Natural frequencies test For the natural frequencies test, the Prosig P8004 hammer equipment showed in Fig. 27.6 is used. First, rubber pads and wood blocks are used to support prisms to reduce the influence from the Earth. Second, the computer programmer is set. Third, the receiver of the equipment is installed in the center of prisms and then the hammer is used to hit a point which is within 5 cm of the receiver. Finally, the frequency at the highest amplitude is recorded.

27.3

Results

27.3.1 Slump tests Slump tests for all mixes are listed in Table 27.5. According to Table 27.5, slump values of mixes 1, 2 and 3 are around 64 mm. Furthermore, values of mixes 4, 5, and 6 are around 83 mm. The reason for this increment in slump values is that gravel has been replaced by fibers. Gravel is recognized as the skeleton of concrete. However, fibers are not as strong as gravel. Thus, values of slump tests will deteriorate when gravel is substituted with fiber.

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Figure 27.6 (A) The receiver for the Prosig P8004 equipment; (B) the hammer for the Prosig P8004 equipment.

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Table 27.5 Slump tests. Mix Slump value (mm)

Mix 1

Mix 2

Mix 3

Mix 4

Mix 5

Mix 6

65

60

67

82

86

80

Figure 27.7 Compressive strength.

27.3.2 Compressive strength As mentioned in the literature review section, adding rubber will incite a reduction of compressive strength. In Fig. 27.7, there is a 17.4% reduction of compressive strength caused by adding 5% rubber between mixes 1 and 2, and then, the compressive strengths of mixes 3, 4, and 5 with rates of 5.3%, 2.2%, and 1.6%, respectively. The compressive strength of mix 5 is the highest. The reason for this is that fiber confines concrete and supports the concrete to endure higher loads. However, the compressive strength of mix 6 drops with a rate of 1.2% to 53.14 Mpa. This phenomenon can be attributed to the fact that too much fiber induces reductions in compressive strength.

27.3.3 Splitting tensile strength According to Fig. 27.8, a significant decline in the splitting tensile strength at a rate of 10% can be observed between mixes 1 and 2. This comes about as a result of the additional rubber. Afterwards, there is a noticeable increase of strength between mixes 2 and 3 with a rate of 3.9%, which is the highest, and then two continuous increases in splitting tensile strength of mixes 4 and 5 with rates of 2.1% and 2.0%, respectively. This can be explained by the fact that the strength increases by adding fiber into concrete. However, those increments stop at mix 5 with 0.2% fiber. Finally, the strength of mix 6 decreases to 2.97 MPa with a rate of 2.0%. According to the values in Fig. 27.8, it can be considered that the splitting tensile strength is reduced when excessive fiber is added to the concrete.

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Figure 27.8 Splitting tensile strength.

Figure 27.9 Flexural strength.

27.3.4 Flexural strength As can be seen in Fig. 27.9, there is a considerable decline of flexural strength with a rate of 13.4% in mix 2, compared with mix 1 which is plain concrete. After that, the strengths of mixes 3, 4, and 5 increase constantly at rates of 0.3%, 0.5%, and 0.2%, respectively. Finally, the strength of mix 6 remains at 5.92 MPa. It can be concluded that additional fiber can improve the flexural strength, because fiber can increase the adhesion of concrete particles when they are broken. Nevertheless, increments stop at mix 5 with 0.2% fiber.

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27.3.5 Self-healing evaluation From previous researches, there has been none that had calculated the increased rates of self-healing. In addition, most research into self-healing concrete does not consider practicality and systems requirements for engineering applications. In this study, a method of measuring self-healing increments is presented, in order to help engineers instigate the durability-based design of concrete structures. First, four prisms were prepared in the same design of concrete, with one named as a RFC. Second, the speed of UPV tests of other prisms was used to subtract the speed of reference. Then values calculated in the second step were divided by the speed of the RFC to obtain decreased speed ratios in different cracks between two specimens. Afterwards, the n day’s decreased speed ratio was used to subtract t day’s decreased speed ratio, so as to obtain the increased self-healing rate, which is shown in Eq. (27.3). Dt 5

ðVt 2 Vrt Þ Vrt

ðVn 2 Vrn Þ Vrn Int 5 Dn 2 Dt Dn 5

(27.3)

where, Vt is the speed of control samples at t day; Vrt is the speed of the reference sample at t day; Vrn is the speed of the reference sample at n day; Vn is the speed of control samples at n day; Dt is the decreased speed ratios by different cracks at t day; Dn is the decreased speed ratios by different cracks at n day; Int is the increase rates of self-healing between t day and n day (n . t).

27.3.5.1 Standardized cracks From Fig. 27.10, self-healing increments of every specimen can be observed. As regards mix 1, the self-healing increment rate of specimen B drops from 0.75% to 0.1% between 30 and 32 days, and then it fluctuates between 0.1%
0.4% up until 42 days. For A and C, self-healing rates fluctuate between 0.2%
0.45% and 0.1%
0.45%, respectively. Moreover, the fluctuation range of self-healing rates of specimens in mix 2 is between 0.2%
1%. Furthermore, the self-healing rates of specimens in mix 3 fluctuate between 0.38%
1%. Additionally, rates of specimens in mixes 4, 5, and 6 all fluctuate between 0.2%
1%. According to the results shown in Fig. 27.10, it can be concluded that self-healing increments of specimens with standardized cracks all fluctuate between 0.1% and 1%. During tests, the authors did not find any healing phenomenon on specimens with standardized cracks.

27.3.5.2 Natural cracks In this study, mixes 1 and 2 with standardized cracks were utilized to compare selfhealing rates with specimens which have natural cracks. Unreinforced concrete

Figure 27.10 (A) Self-healing rates of specimens A, B, and C with standardized cracks of mix 1; (B) self-healing rates of specimens A, B, and C with standardized cracks of mix 2; (C) self-healing rates of specimens A, B, and C with standardized cracks of mix 3; (D) selfhealing rates of specimens A, B, and C with standardized cracks of mix 4; (E) self-healing rates of specimens A, B, and C with standardized cracks of mix 5; (F) self-healing rates of specimens A, B, and C with standardized cracks of mix 6.

Figure 27.10 (Continued).

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specimens are easily broken using a bending machine. Other mixes, except for mixes 1 and 2, can be made for natural cracks because there is fiber inside the concrete which helps to bond concrete particles. Theoretically, mixes 1 and 2 with standardized cracks precracked at an early age are more likely to be healed than other mixes such as mixes 3, 4, 5, and 6, which were cracked at 28 days. It can be concluded that specimens with natural cracks are much more difficult to heal than specimens with standardized cracks. The difference between natural cracks and standardized cracks is the presence of fiber between gaps. As observed in Fig. 27.11, the self-healing rates of specimens with natural cracks are illustrated. In mix 3, self-healing rates of specimens all slightly increase between 30 and 36 days, and then decline from 36 to 42 days. The rate of specimen B of mix 3, which is always the highest, goes up from 3.9% to 4%, and then decreases to 3.5%. For mix 4, the rates of specimens A and C remain at 4% from 30 to 36 days, and afterwards they go down to around 3% up until 42 days. The self-healing rate of specimen B decreases from 3% to 2.8% from 30 to 32 days, then goes back to 3% within 2 days, finally dropping to 2% up until 42 days. With regard to mix 5, rates of specimens A and B slightly increase from 4% to 4.1%. In the meantime, the rates of specimen C increase from 3% to 3.1%. In mix 6, the rates of specimens A and C remain at 3% from 30 to 38 days, then they decrease to 2.8% and 2.5%, respectively, up until 42 days. The rate of specimen B fluctuates around 2.5% between 30 and 38 days, and then drops to 2.4% in the final 4 days. According to the results in Fig. 27.11, it can be concluded that self-healing rates of specimens with natural cracks increase slightly or remain at a certain value from 28 days. Afterwards, increments of self-healing rates stop at around 36 days. Then, they start to decrease. The drops of self-healing rates can be interpreted as there is almost no unreacted cement that can be converted into calcium carbonate. Thus, UPV results are pretty much the same across different days. Compared to the UPV values in Fig. 27.11, they indicate a trend of self-healing rates from which it can be concluded that self-healing rates increase at their highest speed then decrease to a value according to different proportions of fibers. Furthermore, 0.2% fiber performs the highest self-healing rates of specimens with natural cracks in all concrete mixes. The mechanism of identifying self-healing abilities involves comparing control samples with reference samples in the same mix on the same day. According to Eq. (27.3), self-healing rates can be calculated as shown in Figs. 27.11 and 27.12. As demonstrated in Table 27.6, specimens with natural cracks of mix 3 clearly show a self-healing ability. According to UPV results in Table 27.6, it can be observed that differences on speed of UPV tests between control samples and reference samples in the same mix at the same day are smaller from day to day. It can be concluded that cracks are healed to improve the speed of UPV tests. Time refers to the time of passing through the prisms, and speed refers to speed of the UPV tests. Difference refers to percentages of the difference between control samples and reference samples in the same mix on the same day. The increase rate refers to self-healing increments.

Figure 27.11 (A) Self-healing rates of specimens A, B, and C with natural cracks of mix 3; (B) self-healing rates of specimens A, B, and C with natural cracks of mix 4; (C) self-healing rates of specimens A, B, and C with natural cracks of mix 5; (D) self-healing rates of specimens A, B, and C with natural cracks of mix 6.

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Figure 27.11 (Continued).

Figure 27.12 Increased rates on self-healing for all mixes.

Table 27.6 Self-healing increments calculations of specimens in mix 3.

Reference Specimen A Specimen B Specimen C

Time

Speed

Difference Time

Speed

28 days 100.4 114.7 113.9 115.5

/ 4.9801 4.3592 4.3898 4.3290

/ / 12.467% 11.853% 13.074%

/ / 5.0251 / 4.5579 9.298% 4.6253 7.956% 4.4883 10.682%

30 days 99.5 109.7 108.1 111.4

Difference Increase rate / / 3.169% 3.897% 2.391%

Time

Speed

Difference Increase rate

32 days 99.2 109.3 107.7 111.0

/ / 5.0403 / 4.5746 9.241% 4.6425 7.892% 4.5045 10.631%

/ / 3.227% 3.960% 2.443%

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Fig. 27.12 shows self-healing increase rates of specimens with natural cracks in mixes 3, 4, 5, and 6. The self-healing rate of mix 5, which is always the highest of all mixes, increases from 3.5% to 3.8% between 30 and 40 days, then drops slightly to 3.7% at 42 days. For mixes 3 and 6, self-healing rates remain at 3.1% and 2.9% from 30 to 38 days, respectively, and afterwards drop to 3% and 2.7%, respectively. With regards to mix 4, the self-healing rate goes up from 3.5% to 3.7% between 30 and 36 days, then falls to 2.8% within 6 days. In total, self-healing rates initially are around 3.25% at 30 days. Afterwards, they rise to around 3.5% up until 36 days. Finally, they start decreasing. Moreover, it can be observed that mix 5 with 0.2% fiber performs better than other mixes on self-healing. Images showing healed cracks of mixes 3, 4, 5, and 6 are presented in Figs. 27.13
27.16.

Figure 27.13 (A) Existing cracks of specimens with natural cracks in mix 3; (B) self-healing phenomenon of specimens with natural cracks in mix 3.

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Figure 27.14 (A) Existing cracks of specimens with natural cracks in mix 4; (B) self-healing phenomenon of specimens with natural cracks in mix 4.

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Figure 27.15 (A) Existing cracks of specimens with natural cracks in mix 5; (B) self-healing phenomenon of specimens with natural cracks in mix 5.

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Figure 27.16 (A) Existing cracks of specimens with natural cracks in mix 6; (B) self-healing phenomenon of specimens with natural cracks in mix 6.

27.3.6 Natural frequencies The natural frequency test is usually used for detecting cracks in building slabs or bridges. From the recorded data in Fig. 27.17, undulated lines can be seen. With regard to the lines in Fig. 27.17, it can be interpreted that natural frequencies of specimens in all mixes fluctuate randomly between 20 and 23 Hz. However, there is no trend found in natural frequency tests.

Figure 27.17 (A) Natural frequencies of specimens with standardized cracks in mix 1; (B) natural frequencies of specimens with standardized cracks in nix 2; (C) natural frequencies of specimens with standardized cracks in nix 3; (D) natural frequencies of specimens with standardized cracks in nix 4; (E) natural frequencies of specimens with standardized cracks in mix 5; (F) natural frequencies of specimens with standardized cracks in mix 6.

Figure 27.17 (Continued).

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27.4

851

Discussion

27.4.1 Mechanical properties According to the results in Figs. 27.7
27.9, it can be concluded that mechanical properties all decrease when 5% rubber is added to concrete. Subsequently, mechanical properties increase when fiber is added to concrete. This means that adding the fiber can improve the mechanical properties of concrete. More of the fiber induces higher mechanical properties. Finally, mechanical properties stop increasing when 0.2% fiber is added, then they start to drop when 0.25% fiber is added to the concrete. It can be supposed that excessive fiber will induce reductions in mechanical properties.

27.4.2 Self-healing abilities of specimens with standardized cracks According to Fig. 27.10, a fluctuation between 0.1%
1% of self-healing abilities of specimens with standardized cracks can be observed. Self-healing rates of specimens with standardized cracks are very low and can be ignored. This phenomenon can be attributed to the fact that there is no bonding force between fiber and concrete, thus calcium carbonate cannot be formed to fill the gaps. This is the reason why specimens with standardized cracks cannot be healed. The results in Fig. 27.10 reveal that inserting plastic films is not practical for autogenous self-healing tests. This is because plastic films will physically block fiber between gaps, and then fiber cannot bridge separated concrete to enhance the self-healing abilities. The values in Fig. 27.10 also show that additional fiber in specimens with standardized cracks has no function in stimulating self-healing ability when it is merely added to concrete instead of being placed in gaps.

27.4.3 Self-healing abilities of specimens with natural cracks According to Fig. 27.11, the self-healing rates of specimens with natural cracks gradually increase, then they stop increasing at around 36 days, finally decreasing up until 42 days. Compared with control mixes, which are mixes 1 and 2, the selfhealing rates of mixes 3, 4, 5, and 6 are much higher. The self-healing rates of mixes 3, 4, 5, and 6 are between 3% and 4%. The self-healing rates of mixes 1 and 2 are between 0.1% and 1%. Furthermore, there is no crack healing found in mixes 1 and 2. This can be attributed to the advantage of adding fiber. The fiber bridge cracks and creates cores of calcium carbonate during hydration processes. Furthermore, the self-healing values in Fig. 27.12 show that specimens of mix 5 heal the fastest. It can be considered that 0.2% of fiber in rubberized concrete has the best self-healing ability.

27.4.4 Review of making cracks According to tests carried out by the authors in this study, the key factor of selfhealing tests is to generate cracks, because it is difficult to control the crack width

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of concrete specimens without reinforcement. A linear variable differential transformer (LVDT) was installed at the bottom of specimens to restrict the crack width [44]. In that case, the width of cracks was successfully limited from 0.15 to 0.17 mm. Moreover, a clip gauge was used to control the width of cracks [45]. As mentioned in the literature review section, cracks should be smaller than 0.3 mm or they will not be able to be healed [24] [25]. This shows that the use of LVDT or clip gauge can be suitable methods to limit crack width in future studies on selfhealing concrete. Moreover, it is easier to generate cracks in smaller specimens than in large prisms. Small cylinders or prisms can be wrapped in tape to prevent them from being separated into two parts. Thus, cracks can be easily generated.

27.4.5 Review of test methods The UPV test is very sensitive. The main factors influencing UPV results can be concluded as sizes of aggregate, ages of concrete, test temperatures, moisture content of specimens, types of cement, shapes and sizes of specimens, and curing conditions [46]. Furthermore, a 1% greater water content of concrete specimens increases UPV results by 160 m/s. The results of UPV will increase by 34 m/s on average when temperature increases by 10 C [47]. In future studies, every parameter needs to be recorded before starting UPV tests. Hopefully, an equation for calibrating UPV values to exclude negative influences can be provided. There have been other methods utilized by previous researchers to identify self-healing abilities, as described next. 1. Scanning electron microscope SEM was applied to monitor details of cracks by zooming in at a high magnification rate [1]. Using SEM, one can clearly see detailed images of gaps. It is thus easy to measure the width of cracks. However, this method cannot be used to analyze the composition of precipitation. 2. X-ray diffraction It is commonly known that calcium carbonate is white substance. Researchers usually use this phenomenon to preliminarily identify whether there is a self-healing performance. If there is newly formed calcium carbonate between cracks, it means that the cracks are healed. However, it is difficult to distinguish whether the white substance is calcium carbonate. XRD can distinguish calcium carbonate by irradiating specimens [1]. XRD can be combined with SEM to accurately identify the self-healing phenomenon.

In future studies on self-healing concrete, XRD, SEM, and UPV methods could be utilized together to precisely identify self-healing abilities.

27.4.6 Natural frequencies Based on Fig. 27.17, natural frequencies of specimens fluctuate between 20 and 23 Hz. Theoretically, frequencies will be different when variable cracks are obtained initially. This means that, if cracks are healed, frequencies of specimens with cracks

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will change to be the same frequency as the RFC. However, natural frequencies of those specimens in this study randomly fluctuate. The reason for this may be because specimens in this study were too small to cause changes in frequencies. In general, a measurement of natural frequencies usually carry out on a larger size of components or structures (e.g. bridge girders, beams, or panels). Thus, it could be concluded that natural frequencies are not available for inspecting self-healing abilities.

27.5

Conclusion

An improved concrete which contains 5% of 180- and 400-µ rubber and different proportions of Duras EasyFinish fiber was measured for self-healing abilities in this study. The concrete helps to reduce the negative influence of waste rubber. Furthermore, the concrete can be used in construction industries to reduce maintenance fees and enhance durability. Compressive strength, flexural strength, and splitting tensile strength, which can be concluded as mechanical properties, were tested in this study. There were noticeable reductions in mechanical properties when the rubber was added to the concrete. It can be interpreted that adding the crumb rubber in concrete dramatically induces reductions in mechanical properties. Afterwards, each strength was increased by adding fiber. This illustrates that adding Duras EasyFinish fiber can enhance the mechanical properties of rubberized concrete to support higher loads and bond concrete together. However, these increments in mechanical properties stopped when 0.2% fiber was added. This shows that mechanical properties increase to the highest value when 0.2% fiber is added to concrete. For standardized cracks, self-healing rates fluctuated between 0.1% and 1%, which is very low. Additionally, there is no self-healing phenomenon shown in standardized cracks. Thus, it can be interpreted that standardized cracks cannot be healed by adding fiber to rubberized concrete. For natural cracks, self-healing rates go up to around 3.5% at 36 days. Then, rates starting dropping within the following 6 days. According to the data related to natural frequencies and ultrasonic pulse velocities, it is obvious that the difference between control samples and the reference sample in the same mix on the same day is smaller from day to day, which can be recognized as evidence of self-healing. Note that there are some disadvantages of UPV tests, which have been listed in Section 27.4.5. Moreover, the SEM and the XRD methods have been analyzed and suggested to be combined with the UPV test to improve self-healing measurement. With respect to the natural frequency test, the results randomly fluctuated between 20 and 23 Hz. Moreover, there was no change in natural frequencies between specimens with different crack depths. Thus, this method may not be suitable for measuring the self-healing ability. This may result from the smallness of the specimens. In summary, concrete with 5% crumb rubber shows the best mechanical properties that can be utilized to satisfy concrete requirements of practical engineering standards and to reduce negative influence of waste rubber. Moreover, it can be

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concluded that concrete with 0.2% fiber has the best performance on self-healing and could be applied in construction areas. By making cracks, LVDT and clip gauge have been suggested to be used to limit crack width in future studies on selfhealing. Moreover, the SEM and XRD methods have been suggested to assist more accurately the UPV method to measure self-healing abilities. Testing natural frequencies has been discarded as a method for evaluating self-healing abilities of small specimens because there was no change in the natural frequencies between specimens with different crack depths.

Acknowledgments The authors are sincerely grateful to the European Commission for financial sponsorship of the H2020-RISE Project No. 691135 “RISEN: Rail Infrastructure Systems Engineering Network,” which enables a global research network that tackles the huge challenge in railway infrastructure resilience and advanced sensing in extreme environments (www.risen2rail.eu) [48]. In addition, this project is partially supported by European Commission’s Shift2Rail, H2020-S2R Project No. 730849 “S-Code: Switch and Crossing Optimal Design and Evaluation.”

Author contributions X.H. and S.K. developed the concept; X.H. performed experiments and analyzed the data; and S.K. contributed the materials and analysis tools. The authors cowrote this chapter.

Conflicts of interest Authors declare no conflict of interest.

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ˇ c´ Marijana Hadzima-Nyarko and Ivana Milicevi Josip Juraj Strossmayer Univеrsity of Osijek, Faculty of Civil Engineеring and Architecturе Osijek, Osijek, Croatia

28.1

Introduction

Requirements for the use of recycled materials in concrete production, as one of the most commonly used materials in construction, are becoming more common. Currently, a growing problem is the disposal of car tires. This chapter presents an overview of current expressions for the estimation of rubberized concrete compressive strength based on its constituents. Since rubber waste is a very durable material with high resistance to environmental impacts, disposing of used tires is a major concern nowadays. Inadequate disposal may lead to significant aesthetic and ecological problems. Due to their low density and slow degradation, decomposition of waste rubber takes longer than 50 years, and therefore large quantities of rubber tires are cumulatively disposed of each year. In numerous countries, both disposal and waste tire management have become major environmental concerns. This problem can be reduced by replacing the natural fine and/or coarse aggregate in concrete with waste tire rubber. Duе to the previously stated fact of the relatively long waste tire life, there is an increasing interest in replacing the natural river aggregate in concrete mixes with rubber obtained from waste tires, that is, rubbеrized concrete, resulting in environment-friendly concrete with recycled rubber. Based on a literature review, by taking into account all mechanical properties of concrete with waste tire rubber, it is possible to conclude that tire rubber as an aggregate has the most impact on concrete compressive strength. Some authors have provided results of concrete cоmpressive strength reductiоns of 90% compared to the original mixture when 100% replacement of chipped rubbеr was made. When sand was totally replaced with crumb rubber, the reduction in concrete strength was about 80%. In all cases, a decrease in compressive strength was observed, with the decrease being more pronounced as the degree of tire rubber replacement was increased. This reduction effect could be related to the weak strength of the bond between the cement paste and the rubber particles, the increased amount of air trapped between the rubber particles and the cement paste, and the low modulus of elasticity of the rubber compared to natural aggregates. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00028-4 © 2020 Elsevier Inc. All rights reserved.

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According to the literature review, the presence of rubber as an aggregate in concretes decreases the compressive strength. The literature review reveals that a small number of researches has been conducted on the prediction of the compressivе strength of concrete with waste tirе rubber, but a few on a database are large enough to obtain more reliable prediction of compressive strength [1]. With the aim of investigating various mechanical characteristics of concrete with waste tire rubber, a database containing 431 mixtures provided by different researchers is presented. The partial or full replacement of natural aggregate with waste tire rubber and its influence on the compressivе strength of rubberizеd concrete is explored. Since the relationships betweеn concrete properties and concrete components are important for optimizing the quantities of components, a brief state of the art of existing equations is given.

28.2

Literature review

The disposal of used tires has became a major concern due to the fact that rubber waste is highly resistant to most natural environments and is also a very durable material. Large quantities of used rubber tires (about 180 million in the European Union and 275 million in the United States) accumulate each year [2]. As was stated in Dong et al. [3], in the United States, more than 20% of the total amount of waste tires has been recycled into various civil engineering applications, for example, as an additive or modifier in asphalt paving mixtures for pedestrian blocks and Pоrtland cement concrete mixtures which can be applied as nonbearing structures, such as exteriоr walls. The potential applications of rubber tires as an aggregate have been investigated in many studies. The addition of waste tire rubber significantly changes the characteristics of concrete, which could be improved by elastic and deformable waste tire rubber particles. This means that concrete becomes more ductile, as illustrated by the higher post-failure toughness [4]. On the other hand, the hydrophobic nature of rubber weakens the bond between the untreated rubber particles and hydrated cement, resulting in a significant reduction in the compressive and tensile strength of rubberized concrete [5,6]. The replacement of coarse or fine aggregates with waste tire rubber depends on the size [7], which ranges from rubber chips (25
50 mm), to crumb rubber (1
8 mm), to powders (0.075
less than 1 mm). There are already very useful scientific papers related to: 1. The addition of scrap tire rubber affecting the properties of concrete [8]; 2. Fresh or hardened characteristics of rubberized and self-compacting rubberized concrete [9]; 3. Mechanical properties of rubber [9]; 4. Modified bitumen with recycled tire rubber for road asphalt mixtures [10]; 5. Pyrolysis of waste tires [11]; and 6. Addition of waste tire rubber in Portland cement concrete and asphalt [12].

In Ref. [13], the effects of rubber sand on the properties of both rubberized mortar and rubberized concrete were reviewed. The concrete properties investigated included

Equations for prediction of rubberized concrete compressive strength: a literature review

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workability, bleеding, porоsity, dеnsity, shrinkage, impact energy, strength, impact lоad, tоughness, corrosion resistance, ductility, abrasion resistance, carbonation resistance, frеeze/thaw resistance, thеrmal insulation, fire resistance, water absorption, chloride ion penetration, resistance to aggressive environments, sound absоrption, energy absоrption, cracking resistance, and electrical resistance. In this chapter, we investigate only the influence of partial or full waste tire rubber replacement of natural aggregate on the main mechanical property of concrete, that is, compressive strength. The compressive strength of concrete at the age of 28 days (fc,28) is the input value in every calculation of bearing elements and therefore it is one of the most important mechanical properties of concrete, especially for structural applications. Issa and Salem [14] used recycled rubber as a substitute for sand in the following proportions: 15%, 25%, 50%, and 100%. They concluded that concrete in which 25% of the fine aggregate was replaced by waste rubber could be used in the production of load-bearing structural elements. Al-Tayeb et al. [15] replaced the fine aggregate with 5%, 10%, and 20% rubber, while Ozbay et al. [16] replaced the fine aggregate with 5%, 15%, and 20% rubber. Their tests show that the strength and static modulus of elasticity of concrete decrease as the amount of rubber increases. The main conclusion from earlier studies is that using waste tire materials as an alternative to replace part of coarse or fine aggregates reduces the concrete compressive strength. The knowledge of relationships between components and concrete properties is imperative for optimizing the quantities of components as this relationship cannot be modeled by a mathematical formula. First, we present formulas for compressive strength of concrete given in building codes and by researchers in order to provide an insight into the basic components used in the expressions. Then, we investigate a relatively small number of formulas for rubberized concrete given by researchers. Khatib and Bayomy [17] developed a functiоn that quantifies the rеductiоn in strеngth for rubberizеd cоncrete. Ghaly and Cahill IV [18] developed correlations, based on the experimental results, to estimate the concrete strength reduction as a function of the content of rubber in the mixture. Ling [19] also provided, based on experimental testing, a formula which determines the strength reduction factor. In this formula, the dependent factors of rubber content and w/c ratio are taken into account. A review of the literature revealed that not much research has been carried out to predict the compressive strength of rubberized concrete on a large database to obtain more reliable predictions of compressive strength. The aim of this chapter is to compare existing expressions for the values of compressive strength of concrete with waste tire rubber using a database consisting of 431 experimental tests.

28.3

Database description

A systematic search of published articles on the properties of rubber concrete with waste tire rubber up to January 2018 was conducted. Mixtures containing rubber,

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such as rubber mortar or asphalt rubber concrete, were not taken into account as it was intended to test the compressive strength of rubberized concrete. Also, articles written in non-English languages or which are local rather than international were also excluded. In this study, broad keywords such as rubber, concrete, rubcrete, rubber concrete, and rubberized concrete, were used to ensure that there were no omissions. A database of 431 published experimental testing was created (Table 28.1). The parameters of the database were selected according to all available data components of the mixtures and obtained strengths. If any parameter was not available for the measurements, these test data were excluded from the database. There are different ways of adding rubber aggregate to concrete as described in the literature: examples which incorporate waste tire rubber as a prоportion of vоlume or weight of cоncrete, then which substitute aggregates of concrete by weight, and finally which incorporate rubber aggregates by volume. Due to the fact that the density of the natural aggregate in concrete is about 2.5 times higher than the rubber substance, to replace the aggregates with rubber by way of weight replacement, the volume of the resulting mixture would be much higher compared to the original. Therefore, in practical applications, it is preferable to substitute the natural aggregate with rubber by volume. Various characteristics of concrete with rubber waste were investigated by researchers, but in this chapter, only the influence of replacement (partial or full) of natural aggregate with waste rubber in concrete on the compressive strength of rubberized concrete is analyzed. The compressive strength achieved as a function of the percentage of replacement (partial or full) of the aggregate with waste rubber for the whole database in presented in Fig. 28.1. The first conclusion that may be drawn from analyzing Fig. 28.1 is that the addition of waste rubber aggregate leads to the reduction, that is, loss of concrete compressive strength, and this becomes more significant as the proportion of rubber aggregate in the mixture is increased. This reduction is attributed to several causes: G

G

G

G

G

Significantly large relative deformations between rubber and concrete appear and lead to early cracking. There are two reasons for this: first, the Poisson’s ratio of rubber is twice that of concrete and second, the Young’s modulus of rubber is about one-third that of concrete [48]. Because the mоdulus of elasticity of rubber particles is low, high internal tensile stresses perpendicular to the direction of the applied compression load are produced. This causes early failure in cement mortar [5]. Waste rubber particles act as air voids to create cracks. This is followed by crack propagation, and then a decrease in strength [48]. Due to the low permeability of the rubber particles, the interaction between the boundaries of the rubber particles and the cement paste can be significantly degraded. This increases the volume of the weakest phase and the interfacial transition zone [48]. Because the specific gravity of rubber is lower than concrete and the vibration process during concrete mixing and placement, the rubber goes to the top of the concrete, resulting in a nonhomogeneous mixture and concrete strength reduction [9].

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Table 28.1 Experimental database of concrete with tire rubber: list of authors and samples. No.

Authors

Year

References

No. of specimens

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Toutanji Gu¨neyisi et al. Albano et al. Geosglu and Guneyisi Azmi et al. Batayneh et al. Taha et al. Turatsinze and Garros Zheng et al. Ganjian et al. Aiello and Leuzzi El-Gammal et al. Ozbay et al. Paine and Dhir Ghedan and Hamza Son et al. Grinys et al. Rahman et al. Siringi Al-Tayeb et al. Dоng et al. Bala et al. Fiore et al. Geosglu et al. Kumar et al. Mohammadi et al. Mohammed and Azmi Onuaguluchi and Panesar ´ ci´c et al. Topliˇci´c-Curˇ Wang and Huang Youssf et al. Herrera-Sosa et al. Ismail et al. Khan and Singh Mishra and Panda Selvakumar et al. Toma et al. Asutkar et al. Ishwariya Liu et al. Zaoiai et al. Almaleeh et al. Murugan et al.

1996 2004 2005 2007 2008 2008 2008 2008 2008 2009 2010 2010 2010 2010 2011 2011 2012 2012 2012 2013 2013 2014 2014 2014 2014 2014 2014 2014 2014 2014 2014 2015 2015 2015 2015 2015 2015 2016 2016 2016 2016 2017 2018

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [16] [32] [33] [34] [35] [36] [37] [38] [3] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]

5 70 13 16 15 6 9 5 7 4 9 4 4 13 2 6 12 4 17 4 5 5 7 11 12 12 45 10 4 6 12 7 7 4 5 5 4 5 2 9 5 18 6 431

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Figure 28.1 The obtained compressive strength in relation to the percentages of replacement of aggregate with waste rubber based on the dataset of 431 experiments.

The experimental database consists of six parameters which are chosen as the most important parameters for the estimation of compressive strength: cement (kg), w/c ratio, the percentage of fine natural aggregate (NAF), the percentage of coarse natural aggregate (NAC), the percentage of fine rubber (RBF), and the percentage of coarse rubber (RBC). According to Table 28.1, two boundary sizes of waste rubber aggregate were defined in the database: fine rubber with size from 0 to 4 mm and coarse rubber with size from 4 to 16 mm. Accordingly, two subparts were also defined for the aggregate partition: fine aggregate with size between 0 and 4 mm and coarse aggregate with size between 4 and 16 mm. According to the following formula, the total aggregate ratio has to be 100%: NАF ½% 1 NАC½% 1 RBF½% 1 RBC½% 5 100%

(28.1)

whеre, NAF denotes the percentage of fine natural aggregate; NAC denotes the percentage of coarse natural aggregate; RBF denotes the percentage of fine rubber; RBC denotes the percentage of course rubber. Table 28.2 presents the minimum, maximum, and average values of collected parameters in the database of rubberized concrete. Fig. 28.2, in which the histogram of the percentage of RBF substitution of fine aggregate is presented, shows that 33.64 % of all specimens (exactly 145 examples) contain a maximum of 5% of RBF. Most of the samples (89.10%) contain up to 25% of substitution of RBF to the fine aggregate. Only 15 mixtures contain up to 75% substitution of RBF to the fine aggregate. Thirty-two specimens contain a total substitution of fine aggregate (i.e., 100% of RBF).

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Table 28.2 The range of collected parameters in the rubberized concrete database. Parameter

Unit

Minimum value

Maximum value

Average value

w/c ratio Cement NAF (0
4 mm) RBF (0
4 mm) NAC (4
16 mm) RBC (4
16 mm) fc,28

kg % % % % MPa

0.27 270 0 0 0 0 0.78

0.68 629.27 100 100 100 100 85.7

0.48 393.99 84.46 12.99 93.63 4.75 28.90

Figure 28.2 Histogram of RBF and RBC substitution with the fine and coarse aggregate.

The percentages are slightly different for coarse rubber aggregate substitution (Fig. 28.2), where 95.36% of samples contain up to 20% substitution of RBC to the coarse aggregate. Fifty percent substitution of the coarse aggregate with RBC has only four specimens, while 75% substitution of the coarse aggregate with RBC has only two specimens and total replacement of RBC has six specimens. From the database of the cеment, which ranges from 250 to 650 kg, 85.56% of all samples have up to 450 kg, while another approximately10% contain up to 600 kg. Only five mixtures have the maximum amount of cement (650 kg). This is presented in Fig. 28.3. The w/c ratio is presented in Fig. 28.4 and ranges from 0.25 to 0.7. A total of 144 mixtures have a valuе of 0.45, while 110 mixturеs have a w/c ratio value of 0.6.

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Figure 28.3 Cement content in the database.

Figure 28.4 w/c ratio in the database.

28.4

Expressions for compressive strength in the literature

In this sеction, formulas for the cоmprеssivе strеngth of concrete and rubberized concrete given both in building codes and by researchers are presented.

Equations for prediction of rubberized concrete compressive strength: a literature review

28.5

865

Expressions for compressive strength of concrete

The cоmprеssivе strеngth of cоncrеtе at age t depends on the curing conditions, temperature, and type of cement. In accordance with EN 12390, the concrete compressive strength at various ages fcm(t) is given by the expressions (for a mean temperature of 20 C and curing) [61]: fcm ðtÞ 5 β cc ðtÞ 3 fcm

(28.2)

β cc ðtÞ 5 esð12 t Þ

(28.3)

with 28 0:5

where, fcm (t) denotes the mean concrete compressive strength at an age of t days; t denotes the age of the concrete in days; fcm denotes the mean 28-days compressive strength (Table 28.3); β cc(t) denotes a coefficient which depends on concrete age t; s denotes a coefficient which depends on cement type. According to ACI 209 [62], the formula for estimating the compressive strength at any time is as follows: 0
fc t 5

t 0
3 fc 28 a 1 bt

(28.4)

where, (fc’)28 denotes comprеssivе strеngth at 28 days; t denotes the age of the concrete in days; a in days and b are constants. Since the еvaluation of comprеssivе strеngth with timе is a large problem for structural enginееrs, ACI Committее 209 [62] gives the following relationship for moist-cured concrete made with normal Portland cement (i.e., ASTM Type I): 0
fc t 5

t 0
3 fc 28 4 1 0:85t

(28.5)

while for ASTM Type III, the equation is as follows: 0
fc t 5

t 0
3 fc 28 2:3 1 0:92t

(28.6)

According to SRPS U.M1.048 [63], compressive strength is obtained by testing at age t calculated on the value of compressive strength at age 28 in Eq. (28.7): 1 0
0
fc t 5 3 fc 28 rc where, rc is a coefficient given in Table 28.4.

(28.7)

Table 28.3 Characteristics of compressive strength of concrete according to EN 1992-1-1:2004 HRN EN 1992-1-1:2013 [61]. Compressive strength classes fck

Analytical equation

12

16

20

25

30

35

40

45

50

55

60

70

80

90

15

20

25

30

37

45

50

55

60

67

75

85

95

105

20

24

28

33

38

43

48

53

58

63

68

78

88

98

fcm 5 fck 1 8ðMPaÞ

1.6

1.9

2.2

2.6

2.9

3.2

3.5

3.8

4.1

4.2

4.4

4.6

4.8

5.0

fcm 5 0:3 3 fck3 # C50=60

cm fcm 5 2:12 3 ln 1 1 f10 . C50=60

(MPa) fck,cube (MPa) fcm (MPa) fctm (MPa)

2

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Table 28.4 Coefficients rc for the compressive strength estimation [63]. Number of months Coefficient rc

1 1

2 0.91

3 0.87

6 0.81

9 0.78

12 and more 0.75

Namyong et al. [64] proposed a formula for 28-day compressive strength prediction with the standard error of 1.1 N/mm2: fp 5 e½2:9821:588ð c Þ20:00642c17:6888fdðs1gÞg w

(28.8)

where, fp is the prediction comprеssivе strеngth (N/mm2); w/c is the watеr
cemеnt ratio; c is the cеmеnt content (kg/m3); w is the watеr content (kg/m3); c/(s 1 g) is the cеmеnt
aggrеgate ratio.

28.5.1 Expressions for compressive strength of rubberized concrete Khatib and Bayomy [17] developed a characteristic function which quantifies the strength reduction for rubberized concrete mixtures. Among several mathematical functions including various degrees of polynomial functions which were analyzed, they gave the best mathematical function that can resemble the trend of the strength reduction curves: SRF 5 a 1 bð12RÞm

(28.9)

with the condition that: a512b

(28.10)

where, SRF is factor for strength reduction; R is the content of rubber volumetric ratio by total volume of aggregate; a, b, and m are function parameters. The exponent m reflects the degree of curvature of the downward curve. It relates the sensitivity of the mixture to the decrease of strength with the rubber content. When incorporating these parameters in the compressive SRF model for ultrafine crumb and tire-derived fuel chips rubber, the following equation is obtained: SRF 5 0:10 1 0:90ð12RÞ7

(28.11)

Ghaly and Cahill [18] provided experimental testing in order to study the еffеct of the addition of crumb rubber, as a substitute for a portion of fine aggregates (sand), on the strength of concrete. Based on the experimental results, they developed a dimensionless stress reduction factor (Ru) of the rubbеr content in the mixtures with correlations to assess the concrеtе strеngth reduction:

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New Materials in Civil Engineering

Ru 5


ðSu Þr w=c ðr Þ ðSu Þnr

(28.12)

where, (Su)r is the compressive strength of rubberized concrete at 28 days; (Su)nr is the compressive strength of nonrubberized concrete at 28 days; r is the percent of rubber in the mix. The best-fit curve for all data points has the following form: Ru 5 0:0136ðr Þ0:34

(28.13)

However, it has to be noted that the accuracy of the correlations is R2 5 0.39. Ling [19] provided experimental testing in order to calculate the density and strength reduction factors for rubberized concrete blocks by considering the dependent factors of rubber content and w/c ratio. An equation for determining the strength reduction factor (RCS) as shown in Eq. (28.13) was established by including the dependent factors of rubber content and w/c ratio:  
Sr Rcs 5 w=c ðr Þ Su

(28.14)

where, Sr is compressive strength of rubberized concrete block (MPa); Sc is compressive strength of control concrete block (MPa); r is rubber content by total sand volume. For simplicity, the dependent factor of w/c ratio was eliminated by using the average results of the three series of w/c ratio mixtures, thus the average correlation of the reduction strength factor for 28 days has the following form: R28d2csðaverageÞ 5 0:0274 lnðr Þ 1 0:0169:

(28.15)

As wass stated by Ling [19], the equation (only expressed as a function of the percentage of rubber content) corresponds to the reduction strength factors (RCS) in the tested range (within 0%
50% for rubber content and 0.45
0.55 for w/c ratio). Youssf et al. [48] collected 148 crumb rubber concrete mixtures, based on which they developed the model and compared the predictions of the present model to the predictions of previous models. Their model for estimation of concrete compressive strength is in an exponential form, selected among several forms (e.g., linear, logarithmic, exponential, polynomial, power). Compared to the other forms, the advantage of the exponential form is that when the rubber content Rt equals zero (no rubber), the concrete compressive strength is not affected (e0 5 1.0). The initial compressive strength model form was: f 0CRC 5 f 0c eαRt




(28.16)

Equations for prediction of rubberized concrete compressive strength: a literature review 0

869 0

where, fCRC is the compressive strength of the crumb rubber concrete; fc is the compressive strength of the control concrete (without rubber); Rt is the rubber content by volume of the total aggregates; α is a constant. Since the value of eαRt in Eq. (28.16) has to decrease as Rt increases, the constant value “α” used a wide range of negative values ranging from 20.1 to 230.0 to select the appropriate value that resulted in the least root mean square error in the model predictions. They found that the appropriate value is 24.2, so the final form of the equation has become: f 0CRC 5 f 0c e24:2Rt




(28.17)

Youssf et al. [48] stated that model they proposed resulted in prediction of crumb rubber concrete strength with a mean error of only 10.7%. Zain et al. [65] proposed and developed new mathematical models using a nonlinear regression equation for the prediction of concrete compressive strength at different ages. After analyzing the influence of mix constituents on the strength at ages of 7 and 28 days, the proposed model was used to predict the compressive strength at the specified ages:
1:61814 f28 5 0:34262 3 C228:731 3 W 28:0856 3 FA228:3023 3 CA21:9259 3 ρ0:72819 3 w=c

(28.18) where, f28 is the compressive strength of concrete at 28 days; w/c is the water/ cement ratio; C is the quantity of cement in the mix; CA is the quantity of coarse aggregate in the mix; FA is the quantity of fine aggregate in the mix.

28.6

Comparison of existing expressions

A comparison is presented for expressions given by Khatib and Bayomy [17] and Youssf et al. [48]. Since Ghaly and Cahill [18] stated that the accuracy of the correlations is R2 5 0.39, this model will not be considered. The model proposed by Zain et al. [65] could not be compared as there are unsufficient data for the density of mixtures, which was absent in many articles. Using Eq. (28.11), the strength reduction factor SRF is calculated for every mixture and the compressive strength is reduced by this factor. In Fig. 28.5, compressive strength from the database is compared with compressive strength obtained by using expressions given by Khatib and Bayomy [17]. In Fig. 28.5, the target or measured compressive strength is represented by the x-axis, while the y-axis represents the output or calculated compressive strength. Even the coefficient of regression, R, is relatively high, it can be seen that the model proposed by Khatib and Bayomy underestimates the actual compressive strength. Exponential functions for compressive strength of dataset versus expression are given in Fig. 28.6. Using Eq. (28.17), the compressive strength is calculated for every mixture. In Fig. 28.7, the compressive strength from the database is compared with the

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Figure 28.5 Compressive strength of data obtained by using the expressions of Khatib and Bayomy.

Figure 28.6 Exponential functions for compressive strength of dataset versus the expression of Khatib and Bayomy.

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Figure 28.7 Compressive strength of data and that obtained by using the expressions of Youssf et al.

Figure 28.8 Exponential functions for compressive strength of dataset versus the expression of Youssf et al.

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New Materials in Civil Engineering

compressive strength obtained using the expressions given by Youssf et al. [48]. It can be seen in Fig. 28.7 that the coefficient of regression, R, is higher than that obtained by the previous model, indicating that the model proposed by Youssf et al. is slightly better. However, when comparing Figs. 28.5
28.7, one can see that the Youssf et al. model overestimates the actual compressive strength in more situations than the Khatib and Bayomy model. Exponential functions for compressive strength of dataset versus expression are given in Fig. 28.8.

28.7

Conclusion

The main purpose of this investigation was to compare the expressions given in code provisions and by other authors that have been investigating rubberized concrete compressive strength. The current review aims to show the previous researches carried out on the effect of partial or full replacement of natural aggregate in concrete with waste rubber on the mechanical properties of concrete. The compressive strength, as a measure of the concrete’s ability to resist loads which tend to compress it, is a function of the concrete components, for example, the aggregates, the cement matrix, and their relative proportions. Due to the fact that the mechanical properties of concrete are highly dependent on the types and proportions of binders and aggregates, the existing expressions for predicting the compressive strength cover all of the experimental data, as is shown in this chapter. Two models, each defined by expressions which are suitable for the available database, were compared. Comparing the models from Khatib and Bayomy and Youssf et al. the results obtained showed a relatively good match. The model proposed by Khatib and Bayomy underestimates the actual compressive strength, while that proposed by Youssf et al. overestimates the actual compressive strength in some more situations than the Khatib and Bayomy model. Until a more accurate model is proposed, based on the obtained results, it is safer to use the model by Khatib and Bayomy.

Acknowledgment This work has been supported in a part by Croatian Science Foundation under the project UIP-2017-05-7113 Development of Reinforced Concrete Elements and Systems with Waste Tire Powder—ReCoTiP.

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18.

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[2] G. Skripkiunas, A. Grinys, B. Cernius, Deformation properties of concrete with rubber waste additives, Mater. Sci. 13 (3) (2007) 219
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123. Available from: https://doi.org/10.1016/j.conbuildmat.2013.06.072. [4] G. Li, M.A. Stubblefield, G. Garrick, J. Eggers, C. Abadie, B. Huang, Development of waste tire modified concrete, Cem. Concr. Res. 34 (12) (2004) 2283
2289. [5] J.B. Topcu, The properties of rubberized concrete, Cem. Concr. Res. 25 (2) (1995) 304
310. [6] B. Huang, G. Li, S.-S. Pang, J. Eggers, Investigation into waste tire rubber-filled concrete, J. Mater. Civ. Eng. 16 (3) (2004) 187
194. [7] I.O. Toma, O.M. Banu, R.G. Taran, M. Budescu, N. Taranu, Complete characteristic curve of concrete and rubberized concrete, in: Proceedings of the ICAMS 2014—Fifth International Conference on Advanced Materials and Systems, Bucharest, Romania, October 23
25, 2014. [8] R. Siddique, T.R. Naik, Properties of concrete containing scrap-tire rubber—as overview, Waste Manag. 24 (2004) 563
569. [9] K.B. Najim, M.R. Hall, A review of the fresh/hardened properties and applications for plain-(PRC) and self-compacting rubberized concrete (SCRC), Constr. Build. Mater. 24 (2010) 2043
2051. [10] D.L. Presti, Recycled tyre rubber modified bitumens for road asphalt mixtures: a literature review, Constr. Build. Mater., 49, 2013, pp. 863
881. [11] P.T. Williams, Pyrolysis of waste tyres: a review, Waste Manag. 33 (2013) 1714
1728. [12] X. Shu, B. Huang, Recycling of waste tire rubber in asphalt and Portland cement concrete: an overview, Constr. Build. Mater. 67 (2014) 217
224. [13] A.M. Rashad, A comprehensive overview about recycling rubber as fine aggregate replacement in traditional cementitious materials, Int. J. Sustain. Built Environ. 5 (2016) 46
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1307. Available from: https://doi.org/10.1617/s11527-012-9974-3-2012. [16] E. Ozbay, M. Lachemi, U.K. Sevim, Compressive strength, abrasion resistance and energy absorption capacity of rubberized concretes with and without slag, Mater. Struct. 44 (2011) 1297
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28, 2008.

