Handbook on Advanced Concrete Technology [1 ed.] 9781783322565, 9781842657423

Citation preview

Handbook on

Advanced Concrete Technology Editors

N.V. Nayak A.K. Jain

Editors N.V. Nayak A.K. Jain

a Alpha Science International Ltd. Oxford, U.K.

Handbook on Advanced Concrete Technology 646 pgs. | 247 figs. | 141 tbls.

Editors N.V. Nayak Managing Director Gammon Realty Limited Mumbai A.K. Jain Advisor UltraTech Cement Limited Mumbai Copyright © 2012 ALPHA SCIENCE INTERNATIONAL LTD. 7200 The Quorum, Oxford Business Park North Garsington Road, Oxford OX4 2JZ, U.K. www.alphasci.com All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the publisher. ISBN 978-1-84265-742-3 Printed in India

Foreword Oxford Dictionary describes ‘concrete’ as a building material made from a mixture of gravel, sand, cement and water, forming a stone like mass on hardening. From this simple description, concrete has now emerged as a very versatile construction material for a variety of civil engineering applications. Reinforced concrete and pre-stressed concrete have opened new vistas and possibilities for use of concrete. In order to meet various technical and physical needs, concrete technology has advanced considerably over the years. Many varieties of concrete such as light weight, high strength, self-compacting, fibre reinforced, polymer impregnated, high weight, high strength, self-compacting, fibre reinforced, polymer impregnated, high volume fly ash etc. have been developed for specific and custom-oriented use. These materials are indeed very complex in nature and require judicious use of different ingredients including chemical admixtures in production, placing and subsequent care. Concrete of modern day is no more a simple material and it needs appropriate technology and know-how to achieve the desired results. A number of books have been written on concrete technology and on reinforced and pre-stressed concrete which are readily available in the market. But for a practising engineer, for immediate and ready reference book in the form of a “Handbook” is badly lacking. It is heartening to note that this gap is now being filled by this very useful “Handbook on Advanced Concrete Technology”. This “Handbook on Advanced Concrete Technology” is authored by 26 professionals, who are well known in their fields and who have drawn on their knowledge and long experience with concrete. All aspects of concrete including materials, proportioning, production, placing, compaction, acceptance criteria, different varieties and their applications, codal provisions, etc. have been comprehensively covered in 37 chapters. The “Handbook” has been mainly written with an objective to answer the problems faced at construction sites by practising engineers. The comprehensive coverage of the subject would also make the “Handbook” equally useful to the academics, consultants, builders and all other professionals connected with concrete constructions. The most important construction issue emerging to the fore at the beginning of this century is sustainability. It is more pertinent to our country as nearly 7% of National GDP is spent on construction every year and a major share is utilized by reinforced concrete. It is in the figness of things that concrete is producted, placed, compacted and cured in the best possible manner to minimize subsequent repairs and retrofitting. The knowhow on concrete technology covered in

vi

Foreword

the “Handbook”, if properly used at construction sites, would certainly help in building durable and sustainable structures. I congratulate all the authors, editors and publisher for accomplishing this difficult task admirably and hope that this publication would prove very useful to all the professionals connected with modern day construction.

E. Sreedharan Former Managing Director Delhi Metro Rail Corporation Ltd.

Preface The present book is brought out with a purpose to provide insight into the subject of concrete technology based on practical experience gained over the years especially in Indian conditions. The “Handbook on Advanced Concrete Technology” is authored by 26 well known professionals engaged in the art and science of production and application of concrete. The book contains 37 chapters and each chapter has been co-authored generally by two persons and reviewed by one or more professionals in order to make the subject more contemporary and relevant to the actual requirements of construction sites. An effort has been made to write each chapter with a view to include the latest developments in the field of concrete technology in India and abroad. The chapters on concrete making materials like cement, aggregate and water have been written keeping in view the requirements of Standards and qualitative needs of these materials in producing durable concrete. The chemical and mineral admixtures have given new dimension to the concrete technology over the last few decades and made it possible to produce ultra high strength concrete with high performance characteristics. These admixtures have been covered in great detail keeping in view the requirements of Indian and International codes of practices and experience gained in their use at different construction sites. Concrete has emerged as the most versatile material to meet the varied needs of modern day construction. Varieties of concrete have been developed, to mention a few like self compacting concrete, high volume fly ash concrete, fibre concrete, light and heavy weight concrete, polymer concrete and host of others to meet the specific needs of the construction industry. The different types of concrete which are normally required in modern day construction have been discussed in various chapters. It is expected that discussions on these concretes will be found more useful from practical point of view and relevance to the site conditions. The proportioning of various ingredients to produce concrete of requisite characteristics has been described in great detail keeping the requirements of the latest Indian and International codes and prevalent practices in the industry. The examples of concrete mix proportioning with and without inclusion of mineral admixture like fly ash have been provided for ease of understanding of the subject. The selection of appropriate materials, their impact on quality of concrete and proper proportioning in order to develop technically sound and economically optimized concrete mixes have been covered in detail to enable to produce durable and sustainable concrete. Similarly various methods of non-destructive tests on concrete and repairs and maintenance of concrete have been covered to provide guidance on testing and repairs. The main emphasis on selection of topics, authors and subject matter has been to provide latest developments in the respective fields of the Concrete Technology under one publication. It is hoped that the effort would help not only the practicing engineers but the students, researchers,

viii

Preface

and other professionals who are engaged in the pursuit of the subject. The concrete in one form or the other has virtually become an integral part of all modern constructions and so much knowledge has been gained on the subject that it may be extremely difficult to bring out a comprehensive compilation covering all aspects of the subject. Nevertheless, the present endeavor is made keeping in view, the needs of the practicing engineers, students and other professionals in India and abroad and to provide useful insights of the subject. Hence it is hoped that this book will receive very good response. The editors and authors will be very thankful to the readers for any suggestions made to improve the ‘Handbook’. A sincere effort will be made to give due consideration to the suggestions and update the ‘Handbook’ from time to time. Editors N.V. Nayak A.K. Jain

Acknowledgements The editors would like to express sincere thanks to all who have made strenuous efforts in bringing this project to fruition. It would not have been possible without the untiring efforts and complete support of the following, The authors for respective chapters and for the patience with which many changes/modifications required were made in the course of review, editing, checking process, etc. Thanks are also due to family members of the authors for their silent sacrifice during the involvement of the authors in this rigerous task for bringing this handbook. Thanks also due to secretarial staff in typing the manuscript again and again. Special thanks are due to Mrs. Shashikala B. Iyer and Mr. Devendra Joshi of Gammon India Ltd. for the pains taken in organising various review meetings, follow ups, typing, retyping the manuscript, follow up with the publisher, etc. In addition, thanks are due to Ms. Amruta Jadhav of Ultra Tech Cement Ltd. for providing secretarial support during the preparation of the ‘Handbook’. The Chairman and Managing Director, Gammon India Limited, Mr. Abhijit Rajan provided the facilities for the various meetings, discussions, etc. which we acknowledge with gratitude. To bring out the book in the present format, authors have liberally used various publications including the books, journals, articles, etc. Every attempt is made to acknowledge in the text at appropriate places. If anything is left out inadvertantly, then we appologise. We thank the authors and publishers of such publications mentioned or otherwise. We referred to various standards of our country as well as other countries. Our thanks are to these organisations including BIS, IRC, etc. In addition to editors, Mr. S.G. Bapat (Ex-Chief Engineer, NPCIL) and Mr. V.K. Sharma (Ex-Director, NPCIL) helped in review of some of the chapters and in proof reading, which is highly appreciated. Our special thanks and gratitude to Padma Vibhushan Er. E. Sreedharan, former Managing Director of Delhi Metro Rail Corporation Ltd. (DMRC) for writing FOREWORD. It has helped in enhancing the value of the book. Finally thanks are due to publisher, M/s Narosa Publishing House Pvt. Ltd. for coming forward to publish the “Handbook” and having shown full confidence in success of the project. Editors N.V. Nayak A.K. Jain

Contents Foreword Preface Acknowledgements

v vii ix

1. Cement 1.1 Introduction 1.2 Basic Composition of Cement 1.3 Manufacture of Cement 1.4 Physical Properties of Cement 1.5 Chemical Properties of Cement 1.6 Heat of Hydration 1.7 Colour of Cement 1.8 Variety of Cement and their Applications 1.9 Storage of Cement 1.10 Conclusion Appendix I Appendix II Appendix III

1.1 1.1 1.1 1.2 1.9 1.11 1.13 1.13 1.13 1.21 1.26 1.27 1.28 1.29

2. Mineral Admixtures 2.1 Introduction 2.2 Pozzolanic Reaction and Concrete 2.3 Fly Ash as Mineral Admixture in Concrete 2.4 GGBS in Concrete 2.5 Silica Fume in Concrete 2.6 Metakaolin in Concrete 2.7 Green, Durable and Economical Concrete with Fly Ash and GGBS

2.1 2.1 2.1 2.1 2.7 2.12 2.16 2.17

3. Water 3.1 Introduction 3.2 Quality of Water

3.1 3.1 3.1

xii

Contents

4. Coarse Aggregate 4.1 Introduction 4.2 Classification of Aggregates 4.3 Properties of Coarse Aggregates Affecting Concrete Characteristics 5. Fine Aggregate 5.1 5.2 5.3 5.4 5.5

Introduction Grading of Fine Aggregate Crushed Sand Deleterious Material in Fine Aggregates Fine Aggregate Requirement

4.1 4.1 4.1 4.5 5.1 5.1 5.1 5.4 5.4 5.4

6. Manufactured Sand as Fine Aggregate 6.1 Shortage of Natural Sand 6.2 Optimum Shape 6.3 Void Content 6.4 Manufacturing Process 6.5 Optimal Process 6.6 Tests for Manufactured Sand

6.1 6.1 6.1 6.2 6.3 6.4 6.4

7. Use of Copper Slag as Fine Aggregate in Concrete

7.1

7.1

Introduction

7.1

7.2 7.3

What is Copper Slag? Concluding Remarks

7.1 7.8

8. Chemical Admixture 8.1 Definition 8.2 History 9. Concrete Mix Proportioning 9.1 Objectives 9.2 Principles of Concrete Mix Proportioning 9.3 Methods of Concrete Mix Proportioning Annexure A Annexure B Annexure C Annexure D 10. Concrete Production and Supply 10.1 Storage of Materials 10.2 Batching

8.1 8.1 8.1 9.1 9.1 9.1 9.16 9.26 9.29 9.34 9.38 10.1 10.2 10.7

Contents

10.3 10.4 10.5 10.6 10.7

Mixing Arrangements Control Systems Transportation Batching Special Concrete Checklist for Various Items

xiii

10.8 10.8 10.9 10.9 10.10

11. Placing, Compaction and Curing of Concrete 11.1 Introduction 11.2 Placing of Concrete 11.3 Compaction of Concrete 11.4 Curing of Concrete

11.1 11.1 11.1 11.6 11.14

12. Shrinkage and Creep 12.1 Introduction 12.2 Shrinkage 12.3 Creep

12.1 12.1 12.1 12.8

13. Strength and Durability of Concrete 13.1 Introduction 13.2 Strength of Concrete 13.3 Mechanical Properties of Hardened Concrete 13.3 Bond Strength of Concrete and Reinforcement Steel 13.4 Durability of Concrete

13.1 13.1 13.2 13.10 13.30 13.31

14. Reinforcement Cover and Corrosion 14.1 Introduction 14.2 The Corrosion Process 14.3 Black Rust 14.4 Corrosion due to Carbonation 14.6 Corrosion due to Chloride Attack 14.7 Measures for Reducing the Rate of Corrosion 14.8 Improving the Quality of Cover

14.1 14.1 14.1 14.4 14.5 14.7 14.8 14.14

15. Concrete Resistance to Sulphate Attack 15.1 Introduction 15.2 Mechanism of Sulphate Attack 15.3 Mitigation of Sulphate Attack

15.1 15.1 15.1 15.2

16. Alkali Silica Reaction 16.1 Introduction 16.2 Mechanism of Alkali-silica Reaction

16.1 16.1 16.1

xiv

Contents

16.3 Tests for Alkali Reactivity of Aggregate 16.4 Mitigation Measures for Alkali-silica Reaction 16.5 Prevention/Minimizing ASR Effect on Concrete Structure

16.3 16.5 16.10

17. High Strength and High Performance Concrete 17.1 Introduction 17.2 High Performance Concrete – Overview 17.3 High Performance Concrete – Major Characteristics 17.4 Materials Selection and Mix Proportioning – Difficulties

17.1 17.1 17.2 17.2 17.3

18. Self Compacting Concrete 18.1 Introduction 18.2 Definition 18.3 Mix Proportion of SCC 18.4 Test Methods 18.5 Acceptance Test Criteria for SCC 18.6 Production, Placing and Quality Control 18.7 Effects of Vibrations on Freshly Laid Concrete 18.8 Other Characteristics of Self-compacting Concrete vs Normal Concrete 18.9 Advantages of Self-compacting Concrete (SCC) Over Normal Concrete (NC)

18.1 18.1 18.2 18.2 18.5 18.11 18.12 18.13 18.15 18.15

19. Hot Weather Concrete 19.1 Introduction 19.2 Issues Associated with Hot Weather Concreting 19.3 Production of Concrete in Hot Weather 19.4 Placing and Curing

19.1 19.1 19.1 19.6 19.6

20. Cold Weather Concrete 20.1 Introduction 20.2 Issues Associated with Cold Weather Concreting 20.3 Production of Concrete in Cold Weather 20.4 Preparation Before Concreting 20.5 Protection of Concrete Against Freezing 20.6 Protection of Concrete Against Freeze Thaw Effect

20.1 20.1 20.1 20.5 20.7 20.8 20.10

21. Pervious Concrete 21.1 Introduction 21.2 Applications of Pervious Concrete 21.3 Pavement Applications 21.4 Surface Course 21.5 Parking Lots

21.1 21.1 21.2 21.2 21.3 21.3

Contents

21.6 21.7 21.8 21.9 21.10 21.11 21.12 21.13 21.14 21.15 21.16 21.17 21.18 21.19 21.20

Roadways Permeable Bases and Edge Drains Materials Aggregates Cementitious Materials Water Admixtures Percolation Rate of Pervious Concrete Pervious Pavement Construction Placing Consolidation Jointing Testing Pervious Concrete in India Conclusion

xv

21.4 21.4 21.4 21.5 21.5 21.5 21.5 21.6 21.7 21.7 21.8 21.9 21.11 21.11 21.12

22. Lightweight Concrete 22.1 Introduction 22.2 Types of Lightweight Concrete 22.3 No-Fines Concrete 22.4 Lightweight Aggregate Concrete 22.5 Aerated Concrete 22.6 Testing of Lightweight Concrete 22.7 Water Absorption 22.8 Application of Lightweight Concrete

22.1 22.1 22.1 22.2 22.3 22.3 22.4 22.4 22.5

23. Fibre 23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.8

23.1 23.1 23.3 23.4 23.13 23.14 23.17 23.20 23.20

Reinforced Concrete Introduction of Fibre Reinforced Concrete Steel Fibre Reinforced Concrete (SFRC) Properties of Different Steel Fibre Types Synthetic Fibre Reinforced Concrete (SYNFRC) Types of Synthetic Fibres Dosing Glass Fibre Reinforced Concrete Properties of GFRC

24. High-Density Concrete 24.1 Heavy Aggregate 24.2 Special Heavy Aggregate 24.3 Mix Proportioning, Mixing and Placement of High Density Concrete

24.1 24.2 24.2 24.2

xvi

Contents

25. Underwater Concreting 25.1 Methods of Underwater Placements 25.2 Materials 25.3 Admixtures 25.4 Mix Proportioning 25.5 Concrete Production and Testing 25.6 Tremie Equipment and Placement Procedure 25.7 Placement Procedure 25.8 Direct Pumping 25.9 Concrete Characteristics 25.10 Precautions to be Taken in Underwater Placements 25.11 Go-Devils 25.12 Laitance 25.13 Cracking 25.14 Detailing 25.15 Antiwashout Admixtures 25.16 Conclusion

25.1 25.1 25.1 25.2 25.2 25.2 25.3 25.3 25.5 25.5 25.5 25.6 25.6 25.7 25.7 25.7 25.8

26. Mass 26.1 26.2 26.3 26.4 26.5 26.6

26.1 26.1 26.2 26.3 26.3 26.4 26.5 26.7

Concrete and Temperature Rise in Concrete Introduction Causes of Thermal Cracking Methods of Temperature Control Choice of Cement System Materials and Mix Proportioning Calculation for Rise in Temperature Appendix – I

27. Roller Compacted Concrete 27.1 Materials and Mixture Proportioning for RCC 27.2 Materials 27.3 Construction of RCC Dams

27.1 27.1 27.2 27.9

28. Foam 28.1 28.2 28.3 28.4 28.5 28.6

28.1 28.1 28.1 28.2 28.3 28.6 28.7

Concrete Introduction Advantages of Foam Concrete Applications of Foam Concrete Production of Foam Concrete The Technique Special Applications

Contents

29. Acid 29.1 29.2 29.3 29.4 29.5

xvii

Resistant Concrete Introduction Acid Resistant Concrete Mix Design for ARC Concretes New Generation Construction Chemicals for HPC Microsilica Slurry for High Performance Concretes

29.1 29.1 29.2 29.5 29.6 29.8

30. Concrete Composites Containing Polymers 30.1 Introduction 30.2 Polymers as a Compositing Material 30.3 Polymer Concrete (PC) 30.4 Polymer Modified Concrete (PMC) 30.5 Polymer-impregnated Concrete (PIC) 30.6 Future Possibilities of Polymer-Cement Composites

30.1 30.1 30.1 30.3 30.4 30.7 30.10

31. Concrete Roads and High Volume Fly Ash Concrete 31.1 31.1 30.1 History of Concrete Roads 31.2 Advantages of Concrete Roads 31.1 31.3 Some Disadvantages of Concrete Roads and How they can be Overcome 31.2 31.4 Economics of Concrete Roads vis-à-vis Flexible Pavements 31.3 31.5 Design of Concrete Pavements 31.3 31.6 Joints in Concrete Roads 31.6 31.7 White-topping 31.8 31.8 Continuously Reinforced Concrete Pavement (CRCP) 31.9 31.9 High Volume Fly-ash Concrete 31.9 31.10 Construction 31.15 Appendix I 31.22 32. Quality Control and Quality Assurance of Concrete 32.1 Introduction 32.2 What are Quality Assurance (QA) and Quality Control (QC)? 32.3 Concrete Industry Scenario 32.4 Regulatory Framework for QA and QC 32.5 Indigenous Quality Scheme 32.6 Check List

32.1 32.1 32.1 32.2 32.4 32.5 32.15

33. Non 33.1 33.2 33.3

33.1 33.1 33.1 33.5

Destructive Testing of Concrete Introduction The Objective Commonly used Test Methods

xviii

Contents

33.4

Sampling and Relibility

33.22

34. Repair and Maintenance of Concrete 34.1 Introduction 34.2 Why Repairs — Introduction 34.3 Injections for Repairs 34.4 Methods of Repairs 34.5 Preventive Maintenance

34.1 34.1 34.8 34.15 34.25 34.45

35. Concrete and its Environmental Impact 35.1 CO2 Emission 35.2 CO2 Emissions from Cement 35.3 Suggestions to Reduce Environmental Impact 35.4 A Model for the Future

35.1 35.2 35.4 35.4 35.7

36. Current and Expected Future Advances in Concrete 36.1 Advances in Concrete Technology 36.2 Application of Ultra High Strenght Concrete 36.3 Technology Development 36.4 Ultra Thin White Toppings for Roads 36.5 Types of Overlay 36.6 Advantages 36.7 Features

36.1 36.1 36.2 36.3 36.10 36.10 36.11 36.11

37. Summary of Codal Provision for Concrete and Cementitious Materials 37.1 Introduction 37.2 Concrete Making Materials 37.3 Concrete Mix Proportioning 37.4 Batching and Mixing of Concrete 37.5 Durability of Concrete 37.6 Sampling and Strength of Concrete

37.1 37.1 37.1 37.9 37.10 37.14 37.18

About the Authors Index

A.1 I.1

1 Cement A.K. Jain and C.M. Dordi

1.1 INTRODUCTION Cement is one of the most important building materials today. The term ‘Cement’ is generic that can be applied for many organic and inorganic materials. However, the most widely used and versatile variety is Portland cement. The invention of Portland cement brought about a landmark change and provided a satisfactory answer to mankind’s quest for a strong and durable binder for constructions. The history of modern Portland cement can be traced back to only about 175 years, when Joseph Aspdin invented and patented it in 1824. Over time, Portland cement has come to be most accepted from all over the world, replacing the old cementitious materials that were first made by the Romans using a mixture of calcined clay and lime. The growth and development of cement industry in India dates back 1914, when the first cement plant had been commissioned at Porbunder in Gujarat. The growth of the industry, during initial period was sluggish and in 1947, the installed capacity was 2.22mn.t and cement production 1.48mn.t with per capita consumption of 4kg. The growth of the industry remained stinted during the control regime up to early nineteen eighties. The easing of controls was initiated in 1982 culminated in total decontrol in 1989 and the policies of liberalization put the dormant cement industry on a vibrant growth path. The capacity and production which were 29mn.t and 21mn.t in 1981-82 shot up to 226mn.t and 181mn.t in 2008-2009 (2). The Indian cement industry is growing at a fast pace, especially over the last two decades, with rapid modernization and technology up-gradation. The industry is presently growing 9%-10% per annum. India is now occupying the second position in the world (next to China) in both installed capacity and annual production. Despite being the second largest producer in world, the per capita cement consumption is low at about 200kg as compared to the current world average of about 310kg. Thus, there is scope for further rapid growth over the next couple of decades.

1.2 BASIC COMPOSITION OF CEMENT Cement is a chemical compound and hydraulic cements are primarily composed of hydraulic calcium silicates. Hydraulic cements set and harden by reacting chemically with water. Hydration

1.2

Handbook on Advanced Concrete Technology

begins as soon as cement comes in contact with water. Each cement particle forms a type of growth on its surface that gradually spreads until it links up with the growth from other cement particles or adheres to adjacent substance. Such build-up results in progressive stiffening, hardening and strength development. The oxide and compound compositions of Ordinary Portland Cement are shown in Table 1.1 TABLE 1.1 Approximate oxide and compound compositions of OPC (Indicative range of prevailing values – before grinding into cement with permissible additives) Oxide

Approximate Content %

Chemical Compound

CaO

61-67

Tricalcium Silicate

SiO2 Al2O3 Fe2O3

19-25 3.0-8.0 4-6

Dicalcium Silicate Tricalcium Aluminate Tetra Calcium Aluminoferrite

Formula

Abbreviated Formula

Approximate Content %

3CaO.SiO2

C3S

40-55

2CaO.SiO2 3CaO.Al2O3 4CaO2.Al2O3.Fe2O3

C2S C3 A

15-25 5-11

C4AF

14-20

In addition to the four major compounds, there are many minor constituents, such as K2O, Na2O, Mn2O3 and TiO2 in the cement. Their presence is insignificant. However, the limits for some of the minor constituents are laid down by BIS in the relevant Indian Standards (IS) as optional requirement depending upon the usage of cement. The four major compounds have the following properties: (a) Tricalcium silicate (C3S) hydrates and hardens rapidly and is largely responsible for early strength development. (b) Dicalcium silicate (C2S) hydrates and hardens slowly and contributes largely to latter age strength development. (c) Tricalcium aluminate (C3A), hydrates faster than silicates (C3S, C2S) and contributes very little to the strength development. Presence of gypsum regulates setting and hardening behavior of C3A as well as C3S. Cement with C3A content 5% to 8% exerts improved resistance against chloride and sulphate attack, while cement with C3A content below 5% is especially resistant to soils and water containing sulphates alone. (d) Tetracalcium aluminoferrite (C4AF) reduces the clinkering temperature, thereby assisting pyroprocessing. It hydrates rather rapidly but contributes insignificantly to the strength development.

1.3

MANUFACTURE OF CEMENT

Ordinary Portland Cement (OPC) is a complex mixture of several compounds – silicates of calcium, aluminum and iron, silico-aluminates and alumino-ferrites. In association with water these compounds hydrate and form different complex compounds and release lime. The binding property of cement is contributed by one of the main products – calcium silicate hydrate, and this property increases with passage of time, with progressive loss of water by the hardening concrete. Chemical analysis of a conventional Ordinary Portland Cement usually shows it to

Cement

1.3

contain 60 to 68% CaO(lime), 20 to 24% SiO2 (silica), 4 to7% Al2O3 (alumina) and 2 to 4% Fe2O3 (iron oxide). The shortest and least-cost route for making cement is preparing a suitable raw mix derived from intimate mixing of a few naturally occurring raw materials in finely ground form, and then burning the mix in a kiln for making cement clinker. The raw mix contains a combination of naturally occurring calcium carbonate rich rock or mineral as a major component, with one or more additives and corrective materials. Accordingly, the principal raw materials for its manufacture are calcium-rich (calcareous) and silica-alumina-rich (argillaceous) minerals besides supplementary corrective materials containing alumina and/or iron. Table 1.2 summaries the range of naturally occurring minerals and industrial byproducts containing these compound, which comprise these raw materials. TABLE 1.2 Raw materials for making cement with their respective proportions in raw mix Category

Nature and proportion in raw mix (%)

Materials

Principal

Carbonates (75-90%)

Limestone, Chalk, marble, seashell and marl Carbonate sludge of paper, sugar and fertilizer industry waste

Additive (i)

Silica Source Alumino Silicates (5-20%)

Sand, sandstone, quartzite shale, fly ash, foundry sand Clay, soil, shale, phyllites, slate and volcanic rocks fly ash, pottery sludge

(ii)

Lime silicate (5-20%)

Wollastonitic rocks metallurgical slags and nepheline wastes of aluminum industry

Supplementary

Correctives (1-5%)

Sand and sandstones, bauxite, iron ore and laterite Red mud, foundry cinders, pyrite cinder from chemical industry, mill scale

The raw materials used in the manufacture of Portland cement must contain appropriate proportions of lime, silica, alumina and Iron oxide components. The indicative ranges of the different components are given in Table 1.3 TABLE 1.3 Range of chemical of raw mix for Portland cement clinker Chemical Composition

Ranges in percentage

CaO SiO2 Al2O3 Fe2O3 MgO

44-47% 10-14% 2-4% 1-3% 1-3%

During manufacture, analyses of all raw materials are made frequently to ensure consistency of composition of both the inputs and the output for getting the required high quality cement. The cement manufacturing process may appear to be very simple. However, there are a multitude of interlinked steps involved in the continuous process of large volume cement manufacture. In sequence, it starts with the mining of limestone, crushing and grinding of the

1.4

Handbook on Advanced Concrete Technology

limestone and raw materials to form a raw mix, handling and storage of ground raw mixes of different quality in different silos or compartments, extractions and proportioning of the raw meal to get the required kiln feed, storage and grinding of coal, its burning in the kiln with the kiln feed to get clinker, cooling of the clinker with heat recovery and recycling, storage and extraction and grinding of the clinker with gypsum only for OPC and with Pozzoloana or slag for blended cements, and finally, storage of different types of cement in silos. The manufacturing operations involve handling and conveying of large volumes of materials in each stage, major heat recovery from the kiln and its recycling for energy economy, process and operational control devices and mechanisms installed at each stage, equipment for control of dust emissions at each stage and auxiliary mechanical and electrical installations. The combination makes a large cement plant a giant industrial complex spread over tens of acres with a supporting limestone mine spread over hundreds of acres. There are three processes of cement manufacture. Wet Process: This was the first process to start with. In this process, the raw materials are ground in a wet ball mill containing 29% to 33% water. The water provides the medium for easy homogenization of the material. However, the wet raw meal (called slurry) entails a much higher fuel consumption for driving off the water. With the oil crisis cropping up in the 70’s the urge for fuel economy prompted cement plants throughout the world to switch over to Semi-Dry Processes, which had already made their appearance in the sixties. Semi-wet Process: This process is a modification of the Wet Process, wherein the wet slurry in an erstwhile process plant is filter-pressed to get a semi-wet material. Semi-Dry Process: In this process the raw materials are ground in a dry ball mill. The raw –meal powder thus produced is converted into nodules by simultaneously spraying water and the powder on to a rotating inclined pan wherein the nodules spill the pan into a chute for discharge into a raw-meal storage. Vertical Shaft Kiln (VSK) cement plants use this technology. Dry Process: This is the contemporary modern process and the only viable technology at present till a more advanced technology evolves. Cement plants across the country had undergone, over the last two decades, major technological upgradation and modernization, geared primarily towards productivity enhancement, optimization for process decongesting, energy conservation and pollution control. Use of varied types of wastes as substitute raw material or fuel is showing an upward trend, providing the twin advantages of cost reduction in manufacture, besides larger contribution to society in environmental protection, without compromising on product quality. Technologies for continuous commercial level cement manufacture: (a) Vertical Shaft Kiln (VSK) Plants – The plants based on this technology are of 10 to 200 tonnes per day (tpd) individual capacity and are called VSK mini cement plants. More than 300 such plants were set up in the country, mostly during the seventies and eighties. The objective of VSK plants was to meet local demand under acute cement scarcity through small-scale cement manufacture. The process did not permit significant modernization or technological upgradation.

Cement

1.5

(b) Mini Rotary Kiln Plants – Such plants are more than 50 of 200 to 600 tpd individual capacity in the country. Some among them have adopted limited modernization. They also include plants manufacturing white cement, which are of smaller capacity than large capacity plants or major cement plants. Plants of the above two categories contribute to only 7 percent of the total installed capacity of cement production in India. (c) Major Cement Plants – Range in individual capacity from more than 1200 tpd and reaching today upto 15,000tpd. These plants, numbering 125, constitute 93 percent of the total cement manufacturing capacity and 95 percent of the total cement production of the country. The customer, therefore, has overwhelming chances of dealing with a major plant as the cement supplier. Irrespective of the process followed, the main and successive steps in the manufacture of cement are the same. These are illustrated in Fig. 1.1(3) Mixing and conveying Mine

Blending bed

Gantrv

Blending of crushed lime stone

Storage of additives etc

Limestone mining

Proportioning of limestone, clay, ironoxide, etc, and conveying

Crusher

Coal mill

Limestone crushing

Grinding of coal

Kiln with PH & cooler Cement mill Grinding of clinker with gypsum and other additives

Cement silo

Clinker gantry Storage of clinker

Bag house packing house

Raw mill Burning of raw mix with coal or other fuel

Raw meal silo Blending & storage

Storage of cement Cement packing

Fig. 1.1 Flow-sheet of cement manufacturing process

Raw mix grinding

1.6

Handbook on Advanced Concrete Technology

A layman can comfortably understand the manufacturing process of cement by following the sequence given below. For a clear picture, the reader may refer to the items of Facilities and Equipments as given in Fig. 1.2. Sequence for Dry Process Plants (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)

Limestone mining and transport (1,2,3 and 4 in Fig. 1.2) Crushing of limestone (5 in Fig. 1.2) Preparation of stockpile of limestone and pre-homogenisation (6 in Fig. 1.2) Proportioning of limestone with additives/correctives (alumina, ironoxide, silica, etc.) Raw-mix grinding (7 in Fig. 1.2) Raw-meal storage-cum-blending in silo Preheater and Precalciner for preheating and calcinations of raw meal (8 in Fig. 1.2) Burning of raw meal along with coal or other fuels in rotary kiln (9 in Fig. 1.2) Cooler for cooling clinker obtained from kiln (10 in Fig. 1.2) Clinker storage (11 in Fig. 1.2) Hopper for feeding other additives to cement mill (12 in Fig. 1.2) Cement mill (13 in Fig. 1.2) for grinding of clinker (output of kiln) along with predetermined quantity of gypsum and other mineral additives (in case of blended cements)

(m) Storage of cement in silo before packing and dispatch (14 in Fig. 1.2) For simplification, Fig. 1.2 does not show location of bag house or electrostatic precipitators.

Mining of Limestone and its Transport Limestone or rocks of similar chemical composition (marble, chalk, marl and lime-Kankar, etc.) constitute the major raw material for cement making. About 1.5 tonnes of limestone is consumed to produce 1 tonne of cement clinker. A large plant of 1 to 1.5 million tones annual capacity requires about 1.75 to 2.5 million tones of limestone to be annually raised, transported to the plant and stocked. The limestone mine is therefore the lifeline of a plant Limestone mining is a major mining activity and mines are operated following the rules of Indian Mines and Minerals Act, Mines Safety Rules and provisions of the Environmental Protection Acts. Figure 1.2 shows a lime stone quarry (1), a rock drill (2) for making holes in the limestone bed for putting explosives for rock blasting and fragmentation. The blasted rock is hauled by mechanical shovels or excavators in to a dump truck (3) for carrying the material to the crusher.

Limestone Crushing The mined limestone containing mostly boulders and fragments of varying sizes is size-reduced in a primary crusher (5’n Fig. 1.2). The crusher may be located either in the quarry or in the plant.

1

2

25 Rotary kiln

24 Noduliser

3

4

23 Electrostatic precipitator

22 Raw meal storage

Semidry process 21 Raw mill, drier

20

5

ss

ce

pro

23

24

7

8

15

Dr

ce

f5m

ro yp

m

80

9

16

17

18

Rotary kiln

9

ss

Preheater, precalciner

8

10

Raw mill, drier

7

10 Cooler

Prehomogenisation

r tp

s es oc

1

m 50

19

14

f5m

19 Rotary kiln

18 Electrostatic precipitator

17 Slurry storage

16 Storage and homogenisation

WET RPOCESS 15 Slurry thinner

e W

11

Crusher

5 6

13

14 Cement silos, despatch

Feeder

4 12

13 Cement mill

Dumper

3

12 Additives

11 Clinker storage

Fig. 1.2 Schematic diagram of a modern cement plant

6

y -dr

mi

21

Se

22

MANUFACTURE OF CEMENT Facilities and equipment

1 Quarries (limestone, clay) 2 Rock drill

Cement 1.7

1.8

Handbook on Advanced Concrete Technology

Limestone Stockpile Preparation Both high-grade and low-grade crushed limestones are made into stockpiles of 10,000-15,000 tonnes by a device called stacker. For this purpose, ratios of different grades of limestone are predetermined such that extraction of the limestone in slices by a reclaimer will provide the required quality of the reclaimed limestone (6 in Fig. 1.2).

Raw-meal Blending and Storage The finely ground raw material called raw-meal is stored in silo (not shown in Fig. 1.2), wherein both blending of raw-meal of different qualities and storage in different segments of a silo or in different silos, are carried out so that the kiln feed composition is uniform for proper burning in the kiln.

Raw-meal Burning – Pyroprocessing In modern cement plants, the raw meal is preheated to a temperature of 900ºC to 1000ºC. in multi-stage preheaters (8 in Fig. 1.2). The material fed from the top of the preheater passes through stages of cyclones (4 to 6) stages and a precalciner (8 in Fig. 1.2) to the rotary kiln. The kiln (9 in Fig. 1.2) revolves at a speed of around 3 rpm. It is lined with refractory bricks and is slightly inclined away from the preheater to allow the heated raw materials to roll down slowly towards the outlet. Pulverized coal with the requisite amount of combustion air is pumped through burners from the other end. The preheated precalcined raw meal rolls down the kiln and gets heated to a level that, first the carbon dioxide from decarbonation of the limestone is driven off with the combustion gases; then further heating to 1300ºC to 1400ºC sinters the raw meal to form what is called “clinker”. Good quality clinker, generally, comprises small dark greenish blue nodules varying in size from 3 to 30mm. The temperature of clinker at the kiln outlet is nearly 1000ºC. the clinker is made to pass through a cooling arrangement called coolers with circulating cold air to take away the heat. The hot exhaust air from the cooler and the preheater is recycled into the kiln inlet or used for other purposes in the plant (like drying of raw materials, fly ash, slag, gypsum, etc.). The temperature of the clinker at the outlet of the cooler may drop to 95ºC. There are two types of coolers – old or wet process plants have planetary coolers (several cooling tubes fixed to the rotary kiln), while modern dry process plants have stationary grate coolers at the end of the kiln (10 in Fig. 1.2). The grate cooler contains several cooling chambers. As the clinker gets cooled, lumps are formed. Accordingly, a clinker crusher is provided at the end of the grate cooler to crush the clinker to the correct size, prior to grinding.

Clinker Storage In modern plants the clinker from the cooler is stored under a covered shed (11 in Fig. 1.2) or in the clinker gantry (a compartmented part of general covered storage area for all materials in process – Limestone, coal, gypsum, clinker, additives, etc. called Crane Gantry as is the practice with old plants). A common crane lifts the different materials and feeds to different hoppers in required proportions for further processing.

Cement

1.9

Storage of Additives In most plants additives e.g. gypsum, fly ash, slag etc. used for making cement from clinker, are stored separately (12 in Fig. 1.2).

Finish Grinding of Cement This is the final phase in cement manufacturing, wherein the clinker is ground with gypsum, may be natural gypsum or byproduct chemical gypsum from fertilizer or phosphorous industries, is added to the extent of 2 to 5 percent to the clinker in the grinding process. The clinker is then ground in closed-circuit ball mill (13 in Fig. 1.2) or in a combination of a pregrinder or Roll Press with Ball mill that ensure an uniform particle size distribution of cement particles in each batch of cement. A modern cement mill comprises high-pressure twin roller presses and ball mill in tandem with high efficiency separators.

Storage and Packing After grinding, the cement is collected in a hopper and taken to the cement silo through a bucket elevator. Modern cement plants have the facility of multi-compartment cement silo, which is divided into a number of compartments. It facilitates storage of a particular variety of cement in a specified compartment to avoid intermixing of different varieties of cement. The cement from the cement silo is taken to the packing plant. Most cement plants have electronically controlled rotating packing machines with 8 to 12 nozzles. The system has provision to account for the weight of empty bags of different types and ensure a 50-kg net weight of cement bags within +250 g limit. These bags are automatically discharged from the packer on to the conveyor belts to different loading areas. The cement plants now offer a variety of packing bags made out of paper, PP (Polypropylene), HDPE (High density Polyethylene). Grey cement is normally supplied in 50-kg bags but packing in 25-kg bags is also permitted by the Bureau of Indian Standards (BIS). White cement is available in smaller packs of 5 kg and 1 kg also in addition to the normal 50-kg bags.

1.4 PHYSICAL PROPERTIES OF CEMENT Following are the physical properties of cement which determine its quality and fitness for use. • Fineness • Setting time • Soundness • Compressive strength • Fineness: The quality of a cement depends to a great extent on the fineness of grinding. Finely ground cement has a higher initial strength, as it hydrates faster than coarsely ground cement. Fineness is normally expressed in blaine (m2 /kg). It means that if one kg of cement is taken, the sum total of the surface area of all its particles will be so many m2, which is in SI units. For Indian cements, normally it varies between 225

1.10

Handbook on Advanced Concrete Technology

and 325 m2 /kg. Sometimes, it is also expressed in CGS system (cm2 /g), in which case this value is multiplied by 10 and ranges accordingly between 2250 and 3250cm2 /g. Fineness of cement is determined by air permeability (Blaine apparatus) method or sieve analysis. Air permeability or Blaine method: In this method, the air permeability of a bed of cement is measured from the time taken for a certain quantity of air to flow through the cement bed under specified conditions. The specific surface of the cement in cm2 /g is then calculated from the air permeability. This method is mostly used in the cement industry. • Setting time: Cement sets and gives concrete sufficient strength within a reasonable time. Obviously, in order to allow sufficient time for applying the mortar or placing the concrete, cement must not set too quickly. It is equally important that after mixing and placing, setting should be complete within a reasonable period. Accordingly, two terms – initial and final set – are used to describe setting time. Broadly speaking setting refers to a change from a fluid to a rigid state. Setting should be clearly distinguished from hardening, which refers to the gain in strength of a set cement paste. Initial setting time: This is the time during which the cement paste remains in plastic condition and can be moulded into any shape. Therefore, mixing transportation, placing and compaction shall be completed within initial setting time. Concrete should not be disturbed after its initial set. During initial setting, rapid rise in temperature takes place, attaining a peak during the final set. The setting time of cement decreases with rise in ambient temperature. Normally, initial setting time is kept 100-140 minutes against the minimum IS requirement of 30 minutes (min). The higher value is meant to allow for manual operations and the higher ambient temperatures prevailing in most parts of India. Final setting time: This is the duration after which the cement paste loses its plasticity and becomes rigid. It is during this time that the peak temperature of the paste is reached. The cement paste/concrete starts hardening (gaining strength) after the lapse of the final setting time. The final setting is normally kept between 200-300 minutes against the requirement of 600 minutes (max) as per IS. • Soundness: Soundness refers to volume (dimensional) stability of hardened cement. It is necessary that a cement paste, once set, should not undergo any volume change. In particular there should be no expansion, otherwise under restraint conditions, cracks may develop in the hardened paste. The expansion may take place due to delayed or slow hydration on account of the presence of free lime, magnesia and/or calcium sulphate in the cement. Free lime is the CaO in excess of what can chemically combine with other oxides in the kiln. Lime externally added to cement does not produce unsoundness because it hydrates rapidly, whereas free lime in clinker (in inter-crystallised form) hydrates slowly and causes unsoundness. In good quality cement, free lime should not exceed 1 to 1.5 percent. The soundness of cement is determined by either Le Chatelier or autoclave test.

Cement

1.11

Le Chatelier test: Determines the expansion of cement paste owing to presence of free (uncombined) lime using the Le Chatelier apparatus. The apparatus measures length-wise expansion of hardened cement. As per IS, the maximum limit of expansion is 10 mm. However, for most good quality cements, it varies between 1 and 2 mm. This test detects unsoundness due to presence of free lime only. Autoclave test: A cement can be unsound also due to the presence of magnesia (MgO). MgO in crystalling form (Periclase) reacts with water (hydrates) very slowly. It expands after hardening of cement paste and leads to crack formation. Therefore, an accelerated test (autoclave- heating under pressure) is done, which is sensitive to both free lime and free magnesia. The permissible limit (max) as per IS for expansion in autoclave test is 0.8 percent, while most good quality cements give a value of 0.1 to 0.2 percent (max). • Compressive strength: Compressive strength is the most important property of cement. Depending upon its class and type, a cement should attain the compressive strength stipulated in the respective cement standard at 3, 7 and 28 days. Owing to the difference between the test methods, different cement standards are not entirely comparable. In general, a lower water-cement ratio and a higher proportion of cement result in higher strength. Strength tests are not made on neat cement paste, but on 1:3 cement-sand mortar using standard sand (Ennore sand as per IS Specifications) There are several forms of strength test: direct tension, direct compression and flexure. As per IS, direct compression method is used and cement-sand mortars are tested for 3, 7 and 28 day Strength as per standard testing procedure stipulated in IS specifications. In cement-mortar cube testing, the weight of water is determined by normal consistency test, 70.7mm cubes are made and compacted using a vibrating table and the cubes are demoulded after 24 hours (temp. maintained at 27+2ºC and relative humidity at 90 percent) and further cured in water until tested. The cubes are tested at 3, 7 and 28 days, because 3-day strength is nearly half of 28-days strength and 7-day strength is nearly two-third of 28-day strength. The compressive strength is expressed as mega Pascals (MPa) or Newtons per mm2 (N/mm2). Both units are the same. In case it is expressed as kg/cm2 then, values shall be multiplied by 10 (33 MPa = 330 kg/cm2).

1.5 CHEMICAL PROPERTIES OF CEMENT The various chemical characteristics, which influence the quality of cement are: Lime saturation factor (LSF) Insoluble residue (IR) Magnesia (MgO) Sulphuric anhydride (SO3) Loss on ignition (LOI)

1.12

Handbook on Advanced Concrete Technology

Alkalis (as Na2O equivalent) Chlorides (Cl)

Lime Saturation Factor (LSF) LSF is an indicator of the degree of fixation of lime in cement, which in turn denotes the levels of reactivity of the product. LSF is the ratio of the percentage of lime to the sum of the percentages of silica, alumina and iron oxide. This factor should not be greater than 1.02 and shall not be less than 0.66. If this factor is higher towards 1.0, it means the cement has higher tricalcim silicate (C3S). This will not only make the cement to gain early strength but also liberate higher heat of hydration. On the other hand, if the factor is less, say 0.70, the cement has higher dicalcium silicate (C2S), which makes the cement to gain strength slowly. A higher LSF value also denotes higher burnability of clinker, needing higher temperature of burning. Besides, it leads to producing a higher percentage of free lime in clinker. In Indian cement plants, this value is generally kept between 0.85 to 0.95.

Insoluble Residue (IR) All the four compound of cement (C3S, C2S, C3A and C4AF), MgO and CaSO4 are soluble in hydrochloric acid. But free silica (quartz), impurities in gypsum or some types of adulterants are insoluble. BIS restricts the maximum limit of these insolubles at 3 percent in OPC to check any possibility of adulteration. In case performance improver is added, then the permissible limit is 5 percent. In case of PPC, the IR will depend upon the content of fly ash or calcined clay in the cement. Therefore, any value of insoluble residue higher than the prescribed values as per Appendix II indicates adulteration in OPC. However, this is different for blended cements and is determined by a separate formula.

Magnesia (MgO) Excess MgO, present in the form of a synthetic mineral phase periclase, hydrates very slowly and the product of hydration expands, causing, expansion cracks in the concrete. Therefore BIS limits the amount of MgO in cement as per Appendix II.

Sulphuric Anhydride (SO3) Gypsum is added to clinker at the time of grinding to retard the flash setting effect of C3A. It is, therefore, termed as “set retarder” and serves to provide the required workability of cement paste before its application. The quantum of gypsum added depends on the amount of C3A present in clinker and the duration of retardation desired after mixing with water. The gypsum added to the cement is present in the form of sulphuric anhydride. Most of it is consumed during hydration but excess, if any cause expansion in hardened concrete. Therefore, BIS prescribe upper limits as per Appendix-II.

Loss on Ignition (LOI) Loss on ignition of cement is determined by heating a cement sample to 900-1000ºC, until a constant weight is obtained. Normally a high loss on ignition is an indication of pre-hydration

Cement

1.13

and carbonation, which may be caused by prolonged and improper storage or adulteration during transportation. BIS specifies a 4 to 5 percent upper limit for loss on ignition.

Alkalis (as Na2O Equivalent) Alkalis in cement come from raw materials. These are expressed Na2O equivalent and BIS has laid down a limit of 0.6 percent alkalis for low alkalis OPC. A higher percentage of alkali in cement may cause alkali aggregate reaction in concrete, if reactive silica is present in the aggregate being used.

Chlorides (Cl) Chlorides in cement again owe to raw materials used in manufacture. Excess presence of chlorides results in corrosion of reinforcement in RCC. The latest IS 456-2000 Code of Practice for Plain and Reinforced concrete limits the total acid-soluble chloride content in concrete to 0.6 kg/m3 for RCC and 0.4 kg/m3 for prestressed concrete. As per BIS, the permissible limit of chloride in cement is 0.1 percent for normal concrete and 0.05 percent for prestressed concrete. In addition to physical and chemical properties of cement, Heat of Hydration and colour of cement are also sometimes considered by the consumer. These are briefly described below:

1.6

HEAT OF HYDRATION

All cements liberate heat to varying extent during hardening, termed as heat of hydration. Ordinary Portland Cement, during the first four weeks of hardening, liberates nearly total heat of hydration to the extent of about 80-90cal/g. Where great masses of concrete are involved, as in large hydraulic structures (dams, barrages, reservoirs etc.) there is always a risk of thermal expansion and eventual cracks arising from the thermal stresses. In blended cements (PPC, PSC), rate of heat liberation is comparatively lower than Ordinary Portland Cement, to the extent of about 50-60 cal/g. Therefore, for mass concreting works, blended cements would be a preferred choice.

1.7

COLOUR OF CEMENT

Colour of cement mainly depends upon the nature of the raw materials used in the production of cement. Colour of cement has no influence on its physical and chemical properties.

1.8 VARIETY OF CEMENT AND THEIR APPLICATIONS Indian cement industry is at present producing mainly six main varieties of cement like Ordinary Portland Cement (OPC), Portland Pozzoloana Cement (PPC), Portland Blastfurance Slag Cement (PSC), Sulphate Resisting Cement (SRC), Oil Well Cement and White Cement. Under OPC again it is producing high compressive strength grades, e.g., “43” (43MPa* at 28 days) and “53” (53 MPa* at 28 days), to meet special requirements like prestressed concrete, precast products, besides 53S-43S (earlier IRS-T 40) for railway-sleepers.

1.14

Handbook on Advanced Concrete Technology

Indian Standard Specifications for Cements Cements produced in India should compulsorily conform to Indian Standard specifications (Table 1.4) issued by the Bureau of Indian Standards (BIS), the national body for the formulation and implementation of Indian Standards (IS). It is a statutory requirement that each cement bag must conform to BIS specifications for the type and grade printed on it. TABLE 1.4 Indian Standard Specification on Cements(4) Name of Cement Ordinary Portland Cement – 33 Grade High Strength Ordinary Portland Cement • 43 Grade • 53 Grade • 43-S Grade • 53- S Grade Portland Pozzolana Cement – Part – I Fly ash base Portland Pozzolana Cement – Part – II Calcined clay base Portland Blast Furnace Slag cement High Alumina Cement for structural use Rapid Hardening Portland Cement Oil Well Cement Sulphate Resisting Portland Cement Low Heat Portland Cement White Portland Cement Super Sulphated Cement Hydrophobic Cement Masonry Cement

IS No. IS: 269 – 1989 IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS:

8112 – 1989 12269 – 1987 8112 – 1989 (as amended) 12269 – 1987 (as amended) 1489 (Part I) – 1991 1489 (part II) – 1991 455 – 1989 6452 – 1989 8041 – 1990 8229 – 1986 12330-1988 12600 – 1989 8042 – 1989 6909 – 1990 8043 – 1991 3466 – 1988

The physical and chemical characteristics of the above cements are summarized in Appendix I and Appendix II. respectively.

Application of Different Types of Cements The selection of cement for any application is to be made out of the aforementioned types (Table 1.4) of cements only. Some of the varieties, such as OPC, PPC, PSC, SRC and white cement are readily available in the market. The rest have to be obtained on specific order from cement manufacturers. In all types of construction the architect and the builder or the contractor decide upon the type of cement to be used in the different parts of the building. However, the customer or the owner should satisfy himself that the right type and quantity of cement has been used in each stage. Following are a few guideline for the consumer. Design parameters, type of construction, ground conditions, durability requirements and environmental conditions are some of the factors which dictate the selection of appropriate cement for an application. The major factors affecting the choice of cement are as follows –

Cement

1.15

I. Functional requirements of the structure – its load carrying capacity, reversal of stresses, shrinkage, creep and deflection etc. II. Ground condition – Type of soil quality of ground water, alternate wetting & drying, ground water table and swampy lands etc. III. Environmental and exposure conditions – Rainfall, sulphate & chloride attack, chemical fumes and moisture etc. IV. Speed and method of construction – Precast, pre stressed components, slip form, repairs and rehabilitation needs. V. Durability requirements, service life, corrosion, cracks and deterioration etc.

‘33’ Grade Ordinary Portland Cement (OPC – 33) Used for general low-rise civil construction works under normal environmental conditions. The compressive strength of this cement after 28 days, when tested as per IS: 4031-1988 is 33 N/mm2. Due to its low compressive strength, this cement is generally not used where higher grades of concrete, say M-20 and above is required. The use of “33” grade cement has virtually come to an end in the country and its place has been taken by higher grades of OPC and blended cements, such as Portland Pozzoloana Cement and blast furnace slag cement.

‘43’ Grade Ordinary Portland Cement (OPC – 43) It is mostly used for general civil construction works including residential, commercial and industrial buildings, road, bridge, flyovers and irrigation projects in most parts of the country. Its minimum compressive strength at 28 days shall be 43 N/mm2. It is readily available in the market under different brand names and accounts for nearly 15 percent of the cement produced in India today. Its market share is continuously coming down and the share of blended cements (PPC, PSC) is rising instead.

‘53’ Grade Ordinary Portland Cement (OPC – 53) A high-strength OPC, used for high-rise buildings, bridges, flyovers, chimneys, and prestressed concrete structures where high grade concrete (>M30) is normally required. Advent of this grade of cement in the country owes to the improved technology at modern cement plants, which enable production of high quality clinker. This grade was introduced in the country in 1987 by BIS and commercial production started from 1991. ‘53’ grade cement is a high-strength OPC, which provides numerous advantages wherever concrete say of M30 or higher grade is required. ‘53’ grade cement is becoming popular in construction industry and progressively its market share is increasing. It is available on regular basis in the market under different brand names. It accounts for approximately 15 percent of the total production of cement in the country.

Portland Pozzolana Cement (PPC) Useful for general construction works and especially suitable for works in aggressive environmental conditions, employed for water retaining structures, marine works, mass concreting, such as dams, dykes, retaining walls, foundations, and sewage pipes.

1.16

Handbook on Advanced Concrete Technology

Portland Pozzolana Cement utilizes two types of pozzolans: (a) Fly ash – Covered under IS 1489 (part – I) (b) Calcined clay – Covered under IS 1489 (part – II) PPC with fly ash, as Pozzolana, is more popular in the country owing to the easy availability of quality fly ash from modern coal-based thermal power plants. PPC provides improved workability, has less segregation and bleeding, increased water-tightness and reduced tendency of lime to leach out. It produces less heat of hydration and offers greater resistance to the attack of aggressive waters containing sulphates and chlorides. It improves the durability of structure, and also the strength of concrete over a period of time. It protects concrete against alkali-aggregate reaction. The minimum compressive strength of PPC after 28 days, as prescribed by BIS, is 33 N/mm2. However, in the market, PPCs of much higher strength are available. The proposal of introducing higher grade in PPC, on similar lines of OPC, is under active consideration of BIS. The fly ash in PPC helps to improve the impermeability of concrete by converting Ca(OH)2 into calcium silicate hydrate (C-S-H) gels and make concrete denser. However, this pre supposes use of good quality fly ash conforming to IS: 3812-2003. PPC can be used with advantage for masonry mortars and plastering. Indeed, it can be used wherever OPC is usable under normal conditions, but the development of strength at initial stages, say up to 7 days, is lower in the case of PPC as compared to OPC. However, at later stages, beyond 28 days, PPC not only catches up with OPC in strength but also overtakes it. As per the revised code IS:456:2000, a minimum curing period of 10 days is prescribed for PPC and for OPC it is 7 days in dry and hot climates. For improved performance, concrete with PPC calls for sustainable curing. Portland Pozzolana cement is manufactured either by inter-grinding Portland cement clinker, gypsum and fly ash obtained from thermal power plants, or by intimately blending OPC and fly ash. The percentage of fly ash in PPC permitted by IS:1489 varies from 15 percent (minimum) to 35 percent (maximum). The fly ash to be used for manufacturing of PPC should confirm to IS: 3812 (Part I)-2003. PPC accounts for nearly 63 percent of the cement produced in the country. Requires longer curing, ideally 10 days, compared to 7 days for OPC.

Portland Slag Cement (PSC) Used for general civil engineering construction but mainly preferred for construction of marine structures and in coastal areas where excessive amounts of chloride and sulphate salts are simultaneously present. Also can be used with advantage for mass concrete works, thanks to its low heat of hydration. PSC also reduces alkali-aggregate reaction. It is equally suitable for general construction works and can be effectively used wherever OPC or PPC is used. But the curing period for slag cement is longer compared to OPC, a minimum of 10 days is recommended by IS: 456-2000. The slag in PSC has the ability to combine with lime to form stable compounds, which help prevent leaching of lime as efflorescence wherever water seepage takes place.

Cement

1.17

Portland slag cement is manufactured by inter-grinding Portland cement clinker, gypsum and granulated slag obtained from steel and ferro alloys industries. PSC also manufactured by blending OPC with Ground Granulated Blastfurnace Slag (GGBS) through mechanical blenders. The slag is non-metallic product consisting essentially of glass-containing silicates and alumino-silicates of lime and other bases. Granulated slag obtained by rapidly chilling or quenching the molten slag with water or steam and air. The granulated slag used for manufacture of slag cement should conform to IS: 12089-1987. The slag constituent shall not be less than 25 percent and not more than 70 percent in PSC as per IS: 455 – 1989 (as amended). The compressive strength of PSC is equivalent to that of “33” grade OPC. However, slag cements of strength equivalent to “43” and “53” grade OPC are available in the market. The use of slag cement is very popular in the eastern parts of the country due to the easy availability of slag from steel plants. PSC accounts for approximately 7 to 8 percent of the total cement production in the country. Requires longer curing, ideally 10 days, compared to 7 days for OPC.

Masonry Cement Used for making mortar for brickwork and plastering. It has low compressive strength, only 5 MPa when tested as per IS: 4031 (part – 7). It contains air-entraining agents and other mineral admixtures, which improve water retentivity (retention capacity), plasticity and workability of mortars. Because of its property of producing smooth, plastic, cohesive, strong yet workable mortar when mixed with fine aggregates, masonry cement is considered superior to lime mortar, lime-cement mortar or straight cement mortar. The cement is very useful for plaster and masonry works but is not yet popular in construction industry in the country, for the fear that it may inadvertently be used for structural works. In fact it is not available in the market on regular basis. But some cement manufactures are considering to market this cement after proper education of the consumers. Masonry cement is very popular in America, Europe and many South-East Asian countries. In India, it is yet to get due recognition owing to its susceptibility to adulteration by unscrupulous traders and lack of consumer awareness. Not to be used in structural concrete, flooring and foundation work, or for reinforced and prestressed concrete work.

Low Heat Portland Cement (LHC) Used for making concrete for dams and other water retaining structures, bridges, abutments, massive retaining walls, piers and slabs, etc. In mass concreting, there is often considerable rise in temperature from the heat of hydration of the cement with the resultant expansion, and the slow rate at which it is dissipated from the surface. The shrinkage, which takes place on subsequent cooling, sets up tensile stresses in the concrete which may result in cracking. The use of low heat cement is advantageous since it

1.18

Handbook on Advanced Concrete Technology

evolves less heat than OPC, 70 cal/g as against normally 90 cal/g for OPC. This cement is not available in the market on regular basis and has to be obtained on specific order.

Rapid Hardening Cement (RHC) Used for repair and rehabilitation works and where speed of construction and early completion is required due to limitation of time, space or other reasons. Rapid hardening cement is basically OPC but its minimum fineness is 3250 cm2 /g against the minimum of 2250 cm2 /g for normal OPC. Among its special properties are development of high early strength even in cold weather and continued strength gain with time, besides minimum expansion/shrinkage during hardening. Its compressive strength at 1 day is 16 N/mm2 and at 3 days 27 N/mm2. Since higher grade OPC, say “43” and “53”, nowadays freely available in the market to meet these requirements, RHC is not being manufactured and marketed on regular basis. The characteristics of this cement can be effectively met with by any good quality “53” grade OPC available in the market.

White Portland Cement (WPC) Meant for generally nonstructural and mostly decorative use, normally used for flooring, general architectural purposes, such as mosaic tiles, terrazzo, decorative concrete wall paintings, and special effects. White cement has also the advantage that it is not liable to undergo staining, since it has low content of soluble alkalis. White Portland cement is made from raw materials containing very little iron oxide and manganese oxide. Limited quantities of certain chemicals, which improve whiteness of the cement, are added during manufacture. The compressive strength of white cement shall not be less than 90 percent of that of “33” grade OPC and the reflectance of neat cement ring shall not be less than 70 percent as per IS specification. This cement is available in 50-kg, 5-kg and 1-kg packs in the market. Very good quality white cement is now available in the market and its compressive strength is equivalent to that of “53” grade OPC and Whiteness more than 90 percent.

Sulphate Resisting Cement (SRC) Used for underground structures in sulphate-salts abounding environment, effluent treatment plants, sugar and other chemical industries, where civil works are likely to be subjected to sulphate attack. The sulphate salts, present in soil or water, react with tricalcium aluminate hydrate and form tricalcium sulphoaluminate. This compound has a volume more than twice the original volume of C3A hydrate and this induces stresses in concrete leading to cracks and disruption of concrete. Being a specially formulated cement, with lower C3A content ( > > < < < <
> > < < < <
320 < 34

> 4.5 > 80% of corresponding plain cement mortar 0.8

2.4

Handbook on Advanced Concrete Technology

The quality and performance of fly ash can vary even if sourced from the same source as the power station may be using different coals from time to time. Therefore, once a source is validated and selected, one should request a test certificate from the vendor stating both Loss on Ignition and fineness (% residue on a 45µm sieve) for each delivery. The use of fly ash in concrete can be facilitated in two ways; through the use of a pre-blended Portland Pozzolana Cement (PPC) or separately mixed with OPC at the batching plant.

2.3.3 Pozzolanic Reaction of Fly Ash When fly ash is added to concrete the pozzolanic reaction occurs between silica glass (SiO2) and calcium hydroxide (Ca(OH)2) or lime which is a by-product of the hydration of Portland cement. The hydration products fill the interstitial pores reducing the permeability of the matrix. The reaction products formed differ from the products found in Portland cement-only concretes. A much finer pore structure is developed with time presuming there is access to moisture to maintain the hydration process. With a continuing availability of moisture, the lime reacts with fly ash and produces additional hydration products which help in refining the pore-structure. The pozzolanic reaction is as shown below: Calcium hydroxide + silica = Calcium silicate hydrate + water Ca(OH)2 + SiO2 = C-S-H + H2O

Relative strength-for equal 28-day strength

The above reaction is also called as secondary hydration reaction. This considerably improves the long term strength of concrete as compared to normal concrete containing only OPC. It is to facilitate the secondary hydration that prolonged curing, atleast 14 days is recommended for concrete made with fly ash. This effect is shown in Fig. 2.3. 1.4 70% OPC + 30% PFA 1.2 1.0 100% OPC

0.8

Test cubes cured at 20°C in water

0.6

Mixes designed to give equal 28-day cube strengths

0.4 0.2 0.0

0

7

14

Fig. 2.3

21

28

35

42 49 56 Age (days)

63

70

77

Effect of fly ash on strength development

84

91

Mineral Admixtures

2.5

2.3.4 Heat of Hydration The hydration of Portland cement compounds is exothermic, typically with 335 Joules of heat per gram being liberated. When cement is replaced with fly ash in concrete it reduces the temperature rise during hydration period. The peak temperatures in fly ash concrete are lower than equivalent PC concretes. Fly ash is able to reduce the heat of hydration when used as part replacement of cement as the heat of hydration of fly ash is approximately 50% of cement. The effect is a shown in Fig. 2.4. 40

104

35

95

30

86

77 B C

20

68

59

15 A Plain concrete 10

Temperature rise (°F)

Temperature rise (°C)

A 25

50

B 25% fly ash C 30% fly ash

5

41

0

32 6

12

24

36 48 60 72 96 Time since placing-hours

120 144 168

Fig. 2.4 Variation of temperature with time at the center of 15 m3 concrete blocks

2.3.5 Setting time and Formwork Striking times Using fly ash in concrete will increase the setting time compared with an equivalent grade of OPC concrete. When 30 per cent fly ash is used to replace OPC in a mix, the setting time may be increased by up to 2 hours. This increased setting time reduces the rate of workability loss. However, it may result in finishing difficulties in periods of low temperature. In compensation, it will reduce the incidence of cold joints in the plastic concrete. There is a period before the reaction of fly ash in concrete commences (usually the effect is seen after 4 to 7 days) that the gain in strength is slow, but once the reaction has started, the long term strength gain is greater for fly ash concrete. Formwork striking times at lower ambient temperatures may have to be extended in comparison to OPC concrete, especially with thin sections. For soffit formwork, greater care shall be taken.

2.6

Handbook on Advanced Concrete Technology

4.3.6 Curing Hydration reactions between cement and water provide the mechanism for the hardening of concrete. The degree of hydration dictates strength development and other aspects of durability. If concrete is allowed to dry out, hydration will cease prematurely. Fly ash concrete has slower hydration rates and the lack of adequate curing will affect the final product. Thicker sections are less vulnerable than thin concrete sections because heat of hydration will promote the pozzolanic reaction. Usually 10 to 14 days of curing is required for concrete with fly ash depending upon climatic conditions.

4.3.7 Concrete Durability and Fly Ash The durability of concrete is greatly affected by the penetration of gases and fluids into the concrete. The degree of penetration depends on the permeability of the concrete, and since permeability is a flow property it relates to the ease with which a fluid or gas passes through it under the action of a pressure differential. Due to the secondary hydration of fly ash in concrete, eventually the matrix becomes denser and has less voids and inter-capillary pores. This reduces the permeability of the concrete drastically as shown in Fig. 2.5.

2

Log scale-diffusion coefficient (D) cm /s ¥ 10

9

Estimate of chloride diffusion coefficient 100

25 MPa concrete

10

35 MPa concrete 1 50 MPa concrete 70 MPa concrete 60 MPa concrete 0.1 0

10

20

30 % PFA content

40

50

60

Fig. 2.5 Rate of permeability of fly ash concrete

The beneficial properties of using fly ash for sulfate resistance, chloride penetration, permeability, alkali-silica reactivity have been discussed in various chapters of the book.

2.3.8 Carbonation of Concrete As fly ash reacts with calcium hydroxide, the pH of concrete is reduced. However, fly ash also reduces the permeability of concrete appreciably when the concrete is properly designed and cured. When designing concretes for equal 28-day strength the slow reaction rate of fly ash

Mineral Admixtures

2.7

usually means that the total cementitious material is often increased. This increase partially compensates for the reduction in available calcium hydroxide. Also the permeability of the concrete reduces thereby not allowing carbon dioxide to penetrate into the concrete. This leads to the result that the carbonation of fly ash concrete is not significantly different from Portland cement for the concrete of the same grade. The ACI Manual of Concrete Practice (1994) confirms this view: ‘Despite the concerns that the pozzolanic action of fly ash reduces the pH of concrete, researchers have found that an alkaline environment very similar to that in concrete without fly ash remains to preserve the passivity of the steel’. Carbonation is a complex function of permeability and available lime. With properly designed, cured and compacted fly ash concrete, carbonation is not significantly different from other types of concrete. With extended curing and the low heat of hydration properties of fly ash concrete, the resulting low permeability may more than compensate for the reduced lime contents.

2.4

GGBS IN CONCRETE

Blast furnace slag is produced as a by-product during the manufacture of iron in a blast furnace. It results from the fusion of a limestone flux with ash from coke and the siliceous and aluminous residue remaining after the reduction and separation of the iron from the ore. The slag is rapidly cooled with water to form a glassy disordered structure. If the slag is allowed to cool too slowly this allows a crystalline well-ordered structure to form which is stable and non-reactive.

2.4.1

Chemical Composition of GGBS

The chemical composition of slag will vary depending on the source of the raw materials and the blast furnace conditions. The major oxides exist within the slag glass (formed as the result of rapid cooling) in the form of a network of calcium, silicon, aluminium and magnesium ions in disordered combination with oxygen. The oxide composition of ggbs is shown in Table 2.2 where it is compared with that of Portland cement. TABLE 2.2 Typical oxide composition of Portland cement and GGBS Oxide Composition (%)

Portland cement

GGBS

CaO SiO2 Al2O3 Fe2O3 MgO SO3 K2O

64 21 6.0 3.0 1.5 2.0 0.8

40 36 10 0.5 8.0 0.2 0.7

Na2O

0.5

0.4

The use of GGBS in concrete can be facilitated in two ways; through the use of a pre-blended or interground Portland Slag Cement (PSC) or separately mixed with OPC at the batching plant.

2.8

Handbook on Advanced Concrete Technology

When using a PSC, one should request the manufacturer to state the percentage of GGBS in the PSC as this can vary widely (permissible range – 25% - 70% as per IS 455-1989), but generally kept close to 40%. To know the percentage of GGBS in PSC is important when one designs concrete for durability in specific applications like presence of sulphate and chloride bearing salts in soil or water. There is no Indian code available for the use of GGBS as a mineral admixture in concrete. However, a specification for granulated slag for the manufacture of Portland Slag Cement (PSC) is available (IS 12089: 1987), which only deals with the chemical composition of the slag, not the physical requirements. To overcome this, one can use the British Standard (BS EN 15167-1:2006). GGBS for use in concrete should conform to the parameters shown in Table 2.3. TABLE 2.3 Properties of GGBS for use in concrete Property

Standard

Requirement

Glass Content Blaine’s Fineness Compressive Strength

IS 12089:1987 BS EN 15167-1:2006 BS EN 15167-1:2006

> 85% > 275 m2/kg 7-days: > 12.0 MPa 28-days: > 32.5 MPa

Initial Setting time

BS EN 15167-1:2006

Not less than OPC

Soundness (Le Chatelier)

BS EN 15167-1:2006

< 10 mm

2.4.2

Chemical Reactions

In the presence of water, ggbs will react very slowly but this reaction is so slow that on its own, slag is of little practical use as cementitious material. Essentially the hydraulic properties of slag are locked within its glassy structure and in order to release this reactivity some form of ‘activation’ is required. The activators, which are commonly sulphates and/or alkalis, react chemically with ggbs, and increase the pH of the system. Once a critical pH has been reached the glassy structure of the slag is ‘disturbed’, the reactivity is released and the slag will begin to react with water producing its own cementitious gels. In practice activation is achieved by blending ggbs with Portland cement as the latter contains both alkalis (Ca(OH)2, NaOH and KOH) and sulphates. The chemical reactions that occur between the Portland cement, water and ggbs are very complex and are summarized below. OPC + water Æ C-S-H + Ca(OH)2 NaOH KOH GGBS + water + Ca(OH)2 Æ C-(N,K)-S-H NaOH KOH

Mineral Admixtures

2.9

Reaction Rate The rate of chemical reactions is influenced by several factors including: • The quantity and relative proportions of the two components: the lower the proportion of cement, the slower the reaction • Temperature: just as with Portland cement the rate of the reaction increases with an increase in temperature and vice versa • The properties of the two components, in particular chemical composition and fineness

2.4.3 Properties of Concrete in Plastic State Made with Slag Cements Water demand/workability: Unlike fly ash, use of slag does not reduce the water demand significantly. For concrete made with equal slump, the reductions are small and are no more than about 3 per cent. This reduction is related largely to the smoother surface texture of the slag particles and to the delay in the chemical reaction. Stiffening times: As GGSB will react slower with water than Portland cement there will be an increase in the stiffening time of concrete. The extension in stiffening time will be greater at high replacement levels (above about 50 per cent) and at lower temperatures (below about 10°C). Heat of hydration and early age thermal cracking: The rate of heat evolution associated with ggbs is reduced as the proportion of slag is increased. This is of benefit enabling concrete to be poured in higher volumes and reduced temperature rise reduces the likelihood of thermal cracking. The actual temperature reductions that can be achieved in practice depend on many factors including: section size, cement content, proportion of slag, and chemical composition and fineness of the cementitious components. Figure 2.6 shows typical temperature–time profiles for concretes with and without slag and Fig. 2.7 shows the influence on temperature rise of different slag additions at different lift heights.

2.4.4 Properties of Concrete in Hardened State Made with Slag Cements Compressive strength and strength development: Since rate of hydration of ggbs is slower than Portland cement, the early rate of strength development of slag concretes is slower, the higher the slag content the slower the strength development. However, provided adequate moisture is available, the long-term strength of the slag concretes will be higher. This higher later age strength is due in part to the prolonged hydration reaction of the slag cements and to the denser hydrate structure that is formed as a result of the slower hydration reaction. As with Portland cements the rate of the hydration reaction of slag cements is temperature dependent but slag cements have higher activation energy than Portland cements and therefore their reaction rates are more sensitive to temperature change. The influence of temperature on strength development is of significance when considering the behaviour of concrete in situ and there could be a significant difference between actual strength in the structure and that indicated by cubes cured under standard conditions. In thick sections or cement-rich concrete the early temperature rise may well be in excess of 50°C. The effect of this is to accelerate

Handbook on Advanced Concrete Technology

3

0% slag 100% OPC

Peak II

30% slag 70% OPC

Rate of heat output (W/kg)

50% slag 50% OPC 70% slag 30% OPC

Peak S

2

Total heat output (KJ/Hg)

2.10

300 250 200 150 100 50 0 0

10

20

30 40 50 Time (hour)

60

70

1

0 0

10

20

30

40

50

60

70

Time (hour)

Fig. 2.6 Calorimetric curves for OPC and slag cement with 30, 50 and 70% replacement level, cured at 20°C

% of temperature rise in PC concrete

100

Lift height (m)

90

3 80

2 1.5

70

1

60

50

Fig. 2.7

10

20

30 40 50 Replacement level (%)

60

70

80

Influence of ggbs on temperature rise for various lift heights

Mineral Admixtures

2.11

the early strength gain and to impair long-term development for concretes with and without ggbs. However, the amount by which the long-term strength gain is reduced is lower for slag cements than for Portland cements. Conversely in thin sections cast in cold weather conditions the slower rate of gain of strength may lead to some extension in formwork striking times. Surface finish: The slight improvement in workability and small increase in paste volume of ggbs concretes generally makes it easier to achieve a good surface finish. In addition the colour of the concrete will be lighter than that of Portland cement concretes. Formwork pressures: As a result of the extended setting times mentioned earlier the use of slag in concrete will lead to an increase in formwork pressures. The increase in pressure will be particularly pronounced in high lifts (> 4 m) cast at low temperatures (< 5°C) and at low placing rates (< 0.5 m/h). Formwork striking times: The slower rate of gain of early strength of concretes containing high levels of ggbs may require slight extension of formwork striking times. In practice, however, the actual construction process often requires concrete to be cast one day and vertical formwork struck the next. In such cases it is quite likely that the minimum striking times will in any case be extended and that therefore the use of slag may not affect the actual construction process. Also certain admixtures which can accelerate the strength gain can be used in order to achieve the desired striking times. Curing: In situations where durability is recognized as a potential problem then it may be expected that concretes containing slag will require longer curing periods particularly in hot weather conditions and where thin sections are involved.

2.4.5 Durability of Concrete with GGBS The durability of concrete is primarily related to its permeability/ diffusion to liquids and gases and its resistance to penetration by ions such as sulfates and chlorides. In general well-cured slag concretes are more durable than similar Portland cement concretes as the permeability of slag concrete is much lower as compared to normal OPC concrete due to the secondary hydration between the calcium hydroxide and GGBS. The relationship of porosity in concrete with GGBS at different ages under the influence of varying temperatures is as given in Table 2.4 below: TABLE 2.4 Influence of temperature and cement type on pore structure of cement pastes Composition (by weight) Type I Portland cement (OPC)

65% GGBS + 35% OPC

Age (days)

Porosity (%) Cured at 27°C 38°C 60°C

Medium pore size (nm) Cured at 27°C 38°C 60°C

7

25.0

24.0

25.0

13.5

12.0

9.0

28

20.0

22.0

22.0

13.0

15.0

13.5

90 7 28 90

17.0 21.5 12.2 9.5

19.0 19.0 11.5 10.0

24.5 17.0 12.0 10.0

10.5 13.5 2.75 2.30

12.0 13.5 2.75 2.45

17.5 11.0 2.95 2.60

8.0

8.0

8.0

2.35

2.40

2.50

180

2.12

Handbook on Advanced Concrete Technology

The reduction in the porosity of concrete at various ages indicate the effect of secondary hydration and the reduction at any given age for increasing curing temperatures indicate the effect of temperature on the hydration processes.

2.4.6

Carbonation

The ability of the concrete to protect the steel from corrosion depends, among other factors, upon the extent to which the concrete in the cover zone has carbonated. The use of GGBS in concrete mix entails an even greater necessity for good curing. In consequence, poorly cured concrete containing GGBS exhibits very high carbonation. However, when the GGBS content in concrete is below 50% and the concrete is exposed to CO2 at a concentration of 0.03%, there is only a marginal increase in carbonation. Similarly, when the strength of concrete is more than 30 MPa, the percentage increase in carbonation depths against the normal concrete are very insignificant.

2.5

SILICA FUME IN CONCRETE

Silica fume is produced during the high-temperature reduction of quartz in an electric arc furnace where the main product is silicon or ferrosilicon. High-purity quartz is heated to 2000°C with coal, coke or wood chips as fuel and an electric arc introduced to separate out the metal. As the quartz is reduced it releases silicon oxide vapour. This mixes with oxygen in the upper parts of the furnace where it oxidizes and condenses into microspheres of amorphous silicon dioxide. The fumes are drawn out of the furnace through a precollector and a cyclone, which remove the larger coarse particles of unburnt wood or carbon, and then blown into a series of special filter bags. Silica fume is, when collected, an ultrafine powder having the following basic properties: 1. At least 85 per cent SiO2 content 2. Mean particle size between 0.1 and 0.2 micron 3. Minimum specific surface of 15000 m2 /kg 4. Spherical particle shape BIS has issued standards for Silica fume under IS 15388-2003. A comparison of cementitious materials is given in Table 2.5 below.

Available Forms of Silica Fume As the powder is about hundred times finer than ordinary Portland cement there are transportation, storage and dispensing considerations to be taken into account. To accommodate some of these difficulties the material is commercially available in various forms. The differences between these forms are related to the shape and size of the particles and do not greatly affect the chemical make-up or reaction of the material. The main forms are as follows.

Undensified Bulk density: 200–350 kg/m3

Mineral Admixtures

2.13

TABLE 2.5 Comparision of physical & chemical properties of various cementitious materials Portland cement

Fly Ash

GGBS

Silica fume

300–500 1000 2.30

300–500 1000–1200 2.90

15000–20000 200–300 2.20

Physical data for cementitious materials Surface area (m2/kg) Bulk density (kg/m3) Specific gravity

260–300 1300–1400 3.12

Chemical data for cementitious materials percent by mass SiO2 (min) Fe2O3 Al2O3 CaO MgO

20 3.5 5.0 65 1.0

50 10.4 28 3 2

38 0.3 11 40 7.5

85 1.2 0.7 0.2 0.2

Na2O + K2O

0.8

3.2

1.2

1.5

As bulk density is very low, undensified silica fume is usually not used in normal concrete production. Though it may be used for refractory products and formulated bagged materials such as grouts, mortars, concrete repair systems and protective coatings.

Densified Bulk density: 500–650 kg/m3 In the densification process the ultrafine particles become loosely agglomerated, making the particulate size larger. This makes the powder easier to handle, with less dust, than the undensified form. Usually densified silica fume is used in concrete.

Micropelletized Bulk density: 700–1000 kg/m3 Micropelletization involves forming the powder into small spheres about 0.5–1 mm in diameter. The material in this form does not readily break down in conventional concrete mixing and is best suited to intergrinding with cement clinker to produce composite cement.

Slurry Specific gravity: 1400 kg/m3 This material is produced by mixing the undensified or densified powder and water in equal proportions, by weight, to produce stable slurry. Mixing and maintaining stable slurry requires expensive hi-tech equipment and cannot be done easily and, therefore, all slurries should be obtained from one specific supplier to maintain quality. Certain additives are used for keep the slurry stable else there is a possibility of settlement and the properties of slurry will not be uniform.

2.14

Handbook on Advanced Concrete Technology

2.5.1 Effects on Fresh Concrete Due to the nature and size of silica fume, a small addition to a concrete mix will produce marked changes in both the physical and chemical properties. When silica fume is used the water demand is increased due to the high fineness of the material. It also increases the cohesiveness of concrete mix. Concrete made with silica fume will have a lower slump than a similar ordinary concrete due to the greater cohesion. Thus it is imperative to use a high end superplasticiser to maintain the required workability. One of the effective way to negate the water demand is to reduce the sand content in the mix. This will help in slightly reducing the fines. When the mix is supplied with energy, as in pumping, vibrating or tamping, the silica fume particles, being spherical, will act as ball bearings and lubricate the mix giving it a greater mobility than the similar ordinary concrete. Silica fume concrete is often referred to as being thixotropic in nature. To describe this when measuring the slump of a silica fume concrete it must be remembered that the value will only indicate the consistency of the concrete and will not relate to its workability. The most favourable test for such concrete is the flow table, or similar, which gives a reaction to energy input and thus gives a better visual appraisal of the workability of the mix. As the concrete is more cohesive it is less susceptible to segregation, even at very high workabilities such as in flowing or self-compacting concretes. The above mentioned property also leads to producing very less bleed water in concrete mix. This lack of segregation and bleed water also makes it ideal for incorporation into high-fluidity grout. The concrete must therefore be cured, in accordance with good practice, as soon as it has been placed, compacted and finished. The lack of bleed water means that finishing can be commenced much sooner than with ordinary concretes.

2.5.2

Properties of Concrete in Hardened State Made with Silica fume

Compressive Strength and Strength Development As microsilica reacts, and produces calcium silicate hydrates, the voids and pores between cement grains and aggregate particles are closely filled. Coupling this with the physical filling effect it can be seen that the matrix of the concrete will be very homogenous and dense, giving improved strength and impermeability. Generally silica fume concrete exhibit very high compressive strengths. In general, for an equal strength, an increase will be seen in the w/b ratio while for a given w/b ratio an increased strength will result (Fig. 2.8). Silica fume concrete will show a marked reduction in strength gain when subjected to early age drying and this can be as much as 20 per cent. This issue once again highlights the importance of proper curing.

Abrasion and Erosion Low w/b ratio high-strength silica fume concrete shows greatly improved resistance to abrasion and erosion and a large amount of silica fume concrete has been produced to specifically utilize this quality. A large repair project on the Kinzua dam, USA, has been studied and results show good performance of the concrete used. Many hydropower projects in India are utilizing silica fume concrete for this purpose.

Mineral Admixtures

2.15

Compressive strength (MPa) 100

¥ ¥ ¥ ¥

90 80 70

¥ ¥

16% MS

8% MS

60

¥¥

50 40 30 ¥ 20 Reference concrete

10 0 0.3

0.4

0.5

Fig. 2.8

0.6

0.7

0.8

0.9

1.0

1.1

Strength versus w/b ratio

2.5.3 Effects on Carbonation As the concrete sets and hardens the pozzolanic action of the silica fume takes over from the physical effects. The silica fume reacts with the liberated calcium hydroxide to produce calcium silicate and aluminate hydrates. These both increase the strength and reduce the permeability by densifying the matrix of the concrete. Silica fume, having a greater surface area and higher silicon dioxide content, has been found to be much more reactive than pfa or ggbs. The high reactivity and consumption of calcium hydroxide has prompted questions relating to the pH level of the concrete and the corresponding effects on steel passivity and carbonation rate. Studies have shown that the effect on carbonation rate is highly dependent on the quality of the concrete mix produced. Good-quality, well-proportioned, silica fume concrete does not exhibit any greater carbonation than a normal Portland cement concrete. The reduction of pH in a concrete mix is usually from approximately 13.5 to 13.0 and this latter value is well above the level for steel passivity. It has been estimated that a 25 per cent addition of silica fume would be required to use up all the calcium hydroxide produced in a concrete, and studies have shown that at this level the pH still does not drop below 12.0. In normal practice the highest dosage advised for concrete is 15 per cent and this should have no deleterious effects.

2.5.4

Durability of Silica Fume Concrete

The use of a silica fume concrete, with its potential for greater strengths, both compressive and tensile, its more refined pore structure and lower permeability, gives the opportunity of providing a more durable concrete with a longer working lifespan than a conventional concrete in the same environment.

2.16

2.6

Handbook on Advanced Concrete Technology

METAKAOLIN IN CONCRETE

Kaolin is soft, white clay resulting from the natural decomposition of feldspars and other clay minerals. It occurs widely in nature. It is used for making porcelain and chinaware, as a filler in the manufacture of paper and textiles and as a medicinal absorbent. Kaolinite is the principal mineral constituent of kaolin. Kaolin is extracted from the granite using high-pressure water jets. The kaolin slurry is then concentrated and refined using standard mineral processing techniques. The refined product is dried. The ball clays are treated using standard dry processing techniques. When kaolin is heated to a temperature of 450°C dehydroxylation occurs and the hydrated aluminosilicates are converted to materials consisting predominantly of chemically combined aluminium, silicon and oxygen. The rate at which water of crystallization is removed increases with increasing temperature and at 600°C it proceeds to completion. Metakaolin is formed in kilns when kaolin is heated at a temperature between 700°C and 800°C. The calcined product is cooled rapidly and ground to a fine powder. The metakaolin formed in this way has a highly disorganized structure. The physical properties of metakaolin depend very much on the quality of the raw material used, the calcination temperature and the finishing processes. The typical physical properties are listed below: Fineness: > 10 microns (mass % max.)

10

< 2 microns (mass % min.)

5

2

Surface area (m /kg)

9000

Moisture: % Water (mass % max.) 3

Bulk density: (g/cm )

0.5 0.3

BIS has recently circulated draft code on metakaoline under document CED 2 (7668)1 in May’ 2009.

2.6.1 Effect on Fresh Concrete The particle density of metakaolin (2.4) is lower than that of Portland cement (3.1). Thus, when metakaolin is used as a replacement for cement the volume of cementitious material is increased. Reducing the sand content of the mix overcomes the effect of the increased volume of cementitious powder. Concrete mixes made with metakaolin have better cohesiveness, easier to pump and place and it bleeds less than plain concrete. The greater volume of cementitious fines results in the production of sharp edges and high quality surface finish on cast vertical surfaces. The greater volume of cementitious material in metakaolin concrete increases its water demand. The effect of increasing metakaolin content on the water demand of concrete is shown in Fig. 2.9. The increase of water demand is readily offset by the use of a standard plasticizer.

Mineral Admixtures

2.17

260

3

Total water content (L/m )

Plasticized concrete Plain concrete 240

220

200

180

0

5

Fig. 2.9

10 15 20 Metakaolin content (%)

25

30

Effect of metakaolin on water demand

2.6.2 Effect on Hardened Concrete Calcium silicate hydrates and calcium aluminosilicate hydrates are formed as a result of reaction between metakaolin and calcium hydroxide. There is also increase in the hydration of C3S. This results in significant increase in the strength of concrete. The optimum level of replacement of cement with metakaolin is between 5 per cent and 20 per cent.

2.6.3

Durability of Metakaolin Concrete

The formation of insoluble, stable cementitious products in place of potentially soluble calcium hydroxide leads to reduced porosity and permeability. This helps in reducing the amount of attack by external agencies (chlorides, sulfates, acids etc.) and improves resistance to alkali-silica reactivity.

2.7

GREEN, DURABLE AND ECONOMICAL CONCRETE WITH FLY ASH AND GGBS

Traditionally, construction activities have relied on using Portland cement. With huge backlog of infrastructure construction being taken up by the government, the demand and cost of cement is rising sharply. With the cost of cement and subsequently construction increasing rapidly, today the industry demands that the concrete used for building structures should not only have high strength but should also be economical, highly durable and the structure should have higher service life than the previously built structures. Using normal Portland cement alone in concrete has many problems in itself in terms of cost, durability and environmental impact. Production of one ton of normal Portland cement

2.18

Handbook on Advanced Concrete Technology

releases approximately 0.89 ton of CO2 into the atmosphere. The current demand of sustainable construction by reducing the environmental impact and increasing the durability of the structure cannot be achieved only with the use of OPC alone. The use of mineral admixtures like, Fly Ash, Ground Granulated Blast Furnace Slag in making concrete is the solution to the above problem. Fly Ash or GGBS if not utilized have to be disposed off in landfills, ponds or rejected in river systems, which presents serious environmental concerns since it is produced in large volumes. Far to be considered as a waste product, research and development as discussed earlier in the chapter has shown that fly ash & GGBS actually represent a highly valuable concrete material. In order to considerably increase utilization of fly ash & GGBS that otherwise is being wasted, and to have a significant impact on greenhouse gas emissions, it is necessary to advocate the use of concrete that will incorporate these materials as a replacement of cement. A conscious use of these materials, especially the abundantly available fly ash in India, would help to increase cement supply and conserve natural resources. It would also save energy, help the cause of environment, provide superior concrete structures and benefit the users. Table 2.6 gives a summary of effectiveness of various mineral admixtures on concrete. TABLE 2.6 Summary of effectiveness of various mineral admixtures on concrete Mineral admixture

% addition with respect to total cementitious material

Resistance to ASR

Resistance to carbonation

Resistance to chloride attack

Resistance to sulphate attack

Fly Ash

Silica fume

10 to 25% 26 to 50% 50% 50 to 70% 5 to 10%

Good Excellent Very Good Excellent Excellent

Moderate Moderate Moderate Poor Moderate

Good Excellent Very Good Excellent Very Good

Good Good Very Good Excellent Moderate

Metakaolin

10 to 20%

Excellent

Moderate

Very Good

Moderate

GGBS

References 1. John Newman, Ban Seng Choo (2003), “Cementitious Additions” Advanced Concrete technology – Constituent materials, 1st Edition. 2. N. V. Nayak (2006) “Annexure A-15: A Durable Concrete – A Practical View Point” Foundation Design Manual, 5th Edition. 3. V. M. Malhotra, A. A. Ramezanianpour, (1994), “Effect of Fly Ash on Properties of Fresh Concrete” Fly Ash in Concrete, 2nd Edition.

3 Water N.V. Nayak and Manish Mokal

3.1 INTRODUCTION Water is one of the most critical but probably the cheapest constituents of concrete. Water in concrete should be as low as possible, however, minimum amount of water is essential for hydration of cement (minimum water requirement for complete hydration of cement is about 0.27 times the mass of cement). Lower water binder ratio increases the strength and generally improves the durability of concrete. It is therefore desirable to keep water binder ratio as low as possible but adequate to get workable concrete and achieve adequate hydration of cement. The binder includes all cementitious materials such as cement, fly ash, ggbs, silica fume, rice husk ash etc.

Water Requirement The water requirement of fresh concrete is the water content in litres per cubic metre, required to bring the mix to the specified consistency. As the strength of hardened concrete is influenced by water binder ratio, the quantity of water in concrete has therefore significant implications. It has therefore to be chosen with careful consideration. Water content also affects the dimensional stability of hardened concrete. Water requirement is determined by; (a) The properties and contents of other materials in the mix (Table 3.1). (b) The consistency of the concrete. The ingredients that affect water requirement in the concrete mix are shown in Table 3.2.

3.2

QUALITY OF WATER

In general potable water can be used for mixing and curing of concrete. There could be certain impurities both dissolved and/or in suspension. The permissible limits of these impurities are as indicated in Table 3.2.

3.2

Handbook on Advanced Concrete Technology

TABLE 3.1 Material Factors that Influence the Water Requirement of Concrete Material

Factors that Influence Water Requirement • Average Particle Size

Stone Aggregate

• Packing Capacity (a) Shape (b) Grading • Surface Texture • Particle Shape Grading

Sand

Cement Admixture

Ultrafines (below 75 microns) (a) Type, e.g. (clay) (b) Content • Fineness • Compound composition Type Dosage

Water Requirement Decreases with • Increasing Size • Improving Packaging Capacity • Increasing Smoothness • Improving roundness • Improving Particle size distribution • Decreasing content especially clay

• • • •

Decreasing fineness Decrasing C3S and C3A Use of plasticiser and Superplasticizer Increasing Dosage

Note: Fineness Modulus of sand does not influence water requirement if the concrete is properly proportioned.

TABLE 3.2 Tolerable Concentrations of Some Impurities in Mixing Water Impurity

Tolerable Concentration

Sodium and potassium carbonates : 1,000 ppm (total). If this is exceeded, it is advisable to make tests both and bi-carbonates for setting time and 28 days strength Chlorides Sulphuric anhydride Calcium chloride Sodium iodate, sodium sulphate, sodium arsenate, sodium borate

: : : :

10,000 ppm 3,000 ppm 2 per cent by weight of cement in non-pre-stressed concrete Very low *

Salt and suspended particles

: 2,000 ppm. Mixing water with a high content of suspended solids should be allowed to stand in a setting basin before use.

Total dissolved salts Organix material

: 15,000 ppm : 3,000 ppm. Water containing humic acid or such organic acids may adversely affect the hardening of concrete; 780 ppm. of humic acid are reported to have seriously impaired the strength of concrete.

pH

: Shall not be less than 6

In case of such waters there-fore, further testing is necessary. (From Shetty M.S., 2009) * Even the small percentage of impurities can greatly reduce initial strength of concrete. Hence, whenever these are present in water, necessary trial mixes shall be made. * In my opinion, this table is not required and only Table 3.3 based on IS 456 is adequate.

The permissible limits of solids as per IS 456: 2000 are as indicated in Table 3.3.

Water

3.3

TABLE 3.3 Permissible limit for solids as per IS 456 : 2000 Material

Tested as per

Permissible limit Max as % as ppm

Organic Inorganic Sulphates (as So3) Chlorides (as Cl) Suspended

IS IS IS IS

0.02% 0.30% 0.04% 0.2% 0.05% 0.2%

3025 3025 3025 3025

(pt (pt (pt (pt

18) 18) 24) 32)

IS 3025 (pt 17)

200 mg/litre 3000 mg/litre 400 mg/litre 2000 mg/litre for plain concrete work 500 mg/litre for reinforced concrete work 2000 mg/litre

Algae in mixing water causes marked reduction in strength of concrete. Industrial waste water containing acids/alkalis is not suitable for concrete construction. If the pH value of water is less than 6, it shall not be used in concrete. Vegetable oil has a detrimental effect on the strength of concrete particularly at the later ages. As per IS 456: 2000 sea water is not to be used in reinforced concrete. However, different opinions exist among experts. Based on the review of available literature, the following conclusions can be drawn with respect to use of sea water in concrete. Sea water can be used in plain concrete (concrete with no reinforcement) or even embedded steel parts. Sea water shall not be used in prestressed concrete. It is desirable that sea water is avoided in reinforced concrete. Appropriate measures shall be taken to counteract adverse effects of sea water on concrete. The adverse effects of using sea water in concrete include; (a) High risk of corrosion of any steel reinforcement including embedded steel parts. (b) Possible efflorescence and dampness on concrete surfaces (c) Increased risk of alkali aggregate reaction, if reactive aggregates are used and sulphate attack on concrete. Water permitted for making concrete can be used for curing. Sea water and water containing other impurities with higher percentages than indicated in Table 3.3 can be used for curing only after initial 3 days of curing with water suitable for mixing in concrete. For initial 3 days such water is not permitted as it may penetrates concrete and cause corrosion and other detrimental effects. However, during 3 days particularly concrete with low water binder ratio may become reasonable impermeable and hence other waters as noted above can be used for curing. IS 456: 2000 does not permit the use of water containing tannic acid or iron compounds for curing if staining of concrete surface is to be avoided.

References 1. BS EN 1008 (2002) “Mixing water for concrete, specification for sampling, testing and assessing the suitability of water, including water recovered from processes in the concrete industry, as mixing water for concrete”, London: British Standards Institution.

3.4

Handbook on Advanced Concrete Technology

2. Owens Gill (2009) “Fulton’s Concrete Technology-9th Edition” by Cement and Concrete Institute, South Africa. 3. Gambhir M.L. (2004) “Concrete Technology-3rd Edition” by TATA Magrow Hill Publishing, New Delhi. 4. IS 456 (2000) (4th revision with amendment no.3) on “Code of practice for Plain and Reinforced Concrete” by Bureau of Indian Standards, New Delhi. 5. Neville A.M., J.J. Brooks (1994) “Concrete Technology” by ELBS, Longman. 6. Shetty M.S. (2009) (6th Edition) “Concrete Technology – Theory and Practice” by S. Chand & Co. Limited, New Delhi.

4 Coarse Aggregate N.V. Nayak and Ganesh Kaskar

PREAMBLE Concrete is the most widely used man-made construction material in this Universe, and its use is third only next to air and water. It is obtained by judiciously mixing cement with or without other mineral admixtures, aggregates (coarse and fine), non mineral admixtures and water. In some special cases other additional materials like fibres may be added to the mixture. Such a mixture when placed in forms and allowed to cure and hardened is known as concrete. The strength, durability and other characteristics of the concrete are dependent on characteristics of constituent materials, proportions of mix, method of compaction, curing and other controls. With advancement of concrete technology, it is possible to tailor the properties of concrete closer to our requirements by right selection of the constituents’ materials and subsequent processing methods. Hence in various chapters in Part I, we shall discuss about the constituent materials.

4.1 INTRODUCTION The aggregates provide about 75% of the concrete volume. Hence it is a very important constituent. They should meet certain requirements with respect to grading, shape, size and strength. Generally, they are considered inert though there could be some reactions in some cases. Aggregates are grouped into two categories viz coarse aggregates (coarser than 4.75 mm) and fine aggregate (finer than 4.75 mm). We shall cover coarse aggregate in this chapter.

4.2

CLASSIFICATION OF AGGREGATES

Aggregates may be classified based on the following:

4.2

Handbook on Advanced Concrete Technology

(a) (b) (c) (d) (e)

4.2.1

Based on Based on Based on Based on Artificial

source weight shape geology aggregates

For natural aggregates

Classification Based on Source

Coarse aggregates may be naturally available river aggregate (gravel) or crushed aggregate obtained by crushing of rock. In India, gravel is used to a very limited extent (though permitted as per IS 456). In United States 50% of concrete produced of grade M45 and lower is by using gravel aggregate. Gravels meeting the gradation requirement shall be preferred over the crushed aggregate for concrete of grades lower than M45 as they have many advantages. Gravels have very low flakiness and elongation, for given water and cement content gives higher workability, generally have low impact value. All this gives more durable and high strength concrete. However, bond with gravel is lower than crushed aggregate.

4.2.2

Classification Based on Weight

Aggregates are also classified based on weight as normal weight, heavy weight and light weight aggregate. Normal weight aggregates are commonly used aggregate with specific gravity ranging from 2.05 to 3.0 g/cm3 and producing concrete with unit weight ranging from 2400 to 2700 kg/m3. They are generally obtained from natural gravel or by crushing rocks such as granite, basalt, quartz, sandstone and limestone. Heavyweight aggregates are used in producing heavy weight concrete for radiation shield in atomic power station etc. Heavy weight concrete can have density ranging from 2900 kg/ m3 to 6100 kg/m3. Heavyweight aggregates have specific gravity generally greater than 3 g/ cm3. Heavyweight aggregates are generally obtained from barium mineral rocks, iron ores and titanium ores. Heavyweight concrete has higher tendency to segregation, requires higher cement content, causes increased pressure on formwork and results in excessive wear of plant and mixers. The reason for using such concrete for radiation shield is that heavy concrete is generally dense and has a lower cracking tendancy. Normally coarse aggregates that weigh less than 1000 to 1100 kg/m3 are considered as lightweight aggregates. These aggregates can be natural such as diotomite, pumice, volcanic cinder etc or manufactured such as bloated clay, sintered flyash or foamed blast-furnace slag, expanded polystyrene etc. These lightweight aggregate may be used in masonry walls to reduce the weight of the structure. In addition to reduction in the weight, the concrete produced by using light-weight aggregate provides better thermal and sound insulation and improved fire resistance.

Coarse Aggregate

4.3

The lightweight aggregates absorb more water, thus requiring wetting of aggregate before mixing. Such concrete requires higher water cement ratio, concrete becomes stiff faster and also requires higher cover to reinforcement etc.

4.2.3

Classification Based on Shape

Based on shape, aggregates are generally classified in four categories viz rounded, irregular, angular and flaky and elongated aggregates. The rounded aggregates are generally river or seashore aggregates which are fully water worn and completely shaped by attrition. With rounded aggregates workability improves, cement requirement for given workability and strength reduces. But interlocking between particles is poor resulting in poor bond. Hence, there is reservation in use of rounded aggregate in high strength concrete and pavements. Irregular aggregates are partly rounded (pit sands and gravels). They are partly water worn and partly shaped by attrition. Such aggregate requires more cement than rounded aggregate for given workability but interlocking is better than rounded aggregates. Angular aggregates are generally formed by crushing rock. These aggregates have sharp, angular edges/corners. The interlocking between particles is good, thereby providing good bond. Angular aggregates, require higher cement content for given workability and strength. An aggregate is termed as flaky when its least dimension (thickness) is less than three fifth of its mean dimension. The particles is said to be elongated when its greatest dimension (length) is greater than nine fifth of its mean dimension. The mean dimension of the aggregate is the average of the sieve sizes through which it passes and retained. Flakiness of aggregates is generally expressed in terms of flakiness index. Flakiness index is the percentage by weight of particles having least dimension (i.e. thickness) less than three-fifth of their mean dimension. The percent elongated particles present in aggregates is expressed in terms of elongation index which is defined as the percentage by weight of particles present whose greatest dimension (length) is greater than nine-fifth of their mean dimension. Higher flakiness and elongation indices reduce workability and durability of concrete. Generally, flakiness index of coarse aggregates should be less than 25 percent and combined flakiness and elongation indices together should be less than 35 percent for getting good durable concrete.

4.2.4

Classification Based on Geology

Natural aggregates are obtained from gravel or crushing the rock. Depending upon the origin of rock, the aggregates can be classified as of igneous rock, of sedimentary rock or of metamorphic rock. Aggregates from igneous rock, generally, are of good quality as these rocks are very compact and strong. Granite, basalt, dolerite etc. fall in this category. Aggregates from sedimentary rocks may be good to poor. Some limestones and siliceous sandstones have yielded good aggregates. But some other stones may yield porous, soft or light or flaky aggregates.

4.4

Handbook on Advanced Concrete Technology

Many metamorphic rocks, particularly quartzite and gneiss, have yielded good concrete aggregates though some of the metamorphic rocks may indicate foliation which is not desirable in aggregates.

4.2.5 Artificial Aggregates Artificial aggregates use is generally restricted to non structural areas mainly for masonry in-fill walls in buildings. However, good quality artificial aggregate have been used for structural works. Artificial aggregates include broken bricks, air cooled fresh blast furnace slag, bloated clay, sintered fly ash. Air cooled slag aggregate can be used in mass concrete. However, high sulphur content of slag in these aggregates pose a concern to the durability of concrete.

4.2.6 Recycled Aggregate Construction materials are increasingly judged by their ecological characteristics. Concrete recycling gains importance because it protects natural resources and eliminates the need for disposal by using the readily available concrete as an aggregate source for new concrete or pavement subbase layers. Recycling of concrete involves breaking, removing and crushing existing concrete into a material with a specified size and quality. The quality of concrete with recycled concrete aggregates is very dependent on the quality of the recycled material used. Reinforcing steel and other embedded items, if any, must be removed, and care must be taken to prevent contamination by other materials, such as: asphalt, soil and clay balls, chlorides, glass, gypsum board, sealants, paper, plaster, wood and roofing materials which can be troublesome. After removal of contaminants through selective demolition, screening and/or air separation and size reduction in a crusher to aggregate sizes, crushed concrete can be used as: 1. new concrete for pavements, shoulders, median barriers, sidewalks, curbs and gutters and bridge foundations 2. structural grade concrete 3. soil-cement pavement bases 4. lean-concrete or econo-crete bases 5. bituminous concrete The crushing characteristics of hardened concrete are similar to those of natural rock and are not significantly affected by the grade or quality of the original concrete. Recycled aggregates produced from all but the poorest quality original concrete can be expected to pass the same tests required of conventional aggregates. Recycled concrete can be batched, mixed, transported, placed and compacted in the same manner as conventional concrete. Special care is necessary when using recycled fine aggregate. Only to 10% to 20% recycled fine aggregate is beneficial. The aggregate should be tested at several substitution rates to determine the optimal rate. Higher porosity of recycled aggregate compared to natural aggregate leads to a higher water absorption. It is recommended that recycled aggregates be batched in a prewetted and close to

Coarse Aggregate

4.5

a saturated surface dry condition. To achieve the same workability, slump and water-cement ratio as in conventional concrete, the paste content or amount of water reducer have to be increased.

4.3 PROPERTIES OF COARSE AGGREGATES AFFECTING CONCRETE CHARACTERISTICS As noted earlier, coarse aggregate is the predominant constituent of the concrete. Hence, naturally, properties and characteristics of the fresh and hardened concrete are significantly affected by the properties of the coarse aggregate. Of course, characteristic of the concrete are also affected by other constituent materials such as cement, other mineral admixtures, fine aggregate, non-mineral admixtures, water and impurities etc. In addition to characteristic of the constituent materials, concrete performance is also affected by proportioning of the constituent materials, method of mixing, transporting, placing, compaction (vibration) and curing etc. In the following paragraphs, properties of the coarse aggregate affecting the concrete characteristics are highlighted, which can be listed as follows: (a) (b) (c) (d) (e) (f) (g) (h)

Grading Strength, Abrasion resistance, Elastic modulus, Soundness etc. Shape and Surface structure Density, Specific gravity, Absorption and Surface moisture Thermal Properties Fineness modules Alkali – Aggregate Reaction (AAR) Deleterious substances

These characteristics along with their effect on performance of the concrete are described briefly herein below:

4.3.1

Grading

Grading of coarse aggregate can be defined as distribution of the particle sizes between the largest size aggregates to lowest size aggregates. Particle size is determined by sieve analysis. Generally in reinforced cement concrete (RCC), 20 mm down aggregate are used in slabs, beams etc. 40 mm down aggregates may be used in piles, pile caps, piers, pier caps, wells and caissons etc. provided cover to the reinforcements and gap between reinforcements is 45 mm or more. In self compacting concrete, tendency is to use 10 mm or 12 mm down aggregate. In mass concrete like dams etc. 80 mm down or 150 mm down or even bigger size aggregates may be used. If all aggregates are almost of the same size or vary over a small range of sizes then that aggregate will be uniform. In uniform aggregate voids will be more which requires higher cement content. When aggregates vary gradually changing between biggest size to the lowest size then we have the well graded aggregate. Well graded aggregate will have low void ratio and cement requirement will be low. Hence, one should prefer well graded aggregate.

4.6

Handbook on Advanced Concrete Technology

Well graded aggregate with one or more intermediate size fraction absent are known as gap graded aggregate. Gap graded aggregate may serve similar to well graded aggregate. In fact certain advantages are claimed with gap graded aggregates over well graded aggregates viz lower sand (Fine Aggregate) requirement, lower cement requirement and lower drying shrinkage. But such aggregate should be used mainly for relatively low workability. It needs closer supervision & such concrete tends to segregate. For this reason well graded aggregate is preferred to gap graded aggregate. The Road Research Laboratory of the Department of Scientific and Industrial Research, UK have prepared series of concrete curves which are useful for design of concrete mixes. These concrete curves for the aggregate of 40 mm down, 20 mm down and 10 mm down are shown in Fig. 4.3.1, 4.3.2, 4.3.3 respectively. In these figures, Curve nos. 1 to 4 are also designated. When we move from the curve number 1 to 4, fines content increases. This means curve no. 1 is the coarsest and curve no. 4 is the finest aggregate. Hence, curve no. 1 is suitable for coarser mixes. It will yield maximum permissible aggregate cement ratio and hence more economical. Curve no. 4 is the mix with higher workability and requiring more cement and hence costly. 120

Percentage passing

100

80

77 4 65

60 38

40 25 20

16 11

18

32

31 17

26

26

3

51

55

2

40

44

1

35 22

68 60 49

36

17

7 13 12 7 5 8 3 2 0 150 mm 300 mm 600 mm 1.18 mm 2.36 mm 4.75 mm 10 mm IS Sieve sizes

20 mm

40 mm

Fig. 4.3.1 Recommended grading curves for 40 mm nominal maximum size aggregate

4.3.2

Strength, Abrasion Resistance, Elastic Modulus and Soundness of Aggregates

Strength of the concrete can not exceed that of rock/aggregates. Abrasion resistance and elastic modules are generally indicative of strength of the rock. Invariably, rock/aggregate with high strength will have better soundness.

Coarse Aggregate 100

75 65

60 4 42 40 27 21 20

2 0 150 mm

35

35

28

28 23

21

55

50

45

42 3 2 1

35 30

14

12

16

5

3 2 300 mm

9 600 mm

1.18 mm

2.36 mm

4.75 mm

10 mm

20 mm

IS Sieves sizes

Fig. 4.3.2 Recommended grading curves for 20 mm nominal maximum size aggregate 100 90 80

75

70

Percentage passing

Percentage passing

80

4 60

60

60

3 50

46

46

2

40

37

34

33

30 20

28

20 14

19 12

10 0 150 mm

45

1

26 16

30

20

8 4 300 mm

600 mm

1.18 mm

2.36 mm

4.75 mm

10 mm

IS Sieve sizes

Fig. 4.3.3 Recommended grading curves for 10 mm nominal maximum size aggregate

4.7

4.8

Handbook on Advanced Concrete Technology

Generally three tests are specified for determining the strength of the aggregates. They are “Crushing Value Test”, “Impact Value Test” and “Ten percent Value Test”. Methodology of doing these tests are specified in IS:2386 (Part IV)-1963. As per IS:383 (1970), permissible values for these are as follows: TABLE 4.3.1 Test

For Wearing Coat Concrete

For Other Concretes

1. Aggregate Crushing Value 2. Aggregate Impact Value 3. Aggregate Abrasion Value

£ 30% £ 30% £ 30%

£ 45% £ 45% £ 50%

Of the above mentioned three tests, test no 2 viz impact value test is more commonly adopted. Lower Impact Value means higher strength of aggregates. Abrasion test is carried out using Los Angeles machine. Here again lower the abrasion value, higher is the strength of aggregate. The soundness of the aggregate indicates the resistance of aggregate to sever climatic changes such as freezing and thawing, and alternate wetting and drying. The method of testing is defined in IS:2386 (Part IV) (1963). For concrete liable to be exposed to the action of frost and thawing, the aggregate shall pass a sodium or magnesium sulphate accelerated soundness test specified in IS:2386 Part V (1963). As a general guide as per IS:383 (1970), it may be taken that average loss of weight after 5 cycles shall not exceed. • 12% when tested with sodium sulphate (Na2SO4) and • 18% when tested with magnesium sulphate (Mg SO4) Elastic modules is generally related to the strength. Higher the strength, higher is the elastic modules. Elastic modules of hard rock, generally varies between 200 MPa to 300 MPa.

Shape and Surface Texture The different types of shapes of aggregates and their effects on concrete are discussed in para 4.2.3. In IS:383 (1970), surface textures are broadly classified into five categories viz glossy, smooth, granular, crystalline and honeycombed and porous. The strength of the bond between aggregate and cement paste depends upon surface texture. An aggregate with rough texture is preferred to one with smooth surface as it gives a better bond and hence higher strength to concrete.

Density, Specific gravity, Absorption and Surface Moisture The bulk density of aggregates are affected by particle shape, size, grading, moisture content and specific gravity. Classification of aggregates based on weight is indicated in para 4.2.2 above.

Coarse Aggregate

4.9

The specific gravity of an aggregate gives valuable information of its quality and properties. Higher the specific gravity, heavier and stronger is the aggregate. Generally, specific gravity ranges between 2.6 to 2.8g/cm3. In case of rock/aggregate, we have two specific gravities viz apparent specific gravity and true specific gravity. Any aggregate or rock will have pores, the extent of which varies from rock to rock. Apparent specific gravity refers to specific gravity inclusive of voids present. True specific gravity is exclusive of voids present. True specific gravity is always higher than apparent specific gravity. In concrete mix proportioning, we have to consider apparent specific gravity. Rocks have pores or cavities, the extent of which varies from rock to rock. These pores in rock develop due to presence of air bubbles, which are entrapped in rock during its formation or due to decomposition of constituents materials. Porosity can vary from close to zero to 20 percent in general and much higher. When all the pores are full and there is no water on the surface of aggregate, the aggregate is known to be in the saturated – surface dry condition (SSD). On saturation as above and if there is also water on the surface the aggregate is known to be in wet condition. Effective absorption is the moisture content required to bring the oven dry aggregate to SSD condition. The amount of water to bring the aggregate to wet condition and beyond SSD is known as the surface moisture. Water required for effective absorption and surface moisture is known as absorbed water; knowledge of absorbed water is necessary in concrete mix proportioning.

Thermal Properties The thermal properties of the aggregate that are important for concrete technologists are coefficient of thermal expansion, specific heat and thermal conductivity. For majority of rocks, coefficient of thermal expansion falls in the range of (5 to 13) 10 –6 per °C. For hydrated cement, it generally is in the range of (11 to 16) 10 –6 per °C. This difference is generally in order. If there is substantial difference then durability of concrete can get affected particularly for concrete subjected to high temperature differencial viz concrete subjected to freezing and thawing. The specific heat of the aggregate is measure of heat capacity and thermal conductivity is the ability to conduct the heat. These properties are of importance in mass concrete, fire resistant concrete and in concrete where insulation is required.

Fineness Modules The fineness modules is a numerical index giving some idea of mean size of aggregates. The fineness modules of coarse aggregates, generally varies between 5.5 to 8.0. The lower value of fineness means aggregate is finer and so on. The fineness modules is sometimes used to check quality of aggregates received with respect to aggregate used in mix proportioning based on which mix proportioning has been finalized.

4.10

Handbook on Advanced Concrete Technology

Alkali – Aggregate Reaction (AAR) The Alkali – Aggregate Reaction (AAR) or Alkali – Silica Reactivity (ASR) is the reaction between active silica constituents of aggregates and alkalies present in cement etc. The alkalies in cement are Na2O and K2O and generally expressed as equivalent of Na2O. If alkali content of cement equivalent of Na2O is less than 0.6 percent, then it is known as low alkali cement. Majority of cement manufactured in India at present has got alkali content less than 0.6%. AAR occurs when soluble alkalis in cement dissolved in water and into highly caustic liquid which reacts with reactive silica present in aggregate, and produces highly expansive alkali silica gel. This gel exerts hydraulic pressure resulting in cracking, reduced strength and deterioration of concrete in general. The factors promoting AAR are reactive type of aggregate, high alkali content in cement, higher cement content, availability of moisture, ambient temperature, alkali content in other constituent materials such as admixture, alkaline water (even alkali from deicing salt) and ambient temperature (between 10 to 40°C). It is considered that effect of AAR is negligible as long as total alkali content in concrete does not exceed 3 kg/m3 of concrete. TABLE 4.3.2 Limits of Deleterious Materials (Clause 3.2.1) Sl. No. Deleterious Susbtance

Metrod or test

Fine Aggregate Percentage by Weight, Max

Concrete Agfregate Percentage By Weight, Max

Uncrushed

Crushed

Uncrushed

Crushed

(1) (i)

(2) Coal and llgnite

(3) IS: 2386 (Part II)-1963

(4) 1.00

(5) 1.00

(6) 1.00

(7) 1.00

(ii) (iii)

1.00 15.00

1.00 3.00

1.00 3.00

–

–

3.00

–

(v)

Shale

do IS: 2386 (Part I), 1963 IS: 2386 (Part II). 1963 do

1.00 3.00

(iv)

Clay lumps Materials finer than 75-m* IS Sieve Soft progments

1.00

–

–

–

(vi)

Total of percentage of all deletrious materials (expect mica) including Sl. No. (i) to (v) for col 4, G and 7 and Sl No. (i) and (ii) for col 5 only

5.00

2.00

5.00

5.00

–

Note 1

The presence of mica in the fine aggregate has been found to reduce considerably the durability and compressive strength of concrete and further investigations are underway determine the extent of the deleterious effect of mica. If is advisable, therefore, to investigate the mica content of fine aggregate and make suitable allowances for the possible reduction in the strength of concrete or mortar.

Note 2

The aggregate shali not contain harmful organic impurities [tested in accordance with IS: 2386 (Part II)- 1963] in sufficient quantities to affect adversely the strength or durability of concrete. A fine aggregate which falls in the test for organic impurities may be used, provided that, when tested of the effect of organic impurities on the strength of mortar, the relative strength at 7 and 28 days, reported in accordance with 7 of IS: 2386 (Part VI)-1963 is not less than 95 percent.

Coarse Aggregate

4.11

Damage due to AAR are wide spread. Such AAR can be controlled by using low alkali cement, reduced cement, content, use of secondary mineral admixtures such as ggbs, fly ash, silica fume, metakaolin etc, adding crushed stone dust to increase the ratio of silica powder to alkalis present and/or using suitable aggregate.

Deleterious Substances in Aggregates Deleterious substances are those which adversely affect the workability, setting, hardening, strength and/or durability characteristics of concrete. Materials like clot, clay, mica, organic matter, sea shells, alkali etc. are deleterious materials. IS 383 (1970) puts limit on deleterious material as noted in Table 4.3.2 In table 4.3.3 some potentially deleterious constituents found in aggregates as per John Newman et al (2003) are listed. TABLE 4.3.3 Some potentially deleterious constituents found in aggregates Potentially deleterious constituent Clay coatings on aggregate particles Clay lumps and altered rock particles Absorptive and microporous particles Coal and light weight particles Weak or soft particles and coatings Organic matter Mica Chloriedes2 Sulfates Pyrite (iron disulfied) Soluble lead, zinc or cadmium Alkali-reactive constiments Releasable alkalis

i – – – – – ÷÷ – – – – ÷÷ – –

Possible adverse effect in concrete ii iii ÷ – – ÷ – ÷ – – ÷÷ ÷ – ÷ – ÷÷ – – – – – – – – ÷ – – –

iv – ÷ ÷ – ÷ – – ÷ ÷÷ ÷÷ – ÷÷ ÷÷

v – ÷÷ ÷÷ ÷÷ ÷÷ – ÷ – ÷ ÷÷ – – –

1 (i) Chemical Interference with the setting of concrerte (ii) Physical prevention of good bond between the aggregate and the cement paste (iii) Modification of the properties of the fresh concrete to the detriment of the durability and strength of the hardened material (iv) Interaction between the cement paste and the aggregate which continues after hardening, after hardening, sometimes causing expansion and poor cracking of the concrete (v) Weakness and poor drability of the aggregate particles themeselves 2. The main problem with chlorides in concrete is associated with the corrosl of embedded steel. ÷÷ = main effect ÷ = addition effect

5 Fine Aggregate N.V. Nayak and Ganesh Kaskar

5.1 INTRODUCTION As noted in Chapter-4, fine aggregate is finer in size than 4.75 mm. Its size ranges from 4.75 mm to 150 microns. A fraction finer than 150 microns is considered as dust or silt, but it is found in varying percentages in fine aggregate obtained from different sources. Till recently, fine aggregate was basically natural sand. With vast expansion in infrastructure and extensive use of concrete, availability of natural sand has greatly depleted. In any case, availability of natural sand all round the year and meeting the gradation requirement (noted in para 5.2 below) is difficult as gradation of natural sand varies from season to season. Quite often, natural sand is mixed with clay lumps and other impurities also. Because of the scarcity of natural sand, sometimes sand from creek/sea river estuary or desert sand is used as fine aggregate. This practice is not desirable and should be discouraged for all RCC and plastering work until sand from such sources is suitably treated /washed. Such sand contains salts which can add to corrosion of reinforcement steel in RCC and may precipitate on plastered surfaces. Even if such sand is washed, it is difficult to remove all dissolved salts. Further, they also contain sea shells. Sea shells are undesirable as they increase water demand and lead to void formation in concrete. Over the years, sea shells in concrete also undergo delayed reaction resulting in expansion and this causes cracking of concrete and subsequent deterioration. In addition, the dissolved salts in the sea/creek sand may appear in the form of efflorescence over the plastered concrete surfaces affecting its appearance and durability. But natural sand obtained from river and meeting the gradation requirement has many advantages. The particles of such sand are rounded and smooth thereby reducing the water/plasticizer requirement and concrete also becomes more workable and cohesive.

5.2

GRADING OF FINE AGGREGATE

Bureau of Indian Standards (BIS) has laid down in IS 383 : 1970 grading of natural sand as per Table 5.2.1. Thus the natural sand can fall either in any one of the Zones I, II, III or IV. Zone

5.2

Handbook on Advanced Concrete Technology

I is coarsest sand, Zone IV is the finest. Sand falling in Zone II is ideal for concrete work. Sand falling in Zone I & III is acceptable but sand falling in Zone IV or coarser than Zone I is to be eliminated, if possible, from RCC works. TABLE 5.2.1 Fine aggregates (Clause 4.3 of IS 383) Is Sieve Designation

Percentage Passing for Grading Zone I Grading Zone II

Grading Zone III

Grading Zone IV

10 mm 4.75 mm 2.36 mm 1.18 mm 600 micron 300 micron

100 90-100 60-95 30-70 15-34 5-20

100 90-100 75-100 55-90 35-59 8-30

100 90-100 85-100 75-100 60-79 12-40

100 95-100 95-100 90-100 80-100 15-50

150 micron

0-10

0-10

0-10

0-15

Note 1:

For crushed stone sands, the permissible limit on 150-micron IS Sieve is increased to 20 percent. This does not affect the 5 percent allowance permitted in 4.3 (of IS 383) applying to other sieve sizes.

Note 2: Fine aggregate complying with the requirements of any grading zone in this table is suitable for concrete but the quality of concrete produced will depend upon a number of factors including proportions. Note 3: Where concrete of high strength and good durability is required, fine aggregate conforming to any one of the four grading zones may be used, but the concrete mix should be properly designed. As the fine aggregate grading becomes progressively finer, that is, from Grading Zones I to IV, the ratio of fine aggregate to coarse aggregate should be progressively reduced. The most suitable fine to coarse ratio to be used for any particular mix will, however, depend upon the actual grading, particle shape and surface texture of both fine and coarse aggregates. Note 4: It is recommended that fine aggregate conforming to Grading Zone IV should not be used in reinforced concrete unless tests have been made to ascertain the suitability of proposed mix proportion.

The gradings of Zone I to Zone IV are depicted in Fig. 5.2.1 above. Such a table will help in easily identifying the Zone of sand procured. In practice sand is classified as coarse sand, medium sand or fine sand rather than falling in Zone I, II, III, etc. To identify the sand as coarse, medium or fine, fineness modulus can be used as yardstick as noted below: Grading of Sand Coarse Sand Medium sand Fine sand

: : :

Fineness Modulus 3.0 – 3.5 2.4 – 2.9 2.0 – 2.4

Fineness modulus is the sum of the cumulative percentages of weight retained on each of the standard sieves divided by 100. Set of sieves in case of sand are 4.75mm, 2.36mm, 1.18mm, 600 microns, 300 microns and 150 microns. Higher the fineness modulus coarser is the sand. The fineness modulus can be looked upon on a weighted average size of a sieve on which the material is retained, the sieve being counted from the finest. For instance, a fineness modulus of 4.00 can be interpreted to mean that fourth sieve, i.e. 1.18 mm, is the average size (3). Propovics (4) showed FM to be a logarithmic average of the particle size distribution. The average particle

Fine Aggregate

5.3

Grading of Fine Aggregate 120

a

100

b

c Zone II

80

% passing

d

60

Zoen III

Zone IV

Zone I

40

20

Lower limit

0 10

4.75

2.36

1.18 IS Sieve Size (mm)

Upper limit 0.6

0.3

0.15

a - d: Common Upper Limit for Zone III & IV It is however advised that these grading graphs shall be drawn seperately zone wise on tracing sheets. This will enable for easy indentification of zone the fine aggregate belongs to.

Fig. 5.2.1

size doubles for an increase in FM of 1.0 and halves for a decrease in FM of 1.0. A sand of FM of 3.5 will have an average particle size twice that of a sand with FM 2.5. A method of calculating FM is illustrated in Table 5.2.2. below: TABLE 5.2.2 IS Sieve Size

4.75 2.36 1.18 0.60 0.30 0.15 Pan Fineness modulus 270 ÷ 100 = 2.7

Retained by Sieve On Specific Sieve Mass (gm) Percentage of total mass 0 52 156 109 153 65 51 586

0 8.9 26.6 18.6 26.1 11.1 8.7 100

Cumulative percentage of total mass 0 8.9 35.5 54.1 80.2 91.3 100 270

5.4

Handbook on Advanced Concrete Technology

FM does not describe the grading or distribution of particle sizes. However, it is a useful index of particle size – a low FM indicates a sand consisting mainly of fine material, while a high value indicates a sand containing a high proportion of coarse particles. A very fine sand may have FM as low as 0.5. Medium sands have FM between 2.4 to 2.9 while FM of coarse sands range from 3.0 to 3.5.

5.3

CRUSHED SAND

With the availability of natural sand greatly decreasing, use of crushed sand in concrete has appreciably increased. To begin with crushed sand obtained by jaw crushers was used and is still being used. But such sand particles are flaky, rough textured and results in harsh concrete requiring higher water cement ratio or plasticizer for producing workable concrete. Further such sand may contain a lot of dust. But with the use of modern crusher with VSI, one can get better quality sand with rounded particles and also it is feasible to control the dust through suitable mechanical means. Hence, one can obtain sand of required gradation falling in Zone I to III. Therefore, when using the crushed sand, it would be better to specify the sources such as VSI crusher so that final product meets the gradation requirement. In fact use of crushed sand in many cases is desirable to overcome the deficiencies in the grading of natural river sand especially in respect of the fines. Now the tertiary crushers with VSI, are used to produce good quality sand. They are extensively used in Kerala where dredging of rivers is prohibited by law. Their use is also increasing gradually in other States of India. As per IS 383 : 1970 for crushed stone sands, the permisible limits on 150 micron sieve is increased from 10% to 15% for grading zones I, II and III and from 15% to 20% for grading zone IV.

5.4

DELETERIOUS MATERIAL IN FINE AGGREGATES

Deleterious material are defined in Chapter 4. Further, maximum percentage of deleterious materials of fine aggregate as per IS 383 : 1970 have been indicated in Table 4.3.2 of Chapter-4.

5.5

FINE AGGREGATE REQUIREMENT

Fine aggregate requirement should be such that coarse and fine aggregate combined together should produce minimum voids. This should necessitate minimum cement paste requirement. The proportion of coarse to fine aggregate will vary from place to place depending upon particle size distribution of locally available material. The efforts should be directed to arrive at the optimum ratio of fine to coarse aggregate so as to arrive at the best particle packing of aggregate.

References 1. IS 383 : 1970 (Reaffirmed Feb. 97) “Specification for coarse and fine aggregates from natural sources for concrete” by Bureau of Indian Standards, New Delhi

Fine Aggregate

5.5

2. Gambhir M.L. (2004) “Concrete Technology-3rd Edition” by TATA Magrow Hill Publishing, New Delhi. 3. Neville A.M, (2005) “Properties of Concrete” – 4th edition, page 154-155. 4. Popovics, S., (1994) “The use of fineness modulus for the grading evaluation of aggregates for concrete” Road Research Laboratary. U.K, supplementary notes, Vol 18, No. 56

6 Manufactured Sand as Fine Aggregate Sanjay Bahadur and N.V. Nayak

6.1

SHORTAGE OF NATURAL SAND

There is growing shortage of natural sand in many cities. The severity varies from market to market, and in some cases this may not appear to be a priority topic. Eventually, pressure from environmentalists and sand conservationists worldwide will continue to encourage both legislators and construction engineers to look for viable alternatives to natural sand. Cubical sand manufactured from crushed rock is the most desirable fine material for concrete production. It is generally accepted that particle shape depends on the rock type, breakage energy and the type of crusher used. The rocks are crushed using crushers to manufacture coarse aggregates and the fines which are produced are usually flaky and has been used in filling, asphalt etc. Manufactured sand is defined as purpose made crushed fine aggregate produced from a suitable source material. In many places within the country, the problem of non-availability of natural sand is increasing with each passing day. It is further aggravated by the seasonality, inconsistency and volatility that are associated with extraction and supply of natural sand. In the market, the need for good quality manufactured sand is evident and the market has started to move towards the same. The government has banned sand dredging in many parts of the country.

6.2

OPTIMUM SHAPE

The optimum shape of manufactured sand is spherical, next best being cubical. Similarly, an even gradation of the total coarse aggregate fraction is desirable so that the smaller particles can fit between the larger particles, thereby minimizing the voids. Well-shaped aggregates also minimize the incidence and degree of segregation. It has been proven that more than 20kg of cement can be saved for every cubic meter of concrete that is made by replacing a poorly shaped aggregate with a cubical aggregate. In addition, both compressive strength and flexural strength are improved by using cubical aggregates, which also increases workability and reduces

6.2

Handbook on Advanced Concrete Technology

bleeding and shrinkage. The impact of the physical characteristics of the sand used in the concrete mix is even greater than that of the coarse aggregate fractions, both in the concrete’s plastic and hardened states.

