Concrete Technology Theory and Practice

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C H A P T E R Modern Cement Factory Courtesy : Grasim Industries Cement Division

Cement ! General ! Early History of Modern Cement ! Manufacture of Portland Cement ! Wet Process ! Dry Process ! Chemical Composition ! Hydration of Cement ! Heat of Hydration ! Calcium Silicate Hydrates ! Calcium Hydroxide ! Calcium Aluminate Hydrates ! Structure of Hydrated Cement ! Transition Zone ! Water Requirements for Hydration

General

T

he histo ry o f ce me nting mate rial is as o ld as the histo ry o f eng ineering co nstructio n. So me kind o f c e m e n tin g m a te ria ls w e re u se d b y Eg yp tia n s, Ro mans and Indians in their ancient co nstructio ns. It is b e lie ve d th at th e e arly Eg yp tian s m o stly u se d ce me nting mate rials, o b taine d b y b urning g ypsum. No t m uc h lig h t h as b e e n th ro w n o n c e m e n tin g m ate rial, use d in the c o nstruc tio n o f the c itie s o f Harap p a and Mo he njad aro . An analysis o f mo rtar fro m the Gre at Pyramid sh o w e d th at it c o n tain e d 8 1 .5 p e r c e n t c alc ium sulphate and o nly 9.5 per cent carb o nate. The early G re e ks an d Ro m an s u se d c e m e n tin g m ate rials o b taine d b y b urning lim e sto ne s. The re m arkab le h a rd n e ss o f th e m o rta r u se d in e a rly Ro m a n b ric kw o rks, so m e o f w hic h still e xist, is p re se nting sufficient evidence o f the perfectio n w hich the art o f c e m e nting m ate rial had attaine d in anc ie nt tim e s. The superio rity o f Ro man mo rtar has been attributed to th o ro u g h n e ss o f m ixin g an d lo n g c o n tin u e d ramming . The Greeks and Ro mans later b ecame aw are o f the fac t that c e rtain vo lc anic ash and tuff, w he n

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mixe d w ith lime and sand yie ld e d mo rtar p o sse ssing sup e rio r stre ng th and b e tte r d urab ility in fresh o r salt w ater. Ro man builders used vo lcanic tuff fo und near Po zzuo li villag e near Mo unt Ve suvius in Italy. This vo lcanic tuff o r ash mo stly silice o us in nature thus acq uire d the name Po zzo lana. Late r o n, the nam e Po zzo lana w as ap p lie d to any o the r m ate rial, natural o r artificial, having nearly the same co mpo sitio n as that o f vo lcanic tuff o r ash fo und at Po zzuo li. Th e Ro m an s, in th e ab se n c e o f n atural vo lc an ic ash , use d p o w e re d tile s o r p o tte ry as po zzo lana. In India. po w ered brick named surkhi has been used in mo rtar. The Indian practice o f thro ug h mixing and lo ng co ntinue d ramming o f lime mo rtar w ith o r w itho ut the ad d itio n o f Surkhi yie ld e d stro ng and imp e rvio us mo rtar w hich co nfirme d the se cre t o f sup e rio rity o f Ro man mo rtar. It is le arnt that the Ro mans ad d e d b lo o d , milk and lard to the ir mo rtar and co ncre te to ac hie ve b e tte r w o rkab ility. Hae mo g lo b in is a p o w e rful air-e ntraining ag e nt and p lastic ize r, w hich perhaps is yet ano ther reaso n fo r the d urab ility o f Ro man structures. Pro b ab ly they d id no t kno w ab o ut the d urab ility asp e ct b ut use d the m as w o rkab ility ag e nts. The ce me nting mate rial mad e b y Ro mans using lime and natural o r artificial Po zzo lana re taine d its p o sitio n as the chie f b uild ing mate rial fo r all w o rk, p articularly, fo r hyd raulic co nstructio n. Be lid o r, a principal autho rity in hydraulic co nstructio n, reco mmended an initimate mixture o f tiles, sto ne chips, and scales fro m a b lack-smith’s fo rg e, carefully g ro und , w ashed free fro m co al and d irt, d rie d and sifte d and the n mixe d w ith fre sh slake d lime fo r making g o o d co ncre te . When w e co me to mo re recent times, the mo st impo rtant ad vance in the kno w led g e o f c e m e n ts, th e fo re run n e r to th e d isc o ve rie s an d m an ufac ture o f all m o d e rn c e m e n ts is und o ub te d ly the inve stig atio ns carrie d o ut b y Jo hn Sme ato n. Whe n he w as calle d up o n to re b uild the Ed d ysto ne Lig ht-ho use in 1 7 5 6 , he mad e e xte nsive e nq uirie s into the state o f art e xisting in tho se d ays and also c o nd uc te d e xp e rim e nts w ith a vie w to find o ut the b e st mate rial to w ithstand the se ve re actio n o f se a w ate r. Finally, he co nclud e d that lime -sto ne s w hic h c o ntaine d c o nsid e rab le p ro p o rtio n o f c laye y m atte r yie ld e d b e tte r lim e p o sse ssing sup e rio r hyd raulic p ro p e rtie s. In sp ite o f the suc c e ss o f Sm e ato n’s e xp e rim e nts, the use o f hyd raulic lime mad e little p ro g re ss, and the o ld p rac tic e o f mixture o f lime and p o zzo lana remained po pular fo r a lo ng perio d. In 1976 hydraulic cement w as made by calcining no dules o f arg illac e o us lim e -sto ne s. In ab o ut 1 8 0 0 the p ro d uc t thus o b taine d w as c alle d Ro m an c e m e nt. This typ e o f c e m e nt w as in use till ab o ut 1 8 5 0 afte r w hic h this w as o utd ate d b y p o rtland ce me nt.

Early History of Modern Cement The inve stig atio ns o f L.J. Vic at le d h im to p re p are an a rtific ia l h yd ra u lic lim e b y c alc ining an intim ate m ixture o f lim e sto n e a n d c la y. Th is p ro c e ss m ay b e re g ard e d as the le ad ing kno w le d g e to the m a n u fa c tu re o f Po rtla n d c e m e n t. Ja m e s Fro st a lso patented a cement o f this kind in 1 8 1 1 a n d e sta b lish e d a facto ry in Lo nd o n d istrict.

Joseph Aspdin’s first cement works, around 1823, at Kirkgate in Wakefield, UK. Courtesy : Ambuja Technical Literature

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The sto ry o f the inventio n o f Po rtland cement is, ho w e ve r, attrib ute d to Jo se ph Aspd in, a Le e d s b u ild e r a n d b ric kla ye r, e ve n th o u g h sim ila r pro cedures had been ado pted by o ther invento rs. Jo seph Aspdin to o k the patent o f po rtland cement o n 2 1 st O c to b e r 1 8 2 4 . Th e fa n c y n a m e o f p o rtland w as g ive n o w ing to the re se mb lance o f th is h a rd e n e d c e m e n t to th e n a tu ra l sto n e o c c urring at Po rtland in Eng land . In his p ro c e ss Asp d in m ixe d and g ro und hard lim e sto ne s and fin e ly d ivid e d c lay in to th e fo rm o f slu rry an d calcined it in a furnace similar to a lime kiln till the CO 2 w as e xp e lle d . The m ixture so c alc ine d w as th e n g ro u n d to a fin e p o w d e r. Pe rh a p s, a te m p e ra tu re lo w e r th a n th e c lin ke rin g te mp e rature w as use d b y Asp d in. Late r in 1 8 4 5 Isaac Charles Jo hnso n b urnt a mixture o f clay and c h alk till th e c lin ke rin g stag e to m ake b e tte r ce me nt and e stab lishe d facto rie s in 1 8 5 1 . In th e e arly p e rio d , c e m e n t w as use d fo r making mo rtar o nly. Late r the use o f ce me nt w as e xte n d e d fo r m akin g c o n c re te . As th e u se o f Po rtla n d c e m e n t w a s in c re a se d fo r m a kin g co ncre te , e ng ine e rs calle d fo r co nsiste ntly hig he r Oldest surviving kiln, northeast Kent, UK, sta n d a rd m a te ria l fo r u se in m a jo r w o rks. (1847AD). Courtesy : Ambuja Technical Literature Asso ciatio n o f Eng ineers, Co nsumers and Cement Manufac ture rs have b e e n e stab lishe d to sp e c ify stand ard s fo r ce me nt. The Ge rman stand ard sp e cificatio n fo r Po rtland ce me nt w as d raw n in 1877. The British standard specificatio n w as first draw n up in 1904. The first ASTM specificatio n w as issue d in 1 9 0 4 . In India, Po rtland cement w as first manufactured in 1904 near Madras, by the So uth India Ind ustrial Ltd . But this ve nture faile d . Be tw e e n 1 9 1 2 and 1 9 1 3 , the Ind ian Ce me nt Co . Ltd ., w as estab lished at Po rb and er (Gujarat) and b y 1 9 1 4 this Co mpany w as ab le to d eliver ab o ut 1 0 0 0 to ns o f Po rtland cement. By 1 9 1 8 three facto ries w ere estab lished . To g ether they w ere ab le to p ro d uce ab o ut 8 5 0 0 0 to ns o f ce me nt p e r ye ar. During the First Five -Ye ar Plan (1 9 5 1 1 9 5 6 ) ce me nt pro d uctio n in Ind ia ro se fro m 2 .6 9 millio n to ns to 4 .6 0 millio n to ns. By 1 9 6 9 the to tal p ro d uctio n o f ce me nt in Ind ia w as 1 3 .2 millio n to ns and Ind ia w as the n o ccup ying th e 9 th p lac e in th e w o rld , w ith th e USSR p ro d uc in g 8 9 .4 m illio n to n n e s an d th e USA p ro d ucing 7 0 .5 millio n to nne s1 .1 . Tab le 1 .1 sho w s the Gro w th o f Ce me nt Ind ustry thro ug h Plans. Prio r to the manufacture o f Po rtland cement in Ind ia, it w as impo rted fro m UK and o nly a fe w re info rc e d c o nc re te struc ture s w e re b uilt w ith im p o rte d c e m e nt. A thre e sto re ye d structure b uilt at Byculla, Bo mb ay is o ne o f the o ld e st RCC structure s using Po rtland ce me nt in Ind ia. A c o nc re te maso nry b uild ing o n Mo unt Ro ad at Mad ras (1 9 0 3 ), the har-ki-p ahari b rid g e at Harid w ar (1 9 0 8 ) and the Co tto n De p o t Bo mb ay, the n o ne o f the larg e st o f its kind in the w o rld (1 9 2 2 ) are so me o f the o ld e st co ncre te structure s in Ind ia. 1 .2

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Ta ble 1 .1 . Grow t h of Ce m e nt I ndust r y t hrough Pla ns Five Year Plan

At the end o f the Year

Capacity (* )

% ag e Gro w th

Pro d uctio n (* )

% ag e Gro w th Cement

GDP Gro w th

Pre Plan

5 0 -5 1

3 .2 8

2 .2 0

I Plan

5 5 -5 6

5 .0 2

4 .6 0

II Plan

6 0 -6 1

9 .3 0

1 3 .1 2

7 .9 7

1 1 .6 2

7 .1

III Plan

6 5 -6 6

1 2 .0 0

5 .2 3

1 0 .9 7

6 .6 0

3 .4

The re w e re Annual Plans fo r 1 9 6 6 -6 7 , 6 7 -6 8 and 6 8 -6 9 IV Plan

7 3 .7 4

1 9 .7 6

1 0 .4 9

1 4 .6 6

5 .9 7

4 .6

V Plan

7 8 -7 9

2 2 .5 8

2 .7 0

1 9 .4 2

5 .7 8

5 .5

VI Plan

8 4 -8 5

4 2 .0 0

1 3 .2 2

3 0 .1 3

9 .1 8

3 .8

VII Plan

8 9 -9 0

6 1 .5 5

7 .9 4

4 5 .4 1

8 .5 5

6 .9

Annual

9 0 -9 1

6 4 .3 6

0 .9 0

4 8 .9 0

1 .4 9

5 .4

Plans

9 1 -9 2

6 6 .5 6

3 .4 2

5 3 .6 1

9 .6 3

5 .3

VIII Plan

9 2 -9 3

7 0 .1 9

5 .4 5

5 4 .0 8

0 .8 8

4 .1

9 3 -9 4

7 6 .8 8

9 .5 3

5 7 .9 6

7 .1 7

6 .0

9 4 -9 5

8 3 .6 9

8 .8 6

6 2 .3 5

7 .5 7

7 .2

9 5 -9 6

9 7 .2 5

1 6 .2 0

6 9 .5 7

1 1 .5 8

7 .1

9 6 -9 7

1 0 5 .2 5

8 .2 3

7 6 .2 2

9 .5 6

6 .8

9 7 -9 8

1 0 9 .3 0

3 .8 5

8 3 .1 6

9 .1 0

5 .2

IX Plan

(*) Includes mini cement plants Source: Indian Cement Industry Emerging Trends — P. Parthsarathy and S.M. Chakravarthy

Ta ble 1 .2 . Pe r Ca pit a Ce m e nt Consum pt ion of Se le c t e d Count rie s of t he World (1 9 8 2 , 1 9 9 4 a nd 1 9 9 7 ) Co untry

Per Capita Cement Co nsumptio n (Kg .) 1982

USA

1994

1997

256

328

347

92

333

388

Taiw an

590

1285

966

Japan

617

642

622

Malaysia

290

512

831

Thailand

132

491

595

China

Arg e ntina

198

184

1 4 5 (1 9 9 6 )

Brazil

201

165

240

Ve ne zue la

356

222

1 6 9 (1 9 9 6 )

Turke y

251

436

511

W o rld

188

241

India 78 kg (1996), 82 kg (1997)

252

(1 9 9 5 )

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The perusal o f tab le 1.2 sho w s that per capita cement co nsumptio n in Ind ia is much less than w o rld ave rag e . Co nsid e rab le infrastruc tural d e ve lo p me nt is ne e d e d to b uild mo d e rn Ind ia. Pro d uctio n o f mo re ce me nt, kno w le d g e and e co no mical utilisatio n o f ce me nt is the ne e d o f the d ay. The e arly scie ntific stud y o f ce me nts d id no t re ve al much ab o ut the che mical re actio ns that take place at the time b urning . A d e e pe r stud y o f the fact that the claye y co nstitue nts o f limesto ne are respo nsib le fo r the hydraulic pro perties in lime (as estab lished b y Jo hn Smeato n) w as no t take n fo r furthe r re se arc h. It m ay b e m e ntio ne d that am o ng the e arlie r c e m e nt te c hno lo g ists, Vic at, Le Chate lie r and Mic hae lis w e re the p io ne e rs in the the o re tic al and p ractical fie ld . Syste matic w o rk o n the co mpo sitio n and che mical re actio n o f Po rtland ce me nt w as first b eg un in the United States. The stud y o n setting w as und ertaken b y the Bureau o f Stand ard s and sinc e 1 9 2 6 m uc h w o rk o n the stud y o f Po rtland c e m e nt w as also c o nd uc te d b y the Po rtland Cement Asso ciatio n, U.K. By this time, the manufacture and use o f Po rtland cement had spre ad to many co untrie s. Scie ntific w o rk o n ce me nts and fund ame ntal co ntrib utio ns to the chemistry o f Po rtland cements w ere carried o ut in Germany, Italy, France, Sw eden, Canada and USSR, in ad d itio n to Britain and USA. In Gre at Britain w ith the e stab lishme nt o f Build ing Research Statio n in 1 9 2 1 a systematic research pro g ramme w as und ertaken and many majo r c o ntrib utio ns have b e e n m ad e . Early lite rature s o n the d e ve lo p m e nt and use o f Po rtland c e m e nts m ay b e fo und in the Build ing Sc ie nc e Ab strac ts p ub lishe d b y Build ing Re se arc h Statio n U.K. sinc e 1 9 2 8 , “Do c um e ntatio n Bib lio g rap hiq ue ” issue d q uarte rly sinc e 1 9 4 8 in France and “Hand b uch d e r Ze me nt Lite rature ” in Ge rmany.