Influence of cobinders on durability and mechanical properties of alkali-activated magnesium aluminosilicate binders from soapstone

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Z. Abdollahnejad1,2, M. Mastali2, F. Rahim2, Tero Luukkonen2, Paivo Kinnunen2 and Mirja Illikainen2 1 Fibre and Particle Engineering Research Unit, University of Oulu, Oulu, Finland, 2Civil & Environmental Engineering Department, University of Connecticut, CT, United States

29.1

Introduction

The consumption of raw materials in cement manufacturing imposes a high amount of carbon dioxide (CO2) emissions and cost [1]. Thus, the use of inorganic wastes in cementitious products to produce sustainable construction materials has received a great deal of attention. Utilization of inorganic wastes directly in construction materials faces some difficulties, such as low reactivity and binding properties, therefore enormous efforts have been made to minimize these drawbacks [2
5]. One of the effective proposed approaches to use inorganic waste was the use of alkali-activation technology [2
5]. Alkali-activated binders present acceptable mechanical and durability properties, and bring both environmental and economic benefits [6]. However, there are still some drawbacks to these alternative sustainable construction materials, such as large drying shrinkage, low strength, and efflorescence [7]. The magnitude of these problems mainly depends on the used precursors, alkali-activation properties, and curing conditions. Soapstone is a soft talc-rich metamorphic rock commonly used to produce, for instance, carvings, architectural elements, and countertops. Soapstone waste powder is generated during the industrial processing of the mineral and is a problem for a clean ecosystem. Soapstone has two main components, SiO2 and MgO (which may change based on the rock locations), however, magnesium aluminosilicate binders are poorly reactive in alkali activation due to their chemical structure and lack of amorphous components. As a result, their mechanical properties are insufficient for use in construction applications [8]. The mechanical performances of these materials could be improved by using different cobinders, thermal treatment, and high alkalinity [9,10]. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00029-6 © 2020 Elsevier Inc. All rights reserved.

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Cobinders are able to provide missing chemical components crucial for successful alkali-activation reactions and therefore, have beneficial effects on mechanical properties and durability of alkali-activated soapstone. In this study, four cobinders [i.e., metakaolin, lime, rock wool (RW) (stone wool), and silica fume] were used together with soapstone. Metakaolin (MK) is manufactured by heating kaolin up to a temperature of 800 C. It has good characteristics, such as a high specific area and strong coordinative bond [11]. MK possesses high aluminum and silicon contents and could be used as supplementary materials in construction materials to gain high mechanical strength and toughness, and excellent durability performances such as high temperature resistance. The MK production process releases 80%
90% less CO2 emission than ordinary Portland cement (OPC) [12]. RW is an inorganic fibrous substance that is considered as waste, which is formed by steam blasting and cooling molten glass. RW occupies a large space in landfilling, and this approach is not an eco-friendly method for disposal [13]. Recently, mineral wools have been used as a substitute for OPC in concrete to improve the mechanical performances [14] or have been used directly in preparation of alkali-activated binders due to the high calcium, aluminum, and silicon oxides contents [15]. Silica fume with fine noncrystalline silica particles is produced at a temperature of 2000 C in silicon-based industries. The particle size is less than 1 μm, the bulk density is between 130 and 430 kg/m3, the specific gravity of 2.2 g/cm3, and the specific surface area is 15,000
30,000 m2/kg. This precursor is rich in silicon oxide. Since each cobinder is rich in some chemical components, the mentioned four cobinders along with soapstone talc were activated using an alkaline solution, which contains NaOH and sodium silicate. The mixtures were cured using a preheating method (60 C) in the first 24 hours, and then were placed in the controlled environmental conditions (23 C and 35% RH) until 28 days. Then, mechanical strength (compressive and flexural strength), drying shrinkage, and durability (acid attack, high temperature, carbonation, water absorption by immersion and capillary action) of alkali-activated soapstone binders were investigated.

29.2

Experimental plan

29.2.1 Materials and mix design The mix compositions were comprised of soapstone, sodium silicate, sodium hydroxide, and natural sand. In all mixtures, different cobinders such as MK, lime, RW, and silica fume were replaced by 20% in weight of soapstone. The physical and chemical properties of the used binders are listed in Table 29.1. As shown in Table 29.1, soapstone is rich in silicon dioxide, magnesium oxide, and sodium oxide. Enrichment in these chemical components does not necessarily result in obtaining acceptable mechanical strength, therefore different cobinders enriched in

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Table 29.1 Physical and chemical compositions of the used binders. Element/oxide (wt.%)

Soapstone

Metakaolin

Lime

Rock wool

Silica fume

SiO2 Al2O3 Fe2O3 Na2O K2O P2O5 TiO2 MgO SO3 CaO CrO3 MnO NiO Cl LOI (525 C) LOI (950 C) d10 (μm) d50 (μm) d90 (μm)

38.39 1.61 0.14 16.59 0.06 0.03 0.14 37.96 0.35 1.74 0.60 0.21 0.32
21.30
2.60 24.00 124.8

53.00 44.50 0.40 0.30 0.10 0.10 1.40






0.30 0.60 0.60 1.30 7.50

1.69 0.32 0.38
0.05

2.40 0.10 79.7
1.12
0.02
14.21 1.69 8.06 38.26

40.40 15.80 9.20 1.40 0.40 0.10 0.80 12.60
17.40



2.40
0.70 6.90 33.60

93.40 0.75 1.24 0.39 1.25
0.02 1.02
1.39



1.69 2.31 4.70 12.53 21.85

LOI, Loss on ignition.

other chemical components were used to assess the feasibility of increasing strength. MK has high silicon and aluminum oxide contents, lime is rich in calcium oxide, silica fume has a high amount of silicon oxide, and RW has high contents of silicon dioxide, magnesium oxide, aluminum oxide, and calcium oxide. The used alkali solution to activate the binders included sodium hydroxide (10 M) and sodium silicate (SiO2/Na2O 5 2.5), with the ratio of sodium silicate to sodium hydroxide being equal to 2.1. Standard sand was provided by the reviver sand with maximum and minimum sizes of 2 and 0.08 mm, respectively. According to the use of four different cobinders, four mixture compositions were prepared and assessed in this study, in which 20% of soapstone weight in the reference mixture was replaced by different cobinders in these mixtures. The dry ingredients were mixed for 3 minutes at minimum speed. Then, the alkali activator was added to the mixture and it was mixed for a further 3 minutes to achieve good homogeneity. Fresh mortar was filled in the molds (40 3 40 3 160 mm, 100 3 100 3 100 mm, 50 3 50 3 50 mm) and vibrated with using a jolting table for compaction. Then, the samples were cured in an oven at 60 C for 24 hours. After that, the specimens were demolded and placed at the ambient condition [24 C and relative humidity (RH) 35%] until the test date.

Figure 29.1 (A) UPV device, (B) adopted flexural test setup, (C) the used apparatus for measuring the drying shrinkage, (D) the test setup used for measuring water absorption by immersion and apparent porosity, (E) covered cubes with paraffin for water absorption by capillarity, (F) heating the beams in an oven.

Figure 29.1 Continued.

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29.2.2 Test procedures 29.2.2.1 Ultrasonic pulse velocity The ultrasonic pulse velocity (UPV) determines the quality of concrete as a nondestructive assessment by a pulse of the ultrasonic wave, which passes through the concrete. The velocity indicates the air voids or cracks, however, higher velocity indicates good quality of concrete. Pulse velocity of specimens were calculated using the following equation: V5

L T

(29.1)

where V refers to pulse velocity (m/s), L is the distance between two transducers (mm), and T denotes transmission time (μsec).

29.2.2.2 Flexural strength According to the ASTM C78 recommendation [16], the flexural test was carried out using a Zwick Z100 material testing machine. The flexural load applied on beams at a displacement rate of 0.6 mm/min and 200 kN load cell capacity was used, as indicated in Fig. 29.1B. The results were obtained from the average of three beams. The flexural strength of the prismatic beams was calculated using Eq. (29.2): σf 5

3FL 2bh2

(29.2)

where σf refers to flexural strength, F is the maximum flexural load, L is the span length, b is the width, and h is related to the height of the specimen.

29.2.2.3 Compressive strength The split prismatic beams during the flexural test were used for the compressive test, according to the ASTM C116-90 recommendation [17]. The displacement rate of 1.8 mm/min and a load cell with capacity of 200 kN was used. The results were obtained from the average of six tested samples from each mixture.

29.2.2.4 Drying shrinkage According to the ASTM C157 recommendation [18], the length changes of the specimens with dimension 40 3 40 3 160 mm were measured by a digital gauge (see Fig. 29.1C), and the measurement was continued until the length change became constant. The length change measurement started immediately after demolding. The results were obtained from the average of three beams.

Influence of co-binders on durability and mechanical properties

883

29.2.2.5 Water absorption by immersion Based on the ASTM C1585-04 recommendation [19], three cubes with edges of 50 mm for each mixture were initially dried at 100 C for 48 hours. Then, the samples were sunk in water for 48 hours. Afterward, the samples were removed from the water, and the saturated weight was measured. The results were obtained for the average of three tested samples for each mixture. Water absorption (Wm) is calculated based on the following equation: Wm 5

ðMs 2 Md Þ 3 100 Md

(29.3)

where Wm denotes the water absorption, Ms stands for saturated mass, and Md is dry mass. Moreover, three cubic specimens with dimensions of 50 3 50 3 50 mm were immersed into the water, and then weighing buoyancy mass (Mb) with a basket which was fixed to scale (see Fig. 29.1D). The apparent porosity of specimens was calculated using Eq. (29.4): Apparent porosity 5

ðMs 2 Md Þ 3 100 ðM s 2 Mb Þ

(29.4)

29.2.2.6 Water absorption by capillary The lateral surfaces of three cubes with edges of 100 mm were covered by paraffin. The coefficient of water absorption due to capillary action was measured based on BS EN1015
18:2002 recommendation [20]. Fig. 29.1E shows the set up adopted for capillary water absorption. The capillary water absorption coefficient was calculated using Eq. (29.5): ΔB Aw 5  pffi  A t

(29.5)

where, Aw denotes the capillary water absorption coefficient, ΔB is the absorbed mass, A is the surface area, and Ot is the time (min0.5).

29.2.2.7 Acid test An acid test was carried out by immersing the cubic samples (50 3 50 3 50 mm) in the acidic solution for 7 days, which contained 5% sulfuric acid. Cubes were dried at 105 C for 3 days and then weighed.

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29.2.2.8 High temperature Two prismatic beams (40 3 40 3 160 mm) from each mixture were exposed to 800 C for 3 hours. Then, heated beams were left at room temperature to cool down. The flexural and compressive strengths then were evaluated. Fig. 29.1F shows the beams that experienced and did not experience high temperature.

29.2.2.9 Carbonation resistance Three prismatic beams with dimensions of 40 3 40 3 160 mm from each mixture composition were used to determine the carbonation resistance. The specimens were placed in the carbonation chamber for 7 days, where CO2 gas was circulated with a concentration of 5%, 60% RH, and temperature 23 C. After that, the impacts of carbonation were investigated by assessing the flexural and compressive strengths.

29.2.2.10 Thermogravimetric analysis and differential thermal analysis A Precisa PrepASH 129 analyzer was used to perform these analyses. Samples were prepared as powders and were placed in alumina crucibles and tested at an argon atmosphere and a heating rate of 10 C/min. Mass loss was recorded by the gradual increase of temperature during the test.

29.2.2.11 X-ray diffraction This analysis was performed to investigate the crystalline phases of the specimens and the effects of different cobinders on the phase identification of the crystalline. This mineralogical assessment was carried out using a Bruker D8 Advance X-ray diffractometer ˚ ) at 40 mA and 40 kV. The scanning process of with Ni-filtered Cu kα radiation (1.54 A each sample from 13 to 80 degrees was executed at a scan rate of 0.5 seconds per step.

29.3

Results and discussion

29.3.1 Ultrasonic pulse velocity As indicated in Fig. 29.2, the replacement of only 20% soapstone with different cobinders significantly affected the UPV. The minimum and maximum recorded UPV were approximately 1950 and 3500 m/s in the mixtures containing lime and silica fume, respectively. All cobinders increased the UPV compared to the reference mixture with 100% soapstone. This means that these cobinders influence the air voids with packing impacts of particle size distribution and chemical reactions. It is shown in Section 29.3.6, that the replacement of soapstone with lime does not increase the formation of the products due to the chemical reactions.

Influence of co-binders on durability and mechanical properties

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Figure 29.2 Effects of different cobinders on the UPV.

29.3.2 Compressive and flexural strengths The impacts of using different cobinders on the mechanical strength of alkaliactivated soapstone based binders are shown in Fig. 29.3. According to the results, except lime, the addition of cobinders increased flexural strength, so that the maximum increase of the flexural strength was recorded around 4.5 times in the alkaliactivated binders with MK compared to the alkali-activated binders with 100% soapstone. Afterward, the addition of rock wool provided the maximum increase of the flexural strength (3 times). A similar trend was observed for the compressive strength, although this enhancement was greater in the compressive strength than the flexural strength. The maximum increase of the compressive strength was more than 5 times in the alkali-activated binders with MK compared to the alkali-activated binders with 100% soapstone. Then, adding rock wool registered the maximum increase of the compressive strength (3 times). Despite providing lower flexural strength in alkaliactivated binders with lime than the reference mixture, lime could increase the compressive strength higher than the reference mixture (  20%). Differences in the strength due to the use of different cobinders arise from differences in the chemical reactions and products, and particle packing impacts. As indicated in Fig. 29.4, similar crystallinity was observed in alkali-activated soapstone binders with different cobinders after 28 days aging, although the addition of lime also introduced albite in the crystalline phases. The major identified crystalline phases were quartz (SiO2), talc [Mg3 Si4O10(OH)2], magnesite [Mg (CO3)], magnetite (Fe3O4), and albite (NaAlSi3O8).

29.3.3 Drying shrinkage Since the used cobinders are rich in different chemical components, their incorporations in alkali-activated magnesium aluminosilicate binders from soapstone could affect differently the drying shrinkage (see Fig. 29.5). Mastali et al. showed that the molar ratio of calcium to silicon dioxide has the main responsibility of drying

Figure 29.3 Impacts of using different cobinders on (A) flexural strength and (B) compressive strength.

Influence of co-binders on durability and mechanical properties

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Figure 29.4 Effects of different cobinders on X-ray diffraction curves (Q, quartz (SiO2), T, talc (Mg3 Si4O10(OH)2), M, magnesite (Mg (CO3)), Mt, magnetite (Fe3O4), A, albite (NaAlSi3O8)).

Figure 29.5 Drying shrinkage of alkali-activated soapstone binders containing different cobinders.

shrinkage (60%
80% of drying shrinkage) [7]. Then, aluminum to silicon dioxide had high impact on the drying shrinkage (12%
30% of drying shrinkage). These chemical ratios affect the fine pore structure and its effects on the tensile stresses of capillary pores, the strengths, and elastic modulus [7]. Interestingly, it was observed

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New Materials in Civil Engineering

that the recorded drying shrinkage was in the line of strength. Thus, the minimum and maximum drying shrinkage were registered for alkali-activated soapstone binders with MK (5000 μm/m) and lime (22,000 μm/m), respectively. However, the registered drying shrinkage for alkali-activated soapstone binders with rock wool and silica fume are similar (18,000 μm/m) and relatively high. According to the recorded drying shrinkage reported in Reference [7], the drying shrinkage in alkaliactivated binders is limited to the maximum (16,000 μm/m), regardless of curing type, precursor types, and alkali solutions. It could be concluded that there is a high possibility of cracks forming on the surfaces of specimens, therefore it is recommended to use different fibers as one of the easiest solutions to improve mechanical properties and mitigate drying shrinkage.

29.3.4 Water absorption by immersion and capillary These assessments clarify indirectly the influences of using different cobinders on total porosity and the pore network of the mixtures. According to the results indicated in Fig. 29.6, the maximum and minimum total porosity were recorded in alkali-activated soapstone binders with MK (with apparent porosity of 25%) and silica fume (with apparent porosity of 5%), respectively. The obtained results for the pore network (evaluated by water immersion by capillary action) were in the line of total porosity. This means that using different cobinders has a great impact on the pore structure of these alternative binders. For the plain fly ash-based alkaline mortars, the capillary coefficient was reported to be in the range of 0.18
0.51 kg/m2 min0.5 [21,22], therefore it could be concluded that the permeability of alkali-activated soapstone binders with cobinders is relatively low (0.03
0.14 kg/m2 min0.5) and comparable to reinforced one-part alkali-activated slag-based binders with capillary coefficients of 0.14
0.17 kg/ m2 min0.5 [23]. It is worth stating that exposing specimens made with alkaliactivated soapstone binders containing lime to water led to them being destroyed and it was impossible to measure water absorption by immersion and capillary action, as shown in Fig. 29.6E.

29.3.5 Acid resistance The mechanism of deterioration in this evaluation involves with decalcification of C
S
H and the formation of soluble salt calcium acetate, therefore this damage becomes more intensified in alkali-activated binders with a higher amount of calcium and this chemical component plays a critical role in acid resistance [24]. Fig. 29.7 illustrates the mass loss of the mixtures exposed to 5% sulfuric acid for 7 days. According to the results, the maximum mass loss was recorded as more than 90% in alkali-activated soapstone binders with lime. Since the calcium content in other mixtures with cobinders was low, acid attack had no great effect on mass loss (maximum 3%).

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Figure 29.6 Effects of using different cobinders on (A) water absorption by immersion, (B) apparent porosity; (C) water absorption by capillary; (D) capillary coefficients; (E) destroyed specimens made with lime as cobinder.

Figure 29.7 Mass loss when exposed to 5% sulfuric acid.

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29.3.6 High temperature Exposing alkali-activated soapstone binders containing different cobinders to high temperature results in two different impacts, which could either increase or decrease the strength: (1) increasing the chemical reactions of the unreacted particles increases the strength; (2) damaging the bonding properties of the gels formed in the matrix decreases the strength. Therefore and increase or decrease of the strength depends on which scenario dominates. Fig. 29.8 displays the high temperature resistance of the mixtures. Regarding these results, the used cobinders had a significant role on the increase or decrease of the strength against high temperature. Replacing

Figure 29.8 Temperature resistance of alkali-activated binders with different cobinders: (A) flexural strength; (B) compressive strength.

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MK reduced the strength (  35% for the flexural strength and  20% for the compressive strength), while the maximum increases of the compressive and flexural strength were recorded at  70% and more than 2 times in the mixtures with RW and silica fume, respectively. Despite the strength loss of the mixtures containing MK, this mixture indicated acceptable strength even after experiencing high temperature ( . 7 MPa for the flexural strength and  20 MPa for the compressive strength). Mass loss and the formed chemical products under the elevated temperature determined with executing thermogravimetric analysis (TGA) and DTG, respectively, and their results are shown in Fig. 29.9. In general, the mixtures lost a total of around 12%
13% of their initial mass. Moreover, it was revealed that the minimum and maximum mass lost were observed in the mixtures with MK and silica fume, respectively. Mass losses in TGA could be considered in temperatures from 100oC to 800oC. During the first stage, both free water and structurally bonded water are% available% in the composition. Then, the free water could be evaporated up to 100oC, and the weight loss from 100oC to 800oC is attributed to the structural water.% Regarding % endothermic % DTA curves, three major peaks at around 190oC, 580oC, and 780oC % % dehydration % were detected; the large shoulder just below 200oC is attributed to the % of the calcium-rich silicate gel [25]. The second and third destruction phases could be attributed to the decomposition of portlandite [Ca(OH)2] and calcium carbonate or calcite (CaCO3) [26]. Moreover, the DTG curves showed that the reduction of the peak corresponding to the calcium-rich silicate gel was responsible for the generation of Ca(OH)2.

29.3.7 Carbonation resistance Fig. 29.10 depicts the effects of carbonation on the mechanical strength. Exposing alkali-activated materials could either increase or decrease the strength. During the carbonation reaction of alkali-activated soapstone binders with different cobinders, there is enough solid sodium silicate in the polymerization reactions and consumption of the OH2 in the pore liquid could dissolve rapidly and release a large amount of Ca21, OH2, Na1, and SiO223, which can react with a large amount of CO2 and consequently form H2O, CaCO3, NaSO3, and C
S
H gel [27]. The formation of these crystals could introduce internal stresses to the matrix. If there is enough space in the matrix to fill the gaps by these formed crystals, the strength will be increased. Otherwise, the imposed internal stresses form cracks and the strength reduces. According to the results, the formation of crystals (depending on the cobinders these could be mainly MgCO3, NaCO3, SiCO3, and CaCO3) increased both compressive and flexural strength. The increase of the flexural strength was varied in the range of 70 (for MK) to 200% (for silica fume), also, the increase for the compressive strength was from 10 (for MK) to 200% (for silica fume).

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Figure 29.9 Influences of different cobinders on: (A) TGA; (B) DTG.

29.4

Conclusions

This chapter presents the experimental results regarding the influences of replacing soapstone with different cobinders (MK, lime, RW, and silica fume) on the mechanical strength and durability of alkali-activated magnesium aluminosilicate binders from soapstone. The following conclusions can be made based on the results: 1. Replacement of cobinders increases UPV and decreases air voids, with the maximum and minimum increase of UPV observed for silica fume and lime, respectively.

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Figure 29.10 Effects of carbonation on: (A) flexural strength; (B) compressive strength. 2. Except lime, all cobinders increase the mechanical strength. The maximum increase of strength was recorded for MK. 3. Drying shrinkage and total porosity are governed significantly by the type of cobinder, so that the minimum and maximum drying shrinkage were registered for alkali-activated soapstone binders with MK (5000 μm/m) and lime (22,000 μm/m), respectively. 4. The maximum and minimum total porosity were recorded in alkali-activated soapstone binders with MK (with apparent porosity of 25%) and silica fume (with apparent porosity of 5%), respectively. 5. Alkali-activated soapstone binders with cobinders provided very low capillary coefficients in the range of 0.03 (for silica fume) to 0.14 (for MK) kg/m2 min0.5. 6. The maximum mass loss due to acid attack was recorded for alkali-activated soapstone binders with lime (  90%), while mass loss for other cobinders was less than 3%.

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7. The type of cobinder used plays a critical role in high temperature resistance of alkaliactivated soapstone binders. Using RW increases the mechanical strength under high temperature, while replacing MK decreases this strength. 8. Regardless of the cobinder type used, mechanical strengths of alkali-activated soapstone binders increase under carbonation. The maximum and minimum increases of strength observed were due to the use of silica fume and MK, respectively.

Acknowledgment This study received funding from GEOBIZ project, grant ID: 1105/31/2016.

References [1] E. Worrell, L. Price, N. Martin, C. Hendriks, L. Ozawa Meida, Carbon dioxide emission from the global cement industry, Annu. Rev. Environ. Resour. 26 (2001) 303
329. [2] J. Provis, Alkali-activated materials, Cem. Concr. Res. 114 (2018) 40
48. [3] T. Luukkonen, Z. Abdollahnejad, J. Yliniemi, P. Kinnunen, M. Illikainen, One-part alkali-activated materials: a review, Cem. Concr. Res. 103 (2018) 21
34. [4] F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali, Alkali-activated binders: a review part 1. Historical background, terminology, reaction mechanisms and hydration products, Constr. Build. Mater. 22 (2008) 1305
1314. [5] F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali, Alkali-activated binders: a review. Part 2. About materials and binders manufacture, Constr. Build. Mater. 22 (2008) 1315
1322. [6] M.C.G. Juenger, F. Winnefeld, J. Provis, J.H. Ideker, Advances in alternative cementitious binders, Cem. Concr. Res. 41 (2011) 1232
1243. [7] M. Mastali, P. Kinnunen, A. Dalvand, R. Mohammadi Firouz, M. Illikainen, Drying shrinkage in alkali-activated binders—a critical review, Constr. Build. Mater. 190 (2018) 533
550. [8] K.J.D. MacKenzie, S. Bradley, J.V. Hanna, M.E. Smith, Magnesium analogues of aluminosilicate inorganic polymers (geopolymers) from magnesium minerals, J. Mater. Sci. 48 (2013) 1787
1793. [9] T. Luukkonen, Z. Abdollahnejad, J. Yliniemi, M. Mastali, P. Kinnunen, M. Illikainen, Alkali-activated soapstone waste—mechanical properties, durability, and economic prospects, Sustain. Mater. Technol. 22 (2019). Available from: https://doi.org/10.1016/j. susmat.2019.e00118. [10] Z. Abdollahnejad, T. Luukkonen, M. Mastali, P. Kinnunen, M. Illikainen, Development of alkali-activated magnesium aluminosilicate binders from soapstone, in: Sustainable Materials, Systems and Structures, Rovinj, Croatia, 2019. [11] L. Chen, Z. Wang, Y. Wang, J. Feng, Preparation and properties of alkali activated metakaolin-based geopolymer, Materials 9 (2016) 1
12. Available from: https://doi. org/10.3390/ma9090767. [12] P. Rovnanik, Effect of curing temperature on the development of hard structure of metakaolin-based geopolymer, Constr. Build. Mater. 24 (2010) 1176
1183.

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[13] C.H. Chen, R. Huang, J.K. Wu, C.C. Yang, Waste E-glass particles used in cementitious mixtures, Cem. Concr. Res. 36 (2006) 449
456. [14] V.S. Ramachandran, J.J. Beaudoin, Handbook of Analytical Techniques in Concrete Science and Technology: Principles, Techniques and Applications, first ed., William Andrew, 2001. [15] A. Alzaza, M. Mastali, P. Kinnunen, L. Korat, Z. Abdollahnejad, V. Ducman, et al., Production of lightweight alkali activated mortars using mineral wools, Materials 12 (10) (2019) 1695. Available from: https://doi.org/10.3390/ma12101695. [16] ASTM C78/C78M-18, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading), ASTM International, West Conshohocken, PA, 2018, ,www.astm.org.. [17] ASTM C116-90, Test Method for Compressive Strength of Concrete Using Portions of Beams Broken in Flexure (Withdrawn 1999), ASTM International, West Conshohocken, PA, 1990, ,www.astm.org.. [18] ASTM C157/C157M-17, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, ASTM International, West Conshohocken, PA, 2017, ,www.astm.org.. [19] ASTM C1585-04, Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes, ASTM International, West Conshohocken, PA, 2004, ,www.astm.org.. [20] BS EN 1015-18:2002, Methods of Test for Mortar for Masonry. Determination of Water Absorption Coefficient Due to Capillary Action of Hardened Mortar, BSI, UK. [21] M. Mastali, Z. Abdollahnejad, F. Pacheco-Torgal, Fly ash alkaline-based mortars containing waste glass and recycled aggregates submitted to accelerated carbon dioxide curing, Resour. Conserv. Recyc 129 (2018) 12
19. [22] Z. Abdollahnejad, F. Pacheco-Torgal, J.B. Aguiar, Cost-efficient one-part alkaliactivated mortars with low global warming potential for floor heating systems applications, J. Eur. J. Environ. Civ. Eng. (2016). Available from: https://doi.org/10.1080/ 19648189.2015.1125392. [23] Z. Abdollahnejad, M. Mastali, M. Falah, K. Mohammad Shaad, T. Luukkonen, M. Illikainen, Durability of the reinforced one-part alkali-activated slag mortars with different fibers, Waste Biomass Valori., 2020. https://doi.org/10.1007/s12649-020-00958-x. [24] T. Bakharev, J.G. Sanjayan, Y.B. Cheng, Resistance of alkali-activated slag concrete to acid attack, Cem. Concr. Res. 33 (2003) 1607
1611. [25] L. Alarcon-Ruiz, G. Platret, E. Massieu, A. Ehrlacher, The use of thermal analysis in assessing the effect of temperature on a cement paste, Cem. Concr. Res. 35 (2005) 609
613. [26] F.U.A. Shaikh, S.W.M. Supit, Mechanical and durability properties of high volume fly ash (HVFA) concrete containing calcium carbonate (CaCO3) nanoparticles, Constr. Build. Mater. 70 (2014) 309
321. [27] G. Huang, Y. Ji, J. Li, Z. Hou, C. Jin, Use of slaked lime and Portland cement to improve the resistance of MSWI bottom ash-GBFS geopolymer concrete against carbonation, Constr. Build. Mater. 166 (2018) 290
300.

Fly ash utilization in concrete tiles and paver blocks

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S.K. Sahu, S. Kamalakkannan and P.K. Pati Department of Civil Engineering, NIT Rourkela, Odisha, India

30.1

Introduction

Industrialization and urbanization are two continuous phenomena that are unabated all over the world. Their negative impact on the global environment has resulted in the depletion of natural resources due to the rapid growth of the construction industry. Overexploitation of mineral resources is harming the environment, causing deficiencies, endangering society, and advancing toward an unsustainable future. Also, the disposal, management, and utilization of industrial waste produced are of utmost importance. A major concern is the production of enormous quantities of fly ash due from coal-based power generation around the world, but mostly in developing nations. The industrial world produces huge quantities of fly ash, which is a waste from thermal power plants and that needs to be disposed of safely, otherwise it can have disastrous consequences. Disposal of fly ash is a major issue in India due to the scarcity of land, pollution of ground water due to leaching, and health issues faced by people living near dump sites. All these adverse situations that have emerged due to the generation of fly ash are the cause of severe concerns to environmentalists all across the globe. The production of fly ash in India is approximately 200 million tons per year, and currently only 56% of this is utilized, therefore a significant improvement is needed. The effective utilization of fly ash is restricted to the manufacture of bricks, use in pavements, and fly ash-based pozzolanic cement production. Fly ash can be effectively used in concrete paver blocks and concrete tiles, replacing conventional construction materials such as cement and fine aggregate. Concrete tiles and paver blocks are used for various purposes like paving of approaches, paths, parking places, and footpaths. Paver blocks may also find their application in roadways as precast pavements. The increasing costs of construction and building materials, and the need to meet sustainable development, mean that alternative construction methods, techniques, and materials are being used extensively to replace old conventional methods. So as to enhance and improvise the applications of concrete tiles and paver blocks, proper knowledge and understanding of the behavior of the products produced, many of which are produced indigenously, is essential. Society’s expanding reliance on sand has resulted in its depletion, causing real difficulties in construction area which need to be addressed. The studies involving fly ash utilization as a fine aggregate substitution in concrete are inadequate. New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00030-2 © 2020 Elsevier Inc. All rights reserved.

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Siddique [1] replaced fine aggregate with class F fly ash and studied the split tensile, compressive, and flexural behavior of concrete specimens up to 50% replacement level. Test results for abrasion characteristics and compressive strength of concrete were studied by Siddique [2] when sand was substituted with class F fly ash. Parvati and Prakash [3] studied the properties of concrete subjected to elevated temperatures of up to 800
C for partial replacement of fine aggregate with fly ash. The test results showed an increase in shear strength, flexural strength, and compressive strength of the concrete. Singh and Siddique [4] studied the strength and microstructural properties of concrete, which contained coal bottom fly ash as a substitute for fine aggregate. The use of coal bottom fly ash increases the watercement ratio in order to obtain same workability on a level with that of conventional concrete. Kanthi and Kavitha [5] replaced sand with fly ash and examined the results for various strength behaviors of concrete such as flexural strength, modulus of elasticity, and compressive strength. Deo and Pofale [6] published a report on sand replacement with fly ash based on the maximum density and minimum voids method. Fly ash is a highly pozzolanic material, having substantial potential to replace Portland cement, which is a major producer of carbon dioxide and thereby reducing greenhouse gas emissions. Christy and Tensing [7] tested the strength properties of cement mortar in which cement was substituted partially with fly ash by up to 30%, with intervals of 5%. Pathak and Siddique [8] studied the properties of selfcompacting concrete with 30%50% class F fly ash as cement replacement material, when it was subjected to elevated temperature. Huang et al. [9] studied the mechanical properties of concrete containing fly ash in high volumes. A mix design was done for concrete containing 20%80% fly ash for cement replacement. From the test results it was concluded that the setting time and air content were increased as the fly ash percentage was increased. Experimental investigations were carried out by Bahedh and Jaafar [10] to study the permeability and mechanical behavior of ultrahigh-performance concrete (UHPC). The concrete specimens were subjected to autoclaving. The effect of various percentages of fly ash and different autoclaving times were studied. The mechanical properties of fly ash concrete were examined using different percentages of fly ash by Khan and Ali [11]. Efforts were made to enhance the concrete properties with the addition of coconut fibers. Stressstrain curves and load time curves were studied to analyze the compressive, flexural, and split tensile behaviors. A great deal of literature is available on the utilization of fly ash in structural concrete, the manufacture of pozzolanic cement, and as a replacement for fine aggregate. However, when it comes to the application of fly ash in concrete tiles, the available literature is very limited. Mishulovich and Evanko [12] conducted their research on the effect of fly ash use with moderately high carbon content on ceramic tiles. The process involved subjecting the tiles to a temperature that would be sufficient for the oxidation of carbon before the formation of a liquid phase. Partial replacement of cement and sand with fly ash in tile adhesive was investigated by Andic¸ et al. [13]. The statistical relationship between tensile adhesion,

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flexural strength, and compressive strength was studied. Rajamannan et al. [14] studied the effects of the addition of fly ash in ceramic tiles and examined their mechanical properties. The use of fly ash in small amounts improved the strength parameters of the tiles. When it comes to combining beauty and functionality, no product outshines cement concrete tiles. They last far longer than ceramic tiles and can be customized and also provide versatility and flexibility. Cota et al. [15] studied the properties of concrete tiles when quartz aggregate was replaced with waste glass and Portland cement was replaced with metakaolin. Permeability, dynamic modulus, bulk modulus, and length changes were examined due to alkali silica reaction expansion. Metakaolin was used as a replacement for cement in order to achieve lower alkali silica reaction expansion. Wang et al. [16] utilized high alumina fly ash (HAFA) in ceramic tiles and studied the influence of HAFA and sintering temperature on tile properties such as bulk density, flexural strength, and apparent porosity. Also, studies related to the application of fly ash in concrete paver blocks is much less. Paver blocks are increasingly used in office and residential complexes. They are fully engineered products laid with restrained edges and having a granular bedding coarse offering outstanding strength and durability. They are esthetically pleasing, comfortable to walk on, trafficable, and are used in footways, parking bays, drive ways, etc. Karasawa et al. [17] investigated the use of fly ash as a fine aggregate in paver blocks and analyzed its plastic deformation while also measuring the demolding, curing, and flexural strength. Poon and Chan [18] utilized recycled concrete and crushed clay brick as aggregate to produce paver block and examined their compressive strength and tensile strength. Uygunoglu et al. [19] discussed the strength and durability properties of paver block, in which crushed sandstone was replaced with concrete waste and marble waste, and cement was replaced with fly ash. Ceramic tile polishing waste was utilized by Penteado et al. [20] as a replacement for cement and sand in paver blocks. The specimens were subjected to a porosity test, water absorption test, and compression test to analyze the behavior of the concrete. The above survey shows that a significant amount of work has been done on the utilization of fly ash as a replacement for cement and sand. However, research on the utilization of fly ash in the fabrication of concrete tiles and paver blocks has yet to be carried out. Very few studies have focused on the use of fly ash in paver blocks. Abrasion and durability aspects of paver blocks also remain to be covered. Therefore this study was conducted to assess the feasibility of fly ash in high amounts in concrete tiles and paver blocks without affecting the strength parameters as per Indian Standard Specifications. The scope of this study includes preparing mix designs for different percentages of fly ash as a replacement for cement and fine aggregate and evaluating the compressive strength, flexural strength, water absorption, and durability aspects for paver blocks and wet transverse strength and water absorption concrete tiles. Also, the suitability of fly ash as an innovative green construction method will be recommended for road construction purposes.

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30.2

New Materials in Civil Engineering

Experimental procedure

30.2.1 Materials Portland slag cement, which is most commonly used in the region, was used for the study complying with IS 455 [21]. Its specific gravity was 2.87 having initial and final setting times of 70 minutes and 480 minutes, respectively. For fine aggregate, natural sand was used, having a maximum size of 4.75 mm. Its specific gravity was 2.72, conforming to Zone-III, and having a fineness modulus of 2.313. Coarse aggregates used in this study were of 10 mm nominal size, having a specific gravity of 2.72. Both aggregates comply with IS 383 [22]. The grading of both fine and coarse aggregates and their mixture is shown in Table 30.1. Fly ash is classified into two types for use as a mineral admixture as per ASTM C 618 [23]. These are: (1) class ‘F’ fly ash and (2) class ‘C’ fly ash. The major distinction between these classes is the percentages of silica, alumina, and iron and calcium content. Fly ash from two sources was used for present study. Class F fly ash procured from a local source was used in the manufacturing of tiles and paver blocks. The fly ash was subjected to XRD test to obtain its chemical composition. With the help of X’pert-Highscore software, a graph was drawn to analyze the compounds present in the fly ash. The result of the XRD test of fly ash is shown in Fig. 30.1. From the graph it can be seen that silicon oxide is predominantly present in the sample, which mainly contributes to the hydration products. Some percentages of ferric oxide, calcium carbonate, and alumina are also present.

30.2.2 Mix design Generally, for the manufacture of precast concrete paver blocks, dry, low slump mixes are needed. Mix design was done for control mix of M40 grade concrete using IS10262 [24], as per the specifications given by IS 15658 [25] for medium traffic condition, which provided a mix proportion of 1:2.25:2.25 with a water cement ratio of 0.40 and superplasticizer “Conplast SP430 G8 (FOSROC)” conforming to IS 9103 [26]. In this study, two types of replacement were done, cement Table 30.1 Particle size distribution of aggregates and their mixture. Sieve size (mm)

Fine aggregate

Coarse aggregate

Mixture

20 10 4.75 2.36 1.18 0.6 0.3 0.15

100 100 100 98.5 92.5 58.9 16.3 2.9

100 26.08 2.52 0 0 0 0 0

100 63.04 51.26 49.25 46.25 29.95 8.15 1.45

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Figure 30.1 XRD test of fly ash.

Table 30.2 Proportion of fresh concrete mixture for sand replacement with fly ash. Mixture no.

Cement (kg/m3 Þ

Fly ash (%)

Fly ash (kg/m3 Þ

W/C ratio

Sand (kg/m3 Þ

Coarse aggregate (kg/m3 Þ

Superplasticizer (%)

SR-0 SR-10 SR-20 SR-30 SR-40

416.0 416.0 416.0 416.0 416.0

0 10 20 30 40

0 93.65 187.30 280.95 374.60

0.4 0.4 0.4 0.4 0.4

936.50 842.85 749.20 655.55 561.90

936.5 936.5 936.5 936.5 936.5

0.5 1.0 1.1 1.15 1.2

replacement and fine aggregate replacement, where the percentage of fly ash varied from 0% to 40%, at intervals of 10%. The proportion of both coarse and fine aggregate was 50%. The details of concrete mixes with replacement of sand and cement for paver blocks are presented in Tables 30.2 and 30.3, respectively. For casting of tiles, Portland pozzolana cement conforming to IS 1489 [27] was used. The proportions of 1:2:4 (cement:fine aggregate:coarse aggregate) was adopted as standard mix with a watercement ratio of 0.5. Casting of the tiles was done using a suitable rubber mold of required thickness and surface area by vibration table.

30.2.3 Manufacturing of paver blocks and tiles Cement, sand, coarse aggregate, water, and superplasticizer were mixed thoroughly in the concrete mixer. Then it was filled in rubber paver molds and tile molds of

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Table 30.3 Proportion of fresh concrete mixture for cement replacement with fly ash. Mixture no.

Cement (kg/m3 Þ

Fly ash (%)

Fly ash (kg/m3 Þ

W/C ratio

Sand (kg/m3 Þ

Coarse aggregate (kg/m3 Þ

Superplasticizer (%)

CR-0 CR-10 CR-20 CR-30 CR-40

416.0 374.4 332.8 291.2 249.6.0

0 10 20 30 40

0 41.6 83.2 124.8 166.4

0.4 0.4 0.4 0.4 0.4

936.5 936.5 936.5 936.5 936.5

936.5 936.5 936.5 936.5 936.5

0.5 1.15 1.2 1. 1.2

Table 30.4 Paver block and tile details. SI no.

Shape

Thickness (mm)

Plan area (m2 Þ

Length (cm)

Width (cm)

1 2 3 4 5 6 7

Zig zag I Dumbbell Zig zag Pentagon Rectangle Rectangular Tile

80 60 60 60 80 60 28

0.046 0.033 0.036 0.046 0.041 0.046 0.096

30 22.5 26.5 30 24 22.5 31

15 12.5 11 15 20 20 31

different shapes and different thicknesses. All the filled paver molds and tile molds were vibrated using a table vibrator. After casting, all the specimens were finished with a steel trowel and were kept for 24 hours. After 24 hours they were demolded from the paver molds and kept in a water tank for curing. For replacement of cement and sand with fly ash the same procedure was repeated. The sizes and specifications of paver blocks and tiles are shown in Table 30.4. Cast paver blocks and tiles are shown in Fig. 30.2A and B respectively.

30.2.4 Test methods After curing, the paver blocks and tiles were subjected to various testing to determine their strength parameters. The tests were performed following the relevant Indian standards. For paver blocks, compressive strength, flexural strength, water absorption, and durability tests were performed following IS 15658 [25]. For concrete tiles, wet transverse strength and water absorption tests were performed following IS 1237 [28]. The compressive strength of paver blocks was determined at 7 and 28 days using a universal testing machine (UTM) as per IS 15658 [25]. The average strength of four samples at 28 days was taken as the apparent compressive strength of the paver block. The apparent compressive strength of paver block was multiplied with the

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Figure 30.2 (A) Cast paver blocks; (B) cast tiles.

correction factor as described in IS 15658 [25] to obtain the paver block compressive strength. The flexural strength of paver blocks for control mix and for different percentages of sand and cement replacement with fly ash were done as per IS 15658 [25], implementing a three-point bending test. The arrangements for compressive strength and flexural strength test for paver block are shown in Figs. 30.3 and 30.4, respectively. For water absorption, the tile specimens were immersed in

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Figure 30.3 Compressive strength test set up.

water for 24 hours. Saturated weight was considered and oven dried weight was taken after keeping it in the oven at a temperature of 107
C 6 7
C. A freezethaw test was done to determine the durability aspects of the paver blocks. First the specimens were oven dried for 24 hours and the dry weights of the specimens were noted. Then the specimens were completely immersed in 3% sodium chloride solution for 24 hours at a temperature of 23
C 6 3
C. After 24 hours saturation, the specimens were subjected to continuous freezethaw cycles, with one freezethaw cycle taking 24 hours. The cycle consists of 16 6 1 hour of freezing at a temperature of 215
C 6 2
C, followed by 8 6 1 hour of thawing at a temperature of 23
C 6 3
C. After completion of 50 cycles, the specimens were washed with 3% sodium chloride solution to remove spalled particles from the specimen. These spalled particles were filtered and dried. The dry weight of the spalled particles was noted. This was defined as weight loss and is expressed as a percentage of the initial dry weight of the specimen. The freezing chamber is shown in Fig. 30.5 and thawing chamber in Fig. 30.6. The wet transverse strength is one of the physical requirements that needs to be satisfied and represents the stress a concrete tile can withstand in bending. The wet transverse strength of the cement concrete tiles was conducted according to the

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Figure 30.4 Flexural strength test set up.

procedure given in IS 1237 [28]. A MOR (modulus of rupture) testing machine was used for this test and the test set up is shown in Fig. 30.7. Ply wood padding of 3 mm thickness and 20 mm wide was placed between the tiles and each of the supports. The load was applied gradually until the tile broke. The load causing the tile to break was recorded and the wet transverse strength was calculated. For water absorption, the tile specimens were immersed completely in clean water for 24 hours. After 24 hours the specimens were removed from the water and wiped dry and weighed immediately to obtain the saturated mass. The tiles were reweighed after keeping them in an oven at a temperature of 65
C 6 1
C for 24 hours.

30.3

Results and discussion

To analyze the effects of fly ash as a replacement for cement and sand in a higher grade of concrete, concrete tiles and paver blocks were cast and tested for different strength parameters. The analysis of the test results is given below.

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Figure 30.5 Sample inside the freezing chamber.

30.3.1 Test results for sand replacement with fly ash Paver blocks were tested for compressive strength, flexural strength, water absorption, and freezethaw for M40 grade with different shapes and thicknesses. The shape of the paver blocks considered for compressive strength were zig zag (60 mm), zig zag (80 mm), I (60 mm), dumbbell (60 mm), rectangle (60 mm), and pentagon(80 mm).

30.3.1.1 Compressive strength The 7- and 28-day compressive strengths of paver blocks containing fly ash as a sand replacement material are presented in Figs. 30.8 and 30.9, respectively. It is observed that 7- and 28-day compressive strength of all the paver blocks containing fly ash up to 30% is more than the compressive strength of paver blocks without fly ash. When sand is replaced with 10% fly ash, 14.51%, 5.05%, 29.03%, 1.64%, 29.60%, and 23.82% increases in strength are noticed for zig zag (60 mm), zig zag (80 mm), I shape (60 mm), pentagon (80 mm), dumbbell (60 mm), and rectangle (60 mm) shapes, respectively. Seven-day compressive strength of paver blocks for

Fly ash utilization in concrete tiles and paver blocks

Figure 30.6 Sample inside the thawing chamber.

Figure 30.7 Modulus of rupture testing machine.

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Figure 30.8 Sand replacement with fly ash versus 7-day compressive strength of paver block.

Figure 30.9 Sand replacement with fly ash versus 28-day compressive strength of paver block.

all shapes are more than the required target strength with up to 30% sand replacement. That means earlier strength gain is more than that of getting later strength in replacement of sand with fly ash. The maximum compressive strength for all the shapes is noted at 10% replacement of cement with fly ash. Based on the results of this study, the optimum percent of fly ash to be added to enhance the compressive strength is 30%, as the 28-day compressive strength of paver blocks having 30% fly ash for all shapes and thicknesses is above the target strength of 44.13 MPa.

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Figure 30.10 Sand replacement with fly ash versus 7-day flexural strength of paver block.

Figure 30.11 Sand replacement with fly ash versus 28-day flexural strength of paver block.

30.3.1.2 Flexural strength The effects of sand replacement with fly ash on 7- and 28-day flexural strength of concrete paver blocks are presented in Figs. 30.10 and 30.11, respectively. It is observed from Fig. 30.10 that 7-day flexural strength increases as sand replacement increases up to 30%, and after that it decreases. However, 28-day flexural strength increases up to 10% replacement, and after that it decreases as the percentage of replacement increases. Although there is a decrease in flexural strength at 28 days

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afterp10% ffiffiffiffiffi replacement of sand, it is more than the required flexural strength, that is, 0.7 fck 5 4.43 MPa as per IS 456 [29].