6.3 VOID CONTENT The principles of total internal friction and void content apply equally to the fine fraction, but because of the vastly smaller particle size and therefore the greatly increased surface-area-to-volume ratio, any detrimental or undesirable shape or texture properties will be greatly amplified. Similarly, manufactured sand presents an opportunity to control the mineral content in the particles. Natural sand often contains undesirable minerals and clays, and the effect of these materials on both the fresh and the hardened states of concrete can be extremely harmful. For example, the effect of clay particles in fresh concrete is not obvious, as the particles absorb disproportionate volumes of water and hence swell to many times their original size. This swelling occupies a volume in the cement paste in its fresh state. When it hardens, however, the clay particles contract and leave minute voids, which in turn increase the shrinkage and permeability and hence reduces the concrete’s chemical resistance and compressive strength. Other undesirable materials, ranging from basic chlorides to harmful chemicals, can exist in this fine material fraction. The use of manufactured sand, however, reduces the risk of impurities.

Comparison Between Manufactured Sand and Natural Sand Manufactured sand

Natural sand

• A fine aggregate produced by crushing rock, gravel or slag • Can often be flaky & elongated displaying sharper, angular edges a rough surface texture • Higher micro-fines content

• It is the result of natural weathering and abrasion of rock • Mostly alluvial or weathered rock, but can of marine origin also be • Natural sands are rounded and smooth

%

Natural Sand Manufactured Sand

63 mm

5mm

Typical grading curves

Manufactured Sand as Fine Aggregate

6.3

Manufactured Sand Performance Drivers ROCKS INTRINSIC PARAMETERS

PRODUCTS AND APPLICATIONS

Parameters

Has impact on

Petrography of rocks • Structural Geology of deposit • Faults, weathering Rock mechanical parameters • Hardness, crushability • Abrasiveness Particle size distribution Particle shape Micro-fines • Content (12s

10-30mm 6-12s

30-60mm 3-6s

60-180mm 0-3s

70 60 50

15

40

15

30

40 60

40 60

80 100

100

20

10 0.2 0.4 0.6 0.8

15

15

40

40 60

60

80

80

80

100

100

0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 Free-water/cement ratio

Fig. 9.5 Recommended proportions of fine aggreagate according to percentage passing a 600 mm sieve

In the ACI method, the air content of concrete (Table 9.18) is considered to arrive at the absolute volume of mix constituents. The batch weights of the materials per unit volume of concrete is then calculated from the absolute volume. In Indian Standards (IS 10262 and IRC 44), the batch of weights of materials are calculated from the absolute volumes, but the air content of concrete is not considered. It is stated that “the air content of normal (non-air-cntrained) concrete, is not of much significance in mix proportioning procedure”6.

9.20

Handbook on Advanced Concrete Technology

TABLE 9.18 Approximate mixing water (kg/m3 of concrete) requirements for different slumps and maximum sizes of aggregates. (From ACI 211.1) Slump, mm

Maximum Sizes of Aggregates in mm 10

12.5

20

30-50 80-100 150-180 Approximate amount of entrained air in non-air-entrained concrete, percent

205 225 240 3.0

200 215 230 2.5

185 200 210 2.0

30-50 80-100 150-180 Recommended average total air content, percent

180 200 215 8.0

175 190 205 7.0

165 180 190 6.0

25

40

50

75

150

155 170 180 0.5

145 160 170 0.3

125 140 – 0.2

140 155 165 4.0

135 150 160 3.5

120 135 – 3.0

Non - Air - Entrained Concrete 180 195 205 1.5

160 175 185 1.0

Air-Entrained Concrete 160 175 185 5.0

145 160 170 4.5

Note: These quantities of mixing water are maximum for reasonably well-shaped, angular coarse aggregate graded within limits of accepted specifications. The slump values for concrete containing aggregates larger than 40mm are based on slump tests made after removal of particles larger than 40mm by wet screening.

In the British Method, the quantities of ingredients are calculated directly from the wet density of concrete, which is dependent on the specific gravity of the crushed and uncrushed combined aggregates (Fig. 9.2), on S.S.D. basis.

9.3.2 The ACI Mix Proportioning Procedure The ACI 211.1 gives recommended practice for selecting concrete proportions for 28-day compressive strength of 15-45 N/mm2. The relationship between water-cement ratio and the target mean 28-day compressive strength of non-air-entrained and air-entrained concrete are given in Table 9.17 when type I Portland cement is used either alone or together with a pozzolana. For other types of Portland cements (type II, III, IV or V), use of blast furnace slag or very high quantities of pozzolana, Table 9.17 is not applicable. Approximate strength - w/cementitious material ratio relationship should be developed for these cases. The water content is selected from Table 9.18 for the desired slump of concrete and the maximum size of aggregate. The cement content is calculated from water content and the water-cement ratio selected. The coarse aggregate content is next estimated from Table 9.19 for different MSA and fineness modulus of sand. Finally the fine aggregate content is determined by subtracting the sum of the absolute volumes of cement, water, coarse aggregate and air content from the unit volume of concrete.

9.3.3 The British Mix Proportioning Procedure The Department of the Environment (DOE) uses the relationship between free water-cement ratio and the compressive strength of concrete depending on the type of cement

Concrete Mix Proportioning

9.21

TABLE 9.19 Volume of Dry-Rodded Coarse Aggregate Per Unit Volume of Concrete (From ACI 211.1) Maximum Size of Aggregate, mm

Volume of dry-rodded coarse aggregate per unit volume of concrete for fineness module of sand 2.40 0.50 0.59 0.66 0.71 0.76 0.78 0.81 0.87

10 12.5 20 25 40 50 70 150

2.60 0.48 0.57 0.64 0.69 0.74 0.76 0.79 0.85

2.80 0.46 0.55 0.62 0.67 0.72 0.74 0.77 0.83

3.00 0.44 0.53 0.60 0.65 0.70 0.72 0.75 0.81

Note: The values will produce concrete mix with a workability suitable for reinforced concrete construction. For less workable concrete, such as that required for concrete pavement construction, the volumes may be increased by 10%. For more workable concrete, such as when concrete must be placed around congested reinforcing steel, or when placement is to be by pumping, they may be reduced by upto 10%.

(OPC, Sulphate-resisting Portland cement and Rapid hardening Portland cement), type of aggregate (uncrushed and crushed) used (Table 9.20). The water-cement ratio for the target mean compressive strength of concrete is determined using Table 9.20 and Fig. 9.1, and compared with the water-cement ratio specified for durability and the lower of these two values adopted. The water content is selected for different type, MSA and workability of concrete (slump or Vebe time) from Table 9.21. The cement content is calculated from the water-cement ratio and the water content of the concrete mix. The total aggregate content (on S.S.D basis) is next determined by subtracting the total of cement and water content from the wet density of concrete (Fig. 9.2), the wet density being dependent on water content and the relative density of the combined aggregate (on S.S.D. basis) . Finally, the proportions of fine and coarse aggregates are determined from Fig. 9.3, 9.4 & 9.5, depending on the water-cement ratio, MSA, the workability level and the grading of fine aggregate (with percentage of fine aggregate passing a 600 micron sieve). TABLE 9.20 Approximate compressive strengths of concrete mixes made with water-cement ratio of 0.5 (From ‘Design of normal concrete mixes’21) Compressive Strengths (N/mm2) at age (days) 3 7 28 91

Type of Cement

Type of Coarse Aggregate

Ordinary Portland Cement or Sulpate Resisting Portland Cement

Uncrushed

22

30

42

49

Crushed

27

36

49

56

Rapid Hardening Portland Cement

Uncrushed

29

37

48

54

Crushed

34

43

55

61

9.22

Handbook on Advanced Concrete Technology

TABLE 9.21 Approximate water contents (kg/m3) required to give various levels of workability (From ‘Design of normal concrete mixes21‘) Slump (mm)) Vee-Bee time (sec) Maximum Size of Aggregate (mm) 10 20 40

0-10 >12

10-30 6-12

30-60 3-6

60-180 0-3

Uncrushed Crushed Uncrushed Crushed Uncrushed

150 180 135 170 115

180 205 160 190 140

205 230 180 210 160

225 250 195 225 175

Crushed

155

175

190

205

Type of Aggregate

Note 1: When coarse and fine aggregates of different types are used, the water content is estimated by the expression: 2/3 Wf+1/3Wc Where Wf = water content appropriate to type of fine aggregate, and Wc = water content appropriate to type of coarse aggregate

9.3.4

Concrete Mix Proportioning in Accordance with Indian Standard Guidelines

The following basic data are required to be specified for mix proportioning of a particular grade of concrete: (a) Grade designation i.e. the characteristic compressive strength of concrete (that is, below which not more than 5 percent of the test results are expected to fall) at 28 days (fck); (b) Type of cement; (c) Type and maximum nominal size of aggregate to be used; (d) Limitation on the maximum water-cement ratio and the minimum cement content to ensure adequate durability for the type of exposure (see Tables 9.4 & 9.5). (e) Degree of workability desired (for guidance see Table 9.3); (f) Whether an admixture shall or shall not be used and the type of admixture and the condition of use; (g) Standard deviation (S) for compressive strength of concrete. The standard deviation has to be calculated from at least 30 test results, each test result being average of 3 cube test results at 28 days. Where sufficient test results for a particular grade of concrete are not available, the value of standard deviation given in Table 9.1 may be used for design of mix in the first instance. As soon as the results of samples are available, actual calculated standard deviation value shall be used and the concrete mix designed accordingly. When significant changes are made in the production of concrete batches (e.g. changes in the materials used, mix proportioning, equipment or technical control), the standard deviation value shall be separately calculated for such batches of concrete.

Concrete Mix Proportioning

9.23

The calculation of standard deviation shall be brought up-to-date after every change of mix proportioning. The Step-by-step procedure of mix proportioning is as follows: (a) The target mean strength is first determined as follows: f´ck = fck + K.S. where f´ck = target mean compressive strength at 28 days in N/mm2, S = Standard deviation, and K = a statistical value depending upon the accepted proportion of low results and the number of tests. Note: As per IS 456, the characteristic strength is defined as the strength of concrete below which not more than 5 percent of the test results are expected to fall. In such case, K = 1.65 in the above equation. (b) The water-cement ratio or the water-cementitious materials ratio (w/b ratio) for the target mean strength shall be estimated from an established relationship between the water-cement ratio or the water-cementitious materials ratio and the 28-day compressive strength of concrete. Otherwise, the maximum free water-cement ratio given in Table 9.5 (ie. maximum water-cement ratio or water-cementitious materials ratio specified for different exposure conditions) may be used as a starting point. The supplementary cementitious materials i.e. mineral admixtures shall also be considered in this watercement ratio calculation. TABLE 9.22 Preliminary selection of water-cement ratio for the given grade of concrete (From IRC 44) Sl.No. 1 2 3 4 5

Grade of Concrete M 25 M 30 M 35 M 40 M 50

Approximate water-cement ratio 0.50 0.45 0.42 0.38 0.34

6

M 60

0.28

Note: 1.

The supplementary cementitious materials that is, mineral admixtures shall also be considered in water-cement ratio calculations and be referred as water-cementitious materials ratio.

2.

The maximum w/c ratio or w/cementitious material ratio shall be restricted to 0.50 for the respective grade of concrete as per IRC 15.

(c) The water-cement ratio or the water-cementitious materials ratio (w/b ratio) thus selected should be checked against the limiting water-cement-ratio (for plain or reinforced concrete as the case may be) for the requirements of durability (Tables 9.4 & 9.5) and the lower of the two values adopted. The water content per cubic metre of concrete is selected from Table 9.23 for the nominal maximum size of aggregate. This water content

9.24

Handbook on Advanced Concrete Technology

is for concrete without any plasticizer or superplastitizer and corresponding to saturated surface dry angular aggregate, and for 25-50mm slump of concrete. This water content can be reduced by 10kg for sub-angular aggregate, by 20kg for gravel aggregate with some crushed particles, and by 25kg for rounded gravel to produce the same workability. For the desired workability (other than 25-50mm slump), the required water content may be established by trial. For every additional 25mm slump, about 3 percent water may be increased. Alternately, the workability of concrete can be increased by use of chemical admixture (i.e. plasticizer or superplasticizer) conforming to IS 910325. Water - reducing admixtures can reduce water content of concrete by 5-10 percent, whereas superplasticizing admixtures can reduce the water content by more than 20 percent. TABLE 9.23 Maximum Water Content Per cubic meter of concrete without any chemical admixture. (For 25-50 mm slump of concrete) (From IS 10262) Sl.No.

Nominal maximum size of aggregate (mm)

Maximum water content * (kg/m3)

1 2

10 20

208 186

3

40

165

* Water content corresponding to saturated surface dry (S.S.D.)aggregate.

(d) The cement content and the mineral admixture content per unit volume of concrete are calculated from the water-cement ratio and the quantity of water per unit volume of concrete. The cementitious materials content so calculated shall be checked against the minimum cement content of Tables 9.4 or 9.5 for the requirement of durability and greater of the two values adopted. The maximum OPC content shall not be more than 450kg/m3 of concrete as per IS 456. (e) The volume of coarse aggregate per unit volume of total aggregate is next estimated from Table 9.24, for the nominal maximum size of aggregate, grading of fine aggregate (Zone I, Zone II, Zone III or Zone IV), and for a water-cement ratio of 0.50. For other water-cement ratios, the adjustment in the volume of coarse aggregate is to be carried out as follows: For every +/– 0.05 change in water-cement ratio, the volume of coarse aggregate is to be changed to –/+ 0.01m3. For more workable concrete mixes e.g. for pumpable concrete mixes, the volume of coarse aggregate of Table 9.24 is to be reduced by about 10 percent. (f) The volume of fine aggregate per unit volume of total aggregate is automatically known, as the volume of coarse aggregate has been decided. (g) Having estimated the qualities of cement, water, plasticizer or superplasticizer and the mineral admixture, their total absolute volumes are next calculated, using their respective specific gravity values. The absolute volume of (coarse + fine aggregate)

Concrete Mix Proportioning

9.25

TABLE 9.24 Volume of Coarse Aggregate per unit volume of total aggregate (For water-cement ratio of 0.50) (From IS 10262 and IRC 44) Sl. No.

Nominal maximum size of aggregate (mm)

Volume of coarse aggregate * per unit volume of total aggregate for different grading zones of fine aggregate

1 2

10 20

Zone IV 0.50 0.66

Zone III 0.48 0.64

Zone II 0.46 0.62

Zone I 0.44 0.60

3

40

0.75

0.73

0.71

0.69

* Volume is based on aggregates in saturated surface dry condition. Note: 1.

The adjustments of volume of coarse aggregate for other water-cement ratios are to be made as follows: –/+ 0.01m3 for every +/– 0.05 change in water-cement ratio.

2. The volume of coarse aggregate given in the table are suitable for normal workable concrete mixes suitable for reinforced concrete construction. For more workable concrete mixes e.g. that is required when placement is by pump or when the concrete is required to be worked around congested reinforcing steel, it may be desirable to reduce the values by upto 10 percent.

per cubic metre of concrete is next determined by subtracting the total absolute volume of (cement + water + chemical admixture + mineral admixture) from 1 cubic metre. The quantities of coarse and fine aggregate (in kg/m3 of concrete) are next estimated from the above estimate of their total absolute volumes/ cubic metre of concrete, their proportion in concrete (from (e) and (f) and their specific gravity values. (h) The coarse aggregates of different size fractions are then combined in suitable proportions, so as to result in an overall grading conforming to Table 9.10, for the particular nominal maximum size of aggregate. (i) The calculated mix proportions shall be checked by means of trial mixes. Workability of the trial mix no. 1 shall be measured. The concrete mix shall be observed for any segregation or bleeding. If the measured workability of the trial mix No.1 is different from the stipulated value (with the tolerance limit), the mixing water and/or chemical admixture content shall be adjusted accordingly. With this adjustment, the mix proportion shall be recalculated keeping the free water-cement ratio (or the water-cementitious materials ratio) at the pre-selected value, which will comprise trial mix no.2. In addition, two more trial mixes no.3 and 4 shall be made with the water and chemical admixture content same as those of trial mix no.2, and varying the free water-cement ratio by + 10% of the preselected value. Concrete mixes no. 2, 3 and 4 normally shall provide sufficient information including the relationship between water-cement ratio and the 28-day compressive strength of concrete, from which, the concrete mix proportions for the field trial may be arrived at. An illustrative example of concrete mix proportioning using a normal superplasticizer (naphthalene-based) (and without any mineral admixture), for a grade of concrete of M40 suitable for concrete road construction (with a slump of 20-25mm) is given in Annexure “A”. Another illustrative example of concrete mix proportioning using a normal superplasticizer (naphthalene-based) and a mineral admixture (flyash) for a M20 grade high workability pumpable

9.26

Handbook on Advanced Concrete Technology

concrete mix with 20mm MSA is given in Annexure “B”. Such concrete mix is being produced and supplied by our Ready Mixed Concrete plants. The third illustrative example on concrete mix proportioning using all materials, grade of concrete and workability as in Annexure ‘B’, but with 40mm MSA. A fourth illustrative example on mass concrete mix proportioning (suitable for a concrete dam) for M15 grade of concrete (for a slump of 40mm) using 150mm MSA and Portland slag cement (with slag content of 55%) but without a chemical admixture is given in Annexure ‘D’. These examples are merely illustrative to explain the procedure; and the actual mix proportions shall be based on trial batches with the given materials.

ANNEXURE “A” Illustrative example on concrete mix proportioning for M40 grade concrete, suitable for concrete road construction (for a slump of 20-25mm) and using a normal naphthatenebased superplasticizder, and without any mineral admixture. A-1 DESIGN STIPULATIONS 1. 2. 3. 4. 5.

Grade designation Type of Cement Maximum nominal size of aggregate Workability required ment: Durability require (a) Minimum cement content (b) Maximum water-cement ratio 6. Type of aggregate 7. Degree of quality control expected at the site of construction

: : : :

M 40 OPC 43-grade 20mm 20-25 mm slump

: : : :

325kg/m3 0.50 Crushed, angular Good

8. Chemical admixture

: Naphthalene-based superplasticizer, with sp.gr of 1.10 (water-reducing type, able to reduce about 20 percent of the mixing water, maintaining the workability of concrete, 20-25mm slump).

A-2

TEST DATA FOR MATERIALS (a) Specific gravity of OPC (b) Specific gravity of 1. Coarse aggregate 2. Fine aggregate (c) Water absorption: (i) Corse aggregate (ii) Fine aggregate (d) Free (surface) moisture: (i) Coarse aggregate (ii) Fine aggregate (e) Sieve Analysis:

: 3.15 : : 2.74 : 2.62 : 0.5 percent : 1.0 percent : Nil (absorbed moisture also nil) : 2.0 percent

Concrete Mix Proportioning

9.27

1. Coarse Aggregate IS Sieve Size (mm)

Sieve analysis of coarse aggregate fractions (% passing) I 10-20mm II Below 10mm

20 10 4.75

100 2.80 Nil

100 78.30 8.70

Percentage of different fractions

I 60%

II 40%

Combined 100%

60 1.68 –

40 31.3 3.48

100 32.98 3.48

Percentage passing for graded aggregates (as per Table 9.10)

Remarks

95-100 25-55 0-10

Conforming to Table 9.10 (Table 2 of IS 383)

2. Fine Aggregate IS Sieve Size 10mm 4.75mm 2.36mm 1.18mm 600 micron 300 micron 150 micron

A-3

Percentage Passing

Remarks

100 98 93 85 52 20 5

Conforming to grading Zone II as per IS 383

TARGET STRENGTH FOR MIX PROPORTIONING The target average 28-day compressive strength of concrete f´ck = fck + 1.65 × 5 From Table 9.1, standard deviation S = 5N/mm2 (for ‘good’ control) Therefore, the target strength = 40 + 1.65 × S = 48.25 N/mm2

A-4

SELECTION OF WATER - CEMENT RATIO From Table 9.22 (From IRC 44), the approximate water-cement ratio for M40 grade of concrete = 0.38 From durability requirement, maximum water-cement ratio specified = 0.50. Hence water-cement ratio adopted = 0.38

A-5

SELECTION OF WATER CONTENT From Table 9.23, for 20mm MSA and for a slump of 25-50mm, maximum water content = 186kg/m3 of concrete. The superplasticizer proposed to be used is capable of reducing 20 percent mixing water, maintaining the slump of concrete of about 25mm. Therefore, the reduced mixing water content = 186 × 0.80 = 148.8 kg

A-6

CALCULATION OF CEMENT CONTENT Water-cement ratio = 0.38 Cement content = 148.8 / 0.38 = 391.5 kg. Minimum cement content specified for durability = 325kg /m3 of concrete. Hence cement content adopted = 391.5 kg/m3 of concrete

9.28

Handbook on Advanced Concrete Technology

A-7

PROPORTION OF VOLUME OF COARSE AND FINE AGGREGATE PER UNIT VOLUME OF TOTAL AGGREGATE From Table 9.24, volume of coarse aggregate corresponding to 20mm MSA and fine aggregate of grading zone II and for a water-cement ratio of 0.50 = 0.62m3/m3 of total aggregate. In the present case, water-cement ratio is 0.38. The water-cement ratio is lower by 0.12 (0.50-0.38) i.e. 0.12. Therefore Volume of coarse aggregate = 0.62 + ____ × 0.01 0.05

= 0.62 + 0.024 = 0.64 m3/m3 of total aggregate \ Volume of fine aggregate = 1 – 0.64 = 0.36 m3/m3 of total aggregate A-8

CALCULATION OF MIX PROPORTIONS The quantities of materials per unit volume of concrete shall be calculated as follows: (a) Volume of concrete = 1 m3 Mass of cement 1 (b) (i) Volume of cement = ______________ × _____ sp. gr of cement 1000 391.5 = _____ = 0.124m3 3150 148.8 (ii) Volume of Water = _____ = 0.148m3 1000 (iii) Volume of chemical admixture (superplasticizer @ 1.0 percent Mass of superplasticizer 1 by mass of cement = ____________________ × _____ sp.gr of superplasticizer 1000 3.915 3 = _____ = 0.00356 m 1100 __________________ Total = 0.276 m3 (c) Volume of (Coarse + Fine aggregate) = 1 – 0.276 = 0.724 m3/m3 of concrete (d) Mass of Coarse aggregate = 0.724 × Vol. of Coarse aggregate/m3 of total aggregate × sp. gr. × 1000 = 0.724 × 0.64 × 2740 = 1269 kg/m3 of concrete (e) Mass of fine aggregate = 0.724 × Vol. of fine aggregate/m3 of total aggregate × sp.gr × 1000 = 0.724 × 0.36 × 2620 = 683 kg/m3 of concrete

A-9

ESTIMATED QUANTITIES OF INGREDIENTS (in kg/m3 of concrete) Cement

= 391.5

Superplasticizer

= 3.915

Concrete Mix Proportioning

9.29

Coarse aggregate (S.S.D.) = 1269

A-10

Fine aggregate (S.S.D.)

= 683

Water

= 148.8

ACTUAL QUANTITIES OF ALL INGREDIENTS Quantity of Coarse aggregate (dry) = 1269/1.005 = 1262.7 kg. Since coarse aggregate is in dry condition, extra water 1262.7 × 0.5 required for absorption @ 0.5% = ___________ = 6.31 kg. 100 Quantity of fine aggregate (dry) = 683/1.01 = 676.2 kg. The fine aggregate is wet, and the free (surface) moisture (to be deducted from mixing water) = Total quantity of wet fine aggregate-quantity of fine aggregate (S.S.D) = 676.2 × 1.03 – 683 = 13.5kg.

Actual quantities of all ingredients per m3 of concrete are: Cement

=

391.5kg.

Superplasticizer

=

3.91kg

Coarse aggegate(dry)

=

1262.7 kg.

Fraction I (60%)

:

757.6 kg.

Fraction II (40%)

:

505.1 kg.

Fine aggregate (wet) = 676.2 × 1.03 = 696.5 kg. Mixing water = 148.8 + 6.31 – 13.5 = 141.6kg. A-11

The workability of concrete shall be measured in terms of slump, and the water content and the dosage of superplasticizer shall be adjusted for achieving the required workability of concrete, if required. The concrete mix proportions shall be recalculated for the actual water content and checked for durability requirements.

A-12 Two more trial mixes having variation of + 10 percent water-cement ratio of A-11 shall be carried out, and a correlation between three water-cement ratios and their corresponding 28-day compressive strengths shall be plotted, and the actual water-cement ratio required for the target 28-day compressive strength of concrete shall be estimated from this correlation graph. The final mix proportions (using this water-cement ratio) shall be recalculated and recommended for the field trial.

ANNEXURE ‘B’ Illustrative Example on concrete mix proportioning for M20 grade pumpable concrete (for a slump of 100-120mm) and using 20mm MSA, a superplasticizer and 25 percent flyash.

9.30

B-1

B-2

Handbook on Advanced Concrete Technology

DESIGN STIPULATIONS 1. Grade designation 2. Type of cement 3. Maximum nominal size of aggregate 4. Workability required 5. Durability requirement Exposure condition

: : : : : :

M20 OPC 43-grade 20mm 100-120mm slump ‘Moderate’(Reinforced concrete construction)

(i) Minimum cement content (ii) Maximum water-cement ratio 6. Degree of quality control expected at the site of construction 7. Type of aggregate 8. Chemical Admixture

: :

300kg/m3 of concrete 0.50

: : :

Good Crushed, angular Naphthalene-based superplasticizer with sp.gr of 1.1. (water-reducing type, capable of reducing mixing water of concrete by about 20 percent, with about 1 percent dosage, maintaining the workability of concrete of 50mm slump).

9. Mineral admixture

:

It is proposed to use flyash (satisfying the requirement of IS 3812-part 1) as 25 percent by weight of total cementitious materials.

TEST DATA FOR MATERIALS (a) Specific gravity of OPC (b) Specific gravity of: (i) Coarse aggregate (ii) Fine aggregate (iii) Flyash (iv) Superplasticizer (c) Water absorption: (i) Coarse aggregate (ii) Fine aggregate (d) Free (surface) moisture: (i) Coarse aggregate (ii) Fine aggregate (e) Sieve Analysis:

:

3.15

: : : :

2.74 2.62 2.20 1.10

: :

0.5 percent 1.0 percent

: :

Nil (absorbed moisture also nil) 2.0 percent

Concrete Mix Proportioning

9.31

(i) Coarse aggregate (20mm MSA): IS Sieve Size (mm)

Sieve analysis of coarse aggregate fractions (% passing) I 10-20mm II Below 10mm

20 10 4.75

100 2.80 Nil

100 78.30 8.70

Percentage of different fractions

I 60%

II 40%

Combined 100%

60 1.68 –

40 31.3 3.48

100 32.98 3.48

Percentage passing for graded aggregates (as per Table 9.10)

Remarks

95-100 25-55 0-10

Conforming to Table 9.10 (Table 2 of IS 383)

(ii) Fine Aggregate IS Sieve Size

Percentage Passing

10mm 4.75mm

100 98

2.36mm

93

1.18mm 600 micron 300 micron 150 micron

85 52 20 5

Remarks

Conforming to grading Zone II as per IS 383

B-3

TARGET STRENGTH FOR MIX PROPORTIONING The target average 28-day compressive strength of concrete = f´ck = fck + 1.65 × S From Table 9.1, standard deviation S = 4.0 N/mm2 (for ‘good’ control) Therefore, the target strength = 20 + 1.65 × 4 = 26.6 N/mm2

B-4

SELECTION OF WATER-(CEMENT + FLYASH) RATIO From Table 9.5, the water cement ratio for M20 grade of concrete = 0.55 Maximum water - cement ratio for the durability condition ‘Moderate’ is 0.50. Hence water-cement ratio adopted = 0.50

B-5

SELECTION OF WATER CONTENT From Table 9.23, for 20mm MSA and for a slump of 25-50mm, the maximum water content = 186 kg/m3 of concrete. The superplasticizer proposed to be used at dosage rate of 1 percent is able to provide 100-120mm slump, using a lower water content of 167kg/m3 of concrete (about 10% lower).

B-6

CALCULATION OF CEMENT AND FLYASH CONTENTS Water - cement ratio = 0.50 Cement content = 167/0.50 = 334 kg/m3 of concrete Minimum cement content specified for durability

9.32

Handbook on Advanced Concrete Technology

= 300 kg/m3 of concrete (Ref. Table 9.5). Hence cement content adopted = 334 kg. Using flyash in concrete, (cement + flyash) = 334 × 1.10 = 367 kg. so, OPC content = 275 kg/m3 of concrete, and flyash content = 92kg/m3 of concrete. \ Water-binder (w/b ratio) = 167/367 = 0.45 B-7

PROPORTION OF VOLUMES OF COARSE AND FINE AGGREGATE PER UNIT VOLUME OF TOTAL AGGREGATE From Table 9.24, Volume of coarse aggregate corresponding to 20mm MSA and fine aggregate of grading zone II, and for a water-cement ratio of 0.50 = 0.62m3/m3 of total aggregate. For a w/b ratio of 0.45, the corrected volume of coarse aggregate = 0.62 + 0.01 = 0.63 m3/m3 of total aggregates. Now, for high - workability pumpable concrete, the Volume of coarse aggregate shall be reduced by 10 percent. Volume of coarse aggregate becomes 0.63 × 0.90 = 0.567 m3/m3 of total aggregate. \ Volume of fine aggregate = 1 – 0.567 = 0.433 m3/m3 of total aggregate.

B-8

CALCULATION OF MIX PROPORTIONS The quantities of materials per unit volume of concrete shall be calculated as follows: (a) Volume of concrete = 1 m3 (b) Volume of cement, water, flyash and superplasticizer: (i) Volume of cement

275 Mass of cement 1 = _____________ × _____ = _____ = 0.087m3 sp.gr of cement 1000 3150

167 (ii) Volume of water _____ = 0.167m3 1000 Mass of flyash 1 (iii) Volume of flyash = ____________ × _____ sp.gr of flyash 1000 92 = _____ = 0.042 m3 2200 (iv) Volume of Superplasticizer @ 1.0 percent by mass of (Cement + Flysh) Mass of Superplsticizer 1 = ____________________ × _____ . sp.gr of superplazticizer 1000 3.67 3 = _____ = 0.0033m 1100 _______________ Total = 0.299m3 (c) Volume of (coarse + fine aggregate) = 1 – 0.299 = 0.701 m3/m3 of concrete

Concrete Mix Proportioning

(d) Mass of coarse aggregate) (on S.S.D basis)

9.33

= 0.701 × 0.567 × 2740

= 1089 kg/m3 of concrete (e) Mass of fine aggregate(SSD) = 0.701 × 0.433 × 2620 = 795kg/m3 of concrete. B-9

ESTIMATED QUANTITIES OF INGREDIENTS (In kg/m3 of concrete) Cement = 275 Flyash = 92 Superplasticizer = 3.67 Coarse Aggregate(SSD) = 1089 Fine aggregate (S.S.D) = 795

B-10 ACTUAL QUANTITIES OF ALL INGREDIENTS 1089 Quantity of coarse aggregate (dry) = _____ = 1083 kg. 1.005 795 Quantity of Fine aggregate (dry) = ____ = 787kg. 1.01 Since coarse aggregate is in dry condition, extra water required for absorption @ 0.5 percent = 1083 × 0.5/100 = 5.4 kg. The fine aggregate is wet, and the free (surface) water (to be deducted from mixing water) = quantity of wet fine aggregate - fine aggregate (S.S.D.) = 787 × 1.03 – 795 = 810.6 – 795 = 15.6 kg. \ Actual quantities of all ingredients per m3 of concrete: Cement : 275 kg Flyash : 92 kg. Superplasticizer : 3.67 kg. Fine aggregate (wet) : 810.6 kg Mixing water = 186 + 5.4 × 15.6 = : 156.8 kg. Coarse aggregate (dry) : 1083 kg. Fraction I (60%) : 650 kg. Fraction II (40%) : 433 kg. B-11 The workability of concrete shall be measured in terms of slump, and the water content and the dosage of superplasticizer shall be adjusted for achieving the required workability of concrete, if required. The concrete mix proportions shall be recalculated for the actual water content and checked for durability requirements.

9.34

B-12

Handbook on Advanced Concrete Technology

Two more trial mixes having variation of + 10 percent water-cement ratio of B-11 shall be carried out, and a correlation between three water-cement ratios and their corresponding 28-day compressive strengths shall be plotted, and the actual water-cement ratio required for the target 28-day compressive strength of concrete shall be estimated from this correlation graph. The final mix proportions (using this water-cement ratio) shall be recalculated and recommended for the field trial.

ANNEXURE ‘C’ Illustrative Example on concrete mix proportioning for M20 grade pumpable concrete (for a slump of 100-120mm) and using 40mm MSA, a superplasticizer and 25 percent flyash. C-1

DESIGN STIPULATIONS 1. Grade designation 2. Type of cement 3. Maximum nominal size of aggregate 4. Workability required 5. Durability requirement : Exposure condition (i) Minimum cement content (for 40mm MSA) (ii) Maximum water-cement ratio 6. Degree of quality control expected at the site of construction 7. Type of aggregate 8. Chemical Admixture

9. Mineral admixture

: : : :

M20 OPC 43-grade 40mm 100-120mm slump

: ‘Moderate’(Reinforced concrete construction) : 300 – 30 = 270kg/m3 of concrete : 0.50 : Good : Crushed, angular : Naphthalene-based superplasticizer with sp.gr of 1.1. (water-reducing type, capable of reducing mixing water of concrete by about 20 percent, with about 1 percent dosage, maintaining the workability of concrete of 50mm slump). : It is proposed to use flyash (satisfy the requirement of IS 3812-part 1) as 25 percent by weight of total cementitious materials.

C-2 TEST DATA FOR MATERIALS (a) Specific gravity of OPC (b) Specific gravity of:

: 3.15

Concrete Mix Proportioning

(i) Coarse aggregate (ii) Fine aggregate (iii) Flyash (iv) Superplasticizer (c) Water absorption: (i) Coarse aggregate (ii) Fine aggregate (d) Free (surface) moisture: (i) Coarse aggregate (ii) Fine aggregate (e) Sieve Analysis: (i) Coarse aggregate (40mm MSA): IS Sieve Size (mm)

40 20 10 4.75

: : : :

2.74 2.62 2.20 1.10

: 0.5 percent : 1.0 percent : Nil (absorbed moisture also nil) : 2.0 percent

Sieve analysis of coarse Percentage of different fractions aggregate fractions (% passing) Fraction-I Fraction-II Fraction-III I 54% II 27% III 19% 40-20mm 20-10mm 10-4.75mm 95 5 0 –

100 95 30 5

– 100 70 10

51.3 2.7 – –

27 25.6 8.1 1.3

Percentage passing for Desired graded aggregates Grading (Ref. Table 9.10) Combined 100%

19 19 13.3 1.9

97.3 47.3 21.4 3.2

95-100 30-70 10-35 0-5

(ii) Fine Aggregate IS Sieve Size 10mm 4.75mm 2.36mm 1.18mm 600 micron 300 micron 150 micron

C-3

9.35

Percentage Passing 100 98 93 85 52 20 5

Remarks

Conforming to grading Zone II as per IS 383

TARGET STRENGTH FOR MIX PROPORTIONING The target average 28-day compressive strength of concrete = f´ck = fck + 1.65 × S From Table 9.1, standard deviation S = 4.0 N/mm2 (for ‘good’ control) Therefore, the target strength = 20 + 1.65 × 4 = 26.6 N/mm2

9.36

Handbook on Advanced Concrete Technology

C-4

SELECTION OF WATER-(CEMENT + FLYASH) RATIO From Table 9.5, the water cement ratio for M20 grade of concrete = 0.55 Maximum water - cement ratio for the durability condition ‘Moderate’ is 0.50. Hence water-cement ratio adopted = 0.50

C-5

SELECTION OF WATER CONTENT From Table 9.23, for 40mm MSA and for a slump of 25-50mm, the maximum water content = 165 kg/m3 of concrete. The superplasticizer proposed to be used at dosage rate of 1 percent is able to provide 100-120mm slump, using a lower water content of 149kg/m 3 of concrete (about 10% lower).

C-6

CALCULATION OF CEMENT AND FLYASH CONTENTS Water - cement ratio = 0.50 Cement content = 149/0.50 = 298kg/m3 of concrete Minimum cement content specified for durability = 300 kg/m3 of concrete (Ref. Table 9.5). For 40mm MSA, the minimum cement content required = 300 – 30 = 270kg/m3 of concrete. Hence cement content adopted = 298kg/m3. Using flyash in concrete, (cement + flyash) = 298 × 1.10 = 328kg. so, OPC content = 246kg/m3 of concrete, and flyash content = 82kg/m3 of concrete. \ Water-binder (w/b) ratio = 149/328 = 0.45

C-7

PROPORTION OF VOLUMES OF COARSE AND FINE AGGREGATE PER UNIT VOLUME OF TOTAL AGGREGATE From Table 9.24, Volume of coarse aggregate corresponding to 20mm MSA and fine aggregate of grading zone II, and for a water-cement ratio of 0.50 = 0.71m3/m3 of total aggregate. For a w/b ratio of 0.45, the corrected volume of coarse aggregate = 0.71 + 0.01 = 0.72 m3/m3 of total aggregates. Now, for high - workability pumpable concrete, the Volume of coarse aggregate shall be reduced by 10 percent. Volume of coarse aggregate becomes 0.72 × 0.90 = 0.65 m3/m3 of total aggregate. \ Volume of fine aggregate = 1 – 0.65 = 0.35 m3/m3 of total aggregate.

C-8

CALCULATION OF MIX PROPORTIONS The quantities of materials per unit volume of concrete shall be calculated as follows: (a) Volume of concrete = 1 m3 (b) Volume of cement, water, flyash and superplasticizer: 246 Mass of cement 1 (i) Volume of cement = _____________ × _____ = _____ = 0.078m3 sp.gr of cement 1000 3150

Concrete Mix Proportioning

9.37

149 (ii) Volume of water _____ = 0.149m3 1000 Mass of flyash 1 (iii) Volume of flyash = ____________ × _____ . sp.gr of flyash 1000 = 82/2200 = 0.037m3 (iv) Volume of Superplasticizer @ 1.0 percent by mass of (Cement + Flysh) Mass of Superplsticizer 1 = ____________________ × _____ . sp.gr of superplazticizer 1000 3.28 3 = _____ = 0.003m 1100 ______________ Total = 0.267m3 (c) Volume of (coarse + fine aggregate) = 1 – 0.267 = 0.733 m3/m3 of concrete (d) Mass of coarse aggregate = 0.733 × 0.65 × 2740 (on S.S.D basis) = 1305kg/m3 of concrete (e) Mass of fine aggregate(SSD) = 0.733 × 0.35 × 2620 = 672kg/m3 of concrete. C-9

ESTIMATED QUANTITIES OF INGREDIENTS Cement Flyash Superplasticizer Coarse Aggregate(SSD) Fine aggregate (S.S.D)

C-10

= = = = =

(In kg/m3 of concrete) 246 82 3.28 1305 672

ACTUAL QUANTITIES OF ALL INGREDIENTS 1305 Quantity of coarse aggregate (dry) = _____ = 1298kg. 1.005 672 Quantity of Fine aggregate (dry) = ____ = 665kg. 1.01 Since coarse aggregate is in dry condition, extra water required for absorption @ 0.5 percent = 1298 × 0.5/100 = 6.5 kg. The fine aggregate is wet, and the free (surface) water (to be deducted from mixing water) = quantity of wet fine aggregate - fine aggregate (S.S.D.) = 665 × 1.03 – 672 = 685 – 672 = 13.0 kg.

9.38

Handbook on Advanced Concrete Technology

\ Actual quantities of all ingredients per m3 of concrete: Cement Flyash Superplasticizer Fine aggregate (wet) Mixing water = 149 + 6.5 – 13.0 = Coarse aggregate (dry) Fraction I (40-20mm) (54%) Fraction II (20-10mm) (27%) Fraction III (10-4.75mm) (19%)

: : : : : : : : :

246 kg 82 kg. 3.28g. 685kg 142.5 kg. 1298kg. 701 kg. 350 kg. 247 kg.

C-11

The workability of concrete shall be measured in terms of slump, and the water content and the dosage of superplasticizer shall be adjusted for achieving the required workability of concrete, if required. The concrete mix proportions shall be recalculated for the actual water content and checked for durability requirements.

C-12

Two more trial mixes having variation of + 10 percent water-cement ratio of C-11 shall be carried out, and a correlation between three water-cement ratios and their corresponding 28-day compressive strengths shall be plotted, and the actual water-cement ratio required for the target 28-day compressive strength of concrete shall be estimated from this correlation graph. The final mix proportions (using this water-cement ratio) shall be recalculated and recommended for the field trial.

COMPARISON OF ANNEXURE ‘B’ AND ANNEXURE ‘C’ For the same workability (100-120mm slump) and 28-day compressive strength of concrete and using same materials (i.e. cement, aggregates, superplasticizer and flyash), it can be seen that the quantity of cement required for M20 grade concrete using 40mm MSA is (275 – 246)/275 × 100 i.e. about 10% less than that in concrete using 20mm MSA.

ANNEXURE D Illustrative Example on Mass Concrete mix proportioning (Suitable for a concrete dam) for M15 Grade of concrete (for a slump of 40mm) using 150mm MSA, and Portland slag cement (containing 55% slag), but without any chemical admixture. D-1

DESIGN STIPULATIONS Grade of Concrete Cement Workability of concrete required Maximum size of aggregate (Crushed rock) Exposure condition Expected Quality Control to be exercised at site

: M15 : Portland slag cement (with 55% g.g.b.s.) : 40mm slump : 150mm : Moderate : Good

Concrete Mix Proportioning

D-2

TEST DATA ON MATERIALS Cement (i) 28-day compressive strength of PSC (ii) Specific Gravity of PSC

= 57.5 MPa = 2.98

Coarse Aggregate (dry) (i) Specific Gravity (ii) Water absorption (iii) Sieve Analysis: Sieve Size (mm)

150 75 37.5 19 9.5

100 Nil

Fraction II 75-37.5mm 100 35.8 Nil

4.75

Remarks:

: 2.68 : 0.2%

Percentage Passing Fraction I 150-75mm

9.39

Percentage of different fractions

Fraction III Fraction IV I 50% II 30% III 10% IV 10% 37.5-19mm 19-4.75mm

Combined

50

100 50 30.7 10 3.7

100 3.67 Nil

100 97.48 37.10

Nil

30 30 10.7

10 10 10 0.36

10 10 10 9.7 3.7

2.06

The combined grading of coarse aggregate satisfies the requirement of Table 9.11 to some extent, for crushed 150mm MSA.

Fine aggregate (Dry, Crushed Stone): (i) Specific gravity (ii) Water absorption (iii) Sieve Analysis:

: 2.68 : 0.9%

IS Sieve Size

Percentage Passing

Remarks

10mm 4.75mm 2.36mm 1.18mm 600 micron 300 micron

100 97.25 83.75 73.15 58.80 22.65

Conforms to grading zone II as per IS 383 (Ref. Table 9.8)

150 micron

15.95*

* For crushed stone fine aggregate, maximum limit is 20%.

D-3

CONCRETE MIX PROPORTIONS The concrete mix proportioning for M15 grade of concrete is based on 15cm size concrete specimens prepared with concrete wet-screened through 40mm size sieve. As per literature14, such specimens indicate higher compressive strength of concrete, and

9.40

Handbook on Advanced Concrete Technology

therefore, the target mean strength will be about 25% higher i.e. (15+1.65x3.5) × 1.25 = 26.0N/mm2,. (The standard deviation = 3.5N/mm2 from Table 9.1). From Table 9.5, the water-cement ratio selected for M15 grade concrete = 0.60. For the exposure condition ‘moderate’, the maximum water-cement ratio specified in Table 9.5 is also 0.60. The water content of concrete using 150mm MSA for a slump of 40mm selected from Table 9.18 = 125kg/m3 of concrete. \ Cement content = 125/0.60 = 208 kg/m3 of concrete. But for the durability requirement (Table 9.5) for ‘Moderate’ exposure condition, the minimum cement content required = 240kg/m3 of concrete (for 20mm MSA). For higher MSA, this can be reduced by 30kg/m3 of concrete (Ref. Table 9.6). For Portland slag cement (containing 55% slag), the cement requirement is about 10% higher than that for OPC concrete. Therefore PSC content = (240 – 30) × 1.1 = 231 kg/m3 of concrete. The proportion of coarse aggregate (for manufactured fine aggregate of grading zone II) is 77% (percent of total aggregate by absolute volume) from Table 9.14. Therefore, % of fine aggregate = 23% (having the same specific gravity as that of coarse aggregate). Estimated Quantities of all Ingredients, kg/m3 of Concrete By Absolute volume method: Absolute volume of PSC = 231/2.98 × 1000 = 0.0775 m3 Volume of water = 125/1000 = 0.125 m3K Total = 0.202 m3 Absolute volume of (Coarse + fine aggregate) = 1 – 0.202 = 0.798 m3 Mass of coarse aggregate (S.S.D) = 0.798 × 0.77 × 2680 = 1646 kg. Mass of fine aggregate (S.S.D) = 0.798 × 0.23 × 2680 = 492 kg. Quantity of dry coarse aggregate = 1646/1.002 = 1642 kg. Quantity of dry fine aggregate = 492/1.009 = 487 kg. D-4 Actual Quantities of all ingredients per m3 of concrete : Cement (PSC) Fine aggregate (dry) Coarse aggregate (dry): Fraction I (150-75mm) (50%) Fraction II (75-37.5mm) (30%) Fraction III (37.5-19mm)(10%) Fraction IV (19-4.75mm)(10%)

= 231 kg. = 487 kg. = = = =

821kg. 493 kg. 164 kg. 164 kg.

Water (including that for absorption for aggregates) = 125 + (1646 – 1642) + (492 – 487) = 134 kg.

Concrete Mix Proportioning

9.41

The volume of mortar per m3 of concrete : Materials PSC: 231/2.98 × 1000

Volume =

0.0775m3

Fine Aggregates: 487/2.68 × 1000 = 0.1817m3 Water: 134/1000 = 0.134 m3 Total = 0.3932m3 This satisfies the requirement of Table 9.15, which specifies the proportion of mortar in concrete (for crushed aggregate of 150mm maximum size) as 0.39m3/m3 of concrete. D-5

As usual, the workability of concrete shall be measured after wet screening through 40mm I.S. sieve, and the water content is to be adjusted if required. The concrete mix proportions shall be recalculated for the actual water content and checked for the durability requirements.

D-6 Two more trial mixes having variation of + 10% water-cement ratio of D-5 shall be carried out, and a correlation between three water-cement ratios and their corresponding 28-day compressive strength of concrete shall be plotted, and the actual water-cement ratio required for the target 28-day compressive strength of concrete shall be estimated from this correlation graph. The final mix proportions (using this water-cement ratio) shall be recalculated and recommended for field trial.

References 1. IS 456: 2000 Code of practice for plain and reinforced concrete. Bureau of Indian Standards, New Delhi. 2. IS 1343: 1980 Code of practice for prestressed concrete, Bureau of Indian Standards, New Delhi. 3. IRC 15Standard specification and code of practice for construction of concrete roads. The Indian Roads congress, New Delhi. 4. IRC 21Standard specification and code of practice for road bridges. The Indian Roads Congress, New Delhi. 5. ACI 318Building code requirement for reinforced concrete. American Concrete Institute. 6. IS 10262: 1982 Indian Standard guidelines for concrete mix proportioning. Bureau of Indian Standards, New Delhi, 2009. 7. IRC 44Guidelines for cement concrete mix design for pavement, 2008, Indian Roads Congress, New Delhi. 8. IS 12269 : 1987 Specification for 53 grade ordinary Portland cement. Bureau of Indian Standards, New Delhi. 9. IS 8041:1990 Specification for rapid hardening Portland cement. Bureau of Indian Standards, New Delhi. 10. IS 12600:1989 Specification for low heat Portland Cement. Bureau of Indian Standards, New Delhi. 11. IS 1489(Part 1): 1991 Specification for Portland pozzolana cement (flyash based). Bureau of Indian Standards, New Delhi.

9.42

Handbook on Advanced Concrete Technology

12. IS 455Specification for Portland slag cement. Bureau of Indian Standards, New Delhi. 13. B.S.5328Part 1 Guide to specifying concrete. British Standards Institution, London. 14. Neville, A.M. Properties of concrete, Fourth Edition, 2000, Pearson Education Asia Pvt. Ltd. England. 15. IS 383:1970 Indian Standard Specification for coarse and fine aggregates from natural sources for concrete. Bureau of Indian Standards, New Delhi. 16. ACI 211.1Mass concrete Mix proportioning. The Manual of concrete practice, American Concrete Institute. 17. IS 457:1957 Indian Standard Code of practice for general construction of plain and reinforced concrete for dams and other massive structures. Bureau of Indian Standards, New Delhi. 18. ACI 211.4R Guide for selecting proportions for high-strength concrete with Portland cement and flyash. Manual of concrete Practice, American Concrete Institute. 19. MAITI, S.C. Flyash concrete mix proportioning. Proceedings, National Seminar on Utilization of Flyash in Water Resources Sector, 11, 12 April 2001. CSMRS, New Delhi. 20. ACI 211.1 Standard practice for selecting proportions for normal, heavy weight and mass concrete, American Concrete Institute. 21. TEYCHENNE, D.C., NICHOLLS, J.C. FRANKLIN, R.E. and HOBBS, D.W., (1988) Design of normal concrete mixes. Department of Environment, Building Research Establishment, London. 22. ACI 211Recommended practice for selecting proportion for no-slump concrete. American Concrete Institute. 23. Road Research Laboratory. Design of concrete mixes. Road Note No. 4, 1950. Her Majesty’s Stationery Office, London. 24. McINTOSH, J.D. and ERNTROY, H.C, (1955) The workability of concrete mixes with 3/8 inch aggregates. Cement and Concrete Association Research Report No.2, London. 25. IS 9103:1999 Specifications for admixtures for concrete. Bureau of Indian Standards, New Delhi.

10 Concrete Production and Supply Vijay Kulkarni and Ganesh Kaskar

INTRODUCTION Concrete production methods have undergone a sea change in the recent past. A perceptible shift is occurring rapidly from volume batching to weigh batching. Hence, this chapter will essentially focus on concrete produced through batching plants. A typical production process of concrete can be divided into four major areas, namely, storage of material, batching of materials, mixing, and the control process. This chapter covers all the four areas. Exhaustive checklists are included at the end, covering each of these areas. In recent times, a number of innovative technological changes have occurred in batching and mixing plants, making the production process more mechanized, accurate and hassle-free. In the state-of-the-art plants, now available in the country, the entire production process is computer controlled. The principal functions of a stationary concrete production plant comprise of the following: • • • • • • •

Storage of materials – Silos, containers and bins Conveying systems (belt/screw conveyors) Batching arrangement Measuring and recording equipment Electrical, hydraulic and pneumatic drives Mixing equipment Control systems.

Figure 10.1 shows typical plant layout of batching and mixing facility. In Figs. 10.1, 10.2, 10.3 and 10.4, provisional layouts of batching plants (30m3/hr and 60m3/hr) are furnished which may serve as guide in finalizing the layout at site.

10.2

Handbook on Advanced Concrete Technology Cement /SCM Aggregate conveyor

Twin-shaft mixer

Aggregate bins

Fig. 10.1 Typical batching plant and mixing plant

10.1

STORAGE OF MATERIALS

The type of material, its condition and quantity involved, will affect the method of storage on site or at a concrete plant.

10.1.1

Cement

The storage system of cement must prevent the ingress of water or moisture; if not the cement will set and be unusable. The cement silo is the commonly used storage system with a typical capacity of 50 - 100 tonnes. Both Portland cements and cement replacement materials can be stored in this way. Cements are usually transported by a bulk tanker and discharged into the silo with the aid of compressed air. Most silos will be aerated to prevent the compaction of cement that may block the silo gates. In certain semi-urban areas, where cement is not supplied in bulk and is received in 50-kg bags, concrete manufacturer is constrained to use semi-mechanized system of handling and conveyance of this material. In such case, covered storage system is essential to ensure that cement is protected from moisture and dampness. Adequate health and safety precautions are essential in handling of cement and other supplementary cementitious materials.

10.1.2 Aggregate Both coarse aggregate and fine aggregates (sand and/or crusher fines) are stored in bins, either at ground level or at elevated locations. The aggregate is usually stored as single sizes, with bin capacities varying between 40 and 60 tonnes. The bins may have some form of screening to prevent the ingress of water and contaminants, for example leaves. Another method used on sites is to store the aggregates on hard ground (for example, concrete ground slabs), with partitions separating the materials. Overfilling of materials can lead to contamination as illustrated in Fig. 10.5 and hence to be avoided.

Fig. 10.2 Typical layout of 30m3/hr batching plant

Concrete Production and Supply 10.3

Fig. 10.3 Typical layout of 60 m3/hr batching plant (compartment type bins)

10.4 Handbook on Advanced Concrete Technology

Fig. 10.4 Typical layout of 60m3/hr batching plant (in-line bins )

Concrete Production and Supply 10.5

10.6

Handbook on Advanced Concrete Technology

Fig. 10.5

10.1.3

Aggregates from one bin should not spill over the other, leading to inter-mixing

Admixtures

Liquid admixtures are normally stored in either 200 litre drums or larger tanks, typically 1000 litres in volume. Admixtures should be used in rotation and agitated where essential. Due to the small volumes added to the concrete, accurate dispensing must be achieved. Figure 10.6 shows a typical admixture dispenser arrangement, measuring into a sight glass, and discharged through the mixer water supply. However some of the modern batching plants have the provision of measuring the admixture by weigh through load cell operated weigh-hoppers.

Fig. 10.6 Admixture dispenser

10.1.4

Mineral Admixtures

From the consideration of sustainability and improvements in the properties of concrete, especially its long-term durability, present-day concrete necessarily contain one or more mineral admixture. These may include the following: • Fly ash (conforming to IS 3812) • Ground-granulated blast furness slag (conforming to BS 6699 and IS 12089) • High reactive metakaolin (IS draft specification) • Silica fume (conforming to IS 15388)

Concrete Production and Supply

10.7

These materials are not cementitious themselves, but possess either pozzolanic or latent hydraulic properties. Therefore, while using these mineral additives, considerable precautions are needed to ensure that the material used has adequate quality (see checklist). Thus, conformity to specifications is highly essential.

10.1.5 Water Water can be used either directly from the muncipal mains supply or from a well. For ensuring sustainability, it is advisable that water is used sparingly. Many batching plants therefore supplement fresh water supply partly with water obtained from wash water system.

10.2 BATCHING Concrete is typically batched by weight of materials to ensure accurate measurement of mix components.

10.2.1 Batching and Loading Equipment 10.2.1.1 Cement The cement weighers are large enough to cope up with all concrete mixes. The weigh hopper has steep angle walls so that cement does not adhere to the surface. An equalizing hose connects the cement hopper and the mixer drum to ensure a clean working environment. The mechanical weigher uses the knife edge balance principle. Larger plants are equipped with electro mechanical weighing systems. Now-a-days, micro-processor based load cell systems ensure that the material is weighed accurately.

10.2.1.2 Water The plants are either equipped with a water meter or a water batch weigher, precision water measuring device ensures exact addition of mixing water required to be added. A centrifugal pump is often used after the batcher, to pump the water through the nozzles of the power mixer. In certain plants which have microprocessor controlled system the aggregate surface moisture corrections are done automatically and the mix consistency or workability is accurately maintained without visual checks or manual measurements.

10.2.1.3 Aggregates The aggregate weighers are generally identical to the cement weighers in the plant. Aggregates in star bin arrangement are unloaded into the loader from pneumatically opened batching (flaps) gates. The flap gates are operated from the control room either manually or automatically. Vibrators are installed in the sand bins to make sure that the sand flows out freely. The aggregates are unloaded from the compartments by gravitational fall to the weigher below through the batching gates.

10.8

Handbook on Advanced Concrete Technology

The batching gates are larger for rapid weighing. Electromechanical weighers are used for accurate results. If the production plant has twin mixer arrangement the aggregates and cements are alternately discharged into the left and right mixer pans. In an inline aggregate storage production plant (Fig 10.4), the aggregates from the silos drop on to the weigher belt through electro-pneumatic segmental batching gates and are conveyed to the elevator skip or an inclined conveyor which feeds the mixer.

10.2.1.4 Concrete Admixtures Admixtures are batched by either volume or weight and usually added to the water prior to mixing, or at the same time, to give efficient dispersion.

10.3

MIXING ARRANGEMENTS

There are various types of concrete mixers used for the production plant. The two basic types are free fall mixers and power mixers. Earlier, indigenously manufactured plants had free fall mixers. A free fall mixer consists of a rotating drum with blade fixed on the drum’s interior. As the drum rotates, the material inside is lifted and dropped. The drum is loaded and emptied by changing the direction of rotation, dropping a flap or tipping it. Most of the modern plants have power mixer. The power mixer sets in motion the materials positively. The materials get thoroughly mixed by rotating arms. These mixers have shorter mixing time, give better homogeneity, consistency and strength to the concrete. Besides, they have better facility for inspection. Pan mixer, or twin shaft mixer are the most common in this type.

10.4 CONTROL SYSTEMS Most of the currently-available production plants offer automatic systems for control functions. These are required for better quality control, higher economy and superior working conditions. Control systems are capable of controlling the entire process of production, i.e. storing of the desired recipes of different mixes, batching, mixing, delivery tickets, inventory controls, etc. Factors Affecting Uniformity of Fresh Concrete Some factors affecting the uniformity of the freshly batched concrete are summarized in Table 10.1. TABLE 10.1 Factors affecting uniformity of concrete Factor

Possible Effect

Precaution to minimize effect

Poor mixer efficiency

Non uniform concrete and variable properties throughout the mix

Reduce “build up” inside mixer; keep clean. Monitor blade wear and do not overload

Fast addition of materials to drum or truck mixers

Cement balling

Ensure proper mixing and ribbon feeding

Admixtures addition at wrong stage

Reduction in effect of admixture

Add admixture to the mixing water to disperse

Concrete Production and Supply

10.9

10.5 TRANSPORTATION Many factors affect the choice of transportation method. The basic objective while transporting concrete is to pour it into the structure in the shortest possible time and without altering any of its properties in fresh or hardened condition. Considering this, the most common method to transport concrete over distances is to use “Truck Mixers” as they keep the concrete under agitation while in transit. Concrete pump can then be used at the project location to transport concrete at heights and also to move it horizontally. The following checks are necessary when concrete is transported using truck mixers. • Mixer revolution should usually be 4 to 14 rpm. • Water addition is not permitted unless planned, approved and supervised by authorized inspector. • It is preferable that mixer drum is covered with wet hessian cloth in warm/hot weather. • Hopper at the top should be covered during heavy rains. • Drum should be rinsed and cleaned prior to loading concrete • While unloading concrete directly in formwork or on a platform, chute extended fully to minimize the drop of concrete to less than 0.5 m. Truck mixers are found to be suitable for transporting concrete mix in horizontal direction while concrete pumps are found suitable for various job site uses as well as for transportation of mix in both horizontal and vertical directions.

10.6 BATCHING SPECIAL CONCRETE When producing light or heavy concretes, the following precautions need to be taken to ensure a uniform quality of concrete. Light aggregate concrete - is defined as having a plastic density of less than 2000 kg/m3 and may give the following production problems. • The aggregates due to their rounded shape can be a hazard to personnel if spilt on a plant yard. • In windy conditions the material may be blown from the conveyor or storage bins, therefore protection should be in place. • Due to their low density, lightweight materials tend to stick in damp storage bins. • When mixing the material tends to float in the cement paste, so increasing the risk of segregation. In pan mixers the aggregate may breakdown due to its friable nature. Heavy aggregate concrete - is defined as having a plastic density of greater than 2600 kg/m3 and may give the following problems. • The increased density of the aggregate will mean that the volume held in each bin will be reduced. • When mixing there will be more wear and tear on mixer bearings and blades. There is also a risk of segregation when using a heavy coarse aggregate and natural sand, due to their differing densities.

10.10

Handbook on Advanced Concrete Technology

• Volumes of heavy aggregate concrete may have to be reduced due to the truck carrying capacities.

10.7

CHECKLIST FOR VARIOUS ITEMS

A-1 Checklist for Cement The following needs to be checked before using the material: • Source (Factory)/Brand (Manufacturer) • Chloride, sulphates and other aggressive chemicals in environment (Soil investigation report) • Suitability of type of Cement (Ordinary Portland Cement, Pozzolana Portland Cement, Portland Slag Cement, Sulphate Resistant Cement etc.) • Week of manufacture/Date of receipt at site • Actual strength of cement at different ages (1, 3, 7 and 28 days) • Consistency of strength (coefficient of variation from the manufacturer for each month) • Fineness of cement and other physical properties (Manufacturer or Laboratory Test Certificate) • Chemical properties (Manufacturer or Laboratory Test Certificate) • Pozzolana or slag content in blended cements • Tricalcium aluminate percentage in sulphate resisting cement • Chloride content (essential for prestressed concrete) • Heat of hydration (essential for massive concrete structures like dams, massive foundations, turbo generator decks etc.) • Temperature of cement (Hot weather concreting) • If cement is supplied in bulkers, check lock seals on all openings • Physical examination of bags if supplied in bags • Net content of cement bag • Presence of lumps (essential in monsoon) • Cement storage near the mixer or batching plant • Cement warehouse or godown at site (Cement received first, should be consumed first) • Cement storage silo (if cement at site is stored in bulk) for leakproofness and cleanliness • Calibration of weighing devices used to weigh cement when received from the supplier • Calibration of cement weighment system for batching • Working of screw conveyors on storage silo or batching mixing plant • Adequacy of quantity of cement at site

Concrete Production and Supply

10.11

A-2 Checklist for Aggregates The following items are required to be checked before using the material: • Source/Name of the Supplier/Crusher Owner • Type of aggregate (natural rounded, natural irregular, crushed cubical, crushed flaky, crushed gravel etc.) • Maximum size (40 mm, 20 mm, 10 mm, etc.) • Hardness or crushing value (geological nature eg. basalt, granite, limestone, pumice etc.) • Sieve analysis- Grading and consistency of grading of different batches • Presence of oversize aggregates • Presence of finer components of aggregates (passing 4.75 mm size sieve) • Presence of dust (passing 75 micron size sieve) • Presence of deleterious materials (clay coating, clay lumps, mica, alkali slag, coal residues, organic matter, etc.) • Chloride content • Need for washing • Specific gravity • Dry loose bulk density • Surface texture • Chemical stability (alkali aggregate reaction) • Organic matter • Surface moisture content in case of wet aggregates • Absorption (percentage by weight) in case of dry aggregates • Storage in separate heaps or bins • Protection against accidental ingress of soil or dust on which material is unloaded at site • Protection against ingress of snow or ice in extreme cold weather or protection against high ambient temperatures • Temperature in case of extreme (hot & cold) weather • Calibration of weighing system for weigh batching • Adequacy of quantity at site. A-3 Checklist for Chemical Admixtures The following items are required to be checked before using the material: • Source/Brand/Manufacturer • Type to be used (plasticer, superplasticer, retarder, accelerator, air entrainer or combination)

10.12

Handbook on Advanced Concrete Technology

• Reduction of water content/increase of workability/increase of slump retention/reduction of cement content/increase of concrete strength/increase or decrease of setting time of concrete, depending on purpose for use • Performance over reference or control (without chemical admixture) mix • Compatibility with cement proposed to be used in concrete/mortar • Manufacturers/Suppliers recommendations and specifications • Label on the container for proper identification • Shelf-life or expiry date • Storage, in cool place and labels not obliterated • Consistency of the quality (uniformity requirement as per IS 9103) • Correct dosage - not more, not less (dosage within the range specified by manufacturer) • Accurate gauging by dispenser/measuring cylinder of correct size • Dispenser/measuring cylinder washed and cleaned • Specific gravity and solid content • Percentage of chlorides, sulphates and other impurities • Adequacy of quantity at site. A-4 Checklist for Mineral Admixtures The following items are required to be checked before using the material: • • • • • • • • •

Calcined Clay: Source/Manufacturer Manufacturer’s or supplier’s test certificate Physical requirement of fineness, lime reactivity, compressive strength and drying shrinkage (Refer IS 1344) Consistency of physical and chemical properties Storage - protection from rain and moisture Quantity required per batch Size and calibration of weighing system for batching Adequacy of quantity required at site

Fly ash: • Source/Supplier • Manufacturer’s or supplier’s test certificate • Chemical requirements - proportion of chemical oxides, loss on ignition • Physical requirements of fineness, lime reactivity, compressive strength, drying shrinkage and soundness (Refer IS 3812) • Consistency of physical and chemical properties (uniformity requirement as per IS 3812) • Presence of deleterious material (unburnt coal particles)

Concrete Production and Supply

10.13

• Storage - weather tight against dampness to minimise warehouse deterioration • Moisture content • Quantity required per batch considering technical requirement and ambient temperatures • Size and calibration of weighing system for batching • Adequacy of quantity required at site Ground/Granulated Blast Furnace Slag: • Source/Manufacturer • Manufacturer’s or supplier’s test certificate • Chemical and physical requirements (Refer IS 12089 and BS 6699) • Glass content • Proper identification of storage area • Consistency of physical and chemical requirements • Storage - (bins/silos - protection from moisture) • Quantity required per batch • Calibration of weighing system for batching. • Adequacy of quantity required at site Silica Fume: • Source/Supplier/Manufacturer • Manufacturer’s or supplier’s test certificate • Ignition loss, moisture content, fineness, water requirement for standard consistency and activity index (Refer IS 15388) • Availability in solid (powder)/densified solid/slurry form • Measuring and batching devices • Compatible chemical admixture for good dispersion • Safety while handling • Quantity required per batch • Size and calibration of weighing system • Adequacy of quantity required at site A-5: Checklist for Water The following items are required to be checked before using the material: • Source of supply (municipal, bore well at site, tanker water etc.) • Presence of chlorides and sulphates • Presence of oils, acids, alkalis, salts, sugar, organic impurities etc. • Extent of acidity or alkalinity • Potable quality for water/ice • Water storage tanks are above ground level

10.14

Handbook on Advanced Concrete Technology

• Water contamination due to workers taking bath, cleaning utensils or washing clothes • Water storage tanks located in shade (avoid black colour tanks specially in hot weather) • Proper containers for batching water manually • Calibration of water meter if being used for batching water • Water pipeline protection from accidental damage due to movement of vehicles and other traffic on site • Temperature of water in extreme (both hot or cold) weather conditions • Heating arrangement of water in cold weather (sub zero) • Cooling arrangement of water in hot weather using ice flakes • Solids within permissible limits (Refer IS 456) • Water to cement ratio as per environmental conditions • Adequacy of quality of water and ice flakes at site A-6: Checks for Batching The following checks are necessary where concrete materials are batched mainly using computerised batching - mixing plant or RMC plant: • Selection of correct programme of the desired mix and secured by key • Dry mix input per m3, calculation of individual batches and individual input of material moisture • Input of additives in percentage of cement • Calculation and checking of cement and mineral admixture • Adequate speed in case of large pours – bagged cement loading (using a loader) • Adequate back up supply of bulk cement in case of large pours • Tolerance control at each weigher i.e. for aggregates and cement and automatic change over of weighing ranges • Rinsing water correction for admixture • Consistency (workability) correction. Automatic actual value moisture recording and correction • Operation on fully automatic mode (less chances of human error) instead of semi-automatic or manual • Monitoring of batching time automatically or by a batching monitor • Hydraulic operations of all gates for free movement (no jamming) • Screw conveyors (for cement feeding) free and having easy movement (no jamming) • Correct identification of silo to unload cement and mineral admixture (no mixup) • Materials available in adequate quantity for the full requirement/supply • Chances of scraping aggregates from bin bottom avoided due to depleted stock (aggregates in bin bottom generally contain more silt and dust which settles in the bin)

Concrete Production and Supply

10.15

• Drag line (star bin plants), bucket conveyors (vertical plants), conveyors (inline horizontal plants) used to feed/load aggregates in working condition • Power supply connection • Consistency of power supply. Stand by arrangements • Vibrators on moist sand bins in working condition • Temperature of all ingredients • Zero weigher and weigher test circuit • Water addition cross checked by workability • Absorption of water by dry aggregates taken into account while calculating water to be added to the mixer • Bins not overflowing (aggregates of various sizes/grading properly stored) • The partition walls of batching plant (star bin type) adequately strengthened to take full horizontal pressure when adjoining bins are empty • Hoppers of weigh batchers are clean, smooth and levelled. • Silos watertight to prevent lumping of cement which can result in jamming of screw conveyors • Removal of excess materials due to overflowing of the hopper • Batch log based on desired/actual values • Automatically compiled delivery note • Monitoring quality test intervals in relation to each mix (frequency of casting cubes and slump tests etc.) • Day to day plant log • Advance order memory • Text display, standard keys with user control • Various screen masks for information (windows) • Event report memory • Text display, standard keys with user control • Various screen masks for information (windows) • Caliberation done frequenty as per the frequency suggested in IS 4926 • Event report memory • Consumption statistics and log printer A-7 Checklist Mixing Plant The following checks are necessary for concrete mixing using mixing plant. • Availability of all materials • Cleaning and replacing worn out or non functioning parts of drive system, wire rope of skip or belt conveyor system, screw conveyor system and other control and recording systems and hydraulic systems and hydraulic gates • All nuts, bolts and keys tightened

10.16

Handbook on Advanced Concrete Technology

• • • • • • • • • • • • • • •

Interior of the drum, blades cleaned and nozzles functional Worn-out wearing plates and blades replaced if required Grease on all nipples Overloading of the drum Engine or power leader and their control arrangements Experienced batching – mixing plant operator Availability of proper slump cone and proper and adequate number of cube moulds Inspection of mix after unloading – uniform, correct workability (slump) and cohesive (not segregated) Monitoring of mix workability by electrical resistance (dry mix) or by motor output (wet mix) Buttering of the mixer with mortar or rich/oversanded mix in the first batch Mode of operation (manual/semi automatic/fully automatic) Tell tale lamp test circuit Error characteristic field Automatic fault alarm Mixing time in relation to each mix

A-8: Checklist for Concrete Transportation The following checks are necessary when concrete transportation is mechanised: • Type of mechanisation proposed and its adequacy • Distance of travel • Safety requirements for operators and other worker around • Workability drop during the journey • Segregation of mix if yes, remixing • Container/Drum/Bucket transporting clean, watertight and leakproof • Bucket used to transport concrete – side slope not less than 60º with wide, free working and tight closing gate • Bucket discharge gate easy to open and close depending on concrete quantity required to be unloaded • Trained and experienced operators • Operators/Workers using safety apparel • Concrete covered and protected against rain, hot/ cold weather and strong winds • Concrete covered for protection against contamination by oil, dirt, dust etc. • Guard railing provided all round to prevent accidental fall • Concreting sequence and route of travel of equipment • Concrete unloaded without segregation • Adequate workability for placing and compacting or within the range specified

Concrete Production and Supply

10.17

Truck mounted Transit Mixer: The following checks are necessary when concrete is transported using truck mounted transit mixers: • Number of revolutions of drum within prescribed limits • During transportation drum revolutions to be 4 to 14 rpm • Water addition not permitted unless planned, approved and supervised by authorised inspector • No addition of water during transportation (if water allowed to be carried in water tank for washing/cleaning purpose) • Addition of admixture to improve workability with approval • Mixer drum covered with wet hessian cloth in warm/ hot weather • Hopper (to receive concrete) at the top covered during heavy rains • All points necessary for the truck engine and drum drive as recommended by their manufacturers so that no breakdown takes place during the journey • Adequacy of fuel, engine oil and hydraulic oil. • Mobile telephone or wireless set with the driver in working condition (for emergency and technical instructions from the plant or site) • Hydraulic drivers and systems • Drum rinsed and cleaned after unloading concrete completely • Route of travel and concrete unloading (without segregation) • While unloading concrete directly in formwork or on platform, chute extended fully to minimize the drop of concrete to less than 0.5 m.

References 1. Dewar J. D. and Anderson, R. (1992) Manual of Ready Mixed Concrete, Blackie Academic and Professional, Glasgow, UK. 2. Batching, Mixing and Transporting of Concrete, (1996) Ambuja Cement Technical Literature Series. 3. Ready-Mixed Concrete - Code of Practice (2nd Revision), IS 4926: 2003, Bureau of Indian Standards, New Delhi.

11 Placing, Compaction and Curing of Concrete N.V. Nayak and Manish Mokal

11.1 INTRODUCTION Once the concrete is transported near the placing point using transit mixers, dumpers, etc., the subsequent activities involved are placing, compaction and curing the concrete in the designated locations. This chapter deals with various methods of placing, compacting and curing of concrete.

11.2

PLACING OF CONCRETE

A basic requirement of placing is to maintain quality and uniformity of the concrete. Operations should be planned so that the required amounts of consistent quality concrete can be placed. Equipment for placing should be well maintained, and arranged so as to deliver the concrete as close as possible to its final position without segregation, and without damaging or displacing reinforcement, stressing ducts, formwork, etc. The most common equipments used for placing concrete are concrete pump, boom placer, buckets, conveyor belts, tremie and chutes. Important requirements to be checked during placing include the following: • Concrete must be deposited as near as practicable to its final position and not deposited in heaps. • Concrete must be placed such that there is no segregation in the mix and remain homogeneous. • Concrete must be deposited in horizontal layers of a depth that allows for full compaction to take place. Each layer must be thoroughly compacted before the next layer is placed. • Concreting must be carried out continuously to avoid cold joints and horizontal or nearly horizontal placing lines on vertical surfaces.

11.2

Handbook on Advanced Concrete Technology

11.2.1 Pumping of Concrete Pumping is one of the quickest and best methods of placing concrete. With pumping we automatically ensure good quality homogeneous concrete free from segregation, as any concrete which is not properly proportioned and is segregating will get choked in the pump. Advantages of pumping of concrete: • Concrete can be moved horizontally and vertically in one go. • Concrete pump acts as silent quality controller inspector since, it refuses to handle any concrete which is unduly harsh, inadequately mixed, non–cohesive and not correct in consistency. • Saving of large gang of labour for large concrete pours. • Pumped concrete give better finish and strength to concrete structure since it has good cohesion and high workability. • Concrete can be placed in inaccessible places very easily. • Mass concreting can be carried out in a limited time at high speed and without or with minimum cold joints. • Concrete pumps help in speedier completion of contract thus helping in increase cash flow, reduction in project overheads. • Pipeline for delivery of concrete occupies very little space and can be extended or removed easily. • If mobile boom pump is used both horizontal and vertical movements of the boom pumps are possible. This eliminates the need of chutes and avoids segregation of the concrete mix.

11.2.1.1 Mix Design for Pumpable Concrete A concrete which can be pushed through a pipeline is called a pumpable concrete. The pumpable concrete has: • Minimum content of fines of approx. 435 kg/m3 in case of concretes with a max. size aggregate of 20 mm. In case of crushed or flaky material this quantity has to be increased approx. by 10%. • The content of fines and the maximum size of aggregate mix should be as given under Table 11.1: TABLE 11.1 Recommended fines content for various max. size of aggregate Max. size of aggregate (mm)

Recommended fines content per cubic metre of concrete

8 16 20 32 40

525 450 435 400 380

63

325

Fines are defined as cement + supplementary cementitious material + aggregate passing 0.25 mm sieve.

Placing, Compaction and Curing of Concrete

11.3

• Minimum cement content of approximately 260 kg/m3 for concrete with maximum size of aggregate of 20 mm • Approx. water requirement is calculated at w = 30 × k + 25, where k is the fineness modulus of the combined grading of aggregate • Cohesive concrete without segregation and slump more than 90 mm • Combined grading curve as per limits shown in Fig. 11.1 100.0 90.0 80.0 70.0

% Passing

60.0 50.0 40.0 30.0 20.0 10.0 0.0 0.075

0.15

0.3

0.6

1.18 Sieve size

Combined grading

2.36

Upper limit

4.75

10

20

Lower limit

Fig. 11.1 Combined grading curve for pumpable concrete

11.2.1.2 Essential Execution Parameters of Concrete Pumping 1. Laying of pipeline: It is very important to understand the right method of lying of pipeline. A lot of time, money and annoyance will be saved if laying of pipelines is properly planned and carried out with care. The following points are important: (a) Layout of pipeline: While planning the layout of the concrete pipeline, it is essential to ensure that it is the shortest route from the pump to the pouring point. While doing so, ensure that the route has minimum nos. of bends. The co relation for over 100 mm dia. of pipeline is as follows: 1 m vertical rise One 90 Deg. Bend One 45 Deg. Bend One 30 Deg. Bend 1 m rubber pipe

= = = = =

2 3 2 1 2

m m m m m

to 3 m horizontal length horizontal length horizontal length horizontal length steel pipe

11.4

Handbook on Advanced Concrete Technology

(b) Starting distance for vertical pumping: Always provide a horizontal starting distance between the concrete pump and the vertical line. This depends upon the maximum pumping height and pipe diameter. The horizontal starting distance should be approximate 10 to 15% of vertical distance with minimum not less than 1.5 m. (c) Anchoring and Fixtures: Pipeline should be well anchored so that it does not displace due to the motion of the concrete flowing through the pipes. Where bends are installed in pipeline considerable forces are released due to change of direction which result in a considerable movements of the pipeline. (d) Leak proof coupling: Ensure that there are no leaking pipes and coupling joints because concrete bleeding result in plug formation in the pipeline and it hampers the pushing of concrete out of the pipe. There is also a possibility of air getting entrapped in the pipeline which can cause choking of pipeline. (e) High Ambient Temperatures: Shading, covering or even cooling the pipeline is advisable when using long pipelines. The pipeline is to be painted in light or white colour and cover the concrete in the hopper with the wet sacks. With outputs of 20 cum/hr the concrete needs approx. 15 minutes to flow through a pipeline with 125 mm dia., 400 m long, within this time the concrete absorbs considerable heat and if pumping delays occur the concrete can easily start setting causing choking of pipeline. 2. Priming of concrete pump (a) Priming (lubricating): Pipeline must be primed before placing concrete in the pipeline. This sliding layer regenerates itself permanently and created automatically when concrete is pumped continuously by the fine mortar in the mix. This priming is must for newly laid or cleaned pipeline of longer distances (the pipeline of the placing boom are relatively short and it is often possible to start pump without any special priming). (b) Lubricating Mix: The lubricating mix can be prepared with 1:1 ratio of cement and water. Fly ash or GGBS if available should be used replacing 50% cement as they are better lubricators than cement and economical. Where pipeline are more than 100m long we can use 2 part of cementitious material, 1 part of sand and water. Approximately 250 litres of mix is required for 100 m pipeline. Nowadays there are readymade chemicals available with admixture suppliers which can be directly mixed in water and pumped into the pipeline for providing lubrication. Such chemicals save not only in terms of cement consumption but also save the operating cost of batching plant and transit mixer involved in production and transportation of cement slurry. The admixture suppliers suggest recommended dosages for per metre length of pipeline.

11.2.2 Tremie Concrete Tremie is one of the traditional techniques used for the placing of concrete underwater. The basic objectives of the tremie are two-fold:

Placing, Compaction and Curing of Concrete

11.5

• The concrete is piped to the site of placement and therefore segregation and infiltration by surrounding water are avoided • It is inherent in the technique that concrete is placed into the heart of the pour, and therefore the contact surface with the water is minimized. The tremie is suited to the placement of large volumes of concrete from a fixed platform. The tremie consists of a steel pipe mounted vertically in the water. To the top of the pipe is fixed a hopper, to receive the concrete; it also acts as a reservoir for the supply of fresh concrete. The tremie tube should be watertight and have a smooth bore. The integrity of the seals at joints in this pipe is very important, as it ensures that no water is entrained into the fresh concrete during the pouring operation. It is vital that a continuous flow of concrete is achieved and therefore the discharging pipe should be of sufficient diameter to ensure that blockages do not occur. As a guide to the selection of pipe size, the inner diameter should be atleast 6 times the maximum size of aggregate (MSA). It is a common practice to use 200 mm diameter tremie with 20 mm MSA and 250 mm diameter tremie with 40 mm MSA. It is of utmost importance that the end of the tremie is always immersed in the heart of the freshly placed concrete. If the seal is broken then fresh concrete will mix with the water and a discontinuity will occur in the pour.

Tremie

Concrete supplied by skip, pump or chute

Access platforms may be required

Smooth bore tube with watertight quick-action joints Pipe immersed in concrete until completion

Fig. 11.2

Diagrammatical representation of a tremie

It is important to have a correct technique for initial filling or charging of the tremie. The main objective of initial filling is to keep the concrete separated from the water. If the concrete is allowed to fall freely into the tremie pipe, the concrete will segregate and entrain water. To control the condition on the initial charging a steel plug is kept on the top of the tremie pipe. Sufficient quantity of concrete is filled into the hopper and the plug is removed allowing the concrete to flow with force. This force displaces the water in the pipe and concrete is filled at

11.6

Handbook on Advanced Concrete Technology

the bottom in form of a mound. Continuous supply of concrete is maintained into the tremie and the tremie pipe is slowly raised upwards taking care that the bottom of the pipe is always immersed in the concrete till completion of the work. An ideal tremie placement is one where the initial concrete placed is the concrete that ends up on top. This is very important because initial concrete is contaminated with water and slush is allowed to flow out. If this contaminated concrete remains in the structure it will act as a weak link and can cause serious structural and durability issues. In case concrete is to be placed in larger areas under water, multiple tremie pipes will be required. The spacing of the tremie pipes must be such that the concrete doesn’t have to flow too far. Otherwise, segregation will occur and too much area on the sloping face of the flowing concrete will be exposed to water. This can result in weak layers of high water-cement-ratio paste within the structure. A pipe spacing of 4 to 5 times the diameter of the tremie pipe is ideal as shown in Fig. 11.3.

Fig. 11.3 Tremie pipes are spaced so that concrete doesn’t have to flow too far. typical spacing is 4 to 5 times the diameter of tremie pipe

11.3

COMPACTION OF CONCRETE

The purpose of compaction of concrete is to achieve the highest possible density. Vibration is the most common and effective method of compaction. When concrete is placed in the forms, air bubbles can occupy between 5 percent (in highly workable mix) and 20 percent (in low slump mix) of the total volume. The mortar component of the mix if fluidified and the internal friction is reduced thus facilitating dense packing of coarse aggregate particles. Continuous vibration expels most of the entrapped air, but it is not usually possible to remove the entire entrapped air from the concrete. Generally 1.5 to 2 percent entrapped air remains even in well compacted concrete. Vibration must be applied uniformly to the entire concrete mass as otherwise some parts of it would not be fully compacted while other part might be segregated due to over vibration.

11.3.1 Internal vibration Internal or immersion-type vibrators are commonly used to consolidate concrete in walls, columns, beams, and slabs. Flexible-shaft vibrators consist of a vibrating head connected to

Placing, Compaction and Curing of Concrete

11.7

a driving motor by a flexible shaft. Inside the head, an unbalanced weight connected to the shaft rotates at high speed, causing the head to revolve in a circular orbit. The vibrating head is usually cylindrical with a diameter ranging from 20 to 180 mm. The dimensions of the vibrator head as well as its frequency and amplitude in conjunction with the workability of the mixture affect the performance of a vibrator. Table 11.2 shows the range of characteristics and applications for internal vibrators for various applications. TABLE 11.2 Range of characteristics and applications for internal vibrators for various applications Group

Dia. of head mm

Recommended Suggested values of frequency, Eccentric Average Centrifugal vibrations moment, amplitude force, kg mm per minute mm-kg (10-3)

Approximate values of Radius of Rate of action, mm concrete placement m3/h

Application

1

20-40

9000-15,000

3.5-12

0.4-0.8

45-180

80-150

0.8-4

Plastic and flowing concrete in very thin members and confined places. May be used to supplement larger vibrators, especially in prestressed work where cables and ducts cause congestion in forms. Also used for fabricating laboratory test specimens.

2

30-60

8500-12,500

9-29

0.5-1.0

140-400

130-250

2.3-8

Plastic concrete in thin walls, columns, beams, precast piles, thin slabs, and along construction joints. May be used to supplement larger vibrators in confined areas.

3

50-90

8000-12,000

23-81

0.6-1.3

320-900

180-360

4.6-15

Stiff plastic concrete (less than 80-mm slump) in general constructions such as walls, columns, beams, and heavy slabs.

4

80-150

7000-10,500

8-290

0.8-1.5

680-1800

300-510

11-31

Mass and structural concrete of 0-50 mm slump deposited in quantities up to 3 m3 in relatively open forms of heavy construction (power houses, heavy bridge piers, and foundations)

5

130-150

5500-8500

260-400

1.0-2.0

1100-2700

400-610

19-38

Mass concrete in gravity dams, large piers, massive walls, etc. Two or more vibrators will be required to operate simultaneously to mix and consolidate quantities of concrete of 3 m3 or more deposited at one time into the form.

11.8

Handbook on Advanced Concrete Technology

It is important that the internal vibrators are used in the correct manner to achieve satisfactory results. Concrete should not be moved horizontally using vibrators as this will cause segregation. Whenever possible, the vibrator should be lowered vertically into the concrete at regularly spaced intervals and allowed to descend by gravity. It should penetrate to the bottom of the layer being placed and at least 150 mm into any previously placed layer. The height of each layer or lift should be about the length of the vibrator head or generally a maximum of 500 mm in regular formwork. The following procedure should be followed for effective use of internal vibrators: 1. Concrete surface must always be visible. Adequate lighting must be provided for deep and thin sections. 2. When inserting a poker, allow it to penetrate to the bottom of the layer plus about 100 mm into previous layer as quickly as possible under its own weight (Fig. 11.4). If done slowly, the top of layer will get compacted first, making it difficult to the entrapped air in the lower part to escape to surface.

Correct

Fig. 11.4

Incorrect

Correct

Incorrect

Vibrator penetrating about 100mm in previous layer

3. Hold the Poker in concrete for about 20 to 30 seconds. 4. Withdraw slowly. Ensure that the hole made by the poker gets closed up. In case of very stiff concrete, this may not happen. In such case, put the poker back near to the hole and vibrate. 5. Depending upon type of concrete, size of structure etc., put the poker at next location which may be about 500 mm away from the previous location (Fig. 11.5). With small diameter pokers, this distance may be suitably reduced. Areas of no vibration

Vibrator too small

Formwork

Correct size of vibrator

Fig. 11.5 Circles indicate area of vibrator influence

Placing, Compaction and Curing of Concrete

11.9

6. Avoid touching rebars and formwork face. 7. Avoid using poker to shift or to flow the concrete. 8. Avoid putting poker into top of heap of concrete (Fig. 11.6) Incorrect

Correct Third Second

Overcompaction will result in forming of slurry at the lop

Needle vibrator location for conned compaction

First Uncompacted or poorly compacted areas

Heaped up concrete in form Concrete gradually flattened by proper compaction

Fig. 11.6 Incorrect and correct method of compaction in a heaped concrete

9. To attain good durability and finish, concrete must be placed directly in corners and ends of walls. Then flow of concrete, if any, should be from the corners and ends rather than towards them (Fig. 11.7). If cut-outs, openings, void-formers are introduced within the formwork and concrete is required to be placed around them, then correct methods and not incorrect methods as illustrated in Fig. 11.7 need to be followed.

Poker must be moved up and sown to ease flow and achieve compaction

Approx. 30 mm head

Concrete placing and compacting should be done from this end

Void fomer

Concrete must be placed on this side only after it has appeared as shown.

Place and compact concrete from this end after concrete is visible below the bottom flange.

Fig. 11.7 Correct method of placing concrete below a circular or structural void former

10. Make sure that the Poker extends into previous layer by about 100 mm provided previous layer has not reached final set (Fig. 11.4).

11.10

Handbook on Advanced Concrete Technology

11. Full length of Poker head (Metallic Part) should be immersed in concrete, vertically in case of deep structures, inclined in case of flat structures. 12. Avoid running the vibrator when not in concrete, so that the bearings do not get overheated. 13. Avoid sharp bends in the flexible shaft. 14. Avoid wasteful operation of vibrators such as wrong positioning, out of concrete and vibrating already compacted concrete. 15. Avoid excess/over vibration to prevent segregation. Normally the appearance of concrete surface just vibrated gives indication that compaction has been done. A thin film of mortar on surface is a sign. Also air bubbles stop coming to top when compaction is achieved. Experienced operators can recognize the typical pitch (whine) of poker indicating completion of compaction. 16. Special precautions will be required where rebars are congested and lot of embedded parts/inserts exist. 17. Normally, poker should be inserted vertically into concrete. However, for slabs, it may be placed in inclined manner without flexible shaft getting sharply bent. 18. After concreting, clean the vibrator and maintain for next operation. When not in use, store the vibrator horizontally on suitable racks. 19. In case of columns, and walls, it is advisable to tamp the outside face of shutters simultaneously along with vibrator by wooden mallets. This will remove air bubble marks on the surface. 20. The flexible shafts of vibrator get damaged frequently. Keep enough spare needles with shaft readily available during concreting.

11.3.2 External Vibration External vibrators can be form vibrators, vibrating tables, or surface vibrators such as vibratory screeds, plate vibrators, vibratory roller screeds, or vibratory hand floats or trowels. Form vibrators, designed to be securely attached to the outside of the forms, are especially useful (1) for consolidating concrete in members that are very thin or congested with reinforcement, (2) to supplement internal vibration, and (3) for stiff mixes where internal vibrators cannot be used. Form vibrators (Fig. 11.8) can be either electrically or pneumatically operated. The spacing should be such that the intensity of vibration is uniformly distributed over the form; optimum spacing is best found by experimentation. Sometimes it may be necessary to operate some of the form vibrators at a different frequency for better results; therefore, it is recommended that form vibrators be equipped with controls to regulate their frequency and amplitude. Duration of external vibration is considerably longer than for internal vibration—generally between 1 and 2 minutes. The following procedure should be followed for effective use of external vibrators: 1. As a general rule, when the thickness of the concrete in the form exceeds 150 mm, use vibrators (staggered) on both sides of the form. In columns the reinforcement steel will aid in vibration transfer to the center of the column.

Placing, Compaction and Curing of Concrete

11.11

Fig. 11.8 Rotary form vibrator

2. The sinusoidal vibration waves are strongest at the vibrator and they move away in a circular pattern (like the waves when a stone is thrown into water) and reach a 100-150 cm radius. Generally a 100 cm radius from the vibrator can be considered as an effective vibration area. Some of the vibration will travel upto 150 cm radius. At the 150 cm radius the vibration from the next vibrator should overlap the first. 3. For determining the number of form vibrators required for a particular case, the following formula is used: (a) For plastic mixtures in beam and wall forms CF = 0.5(MF + 0.2MC) where CF = centrifugal force; MF = mass of form; and MC = mass of concrete. (b) For stiff mixtures in pipe and other rigid forms CF = 1.5(MF + 0.2MC) The centrifugal force generated by form vibrator is specified in the data sheet. 4. Placing of vibrators: If the thickness of wall is 150 mm, vibrators need to be fixed as shown in Fig. 11.9. If it is over 150 mm, vibrators have to be fixed on both sides and staggered for effective vibration as per Fig. 11.10.

Direction of vibration

2m 1m 1m

2m

F-Front side, B-back side

Fig. 11.9 Placement of external vibrators for wall thickness less than 150 mm

11.12

Handbook on Advanced Concrete Technology

2m 1m 1m

2m

Front Side

Back Side

Fig. 11.10 Placement of external vibrators for wall thickness more than 150 mm

5. Vibration procedure: Place vibrators to be used in their lowest position. Do not start vibrators until the concrete reaches them or is about 15 cm above them. The time required to vibrate varies depending on concrete slump, additives, stiffness of form, vibrator force, etc. When no more air bubbles are breaking on the surface, and a glistening surface appears on top of the concrete, it can be considered that proper compaction is achieved. Once the vibration of one layer is completed, the vibrators can be taken out and fixed on the second layer as shown in Fig. 11.11.

2

1

2

1

2

1

Fig. 11.11 Sequence of external vibrator compaction

Attaching a form vibrator directly to the form generally is unsatisfactory. Rather, the vibrator should be attached to a steel plate that in turn is attached to steel I-beams or channels passing through the form stiffeners themselves in a continuous run. Loose attachments can result in significant vibration energy losses and inadequate consolidation. Vibrating tables are used in precast plants. They should be equipped with controls so that the frequency and amplitude can be varied according to the size of the element to be cast and the consistency of the concrete. Stiffer mixtures generally require lower frequencies (below 6000 rpm) and higher amplitudes (over 0.13 mm) than more workable mixtures. Increasing the frequency and decreasing the amplitude as vibration progresses will improve consolidation. Surface vibrators, such as vibratory screeds, are used to consolidate concrete in floors and other flatwork. Vibratory screeds give positive control of the strikeoff operation and save a great

Placing, Compaction and Curing of Concrete

11.13

deal of labour. When using this equipment, concrete need not have slumps in excess of 75 mm. For greater than 75 mm slumps, care should be taken because surface vibration of such concrete will result in an excessive accumulation of mortar and fine material on the surface; this may reduce wear resistance. For the same reason, surface vibrators should not be operated after the concrete has been adequately consolidated. Because surface vibration of concrete slabs is least effective along the edges, a spud or poker-type vibrator should be used along the edge forms immediately before the vibratory screed is applied. Vibratory screeds are used for consolidating slabs up to 250 mm thick, provided such slabs are non reinforced or only lightly reinforced (welded-wire fabric). Internal vibration or a combination of internal and surface vibration is recommended for reinforced slabs.

11.3.4

Consequences of Improper Vibration

Following are some of the defects caused by under vibration: 1. Honeycombing 2. Excessive amount of entrapped air voids, often called bug holes 3. Sand streaks 4. Cold joints 5. Placement lines and 6. Subsidence cracking Honeycomb results when the spaces between coarse aggregate particles do not get filled with mortar. Faulty equipment, improper placement procedures, a concrete mix containing too much coarse aggregate or congested reinforcement can cause honeycomb. Excessive entrapped air voids are similar to, but not as severe as honeycomb. Vibratory equipment and operating procedures are the primary causes of excessive entrapped air voids, but the other causes of honeycomb apply too. Sand streaks results when heavy bleeding washes mortar out from along the form. A wet, harsh mixture that lacks workability because of an insufficient amount of mortar or fine aggregate may cause sand streaking. Cold joints are a discontinuity resulting from a delay in placement that allowed one layer to harden before the adjacent concrete was placed. The discontinuity can reduce the structural integrity of a concrete member if the successive lifts did not properly bond together. The concrete can be kept alive by revibrating it every 15 minutes or less depending on job conditions. However, once the time of initial setting approaches, vibration should be discontinued and the surface should be suitably prepared for the additional concrete. Placement lines or “pour” lines are dark lines between adjacent placements of concrete batches. They may occur if, while vibrating the overlying layer, the vibrator did not penetrate the underlying layer enough to knit the layers together. Subsidence cracking may occur at or near the initial setting time as concrete settles over reinforcing steel in relatively deep elements that have not been adequately vibrated. Revibration at the latest time that the vibrator will sink into the concrete under its own weight may eliminate these cracks.

11.14

Handbook on Advanced Concrete Technology

Defects from over vibration include: 1. Segregation as vibration and gravity causes heavier aggregates to settle while lighter aggregates rise 2. Sand streaks 3. Loss of entrained air in air-entrained concrete 4. Excessive form deflections or form damage and 5. Form failure caused by excessive pressure from vibrating the same location too long and/or placing concrete more quickly than the designed rate of pour. Under vibration is more often a problem than over vibration.

11.4

CURING OF CONCRETE

A typical definition of curing (BS 8110, 1997) is ‘the process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature regime’. Curing has a strong influence on the properties of hardened concrete; proper curing will increase durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicers and reduce cracking in concrete. Exposed slab surfaces are especially sensitive to curing as strength development and freeze-thaw resistance of the top surface of a slab can be reduced significantly when curing is defective. Flat surfaces are especially prone to plastic and drying shrinkage cracking if adequate curing is not done. When Portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed influences the strength and durability of the concrete. Freshly mixed concrete normally contains more water than is required for hydration of the cement; however, excessive loss of water by evaporation can delay or prevent adequate hydration. The surface is particularly susceptible to insufficient hydration because it dries first. If temperatures are favorable, hydration is relatively rapid in the first few days after concrete is placed; however, it is important for water to be retained in the concrete during this period, that is, for evaporation to be prevented or substantially reduced. With proper curing, concrete becomes stronger, more impermeable, and more resistant to stress, abrasion, and freezing and thawing. The improvement is rapid at early ages but continues more slowly thereafter for a long period. Fig. 11.12 shows the strength gain of concrete with age for different moist curing periods. Curing should continue for minimum 10 days for concrete made with ordinary Portland cement and light units like slabs (150 to 200 mm) etc and for 14 days for blended cements and for heavy structures such as rafts, heavy footings, heavy beams, heavy columns etc. This is known as curing period. Curing period starts from the time of completing the concreting and finishing the same till completion of curing.

11.4.1

Curing Methods and Materials

Concrete can be kept moist (and in some cases at a favorable temperature) by three curing methods:

Placing, Compaction and Curing of Concrete

11.15

60

Compressive strength, MPa

50 In air after 28 days moist curing

40

In air after 7 days moist curing

6

In laboratory air entire time

30

4

20 2 10 0 0 7 28

90

Age at test, days

Compressive strength, 1000 psi

8

Moist-cured entire time

0 365

Fig. 11.12 Effect of moist curing time on strength gain of concrete

1. Methods that maintain the presence of mixing water in the concrete during the early hardening period. These include ponding or immersion, spraying or fogging, and saturated wet coverings. These methods afford some cooling through evaporation, which is beneficial in hot weather. 2. Methods that reduce the loss of mixing water from the surface of the concrete. This can be done by covering the concrete with impervious paper or plastic sheets, or by applying membrane-forming curing compounds. 3. Methods that accelerate strength gain by supplying heat and additional moisture to the concrete. This is usually accomplished with live steam, heating coils, or electrically heated forms or pads. 4. Forms provide satisfactory protection against loss of moisture if the top exposed concrete surfaces are kept wet. Thus as long as the forms are in place the curing of concrete is not an issue. Also concrete which is covered on all sides by earth, eg., retaining walls, piles needs no curing as there is no scope for the moisture to escape but if the earth is very dry then it can absorb moisture from the concrete surface. (a) Ponding and Immersion: On flat surfaces, such as pavements and floors, concrete can be cured by ponding. Earth or sand dikes around the perimeter of the concrete surface can retain a pond of water. Ponding is an ideal method for preventing loss of moisture from the concrete; it is also effective for maintaining a uniform temperature in the concrete. The curing water should not be more than about 11ºC (20ºF) cooler than the concrete to prevent thermal stresses that could result in cracking. The curing must start as soon as the finishing of concrete is complete. Immediately after finishing the concrete surface must be covered by tarpaulin or plastic sheet to avoid loss of moisture. Ponding must be started after the concrete has attained final set and the labour can walk on the surface for preparing the dikes.

11.16

Handbook on Advanced Concrete Technology

The most thorough method of curing with water consists of total immersion of the finished concrete element. This method is commonly used in the laboratory for curing concrete test specimens. (b) Fogging and sprinkling: Fogging and sprinkling with water are excellent methods of curing when the ambient temperature is well above freezing and the humidity is low. A fine fog mist is frequently applied through a system of nozzles or sprayers to raise the relative humidity of the air over flatwork, thus slowing evaporation from the surface. Fogging is applied to minimize plastic shrinkage cracking until finishing operations are complete. Once the concrete has set sufficiently to prevent loss of water ordinary lawn sprinklers are effective if good coverage is provided and water runoff is of no concern. Soaker hoses are useful on surfaces that are vertical or nearly so. If sprinkling is done at intervals, the concrete must be prevented from drying between applications of water by using burlap or similar materials; otherwise alternate cycles of wetting and drying can cause surface crazing or cracking. (c) Wet covering: Fabric coverings saturated with water, such as burlap, cotton mats, rugs, or other moisture-retaining fabrics, are commonly used for curing. Burlap must be free of any substance that is harmful to concrete or causes discoloration. New burlap should be thoroughly rinsed in water to remove soluble substances and to make the burlap more absorbent. Wet, moisture-retaining fabric coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage. During the waiting period other curing methods are used, such as fogging or the use of membrane forming finishing aids. Care should be taken to cover the entire surface with wet fabric, including the edges of slabs. The coverings should be kept continuously moist so that a film of water remains on the concrete surface throughout the curing period. Use of polyethylene film over wet burlap is a good practice; it will eliminate the need for continuous watering of the covering. Periodically rewetting the fabric under the plastic before it dries out should be sufficient. Alternate cycles of wetting and drying during the early curing period may cause crazing of the surface. (d) Membrane-Forming Curing Compounds: Liquid membrane-forming compounds consisting of waxes, resins, chlorinated rubber, and other materials can be used to retard or reduce evaporation of moisture from concrete. They are the most practical and widely used methods of curing not only for freshly placed concrete but also for extending curing of concrete after removal of forms or after initial moist curing. However, the most effective methods of curing concrete are wet coverings or water spraying that keeps the concrete continually damp. Curing compounds should be able to maintain the relative humidity of the concrete surface above 80% for minimum of 10 days to sustain cement hydration. Membrane-forming curing compounds are of two general types: clear, or translucent; and white pigmented. Clear or translucent compounds may contain a fugitive dye that

Placing, Compaction and Curing of Concrete

11.17

makes it easier to check visually for complete coverage of the concrete surface when the compound is applied. The dye fades away soon after application. On hot, sunny days, use of white-pigmented compounds is recommended; they reduce solar-heat gain, thus reducing the concrete temperature. Curing compounds should be applied by hand-operated or power-driven spray equipment immediately after final finishing of the concrete. The concrete surface should be damp when the coating is applied. On dry, windy days, or during periods when adverse weather conditions could result in plastic shrinkage cracking, application of a curing compound immediately after final finishing and before all free water on the surface has evaporated will help prevent the formation of cracks. Power-driven spray equipment is recommended for uniform application of curing compounds on large paving projects. Spray nozzles and windshields on such equipment should be arranged to prevent wind-blown loss of curing compound. When there is any overlay of concrete, bitumen or paint coming on the top of concrete surface, it is recommended to use resin based curing compound as wax based curing compounds act as bond breaker and will not allow proper bonding between old concrete and the overlay. In case wax based curing compounds are used, they must be thoroughly washed away by mechanical broom or any other means before overlay is done. (e) Steam Curing: Steam curing is advantageous where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold weather. Two methods of steam curing are used: live steam at atmospheric pressure (for enclosed cast-in-place structures and large precast concrete units) and high-pressure steam in autoclaves (for small manufactured units). Only live steam at atmospheric pressure will be discussed here. A typical steam-curing cycle consists of (a) an initial delay prior to steaming – 3 to 4 hours (b) a period for increasing the temperature – 2 to 4 hours (c) a period for holding the maximum temperature constant – 5 to 15 hours (d) a period for decreasing the temperature – 2 to 4 hours A typical atmospheric steam-curing cycle is shown in Fig. 11.13. Steam curing at atmospheric pressure is generally done in an enclosure to minimize moisture and heat losses. Tarpaulins are frequently used to form the enclosure. Application of steam to the enclosure should be delayed until initial set occurs or delayed at least 3 hours after final placement of concrete to allow for some hardening of the concrete. Steam temperature in the enclosure should be kept at about 75°C until the desired concrete strength has developed. Excessive rates of heating and cooling should be avoided to prevent damaging volume changes. Temperatures in the enclosure surrounding the concrete should not be increased or decreased more than 10°C to 20°C per hour depending on the size and shape of the concrete element.

11.18

Handbook on Advanced Concrete Technology 90

Pre-steaming Temperature rise period period

Optimum temperature period

Cooling period

Temperature – °C

80 70 60 50 40 30 20 10 0

Time-hour

24

Fig. 11.13 Typical steam curing cycle

The curing temperature in the enclosure should be held until the concrete has reached the desired strength. The time required will depend on the concrete mixture and steam temperature in the enclosure. It is found that mixes with low water cement ratio respond more favorably to steam curing than mixes with higher water-cement ratio. Steam curing should be followed by wet curing for at least 7 days. This supplementary curing is found to increase the strength of steam cured concrete by about 20-30%.

References 1. Karl Ernst V. Eckardstein (1965) “Pumpable Concrete” Pumping Concrete and Concrete Pumps, 1st Edition. 2. John Newman, Ban Seng Choo (2003), “Underwater Concrete” Advanced Concrete technology – Processes, 1st Edition. 3. Steven H. Kosmatka, Beatrix Kerkhoff, and William C. Panarese (2003), “Placing and Finishing Concrete and Curing Concrete” Design and Control of Concrete Mixtures, 14th Edition. 4. Guide for consolidation of concrete, ACI 309R-05. 5. M. S. Shetty (2005) “Fresh Concrete” Concrete Technology, Theory and Practice, 6th Edition.

12 Shrinkage and Creep Manish Mokal and N.V. Nayak

12.1 INTRODUCTION Volume change is one of the most detrimental properties of concrete, which affects the long term strength and durability. Concrete changes slightly in volume for various reasons, and understanding the nature of these changes is useful in planning or analyzing concrete work. If concrete were free of any restraints to deform, normal volume changes would be of little consequence except in some special cases; but since concrete in service is usually restrained by foundations, sub grades, reinforcement, or connecting members, significant stresses can develop. This is particularly true of tensile stresses. Cracks develop because concrete is relatively weak in tension but quite strong in compression. Controlling the variables that affect volume changes can minimize high stresses and cracking. Volume change is defined merely as an increase or decrease in volume. Most commonly, the concrete volume changes can be divided into changes occurring during the plastic state and those occurring during the hardened state of concrete. The various reasons for volume changes are shrinkage, creep, expansion and contraction of concrete due to temperature and moisture cycles, sulphate attack and disruptive expansion of alkali-aggregate reactions. In this chapter we shall discuss the volume change on account of the inherent properties of concrete viz; shrinkage and creep.

12.2

SHRINKAGE

The term shrinkage is used to describe the volume reduction occurring due to loss of moisture at various stages in the life cycle of concrete. Shrinkage can be classified in the following categories:

12.2

Handbook on Advanced Concrete Technology

12.2.1 Plastic Shrinkage 12.2.1.1 The Mechanism of Plastic Shrinkage Plastic shrinkage in compacted concrete is caused by the loss of water before it sets. The volume of concrete is reduced by the amount of water removed which may be due to evaporation, absorption by aggregate or by subgrade. Plastic shrinkage is not harmful in itself, but where shrinkage is restrained the concrete may crack. In case of floors and pavements where the surface area exposed to drying is very large as compared to the depth, and concrete is placed in conditions of direct sun, the extent of evaporation may be very high. Such condition causes the surface of concrete to dry very fast. The magnitude of shrinkage is affected by the amount of water lost from the surface which is governed by the ambient temperature, relative humidity, and wind velocity. The rate of loss of water does not necessarily predict the occurrence of plastic shrinkage as much depends upon the stability of the mix. Basically, if the amount of water lost to evaporation is greater than the rate of bleed, there is a net reduction in volume at the surface of the concrete. The surface layer of concrete tries to shrink but is restrained by underlying layers that are not subject to the same reduction in volume. Restraint can also be partly provided by the reinforcement and friction at the surface of the formwork or sub-base. The result of the restraint is that tensile stresses develop in the surface layer. As the concrete is still in a plastic state and has very little strength, cracks develop at the surface. This is known as plastic shrinkage cracking.

12.2.1.2 Visual Appearance The width of plastic shrinkage cracks can be up to 3 mm, and length between 50 mm to 3 m. The depth is generally 20–50 mm rapidly tapering down within the concrete. In rare occasion they may extend through the full depth of a member. The pattern of cracks generally appear at an approximate angle of 45° to the direction of casting and run parallel to one another (see Figure 12.1). The distance between cracks is variable but could be 1 to 2 m. Cracks may also form randomly as a large map pattern (see Figure 12.2). These different patterns may be influenced by the direction in which finishing operations have been carried out or by physical features such as deep tamping marks. It may be noted that plastic shrinkage cracks do not normally extend to the edge of a slab as this is able to shrink without restraint. These cracks can form in both unreinforced and reinforced concrete.

Fig. 12.1 Plan of diagonal cracking

Fig. 12.2 Plan of map cracking

Shrinkage and Creep

12.2.1.3

12.3

Minimizing Plastic Shrinkage

The following factors affect the extent of plastic shrinkage: • Lower the water cement ratio less will be the bleed water and higher will be the plastic shrinkage • Higher the water content in the mix less will be the plastic shrinkage • Higher ambient temperature and wind velocity will cause higher evaporations causing more plastic shrinkage • Lower the relative humidity faster will be the evaporation causing more plastic shrinkage • Higher the cement content of the mix, greater will be the plastic shrinkage Figure 12.3 gives the rate of evaporation from the concrete surface based on the ambient temperature, relative humidity, concrete temperature and wind velocity. If the rate of evaporation is higher than 1 kg/m2 /hr, adequate measures need to be taken for minimising the plastic shrinkage cracks. Plastic shrinkage can be reduced by preventing loss of moisture from surface of compacted concrete. This can be achieved by covering the surface immediately with plastic sheet after the finishing operation is over preferably within 10 minutes, spraying the surface with curing compound, spraying of fog/mist on the surface, working at night and pre-wetting soil before placing concrete on it.

12.2.1.4 Remedial Measures Plastic shrinkage cracks rarely pass through the full section. As the cracks form in concrete when the paste is still in a plastic state, they run through the paste and around the pieces of aggregate and are not generally wider than 0.5 mm. The two faces of the crack still remain interlocked through the aggregate, restricting vertical movements, unlike a crack that passes through the aggregate. Normally the structural integrity is not compromised, except closer to local shear forces, e.g. at flared column head. These can be restored with a low-viscosity resin injection. In general cracks need to be sealed, especially on slabs, to reduce the risk of reinforcement corrosion. Often the best remedy is to brush dry cement (damped later) or a wet grout into the cracks before deleterious material blocks the openings. This should be carried out as soon as possible, to encourage autogenous healing. In pavement slabs where the crack opening exceeds 0.5 mm a low-viscosity resin can be injected before the surface is trafficked.

12.2.2 Autogenous Shrinkage When no moisture movement to or from the cement paste is permitted, some shrinkage may occur. This shrinkage is the consequence of withdrawal of water from the capillary pores by the hydration of the unhydrated cement, a process known as self desiccation. Shrinkage of such a conservative system is known as autogenous shrinkage and it occurs in the interior of concrete mass. Autogenous shrinkage tends to increase at higher temperatures, with higher cement content, possibly finer cements and with cement having higher C3A and

12.4

Handbook on Advanced Concrete Technology 40

Relative humidity, per cent

Concrete temperature, °C

100 90

35

80 70 30

60 50

25

40 20

30

15

20

10

10

5 5

10

15 20 25 30 Air temperature, °C

35 Wind velocity, km/h 4 40

2

Rate of evaporation, Kg (m /hr)

Metric

35 3 30 25 2

20 15

1

10 5 0

0

Fig. 12.3 Rate of evaporation of water from concrete surface

C4AF contents. As self-desiccation is greater at lower water/cement ratios, autogenous shrinkage could be expected to increase but this may not occur because of the more rigid structure of hydrated paste at low water/cement ratios. A higher content of supplementary cementitious material like fly ash in the mix leads to lower autogenous shrinkage. As autogenous shrinkage is relatively small, except at extremely low water/cement ratios, for practical purposes (other than in large mass concrete structures) need not be distinguished from shrinkage caused by drying out of concrete. The latter is known as drying shrinkage and normally includes that contraction which is due to autogenous volume change.

Shrinkage and Creep

12.2.3

12.5

Drying Shrinkage

12.2.3.1 The Mechanism of Long-term Drying Shrinkage Usually the amount of water included in a concrete mix is greater than that required to take part in the hydration reaction with cement. This is done in order to achieve necessary workability and compaction. The uncombined water is held within the capillary pores that form within the cement paste. If during the service life of concrete it is exposed to conditions of low relative humidity and high ambient temperature, moisture will be lost from the surface. This loss of moisture from the concrete causes the concrete to decrease in volume. This is called drying shrinkage. If the shrinkage movement is opposed by some external or internal restraint, stresses will develop in the concrete. When these restraint stresses exceed the tensile capacity of the concrete, cracks will develop. Thin members with a large surface area such as slabs are particularly vulnerable. The time at which shrinkage cracks occur will depend on the rate of drying caused by the environment. The rate at which moisture is lost is fairly slow and the rate of shrinkage is time dependent. The surface layers of a member lose water faster as compared to the inner layers and thus the rates of shrinkage vary. The interior therefore acts as a restraint and may induce cracking. The influence of reinforcement may be adverse during the early stages of drying shrinkage because it can act as an internal restraint.

12.2.3.2

Factors Affecting Drying Shrinkage

The following factors affect the drying shrinkage in concrete: • Lower the relative humidity, higher will be the rate of drying causing higher drying shrinkage (See Fig. 12.8) • Higher temperature will lead to higher evaporation leading to higher drying shrinkage • Higher wind velocity will lead to more evaporation leading to higher drying shrinkage • Higher the stiffness of the aggregates, higher is the restraining effect leading to lower drying shrinkage (See Fig. 12.9) • Higher the aggregate/cement ratio, the overall shrinking volume fraction of the concrete decreases leading to lower drying shrinkage (See Fig. 12.10) • Higher amount of water in the mix, increases the amount of evaporable water, and thus the potentiality to suffer higher drying shrinkage • Higher the cement paste content, higher is the shrinking phase of the material leading to more drying shrinkage (See Fig. 12.11) • Reducing the w/c ratio will lead to a considerable decrease in the shrinkage strains and the porosity of the cement paste leading to lower drying shrinkage (See Fig. 12.11) • Better curing helps in retaining the water in the body of the concrete thus reducing the drying shrinkage • Large surface area of concrete members leads to higher drying shrinkage

12.6

Handbook on Advanced Concrete Technology

Effective control of these parameters while designing the concrete mix, at the time of placement and subsequent curing will minimize the amount of drying shrinkage. The pattern of shrinkage as a function of cement content, water content and water cement ratio can be estimated from Fig. 12.4.

0.7

Water: cement ratio 0.6 0.5

1600

3

270 l/m 1200

3

Reduction in length × 10

–6

240 l/m

0.4

800 3

210 l/m

3

180 l/m 400

0.3 3

150 l/m 3

120 l/m 0 250

300

Fig. 12.4

350 400 450 500 3 Cement content (kg/m )

550

600

Estimation of drying shrinkage

12.2.3.4 Visual Appearance There is no typical pattern of drying shrinkage cracking as the cracks form at any location where there is a restraint to shrinkage movement but are usually approximately perpendicular to the direction of restraint. Figure 12.5 shows drying shrinkage cracks on a wall. The widths of the cracks will depend on the extent to which the concrete has been allowed to dry out and the length of the member or the distance between positions of restraint. Because the cracks form after the concrete has gained full strength, the cracks can pass through weak aggregate.

Shrinkage and Creep

12.7

Fig. 12.5 Drying shrinkage cracks on slab

Typical locations where drying shrinkage cracks are likely to occur are: • Ground slabs where one horizontal dimension is much greater than the other. The cracks form across the middle of the slab, parallel to the shorter side. Cracks sometimes form diagonally across corners • Suspended slabs supported on stiff edge beams. The location of cracking can be influenced by voids in the slab such as those left for services or stairwells • At significant changes in cross-section like T-beams, flared columns, etc • Slender members (high surface area to volume ratio) shrink more than thick, bulky members • Shrinkage of concrete between movement joints causes joints to open or get wider. Joints must be designed to accommodate the widening caused by shrinkage The following are some of the remedial measures to minimize drying shrinkage cracks: • In concrete shrinkage takes place in the hardened cement paste while the aggregate dilute the paste and reduce overall shrinkage (shrinkage of cement paste is about 6 times that of concrete). Thus greater the paste content higher is the shrinkage. The mix should be designed for a optimum paste content in order to minimize drying shrinkage. • Shrinkage is roughly a linear function of original water content in concrete. Reduction in mixing water will reduce drying shrinkage. • Water cement ratio has little effect on long term drying shrinkage. However, leaner concretes (higher w/c ratio) tend to shrink more rapidly than richer concrete. • Cement type can affect drying shrinkage. Higher C3A plus C4AF content and higher fineness of cement tend to increase rate of drying shrinkage. • The effect of admixtures on shrinkage is highly specific and if admixtures are to be used for shrinkage sensitive structures, tests using the specific materials are recommended.

12.8

Handbook on Advanced Concrete Technology

12.2.4

Carbonation Shrinkage

In addition to shrinkage upon drying, the surface of the concrete undergoes shrinkage due to carbonation. Carbon dioxide in the atmosphere reacts in presence of water with calcium hydroxide liberated during cement hydration. Calcium hydroxide gets converted to calcium carbonate and also some other cement compounds are decomposed. The carbonation reaction is possible even at the normal atmospheric pressure. The rate of penetration of carbon dioxide depends on the moisture content of the concrete and the relative humidity of the ambient medium. Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place. As the new product is less in volume than the product replaced, shrinkage takes place. The amount of shrinkage due to carbonation is higher at intermediate humidities, but not at 100% or less than 25%. In the latter case, there is insufficient water in the pores within the cement paste for CO2 to form carbonic acid which in turn reacts with calcium hydroxide. On the other hand, when the pores are full of water, the diffusion of CO2 into the paste is very slow. As the magnitude of carbonation shrinkage is very small as compared to the drying shrinkage, it is not of much significance. However, the sequence of drying and carbonation, greatly affects the total magnitude of shrinkage. Simultaneous drying and carbonation produces lower total shrinkage than when drying is followed by carbonation. Carbonation of concrete also results in increased strength and reduced permeability because water released by carbonation promotes the process of hydration and also calcium carbonate reduces the voids within the cement paste. But carbonation reduces the alkalinity of concrete which gives a protective coating to the reinforcement against rusting. If the depth of carbonation reaches up to the reinforcement, the reinforcement becomes liable for corrosion.

12.3

CREEP

Creep is defined as the gradual increases in strain or deformation with time under a constant applied stress, after taking into account other time dependent deformations not associated with stress, due to shrinkage, swelling and thermal deformation. Reduction of stress under constant strain also causes creep. Creep is of practical significance since it can be two or three times the initial elastic strain after one year under load, which represents approximately 70 per cent of 20-year creep. It is estimated that 26 percent of the 20 year creep occurs in 2 weeks, 55 percent of the 20 year creep occurs in 3 months and 76 percent of the 20 year creep occurs in one year. If creep after one year is taken as unity, then the average value of creep at later ages is: 1.14 after 2 years 1.20 after 5 years 1.26 after 10 years 1.33 after 20 years 1.36 after 30 years

Shrinkage and Creep

12.9

Figure 12.6 shows the components of time-dependent strain of drying concrete, which applies to general structural elements. The creep of concrete under condition of no moisture movement to or from the ambient medium is termed as the basic creep and the additional creep caused by drying is termed as drying creep. The total creep consists of basic-plus-drying creep.

Drying creep

Total creep

Strain

Basic creep

Shrinkage Initial elastic strain

to

Age

t

Fig. 12.6 Components of strain at age t of drying concrete subjected to load at age, to

Creep is a partly reversible phenomenon. When a sustained load is removed after some time, there is an immediate recovery of elastic strain (generally smaller than the initial elastic strain because the modulus of elasticity has increased), followed by a gradual decrease in strain called creep recovery (Fig. 12.7). The recovery reaches its maximum value more rapidly and is much smaller than the preceding creep so most of the creep is irreversible in nature. Basic creep recovery is about 25 per cent of the preceding creep while drying creep recovery is almost negligible.

Creep Elastic recovery recovery

Strain

Creep

Residual deformation

Initial elastic strain

Age

Fig. 12.7 Elastic and creep recovery on removal of stress

12.10

Handbook on Advanced Concrete Technology

12.3.1 Mechanism of Creep Under external loads, adsorbed water in the hydrated cement paste is removed from or moves within the paste. This causes changes in the gel structure such as: (a) Reduction in inter particle spacings (b) Reduction of layer thickness (c) Displacement of gel layers, resulting in new inter particle bonds Some changes in gel structure are reversible while others are permanent. However, the above mechanism applies at compressive stresses not exceeding 40% of the ultimate strength. At higher stresses, microcracking occurs causing strains much larger than those induced by the movement of moisture.

12.3.2 Factors Affecting Creep The following factors affect the drying shrinkage in concrete: • Higher the ambient humidity, higher will be the rate of drying causing higher creep (See Fig. 12.8) Very dry air

1 0.5 0

0 100

90

80

70

60

Relative humidity of air, % (a)

Fig. 12.8

50

40

100

90

Very dry air

1.5

Very moist air

10

2

Normal air

2.5

Creep conefficient

Very moist air

20

Water curing

–5

Drying shrinkage, × 10

30

3

Water curing

Normal air

50 40

3.5

80

70

60

50

40

Relative humidity of air, % (b)

Effect of relative humidity on (a) drying shrinkage, (b) creep

• Higher the stiffness of the aggregates, lower will be the creep as aggregate undergo very little creep (See Fig. 12.9) • Higher the aggregate/cement ratio, lower will be the creep as the overall volume fraction of the concrete undergoing significant creep decreases (See Fig. 12.10). Also concrete made with stiffer aggregate will creep less at same aggregate/cement ratio.

Shrinkage and Creep Sandstone Gravel Basalt Granite Limestone Quartz

1500

1000

Creep, 10

–6

1000

500

500

0

0 10

100

1000

10000

10

100

Fig. 12.9

10000

Effect of aggregate stiffness on (a) drying shrinkage, (b) creep 0.50 w/c ratio

1

400

0.35 w/c ratio

Range for normal concrete

6

0.8

Creep/N/mm ¥ 10

Range for normal concrete

300

2

0.6 0.4 0.2 0

1000

Time since loading, days (b)

Time after exposure, days (a)

Shrinkage ratio

Drying shrinkage, 10

–6

1500

12.11

0

20

60

40

80

100

200

100

0

0

20

40

60

80

Content of aggregate and unhydrated cement, %

Volume concentration of aggregate, %

(a)

(b)

100

Fig. 12.10 Effect of aggregate content on (a) drying shrinkage, (b) creep

• Higher the cement content, lower will be the creep (See Fig. 12.11). • Higher w/b ratio will increase the creep as a poorer paste structure will have lower strength, stiffness and impermeability thus undergoing more creep (See Fig. 12.11) • Higher the loading stress and time higher will be the creep (See Fig. 12.12)

12.12

Handbook on Advanced Concrete Technology 1.2

3

Drying shrinkage or creep coefficient, kb

Factor

Shrinkage

1 Creep

0.8 100

200

300

400

500 3

600

Cement content, kg/m 500 400

2

300 200

1

0 0.2

0.4 0.6 0.8 Water–cement ratio (b)

Cement content, kg/m (a)

1.0

Fig. 12.11 Effect of (a) cement content, (b) water/cement ratio on drying shrinkage and creep Loaded at 3 months Loaded at 28 days

Pa

1400

8M

1200

Pa

6M

Pa

Creep, 10

–6

1000

6M

800

4 MPa

600

4 MPa

400

2 MPa

200 0 10

100 1000 Time under load, days

10000

Fig. 12.12 Effect of magnitude of stress and time on creep

• Higher the strength of concrete, lower will be the creep • Higher moisture content in the concrete, higher will be the creep as creep is associated with the movement of water within the hydrated cement paste. • Effect of admixtures differs widely. Specific material should be tested for creep sensitive structures • Because creep is associated with the movement of water within and out of the hydrated cement paste, creep decreases with increasing volume to surface ratio

12.3.3 Effects of Creep In general the effects of creep are disadvantageous in concrete structures. The major effects of creep are:

Shrinkage and Creep

12.13

• • • •

Increases in deflection of beams Loss of prestress in prestressed concrete beams Increases in deflection of eccentrically slender columns which could lead to buckling. In mass concrete, creep may be a cause of cracking when restrained concrete undergoes a cycle of temperature change due to heat of hydration and subsequent cooling. • Cause excessive deflection in tall buildings and long bridges • Cracking of partitions and failure of rigidly fixed external cladding can occur due to differential movement. The structural designer must allow for the detrimental effect of creep when designing the structure.

References 1. John Newman, Ban Seng Choo (2003), “Elasticity, Shrinkage, Creep and Thermal Movement” Advanced Concrete technology – Concrete Properties, 1st Edition. 2. P. K. Mehta, Paulo J. M. Monteiro (2005), “Dimensional Stability” Concrete – Microstructure, Properties and Material, 3rd Edition. 3. Steven H. Kosmatka, Beatrix Kerkhoff, William C. Panarese (2002), “Volume Changes of Concrete” Design and control of concrete mixtures, 14th Edition. 4. M. S. Shetty (2005) “Elasticity, Creep and Shrinkage” Concrete Technology, Theory and Practice, 6th Edition. 5. A. M. Neville (2005) “Elasticity, Shrinkage and Creep” Properties of Concrete, 4th Edition. 6. Brian Addis (1998) Fundamentals of concrete, published by Cement and Concrete Institute, South Africa.

13 Strength and Durability of Concrete Mukul Dehadrai, N.V. Nayak and Ganesh Kaskar

13.1 INTRODUCTION The strength and durability of concrete are perhaps the two most important aspects that the designer should keep in mind while specifying materials. While strength of concrete is typically paid more attention to, durability is an equally important factor to consider. For a multiphase material like concrete, each of these properties is derived from several factors put together and the effect of these constituents on the overall macro properties of concrete is important to study. Strength, for example has an inverse relationship with porosity of the cement paste which in turn is directly related to the water cement ratio of the concrete. The aggregate in normal weight concrete is largely dense and the strength usually relies upon the porosity of the cement paste and the cement paste-aggregate boundary which is also known as the Interfacial Transition Zone (ITZ) (Garboczi and Bentz, 1999). Besides material characteristics, strength of concrete is also dependent on the mixing and placement of the constituents. Compaction of the concrete plays an important role in determining the density of concrete in the field. Also, testing for compressive strength of concrete will respond differently to different rates of loading. Such factors are important while considering concrete strength and should be taken note of. Durability of concrete is also becoming an important factor to consider when designing for significant service life. Concrete in itself is an integrally durable material. Sound concrete will resist most durability issues due to its high density and low porosity. In other cases, action from the elements will manifest in the form of physical and chemical attacks on the concrete. These are aimed either at the cement paste or the aggregate in general – however, in the case of reinforced concrete damage can also be sustained from the attack on steel reinforcement. The long term effects of such agencies on the constituents must be understood to ensure problem free service life of concrete in structures.

13.2

Handbook on Advanced Concrete Technology

13.2 STRENGTH OF CONCRETE 13.2.1 Definition and Importance Largely defined, Strength of a material is its ability to withstand an applied stress without failure. Although this is not a very precise term to describe it as a property of concrete as it needs to be qualified depending on the mode of stress under which the failure has occurred. These include compressive, tensile, flexural, shear or even torsion forces. It is also necessary to define the shape and dimensions of the sample being tested in certain cases. Because the compressive strength of concrete is usually much greater than its tensile strength, it is widely considered to be the most important property of concrete. Most structures are designed to take advantage of the immense compressive strength concrete has to offer. Though concrete can be subjected to a variety of different stresses including compressive, tensile and shear, compressive strength remains the value of choice for comparing performance of concrete mixture proportions as well. Compressive strength is also preferred as it is the easiest test to perform in the laboratory with standards supporting procedures for testing concrete cylinders and cubes at 28 days to determine if the concrete has reached its target strength as anticipated. Concrete compressive strength depends on a lot of factors as shown in Fig. 13.1 and as mentioned earlier can be related to properties of the material constituents as well as the concrete placement and compaction procedures being followed on site. The factors affecting strength of concrete will be discussed in the following sections of this chapter.

Concrete strength

Sample testing parameters type of applied stresstension/compression rate of applied stress

Strength of elementary phases in concrete

Parameters of the specimen being tested physical geometry physical size (dimensions in each plane) moisture content in the semple

Cement paste porosty

Aggregate porosity

ITZ Porosity

Fig. 13.1 Factors affecting strength of concrete (after Mehta and Monteiro, 1993)

13.2.2

Early Age Strength Development (Transition from Fluid to Solid Phase)

Development of early age strength of concrete has been studied by many researchers. This has been done with the use of both destructive as well as non-destructive methods (Neville 1973).

Strength and Durability of Concrete

13.3

Studying the development of elastic properties can provide a tool to better understand the nature of the response of concrete to stress.