Manufacture of Portland Cement The raw materials req uired fo r manufacture o f Po rtland cement are calcareo us materials, such as lime sto ne o r chalk, and arg illace o us mate rial such as shale o r clay. Ce me nt facto rie s are estab lished w here these raw materials are availab le in plenty. Cement facto ries have co me up in many reg io ns in Ind ia, eliminating the inco nvenience o f lo ng d istance transpo rtatio n o f raw and finishe d mate rials. The p ro c e ss o f manufac ture o f c e me nt c o nsists o f g rind ing the raw mate rials, mixing the m intim ate ly in c e rtain p ro p o rtio ns d e p e nd ing up o n the ir p urity and c o m p o sitio n and b urning them in a kiln at a temperature o f ab o ut 1300 to 1500°C, at w hich temperature, the mate rial sinte rs and p artially fuse s to fo rm no d ular shap e d clinke r. The clinke r is co o le d and g ro und to fine po w d e r w ith ad d itio n o f ab o ut 3 to 5 % o f g ypsum. The pro d uct fo rme d b y using this pro ce d ure is Po rtland ce me nt. The re are tw o pro ce sse s kno w n as “w e t” and “d ry” pro ce sse s d e pe nd ing upo n w he the r the mixing and g rinding o f raw materials is do ne in w et o r dry co nditio ns. With a little chang e in the ab o ve p ro ce ss w e have the se mi-d ry p ro ce ss also w he re the raw mate rials are g ro und d ry and the n m ixe d w ith ab o ut 1 0 -1 4 p e r c e nt o f w ate r and furthe r b urnt to c linke ring te mp e rature . Fo r many ye ars the w e t p ro c e ss re maine d p o p ular b e c ause o f the p o ssib ility o f mo re ac c urate c o ntro l in the mixing o f raw mate rials. The te c hniq ue s o f intimate mixing o f raw mate rials in p o w d e r fo rm w as no t availab le the n. Late r, the d ry p ro ce ss g aine d mo me ntum w ith the mo d e rn d e ve lo p me nt o f the te chniq ue o f d ry mixing o f p o w d e re d mate rials using co mp re sse d air. The d ry p ro ce ss re q uire s much le ss fue l as the mate rials are alre ad y in a d ry state , w he re as in the w e t pro ce ss the slurry co ntains ab o ut 3 5 to 5 0 pe r ce nt w ate r. To d ry

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the slurry w e thus re q uire m o re fue l. In Ind ia m o st o f the c e m e nt fac to rie s use d the w e t p ro ce ss. Re ce ntly a numb e r o f facto rie s have b e e n co mmissio ne d to e mp lo y the d ry p ro ce ss me tho d . Within ne xt fe w ye ars mo st o f the ce me nt facto rie s w ill ad o p t d ry p ro ce ss syste m. In the w e t p ro ce ss, the lime sto ne b ro ug ht fro m the q uarrie s is first crushe d to smalle r frag ments. Then it is taken to a ball o r tube mill w here it is mixed w ith clay o r shale as the case may b e and g ro und to a fine co nsiste ncy o f slurry w ith the ad d itio n o f w ate r. The slurry is a liq uid o f creamy co nsistency w ith w ater co ntent o f abo ut 35 to 50 per cent, w herein particles, crushed to the fineness o f Ind ian Stand ard Sieve numb er 9, are held in suspensio n. The slurry is p um p e d to slurry tanks o r b asins w he re it is ke p t in an ag itate d c o nd itio n b y m e ans o f ro tating arms w ith chains o r b lo w ing co mpre sse d air fro m the b o tto m to pre ve nt se ttling o f lim e sto ne and c lay p artic le s. The c o m p o sitio n o f the slurry is te ste d to g ive the re q uire d che mical co mp o sitio n and co rre cte d p e rio d ically in the tub e mill and also in the slurry tank b y b lending slurry fro m different sto rag e tanks. Finally, the co rrected slurry is sto red in the final sto rag e tanks and ke p t in a ho mo g e ne o us co nd itio n b y the ag itatio n o f slurry. The co rre cte d slurry is sp raye d o n to the up p e r e nd o f a ro tary kiln ag ainst ho t he avy hang ing chains. The ro tary kiln is an impo rtant co mpo ne nt o f a ce me nt facto ry. It is a thick steel cylind er o f d iameter anything fro m 3 metres to 8 metres, lined w ith refracto ry materials, mo unte d o n ro lle r b e aring s and capab le o f ro tating ab o ut its o w n axis at a spe cifie d spe e d . The le ng th o f the ro tary kiln may vary anything fro m 3 0 me tre s to 2 0 0 me tre s. The slurry o n b eing sprayed ag ainst a ho t surface o f flexib le chain lo ses mo isture and b eco mes flakes. These flake s pe e l o ff and fall o n the flo o r. The ro tatio n o f the ro tary kiln cause s the flake s to mo ve fro m the up p e r e nd to w ard s the lo w e r e nd o f the kiln sub je cting itse lf to hig he r and hig he r temperature. The kiln is fired fro m the lo w er end. The fuel is either po w ered co al, o il o r natural g ass. By the time the mate rial ro lls d o w n to the lo w e r e nd o f the ro tary kiln, the d ry mate rial

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Fig. 1.1. Diagrammatic representation of the dry process of manufacure of cement. (Courtesy : Grasim Industries Cement Division)

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A view of Limestone quarry, raw material preparation : The prime raw material limestone after blasting in mines is broken into big boulders. Then it is transported by dumpers, tippers to limestone crusher where it is crushed to 15 to 20 mm size.

STACKER FOR CRUSHED LIMESTONE RECLAIMER FOR CRUSHED LIMESTONE

After crushing, the crushed limestone is piled longitudinally by an equipment called stacker. The stacker deposits limestone longitudinally in the form of a pile. The pile is normally 250 to 300 m long and 8-10 m height. The reclaimer cuts the pile vertically, simultaneously from top to bottom to ensure homogenization of limestone.

Reclaimer for homogenization of crushed limestone.

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und erg o es a series o f chemical reactio ns until finally, in the ho ttest part o f the kiln, w here the te mp e rature is in the o rd e r o f 1 5 0 0 ° C, ab o ut 2 0 to 3 0 p e r ce nt o f the mate rials g e t fuse d . Lime, silica and alumina g et reco mbined. The fused mass turns into no dular fo rm o f size 3 mm to 2 0 mm kno w n as clinke r. The clinke r d ro p s into a ro tary co o le r w he re it is co o le d und e r co ntro lle d co nd itio ns The clinke r is sto re d in silo s o r b ins. The clinke r w e ig hs ab o ut 1 1 0 0 to 1 3 0 0 g ms p e r litre . The litre w e ig ht o f clinke r ind icate s the q uality o f clinke r. The co o le d clinke r is the n g ro und in a b all mill w ith the ad d itio n o f 3 to 5 p e r ce nt o f g yp su m in o rd e r to p re ve n t flash -se ttin g o f th e c e m e n t. A b all m ill c o n sists o f se ve ral co mpartme nts charg e d w ith pro g re ssive ly smalle r hard e ne d ste e l b alls. The particle s crushe d to the req uired fineness are separated b y currents o f air and taken to sto rag e silo s fro m w here the ce me nt is b ag g e d o r fille d into b arre ls fo r b ulk sup p ly to d ams o r o the r larg e w o rk site s. In the mo d e rn p ro ce ss o f g rind ing , the p article size d istrib utio n o f ce me nt p article s are maintaine d in such a w ay as to g ive d e sirab le g rad ing p atte rn. Just as the g o o d g rad ing o f ag g re g ate s is e sse ntial fo r making g o o d c o nc re te , it is no w re c o g nise d that g o o d g rad ing patte rn o f the ce me nt particle s is also impo rtant. The Fig . 1 .1 sho w s the flo w d iag ram o f d ry pro ce ss o f manufacture o f ce me nt.

Dry Process In the d ry and se mi-d ry p ro c e ss the raw mate rials are c rushe d d ry and fe d in c o rre c t pro po rtio ns into a g rind ing mill w here they are d ried and red uced to a very fine po w d er. The d ry po w der called the raw meal is then further blended and co rrected fo r its rig ht co mpo sitio n and mixed by means o f co mpressed air. The aerated po w der tends to behave almo st like liq uid and in ab o ut o ne ho ur o f ae ratio n a unifo rm mixture is o b taine d . The b le nd e d m e al is furthe r sie ve d and fe d into a ro tating d isc c alle d g ranulato r. A q uantity o f w ater abo ut 12 per cent by w rig ht is added to make the blended meal into pellets. This is d o ne to p e rm it air flo w fo r e xc hang e o f he at fo r furthe r c he m ic al re ac tio ns and co nve rsio n o f the same into clinke r furthe r in the ro tary kiln. The eq uipments used in the dry pro cess kiln is co mparatively smaller. The pro cess is q uite e c o n o m ic al. Th e to tal c o n sum p tio n o f c o al in th is m e th o d is o n ly ab o ut 1 0 0 kg w h e n c o m p are d to the re q uire m e nt o f ab o ut 3 5 0 kg fo r p ro d uc ing a to n o f c e m e nt in the w e t p ro ce ss. During March 1 9 9 8 , in Ind ia, the re w e re 1 7 3 larg e p lants o p e rating , o ut o f w hich 4 9 plants used w et pro cess, 1 1 5 plants used d ry pro cess and 9 plants used semi-d ry pro cess. Since the time o f partial lib eralisatio n o f cement ind ustry in Ind ia (1 9 8 2 ), there has b een an upg radatio n in the q uality o f cement. Many cement co mpanies upg raded their plants bo th in re spe ct o f capacity and q uality. Many o f the re ce nt plants e mplo ye d the b e st e q uipme nts, such as cro ss b e lt analyse r manufacture d b y Gamma-Me trics o f USA to find the co mp o sitio n o f lim e sto n e at th e c o n ve yo r b e lts, h ig h p re ssure tw in ro lle r p re ss, six stag e p re h e ate r, precalciner and vertical ro ller mill. The latest pro cess includ es stacker and reclaimer, o n-line Xray analyser, Fuzzy Lo g ic kiln co ntro l system and o ther mo d ern pro cess co ntro l. In o ne o f the recently built cement plant at Reddypalayam near Trichy, by Grasim Indistries, emplo yed Ro bo t fo r auto matic co llectio n o f ho urly samples fro m 5 different places o n the pro cess line and help analyse the ame, thro ug ho ut 2 4 ho urs, unto uched b y men, to avo id human erro rs in q uality co ntro l. With all the ab o ve so phisticated eq uipments and co ntro ls, co nsistent q uality o f clinker is pro d uce d . The me tho d s are c o mmo nly e mp lo ye d fo r d ire c t c o ntro l o f q uality o f c linke r. The first me tho d invo lve s re fle cte d lig ht o p tical micro sco p y o f p o lishe d and e tche d se ctio n o f clinke r,

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RAW MILL The proportioned raw materials are transported by belt conveyor to Raw Mill for grinding into powder form before burning.

RAW MEAL SILO After grinding, the powdered raw mix, is stored in a raw meal-silo where blending takes place. Blending is done by injecting compressed air. Generally blending ratio is 1:10. This powder material (Raw meal) is fed to the kiln for burning.

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ROBO LAB Consists of automatic sampling and sending station located at different locations in the plant. Samples are being sent through pneumatic tubes to Robo lab. This avoids human error in sampling and ensures accurate quality in semi finished and finished products. 1st of its kind in India has been used at Grasim Cement plant at Reddypalayam.

Robot receiving samples.

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Close circuit grinding technology is most modern grinding system for raw mix as well as for clinker grinding. The systems are in compound mode and are equipped with high efficiency Roller press and separators. The above mentioned system enables to maintain low power consumption for grinding as well as narrow particle size distribution. With this circuit, it is possible to manufacture higher surface area of product as per customers, requirement.

Multi-compartment silo.

Electronic packers : it has continuous weighing system and it ensures that the bags separating from the nozzles have accurate weight of cement. The weight of filled bag is also displayed on the packer.

Cross section of multi-compartment silo.

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Jumbo bag transportation.

fo llo w e d b y p o in t c o u n t o f a re a s o c c u p ie d b y vario u s c o m p o u n d s. Th e se co nd me tho d , w hich is also ap p licab le to p o w d e re d c e m e n t, in vo lve s X-ra y d iffra c tio n o f p o w d e r sp e c im e n . Ca lib ra tio n c u rve s b a se d o n kn o w n m ixtu re s o f p u re c o m p o u n d s, h e lp to e stimate the co mpo und co mpo sitio n. As a ro ug h and re ad y me tho d , litre w e ig ht (b ulk d e nsity) o f clinke r is mad e use o f to asc e rtain th e q u ality. A litre w e ig h t o f a b o u t 1 2 0 0 g m s. is fo u n d to b e satisfacto ry. Jumbo bag packing.

It is im p o rta n t to n o te th a t th e stre n g th p ro p e rtie s o f c e m e n t a re c o nsid e rab ly influe nc e d b y the c o o ling rate o f c linke r. This fac t has o f late attrac te d the atte ntio n o f b o th the c e m e nt m anufac ture rs and m ac hine ry p ro d uc e rs. The e xp e rim e ntal re sults re p o rte d b y Enke g aard are sho w n in tab le 1 .3 .

Ta ble 1 .3 . I nflue nc e of Ra t e of Cooling on Com pre ssive St re ngt h 1 .3 Type o f cement

Co o ling co nd itio ns Q uick

No rmal Ce me nt

Co mpressive Streng th MPa 3 d ays 9 .9

7 d ays

2 8 d ays

1 5 .3

26

Mo d e rate

9 .7

2 1 .0

27

Slo w

9 .7

1 9 .3

24

Ve ry slo w

8 .7

1 8 .7

23

Hig h e arly

Q uick

1 0 .2

1 8 .8

29

stre ng th

Mo d e rate

1 4 .2

2 6 .7

33

ce me nt

Slo w

1 0 .2

2 1 .0

29

9 .1

1 8 .1

28

Ve ry Slo w

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It can be seen fro m the table that a mo derate rate o f co o ling o f clinker in the ro tary co o ler w ill re sult in hig he r stre ng th. By mo d e rate co o ling it is imp lie d that fro m ab o ut 1 2 0 0 ° C, the clinke r is b ro ug ht to ab o ut 5 0 0 ° C in ab o ut 1 5 minute s and fro m the 5 0 0 ° C the te mp e rature is b ro ug ht d o w n to no rmal atmo sp he ric te mp e rature in ab o ut 1 0 minute s. The rate o f co o ling influences the d eg ree o f crystallisatio n, the size o f the crystal and the am o unt o f am o rp ho us m ate rials p re se nt in the c linke r. The p ro p e rtie s o f this am o rp ho us mate rial fo r similar che mical co mp o sitio n w ill b e d iffe re nt fro m the o ne w hich is crystalline d .

Chemical Composition The raw m ate rials use d fo r the m anufac ture o f c e m e nt c o nsist m ainly o f lim e , silic a, alumina and iro n o xide. These o xides interact w ith o ne ano ther in the kiln at hig h temperature to fo rm mo re co mple x co mpo und s. The re lative pro po rtio ns o f the se o xid e co mpo sitio ns are respo nsib le fo r influencing the vario us pro perties o f cement; in additio n to rate o f co o ling and fine ne ss o f g rind ing . Tab le 1 .4 sho w s the ap p ro ximate o xid e co mp o sitio n limits o f o rd inary Po rtland ce me nt.

Ta ble 1 .4 . Approx im at e Ox ide Com posit ion Lim it s of Ordina r y Por t la nd Ce m e nt O xid e

Per cent co ntent

CaO

6 0 –6 7

SiO 2

1 7 –2 5

Al2 O 3

3 .0 –8 .0

Fe 2 O 3

0 .5 –6 .0

Mg O

0 .1 –4 .0

Alkalie s (K2 O , Na 2 O )

0 .4 –1 .3

SO 3

1 .3 –3 .0

Ind ian stand ard spe cificatio n fo r 3 3 g rad e ce me nt, IS 2 6 9 -1 9 8 9 , spe cifie s the fo llo w ing che mical re q uire me nts: (a ) Ratio o f pe rce ntag e o f lime to pe rce ntag e o f silica, alumina and iro n o xid e ; kno w n as Lime Saturatio n Facto r, w he n calculate d b y the fo rmula

CaO − 0.7 SO 3 No t g re ate r than 1 .0 2 and no t le ss than 0 .6 6 2.8 SiO 2 + 1.2 Al 2 O3 + 0.65 Fe 2 O3 (b ) Ratio o f pe rce ntag e o f alumina to that o f iro n o xid e

No t le ss tan 0 .6 6

(c ) We ig ht o f inso lub le re sid ue

No t mo re than 4 pe r ce nt

(d ) We ig ht o f mag ne sia

No t mo re than 6 pe r ce nt

(e ) To tal sulp hur co nte nt, calculate d as sulp huric w he n

N o t m o re

anhyd rid e (SO 3 )

(f ) To tal lo ss o n ig nitio n

th a n 2 . 5 %

C3 A is 5 % o r le ss. No t mo re than 3 % , w he n C3 A is mo re than 5 % No t mo re than 5 pe r ce nt

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As me ntio ne d e arlie r the o xid e s p e rse nt in the raw mate rials w he n sub je c te d to hig h c lin ke rin g te m p e ratu re c o m b in e w ith e ac h o th e r to fo rm c o m p le x c o m p o u n d s. Th e id e ntificatio n o f the majo r co mp o und s is larg e ly b ase d o n R.H. Bo g ue ’s w o rk and he nce it is called “Bo g ue’s Co mpo unds”. The fo ur co mpo unds usually reg arded as majo r co mpo unds are liste d in tab le 1 .5 .