30.3.1.3 Water absorption Water absorption of paver blocks for sand replacement with fly ash for different shapes and different thicknesses is presented in Fig. 30.12. From Fig. 30.12 it is observed that water absorption of paver block for all shapes with 10%30% fly ash is less than the water absorption for concrete tiles. For 40% sand replacement, water absorption for all the shapes of paver blocks is greater than the water absorption percentages at 0% sand replacement, however it is less than the prescribed limit, that is, 6% as mentioned in IS 15658 [25]. Water absorption being related to the pore system of hardened concrete proves that the incorporation of fly ash up to 30% fills maximum pores of the hardened concrete, making the paver blocks less porous.

30.3.1.4 Freezethaw durability The durability of the paver blocks for different percentages of sand replacement were tested by a freezethaw test as per IS 15658 [25]. The test results are shown in Fig. 30.13. The maximum weight loss of blocks was 0.36% for 30% sand replacement, whereas the minimum percentage of weight loss of 0.085 occurred at 0% sand replacement. As per IS 15658 [25], the weight loss after test cycles should not be more than 1% of the initial weight. Thus the specimens are considered to satisfy the durability aspects.

Figure 30.12 Sand replacement with fly ash versus water absorption of paver block.

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Figure 30.13 Sand replacement with fly ash versus weight loss due to the freezethaw cycle.

Figure 30.14 Cement replacement with fly ash versus 7-day compressive strength.

30.3.2 Test results for cement replacement with fly ash Paver blocks were tested for compressive strength, flexural strength, and water absorption for M40 grade concrete with different shapes and thicknesses. The shapes of the paver blocks considered for compressive strength are zig zag (60 mm), zig zag (80 mm), I (60 mm), and dumbbell (60 mm).

30.3.2.1 Compressive strength Figs. 30.14 and 30.15 manifest the effect of fly ash on 7- and 28-day compressive strength of paver block when cement is replaced with fly ash. It is observed that the

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Figure 30.15 Cement replacement with fly ash versus 28-day compressive strength.

Figure 30.16 Cement replacement with fly ash versus 7-day flexural strength.

compressive strength of paver blocks for all shapes and thicknesses at 7 and 28 days is increased as a percentage of cement replacement increases up to 10%. For 20% and 30% replacement of cement, the compressive strength is more than control concrete. Seven days’ compressive strength of paver block for all shapes is more than the required target strength with up to 30% sand replacement. That means the earlier strength gain is more than that of the later strength in replacement of cement with fly ash. For 40% replacement of cement with fly ash, 7 and 28 days’ strengths

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are less than 0% replacement of cement but 28-day strength at 40% replacement is more than the required target strength of paver block.

30.3.2.2 Flexural strength Seven- and 28-day flexural strengths of paver blocks for cement replacement with fly ash are presented in Figs. 30.16 and 30.17, respectively. Maximum 7-day flexural strength is observed at 10% replacement of cement with fly ash. For 20% replacement, the 7-day strength is greater than the strength of paver blocks having 0% fly ash. For 30% and 40% replacements, the strength is almost equal to the strength for control concrete. For 28-day flexural strength, the maximum value is obtained at a 10% cement replacement. The 28-day strength of paver blocks up to 40% cement replacement fulfills the minimum strength requirement, that is, 4.43 MPa.

30.3.2.3 Water absorption Fig. 30.18 shows the water absorption of paver blocks for cement replacement with fly ash. It is observed that water absorption for all shapes is less than the maximum prescribed limit of 6% as per IS 15658 [25]. Maximum enhancement in water absorption occurred at 10% replacement of cement. The water absorption for paver blocks having fly ash up to 40% is less than the water absorption of paver blocks having 0% fly ash. This signifies that the application of fly ash as a substitute for cement reduces the porosity of the paver blocks.

30.3.3 Test results for concrete tiles 30.3.3.1 Wet transverse strength Fig. 30.19 presents the results for wet transverse strength of concrete tiles for sand and cement substitution with fly ash. It is seen that the maximum value for wet

Figure 30.17 Cement replacement with fly ash versus 28-day flexural strength.

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Figure 30.18 Cement replacement with fly ash versus water absorption.

Figure 30.19 Fly ash percentage versus wet transverse strength.

transverse strength occurs at 20% fly ash for both cement and sand replacement. For tiles with 0% fly ash, wet transverse strength is 2.945 MPa. A 4.76% and 24.34% increase in strength over control tiles having 0% fly ash is noted for 10% and 20% replacements of fine aggregate with fly ash, respectively. The corresponding values for cement replacement with fly ash are 3.44% and 19.84%. Up to 30% replacement, the tiles possess the required wet transverse strength as per IS 1237 [28]. that is, 3MPa.

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Figure 30.20 Fly ash percentage versus water absorption.

30.3.3.2 Water absorption The effect of addition of fly ash on concrete tiles as a replacement of fine aggregate and cement is shown in Fig. 30.20. Water absorption for sand replacement with fly ash is at 2.94%, 2.00%, 5.73%, and 5.82% for 0%, 10%, 20%, and 30% fly ash, respectively. For cement replacement, the corresponding values are 2.94%, 2.41%, 4.41%, and 5.23%, respectively. For sand replacement with fly ash, a 32% improvement is achieved for concrete tiles having 10% fly ash as compared to control concrete tiles, whereas an 18% improvement in water absorption is noted for 10% cement replacement with fly ash. Water absorption percentages for concrete tiles up to 30% replacement are within the prescribed limit as per IS 1237 [28], that is, 10%.

30.4

Conclusion

An experimental investigation was carried out to examine the effect of the addition of fly ash in concrete tiles and paver blocks. Strength and durability parameters were studied to recommend the use of fly ash-based tiles and paver blocks, promising a green construction material. The conclusions are as follows: G

G

G

When fly ash is utilized as a substitute material for fine aggregate and cement, the maximum strength parameters are obtained at a 10% replacement level. The paver blocks possess the required flexural strength and compressive strength up to 30% substitution of fine aggregate and cement with fly ash. Application of fly ash enhances the water absorption property of paver blocks significantly. This, in turn, indicates that the porosity of paver blocks is decreased with the inclusion of fly ash, hence promising the possibility of forming less microcracks.

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G

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The maximum weight for a freezethaw test is 0.36% at 30% replacement of sand, which is still below the permissible limit of 1%. This signifies that paver blocks having sand replaced with fly ash up to 30% possess the necessary strength to withstand weathering due to freeze and thaw action. The wet transverse strength of concrete tiles is improved significantly up to 20% replacement of cement and sand. At 30% replacement the concrete tiles also have the required wet transverse strength. The application of fly ash has effectively enhanced the water absorption for 10% substitution of fine aggregate and cement. For concrete tiles with 20% and 30% fly ash, the water absorption is well within the prescribed limit.

It can be concluded that paver blocks with 30% fly ash both as a cement and sand replacement material can be used in footpaths, parking areas, kerbs, etc. providing a new and sustainable construction method. Fly ash-based concrete tiles up to 30% can be used as flooring tiles, ensuring the prevention of mineral resources and promising a cleaner, greener, better, and healthier future.

References [1] R. Siddique, Effect of fine aggregate replacement with class F fly ash on the abrasion resistance of concrete, Cem. Concr. Res. 33 (2003) 18771881. [2] R. Siddique, Effect of fine aggregate replacement with class F fly ash on the abrasion resistance of concrete. 33 (2003b) 539547. [3] V.K. Parvati, K.B. Prakash, Feasibility study of fly ash as a replacement for. 4 (2013) 8790. [4] M. Singh, R. Siddique, Strength properties and micro-structural properties of concrete containing coal bottom ash as partial replacement of fine aggregate, Constr. Build. Mater. 50 (2014) 246256. [5] A. Kanthi, M. Kavitha, Studies on partial replacement of cement by bagasse ash in concrete, Eur. J. Adv. Eng. Technol. 1 (2015) 8992. [6] S.V. Deo, A.D. Pofale, Parametric study for replacement of sand by fly ash for better packing and internal curing, Open. J. Civ. Eng. 05 (2015) 118130. [7] C. Christy, D. Tensing, Effect of class-F fly ash as partial replacement with cement and fine aggregate in mortar, Indian. J. Eng. Mater. Sci. 17 (2010) 140144 (2010) 17, 140144. [8] N. Pathak, R. Siddique, Properties of self-compacting-concrete containing fly ash subjected to elevated temperatures, Constr. Build. Mater. 30 (2012) 274280. [9] C.H. Huang, S.K. Lin, C.S. Chang, H.J. Chen, Mix proportions and mechanical properties of concrete containing very high-volume of class F fly ash, Constr. Build. Mater. 46 (2013) 7178. [10] M.A. Bahedh, M.S. Jaafar, Ultra high-performance concrete utilizing fly ash as cement replacement under autoclaving technique, Case Stud. Constr. Mater. (2018) 9. [11] M. Khan, M. Ali, Improvement in concrete behaviour with fly ash, silica-fume and coconut fibres, Constr. Build. Mater. 203 (2019) 174187. [12] A. Mishulovich, J.L. Evanko, Ceramic tiles from high-carbon fly ash. Cent. Appl. Energy Res.

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¨ . Andic¸, K. Ramyar, O ¨ . Korkut, Effect of fly ash addition on the mechanical proper[13] O ties of tile adhesive, Constr. Build. Mater. 19 (2005) 564569. [14] B. Rajamannan, S.C. Kalyana, G. Viruthagiri, N. Shanmugam, Effects of fly ash addition on the mechanical and other properties of ceramic tiles, Int. J. Latest Res. Sci. Technol. 2 (1) (2013) 486491. [15] F.P. Cota, C.C.D. Melo, T.H. Panzera, A.G. Arau´jo, P.H.R. Borges, F. Scarpa, Mechanical properties and ASR evaluation of concrete tiles with waste glass aggregate. Sustain, Cities Soc. 16 (2015) 4956. [16] H. Wang, M. Zhu, Y. Sun, R. Ji, L. Liu, X. Wang, Synthesis of a ceramic tile base based on high-alumina fly ash, Constr. Build. Mater. 155 (2017) 930938. [17] A. Karasawa, S. Suda, H. Naito, H. Fujiwara, Application of fly ash to concrete paving block. (2003). [18] C.S. Poon, D. Chan, Paving blocks made with recycled concrete aggregate and crushed clay brick, Constr. Build. Mater. 20 (2006) 569577. [19] T. Uygunoglu, I.B. Topcu, O. Gencel, W. Brostow, The effect of fly ash content and types of aggregates on the properties of pre-fabricated concrete interlocking blocks (PCIBs), Constr. Build. Mater. 30 (2012) 180187. [20] C.S.G. Penteado, E.Vd Carvalho, R.C.C. Lintz, Reusing ceramic tile polishing waste in paving block manufacturing, J. Clean. Prod. 112 (2016) 514520. [21] IS: 455, Portland slag cement—specification, Bureau of Indian Standards, New Delhi, 1989. [22] IS: 383, Specifications for coarse and fine aggregates from natural sources for concrete, Bureau of Indian Standards, New Delhi, 1970. [23] ASTM C 618, Standard specification for coal fly ash and raw or calcined natural Pozzolan for use in concrete, Annu. B. ASTM Stand (2019). [24] IS: 10262, Recommended guidelines for concrete mix design, Bureau of Indian Standards, New Delhi, 2009. [25] IS: 15658, Precast concrete block for paving—specification, Bureau of Indian Standards, New Delhi, 2006. [26] IS: 9103, Concrete admixtures—specification, Bureau of Indian Standards, New Delhi, 1999. [27] IS: 1489 (Part 1), Portland Pozzolana cement—specification, Bureau of Indian Standards, New Delhi, 1991. [28] IS: 1237, Cement concrete flooring tiles—specification, Bureau of Indian Standards, New Delhi, 2012. [29] IS: 456, Indian standard plain and reinforced concrete code of practice, Bureau of Indian Standards, New Delhi, 2000.

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

31

Vahid Monfared Department of Mechanical Engineering, Zanjan Branch, Islamic Azad University, Zanjan, Iran

31.1

Introduction

The use of short-fiber (chopped) composites (SFCs) is growing in aircraft and aerospace industries, automotive, and other applications due to their many advantages. One of the dangerous phenomena for composites is the creep phenomenon. Therefore, the SFC’s behavior must be precisely analyzed to prevent dangerous and unwanted events. Many investigations have been carried out to predict the behavior of these SFCs. The mechanical properties of continuous-fiber composites are widely known. As the reader will see, for various reasons, many composites are not reinforced by continuous fibers but by short fibers. The properties of short-fiber composites are very different from those of their continuous-fiber counterparts, and in this chapter a number of models for the mechanical performance of short-fiber composites will be presented. Most examples of the application of these models, and of the effects of processing, will be polymer matrix systems as there is a wealth of information available on these compared with metal and ceramic matrix composites [1].

31.1.1 Advantages of short-fiber composites The reader will recall from the previous knowledge on reinforcements that, as a general rule, continuous fibers are more expensive than other forms of reinforcement. Furthermore, the manufacturing processes for continuous fiber-reinforced composites tend to be slow and inflexible. Let us look at these two points in more detail for polymer matrix composites (PMCs). Hand lay-up, filament winding, autoclave, and vacuum bag processing techniques, generally associated with continuous fiber-reinforced thermoset composites, are suitable for short runs or one-off requirements of high-performance high-priced products. Such techniques are however restrictive when very large numbers of articles are required. For many applications the development of peak strength or stiffness properties is not the main requirement. Where this is the case, there is frequently a desire to manufacture the product in numbers which make the processing techniques employed for continuous fibers prohibitively lengthy, and expensive, in terms of machine time and man-hours. The use of pultrusion to produce New Materials in Civil Engineering. DOI: https://doi.org/10.1016/B978-0-12-818961-0.00031-4 © 2020 Elsevier Inc. All rights reserved.

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continuous-fiber high-performance articles is a fairly rapid process, but can only be applied where a constant profile is required. For large numbers of articles with complex shapes, injection, compression, and transfer molding are favored. The price to be paid for the use of such mass-production techniques is a shortening of fiber length. This reduction in fiber length is partly due to the requirements of the processing technique, but some processes which involve mechanical shearing and mixing actions also promote considerable fiber breakage. Fiber damage is particularly noticeable for injection molding, extrusion, and mixing of polyester molding compounds.

31.1.2 Size of the fibers At this stage it should be clearly stated that there is not a fiber length which is constant for all materials and below which the fibers may be termed “short.” Instead the fiber length should be considered relative to a parameter known as the critical fiber length. The critical fiber length is a function of the matrix and the reinforcement, and as such varies considerably from composite to composite. It is therefore possible for fibers of, say, 5 mm length to be classified as short in one system and not in another. However, there is always a clear distinction between continuousfiber composites and any other type. The behavior of very short fibers is dominated by end effects and they do not therefore act as good reinforcing agents. Discontinuous fibers are normally supplied by manufacturers in standard lengths for different processing routes. Typical length parameters are indicated for a number of well-known polymeric materials and processes in Table 31.1. Note that the processed fiber length may be significantly less than the preprocessed fiber length. Given that a typical fiber may have a diameter of approximately 10 μm, it is clear that high levels of length degradation are required to reduce them to “particles.” This is just as well since, as shown by Table 31.1, processing techniques such as injection molding have a devastating effect on fiber length. For example, the micrograph in Fig. 31.1 shows fibers separated from a polymer matrix by burning. The composite had been injection molded, and marked differences in fiber length are apparent. The initial length of the fibers prior to molding was 6 mm. The histogram in Fig. 31.2 shows the distribution of fiber length for a Table 31.1 Typical size parameters for discontinuous short fibers [1].

Material Polyester-epoxy-CSM Polyester SMC Molding BMC compounds Injection molding thermoplastics

Preprocessed fiber length (mm)

Processed fiber length (mm)

50 25 12

50 25 ,4

3

,3

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Figure 31.1 Light micrograph showing the wide variation in fiber length after injection molding (the fibers have been separated from the thermoplastic matrix) [1,2].

Figure 31.2 Histogram of the lengths of fibers extracted from a thermoset injection molding (the initial fiber length was 6 mm) [1,2].

sample taken from an injection-molded thermoset. Initial fiber length was again 6 mm, and the histogram shows that in this sample none of the fibers remained unscathed during the processing operation. The degree of fiber length degradation

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depends on several process parameters such as screw design, shear rates, melt viscosity, and fiber volume fraction. Other processes such as extrusion can be just as damaging to fibers. For example, extruding glass fibers of initial length 6 mm and of weight fraction 28% in a polypropylene (PP) matrix through a circular die of diameter 3 mm results in a mean fiber length of only 0.5 mm, although some fibers as long as 2 mm may be found. It is clear from the above discussion that dependent upon the type of material used, and the method chosen to process it to its final shape, a wide variety of fiber lengths will be present. Whilst fibers even down to 50 μm in length may retain some ability to reinforce, it is the fact that actual fiber length and its distribution are uncertain that can cause design problems [1].

31.1.3 Fiber orientation The length of fibers and their orientations have equal importance. Although the orientation effects for short-fiber systems are not in general as marked as those described in many references for off-axis loading of unidirectional composites, they are not negligible. The fiber orientation depends on the processing route. When continuous fibers are used the lay-up can be controlled to give predictable end properties for the composite in terms of stiffness and strength. Chopped strand mat (CSM) and sheet molding compounds (SMCs), because of their nature and the methods of processing into panel form, give properties which are essentially isotropic in the plane of the sheet, unless additional reinforcement is added. The properties, however, are very different normal to the plane of the sheet and composites with this type of anisotropy are sometimes referred to as two-dimensional (2D) or in-plane randomly orientated composites. On the other hand, where fibers are shorter, and processing methods involve flow of material in a mold, changes in fiber orientation throughout a molding are inevitable. Such is the case for bulk molding compounds (BMCs) and for the wide range of reinforced thermoplastics currently available. Where thick or variable sections and several injection points are involved, the orientation of the fibers may be impossible to predict. In any case the properties of the material could differ markedly from area to area within the same molding. Changes in fiber orientation are related to a number of factors, such as the geometrical properties of the fibers, viscoelastic behavior of the fiber-filled matrix, mold design, and the change in shape of the material produced by the processing operation. In many processing operations the polymer melt, or charge, undergoes both elongational (or extensional) flow and shear flow. The effect of these flow processes on fiber orientation is shown in Fig. 31.3 for simple 2D deformation. During extensional flow the fibers rotate toward the direction of extension. With large extensions a high degree of alignment may be produced. In shear flow, there is a similar tendency for fibers to rotate toward the direction of shear. The viscosity of the matrix affects fiber orientation mainly through its influence on mold filling, which determines the distribution of elongational and shear flow fields.

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Figure 31.3 Schematic diagram of the changes in fiber orientation during flow: (A) initial random distribution; (B) rotation during shear flow; (C) alignment during elongational flow [3].

Typical flow patterns obtained when injection molding single-gated and double-gated molds are presented in Fig. 31.4 for short glass fiber (SGF)-reinforced PP. First, let us consider the single-gated mold. As the material passes through the gate it experiences large elongational and compressive fields. Solidification first takes place at the mold surfaces, forming a skin. The mold is then filled by material which flows through the core region to the advancing flow front (Fig. 31.4A). A velocity profile is established within the core, and the deformation field in the region of the solidifying skin involves significant elongational flow. Solidification of the core occurs when the mold is full, under completely different conditions to the skin, to give the flow pattern shown in Fig. 31.4B. It is usually found that there is a pronounced preferred orientation of the fibers parallel to the flow direction in the outer layers, and a more random distribution in the core. The flow patterns are more complex with the twin-gated mold. There is “jetting” of the flow from each gate and the fibers do not mix across the center of the component, thus creating a region of weakness which is usually referred to as a weldline or knit-line (Fig. 31.4C and D). In some circumstances the jetting flow may rebound off the mold wall facing the gates and cause an accumulation of fibers at

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Figure 31.4 Flow patterns in injection moldings of short glass fiber-reinforced polypropylene: (A) diagram of the mold filling process; (B) single-gated mold; (C and D) twin-gated molds [4].

the far end of the mold, as seen in Fig. 31.4C. There may also be an increase in fiber concentration at the gate mouths.

31.1.4 Stress and strain fields at embedded fibers in matrix In the analysis of continuous-fiber composites in the many references, any effects associated with fiber ends were neglected. As the aspect ratio (l/D where l and D are fiber length and diameter, respectively) of the fibers decreases the end effects become progressively more significant and the efficiency of the fibers in stiffening and reinforcing the matrix decreases. As well as affecting stiffness and strength, fiber ends play an important role in the fracture of short-fiber composites, and may also contribute to fracture processes in continuous-fiber composites, since the long fibers may break down into discrete lengths. Briefly, the effect of deformation on

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Figure 31.5 Effect of deformation on the strain around a fiber in a low-modulus matrix (unit cell): (A) continuous fiber; (B) short fiber [1].

the strain around an embedded fiber in a low-modulus matrix for continuous and short fibers is shown graphically in a model of a unit cell (UC) in Fig. 31.5. The large shear stresses at fiber ends can produce undesirable effects such as: 1. Interfacial shear debonding; 2. Cohesive failure of matrix or fiber; and 3. Matrix yielding.

31.1.5 Critical fiber length and average fiber stress The maximum strain that can be achieved in a fiber is that applied to the matrix, εm , and in that situation the tensile stress in the fiber, σTf , is given by σTf 5 εm Ef

(31.1)

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Figure 31.6 Variation of tensile stress in a fiber and shear stress at the interface [5].

where Ef is the Young’s modulus of the fiber and it is assumed that the fiber has deformed elastically. Also, 
 σ Tf 5 1 2 lc =2l σ^ Tf

(31.2)

In which, “σ Tf ” is the average fiber tensile stress and “σ^ Tf ” is the fiber tensile stress. Figs. 31.6 and 31.7 are sufficient to explain the fiber and matrix behaviors in the shortfiber composite graphically. That is, variation of tensile stress in a fiber and shear stress at the interface shown in Fig. 31.6, and axial strain in a fiber as a function of position along the fiber at different loadings depicted in Fig. 31.7. For more information and illustrations, refer to Ref. [1].

31.1.6 Stiffness and strength Having obtained an understanding of the concept of critical fiber length and how it determines the average stress in a fiber, we are in a position to study the mechanical properties of short-fiber-reinforced composites. For the consideration of stiffness and strength of these composites, it is convenient to subdivide such materials into three main classes. 1. Aligned systems; 2. 2D composites; 3. Variable fiber orientation.

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Figure 31.7 Axial strain in a fiber as a function of position along the fiber at different loadings [6].

Figure 31.8 Ratio of the average stress in a short fiber σ Tf to that in a continuous fiber σ^ Tf as a function of length according to Eq. (31.2) [1].

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Figure 31.9 Young’s modulus as a function of the angle between the stress axis and the direction of alignment of fibers in short-fiber glass fiber-reinforced epoxies [7
9].

Figs. 31.8
31.10 show graphically the explanation of this section simply. Finally, short-fiber composites are used because they tend to be less expensive and more amenable to mass-production techniques than continuous-fiber composites. We have seen that the length and orientation of the fibers in a shortfiber composite play a major role in determining the mechanical properties. The tensile stress in a short fiber is not constant, with the maximum stress occurring away from the fiber ends. The concept of a critical fiber length (lc ), which is the minimum length required for the stress to reach the fracture stress of the fiber, was introduced and used particularly in the analysis of the strength of aligned short-fiber composites. Fiber orientation depends in a complex manner on the processing route. For ease of discussion the composites were classified as aligned systems, 2D composites, or variable fiber orientation composites. As a general rule, analyses for stiffness and strength are better established for the aligned systems.

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Figure 31.10 Effect of fiber length on strength of aligned short-fiber glass
epoxy composite. Dashed and full lines are the predictions for different values lc [10].

31.1.7 Short-fiber thermoset composites Short-fiber thermoset composites (SFTCs) are suitable for hand lay-up and resin transfer molding technologies, being ideal materials for cost-sensitive applications requiring moderate to high mass-production rates. Unfortunately, the technological flexibility of this class of composites only applies to reinforcements in the form of CSM and is lost when components incorporating highly oriented short fibers are requested [11]. Consequently, CSM is the only reinforcement architecture extensively adopted in the fabrication of industrial SFTC parts. Since the success on the market of the applications concerned is strongly cost-dependent, E-glass fibers embedded in polyester resin (the cheapest combination of fiber and matrix) are almost universally employed [12,13], although the use of vinyl ester resins has increased in recent years [14,15]. For example, as a result [12], the influence of the reinforcement structure was shown to be important in the variation of elastic modulus and failure stress as a function of strain rate [12]. In Fig. 31.11, typical stress
strain curves for CSM and SMC composites having approximately the same fiber content by volume (Vf  0:17) are shown. Beyond the proportional limit, the unloading curve (Fig. 31.12) is concave, a small permanent deformation is retained at its completion, and a hysteresis loop appears. Also, progression of the failure modes in a SFTC is shown graphically in Fig. 31.13. The first damage in an SFTC, occurring at 30
50% of the tensile strength (0.3
0.6%elongation) and approximately coinciding with the departure from linearity of the stress/ strain curve, is debonding at the fiber/matrix interface

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Figure 31.11 Stress
strain behavior of CSM and SMC with approximately the same fiber content by volume (Vf  0:17) [11].

Figure 31.12 Behavior of a short-fiber composite under loading
unloading cycles [11].

of fiber bundles oriented perpendicular to the loading direction (grey areas in Fig. 31.13A) After onset, the failures propagate along the original strands with increasing appliedstress, until other strands, oriented at smaller angles to the load direction, are reached and debonding is initiated at their periphery (Fig. 31.13B).

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Figure 31.13 Progression of the failure modes in a short-fiber thermoset composite. Load direction: x; gray areas: fracture surfaces [11].

Besides driving the propagation of the early cracks, the stress increase also causes further debonding within the whole material volume.From the transverse cracks, delamination begins to take place before final failure: this phenomenon is comparable to damage propagation along the periphery of fiber bundles oriented parallel to the applied load (Fig. 31.13C). Thus, the study of continuous fiber laminates can give valuable information on the behavior of SFTCs [11]. Starting from the 1970s, a new class of SFTCs, namely SMCs, appeared on the market [16,17]. Its development was strongly supported by the automotive industry, interested in the availability of low-cost materials having moderate mechanical properties, very high production rates, the ability to reproduce complicated shapes, and good esthetics. The suitability of SMCs not only for closed die molding [18,19], but also for injection molding, has favored its adoption in the extensive production of automotive parts [20,21]. The annual rate of consumption of SMCs in North America was higher than 100,000 tons in the 1990s [22]. In the same period, more than 3000 SMC parts were produced per day by the automotive industry in Europe [21]. It is expected that the rate of increase will be only moderate in the future, mainly on account of increasingly stringent national regulations requiring recyclability of materials for car components. Probably, this will accelerate the

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New Materials in Civil Engineering

development of formulations and industrial production methods based on low-cost thermoplastic matrices, which should progressively replace thermoset-based SMCs in the next few decades. The study of the monotonic and fatigue behavior of SFTCs has closely reflected industrial needs. Until the 1970s, most attention was devoted to identifying the failure modes and macroscopic response of CSM [23
27]. Subsequently, much experimental work was carried out on SMC [28
38], whereas relatively little effort was expended on CSM [11,39
42]. SFTCs in fatigue exhibit microscopic damage similar to that which is observed under monotonic loading conditions, so that a distinction between a fatigue and a monotonic failure can be made only with difficulty from a fractographic analysis. Common fundamental damage mechanisms also exist in the damage development within these composites and continuous-fiber laminates. However, the latter materials fail by fiber breakage, whereas fractured fibers are seldom found in SFTCs, in which bundle pull-out is the main event driving final collapse. Unfortunately, a comprehensive analysis concerning the influence of bundle aspect ratio on the monotonic and fatigue response of SFTCs is lacking at this time. This needs to be done, since many attempts based on more flexible resins and modified fiber
matrix interfaces have been unsuccessful in improving their fatigue sensitivity. From the data available, the main factor influencing the fatigue performances of an SFTC is the fiber type: as with their continuous-fiber counterparts, these materials show better fatigue resistance when higher modulus fibers are employed. In fact, the use of carbon instead of E glass not only lowers the fatigue sensitivity, but also results in a higher residual modulus, helping in stiffness-critical applications. Initial strain versus fatigue life for different composites has been depicted in Fig. 31.14 for more clarification. As with classical laminates, the S/N curve of an SFTC displays some variations in slope, and seems to approach an asymptotic value at large numbers of cycles. Whether this is an indication of an endurance limit as conventionally defined is not known. According to some authors, strain, rather than stress, is the actual parameter governing the material response under a cyclically varying load; the variations in slope are a consequence of changes in failure modes; the endurance limit actually exists, and is mainly dependent on the matrix. However, the typical trends of the S/N curves have also been modeled effectively from the hypothesis of a single failure mechanism developing throughout the loading history, which seems to be confirmed by the strict correlation between the statistical distribution of the monotonic and fatigue data. Clearly, the parameters presently adopted to determine the fatigue sensitivity of a composite only hold in the range of moderately high fatigue lives. These parameters become questionable at very large numbers of cycles, where the material response should be assessed for fatigue-critical applications. Establishing whether an endurance limit actually exists, and the factors affecting it, will require a costly and time-consuming testing stage and a thorough identification of the failure modes involved in the progressive material degradation [11].

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

933

Figure 31.14 Initial strain versus fatigue life for different composites [11].

31.1.8 Different research works It should be mentioned that some different research works are available with special views and insights for analyzing CSM in various Ref. [43
55]. In addition, some applied research papers are about the analysis of the creeping short-fiber composite analytically, numerically, and experimentally [56
73]. For example, a semianalytical formulation has been presented for obtaining the viscosity of solids (such as metals) using the steady-state creep model of the shortfiber composites. For achieving this aim, fluid mechanics theory was used for determining the viscosity. Sometimes, obtaining the viscosity is experimentally difficult and intricate. Therefore the present model may be beneficial to obtain the viscosity of metals [71].

31.2

Analytical methods

Short-fiber-reinforced polymers (SFRPs) are very attractive because of their ease of fabrication, relatively low cost, and mechanical properties which are superior to those of relevant polymer resins. Owing to the partial orientation distribution of the fibers in final components, SFRP composites show direction-dependence, namely anisotropy in their mechanical properties. The fiber-length distribution (FLD) and the fiber-orientation distribution (FOD) in SFRP composites play an important role in determining the composite mechanical properties. Also, the FLD and the FOD have been modeled with suitable probability density functions and the laminate

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New Materials in Civil Engineering

analogy approach is used to derive the expression of the elastic modulus of SFRP as a function of any given direction, the FLD and the FOD. The directiondependence, that is, the anisotropy, of the elastic modulus of SFRP has been studied in detail by taking into consideration the effects of the FLD and the FOD. The present theory is applied to existing experimental results, and the agreement is found to be very satisfactory. As an important result, it has been shown that the modulus of SFRP in any direction increases with the increase in the fiber volume fraction [74]. The composite elastic modulus versus direction angles is depicted in Fig. 31.15. If the electrical resistance of a material depends upon external straining, the material exhibits “piezoresistivity.” The piezoresistive behavior has been realized in an electrically conductive elastomer composite where the microstructure of conductive fillers can be changed under finite deformation of elastomer, resulting in a change in the composite resistivity. The piezoresistive behavior of a conductive short-fiber/elastomer matrix composite has been analyzed by applying a percolation model. A fiber reorientation model is applied to the composite system with the aim of predicting the relationship between the applied finite strain and the reorientation of conductive short fibers. It is found that the piezoresistive behavior of a conductive short-fiber/elastomer composite is attributed to the degeneration of the initially percolating network under finite strain. Some numerical results are then compared with our previous experimental data, showing a reasonably good agreement [75].

Figure 31.15 The composite elastic modulus versus the direction angle Θ (angle of fiber with axis 1) for various fiber volume fractions and any value of Φ (angle the projection of the fiber on to the 2
3 plane) [74].

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

935

Therefore an analytical modeling was developed to study the piezoresistive behavior of a conductive short-fiber-reinforced elastomer composite. The analytical prediction was deduced from a fiber percolation model when the change in the threshold volume fraction (f  ) of fiber under straining is taken into account. The change in the threshold volume fraction of fiber is mainly attributed to the change in microstructure of a composite under straining. The reorientation distributions of fibers due to straining were computed using a fiber reorientation model. It was found that the threshold fiber volume fraction increases as the applied strain increases. It was also found that an initially conductive composite becomes nonconductive around the critical strain, exhibiting as witching behavior [75]. On the basis of the analytical formulation, the computational process of the piezoresistive behavior is illustrated in Fig. 31.16. An analytical model was developed to study the influence of thermal residual stresses on the elastic and yield behaviors of aligned short-fiber-reinforced metal matrix composites. Three-dimensional (3D) solutions of elastic stress field in the matrix of a circular cylindrical UC were first obtained based on the theory of elasticity. On this basis, an alternative analysis of the stress transfers between the matrix and the fiber was given using the shear lag analysis, which provides the expressions for the stress distributions in the matrix and the fiber. The thermal residual stresses and the distributions of the stresses under tensile and compressive loadings as a function of the material parameters, such as fiber volume fraction,

Figure 31.16 The computational procedure of the present model [75].

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New Materials in Civil Engineering

fiber aspect ratio, and matrix yield stress, were calculated. Furthermore, simple expressions for the composite elastic modulus and yield strength under tensile and compressive loadings were derived. These expressions were used to study the influence of the thermal residual stresses and the material parameters on the elastic module and yield strengths under tensile and compressive loadings. Finally, the model predictions were compared with the experimental results on the SiCw/Al
Li T6 composite and several SiCw/Al T6 composites in literature, and also with the other theoretical models [76]. An image analysis method has been presented for measuring fiber orientations in short-fiber-reinforced composites and a mathematical procedure for predicting the elastic modulus of the composites according to the measured FOD. This method determines the FOD from the ratio of fiber matrix perimeter length between two orthogonal planar cross sections of polished composite samples. The analysis algorithm of the method is much simpler than previously reported methods and is efficiently applied to composites with axially symmetric FOD. To verify the theory, FOD measurement and tensile testing were performed on Al2O3/Al composites fabricated by squeeze casting. The elastic modulus values determined by the tests were compared with the theoretical value, and the agreement was satisfactory [77]. A new class of short-fiber composite, in which the ends of the short fibers were enlarged, has been studied. Because of their geometry, these short fibers were named bone-shaped short (BSS) fibers. It was found in several composite systems that the BSS fibers can simultaneously improve both the strength and toughness of composites, and the mechanisms for such improvements vary with the mechanical properties of the composite constituents. The strength increase resulted from the effective load transfer from the matrix to the fibers through mechanical interlocking at the enlarged fiber ends. The toughness increase resulted from one or several mechanisms, including: reduction in stress concentration in a brittle fiber-reinforced composite with weak fiber/matrix interfacial bonding; higher fiber pullout resistance when the BSS fibers bridging a matrix crack are pulled out, with the enlarged ends attached and perhaps deformed; and plastic deformation of ductile fibers. Both experimental and theoretical studies have been conducted on composite mechanical properties and fractography, fiber pullout, and stress analysis. Also, recent developments in BSS-fiber composites as well as discussions on current issues and future directions in this emerging field have been studied and reviewed [78]. Note that short fibers in a real composite are usually not well aligned. It has been shown that the fiber orientation can significantly affect the pullout energy of BSS and CSS (conventional short straight) fibers. It is not clear how fiber orientation distribution affects the strength and toughness of the composites. This issue has not been investigated and should be addressed by computational modeling and carefully designed experimental work in the future [78]. Figs. 31.17
31.24 are about composite sample, set up of our-point bending flexural test, stress
strain curves of BSS- and CSS-fiber composites, and fracture surface using SEM. The geometry of the tensile specimen and DCB specimen are shown in Figs. 31.17 and 31.18, respectively. Four-point bending tests were performed on steel wire-reinforced cement (see the setup in Fig. 31.19).

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

Figure 31.17 Dimensions of the composite sample for tensile testing [78].

Figure 31.18 Sketch of the DCB specimen [78].

Figure 31.19 Set up of our-point bending flexural test [78].

937

Figure 31.20 BSS Ni-fiber-reinforced polyester matrix composites (the short fibers are well aligned) [78].

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

939

Figure 31.21 Stress
strain curves of bone-shaped and straight short Ni fiber composites and polyester matrix. A 0.6% MEKP hardener was used to harden polyester matrix for all samples. Ni fiber length 5 2.5 mm, diameter 5 76.2 m. Fiber volume fraction 5 1.7% [78].

Fig. 31.20 shows a BSS Ni-fiber-reinforced composite with the fibers well aligned. The fiber length is 2.5 mm and fiber volume fraction is 1.7%. The stress
strain curves of both BSS Ni-fiber composites and conventional straight short (CSS) Ni-fiber composites are shown in Fig. 31.21. It is obvious that the strength of BSS-fiber composite samples is significantly higher than that of CSS-fiber composite samples. It can also be seen that the Young’s modulus of the BSS-fiber composite is higher than that of the CSS-fiber composite. The average strengths of the BSS-fiber composite samples, CSS-fiber composite samples, and matrix are 24.43, 22.17, and 20.65 MPa, respectively. The average strength of the BSS-fiber composite samples improves by 10.2% over that of conventional CSSfiber composite samples [78]. Fig. 31.23 compares SEM micrographs of fracture surfaces of BSS- and CSSnickel fiber-reinforced polyester matrix composites. These samples were tested under tensile mode and the stress
strain curves are shown in Fig. 31.21. It can be seen from Fig. 31.23A that the fracture of the BSS-fiber composite sample originated from an area where enlarged fiber ends were clustered. Small cracks initiated from several enlarged ends and coalesced later to form a larger unstable crack, which led to composite failure. BSS fibers were broken before being pulled out. The annealed Ni-fibers usually deformed plastically before final failure, which consumed energy and consequently improved toughness. Therefore the BSS fibers effectively bridged the matrix cracks before they broke, which explains the high

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New Materials in Civil Engineering

Figure 31.22 Stress
strain curves of BSS- and CSS-fiber composites and polyester matrix for (A) l 5 3 mm and (B) l 5 4.5 mm [78].

yield strength and Young’s modulus of BSS-fiber composites. However, the BSSfiber composites have a smaller final strain because the fibers were broken before being pulled out. If the ends were made smaller, it might be possible to allow them to be pulled out with the fiber, which will further improve the toughness. The fracture surface of a CSS-fiber composite sample is shown in Fig. 31.23B. Due to the weak interface, the fibers were easily pulled out, which resulted in a lower yield strength and Young’s modulus [78].

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

941

Figure 31.23 Fracture surface of (A) BSS- and (B) CSS-Ni fiber-reinforced polyester matrix composites [78].

The fractographs of BSS-and CSS-polyethylene fiber-reinforced polyester matrix composites under tensile load are shown in Figs. 31.24 and 31.25. Figs. 31.24 shows the fractographs of several CSS-fiber composite samples. As shown in Figs. 31.24A, a crack was initiated at a fiber end (see the arrow mark). The small crack propagated slowly at the beginning, leaving a smooth area. Figs. 31.24B shows a case of initial crack formation from clustered fiber ends (see arrows in thefigure). No smooth zone associated with slow crack propagation can be seen, apparently because the crack grew to an unstable size by coalescence of several smaller cracks. In both cases, once the crack reached a critical size, it propagated through the entire cross-section of the sample, pulling out fibers in its wake and leaving a relatively flat fracture surface with river marks, similar to the fracture surface of a brittle material. In contrast to BSS fibers, the pullout of CSS fibersdid not result in much matrix damage (Figs. 31.24C), which means less energy was consumed during the pullout process [78]. For the BSS-fiber composites, the crack usually initiated at the fiber ends (Figs. 31.25A), or at surface flaws on the specimen (see

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New Materials in Civil Engineering

Figure 31.24 SEM micrographs of fracture surfaces of CSS-fiber composites [78].

arrows in Figs. 31.25B). The main crack propagated by coalescing with smaller cracks formed at nearby BSS-fiber ends. These smaller cracks were often not on thesame plane as the main crack, resulting in a very rough fracture surface. The disk-shape of the fiber ends as well as the interaction of these fiber ends promoted crack formation in the matrix [78]. Although the bulk polyester matrix can sustain relatively large deformations, it behaved in a brittle manner once a crack was formed, as evidenced by the river-marks in fractographs of both BSS- and CSS-

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

943

Figure 31.25 SEM micrographs of fracture surfaces of BSS-fiber composites [78].

fiber composites (Figs. 31.24 and 31.25). As the crack propagated, some BSS fibers bridging the crack were pulled out, which resulted in extensive matrix damage (Figs. 31.25C), consuming large amounts of energy. Based on the law of mixture and the large strain axisymmetric elasto-plastic finite element (FE) method (FEM) (Fig. 31.26), the analytical expressions for the stress
strain response of short-fiber-reinforced metal matrix composite were

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New Materials in Civil Engineering

Figure 31.26 (A) Schematic illustration of the fibers arrangement in the composite, (B) a circular unit cell representative, (C) FEM meshes in the unit cell, (D) the representative of the unit cell at different K values for VF 5 0:184 (the fiber volume fraction and the fiber end distance) and AF 5 5 (the aspect ratios of the fiber and the unit cell) [79].

derived and a new method of defining the yield behavior of short-fiber-reinforced metal matrix composite was proposed. The effects of the material parameters (fiber volume fraction, fiber aspect ratio, fiber end distance, and matrix strain hardening coefficient) on the deformation behavior of the composite were also investigated. It was demonstrated that there is a close relationship between the stress
strain partition parameter and the deformation behavior of the composite. The effect of the material parameters on the initial yield behavior can be revealed well by this method. The predicted elastic modulus and yield stress are in good agreement with the experiments [79]. The analytical expressions for the stress
strain response of the composite have been derived based on the law of mixture. A new method has been proposed to define the yield responses of the composite by examining the variations and its second-order derivations. The proportion limit, the elastic modulus, the initial yield stress, and the final yield stress can be predicted well by using this method. With the increases of the aspect ratio and the volume fraction, the proportion limit first decreases and then increases. This behavior is thought to be a consequence of the competitive effects of local stress concentrations and local plastic constraints due to the presence of the fiber. Briefly, important and applied formulations have been derived in Ref. [79] as the following (Eqs. 31.3
31.14): q52

ð1 2 VF ÞEF EC 1 VF EM EC 2 EM EF ð1 2 VF ÞEM 1 VF EF 2 EC

(31.3)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

   21    q2EF q 2 EF EC 5 ð12VF Þ 3 VF EM 1VF 1 VF E F q2EM q 2 EM

945

(31.4)

Also, σC is the normal stress of the composite and is as follows, 

  EF q 2 E M σC 5 ð1 2 VF Þ 1 VF σM EM q 2 EF

(31.5)



In which, q BεC , σC 5 EC εC , σM 5 EM εM , and σF 5 EF εF , and EC is the elastic modulus of the composite. Also, the elasto-plastic tangent modulus of the composite HC can be derived as follows: "
n21  n21 # 2q dσC VF λn εC =ð12VF Þ εC EF 1 λn HC 5 5 EF 2 q dεC 1 2 VF 12VF σM 5 λεnM

(31.6) (31.7)

The law of mixture has been applied to describe the tensile stress
strain behaviors of dual-phase materials and metal
ceramic graded composites. For the composite studied, the law of mixture can be written as: σC 5 ð1 2 VF ÞσM 1 VF σF

(31.8)

εC 5 ð1 2 VF ÞεM 1 VF εF

(31.9)

where σC , εC , σM , εM , σF , and εF are the axis stresses and strains of the composite, the matrix, and the fiber, respectively. The relationship between the stresses and strains of the matrix and the fiber can be described by a parameter generally called the stress
strain partition parameter, q5

σM 2 σF εM 2 εF

(31.10)

In this analysis, the stress
strain relations of the matrix and the fiber can be calculated using a volume average approach as follows: σF 5

NF 1 X σ2F ΔVFJ VFT J51 J

(31.11)

σM 5

NM 1 X σ2M ΔVMJ VMT J51 J

(31.12)

where σ2F and σ2M are the average axis stresses obtained from the Gaussian inteJ J gration points within each FE of the fiber and the matrix, respectively; ΔVFJ ,

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New Materials in Civil Engineering

ΔVMJ and NF , NM are the volume and number of elements of the fiber and the matrix, respectively; VFT , VMT are the volumes of the fiber and the matrix, respectively. The expressions for εF , εM have similar forms to those for σF and σM . 0 Also, σMY is the average matrix stress corresponding to the initial yield stress of the composite, which will be given by the following FE calculation:    E F q 2 EM 0 σCY 5 ð1 2 VF Þ 1 VF σMY EM q 2 EF

(31.13)

And the yield strength of the composite σCY can be expressed as follows:    E F q 2 EM σCY 5 ð1 2 VF Þ 1 VF σMY EM q 2 EF

(31.14)

A cylindrical cell model based on continuum theory for plastic constitutive behavior of short-fiber/particle-reinforced composites has been proposed, in which the composite is idealized as uniformly distributed periodic arrays of aligned cells, and each cell consists of a cylindrical inclusion surrounded by a plastically deforming matrix. In the analysis, the nonuniform deformation field of the cell is decomposed into the sum of the first-order approximate field and the trial additional deformation field. The precise deformation field is determined based on the minimum strain energy principle. Systematic calculation results are presented for the influence of reinforcement volume fraction and shape on the overall mechanical behavior of the composites. The results are in good agreement with the existing FE analyses and the experimental results. Therefore the analytical constitutive relation of short-fiber/particle-reinforced composites is achieved by a simulation method [80]. A generalized mathematical model of energy absorption of randomly oriented short-fiber composite has been developed on the basis of the result of a single-fiber pull-out test and some assumptions obtained from experimental investigation of the fracture of the composite of Aramid short fiber. The concept of the pull-out test is incorporated into the model, such that the fibers intersecting a fracture plane (where the fracture of composite occurs) would pull out or fractures like a single-fiber pullout test. The energy contribution of each fiber in the fracture plane is calculated for different embedded lengths and orientations to estimate the total fracture energy. According to the model, it is found that energy absorption depends upon the intrinsic properties of the fiber and matrix, volume fraction, critical embedded length of the fiber, fiber length, and orientation of the fibers in the matrix. Analytical results obtained by the model are compared with the experimental results of Aramid shortfiber composite [81]. An analytical procedure was proposed and validated for predicting the elastic anisotropy and thermal expansion behavior of a short-fiber polymer composite, where both the fiber and the polymer matrix possess anisotropic material properties. The modeling strategy is to consider the fiber composite as an aggregate of units of structure, with an averaging scheme which takes into account the state of orientation. Validation of this strategy required the accurate determination of the unit properties

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

947

and a satisfactory orientation averaging procedure. First, the unit properties were determined for very highly aligned samples with either an isotropic or anisotropic fiber (glass or carbon) and either an isotropic or anisotropic matrix (Nylon or liquid crystalline polymer). The fiber orientation and length distribution were determined by image analysis together with measurements of their elastic and thermoelastic properties. In combination with FE calculations, these results provided the basis for validation of appropriate analytical schemes to predict the unit properties. The orientation averaging procedures were validated by measurements on a model ribbed box component manufactured from a carbon fiber-filled liquid crystalline polymer (anisotropic fiber and anisotropic matrix). Measurements of Young’s modulus and the coefficient of thermal expansion were combined with the determination of fiber orientation by image analysis. The key result was that if the fiber orientation level was high, the best prediction was to assume that the matrix orientation was identical to that of the fibers. For a lower degree of alignment, a better prediction was obtained by assuming that the matrix was isotropic [82]. Mechanical analysis was made based on the micromechanical model of short inorganic fiber-reinforced polymer composites, and an expression for tensile strength was derived by introducing an interfacial strength factor. This equation was applied to estimate the tensile strength of short inorganic fiber-reinforced polymer composites. The results showed that the relative tensile strength increased nonlinearly with increasing fiber volume fraction. Finally, the equation was preliminarily verified using the measured tensile strength of both SGF-reinforced polycarbonate/acrylonitrile
butadiene
styrene copolymer composites and short carbon fiber-reinforced polyamide composites reported in literature. Good agreement was found between the predictions and the experimental data (Fig. 31.27). The results showed that the relative tensile strength increased nonlinearly with increasing fiber volume fraction. Finally, the equation was preliminarily verified using the measured tensile strength of both SGF-reinforced PC/ABS composites and short carbon fiber-reinforced polyamide composites reported in the literature; good agreement was found between the predictions and the experimental data [83]. As shown in Fig. 31.27, the interfacial strength between the fiber and matrix plays a quite important role. Fiber-reinforced gypsums are prevalent building materials in which short fibers with high tensile strength are embedded into a gypsum matrix to produce supplemental strong and lightweight construction materials. Due to facing a growing risk of death and economic disaster in earthquake-prone areas, quake-resistant materials and structures should be employed for building constructions. Gypsum-based composites, as a unique candidate for this purpose, reduce the risks and produce more suitable construction materials for residential buildings. Therefore the tensile strength of gypsum composites with different volume fraction of PP and polyparaphenylene terephthalamide (PPTA) fibers up to 15% were studied. Stress transfer ability from matrix to fibers was analyzed using theoretical shear lag analyses, scanning electron microscopy (SEM), and pull-out tests. The interfacial characteristics were also studied by SEM. The ability of the composites to withstand longitudinal tensile load was also studied by tensile tests of dog-bone-shaped, random-

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New Materials in Civil Engineering

Figure 31.27 Relative tensile strength versus short carbon fiber volume fraction of reinforced polyamide composites [83].

oriented fiber-reinforced gypsum. The tensile strength of randomly oriented shortfiber-reinforced gypsum was evaluated by a mathematical model. The obtained results from the model and experimental results have been compared and discussed [84]. An expression of Young’s modulus of short inorganic fiber-reinforced polymer composites was derived based on the tensile strength equation, and the factor affecting the Young’s modulus was analyzed. This equation was applied to estimate the Young’s modulus of short inorganic fiber-reinforced polymer composites. The results showed that the relative Young’s modulus increased nonlinearly with increasing fiber volume fraction, while it increased linearly with an increase in the fiber length
diameter ratio. Finally, the equation was verified preliminarily by using the measured Young’s modulus of the SGF-reinforced polycarbonate/acrylonitrile
butadiene
styrene copolymer composites and the PP reinforced, respectively, with SGF and short carbon fiber reported in the literature, good agreement was found between the predictions and the experimental data [85]. A 3D “tension
shear chain” theoretical model has been developed to predict the mechanical properties of unidirectional short-fiber-reinforced composites, and especially to investigate the distribution effect of short fibers. The accuracy of its predictions on effective modulus, strength, failure strain, and energy storage capacity of composites with different distributions of fibers are validated by simulations of FEM. It was found that besides the volume fraction, shape, and orientation of the reinforcements, the distribution of fibers also plays a significant role in the

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

949

mechanical properties of unidirectional composites. Two stiffness distribution factors and two strength distribution factors are identified to completely characterize this influence. It is also noted that stairwise staggering (including regular staggering), which is adopted by nature, could achieve overall excellent performance. The proposed 3D tension
shear chain model may provide guidance to the design of short-fiber-reinforced composites [86]. An analytical model has been proposed to predict the effective properties of macrofiber composites (MFCs) using an equivalent layered approach. The assumptions are based on “rule of mixtures,” series and parallel capacitance theory, along with a uniform electric field across the electrodes. Also, a FE model based on a UC approach is carried out to evaluate the effective properties using periodic boundary conditions (Figs. 31.28 and 31.29). Simulated results based on the proposed analytical and numerical approaches are validated with an asymptotic expansion homogenization method (AHM), and the data available for a particular fiber volume fraction from the manufacturer [87]. MFC is an active composite made up of one active layer and two electrode, acrylic and kapton layers, respectively. The active layer of MFC consists of rectangular piezoceramic fiber embedded in passive epoxy matrix. MFCs are widely operated in two response modes, namely longitudinal (d33 type MFC) and transverse (d31 type MFC). The modes differ in their electrode pattern and poling direction. In d33 type MFCs, the poling and applied electric field directions are in alignment with the longitudinal direction of the PZT fiber phase. This configuration results in longitudinal strain owing to d33 coupling constant. Meanwhile in d31 type MFCs, the poling direction and applied electric field are perpendicular to the fiber direction that renders transverse strain due to d31 coupling constant—refer to Fig. 31.28 for a schematic representation of MFCs [87]. Fig. 31.29 shows the variation of electric field across the electrodes for d31 and d33 type MFCs based on the numerical model. Lateral stiffness of a high-rise building is significantly influenced by the design of coupling beams to spread plasticity over the system height. Design and reinforcement detailing should be performed to retain strength and a significant percentage of stiffness during large deformations into a plastic range. The cracking and lowtoughness problems of high-strength concrete can be overcome by the addition of short randomly distributed steel fibers. These steel fibers provide a crack bridging the interference plan between shear walls and coupling beam. An alternative design has been proposed using an analytical model for high-strength fiber-reinforced concrete (HFC). This is to reduce the reinforcement congestion and construction difficulties, in which the fiber composite enables the use of straight bars as partial or total replacement of diagonal bars. An analytical relationship is proposed, herein, to generate the complete stress
strain curve of HFC subjected to uniaxial compression. The fiber generates a passive confinement inside the composite that prevents the concrete from spilling out during cycles of seismic load. Based on nonlinear fracture mechanics, a continuum approach is developed, as a linear elastic-strain softening material, for modeling the tensile behavior of HFC. The model accounts for composite inelasticity and ductility. It also slows down crack growth, fiber

Figure 31.28 Schematic representation of longitudinal (d33 ) and transverse (d31 ) type macrofiber composite [87].