L/3

L/3

Two point L

Cube

Fig. 13.2

L/3

L/3

L/3

Mid point L

Cyinder

Failure modes in concrete (after beushausen (1912))

Upon mixing of various constituents, the cement phase can be imagined to be a system of cementitious particles which are suspended in water. At this point the paste behaves almost like a fluid with no measureable elastic moduli, and hence no ability to withstand any type of stresses. The continued hydration and dissolution of the cement particles creates a connected network of solids with C-S-H being the major component. This connection has been termed as Percolation (Winslow et. al, 1994; Bentz et.al 1995) of the solids. While the connectivity of solids is under way, the water phase in the system depercolates into smaller water filled capillary spaces. This was noted by Powers et. al, (Powers 1968) many years ago and has also been shown by Bentz et.al using computer simulations. Other experimental methods such as rheology measurements, shrinkage and UPV (Ultrasonic Pulse Velocity) have confirmed this as well (Dehadrai et. al, 2007). The longitudinal wave velocity is seen to make a distinct jump at the point of percolation signaling onset of a percolated solid phase in the cement paste. The generation of the network of solids enables the cement paste to ultimately provide a resistance to any type of stress it is subjected to. This signals the onset of solidification and the subsequent development of elastic properties such as elastic modulus. The water filled spaces get ultimately dried up due to self dessication during continued hydration and generate the porous

13.4

Handbook on Advanced Concrete Technology

system in cement paste. The capillary porosity percolation in a hydrating cement paste has a large influence on the long term transport properties of concrete as well as the durability. Connected water phase Hydration

Connected solid phase

Fig. 13.3 Schematic showing percolation of solids and depercolation of water phase as hydration of cement progresses (Dehadrai et. al, 2007)

Plain cement paste w/c = 0.30 Deaired paste Paste with dissolved air

1000

0.8

3000

0.6 0.4 2000 w/c = 0.30 ultrasonic velocity

0.2

CEMHYD3D

0.0

0 0

2

4 6 Time (hours) (a)

8

10

0

4

8 12 Time (hours)

1000 16

(b)

Fig. 13.4 (a) Measurement of percolation in cement paste using ultrasonic methods of nondestructive measurement (b) comparison with 3D hydration modeling CEMHY3D Courtesy NIST (Dehadrai et. al, 2007)

Ultrasonic velocity (m/s)

2000

4000

Initial set

3000

Percolated solid fraction

1.0

Initial set

Ultrasonic velocity (m/s)

4000

Strength and Durability of Concrete

13.2.3

13.5

Elastic Modulus Development

Concrete elastic modulus is a crucial mechanical property and is extensively used by engineers in design and analysis of structures. It directly influences the stress response of concrete and as such may be taken as a measure of its strength. Although several attempts have been made at linking Elastic moduli to strength the relationships were found to be largely empirical.

13.2.3.1

Measuring Elastic Properties of Hydrating Cement Paste

Knowledge of the micro-structural evolution of cementitious elements at early age is essential for forecasting their performance (Ye et. al, 2004). There are a few points to remember in this regard: • Concrete gets its strength from hydration of cementitious elements in the mixture. While concrete sets, the only material that changes its phase is the cement paste. • Hydration causes the cement paste to change from a fluid with no ability to withstand stress to a hardened matrix with measureable elastic properties. Therefore to understand the development of elastic modulus in concrete it is necessary to concentrate on the evolution of cement paste properties. When mixed, cement paste behaves as a fluid with no elastic modulus. With the progress of hydration, the continued development of C-S-H phases a complex network of solids is formed which starts to give the cement paste matrix its elastic properties. Such material develops the ability to withstand stresses. As hydration progresses more and more C-S-H phases are formed giving the material more solid fractions. This phenomenon has been described in literature and several attempts have been made at measuring and quantifying this process. Despite the huge amount of literature devoted to various experimental techniques (Willis et. al, 1998; Kriechbaum, 1994; Diamond, 2000) to measure the early-age property development of cementitious material, applying and modeling these techniques is still not easy. Recently, one of the nondestructive techniques, i.e., ultrasonic pulse velocity (UPV) measurements, has attracted increasing attention for monitoring the early-age behavior of cementitious materials (Boumiz et. al, 1996; Keating and Hannant, 1989; Sayers and Dahlin, 1993; Gimet et. al, 1999). It was seen using UPV measurements that the conduction velocity of longitudinal waves increases as percolation occurs and connectivity of the solid fraction starts to occur, as shown in Fig. 13.5. The point at which percolation of solids starts to occur; also known as the Percolation Threshold, (Bentz et. al, 1995) is the point at which the ultrasonic wave velocity has been observed to increase. The use of longitudinal wave velocity to measure elastic modulus of hardened concrete at later ages is a common practice and has also been a standardized test method. The following relationship is used in this context _________________

(1 – m) E VL = __ ______________ r (1 + m) (1 – 2m)

÷

This has also been shown to work at very early ages (Dehadrai et al. 2007) where the evolution of Poisson’s ratio as well as the change in density can be taken into consideration to

Handbook on Advanced Concrete Technology 4000

Initial set

Percolated solid fraction

1

0.8

3000 0.6

0.4 w/c = 0.30 Ultrasonic velocity 0.2

2000

Ultrasonic velocity (m/s)

13.6

CEMHYD3D

0

1000 0

4

8 Time (hours)

12

16

Fig. 13.5 Percolation threshold signaling onset of property development in cement paste (Sant et al. 2007, Bentz et al., 1995)

measure evolution of dynamic modulus. The change in dynamic modulus is seen to be drastic in the first 24 hours as noted by many researchers after which the increase is seen to occur at a much slower rate. This of course is controlled by hydration dynamics particularly the degree of hydration in cementitious mixtures, especially where replacements are common. Researchers (Boumiz et. al, 1996, Dehadrai et. al, 2007) also showed that the higher concentration of solids (lower w/c ratios) in the mix resulted in higher moduli. The use of this theory for more practical and field type applications is currently the subject of many researchers.

Dynamic modulus (GPa)

40

24 Hour tests

ASTM C597

30

20

VL =

÷

(1 – m) E r (1 + m)(1 – 2m)

10 w/c = 0.30 + 5%SRA w/c = 0.30 0 0

24

48

72

96

120

144

168

Time (hours)

Fig. 13.6 Evolution of dynamic modulus over a period of 7 days (Dehadrai et.al)

Other properties such as Poisson’s ratio evolution have also been observed for a hydrating cement paste. Poisson’s ratio of the material also evolves as hydration progresses. Cement paste

Strength and Durability of Concrete

13.7

is fluid at very early ages its Poisson’s ratio can be assumed to be at 0.5 which then evolves from v = 0.5 in the fluid phase to about v = 0.25. The change in this value was found to be minimal beyond 24 hours. This was shown by Boumiz et. al, using ultrasonic measurements. Using shear wave transmission development of shear modulus (Voigt et. al, 2004) was also shown to occur as the cement paste changed from fluid to a solid phase.

Initial set

0.5

Poisson’s ratio

0.4

0.3

0.2 w/c = 0.35 w/c = 0.40 0.1 0

4

8

12 16 Time (hours)

20

24

Fig. 13.7 Evolution of Poisson’s ratio for hydrating cement paste (Boumiz et. al, 1996)

All in all, using these nondestructive techniques it has been shown that the cement paste hydrates to gain dynamic, shear moduli (Voigt and Shah, 2004; Voigt et. al, 2005) at which point it starts to behave as a complete solid which is capable of withstanding stresses. In the case of concrete, the cement paste matrix forms bridges and percolated solid continua with the aggregate in the system, thereby giving the concrete its elastic properties (Byfors 1980). Using such early age assessment techniques will give the user enough evidence to forecast E for a given mixture. Beyond the 24 hour period, E modulus measurement has also been carried out using ultrasonic and sonic resonance techniques (ASTM E1875) which is discussed in a separate chapter. It is important to note that the testing of fresh concrete using dynamic modulus methods proves favorable owing to the fact that such testing involves application of extremely minute stresses and the response is recorded.

13.2.3.2 Measurement of Elastic Properties of Hardened Concrete Elastic modulus in pure strength of material terms is the ratio between stress and strain but the stress–strain relationship for concrete is non-linear and the material is not strictly elastic. Thus, the concept is not strictly applicable; while, for structural design assessment three types

13.8

Handbook on Advanced Concrete Technology

(Neville 1973) of E-value are used, namely secant modulus tangent modulus and initial tangent modulus. The secant and tangent moduli can be determined from the stress–strain relationship from a short-term static test in which a specimen is loaded in uniaxial compression. A procedure for determining secant modulus is described in BS 1881: Part 121. The procedure described is as follows: 150 dia. × 300 mm specimen is loaded to 33 per cent of ultimate stress and the slope of stress/strain relationship is measured after conditioning to near linearity by progressively loading and unloading. The stress response of the concrete is measured along with longitudinal strain using electronic strain gauges. The response of stress vs. strain is plotted and the dynamic modulus is obtained.

Test specimen

Compressive strength testing machine Strain indicators

Fig. 13.8 Experimental setup for determining Elastic Modulus using uniaxial compressive testing

s

ulu

od nm ctio Se

Stress

ulu

s

od Initial tm tangent en g modulus Tan

Strain

Fig. 13.9 Determining E values of concrete using uniaxial compression testing (Mindess and Young, 1981)

Strength and Durability of Concrete

13.9

Where data such as the concrete strength and the density are available, more practical estimates of the modulus of elasticity can be made using empirical relationships as described in ACI 318 __

Ec = W1.5 c × 0.33÷f c¢ Where Wc is the weight of concrete in Ib/ft3 fc¢ is the specified concrete cylinder strenght in psi Some more relationships are described in the following section.

13.2.3.3

Calculating the Elastic Modulus Using Empirical Relationships

ACI recommends an empirical relationship which is widely used and is as follows: Where

Ecs = 0.043 [g3 (f c¢)] 0.5 Ecs is the static modulus of concrete in MPa

g = Density of concrete in kg/m3 f c¢ = Strength of concrete in MPa for Cylinders The following expressions are valid for f’c being defined for cylinder samples. For known values of density the above equation can be used in the form of following relationships: __

• Ecs = 4740 ÷f c¢ with g = 2300 kg/m3 __

• Ecs = 5060 ÷f c¢ with g = 2400 kg/m3 __

• Ecs = 5370 ÷f c¢ withy g = 2500 kg/m3 Ecs values should be rounded to the nearest tenth digit. For fc¢ as cube strength of concrete the equations above will get modified as follows: __

• Ecs = 4240 ÷f c¢ with g = 2300 kg/m3 __

• Ecs = 4530 ÷f c¢ with g = 2400 kg/m3 __

• Ecs = 4800 ÷f c¢ with g = 2500 kg/m3 In the Indian context, the density of concrete generally varies between 2400 to 2500 kg/m3 and hence the static elastic modulus of concrete can be defined by __

• Ecs = 4700 ÷f c¢ where f¢c is the cube strength of concrete and is in MPa at 28 days. IS 456:2000 recommends the following relationship for elastic modulus: __

Ec = 5000÷f c¢ where f c¢ is the cube strength of concrete and is in MPa at 28 days. It should be noted that these relationships are largely empirical and as such have limitations. The formulae do not account for other factors such as w/c ratio of concrete and resultant porosity of concrete at later ages.

13.10

Handbook on Advanced Concrete Technology

13.3 MECHANICAL PROPERTIES OF HARDENED CONCRETE This section addresses properties such as compressive strength and tensile strength with a more detailed perspective. The relevance of each is discussed with a brief overview of the prevailing methods of measurement of these properties on hardened concrete. The influence of factors on measured values is also briefly mentioned.

13.3.1 Compressive Strength 13.3.1.1 Relevance The compressive strength of concrete is very important, as concrete is used more often in compression than in any other way. It is rather difficult to give average values of the compressive strength of concrete, as its values depend on so many factors, but the existence of widely accepted methods have made it a practical piece of information for field use. The available aggregates are so varied, the methods of mixing and manipulation so different, that the tests must be studied before any conclusions can be drawn on the measured values. Correct measurement techniques along with statistical analysis will produce dependable results which will take into account all influencing factors. Compressive strength test results are primarily used to determine that the concrete mixture as delivered meets the requirements of the specified strength in the job specification. This step is crucial as the structural designer assumes this value to be true while designing reinforced concrete structures. Strength test results from cast cylinders or cubes may be used for quality control, acceptance of concrete, or for estimating the concrete strength in a structure for the purpose of scheduling construction operations such as form removal or for evaluating the adequacy of curing and protection afforded to the structure. IS-456: 2000 dictates that any test result should be the average of at least 3 standard-cured strength specimens made from the same concrete sample and tested at the same age. In most cases strength requirements for concrete are at an age of 28 days. Design engineers use the specified strength Ģc to design structural elements. This specified strength is incorporated in the job contract documents. However the concrete mixture is designed to produce an average strength higher than the specified strength such that the risk of not complying with the strength specification is minimized. Acceptance criteria based on these results help the designers in rejecting or accepting the concrete that they need. It is important to understand that an individual test falling below Ģc does not necessarily constitute a failure to meet specification requirements. When the average of strength tests on a job are at the required average strength, the probability that individual strength tests will be less than the specified strength is about 10% and this is accounted for in certain acceptance criteria (ASTM C39). When strength test results indicate that the concrete delivered fails to meet the requirements of the specification, it is important to recognize in some cases that the failure may be in the testing, not the concrete. This is especially true if the fabrication, handling, curing and testing of the cylinders are not conducted in accordance with standard procedures.

Strength and Durability of Concrete

13.11

Historical strength test records are used by the concrete producer to establish the target average strength of concrete mixtures for future work.

13.3.1.2 Testing for Compressive Strength Testing for compressive strength can yield accurate and relevant results if procedures for sample preparation and testing are followed as mentioned in the relevant standards.

13.3.1.3 Testing Cubes for Compressive Strength Although the cube strength does not represent the actual strength of concrete in the structure, it is an important test to consider while establishing quality control measures. Cube strength tests are a good indicator of the potential strength of the mix and is a good measure of variance on the strength achieved on site. Cube strength values can also be referred to while deciding upon de-shuttering schedules. Strength of concrete or compressive strength of concrete is defined as per IS-456: as follows: • The characteristic compressive strength of a cube measuring 15cm in size at the end of 28 days for concrete. • The characteristic compressive strength of a cube measuring 10cm in size at the end of 28 days for mortar. Some points to remember while testing cubes for compressive strength are as follows: 1 At least 3 samples per batch of concrete are cast to monitor strength development over time to give good statistical significance to the value of strength upon testing. 2 The variance in one sample of 3 cubes should not exceed more than 15% of the average strength to maintain statistical validity of the test results. 3 The cubes are cast in accordance with the relevant standards and care is taken to achieve proper compaction of the concrete as influence of voids and honeycombing due to improper compaction can result in false values. 4 The rate of loading is al so maintained to be no more than 14 N/mm2. 5 The measured results must follow certain acceptance criteria need to be referred to for correctly accepting and rejecting the measured results. The acceptance criteria according to IS-456: 2000 states that: 6 The mean strength determined from any group of four consecutive test results should comply with the following condition: For mixes of M-15 grade Mean Strength

≥ ƒ¢c + 0.825 × Standard Deviation OR ≥ ƒ¢c + 3 N/mm2

For mixes of M-20 grade and above Mean Strength

≥ ƒ¢c + 0.825 × Standard Deviation OR ≥ ƒ¢c + 4 N/mm2

13.12

Handbook on Advanced Concrete Technology

Any individual test result should not have a value less than Ģc Р3 N/mm2 for concrete of grade M-15 Ģc Р4 N/mm2 for concrete of grade M-20 and above Where Ģc is the Target Mean Strength for which the concrete was designed. Apart from the statistical validity, visual assessment of the failure should be done to ascertain the cubes were tested in a correct fashion. The failure mode of the cube gives indications of tensile failures if loaded incorrectly. Such tests should be repeated to avoid errors in measurement.

13.3.1.4 Testing Cylinders for Compressive Strength One of the more common methods of testing compressive strength (especially internationally) is the use of cylindrical samples. The main difference between the cylinder and cube testing procedures is the use of capping. Cylinder ends are usually not found to be plane or parallel enough to mate properly with the platens of the compression testing machines and hence a capping is applied to correct this aberration whereas cube samples are usually flipped on their sides and do not require such preparation. This is usually done with test specimens that are not plane to within 0.05mm. The capping is done in accordance with IS 516 and can be done using neat cement, sulphur or hard plaster. IS 516 also indicates that the capped surfaces should be prepared such that they do not deviate from a plane by more than 0.05mm.

Fig. 13.10 Difference between testing cylinders with and without anti-friction pads

The use of antifriction pads to reduce end restraint can also change the failure mode of the cylinder in compression. Testing is carried out on specimens measuring 150 × 300 mm (IS 516:1959) and in some cases on samples measuring 100 × 200 mm. The smaller specimens tend to be easier to make and handle in the field and the laboratory. The diameter of cylinder is also typically at least 3 times the nominal maximum size of the coarse aggregate used in the concrete. It is important to note that the cylinders should be centered in the compression-testing machine and loaded to complete failure while maintaining the loading rate on hydraulic machine

Strength and Durability of Concrete

13.13

as prescribed. Also, the type of break should be recorded. A common break pattern is a conical fracture.

Type 1

Type 4

Type 2

Type 5

Type 3

Type 6

Fig. 13.11 Modes of failure under compressive load (ASTM C39)

Corrections using factor as prescribed by IS 516:1959 are applied to the obtained strength. This factor can be obtained from the height/diameter curve. A product of the correction factor and the measured strength yields the corrected compressive strength. This is done so as to obtain the corresponding compressive strength of a cylinder having the height/diameter ratio of 2.0.

13.3.2 Relating Cube Strength with Cylinder Strength Both types of samples, cubes as well as cylinders are commonly used for compression testing, both in laboratories as well as on-site. However with cubes, the lateral restraint by the machine platens is a significant factor in resisting failure and consequently the resulting strength is higher than, say, a 150 mm × 300 mm cylinder. The European concrete standard, EN 206-1, put this difference at about 20 per cent for normal weight concrete in their dual classification of strength class. EN 206-1 uses the strength of a 150 mm × 300 mm cylinder. Typically most experts describe the relationship between the cube strength and the cylinder strength as complex. The effects of numerous factors, combined with inherent variations associated with concrete, complicate comparisons between the two types of tests. The factors governing this relationship are described further.

13.3.2.1

Casting and Testing Procedures

Two main factors are associated with the compression testing process: Type of mold The cylinder/cube strength ratio can be affected by the type of mold used to cast cylinder specimens. Cubes are always cast in rigid molds (i.e., made of steel or cast iron), but cylinders can

13.14

Handbook on Advanced Concrete Technology

be cast in either rigid or non-rigid molds. Cylinders cast in cardboard molds have shown a slight reduction in strength (up to 3.5 percent) from those cast in steel or cast iron (Price 1951). Capping and surface planeness Method of capping and out-of-plane surfaces can affect the cylinder/cube strength ratio as well. Method of capping is known to have some effect on cylinder strength (Richardson 1990; Grygiel and Amsler 1977; Saucier 1972) thus affecting the cylinder/cube strength ratio. Planeness of cube specimens is as important as capping of cylinder specimens. Out-of-plane mold surfaces have shown to reduce the strength of concrete cubes by as much as 15 percent (Westley 1966). Care must be taken to ensure that mold and specimen tolerances are met, or the cube strength and cylinder/cube strength ratio could be misleading.

13.3.2.2

Specimen Geometry

Various aspects of specimen geometry are recognized to affect compressive strength test results. The most significant factor is the specimen’s height-to-maximum lateral dimension (h/d) ratio. Although desirable, true uniaxial stress throughout a specimen is not possible in practice. Frictional effects between the specimen and machine platens produce lateral stresses in specimens. This would result in a multiaxial state of stress that tends to increase the strength exhibited by test specimens. Lateral stresses affect specimen stress state in a cone-shaped or pyramid-shaped region to a depth of about 0.866d at each end. As such cylinder specimens measuring 150 × 300 mm (6 × 12 in.) with h ~ 1.7 d will have a region not usually experiencing these multiaxial stresses. Cubes are affected by multiaxial stresses throughout the specimen. Both types of samples tested for compressive strength are shown in Fig. below. It can be expected that cubes will exhibit greater strengths than cylinders for otherwise identical concretes. Past research has shown this to be the case (Gonnerman 1925; Cormack 1956). It is evident from relationships that the strength ratio is more sensitive to h/d ratios for h/d < 1.5 than for h/d > 1.5.

2d

0.268d

0.866d

d

30°

Central region unaffected by lateral stress

Specimen affected by lateral stress throughout its height 30°

Fig. 13.12 Effect of geometry on specimen type

The effect of d is based on the probability of having a weak unit of concrete in any cross-section. The larger a specimen’s d, the more likely it contains a cross-section with an

Strength and Durability of Concrete

13.15

element of low strength that governs failure of the specimen. Neville (Neville 1966) suggested that the relationship between strength fc¢ and lateral dimension d is 1 fc¢ μ __2 d

13.3.2.3 Level of Strength Nominal strength of concrete has been shown to affect the cylinder/cube strength ratio. Research by Evans (1944) indicates that this ratio decreases with decreasing concrete strength. Table 13.1 indicates that the cylinder/cube strength ratio ranges from 0.77 to 0.96, depending on concrete strength level. Data relating strength to cylinder/cube strength ratio (Evans 1944) Cubes

Strength (MPa)

9 15.2 20 24.8 27.6 29 29.6 35.8 36.5 42.1 44.1 48.3 52.4

Cylinder/Cube ratio

Cylinders 6.9 11.7 15.2 20 24.1 26.2 26.9 31.7 34.5 36.5 40.7 44.1 50.3

0.77 0.77 0.76 0.81 0.87 0.91 0.91 0.98 0.94 0.87 0.92 0.91 0.96

60 Cubes Cylinders

Strength (MPa)

50

40

30

20

10

0 0.7

0.75

0.8 0.85 0.9 Cylinder/cube strength ratio

0.95

1

Fig. 13.13 Relation of design strength to cylinder/cube strength ratio

13.16

Handbook on Advanced Concrete Technology

The relationship between strength and cylinder/cube strength ratio is noticeable, and should be considered whenever the two geometries are compared.

13.3.2.4 Direction of Loading and Machine Characteristics Like Rate of Loading On another note, compression loads are applied in the direction of casting for cylinders, and perpendicular to the direction of casting for cubes. Because both cylinders and cubes are cast and consolidated in multiple layers, direction of loading is an important factor in the relationship between cylinder strength and cube strength. When cylinders are loaded, each casting layer occupies an entire cross-section, and receives the total load from the testing machine. When cubes are loaded, each layer extends from top to bottom, and receives a portion of the total load. The effect of flipping a cube on its side depends on two related factors: aggregate segregation and plate fixity. Segregation is important because it can cause variations in strength and elasticity between the casting layers (Tarrant 1954). When cubes are flipped, these variations exist in every cross-section perpendicular to the direction of loading. Each casting layer receives its load from a different part of the plate. If non-segregating concrete is used, then strength across a plane should not vary significantly. Direction of loading

Direction of loading

Casting layers

Fig. 13.14 Effect of direction of loading on cylinder and cube samples

The second effect - platen fixity - comes into play only when segregation occurs. Tarrant showed that when the upper platen is not allowed to rotate, the strongest layers carry most of the load. This results from weak layers yielding more than strong layers. The weaker layers tend to fail first resulting in the stronger layers taking up all the load – in essence attributing the failure usually to the weakest layers. If the upper platen is free to rotate, it adjusts to deformations that vary between casting layers. Each layers carries the same amount of load, and failure occurs when capacity of the weakest layer is reached. Strength of the stronger layers is never fully used. As the platen is free to rotate the total measured load in the concrete cube is lower. Experimental results have shown the effects of segregated concrete on cube and cylinder strength. Sigvaldason (1966) found that segregating concretes generally gave smaller cylinder

Strength and Durability of Concrete

13.17

strength/cube strength ratios than more uniform concrete. His results showed the cylinder/cube strength ratio to be 0.71 to 0.77 for segregated concrete, and 0.76 to 0.84 for non-segregated concrete. Also, cube strength was more sensitive to the method of end loading. Sigvaldason observed strength variations of about 7 percent for cubes and 1 percent for cylinders for varying end conditions (pinned-pinned,pinned-fixed, and fixed-fixed). Neville (1959) found no statistical difference between strengths of cubes of non-segregating concrete loaded perpendicular and parallel to the direction of casting.

13.3.2.6 Aggregate Grading Aggregate grading is also known to affect concrete strength in any structure or specimen. The effect is magnified for compression test specimens due to the relative size of aggregate particles to specimen dimensions. Aggregate grading affects specimen strength through the “wall effect” (Neville 1959). In concrete specimens a greater volume of space exists between the aggregate and mold wall than between aggregate particles within the specimen. The extra mortar at the walls increases the specimen’s compressive strength. Because this is a surface effect, compressive strength increase is greater for specimens with larger surface/volume ratios. Comparing the surface/volume ratio for standard cylinders and cubes indicates how aggregate grading may affect the cylinder/cube strength ratio. The surface/volume ratio of 150 × 300 mm (6 × 12 in.) cylinders is 0.033/mm, but it is 0.040/mm for 150 mm (6-in.) cubes, suggesting that cubes are more sensitive to changes in aggregate grading than cylinders. In fact, Gyengo’s research (1938) showed 1) that cylinder strength is less affected by changes in aggregate grading, and 2) that the cylinder/cube strength ratio decreases with increasing coarseness of aggregate grading.

13.3.3 Relating Field Extracted Core Strength to Cube Strength In-situ measurement of concrete strength is carried out by drilling a core of specified diameter and testing it as standard laboratory cylinder. It is widely accepted as one of the means of evaluating the structural capacity of a concrete member. As such, cores do not serve the same purpose as cylinders. Strength of standard cylinders represents the quality of concrete delivered. Cylinder compressive strength represents the quality of concrete batching, mixing and transportation as well as the sampling, preparation, handling, curing and testing of the cylinders. Strength of cores represents in addition the quality of placement, consolidation, and curing, and techniques for obtaining and testing cores. Therefore, the relationship between core and cylinder strength varies because of the characteristics that each specimen represents. It is important to maintain reasonably even ends to perform compression strength testing. Capping as described in IS 516:1959 proves to be an effective method to achieve the plane surfaces. All cores are tested in accordance with IS 516 : 1959 and the measured compressive strength is corrected using the similar correction factors obtained for the height/diameter ratios. Equivalent cube strength can also be obtained by multiplying the corrected compressive strength by 1.25.

13.18

Handbook on Advanced Concrete Technology

Strength values obtained are generally affected by a few factors: • Coring Direction • Cores obtained by drilling in the direction of concrete casting may sometimes yield higher strength numbers than those obtained by drilling in direction perpendicular to casting. • Consolidation of concrete • Effect of curing • Location of coring

13.3.4 Tensile Strength 13.3.4.1 Relevance Tensile strength values are associated with the characteristic compressive strengths and explicitly defined in most codes. The application of the tensile strength as design parameter in international design codes varies. Direct application is mainly used in the following structural design situations: • Ultimate limit state: • Shear strength and punching shear strength of slabs • Bond anchorage and overlap splicing of reinforcement • Shear transfer in cracks • Minimum reinforcement • Service limit state: • Crack formation • Crack spacing • Tension stiffening • Analysis • Tensile stress-strain relationship in non linear finite element analysis When the tensile strength is applied as a design parameter in the ultimate limit state, the major concern is to maintain adequate safety. It is generally realized that the effective minimum tensile strength of concrete members may be substantially reduced by environmental effects (restrained shrinkage and temperature differences) , construction joints and other crack initiators. Transfer of longitudinal force resultants by the tensile strength of plain concrete is therefore usually not relied upon. However, in cases where the average tensile strength in a certain volume is decisive, such as for anchorage of reinforcement, and/or when the tensile stresses are oriented in directions where the concrete is more protected from detrimental effects, such as in slabs subjected to shear, the tensile strength is reliable and should be recognized as a basic strength parameter in ultimate limit state. The tensile strength of normal weight concrete is usually defined in codes by a simple function of the compressive strength. It is well known that tensile strength is more variable than the compressive strength. It is more influenced by the shape and surface texture of the aggregates than the compressive strength. It is, however, also observed that for example the shear

Strength and Durability of Concrete

13.19

resistance of slabs without shear reinforcement made of concrete with increasing strength cannot be satisfactorily expressed by the increase of the tensile strength alone. It may be assumed that also the increasing brittleness of concrete of increasing strength has a certain influence on the resistance. This can be taken into account either by defining a modified function of the compressive strength with more moderate increase of the tensile strength or by substituting the tensile strength with a different formula in the shear resistance equation.

13.3.4.2 Testing for Tensile Strength Researchers have conducted several theoretical and experimental studies to determine the tensile strength of concrete. The stress obtained by breaking a specimen which is subjected to unilateral tension would yield the most accurate values of tensile strength. However, this test is experimentally difficult and less practical. Particularly, after the production of strong epoxy based adhesives, the uniaxial tensile tests are done with few troubles. Many experimental researches conducted in the past to determine the uniaxial tensile strength failed because of unexpected crushing which occurred as a result of local stress concentrations. Another difficulty in uniaxial tensile tests is that the test specimen is under the influence of moment effects during the tensile test due to eccentricity. Zhou reported that an increase in load eccentricity may decrease the tensile strength. The tensile strengths of concrete were investigated for traditional concretes, but with the recent developments in concrete technology, compressive strength of concrete has highly increased. Despite this, the properties of high strength concretes have not been investigated yet as well as those of traditional concretes. Especially the studies conducted to determine the tensile strength are rather limited. Tension force pin Grip

Steel plate

Specimen L

Boundary stresses are complicated

L-Uniform stress distribution zone

Fig. 13.15 Grips dog bone test end plate loading method

13.3.4.2.1

Split Tensile Testing

This test is standardized by the ASTM C 496 and is a common method for estimating the tensile strength of concrete through an indirect tension test. This method was developed in Brazil and Japan (Riley 1995), and is used in some states of the United States.

13.20

Handbook on Advanced Concrete Technology

To perform this test a cylindrical specimen is placed on its side and loaded in diametrical compression to induce transverse tension in the material. The load is applied continuously at a constant rate within the splitting tension stress range of 0.7 to 1.3 MPa until the specimen fails. The compressive stress produces a transverse tensile stress, which is uniform along the vertical diameter. The splitting tensile strength is computed from the formula:

Where,

2P T = ____2 pld T P l d

= = = =

tensile strength failure load length of the specimen diameter of the specimen Load

1/8 by in. plywood (typ.)

Supplementary steel bar 6 12 in. Concrete cylinder Plane of tensile failure Bed plate of testing machine

Fig. 13.16 Typical arrangement of splitting tensile test

13.3.4.2.2

Flexural Testing

Tensile strength is most often evaluated using a flexure test, in which plain concrete is loaded in bending (Riley 1995). Designers of pavements also use a theory based on flexural strength. Therefore, laboratory mix design based on flexural strength tests may be required, or a cementitious material content may be selected from past experience to obtain the needed design MR (Modulus of Rupture). Some also use MR for field control and acceptance of pavements. Very few use flexural testing for structural concrete. Agencies not using flexural strength for field control generally find the use of compressive strength convenient and reliable to judge the quality of the concrete as delivered. There are certain things to remember while testing concrete using these methods. Flexural tests are extremely sensitive to specimen preparation, handling, and curing procedure. Beams are very heavy and can be damaged when handled and transported from the jobsite to the lab. Allowing a beam to dry will yield lower strengths. Beams must be cured in a standard manner, and tested while wet. Meeting all these requirements on a jobsite is extremely difficult often resulting in unreliable and generally low MR values.

Strength and Durability of Concrete

13.21

A short period of drying can produce a sharp drop in flexural strength. Many state highway agencies have used flexural strength but are now changing to compressive strength or maturity concepts for job control and quality assurance of concrete paving. Cylinder compressive strengths are also used for concrete structures. The data point to a need for a review of current testing procedures. They suggest also that, while the flexural strength test is a useful tool in research and in laboratory evaluation of concrete ingredients and proportions, it is too sensitive to testing variations to be usable as a basis for the acceptance or rejection of concrete in the field. IS 519:1959 describes standard specifications for testing concrete with center-point and third-point loadings. Fig. 13.17 indicates that the mid-point loading results in a moment diagram that is a triangle whereas the third-point loading results in a diagram that is a trapezoid. To equal maximum moments, the third-point loading would have to be 50 percent greater than the mid-point loading. Not surprisingly, the two tests yield different results. P L/2

P L/2

L/2

L/3

Concrete test beam

P/2

L

M = PL/4 (a)

L/2

P/2

P/2

L/3

Concrete test beam

P/2

P/2

L

P/2

M = PL/6 (b)

Fig. 13.17 Flexure test and moment diagrams: (a) center-point loading, (b) third-point loading

Higher strengths are obtained from the mid-point loading. The theoretical maximum tensile strength or modulus of rupture (R or f’r) for the third-point loading is calculated from a simple formula: PL R = ___2 bd P = maximum load, L = span length, b = specimen width, and d = specimen depth. The above formula is valid only if the fracture in the tension surface is within the middle third of the span length. If the fracture is outside by more than 5 percent of the span length, a modified formula is used. 3Pa R = ____2 bd Where a is the distance from the nearest support

13.22

Handbook on Advanced Concrete Technology

It should be noted that the results from the modulus of rupture test tend to overestimate the tensile strength of concrete by 50 to 100 percent, mainly because the flexure formula assumes a linear stress-strain relationship in concrete throughout the cross section of the beam.

13.3.5

Factors Affecting Mechanical Properties of Concrete

The mechanical properties of concrete depend on several factors, some of which are being briefly discussed in this section.

13.3.5.1

Cement

Particularly, the cement content in the concrete is known to affect the strength properties at later ages. Higher the cement content, higher the expected compressive strength. Nevertheless, increase in cement content increases the propensity of other issues like elevated heat release, increased shrinkage in concrete and resulting cracking. Reduction of durability issues with concrete in the future dictates the use of optimum cement content in the concrete mixtures.

Mean 28 day cube strength N/mm

2

70

60

20 mm Aggregate portland cement 75 mm slump

50

40

30

20

10 200

250

300 350 3 Cement kg/m

400

Fig. 13.18 Influence of cement content on compressive strength of concrete (Dewar 1986; Deacon and Dewar 1982)

With regards to cement, the strength of concrete depends on the chemical composition and the fineness of the cement being used. C3S (Tricalcium Silicate) and C2S (Dicalcium Silicate) contribute equally to the development of the ultimate strength. C3S contributes more towards early age strength and in cements with higher C3S contents; the early age strength development is usually faster (within first 3 to 4 weeks). The early age strength characteristics will change with the influence of C3A (Tricalcium Aluminate), with high amounts leading to a phenomenon known as ‘flash set’. Gypsum content in the cement will keep this under control.

Strength and Durability of Concrete 70

13.23

10,000

60

2

Compressive strength (MPa)

Compressive strength (Ib/in. )

8000

C3S 50

6000

40

30 4000 C2S

20

C3A + CSH2

10

2000

– C4AF + CSH2 0

20

40 60 Time (days)

80

0 100

Fig. 13.19 Compressive strength development of cement phases (Mindess and Young 1993)

Compressive strength at early ages (7 days) will increase with an increase in C3S content for well-cured concretes. At 28 days, however, C3S has little influence on the strength of concrete (over a C3S range from 35% to 65% C3S. Finely ground cement is thus a potentially viable option to improve certain properties of concrete like high early strength, which may reduce (by 15%) the required cement content in concrete (Gebhardt 1993). One of the main reasons for the move towards finer cements has been the ever increasing emphasis on high early-age strengths and fast track construction by much of the industry. Finer cements, with their higher surface area, are more reactive at early ages, producing the desired higher early-age strengths. Since most cement producers are hesitant to produce a wide range of products, the same cements that are manufactured for high early-age strength applications (high rise construction, etc.) are also employed in pavements and bridge decks, where long term durability may be much more critical than early-age strength (Tennis and Bhatty 2005) In the early 1950s, Brewer and Burrows were perhaps the first to point out the critical linkage between cement fineness and concrete durability (Brewer and Burrows 1951). This point was reiterated by Houk et. al, (1969) in the late 1960s when examining cements for use in the Dworshak Dam. Burrows and others have continued this advocacy for the use of coarser cements in recent years (Burrows 1998; Bentz and Haecker 1999), but as most research indicates, the trend continues much as before. Very long-term strength development, however, may be detrimentally affected by increased fineness and C3S content.

13.24

Handbook on Advanced Concrete Technology

13.3.5.2 Aggregates Concrete quality is affected by numerous parameters like mineralogy, particle size distribution, maximum size, shape, reactivity related with the aggregate used. Typically, the properties of aggregate that can affect the strength would include: • • • • •

Absoprtion, Porosity, and Permeability Surface Texture Strength and Elasticity Density and Specific Gravity Aggregate Voids

• Hardness • Particle Shape • Coatings • Undesirable Physical Components The strength of the Interfacial Transition zone (ITZ) also has a small but measureable effect on the compressive strength of the concrete. The influence of ITZ is much larger on the tensile and fracture properties of concrete. For normal weight aggregates, due to the inherent size differences between cement and aggregates, a ‘‘wall effect” exists, so that there is a deficiency of cement particles near the aggregate surface relative to their concentration in the bulk (non-ITZ) cement paste.

Ettringite

CH

CSH

Aggregate Transition zone

Bulk cement paste

Fig. 13.20 Interfacial transition zone for aggregate cement paste interface (after Mindess and Young 1993)

The ITZ has a higher concentration of ettringite and portlandite. These are the first phases to precipitate in the initial stages of hydration and they nucleate on the available surfaces i.e.

Strength and Durability of Concrete

13.25

also on the aggregate surfaces. The concentration of CSH is much less in the ITZ, which is key to developing compressive strength. Typically, the bond thickness of this ITZ region which measures roughly 20-40 mm influences the compressive strength to a large extent. The bond strength depends on the surface properties of the aggregates, extent of bleeding in the concrete and to some extent the chemical bonding between the aggregate and cement paste. It is largely very difficult to measure the ITZ bond strength, but decrease in the porosity of the ITZ has been related to the overall increase in compressive strength. Maximum size of aggregate can also affect strength of the concrete. The larger maximum size of aggregate affects the strength in several ways. Under a compressive load the larger aggregate particles tend to create larger stress concentrations leading to an overall decrease in compressive strength. Larger size of aggregate provides a larger degree of restraint to volume changes in a hydrating cement paste by inducing additional stresses, which may weaken the concrete. The concrete mixtures with larger coarse aggregate sizes also exhibit more variability, especially in the case of high strength concrete. 8000

2

Compressive strength (Ib/in. )

7000

w/c Ratio = 0.40 = 0.55 = 0.70

6000

5000

4000

3000

2000 No. 4

3 3 1 1 3 8 4 2 Maximum size aggregate (in.)

6

Fig. 13.21 Influence of Maximum size of aggregate on concrete strength (Cordon and Gillespie 1963)

In case of tensile and fracture strength properties, larger aggregates will generally lead to an increase. This happens due to the greater tendency towards crack bridging and a greater path length available for the cracks to pass around the aggregate particles.

13.26

Handbook on Advanced Concrete Technology

13.3.5.3 Admixtures Chemical Admixtures Chemical admixtures have a minimal direct effect on the strength of concrete. As such chemical admixtures will influence the porosity of the concrete which in turn can affect the strength properties to a great extent. In cold-regions where air entraining is necessary, the increased porosity has been linked to decrease in compressive strength. The advantage to using most chemical admixtures is linked to a lower w/c ratio with higher workability. Such concretes usually exhibit a definite increase in strength Water reducing admixtures, especially super-plasticizers facilitate in maintaining the workability of a low w/c design concrete, which will result in high compressive strength at later ages. These admixtures assist in more uniform dispersion of the cement particles with lower water content, thereby eliminating large pores in hardened concrete as well. The strength gain of concrete mixtures is most affected by use of accelerating admixtures. Accelerators like Calcium Chloride (CaCl2), which is a common accelerator, quickens hydration in concretes even in lower than ambient temperatures. Typically the dose followed is within 1 to 4% which also changes the rate of strength gain in the first few hours. Although, strength gain can be quite rapid in the beginning, compressive strength at later ages may be lower (Ramachandran 1995). Similarly, the use of water reducers will retard the hydration reaction in the early ages. The delayed hydration leads to slower gain in early strength, and as mentioned earlier water reducers can also affect the porosity of the concrete. This influence depends upon the degree of hydration and the water cement ratio of the concrete. At longer ages, when the degree of hydration is not significantly affected by admixtures, the porosity of cement paste is reduced as the water cement ratio is decreased. The reduction in water content, caused by water reducing agents results in a net increase in strength at 28 days (Ramachandran 1995).In general this increase in strength is greater than would be expected simply from the reduction in water (Rixom 1978). Mineral Admixtures The use of mineral admixtures results in a modification of the structure of hardened cement paste and may result in changes within the interfacial transition zone. For normal strength concretes, mineral admixtures such as fly ash and blast furnace slag can be used to replace portland cement with little effect on strength. While the increase in long term strength gain is a key advantage of adding these materials, the characteristic slow rate of early age strength gain is also quite common. The addition of these mineral admixtures has proven instrumental in improving the durability of concrete in addition to long term strength gain. In the appropriate proportions mineral admixtures like silica fume not only combine with Calcium Hydroxide, resulting from the hydration of C3S and C2S, but also fill in voids between the cement particles, reducing the flaw size available for crack initiation. The increase in long term strength is a function of the curing conditions on site and should be given its due importance while placing high strength concretes with large quantity of binder replacements. Thus, due to the physical effect of filling the large voids with fine size mineral admixture particles and formation of the cementitious compounds by pozzolanic reaction products which

Strength and Durability of Concrete

13.27

cause pore refinement and also reduces micro-cracking in the transition zone, significant improvements in strength and durability are achieved. For further discussion on this topic the reader is referred to chapter No: 2 of this book. 30% RHA 65% Slng 15% SF 30% Flyash-Type C 30% Flyash-Type F

Relative compressive strength

150 125

Control

100 75 50 25 0 0

14

28 42 Moist curing time (time)

56

Fig. 13.22 Influence of Mineral Admixtures on strength of concrete (Mindess and Young 1993) (RHA = rice husk ash; SF = silica fume; C ash = Class C fly ash; F ash = Class F fly ash).

13.3.5.4

w/c Ratio

The two important factors which adversely affect both strength and imperviousness of concrete are the presence of large pores in the hydrated cement paste, and the micro-cracks at the aggregate-cement paste transition zone or interfacial zone. It is generally accepted that the strength of concrete is largely a function of the cement paste which is primarily a function of paste porosity. S __ = e–kp S0 S = strength S0 = intrinsic strenght (i.e. at zero porosity) S __ = e–kp p = fractional porosity S0 k depends on material Even though the strength of concrete is dependent largely on the capillary porosity or gel/ space ratio of the paste, these are not easy quantities to measure or predict. They are, therefore, not suitable for use in the mix design procedures. Fortunately, however, the capillary porosity of a properly compacted concrete at any degree of hydration is determined by the water/cement ratio.

13.28

Handbook on Advanced Concrete Technology

Compressive strength

Higher strength with low w/c achieved using admixtures

Vibration

Hand compaction

Well compacted concrete

Insufficient compaction

w/c Ratio

Fig. 13.23 Relationship between strength and water/cement ratio of concrete (Neville 1973)

The role of water-binder ratio on the strength properties of concrete is also important to discuss. A finer description of this role of the w/b (water-binder) ratio in determining strength was given by Gilkey (1961): “For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregate, and water (under the same mixing, curing, and testing conditions) is influenced by: • ratio of cement to mixing water, • ratio of cement to aggregate, • grading, surface texture, shape, strength, and stiffness of aggregate particles, While the w/b ratio has been related to the strength of the concrete it is the porosity or gel-space ratio which truly influences ultimate strength. As mentioned earlier, the strength depends on other factors such as the degree of hydration, curing etc. it is only correct to relate the strength to the concentration of the solid products of hydration of cement in the space available for these products.

Fig. 13.24 Water-binder ratio affecting inter-particle space in cement paste

Strength and Durability of Concrete

13.29

In high w/b ratio concretes, the cement particles have more capillary water in between them. Bentz (2008) referred to it as the distance between the cement particles which is more fundamental. The lower the w/c ratio, the lesser the capillary space between the cement particles, the faster the cement hydration products fills in these spaces. Most importantly, as a result of this, the concrete will also be much stronger. As the cement hydrates to occupy more than twice its original volume, the water that is used up in the hydration process reduces the capillary voids and fills it with solid (gel) products. The relationship between the gel/space ratio and compressive strength was first described by Powers (1968). The compressive strength was tested by Powers and was found to be related to the gel space ratio using f c¢ = 234 r 3 Where fc¢ is the compressive strength in MPa and r is the gel space ratio of the concrete. In other words, these empirical relations state that the strength of the cement paste depends primarily on the physical structure of the gel; although the chemical composition of the cement and its effects should not be neglected. It should also be noted that most of the experimental work related to porosity or gel-space ratio of hardened cement paste, has been performed on specimens of neat cement paste. The characteristics of porosity would be different for concrete with different admixtures or aggregates. Also, the methods with which porosity have been measured in the past have shown significant variance in values.

13.3.5.5

Other Factors

Curing The curing of concrete in an efficient manner is key to obtaining design strength. Curing after placement becomes more and more critical with high strength and high performance concretes. The hydration of cement virtually ceases when the relative humidity within capillaries drops below 80% (Neville). Under an efficient curing method such as water curing, the relative humidity is maintained above 80% to continue the hydration of cement. Conversely, concrete will lose water or moisture through evaporation and become dry in absence of a proper curing. The evaporation decreases the relative humidity by reducing the amount of available moisture, and thereby retards the hydration of cement. In severe cases, the hydration is eventually stopped. When hydration is stopped, sufficient calcium silicate hydrate (CSH) cannot develop from the reaction of cement compounds and water. While CSH is the major strength-providing reaction product of cement hydration, it also acts as a porosity reducer and thereby results in a dense microstructure in concrete. Without adequate calcium silicate hydrate, the development of dense microstructure and refined pore structure is interrupted. A more continuous pore structure may be formed in cover concrete, since it is very sensitive to drying. The continuous pore structure formed in cover concrete may allow the ingress of deleterious agents, and thus would cause various durability problems. Moreover, the drying of concrete surfaces results in shrinkage cracks that may aggravate the durability problems. Therefore, efficient curing is inevitable to prevent the moisture movement or evaporation of water from concrete surface. It can be accomplished by keeping the concrete element completely saturated or as much

13.30

Handbook on Advanced Concrete Technology

saturated as possible until the water-filled spaces are substantially reduced by hydration products (Mather 1987). For this, an extra amount of water must be added to replenish the loss of water due to evaporation or self dessication. A proper curing regime greatly contributes to reduce the porosity and drying shrinkage of concrete, and thus to achieve higher strength and greater resistance to physical or chemical attacks in aggressive environments. Therefore, a suitable curing method such as water ponding, spraying of water, or covering with wet burlap and plastic sheet is essential in order to produce strong and durable concrete. Typically, water curing will produce the highest compressive strength concrete. This however is impractical on construction sites; and the use of wrapped wet burlap is done to keep the concrete moist. The use of plastic sheets will control the evaporation in hot dry climates. Some specifications recommend use of paraffin wax based curing compounds to arrest loss of water from the concrete due to evaporation Inadequate curing can lead to durability related issues which are as a result of high porosity or cracks in the cement paste matrix.

13.4 BOND STRENGTH OF CONCRETE AND REINFORCEMENT STEEL By definition, bond is the force transfer between two materials (MacGregor 1997). Bond behavior involves the bond stresses, transfer mechanisms and ultimately the complete failure mode. Bond stress is defined as the shear stress at the steel-concrete interface, which by transferring the load between the reinforcing bar and surrounding concrete, modifies the steel stresses (Park and Paulay 1975). A good bond between steel deformed reinforcing bar and concrete in concrete structures is crucial for structural and serviceability performance. If this bond is inadequate, behavior and failure characteristics will be altered. The bond mechanism allows the forces to be transferred between the concrete and steel. In concrete construction, many types of contaminants are present on the site, such as form oil for coating the forms and bond breaker used in tilt-up construction. The reinforcement could be contaminated during construction if care is not taken. If contaminated, there is concern regarding the bond strength and specifications are in place to guide the action to be taken. Often, reinforcing bar is subjected to various construction contaminants, such as form oil or mud during concrete construction, and the specifications require the reinforcing bar to be cleaned prior to placing the concrete. Previously, several studies have been performed in regard to the variables that affect bond strength, including the effect of the amount of concrete cover, casting position, slump and consolidation on epoxy-coated reinforcing bar (Darwin et.al 1994), and the effect of rust and scale (Johnston and Cox 1940). It was found that up to a certain degree of rust, the bond was not significantly reduced and the specifications were relaxed. These specifications now recognize that slightly rusted steel reinforcing bar does not cause a significant reduction in the bond stress and allows contractors to use this reinforcing bar without having to first remove the rust. This saves the contractor considerable time and money without significantly affecting the quality

Strength and Durability of Concrete

13.31

and strength of the RC structure. However the potential of reinforcement corrosion would be governed by adequate mixing, curing and placing techniques and a low w/c concrete. The average bond stress is typically represented as µavg Dfsdb mavg = _____ 4lb where: mavg = average bond stress, Dfs = change of steel stress over unit length, db = diameter of reinforcing bar, and lb= embedment length. IS 456:2000 denotes the design bond stress as Tbd and provides recommendations for the design bond stress in limit state method for plain bars. There are several mechanisms that transfer the load between the concrete and steel. The three primary mechanisms are chemical adhesion, mechanical interlock and frictional resistance. Each method contributes to the overall bond strength in varying amounts depending on the type of reinforcing bar and conditions under which the concrete is placed. For deformed steel reinforcing bar, the greatest contribution comes from the mechanical interlock, with the frictional and chemical bonds both helping to a lesser extent. The bearing of concrete on the steel ribs causes the mechanical interlock. As the forces are transferred, the concrete is placed under a shearing stress; thus eventually causing bond failure. With plain (smooth) reinforcing bar, the chemical and frictional bonds would be the primary mechanisms, with the mechanical interlock almost non-existent.

13.5

DURABILITY OF CONCRETE

With more and more money being spent on new structures it becomes imperative to understand their life cycle performance. As such most structures are built of reinforced concrete (being the cheapest construction materials) and the durability of the structure ultimately relies upon the durability of its material: concrete. The emphasis in the next few decades will be on long-term performance standards, materials, optimization techniques and the understanding of how all this contributes towards a problem free structural service life. The knowledge exists for selection of materials, proportion them appropriately, mix them thoroughly, transport and place them without segregation, and cure them to minimize cracking and optimize long-term strength development and durability. Implementing this knowledge is a major challenge, but one that will eventually result in a more permanent, low-maintenance infrastructure. While it is important to note that most contractors still look at the strength (compressive strength) of concrete as the criterion for a good concrete, more and more engineers have become aware of the properties such as permeability to chlorides, and other salts. Rapid Chloride Permeability Testing (RCPT) is now becoming more acceptable as a form of durability testing. As mentioned earlier, the key causes of a less durable concrete could be the manifestation of a number of defects, both material as well as related to skill of the placing labor. However, detection of these defects is critical to understand the service life implications on concrete and the structure as a whole.

13.32

Handbook on Advanced Concrete Technology

Several factors are found to be influencing durability of concrete which could be categorized into internal and external sources.

13.5.1

Durability - Influences from Internal Sources

13.5.1.1

Mix Composition/Variation

Mixture composition of the concrete can include several factors: • • • • • • • • • •

water/cement ratio cement content cement type/composition aggregate type chemical inertness (ASR potential) physical inertness (soundness) vmoisture movement elastic/thermal properties admixtures Impurities

13.5.1.2 Concrete properties The properties of the concrete itself which can affect the overall durability properties will include: • • • • • • • • • • • • • • • •

alkalinity permeability/porosity/absorption elasticity/creep/shrinkage/thermal properties moisture condition strength (compression/tension) degree of hydration/age void distribution air entrainment bleeding potential discontinuity of voids which can be related to permeability Construction reinforcement metal inserts compaction curing contamination

Strength and Durability of Concrete

13.5.2

13.33

Durability - Influences from External Sources

13.5.2.1 Structural/Architectural • loading • detailing • position/protection Exposure • severity/rate/duration/cycling/extremes/shock Ambient conditions • temperature/humidity/frost/rain/sun/wind/extremes of conditions/cycling/number of cycles Liquid • pressure/quantity/rate of permeation Wear • accident/abrasion/attrition/rate of wear Chemicals • acids/alkalis/pH/concentration/in air/stationary or mobile Quality Controls

Fig. 13.25 Column showing porous concrete due to improper mix and incorrect construction practices

13.34

Handbook on Advanced Concrete Technology

Fig. 13.26 RCC column displaying porosity

To achieve good durability of concrete the following factors should be properly controlled: (a) Structural design. (b) Study of environment in which the structure is being constructed. Temperature, humidity and chemical conditions also need to be examined. (c) Concrete Specifications such as maximum water to cement ratio, maximum cement content, type of cement and grade of concrete. (d) Quality of concrete cover around the steel reinforcement and embedments. This includes the quality of concrete cover blocks as well. Figure 13.27 below shows deterioration of concrete due to poor quality of concrete around the steel reinforcement.

Fig. 13.27 Poonam chambers, Worli, Mumbai collapsed on 16th September, 1997

Strength and Durability of Concrete

13.35

(e) Selection of all materials of concrete and good concrete mix design. (f) Workability and cohesiveness of concrete mix. (g) Batching, mixing, transporting, placing, compacting and most important curing. Concrete in plastic form should be uniform and care should be taken to prevent segregation. Fig. 13.28 shows large voids due to segregation of concrete at the column bottom. Water tight reinforced structure containing discontinuous cracks, microcracks and pores Stage 1. No visible damage

Stage 2. Initiation and slow propogation of damage (visible at places)

Stage 3. Faster propogation of deterioration (visible damage at many places)

Stage 4. Propogation of damage at increased rate (deterioration beyond repairs)

Gradual loss of water tightness as cracks, microcracks and pores become more interconnected

Expansion of concrete cracks due corrosion of steel, chemical attack and freezing of water

Spalling of concrete, loss of mass. reduction of strength and stiffness (repair/ rehabilitation inevitable)

Collapse

Weathering effects (cyclic wetting and drying or heating and cooling). loading effects (cyclic loading, impact loading or overloading) leaching effect (excess lime)

Penetration of water, oxygen, chlorides, sulphates and carbon dioxide

Increased penetration of water, oxygen, chlorides, sulphates and carbon dioxide

Excessive penetration of water, oxygen, chlorides, sulphates and carbon dioxide

Fig. 13.28 Stagewise deterioration of concrete

Concrete structures are often tampered with or modified. Structures are often overloaded without any consideration about its loads carrying capacity. Fig 13.27 shows a structural collapse probably attributable to poor maint enance and negligence.

13.5.3

Causes and Preventive Measures

Deterioration of concrete can take place basically due to porosity. Concrete has porosity of several types: • Capillary pores • Entrapped air • Honey combs • Cracks

13.36

Handbook on Advanced Concrete Technology

Also, there can be micro or macro pores present in concrete. Micro pores are in form of capillary pores in the cement gel. Macro pores can be due to entrapped air as a result of stiff workability or poor compaction. Honey-combing as a result of segregation or use of non-cohesive mix causes large voids. Leaching of excess lime also causes porosity.

Capillary Pores Capillary pores in concrete can be as large as 5mm in diameter. Number and size of pores depend on water to cement ratio (w/c) used and the extent of chemical hydration that has taken place. The relation between the age of concrete at which capillary pores get blocked (concrete becomes almost impermeable) and w/c is given in the following table. w/c

Age at which capillary pores become blocked

0.40 0.45 0.50 0.55 0.60 0.70 Over 0.70

3 days 7 days 14 days 3 months 6 months over 1 year infinity

This table clearly shows that as w/c increases the concrete remains permeable for a longer period thereby permitting ingress of moisture and chemicals to penetrate it, leading to corrosion of reinforcement and creating other durability problems. Concrete with w/c = 0.7 remains vulnerable to chemical and moisture ingress for over one year while concrete with w/c = 0.4 almost becomes impermeable within 3 days of casting.

Permeability of Concrete Concrete produced with low water to cement ratio displays low coefficient of permeability of concrete as compared to concrete produced with high water cement ratio. Table below gives the coefficient of permeability of concrete made using same materials but with different w/c ratios. Coefficient of Permeability for different w/c Sr. No.

w/c

Coefficient of Permeability (Valenta)

1. 2. 3.

0.35 0.50 0.65

1.05 × 10–3 10.30 × 10–3 1000 × 10–3

The above table clearly indicates that lower the w/c, lower is the permeability. The coefficient of permeability increases about 1000 times with the increase in w/c from 0.35 to 0.65. Table on next page also shows the permeability and porosity in air of cement pastes hardened upto 90 days using Ordinary Portland Cement with three different w/c.

Strength and Durability of Concrete Permeability (m2 × 10–17) Curing (days)

13.37

Porosity (%)

1

3

7

28

90

5.60 18.70 214.00

0.30 0.59 14.70

0.12 0.07 2.35

0.00 0.07 0.19

0.00 0.00 0.00

1

3

7

28

90

9.80 16.80 20.80

5.90 11.10 14.50

W/C 0.32 0.40 0.50

20.80 33.30 43.50

19.7 28.6 37.8

14.4 20.9 32.2

From the above table, it is very clear that w/c remaining constant permeability and porosity rapidly reduces with increase in curing period. At the same time, with increase of w/c the permeability and porosity also increase at the same curing period.

Permeability and Porosity of Concrete Made from Pozzolanic Cements Permeability of pozzolanic cement pastes which is initially higher as compared to OPC tends to become lower as the curing period proceeds. Even though pozzolanic pastes are always more porous than those made up of OPC, the permeability of pozzolanic cement pastes is identical to that of OPC after a lapse of time. For the first 7 to 15 days cement hydration only involves the clinker and gypsum fractions. Pozzolanic material of flyash will hydrate later on at a slower rate and within an already rigid structure. Some lime reaction products are formed mostly through complex process of dissolution, transportation and precipitation. Mass precipitation into the pores previously formed by hydration of the clinker fraction is not able to fill the larger pores completely but blocks smaller capillaries connecting larger pores or, at least reduce their openings considerably. As a consequence, porosity of pozzolanic cement pastes remains higher than or at the most becomes the same as OPC, but the permeability becomes lower. Concrete made up with 35% flyash replacing OPC cements has turned out to be 2 to 5 times less permeable than concrete manufactured with OPC. Concretes made using pozolanic cement have a better flexural/compressive strength ratio and reduced tendency towards cracking than concrete made using OPC.

Leaching Water can decompose any of the hydrated compounds present in concrete. If concrete comes in continuous contact with water or moisture, the free lime occuring in hardened concrete being easily soluble is the first compound to be attacked and will leach out. This lime extraction to the concrete surface increases both porosity and permeability. The soluble calcium hydroxide leaches through the capillary pores of concrete and leaves a passage for other pollutants such as water, chlorides and sulphates to enter. This also causes alkalinity of concrete to drop initiating corrosion of steel within the concrete.

Cracks in Concrete In modern concrete structures not enough attention is being paid to the fundamental principles of concrete technology governing cracking. In general cracks in concrete range in widths from 0.1 mm to 1.0 mm and are primarily caused due to the following:

13.38

Handbook on Advanced Concrete Technology

• Temperature gradient including frost action. • Humidity gradient (Drying Shrinkage). • Rapid drying conditions (Plastic Shrinkage). • Structural overloading, cyclic or impact loading. • Inadequate structural design and detailing. • Chemical causes including corrosion of reinforcement. Concrete starts cracking at an early age when it is still in plastic stage. When freshly hardened concrete is exposed to temperature and humidity gradient it experiences thermal and drying shrinkage strains. One of these two gradients will have more dominating effect on concrete depending on the following. • Temperature and humidity of the environment. • Size of the structural element. • Temperature of concrete. • Physical and Chemical properties of concrete materials. • Mix proportion of concrete materials. Under the restraining conditions in hardened concrete, shrinkage strain causes a tensile stress. The concrete material will develop cracks when this induced tensile stress exceeds the tensile strength of concrete. However, due to viscoelastic behaviour (creep) of concrete material some of the stress is relieved and it is the residual stress, after the relaxation due to creep that will be responsible for cracking. Cracks on concrete surfaces may or may not influence the strength. However, cracks on concrete surfaces will seriously effect the durability of concrete specially when it is exposed to aggressive environment and number of cyclic loading conditions. Under such conditions cracks wider than 0.3 mm seldom heal. Many standards recommend 0.15 mm as the maximum crack width at the tensile face of reinforced concrete structure subjected to alternate drying and wetting conditions or is located in the tidal zone and subjected to sea water sprays. In concrete design and construction practice, crack widths are generally controlled by proper deployment of primary reinforcement and by use of secondary reinforcement. However, it is well established that reinforcement steel does not prevent cracking or reduce cracking. It simply transforms a few wide cracks into many fine cracks and micro cracks. Deterioration of concrete takes place in stages. The speed of deterioration and damage to concrete will greatly depend on the following factors: • Continuous or discontinuous cracks, microcracks and pores. • Weathering effects such as cyclic heating and cooling or wetting and drying. • Cyclic loading or impact loading. • Environmental action of aggressive chemical ions and their penetration. If cracks and pores are continuous deterioration due to penetration of aggressive chemical ions will be faster. When the cyclic weathering and loading conditions are very repetitive the discontinuous pores and cracks will become continuous and aggressive chemical ions will cause deterioration at first, gradually and later at an increasing rate. The model given in Fig 28. below shows stagewise concrete deterioration due to environmental and other effects.

Strength and Durability of Concrete

13.39

13.5.4 Other Factors 13.5.4.1 Abrasion Resistance Subjection of concrete to surface damage by abrasion may occur from such diverse means as the trafficking of a floor to the flow of water over a weir or against a sea wall. The latter may be enhanced by attrition due to inclusion of sand or stone as on a beach. Generally resistance to abrasion can be improved by a good quality high strength concrete. Important factors would be a high cement content and low water/cement ratio. If attrition or abrasion is anticipated the use of harder aggregate should perhaps be considered, although the use of a harder aggregate will only help abrasion resistance through the sand portion. The top 3mm of the cement paste influence the abrasion resistance, so coarse aggregate has little or no affect. As this aspect of durability relates to the immediate surface of the concrete it is not only the concrete quality that requires attention but also workmanship and curing. Vacuum de-watering and power floating can improve the abrasion resistance of floor.

13.5.4.2 Fire Resistance Generally concrete has good fire resistant properties. That is it performs satisfactorily over relatively long periods of exposure to elevated temperature. It therefore affords excellent cover to reinforcement and structural steelwork. Protective properties increase with section size and cover. However, when exposed to heat the temperature gradient throughout the depth may eventually cause spalling especially at narrow sections and arises. Incidence of spalling is increased if the moisture content is high. Up to 300ºC, the residual strength of concrete is not significantly reduced. Above 600ºC considerable strength losses are likely to occur. The action of heat on reinforced concrete is complex as it is a composite of two very different materials. If steel becomes exposed and heated, it quickly loses strength. For ordinary reinforcing steels only approximately 50% of the original yield strength remains at about 600°C. However almost complete recovery can occur on cooling. For prestressing steel, the strength of the tendons is halved at a lower temperature - approximately 400°C. The effects of the loss are more complex due to additional relaxation in the wire tension. Thus the integrity of prestressed members can pose problems if exposed to severe fire. The aggregate type, cement type and thickness of concret all affect fire resistance. Due to their inherent nature and method of production most lightweight aggregates have excellent resistance to heat. Calcareous aggregates (e.g. limestone) exhibit superior properties than siliceous materials such as quartz and flint. Refractory concretes, which provide improved heat resistant properties, can be made using High Alumina Cement.

13.5.4.3 Frost Action on Concrete Effects of freezing on both fresh and hardened concrete will obviously relate to concreting in winter periods and are dealt with separately below.

13.40

Handbook on Advanced Concrete Technology

Fresh concrete In plastic concrete, if the temperature is reduced such that the free water is turned to ice, the cement hydration process can be retarded or even stopped. As the water freezes a volume increase will occur. This will be detrimental if left, but if the concrete can be re-vibrated as the temperature rises and hydration proceeds, development of strength may continue normally. Permanent damage to the concrete is likely to result if it has stiffened but is not sufficiently mature to withstand the action of freezing forces. In order to reduce or eliminate problems in cold weather, one or more of the following may be considered: • Insulation of the structure or concrete surface • Increased curing time prior to removal of formwork • Increase cement content or use of rapid hardening cement • Use of accelerating admixture • Heating aggregates and/or water Hardened Concrete Once the concrete has achieved a greater degree of maturity it can still be damaged by frost action. Water within the pores of the paste freezes and the change in state to ice results in approximately a 9% volume increase, thus causing pressure and subsequent disruption if the tensile strength of the concrete is exceeded. A second complex additional force within the concrete may be caused by diffusion of water through a process known as osmosis. This results in additional water gravitating toward the zone of freezing. Thus in a slab, water will move up toward to the already frozen surface layer providing additional saturation of the concrete and aggravating the problem. It is thought that the use of de-icing salts increase osmotic effects by being absorbed into the upper part of the concrete. Frost Resistance Dry concrete may be considered resistant to frost attack as deterioration is a function of the degree of water saturation and the maturity of the cement paste. The use of air entraining agents is the most common method of improving frost resistance of hardened concrete. These admixtures form small voids within the concrete which acts as pressure reliefs to the expansion forces, thus minimising disruption.

13.5.4.4

Carbonation

Carbonation is the process by which carbon dioxide, present in the atmosphere, together with moisture (forming carbonic acid) reacts with the calcium hydroxide (alkali) within the hydrated cement to form calcium carbonate. This has the effect of causing a reduction in the alkalinity of the concrete surface. For good quality concrete carbonation proceeds very slowly into the concrete. The reverse is true of porous concrete and if the depth penetrates to the steel, the concret all kalinity which provides protection against rusting, is lost. This is of obvious significance.

Strength and Durability of Concrete

13.41

Treating a freshly broken piece of concrete with phenolphthalein indicator can make a simple test for carbonation. The portion of the concrete, which is still alkaline, turns pink, and the carbonated portion remains uncoloured.

13.5.4.5 Marine Environment To ensure durability, concrete exposed to sea water is required to be of particularly good quality as deterioration may occur from one or more of three specific sources not normally considered with general construction. (a) Abrasion - wave action forces against a sea wall, possibly enhanced with the presence of sand or shingle. (b) Sulfate attack - from salts within the sea water. (c) Corrosion of reinforcement - especially within the splash zone. It can be seen that the marine environment may be particularly aggressive with respect to concrete. Therefore inadequacies in the material itself or construction detail, such as joints and workmanship, will soon become obvious. The majority of problems are likely to occur within the splash zone and probably to a lesser extent the intertidal zone (with high and low water levels). Generally concrete which is continually immersed is rarely affected. An increase in levels of chlorides and wetting/drying cycles may enhance the probability of reinforcement corrosion. Depth of cover to the steel is therefore of prime importance. It follows that low concrete permeability is beneficial, necessitating a low water/cement ratio and sufficiently high cement content. Portland cement is generally satisfactory. Sulfate attack appears to be a lesser problem, and provided the concrete is of good quality and low permeability expansive reaction is unlikely to occur. Leaching of the calcium sulpho-aluminate which causes the disruption may be the reason. In conclusion therefore, even in such an aggressive environment provided the necessary precautions are taken to produce a high quality construction, any potentially deleterious effects on the concrete can be minimized.

13.5.4.6 Sulfate and Other Chemical Attack Deterioration of hardened concrete can occur from attack by aqueous solutions of sulfates. These solutions permeate and react with the calcium aluminate hydrate and calcium hydroxide in the cement paste forming calcium sulpho-aluminate and calcium sulfate respectively. The resultant materials occupy a greater volume thus causing expansion and disruption. Attack is often recognised by the white deposits formed. Calcium, magnesium and sodium sulfate salts can all occur in clay soils, and these dissolve in the groundwater. Factors influencing attack are: • nature and concentration of the sulfate solution • flow of the groundwater and its variation • rate of reaction with the hardened concrete

13.42

Handbook on Advanced Concrete Technology

13.5.4.7 Concrete Quality Appropriate measures to reduce the risk include: Reduce permeability - higher cement content reduce water/cement ratio use of admixtures Use Replacements – Replacement of cement with the use of Metakaoline, Silica Fume introduce finer particles in the system which hydrate and form a dense packing material thereby reducing the overall permeability. Reduce C3A - Use of sulfate resisting cement or ggbs and PFA cement replacement materials. Reduce CA(OH) 2 use of ggbs or pfa (complex chemical reaction) Whilst the various properties of hardened concrete make it an ideal construction material for many purposes in a wide range of demanding conditions, the inherent nature of the cement paste make it vulnerable to certain chemicals. Generally the degree of attack is a function of the strength of the chemical, deterioration increasing with concentration. Hydrated Portland cements are quite highly alkaline and therefore present no particular hazard to alkaline solutions, whereas acidic materials constitute potential problems. Acidic conditions may arise in the ground from peat or other decomposed vegetation. Many other chemicals, products and wastes may cause deterioration to varying degrees and it is necessary to assess each situation individually and design the concrete accordingly. In all cases low permeability is a prime consideration.

13.5.4.8 Corrosion of Steel Reinforcement Probably the most common cause of failure in reinforced concrete, although the action is quite complex. It can be defined as the destructive attack of a metal within its environment by means of an electrolytic reaction, (an electrolyte is a solution which conducts electricity - in concrete this is the pore solution surrounding the steel bar). Electrolytic action - if a metal is placed in an electrolyte, metal ions will pass into solution, with an electrical current between the two due to an excess of electrons left in the metal. The flow ceases when an equilibrium point is reached. If two different metals are placed in the electrolyte a current flows between them releasing metal ions into the solution. For a bar cast into concrete, small “corrosion cells” form along the bar as a result of changes in the surrounding electrolyte and slight differences in the bar surface. The more negatively charged areas are called anodic regions and positively charged areas cathodic regions. If a current flows, metal ions are dissolved into the electrolyte at the anodic points causing corrosion. The remaining electrons flow due to effects of the electrical charge to the cathodic points. Oxygen must be available at the anode for the ferrous hydroxide corrosion product formed to become “rust”. The products of this reaction occupy more volume than the original steel thus causing pressure and bursting stresses in the concrete cover.

Strength and Durability of Concrete

13.43

The reactions that occur during the corrosion of reinforcement are electrochemical in nature and as such the process can be compared to the working of a battery. The development of a cell with the pore solution in the concrete acting as the electrolyte – while the site with loss of metal acts as an anode while the dormant site contributing to the reaction acts as a cathode. This is shown in Fig. 13.29 below.

Anode

Cathode

Fig. 13.29 Existence of the anode-cathode interface resulting in corrosion of reinforcement (Dehadrai)

The highly alkaline nature of uncarbonated concrete fortunately inhibits corrosion and the reinforcement surface is said to be “passivated”. If the alkalinity is reduced, for example by carbonation, then electrolytic action can proceed. The presence of chlorides, even in relatively small concentrations, changes the nature of the electrolyte and effectively de-passivates the steel hence accelerating corrosion. Chlorides are often implicated where significant corrosion has occurred. The following factors influence the potential for corrosion: • presence of water and oxygen • inadequate cover to the steel • permeability of the concrete • presence of chlorides • carbonation The most effective means of providing protection against corrosion are therefore adequate cover and low concrete permeability. Preventive Measures To avoid corrosion of steel, following preventive measures are to be taken. • Concrete mix should be designed with as low a water cement ratio as possible depending on environmental conditions in which the structure is proposed. Some guidelines as per Bureau of Indian Standards are given in subsequent paras and they must be followed.

13.44

Handbook on Advanced Concrete Technology

• Concrete should be made in such a manner that voids due to entrapped air or segregation do not occur. • Plastic and drying shrinkage cracking of concrete should be avoided by taking adequate care in designing concrete mixes and by proper construction practices, specially curing. • Concrete mix should have good workability and cohesiveness and must be placed and compacted properly. • Protective coating on steel can be considered as a second line of defence against corrosion.

13.5.4.9

Chloride

Harmful Effects of Chloride The chloride ions in concrete can have harmful effect on concrete as well as on reinforcement. In the first case chloride penetration brings about concrete swelling of 2 to 2.5 times larger than that observed with water penetration. This causes slight reduction of concrete strengths as well as causes leaching of concrete making it more porous and vulnerable. In the second case presence of chloride near the reinforcement steel is extremely dangerous. If the chloride to hydroxide ratio near the reinforcement steel drops below 0.3 passivation is destroyed and corrosion is inevitable. Chlorides have therefore to be prevented from entering into concrete. Chlorides can be present in concrete materials and are termed as “domestic”chlorides or chlorides can be present in the environment around the concrete structure and are termed as “foreign” chlorides.

Limitation of Chlorides in Materials The amount of chlorides permitted in concrete so far as corrosion of reinforcement is concerned is limited to acid-soluble chloride content of 0.10% by weight of cement, at the time of placing concrete. The limit for presetressed concrete is 0.05%. This is recommended in IS:456 Code of Practice for Plain and Reinforced Concrete.

Tricalcium Aluminate (C3A) There has been a lot of discussion about the limits of chloride in concrete about “domestic” chloride versus “foreign” chloride. If there is uniform distribution of chlorides, corrosion may be minimal. Further, even if the chloride is initially uniformly distributed, a non-uniform distribution eventually may result, due to movement of water containing chloride in solution. Some of the “domestic” chloride can become chemically fixed by reactions with C3A components of the Portland cement forming calcium chloroaluminate hydrates. This not only explains the good performance of portland cement containing high amounts of calcium aluminate, but also advocates such cements as a solution to the problem. It is not advisable to use Sulphate Resistant Cements in environment where excessive chlorides are present as Sulphate Resistant Cements have low C3A content and therefore less ability to form calcium chloro-aluminate hydrates. Further, concrete with sulfate resisting cement

Strength and Durability of Concrete

13.45

has higher permeability than concretes made with OPC and as such may present a long term durability problem.

13.5.4.10

Chloride Diffusion

To protect reinforcement from chlorides penetration, it is essential to produce impermeable concrete (concrete having a low water to cement ratio) and give thicker cover to reinforcement steel. Figure 13.30 shows precast concrete piles of M 32.5 concrete grade made using pozzolana Portland cement and concrete cover to reinforcement was 75 mm. Piles were driven in soils containing very high concentration of chlorides in groundwater and sub soil.

Fig. 13.30 Precast concrete piles of M 32.5 grad-Trombay, Mumbai.

Studies have been undertaken and it is observed that with all mix parameters remaining the same, reduction of w/c reduces chloride ion penetration into the concrete to a considerable extent. Table below shows that chloride diffusion in concrete mixes reduces considerably as the w/c reduces. Chloride diffusion in concrete mixes for different w/c Sr. No.

w/c

Chloride diffusion × 10–8 cm2/sec

1. 2.

0.40 0.50

1.05 × 10–3 10.30 × 10–3

3.

0.61

1000 × 10–3

13.5.4.11

Blended Cements in Chloride Environment

It is also recommended to use blended cements containing Pozzolanic materials or slag as the chloride diffusion through cement pastes of these cements is at a very slow rate than compared to OPC and sulphate resisting cements. Table given below shows chloride diffusion in various types of cement pastes having constant w/c = 0.5 at 25°C.

13.46

Handbook on Advanced Concrete Technology

Type of Cement Chloride

Diffusion cm2/sec × 10–8

OPC Pozzolana Cement (70% OPC & 30% flash) Slag Cement (35% OPC & 65% slag) Sulphate Resistant Cement

4.47 1.47 0.41 10.00

The above table clearly shows that cement with 65% slag is most suitable while sulphate resistant cement is least suitable in chloride environment. Pozzolanic material if present around 33% is considered to be very effective in reduction of chloride diffusion into concrete. However, percentage of pozzolana being restricted to 35% in IS 1489 and slag being restricted to 70% in IS 455 concrete can be manufactured using pozzolana or slag as mineral additives. Therefore it is possible to use higher percentage of such materials in very aggressive environment wherein high proportions of chloride are present. Figure 13.31 shows precast concrete liners for sewerage outfall tunnel cast using 30% cement and 70% ground granulated slag to produce concrete of M 45 grade.

Fig. 13.31 Construction of precast concrete liners

13.5.5

Alkali Silica Reaction

Whenever there are sufficiently high levels of alkalinity in a concrete structure, the alkalis can react with certain forms of silica in the aggregate, producing an alkali-silicate gel. This gel may absorb water and swell causing the concrete to crack in a random characteristic pattern often referred to as ‘map’ cracking. These cracks generally do not develop until the concrete is about 5 years old. Figure 13.32 shows a column severely affected by ASR. The alkali-silica reaction is, essentially an attack by sodium or potassium hydroxide solution on silica, producing an alkali silicate gel. The rate of this attack will depend on the relative concentration of these hydroxides in the pore solution and it is only at the upper end of the pH range that significant attack develops.

Strength and Durability of Concrete

13.47

Fig. 13.32 Photograph of concrete column affected by Alkali Silica reaction

The gel rapidly takes up calcium, the most likely source being the portlandite (Ca(OH)2) produced by the binder hydration reactions, gels analyzed in concrete are usually found to contain calcium, sodium, potassium silicates and be of a variable composition. Such gels are capable of taking pore solution into their structure and expanding. It is this expansive force which creates internal stresses within the concrete and can cause cracking. The severity of the expansive force varies both with the composition of the gel, and with the total amount of gel present in the concrete, in a way, which is not fully understood. The amount of gel also depends on the amount of available reactive silica and, therefore, up to a point an increase in the amount of reactive silica produces an increase in expansion. Above a certain proportion of reactive silica to alkali, however, the concentration of hydroxide in solution is insufficient to maintain the same degree of attack and the expansion decreases again. This is the reason for the critical, or ‘pessimum’, proportion of reactive aggregate Three conditions are necessary for ASR to occur: • A sufficiently alkaline solution in the pore structure of the concrete • aggregate or aggregate combination susceptible to attack by this alkaline solution. • A sufficient supply of water If any of these conditions are absent, then ASR may not occur.

13.5.5.1

Alkali Sources

Cements The main source of alkalis in concrete is from the Portland cement and other cementitious materials.

13.48

Handbook on Advanced Concrete Technology

Aggregates Salt contaminated aggregates will contain alkali metal ions (sodium and potassium) also. The salt content of such aggregates should be taken into account when calculating the levels of alkalis in concrete.

13.5.5.2 Additional Sources In addition to the cement and aggregates, other sources of alkalis may also be available, namely, admixtures and mixing water. The alkalis content of these additional materials should also be taken into account.

13.5.5.3 Preventive Measures The following measures will have to be followed for preventing the alkali silica reaction: • Use non reactive aggregates from alternate source. • Use low alkali cement as suggested above. • Control total alkali content in concrete. • Reduce water content in concrete by using proper chemical admixtures. • Use blended cements or use mineral additives as part replacement of cement. • Use alkali silica reaction inhibiting salts. • Control dampness around the structure.

Conclusions If durable concrete structures are to be produced all controls listed earlier must be properly exercised. Certain common parameters emerge and must be noted. • It is very important to control the water to cement ratio while designing and producing concrete mixes. Low water to cement ratio gives low permeability of water and other aggressive chemicals. It also means high strength. This is probably the only reason why concretes in many developed countries are not considered to be of structural grade if their strengths are below M40. • Concrete must have good workability and cohesiveness. This reduces the chances of porosity due to entrapped air and honeycombing. • All steps of concrete making must be properly supervised and controlled. Each step has a significant role to play in durability and therefore cannot be ignored. • There are several reasons due to which concrete cracks. It is not possible to completely avoid cracking. It is difficult to cover all such reasons in this write up. However proper design, selection of materials including the chemical admixtures, curing and protection against wind, low humidity and high ambient temperatures, is necessary to reduce cracking. • Services, like electrical conduits, water and drainage lines embedded within the concrete structure or tampering and modifying a concrete structure can pose a serious threat to durability and hence before taking any action durability aspects must be considered.

Strength and Durability of Concrete

13.49

• Blended cements, mineral additives and chemical additives have a significant role to play in durability and advantages in their use should be fully exploited. • Minimum cement content and maximum water to cement ratio will depend on the environmental conditions prevailing around the proposed structure.

References 1. Digital simulation of the aggregate-cement paste interfacial zone in concrete-E.I Garboczi, DP Bentz-Journal of material Research, 1999 2. Properties of concrete-AM Neville-1973 3. Concrete-PX Mehta, PJM Monteiro-1993-Prentice-Hall Englewood Cliffs, NU 4. Jones, R., Non-Destructive Testing of Concrete, Cambridge University Press, London, 103 pp. (1962). 5. Identifying the Fluid-to-Solid Transition in Cementitious Materials at Early Ages Using Ultrasonic Wave Velocity and Computer Simulation-M Dehadrai, G Sant, D Bentz, J Weiss-concrete.org 6. Winslow, D.N., Cohen, M.D., Bentz, D.P., Snyder, K.A., and Garboczi, E.J., “Percolation and Pore structure in Mortars and Concrete”, Cement and Concrete Research, Vol. 24(1), 25-37, 1994 7. Bentz, D.P., Hwang J.T.G., Hagwood, C., Garboczi. E.J., synder, K.A., Buenfeld, N., and scrivener, K.L., Interfacial Zone Percolation in Concrete: Effects of Interfacial Zone Thickness and Aggregate Shape, MRS Proceedings, Vol. 370, Microstructure of Cement-Based System/Bonding and Interfaces in Cementitous Materials, 43-442. 1995. 8. Ye G., Lura P., K. Van Breugel and A.L.A Fraaij, “Study on the development of the microstructure in cemen-based materials by means of numerical simulation and ultrasonic pulse velocity measurement,” Cem.Concr.Compos. (2004) 9. The properties of fresh concrete-TC Powers-1968 10. S. Diamond, Mercury porosimetry; an inappropriate method for the measurement of pore size distribution in cement-based materials, Cem.Concr.Res. 30(10) (2000) |1517-1525|. 11. Detecting the Fluid-to-Solid Transition in Cement Pastes: Comparing experimental and numerical techniques-sant Gaurav; Dehadrai Mukul; Bentz Dale; Lura Pietro; Ferraris Chiara F.; Bullard Jeffrey W.; Weiss Jason-Concrete international ISSN 0162-4075 CODEN CIDCD2, 2009, vol. 31, American Concrete Institute, Farmington Hills, MI 12. B.Boumiz, C. Vernet, F.T. Cohen, Mechanical properties of cement pastes and mortars, Adv. Cem. Based Mater. 3 (1996) 94-106. H.W. Reinhardt, C.U. Grosse, Setting and hardening of concrete continuously monitored by elastic waves, The Online Journal of Nondestructive Testing and Ultrasonics. 13. J. Keating, D.J. Hannant, Correlation between cube strength, ultrasonic pulse velocity and volume change for oil well cement slurries, Cem.Concr.Res. 19(1989) 715-726. 14. C.M.Sayer, A.Dahlin, Propagation of ultrasound through hydrating cement parts at early times, Adv. Cem. Based Master.1(1993) 12-21. 15. Voigt, T and Shah, 5.P., “Properties of early age Portland cement motor monitored with a shear wave reflection method, ACI Materials Journal 101 (6) 473-482 (2004). 16. Voigt, T, Ye, G, Sun, Z.; Shah SP.; Van Breugel, K. “Early age microstructure of Portland cement mortar investigated by ultrasonic shear waves and numerical simulation Cement and Concrete Research, v 35, n 5 p 858-866, (2005)

13.50

Handbook on Advanced Concrete Technology

17. Voigt, T., Akkaya, Y. and Shah, S.P., ‘Determination of early age mortar and concrete strength by ultrasonic wave reflections’. Journal of Materials in Civil Engineering 15(3) 247-254 (2003). 18. Byfors, J. “Plain Concrete, Prentice-Hall (1981). 19. Nondestructive testing of concrete: history and challenges, concrete Technology-Past, Present and Future, NU Carino-VM Malhotra-Symposium, Ed Mehta, 1994 20. Price, W.H. “Factors influencing Concrete Strength.” Journal of the American Concrete Institute, Vol. 47, Feb. 1951, pp. 417-432. 21. Richardson, D.N. “Effects of Testing Variables on the Neoprene Pad and Sulphur Mortar-Capped Concrete Test Cylinders”. ACI Materials Journal, Vol. 87, No 5 (September-October 1990), pp 1977. 22. Westley, J.W. “Some Experiments on Concrete Cubes with Non-Plane Surfaces. “Magazine of Concrete Research, Vol. 18, No. 54 (March 1966), pp 35-37. 23. Gonnerman, H.F. “Effect of Size and Shape of Test Specimen on Compressive Strength of 24. Concrete. “Proc ASTM, Vol. 25, Part 2, 1925, pp. 237-250. 25. Neville, AM. “A General Relation for Strengths of Concrete Specimens of Different Shapes and Sizes”. 26. Journal of the American Concrete Institute, Vol. 63, No. 10 (October 1966), pp 1109-1195. 27. Plowman, 1M., Smith, W.F., and Sheriff, T. “Cores, Cubes, and the Specified Strength of Concrete.” The Structural Engineer, Vol. 52, No. 11 (Nov. 1914), pp. 421-426. 28. Tarrant, A.G. “Frictional Difficulty in Concrete Testing”. The Engineer, Vol. 198, No. 5159 (December 1954), pp. 801-802. 29. Neville, A.M. The Influence of the Direction of Loading on the Strength of Concrete Test Cubes. ASTM Bulletin No. 239, July 1959, pp. 63-65. 30. Gyengo, T. “Effect of Type of Test Specimen and Gradation of Aggregate on Compressive Strength of concrete”. Journal of the American Concrete Institute, Vol. 34, Jan-Feb. 1938, pp. 269-282 31. Riley, O. (1995), Time to Rein in the Flexure Test,” Concrete International, Vol. 17. No. 6, pp. 42-44 32. Mahta, P.K., and Monteiro, P. (1993), “Concrete, Structure, Properties and Materials,” 2nd ed., Prentice-Hall, Inc., Engleqood Cliffs, New Jersey, pp. 66-71. 33. Mindess, S., Young, F., and Darwin, D. (2003), “Concrete,” 2nd ed., Prentice-Hall, Inc., Englewood Cliffs, New Jersey, pp. 315-327, 417-432. 34. Structural properties of high strength concrete and its Implication for precast prestressed concrete - S.H. Ahmad and S.P. Shah, PCI Journal, Vol. 30, No. 6, pp. 92-119 (1985) 35. Dewar, J.D. Mix design for ready-mixed concrete. The Municipal Engineer, London, February 1986, 35-43. 36. Deacon, C. and Dewar. J.D. Concrete durability-specifying more simply and surely by strength. 37. Concrete, London, February 1982. 38. Gebhardt, R.F., “Why a Performance Standard for Hydraulic Cement, Concrete, and Aggregates, Vol. 15, No. 2, Winter 1993, pages 119 to 123. 39. Tennis, P.D., and Bhatty, J.I., “Portland Cement Characteristics-2004,” Concrete Technology Today, Portland Cement Association, Vol. 26, No. 3, Dec. 2005.

Strength and Durability of Concrete

13.51

40. Brewer, R.W., “Coarse-Ground Cement Makes More Durable Concrete,” Journal of the American Concrete Institute, Vol. 22, No. 5, 353-360, 1951. 41. Bentz, D.P., and Haecker, C.I., “An Argument for Using Coarse Cements in High Performance Concretes,” Cement and Concrete Research, Vol. 29, No. 4, 615-618, 1999. 42. Burrows, R.W., Kepler, W.F., Hurcomb, D., Schaffer, J., Sellers, G., “Three Simple Tests for Selecting Low-Crack Cement,” Cement and Concrete Composites, Vol. 26, No. 5, 509-519, 2004. 43. Erdogu K, Tokyay M, Turker P (1999). Comparison of intergrinding and separate grinding for the production of natural pozzolan and GBFS incorporated blended cement and Concrete Research 29-743-746. 44. Peris-Mora E, Paua. l, Monzo. I 1994. Influence of different sized fractions of a fly ash on workability of mortars, Cement and Concrete Research 23(4) 917-924. 45. Tasdemir C (2003). Combined effects of mineral admixtures and curing conditions on the sorptivity coefficient of concrete, Cement Research 33 1637-1642. 46. Early-Age Properties of Cement-Based Materials. I: Influence of Cement Fineness-DP Bentz, G Sant, J Weiss-Journal of Materials in Civil Engineering. 2008 47. Kaplan, M.F., Flexural and Compressive Strength of Concrete Affected by the Properties of Concrete Affected by the Properties of Coarse Aggregate, ACI J.Proceedings, 55,1193-1202, 1959. 48. Walker, S. Ve Bloem, D.L., Effect of Aggregate Size on the Properties of Concrete, ACI J. Proceedings, 57, 3, 283-291, 1960. 49. William, A.C. ve Aldridge, H.A., Variable in Concrete Aggregates and Portland Cement Paste Which Influence the Strength of Concrete, ACI J. Proceedings, 60, 8, 1029-1038, 1963. 50. Barnes, B.D., Diamond, S., and Dolch, W.L., Micromorphology of the Interfacial Zone Around Aggregates in Portland Cement Mortar, Journal of the American Ceramic Society, Vol. 62(1-2), 21-24, 1979. 51. Bentz D.P, Garboczi EJ. Simulation studies of the effects of mineral admixtures on the cement paste-aggregate interfacial zone. ACI Mater Journal 1991; BB(5):518-29. 52. Winslow DN, Cohen MD, Bentz DP, Snyder KA, Garboczi EJ. Percolation and pore structure in mortars and concretes. CemConcr Res 1994; 24:25-37. 53. Variables in concrete aggregates and Portland cement paste which influence the strength of concrete- WA Cordon, HA Gillespie, Journal of the American Concrete Institute, Vol. 60, No.8, pp. 1029-1052 (1963).] 54. Water-Cement Ratio Versus Strength-Another Look-HU Gilkey-ACI Journal Proceeding, 1961 55. C. Shi, J.Stegemann, and R. Caldwell, “Effect of Supplementary Cementing Materials on the Specific Conductivity of Pore Solution and Its implications on the Rapid Chloride Permeability Test Results,” (AASHTO T277 and ASTM C 1202) July-August 1998, pp. 389-394. 56. Dhir, R.K., Hewlett, P.C., and Chan, Y.N. (1987). “Near-surface characteristics of concrete: assessment and development of insitu test test method.” Magazine of Concrete Research, 39(141), 183-195. 57. Hooton, R.D., Mesic, T., and Beal, D.C. (1993). “Sorptivity testing of concrete as an indicator of concrete durability and curing efficiency.” Proceeding of the 3rd Canadian Symposium on Cement and Concrete, Ottawa, Ontario, 264-275.

13.52

Handbook on Advanced Concrete Technology

58. Geiker, M.(2004). “On the ingress of chlorides in cracked. “ACBM Semi-Annual Meeting, Evanston, Illinois. 59. Parrot, L. J. (1988). “Moisture profiles in drying concrete”. Advances in Cement Research, 1(3) 164-170. 60. Akita, H., Fujiwara, T., and Ozaka, Y.(1997). “A practical procedure for the analysis of moisture transfer within concrete due to drying.” Magazine of Concrete Research, 49(179), 129-137.

14 Reinforcement Cover and Corrosion Manish Mokal and N.V. Nayak

14.1 INTRODUCTION Corrosion of reinforcing steel in concrete is the most significant deterioration process affecting reinforced concrete structures. Statistics indicate that 40 percent of the failure of structures occurs due to corrosion of reinforcement. Corrosion is an electrochemical process whereby a metal undergoes a reaction with oxygen and water present in the environment to form compounds. Steel reinforcement has a natural tendency to corrode if access to oxygen is possible in a moist environment. This is because it is formed of metals found naturally occurring as ores to which they wish to revert. Carbonation and presence of chlorides are the two major causes of corrosion in reinforcement. Carbonation is a problem that mainly affects buildings. Chlorides affect structures that are exposed to marine environments or de-icing salts (de-icing salt are not used in India). Presence of acidic gases such as sulphur dioxide, aggressive ions such as sulfates, fluorides and bromides and stray electrical currents can be some other reasons for corrosion of reinforcement though the percentage of such occurrence is very less. Fig. 14.1 gives an indication regarding the degree of corrosive environment in different parts of India. Corrosion causes loss of rebar cross-section and corrosion products are formed. As the corrosion products occupy a larger volume than the original metal from which they were derived (Fig. 14.2) they tend to generate tensile stresses in concrete causing cracking and spalling of the concrete cover. Very often the first indication of a problem is the appearance of a crack following the line of reinforcement (Fig. 14.3).

14.2 THE CORROSION PROCESS Concrete is alkaline in nature with pH between 12-13. The alkaline condition leads to formation of a ‘passive’ layer on the steel surface. The passive layer is a very dense, impenetrable film,

14.2

Handbook on Advanced Concrete Technology Corrosion map of india*

Delhi

Kolkata

Mumbai Hyderabad Extremely severe Severe Bangalore

Moderate Mild Negligible

Fig. 14.1 Map indicating degree of corrosive environment in India (Source: Central Electro Chemical Research Institute)

Fig. 14.2 Expansive corrosion products on steel in chloride contaminated concrete

which, if fully established and maintained, prevents further corrosion of the steel. The passive layer is a coating that forms itself initially and maintains and repairs itself as long as the passivating (alkaline) environment is there to regenerate it if it is damaged.

Reinforcement Cover and Corrosion

14.3

Fig. 14.3 Corrosion induced by carbonation of a concrete beam which has resulted in a crack in the concrete cover following the line of the reinforcement

However, the passivating environment is not always maintained. Two conditions can break down the passivating environment in concrete without attacking the concrete first. One is carbonation and the other is chloride attack. These will be discussed subsequently. Once the passive layer breaks down then areas of rust will start appearing on the steel surface. The chemical reactions are the same whether corrosion occurs by chloride attack or carbonation. When steel in concrete corrodes it dissolves in the pore water and gives up electrons: (3.1) The anodic reaction: Fe Æ Fe ++ + 2e– The two electrons (2e–) created in the anodic reaction must be consumed elsewhere on the steel surface to preserve electrical neutrality. This is a reaction that consumes water and oxygen: (3.2) The cathodic reaction: 2e – + H2O + ½O2 Æ 2OH– It may be noticed that hydroxyl ions are generated in the cathodic reaction. These ions increase the local alkalinity and therefore will strengthen the passive layer, warding off the effects of carbonation and chloride ions at the cathode. Note that water and oxygen are needed at the cathode for corrosion to occur. The anodic and cathodic reactions are only the first steps in the process of creating rust. Several more stages must occur for ‘rust’ to form which are explained in Fig. 14.4. Unhydrated ferric oxide Fe2O3 has twice the volume of original steel it replaces when fully dense. Upon hydration it further swells and becomes porous. Thus the volume increase at the steel/concrete interface is six to ten times as indicated in Fig. 14.5. This leads to the cracking and spalling that we observe as the usual consequence of corrosion of steel in concrete and the red/brown brittle, flaky rust we see on the bar and the rust stains we see at cracks in the concrete. It can be seen that both oxygen and water is necessary for corrosion to occur. Thus, there is no corrosion in dry condition, probably below a relative humidity of 60 percent; nor there is

14.4

Handbook on Advanced Concrete Technology

Ionic current

2+

Fe Æ Fe 2e

– – 1 O + H2O + 2e Æ 2OH 2 2

–

Electronic current The anode 2+

The cathode –

Fe + 2OH Æ Fe(OH)2 Ferrous hydroxide 4Fe(OH)2 + O2 + 2H2O Æ 4Fe(OH)2 Ferric hydroxide 2Fe(OH)2 Æ Fe2O3 . H2O + 2H2O Hydrated ferric oxide (rust)

Fig. 14.4 Anodic, cathodic, oxidation and hydration reactions for corroding steel Fe Progressive anodic and cathodic reactions of iron in presence of water and oxygen

FeO Fe3O4 Fe2O3 Fe (OH)2 Fe (OH)3 Fe (OH)3 3H2O

0

1

2

3

4

Volume, cm

5

6

7

3

Fig. 14.5 Relative volumes of iron and its oxides

any corrosion in concrete when fully immersed in water, because there is very little oxygen in dissolved water except when water can entrain air, for example by wave action or in tidal zone. The optimum relative humidity for corrosion is 70 to 80 percent. At higher relative humidity, the diffusion of oxygen through the concrete is considerably reduced.

14.3 BLACK RUST It is known that for corrosion to occur presence of oxygen and water is must. But there is a different mechanism of corrosion found in conditions where oxygen is absent. If the anode and cathode are well separated (by several hundred millimetres) and the anode is starved of oxygen (say by being underwater) the iron as Fe2+ will stay in solution. This means that there will be no expansive forces as described earlier to crack the concrete as a result corrosion may not be detected. The product formed in such conditions is called black rust due to the colour of the liquid. This type of corrosion is found under damaged waterproof membranes and in some underwater or other water saturated conditions. This kind of corrosion is potentially dangerous as there is

Reinforcement Cover and Corrosion

14.5

no indication of corrosion by cracking and spalling of the concrete and the reinforcing steel may be severely weakened before corrosion is detected. Rebars may be hollowed out in such deoxygenated conditions particularly under membranes or when water is permanently ponded on the surface. Examples of rebars attacked in this way are shown in Fig. 14.6. These bars were taken from underneath damaged waterproof membranes.

Fig. 14.6 Reinforcing bars taken from under the end of a waterproofing membrane. They have been subjected to low oxygen conditions and therefore one of them shows severe local wasting. There was no expansive oxide growth.

14.4 CORROSION DUE TO CARBONATION When atmospheric carbon dioxide (CO2) dissolves in the cement pore solution, carbonic acid (H2CO2) is formed. Unlike most other acids the carbonic acid does not attack the cement paste, but just neutralizes the alkalies (calcium hydroxide) in the pore water, mainly forming calcium carbonate that lines the pores. Calcium hydroxide (Ca(OH)2) will be converted into calcium carbonate (CaCO3). As the levels of the alkaline solid phases are depleted, the pH value of concrete falls. This process is termed carbonation. The pH value of pore water in concrete is generally 12.5 to 13.5. The high alkalinity forms a thin passivating layer around the steel reinforcement and protects it from action of oxygen and water. As long as the steel is in highly alkaline environment, it will not corrode. When carbonation occurs the pH value of pore water reduces from 13 to around 9.0. When all the Ca(OH)2 has been carbonated, the pH will reduce to about 8.3. In such low pH value, the protective layer gets destroyed and the steel is exposed to corrosion. However, presence of moisture and oxygen is required for initiation of corrosion. The rate of carbonation depends on the following factors: • Level of pore water i.e., relative humidity – higher the relative humidity, higher will be the rate of carbonation

14.6

Handbook on Advanced Concrete Technology

• Grade of concrete – higher the grade of concrete, lower will be the rate of carbonation • Permeability of concrete – higher the permeability of concrete, higher will be the rate of carbonation • Age of concrete – Higher the age of concrete, lower will be the rate of carbonation • Extent of carbon dioxide in air – Higher the carbon dioxide in air, higher will be the rate of carbonation

14.5.1

Carbonation of Concrete Containing Supplementary Cementitious Material

Nowadays supplementary cementitious material like fly ash and ground granulated blast furnace slag (GGBS) are being widely used in concrete. It is important to understand the carbonation behavior of such concrete. The silica in the fly ash reacts with Ca(OH)2 resulting from the hydration of cement. As a result it leads to lower Ca(OH)2 content in the hardened cement paste so that a smaller amount of CO2 is required to remove the Ca(OH)2. It follows that concrete with supplementary cementitious material will be prone to rapid carbonation. However it must be understood that the silica reacts with Ca(OH)2 to produces more C-S-H gel thereby making the cement paste denser and thereby reducing the diffusivity of gases into concrete and thus slowing down the carbonation process. It is important to understand the relation of the two processes which will together determine the rate of carbonation. Curing is an important factor which is necessary for the pozzolanic reaction to take place. Figure 14.7 gives depth of carbonation at different w/b ratio for various combinations of concrete blended with supplementary cementitious material. Figure 14.8 gives the depth of carbonation with respect to the strength (grade of concrete). Depth of carbonation (mm)

12 10 8 6 4 2 0 OPC W/b-0.4

OPC + 25% Fly ash W/b-0.5

OPC + 25% Slag W/b-0.6

OPC + 50% Slag

W/b-0.7

Fig. 14.7 Depth of carbonation at different w/b ratio for variaous combinations of concrete blended with supplementary cementitious material

Reinforcement Cover and Corrosion

14.7

Carbonation depth (mm)

15 M15 12 M25 9 M35

6

3

M45

0

8

10 No of years

15

20

Fig. 14.8 Depth of carbonation with respect to strength (grade) of concrete

14.6 CORROSION DUE TO CHLORIDE ATTACK Chlorides can come from several sources. They can be cast into the concrete or they can diffuse in from the surrounding environment. Chlorides cast into concrete can be due to: • deliberate addition of chloride set accelerators (calcium chloride CaCl2 was widely used until the mid-1970s); • chlorides in cement, admixture etc. • use of seawater in the mix; • contaminated aggregates Chlorides can diffuse into concrete due to: • sea salt spray and direct seawater wetting; • deicing salts; • use of chemicals (structures used for salt storage, brine tanks, aquaria, etc.) The oxides which make up the passive film on iron are stable in the alkaline environment in concrete even when chloride ions are present. As the chloride concentration increase, the anodic curve which is vital to maintain passivity is altered. The effect is to make the potential more negative with the consequent effect on the corrosion current. The process first involves the oxidation of iron to ferrous ions (Fe2+): 2Fe Æ 2Fe2+ + 4e– The cation combines with chloride ions to form chloride or oxychloride compounds (FeCl2 and FeOCl). 2Fe2+ + 4Cl– Æ 2FeCl2 The process then becomes self-propagating, due to the acidic conditions created and the recycling of chloride ions.

14.8

Handbook on Advanced Concrete Technology

2FeCl2 + 4H2O Æ 2Fe(OH)2 + 4HCl Alternatively 2FeCl2 + 4H2O Æ 2Fe(OH)2 + 4H+ + 4Cl– And 4FeOCl + 2H2O Æ 2Fe(OH)2 + 2Cl– There follows a consequent recycling of the liberated chloride ions. Although corrosion products are being produced so too is hydrogen chloride and hydrogen (H+) or hydronium ions (H3O +). Hydrochloric acid (HCl) is effectively formed (Fig. 14.9). The presence of chloride ions promote the continued dissolution of iron. The risk of corrosion initiation as a function of the chloride content in concrete may be estimated from the data in Fig. 14.10. Salt –

–

CI Concrete

CI –

CI

H2O HCI

Passive film steel

Fe

2+

Fig. 14.9 Schematic representation of the process of corrosion initiation arising from chloride contamination 80%

Risk of corrosion (%)

70% 60% 50% 40% 30% 20% 10% 0% 1.5%

Chloride content (wt% cement)

Fig. 14.10 Risk of corrosion initiation as a function of the chloride content in concrete

14.7 MEASURES FOR REDUCING THE RATE OF CORROSION Corrosion protection strategies for steel reinforcing bars can be grouped into four categories: design, concrete, corrosion inhibitors and reinforcement type.

Reinforcement Cover and Corrosion

14.9

The design category includes such items as: • Concrete cover (Fig. 14.11 gives the minimum covers for worst exposure conditions in various national codes. Cover specified in IS 456 is the highest) • Maximum allowable crack width in service • Reinforcement distribution (crack control provisions) • Rigid overlays (silica fume concrete, latex modified concrete, dense concrete, polymer concrete) • Protective coating to the surface of concrete (impermeable membranes like bituminous paints, anti-carbonation paints) India is 456-1978 UK Sweden Netherlands France CEB/FIP model code USSR 0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

Cover (mm)

Fig. 14.11 Minimum covers for worst exposure conditions in various national codes

The concrete category includes such items as: • • • • • • •

Water cement ratio Permeability of concrete Dense concrete matrix Pozzolans (fly ash, GGBS, silica fume, metakaolin, rice husk ash) Latex, epoxy and polymer admixtures Cement type Aggregate gradation

The corrosion inhibitors category includes such items as: • Organic corrosion inhibitors • Inorganic corrosion inhibitors • Mixed corrosion inhibitors

80

14.10

Handbook on Advanced Concrete Technology

The reinforcement category includes such items as: • • • • • • •

Epoxy coated bars Galvanized bars Nickel clad bars Copper clad bars Stainless steel bars Corrosion resistant alloyed bars Non-metallic bars

In order to increase the service life of any structure it is important to prolong the initiation of corrosion in the reinforcement. Once the corrosion is initiated, the propagation depends upon the availability of oxygen in the surrounding. Various means for prolonging the initiation of corrosion and their effectiveness are: • Increasing the cover of concrete (Fig. 14.12 and Fig. 14.13) The time required for the chloride ions or carbonation effect to reach the reinforcement is more with increase in the cover of concrete. Thus increasing the cover will prolong the initiation of corrosion. However, too much increase in the cover is also not good. 12

No.of years

10 8 6 Propagation

4

Initiation

2 0 40 mm

50 mm

60 mm

70 mm

80 mm

Depth of cover (mm)

Fig. 14.12 Effect of depth of cover on corrosion of reinforcement for concrete with w/b of 0.40

• Reducing the water binder ratio (Fig. 14.14) The permeability of concrete reduces with reduction in water binder ratio. Thus the time required for chloride ions to reach the reinforcement will be more as the water binder is lowered. • Use of supplementary cementitious material (Fig. 14.15) Use of supplementary cementitious materials like fly ash, GGSB, silica fume, metakaolin and rice husk ash reduces the permeability of concrete and makes the concrete matrix denser. Thus time required for initiation of corrosion in concrete with supplementary cementitious material will be more.

.

Reinforcement Cover and Corrosion

Depth

30

14.11

Concrete cover nominal thickness

20 Concrete cover half nominal thickness

10

2 5

10 15

25

50

100 Time

Fig. 14.13 Reduction of initiation time of corrosion due to local reduction in concrete cover 14 12

No. of years

10 8 Propagation

6

Initiation

4 2 0 0.3

0.35

0.4

0.45

0.5

0.55

Water/binder ratio

Fig. 14.14

Effect of water/binder ratio on corrosion of reinforcement

• Use of corrosion inhibitors in concrete (Fig. 14.16) Corrosion inhibitors are either inorganic or organic and are classified based on their protection mechanism. An active type of inhibitor (anodic) facilitates the formation of an oxide film on the surface of the reinforcing bar. Passive systems protect by reducing the rate of chloride ion migration. Calcium nitrite is an inorganic inhibitor. • Use of fusion bonded epoxy coated reinforcement, hot dip galvanized reinforcement or stainless steel (Fig. 14.17) Corrosion resistance of reinforcing bars can be improved by applying an organic coating (fusion bonded epoxy coating - FBEC) to isolate it from the corrosive medium as they have high resistance to alkali attack. Disadvantages of FBEC are: 1. In case of any pinholes, cracks in coating due to bending, handling, the exposed area becomes susceptible to severe corrosion. However, extensive accelerated testing done

14.12

Handbook on Advanced Concrete Technology 25

No. of years

20

15 Propagation 10

Initiation

5

0 Only OPC

OPC + 25% Fly ash

OPC + 35% Fly ash

OPC + 50% Slag

OPC + 75% Slag

OPC + 7% Silicafume

Cementitious material

Fig. 14.15 Effect of addition of various supplementary cementitious materials on corrosion of reinforcement for concrete with w/b of 0.40 18 16

No. of years

14 12 18 8

Propagation

6

Initiation

4 2 0 No calcium nitrite 10 Lit. calcium nitrite/cum.

15 Lit. calcium nitrite/cum.

20 Lit. calcium nitrite/cum.

Corrosion inhibitor

Fig. 14.16 Effect of addition of corrosion inhibitors on corrosion of reinforcement for concrete with w/b of 0.40

indicates that there is no such phenomenon. Thus there is still some ambiguity regarding this point. 2. The bond strength between concrete and FBEC reinforcement is lower than an uncoated reinforcement. Hot-dipped galvanized steel is produced by dipping clean and fluxed steel into a bath of molten zinc. Zinc is a highly reactive metal that coats the steel and also reacts with steel to

Reinforcement Cover and Corrosion

14.13

30

No. of years

25 20 15 Propagation 10

Initiation

5 0 Normal reinforcement

Epoxy coated reinforcement

Stainless steel reinforcement

Different types of reinforcement

Fig. 14.17 Effect of different reinforcement on corrosion of reinforcement for concrete with w/b – 0.40

form hard iron/zinc alloys. The zinc forms a sacrificial anode and is itself consumed to protect the steel but its reaction products are not as expansive as those of steel and they are soluble. Under-film corrosion of galvanized steel is not possible. The main advantages of galvanized steel are: • It offers additional protection where the risk of corrosion is high (inland where carbonation is a problem as well as in coastal regions where chloride ingress is the dominant problem) • Where cover is less than ideal (in precast units) • Where rust staining cannot be tolerated • Where steel is partly embedded in concrete (holding down machine bolts) • It has longer life in cracked carbonated concrete than uncoated bar • It delays the initiation of corrosion and cracking • It tolerates higher chloride migration levels than uncoated steel Some misconceptions of hot-dip galvanizing are that: • Bars cannot be bent after galvanizing • If the coating is scratched, protection fails • The heat of the molten zinc reduces the strength of high-tensile bars • High-tensile bars will suffer hydrogen embrittlement (galvanising is not generally recommended for steels with a tensile strength above 800 N/mm2) • Hot-dip galvanized bars will accelerate corrosion • Hot-dip galvanized bars cannot withstand the high pH of concrete

14.14

Handbook on Advanced Concrete Technology

Galvanized and ungalvanized bars should not be mixed together as they can cause accelerated depletion of the galvanizing. If they are being used in the same structure (for instance on a bridge deck but not the substructure or vice versa), then care should be taken to ensure complete electrical isolation of the galvanized and ungalvanized bars. Stainless steels are iron-based alloys, containing at least 10.5% chromium, whose corrosion resistance increases with alloying metal contents. Some of the most expensive grades also contain nickel. Stainless steel depends upon oxygen to oxidize the highly reactive chrome which forms an impermeable protective barrier against corrosion. It is important to note that the bond of stainless steel with concrete is inferior to that with normal reinforcement. Stainless steel reinforcement is ideal: • In conditions where there may be high chloride concentrations and/or cover depth may be compromised. • Where the risk of failure due to corrosion cannot be permitted. • Where the service life of a structure is very long and maintenance is not possible especially in sub-structure in severe exposure conditions. Stainless steel reinforcement is unsuitable: • Where there is no oxygen present. • Where a less costly alternative would suffice. • Where the reinforcement will incur high fatigue.

14.8 IMPROVING THE QUALITY OF COVER The first requirement for maximum durability against corrosion is low permeability concrete, especially the cover to the reinforcing steel. As cover to the reinforcement is the first line of defense against ingress of harmful chlorides into the concrete, it is very important to take adequate steps to ensure that the quality of cover is good. The following measures can be taken: • Ensure that the concrete in the cover zone is properly compacted and is devoid of any pinholes and honeycombing. Any such defect observed must be immediately repaired and adequate steps taken in order to ensure that the reoccurrence is reduced • Concrete cover blocks should be made of same grade of concrete and properly cured before put to use. This will ensure that the cover blocks do not crush during handling, fixing or concreting and are able to maintain the required cover • Plastic spacers can be used instead of concrete cover blocks • The cover blocks are securely fixed to the reinforcement with binding wires so that they do not get dislodged during concreting. It is important to note that in case the binding wire is longer than required it should be bent inwards so that there is no portion projecting outwards. The projected wire if exposed may act as a source of corrosion initiation. This situation is particularly true in piles.

Reinforcement Cover and Corrosion

14.15

• Use of vacuum dewatering technique on flat concrete surfaces will help in removing the water from the cover area thereby increasing its impermeability and its ability to resist corrosion. Corrosion of steel in concrete can be modeled as a two-stage process. The initiation phase is the time required for sufficient accumulation of aggressive species at the rebar surface to initiate corrosion. The duration of the propagation phase is the total time until concrete cracking, caused by formation of corrosion products from the steel reinforcement, which occupy a larger volume than the parent steel. The service life of a concrete structure, degraded by corrosion, is considered to be the sum of the initiation and propagation periods. The graphs indicated earlier are obtained from modeling done in Life-365 software designed by a consortium of Concrete corrosion inhibitor association, National ready mix concrete association, Slag cement association and Silica fume association of USA.

References 1. John Newman, Ban Seng Choo (2003), “Reinforcement corrosion” Advanced Concrete technology – Concrete Properties, 1st edition 2. John P. Broomfield (2007) “Corrosion of steel in concrete” Corrosion of steel in concrete, Understanding, investigation and repair, 2nd Edition 3. P. K. Mehta, Paulo J. M. Monteiro (2005) “Durability” Concrete – Microstructure, properties and material, 3rd Edition 4. M. S. Shetty (2005) “Durability of Concrete” Concrete Technology, Theory & Practice, 6th Edition 5. A. M. Neville (2005) “Durability of concrete” Properties of Concrete, 4th Edition 6. N. V. Nayak (2006) “Annexure A-15: A Durable Concrete – A Practical View Point” Foundation Design Manual, 5th Edition 7. J. L. Smith, Y. P. Virmani (2000) “Corrosion Control Measures” Materials and methods for corrosion control of reinforced and prestressed concrete structures in new construction, Publication No. 00-081 of U.S. Department of transportation, Federal Highway Administration 8. Life 365 – Service life prediction model for reinforced concrete exposed to chlorides, Version 2.0.1

15 Concrete Resistance to Sulphate Attack Manish Mokal and N.V. Nayak

15.1 INTRODUCTION The most common sulphates present in soils are in the form of calcium, sodium, potassium and magnesium. They occur in soil or ground water. Because the solubility of calcium sulphate is low, ground water contains more of other sulphates. Ammonium sulphate is present in agricultural soil and water from the use of fertilizers or from sewage and industrial effluents. Decay of organic matters in marshy land, shallow lakes often leads to formation of H2S, which is transformed into sulphuric acid by bacterial action. Water used in concrete cooling towers can also be a potential source of sulphate attack on concrete. Thus sulphate attack is a common occurrence in natural or industrial situations.

15.2

MECHANISM OF SULPHATE ATTACK

All forms of sulphates have a deleterious action on concrete, but the mechanism and severity of attack vary according to the type of sulphate present in soil or water. The damage is of two forms: expansion and cracking due to formation of expansive compounds and loss of strength due to decalcification of the cement hydrates, especially Calcium Silicate Hydrates. Calcium sulphate reacts only with hydrated calcium aluminate to form calcium sulphoaluminate (ettringite). Sodium sulphate reacts with free calcium hydroxide in the concrete to form calcium sulphate, which then reacts with the aluminates. The reactions can be written as: (Cal. hydroxide + Sodium sulphate solution) = (Cal. sulphate + Sodium hydroxide + water) Calcium sulphate formed by the above reaction has more than double the volume of calcium hydroxide. (Cal. aluminate hydrate + Cal. sulphate + Water) = (Cal. sulphoaluminate + cal. Hydroxide + Water)

15.2

Handbook on Advanced Concrete Technology

Calcium sulphoaluminate (i.e., Ettringite) formed by the combination of hydrated calcium aluminate and gypsum also has volume double that of its original elements. The changes in solid volume for other reactions with sulphates are given in Table 15.1. This tremendous increase in volume exerts tensile stresses on the surrounding hydrated paste thereby causing it to crack. TABLE 15.1 Molecular volumes of concrete compounds Compound Calcium hydroxide Magnesium hydroxide Calcium sulphate (Gypsum) Calcium aluminate hydrate Calcium sulphoaluminate (Ettringite)

Molecular weight

Density

Molecular volume

74.1 58.3 272.2 668.3 1236.6

2.23 2.38 2.32 1.81 1.73

33.2 24.5 74.2 369.2 714.9

The effects of magnesium sulphate are more far deleterious than other sulphates and as it decomposes the hydrated calcium silicates, in addition to reacting with the aluminates and calcium hydroxide. The magnesium sulphate reacts with hydrated calcium silicates in the following manner: (Cal. silicate hydrates + Mag. Sulphate) = (Cal. sulphate + Mag. Hydroxide + Silica gel) This is not the end of the reaction since magnesium hydroxide and silica gel can react very slowly to form hydrated magnesium silicate. This hydrated magnesium silicate has no binding power, in contrast to silica gel and its formation represent the final stage in the deterioration of concrete attacked by magnesium sulphate solutions, though it may only be reached after long periods. It is thus seen that concrete subjected to attack from sulphates may suffer from two types of damages: volumetric expansion due to formation of gypsum and ettringite that leads to cracking and loss of strength of the matrix due to degradation of calcium-silicate-hydrate (C-S-H).

15.3

MITIGATION OF SULPHATE ATTACK

The classification of severity of sulphate exposure and the recommendations regarding the type of cement, water cement ratio as specified in IS 456 are given in Table 15.2. Notes: • Cement content given in the table is irrespective of grade of cement • Use of supersulphated cement is generally restricted where the prevailing temperature is above 40°C • Supersulphated cement gives an acceptable life provided that the concrete is dense and prepared with a water-cement ratio of 0.4 or less, in mineral acids, down to pH 3.5 • The cement contents given in col. 7 of this table are the minimum recommended. For SO3 content near the upper limits of any class, cement contents above these minimum are advised

Concrete Resistance to Sulphate Attack

15.3

TABLE 15.2 Requirements of concrete exposed to sulphate attack Concentration of sulphates, Expressed in SO3

Dense fully compacted concrete with 20 mm maximum size aggregate complying to IS 383 Minimum cement Maximum water content kg/cum** cement/binder ratio**

Class

Total SO3 in soil %

SO3 in 2:1 water: soil extract g/l*

In ground water g/l

Type of cement

1 2

< 0.2 0.2 – 0.5

< 1.0 1.0 – 1.9

< 0.3 0.3 – 1.2

OPC, PSC, PPC OPC, PSC, PPC SSC, SRC

280 330 310

0.55 0.5 0.5

3

0.5 – 1.0

1.9 – 3.1

1.2 – 2.5

SSC, SRC, PPC, PSC

330 350

0.5 0.45

4 5

1.0 – 2.0 > 2.0

3.1 – 5.0 > 5.0

2.5 – 5.0 > 5.0

SSC, SRC SRC, SSC with protective coating

370 400

0.45 0.4

* The water soluble SO3 is more critical than the total SO3 in the soil; it is the water soluble SO3 that reacts with cement hydrates. ** The above recommendation does not account for the presence of chlorides & their effect on concrete. 1 g/l = 1000 ppm OPC – Ordinary Portland cement PSC – Portland Slag cement PPC – Portland Pozzolana cement SSC – Supersulphated cement SRC – Sulphate resisting cement Protective coating – Asphalt, chlorinated rubber, epoxy or polyurethane

• For severe conditions, such as thin sections under hydrostatic pressure on one side only and sections partly immersed, considerations should be given to a further reduction of water-cement ratio • Portland slag cement conforming to IS 455 with slag content more than 50 percent exhibits better sulphate resisting properties • Where chloride is encountered along with sulphates in soil or ground water, ordinary portland cement with C3A content from 5 to 8 percent shall be desirable to be used in concrete, instead of sulphate resisting cement. Alternatively, portland slag cement conforming to IS 455 having more than 50 percent slag or a blend of ordinary portland cement and slag may be used provided sufficient information on performance of such blended cements in these conditions The purpose of classification of the severity of the sulphate exposure, shown in Table 15.2, is to suggest preventive measures. The following measures can be adopted to mitigate the sulphate attack: (a) Use of sulphate resisting cement – The most efficient method of resisting sulphate attack is to use cement with low C3A content. However, it must be noted that use of sulphate resisting cement alone is not adequate under severe conditions as not only

Handbook on Advanced Concrete Technology

calcium sulphate but other sulphates are also present. Therefore, although sulphate resisting cement does not contain enough C3A for the formation of expansive ettringite, the Ca(OH)2 present and possibly also C-S-H are vulnerable to attack especially by magnesium sulphates. Fig. 15.1 indicates the effect of C3A content in portland cement on rate of deterioration of concrete exposed to sulphate bearing soils. 120 IV:AV

III

II

I

me nt 225 cont kg/ 3 ent: m

100

80

Ce

Rate of deterioration, per cent/ year

15.4

60

40

20

390 0 0

2

4

8 6 C3A content, per cent

10 12

Fig. 15.1 Effect of C3A content in portland cement on rate of deterioration of concrete exposed to sulphate bearing soils

In case chlorides are also present along with sulphates it is not advisable to use sulphate resistant cement. Table 15.3 gives the chloride diffusion values of concrete made with different cements. The chloride diffusion values indicate the amount of permeability of the concrete. It can be seen that concrete made with slag cement has the least permeability and concrete made with sulphate resistant cement has almost 24 times higher permeability. It is thus advisable to use slag cement or pozzolana cement or even ordinary portland cement instead of sulphate resistant cement especially when chlorides are also present in the environment. Higher the impermeability of concrete higher will be the time required for the chloride ions to reach the reinforcement steel, thus delaying

Concrete Resistance to Sulphate Attack

15.5

the deterioration of the reinforcement. Similarly, a concrete with low water cement ratio also demonstrates higher resistance to sulphate attack as the permeability of concrete is reduced. Fig. 15.2 shows the effect of water-cement ratio on rate of deterioration of concrete made of OPC and exposed to sulphate bearing soils. 120

Rate of deterioration. per cent/ year

100

80

60

40

20

0.85

0.75

0.65

0.55

0.45

0.35

0

w/c ratio

Fig. 15.2 Effect of water cement ratio on rate of deterioration of concrete made of OPC and exposed to sulphate bearing soils TABLE 15.3 Chloride ion diffusion for various cements Type of cement

Chloride diffusion sq.cm/s × 108 with w/c = 0.5 at 25°C

OPC Pozzolana cement (70% OPC & 30% Fly Ash) Slag cement (35% OPC & 65% Slag) Sulphate resistant cement Note: Value in ( ) are multiples of that with respect to slag cement

4.47 (10.9) 1.47 (3.6) 0.41 (1.0) 10.00 (24.4)

15.6

Handbook on Advanced Concrete Technology

(b) Use of air entraining agent – Air-entrained concrete produced by the introduction of air-entraining agents contains billions of microscopic air cells distributed uniformly in the body of concrete. This air is different from the entrapped air that remains in concrete due to insufficient compaction which is concentrated in bigger size at random locations. Use of air entrainment to the extent of about 6% has beneficial effect on the sulphate resisting qualities of concrete. The beneficial effect is possibly due to reduction of segregation, improvement of workability, reduction in bleeding and in general better impermeability of concrete. It may be noted that with 1% increment of air, there will be a loss of strength by 4-6%. However, the loss of strength can be compensated by reducing the water-cement ratio which will also help in reducing the permeability of concrete thus increasing the durability as discussed in (a) above. (c) Use of pozzolana – Incorporation of or replacing a part of cement by a pozzolanic material reduces the sulphate attack. Addition of pozzolana converts the calcium hydroxide into secondary hydration products. This pozzolanic action is responsible for impermeability of concrete. Secondly, the removal of calcium hydroxide reduces the susceptibility of concrete to attack by sulphates. Fig. 15.3 and 15.4 shows the effect of fly ash and slag respectively on expansion of mortar bars under ultimate alkaline environment (pH – 12). Table 15.4 gives effectiveness of supplementary cementitious material in concrete when exposed to various environmental conditions.

Mortar Expansion (microstrain)

2,000

1,500 GP

1,000

500 40% FA 20% FA 0 0

10

20

30

40

50

Immersion Period in 5% Na2SO4 (weeks)

Fig. 15.3 Effect of fly ash on expansion under alkaline environment (pH – 12)

Concrete Resistance to Sulphate Attack

15.7

2,000

Mortar Expansion (microstrain)

1,500

GP 1,000

40% Slag 500

60% Slag 80% Slag 0 0

10 20 30 40 Immersion Period in 5% Na2SO4 (weeks)

50

Fig. 15.4 Effect of slag on expansion under alkaline environment (pH – 12) TABLE 15.4 Effectiveness of different supplementary cementitious material (SCM) in concrete Mineral admixture

Resistance to ASR

Resistance to carbonation

Resistance to chloride attack

Resistance to sulphate attack

Silica fume

10 to 25% 26 to 50% 50% 50 to 70% 5 to 10%

Good Excellent Very Good Excellent Excellent

Moderate Moderate Moderate Poor Moderate

Good Excellent Very Good Excellent Very Good

Good Good Very Good Excellent Moderate

Metakaolin

10 to 20%

Excellent

Moderate

Very Good

Moderate

Fly Ash GGBS

% addition with respect to total cementitious material

It is seen that for extreme exposure conditions like marine environment, GGBS or fly ash with higher permissible ranges is preferred to silica fume. (d) High pressure steam curing – High pressure steam curing improves the resistance of concrete to sulphate attack. This improvement is due to the change of C3AH6 into a less reactive phase and also to the removal and reduction of calcium hydroxide by the reaction of silica which is invariably mixed when high pressure steam curing method is adopted.

15.8

Handbook on Advanced Concrete Technology

However, it is important to note that there is a reduction in long term strength of steam cured concrete probably due to presence of fine cracks caused by the expansion of air bubbles in the cement paste which induce tensile stresses in the surrounding cement paste.

References 1. A. M. Neville (2005) “Durability of concrete” Properties of Concrete, 4th Edition. 2. N. V. Nayak (2006) “Annexure A-15: A Durable Concrete – A Practical View Point” Foundation Design Manual, 5th Edition. 3. M. S. Shetty (2005) “Durability of Concrete” Concrete Technology, Theory and Practice, 6th Edition. 4. Peter C. Hewlett (2003) “Resistance of Concrete to Destructive Agencies” Lea’s Chemistry of cement and concrete, 4th Edition. 5. IS 456:2000, Plain and reinforced concrete – Code of practice, Fourth Revision. 6. Mick Ryan (1999) Sulphate Attack and Chloride Ion Penetration: Their Role in Concrete Durability, QCL Group Technical Note. 7. (2002) Sulfate-resisting Cement and Concrete, Cement Concrete and Aggregates, Australia.

16 Alkali Silica Reaction Manish Mokal and N.V. Nayak

16.1 INTRODUCTION In general aggregates used in concrete are considered to be inert. Under certain conditions, however, some components in aggregates, especially siliceous minerals, may react with the sodium and potassium hydroxides in the alkaline pore solution of concrete to form a gel. This gel can absorb water and swell excessively. The swelling gel exerts pressures on the hardened concrete thereby creating tensile stresses in concrete. If the tensile stresses developed exceed the tensile strength of the concrete, extensive cracking may become apparent on the surface. This phenomenon is known as alkali-aggregate reaction (AAR). The term ‘alkali-aggregate reaction’ is used to encompass a number of forms of reaction. The predominant of these throughout the world is alkali-silica reaction (ASR). Other forms are alkali-carbonate reaction and alkali-silicate reaction. The alkali-carbonate reaction occurs when alkalis react with fine grained argillaceous dolomitic limestone aggregate containing calcite and clay. The alkali-carbonate reaction has been a greater cause for concern in Canada and China than elsewhere. The alkali-silicate reaction is significantly less prevalent and less well researched. Attention in most continents is focused on alkali-silica reaction and therefore ASR is more universally referred to than AAR, almost to the extent of excluding consideration of the other forms. The reaction occurs to a limited extent in many concretes but the incidence of damage due to ASR is exceedingly rare. The reaction may become apparent at some time in the life of a structure but the reaction exhausts itself, leaving the structure in a deteriorated but serviceable condition. But the cracks developed due to ASR facilitate the entry of harmful chemicals like sulphates & chlorides thus causing a serious concern to the durability of the structure. Table 16.1 lists the minerals and rocks which are known to participate in each reaction.