Ta ble 1 .5 . Bogue ’s Com pounds Name o f Co mpo und Tricalcium silicate

Fo rmula

Ab b reviated Fo rmula

3 CaO .SiO 2

C3 S

Dicalcium silicate

2 CaO .Sio 2

C2 S

Tricalcium aluminate

3 Cao .Al2 O 3

C3 A

Te tracalcium alumino fe rrite

4 CaO .Al2 O 3 .Fe 2 O 3

C4 AF

It is to be no ted that fo r simplicity’s sake abbreviated no tatio ns are used. C stands fo r CaO , S stand s fo r SiO 2 , A fo r Al2 O 3 , F fo r Fe 2 O 3 and H fo r H2 O . The eq uatio ns sug g ested b y Bo g ue fo r calculating the percentag es o f majo r co mpo unds are g ive n b e lo w. C 3 S = 4 . 0 7 (CaO ) – 7 . 6 0 (SiO 2 ) – 6 . 7 2 (Al2 O 3 ) – 1 . 4 3 (Fe 2 O 3 ) – 2 . 8 5 (SO 3 ) C2 S

= 2 . 8 7 (SiO 2 ) – 0 . 7 5 4 (3 Ca O . SiO 2 )

C3 A

= 2 . 6 5 (Al2 O 3 ) – 1 . 6 9 (Fe 2 O 3 )

C 4 AF= 3 . 0 4 (Fe 2 O 3 ) The o xid e sho w n w ithin the b racke ts re p re se nts the p e rce ntag e o f the same in the raw mate rials.

Ta ble 1 .6 . T he Ox ide Com posit ion of a T ypic a l Por t la nd Ce m e nt a nd t he Cor rosponding Ca lc ula t e d Com pound Com posit ion. O xid e co mpo sitio n Per cent

Calculated co mpo und co mpo sitio n using Bo g ue’s eq uatio n per cent

Ca O

63

C3 S

5 4 .1

SiO 2

20

C2 S

1 6 .6

Al2 O 3

6

C3 A

1 0 .8

Fe 2 O 3

3

C 4 AF

Mg O SO 2 K2 O Na 2 O

9 .1

1 .5 2



1 .0 



In ad d itio n to the fo ur majo r co mp o und s, the re are many mino r co mp o und s fo rme d in the kiln. The influe nce o f the se mino r co mp o und s o n the p ro p e rtie s o f ce me nt o r hyd rate d co mpo und s is no t sig nificant. Tw o o f the mino r o xid e s name ly K2 O and Na 2 O re fe rre d to as alkalis in cement are o f so me impo rtance. This aspect w ill b e d ealt w ith later w hen d iscussing alkali-ag g re g ate re ac tio n . Th e o xid e c o m p o sitio n o f typ ic al Po rtlan d c e m e n t an d th e co rre sp o nd ing calculate d co mp o und co mp o sitio n is sho w n in tab le 1 .6 .

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C2S

C3A + C4AF

C3S

Schematic presentation of various compounds in clinker Courtesy : All the photographs on manufacture of cement are by Grasim Industries Cement Division

Tricalcium silicate and d icalcium silicate are the mo st imp o rtant co mp o und s re sp o nsib le fo r stre ng th. To g e the r the y co nstitute 7 0 to 8 0 p e r ce nt o f ce me nt. The ave rag e C3 S co nte nt in mo d e rn ce me nt is ab o ut 4 5 pe r ce nt and that o f C2 S is ab o ut 2 5 p e r ce nt. The sum o f the co ntents o f C3 A and C4 AF has d ecreased slig htly in mo d ern cements. The calculated q uantity o f the c o mp o und s in c e me nt varie s g re atly e ve n fo r a re lative ly small c hang e in the o xid e c o m p o sitio n o f th e raw m ate rials. To m an u fac tu re a c e m e n t o f stip u late d c o m p o u n d co mpo sitio n, it b e co me s ab so lute ly ne ce ssary to clo se ly co ntro l the o xid e co mpo sitio n o f the raw materials. An increase in lime co ntent b eyo nd a certain value makes it difficult to co mb ine w ith o the r c o mp o und s and fre e lime w ill e xist in the c linke r w hic h c ause s unso und ne ss in cement. An increase in silica co ntent at the expense o f the co ntent o f alumina and ferric o xide w ill make the cement difficult to fuse and fo rm clinker. Cements w ith a hig h to tal alumina and hig h ferric o xide co ntent is favo urable to the pro ductio n o f hig h early streng ths in cement. This is p e rhap s d ue to the influe nc e o f the se o xid e s fo r the c o m p le te c o m b ining o f the e ntire q uantity o f lime p re se nt to fo rm tricalcium silicate . The ad vancement mad e in the vario us spheres o f science and techno lo g y has helped us to reco g nise and und erstand the micro structure o f the cement co mpo und s b efo re hyd ratio n and after hydratio n. The X-ray po w der diffractio n metho d, X-ray fluo rescence metho d and use o f po w erful electro n micro sco pe capable o f mag nifying 50,000 times o r even mo re has helped to re ve al the crystalline o r amo rp ho us structure o f the unhyd rate d o r hyd rate d ce me nt. Bo th Le Chatelier and To rneb o hm o b served fo ur different kinds o f crystals in thin sectio ns o f ce me nt clinke rs. To rne b o hm calle d the se fo ur kind s o f crystals as Alite , Be lite , Ce lite and Fe lite . To rne b o hm’s d e scrip tio n o f the mine rals in ce me nt w as fo und to b e similar to Bo g ue ’s d e sc rip tio n o f the c o mp o und s. The re fo re , Bo g ue ’s c o mp o und s C3 S, C2 S, C3 A and C4 AF are so me time s calle d in lite rature as Alite , Be lite , Ce lite and Fe lite re sp e ctive ly.

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Ce me nt and hyd ratio n o f Po rtland ce me nt can b e sche matically re p re se nte d as b e lo w :

Hydration of Cement Anhyd ro us c e m e nt d o e s no t b ind fine and c o arse ag g re g ate . It ac q uire s ad he sive pro perty o nly w hen mixed w ith w ater. The chemical reactio ns that take place betw een cement and w ate r is re fe rre d as hyd ratio n o f ce me nt. The chemistry o f co ncrete is essentially the chemistry o f the reactio n betw een cement and w ate r.O n ac c o unt o f hyd ratio n c e rtain p ro d uc ts are fo rme d . The se p ro d uc ts are imp o rtant b e c a u se th e y h a ve c e m e n tin g o r ad he sive value . The q uality, q uantity, c o n tin u ity, sta b ility a n d th e ra te o f fo rmatio n o f the hydratio n pro ducts are imp o rtant. An h yd ro u s c e m e n t c o m p o u n d s w h e n m ixe d w ith w ate r, re ac t w ith e a c h o th e r to fo rm h yd ra te d c o mp o und s o f ve ry lo w so lub ility. The hyd ratio n o f c e me nt c an b e visualise d in tw o w a ys. Th e first is “th ro u g h so lutio n” mechanism. In this the cement c o m p o u n d s d isso lve to p ro d u c e a su p e rsatu rate d so lu tio n fro m w h ic h d iffe re n t h yd ra te d p ro d u c ts g e t p re c ip itate d . The se c o nd p o ssib ility is

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that w ate r attac ks c e m e nt c o m p o und s in the so lid state c o nve rting the c o m p o und s into hydrated pro ducts starting fro m the surface and pro ceeding to the interio r o f the co mpo unds w ith time. It is pro bable that bo th “thro ug h so lutio n” and “so lid state” types o f mechanism may o ccur during the co urse o f reactio ns b etw een cement and w ater. The fo rmer mechanism may p re d o m inate in the e arly stag e s o f hyd ratio n in vie w o f larg e q uantitie s o f w ate r b e ing availab le , and the latte r me chanism may o p e rate d uring the late r stag e s o f hyd ratio n.

Heat of Hydration The re actio n o f ce me nt w ith w ate r is e xo the rmic. The re actio n lib e rate s a co nsid e rab le q uantity o f heat. This lib eratio n o f heat is called heat o f hyd ratio n. This is clearly seen if freshly mixe d ce me nt is p ut in a vaccum flask and the te mp e rature o f the mass is re ad at inte rvals. The stud y and c o ntro l o f the he at o f hyd ratio n b e c o m e s im p o rtant in the c o nstruc tio n o f c o n c re te d am s an d o th e r m ass c o n c re te c o n stru c tio n s. It h as b e e n o b se rve d th at th e te mp e rature in the inte rio r o f larg e mass co ncre te is 5 0 ° C ab o ve the o rig inal te mpe rature o f the co ncre te mass at the time o f p lacing and this hig h te mp e rature is fo und to p e rsist fo r a pro lo ng ed perio d . Fig 1 .2 sho w s the pattern o f lib eratio n o f heat fro m setting cement1 .4 and d uring e arly hard e ning p e rio d . O n mixing cement w ith w ater, a rapid heat evo lutio n, lasting a few minutes, o ccurs. This h e at e vo lutio n is p ro b ab ly d ue to th e re ac tio n o f so lutio n o f alum in ate s an d sulp h ate s (ascend ing peak A). This initial heat evo lutio n ceases q uickly w hen the so lub ility o f aluminate is d epressed b y g ypsum. (d ecend ing peak A). Next heat evo lutio n is o n acco unt o f fo rmatio n o f e ttring ite and also may b e d ue to the re actio n o f C3 S (asce nd ing p e ak B). Re fe r Fig . 1 .2 . Diffe re nt co mp o und s hyd rate at d iffe re nt rate s and lib e rate d iffe re nt q uantitie s o f he at. Fig . 1.3 sho w s the rate o f hyd ratio n o f pure co mpo und s. Since retard ers are ad d ed to co ntro l the flash se tting pro pe rtie s o f C3 A, actually the e arly he at o f hyd ratio n is mainly co ntrib ute d fro m the hydratio n o f C3 S. Fineness o f cement also influences the rate o f develo pment o f heat b ut no t the to tal he at. The to tal q uantity o f he at g e ne rate d in the co mp le te hyd ratio n w ill d e p e nd up o n the re lative q uantitie s o f the majo r co mp o und s p re se nt in a ce me nt. Analysis o f he at o f hyd ratio n d ata o f larg e num b e r o f c e m e nts, Ve rb e c and Fo ste r1 .5 co mp ute d he at e vo lutio n o f fo ur majo r co mp o und s o f ce me nt. Tab le 1 .7 . sho w s the he ats o f hyd ratio n o f fo ur co mp o und s.

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Ta ble 1 .7 . H e at of H ydra t ion 1 .5 Co mpo und

Heat o f hyd ratio n at the g iven ag e (cal/ g ) 3 days

90 days

13 years

C3 S

58

104

122

C2 S

12

42

59

C3 A

212

311

324

69

98

102

C4 AF

Since the he at o f hyd ratio n o f ce me nt is an ad d itive p ro p e rty, it can b e p re d icte d fro m an e xpre ssio n o f the type H = aA + b B + cC + d D Where H re p re se nts the he at o f hyd ratio n, A, B, C, and D are the pe rce ntag e co nte nts o f C3 S, C2 S, C3 A and C4 AF. and a , b , c and d are co e fficie nts re p re se nting the co ntrib utio n o f 1 p e r ce nt o f the co rre sp o nd ing co mp o und to the he at o f hyd ratio n. No rmal cement g enerally pro duces 89-90 cal/ g in 7 days and 90 to 100 cal/ g in 28 days. The hyd ratio n p ro c e ss is no t an instantane o us o ne . The re ac tio n is faste r in the e arly perio d and co ntinues idenfinitely at a decreasing rate. Co mplete hydratio n canno t be o btained under a perio d o f o ne year o r mo re unless the cement is very finely g ro und and reg ro und w ith e xce ss o f w ate r to e xpo se fre sh surface s at inte rvals. O the rw ise , the pro d uct o b taine d sho w s unattacked co res o f tricalcium silicate surro und ed b y a layer o f hyd rated silicate, w hich b eing re lative ly imp e rvio us to w ate r, re nd e rs furthe r attack slo w. It has b e e n o b se rve d that afte r 2 8 d ays o f curing , ce me nt g rains have b e e n fo und to have hyd rate d to a d e p th o f o nly 4 µ. It has also b e e n o b se rve d that co mp le te hyd ratio n und e r no rmal co nd itio n is p o ssib le o nly fo r ce me nt p article s smalle r than 5 0 µ. A g rain o f cement may co ntain many crystals o f C3 S o r o thers. The larg est crystals o f C3 S o r C2 S are ab o ut 4 0 µ. An ave rag e size w o uld b e 1 5 -2 0 µ. It is pro b ab le that the C2 S crystals p re se nt in the surface o f a ce me nt g rain may g e t hyd rate d and a mo re re active co mp o und like C3 S lying in the inte rio r o f a ce me nt g rain may no t g e t hyd rate d . The hyd rate d p ro d uct o f the ce me nt co mp o und in a g rain o f ce me nt ad he re s firmly to the unhyd rated co re in the g rains o f cement. That is to say unhyd rated cement left in a g rain o f cement w ill no t red uce the streng th o f cement mo rtar o r co ncrete, as lo ng as the pro d ucts o f hyd ratio n are w e ll co mp acte d . Ab rams o b taine d stre ng th o f the o rd e r o f 2 8 0 MPa using m ixe s w ith a w ate r/ c e m e nt ratio as lo w as 0 .0 8 . Esse ntially he has ap p lie d tre m e nd o us pressure to o b tain pro per co mpactio n o f such a mixture. O w ing to such a lo w w ater/ cement ratio , h yd ratio n m u st h ave b e e n p o ssib le o n ly at th e su rfac e o f c e m e n t g rain s, an d a co nsid e rab le p o rtio n o f ce me nt g rains must have re maine d in an unhyd rate d co nd itio n. The p re se nt d ay Hig h Pe rfo rm anc e c o nc re te is m ad e w ith w ate r c e m e nt ratio in the reg io n o f 0.25 in w hich case it is po ssib le that a co nsiderab le po rtio n o f cement g rain remains unhyd rate d in the co re . O nly surface hyd ratio n take s p lace . The unhyd rate d co re o f ce me nt g rain can b e d e e me d to w o rk as ve ry fine ag g re g ate s in the w ho le syste m.

Calcium Silicate Hydrates Durin g th e c o urse o f re ac tio n o f C3 S an d C 2 S w ith w ate r, c alc ium silic ate h yd rate , ab b re viate d C-S-H and calcium hyd ro xid e , Ca(O H)2 are fo rme d . Calcium silicate hyd rate s are the mo st impo rtant pro ducts. It is the essence that determines the g o o d pro perties o f co ncrete.

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It make s up 5 0 -6 0 p e r ce nt o f the vo lume o f so lid s in a co mp le te ly hyd rate d ce me nt p aste . The fact that te rm C-S-H is hyp he nate d sig nifie s that C-S-H is no t a w e ll d e fine d co mp o und . The mo rpho lo g y o f C-S-H sho w s a po o rly crystalline fib ro us mass. It w as co nsid e re d d o ub tful that the pro d uct o f hyd ratio n o f b o th C3 S and C2 S re sults in the fo rmatio n o f the same hyd rate d co mp o und . But late r o n it w as se e n that ultimate ly the hyd rate s o f C3 S and C2 S w ill turn o ut to b e the sam e . The fo llo w ing are the ap p ro xim ate e q uatio ns sho w ing the re actio ns o f C3 S and C2 S w ith w ater. 2 (3 CaO .SiO 2 ) + 6 H2 O → 3 CaO .2 SiO 2 . 3 H2 O + 3 Ca(O H)2 o r it can b e w ritte n as 2 C3 S

+

6H →

C3 S2 H3

+ 3 Ca(O H)2

The co rre sp o nd ing w e ig hts invo lve d are 100 Similarly,

+

24

→

75

+

2 (2 CaO .SiO 2 ) + 4 H2 O → 3 Cao .2 SiO 2 . 3 H2 O +

49. Ca(O H)2

o r it can b e w ritte n as 2 C2 S

+

4 H →

C3 S2 H3

+ Ca (O H)2

The co rre sp o nd ing w e ig hts invo lve d are 100

+

21

→

99

+

22

Ho w ever, the simple eq uatio ns g iven abo ve do no t bring o ut the co mplexities o f the actual re actio ns. It c an b e se e n that C3 S p ro d uc e s a c o m p arative ly le sse r q uantity o f c alc ium silic ate hyd rate s and mo re q uantity o f Ca(O H)2 than that fo rme d in the hyd ratio n o f C2 S. Ca(O H)2 is no t a d e sirab le p ro d uc t in the c o nc re te m ass, it is so lub le in w ate r and g e ts le ac he d o ut making the co ncre te p o ro us, p articularly in hyd raulic structure s. Und e r such co nd itio ns it is use ful to use ce me nt w ith hig he r p e rce ntag e o f C2 S co nte nt. C3 S re ad ily re acts w ith w ate r and pro d uce s mo re he at o f hyd ratio n. It is re spo nsib le fo r e arly stre n g th o f c o n c re te . A c e m e n t w ith m o re C 3 S c o n te n t is b e tte r fo r c o ld w e ath e r c o nc re ting . The q uality and d e nsity o f c alc ium silic ate hyd rate fo rme d o ut o f C3 S is slig htly infe rio r to that fo rme d b y C2 S. The e arly stre ng th o f co ncre te is d ue to C3 S. C2 S hyd rates rather slo w ly. It is respo nsib le fo r the later streng th o f co ncrete. It pro d uces le ss he at o f hyd ratio n. The c alc ium silic ate hyd rate fo rm e d is rathe r d e nse and its sp e c ific surface is hig he r. In g e ne ral, the q uality o f the p ro ud ct o f hyd ratio n o f C2 S is b e tte r than that p ro d uc e d in th e h yd ratio n o f C 3 S. Fig 1 .4 sh o w s th e d e ve lo p m e n t o f stre n g th o f p ure co mp o und s.