Figure 31.29 Linear and nonlinear electric field distribution of d31 and d33 type macrofiber composites [87].

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

951

debonding and pullout mechanisms, and also attenuates fracture energy and element size effect. There is a wide variation in the code limit for predicting maximum shear stress. For this reason, based on experimental results, a proposed strut-and-tie model is developed to determine the contribution of fiber composite in the shear resistance of short-coupling beams. Comparing the analytical results with experimental results, the adopted analytical model shows a good agreement. A nonlinear FE model is proposed to examine the effect of using HFC on 40-story high-rise building [88]. The calculation of the effective viscoelastic properties of a short-fiber-reinforced composite has been carried out. The orientation distribution of the fibers is described by a scatter parameter, varying from perfectly aligned fibers to randomly oriented ones. Both matrix and fibers are assumed to be isotropic. The viscoelastic behavior is described using fraction-exponential operators of Scott Blair
Rabotnov. The results are obtained in closed form [89]. The main challenge appearing in using an elastic-viscoelastic analogy is to obtain analytical formulas for inverse Laplace trans-form. This difficulty constitutes the main reason to use the oversimplified dashpot-spring models. Unfortunately, the simplest models are not sufficiently flexible to match experimental data for real materials. Recycled mixed postconsumer and postindustrial plastic wastes consisting of HDPE, LDPE, and PP were injection molded with SGF (10%
30% by weight) to produce new-generation composite materials. Intensive experimental studies were then performed to characterize the tensile, compression, and flexural properties of glass fiberreinforced mixed plastics composites. With the addition of 30 wt.% of glass fiber, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibers in the longitudinal direction at the outer layers. The randomness and length of the glass fibers were accounted to modify the existing rule of mixture and fiber model analysis to reliably predict the elastic and strength properties of glass fiber-reinforced mixed plastics composites [90]. Randomly oriented strand composites were promising materials for complexshaped parts, especially in the aerospace and automotive industries. However, test data from small specimens tend to have a high level of scatter as these materials contain several centimeters of chopped carbon fiber strands, in which the mechanical properties of ultrathin chopped carbon fiber tape reinforced thermoplastics (UTCTT) have been studied, which contain chopped thermoplastic thin-ply prepreg tapes produced using a paper-making technique. A model was proposed for accurately evaluating the flexural modulus and its scatter, which was verified by comparison with experimental data. The predictions of the model showed excellent agreement with the experimental results. This method makes it possible to quantify the scatter of the flexural modulus and is useful for designing geometries of not only standard test specimens, but also complex parts for actual applications [91]. The impact behavior of flax fiber-reinforced polymer (FFRP)-strengthened coconut fiber-reinforced concrete (CFRC) slabs was investigated through experimental and theoretical studies. Plain concrete, CFRC, and FFRP-CFRC slabs were built

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New Materials in Civil Engineering

and tested under impact loadings. Impact results showed FFRP-CFRC specimens had better performance in aspects of energy absorption and keeping the integrity of the concrete, compared with the PC and CFRC specimens. Another impact test was carried out to discover the more effective wrapping configuration between three different wrapping designs of the FFRP strengthened CFRC slab, and their parameters, that is, impact force history, strain history of FFRP, deflection history, energy absorption, and damage pattern were discussed to evaluate the impact resistance. After the experimental study, a theoretical analysis method was used to predict the maximum impact force as well as the maximum deflection, the results of which showed good agreement with the experimental results [92]. A mechanical behavior and vibration analysis of short bamboo fiber-based polymer composite beam structures has been carried out. Bamboo fiber has been widely used for many applications due to its low cost, light weight, short growth cycle, and high availability. Two different weight fractions (10 and 15 wt.%) of short bamboo fibers are considered during the fabrication of short bamboo fiber-reinforced polyester composite beams (SBFRPCB). The mechanical properties like tensile strength and tensile modulus are measured. The vibration analysis using an analytical method and a FEM are carried out to obtain natural frequencies and mode shapes of SBFRPCB structures [93]. The successful fabrications of a new class of polyester-based composites reinforced with short bamboo fibers have been done. The present investigation revealed that 15 wt.% fiber loading shows superior hardness, tensile strength, and impact strength. Meanwhile flexural strength was better in 10 wt.% of fiber loading. The tensile test and impact test of short bamboo fiber-reinforced polyester composite has been done. It has been concluded that the poor interfacial bonding is responsible for low mechanical properties. Possible use of these composites such as pipes carrying coal dust, industrial fans, helicopter fan blades, desert structures, and lowcost housing is recommended. It is observed that the frequency is altered as bamboo fiber weight percentage from between the theoretical and FEA investigations. Again, by changing the boundary conditions, the magnitude of natural frequency is also changed. Similar results are also observed in clamped
clamped end conditions. In this situation, the vibration parameters are altered with respect to the weight fraction of bamboo fiber and boundary conditions. Fig. 31.30 shows (A) the meshing model and (B
G) the first three mode shapes and corresponding natural frequencies of SBFRPCB (10 wt.%) in fixed
fixed and fixed
free end conditions, respectively. It is observed that the stiffness of the beam increased with an increase in the wt.% of short bamboo fibers. Therefore the corresponding natural frequencies are increased. Although several works have been published in the literature on agave fibers and their biocomposites, accurate information about the choice of both the fibers and the manufacturing processes that allow the user to optimize the biocomposite properties in terms of strength and stiffness are not yet available; also, no theoretical models that can be used for an accurate evaluation of the mechanical properties of these biocomposites are reported. To this aim, a contribution has been given by considering green epoxy biocomposites reinforced by both short and discontinuous

Figure 31.30 (A) Meshing model; (B
G) first three mode shapes in fixed
fixed and fixed
free end conditions, respectively [93].

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sisal agave fibers arranged in proper MAT-type fabrics. In particular, an optimized manufacturing process that allows obtaining good-quality biocomposites is proposed. A detailed analysis of the experimental results, obtained through preliminary pull-out and tensile tests carried out, along with an accurate analysis of the damage process performed by SEM micrographs, have allowed the development of reliable theoretical models that permit the evaluation of the mechanical properties of the analyzed biocomposites. Finally, the comparison with the best performing short/discontinuous-fiber biocomposites reported in the literature has shown how the implemented biocomposites exhibit comparable tensile strength and significantly higher stiffness, also with respect to biocomposites reinforced by more stiff and more expensive fibers (flax, hemp, etc.) [94]. An approach to thermomechanical analysis of effective properties of transversely isotropic, parallel, short-fiber, random composites with various models of anisotropic interphases has been presented. This is founded on a novel combination of two basic prior developments. One is the concept of the energy-equivalent inhomogeneity, and extended to include thermal effects. It transforms the problems with interphases to problems without interphases. The other development is the method of conditional moments, introduced long ago to analyze random composites without interphases. As such, the method is a perfect match for the idea of equivalent inhomogeneity. Together they provide a powerful tool for the analysis of problems difficult to solve using other approaches. In addition, it leads to closed-form relations obtained for the effective elastic stiffness tensor and the effective coefficient of thermal expansion of composites. To demonstrate the versatility of the approach, a composite containing randomly distributed unidirectional short fibers with anisotropic interphases is discussed here for the first time. As a result, the numerical examples could not be evaluated comparatively. Comparisons are made only in a few special cases of infinite fibers, for which some earlier solutions are available, and the agreement was found to be remarkably good. Both Gurtin
Murdoch and spring layer models of interphases have been included [95]. This research work [95], for the first time, presented the energy-based equivalent inhomogeneity approach to thermal problems and to anisotropic interphase. Other shapes of the inhomogeneities, other models of interphases, or even other physical problems (e.g., flow through porous media, electric conductivity, etc.) can also possibly be handled by this approach.

31.3

Numerical methods

A micromechanics FE model has been developed for stress transfer in short-fiber composites, incorporating a heterogeneous interphase region. The specimen consists of a single fiber under stress embedded in an epoxy matrix. Considering a heterogeneous and compliant interphase, a generalized computational procedure has been developed that enables imperfect adhesion or loss of interphasial strength simulations (Fig. 31.31). Varying the global variables of the problem, parametric studies were performed to study the influence of the model parameters on the load transfer

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Figure 31.31 Composite model [96].

characteristics from the fiber to the matrix. Numerical results of the stress distribution have been determined as a function of geometric and material variables. The effects of both the imperfect adhesion between the fiber and the matrix, and the loss of interphase compliance on stress state were demonstrated and discussed. The results showed that the interphase plays a significant role in stress transfer characteristics of fibrous composites, and extension of the load transfer zone is restricted very close to the loaded fiber end [96]. A generalized computational model has been developed for the investigation of the load-carrying characteristics of fibrous composites incorporating a heterogeneous interphase region. This model enables simulations of both imperfect adhesion and interphasial weakness by proper formulation of the interphase properties. Then, parametric studies were conducted to demonstrate how the interphase strength and the adhesion efficiency affect the local stress state. FE calculations of the stress state show that the interphase leads to a substantial change in load transfer characteristics depending on the variation of interphase properties. The changes in the interphase strength and the adhesion efficiency can drastically alter the stress state

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in the interphase and thus the load transfer characteristics of the composite. This is important to the structural properties and performance of a composite since constituent stresses affect the fiber
matrix adhesion and interaction. Hence, optimizing the interphasial stiffness is an attractive option for optimizing composite materials for specific mechanical requirements. The present model is general and may be used for further analysis and hence provide a basis for further understanding of the toughening mechanism for fiber bridging [96]. Composite materials reinforced by BSS fibers enlarged at both ends are well known to have significantly better strength and toughness than those reinforced by CSS fibers with the same aspect ratio. Comparing the fracture characteristics of double-cantilever-beam specimens made of BSS and CSS fiber composites reveals the distinct mechanisms responsible for the toughness enhancement provided by the BSS fiber reinforcement. Enlarged BSS fiber ends anchor the fiber in the matrix and lead to a significantly higher stress to pull out than that required for CSS fibers, altering crack propagation characteristics. To study BSS fiber-bridging capability further, the effects of increasing the size of the enlarged fiber end on the pull-out characteristics have been examined and identified the sequence of failure mechanisms involved in the pull-out process. However, large microcracks initiated at the enlarged ends can potentially mask the toughening enhancements provided by BSS fibers. To understand the influence of fiber-end geometry on debond initiation at the fiber ends, the interfacial stresses around fiber ends varying in geometry have been analyzed using an elastic finite-element model [97]. Analysis for understanding the various processes has been done using a FEM that can occur and contribute to stress transfer between the matrix and fibers in short-fiber composite materials. These processes are elastic stress transfer, plastic stress transfer, matrix yielding (mode α), interfacial debonding (mode β), matrix cracking (mode γ), fiber pull-out, and fiber fragmentation. This discussion will cover the theory underlying each process, description of FE models, and analysis of stress transfer [98]. The model (Fig. 31.32) is described using cylindrical polar coordinates. The axis of the fiber defines the z-axis. The shaded region, OAFE, represents a fiber having a length of 2L and is entirely embedded in a matrix material (unshaded region); both fiber and matrix materials are homogeneous and isotropic. The FE model possesses both axisymmetry (about OD) and mirror symmetry (about OB), so only a quarter of the model is needed for the analysis. The mesh contains quadrilateral elements. Larger elements are assigned where little changes in stresses are expected, for example, at regions in the matrix and fiber which are far away from the fiber end, and progressively smaller elements are introduced where the stress gradient gets larger, such as around the fiber ends. In practice, many more elements are used in the mesh. It then follows that those models for studying stress transfer processes are derived from one of these two basic designs and are distinguished by having different boundary conditions. Axisymmetry is implemented by constraining OD (Fig. 31.32). In many models mirror symmetry is implemented by constraining OB (Fig. 31.32). Natural fibers of plant origin, used as reinforcement in PMC materials, exhibit highly anisotropic elastic properties due to their complex internal structure.

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Figure 31.32 Sketch of a 3D axisymmetric FE model represented in 2D for an upper right quadrant [98].

Mechanical properties can be evaluated not only by tests but also by mechanical models reflecting the principal morphological features of fibers. Such a FEM model is applied to estimate the elastic properties of a UC of a short-fiber-reinforced composite, an elementary flax fiber embedded in a polymer matrix. An orientation averaging approach is used for the prediction of the stiffness of short flax fiberreinforced PMC. The numerical estimates of Young’s modulus are compared to the test results of extruded flax/PP composite [99]. A numerical analysis has been proposed for predicting fiber motion during injection molding of short-fiber-reinforced composites using the moving particle semiimplicit (MPS) method. Its meshless and Lagrangian nature enables us to track individual fibers and to easily represent free surfaces, in which the mechanism of fiber orientation in a T-shaped bifurcation was investigated experimentally and numerically. The fiber orientation of injection-molded glass-fiber/PP composite was observed by X-ray CT. Despite the symmetric mold shape, there was asymmetric fiber orientation due to the mold filling process. Fiber motion in the bifurcation was then analyzed by the proposed simulation, and the fiber orientation was quantitatively evaluated in each small region. The prediction agreed well with the experiment, and the associated mechanism of fiber orientation is discussed. Furthermore, this approach explicitly demonstrates the interaction between fibers, which is an advantage of the proposed approach [100]. The tensile strength of gypsum matrix composites containing different volume fractions of randomly distributed short aluminum fibers with 6, 9, and 12 mm length has been modeled using multiscale FE analysis. In order to simulate the tensile behavior of the composites, each specimen with a particular fiber volume fraction was considered as a combination of numerous cubic elements each of which contains short fibers with variable local fiber volume fraction and independent arbitrary orientations. Numerical models were conducted based on the micromechanical methods of Cox (combined with the Halpin
Tsai approach), by applying different

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random distribution functions. The results have been verified by experimental data obtained from tensile tests carried out on special briquette-shaped composite specimens. The numerical analysis gives satisfactory agreement with the experimental results. By increasing the bonding strength of aluminum fibers to the matrix (i.e., by anodizing) agreement between experimental and numerical results is more pronounced [101]. The analysis of size effects in short-fiber-reinforced composites has been done numerically. The microstructure of such composites often represents the first hierarchy level of a bioinspired material. For modeling fiber cracking as well as debonding between fiber and matrix material, a fully 3D cohesive zone model is applied. It was shown that this captures the size effect associated with material failure of a single fiber. Furthermore, this scaling effect strongly depends on the shape and orientation of the assumed preexisting crack. For this reason, a 2D description can usually only predict the size effect qualitatively. Based on the aforementioned findings, a representative volume element (RVE) containing ceramic fibers embedded within a polymer matrix is considered. Similar to the single fiber, the RVE also shows a pronounced size effect. However, the underlying physical process is significantly more complex. More explicitly, the size effect of the RVE is a superposition of that related to the isolated fibers as well as of that induced by debonding of the fibers from the matrix material (Fig. 31.33). For estimating the different effects, a perfect bond is also modeled [102]. The parameters given in Tables 31.2 and 31.3 show that the critical separation at which the respective cohesive zone fails, is very large for the polymer matrix (6 μm), compared to approximately 1 μm for the fiber and the interface, that is, the fiber fails before the matrix and the minimum applied displacement to tear the RVE apart is 6 μm. The FE model of the specimen having a center crack and that showing a surface crack are significantly different: while for a center-cracked fiber an axisymmetric model similar to that shown in Fig. 31.34 is sufficient, a fully 3D model must be used for capturing the surface cracked fiber as shown in Fig. 31.35. The crack front radius for the latter is assumed to be equal to the fiber’s radius. Again, the FE mesh is the same for all different sizes with 38,000 continuum and 1400 cohesive elements resulting in 130.500 degrees of freedom. The loading is a prescribed displacement in longitudinal direction at the top surface, while the side surfaces are fixed (uniaxial straining). One of the fibers has a precrack at the surface with an area of 20% of the initial cross-section area (see Fig. 31.36). Fig. 31.37 presents the stress
displacement response of the RVE for selected sizes including perfectly bonded and debonding interface. For a small RVE with fiber debonding (Fig. 31.37), the mechanical behavior is similar to that for the perfect bond (Fig. 31.37). However, some interface debonding also occurs, which reduces the peak stress. For the larger RVE, debonding is the main failure mechanism. The slight decrease after the peak stress is caused by a gradual separation of the interface. After some debonding, matrix damage starts,

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Figure 31.33 Modeling material failure by means of a cohesive zone approach. The traction vector Ts is a monotonically decreasing function depending on the displacement discontinuity (amplitude) ⟦u⟧. A fully open crack is stress-free, that is, Ts 5 0 [102].

Table 31.2 Material parameters characterizing ceramic fibers [102]. Young’s modulus Poisson’s ratio Fracture energy Cohesive strength Cohesive elastic stiffness Critical separation Flaw tolerant size

E V Gf σc c δc hft

250,000 MPa 0.27 640 J/m2 600 MPa 50,000 MPa/μm 1.067 μm 440 μm

which increases the rate of interface debonding and leads to a strong decrease in the resulting stress. Again, the last branch of the curves (after 2 μm displacement) is due to matrix cracking after complete debonding.

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Table 31.3 Material parameters characterizing the polymer and the interface between fiber and matrix [102].

Polymer Young’s modulus Poisson’s ratio Fracture energy Cohesive strength Cohesive elastic stiffness Critical separation

EP vpp Gf,p σc,P c,p δc,P

3000 MPa 0.33 40 J/m2 120 MPa 3000 MPa/μm 6.0 μm

Gf, int σc,int cint δc,int

20 J/m2 40 MPa 3000 MPa/μm 1.0 μm

Interface between polymer and ceramic Fracture energy Cohesive strength Cohesive elastic stiffness Critical separation

The effect of the RVE size on the peak stress and the fracture energy is shown in Fig. 31.38. The failure of small perfectly bonded RVEs can be assessed by a strength criterion, that is, by the average strength of the fiber and the matrix. The peak stress decreases for larger RVE sizes. However, due to the suppressed bending of the fiber in the composite, the transition regime occurs for larger RVEs and is more pronounced than for a single fiber. The fracture energy of the composite is hardly affected by the RVE size (see Fig. 31.38B). As expected, for a debonding interface, the peak stress is always smaller than for the perfectly bonded interface. The peak stress decreases significantly when debonding is the main failure mechanism, that is, for sizes of 10 μm and above (see Fig. 31.38A) [102]. In this case, the fracture energy (Fig. 31.38B) increases strongly due to high deformation capacity, which is caused by the gradual debonding of the interface. The size effect associated with material failure in short-fiber-reinforced composites has been investigated by FE simulations of RVEs. Fiber cracking as well as debonding between fiber and matrix material was captured by adopting a fully 3D cohesive zone model. The numerical results have shown a pronounced size effect of the RVE. The two sources identified for this effect are breaking and debonding of the fibers. For isolating these effects and for quantifying their influence, perfect bonding was numerically enforced. Furthermore, the size effect of the single fibers was separately investigated. It was shown that a single fiber reaches its theoretical strength, if the diameter is smaller than a certain threshold. This threshold depends strongly on the shape and orientation of the preexisting microcracks [102]. A center crack, as assumed in the one-dimensional model, leads usually to an overestimation of the flaw tolerance. For this reason, a fully 3D model is usually required. In summary, the diameter of the fibers implicitly defines their relative strength. Analogously, by varying the length of the fibers, the fracture energy as well as the average strength related to fiber debonding can be modified. For this reason, an optimization of these two parameters (length and diameter of the fibers) with

Figure 31.34 Detail of the deformed 2D cracked specimens (double symmetry is utilized); (A) small specimen (W 5 1 μm) showing an almost homogeneous crack opening and (B) large specimen (W 5 600 μm) showing a pronounced opening at the crack tip [102].

Figure 31.35 Deformed 3D edge-cracked fiber; (A) small diameter (2 μm) showing an almost homogeneous crack opening and (B) large diameter (40 μm) showing a pronounced opening at the crack tip and compressive stress at the back face [102].

Figure 31.36 Arrangement of unidirectional short fibers in the composite and finite element mesh of the representative volume element [102].

Figure 31.37 Stress
displacement response of the RVE for selected sizes including perfectly bonded and debonding interface [102].

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Figure 31.38 Size effect of the 3D RVE (initial crack width 10%) for perfectly bonded and debonding fibers: (A) averaged peak stress and (B) averaged fracture energy [102].

respect to optimized macroscopic fracture properties of the fiber-reinforced composites seems to be promising. The growing usage of short-flax-fiber-reinforced polymer composites in such applications as the automotive industry necessitates the prediction of their mechanical response up to and beyond the limit of elasticity. Due to the imperfect, mechanical interlocking-dominated adhesion of natural fibers to most polymers, both fiber debonding and matrix yielding contribute to the nonlinear deformation. The deformation under an active loading of a short misaligned fiber composite was modeled by the orientation averaging approach, employing an analytical description of the behavior of a UC, the parameters of which are determined using an FEM analysis of UC response under selected loading modes. The model is applied to the prediction of stress
strain diagrams in tension of flax/PP composites with different fiber volume fractions [103]. The effective elastic properties of composites reinforced by spatially randomly distributed short cylindrical fibers with certain aspect ratios have been investigated using the numerical homogenization method, in which a modified RSA algorithm is proposed to generate the periodic RVEs. The periodic boundary conditions are introduced and the periodic RVEs thus created are analyzed to obtain the mechanical properties of composites by using the FE package ABAQUS. The ABAQUS Python Interface is used to introduce the periodic boundary conditions and to obtain the average stresses and strains of RVEs. The simulation results show that the periodic boundary conditions guarantee the continuity of strain and stress fields on the boundaries of RVEs. In the case of a fiber aspect ratio of 15 and fiber volume fraction of 10%, it is sufficient to consider the size of RVE as L/l 5 2.5, in which an approximately random fiber orientation exists. The effective elastic properties of composites obtained by the numerical homogenization agreeing well with those obtained from the traditional equations for composites based on the Halpin
Tsai estimation and with those measured from the

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uniaxial tensile experiments shows the validation of the numerical homogenization for effective elastic properties of composites reinforced by spatially randomly distributed short fibers [104]. Here, the modified random sequential adsorption algorithm [104] combining special conditions has been used to generate the periodic RVEs of composites reinforced by the spatially randomly distributed fibers up to the desired volume fractions, and the flowchart is given in Fig. 31.39. In general, these algorithms can generate up to 15% volume fractions with identical fibers. Here, the identical fibers are considered only and the generated periodic RVE and mesh of composites reinforced by the spatially randomly distributed fibers with aspect ratios of 15 and fiber volume fraction of 10% are shown in Fig. 31.40. As follows from the formulation of the periodic boundary conditions, the simulations have predefined displacement constraints satisfying the displacement continuity condition that yields the traction continuity condition as shown in Fig. 31.41 [104] in the form of Von Mises stresses calculated at the centroid of the FEs in two opposite surfaces, face top and face bottom. The values of the stresses and strains are extrapolated from the respective integration points using the shape functions of the element or elements sharing the nodes [104]. Fig. 31.42 shows the relative difference and average difference of the extrapolated Von Mises stresses, the extrapolated maximum principal stresses, and the extrapolated maximum principal strains on the opposite nodes with the peer-topeer comparison, respectively. Briefly, a technique to evaluate effective elastic properties related to spatially randomly distributed short-fiber-reinforced composites having periodic microstructures has been presented. One primary aspect of this is the development of an appropriate microcell model for the FE-analysis in ABAQUS with the periodic boundary conditions. Using the modified random sequential adsorption algorithm, periodic RVEs are generated. Taking both definitions of RVE into account, the size of RVEs of composites reinforced by spatially randomly distributed short fibers with an aspect ratio of 15 is selected as L/l 5 2.5 (L is the length of the cubic RVE and l is the length of the identical fiber). The homogenized effective elastic material properties are in good agreement compared with the estimations obtained by using the traditional equations for short random-fiber composites based on the Halpin
Tsai theory and from the uniaxial tensile experiments. Meanwhile, it confirms that the mechanical behavior of spatially randomly distributed short-fiber-reinforced composites is approximated to that of an isotropic material, resulting from the approximately random distribution of fiber orientation in the RVEs. The developed Python-Scripts in combination with the ABAQUS batch processing provide a powerful tool for the rapid calculation of effective material properties for composites reinforced by spatially randomly distributed short fibers with any number of phases, that is, there are no restrictions regarding the number of materials, geometry, and material symmetry [104]. The simulation image processing of thermoplastic reinforced with short fibers has been done with an established code in Matlab for the determination of the sizes, orientation, and ambiguity of the fibers. It gives a general idea about what happens before crystallization in the mold. Generally, the simulation of short-fiber orientation involves

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Figure 31.39 Flowchart of the periodic RVE generation based on the modified RSA algorithm [104].

coupled analysis of flow, temperature, moving free surface, and fiber kinematics. For the governing equation of the flow, a Hele
Shaw flow model along with the generalized Newtonian constitutive model has been widely used. The kinematics of a group of fibers is described in terms of the second-order fiber orientation tensor [105]. An unstructured mesh Galerkin FEM has been used to obtain estimates of the viscoelastic moduli of SGF-reinforced polymer composites. Periodic Monte Carlo

Figure 31.40 RVE and mesh of composites reinforced by spatially randomly distributed fibers with aspect ratios of 15: (A) RVE; (B) mesh [104].

Figure 31.41 Von Mises stresses calculated at the centroid of the finite elements in two opposite surfaces: face top and face bottom [104].

Figure 31.42 Difference of the extrapolated Von Mises stresses, the extrapolated maximum principal stresses and the extrapolated maximum principal strains on the opposite nodes: (A) relative difference; (B) average difference [104].

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models with 125 identical inclusions are studied. Both spheroidal and spherocylindrical inclusions are considered. The estimates are compared against predictions of the dilute approximation, Mori
Tanaka (MT), and self-consistent (SC) models. It is shown that at small fiber fractions, the dilute approximation (Eshelby) model is exact. However, for the axial stiffness the dilute regime is limited to fiber volume loadings of a few tens of a percent, while typical SGF polymer composites have fiber loadings from 10% to 20%. It is found that in this concentrated regime, both MT and SC models give excellent predictions for all but the axial stiffness modulus. To assess the feasibility of reliable stiffness and vibration damping design of composite structures from short-fiber-reinforced polymers, Monte Carlo models with various fiber orientation distribution (FOD) states are studied. It is shown that the quick Voigt (constant strain) orientation averaging procedure gives excellent viscoelastic stiffness predictions provided that the FE estimates are used for the required moduli of the basis FOD state with fully aligned fibers [106]. Micromechanics analysis using the RVE approach implemented with the FEM has been widely used for computing material properties of unidirectional fibrous PMCs. However, little attention has been given to viscoelastic RVEs of discontinuous fiberreinforced composites. In this research, a RVE-based FE algorithm for evaluating the effective viscoelastic creep behaviors of aligned short-fiber composites has been developed. A parametric study including considerations of fiber volume fraction, fiber aspect ratio and fiber packing geometry is performed through the proposed algorithm. Computed results indicate that increasing the fiber volume fraction decreases the mechanical compliance of the overall compound, and the effect of fiber reinforcements is particularly significant in the direction of fiber alignment. Additionally, increasing the fiber aspect ratio reduces the creep compliance coefficient along the direction of fiber alignment more than coefficients along other directions. The fiber packing geometry affects the values of axial compliance properties at low fiber volume fraction and its impacts become less as the fiber volume fraction increases. We also provide an application to simulate the equivalent viscoelastic creep response out of the RVE approach through ABAQUS user-defined material subroutine, and the maximum absolute error between the two sets of data is only 1% [107]. In addition to the fiber volume fraction and aspect ratio, various fiber-packing geometries also yield different material responses. In particular, we consider a regular array-packing geometry and a staggered array-packing geometry (Figs. 31.43 and 31.44), such as those investigated by Tucker and Liang for elastic stiffness of short-fiber composites previously. Notice, in Fig. 31.43, that the green area denotes the polymer matrix and the yellow circles are fiber reinforcements. Additionally, the purple and yellow circles appearing in Fig. 31.44 indicate that fibers are packed in a staggered array. Also, the staggered fibers are centered along the length of neighboring fibers in the staggered array RVEs considered here [107]. It is important to note that discontinuous fiber-filled polymers with the assumed perfectly unidirectional fiber alignment appearing in Figs. 31.43 and 31.44 are difficult if not impossible to fabricate experimentally, to our knowledge. Interesting results for the RVE subjected to a longitudinal shear load are given through Fig. 31.45.

Figure 31.43 Unidirectional short fibrous composites in regular array fiber-packing geometry [107].

Figure 31.44 Unidirectional short fibrous composites in staggered array fiber-packing geometry [107].

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From the results shown in Fig. 31.46, it is clearly seen that the uniformly meshed RVE responds homogeneously, while the mesh of the regular array packed twomaterial RVE exhibits different displacements spatially due to the heterogeneous material definition of the matrix and fiber phases. Moreover, the legends showing the strain responses of the integration point between the subfigures of Fig. 31.46 also varies significantly, such that the two-material RVE yields a directly different strain response, while the one-material UC exhibits a similar response over the entire domain [107]. This indicates that the one-material homogeneous model defined by our modified subroutine can yield equivalent homogeneous creep response out of creep data obtained by the two-material defined RVE. One advantage of the RVE approach [107] is that the heterogeneous medium of a composite material is replaced by a homogeneous continuum while the anisotropic material behaviors are retained. Equivalent homogenized material behaviors such as elastic constants, damage, and failure mechanisms of fiber-reinforced polymers are seen among the existing literature (e.g., [108
112]).

31.4

Experimental methods

The thermal expansion behavior of short carbon fiber/epoxy composites was investigated with respect to fiber length and fiber volume fraction. The effective coefficient of thermal expansion of short carbon fiber/epoxy composites was calculated based on the Halpin
Tsai equation for module and the linear coefficient equation. From the investigation, it has been found that the coefficient of thermal expansion is more dependent on fiber volume fraction than fiber length. Therefore the ferrule for optical connector that has compatible coefficient of thermal expansion with the optical fiber was manufactured with short carbon fiber/epoxy composite by controlling the fiber volume fraction [113]. For short-fiber polymer composites such as CSM laminates, glass mat thermoplastics (GMT), and SMCs, conventional methods to measure fracture energy are often not valid since linear elastic fracture mechanics (LEFM) criteria are not fulfilled. For the case of a long bridged crack with a small damage zone perpendicular to the crack, a crack-bridging approach may instead be used. The bridging law is an important material parameter. The bridging law in combination with stress analysis can be used to address problems where LEFM is invalid. The bridging law of the material is related to material composition and fiber architecture. It is therefore of interest to develop an experimental method for determination of bridging law and fracture energy in short-fiber polymer composites. A method based on a large double-cantilever beam (DCB) specimen loaded by pure bending moments has been used. Commercial GMT and SMC materials have been investigated as well as CSM laminates. Fracture energy and bridging law data were determined. All materials demonstrate softening bridging laws and this is discussed based on observed mechanisms of failure. A DCB method to measure the fracture energy and bridging law for short-fiber polymer composites was investigated. Fracture energy and bridging-law data are

Figure 31.45 Strain contour for a short-fiber composite regular array RVE: (A) instantaneous strain contour; (B) long-term strain contour resulting from longitudinal shear stress load [107].

Figure 31.46 Long-term strain contour for a short-fiber composite regular array RVE subjected to longitudinal axial stress of magnitude of 1: (A) two-material RVE model; (B) one homogenized material model [107].

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determined although further work is needed to produce data which are strictly valid. Softening bridging laws were observed for all materials investigated, due to pullout of fibers. As the crack opening increases, more fibers become completely pulled out and no longer contribute to the bridging stress [114]. In contrast to assumptions in many theoretical studies, the maximum local bridging stress is higher than the material tensile strength due to differences in failure mechanisms. Other experimental observations of fiber volume fraction, matrix, and fiber length effects correlate well with predictions from brittle matrix composite models, based on the assumption of fiber pull-out as the dominating mechanism of energy absorption. Research on short fibers/rubber foam composites is rarely found in the literature. Therefore the microcellular rubber foams unfilled (MF), strengthened by pretreated short fibers (MFPS) and untreated short fibers (MFUS) have been prepared, respectively. The microstructure and mechanical properties of the three composites have been studied via SEM and mechanical testing, respectively. The SEM results show that both pretreated and untreated short fibers disperse uniformly in the composites and in bidimensional orientation. Moreover, the pretreated short fibers have much better adhesion with the rubber matrix than untreated ones. The experimental results also indicate that the introduction of short fibers is mainly responsible for the great enhancement of most mechanical properties of the microcellular rubber foams, and the good interfacial adhesion of the short fibers with the matrix contributes to the more extensive improvement in the mechanical properties. It is also found that the reinforcement effect of short fibers to compressive modulus strongly depends on the density of microcellular rubber foams, the orientation of short fiber and the deformation ratio. The compressive modulus of microcellular rubber foams at the normalized density less than 0.70 and beyond 0.70 is predicted by the modified simple blending model and the Halpin
Kerner model, respectively. The theoretically predicted values are in good accordance with the experimental results. Fig. 31.47 shows the typical microstructures of MF, MFUS, and MFPS specimens with the same normalized density, respectively. First, as shown in Fig. 31.47A
C, pretreated short fibers are detached into singles, distributing evenly in the composites; most of the fibers are in bidimensional orientation because of the process molding technique. The same happens in the microcellular rubber foams filled with untreated short fibers. Second, the topologies of the cross-section planes show that the pretreated short fibers have good adhesion with the rubber matrix, while the untreated short fibers are surrounded with gas cells. This implies that pretreating short fibers using the adhesive efficiently improves the compatibility between the matrix and short fiber, and therefore avoids the gas cells surrounding the short fiber, while untreated short fibers easily induce the foaming on their surfaces due to their high surface energy. For application, it is necessary for short fibers to stay in the rubber matrix (planar walls and vertical struts) to guarantee the reinforcement and, hence, using pretreated short fibers has greater advantage [115]. In addition, Fig. 31.47 also suggests that almost all gas cells, with about 0.1 mm average cell diameter, distribute uniformly in the three composites. Moreover, the thickness of the cell walls is very uniform. All these illustrate that the curing characteristic of vulcanizes well matches the decomposition characteristic of the

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Figure 31.47 SEM micrographs of razor cut surfaces of microcellular rubber foams: (A) MF with normalized density 0.65, (B) MFUS-with normalized density 0.67, and (C) MFPS-with normalized density 0.68 [115].

blowing agent H, and that the short fibers have no influences on the cure process of microcellular rubber foams, nor on the main foaming dynamics of the composites. Moreover, the cell shapes in MFUS and MFPS (Fig. 31.46B and C) are somewhat more irregular than those in MF (Fig. 31.47A), demonstrating that the

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morphology of the cell structure is affected by the presence of the short fibers. It is tempting to speculate that when the composites are vulcanized, blowing agent H generates gas and the gas swells out isotropically in MF because of the homogeneous matrix, which is basically the same as the foaming mechanism of plastic foams and sponges. However, for MFUS and MFPS, the gas cells’ expansion may be isotropic because of the incorporation of short fibers to the rubber matrix [115]. Representative tensile stress
strain curves of MF, MFUS, and MFPS with almost the same normalized density of 0.67 (typical density in references) are presented in Fig. 31.48. The figure well exhibits that MF specimens’ stress increases are low and linear with the rising tensile strain. In comparison, the stress of MFPS elevates sharply with tensile strain followed by a sustained plateau region where the yielding happens due to the interfacial debonding or interfacial region being destroyed. For MFUS, the stress
strain curve is between those of MFPS and MF, but more similar to that of MFPS. The tensile modulus of microcellular rubber foam specimens as a function of normalized density is summarized in Fig. 31.49 [115]. The tensile strength of microcellular rubber foams as a function of normalized density is shown in Fig. 31.50. By comparing between the theoretical values predicted and the experimental results shown in Fig. 31.51, it is obvious that the predicted theoretical values are almost in agreement with the experimental data, though slightly higher than the research results [115]. The lower the normalized density is, the larger the

Figure 31.48 Comparison of tension behavior for microcellular rubber foam specimens with the same normalized density (0.67) [115].

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Figure 31.49 Tensile modulus for microcellular rubber foam specimens as a function of normalized density [115].

discrepancy between the predicted values and the experimental data. It should be stressed that the above model does not take into consideration the impact of shortfiber orientation, the adhesive bonding, and other factors on the compressive modulus. Greater effort is thus needed to propose a more advanced model including all these factors. Finally, in the experiments, MF, MFPS and MFUS are prepared, respectively. The SEM results show that both pretreated and untreated short fibers disperse uniformly in the composites and in bidimensional orientation, and the pretreated short fibers have much better adhesion with the rubber matrix than untreated ones. The experimental results also indicate that the introduction of short fibers into microcellular rubber foams is mainly responsible for the great enhancement of most mechanical properties such as tensile modulus, hardness, and tear strength; the good interfacial adhesion of the short fibers with the matrix contributes to the further improvement in the mechanical properties; the rebound resilience almost keeps constant at a given normalized density; while the tensile strength is decreased. It is also found that the reinforcement effect of short fibers on the compressive modulus strongly depends on the density of microcellular rubber foams, the orientation of short fibers, and the deformation ratio. The compressive modulus of microcellular rubber foams at the normalized density less than 0.70 and beyond 0.70 is predicted by the modified simple blending model and the Halpin
Kerner model, respectively. The theoretical predicted values are in accordance with the experimental results. Nowadays, researches on nanoclay/rubber composites have attracted great interest

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Figure 31.50 Tensile strength for microcellular rubber foams specimens as a function of normalized density [115].

Figure 31.51 Comparison of the relative compressive modulus of MFUS and MFPS at the normalized density beyond 0.70 to the theoretical value predicted from the Halpin
Kerner model [115].