16.2

MECHANISM OF ALKALI-SILICA REACTION

The terms alkali-aggregate reaction or alkali-silica reaction are universally used and accepted. However, these terms offer a somewhat misleading picture of the chemical processes involved.

16.2

Handbook on Advanced Concrete Technology

TABLE 16.1 Minerals, rocks and other substances that are potentially deleteriously reactive with alkalis in concrete Minerals Opal; Tridymite; Cristobalite Chalcedony, cryptocrystalline, microcrystalline or glassy quartz; Coarse-grained quartz that is intensely fractured, granulated and strained internally or rich in secondary inclusions; Siliceous, intermediate and basic volcanic glasses; Vein quartz Rocks Rock

Reactive component

Granodiorite Charnockite

Strained quartz; microcrystalline quartz

Granite Pumice Rhyolite Andesite

Igneous

Dacite

Silicic to intermediate silica-rich volcanic glass; devitrified glass; tridymite

Latite Perlite Obsidian Volcanic tuff Basalt

Chalcedony; cristobalite; palagonite; basic volcanic glass

Gneiss

Metamorphic

Schist

Strained quartz; microcrystalline quartz

Quartzite

Strained and microcrystalline quartz; chert

Hornfels Phyllite Cataclasite

Strained quartz; microcrystalline to cryptocrystalline quartz

Mylonite Argillite Sandstone

Strained and microcrystalline quartz; chert; opal

Greywacke

Strained and microcrystalline to cryptocrystalline quartz

Siltstone

Strained and microcrystalline to cryptocrystalline quartz; opal

Shale Sedimentary

Tillite

Strained and microcrystalline to cryptocrystalline quartz

Chert, Flint

Cryptocrystalline quartz; chalcedony; opal

Diatomite

Opal; cryptocrystalline quartz

Argillaceous dolomitic limestone and calcitic

Dolomite; clay minerals exposed by dedolomitisation

dolostone Quartz-bearing argillaceous calcitic Dolostone Other substances Synthetic glass; silica gel

Siliceous aggregate are not directly attacked by alkali ions as might be presumed. Instead, hydroxyl ions (OH–) have been found to initiate damaging reaction with susceptible aggregate. Research has shown that the reaction depends on the formation of high concentrations of hydroxyl ions in solution and not on the concentration of alkali ions. The effect of higher

Alkali Silica Reaction

16.3

alkalies in cement lies in the large concentration of hydroxyl ions formed in concrete during hydration process. The alkaline hydroxides in pore water derived from the alkalies (Na2O and K2O) in the cement starts attacking the siliceous minerals in the aggregates. As a result, an alkali-silicate gel is formed, either in the planes of weakness or pores in the aggregate (where reactive silica is present) or on the surface of the aggregate particles. This may destroy the bond between the aggregate and the surrounding hydrated cement paste. The gel is of unlimited swelling type; it imbibes water with a consequent tendency to increase the volume. As the gel is confined by the surrounding hydrated cement paste, internal pressures result and may eventually lead to expansion, cracking and disruption of the hydrated cement paste. Thus, expansion appears to be due to hydraulic pressure generated through osmosis, but expansion can also be caused by the swelling pressure of the still solid products of the alkali-silica reaction. For this reason, it is believed that it is the swelling of the hard aggregate particles that is most harmful to concrete. Some of the relatively soft gel is later leached out by water and deposited in the cracks already formed by the swelling of the aggregate. The size of siliceous particles affects the speed with which reaction occurs, fine particles (20 to 30m) leading to expansion within a month or two, larger ones only after many years. Fig. 16.1 gives a graphical representation of the alkali-silica reaction mechanism. It is believed that the gel formation takes place only in presence of Ca++ ions. This is important from the point of view of mitigation of the alkali-silica reaction by inclusion of pozzolana, which consumes Ca(OH)2. The alkali-silica reaction occurs only in presence of water. The minimum relative humidity required in the interior of concrete for the reaction to proceed is about 85% at 20°C. At higher temperatures, the reaction can take place at a slightly lower relative humidity. Generally, a higher temperature accelerates the progress of the alkali-silica reaction but does not increase the total expansion induced by the reaction. The reaction can be disruptive and manifest into cracking. The crack width can range from 0.1 mm to as much as 10 mm in extreme cases and about 25-50 mm deep. Hence, in most cases, the alkali-silica reaction adversely affects the appearance and serviceability of the structure. But the cracks facilitate the ingress of harmful chemicals thus accelerating the deterioration of concrete and further corrosion of reinforcement steel. The pattern of surface cracking induced by the alkali-silica reaction is irregular, somewhat reminiscent of a huge spider’s web. However, the pattern similar to that caused by sulphate attack or by freezing and thawing or even by severe plastic shrinkage. The difference lies in the fact that many cracks caused by the reaction can be seen to pass through individual aggregate particles and also through the surrounding hydrated cement paste.

16.3 TESTS FOR ALKALI REACTIVITY OF AGGREGATE If petrographic examination finds that an aggregate is potentially reactive, then either further testing is required to prove it or not, or it must be assumed that the aggregate is reactive and appropriate precautions taken.

16.4

Handbook on Advanced Concrete Technology K

Aggregate

Alkali silica get

SiO2

Cement paste

No Step 1 – Alkalies react with silica at the particle Surface to form alkali–Silica "GET". H2O

NaOH

SiO2

H2O

H2O KOH

Colloidal silica disperses In get

Step 2a–As the alkali-silica reactions continue, alkali hydror-ides cause collodial silica to be peptized in the get

SiO2

Fluid alkali silica get confined by cement paste

H2O

SiO2

Silica in solution H2O Step 2b–Solution draws water from cement paste.

SiO2

Sollid alkali silica get Cracks in cement paste

Step 3a–If get is fluid and confined –hydraul pressure is exerted on the cement paste.

Step 3b–If get is solid–swelling of get causes exerted on the cement paste.

Fig. 16.1 Graphical representation of the alkali-silica reaction mechanism

Tests for determining the reactivity of aggregates generally involve subjecting the aggregate to conditions designed to accelerate the reaction so that results can be obtained in a more practical time frame. These include tests on the aggregates themselves, tests on mortar bars, and tests on concretes. Tests at 38°C have been shown to produce reliable results (Grattan-Bellew, 1989); tests at 60°C can produce results more quickly. Concrete tests are more reliable than chemical and mortar bar tests but expansion is measured for prolonged periods.

16.3.1

Chemical Test

IS 2386 (Part 7)/ASTM C289, known as the “quick chemical test” or the “Mielenz test” (Mielenz and Benton, 1958), has been found to be a satisfactory initial method for determining the potential reactivity of aggregate. The test categorizes aggregates as “innocuous”, “potentially deleterious” or “deleterious”. IS 2386 (Part 7)/ASTM C289 is sensitive to sample preparation

Alkali Silica Reaction

16.5

and requires strict adherence to the test method and should be performed by an experienced laboratory.

16.3.2

Mortar Bar Tests

Mortar bar tests allow a coarse aggregate to be tested by crushing it, separating the crushed material into size fractions and recombining them to a specified grading. Sand may be tested either “as supplied” or to the specified grading requirement. Testing to the specified grading can alter the proportions of reactive minerals in the material, and thereby give misleading results.

IS 2386 (Part 7)/ASTM C227 The test requires measurement of the length change of mortar bar specimens stored over water at 38ºC for six months, but often measurements have to be carried out for two years to obtain reliable results. IS 2386 (Part 7)/ASTM C227 has proved to be a suitable test method for determining the reactivity of aggregates but requires experience to obtain meaningful results.

ASTM C1260 ASTM C1260, better known as the “rapid mortar bar test” or the “NBRI test”, is based on a procedure developed by the National Building Research Institute [NBRI] in South Africa to overcome ASTM C227’s shortcomings like duration of test, test conditions affecting the results, and slowly-reacting aggregates not being detected. It involves storing mortar bars in highly alkaline solutions at 80°C for a minimum period of two weeks and measuring their expansion. The aggressive nature of the test causes ASR expansion of aggregates that are known to be non-reactive in concrete. To avoid taking unnecessary precautions to prevent ASR damage with such aggregate, the method is recommended only as a screening test, with aggregates that test as reactive being subjected to further testing before use. Guidelines for acceptance of aggregate given in appendix to ASTM C1260-01 suggests that expansions less than 0.10% at 14 days usually indicate innocuous behaviour, expansions greater than 0.20% at 14 days indicate potentially deleterious expansion and expansions between 0.10% and 0.20% require extra information to be obtained.

16.4

MITIGATION MEASURES FOR ALKALI-SILICA REACTION

The various measures for mitigation of alkali-silica reaction are as under:

16.4.1 Use of Non-reactive Aggregate Alkali-silica reaction can be prevented by specifying that non-reactive aggregate shall be used in the mix. Table 16.2 lists the non-reactive aggregates generally specified. Provided that the aggregate are chosen from the list and are correctly identified, and a source inspection is also carried out to see that there are no local concentrations of reactive constituents, the concrete mix should be non-reactive.

16.6

Handbook on Advanced Concrete Technology

TABLE 16.2 List of non-reactive aggregate types 1. 2. 3. 4. 5. 6. 7.

Air-cooled blastfurnace slag Andesite Basalt Diorite Dolerite Dolomite (3) Expanded clay/shale/slate

8. 9. 10. 11. 12. 13. 14.

Fledspar Gabbro Gneiss Granite Limestone Marble Microgranite

15. 16. 17. 18. 19. 20. 21.

Quartz (1, 2) Schist Sintered pulverized fly ash Slate Syenite Trachyte Tuff

1 Feldspar and quartz are not rock types but are discrete mineral grains occurring principally in fine aggregate 2 Not quartzite and not highly strained quartz 3 Possibility of alkali-carbonate reaction

16.4.2

Limiting the Total Alkali Content in the Mix

If the sole source of alkalis in concrete is Portland cement, then limiting the alkali content in the cement would prevent the occurrence of deleterious reactions. The minimum alkali content of cement at which expansive reaction can take place is 0.6% of the Na2O equivalent. This is calculated from the actual Na2O content plus 0.625 times the K2O content of the clinker. The limit of the equivalent soda content of 0.6% defines the low-alkali cement. Na2O equivalent = Na2O + 0.625 × K2O The alkali-silica reaction takes place only at high concentrations of OH– that is at high values of pH in the pore water. The pH of the pore water depends on the alkali content of the cement. Specifically, high-alkali cement leads to a pH between 13.5 and 13.9, while low-alkali cement results in a pH of 12.7 to 13.1. An increase in pH of 1.0 represents a ten-fold increase in hydrogen ion concentration. Thus the hydrogen ion concentration with low-alkali cement is about 10 times lower than when high-alkali cement is used. This is the rationale of using low-alkali cement with potentially reactive aggregate. Fig. 16.2 below gives the relationship between the total alkali content in concrete and the expansion of test specimen with age. The prevention of a deleterious alkali-silica reaction by limiting the alkali content in cement is valid only when two conditions are satisfied viz., there is no other source of alkalis in the concrete; and the alkalis do not become concentrated in some locations, at the expense of others. Such concentration may be caused by moisture gradients or by alternating wetting and drying. This can also occur when an electric current is passed through concrete as in the case of cathodic protection to prevent corrosion of embedded steel. Other internal sources of alkalies are some admixtures or even the mixing water. Alkalies present in fly ash and GGBS should also be included in the calculation, but only taking a proportion of the actual amount of alkalies in these cementitious materials. BS 5328: Part 4: 1990 uses 17% for fly ash and 50% for GGBS of the total alkalies present in the cementitious material. Because of the varied origin of alkalies, it is logical to limit the total content of alkalies in concrete. To avoid alkali aggregate reaction, BS 5328: Part 1: 1991 limits the total alkalies (expressed as Na2O equivalent) to a maximum of 3.0 kg/m3 of concrete.

Alkali Silica Reaction

16.7

0.4 ALKALI CONTENT 3

kg/m 5.45 5.45 4.56 4.04 3.62 3.20

Expansion (per cent)

0.3

3

lb/yd 9.18 8.46 7.68 6.81 6.10 5.39

0.2

0.1

0 1

10

100 Time (days)

Fig. 16.2 Relationship between alkali content in concrete and expansion of test specimen

16.4.3

Pessimum Content of Reactive Silica

The expansion caused by the alkali-silica reaction is greater with greater content of reactive silica, but only up to certain content of silica; at higher contents, the expansion is smaller. The proportion of reactive material, or reactive aggregate, corresponding to the peak expansion is called the pessimum content. This is illustrated in Fig. 16.3. There is thus a pessimum content of silica. This pessimum content is higher at lower water/cement ratios and at higher cement contents. The ratio of reactive silica to the alkalies corresponding to the maximum expansion usually lies in the range of 3.5 to 5.5. Thus if the amount of reactive silica content in the concrete is varied then the silica/alkali ratio can be moved away from the pessimum. It is found that expansion due to the alkali-silica reaction can be reduced or eliminated by the addition of reactive silica in a finely powdered form to the mix. In the range of low silica contents, the greater quantity of silica for a given amount of alkalis increases the expansion, but with higher values of silica content, the situation is reversed as shown in Fig. 16.3. Also greater the surface area of the reactive aggregate the lower the quantity of alkalis available per unit of this area, and the less alkali-gel can be formed. A probable explanation for the pessimum behaviour is as follows: Region A: The reactive silica content is low and gel growth after the concrete has hardened is of insufficient intensity to induce cracking. So gel growth occurs without any adverse effect on the concrete. Region B: The reaction continues after the concrete has hardened and the intensity of the reaction is sufficient to induce cracking. The expansion ceases when all the reactive silica is depleted or when the reaction falls to a sufficient low level. In this region there is an excess of alkalis and the composition of the reaction is probably independent of the alkali/reactive silica ratio.

16.8

Handbook on Advanced Concrete Technology Reactive aggregate content: % by mass of total aggregate 20 40 60 80

0

B

Expansion

A

C

D

Region

Effect

A, D B

Reaction but no cracking Reaction, cracking, excess of alkalis Reaction, cracking, excess of reactive silica

C

0

1

100

3 2 4 5 Reactive aggregate content: % by mass of total aggregate

6

Fig. 16.3 Relation between expansion after 224 days and reactive silica content in the aggregate

Region C: The reaction continues after the concrete has hardened, cracking occurs and expansion ceases when the alkalis in the pore water are reduced to a threshold level or are depleted. In this region there is an excess of reactive silica and the alkali/ silica ratio of the reaction product and the uptake of the water by the reaction product decreases with decreasing alkali/ reactive silica ratio. Region D: The reactive silica content is so high and the reaction is so rapid that by the time the concrete has hardened the rate of gel growth is too slow to induce cracking. Copious quantities of gel can be formed without any adverse effect on the concrete.

16.4.4

Use of Supplementary Cementitious Materials

Although their predominant use has been to improve other aspects of concrete performance, SCMs can be used, specifically to reduce ASR expansion in concrete containing reactive aggregate. Several mechanisms contribute to this effect, their relative importance depending on the concrete composition and the nature and amount of the SCM: • The total alkali content of the concrete will be reduced if the SCM has lower alkali content than the cement it replaces. Not all the alkali in SCM is necessarily available to enter the pore solution, so the reactive alkali content of the concrete may be reduced even if the total alkali content is not.

Alkali Silica Reaction

16.9

• Reaction of the SCM with the calcium hydroxide produced during cement hydration will reduce the calcium hydroxide available to maintain the very high pH necessary for ASR, as discussed in 6.5.2 above. • The product of the reaction between SCM and calcium hydroxide binds alkalis so that they are unavailable to participate in ASR. • The concrete will be less permeable, reducing the ingress of moisture and the diffusion of alkalis to reactive minerals. • The concrete may be stronger and better able to withstand expansive forces without cracking. Where SCM is added to concrete to protect against ASR, the level of cement replacement will depend on the SCM used. Approximate replacement levels needed to minimize ASR damage are: • 7-8% of total binder content for silica fume (Fig. 6.4) • 10-15% for metakaolin • 20-25% for fly ash • more than 50% for blast furnace slag (Fig. 6.5) 0.25

0.15

PC

0.10

2.5 SF 5SF

0.05

7SF Limit

0.00 -0.05 0

365

730

1095

Time (days)

Fig. 6.4 Effect of silica fume on alkali-silica reaction 0.20

Per cent expansion

Length Change (%)

0.20

0.15

Type I portland 30% slag cement

0.10

40% slag cement 50% slag cement

0.05 0.00 1

2

3 Age, months

4

6

Fig. 16.5 Effect of slag cement on alkali-silica reaction

16.10

Handbook on Advanced Concrete Technology

The level of replacement needed to control ASR expansion will vary with the aggregate, the cement, the presence of other SCM, and the chemistry, mineralogy and particle size distribution of the particular SCM product(s) used, and must be determined by testing. Concrete containing SCM must be thoroughly cured to ensure that the cement is fully hydrated, the SCM is fully reacted and the desired long-term properties achieved. This may necessitate extended curing periods for SCMs that hydrate more slowly than Portland cement.

16.5 PREVENTION/MINIMIZING ASR EFFECT ON CONCRETE STRUCTURE Once it is realized that concrete structure is being affected by ASR, one should take immediate action to see that further deterioration by ASR is minimized. As a first step all the cracks should be grouted to prevent entry of harmful chemicals into the concrete as it would further accelerate the deterioration of concrete. Of course grouting the cracks will be only an intermediate solution as the ASR would still be continuing inside the concrete causing further cracking. As discussed in the earlier sections, humidity levels inside the concrete needs to be more than 75-80% for ASR to proceed. Therefore, to inhibit expansion, moisture has to be prevented or restricted from entering the concrete. At the same time, the concrete must be able to dry out and its internal relative humidity reduce below 75%. Structures or elements of structures that can be treated as a whole with a hydrophobic substance such as a silane or siloxane should over a period of time dry out sufficiently for the reaction and expansion to stop provided that no part of the concrete is continuously in contact with free water or wet soil. Since the hydrophobic material allows free movement of water vapour in and out of concrete, it will not dry out sufficiently for the reaction and expansion to stop if the mean annual ambient relative humidity is above a value of between 75% and 85%. Ventilated cladding, which prevents the concrete from being wetted directly, might also allow it to dry out. Once the concrete has dried out sufficiently for ASR to stop (established by regular monitoring for dimensional changes over a period of atleast two seasons) the final crack repair procedures can be carried out and the surface coated if required. The method of filling, grouting or covering the cracks, and the material to be used, will depend on the purpose of the repair and the width, length and distribution of the cracks. Structures do not look attractive after cracks have been repaired; aesthetic considerations will dictate whether to coat the repaired surfaces. Several structures affected by ASR in South Africa and Canada have been treated with silane and the reaction successfully stopped. There is uncertainty about the effectiveness of treating, for example, the wing wall of a bridge with silane to stop further expansion (because water can enter via the fill). Where there is concern about corrosion of reinforcing steel, a migrating corrosion inhibitor could be applied to the concrete surface prior to the application of the silane.

References 1. A. M. Neville (2005) “Durability of concrete” Properties of Concrete, 4th Edition. 2. M. S. Shetty (2005) “Durability of Concrete” Concrete Technology, Theory & Practice, 6th Edition.

Alkali Silica Reaction

16.11

3. Alkali-silica reaction in concrete, D. W. Hobbs. 4. C.J. Munn, S.A. Freitag, N.B. Milestone, A. Happy, W. South, I. Brown, D.P. Barnard (2003) “Principles of Minimising the Risk of ASR” Alkali Silica Reaction - Minimising the Risk of Damage to Concrete Guidance Notes and Recommended Practice, 2nd Edition, Cement and Concrete Association of New Zealand. 5. Oberholster, B (2009) “Alkali-Silica Reaction” Fulton’s Concrete Technology, 9th Edition, cement and concrete Institute, South Africa.

17 High Strength and High Performance Concrete A.K. Jain and C.M. Dordi

17.1 INTRODUCTION IS 456:2000 Plain and Reinforced Concrete - Code of Practice specifies three groups of concrete based on different grades. Ordinary concrete includes concrete grades from M-10 to M-20, Standard concrete from M-25 to M-60 and High Strength Concrete from M-65 to M-100. Under note 2, code further mentions “For concrete of compressive strength greater than M-60(High Strength Concrete) design parameters given in the standard may not be applicable and the values may be obtained from specialized literatures and experimental results”. It is therefore necessary to take help from standard published works and experimental results in the design and production of High Strength Concrete. High Strength Concrete (HSC) is normally perceived as a concrete with a limited use such as construction of columns in high rise buildings, or off-shore platforms. Nevertheless, High Strength Concrete is primarily used in critical components of the structure and mostly under aggressive environmental conditions. Over the years, it has been felt, that apart from the prescriptive specifications of strength alone, concrete in such situations should also be brought under the ambit of desired performance. Therefore the concept of High Performance Concrete (HPC) emerged in 1990’s when the expression was first time coined by Prof. Roger Lacroix and Prof Yves Malier. High Performance Concrete is now viewed as an emerging type of concrete whose applications are growing both in volume and in diversity. In fact High Strength Concrete has now become synonymous with High Performance Concrete. In this chapter therefore the High Strength Concrete and High Performance Concrete both will be clubbed together and referred as “High Performance Concrete”.

17.2

17.2

Handbook on Advanced Concrete Technology

HIGH PERFORMANCE CONCRETE - OVERVIEW

The HPC is the concrete, which ensures long-time durability in structures exposed to aggressive environments. Durability of concrete is its ability to resist weathering action, chemical attack, abrasion and all other deterioration processes. Weathering includes environmental effects such as exposure to cycles of wetting and drying, heating and cooling, as also freezing and thawing. Chemical deterioration process includes acid attack, expansive chemical attack due to sulphate reaction, alkali aggregate reaction, corrosion of steel in concrete due to moisture and chloride ingress. In 1998, the term High Performance Concrete has been redefined by American Concrete Institute (ACI) as follows – “Concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing and curing practices”. The philosophy of HPC concrete design should be STRENGTH through DURABILITY rather than DURABILITY through STRENGTH. HPC is required as a construction material in structures constructed in very severe environment. The structures like tunnels in sea beds, tunnels and pipes carrying sewage, offshore piers and platforms, confinement structures for solid and liquid wastes containing toxic chemicals and radioactive elements, jetties and ports, sea link bridge piers and superstructures and high rise buildings, chimneys and towers, foundations and piles in aggressive environment. Though normal concrete has performed reasonably well in the past in favourable environment, if designed and constructed properly. However, unacceptable rates of deterioration in many recently constructed buildings, bridges and infrastructure projects exposed to hostile environments have caused great concern the world over. It has shown that earlier existing criteria were no longer adequate for ensuring long term durability and concrete mixes have to be designed for high performance rather than High Strength alone. Therefore a need has arisen for defining the essential characteristics in order to meet major specifications of HPC. In addition the guidelines for producing the material with conventional practices has also to be developed and examined.

17.3

HIGH PERFORMANCE CONCRETE – MAJOR CHARACTERISTICS

Concrete to be classified as HPC should be able to comply with requirements of impermeability and dimensional stability as stated below: Impermeability: It is extremely important for even ordinary concrete to be dense, defect free and impermeable. Penetration of moisture and harmful chemical ions can seriously affect performance of concrete. Corrosion of steel or expansive reaction within concrete reduces durability of concrete within a few years of service life. Since impermeability of concrete is the first line of defence, it is extremely important for HPC to have a very low coefficient of permeability in the range of 1 × 10 –14 m/sec.

High Strength and High Performance Concrete

17.3

Measurement of water permeability by traditional tests are very cumbersome and often do not give reproducible data. Chloride-ion permeability test (AASHTO 277) is found to be more practical. ASTMC 1202 classifies chloride-ion penetrability as given in Table 17.1. TABLE 17.1 Chloride-ion Permeability Chloride-ion Penetrability

Charge passed (Coulombs)

High Moderate Low Very Low

> 4000 2000 to 4000 1000 to 2000 100 to 1000

Negligible

> 100

When concrete permeability coefficient is very low, a chloride-ion permeability test (AASHTO 277) is the most appropriate. In this test the rate of permeation of chloride ions is expressed in terms of Coulombs (C). When the concrete mix shows 500 C or less current flow in a 6 hours chloride permeability test, it is considered to be virtually impermeable. Dimensional or Volume Stability High dimensional or volume stability will depend on the following main characteristics of concrete: • High elastic modulus • Low thermal strain • Low drying shrinkage • Low creep If the above characteristics of concrete are not taken care of, undesirable stress effects can result in volume changes under restrained conditions. Conventional materials can produce concrete of high compressive strengths (above 60 MPa) but the increase in elastic modulus is not proportional. The improvement in elastic modulus of concrete can only be achieved when suitable materials in correct proportions are incorporated in concrete mixes. Creep and drying shrinkage strains in normal concrete can be as high as 0.08% each. With proper materials and mix proportions, it is possible to reduce the 90 days drying shrinkage strain to less than 0.04%. Creep and drying shrinkage are highly dependent on the aggregate type and content. To achieve high dimensional stability it is desirable to reduce the magnitude of strains by limiting the total volume of the cement paste in concrete and by using coarse aggregate which has high strength and high elastic modulus. With availability of good quality chemical admixtures and mineral additives, it is possible to reduce the volume of the cement paste. Drying shrinkage is more influenced by excessive mixing water rather than due to cement. (Refer clause 6.2.4 IS 456-2000).

17.4 MATERIALS SELECTION AND MIX PROPORTIONING – DIFFICULTIES Conventional concrete and HPC differ considerably from each other. HPC is a multicomponent material like conventional concrete but its components are many more than those used in ordinary

17.4

Handbook on Advanced Concrete Technology

concrete. Therefore special attention is required for the selection of concrete components and their proportions. It is also equally important that the production methods, like mixing, handling, transporting, placing, compacting and curing, are also executed properly. • Selection of appropriate materials: The selection of material is a problem because cements and cementitious materials and aggregates are available with wide variations of compositions and properties. There are no clear guidelines as to the selection of binder and aggregate types most suitable for use in HPC. The situation is further complicated by the fact that HPC also requires use of chemical admixtures and mineral additives simultaneously and that there is a prolification of these admixtures in the market with no simple rules by which one can easily make a judicious choice. • Selection of mix proportions of various ingredients: The relationship between strength and water-cement ratio which is the backbone of mix-proportioning methods for ordinary concrete mixes may not meet all the requirements of HPC for impermeability and volume stability. In HPC since very low water-cement ratios are desirable, the type and dosage of mineral additives have a very great influence on the strength and other characteristic properties of concrete. Size, grading and type of aggregates and their proportions also have great influence. The dimensional stability of HPC also need to be carefully looked into. In conventional mix proportioning methods, these aspects are not given a serious consideration. • Durability and Dimensional Stability: In ordinary concrete the main requirements are generally strength and workability. In HPC besides strength and workability requirements, other characteristic properties have to be given equal if not more importance. In order to optimise characteristic properties such as long term durability in a given environment and dimensional stability, extensive testing programme may be necessary. • Sequence of Mixing Component Materials: The fourth and final difficulty in HPC is to finalise sequence in which the component materials are to be added during the mixing operation, the efficiency of the mixer, the method of transportation and placement and most important of all curing methodology. The above steps of concrete production are often not given due importance in ordinary concrete but they cannot be overlooked in HPC as they have considerable influence on the microstructure and properties of the ultimate product. To understand HPC better, it is important to first have an overview of the composition of concrete. According to a simple model, concrete is a composite material consisting of two components - the aggregate skeleton and cement paste. The cement paste is the binder (glue) for the aggregate mass and which constitutes the discontinuous phase. As a first approximation, the permeability, strength, dimensional stability, workability and other properties of concrete depend both on the aggregate to binder (cement + cementitious materials) ratio and the quality of each of the components which contribute to this ratio. A dry mixture of well graded fine and coarse aggregates is needed which contains around 21 to 22 percentage voids and to be filled up with the binder paste. In actual practice, to

High Strength and High Performance Concrete

17.5

produce workable concrete mixtures, at least 25 percent cementitious paste by volume is needed. Cement pastes hydrated with large amounts of water are generally weak and permeable because of presence of high volume of capillary pores, large crystals of hydration products specially calcium hydroxide and above all microstructural inhomogeneties. In normal concrete, when water to cement ratio is greater than 0.40, the presence of coarse aggregate particles in cement paste matrix gives rise to non-homogeneous distribution of water during placement and consolidation. Local regions of high water/cement ratio adjacent to the aggregate surface are formed due to the aggregate “wall effect”. This causes weak interfacial zone between cement paste and aggregate, resulting in drop in strength and reduction of impermeability. For low permeability and high strength of concrete it is desirable to reduce both water and aggregate content. However, this has an undesirable effect as the stiff consistency of the mix results in difficulty in compaction. On the other hand, on increase of cement paste content in concrete, the strength and impermeability are improved but dimensional stability is impaired. Studies have shown that about 35 percent of cement paste by volume, in HPC, represents an optimum solution in balancing the conflicting requirements of strength, workability and most importantly dimensional stability.

Volume Stability Mismatch of elastic moduli or coefficient of thermal expansion between the cement paste and aggregate will cause cracking when the structure is subjected to frequent cycles of temperature variations. For a fixed cement paste to aggregate ratio, the use of aggregate with a very low elastic modulus results in higher creep and shrinkage of concrete. Hence, such types of aggregate must not be used in HPC. From the point of long term dimensional stability, concrete mixtures containing coarse aggregates derived from limestone or basalt are generally known to perform very well in HPC. Fine aggregates (< 0.478 cm) with a medium - to - high fineness modulus between 2.5 to 3.0 are generally considered adequate. Coarse aggregate should have equidimensional particles (not flaky/elongated) obtained by crushing dense basalt or limestone or a plutonic type igneous rock (such as granite, syenite, diorite and diabase). Single size aggregate is preferred to downgraded aggregate. Larger than 25 mm MSA (maximum size aggregate) generally impairs the strength and impermeability of concrete and therefore are not recommended for use in HPC. Generally, 10 to 15mm MSA size is considered optimum for HPC. In HPC the aggregate - cement paste interfacial zone is strong, therefore the aggregate can be the weak link as far as strength is concerned. This is not of concern in case of conventional concretes. In HPC any internal defect present within the aggregate particles such as microcracks, large pores and inclusions of soft minerals, can influence the strength adversely. In most rocks reduction of MSA to 10 to 15 mm often eliminates such internal defects if any. It has been observed that in concrete specimens loading and unloading cycles in the elastic range produces much wider hysteresis loops in case of concrete containing certain granite and gravel aggregates. Large residual strain on unloading is indicative of either a weak interfacial zone or presence of weakness within the aggregate particle. It is therefore important to properly evaluate the coarse aggregate proposed to be used in HPC mixes. It may be noted that all

17.6

Handbook on Advanced Concrete Technology

granite and gravel aggregates may not be unsuitable for use in HPC mixtures. For example, in the North Western United States, a glacial gravel has been successfully used to make 120 MPa concrete.

Cement It is a known fact that given high-quality aggregates the strength and permeability of all concrete mixtures will depend on the physical and chemical properties of cement, the type and dosage of chemical admixtures and mineral additives, the original water-cement ratio and the degree of hydration. Minimum water demand for a given consistency of the cement paste is a primary consideration in the selection of cement for making HPC. Commercial cements meeting the requirements of BIS code of practice for 53 grade Ordinary Portland Cements (IS 12269) may vary considerably in chemical composition and fineness both of which influence water requirement for normal consistency. Physicochemical interactions between some cements and water reducing admixtures are known to cause rapid stiffening or slump loss. Concrete may require retempering with additional water. For example, some superplasticisers containing significant amounts of free sulphate, may cause abnormal stiffening of cement paste made with a high C3A Portland cement. The loss of workability is due to formation of ettringite, a calcium sulphoaluminate hydrate, which is known to immobilize large amount of free water by surface adsorption. Potland cements with higher C3A, high alkali and sulphate contents are more prone to slump loss problem. Blended Cement would perhaps be a better option to avoid loss of workability. Another alternative would be to use pozzolanic (fly ash) material or ground granulated blast furnace slag (ggbs) as mineral additives. Besides other advantages, mineral additives reduce heat of hydration and thereby reduce rapid loss of slump.

Role of Mineral Additives Mineral additives are fine powders mainly composed of silicate glasses or non crystalline silica which in the presence of moisture, calcium and hydroxyl ions, slowly hydrate to form cementing products. The most commonly used mineral additives in our country are siliceous by-products or wastes such as ground granulated blast-furnace slag, flyash and condensed silica fume (micro silica). Incorporation of mineral additives can lead to many technical benefits and hence their presence as an ingredient in HPC is considered unavoidable. The most important benefits are listed below: • These admixtures improve the rheological properties of concrete such as cohesiveness and stability. In normal concrete micro-structural non-homogeneity specially in the cement paste and aggregate interfacial zone causes bleeding and segregation. Mineral additives if present reduce bleeding and segregation. • Due to high ambient temperature and presence of super plasticizers, HPC mixes show a tendency to lose their workability rapidly. Presence of mineral additives helps in increasing the time of set and to reduce the rapid drop of slump thereby allowing adequate concrete compaction.

High Strength and High Performance Concrete

17.7

• The fine particles of a less reactive solid (compared to Portland cement) when well dispersed in the cement paste provide numerous nucleation sites for the precipitation of hydration products. Studies have shown that at a given water-cement ratio and degree of hydration, the hydrated cement pastes containing mineral additives (less reactive solids) are generally composed of poorly crystallined products with homogeneous distribution of fine pores. This improves impermeability of concrete and the strength development with age. The microstructure of cement paste containing mineral additive is far superior to a similar cement paste without any mineral additive. • Cement paste microstructure without mineral additives are found to be heterogeneous and having local areas of high concentration of large pores as well as large crystals of calcium hydroxide from which microcracks usually originate creating loss of durability.

Quality of Mineral Additives In order to enhance the properties of HPC, the mineral additives should be properly investigated for their performance. The essential factors being fineness, particle size, pozzolanic and/or cementitious characteristics, degree of uniform dispersion and curing conditions. High quality flyash and blast furnace slag are usually cheaper than Portland cement and may be used in quantities ranging between 20 to 30 percent by mass of total cementitious material. Larger dosages are usually avoided in HPC due to lower strength gain at 3 and 7 days and also to maintain alkaline characteristic of concrete which is important for long term durability of reinforced concrete structures. When early strengths are desirable, specially in cold weather conditions, micro silica (highly pozzolanic material) or a combination of micro silica with fly ash or slag is advantageous. Micro silica or condensed silica fume has 0.1 micron average particle size and 20m2 /g surface area. It is an expensive material and also requires higher dosage of superplasticisers, for proper dispersion. It is generally not recommended to use more than 10% micro silica in HPC. The optimum solution would be to use a combination of 8% to 10% micro silica with 15% to 20% fly ash or slag by volume. This gives excellent particle packing in concrete.

Role of Chemical Admixtures HPC in addition to Portland cement also consists of mineral additives which are very fine and need to be uniformly dispersed in the concrete mix. Chemical admixtures therefore are mainly used for dispersion of all fine particles in the mix, reduction of water content in HPC while maintaining the desired workability. The other objectives for use being improving consistency, controlling the time of set and providing protection against deterioration by freezing and thawing cycles. Conventional water reducing chemical admixtures such as derivatives of sulphonated lignin are able to reduce water content of concrete mixtures by 5 to 10% at normal dosages. At high dosages conventional water reducing admixtures cause excessive retardation. Hence high range water reducing admixtures popularly known as superplasticisers are used. They provide high consistency to concrete at very low water content without causing excessive set retardation.

17.8

Handbook on Advanced Concrete Technology

These superplasticisers are high molecular weight anionic surfactants derived from sulphonated formaldehyde of naphthalene or melamine. Generally, the choice is between derivatives of naphthalene or melamine. Clear preference does not exist in favour of either of the two groups of superplasticisers. Some studies have shown that melamines produce a higher water reduction but tend to cause a more rapid slump loss. Melamines also are more compatible with air entraining admixtures and therefore preferred to be used in cold ambient temperature environment. Naphthalene derivatives are more suitable in warm ambient temperatures. Superplasticisers are generally used in quantities ranging from 0.8 to 2 percent solid by weight of cementitious materials. They produce powerful dispersing effect on the fine cementitious materials present in the cement paste. This effect can be short-lived and therefore a portion say one-third or one-fourth of the superplasticiser dosage is added later when the concrete mix has arrived at the job site. The choice of superplasticiser is usually governed by cost, dosage and compatibility with other components in the cement paste. It is quite common to achieve the desired properties in concrete by using a combination of two chemical admixtures for example a superplasticiser can be used with normal water reducing admixture when cost effectiveness is desired in addition to set retardation.

Mix Proportioning Mix proportioning is the process of determining the right combination of various easily available materials that will produce a concrete mix with the specified characteristics and at the lowest possible cost. Whether conventional concrete or HPC mix is being proportioned, the finalisation of the individual constituents of the mix is not easy as it involves the art of balancing various conflicting parameters or requirements. Whatever be the computational method deployed to ascertain the mix proportions, it is important that the proportions should be cross-checked by actual laboratory and/or field trial before venturing into full scale concrete production. At best the computational method permits the determination of the proportions of various components for the first trial batch. Accuracy of the method used to determine mix proportions would certainly help reduce the number of trials. Therefore careful thought must be given before opting for any method. The procedures of mix design adopted for conventional concrete cannot be used for HPC as the following concrete parameters have to be complied with. • Concrete strength specified are generally more than 60 MPa. • Workability specified is very high 200 to 250 mm slump. • Properties of concrete are extremely sensitive to aggregate characteristics and water content of the concrete mix. • Extremely low permeability is required to face extreme environmental conditions. The following considerations are therefore generally required to develop the procedure for mix proportioning of HPC: • Cement paste to aggregate ratio. • Strength and quality of aggregate • Water content

High Strength and High Performance Concrete

17.9

• Cement content • Type and dosage of mineral additive • Type and dosage of chemical admixture • Fine to coarse aggregate ratio It is assumed that by using suitable coarse aggregates of maximum aggregate size of 10 mm to 15 mm, adequate dimensional stability of HPC concrete (e.g. elastic behavior, creep and shrinkage) can be obtained at a fixed cement paste to aggregate ratio of about 35% by volume. Strength is not the sole criteria for HPC. However, as concrete with strengths over 60 MPa generally show very low permeability, that is < 10-14 m/s, and satisfy weathering characteristics as well as abrasion resistance. Concrete strength is used as a basis of mix proportioning and quality control for HPC. It is therefore desirable that minimum concrete strength should be above 60 MPa. It is possible to achieve high concrete strengths by improving strength of the cement paste which can be controlled through the water content and dosage of admixtures. In order to classify HPC based on high strengths, it may be convenient to divide the 65 to 120 MPa strengths into several strength grades (See Table 17.2). TABLE 17.2 Relationship between average compressive strength and maximum water content per m3 of concrete: Strength Grade A B C D

Average Compressive Strength at 28 days (MPa) 65 75 90 105

Maximum Water Content (kg/m3) 160 150 140 130

E

120

120

The water content of the mix for conventional concrete depends on aggregate MSA and the workability (slump) of concrete. As HPC is produced with a narrow range of MSA (10 to 15 mm) and slump (200 to 250 mm) both these parameters need not be taken into consideration while finalising water content of HPC. A general inverse relationship exists between water content and concrete strength in case of HPC. This has been observed by reviewing various HPC mixes produced using various types of materials worldwide. Table 17.2 is used to estimate the maximum content of mixing water. This table is derived on experience with high slump superplasticised concrete mixtures containing 12 to 19 mm MSA. This relationship can be exploited for prediction and control of concrete compressive strength. As stated earlier the cement paste volume is fixed at about 35% for HPC. The cement paste contains anhydrous cement, mineral additives, water and entrapped air. As HPC cementitious paste consists of mineral admixtures and cement, the concrete mixes require thorough mixing for homogenisation of the cement paste. The HPC mixes tend to trap around 2% air by volume when no air entraining admixtures are used. Therefore if the volume of water (Table 17.2) and air content (2%) are known, the balance volume of cement paste will be contributed by cement and mineral additives.

17.10

Handbook on Advanced Concrete Technology

The following options are available for mix proportioning: • Only Portland Cement - (Option 1): Portland cement alone may be used without any mineral additives. In the strength range of 65 MPa to 120 MPa this option should only be exercised when absolutely unavoidable. Absence of mineral additives in the HPC mix will not provide some important technical benefits for example better handling and easier compaction of plastic concrete, improved resistance to thermal cracking, and above all better long term durability against corrosive environment. • Portland Cement + flyash or ground granulated blast furnace slag - (Option 2): In HPC, experience has shown that approximately 25% Portland cement should be substituted by good quality flyash or slag. As a first approximation it is best to assume a 75:25 volumetric proportion between Portland cement and mineral additive. The blend of Portland cement and cementitious material is advantageous as it reduces heat of hydration, improves workability, enhances the microstructural characteristics of well hydrated cement pastes, improves strength and reduces permeability. The last two characteristics are usually available only after at least 7 days moist curing at normal temperatures. Sometime even longer curing period may be necessary to give full advantage of mineral additives. • Portland cement + flyash or slag + condensed silica fume - (Option 3): Addition of flyash or slag as mineral additive can cause lower concrete strengths in early age. In extremely cold temperatures or otherwise, when early concrete strengths are desirable, in addition to flyash/slag, micro silica or condensed silica fume can be successfully used. Micro silica or condensed silica fume can be used alone or in combination with flyash/slag. For instance instead of 25% flyash, a mixture of about 10% silica fume and about 15% flyash may be used in combination. • Chemical Admixtures: It is recommended that superplasticisers or high range water reducing chemicals should only be used in HPC. Superplasticiser are produced using either the derivatives of naphthalene or melamine. The selection of the derivative type would depend on the ambient temperature conditions as discussed earlier. For the first trial batch 1% (solid content) superplasticiser by weight of cementitious material, is recommended. It is important to know the specific gravity of the superplasticiser solution and the weight fraction of the solids present in the solution. The dosage of the superplasticiser will have to be calculated considering weight of solids present in the solution. While, the water content in the superplasticiser must be accounted for in the water to binder ratio of the mix. A typical calculation is given below to determine the actual quantity of liquid superplasticiser required to be added and water content of the superplasticiser:

Known Parameters (a) Total cementitious material in the mix = 500 kg/m3 (b) Dosage of superplasticiser = 1 % of cementitious materials (solid content) by weight.

High Strength and High Performance Concrete

17.11

(c) Specific gravity of superplasticiser = 1.21g/cm3 (d) Solid content of superplasticiser = 41%

Calculations (a) 1 % superplasticiser (solid content) = 500 × 0.01 = 5 kg/m3 - (a) (a) 5 (b) Weight of solution = ___________ = ____ = 12.20 kg/m3 - (b) Solid content 0.41 to be added to the mixer per m3 of concrete. (b) 12.20 (c) Volume of the = _____________ = _____ = 10.1 lit/m3- (c) 1.21 Specific gravity solution to be added to the mixer per m3 of concrete. (d) Weight of water in the superplasticiser solution per cubic meter of concrete = (c) × Specific Gravity × (1-0.41) = 10.10 × 1.21 × 0.59 = 7.2 kg/m3

Ratio Between Fine and Coarse Aggregate In conventional concrete the ratio between fine and coarse aggregates would generally depend on the aggregate grading, shape, MSA, the rheology of the cement paste and the desired workability of concrete. Due to the relatively high cement paste volume in HPC mixes, it is recommended that more than 40% fine aggregates by volume of total aggregates may not be necessary. It is therefore recommended that, for the first trial batch, a 2:3 ratio between the fine and coarse aggregates may be followed.

Steps in Mix Proportioning Following important steps are needed in Mix proportioning of HPC Step No. 1 2 3 4 5 6 7 8 9 10

Activity Choice of Strength of Concrete Estimation of mixing water Volume of cement paste components Estimation of Aggregate content Calculation of batch weights Dosage of superplasticiers Moisture correction First Trial Batch Adjustments by more trials Finalise mix composition

17.12

Handbook on Advanced Concrete Technology

Corrective Measures to be Taken in HPC Mixes Sr. No. Deficiency

Recommendation

1

Workability too less

Gradually increase the superplasticiser dosage, desired workability will be achieved.

2

Tendency for mix to segregate

Increase fine aggregate of higher fineness. If the mix has no mineral additives, then this problem can also be resolved by using adequate proportion of silica fume or fly ash or slag or combination of any two of them

3

Set retardation of the mix

This deficiency is generally observed when ambient temperatures are very low. Study various other combinations of cements and superplasticisers which quicken the setting of the mix. Set activators or accelerators are available and can be dosed separately in addition to supeplasticisers. Modified superplasticisers containing set accelerators are also available commercially and can be used.

4

Stiffening the mix

Study compatibility of cement superplasticizer, select a better combination. In case stiffening of concrete consistency is high, there is likely hood of high content of reactive C3A in the cement. Another reason could be high ambient temperatures which can cause rapid hydration of cement resulting in stiffening of the mix. This can be offset by using ice flakes, along with chilled water in concrete mix as part of the water content. Lowering the temperature of other mix constituents will also help to a great extent. In some cases increase of superplasticisers may also help. However, if everything fails an increase in the water content may become unavoidable.

5

Low 28 days compressive strength

Examine the fracture surface of concrete specimens and the strain-stress curves. These may provide some clue of the weakest component of the composite. In the case of frequent debonding between cement paste and coarse particles, whether the cement paste in the transition zone need strengthening or the aggregate is too smooth. In the first case, reduction in water content and/or incorporation of fine particles of a suitable mineral additive (silica fume) will solve the problem. In the second case crushed aggregates having rough texture should be considered.

6

High 28 days compressive strength Reduce cement of the mix

Sequence of Batching Various Materials in HPC The reason for properly sequencing the addition of various constituents of HPC, into the mixer, is to homogenise the mix. Homogenisation of the mix prevents local concentration of voids and helps dispersion of fine particles uniformly in the mix. This prevents microstructural weak links and in turn improves performance of concrete. Bearing the above in mind, the general sequence to be followed is given below: add first coarse and fine aggregates in the mixer. next add cementitious materials. then add 80% water. finally the superplasticiser is added with balance quantity of water.

High Strength and High Performance Concrete

17.13

Another alternative could be using high speed mixers to prepare superplasticised cementitious paste and then adding aggregates in the mixer.

Transporting, Placing, Compacting, Finishing and Curing of HPC Segregation of the mix can take place while transporting and placing concrete if adequate care and precautions are not taken. HPC is generally designed as a flowing concrete mix. However, due to either presence of considerable quantity of cementitious materials or due to any non uniformity in the mix, there may still be possibilities of entrapped air pockets within the mix. Therefore, proper consolidation is necessary to eliminate any chances of entrapped air void left in concrete. Concrete mixes with high cementitious material content, specially comprising of silica fume, often tend to be very sticky and difficult to render and finish. Proper vibratory screeds will have to be used to give smooth, dense and impermeable skin on top surface of concrete. All other precautions necessary for conventional concrete mixes should also be taken for HPC while transporting, placing compacting and finishing. Proper curing by maintaining adequate humidity and temperature conditions for a certain specified time period is absolutely necessary in case of HPC. Curing helps development of strength and impermeability. As long as the concrete temperatures are sufficiently above freezing (i.e >5°C) external moist curing provides the best results. Because HPC mixes have high cementitious material content, there is a tendency to assume that they require a longer curing period than ordinary concrete. Studies have shown that good continuous curing for a 7 days period was sufficient to make the concrete impervious enough hence further moist curing was not necessary to enhance the compressive strength. In warm/hot weather it is extremely important to do early curing and protect the freshly laid concrete from rapid early drying conditions.

Conclusions HPC mixes need not be used in all structures. HPC mixes require superior quality materials, methods and supervision besides being costlier at first instance. In important structures HPC mixes are the best answer to improve performance and reduce the maintenance cost. However, there is no answer which gives clearly a demarcating point as to where does normality of concrete ends and high performance starts. Country to country even normal concretes are defined differently. From time to time even the definition of normal concrete keeps changing in the same country. It is likely that concrete regarded as HPC today will in future be considered as normal and even HPC will be redefined in future. Concrete technology is very dynamic and always displaying new, interesting and often exciting phases. The traditional approach to durability that is minimum cement content, maximum w/c ratio and type of cement is being questioned by researchers and technologists. Today studies are being conducted on concrete durability and new dimensions such as particle packing, interfacial zones, transport mechanisms, binding capacity are the hot topics being looked into along with the need for sustainability.

17.14

Handbook on Advanced Concrete Technology

References 1. Gujarat Ambuja Technical Literature Series 83, A to Z Answers on High Performance Concrete. 2. N. Bhanumathidas & N. Kalidas, (Jul., 2002) Fly ash for Sustainable Development. 3. P.C. Basu - NPP Containment Structure, (Oct., 2001) Indian experience in silica fume based HPC, Indian Concrete Journal, pp 656-666. 4. S. Saini, S.S. Dhuri, D.K. Kanhere and S.S. Momin, (Oct., 2001) High Performance Concrete for an urban viaduct in Mumbai, Indian Concrete Journal, pp 634-644. 5. R.C. Lewis and S.A. Hasbi, (Oct., 2001) - Use of silica fume concrete: Selective case studies, Indian Concrete Journal, pp 645-655. 6. P.C. Basu, (Sept., 1999) - Performance requirements of HPC for Indian NPP Structures, Indian Concrete Journal, pp 539-547. 7. Amit Mittal and R.R. Kamath, (Sept., 1999) - Properties of HPC for PC dome of NPP, Kaiga, Indian Concrete Journal, pp 561-570. 8. Gujarat Ambuja Technical Literature Series No. 47, (Nov., 1999) Concrete for the next Milennium. 9. P. Kumar Mehta and Pierre, (1990), Claude C. Aitcin, Principles underlaying Production of High Performance Concrete - Cement, Concrete and Aggregates Winter, pp 70-78. 10. P. Kumar Mehta and P.C. Aitcin, (May, 1990) Microstructural Basis for selection of Materials and Mix Proportions of High Strength Concrete, Proceedings Berkeley Conference of High Strength Concrete 11. G.R. Mass, (1989) Premixed cement paste, Concrete International. Vol. 11, No.11, pp 82-85. 12. J Asselanis, P C. Aitcin and P. Kumar Mehta (1989) effect of curing conditions on compressive strength and elastic modulus of very High Strength Concrete, Cement, Concrete and Aggregates - Vol. 10, No.1, pp 80-83.

18 Self Compacting Concrete N.V. Nayak, S.G. Bapat and Himanshu Kapadia

18.1 INTRODUCTION Self Compacting Concrete (SCC) was first developed in Japan around the year 1980. Professor H. Okamura of University of Tokyo, Japan is mainly responsible for initiating and initial development of such concrete. The need for development of such concrete arose from scarcity of skilled manpower in Japan during this period. Though it was developed to overcome deficiency of the skilled manpower, subsequently it is observed that SCC not only reduces the requirement of man power, both skilled and unskilled, but it also results in more durable concrete. Though the concept, originally, was thought to be a tool to enhance long term durability of structures, having members with congested reinforcements, the excellent user-friendly characteristics of SCC are of great attraction today in traditional construction industry. SCC has a big role to play because of the sustainable benefits in construction, both qualitatively and quantitatively. Self-compacting concrete can be taken as greatest technical advancement and most revolutionary development in concrete technology over the years, at least from 1980 till date. This is the concrete of future, as it will be replacing the normal concrete, because of many advantages as noted later in the chapter. SCC is used in many other countries, both developed and developing countries, such as Canada, Sweden, Netherlands, Taiwan, Thailand, USA, Austria, Korea, France, U.K. Germany, etc. In India, it has been used in limited way since the year 2003. It has been used in Tarapur, Kaiga and Kota Atomic Power Projects, Delhi Metro Projects, Bandra-Worli Sea Link Project etc. SCC is sometime also referred to as Self-Consolidating Concrete or Self Leveling Concrete though widely it is referred as Self-Compacting Concrete (SCC). However, the appropriate terminology is SCC, as aptly defined further. In future, such concrete may only be designated as SCC.

18.2

18.2

Handbook on Advanced Concrete Technology

DEFINITION

A concrete, that is capable of self compacting (self consolidating), occupies all the space in the form without any external efforts (in the form of mechanical vibration, floating, poking etc.) is termed as Self Compacting Concrete. The guiding principle behind the self-compacting is that the sedimentation velocity of a particle is inversely proportional to the viscosity of the floating medium in which the particle exists. For a concrete to be self compacting, to occupy the full space, flowing through the congestion in the form without any external efforts, it has to have an acceptable level of passing ability, filling ability, flow-ability and stability.

18.3 MIX PROPORTION OF SCC The concrete is a heterogeneous mixture of cement, coarse aggregate, fine aggregate, water and chemical admixture. All these ingredients have different specific gravities. Hence, it is difficult to keep these constituents in a cohesive form. When the concrete has a consistency of fluid, the material of higher mass tend to settle down. This problem is tackled by adding more amount of fines (material passing 100 microns) per cubic meter of concrete and by using superplasticizer. Superplasticizers reduce the water demand of highly fluid mix while using as less water as possible. This is a key to produce high density and high strength concrete. Ideally, SCC can be produced by using only a superplasticizer. But small variation in the parameters of ingredients like grading, moisture content, absorption, batching tolerances etc can change the characteristic of SCC, leading to mix becoming unstable and sticky. This problem is overcome by varying the viscosity of the mix by adding viscosity modifying agent (VMA), which acts as a stabilizing agent. It is a pseudo-plastic agent which thickens the paste and keeps the ingredients in suspension for a longer time and provide segregation resistance to the concrete under static condition. General guideline for mix proportion for normal concrete vis-a-vis SCC is given in Fig. 18.1 and Fig. 18.2. In general, SCC has got less coarse aggregate, more fine aggregate and needs super plasticizer and viscosity modifying agent (VMA) as admixtures (Ref. Fig. 18.1 and Fig. 18.2). Normal mix 10%

Cement

18%

Water

2%

25%

45%

Fine aggregate

Air

Coarse aggregate

Fines 10%

18%

2% 8%

26%

36%

SCC

Fig. 18.1 Constituents of normal vs self compacting concrete (by volume) (design and control of concrete mixes by Steven H. Kosmatka, Beatrix Kerkhoff and William C. Panarese).

Self Compacting Concrete

18.3

Admixture

18%

46%

0.01% 20%

Water

Traditional concrete

24%

12%

Coarse aggregate

Sand

Fines

Self compacting concrete

28%

34%

18%

Fig. 18.2 Constituents of normal vs self-compacting concrete, by volume (Concrete journal July/Aug’ 2001 Edition, by Rob Gaimster and Jhon Gibbs)

Some broad guidelines for proportioning of various components of SCC are given below.

18.3.1 Coarse Aggregate Aggregate which are bigger than 4.75 mm (4mm in many European Countries) are considered as coarse aggregate. Generally, coarse aggregate size is limited to 20mm, though often maximum size of the aggregate used is between 10-12mm, considering congestion of reinforcement. There are rare occasions, wherein 40mm down aggregate has been used in SCC. Regarding the characteristics of different types of aggregate, crushed aggregate tend to improve the strength, because of the interlocking angular particles, but at the same time reducing the flowability, while rounded aggregate improve the flow because of the lower internal friction. Gap graded aggregate are found to be better than continuously (well) graded aggregate, as well graded aggregates experience greater internal friction and give reduced flowability. The amount of coarse aggregate is generally less than 50% by weight of total aggregates (coarse and fine aggregate combined). Normal range of coarse aggregate is generally 700 to 800 kg per cum of concrete. By volume, coarse aggregate content is 50 to 60% of the total aggregate volume. The actual volume of the coarse aggregate depends upon the characteristics and maximum size of the coarse aggregate. The lower the maximum size of aggregate, the higher is the proportion of coarse aggregate and viseversa. Similarly, with rounded aggregate higher percentage of coarse aggregate can be used than with crushed coarse aggregate.

18.3.2 Fine Aggregate (Sand) Aggregate smaller than 4.75mm (4mm in many European Countries) and up to 0.075mm (0.125mm in European Countries) are considered as fine aggregate (sand). Sand in SCC is generally more finer than in normal concrete. The ratio of weight of sand to coarse aggregate

18.4

Handbook on Advanced Concrete Technology

could be in the range of 1.1 to 1.6. The ratio is on the lower side of this range, if filler dust is used in SCC. The volume of sand content should be in the range of 40% to 50% of the mortar/paste volume. Both natural and crushed sand meeting the particle size range can be used.

18.3.3

Fines/Dust

Particles, which are smaller than 0.075mm (0.125mm in European Countries) are considered as dust/fines. The amount of dust to be used depends on various factors including whether secondary cementing materials like GGBS, fly ash and/or silica flume are used or not. If such secondary cementing materials are used, then requirement of fines reduces. In some cases, the need for fine is completely eliminated by using secondary cementing materials. Increased dust helps in preventing the segregation. In absence of secondary cementing material the amount of fine is generally 160 to 240 lt or 450 to 600 kg per cum of concrete.

18.3.4

Cement

The amount of cement, generally used in SCC, is same as normal concrete (NC). Though some times little more cement is used, it may be because of the initial usage of such concrete. The addition of secondary cementing material (SCM) like fly ash, GGBS, silica flume is beneficial.

18.3.5 Water From durability consideration water cement ratio should be restricted as in case of normal concrete and it should preferably be less than 0.4. Water powder volume ratio is generally 0.8 to 1.10 In Table 18.1 mix proportions used on three projects for M30, M35, and M80 are given as example for guidance. TABLE 18.1 Mix Proportioning of SCC for M30, M35, M45 and M80 S.No.

M30

kg/m3 M35

M80

Ingredient

Remarks

1. 2. 3. 4.

Cement (OPC 53 Gr.) Fly Ash Micro silica Coarse Aggregate (CA) – 20mm -10mm

225 225 NIL 354} 708 355}

330 150 NIL 455} 764 309}

400 160 40 284} 711 427}

5

Fine Aggregate (FA)

288 River 684 Crushed

917

970

6. 7.

Water (W/B, ratio) PC Admixture As % of Binder by weight

165 *0.8% Type G

163 (0.34) *0.8%

135 (0.225) 1.2

8.

VMA

0.60%

*0.2%

0.2

9.

Ratio FA/CA

1.37

1.2

1.364

Crushed Sand

Self Compacting Concrete

18.5

18.4 TEST METHODS As noted earlier there are three basic characteristics of self-compacting concrete namely, viz filling ability (flow-ability), passing ability (free from blocking by reinforcement) and resistance to segregation (stability/homogeneity). Therefore, it is very necessary to carryout tests to assess these characteristics. But unfortunately so far not a single test has been standardized as “code” by any country. At the same time, number of tests have been developed and used by many organizations. All these tests are mainly adhoc in nature and none has been standardized. EFNARC specifications and guidelines detail some of the tests for qualifying SCC. These tests have been developed on the basis of field experience. The most commonly used tests are Slump Flow Test (along with T50cm Test), V Funnel Test, L Box Test, ‘U’ Box Test and Fill Box Test. In addition, there are other tests like ‘J’ ring test, etc. However, only the first five tests mentioned above are briefly described herein below:

18.4.1 Slump Flow Test and T50cm Test This is one of the simplest test initially developed in Japan for assessment of under water concrete. It is most commonly used test for SCC and gives a good assessment of filling ability. However, it gives no indication of the ability of the concrete to pass between the reinforcement without blocking, but may give some indications on resistance to segregation. This test can be used at site to assess the consistency of supply of Ready Mixed Concrete, the mix of which has been proportioned after assessment of various characteristics based on five tests stated above. The method of execution of slump flow test is illustrated in Fig. 18.3(a). The brief procedure for execution of this test is as given below: • Position the slump cone at the center of the leveled flow table. [Fig. 18.3(a)] • Pour the concrete with a scoop from top without tamping to fill the slump cone completely. Strike off excess concrete. • Lift the cone vertically without any jerks and allow the concrete to flow freely. • Note the time required for the concrete to cover 50 cm diameter spread circle. (T50cm time is the time required for the concrete to cover 50 cm dia. spread circle from the time the slump cone is lifted). [Fig.3(b)]. As per present guide lines, T50 time should be 2 to 5 sec. • Measure the average flow diameter of concrete after concrete stops flowing [Fig. 18.3(c)]. • The minimum slump flow required at the time of pour, according to the guidelines, is 650 to 800 mm depending upon site conditions. This value in mm is known as slump flow value (mm). Thus, from this test, slump flow value (mm) and T50cm slump flow time (sec) are obtained. Thus, slump flow is the mean diameter in two perpendicular directions of the concrete spread, after the concrete had stopped flowing. The T50cm slump flow time is the time taken

18.6

Handbook on Advanced Concrete Technology

(a) Slump cone over flowtable

(c) Measuring the flow

(b) Conc. spreads over flowtable

(d) Aggregate dispersion on surface

Fig. 18.3 Slump flow test

for the concrete to spread by 50 cm. High slump flow value indicates greater flowability and lesser resistance to segregation. T50cm time less than the minimum range value specified also indicates that the viscosity is very low, which may lead to segregation. If T50cm time value is more than maximum range value specified, it indicates very stiff and non flowable concrete. The permissible range of values for slump flow and T50cm test time are indicated in Table 18.2. TABLE 18.2 Range of Test Values Considered Acceptable. S.No.

Test

Unit

3. 4.

(a) Slump Flow (b) T50 cm Slump Flow Time (a) V. Funnel – T0 (min) (b) V. Funnel – T5 (min) U Box (h1-h2) L Box (h2/h1)

mm Sec.** Sec. Increase over (2a) in Sec. Mm Ratio

650-800 2-5 8-12* 0-3 1800 mm

Minimum concrete temperature as placed and maintained 1

–

13°C

10°C

7°C

5°C

16°C 18°C 21°C

13°C 16°C 18°C

10°C 13°C 16°C

7°C 10°C 13°C

28°C

22°C

17°C

11°C

Minimum concrete temperature as mixed for indicated air temperature 2 3 4

Above –1°C –18 to –1°C Below –18°C

Maximum allowable gradual temperature drop in first 24 hr after end of protection 5

–

Heating of mixing water – The temperature of water can be raised to a maximum of 80°C. Premature contact of very hot water and concentrated quantities of cement can cause flash set and cement balls in mixers. When hot water nearing 80°C is used it is advisable to add the hot water on the aggregate first and then add the cement into the mixer. Pipeline and storage tank need to be properly insulated to avoid any heat loss from the water till it reaches the batching plant mixer. Heating of aggregate - When aggregates are free of ice and frozen lumps, the desired temperature of the concrete during mixing can usually be obtained by heating only the mixing water, but when air temperatures are consistently below - 4°C, it is usually necessary to also heat the aggregates. Heating aggregates to temperatures higher than 15°C is rarely necessary if the mixing water is heated to 60°C. If the coarse aggregate is dry and free of frost, ice, and frozen lumps, adequate temperatures of freshly mixed concrete can be obtained by increasing the temperature of only the sand, which seldom has to be above about 40°C, if mixing water is heated to 60°C. The heating of aggregate can be done either passing hot steam into the aggregate bins or by passing hot air. Use of hot steam is thermally more efficient than hot air though it does create a problem of moisture variation in the aggregate. The exact temperature of water and aggregate required for producing the desired temperature of concrete can be calculated from the following formula: 0.22 (Ts◊Ws + Ta◊Wa + Tc◊Wc + Tw◊Ww + Ts◊Wws + Ta◊Ww T = _______________________________________________ 0.22(Ws + Wa + Wc) + Wwa + Wws Where, T = final temperature of concrete mixture (deg C) Tc = temperature of cement (deg C) Ts = temperature of fine aggregate (deg C)

Cold Weather Concrete

20.7

Ta = temperature of coarse aggregate (deg C) Tw = temperature of added mixing water (deg C) Wc = weight of cement (kg) Ws = saturated surface-dry weight of fine aggregate (kg) Wa = saturated surface-dry weight of coarse aggregate (kg) Ww = weight of mixing water (kg) Wws = weight of free water on fine aggregate (kg) Wwa = weight of free water on coarse aggregate (kg) It is important to note that there will be certain loss of concrete temperature due to the cold ambient conditions. It is necessary to calculate the amount of temperature loss during transit and produce the concrete at a higher temperature at the batching plant accordingly. The approximate loss in temperature is as given in the following formula. T = 0.25 (tr - ta) Where, T = temperature drop to be expected during a 1-hr delivery time, deg F or C. (This value must be added to tr to determine the required temperature of concrete at the plant.) tr = concrete temperature required at the job, deg C ta = ambient air temperature, deg C It is always advisable to insulate the transit mixers with some insulating material in order to prevent excessive heat loss of the concrete during transportation. All else being equal, low slump and/or low water cement ratio mixes are particularly desirable in cold weather for flatwork. This reduces bleeding and decreases setting time. Bleed water may remain on the surface for a period sufficiently long to interfere with proper finishing. If the bleed water is mixed into the concrete during trowelling, the surface will have a lower strength and may become prone to dusting and subsequent freeze thaw damage if exposed.

20.4

PREPARATION BEFORE CONCRETING

Preparation before concreting consists primarily of ensuring that all surfaces that will be in contact with newly placed concrete are at temperatures that cannot cause early freezing or seriously prolong setting of the concrete. All snow, ice, and frost must be removed so that it does not occupy space intended to be filled with concrete. Hot-air jets can be used to remove frost, snow, and ice from forms, reinforcement, and other embedments. Unless the work area is sheltered, this work should be done just prior to concrete placement to prevent refreezing. Concrete should not be placed on frozen subgrade material. The subgrade sometimes can be thawed acceptably by covering it with insulating material for a few days before the concrete placement, but in most cases external heat must be applied. Ordinarily, the temperatures of these contact surfaces, including subgrade materials, need not be higher than a few degrees above freezing, say 2°C, and preferably not more than 5°C higher than the minimum placement temperatures given in Table 20.2.

20.8

Handbook on Advanced Concrete Technology

20.5 PROTECTION OF CONCRETE AGAINST FREEZING To prevent early-age freezing, protection must be provided immediately after concrete placement. Arrangements for covering, insulating, housing, or heating newly placed concrete should be made before placement, typically as shown in Fig. 20.5, 20.6 & 20.7. The protection that is provided should be adequate to achieve, in all sections of the concrete cast, the recommended temperature and moisture conditions.

Fig. 20.5 Maintaining the temperature of work area by covering it with insulating material and providing external heat inside

Fig. 20.6 With air temp. down to –23 °C, concrete was cast in this insulated column form made of 19mm high-density plywood inside, 25mm rigid polystyrene in the middle and 13mm rough plywood outside

Cold Weather Concrete

20.9

Fig 20.7 Covering the foundation with insulating material in order to maintain the temperature of concrete

In cold weather, the temperature of the newly placed concrete should be kept close to the values shown in Line 1 of Table 20.2 for the lengths of time indicated in Table 20.3 for protection against early-age freezing. The length of the protection period may be reduced by: 1. using higher cement in excess of the design cement content; 2. using an accelerating admixture (preferably chloride free accelerators); or 3. by maintaining higher concrete temperatures. TABLE 20.3 Length of protection period required to prevent damage from early-age freezing of air entrained concrete Line

Exposure

Protection period at temperature indicated in Line 1 of Table 20.2, days OPC

60 kg/m³ of extra cement or accelerating admixture or maintaining higher concrete temperature

1

Not exposed

2

1

2

Exposed

3

2

Line 1 of Table 20.2 refers to concrete that will be exposed to little or no freezing and thawing in service or during construction, such as in foundations and substructures. Line 2 refers to concrete that will be exposed to the weather in service or during construction. At the end of the protection period, concrete should be cooled gradually to reduce crack-inducing differential strains between the interior and exterior of the structure. The temperature drop of concrete surfaces should not exceed the rates indicated in Table 20.2. This can be accomplished by slowly reducing sources of heat, or by allowing insulation to remain

20.10

Handbook on Advanced Concrete Technology

until the concrete has essentially reached equilibrium with the mean ambient temperatures. Insulated forms, however, can present some difficulties in lowering the surface temperatures. Initial loosening of forms away from the concrete and covering with polyethylene sheets to allow some air circulation can alleviate the problem. As shown in Table 20.2, the maximum allowable cooling rates for surfaces of mass concrete are lower than for thinner members.

20.6 PROTECTION OF CONCRETE AGAINST FREEZE THAW EFFECT As the temperature of saturated concrete is lowered, the water held in the capillary pores in the hardened cement paste freezes and expansion of concrete takes place. If subsequent thawing is followed by re-freezing, further expansion takes place, so that repeated cycles of freezing and thawing have a cumulative effect. Ice formation results in a 9 per cent volume expansion of water. Therefore, if ice formation is initiated in a pore, water will be expelled from this cavity. If this water flow is hindered, hydraulic pressures will be generated within the solid matrix. This phenomenon explains why dilations are observed during freezing. It is also consistent with the fact that air entrainment can improve the frost resistance of concrete. The presence of air bubbles reduces the distance that water has to travel and contributes to ease the dissipation of internal pressures. Air bubbles can be introduced by air entrainment using air entraining admixtures. Although air entrainment greatly enhances the resistance of concrete to cycles of freeze thaw, it is vital that the concrete has a low water cement ratio so that the volume of capillary pores is small. It is also essential that substantial hydration takes places before exposure to freezing. Such concrete has a low permeability and imbibes less water in wet weather. It may be noted that addition of air in concrete reduces the strength but it can be compensated by reducing the water cement ratio.

References 1. Cold weather concreting, ACI 306R-88. 2. A. M. Neville (2005) “Effect of freezing and thawing and of chlorides” Properties of Concrete, 4th Edition. 3. John Newman, Ban Seng Choo (2003), “Hot and Cold Weather Concrete” Advanced Concrete technology – Concrete Properties, 1st Edition.

21 Pervious Concrete Himanshu Kapadia, Amit Datta and S.G. Bapat

21.1 INTRODUCTION The term “pervious concrete” typically describes a zero slump, open-graded material consisting of Portland cement, coarse aggregate, little or no fine aggregate, admixtures and water. The combination of these ingredients will produce a hardened material with connected pores (Fig. 21.1), ranging in size from 2 mm to 8 mm, that allows water to pass through easily. The void content can range from 18% to 35%, with typical compressive strengths of 2.8 MPa to 28 MPa. The drainage rate of pervious concrete pavement will vary with aggregate size and density of the mixture, but will, generally, fall into the range of 81 to 730 l/min/m2

Fig. 21.1 Pervious concrete

Concern has been growing in recent years towards reducing the pollutants in water supplies and the environment. In the 1960s, engineers realized that runoff from developed real estate had the potential to pollute surface and groundwater supplies. Further, as land is developed, runoff leaves the site in higher rates and volumes, leading to downstream flooding and bank erosion. Pervious concrete pavement reduces the impact of development by reducing runoff rates and protecting water supplies.

21.2

Handbook on Advanced Concrete Technology

Continual urbanization has led to the increase in impervious surface area, further leading to blockage in percolation of precipitation from rainfall. Result is excess in surface run off, leading to floods and bank erosions. UK government has recently issued Flood and Water Management Act 2010, covering requirement for Sustainable Urban Drainage System (SUDS) for all new construction works. To meet this requirement, pervious concrete is the solution. The concrete, having 15% to 35% interconnected pores by volume, allows direct infiltration of water through its structure and will effectively solve drainage problems and reduce the risk of flash floods.

21.2

APPLICATIONS OF PERVIOUS CONCRETE

Pervious concrete has been used in wide range of applications, including: • Pervious pavement for parking lots (Fig. 21.1); • Rigid drainage layers under exterior wall areas; • Greenhouse floors to keep the floor free of standing water; • Structural wall applications where lightweight or better thermal insulation characteristics, or both are required; • Pavements, walls, and floors where better acoustic absorption characteristics are desired; • Base course for city streets, county roads, driveways, and airports; • Surface course for parking lots, tennis courts, zoo areas, and animal barns and stalls; • Bridge embankments; • Swimming pool decks; • Beach structures and seawalls; • Sewage treatment plant sludge beds; • Solar energy storage systems; • Wall linings for drilled water wells; and • Artificial reefs where the open structure of pervious concrete mimics the reef structure. Typically, un-reinforced pervious concrete is used in all these applications because of the high risk of reinforcing steel corrosion due to the open pore structure of the material.

21.3

PAVEMENT APPLICATIONS

Advantages of pervious concrete pavements over conventional concrete pavements include: • Controlling storm water pollution at the source; • Increasing facilities for parking by eliminating the need for water-retention areas; • Controlling storm water runoff; • Reducing hydroplaning on the surface of roads and highways; • Creating additional lift to the aircraft during takeoff due to the cooling effect;

Pervious Concrete

21.3

• • • •

Reducing glare on the road surfaces to a great extent, particularly when wet at night; Reducing the interaction noise between the tire and the pavement; Eliminating or reducing the size of storm sewers; and Allowing air and water to reach roots of trees, even with the pavement within the tree drip line Engineers have specified pervious concrete in pavements as: • Surface course; • Permeable base and edge drains; and • Shoulders. The success of pervious pavement systems has been mixed. In some areas, pervious concrete pavement systems have been applied successfully, while in others, they have clogged in a short time. Many failures can be attributed to contractor’s inexperience, higher compaction of soil than specified and improper site design. For a pervious concrete pavement to work successfully: • Permeability of soils should be verified. A percolation rate of 13 mm/h, and a soil layer of 1.2m or more are generally recommended. • Construction site runoff and heavy equipment should be kept away from entering the pervious pavement area. The pervious concrete pavement should not be placed into service until all disturbed land that drains to it has been stabilized by vegetation. Strict erosion and sediment controls during any construction or landscaping activity are essential to prevent clogging of the system and should be incorporated into the construction site storm water management plan; • Construction traffic (primarily vehicular) should be directed away from the pervious pavement area during construction to prevent compaction of underlying soil layers and loss of infiltrative capacity; and • Maintenance may be performed on a regular basis.

21.4

SURFACE COURSE

Pervious concrete may be used as a surface course for parking lots and minor road strips. The use of pervious concrete in surface course is for three basic purposes: • Places which frequently encounters heavy storms that cause quick accumulation of large amounts of storm water and the use of pervious concrete reduces the runoff volume. • Designers prefer that the storm water be retained on the site to recharge the groundwater system. • The cost effectiveness of using pervious concrete over conventional concrete is greatly enhanced with the elimination of storm sewers.

21.5

PARKING LOTS

The concept of pervious concrete for parking lots has been developed as a means of handling the enormous quantities of water running off a parking lot during a storm. Pervious concrete

21.4

Handbook on Advanced Concrete Technology

allows the water to percolate into the ground under the pavement. The Environmental Protection Agency (EPA) of the US has adopted a policy that recommends the use of pervious pavements as a part of their Best Management Practice (BMP) as a way for communities to mitigate the problem of storm water runoff. Pervious concrete parking lots have also been selected as an integral solution to the problem of hot pavements in the Cool Communities program. Generally, the air temperature over pervious concrete parking lots is cooler than the temperature over asphalt parking lots. This option is a unique and attracting feature among the designers to specify pervious concrete in Parking Lots. In addition, pervious concrete is considered a non-pollutant to the environment. The practical range of design thicknesses for pervious concrete pavements is from 125 mm to 250 mm for plain parking lots.

21.6 ROADWAYS Pervious concrete for roadways is usually considered for two applications: 1. as a drainable base, or sub-base material; and 2. as a roadway surface, or friction-course. In both categories, the drainage characteristics are required properties, but strength requirements may vary depending on the location of the material in the pavement section. The practical range of design thicknesses for pervious concrete is from 150 mm to 250 mm for plain roadway pavements. Many highways in Europe are being constructed using an overlay of latex modified pervious concrete that allows for pavement drainage and tire-noise reduction. The latex modification results in better mechanical properties.

21.7 PERMEABLE BASES AND EDGE DRAINS A pervious concrete base drains water that would normally accumulate beneath a pavement. This type of construction helps to reduce pumping of sub-grade materials that could lead to the failure of the pavement. In some States in United States of America, the departments of transportation have created standards for constructing drainable bases and edge drains using pervious concrete. Pervious concrete, in these applications, is usually of a lower strength (7 MPa or less) and is used in conjunction with a non-woven geo-textile fabric. A similar system can be used in slope stabilization.

21.8

MATERIALS

Pervious concrete, also known as porous, gap-graded, permeable, or enhanced porosity concrete, mainly consists of normal Portland cement, uniform-sized coarse aggregate, and water. This combination forms an agglomeration of coarse aggregates surrounded by a thin layer of hardened cement paste at their points of contact. This configuration produces large voids between the coarse aggregate, which allows water to permeate at a much higher rate than conventional concrete. Pervious concrete is considered a special type of porous concrete. Porous concrete can be classified into two types:

Pervious Concrete

21.5

1. The porosity is present in the aggregate component of the mixture (lightweight aggregate concretes) and 2. porosity is introduced in the non-aggregate component of the mixture (pervious concrete) Lightweight aggregate concrete can be produced by using extremely porous natural or synthetic aggregates. Pervious concrete has little or no fine aggregate in the mixture. Another distinction between these two types of porous concrete is based mainly on the void structure. Lightweight aggregate concretes contain large percentages of relatively non-connected voids. Pervious concrete, however, contains high percentages of interconnected voids, which allows the rapid passage of water through the body of concrete.

21.9

AGGREGATES

Aggregate grading used in pervious concrete is typically either single-sized coarse aggregate or grading between 19 mm and 9.5 mm. Rounded and crushed aggregates, both normal and lightweight, have been used to make pervious concrete. The aggregate used should meet requirements of IS 2836 and IS 383. Fine aggregate should not typically be used in pervious concrete mixtures because they tend to compromise the connectedness of the pore system.

21.10

CEMENTITIOUS MATERIALS

Portland cement is used as the main binder but fly ash, GGBS and silica fume can also be used in conjunction based on the strength & durability requirements.

21.11 WATER Water quality for pervious concrete is governed by the same requirements as for conventional concrete. Pervious concretes should be proportioned with a relatively low water to cement ratio in the range of 0.30 to 0.40, because an excess amount of water will lead to drainage of the paste and subsequent clogging of the pore system. The addition of water, therefore, has to be monitored closely in the field.

21.12

ADMIXTURES

Three types of admixtures are required for producing pervious concrete: • A Water-reducing admixture (high-range or medium range) is used depending on the w/c ratio. Admixtures act as a superior cement dispersant allowing water reduction and efficient cement hydration to maximize the strength of pervious concrete mixes. Strength is sometimes a challenge because pervious concrete has a high void content which negatively affects strength. • Retarding admixtures are used to stabilize and control cement hydration. Retarding admixtures are preferred when dealing with stiff mixes, such as pervious concrete, especially in hot weather applications. Retarding admixtures can act as lubricants to help discharge of concrete from a mixer and can improve handling and in-place performance characteristics.

21.6

Handbook on Advanced Concrete Technology

• A viscosity modifying agent (VMA) is also required since pervious concrete is a harsh mix, VMA adds body and helps to lubricate these mixes. The result is better flow and faster discharge time from a truck and easier placement and compaction. In addition, the use of VMA is an insurance policy to help prevent paste to drain down and significantly increases compressive and flexural strength in low compaction mixes. Typical mix proportion of pervious concrete is given in Table 21.1. TABLE 21.1 Typical mix proportioning of pervious concrete Material

Conventional Concrete

Pervious Concrete

356 1096

313 1542

764 169 0.47 0.5% ----2 – 4% ---

--99 0.31 0.5% 0.5% 0.2 – 0.3% --28%

2350 – 2400

1900 – 2000

3

Cement (kg/m ) Coarse Aggregate(kg/m3) Fine Aggregate (kg/m3) Water (l) w/c Ratio PCE based Admix (% of cement) wt) Hydration Control Admix (% of cement)wt) VMA (% of cement wt) Air Content Air Voids Unit Weight (kg/m3)

21.13 PERCOLATION RATE OF PERVIOUS CONCRETE One of the most important features of pervious concrete is its ability to percolate water through the matrix. The percolation rate of pervious concrete is directly related to the air void content. Tests have shown that a minimum air void content of approximately 15% is required to achieve significant percolation. Because the percolation rate increases as air void content increases and consequently, compressive strength decreases, the challenge in pervious concrete mixture proportioning is achieving a balance between an acceptable percolation rate and an acceptable compressive strength. The permeability of pervious concrete can be measured by a simple falling head permeameter as shown in Fig. 21.2. Using this approach, the sample is enclosed in a latex membrane to avoid the water flowing along the sides of the specimen. Water is added to the graduated cylinder to fill the specimen cell and the draining pipe. The specimen is preconditioned by allowing water to drain out through the pipe until the level in the graduated cylinder is the same as the top of the drain pipe. This minimizes any air pockets in the specimen and ensures that the specimen is completely saturated. With the valve closed, the graduated cylinder is filled with water. The valve is then opened, and the time in seconds t required for water to fall from an initial head h1 to a final head h2 is measured. The equipment is calibrated for an initial head of 290 mm and a final head of 70 mm. The coefficient of permeability k in m/s can be expressed as: k = A/t, where A = 0.084 m

Pervious Concrete

21.7

95 mm

300 mm

Graduated cylinder

Drain pipe

Top of the

10 mm

150 mm

sample

Sample

100 mm

O-Ring Valve

Fig. 21.2 Apparatus for measuring permeability of pervious concrete by a simple falling-head permeameter

21.14 PERVIOUS PAVEMENT CONSTRUCTION Subgrade Preparation and Layout A well-prepared, uniform sub-grade at the correct elevation is essential to the construction of a quality pavement. The top 150 mm of the sub-grade should be composed of granular or gravelly material with not more than a moderate amount of 10% silt or clay. The sub-grade should not be disturbed; muddy, saturated, or frozen when placement begins. The sub-grade soils should be moistened before concrete placement. Failure to provide a moist sub-base will result in a reduction in strength of the pavement and can lead to a premature pavement failure. To ensure uniform compaction, wheel ruts should be raked and compacted again before concrete placement operations. If the sub-grade soil properties require that an aggregate recharge bed be incorporated into the drainage design of the site, it should be placed on the prepared sub-grade, compacted, and trimmed to the proper elevation.

21.15

PLACING

A well-planned project layout can expedite construction operations, permit efficient use of placement equipment, and provide access for concrete delivery trucks (refer Fig. 21.3). A

21.8

Handbook on Advanced Concrete Technology

drawing showing the location of all joints and the placement sequence should be available before construction begins. Locations of fixed objects should be established with the joint pattern and construction methods in mind. The placement of pervious concrete needs to be completed as quickly as possible as the water in the mixture is very less and tends to hydrate/set fast. If a Hydration control admixture is being used then that would take care of the quality of concrete owing to the delay in placement. Any time the fresh material is allowed to sit exposed to the elements is time that it is losing water needed for curing. Drying of the cement paste can lead to a raveling failure of the pavement surface. All placement operations and equipment should be designed and selected with this in mind and scheduled for rapid placement and immediate curing of the pavement.

Fig. 21.3 Placement of pervious concrete by reat-discharge mixer truck.

Concrete should be deposited as close to its final position as practical. This is commonly done by direct discharge from the chute of the mixer truck directly onto the sub-grade. For placements where mixers cannot reach, or where the sub-grade disturbance is to be minimized, a conveyor may be used. Because pervious concrete mixtures are typically harsh (zero slump), pumping is not recommended. After depositing concrete, it should be cut to a rough elevation with a rake or similar hand tool. Pervious concrete along the forms should be compacted by hand tamp to ensure that the edges maintain structural integrity after the forms are removed and the concrete is put into service. During compaction of the concrete, the outside edge of the tamp should be kept on the form to ensure that the concrete is not compacted below the form elevation.

21.16

CONSOLIDATION

Immediately after strike off, the riser strips are removed on each form and the concrete is compacted to the form’s elevation with a weighted roller. A hand tamp may be used along the

Pervious Concrete

21.9

edges to facilitate compaction along the forms. The roller is used to compact the concrete to create a strong cement paste bond between aggregate particles and to provide an acceptable surface smoothness. The roller should be of adequate width to ride on the forms and should provide a minimum of 0.07MPa vertical force. The average roller of the size needed to span a 3.7 m lane width and weighs approximately 270 kg to 320 kg. A smaller landscape roller or custom-built rolling tool can be used in tight areas and for smaller placements; the roller should weigh approximately 90 kg to 140 kg. Landscape rollers are not recommended for larger placements due to the extended rolling time required that can lead to raveling failures. Some situations require extra effort to ensure a quality pavement. In areas where ride quality is of special concern, the pavement should be cross-rolled to smooth out any vertical deviations in surface elevation. Adjacent to sidewalks and at exposed pavement edges, the concrete should be tooled to provide a smooth corner. After strike off, compaction and edging, no other finishing operations should be performed (refer Figs. 21.4, 21.5 and 21.6).

Fig. 21.4 Low Compaction by means of hand roller with little or no ballast or a friction screed

Pervious concrete mixes are generally stiff mixes, having almost zero slump values. However, the slump test is not a proper indication for the workability of pervious concrete and should not be prescribed as in acceptance measure. The fresh density is lower than that of normal concrete and is in the range of 1900 kg/m3 to 2200 kg/m3. The fresh density reasonably correlates to the hardened void ratio. Hence, fresh density value can act as a viable quality control measure for pervious concrete.

21.17

JOINTING

Contraction joints should be installed as indicated by the plans. They should have a depth of 1/3 to 1/4 of the thickness of the pavement. Joints can be installed in the fresh concrete with

21.10

Handbook on Advanced Concrete Technology

Fig. 21.5

Fig. 21.6 Note Figs. 21.5 and 21.6 - high Compaction by means of a heavy hand roller with a ballast or filled with water, sand, etc. or a high density paver

tools or saw cut after the concrete hardens. Tooled joints, however, produce the best results. Conventional concrete jointing tools cannot be used for pervious concrete. A specially designed rolling jointer with a blade that is at least 1/4 (preferably 1/3) the thickness of the slab and with enough weight to force the blade to cleanly cut the joint may be used (refer Figure 21.7). In placements with wide lane widths, a longitudinal joint may be cut with the compacting roller. If saw cut, the procedure should begin as soon as the pavement has hardened sufficiently to prevent damage to the surface. Only enough plastic cover material to saw cut the required areas should be removed. If early entry saws with vacuum systems to collect dust are not being used, after sawing, the exposed areas should be soaked with water, which will flush the pores of the fines generated by sawing and ensure that sufficient water is present for proper curing. It is important to immediately recover the exposed area with a plastic curing material as soon as saw cuts have been made.

Pervious Concrete

21.11

Fig. 21.7 Jointing down with a roller jointer (pizza cutter)

21.18 TESTING As is done in regular pavement sub-grade testing, particle size analysis, soil classification, standard or modified proctor density test etc have to be conducted in case of pervious concrete construction. In addition, a double ring in filtro-meter test as per ASTM D 3385 or other suitable test should be performed to test soil permeability. As it is not possible to assess in-place consolidation level of pervious concrete pavement, lot of care is required to interpret the properties of pervious concrete specimens prepared in laboratory or in the field. These specimens can be considered adequate for ensuring that the5 concrete supplied meets the specifications. As per ACI Committee 522.1, the jobsite acceptance should be based on unit weight of fresh concrete, with acceptance tolerance of (+) or (–) 80 kg/m3 from the design density. Once the pavement is constructed, cores can be taken and tested for thickness and unit weight. The slump and air content tests are not applicable for pervious concrete. ASTM C 1701 gives the test method for calculating infiltration rate of in-situ pervious concrete. A low infiltration rate value on a new pervious concrete pavement indicates that the voids are sealed with cement paste during construction due to either improper mix proportion or defective construction practice followed. This test is required to be conducted at number of locations and average value is to be calculated. In case of pervious concrete, the acceptance of concrete should not be based on compressive strength, as it varies widely with amount of compaction. In case of taking cores, the pavement can disturb the cement paste matrix and lead to lower strength value. MT Bassuonl and M Sonebi reported in their research that they observed quite different trend of effect of w/c ratio on strength. It was observed that at constant quantity of coarse aggregate and cement, the increase in w/c ratio from 0.28 to 0.40, exhibited increase in compressive strength. This is contrary to the trend exhibited by normal concrete. The reason is that the increase in w/c ratio produced higher volume of paste, which was in excess of the paste required to encapsulate the aggregate. Surplus paste clogged the open pore structure, reducing the void ratio. Thus there was increase in compressive strength.

21.19

PERVIOUS CONCRETE IN INDIA

This technique can be adopted in India for construction of parking lots, footpaths, rural roads etc. In coming two decades there will be significant housing projects in India and the roads

21.12

Handbook on Advanced Concrete Technology

around the apartments and surfacing inside the compounds can be made with pervious concrete. The water demand in cities is going up day-by-day due to urban migration. Due to depletion of ground water level, the water shortage will be experienced. The pervious concrete if adopted, as mentioned above, will allow the water to percolate into the ground, which will help in increasing the ground water level. Another advantage in Indian scenario is that the pervious concrete construction is mostly manual and can be done without any heavy equipment. The labour cost in India being less, the construction cost of pervious concrete will be lower. Considering these advantages, this technique will become popular in India.

21.20

CONCLUSION

While a lot of care needs to be taken and is not an easy task to produce and successfully place pervious concrete, it is equally difficult to maintain a pervious concrete pavement and this has prevented wide acceptance of pervious concrete. Specifically, clogging of the pores prevents storm water from percolating through the concrete. It follows that if storm water is not able to drain through the pervious concrete layer, then it is no longer pervious, and the design benefit assumptions are no longer valid. If a pavement is in a harsh environment, such as a coastal area like Mumbai or anywhere that would cause heavy accumulations of fines, it may be necessary to perform this through preventive maintenance more frequently.

References 1. Debo, T. N., and Reese, A. J., (2002) Municipal Storm Water Management, 2nd Edition, CRC Press, 976pp. 2. Ferguson, B., (1994) Storm water Infiltration, CRC, 288 pp. 3. Ferguson, B., (1998) Introduction to Storm water: Concept, Purpose, Design, Wiley, 272 pp. 4. The journal of design and build institute of America – Oct 2006, pp 13–14. 5. Medico, J. J., Jr., (1975) “Porous Pavement,” U.S. Patent No. 3870422. 6. Meininger, R. C., (1988) “No-Fines Pervious Concrete for Paving,” Concrete International, V. 10, Aug., pp. 20-27. 7. Mark A. Bury, Christine A. Mawby, Dale Fisher, Making Pervious Concrete Placement Easy using a Novel Admixture System – Concrete in Focus article – 2006. 8. “Thickness Design Procedure for Concrete Highway and Street Pavements”,- Proceedings of Fifth International Conference of Concrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, Ind.: 1993. 9. ACI 522 – R – Pervious Concrete: 2010. 10. Karthik H. Obla – Pervious concrete – An Overview – The Indian Concrete Journal – pp 9-18: 2010. 11. M. T. Bassuonl and M. Sonebi (2010) “Pervious Concrete”: A sustainable drainage solution, Concrete, pp 14-16.

22 Lightweight Concrete Himanshu Kapadia, Amit Datta and S.G. Bapat

22.1 INTRODUCTION Lightweight concrete can be defined as a type of concrete which includes an expanding agent, which increases the volume of the mixture while giving additional qualities such as nail-ability and lessens the dead weight. It is lighter than the conventional concrete. The use of lightweight concrete has been widely spread across various types of constructions. The main characteristics of lightweight concrete are its low density and low thermal conductivity. Its advantages are that there is a reduction of dead load, faster building rates in construction and lower haulage and handling costs. Lightweight concrete maintains its large voids and does not form laitance layers or cement films when placed. However, proper water cement ratio is vital to produce adequate cohesion between cement and water. Insufficient water can cause lack of cohesion between particles, thus loss in strength of concrete. Likewise, too much water can cause cement to run off aggregate to form laitance layers, subsequently weakening in strength. It is lighter than the conventional concrete with a dry density from 300 kg/m3 up to 1840 kg/m3, i.e. 87% to 23% lighter. It was first introduced by the Romans in the second century where ‘The Pantheon’ has been constructed using pumice, the most common type of aggregate used during that particular period. From there on, the use of lightweight concrete has been widely spread across other countries such as USA, United Kingdom and Sweden. The building of ‘The Pantheon’ of lightweight concrete material is still standing eminently in Rome until now for about 18 centuries (Fig. 22.1). It shows that the lighter materials can be used in concrete construction and has an economical advantage.

22.2 TYPES OF LIGHTWEIGHT CONCRETE Lightweight concrete can be prepared either by injecting air/gases in its composition or it can be achieved by omitting the finer sizes of the aggregate or even replacing them by a hollow, cellular or porous aggregate. Particularly, lightweight concrete can be categorized into three groups:

22.2

Handbook on Advanced Concrete Technology

Fig. 22.1 The Pantheon in Rome

• No-fines concrete • Lightweight aggregate concrete • Aerated/Foamed concrete

22.3

NO-FINES CONCRETE

No-fines concrete can be defined as a lightweight concrete composed of cement and coarse aggregate. Uniformly distributed voids are formed throughout its mass. The main characteristics of this type of lightweight concrete is that it maintains its large voids without forming laitance layers or cement film when placed. Figure 22.2 shows one example of No-fines concrete.

Fig. 22.2 No fines concrete

No-fines concrete is usually used for both load bearing and non-load bearing for external walls and partitions. With increase in cement content, the strength of no-fines concrete increases. However, it is sensitive to the water composition. Insufficient water can cause lack of cohesion between the particles and therefore, subsequent reduction of strength of the concrete. Likewise, too much water can cause cement film to run off the aggregate to form laitance layers, leaving the bulk of the concrete deficient in cement and thus weakens the strength.

Lightweight Concrete

22.4

22.3

LIGHTWEIGHT AGGREGATE CONCRETE

Porous lightweight aggregate of low specific gravity is used in this lightweight concrete instead of conventional aggregate. The lightweight aggregate can be natural aggregate such as pumice, scoria and all of those of volcanic origin and the artificial aggregate such as expanded blast-furnace slag, vermiculite and clinker aggregate. The main characteristic of this lightweight aggregate is its high porosity, which results in a low specific gravity. The lightweight aggregate concrete can be divided into two types according to its application. One is partially compacted lightweight aggregate concrete and the other is the structural lightweight aggregate concrete. The partially compacted lightweight aggregate concrete is mainly used for two purposes viz. for precast concrete blocks or panels and for cast in-situ roofs and walls. The main requirement for this type of concrete is that it should have adequate strength and a low density to obtain the best thermal insulation and a low drying shrinkage to avoid cracking. Structural lightweight aggregate concrete is fully compacted, similar to that of the normal reinforced concrete of conventional aggregate. It can be used with steel reinforcement and has a good bond between the steel and the concrete. The concrete should provide adequate protection against the corrosion of the steel. The shape and the texture of the aggregate particles and the coarse nature of the fine aggregate tend to produce harsh concrete mixes.

22.5

AERATED CONCRETE

Aerated concrete does not contain coarse aggregate, and can be regarded as an aerated mortar. Typically, aerated concrete is made by introducing air or other gas into a cement slurry and fine sand. In commercial practice, the sand is replaced by pulverized fuel ash or other siliceous material and lime maybe used along with cement. There are two methods to produce the aerated concrete. The first method is to inject the gas into the mixture during its plastic condition by means of a chemical reaction. The second method is to introduce air, either by mixing-in stable foam or by whipping-in air, using an air-entraining agent. The first method is usually used in precast concrete factories where the precast units are subsequently autoclaved in order to produce concrete with a reasonably high strength and low drying shrinkage. The second method is mainly used for in-situ concrete, suitable for insulation in roof screeds or pipe lagging. Figure 22.3 shows the aerated concrete. Advantages of use of light weight concrete are: • Rapid and relatively simple construction. • Economical in terms of transportation as well as reduction in manpower. • Significant reduction of overall weight results in saving structural frames, footing or piles. • Most of lightweight concrete have better nailing and sawing properties than heavier and stronger conventional concrete. • Better thermal and sound insulation compared to conventional concrete.

22.4

Handbook on Advanced Concrete Technology

Fig. 22.3 Aerated concrete

22.6 TESTING OF LIGHTWEIGHT CONCRETE Compressive Strength Compressive strength is the primary physical property of concrete and is the one most used in design. It is one of the fundamental properties used for quality control of light weight concrete. Compressive strength may be defined as the measured maximum resistance of a concrete specimen to axial loading. It is found by measuring the highest compression stress that a test cylinder or cube will support. The required compressive strength of lightweight concrete is 3.45 MPa at 28 days as a non-load bearing wall. The compressive strengths obtained from these mixtures carried out are higher than 3.45 MPa and therefore it is acceptable to be produced as non-load bearing structure. However, for structural concrete the strength specified in the design has to be met with.

22.7 WATER ABSORPTION These properties are particularly important in concrete, as well as being important for durability. It can be used to predict concrete durability to resist corrosion. Absorption capacity is a measure of the porosity of an aggregates; it is also used as a correlation factor in determination of free moisture by oven-drying method. The absorption capacity is determined by finding the weight of surface-dry sample after it has been soaked for 24 hr in water and again finding the weight after the sample has dried in an oven; the difference in weight, expressed as a percentage of the dry sample weight, is the absorption capacity.

Lightweight Concrete

22.5

Absorption capacity can be determined using BS absorption test. The test is intended as a durability quality control check and the specified age is 28-32 days (S.G. Millard). Test procedure has been given in BS 1881: Part 122. Density of Light weight concrete varies from 400 kg per cum to 1800 kg per cum depending upon aggregates as well as amount of foam.

22.8

APPLICATION OF LIGHTWEIGHT CONCRETE

Lightweight concrete has been used since last eighteen centuries by the Romans. The application on the ‘The Pantheon’, where it used pumice aggregate in the construction of cast in-situ concrete is the proof of its usage. In USA and England, in the late nineteenth century, clinker was used in the construction. The examples are the ‘British Museum’ and other low cost housing. The lightweight concrete was also used in construction during the First World War. The United States used mainly for shipbuilding and for manufacturing of concrete blocks. The foamed blast furnace-slag and pumice aggregate were introduced in England and Sweden for block making around 1930s. Nowadays with the advancement of technology, lightweight concrete has expanded its uses. For example, considering its outstanding insulating characteristics, it is widely used as loose-fill insulation in masonry construction. It enhances fire ratings, reduces noise transmission and does not rot and resists termites. It is also used for vessels, roof decks, roof insulation and other applications. The lightweight concrete has been successfully used for the superstructure of bridge over the river Mettaponi in Virginia State and the new Benicia-Martinez Bridge across the Carquinez Strait in California. Both are long span bridges. One has to be very careful in selecting the aggregate and proportioning the mix for light weight structural concrete. The precautions required to be taken during mix proportioning and production are listed below: • From each test mix proportioning, two samples are required to be tested, and the results are to be used to determine values for the modulus of elasticity, creep, and shrinkage. • During production, five samples are to be tested to determine the modulus of elasticity, creep, shrinkage, and density. • During production, one compressive strength cube/cylinder sample for each concrete type is required to be cast for day’s placement up to a maximum of 50 m3. For each compressive strength cylinder sample, two cylinders are required for the permeability test. • During production, air content and temperature are required to be monitored. Both these bridges are pre-stressed concrete bridges, one of which is segmental construction. Careful selection of aggregate, systematic mix proportioning, proper quality assurance and quality control measures etc. will lead to successful application of light weight concrete for structural work. The main advantage will be reduction in dead weight. The overall design will thus be economical.

22.6

Handbook on Advanced Concrete Technology

References 1. Mat Lazim Zakaria,(1978). Bahan dan Binaan, Dewan Bahasa dan Pustaka. 2. Mohd Roji Samidi,(1997). First report research project on lightweight concrete, University Teknologi Malaysia, Skudai, Johor Bahru. www.pearliteconcreteforrorepair.com, 1995. 3. Formed Lightweight Concrete. www.pearliteconcreteforrorepair.com 4. Shan Somayuji (1995), Civil Engineering Materials, N.J Prentice. 5. Norizal, Production of Foamed Concrete. USM. www.hsp.usm.my/Norizal/hbp.htm, 2010. 6. A.M. Neville (1985), Properties of concrete, Pitman. 7. Liew Chung Meng, Introduction to Lightweight Concrete. www.maxpages.com., 2010. 8. Cellular Lightweight Concrete, Plan City/NCS LLC. www. Neoporsystem.com, 2010. 9. Flying Concrete-Introduction to Lightweight Concrete, by US Department of Interior Bureau of Reclamation. www.geocities.com, 2010. 10. Application on Litebuilt @ Aerated and Composite Concrete by PTY LTD., 2010. 11. J.H. Bungey and S.G. Millard, “Testing Concrete in Structures”, (3rd Edn), Chapman and Hall, 1996. 12. Bomhard, Light Weight Concrete Structures, Potentialities, Limits and Realities, in Lightweight Concrete. Concrete Society, The Construction Press Ltd, Lancaster, England, 1980, pp 277-307. 13. SP-23 (S and T). Handbook on Concrete Mixes (Based on Indian Standards) Bureau of Indian Standards, 1982.

23 Fibre Reinforced Concrete Himanshu Kapadia, Ganesh Chaudhari and S.G. Bapat

23.1 INTRODUCTION OF FIBRE REINFORCED CONCRETE Fibre reinforced concrete is a type of concrete that includes fibrous substance that increases its structural strength, ductility and durability. Fibre reinforced concrete has small distinct Fibres that are homogeneously dispersed and oriented randomly in order to work as three dimensional reinforcements. Commonly used Fibres are steel Fibres, and synthetic Fibres (Fig. 23.1) Glass Fibres, Aramid Fibres and other types of natural Fibres are still in predevelopment stage and a consistent commercial acceptability as compared to Steel and Synthetic is yet to be established. The characteristics of Fibre reinforced concrete are changed by the alteration of quantities of concretes, Fibre substances, geometric configuration especially Length, Diameter and type of anchorage, dispersion, direction and concentration. Portland cement concrete is believed to be a comparatively brittle substance. When un-reinforced concrete is exposed to tensile stresses, it is likely to fracture and fail. The failure mode is moreover very brittle. After the reinforcement of concrete by steel; it becomes a composite group in which the steel endures the tensile stresses. When concrete is reinforced by using Fibre in the mixture, it further increases the tensile strength of the composite system. Research has revealed that the ductility of concrete can be improved tremendously by the addition of Fibre reinforcement and after cracking of concrete there is a post crack tensile capacity available which is not in case of un-reinforced concrete. Since the stretching ability under load of reinforcing Fibre is greater than concrete, initially the composite system will function as un-reinforced concrete. However, with additional loading the Fibre reinforcement will be activated, to hold the concrete mix together. The characteristics of concrete depend upon the kind of Fibre utilized, volume proportion of the Fibre, and the ratio of length and diameter of the Fibre. These conditions will improve the mechanical properties, including toughness, ductility, post crack tensile strength, and shear resistance and post crack loading limit of the Fibre reinforced concrete. Steel fibres are generally used since they can be taken into consideration also for the load bearing capacity in the cracked state of concrete, plastic Fibres have recently been introduced in the field of

23.2

Handbook on Advanced Concrete Technology

Fig. 23.1

Types of fibres

reinforcement, mainly to prevent the plastic shrinkage or reinforcing the concrete in fresh state. It is considered that the contribution of plastic Fibres to increase the static strength of concrete is limited. Nylons have the characteristics of a plastic material, and presently have a limited application in the slab technology. It is generally believed that nylon Fibres possess strength, which is greater than the welded wire fabric in such slabs. For designs in the serviceability limit state and the ultimate limit state, it is essential to define the performance of the fibre reinforced concrete. The performance of a fibre reinforced concrete is mainly described in terms of its material property. Whereas the compression strength is equally determined as for traditional reinforced concrete, the determination of the post crack flexural strength and the post crack tensile strength has to be derived by special tests such as beam test or panel tests In suspended slabs, bolts are anchored/driven in concrete to support the loads at soffit. Tensile strength of un-reinforced concrete is relied upon to transmit and sustain the load. The concrete in between reinforcing bars is plain and un-reinforced. It has a limited capacity to transfer the tensile force. When fibre reinforced concrete is used in slab construction, the fibres are aligned in the direction of tensile force. This enhances the strength of the concrete in direct tension.

Fibre Reinforced Concrete

23.3

This chapter covers Steel and Synthetic fibres in details, viz. properties, performance, application areas, dosing, mixing, workability and quality control.

23.2 STEEL FIBRE REINFORCED CONCRETE (SFRC) A variety of fibre materials, other than steel, glass, or natural fibres have been developed for use by the construction industry for fibre reinforced concrete. These fibres are categorized as synthetic fibres for use in synthetic fibre reinforced concrete, SNFRC for identification. Concrete is well known for its beneficial properties. The workability as fresh concrete is given and as hardened concrete it shows very strong compressive strength. The weak tensile strength will be compensated by providing reinforcement. In the past decades steel fibre reinforced concrete has evolved as a widely used alternative to improve tensile properties and toughness of plain or conventionally reinforced concrete. Tremendous research has been done and many projects of almost any size have been executed in various countries including India. Steel fibres are well known for their ability to transfer stresses at very small crack widths, so that ductility and post crack strength will be provided to the concrete. Hence Steel fibre reinforced concrete is widely used for applications like industrial floors, foundation slabs, tunnelling or pre-cast elements. Relating to these applications, SFRC is supposed to be a well established building material and a meaningful alternative to plain or reinforced concrete. Unlike traditional steel reinforcement, steel fibres are a discontinuous and 3-dimensionally orientated reinforcement, once it is mixed into the concrete. A variety of fibre types are available. They can be made from different materials. They are having different shapes and different sizes. However, their effect on concrete properties varies to the great extent. Steel fibre reinforced concrete is defined as a concrete, containing discontinuous discrete steel fibres, when incorporated improves its Crack resistance, Ductility, Energy absorption and Impact resistance characteristics and offers a long term post crack tensile strength. As a result of this, SFRC has ability to arrest cracks. Steel Fibre Reinforced Concrete (SFRC) possesses increased extensibility and tensile strength, both at first crack and at ultimate. Particularly under flexural loading, fibres are able to hold the matrix together even after extensive cracking. Steel fibres, transform a brittle concrete into a ductile material. As the concrete cracks they transfer the tension across the crack and guarantees a post crack load bearing capacity (See Fig. 23.2). Secondary crack

Friction Compression

Secondary crack

Fig. 23.2 Transfer of tension across the crack (redistribution of stresses)

23.4

Handbook on Advanced Concrete Technology

Therefore steel fibre reinforced concrete shall never be simplified as a “concrete with steel fibres”. SFRC has to be seen as a material which is composed from an appropriate concrete composition, a suitable fibre type and the corresponding dosage to meet the given requirements.

23.3 PROPERTIES OF DIFFERENT STEEL FIBRE TYPES In general steel fibres are divided into 5 classes which are described below. • group I : Cold-drawn wire • group II : Cut sheet • group III : melt extracted • group IV : shaved cold drawn wire • group V : milled from blocks However most effective and common fibre type is to be found in group I: cold drawn wire. Common fibre types can be seen in Fig. 23.3.

Milled form blocks

Straight fibres

Glued and hooked end fibres

End hooked

Fig. 23.3 Common fibre types

Aspect Ratio Aspect ratio is length /Diameter. As length of the fibre increases, it increases the length covered by fibre in the concrete. As the diameter of the fibre reduces, number of fibres per given weight increases, thereby increasing network of wire in the meter cube of concrete. Refer following typical data, which compares three different types of fibres of a particular manufacturer based on Aspect ratio.

Fibre Reinforced Concrete

23.5

RL-45/50-BN – l/d = 45 L = 147 m/kg RC-65/60-BN – l/d = 65 L = 200 m/kg RC-80/60-BN – l/d = 80 L = 276 m/kg The properties will differ from manufacturer to manufacturer. In all the above three types all fibres have same metallurgical properties. But 1 kg of fibres gives different length per kg, as we can see from the data. Aspect ratio is changing from 45 to 80 which is helping network per kg going from 147 m to 276 m. We can approximately say that 2 kg of RL 45/50 (45 Aspect Ratio) will give Equivalent performance of 1 kg of RC 80/60 BN (80 Aspect Ratio). Like with any kind of reinforcement, it is important to provide as much reinforcement in a section as needed. But comparing a fibre dosage with another fibre dosage would not lead to the right conclusions as it depends on number of variables. The reason is that different factors are influencing the fibre performance in a meaningful way. The main influencing factors are: • • • •

The The The The

shape (straight, hooked, undulated, crimped, Twisted, coned) length (12.7 mm to 63.5 mm) diameter (0.4 mm to 1.05mm) tensile strength (1000 N/mm² – 2500 N/mm²)

The usual amount of steel Fibres ranges from 10 kg/m³ of concrete for higher aspect ratios (80), to 80 kg/m³ for lower aspect ratio (50). The dosages of fibres are in general inversely proportional to Aspect ratio. Higher the aspect ratios lower the dosage, lower the aspect ratio higher the dosage (Fig. 23.4). Dosage also varies as per application and loadings. 16 14

80

Performance 12 of the steel 10 fibre 8 reinforced concrete 6

65 45 < 45

4 Cut wires

2 0 10

20

Fig. 23.4

30

40

50

3

Fibre dosage [kg/m ]

In case of the same type of anchorage, especially the length and the diameter are having the biggest influence on the final fibre performance. It can be stated that fibre performance increases by increasing the fibre length and decreasing the diameter. Higher the number of fibres more is the anchorage length available.

23.6

Handbook on Advanced Concrete Technology

Glued fibres are specially developed to enable a homogenous fibre distribution in concrete, especially for high performing fibres where a huge amount of fibres for each kg is given. The risk of fibre balling will be avoided effectively by using glued fibre types. The following criteria are the decisive points to finally reach high post crack strength. • • • • • •

hooked ends as thin as possible as long as possible high slenderness adapted tensile strength to the concrete strength optimized concrete recipe

The higher the amount of fibres and the longer the fibre, the bigger is the possibility that a fibre contains a crack. Table 23.1 gives an arithmetic model for comparing the performance of SFRC Slab on grade using various fibre geometries, for slab on grade with 3 t to 7 t UDL on a sub base of 5 % CBR TABLE 23.1 Sr. No.

Aspect Ratio (L/D) Length/Diameter

Type

Length mm

Diameter mm

Length per kg M

Dosage per m3 kg

1 2 3

48 67 80

Loose Glued Glued

50 60 60

1.05 0.9 0.75

147 200 288

31.5 20 15

• The dosage of fibres varies invariably as per Aspect Ratio, Tensile strength, Anchorage. These are case specific results and presented for understanding of concept only.

Practicality of Steel Fibre Reinforced Concrete Several procedures for proportioning SFRC mixes are available, which emphasize the workability of the resulting mix. However, there are some considerations that are particular to SFRC. In general, SFRC mixes contain higher ratios of fine to coarse aggregate than in ordinary concretes, and so the mix proportioning procedures that apply to conventional concrete may not be entirely applicable to SFRC. Commonly, to reduce the quantity of cement, up to 35% of the cement may be replaced by fly ash. In addition, to improve the workability of higher fibre volume mixes, water reducing admixtures and, in particular, super plasticizers are often used, in conjunction with air entrainment. The range of proportions for SFRC is given in Table 23.2. For steel fibre reinforced Shot-Crete, different considerations apply, with most mix proportions being arrived at empirically. Typical mix proportion for steel fibre Shot-Crete is given in Table 23.3.

Fibre Reinforced Concrete

23.7

TABLE 23.2 Range of proportions for normal weight fibre reinforced Concrete Ingredients Cement (kg/m3) w/c ratio Fine/coarse aggregate (%) Entrain air (%) Fibre content kg/m3 *

10 mm MSA

20 mm MSA

355-590 0.35-0.45 45-60 4-7 10-35

300-535 0.4-0.5 45-55 4-6 10-35

* These are general mix proportions as such there is no direct relation with aggregate size and dosage of fibres. Dosage of fibres clearly relates to Aspect ratio, anchorage and Tensile strength

TABLE 23.3 Typical steel fibre reinforced Shot-Crete mixes* Ingredients

10 mm MSA (kg/m3)

Cement Blended sand (< 6.35 mm) 9.5mm aggregate Steel fibres Accelerator w/c ratio

445 697-880 700-875 25-70 Varies 0.40-0.45

* This is general mix proportion as such there is no direct relation with aggregate size and dosage of fibres. Dosage of fibres clearly relate to Aspect ratio, Anchorage and Tensile strength.

Batching, Mixing and Producing SFRC One of the basic concerns in SFRC is to introduce sufficient volume of fibres to be uniformly dispersed (Fig. 23.5) to achieve the desired improvements in mechanical behaviour, while retaining sufficient workability in the fresh mix to permit proper mixing, placing and finishing. The performance of the hardened concrete is enhanced more by fibres with a higher aspect ratio, since this improves the fibre-matrix bond. On the other hand, a non collated/unglued high aspect ratio adversely affects the workability of the fresh mix. In general, the problems of both workability and uniform distribution increase with increasing fibre length and volume. One of the main difficulties in obtaining a uniform fibre distribution is the tendency of loose steel fibres to ball or clump together. Clumping/Balling may be caused by a number of factors: • The loose fibres may already be clumped together before they are added to the mix and normal mixing action will not break down these clumps. • Fibres may be added too quickly to allow them to disperse in the mixer. • Too high a volume of fibres added. • The mixer itself may be too much worn out or inefficient to disperse the fibres • Introducing the fibres to the mixer before the other concrete ingredients In view of this, care must be taken in the mixing procedures. Most commonly, when using a transit mixer or revolving drum mixer, the fibres should be added last to the wet concrete. The concrete alone, typically, should have a slump of 15-25 mm greater than the desired slump of the SFRC or 50 mm to 60 mm in case of SFRS. The use of collated/glued fibres held together

23.8

Handbook on Advanced Concrete Technology

Fig 23.5

Homogeneous dispersion of fibres in x ray and on project

by a water-soluble component which dissolves during mixing largely eliminates the problem of clumping/balling. SFRC initially appears to be very stiff because the fibres tend to inhibit flow; however use of vibrators or high-range water-reducing admixtures (HRWR) allows easy placing of such seemingly unworkable concrete. Pavement quality concrete (PQC) and some special structural applications may benefit from less workable, but much higher quality concrete with the water-cement ratios in the range of 0.35 to 0.43. At the upper end of the water-cement spectrum, tests have shown that further addition of water causes an increase in slump without a change in workability under vibration. This water addition reduces the quality of the mixture without improving the placeability and it can give rise to excessive bleeding and segregation. Additional water may increase the slump of the SFRC without increasing its workability and placeability under vibration. The finishing operations with SFRC are essentially the same as for ordinary concrete, though perhaps more care must be taken regarding workmanship. A good Fibre mixture generally has proportions of sand and admixtures which make it well-suited for pumping. Gradations suited to SFRC are also compatible with pumping. Although a mixture may appear stiff and unworkable, it may pump surprisingly well. Because of its composition, an SFRC mixture will move through the line without slugs and has been reported to pump more easily and with less trouble than conventional concrete. Some important points about pumping SFRC are: • Use a pump capable of handling the volume and pressures. • Use of fibres with given length limitations. (Lmin > 2 × Max. Size Aggregate (25 mm to 28 mm) & Lmax < 2/3rd Inner dia of pipe 100 mm to 110 mm) (Fig. 23.6) • Avoid flexible hose if possible • Provide a screen over the pump hopper to prevent any Fibre balls from entering the line. 50 mm × 75 mm mesh is usually adequate. • Do not try to pump a fibrous mix that is too wet. Pump pressures can cause the fluid paste and fine mortar to squeeze out ahead of the rest of the mixture, resulting in a mat of Fibres and coarse aggregate without mortar. It must be noted that this is the result of a mixture that is too wet, not too dry. The similar type of plugging can occur with conventional concrete, with the plug consisting of coarse aggregate devoid of paste and fine mortar.

Fibre Reinforced Concrete

Concrete pipe hose inner diameter (100mm)

Fig. 23.6

23.9

Fibre length should be < 66 mm

Minimum and maximum length of fibre

In addition to selection of appropriate fibres it is very much necessary to have consistent concrete with continuous gradation. What fibres need is concrete with enough paste around the aggregates. Following graph (Fig. 23.7) and Table 23.4 gives tentative guidelines for sieve curve for various aspect ratio and dosages. However these are just guidelines and it is advisable to create an effective sieve curve based on project experience. Sieve curves - 16 mm (DIN 1045) 120 100

%

80 60 40 20 0 0

0,125

0,25

0,5 1 2 Sieve diameter (mm)

4

Fig. 23.7 Sieve curve

8

16

C 16 B 16 A 16 AA BB CC

23.10

Handbook on Advanced Concrete Technology

TABLE 23.4 Aspect Ratio 80 65 45

Dosage 40 CC CC BB

One researcher studied structural behaviour of steel fibre reinforced concrete wall panels in two-way in-plain action. M-30 grade concrete was used with cement content of 380 kg of fly ash based PPC. The dosage of steel fibre having aspect ratio 60 was 0.50%. 16 wall panels with normal concrete and 16 wall panels with steel fibre were cast. The panels were subjected to uniformly distributed vertical load applied at small eccentricity of t/6 to simulate a typical wall under load. Steel fibres of 0.38 mm dia meter and aspect ratio 60 were used. The finding of the study is that the SFRC wall panels exhibit softening behaviour and fell in a more ductile manner than the normal concrete wall panels. Thus, the SFRC wall panels can be considered advantageously in high rise building construction, which will lead to reduction in mask. The structural response in seismic condition will improve. Another researcher studied compression and tension behaviour of Slurry Infiltrated Fibre Concrete (SIFCON) produced with low tensile strength steel fibres. 42 cubes size 150 × 150 × 150 mm and 21 cylinders of 150 × 300 mm were cast and tested. Out of these specimens, 3 cubes and 6 cylinders were cast as control specimens. The test results indicated that the SIFCON specimens produced with low tensile strength steel fibres showed better performance under both compression and tension. The grade of concrete was M-25 and the results indicated that split tensile strength for 6% to 12% fibre volume and 40-60 aspect ratio is 120% to 246% over control mix. Similarly, the compressive strength goes up about 10% to 26% over control mix. This aspect is worth studying in details for steel fibre as well as for PP fibres.

Application Area for Steel Fibre Reinforced Concrete Generally, when used in structural applications, steel Fibre reinforced concrete should only be used in a supplementary role to inhibit cracking, to improve resistance to impact or dynamic loading, and to resist material disintegration. In structural members where flexural tensile or axial tensile stresses will occur, such as in beams, columns, suspended slabs (i.e., not slabs on grade), the reinforcing steel must be capable of resisting the tensile stresses. The following are some examples of general structural and non-structural uses of SFRC: 1. Hydraulic structures – Dams, spillways, stilling basins, and sluiceways as new or replacement slabs or overlays to resist cavitation damage 2. Airport and highway paving and overlays – Particularly where a thinner-than-normal slab is desired 3. Industrial floors – For impact resistance and resistance to thermal shock 4. Refractory concrete – Using high-alumina cement in both cast-able and shotcrete applications

Fibre Reinforced Concrete

23.11

5. Foundation slabs for residential buildings 6. Bridge decks – As an overlay or topping where the primary structural support is provided by an underlying reinforced concrete deck 7. In shotcrete linings – For underground support in tunnels and mines, usually with rock bolts 8. In shotcrete coverings – To stabilize aboveground rock or soil slopes, e.g., highway and railway cuts, and embankments 9. Thin shell structures – Shotcreted “foam domes” 10. Explosion-resistant structures – Usually in combination with reinforcing bars 11. A possible future use in seismic-resistant structures Steel fibre reinforced concrete is mainly used in industrial floors. Tunnelling is also a wide application area for SFRS. For example shot Crete as temporary lining or pre-cast segmental linings. In case of housing SFRS is used for foundation slabs and for strip foundations. Steel fibre reinforced industrial floors have become a state of the art in many countries around the world. Relevant design methods assume specific models for the interaction between floor slab and sub base. In most countries, industrial floors are considered as “minor structures“, at least from a structural point of view. However, this may not lead to disrespect to proper designing and planning of the floor. In principle, two different types of execution of floors are distinguished: saw cut floor and “joint less” floors. Saw cuts are introduced at short spacing to enforce a controlled cracking of the slab. Restraint stresses will be eliminated by that. Typically joint spacing is located between 4 m and 10 m. Mostly this will be determined in dependency of the slab thickness and the arrangement of the columns of the main structure. Spalling and subsequent maintenance of saw cut joints (due to load repetitions of heavy vehicles) can be prevented by execution of a so called “joint less” floor. In this case no saw cuts are foreseen but special joint profiles are introduced at a distance of around 25 m to 40 m. In addition to giving special attention in regards to designing, detailing and execution, a role of experienced flooring contractor is also an important factor.

Fig. 23.8

Example of a joint less floor for warehouse

23.12

Handbook on Advanced Concrete Technology

Fig. 23.9

Example of a saw cut floor for a Port project

No doubts that SFRC will be applied in the near future also for new and more challenging tasks. Especially in the combination with traditional reinforcement, this building material shows a lot of advantages and further application areas. Crack width design, which means a calculated limitation of the maximum crack, is just one aspect to be mentioned here. The following description of one beautiful construction executed in combined reinforcement should state this in one enclosed example. Steel fibres are also used to reinforce high strength concrete. The aim is to transfer this so very brittle material “high strength concrete” into a material with a ductile behaviour. Only a ductile material is allowed to be designed as load bearing element.

Shot Crete Shell Combined reinforcement was used in this example for a thin shell structure (Fig. 23.10) in the oceanographic park Valencia (Spain). Due to the curvature and the limited shell thickness of 6 cm to 12 cm, it would have been very difficult to install a complicated traditional reinforcement in an accurate and safe way. A maximum diameter of 8mm rebar was preferred due to the demanded concrete cover. Hence the traditional reinforcement had to be placed close to the neutral axis of the section. Whereas the post- crack strength of the steel fibre reinforced concrete was also considered for the ultimate limit state. Concrete was applied by Shot-Crete technology. With the chosen combination, both serviceability and ultimate limit state criteria could be met with.

Technical and Economical Advantages of SFRC Steel fibre reinforced concrete has proven over the years to be a reliable construction material. Steel fibres are well known for their ability to transfer stresses already at very small crack openings, so that ductility and post crack strength will be provided to the concrete. Hence, Steel fibre reinforced concrete is widely used for applications like industrial floors, foundation slabs, tunnelling or pre-cast elements. Relating to these applications, SFRC is supposed to be a well established building material and a meaningful alternative to plain or reinforced concrete. Meanwhile a number of guidelines and recommendations are available to design SFRC.

Fibre Reinforced Concrete

23.13

Fig. 23.10 The final shell: one of the most beautiful examples of combined reinforcement

Reduced construction time, simplified reinforcement drawings, no stockyard, enhanced job safety and increased durability and ductility are only some main benefits of SFRC, which are mentioned in that context. At the same time it needs some special knowledge to understand, design and execute this special building material.

23.4 SYNTHETIC FIBRE REINFORCED CONCRETE (SYNFRC) Synthetic fibres are man-made fibres resulting from research and development in the petrochemical and textile industries. SNFRC utilizes fibres derived from organic polymers which are available in a variety of formulations. Fibre types that have been tried in Portland cement concrete based matrices are: acrylic, aramid, carbon, nylon, polyester, polyethylene and polypropylene. For many of these fibres, there is little reported research or field experience, while others are found in commercial applications and has been the subject of extensive reporting. Synthetic fibres are said to be melted when the crystalline portions of the polymers that they are made of are converted on heating from a solid to a glassy or liquid state. The temperature at which this physical change occurs is called the melting point. If on heating a fibre decomposes before it melts, it is because one of many possible chemical reactions occur at a lower temperature before reaching the melting point. A typical type of decomposition is oxidation. Oxidation is caused by the chemical reaction of the fibre with the oxygen in the air. The temperature at which decomposition occurs is called the decomposition temperature. Decomposition is usually noticed because the fibre quickly changes colour, fumes or undergoes an obvious chemical change. Around the millennium, suppliers started to offer macro synthetic fibres. Some applications for mining and Pre-cast elements were executed with this new construction material. Macro synthetic fibres, sometimes, have dimensions comparable to steel fibres but nevertheless have very different material properties. Plastic fibres are available in very wide range in terms of their material properties like shape, diameter, length, anchorage, tensile strength and Young’s modulus.

23.14

Handbook on Advanced Concrete Technology

In general Young’s modulus is lower than that of a hardened concrete, which limits the use of such fibres for permanent structures as long as higher deformations are acceptable. Furthermore, the long term behaviour shows a high trend to creep behaviour. Macro synthetic fibres start loosing their mechanical properties at 50°C, which has also to be taken into consideration when fire resistance is a desired requirement. There are some definite applications, where a plastic fibre supersedes other materials in terms of providing benefits to the concrete. These items are mentioned in subsequent paragraphs.

23.5 TYPES OF SYNTHETIC FIBRES Acrylic Acrylic fibres contain at least 85 percent by weight of acrylonitrile units. Generally, acrylic fibres used in the textile industry have a tensile strength ranging from 207 MPa to 345 MPa. However, special high tenacity acrylic fibres have been developed to replace asbestos fibre in many fibre reinforced concrete products. These fibres have tensile strengths of up to 1000 MPa.

Aramid Aramid (aromatic polyamide) is a high-modulus, manmade polymeric material that was first discovered in 1965. Attempts to incorporate Aramid fibre into concrete as a form of reinforcement began by the late 1970s. It has been concluded that the mechanical properties of a cement matrix reinforced with aramid fibres are sufficiently attractive to warrant further studies. However, the high cost of aramid fibres has been a limitation to commercial acceptance. Aramid fibres have the following properties: • They are two and a half times as strong as E-glass fibre and five times as strong as steel fibres per unit weight. • The strength of aramid fibre is unaffected up to 160° C and exhibit dimensional stability up to 200° C and is creep resistant • Aramid strand with different numbers of fibres of varying diameter is also available.

Carbon Carbon fibres were developed primarily for their high strength and stiffness properties for applications within the aerospace industry. Compared with most other synthetic fibre types, carbon fibres are expensive and, as previously mentioned with aramid fibres, this has limited commercial development. However, laboratory research has continued to determine the physical properties of carbon fibre reinforced concrete (CFRC). Properties of CFRC: • They have high tensile strength and elastic modulus • They are also inert to most chemicals

Fibre Reinforced Concrete

23.15

• They are manufactured as either HM (high modulus) fibres or HT (high-tensile strength) fibres and are dependent upon material source and extent of hot-stretching for their physical properties. • They are available in a variety of forms. It has been shown that carbon fibres can be made from petroleum and coal pitch, which is less expensive than the poly-acrylonitrile fibre used to make PAN based carbon fibre.

Nylon Nylon is a generic name that identifies a family of polymers characterized by the presence of the amide functional group—CONH. Various types of nylon fibres exist in the marketplace for use in apparel, home furnishings, industrial, and textile applications. A nylon fibre’s properties are imparted by the base polymer type (molecular weight, end groups, residual monomer, etc.), addition of different levels of additives (light and heat stabilizers, delusterants, etc.), manufacturing conditions (spinning, drawing, texturing, etc.), and fibre dimensions (cross-sectional shape and area, fibre length, etc.). Currently, only two types of nylon fibre are marketed for fibre reinforced concrete. They are nylon 6 and nylon 66. Nylon fibres are spun from nylon polymer. The polymer is transformed through extrusion, stretching, and heating to form an oriented, crystalline, fibre structure. In addition to conventional yarns produced by standard drawing, nylon fibre properties may be enhanced by special treatments including over finishing, heating, air texturing, etc. Nylon fibres are available as multifilament yarns, monofilament, staple, and tow. For concrete applications, high tenacity (high tensile strength) heat and light stable yarn is spun and subsequently cut into shorter lengths. Nylon fibres exhibit good tenacity, toughness, and excellent elastic recovery. Nylon is very heat stable and is readily used in commercial applications requiring this property, such as tires. Nylon is hydrophilic, with a moisture regain of 4.5 percent. The moisture regain property does not affect concrete hydration or workability at low prescribed contents ranging from 0.1 to 0.2 percent by volume, but should be considered at higher fibre volume contents. Nylon is a relatively inert material, resistant to a wide variety of organic and inorganic materials including strong alkalis. It has been shown to perform well under accelerated aging conditions. While a number of options are available on the type of fibre to be used, it is very important to choose the right fibres for the appropriate application. Care should be taken to design the structure for the appropriate load coming over it fully understanding the limitations of the fibres that are involved. Though the right design of the structure is done and the fibre is chosen, much lies in the hands of the engineer that executes. The mix proportioning, batching, mixing, placing and finishing all play an important role in order to achieve a long-lasting service life of the structure.