Calcium Hydroxide The o the r p ro d ucts o f hyd ratio n o f C3 S and C2 S is calcium hyd ro xid e . In co ntrast to the C-S-H, the calcium hyd ro xid e is a co mpo und w ith a d istinctive hexag o nal prism mo rpho lo g y. It c o nstitute s 2 0 to 2 5 p e r c e nt o f the vo lum e o f so lid s in the hyd rate d p aste . The lac k o f d urab ility o f c o nc re te , is o n ac c o unt o f the p re se nc e o f c alc ium hyd ro xid e . The c alc ium hyd ro xid e also reacts w ith sulphates present in so ils o r w ater to fo rm calcium sulphate w hich further reacts w ith C3 A and cause d eterio ratio n o f co ncrete. This is kno w n as sulphate attack. To re d uce the q uantity o f Ca(O H)2 in co ncre te and to o ve rco me its b ad e ffe cts b y co nve rting it into ce me ntitio us p ro d uct is an ad vance me nt in co ncre te te chno lo g y. The use o f b le nd ing

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m ate rials suc h as fly ash, silic a fum e and suc h o the r p o zzo lanic m ate rials are the ste p s to o ve rco me b ad e ffe ct o f Ca(O H)2 in co ncre te . This asp e ct w ill b e d e alt in g re ate r d e tail late r. The o nly ad vantag e is that Ca(O H)2 , b e ing alkaline in nature maintain pH value aro und 1 3 in the co ncre te w hich re sists the co rro sio n o f re info rce me nts.

Calcium Aluminate Hydrates The hyd ratio n o f aluminates has b een the sub ject o f numero us investig atio ns, b ut there is still so me unce rtainty ab o ut so me o f the re p o rte d p ro d ucts. Due to the hyd ratio n o f C3 A , a c alc ium aluminate syste m CaO – Al2 O 3 – H2 O is fo rme d . The c ub ic c o mp o und C3 AH6 is p ro b ab ly the o nly stab le co mp o und fo rme d w hich re mains stab le up to ab o ut 2 2 5 ° C. The re ac tio n o f p ure C3 A w ith w ate r is ve ry fast and this may le ad to flash se t. To pre ve nt this flash se t, g yp su m is a d d e d a t th e tim e o f g rin d in g th e c e m e n t c lin ke r. Th e q u an tity o f g yp su m ad d e d h as a b e a rin g o n th e q u a n tity o f C 3 A p re se nt. The hydrated aluminates do no t c o ntrib ute anything to the stre ng th o f co ncrete. O n the o ther hand, their p re se nce is harmful to the d urab ility o f c o n c re te p artic ularly w h e re th e c o nc re te is like ly to b e attac ke d b y sulp hate s. As it hyd rate s ve ry fast it m ay c o ntrib ute a little to the e arly stre ng th. O n hyd ratio n, C4 AF is b e lie ve d to fo rm a system o f the fo rm CaO – Fe 2 O 3 – H2 O . A hydrated calcium ferrite o f the fo rm C3 FH6 is co mparatively mo re stab le. This hyd rated pro d uct also d o es no t co ntrib ute anything to the stre ng th. The hyd rate s o f C4 AF sho w a c o m p arative ly hig he r re sistanc e to the attac k o f sulp hate s than the hyd rate s o f calcium aluminate . Fro m the stand p o int o f hyd ratio n, it is c o nve nie nt to d isc uss C3 A and C4 AF to g e the r, b e cause the p ro d ucts fo rme d in the p re se nce o f g yp sum are similar. Gyp sum and alkalie s g o into so lutio n q uickly and the so lubility o f C3 A is depressed. Depending upo n the co ncentratio n o f aluminate and sulp hate io ns in so lutio n, the p ricip itating crystalline p ro d uct is e ithe r the c alc ium alum inate trisulp hate hyd rate (C6 A S 3 H3 2 ) o r c alc ium alum inate m o no sulhp hate hyd rate (C4 A S H18 ). The calcium aluminate trisulp hate hyd rate is kno w n as e ttring ite . Ettring ite is usually the first to hydrate and crystallise as sho rt prismatic needle o n acco unt o f the hig h sulphate/ aluminate ratio in so lutio n phase during the first ho ur o f hydratio n. When sulp hate in so lutio n g e ts d e p le te d , the aluminate c o nc e ntratio n g o e s up d ue to re ne w e d h yd ratio n o f C 3 A an d C 4 AF. At th is stag e e ttrin g ite b e c o m e s u n stab le an d is g rad u ally co nve rte d into mo no -sulp hate , w hich is the final p ro d uct o f hyd ratio n o f p o rtland ce me nts co ntaining mo re than 5 pe rce nt C3 A. The amo unt o f g yp sum ad d e d has sig nificant b e aring o n the q uantity o f aluminate in the c e m e nt. The m ainte nanc e o f alum inate -to -sulp hate ratio b alanc e the no rm al se tting

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b e havio ur o f ce me nt p aste . The vario us se tting p he no me na affe cte d b y an imb alance in the A/ S ratio is o f practical sig nificance in co ncre te te chno lo g y. Many theo ries have been put fo rw ard to explain w hat actually is fo rmed in the hydratio n o f c e m e n t c o m p o u n d s w ith w ate r. It h as b e e n said e arliiar th at p ro d u c t c o n sistin g o f (CaO .SiO 2 .H2 O ) and Ca(O H)2 are fo rme d in the hyd ratio n o f calcium silicate s. Ca(O H)2 is an unimpo rtant pro duct, and the really sig nificant pro duct is (CaO .SiO 2 .H2 O ). Fo r simplicity’s sake this pro d uct o f hyd ratio n is so metime called to b ermo rite g el b ecause o f its structural similarity to a naturally o ccurring mine ral to b e rmo rite . But ve ry co mmo nly the p ro d uct o f hyd ratio n is re fe rre d to as C – S – H g e l. It may no t be exactly co rrect to call the pro duct o f hydratio ns as g el. Le chatelier identified the p ro d ucts as crystalline in nature and p ut fo rw ard his crystalline the o ry. He e xp laine d that the p re cip itate s re se mb le crystals inte rlo cke d w ith e ach o the r. Late r o n Michae lis p ut fo rw ard his c o llo id al the o ry w he re in he c o nsid e re d the p re c ip itate s as c o llo id al mass, g e latino us in nature . It is ag re e d that an e le me nt o f truth e xists in b o th the se the o rie s. It is acce p te d no w that the pro duct o f hydratio n is mo re like g el, co nsisting o f po o rly fo rmed, thin, fibro us crystals that are infinitely small. A variety o f transitio nal fo rms are also b elieved to exist and the w ho le is seen as b und le o f fib res, a fluffy mass w ith a refractive ind ex o f 1 .5 to 1 .5 5 , increasing w ith ag e . Since the g el co nsists o f crystals, it is po ro us in nature. It is estimated that the po ro sity o f g el is to the extent o f 28% . The g el po res are filled w ith w ater. The po res are so small that the sp e c ific surfac e o f c e m e nt g e l is o f the o rd e r o f 2 m illio n sq . c m . p e r g m . o f c e m e nt. The p o ro sity o f g e l c an b e fo und o ut b y the c ap illary c o nd e nsatio n me tho d o r b y the me rc ury p o ro sime try me tho d .

Structure of Hydrated Cement To und e rstand the b e havio ur o f co ncre te , it is ne ce ssary to acq uaint o urse lve s w ith the struc ture o f hyd rate d hard e ne d c e m e nt p aste . If the c o nc re te is c o nsid e re d as tw o p hase mate rial, name ly, the p aste p hase and the ag g re g ate p hase , the und e rstand ing o f the p aste phase b e co me s mo re impo rtant as it influe nce s the b e havio ur o f co ncre te to a much g re ate r e xte nt. It w ill b e d iscusse d late r that the stre ng th, the p e rme ab ility, the d urab ility, the d rying shrinkag e, the elastic pro perties, the creep and vo lume chang e pro perties o f co ncrete is g reatly influenced by the paste structure. The ag g reg ate phase tho ug h impo rtant, has lesser influence o n the p ro p e rtie s o f co ncre te than the p aste p hase . The re fo re , in o ur stud y to und e rstand co ncrete, it is impo rtant that w e have a d eep und erstand ing o f the structure o f the hyd rated hard e ne d ce me nt p aste at a p he no me no lo g ical le ve l.

Transition Zone Co ncrete is g enerally co nsid ered as tw o phase material i.e ., paste phase and ag g reg ates p hase . At macro le ve l it is se e n that ag g re g ate p article s are d isp e rse d in a matrix o f ce me nt paste. At the micro sco pic level, the co mplexities o f the co ncrete b eg in to sho w up, particularly in the vicinity o f larg e ag g re g ate p article s. This are a can b e co nsid e re d as a third p hase , the transitio n zo ne , w hic h re p re se nts the inte rfac ial re g io n b e tw e e n the p artic le s o f c o arse ag g reg ate and hardened cement paste. Transitio n zo ne is g enerally a plane o f w eakness and, the re fo re , has far g re ate r influe nce o n the me chanical b e havio ur o f co ncre te . Altho ug h transitio n zo ne is co mpo se d o f same b ulk ce me nt paste , the q uality o f paste in the transitio n zo ne is o f p o o re r q uality. Firstly d ue to inte rnal b le e d ing , w ate r accumulate b elo w elo ng ated , flaky and larg e pieces o f ag g reg ates. This red uces the b o nd b etw een paste

Cement "

23

and ag g re g ate in g e ne ral. If w e g o into little g re ate r d e tail, the size and c o nc e ntratio n o f crystalline co mpo unds such as calcium hydro xide and ettring ite are also larg er in the transitio n zo ne . Suc h a situatio n ac c o unt fo r the lo w e r stre ng th o f transitio n zo ne than b ulk c e me nt paste in co ncre te . D u e to d ryin g sh rin kag e o r te m p e ratu re variatio n , th e tran sitio n zo n e d e ve lo p s micro cracks e ve n b e fo re a structure s is lo ad e d . Whe n structure is lo ad e d and at hig h stre ss levels, these micro cracks pro pag ate and big g er chracks are fo rmed resulting in failure o f bo nd. The re fo re , transitio n zo ne , g e ne rally the w e ake st link o f the c hain, is c o nsid e re d stre ng th limiting p hase in co ncre te . It is b e cause o f the p re se nce o f transitio n zo ne that co ncre te fails at co nsid e rab ly lo w e r stre ss le ve l than the stre ng th o f b ulk p aste o r ag g re g ate . So me time s it may b e ne ce ssary fo r us to lo o k into the structure o f hard e ning co ncre te also . The rate and e xte nt o f hyd ratio n o f ce me nt have b e e n inve stig ate d in the p ast using a varie ty o f te c hniq ue s. The te c hniq ue s use d to stud y the struc ture o f c e me nt p aste inc lud e measurements o f setting time, co mpressive streng th, the q uantity o f heat o f hydratio n evo lved, the o p tic al and e le c tro n m ic ro sc o p e stud ie s c o up le d w ith c he m ic al analysis and the rm al analysis o f hyd ratio n p ro d ucts. Co ntinuo us mo nito ring o f re actio ns b y X-ray d iffractio ns and co nd uctio n calo rime try has also b e e n use d fo r the stud y. Me asure m e nts o f he at e vo lve d d uring the e xo the rm ic re ac tio ns also g ive s valuab le insig ht into the nature o f hyd ratio n re ac tio ns. Sinc e ap p ro xim ate ly 5 0 % o f a to tal he at

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" Concrete Technology

e vo lutio n o c c urs d uring the first 3 d ays o f h yd ratio n , a co ntinuo us reco rd o f the rate o f he at lib e ratio n d uring this tim e is e xtre m e ly u se fu l in und erstand ing the d eg ree o f hyd ratio n and the re sultant stru c tu re o f th e h ard e n in g cement paste. Fig . 1.5 sho w s th e c o m p o sitio n o f c e m e n t p aste s at d iffe re nt stag e s o f hyd ratio n.

Schematic representation of two fresh cement pastes having a water/cement ratio of 0.65 and 0.25.

The m e c hanic al p ro p e rtie s o f the hard e ne d c o nc re te d e p e nd m o re o n the p hysic al structure o f the pro ducts o f hydratio n than o n the chemical co mpo sitio n o f the cement. Mo rtar and c o nc re te , shrinks and c rac ks, o ffe rs varying c he mic al re sistanc e to d iffe re nt situatio ns, c re e p s in d iffe re nt m ag nitud e , and in sho rt, e xhib its c o m p le x b e havio ur und e r d iffe re nt co nd itio ns. Eve ntho ug h it is d ifficult to e xp lain the b e havio ur o f co ncre te fully and e xactly, it is p o ssib le to e xp lain the b e havio ur o f co ncre te o n b e tte r und e rstand ing o f the structure o f the hard e ne d ce me nt p aste . Just as it is ne ce ssary fo r d o cto rs to und e rstand in g re at d e tail the anato my o f the human b o d y to b e ab le to d iag no se d ise ase and tre at the p atie nt w ith medicine o r surg ery, it is necessary fo r co ncrete techno lo g ists to fully understand the structure o f hard e ne d c e m e nt p aste in g re at d e tail to b e ab le to ap p re c iate and re c tify the ills and d e fe cts o f the co ncre te .

(a) Water/cement ratio 0.6

(b) Water/cement ratio 0.5

Fig. 1.6. Diagram representing paste structures. c represents capillary cavities.1.6

Fig. 1.7. Microscopic schematic model representing the structure of hardened cement paste.

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25

Fo r simplicity’s sake w e w ill co nsid er o nly the structure o f the paste phase. Fresh cement paste is a plastic mass co nsisting o f w ate r and ce me nt. With the lapse o f time , say o ne ho ur, the hard ening paste co nsists o f hyd rates o f vario us co mpo und s, unhyd rated cement particles and w ate r. W ith furthe r lap se o f time the q uantity o f unhyd rate d c e me nt le ft in the p aste d e cre ase s and the hyd rate s o f the vario us co mp o und s incre ase . So me o f the mixing w ate r is use d up fo r c he mic al re ac tio n, and so me w ate r o c c up ie s the g e l-p o re s and the re maining w ater remains in the paste. After a sufficiently lo ng time (say a mo nth) the hydrated paste can be co nsidered to be co nsisting o f abo ut 85 to 90% o f hydrates o f the vario us co mpo unds and 1 0 to 1 5 pe r ce nt o f unhyd rate d ce me nt. The mixing w ate r is partly use d up in the che mical re actio ns. Part o f it o ccup ie s the g e l-p o re s and the re maining w ate r unw ante d fo r hyd ratio n o r fo r filling in the g e l-p o re s cause s cap illary cavitie s. The se cap illary cavitie s may have b e e n fully filled w ith w ater o r partly w ith w ater o r may be fully empty depending upo n the ag e and the amb ient temperature and humid ity co nd itio ns. Fig ure 1.6 (a ) and (b ) schematically d epict the structure o f hydrated cement paste. The dark po rtio n represents g el. The small g ap w ithin the d ark p o rtio n re p re se nts g e l-p o re s and b ig sp ace such as marke d “c ” re p re se nts capillary c avitie s. 1 .6 Fig . 1 .7 re p re se nts the m ic ro sc o p ic sc he m atic m o d e l o f struc ture o f hard e ne d ce me nt paste .

Water Requirements for Hydration It has b een b ro ug ht o ut earlier that C3 S req uires 24% o f w ater b y w eig ht o f cement and C2 S re q uire s 2 1 % . It has also b e e n e stimate d that o n an ave rag e 2 3 % o f w ate r b y w e ig ht o f cement is req uired fo r chemical reactio n w ith Po rtland cement co mpo unds. This 23% o f w ater che mically co mb ine s w ith ce me nt and , the re fo re , it is calle d b o und w ate r. A ce rtain q uantity o f w ater is imb ib ed w ithin the g el-po res. This w ater is kno w n as g el-w ater. It can b e said that b o und w ate r and g e l-w ate r are c o m p lim e ntary to e ac h o the r. If the q uantity o f w ate r is inad e q uate to fill up the g e l-po re s, the fo rmatio ns o f g e l itse lf w ill sto p and if the fo rmatio n o f g el sto ps there is no q uestio n o f g el-po res b eing present. It has b een further estimated that ab o ut 1 5 p e r ce nt b y w e ig ht o f ce me nt is re q uire d to fill up the g e l-p o re s. The re fo re , a to tal 38 per cent o f w ater b y w eig ht o f cement is req uired fo r the co mplete chemical reactio ns and to o ccupy the space w ithin g e l-po re s. If w ate r e q ual to 3 8 pe r ce nt b y w e ig ht o f ce me nt is

Fig. 1.8. Diagrammatic representation of the Hydration process and formation of cement gel.