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[115]. Extensive work on the nanoclay/microcellular rubber foams is interesting and promising Composites were prepared by mixing biodegradable poly(ε-caprolactone) thermoplastic (PCL) with short flax fiber bundles. In order to improve fiber/matrix adhesion, poly(ε-caprolactone)-g-maleic anhydride (MA) copolymer (PCL-g-MA) compatibilizer was prepared in an internal mixer. The grafting reaction of MA onto PCL polymer was carried out in the presence of dicumyl peroxide as an initiator. Mechanical properties were analyzed as a function of compatibilizer concentration and fiber amount. In addition, thermal properties of flax/PCL and flax/PCL-g-MA composites were also examined by thermogravimetric (TG) analysis. Composites fabricated with flax fiber bundles and PCL-g-MA matrix showed the highest tensile and flexural strength. SEM of fractured surfaces confirmed the adhesion improvement between flax fiber bundles and PCL-g-MA matrix. Results obtained by TG analysis showed that fiber addition and matrix modification slightly reduced the thermal stability of composites. The correlation between the experimental mechanical properties of short flax fiber bundle reinforced PCL composites with values calculated by various empirical models has also been analyzed. For composites based on PCL-g-MA matrix, a good agreement was found between empirical model and experimental values for all fiber contents. However, for composites based on the PCL matrix a good agreement only existed until 20 wt.% flax fiber content, and beyond this value, experimental strength fell well below predictions [116]. In order to enhance the mechanical properties of B4C without density increase, the short carbon fibers M40, M55J, and T700 reinforced B4C ceramic composites were fabricated by a hot-pressing process. The addition of the carbon fibers accelerates the densification of the B4C, decreases their densities, and improves their strength and toughness. The enhancement effects of the three kinds of carbon fibers were studied by investigating the density, Vickers hardness, and the mechanical properties such as flexural strength, flexural modulus, and fracture toughness of the composites. The fiber type has a great influence on the mechanical properties and enhancement of the short carbon fiber-reinforced B4C composites. A flexible carbon fiber with high strength and low modulus such as T700 is appropriate to reinforce the B4C matrix ceramic composites [117]. The effect of fiber diameter change on the toughness of short-fiber polymerbased composites was studied by assuming that the main energy absorption upon fracture is the fiber pull-out mechanism. An expression is derived for a characteristic fiber length, distinct from the critical length, above which an increase in fiber diameter always yields an increase in composite toughness. Experimental validation was provided through impact testing of composites made of short polyethylene terephthalate fibers in a PP matrix [118]. SEM photographs of the fracture surfaces of impacted specimens (Fig. 31.52) clearly reveal ample evidence for extensive fiber pull-out and apparent low adhesion [118]. Thermal-ratcheting behavior in short-fiber-reinforced metal matrix composites has been investigated. The internal stresses due to mismatch in thermal expansion coefficients between the fiber and matrix in the composites can introduce

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Figure 31.52 Scanning electron micrograph [118] of the fracture surface of PET/PP composites following Izod impacting. Fiber pull-out is prominent, and clean fiber surfaces and holes in the matrix point to relatively poor interfacial adhesion, as indeed confirmed by single fiber pull-out tests. The fiber diameter is 17 μm.

anomalous deformation under thermal-cycling conditions with or without external mechanical loads. A more comprehensive understanding of such thermal-ratcheting behavior in short-fiber-reinforced composites was presented subject to a wide range of external uniaxial loads based on a mean-field micromechanical model considering the local mass transfer by diffusion along the fiber
matrix interface. The validity of the model has been verified through systematic experiments with directionally solidified Al
Al3Ni eutectic composites [119]. The crack initiation and propagation of short carbon fiber-reinforced geoPMCs (Cf /geopolymer composites) during bending test were observed in situ by environmental SEM (ESEM). Many microcracks initiate, and then propagate on the side of the beam sample with the increase in the bending load. A nearly elastic response of the load
displacement curve and significant deformation of the composites are observed at the initial stages. The propagation of the microcracks ceases, and these cracks tend to close to some extent while the main crack forms. The fiber bridging effect in the micro- and main cracks effectively maintains the composite integrity and makes the composites exhibit a noncatastrophic fracture behavior. A simple mode for the damage behavior of the composites during the bending test is discussed [120]. The typical load
displacement curve for the Cf /geopolymer composites is given in Figs. 31.53 and 31.54. The composites exhibit a significant deformation and an

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Figure 31.53 Load
displacement curve for the Cf /geopolymer composites [120].

obvious noncatastrophic fracture behavior during the bending test, which is regard as a great toughening effect for the short carbon fiber with such a low volume percentage (3.5 vol.%). The composites exhibit a nearly elastic response in the initial stages (stages I and II), though a change appears at a load of about 6 N, which is similar to that of unidirectional continuous fiber-reinforced composites. Beyond the elastic limit, the applied load produces plastic deformation until the maximum load is reached. Then the load gradually decreases with the increasing displacement, and forms a long tail (stage III) [120]. The typical load
displacement curve for the Cf /geopolymer composites is given in Fig. 31.53. Fig. 31.54 shows a series of ESEM images of crack initiation and propagation process on the side of the beam sample of the composites, which corresponds to the test points in the load
displacement curve in Fig. 31.53. At the first elastic stage (stage I), no crack is found on the beam sample as shown in Fig. 31.54A. However, at the beginning of the second elastic stage (stage II), a lot of microcracks (Fig. 31.54B) appear on the side of the beam sample. Other new microcracks will occur on the beam surface, as shown in Fig. 31.54B [120]. This interesting phenomenon indicates that the stress distribution in the matrix has been well changed due to the enhancement effect of the reinforced fibers. Studies on this phenomenon will be carried out in the future. Though the formation of these microcracks reduces the matrix elastic modulus, as indicated by the load
displacement curves in Fig. 31.53, the sample keeps a nearly elastic deformation behavior accompanied by the propagation of microcracks (Fig. 31.54C). Under the increasing bending load, the microcracks grow at similar rates. This unconventional fracture behavior is supposed attributed to the following reasons.

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Figure 31.54 Series of ESEM images (A
E) of crack initiation and propagation process on the side of a beam sample of the Cf /geopolymer composites corresponding to the position A
E of the load
displacement curve in Fig. 31.53 separately [120].

The short carbon fibers used in this study have a length of 7 mm and the gap lengths are 300
500 μm, as shown in Fig. 31.54C. Hence, the fibers are long enough to bridge several microcracks together. As discussed above, the fibers have a far higher mechanical strength than that of the matrix. Thus, the bridging fibers in the microcracks are difficult to fracture, which is helpful in keep maintaining the composite integrity and retarding the formation of a main crack [120]. The significant deformation of the composites can be attributed to the large number of microcracks during the bending test.

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It can be seen from Fig. 31.55A that the microcracks are generated on nearly the whole span side surface of the beam sample. The gaps between two neighbor microcracks are 300
500 μm. The gaps become broader as they become more distant from the main crack, which means that the stress will be less as the stress locations are far from the crosshead. It is also found that the fracture path is not straight, as shown in Fig. 31.55B and C. A lot of crack deflection (Fig. 31.55D) and crack branching (Fig. 31.55E) are found, which undoubtedly leads to an increase in the fracture toughness [120]. Therefore in situ crack growth observation during the three-point flexural test shows that lots of microcracks form on the whole surface of the beam sample. With the increase in the bending load, the beam sample keeps a nearly elastic deformation behavior at the initial stages and exhibits an obvious displacement. The beam sample produces pseudoplastic deformation as the maximum load is reached. The propagation of the microcracks ceases, and they tend to close to some extent while the main crack forms because the stress in the microcrack area is somewhat relaxed and the stress in the main crack area greatly increased. The fiber bridging effect in the micro- and main cracks effectively keeps the composites integrity and bears the load during the bending test, which makes the composites exhibit a noncatastrophic fracture behavior [120]. The effective thermo-electro-elastic properties of MFCs have been evaluated by means of an analytical model based on “equivalent layered approach” using the concepts of “rule of mixtures” and “series and parallel capacitance theory.” To account for the complex electric field distribution and the constituents’ shape and position, a numerical model based on FE calculations is developed using a “unitcell approach.” The piezoelectric charge constants of MFCs are experimentally evaluated by applying electrical load in various thermal environments (27 C
70 C). The proposed models are validated with the results available in the literature, experimental values, and data from the manufacturer. Also, the effect of the thermal environment on the effective constants of MFCs for various volume fractions of piezoceramic phase is investigated [121]. Natural materials as fillers of polymers are of increasing potential in modern industrial areas. Their price as well as very good mechanical properties speak for their use. The industrial application of natural fillers is also predetermined by economic aspects and a relationship to the environment—a prioritization of renewable resources when it is necessary to find adequate utilization for secondary materials coming into being, for example, in the case of processing commodities of an oil palm—palm oil. It describes the basic strength characteristics of a short-fiber chaotic system with fibers gained from the waste—empty fruit bunches (EFBs)—which come into being during the process of oil pressing. The experimental approach describes undemanding preparation of composite systems without using special technologies with a respect to the needs of developing countries and simple applications above all. The experimental program compares the composite systems with nontreated fibers with the system with chemically treated surface of the fibers by 6% solution of NaOH. Electron microscopy was used for the evaluation of the interfacial interaction. The experiment describes short-fiber composites, studies the

Figure 31.55 The side of a bar specimen of the Cf /geopolymer composites after a three-point flexural test (A) and series of images of fiber pulling-out (B), fiber bridging (C), crack deflection (D), and crack branching (E) corresponding to zones 1
4 in image (A) separately [120].

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influence of the length (3, 6, and 9 mm), and the influence of the fiber concentration (2.5
10.0 vol.%) on observed characteristics—tensile strength, module of elasticity, hardness, and two-body abrasion. The chemical treatment of the fiber surface by the solution of NaOH influenced the surface structure of the fibers and it optimized the interaction of the fibers with used matrix. The inclusion of short EFB fibers increased the modulus of elasticity up to 47%, and decreased the tensile strength to 26%. EFB fibers did not significantly decrease the ability of the material to resist two-body abrasion. A described composite system and a production process can be marked as sustainable manufacturing [122]. An investigation was presented on increasing the fracture toughness of epoxy/short carbon fiber (SCF) composites by alignment of SCFs using an externally applied alternating current (AC) electric field. First, the effects of SCF length, SCF content, and AC electric field strength on the rotation of the SCFs suspended in liquid (i.e., uncured) epoxy resin are investigated. Second, it was shown that the mode I fracture toughness of the cured epoxy composites increases with the weight fraction of SCFs up to a limiting value (5 wt.%). Third, the toughening effect is greater when the SCFs are aligned in the composite normal to the direction of crack growth. The SCFs increase the fracture toughness by inducing multiple intrinsic and extrinsic toughening mechanisms, which are identified. Based on the identified toughening mechanisms, an analytical model is proposed to predict the enhancement to the fracture toughness due to AC electric field alignment of the SCFs [123]. The effects of hydrothermal aging on moisture absorption and mechanical durability of short carbon fiber-reinforced polyamide 6 (CF/PA6) composites have been analyzed by immersion in water at 20 C, 40 C, and 60 C. Tensile strength, Young’s modulus, and impact strength of PA6 and CF/PA6 specimens are monitored, and a predictive model based on Arrhenius methodology has been utilized to estimate the retention of tensile strength. By gravimetric experiments, moisture uptake and diffusion coefficients of PA6 and CF/PA6 composites are analyzed by a Fickian model, with good exponentially decaying fit with tensile strength [124]. The effect of addition of multiwalled carbon nanotubes (MWCNTs) has been measured with different weight percentage values (0.5%, 1%, and 1.5%) on the vibration and morphological properties of SGF (10, 20, and 30 wt.%) reinforced PP, and the PP foam composites. Nanocomposites were first compounded using the melt-compounding technique in a twin-screw extruder. Then, azodicarboxamide was added as a foam agent to investigate the foaming behavior of the samples. The free vibration behavior of the samples under clamp-free boundary conditions was studied via operational modal analysis (OMA). In order to confirm the experimental results, the Euler
Bernoulli beam theory was employed to calculate the values of the natural frequencies. The results of analyses indicated that the theoretical natural frequencies had a significant correlation with the experimental results within an 18% mean error level. The experimental results showed that addition of SGF particles to the PP and PP foam would significantly increase the natural frequencies, but this would in return decrease the damping factor. The results also indicated that, with the addition of MWCNTs to the samples, the damping factor and natural frequencies of the PP and SGF/PP composites would increase. The highest increase in

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the natural frequencies amounted to 118.8%, which resulted from a combination of 20 wt.% SGF and 1 wt.% MWCNTs. On the other hand, the highest increase in the damping factor amounted to 68.9%, which resulted from a combination of 20 wt.% SGF and 1.5 wt.% MWCNTs [125]. Fig. 31.56 presents a schematic view of the vibration test system. Fig. 31.57A and B show the typical stabilization diagrams of the estimated state space model for the PGF-CNT1 and PFGF2 samples [125]. The natural frequencies and damping ratios for all the samples were estimated based on the stabilization diagrams. Therefore the effects of different amounts of MWCNTs (0.5
1.5 wt.%) on the vibration and morphology of the SGF (10
30 wt.%)/PP composites and SGF/PP composite foams were examined. For this purpose, the Euler
Bernoulli beam theory was used to validate the experimental natural frequencies, while the morphology of the samples was investigated via SEM micrographs. The most important findings of the present study can be summarized as follows: 1. Both the natural frequencies and damping ratios of the neat PP and PP foam were enhanced as different amounts of MWCNTs were added to the samples. In comparison with the other amounts, the addition of 1 wt.% of MWCNTs showed the maximum improvement in the natural frequencies, while the addition of 1.5 wt.% of MWCNTs showed the maximum improvement in the damping ratios of the samples. 2. The natural frequencies of the PP and PP foam gradually grew as the SGF content was increased. In contrast, the damping ratios of the neat PP and PP foams decreased as the SGF content was increased. 3. Adding the MWCNT particles to the SGF/PP composite and SGF/PP composite foam led to larger improvements in the natural frequencies of the samples. The maximum improvement occurred for 1 wt.% of MWCNTs. 4. As the MWCNTs were added to the SGF/PP composite and SGF/PP composite foam, the damping ratios of the samples in both the first and second modes improved significantly. The maximum improvement occurred for 1.5 wt.% of MWCNTs. 5. By combining the SGF and MWCNT particles and adding the combination to the PP matrix, we could achieve PGF
CNT hybrid nanocomposites which simultaneously have high natural frequencies (stiffness) and high levels of dissipating vibration energy.

Figure 31.56 Schematic view of the vibration test system [125].

Figure 31.57 A stabilization diagram of the samples: (A) PGF-CNT1 and (B) PFGF2 [125].

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6. The natural frequencies of the foam samples were lower, and their damping ratios were higher, than those of the solid samples in both the first and second modes. 7. The theoretical natural frequencies had a significant correlation with the experimental results within the 18% mean error level. 8. The main advantage of OMA is that it reduces the loading effects of the force transducer via stimulation of a structure through environmental loads. In addition, use of a laser to record the acceleration rate would help eliminate the loading effects of the accelerometer. 9. The SEM and TEM images showed that the MWCNT particles were well dispersed in the PP matrix at low loading level contents, and that the enhancement of the interface between the matrix and fillers was obtained. In addition, some agglomerates were detected once 1.5 wt.% of MWCNTs was added. 10. The SEM images of the foam samples showed that the MWCNT particles brought a significant decrease, and a narrower distribution of the cell size, to the SGF/PP composite foams. 11. The best foam qualities of the PFGF-CNT1 sample were obtained when 1 wt.% of MWCNTs was added to the sample [125].

In addition, a large number of studies have examined the effects of MWCNTs on the mechanical and structural properties of polymeric foams [126
128]. The average response and isotropy of the 3D RVEs for random short-fiberreinforced elastomer composites (SFECs) have been explored by a FEM with different fiber volume fractions (Vf ) and RVE sizes. The RVEs were loaded spatially by directly applying stretches in nine loading directions and the homogenized response in each direction was obtained. The coefficient of variation of the responses of an RVE over different loading directions was used to represent the anisotropy of the RVE. The orientation tensor error was used to represent the anisotropy of all the fibers and was compared with the anisotropy of the RVEs. The simulation results for the SFEC were compared with the elastic modulus obtained by traditional empirical equations based on Halpin
Tsai estimations. The results show that the anisotropy of the RVEs decreases with an increase in the RVE size and is higher for RVEs with higher Vf . The anisotropy of the fibers decreases with an increase in the Vf . A method of averaging responses of each RVE over all loading directions greatly reduces the variation in response over different RVEs, which can be used to improve the prediction accuracy more efficiently than increasing the RVE size [129]. The fracture prediction of U-notched single-edge notched bend (SENB) specimens made of the SGF-reinforced polyamide 6 (SGFR-PA6) with variable moisture has been investigated experimentally and theoretically. In the experimental program, numerous rectangular specimens weakened by edge U-shaped notches of different tip radii are tested for fracture under pure mode I loading, and the load-carrying capacity (LCC) of the specimens is experimentally measured. In the theoretical program, in order to avoid time-consuming and complex elastic-plastic failure analysis, and due to the significant strain-hardening and the substantial strain-to-failure of the composite material tested, the prediction of the experimentally obtained LCCs is approached in order to test its validity using the fictitious material concept (FMC), proposed most recently by the first author, in combination with the well-known theory of critical distances (TCD).

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According to the new proposed failure model, since the fictitious linear-elastic material is used, TCD can be applied by means of its basic formulation without considering any previous calibration of the corresponding critical distance. It is revealed that the FMCTCD combined criterion can predict the experimental results well [130]. The stress
strain curves (engineering variables) of the SGFR-PA6 tested in this research are shown in Fig. 31.58 [130]. The tests were conducted at room temperature (20 C) using an Instron 8501 universal testing machine, the strain being measured with an axial Instron extensometer with a 10 mm gauge length. These curves are used in Section 31.3 to compute the tensile strength value of the fictitious material. A 3D view of the SENB sample is illustrated in Fig. 31.59. Also, some of the tested SENB specimens after the fracture tests are shown in Fig. 31.60. Fig. 31.61 gathers the results obtained after applying the EMC-PM (PEMC-PM) and the FMC-PM (PFMC-PM) models to the different U-notched SGFR-PA6 specimens, including the minimum and maximum deviations of the predictions from the experimental results [130]. Finally, the experimental failure research was conducted on some U-notched rectangular specimens with various notch tip radii made of SGF-reinforced polyamide 6 (SGFR-PA6) and subjected to pure mode I loading via symmetric three-point bending. The experiments were performed for various fiber and moisture contents and the effects of these two parameters were studied on the failure of the specimens. Because both the strain-to-failure and the strain-hardening for the SGFR-PA6 materials are simultaneously high, the equivalent material concept (EMC) was considered to be inefficient and hence, the fictitious material concept (FMC) was employed to be linked with the point method (PM), that is, the simplest and perhaps the most well-known failure prediction method in the theory of critical distances (TCD), for predicting the experimentally obtained critical loads of the notched specimens. It was found that the FMC-PM combined criterion is generally capable of predicting the experimental results well, with no need to perform time-consuming and complex nonlinear failure analyses. It seems that the FMC-PM criterion works well not only on ductile steels, but also on short-fiber-reinforced polymers like SGFR-PA6 [130]. Of course, a large number of failure studies on various engineering materials, notch shapes, and loading conditions are required to comprehensively demonstrate the effectiveness of the FMC in the context of the nonlinear failure of notched components.

31.5

Constitutive and fundamental researches

In addition, some constitutive and fundamental researches have been done to analyze the SFCs using fictitious fiber and/or imaginary fiber techniques. The solution base of the proposed analytical method is known as the fictitious fiber technique [131] and imaginary fiber technique [64,132
136], the details of which are given in Ref. [64,131
138]. For example, one of the most effective researches has been carried out by Weng and Sun [131]. They used a fictitious fiber technique for studying the effects of the fiber length on elastic moduli of randomly oriented chopped-fiber composites. This

Figure 31.58 The stress
strain curves of the tested SGFR-PA6 materials: (A) 10 wt.% specimens, (B) 50 wt.% specimens [130].

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Figure 31.59 Geometry of the SENB specimen [130].

technique is similar to the imaginary fiber technique for analyzing short-fiber composites. The mentioned solution method based on fictitious fiber technique [131] is similar to Hsueh’s elastic solution [132
136], known as the imaginary fiber technique [132
136]. The application of the fictitious and imaginary fiber techniques is very important to analyze the matrix located at the top of the fiber. Also, this technique can help in the analysis of a complete composite. One of the important applications of the fictitious and imaginary fiber techniques has been presented. When considering the fictitious and imaginary fiber technique, we can simply solve and analyze the short-fiber composites because the matrix (located at the top of the fiber) is divided into the two regions of the matrix and imaginary fiber. Also, by dividing it into two sections, more boundary conditions are applied for analyzing the short-fiber composites. Therefore more exact results are obtained using this technique [64,131
138].

31.6

Solved problems

Problem 31.6.1. Define the critical fiber length. An aligned short-fiber composite consists of 40 vol.% carbon fibers of length 2 mm and diameter 7 μm in a polycarbonate matrix. The tensile strength of the fibers and the shear strength of the fiber
matrix interface are 2.5 GPa and 12.5 MPa, respectively. Calculate the critical fiber length, lc , and then estimate the longitudinal tensile strength of the composite given that the stress on the matrix at the failure strain of the fibers is 30 MPa [1]. Answer: lc 5 0:7 mm, therefore l . lc . Longitudinal strength of composite is 843 MPa. Problem 31.6.2. Explain why the stiffness of a discontinuous fiber composite is less than that of a continuous-fiber composite with the same constituents and same

Figure 31.60 Some of the SENB specimens after the fracture tests: (A) 10 wt.% 22% moisture, (B) 50 wt.%
sat [130].

Figure 31.61 Results obtained using EMC-PM and FMC-PM models [130]: (A) SGFR-PA6 with 10 wt.% and 2% moisture, (B) SGFR-PA6 with 10 wt.% and 5% moisture, (C) SGFR-PA6 with 50 wt.% and 2% moisture, and (D) SGFR-PA6 with 50 wt.% and 4% moisture (saturated).

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proportion of reinforcement [1]. Calculate the longitudinal Young’s modulus, E11 , for an aligned short-fiber carbon
epoxy composite with 42 vol.% reinforcement given that Young’s modulus of the fiber and matrix are 390 and 5.5 GPa, respectively, and that the length efficiency parameter, ηL , is 0.84. Answer: 141 GPA. Note: Indicate whether statements are true or false (Problems 6.3
6.9). Problem 31.6.3. A short fiber composite is one with a fiber length less than 10 mm. A. True B. False

Answer: B Problem 31.6.4. In the context of short-fiber composites, the aspect ratio is defined as l2 =D and thus has units of length (l and D are fiber length and diameter, respectively). A. True B. False

Answer: B Problem 31.6.5. If the fiber length is less than lc , where lc is the critical fiber length, the tensile stress in the fiber never reaches the fiber fracture stress. A. True B. False

Answer: A Problem 31.6.6. 2D composites have in-plane randomly orientated fibers [1]. A. True B. False

Answer: A Problem 31.6.7. If the Young’s modulus of the fibers is much greater than that of the matrix the effective modulus of an aligned short fiber composite is given by EðshortÞ 5 ηL Econtinuous [1]. A. True B. False

Answer: A Problem 31.6.8. In contrast to the behavior of aligned continuous-fiber composites, the tensile strength of an aligned discontinuous (short) fiber composite is independent of the angle between the fiber and the loading axis. A. True B. False

Answer: B

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Problem 31.6.9. In variable fiber orientation composites, such as injection-molded materials, the safest procedure is to obtain upper and lower bounds for strength and then base design on the lower bound unless experience suggests this is unduly pessimistic. A. True B. False

Answer: A Note: For each of the statements of questions 6.10
6.13, one or more of the completions given are correct. Mark the correct completions [1]. Problem 31.6.10. The main reasons for the widespread use of short-fiber composites are It is easier to align short fibers than continuous fibers; Short fibers are generally less expensive than continuous fibers; Short-fiber composites are always isotropic; Manufacturing processes for continuous-fiber composites tend to be slow and inflexible; E. The longitudinal strength of an aligned short-fiber composite is greater than that of an aligned continuous-fiber composite, with the same volume fraction of fibers.

A. B. C. D.

Answer: B, D Problem 31.6.11. If the fiber length l . lc , where lc is the critical length, the average fiber stress is A. B. C. D. E.

Less than the fiber fracture stress; More than the fiber fracture stress; Proportional to the fiber fracture stress; Independent of l; Increases with increasing l.

Answer: A, C, E Problem 31.6.12. Young’s modulus E11 parallel to the fibers in an aligned shortfiber composite is given by:


 A. E11 5 ηL Ef ϑf 2 Em 1 2 ϑf where ϑ is volume fraction, subscripts f and m refer to fiber and matrix, respectively, and
 ηL is the length efficiency parameter; B. E11 5 ηL Ef ϑf 1 Em 1 2 ϑf ; C. E11 5 ηL Ef ϑf 1 Em ϑm ; D. A modified law of mixtures incorporating a length efficiency parameter; E. A modified maximum stress criterion incorporating a length efficiency parameter.

Answer: B, C, D Problem 31.6.13. The longitudinal strength σ^ 1T of an aligned short brittle fiber composite A. is greater than that of an aligned continuous-fiber composite; B. is less than that of an aligned continuous-fiber composite; C. is approximately half that of an aligned continuous-fiber composite for l 5 lc where l and lc are fiber length and critical fiber length, respectively;

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D. is approximately twice that of an aligned continuous fiber composite for l 5 lc where l and lc are fiber length and critical fiber length, respectively; E. approaches that of an aligned continuous-fiber composite when l . 10lc ; F. approaches that of a random short-fiber composite when l . 10lc .

Answer: B, C, E Note: Each of the sentences in questions 6.14
6.17 consists of an assertion followed by a reason [1]. Answers: A. if both assertion and reason are true statements and the reason is a correct explanation of the assertion; B. if both assertion and reason are true statements but the reason is not a true explanation of the assertion; C. if the assertion is true but the reason is a false statement; D. if the assertion is false but the reason is a true statement; E. if both the assertion and reason are false statements.

Problem 31.6.14. A weld-line may be found in a component which has been injection molded using a twin-gated mold because jetting of the flow from each gate may occur, thus restricting mixing of the fibers across the center of the component. Answer: A Problem 31.6.15. The critical length lc may readily be calculated from micrographs because lc is simply the average length of the fibers aligned with the stress axis. Answer: E Problem 31.6.16. The average stress in a short fiber is less than the maximum stress that can be achieved in a continuous fiber because the stiffness of short fibers is usually a factor of four less than that of a continuous fiber. Answer: C Problem 31.6.17. Porous carbon
carbon composite, known as carbon-bonded carbon fiber (CBCF), is a good example of an aligned short-fiber composite because there is a random arrangement of the fibers in the xy plane and some alignment of the fibers in planes zx and xy normal to that plane. Answer: D Note: Select the correct word from each of the groups given in italics (inside brackets, into braces) in the following passage [1,11,74,78,83,139]. Problem 31.6.18. The shear stress around a short fiber is (zero/maximum/minimum) at the fiber ends and (almost zero/maximum/minimum) in the center. The tensile stress in a fiber is (zero/maximum/minimum) at the fiber ends, and (zero/maximum/minimum) at the center of the fiber. This means the reinforcing efficiency of short fibers (remains constant/increases/decreases) with decreasing fiber length. It is often useful to consider fiber length in units of the critical fiber length which is the (minimum/maximum) fiber length required for the stress to reach the fracture stress of the fiber. Answer: maximum, almost zero, zero, maximum, decreases, minimum.

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New Materials in Civil Engineering

Problem 31.6.19. The chopped fibers (generally, always) lie parallel to the surface of the mold and are oriented (randomly, nonrandomly) in planes parallel to the surface. Answer: generally, randomly Problem 31.6.20. Properties of a discontinuous-fiber-reinforced composite can be (isotropic, orthotropic) that is, they do not change with direction within the plane of the sheet. Answer: isotropic Problem 31.6.21. Chopped-strand milled (CSM) fibers continuous-fiber glass strands can be chopped to specific lengths or hammer-milled into very short fiber lengths normally (0.4
6.5 mm, 0.5
1.5 mm). Answer: 0.4
6.5 mm Problem 31.6.22. In the short fibers, the (minimum, maximum) fiber stress occurs at the middle of the fiber length. Answer: maximum Problem 31.6.23. It is expected from micromechanics, that the strength along the fiber direction in a short-fiber composite can be improved by (increasing, decreasing) the fiber (or bundle) aspect ratio or the fiber volume fraction. Answer: increasing Problem 31.6.24. The fatigue behavior of short-glass-fiber-reinforced thermosets is practically (independent, dependent) of the frequency within the range of “f” values usually employed. Answer: independent Problem 31.6.25. Owing to the partial fiber orientation, short-fiber composites will demonstrate, more or less (anisotropy, isotropy) in their mechanical properties. Answer: anisotropy Problem 31.6.26. In a short fiber, a strong interface is desired to effectively transfer load from matrix to fiber. A strong interface can diminish the (ineffective, effective) length at both ends of the fiber and, so can increase the effective length that carries load. Answer: ineffective Problem 31.6.27. Crack bridging in weakly bonded continuous filament composites has proved to be useful in enhancing the composite fracture toughness, its usefulness in short-fiber composites is usually limited, because a (weak, strong) interface significantly decreases the fiber length that carries load. Answer: weak Problem 31.6.28. For BSS-fiber composites, bone-shaped short, a weak interface can be used to significantly (reduce, increase) stress concentration without sacrificing the strength. For brittle-ceramic
fiber composites, reduction of stress concentrations will improve the toughness since the fibers can effectively bridge cracks (without, with) premature fracture. For (ductile, brittle) BSS fiber-reinforced composites, the weak interface allows the fiber to debond and (plastically, elastically) deform over the entire fiber length, which consumes large amounts of energy and consequently improves toughness. Answer: reduce, without, ductile, plastically

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

999

Problem 31.6.29. It may be seen that the relative tensile strength of the composites increases (nonlinearly, linearly) with an increase of the volume fraction of the SGF, and the estimations of the relative tensile strength closely matches the experimental measured data. Answer: nonlinearly Problem 31.6.30. The (interfacial, outerfacial) strength between the fiber and matrix plays a quite important role to predict composite behavior compared to the other parameters. Answer: interfacial Problem 31.6.31. What are the boundary conditions (B.C.’s) in the following short fiber composite UC model considering both creeping the fiber and matrix (Fig. 31.62)? Answer: The boundary conditions (B.C.’s) for the mentioned model are as follows: u_ 1 ð0; x2 Þ 5 2 u_ 1 ð 2c; x2 Þ 5 u_ 1fT

(31.15)

Figure 31.62 Schematic illustration of the fiber and matrix boundaries after creeping under low axial tensile stress in x1 direction (plane stress problem, front view of a unit cell) [67].

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New Materials in Civil Engineering

   c l u_ 1 l 2 ; x2 5 2 u_ 1 c 2 ; x2 5 u_ 1mT 2 2

(31.16)

    2D D u_ 2 x1 ; 5 2 u_ 2 x1 ; 5 u_ 2mL 2 2

(31.17)

    2b b u_ 2 x1 ; 5 2 u_ 2 x1 ; 5 u_ 2fL 2 2

(31.18)

u_ 1 ð0; x2 Þ 5 u_ 2 ðx1 ; 0Þ 5 0

(31.19)

b u_ 2 ðx1 ; 2 bÞ 5 u_ 2 ðx1 ; bÞ 5 2 u_ 1c c

(31.20)

c u_ 1 ðc; x2 Þ 5 u_ 1 ð 2c; x2 Þ 5 2 u_ 2b b

(31.21)

u_ 1x 5 u_ 2y ; u_ 1y 5 2 u_ 2x

(31.22)

In the above boundary conditions, the subscripts f, T, and m indicate the fiber, top, matrix, respectively. The displacement rates (u_ 1 ; u_ 2 ) are considered for the creeping fiber deformation rates. (u1 ; u2 ) and (w1 ; w2 ) are, respectively, displacement fields in the creeping fiber and matrix. Moreover, subscripts 1 and 2 are related to x1 and x2 directions, in which u1c , u2b , w1l , and w2D are boundary values in the outer surfaces. Also: u_ 3 ðx1 ; x2 ÞD0

(31.23)

w_ 1 ðc; x2 Þ 5 w_ 1 ð 2c; x2 Þ 5 u_ 1c

(31.24)

w_ 2 ðx1 ; bÞ 5 w_ 2 ðx1 ; 2 bÞ 5 u_ 2b

(31.25)

w_ 1 ðl; x2 Þ 5 w_ 1 ð 2l; x2 Þ 5 w_ 1l

(31.26)

w_ 2 ðx1 ; DÞ 5 w_ 2 ðx1 ; 2 DÞ 5 w_ 2D

(31.27)

Displacement rates in fiber and matrix are u_ 1 ; u_ 2 and w_ 1 ; w_ 2 , respectively, in which superscripts 1 and 2 indicate the displacement rates in x1 and x2 directions, in which u_ 1c is the displacement rate in x1 2 direction for outer surface ðx1 5 6 cÞ, as well as u_ 2b is the displacement rate in x2 2 direction for outer surface ðx2 5 6 bÞ. In addition, the relation between u_ 1c and_u 2b is presented by the incompressibility condition ðΔV 5 0Þ in crept short fiber. Therefore the mentioned condition gives b u2b 5 2 u1c c

(31.28)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

w2D 5 2 u2b 5

D w1l l

b w2D D

b u_ 2b 5 2 u_ 1c c w_ 2D 5 2 u_ 2b 5

D w_ 1l l

b w_ 2D D

1001

(31.29)

(31.30)

(31.31)

(31.32)

(31.33)

x u_ 2x 5 2 u_ 1c 0 # x # b c

(31.34)

y u_ 1y 5 2 u_ 2b 0 # y # c b

(31.35)

Therefore, ideal displacement rates can be averagely given by u_ 2 5

2 u_ 1c x2 c

(31.36)

u_ 2b x1 b

(31.37)

u_ 1 5 2

Thus, using such a procedure, the creep strain rates of the composite may be obtained as ε_ 1f 5

2 u_ 2b u_ 1c 5 c b

(31.38)

ε_ 2f 5

2 u_ 1c u_ 2b 5 b c

(31.39)

Subscripts 1; 2; andf indicate x1-direction, x2-direction and fiber, respectively. Note that value of u2b; u1c; and u3a are approximately very small and equal to zero in steady-state creep of the short fiber (Eqs. 31.15
31.39). Also, u3a is the displacement value in x3-direction for outer surface ðx3 5 6 aÞ. In addition ε_ 1 ; ε_ 2 ; ε_ 3 are principal strain rates in the 1, 2, and 3 directions. Problem 31.6.32. What are the compatibility equations for plane stress of the above problem?

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New Materials in Civil Engineering

Answer: For a plane stress problem, the compatibility equations reduce to the following conditions (Eqs. 31.40
31.56): @2 ε_ 33 5 0; ε_ 13 5 ε_ 23 5 0; ε_ 33 6¼ 0 @x22

(31.40)

@2 ε_ 33 5 0; ε_ 13 5 ε_ 23 5 0; ε_ 33 6¼ 0 @x21

(31.41)

@2 ε_ 33 5 0; ε_ 13 5 ε_ 23 5 0; ε_ 33 6¼ 0 @x1 @x2

(31.42)

2

@2 ε_ 12 @2 ε_ 11 @_ε22 2 2 2 5 0; ε_ 13 5 ε_ 23 5 0; ε_ 33 6¼ 0 @x1 @x2 @x22 @x1

(31.43)

Problem 31.6.33. Determine the u_ 1 ; u_ 2 of Problem 31.6.31 (please use the complex variable method CVM for analyzing the problem). Answer: The steady-state creep problem in composites is studied for two cases (constant h and variable h) by a complex variable method. According to the rela_ tionship between the shear stress and the shear strain rate (τ~γ), γ_ 1 γ_ γ_ 5 2 5 3 5 h ðh is a constant or variable parameterÞ τ1 τ2 τ3

(31.44)

Parameter h may be given by the following, h5

3_ε e σe

(31.45)

We know that the equivalent strain rate ε_ e has a direct relation with time ΩðtÞ and equivalent stress Γðσe Þ, that is, ε_ e 5 AΩðtÞΓðσe Þ

(31.46)

Parameter A is a constant for obtaining the creep constitutive equations. ε_ e 5 ΩðtÞΓðσe Þ; considering A 5 1

(31.47)

1 for constant h: x1 5

z z z z 1 ; x2 5 i 2 i; U 5 u_ 1 1 iu_ 2 2 2 2 2

(31.48)

In which, z 5 x1 1 ix2 ; z 5 x1 2 ix2 ; i2 5 2 1

(31.49)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1003

I U 5 u_ 1 2 iu_ 2 5

[ðzÞdz 1 ρðzÞ 1 ϖðz; z Þ

(31.50)

c

 U 5 u_ 1 1 iu_ 2 5 ρðzÞ 1 i ϖðz; z Þ 2 2πRes[ðzÞjz5z0

(31.51)

u_ 1 5 ρðzÞ

(31.52)

u_ 2 5 ϖðz; z Þ 2 2πRes[ðzÞjz5z0

(31.53)

Also ðzÞ, [ðzÞ, U, U , ϖðz; z Þ and all other functions are the analytic functions. 2 for variable h: Ψ 5 z 2 z1 z; ƈ 5 z Tðz; z Þ 1 zðz u_ 1 5 u_ 1 ðz 2 z1 z Þ 1
u_ 2 5 u_ 2 ðzÞ 1 ʞðz; z Þ

(31.54) (31.55) (31.56)

In which,
T ðz; z Þ, zðzÞ, ʞðz; z Þ, and all other functions are the analytic functions. The behavior of the creeping fiber displacement rates u_ 1 and u_ 2 is graphically shown in Figs. 31.63A
D and 31.64A
D. The behaviors of u_ 1 in 3D coordinates with respect to z and z for the low stresses and temperatures (constant h) are shown in Fig. 31.63A
D. For example, linear behavior of u_ 1 in 3D coordinate with respect to z and z is shown in Fig. 31.63A. Moreover, Fig. 31.63A
D are presented for four different analytic functions ρðzÞ. According to Fig. 31.63A
D, it is concluded that the creep displacement rate behaviors based on the analytic functions of Fig. 31.63B and D are approximately correct and exact. That is, selecting the even polynomial functions as the analytic functions yields the correct behavior in comparison with the odd functions of Fig. 31.63A and C. Also, Fig. 31.63B and D demonstrate a uniform behavior for u_ 1 , and also symmetric behavior is seen in these figures, in which these symmetric behaviors in the diagrams prove the correct behavior of the creep (identical tensile and compressive creep deformations of the UC due to the symmetric UC). Tensile deformations on the top and down of the fiber and matrix along with the compressive deformations on the left and right sides of the fiber and matrix are the correct behaviors of the creep (see Fig. 31.63B and D). Prediction of the creep behavior shown in Fig. 31.63B and D is more accurate than the prediction presented in Fig. 31.63A and C. That is, more accurate results for predicting the creep behaviors are because of the choosing of the even analytic polynomial functions for ρðzÞ. Therefore choosing the odd analytic functions has not resulted in the more accurate results comparing the even analytic polynomial functions for predicting the displacement rate u_ 1 .

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New Materials in Civil Engineering

Figure 31.63 (A) Linear behavior of u_ 1 with respect to z, z for ρðzÞ 5 zcosðzÞ:(B) Nonlinear behavior of u_ 1 with respect to z, z for ρðzÞ 5 z2 : (C) Nonlinear changes of u_ 1 with respect to z, z for ρðzÞ 5 SinðzÞ. (D) Linear behavior of u_ 1 with respect to z, z for ρðzÞ 5 z4

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1005

Figure 31.64 (A) Nonlinear changes of u_ 2 with respect to z, z for ρðzÞ 5 zcosðzÞ: (B) Nonlinear behavior of u_ 2 with respect to z, z for ρðzÞ 5 z2 . (C) Nonlinear changes of u_ 2 with respect to z, z for ρðzÞ 5 z4 : (D) Nonlinear changes of u_ 2 with respect to z, z for ρðzÞ 5 SinðzÞ.

Here, the behaviors of u_ 2 in 3D coordinates with respect to z and z for the low stresses and temperatures (constant h) are presented in Fig. 31.64A
D. In addition, the linear behavior of u_ 2 in 3D coordinate with respect to z and z is shown in Fig. 31.64D. Moreover, Fig. 31.64A
D are presented for four analytic

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New Materials in Civil Engineering

functions of ρðzÞ. In accordance with Fig. 31.64A
D, it is found that the creep displacement rate behaviors based on the analytic polynomial functions of Fig. 31.64B and C are approximately accurate and correct. That is, choosing the even polynomial functions as the analytic functions shows the correct behavior in comparison with the odd functions of Fig. 31.64A and D. In addition, Fig. 31.63B and C show a uniform behavior for u_ 1 , and also symmetric behavior is seen in these figures, in which these symmetric behaviors in the diagrams show the correct behavior of the creep. Tensile deformations on the top and bottom of the fiber and matrix along with the compressive deformations on the left and right sides of the fiber and matrix are acceptable behaviors of the creep (see Fig. 31.63B and C). Prediction of the creep behavior shown in Fig. 31.63B and C is more accurate than the prediction presented in Fig. 31.63A and D. That is, more accurate results for predicting the creep behaviors are because of choosing the even analytic polynomial functions for ρðzÞ. Therefore selecting the odd analytic functions has not resulted in more accurate results in comparison with the even analytic polynomial functions for predicting the displacement rate u_ 2 . Problem 31.6.34. Assume the clamp shown in Fig. 31.65 is constructed from a unidirectional composite with elastic properties E1 5 30:3 3 106 psi, E2 5 2:8 3 106 psi, G 5 0:93 3 106 psi, and ϑ12 5 0:21. The compressive force at points B and C of the clamp is 1.5 kip, and screw ED can only experience a tensile force. Find the σy , displacement and strain fields [140]. Answer: For this problem we will establish the displacement field corresponding to point F, located on plane a
a on the clamp. Thermal and hygral effects are neglected. The fiber orientation is defined in Fig. 31.65. The loads acting on section a
a are established by defining an appropriate free body diagram (FBD). Two possible FBDs can be used, as shown in Figs. 31.66
31.68. The loads acting on section a
a are established by defining an appropriate FBD. Two possible FBDs can be used, as shown in Figs. 31.66 and 31.67: one for portion AC of the clamp, or one for portion AB. In either case the unknown tensile force in the screw must be determined. Equilibrium equations yield: FD 5 3:5kip

(31.57)

N 5 2kip

(31.58)

M50

(31.59)

σy 5

2 2:0 5 2 3:55ksi ð0:75Þð0:75Þ

(31.60)

Having established the state of stress at point F, the resulting displacement field can be defined. Since displacements are required, the compliance matrix relating strain to stress must be defined. The relationship between strain and stress is

Figure 31.65 Composite clamp assembly [140].

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New Materials in Civil Engineering

Figure 31.66 Possible FBDs for composite clamp.

Figure 31.67 FBD for internal reactions at section a
a.

Figure 31.68 State of stress at point F of the composite clamp.

8 9 8 9 < εx =    < S 12 =
 0 εy 5 S 23:55 3 103 5 S 22 σy0 :γ ; : ; S 26 xy

(31.61)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1009

Therefore using the off-axis strain
stress relationship for a case of plane stress has S 12 5 2 0:1328 3 1026 ; S 22 5 0:2399 3 1026 ; S 26 5 0:2857 3 1026

(31.62)

8 9 8 9 < εx = < 471:4 = εy 5 2 851:6 μin=in :γ ; : ; 2 1014:2 xy

(31.63)

The displacement fields are found to be, U 5 ð471:4x 2 507:1yÞ 3 1026

(31.64)

V 5 ð 2507:1x 2 851:6yÞ 3 1026

(31.65)

These numerical results would be different had another material and/or a different fiber orientation been used. For example, a fiber orientation of θ 5 160 degrees, instead of 260 degrees, results in 8 9 8 9 < εx = < 471:4 = εy 5 2 851:6 μ in=in :γ ; : ; 2 1014:2 xy 160

(31.66)


 Since only the shear term S 26 changes sign in going from 260 to 160 degrees, γ xy is the only affected strain. This sign change is reflected in the displacement field, which would be U160 5 ð471:4x 1 507:1yÞ 3 1026

(31.67)

V160 5 ð 2507:1x 1 851:6yÞ 3 1026

(31.68)

Problem 31.6.35. Assume a 72-in diameter, closed-end pressure vessel is designed to operate under an applied pressure of 100 psi. A unidirectional composite reinforcement is to be circumferentially wound around the vessel at selected intervals along the span. Due to space limitations the reinforcement has a cross-sectional area of 0:5in2 . The vessel is shown in Fig. 31.69. Two materials are considered for the reinforcement. We wish to define the reinforcement spacing (s) as a function of the arbitrary fiber orientation angle θ, assuming that the reinforcements sustain all forces typically expressed as the circumferential stress in the vessel. In addition, the normal strain in the circumferential direction is not allowed to exceed 6000 μin=in. The two materials selected have the following elastic properties (Table 31.4). Find stress and strain fields, s, compliance and stiffness matrices S, and diagram of reinforcement spacing versus fiber orientation [140].

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New Materials in Civil Engineering

Figure 31.69 Schematic of composite reinforced pressure vessel. Table 31.4 Two material properties. Property 6

E1 (10 psi) E2 (106 psi) G12 (106 psi) v12

Material 1

Material 2

30.3 20.80 0.93 0.21

8.29 2.92 0.86 0.26

Figure 31.70 FBD for reinforced pressure vessel.

Answer: According to Fig. 31.70, the summation of the forces in the x-direction results in X

Fx 5 0 5 2σx ðAÞ 2 Pð6Þð12ÞðsÞ 5 2σx ð0:5Þ 2 100ð72ÞðsÞ

(31.69)

From this we establish the relationship between reinforcement stress and spacing as σx 5 7200s. There are two possible approaches to solving this problem. Either by

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1011

using the stiffness matrix or the compliance matrix and solving one of the following sets of equations, 8 9 8 9 8 9 8 9 < 6000 = < σx =  < 6000 = < σx =   εy εy 3 1026 or 3 1026 5 S 0 5 Q 0 : γ ; : γ ; : ; : ; 0 0 xy xy

(31.70)

Solving this equation for s, s5

8:33 3 1027 S 11

(31.71)

Since only S 11 is involved in the solution, the sign of the fiber orientation in the reinforced does not influence the solution (Fig. 31.71 and Table 31.5). Problem 31.6.36. The composite reinforced pressure vessel (Problem 31.6.35) is used to illustrate the effects of thermal and hygral strains on analysis. The relation between the normal stress in the composite reinforced and spacing (σx 5 7200s) defined as in Problem 31.6.35 is used again, as is the constraint εx # 6000μ in=in. Determine the stress and strain fields, s, compliance and stiffness matrices S, α, β, and diagram of reinforcement spacing versus fiber orientation considering various conditions [140]. Answer: As before, two possible equations can be solved: 8 9 2 < σx = Q 11 0 5 4 Q 12 : ; 0 Q 16

Q 12 Q 22 Q 26

9 8 9 8 9 1 30 8 Q 16 < 6000 3 1026 = < αx = < βx = εy 2 αy ΔT 2 β y M A Q 26 5@ : ; : ; :β ; αxy γ xy Q 66 xy

(31.72) or

Figure 31.71 Reinforced spacing as a function of fiber orientation.

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New Materials in Civil Engineering

Table 31.5 Values of S 11 and s for two materials and various angles. Angle (θ)

0 30 45 60 90

Material 1

Material 2

S11 ð 3 1026 Þ

s (in)

S11 ð 3 1026 Þ

s (in)

0.033 0.239 0.363 0.402 0.357

25.20 3.47 2.29 2.07 2.33

0.121 0.295 0.391 0.406 0.343

6.90 2.82 2.13 2.05 2.43

Table 31.6 Values of αx and β x for two materials.

α1 α2 β1 β2

Material 1

Material 2

3.4 3 1026 in/in/ F 5.0 3 1026 in/in/ F 0.0 0.20

12.0 3 1026 in/in/ F 8.0 3 1026 in/in/ F 0.0 0,40

8 9 2 26 < 6000 3 10 2 αx ΔT 2 β x M = S 11 εy 2 αy ΔT 2 β y M 5 4 S 12 : ; γ xy 2 αxy ΔT 2 β xy M S 16

S 12 S 22 S 26

38 9 S 16 < σx = S 26 5 0 : ; 0 S 66

(31.73)

Therefore the problem reduces to solving 6000 3 1026 2 αx ΔT 2 β x M 5 S 11 ðσx Þ 5 S 11 ð7200sÞ

(31.74)

αx and β x are functions of fiber orientation and are established from αx 5 m2 α1 1 n2 α2 and β x 5 m2 β 1 1 n2 β 2 . For the two materials considered in Problem 31.6.35, the thermal and hygral coefficients of expansion are assumed to be as described in Table 31.6. The variation of αx and β x with selected fiber angles θ for each material is tabulated here (Table 31.7). For this problem it is assumed that ΔT 5 2 280℉ and the average moisture content is M 5 0:05. Manipulation of the governing equation for reinforcement spacing results in
 8:33 3 1027 2 1:389 3 1024 αx ΔT 1 β x M s5 S 11

(31.75)

In this expression αx , β x , and S 11 are functions of θ. Individual contributions of thermal and hygral effects on the reinforcement spacing are illustrated by separating

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1013

Table 31.7 Values of αx and β x for two materials at various degrees. Material 1

Material 2

θ

αx(µin/in/ F)

βx

αx(µin/in/ F)

βx

0 30 45 60 90

3.40 7.05 10.70 14.40 18.00

0.00 0.05 0.10 0.15 0.20

5.00 6.60 8.50 10.25 12.00

0.00 0.10 0.20 0.30 0.40

Figure 31.72 Effects of temperature on reinforcement spacing, M 5 0.

them and examining one at a time. Fig. 31.72 shows the effect of temperature compared to the solution for Problem 31.6.35 for both materials. The effects of ΔT 5 2 280℉ (and M 5 0) increase the required reinforcement spacing for each material considered. The effects of moisture alone are presented in Fig. 31.73 for M 5 0:5 and ΔT 5 0, and are compared to the results of Problem 31.6.35. The negative reinforcement spacing indicates that the constraint on εx has been violated. This does not imply that no reinforcement is required, since the constraint on εx , is an artificially imposed failure criteria. The swelling strains produced from inclusion of hygral effects will reduce the strain in the reinforcement. The combination of thermal and hygral effects on predicted reinforcement spacing are shown in Fig. 31.74. The originally predicted spacing from Problem 31.6.35 is not presented in this figure. Thermal and hygral effects can influence the state of stress in a composite, and have an effect on lamina failure.