Micro Synthetic Fibre Reinforced Concrete and its Application Area Micro fibre concrete with polypropylene fibres are mainly produced and used to reduce plastic shrinkage in fresh concrete. During the hardening process of concrete the so called dissipation of hydration heat induces tensile stresses which are even higher due to evaporation of

23.16

Handbook on Advanced Concrete Technology

water. If those stresses are exceeding a certain extent, micro cracks are developed. Micro fibre concrete with polypropylene fibres reduces effectively the early shrinkage behaviour in first 10 hours of placement. The reason is that these types of fibres are able to hold back some water and slow down the evaporation process. They also are able to pick up some limited tensile stresses; however, typical replacement of reinforcement is not foreseen. These types of fibres work better to reduce plastic shrinkage cracks and are often added, in addition to the reinforcement of concrete. A further range of use is for improving the fire behaviour of concrete structures under very high temperature. Concrete is supposed to be fire resistant. Addition of micro fibres can be an effective way to enhance the fire resistance of concrete. Additions of micro synthetic fibres have positive influence on properties like impact resistance, shutter resistance and abrasion resistance of concrete. These fibres control exclusive spalling in high performance concrete, when exposed to fire. This is now adopted as standard method on many tunnel projects throughout the world. It is supposed that these very thin fibres are melting fast and hence they will create some channels where the vapour pressure, which usually exceeds the concrete tensile strength, can be released. Very good results can be achieved by combination of these fibres with steel fibres.

Macro Synthetic Fibre Reinforced Concrete and its Application Area Macro fibre concrete with polypropylene fibres are mainly used in lightly loaded applications, where the concrete is calculated as un-cracked concrete, just to improve the concrete in terms of their crack behaviour and to improve the resistance against the thermal shrinkage process. The reason that these fibres are mainly used in lightly loaded structures is that under long term loadings these fibres tend to creep and hence, a design in the cracked state under long term loadings does make the structure unsafe. Also for lightly loaded outside areas these types of fibres are taken to reinforce the concrete. Again these are structures which mainly remain in the un-cracked state. Macro synthetic fibres do not corrode. Hence in case of macro fibres, rusty spots do not appear on the surface. Additionally, macro synthetic fibres can be effectively used in applications like temporary linings, such as for mines, when larger deformations are allowed and acceptable.

Material Properties The following table illustrates the main differences in terms of the material and the material properties of steel and plastic fibres.

Fibre Reinforced Concrete Steel mesh/steel fibre

23.17

Micro/macro polymer fibre extruded polypropylene/polyethylene

Material

Typical length of fibres

30 – 60 mm

Micro: 6 – 20 mm

Typical diameter of fibres

0.5 – 1.0 mm

Micro: 0.015–0.030 mm Micro: 0.5–1.0 mm

Micro: 30 – 65 mm

Young’s modulus

210 000 MPa

3 000 – 10 000 MPa

Tensile strength

500 – 2 000 MPa

300 – 600 MPa

7 850 Kg/m3

910 kg/m

Melting point (°C)

1500°C

160°C does not reingorce

Creep behaviour tension (Tg glastransition temperature)

+ 370°C

– 20°C

Density

3

Application Area The following table illustrates the main application areas for steel and plastic fibres.

Dramix steel fibres

Synmix R macro synthetic fibres

Duomix R micro synthetic fibres

Plastic shrinkage reinforcement Anti-spalling aid at fire Non load bearing reinforcements Precast: handling and transportation reinforcement Flooring: temperature and shrinkage reinforcement

Temporary linings (such as in mines) allowing large deformations Crack controlling reinforcement Structural reinforcements Heavy impact Fatigue

23.6

DOSING

There are different ways to dose steel fibres into the concrete. A main difference can be seen between “Adding of fibres” and “dosing of fibres“, which will be carried out with automatic equipment. With dosing equipment a uniform distribution of steel fibres will be achieved and

23.18

Handbook on Advanced Concrete Technology

the Quality control in regards to the right amount of steel fibres will be secured. Manufacturers developed a special equipment to match these requirements. Nevertheless, in most cases the addition of fibres is still made by means of conveyor belts, blower blast equipment or by dosing fibres manually. In order to avoid problems of fibre balling related to adding loose fibres with a high aspect ratio (l/d), glued fibre technology has been developed. Fibres with a higher aspect ratio (length/diameter) than 60 are glued together to bundles. After adding glued steel fibres into the concrete, the bundles spread evenly at “macro-level”.

Fig. 23.11 Fibre addition: manually

Fig. 23.12 Addition of fibres with conveyor belt

Fig. 23.13 Automatic booster: automatically dosing of steel fibres

Recommendation for handling, dosing, mixing and quality control in table form:

Fibre Reinforced Concrete Dramix

Glued

Handling

Before adding fibres

Dosing

“

“

23.19

“

Duomix

Synmix

Loose

-Gloves and eye protection must be used!

-Gloves and eye protection must be used!

-Gloves and eye protection must be used!

-Keep day

-Keep day

-Keep day

-No stacking

-No stacking

-No stacking

Maximum dosage depends on: -Concrete cmposition -Placing method -Type of application Bekaert recommendations: -Preferably use a central batching plant mixer -A continuous grading and sieve curve -Sufficient fines and mortar content -Optimum slump before fibre addition > 12cm Note: -Depending on dosage and fibre type, fibres reduce the slump -Adjust required consistency only with (super) plasticizers

Bekaert recommendations: -Preferably use a central batching plant mixer -A continuous grading and sieve curve -Optimum slump before fibre addition > 12cm Note: -Depending on dosage and fibre type, fibres reduce the slump -Adjust required consistency only with (super) plasticizers

Bekaert recommendations: -Preferably use a central batching plant mixer -A continuous grading and sieve curve -Optimum slump before fibre addition > 12cm Note: -Depending on dosage and fibre type, fibres reduce the slump -Adjust required consistency only with (super) plasticizers

-Bage are non degradable Introduce fibres with sand and aggregates

Introduce fibres with sand and aggregates

Introduce fibres with sand and aggregates

Plant mixer

Add fibres to fresh mixed concrete Never add fibres as a first component

Add fibres to fresh mixed concrete Never add fibres as a first component

Add fibres to fresh mixed concrete Never add fibres as a first component

Truck mixer

Never add fibres as a first component Never fill drum completely with concrete in order to achieve even fibre distribution Add fibres continuously at a maximum of 40 Kg/min

Never add fibres as a first component Never fill drum completely with concrete in order to achieve even fibre distribution -Speed : add fibres continuously at a maximum of 5 kg/min

Never add fibres as a first component Never fill drum completely with concrete in order to achieve even fibre distribution -Speed : add fibres continuously at a maximum of 5 kg/min

-Mixing time depends on the efficiency of the mixing equipment

-Mixing time depends on the efficiency of the mixing equipment

If blower blast equipments are used (not needed for dramix fibres) the suitability should be tested on beforehand

Mixing

Mixing time depends on the efficiency of the mixing equipment Do: drum rotation speed >12 rpm -Mixing time: after adding all fibres,mix 3 1 minute/m concrete and not less than 5 min

Quality control

Pumping

Before using fibre concrete, a preliminary test must be done -Workability -Air content -Separation of fibre bunde when using glued fibres -Homogenious fibre distribution in the concrete

Do: drum rotation speed >12 rpm -Mixing time: after adding all fibres,mix 3 1 minute/m concrete and not less than 5 min

Before using fibre concrete, a preliminary test must be done -Workability

Do: drum rotation speed >12 rpm -Mixing time: after adding all fibres,mix 3 1 minute/m concrete and not less than 5 min

Before using fibre concrete, a preliminary test must be done -Workability -Air content

-Air content -Homogenious fibre distribution in the concrete

Hose diameter > 1.5 x fibre length

Hose diameter > 1.5 x fibre length

Hose diameter > 1.5 x fibre length

For complicated pump lines or concrete compositions, a trial is recommended prior to execution

For complicated pump lines or concrete compositions, a trial is recommended prior to execution

For complicated pump lines or concrete compositions, a trial is recommended prior to execution

Quality Control • Checking of the delivery note • Consistency (e.g. slump test) • Particularities (e.g. bleeding of concrete, fibre concentration) • Fibre distribution

23.20

Handbook on Advanced Concrete Technology

• Checking of the delivery note e.g. counting of the empty fibre-bags • Checking of the length, diameter and shape of the fibres • Wash out test for control of the fibre amount

23.7 GLASS FIBRE REINFORCED CONCRETE Glass Fibre reinforced composite (GFRC) materials consist of high strength glass Fibre embedded in a cementitious matrix. In this form, both Fibres and matrix retain their physical and chemical identities, yet they produce a combination of properties that can not be achieved with either of the components acting alone. In general, Fibres are the principal load-carrying members, while the surrounding matrix keeps them in the desired locations and orientation, acting as a load transfer medium between them and protects them from environmental damage. In fact, the Fibres provide reinforcement for the matrix and other useful functions in Fibre-reinforced composite materials. Glass Fibres can be incorporated into a matrix, either in continuous lengths or in discontinuous (chopped) lengths.

23.8 PROPERTIES OF GFRC Mechanical properties of GFRC composites depend upon fibre content, polymer content (if used), water-cement ratio, porosity, sand content, fibre orientation, fibre length, and curing. The primary properties of spray-up GFRC used for design are the 28-day flexural Proportional Elastic Limit (PEL) and the 28-day flexural Modulus of Rupture (MOR). The PEL stress is a measure of the matrix cracking stress. The 28-day PEL is used in design as the limiting stress to ensure that long-term, in-service panel stresses are maintained below the composite cracking strength. In addition, de-moulding and other handling stresses should remain below the PEL of the material at the specific time that the event takes place. As seen from generalized load-deflection curve of GFRC composite subjected to flexural test, young (28- day old) GFRC composites typically possess considerable load and strain capacity beyond the matrix cracking strength (PEL). The mechanism, which is primarily responsible for this additional strength and ductility, is fibre pull-out. Upon first cracking, much of the deformation is attributed to fibre extension. As load and deformation continue to increase, and multiple cracking occurs beyond the proportional elastic limit, fibres begin to de-bond and subsequently slip or pull-out to span the cracks and resist the applied load. Load resistance is developed through friction between the glass fibres and the cement matrix as the fibres de-bond and pull-out. GFRC made of cement, AR-glass fibres, sand, and water is a non-combustible material and when used as a surface material, its flame spread index is zero. Single skin GFRC panels can be designed to provide resistance to the passage of flame, but fire endurances of greater than 15 minutes are primarily dependent upon the insulation and fire endurance characteristics of the dry wall or back-up core

Recent Developments for Improvement of GFRC Durability Even though polymer additions to AR-GFRC have been shown to reduce the rate at which GFRC composites lose strength and ductility, commercially available GFRC systems will still

Fibre Reinforced Concrete

23.21

experience reductions in strength and ductility at a rate, which is environment dependent. Over the past few years, several new methods of improving the long-term durability of GFRC have been developed. All of these methods involve either specially formulated chemical coatings on the glass fibres or modification of the cement matrix.

Glass Fibre Modifications Since the introduction of alkali-resistant glass fibre in 1971, several attempts have been made to further improve glass fibres for use in GFRC. Most of these attempts have been directed towards improving commercially available AR-glass fibres by application of special fibre coatings. These special coatings are intended to reduce the affinity of the glass fibres to calcium hydroxide, the hydration product, which is primarily responsible for composite embitterment. Some second generation AR glass fibres, which are currently commercially available, are examples of the potential benefits of fibre coatings. Long term durability data for composites manufactured with these fibres indicate that strength and ductility decreases at slower rate than conventional AR-glass composites. However, there is still some loss in strength and toughness indicated by current test results. Since predictions of long-term material properties are based on a correlation of accelerated aging data with natural aging data, it is still too early to make an accurate prediction of how effective these fibres will ultimately be for improving the long-term strength and ductility. A method called “silica fume slurry infiltration” was developed to incorporate silica fume directly into the spaces between individual glass filaments in a fibre glass roving. It was discovered that by hand-dipping the roving into commercially dispersed silica fume slurry, the spaces between the individual glass filaments could be adequately filled with silica fume. Results of tests performed on aged composites containing 3 percent AR-glass fibre by weight and fabricated using silica fume slurry infiltration indicated a substantial decrease in the rate at which strength loss takes place. It has not been determined whether this manufacturing method is commercially feasible.

Cement Matrix Modifications Over the years, several researchers have approached the GFRC strength durability problem by altering the cement matrix. Most of these efforts were geared towards trying to reduce or eliminate the formation of calcium hydroxide produced during hydration. Development of high alumina cement (HAC) and super sulphated cement represented early attempts at trying to modify the cement matrix. Although both of these cements were somewhat effective in improving the long-term strength durability of GFRC composites, other undesirable effects, such as increased porosity and strength loss of the cement matrix, were evident. A more recent development is the use of lime reactive materials as cement additives. Silica fume and metakaolin used in standard Portland cement have proved to be effective agents for early reaction and elimination of calcium hydroxide. However, in order to significantly reduce the levels of calcium hydroxide, very large percentages (greater than 20 percent) of the materials must be used. Methods have been developed to incorporate large percentages of silica into the cement matrix without dispersion problems. However, incorporation of large percentages of silica

23.22

Handbook on Advanced Concrete Technology

fume has not shown to be a very cost effective method of improving the long-term durability or aged strain capacity of GFRC. Another new development regarding improved long-term strength durability of GFRC is CGC cement. This cement is claimed not to produce calcium hydroxide during hydration. Tests performed on GFRC composites fabricated using CGC cement and AR-glass fibres indicated that initial 28-day strengths and ultimate strains are essentially retained after exposure to accelerated aging conditions. However, use of CGC cement in composites fabricated using E-glass fibres was unsuccessful because of the alkali attack on the glass fibres. Primary curing, after manufacture of sprayed or cast CGC cement, is very important. Temperature must be automatically controlled using temperature sensors at the heat sources (usually steam). In the winter months, pre-curing is an effective way of saving time within the curing regime up to the final trowel finishing. The heating rate for primary curing must be maintained as noted to achieve optimum properties. The secondary curing, after steam curing, should be done indoors or in a protected area. In case of products stored outside, items should be covered with a plastic sheet during first 7 days after de-moulding to prevent adverse drying from direct sunlight and wind.

References 1. Shah, S. P., and Rangan, B. V. (1971) “Fibre Reinforced Concrete Properties,” ACI JOURNAL, Proceedings, Vol. 68, No. 2, pp. 126-135 2. Hoff, George C. (1986) “Use of Steel Fibre Reinforced Concrete in Bridge Decks and Pavements,” Steel Fibre Concrete, Elsevier Applied Sciences Publishers, Ltd., pp. 67-108. 3. Ramakrishnan, V.; Coyle, W. V. ; Kopac, Peter A. ; and Pasko, Thomas J., Jr. (1981) “Performance Characteristics of Steel Fibre Reinforced Super plasticized Concrete,” Developments in the Use of Super plasticizers, SP-68, American Concrete Institute, Detroit, pp. 515-534. 4. Johnston, C. D. (1974) “Steel Fibre Reinforced Mortar and Concrete—A review of Mechanical Properties,” Fibre Reinforced Concrete, SP-44, American Concrete Institute, Detroit, pp. 127-142. 5. Morgan, D. R.; McAskill, N.; Richardson, B. W.; and Zellers, R. C. (1989) “A Comparative Evaluation of Plain, Polypropylene Fibre, Steel Fibre, and Wire Mesh Reinforced Shotcrete,” Transportation Research Board, Washington D. C. 6. Banthia, N., and Ohama, Y. (1989) “Dynamic Tensile Fracture of Carbon Fibre Reinforced Cements,” Proc., Int’l Conf. on Recent Developments in FRC, Cardiff, pp. 251-260. 7. Jakel, G. R. (1975) “Fibre Reinforced Concrete Products and their Formation; Polyesters, Cellulose Pulp,” U.S. Patent #US3899344. 8. Nagabhushanam, M.; Ramakrishnan, V.; and Vondran, G. (1989) “Fatigue Strength of Fibrillated Polypropylene Fibre Reinforced Concrete,” Transportation Research Record 1226, National Research Council, Washington D.C., pp. 36-47. 9. Deutscher Beton- und Bautechnik-Verein e.V.: DBV-Merkblatt “Stahlfaserbeton”. Fassung October 2001. 10. Ganesh P. Chaudhari, Design of Durable SFRC Industrial floor ACI Seminar 2008, Rantagiri, India 11. Österreichische Vereinigung für Beton- und Bautechnik “Richtlinie Faserbeton” Fassung July 2008

Fibre Reinforced Concrete

23.23

12. DIN EN_14889-1: Fasern für Beton Teil 1: Stahlfasern - Begriffe, Festlegungen und Konformität. 13. DIN EN_14889-2: Fasern für Beton Teil 1: Polymerfasern - Begriffe, Festlegungen und Konformität. 14. Die Bibliothek der Technik Band 136: „Stahlfaserbeton: Ein neuer Baustoff und seine Perspektiven“, [Hochttief/Bekaert]. 15. RILEM TC 162-TDF: “Test and design methods for steel fibre reinforced concrete Background and experiences-”, Chairlady L. Vandewalle, March 2003. 16. The Concrete Society: Technical Report No.63 “Guidance for the design of steel-fibre-reinforced concrete”, March 2007. 17. G. Vitt (2008) Combined reinforcement – practical experiences, BEFIB 2008, 17-19 September 2008, Chennai. 18. A. Lambrechts, brochure: Steel- and Synthetic Fibre Reinforced Concrete. 19. Bekaert, brochure: Recommendations for handling, dosing and mixing. 20. Trevor Atkinson (2010) Fibres – the traditional method of reinforcement? Concrete, pp 45-46. 21. A Richardson and C McIntyre (2010): The effect of fibre concrete on the ultimate pull-out force of anchor bolts; Concrete, pp 48-50. 22. N. Ganesan, P. V. Indira, S. Rajendra Prssad (2010) Structural behaviour of steel fibre reinforced concrete wall panels in two-way in-plane action, Indian Concrete Journal, pp 21-28. 23. C. Sashidhar, H. Sudarsana Rao, N. V. Ramana, K. Gnaneswar (2010) Compression and tension behaviour of SIFCON produced with low tensile strength steel fibre, Indian Concrete Journal, pp 31-36.

24 High-Density Concrete A.K. Laharia and D.K. Jain

High-density or heavy concrete is principally used as biological shield for protection against ionizing radiation, attenuating them to a level considered to be safe for humans. Shielding is required for protection against radiation in nuclear reactors, reprocessing facilities, spent fuel storage and numerous medical facilities handling radiation. Concrete is a widely used construction material because of its excellent strength, reasonable durability, low cost and flexibility to cast in various forms. The capabilities of concrete are well known as a structural material, due to its high compressive strength and compatibility with reinforcement steel. It also forms a good shielding material against both neutrons and gamma radiation due to high density and availability of bound water. Therefore, it is a universally used shield material in stationary power reactors. Since effectiveness of biological shield or the ability to attenuate a, b, g, protons and neutrons is almost directly proportional to its density, high density concrete is used in high radiation zones or where space is at premium. Normal concrete has a density of about 2400 kg/m3. The high density concrete used in nuclear power plants has a density of about 3600 kg/m3. Under special circumstances, at few locations super high density concrete having density of 4100-5500 kg/m3 is also used. Super Heavy Aggregates in the form of steel shots and cuttings are used to produce Super High Density Concrete. Almost any kind of concrete can be used for reactor shields, the exact type will depend upon the design of the reactor. There are several properties of interest which one must consider in the use of concretes for radiation shielding in addition to its mechanical and chemical properties like: • Density • Hydrogen content • Dehydration with temperature • Internal heat generation as a result of attenuation of radiation • Secondary gamma rays produced due to attenuation of neutrons

24.2

Handbook on Advanced Concrete Technology

Borated concrete is sometimes used to increase the effectiveness of concrete especially for shielding against low energy neutrons as it suppresses secondary gamma rays. Boron is added in concrete in the form of Boron carbide (B4C). Usually addition of one or two percent of Boron carbide is sufficient. Addition of boron carbide, however, retards the hardening of concrete for the first 3 to 7 days. Thereafter, hardening proceeds normally. For thermal neutron shielding at elevated temperatures (400°-480°C), Serpentine concrete is used. Serpentine is a group of common rock-forming hydrous magnesium phyllosiliate ((Mg,Fe)3Si2O5(OH) 4) minerals. Serpentine aggregates hold their water of crystallization up to about 500°C, which helps to attenuate neutrons.

24.1

HEAVY AGGREGATE

In general, High Density Concrete is produced using Ordinary Portland cement and coarse and fine aggregate produced from hematite ore. Heavy coarse & fine aggregate, generally, manufactured from hematite ore or any other approved iron ore after formal approval of quarry. The ore used for production of aggregates should not be of laminated variety. The iron ore boulders should also be free from laterite or other coverings/coatings. The iron ore boulders are generally brought to site and kept in stacks of defined capacity for sampling at site before crushing. The stacks are identified by proper numbering and a statement showing the arrivals and stock position. Generally 5 numbers of boulders from stacks of 50 t each are collected as samples from each stack and tested as per the specifications. On testing if material is not found satisfactory, whole stack is removed. A stack is declared to have been approved for crushing when the average value of the specific gravity of the samples is (saturated surface dry basis) 4.65g/cc or above and no single value is less than 4.50g/cc. The aggregate produced should be free from an excess of thin elongated particles, soft pieces, vegetation and other deleterious matter.

24.2

SPECIAL HEAVY AGGREGATE

The special heavy aggregates may consist of steel balls/ steel shots round in shape and well rusted to make the surface rough. The maximum size of steel shots should be 5 mm down to 0.5 mm and the aggregates should be well graded. The stock piles of heavy aggregates shall be formed so as to prevent segregation. The deposition and the removal should be done in a manner to maintain the uniformity of grading. The specific gravity of special heavy aggregate should not be less than 7.50g/cc.

24.3 MIX PROPORTIONING, MIXING AND PLACEMENT OF HIGH DENSITY CONCRETE The mix proportioning is an important aspect in case of heavy density concrete because aggregate being heavy, the chances of segregation of particles are high. In order to maintain the cohesiveness of mix and proper placeability of concrete, the paste content is maintained high in case of Heavy Density Concrete.

High-Density Concrete

24.3

Use of proper mixer is also important in case of High Density Concrete due to possibility of blade damage. Where concrete is required to be pumped, for placement at higher elevations, special care is required while selecting the concrete pump as the pump will witness higher pressures in proportion to the increase in density. In High Density Concrete, to avoid segregation while maintaining high mobility (to make concrete pumpable), addition of mineral admixtures like micro silica/fly ash is required.

Specifications of Heavy Aggregate Generally the specifications followed for normal aggregate are also followed for high density concrete, except for requirements as mentioned above.

Properties of Heavy Fine and Coarse Aggregates Heavy coarse and fine aggregates, to be used in high density and special high density concrete, are manufactured from hematite ore or from other approved iron ores. Properties of fine and coarse aggregates, as for normal concrete, shall conform to IS 383.

Water Content in Neutron Shields For neutron shields, the fixed water content in normal concrete should not be less than 5% by weight. For heavy concretes the minimum fixed water content in percent should not be lower than 11.5/r, where r is the density of the heavy concrete in gm/cc. The dehydration rate is approximately 2 to 2.3% of the existing water content per year. The half value thickness of heavy concrete with density 3.5gm/cc for 2Mev gamma photon is 4.5cm.

Typical Mix for High Density Concrete Mix proportioning for High density as well as special high density concrete is carried out in the same way as followed for normal concrete. Typical mix proportioning is given below: Grade of Concrete: H 25/ 20 (H- stands for heavy; 25 is the characteristic strength in MPa and 20 is the maximum of aggregate in mm) Slump required w/b ratio Cement Coarse Aggregate Fine Aggregate Water Chemical Admixture

: : : :

Min 125 mm (for pumped concrete) 0.5 330 kg Size 20mm – 10 mm – 1080 kg Size < 10 mm – 720 kg : 1473 kg : 165 kg : 8.0 kg

Source for Heavy Aggregate The hematite ore, having good iron content, can be procured from any good iron ore mine. Some of the sources are Chomu near Jaipur (Rajasthan), Bellary and Kudremukh in Karnataka.”

24.4

Handbook on Advanced Concrete Technology

References 1. Kan YC, Pei KC, Chang CL (2004) Strength and fracture toughness of heavy concrete with various iron aggregate inclusions. Nuclear Engineering and Design, 228:119-127. 2. Akkurt I, Basyigit C, Kilincasuan S (2006) Radiation shielding of concrete containing different aggregates. Cement and Concrete Composites, 28:153-157. 3. Gensel O, Brostow W, Ozel C, Filiz M, “Concrete Containing Hemalite for use as Shielding Barriers”, ISSN 1392–1320 Materials Science (Medžiagotyra). Vol. 16, No. 3. 2010. 4. Engineering Compendium on Radiation Shielding: Volume 3: Shield Design and Engineering (Hardcover) by Jaeger, R G, Blizard E.P., Chilton A. B., published by Springer. 5. Kaplan, M.F., Concrete Radiation Shielding 1989 - nuclear physics, concrete properties, design and construction, Published by Longman Scientific and Technical, Wiley. 6. Ignalina Source Book – Lithuanian International Nuclear Safety Centre.

25 Underwater Concreting Himanshu Kapadia, Amit Datta and S.G. Bapat

Typical underwater concrete placements include nonstructural elements such as cofferdams or caisson seals, and structural elements such as bridge piers, dry-dock walls and floors, piles, water intakes, etc. Concrete placed under water has also been used to add weight to sink pre-cast tunnel sections, to join tunnel sections once in place, and to repair erosion or cavitation damage to major hydraulic structures.

25.1

METHODS OF UNDERWATER PLACEMENTS

The tremie is currently the most used technique to place concrete under water, but use of direct pumping is increasing. Basic technique—Successful placement of concrete under water requires preventing flow of water across or through the placement site. Once flow is controlled, either tremie or pump placement consists of the following three steps: 1. The first concrete placed is physically separated from the water by using a go-devil or pig in the pipe, or by having the pipe mouth sealed and the pipe dewatered; 2. Once filled with concrete, the pipe is raised slightly to allow the go-devil to escape or to break the end seal. Concrete will then flow out and develop a mound around the mouth of the pipe. This is termed establishing a seal; and 3. Once the seal is established, fresh concrete is placed into the mass of existing concrete. The exact flow mechanism that takes place is not precisely known, but the majority of the concrete apparently is not exposed to direct contact with the water (Gerwick, Holland, and Kommendant 1981).

25.2 MATERIALS Aggregate The maximum size of aggregate used in placements for Reinforced structural elements under water is usually 20 mm. Larger aggregate of 25 mm down can be used depending on availability, spacing of reinforcement and meeting desired workability requirement of the concrete.

25.2

25.3

Handbook on Advanced Concrete Technology

ADMIXTURES

To improve the characteristics of fresh concrete, especially flowability, admixtures are used in concrete placed under water (Williams 1959). For example, an air-entraining admixture can be beneficial because of the increased workability that can be achieved with its use. Water-reducing or water-reducing and retarding admixtures are particularly beneficial in reducing water content to provide a cohesive yet high-slump concrete. Retarding admixtures are beneficial in a large monolithic placement. In massive placement, use of high range water-reducing admixture (HRWR) is not recommended as there will be high slump and also the slump is maintained for longer period by use of HRWR admixture. The use of HRWR for smaller volume placements, in which flow distances are not critical, may be acceptable. Admixtures are also available to prevent washout of cementitious materials and fines from concrete placed underwater.

25.4 MIX PROPORTIONING Pozzolans are generally used because they improve flow characteristics. Relatively rich mixtures, 356 kg/m3 cementitious materials, or more and a maximum w/c ratio of 0.45 are recommended. Fine aggregate content of 45% to 55% by volume of total aggregate and air contents of up to 5% are generally recommended. A slump of 150 mm to 200 mm is necessary and occasionally a slightly higher range is needed when embedded items obstruct the flow or when relatively long horizontal flow is required. If possible, the final selection of a concrete mixture should be based on test placement made under water in a placement box or in a pit that can be dewatered after the placement. Test placements should be examined for concrete surface flatness, amount of laitance present, quality of concrete at the extreme flow distance of the test placement, and flow around embedded items.

25.5 CONCRETE PRODUCTION AND TESTING Sampling should be done as near to the tremie hopper as possible to ensure that concrete with the proper characteristics is arriving at the tremies. Once a concrete mixture has been approved, slump, air content, unit weight, and compressive strength testing should be adequate for production control. Because of the importance of the flowability of the concrete successful placement, slump and air content tests should be performed more frequently than is usually done for concrete not placed under water. Compressive strength specimens should be available for testing at early ages to determine when the concrete has gained enough strength to allow dewatering of the structure. Hence, additional samples are required to be cast. The concrete temperature should be kept as low as practical to improve placement and structural qualities. Depending on the volume of the placement and the anticipated thermal conditions within the placement, maximum temperatures in the range of 16° C to 32° C are normally specified. While concrete placed under water obviously should not be freeze, a minimum concrete temperature of 5° C should be maintained. Heating either water or aggregate can cause erratic slump-loss, and hence extreme care should be taken when such procedures are used to raise the concrete temperature.

Underwater Concreting

25.3

25.6 TREMIE EQUIPMENT AND PLACEMENT PROCEDURE Tremie Pipes The tremie should be fabricated of heavy-gauge steel pipe to withstand handling stresses. In deep placements, buoyancy of the pipe can be a problem if an end plate is used to gain the initial tremie seal. Use of pipe with thicker walls or weighted pipe can overcome buoyancy problems. Tremie pipes should have a diameter large enough to avoid aggregate-induced blockages. Pipes in the range of 200 mm to 300 mm diameter are adequate for the range of aggregate recommended herein. For deep placements, the tremie should be fabricated in sections with joints that allow removal of upper sections with progressive placement. Sections can be joined by flanged, bolted connections, (with gaskets) or screwed together. Whatever joint technique is selected, joints between tremie sections should be watertight and should be tested for water-tightness before beginning of placement. The tremie pipe should be marked to allow quick determination of the distance from the surface of the water to the mouth of the tremie. The tremie should have a suitably sized funnel or hopper to facilitate transfer of concrete from the delivery device to the tremie. A stable platform should be provided to support the tremie during placement. Floating platforms are generally not suitable. The platform should be capable of supporting the tremie while sections are being removed from the upper end of the tremie.

25.7 PLACEMENT PROCEDURE All areas in which there is to be perfect bond between cured concrete and fresh concrete, the surfaces should be thoroughly cleaned immediately before concrete placement. Pipe spacing should be in the order of one pipe for every 300 ft2 (28 m²) of surface area or pipe spacing to be between 4 m to 6 m. These spacings are recommended, however, concrete has been placed that flowed as far as 21 m with excellent results. For most large placements, it will not be practical to achieve a pipe spacing as close as 5 m on centers simply because it would be impractical to supply concrete to the number of tremies or pumps involved. Actual pipe spacing should be established on the basis of the thickness of the placement, congestion due to reinforcing steel, available concrete production capacity and available capacity to transfer concrete to the tremies. The placement method selected should also be considered. Tremies with end plates should be filled with concrete before being raised off the bottom. The tremie should then be raised a maximum of 150 mm to initiate flow. These tremies should not be lifted further until a mound is established around the mouth of the tremie pipe. Initial lifting of the tremie should be done slowly to minimize disturbance of material around the mouth of the tremie. Tremies with a go-devil should be lifted a maximum of 150 mm to allow water to escape. Concrete should be added to the tremie slowly to force the go-devil downward. Once the go-devil reaches the mouth of the tremie, the tremie should be lifted enough to allow the go-devil to escape. After that, a tremie should not be lifted again until a sufficient mound is established around the mouth of the tremie. Tremies should be embedded in the fresh concrete for 1.0 m to 1.5 m. Exact embedment depth will depend on placement rate and setting time of

25.4

Handbook on Advanced Concrete Technology

the concrete. All vertical movements of the tremie pipe should be done slowly and carefully to prevent loss of seal. If loss of seal occurs in a tremie, placement through that tremie should be stopped immediately. The tremie should be stopped, the end plate should be replaced, and flow should be restarted as described above. To prevent washing of concrete already in place, a go-devil should not be used to restart a tremie after loss of seal. Concrete placement should be as continuous as possible through each tremie. Excessive delays in placement can cause the concrete to stiffen and resist flow when placement resumes. Placement interruptions of up to approximately 30 min should allow restarting without any special procedures. Interruptions of between 30 min and the initial setting time of the concrete should be treated by removing, resealing, and restarting the tremie. Interruptions of duration greater than the initial setting time of the concrete should be treated as a construction joint. If a break in placement results in a planned or unplanned horizontal construction joint, the concrete surface should be green-cut after it sets. Green-cutting by a diver is difficult but can be accomplished where there is no practical alternative for cleaning. The concrete surface should be water-jetted immediately before resuming concrete placement. Recommendations on the rate of concrete rise are generally in the range of 0.3 m/ hr. to 3 m/ hr. Calculation of a projected rate is somewhat difficult because the exact flow pattern of the concrete will not be known. The most logical approach is to compare concrete production with the entire area that is being supplied. As with pipe spacing, achieving the recommended values can be difficult. Concrete has been successfully placed under water at rates of approximately 150 mm of rise per hr. (Gerwick, Holland, and Kommendant 1981). The volume of concrete in place should be monitored throughout the placement. Under runs (using less concrete than anticipated) are indicative of loss of tremie seal, because the washed and segregated aggregates will occupy a greater volume. Overruns (using more concrete than anticipated) are also indicative of loss of concrete from the forms. Once the placement scheme has been developed, flow distances and rates of rise can be calculated. If flow distances seem excessive or if the rate of concrete rise is too low, make a judgment as to the suitability of the available plant or the necessity for breaking the placement into smaller segments. Tremie blockages that occur during placement should be cleared extremely carefully to prevent loss of seal. If a blockage occurs, the tremie should be quickly raised 150 mm to 610 mm and then lowered in an attempt to dislodge the blockage. The depth of pipe embedment should be closely monitored during all such attempts. If the blockage cannot be cleared easily, the tremie should be removed, cleared, resealed, and concreting can then be resumed. The pipe delivering concrete should remain fixed horizontally while concrete is flowing. Horizontal movement of the pipe will damage the surface of the concrete already placed and create additional laitance and lead to loss of seal. Horizontal distribution of the concrete is accomplished by flow of the concrete after exiting the pipe or by halting placement, moving the pipe, reestablishing the seal, and resuming placement. Two methods are typically used to achieve horizontal concrete distribution in large placements viz. the layer method and the advancing slope method. In the horizontal layer method, the entire area of the placement is concreted simultaneously using a number of tremies. With the advancing slope method, one portion of the placement is brought to finished grade and then the tremies are moved to bring

Underwater Concreting

25.5

adjacent low areas to grade. Work normally progresses from one end of a large placement to the other. Concrete slopes from nearly flat to 1:6 (vertical to horizontal) can be expected. To evaluate the underwater placement, the following techniques can be used: • Coring in areas of maximum concrete flow or in areas of questionable concrete quality; • After dewatering, accurately surveying the concrete surface to evaluate the adequacy of the concrete mixture and the placement plan; and • After removal of forms or sheet piling, inspecting the exterior surface of the concrete with divers for evidence of cracking, voids, or honeycomb.

25.8

DIRECT PUMPING • Tremie placement techniques are also applicable to direct pump placement under water. The following minor differences, however, are worth noting: • The mechanism causing concrete flow through the pipeline is pump pressure rather than gravity; • The concrete should be proportioned for flow after leaving the pipe rather than simply for pumping; • Pipes are typically smaller than those used for tremies. Rigid sections should always be used for the portion actually embedded in the concrete; • The pump action can cause some lateral movement of the pipe where it is embedded in the fresh concrete; this movement can contribute to laitance formation by drawing fines to the pipe-concrete interface; and • A relief valve (air vent) will be required near the highest point in the pipeline to prevent development of a vacuum blockage.

25.9

CONCRETE CHARACTERISTICS

Concrete placed under water is expected to be of excellent quality. Curing conditions are excellent and drying shrinkage is minimal. Compressive strengths of the rich mixtures used will often be in the range of 28 MPa to 55 MPa. Other structural properties do not differ from those of similar concretes placed in dry. In-place unit weight, often critical in massive placements to offset hydrostatic uplift, will be close to that measured for the fresh concrete before placement. If laitance is entrapped in the concrete, however, unit weight can be significantly below that of the fresh concrete. Although there have been recent attempts to ascertain the quality and homogeneity of concrete placed under water using nondestructive techniques (Laine et al. 1980), coring is still the recommended technique for evaluation of questionable areas.

25.10

PRECAUTIONS TO BE TAKEN IN UNDERWATER PLACEMENTS

Inspection of concrete placement under water is difficult. The water itself will become increasingly murky as the placement progresses and the surface of the fresh concrete will not

25.6

Handbook on Advanced Concrete Technology

support the weight of a diver. Therefore, pre-placement inspection becomes extremely important and should concentrate on reviews of the proposed procedures and equipment and the proposed concrete mixture. Inspection during the placement will be limited to observing all phases of the concrete production, transportation and placement procedures. Because the success of an underwater placement depends largely on the concrete itself, sampling and testing during the placement are critical to ensure compliance with approved mixtures and required concrete characteristics viz. slump, air content, temperature etc. An inspection plan, detailing locations and frequency of soundings etc. should be developed. Soundings should be taken over the entire area of the placement at regular intervals, such as every hour or every 75 m3 etc. Locations for taking soundings should be marked on the structure to ensure that all soundings are made at the same location. Additionally, soundings should be taken on a more frequent basis adjacent to each tremie to monitor pipe embedment. Data obtained from soundings should be plotted immediately to monitor the progress of the placement. The most common cause of loss of seal is excessive vertical movement of the pipe to clear a blockage or to remove a pipe section. With either placement method, the loss of seal will result in washing and segregation. A related and similar problem is the failure to establish a satisfactory seal at the beginning of a placement.

25.11

GO-DEVILS

The use of go-devils has traditionally been advocated as a technique for sealing tremies or pump lines. Although the technique is effective, the water that is forced out of the pipe ahead of the go-devil can wash and scour the material underlying the placement area. This condition can be alleviated by the placement of a layer of properly graded rock before the start of concreting. When a pipe is relocated during a placement, the water forced out of the pipe will wash previously placed concrete, resulting in extreme segregation, laitance formation and possibly entrapped zones of un-cemented aggregate. Therefore, the use of a go-devil at the beginning of a placement is acceptable, but not to restart a tremie or pumping line during a placement.

25.12

LAITANCE

Because it is physically impossible to separate the concrete and the water completely, certain amount of laitance will be formed. If the seal is lost or if the concrete is disturbed in any way, additional laitance will be formed when starting or restarting pipes. The laitance will flow to and accumulate in any low areas on the surface of the concrete. Such accumulations can prevent sound concrete from filling an area and can become entrapped by subsequent concrete flows. In either case, the zones of laitance will be more permeable and lower in strength. Problems with laitance can be avoided by using pumps or air-lifts during the placement to remove unsuitable material as it accumulates. Another way of reducing laitance problems is to discard several inches of concrete from the form. This can only be done where the top of the form coincides with the top of the placement.

Underwater Concreting

25.13

25.7

CRACKING

Problems associated with heat development and subsequent cracking in massive underwater placements have generally not been resolved. The following characteristics, however, of underwater placements should be considered. • Cement content—Underwater concrete mixtures have traditionally used high cement contents of 385 kg/m3 or more to compensate for cement washout and to provide the necessary flow characteristics to the concrete. Measurements made on one large placement indicated maximum internal concrete temperature in excess of 35°C above the placement temperature of 16°C. • Placement environment—Tremie concrete is usually placed in locations that act as excellent heat sinks. The temperature of the water surrounding the concrete will normally vary little; thus, the outside of the concrete mass cools quickly, developing steep temperature gradient. • Volume—To eliminate construction joint preparation under water, placements tends to be large monoliths placed over short periods of time. • Restraint—Underwater placements are frequently made on rock or contain many piles with the concrete acting as a pile cap. In either case, a high degree of restraint can be present. Of the methods recommended for controlling cracking in mass concrete, modifying the materials or mixture proportions appears to have the greatest potential for application in underwater placements. In particular, use of low heat cements, replacement of 15% to 30% of the cement with a suitable pozzolan, and cooled aggregate and water are recommended. It is conceivable, but as yet untried, to provide internal cooling using the water available at the site or to include insulation in the fabrication of forms used in structural placements.

25.14

DETAILING

Concrete placed under water moves to its final position in the structure by gravity, without vibration and inspection. Therefore, all formwork, reinforcing steel and pre-cast elements to be filled with concrete should be detailed with underwater placement in mind and incorporate the following: • Reinforcing steel should be sized and placed to allow the maximum possible openings between bars so that concrete flow will not be impeded; • Forms should be adequately sealed to prevent loss of concrete or mortar; and • Forms and reinforcing steel should not trap laitance in areas intended to be filled with concrete.

25.15

ANTIWASHOUT ADMIXTURES

Chemical admixtures intended for use in concrete placed underwater have been developed (Saucier and Neeley 1987; Khayat, Gerwick, and Hester 1990). These antiwashout admixtures make the concrete more cohesive and thus less prone to washout of cement or fines from the

25.8

Handbook on Advanced Concrete Technology

concrete during placement. These admixtures were developed for use in situations where freshly placed concrete, may be exposed to flowing water during or after placement where concrete placement is not thick enough to permit the required tremie pipe embedment, or where the wash-out of cement may cause an environmental problem. A Corps of Engineers test method (CRD-C 61) has been developed to evaluate the effectiveness of these admixtures (Neeley 1988). Because of the thixotropic nature of the concrete treated with these admixtures, they should be used with caution for massive placements in which the concrete is expected to flow for long distances once it exits the tremie pipe. Trial placements should be conducted to verify that the concrete proportioned with the antiwashout admixture can maintain adequate slump life and can flow for the required distance. Applications of these antiwashout admixtures include underwater paving of a canal (Kepler 1990; Klemens 1991) and underwater repair of a dam (Neeley and Wickersham 1989).

25.16

CONCLUSION

The underwater concrete operation is a complex one in which a lot of due consideration needs to be given. In most countries, there are strict environmental limitations, which call for sophisticated methods of placing and materials to safely place without any environmental impact. This makes the situation even more complex. Another area, where a lot of studies and sophistication is required, is the repair of the concrete underwater. These are done with the help of expert divers and needs a better level of understanding the materials that has to be used for relevant conditions.

References 1. Gerwick, B. C., 1964, “Placement of Tremie Concrete,” Symposium on Concrete in Aqueous Environments, SP-8, American Concrete Institute, Farmington Hills, Mich., pp. 9-20. 2. Gerwick, B. C., and Holland, T. C, 1983, “Cracking of Mass Concrete Placed Under Water,” Concrete International: Design and Construction, V. 5, No. 4, Apr., pp. 29-36. 3. Gerwick, B. C.; Holland, T. C.; and Kommendant, G. J., 1981, “Tremie Concrete for Bridge Piers and Other Massive Underwater Placements,” Report No. FHWA/RD-81/153, Federal Highway Administration, Washington, D.C., 203 pp. 4. Holland, T. C., and Turner, J. R., 1980, “Construction of Tremie Concrete Cutoff Wall, Wolf Creek Dam, Kentucky,” Miscellaneous Paper No. SL-80-10, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., 85 pp. Kepler, W. F., 1990, “Underwater Placement of a Canal Lining,” Concrete International, V. 12, No. 6, June, pp. 54-59. 5. Khayat, K. H.; Gerwick, B. C.; and Hester, W. T., 1990, “High-Quality Tremie Concretes for Underwater Repairs,” Proceedings, Paul Klieger Symposium on Performance of Concrete, SP-122, D. Whiting, ed., American Concrete Institute, Farmington Hills, Mich., pp. 125-138. 6. American Concrete Institute – Manual Practice – 304R 7. Williams. J. Wayman, Jr., 1959,” TremieConcrete Controlled with admixtures”, ACI Journal, Proceedings, V-55 No. 8, pp839-850. 8. Laine et.al.,” Probing Concrete with radio waves,” Proceedings ASCE, V. 106 GT 7.pp 759-766.

Underwater Concreting

25.9

9. Saucier, K.L., and Neeley, B.D., 1987,” Antiwashout Admixtures in Underwater Concrete,” Concrete International, V. 9, No. 5, May, pp 42-47. 10. Neeley., B.D., 1988,” Evaluation of Concrete mixtures for use in under water repairs,” Technical Report No. REMR-CS-18,U.S Army Engineers Waterways Experiment Station, Vicksburg, Miss., 124 pp. 11. Neeley., B.D., and Wickersham, J., 1989,”Repair of Red Rock Dam,” Concrete International, V. 11, No. 10, Oct., pp36-39. 12. Klemens, T.L, 1991,” Who Says You Can’t pave under water?” Highway and highway construction. V, 134, No. 10 pp64-66. 13. Kepler, W.F., 1990,” Underwater Placement of a Canal Lining,” Concrete International. V. 12 No. 6, June, pp54-59.

26 Mass Concrete and Temperature Rise in Concrete A.K. Jain and N.V. Nayak

26.1 INTRODUCTION There are concrete structures, which, apart from imposed and dead loads, are also required to be specially designed for non-load, ‘secondary’ forces. For example, superstructures of long-span concrete bridges may experience significant secondary stresses due to seasonal and daily variation in ambient temperature, as well as shrinkage of concrete. Apart from such atmospheric temperature variations, thermal effects due to heat rise inside concrete following the hydration of cement can also become prominent in certain types of structures. Hydration of cement causes exothermic rise in temperature of the concrete. In concrete members of relatively large sizes, the rise in temperature can be of the order of 40 to 50 degrees Celsius above the placing temperature, unless the heat is quickly dissipated. Significant tensile stress may develop from the dimensional changes associated with variation of temperature within the concrete. Mass concrete is defined by ACI Committee 116 as “Any large volume of cast-in-place concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and attendant volume change to minimize cracking”. Such cracking may cause loss of structural integrity and monolithic action, may also cause excessive seepage and be aesthetically objectionable. Salient mass concrete practices, which are outlined herein, were initially developed in connection with construction of concrete dams. Mass concrete is, therefore, easily identified with concrete dams and similar hydraulic structures. However, temperature induced cracking can be experienced in other concrete members, like thick foundations, bridge piers and abutments, pile caps, thick walls and underground concrete linings in tunnels etc. to that extent they are all mass concrete, and should be designed and constructed appropriately. Thus ‘Mass Concrete’ includes not only low cement content concrete used in dams and other massive structures but also moderate to high cement content concrete in structural members that require special considerations to handle heat of hydration and subsequent temperature rise.

26.2

26.2

Handbook on Advanced Concrete Technology

CAUSES OF THERMAL CRACKING

More than the rise in temperature in concrete, it is the temperature differential or thermal gradient inside and across the mass that causes cracking. With the generation of heat of hydration of cement, temperature of concrete rises. Because concrete has low thermal conductivity, the heat generated within the mass dissipates rather slowly. While the outside surface cools down rapidly, the inside temperature takes time to come to equilibrium with the ambient. The width and depth of cracks depend upon the temperature gradient between the hot internal concrete and the cooler concrete surface. Heat escapes from a mass of concrete at a rate which is inversely proportional to the square of its least dimension. Hence, the importance of the size or dimension of the structural member becomes obvious. By way of example, a 150 mm thick concrete wall can become thermally stable in about 2 hours; a 1.5 m thick wall will take about 7 to 10 days to reach a similar condition. The time taken in case of a 50 m thick arch dam may be two years and upto 200 years in case of a 150 m thick concrete gravity dam. Figure 26.1 below Indicates the general pattern of time-temperature history of mass concrete. 1

3

2

Temperature

T2

T3 T1 Time 1. Initial cooling period. 2. Stabilization period. 3. Final annual temperature cycle. T1 – Initial placement cycle. T2 – Peak temperature. T3 – Natural and design closure temperature

Fig. 26.1

Time temperature history of mass concrete

A definite member size beyond which a concrete structure should be classified as mass concrete is not readily available. However, ACI 211.1 states that “Many large structural elements may be massive enough that heat generation should be considered, particularly when the minimum cross – sectional dimensions of a solid concrete member approaches or exceed 2 to 3 ft (600 to 900 mm) or when cement contents above 600 lb per cubic yard) 365 kg/m3) are being considered”.

Mass Concrete and Temperature Rise in Concrete

26.3

If the rise in temperature and subsequent cooling were uniform throughout the mass of concrete, thermal stresses would not result. Because of differential temperature across the section, the expansion inside the mass which is at higher temperature will be restrained by the outside surface which may have experienced some degree of cooling off. Thus tensile stresses are generated at the outside surface while stresses inside are compressive in nature. If the tensile stresses exceed the tensile strength of concrete, cracks appear. As a general guidance, temperature differential between the internal and the external surface of the order of 20 degrees Celsius or more causes cracking. Cracks also occur, when the thermal movements are restrained externally, e.g. concrete tunnel lining in contact with rough rock surface.

26.3 METHODS OF TEMPERATURE CONTROL For general constructions, any concrete structure having lateral dimensions of 0.75 m or more may need appropriate mass concreting practice. The main emphasis is on temperature control, which can be achieved by number of measures. ACI Committee 207 lists the following: • Selection of low-heat-of-hydration or Portland Blended cements like PSC and PPC • Use of low cement content –200 to 450 lb per cubic yard (120 to 265 kg/m3) • Use of large size aggregates 75 to 150 mm and high aggregate content up to 80% of total aggregate. • Pre-cooling of mix water and aggregates to lower the temperature of fresh concrete. The temperature of concrete can be brought down upto 50°F (10°C) through proper measures. • Post-cooling of hardened concrete, through the embedded cooling pipes • Appropriate block dimensions for placement, lift heights and placing schedules, the lift height may be restricted to 1.5 m • Use of Pozzolans. The heat of hydration of a pozzolan is approximately 25% to 50% of that of OPC. • Use of steel forms for rapid heat dissipation. Some of these are elaborated as under,

26.4

CHOICE OF CEMENT SYSTEM

The only source of rise in temperature of fresh concrete is the heat of hydration of cement. The choice of cement having low heat of hydration is the primary step in controlling thermal cracking in mass concrete. National Specifications like ASTM type IV or Indian Standard IS: 12600 lay down the requirements of low-heat OPC, which were developed for such application. Low heat cement conforming to IS: 12600 is not commercially marketed in India; nevertheless, the limit of heat of hydration for such cement – 65 cal/g (272 Joules/g) at 7 days and 75 cal/g (314 Joules/g) at 28 days – serve as benchmark for other cements, which fulfill the requirements. Quality blended cements such as PPC and PSC meet this requirement. The heat of hydration of Portland cement depend upon its chemical composition – mainly C3S and C3A contents, and fineness of grinding. In modern cement plants, careful tailoring of chemical composition and control of particle size distribution (PSD) achieve desirable setting

26.4

Handbook on Advanced Concrete Technology

and strength properties. Thus, the cements can have lower C3A content and need not be ground to high Blaine’s fineness. Such cements can have low heat of hydration, without compromising on strength characteristics. The strength characteristics are much better controlled through particle size distribution (PSD) rather than Blaine’s fineness. Use of Pozzolana and granulated blast furnace slag in the manufacture of composite (blended) cements is particularly useful. Use of Pozzolana in concrete dams in the USA had started with the primary objective of controlling the heat rise. This is now well accepted throughout the world. Composite cements also render the concrete more durable and resistant to aggressive elements. The Table 26.1 gives typical values of heat of hydration of some popular cements being produced in India, both OPC and Blended cements. TABLE 26.1 Heat of Hydration Data of Indian Cement (Typical)

43 Grade OPC

53 Grade OPC

Blended Cements PPC/PSC with ª20% fly ash or ª50% slag respectively

Cal./g

Joules/g

BTU/lb.

65 80 95 75 85 105 64 74

272.155 334.96 397.8 293.09 355.9 439.63 268 309.83

117 144 456 126 153 189 115.2 133.2

3 Day 7 Day 28 Day 3 Day 7 Day 28 Day 7 Day 28 Day

Note 1 Calorie = 4.187 Joules = 0.00397 BTU 1 BTU = 252.0 Cal = 1055 J 1 cal/gm = 1.8 BTU/lb = 4.187 Joules/g

If the concrete aggregates are reactive to alkalies in the hydrated cement, the alkali silica reaction (ASR) is more likely in humid environments as in hydraulic structures. Use of ordinary Portland cement with additional stipulation of its alkali content expressed as (Na2O + 0.658 times K2O) being less than 0.6 percent becomes necessary. Blended or composite cements provide additional safeguard as equivalent alkali content in these cement is lesser than OPC. For more details, specialist literature should be referred.

26.5 MATERIALS AND MIX PROPORTIONING Apart from the heat of hydration of cement, its content in the concrete mix also decides the total heat evolved. As a rule of thumb, it is estimated that the temperature rise under adiabatic conditions is 12°C per 100 kg of cement per cubic meter of concrete. The aim should be to lower the cement content in the concrete mix as much as possible. In such situation, the nominal maximum size (MSA) of aggregate becomes important consideration. Theoretically, the larger the maximum aggregate size, the less cement is required in a given volume of concrete to achieve the desired quality. In large plain concrete constructions like dams, use of 80 mm or

Mass Concrete and Temperature Rise in Concrete

26.5

even 150 mm MSA is common. In some major concrete dams presently under construction in India, use of 150 mm MSA allowed M15 grade concrete to be made with cement content of 160 to 170 kg/cubic meter.

26.6

CALCULATION FOR RISE IN TEMPERATURE

In massive structures of high volume-to-surface ratio an estimate of the adiabatic temperature rise can be made as under: CH T = ___ S Where T = Temperature rise in degrees centigrade of the concrete due to heat generation of cement C = Proportion of cement in concrete, by weight H = Heat generation due to hydration of cement cal/g (at 7 days) S = Specific heat of concrete, cal/g/°C Example Assume a concrete weighing 2400 kg/m3 contains 200 kg cement and 50 kg pozzolan (fly ash) per cubic meter. Assume heat of hydration of pozzolan 40% that of the cement 200 + (0.4 × 50) C = _______________ = 0.0916 2400 The heat of hydration of cement (7 days) is assumed 75 cal/g. Specific heat of concrete normally ranges 0.35 to 0.45 cal/g/°C depending upon aggregates. In this example, it is assumed 0.4 0.0916 × 75 T = __________ 0.4 = 17.17°C Therefore if concrete is placed at 30°C, it can be expected to have a temperature of approximately 47.17°C at the interior of the member at 7 days if there is no heat loss. For relatively large pours, set-retarding admixtures are used to prevent formation of cold joints between successive placements, especially in Roller compacted concrete dams. It has been a practice in India to use air-entraining admixtures, to improve the workability of concrete in dam construction, though predominantly air entrainment is used to counter the effect of freezethaw. Mass concrete constructions may not be subjected to imposed loads at early ages. It is prudent to design such concrete mixes to attain the desired strength at a later age, may be 90 days, or even one year. This again allows lowering the cement content and facilitating the use of composite cement with larger proportion of cement replacement materials.

Precautions • For reinforced concrete elements of least dimensions of about 0.5 m or more, thermal cracking can result due to external restraints to thermal movements. This can occur in

26.6

Handbook on Advanced Concrete Technology

walls placed against existing foundation. Apart from controlling the generation of heat of hydration, the strategy should be to prevent heat loss from the external surface and prevent it from cooling too fast. For this, formwork should be insulated (tenting, quilts, or sand on polyethylene sheeting) and its removal delayed. Even hot water can be used for curing to keep surface warm. • Pre – cooling of fresh concrete is achieved by cooling of aggregates and replacing mix water by ice. The rate of hydration of cement is lower, the lower the concrete temperature. Depending upon the conditions, placing concrete at a low temperature of about 10 to 14°C result in the peak temperature being not much above the ambient. This reduces the incidence of thermal cracking. (Appendix – I gives calculation for determining the temperature of concrete and plant capacities). • Embedded pipe cooling allows temperature rise to be controlled in restrained zones. It consists of a series of evenly spaced pipe coils through which refrigerated or cold water is circulated. Such post-cooling may not be required when pre-cooling of concrete has been adopted. • In order to have as much of the total heat of hydration dissipated before the next placement, it is common practice in concrete dam construction to restrict the height of each lift, and to allow suitable time interval before the next placement. IS 14591-1991 (Temperature Control of Mass Concrete for Dams-guidelines) specifies minimum elapsed time between placing of successive lifts in any block restricted to 72 hours. The lift joints, however, need careful preparation before it receives the next layer of concrete. • Use large aggregates of size 75 to 150 mm and increase percentage of coarse aggregates up to 80% of total aggregates. • Keep low cement content and supplement with high quality Pozzolan/ggbs. • Usual care should be taken in all aspects of workmanship in batching, mixing, placing, compaction and curing as per sound engineering practices in general concrete constructions. Mass concrete needs special care in handling, placing and curing. As the interior concrete increases in temperature, the surface concrete may be cooling and contracting. This causes tensile stress and cracks at the surface if the temperature differential is too great. The width and depth of cracks depends upon the temperature gradient between the hot internal concrete and the cooler concrete surface.

References 1. ACI 116R – 16; Concrete and Concrete Terminology. 2. ACI 211.1 – 91; Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete. 3. ACI 207. IR – 96; Mass Concrete. 4. IS 12600 – Specificarion for Low Heat Portland Cement. 5. IS 14591 – Temperature Control of Mass Concrete for Dams-Guidelines. 6. SP 23 – 1982 – Handbook on concrete Mixes.

Mass Concrete and Temperature Rise in Concrete

26.7

APPENDIX – I Calculation of Temperature of Concrete and Plant Capacities How to determine the amount of ice and chilled water required per cubic meter of concrete to achieve a temperature of 20°C at placing point and the plant capacities for daily production of 1000 cubic meter of concrete The individual weight, water absorption and free moisture content of each ingredient is given below; Ingredient

Weight in SSD condition (kg/cum.)

Water absorption %

Free moisture content %

Cement Fly Ash 20 mm aggregate 10 mm aggregate Sand

280 100 702 468 730

– – 0.9 1.0 1.9

– – 0.3 0.5 3.0

Water

141

–

–

Calculation of individual weight after water correction Material

Wt/cum

Cement Fly Ash 20 mm 10 mm Natural sand

280 100 702 468 730

Free Water

141

Water absorption %

0.9 1.0 1.9

Free moisture content %

Excess moisture content %

0.3 0.5 3

Water correction (l)

Corrected wt. (kg)

–0.6 –0.5 1.1

4.2 2.3 –8.0

280.0 100.0 697.8 465.7 738.0

Total

–1.5

139.5

Temperatures of individual material is as given below; Material Cement Fly Ash 20 mm 10 mm Natural sand Free Water Excess water in natural sand

Weight

Temperature°C

280.0 100.0 697.8 465.7 738.0 139.5

40 35 33 33 35 6

1.1

35

Calculation of temperature of fresh concrete-based on the formula given in SP23. 0.22 (Ta.Wa + Tc.Wc) + (Ww-Wi)Tw + Wwa.Twa – 79.6.Wi Temperature of concrete (T) = __________________________________________________ 0.22(Wa + Wc) + Ww + Wi + Wwa Where T = temperature of freshly mixed concrete (°C)

26.8

Handbook on Advanced Concrete Technology

Ta, Tc, Tw, Twa = temperature of aggregate, cement, added mixing water and free water on aggregate. Wa, Wc, Ww, Wwa, Wi = weight of aggregate, cement, added mixing water, free water on aggregate, ice. Case 1: Based on Case 2: Based on

Mixing water is at ambient temperature i.e. 27°C the above formula the temperature of fresh concrete = 33.3°C Temperature of mixing water is 6°C the above formula the temperature of fresh concrete = 28.8°C

In order to achieve 20°C at placing point, we need to produce the concrete approx. 17°C at batching plant. To achieve this, part of the water is replaced by ice. Case 3: Temperature of water is 6°C and part water is replaced by ice When 89 kg of ice is added as replacement of water the temperature achieved = 16.9°C Calculation of plant capacities (Formulae given in ACI 207.4R) Ice plant capacity = Amount of ice (kg) × Total production per day × 10% extra = 89 × 1000 × 1.1 = 97900 kg = 98 tons per day (TPD) Maximum concrete placement rate (cum/hr) × heat to be removed (kJ/cum) Chilling plant capacity = ________________________________________ 12660 (kJ/hr) Suppose the chilling plant is operating for 16 hours per day, the chilling plant capacity 1000 × 16 × (51 × 4.18 × (27 – (6 –2*)) = __________________________________ 12660 = 27 Ton refrigeration (TR) * In order to achieve 6°C in batching plant, we need to produce water at 4°C in the chilling plant Specific heat of water = 4.18 kJ/kg °C.

27 Roller Compacted Concrete Sunil Sapre, Someshekar Shivgunde and Himanshu Kapadia

Roller Compact Concrete (RCC) is defined as “concrete compacted by using roller and in its unhardened state, will support a (vibratory) roller while being compacted. RCC is usually mixed using high-capacity continuous mixing or batching equipment, delivered with trucks or conveyors, and spread with one or more bulldozers in layers prior to compaction. RCC can use a broader range of materials than conventional concrete.

27.1 MATERIALS AND MIXTURE PROPORTIONING FOR RCC Mixture proportioning methods and objectives for RCC differ from those of conventional concrete. RCC must maintain a consistency that will support a vibratory roller and haul vehicles, while also being suitable for compaction by a vibratory roller or other external methods. The aggregate grading and paste content are critical parts of mixture proportioning. Specific testing procedures and evaluation methods have been developed that are unique to RCC technology. The cementitious material content for RCC dams has varied over a broad range from 59 kg/m3 to more than 297 kg/m3. At one end of the spectrum, the 75 mm nominal maximum size aggregate, interior mixture at Willow Creek Dam contained 60.5 kg/m3 of cementitious material. The mixture containing 47.5 kg/m3 of cement plus 19.0 kg/m3 of fly ash, averaged a compressive strength of 18.2 MPa at 1 year. In comparison, the 50 mm NMSA interior mixture at Upper Stillwater Dam contained 251.6 kg/m3) of cementitious material, consisting of 79.5 kg/m3 of cement plus 172.0 kg/m3 of fly ash and averaged a strength of 42.6 MPa at 1 year. Many RCC projects have used cementitious materials content between 104 and 178 kg/m3 and produced an average compressive strength between 13.8 and 20.7 MPa at an age of 90 days to 1 year. An essential element in the proportioning of RCC for dams is the amount of paste. The paste volume must fill or nearly fill aggregate voids and produce a compactable, dense concrete mixture. The paste volume should also be sufficient to produce bond and water tightness at the horizontal lift joints, when the mixture is placed and compacted quickly on a reasonably

27.2

Handbook on Advanced Concrete Technology

fresh joint. Experience has shown that mixtures containing a low quantity of cementitious materials may require added quantities of nonplastic fines to supplement the paste fraction in filling aggregate voids. Certain economic benefits can be achieved by reducing the processing requirements on aggregates, the normal size separations and the separate handling, stockpiling and batching of each size range. However, the designer must recognize that reducing or changing the normal requirements for concrete aggregates must be weighed against greater variation in the properties of the RCC that is produced and should be accounted for by a more conservative selection of average RCC properties to be achieved.

27.2 MATERIALS Cementitious Materials Portland cement—RCC can be made with any of the basic types of portland cement. For mass applications, cements with a lower heat generation are beneficial. They can include 33 Grade OPC cements, PPC – Portland Pozzolana cement (fly ash based) and PSC – Portland Slag cement). Strength development for these cements is usually slower than 53 Grade cements at early ages, but higher strengths than RCC produced with 53 G cements are ultimately produced. Heat generation due to hydration of the cement is typically controlled by use of lower heat of hydration cements, use of less cement, and replacement of a portion of the cement with pozzolan or a combination of these. Reduction of peak concrete temperature may be achieved by other methods, such as reduced placement temperatures.

Pozzolans The use of pozzolan will depend on required material performance as well as on its cost and availability at each project. Use of a pozzolan in RCC mixtures may serve one or more of the following purposes: 1. as a partial replacement for cement to reduce heat generation 2. as a partial replacement for cement to reduce cost; and 3. as an additive to provide supplemental fines for mixture workability and paste volume. The rate of cement replacement may vary from none to 80 percent, by mass. RCC mixtures with a higher content of cementitious material often use larger amounts of pozzolan to replace portland cement in order to reduce the internal temperature rise that would otherwise be generated and consequently reduce thermal stresses. In RCC mixtures that have a low cement content, pozzolans have been used to ensure an adequate amount of paste for filling aggregate voids and coating aggregate particles. Pozzolan may have limited effectiveness in low cementitious content mixtures with aggregates containing deleterious amounts of clay and friable particles. While the pozzolan enhances the paste volume of these mixtures, it may not enhance the long-term strength development because of insufficient availability of calcium hydroxide released from the portland cement for a pozzolanic reaction.

Roller Compacted Concrete

27.3

Class F fly ash and Ground granulated Blast furnace slag are recommended to be used in Roller compacted concretes. Class F Fly ash, especially at cool temperatures, generally delay the initial set of RCC mixtures, contributing to low early strength, but extending the working life of the freshly compacted lift joint. In high pozzolan content RCC mixtures, the heat rise may continue for up to 60 to 90 days after placing.

Aggregates The selection of aggregates and the control of aggregate properties and gradings are important factors influencing the quality and uniformity of RCC production. A basic objective in proportioning any concrete is to incorporate the maximum amount of aggregate and minimum amount of water into the mixture, thereby reducing the cementitious material quantity, and reducing consequent volume change of the concrete. This objective is accomplished by using a well-graded aggregate with the largest maximum size which is practical for placement. However, in RCC mixtures, the potential for segregation and the means of compaction must also be primary considerations in selecting the maximum size of aggregate. The combined aggregate gradation should be selected to minimize segregation. The key to controlling segregation and providing a good compactable mixture is having a grading that is consistent and contains more material passing the 4.75 mm sieve than typical in conventional concrete of similar nominal maximum size aggregate. In conventional concrete, the presence of any significant quantity of flat and elongated particles is usually undesirable. However, RCC mixtures appear to be less affected by flat and elongated particles than conventional concrete mixtures. This peculiarity is because vibratory compaction equipment provides more energy than traditional consolidation methods, and because the higher mortar content in RCC mixtures tends to separate coarse aggregate particles. Field tests with amounts of 40% flat and elongated particles on any sieve with an average below approximately 30%, with a ratio of 1:5, have shown flat and elongated particles to be no significant problem. The use of manufactured aggregate (crushed stone) has been found to reduce the tendency for segregation, as compared to rounded gravels.

Coarse Aggregate The selection of a nominal maximum size aggregate should be based on the need to reduce cementitious material requirements, control segregation and facilitate compaction. Most RCC projects have used a nominal maximum size aggregate of 37.5 mm to 75 mm. NMSA has little effect on compaction when the thickness of the placement layers is more than 3 times the NMSA, segregation is adequately controlled, and large vibratory rollers are used for compaction. Grading of coarse aggregate usually follows IS 383 size designations where close control of grading of coarse aggregate and RCC production are desired, size separations should follow normal concrete practice. However, as the size range increases, it becomes increasingly more difficult to avoid segregation of the larger particles during stockpiling and handling of this aggregate. Aggregate for RCC have used a single stockpile or been separated into as many as five aggregate sizes. RCC mixtures for over topping protection for embankment dams frequently use a NMSA of 25 mm as the concrete section is thinner.

27.4

Handbook on Advanced Concrete Technology

Fine Aggregate The grading of fine aggregate strongly influences paste requirements and compactability of RCC. It also affects water and cementitious material requirements needed to fill the aggregate voids and coat the aggregate particles. For those mixtures having a sufficient cementitious materials content and paste volume, IS 2386 & IS 383 fine-aggregate grading can be satisfactorily used. This can be determined when the mixtures are proportioned.

Fines In low cementitious materials content mixtures, supplemental fines, material passing the 0.075mm sieve, are usually required to fill all the aggregate void spaces. Depending on the volume of cementitious material and the NMSA, the required total minus 0.075 mm fines may be as much as 10% of the total aggregate volume, with most mixtures using approximately 3 to 8%. Characteristics of the fines and fines content will affect the relative compactability of the RCC mixture and can influence the number of passes of a vibratory roller required for full compaction of a given layer thickness. Regardless of whether it is accomplished by adding aggregate fines, cement, pozzolan, or combination of these, most compactable RCC mixtures contain approximately 8 to 12% total solids finer than the 0.075mm sieve by volume or 12 to 16% by mass. The fines fill aggregate void space, provide a compactable consistency, help control segregation, and decrease permeability. Including aggregate fines in low cementatious paste mixtures allows reductions in the cementitious materials content. Excessive additions of aggregate fines after the aggregate voids are filled typically are harmful to the RCC mixture because of decreases in workability, increased water demand and subsequent strength loss. When adding aggregate fines to a mixture, another consideration is the nature of the fines. Crusher fines and silty material are usually acceptable. However, clay fines, termed plastic fines, can cause an increase in water demand and a loss of strength, and produce a sticky mixture that is difficult to mix and compact.

Chemical Admixtures Chemical admixtures have known to play an important role in the production of roller compacted concretes. They have been effective in RCC mixtures that contain sufficient water to provide a more fluid paste. Water reducing and water-reducing and retarding are the most commonly used chemical admixtures. Water-reducing admixtures, used at very high dosages, but not exceeding optimum admixture dosage, have shown to reduce water demand, increase strength, retard set, and promote workability in some RCC mixtures. Air-entraining admixtures are not commonly used in RCC mixtures because of the difficulty in generating the air bubbles of the proper size and distribution when the mixture has a no-slump consistency. RCC exhibiting a fluid paste consistency has generally been necessary for air-entraining admixtures to perform.

Mixture Proportioning A goal of mass-concrete mixture proportioning, which is also applicable to RCC mixture proportioning, is to provide a maximum content of coarse aggregate and a minimum amount

Roller Compacted Concrete

27.5

of cement while developing the required plastic and hardened properties at the least overall cost. Optimum RCC proportions consist of a balance between good material properties and acceptable placement methods. This includes minimizing segregation. In implementing a specific mixture-proportioning procedure, the following considerations regarding plastic and hardened properties should be addressed:

Workability • Sufficient workability is necessary to achieve compaction or consolidation of the mixture and also to provide an acceptable appearance when RCC is to be compacted against forms. • Workability is most affected by the paste portion of the mixture including cement, pozzolan, aggregate fines, water, and air. RCC mixtures with the degree of workability necessary for ease of compaction and production of uniform density from top to bottom of the lift, for bonding with previously placed lifts, and for support of compaction equipment, generally have a Ve Be time of 10 to 45 sec. • The water demand for a specific level of workability will be influenced by the size, shape, texture and gradation of aggregates and the volume and nature of cementitious and fine materials.

Strength • RCC strength depends upon the quality and grading of the aggregate, mixture proportions, as well as the degree of compaction. • There are differing basic strength relationships for RCC, depending on whether the aggregate voids are completely filled with paste or not. • The water-cement ratio (w/c) law, as developed by Abrams in 1918, is only valid for fully consolidated concrete mixtures. Therefore, the compressive strength of RCC is a function of the water : cementitious materials ratio (w/cm) only for mixtures with a VeBe time less than 45 sec, but usually in the 15 to 20 sec range. For drier consistency (all voids not filled with paste) mixtures, compressive strength is controlled by moisture-density relationships. • With the same aggregate, the moisture content necessary to produce maximum compressive strength is less than the moisture required to produce an RCC mixture with a VeBe time in the range of 15 sec. There is little or no change in optimum moisture content with varying cementitious contents. • The design strength is usually not determined by the compressive stresses in the structure, but is more dependent on the required tensile strength, shear strength, and durability. These are usually dictated by dynamic and static structural analysis, combined with an analysis of thermal stresses.

27.6

Handbook on Advanced Concrete Technology

Segregation • A major goal in the proportioning of RCC mixtures is to produce a cohesive mixture while minimizing the tendency to segregate during transporting, placing, and spreading. Well-graded aggregates with a slightly higher fine aggregate content than conventional concrete are essential for NMSA greater than 37.5 mm. • If not proportioned properly, RCC mixtures tend to segregate more because of the more granular nature of the mixture. This is controlled by the aggregate grading, moisture content and adjusting fine content in lower cementitious content mixtures. • Higher cementitious content mixtures are usually more cohesive and less likely to segregate.

Permeability • Mixtures that have a paste volume of 18 to 22% by mass will provide a suitable level of impermeability, similar to conventional mass concrete in the un-jointed mass of the RCC. • Most concerns regarding RCC permeability are directed at lift-joint seepage. Higher cementitious content or high-workability mixtures that bond well to fresh lift joints will produce adequate water tightness. • However, lower cementitious or low workability content mixtures are not likely to produce adequate water tightness without special treatment, such as use of bedding mortar between lifts. Where a seepage cutoff system is used on the upstream face, the permeability of the RCC may be of little significance except as it may relate to freeze/thaw durability of exposed surfaces.

Heat Generation • To minimize the heat of hydration in RCC, care should be taken in the selection and combination of cementing materials used. In cases where pozzolan is used, it may be worthwhile to conduct heat of hydration testing on various percentages of cement and pozzolan to identify the combination that generates the minimum heat of hydration, while providing satisfactory strength, prior to proportioning the mixture. • The amount of cementitious material used in the mixture should not be more than necessary to achieve the necessary level of strength. • Proportioning should incorporate those measures which normally minimize the required content of cementitious material, such as appropriate MSA and well-graded aggregates.

Durability The RCC mixture should provide the required degree of durability based on materials used, exposure conditions and expected level of performance. RCC should be free of damaging effects of alkali-aggregate reactivity by proper evaluation and selection of materials.

Roller Compacted Concrete

27.7

Consideration should be given to higher cementitious material contents where air-entrained RCC can not be achieved, where RCC may be exposed to erosion by flowing water, or where protective zones of conventional concrete cannot be incorporated into the structure. TABLE 27.1 Mix Design followed at Shimajigawa Dam, Japan Materials

Value

MSA Cement Fly ash Water Sand-Aggregate ratio FIne Aggregate Coarse Aggregate Air

80 mm 84 kg/M3 36 kg/m3 105 kg/m3 34% 752 kg/m3 1482 kg/m3 150%

Design and Construction Considerations Water Tightness and Seepage Control Achieving water tightness and controlling seepage through RCC dams are particularly important design and construction considerations. Excessive seepage is undesirable from the aspect of structural stability and because of the adverse appearance of water seeping on the downstream dam face, the economic value associated with lost water and possible long-term adverse impacts on durability. RCC that has been properly proportioned, mixed, placed and compacted should be as impermeable as conventional concrete. The joints between the concrete lifts and interface with structural elements are the major pathways for potential seepage through the RCC dam. This condition is primarily due to segregation at the lift boundaries and discontinuity between successive lifts. It can also be the result of surface contamination and excessive time intervals between lift placements. Seepage can be controlled by incorporating special design and construction procedures that include contraction joints with water-stops making the upstream face watertight, sealing the interface between RCC layers and draining and collecting the seepage.

Upstream Facing RCC cannot be compacted effectively against upstream forms without the forming of surface voids. An upstream facing is required to produce a surface with good appearance and durability. Many facings incorporate a watertight barrier. Facings with barriers include the following: • Conventional form work with a zone of conventional concrete placed between the forms and RCC material. • Slip-formed interlocking conventional concrete elements. RCC material is compacted against the cured elements. • Precast concrete tieback panels with a flexible waterproof membrane placed between the RCC and the panels.

27.8

Handbook on Advanced Concrete Technology

A waterproof membrane sprayed or painted onto a conventional concrete face is another method; however, its use has been limited since such membranes are not elastic enough to span cracks that develop and because of concerns about moisture developing between the membrane and face and subsequent damage by freezing.

Horizontal Joint Treatment Bond strength and permeability are major concerns at the horizontal lift joints in RCC. Good sealing and bonding are accomplished by improving the compactibility of the RCC mixture, cleaning the joint surface, and placing a bedding mortar (a mixture of cement paste and fine aggregate) between lifts. When the placement rate and setting time of RCC are such that the lower lift is sufficiently plastic to blend and bond with the upper layer, the bedding mortar is unnecessary; however, this is rarely feasible in normal RCC construction. Compactibility is improved by increasing the amount of mortar and fines in the RCC mixture. The lift surfaces should be properly moist cured and protected. Cleanup of the lift surfaces prior to RCC placement is not required as long as the surfaces are kept clean and free of excessive water. Addition of the bedding mortar serves to fill any voids or depressions left in the surface of the previous lift and squeezes up into the voids in the bottom of the new RCC lift as it is compacted. A bedding mix consisting of a mixture of cement paste and fine and 10mm MSA is also applied at RCC contacts with the foundation, abutment surfaces, and any other hardened concrete surfaces.

Seepage Collection A collection and drainage system is a method for stopping unsightly seepage water from reaching the downstream face and for preventing excessive hydrostatic pressures against conventional concrete spillway or downstream facing. It will also reduce uplift pressures within the dam and increase stability. Collection methods include vertical drains with water-stops at the upstream face and vertical drain holes drilled from within the gallery near the upstream or downstream face. Collected water can be channeled to a gallery or the dam toe.

Non-overflow Downstream Facing Downstream facing systems for non-overflow sections may be required for aesthetic reasons, maintaining slopes steeper than the natural repose of RCC, and freeze-thaw protection in severe climate locations. Facing is necessary when the slope is steeper than 0.8H to 1.0V when lift thickness is limited to 30 cm or less. Thicker lifts require a flatter slope. Experience has demonstrated that these are the steepest un-compacted slopes that can be practically controlled without special equipment or forms. The exposed edge of an un-compacted slope will have a rough stair stepped natural gravel appearance with limited strength within 30 cm of the face. Downstream facing systems include conventional vertical slip-forming placement and horizontal slip-forming similar to that used on the upstream face. When this type of slope is used, the

Roller Compacted Concrete

27.9

structural cross section should include a slight overbuild to account for deterioration and unraveling of material loosened from severe weather exposure over the project life

Transverse Contraction Joints • Transverse contraction joints are required in most RCC dams. The potential for cracking may be slightly lower in RCC because of the reduction in mixing water and reduced temperature rise resulting from the rapid placement rate and lower lift heights. In addition, the RCC characteristic of point-to point aggregate contact decreases the volume shrinkage. • Thermal cracking may, however, create a leakage path to the downstream face that is aesthetically undesirable. Thermal studies should be performed to assess the need for contraction joints. Contraction joints may also be required to control cracking if the site configuration and foundation conditions may potentially restrain the dam. If properly designed and installed, contraction joints will not interfere or complicate the continuous placement operation of RCC.

Waterstops Standard water-stops may be installed in an internal zone of conventional concrete placed around the joint near the upstream face. Water-stops and joint drains are installed in the same manner as for conventional concrete dams. Around galleries and other openings crossing joints, water-stop installation will require a section of conventional concrete around the joint.

27.3 CONSTRUCTION OF RCC DAMS The layout, planning, and logistics for construction with RCC are somewhat different than for conventional mass-concrete construction. Instead of vertical construction with independent monolith blocks, RCC construction involves placing relatively thin lifts over a large area. Conventional mass-concrete placement usually requires a high ratio of man hours to volume placed due to labor-intensive activities, such as forming faces, joint preparation, and consolidating concrete with internal vibrators. RCC typically has a lower ratio of man hours to volume placed because of the use of mechanical equipment for spreading and compacting the mixture, less forming, and reduced joint cleanup. Interruptions and slowdowns, generally, cause reduced joint and RCC quality, as well as increased costs.

Aggregate Production and Plant Location Aggregate stockpiles and the concrete plant location for RCC can be even more important than for conventional concrete. Typically, large stockpiles are provided prior to starting RCC placement. Some of the reasons for this are: • Temperature control: Producing aggregate during the winter so that they are stockpiled cold for later use • Rapid placement rate—The rate of aggregate use during RCC placing may exceed the capacity of an aggregate production plant. Large aggregate stockpiles also have the benefit of more stable moisture contents, which reduce variations in RCC consistency.

27.10

Handbook on Advanced Concrete Technology

• Inadequate cementitious material delivery and storage has limited RCC production on some projects. A steady flow of these materials is necessary for optimum production and consistent RCC quality. • The RCC plant layout and location should be selected to minimize energy requirements and be appropriate for the terrain, whether the RCC is transported by conveyor or haul vehicles. • The plant should be located on a raised area and graded so that spillage and wash water drain away without creating a muddy area, especially if vehicular haul is used. • A plant location adjacent to the RCC structure minimizes transport time, which is critical to RCC quality and reduces transport equipment needs. The plant should have a bypass or belt discharge that allows for wasting out-of-specification RCC without delivering it to the dam.

Proportioning and Mixing The RCC method changes the production-controlling elements of mass-concrete placements from the rate of placement for conventional mass concrete to the output of the concrete plant and delivery system for RCC. Rapid and continuous delivery of RCC is important to mass applications. Mixers for RCC need to accomplish two basic functions: • the mixers should thoroughly blend all ingredients, and • should provide sufficient capacity for high placing rates typical in RCC. Typical placing rates are 76 m3/hr for small size projects, 190 to 380 m3/hr for medium projects, and 570 to over 760 m3/hr) for large projects. Variations in free moisture content of the aggregates can be particularly troublesome at plant start-up. Providing too little water in the initial mixtures is particularly undesirable because Initial mixtures are frequently used for covering construction joints or foundation areas where the RCC should be on the wet side for improved workability and bond. It is better to start with higher moisture content and to subsequently reduce it to the desired consistency than to start with a mixture that is too dry. Accurately introducing the specified quantities of materials into a mixer is only one part of the mixing process. Uniformly distributing and thoroughly blending materials, and discharging them in a continuous and uniform manner are also essential for providing quality RCC.

Batching and Drum Mix Methods RCC has been successfully produced with conventional batch type plants and drum mixers. Lower production, bulking, sensitivity to the charging sequence, slow discharge and buildup in the mixer are common problems in RCC production when compared to batching of conventional plant and transmit mixed concrete. • Proper ribboning or sequencing and feed rates of the aggregates and cementitious materials, as they are fed into the mixer, are important factors in minimizing mixing time and buildup for both drum type batch operations and continuous mixers.

Roller Compacted Concrete

27.11

• The timing of adding water to the mixture and the angle of its introduction has been critical in drum mixers. • Mixture uniformity should be maintained at all production rates that will be used. Continuous mixers typically work efficiently above a minimum production rate, and up to production levels that are two to three times that of the minimum rate. • Variations in production requirements, such as near abutments around galleries or other confined areas, can be accommodated on large projects with multiple mixers by shutting down some of the mixers until the higher production rate is needed again. • The accuracy of the concrete plant and methods for control of the mixture during production should be studied for cost effectiveness and mixture strength requirements.

Transporting and Placing The process of mixing, transporting, placing, spreading and compacting should be accomplished as rapidly as possible and with as little re-handling as possible. The time lapse between the start of mixing and completion of compaction should be considerably less than the initial set time of the mixture under the conditions in which it is used. A general rule for mixtures with little or no pozzolan is that placing (depositing), spreading and compacting should be accomplished within 45 min of mixing and preferably within 30 min of mixing. This limit is applicable at mixture and weather conditions of approximately 21°C and mixtures that are non-retarded. The time can be extended for cooler weather and should be reduced in warmer weather. Low humidity, windy conditions and multiple handling can decrease workability and reduce the allowable time for completing compaction to less than 45 min.

Segregation Considerations The maximum size of the aggregate and the tendency for the mixture to segregate are major factors in selecting equipment used to transport RCC from the mixing plant to the placement area. A 38 mm NMSA concrete can be transported and placed in non-agitating haul units designed for aggregate hauling and earth moving, without objectionable segregation. Conveyor systems must be designed to minimize segregation at transfer points. RCC mixtures with a 75 mm NMSA have a greater tendency to segregate when they are dumped onto hard surfaces, but with care and proper procedures, these mixtures have been hauled, dumped and remixed successfully. Severe segregation can occur during the transportation and placing of large NMSA, and drier consistency mixtures. Design of wetter consistency mixes also reduces the tendency of mixes to segregate.

Transporting Methods The two principal methods of transporting RCC are by conveyor and by hauling vehicles. Transport by bucket or dinky have been used, but these slows the rate of production and are more prone to cause segregation. However, if such a system is already available (or necessary) for large volumes of conventional concrete, it can also be used for the RCC.

27.12

Handbook on Advanced Concrete Technology

Conveyors Transport by continuous high-speed conveyors from the concrete plant directly to the mass RCC placement in particular for dams is ideal. The overall economics, including direct and indirect costs of alternate delivery systems, as well as reliability, the final quality and schedule, should be considered when deciding whether to use or require a conveyor delivery system. Conveyor systems that work well with a conventional concrete may not work well with a low cementitious, drier, larger-aggregate or high-fines RCC. Clogged transfers, segregation at the discharge, severe wear at transfers, segregation over rollers, slow belts, not being able to start or stop a loaded belt, drying, loss of paste and contamination of the RCC lift surface from material dropping off the return side of belts are the most common potential problems associated with conveyor transport.

Haul Vehicles If vehicles are to be used for transporting RCC, a thorough preliminary study should be made of the haul road system. Problems that may prevent hauling by road include steep and rough terrain, lack of road-building material, plant location, schedule and environmental considerations. If the concrete plant is located upstream of a dam, the method of bringing the road through or over the upstream face, system must be worked out in detail. From a scheduling standpoint, construction of roads should be completed prior to start of RCC placement. Raising the roads fast enough to keep up with the rate of rise of the dam may require so much time that it becomes an inefficient system at higher elevations. To avoid slowing the mixing and placing operations, raising the haul roads during a 2 to 4 hr/day shutdown period, while maintenance and other work is being performed, should be considered.

Placing and Spreading A preferred technique of placing RCC in a dam is to advance each lift from one abutment to the other. An exception is where the distance from abutment to abutment is shorter than the distance from the upstream to the downstream face, such as at the bottom of dams in narrow canyons. In this case, placement can be started by working in the upstream-downstream direction. Motor graders have been used on some RCC projects for spreading RCC. They are difficult to maneuver in small areas and at abutments. The tires and blade can damage compacted surfaces. There also is a tendency to overwork and rework the surface. Tracked dozer equipment has proven to be best for spreading RCC. Tracked dozers are fast, sufficiently accurate, and contribute to uniformly compacted RCC. By careful spreading, a dozer can remix RCC and minimize segregation that occurs from dumping. Careful attention should be given to ensure that remixing is occurring and that the dozer is not simply burying segregated material. Dozers using U-shaped blades are typically modified by welding extension plates on the edges of the blades to limit segregation that can occur as RCC rolls off of the edge during spreading. Dozers should have at least hydraulic tilt capability and preferably both tilt and angle hydraulic capability. Operating the dozer on a compacted surface will damage the RCC. When it is necessary for the dozer to drive onto compacted RCC, the operator should limit the movement to straight back and forth travel, or travel on rubber mats, or both, such as lengths of old conveyor belts.

Roller Compacted Concrete

27.13

If the roller operator simply stays back from the conventional concrete far enough to avoid sinking into it or shoving it up, the two mixtures may not adequately compact or bond together. Conventional concrete is usually needed for appearance and possibly durability of the exposed face. The minimum amount that can be stacked against the form, approximately 50 to 150 mm wide, will provide a conventional concrete for appearance. If the RCC has a wetter consistency, and especially if it has a delayed set, it is possible to place the conventional concrete mixture after the RCC. The facing concrete still needs to have a relatively low slump when RCC compaction is performed, but it can still be possible to immersion vibrate the interface region of the RCC and conventional concrete. The most common compacted lift thickness has been 300 mm. The trend is to use the thickest lifts compatible with the RCC mixture and the spreading and compaction equipment to achieve the specified minimum density. In Japan, thicker lifts from approximately 0.5 to 1.0 m have been compacted in one lift after being spread by dozers in several layers. This is only possible with the help of specially designed poly carboxylic ether based admixtures which makes the concrete vibration sensitive and therefore the paste is released at a faster rate. A 300 mm thickness is convenient to work with in the field, if no such special admixtures are available. Another factor influencing lift thickness is the maximum allowed exposure time before covering one lift with the subsequent lift. Each project should be studied to optimize the benefits of various lift thicknesses. Thicker lifts mean longer exposure times but fewer lift joints and fewer potential seepage paths. Thinner lifts result in more potential lift joints but allow the joints to be covered sooner, resulting in improved bond. Mix proportions will also affect the workability and consequently the ability to achieve uniform density, for the full lift thickness.

Recommendations Based on various experiences the following recommendations can be drawn on equipment and methods used for RCC construction. • Batching and mixing: A continuous, weighing plant for speed and the ability to record input mix parameters. The plant should feed to a gob hopper if necessary, for intermittent loads feeding hauling vehicles. • Transporting: End dump trucks that can dump into an intermediate hopper or directly in front of the dozer. Front-end loaders only for operating along sloped sidewalls. Conveyors if the entire placement can be reached with few set-ups. • Spreading: A dozer equipped with a spreader box and a side edge compactor for unformed, sloping faces. • Compaction: roller.

A double-drum roller for wide, open areas; otherwise a single-drum

• Edge Compaction: A small track-hoe equipped with a vibrating plate compactor. The plate should be about 3ft (0.9m) long, 1ft [0.3m] (one lift) high, and have about 6 inches (15cm) of top down-pressure.

27.14

Handbook on Advanced Concrete Technology

Conclusion Roller compacted concrete, though an old technology, requires a lot of important consideration in each and every step right from choosing the raw materials to the use of particular equipment for a job. Roller compacted concrete (RCC) dams have become popular recently due to their cost-effectiveness compared with traditional dams built with conventional mass concrete (CMC). Since the first RCC dams were constructed more than 25 years ago, a number of different design concepts and innovations have been developed to fulfill the objectives of the structure. The selection of the type of RCC dam concept that better suits the conditions of each project should be carefully evaluated. The optimization in the use of the materials available is a main aspect to consider for that selection. The latest being the Vis-RCC technology - Vibration Sensitive RCC, which reduces the construction time manyfold.

References 1. Dunstan, M. R. H., “A Method of Design for Mix Proportions of Roller-Compacted Concrete to Be Used in Dams,” Transactions, 15th International Congress on Large Dams, International Commission on Large Dams (ICOLD), Paris, V. 2, pp. 713-738. 2. Dolen, T. P., 1991, “Mixture Proportioning Concepts and Quality Control for RCC Dams,” International Symposium on Roller-Compacted Concrete for Dams, Beijing,. pp. 440-447. 3. Schrader, E. K., and Namikas, D., 1988 “Performance of Roller-Compacted Concrete Dams,” 16th International Congress on Large Dams, San Francisco, , International Commission on Large Dams (ICOLD), V. III, Q62, pp. 339-364. 4. Schrader, E. K., 1994. “Roller-Compacted Concrete for Dams, State of the Art,” International Conference on Advances in Concrete Technology, Athens, Greece; 2nd Edition, CANMET, Ottawa, Canada. 5. “Technical Guide to RCD Construction Method,” Technical Center for National Land Development, Japan, 1981. 6. Dolen, Timothy P., 2003 “Long-term Performance of Roller Compacted Concrete at Upper Stillwater Dam, Utah, U.S.A., International Symposium on Roller Compacted Concrete,” Madrid, Spain. 7. Pauletto, M., Dunstan M.R.H. and Ortega, F., 2003 Trial mix programme and full-scale trials for Olivenhain RCC dam, USA, Proceedings of the 4th International Symposium on RCC Dams, pages 329-338, Madrid, Spain. 8. Maia, M.C. and Silva Matos, D., 2006 Pedrógão nears completion in Portugal, The International Journal on Hydropower and Dams, Volume 13, Issue 5.

28 Foam Concrete Samir Surlaker and B.V.B. Pai

28.1 INTRODUCTION There are two names normally used to describe this category of light weight concrete as foam concrete or foamed Concrete. The more appropriate name should be foam concrete as concrete is not foamed but only preformed foam is used with normal concrete or for that matter most commonly to a premixed cement slurry or cement and sand mortar or cement with many supplementary cementitious materials like PFA, GGBSF, Metakaolin and/or silica fume to impart mechanical properties. Foam concrete has been used all the world over since 1920 where compressive strengths were not criteria but applications were that of filler material having low weight, thermal and accoustic insulations. They are used for multiple uses as backfill materials and trenches and for sub bases for roads. The densities can be easily controlled by controlling the quantity of preformed foam and only concern is to control the stability of foam and to avoid coalescence of air voids to create larger bubble sizes and wrong spacings. The modern technologies can produce stable monocellular foams with controlled entrapments to exactly determine the densities depending upon specific application.

28.2 ADVANTAGES OF FOAM CONCRETE The greatest advantage of foam concrete is being lightweight while maintaining almost all properties of conventional concrete. It can be used for internal as well as external applications The production of foam concrete is relatively simple and does not need special machineries except to produce a performed foam with different foaming agents available. Few advantages are enlisted as under: • Foam concrete is easy to place and finish • Reduction of dead weight of structure • Can be precast or cast in-situ • Excellent insulation – Thermal and Accoustic • Light weight filling material

28.2

Handbook on Advanced Concrete Technology

• • • • •

28.3

Excellent as underlays in flooring Can be impregnated or coated Resistant to fire Nails/fittings can be driven in foam concrete elements easily Easy to produce cut, saw and carry With all these advantages, foam concrete is a material which can be used to advantage in construction

APPLICATIONS OF FOAM CONCRETE

Conventionally foam concretes can be used for several non structural applications as under: • Backfills • Light weight blocks, bricks etc • Light weight in-situ walls • Filling of cavities in building utilities • Insulation concrete for roofs before waterproofing • Trench reinstatements • Flooring underlays • Walls and floors of low cost housing with insulation properties • Roofings for low cost housing • Concretes where fire proofing properties/thermal insulations are required • Filling of cavity walls • Light weight precast elements for internal and external purposes For different applications, different densities are desired. To achieve these densities it requires different mix composition and above all different foam quantities to be introduced. With new machineries, all these parameters can easily be calculated before hand and can be achieved in practice. Few examples are given below: Density : 300-600 kg/m3 Material : Only cement + foam Applications : Thermal and acoustic insulations, cavity walls blocks etc Density : 600-1000kg/m3 Material : Cement + Fillers + foam Application : Slabs, flooring bases, internal walls and blocks etc. Density : 1000-1600 kg/m3 Materials : Cement+ fillers +sand+ foam+ light weight beads/fibres Application : External application for walls and structural elements

Foam Concrete

28.3

: 1600- 1800 kg/m3 : Cement + sand + supplementary cementitious materials + foam beads/ fibres Application : Medium weight elements in situ casting of wall, reinforced element precast and in situ, including structural application.

Density Material

For the production of foamed concrete, cement, fillers and water are thoroughly mixed. The composition of the mortar is important for the resulting compressive strength, however, it is mainly depending on the specific gravity. Lower specific gravity corresponds with lower strength and better insulation properties.

28.4 PRODUCTION OF FOAM CONCRETE Production of foam concrete is a simple process and can be done in normally available mortar/concrete mixers. The additional requirement is a Foam Generator which makes a preformed mono cellular foam which can be added to cement slurry or cement + fillers slurry so that it is homogenously dispersed in the mix. Placing can be done as in conventional concrete with a precaution to see the pours are limited to 0.5 to 0.75 m height so that the foam concrete is not unduly compacted to loose its designed density. Fully integrated plants are available now which include foam Generator in the assembly so as one single unit can produce foam, mix ingredients and eventually pump the foam concrete in form works. A typical assembly is shown in Fig. 28.1. The foam generator can be incorporated into any existing standard production processes without problem.

1 binders adders aggregate madrigals 3

5 2 4

1

Foam concentrate

4 Mixer

2

Control unit

5 Basic materials

3

Mc-foam generator II

6 Transport vehicle

6

Fig. 28.1 Typical assembly for foam concrete

Foam Generators are normally of two, types. The first one is which produces only foam by combination of water, foaming agent and air and this is added to already prepared Mix. Figure 28.2 shows foam Generator and Aeration section. If only cement and finer materials are part of mix, a simple hand held foam Generator is available. The functional principle is

28.4

Handbook on Advanced Concrete Technology

shown in Fig. 28.3, Fig. 28.4 shows the set up of second type of Foam Generator. In this case the cement slurry and foam are mixed at the head and foam concrete is the result. This is very suitable for low density foams and with its easy to carry assembly. This is extremely suitable for low volume foam concretes. Air

Mixture of water and foam concentrate

Fig. 28.2 The centre piece of foam generator the aeration section

Pump

Compressed air

MC-Foam generator III

Fig. 28.3 The functional principle of foam generator

Fig. 28.4 Foam genrator assembly

Foam Concrete

28.5

Generally foaming agent used in the two types are different. Materials required are mostly cement, fine rounded sand, fine minerals supplementary cementitious materials foaming agent and water. Several Mix designs are shown it Table 28.1. Which can be used as a guideline for initial mix designs. First step is to decide the wet density and therefore what quantity of Foam is required for one m3 of foamed Concrete. TABLE 28.1 Properties of Foam Concrete Cast density

400

500

600

700

800

Cube compressive strength (28 days) • attainable maximum (28 days) Tensile strength (28 days) • attainable maximum (28 days) Flexural strength (28 days) Modulus of elasticity (compression 28 days) Shrinkage (laboratory) Shrinkage (in actual practice) Water absorption Water vapour diffusion resistance factor • between 50% - 100% RH • between 70% - 100% RH Heat conduction on coefficient • absolutely dry material • at 70% RH • at 95% RH

0.5 1.3 0.05 0.10 0.10 300 5.5 1.5 75

1.0 2.0 0.10 0.20 0.15 650 5.5 1.3 50

2.0 3.0 0.20 0.30 0.35 1,200 4.5 1.2 33

3.0 3.0 0.25 0.35 0.44 1,850 4.5 1.2 22

4.5 4.5 0.30 0.45 0.50 2,200 4.0 1.2 15

2.5 5.0

3.5 6.0

4.0 7.0

4.5 8.0

5.5 9.0

0.09 0.11 0.14

0.10 0.13 0.17

0.12 0.15 0.20

0.14 0.18 0.23

0.17 0.22 0.22

The highly compact device is used in a steady state in ready-mixed concrete and mortar plants, whereby only electricity, water and sufficient air pressure form the technical requirements. The method can be incorporated into existing production processes for the production of foam with ease. In addition, the simple start-up of operation is economical and controllable. Foaming agents should have following properties: • Free of chloride • High stability of the pores – See Fig. 28.5 • Good workability of concrete/mortar • Improves the homogeneity of the concrete/mortar • Good water retention • Specific gravity can be adjusted according to the requirements.

28.6

Handbook on Advanced Concrete Technology

Fig. 28.5 Mono cellular foam

Foaming agents should produce: • Trowel-ready mortar • Insulation mortar • Foamed concrete • Filling grouts • Levelling mortar • Light weight concrete • Polystyrene concrete A good foaming agent should result in: • Mono cellular foam i.e uniform pore size • Specifically controllable pore size • Very stable foam with low quantity of additive • Trouble-free use even with highly absorbent aggregates • Distinct savings through more economical mortar mixtures

28.5 THE TECHNIQUE The new technology, a mixture of water and foam concentrate is charged directly with air via a porous membrane, thus forming a highly stable homogenous, mono cellular foam. The modified generator is combined with a conventional pump (e.g. screed pump), which makes it possible to foam mineral slurries. Because the machine is mobile, it can be used directly at the point of application, thus providing considerable logistic advantages for the company carrying out the work. Costs are reduced because only the mineral slurry has to be transported and does not reach its full volume until it is foamed on site. The “foam mortar” produced in this manner can be adjusted to extremely low bulk densities. Areas of application include the packing or injection of voids in rock, concrete and mansonry. This technology can also be used where it is not possible to pump ready-made material because of excessive distances. It is, therefore, suitable for backfilling and packing operations in the mining and tunnel construction sectors.

Foam Concrete

28.7

In this new technology a mixture of water and foam concentrate is fed, using air, directly above a porous membrane with a defined pore size. This creates homogenous mono-cellular foam of high stability. This method can even do without the use of stabilizers. For concrete and mortar with pre-fabricated foam. The complete system developed consists of the Foam Generator itself and special foam concentrations. Water and air pressure are varied via manually adjustable valves. An additional central pump feeds the required amount of foam concentrate. This way the desired foam consistency can easily be set. The Foam Generator can be controlled manually or electronically, whereby the amount of foam required is recalled via a time control unit. Simplicity of handling and low maintenance is ensured. Adding preformed foam to the foam Generator allows the production of fresh mortar that is of high quality and is economical at the same time. Time-consuming mixing of the air-entraining agent become obsolete, which means shortened mixing times and increased productivity. The mono–cellular foam produced with the foam Generator produces a smooth mortar with very stable air voids that can be applied with ease. Stabilizers are therefore no longer required. By selecting a suitable foam concentrate the mortar’s workability can be optimized. The working time can be controlled with precision with mortar retardants, which are harmonized with the respective starting substances. With the Foam Generator a ready-to-install and pumpable porous lightweight concrete can be produced directly at the ready-mixed concrete plant by adding foam to a pre-fabricated concrete or mortar mixture. Through the added foam quantity, bulk density in the range of 400 to 2000 kg/m3 and correlating compressive strengths of 2 to 20 N/mm2 can be set with precision. The diverse range of application possibilities of porous lightweight concrete consists of fillings (sewage systems, pipes, cesspits, underground tanks etc.) to insulation layers in the restoration of old buildings and even in cattle sheds right down to levelling layers and internal partitioning walls. In contrast to normal mortar, no air voids are added to lightweight masonry mortar in order to achieve better workability. Stable air voids in connection with suitable light aggregates are a vital component of the mortar composition in order to maintain low bulk density and to guarantee insulation properties. While this can seldom be realized with standard procedures, the system allows the problem-free production of a quality lightweight mortar. In this instance it is not the compressive strengths but the low bulk density, i.e high insulation and good flow property, that is the main concern. In the production of no-fines lightweight concrete, aggregates and pre-fabricated lightweight concrete components, through, the use of foam that is pre-fabricated with the Foam Generator, the quality improvement and a higher cost-effectiveness can be achieved. In this area especially the ease of workability, the low bulk density and as a result the improved heat insulation, as well as a better finish of the precast concrete blocks or components must be stressed.

28.6

SPECIAL APPLICATIONS

For some special application areas further developments are available, the Foam Generator is combined with a standard concrete pump (e.g. a screed pump), which can then form the direct foaming of binder pastes. The machine is a mobile unit and can therefore be used directly on the site of application. This is shown in Fig. 28.4. This translates into considerable logistic

28.8

Handbook on Advanced Concrete Technology

advantages for the executing company. It saves costs since only the binder glues need to be transported and only on-site is the final large volume then achieved through the foaming of the components. The Foam Mortar thus prepared can be set at extremely low bulk densities. Application areas here are filling or pressure-grouting of cavities in stone, concrete or brickwork that need to be removed. Further application areas for this technology exist where the conveyance of the prefabricated material via pumps is impossible due to distances being too long, e.g. in the filling of pits etc.

Conclusion A lot of research is going on for developing structural light weight concretes and development of such high strength, high flexure foam concretes can be of tremendous use to civil engineering industry to enable to reduce weights of structural components. It is reported that foam concretes of over 50 MPa are already designed at laboratory levels and it would be a matter of time when the success can be translated to construction sites. This is made possible by use of very low w/c ratio of around 0.3 and supplementary cementations materials like microsilica especially in slurry form, alumino silicates, finely ground granulated blast furnace slags with higher reactivity levels to impart desired mechanical properties. The advent of precision machineries with electronic controls with respect of micro bubble sizes and monocellular foam outputs, the desired properties are achievable. It can be a material of future. All in all, foam concrete is a very versatile, light weight material and can be employed in almost all civil engineering applications with a sound judgment. It has the potential of future lightweight structural material.

References 1. Technical literatures on Foam concrete Ltd, UK, MC-Bauchemie, Germany, Edama GmbH, Germany. 2. Foam Concrete by Lynton Cox and Simon Van Dijk, Feb 2002, Concrete. 3. Air void system of Foamed concrete and its effect on mechanical properties by Tiong-Huan Wee et al., Jan-Feb 2006 ACI Materials Journal.

29 Acid Resistant Concrete Samir Surlaker and B.V.B. Pai

29.1 INTRODUCTION Adopting the definition of ACI for High Performance Concrete, High Performance Concrete is a concrete that meets special combination of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing and curing practices. Thus a High Performance Concrete is a concrete in which certain characteristics are developed for a particular application and environment. Examples of characteristics that may be considered critical in an application are: ease of placement, compaction without segregation, early age strength, long-term mechanical properties, permeability, density, heat of hydration, toughness, volume stability and long life in severe environments. This implies that either improvement of rheology or durability property qualify the concretes to be termed as High Performance Concrete. Though both these properties are desirable at the same time, the judicious balance between the two is a more common approach under actual site conditions. Table 29.1 shows some properties and areas of application of High Performance Concrete. TABLE 29.1 Properties of High Performance Concrete Property

Type of Concrete

Areas of application

Compressive Strength

High strength Concrete

Constructuion Elements in High Rise Buildings

Workability

Self Compacting Concrete

Precast Industry Filigree Construction Elements

Durabilty

High Resistance Concrete Chemical-Mechanical High Density Concrete with Low Permeability

Natural Draught Cooling Tower Tank Bund Areas Marine Structures

Density

Durability is clearly stated as a function of exposure class, design and detailing, concrete composition and placement, workmanship and the achievement of specified cover. Depending

29.2

Handbook on Advanced Concrete Technology

upon exposure conditions, the durability of the exposed concrete is determined and service life is defined accordingly. In view of this requirement, it would be necessary to produce concrete with low water cement ratio to achieve virtual impermeability. Air and gas diffusion coeffecients also influence the deterioration process. Therefore higher density concrete matrices would be required for High Performance Concrete. The word Performance will be the key word in defining, designing and specifying HPC concrete. Acid Resistant Concrete is a particular case of HPC.

29.2

ACID RESISTANT CONCRETE

In general, concrete is susceptible to damage by action of acids. The degradation mechanism involves dissolution of soluble constituents of cement paste destroying its crystalline structure. The major factor contributing to destruction of concrete is its permeability and the concentration and type of acids. Ordinary Portland Cement based concretes are more vulnerable to attacks on account of high quantity of calcium hydroxide released during hydration of Calcium Silicates. It should be noted that the Binder is damaged by acid attack. Fig. 29.1a shows binder damage after acid attack as viewed through a microscope.

Fig. 29.1a Binder damage after acid attack as viewed through a microscope.

The chemical reaction can be expressed as follows: 1. Formation of Calcium Hydroxide C3S, C2S +H2O Æ CSH + Ca (OH)2 • Reaction of Calcium Hydroxide Ca(OH)2 + ‘acid’ Æ Ca++ (solution) It is also an established fact that concretes made with pozzolanas like Blast Furnace slag or fly ash in which Calcium Hydroxide is combined in a less soluble form offers a greater degree of resistance. Therefore the initial step for producing an acid resistant concrete is formulation

Acid Resistant Concrete

29.3

of Acid Resistant Binder. The testing for Acid Resistance of the Binder should be conducted by immersing the mortar and concrete specimen in water containing sulphuric acid of 2.5 pH over a period of days. The acid containing water is periodically changed to arrest the lowering of salt concentration which in turn would provide the protection to the surface of the specimen. In order to avoid the building of protection film on the Test Specimen, they are cleaned weekly. The loss of weight of the test specimen can be measured and degree of deterioration optically assessed.

ARC Specimen Normal concrete specimen

Fig. 29.1b Normal cement binder versus ARC binder

The comparative performance of Acid Resisting Concrete Binder (ARC) subjected to testing for 75 days Sulphuric acid (pH 2.5) is shown in Fig. 29.1b which shows the physical appearance of normal cement binder versus Acid Resistant Binder. It is seen from the photo that only the top surface is mildly deteriorated and by designing the concrete for high density and lower permeability will lead to production of acid resisting High Performance Concrete. After taking care of chemical properties of the binding material, the subsequent aim should be to design the concrete mix with low permeability coupled with high density. Physically this can be achieved by resorting to excellent aggregate gradation curve as per Fueller and Thompson. Figure 29.2 shows ideal particle size distribution for achieving very dense spherical aggregate packing. It can be observed from the aggregate grading curve that there is a smooth transition right from largest aggregate to smallest grain of microsilica slurry. Initially, it was contemplated that the aggregate size 2/8 mm fraction should be 35% of the total aggregates but after due trials, the gap grading technique was used in aggregate gradation bringing down 2/8 mm fraction to 15% and increasing 8/16 mm fraction to 50%. 100 90 80 70 60 50 40 30 20 10 0

Ideal partical size distribution

1 1 1 4056 1024 512

1 256

1 125

Fig. 29.2

1 54

1 32

Calculation according to Fuller and Thompson

1 16

1 8

1 4

Graphical depiction of sieve line

29.4

Handbook on Advanced Concrete Technology

ARC was considered for a prestigious project in Germany. The above concrete was initially conceptualised theoretically by Prof.Dr.Ing.Bernd Hillemeier, of TU Berlin for the Cooling Tower Shell of Browncoal Power Station in Neideraussem. The natural Cooling Tower of 950 MW class in BoA Nierderaussem is of about 200 m height with top diameter of about 86m and the Bottom diameter of about 136 m. The Cooling water circulation is about 91,000 m3/hr which at the moment would qualify this Cooling Tower to be one of the tallest and biggest Cooling Towers in the world. The total concrete quantity used in this construction was about 32000 m3, out of which about 17,650 m3 was HPC used in Cooling Tower Shell construction. The consumption of reinforcement was about 3,650 MT of steel. Figure 29.3 shows the cooling tower.

Fig. 29.3 Cooling tower constructed with acid resistant concrete

It was concluded that to achieve acid resistant concrete, it would not only be sufficient to achieve higher density but also to achieve higher compressive strength. It was estimated that 28 days HPC strength would be in the range of M 85. This strength was necessary in spite of the fact that strength required by structural design was only M 35. Therefore this concrete was designated as SRB 85/35 wherein SRB stands for Saeure Resistant Beton (Acid Resistant Concrete). This SRB 85/35 distinguishes itself from other high strength concretes by an unusual low composition of cement content of about 250 kg/m3 and FlyAsh and Microsilica with a special attention paid to the gradation of aggregates. It is worthwhile to note that the main reason for developing this concrete was to avoid coating of concrete under severe exposed conditions. If the Cooling Tower was not constructed from Acid Resistant Concrete (ARC), the surface area to be coated would be in the region of 66000m2 and would have costed 10% of the cost of production. In addition to recurring coating costs over the design life of the structure and undue interruptions in normal functioning of the plant.

Acid Resistant Concrete

29.5

29.3 MIX DESIGN FOR ARC CONCRETES The following are the concepts used in the design of HPC – SRB 85/35 • Very dense spherical aggregate packing, well graded up to the finest range in graded steps up to the binding constituents. This minimizes the binding material content by about 50% as compared to the customary and conventional HPCs. Figure 29.2 (on page 29.3) shows the graphical depiction of the sieve line. • Maximum resistance of Binder – material - matrix to the dissolved as well as gaseous attacking constituents with enhanced resistance against chloride diffusion and virtual impermeability against water and gases. Figure 29.4 shows the working principles of higher density against the acid attacks compared to traditional aggregate. The special mixing ratio adopted for Binder Matrix is as follows: 70% 20% 10%

CEM 1 42.5 R HS/NA Flyash Microsilica

“Normal” concrete

Acid attack

New SRB

Fig. 29.4 Working principle of acid resistant concrete

• The dry substance of Microsilica slurry (Silica Suspension) consists of specific surface of approx. 50m2 /g. This unusually high specific surface was achieved by mixing silica fume with colloidal silicic acid. The ball shaped particles with diameters of 50 to 100 times smaller than cement optimally fill all the microscopic voids in concrete and thereby reduce the volume of pores. It should be noted that the presence of silica suspension also result in pozzolanic reaction. • The Testing for Acid Resistance was conducted by immersing the mortar and concrete specimen in water containing sulphuric acid. More than 800 specimens were tested with a combination of several Binder material compositions. Figure 29.4a shows the testing apparatus. Typical Mix Design of the concrete is shown as Table 29.2 and Fig. 29.1b shows difference between deterioration of the normal concrete specimen and the special composition concrete mixed for this ARC.

29.6

Handbook on Advanced Concrete Technology

Vorversuch zur auslagerung von betonplatten in kühltürme Zulaufschlauch Vergleichsbeton

Glasbehälter

Sprühköpfe

Schwefelsäurehaltiges

RWE/TUBeton

Pumpe wasser

Fig. 29.4a Testing apparatus for acid resistant concrete

TABLE 29.2 Typical Mix Design for Acid Resistant Concrete Meterial

Concrete composition Name or value

Quantity [kg/m3]

Cement Fly ash Silica suspension Water Plasticizer

CEM I 42, 5 R-HS/NA EFA KM/C Centrilit Fume S 2 w/c-Value: 0, 40 Muraplast FK 62.30-W.T.

251 74 8 M.-% or. 26/[52](1) 123 litres/m3 2.5 M.-%(2)

Siliceous Sand Sand (Rheinsiche Baustoffwerke) Gravel (Rheinische Baustoffwerke) Gravel (Rheinsiche Baustoffwerke)

0.1 – 0.6 mm 0/2 mm 2/8 mm 8/16 mm

50 623 294 967

(1) (2)

M.-% based on cement content + fly ash M.-% based on total binder content consisting of ement, fly ash ans solid component of silica suspension

29.4 NEW GENERATION CONSTRUCTION CHEMICALS FOR HPC High Performance Concrete when designed for low permeability makes it necessary to use very low water-cement ratio coupled with adequate workability for the sake of compaction. Under such conditions, it is not advisable to increase the cement content of the mix, as it could create problems with thermal expansions. Therefore, High Range Water Reducing Agents are normally incorporated in High Performance Concretes. In case of Self Compacting Concretes, the Super Plasticizers are also required to be added to give the mix a flowable consistency leading to self consolidation. In Self Compacting Concrete, the viscosity of the mix plays a major role and therefore stabilizers have also to be incorporated in Self Compacting Concretes.

Acid Resistant Concrete

29.7

Previously it was believed that the action of Plasticizers and Super Plasticizers was due to reduction of surface tension of water as well as due to reduction of internal friction of solid components of the concrete. Super Plasticizers are normally surface active agents adsorbed on cement grains and cause deflocculation of the agglomerates of cement particles. The major mechanism was the adsorption of Super Plasticizer on the cement grain leading to electrostatic repulsion. This can be measured by Zeta potentials which substantiated this mechanism of action of plasticizer. Following are the bases which are commonly used as Super Plasticizers. • Modified Lignosulfonate (MLS) • Napthalenesulfonate (SNF) • Melaminesulfonate (SMF) • Acrylic Polymer based (AP) as follows: • Coopolymer of Carboxylic acrylic acid with Acrylic Ester (CAE) • Cross Linked Acrylic Polymer (CLAP) • Polycarboxylatethers (PCE) • Combinations of above. In HPC concrete formulations we can incorporate the new generation super plasticizer based on Polycarboxylates with a new Molecule design. The PCE based Super Plasticizers are by far superior to the conventional ones with respect to initial slump as well as slump retention with time. The efficient working of these plasticizers is due to the new type of molecule designs. Polycarboxylatether builds the main chain with short carboxylate side chains which are adsorbed on the cement particle surfaces. Adsorption and Steric Hinderance are now considered as the main criteria for dispersion of acrylic polymer family of Super Plasticizer. Figure 29.5 shows Steric Hinderance Effect.

Fig. 29.5 Steric hinderance effect

29.8

Handbook on Advanced Concrete Technology

These long chained polymers work as spacers as against the normal melamine naphthalene combinations. The working of PCE’s is that they provide very high dispersing effect on account of Steric Hinderance. The outstanding property of dispersion of acrylic family admixture is due to spacer effect of physical separation of cement grains due to long chain. Further, the lower loss slump with respect to time is also due to this physical phenomenon. In addition, since the adsorbed chains on cement grains are small, there is no retardation effect and therefore this admixture poses no hurdles to developments to high early strengths. This would not be feasible with High Range Water Reducing Agent (HRWRA) combined with retarders for the sake of giving longer workability periods. Cement and Superplasticizers compatibility plays a very major role in High Performance Concrete as the problem is more acute at low water cement ratios. For the cement, the content of C3A and C4AF, the reactivity of C3A, the content of Calcium Sulphate and the final form of Calcium Sulphate are most important and for the Super Plasticisers, the guiding factors are molecular chain length, position of sulphate group in the chain counter ion type and the presence of residual sulphate. These factors affect the cement deflocculation properties. It is preferable to use a very low C3A content cement for Acid Resistant Concrete and the cement used in the above concrete formulations is the OPC with low alkaline content and 0% C3A. The Fly Ash used is from High Temperature firings with very low water requirements. It is therefore of utmost importance that compatibility of cements and Super Plasticizers is tested in Laboratory to establish best combinations from rheological point of view. When the incompatibility is observed either as flash set, false set or quick loss of slump with respect to time, it is preferable to do trials with different sets of cement admixture combination. If the problem still persists, then one should check the cement for C3A content, type of gypsum present and Blaine’s fineness. The blending of cement and Fly Ash or Slag can be tried in addition to small doses of Hydrocaboxylic retarder. Water content should be progressively increased to get the desired workability and strength.

29.5 MICROSILICA SLURRY FOR HIGH PERFORMANCE CONCRETES The term “Microsilica” is adopted to characterise the silica fumes which are used for the production of concrete. Microsilica or Condensed Silica Fume (CSF) is a by product resulting from reduction of high purity quartz with coal in the Electric Arc Furnaces used in manufacture of Silicon, Ferrosilicon and other alloys of silicon. CSF which is highly pozzolanic has high content of amorphous silicon dioxide in the range of 85 – 98% in the form of very fine spherical particles averaging diameters about 0.1 – 0.5 micron. The Microsilica is initially produced as an ultra fine undensified powder with the following basic characteristics. • At least 85% SiO2 content • Mean particle size about 0.1 to 0.2 µm. • Minimum specific surface area of 15000 m2 per kg. • Spherical particle shape. As the Microsilica is about 100 times finer than ordinary cement, there are serious problems associated with its transportation, storage, handling and dispensing in the concrete. Basically CSF is available in four production forms as under:

Acid Resistant Concrete

• • • •

Undensified Densified Micro pellatised Slurry Form

-

Bulk Density Bulk Density Bulk Density Specific Gravity

-

29.9

200 – 300 kg/m3 500 – 600 kg/m3 600 – 800 kg/m3 approx. 1.35 to 1.40

– The dispersion of Silica Fume in HPC is the most important consideration while selecting the form of Microsilica. Based on data obtained by several researchers, with different forms of Microsilica there is no reason to advocate for or against the use of any specific form of Silica Fume. The decision will therefore depend upon the type of requirement of the actual user and the problem associated with handling, storing and essentially dispersing the Microsilica in the concrete mix. The Microsilica slurries are produced by mixing Microsilica powder and water in equal proportions by weight. The 50:50 Microsilica water slurry requires grinding, high dispersion, continuous recirculation, agitation and stablilisation for a close pH control to prevent the settlement. In addition to ingredients of concretes like cement, aggregates, water and admixtures, other pozzolanic materials like Fly Ash, Blast Furnace Slag, Metakaolin, Condensed Silica Fume, Rice Husk Ash etc may have to be incorporated in Acid Resistant Concrete. In present formulations we have used Microsilica in slurry form to achieve higher density in Acid Resisting Concrete. The term Microsilica is adopted to characterise the silica fume which are used in the production of concrete. There are three main reasons for the incorporation of Silica Fume as an additive for HPC. Microsilica has a filler effect i.e very fine particle distribute itself in the space between the materials in the concrete in a homogenous way to give rise to more dense concrete. Silica Fume improves the strength of the transition zone between cement paste and aggregates. CSF is highly pozzolanic in combination with Portland cement. During cement hydration there is surplus of Calcium Hydroxide. The Added Condensed Silica Fume’s SiO2 reacts with surplus of Calcium Hydroxide. This results in greater amounts of Calcium Silicate Hydrate, which are denser and stronger than Calcium Hydroxide. The pozzolanic reaction and the filler-effect lead to a compaction of the cement paste and the conversion of CH crystals into CSH gel leads to a homogeneous paste. This phenomenon of dense packing in the interface zone of aggregates also contributes to increase in strength of the concrete on account of aggregates fully contributing their strength to the set concrete. Therefore the high strength of concrete with silica fume is greater than those of the matrix, indicating the contribution of the aggregate to the total strength. Silica fume is incorporated in these concrete formulations especially to take advantage of this Microfiller effect. Silica Fume particles densify the packing by filling spaces between cement grains leading to virtually impermeable concrete. This physical mechanism of dense packing and increase of bond between aggregate and hydrated cement paste depends on thorough dispersion of Silica fume in the matrix. Sequence of addition of materials and the form of Microsilica will play a major role in contributing to this factor. Adequate dispersion of Microsilica in the concrete is vital as cases have been found of damaging expansion in concrete which did not contain reactive aggregates. These were caused

29.10

Handbook on Advanced Concrete Technology

by reaction between the alkaline pore solution and agglomeration of Silica fume. Handling of fine powders also poses potential health risks in addition to adequate dispersing of fine powder in the concrete mix. Considering this option and various experiences made, slurry versions are used in most applications such as Precast, RMC and at Site. Slurry is the easiest and most practical way to introduce Microsilica into concrete mixes. The dispersion of Microsilica Slurry is much easier in the concrete and the mixing time required is roughly the same as that used for ordinary concrete without Microsilica. Experience shows that slurry forms of Microsilica (50:50 with water) have all the benefits in transportation, dispensing methods, mixing times and dispersions to get the desired effect in ARC. Until now the slurries were made out of undensified Microsilica When the slurries are to be prepared from densified Microsilica, there are other problems associated with stability of slurry due to compactness and agglomerations of Microsilica grain. The Microsilica slurries are produced by mixing Microsilica powder and water in equal proportions by weight. The manufacture of stable Microsilica slurry is a technology in itself. It consists of several other compatible additives incorporated during manufacture. The commercial and proprietary slurry formulations contain dispersing agents, superplasticizers, corrosion inhibitors, stabilizers, etc.

Conclusion Over a period of time, the performance based specification would replace prescriptive specifications. As knowledge of different materials of construction becomes more conclusive in terms of performance. Substitution of cement would be practiced and green concretes will be adopted.The performance criteria will therefore focus on other properties like resistance to chemical/mechanical attacks, lower permeabilities, higher densities, ease of placement and self consolidation, high modulus of elasticity etc. Test methods are being developed to simulate the durability requirement so that these values are measurable with respect to performance. These specifications would mean that every concrete will have to be individually designed, tested for rheology and durability to ensure higher degree of success in known aggressive environments. The designer and the concrete supplier will have to work as a team in determining basic material proportion and a thorough study of service conditions will have to be undertaken prior to design of structure. With the acceptance of High Performance Concrete, simultaneous acceptance will have to be made of high efficiency mixing devices, controlled permeability formworks and efficient methods of curing. Modes of measurements of flowability etc will have to be suitably modified to simulate site conditions. Since admixtures are added in very small quantities, mixers used for trials should have adequate dispersing capacity. Mode of addition of various materials will play a major role in determining the flowability and fresh concrete properties of concrete. For every new project, the concrete will have to be properly and individually designed with respect to compressive strength, density, chemical and mechanical stresses and thermal variations. The concrete will therefore have to be designed with a few distinguishing properties rather than from the view point of structural requirements alone. Different models will have to be considered for design of HPC concrete mixes and the conventional methods of specifying the concrete would be inadequate for such concretes.

Acid Resistant Concrete

29.11

References 1. A.M. Neville (1996), Properties of Concrete, Fourth Edition. 2. Charles Goodspeed, et al (1996), High Performance Concrete (HPC) defined for Highway Structures, Version 1.1. 3. Eugen Kleen (1999), Technical Director, MC-Bauchemie Mueller GmbH and Co. KG, Germany, The Superlative Cooling Tower, High Performance Concrete with Higher Resistance against Acid Attack. 4. Joachim Budnik and Ulrich Starkmann (1999), Der Naturzugkuehlturm Niederaussem Heitkamp, Printed in German Concrete Magazine “Beton”. 5. Technical Report No. 41, Microsilica in Concrete, Report by Concrete Society Working Party, 1993. 6. Special issue on High Performance Concrete for Nuclear Power Plants, Indian Concrete Journal, Vol. 73, September 1999. 7. H. Reul, Handbuch der Bauchemie, 1991.

30 Concrete Composites Containing Polymers A.K. Chatterjee

30.1 INTRODUCTION It is quite widely acknowledged that conventional Portland Cement Concrete, inspite of being the most extensively used construction material, suffers from certain serious property limitations, such as, low flexural strength, low failure strain, low resistance to aggressive chemicals caused by the presence of voids and pores in the structure. These drawbacks are well recognized by the design and construction engineers, who in most situations adopt practical means to ensure the durability of the concrete structure for a given exposure condition. In certain situations one has to deviate from the conventional approaches and the specific problems are tackled by using concrete formulations which contain an organic polymer or resin instead of or in conjunction with Portland cements. These composite materials offer the advantages of higher flexibility, improved durability, better resistance to corrosion, reduced water permeability and so on. This chapter deals with a broad introduction to this class of concrete composites, more from the production and application aspects than from the perspective of polymer chemistry and materials science.

30.2

POLYMERS AS A COMPOSITING MATERIAL

Polymers are formed by reacting a number of monomers together, a process known as polymerization. It is therefore necessary for an engineer to differentiate between a monomer and a polymer and also to understand the outline of the polymerization process. Monomers are basically hydrocarbon compounds derived in the first instance from crude oil, coal and natural gas. These hydrocarbons comprise molecules of differing chain lengths and boiling points and form the building blocks of polymers. When the polymerization reaction

30.2

Handbook on Advanced Concrete Technology

produces many hundreds or thousands of repeating units, a macromolecule is formed of suitable molecular weight (25,000 +). A typical schematic representation of the polymerization process is shown in Fig. 30.1. The readers may refer to the reference No.1 under “Further Reading” for details of polymer chemistry. • A monomer is the basic building block of a polymer. • A polymer is a substance, the molecules of which consist of large number of low mass base units or monomer residues which are connected by primary bonds

A–A–A–A–A Or A n • An example: Polyvinyl Chloride

CHCI – CH2 – CHCI – CH2 Or CHCICH2

n

Fig. 30.1 Hydrocarbon families in the perspective of monomers and polymers

Fundamental Classification of Polymers Polymers are generally classified in three groups: • thermoplastics, • thermosets, • elastomers. The thermoplastics are those that can be remelted and reformed; the thermosets are the ones that behave in a opposite manner, meaning that they cannot be remelted and reformed. Elastomers are macromolecules which, when deformed by a weak applied stress, will return rapidly to approximately their initial shape and dimensions after the stress is removed. The “addition” polymerization in which monomer A or, say, ethene is polymerized to polymer A or polyethene, for example, with no byproduct being formed, normally results in thermoplastics. The most commonly used ones include members of the olefin, vinyl and styrene families. The “condensation” polymerization in which two co-monomers combine to form the desired polymer and a byproduct (usually water), i.e. monomer A + monomer B Æ polymer C + byproduct. Polymers formed through condensation can be thermoplastic or thermosetting but all thermosets are based on condensation reaction. Epoxy resins and epoxides, polyester resins, phenol formaldehyde, etc. are typical examples of thermoset polymers. So far as the elastomers are concerned, there are a large number of materials that include natural rubber and synthetic polymers. Elastomers can be thermoplastic (e.g. styrenes, olefins, polyesters, polyamides and polyurethanes) or vulcanized – a process that prevents subsequent remelting.