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o nly use d it can b e no tice d that the re sultant p aste w ill und e rg o full hyd ratio n and no e xtra w ate r w ill b e availab le fo r the fo rmatio n o f und e sirab le capillary cavitie s. O n the o the r hand , if m o re than 3 8 p e r c e nt o f w ate r is use d , the n the e xc e ss w ate r w ill c ause und e sirab le cap illary cavitie s. The re fo re g re ate r the w ate r ab o ve the minimum re q uire d is use d (3 8 p e r cent), the mo re w ill b e the und esirab le capillary cavities. In all this it is assumed that hyd ratio n is taking p lac e in a se ale d c o ntaine r, w he re mo isture to and fro m the p aste d o e s no t take p lace . It can be seen that the capillary cavities beco me larg er w ith increased w ater/ cement ratio . With lo w er w / c ratio the cement particles are clo ser to g ether. With the pro g ress o f hyd ratio n, w hen the vo lume o f anhydro us cement increases, the pro duct o f hydratio n also increases. The inc re ase in the vo lum e o f g e l d ue to c o m p le te hyd ratio n c o uld fill up the sp ac e e arlie r o ccupied by w ater upto a w / c ratio o f 0.6 o r so . If the w / c ratio is mo re than 0.7, the increase in vo lume o f the hyd rate d p ro d uct w o uld ne ve r b e sufficie nt to fill up the vo id s cre ate d b y w ater. Such co ncrete w o uld ever remain as po ro us mass. This is to say that g el o ccupies mo re and mo re space, that o nce o ccupied b y mixing w ater. It has b een estimated that the vo lume o f g e l w o uld b e ab o ut tw ice the vo lume o f unhyd rate d ce me nt. The d iag rammatic re pre se ntatio n o f pro g re ss o f hyd ratio n is so w n in Fig . 1 .8 . Fig . 1 .8 (a ) re p re se nts the state o f c e m e nt p artic le s im m e d iate ly w he n d isp e rse d in an aq ue o us so lutio n. During the first few minutes, the reactio n rate is rapid and the calcium silicate hydrate fo rms a co ating aro und the cement g rains See Fig . 1 .8 (b ). As hyd ratio n pro ceed s, hyd ratio n pro ducts, including calcium hydro xide are precipitated fro m the saturated so lutio n and b ridg e the g ap b e tw e e n the ce me nt g rains, and the p aste stiffe ns into its final shap e , se e Fig . 1 .8 (c ). Furthe r hyud ratio n invo lving so me c o mp le x fo rm o f d iffusio n p ro c e ss re sults in furthe r d e p o sitio n o f the ce me nt g e l at the e xp e nse o f the unhyd rate d ce me nt and cap illary p o re w ate r Fig . 1 .8 (d ). What has b een d escrib ed b riefly is the appro ximate structure o f hard ened cement paste o n acco unt o f the hyd ratio n o f so me o f the majo r co mpo und s. Very little co g nisance is taken o f th e p ro d uc t o f h yd ratio n o f th e o th e r m ajo r an d m in o r c o m p o un d s in c e m e n t. Th e mo rpho lo g y o f pro d uct o f hyd ratio n and the stud y o f structure o f hard ened cement paste in its e ntire ty is a sub je ct o f co ntinue d re se arch. The d evelo pment o f hig h vo ltag e electro n micro sco py, co mb ined w ith d evelo pments o f skill in m akin g ve ry th in se c tio n s is m akin g p o ssib le h ig h re so lu tio n p h o to g rap h y an d d iffrac to m e try w h ile at th e sam e tim e re d uc in g d am ag e to th e sp e c im e n w h ile un d e r o b se rvatio n. The scanning e le ctro n p ro vid e s ste re o g rap hic imag e s and a d e taile d p icture o f struc ture o f c e me nt p aste . The se fac ilitate furthe r to und e rstand ag g re g ate c e me nt b o nd , micro fracture and po ro sity o f ce me nt g e l.

R EFER EN C ES 1.1

CRI Foundation Souvenir, March 1970.

1.2

Information supplied by Associated Cement Company, India, Sept. 1978.

1.3

Enkegaard, The Modern Planetary Cooler, Cement Technology, March/April 1992.

1.4

W. Lerch, Proceedings of ASTM, Vol. 46–1946.

1.5

G.J. Verbeck and C.W. Foster, Proceedings of ASTM, Vol. 50–1958

1.6

T.C. Powers, The Physical Structure and Engineering Properties of Concrete, Portland Cement Association Research Department Bulletin 90, July 1958.

1.7

Grasim Industries Cement Division : Technical Literature.

2

C H A P T E R Part view of Cement Factory Courtesy : Grasim Industries Cement Division

! Types of Cement ! ASTM Classification ! Ordinary Portland Cement ! Rapid Hardening Cement ! Extra Rapid Hardening Cement ! Sulphate Resisting Cement ! Portland Slag Cement (PSC) ! Quick Setting Cement ! Super Sulphated Cement ! Low Heat Cement ! Portland Pozzolana Cement ! Air-Entraining Cement ! Coloured Cement (White Cement) ! Hydrophobic cement ! Masonry Cement ! Expansive Cement ! IRS-T 40 Special Grade Cement ! Oil-Well Cement ! Rediset Cement ! High Alumina Cement ! Refractory Concrete ! Very High Strength Cement ! Fineness Test ! Standard Consistency Test ! Setting Time Test ! Strength Test ! Soundness Test ! Heat of Hydration ! Chemical Composition Test

Types of Cement and Testing of Cement I

n th e p re vio u s c h a p te r w e h a ve d isc u sse d vario us p ro p e rtie s o f Po rtland c e m e nt in g e ne ral. W e h a ve se e n th a t c e m e n ts e xh ib it d iffe re n t p ro p e rtie s and characte ristics d e p e nd ing up o n the ir che mical co mp o sitio ns. By chang ing the fine ne ss o f g rind ing o r the o xid e c o m p o sitio n, c e m e nt c an b e m ad e to e xh ib it d iffe re n t p ro p e rtie s. In th e p ast c o ntinuo us e ffo rts w e re m ad e to p ro d uc e d iffe re nt kind s o f c e m e nt, suitab le fo r d iffe re nt situatio ns b y c h a n g in g o xid e c o m p o sitio n a n d fin e n e ss o f g rin d in g . W ith th e e xte n sive u se o f c e m e n t, fo r w id e ly varying co nd itio ns, the typ e s o f ce me nt that c o u ld b e m a d e o n ly b y va ryin g th e re la tive p ro p o rtio n s o f th e o xid e c o m p o sitio n s, w e re n o t fo und to b e sufficie nt. Re co urse s have b e e n take n to ad d o n e o r tw o m o re n e w m ate rials, kn o w n as ad d itive s, to the clinke r at the time o f g rind ing , o r to the use o f entirely d ifferent b asic raw materials in the manufacture o f ce me nt. Th e u se o f a d d itive s, c h a n g in g c h e m ic a l co mpo sitio n, and use o f d iffe re nt raw mate rials have re sulte d in the availab ility o f many typ e s o f ce me nts

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to cater to the need o f the co nstructio n industries fo r specific purpo ses. In this chapter w e shall d e al w ith the p ro p e rtie s and use o f vario us kind s o f ce me nt. The se ce me nts are classifie d as Po rtland cements and no n-Po rtland cements. The d istinctio n is mainly b ased o n the metho d s o f manufac ture . The Po rtland and No n-Po rtland c e me nts g e ne rally use d are liste d b e lo w : Ind ian stand ard sp e cificatio n numb e r is also g ive n ag ainst the se e le me nts.

Types of Cement (a ) O rd inary Po rtland Ce me nt (i )

O rd inary Po rtland Ce me nt 3 3 Grad e – IS 2 6 9 : 1 9 8 9

(ii) O rd inary Po rtland Ce me nt 4 3 Grad e – IS 8 1 1 2 : 1 9 8 9 (iii) O rd inary Po rtland Ce me nt 5 3 Grad e – IS 1 2 2 6 9 : 1 9 8 7 (b ) Rap id Hard e ning Ce me nt

– IS 8 0 4 1 : 1 9 9 0

(c ) Extra Rap id Hard e ning Ce me nt

–

–

(d ) Sulp hate Re sisting Ce me nt

– IS 1 2 3 3 0 : 1 9 8 8

(e ) Po rtland Slag Ce me nt

– IS 4 5 5 : 1 9 8 9

(f )

–

Q uick Se tting Ce me nt

–

(g ) Sup e r Sulp hate d Ce me nt

– IS 6 9 0 9 : 1 9 9 0

(h ) Lo w He at Ce me nt

– IS 1 2 6 0 0 : 1 9 8 9

( j)

– IS 1 4 8 9 (Part I) 1 9 9 1 (fly ash b ase d )

Po rtland Po zzo lana Ce me nt

– IS 1 4 8 9 (Pa rt II) 1 9 9 1 (c a lc in e d c la y b ase d ) (k) Air Entraining Ce me nt

–

(l)

– IS 8 0 4 2 : 1 9 8 9

Co lo ure d Ce me nt: White Ce me nt

–

(m ) Hyd ro p ho b ic Ce me nt

– IS 8 0 4 3 : 1 9 9 1

(n ) Maso nry Ce me nt

– IS 3 4 6 6 : 1 9 8 8

(o ) Exp ansive Ce me nt

–

(p ) O il We ll Ce me nt

– IS 8 2 2 9 : 1 9 8 6

(q ) Re d ise t Ce me nt

–

(r) Co ncre te Sle e p e r g rad e Ce me nt

– IRS-T 4 0 : 1 9 8 5

(s) Hig h Alumina Ce me nt

– IS 6 4 5 2 : 1 9 8 9

(t)

–

Ve ry Hig h Stre ng th Ce me nt

– –

–

ASTM Classification Befo re w e discuss the ab o ve cements, fo r g eneral info rmatio n, it is necessary to see ho w Po rtlan d c e m e n t are c lassifie d un d e r th e ASTM (Am e ric an So c ie ty fo r Te stin g Mate rials) stand ard s. As p e r ASTM, ce me nt is d e sig nate d as Typ e I, Typ e II, Typ e III, Typ e IV, Typ e V and o the r mino r typ e s like Typ e IS, Typ e IP and Typ e IA IIA and IIIA.

Type I Fo r use in g eneral co ncrete co nstructio n w here the special pro perties specified fo r Types II, III, IV and V are no t re q uire d (O rd inary Po rtland Ce me nt).

Type II Fo r use in g eneral co ncrete co nstructio n expo sed to mo derate sulphate actio n, o r w here mo d e rate he at o f hyd ratio n is re q uire d .

Types of Cement "

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Type III Fo r u se w h e n h ig h e a rly stre n g th is re q uire d (Rap id Hard e ning Ce me nt).

Type IV Fo r use w h e n lo w h e at o f h yd ratio n is re q uire d (Lo w He at Ce me nt).

Type V Fo r use w he n hig h sulp hate re sistance is re q uire d (Sulp hate Re sisting Ce me nt). ASTM stand ard also have c e m e nt o f the type IS. This co nsist o f an intimate and unifo rm b le nd o f Po rtland Ce m e nt o f typ e I and fine g ranulate d slag . The slag c o nte nt is b e tw e e n 2 5 and 7 0 p e r ce nt o f the w e ig ht o f Po rtland Blast-Furnace Slag Ce me nt.

Type IP Cross Section of Multi-compartment Silo for This c o nsist o f an intim ate and unifo rm storing different types of cement. b le nd o f Po rtland Ce m e nt (o r Po rtland Blast Courtesy : Grasim Industries Cement Division Furnac e Slag Ce m e nt) and fine p o zzo lana in w hic h the p o zzo lana c o nte nt is b e tw e e n 1 5 and 4 0 p e r ce nt o f the w e ig ht o f the to tal ce me nt.

Type IA, IIA and IIIA The se are typ e I, II o r III ce me nt in w hich air-e ntraining ag e nt is inte rg ro und w he re aire ntrainme nt in co ncre te is d e sire d .

Ordinary Portland Cement O rd inary Po rtland c e me nt (O PC) is b y far the mo st imp o rtant typ e o f c e me nt. All the d iscussio ns that w e have d o ne in the p re vio us chap te r and mo st o f the d iscussio ns that are g o ing to b e d o ne in the co ming chap te rs re late to O PC. Prio r to 1 9 8 7 , the re w as o nly o ne g rad e o f O PC w hich w as g o ve rne d b y IS 2 6 9 -1 9 7 6 . Afte r 1 9 8 7 hig he r g rad e ce me nts w e re intro duced in India. The O PC w as classified into three g rades, namely 33 g rade, 43 g rade and 53 g rade depending upo n the streng th o f the cement at 28 days w hen tested as per IS 40311 9 8 8 . If the 2 8 d ays stre ng th is no t le ss than 3 3 N/ mm 2 , it is calle d 3 3 g rad e ce me nt, if the streng th is no t less than 43N/ mm 2 , it is called 43 g rad e cement, and if the streng th is no t less the n 5 3 N/ m m 2 , it is c alle d 5 3 g rad e c e m e nt. But the ac tual stre ng th o b taine d b y the se ce me nts at the facto ry are much hig he r than the BIS sp e cificatio ns. Th e p h ysic al an d c h e m ic al p ro p e rtie s o f 3 3 , 4 3 an d 5 3 g rad e O PC are sh o w n in Tab le 2 .5 and 2 .6 . It has b een po ssib le to upg rad e the q ualities o f cement b y using hig h q uality limesto ne, mo d e rn e q uip me nts, c lo se r o n line c o ntro l o f c o nstitue nts, maintaining b e tte r p artic le size d istrib utio n, fine r g rind ing and b e tte r p ac king . Ge ne rally use o f hig h g rad e c e me nts o ffe r m any ad vantag e s fo r m aking stro ng e r c o nc re te . Altho ug h the y are little c o stlie r than lo w g rad e ce me nt, the y o ffe r 1 0 -2 0 % saving s in ce me nt co nsump tio n and also the y o ffe r many o the r hid d e n b e ne fits. O ne o f the mo st imp o rtant b e ne fits is the faste r rate o f d e ve lo p me nt

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o f stre ng th. In the mo d e rn c o nstruc tio n ac tivitie s, hig he r g rad e c e me nts have b e c o me so p o p ular that 3 3 g rad e c e me nt is almo st o ut o f the marke t. Tab le 2 .9 sho w s the g rad e s o f ce me nt manufacture d in vario us co untrie s o f the w o rld . The manufac ture o f O PC is d e c re asing all o ve r the w o rld in vie w o f the p o p ularity o f b le n d e d c e m e n t o n ac c o u n t o f lo w e r e n e rg y c o n su m p tio n , e n viro n m e n tal p o llu tio n , e co no mic and o the r te chnical re aso ns. In ad vance d w e ste rn co untrie s the use o f O PC has co me d o w n to ab o ut 4 0 per cent o f the to tal cement pro d uctio n. In Ind ia fo r the year 1 9 9 8 9 9 o ut o f the to tal ce me nt pro d uctio n i.e. , 7 9 millio n to ns, the pro d uctio n o f O PC in 5 7 .0 0 millio n to ns i.e. , 7 0 % . The pro d uctio n o f PPC is 1 6 millio n to ne i.e. , 1 9 % and slag ce me nt is 8 millio n to ns i.e. , 10% . In the years to co me the use o f O PC may still co me d o w n, b ut all the same the O PC w ill re main as an impo rtant type fo r g e ne ral co nstructio n. The d e tail te sting me tho d s o f O PC is se p arate ly d iscrib e d at the e nd o f this chap te r.

Rapid Hardening Cement (IS 8041–1990) This c e me nt is similar to o rd inary Po rtland c e me nt. As the name ind ic ate s it d e ve lo p s stre ng th rap id ly and as suc h it m ay b e m o re ap p ro p riate to c all it as hig h e arly stre ng th c e m e n t. It is p o in te d o u t th at rap id h ard e n in g c e m e n t w h ic h d e ve lo p s h ig h e r rate o f d e ve lo p me nt o f stre ng th sho uld no t b e co nfuse d w ith q uick-se tting ce me nt w hich o nly se ts q uickly. Rapid hard ening cement d evelo ps at the ag e o f three d ays, the same streng th as that is e xp e cte d o f o rd inary Po rtland ce me nt at se ve n d ays. The rapid rate o f develo pment o f streng th is attrib uted to the hig her fineness o f g rinding (spe cific surface no t le ss than 3 2 5 0 sq . cm pe r g ram) and hig he r C3 S and lo w er C2 S co ntent. A hig her fineness o f cement particles expo se g reater surface area fo r actio n o f w ater and also hig he r p ro p o rtio n o f C3 S re sults in q uic ke r hyd ratio n. Co nse q ue ntly, c ap id hard e ning ce me nt g ive s o ut much g re ate r he at o f hyd ratio n d uring the e arly p e rio d . The re fo re , rap id hard e ning ce me nt sho uld no t b e use d in mass co ncre te co nstructio n. The use o f rap id he ad ing ce me nt is re co mme nd e d in the fo llo w ing situatio ns: (a ) In p re -fab ricate d co ncre te co nstructio n. (b ) Whe re fo rmw o rk is re q uire d to b e re mo ve d e arly fo r re -use e lse w he re , (c ) Ro ad re p air w o rks, (d ) In co ld w e athe r co ncre te w he re the rap id rate o f d e ve lo p me nt o f stre ng th re d uce s the vulne rab ility o f co ncre te to the fro st d amag e . The physical and chemical req uirements o f rapid hard ening cement are sho w n in Tab les 2 .5 and 2 .6 re sp e ctive ly.