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New Materials in Civil Engineering

Figure 31.73 Effects of moisture on reinforcement spacing with ΔT 5 0.

Figure 31.74 Effects of ΔT and M on reinforcement spacing.

Problem 31.6.37. Assume the compliance matrix for a particular material is [140] 2 6 6 6 ½ S 5 6 6 4

7 11 28 12 5 28

11 25 2 15 25 8 2 15

2 8 12 5 2 15 25 8 2 2 30 5 2 30 15 19 5 19 2 1 12 11 12

28 2 15 12 11 12 2

3 7 7 7 7 3 1029 7 5

(31.76)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1015

Assume the thermal and hygral coefficients of expansion are constant over the ranges of ΔT and ΔM of interest and are 8 9 8 9 < α1 = < 10 = 5 5 μin=in= F α : 2; : ; 2 α3

(31.77)

8 9 8 9 < β 1 = < 0:1 = 5 0:4 μin=in= F β : 2; : ; 0:2 β3

(31.78)

Determine the stress and strain fields as well as a diagram of normal strain versus temperature. Answer: The strains are represented as 8 9 2 ε1 > > > > > 6 > > > > ε2 > > > > 6 > > = 6

6 3 56 6 > ε4 > > > 6 > > > > 6 > > 4 > ε5 > > > > > : ; ε6

7

11

11 28 12

25 2 15 25

5

8

28

12

5

28

2 15 25 8 2 15 2 30 5 12 2 2 30 15 19 11 5

19

21

12

2 8 2 15 12 11 12 2 8 9 9 8 10 0:10 > > > > > > > > > > > > > > > > > > > > 5 0:40 > > > > > > > > > > > < 2 = = < 0:20 > 3 1026 ΔT 1 1 ΔM > > > 0 > > > > > > 0 > > > > > > > > > > > > > 0 > 0 > > > > > > > > ; : > ; : 0 0

38 9 σ > > > 1> > 7> > > σ2 > > > 7> > > > 7> 7< σ3 = 7 7> σ > 7> > > 4> > 7> > > 5> σ5 > > > > ; : > σ6

(31.79)

Assume a state of plane stress in which the only nonzero stresses are σ1 5 20ksi, σ2 5 10ksi, σ6 ð 5 τ 12 Þ 5 5ksi. The strains are 8 9 8 ε1 > > > > > > > > > > > > ε > > > 2 > < = > < > ε3 5 > > > ε4 > > > > > > > ε5 > > > > > > : ; > : > ε6

9 9 8 9 8 210 > 10 > 0:10 > > > > > > > > > > > > > > > 395 > 5 > 0:40 > > > > > > > > > > = = = < > < 2 250 2 0:20 26 26 3 10 1 3 10 ΔT 1 ΔM 545 > 0 > 0 > > > > > > > > > > > > > > > 120 > 0 > 0 > > > > > > > > > > ; ; ; : > : 2 300 0 0

(31.80)

From this we observe that the shear strains (ε4 ; ε5 ; ε6 ) remain constant for any ΔT and ΔM values considered. The normal strains are affected by both ΔT and ΔM. It is also observed that even though a state of plane stress exists, the

1016

New Materials in Civil Engineering

out-of-plane shear strains (ε4 and ε5 ) are present. In order to evaluate the effects of ΔT and ΔM, assume they are limited to the ranges of 2300℉ # ΔT # 300℉ and 0 # ΔM # 0:1. A plot of the variation of ε1 as a function of ΔT and ΔM is presented in Fig. 31.75. The results are seen to be linear for each case, and plots of either of the remaining normal strain components would also be linear (the magnitudes are different). These results are somewhat fictitious, since moisture and stiffness and compliance are coupled to temperature. Problem 31.6.38. Consider the case of uniaxial tension shown in Fig. 31.76. Determine the displacement fields, matrix of S, and strain field [140]. Answer: For this problem the strain
stress relationship is 8 9 2 < εx = S 11 εy 5 4 S 12 :γ ; S 16 xy

S 12 S 22 S 26

38 9 S 16 < 0 = S 26 5 σ0 : ; 0 S 66

(31.81)

From these equations it is obvious that there is shear
extension coupling for any angle of θ other than 0 or 90 degrees. Even under a simple uniaxial load, the deformation will be similar to that of a state of stress including shear. Assume specimen dimensions as shown in Fig. 31.76, σ0 5 50ksi, θ 5 45degrees, and material properties of E1 5 25 3 106 psi, E1 5 1 3 106 psi, G12 5 0:5 3 106 psi, and ϑ 5 0:25. These results are as follow: S11 5

1 1 5 4 3 1028 ; S22 5 5 1 3 1026 E1 E2

S12 5 2 ϑ12 S11 5 2 1 3 1028 ; S66 5

Figure 31.75 Effects ΔT and ΔM on ε1 .

1 5 2 3 1026 G12

(31.82)

(31.83)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1017

Figure 31.76 Off-axis lamina.

At θ 5 45degrees, sine and cosine terms are identical (m 5 n 5 0:707). Therefore 8 9 8 9 < εx = < 2 2:45 3 1027 σ0 = εy 5 2 7:55 3 1027 σ0 :γ ; : ; 2 4:8 3 1027 σ0 xy Since σ0 5 50ksi, 8 9 8 9 < εx = < 2 0:01225 = εy 5 0:03775 in=in :γ ; : ; 2 0:024 xy

(31.84)

(31.85)

The displacement field is obtained from the definitions of axial and shear strain as follows εx 5

@U ! U 5 2 0:01225x 1 f ð yÞ @x

(31.86)

εy 5

@U ! V 5 0:03775x 1 gð xÞ @y

(31.87)

γ xy 5

@V @U 0 0 1 ! f ðyÞ 1 g ðxÞ 5 2 0:024 @x @y

(31.88)

Also f ðyÞ 5 C1 1 C2 y; gðxÞ 5 C3 1 C4 x

(31.89)

1018

New Materials in Civil Engineering

According to the above equations C1 5 C3 5 0; C4 5 C2

(31.90)

U 5 2 0:01225x 2 0:012y

(31.91)

V 5 0:03775y 2 0:012x

(31.92)

Problem 31.6.39. Assume the lamina shown in Fig. 31.77 is subjected to the multiaxial state of stress indicated. Also, the material properties and failure strengths are assumed to be as shown in Table 31.8 [140]. Find the principal direction strains, and the failure strains. Answer: The stresses in the principal material directions are, 9 2 9 8 38 m2 n2 2mn > > > = < σ1 = < 2 10 > 6 7 15 σ 2 5 4 n2 m2 2 2mn 5 > > > > ; ; : : 10 τ 12 2 mn mn m2 2n2 9 9 8 2 38 0:25 0:75 0:866 > = = > < 17:41 > < 2 10 > 6 7 5 2 12:41 ksi 5 4 0:75 0:25 2 0:866 5 15 > > > ; ; > : : 5:825 2 0:433 0:433 2 0:5 10 (31.93) When the preceding stresses are compared to these failure strengths, it is evident that no failure has occurred according to the maximum stress theory. For completeness,

Figure 31.77 Multiaxial state of stress.

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1019

Table 31.8 The material properties and failure strengths. E1 5 8.42 3 106 psi X 5 136 ksi S 5 6 ksi

E2 5 2.00 3 106psi X0 5 280 ksi

G12 5 0.77 3 106 psi Y 5 4 ksi

v12 5 0.293 Y0 5 20 ksi

however, the failure strains should also be checked. The stresses just given can be used to determine the principal direction strains by using the compliance matrix so that 9 2 8 38 S11 S12 0 > > = < < ε1 > 6 7 ε2 5 4 S12 S22 0 5 > > > ; : : 0 0 S66 γ 12 2 1:2 2 0:35 6 5 4 2 0:35 5 0

0

9 17:41 > = 2 12:41 > ; 5:825 8 9 8 9 3 0 > > < 17:41 > = < 2536 > = 7 0 5 3 1027 2 12:41 3 103 5 2 6814 μin=in > > > > : ; : ; 12:9 5:825 7518

(31.94) The failure strains associated with this material are Xε 5

X Y S 0 5 16; 152μin=in; Yε 5 5 10; 000μin=in; Sε 5 5 7792μin=in E1 E2 G12 (31.95)

A comparison of the failure strains with those resulting from the applied state of stress shows that failure does not occur. In each case the shear term (either strain or stress) was the closest to failure. This result brings to light some interesting aspects of shear failures, and the general importance of shear stresses. Problem 31.6.40. The maximum stress and Tsai
Hill theories are investigated for pure shear. The lamina under consideration is assumed to have an arbitrary fiber orientation of either 2 θ or 1θ, as shown in Fig. 31.78 [140]. The material is glass/epoxy with E1 5 7:8 3 106 psi, E2 5 2:6 3 106 psi, G12 5 1:25 3 106 psi, ϑ12 5 0:25, and failure 0 0 strengths X 5 X 5 150ksi, Y 5 4ksi, y 5 20ksi, and S 5 8ksi. Analyze the maximum stress criterion, Tsai
Hill criterion, and then find the stress fields, and finally compare the maximum stress and Tsai
Hill failure criteria for pure shear schematically. Answer: The stresses in the 1
2 plane based on an applied shear stress of 2τ are, 8 9 9 8 9 8 < σ1 = < 0 = < 2 2mn = 5 ½Tσ  τ σ 0 5 2mn : 2; ; : ; : 2 τ 12 2τ n 2 m2

(31.96)

The tensile and compressive components of stress change with θ. For a positive angle, σ1 is compressive and σ2 tensile. For a negative angle, σ1 is tensile and σ1 compressive. The shear stress τ 12 will not change sign as θ changes from positive

1020

New Materials in Civil Engineering

Figure 31.78 Pure shear with an arbitrary fiber orientation. 0

to negative, but it will change sign based on the angle itself. Since X 5 X , the sign of σ1 is not significant, but the sign of σ2 will dictate which failure strength (Y or Y0 ) is used for the 2-direction. Maximum stress criterion. In the maximum stress criterion, four failure condi0 tions must be checked. For materials in which X 6¼ X , a fifth condition is required. Each condition is a function of θ: σ1 5 150000 5 2 2mnτ σ2 5 4000 5 2mnτ σ2 5 20000 5 2mnτ

τ 5 75000=mn

τ 5 2000=mn

ðfor all θÞ

ðfor 1 θÞ

τ 5 10000=mn

ðfor 2 θÞ


 τ 12 5 8000 2 n2 2 m2 τ 5 8000=ðn2 2 m2 Þ ðfor all θÞ

(31.97) (31.98) (31.99) (31.100)

Tsai
Hill criterion. The Tsai
Hill criterion requires only one equation to establish failure. Substituting the stresses and failure strengths for this case into the following equation yields the failure criteria. The governing equation depends on the sign of σ2 , since it is the only stress component having two failure strengths (for this material). σ21 σ1 σ2 σ2 τ2 2 2 1 22 1 12 51 2 X X Y S2

(31.101)

1θ: σ1 is compressive and σ2 is tensile, and the failure equation is 

ð22mnτ Þ 150

2


2 2 ! 2   n 2m2 ð2mnτ Þð 22mnτ Þ 2mnτ 2 2 1 1 τ2 5 1 4 8 1502 (31.102)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1021

    2
2 150 2 150 2
2 2 ð22mnτ Þ 2 ð2mnτ Þð 22mnτ Þ 1 ð2mnτ Þ 1 n 2m2 τ 2 5 150 3 103 4 8 2

(31.103) h
2 i τ 2 5633m2 n2 1 351:6 n2 2m2 5 2:25 3 1010

(31.104)

2θ: σ1 is tensile and σ2 is compressive, and the failure equation is 

ð2mnτ Þ 150

2


2 2 ! 2   n 2m2 ð2mnτ Þð 22mnτ Þ 22mnτ 2 2 1 1 τ2 5 1 20 8 1502 (31.105)

  150 2 ð22mnτ Þ2 20   2
2 150 2
2 1 n 2m2 τ 2 5 150 3 103 8

ð2mnτ Þ2 2 ð2mnτ Þð 22mnτ Þ 1

h
2 i τ 2 233m2 n2 1 351:6 n2 2m2 5 2:25 3 1010

(31.106)

(31.107)

Solutions for the maximum stress criteria result in sign changes for τ similar to the available methods. Solutions to the Tsai
Hill criteria yield two roots for τ for each angle. To compare these theories, absolute value jτ j versus θ is plotted in Fig. 31.79. The Tsai
Hill theory produces a more uniform curve of jτ j versus θ than the maximum stress theory. For negative fiber angles the stress required to produce failure is greater than for positive angles. Depending on the fiber orientation angle, the maximum stress criterion will be controlled by either σ2 or θ. The regions in which either shear or normal stress control failure for the maximum stress criterion are established by examination of the failure criteria at each angle. For example, θ 5 10degrees, for which cos10 5 0:9848 and sin10 5 0:1736. Comparing the σ2 and the shear it is easy to see that σ2 5 τ5

2000 5 11; 100psi ð0:9848Þð0:1736Þ

8000 5 8500psi ð0:030 2 0:9698Þ

(31.108)

(31.109)

Therefore the failure is shear controlled at this angle. From Table 31.9, the primary difference between failure theories is the form of the interactive term, F12 . The Ashkenazi [141] and Chamis [142] theories require 0 0 experimentally determined parameters not generally defined when X, X , Y, Y , and

1022

New Materials in Civil Engineering

Figure 31.79 Comparison of maximum stress and Tsai
Hill failure criteria for pure shear. of interactive failure
Table231.9 Summary  theories governed by Fij σi σj 5 1 F11 σ1 1 F22 σ22 1 F66 τ 212 1 2F12 σ1 σ2 5 1 [140
145]. Theory Ashkenazi Chamis Fisher Tsai
Hill Norris 

F11

F22

1 X2 1 X2 1 X2 1 X2 1 X2

1 Y2 1 Y2 1 Y2 1 Y2 1 Y2

F12  1 4

1 2 U2 2 X2 K1 K 01 2 2XY K 2 2XY 2 2X1 2 K 2 2XY

2

1 Y2

2

 1 S2

F66 1 S2 1 S2 1 S2 1 S2 1 S2

An additional condition in the Norris theory is that σ21 5 X 2 and σ21 5 Y 2

S are established. In the case of uniaxial tension applied to a lamina with a fiber orientation of 45degrees, it is easily shown that very little difference in predicted failure load exists between the theories. This is due primarily to the magnitude of F12 as compared to the other terms. For fiber orientations other than 45degrees the same conclusion may not be valid. The influence of an interactive term on lamina failure is better observed in the Tsai
Wu theory developed in the next section. The Tsai
Hill theory can also be formulated based on strains, by incorporating the appropriate relationships between principal direction strains and stresses and the failure strains (Xε 5 X=E1 ; etc:), and substituting into Eq. 31.101. Problem 31.6.41. The fibers of a carbon/epoxy composite are 8 μm in diameter and are coated with a 1 μm thick epoxy coating [146]. Determine the maximum fiber volume ratio, Vf , that can be achieved by bonding them with a similar epoxy matrix. Answer: V f 5 0:5804

(31.110)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1023

Problem 31.6.42. Determine the transverse modulus E2 , of a unidirectional carbon/epoxy composite with the properties [146], E2f 5 14:8 GPa ð2:15 MsiÞ

(31.111)

Em 5 3:45 GPa ð0:5 MsiÞ

(31.112)

ϑm 5 0:36

(31.113)

V f 5 0:65

(31.114)

using the mechanics of materials approach and the Halpin
Tsai relationship with ξ 5 1. Answer: Mechanics of materials: E2 5 7:56GPa

(31.115)

Halpin-Tsai: E2 5 8:13 GPa

(31.116)

Problem 31.6.43. Determine the in-plane shear modulus G12 , of a glass/epoxy composite with the properties [146], Gf 5 28:3 GPa ð4:10 MsiÞ

(31.117)

Gm 5 1270 GPa ð184 MsiÞ

(31.118)

V f 5 0:55

(31.119)

using the mechanics of materials approach and the Halpin
Tsai relationship with ξ 5 1. Answer: Mechanics of materials: G12 5 2:68 GPa

(31.120)

Halpin-Tsai: G12 5 3:84 GPa

(31.121)

Problem 31.6.44. Given a glass/epoxy composite containing short aligned fibers of lengthland radius r, with Ef 5 69 GPa ð10 MsiÞ, Em 5 3:45 GPa ð0:5 MsiÞ,ϑm 5 0:36, and Vf 5 0:50, determine l/r by Cox’s and Halpin’s approaches so that E1 (discont.)/E1 (cont.) 5 0.9 [146]. Answer: Cox: l=r 5 58:4

(31.122)

Halpin: l=r 5 75:45

(31.123)

1024

New Materials in Civil Engineering

Problem 31.6.45. A woven carbon/epoxy composite has identical moduli and Poisson’s ratios in the warp and fill directions, E
1 5E2 5 E and ϑ12 Dϑ21 5 ϑ. Determine
 the maximum values of Poisson’s ratio ϑxy max shear coupling coefficient ηsx max , and the corresponding angles θ to the principal material directions [146]. Answer:





ϑxy

ηsx





max

5

E 2 2ð1 2 ϑÞG12 at θ 5 45degrees E 1 2ð1 2 ϑÞG12

 1=2 1 E 2 2ð1 1 ϑÞG12 π 1 21 E pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi at θ 5 2 tan 5 max 4 2 2 2ð11ϑÞG12 2ðHϑÞEG12

(31.124)

(31.125)

Problem 31.6.46. Determine the longitudinal modulus E1 and longitudinal tensile strength F1t of a unidirectional silicon carbide/ceramic composite with the properties [146], V f 5 0:4

(31.126)

Ef 5 172 GPa ð25 MsiÞ

(31.127)

Em 5 97 GPa ð14 MsiÞ

(31.128)

Fft 5 1930 MPa ð280 ksiÞ

(31.129)

Fmt 5 138 MPa ð20 ksiÞ

(31.130)

Assume linear elastic behavior for both fiber and matrix. Everything else being equal, how does the strength F1t vary with fiber modulus E1f ? (Note: Strength is defined here as the composite stress at failure initiation of one of the phases.) Answer: E1 5 127 GPa ð18:4 MsiÞ

(31.131)

F1t 5 180:7 MPa ð26:2 ksiÞ

(31.132)

The strength F1t increases linearly with E1f , up to the point where εuft 5 εumt , thereafter it decreases asymptotically to the value of 772 MPa. Problem 31.6.47. A unidirectional E-glass/epoxy composite is loaded in transverse tension. Obtain the stress concentration factor from Fig. 31.80 and calculate the transverse tensile strength based on the maximum stress and maximum strain criteria, for the constituent and composite properties [146], V f 5 0:65

(31.133)

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1025

Figure 31.80 Stress concentration factor in matrix of unidirectional composites with square fiber array under transverse tension.

Ef 5 69 GPa ð10 MsiÞ

(31.134)

Em 5 3:45 GPa ð0:5 MsiÞ

(31.135)

ϑm 5 0:36

(31.136)

Fmt 5 104 MPa ð15ksiÞ

(31.137)

(Hint: Neglect residual stresses [146].) Answer: Maximum stress criterion:F2t 5 54:7MPað7:9ksiÞ

(31.138)

Maximum strain criterion:F2t 5 87:8 MPa ð12:7 ksiÞ

(31.139)

Problem 31.6.48. Determine the shear strength Fs of an off-axis lamina with principal material axis at θ 5 45degrees using the maximum stress, maximum strain, Tsai
Wu, Tsai
Hill, and Hashin
Rotem theories for a woven carbon/epoxy composite (AGP370-5H/3501-6S, Table 31.10, Fig. 31.81) [146]. Answer: Fsð1Þ 5 900 MPa ðMax:stress and Hashin 2 RotemÞ

(31.140)

Table 31.10 Properties of typical fabric composites (two-dimensional) [146].

Property Fiber volume ratio, Vf Density, ρ, g/cm3 (lb/in.3) Longitudinal modulus, E1, GPa (Msi) Transverse modulus, E2, GPa (Msi) In-plane shear modulus, G12, GPa (Msi) Major Poisson’s ratio, v12 Minor Poisson’s ratio, v21 Longitudinal tensile strength, F1t, MPa (ksi) Transverse tensile strength, F2t, MPa (ksi) In-plane shear strength, F6, MPa (ksi) Ultimate longitudinal tensile strain, ε001t Ultimate transverse tensile strain, ε002t

Woven glass/ epoxy (7781/ 5245C)

Woven glass/ epoxy (120/ 3501-6)

Woven glass/ epoxy (M10E/ 37S3)

Kevlar 49 fabric/ epoxy (K120/ M10.2)

Carbon fabric/epoxy (AGP370- 5H/35016S)

0.45 2.20 (0.080) 29.7 (4.31)

0.55 1.97 (0.071) 27.5 (3.98)

0.50 1.90(0.068) 24.5 (3.55)



29 (4.2)

0.62 1.60(0.58) 77(11.2)

29.7 (4.31)

26.7 (3.87)

23.8 (3.45)

29 (4.2)

75 (10.9)

5.3 (0.77)

5.5 (0.80)

4.7 (0.68)

18 (2.6)

6.5 (0.94)

0.17 0.17 367 (53)

0.14 0.13 435 (63)

0.11 0.10 433 (62.8)

0.05 0.05 369 (53.5)

0.06 0.06 963 (140)

367 (53)

386 (56)

386 (55.9)

369 (53.5)

856(124)

97.1 (14.1)

55 (7.9)

84(12.2)

113 (16.4)

71 (10.3)

0.025

0.019

0.022




0.013

0.025

0.018

0.021




0.012 (Continued)

Table 31.10 (Continued)

Property Longitudinal compressive strength, F1c, MPa (ksi) Transverse compressive strength, F2c, MPa (ksi) Longitudinal thermal expansion coefficient, α1, 1026/  C (1026/ F) Transverse thermal expansion coefficient, α2, 1026/ C (1026/ F) Longitudinal moisture expansion coefficient, β 1 Transverse moisture expansion coefficient, β 2

Woven glass/ epoxy (7781/ 5245C)

Woven glass/ epoxy (120/ 3501-6)

Woven glass/ epoxy (M10E/ 37S3)

Kevlar 49 fabric/ epoxy (K120/ M10.2)

Carbon fabric/epoxy (AGP370- 5H/35016S)

549 (80)




377 (54.6)

129(18.7)

900 (130)

549 (80)




335 (48.6)

129(18.7)

900(130)

10.0 (5.6)










3.4 (1.9)

10.0 (5.6)










3.7 (2.1)

0.06










0.05







0.05

0.06




1028

New Materials in Civil Engineering

Figure 31.81 Lamina model.

Figure 31.82 Lamina model.

5 850 MPa ðMax:strainÞ

(31.141)

5 540 MPa ðTsai 2 WuÞ

(31.142)

5 543 MPa ðTsai 2 HillÞ

(31.143)

Problem 31.6.49. A 45degrees off-axis lamina is loaded under pure shear τ s and subjected to a temperature rise ΔT (Fig. 31.82 [146]). Find a relationship between τ s , ΔT, α1 , and α2 and the engineering properties of the lamina such that there is no shear deformation.

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1029

Answer: 

11ϑ12 11ϑ21 τ s 5 ðα2 2 α1 ÞΔT 1 E1 E2

21 (31.144)

Problem 31.6.50. Determine the FPF strength of a ½0=90s laminate under uniaxial tension or compression based on 1. The maximum stress criterion; 2. The Tsai
Wu criterion (material: AS4/3501-6 carbon/epoxy, Table 31.11 [146]).

Answer: Maximum stress criterion: F xt 5 438 MPa

(31.145)

F xc 5 922 MPa

(31.146)

Tsai
Wu criterion: F xt 5 436 MPa

(31.147)

F xc 5 1282 MPa

(31.148)

Problem 31.6.51. What is the definition of a composite? Answer: A composite is a material construction that consists of at least two macroscopically identifiable materials that work together to arrive at a better result [147]. Problem 31.6.52. Name three advantages of composites. Answer: 1. Weight saving; 2. Considerable freedom in geometry, choice of materials, choice of process; 3. Low total maintenance cost.

Problem 31.6.53. Name three disadvantages of composites. Answer: 1. Stiffness and failure behavior can be inconvenient; 2. Limited knowledge on behavior of details and connections; 3. Often high investment cost.

Problem 31.6.54. What is the function of fibers in a composite? Answer: Usually, fibers in a composite determine stiffness and strength to a large extent—A polymer to which oriented fibers are added becomes much stronger and stiffer in fiber direction than perpendicular to fiber direction. Problem 31.6.55. What is an autoclave? Answer: This is a fairly large oven, capable of high temperatures and internal pressure, which can be used to apply the optimal curing conditions to a product.

Table 31.11 Properties of typical unidirectional composites (two-dimensional) [146]. E-glass/epoxy

5-glass/epoxy

Kevlar/epoxy (Aramid 49/ epoxy)

Carbon/epoxy (AS4/3501-6)

Carbon/epoxy (IM6G/3501-6)

0.55 1.97 (0.071) 41 (6.0) 10.4 (1.50) 4.3 (0.62) 0.28 0.06 1140 (165)

0.50 2.00 (0.072) 45 (6.5) 11.0(1.60) 4.5 (0.66) 0.29 0.06 1725 (250)

0.60 1.38 (0.050) 80 (11.6) 5.5 (0.80) 2.2 (0.31) 0.34 0.02 1400 (205)

0.63 1.60 (0.058) 147 (21.3) 10.3 (1.50) 7.0 (1.00) 0.27 0,02 2280 (330)

0.66 1.62 (0.059) 169 (24.5) 9.0 (1.30) 6.5 (0.94) 0.31 0.02 2240 (325)

39 (5.7)

49 (7.1)

30 (4.2)

57 (8.3)

46 (6.7)

89 (12.9) 0.028 0.005 620 (90)

70 (10.0) 0.029 0.006 690 (100)

49 (7.1) 0.015 0.005 335 (49)

76 (11.0) 0.015 0.006 1725 (250)

73 (10.6) 0.013 0.005 1680 (245)

128 (18.6)

158 (22.9)

158 (22.9)

228 (33)

215 (31)

7.0 (3.9)

7.1 (3.9)


2.0 (
1.1)


0.9 (
0.5)


0.9 (
0.5)

26 (14.4)

30 (16.7)

60 (33)

27 (15)

25 (13.9)

0

0

0

0,01

0

0.2

0.2

0.3

0.2

—

Property Fiber volume ratio, Vf Density, ρ, g/cm3 (lb/in.3) Longitudinal modulus, E1, GPa (Msi) Transverse modulus, E2, GPa (Msi) In-plane shear modulus, G12, GPa (Msi) Major Poisson’s ratio, v12 Minor Poisson’s ratio, v21 Longitudinal tensile strength, F1t, MPa (ksi) Transverse tensile strength, F2t, MPa (ksi) In-plane shear strength, F6, MPa (ksi) Ultimate longitudinal tensile strain, ε001t Ultimate transverse tensile strain, ε002t Longitudinal compressive strength, F1c, MPa (ksi) Transverse compressive strength, F2c, MPa (ksi) Longitudinal thermal expansion coefficient, α1, 1026/ C (1026/ F) Transverse thermal expansion coefficient, α2, 1026/ C (1026/ F) Longitudinal moisture expansion coefficient, β 1 Transverse moisture expansion coefficient, β 2

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1031

Problem 31.6.56. Name three failure mechanisms that can occur with composites, including their possible cause and the measures you can take against the occurrence of these mechanisms [147]. Answer: Splitting: if many fibers run in a single direction, and the connection transverse to the fibers is not satisfactory, a composite will be susceptible to splitting. Cracks will develop parallel to the fibers, and through the thickness of one or multiple plies. Splitting can occur because of in-plane bending, or a wedge effect of a support or connection. A good remedy is to build up the laminate by alternating plies with different fiber orientations. Delamination: is similar to splitting, but here the crack develops between two plies in the plane of the laminate. This failure mechanism can easily occur, since the shear stresses between plies can be high and usually the inter-ply interface is not reinforced. A remedy is preventing high shear stresses between plies. If this is not possible, reinforcement can be directed through-the-thickness, for example, stitching plies together, or applying “Z-pinning.” Buckling: macroscopic- or Euler-buckling is a structural property which can develop regardless of material in long, slender structural elements loaded in compression. The possibility of damage because of buckling should be considered in design. This can be the buckling of fibers, bundles, and plies that buckle under load (often in this order). Resistance against macroscopic buckling can be increased by using a stiffer material or structure, or by reducing the free buckling length. This can be done by reducing the size of panels or using (thicker) sandwich layers. Problem 31.6.57. Name three advantages and four disadvantages of sandwich materials. Answer: Advantages: Often good combination of stiffness and weight; thermal and acoustic insulation; suitable as antibuckling element. Disadvantages: sensitive to delamination of skin and core; connections require additional attention; integration in a product requires additional attention, for example, degassing in a vacuum process; shear deformation can be considerable.

Figure 31.83 Shear fracture.

1032

New Materials in Civil Engineering

Figure 31.84 Tensile fracture.

Figure 31.85 Splitting [147].

Problem 31.6.58. A pin-loaded hole joint is sensitive to different failure mechanisms. Give a possible remedy for the failure of a pin-loaded hole joint [147]: (1) shear fracture, (2) tensile fracture, (3) splitting. Answer: 1. Shear fracture: This can be avoided by applying 6 45degrees plies next to the pin-hole joint (Fig. 31.83). 2. Tensile fracture: This can be avoided by applying additional UD layers next to the joint, with the fibers in load direction (Fig. 31.84). 3. Splitting: This can be avoided by applying additional layers next to the joint, transverse to the load direction (Fig. 31.85).

Problem 31.6.59. Obtain the local Tsai
Hill (failure theory) formulation in a creeping short-fiber composite at z 5 0 in the creeping matrix (creeping composite) (Fig. 31.86).

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1033

Figure 31.86 Unit cell model.

Answer: The shear stress at z 5 0 in a creeping matrix is approximately zero ðτD0Þ. Also, Tsai
Hill failure theory is σ1 2 2 σ1 σ2 1

 2  2 X X σ22 1 τ 212 5 X 2 Y S

(31.149)

Therefore the local Tsai
Hill (failure theory) formulation in a creeping shortfiber composite (creeping matrix) at z 5 0 in the creeping matrix is as follows:  2 X σ1 2 σ1 σ2 1 σ22 5 X 2 Y 2

(31.150)

Problem 31.6.60. Find the local Tsai
Wu (failure theory) formulation in a creeping short-fiber composite at the outer surface of the creeping matrix (i.e., at z 5 z0 ; r 5 b) (Fig. 31.87). Answer: The shear stress at z 5 z0 ; r 5 b in the creeping composite (creeping matrix) is approximately zero ðτD0Þ. Also, Tsai
Wu failure theory is F11 σ1 2 1 2F12 σ1 σ2 1 F22 σ22 1 F66 τ 212 1 F1 σ1 1 F2 σ2 5 1

(31.151)

Therefore the local Tsai
Wu (failure theory) formulation in a creeping shortfiber composite at the outer surface of the creeping composite (i.e., at z 5 z0 ; r 5 b) is as follows: F11 σ1 2 1 2F12 σ1 σ2 1 F22 σ22 1 F1 σ1 1 F2 σ2 5 1

(31.152)

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New Materials in Civil Engineering

Figure 31.87 Unit cell edges in the second stage creep.

Problem 31.6.61. Propose a method (or different formulation) based on Tsai
Wu (failure theory) formulation in order for suitable agreement between the experimental and theoretical results. [according to the Tsai
Wu (failure theory) formulation for different design of a composite.] Answer: Tsai
Hill failure theory is  2  2 X X 2 σ1 2 σ1 σ2 1 σ2 1 τ 212 5 X 2 Y S 2

(31.153)

By dividing both sides by X 2 we have σ 2 1

X

1

σ 2 2

Y

2

σ1 σ2 τ 12 2 1 51 X2 S

(31.154)

Also, for safe design we have σ 2 1

X

1

σ 2 2

Y

2

σ1 σ2 τ 12 2 1 ,1 X2 S

(31.155)


2 It is approximately clear that just the value of the parameter of “ σ2 =Y ” may be an important and main value and a reason for different results between the experimental and

Problems in short-fiber composites and analysis of chopped fiber-reinforced materials

1035

theoretical methods probably (see Eqs. 31.153
31.159). Generally, the real values in experiments are less than the theoretical values in such problems. Therefore this parameter (related to resin strength) must be decreased by multiplying on a golden parameter (coefficient) in order for suitable agreement between the experimental and theoretical results
(locally improving the results). This golden parameter (coefficient) can be  “1 2 ϑ=3 ,” in which ϑ is the Poisson’s ratio. Therefore we have the following parameter for possible (probable) improvement,   ϑ 32ϑ golden coefficient ðparameterÞ 5 @ 5 1 2 5 3 3

(31.156)

Finally, Tsai
Hill failure theory may be in the following form to present the different results: σ 2 1

X

1@

σ 2 2

Y

2

σ1 σ2 τ 12 2 1 51 X2 S

(31.157)

or σ 2 1

X



 ϑ σ2 2 σ1 σ2 τ 12 2 1 12 2 2 1 51 3 Y X S

(31.158)

  ϑ σ2 2 σ1 σ2 τ 12 2 1 12 2 2 1 ,1 3 Y X S

(31.159)

or σ 2 1

X

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1043

[143] L. Fischer, Optimization of orthotropic laminates, Eng. Indus. Ser. B 89 (3) (1967) 399
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288. [145] C.D. Noms, Strength of orthotropic materials subjected to combined stress, U.S. Forest Products Laboratov Report #IR16, 1950. [146] Isaac M. Daniel, Ori lshai, Engineering Mechanics of Composite Materials, second ed., Oxford University Press, New York, Oxford, 2006. [147] R.P.L. Nijssen, Composite Materials: An Introduction, A VKCN Publication, 2015.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A AA BFS cement (ASC), 474 AA FA/slag (AAFS), 467 AABs. See Alkali-activated binders (AABs) AAGU. See GGBFS ultrafine palm oil fuel ash (AAGU) AAMs. See Alkali-activated materials (AAMs) AAS. See Alkali-activated slag systems (AAS); Alkaline activator solution (AAS) AAS concrete (AASC), 468, 472, 477 478 AAS FA-based concrete mixes (AASFC), 472 473 AAS pastes (AASPs), 467 AAS-based mortars (AASm), 475 AAS/silica fume (AASS), 478 AASC. See AAS concrete (AASC) AASFC. See AAS FA-based concrete mixes (AASFC) AASm. See AAS-based mortars (AASm) AASPs. See AAS pastes (AASPs) AASS. See AAS/silica fume (AASS) ABAQUS Python Interface, 965 966 Abram cone, 284 285, 304 305 AC. See Alternating current (AC) Acceleration period, 617 Accelerometer sensor (AS), 99 ACCs. See Advanced cementitious composites (ACCs) ACI. See American Concrete Institute (ACI) Acid resistance of AAC mixes, 474 475 for alkali-activated soapstone binders, 888 889 Acid test for alkali-activated soapstone binders, 883 ACO. See Ant colony optimization (ACO)

Activator modulus (Ms), 464 Activator powders, 601 602 Active crack, 304 Activity concentration index. See External risk index Activity indices, 681 Adaptability in Fac¸ade function, 367 368 Adaptive fac¸ades, 367 Additives, 801 802 manufacturing, 540 of UHPFRC, 298 302 Admixed inhibitors, 661 662 Admixtures, 663 ADSC. See Alternating DSC (ADSC) Advanced cementitious composites (ACCs), 41 AEMM. See Age-adjusted effective modulus (AEMM) AFD. See Aluminum filter dust (AFD) Affine function, 72 AFm. See Al2O3 Fe2O3-mono (AFm) AFM. See Atomic force microscopy (AFM) AFt. See Al2O3-Fe2O3-tri (AFt) Age-adjusted effective modulus (AEMM), 480 Aggregates, 3, 7 8, 301, 301f, 613. See also Clay-expanded aggregates; Coarse aggregates; Recycled aggregate Aging indices, 708 mechanism of thermochromic asphalt binders, 712 resistance evaluation of asphalt, 707 712 Agricultural waste, 437 AHM. See Asymptotic expansion homogenization method (AHM) AI. See Artificial intelligence (AI) Air polluting effect, 53

1046

Al2O3-Fe2O3-tri (AFt), 402 Al2O3 Fe2O3-mono (AFm), 402 Albite, 184 185 AlexNet, 88 Algorithmic thinking. See Parametric design Alkali activation, 529 Alkali dosage, 535 Alkali-activated binders (AABs), 592 594, 600 601, 606 609 alkaline activators, 593 594 alternative for Portland cement, 592 593 cost analysis, 606 609, 608f rice husk ash, 597 602 SCBA, 602 605, 603f silica fume, 597 waste glass, 594 597 Alkali-activated cementitious systems, 461 462 Alkali-activated composites with alternative aggregates, 472 473 with alternative binders, 469 470 behaviour incorporated with fibers, 477 478 with different activators, 471 472 durability studies on, 473 475 acid resistance of AAC mixes, 474 475 chloride resistance of AAC mixes, 473 474 sulfate resistance of AAC mixes, 475 elevated-temperature performance of, 475 477 future trends for, 482 workability and strength characteristics of geopolymers and, 465 469 Alkali-activated concrete (AAC), 459, 472 AAS, 463 464 alkali-activated cementitious systems, 461 462 behaviour of rebar-reinforced structural elements made from, 479 480 effect of dosage and modulus of activator solutions, 464 465 geopolymeric cementitious systems, 460 461 requirements for alkali activation of ground granulated BFS, 463 Alkali-activated materials (AAMs), 529 530

Index

Alkali-activated slag systems (AAS), 463 464, 480 Alkali-activated soapstone binders, 877 materials and mix design, 878 881 physical and chemical compositions, 879t test procedures, 882 884 Alkali-resistant glass fibers (AR-glass fibers), 280 281 Alkaline activators, 593 594 cations, 463 464 cement, 462 Alkaline activator solution (AAS), 464 Alloys, 660 661 Alternate fine aggregates, 8 9 Alternating current (AC), 665 666, 986 Alternating DSC (ADSC), 229 Alumina (Al2O3), 5, 159, 414, 463, 513, 615 Aluminosilicate precursor, 530f, 531 534 source, 461 Aluminum, 438 439 powder, 537 Aluminum filter dust (AFD), 175 Aluminum oxide. See Alumina (Al2O3) American Concrete Institute (ACI), 279 American Society for Testing and Materials (ASTM), 225 226 Amine-based organic compounds, 661 662 Ammonium acetate technique, 418 Ammonium molybdate ((NH4)6MO7O24), 185 186 Amorphous cellulose, 585 Analytical methods of SFCs, 933 954 Andreasen particle distribution method, 298 299 Angle-ply laminates effect of control voltage on frequency ratio embedded with piezoelectric layer, 391 392 influence of temperature rise on frequency ratio embedded with piezoelectric layer, 390 Anhydrate (CaSO4), 159 Anhydrous Portland cement, 615 ANN. See Artificial neural network (ANN) Anodic inhibitors, 664 665 Anorthite (Ca(Al2SiO8)), 200 201, 755 756

Index

Ant colony optimization (ACO), 83 Apparent repair analysis, 805 811 Aqueous free radical copolymerization, 317, 318f AR-glass fibers. See Alkali-resistant glass fibers (AR-glass fibers) Arbitrary phases, cement hydration in, 616 617 Artificial aggregates of FA, 683 686 recent developments in, 685 686 Artificial gypsum, 159 160 in clay-based ceramic applications, 173 175 bricks, 173 porcelain stoneware tiles, 175 stoneware tiles, 173 175 Artificial intelligence (AI), 80 Artificial neural network (ANN), 80 81 AS. See Accelerometer sensor (AS) ASC. See AA BFS cement (ASC) Ashes, 158 159 in clay-based ceramic applications, 164 173, 165t bricks, 164 169 clay-expanded aggregates, 172 173 porcelain stoneware tiles, 171 stoneware tiles, 170 171 Aspect ratio, 278 Asphalt, 691, 702 703 Asphalt pavement, 691 692 surface distress application of CI frameworks in PMS, 97 102 CI methods, 80 84, 84f methodology and application, 84 97 Associated design. See Parametric design “Associated flow” model, 736 741 ASTM. See American Society for Testing and Materials (ASTM) Asymptotic expansion homogenization method (AHM), 949 Atomic force microscopy (AFM), 228 231, 668 Aureobasidium mold, 340 Aureobasidium pullulans molds, 356 Average fiber stress, 925 926 Average percent recovery, 703 705 Average-pooling, 86, 87f

1047

B B.O.C. Bonneshof Office Center in Dusseldorf, 365 367, 366f BA. See Bat algorithm (BA); Bottom ash (BA) Bacillus, 53 Bacillus bacillus, 801 802, 805, 814 820 Bacillus pasteurii, 54, 59f B. pasteurii ATCC 11859, 58 Bacteria as self-healing agent, 403 404 Bacterial concrete, 51 61 Bacterial nanocellulose (BNC), 585 Bacterially induced carbonate precipitation, 57 58 Bagasse from beer (BB), 172 173 BASF Microair 210, 538 539 Bat algorithm (BA), 83 BB. See Bagasse from beer (BB) BBA. See Bovine bone ash (BBA) BBR test. See Bending beam rheolometer test (BBR test) Bearing capacity of EPS geofoam, 136 137 Bending beam rheolometer test (BBR test), 706 707 Bentonite, 583, 584f BF dust. See Blast furnace dust (BF dust) BFS. See Blast furnace slag (BFS) BG. See Borogypsum (BG) BIM. See Building information modeling (BIM) Bingham model, 284 Bioconcrete culturing of cells for use in, 59 60 durability studies on, 57 58 Bio-inspired computation method. See Swarm intelligence (SI) Biobased fac¸ade materials, 335 347 engineered wood products, 340 342 modified wood, 337 340 natural wood, 335 337, 336f Biobased materials, 333 334 Biochar, 624 625 Bioinspiration for fac¸ade design, 355 358 Biological self-healing mechanism, 400 401 Biomass, 164 energy valorization, 159 FA, 685 Biomimetics, 51 61

1048

Biomimicry, 355 356 Biomineralization, 53, 57 58 Bituminous pavement temperature, adjustment of, 713 714 Black rice husk ash (BRHA), 494, 497 498, 500 chemical composition, 514t morphologies, 514f Blast furnace dust (BF dust), 157 158 Blast furnace slag (BFS), 1 2, 25, 407, 463, 614 requirements for alkali activation, 463 Blast oxygen furnace slag (BOF slag), 460 Blast protection, 149 150 Blended cements, 618 of FA, 683 Blowing agents, 535 538 BMCs. See Bulk molding compounds (BMCs) BNC. See Bacterial nanocellulose (BNC) BOF slag. See Blast oxygen furnace slag (BOF slag) Bone-shaped short fibers (BSS fibers), 936 BSS-polyethylene fiber-reinforced polyester matrix composites, 941 943 composite materials reinforced by, 955 956 fracture surfaces, 943f Ni-fiber-reinforced composite, 939, 941f stress strain curves, 940f Boolean logic, 81 Boric acid, 160 Borogypsum (BG), 160 Boron oxide (B2O3), 160 Bottom ash (BA), 158 159, 494 Bottom-up approach, 410 411 Bovine bone ash (BBA), 171 Box-test, 305, 305f Bp M-3 (mutant), 55 Breaking strength (BS), 186 187 BRHA. See Black rice husk ash (BRHA) Brick making artificial gypsum in clay-based ceramic applications, 173 175 ashes in clay-based ceramic applications, 164 173 glass waste in clay-based ceramic applications, 195 203

Index

mineral slags and metallurgy waste in clay-based ceramic applications, 175 180 organic waste in clay-based ceramic applications, 203 208 ornamental rock waste in clay-based ceramic applications, 189 195 review of studies into incorporation of waste materials in, 164 208 sludge in clay-based ceramic applications, 180 189 Bridge abutments, EPS in, 138 144 basic design concepts, 140 144 cases histories and performance, 139 BRUKER AXS S4 PIONEER XRF microscopy machine, 509 BS. See Breaking strength (BS) BSS fibers. See Bone-shaped short fibers (BSS fibers) Buckling, 1031 Building information modeling (BIM), 350 351 Bulk molding compounds (BMCs), 922 Buoyancy of EPS geofoam, 136 C CA. See Cat algorithm (CA) CAD software. See Computer-aided design software (CAD software) Cadmium, 680 CAE software. See Computer-aided engineering software (CAE software) Cafe´ Fratelli, 361, 361f Calamine (Zn4Si2O7(OH)2
H2O), 175 178, 200 201 Calcite (CaCO3), 53 54, 162, 175 178, 402, 826 Calcium carbonate. See Calcite (CaCO3) Calcium hydroxide (CH), 443, 473, 476, 615 616 Calcium nitrate-based corrosion inhibitor, 662 663 Calcium oxide (CaO), 6, 159, 513 Calcium silicate hydrate (CSH), 471, 615 616 gel, 671, 781 Calcium sulfoaluminate, 616 Calcium sulfoaluminate cements (CSAs), 672

Index

Calcium silicate hydrates gel (C S H gel), 462 464, 468 California bearing ratio mold (CBR mold), 749 CAM. See Computer-aided manufacturing (CAM) CaO SiO2 Al2O3 system, 7, 7f Carbon dioxide (CO2) emissions, 877 Carbon fibers (CFs), 23 24, 282, 282t, 283f, 478, 784 Carbon FRP composites (CFRP composites), 223 224, 644 Carbon nanotubes (CNTs), 376 377, 412, 415, 777 778 cement-based composites with, 784 785, 788 789 Carbon sink, 333 Carbon-PAN. See Carbon-polyacrylonitrile (Carbon-PAN) Carbon-polyacrylonitrile (Carbon-PAN), 282 Carbonation resistance in alkali-activated soapstone binders, 884, 891, 893f Carbonyl index (CI), 708 Carrier-attached Bacillus bacillus, 809 Cast tiles, 902t manufacturing, 901 902 Casting concrete, 830 831 Cat algorithm (CA), 83 Cation exchange capacity (CEC), 418 Cationic surfactant, 539 Cattenom Nuclear Power, 297 Caustic soda (NaOH), 161 CBMs. See Cement-based materials (CBMs) CBR mold. See California bearing ratio mold (CBR mold) CBRI. See Central Building Research Institute (CBRI) CC. See Conventional concrete (CC) CCA. See Corn cob ash (CCA) CCT. See Continuous curvelet transform (CCT) CD. See Cracking distress (CD) CEC. See Cation exchange capacity (CEC) Cellular concrete. See Foam/foamed concrete Cellulose, 584 585 Cellulose ethers (CEs), 584 585 Cellulose nanocrystals (CNCs), 585 Cellulose nanofibers (CNFs), 585