Concrete Composites Containing Polymers

30.3

The common applications of different polymers traditionally have been as plastics, fibres, elastomers, coating and adhesives. A very useful and interesting extension of the use of polymer is as an ingredient of concrete composite. The varieties of composite that can thus be created is shown in Fig. 30.2. The distinction between these three classes is important to the design engineer in the selection of the appropriate material for a given application. Composites

Polymer Concrete (PC)

Polymer Impregnated Concrete (PIC)

Polymer Modified Concrete (PCM)

Fibre Reinforced Polymer-Modified Concrete Prime Objective To overcome the intrinsic problems of low ductility, high brittleness and strong cracking propensity of normal RCC

Fig. 30.2 Extending polymer to concrete

30.3

POLYMER CONCRETE (PC)

This class of composite has been in commercial use since 1950s. It is a mixture of aggregates with a polymer as the sole binder. To minimize the quantity of polymer, which is an expensive binder, it is very important to achieve the maximum possible dry-packed density of the aggregate. The properties of PC are largely dependent on the amount and properties of polymer in the concrete. This is illustrated in Fig. 30.3. A PC formulation made with methyl methacrylate (MMA) approximates a brittle material showing an almost linear stress-strain relationship with high ultimate strength, but the addition of butyl acrylate produces a more ductile material. Similarly, one may also mention about polyester concretes that are viscoelastic and fail under a sustained compressive loading at stress levels greater than 50 per cent of the ultimate strength. Sustained loadings at a stress level of 25 per cent do not reduce the ultimate strength capacity for a loading period of 1000 h. The illustrative properties of PC are given in Table 30.1, as compared to the other two composites.

Uses of Polymer Concrete (PC) Due to good chemical resistance and high initial strength as well as the modulus of elasticity, the industrial use of PC has been mainly in overlays and repair jobs. The thermal and creep characteristics of these materials are usually not favourable for structural applications.

30.4

Handbook on Advanced Concrete Technology Stress - Strain Diagram

(a) 8000

60 (b) (c)

50

7000

8

40

5000 30 4000 3000

Stress. Pa × 10

Stress, psi

6000

20 Polymer Composition

2000

(a) MMA Only

10

(b) 95 wt. % MMA, 5% butyl acrylate

1000

(c) 80 wt. % MMA, 20% bufyl acryat 0 0

2000

4000

6000

8000

100,00

Strain, m in/in (m m/m)

Fig. 30.3 Manipulation of PC products and properties

Polymer/Resin Cements These types of cements are made using different base resins such as phenolic, furane, epoxy and polyester and is largely used as acid, alkali or solvent resistant bedding and joint lining in various industries, depending on the chemicals handled. These cements are specially used in reaction vessels where agitation is involved or if the length to depth dimentional difference of the vessel is more than 3m. Generally, the phenolic and furane based cements are resistant to alkalis and solvents. The polyester/vinyl ester based cements show resistance to acids and alkalis. The epoxy based cement can be prepared with quick setting properties with resistance to alkalis and dilute acids. Sometimes carbon-filled phenolic base is used for resistance to acids and solvents.

30.4

POLYMER MODIFIED CONCRETE (PMC)

The polymer modified concrete is a composite in which 10 – 15 per cent by weight of the cement binder is replaced by a synthetic organic polymer. It is produced by incorporating a monomer, prepolymer-monomer mixture or a dispersed polymer (latex) into a cement concrete

Concrete Composites Containing Polymers

30.5

TABLE 30.1 Typical Properties of Polymer Containing Concrete Composites Material

Tensile strenght MPa

Modulus of Elasticity GPa

Compressive Strenght, MPa

Shear Bond Strength, KPa

Water Sorption, %

Polymer impregnated concrete

10.5

42

140

–

0.6

Polymer impregnated concrete +++

14.7

49

273

–

Polymer cement concrete

5.6

14

38

≥ 4.550

Portiand cement concrete

2.5

24.5

35

875

Freezethaw Resistance, No. of Cycles/% Wt. Loss

Acid Resistance**

3.500/2

10

£ 0.6

–

≥ 10

–

–

4

700/25

–

5.5

*The values given represent average values; the properties of commercial products may vary over a wide range, depending on formulation and production process **Improvement factor in relation to portland cement concrete ***Concrete autoclaved before impregnation

mix. To effect the polymerization of the monomer, a catalyst is added to the mixture. The process technology used is very similar to that of conventional concrete. Therefore, polymer-modified concrete can be cast-in-place in field applications. Modification of concrete with a polymer latex, which is a colloidal dispersion of polymer particles in water is often called latex-modified concrete (LMC). LMCs are generally more cost-beneficial than PMCs. Consequently a great variety of latexes are now available for use in polymer cement concretes and mortars. The most common latexes are based on polymethyl methacrylate (also called acrylic latex), polyvinyl acetate, vinyl chloride copolymers, polyvinyledene chloride, styrene- butadiene copolymer, nitrila rubber and natural rubber. Each polymer yields characteristics properties. The acrylic latex provides a very good water-resistant bond between the polymer and the concrete components, whereas latexes of styrene-based polymers results in a high compressive strength. Poor properties of certain products have been attributed to the incompatibility of certain polymers with some of the concrete ingredients. The polyvinylidene chloride based latex is now seldom used because of the risk of corrosion of steel in concrete. Similarly, there is reduced usage of polyvinyl acetate base latex due to its yielding low wet strength in the concrete. Hence elastomeric polymers based on styrene butadiene and polyacrylate copolymers have become more popular. Since a latex holds polymer particles in aqueous suspension with the help of surface active agents, there is a tendency in modified concrete to incorporate large amounts of entrained air. It, therefore, becomes necessary to add air detraining agents to commercial latexes. LMC is made with as low an addition of extra mixing water as possible; the spherical polymer molecules and the entrained air associated with the latex usually provide excellent workability. Typically, the water-cement ratios are in the range of 0.40 – 0.45 and the cement contents are of the order of 390 – 420 kg/m3.

30.6

Handbook on Advanced Concrete Technology

Curing of latex polymer cement concrete is different from that of conventional concrete, because the polymer forms a film on the surface of the product, retaining some of the internal moisture needed for continuous cement hydration. Because of the film-forming feature moist curing of the latex product is generally shorter than for conventional concrete. Hardening of a latex takes place by drying or loss of water. Hence, dry curing is undertaken for LMC. The material cured in air is believed to form a continuous and coherent polymer film which coats the cement hydration products, aggregate particles and even the capillary pores (see Fig. 30.4).

Fig. 30.4 SEM Micrographs showing polymer films in various hardened PMC’s

Properties and Uses Generally PMC/LMC exhibits excellent bonding to steel and to old concrete. It also shows good ductility, resistance to penetration of water and aqueous salt solutions and resistance to freezethaw damage in colder climates. The modulus of elasticity may or may not be higher than that of unmodified concrete, depending on the polymer latex used. Generally, as the polymer forms a low modulus phase with the polymer cement concrete, the creep is higher than that of plain concrete and decreases with the type of polymer latex used in the following order :

Concrete Composites Containing Polymers

30.7

• polyacrylate • styrene-butadiene copolymer • polyvinyledene chloride • unmodified cement The drying shrinkage of polymer cement concrete is generally lower than that of conventional concrete. The amount of shrinkage depends on the water-cement ratio, cement content, polymer content and curing conditions. PMC/LMC is susceptible to higher temperature effects than ordinary cement concrete. For example, creep increases with temperature to a greater extent than in ordinary cement concrete, whereas flexural strength and modulus of elasticity decrease. These effects are greater in materials made with elastomeric latex than in those made with thermoplastic polymers. The typical properties of PMC are given in Table 30.1. The main application of PMC/LMC is in floor surfacing as it is non-dusting and relatively cheap. Because of lower shrinkage, good resistance to permeation by various aqueous solutions and good bonding properties to old concrete, it is particularly suitable for thin (25 mm) floor topping, concrete bridge deck overlays, anti-corrosive overlays, concrete repairs and patching. Because of good adhesive properties, latex-modified mortars are needed for laying bricks, in prefabricated panels and in stone and ceramic tiles.

Fibre-Reinforced Polymer Modified Concrete (PMC)/Latex Modified Concrete (LMC) For further improvement of the flexural behaviour of the PMC composites various types of fibres are sometimes used. The incorporation of glass fibres, steel fibres and polypropylene fibres is relatively common. Attempts have been made to use carbon fibres in such composites with significantly improved ductility. The addition to cement mortar of 0.37 per cent of short pitch-based carbon fibres together with styrene-butadiene base latex and an antifoam agent increased the flexural strength by 54 per cent and the compressive strength by 30 per cent at 28 days of curing. In this total increase of flexural strength the fibres contributed 33 per cent without much effect on the compressive strength.

30.5

POLYMER-IMPREGNATED CONCRETE (PIC)

The concept underlying PIC is that if voids are responsible for low strength as well as poor durability of concrete in severe environments, then eliminating them by filling with a polymer should improve the characteristics of the material. In line with this concept, PIC is made by impregnation of precast hardened Portland cement concrete with low viscosity monomers (in either liquid or gaseous form) that are converted to solid polymer under the influence of UV radiation or heat or chemical catalysts. It is produced by drying conventional concrete, displacing the air from the open pores by vacuum and pressure, saturating the open pore structure by diffusion of low viscosity monomers (~ 10 cps) and in-situ polymerization of the monomer. A

30.8

Handbook on Advanced Concrete Technology

large proportion of the void volume is filled with polymer, which forms a continuous reinforcing network. A concrete structure may be impregnated to varying depths or in the surface layer only, as required for a given application. The main disadvantages of polymer impregnation are the relatively high cost of the monomers used and the complexity of the fabrication process.

Polymers Used in Polymer Impregnated Concrete (PIC) Some of the most widely used monomers for polymer concrete system include : • styrene (S) • butyl acrylate (BA) • vinyl acetate (VAc) • acrylonitrile (AN) • methyl acrylate (MA) These monomers may be used alone or in mixture. Unsaturated polyester styrene also is a common system. PIC based on epoxy polymers are more expensive and less often used, although their properties are superior. Trimethylpropane trimethylacrylate (TMPTMA) serves as a cross- linking agent used also to decrease the polymerization time. The initiators most commonly used in the polymerization process are : • 2, 2’ - azobis – (isobutyronitrile) • 2, 2’ – azobis – (2, 4 dimethylvaleronitrile) • benzoyl peroxide • lauroyl peroxide • methyl ethyl ketone peroxide Often promoters are used, besides the initiators, for increasing the rate of decomposition of the peroxide initiators.

Sequence of Operation The process starts with the casting of concrete. Since the quality of concrete before penetration is not important from the stand point of properties of the end product, no special care is needed in the selection of materials and proportioning of concrete mixtures. The thickness for impregnation is generally limited to about 150 mm, since it is difficult to fully penetrate thick sections. Following the removal of elements from forms, conventional curing for even 7 days may be adequate. For fast production schedules thermal curing may be adopted. The time and temperature needed for removal of free water from the capillary pores of moist-cured products depend on the thickness of the elements. Temperature of the order of 150ºC can accelerate the drying process. The in-situ penetration of concrete in the field may be achieved by surface ponding, but the precast elements are directly immersed in the monomer-catalyst mixture. To prevent loss of monomer by evaporation during handling and polymerization, the impregnated elements must be effectively sealed in steel containers or several layers of aluminium foil. In the

Concrete Composites Containing Polymers

30.9

rehabilitation of bridge decks this has been achieved by covering the surface with sand. Finally, the thermo-catalytical polymerization is the preferred technology. The time for complete polymerization in the sealed elements exposed to steam, hot water or hot air, or infrared heat at 80º + dh may vary from a few to several hours.

Properties and Applications of Polymer Impregnated Concrete (PIC) The typical properties of PIC are shown in Table 1 in which one may observe that the highest strength can be achieved in autoclaved concrete. As an illustration the stress-strain curve of a PIC made with PMMA is shown in Figure 5 as compared to a plain un-impregnated concrete. 17 16 15

Polymer impregnated Concrete 3” Dia × 6” Cylinder PMMA, loading 5.4 wt%

Fracture 110 100

14

90

13

80

11 70

6

10

E = 5.5 × 10 psi

9

60

8

Plain unimpregnated Concrete 3” Dia × 6” Cylinder

7 6

Pa

–3

Stress (psi × 10 )

12

50 40

5 30

4

Fracture

3

20 6

2

E = 1.8 × 10 psi

10

1 1000

2000 3000 Compressive Strain, m in/in (m m/m)

4000

Fig. 30.5 Effects of polymerization in PIC

Impregnation of concrete with polymers results not only in a remarkable improvement in tensile, compressive and impact strength, it also has significant effect on sealing of continuous pores resulting in almost 99 per cent decrease in water and salt permeability, about 30 per cent increase in the coefficient of thermal expansion, decrease of approximately 17 per cent in specific heat and almost 13 per cent decrease in thermal diffusivity. The pore-sealing effect also minimizes changes in properties such as dielectric constant and dielectric loss that are sensitive to moisture content. Applications of concrete impregnated to a depth in building and construction including structural floors, high performance structure, food-processing buildings, sewer pipes, storage

30.10

Handbook on Advanced Concrete Technology

tanks for seawater, desalination plants, distilled water plants, marine structures, tunnel liners, swimming pools, etc. Partially impregnated concrete is used for the protection of bridges and concrete structure against deterioration and repair of deteriorated building structures, such as ceiling slabs, underground garage decks and bridge decks.

30.6

FUTURE POSSIBILITIES OF POLYMER-CEMENT COMPOSITES

In this chapter an attempt has been made to capture the present status of using polymers as an ingredient in making concrete composites. The progress in the development of “ductile cements” has not been dealt with as it is a wide enough subject to be treated separately. From the developments that are being done in the fields of Macro-Defects-Free cements (MDF) or composites with cements other than Portland varieties, it appears that in near future a polymer-based cementitious composite is likely to emerge as an engineering material closely competing with metals. A spring made with MDF cement patented by the researchers of the Imperial Chemical Industries (ICI) a couple of decades ago continues to be an indicator of the potential that exists in the family of polymer-cement composites (see Figure 6) to result in a ductile cement.

Fig. 30.6 Spring made with MDF cement

Concrete Composites Containing Polymers

30.11

References 1. Ger Challa (1993) Polymer Chemistry : An Introduction, Ellis Harwood Limited, Sussex, UK. 2. Hugh McArthur and Duncan Spalding (2004) Properties, Uses, Degradation and Remediation, Harwood Publishing, UK. 3. P.K. Mehta and P.J.M. Monteiro, Concrete (2008) Microstructure, Properties and Materials, Indian Reprint, ICI, Chennai. 4. J.A. Mason (1981) Applications in Polymer Concrete, ACI Publication SP – 69, American Concrete Institute, Detroit, Michigan. 5. Proceedings of International Congress on Polymers in Concrete (ICPIC), Korea, 2007. 6. Guide for Use of Polymers in Concrete, ACI 548, R-09, American Concrete Institute, Detroit, Michigan, 2009. 7. Y. Ohama (Editor) (1997) Polymers in Concrete, E & FM Spon, UK.

31 Concrete Roads and High Volume Fly Ash Concrete L.R. Kadiyali and A.K. Jain

30.1 HISTORY OF CONCRETE ROADS Concrete roads have a history of about hundred years. Notable among the early concrete roads are the German autobahns. In India too, concrete roads were built in the 1920s and 1930s. The Marine Drive built in the 1930s in Mumbai is performing well even now. However, the abundant availability of bitumen and the scarcity of cement in India led to a virtual abandonment of concrete roads during the period 1950–1990. The plentiful availability of cement in recent years has led to a revival of interest in concrete roads since 1990.

31.2 ADVANTAGES OF CONCRETE ROADS Concrete roads enjoy several advantages over the conventional flexible pavements. Some of these are: 1. Long life : Well designed concrete roads last for 30-40 years. In fact, the trend world-wide is to aim at a life of 60 years. 2. Maintenance-free : A good concrete road hardly needs any maintenance. In the case of performance plain jointed pavements, the only maintenance needed is at the joints, where sealants have to be renewed. 3. Availability of binder

: Cement is available in plenty in India, and will be available for many years to come. On the other hand, bitumen, derived from petroleum crude will become scarcer and costlier in the coming years.

4. Less prone to : Being dense, concrete does not permit entry of water, and this damage by water prevents the water reaching the subgrade and lowering its strength. On the other hand, water can enter through voids in a bituminous surface.

31.2

Handbook on Advanced Concrete Technology

5. Good abrasion resistance

: Because of its high strength, concrete has a high abrasion resistance and shows superior performance at curves and gradients.

6. Good riding quality 7.

8.

9. 10.

11.

12.

13.

: The riding quality of a concrete pavement is maintained over its entire life, thus leading to savings in fuel consumption, tyre wear and spare parts consumption. Fuel savings due : Concrete pavements hardly deflect due to their rigidity, whereas a to rigid action flexible pavement deflects considerably. As a result, a loaded tyre has to continuously overcome the ripples in front of the deflection bowl, leading to 15-20 per cent extra consumption of fuel. Effect of spillage : A bituminous surface gets damaged by the oil spillage from the engine and other parts, whereas concrete is not affected. Thus, concrete is a preferred choice at airport apron and taxi-ways, bus depots, bus stands, truck laybyes, toll plazas and parking areas. Design precision : Since the properties of concrete are well understood, the design can be done precisely with accurate determination of stresses. Good reflectivity : Concrete being light in colour, reflects street illumination better than the dark surface of a bituminous road, and hence a saving of about 10 per cent in electricity can be achieved. Conservation of : Concrete pavements require less stone aggregates for initial stone aggregates construction than a flexible pavement, and practically no stone aggregates at all for maintenance. Thus, concrete pavement is a sustainable option. Saving in energy : Bituminous mixes involve the heating of bitumen and aggregates, consumption in which consumes large quantity of diesel. In addition, since the constructions quantity of stone aggregates needed for flexible pavement is more, the production (quarrying and crushing) of stone aggregates and their transport involve higher energy consumption than for a concrete pavement (Ref 2). The energy saved in constructing concrete roads can be of the order of 30-50 per cent. Economics : Contrary to commonly held perception, concrete pavements are cheaper than flexible pavements for a highly trafficked road even initially. Over the life-cycle, concrete pavements emerge as the cost effective option for all types of roads.

31.3 SOME DISADVANTAGES OF CONCRETE ROADS AND HOW THEY CAN BE OVERCOME Great care is needed in designing, detailing and construction of concrete roads. This will prevent premature failures, which can invite public criticism. Several utility services such as electric cables, telephone cables, water pipes, sewers and storm-water drains are located below the road pavement. These may require repairs or

Concrete Roads and High Volume Fly Ash Concrete

31.3

remodelling, which entails cutting the pavement and reinstatement. While this may be easily done in a flexible pavement, it is difficult in a concrete pavement. Use of modern aids like trench-less technology and provision of pipes to house the utilities along and across the pavements can overcome this handicap. Concrete requires to be cured for 10 to 14 days, and thus the road cannot be used by traffic. Traffic diversions are needed. However, modern “fast track” concrete has been developed in countries abroad and roads are opened to traffic in a short time of 2 to 3 days. Concrete roads generate more noise than bituminous surfaces, and in urban locations high noise levels are not tolerated. In countries abroad, this problem is overcome by modern techniques whereby “whisper concrete” with low noise is now adopted.

31.4 ECONOMICS OF CONCRETE ROADS VIS-À-VIS FLEXIBLE PAVEMENTS With the present prices of bitumen and cement, a concrete road is in fact cheaper in initial cost for highway traffic. Recent analysis (Ref 1) has clearly established this fact, as can be seen in Fig. 31.1. CBR - 2, Aggregate Zone - 1

Cost of Pavement (Rs – Lakhs / Two lanes / km)

250 200 150 100 50

Bitumen 0 Price:Rs/T Cement Price:Rs/T

30000

35000 4000

40000

30000

35000 4500

40000

30000

35000 5000

40000

Cost_ Flexi Cost_ Rigid

Fig. 31.1 Initial cost of new pavement (2 lane × 1km Long)

When life-cycle-cost is considered, the cost advantage tilts further in favour of concrete roads as seen in Fig. 2. Added to the above is the savings in fuel consumption of commercial vehicles (15-20 per cent), savings in electricity for street lighting (above 10 per cent) and saving in energy consumption during construction (Ref 2).

31.5

DESIGN OF CONCRETE PAVEMENTS

The current Indian practice for the design of plain jointed concrete pavements is based on the determination of stresses due to loads and combining them with the curling stresses due to

31.4

Handbook on Advanced Concrete Technology CBR - 2, Aggregate Zone - 1

Life Cycle Cost of Pavement e(Rs - Lakhs / Two Lanes / hm)

350 300 250 200 150 100 50

Bitumen 0 Price:Rs/T Cement Price:Rs/T

30000

35000 4000

40000

30000

NPV_ Flexi

35000 4500

40000

30000

35000 5000

40000

NPV_ Rigid

Fig. 31.2 Life cycle cost of new pavement (2 lane × 1 km long)

temperature gradient across the slab cross-section (Ref 3). The salient features of the present guidelines are: 1. The computation of load stresses is done with the 98th percentile axle load, determined from an axle load spectrum study. 2. The axle load value is multiplied by a load safety factor, whose values are 1.2, 1.1 and 1.0 for (i) important roads, (ii) roads having lower proportion of truck traffic, and (iii) residential and other streets respectively 3. The load stresses are determined on the basis of Westergaard’s analysis, for placement of single and tandem axles along the edge 4. The tyre pressure is taken as 0.8 MPa 5. A design period of 30 years is considered 6. In the absence of any data, the rate of growth of commercial traffic may be taken as 7.5 per cent per annum 7. The Modulus of Subgrade Reaction (k value) is either determined from a plate load test, or taken (on an approximate basis) from standard tables relating k value to the CBR value of soil subgrade 8. For modern concrete pavements, a Dry Lean Concrete (DLC) sub-base of 100-150 mm thickness is recommended 9. A separation layer of polythene sheet is provided between the DLC and the concrete slab 10. A good drainage layer is provided below the pavement 11. Temperature stresses are calculated on the basis of Bradbury’s coefficients 12. The thickness arrived at is checked for fatigue consumption on the basis of Miner’s Hypothesis

Concrete Roads and High Volume Fly Ash Concrete

31.5

13. 25 per cent of the commercial traffic is assumed to travel along the pavement edge 14. Contraction joints are provided with dowel bars to transfer the load from one slab to the next 15. Tie bars are provided along the longitudinal joints 16. The concrete recommended is of grade M-35 or M-40 For highways, a thickness of about 300 mm is the usual practice. For less important roads, a thickness of 250 mm is generally found adequate. For Rural Roads, a thickness of 180-200 mm is found adequate. Figures 31.3, 31.4, 31.5 and 31.6 give typical cross-sections of new concrete pavements for (i) divided carriageway National Highways (ii) two lane National Highway/State Highways (iii) two lane Major District Road or sub-arterial city street and (iv) single lane Rural Road respectively. CL

30000 ROW

30000 ROW

25500 Paved Shoulder 1500 1500

Medan 5000 700

250

250

7000

Paved Shoulder 1500 1500

New carriageway New carriageway Hard gronular Shoulder Finished Grode Level Detail-a 3.0% 3.0% (Moorum) 3.0% 2.5% 2.5% 3.0%

Sl

op

2

pe

1.

New S embankment Cement concrete th 300 mm Dcl th 150 mm Gsb th 150 mm Subgrade th 500 mm 11 No’ s Hope Telecom Ducts of 40/33 mm size

New embankment

lo

Medion Filling With Selected Earth

e

1.

2

Cement concrete th 300 mm Dry lean concrete th 150 mm GSB th 150 mm Subgrade th 500 mm

Fig. 31.3 Typical cross-section of concrete road for divided carriageway national highways 7000

1000 1500 CartPaved -hen shou- shoulder -lder

1500 1000

Carriageway 2.5%

CL

2.5%

CartPaved -hen shoulder shou-lder

2:1

2:1 300 Thk., m-40 Grade concrete 150 Thk., day lean concrete m - 40 150 Thk., Dratnage layer Note: All dimensions are in millimetres unless other wise noted

Fig. 31.4 Two lane national highway/state highway in concrete

31.6

Handbook on Advanced Concrete Technology 1000 1500 EartPaved -hen shou- shoulder -lder

7000 Carriageway 2.5%

CL

1500 1000 EartPaved -hen shoulder shou-lder

2.5%

2:1

2:1 250 Thk., m-40 Grade concrete 100 Thk., day lean concrete m - 40 150 Thk., Dratnage layer Note: All dimensions are in millimetres unless otherwise noted

Fig. 31.5 Two-lane major district road/city sub-alterial street 1625 1000 Gravel shoulder

3750 Carriageway 2.5%

CL

1625 1000 2.5%

Gravel shoulder

2:1

2:1 200 Thk., m-35 Grade concrete 100 Thk., dry lean concrete m - 10 150 Thk., Dratnage layer Note: All dimensions are in millimetres unless other wise noted

Fig. 31.6 Rural road in concrete

A typical design calculations for a concrete pavement is enclosed in Appendix I.

31.6 JOINTS IN CONCRETE ROADS For jointed concrete pavements, the following types of joints are needed: 1. Contraction joints

: To prevent irregular cracks and to relieve warping stresses at a spacing of 3 to 4.5 m transversely

2. Longitudinal joints

: Provided for pavements wider than 4 m to allow for transverse warping

3. Expansion joints

: These are provided transversely only at locations where the slab abuts a permanent structure such as abutment of bridge/culverts

A layout of these joints for a 2-lane road is shown in Fig. 31.7. At transverse joints, dowel bars are needed to transfer the load from one slab to the other. The diameter of the round plain dowel bar varies from 25 to 32 mm, its length is about 500 mm, and the spacing is in the range of 200 to 300 mm depending upon the slab thickness. The dowel bar is bonded to concrete for half its length and has a plastic sleeve at the other for free movement (Fig. 31.8). The joints are sawn to a depth of d/3 or d/4, d being the slab

Concrete Roads and High Volume Fly Ash Concrete

31.7

thickness, it being assumed that the weak plane caused by the sawn cut enables the crack to extend to full depth. 7000 Carriageway

3500

1500 Shoulder

3500

24 Nos. 32 mm dia. dowel bars 550 mm long

350

Construction joints

Construction joints @4500 mm c/c

175

Median

Longitudinal joints with tie bars 12 mm dia. 450 mm long

500 350

Fig. 31.7 Layout of joints in a two lane road Top of the groove is widened for sealing purpose

Ms round dowel bar 1/4 to 1/3d

d Plastic sheathing Contraction joint with dowel bar (a)

Fig. 31.8 Details of contraction joint

31.8

Handbook on Advanced Concrete Technology

Tie rods are used across longitudinal joints to prevent the adjoining slabs from separating, especially at fills or curves. They are generally round or deformed bars, of dia 8-14 mm, length 300-650 mm, spacing 400-600 mm. The joints are sealed by a sealant of the following types: Cold-poured sealant • Polysulphide • Silicone • Polyurethane Hot-poured sealant • Rubberised bitumen • Preformed sealants • Strips of a synthetic material

31.7 WHITE-TOPPING As bitumen is becoming scarcer and costlier, highway authorities are considering to convert the black-topped roads to concrete roads by provided an overlay. This is known as white-topping. White-topping is classified into three types (Ref 4): • Conventional White-topping • Thin White-topping (TWT) • Ultra Thin White-topping (UTWT) Conventional White-topping is a concrete pavement designed and constructed over an existing pavement without consideration of any bond between the concrete overlay and the underlying bituminous layer. Its thickness can be in the range of 200-300 mm. The other two types of white-topping, viz thin (TWT) and ultra-thin (UTWT) are based on the consideration that a bond exists between the concrete overlay and the underlying bituminous layer. The thickness of the overlay is: 100-200 mm for TWT 50-100 mm for UTWT The bond between the concrete layer and the bituminous layer is achieved by milling. A minimum thickness of 75 mm of bituminous layer is necessary. The joints are at shorter spacing 0.6 to 1.2 m. Concrete is of high strength (M-45 or M-50) and is provided with fibres. The composite action enables the neutral axis to shift downward, with the result that much of the area of the concrete slab comes under compression. Many white-topping projects have been implemented in India, particularly in Mumbai, Pune and Bangalore. Since the panel size of TWT and UTWT is small, the curling stresses developed are small. The joints are sawn to a depth of about 1/3rd the slab depth and a width of 3-5 mm and need not be sealed. There is no need to provide dowel bars and the aggregate interlock and the firm support provided by the underlying pavement can take care of load transfer.

Concrete Roads and High Volume Fly Ash Concrete

31.9

Drainage is an important consideration for the efficient functioning of the white-topped pavement. Photos of the Ultra-thin white-topping in Pune and Mumbai cities are enclosed (Photos 1 and 2).

Photo 1 A view of ultra-thin white topping in Pune

Photo 2 A view of white topping in Mumbai

31.8 CONTINUOUSLY REINFORCED CONCRETE PAVEMENT (CRCP) The presence of joints in the conventional jointed concrete pavement results in a jerky ride, besides needing maintenance such as resealing. These disadvantages can be overcome by constructing a pavement of any length without contraction or expansion joints. This is achieved by providing reinforcement at 0.6-0.8 per cent of the cross-sectional area at mid-height, to take care of randomly occurring cracks. The fine cracks do not affect the structural integrity of the pavement slab. A slight reduction in slab thickness is also possible, though some authorities do not recommend it. CRCP has not been taken up in India so far, though there exists a good case for it in the future Expressways being planned.

31.9 31.9.1

HIGH VOLUME FLY-ASH CONCRETE Introduction

In 1985, the Advanced Concrete Technology Group at CANMET under the direction of Prof. V.M. Malhotra initiated studies on structural concrete incorporating high volumes (>50%) of low calcium fly ash. The purpose of this research was to develop concrete with adequate early-age strength and workability, low temperature rise and high later-age strength for Federal Agency in Ottwa, Canada for pavement construction (7).

31.9.2

Definition

The term High Volume Fly Ash (HVFA) concrete was coined by Prof. V.M. Malhotra at CANMET in the 1980s. This concrete has very low water content and at least 50% of the Portland cement by mass is replaced with ASTM class F or class C fly ash. High-early strength is obtained by reducing the water-to-cementitious materials ratio of the concrete mixture to 0.4 or less. Because of the low water content, a high range water reducer (superplasticizer) is used, which

31.10

Handbook on Advanced Concrete Technology

is also necessary when slump values ranging from 150 to 200 mm are required. In situations where high early-strength and high slump are not required, the use of a superplasticizing admixture is not necessary. The HVFA concrete has excellent workability, low heat-of-hydration, adequate early-age strength and very high later-age strength, low drying shrinkage and excellent durability characteristics that are essential for enhancement of sustainability of modern concrete construction.

31.9.3 Characteristics of HVFA Concrete (a) Minimum 50% fly ash by mass of the cementitious materials, which is usually less than 400kg/m3. (b) Low water content, generally less than 130kg/m3. (c) Portland cement content, generally less than 200kg/m3. (d) A superplasticizer is generally required in the production of HVFA concrete due to low water to cementitious ratio. (e) The fresh concrete mixtures show excellent workability, pumpability and finishability. By adjusting mixture proportions, it is possible to produce self-compacting HVFA concrete without the use of viscosity-modifying admixtures. (f) HVFA concrete has lower strength compared to normal concrete at 3, 7 and 28 days but similar strength at 56days and considerably higher strength at one year. (g) HVFA concrete has homogeneous microstructure and very strong interfacial bond between cement paste and aggregate/steel. It exhibits considerably less strain and cracking from drying and thermal shrinkage. (h) The special characteristic of HVFA concrete is its very low electrical conductivity thus resulting in little or no threat to reinforcement corrosion and greatly enhanced durability. (i) HVFA concrete is very environment friendly product because of considerably less Portland cement in concrete mixtures.

31.9.4 HVFA Concrete Mixture Proportioning The correct mixture proportioning is very important if HVFA concrete is to have the desired properties. As with normal Portland cement concrete, mixture proportions for HVFA concrete depend on the strength and workability requirements. However, for HVFA concrete it is essential that the basic principles of very low water content and very low w/b are maintained. In order to achieve superior durability characteristics, it is necessary that high slumps and workability be achieved with the use of a suitable superplasticizer. It is recommended that laboratory trial mixture should be made using local materials, like aggregates, cement, fly ash and chemical admixtures to evaluate the workability and strength development. However, for the first trial batch, the typical suggested mixture proportions for low, moderate and high-strength HVFA concrete are shown in Table 31.1(8).

Concrete Roads and High Volume Fly Ash Concrete

31.11

TABLE 31.1 Typical Proportion for HVFA Concrete Strength class 28 days

Low strength 20 MPa

Moderate strength 30 MPa

Water ASTM Type I/II cement (OPC 43/53 grade)

120-130 125-130

115-125 155-160

115-120 180-200

ASTM class F fly ash (fly ash conforming to IS 3812 Class I)

125-130

215-220

220-225

1170 + 10

1200 + 10

1110 + 10

800 + 10

750 + 10

750 + 10

+

+

+

Coarse aggregate (MSA 20mm) Fine aggregate Superplasticizer

High Strength 40 MPa

Note: 1. The above figures are for typical mixture proportion kg/m3. 2. + the amount of superplasticizer will depend upon the slump requirement and the type of superplasticizer used. To achieve slumps of about 175mm, the amount of Naphthalene-based superplasticizer will range from 3 to 5 l/m3 of concrete. For acrylic superplasticizers, the dosage will be much lower. For slip formed pavements, where the slump requirement is 40-50 mm, the amount of superplasticizer needed would be considerably less.

31.9.5

Properties of Fresh and Hardened HVFA Concrete

The properties of fresh concrete include slump, air content, bleeding, time-of-set and autogenous temperature rise. The properties of hardened concrete include compressive, flexural and splittingtensile strengths, Yong’s modulus of elasticity, drying shrinkage and creep.

31.9.5.1 Properties of Fresh Concrete Slump Because of the very low unit water content the slump of the HVFA concrete is low. Therefore, it is essential to use a superplasticizer when a high slump is needed for placement. In general, the use of Naphthalene and Melamine-based superplasticizing admixtures result an immediate increase in slump but most of the increase is lost within 30 to 60 minutes. It would be desirable to use retarder cum superplasticizing admixtures in HVFA concrete.

Air Content Though air entrainment is not essential for concrete structures in most parts of India, but there is no difficulty in entraining air in HVFA concrete. 5% to 7% air can easily be incorporated in HVFA concrete. However, when the carbon content in fly ash is high (more than 4% or when activated carbon particles are present) the dosage of air entraining admixture will increase. In such cases either the source of fly ash should be changed or dosage of air entraining admixture should be increased.

Workability Both the low and high-slump HVFA concrete possess excellent cohesiveness and workability. The HVFA concrete mixtures therefore exhibit very good pumpability, compactability and

31.12

Handbook on Advanced Concrete Technology

finishability characteristics. HVFA Concrete shows excellent workability even without the air entrainment in warm climates.

Bleeding Due to low unit water content, the bleeding of the HVFA concrete ranges from negligible to very low. It is therefore important that curing of HVFA concrete be commenced as soon as possible after placement. The problem is acute in hot and windy climates and in such situations, it is recommended to use chemical curing compounds and cover especially the flat surfaces with polythene sheets to prevent plastic shrinkage cracking.

Time–of-set In general, the final time-of-set of HVFA concrete may be two to three hours longer when compared with control Portland cement concrete. This is not considered significant from the point of view of construction scheduling except for a little delay in making saw-cut joints in concrete pavement after placement. On the other hand, the somewhat delayed setting of the HVFA concrete can be beneficial in hot weather because it allows more time for transporting and placing of concrete. In cold weather, excessive set retardation can be avoided by using a suitable set-accelerating agent.

Adiabatic Temperature Rise In mass concrete and rather massive structural-concrete members, it is important that temperature rise due to the hydration of cement be kept as low as possible to avoid thermal cracking. For example, in a large HVFA Concrete block, 3.05 × 3.05m, the maximum temperature reached was 54ºC (a rise of 35ºC when the concrete placement temperature was 19ºC). This was considerably less than the 83ºC (a rise of 65ºC when the concrete placement temperature was 18ºC) in a block of the same size made of concrete with ASTM type I cement only. In this investigation, the total amount of cementitious material by mass was the same for two types of concretes (9).

31.9.6

Properties of Hardened Concrete

In general, the mechanical properties of HVFA concrete has superior attributes because of its low unit water content, low water-to-cementitious materials ratio, low porosity, very dense and uniform microstructure, low thermal and drying shrinkage, fewer cracks and high durability.

Compressive Strength HVFA concrete can achieve adequate early-age strength and considerably high late-age strength with proper selection of the mix proportions and the type of cement. In one investigation, (Fig 31.9 Fly ash 3), A HVFA concrete mixture containing 155 kg/m3 of ASTM type I Portland cement, 215 kg/m3 of ASTM class F fly ash, and 115 kg/m3 of water (0.31w/b) gave 10MPa at 1 day, 35 MPa at 28 days and 60 MPa at one year(10).

Concrete Roads and High Volume Fly Ash Concrete

31.13

70 W/(C + F ) = 0.31 Cement: 155 kg/m

60

3

Fly Ash 3

3

Compressive Strength, MPa

Fly Ash: 215 kg/m

Fly Ash 5 Fly Ash 1

50

Fly Ash 2 Fly Ash 4 Fly Ash 6

40

30

Cement: Fly Ash: Coarse Agg.: Fine Agg.: A.E.A.: S.P.:

20

10

ASTM Type 1 Low calcium variety Crushed Limestone (19-mm max.) Natural Sand Synthetic Resin Type Naphthalene Based Product

Specimen Size: 152 × 305-mm cylinders 0 0

100

200

300

400

500

Age, days

Fig. 31.9 Compressive strength development of HVFA concrete

In winter, especially when concreting at temperature below 15ºC, extra precaution must be taken to keep proper curing temperature for adequate strength development by providing surface insulation or heating enclosures.

Flexural and Splitting-tensile Strength In general the ratios of the flexural and splitting tensile strength to compressive strength at 28 days of HVFA concrete are either comparable or higher than those for conventional Portland Cement Concrete. For conventional Portland Cement Concrete, the flexural strength (basis for the design pavements) reaches a maximum value between 14 and 28 days, and beyond this age there is no significant increase. On the contrary, the flexural strength of HVFA concrete keeps on increasing with age because of the pozzolanic reaction of fly ash, and strengthening of the interfacial bond between cement paste and aggregate. The flexural and tensile strengths may not reach a maximum value even at one year. The increase in flexural strength between 28 days and one year in case of HVFA concrete can be as high as 40% depending upon the mixture proportions. Thus HVFA concrete provides significant advantage over conventional concrete for use in pavements. The HVFA concrete also reduces cost by about 20% as compared to normal concrete.

Yong’s Modulus of Elasticity The Yong’s modulus of elasticity of HVFA concrete is approximately 35 and 38 GPA at 28 and 91 days respectively (11). The high modulus achieved may be due to the fact that a considerable portion of unreacted fly ash consisting of glassy spherical particles acts as a fine aggregate.

31.14

Handbook on Advanced Concrete Technology

The very low porosity of the transition zone between the cement mortar and coarse aggregates also contributes to the high values of the modulus in the HVFA concrete.

Drying Shrinkage and Creep Due to the lower content of cement paste and water, the drying shrinkage of HVFA concrete is lower than that of conventional Portland cement concrete. The true drying shrinkage of concrete in structures would be even lower because the laboratory test method tends to overestimate the drying shrinkage. The creep of HVFA concrete is also low. The low creep strains of HVFA concrete, once again, is probably due to the unreacted fly ash particles in concrete acting as fine aggregate and the stronger interfacial transition zone providing high restraint against creep. Also the very low water content of the concrete would have the effect of reducing the strains. The low creep values of HVFA concrete are of significant benefit for high-strength reinforced concrete columns in tall buildings

31.9.7 Construction Practices The recommended practice for HVFA concrete construction is essentially the same as used for conventional concrete. However, the superior workability characteristics of the HVFA concrete have a positive impact on the placement, consolidation and finishing operations. The negative impact can be due to lack of proper curing without which most of the engineering benefits associated with the use of HVFA concrete would not be realized.

31.9.8 Placement and Consolidation of HVFA Concrete Due to high volume of fines and low water content, HVFA concrete is generally very cohesive and shows no tendency for bleeding and segregation. Fresh concrete mixtures possess excellent pumpability and workability characteristics even at a slump as low as 75 mm. It is therefore not necessary to require more than 100 to 125 mm slump for commonly used methods of placement, that is pumping or truck chutes. The material moves well to fill space without much effort. Due to the low cement content, the setting and hardening properties of HVFA concrete would be seriously affected if any extra water is added than that specified by the original mixture proportions.

31.9.8.1 Placement of Slabs on Grade As with conventional concrete, before the flat work placement, the form work and sub grade should be properly moistened to prevent water loss from the HVFA concrete mixtures that already contain low water content. Also as HVFA concrete shows little bleeding after the finishing operations, the flat work surfaces must be protected immediately against water loss by covering with a heavy plastic sheet. Alternatively fogging may be used during the concrete placement. Unless these precautions are taken, HVFA concrete would be prone to plastic shrinkage cracking especially in hot and windy weather.

Concrete Roads and High Volume Fly Ash Concrete

31.9.8.2

31.15

Finishing of Slabs

HVFA concrete can be finished immediately or very quickly after the placement as there is a little or no bleed water, thus saving construction time. On the other hand, with very large slabs, there would be some delay in cutting of contraction joints because of two or three hours of delay in the final setting time of concrete.

31.9.8.3 Curing of HVFA Concrete In order for HVFA concrete to achieve the desired strength, crack resistance and durability characteristics it is necessary to provide proper curing. The strength development at early ages up to 7 days comes mainly from the hydration of Portland Cement which is severely retarded at low temperatures. Therefore it is essential the temperature of freshly placed concrete is about 15ºC. HVFA concrete continues to gain considerable strength and a corresponding decrease in permeability for at least up to one year. This is attributable to the continued in-situ hydration of fly ash particles even when all the calcium hydroxide present in the system has been consumed. This is why good moist curing for at least a week after the placement, followed by applications of a membrane curing compound that helps to seal in the moisture for long time, is essential to take full advantage of the strength and durability potential of HVFA concrete.

31.10 CONSTRUCTION 31.10.1 Types of Construction Before the advent of paving machineries, concrete roads were constructed with manual means. Activities like mixing of concrete, rodding of concrete (for compaction) and finishing the surface with wooden floats etc were done by hand tools. Things got improved with the availability of small drum mixers, needle vibrators and vibratory screeds. A village road work under progress can be seen using such facility in Photos 3 and 4 using this technology.

Photo 3: A village road under construction

Photo 4: A plate vibrator being used for compacting a concrete panel

31.10.2 Brief Description of Manual Construction Methods 31.10.2.1 The Steps Involved in Manual Construction of Concrete Road (i) Fixing of side-shuttering to the base to the correct height using steel spacers or shims. The side shuttering shall be fixed to base with the help of steel dowel pins. Side bracings

31.16

(ii)

(iii)

(iv)

(v)

(vi) (vii)

Handbook on Advanced Concrete Technology

are also provided so that during concreting the shuttering will not tilt or bulge. The inside surface of shuttering shall be coated with mould releasing oil. The base is cleaned with broom or compressed air and a 125 micron thick plastic sheet is fixed to function as antifriction and separation layer. The dowel bar cage shall be fixed at appropriate locations, as recommended in the design. Similarly tie rods shall be fixed at specified locations through the holes made in the side-shuttering. Concrete mix is discharged on the carriageway and spread to a thickness above the shuttering with extra thickness known as surcharge so that after compacting the mix, the desired thickness is achieved. The concrete shall be compacted with needle vibrators keeping the needle vertical. The vibration shall be given at close interval like 30 cm apart depending upon the thickness of the slab and consistency of the mix. The needle shall be placed all along the side shuttering but shall not be allowed to touch the side shuttering. By adopting this technique, honeycombing of sides can be avoided. The surface shall be furnished with a wooden tamper bar or a screed vibrator. It is always necessary to keep a wooden bench to cross-over the slab during finishing and texturing operation. Otherwise masons tend to walk on the concrete surface. The surface shall be wiped with a squeezed moist hessian cloth to remove the bleeding water, if any. The surface shall be textured with brush or tine texture depending upon the specification.

31.10.2.2

Suggestions for Better Results Using Manual Construction Methods

When constructing rural roads where there are not many resources for bringing advanced machineries, with little extra fund, the small plants listed below can be procured to help in constructing very good quality roads. (i) Mixers: If we cannot arrange 12-15 cum/hr capacity mini batching plants, even 7/10 or 10/14 cft mixers can be used to produce mix on the basis of weigh batching. This helps in saving enormous quantity of cement. (ii) Screed Vibrators: This is a very useful tool for road work. As a large area of road has to be levelled and finished, finishing the surface with vibratory screed will improve the surface regularity of the road. The screed vibrator can be even fabricated locally. However, screed vibrators manufactured in a factory has many advantages. A typical Photo 5: A view of a screed screed vibrator is shown in Photo 5. vibrator (manufacturer : M/S multiquip)

Concrete Roads and High Volume Fly Ash Concrete

31.17

(iii) Needle Vibrator: These vibrators are readily available in the market which can operate with petrol or electricity. (iv) Joint Cutting Saw: The transverse joints made by inserting steel or plastic strips bulge and affects surface regularity seriously. Therefore, concrete saw is a very useful tool even in low volume concrete road works. The above plant and machineries suggested can help in producing excellent road.

31.10.3

Mechanised Paving

Till 1990s in India the concrete roads were constructed by and large by using small plants and machineries. Two types of constructions are possible like with mechanised pavers such as: (i) Fixed form construction (ii) Slip form construction

31.10.3.1

Fixed Form Paving

Although a train of concrete pavers, vibrators, dowel bar inserts, finishers etc are used in this method, these machines are made to move on the side-shuttering or fixed forms. Shown in Photo 6 is an airport site where fixed form paving was adopted. Normally the concrete train is made to move on railings fixed outside the side-shuttering made of steel sections as seen in Photo 7. Excellent roads and airports have been constructed with this method, but the time and cost overheads of fixing side shuttering makes it un- attractive. In some projects, as many as 20-25 persons were required for fixing shuttering each day. Normally if 100 m paving is planned each day, 200 m shuttering is required each day for both sides. Normally at least 600 m of shuttering material has to be available with the contractor for use.

Photo 6 : A view of paver, vibrators etc, used in fixed-form paving

Photo 7 : Side shuttering-cum-railing used in fixed-form paving

The steps involved are: (i) Fixing of side-shuttering-cum-railing firmly to base with steel dowel pins and side bracing (ii) Checking the top level of the shuttering to an accuracy of ±2 mm by raising or lowering the shuttering. Steel plates are required to be used while raising the level. Any gap under shuttering shall be filled with lean cement mortar. The inside surface of shuttering shall be applied with mould release oil.

31.18

Handbook on Advanced Concrete Technology

(ii) The surface base shall be cleaned with brush and compressed air. Any distress caused by construction traffic shall be corrected with appropriate mix. (iii) A plastic sheet of 125 micron thickness shall be placed as a separation and antifriction layer on the base, a few hours before concreting. It is a good engineering practice that a fine spray of water is applied on the base surface before placing plastic sheet. The plastic shall be fixed using concrete nails or other suitable method, but wrinkles shall not be formed in the plastic sheet. (iv) Tie rods, if specified shall be inserted from sides of the shuttering. Holes shall be provided on the side shuttering as specified. (v) Production of mix in large projects would be in the order of 100 cum/hr or more. It is important that the mix production should match with paving speed such that paver can pave 1 m per a minute. This speed helps in producing better surface with improved riding quality. (vi) Normally fixed-form paving is used in large projects, and hence the quantum of concrete used would be more. Therefore, use of water tight dumpers would be desirable for transporting and discharging concrete mix. Transit mixers also can be used for this purpose but it takes too much time for discharging the mix. This not only affects the progress of work, but sometimes the mix may dry out during such placement especially during hot and dry season. If dowels are fixed manually with the help of steel cages, care has to be taken to avoid discharging mix on the dowels. As a precaution the mix has to be deposited on the dowel assembly manually with the help of hand shovel with all care not to disturb dowels. As explained in manual construction, the mix shall be spread between side shuttering with required surcharge with spreader unit so that after compacting the mix, the level achieved is nearly as per requirement. (vii) Paving caravan shall be moved one after another to spread, vibrate and compact, dowel insertion, finishing, texturing and curing the surface. Paving operation in progress can be seen in a project in Photo 7.

31.10.3.2

Slipform Paving

To avoid fixing of side-forms on a day-to-day basis at high cost and additional time, a slipform paver was developed as early as 1949 in the USA and a 3 m (10 ft) wide pavement was constructed in Iowa. About 1.5 m to 2.5 m short plates fixed to the paver are sufficient to retain the edges of wet concrete in shape without collapsing as the mix used remains stiff. The slipform paver is operated using normally 4-electronic sensors connected to string line fixed on both sides to regulate slab thickness, alignment and camber/grade. The workability (slump) of mix in this case has to be controlled so that mix shall not subside or collapse. Some early units were provided with 2-3 units comprising of paver, finisher along with dowel-bar-inserter and separate unit for texturing and curing. Lately the slip form pavers are equipped only with a single unit handling paving and finishing operations. The modern slip form pavers have inbuilt mechanism for inserting dowel and tie rods which is a big advantage. Photos of Slip form pavers in use are given in Photos 8, 9 and 10.

Concrete Roads and High Volume Fly Ash Concrete

Photo 8 : A slip form vibrator-cumpaver in operation

31.19

Photo 9 : A view of modern slip-form paver

Photo 10 : Concrete being spread by slip-form paver

The various steps involved in slip form paving are: (i) Concrete roads constructed on our National Highways have a base of dry lean concrete (DLC). This layer is kept slightly wider than the cement concrete slabs in order to provide some off-set. If the off-set is of the order of about 1.0 m or so, the track of paver can move on it. The sensors are designed to take care of undulations below the track while finishing the surface. But too much undulation would affect the surface regularity of concrete pavement. On account of cost considerations, the off-set of DLC is normally restricted to 300 to 500 mm depending upon the design engineer. . In such situations, it is necessary to level and compact the strip of shoulder beyond DLC edge so that it does not yield during the movement of tracks. This is an important preparation which has to be carried out to avoid bad riding quality. (ii) Fixing of side shuttering is not required in this case. String line has to be fixed so that electronic sensors of the paver can move on them. The string line has to be fixed in advance very accurately with a tolerance of ±2 mm so that paved slabs, their camber and alignment remain within the acceptable limit. (iii) Concrete mix has to be dumped on the carriageway in the method suggested by the manufacturer of the paver unit. (iv) Present day pavers are capable of handling mix dumped on the carriageway by spreading it, compacting it to form a pavement mould, and subsequently inserting dowels, tie rods, finishing it and texturing and applying curing compound.

31.20

Handbook on Advanced Concrete Technology

After the concrete slabs have been constructed the remaining activities are equally important. These are: (i) Blotting the surface: Any water rising to the concrete surface on account of bleeding has to be blotted. It is normally done by a hessian cloth which is dragged on the surface. (ii) Texturing: Texturing is done almost when paving operation is completed. There are two types of texturing which are being practised. The MoRTH specification recommends brush texture, but with the experience we have in India, it is now seen the brush texture gets eroded fast. Therefore, the present thrust is to use tine texture. Tine texture can be provided longitudinally or transversely. The details of tine grooves are given in Fig. 31.10. Land area

4 4

19

19

Fig. 31.10 Details of tine texture

(iii) Curing: Water curing is the best method of curing. But there are many places where abundant quantity of water for such purpose is not available. Use of synthetic curing compound is used in all latest Highway Projects as recommended by MoRTH Specifications. The liquid curing compound is helpful in encasing the concrete mould from loosing water required for hydration action in the initial period. Subsequently the pavement is cured by using wet hessian or water ponding etc. Curing of concrete slabs at least for 14 days is recommended. (iv) Joints: Concrete slabs constructed in long lengths tend to crack. To avoid such random cracks, joints are cut with saw at specified locations in transverse direction so that the crack can take place at the cut joints, which can be sealed with a sealant subsequently. A joint cutting saw in action can be seen in photo 11. When the slab width is more than 3.5 m, it is desirable to provide a longitudinal joint. Only when a slab abuts with a permanent structure like bridge or culvert, an expansion Photo 11 : A concrete saw in action joint is introduced.

Concrete Roads and High Volume Fly Ash Concrete

31.21

(v) Joint Cutting: Most of the cracks that have appeared in concrete pavements constructed recently are due to delayed cutting of joints. Joints have to be cut when the concrete is mature. A sawing window (Fig. 31.11) has to be developed based on laboratory tests. The use of maturity meters also needs to be popularised. Sawing window Too late (cracking)

Concrete strength

Too early (raveling)

Restraint stress equals concrete strength Minimum strength to avert excessive saw cut raveling

Time

Fig. 31.11

Sawing window

31.10.4 New Trends in Concrete Pavement Construction We in India have made a new beginning in construction of concrete roads since about 16-17 years. The advanced countries had already started construction of concrete roads since about 100 years. Therefore, they have developed the road construction technology to perfection. Even the Autobahn constructed after 1932 in Germany even before World War II is still serving satisfactorily. There is a need in India to carryout R&D so that we can improve the technology to suit our site conditions, materials, weather etc. Some of the new trends in constructing concrete roads in advanced countries are as under: (i) The Dry Lean Concrete base is dispensed with and bituminous layer is being used in its place in countries like Germany. (ii) The plastic sheet being used as antifriction and separation layer is being replaced with 4-5 mm thick geosynthetic sheet.. (iii) Brush texture is being replaced with tine texture from durability consideration and reduced noise pollution (iv) Use of whisper concrete is being tried to reduce the noise generation. The technique involves application of retarder on the freshly laid concrete surface and washing the surface next day with a jet of water to expose the concrete surface. Such a surface is found to generate less noise. (v) Use of thin premoulded synthetic board as a sealing strip which would expand and remain in the joint groove without any maintenance.

31.22

Handbook on Advanced Concrete Technology

(vi) Recycling of old concrete for use in lower construction is done in advanced countries. (vii) The HVFA concrete is ideally suitable for pavement construction. The continued gain of compressive and flexural strength of HVFA concrete is very beneficial in the performance of rigid pavements. In addition, large volumes of fly ash can be used in HVFA concrete, otherwise the disposal of the same would pose serious environmental and ecological problems.

APPENDIX I ILLUSTRATIVE EXAMPLE OF SLAB THICKNESS DESIGN Example A cement concrete pavement is to be designed for a two lane two way National Highway in Karnataka State. The total two way traffic is 3000 commercial vehicles per day. The design parametres are: Compressive strength of cement concrete = 45 MPa CBR of subgrade = 6% Corresponding Modulus of subgrade reaction = 45 kPa/mm (from Table 2) Thickness of DLC subbase = 150 mm Effective modulus of subgrade reaction = 242.5 kPa/mm (from Table 4) Elastic modulus of concrete = 30,000 MPa Poison’s ratio = 0.15 Coefficient of thermal expansion of concrete = 10 X 10 –6 /oC Tyre pressure = 0.8 MPa Rate of traffic growth, r = 6% The axle load spectrum obtained from axle load survey is given in Table: Single Axle Loads Axle Load class, kN Percentage of axle loads

Tandem Axle Loads Axle Load class, kN Percentage of axle loads

190-210 170-190 150-170 130-150 110-130 90-110 Less than 90

0.6 1.5 4.8 10.8 22.0 23.3 30.0

340-380 300-340 260-300 220-260 180-220 140-180 Less than 140

0.3 0.3 0.6 1.8 1.5 0.5 2.0

Total

93.0

Total

7.0

The 98 percentile of the load spectrum of single axle load has been determined as it is more severe; the 98 percentile value computed is 180 kN. Design The design is carried out for load and temperature stresses at the edge and corner.

Concrete Roads and High Volume Fly Ash Concrete

31.23

Present Traffic = 3000 cvpd, Design life = 30 yrs, r = 6%

[

]

(1.06)30 – 1 Cumulative repetition in 30 years. = 3000 × 365 __________ 0.06 = 86, 568, 714 commercial vehicles Design Traffic = 25% of the total repetitions of commercial vehicles = 21, 642, 179 Front axles of the commercial vehicles carry much lower loads and cause small flexural stress in the concrete pavements and they need not be considered in the pavement design. Only the rear axles, both single and tandem, should be considered for the design. Assuming that mid point of the axle load class represents the group, the total repetitions of the single axle and tandem axle loads are as follows: Single Axles Load in kN

Tandem Axles Expected repetitions

20 18 16 14 12 10

129853 324633 1038825 n 2337355 4761279 5042628

Less than 10

6492654

Load in tonnes

Expected repetitions

36 32 28 24 20 16

64927 64927 129853 389559 324633 108211

Less than 16

432844

(i) First Trial Trial thickness, h = 280 mm, Effective modulus of Subgrade reaction = 242.5 kPa/mm, design period = 30 yrs, Flexural strength of concrete = 4.5 MPa, Load safety factor = 1.2. The edge stresses computed are given in Table 12. Fatigue life consumed for different axles is worked out and presented in Table given below. Axle load (AL), kN

AL x 1.2

Stress, MPa from charts

Stress ratio

Expected repetition, n

Fatigue life, N

Fatigue life consumed

240 216 192 168

2.45 2.25 2.04 1.82

0.55 0.51 0.46 0.41

129853 324633 1038825 2337355

1.24x105 4.85x105 1.4335 x107 Infinity

1.047 0.669 0.072 0.00

360

432

1.74

0.39

64927

Infinity

0.00

320

384

1.84

0.40

35560

Infinity

0.00

Single Axle 200 180 160 140 Tandem Axle

Cumulative fatigue life consumed = 1.788 The design is unsafe since cumulative fatigue life consumed is more than 1.0. Therefore, redesign taking higher thickness.

31.24

Handbook on Advanced Concrete Technology

(ii) Second Trial Assume a trial thickness, h = 300 mm. Axle load

AL x 1.2 (AL), kN

Stress, MPa

Stress ratio from charts

Expected repetition, n

Fatigue life, N

Fatigue life consumed

240 216 192

2.23 2.04 1.85

0.50 0.46 0.42

129853 324633 1038825

7.62 x 106 1.4335 x 107 Infinity

360

432

1.59

0.36

64927

Infinity

0.00

320

384

1.45

0.33

35560

Infinity

0.00

Single Axle 200 180 160

0.17 0.022 0.00

Tandem Axle

Cumulative fatigue life consumed = 0.192 Since the cumulative fatigue life consumed is less than 1.0, the design is safe based on fatigue criterion. Check for temperature stresses CE at Edge warping stress = ______ 2 L = 4500 mm B = 4000 mm l = 730.5 mm (see below under corner stress) B/l = 5.48 \

L/l = 6.16; (taken higher of the above two values)

for

L/l = 6.16, C = 0.9376 from Fig. 2. The temperature differential is 21°C for Karnataka region. 0.9376 × 3 × 104 × 10 × 10 –6 × 21 = ______________________________ = 2.95 MPa 2

For a Pasternak foundation condition, 85% of the temperature stress computed above is taken in the design. Therefore, warping stress to be considered in the design = 0.85 × 2.95 = 2.507 MPa Total stress = wheel load stress (98 percentile) + temperature stress = 2.04 + 2.507 = 4.547 MPa which is more than the flexural strength of concrete slab i.e., 4.5 MPa. So the pavement thickness of 300 mm is unsafe under the combined action of wheel load and temperature stresses.

Concrete Roads and High Volume Fly Ash Concrete

31.25

Therefore, redesign taking higher thickness. (iii) Third Trial Assume a trial thickness, h = 310 mm Axle load (AL), kN

AL x 1.2

Stress, MPa from charts

Stress ratio

Expected repetition, n

Fatigue life, N

Fatigue life consumed

200 180 160

240 216 192

2.12 1.95 1.76

0.48 0.44 0.40

129853 324633 1038825

2.4x106 Infinity Infinity

0.054 0.00 0.00

Tandem Axle 360

432

1.53

0.35

64927

Infinity

0.00

320

384

1.39

0.31

35560

Infinity

0.00

Single Axle

Cumulative fatigue life consumed = 0.054 Since the cumulative fatigue life consumed is less than 1.0, the design is safe based on fatigue considerations. Now temperature stress can be computed. Check for temperature stresses CE at Edge warping stress = ______ 2 L = 4500 mm B = 4000 mm l = 748.69 mm (see below under corner stress) B/l = 5.34 \

L/l = 6.01; (taken higher of the above two values)

for

L/l = 6.01, C = 0.921 from Fig. 2. The temperature differential is 21°C for Karnataka region.

0.921 × 3 × 104 × 10 –6 × 21 = ________________________ = 2.90 MPa 2 For a Pasternak foundation condition, 85% of the temperature stress computed above is taken in the design. Therefore, warping stress to be considered in the design = 0.85 × 2.90 = 2.465 MPa Total stress = wheel load stress (98 percentile) + temperature stress = 1.95 + 2.465 = 4.415 MPa which is less than the flexural strength of concrete slab i.e., 4.5 MPa. So the pavement thickness of 310 mm is safe under the combined action of wheel load and temperature stresses. Corner stress is not critical in a dowelled pavement. However, for non-dowelled pavement, the slab should be checked for load stress at corner.

31.26

Handbook on Advanced Concrete Technology

Check for Corner Stress The corner stress can be calculated from the following formula: __

Corner Stress: 3P/h

2

[ ( )] a÷2 1 – ____ 1

1.2

The 98% percentile axle load is 180 kN. The wheel load, therefore, is 90 kN (= 90,000 N). ____________ 1000 × E h3 Radius of relative stiffness, l = 4 ___________ 12 (1 – m2) k

÷

E = 30,000 MPa h = 310 mm m = 0.15 k = 242.5 kPa/mm Tyre pressure = 0.8 MPa ___________________

\

1000 × 3 × 104 × 3103 l = ___________________ 12(1 – 0.152) 242.5

÷

= 748.69 mm a = radius of area of contact of wheel. Considering a single axle dual wheel,

[

(

S _________ P P __ a = 0.8521 × _____ q × p + p 0.5227 × q

)]

0.5 0.5

where P = Wheel load on one tyre in Newtons (N) = wheel load × load safety factor S = C/c dist between two tyres = 310 mm q = tyre pressure in MPa

[

(

45,000 × 1.2 45,000 ___ 310 ______ 1 a = 0.8521 × ___________ + ____ × p 0.8 × p 0.5227 0.8 = [18305.8 + 35455.3] 0.5 = 231.86 mm P = ZP1 = 90,000 N \

[ (

__

3 × 90,000 231.86 × ÷2 Corner Stress = __________ 1 – ___________ 2 748.69 310

)] 1.2

)]

0.5 0.5

Concrete Roads and High Volume Fly Ash Concrete

31.27

3 × 90,000 = __________ [1 – 0.371] 3102 = 2.1 MPa As the corner stress is less than the flexural strength of the concrete, (i.e.,4.5 MPa), therefore assumed pavement thickness of 310 mm is safe.

References 1. Handbook of Cement Concrete Roads, Cement Manufacturers’ Association, New Delhi, 2010. 2. L.R. Kadiyali, Expert Talk on “Ways to Reduce Energy Consumption in Road Construction”, Accepted for the Indian Roads Congress Annual Session, Nagpur, 2010. 3. Guidelines for the Design of Plain Jointed Rigid Pavements for Highways, IRC: 58-2002, Indian Roads Congress, New Delhi, 2002. 4. Tentative Guidelines for Conventional Thin and Ultra-thin White-topping, IRC:SP:76-2008, Indian Roads Congress, New Delhi, 2008. 5. Guidelines for Maintenance, Repair and Rehabilitation of Cement Concrete Pavements, IRC: SP: 83-2008, Indian Roads Congress, New Delhi, 2008. 6. National Workshop on Sustainable Concrete Pavements: Practices, Challenges and Directions, Indian Institute of Technology, Madras, Chennai, 2010. 7. V.K. Malhotra and P.K. Mehta (Jan 2008), - High Performance, High Volume Fly ash Concrete for Building Sustainable and Durable Structures (third edition, pp. 41-42). 8. Mehta, P.K. “A simple method of proportioning sustainable concrete mixtures”; Presented at the spring convention of the American Concrete Institute, Washington D.C., March 2004. 9. Bisaillon, A., Rivest, M. and Malhotra, V.M., “Performance of High-Volume Fly Ash Concrete in Large Experimental Monoliths”, ACI Materials Journal Vol. 91, No. 2, March – April 1994, pp. 178-187. 10. Malhotra V.M., “CANMET Investigations Dealing with High-Volume Fly Ash Concrete”; CANMET Publications: Advances in concrete Technology, MSL 94-1 (IR), 1994, pp. 445-482. 11. Bilodeau, A. and Malhotra, V.M. “High-Volume Fly Ash system: Concrete Solution for Sustainable Development”; ACI Materials Journal, Vol. 97, No. 1, Jan-Feb 2000, pp. 41-48.

32 Quality Control and Quality Assurance of Concrete Vijay R. Kulkarni

32.1 INTRODUCTION The term quality has been defined differently by different experts. For example, Deming defines it “Meeting the Customers’ needs”; Juran says it is “Fitness for Use”, whereas Crosby defines it as “Conformance to Requirements”. One more definition of quality can be “Satisfaction of stated and implied needs”. Quality can have separate meaning. It can have subjective meaning, when it is used to indicate elegance or luxury. Quality can be relative, when the term is used to indicate grade (e.g. 5-star hotel). It can have objective meaning when it is used to indicate a specific requirement or fitness for purpose (e.g. M20/M30 grade of concrete). The term quality has a much wider and an all-encompassing significance when applied to any multi-dimensional activities. In the context of construction industry, quality is not only the slated and implied needs of the users/owners — who should be assured of the required serviceability and safety, without undue maintenance — but also the slated and implied needs of the client/promoters who should be assured of adequate returns on their investments. Quality in construction can be said to have been achieved if it is achieved without time and cost overruns, ensuring the required serviceability, durability and safety, without undue maintenance.

32.2 WHAT ARE QUALITY ASSURANCE (QA) AND QUALITY CONTROL (QC)? Quality control (QC), sometimes called process control, is defined as “the operational techniques and activities that are used to fulfill requirements of quality”. It is the sum total of activities performed by the seller (producer) to make sure that a product meets contract specifications and requirements1. The main problem about quality control is that it is an “after event” operation, designed to prevent defective items from passing through the system. What happens when a product fails?

32.2

Handbook on Advanced Concrete Technology

The consumer’s risk is that a bad batch may get accepted, while the producer’s risk is that a good batch may be rejected and the production cost is increased due to rejection. Quality assurance (QA) can be defined as all those planned activities and systematic actions necessary to provide adequate confidence that a product or service will satisfy the given contract specific requirements1. Quality assurance provides consistency, while quality control is a part of quality assurance. Quality assurance involves the whole management team and their staff together with independent supervision. Within an organization, quality assurance serves as a management tool; in contractual situations, quality assurance serves to provide confidence in the supplier. High performance quality does not mean increased costs. Better controls can lead to savings by way of optimizing the use of materials, leading to lower wastage. This means material savings, leading to overall economy. According to Bryant Mather, “The best concrete for any given purpose is the one that does the job satisfactorily at lowest cost, having due regard for both initial cost and maintenance and repair.”2 Unlike steel, aluminum, glass, etc, concrete is not a factory-produced material. It is produced at site or in a commercial batching plant and supplied in semi-finished state. The quality of the local ingredients is prone to variations. Further, variations are also likely in the production process. In addition, quality of concrete is also likely to be affected due to inadequate precautions during transportation, placement, compaction and finishing of concrete. In view of these factors, adoption of correct and strict procedures during quality control and quality assurance of concrete assumes greater importance.

32.3

CONCRETE INDUSTRY SCENARIO

Concrete construction scenario in India has witnessed significant changes during 2000-2010 decade. Traditionally, any construction involving major use of concrete has been a labour-intensive activity, and even today, an overwhelming majority of concrete produced in the country is site-mixed, and most of it is volume-batched. However, thanks to the liberalization of the Indian economy and emphasis on the development of physical infrastructure, concrete construction scenario in India - especially in urban India - is undergoing welcome transformation in the recent years. The demand for higher speed of construction, especially for residential and commercial buildings flyovers, highways, roads, aviation, etc. in metropolitan and other big cities of India, necessitated adoption of mechanized and semi-mechanized techniques of construction. The need for large volumes of concrete as well as faster speed of concrete construction was felt. This was conducive for the development of ready-mixed concrete (RMC). The concept of RMC is not new to India. Captive RMC plants arrived in the country in 1950s; but remained confined for application in mega projects. Thus, India missed ‘commercial’ RMC technology for nearly five decades! Early 1990s witnessed the beginning of RMC industry in India. The growth of RMC started with metropolitan cities, then spread to other major cities, and is now trickling down to tier II and III cities. No authentic data is available on the RMC

Quality Control and Quality Assurance of Concrete

32.3

industry in India. Based on the rough estimates, it is reported that as on December 2008, there were around 450-500 RMC plants producing about 25-30 million m3 of concrete per annum3. In addition, a large number of batching and mixing plants belonging to medium and large construction companies also operates as captive plants on a number of projects throughout the country. With the current emphasis on building physical infrastructure in the country, there seems to be a phenomenal increase in captive batching and mixing plants. The discussion in this article pertains mainly to QA and QC procedures for concrete produced either from ready-mixed concrete plants or captive batching/mixing plants.

IS 4926:2003 Recommendations on QA and QC There is a separate BIS standard on ready-mixed concrete, namely IS 4926. In accordance with the provisions of IS 4926, QA and QC of concrete is divided into following three components4: • Forward control • Immediate control • Retrospective control.

Forward Control It includes control of material storage, monitoring purchased material quality, modification of mix design, plant maintenance, transfer and weighing equipment, etc. The main features of each of these items are briefly described below. • Control of purchased material quality: It is ensured through: – Visual checks, – Sampling and testing, – Certification from material suppliers • Control of material storage: It is done in such a way to: – Prevent contamination – Ensure suitable transfer and feed system – Protect materials • Mix design and modifications – Producer to maintain records of all mix design and modifications • Transfer and weighing equipment: This is to be in accordance with Annex E of the code and should include: – Documented calibration procedure including corrective action in place – Daily production records. • Plant maintenance: This is to be in accordance with Annex F of the code and should include: – Documented plant maintenance procedures – Regular plant inspection carried out, faults rectified and recorded

32.4

Handbook on Advanced Concrete Technology

• Plant mixers and truck mixers: This is to be in accordance with Annex F of the code and should include: – Regular maintenance to be performed as per requirements of manufacturer and recorded.

Immediate Control It mainly involves both production and product control as described below: • Production control – Essential to satisfy client requirements and code specifications. – Workability to be controlled on a continuous basis with no additional water – Each load to be inspected and printed record made • Product control – Random sample to be tested for workability; and wherever appropriate, for density, temperature and air content – Crucial issue: Maintain required w/b ratio – Corrections essential for moisture content of aggregates and changes in their grading.

Retrospective Control It mainly involves assessment of factors influencing control of quality not assessed during production. It covers mix performance, stock control of materials and procedures to help diagnose and correct faults arising out of complaints received from customers.

32.4

REGULATORY FRAMEWORK FOR QA AND QC

Although the IS 4926:2003 has specified comprehensive procedures for ensuring QA and QC of concrete, it is difficult for customers of concrete to check each and every provision. What is needed is a simple mechanism by which the customer should get assurance about the quality of concrete being supplied to him. Different countries in the world have developed a regulatory framework for ensuring QA and QC of concrete involving certification of production facility. For example, the National Ready Mixed Concrete Association (NRMCA), USA, developed the Plant Certification System way back in 19655. This system based on ASTM C 946, was revised 10 times since inception, the latest revisions being done in December 2007. Most of the contracts executed in the USA specify adherence to the NRMCA’s plant certification system. Further, based on AASHTO guidelines, NRMCA developed guidelines for quality control and quality assurance of concrete for the benefit of the users of RMC7. In Canada, the Ready Mixed Concrete Association of Ontario (RMCAO) has evolved and is implementing Quality Scheme for RMC8. This scheme involves audit of each production facility, which is inspected to ensure that by virtue of equipment, facilities, materials, statistical control and procedures, proper “capability” of producing quality concrete exists. In the U.K., the certification of RMC facilities was started by the British Ready Mixed Concrete Association. Later, as the industry

Quality Control and Quality Assurance of Concrete

32.5

matured, an independent organization, namely, Quality Scheme for Ready-Mixed Concrete (QSRMC) started providing product quality certification for design, production and supply of RMC9. The scheme is based on ISO 9001: 2000, BS EN 206-1:2000 and BS EN 8500-Part I and Part II10,11,12. It contains robust regulations and rigorous assessment procedures; however, it is quite complex and onerous. In line with the international practice and taking cognizance of the typical Indian conditions including the provisions in Indian standards, the Ready Mixed Concrete Manufacturers’ Association (RMCMA) developed a regulatory framework in 2008 and the same is being implemented presently3,13. Here, it would be appropriate to clarify one aspect. India, being a late starter in RMC business, had the advantage of having the latest generation technology of batching and mixing plants, the use of which augurs well for producing quality concrete. With the state-of-the-art plant and equipment, a considerable level of sophistication has been brought into the production of concrete. For example, the erstwhile electrical and mechanical weighing system has been replaced with the load-cell based electronic system, the efficiency of the mixer has been vastly improved with the introduction of power mixers and the production process has been fully automated with computer-controlled. All this has led to improvements in both the quality and uniformity of the product. However, the crucial question is: will the mere use of state-of-the-art plant and equipment result in ensuring quality concrete? The answer is certainly ‘no’. Thus, what is needed is a framework of quality, providing assurance that the modern tools and equipment being used are capable of producing quality concrete on the one hand and the producer is capable of implementing a Quality Plan to provide assurance about the quality of his products, on the other hand.

32.5

INDIGENOUS QUALITY SCHEME

The quality scheme of concrete developed by the National Experts Committee of RMCMA rests on two strong pillars, namely, best international practices that are suitable for Indian conditions and strict observance of the codes of the Bureau of Indian Standards. The quality scheme can be divided in two parts: • Audit-based certification of RMC production facilities • Guidelines for quality control and quality assurance. Two detailed manuals were prepared covering the above-mentioned two parts. These manuals, known as Quality Manual Part I and II are available in the public domain and can be downloaded from www.rmcmaindia.org. The activities in a typical production facility can be grouped in six main areas, namely, material and storage, handling system, batching equipment, central mixer, ticketing system, delivery fleet and testing laboratory. These activities can further be sub-divided into a number of areas as shown in Fig. 32.1. The QC Manual Part I developed by the RMCMA contains an exhaustive “Check List” covering all the areas covered in Fig. 32.1. It contains some 125 items. Out of these, conformance with some 110 items is considered to be strictly essential for achieving good quality concrete, and hence for getting the certification by the RMCMA.

32.6

Handbook on Advanced Concrete Technology QC Manual part I: production facilities

Material storage & handling

Batching equipment

Cement

Scales

Aggregates

Weigh

Water

Batchers

Admixtures

Devices for water

Central mixer

ticketing system

Uniformity of concrete

Provisions on delivery ticket

Timing device

Truck mixers Agitators Nonagitating units

Admixture dispensers Accuracy of plant batching

Delivery fleet

Testing laboratory

Batching system Recorders

Fig. 32.1 Main activities of a typical RMC facility

While developing the Check List, it was ensured that the provisions in the same meet most of the stipulations in the Indian Standard, IS 4926: 2003 (second revision) and the other relevant codes on concrete such as IS 456, IS 383, IS 9103, etc. In fact, in certain cases, the RMCMA requirements are more stringent than those of IS 4926:2003 and other codes.

Audit Procedure With a view to bring in transparency, enhanced credibility and to win over the confidence of customers, it was considered essential by the RMCMA to introduce a yearly audit of the RMC production facility by an external auditor. For this purpose a detailed audit procedure was drawn and the selection criteria for auditors were also finalized. These are included in the Quality Manual Part I. The scheme is open for members of RMCMA as well as others. Before undertaking audit, the auditor produces an undertaking that he has no business relationship with the Company whose plant he would be auditing. The audit is carried out on the stipulated date and shortcomings, if any, are conveyed to RMCMA in writing. Then RMCMA requests the producer to rectify the shortcoming within a period of two months. The auditor revisits facility and satisfies himself about the rectifications made by the producer. Once the auditor gives his okay, RMCMA seeks an undertaking from CEO/Director of producer Company, stating that the “Company would abide by the provisions in the Check List during certification period” On receipt of audit report and the undertaking, certification is granted to producer’s facility. It is noteworthy that under the RMCMA scheme, the customers have the right to visit RMC facility any time during the certification period and cross check compliance in accordance with the Check List.

Quality Control and Quality Assurance of Concrete

32.7

The RMCMA auditors are experienced professionals, who are carefully chosen based on the selection criteria laid down in the Quality Manual Part I. The auditors undergo orientation-cum-training program, including field work involving mock audit of RMC facility. Only after due accreditation, the auditors are permitted to undertake audits. The RMCMA Quality scheme is subject to review. Based on the inputs received from National Experts and customers, both the checklist items and audit procedure can be improved and modified.

Adherence to Statutory Requirements One more crucial feature of the RMCMA quality scheme is its adherence to the prevailing statutory norms in India. Before undertaking any audit, the auditor seeks and verifies certificates of compliance on the following three aspects from the RMC producer: • Permission and consent to operate RMC facility from state Pollution Control Board • Permission from factory inspector confirming adherence to health and safety norms • Permission/license to operate plant from local body/municipal authority. This feature of the quality scheme ensures that the certified RMC facility conforms to the requirements of health, safety and environment.

Guidelines for QA and QC The RMCMA certification through external audit would provide guarantee to the user that the plant and equipment used in the production process of the certified plant conform to the requirements of the relevant IS codes as well as with the statutory norms. However, it needs to be pointed out that availability of proper plant and equipment is only one of the factors - although a very vital one - that controls quality of concrete. No claim is made by the RMCMA that certification of RMCMA will necessarily assure delivery of high quality concrete. The RMCMA certificate should therefore be accepted precisely for what it is - evidence that a certified production facility do possess capabilities to produce quality concrete. The existence of these capabilities will reduce the likelihood of deficiencies in quality of concrete supplied by certified facilities. Realizing that mere certification based on the Check List may not be sufficient to instill assurance on quality amongst customers, RMCMA prepared detailed Guidelines for QA and QC of concrete (Quality Manual Part II). The guidelines recommends establishment of a QC Plan by RMC producer, which would incorporate the following information: • Source and properties of all ingredients of concrete • Process control including mix design • Data base on fresh and hardened properties of concrete • Statistical analysis of the properties of concrete. Quality Manual Part II ensures that the minimum benchmarks suggested in the guideline document are based on the relevant provisions in BIS codes such as IS 456, IS 4926, IS 383, IS 9103, IS 3812, etc. In fact, certain benchmarks in the guidelines far exceed the provisions in different codes.

32.8

Handbook on Advanced Concrete Technology

Based on the Guidelines contained in the Quality Manual Part II, each RMC plant should develop its own QA-QC Plan and documentation and make the same available to its customers on demand. Further, RMC producers can develop their own quality norms over and above the benchmark provisions in the guidelines. There would always be a room for continual improvement in quality and one should welcome it. Such documentation on quality will go a long way in ensuring a higher confidence level amongst customers. With a view to enable site personnel to enforce QA-QC of RMC, the RMCMA has developed tables and graphs. Some samples are enclosed here as Tables 32.1 to 32.10 and Figs. 32.2, 32.3 and 32.4 in this chapter. More details are available in Quality Manual Part II. TABLE 32.1 Selected physical properties of cement Property

Manufacturer I

Manufacturer II

Manufacturer III

PPC

OPC 53 grade

PSC

Date of testing Type of cement

Provisions of Test results* Provisions of Test results* Provisions of Test results* IS 1489 IS 12269 IS 455 Fineness, m2/kg Min. compressive strength, MPa 3-day 7-day 28-day Setting time, minute Initial Final Soundness, mm Loss on ignition, % % of mineral admixture (fly ash

300 (min)

225 (min)

225 (min)

16 22 33

27 37 53

16 22 33

30 (min) 600 (max) 10 (max)

30 (min) 600 (max) 10 (max) 4 (max)# –

30 (min) 600 (max) 10 (max)

or slag) in PPC or PSC

fly ash

15-35 %

35-70 % slag

* Based on data from manufacturer’s certificate or in-house testing or testing done in a third-party lab. # If performance improver is not used and 5(max) if performance improver is used.

TABLE 32.2 Physical requirements of fly ash conforming to IS 3812 and results of selected tests on samples Sr. Property No. 1 2 3 4 5

Particles retained on 45 m sieve* Blaine’s fineness, m2/kg# Lime reactivity, MPa# 28-day compressive strength, MPa# Soundness, %#

IS Requirements Frequency of test suggested by RMCMA 34% (max)

Each consignment

320 (min)

3-monthly/change of source

4.5 (min) Not less than 80% of control

3-monthly/change of source 3-monthly/change of source

0.8 (max)

3-monthly/change of source

* Test conducted on each consignment before acceptance # Based on data furnished by supplier by conducting tests in a third-party lab.

Sample 1

Sample 2

Date

Date

Test report

Test report

Quality Control and Quality Assurance of Concrete

32.9

TABLE 32.3 Results of initial laboratory trials* on chemical admixture Property

Control

Concrete with admixture

concrete

Manufacturer I

Manufacturer II

Adverse effects, if any, Manufacturer III

observed during trials

Name of manufacturer Name of brand Generic type Water content, % of control sample Slump 0 min 30 min 60 min 90 min Setting time, minute Initial Final Compressive strength, % of control sample 1-day 3-day 7-day 28-day Air content, % max over control Observations on cement-admixture compatibility, if any * Laboratory trials can be conducted in plant lab or in a third-party lab (in case facilities are not available in-house)

TABLE 32.4 Uniformity requirement of admixtures conforming to IS 9103 and results of selected tests on samples Sr. Uniformity test

Requirements as per

No.

IS 9103

Suggested frequency

Sample 1 Date report

1 Relative density

Within 0.02 of the value stated by the manufacturer

Every batch/consignment

2 Dry mat. content of admixture Liquid Solid

0.95 T £ DMC £ 1.05 T where, T = Manufacturer’s stated value in % by mass DMC = Test result in % by mass

Each new batch before acceptance

3 Ash content

0.95 T £ AC £ 1.05 T where, AC = Test result in % by mass

–

Sample 2 Test

Date report

Test

Contd...

32.10

Handbook on Advanced Concrete Technology

Contd... 4 Chloride ion content

Within 10 % of the value or within 0.2 %, whichever is greater as stated by the manufacturer

Each new batch before acceptance

5 pH

6 (min.)

Each new batch

Note

While the test at Sr. No. 1 can be done on each batch/consignment of admixture, the remaining tests mentioned at Sr. No. 2 to 5 can be done in a third-party lab by the supplier and results furnished to RMC producer.

TABLE 32.5 Permissible limits for solids and results of tests on samples of fresh and recycled water Sr. Solids No.

Permissible limits as specified in IS 456, max., mg/l

1 Sulphates as SO3 2 Chlorides as Cl Reinforced concrete 3 pH 4 Organic 5 Inorganic

Combined water (Fresh + Recycled) Sample 1 Sample 2 Sample 1 Date Test Date Test Date Test report report report

Sample 2 Date Test report

400

500 Not less than 6 200 3000

6 Suspended matter 2000 Note

While the tests mentioned at Sr.No. 1, 2 and 3 can be done quickly at plant with the help of a ready-made kit, the remaining 1 tests (Sr. No. 4, 5 and 6) can be done in a third-party lab at the frequency suggested in IS 4926

TABLE 32.6 Physical properties of aggregates Property

Frequency of testing Permissible limits, if any, as Sample 1* as per IS 4926 (Low test rate)

specified in IS 383

Impact value

As specified

Los Angle’s abrasion value

Yearly/Source change

- Not more than 30 for wearing surfaces - Not more than 45 for non-wearing surfaces - Not more than 30 for wearing surfaces - Not more than 50 for non-wearing surfaces

Soundness

Yearly/Source change Chloride content Six monthly Potential AAR 5 Yearly/Source change * Based on tests conducted in-house or in a third-party lab.

Date

Sample2* Test report

Date

Test report

Quality Control and Quality Assurance of Concrete

32.11

TABLE 32.7 Additional physical properties of aggregates Property

Test frequency

Test frequency suggested by

Sample 1*

suggested by IS 4926

Guideline

Date

Gradation Moisture content

Monthly

Weekly or source change Daily twice (three times in monsoon)

Silt content for fine aggregates

Monthly

Each lot

Water absorption

3 monthly

Once in month; or source change

Particle density, Bulk density Flakiness

3 monthly 6 monthly 6 monthly

3 monthly 6 monthly 6 monthly

Chloride content

6 monthly

6 monthly

Sample 2* Test result

Date

Test result

* Based on tests conducted at plant lab.

TABLE 32.8 Typical sample report of fresh concrete Name of Company: _________________________ ________________________________ Location: ______________________________ Name of client/project: __________________

Date: _________________ Truck No.: ______________ Ticket No.____________ Total quantity:________m3

Time history • Time • Time • Time • Time • Time

Sampled at: • End of chute End of pump hose Others

batched: ___________ arrival at job site: _______ discharged: _________ sampled: _______ tested: _______

Ambient temperature: ________°C Concrete temperature: ________°C Slump:_________mm Unit weight: ___________kg/m3

No. of cubes made: __________ Cubes stored at: _____________ Cube prepared by: Mr.____________ Name of authorized person: Mr.___________ Signature: ____________________

32.12

Handbook on Advanced Concrete Technology

TABLE 32.9 Production control: Suggested frequencies of inspection, maintenance/calibration Items

Check for

Frequency

Frequency

prescribed by IS 4926

prescribed Date Operator Date Operator of operator, by RMCMA name and name and if any sign sign

Cementitious materials

Visual Inspection Weekly for weather-tightness and leaks

Weekly

Aggregate

Visual Inspection for segregation and contamination

–

Daily

Conveyor belts Visual Inspection and rollers for wear and alignment

Weekly

Weekly

Central mixer

Weekly

Daily

Visual Inspection of

blades and built up Trucks Visual Inspection of Weekly blades and built up Scale 1. Mechanical/knife calibration for edge systems 2 monthly all weighing and measuring equipment 2. Electrical/load 3 monthly cell systems Water meters Calibration Monthly Admixture Calibration Monthly dispensers Gear boxes Oil change Quarterly and oil baths

Plant inspection

Observation

Weekly

Monthly

Monthly Monthly Monthly Quarterly

Slump, mm

Upper limit

Lower limit

Slump, mm

140 120 100 80 60 1

3

5

7

9

11

13

15 17 Sample No.

19

21

23

25

27

29

Fig. 32.2 Typical variation in the slump of a pumpable mix (specified value: 100 mm)

Quality Control and Quality Assurance of Concrete TABLE 32.10 Concrete mix design information Name of RMC Producer: _____________________________________________________ Name of Client/Contractor:___________________________________________________ Site: ____________________________________________________________________ Mix code Characteristic strength, N/mm2 Target strength, N/mm2 Minimum cement content, kg/m3 (if specified) Mineral additives, kg/m3 (if specified) • Pulverized fuel ash • Slag • Silica fume • Others (mention type) Cement type and grade Nominal maximum aggregate size, mm Maximum free water-binder ratio (if specified) Aggregate/cement ratio (if specified) Target workability at plant, (Slump, mm) Target workability at site, (Slump, mm) Maximum temperature of concrete at the time of placing (if specified) Class of sulphate resistance (if applicable) Exposure condition (if applicable) Class of finish (if applicable) Mix application Method of placing Any other requirements (if applicable) Laboratory compressive strength, MPa 7-day 28-day Source Adapted from IS 4926

Compressive strength, MPa

7-day strength

28-day strength

40 35 30 25 20 15 10 1

3

5

7

9

11

13 15 17 19 Sample No.

21 23

25 27

Fig 32.3 Typical variation of 7-day and 28-day strengths of M25 concrete

32.13

32.14

Handbook on Advanced Concrete Technology 28-day strength

Mean of 4 consecutive

fck - 3

fck + 3

Comp. strength, MPa

40 35 30 25 20 15 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 Sample No.

Fig. 32.4 Variation of 28-day strength of concrete along with the variation in the mean of 4 non-overlapping consecutive test results

Controlling Variations in Production In a typical construction project involving use of large volumes of concrete - obtained either from a commercial RMC plant or a site-based captive plant - the first step in the QA and QC process is to get the facility certified from RMCMA based on the Quality Manual Part I. A detailed characterization of the physical and material properties of all ingredients of concrete is the next step. It is then essential to conduct laboratory trials and finalize the mix proportions, adhering to the provisions of various Indian codes. Once the mix proportions are frozen and approved by the client, the crux of the QA and QC process then lies in controlling the variations arising out of day-to-day variations in the properties of ingredients, process control parameters and properties of concrete in the fresh and hardened states. With a view to control variations in the properties of ingredients, the IS 4926:2003 specifies the frequency of testing of materials. For example, in case of aggregates, its moisture content has a considerable bearing on the water/binder ratio and hence the strength of concrete. Therefore, moisture content needs to be monitored on a daily basis. Further, the variation in grading affects the workability and tendency of segregation and bleeding in fresh state and compressive strength in the hardened state. Therefore this property needs to be monitored on a weekly basis. Fly ash is another ingredient which is being now used by many concrete producers. Since fly ash is mostly used as a part replacement of cement, it is essential that it has pozzolanic character. This can be ensured by a quick test of wet sieving of fly ash sample from each bulker and ensuring that the percentage retained on 45 micron sieve is less than 34 percent as specified in IS 3812. In case of chemical admixture, IS 9103 specifies tests to check the uniformity requirements of the material. These tests should be performed at regular interval as suggested by the code. One more issue which has come to the forefront in recent times is the problem of cement-admixture

Quality Control and Quality Assurance of Concrete

32.15

compatibility, which can seriously affect slump and slump retention. It is essential to resolve the compatibility problem well in advance when conducting laboratory tests. As far as concrete is concerned, it is imperative to closely monitor its four important properties, namely, workability, plastic density, temperature and 7 and 28-day compressive strengths. Amongst these four properties, the first and the last property are more crucial. Any failure to achieve the specified values may result in rejection of concrete by the client. While the shortcomings in workability, if any, are visible at the time of delivery, those in compressive strength would be known only later, say after 7 or 28 days. For monitoring trends in compressive strength and also to provide early warning on changes, a technique known as Cusum (Cumulative summation) technique is used by experienced concrete producers. This technique assists in quick detection of changes in the properties, and indicates when action should be taken to increase the probability of meeting the specifications or to reduce the material cost of the concrete. In absence of such technique, it will be a good practice to monitor concrete’s early strength (3-day or 7-day), in addition to the 28-day strength.

Transporting, Placement, Compaction and Finishing The quality of concrete will also be dependent on the precautions taken during transportation, placement, compaction and finishing. Guidelines on these site-based activities are available both in the codes and in textbooks. For hot-weather and cold-weather conditions, different sets of precautions are needed. These are also dealt within other chapters.

32.6

CHECK LIST

The following check list provides quick tips to ensure QA and QC of concrete: 1. Ensure that the concrete is procured from RMCMA-certified facility 2. Ensure that the RMC facility strictly adheres to the statutory requirements in respect of pollution control, health and safety norms and permission from local authorities. 3. Ensure that the concrete producer follows a well-established Quality Plan 4. Ensure that physical and chemical properties of all ingredients of concrete, namely, cement, aggregates, water, chemical admixtures, etc. are tested at code specified frequency and the results of the tests are available for inspection. 5. Ensure that mix proportioning of various mixes is carried out by rational methods and that the durability criteria in respect of minimum cementitious content, w/b ratio, grade of concrete, etc as specified in IS 456:2000, is followed. It is essential to get the mix proportions approved from client/ client’s representative. 6. Ensure that all process control parameters, especially in respect of scale calibration of all weighing and measuring equipments, water meter, admixture dispensers, checks for weather-tightness and leaks of silos, built up on mixer blades, etc. are carried out regularly in accordance with the provisions in IS 4936:2003. 7. Ensure that all ingredient materials are received from approved sources. In case there is a change in source of any ingredient, appropriate corrections needs to be carried out in mix proportions.

32.16

Handbook on Advanced Concrete Technology

8. Ensure that test certificates are made available for factory-produced materials like cement and admixtures. Insist on certificate for each batch of these materials received in plant. Verify these certificates and ensure that the uniformity requirements of various parameters are within code-specified limits. 9. Ensure that moisture content of aggregates is monitored on day-to-day basis and appropriate corrections made in the water content of the mix. Check for the deleterious materials content (mainly silt content in sand) and ensure that the values lie within the code-specified limits. 10. When any supplementary cementitious materials like fly ash, ggbs, silica fume, etc are used as cement replacement materials, ensure that the uniformity requirements of various parameters are within the code-specified limits. For fly ash, a quick test of wet sieving can be carried out in a laboratory and the percentage retained on 45 micron sieve can be used for acceptance of the material to ensure that the fly ash used in concrete has pozzolanic character. 11. Ensure that the workability and compressive strengths are closely monitored and the values are within the IS 456-specified acceptance criteria. 12. Ensure that the data base on various properties of ingredients and concrete is properly maintained (preferably in Excel-based sheets) and is available for checking. 13. For monitoring the trends in compressive strength and workability, make use of Cusum-centric systems so that there is effective control on variations and corrective actions can be taken quickly. 14. Employ skilled personnel to carry out production and quality control functions. Provide systematic training to new entrants. 15. Ensure that adequate precautions are taken during transportation, placing, compacting and finishing concrete.

References 1. Newman, Ken, (1992) Common quality in concrete construction, in Quality Assurance in Concrete Construction, Compilation no. 16, American Concrete Institute, Detroit, Michigan 48219, USA, pp. 16-27. 2. Mather, Bryant, (1992) Selecting relevant levels of quality in concrete construction, in Quality Assurance in Concrete Construction, Compilation no. 16, American Concrete Institute, Detroit, Michigan 48219, USA, pp. 9-15. 3. _____ “Forward” Certification of Ready Mixed Concrete Facilities, Quality Manual Part I, Ready Mixed Concrete Manufacturers’ Association, Mumbai, p. 41. 2008. 4. _____ Ready-Mixed Concrete – Code of Practice (Second Revision), IS 4926: 2003; Bureau of Indian Standards, New Delhi. 5. _____ Certification of Ready Mixed Concrete Production Facilities, Plant Certification Check List, QC Manual Section 3, 10th Revision, December 2007, National Ready Mixed Concrete Association, Silver Spring, Maryland, USA. 6. _____ Specifications for Ready Mixed Concrete, ASTM C 94/C94 M, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA, USA. 2011.

Quality Control and Quality Assurance of Concrete

32.17

7. _____ Guideline Manual for Quality Control and Quality Assurance, NRMCA Publication No. 190, National Ready Mixed Concrete Association, Silver Spring, Maryland, USA. 2007. 8. _____ Seals of Quality, Ready Mixed Concrete Association of Ontario, 365 Brunel Road, Unit 3, Mississauga, Ontario, Canada. 2006. 9. _____Quality and Product Conformity Regulations (2006) The Quality Scheme for Ready Mixed Concrete (QSRMC), U K, pp. 1-99. 2006. 10. ______ Concrete – Part 1: Specification, Performance, Production and Conformity, EN 206-1:2000, European Committee for Standardization, Brussels. 2000. 11. ______ Concrete - Complementary British Standard to BS EN 206-1, Part 1: Method of Specifying and Guidance for the Specifier, BS 8500-1, British Standards, UK. 2006. 12. ______Concrete- Complementary British Standard to BS EN 206-1, Part 2: Specification for Constituent Materials and Concrete, BS 8500-2:2002, British Standards, UK. 2006. 13. _____ Guidelines on Quality Control and Quality Assurance of Ready Mixed Concrete, Ready Mixed Concrete Manufacturers’ Association, Mumbai, p. 61. 2008.

33 Non Destructive Testing of Concrete A.S. Parulekar and Chetan Raikar

33.1 INTRODUCTION Non destructive test for any material is by definition, such test which can be done on the material while the material continues to remain in service for which it is intended. With reference to the concrete as a material, non-destructive testing can be broadly further classified into – (a) Those non-destructive tests which can be directly carried out on the structural elements without extraction of samples. (b) Those non-destructive tests which can be considered as partially destructive, as the further tests are done in laboratory of the samples extracted, but without affecting the serviceability of the structural component.

33.2 THE OBJECTIVE It is expected that the NDT of concrete is carried out with a specific objective in mind. To get the desired results, the objectives vary depending upon the situation in which ND testing is resorted to. The following are few typical objectives for which ND testing is carried out:

33.2.1 For Existing Structures 33.2.1.1 General Assessment of Concrete Properties This is the most generalized objective for ND testing. It is generally adopted in case of existing old structures where no data is available. A few typical situations where such general assessment is required are – (a) As a back-up data for stability certification--. Any structural engineer likes to depend upon some quantifiable data apart from his visual observations before certifying any

33.2

Handbook on Advanced Concrete Technology

structure for its stability as per the statutory requirements. Such Non Destructive Testing can be on sample basis or extensive; the same depends upon the individual Structural Engineer’s confidence level about the condition of the structure. (b) As a back – up data for designing repair/rehabilitation scheme: In case of old structures, general assessment is done to get a clear picture of the concrete quality and its uniformity. Normally such testing is on sample basis. It helps in designing and deciding upon extent of a particular rehabilitation approach. For example, if Non Destructive Testing indicates that the general cover thickness is less and/or the cover concrete is permeable, the rehabilitation scheme shall include a good quality anti – carbonation paint.

33.2.1.2 Acceptance/Rejection of Concrete for Changed Demands Many times it is needed that the existing structural elements will be subjected to increased loads, either due to extension of structures or change of use/renovation etc. In such cases, the Structural Engineer is expected to check the capacities of various members for the expected loads. For establishing the capacities, he needs to know the in–situ strength of concrete and the reinforcement. In many cases, if the drawings are not available, Non Destructive Test results are the only source of getting this data. Even if drawings are available, the Structural Engineer will like to confirm the data especially the concrete grade before taking the final decision.

33.2.1.3

Durability Assessment

In this, the structure is assessed purely from the point of view of durability i.e. expected service life. The estimate of expected service life from this study involves a very detailed study over a much larger period of time. But such studies are extensively reported and can be used as guidelines. In normal circumstances, these parameters can be tested to get a general idea about the state of concrete as well as to define the priorities for maintenance/repairs based on the comparative study of different structural elements/structures. The various parameters which are assessed are depth of penetration of atmospheric elements like chlorides, carbon dioxide etc., thickness of cover concrete, permeability of cover concrete and corrosion potential study.

33.2.1.4 Detection of Defects This particular application requires a detailed testing scheme and generally limited to specific structural elements of more importance such as precast bridge girders, gantry girders, silo walls, water tank shaft walls etc. This testing is done to identify the defective portion of the entire structural element, when there is any doubt. The defect can be in the form of internal voids or delamination. To detect such defects, the entire structural elements are tested at locations marked on grids at a fixed interval. The data such obtained is analyzed and further areas of investigation are identified. In the suspect areas, the grids are made finer, thus converging towards the defect.

Non Destructive Testing of Concrete

33.3

In such studies, the most commonly tested parameter is the integrity of concrete.

33.2.1.5 Damage Assessment After Earthquake/Storm etc. Though visual assessment of cracks and deflection is most of the times adequate to assess the structural damage post–earthquake; non – destructive testing helps in assessment of root cause of the damage. For example, if more damage is observed in a particular area for some specific elements, Non Destructive Testing can establish that the concrete quality of those elements itself was poor as compared to other elements before earthquake/storm etc. The most common parameters which are tested in such assessment is in–situ strength as well as integrity of concrete.

33.2.1.6 Damage Assessment After Fire/Blast etc. Non Destructive testing plays a major role in the assessment of damage due to fire in concrete structures. The assessment is done in two ways – (a) Comparative assessment is done to determine the fire affected areas. For this, testing is done at various placevbs expected to be fire affected. Also, the testing is done on similar concrete elements away from the affected places, preferably in the same structure but apparently unaffected by the fire/blast etc. These test results are considered as benchmark results. Based on these, the affected areas or structural elements can be classified as severely affected, moderately affected and less affected etc. (b) Using some specific techniques of integrity testing, the depth upto which the fire has affected for any concrete element can be estimated, which is very useful while designing the rehabilitation scheme.

33.2.1.7

Assessment of the Crack Depths

This is a peculiar and specific application in which the surface cracks are assessed for their estimated depths. The technique of indirect Ultrasonic Pulse Velocity (Ref 3.2) is used for this. The estimated depth of crack from the concrete surface is calculated using principles of trigonometry. For large scale assessment of crack depths in important structure, it is recommended that validation of test results shall be done on a artificial crack made in a concrete test block.

33.2.1.8 Thickness Assessment In existing structures, many times it is required to assess the thickness of concrete where the other face is not accessible. The typical situations are of concrete parameters, base rafts, retaining walls, tunnel lining, canal lining etc. In such situations, Non Destructive testing can be used to know the thickness. For such application, pulse–echo techniques are used along with core extraction for confirmation.

33.4

Handbook on Advanced Concrete Technology

33.2.2 For New Construction/During Construction 33.2.2.1 Acceptance/Rejection of Concrete Generally, the cube compressive strength tests are used in the construction practice, for acceptance of concrete as per IS 456:2000. However, there are many situations when the cube test results cannot be used for the same. The typical situations are: • Cubes are not taken during concreting. • Cubes are misplaced or wrongly numbered. • Cubes are cast by untrained people. • There is a doubt that cube concrete doesn’t represent the actual concrete. In all such cases, cube test results cannot be relied upon. Also, when cube test results fail, there is a doubt whether the cube concrete represents the actual concrete or not. In all these situations, Non Destructive testing is used as a handy tool. It is preferable that Non Destructive test results representing such doubtful concrete shall be compared to the benchmark values of concrete of same / similar age but independently accepted using cube test results. When such benchmark concrete is not available, it is necessary that required numbers of core tests are taken to establish the equivalent cube compressive strength.(Ref 3.7). It is to be noted that analysis of non destructive test results is based on statistical parameters. Hence, although acceptance of concrete can be done based on the non destructive test results of sufficient numbers, the rejection shall not be only based on Non Destructive test results, but shall be supported by load test, wherever applicable.

33.2.2.2 Identification/Confirmation of Doubtful Concrete As a part of quality control measures, when there is any doubt in the quality for particular concrete, the same can be confirmed using Non Destructive testing techniques by simple comparative assessment using benchmark concrete. The reasons for such doubts can be many, such as cement deficiency, excess water, malfunctioning of admixture, slurry leakage through shuttering, improper mixing, inadequate curing etc. Normally, the test parameters used for such comparative testing are in-situ surface strength as well as integrity. Also, Non Destructive techniques can be used to estimate the crack depths of surface cracks, wherever required.

33.2.2.3 Establishing Uniformity of Concrete Non destructive testing can be used as an effective quality assurance tool to establish uniformity of concrete across a large set of structural elements in a particular structure or group of structures built with same concrete. The testing can be done on large number of locations without affecting the normal progress of work. Various statistical parameters like average, standard deviation, co–efficient of variation are established and can be used as control parameters in Quality Assurance Procedure (QAP).

Non Destructive Testing of Concrete

33.5

33.2.2.4 Collection of Data for Long Term Monitoring This is a Non Destructive testing application which is desirable but not in the current practice. In this, various Non Destructive tests are done on a random sample basis at particular locations spread over the entire structure. The test results are documented and preserved as reference results for further testing. If such testing is carried out at same locations after a specific interval repeatedly, the same will serve as very useful data in theoretical service life prediction for the structures. It is recommended that such collection of data shall be done in atleast important structures like nuclear structures, major bridges, high rise buildings etc.

33.3 COMMONLY USED TEST METHODS 33.3.1 Core Test 33.3.1.1 Principle What can be better than taking a part of concrete under investigation, observing it closely and be able to test under a cube testing machine? It is a partially destructive test in which cylindrical cores of diameter 75 mm to 150 mm and 100 to 200mm length are extracted out by core cutter machine and tested for its crushing strength after preparations. The test is the most direct test among all the NDT tools. It gives in-situ crushing strength of concrete which can be converted into in-situ cube strength. Core extraction can also be used to determine various other parameters like density, modulus of elasticity, cement content, permeability, various chemical tests and microscopic examinations. The cores are also very useful for visual inspection. The aggregate matrix, voids, micro porosity etc can be observed which gives insight into real concrete inside.

33.6

Handbook on Advanced Concrete Technology

Test Apparatus The cores are extracted using a simple but powered drilling machine with cylindrical bits with diamonds attached at the end as a cutting media. The machine is mounted on the concrete surface with the help of mechanical anchor fasteners, perpendicular to the surface from which the core is to be extracted, the cutting end is supplied with water for cooling through in-built arrangement.

33.3.1.3

Influencing Factors

(a) Diameter of core/Length of core: There are certain important aspects that need to be taken care while performing the test. The diameter of the core should be at least three times the maximum size aggregate. The lower diameter will tend to give lower results due to drilling damage to the concrete matrix. The length of the core should be two times the diameter; however it is not always possible to get the cores of length/diameter ratio of two. There are correction factors that are applied to cores of different length/diameter ratios. The cores of different L/D ratios will need correction factor to find equivalent standard cylinder strength with L/D of two. L/D ratio of less than one is not preferred.

Correction factor

1 – 00

0 – 96

0 – 92

0 – 88 1–0

1–2

1–4

1–6

1–8

2–0

Height Ratio – Diameter

For all practical purposes a correction factor of 1.25 is taken to convert the cylinder strength into equivalent cube strength. There are other formulae and graphs by different standards that can be used for more accurate conversions with prior agreement between the parties concerned. (b) Location of core test: The location of the cores should be decided keeping in mind the aim of core test. The location must be representative of the average in-situ strength and within member variability of strength must be taken into consideration. For example, in column , for purpose of finding 28 days standard cube strength for determining the concrete grade, it is advisable to take core around 300 mm above the bottom. Whereas, if minimum possible in situ strength is to be determined, the core should be taken around 300 mm below the top of the column. In case of deep beam similar logic can be used. But for normal beams the cores must be taken in upper half preferably one third from top.

Non Destructive Testing of Concrete

33.7

(c) Presence of Reinforcement: Care should be taken to avoid reinforcements. Cover meter can be used to locate the bars. It is usual to find rebar in the core samples. These are usually on one or the other side. The part of the core beyond rebar is cut off and only the concrete portion is taken for the test. Where it is not possible to avoid the bars correction factors are applied. The core with rebar parallel to the axis of cores can not be used as the effect of the rebar will be too large to ignore. The rebar perpendicular to the axis can be tolerated as long as the correction factor is less than 10%. (Δr h) Corrected strength = measured strength × 1.0 + 1.5 ______ (Δc I) Where Δr = bar diameter

[

]

Δc = core diameter h = distance of bar axis from nearer end of core l = core length (uncapped). Multiple bars within a core can similarly be allowed for by the expression

[

S (Δr h) corrected strength = measured strength × 1.0 + 1.5 _______ Δc l

]

If the spacing of two bars is less than the diameter of the larger bar, only the bar with the higher value of (Δr h) should be considered.

33.3.1.4

Limitations

The test usually gives results within +/– 12/ √– n% of actual in situ strength at 95% confidence limit, where n is the number of samples. For one sample, the in built inaccuracy is 12%. Hence there should be minimum three samples to bring down the uncertainty level to around 7% which is usually acceptable. This uncertainty is other than variability of strength of in-situ concrete which may be up to +/– 15% for normal concrete for 100 mm diameter core. Average of three (more numbers for smaller diameter) cores is computed as the representative value for the relevant part of the structure. Where it is not possible to take more than one core from the member for reasons of safety, looks, structural risks, possibility of smaller diameter cores should be checked with. But when using only one sample for strength determination help of other test such as rebound hammer, ultrasonic pulse etc should be taken to increase the confidence in the results. The effect of testing dry core or wet core (saturated) should be taken into account to predict the standard cube strength as the cubes are tested in saturated conditions whereas the cores are tested dry. The cores must be handled very carefully during transportation. A bad handling may even cause fracture and thus rejection of the core. A bad capping or trimming of the two ends may also cause wrong results, usually on negative side.

33.8

Handbook on Advanced Concrete Technology

33.3.2 Rebound Hammer Test 33.3.2.1 Principle This is one of the fastest and cheapest NDT tool with very easy application. The principle lies in the fact that harder the material more will be the rebound of any object striking it. If a ball is dropped on two different floors made up of clay and concrete; the rebound of the ball will be more on the concrete floor than the clay floor. This is due to elastic reaction of the ball as well as the floor material under impact. Concrete within a small range of deflection is highly elastic. This property is used in assessment of hardness and in turn strength of concrete, by impacting spring loaded hammers and measuring the rebound. The relationship curves established by large scale data collections and tests are used for deducing in situ strength. The tool is very useful in quick and fast scanning of large part of structures in a very cost effective way. The analysis of the data can be used to identify suspect members or zones and more costly or tedious tests can be conducted on identified locations, thus saving cost and time.

33.3.2.2 Test Equipment and Procedure The most common equipment is the Schmidt hammer which was invented in late 1940s. The equipment consists of a casing, spring loaded weight that slides on a guide bar, locking and release mechanism and a pointer to measure the rebound. The equipment is very simple and handy to use at any location as it weighs only about 2 kg. The modern version transferring the measured rebound to the digital output with a facility of data storage and printouts is also available. The impact energy transferred to the concrete is roughly 2.2 Nm and it leaves a small indentation on concrete surface due to local crushing of concrete. The indentation however is very small and does not harm the structure. Concrete surface

Window and scale

Impact spring

Release catch

Rider on guide rod

Hammer guide

Plunger Hammer mass

Housing

Compression spring Locking button

Fig. 33.1 Typical rebound hammer

The test is necessarily conducted on any formed concrete surface. The typical steps are as follows: (a) Remove any cladding, plaster etc. and expose the bare concrete formed surface in an approx. area of 300 × 300mm

Non Destructive Testing of Concrete

33.9

(b) Make the surface smooth using carborandum grinding stone which is generally supplied with the equipment .This is very important so as to ensure the uniform contact surface at the tip. (c) Mark twelve numbers of test locations at a center to center distance of approx. 25 to 30 mm in any geometrical pattern like rectangular grid or in diamond pattern. (d) Carry out the test at each test point and note 12 readings, (e) Discard two extreme readings and take the average of 10 readings. This number is the test result of the location The test results are expressed in term of rebound number. The number is typically from 10/12 for very weak concrete to 45/50 for higher concrete grades

33.3.2.3

Caliberation for Compressive Strength

The relationship between rebound number and standard cube strength of concrete is established in the laboratory and is given as a graph for use at sites.

2

Cube compressive strength (N/mm )

50

40

30

20

10 10

20

30 Rebound number

40

50

Fig. 33.2 Typical rebound number/compressive strength calibration chart

The above is plot of rebound numbers and corresponding cube crushing strength from the results obtained in laboratory test. The cubes are held in the cube testing machine and restraining load of 15% of expected concrete strength or minimum 7 N/mm2 is applied. The rebound numbers are taken on the vertical surface in the same way as the actual test on any concrete surface. The rebound number is recorded for the corresponding actual crushing strength of the cube tested after drying for 24 hours in the oven. This is required as the in situ concrete to be tested at sites will, usually be in dry condition. The rebound hammer equipment also has graphs on them showing relationships between rebound number and strength for three position of the test e.g. vertically up, horizontal and inclined at 45º. The gravity affects the plunger’s rebound and thus rebound number too. Hence the three graphs are obtained.

Handbook on Advanced Concrete Technology

7000 6000 5000 4000 3000 2000

±900 ±850 ±800 ±750 ±700 ±600

1500 20 25 30 35 40 45 50 55 Rebound value R

±950

8500 8000 7000

±1000

6000

±950

5000

±900 4000

Dispersion [psi]

±950

fckcyl.m [psl] (Cylinder Δ6” ¥ 2”)

fckcyl.m [psl] (Cylinder Δ6” ¥ 2”)

8500 8000

Dispersion [psi]

33.10

±850 3000 ±750 2000

±650 1500 20 25 30 35 40 45 50 55 Rebound value R

The graphs on the hammers are provided by the manufacturer based on very large number of test results for wide range of material properties. The use of such graphs must be made with caution because they would tend to overestimate or underestimate in situations like use of different type of aggregates. Hence it is highly recommended that the test agencies must develop their own charts using local materials and with local practices. This would provide a much more reliable test prediction.

33.3.2.4

Influencing Factors

(a) The cement paste thickness at test point: The rebound hammer readings vary for the same concrete as the rebound number is affected by some factors which create variation in the same member at nearby locations. It should be remembered that rebound number actually measures the hardness property within a very shallow depth from the surface. It is presumed that the effect is only up to 30 millimeter. If the plunger is pressed directly against a large aggregate, it would give higher rebound number than when plunger strikes cement paste. This is the reason for which 12 readings are taken at close interval to generate one test result so that this influence is taken care statistically. (b) Surface Finish: The smooth and well finished surface shows more hardness then rough or grainy surface. Steel finish gives more rebound than ply finish and wood finish is lowest. (c) Edge Distance: The numbers taken away from the edge would be higher than taken at edges and therefore the test should not be carried out within 75 millimeter from the edge. (d) Aggregate type: The aggregate type has great effect on the variation of rebound numbers. Harder aggregate tend to give overestimation of strength, whereas a weak limestone may give underestimation of the concrete strength.

Non Destructive Testing of Concrete

33.11

(e) Carbonation: The other factor that affects the rebound number greatly is carbonation of the cover zone. The carbonation of concrete tends to make the concrete harder in the cover zone, thus overestimating the in-situ compressive strength obtained by this test at that location . (f) Type of cement and sand: Though the cement and sand type will theoretically affect the rebound number, the effect can be ignored for all practical purposes unless special quick setting cement or special light weight sand is being used in making concrete.

33.3.2.5

Limitations

Use of this test in early age of concrete must be done with care and knowledge that variation in results would be much more than in matured concrete. However, this test is widely used in pre-cast industry to check the readiness of the concrete for lifting, handling, transport etc. The rebound hammer can also be used in high rise construction for ascertaining the quality of the columns when the upper slab is being cast before the columns have reached 28 days of age. Determination of strength using this tool must be done with caution and full understating of the factors influencing the interpretations so that assessment is as close to the real in-situ strength as possible. Proper corrections must be applied for conditions that defer from the standard testing conditions. Large number of data must be taken for statistical analysis. The test is very good tool in checking uniformity of concrete production at any site over time. It is very fast, reliable and cheap tool to be used in isolating weaker member out of large number of members in a structure. It is also a good tool in evaluation of general health of concrete structures.

33.3.3 Ultrasonic Pulse Velocity Test 33.3.3.1 Principle The ultrasonic waves are waves that have higher speed than sound wave. These waves can pass through solids and can be detected at far ends. The velocity and scatter of the waves depend mainly on the density and the elastic property of the medium. It is more in denser medium and in stronger medium. There are three kinds of waves generated due to applied impulse, surface waves, transverse waves and longitudinal waves. Surface waves are slowest and travel along the surface; transverse waves travel perpendicular to the direction of propagation and are faster. These are also called shear waves. Longitudinal waves are the waves of most importance and they travel in forward direction taking the wave front farther and farther. These waves can travel far into the medium and its behavior can give much useful information about the interior of the concrete including its elastic property, density, strengths, voids, compaction etc. when used by experienced engineer. The test consists of inducing short bursts of waves; of frequency range 20 to 150 kHz; using an acoustic-electro transducer and detecting the pulses at some fix distance by the receiver across the body of the concrete member. The equipment measures the time taken by the waves and the same is recorded. Velocity can be determined thereafter. The equipments have display units that show the time in microseconds. The velocity ranges from 3.5 to 4.8 km per second for reasonably good concrete depending on grade of concrete, type of aggregate and moisture content.

33.12

Handbook on Advanced Concrete Technology

Fig. 33.4 Ultrasonic pulse velocity instrument

33.3.3.2 Test Methodology There are three arrangements in which the transducers can be placed. Direct – opposite faces Semi direct – adjacent faces at right angles Indirect – Same face

Direct Transmission The transducers are placed directly opposite to each other with the help of acoustic jelly for better acoustic connection between transducer surface and the concrete. This is the most preferred way of measurement as the signal strength is highest. Depending on concrete density, type of aggregate, type of equipment etc the waves can be detected up to 1.5 meter across the mass of the concrete. Three distinct stable readings should be taken at the selected points and noted down. The average should be considered as the test result of that point. This is the arrangement which gives most reliable results and should be preferred for all important assessment. The data can be used for estimation of integrity, uniformity and porosity, large voids within the body of concrete and structural cracks.

Semi Direct Transmission The transducers are placed on adjacent faces usually in absence of possibility of getting the direct opposite faces. The faces having too wide angle, say more than 120°, and large distance between transducers, say more than 500 mm, would induce large error in the measurements due to interferences of other wave fronts. The signal strength of these waves is just about one third of the direct wave for the same distance.

Indirect Transmission The arrangement is least preferred for any useful information about soundness of concrete as it records the surface waves and the scattered waves. The path of travel is uncertain and

Non Destructive Testing of Concrete

33.13

therefore velocity measurements would have large variation and equally large error. The method, though, is useful in determining crack depth and lamination or presence of weak layers within reasonable depth.

33.3.3.3 Applications (a) General Quality of Concrete: The most useful and widely used application of the USPV test is to assess the quality of the concrete from the point of view of soundness and integrity. IS 13311(part 1) classifies the concrete based on the test result obtained from direct transmission which is as follows: TABLE 33.2 Velocity Criterion for Concrete Quality Grading Si. No. 1. 2. 3. 4. Note

Pulse Velocity by Cross Probing (km/sec) Above 3.5 to 3.0 to Below

4.5 4.5 3.5 3.0

Concrete QUality Grading Excellent Good Medium Doubtful

In case of “doubtful” quality it may be necessary to carry out further tests.

The USPV test is simple and reasonably cheap. Large number of test readings can be collected about the structure and statistical analysis can be done. Also, the test can be repeated at a particular location on any given time at a later date. (b) Strength Prediction: The strength of concrete has direct relation with the pulse velocity for well compacted concrete and for given aggregate type. However the range of velocity is very small for wide range of strengths for reasonably well compacted concrete. The relationship is greatly influenced by aggregate types, method of mixing and compaction, curing, air content and moisture content. It is recommended that the test agencies must develop relationships by lab experiments for local materials and practices. This can be done by casting cubes with different w/c ratio, different a/c ratio and different aggregate types. The number of cubes must be sufficient to represent at least 30 set of readings for each parameter for statistical analysis. The pulse velocity readings can be taken on the cubes and the cubes shall be then crushed. The results can be related in various ways as required. These charts would help in prediction of in situ strengths with acceptable level of variation. (c) Measurement of crack depth: There is one common use of the test and that is measurement of cracks. The indirect arrangement is used for this. The transducers are placed on either side the cracks at known distances. The travel time of the pulse is noted. Due to presence of the crack the pulse takes longer time to reach the receiver as it has to duck below the crack. The waves can not jump through air gap. This delay would reflect in the velocity measurement and can be used to calculate the depth of the crack However presence of moisture or dust that can bridge the crack, severely affect the test results. Hence lot of caution is required.

33.14

Handbook on Advanced Concrete Technology x

x

h

Fig. 33.5 Crack depth measurement

(d) Identification of weak portion: Pulse velocity measurements can be effectively used for identifying weak and porous zones of concrete or any portion that is over stressed and cracked/crushed internally as the pulse velocity in such region will be considerably lower than that in reasonably good concrete. A contour can be drawn of the velocities as shown in figure below to know fire damage, internal cracking or zone of weakness for large members such as lift well, deep beams, wide columns etc.

3.90 4.00

4.00 4.20 4.30

4.00 4.10 4.20 4.30

Fig. 33.6 Typical pulse velocity beam contours (km/s.)

33.3.4

Influencing Factors

(a) Moisture content: The pulse velocity is affected by the moisture content. A fully saturated concrete gives up to 5% higher velocity than dry identical concrete. This must be considered when using pulse velocity for determining equivalent standard cube strength. It is recommended that the velocity measurement shall be taken on cores from the structure before crushing the cores, to establish much reliable relationships for testing of important structures. (b) Reinforcement steel: Wherever there is steel in the path of the pulse, suitable corrections are to be applied. The wave travels much faster in steel than in concrete. Steel parallel to the path and near the transducer location will have considerable effect on the results and may give high values. Steel across the path of travel of pulses have small effects depending on the diameter and number of bars. There are standard correction methods that are applied to test results.

33.3.3.5

Limitations

The reliability of this test for checking soundness is very good but not so good for strength determination where the accuracy is not greater than +/– 20%. Also, huge laboratory experimentation is required to obtain the correlation as the range of velocity is very small for a wide spectrum of strengths. Moreover, the relationship is greatly influenced by various independent factors, making it virtually impossible to get a valid relation. Hence, though it is

Non Destructive Testing of Concrete

33.15

theoretically possible, strength prediction becomes virtually impossible, using only USPV test. Hence, the test must be conducted in combination to rebound hammer test at least and with few cores, if possible.

33.3.4 Half Cell Potentiometer Test 33.3.4.1 Principle The health of RCC structure is practically defined by the state or reinforcement bars within. Corroded bars reduce the capacity of the structure and compromise with factor of safety. Once the corrosion sets in, the rate keeps exponentially increasing. Hence it is very important to assess the region of concrete having expected corrosion or environment leading to reduced passivity. The concrete cover becomes less passive over time due to carbonation or effect of air-borne salts as well as chlorides in concrete. This can be measured by measuring potential difference between surface of the concrete and the rebar inside for a given applied potential using standard cell. The areas that have corrosion or such environment would show less potential difference due to higher conductivity of the concrete.

33.3.4.2 Test apparatus and Methodology The test is very fast and cheap. Large areas can be covered in reasonably less time. The instrument is very handy. It consists of copper-copper sulphate cell and high impedance voltmeter. Voltmeter + – Copper rod Saturated copper sulphate

Fig. 33.7

Porous plug

Reinforcement potential measurement

The area to be scanned needs to be wetted for better connectivity of the cell to the concrete. Voltmeter is connected to the steel bar at one point. The steel mesh is usually well connected electrically within concrete. The readings are taken in grid pattern with grids of 100 cm/50 cm/10 cm as per accuracy required . The results are plotted as contours of iso-potential lines. The region of the low potential is thus identified.

33.3.4.3

Influencing Factors and Limitations

The potential measurement is affected by factors such as moisture content in the cover zone. Higher moisture content would give lower potential. A complete dry cover may on other

33.16

Handbook on Advanced Concrete Technology – 200

– 200

– 200 –300

– 100 – 100 – 200

– 200 –300

– 100

–300

Fig. 33.8

Typical half-cell potential contours

hand give very high potential. Therefore absolute values can not be used for any numerical calculations. The islands of low potential are of interest and can be studied further. Below is the table which is used for assessing the corrosion potential of a cover zone. However it should be noted that it only indicates the likelihood of corrosion and not the state of corrosion of steel. Hall-cell potential (mV) relative to copper/copper sulphate reference electrode

Percentage chance of active corrosion

< – 350 – 200 to – 350 > – 200

90% 50% 10%

General Guideline for Interpretation of Half Cell Potential Test Results

33.3.5 Pull Out Test 33.3.5.1 Principle An insert embedded in concrete requires some force to pull it out. The force will depend on the depth of embedment, shape and strength of concrete. If the first two are standardized and the relation between pull out force and the standard cube strength is developed in laboratory for given materials and practices in use, it is possible to find in situ strength of the concrete. There are basically two types of tests under this category. One type is in which inserts are embedded in concrete during casting and the other type is when a hole is drilled and insert is placed in hardened concrete. It is obvious then that first category requires planning before casting. Therefore second category is more frequently used as NDT tool whereas first category is suitable for research and development projects.

33.3.5.2 Test Methodology There are many proprietary variations in this test in the shape of embedment and equipments for pulling out. But basically all tests require a small portion of concrete to be destroyed. The method is less costly and less accurate than core test but more accurate than rebound hammer or ultrasonic test for estimation of strength.

Non Destructive Testing of Concrete

33.17

A typical arrangement is shown below: 25 mm

Failure cone Form

25 mm

55 mm

Cut off

Removable stem

Fig. 33.9

33.3.5.3

8.5 mm Anchor plate

Reaction ring

Pull out arrangement (lok test insert)

Influencing Factors and Limitations

Various research programs have established that the test results are relatively independent of the type of natural aggregate, curing conditions and type of cement. There exists a consistent relation between cube crushing strength and shear and tensile strength. So the established relation between pulling force and the standard cube strength can be used for wide variety of materials and wide range of well formed concrete. Still it would always be advisable, that the test agency must develop the relationship for local materials and practices for better results and reliability. Some precautions are required in the test such as the distance between two test locations should be four times the fracture diameter. The location should be at least three fracture diameters away from the edges. Also there should be no reinforcement within 35 mm of the embedment.Also,the loading rate has effect on the required pulling force It is generally accepted that an accuracy level of +/– 20% is achieved by this test in predicting the actual in situ strength by taking average of 4 readings. Accuracy level of +/– 10% is reported when the relationships have been established for particular type of concrete locally. The dry or moist state of concrete has been found to have little effect on accuracy and relationships between pull force and standard cube strength. However, it must be noted that the calibration process must take into account that the in situ test done on hardened concrete in the structure and that done during calibration on cubes or slabs will have effect of variation in curing history, compaction and within member variability. The standard cube strength relating to the pull force will actually represent the in situ equivalent wet cube strength and for specification compliance must be converted into potential standard cube strength by use of such established charts for similar concrete without correction for moisture content.

33.18

Handbook on Advanced Concrete Technology

33.3.6 Carbonation Test 33.3.6.1 Principle The free alkali in the concrete turns into carbonates by the attack of carbon dioxide found in the atmosphere, in presence of moisture. This decreases the alkalinity in the cover zone and increases the possibility of corrosion. Hence, in old concrete it is required to check the depth of carbonation for determining the requirement of suitable protection methods. It is also required in any NDT plans for suitable correction factors and for interpretations of the results of other tests as depth of carbonation has significant effects on many tests.

33.3.6.2 Test Methodology There are two ways to check carbonation. One is to check the cut surface of cores, pulled off concrete from other tests and the other is by drawing powder from the concrete by step drilling. Phenolphthalein indicator is used on slightly moist powder or cut surface. The change in color to purple red shows absence of carbonation. No change means concrete has been carbonated and may have PH value close to 9 e.g. less alkaline than the uncarbonated concrete. Drilling must be done gradually in increments of 5 mm. any lower increment is impractical. Thus, the accuracy of measurement of depth by this method is not good. However it is sufficient for all practical purposes. Where more accurate determination is required it is suggested that small core be taken by dry cutting. The core surface should be cleaned by blowing air and indicator should be spread after moistening if the surface is dry. The indicator works only in presence of moisture.

33.3.7 Cover Meter and Rebar Locator Test 33.3.7.1 Applications Though, it is not a test of concrete parameter directly, many times the cover thickness and the location of the bars are required to be determined. Such cases may involve specification compliance check on adequacy of cover. The best protection to the reinforcement is provided by the cover concrete and therefore it should be checked . The location of bars may be needed to be known before tests such as core test, ultrasonic pulse velocity test, pull out and pull off test etc. In the cases involving tests on old buildings where the drawings are not available, the diameter and the number of bars may be needed to be known for assessing the existing structural capacity for determining the strengthening measures for extension, change of use etc.

33.3.7.2

Principle and Test Methodology

The cover meter works on principle of magnetic induction. The presence of steel reinforcement under moving magnetic field produces induced voltage that is recorded by the equipment. The strength of induced voltage depends on the bar diameter and the distance of the bar from center of the probe. The probe is run on the surface of the concrete in perpendicular direction to the bar axis and points of highest voltage are marked. A typical arrangement is shown in Fig. 33.10.

Non Destructive Testing of Concrete Batteries

33.19

Calibration voltage

Amplifier

Magnetic core

Moving coil meter

Generator coil Coil

Magnetic flux

Fig. 33.10 Typical simple cover meter circuitry

There are many types of instruments available with varying degree of accuracy, sophistication and complexity. Simple cover meter can give location of bars and measurement of cover with accuracy level of about +/– 20 %. In case of high end instrument the accuracy may be as high as +/– 10%.