Extra Rapid Hardening Cement Extra rap id hard e ning ce me nt is o b taine d b y inte rg rind ing calcium chlo rid e w ith rap id hardening Po rtland cement. The no rmal additio n o f calcium chlo ride sho uld no t exceed 2 per cent by w eig ht o f the rapid hardening cement. It is necessary that the co ncrete made by using e xtra rap id hard e ning c e me nt sho uld b e transp o rte d , p lac e d and c o mp ac te d and finishe d w ithin ab o ut 2 0 minute s. It is also ne ce ssary that this ce me nt sho uld no t b e sto re d fo r mo re than a mo nth. Extra rap id hard e ning c e me nt ac c e le rate s the se tting and hard e ning p ro c e ss. A larg e q uantity o f he at is e vo lve d in a ve ry sho rt tim e afte r p lac ing . The ac c e le ratio n o f se tting , hard ening and evo lutio n o f this larg e q uantity o f heat in the early perio d o f hyd ratio n makes the cement very suitable fo r co ncreting in co ld w eather, The streng th o f extra rapid hardening

Types of Cement "

31

cement is ab o ut 2 5 per cent hig her than that o f rapid hard ening cement at o ne o r tw o d ays and 1 0 –2 0 per cent hig her at 7 d ays. The g ain o f streng th w ill d isappear w ith ag e and at 9 0 d ays the stre ng th o f e xtra rap id hard e ning ce me nt o r the o rd inary p o rtland ce me nt may b e ne arly the same . The re is so me e vid e nce that the re is small amo unt o f initial co rro sio n o f re info rce me nt w hen extra rapid hard ening cement is used , b ut in g eneral, this effect d o es no t appear to b e pro g ressive and as such there is no harm in using extra rapid hard ening cement in reinfo rced co ncre te w o rk. Ho w e ve r, its use in pre stre ss co ncre te co nstructio n is pro hib ite d . In Russia, the atte mp t has b e e n mad e to o b tain the e xtra rap id hard e ning p ro p e rty b y g rind ing the cement to a very fine d eg ree to the extent o f having a specific surface b etw een 5000 to 6000 sq . cm/ g m. The size o f mo st o f the particles are g enerally less than 3 micro ns2.1 . It is fo und that this very finely g ro und cement is d ifficult to sto re as it is liab le to air-set. It is no t a co mmo n ce me nt and he nce it is no t co ve re d b y Ind ian stand ard .

Sulphate Resisting Cement (IS 12330–1988) O rd inary Po rtland ce me nt is susce p tib le to the attack o f sulp hate s, in p articular to the actio n o f mag ne sium sulp hate . Sulp hate s re act b o th w ith the fre e calcium hyd ro xid e in se tc e m e nt to fo rm c alc ium sulp hate and w ith hyd rate o f c alc ium alum inate to fo rm c alc ium sulp ho aluminate , the vo lume o f w hich is ap p ro ximate ly 2 2 7 % o f the vo lume o f the o rig inal aluminates. Their expansio n w ithin the frame w o rk o f had ened cement paste results in cracks and sub seq uent d isruptio n. So lid sulphate d o no t attack the cement co mpo und . Sulphates in so lutio n p e rme ate into hard e ne d co ncre te and attack calcium hyd ro xid e , hyd rate d calcium aluminate and e ve n hyd rate d silicate s. Th e ab o ve is kn o w n as su lp h ate attac k. Su lp h ate attac k is g re atly ac c e le rate d if acco mpanied by alternate w etting and drying w hich no rmally takes place in marine structures in the zo ne o f tid al variatio ns. To re me d y the sulp hate attack, the use o f ce me nt w ith lo w C3 A co nte nt is fo und to b e e ffe c tive . Suc h c e m e nt w ith lo w C3 A and c o m p arative ly lo w C4 AF c o nte nt is kno w n as Sulp hate Re sisting Ce m e nt. In o the r w o rd s, this c e m e nt has a hig h silic ate c o nte nt. The sp e cificatio n g e ne rally limits the C3 A co nte nt to 5 pe r ce nt. Te tracalcium Alumino Fe rrite (C3 AF) varie s in No rmal Po rtland Ce me nt b e tw e e n to 6 to 12% . Since it is o ften no t feasib le to red uce the Al2 O 3 co ntent o f the raw material, Fe 2 O 3 may b e ad d e d to the mix so that the C4 AF co nte nt incre ase s at the e xpe nse o f C3 A. IS co d e limits the to tal co nte nt o f C4 AF and C3 A, as fo llo w s. 2 C3 A + C4 AF sho uld no t e xce e d 2 5 % . In many o f its physical pro perties, sulphate resisting cement is similar to o rdinary Po rtland cement. The use o f sulphate resisting cement is reco mmended under the fo llo w ing co nditio ns: (a ) Co ncre te to b e use d in marine co nd itio n; (b ) Co n c re te to b e u se d in fo u n d atio n an d b ase m e n t, w h e re so il is in fe ste d w ith sulp hate s; (c ) Co ncre te use d fo r fab ricatio n o f pipe s w hich are like ly to b e b urie d in marshy re g io n o r sulp hate b e aring so ils; (d ) Co ncre te to b e use d in the co nstructio n o f se w ag e tre atme nt w o rks.

Portland Slag Cement (PSC) (IS 455–1989) Po rtlan d slag c e m e n t is o b tain e d b y m ixin g Po rtlan d c e m e n t c lin ke r, g yp sum an d

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g ranulate d b last furnac e slag in suitab le p ro p o rtio ns and g rind ing the m ixture to g e t a tho ro ug h and intimate mixture b e tw e e n the c o nstitue nts. It may also b e manufac ture d b y separately g rinding Po rtland cement clinker, g ypsum and g ro und g ranulated blast furnace slag and late r m ixing the m intim ate ly. The re sultant p ro d uc t is a c e m e nt w hic h has p hysic al p ro p e rtie s sim ilar to th o se o f o rd in ary Po rtlan d c e m e n t. In ad d itio n , it h as lo w h e at o f hyd ratio n and is re lative ly b e tte r re sistant to chlo rid e s, so ils and w ate r co ntaining e xce ssive am o unt o f sulp hate s o r alkali m e tals, alum ina and iro n, as w e ll as, to ac id ic w ate rs, and the re fo re , this c an b e use d fo r m arine w o rks w ith ad vantag e . Th e m an u fac tu re o f b last fu rn ac e slag cement has b een d evelo ped primarily to utilize b last furnace slag , a w aste pro d uct fro m b last fu rn ac e s. Th e d e ve lo p m e n t o f th is typ e o f c e m e nt has c o nsid e rab ly inc re ase d the to tal o utput o f cement pro d uctio n in Ind ia and has, in ad d itio n, pro vid ed a sco pe fo r pro fitab le use fo r an o the rw ise w aste p ro d uct. During 9 8 -9 9 Ind ia p ro d uc e d 1 0 % slag c e m e nt o ut o f 7 9 millio n to ns. Th e q u an tity o f g ran u late d slag m ixe d w ith po rtland clinker w ill rang e fro m 25-65 per c e n t. In d iffe re n t c o u n trie s th is c e m e n t is kno w n in d ifferent names. The q uantity o f slag m ixe d also w ill vary fro m c o untry to c o untry Schematic representation of production of the m axim um b e ing up to 8 5 p e r c e nt. Early blast furnace slag. stre ng th is m ainly d ue to the c e m e nt c linke r fractio n and later streng th is that due to the slag fractio n. Separate g rinding is used as an easy m e ans o f ve rying the slag c linke r p ro p o rtio n in the finishe d c e m e nt to m e e t the m arke t d e mand . Re ce ntly, und e r Bo mb ay Se w ag e d isp o sal p ro je ct at Band ra, the y have use d 7 0 % g ro und g ranulated b last furnace slag (GGBS) and 30% cement fo r making g ro ut to fill up the tre nch aro und p re cast se w e r 3 .5 m d ia e mb e d d e d 4 0 m b e lo w MSL. Po rtland b last furnac e c e m e nt is sim ilar to o rd inary Po rtland c e m e nt w ith re sp e c t to fine ne ss, se tting tim e , so und ne ss and stre ng th. It is g e ne rally re c o g nise d that the rate o f hard e ning o f Po rtland b last furnace slag ce me nt in mo rtar o r co ncre te is so me w hat slo w e r than that o f o rdinary Po rtland cement during the first 28 days, but thereafter increases, so that at 1 2 mo nths the stre ng th b e co me s clo se to o r e ve n e xce e d s tho se o f Po rtland ce me nt. The he at o f hyd ratio n o f Po rtland b last furnac e c e me nt is lo w e r than that o f o rd inary Po rtland ce me nt. So this ce me nt can b e use d in mass co ncre te structure s w ith ad vantag e . Ho w e ve r, in c o ld w e athe r the lo w he at o f hyd ratio n o f Po rtland b last furnac e c e me nt c o up le d w ith mo d e rate ly lo w rate o f stre ng th d e ve lo p me nt, can le ad to fro st d amag e . Exte nsive re se arch sho w s that the p re se nce o f GGBS le ad s to the e nhance me nt o f the intrinsic p ro p e rtie s o f the co ncre te b o th in fre sh and hard e ne d state s. The majo r ad vantag e s curre ntly re co g nise d are : (a ) Re d uce d he at o f hyd ratio n; (b ) Re fine me nt o f p o re structure ; (c ) Re d uce d p e rme ab ility;

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(d ) Incre ase d re sistance to che mical attack. It is se e n that in Ind ia w he n the Po rtland b last furnace slag ce me nt w as first intro d uce d it m e t w ith c o nsid e rab le susp ic io n and re sistanc e b y the use rs. This is just b e c ause so m e manufacture rs d id no t use the rig ht q uality o f slag . It has b e e n p o inte d o ut that o nly g lassy g ranulated slag co uld b e used fo r the manufacture o f slag cement. Air-co o led crystallined slag canno t be used fo r pro viding cementitio us pro perty. The slag w hich is used in the manufacture o f vario us slag ce me nt is chille d ve ry rap id ly e ithe r b y p o uring it into a larg e b o d y o f w ate r o r b y sub je cting the slag stre am to je ts o f w ate r, o r o f air and w ate r. The purpo se is to co o l the slag q uickly so that crystallisatio n is prevented and it so lidifies as g lass. The pro duct is called g ranulated slag . O nly in this fo rm the slag sho uld be used fo r slag cement. It the slag prepared in any o the r fo rm is use d , the re q uire d q uality o f the ce me nt w ill no t b e o b taine d . Po rtland slag ce me nt e xhib its ve ry lo w d iffusivity to chlo rid e io ns and such slag ce me nt g ive s b e tte r re sistance to co rro sio n o f ste e l re info rce me nt.

Ta ble 2 . 1 . Diffusion of chloride ions at 2 5 °C in c e m e nt pa st e s of w /c 0 .5 Type o f cement SRPC*

Diffusivity (x 1 0 –9 cm 2 / s) 1 0 0 .0

O PC

4 4 .7

7 0 % O PC/ 3 0 % Fly ash

1 4 .7

3 5 % O PC/ 6 5 % GGBS

4 .1

SRPC* – Sulphate resisting Portland cement.

Application of GGBS Concrete In recent years the use o f GGBS co ncrete is w ell reco g nised . Co mb ining GGBS and O PC at mixe r is tre ate d as e q uivale nt to facto ry mad e PSC. Co ncre te w ith d iffe re nt p ro p e rtie s can b e mad e b y varying the p ro p o rtio ns o f GGBS. While p lacing larg e p o urs o f co ncre te it is vital to minimise the risk o f e arly ag e the rmal cracking b y co ntro lling the rate o f temperature rise. O ne o f the accepted metho d s is thro ug h the use o f GGBS co ncre te co ntaining 5 0 % to 9 0 % GGBS. Ge ne rally, a co mb inatio n o f 7 0 % GGBS and 3 0 % O PC is re co mme nd e d . Re sistance to che mical attack may b e e nhance d b y using GGBS in co ncre te . Re sistance to acid attack may b e imp ro ve d thro ug h the use o f 7 0 % GGBS. To co unte r the p ro b le m o f sulp hate and chlo rid e attack 4 0 % to 7 0 % GGBS may b e use d . The re is a g e ne ral co nse nsus amo ng co ncre te te chno lo g ists that the risk o f ASR can b e minimised b y using at least 50% GGBS. GGBS co ncrete is also reco mmend ed fo r use in w ater re taining struc ture s. Ag g re ssive w ate r c an affe c t c o nc re te fo und atio ns. In suc h c o nd itio ns GGBS co ncre te can p e rfo rm b e tte r.

Quick Setting Cement This ce me nt as the name ind icate s se ts ve ry e arly. The e arly se tting p ro p e rty is b ro ug ht o ut b y re d ucing the g yp sum co nte nt at the time o f clinke r g rind ing . This ce me nt is re q uire d to b e mixe d , p lace d and co mp acte d ve ry e arly. It is use d mo stly in und e r w ate r co nstructio n w he re p um p ing is invo lve d . Use o f q uic k se tting c e m e nt in suc h c o nd itio ns re d uc e s the p ump ing time and make s it e co no mical. Q uick se tting ce me nt may also find its use in so me typ ical g ro uting o p e ratio ns.

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Super Sulphated Cement (IS 6909–1990) Sup e r sulp hate d ce me nt is manufacture d b y g rind ing to g e the r a mixture o f 8 0 -8 5 p e r c e nt g ranulate d slag , 1 0 -1 5 p e r c e nt hard b urnt g yp sum , and ab o ut 5 p e r c e nt Po rtland cement clinker. The pro duct is g ro und finer than that o f Po rtland cement. Specific surface must no t be less than 4000 cm 2 per g m. The super-sulphated cement is extensively used in Belg ium, w he re it is kno w n as “c im e nt m e tallurg iq ue sursulfate .” In Franc e , it is kno w n as “c im e nt sursulfate ”. This cement is rather mo re sensitive to deterio ratio n during sto rag e than Po rtland cement. Super-sulphated cement has a lo w heat o f hydratio n o f abo ut 40-45 calo ries/ g m at 7 days and 4 5 -5 0 at 2 8 d ays. This c e m e nt has hig h sulp hate re sistanc e . Be c ause o f this p ro p e rty this c e m e nt is p artic ularly re c o m m e nd e d fo r use in fo und atio n, w he re c he m ic ally ag g re ssive co nd itio ns e xist. As sup e r-sulp hate d ce me nt has mo re re sistance than Po rtland b last furnace slag ce me nt to attack b y se a w ate r, it is also use d in the marine w o rks. O the r are as w he re super-sulphated cement is reco mmend ed includ e the fab ricatio n o f reinfo rced co ncrete pipes w hich are like ly to b e b urie d in sulp hate b e aring so ils. The sub stitutio n o f g ranulate d slag is re spo nsib le fo r b e tte r re sistance to sulphate attack. Sup e r-sulp hate d c e m e nt, like hig h alum ina c e m e nt, c o m b ine s w ith m o re w ate r o n hyd ratio n than Po rtland cements. Wet curing fo r no t less than 3 d ays after casting is essential as the premature d rying o ut results in an und esirab le o r p o w d e ry surface laye r. Whe n w e use sup e r sulp hate d cement the w ater/ cement ratio sho uld no t b e less than 0 . 5 . A m ix le a n e r th a n a b o u t 1 : 6 is a lso n o t re co mme nd e d .