1049

Cement, 1 10, 41 42, 45, 499 500, 614, 901 902 composition, 5 7 hydration, 615 616, 616t matrices, 625 for self-healing concrete, 828 test results for cement replacement with fly ash, 911 913 compressive strength, 911 913 flexural strength, 913 water absorption, 913 types, 4 5 Cement paste binder, 494 superplasticizer effect on, 321 323 flow profile, 321, 322f, 323f yield stress of cement paste, 322 323, 323f Cement-based composites, 777. See also Nanocement-based composites increasing rate of properties, 778t smart and multifunctional cement-based composites, 778 784 Cement-based materials (CBMs), 614 advantages and disadvantages, 621 630 Cement-based matrix of FRC, 275 278 of UHPFRC, 298 302 Cement-generated environmental problems, 396 Cementitious construction materials biomimetics and bacterial concrete, 51 61 cement and concrete, 1 10 fiber-reinforced concrete polymer composites, 23 25 FRC, 16 23 geopolymer concrete, 13 16 HPC, 10 13 LWC, 25 40 UHSC, 40 51 Cementitious materials, 624 self-healing mechanism in, 399 409 Cement polymer composites (CPCs), 625, 629 630 Cenosphere of FA, 685, 685f Central Building Research Institute (CBRI), 20 21 Centrifuge tests, 149 150

1050

Ceramic tile polishing waste, 899 CEs. See Cellulose ethers (CEs) CFA, 169 CFRC slabs. See Coconut fiber-reinforced concrete slabs (CFRC slabs) CFRP composites. See Carbon FRP composites (CFRP composites) CFs. See Carbon fibers (CFs) CH. See Calcium hydroxide (CH) CH plasticity. See Clay of high plasticity (CH plasticity) Chapelle test, 681 Chemical etching, 686 Chemically innovative engineered cement substitutes, 619 Chemisorption, 664 665 Chloride, 680 binding, 11 chloride-induced rebar corrosion in concrete, 659, 659f diffusion, 451 454 water absorption vs. square root of time, 452f WMP fibers effects on chloride penetration depth, 453f resistance of AAC mixes, 473 474 Chopped composites. See Short-fiber composites (SFCs) Chopped strand mat (CSM), 922, 929 laminates, 973 stress strain behavior, 930f CI. See Carbonyl index (CI); Computational intelligence (CI) Circular Pavilion, 360, 361f Clay of high plasticity (CH plasticity), 745, 755 Clay-based ceramic products development of waste treatment, 157f industrial waste materials as aggregate in clay ceramics, 158 164 review of studies into incorporation of waste materials in brick making, 164 208 waste generation by economic activities and households, 156f Clay-expanded aggregates ashes in clay-based ceramic applications, 172 173

Index

glass waste in clay-based ceramic applications, 203 mineral slags and metallurgy waste in clay-based ceramic applications, 180 organic waste in clay-based ceramic applications, 207 208 ornamental rock waste in clay-based ceramic applications, 195 sludge in clay-based ceramic applications, 188 189 Clays, 418 419, 613 Climatic design, 354 355, 354f Close infrared region, 230 CLT. See Cross-laminated timber (CLT) CMAI. See Complex modulus aging index (CMAI) CMOD. See Crack mouth opening displacement (CMOD) CNCs. See Cellulose nanocrystals (CNCs) CNFs. See Cellulose nanofibers (CNFs) CNN. See Convolutional neural network (CNN) CNTs. See Carbon nanotubes (CNTs) Co-60 radionuclide, 624 625 Coal energy production, 159 Coarse aggregate for self-healing concrete, 828 829 Coarse aggregates, 7 9, 45, 500, 901 902 gradation, 502t, 503f Coarse ash/bottom ash, 8 Coarse natural aggregate (NAC), 862 Coating, 405 406 Cobalt, 419 hexacyanoferrate solutions, 632 Cobinders, 878 Coconut fiber-reinforced concrete slabs (CFRC slabs), 951 952 Coefficient of thermal expansion (CTE), 187 188, 257, 260 261 Coffee waste (CW), 207 Cohesive zone approach, 958, 959f Cold twisted deformed (CTD) bars, 10 Colemanite, 160 Colorants, 157 Columbic adsorption, 664 Compact reinforced concrete (CRC), 297 Compaction process, 503 505 Comparative analysis, 597 Complex modulus aging index (CMAI), 708

Index

Composite materials, 221, 222f, 625 reinforced by BSS fibers, 955 956 Composites, 980 model, 954 955, 955f piezoelectric plates, 375 376 sandwich beams, 237 238 Compressing, 65 Compression, 751 Compressive strength (Cs), 285 286, 286f, 442 443, 777 778, 832 activity index, 520 in alkali-activated soapstone binders, 882, 885 cobinders impact, 886f effect of bacteria, 60 61 of EPS geofoam, 122 123 in fly ash utilization, 906 908, 911 913 for self-healing concrete, 837, 837f test, 505 Computational design. See Parametric design Computational homogenization, 721 722 Computational intelligence (CI), 80 methods of asphalt pavement surface distress, 80 84, 84f Computer numerical control machine, 348 349, 349f Computer-aided design software (CAD software), 348 Computer-aided engineering software (CAE software), 348 349 Computer-aided manufacturing (CAM), 348 349 Concrete, 1 10, 395, 420 421, 436, 459, 475 476, 591, 613, 797 798, 825, 859 based on innovative cement-based materials, 635 636 composites, 435 damage, 660 design, 829 830, 832t durability, 396 397 fiber in, 826 mix design and proportion, 502 503 pavement, 494 rheology, 324 325, 325f effects of dry extract superplasticizers on fluidity concrete, 325 effects of superplasticizer type, 324

1051

effects of superplasticizers on fluidity concrete, 324 tests, 832 835 Concrete compressive strength, superplasticizer effect on, 326 327 Concrete corrosion, 657 658 assessing techniques, 666 668 DC polarization measurements, 667 electrochemical impedance spectroscopy, 667 668 potential measurement, 666 667 transient methods, 668 inhibitors, 661 663 Concrete incorporating waste metalized plastic fibers, 439 454 fresh properties density, 439 postconsumer waste metalized plastic films, 440f workability, 439 442 hardened properties chloride diffusion, 451 454 compressive strength, 442 443 flexural strength, 444 447, 445f, 446f impact resistance, 447 448 sorptivity and water absorption, 448 451 splitting tensile strength, 443 444 Concrete tiles, 897, 902t test results for water absorption, 915 wet transverse strength, 913 914 Conditioning, 622 Conductive fillers, 782 Confined resin simulation, 734 Confined steel-reinforced resin, 736 741 Conplast SP430 G8 (FOSROC), 900 901 Conservative approach, 274 275 Constitutive and fundamental researches of SFCs, 990 992 Constitutive relations, 381 382 Construction waste, 797 798 Containers for radioactive waste based on innovative cement-based materials, 630 635 Continuous curvelet transform (CCT), 71 Continuous ridgelet transform (CRT), 69, 72 Continuous wavelet transform (CWT), 68 Contourlet transform, 73 75, 74t, 75t

1052

Conventional anticorrosive coatings, 405 Conventional cement concrete, 41 Conventional concrete (CC), 11 12 Conventional FRP composites, 238 239 Conventional pavement, 493 Conventional reinforcement, 289 292 Conventional short straight fibers (CSS fibers), 936 CSS-polyethylene fiber-reinforced polyester matrix composites, 941 943 fracture surfaces, 942f Ni-fiber composites, 939, 941f stress strain curves, 940f Convolutional layers, 86 Convolutional neural network (CNN), 81, 86 88 structure for pavement distress detection and classification, 89f Cool pavement, 691 692 Copper, 419 Copper slag (CS), 460 Core panel system of EPS, 150 Corn cob ash (CCA), 606 Corrosion, 657 660 chloride-induced rebar corrosion in concrete, 659f concrete corrosion assessing techniques, 666 668 inhibitors, 661 663 durability studies of concrete with admixtures, 670 673 limitation of inhibitors, 663 mechanism of inhibition, 664 665 monitoring techniques for evaluation of inhibitor efficiency and corrosion rate, 665 666 product, 660 661 analysis techniques, 668 670 severity, 660 661 surface characterization of metals/rebars after corrosion, 668 techniques to assess inhibitor performances, 665 666 Corrosion-resistant TMT bars, 10 Cost analysis, 606 609, 608f CPCs. See Cement polymer composites (CPCs)

Index

Crack mouth opening displacement (CMOD), 289, 401 Crack(s), 825, 827 828, 832, 851 852 detection, classification, and quantification, 97 distribution, 36 repair, 808t effect, 809 Cracking process, 304 Cracking distress (CD), 100 CRC. See Compact reinforced concrete (CRC) Creep behavior, 1003, 1006 of EPS geofoam, 127 129 Creep test, 228 Creeping fiber displacement rates, 1003 Critical active cracking, 304 Critical fiber length, 920, 925 926 Cross-laminated timber (CLT), 334, 340 342, 341f Cross-ply laminated plates, 376 control voltage effect on the frequency ratio of embedded with piezoelectric layer, 389 influence of temperature rise on frequency ratio embedded with piezoelectric layer, 387 389 linear static analysis of, 384 CRT. See Continuous ridgelet transform (CRT) Crumb rubber concrete, 826 Crumb rubber for self-healing concrete, 829 Crushed concrete, 614 Crushed quartz, 42 CS. See Copper slag (CS) Cs-134 radionuclide, 624 625 CSA. See Cuckoo search algorithm (CSA) CSAs. See Calcium sulfoaluminate cements (CSAs) CSH. See Calcium silicate hydrate (CSH) C S H gel. See Calcium silicate hydrates gel (C S H gel) CSM. See Chopped strand mat (CSM) CSS fibers. See Conventional short straight fibers (CSS fibers) CTE. See Coefficient of thermal expansion (CTE)

Index

Cuckoo search algorithm (CSA), 83 Culturing of cells for use in bio-concrete, 59 60 Curing condition, 505, 507f methods, 3 4 of UHSC, 42 43 Curvelet transform, 70 71, 71t CW. See Coffee waste (CW) CWT. See Continuous wavelet transform (CWT) Cyclic ester, 693 Cyclic/dynamic loading, behavior under, 124 125 Compressive strength, expressions for, 870f of concrete, 865 869 in literature, 864 of rubberized concrete, 867 869 D Damage mechanics, 729 Damage variable, 729 733 Damage visual enhancement, 399 400 Daubechies filters in ridgelet transform, 70, 70t DBD. See Deep borehole disposal (DBD) DC. See Direct current (DC) DCB method. See Double-cantilever beam method (DCB method) DE. See Diatomaceous earth (DE); Differential evolution (DE) De Meyer filter, 68, 71 Deceleration period, 617 Deep borehole disposal (DBD), 635 636 Deep learning (DL), 81, 85 86 Deep network, 85 Deformed 3D edge-cracked fiber, 958, 962f Deformed bar. See Reinforcing bars (Rebar) Defuzzification, 82 Degreasing raw materials, 157 Delamination, 1031 Denoising, 65 DenseNet-201, 88 Densified small particles, 296 297 Density, 439, 548 Design for disassembly concept (DfD concept), 341, 359 Desulfurization slag powder (DSP), 470 DFB. See Directional filter banks (DFB)

1053

DfD concept. See Design for disassembly concept (DfD concept) Diameter ratio effect (DR effect), 761 765 Diatomaceous earth (DE), 582 583 Diatomite. See Diatomaceous earth (DE) Dicalcium silicate, 615 Differential evolution (DE), 83 Differential scanning calorimeter (DSC), 228 229, 244, 697 699 analysis, 229 Differential TGA curves (DTG curves), 697, 698f, 891 Differential thermal analysis, 884 Digitalization, 348 349 Dimensionless stress reduction factor, 867 868 1,3-Dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), 339 Dioctahedral phyllosilicate, 417 Direct current (DC), 665 666 polarization measurements, 667 Direct foaming method, 535 540 blowing agents, 536 538 surfactants, 538 540 Direct ink writing (DIW), 529 530 Direct iteration technique, 377 Directional filter banks (DFB), 73 Discarded tires, 825 Discontinuous fibers, 920 typical size parameters, 920t Discrete ridgelet transform, 69 Discrete shearlet transform (DST), 72 73 Discrete wavelet transform (DWT), 68 Dispersion fluidification phenomenon, 319 Displacement fields, 378 380 relations, 378 381 Disposal in deep geologic repository, 622 Dissolution process, 461 Distress manifestation index (DMI), 100 Diversification of EPA, 93 96 DIW. See Direct ink writing (DIW) DL. See Deep learning (DL) DMDHEU. See 1,3-Dimethylol-4,5dihydroxyethyleneurea (DMDHEU) DMI. See Distress manifestation index (DMI) DMTA. See Dynamic mechanical thermal analysis (DMTA)

1054

Dormant period, 616 Double-cantilever beam method (DCB method), 973 DR effect. See Diameter ratio effect (DR effect) Dry extract superplasticizer effects on fluidity concrete, 325 Drying shrinkage in alkali-activated soapstone binders, 882, 885 888, 887f DSC. See Differential scanning calorimeter (DSC) DSP. See Desulfurization slag powder (DSP) DST. See Discrete shearlet transform (DST) DTG curves. See Differential TGA curves (DTG curves) Durability studies on bioconcrete, 57 58 water absorption tests, 58 of concrete with admixtures, 670 673 Duras EasyFinish fiber, 826, 829, 831f, 831t DWT. See Discrete wavelet transform (DWT) Dynamic characterization of EPS geofoam, 125 127 Dynamic mechanical thermal analysis (DMTA), 228 229, 231 232 E E-glass fibers, 280 281 EAFD. See Electric arc furnace dust (EAFD) EC. See Evolutionary computation (EC) ECC. See Engineered cementitious composite (ECC) Eco-friendly biocement composites, 614 EDX. See Energy-dispersive X-ray (EDX) EFBs. See Empty fruit bunches (EFBs) Effective modulus method (EMM), 480 EIS. See Electrochemical impedance spectroscopy (EIS) Elastic range, 123 Electric arc furnace dust (EAFD), 178 Electrical conductivity test, 681 Electrical double layer, changes in, 664 Electrical resistivity of cement-based composites, 782 Electrochemical impedance spectroscopy (EIS), 666 668

Index

Electromagnetic wave-shielding/absorbing cement-based composites, 784 Electron microscopy, 984 986 Electroplating sludge, 184 185 Electrorestrictive materials, 375 Elemental analysis, 814 Elemental silicon, 537 Elevated-temperature performance of alkaliactivated composites, 475 477 Embankments, EPS in, 131 138 case histories and performance, 132 134 Manchester railway embankment, 133 134 road embankment, 132 133 Watford Junction replacement station platform, 134 design procedure and notes, 135 138 practical issues, 134 135 layout of blocks, 134 135 longitudinal geometry, 135 site preparation, 135 EMC. See Equivalent material concept (EMC) EMC-PM model, 990, 994f EMM. See Effective modulus method (EMM) Emperor penguin algorithm (EPA), 84 85, 93 97, 96f Empty fruit bunches (EFBs), 984 986 Encapsulation in cement, 624 Energy efficiency, 365 367 expressions, 382 problems in cement industries, 398 spectrum analysis, 818f valorization, 158 159 Energy absorption, mathematical model of, 946 Energy-dispersive X-ray (EDX), 511 512 Engineered cementitious composite (ECC), 406, 415 Engineered wood products (EWP), 340 342 Enterprise resource planning (ERP), 348 349 Environmental durability of FRP composites in civil structures, 241 261 freeze thaw resistance, 259 261 humid environments, 249 256 hydrothermal response, 249 253

Index

hygrothermal behavior, 253 256 temperature, 242 248 elevated temperatures, 245 248 low and cryogenic temperatures, 243 245 thermal shock, 257 259 UV irradiation, 256 257 Environmental impact of EPS geofoam, 130 Environmental SEM (ESEM), 981 Environmental suitability nanomaterialbased concretes, 421 422 EP. See Evolutionary programming (EP) EPA. See Emperor penguin algorithm (EPA) Epoxy, 257 258 coatings, 405 406 epoxy-coated bars, 10 epoxy/SCF composites, 986 resin, 718, 722 EPS. See Expanded polystyrene (EPS); Extracellular polymeric substances (EPS); Extruded polystyrene (EPS) EPS20 geofoam, 128 129 Equivalent material concept (EMC), 990 ERP. See Enterprise resource planning (ERP) Escherichia coli, 55 ESEM. See Environmental SEM (ESEM) Esterification, 317, 318f Ettringite, 615 616 Eu-152 radionuclide, 624 625 Euler-buckling, 1031 Euplectella, 57 Evolutionary computation (EC), 80, 82 83 Evolutionary programming (EP), 83 Expanded polystyrene (EPS), 565, 567t geofoam, 117, 117f in bridge abutments and retaining structures, 138 144 density, 120 121, 121t, 123t design manuals, 119 120 in embankments, 131 138 history, 118 119 issues, 129 131 in other uses, 149 150 production procedure, 118f properties, 120 131 standards, 120t in utility protection, 144 149 geofoam inclusion

1055

effect of geobeads inclusion, 745 755 effect of geofoam granules column, 755 774 lightweight concrete, 566 570, 573 574 volume change, 571f Expanded vermiculite, 584 Expansive cement, 5 Expansive soil, 745 747, 746t, 756t Experimental methods of SFCs, 973 990 Extenders, 580 585 low-density materials, 581 583 water extenders, 583 585 External glazing, 343 External risk index, 173 175 External shear strength of EPS geofoam, 123 124 Extra rapid hardening cement, 4 Extracellular polymeric substances (EPS), 61 Extrinsic stimuli of fac¸ade adaptability, 367 368 Extruded polystyrene (EPS), 548 Extruded polystyrene geofoam (XPS geofoam), 117 F FA. See Firefly algorithm (FA); Fly ash (FA) FA-based geopolymer (FA-GPm), 475 concrete, 461 FA-GPm. See FA-based geopolymer (FAGPm) FabCrete. See Textile reinforced concrete (TRC) Fac¸ade design, 348 364 BIM, 350 351 bioinspiration for fac¸ade design, 355 358 climatic design, 354 355, 354f digitalization, 348 349 modular design and prefabrication, 352 353 parametric design, 351 352 prestige, symbolism, and individualization, 361 362 sustainable, restorative, and regenerative aspects, 363 364 urban mining and design for disassembly, 358 360 Fac¸ade function, 365 369 adaptability, 367 368

1056

Fac¸ade function (Continued) energy efficiency, 365 367 fac¸ade leasing, 368 369 Fac¸ade systems, 334 335 Far infrared region, 230 Fast discrete curvelet transform (FDCT), 71 Fatigue behavior of GFRP bars, 248 Fatigue test, 226 FBD. See Free body diagram (FBD) FDCT. See Fast discrete curvelet transform (FDCT) FE. See Finite element (FE) FE-SEM. See Field emission scanning electron microscopy (FE-SEM) Feed-forward neural network (FFNN), 81 FEMs. See Finite element methods (FEMs) Ferric oxide (Fe2O3), 5 6, 615 Ferronickel slag (FNS), 470 Ferrosilicon (FeSi), 537 FFNN. See Feed-forward neural network (FFNN) FFRP. See Flax fiber-reinforced polymer (FFRP) FFRP-CFRC slabs, 951 952 FGMs. See Functionally graded materials (FGMs) FGW. See Fiberglass waste (FGW) Fib Model Code 2010, 288 289 Fiber reinforced polymer (FRP), 643 composite materials, 23 25 Fiber-length distribution (FLD), 933 934 Fiber-orientation distribution (FOD), 933 934, 936, 967 971 Fiber-reinforced cementitious composites, 437 Fiber-reinforced concrete (FRC), 16 23, 273 294, 292f, 436, 454 applications, 275f constituent materials, 275 284 cement-based matrix, 275 278 fibers, 278 284 new trends and applications, 293 294, 294f, 295f with nonmetallic fibers, 20 partial safety factors, 292t polymer composites, 23 25 carbon fiber mat roll, 24f fiber reinforced polymer composite laminates, 23 25

Index

rheology and mechanical properties, 284 288 slurry infiltrated fibrous concrete (SIFCON), 22 23 steel fiber-reinforced concrete (SFRC), 18 22 technical codes for design, 288 293 Fiber-reinforced gypsums, 947 948 Fiber-reinforced polymer composites (FRP composites), 222 applications in structural fields, 223 224 assessment by mechanical, chemical, and thermal behaviors, 224 233 macro characterization, 224 228 micro characterization, 228 233 composite materials, 221, 222f environmental durability of FRP composites in civil structures, 241 261 evaluation of special structural properties, 233 241 toughening mechanisms through implications of nanofillers, 238 241 vibrational properties, 233 238 technological superiorities, 222 223 Fiber(s), 42 aspect ratio, 971 in concrete, 826 efficiency, 278 of FRC, 278 284 carbon fibers, 282, 282t, 283f characteristics, 278f glass fibers, 279 281, 280t, 281f natural fibers, 282 284, 283f, 284t polymeric fibers, 281, 281f, 282t steel fibers, 279, 280f orientation, 922 924, 928 changes in, 923f flow patterns in injection moldings, 924f packing geometry, 971 pull-out mechanism, 980 reorientation model, 934 for self-healing concrete, 829 volume fraction, 971 Fiberglass waste (FGW), 199 Fictitious material concept (FMC), 989 990 Field emission scanning electron microscopy (FE-SEM), 511 512, 512f

Index

Fiji ImageJ, 544 545 Fine aggregates, 7 9 alternate fine aggregates, 8 9 coarse aggregates, 9 for self-healing concrete, 828 829 Fine natural aggregate (NAF), 862 Fingerprint region, 230 Finite element (FE), 943 944 model, 958 on confined resin tests, 734, 735f for unconfined resin simulation, 733, 734f Finite element methods (FEMs), 350, 385, 943 944 Finite ridgelet transform. See Discrete ridgelet transform Finite strip method (FSM), 376 Fire resistance, 552 Fire risk of EPS geofoam, 130 Fired clay-based ceramics, 157 Firefly algorithm (FA), 83 First-order numerical homogenization, 721 First-order shear deformation plate theory (FSDT), 376 377 Five-dimensional representation (5D representation), 350 Fixing of EPS geofoam, 130 131 FL. See Fuzzy logic (FL) Flax fiber-reinforced polymer (FFRP), 951 952 FLD. See Fiber-length distribution (FLD) Flexibility, 415 Flexural strength (Fs), 287 288, 777 778, 826 in alkali-activated soapstone binders, 882, 885 cobinders impact, 886f of EPS geofoam, 136 137 in fly ash utilization, 909 910, 913 for self-healing concrete, 838, 838f Flexural tenacity, 302 303 Flexural test, 226 227 Flexural toughness, 19 Flower pollination algorithm (FPA), 83 Fluid mechanics theory, 933 Fluidity concrete dry extract superplasticizer effects on, 325 effects of superplasticizers on, 324 Fluorapatite (Ca5(PO4)3F), 160

1057

Fluxes, 157 Fly ash (FA), 1 2, 158 159, 299, 300f, 459, 494, 529, 531 534, 582, 594 596, 618, 677, 897 898 applications, 681 682 characterization, 679 681 from application perspective, 681 chemical, 680 microstructural, 680 mineral, 680 681 physical, 679 680 chemical composition and physical properties, 645t developments in industrial fly ash applications, 682 686 in India, 897 proportion of fresh concrete mixture, 901t, 902t rationale for use, 681, 682f use as fine particles, 681 682 use for chemically active minerals, 682 XRD test of, 901f Fly ash utilization, 897 899 experimental procedure manufacturing of paver blocks and tiles, 901 902 materials, 900 mix design, 900 901 particle size distribution, 900t test methods, 902 905 sustainable development, 678 679 test results for cement replacement with fly ash, 911 913 compressive strength, 911 913 flexural strength, 913 water absorption, 913 test results for concrete tiles water absorption, 915 wet transverse strength, 913 914 test results for sand replacement with fly ash, 906 910 compressive strength, 906 908 flexural strength, 909 910 freeze thaw durability, 910 water absorption, 910 FMC. See Fictitious material concept (FMC) FMC-PM model, 990, 994f FNS. See Ferronickel slag (FNS) Foam

1058

Foam (Continued) concrete technology, 529 stability, 546 548 Foam/foamed concrete, 26 31 applications, 26 27 characteristics, 29 experimental investigations, 29 31 details of mix, 30 ingredients, 30 mixing, casting, and placing procedures, 30 31 production of foam, 30 material constituents, 27 28 mix proportioning, 28 29 strength ranges, 29 Foaming agent types, 534 535 FOD. See Fiber-orientation distribution (FOD) Food packaging products, 438 439 Footprint of uncertainty (FOU), 91 Four-dimensional representation (4D representation), 350 Fourier transformation infrared spectroscopy (FTIR spectroscopy), 228 230, 418, 708 710 of thermochromic powders, 693 694 FPA. See Flower pollination algorithm (FPA) Fractography, 232 FRASC. See Freshwater and river sand (FRASC) FRC. See Fiber-reinforced concrete (FRC) Free body diagram (FBD), 1006, 1008f, 1010f Free swell index (FSI), 745 Freeze-casting, 540 Freeze thaw durability in fly ash utilization, 910 resistance, 259 261 test, 904 Frequency domain methods, 66 Fresh state of FRC, 284 285 properties, 647 of UHPFRC, 304 306 Freshwater and river sand (FRASC), 468 469 Freshwater and sea sand (FSASC), 468 469 FRP. See Fiber reinforced polymer (FRP)

Index

FRP composites. See Fiber-reinforced polymer composites (FRP composites) FSASC. See Freshwater and sea sand (FSASC) FSDT. See First-order shear deformation plate theory (FSDT) FSI. See Free swell index (FSI) FSM. See Finite strip method (FSM) FTIR spectroscopy. See Fourier transformation infrared spectroscopy (FTIR spectroscopy) Fully 3D cohesive zone model, 958 Fully connected layers, 87 Functional nanomaterials advantages and disadvantages of nanomaterials for self-healing concrete, 420 economy of nanomaterial-based selfhealing concretes, 420 421 environmental suitability and safety features of nanomaterial-based concretes, 421 422 nanomaterial-based self-healing concrete, 413 419 nanomaterials, 410 413 self-healing concrete, 398 409 sustainability of nanomaterial-based self-healing concrete, 419 420 of traditional OPC-based concrete, 396 398 Functionally graded materials (FGMs), 377 Furfurylation process, 339, 339f Fusion method, 594 Fuzzification, 82 Fuzzy inference engine, 82 Fuzzy logic (FL), 80 82 Fuzzy rule base, 82 Fuzzy sets, 81 82 Fuzzy systems, 82 type I fuzzy sets, 82 type II fuzzy sets, 82 G GA. See Genetic algorithms (GA) Galerkin technique, 376 Galvanic sludge wastes (GSWs), 161 Galvanization, 161

Index

Galvanized bars, 10 Gaseous CO2, 421 422 Gaussian noise, 65 GB inclusion. See Geobeads inclusion (GB inclusion) GBFS. See Granulated blast-furnace slag (GBFS); Ground blast furnace slag (GBFS) GBM. See Glass bottle mixture (GBM) GC Osaka Building, 344 345, 345f GE composite. See Glass fiber/epoxy composite (GE composite) GE composites. See Glass/epoxy composites (GE composites) Gel phase, 616 General type 2 fuzzy system (GT2FS), 91, 92f Genetic algorithms (GA), 83 Geobeads inclusion (GB inclusion), 745 effect of, 745 755 experimental program, 749 751 LCA, 749 750 specimen preparation, 750 test procedure, 751 material characteristics, 745 748 results and discussions, 751 755 Geocements, 461 462 Geofoam, 117 Geofoam granules column (GGC), 755 effect of, 755 774 densities, 765 769 experimental procedure, 760 761 experimental setup, 758 760 column preparation, 759 760 LCA, 758 specimen preparation, 758 759 material properties, 755 758 soil characteristics, 755 756 effect of placement condition, 769 772 results and discussions, 761 772 variation of moisture content on specimens, 772 774 Geopolymer cement concretes (GPCC), 13 Geopolymer concrete (GPC), 13 16, 472, 479 480 building blocks and paver blocks, 16 development of structural grade, 14 16 mix composition for, 47t strength characteristics of mixes, 16t

1059

Geopolymeric cementitious systems, 460 461 Geopolymerization, 157 158 Geopolymers, 460 462 binders, 597 600 innovation, 592 593 mixes, 604f production mechanisms, 602f workability and strength characteristics of, 465 469 GFRPs. See Glass fiber-reinforced polymers (GFRPs) GGBFS. See Ground granulated blast furnace slag (GGBFS) GGBFS ultrafine palm oil fuel ash (AAGU), 469 470 GGC. See Geofoam granules column (GGC) GHG. See Greenhouse gas (GHG) GHPC. See Green high-performance concrete (GHPC) Glass, 162 163 Glass bottle mixture (GBM), 200 201 Glass composites, timber and, 342 346 Glass fiber-reinforced polymers (GFRPs), 279 280 composites, 223 224, 644 Glass fiber/epoxy composite (GE composite), 239 Glass fibers, 23 24, 279 281, 280t, 281f Glass mat thermoplastics (GMT), 973 Glass textile, 34 Glass transition temperature (Tg), 239 240 Glass waste, 162 163 in clay-based ceramic applications, 195 203, 196t bricks, 199 201 clay-expanded aggregates, 203 porcelain stoneware tiles, 202 203 stoneware tiles, 201 202 Glass waste sludge (GS), 199, 201 Glass/epoxy composites (GE composites), 258 Global stability analysis, 135 Global warming, 459 Glued laminated timber beams (GLT beams), 340 341 Glulam, 334 GMT. See Glass mat thermoplastics (GMT) Gold (Au), 659

1060

Gold cadmium alloy (Au-Cd alloy), 404 GoogleNet, 88 GPC. See Geopolymer concrete (GPC) GPC solids (GPS), 14 GPCC. See Geopolymer cement concretes (GPCC) GPR. See Ground penetration radar (GPR) GPS. See GPC solids (GPS) GPW. See Production of granite waste (GPW) Granite, 162 Granulated blast-furnace slag (GBFS), 472 Graphene, 415 cement-based composites with, 785 786 Graphite, 777 778, 784 Gravel particles, 828 829 Gravimetric analyzers, 418 Green cement-based products, 614 Green fac¸ades, 346 347, 347f Green high-performance concrete (GHPC), 1 2 Green innovative CBM, 614 Green walls, 346 347, 347f Greenhouse gas (GHG), 396, 459, 592 emissions, 617 Green Lagrange strain fields, 376 377 Grinding procedure, 500 501, 503t, 504f Ground blast furnace slag (GBFS), 469 470 Ground granulated blast furnace slag (GGBFS), 13 14, 459, 463, 594 596 Ground penetration radar (GPR), 99 GS. See Glass waste sludge (GS) GSWs. See Galvanic sludge wastes (GSWs) GT2FS. See General type 2 fuzzy system (GT2FS) GW. See Gypsum waste (GW) Gypsite, 629 Gypsium, 629 Gypsum (CaSO4. 2H2O), 159, 175 178, 613 gypsum-based composites, 947 948 Gypsum waste (GW), 173, 174t H HAC. See High alumina cement (HAC) HAFA. See High alumina fly ash (HAFA) Halochromic materials, 375 Halpin Kerner model, 978 980 Halpin Tsai

Index

approach, 957 958 equation, 973 theory, 966 Hardened state of FRC, 285 288 properties, 647 650 of UHPFRC, 306 309 Harmony search algorithm (HSA), 83 HBA. See Honey bee algorithm (HBA) hcp. See Hydrated binding cement paste (hcp) HDC. See High durability concrete (HDC) Healing agents, 399 400 Heliantus, 357 Hematite, 755 756 Hemicellulose, 585 Hemihydrate (CaSO4 0.5H2O), 159 Herschel Bulkley model, 466 HFC. See High-strength fiber-reinforced concrete (HFC) HFMs. See High-fidelity models (HFMs) HGMSs. See Hollow glass microspheres (HGMSs) High alumina cement (HAC), 5, 474 High alumina fly ash (HAFA), 899 High durability concrete (HDC), 10 11 High temperature in alkali-activated soapstone binders, 884, 890 891 High Volume Hybrid Fiber Reinforced Composites (HVHFRC), 41 High water-reducing admixtures. See Superplasticizer High yielding strength deformed bars, 10 High-energy ball-milling technique, 410 411 High-fidelity models (HFMs), 721 High-magnesium nickel slag (HMNS), 467 High-molecular-weight PNSs, 324 High-performance cement technique, 621 High-performance concrete (HPC), 1 2, 10 13 characterization and design philosophy, 11 13 High-performance FRC (HPFRC), 296 297 High-pressure steam curing, 42 43 High-range water reducer (HRWR), 415 416 High-strength concrete (HSC), 1 2, 412 G

Index

High-strength fiber-reinforced concrete (HFC), 949 951 High-strength friction grip bolts (HSFG bolts), 717 Higher order shear deformation theory (HSDT), 376 Hill Mandel computational homogenization method, 721 722 Hitachi HT7700 TEM, 511 HM. See Hybrid method (HM); Hydration modulus (HM) HMA. See Hot mixture asphalt (HMA) HMC. See Hygroscopic moisture content (HMC) HMNS. See High-magnesium nickel slag (HMNS) Hollow fibers, 399 401 Hollow glass microspheres (HGMSs), 565, 575, 575f density variation of composite, 577f as insulation agents, 575 577, 576t for oil well cements, 577 580, 579t strength variation of polymeric composite, 577f thermal conductivity change with density change, 578f HOMO, 663 Honey bee algorithm (HBA), 83 Hook effect, 684, 684f Hopfield’s recurrent network, 81 Hot mixture asphalt (HMA), 713 714 HPC. See High-performance concrete (HPC) HPFRC. See High-performance FRC (HPFRC) HRWR. See High-range water reducer (HRWR) HS. See Hybrid systems (HS) HSA. See Harmony search algorithm (HSA) HSC. See High-strength concrete (HSC) HSDT. See Higher order shear deformation theory (HSDT) HSFG bolts. See High-strength friction grip bolts (HSFG bolts) HVHFRC. See High Volume Hybrid Fiber Reinforced Composites (HVHFRC) Hybrid FRC, 20 Hybrid laminated composite plates, 376 Hybrid method (HM), 80, 84 Hybrid systems (HS), 99

1061

Hydrated binding cement paste (hcp), 443 Hydration, 3 4 Hydration modulus (HM), 463 Hydrogen peroxide (H2O2), 534 537 Hydrophobic cement, 5 Hydrothermal method, 594 Hydroxyethyl cellulose, 573 HygroScope, 367 368 Hygroscopic moisture content (HMC), 755 I IAEA. See International Atomic Energy Agency (IAEA) IBS. See Image-based systems (IBS) Ice-templating, 540 ICP. See Inductively coupled plasma (ICP) IEA. See International Energy Agency (IEA) IGU. See Insulating glass units (IGU) Ilmenite, 416 ILSS. See Interlaminar shear stress (ILSS) Image compression, 65 Image enhancement, 65 66 Image resolution enhancement, 68 Image-based systems (IBS), 99 Imaginary fiber technique, 990 992 Impact resistance, 447 448 Imperfect ditch method. See “Imperfect trench” method “Imperfect trench” method, 144 145, 147f Impregnation, 339 In situ polymerization, 693 In-plane randomly orientated composites. See Two-dimensional composites (2D composites) Inception-v3, 88 Incident electron beam, 232, 233f Incorporated wood fibers, 627 Individualization, 361 362 Induced trench method. See “Imperfect trench” method Inductively coupled plasma (ICP), 418 Industrial activities, 155 Industrial floors, 21 Industrial waste materials as aggregate in clay ceramics, 158 164 artificial gypsum, 159 160 ashes, 158 159 glass waste, 162 163 metal slags and metallurgy waste, 160

1062

Industrial waste materials as aggregate in clay ceramics (Continued) organic waste, 163 164 ornamental rock waste, 162 sludge, 161 Industrialization, 897 Initial hydration, 616 Initial rate of absorption (IRA), 201 Injection bolts, 717, 718f, 719f Injection materials, 718 computational homogenization, 721 722 experiments, 722 733, 724f confined specimens, 723f, 728 729 material tests, 722 unconfined specimens, 722 727, 723f numerical simulation of resin, 733 734 confined resin simulation, 734 unconfined resin simulation, 733 numerical simulation of steel-reinforced resin, 734 741 results, 729 733 “Ink”, 540 InnoRenew CoE building, 348, 349f Innovative cement-based materials, 617 636 blended cements, 618 cement hydration, 615 616, 616t in arbitrary phases, 616 617 chemically innovative engineered cement substitutes, 619 concrete based on innovative cementbased materials, 635 636 containers for radioactive waste based on, 630 635 fly ash, 618 marble waste as filler material, 618 ordinary Portland cement in environment protection, 619 620 Portland cement, 615, 615t stone crusher waste as fine aggregates, 618 Innovative green construction method, 899 Insulating glass units (IGU), 344 Insulation of EPS geofoam, 130 Intelligent reinforced concrete (IRC), 404 Intensification of EPA, 93 96 Interface friction angle, 124 Interfacial transition zone (ITZ), 649, 684, 783

Index

Interlaminar shear stress (ILSS), 224 225 test, 227 228 Internal shear strength of EPS geofoam, 123 124 Internal stability, 135 International Atomic Energy Agency (IAEA), 620 International Energy Agency (IEA), 395 International roughness index (IRI), 100 Internet of Things (IoT), 350 Interval type 2 fuzzy system (IT2FS), 91, 92f Intrinsic stimuli of fac¸ade adaptability, 367 368 Iodine, 621 IoT. See Internet of Things (IoT) IRA. See Initial rate of absorption (IRA) IRC. See Intelligent reinforced concrete (IRC) IRI. See International roughness index (IRI) Iron, 460 Iron oxide (Fe2O3), 159, 513 nanoparticles, 419 Isogeometric technique, 377 IT2FS. See Interval type 2 fuzzy system (IT2FS) ITZ. See Interfacial transition zone (ITZ) J Joints and connections, 717 K Kaolin, 179, 185 186 Kaolinite. See Nanokaolin Kaolinite-enriched rocks, 417 Kernite, 160 KHA. See Krill herd algorithm (KHA) Kilns, 552 Kinetic energy of plate, 382 Knit-line. See Weld-line Komagataeibacter xylinus, 585 Krill herd algorithm (KHA), 83 L l/b. See Liquid to binder (l/b) L/D ratio effect, 760 765 Lactose mother liquor (LML), 54 Lagrangian equations, 376 Laminated composite plates

Index

dynamic and transient analyses, 384 385 formulation, 377 384 linear static analysis of cross-ply laminated plates, 384 nonlinear vibration analysis of composite plates, 385 392 piezoelectric, 378f Laminated FRP composites, 222 223 Laminated-veneer lumber (LVL), 334, 341 342 Laminates, 24 25 Laplacian pyramid (LP), 73 74 Large consolidation apparatus (LCA), 749 750, 758 LAS test. See Linear amplitude sweep test (LAS test) LB. See Luria Bertani (LB) LBD. See Light-burnt dolomite (LBD) LC-AS. See Lithium carbonate-aluminum sulfate (LC-AS) LCA. See Large consolidation apparatus (LCA); Life cycle assessment (LCA) LCC. See Load-carrying capacity (LCC) LD slag. See Linz-Donawitz slag (LD slag) LDS. See Load distribution slab (LDS) Leaching, 339 Lead, 680 Lead-zirconate-titanate (PZT), 375 Leadership in Energy and Environmental Design (LEED), 364 Learning network weights, 85 86 LECA. See Lightweight expanded clay aggregate (LECA) LEED. See Leadership in Energy and Environmental Design (LEED) LEFM. See Linear elastic fracture mechanics (LEFM) LFA. See Lime FA (LFA) LFM. See Low-fidelity model (LFM) Life cycle analysis of self-healing concrete, 399 Life cycle assessment (LCA), 170 171, 343 344, 399 Light-burnt dolomite (LBD), 471 472 Lightweight aggregates (LWAs), 172 Lightweight cement-based materials, 565 extenders, 580 585 low-density materials, 581 583 water extenders, 583 585

1063

lightweight/high-strength aggregates, 575 580 HGMS as insulation agents, 575 577, 576t HGMS for oil well cements, 577 580, 579t lightweight/low- strength aggregates, 566 574 hardened state properties, 571 574 production, 566 571 Lightweight concrete (LWC), 25 40 foam concrete, 26 31 lightweight aggregate concrete, 31 32 low-density concretes and associated aggregates, 32 no-fines concrete, 31 TRC, 33 40 types, 26 Lightweight embankments, 144 Lightweight expanded clay aggregate (LECA), 554 Lightweight materials, 565 Lignin, 585 Lime, 613, 615 Lime FA (LFA), 474 Limestone (LS), 162, 398, 613 Linear amplitude sweep test (LAS test), 706 Linear Drucker Prager model, 733 material parameters, 734t plastic criterion, 721 Linear drying shrinkage (LS), 186 187 Linear elastic fracture mechanics (LEFM), 973 Linear static analysis of cross-ply laminated plates, 384 Linear variable differential transformer. See also Linear variable displacement transformers (LVDTs) Linear variable displacement transformers (LVDTs), 722, 851 852 Linear variation model, 292 Linz-Donawitz slag (LD slag), 460 Lion optimization algorithm (LOA), 83 Liquid crystalline polymer, 946 947 Liquid nitrogen (LN2), 243 Liquid sodium silicate (LSS), 464 Liquid to binder (l/b), 14 Lithium carbonate-aluminum sulfate (LCAS), 672

1064

Living Building Challenge certification program, 364 LMF. See Lower membership function (LMF) LML. See Lactose mother liquor (LML) LOA. See Lion optimization algorithm (LOA) Load distribution slab (LDS), 137 Load-carrying capacity (LCC), 989 990 Local receptive field, 86 Loss function, 86 Lotus effect, 52, 340 Low heat cement, 5 Low-density concretes and associated aggregates, 32 moderate-strength lightweight concrete and associated aggregates, 32 structural lightweight concrete and associated aggregates, 32 materials, 581 583 Low-fidelity model (LFM), 721 Lower membership function (LMF), 91 LP. See Laplacian pyramid (LP) LS. See Limestone (LS); Linear drying shrinkage (LS) LSS. See Liquid sodium silicate (LSS) LUMO, 663 Luria Bertani (LB), 59 LVDTs. See Linear variable displacement transformers (LVDTs) LVL. See Laminated-veneer lumber (LVL) LWAs. See Lightweight aggregates (LWAs) LWC. See Lightweight concrete (LWC) M M&R. See Maintenance and rehabilitation (M&R) MA. See Maleic anhydride (MA) MAAS. See MgO-modified AAS (MAAS) Macro Defect Free cement (MDF cement), 41 Macrocrack, 304 Macrodefect free (MDF), 296 297 Macrofiber composites (MFCs), 949 effective thermo-electro-elastic properties of, 984 Macrofibers of UHPFRC, 302 304 Macroscale Cauchy stress, 721 722 Macroscopic-buckling, 1031

Index

Magnesia (MgO), 5 6, 463, 513, 615, 877 Magnesium carbonate (MgCO3), 619 Magnesium oxide. See Magnesia (MgO) Magnetorestrictive materials, 375 Maintenance and rehabilitation (M&R), 79, 97, 102 Maleic anhydride (MA), 980 Manchester railway embankment, 133 134 Manufactured sand, 8 Marble, 162 Marble powder (MPW), 193 Marble waste, 614 as filler material, 618 Marine aerosols, 657 658 MAs. See Mineral admixtures (MAs) Masonry cement, 5 MATEST compression strength machine, 505 Max-pooling, 86, 87f Maximum dry density (MDD), 750 Maximum stress, 1019 criterion, 1019 1021, 1022f MC2010 fib, 289 MDD. See Maximum dry density (MDD) MDF. See Macrodefect free (MDF) MDF cement. See Macro Defect Free cement (MDF cement) Mechanical analysis, 947 Mechanical testing, 255 256 Membership function (MF), 81 82 Mercury intrusion porosimetry, 545 Meshing model, 952, 953f Metakaolin (MK), 417, 529, 878 Metal reactivity reduction, 665 Metal slags, 160 Metalized plastic films, 438 439 Metallurgy waste, 160 in clay-based ceramic applications, 175 180, 176t bricks, 175 179 clay-expanded aggregates, 180 porcelain stoneware tiles, 180 stoneware tiles, 179 Methyl methacrylate (MMA), 400 401 Methyl stearate, 693 694, 706, 712 MF. See Membership function (MF); Microcellular rubber foams (MF) MF short fibers (MFPS), 975 preparation, 978 980

Index

relative compressive modulus, 979f stress strain curves, 977 MF untreated short fibers (MFUS), 975 preparation, 978 980 relative compressive modulus, 979f stress strain curves, 977 MFCs. See Macrofiber composites (MFCs) MFPS. See MF short fibers (MFPS) MFSDT. See Modified FSDT (MFSDT) MFUS. See MF untreated short fibers (MFUS) MgO-modified AAS (MAAS), 467 Microbes for self-healing, 801 802 Microbiology, techniques used in, 58 59 growing strain in alkalophilic conditions, 58 59 Microcellular rubber foams (MF), 975 preparation, 978 980 stress strain curves, 977 Microcomputed tomography (μCT), 541 characterization, 544 545 Microcrack, 304, 436 437, 494 Microcracking, 36 Microencapsulation, 401 402 Microfibers, 18 of UHPFRC, 302 304, 302f Micromechanics analysis, 971 Micropores, 494 Microscale displacement, 721 722 Microscopic repair analysis, 811 814 Microsilica. See Silica fume (SF) Mid-infrared region, 230 Migrating organic-based corrosion inhibitors, 661 662 Milling techniques, 410 Mineral admixtures (MAs), 11 12 Mineral slags in clay-based ceramic applications, 175 180, 176t bricks, 175 179 clay-expanded aggregates, 180 porcelain stoneware tiles, 180 stoneware tiles, 179 Mining, 160 Mining mud, 161 Mixed boundary conditions, 721 722 Mixed corrosion inhibitors, 661 662 Mixed design method of porous concrete pavement, 499 Mixing concrete, 830