33.3.7.3

Influencing Factors and Limitations

The measurement of cover is affected by the bar diameter. Higher diameter bar would give lower cover. Therefore, the common instruments give setting for two cover ranges. The higher end instruments have setting for different diameter of bars, type of steel etc. The estimation of cover depth and bar diameter are inter dependent. Bar diameter can be known accurately if the cover is known. Cover can be known more accurately if the bar diameter is known. But in practice, both are unknown and the accuracy level decreases when both are being predicted based on some assumptions. Bundling of bars, layering of bars (as in beam bottom), presence of binding wire, steel-inserts etc. affect the results. In case of bundling, the approximate equivalent diameter would be assessed. But in case of layering the results may be confusing. The test may be more useful for slabs, walls, columns than beams and beam-column junctions.

33.3.8 Initial Surface Absorption Test (ISAT) 33.3.8.1 Principle The test is basically a measure of permeability of concrete. There are many test methods by which permeability can be tested and Initial Surface Absorption Test (ISAT) is one of the popular method for the same. Initial surface absorption is defined as the rate of flow of water into concrete per unit area at a stated internal from the start of the test at a constant applied head and temperature, which is expressed in ml m2 /s at a stated time.

33.20

Handbook on Advanced Concrete Technology

This rate is initially high due to capillary action, when water comes into contact with dry concrete. The rate decreases, as the waterfilled length of capillaries increase. A relationship can be established in the form of equation P = at –n Where

p = initial surface absorption t = time a and n: Constants

By measuring ‘p’ at various ‘time’ intervals, constants ‘a’ and ‘n’ can be calculated for a particular concrete. From this established relation, ‘p’ can be woked out for any given time period.

33.3.8.2

Equipment and Test Methodology

The standard equipment consist of a cap, which can be clamped and sealed onto the concrete surface as shown in Fig. 33.11.

Reservoir Glass capillary and scale Flexible tubing

Concrete surface

Outlet

Inlet

Tap

Watertight cap clamped to surface

Fig. 33.11 Initial surface absorption test

The inlet to the cap is connected to a reservoir and an outlet is connected to a capillary tube with a scale. The water contact area must be at least 5000 mm2. The reservoir and horizontal capillary must be set at 200 ± 20 mm above the surface to be tested. At the start of the test, the cap must be fixed to the surface and sealed to provide a water tight assembly. The test is started when the reservoir tap is opened and water at 20 ± 2°C is allowed to flow into the cap. The capillary should be disconnected from the outlet tube until all the air has been expelled. The reservoir head must be maintained and the capillary is adjusted shortly before the ‘measurement time’ so that it fills with water before fixing horizontally at the same level as reservoir surface. Measurements are made by closing the inlet tap and watching for movement of the capillary for 30 seconds to 120 seconds. The measured reading is then factored to give the number of scale units moved in one minute, which is the value of initial surface absorption. This procedure should be performed at 10 min,30 min,one hour and two hours after the start of test, on minimum three separate samples or sample locations.

Non Destructive Testing of Concrete

33.3.8.3

33.21

Influencing Factors and Limitations

The most important influencing factor in getting reliable results is the moisture level of the specimen at the surface. Though oven dry specimen give most reliable results in laboratory, it is very difficult to achieve that condition in case of in-situ testing. To overcome this, it is recommended that minimum dry period at 48 hours is specified. Also, various drying techniques are adopted to achieve near oven dry condition. Another limitation in in-situ use is to achieve a watertight fixing of the cap. This is absolutely must to get the correct readings.

33.3.9 Impulse Radar Systems 33.3.9.1 Principle and Applications In Radar Systems, electromagnetic waves, typically in the frequency range 500 MHz to 1 GHz are passed into solids. The speed and attenuation of the signal is influenced by the electrical properties of solid materials. Reflections and refractions of the radar wave occur at interfaces between different materials and returning signal is interpreted to provide an evaluation of properties and geometry of sub-surface features. The main application of impulse radar systems is to identify sub-surface defects and features in any material, like delaminations, embedded objects, sub-surface utilities etc. The influencing depth and the output accuracy is determined by the frequency of transmitter antenna. A 500 MHz antenna is more appropriate for deeper investigations, in which case the system is called ‘Ground Penetrating Radar Systems’. A 1 GHz antenna is used for relatively smaller depths but higher accuracy, which is used for concrete applications (upto 500 mm thick), in which case the system is called ‘Surface Penetrating Radar Systems’. Power supply Radar pulse generator & control unit Transmitter

Data storage & display

Receiver Scan

Air Material 1 Anomaly

Material 2

Fig. 33.12 Investigation of sub surface anomaly using radar

33.22

Handbook on Advanced Concrete Technology

33.3.9.2 The Equipment The typical impulse radar equipment comprises of a pulse generator and receiver connected to an transmitting antenna, which is held in contact with the concrete surface. Alternatively, a focused beam horn antenna with an air gap of about 300 mm between the horn and concrete surface, is adopted. There are many commercially available systems with various combinations of antennas, sensors and software, including a hand-held radar scanner. While performing the test, the antenna is moved along with concrete surface and received data is processed through microprocessors.

33.3.9.3

Influencing Factors and Imitations

The rate of propagations of radar waves through concrete is governed by very complex mathematical expressions. For assessing the depth of any sub-surface object like reinforcement, conduit or delamination, it is necessary to have the knowledge of this speed. This speed or rate of propagation is largely dependent upon the relative permittivity of concrete, which in turn is predominantly dependent upon the moisture content of the concrete. Another important limitation is the complexity of the results obtained. The interpretation of radar results indicating sub-surface features are not user friendly. The radar ‘picture’ obtained doesn’t resemble the form of embedded features, but often represented by complex pattern due to the diverging nature of the electromagnetic pulse. The use of modern signal processing can simplify the image, but still it needs a very experienced and expert interpreter.

33.4

SAMPLING AND RELIBILITY

The analysis of any Non-Destructive Test results is done using statistical parameters. Hence, as a general rule, more number of samples will always give more reliable results. However, any test plan must give reasonably accurate assessment in reasonable time and within reasonable cost. Hence, it is necessary to understand the effect of sample volume, sample selection and type of test that can give assessment within acceptable limits for optimization of cost. A test plan that is based merely on economy with disregards to reliability will be waste of money and efforts for the results would be misleading. The selection of type and combination of test must be done keeping following in mind. (a) Statistically 30 or more samples are required achieving 95 % confidence. Say for example, the in situ strength of concrete in any structure is to be determined with 95% confidence limit, then 30 or more comparable locations must be tested of the concrete with same expected in-situ strength. (b) However, it is not possible to execute core test at this scale due to safety and economy considerations. Hence, rebound hammer test which is cheap and fast, shall be done on three times more locations than core tests, where cores shall be taken for establishing correlation between the rebound number and in-situ strength. (c) Also, when the two or more methods indicate similar trend of estimation, the overall confidence in the estimation increases. Say for example, for a concrete grade M 35 the

Non Destructive Testing of Concrete

33.23

core test are indicating equivalent grade as M 30 and the rebound hammer readings are also leaning on negative side then there is fairly good chance that the concrete is below the specification. Any contradiction in results of parallel tests would though confuse the interpretations, but at least ensure that the results are studied more carefully before making grossly wrong judgment. In case of any building the columns are most important so more of columns must be covered in the test plan. Next important elements are beams and least are the slabs. The consequences of failure of the individual structure are what determine its importance. The extent of the testing is also affected by the state of the structure. A very old or badly damaged structure would require more ND tests at shorter distance than relatively newer and seemingly good structure. Hence, it can be concluded that the test plan prepared by way of using personal judgment of the experienced NDT engineer keeping importance of the test, budget, time etc in mind, is the best plan.

References 1. IS 13311 (Part 1): 1992 – Indian Standard for Non-Destructive Testing of concrete – Methods of Test –Part 1- Ultrasonic Pulse Velocity. 2. IS 13311 (PART 2): 1992 – Indian Standard for Non-Destructive Testing of Concrete – Methods of test – Part 2-Rebound Hammer. 3. J.H. Bungey and S.G. Millard, (1996) ‘Testing of Concrete in structures”, Blackie Academic and Professional. 4. BS 1881: part 201: 1986 Guide to the use of non-destructive methods of test for hardened concrete, British Standards Institution, London. 5. BS1881 Part 202: 1986 Recommendations for surface hardness testing by Rebound hammer. British Standards Institution, London. 6. BS1881: Part 203: 1986 Recommendations for measurement of velocity of ultrasonic pulses in concrete, British Standards Institution, London. 7. BS1881: Part 120: 1983 Method for determination of the compressive strength of concrete cores, British Standards Institution, London.

34 Repair and Maintenance of Concrete Samir Surlaker and A.S. Parulekar

34.1 INTRODUCTION Concrete is the most versatile man-made construction material of our times on account of its flowability in most complicated forms while wet and its strength and durability characteristics when set or hardened. Concrete constructions are economical considering the longetivity of the structures. Durability of concrete is therefore a function of the performance of concrete with respect to time. The concrete is said to be durable when it can withstand the conditions, for which it is designed, over a period of time without getting damaged or deteriorated. Strength alone is not an indication of concrete durability. Protected concrete has shown more resistance against carbonation and chloride ingress when compared to unprotected concrete. It is a myth to believe that concrete is a maintenance free construction material, if subjected to different exposure conditions. Perpetual maintenance is key to durability Protective Coatings enhance the protective quotient of concrete cover.

34.1.1

Concept of Cover

Due to the innumerable factors like congestion of reinforcement, geometry of element, slump of concrete, etc., the cover of concrete is not of the same quality as that of core concrete. This weakness of concrete cover makes the concrete more vulnerable to deterioration. It is not enough to prescribe the thickness of cover alone without specifying the quality of concrete cover. The concrete cover is, therefore, the first line of defence in corrosion protection. Given actual working conditions, it is difficult to create a cover which will be resistant to carbonation or chloride ingress. Therefore the last step of Concrete Repair which is anti carbonation or chloride ingress resistant coating, ensures the durability of Repairs. Concrete is heterogeneous material and therefore, non-homogenous. Such non-homogeneity occur both at macro and micro levels. The cover has many non visible micro cracks and these

34.2

Handbook on Advanced Concrete Technology

act as avenues for water and gas penetration. It is stated that in an uncontrolled permeability form work, the water cement ratio is 0.10 more and the cement content is about 45 kg/m3 less compared to the original concrete mix and therefore, the cover is most vulnerable to attacks. Under pore solution of pH values of up to 12.5, the reinforcement in the concrete remains in passivating conditions and does not initiate the process of corrosion. Fig. 34.1 shows passivation of reinforcement and concept of cover. When the pH value of the pore water is reduced to less than 9.5, the reinforcement is no longer in the passivating range and corrosion occurs. Corrosion also occurs due to ingress of chlorides etc.

CO2

H2O

Concrete Surface Extent of Carbonation H2O H2O H2O HO O2 O2 2– O2 – – O2 – – Cl Cl Cl Cl Cl – – – – – – Cl Cl Cl– Cl Cl – Cl– O2 Cl Cl – – O O2 Cl– Cl – – – 2 H O Cl Cl 2 Cl H O Cl H2O Cl Cl– Cl– – 2 – H2O – Cl H2O O2 O2 O2 Cl Cl SO2

O2

–

Cl

H2O

O2

–

Cl

H2O

Passive Layer Reinforcement

Fig. 34.1 Passivation of reinforcement and concept of cover

34.1.2 Possible Causes of Deterioration Before getting to the investigations about a distress to the structure, it is necessary to understand the factors controlling the deterioration of concrete. A knowledge of why and in depth investigation can only lead to proper diagnosis. There are two stages at which the deterioration of concrete can start: • Before and during Construction. • During the service conditions of the structure. To a great extent, through proper measures, it is possible for us to arrest or minimize the deterioration of concrete in first stage i.e. before or during construction. However in second case, most of the factors affecting the durability of concrete are beyond our control. At this stage we can only take steps to maintain and take immediate action to mitigate the effects. Table 34.1a shows possible causes of deterioration. Table 34.1b shows common causes of defects as per EN 1504.

Repair and Maintenance of Concrete

34.3

TABLE 34.1a Possible causes and signs of detecriation in concrete and reinforcement Cause of Deterioration Before and During Construction

Environmental attacks

Signs

Design errors

Incorrect joint, spacing restraints, incorrect load calculations, excessively slender designs

Craking

Mix Design

Poor aggregate grading, too high or low cement content, incorrect water/ cement ratio

Cracking, increase in permeaility, surface sanding, increased carbonation rates (spalling as a result of corrosion)

Poor Workmanship

Addition of water (incorrect w/c ratio) Poor compaction, Low cover to steel, Poor shuttering, Omitted or insufficient curing

Pores, voids, cracks, blemishes, exposed reinforcement, low strenght surface sanding, premature carbonation of reinforcement

Excessive mechanical stresses

Static or dynamic overloading i.e. collision, explosion, abrasion etc.

Disintegration, cracking, wear

Thermal stresses

Temperature changes, freeze/thaw cycles, fire damages

Cracking, reinforcement corrosion, surface sanding

Chemical attack

Aggressive gases (CO2, SO2) corrosive soils or water, acids, salts

Disintegration, surface sanding carbonation, reinforcement corrosion

Biological effects

Plant Micro-organisms

Cracking, expansion effects, flaking

TABLE 34.1b Common causes of defects (EN 1504) Defects in Concrete

Mechanical

Impact Overload Movement e.g. settlement Explosion Vibration

Chemical

Reinforcement Corrosion

Physical

Alkali-Aggregate reaction

Freeze/Thaw Thermal

Aggressive Agents e.g. Sulphates, Soft water, Salt Biological Activities

Salt Crystalisation Shrinkage Erosion Wear

Carbonation

Corrosive Contaminants

At Mixing Sodium Chloride Calcium Chloride

Stray currents

From External Environment Sodium Chloride Other Contaminants

34.4

Handbook on Advanced Concrete Technology

34.1.2.1

Deterioration Before and During Construction

There are three areas where prevention can be practiced: (a) Design Errors (b) Concrete Mix Proportioning. (c) Poor Workmanship. Design should not only take care of loads to be carried but designer should also be aware of concreting in slender elements. Joints should be adequately designed. Concrete mix proportioning should be of proper grading and slump should enable placement in complicated forms and congested reinforcement. Water cement ratio should be controlled. Proper erection of form work of controlled permeability with bar spacers is desired. Fig. 34.2 shows lack of cover and honeycombing. Curing should be proper.

Fig. 34.2 Insufficient cover and honey combing

34.1.2.2

Deterioration During Service Conditions

Following environmental attacks deteriorate concrete and cause corrosion of reinforcement: (a) (b) (c) (d)

Mechanical Stresses Thermal Stresses Chemical Attacks Biological Effects

34.1.3 Disintegration of Concrete Disintegration of concrete takes place due to internal processes like alkali silica reaction (ASR) or external chemical attacks like sulphates, salts or soft water. Biological activities also deteriorate

Repair and Maintenance of Concrete

34.5

concrete. Concrete gets damaged due to physical effects like abrasion, impacts, vibrations etc. Repair should include concrete disintegration possibilities and methods to counteract this.

34.1.4

Mechanism and Principles of Corrosion

The alkaline environment of concrete protects the embedded reinforcement against corrosion. The good quality concrete with low water cement ratio lowers the permeability, minimizing the penetration of chloride ion, carbon dioxide, oxygen and water. Chloride ions in the paste tend to destroy the protective film formed on the steel by the alkaline environment. Higher the chloride ion concentration lower is the toleration of corrosion. Corrosion is an electrochemical process and most common form of corrosion in concrete is in an aqueous medium. In the presence of aqueous medium, which acts as an electrical conductor, anode is formed where the electrochemical oxidation takes place and cathode is formed where electrochemical reduction occurs. Therefore, at cathode, the reduction takes place lowering the size of reinforcement and therefore, the structural ability to carry the stresses. Availability of oxygen, water and chloride ions are the basic requirements for corrosion Corrosion influenced by Carbonation is depicted below: Ca (OH)2 + CO2 Æ CaCO3 + H2O (pH – 12 – 13) Anode:

(pH < 9)

Fe Æ Fe2f + 2e–

2e– + ½ O2 + H2O Æ 2OH Cathode _______________________ Fe2+ + 2OH Æ Fe(OH)2 2Fe(OH)2 + ½ O2 Æ 2 FeOH + H2O Corrosion influenced by Chloride is depicted below: Anode:

Fe Æ Fe2 + 2e–

Cathode:

– + ½ O2 + H2O Æ 2OH– 2e ________________________ Fe2+ + 2OH– Æ Fe(OH)2

Fe(OH)2 + 2Cl– Æ FeCl2 + 2OH– pH < 12.6

34.1.4.1 What is Carbonation of Concrete Carbonation is the effect of CO2, from the atmosphere reacting with alkaline component in concrete, Ca(OH)2, in the presence of moisture, thereby converting the calcium hydroxide to CaCO3. The calcium carbonate is slightly soluble in water. The pH value of the pore water is generally between 12.5 to 13.5, but due to carbonation the pH value is reduced to less than 9. The reinforcement, therefore, is no longer in the passivating range and corrosion occurs. The corrosion is accelerated further in presence of moisture and oxygen.

34.6

Handbook on Advanced Concrete Technology

Ca(OH)2+ CO2+ H2O = CaCO3 + 2H2O Relative humidity is one of the major factors influencing the rate of carbonation. Figure 34.3 shows process of carbonation. The process of carbonation continues over a period of time and the depth of carbonation reaches the depth of cover. The factors influencing the depth of carbonation are: (a) (b) (c) (d) (e) (f)

Depth of Cover Permeability of Concrete Grade of Concrete Time Whether the concrete is protected or unprotected The environmental influences.

The ultimate result: Cracking, spalling and corrosion. The carbonation of concrete

Ca(OH)2

+ CO2

Calcium + Carbon hydroxide dioxide

+

H2O

CaCO3

+

2H2O

+

Water

Calcium carbonate

+

2 × Water

1st: phase Diffusion inwards of carbon dioxide 2nd: phase reaction between carbon dioxide and water molecules 3rd: phase reaction between resultant carbonic acids and the alkaline components of concrete

Relative humidity

Rate of carbonation

Less than 30%

Low

40% to 75%

high

more than 75%

Low

pH-value 0

1

2

4

3

5

Acid

6

7

8

9

10

Neutral

Protective coating required over concrete in the case of very severe attack by acid media

CaCO3 Carbonated concrete

11

12

13

14

Alkaline

Ca(OH)2 Alkaline concrete active corrosion protection for reinforcement

Fig. 34.3 Carbonation of concrete

34.1.4.2 Chlorides in Concrete Other major materials which can destroy the passivation of steel in concrete is the chloride ion. The chlorides, which are present in concrete, are at two different stages:

Repair and Maintenance of Concrete

34.7

• At the time of casting - In aggregates, in water and in admixtures • In service conditions - Concretes exposed to marine atmospheres and due to deicing or other salts. The ranges of chloride concentration with respect to rate of corrosion are: Low — Medium — High —

Upto 0.4% by weight of Cement 0.4 to 1.0% by weight of Cement Above 1% by weight of Cement.

The corrosion of reinforcement can occur irrespective of the carbonation in concrete. Combined with carbonation, the deterioration is faster comparatively. It is of utmost importance to determine, whether the corrosion is induced due to carbonation or chlorides as the remedial measures shall be different in both the cases. Fig. 34.4 shows pitting corrosion by chloride.

Chloride cause beginning of pitting corrosion

Chloride cause complete dissolving of steel Chloride corrosion is invisible on surfaces of concrete components

Fig. 34.4

Corrossion Bv Chlorides (pitting corrossion)

34.1.5 Corrosion and Cracking Corrosion is basically an electrochemical process and involves consumption of the anode. In this case, iron oxide is formed and deposited at cathodic portions. The rate of corrosion determines the speed of deterioration and the degree of corrosion at the particular moment determines whether the damage is structural or non structural. Physically speaking, the rusted reinforcement occupies a volume of about 2.5 times that of normal re-bar and this creates internal stresses leading to cracking of the protective cover. These cracks are relatively easy to identify as they tend to follow the line of reinforcement. Fig. 34.5 shows physical process of concrete cracking due to corrosion. The degree of rusting determines whether the present existing diameter of bar is capable of carrying the structural stresses at that moment of time. This will classify the repairs into structural and cosmetic, Structural repairs are undertaken to restore structural stability while cosmetic repairs are undertaken to restore durability. It is a normal practice to add reinforcement, if the deterioration is more than 15% of the re-bar area.

34.8

Handbook on Advanced Concrete Technology

Good quality concrete (pH = 12-12) steel is passivated

Carbon dioxide enters, pH begins to drop. steel is not yet effected

pH at steel drops below 9.5, corrosion begins

Volume expansion of rust causes cracking and spalling

Fig. 34.5 Physical process of concrete cracking

34.2 WHY REPAIRS — INTRODUCTION Many a times even after taking all the due precautions and all quality control measures the structures deteriorate. One of the practical reasons is due to the fact that at the time of planning or at the design stage it is very difficult to estimate fully the real stresses in practice that the concrete will have to carry. It is also not possible to suggest all the different types of coatings and membranes, also on account of economical considerations. The concrete then becomes vulnerable to water ingress and other aggressive chemicals. Such problems can only be identified by being vigilant and by inspections conducted from time to time. Most of these problems can be identified at an early date by visual inspection. The maintenance on earlier date can not only be economical but also will preserve the durability of concrete. Rest of the defects could be rectified by usage of particular coating or minor repairs. If neglected, the problems manifest themselves into serious defects like peeling of plasters, corrosion of reinforcement and in some cases the degree of corrosion is so high that the repair is of structural nature. Such repairs are normally costly but are necessary for preserving the structures. The reports of maintenance and the records should be preserved to determine the suitability as well as the durability of the repair materials. Rehabilitation is, in fact, the extension of waterproofing and requires very high degree of expertise. In addition to having knowledge of structural behavior the rehabilitation expert should also possess thorough knowledge of material science, not only in terms of advantages but also the disadvantages and the limitations of the materials employed therein.

34.2.1

Principles of Repair

There are basically two principles for repairs – Principle # 01 :- Set up a cover that is sufficient in density and thickness and Principle # 02 :- Leave the cover and apply a surface protection system. Figure 34.6 shows both the principles. The application of protective coating is an attempt to increase the effective cover. Therefore, the principle of equivalent cover is very

Repair and Maintenance of Concrete

34.9

valuable in repair strategies. Repaired concrete structures fail within a very short period of time, if all steps are not properly followed or compromised due to economic limitations. This is a serious cause for concern, in present day unqualified repair scenario.

Principle #1:

Principle #2:

Set up a cover that is sufficient in density and thickness

Leave the cover as it is, or set up a minimum required cover and apply a surface protection system

Fig. 34.6 Principles of repair

34.2.2 Rehabilitation Strategies Rehabilitation engineering is a specialized field which calls in for skills and abilities beyond design and construction engineering. The engineer in charge for rehabilitation should have qualities of investigator, structural designer, material technologist and awareness of application techniques. The systematic approach to deteriorated structure is absolutely necessary and it should be a balance between Technology, Management and Economics. The first task when a structure shows signs of cracking, spalling, rust staining or any other sign is to determine whether the damage is structural or non-structural. Figure 34.7 gives Rehabilitation matrix. The matrix suggests three major components of the engineering process. (a) Concrete condition survey (b) Structural aspects investigation (c) Repair programme and materials. EN 1504 specifies following options in deciding appropriate actions • Do nothing • Re-Analysis of structural capacity, possibly resulting in downgrading of the function of the structure. • Prevention or reduction of further deterioration without improvement of the structure. • Improvement, strengthing or refurbishment of parts or all of the structure. • Reconstruction of parts or all of the structure • Demolition of parts or all of the structure

34.10

Handbook on Advanced Concrete Technology I. Concrete Condition A. Defects B. Damage C. Deterioration

II. Structure Aspects A. Design Systems B. Facility Type C. Constraints

III. Repair Program A. Structural Members B. Action Plan C. Plans & Specs

Fig. 34.7 Rehabilitation matrix

DIN 1504 states various principles for remedial actions, with respect to deterioration processes. These tables can form the best guideline for Repairs and Rehabilitation. Tables 34.2 and 34.3 show the details.

34.2.3 Basic Steps of Repair When a structure shows signs of distress or deterioration, the following steps should be taken, in principle. The steps are as under: (a) Preliminary Investigation, detailed Investigation (b) Diagnosis (c) Laying out specifications for repairs (d) Selection of Materials (e) Surface Preparations (f) Actual Repairs (g) Periodical maintenance (h) Maintenance of Reports/Listing dockets etc for future repairs

34.2.4

Preliminary Investigation and Detailed Investigation

The first step is to conduct visual inspection of the damaged structure and to inspect the records. Figure 34.8 shows a typical flow chart for inspection of corroded steel in concrete. Further survey to be conducted, after preliminary investigation, are de-lamination surveys, crack surveys, compressive strength, Tests etc. Preliminary investigation and diagnosis are carried out to ascertain the nature of damage and the extent of distress to establish feasibility of repairs. Destructive as well as non-destructive methods are at our disposal for determining the extent of distress. Table 34.4 shows the examination methods and the assessments to determine the mode of repairs. Figure 34.9 shows a compact kit for evaluation of damage.

Repair and Maintenance of Concrete

34.11

TABLE 34.2 Deterioration process and remedial actions according to EN 1504(27.16) Observation

Cause of Defects

Principle of Remedial Actions

1. Mechanical: Impact, Overload Moment (settlement), Explosion Defects in Concrete

Cracks, Spalling, Delamination, 2. Chemical: Alkali-aggregates Disintegration of the matrix. reaction, Aggressive agents (sulphates, Soft Water, acids, salts). Biological Activities. 3. Physical: Freeze/thaw. Thermal-fire Salts crystallisation, Shrinkage, Erosion wear.

Reinforcement Corrosion

Uniform Corrosion, pitting Corrosion. Stress corrossion, Cracking

Concrete restoration (CR), Structural Strengthening (SS) Protection against ingress (PI). Moisture Control (MC) Increasing Resistance to chemicals (RC) Protection against Ingress (PI) Moisture Control (MC) Increasing Physical resistance (PR) Structural Strengthening (SS)

Carbonation of Concrete.

Preserving or restoring passivity (RP) Control of anodic areas (CA)

Corrosive contaminants: Sodium Chloride, Calcium Chloride, Others

Cathodic Control (CC) Cathodic Protection (CP). Control of anodic areas (CA) Preserving or restoring passivity (RP).

Stray Currents.

Increasing resistivety (IR)

TABLE 34.3 Principal and remedial actions according to EN 1504 Principal

Surface Protection

Methods based on the principle (examples)

Protection ingress (PI): Reducing or preventing the Surface impregnation, surface coating, Bandaging ingress of adverse agents, e.g. water, other liquids, cracks, Filling cracks. Converting cracks to joints. vapour, gas chemicals, and biological agents Erecting external panels. Applying membranes Physical resistance (PR): Increasing resistance to physical or mechanical attack.

Overlays, Coatings, Impregnation.

Resistance to chemicals (RC): Increasing resistance to chemical attack

Repair

Moisture control (MC): Adjusting and maintaining the moisture content in the concrete within a specifies range of values

Hydrophobic impregnation Surface coating. Sheltering or overcladding.

Concrete restoration (CR): Restoring to the originally designed shape and function

Hand-applied mortar, Recasting with concrete Spraying concrete or mortar Replacing elements

Cathodic Control (CC): Creating Conditions in which potentially cathodic areas of reinforcement are unable to drive an anodic reaction.

Reducing oxygen supply at the cathode by saturation or surface coating.

Preserving or restoring passivity (RP): Creating chemical conditions in which the surface of the reinforcement is maintained in, or is returned to, a passive condition.

Increasing cover with additional concrete or mortar. Replacing contaminated or carbonated concrete. Electrochemical realkalisation of carbonated concrete. Realkalisation of carbonated concrete by diffusion. Electrochemical chloride extraction.

Cathodic protection (CP)

Applying electrical potential.

Control of anodic areas (CA): Creating conditions in which potentially anodic areas of reinforcement are unable to participate in corrosion reaction.

Painting reinforcement with coatings conatining active elements (eg zinc). Painting reinforcement with barrier pigments coatings Applying penetrating. corrosion inhibitors to the concrete surface.

Structural Strenthening Structural strengthing: (SS): Increasing or restoring the structural load bearing capacity of an element of the concrete structure.

Adding or replaing embedded or external reinforcing steel bars. Installing bonded rebars in preformed or drilled holes in the concrete Plate bonding. Adding mortar or concrete. Injecting cracks or voids Filling cracks or voids. Prestressing-post-tensioning.

34.12

Handbook on Advanced Concrete Technology VISUAL INSPECTION

Cracking

Crack Spalling

Cracking and rust staining

Examine for Sulphide inclusions

Examine Steel No rust

Rust Staining

Rust

Absent at point of staining

Present

Structural Appraisal CORROSION OF REINFORCEMENT

Carry out investigation of concrete Determine Cl Content On-site

Lab

Measure carbonation Phenolphthalein on freshly fractured concrete

Determine cement content On site No method

Fig. 34.8 Flowchart for inspection of corrode steel in concrete

Fig. 34.9 Compact diagnosis kit

Lab

Repair and Maintenance of Concrete TABLE 34.4 damage

34.13

Possible Diagnosis, methods of examination and tools for assessment of concrete

Possible Defect

Examination Method/tools

Assesment/result

Cavities

Hammer Testing

Hollow sound = cavity = knock off

Compressive Strength

Drilling core/test machine/test Harmmer

Nominal strength strength N/mm2

Adhesive Strength in tension

Heroin Adhesive strength tester

Nominal strength N/mm2 (1.5 N)

Surface Strength

Scraping test/knife/screwdriver

Chip off, crack = fine mortar after sand-blasting

Cracks

Rulers/scales, moisten with water

Record of width/length in mm/running metre (distribution over building)

Concrete Cover

Rule/locating device/thermography

cover in mm (number and distribution over building)

Carbonation

Indicator paper, spraying with phenolphatalein

No disoloration = concrete is carbonated

Chloride

Silver nitrate test (Potassium Dichromate)

Indicator solution remain yellow = chloride red to brown coloration = healthy concrete

Corrosion of reinforcement

Visual inspection

Low concrete over, flaking/rusting of the steel

Watertightness

Test device (drilling core)-Test tube

Depth of penetration in cm water loss

Moisture content

Heating of surface/CM device

Light discoloration, humidity content in wgt.-% (< 6%)

Chedmical Attack

Chemical Analysis

Oil and fat pollution

Wetting-sample with water

Hydrophobic effect oil/fat

Additional examination for light weight concrete

sampling/laboratory test

Determination of bulk density

There are several approaches available for identifying corrosive environments and active corrosion of reinforcement. There are several methods that can determine specifically the areas of potential re-bar corrosion. The concrete cover meters determine the depth of cover and the present diameter of re-bar. Phenophthaline test can determine the carbonation state. Chloride tests sets are available to determine the amount of chlorides in the concrete. Corrosion rate meters are available which can determine the size and orientation of the reinforcing steel. They measure not only the presence of corrosion but also measure the rate of corrosion. Half cell potential testing equipment, based on the principle of difference in chemical potentials at various points on the steel, generate the flow of current from anode to cathode. Detecting and measuring these potential data determine areas of corrosion activity. The potential map can be plotted with this device giving the over all picture. The aim of investigation is primarily to determine the extent of damage or distress, whether the damage is structural or non structural. As unless, the cause of distress is established, the remedial measures shall have no meaning as it is the cause that is to be rectified rather than the surface appearance of the damaged structure. The things to be ascertained are: (a) Whether the concrete is carbonated. (b) Whether the chloride levels are high

34.14

Handbook on Advanced Concrete Technology

(c) (d) (e) (f) (g) (h) (i)

Depth of carbonation and cover depth Permeability of concrete Degree of corrosion Present load carrying capacity of structure Whether the defects are localized or on total area Appearance of cracks and types of cracks Whether designed loads and service loads are same

Important information should be obtained about the age of structure, type of construction, structural design assumptions, present loading conditions, whether already the repairs were carried out and their durability. Investigation for structural repairs should essentially include the compressive strength tests, cement content tests and chloride concentration test. Test results of NDT should be cross checked before condemning the concrete. Detail of NDT testing is covered elsewhere. Interpretation of the results is very crucial and the experts job is extreme care and judgments should be exercised in the interpretation of results.

34.2.5

Diagnosis

Diagnosis is actually interpretation of the results obtained from the investigations. The interpretation requires sound knowledge and experience in this field and should essentially be done by qualified engineers. Since the diagnosis is done on the basis of random sampling, it is necessary to carry out further examinations during the course of repairs, particularly with regard to depth of carbonation and the existence of cavities, hollows etc. Only proper diagnosis can form the base for permanent and durable repair of the structure in question.

34.2.6

Specifications

Since the field of repairs and maintenance is a specialized one, it is very important that proper specifications are laid down for carrying out the remedial measures. The specifications should include: (a) (b) (c) (d) (e) (f) (g) (h) (i)

Materials for repairs. Calculations for extra reinforcement for structural repairs Materials for injecting the cracks Guideline for surface preparations Work rhythms and cycles Steps for repairs Precaution to be taken while using the materials as well as the curing procedures etc. Supervision and quality control at site Scope of work and quantities

Only proper specifications can lead to proper repairs with minimum clashes between the owner and the contractor. Methods of measurement should be unambiguous.

Repair and Maintenance of Concrete

34.3

34.15

INJECTIONS FOR REPAIRS

Cracks occur in the concrete despite the fact that quality is controlled. Cracks are one of the signs that give the indication of damaged or distressed structure. However, it is fortunate that all cracks are not a sign of structural failure. Basically the cracks have to be repaired for two reasons viz. for structural purposes and for durability purposes. The selection of material for injection requires thorough understanding of the properties of the material and functions that such a repair has to perform. In all the cases, it is imperative that the cause of crack is properly determined otherwise the selection of material can be totally faulty.

34.3.1

Introduction

Injections is first step in repair programme. Basically, the injections can be of three categories. First, injections that are undertaken to restore the structural stability. Second, injections that are undertaken to protect the reinforcement to avoid the moisture and air entering the concrete and to lower the rate of corrosion. Third, the injections that are undertaken to stop the water entering the structure. The repair of cracks alone cannot guarantee the structural stability or durability of concrete and therefore, it should necessarily be complimented with other treatments as per the established practices of civil engineering. Under all circumstances, it is advisable to trust these type of jobs to experienced contractors having the knowledge of materials as well as experience in the use of several equipment.

34.3.2 Why Cracks Occur? The cracks can occur in structures at two different stages. Firstly during the construction and secondly during the service conditions of the structure. To a great extent, through proper measures, it is possible to arrest or minimise the cracks in the first case. But in the second case, many of the factors are beyond control and therefore the cracks should be treated at the earliest to minimise the damage. In general, the major factors effecting the formation of structural cracks are: (a) Errors in stress calculations (b) Faulty construction, form work alignment, removal etc (c) Excess loading under service conditions (d) Settlements (e) Unforeseen physical damage like fires, explosions etc (f) Lowering of section of reinforcement in the second stage of corrosion Non structural cracks are mainly due to: (a) Plastic shrinkage cracking - rapid evaporation of water (b) Drying shrinkage cracking (c) Plastic settlement cracking - settlement of concrete in formwork (d) Cracking caused due to poor workmanship (e) Alkali aggregate reaction

34.16

Handbook on Advanced Concrete Technology

In both the cases, repairs are necessary to be undertaken. The first type structural cracks, can lead to structural failure. The second type non structural cracks lower the durability of concrete. Non structural cracks, if neglected, can lead to corrosion of structural reinforcement, thereby converting themselves into structural cracks.

34.3.3 Reasons for Crack Injection Injection is the first step of rehabilitation, both when structural distress is encountered as well as when leakages are detected and durability compromised. The injection technique is often the only viable solution in order to repair damaged structures and thus avoid any further ensuing consequential damage. Cracks will cause even greater problems, if they are not repaired appropriately and promptly. Early crack injection avoids serious consequential damage to the buildings. In many cases cracks may reach far into the concrete or even divide entire structural components, thus threatening the stability of the whole structure. For example, chlorides penetrating unhindered into the interior may endanger structural integrity and durability. Cracks, therefore, lower the durability of buildings. Moisture ingress restricts the usage of buildings and may ultimately lead to adverse economic effects inclusive of collapse of structures. The combined effect of moisture ingress and frost may even aggravate existing damage. Due to its increased volume, ice causes more serious crack damage and large-area frost heave. Cracks can endanger the stability of entire buildings. Apart from the structural aspects, the entire appearance of a building – an important criterion for the valuation and assessment of buildings – is considerably impaired by cracks and deterioration that is brought out by water entry and chemicals ingress. Table 34.5 shows various reasons for Injection. TABLE 34.5 Various reasons for crack injection Crack Injection • • • • • • • •

Structural injection for dry cracks Structural injection for damp cracks Sealing of cracks and cavities for waterproofing Sealing against pressurized water Injections for imparting stability in Masonary structures Frictional Sealing of losse masonary External sealing using curtain injection technology Grid injection for dampness

34.3.4 The Process of Injection After completion of diagnosis and selection of materials for injection the work of injection passes through following stages. (a) Preparation of the crack (b) Location of points for injection (c) Fixing of injection nozzles

Repair and Maintenance of Concrete

34.17

(d) Surface sealing of cracks (e) Injection of resin (f) Removal of packers and plugging (g) Removal of sealing material (h) Final surface treatment after injection resin/grout hardens The preparation of crack is required to ensure perfect bonding of the injection material to both the crack surfaces. The preparation of crack should aim at removal of dirt, loose material and moisture in the crack, if the system chosen is not compatible with moisture. This can be done with compressed air and solvents depending upon the width of the crack and contamination. The work rhythm is of utmost importance. In case of vertical cracks, the injection should start from the bottom most point and it should be continued until the resin flows out of immediate top point. Then the lower nipple should be sealed. In case of horizontal joints different patterns are possible. Figure 34.10 shows the sequence.

11 10 9 8 7 For vertical cracks

6 5 4 3 2 1 (a)

CL 9

For Horizontal cracks

7

5

3

1

4

2

6

8

CL 10

9

8

7

6

1

2

(b)

Fig. 34.10 Sequence of injection

3

4

5

34.18

Handbook on Advanced Concrete Technology

34.3.5

Guideline for Material Selection

The selection of material for injection in the crack largely depends upon the investigation and primarily on the following factors: (a) Pattern of cracks (b) Width of the crack (c) Movements in the crack faces 1. Due to temperature variations 2. Due to dynamic loadings (d) Moisture in the crack (e) Dirt in the crack The pattern of the cracks decides the reason for cracking, which in turn reflects on the selection of base material. Width of the crack has direct bearing on the viscosity of the material required. It depends on the movements in the crack, which reflects the type of material required whether it should act as structural injection or just an elastic seal. When the injection is a structural one it should be able to transfer stresses from one crack face to the other The moisture in the crack calls for a water compatible system of injection. Existence of dirt in the crack will guide the crack preparation system.

34.3.5.1 Materials for Injections – Properties All cracks are different. They vary depending on the construction material, cause, location and environment. One single system is not able to achieve durable and reliable results. Various solutions based on different materials which are tailored to specific application needs, are now available to users. A range of solutions is essential depending upon job and site conditions. Table 34.6 shows selection of materials with respect to job and site conditions. TABLE 34.6 Selection of Materials How to choose the right product-Job Conditions/Site Conditions I Sealing Coming from: • movement cracks • stationary cracks • construction joints • expansion joints • bad construction All needs flexible filling III Wet Cracks • movement cracks • stationary cracks • construction joints • expansion joints • bad construction Need Hydrostructural Resins and gels for flexible filling

II Strengthening of structures Cracks coming from: • mechanical damages, damages caused by earth movements • general wear and tear • Bad construction

All needs rigid filling IV Dry Cracks • Non moving cracks • mechanical damages • damages caused by earth movements • general wear and tear • bad construction Needs Duromers for Rigid filling

Repair and Maintenance of Concrete

34.19

Very low-viscosity, highly cross-linked series of duromers easily penetrate into the crack, also filling the so called crack root. This ensures a seamless, rigid bonding of the crack edges. These highly cross-linked duromer resins are also the right choice for critical joints, where all important static forces are being transmitted. The properties of the injection material must be compatible with the parent material. The structural mechanics of the injected element should remain unaffected. This is an important aspect when carrying out construction works on concrete or masonry buildings that are classified as historical monuments. Table 34.7 shows salient properties of injection materials. TABLE 34.7 Properties of Injection Materials Properties • • • • • • • •

Good compressive and flexural strengths Excellent bonding properties Lower viscocities Workable at wide range of temperatures Compatibility with moisture Non shrinking Low modulus of elasticity at higher temperatures Longer pot life and workability time.

34.3.5.2 Materials for Injections – Selection There are four different categories for classification of materials: (a) Duromer Resins: Duromer Resins have been used for many years for the rigid injection of cracks in dry structural sections. Epoxy resins of very low viscosity can be injected into the finest of cracks. This guarantees a strong permanent bridging of both sides of the crack, thus restoring bearing capacity for the designed loads planned. This process can also be carried out where the structure is also subject to vibration. Duromer resins of varying viscosity can be used for injection and impregnation, depending on the width of the crack. (b) Elastomer Resins: If rigid injection material is used the cracks become visible after injection or new cracks develop adjacent to old cracking. Use of quality elastomer resins is a solution for avoidance of such occurrences. Elastomer resins can be designed with distinct pore structures. An homogenous closed cell structure is formed which enables safe sealing. The integrated compression and decompression reserves in the cell structure absorb the expansion and the contraction in the crack. The effectiveness and permanency of the seal is guaranteed by the ability to expand and contract in symphony with the crack movement while maintaining the tenacious bond to crack surfaces. (c) Hydrostructural Resins: Hydrostructural resins cure to form elastic membrane and impervious seals when encountered with water. This property is very useful when the

34.20

Handbook on Advanced Concrete Technology

external of building is not accessible like in buried structures and when the external waterproofing envelope is damaged. By drilling through the structure to the interface between existing waterproofing membrane or protection board, hydrostructure gels can be injected to re-create the seal. Hysdrostructure resins can be designed for high chemical and mechanical resistances. They guarantee permanent elasticity, tenacious and isotopic bonding and unique skin effect. Figure 34.3 shows water reactive PU Foam. (d) Cement Suspensions: These are the oldest of the injection materials well known to all. Cement suspensions are insensitive to varying moisture levels in the building. Even large volumes of injection are possible with cementatious suspensions, suitably modified to low viscosities and having non shrink properties. They ensure a high degree of efficiency enabling a reduction in restoring costs. Table 34.8 shows selection with respect to crack widths. Table 34.9 shows typical physical values. TABLE 34.8 Showing selection of materials Type of Cracks

Width

Movement

Water

Type of Material

Mode of Application and /Or Principle

Shrinkage Cracks in Concrete

£ 0.2 mm

no

no

Two Component Epoxy Injection

Surface Treatment which works Through Capilary Action

Shrinkage Cracks in Plaster

£ 0.2 mm

no

generally not

One component flexible paint on acrylic base

Coat with roller or brush.

Structural cracks in Concrete, brickwork

0.2 – 1 mm

no

no

Two Component epoxy Injection low viscocity

Low pressure injection, shorter cracks with high pressure injection

Structural cracks in Concrete, brickwork

1 – 2 mm

no

no

Two Component epoxy Injection & solvent free epoxy

Low pressure injection,

Structural cracks in Concrete, brickwork

2 – 5 mm

no

no

Solvent free epoxy thixotropic

Low pressure injection, with hand pump.

Structural cracks in Concrete, brickwork

≥ 5 mm

no

Dry/wet

Polymer modified cement based grout

Grout with injection, injection grout, by gravity or hand pump

Structural cracks in Concrete, brickwork

≥ 15 mm

no

Dry/wet

Non shrink Grout

Cut and fill non shrink mortar

Moving cracks in Concrete, brickwork

0.2 – 1 mm

due to temperature changes

Dry/wet

Two component Polyurethane injection and flexible paints. When wet joint, primary injection with Polyurethane gel forming

High pressure injection with (low pressure injection also possible). Then coat with roller/brush.

Butt joint in prestressed concrete (Coupling Joints)

0.2 – 2mm

Vibration

Dry/wet

Two component Polyurethane injection and joint sealant, when wet joint, primary injection with polyurethane gel forming

For jopints pressure injection for floors, seal joints with the sealant, gums or spatulas

Moving cracks in Concrete, brickwork and floors

≥ 2 mm

Vibration

Dry/wet

sealants on different basis including flowable grades

sealant, gums or spatulas, for horizontal surface flowable grade of joint sealant can be used

Repair and Maintenance of Concrete

34.21

TABLE 34.9 Typical values of commonly used injection material Type if Material Base

Solvent free Epoxy Solvent Free Epoxy Injection Epoxy Injection Polyurethane Unfilled Epoxy Filled Resin Resin gelforming

Sp. Gravity and consumption/ ltr. in kg Pot Life 23°C of 1 Kg Compr. Str. N/mm2 24 Hrs 72 Hrs Viscosity at 23° in mPas

Polyurethane Normal

1.1

1.1

1.08

1.05

25 CC

1.02

40 M

40 M

40 M

40 M

5.20 sec after water contact

50 M

33 71

29 33

78 80

78 80

N.A

Net measurable as soft elstic

1200

1000

100

100

app.100

285

Note All above material are resin and two component formulation. The values are indicative and vary from manufacture.

TABLE 34.10 Pre-treatment methods in the Repairs Method of Operation Mechanical Manual

Wire Brush steel wire broom

Mechanical

Rotating wire brush, compressed air needle gun, grinding

Blasting

Sand-blasting, dust free sand blasting, high pressure water blasting

34.3.6

Procedure Thermal

Chemical Not recommedded as surface preparation method as chemicals can create unknown problems for concrete/steel

flame blasting

Machinery For Injections

The equipment required for crack injection can range from a simple bucket with an outlet to most sophisticated pneumatically compressed machines capable of producing about 500 bar pressure and with hand controlled nozzles with a mixing assembly to mix the two components at the point of injection. The sophisticated machinery are designed to provide better working pressures, better nozzle nipple combinations and to take care of pot life considerations. Following equipment are normally used. (a) Hand guns, sealant guns or grease guns in which two components are mixed and filled into the guns and a pressure of about 6 - 10 bars can be exerted.

34.22

Handbook on Advanced Concrete Technology

(b) Foot-pumps can be employed, attached to single vessel containers, into which the premixed two components are added. These are normally suitable for small quantities of material. (c) Machines are available in which the two components are separately introduced in two containers and automatically controlled quantities can be mixed at mixing assembly near the nozzles. This arrangement solves the pot life problems. These machines are connected to pneumatic or mechanical transmissions for creating pressures to the tune of 500 bars. Since the exact quantity of different components can be pre-set, these machines are very suitable for continuous injection operations. The simplest of the injection method is the brush injection. The resin is brushed on the non moving surface cracks and is absorbed in capillary action. In case of pressureless injection, the material is poured into the nipples or pipes acting as nipples, the use of such injection depends largely on the dimensions of the crack. In case of structural cracks of the width 0.2 - 1.0 mm, it is advisable to resort to low pressure injection. This low pressure can either be created with hand guns (sealant guns, grease guns etc) or a normal compressor used at site. The pressure developed is around 6 - 10 bars. Depending upon the crack widths and depths, high pressure injections can be resorted to for structural crack repairs. It is possible to develop pressures to the tune of 500 bars using mechanical or pneumatic transmissions. The injection method should be clearly specified prior to the commencement of the work and should be supervised to conform with the specifications. After the injection resin or grout has hardened and after the removal of the nipples, the surface sealing material, which is normally quick setting hydraulic system or thermoplastic resin should be scrapped off completely and the surface should be prepared for further cosmetic or strengthening treatment. Figure 34.11 shows various types of machinery.

Hand lever press

Pedal press

Electrical membrane pump

1-Component

Two component machine

Fig. 34.11 Machineries for injection

Repair and Maintenance of Concrete

34.3.6.1

34.23

Selection of Packers for Injection

Packers or Nipple Systems are the link between the structure and face of the crack and the injection nozzle. Packers must be of adequate size to guarantee the flow of injection resin to the desired place with or without being displaced or de-bonded due to injection pressures or rebounds. The critical selection depends upon the access to crack, quality of surface, surface condition as well as pressures used in injection process. There are normally three types of Packers used under general conditions: (a) Adhesion Packers: For the injection of dry cracks, cavities and substrates with Epoxy and polyurethane resins where surface conditions are suitable and flat. (b) Drill or Bore Injection Packers: for the injection of dry, moist and water bearing (pressurized and Non-presurrized) cracks, cavities and substrates with epoxy, Acrylic and Polyurethane resins (c) Hammer Packers: for the injection of cement injections and acrylic gels. The main differences are valve openings, dimensions and pressures. The first step is selection of Packers. They should be able to be connected to the injection nozzle, so that the pressure should not be lost. Thereafter, it should be possible to tie or seal the Packers, so that the resin is not lost and they should be removable to enable the surface smoothening. The spacing of injection points depend upon the width of crack as well as the porosity of concrete. However, as a thumb rule, in case of adhesion packer, the spacing should be about 50% of concrete cross section. Figure 34.12a shows installation. Figure 34.12b shows different type of packers. Adhesion Packers: Fixed onto the element surface at a distance equal to the element thickness (usually with surface sealing)

Injection Packers: Placed in pre-drilled holes. Spaced at a distance of approximately half the element thickness on alternate sides of the crack

Fig. 34.12a Installation of packers

34.24

Handbook on Advanced Concrete Technology

Fig. 34.12b Showing different packers/nipples

34.3.7 Precautions 34.3.7.1 Viscosity and Reaction Times Two major factors influencing the success of injection from material point of view are viscosity and reaction time. It is a balance in design between these two parameters that makes the difference between two products. Viscosity is the guiding factor for selection with respect to crack widths. The loss of viscosity with respect to time also contributes to selection of machinery. Lower the viscosity higher is the penetrability. The loss of viscosity controls the work rhythm and also contributes to pot life. Low viscosity injection products can be applied at low pressure. Figure 34.13 shows that the low viscosity affects the selection of one or two components machineries. Reactivity of resins is a key parameter when water under pressure is encountered. The reactivity can be controlled by designed addition of accelerators and setting can come in as low as 7 seconds for quick water stoppage. The technical efficiency with regard to the long pot life and the fast reaction inside the crack is also unique. Viscosity in Cps 1K

1000

A+B 500 300 200 100

A 0

10

B Time in min

Fig. 34.13 Injection system of 1 component and two components injection (viscosity)

Repair and Maintenance of Concrete

34.3.7.2

34.25

Secondary Injections

Another controversy in Injection application is the claim about single operation injection to solve the leakage problem. This is not possible and water sealing must be done in two steps: Primary Injection and Secondary Injection in most of critical cases with very few exceptions, the Primary Injection is to arrest the aggressive water inflow and is normally carried out by foaming PU resins which expand 30 – 40 times when water is encountered. Its aim is to primarily stop excess water to facilitate further working and it still leaves fine water paths. The Secondary Injection is aimed at filling these water paths either by elastic or rigid materials.

34.3.7.3

Cleaning and Safety Instructions

The injection resin must be removed by allowing the reservoir to empty. Please do not allow the pump to run when no liquid is present. Sufficient solvent should be placed in the reservoir to fill the pump and the delivery line. The solvent is then pumped through in order to clean the pump and the delivery line in a rotational manner. After the first cleaning the machine should be cleaned again with clean solvent in the aforementioned manner. The pump should then be filled with oil to prevent damage to the seals. Hand gloves must be worn during full injection process including mixing. Protective clothing and goggles should be worn during all injection works. The injection gun should in no case be directed to other person or to own body. During interruptions of work the injection gun should be protected from dirt by laying it into an empty pack or by cleaning. Before application, please read the safety information given on pack label.

34.3.8

Conclusion

The modern injection technology coupled with proper equipment can solve almost all types of rehabilitation problems thereby providing economical solution in comparison to demolition and reconstruction of structures. The specifications should be very clear and unambiguous. The specifications should at least cover points like material, viscosity, techniques to be adopted, the equipment to be employed, type of nozzles and spacings, pressure to be applied etc. The repair of cracks is a part of repairs of damaged and distressed structure and is not a substitute to other remedial measures required to be adopted for successful rehabilitation. As a part of maintenance of structures, it is better that the first occurrence of cracks is immediately reported to the expert and his advice taken rather than to hide the defect by plastering, filling with crack fillers etc. The job of application must be entrusted to trained and professional applicators with certification from Manufactures and independent building institutes.

34.4

METHODS OF REPAIRS

Methods of repairs depend upon the investigations and assessments made by competent engineers. When the structure is distressed or damaged the normal visual signs are: (a) Rust stains or rust spots (b) Cracks-different patterns and sizes

34.26

Handbook on Advanced Concrete Technology

(c) Peeling of plasters (d) Spalling of concrete (e) Rusted reinforcement, if exposed It is the primary task to determine whether the damage is: • Structural • Non Structural Structural Repairs: Are undertaken to restore the structural stability to carry the present stresses under the service conditions. This also involves various strengthening and rehabilitation techniques to enhance the load carrying capacities. Structural calculation and increasing of sections and additional reinforcement forms a part of the system. Non Structural Repairs: Are undertaken to restore the long term durability, but does not increase the load carrying capacity of the structure in question.Non structural repairs due to corrosion of reinforcement are done by surface or patch Repairs. Even in this case there will be a slightly different approach when corrosion is due to carbonation or chlorides A non structural repair or cosmetic repair, if not carried out at appropriate time can lead to structural distress.

34.4.1 Patch Repairs of Reinforced Concrete Patch Repairs are normally undertaken to reinstate the durability of the structure rather than structural integrity. Normally patch repairs are most common in building Repairs and when corrosion is mostly due to carbonation of concrete. After laying the specifications, following major application steps are involved: (a) Damaged concrete removal – surface preparation (b) Reinforcement preparation (c) Inhibitor coat for reinforcement (d) Bond coat (e) Concrete reinstatement (f) Surface repairs (g) Protective coating for full surface Figure 34.14 shows the steps for repair and Figs. 34.15a and b show old and new systems

34.4.1.1 Damaged Concrete Removal/Surface Preparation The damaged or deteriorated areas should be clearly marked and previously inspected by contractor to categorize them under various heads as under : (a) cracked areas (b) spalled areas (c) hollow sounding areas

Repair and Maintenance of Concrete

34.27

Fig. 34.14 Conventional steps of repair

(d) (e) (f) (g)

carbonated concrete areas/depths high chloride concentration areas/depths low cover depth areas Accessible/non accessible areas

Concrete can be removed partially or totally. Sound areas of carbonated concrete does not need removal. Concrete can be removed to desired depth by conventional methods like chisel and hammer, cutters, grinding wheels etc. Concrete removal should be to an adequate depth so that there are no feather edges. Areas in plan should be of clear geometrical shapes and if possible dovetail in depth. New technologies involve hydro demolition, hydro milling etc. in addition to pneumatic chipping hammers.

34.4.1.2 Reinforcement Preparation The aim of reinforcement preparation is to remove the rust and prepare the surface to take up corrosion inhibitor coat for future protection. Mechanical methods are most suitable like rotary wire brushes. Small hand held grit blasting equipment gives excellent surface. If water blasting was used for concrete removal, most of the rust also gets removed in the process. Water blasting is also environmental friendly. In chloride contaminated concrete it is advisable to clean behind the bars to expose reinforcement fully. Chemical rust removers/converters are not recommended in Technical report 38 of concrete society as they may contaminate or damage the concrete. Chemicals need careful control and have to be totally removed prior to further treatments, Table 34.10 shows various methods. If the section of reinforcement is reduced and if the repair is of structural nature, the existing reinforcement should be supplemented as per structural calculations. It should be clearly mentioned in the specifications. The mode of replacing or supplementary should be pre agreed. Extra reinforcement can be in form of: (a) Straight laps (b) Anchored Reinforcement

34.28

Handbook on Advanced Concrete Technology Diagrammatic view of need for and limits to exposure of reinforcement steel: unsound concrete and areas over corroded reinforcement are removed. concrete broken out. Area of reinforcement steel to be treated with a protective coat Carbonated concrete (solid, over non corroded steel is not removed)

Hammer testing the concrete surface for cavities and chiseling of all loose portions to expose sound core concrete and expose rusted reinforcement

Clean rusted reinforcement and exposed concrete surface by sand blasting, mechanical devices or any other established methods.

Application of two coats of mineral based polymer modified corrosion inhibiting primer.

Fig. 34.15a Stages of repair work-old system

Application of Polymer based bond coat.

Application of polymer modified ready to use coarse mortar or mortar of low permeability or guniting with guniting aid.

Application of polymer modified ready to use fine mortar over the whole surface

Final Anticarbonation protection coat to concrete depending upon protection required on full surface.

Repair and Maintenance of Concrete

Reprofiling with coarse ready to use polymer mortar.

Corrosion inhibitor & bond coattwo in one.

Smoothening full surface with fine ready to use polymer mortar – If required

34.29

Mandatory anticarbonation coating on full surface

Fig. 34.15b Stages of repair work-new system

(c) Using connectors (d) Welding the extra steel (e) Any other technique. Both the old and new reinforcement should be clean to take the further treatments.

34.4.1.3 Inhibitor Coat for Reinforcement Several methods are suggested for reinforcement coating based on principles of encapsulation, alkaline protection and cathodic protection. Figure 34.16 shows the principles and materials. Coating can be broadly as under: (a) (b) (c) (d) (e) (f)

Cement Slurry Cement Slurry modified by polymers Ready to use propreitory corrosion inhibitors Epoxy coating Zinc chromate primers Zinc rich epoxy coating

In case of chloride contaminated concrete and concretes in marine condition Zinc rich epoxies provide cathodic protection. In case of carbonation contaminated concrete alkaline polymer modified mineral slurries are recommended as corrosion inhibitors. Figure 34.17 shows a well coated reinforcement.

34.30

Handbook on Advanced Concrete Technology

Natural protection by alkali Environment created by cement matrix

Alkaline protection

Alkaline slurry coating enhances the alkaline environment around the bar

Zine applied to bar surface sacrifices itself to protect the steel Zine applied to concrete surface sacrifices itself to protect the steel

Cathodic protection

Surface installed anode connected to bar with impressed current changes the flow of electrical current to protect the steel bar

Epoxy encapsulation electrically insulates reinforcing bar ?

Electrical insulation

(Ref: peter H. Emmonds, repair & maintenance, © R.S. Md Cp kingston, MA)

Fig. 34.16 Principles and materials for steel protection

34.4.1.4

Bond Coat

Success of repairs is totally dependent on bonding of old concrete to new concrete reinstatement. Only when the full element behaves monolithic, the repairs are durable. Surface preparation should be perfect for bonding as mechanical keying is one important component of bonding. The Bond coats can be based on: • Solvent free epoxy • Polymer modified slurry mortar In both cases the work should be done wet in wet. In case of solvent free PCC mortar repairs the polymer modified bonding coat is preferable as modulus of elasticity of full system is compatible. In case of epoxies due to differential expansion due to thermal stresses the contact surfaces have possibility of de-bonding.

34.4.1.5 Concrete Reinstatement Concrete Reinstatement or concrete replacement is key part of repairs. The concrete or mortar which is to be used for this purpose should be with lowest permeability, good bonding

Repair and Maintenance of Concrete

34.31

Fig. 34.17 Well coated reinforcement

characteristics, non shrink and dimensionally stable. The compressive strength should be about 10-15% more than concrete to be repaired and modulus of elasticity should be similar to that of base concrete. Replacement can be done by following methods: (a) Conventional concrete or mortar of low permeability (b) Polymer concrete (For overlays) or polymer mortars prepared by: 1. Addition of polymers at site 2. Prepacked one component mortars (c) Pure polymers (Resins) used as binders in lieu of cement Repair mortars should be applied Wet on Wet when bonding coat is used. The sequence for application should be: (a) Prepare the surface (b) Soak or wet the surface (c) Apply bond coat (d) Apply repair mortar by Hands or pneumatic means If layers are thick, work can be done in 2 or 3 layers. The same sequence is to be followed for subsequent layers. Curing is absolutely essential for minimum of 3 days. For prepacked factory manufactured mortars the dispersible or re-dispersible powders are normally preferred as polymerization of monomers in alkaline environment of hydrating cement constitutes problems. The redispersible powders are already polymerized versions and the choice is rather limited to acrylic powder and vinyl acetate ethylene copolymers. For preparation of site mixed mortars dispersions of styrene butadiene rubbers (SBRs), acrylics and vinyl copolymers are mostly used. The dispersions harden physically by withdrawing water from the mortar during its drying process and by bonding the polymer particles together. In a few cases, the thermosetting plastics like epoxy dispersions are employed and they harden chemically through polyaddition. It is the selection of the polymer additive component that eventually determines the quality of Repairs.

34.32

Handbook on Advanced Concrete Technology

In the second category of pure polymers, epoxy resins are most commonly used. These epoxy resins are filled with inert fillers to give rise to final mortars. Polyesters and polyurethanes are also used and are supplied by few manufactures. Pure polymer mortars are characterized by very high bond and compressive strengths and lowest shrinkage. The two major disadvantages are : high difference in the moduli of elasticity and the costs. On account of economic reasons, the use is limited to a few specialized repair sectors wherein the potential of the high strengths and chemical and abrasion resistance could be tapped, viz, chemical plant flooring, etc.

34.4.1.6

Surface Repairs

After completion of patch repairs by coarse PCC/Resin mortars the surface repairs are undertaken. The main reasons for these repairs of normally 2-3 mm thickness are aimed at filling the blow holes and bringing the entire surface to same level and texture. Since it is difficult to achieve 2-3 mm thickness by cement sand mortars, it is preferable to use one component polymer modified prepacked mortars which can be drawn to zero thicknesses. In many case such mortars are self cured as curing is very critical. The surface texture should be accurately levelled to take surface protection systems.

34.4.1.7 Protective Coating for Full Surfaces The last and most important step is the application of Protective Coating over full surface of repaired and original area. Unfortunately most of the specifications for building Repairs do not specify Anti-carbonation Coatings for full area thereby jeopardizing the full repairs. The concept of Repair is to stop or retard the corrosion process already initiated in RCC Structures. To stop further entry of CO2, SO2, moisture and chlorides in few cases, the entire surface has to be coated with a system to maintain the statusquo of corrosion ongoing in structures. Selection of coating depends on exposure conditions. There are innumerable protective coating available for such application. The selection depends upon user requirements and exposure conditions. Most common properties required to arrest further corrosion are: (a) Carbonation resistant (b) Breathability (c) Crack bridging (d) UV resistance (e) Low permeability to water In case of structures in contact with Chlorides – it may be required that the coatings are resistant to chloride ion penetration. Details about properties and selection of protective coating are dealt in this chapter elsewhere.

34.4.2 Concrete Repair-material Factor The selection of material is the most important step in repair and rehabilitation programme. The rehabilitation engineer is confused with infinite number of proprietary materials available in the market. The selection of material has a chemical angle and the manufacturers literatures

Repair and Maintenance of Concrete

34.33

normally highlight the composition of materials rather than performance characteristics. Any approach, which begins with predetermined materials as a base for designing Repair Systems will lead to definite failures. It is preferable to have all the materials based on same generic polymer as the materials are more compatible. The materials should have features like very good bonding characteristics, good strength development characteristics, the materials should be non-shrinking type and above all they should be as impermeable as possible .The repaired surfaces should be finally coated with coatings in which the basic property should be that the protective coating should have high resistance towards diffusion of carbon dioxide, which will further protect the concrete from the effects of carbonation. Simple tests would be Compressive Strength, Flexural Strength, Air Entrainment and Chloride Content in the Polymer Mortars. For Uniformity and Equivalence testing, one should go in for Specific Gravity, Solid Content and IR Spectrum. Increased Solid Content in no way means better quality. Solid Content increase may be due to addition of any other non relevant ingredients. Therefore, Performance Tests are the deciding factors and one should avoid prescriptive specifications.

34.4.3

Polymers for Repairs

There are thousands of polymer and every polymer is not suitable for repair. Follows a classic paragraph from the Technical Report No.39 of Concrete Society, UK titled Polymers in Concrete quoted as verbatim: The selection of Polymer Dispersion is really a matter for the formulating Chemist. The Engineer should only specify the performance he requires from the Polymer Modified Mortar or Concrete. Specifying the generic polymer type will not assist towards obtaining the most suitable product, but plays into the hands of commercial interests who seek to promote Polymers produced from a particular feedstock. The polymers to be used in conjunction with cement concrete and mortar shall primarily be compatible with water and cementatious hydraulically setting systems. The polymers should not interfere with setting behavior of cements and must be capable of hardening in alkaline environment, in which cement sets. The addition of polymers should not affect the workability of the mortars adversely and should not make the mortars too viscous or too sticky. Further, the polymer addition should not stiffen the mortars early for enabling the practical application. The durability of repair lies in establishing the fact that the polymers used are resistant to saponification. Polymers should be free from chlorides and other deleterious materials and should not attack the reinforcement to promote corrosion. Polymers should impart following properties to PCCs: (a) (b) (c) (d) (e)

Higher bond and adhesive strengths Lower shrinkage characteristics Increased bending tensile strengths Controlled thermal and saturation expansion Increased chemical and abrasion resistance

34.34

Handbook on Advanced Concrete Technology

(f) Lowering the permeability and chloride ingress (g) Increased resistance to carbonation. The Properties of PCC are more sensitive to environmental changes compared to ordinary cement-sand mortars. The strengths also vary considerably depending upon the mode of curing and usage effected. There are notable variations in flexural strengths. The parameters mostly affected are the elastic modulus and creep. While using polymers it should be noted that polymer dispersion is water based and the ratios of mixing vary from manufacturer to manufacturer depending upon solid contents and the quality. Ultimately, it is the proportion of solid content to cement content that reflects on the quality of PCC. Typical mixing ratios for various applications are shown in Table 34.11. TABLE 34.11 Typical ratios for various applications Type of Application Bonding Slurry For Bonding New layers to hardened bases Patching & Repair Mortars

Mixing Ration (Parts By Volume)

Above 10mm thick Above 30mm thick

Cement: Polymer: Cement: Polymer: Cement: Polymer: Polymer: Polymer:

Sand 1:1 Water 1:2 Sand 1:2 Water 1:2 Sand 1:3 Water 1:3 Water 1:4 Water 1:6

Cement Mortars Lime and lime cement Mortars Joint Mortars Bonding Mortors

Cement: Polymer: Polymer: Polymer: Polymer: Polymer:

Sand 1:2 Water 1:2 Water 1:5 Water 1:10 Water 1: 2 Water 1: 2

Upto 10mm thick Above 10mm thick

Cement screeds with high abrasion resistance, high elasticity & less dust formation Leveling & Smoothing mortars With increased oil & petrol resistance Plastic Reinforced mortars For plaster, bonding & Joint mortars with better bonding And higher weather resistance

34.4.4 Uniformity and Equivalence tests for Polymer Since there is a lack of awareness about the type of polymer additives, there are instances wherein different polymers and different solid contents are supplied which are different from approved samples. It is imperative to note that since polymer dispersions are based on water, they can have different concentrations. One should never compare the dispersions on unit price basis without considering solid contents and the type of polymer employed. Under such circumstances tests should be resorted to. The composition of different samples can be qualitatively compared by Infrared spectrometry A typical spectrum for acrylic dispersion is represented in Fig. 34.18 which shows an infrared spectrogram. It should be retained as a reference spectrum. The spectrum of the supply product can be compared with this reference spectrum to ascertain that the same product composition is supplied. Every batch should be tested to determine the solid content by simple method of oven

Repair and Maintenance of Concrete

34.35

drying and specific gravity. Another most important test is Air entrainment test to determine the air content of the mortar. It reflects the quality of polymer. Such test procedures should be included in the Indian codes of practice as these would ensure the quality and above all dispel all the doubts from the minds of actual users, architects and designers.

Fig. 34.18 Infrared spectrometer and spectrum

34.4.5

Composition of Polymer Mortars

The general composition of Polymer Mortars is a good quality cement as Binder and well graded fine and quartz aggregates. Some special fillers are added to make the mortar more impermeable like Fly Ash, Microsilica etc. Polymer Modifiers in powder and liquid forms like acrylics, SBR’s etc are added to provide enhanced bonding and to increase water retaining capacity. Fibre reinforcement is sometimes added to control the shrinkage cracking and to increase toughness to impact and abrasion. Miscalleaneous chemical modifiers are added to modify the behaviour of repair materials and they include supplementary cementatious materials, Accelerators, Retarders, Shrinkage Compensative additives, Water Reducers, Flowability Agents, Expanding agents, Air Entraining Admixtures etc. The use of all these modifiers can control and has effect on constructability, permeability and durablilty. The repair material ingredients should be optimised for effectiveness and the ingredients should be limited as more complex materials cause more problems. They must be non-injurious to the parent material through compatibility. The permeability of the repair material should be balanced so that they are impermeable to liquids but freely transmits water vapour. If impermeable materials (like epoxy mortars) are used for large patches, the moisture vapour will be entrapped between the concrete and the topping. The entrapped moisture will cause the failure either at the bond line or within the weaker of the two materials.

34.36

Handbook on Advanced Concrete Technology

The dimentional stability of the repair mortar is one of the primary requirement for successful repair. The differential shrinkage between old concrete and new repair mortars lead to bond failure after repair. In this case the repair material must also be essentially shrinkage free or should be able to shrink without losing bond. The coefficient of thermal expansion should be similar when composite materials are used. The modulus of elasticity of the repair material is very important criteria for selection as materials with low modulus of elasticity deform more under given unit load than materials with high modulus elasticity. The repair can fail when the load is applied parallel to the bond line. Figure 34.19 shows the deformations of high and low modulus materials which is a measure of stiffness. Load transfer has to be properly designed in addition of specifying materials.

High modulus Low Modulus

High modulus

Low modulus

Fig. 34.19 Modulus of elasticity (stiffness)

These mortars can be site mixed or prepacked. Prepacked mortars give better quality control. If site mixed then, Supervision from consultants with respect to consumption as well as application can only guarantee the right usage. Further, consistent supervision and joint responsibility is crucial for executing successful repair. Eternal vigilance is the price of durable repairs. The most important conclusion is summarized in Report of concrete society working party, Technical Report No.38, Titled ‘Patch Repair of Reinforced Concrete subject to reinforcement corrosion, Model Specification and Method of Measurement.’ In the section 13 on materials, the compatibility and responsibility is highlighted and the paragraph is stated here in verbatim as follows: It is recommended that all elements of a repair and protection system (reinforcement protective system, bonding aid or primer, repair mortar, surface filler, fairing coat and protection system) are obtained from a single manufacturer. Otherwise there can be divided responsibilities, if problems arise. If use of products from more than one manufacturer is unavoidable then the combination should be the subject of trials by both manufacturers to check compatibility. Evidence of previous successful use of the materials in combination would also be useful.

Repair and Maintenance of Concrete

34.4.5.1

34.37

Properties and Testing

The best way to ensure the advantages of polymers is to conduct a series of tests on the polymer modified mortars and to compare the results with mortars without polymers. In most of the practical cases it suffices only to conduct the tests on polymer modified concrete, if the base concrete values are known. It is normally sufficient that the repair mortar have 10 to 15 percent higher strength than the base concrete. In case of structural mortars also the design strengths can be achieved. Following major tests can be conducted: (a) (b) (c) (d) (e)

Compressive strength Flexural strength Air entrainment Alkali resistance Chloride content test.

In (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k)

case of specialized repairs, following tests may also be conducted: Water permeability Vapour permeability Carbonation resistance Wear resistance Chloride ion penetration Shrinkage characteristics Bond and shrinkage tests in typical repair case UV resistance Modulus of elasticity Dynamic modulus of elasticity Coefficient of thermal expansion

The number of tests to be conducted can be endless and only those tests need be conducted which have a direct bearing on the given repair situation.

34.4.5.2

Durability of Repairs

Durability of Repairs has been a matter of major concern during recent times. Experience has shown that the durability of these repairs is not up to the mark and several failures of repaired sections are evident. The failures of the repaired sections can be attributed to both materials as well as workmanship. While workmanship can be improved by efficient supervision and vigilance, the onus of good repairs also falls on the quality of materials employed in the rehabilitation of structures as no repair can be better than the materials used. Figure 34.20 shows a typical repair failure. Figure 34.21 shows a flow diagram for Durability of repairs. In many instances, serious cracking along the junction of repairs after 2/3 months is observed especially in monsoon and in most of these cases the cracks were stained with rust and this was clearly due to non application or inadequate application of Corrosion Inhibitor in

34.38

Handbook on Advanced Concrete Technology

Fig. 34.20 Repair failures

the Repair System. i.e the corrosion was not arrested. There can be misuse of products at two stages firstly the specification stage and secondly at application stage. At first stage an interaction between construction chemical manufacturers and consultants can solve the problem. At second stage of application training and supervision are part of solution. Application involving polymers and chemicals should be entrusted to trained skilled workers not only with respect to mixing type and ratios but also with respect to application type, time and curing. The durability of repairs is the function therefore of right materials used in right way. One has to be very cautious about selection of material and application technologies and the decision should be made depending upon performance criteria of the materials, rather than ingredients of the repair material. Such failures can be controlled by establishing proper quality control procedures. A Quality Assurance System has to be developed for ensuring the quality and durability of repairs. Quality Assurance System should involve Building Owner, Consultant, Neutral parties, Test Institutes, Qualified Contractors and reliable material supplier.

34.4.6 Other Techniques for Strengthening Repair involves to replace or correct deteriorated, damaged or faulty materials components or elements of a structure. In rehabilitation and retrofitting, we not only restore the concrete to take loads for present service conditions but we can also enhance load carrying capacities while maintaining the durability of structures. Following methods are adopted: (a) Section enlargement/Jacketing (b) External pre-stressing (c) Externally bonded reinforcement In addition to this, there are several specialized techniques used like shear strengthening, use of shear collars, confinement strengthening, span shortening techniques etc.

Repair and Maintenance of Concrete

REPAIRED CONCRETE STRUCTURE Repair

Weakened bond between repair material of concrete surface Differential volume changes Loading of element

Cracking

Increase in permeability along the perimeter of repair

Increase in permeability

Presence of H2O, Cl etc

Penetration of H2O, CO2, Cl etc.

(1) Depassivation of steel reinforcement (2) Formation of rust products

(1) Accumulation and expansion of rust products (2) Loss of bond between reinforcement and repair material

Expansion, more cracking, enlargement of existing cracks, spalling

Expansion, cracking spalling of existing concrete adjacent to the repair

REPAIR FAILURE

Fig. 34.21 Durability of repairs. flowchart

34.39

34.40

Handbook on Advanced Concrete Technology

34.4.6.1 Section Enlargement – jacketing This is one of the oldest techniques used to increase the cross section of element tying additional reinforcement which would lead to enhancing its load bearing capacity. The main thing is to bond the new material to the old element fully so that perfect load transfer and monolithic behaviour is guaranteed. This is achieved by means of thorough surface preparation, bonding agents as well as shear and other connections. The materials can be: (a) (b) (c) (d) (e) (f)

Conventional concrete/mortar Cementatious grouts Polymer modified cement mortars Micro concretes Shotcreting dry or wet process Polymer mortars

The selection of material will determine not only the structural calculations but also placement method to be adopted .

Conventional Concrete/Mortar Jacketing can be done by erection of form works and very plasticized concrete or mortar can be cast depending on the thickness to be achieved, Post box windows to be kept for vibration and observation. Vibrations can be avoided if modern technology of self consolidating concrete is adopted.

Cementatious Grouts Here the form work is erected and single size prepacked aggregate is filled in the forms. Cementatious or polymer cement grout is introduced through tubes to fill the voids. This is very efficient method to reduce the bulk shrinkage. This method avoids vibration also.

Polymer Modified Cement Mortars Bonding in jacketing can be enhanced by using polymer modified cement mortars or grouts. The flexural strengths can also be increased by addition of polymers.

Micro Concretes Micro concretes are in fact factory manufactured one component, ready to use, prepacked polymer mortars with excellent grading curve and aggregates up to 6 - 8 mm size. They contain inbuilt plasticizer and non shrink agents. By virtue of this very high strengths in the range of 80-90 N/mm2 are achieved. These strengths help to design section as composite section as well as lower thickness can be achieved because of its factory quality control, it is very popular material for Jacketing.

Shotcreting Shotcreting and guniting are also old methods which apply pneumatically apply the mortars or concrete, so that the compaction is very high. It is very suitable for large areas and for vertical and overhead areas. Figure 34.21 shows a typical repair. There are two processes:

Repair and Maintenance of Concrete

34.41

• Dry process • Wet process. Dry Process: The material like cement, aggregates etc are dry mixed and water is added at the nozzle. Accelerators are usually added to reduce the rebound and to make higher thicknesses in single pass. Supplementary cementatious materials can be added for lowering the shrinkage. Wet Process: Any normal concrete which can be pumped is suitable for wet spraying process. All the materials are wet mixed and pneumatically applied through the nozzles. Polymers can be effectively added in wet process. In both the processes as of today, supplementary cementatious materials like microsilica, PFA, GGBS is added. Also different type of fibres are used for shrinkage reduction. Surfaces should be properly prepared and reinforcement mesh is added to complete the system. Bonding agents can be surface applied.

Polymer Mortars Resin based polymer mortars can be employed for repairs of structural elements. Structural designs can be done as composite structure and thinner sections are possible.

External Pre-stressing Normally, this technique is used where the damage is due to undesirable or excessive deformation. The technique of pre-stressing is same as through tendons but since it is done externally better inspection is possible. The flexural capacity of these structural member is increased especially in case of pre-stressed elements. This requires shear transfer mechanism and end bearing as an assembly. The existing cracks should be filled and grouting of strands should be done with care. Protection of the total assembly is very important as this is externally created.

34.4.6.3

Externally Bonded Reinforcement

Working principle of this system is plate bonding technique. When the steel plate is bonded to concrete with epoxy adhesive the structure behaves like composite structure. Two systems are prevalent: (a) Steel Plate Bonding (b) GFRP/CFRP Bonding.

Steel Plate Bonding It is an efficient method of increasing flexural capacity in beams when application is on tension side. The success depends upon the epoxy adhesive which transfers load from concrete to steel. Its ease of constructability was reason for major success as well as plate bonding does not increase the section. Disadvantage were cutting of steel plates to suit the geometry, its weight and problem of corrosion.

34.42

Handbook on Advanced Concrete Technology

34.4.7 Fibre Reinforced Plastic Bonding This is in fact an extension of steel plate bonding technique with tremendous advantage of light weight, ease of cutting and mouldability to suit any element with high chemicals resistance. Two systems are popular: • Glass Fibre reinforced plastic (GFRP) • Carbon Fibre reinforced plastic (CFRP) FRP composites contain fibres of high tensile strength in polymer matrix of epoxy or vinyl ester. The rapid acceptance of this material is due to serviceability and ease of application without disturbing the structure. Fibre wrapping of columns provides passive confinement with increase of ductility and strength. Shear strengths are also increased. Wrapping gives excellent protection against explosions. Figure 34.22 shows a CFRP wrap on column. FRP plates can be bonded with epoxies to increase flexural strengths. The limitation is the use of epoxy which can change its charactertics during thermal variation and fire.

Fig. 34.22 Strengthing of column by CFRP wrap

34.4.7.1 Corrosion Inhibiting Admixtures The Corrosion Inhibitor is an admixture that is used in concrete to keep the metal embedements in concrete free from corroding. Corrosion Inhibitor are also available as surface applied liquids and slurries which fight corrosion in a unique way. Corrosion Inhibitors extend the time before initiation and reduce the rate of corrosion in propogation period. The mechanism of inhibitor will vary depending upon the chemical nature of inhibitor and factors causing corrosion. The chemical composition of corrosion inhibitor may be organic, inorganic or mixed. Corrosion Inhibitor are divided in three types – anodic, cathodic and concrete mixed. Mixed inhibitors contain one or two types of molecules with proton and electron acceptor groups. Studies indicate that Calcium Nitrite is one of the most widely used corrosion inhibitor, base of which is inorganic in nature. With the addition of nitrite (Na+ or Ca+) to the concrete a competing oxidation reaction is initiated at the surface of the steel which regenerates the passivating layer with Fe2O3:

Repair and Maintenance of Concrete

34.43

2Fe ++ + 2 OH– + 2NO2– Æ 2NO + Fe2O3 + H2O Anodic Inhibitor When NO2– is added the change in pH values is the same, but the corrosion potential remains in passive values. So, the NO2– avoids corrosion as much in alkaline as in neutral media. Nitrite ions rapidly oxidize ferrous ion to ferric, blocking further passage of ferrous ion from steel into the electrolyte. It is claimed that nitrates should be even better inhibitor than nitrites since two moles of Ferrous iron are oxidized of Ferric per mole of Nitrogen oxide added to Nitrate, Inhibitor mechanism is explained. Nitrate will give an extra reduction step compared to nitrite: 2Fe6 (OH)2 (S) + NO3– + H2O = 2Fe(OH)3 (S) + NO2– Eo = 0.57V Nitrates are more easily available and more economical and this would make the usage more common.

34.4.7.2

Modern Techniques

Since the corrosion process is Electrochemical, the cathodic protection system are popular measures to reverse the electrolytic process. They can be used as remedial measures as well as can be embedded during construction to monitor the future corrosion. The working of Cathodic protection Process is shown in Fig. 34.23. The cathodic Protection System should be properly designed for continuity of current circulation. There are two basic types of cathodic protection viz. Impressed current cathodic and Galvanic Cathodic Protection.

Cathodic protection with elgard anode mesh

Installation of elgard sacrificial Anode Mesh

Fig. 34.23 Cathodic protection

Few other techniques like Migrating Corrosion Inhibitors which are patented are also available for the user. The principle behind migration of MCI® (Migration Corrosion Inhibitor) through concrete, as stated in the literature of Cortex Corporation is as under – “The migration of MCI molecules is based on their ability to diffuse in both vapor or liquid form. They migrate

34.44

Handbook on Advanced Concrete Technology

into even the smallest pores and cavities, seeking out either positively or negatively charged metal surfaces (cathodic/anodic). The creation of a protective monomolecular layer on metals embedded in concrete is based on the physical adsorption mechanism, which means that MCI can inhibit further corrosion of heavily corroded or scaled internal bars”. They are both protective mechanism on organic basis and environmentally safe. Their protection mechanism are anodic and cathodic and they are migratory. They are available as liquid concrete admixture as well as powders and in a slurry form for different applications.

34.4.7.3

Realkalization and Desalination

These are non destructive electrochemical treatments to stop reinforcement corrosion. While realkalization is aimed at Carbonated concrete, desalination is employed for chloride contaminated concrete. NORCURETM realkalization. Is a patented electrochemical treatment for reinforced concrete owned by Norwegian Concrete Technologies (NCT). It halts and permanently prevents future reinforcement corrosion in carbonated concrete by reinstating it’s alkalinity. The method increases the pH of the carbonated concrete to greater than 10. which is sufficient to maintain a passive oxide film on the steel. Realkalization is performed by applying an electric field between the reinforcement steel in the concrete and externally mounted anode of either a mild steel or titanium mesh. The anode is surrounded in an alkaline electrolyte, usually a sodium carbonate solution, which is held in a reservoir in contact with the concrete surface. During treatment, the alkaline solution is transported into the carbonated concrete by means of electro-osmosis. Simultaneously, electrolysis produces a very alkaline environment at the reinforcement surface. In some situations, electro-osmosis can not be used and the introduction of sodium carbonate is not desirable. In these cases, variation of the treatment allows realkalisation by electrolysis alone. NORCURETM Desalination is a patented electrochemical treatment for reinforced concrete owned by Norwegian Concrete Technologies (NCT). It halts and permanently prevents future reinforcement corrosion in chloride contaminated concrete by removing chlorides from the concrete and simultaneously increases the pH level in the surroundings of the reinforcement steel sufficiently to permanently passivate the reinforcement. Desalination is performed by applying an electric field between the reinforcement steel in the concrete and temporary externally mounted anode of either a mild steel or titanium mesh. The anode is surrounded by electrolyte, generally of potable water, which is held in a reservoir in contact with the concrete surface. The reservoir is either a spray applied cellulose fiber, a system of two layers of felt cloth or a light weight shuttering/tank arrangement. During treatment, the chlorides are transported out of the concrete, towards the positively charged external electrode mesh by means of migration. The advantage over conventional repair techniques is that the treatment is non-destructive in the sense that only already spalled and cracked concrete has to be repaired with nominal cleaning of exposed reinforcement. In situation where damage due to chloride contamination is at a pre-spall stage of development, little or no concrete removal is necessary. The surrounding, therefore, are only slightly affected by the treatment, and noise and dust disturbance is minimal. The process utilizes environmentally safe materials. Large areas can be treated at a time, and the treatment provides long term durability.

Repair and Maintenance of Concrete

34.5

34.45

PREVENTIVE MAINTENANCE

Premature failure of building components in a structure is not uncommon. To stem this, preventive maintenance has come into vogue, of late. Preventive maintenance is a part of asset management system. It seeks to forestall the deteriorating process of building components. Minor repairs and replacements are categorized under preventive maintenance activity. The preventive maintenance programme precludes major repair work or replacement of substantial part of the structure. The prime idea behind preventive maintenance is that structures are slated to be durable, if properly maintained. Conversely, lack of proper maintenance will lead to early failure of its components, which in turn, will result in reducing the span of its existence. An effective preventive maintenance programme’s essential requisite is skilled and qualified personnel who keep abreast of changing technology in the construction industry. There are assorted instances of structural failures due to ignorance of building techniques on the part of construction personnel and neglect of preventive maintenance on the part of owners. Making house owner aware will be key to this initiative.

34.5.1 Why Preventive Maintenance? What is more, owners or managers of housing complex are mostly averse to carrying out any preventive maintenance. For they do not know its potential. They do not take such a programme seriously since the costs involved do not produce ready returns. They must try and understand the cost benefit advantage they give in the long run. Good sense will only dawn with the occurrence of interference with function of the structure due to deterioration. Cumulative experience will tell that in the absence of periodic inspection and regular maintenance, hardly any building can function for the expected span of time. What preventive maintenance can do is a legitimate question. Prevention of failures has a wide ramification. With regular repair, joints in walls will not leak excessively or too early. Expansion joints require periodic maintenance to keep away water. Minor and short term defects can prove very costly later with serious damages to the system, if not corrected early. Preventive maintenance takes care of potential failures to arrest actual failures. It arrests progress of deteriorating process.

34.5.2 Impact of Materials and Systems Quality of materials, type of system, the system’s design as well as building techniques govern the frequency, extent and costs of preventive maintenance. Materials selected and construction methods adopted largely influence the extent of preventive maintenance required during the service life of the structure. And it is an aphorism to say that the success of a preventive maintenance programme hinges on the quality of original construction alone. The quality of construction should be weighed against the anticipated preventive maintenance requirements. This has to be done at the initial stage of project development. Analysis of the life cycle cost of a structure is an important aspect of preventive maintenance. Life cycle cost analysis help to determine what quality of construction will be cost effective. And performance requirements should be the criterion for material selection.

34.46

Handbook on Advanced Concrete Technology

34.5.3 Weather Factor For detailing building construction, knowledge of climate and other environmental influences that degrade building system is a must. If improperly detailed, the system will need frequent repairs and maintenance, making it high-cost. If water is allowed to penetrate, it can cause considerable damage. This is very true of paved areas, masonry walls and other building components. Besides, where rainfall is substantial, design against moisture should be a major requirement. Deteoriation of concrete is a natural process due to carbonation, acid rains etc. Quality of construction and workmanship has attracted a lot of complaints. Well-detailed construction and good quality materials alone cannot ensure efficient performance of the building systems without high quality workmanship. The environmental exposure is major cause for deterioration of concrete.

34.5.4

Inspection

While looking for potential failures, task of inspection is to decide on the spot what type of repairs or maintenance are needed. If the preventive maintenance personnel is not in a position to make a decision, it will result in extensive damage to the building envelope as well as building interiors. The preventive maintenance staff has a demanding task in inspecting and analyzing existing conditions and for taking necessary action without delay, if the failure is imminent. A detailed inspection check list should be created, which includes course of action. Once the problem is identified, it will be possible to spell out the repairs. Inspection can be the starting point of high-quality preventive maintenance and repair work. Inspection, to be effective, is to be systematic. The inspector should be qualified and capable of reading signs of potential damage and should be conversant with the indications which call for maintenance work and system repairs. He must also know where to look for signs. Besides, the amount of damage has to be assessed. In new construction, inspectors should make sure that the construction complies with the construction documents. Apart from inspecting the repair and maintenance work, the preventive maintenance task expect the inspectors to find the problem, mark its severity, and also to prescribe the course of action to solve the problem. Cause of degradation is to be detected to prevent its recurrence.

34.5.5 Corrective Steps A useful approach to preventive maintenance is to rectify problems that are caused by faulty initial design and construction lapses. The final step in inspection after completing the project, generally resolves this aspect. However, yet there may be areas which are susceptible to failure. In such cases, modifications have to be made so that components in these areas are not exposed to above normal degradation potential. In the event of those areas, susceptible to premature failure, they must be clearly marked and inspected continuously. In preventive maintenance, knowing how to perform the work needed to prevent the possibility of failures is a part of the process. It has also to be effectively managed. Other aspects are setting objectives of the programme, its organisation, estimating investment costs and also fix the break-even point of the programme. It is a known fact that failures are

Repair and Maintenance of Concrete

34.47

always more costly than performance of preventive maintenance programme. But managers may be reluctant to accede unless there are visible signs of deterioration and imminent failure. These days, concrete failures are numerous. Spalling and cracking of concrete, rusting of rebars, and a number of impairments are not difficult to find. The preventive maintenance activities consist of regularly performed inspections While the activities can be initiated by minor failures, repeated failures point to the need for capital outlay for replacement or minor repair. The elements of the cycle of preventive maintenance programme are inspection, work generated through inspection, information system and preventive maintenance procedures. The information system takes care of budgeting, costing and scheduling activities. It also sets the objectives, records finished jobs and charts out the benefits of the programme. The preventive maintenance field is complex. It needs a reference and recording system, procedures for inspection, design and implementation. It also needs qualified staff for budgeting, accounting and execution. Hence, organisation and efficiency is imperative.

34.5.6 Planning and Organisation Effective preventive maintenance programme calls for careful planning, organization and implementation. It involves something more than mere reaction. Substantial efforts are needed in the orgainsation of an effective programme, ensuring efficient and harmonious working different constituents of the programme. An efficient programme begets benefits and is cost-effective. It is necessary to outline the objectives of the programme on the basis of inspection report. Most people need to be convinced about the importance of preventive maintenance. Unqualified workers following poorly structured procedures may exhibit a high level of activity as a result of short notice responses to failure. Preventive maintenance programme has to be pursued on a continuous basis and monitored and documented for future reference. Periodic review of the programme will be useful. It will show how far the objectives are met. An effective preventive maintenance programme should outline areas to be maintained in order of priority defining responsibilities of staff.

34.5.7

Leakages and Waterproofing

Since waterproofing is a major part of preventive maintenance it is briefly discussed here. To keep unwanted water out, is waterproofing. Hence, the term waterproofing needs to be used with some limitations. Very often cases of damp-proofing are referred to as waterproofing. What is to be noted here is that the passage of water through the structure not only results in peeling of plaster or paints but also it has wider implications. The passage of water through concrete in the first stages can lead to corrosion of the embedded rebars leading to structural failure at a later stage. The water can also carry chlorides and sulphides accelerating deterioration. The ingress of water in tanks holding potable water can lead to serious health hazards. The dampness can decay the wood in the buildings. Hence, it behaves well to diagnose the cause of water leaks at an early date in order to take corrective measures. An early or timely treatment will not only provide longevity to the structures but also will save financial losses.

34.48

Handbook on Advanced Concrete Technology

Waterproofing failures are caused quite often by faulty materials, inappropriate selection of specific products and systems for a given application and/or from poor detailing and incorrect installation procedures. One thing to remember is that cost savings will not always benefit in any way. Besides, the practice of calling for a guarantee will not necessarily absolve the designer or the specifier from liability for failure of waterproofing installations. For effective waterproofing, the specifier should consider a combination of materials and methods of installation. A combination of internal and external coating systems would be useful.

34.5.8 State of Waterproofing in India Waterproofing does not seem to receive the attention it rightfully deserves in this country. It is rather taken as a separate job to plug leakage or stop seeping before rains. The leakage and seepage in different concrete structures are the most chronic and common problem occurring from time to time after the completion of construction or at a later stage. The roof and flooring form the most important elements of a building since security of the entire structure depends upon the integrity of its top and bottom. Hence, a sound roof and a dry floor are vital. However, a large number of roofs, basements and other water retaining and water excluding parts of the structure remain damp or leak. Despite the fact that several decisions are to be made regarding selection of materials and their application, waterproofing systems are considered solely on the basis of cost rather than on performance. With the advent of polymer technology and new building chemicals, water proofing has taken long strides. It calls for use of water reducing agents and polymer based admixtures in the concrete mix to achieve dense and impermeable concrete. Different types of materials like flexible sheeting, polymer modified and epoxy based products have been developed to provide better effect. One lingering problem in the field of waterproofing in India is the practice to ask/guarantees and equally common with contractors to provide same. This does not in any way prevent failures or the contractors backing out of his obligation. The need for putting up minimum price to win the contract leads watering down the quality of the job. The best way from this situation is to have a system of quality Assurance for waterproofing. Preventive maintenance has to be built in to planning in the case of new structures.

34.5.9 Maintenance of Records Maintenance of proper reports about the damages and subsequent repairs can offer a lot of information. This is crucial for future diagnosis of subsequent deterioration. The records about the types of materials used at previous repairs along with other relevant technical data will clearly show the suitability for future usage as well as the durability of such materials. Documentation of this kind will go a long way to ascertain the quality of repairs, which would provide pointers for future from the past failures. Surface protection through periodical repairs and waterproofing deserve utmost attention if the life and durability of the concrete structure is to be maintained. Over the period of time, any concrete structure is prone to deteriorate due to various causes and timely attention is a must. Any neglect or postponing the maintenance and repair needs to another day will only lead to further deterioration. This will not only prove to

Repair and Maintenance of Concrete

34.49

be costly but make repairs difficult and complex. Full record of minor and major repairs and surface treatments to be maintained for durability checking.

34.5.9.1 Emphasis on Protection Concrete is constantly under attack of environment pollution as well as moisture ingress and penetration of chlorides or other deleterious materials. It is, therefore, clear that concrete needs to be protected with coatings which can resist carbonation and ingress of water in any form. Figure 34.24 shows mechanism of water entry into concrete. Concrete is fundamentally wettable material and has natural affirmation towards water. Therefore, in addition to porosity and permeability of concrete, sorptivity must also be ascertained. A reliable way to enhance the durability of concrete is to provide surface protection to act as physical and chemical barrier to create equivalent cover.

Fig. 34.24 Mechanisms of water entry

Surface treatments are also employed to increase the aesthetics and visual appearance of concrete. Several benefits can be obtained by incorporating the surface treatments in a well designed manner, right from conception of project. Although the main function of any surface protectant is moisture ingress control either by physical barrier concept or conversion of capillaries to non wettable, the coatings can also be designed for resisting chemicals. Chloride ingress can be effectively controlled by surface protectants. Further, the diffusivity of carbon dioxide, sulphur dioxide and oxygen can be lowered for corrosion control. Root growths and mould growths can be prevented in concretes under damp conditions.

34.5.10 Corrosion and Equivalent Cover There are two stages of corrosion, the first one is Initiation and second is Propagation. In initiation, the protective passive layer on the steel surface is destroyed either by Chloride

34.50

Handbook on Advanced Concrete Technology

penetration or carbonation. In propagation, the rate of corrosion is governed by oxygen, water transport, chlorides or internal electrical resistance. The Discussion document of Concrete Society, which gives the Tuuti 2-phase model of reinforcement corrosion is shown in Fig. 34.25. The design life is expressed as initiation period and propagation period. Since the corrosion goes through stages of initiation and propagation and acceleration period, it is possible to coat the concrete at any stage and its design life is longer, as the concrete is structurally sound. The protection can be done at any stage of life of concrete before and after repairs after determining the structural stability and corrosion threshold. Significant level of damage

n

sio

ro or

C

Carbonation or chloride penetration to rebar

Rate of corrosion is represented by the slope End of design working life

Initiation period (ti)z

Propagation period (tp)

Source: Concrete society – Developments in durability, deign & Performance – Based specification of concrete

Fig. 34.25 Tuuti 2-phase model of reinforcement corrosion

34.5.11 Why Surface Treatments? When concrete is exposed to aggressive environmental conditions in presence of water, the corrosion controlling constituents are carried to the reinforcement steel. Surface protection can be, therefore, act as Physical or Chemical barrier right at the interface of structure-environment matrix arresting the very beginning of process of initiation. The surface protection coatings which can inhibit water ingress from outside are beneficial in stopping the corrosion at initial stage and retarding the propagation stage. Surface protection coats can also help retard the alkali – silica reaction by depriving water to the reaction. Since the outermost skin of the concrete is directly exposed to attacks, the cover plays the most crucial role in the protection of concrete, be it by carbonation, chlorides or sulphates ingress. It is imperative to note here that cover is a very important element for protection of concrete and merely covering or repairing the cover to hide surface defects can lead to earlier deterioration Fig. 34.26 shows cracks and capillaries in concrete.

34.5.12 Coatings for Concrete There is a marked difference between paints and protective coatings for concrete. Whereas the paints are meant to beautify the concrete, the protective coatings are meant to protect the

Repair and Maintenance of Concrete

34.51

Capillaries

Cracks

Fig. 34.26 Cracks and capillaries in concrete

concrete under service conditions. It should be noted that coatings to be applied on concrete should not only be selected by considering only the aesthetic effect, but the selection must be based on the protection criteria. It is possible to have both aesthetic and protection judiciously combined in a single well designed coating. Figure 34.27 shows the properties and requirements of flexible protective coatings. Generally, the protective coatings should have the following basic characteristics. CO2

Reduces Carbonation

Water repellant

Crack Bridging

Open for water vapour diffusion

Ultra Voilet Radiations

Fig. 34.27 Properties of protective coatings

(a) Impermeability to water and Breathing Characteristics: The coatings should be permeable to water vapour but they should be impermeable to water and other gases like oxygen, carbondioxide etc. This means the coating should be breathable. Breathability is that property which enables the water vapour to move in and out of concrete with the fluctuations of temperature and humidity. This is a balancing point in well designed coating. (b) Carbon Dioxide diffusivity – Carbonation: The protective coatings should also be tested for diffusion of carbon dioxide to designate them as anti carbonation coatings. It is not sufficient

34.52

Handbook on Advanced Concrete Technology

that protective coatings are tested for carbonation initially but they should also be tested for carbonation after exposure to weathering conditions and cycles of loading. (c) Stability to ultra-violet radiations: For exterior application, it is mandatory that the coatings should be stable to ultra-violet radiations. The coatings should remain flexible even after exposure to sunlight without degrading. For example, epoxies do not posses adequate UV resistance. Acrylics are fully resistant to UV radiations by virtue of their basic formulation. (d) Flexibility and Crack Bridging Properties: This is one of the most important characteristics for protective coatings, especially in dynamically loaded structures like Bridges. The ingress of moisture, oxygen, carbon dioxide etc is not only through continuous capillaries but also through the cracks. Crack bridging capacity is a function of flexibility and thickness of coatings. Polymer modified cementatious flexible mortars can be useful in crack bridging capacity, which forms the base of concrete protection, especially in case of bridge decks, splash zones, tall structures, cooling towers etc in industrial atmosphere. The coatings should be self curing. These coatings also have lower chloride penetration characteristics.

34.5.13

Guidelines for Specification

The ability of coating to resist the ingress of CO2 and inhibit the carbonation imparts the designation of Anti carbonation Coating. The diffusion is measured using Ficks Law of Diffusion. For the sake of expressing, how many time a coating is impermeable compared to air, is expressed as mCO2. Thereafter the equivalent air layer thickness “R” is calculated as R = mCO2 × S. S is the film thickness in meter and here comes the involvement of film thickness. For different formulation, µCO2 can be very varied depending upon the formulations. This is the main reason why dry film thickness alone cannot be criteria for coating selection and holistic specification should involve all three parameters. R should be greater than 50m. Taking an example of an acrylic protective coating mCO2 = 3.02 × 106 for a Dry Film Thickness of 162 m, the equivalent air thickness ‘R’ (R = m × S) R = 3.02 × 106 × 162 × 10 –6 = 489 m which is > 50 m. Under such conditions the specifier can reduce the film thickness to 125 m and arrive at a value: R = 3.02 × 106 × 125 × 10 –6 = 377 m which is still adequate. Therefore the carbonation protection quotient depends not only on film thickness in isolation but are directly related to mCO2. Many a times for the sake of still clearest understanding of CO2 diffusion, this value can also be expressed as equivalent thickness of concrete layer or equivalent cover, which would have same resistance. Equivalent thickness of concrete expressed as R Sc = __________ m Concrete For a concrete of strength 30 N/m2, average m value is approx. 400. So for the same solvent containing acrylic coating mentioned in the above examples with a Dry Film Thickness of 125 m and R = 377 m.

Repair and Maintenance of Concrete

34.53

377 Sc = ______ = 0.95 400 m which means an equivalent cover of 95 cm. This is the correlation between the CO2 diffusion coefficient, equivalent air layer thickness, film thickness and equivalent concrete cover. Further, it is imperative that anti carbonation coating also have to be breathable, otherwise the internal moisture will lead to active corrosion in concrete but also due to fluctuation in temperatures, moisture trapped in concrete will damage the coatings with pore pressure of water in capillaries. This value is expressed as follows: g/m2 /day or cm2 /s as diffusion coefficient vapour translation rate. If expressed as equivalent air layer thickness for the same coating as indicated above for which m H2O is 12.8 × 103 considering Dry Film Thickness of 163 m, equivalent air layer thickness. Sd = m × S in meters = 12.8 × 103 × 163 × 10 –6 = 2.08 m < 4 m (requirement) for a Dry Film Thicnkess of 125 m, the calculation will be Sd = 12.8 × 103 × 125 × 10 –6 = 1.6 < 4 m indicating that is adequately breathable In addition to this, two characteristics of water absorbtion should be specified. Eventually when dealing with structures which vibrate like bridges the coating must vibrate with dynamic of structure and be elastomeric with crack bridging capacity, Crack bridging capacity is a function of elasticity as well as film thickness and this must also be specified.

34.5.14 Specifying the Protective Coating Specifying protective coating is one of the difficult function qualified architects and engineers have to perform. There are so many unknown factors to be considered, it becomes almost difficult to specify single coating. Some resort to generic raw material bases while others give more weightage to wet and dry film thickness. This creates ambiguity as high quality coating may perform better even at lower film thicknessess and when minimum thicnkesses are specified, they become uneconomical while in reality they may be cheapest with respect to performance. First and foremost thing is that the specifier must clearly understand the protection requirements. Table 34.12 shows different types of coating and their effectiveness. It is, therefore, definite that specification will change from project to project. Specifications should also depend upon the life span durability and protective quotient with respect to time. Cost calculations must be time dependent. A lot of confusion occurs on specifying the dry and wet film thicknesses on concrete surfaces which has relatively high undulations. It is difficult to measure the Dry Film Thickness. There are instruments like Paint Identification Gauges etc which can measure the Dry Film by cutting but accuracy is limited when thickness are in the range of 200 – 500 m. The easiest way

34.54

Handbook on Advanced Concrete Technology

TABLE 34.12 Comparison of different generic coating systems Coating Systems

Equivalent air layer thickness (m)

Coloured methacrylate Ethylene Copolymner Hypalon Chlorinated rubber Water dispersed epoxide Epoxy (Solvent dispersed) Concrete (35 II/mm2) Concrete (35 II/mm2) Silicone impregnated Thin layer cementitious render Standard exterior emulsion paint Ref:

R CO2 308 1100 1625 4477 101 179 3.58 2.67 2.40 2.07

SD 1.72 0.15 21.2 15 61.5 12.6 0.3 0.3 0.2 0.46

P.C. Hewelett, Methods of Protecting Concrete, International Conference, Scotland, Sept 1990

is to measure the Wet Film Thickness and knowing the solid contents of protective coatings the Dry Film Thickness can be easily calculated. Following formula gives the basis of calculation for theoretical coverage rate given volume solids and Dry Film Thickness: 10 × % solids by volume M2 = _________________________ = _____ dry film thickness in microns Litre 10 × % solids content by volume M2 = ________________________________________ = ___ dry film thickness in microns × specific gravity Kg Further referring to Table 34.13 in which relationship between solid contents by volume and dry and wet films are stated, coverage can be calculated as under: Example Volume solids 90% - required DFT 125 mm Therefore a 139 mm wet film thickness is required to achieve 125 mm DFT at 90% volume solids. which gives a theoretical coverage rate of 7.2 m2 /litre at a wet film thickness of 139 mm. Rather than specifying coatings by generic names and film thickness, it is preferable to specify them by performance requirements. The generic name can be stated as part of specification viz acrylic as it indicates that coating is resistant to UV radiations by virtue of its raw material base and it is proven to be the best with respect to all the qualities as well as economy. The performance properties, normally specified, are diffusion resistance against CO2 and Diffusion resistance against H2O vapour, when designated as anti carbonations breathable coatings. The equivalent air thickness ‘R’ in metre should be > 50 m and water vapour diffusion which is also expressed as Diffusion equivalent Air Layer thickness SD which denotes breathability is normally expected to have a value < 4 metres. It is also expressed as g/m2 /day in other specifications. It is a trend all over that Performance Specifications are riding over prescriptive specifications.

Repair and Maintenance of Concrete

34.55

TABLE 34.13 Calculation chart, dry film thickness and theoretical coverage rates DFT um 20 30 40 50 60 70 80 90 100 125 150 200 250 300 350 400 450 500 Ref:

% Solids Content by Volume 100 20 50 30 33 40 25 50 20 60 17 70 14 80 13 90 11 100 10 125 8.0 150 6.7 200 5.0 250 4.0 300 3.3 350 2.9 400 2.5 450 2.2 500 2.0

90 22 45 33 30 44 23 56 18 67 15 78 13 89 11 100 10 111 9.0 139 7.2 167 6.0 222 4.5 278 3.6 333 3.0 389 2.6 444 2.3 500 2.0 556 1.8

80 25 40 38 27 50 20 63 16 75 13 88 11 100 10 113 8.9 125 8.0 156 6.4 188 5.3 250 4.0 313 3.2 375 2.7 438 2.3 500 2.0 563 1.8 625 1.6

70 29 35 43 23 57 18 71 14 86 12 100 10 114 88 129 7.8 143 7.0 179 5.6 214 4.7 286 3.5 357 2.8 423 2.3 500 2.0 571 1.8 643 1.6 714 1.4

60 33 30 50 20 67 15 83 12 100 10 117 86 133 75 150 6.7 167 6.0 208 4.8 250 4.0 333 3.0 417 2.4 500 2.0 583 1.7 667 1.5 750 1.3 833 1.2

50 40 25 60 17 80 13 100 10 120 83 140 71 160 63 180 5.6 200 5.0 250 4.0 300 3.3 400 2.5 500 2.0 600 1.7 700 1.4 800 1.3 900 1.1 1000 1.0

40 50 20 75 13 100 10 12.5 80 150 67 175 57 200 50 225 4.4 250 4.0 313 3.2 375 2.7 500 2.0 625 1.6 750 1.3 875 1.1 1000 1.0 1125 0.9 1250 0.8

30 67 15 100 10 133 7.5 167 60 200 50 233 43 267 38 300 3.3 333 3.0 417 24 500 2.0 667 1.5 833 1.2 1000 1.0 1167 0.9 1333 0.8 1500 0.7 1667 0.6

The Application and measurement of protective Coatings for Concrete the Conctete Repair Association, Reprint Feb 2001

125

125

139

156

179

208

250

313

417

8.0

7.2

6.4

5.6

4.8

4.0

3.2

2.4

34.56

Handbook on Advanced Concrete Technology

34.5.15

Importance of Surface Preparation

First and foremost thing before application of any coating is to be aware of the internal moisture. The selection of the coating should be decided depending upon the moisture content of the concrete at the time of application. Otherwise, the coatings will crack, peel off and blistered when the moisture in the capillaries tries to escape as water vapour. Surface preparation plays a key role in the success of coating performance. Adequate surface preparation ensures the durability of coatings. For this reason, particularly, high importance is attached to the substrate. The strength of the substrate must be high enough to accept the mechanical loads. The condition of the substrate must be tested before each surface treatment. The following check list should be followed. (a) Internal Moisture Content (b) Surface Pull-off Strength (c) Tensile Strength (d) Existing finishes (e) Dirt, oil and other contamination (f) Remnants of curing compounds/mould release agents (g) Chloride content (h) Surface liatance or efflorescence (i) External factors affecting moisture (j) Cavities, cracks, blow holes and textures. The pull off strength of the substrate must, after preparation achieve a minimum of 1.5 N/mm2. Permissible residual moisture content will vary according to the coating system to be applied, Generally 7% for epoxy resins systems and not more than 4% for polyurethane systems. Therefore, for concretes which are freshly cast and considering the time required for concrete to dry, epoxy and PU coatings would be unsuitable because of obstruction of water vapour transport. For proper adhesion of any surface coatings, the substrate must be free from all adhesion-inhibiting substances such as oil, grease, release agents as well as cement laitance and dust. After all the surface preparation measures, the surface is to be swept for remaining dust and loose particles. This is best achieved with an industrial vacuum cleaner. All blow holes should be broken and filled with non-shrink fine polymer mortars called cosmetic mortar. This is mandatory and no coating will be effective unless blow holes are filled as these would be avenues for entry of CO2, SO2, Water etc. A dust bonding primer coat should be applied for increasing adhesion prior to coating.

34.5.16

Conclusion

The protection of concrete should start right at the time of design and strategies have to be decided for internal as well as external protection. The internal protection consists of low water cement ratios, addition of pore blocking integral waterproofers. addition of corrosion

Repair and Maintenance of Concrete

34.57

Fig. 34.28 Well maintained buildings

inhibiting admixtures, coating of reinforcements especially for structures liable to chloride ingress, provision of proper quality specified cover etc. The external protection, which normally includes surface sealers, impregnations, coatings and thin polymer overlays also work on the principle of providing equivalent extra cover. The two major factors that induce corrosion are water and oxygen, therefore the surface protection should primarily satisfy these requirements, in addition to enabling the concrete to breathe as entrapped moisture in concrete trying to escape can offset the protection process. Less permeable concrete, proper provision of cover and additional protection strategy only can guarantee durable concretes to withstand the stresses in severe exposure conditions. Studies reveal that protected concretes are definetely more durable than the unprotected concretes. The depth of carbonation is much lower in protected concrete for a period of time signifying with durability. Under given conditions of severe exposure to aggressive atmospheres, sunlight, rain and extreme temperatures, it seems that acrylic elastomeric coatings ensure the best results. The breathing capacity is the most important characteristic specially for newly laid concretes. For dynamically loaded structures, crack bridging characteristics subjected to several alternative cycles withstanding is mandatory. The reports and the records of maintenance should be preserved to determine the suitability as well as the durability of the repair materials. Rehabilitation is, in fact, the extension of waterproofing and requires very high degree of expertise. In addition to having knowledge of structural behavior, the rehabilitation expert should also possess thorough knowledge of material science, not only in terms of advantages but also the disadvantages and the limitations of the materials employed therein.

34.58

Handbook on Advanced Concrete Technology

Testing of protective coatings for water vapour diffusion and carbon dioxide diffusion should be taken at the same time as these are contradictory characteristics. The water absorption test can be conducted as per conventional practices. Since crack bridging capacity is a function of dry film thickness the specification should clearly mention the desired thickness for protective coating. Rather than going by the generic classification and the base of the protective coating, it is preferable to approach the selection by performance criteria and full knowledge of the stresses expected on the applied coating. Perpetual Maintenance is the key to long term durability of Structures. Figure 34.28 shows two well coated and well maintained buildings.

References 1. North Cadsawan (1993), Weathering the Storm, Concrete Repair Digest. 2. Thomas L. Rewerts (1993), Beware of Internal Moisture when coating concrete, Concrete Construction. 3. John Lamb (1993), Acrylic Elastomeric Coating, The Applicator. 4. C.D Pemoroy (1990), Benefits of Concrete as Construction material, International Conference, UK. 5. P.C. Kreijger (1990), Inhomogenity in Concrete, Protection of Concrete, E&F.N. Spon. 6. H. Reul, Handbuch der Bauchemie, 1991. 7. Grant T. Halvorse (1993), Concrete Cover, Concrete Construction. 8. Technical Literatures of EmceColor-flex and Betonflair, MC-Bauchemie (India) Pvt Ltd, 2010. 9. P.C. Hewlett (1990), Methods of Protecting concrete, International Conference, Scotland. 10. M. Leeming (1990), Surface Treatment for the protection of Concrete, Ove Anup and Partners, UL. 11. A guide to the use of waterproofing,damp proofing, Protective and decorative barrier systems for Concrete, ACI Committee 515-1985. 12. Discussion Document – Developments in Durability Design and Performance Based Specification of Concrete, Concrete Society, 1996. 13. Guide to Surface Treatments for Protection and enhancement of concrete, Concrete Society, 1997. 14. The Application and Measurement of Protective Coatings for Concrete, The Concrete Repair Association, Feb 2001. 15. DAfStb, Richtlinie fur Schutz und Instandsetzung von Betonbauteilen, August 1990. 16. F. Vazquez, Corrosion Inhibitors – Application of Admixtures in Concrete. RILEM REPORT 10, E&F.N. SPON 1995. 17. Xila Liu and Shuke Miao – A Probobilistic Prediction Method for Effective Service Lifetime of Reinforced Concrete Structures, China Civil Engineering Journal 1990. 18. Grant T. Halvorsen (1993), Concrete Cover – Concrete Construction. 19. Martin S. McGovern (1996), ‘Current’ technologies curb rebar corrosion – Concrete Repair Digest. 20. Technical literature of MC-Corrodur of MC-Bauchemie (India) Private Limited, 2010.

Repair and Maintenance of Concrete

34.59

21. Technical literature of NorcureTM. 22. Corrosion of Metals in Concrete reported by ACI Committee 222, 2010. 23. A.D. Hammond (1997), Electro Chemical Techniques for the repair of concrete, E&F.N. SPON. 24. Literature of ElgardTM Corporation. 25. Literature of CortexTM Corporation. 26. Peter Pullar-Strecker – Corrosion Damaged Concrete – Assessment and Repair, CIRIA 1988. 27. Peter H. Emmons, A.M. Vaysburd and Jay Thomas (2000), strengthening concrete structures Advanced composites. 28. Hans Beushausen and Mark Alexander (2009), Concrete repair chapter 27, Fultons concrete technology handbook. 29. Emmons, Peter H. (1992), Concrete repair and Maintenance – Problem analysis, repair strategy techniques. 30. Tracy, Robert G. and Fling Russel (1990), Rehabilitation Strategies, Repair and rehabilitation of concrete structures. 31. Warner, James (1984), Selecting Repair materials, Concrete repair and Restoration. 32. Patch Repair of reinforced concrete, Technical Report No. 38, Concrete Society, London, 1991. 33. Polymers in Concrete, Technical Report No. 39, Concrete Society, London, 1994. 34. Defects in Building, Department of the Environment, Property Service Agency, UK, 1976. 35. Matulionis Raymond and Freitag, Jaon C. (1990), Preventive Maintenance building building, Von Nostrand Reinhold, New York. 36. Dur and Green, Protection of concrete, Span Publication. 37. C.D. Pemoroy (1990), Benefits of concrete as construction material, International Conference, UK. 38. The Application and Measurement of protective coatings for concrete, The concrete repair Association, 1997. 39. American Concrete Institute, Concrete Repair manual, Vol. 1 & 2, Third edition, Published by ACI & ICRI, 2009. 40. Emmons, P.H. (1994), Concrete repair and Maintenance illustrated, Kingston, Massachusetts: R.S. Means company. 41. Vaysburd, A.M. and Emmons, P.H. (2004), Corrosion inhibitors and other protective systems in concrete repair, concepts or misconcepts, cement and concrete composites, Vol. 26, No. 3, PP. 225-263. 42. Peter H., Emmons, Gajanan M. Sabnis (2001), – Concrete repair and Maintenance, Galgotia Publication Delhi. 43. Raymond C. Matuliois and Joan C., Freitag editors (1991), Preventive Maintenance of Buildings. Van Nostrand Reinfold Publication. 44. John P. Broumfield (1997), Corrossion of Steel in Concrete by E&F.N. Spon. 45. A.M. Vaysburd, B. Bissannette (2010), Durability of concrete repair and research: Some Random thoughts, Journal of 3r’s Vol. 1. 46. Surface Preparation and coating of concrete published by SSPC, 2004.

35 Concrete and its Environmental Impact Himanshu Kapadia and Amit Datta

Concrete is second only to water as the most consumed substance in the world. The world’s yearly cement production of nearly 3.0 billion tons accounts for about 7% of the global loading of carbon dioxide into the atmosphere. Portland cement, the principal hydraulic cement in use today, is not only one of the most energy-intensive materials of construction but also is responsible for a large amount of greenhouse gases. Producing a ton of Portland cement requires about 4 GJ of energy and Portland cement clinker manufacture releases approximately 1 ton of carbon dioxide into the atmosphere. Furthermore, mining large quantities of raw materials such as limestone and clay, and fuel such as coal, often results in extensive deforestation and top-soil loss. Ordinary concrete, typically, contains about 12% to 15% cement and 80% aggregate by mass. This means that globally, for concrete making, we are consuming sand, gravel, and crushed rock at the rate of 10 to 11 billion tons every year. The mining, processing, and transport operations, involving such large quantities of aggregate, consume considerable amounts of energy, and adversely affect the ecology of forested areas and riverbeds. The concrete industry also uses large amounts of fresh water. The mixing water requirement alone is approximately 1 trillion litre every year. Reliable estimates aren’t available, but large quantities of fresh water are also being used as wash-water by the ready mixed concrete industry and for curing concrete. Besides the three primary components, i.e., cement, aggregates, and water; numerous chemical and mineral admixtures are also incorporated into the concrete mixtures. They too represent huge inputs of energy and materials into the final product. Leaving aside mixing, transporting, placing, consolidation and finishing of concrete, all these operations are energy-intensive. Fossil fuels are the primary source of energy today and the environmentalists are seriously debating on the costs associated with the use of fossil fuels. Finally, the lack of durable materials also has serious environmental consequences. Increasing the service life of products is a long-term and easy solution for preserving the earth’s natural

35.2

Handbook on Advanced Concrete Technology

resources. Concrete structures are, generally, designed for a service life of 100 to 150 years or so. But experience shows that in urban and coastal environments many structures begin to deteriorate in 20 to 30 years or even in lesser time due to various reasons. In the April 1998 issue of ASCE News, the American Society of Civil Engineers gave the Nation’s infrastructure (U.S. Infrastructure) an average grade of D and estimated that it would take $1.3 trillion to fix the problems. The cost to repair or to replace several hundred thousand concrete bridge decks alone would be $80 billion, whereas the present annual federal funding for this purpose is about $5 to $6 billion. Considering the funding constraints, Freyermuth5 has suggested that in the future structures may be designed and built for a minimum service life of 100 to 120 years, and major bridges in urban environments should have at least 150 years of useful life. The trend towards designing infrastructure is based on life-cycle cost. This will not only maximize the return on the available capital but also lead to conserving the available natural resources. The need for reducing the environmental impact of concrete is recognized in a recent report of the Strategic Development Council. An abbreviated version of the report, “Vision 2030: A Vision for the U.S. Concrete Industry,” was published in Concrete International, March, 2001. According to this report, concrete technologists are faced with the challenge of leading future development in a way that protects environmental quality while projecting concrete as a construction material of choice. Public concern will be to responsibly address the issue of climate change, resulting from the increased concentration of global warming gases.

35.1

CO2 EMISSION

What is CO2? Carbon dioxide is a colourless gas that makes up a minor part of the Earth’s atmosphere - approximately three parts in 10,000. It is formed in the decay of organic materials, the respiration of plant and animal life, and the natural and human-induced combustion of carbon-based materials and fuels. Carbon dioxide is removed from the atmosphere through photosynthesis and ocean absorption. For industrial usage, CO2 is recovered from lime kilns, flue gases, ammonia synthesis, and other sources.

What is the Role of CO2 in the Earth’s Atmosphere? Carbon dioxide is one of a naturally occurring greenhouse gases (others include water vapour, methane, and nitrous oxide) that keep the Earth warm enough to support life. These gas molecules absorb much of the sun’s energy that is re-radiated by the Earth’s surface, and reflect this energy back to the Earth as heat. The gas molecules function like an insulating blanket, or like glass panes of a greenhouse, transmitting sunlight but holding in heat - hence the term “greenhouse gases.”

What is the link Between Greenhouse Gases and Global Warming? Scientific studies show that a variety of human activities release greenhouse gases. The most significant factor is the burning of fossil fuels for producing electrical energy, heating and

Concrete and its Environmental Impact

35.3

transportation. Over the past century, rapid population growth accompanied by rising energy consumption and industrial production have led to higher concentrations of greenhouse gases in the atmosphere. As more heat is being trapped in the atmosphere, global temperatures are gradually rising. Most experts agree that average global temperatures have risen by about 1 degree Celsius over the past 100 years, and could rise by up to 5 degrees Celsius by the middle of the 21st century. Outgoing solar radiation

Outgoing infrared radiation

Radiated by earth’s surface and atmosphere, evaporation and other atmosphere processes

Incoming solar radiation

Reflected by atmosphere Greenhouse effect Reflected by earth’s surface Absorbed by atmosphere Absorbed by earth’s surface

The greenhouse effect

Why is Global Warming a Concern? If current warming patterns continue and average global temperatures rise even by a few degrees Celsius, the impact on natural ecosystems and human life would be devastating. Melting polar ice caps and mountain glaciers would result in rising sea levels and coastal flooding. Precipitation and weather patterns would change dramatically, bringing extremes of drought and rainfall. The changing climate would alter forests, crop yields, and water supplies, and could lead to famine. Many plant and animal habitats would be threatened, and some species would likely become extinct. As more and more people come to understand the threat of global warming, individuals and governments are making efforts to reduce greenhouse gas emissions. At the United Nations Climate Change Conference in Kyoto in 1997, the major industrial nations agreed to binding targets for reductions in greenhouse gas emissions. Some progress has been made, but much more is needed.

35.4

Handbook on Advanced Concrete Technology

35.2 CO2 EMISSIONS FROM CEMENT Production Carbon dioxide is released during the production of clinker, a component of cement, in which calcium carbonate (CaCO3) is heated in a rotary kiln to induce a series of complex chemical reactions (IPCC Guidelines).Specifically, CO2 is released as a by-product during calcination, which occurs in the upper, cooler end of the kiln, or a pre-calciner, at temperatures of 600°C -900°C, and results in the conversion of carbonates to oxides. The simplified stoichiometric relationship is as follows: CaCO3 + heat = CaO + CO2 At higher temperatures in the lower end of the kiln, the lime (CaO) reacts with silica, aluminum and iron containing materials to produce compounds in the clinker, an intermediate product of cement manufacture. The clinker is then removed from the kiln to cool, ground to a fine powder, and mixed with a small fraction (about five percent) of gypsum to create the most common form of cement known as Portland cement. Masonry cement is generally the second most common form of cement used in US and Western countries. Because masonry cement requires more lime than Portland cement, therefore masonry cement generally results in additional CO2 emissions

35.3 SUGGESTIONS TO REDUCE ENVIRONMENTAL IMPACT The environmental impact of the concrete industry can be reduced through resource productivity by conserving materials and energy for concrete-making and by improving the durability of concrete products. The task is most challenging but can be accomplished if pursued diligently. It is necessary to examine how the concrete industry will have to restructure when the business paradigm shifts its emphasis from a culture of acceleration to a culture of resource productivity.

Cement Conservation Cement conservation is the first step in reducing the energy consumption and greenhouse-gas emissions. Resource productivity consideration will require us to minimize Portland cement use while meeting the future demands for more concrete. This must be the top priority for a viable concrete industry. Except for blended Portland cements containing mineral additions, no other hydraulic cement seem to satisfy the setting, hardening, and durability characteristics of Portland cement-based products. Although there is steady growth in the use of Portland cement blends containing cementitious or pozzolanic by-products, such as ground granulated blast-furnace slag and fly ash, vast quantities of these by-products still end up either in low-value applications such as landfills and road sub bases, or are simply disposed by ponding and stockpiling. The world cement consumption rate is expected to reach about 4 billion tons by end 2020, and there are adequate supplies of pozzolanic and cementitious by-products that can be used as cement substitutes, thus eliminating the need for the production of more Portland cement clinker. Interestingly, as will be discussed below, Portland cement blends containing 50% or more granulated blast furnace slag or fly ash can yield much more durable concrete products than neat Portland cement and this would also contribute to natural resource conservation. The

Concrete and its Environmental Impact

35.5

slower setting and hardening rate of concrete containing a high-volume of a mineral admixture can be compensated for, to some extent, by reducing the water cementitious materials ratio with proper selection of a super plasticizer and also continuing curing to a little longer period. Nevertheless, for most structural applications, somewhat slower construction schedules ought to be acceptable when resource maximization and not only labour productivity, becomes the most important industry goal.

Alternative Sources of Energy The most significant way to reduce CO2 emissions is improving the energy efficiency of the cement kiln operation. Indeed, dramatic reductions in energy use have been realized in recent decades, as discussed above. Switching to lower-CO2 fuels such as natural gas and agricultural waste (peanut hulls, rice husk, etc.) can also reduce emissions. Another strategy, which addresses the CO2 emissions from calcining limestone, is to use waste lime from other industries in the kiln. Substitution of fly ash for some of the ordinary Portland cement in concrete can have a very large effect.

Use of Additives to Reduce Carbon Emission During Grinding Process The cement grinding aids are the additional materials, admixed in small amounts during the cement grinding process which helps in increasing fly ash level without affecting other properties of cement and reduce clinker quantity as CO2 is emitted during clinker production and not in cement production. By reducing clinker percentage we can automatically reduce CO2 emission.

Aggregate Conservation In North America, Europe and Japan, about two-thirds of the construction and demolition waste consists of masonry and old concrete rubble. This presents a great opportunity for the concrete industry to improve its resource productivity by using coarse aggregate derived from construction and demolition wastes. In many parts of the world, dredged sands and mining wastes can be processed for use as fine aggregate. Recycling these wastes in spite of some processing cost is becoming economical, particularly in countries where land is scarce and waste disposal costs are very high. In addition, virgin aggregate deposits have already been depleted in many areas, and hauling aggregates over long distances can be much more expensive than using a free or a low-cost source of local recycled aggregate. Recycled concrete, in some cases, is being used as a road fill, which is better than landfill but it is “down-cycling”, in the sense that virgin aggregate continues to be used for making new concrete. Lauritzen6 has estimated annual worldwide generation of concrete and masonry rubble at roughly 1 billion tons. At present, only small quantities of aggregate derived from recycled concrete and masonry are being used. Due to environmental considerations and the high cost of waste disposal, however, most countries in Europe have established short-term goals aimed at recycling 50% to 90% of the available construction and demolition waste. Recycled-concrete aggregate, particularly the recycled masonry aggregate, has a higher porosity than natural aggregate. Therefore, with a given workability, the water

35.6

Handbook on Advanced Concrete Technology

requirement for making fresh concrete tends to be high and mechanical properties of hardened concrete are adversely affected. The problem can be resolved by using blends of recycled and natural aggregate or by using water-reducing admixtures and fly ash in concrete.