Low Heat Cement (IS 12600-1989) It is w e ll kno w n that hyd ratio n o f c e m e nt is an e xo the rm ic ac tio n w hic h p ro d uc e s larg e q uantity o f he at d uring hyd ratio n. This asp e ct has b e e n d iscusse d in detail in Chapter 1. Fo rmatio n o f cracks in larg e b o dy o f co ncre te d ue to he at o f hyd ratio n has fo cusse d the atte ntio n o f the c o nc re te te c hno lo g ists to p ro d uc e a kind o f ce me nt w hich p ro d uce s le ss he at o r the same am o unt o f he at, at a lo w rate d uring the hyd ratio n pro cess. Cement having this pro perty w as develo ped in U. S. A. d u rin g 1 9 3 0 fo r u se in m a ss c o n c re te co nstructio n, such as d ams, w he re te mp e rature rise b y the he at o f hyd ratio n can b e co me e xce ssive ly larg e . A Law heat cement is made use of in lo w -heat evo lutio n is achieved by reducing the co ntents construction of massive dams. o f C3 S and C3 A w hich are the co mpo und s evo lving the maximum he at o f hyd ratio n and incre asing C2 S. A re d uctio n o f te mp e rature w ill re tard the che mical actio n o f hard e ning and so furthe r re strict the rate o f e vo lutio n o f he at. The rate o f evo lutio n o f heat w ill, therefo re, be less and evo lutio n o f heat w ill extend o ver a lo ng er perio d. The re fo re , the fe ature o f lo w -he at ce me nt is a slo w rate o f g ain o f stre ng th. But the ultimate streng th o f lo w -heat cement is the same as that o f o rdinary Po rtland cement. As per the Indian Stand ard Spe cificatio n the he at o f hyd ratio n o f lo w -he at Po rtland ce me nt shall b e as fo llo w s: 7 d ays — no t mo re than 6 5 calo rie s p e r g m. 2 8 d ays — no t mo re than 7 5 calo rie s p e r g m.

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The sp e cific surface o f lo w he at ce me nt as fo und o ut b y air-p e rme ab ility me tho d is no t le ss than 3 2 0 0 sq . cm/ g m. The 7 d ays stre ng th o f lo w he at ce me nt is no t le ss than 1 6 MPa in c o ntrast to 2 2 MPa in the c ase o f o rd inary Po rtland c e m e nt. O the r p ro p e rtie s, suc h as se tting time and so und ne ss are same as that o f o rd inary Po rtland ce me nt.

Portland Pozzolana Cement (IS 1489–1991) The histo ry o f po zzo lanic material g o es back to Ro man’s time. The descriptio ns and details o f po zzo lanic material w ill b e dealt separately under the chapter ‘Admixtures’. Ho w ever a b rief d e scrip tio n is g ive n b e lo w. Po rtland Po zzo lana ce me nt (PPC) is manufacture d b y the inte rg rind ing o f O PC clinke r w ith 1 0 to 2 5 pe r ce nt o f po zzo lanic mate rial (as pe r the late st ame nd me nt, it is 1 5 to 3 5 % ). A p o zzo lanic m ate rial is e sse ntially a silic io us o r alum ino us m ate rial w hic h w hile in itse lf po sse ssing no ce me ntitio us pro pe rtie s, w hich w ill, in fine ly d ivid e d fo rm and in the pre se nce o f w ate r, re ac t w ith c alc ium h yd ro xid e , lib e rate d in th e h yd ratio n p ro c e ss, at o rd in ary temperature, to fo rm co mpo unds po ssessing cementitio us pro perties. The po zzo lanic materials g enerally used fo r manufacture o f PPC are calcined clay (IS 1 4 8 9 part 2 o f 1 9 9 1 ) o r fly ash (IS 1 4 8 9 p art I o f 1 9 9 1 ). Fly ash is a w aste mate rial, g e ne rate d in the the rmal p o w e r statio n, w he n po w d e re d co al is use d as a fue l. The se are co lle cte d in the e le ctro static pre cipitato r. (It is called pulverised fuel ash in UK). Mo re info rmatio n o n fly ash as a mineral admixture is g iven in chapte r 5 . It m ay b e re c alle d that c alc ium silic ate s p ro d uc e c o nsid e rab le q uantitie s o f c alc ium hyd ro xid e , w hic h is b y and larg e a use le ss m ate rial fro m the p o int o f vie w o f stre ng th o r d urab ility. If suc h use le ss m ass c o uld b e c o nve rte d into a use ful c e m e ntitio us p ro d uc t, it c o nsid e rab ly im p ro ve s q uality o f c o nc re te . The use o f fly ash p e rfo rm s suc h a ro le . The po zzo lanic actio n is sho w n b e lo w : Calcium hyd ro xid e + Po zzo lana + w ate r → C – S – H (g e l) Po rtland po zzo lana cement pro d uces less heat o f hyd ratio n and o ffers g reater resistance to the attack o f ag g re ssive w ate rs than o rd inary Po rtland ce me nt. Mo re o ve r, it re d uce s the le ac hing o f c alc ium hyd ro xid e w he n use d in hyd raulic struc ture s. It is p artic ularly use ful in marine and hydraulic co nstructio n and o ther mass co ncrete co nstructio ns. Po rtland po zzo lana c e m e nt c an g e ne rally b e use d w he re o rd inary Po rtland c e m e nt is usab le . Ho w e ve r, it is impo rtant to appreciate that the ad d itio n o f po zzo lana d o es no t co ntrib ute to the streng th at early ag es. Streng ths similar to tho se o f o rdinary Po rtland cement can b e expected in g eneral o nly at later ag es pro vided the co ncrete is cured under mo ist co nditio ns fo r a sufficient perio d. In Ind ia there is apprehensio n in the mind s o f the user to use the Po rtland po zzo lana cement fo r struc tural w o rks. It c an b e said that this fe ar is no t justifie d . If the Po rtland p o zzo lana ce me nt is manufacture d b y using the rig ht typ e o f re active p o zzo lanic mate rial, the Po rtland po zzo lanic cement w ill no t b e in any w ay inferio r to o rd inary Po rtland cement except fo r the rate o f d e ve lo p me nt o f stre ng th up to 7 d ays. It is o nly w he n infe rio r p o zzo lanic mate rials, w hich are no t o f re active type and w hich d o no t satisfy the spe cificatio ns limit fo r po zzo lanic mate rials, are use d the ce me nt w o uld b e o f d o ub tful q uality. The ad vantag e s o f PPC can b e summe rise d as fo llo w s. Te chnically PPC has co nsid e rab le ad vantag e s o ve r O PC w he n mad e b y using o p timum p e rce ntag e o f rig ht q uality o f fly ash.

Advantages of PPC (a ) In PPC, co stly clinker is replaced b y cheaper po zzo lanic material - Hence eco no mical.

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(b ) So lub le c alc ium hyd ro xid e is c o nve rte d in to in so lu b le c e m e n titio u s p ro d u c ts resulting in impro vement o f permeability. He n c e it o ffe rs, a lro u n d d u ra b ility c harac te ristic s, p artic ularly in hyd raulic structure s and marine co nstructio n. (c ) PPC c o nsum e s c alc ium hyd ro xid e and d o e s no t p ro d uce calcium hyd ro xid e as much as that o f O PC. (d ) It g e ne rate s re d uc e d he at o f hyd ratio n and that to o at a lo w rate . (e ) PPC b e ing fine r than O PC and also d ue to po zzo lanic actio n, it impro ves the po re size d istrib u tio n an d also re d u c e s th e micro cracks at the transitio n zo ne . (f ) Re d uc tio n in p e rm e ab ility o f PPC o ffe rs many o the r alro und ad vantag e s. (g ) As th e fly a sh is fin e r a n d o f lo w e r d e nsity, the b ulk vo lume o f 5 0 kg b ag is slig htly m o re than O PC. The re fo re , PPC g ive s mo re vo lume o f mo rtar than O PC. (h ) The lo ng te rm stre ng th o f PPC b e yo nd a c o up le o f mo nths is hig he r than O PC if e n o u g h m o istu re is a va ila b le fo r co ntinue d po zzo lanic actio n. Schematic representation of the formation

All the ab o ve ad vantag e s o f PPC are mainly of fly ash. d ue to the slo w co nve rsio n o f calcium hyd ro xid e in the hyd rate d c e me nt p aste into c e me ntitio us p ro d uc t. In o ne inve stig atio n, 2 0 p e r c e nt calcium hyd ro xid e in o ne year o ld O PC paste w as fo und to b e o nly 8 .4 p e r ce nt calcium hyd ro xid e in a sim ilarly hyd rate d p aste c o ntaining 3 0 p e r ce nt p o zzo lana. It may b e no te d that d ue to the d ilutio n an d le ac h in g also c e rtain re d uc tio n in calcium hyd ro xid e may have take n p lace . Giving co nsideratio n to that effect, the calcium hydro xide sho uld have b e e n 1 4 % . But the fac t is that o nly 8 .4 % has re m aine d g o e s to p ro ve that 5 .6 % o f c a lc iu m h yd ro xid e w a s c o n ve rte d b y th e p o zzo lan ic ac tivity. Fig . 2 .1 sh o w s th e typ ic al re d uctio n o f Ca(O H)2 . A fe w o f the d isad vantag e s are that the rate o f d e ve lo p m e n t o f stre n g th is in itially slig h tly slo w e r than O PC. Se co nd ly re d uctio n in alkalinity re d u c e s th e re sista n c e to c o rro sio n o f ste e l re info rc e m e nt. But c o nsid e ring the fac t that PPC sig nific antly im p ro ve the p e rm e ab ility o f co ncrete, increases the resistance to co rro sio n o f reinfo rcement. The setting time is no minally lo ng e r.

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Status of PPC in India O ve r 6 0 millio n to ne s o f fly ash is g e ne rate d fro m o ve r 7 5 the rmal p o w e r statio ns. But the q ualitie s o f suc h fly ash are g e ne rally no t satisfac to ry to b e use d in PPC. In w e ste rn co untrie s fly ash g e ne rate d in the rmal p o w e r p lants are furthe r p ro ce sse d to re nd e r it fit fo r using in PPC. Be cause o f the po o r q uality o f fly ash, lack o f aw are ne ss and fe ar psychics o n the p art o f use rs, PPC is no t p o p ular. In Ind ia o nly 1 9 % o f to tal c e me nt p ro d uc tio n is PPC. (1 9 9 8 -1 9 9 9 ) and ab o ut 1 0 % is slag cement. Go vernment o f Ind ia has set up an o rg anisatio n called Fly Ash missio n to pro mo te the use o f fly ash as mineral ad mixture o r in manufacturing PPC. It has b een realised b y all experts in the w o rld that mo re and mo re b lend ed cement has to b e use d fo r sustainab le d e ve lo p me nt o f any co untry. Due to the sho rtag e o f electrical po w er, many cement facto ries have their o w n dedicated the rmal p o w e r p lant. The y use the ir o w n fly ash fo r manufacturing PPC. As the y kno w the imp o rtanc e o f the q ualitie s o f fly ash, the y take p artic ular c are to p ro d uc e fly ash o f g o o d q ualities to b e used in PPC. The PPC pro duced b y such cement plant is o f superio r q uality. The c he mic al and p hysic al q ualitie s o f p ro p e rtie s o f suc h PPC sho w muc h sup e rio r value s than w hat is p re scrib e d in BIS stand ard . The p hysical and che mical p ro p e rtie s o f PPC as g ive n in IS: 1 4 8 9 (p art-I) 1 9 9 1 is g ive n in tab le 2 .5 Birla Plus, Suraksha, Silicate Cement, Birla Bo nus are so me o f the b rand names o f PPC in Ind ia.

Grading of PPC In many co untrie s, PPC is g rad e d like O PC d e p e nd ing up o n the ir co mp re ssive stre ng th at 2 8 d ays. In Ind ia, so far PPC is c o nsid e re d e q uivale nt to 3 3 g rad e O PC, stre ng thw ise , altho ug h so me b rand o f PPC is as g o o d as even 5 3 g rad e O PC. Many cement manufacturers have re q ue ste d BIS fo r g rad ing o f PPC just like g rad ing o f O PC. The y have also re q ue ste d fo r upper limits o f fly ash co ntent fro m 25% to 35% . Recently BIS has increased the fly ash co ntent in PPC fro m 1 0 –2 5 % to 1 5 –3 5 % .

Application Po rtland po zzo lana cement can be used in all situatio ns w here O PC is used except w here hig h e arly stre ng th is o f sp e c ial re q uire me nt. As PPC ne e d s e no ug h mo isture fo r sustaine d po zzo lanic activity, a little lo ng er curing is d esirab le. Use o f PPC w o uld b e particularly suitab le fo r the fo llo w ing situatio ns: (a ) Fo r hyd raulic structure s; (b ) Fo r mass co ncre te structure s like d am, b rid g e p ie rs and thick fo und atio n; (c ) Fo r marine structure s; (d ) Fo r se w e rs and se w ag e d isp o sal w o rks e tc.

Air-Entraining Cement Air-e ntraining ce me nt is no t co ve re d b y Ind ian Stand ard so far. This ce me nt is mad e b y mixing a small amo unt o f an air-entraining ag ent w ith o rdinary Po rtland cement clinker at the time o f g rind ing . The fo llo w ing typ e s o f air-e ntraining ag e nts co uld b e use d : (a ) Alkali salts o f w o o d re sins. (b ) Synthe tic d e te rg e nts o f the alkyl-aryl sulp ho nate typ e . (c ) Calcium lig no sulp hate d e rive d fro m the sulp hite p ro ce ss in p ap e r making . (d ) Calcium salts o f g lue s and o the r p ro te ins o b taine d in the tre atme nt o f animal hid e s.

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These ag ents in po w der, o r in liq uid fo rms are added to the extent o f 0.025–0.1 per cent b y w e ig ht o f ce me nt clinke r. The re are o the r ad d itive s includ ing animal and ve g e tab le fats, o il and the ir ac id s c o uld b e use d . We tting ag e nts, aluminium p o w d e r, hyd ro g e n p e ro xid e c o uld also b e use d . Air-e ntraining c e me nt w ill p ro d uc e at the time o f mixing , to ug h, tiny, d isc re te no n-c o ale sc e ing air b ub b le s in the b o d y o f the c o nc re te w hic h w ill m o d ify the p ro p e rtie s o f p lastic c o nc re te w ith re sp e c t to w o rkab ility, se g re g atio n and b le e d ing . It w ill mo d ify the p ro p e rtie s o f hard e ne d co ncre te w ith re sp e ct to its re sistance to fro st actio n. Airentraining ag ent can also b e ad d ed at the time o f mixing o rd inary Po rtland cement w ith rest o f the ing re d ie nts. Mo re ab o ut this w ill b e d e alt und e r the chap te r “Ad mixture s.”

Coloured Cement (White Cement IS 8042–1989) Fo r m anufac turing vario us c o lo ure d c e m e nts e ithe r w hite c e m e nt o r g re y Po rtland ce me nt is use d as a b ase . The use o f w hite ce me nt as a b ase is co stly. With the use o f g re y ce me nt o nly re d o r b ro w n ce me nt can b e p ro d uce d . Co lo ure d c e m e n t c o n sists o f Po rtlan d c e m e n t w ith 5 -1 0 p e r c e n t o f p ig m e n t. Th e p ig me nt canno t b e satisfacto rily d istrib ute d thro ug ho ut the ce me nt b y mixing , and he nce , it is usual to g rind the ce me nt and p ig me nt to g e the r. The p ro p e rtie s re q uire d o f a p ig me nt to be used fo r co lo ured cement are the durability o f co lo ur under expo sure to lig ht and w eather, a fine state o f d ivisio n, a che mical co mpo sitio n such that the pig me nt is ne ithe r e ffe cte d b y the ce me nt no r d e trime ntal to it, and the ab se nce o f so lub le salts. The pro cess o f manufacture o f w hite Po rtland cement is nearly same as O PC. As the raw materials, particularity the kind o f limesto ne req uired fo r manufacturing w hite cement is o nly availab le aro und Jo d hp ur in Rajasthan, tw o fam o us b rand s o f w hite c e m e nt nam e ly Birla White and J.K. White Ce me nts are manufacture d ne ar Jo d hp ur. The raw mate rials use d are hig h p urity lime sto ne (9 6 % CaCo 3 and le ss than 0 .0 7 % iro n o xid e ). The o the r raw mate rials are c hina c lay w ith iro n c o nte nt o f ab o ut 0 .7 2 to 0 .8 % , silic a sand , flo ursp ar as flux and se le nite as re tard e r. The fue ls use d are re fine d furnace o il (RFO ) o r g as. Se a she lls and co ral can also b e use d as raw mate rials fo r p ro d uctio n o f w hite ce me nt. The pro perties o f w hite cement is nearly same as O PC. Generally w hite cement is g ro und fine r than g re y ce me nt. White ne ss o f w hite ce me nt as me asure d b y ISI scale shall no t b e le ss than 7 0 % . W hite ne ss c an also b e m e asure d b y Hunte rs Sc ale . The value as m e asure d b y Hunte rs scale is g e ne rally 9 0 % . The stre ng th o f w hite ce me nt is much hig he r than w hat is state d in IS co d e 8 0 4 2 o f 1 9 8 9 . A typ ical te st re sult o f Birla White is sho w n in Tab le 2 .2 .

Ta ble 2 .2 . Typic a l Prope r t ie s of Birla Whit e Por t la nd Ce m e nt 2 .2 Characteristics

IS: 8 0 4 2 . 1 9 8 9

Birla White

Max 2 .0

0 .6 0

1 . CHEMICAL

a . Inso lub le re sid ue % b . Iro n O xid e %

Max 1 .0

0 .2 0

c . Mag ne sium O xid e %

Max 6 .0

0 .8 0

d . Sulp hur Trio xid e %

Max 3 .0

2 .9 0

e . Alumina/ Iro n O xid e %

Min 0 .6 6

9 .0 0

f.