1065

Mjo¨sa Tower, 342, 343f MK. See Metakaolin (MK) MLGs. See Multilayer graphenes (MLGs) MLP. See Multilayer perceptron network (MLP) MMA. See Methyl methacrylate (MMA) Mn-54 radionuclide, 624 625 Moderate-strength lightweight concrete and associated aggregates, 32 Modified FSDT (MFSDT), 377 Modified random sequential adsorption algorithm, 966 Modified wood, 337 340 Modular design, 352 353 Modulus of rupture (MOR), 164 Mohr Coulomb formulation, 123 124 Moisture contents of aggregates, 829t ingression, 249 Molecular dynamics, 663 Molecular nanotechnology, 410 411 Molecular-level processing, 410 411 Monte Carlo modeling, 636, 967 971 Montmorillonite, 755 756 MOR. See Modulus of rupture (MOR) Mori Tanaka models (MT models), 967 971 Mortar composite, 626 627 MPW. See Marble powder (MPW) MRA. See Multiresolution analysis (MRA) Ms. See Activator modulus (Ms) MSCR test. See Multiple stress creep recovery test (MSCR test) MSFRC. See Multiscale FRC (MSFRC) MSW. See Municipal solid waste (MSW) MSWBA. See Municipal solid waste incineration bottom ash (MSWBA) MSWFA. See Municipal solid waste FA (MSWFA) MT models. See Mori Tanaka models (MT models) Multibarrier container and dimensions, 636, 637f disposal concept, 635, 635f system, 622, 623f Multibolt FRP joints, 246 Multilayer graphenes (MLGs), 783 Multilayer perceptron network (MLP), 81

1066

Multiple stress creep recovery test (MSCR test), 703 705 Multiresolution analysis (MRA), 72 Multiscale FRC (MSFRC), 297 Multiwalled carbon nanotubes (MWCNTs), 239 240, 415, 779, 784 785, 986 989 Municipal solid waste (MSW), 158 159, 163 164 Municipal solid waste FA (MSWFA), 168 169 Municipal solid waste incineration bottom ash (MSWBA), 168 169 Mushroom (Pleurotus pulmonarius), 620 MWCNTs. See Multiwalled carbon nanotubes (MWCNTs) N NA. See Nutrient agar (NA) NAC. See Coarse natural aggregate (NAC) NAF. See Fine natural aggregate (NAF) Nano-boron nitride (Nano-BN), 778 cement-based composites with, 788 Nano-BRHA, 494 495 morphologies, 514f particle size of, 512 513 Nano-carbon black (NCB), 778 cement-based composites with NCB composite fillers, 788 789 Nano-graphite platelets (NGPs), 778 779 Nano-TiO2, 777 778, 780, 783 cement-based composites with, 786 787 Nano-ZrO2, 780, 783 cement-based composites with, 787 788 Nanoalumina, 412, 414 Nanocement-based composites, 784 789 with carbon nanotube/nanocarbon black composite fillers, 788 789 with CNTs, 784 785 with graphene, 785 786 with nano-boron nitride, 788 with nano-SiO2, 786 with nano-TiO2, 786 787 with nano-ZrO2, 787 788 Nanoclay, 418 419 Nanocomposites, 986 987 Nanoconcrete production, 411f, 412 Nanofillers, 777 778

Index

toughening mechanisms through implications of, 238 241 Nanoiron, 419 Nanokaolin, 417 418 Nanomaterial-based self-healing concrete economy, 420 421 Nanomaterials, 395 396, 410 411, 494 495, 498, 777 778 nanoconcrete production, 412 nanomaterial-based concrete, 411 environmental suitability and safety features of, 421 422 nanomaterial-based self-healing concrete, 413 419 CNTs, 415 economy of, 420 421 nanoalumina, 414 nanoclay, 418 419 nanoiron, 419 nanokaolin, 417 418 nanosilica, 413 414 nanosilver, 419 PCE, 415 416 sustainability of, 419 420 titanium oxide, 416 417 production, 410 411 significance as self-healer, 412 413 Nanometakaolin, 417 418 Nanonucleation effect, 781 Nanoparticles (NPs), 239, 410 412, 777 778 Nanopolycarboxylates, 412 Nanosilica (Nano-SiO2), 412 414, 494 495, 574, 777 778, 780, 783 cement-based composites with, 786 nano-SiO2-coated TiO2, 787 porous concrete pavement containing, 498 Nanosilver, 419 Nanotechnology, 419 420, 777 778 NaOH. See Sodium hydroxide (SH) N A S H gel. See Sodium alumino silicate hydrate gel (N A S H gel) Natural cellulose macrofibers, 585 Natural cracks, 832, 839 847 self-healing abilities of specimens with, 851 Natural fibers, 20, 282 284, 283f, 284t, 956 957

Index

Natural frequencies, 828, 850f, 852 853 Natural materials, 984 986 Natural river sand, 8 Natural stone waste (NSPW), 189 193 Natural wastes, 614 Natural wood, 335 337, 336f fac¸ade with timber fac¸ade cladding elements, 337f plywood and wood shingles implemented on buildings, 336f Naturally Occurring Radioactive Materials (NORM), 185 186 NCB. See Nano-carbon black (NCB) NCD. See Non-cracking distress (NCD) Newmark time-stepping technique, 376 Newton-Raphson iteration technique, 376 NGPs. See Nano-graphite platelets (NGPs) Nickel, 419, 782 Nitinol, 404 Nitrite-based inhibitors, 663 No-fines concrete. See Porous concrete Non-cracking distress (NCD), 100 Non-linear activation function, 85 Nonionic surfactants, 539 Nonlinear vibration analysis of composite plates, 385 392 control voltage effect on frequency ratio, 389 of angle-ply laminates embedded with piezoelectric layer, 391 392 temperature rise influence on frequency ratio, 387 389 of angle-ply laminates embedded with piezoelectric layer, 390 Nonmetallic fibers, fiber-reinforced concrete with, 20 Nonrecoverable creep compliance (Jnr), 703 705 Nonrenewable natural resources, 459 Nonrenewable raw materials, 435 Nonsteady-state migration (NSSM), 473 Nonyielding walls, 140 NORM. See Naturally Occurring Radioactive Materials (NORM) Normal strength concrete (NSC), 1 2 NPs. See Nanoparticles (NPs) NRC. See Nuclear Regulatory Commission (NRC) NSC. See Normal strength concrete (NSC)

1067

NSPW. See Natural stone waste (NSPW) NSSM. See Nonsteady-state migration (NSSM) Nuclear Regulatory Commission (NRC), 629 Numerical homogenization method, 718 721 Numerical methods of SFCs, 954 973 Nutrient agar (NA), 58 Nyctinasty, 357 Nylon, 946 947 O Off-axis elastic constant matrix, 381 Oil sludge, 161 Oil-well cement, 5 Olive pomace ash (OP ash), 164 168 OMA. See Operational modal analysis (OMA) OMC. See Optimum moisture content (OMC) OP. See Open porosity (OP) OP ash. See Olive pomace ash (OP ash) Opaque coatings, 339 OPC. See Ordinary Portland cement (OPC) OPC-based concrete (OPCC), 476 Open porosity (OP), 545 determination, 541 542 Operational modal analysis (OMA), 986 987 Optical fibers, 375 Optical microscopy characterization, 542 Optimum moisture content (OMC), 750 Ordinary Portland cement (OPC), 2, 395, 421 422, 471 472, 499 500, 615, 643, 671, 878 chemical composition, 514t and physical properties, 645t in environment protection, 619 620 morphologies, 514f OPC-based mixtures, 624 particle size of, 512 513 Organic waste, 163 164 in clay-based ceramic applications, 203 208, 204t bricks, 203 207 clay-expanded aggregates, 207 208 porcelain stoneware tiles, 207 stoneware tiles, 207 Ornamental rock waste, 162

1068

Ornamental rock waste (Continued) in clay-based ceramic applications, 189 195, 190t bricks, 189 193 clay-expanded aggregates, 195 porcelain stoneware tiles, 194 195 stoneware tiles, 193 194 Osmotic pressure, 614 Our-point bending flexural test, 936, 937f Oxalis, 357 Oxidation, 614 Oxide composition of cement, 7 8 calculation of compound composition of cement, 5 6 Oxides of calcium (CaO), 463 P PAI. See Phase angle aging index (PAI) Palm oil fuel ash (POFA), 437, 494 Paper mill sludge, 163 Paper mill sludge compost (PMSC), 206 Paper mill waste (PMS), 203 Parabolic function, 228 Parametric design, 351 352, 352f Partial electrochemical reactions, participation of inhibitor in, 665 Particle morphology, 513 Particle size distribution, 679 680 Particle swarm optimization (PSO), 83 Passive film, 660 661 Paste-like matrix, 799 800 PAV test. See Pressure aging vessel test (PAV test) Pavement composition considerations of EPS geofoam, 137 Pavement condition index (PCI), 100 Pavement conditions, 79 Pavement management system (PMS), 79 application of CI frameworks in, 97 102 condition assessment, 99 100, 99f condition prediction, 100 102 inventory definition, 97 99 M&R operations analysis, 102 subsystems, 98f Pavement surface characteristics, CI methods, 66 75 Contourlet transform, 73 75, 74t, 75t Curvelet transform, 70 71, 71t pavement texture evaluation system, 67f

Index

Ridgelet transform, 69 70, 70t Shearlet transform, 72 73, 73t wavelet transforms, 67 68, 68t Pavement system design, 135 Paver blocks, 897, 902t abrasion and durability aspects, 899 manufacturing, 901 902 PBO. See Polypara-phenylene benzobisoxazole (PBO) PC. See Portland cement (PC) PCEs. See Polycarboxylate ethers (PCEs) PCFA. See Powder coal fly ash (PCFA) PCI. See Pavement condition index (PCI) PCL. See Poly(ε-caprolactone) thermoplastic (PCL) PCL-g-MA compatibilizer. See Poly (ε-caprolactone)-g-MA copolymer compatibilizer (PCL-g-MA compatibilizer) PCMs. See Phase change materials (PCMs) PE. See Polyester (PE) Pelletized aggregates, 32 Penetration, 700 Percent swell (PS), 752 753, 760 761 Percent swell reduction factor (PSr), 752 753 Percentage of coarse rubber (RBC), 862 863, 863f Percentage of fine rubber (RBF), 862, 863f Periodic boundary conditions, 721 722 Perlite, 32 Permafrost regions of EPS geofoam, 130 Permeability, 614 coefficient, 505 508 test of concrete, 505 508 Permeable concrete. See Porous concrete Pervious concrete. See Porous concrete PET. See Polyethylene terephthalate (PET) Petroleum waste sludge, 161 PFA. See Pulverized fuel ash (PFA) PG. See Phosphogypsum (PG) PGFRP profiles. See Pultruded GFRP profiles (PGFRP profiles) pH-sensitive polymers, 375 Phase angle aging index (PAI), 708 Phase change materials (PCMs), 781 782 Phase imaging, 230 231 Phosphogypsum (PG), 160 Photooxidation aging, 707 708

Index

Physical barrier formation, 664 665 Piezoelectric materials, 375 nonlinear vibration analysis of composite plates, 385 392 Piezoresistivity, 934 PITCH method, 282 Plain and ribbed mild steel bars, 10 Plain concrete, 951 952 Plastics, 435, 437 438 Platinum, 659 Pleurotus pulmonarius. See Mushroom (Pleurotus pulmonarius) Plywood, 341 342 PM. See Point method (PM) PMCs. See Polymeric matrix composites (PMCs) PMH1 Improvement Project. See Port Mann/ Highway 1 Improvement Project (PMH1 Improvement Project) PMMA. See Polymethyl methacrylate (PMMA) PMS. See Paper mill waste (PMS); Pavement management system (PMS) PMSC. See Paper mill sludge compost (PMSC) PMSs. See Polymelamine sulfonates (PMSs) PNS. See Polynaphthalene sulfonates (PNS) POFA. See Palm oil fuel ash (POFA) Point method (PM), 990 Poisson noise, 65 Poisson’s ratio of EPS geofoam, 121 122 Poly(ε-caprolactone) thermoplastic (PCL), 980 Poly(ε-caprolactone)-g-MA copolymer compatibilizer (PCL-g-MA compatibilizer), 980 Polycarboxylate ethers (PCEs), 45 46, 301 302, 315 317, 317f, 415 416 Polycarboxylates, 321 Polycarboxylic ethers. See Polycarboxylate ethers (PCEs) Polyester (PE), 632 Polyethylene terephthalate (PET), 404 405, 438 439 Polymelamine sulfonates (PMSs), 315 316, 316f Polymeric fiber, 279, 281, 281f, 282t, 303, 437

1069

Polymeric matrix composites (PMCs), 222 223, 919 Polymeric-based plastics, 437 438 Polymers, 222, 438 coatings, 405 406 polymer-based corrosion inhibitors, 663 polymer-based inhibitors, 661 662 polymer cement composite, 627 629 Polymethyl methacrylate (PMMA), 632, 801 802 Polynaphthalene sulfonates (PNS), 315, 316f Polypara-phenylene benzobisoxazole (PBO), 33 Polyparaphenylene terephthalamide fibers (PPTA fibers), 947 948 Polypropylene (PP), 436, 920 922 Polystyrene (PS), 566 Polyurethane (PUR), 257 258 Polyvinyl alcohol (PVA), 625 fibers, 303 Pooling layers, 86 Porcelain stoneware tiles, 187 188 artificial gypsum in clay-based ceramic applications, 175 ashes in clay-based ceramic applications, 171 glass waste in clay-based ceramic applications, 202 203 mineral slags and metallurgy waste in clay-based ceramic applications, 180 organic waste in clay-based ceramic applications, 207 ornamental rock waste in clay-based ceramic applications, 194 195 sludge in clay-based ceramic applications, 187 188 Porous alkali-activated materials, 529 540 alkali activation conditions affecting properties of, 531 535 alkali dosage, 535 aluminosilicate precursor types, 531 534 effects of setting time, 535 foaming agent types, 534 535 aluminosilicate precursors, 530f characterization of porosity in alkaliactivated materials, 541 546 μCT characterization, 544 545 mercury intrusion porosimetry, 545

1070

Porous alkali-activated materials (Continued) open and total porosity determination, 541 542 optical microscopy characterization, 542 SEM characterization, 542 544 ultrasonic pulse velocity measurement, 545 546 functional properties and applications application in water and wastewater treatment, 552 554 fire resistance, 552 sound absorption, 550 552 thermal conductivity, 549 550 production methods, 535 540 direct foaming, 535 540 sacrificial filler and replica method, 540 3D printing, 540 properties, 546 549, 547t durability properties, 549 foam stability, 546 548 mechanical properties, 548 549 synthesis conditions of, 532t Porous concrete, 496, 529 Porous concrete pavement, 493 chemical composition, 513 514 compressive strength, 517 520 compressive strength activity index, 520 relationship between compressive strength and curing age, 519 relationship between compressive strength and density, 518 519 concrete mix design and mix proportions, 516 containing nanosilica, 498 description of problem, 494 495 experimental plan compaction process, 503 505 compressive strength test, 505 concrete mix design and proportion, 502 503 curing condition, 505 FE-SEM, 511 512 grinding procedure, 500 501 permeability test, 505 508 sound absorption test, 508 509 TEM, 511

Index

workability, 503 XRD test, 509 510 XRF, 509 literature review, 496 499 BRHA, 497 498 mixed design method, 499 nanomaterials, 498 pavement structure, 496f problems regarding porous concrete structure, 497 materials BRHA, 500 coarse aggregates, 500 OPC, 499 500 water, 500 mineralogical and phase identification, 515 particle morphology, 513 particle size of OPC and nano-BRHA, 512 513 permeability, 521 522 relationship between permeability and compressive strength, 521 522 significance of research, 495 sound absorption, 522 523 workability, 516 517 Port Mann/Highway 1 Improvement Project (PMH1 Improvement Project), 132 133 Portland cement (PC), 4, 435 436, 459, 529, 591, 613, 615, 615t, 898 alternative for, 592 593 Portland Pozzolana cement (PPC), 682 683 Portland slag cement (PSC), 4, 900 Positive arching, 145 Post and beam cover systems, 144 Postconsumer materials, 617 Potassium hydroxide, 461 Potassium oxide (K2O), 5 6, 513 Potential measurement, 666 667 Powder coal fly ash (PCFA), 596 Powdered X-ray diffraction (PXRD), 418 Pozzolanic admixtures, 573 Pozzolanic ash, 681 Pozzolanic cement production, 897 899 Pozzolanic hydration process, 444 Pozzolanic materials, 582 Pozzolanic mineral admixtures, 573 PP. See Polypropylene (PP)

Index

PPC. See Portland Pozzolana cement (PPC) PPTA fibers. See Polyparaphenylene terephthalamide fibers (PPTA fibers) Prairie climate, 357 Pre-puffs, 118 Pre-trained models, 88 Precast products, 20 Precisa PrepASH 129 analyzer, 884 Preconsumer materials, 617 Prefabricated crack method, 805 apparent repair analysis, 805 811 crack repair, 808t microscopic repair analysis, 811 814 Prefabrication, 294, 352 353 Present serviceability index (PSI), 100 Pressure aging vessel test (PAV test), 692 Pressurized water reactors (PWRs), 629 630 Prestige, 361 362 Probe, 231 Processed fly ash, 686 Production of granite waste (GPW), 162 Proportioning, 2 3 PS. See Percent swell (PS); Polystyrene (PS) PSC. See Portland slag cement (PSC) Pseudomonas aeruginosa ATCC 27853, 58 PSI. See Present serviceability index (PSI) PSO. See Particle swarm optimization (PSO) Pull-out test, 946 Pultruded GFRP profiles (PGFRP profiles), 233 234 Pultruded glass fiber-reinforced vinyl ester matrix composite, 249 Pulverized fuel ash (PFA), 624, 677 PUR. See Polyurethane (PUR) PVA. See Polyvinyl alcohol (PVA) PWRs. See Pressurized water reactors (PWRs) PXRD. See Powdered X-ray diffraction (PXRD) PZT. See Lead-zirconate-titanate (PZT) Q QA. See Quality assurance (QA) QC. See Quality control (QC) QP. See Quartz powder (QP) Quality assurance (QA), 119 120 Quality control (QC), 119 120 Quantum mechanical analysis, 663

1071

Quarry rock dust, 618 Quartz (SiO2), 175 178 Quartz powder (QP), 34 Quartz sand, 45 Quick setting cement, 4 R RAC. See Recycled aggregate concrete (RAC) Radial Basis Function (RBF), 81 Radical copolymerization, 316 Radioactive waste, 621 Radionuclides, 624 625 Radon transform, 69 Ramberg Osgood relationship, 721, 729, 733t Randomization function, 97 Randomly oriented strand composites, 951 Rapid chloride ion penetration test (RCPT), 669 Rapid chloride permeability (RCP), 51, 473 Rapid hardening cement, 4 Rate of creep method (RCM), 480 RBC. See Percentage of coarse rubber (RBC) RBF. See Percentage of fine rubber (RBF); Radial Basis Function (RBF) RC. See Reinforced concrete (RC) RCAs. See Recycled concrete aggregates (RCAs) RCLDS. See Reinforced concrete load distribution slab (RCLDS) RCM. See Rate of creep method (RCM) RCP. See Rapid chloride permeability (RCP) RCPT. See Rapid chloride ion penetration test (RCPT) RCS. See Strength reduction factor (RCS) Reaction-diffusion model, 671 Reactive powder concretes (RPCs), 40, 297 Rebar corrosion, 657 659 Rebar-reinforced structural elements, behaviour of, 479 480 Recurrent neural network (RNN), 81 Recycled aggregate, 797 800, 802 804 effects of, 817 820 Recycled aggregate concrete (RAC), 797 801 Recycled CFRP and GFRP fibers, 645 analysis, 650 652

1072

Recycled CFRP and GFRP fibers (Continued) experimental plan, 645 647 materials and mix designs, 645 646 proportions of materials used in mix compositions, 646t test procedure, 646 647 results, 647 650 Recycled concrete aggregates (RCAs), 472 components, 804f composition statistics, 803t grading composition, 805t physical properties, 803t Recycling of EPS geofoam, 130 of nonbiodegradable wastes, 435 Red mud (RM), 161 Rediset cement, 5 “Reduce-reuse-recycle” paradigm, 359 REED. See Restorative Environmental and Ergonomic Design (REED) Reference concrete (RFC), 829 830 Reinforced concrete (RC), 223 224, 613 Reinforced concrete load distribution slab (RCLDS), 131 132 Reinforced masonry, 9 Reinforcement steel. See Reinforcing bars (Rebar) Reinforcing bars (Rebar), 9 10 chemical composition of reinforcements, 10 types, 10 Reinforcing steel. See Reinforcing bars (Rebar) Reissner-Mindlin theory, 376 Relative flexibility, 142 Relative humidity (RH), 253 254, 879 Representative volume element (RVE), 958, 960 advantage, 973 with fiber debonding, 958 959 and mesh of composites, 968f micromechanics analysis using, 971 periodic, 965 966, 967f size effect, 960, 965f stress displacement response, 964f Research Institute in Cerdanyola del Valle`s, 368, 368f Resin, numerical simulation of, 733 734

Index

confined resin simulation, 734 unconfined resin simulation, 733 ResNet, 88 Restorative Environmental and Ergonomic Design (REED), 363 Retaining structures, EPS in, 138 144 basic design concepts, 140 144 cases histories and performance, 139 Reversible thermochromic materials, 693 RFC. See Reference concrete (RFC) RH. See Relative humidity (RH) RHA. See Rice husk ash (RHA) RHA-based activators (RHAAs), 597 Rheological parameters, 284 285 Rhyl Pont y Ddraig lifting bridge, 223 224, 224f Rice husk, 495 Rice husk ash (RHA), 164, 301, 494, 597 602 Rice straw ash (RSA), 170 Ridgelet transform, 69 70, 70t RIGAKU Smartlab X-ray Diffractometer, 509 Rigid plastic model, 292 RM. See Red mud (RM) RNN. See Recurrent neural network (RNN) Road embankment, 132 133 Rock wool (RW), 878 Rovings, 24 25 RPCs. See Reactive powder concretes (RPCs) RSA. See Rice straw ash (RSA) Rubber waste, 857 Rubberized concrete, 825 826 Rubberized concrete compressive strength, 857 comparison of existing expressions, 869 872 compressive strength of concrete, 866t database description, 859 863, 863t expressions for compressive strength, 870f, 870f of concrete, 865 869 in literature, 864 of rubberized concrete, 867 869 literature review, 858 859 Runge-Kutta approach, 376 Rutile, 416 Rutting process, 703 705

Index

RVE. See Representative volume element (RVE) RVE-based FE algorithm, 971 RW. See Rock wool (RW) S S/N ratio. See Signal to noise ratio (S/N ratio) S/S process. See Solidification/stabilization process (S/S process) SA. See Soil alone (SA) Sacrificial filler and replica method, 540 Saline clay (SC), 188 Salt and pepper noise, 65 Sand, 8, 901 902 Sand replacement with fly ash, test results for, 906 910 compressive strength, 906 908 flexural strength, 909 910 freeze thaw durability, 910 water absorption, 910 SANS. See Small-angle neutron scattering (SANS) SAP. See Superabsorbent polymer (SAP) SBA. See Sugarcane bagasse ash (SBA) SBFRPCB. See Short bamboo fiberreinforced polyester composite beams (SBFRPCB) SBS test. See Short beam shear test (SBS test) SC. See Saline clay (SC); Sodium carbonate (SC) SC models. See Self-consistent models (SC models) Scanning electron microscopy (SEM), 228 229, 232, 468 469, 542 544, 626 627, 650, 668, 786, 834, 852, 947 948 characterization, 542 544 SCBA. See Sugarcane bagasse ash (SBA) SCC. See Self-compacting concrete (SCC) SCMs. See Supplementary cementing materials (SCMs) Scoria aggregate, 32 SDR. See Span-to-depth ratio (SDR) Sea sand, 8 Sealing and support matrix (SSM), 635 636 Seawater and river sand ASC (SRASC), 468 469

1073

Seawater and sea sand ASC (SSASC), 468 469 Segregation test, 285, 286f Seismic loading of EPS geofoam, 136 Self-adjusting cement-based composites, 781 782 Self-cleaning effect, 52 Self-compacting concrete (SCC), 275 276, 276f, 416, 644 Self-consistent models (SC models), 967 971 Self-damping cement-based composites, 783 Self-healer, nanomaterials significance as, 412 413 Self-healing cement (SHC), 581 582 SHC-based composites, 780 781, 781t Self-healing concrete, 825 827 advantages and disadvantages of nanomaterials for, 420 assessment, 827 828 Bacillus bacillus, 802 comparison of various self-healing concrete strategies, 408t compressive strength, 837, 837f energy spectrum analysis, 818f flexural strength, 838, 838f life cycle analysis of, 399 making cracks, 851 852 materials cement, 828 crumb rubber, 829 fiber, 829 fine aggregate and coarse aggregate, 828 829 water, 828 mechanical properties, 851 methods casting concrete, 830 831 concrete design, 832t concrete tests, 832 835 design of concrete, 829 830 making cracks, 832 mixing concrete, 830 moisture contents of aggregates, 829t natural frequencies, 850f, 852 853 NC section element components, 819t prefabricated crack method, 805 preparation and maintenance method, 805 RC section element components, 817t

1074

Self-healing concrete (Continued) recycled aggregate, 802 804 effect, 817 820 self-healing abilities of specimens with natural cracks, 851 with standardized cracks, 851 self-healing characteristics apparent repair analysis, 805 811 crack repair, 808t microscopic repair analysis, 811 814 self-healing evaluation, 839 847, 841f natural cracks, 839 847 self-healing increments calculations, 844t standardized cracks, 839 self-healing mechanism in cementitious materials, 399 409 bacteria as self-healing agent, 403 404 coating, 405 406 expansive agents and mineral admixtures, 402 403 hollow fibers, 399 401 microencapsulation, 401 402 shape memory materials as self-healer, 404 405 self-repairing weakening principle, 814 817 slump tests, 835 836 splitting tensile strength, 837, 838f sustainability of smart concrete, 398 test methods, 852 Self-healing evaluation, 839 847, 841f natural cracks, 839 847 self-healing increments calculations, 844t standardized cracks, 839 Self-healing technology, 395 Self-heating cement-based composites, 782 Self-organizing neural network (SONN), 81 Self-organizing strategy, 83 Self-repairing weakening principle, 814 817 Self-restoration process, 404 405 Self-sensing cement-based composites, 778 780 SEM. See Scanning electron microscopy (SEM) SEM-EDS electron microscopy, 811 Semi-supervised learning, 81 Semianalytical formulation, 933

Index

SENB. See Single-edge notched bend (SENB) SERC. See Structural Engineering Research Centre (SERC) Serviceability limit state (SLS), 289 Settlement of EPS geofoam, 137 Sewage sludge (SWS), 180 SF. See Silica fume (SF) SFA. See Silica fume-based activators (SFA) SFCs. See Short-fiber composites (SFCs) SFECs. See Short-fiber-reinforced elastomer composites (SFECs) SFRC. See Steel fiber-reinforced concrete (SFRC) SFRPs. See Short-fiber-reinforced polymers (SFRPs) SFRS. See Steel fiber reinforced shotcrete (SFRS) SFTCs. See Short-fiber thermoset composites (SFTCs) SG-B. See Sodium gluconate-borax (SG-B) SGF. See Short glass fiber (SGF) SGF-reinforced polyamide 6 (SGFR-PA6), 989 990 stress strain curves, 991f SGFR-PA6. See SGF-reinforced polyamide 6 (SGFR-PA6) SH. See Sodium hydroxide (SH) Shape memory alloy (SMA), 375, 404, 820 Shape memory materials as self-healer, 404 405 Shape memory polymers (SMPs), 404 405 SHC. See Self-healing cement (SHC) Shear strength of EPS geofoam, 123 124 Shearlet transform, 72 73, 73t Sheet molding compounds (SMCs), 922, 930f, 973 Shewanella species, 55 Short bamboo fiber-reinforced polyester composite beams (SBFRPCB), 952 Short beam shear test (SBS test), 227 228 Short glass fiber (SGF), 923 Short-cut super-fine stainless wire (SSSW), 779 Short-fiber composites (SFCs), 919 advantages, 919 920 analytical methods, 933 954 behavior under loading unloading cycles, 930f

Index

constitutive and fundamental researches, 990 992 critical fiber length and average fiber stress, 925 926 experimental methods, 973 990 fiber orientation, 922 924, 928 changes in, 923f flow patterns in injection moldings, 924f numerical methods, 954 973 research works, 933 size of fibers, 920 922 lengths of fibers, 921f variation in fiber length, 921f solved problems, 992 1035 stiffness and strength, 926 928 stress and strain fields at embedded fibers in matrix, 924 925 Short-fiber thermoset composites (SFTCs), 929 932 progression of failure modes, 931f Short-fiber-reinforced composite, 951 Short-fiber-reinforced elastomer composites (SFECs), 989 Short-fiber-reinforced polymers (SFRPs), 933 934, 967 971 Short-term thermal oxidation aging, 707 708 SI. See Sulfoxide index (SI); Swarm intelligence (SI) SIFCON. See Slurry infiltrated fibrous concrete (SIFCON) Signal to noise ratio (S/N ratio), 206 Sika AER5, 538 539 Sika Lightcrete 02, 538 539 Silica. See Silicon dioxide (SiO2) Silica fume (SF), 1 2, 45, 285, 299, 300f, 469, 531, 582 583, 597, 878 silica yield, 602f Silica fume-based activators (SFA), 597 Silicic acid, 680 Silicon carbide (SiC), 537 Silicon dioxide (SiO2), 5 6, 159, 414, 463, 513, 605, 615, 680, 877 composition in SCBA, 606f silica-fume admixed AASC, 474 silica-rich industrial by-products, 537 Silver, 419, 659 Silver nanoparticles (Ag NPs), 419

1075

Single-edge notched bend (SENB), 989 990, 992f Single-walled carbon NTs (SWCNTs), 415 Sintered fly ash lightweight aggregates, 683 685 Sintering, 685 Six-dimensional representation (6D representation), 350 Skeleton raw materials, 157 Slag, 299 300, 301f, 529 Slate, 162 Slot-trench cover systems, 144 SLS. See Serviceability limit state (SLS) Sludge, 161 in clay-based ceramic applications, 180 189, 181t bricks, 180 185 clay-expanded aggregates, 188 189 porcelain stoneware tiles, 187 188 stoneware tiles, 185 187 Slump flow test, 304 305, 305f, 439 441, 646, 833f, 835 836 Slurry infiltrated fibrous concrete (SIFCON), 22 23 Slurry mortar composite, 626 SM. See Smectite (SM) SMA. See Shape memory alloy (SMA) Small-angle neutron scattering (SANS), 671 Smart and multifunctional cement-based composites, 778 784 electromagnetic wave-shielding/absorbing cement-based composites, 784 self-adjusting cement-based composites, 781 782 self-damping cement-based composites, 783 self-healing cement-based composites, 780 782, 781t self-sensing cement-based composites, 778 780 wear-resisting cement-based composites, 783 Smart concretes, 395, 777 sustainability, 398 Smart materials, 375 Smart self-healing concrete, 421 Smart structures, 375 Smart/multifunctional cement-based composites, 777

1076

SMCs. See Sheet molding compounds (SMCs) Smectite (SM), 179 SMPs. See Shape memory polymers (SMPs) Soapstone, 877 Sodium alumino silicate hydrate gel (N A S H gel), 466, 468 Sodium benzoate, 661 662 Sodium carbonate (SC), 464 Sodium gluconate-borax (SG-B), 672 Sodium hydroxide (SH), 461, 472, 535 Sodium hypochlorite, 538 Sodium nitrate, 661 662 Sodium nitrite, 661 662 Sodium oxide (Na2O), 467, 513 Sodium perborate, 538 Sodium silicate, 535, 594 596, 600 Softening point, 700 Softmax classifier, 87 Soil alone (SA), 751 Soil arching process, 145 Solar radiation, 358 Solid waste materials, 435 Solidification, 923 Solidification/stabilization process (S/S process), 617 618, 624 Solution process, 382 384 Solvent risk of EPS geofoam, 129 130 SONN. See Self-organizing neural network (SONN) Sorptivity, 448 451 Sound absorption of porous AAM, 550 552 of porous concrete, 522 523 test, 508 509 impedance tube apparatus, 509f Sound transmission loss (STL), 548 SP. See Swelling pressure (SP) Span-to-depth ratio (SDR), 227 228 Spatial domain methods, 66 Specific adsorption, 664 Speckle noise, 65 Spectrophotometry, 695 696 Spectrum, 230 Spiral movement in N-dimensional space, 95 Spiral-like functions, 97 Splitting tensile strength, 826, 1031, 1032f for self-healing concrete, 837, 838f Sporosarcina pasteurii, 53, 55

Index

SqueezNet, 88 SRASC. See Seawater and river sand ASC (SRASC) SS. See Steel-making slag (SS) SSASC. See Seawater and sea sand ASC (SSASC) SSM. See Sealing and support matrix (SSM) SSSW. See Short-cut super-fine stainless wire (SSSW) Stabilization mechanisms, 535 536 Stainless steel bars, 10 Standard curing, 42 Standard sand, 45 Standardized cracks, 832, 833f, 839 self-healing abilities of specimens with, 851 Steady state period, 617 Steam curing process, 42 Steel, 460, 475 476 Steel fiber-reinforced concrete (SFRC), 18 20. See also Textile reinforced concrete (TRC) applications, 20 22 manhole covers and frames, 21f Steel fiber reinforced shotcrete (SFRS), 22 Steel fibers, 45, 277t, 279, 280f, 302 303, 784, 826 Steel reinforcement corrosion, 660 Steel-making slag (SS), 178 Steel-reinforced resin, 718, 720f numerical simulation of, 734 741 confined steel-reinforced resin, 736 741 unconfined steel-reinforced resin, 734 736 Stern Geary equation, 667 STL. See Sound transmission loss (STL) Stone crusher waste as fine aggregates, 618 Stone dust, 614 Stoneware tiles. See also Porcelain stoneware tiles artificial gypsum in clay-based ceramic applications, 173 175 ashes in clay-based ceramic applications, 170 171 glass waste in clay-based ceramic applications, 201 202 mineral slags and metallurgy waste in clay-based ceramic applications, 179

Index

organic waste in clay-based ceramic applications, 207 ornamental rock waste in clay-based ceramic applications, 193 194 sludge in clay-based ceramic applications, 185 187 Strain energy, 382 Strength reduction factor (RCS), 868 Strength vs. crack opening curves, 289 Stress strain behavior of EPS geofoam, 121, 122f Stress strain partition parameter, 945 Stress strain relations, 381 Structural Engineering Research Centre (SERC), 20 Structural lightweight concrete and associated aggregates, 32 Styrene butadiene styrene copolymer (SBS) modified asphalt, 700 Subsampling layer, 86 Sugarcane bagasse ash (SBA), 164, 470, 602 605, 603f, 606f Sulfate resistance of AAC mixes, 475 Sulfate resisting cement, 4 Sulfonated superplasticizers, 320 Sulfoxide index (SI), 708 Sulfur trioxide (SO3), 615 Super sulfated cement, 4 5 Superabsorbent polymer (SAP), 413 Superfluidifying agents, 275 276 Superplasticizer, 1 2, 285, 304, 315, 321f, 645, 901 902 absorption by cement particles, 319f action mechanisms, 318 321 action principle of steric, 319f chemical structure, 315 317 dispersion of cement particles by, 320f effect on cement paste, 321 323 effect on concrete, 320f compressive strength, 326 327 rheology, 324 325 PCEs, 317, 317f PMSs, 316, 316f PNS, 315, 316f VCPs, 316, 317f Supervised learning, 81 Supplementary cementing materials (SCMs), 435 436, 618

1077

Surface characterization of metals/rebars after corrosion, 668 Surfactants, 535 536, 538 540 Sustainability, 435 of nanomaterial-based self-healing concrete, 419 420 of smart concrete, 398 of traditional OPC-based concrete, 396 398 Sustainable concrete composites, 436 437 applications, 454 concrete incorporating waste metalized plastic fibers, 439 454 general appraisal, 435 436 waste metalized plastic fibers, 437 439 Sustainable design framework, 364 Sustainable development of FA utilization, 678 679 factors governing strategies for, 679f Sustainable energy, 420 Sustainable use, 800 801 Swarm intelligence (SI), 80, 83 SWCNTs. See Single-walled carbon NTs (SWCNTs) Swelling phase, 751 Swelling pressure (SP), 751, 761 Swelling pressure reduction factor (SPr), 753 755 SWS. See Sewage sludge (SWS) Symbolism, 361 362 T T123 nickel particles, 782 T1FS. See Type 1 fuzzy system (T1FS) T287 nickel particles, 782 T2FL. See Type-II fuzzy logic systems (T2FL) T2FS. See Type 2 fuzzy system (T2FS) Tamedia building, in Zurich, 344 345, 345f TCD. See Theory of critical distances (TCD) TCLP. See Toxicity Characteristic Leaching Procedure (TCLP) TCO. See Total cost of ownership (TCO) TDI. See Toluene-di-isocyanate (TDI) TEM. See Transmission electron microscope (TEM) Temperate forests, 357 Temperature-modulated differential scanning calorimetry (TMDSC), 229

1078

Tensile strength, 287, 287f Tensile testing, 225 226, 936, 937f Tension-crack opening, 292 Tetracalcium aluminoferrite (C4AF), 7, 615 Textile reinforced concrete (TRC), 33 40. See also Steel fiber-reinforced concrete (SFRC) characteristics, 34 36 debonding characteristics of textiles in, 36 39 TFOT. See Thin-film oven test (TFOT) TG. See Thermogravimetry (TG) TGA. See Thermogravimetric analysis (TGA) Theory of critical distances (TCD), 989 990 Theory of elasticity, 125 126 Thermal conductivity, 574 Thermal oxidation, 707 708 Thermal shock, 257 259 Thermal treatment, 338 Thermally modified wood (TMW), 338, 338f Thermochemical method, 594 Thermochromic asphalt binders, 692 adjustment of bituminous pavement temperature, 713 714 antiaging properties, 707 712 optical and thermal properties, 699 700 performance characterization, 699 712 physical properties, 700 701 recommendations for future research and applications, 714 715 rheological properties, 702 707 fatigue performance, 706 low-temperature performance, 706 707 rutting, 703 705 viscoelastic properties, 702 703 three-component organic reversible thermochromic materials, 693 699 Thermochromic materials, 692, 713 714 three-component organic reversible, 693 699 Thermochromic microcapsule materials, 693 Thermogravimetric analysis (TGA), 697, 698f, 884, 891, 980 mass losses in, 891 Thermogravimetry (TG), 786 Thermomechanically treated (TMT) bars, 10 Thigmonasty, 357

Index

Thin-film oven test (TFOT), 692 Three-component organic reversible thermochromic materials, 693 699 colors of thermochromic powders, 695f components and structures, 693 694 FTIR spectra of thermochromic powders, 694f molecular structures of electron donors, 696f thermal and optical properties, 695 699 thermochromic mechanism, 694 695 Three-component thermochromic compounds, 693 Three-dimension (3D) axisymmetric FE model, 956, 957f printing, 529 530, 540 “tension shear chain” theoretical model, 947 948 Three-dimensional representation (3D representation), 350 Three-dimensional silicon aluminate structures, 461 Three-point short beam shear test, 227, 227f 3D inspector system (3DIS), 99 Timber, 333 334 for adding stories to existing buildings, 342f biobased fac¸ade materials, 335 347 and glass composites, 342 346 green walls and green fac¸ades, 346 347 timber-behind-glass, 343 346 trends and perspectives, 348 369 Time-dependent behavior of EPS geofoam, 127 129 Tincal, 160 Ti O Si bonds, 786 787 Titania. See Titanium oxide (TiO2) Titanium oxide (TiO2), 416 417 Titanium oxide nanoparticles, 412 TMDSC. See Temperature-modulated differential scanning calorimetry (TMDSC) TMW. See Thermally modified wood (TMW) Toluene-di-isocyanate (TDI), 402 Top-down approach, 410 Total cost of ownership (TCO), 368 369 Total fracture energy, 302 303 Total porosity determination, 541 542

Index

Toughening mechanisms through implications of nanofillers, 238 241 Toxicity Characteristic Leaching Procedure (TCLP), 173 175 Traditional OPC-based concrete sustainability cement-generated environmental problems, 396 concrete durability, 396 397 energy problems in cement industries, 398 Training from scratch, 88 Transfer learning, 88, 90f Transform-based image enhancement techniques, 66 Transformation matrix, 381 Transient methods, 668 Transition temperature, 694 695 Transmission electron microscope (TEM), 228 229, 232 233, 511, 511f, 668 “Transmission line” model, 667 TRC. See Textile reinforced concrete (TRC) Tricalcium aluminate, 615 Tricalcium silicate, 615 Triton X-100, 539 Tsai Hill failure theory, 1019 1020, 1022f, 1033 1035 TSS. See Two-step sintering (TSS) Two-dimensional composites (2D composites), 922 Two-dimensional comprehensive model, 660 Two-step sintering (TSS), 170 Type 1 fuzzy system (T1FS), 88 Type 2 fuzzy system (T2FS), 88, 92f, 94f Type-II fuzzy logic systems (T2FL), 84 85, 88 93 Type-reduction, 91 U UC. See Unit cell (UC) UHPC. See Ultra high-performance concrete (UHPC) UHPFRC. See Ultrahigh-performance fiberreinforced concrete (UHPFRC) UHSC. See Ultra-high strength concrete (UHSC) Ulexite, 160 ULS. See Ultimate limit state (ULS) Ultimate limit state (ULS), 289 Ultimate tensile strength (UTS), 243

1079

Ultra high-performance concrete (UHPC), 294 298, 412 414, 898 constituent materials, 298 304 materials used in UHPC and contribution, 296t stress strain diagram, 307f Ultra-high strength concrete (UHSC), 1 2, 18, 40 51 benefits, 44 characterization of materials and development of mix, 44 45 criteria for material selection, 41 42 curing, 42 43 equipment used, 46, 48f mechanical properties, 48 51, 49t mechanism of production, 41 mix proportion, 46, 47t selection of parameters for UHSC components, 43t specimen preparation, 46 48 Ultrahigh-performance fiber-reinforced concrete (UHPFRC), 297 mix design of conventional, 302t new trends and applications, 309 310 rheology and mechanical properties, 304 309 Ultramacropores, 535 Ultrasonic pulse velocity (UPV), 828, 834, 834f, 881f, 882, 884 cobinder’s effects, 885f measurement, 545 546 Ultrasonic sensor (US), 99 Ultrathin chopped carbon fiber tape reinforced thermoplastics (UTCTT), 951 Ultraviolet (UV), 692 irradiation, 256 257, 707 708 protection of EPS geofoam, 129 radiation, 692 UMF. See Upper membership function (UMF) Unconfined resin simulation, 733 Unconfined steel-reinforced resin, 734 736 Uniaxial compressive behavior, 729 Unidirectional short fibers, 958, 963f Unidirectional short fibrous composites, 972f Unit cell (UC), 924 925, 1033f Universal testing machine (UTM), 902 904 Unstructured mesh Galerkin FEM, 967 971

1080

Unsupervised learning, 81 Upper membership function (UMF), 91 UPV. See Ultrasonic pulse velocity (UPV) Urban heat island effect, 691 Urban mining and design for disassembly, 358 360 Urbanization, 897 Urease, 403 US. See Ultrasonic sensor (US) UTCTT. See Ultrathin chopped carbon fiber tape reinforced thermoplastics (UTCTT) Utility protection, EPS in, 144 149 cases histories and performance, 144 145 design considerations, 148 149 practical issues, 145 148 UTM. See Universal testing machine (UTM) UTS. See Ultimate tensile strength (UTS) UV. See Ultraviolet (UV) V V-funnel test, 306, 306f, 646 van der Waals forces, 415 VCPs. See Vinyl copolymers (VCPs) Vebe` consisto-meter, 284 285 VeBe time test, 439 441 VECD analysis. See Viscoelastic continuum damage analysis (VECD analysis) Ventilated timber-behind-glass solutions, 344 Vermiculite, 32, 755 756 VGG model, 88 Vibration characteristics of composite plates, 376 Vibration test system, 987, 987f Vine shoot (VS), 203 Vinyl copolymers (VCPs), 315 316, 317f Viscoelastic continuum damage analysis (VECD analysis), 706 Viscosity modifying agent (VMA), 23 Viscosity tests, 700 Visible light, 695 VMA. See Viscosity modifying agent (VMA) Von Karman nonlinear components of strain, 380 381 Von Karman strains, 376 377 Von Mises stresses, 966, 968f VS. See Vine shoot (VS)

Index

W W/C ratio. See Water cement ratio (W/C ratio) WAC. See Water absorption capacity (WAC) WAS. See Washing aggregate sludge (WAS) Washing aggregate sludge (WAS), 188 Waste, 621 FRP materials, 644 management hierarchy, 438f materials, 617 metalized plastic fibers, 437 439 concrete incorporating, 439 454 engineering properties of WMP fibers, 439t tire rubber, 858 experimental database of concrete with, 861t Waste expanded polystyrene beads, 747 748 geofoam, 756 758 Waste foundry sand (WFS), 179 Waste glass, 594 597 BSEM/EDX images of slag pastes, 595f Waste metalized plastics (WMPs), 438 Wastewaters sludge, 161 Water, 500, 901 902 absorption, 448 451 by capillary, 883, 888 by immersion, 883, 888 percentage of, 50 extenders, 583 585 in fly ash utilization, 910, 913, 915 permeability of concrete, 50 51 for self-healing concrete, 828 tests, 58 Water absorption capacity (WAC), 180 184 Water cement ratio (W/C ratio), 495, 497, 573 Watford Junction replacement station platform, 134 Wave attenuation, 149 150 Wavelet transforms, 67 68, 68t WBA. See Wood BA (WBA) Wear-resisting cement-based composites, 783 Weld-line, 923 924 Wet transverse strength, 904 905

Index

Wet transverse strength in fly ash utilization, 913 914 WFS. See Waste foundry sand (WFS) WMPs. See Waste metalized plastics (WMPs) Wolf Trap Pedestrian Bridge, 223 224, 225f Wood, 333 Wood BA (WBA), 168 Wood-plastic composites (WPCs), 341 342 Workability of concrete, 503 of concrete incorporating waste metalized plastic fibers, 439 442 of geopolymers and alkali-activated composites, 465 469 WPCs. See Wood-plastic composites (WPCs) Wraps, 24 25 X XPS geofoam. See Extruded polystyrene geofoam (XPS geofoam) X-ray CT, 36 38, 38f, 39f X-ray diffraction (XRD), 468 469, 497 498, 626 627, 668, 745, 746f, 755 756, 786, 852

1081

in alkali-activated soapstone binders, 884 test, 509 510, 510f X-ray fluorescence spectroscopy (XRF spectroscopy), 418, 509, 510f X-ray photoelectron spectroscopy (XPS), 668 XRD. See X-ray diffraction (XRD) XRF spectroscopy. See X-ray fluorescence spectroscopy (XRF spectroscopy) Xyhlo biofinish technology, 340, 340f Y Yield stress of cement paste, 322 323, 323f Yielding walls, 140 Young’s modulus, 925 926, 928f, 946 947 BSS-fiber composite, 939 expanded polystyrene, 120 122 with geofoam density, 125f of rubber, 860 Young Laplace equation, 534 535 Z Z-pinning, 1031 Zeiss Crossbeam 340, 511 512 Zero-energy buildings (ZEB), 365 367 Zn-65 radionuclide, 624 625