Water Conservation So far, fresh water is abundantly available almost everywhere, and is being freely used for all purposes by the concrete industry. In fact, construction practice codes routinely recommend the use of potable water for concrete mixing and curing. But now, the situation has changed. Although there is a lot of water on earth but less than 3% is fresh and most of that is either locked up in fast-melting glaciers and ice caps or is too deep in the earth to retrieve. In recent press reports, the Indian government expressed deep concern over a future water shortage in the country. Because, due to global warming, the Himalayan glaciers, which are the primary source of water for Indian rivers, have receded by 30 m (100 ft) during the past 2 years alone. Due to growing agricultural, urban, and industrial needs, water tables in every continent are falling. Increasing pollution of the water in our rivers, lakes and streams compounds the problem. It is suggested that with water, as with energy, the only practical, large-scale solution is to use what resources we have far more efficiently. Regrettably, we’re making the same mistake with water as with energy. We’re depleting non-renewable water resources rapidly and seeking yet more water. As one of the largest industrial consumers of fresh water, it’s imperative for the concrete industry to use water more efficiently. In addition to approximately 100 million m3 wash-water used by the ready mixed concrete trucks, globally mixing water requirement of 1 trillion can be cut in half by better aggregate grading and by greatly expanding the use of mineral admixtures and superplasticizers. Also, the disposing of wash concrete in ready mix plants to clean the transit mixture is the major concern these days. This can be avoided by the use of hydration control admixture as it will retain the concrete in plastic state up to required time (over night) and then the balance quantity can be reused with the fresh concrete next day after adjustment of water..

Treating and Recycling of Wash Water Production of large amounts of waste wash water coming from ready-mixed concrete plants leads to problems of environmental impact. National laws, usually, prohibit the disposal of such types of water, due to their extremely high pH value and suspended matter quantity, and require the water to be treated prior to discharge. In general waste water is being filtered for further usage. The results have shown that mortar and concrete prepared with recycled water exhibit 28-day mechanical strength in no way lower than 96% of the reference materials and in some cases, even better. Moreover, the use of wash water in concrete leads to a reduction of the concrete capillary water absorption and mortar micro porosity, which surely improves the durability of the material. This effect can be ascribed to the filling action of the fines present in the wash water and to the slight reduction of the actual water/cement ratio.

Concrete and its Environmental Impact

35.7

Use of Curing Membranes Use Of curing membranes will greatly help in saving of precious potable water as more than 30% water will get wasted during curing process apart from regular quantity of water that is required for curing. Pumping of water again consumes energy for all high rise buildings.

Concrete Durability In addition to the steps outlined above, improving concrete durability presents a long-range solution and a major breakthrough for improving the resource productivity of the concrete industry. For example, the resource productivity of the concrete industry will jump by a factor of 2-3 if most structural concrete elements are built to last for 100 years instead of 30-50 years. Why do modern reinforced concrete structures sometimes begin to deteriorate in 20 years or less, whereas there are buildings and sea walls made of unreinforced Roman concrete that continue to be in good condition even after almost 2000 years? Primarily, because our Portland-cement concrete mixtures are highly crack-prone and therefore become permeable during service. The embedded steel reinforcement in permeable concrete corrodes easily, causing progressive deterioration of the structure. Today’s construction practice, driven by a culture of ever-accelerating construction speeds, uses concrete containing a relatively large amount of high-early strength Portland cement. As a result, the extensibility or crack-resistance of modern concretes is poor because of the high tensile stresses induced initially by substantial thermal contraction and drying shrinkage, and too little creep relaxation. If durability and sustainability are important goals, current construction practice and the codes of recommended practice must undergo a paradigm shift to achieve crack-free concrete structures in preference to high speeds of construction. In fact, technology is available in the form of somewhat slower-hardening by using blended Portland cements containing 50% to 60% fly ash or granulated blast-furnace slag. Mixture proportions to be done properly considering properties, and applications of high-volume fly-ash super plasticized concrete mixtures. If the mixing water content and the total cementitious materials in concrete are further reduced with the help of a super plasticizer/ hyper super plasticizers, it is possible to eliminate all or most of the shrinkage and cracking, and produce a highly durable concrete.

35.4

A MODEL FOR THE FUTURE

The high-volume fly ash concrete system provides a model for the future for making concrete mixtures that shrink less, crack less, and would be far more durable and resource-efficient than conventional Portland-cement concrete. The ability to design and build structural members to achieve service life of structure without any maintenance for 100-150 years or more instead of 20-30 years will in the long run increase the concrete industry’s resource productivity by tenfold. Meanwhile, by substituting recycled materials for natural materials, as described in this article, it should be possible to substantially improve the resource productivity of the concrete industry immediately. Unquestionably, the greatest challenge that the concrete industry faces during the 21st century is to achieve a sustainable pattern of growth. The task is formidable but the ideas and examples cited in this

35.8

Handbook on Advanced Concrete Technology

article show that it can be accomplished provided we make a paradigm shift from the culture of accelerating construction speeds to a culture of conservation of energy and material. The policy makers and specifiers should be pursued for this shift.

Recommendations 1. It is recommended that attempts are made to reduce the volume of excess ready-mixed concrete ordered by: (a) Planning of pours to allow amalgamation of deliveries. (b) Accurate measurement of the volumes required. (c) Preparing non-critical uses for any surplus material. 2. The remaining surplus concrete should be allowed to harden where it can be kept as free of contamination as possible to gain the best value at disposal to a crusher. 3. Recycled aggregate should be used wherever possible. 4. Supplementary cementitious materials like fly ash, ggbs, rice husk ash, metakaoline, silica fume, etc. should be used to the possible extent to replace Portland cement in concrete. 5. The concrete structures should be planned and constructed for useful life of 100 years and more by choosing proper grade and mix proportions of concrete reinforcement cover and by adopting appropriate construction practices. 6. The use of curing compounds and superplasticers should be increased in concrete to conserve water for curing and mixing respectively.

References 1. 2. 3. 4.

Michael J. Gibbs. Peter Soyka and David Conneely (ICF Incorporated). Dina Kruger (USEPA). Sandrolini Franco; Franzoni Elisa (1).

36 Current and Expected Future Advances in Concrete Himanshu Kapadia and B.V.B. Pai

The industrialized and developing world is facing the issues related to new construction as well as repair and rehabilitation of existing facilities. Rapid construction and long term durability are requirements on most projects. Initial and life-cycle costs play a major role in today’s infrastructure development. There have been number of notable advancements made in concrete technology in the last fifty years. Some of these advances have been incorporated in routine practices. But, in general the State-of-practice has lagged far behind the state-of-art. This is particularly true for public sector projects. There is an increasing concern in most of the world that it takes unduly long time for successful concrete research products to be utilized in practice. Even though some advances have been made in quick implementation of new concrete technology, significant barriers to innovation and implementation remain.

36.1

ADVANCES IN CONCRETE TECHNOLOGY

Numerous advances in all areas of concrete technology including materials, mixture proportioning, recycling, structural design, durability requirements, testing and specifications have been made. Innovative contracting mechanisms have been considered, explored and tried. Some progress has been made in utilizing some of these technology innovations, but largely these remain outside routine practice.

Innovative Solutions to Address Future Trends Adaptation for a broader range of cements and aggregates • Less material and energy consumption in construction and finished buildings • Lower construction costs and faster construction processes • Construction of structures with longer lifespan

36.2

Handbook on Advanced Concrete Technology

The admixtures based on a modified Polycarboxylic Eather Based technology, in principle, consist of polymer blocks of slump retention, early strength, final strength and for water reduction in concrete. In brief, building polymer block controls the adsorption speed of the PCE to the cement surface while it is not retarding. Another Building block is enhancing the natural hydration process, without influencing the hydration products. On the contrary, other building block for strength enhancing effect, based on influencing the hydration process, has enabled concrete technologists to design concrete for longer retention without compromising on other properties of concrete. The use of self-consolidating concrete (SCC) and ultra-high strength concrete has contributed for growth in concrete industry. New concept of zero energy system coming into market for Precast concrete, wherein we can eliminate or reduce Vibration and thermal curing to achieve high early strength,

Ultra-high Strength M80 Above Always architects / Consultants dream about having slender colums to have more useable or carpet area instead of Bulkier colum which occupies more space and sometimes become obstacle for places like auditorium

36.2

APPLICATION OF ULTRA HIGH STRENGHT CONCRETE

Trend of Architectural Design

Small

Small

Large

Research and development on high-strength concrete have continuously been done for more than 40 years. This Topic describes the past successful technology developments on materials to obtain the workable high-strength concrete and introduces recent enhanced performances as to strength, durability and fire resistance of high-strength concrete. Furthermore, some applications of high-strength concrete to structural columns of high-rise buildings and an application of high-strength steel fiber reinforced concrete to a pre-stressed concrete bridge.

Current and Expected Future Advances in Concrete

36.3

Realizing higher strength concrete has been a target or a dream for researchers and engineers engaged in construction industry, in the early 1960’s, a super plasticizer was invented in Japan. By the inclusion of the super plasticizer, high-strength concrete could be realized by reducing w/c to under 30%. The high-strength concrete was, however, applied only to the factory products because it had a large loss in slump. Recent successful researches and developments on materials and construction methods have led to the cast-in place high-strength concrete with good workability, the strength of more than 100 MPa and higher durability. The high-strength concrete has been applied to a number of high-rise buildings and diaphragm walls. The high-strength concrete however has two defects. One is the occurrence of thermal cracks due to the heat of hydration of a large amount of cement content. To overcome this, many types of low heat cements were developed and utilized. The other is the poor resistance to spalling during fire attack. Inclusion of some synthetic fibers is much effective for reducing the spalling of high-strength concretes. Furthermore, high-strength steel fiber reinforced concrete (referred to as “RPC”) based on a densest packing theory with heat curing was investigated to exhibit compressive strength of more than 200 MPa with great ductility. Including above mentioned, this chapter describes recent developments and applications of high-strength concrete in Japan

36.3 TECHNOLOGY DEVELOPMENT A lot of technology developments have been done to realize the high strength concrete in terms of material, mix proportion and construction method. High-strength concrete shows a small yield stress by the inclusion of the chemical admixture, but it shows a high plastic viscosity due to a low water-cement (binder) ratio in terms of rheological aspect. This high plastic viscosity, generally, makes it difficult to achieve the easy placement. A new type of polycarboxylate based air-entraining and high-range water-reducing admixture has been developed. The admixture imparts the high-strength concrete with high deformability and reduces its plastic viscosity in the period immediately after initial mixing until placement, even when the water-binder ratio is 20% or below Silica fume is a by-product from the Ferro alloys industry. It consists of extremely fine amorphous silica particles. It must always be used to achieve the concrete with the strength of more than 80 MPa. Replacement of the cement by silica fume increases the strength of the concrete and enhances the durability of the concrete. The reasons for this interesting property may be attributed to a micro filler effect and a pozzolanic reaction of silica fume in the cement based products. The particles of silica fume are about 100 times smaller than the cement grains. A mean diameter is approximately 0.1µm. Therefore, it is said that the particle of silica fume is smaller than that of smoke of cigarette. The micro filler effect means that silica fume particles are easily introduced into the space between cement grains, thus reducing the space available for water and producing dense structure of hydration products. The pozzolanic reaction means that silica fume particles react chemically with calcium hydroxide to produce well crystallized CSH gel and to enhance durability Cement content of mix proportions of high-strength concrete is extremely higher compared with normal strength concrete. Binary cement and tertiary cement, which contain cement and

36.4

Handbook on Advanced Concrete Technology

mineral admixtures, are commonly used to prevent the thermal cracks due to heat of hydration. Ground granulated blast furnace slag and flyash are sometimes utilized as mineral admixtures. Also, low-heat or moderate-heat Portland cement containing relatively high amount of belite is often used to prevent thermal cracks and to reduce an autogenous shrinkage. These properties are desirable for the massive high-strength concrete.

Technical Challenges Generally Associated with UHSC • • • • •

Dispersibility of cement very poor at low water cement ratios Mixing time tends to be exceptionally long. The viscosity of concrete is very high, especially if polymer fibers are applied. Thixotropy of concrete tends to be high. Slump and flow of concrete eventually increase over time, bearing the risk of segregation.

Mix Design of Ultra High Strength Concrete Materials Cement: Additive: Aggregate: Admixture: Fibre:

Ordinary Portland, Low heat, Silica fume premixed Silica fume (Powder type, Cement mixed type) Andesite crushed sand and stone Hyper plasticizer for Ultra High Strength Concrete Polypropylene or polyethylene Fibre

Super Slump Retaining Concrete Often we have observed either high flow concrete which tends to segregate while vibrating or very low slump which makes concrete pumpable during concreting specially for Ready mix concrete which is coming from far off distance. We also observe many a time that site engineer hold Transit mixture till slump comes down around 120 to 140 mm to avoid segregation which results in wastage of Transit mixture Fuel (approximately around 4 lts). It is also observed many manufacturers either use additional water in transit mixture or re-dose with same admixture which has higher redar content which affects setting time. Super slump retaining Concrete technology brings benefits to all concrete producers in terms of slump retention without affecting strength development. It assures concrete required workability at the time of placing which gives total freedom to site engineer wherein he can place concrete right from mixing time up to 120 min. This also avoids in over designing concrete mix in terms of high workability considering slump loss

Self Curing Concrete Self-curing concrete is a new concept on curing which has emerged within the last 5 years where an internal polymer based curing admixture can be used, eliminating the external curing

Current and Expected Future Advances in Concrete

0

30

60

90

36.5

120 min

Workability

Super slump retention concrete

Transportation time [min]

Product Name: Glenium 140 Suretec manufactured by BASF, dosage at 0.8% procedure. In order to investigate the feasibility of this method and to develop sufficient technical background in this area, a research project has been carried out at the University of New South Wales. Results for different curing methods including water based hydrocarbon resin curing membrane and an Internal Curing Admixture (ICA) with high solids content (64%) are presented in this paper. Mixes with several different binder types namely, OPC, OPC/ Fly ash and OPC/Slag were investigated in the laboratory and in the field. With ICA, a more dense microstructure was observed. Reduced porosity, permeability, water absorption and improved compressive strength developments were also recorded. ICA performed similar to a good quality curing membrane. Recent comparative field trials on typical concrete slabs cured by ICA and other methods confirmed the results obtained in the laboratory investigation. The object of curing is to keep concrete saturated or as nearly saturated as possible, until the water filled spaces in the fresh cement paste have been substantially reduced by the products of hydration of the cement . For sufficient hydration to take place, the humidity in the pores need to be maintained above 80% and below this level, the degree of hydration can be affected significantly. The purpose of curing is mainly to ensure low permeability and as a result better durability of a structure. Curing also improves compressive strength, flexural strength, abrasion resistance, reduces porosity, long-term shrinkage, plastic cracking and enhances resistance to reinforcement corrosion by improving the quality of the concrete in the cover zone

36.6

Handbook on Advanced Concrete Technology

Concrete curing practices have changed through the years and in many cases, the shift has been from “external water-adding” to “external water-retaining” techniques. Water adding techniques include immersion or ponding, water mist or spray, and wet hessian or burlap. Some water retaining techniques are plastic sheeting, delayed removal of form work, and curing membranes. More recently, advancement in self-curing concrete has emerged as a potential alternative to traditional ‘external curing methods’ Self-curing is an ‘internal curing system’ where a water soluble polymer is added to the concrete mix. This method overcomes the difficulty in ensuring that effective curing procedures are employed by the construction personnel as the internal curing admixture is a component of the mix. The mechanism of self-curing can be explained as follows: Continuous evaporation of moisture takes place from an exposed surface due to the difference in chemical potentials (free energy) between the vapour and liquid phases. The polymers added in the mix, mainly form hydrogen bonds with water molecules, and reduce the chemical potential of the molecules which in turn reduces the vapour pressure. Physical moisture retention also occurs. This reduces the rate of evaporation from the surface. In a previous research on internal curing admixtures, it was established that some polymers with molecular weights less than 5000, enhanced the degree of hydration of the cements in concrete mixes significantly. The drying shrinkage, up to 54 days, of concrete cured using internal admixtures was identical to externally cured specimens using membranes. Water absorption of internally cured specimens was considerably lower than those cured externally with membranes. Bonding characteristics (with the substrate) of fresh concrete, cured using internal admixtures, were superior to those cured externally At lower dosages of internal curing admixtures, improvement in strength and permeability characteristics have been achieved Study conducted on following (i) To assess the effect of internal curing admixtures on the properties of concrete made of Ordinary Portland Cement (OPC), fly ash and slag binders, (ii) To evaluate membrane method of curing and internal curing in terms of their ability to reduce evaporation of water from concrete surfaces under simulated environmental conditions, (iii) To relate the properties of self-cured concrete to those cured by traditional methods and to assess the efficiency of these internal curing materials and (iv) To transfer the internal curing techniques to field applications. Conclusions drawn from the results available to date are as follows: • At a dosage of 5 l/m3, the strength developments of slabs with the Internal Curing Admixture are similar to those cured using a water based hydrocarbon resin membrane. This result was verified both in the laboratory and in field trials. • Porosity and absorption values obtained with the Internal Curing Admixture are comparable to that cured with the water based hydrocarbon resin membrane both in the laboratory and in field trials. • Sorptivity of slabs cured using ICA is comparable to that of the membrane in the laboratory and marginally higher in the field tests.

Current and Expected Future Advances in Concrete

36.7

• Improved microstructure of the matrix is evident with the internal curing admixture when compared with no curing. • ICA can significantly reduce plastic shrinkage cracking of slabs compared with no curing, and is comparable to a good quality curing membrane for the control of plastic shrinkage. Rheomac 730 is an internationally known brand and dosage would be around 1.5% by weight of cement. Internal curing admixtures provide a suitable means of ensuring adequate curing and this method may eliminate the need for external curing procedures. Long haul concrete Long haul concrete is a unique system developed out of chemical products that is able to control the hydration of cement up to 72 hours without loss of concrete quality. The system consists of two products, a Stabilizer to halt the hydration, and an Activator to restart the hydration process when required. • Stabilizer is unlike conventional chemical retarders that do not completely stop the hydration process, as it acts on all mineral components of the cement, gypsum, PFA, slag and silica fume providing a long lasting effect. It maintains the freshness of newly mixed concrete until activated. • The duration of stabilizing activity depends on the dose. The stabilizing effect will wear off naturally if no Activator is added. Increasing the dosage of Activator-reduces the setting time and accelerates the hardening process. Key Benefits: • Used to stabilize ready-mixed concrete, Stabilizer provides the following benefits: • Controls/retains the slump of fresh concrete over extended hauling times. • Controls/reduces concrete temperature rise resulting from hydration over extended hauling times. • Eliminates the need for portable batch plants. • Ready-mix concrete covered the long distances. • Reduces or eliminates the costs associated with ice used to reduce concrete temperature on projects where a maximum concrete temperature is specified or re dosing of admixture Application in RMC/Site batched concrete: • Can be used for developing concrete mix with >3 hours retention specially for Tunnnel lining where transit mixer has to stand more than 10 hrs before shot creting. • Can be used to extend the life of regular concrete mix in emergency. (It can be added at any time before the concrete mix has started initial set). • Can be used to keep alive the returned material and reuse at same grade or one grade lower.

36.8

Handbook on Advanced Concrete Technology

• Can be used to clean the transit mixer at the end of the day by mixing small quantity with water. • Can be added to the transit mixer post delivery of concrete, will keep the material retained in mixer live and no washing required while waiting for next load. • Saving the trouble of throwing unused concrete and recycling of wash water helps in preventing Environmental damage too. • Can be used wherever traffic restriction are enforced for movement of transit mixers (Metro constructions), where contractor can stock total days requirements.

How to Use • when the concrete is designed for 01 hour but while in transit, the pot life needs to be extended by another 03 hours• From the batch card, identify the mix design and admixtures used. • Before stabilizing the concrete, immediately measure and record the concrete temperature. • Determine the total amount of Portland cement per cubic metre corresponding to the mix design of the fresh concrete. • Determine the total Stabilizer dosage in kg per cubic meter i.e. 0.19% wt. of binder/100kg/hour of retention (that means for 320kg mix, required 1.82kg/cu.m/for 03 hrs retention). • Dispense into the concrete of at least 120mm slump and mix for 7 to 9 minutes at normal mixing speed. (If slump is less, then first add small amount of admixture to bring it to 120mm). • Ensure that required quantity is mixed thoroughly in ready mix truck during transit, Transit mixer must be turning as slowly as possible or stop the rotation, if it is rotated the film around the cement particle will be broken and initiates, which would result in loss of slump much earlier than designed. Reactivating the mix: • After 03 hours the effect will start wearing automatically. • The setting time may be increased by 5-6 hours, but ultimate 28 days strength does not change. • If the delayed setting times are acceptable, then Activator- need not be used. • If it is unacceptable, 0.75%/100kg will suffice the need. • Dispense the required amount of Activator into the stabilised concrete and mix for 7 to 9 minutes at normal mixing speed What you may need to know more: • It does not reduce the strength over control mix. • Complies with ASTM C-494-86, Type B and D. • The stabilized mix can be activated at any time by using the Activator.

Current and Expected Future Advances in Concrete

36.9

Accelerated Hardening Concrete The construction industry in general and concrete producers in particular are committed to successfully addressing the four main challenges for a sustainable construction industry, efficient processes energy reduction, material optimization, high Performance specs. In today’s competitive environment, especially during periods of weak macroeconomics, the industry’s major concern is to balance this commitment with the best possible cost effectiveness. Therefore, accelerated concrete hardening at early ages is a target for saving time and money. High early strength development is of key importance for precast producers. In a capital intensive industrialized production process with high quality control, the critical economic factors are fast re-utilization of formworks and a high, continuous output with the lowest possible production effort. The need for fast strength development makes the best suitable mix design a pre-requisite. To meet all these partially contradicting requirements, a new solution is offered to the industry by the Crystal Speed Hardening concept developed with a concept based on a unique technology of concrete hardening acceleration. The Crystal Speed Hardening concept enables more sustainable concrete production. Thanks to the unique technology of the new hardening accelerator. A previously unattainable hardening boost at all temperature levels is achieved in the early ages (6-12 hrs) due to the powerful seeding of the mix design, which is also able to support the final performance of the specified concrete. The Crystal Speed Hardening concept exceeds and strengthens all existing solutions for key industry needs. It allows the concrete industry to achieve better process economy, higher concrete quality and increased energy efficiency in order to save time and money and reduce CO2 emissions. X-SEED is the essential component of the Crystal Speed Hardening concept. It consists of synthetically produced nano particles suspended in a liquid and boosts the hardening properties of the concrete mix. While traditional acceleration methods such as heat application or common accelerators typically affect concrete’s cost It allows the concrete industry to achieve better process economy, higher concrete quality and increased energy efficiency in order to save time and money and reduce CO2 emissions. The essential component of the Crystal Speed Hardening concept is, it consists of synthetically produced nano particles suspended in a liquid and boosts the hardening properties of the concrete mix. While traditional acceleration methods such as heat application or common accelerators typically affect concrete’s cost and durability, X-SEED brings a level of hardening acceleration to the mix that supports natural hydration and long-term performance properties by offering attractive overall cost saving potential.

Mechanism of Action In standard cement hydration, the main clinker phases C3S and C2S react with water to form Calcium Silicate Hydrate (CSH) crystals and Calcium Hydroxide. The nucleation of the CSH crystals is an exothermic reaction, which occurs on the cement grain surface and requires overcoming some activation barriers for further growth

36.10

Handbook on Advanced Concrete Technology

With This new system, it is possible to suspend extra fine, synthetically produced CSH crystals in a ready to use admixture suspension and use them as seeding material in the pore solution between the cement grains. The active CSH crystals can virtually grow without energy barrier. This method is known as crystal seeding. It was found that the crystals show preferential growth behavior in between and not on the surface of the cement grains. Therefore the growth of the crystal structure is far quicker and earlier hardenings as well as an earlier strength development are observed.

Conclusions Crystal Speed Hardening is designed to support the concrete industry in meeting key sustainable construction targets: efficient processes energy reduction material optimization high quality specifications. The centerpiece of this concept is X-SEED, a unique hardening accelerator which enables a quantum leap in high early strength development at low, ambient and heat curing temperatures. Crystal Speed Hardening benefits from the principle of Crystal Seeding in all types of cementitious materials. The need for speed is satisfied by the excellent performance of X-SEED. Crystal Speed Hardening adds values to concrete by exploring various options of hardening acceleration and has the potential to move the market up to the next level of advanced construction practice.

36.4 ULTRA THIN WHITE TOPPINGS FOR ROADS Background Indian roads have experienced 8 fold increases in terms of Length from 1951 to 2000, from 4 L km to 33 L km where number of vehicles from 3 L to 500 L. 80 - 90% of National and State Highways are not structurally adequate for permissible axle load of 10.2 tonnes and Maintenance of existing roads has not reached even 50-55%. About 19,250 km of NH require Strengthening - Cost about 14,450 Cr and would take about 10 to 15 years. India loses about 20,000 - 30,000 Cr every year due to surface roughness and Vehicle operating costs Need of the hour – Strengthen existing roads with an OVERLAY of suitable thickness and Specification, with Increased Strength and Life time

36.5 TYPES OF OVERLAY • Flexible Overlay (Bituminous) • Cement Concrete Overlay Practice in INDIA so far (Bituminous) • Abundant Supply of Bitumen • Resource Constraint (Stage Wise Construction) • Traffic Volume and Loads were Manageable

Current and Expected Future Advances in Concrete

Shortcomings of Flexible Overlay • The Design life is hardly 10-15 years (Actually 5 yrs) • Weak Sub-grade, leading to settlement and deformations etc • Uncertain future of Bitumen Supply • Rising Cost of Bitumen • Durability Cement Concrete Overlays

36.6

ADVANTAGES • • • • • • •

Practically Maintenance-free long life (Durability) Good Riding quality Hard Surface No effect of spillage of oil Cost Economics Fuel savings by trucks Practically impervious to Water and other harmful chemicals

Cement Concrete Overlays (Commonly known as “White Topping”) Thickness Range Conventional White Topping: 200 - 300 mm Thin White Topping: 125 - 200 mm Ultra-Thin White Topping: 50 - 125 mm

36.7

FEATURES • Smaller Dimension of Panels • High Strength Concrete • Use of Fibers Thin slabs (50 to 100 mm) Short square slabs (0.6 to 1.8 m)

Milled surface

Existing HMA

pavement

36.11

36.12

Handbook on Advanced Concrete Technology

Mechanism of Load Transfer

Current Situation • Ultra Thin White Topping has been approved and included in Indian Roads Congress Codes (IRC 76) • Number of experimental patch works conducted at Mumbai By GBMC, in Thane by TMC, in Pune by PMC and At Central Road Research Institute Delhi • This concept successfully accepted by other States, viz Karnataka and Goa.

Conclusion • Number of New technologies that are discussed above will be only successful if they are cost effective. Another important aspect, which is key to success is long durability characteristics or sustainability. • To assure cost effectiveness, life cycle cost is to be considered and not the initial cost.

37 Summary of Codal Provision for Concrete and Cementitious Materials A.K. Jain and Jayant Kulkarni

37.1 INTRODUCTION A summary of various codal provisions is compiled relating to concrete making materials, its production, testing in fresh and hardened state, acceptance criterion and durability characteristics for the ease of field engineers and practitioners. The summary mainly includes those provisions which are frequently used for in production and quality control of concrete. There are many other provisions in the codes, which are mostly specific to a particular situation, for that it is recommended to refer to the relevant code.

37.2

CONCRETE MAKING MATERIALS

Codal provision for cement, water, aggregates, cementitious materials (fly ash, ggbs, silica fume, matakaoline), and chemical admixtures are included in this section;

37.2.1

Cement

In India all types of cement to be produced, sold and used in construction work have compulsorily to conform to a standard issued by Bureau of Indian Standards (BIS). No Cement in India can be produced and sold without BIS certification. BIS has issued Standards for different types of cement as per Table 37.1

37.2.2 Water IS: 456 2000 (clause 5.4), governs the quality of water to be used for production and curing of concrete. The permissible limits of solids are given in Table 37.4

37.2

Handbook on Advanced Concrete Technology

TABLE 37.1 Indian Standards on Cement Name of Cement standard

IS No.

Ordinary Portland Cement – 33 Grade Ordinary Portland Cement • 43 Grade • 53 Grade • 43-S Grade • 53-S Grade Portland Pozzolana Cement – Part – (I) Fly ash based Portland Pozzolana Cement – Part – (II) Calcined clay based Portland Blast Furnace Slag Cement High Alumina Cement for structural use Rapid Hardening Portland Cement Oil Well Cement Sulphate Resisting Portland Cement Low Heat Portland Cement White Portland Cement Super Sulphated Cement Hydrophobic Cement

IS: 269 – 1989 IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS: IS:

Masonry Cement

IS: 3466 – 1988

8112 – 1989 12269 – 1987 8112 – 1989 (as amended) 12269 – 1987 (as amended) 1489 (Part I) – 1991 1489 (part II) – 1991 455 – 1989 6452 – 1989 8041 – 1990 8229 – 1986 12330 – 1988 12600 – 1989 8042 – 1989 6909 – 1990 8043 – 1991

The physical and chemical characteristics of these cement are given at table – 2 and table -3 respectively.

TABLE 37.2 Physical Characteristics of Various Types of Cement SI. Types of Cement No

IS Number

Fineness m2/kg (Min)

Setting time, Minutes

Initial (Min)

Final (Max)

Soundness

Compressive Strength,MPa

Le Autoclave, 3 Days Chatelier % mm

7 Days

28 days

1

OPC 33 Grade

269:1989

225

30

600

10

0.8

16

22

33

2

OPC 43 Grade

8112:1989

225

30

600

10

0.8

23

33

43

3

OPC 53 Grade

12269:1987

225

30

600

10

0.8

27

37

53

4

PPC (Fly ash based)

1489 (Part I): 1991

300

30

600

10

0.8

16

22

33

5

PSC (Slag Cement)

455: 1989

225

30

600

10

0.8

16

22

33

6

SRC (Sulphate Resisting Cement)

12330:1988

225

30

600

10

0.8

10

16

33

7

Rapid Hardening Cement

8041:1990

325

30

600

10

0.8

27

–

–

8

Rly Sleeper Class Cement (43-S/53-S)

370

60

600

5

0.8

–

37.5

–

9

Low Heat Portland Cement

320

60

600

10

0.8

10

16

35

12600:1989

10 Masonry Cement

3466:1989

*

90

1440

10

1

NS

2.5

5

11 High Alumina Cement

6452:1989

225

30

600

5

NS

35

NS

NS

Contd...

Summary of Codal Provision for Concrete and Cementitious Materials

37.3

Contd... 12 Super Sulphated Cement

6909:1990

400

30

600

5

NS

15

22

30

13 White Cement

8042:1989

225

30

600

10

0.8

14.4

19.8

29.7

{(i) The drying shrinkage for PPC 0.15% (max). (ii) In the case of low heat cement, the heat of hydration for 7days 272 kj/kg (max) and for 28 days 314 kj/kg}(max) NS – Not Specified, * Residue on 45 – micron IS Sieve, Max percent (by wet sieving) is 15.

TABLE 37.3 Chemical Characteristics of Various Types of Cement SI. Type of Cement No.

Lime Saturation Factor, %

Alumina Iron Ratio % Min

Insoluble Residue % Max

Magnesia % Max

Sulphuric Anhydride %

Loss on Ignition % max

1

OPC 33 Grade (IS 269 – 1989)

0.66 Min 1.02 Max

0.66

4#

6

2.5 Max when C3A5

5

2

OPC 43 Grade (IS 8112 – 1989)

0.66 Min 1.02 Max

0.66

3#

6

2.5 Max when C3A5

5

3

OPC 53 Grade (IS 12269 – 1987)

0.8 Min 1.02 Max

0.66

3#

6

2.5 Max when C3A5

4

4

Sulphate Resisting Cement (IS 12330 – 1988)

0.66 Min 1.02 Max

NS

4

6

2.5 Max

5

5

Portland Pozzolana Cement (IS 1489- 1991) Part – 1@

NS

NS

4 (100 – X) X + _________ 100

6

3 Max

5

6

Rapid Hardening Cement (IS 8041 – 1990)

0.66 Min 1.02 Max

0.66

4

6

2.5 Max when C3A5

5

7

Slag Cement (IS 455- 1989)*

NS

NS

4

10

3 Max

5

8

High Alumina Cement (IS 6452 – 1989)

NS

NS

NS

NS

NS

NS

9

Super Sulphated Cement (IS 6909- 1990)

NS

NS

4

10

6 Min

NS

NS

0.66

4

6

2.5 Max when C3A5

5

0.8 Min 1.02 Max

0.66

2

5

3.5 Max

4

10 Low Heat Cement (IS 12600-1989) 11 43-S/53-S

X-Declared percentage of fly ash, NS – Not Specified, * The Slag should not be less than 25% and not more than 70% @ The fly ash should not be less than 15% and not more than 35% # When performance improver not added. When P.I. added max 5%

TABLE 37.4 Permissible Limit for Solids SI. No.

Tested as per

(i) (ii) (iii)

Organic Inorganic Sulphates (as SO3)

(iv) (v)

Permissible limit, Max

IS 3025 (Part 18) IS 3025 (Part 18)

200mg/l 3000 mg/l

Chlorides (as Cl)

IS 3025 (Part 24) IS 3025 (Part 32)

400 mg/l 2000 mg/l for concrete not containing embedded steel and 500 mg/l for reinforced concrete work

Suspended matter

IS 3025 (Part 17)

2000 mg/l

37.4

Handbook on Advanced Concrete Technology

37.2.3 Coarse and Fine Aggregate The quality for coarse and fine aggregates from Natural sources for concrete are governed by IS 383-1970. The major provisions are as under;

37.2.3.1 Deleterious Materials The limits of deleterious materials for coarse and fine aggregates are given in Table 37.5. TABLE 37.5 Limits of Deleterious Materials SI. No.

Deleterious Substance

(1)

Method of Test

(2)

(3)

Fine Aggregate percentage Coarse aggregate percentage by Weight, Max by Weight, Max Uncrushed Crushed Uncrushed Crushed (4) (5) (6) (7)

(i)

Coal and lignite

IS:2386 (Part II) 1963 do IS:2386 (Part I) 1963

1.00

1.00

1.00

1.00

(ii) (iii)

Clay Lumps Materials tiner than 75-m IS Sieve

1.00 3.00

1.00 15.00

1.00 3.00

1.00 3.00

(iv)

Soft Fragments

IS:2386 (Part III) 1963

–

3.00

–

(v)

Shale

do

1.00

–

–

–

(vi)

Total of percentages of all – deleterious materials (except mica) including SI. No. (i) to (v) for col 4, 6 and 7 and SI. No. (i) and (ii) for col 5 only.

5.00

2.00

5.00

5.00

–

Note (i) The presence of mica in the fine aggregate has been found to reduce considerably the durability and compressive strength of concrete and further investigations are underway to determine the extent of the deleterious effect of mica. It is advisable, therefore, to investigate the mica content of fine aggregate and make suitable allowances for the possible reduction in the strength of concrete or mortar. (ii) The aggregate shall not contain harmful organic impurities [tested accordance with IS: 2386 (Part II)-1963] in sufficient quantities to affect adversely the strength or durability of concrete. A fine aggregate which fails in the test for the effect of organic impurities may be used, provided that, when tested for the effect of organic impurities on the strength of mortar, the relative strength at 7 and 28 days, reported in accordance with clause 7 of IS: 2386 (Part VI )-l963 is not less than 95 percent.

37.2.3.2 Grading of Coarse Aggregate The coarse aggregates should be supplied in the nominal sizes given in Table 37.6.

37.2.3.3 Grading of Fine Aggregates The grading of fine aggregates shall be within the limits given in Table 37.7

37.2.3.4 Grading of All-in-Aggregates If combined aggregates are available they need not be separated into fine and coarse, but necessary adjustments may be made in the grading by the addition of single-sized aggregates. The grading of all-in-aggregates is given in Table 8

Summary of Codal Provision for Concrete and Cementitious Materials

37.5

TABLE 37.6 Coarse Aggregate IS Sieve Designation

Percentage Passing for Single-sized Aggregate of Nominal Size 63mm 40mm 20mm 16mm 12.5mm 10mm (2) (3) (4) (5) (6) (7)

(1)

Percentag Passing for Graded Aggregate of Nominal Size 40mm 20mm 16mm 12.5mm (8) (9) (10) (11)

80mm

100

–

–

–

–

–

100

–

–

–

63mm

85 to 100

100

–

–

–

–

–

–

–

–

40mm

0 to 30 85 to 100

100

–

–

–

95 to 100

100

–

–

20mm

0 to 5

0 to 20

85 to 100

100

–

–

30 to 70

95 to 100

100

100

16mm

–

–

–

85 to 100

100

–

–

–

90 to 100 –

12.5mm

–

–

–

–

85 to 100

100

–

–

–

90 to 100

0 to 5

0 to 5

0 to 20

0 to 30

0 to 45

85 to 100

10 to 35

25 to 55

30 to 70

40 to 85

4.75mm

–

–

0 to 5

0 to 5

0 to 10

0 to 20

0 to 5

0 to 10

0to 10

0 to 10

2.36mm

–

–

–

–

–

0 to 5

–

–

–

–

10mm

TABLE 37.7 Fine Aggregates Grading IS Sieve Designation Grading Zone I

Percentage Passing for Grading Zone II Grading Zone III

Grading Zone IV

10 mm

100

100

100

100

4.75mm

90-100

90-100

90-100

95-100

2.36mm

60-95

75-100

85-100

95-100

1.18mm

30-70

55-90

75-100

90-100

600 micron

15-34

35-59

60-79

80-100

300 micron

5-20

8-30

12-40

15-50

150 micron

0-10

0-10

0-10

0-15

Note 1

For crushed stone sands, the permissible limit on 150-micron IS Sieve is increased to 20 percent. This does not affect the 5 percent allowance permitted in 4.3 applying to other sieve sizes.

Note 2

Fine aggregate complying with the requirements of any grading zone in this table is suitable for concrete but the quality of concrete produced will depend upon a number of factors including proportions.

Note 3 Where concrete of high strength and good durability is required, fine aggregate conforming to any of the four grading zones may be used, but the concrete mix should be properly designed. As the fine aggregate grading becomes progressively finer, that is from Grading Zones I to IV, the ratio of fine aggregate to coarse aggregate should be progressively reduced. The most suitable fine to coarse ratio to be used for any particular mix will however, depend upon the actual grading, particle shape and surface texture of both fine and coarse aggregates. Note 4

It is recommended that fine aggregate confirming to Grading Zone IV should not be used in reinforced concrete unless tests have been made to ascertain the suitability of proposed mix proportions.

37.6

Handbook on Advanced Concrete Technology

TABLE 37.8 All-in-aggregate Grading IS Sieve Designation

Percentage Passing for All-in-aggregates of 40 mm Nominal Size 20mm Nominal Size

80 mm 40 mm 20 mm 4.75 mm 600 micron

100 95 to 100 45 to 75 25 to 45 8 to 30

100 95 to 100 30 to 50 10 to 35

150 micron

0 to 6

0 to 6

37.2.4

Cementitious Materials

Cementitious materials like fly ash, ggbs, silica fume, metakaoline and rice husk ash have become integral part of modern concrete. In India, BIS has formulated codes for fly ash and silica fume. The draft code for metokaoline is under circulation while there are no codes for ggbs and rice husk ash. Though there is a code for granulated slag (IS 12089-1987) but it is for manufacture of Portland Slag Cement. The quality of ggbs to be used directly in concrete is still governed by British Standard, B.S. 6699-1992. Major provisions of these codes are described as under;

37.2.4.1 Fly Ash Specifications of fly ash are governed by IS 3812 (Part 1): 2003. Part 1 of the code 3812 includes specifications of fly ash for use as pozzolana in cement, cement mortar and concrete. The chemical and physical requirements of fly ash are given in Table 9 and Table 10 respectively. TABLE 37.9 Chemical Requirements SI. No.

Characteristic

(1)

(2)

(i)

Silicon dioxide (SiO2) plus aluminum oxide (Al2O3) plus iron oxide (Fe2O3) in percent by mass, Min Silicon dioxide (SiO2) in percent by mass, Min (1)Reactive silica in percent by mass, Min Magnesium oxide (MgO) in percent by mass, Max Total Sulphur as sulphur trioxide (SO3) percent by mass, Max Available alkalies as sodium oxide (Na2O) in percent by mass, Max Total Chlorides in percent by mass, Max Loss on ignition in percent by mass, Max

Requirements Silicious Calcareous Pulverized Pulverized Fuel Ash Fuel Ash (3) (4)

Method of Test Ref. No. Annex IS No.

(5)

(6)

70

50

–

IS 1727

35 20 5.0 3.0

25 20 5.0 3.0

– B – –

IS 1727 – IS 1727 IS 1727

1.5

1.5

–

IS 4032

0.05

0.05

–

IS 124232

5.0

5.0

–

IS 1727

1. Optional test 2. For the purpose of this test, wherever reference to cement has been made, it may be read as pulverized fuel ash for annex ‘B’ refer to IS 3812 (Part 1): 2003

Summary of Codal Provision for Concrete and Cementitious Materials

37.7

TABLE 37.10 Physical Requirements of fly ash SI. No. (1)

Characteristic (2)

(i) (ii) (iii) (iv)

(v) NOTE

Requirements (3)

Fineness-specific surface in m2/kg by Blaine’s permeability method, Min (1) Particles retained on 45 micron IS Sieve (wet Sieving) in percent, Max Lime reactivity – Average compressive strength in N/mm2, Min Compressive strength at 28 days in N/mm2, Min

320 34 4.5 Not less than 80 percent of the strength of corresponding plain cement mortar cubes

Soundness by autoclave test-Expansion of Specimen in percent, Max

0.8

Fly ash of fineness 250m2/kg(Min) is also permitted to be used in the manufacture of Portland pozzolana cement by intergrinding it with Portland cement clinker if the fly ash when ground to fineness of 320 m2/kg or to the fineness of the resultant Portland pozzolana cement whichever is lower, meets all the requirements specified in table 9 and 10 as above. (1)

Optional test

37.2.4.2 Ground Granulated Blast Furnace Slag (ggbs) The specifications of ggbs for use with Portland cement are given in BS 6699-1992. The physical and chemical requirements are compiled as under; TABLE 37.11 Physical Requirements SI. No.

Characteristic

Requirement

1

General

ggbs shall be obtained by quenching molten iron blast furnace slag in water or steam, to produce a glassy granular product which is then dried and ground to a fine powder.

2 3 4 5 6

Fineness Glass content Initial Setting Time Soundness Compressive strength

Not less than 275m2/kg Not less than 67% Not less than the setting time of Portland cement Not more than 10mm A combination of 30% of Portland cement of class 42.5 and 70% ggbs when intimately mixed together and tested as per EN 196: Part 1 shall have compressive strength; (a) 7 days: Not less than 12N/mm2 (b) At 28 days: Not less than 32.5 N/mm2

TABLE 37.12 Chemical Requirement SI. No.

Characteristic

Requirements

1 2 3 4 5 6

Insoluble Residues Magnesia Sulphur loss on Ignition Manganese Chlorides

Not Not Not Not Not Not

7

CaO + MgO Chemical Modulus ____________ SiO2

shall be not less than 1.0

more more more more More more

than than than than than than

1.5% 14% 2.5% as SO3 3% 2% 0.1%

37.8

Handbook on Advanced Concrete Technology

37.2.4.3

Silica Fume

The Physical and chemical requirements of silica fume are specified in IS 15388: 2003 TABLE 37.13 Chemical Requirements SI. No. (1)

Characteristic (2)

(i) (ii) (iii)

SiO2, percent by mass, Min Moisture content, percent by mass, Max Loss on ignition, percent by mass, Max

(iv)

Alkalies as Na2O, Percent, Max

Requirements (3)

Test Method (4)

85.0 3.0 4.0

IS 1727 see Note 1 IS 1727

1.5

See Notes 2 and 3

Notes 1. For determination of moisture content, dry a weighed sample as received to constant mass in an oven at 105°C to 110°C. Express in percentage, the loss in mass and record as moisture content. 2. Requirement of limiting alkalies shall be applicable in case silica fume is to be used in concrete containing reactive aggregate. 3. For determination of alkalies, method of test used for determination of this in cement may be adopted.

TABLE 37.14 Physical Requirements SI. No.

Characteristic

Requirement

Method of test, Ref to Annex IS No. (4) (5)

(1)

(2)

(3)

(i) (ii)

2

Specific surface m /g, Min (see Note1) Oversize Percent retained on 45 micron IS sieve, Max (see Note 1)

15 10

A of IS 15388 –

– 1727

(iii)

Oversize percent retained on 45 micron IS Sieve, variation from average percent, Max (see Notes 1 and 2)

5

–

1727

(iv)

Compressive strength at 7 days as percent of control sample, Min (see Note 3)

85.0

–

1727

Notes 1. Any one of the tests specified in (i) or (iii) indicated may be adopted. 2. For (iii) the average shall consist of the ten preceding tests or all of the preceding tests if the number is less than ten. 3. In the test method for determination of compressive strength of silica fume cement mortar in accordance with IS 1727, the value of factor N may be taken as one.

37.2.4.4

Metakaoline

The draft code for metakaoline has been circulated by BIS in May’ 2009. Physical and chemical characteristic requirements in draft code are as under; TABLE 37.15 Physical Requirements SI. No.

Characteristic

(i) (ii) (iii)

Specific surface m2/kg, Min Oversize Percent retained on 45 micron IS sieve, (wet sieving) Max Oversize percent retained on 45 micron IS Sieve, variation from average percent,

(iv)

Compressive strength at 7 days as percent of control sample, MPa, Min

Requirements 9000 1.5 1.0 85.0

Summary of Codal Provision for Concrete and Cementitious Materials

37.9

TABLE 37.16 Chemical Requirements SI. No.

Characteristic

Requirements

(i) (ii) (iii)

SiO2 + Al2O3 + Fe2O3 percent by mass, Min Moisture content, percent by mass, Max Loss on ignition, percent by mass, Max

(iv)

Alkalies as Na2O, Percent, Max

94.0 3.0 2.0 1.5

37.3 CONCRETE MIX PROPORTIONING Concrete Mix proportioning is mainly governed by IS 10262: 2009 and IS 456: 2000. The major provision in these codes relating to concrete mix Proportioning are given below;

37.3.1

Assumed Standard Deviation

Where sufficient test results are not available, the value of standard deviation given in Table 37.17 may be assumed. TABLE 37.17 Assumed Standard Deviation SI.No. (1)

Grade of Concrete (2)

(i) (ii) (iii) (iv) (v) (vi) (vii) (viii)

M M M M M M M M

(ix) (x) (xi)

M – 50 M – 55 M – 60

Note

– – – – – – – –

10 15 20 25 30 35 40 45

Assumed Standard Deviation N/mm2 (3) 3.5 4.0

5.0

The above values correspond to the site control having proper storage of cement; weigh batching of all materials; controlled addition of water; regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength. Where there is deviation from the above, values given in the above table shall be increased by 1 N/mm2.

37.3.2 Selection of Water Content The quantity of maximum mixing water per unit volume of concrete may be determined from Table 37.18. The water content in Table 18 is for angular coarse aggregate and for 25-50mm slump range. It can be reduced by approximately 10kg for sub-angular aggregates and 25kg for rounded gravel for same workability. For higher slump, the required water content may be established by trial or an increase by about 3% for every 25mm slump or alternatively by use of chemical admixture conforming to IS 9103.

37.10

Handbook on Advanced Concrete Technology

TABLE 37.18 Maximum Water Content per Cubic Metre of Concrete for Nominal Maximum Size of Aggregate

(1)

Nominal Maximum Size of Aggregate mm (2)

Maximum Water Content(1) kg (3)

(i) (ii)

10 20

208 186

(iii)

40

165

SI. No.

Note

These quantities of mixing water are used in computing cementitious material contents for trial batches. (1)

37.3.3

water content corresponding to saturated surface dry aggregate

Estimation of Coarse Aggregate Proportion

Approximate values for aggregate volume are given in Table 37.19 for water cement ratio of 0.5, which may be suitably adjusted for other water cement ratios. TABLE 37.19 Volume of Coarse Aggregate per unit Volume of Total Aggregate for Different Zones of Fine Aggregate Volume of Coarse Aggregate(1) per unit Volume of Total Aggregate for Different Zones of Fine Aggregate Zone IV Zone III Zone II Zone I (3) (4) (5) (6)

SI. No.

Nominal Maximum Size of Aggregate

(1)

mm (2)

(i) (ii)

10 20

0.50 0.66

0.48 0.64

0.46 0.62

0.44 0.60

(iii)

40

0.75

0.73

0.71

0.69

(1)

Volumes are based on aggregates in saturated surface dry condition.

37.3.4 Nominal Mix Concrete Nominal mix concrete may be used for concrete of M-20 or lower. The proportioning of materials for nominal mix concrete shall be as per Table 37.20.

37.4 BATCHING AND MIXING OF CONCRETE In batching concrete, the quantity of both cement and aggregate shall be determined by mass: admixture, if solid by mass: liquid admixture may however be measured in volume or mass: water shall be weighed or measured by volume. The grading of coarse and fine aggregate should be checked as frequently as possible.

37.4.1 Tolerance Limits for Measurement of Constituents (a)

Cement and Cementitious materials

+ 2 Percent

(b)

Coarse and fine aggregates

+ 3 Percent

(c)

Chemical Admixtures and Water

+ 3 Percent

(Note – The figures are percent of the quantity of the constituent being measured)

Summary of Codal Provision for Concrete and Cementitious Materials

37.11

TABLE 37.20 Proportions of Nominal Mix Concrete (Clauses 9.3 and 3.3.1 IS 456: 2000) Grade of Concrete

(1) M M M M M

– – – – –

Total Quantity of Dry Aggregates by mass per 50kg of cement, to be taken as the sum of the individual masses of fine and Coarse aggregates, kg, Max (2)

5 7.5 10 15 20

800 625 480 330 250

Proportion of Fine Aggregate to Coarse Aggregate (by mass)

Quantity of water per 50kg of cement, Max

(3)

(4)

Generally 1:2 but subject to an upper limit of 1:11/2 and a lower limit of 1:21/2

60 45 34 32 30

Note 1 The proportioning of the fine to coarse aggregate should be adjusted from upper limit to lower limit progressively as the grading of fine aggregates becomes finer and the maximum size of coarse aggregate becomes larger. Graded coarse aggregate shall be used. Example For an average grading of fine aggregate (that is Zone II of Table 4 of IS 383). The proportions shall be 1:11/2, 1:2 and 1:21/2 for maximum size of aggregates 10mm, 20mm and 40mm respectively. 2. Quantity of water required from durability point of view may be less than the value given above.

37.4.2 Water Cement Ratio It is important to maintain the water cement ratio constant at its correct value. Determination of moisture contents of the both fine and coarse aggregates shall be done as frequently as possible. In case of nominal mixes, the amount of surface water may be estimated from the values in the Table 37.21. TABLE 37.21 Surface Water Carried by Aggregate (Clause 10.2.5 of IS 456: 2000) SI. No. (1) (i) (ii) (iii) (iv) (1)

Aggregate (2) Very wet sand Moderately wet sand Moist sand Moist gravel or crushed rock(1)

Approximate Quantity of Surface Water Percent by Mass l/m3 (3) (4) 7.5 5.0 2.5 1.25-2.5

120 80 40 20-40

Coarser the aggregate, less the water it will carry.

37.4.3

Mixing and Mixing Time

Concrete shall be mixed in a mechanical mixer. The mixers shall be fitted with water measuring devices. The mixing shall be continued until there is a uniform distribution of the materials and the mass is uniform in colour and consistency. The mixing time shall be at least 2 minute. For more efficient mixers, manufacture’s recommendations shall be followed.

37.12

Handbook on Advanced Concrete Technology

37.4.4 Workability Workability should be checked at frequent intervals. Workability requirement depends on placing conditions. Degree of workability for placing conditions is given in Table 37.22. TABLE 37.22 Requirement of Workability for Different Placing Conditions Placing Conditions (1) Blinding concrete; Shallow sections; Pavements using pavers Mass concrete; Lightly reinforced sections in slabs, beams, walls, columns; Floors; Hand placed pavements Canal lining; Strip footings Heavily reinforced sections in slabs, beams, walls, columns; Slipform work; Pumped concrete Trench fill; In-situ-piling Tremie concrete Note

Degree of Workability (2)

Slump (mm) (3)

Very low

Low

25-75

Medium

50-100

Medium

75-100

High

100-150

Very high

For most of the placing conditions, internal vibrators (needle vibrators) are suitable. The diameter of the needle shall be determined based on the density and spacing of reinforcement bars and thickness of sections. For tremie concrete, vibrators are not required to be used.

The workability shall be within the following limits on the specified value (IS 4926: 2003) Slump

+ 25mm or + 1/3 of the specified value, whichever is less.

Compacting factor

+ 0.03 where the specified value is 0.90 or greater + 0.04 where specified value is less than 0.90 and greater than 0.80 + 0.05 where the specified value is 0.80 or less

37.4.5 Form Work The form work shall be designed and constructed so as to remain sufficiently rigid during placing and compaction of concrete, and shall be such as to prevent loss of slurry from the concrete. The tolerances on the shapes, lines and dimensions shall be within the limits given in Table 37.23.

Summary of Codal Provision for Concrete and Cementitious Materials

37.13

TABLE 37.23 Tolerances in Form Work (a) Deviation from specified dimensions of cross-section of columns and beams (b) Deviation from dimensions of footings 1. Dimensions in Plan 2. Eccentricity 3. Thickness

+12 mm – 6 mm +50 mm –12 mm 0.02 times the width of the footing in the direction of deviation but not more than 50 mm + 0.05 times the specified thickness

37.4.6 Curing of Concrete Curing is process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature regime. Exposed surfaces of concrete shall be kept continuously in a damp or wet condition by ponding or by covering with a layer of sacking, canvas, hessian or similar materials and kept constantly wet for a period; (IS 456: 2000) Type of Binder

Normal Weather (days)

Dry and Hot Weather (days)

7 10

10 14

OPC based concrete Blended cement or mineral admixtures based concrete

37.4.7 Stripping Time Forms shall not be released until the concrete has achieved strength of at least twice the stress to which the concrete may be subjected at the time of removal of form work. Where ambient temperature does not fall below15°C and where OPC is used and adequate curing is done, the stripping period may be adopted as per Table 37.24 (IS: 456) TABLE 37.24 Stripping time of form Work Type of Form Work Vertical form work to columns, walls, beams Soffit form work to slabs (Props to be refixed immediately after removal of form work) Soffit formwork to beams (props to be refixed immediately after removal of form work) Props to Slab: 1. Spanning up to 4.5m 2. Spanning over 4.5m Props to beams and arches: 1. Spanning up to 6m 2. Spanning over 6m

Minimum Period Before Striking Formwork 16-24 h 3 days 7 days

7 days 14 days 14 days 21 days

37.14

Handbook on Advanced Concrete Technology

37.5

DURABILITY OF CONCRETE

A durable concrete is one that performs satisfactorily in the working environment during its anticipated exposure conditions during service life. The factors influencing durability includes; (a) The environment (b) The cover to embedded steel (c) The type and quality of constituent materials (d) The cement content and water/ cement ratio of the concrete (e) Workmanship to obtain full compaction and efficient curing and (f) The shape and size of the member The general environment to which the concrete will be exposed during its working life is classified into five levels of severity that is mild, moderate, severe, very severe and extreme as described in Table 37.25. TABLE 37.25 Environmental Exposure Conditions (Clauses 8.2.2.1 and 35.3.2) SI. No.

Environment

Exposure Conditions

(i)

Mild

Concrete surface protected against weather or aggressive conditions, except those situated in coastal area

(ii)

Moderate

Concrete surface sheltered from severe rain or freezing whilst wet Concrete exposed to condensation and rain. Concrete continuously under water Concrete in contact or buried under non aggressive soil/ground water

(iii)

Severe

Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed in sea water Concrete exposed to coastal environment

(iv)

Very severe

Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet Concrete in contact with or buried under aggressive sub-soil/ground water

(v)

Extreme

Surface of members in tidal zone Members in direct contact with liquid/solid aggressive chemicals

37.5.1 Concrete Mix Requirements The approximate values for minimum cement content and the maximum free water-cement ratio are given in Table 37.26 for different exposure conditions The minimum cement content and maximum water-cement ratio apply for 20mm nominal maximum size aggregate. For other sizes of aggregate they should be changed as per Table 37.27.

37.5.2 Requirement of Concrete Cover The protection of the steel in concrete against corrosion depends upon an adequate thickness of good quality concrete cover. The nominal cover to the reinforcement shall be provided as per Table 37.28.

Summary of Codal Provision for Concrete and Cementitious Materials

37.15

TABLE 37.26 Minimum Cement Content, Maximum Water-cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal weight aggregates of 20mm Nominal maximum Size SI. No.

Exposure

(3)

Plain Concrete Maximum Free Water-Cement ratio (4)

(i)

Mild

220

0.60

–

300

0.55

M-20

(ii)

Moderate

240

0.60

M-15

300

0.50

M-25

(iii)

Severe

250

0.50

M-20

320

0.45

M-30

(iv)

Very Severe

260

0.45

M-20

340

0.45

M-35

(v)

Extreme

280

0.40

M-25

360

0.40

M-40

Minimum Cement Content kg/m3 (1)

(2)

Minimum Grade of Concrete (5)

Minimum Cement Content kg/m3 (6)

Reinforced Concrete Maximum free water cement ratio (7)

Minimum Grade of Concrete (8)

Notes 1. Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions (mineral admixtures). The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1489 (Part 1) and IS 455 respectively. 2. Minimum grade for plain concrete under mild exposure conditions is not specified.

TABLE 37.27 Adjustments to Minimum Cement Contents for Aggregates Other Than 20mm Nominal Maximum Size SI. No.

Nominal maximum Aggregate Size

(1)

mm (2)

Adjustments to Minimum Cement contents in Table 26 (above) kg/m3 (3)

(I) (II)

10 20

+40 0

(III)

40

–30

TABLE 37.28 Nominal Cover to meet Durability Requirements Exposure

Nominal concrete Cover in mm not less than

Mild Moderate Severe Very Severe

20 30 45 50

Extreme

75

Note For main reinforcement up to 12mm diameter bar for mild exposure the nominal cover may be reduced by 5mm. Unless specified otherwise, actual concrete over should not deviate from the required nominal cover by +10mm For exposure conditions ‘severe’ and ‘very severe’, reduction of 5 mm may be made, where concrete grade is M35 and above

37.5.3 Exposure to Sulphate Attack The recommendation for the type of cement, maximum free water/cement ratio and minimum cement content, which are required at different sulphate concentrations in near neutral ground water having pH of 6 to 9 are given in Table 37.29.

37.16

Handbook on Advanced Concrete Technology

TABLE 37.29 Requirements for concrete Exposed to sulphate attack Concentration of Sulphates, Expressed as S03

SI. No.

Class

In Soil ________ Total SO3

SO3in

In Ground Water

Type of Cement

2:1 Water: Soil Extract Percent

g/l

(3)

(4)

Dense, Fully Compacted Concrete. Made with 20 mm Nominal Maximum Size Aggregates Complying with IS 383 Minimum Cement Maximum Free Content kg/m3 Water Cement Ratio

g/l

(1)

(2)

(7)

(8)

(i)

1

Traces (< 0.2)

Less than 1.0

Less than 0.3

Ordinary Portland Cement or Portland Slag Cement or Portland pozzolana Cement

280

0.55

(ii)

2

0.2 to 0.5

1.0 to 1.9

0.3 to 1.2

Ordinary Portland Cement or Portland Slag Cement or Portland Pozzolana Cement Supersulphated cement or sulphate resisting Portland cement

330

0.50

310

0.50

330

0.50

350

0.45

370

0.45

400

0.40

(iii)

3

0.5 to 1.0

1.9 to 3.1

(5)

1.2 to 2.5

2.5 to 5.0

(6)

Supersulphated cement or sulphate resisting Portland cement Portland pozzolana cement or Portland Slag cement

(iv)

4

1.0 to 2.0

3.1 to 5.0

Supersulphated or sulphate resisting Portland cement

(v)

5

More than 2.0

More than 5.0 More than 5.0 Sulphate resisting Portland cement or supersulphated cement with protective coatings

Notes 1. Cement content given in this table is irrespective of grades of cement. 2. Use of supersulphated cement is generally restricted where the prevailing temperature is above 40°C. 3. Supersulphated cement gives an acceptable life provided that the concrete is dense and prepared with a water-cement ratio of 0.4 or less, in mineral acids, down to pH 3.5. 4. The cement contents given in col 7 of this table are the minimum recommended. For SO3 contents near the upper limit of any class, cement contents above these minimum are advised. 5. For severe conditions, such as thin sections under hydrostatic pressure on one side only and sections partly immersed, considerations should be given to a further reduction of water-cement ratio. 6. Portland slag cement conforming to IS 455 with slag content more than 50 percent exhibits better sulphate resisting properties. 7. Where chloride is encountered along with sulphates in soil or ground water, ordinary Portland cement with C3A content from 5 to 8 percent shall be desirable to be used in concrete, instead of sulphate resisting cement.

Alternatively, Portland slag cement conforming to IS 455 having more than 50 percent slag or a blend of ordinary Portland cement and slag may be used provided sufficient information is available on performance of such blended cements in these conditions.

For the very high sulphate concentrations in class 5 conditions, some form of lining such as polyethylene or ploychloroprene sheet: or surface coating based on asphalt, chlorinated rubber, epoxy: or polyurethane materials should also be used to prevent access by the sulphate solution.

Summary of Codal Provision for Concrete and Cementitious Materials

37.17

37.5.4 Chlorides in Concrete The total amount of chloride content (as Cl) in the concrete at the time of placing is given in Table 37.30. TABLE 37.30 Limits of Chloride Content of Concrete (Clause 8.2.5.2 of IS 456: 2000) SI. No.

Type of Use of Concrete

(1)

(2)

Maximum Total Acid Soluble Chloride Content Expressed as kg/m3 of Concrete (3)

(i)

Concrete containing metal and steam cured at elevated temperature and pre-stressed concrete

0.4

(ii)

Reinforced concrete or plain concrete containing embedded metal

0.6

(iii)

Concrete not containing embedded metal or any material requiring protection from chloride

3.0

37.5.5 Sulphates in Concrete The total water soluble sulphate of the concrete mix, expressed as SO3, should not exceed 4 percent by mass of the cement in the mix. The sulphate content should be calculated as the total from the various constituents of the mix. The 4 percent limit does not apply to concrete made with supersulphated cement conforming to IS 6909.

37.5.6

Alkali-aggregate Reaction

Precautions recommended in the code are as under; (a) Use of non-reactive aggregate from alternate sources. (b) Use of low alkali Ordinary Portland Cement having total alkali content not more than 0.6 percent (as Na2O equivalent). (c) Use of fly ash conforming to IS 3812 (Part I) or ggbs conforming to IS 12089 as part replacement of OPC (having total alkali content as Na2O equivalent not more than 0.6 percent), provided fly ash content is at least 20 percent or slag content is at least 50 percent. (d) Measures to reduce the degree of saturation of the concrete during service such as use of impermeable membrane. (e) Limiting the cement content in the concrete mix.

37.5.7

Freezing and Thawing

When concrete lower than grade M-50 is used, the mean total air content by volume of the fresh concrete at the time of delivery into the construction should be: Nominal Maximum Size

Entrained Air

Aggregate (mm) 20

Percentage (%) 5±1

40

4±1

37.18

Handbook on Advanced Concrete Technology

37.6

SAMPLING AND STRENGTH OF CONCRETE

Samples from fresh concrete shall be taken and cubes shall be made cured and tested at 28 days. In order to get relatively quicker idea of the quality of concrete, optional test at 7 days may be carried out in addition to 28 days compressive strength test.

37.6.1

Frequency of Sampling

The minimum frequency of sampling of concrete of each grade shall be in accordance with the following (IS 456: 2000) Quantity of concrete in the work m3 1-5 6-15 16-30 31-50 51 and above

Number of samples 1 2 3 4 4 plus one additional sample for each additional 50m3 or Part thereof.

Note

At least one sample shall be taken from each shift. When concrete is produced at continuous production unit, such as ready-mixed concrete plant, frequency of sampling may be agreed upon mutually by suppliers and purchasers.

37.6.2 Acceptance Criteria 37.6.2.1 Compressive Strength The concrete shall be deemed to comply with the strength requirements when both the following conditions are met; (a) The mean strength determined from any group of four non overlapping consecutive test results complies with appropriate limits of col-2 of Table 37.31. (b) Any individual test results complies with the appropriate limits in col-3 of Table 37.31

37.6.2.2 Flexural Strength When both the following conditions are met, the concrete complies with the specified flexural strength; (a) The mean strength determined from any group of four consecutive test results exceeds the specified characteristic strength by at least 0.3 N/mm2. (b) The strength determined from any test result is not less than the specified characteristic strength by less 0.3 N/mm2.

37.6.3

Compliance Requirement

The concrete is deemed to comply the characteristic strength if the conditions given in Table 37.31 are met with;

Summary of Codal Provision for Concrete and Cementitious Materials TABLE 37.31 IS 456: 2000

37.19

Characteristic Strength Compliance Requirements (Clauses 16.1 and 16.3) of

Specified Grade (1)

Mean of the Group of 4 Non-Overlapping Consecutive Test Results in N/mm2 (2)

M-20 or above

> fck + 0.825 x established standard deviation (rounded off to nearest 0.5N/mm2) or fck + 3N/mm2, whichever is greater > fck + 0.825 x established standard deviation

M-15

Individual Test Results in N/mm2 (3) > fck-3N/mm2

> fck-3 N/mm2

2

(Rounded off to nearest 0.5 N/mm ) or fck + 3 N/mm2, whichever is greater Note In the absence of established value of standard deviation, the values given in Table 37.17 may be assumed, and attempt should be made to obtain results of 30 samples as early as possible to establish the value of standard deviation.

Reference 1. All BIS codes on Concrete and Cementitious materials (Part I) and relevant BIS codes on Concrete Chemical Admixtures (Part II) and also foreign codes on Fibres, Fibre Reinforced Concrete, Sprayed Concrete and Shotcrete (Part III) are listed for easy references. PART I List of BIS Codes on Concrete and Cementitious materials SI. No. IS Number/DOC Number STANDARDS PUBLISHED

Title

Reaffirm Date

No. of Amd. 8

1.

IS 269: 1989

Specification for ordinary Portland cement, 33 grade (fourth revision)

Oct 2008

2.

IS 383: 1970

Specification for coarse and fine aggregates from natural sources for concrete (second revision)

Jan 2007

3.

IS 455: 1989

Specification for Portland slag cement (fourth)

Jul 2009

6

4.

IS 456: 2000

Code of practice for plain and reinforced concrete (fourth revision)

Aug 2005

3

5.

IS 457: 1957

Code of practice for general construction of plain and reinforced concrete for dams and other massive structures

Jul 2009

2

6.

IS 516: 1959

Method of test for strength of concrete

Oct 2008

2

7.

IS 650: 1991

Specification for standard sand for testing of cement (second revision)

Oct 2008

9.

*IS 1343: 1980

Code of practice for prestressed concrete (first revision)

Feb 2004

1

10.

IS 1344: 1981

Specification for calcined clay pozzolana (second revision)

Oct 2008

1 Contd...

37.20

Handbook on Advanced Concrete Technology

Contd... 11.

IS 1489(Part): 1981

Specification for Portland pozzolana cement: Part 1 Flyash based (third revision)

Jul 2009

5

12.

IS 1489(Part2): 1991

Specification for Portland-pozzolana cement: Part 2 Calcined clay based (third revision)

Jul 2009

4

13.

IS 1727: 1967

Methods of test pozzolanic materials (first)

Oct 2008

1

14.

IS 2386(Part 1): 1963

Methods of test for aggregates for concrete: Part 1 Particle size and shape

Jan 2007

3

15.

IS 2386(Part 2): 1963

Methods of test for aggregates for concrete Part 2 Eatimation of deleterious materials and organic impurities

Jan 2007

16.

IS 2386(Part 3): 1963

Methods of test for aggregates for concrete: Part 4 Mechanical properties

Jan 2007

17.

IS 2386(Part 4): 1963

Methods of test for aggregates for concrete: Part 5 Soundness

Jan 2007

18.

IS 2386(Part 5): 1963

Methods of test for aggregates for concrete: Part 5 aggregatesm

Jan 2007

2

19.

IS 23863(Part 6): 1963

Methods of test for aggregates for concrete: Part 6 Measuring mortar making properties of fine aggregates

Jan 2007

2

20.

IS 2386(Part 7): 1963

Methods of test for aggregates for concrete: Part 7 Alkali aggregate reactivity

Jan 2007

2

21.

IS 2368(Part 8): 1963

Methods of test for aggregates for concrete: Part 8 Petrographic examination

Jan 2007

1

22.

IS 2430: 1986

Methods for sampling of aggregates for concrete (first revision)

Jul 2009

23.

IS 2502: 1963

Code of practice for bending and fixing of bars for concrete reinforcement

Oct 2008

24.

IS 2645: 2003

Integral waterproofing compounds for cement mortar and concrete-Specification (second)

Dec

25.

IS 2770(Part 1): 1967

Methods of testing bond in reinforced concrete: Part 1 Pull-out test

Jan 2007

26.

IS 3085: 1965

Method of test for permeability of cement mortar and concrete

Jan 2007

27.

IS 3370(Part 1): 2009

Code of practice for concrete structures for storage of liquids: Part 1 General requirements

28.

IS 3370(Part 2): 2009

Code of practice for concrete structures for storage of liquids: Part 2 Reinforced concrete structures (First Revision)

29.

IS 3370(Part 3): 1967

Code of practice for concrete structures for the storage of liquids Part 3 Prestressed concrete

Oct 2008

1

30.

IS 3370(Part 4): 1967

Code of practice for concrete structures for the storage of liquids: Part 4 Design tables

Oct 2008

2 Contd...

Summary of Codal Provision for Concrete and Cementitious Materials

37.21

Contd... 31.

IS 3466: 1988

Specification for masonry cement (second revision)

Oct 2008

32.

IS 3535: 1986

Methods of sampling hydraulic cement (first)

Oct 2008

33.

IS 3558: 1983

Code of practice for use of immersion vibrators for consolidating concrete (first revision)

Oct 2008

34.

IS 3812(Part 1): 2003

Specification for pulverized fuel ash Part 1 For use as pozzolana in cement, cement mortar and concrete (second revision)

Dec

35.

IS 3812(Part 2): 2003

Specification for pulverized fuel ash Part 2 For use as admixture in cement mortar and concrete (second revision)

Dec

36.

IS 4013(Part): 1996

Methods of physical tests for hydraulic cement: Part 1 Determination of fineness by dry sieving (second revision)

Aug 2005

37.

IS 4031(Part): 1999

Methods of physical tests for hydraulic cement: Part 2 Determine of fineness by specific surface by Blaine air permeability method (second)

Oct 2008

38.

IS 4031(Part 3): 1988

Methods of physical tests for hydraulic cement: Part 3 Determination of soundness (first revision)

Jul 2003

39.

IS 4031(Part 4): 1958

Methods of physical tests for hydraulic cement: Part 4 Determination of consistency of standard cement paste (first revision)

Jul 2009

40.

IS 4031(Part 5): 1988

Methods of physical tests for hydraulic cement: Part 5 Determination of initial and final setting times (first revision)

Jul 2009

41.

IS 4013(Part 6): 1968

Methods of physical tests for hydraulic cement: Part 6 Determination of compressive strength of hydraulic cement (other than masonry cement) (first revision)

Jul 2009

42.

IS 4031(Part 7): 1988

Methods of physical tests for hydraulic cement: Part 7 Determination of compressive strength of masonry cement (first revision)

Jul 2009

43.

IS 4031(Part 8): 1988

Methods of physical tests for hydraulic cement: Part 8 Determine of transverse and compressive strength of plastic mortar using prism

Jul 2009

44.

IS 4031(Part 9): 1988

Methods of physical tests for hydraulic cement: Part 9 Determine of heat of hydration (first)

Jul 2009

45.

IS 4031 (Part 10): 1988

Methods of physical tests for hydraulic cement: Part 10 Determination of drying shrinkage (first revision)

Jul 2009

4

Contd...

37.22

Handbook on Advanced Concrete Technology

Contd... 46.

IS 4031(Part 11): 1988

Methods of physical tests for hydraulic cement: Part 11 Determination of drying shrinkage (first revisions)

Jul 2009

47.

IS 4031(Part 12): 1988

Methods of physical tests for hydraulic cement: Part 12 Determination of air content of hydraulic cement mortar (first revision)

Jul 2003

48.

IS 4031(Part 13): 1988

Methods of physical tests for hydraulic cement: Part 13 Measurement of water retentivity of masonry cement (first revision)

Jul 2009

49.

IS 4031 (Part 14): 1989

Jul 2009

50.

IS 4031 (Part 15): 1991

Methods of physical tests for hydraulic cement: Part 14 Determination of false set Methods of physical test for hydraulic cement: Part 15 Determination of fineness by wet sleving

51.

IS 4305: 1967

Glossary of terms relating to pozzoiana

Jul 2009

52.

IS 4634: 1991

Methods for testing performance of batch-type concrete mixers (first revision)

Jul 2009

53.

IS 4846: 1968

Oct 2008

54.

IS 4926: 2003

Definitions and terminology relating to hydraulic Ready mixed concrete-Code of practice (second revision)

55.

IS 5512: 1983

Specification for flow table for use in tests of hydraulic cements and pozzolanic materials (first)

Oct 2008

56.

IS 5513: 1996

Specification for vicat apparatus (second revision)

Aug 2005

57.

IS 5514: 1996

Specification for apparatus used in Le-Chatelier test (first revision)

Aug 2005

58.

IS 5515: 1963

Specification for compaction factor apparatus (first revision)

Oct 2008

59.

IS 5516: 1996

Specification for variable flow type air-permeability apparatus (Blaine type) (first revision)

Aug 2005

60.

IS 5525: 1969

Recommendations for detailing of reinforcement in reinforced concrete works

Oct 2008

61.

IS 5536: 1969

Specification for constant flow type air-permeability apparatus (Lea and Nurse type)

Jul 2009

62.

IS 5816: 1999

Method of test for splitting tensile strength of concrete (first revision)

Oct 2008

63.

IS 6452: 1983

Specification for high alumina cement for structural use (first revision)

Jul 2009

64.

IS 646(Part 1): 1972

Glossary of terms relating to cement concrete: Part 1 Concrete aggregates

Jan 2007

Jul 2009

Dec

Contd...

Summary of Codal Provision for Concrete and Cementitious Materials

37.23

Contd... 65.

IS 6461(Part 2): 1972

Glossary of terms relating to cement concrete: Part 2 Materials (other than cement and aggregate)

Jan 2007

68.

IS 6461(Part 3): 1972

Glossary of terms relating to cement concrete: Part 3 Concrete reinforcement

Jan 2007

69.

IS 6461 (Part 4): 1972

Glossary of terms relating to cement concrete: Part 4 Types of concrete

Jan 2007

70.

IS 6461 (Part 5): 1972

Jan 2007

71.

IS 6461(Part 6): 1972

Glossary concrete: Glossary concrete:

72.

IS 6461(Part 7): 1973

Glossary of terms relating to cement concrete: Part 7 Mixing, laying, compaction, curing and other construction aspects

Jan 2007

73.

IS 6461(Part 8): 1973

Glossary of terms relating to cement concrete: Part 8 Properties of concrete

Jan 2007

74.

IS 6461(Part 9): 1972

Glossary of terms relating of cement concrete: Part 9 Structural aspects

Jan 2007

75.

IS 6461(Part 10): 1973

Glossary of terms relating of cement Jan 2007 concrete: Part 10 Tests and testing apparatus

76.

IS 6461(Part 11): 1973

Glossary of terms relating to cement concrete: Part 11 Prestressed concrete

Jan 2007

77.

IS 6461 (Part 12): 1973

Glossary of terms relating to cement concrete: Part 12 Miscellaneous

Jan 2007

78. 79. 80.

IS 6491: 1972 IS 6909: 1990 IS 6925: 1973

Method of sampling fly ash Aug 2005 Specification for supersulphated cement Aug 2005 Methods of test for determination of Oct 2008 water soluble chlorides in concrete admixtures

81.

IS 7246: 1974

Recommendation for use of table vibrators for consolidating concrete

Oct 2008

82.

IS 7320: 1974

Oct 2008

83.

IS 7325: 1974

Specification for concrete slump test apparatus Specification for apparatus for determining constituents of fresh concrete

84.

IS 786(Part 1): 1975

Jan 2007

85.

IS 8041: 1990

Code of practice for extreme weather concreting: Part 1 Recommended practice for cold weather Specification for rapid hardening Portland cement (second revision)

86.

IS 8042: 1989

Specification for white Portland cement (second revision)

Jul 2009

87.

IS 8043: 1991

Specification for hydrophobic Portland cement (second revision)

Jul 2009

of terms relating to cement Part 5 Formwork for concrete of terms relating to cement Part 6 Equipment, tools and plant

Jan 2007

Oct 2008

Jul 2009

Contd...

37.24

Handbook on Advanced Concrete Technology

Contd... 88.

IS 8112: 1989

Specification for 43 grade ordinary Portland cement (first revision) Dimensions and materials of cement rotary kilns, components and auxiliaries (dry process with suspension preheater)

89.

IS 8125: 1976

90.

IS 8142: 1976

91.

IS 8229: 1986

92.

IS 8425: 1977

93.

IS 9012: 1978

94.

IS 9013: 1978

Method of making, curing and determining compressive strength of accelerated cured concrete test specimens

95. 96.

Jul 2009 Oct 2008

Method of test for determining setting time Jan 2007 of concrete by penetration resistance Specification for oil-well cement (first revision) Oct 2008 Code of practice for determination of specific surface area of powders by air permeability Recommended practice for shotcreting

Aug 2005

Jan 2007

1

Oct 2008

1

IS 9103: 1999 IS 9142: 1979

Specification for admixtures for concrete (first) Oct 2008 Specification for artificial light weight Jan 2007 aggregates for concrete masonry units

2

97.

IS 9284: 1979

Method of test for abrasion resistance of concrete

Jan 2007

98.

IS 9376: 1979

Specification for apparatus for measuring aggregate crushing value and ten percent fines

Oct 2008

99.

IS 9377: 1979

Specification for apparatus for aggregate impact

Oct 2008

100.

IS 9399: 1979

Specification for apparatus for flexural testing of concrete

Oct 2008

101.

IS 9459: 1980

Specification for apparatus for use in measurement of length change hardened cement paste, mortar and concrete

Oct 2008

102.

IS 9799: 1981

Specification for pressure meter for determination of air content of freshly mixed concrete

Oct 2008

103.

IS 10070: 1982

104.

IS 10078: 1982

Specification for machine for abrasion testing Oct 2008 of coarse aggregates Specification for jolting apparatus for testing Oct 2008

105.

IS 10079: 1982

Specification for cylindrical metal measures for use in tests of aggregates and concrete

Oct 2008

106.

IS 10080: 1982

Specification for vibration machine of casting standard cement mortar cubes

Oct 2008

4

107.

IS 10086: 1982

Specification for moulds for use in tests of cement and concrete

Oct 2008

4

108.

IS 10262: 2009

Guidelines for concrete mix proportioning (First Revision)

Contd...

Summary of Codal Provision for Concrete and Cementitious Materials

37.25

Contd... 109.

IS 10510: 1983

Specification for vee-bee consistometer

110.

IS 10850: 1984

111.

IS 10890: 1984

Specification for apparatus for measurement Oct 2008 of water retentivity of masonry cement Specification for planetary mixer used in tests Oct 2008 of cement and pozzolana

112.

IS 11262: 1985

Specification for calorimeter for determination Oct 2008 of heat of hydration of hydraulic cement

113.

IS 11263: 1985

Specification for cylinder measures for determination of air content of hydraulic cement

Oct 2008

114.

IS 11578: 1986

Method for determination of specific surface area of powder and porous particle using low temperature gas absorption techniques

Jan 2007

115.

IS 11993: 1987

Code of practice for use of screed board concrete vibrators

Oct 2008

116.

IS 12089: 1987

Specification for granulated slag for manufacture of Portland slag cement

Oct 2008

117.

IS 12119: 1987

General requirements for pan mixers for concrete

Oct

118. 119. 120. 121. 122.

IS IS IS IS IS

Specification for 53 grade ordinary Portland Criteria for design of RCC hinges Specification for sulphate resisting Portland Method for colorimetric analysis of hydraulic Specification for low heat Portland cement

Oct 2008 Dec Jul 2009 Jul 2009 Jul 2009

123.

IS 12803: 1989

Methods of analysis of hydraulic cement by X-ray

Oct 2008

124.

IS 12803: 1989

Method of analysis of hydraulic cement by atomic absorption spectrophotometer

Oct 2008

125.

IS 12870: 1989

Methods of sampling calcined clay pozzolana

Aug 2005

126.

IS 13311(Part 1): 1992

Methods of non-destructive testing of concrete: Oct 2008 Part 1 Ultrasonic pulse velocity

127.

122169: 1987 12308: 1987 12330: 1988 12430: 1988 12600: 1989

IS 13311(Part 2): 1992

Oct 2008

9 6 5

Methods of non-destructive testing of concrete: Oct 2008 Part 2 Rebound hammer

128.

IS 14345: 1996

Specification for autoclave apparatus

Aug 2005

129.

IS 14687: 1999

Guidelines for false work for concrete structures

Jul 2009

130.

IS 14858: 2000

Requirements for compression testing machine Aug 2005 used for testing of concrete and mortar

131.

IS 14959(Part 1): 2001

Method of test for determination of water Jan 2007 soluble and acid soluble chlorides in mortar and concrete: Part 1 Fresh mortar and concrete

1

Contd...

37.26

Handbook on Advanced Concrete Technology

Contd... 132.

IS 14959(Part 2): 2001

Method of test for determination of water soluble and acid soluble chlorides in mortar and concrete: Part 2 Hardened mortar and concrete Silica fume-Specification

133.

IS 15388: 2003

134.

SP 16(S and T): 1980

Design aids for reinforced concrete to IS 456: 1978

135.

SP 23(S and T): 1982

Handbook on concrete mixes (Based on Indian Standards)

136.

SP 16(S and T): 1983

Explanatory handbook on Indian Standard code of practice for plain and reinforced concrete (IS)

137.

SP 16(S and T): 1987

Handbook on concrete reinforcement and detalining

Jan 2007

Dec

FINALISED DRAFTS UNDER PRINT 1.

DOC. CED 2(7494)

Second Revision of IS 1343

2.

DOC. CED 2(7762)

Amendment No. 4 to IS 2386 (Part 1): 1963

DRAFTS COMPLETED WIDE CIRCULATION 1.

DOC. CED 2(7674)

Draft Amendment No. 4 to IS 456: 2000

2.

DOC. CED 2(7672)

Draft Indian Standard Specification for Ordinary Portland Cement, 33 Grade (Fifth Revision of IS)

3.

DOC. CED 2(7673)

Draft Indian Standard Specification for Ordinary Portland Cement, 43 Grade (Second Revision of IS 8112)

4.

DOC. CED 2(7674)

Draft Indian Standard Specification for Ordinary Portland Cement, 53 Grade (First Revision of IS 12269)

5.

DOC. CED 2(7697)

Draft Indian Standard Pulverized Fuel Ash-Specification: Part 1 For use as Pozzolana in Cement, Cement Mortar and Concrete [Third revision of IS 3812 (Part 1)]

6.

DOC. CED 2(7698)

Draft Indian Standard Pulverized Fuel Ash-Specification: Part 2 For use as Admixture in Cement Mortar and Concrete [Third revision of IS]

DRAFT STANDARDS FORMULATED 1.

DOC. CED 2(7668)

Draft Indian Standard Metakaolin for use as Minera Admixture in Hydraulic Cement System

2.

DOC. CED 2(7670)

Draft Indian Standard Specification for Clinker

PART II List of BIS Codes on Concrete Chemical Admixtures 1

IS 9103: 1999

Concrete Admixture Specification

Summary of Codal Provision for Concrete and Cementitious Materials PART III Shotcrete S. No.

37.27

List of foreign codes on Fibres, Fibre Reinforced Concrete, Sprayed Concrete and Standard

Title

Country/Region

Status

1

ASTM A820-06

Standard Specification for Steel Fibers for Fiber-Reinforced Concrete

USA

Active

2

ASTM C1116-10

Standard Specification for Fiber-Reinforced Concrete

USA

Active

3

ASTM C1399-10

Standard Test Method for Obtaining Average Residual-Strength of Fiber-Reinforced Concrete

USA

Active

4

ASTM C1550-10a

Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel)

USA

Active

5

ASTM C1609 -10

Active

6

ACI 506 -05

Standard Test Method for Flexural Performance USA of Fiber-Reinforced Concrete (Using Beam With Third-Point Loading) Guide to shotcrete USA

Active

7

ACI 544.4R

Design Considerations for Steel Fiber Reinforced Concrete

USA

Active, but under revision

8

ACI 544.XR

Elevated and Pile Supported Steel Fiber Reinforced Concrete Slab Applications

USA

New draft, under development

9

NBN B-15-238

Test on fibre reinforced concrete - bending test on prismatic samples (4 point bending)

Belgium

Active

10

JSCE SF-4

Method of tests for steel fiber reinforced concrete

Japan

Active

11

NF P 18-409

Béton avec fibres métalliques - Essai de France flexion (with steel fiber concrete - bending test)

Active

12

CUR 35

Test on fibre reinforced concrete (4 point bending)

Netherlands

Active

13

UNI 11037

Fibre d’acciaio da impiegare nel confezionamento di conglomerato cementizio rinforzato (Steel fibers for use in reinforced concrete)

Italy

Active

14

UNI 11039

Steel fibre-reinforced concrete—Part 1: definitions, classifications, specifics and conformity; Part II: Test method used to determine the early crack strength and ductility indexes

Italy

Active

15

IRC SP-46

Steel Fibre Reinforced Concrete for pavements India

Active

16

EN 14651

Test method for metallic fibre concrete. Europe Measuring the flexural tensile strength (limit of proportionality (LOP), residual)

Active

17

EN 14488

Testing Sprayed Concrete (8 parts)

Europe

Active

18

EN 14889-1

Fibres for concrete - Part 1: Steel fibresDefinition, specifications and conformity (CE label)

Europe

Active

Contd...

37.28

Handbook on Advanced Concrete Technology

Contd... 19

FIB Model Code 2010

Fibre Reinforced Concrete - Design code for concrete structures

Europe

New draft, under development

20

ÖVBB Richtlinie Faserbeton DAfStb Richtlinie Stahlfaserbeton

Guideline of Fibre concrete (4 point bending test) Guideline of Steel Fibre Reinforced Concrete (4 point bending test)

Austria

Active

Germany

Active

22

DBV-Merkblatt Stahlfaserbeton

Recommendations on Steel Fibre Concrete

Germany

Active

23

SIA 162/6

Recommendations on Steel Fibre Reinforced Concrete

Switzerland

Active

24

NB Publication No. 7

Specifications for sprayed concrete

Norway

Active

25

CNR-DT 204/2006 Guide for the design and construction of fibre Italy reinforced concrete structures

Active

26

EHE-08, Annex 14

Recommendations for using concrete with fibres - Code on structural concrete

Spain

Active

27

NZS 3101:2006

Test and design methods for steel fibre reinforced concrete - Concrete Structures Standard

New Zealand

Active

28

RILEM TC 162-TDF

Test and design methods for steel fibre reinforced concrete

International

Active

21

Index

A Abrasion 2.14, 4.5, 4.8, 6.2, 37.24 Abrasion Resistance 4.5, 13.39, 37.24 Absorption 4.4, 8.11, 9.33, 10.15, 23.3, 37.25 Acceptance Test Criteria 18.11 ACI 211.1 9.11, 9.14, 26.6 ACI 305 19.1, 19.7 ACI 306 20.1, 20.10 ACI 522 21.12 Acid Resistant Concrete 29.1, 29.6, 29.11 Acrylic 23.13, 29.7, 34.54 Acrylic Latex 30.5 Activator 2.8, 36.7, 36.8 Adiabatic Temperature Rise 26.5, 31.12 Admixture Dispenser 8.12, 32.15 Aggregate 1.13, 4.1, 4.2, 4.3, 4.10, 9.28, 29.3 Aggregate Grading 5.2, 21.5, 37.9 Alkali Aggregate Reactivity (AAR) 27.6 Alkali Silica Reaction (ASR) 16.1, 34.4 Alkali Sources 13.47 Alkalis 1.12, 1.13, 13.48, 34.44 All-in-aggregate 37.4, 37.6 All-in-aggregate 37.4, 37.6 Flakiness Index 4.3 Flaky & Elongated Aggregate 4.3, 6.2, 10.11 Heavy Aggregate 4.2, 24.2, 24.3 Heavy Coarse Aggregate 10.9 Light weight aggregate 4.2 Pumice Aggregate 22.5 Siliceous Aggregate 16.2 Super Heavy Aggregate 24 Air Entraining 1.17, 9.11, 36.3 Alkali Aggregate Reactivity (AAR) 27.6, 37.20

Alkali Silica Reaction (ASR) 16.4, 34.4 Alkali Sources 13.47 Alkalis 1.12, 16.2, 34.44 Alumina 1.2, 13.22, 15.1, 37.22 Anodic Reaction 14.13 Antiwashout Admixture 25.7, 25.9 Aramid Fibres 23.1, 23.14 ARC 1.4, 5.5, 20.4, 23.22, 29.5, 36.12, 37.13 Areated Concrete 22.3, 22.4 Argillaceous 1.3, 16.2 Artificial Aggregate 4.2, 22.3 Artificial Reefs 21.2 Aspect Ratio 21.4, 23.6, 23.18 ASTM C94 7.8, 13.10, 13.51 Attenuating 24.1 Audit 32.7, 34.4, 36.2 Auditor 3.26, 3.27, 3.62 Autoclave Test 1.10, 37.7 Autogenous Shrinkage 12.3, 36.4 B Backfill 28.1, 28.6 Bandra Worli Sea Link 18.1 Batching 2.4, 2.7, 10.10, 10.14, 27.10, 37.10 Benzoyl Peroxide 30.8 Biological Shield 24.1 Black Rust 14.4 Blaine’s Fineness 2.8, 26.4, 32.8 Blended Cement 1.6, 1.6 26.4, 37.16 Blended Portland Cement 35.2, 35.7 Blockage 11.5, 18.5, 25.4, 25.6

I.2

Index

Blotting 31.20 Bond Strength 13.25, 18.13, 27.8 Boom Placer 11.1 Butyl Acrylate 30.3, 30.8 C Cement1.1, 1.8, 1.15, 9.10, 29.6, 37.27 C3A 1.2, 1.28, 37.16 C4AF 1.2, 12.4, 29.8 Calcareous 1.3, 37.6 Calcined Clay 1.1, 1.14, 37.25 Calcium Sulphoaluminate 1.18, 17.6 High Alumina Cement 1.14, 1.20, 37.22 Hydrophobic Portland Cement 1.19 Low Heat Portland Cement 1.14, 1.17, 37.25 Masonry Cement 11.14, 37.2, 37.25 Oil Well Cement 1.13, 37.24 PPC 1.12, 1.16, 37.3 PSC 1.13, 9.8, 37.2 Rapid Hardening Cement 1.18, 13.40 Sulphate Resistant Cement 10.10, 13.46, 15.5 Super Sulphated Cement 1.4, 37.3 White Cement 1.5, 1.18, 37.3 Cement Manufacturing Process 1.3, 1.5 Limestone 1.3, 1.8, 35.5 Mining 1.3, 1.4, 10.11, 35.5 Crushing 1.3, 1.5, 10.11, 37.24, Raw Mill 1.5, 1.7 Blending 1.5, 1.7, 29.8 Burning1.3, 35.2 Calibration10.10, 32.12, 33.19 Capillary Pores 2.6, 13.37, 30.8 Capping 13.12, 37.7 Carbon 1.3, 2.2, 14.6, 34.1, 34.49, 36.6 Carbonates 1.3, 33.18, 35.4 Carbonation 1.8, 1.13, 14.13, 34.57 Carbonation Shrinkage 12.8 Carbon-di-oxide 2.7 Cathodic Reaction 14.3, 14.4 CBR Value 31.4 Cement Slurry 2.13, 28.1, 34.29 Certification 2.13, 2.8, 34.29

Checklist 10.1, 10.7, 10.16, 32.7 Chemical Admixture 8.1, 8.13, 13.26, 37.26 Chilling Plant 19.6, 26.8 Chloride Diffusion 2.6, 7.8, 13.45, 29.5 Chlorides 1.12, 1.13, 13.44, 34.7, 37.26 Classification 2.3, 4.1, 15.3, 37.27 Coarse Aggregate 4.1, 4.5, 9.27, 17.5, 21.4, 37.24 Co-efficient of Variation 33.4 Cold Joint 2.5, 8.5, 19.2, 26.5 Cold Weather Concrete 20.1, 20.3, 20.10 Compaction 1.10, 2.2, 15.6, 27.3, 37.23 Compressive Strength 1.9, 7.7, 9.11, 17.9, 33.23, 37.24 Concrete 1.1, 8.8, 11.14, 21.4, 26.2, 34.34, 37.28 Concrete Composites 7.9, 13.51, 34, 69 Concrete Cover 10.6, 14.11, 37.15 Concrete Overlay 8.5, 36.11 Concrete Production 2.13, 6.1, 25.4, 36.9 Concrete Pump 10.9, 11.4, 36.4 Concrete Roads 9.31, 31.1, 31.17, 31.27 Contraction Joint 21.9, 27.7, 31.6, 31.15 Control System 10.1, 10.8 Cooling 1.6, 10.14, 25.7, 29.4 Copper Slag 7.1, 7.5, 7.9 Core Test 33.4, 33.18, 34.13 Corosion Inhibiting Admixtures 8.4, 34.42, 34.57 Corrosion1.13, 12.3, 17.2, 37.14 Corrosion Due to Carbonation 14.5 Corrosion Due to Chloride Attack 14.7 Corrosion Inhibitors 14.8, 29.10, 34.59 Corrosion Potential 33.2, 34.43 Corrosion Process 14.1, 34.43 Corrosion Product 13.42, 14.1, 14.15 Cover Concrete 13.29, 34.58 Cover Meter 33.7, 34.13 Crack Depth 33.3, 33.14 Cracks 1.10, 14.11, 3416, 36.4 Creep 1.15, 12.9, 12.13, 35.7 Crushed Sand 5.4, 18.4, 36.4 Crystal Speed Hardening Concept 36.9 C-S-H gel 1.16, 14.6

Index

Cube Strength 2.4, 13.49, 33.17 Curing 1.6, 3.3, 13.18, 35.7, 37.24 Curing Compound 11.12, 31.15, 35.8 Curing Temperature 2.10, 36.10 Cutting Saw 31.17, 31.20 Cylinder Strength 13.13, 13.16, D Damage Assessment 33.3 Defects 1.20, 34.10, 36.3 De-flocculation 8.11 Deleterious Substances 4.5, 4.11 Delhi Metro 18.1 Density 1.9, 13.5, 24.3, 37.12 Direction of Loading 13.16, 13.50 Drainable Base 21.4 Dry Lean Concrete (DLC) 31.4, 31.19 Dry Process 1.4, 1.7, 37.24 Drying Shrinkage 1.27, 12.5, 37.22 Ductility 23.1 23.21, 37, 37 Durability 1.14, 13.51, 29.1, 37.15 Durability of Concrete 2.6, 13.7, 37.14 E Edge Compaction 27.13, Effects of Creep 12.12 Elastic Modulus 4.5, 17.3, 34.34 Elastic Properties of Hydrating Cement 13.5 Elastomers 30.2, 30.3 Electrostatic Repulsion 8.11, 29.7 Elongation Index 4.3 Erosion 2.4, 21.2 27.7 Ettringite 13.24, 15.2, 17.6 Exothermic 2.5, 36.9 Expansion Joint 31.6, 31.9, 34.45 Exposure Conditions 1.15, 34.32, 37.15 External Vibration 11.10 F Ferrosilicon 2.12, 29.8 Fibre Reinforced Concrete 23.1, 23.3, 23.23, 37.28 Fine Aggregate 1.17, 5.2, 11.13, 37.20 Fineness Modulus 3.2, 7.2, 17.5

I.3

Fire Resistance 4.2, 36.2 Flexible Overlay 36.10 Flexible Pavement 31.1, 31.2, 31.3 Flexural Test 13.20, 23.20 Flyash 4.2, 9.31, 37.20 Foam Concrete 28.1, 28.8 Foam Generators 28.3 Fogging 11.15, 31.14 Formwork Striking Time 2.5, 2.11 Fossil Fuel 33.1, 35.2 Free Fall 10.8, 18.16 Friction Course 21.4 Frost Action 13.28, 13.40 Frost Resistance 13.40, 20.10 Fusion Bonded Epoxy Coating 14.11 G Gamma Radiation 24.1 Geometric Configuration 23.1 GFRC 23.20, 23.22 Glass Fibres 23.1, 23.21, 30.7 Global Warming 35.2, 35.6 Glued Fibres 23.6, 23.19 Go-devil 25.1, 25.3, 25.6 Grading 3.2, 3.5, 9.9, 9.25, 37.11 Grading Curve 4.6, 9.18, 34.40 Green House Gases 1.26 Grinding of Cement 1.9 Ground Granulated Blast Furnace Slag 2.1, 28.8, 37.15 H Half Cell Potentiometer 33.15 Hand Roller 21.9, 21.10 Hardened Concrete 1.12, 13.26, 35.6 Heat of Hydration 1.12, 26.1, 37.25 Heat Reflective Paint 19.6 Heating 1.6, 17.2, 35.2 Heavy Aggregate 10.9, 24.2, 24.3 Heavy Coarse Aggregate 10.9 Hessian Cloth 10.9, 19.7, 31.20 High Density Concrete 24.1, 24.3, 29.1

I.4

Index

High Performace Concrete 17.9, 17.11 High Strength Concrete 4.2, 13.25, 36.11 High Volume Fly Ash Concrete 31.1, 31.25, 35.7 High Volume Flyash Concrete 31.27, 35.7 Homogeneity 10.8, 34.1 Honey Combing 13.36, 34.4 Hooked End Fibres 32.4 Hostile Environment 17.2 Hot Weather Concrete 19.1, 19.7 Hydrocarbons 30.1 Hydrogen Content 24.1 Hyper Plasticizer 8.2, 36.4 I Impermeability 1.16, 17.5, 34.51 Impulse Radar System 33.21 Infrared Spectroscopy 8.13 Insulable Residue 1.11, 1.28, 37.7 Integral Water Proofing 8.4 Interfacial Transition Zone 13.1, 31.14 Internal Curing Admixture 36.5, 36.6, 36.7 Initial Surface Absorption Test 33.19, 33.20 Internal Vibration 11.6, 11.13 Ionising Radiation 24.1 IS 3812 1.16, 32.7, 37.26 IS 456 1.13, 9.41, 37.26 IS 4926 10.15, 32.10, 37.22 K K Value 31.4, 33.4 Kaiga 17.14, 18.1 Kiln feed 1.4, 1.8 Kota 18.1 L Latex Modified Concrete 14.9, 30.7 Le Chatelier Test 1.11, 37.22 Leaching 1.16, 13.36, 13.44 Life Cycle Cost 31.3, 34.45, 36.12 Light Weight Aggregate Concrete 4.2 Light Weight Concrete 22.3, 22.6, 34.13 Lignosulphonate 8.5 Lime Saturation Factor 1.11, 1.12

Long Chained Polymers 29.8 Long Haul Concrete 36.7 Loss on Ignition 1.11, 10.12, 37.9 Lubricating 8.11, 11.4 M Machine Characterstics 13.16 Magnesia 1.10, 1.11, 37.7 Manual 1.10, 23.18, 34.59 Manufactured Sand 6.1, 6.2, 6.6 Manufacturing Process 1.3, 6.3 Marine Environment 9.4, 13.41, 15.7 Masonry Cement 1.14, 37.2, 37.25 Mass Concrete 1.16, 19.5, 26.3, 37.12 Material Optimization 36.9, 36.10 MDF Cement 30.10, Mechnical Properties 13.10, 13.49, 21.4 Membrane Curing 31.15 Membranes 14.4, 34.8, 36.6 Membranes 14.4, 34.8, 36.6 Metakaoline 2.16, 35.8, 37.8 Metakaoline 2.16, 35.8, 37.8 Methyl Acrylate 30.8 Micro Silica 8.3, 1.17, 24.3 Microstructure 12.13, 17.7, 36.7 Milled Foam Blocks 23.4 Mineral Additives 1.6, 17.6, 32.13 Mineral Admixture 1.17, 9.8, 35.6, 37.15 Mitigation 15.2, 16.5 Mix Design 7.3, 13.17, 32.13, 36.9 Mixing Arrangement 10.8 Mixing Time 10.8, 27.10, 37.11 Mixing Water 3.2, 10.8, 17.11, 37.10 Mobile Boom Pump 11.2 Modes of Failure 13.13 Modulus of Rupture 13.20, 23.20 Monocellular Foam 28.1, 28.8 Monolithic Action 26.1 Mortar Bar Test 16.4, 16.5 Mould 1.10, 31.17, 37.24 MSA 9.2, 9.35, 27.4, 31.11

Index

N Neutron Shield 24.2, 24.3 No-fines Concrete 22.2, Noise Transmission 22.5 Non Destructive Testing 13.49, 33.4, 37.25 Sampling 3.3, 37.18, 37.25 Reliability 27.12, 33.22 Noxiousness 6.4, 6.5 Nuclear Reactor 24.1 Nylon 23.2, 23.15, O Optimum Process 6.4 Optimum Shape 6.1 Osmosis 13.40, 20.4, 34.44 Overlay 8.5, 30.3, 36.10, 36.11 P Pantheon 22.1, 22.5 Parameters of Concrete Pump 11.3 Particle Size Distribution 1.9, 6.4, 29.3 Passive Layer 14.1, 14.3, 34.49 Percolation Rate 21.3, 21.6 Pervious Concrete 21.1, 21.2, 21.9, 21.12 Pervious Pavement 21.2, 21.3, 21.7 Pessium Content 16.7 pH Value 3.3, 33.18, 35.6 Physical Properties 1.9, 7.2, 32.11 Physical Properties of Cement 1.9, 32.8 Placement Lines 11.13 Plastic Cracking 36.5 Plastic Shrinkage 11.16, 12.13, 36.7 Plate Vibrator 11.10, 31.15 Pointing 1.26 Poisson’s Ratio 13.5, 13.6, 13.7 Pollutants 1.21, 21.1 Polymer Fibre 23.17 Polymer concrete 14.9, 30.3, 34.31 Polymer Modified Concrete 30.3, 30.4, 34.37 Polymer impregnated concrete 30.3, 30.8, 30.9 Polymerization 30.1, 30.8, 34.31 Polymers 8.12, 30.3, 34.33, 36.6

Polysulphide 31.8 Polyurethane 15.3, 34.20, 37.16 Porosity 1.20, 13.1, 13.37, 36.6 Portland Cement 1.1, 1.16, 9.6, 31.14, 37.26 Pozzolonic Reaction 2.1 PQC 23.8 Prediction 14.15, 33.5, 34.48 Prescriptive Specifications 17.1, 34.54 Preventive Measures 13.35, 15.3 Priming of Concrete Pump 11.4 Process Control 6.4, 32.15 Product Control 32.4 Production Control 25.2, 32.12 Production Plants 10.8 Production Process 7.7, 28.5, 36.9 Pulse-Echo Technique 33.3 Pumice Aggregate 22.5 Pumpable Concrete 7.3, 9.34, 11.18 Q Quality Assurance (QA) 32.1, 32.2 Quality Assurance Procedure (QAP) 33.4 Quality Control (QC) 32.1 Quality of Cover 14.14 Quality of Water 30.1, 37.1 Quality Scheme 32.4, 32.5, 32.17 R Radiation 4.2, 24.1, 35.3 Rate of Corrosion 14.8, 34.15, 34.50 Rate of Loading 13.11, 13.16 Rate of Setting 19.2 Ready Mix Concrete 7.5, 7.9, 36.7 Rebar Locator 33.18 Rebound Hammer Test 33.8, 33.22 Recycled Aggregate 4.4, 35.8 Recycled Concrete 4.4, 35.5, Recycled Concrete Aggregate 4.4, 35.5 Reflectivity 31.2 Rehabilitation 1.5, 33.2, 36.1 Reinforcement Cover 14.1, 14.7, 35.8 Relative Humidity 1.11, 12.8, 16.10, 34.6

I.5

I.6

Index

Reliability 27.12, 33.17, 33.22 Resin Cement 30.4 Retrospective Control 32.3, 32.4 Rheological Properties 17.6 Rheology 6.3, 17.11, 29.10 Rheology 6.3, 17.11, 29.10 Rice Husk Ash 1.26, 14.10, 37.6 Rice Husk Ash 1.26, 14.9, 37.6 Riding Quality 31.2, 36.11 Rigid Drainage Layer 21.2 Rigid Pavement 31.22, 31.27 Rise in Temperature 1.10, 26.1, 26.5 Roller Compacted Concrete (RCC) 27.14 Roof Insulation 22.5 Rotary Form Vibrator 11.11 Rubbarised Bitumen 31.8 Rubber 11.3, 30.5, 37.16 S Sand Streaks 11.13, 11.14 Screed Pump 28.6, 28.7 Sea Walls 35.7 Sealants 4.4, 34.4 Secondary Forces 26.1 Secondary Stresses 26.1 Segregation 1.16, 8.4, 11.8, 18.12, 27.11, 34.4 Self Consolidating Concrete 18, 36.2 Constituents 1.2, 13.1, 37.23 Fill Box Test 18.5, 18.10 Filling Ability 18.2, 18.8, 18.11 Flowability 18.3, 25.2, 34.35 J’ Ring Test 18.5 Passing Ability 18.2, 18.9, 18.11 Seggregation Resistance 18.2, 18.7 Slump Flow 9.1, 18.5, 18.11, 18.13 Stabilizing Agent 18.2 U Box Test 18.5, 18.9 V Funnel Test 18.5, 18.13 Viscosity Modifying Agent 18.2, 18.12, 21.6 Self Curing Concrete 36.4, 36.6 Semi-wet Process 1.4

Sequence of Mixing 17.4 Setting Time 1.9, 1.27, 19.2, 37.34 SFRC 23.3, 23.7, 23.22 Shape 1.10, 5.2, 13.50, 37.20 Shear Modules 13.7 Shrinkage reducing Admixtures 8.4, 8.8 Silica Fume 2.1, 2.14, 2.18, 36.3, 37.26 Silica Suspension 29.5, 29.6 Silicon 2.7, 2.16, 37.6 Silicone 31.8, 34.54 Silo 1.4, 1.9, 33.2 Slipform 9.5, 31.18, 37.12 Slump 2.9, 7.5, 8.10, 9.21, 19.2, 26.637.23 Slump Loss 8.2, 9.2, 36.4 Slump Retention 7.5, 32.15, 36.5 Slurry 1.4, 10.13, 29.9, 37.12 Solar Radiation 19.1, 35.3 Soundness 1.9, 4.6, 37.2, 37.21 Spalling 13.35, 34.6, 36.3 Special Grade Cement 1.20 Specific Heat 4.9, 26.5, 30.9 Specimen Geometry 13.14 Spent Fuel Storage 24.1 Split Tensile Test 13.19 Spreading 1.23, 27.12, 31.19 Stability Certification 33.1 Stabilizer 23.15, 29.10, 36.8 Standard Deviation 7.5, 9.3, 9.15, 37.9 Statistical Analysis 13.10, 33.11, 33.13 Statutory Requirements 32.7, 33.2 Steady Flow 27.10 Steam Curing 11.17, 11.18, 23.22 Steel Fibres 23.1, 23.7, 23.8, 37.27 Steric Hindrance Effect 8.11 Storage of Cement 1.5, 1.23, 37.9 Straight Fibres 23.4 Strength Development 1.2, 2.9, 13.11, 36.9, 36.10 Strength Gain 1.18, 11.15, 17.7, 20.3 Strength Grade 1.13, 17.9 Stretching Ability 23.1

Index

Stripping Time 37.13 Structural Integrity 11.13, 26.1, 34.26 Styrene 4.2, 30.5, 34.31 Subsidence Cracking 11.13 Sulphate Attack 1.2, 1.5, 15.5, 16.3, 31.16 Sulphonated Melamine formaldehyde 8.6 Sulphonated Naphthalene formaldehyde 8.6 Sulphuric Anhydride 1.11, 1.12, 37.3 Super Heavy Aggregate 24.1 Super High Density Concrete 24.1 Super Plasticizer 6.3, 23.6, 29.7, 36.3 Super Slump Retainng Concrete 36.4, 26.5 Supplementary cementitious material 9.33, 15.6, 35.8 Flyash 4.2, 9.9, 9.13, 9.31, 9.36, 17.7, 37.20 Particle Shape 2.1, 5.2, 37.5 Density 2.9, 13.9, 21.9, 27.13, 37.12 Physical Requirements 2.3, 10.13 Chemical Requirements 2.2, 37.9 GGBS 1.17, 2.13, 13.42, 34.41, 37.17 Chemical Composition 1.3, 34.42 Chemical Reaction 1.21, 13.42, 35.4 Silica Fume 2.1, 9.13, 17.10, 29.9 Available Forms 2.12 Physical & Chemical Properties 2.13 Surface Absorption Test 33.19, 33.20 Surface Course 21.2, 21.3 Synthetic Fibres 23.1, 23.14, 23.17 T Technology Innovations 36.1 Temperature Control 26.3, 26.6 Tensile Capacity 12.5, 23.1 Tensile Strength 13.2, 13.19, 13.5, 31.13, 37.27 Testing 1.11, 13.1, 13.20, 33.1, 33.15, 37.27 Compressive strength 1.9, 9.21, 13.17, 37.24 Flexural Strength 6.1, 30.7, 34.34, 37.18 Tensile Strength 13.2, 23.1, 23.16, 37.27 Modulus of elasticity 12.9, 31.13, 34.37 Texturing 29.15, 31.18, 31.20 Thermal Cracking 2.9, 26.2, 31.12

Thermal Cracks 16.3, 36.4 Thermal Insulation 21.2, 28.2 Thermal Properties 4.5, 4.9, 13.32 Thermal Strain 17.3 Thermoplastics 30.2 Thermosets 30.2 Tolerance Control 10.14 Tooled Joints 21.10 Transducers 33.12, 33.13 Transit Mixer 10.17, 23.7, 36.8 Transmission 13.7, 33.12, 34.22 Transportation 1.10, 1.16, 14.15, 36.5 Transverse Joints 31.6, 31.17 Tremie 9.5, 11.5, 25.3, 25.6, 37.12 Tunnel Lining 18.16, 33.3 Typical Plant Layout 10.1 U Ulta sonic Pulse Velocity 13.5, 33.3, 33.12 Ultra High Strength 36.2, 36.4 Ultrathin White Topping 31.8, 31.9 Underlays 28.2 Underwater concreting 25.1, 25.7 Unhydrated Ferric oxide 14.3 Uniaxial Tensile Test 13.19 Uniformity Requirement 10.12, 32.9, 32.16 V VeeBee time 9.16, 9.12 Vibrator Influence 11.8 Viscosity 6.3, 18.6, 18.12, 34.24, 36.4 Void Content 6.2, 21.1, 21.6 Volume Stability 11.14, 17.3, 29.1 W Water Demand 2.3, 2.17, 27.4, 27.5 Water Repellents 8.7 Water Requirement 3.1, 10.13, 35.6 Water Stops 27.7, 27.9 Water to binder Ratio (w/b) 17.10 Water to cement ratio (w/c) 13.36 Westeryaard’s Analysis 31.4

I.7

I.8

Index

Wet Process 1.4, 1.8, 34.41 Whisper Concrete 31.3, 31.21 White Topping 31.8, 31.9, 36.12 Wind Speed 19.1, 19.5

Y Yield Stress 36.3 Yield Value 18.12 Young’s Modulus 23.13, 23.17

X X-SEED 36.9, 36.10

Z Zero Energy System 36.2

About the Authors Shri Raj K. Agarwal, B. Sc, L. Lb. is Managing Director, Marketing and Transit (India) Pvt. Ltd., New Delhi. He has been working with the project authorities of hydroelectric projects in the country for the last 15 years. He is also interacting with the cement manufacturers of the country, and providing guidance to both cement manufacturers and Construction Industries, for the use of proper construction materials, in order to achieve longer service life and durable concrete structures. He has contributed effectively in several projects towards use of proper cement and concrete mix in structures of hydroelectric projects (for concrete grades M15 to M 60), including use of Portland slag cement (instead of OPC) from durability point of view. Mr. Sanjay Bahadur, 48, has done his B.E. (Civil) from Delhi College of Engineering. He has rich experience of 26 years in construction industry. He was with L & T-ECC Construction Group, India and Iraq for about 6 years. He is associated with Unitech Group since 1993 and has worked as Managing Director of Unitech Prefab Ltd., Mumbai heading the ‘Ready Mix Concrete’ (RMC), Concrete Blocks and Pavers Operations in India and Chief Executive Officer (Construction Chemicals/Paints) of Pidilite Industries Ltd., Mumbai. He is presently providing consultancy in building material. Shriram Gajanan Bapat is Civil Engineering Graduate from University of Pune of 1967. After working for Maharashtra P.W.D. for brief period of four years, he joined Power Projects Engineering Division of Department of Atomic Energy, which was subsequently converted as Nuclear Power Corporation of India Limited (NPCIL). He worked there in various capacities as Design Engineer, Construction Engineer, Project Engineering etc. He is responsible for adopting innovative techniques in construction like use of fly ash concrete, self-compacted concrete, mechanical threaded couplers for joining rebars for reducing congestion etc. He retired on 31st March 2007 as Chief Engineer (Civil-Design). Presently, he is working as a Freelance Consultant in the field of Civil Engineering Designs and Construction related activities. Anjan Kumar Chatterjee is a Materials Scientist and has been in research, teaching and business management in the fields of cement, concrete and advanced materials for nearly five decades, a significant part of which was as the Wholetime Director of ACC Limited. His research works include phase equilibria of oxy-fluoride systems, manufacture of mineralized clinker, production of calcium sulfoaluminate cements, oilwell cements, blended cements, high-

A.2

About the Authors

strength and high-performance concretes, innovative uses of fly ashes and other environmental amelioration technologies. He has served the United Nations Industrial Development Organization as a Cement Specialist for projects in the Peoples Republic of China and Nigeria He has a large number of publications to his credit. He is a Fellow of the Indian National Academy of Engineering and holds affiliation to many other professional bodies. He is on the editorial board of Cement and Concrete Research and has launched in 2010 an International Journal on Repair, Restoration and Renewal Engineering. Presently he is holding the post of Managing Director of Conmat Technologies Private Limited, a consulting outfit in Kolkata, India and concurrently he is also the Director-in-Charge of Dr. Fixit Institute of Structural Protection and Rehabilitation, Mumbai. Mr. Ganesh Chaudhari is a Civil engineer with 14 years exposure to construction technology and steel reinforcement industry. Having begun his career as a pre-stressing engineer at Freyssinet, moved on to sales and then into marketing at TATA STEEL. He has been involved in the design and application of steel fiber reinforced concrete for the last 7years. Further more; he has presented his papers in many international and national seminars at prominent institutes like NCBM, ICI, IIT, ACI and SERC Chennai. He is currently heading the building product business all over West Asia for Bekaert – No#1 steel wire manufacturers in the world. Mr. Ganesh Chaudhari is a Graduate engineer with Masters in Systems and Marketing. He has also completed his Post Graduate course in operation research and Management. Shri. Amit Datta, born in 1966, completed Diploma in Civil Engineering from Calcutta Engineering College in Kolkata in1986 and for last 25 years, servicing an Indian Construction and Construction Chemical Industry. General Manager-Sales and Marketing with a French Multinational Company called CHRYSO, he is actively participating in Civil Engineering Industry and sharing his experience to Civil Engineering Industry. Mukul Dehadrai is an engineering professional with an extensive background on durability, testing and performance of civil engineeringmaterials, both in the laboratory as well as onsite. He received his degree in Materials Engineering from Purdue University, USA and is an author of peer reviewed papers for international journals; conferences on topics related to the early age testing and property development in concrete and has served as a consultant on projects in the infrastructure as well as real estate domains. He currently manages his own consulting firm which provides concrete technology and durability consulting to industry majors. He serves as an active member of several committees in the American Concrete Institute (ACI) and the American Society of Testing and Materials (ASTM). He is also a serving member of the National Association of Corrosion Engineers. Bert Dijkema, born in Groningen, the Netherlands on 22 May 1958 he obtained his B Sc in Structural Engineering at the HTS Groningen in June 1983 and his B Comm at the same institution in June 1984. Bert started his career as assistant technical manager for Ash Resources (Pty) Ltd in Johannesburg, South Africa on 1 December 1984. He conducted research into the

About the Authors

A.3

effects of fly ash on the durability of concrete between 1985 and 1988. Between 1988 and 1998 he was manager technical sales and marketing manager. From 1998 until 2005 he was the Managing Director for Ash Resources and during this period he promoted the use of fly ash in concrete to enhance durability in the Middle East. Bert has given a number of papers at International conferences on the effects of fly ash on concrete durability. From 2005 to present, Bert Dijkema is working for the Chryso Group in Paris, France where he has the function as International Development Director and is as such responsible for the development of the Company and its products in India. Chryso is a leading manufacturer of concrete admixtures with many high technology and patented products in the new generation (PCP) type of products. C.M. Dordi is B. Tech honours (1971) and M.Tech (1973) from IIT Bombay. He is currently working with Ambuja Cements Ltd (ACL) as Corporate Head (PQM & CSG). He has total experience of 37 years. He was as Head of Customer Support Group (West) in ACL responsible for conducting and/ or organizing 670 Ambuja workshops covering over 16,500 professionals on concrete mix design and its various associated topics. Under his authorship and guidance, ACL have published 145 technical booklets. He has delivered so far over 825 lectures in India and abroad and authored 109 technical papers in various conferences and seminars. He is at present an external Member for Senate, IIT Bombay for the years from 2010 to 2012. A.K. Jain studied Civil Engineering at University of Roorkee (India) and Masters in Construction Management at IIT Delhi. He served in CBRI Roorkee, Irrigation Dept., (U.P.) and Ministry of Defence as Class I officer in Military Engineer Services in various capacities. He has long association with cement and concrete businesses of M/s Grasim Industries Ltd and Ultra Tech Cement. His main areas of interest are cementitious materials, concrete mix optimization, concrete durability, good construction practices and cement based new products. He is member of BIS Committees on cement and concrete and active member of various professional bodies. He has presented number of technical papers in National and International Seminars. D.K. Jain is a graduate in Civil Engineering from IIT, Roorkee. He is presently working as Chief Engineer (Civil) in Nuclear Power Corporation of India Limited. He has over 35 years of experience in the field of Design and Construction of Nuclear Power Plants. He has significant contributions to his credit in the field of Construction and has been associated with almost all nuclear power plants that have been set up so far in India. He has been involved in the development and successful introduction of various new technologies in construction, including new generation concretes, for speeding up the construction as well as improving the quality.

A.4

About the Authors

Dr. L.R. Kadiyali obtained his degree in Civil Engineering from Bombay University, securing the first rank and obtaining two gold medals. He joined Ministry of Transport, Govt. of India on the basis of All-India Engineering Services Examination and served in various capacities and retired as Chief Engineer. During his tenure, he was involved in the planning and development of National Highways. He was associated with the design of the Mumbai-Pune Expressway. He has been associated with the preparation of long term road development plans, 1961-1981, 1981-2001 and Vision Document for 2021. He was deputed by the Govt. of India to England for Post-graduate studies at the Newcastle University. On his return, he obtained his PhD in Civil Engineering from the Regional Engineering College, Warangal. He has authored several Papers on Highway Engineering and has been awarded medals for five Papers by the Indian Roads Congress (IRC). After retirement, he has founded his own Consultancy firm and has designed several highway projects. He has authored two text books on Highway Engineering and Traffic Engineering. He has recently been awarded the life-time achievement award for his contribution to concrete technology by the Indian Concrete Institute. Recognizing his work on concrete roads, he has been elected as honorary member of the American Concrete Pavement Association. Mr. Himanshu Kapadia obtained his Civil Engineering (Honors) from Ravishankar University Raipur in 1988. Since 1995, he has been working in the Construction Chemicals Industry. He has been instrumental in technology transfer and introduction of product systems for a variety of concrete improvement, repair and protection systems. Currently he is based in Singapore and heads BASF Construction Chemicals in ASEAN countries. He was also the founder President of Construction Chemicals Manufacturers Association, India. Ganesh Kaskar holds Masters Degree in Civil Engineering from the prestigious Indian Institute of Technology, Mumbai. His specialization is in Concrete Technology. He brings with him rich experience of around 3 decades in the Civil Construction field, out of which he devoted more than16 years to the Readymixed Concrete industry. He has a long association with RMC India since 1996 and currently holds position as the Executive Director. He was actively involved in setting up the first commercial readymixed concrete operations in India. His earlier assignments in India, have been with major companies like ACC and Tata Consulting Engineers. He was elected President of the Readymixed Concrete Manufacturers’ Association, Mumbai during the period 2007 – 2009. Jayant Kulkarni studied his Civil Engineering at – V.J.T.I., Mumbai University (1976), M.E.(Struct) - V.J.T.I., Mumbai University (1978), Interior Design – J. J. School, Mumbai University (1979), P.G. Diploma - Indian Aesthetics – Mumbai University (2002) and gained rich experience in Structural Design, Project Management and Repairs/Rehabilitation of Structure. He is currently Managing Director of M/s. Epicons Consultants Pvt. Ltd., Thane. He is also a Founder Member and Trustee of ‘We Need You’ Society, working in the field of Education for Poor, Women Upliftment, Cultural Development and Aids Awareness at Ghansoli, Thane.

About the Authors

A.5

Mr. Vijay Kulkani began his career as a design engineer with PWD Maharashtra. After a brief stint with academics, he joined ACC Ltd., where he was involved in R & D work in concrete, NDT and repair and rehabilitation of concrete structures. He was a part of the R and D project on high-performance concrete, jointly undertaken by ACC Ltd and Atomic Energy Regulatory Board. He also undertook quality audit of the Mumbai-Pune Expressway. Mr. Kulkarni was the Editor of the Indian Concrete Journal for more than a decade and authored and published many technical booklets published by ACC. Presently, as Principal Consultant to Ready Mixed Concrete Manufacturers Association, he evolved Quality Scheme for ready-mixed concrete and is involved in its implementation. He received the “Eminent Editor” award and a gold medal from Indian Roads Congress for best technical paper. Mr. Kulkarni was the President of the Indian Concrete Institute for the years 2009-2011. A.K. Laharia is a graduate in Civil Engineering and Post Graduate in Structural Engineering. He is presently working as Additional Chief Engineer (Civil) in Nuclear Power Corporation of India Limited. He has experience of more than 21 years in the field of Design and Construction of Nuclear Power Plants. He has made significant contributions in development of various types of concretes like High Performance Concrete, High Volume Fly Ash Concrete, Self Compacting Concrete, Heavy Density Concrete and Super Heavy Density Concrete being used in Indian Nuclear Power Plants. Dr. S.C. Maiti obtained his PhD in structural engineering from the Indian Institute of Technology (IIT), Kharagpur in 1971. He has 32 years of experience in concrete technology at the National Council for Cement and Building Materials (NCCBM) and retired as joint director in 2003. He was a member of the panel for revision of IS 456: 2000. He has supervised reconstruction of 2,00,000 houses in earthquake affected villages of Gujarat, ensuring quality in construction. Presently, he is a technical consultant in Delhi. Manish Mokal graduated in 1997 from University of Mumbai and completed Masters degree in structures. He is currently employed as Senior Manager at Gammon India Limited, where he is incharge of Quality Control Department in Corporate Office. He has worked on several prestigious infrastructure projects like Bandra Worli Sea Link Project, Nuclear Power Plant, Airport etc. and published number of technical papers on High Performance Concrete and High Volume Fly Ash Concrete. His main areas of interests relate to high strength concrete, durability of concrete and concrete structures. N.V. Nayak graduated in Civil Engineering from the University of Bombay in the year 1959. He secured his M. Tech. in Civil Engineering from the Indian Institute of Technology, Bombay and his PhD from the University of Wisconsin, U.S.A. in the year 1970. He has about 8 years of Teaching, 8 years of Consultancy and around 35 years of Practical Field experience. He is presently Advisor – Gammon India Ltd., Managing Director – Gammon Realty Ltd., Chairman – Geocon International Pvt. Ltd. He has published many papers

A.6

About the Authors

in National and International journals. He is also Examiner of Ph. D candidates of Mumbai University and IIT Bombay mainly on subjects related to Geotechnical/Foundation Engineering and in Concrete Technology. Mr. B.V.B. Pai is a civil engineer by profession and has worked in the Cement Industry for the past 40 years. Most of his experience has been garnered at M/s ACC Ltd where he has served in various capacities till the year 2000. During his tenure at M/s ACC Ltd he has handled assignments in the development of special products like Rapigrout, Shrinkomp, Elecrete etc. He has been associated with the RMC Business (the Bandra-Kurla Complex) and evaluation of HPC,HSC and Fibers. Utilization of Fly-ash, making of products like bricks, blocks, aggregates and light-weight material is a special interest. He has also been Editor-in-chief of The Indian Concrete Journal from 1994 to 2000. His current activities as advisor to BASF Ltd, ALCON Ltd and various other companies. Current interests are in the field of particle packing of binders and specialty admixtures. Arvind Parulekar studied his Civil Engineering at – V.J.T.I., Mumbai University (1987), M.E. (Struct) – M. S. University, Baroda (1989) and gained rich experience in the field of Structural Design, Detail Engineering, Repair/Rehabilitation and Non-destructive Assessment of Structures. He is currently Director of M/s. Epicons Consultants Pvt. Ltd and has more than 800 consultancy assignments to his credit of various nature, related to Structural Engineering. Chetain Raikar is Civil Engineering graduate of 1986. Professional experience in the fields of investigation of structures, structural designing and material testing. Founder of R and D Centre and material testing laboratory of Structwel Designers and Consultants Pvt. Ltd. for Engineering Material Testing, NDT and structural testing, which has expanded to a total of 6 laboratories. He has expertise in Stability and damage level assessments and repairs and restoration of structures, Structural up gradation of structures with particular reference to earthquake adequacy assessment, Conservation of heritage buildings, Structural Design and PMC of various Residential, Commercial and Industrial Projects etc. Certified Trainer of American Concrete Institute, USA for courses in concrete testing. Has two patents in innovation in testing machine development. Has authored and presented more than 100 papers in National and International conferences. Sunil D. Sapre is presently working as Dy. Director/Vice President (Projects) with Patel Engineering Ltd. in Mumbai. He graduated from University of Mumbai in 1980 and completed Post Graduate Diploma in Construction Management in 1987. He has 30 years extensive experience in the construction industry out of which last 20 years in the Hydro Power Sector in India. He was actively associated with First Underwater Double Lake Tap in Asia at Koyna Hydro Electric Project and First Roller Compacted Concrete (RCC) Dams in India at Ghatghar Hydro Electric Pumped Storage Scheme, both in Maharashtra at a very senior level from the Contractor’s end. Presently heading projects worth INR 1000 + crs.

About the Authors

A.7

Starting with Tendering and Estimating in Patel Engineering in 1991, he went on to handle Project Management, Business Development and Contracts Management extensively and has been doing so till date. Samir Surlaker studied civil and structural engineering from VJTI, affiliated to University of Mumbai. He started his career as teacher in V.J.T.I in Concrete Technology and Structural Engineering.He is currently Managing Director of MC-Bauchemie India, a German Collaboration construction chemicals manufacturer. He is expert in the field of concrete technology having over 30 years experience internationally in the field of construction chemicals. He was actively involved in advanced research in compatibility of Admixtures with various cements, supplementary cementations materials and inter admixtures compatibility. He is involved in development of special concrete, like Foam concretes, Acid resistance concretes, SCC, HPC etc. He has contributed several papers at seminars and written many articles in promoting construction chemicals usage in the field of waterproofing and repairs and rehabilitation of concrete. He is member of BIS-CED committees. He is member of several Professional Associations and actively involved in delivering lectures in his chosen field with a view to increase the awareness regarding durability of Concrete. He is recipient of ICI Mumbai 2006 award for outstanding contribution to concrete technology.