0 .6 6 -1 .0 9

0 .9 0

Lime Saturatio n Facto r

g . Lo ss o n Ig nitio n %

< 3%

Types of Cement "

39

2 . PHYSICAL

a . De g re e o f White ne sse s % ISI scale

Min 7 0

Hunte rs scale

88+ 91+

b . Fine ne ss, Blaine M / kg . 2

Min 2 2 5

450*

(Sp e cific Surface )

c . Se tting Time 1 . Intial-minute s

Min 3 0

80

2 . Final-minute s

Max 6 0 0

120

3 d ays (Mp a)

Min 1 4 .4

45

7 d ays (Mp a)

Min 1 9 .8

55

2 8 d ays (Mp a)

Min 2 9 .7

67

1 . Le chate lie rs me tho d (mm)

Max 1 0

1 .0 0

2 . Auto clave e xpansio n %

Max 0 .8

Ne g lig ib le

—

1 .0 0

d . Co mp re ssive Stre ng th (Ce me nt and Std . Sand Mo rtar 1 :3 )

e . So und ne ss

f.

Re te ntio n o f 6 3 micro n sie ve %

Hydrophobic cement (IS 8043-1991) Hydro pho bic cement is o btained by g rinding o rdinary Po rtland cement clinker w ith w ater re p e llant film-fo rming sub stance such as o le ic acid , and ste aric acid . The w ate r-re p e llant film fo rmed aro und each g rain o f cement, red uces the rate o f d eterio ratio n o f the cement d uring lo ng sto rag e , transp o rt, o r und e r unfavo urab le co nd itio ns. The film is b ro ke n o ut w he n the c e me nt and ag g re g ate are mixe d to g e the r at the mixe r e xp o sing the c e me nt p artic le s fo r no rmal hyd ratio n. The film fo rming w ater-repellant material w ill entrain certain amo unt o f air in the bo dy o f the co ncrete w hich incidentally w ill impro ve the w o rkability o f co ncrete. In India certain places such as Assam, Shillo ng etc., g et plenty o f rainfall in the rainy seaso n had have hig h humidity in o ther seaso ns. The transpo rtatio n and sto rag e o f cement in such places cause deterio ratio n in the q uality o f cement. In such far o ff places w ith po o r co mmunicatio n system, ce me nt p e rfo rce re q uire s to b e sto re d fo r lo ng time . O rd inary ce me nt g e ts d e te rio rate d and lo se s so me if its stre ng th, w he re as the hyd ro p ho b ic ce me nt w hich d o e s no t lo se stre ng th is an answ e r fo r such situatio ns. The p ro p e rtie s o f hyd ro p ho b ic c e m e nt is ne arly the sam e as that o rd inary Po rtland cement except that it entrains a small quantity o f air bubbles. The hydro pho bic cement is made actually fro m o rd inary Po rtland ce me nt clinke r. Afte r g rind ing , the ce me nt p article is sp raye d in o n e d ire c tio n a n d film fo rm in g m a te ria ls su c h a s o le ic a c id , o r ste a ric a c id , o r p e ntac hlo ro p he no l, o r c alc ium o le ate are sp raye d fro m ano the r d ire c tio n suc h that e ve ry particle o f cement is co ated w ith a very fine film o f this w ater repellant material w hich pro tects the m fro m the b ad e ffe c t o f m o isture d uring sto rag e and transp o ratio n. The c o st o f this ce me nt is no minally hig he r than o rd inary Po rtland ce me nt.

Masonry Cement (IS 3466 : 1988) O rd inary ce me nt mo rtar, tho ug h g o o d w he n co mpare d to lime mo rtar w ith re spe ct to

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stre ng th and se tting p ro p e rtie s, is infe rio r to lime mo rtar w ith re sp e ct to w o rkab ility, w ate rre te ntivity, shrinkag e p ro p e rty and e xte nsib ility. Maso nry ce me nt is a typ e o f ce me nt w hich is p articularly mad e w ith such co mb inatio n o f materials, w hich w hen used fo r making mo rtar, inco rpo rates all the g o o d pro perties o f lime mo rtar and d iscard s all the no t so id e al p ro p e rtie s o f ce me nt mo rtar. This kind o f ce me nt is mo stly use d , as the name ind icate s, fo r maso nry co nstructio n. It co ntains ce rtain amo unt o f air-e ntraining ag e nt and mine ral ad mixture s to imp ro ve the p lasticity and w ate r re te ntivity.

Expansive Cement Co ncre te mad e w ith o rd inary Po rtland ce me nt shrinks w hile se tting d ue to lo ss o f fre e w ate r. Co ncre te also shrinks co ntinuo usly fo r lo ng time . This is kno w n as d rying shrinkag e . Cement used fo r g ro uting ancho r b o lts o r g ro uting machine fo und atio ns o r the cement used in g ro uting the p re stre ss co ncre te d ucts, if shrinks, the p urp o se fo r w hich the g ro ut is use d w ill b e to so me e xte nt d e fe ate d . The re has b e e n a se arch fo r such type o f ce me nt w hich w ill no t shrink w hile hard e ning and the re afte r. As a matte r o f fact, a slig ht e xp ansio n w ith time w ill p ro ve to b e ad vantag e o us fo r g ro uting p urp o se . This typ e o f ce me nt w hich suffe rs no o ve rall chang e in vo lume o n d rying is kno w n as e xp ansive ce me nt. Ce me nt o f this typ e has b e e n d e ve lo p e d b y using an e xp and ing ag e nt and a stab ilize r ve ry care fully. Pro p e r mate rial an d c o n tro lle d p ro p o rtio n in g are n e c e ssary in o rd e r to o b tain th e d e sire d e xp an sio n . Ge ne rally, ab o ut 8 -2 0 p arts o f the sulp ho aluminate clinke r are mixe d w ith 1 0 0 p arts o f the Po rtland ce me nt and 1 5 p arts o f the stab ilize r. Since e xp ansio n take s p lace o nly so lo ng as co ncre te is mo ist, curing must b e care fully co ntro lle d . The use o f e xpand ing ce me nt re q uire s skill and e xp e rie nce . O ne type o f expansive cement is kno w n as shrinkag e co mpensating cement. This cement w he n use d in c o nc re te , w ith re straine d e xp ansio n, ind uc e s c o m p re ssive stre sse s w hic h ap p ro ximate ly o ffse t the te nsile stre ss ind uce d b y shrinkag e . Ano the r similar typ e o f ce me nt is kno w n as Se lf Stre ssing c e me nt. This c e me nt w he n use d in c o nc re te ind uc e s sig nific ant co mpressive stresses after the drying shrinkag e has o ccurred. The induced co mpressive stresses no t o nly co mpensate the shrinkag e but also g ive so me so rt o f prestressing effects in the tensile zo ne o f a fle xural me mb e r.

IRS-T 40 Special Grade Cement IRS-T-4 0 sp e c ial g rad e c e m e n t is m anufac ture d as p e r sp e c ific atio n laid d o w n b y ministry o f Railw ays und er IRST4 0 : 1 9 8 5 . It is a ve ry fin e ly g ro u n d cement w ith hig h C3 S co ntent d esig ned to develo p hig h early streng th req uired fo r manufacture o f co ncre te sle e p e r fo r Indian Railw ays. This cement can also be u se d w ith a d va n ta g e fo r o th e r ap p lic atio ns w he re hig h e arly stre ng th co ncrete is req uired. This cement can b e use d fo r p re stre sse d co ncre te e le me nts, h ig h rise b u ild in g s, h ig h stre n g th co ncre te .

IRS-T 40 special grade cement was originally made for manufacturing concrete sleeper for railway line.

Types of Cement "

41

Oil-Well Cement (IS 8229-1986) O il-w ells are d rilled thro ug h stratified sed imentary ro cks thro ug h a g reat d epth in search o f o il. It is likely that if o il is struck, o il o r g as may escape thro ug h the space b etw een the steel casing and ro ck fo rmatio n. Ce me nt slurry is use d to se al o ff the annular space b e tw e e n ste e l casing and ro ck strata and also to seal o ff any o ther fissures o r cavities in the sedimentary ro ck laye r. The ce me nt slurry has to b e p ump e d into p o sitio n, at co nsid e rab le d e p th w he re the prevailing temperature may be upto 175°C. The pressure req uired may g o upto 1300 kg / cm 2 . The slurry sho uld re m ain suffic ie ntly m o b ile to b e ab le to flo w und e r the se c o nd itio ns fo r perio ds upto several ho urs and then hardened fairly rapidly. It may also have to resist co rro sive co nditio ns fro m sulphur g ases o r w aters co ntaining disso lved salts. The type o f cement suitable fo r the ab o ve c o nd itio ns is kno w n as O il-w e ll c e m e nt. The d e sire d p ro p e rtie s o f O il-w e ll cement can b e o b tained in tw o w ays: b y ad justing the co mpo und co mpo sitio n o f cement o r b y ad d ing re tard e rs to o rd inary Po rtland ce me nt. Many ad mixture s have b e e n p ate nte d as re tard e rs. The co mmo ne st ag e nts are starche s o r ce llulo se pro d ucts o r acid s. The se re tard ing ag ents prevent q uick setting and retains the slurry in mo b ile co nditio n to facilitate penetratio n to all fissure s and c avitie s. So m e tim e s w o rkab ility ag e nts are also ad d e d to this c e m e nt to incre ase the mo b ility.

Rediset Cement Acclerating the setting and hardening o f co ncrete b y the use o f admixtures is a co mmo n kno w led g e. Calcium chlo rid e, lig no sulfo nates, and cellulo se pro d ucts fo rm the b ase o f so me o f ad mixture s. The limitatio ns o n the use o f ad mixture s and the facto rs influe ncing the e nd p ro p e rtie s are also fairly w e ll kno w n. Hig h alumina cement, tho ug h g o o d fo r early streng ths, sho w s retro g ressio n o f streng th w he n e xp o se d to ho t and hum id c o nd itio ns. A ne w p ro d uc t w as ne e d e d fo r use in the precast co ncrete industry, fo r rapid repairs o f co ncrete ro ads and pavements, and slip-fo rming . In b rie f, fo r all jo b s w he re the tim e and stre ng th re latio nship w as im p o rtant. In the PCA labo rato ries o f USA, investig atio ns w ere co nducted fo r develo ping a cement w hich co uld yield hig h stre ng ths in a matte r o f ho urs, w itho ut sho w ing any re tro g re ssio n. Re g se t ce me nt w as the result o f investig atio n. Asso ciated Cement Co mpany o f India have develo ped an eq uivalent ce me nt b y name “REDISET” Ce me nt.

Properties of “Rediset”2.3 (i ) The ce me nt allo w s a hand ling time o f just ab o ut 8 to 1 0 minute s. (ii ) The stre ng th p atte rn o f REDISET and re g se t in mo rtar and co ncre te is g ive n b e lo w :

Ta ble 2 .3 . Com pre ssive St re ngt h M Pa 2 .3 4 ho urs

2 4 ho urs

2 8 d ays

ACC “REDISET” mo rtar, 1 :3 mix ... ...

20

42

4 2 (Actual te sts)

ACC “REDISET” 1 : 5 .5 mix co ncre te ... ...

21

25

3 2 (Actual te sts)

USA Re g se t mo rtar 1 : 2 .7 5 mix ... ...

7 .0

18

4 2 (Fro m lite rature )

USA Re g se t co ncre te , 6 b ag s ... ...

9 .0

16

4 2 (Fro m lite rature )

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" Concrete Technology

(iii ) The streng th pattern is similar to that o f o rd inary Po rtland cement mo rtar o r co ncrete afte r o ne d ay o r 3 d ays. W hat is ac hie ve d w ith “REDISET” in 3 to 6 ho urs c an b e achie ve d w ith no rmal co ncre te o nly afte r 7 d ays. (iv ) “REDISET” releases a lo t o f heat w hich is advantag eo us in w inter co ncreting but excess he at lib e ratio n is d e trime ntal to mass co ncre te . (v ) The rate o f shrinkag e is fast b ut the to tal shrinkag e is sim ilar to that o f o rd inary Po rtland ce me nt co ncre te . (vi ) The sulp hate re sistance , is ho w e ve r, ve ry p o o r.

Applications “REDISET” can b e use d fo r: (a ) ve ry-hig h-e arly (3 to 4 ho urs) stre ng th co ncre te and mo rtar, (b ) p atch re p airs and e me rg e ncy re p airs, (c ) q uick re le ase o f fo rms in the pre cast co ncre te pro d ucts ind ustry, (d ) palle tisatio n o f iro n o re d ust, (e ) slip -fo rme d co ncre te co nstructio n, (f ) co nstructio n b e tw e e n tid e s.

High Alumina Cement (IS 6452 : 1989) Hig h alumina cement is o btained by fusing o r sintering a mixture, in suitable pro po rtio ns, o f alumina and calcareo us materials and g rind ing the resultant pro d uct to a fine po w d er. The raw mate rials use d fo r the manufacture o f hig h alumina ce me nt are lime sto ne and b auxite . These raw materials w ith the req uired pro po rtio n o f co ke w ere charg ed into the furnace. The furnac e is fire d w ith p ulve rise d c o al o r o il w ith a ho t air b last. The fusio n take s p lac e at a temperature o f ab o ut 1550-1600°C. The cement is maintained in a liq uid state in the furnace. Afte rw ard s the mo lte n ce me nt is run into mo uld s and co o le d . The se casting s are kno w n as p ig s. Afte r co o ling the ce me nt mass re se mb le s a d ark, fine g e y co mp act ro ck re se mb ling the structure and hard e ne ss o f b asalt ro ck. The p ig s o f fuse d ce me nt, afte r co o ling are crushe d and the n g ro und in tub e mills to a fine ss o f ab o ut 3 0 0 0 sq . cm/ g m.

Hydration of High Alumina Cement The im p o rtant re ac tio n d uring the se tting o f the hig h alum ina c e m e nt (HAC) is the fo rmatio n o f mo no calcium aluminate d e cahyd rate (CAH10 ), d icalcium aluminate o ctahyd rate (C2 AH8 ) and alumina g el (AHn). These aluminates g ive hig h streng th to HAC co ncrete but they are metastable and at no rmal temperature co nvert g radually to tricalcium alumina hexahydrate (C3 AH6 ) and g ib b site w hich are mo re stab le . The chang e in co mp o sitio n is acco mp anise d b y a lo ss o f stre ng th and b y a chang e in crystal fo rm fro m he xag o nal to cub ical fo rm w ith the release o f w ater w hich results in increased po ro sity o f co ncrete. The precise manner in w hich the se c hang e s take p lac e d e p e nd s o n the te mp e rature , w ate r/ c e me nt ratio and c he mic al e nviro nme nt. The chang e in co mpo sitio n acco mpanied b y lo ss o f streng th and chang e in crystal fo rm fro m he xag o nal to cub ic shape is kno w n as co nve rsio n. Exp e rime ntal e vid e nce sug g e sts that in the imp o rtant re actio n o f the co nve rsio n fro m CAH10 to C3 AH6 and alumina hyd rate, temperature effects the d eco mpo sitio n. The hig her the temperature, the faster the rate o f co nversio n. Experimental stud ies have also sho w n that the

Types of Cement "

43

hig he r the w ate r/ c e m e nt ratio , the g re ate r is the rate o f c o nve rsio n. The hyd ratio n and co nve rsio n can b e sho w n as fo llo w s: CA + 1 0 H  → CAH10 ;

...(1 )

→ C3 AH6 + 2 AH3 + 1 8 H 3 CA H10 

...(2 )

It sho uld b e no te d that this re actio n lib e rate s all the w ate r ne e d e d fo r the co nve rsio n pro cess to co ntinue. The co nversio n reactio n w ill result in a red uctio n in vo lume o f the so lid s and an increase in the po ro sity, since the o verall d imensio ns o f specimens o f cement paste o r co ncre te re main se nsib ly co nstant.

High Alumina Cement Concrete The use o f hig h alumina ce me nt co ncre te co mme nce d in the U.K. in 1 9 2 5 fo llo w ing its intro d uc tio n in Franc e w he re it had b e e n d e ve lo p e d e arlie r to m ake c o nc re te re sistant to che mical attack, p articularly in marine co nd itio ns. The cap ab ility o f this co ncre te to d e ve lo p a hig h e arly stre ng th o ffe rs ad vantag e s in struc tural use . Ho w e ve r, its hig h c o st p re ve nte d extensive use o f hig h alumina cement fo r structural purpo ses. All the same during 1930’s many structures w ere built in Euro pean co untries using hig h alumina cement. Fo llo w ing the co llapse o f tw o ro o f b e ams in a sc ho o l at Ste p ne y in U.K. in Fe b ruary 1 9 7 4 , the Build ing Re se arc h Estab lishme nt (BRE) o f U.K. starte d fie ld stud ie s and lab o rato ry te sts to e stab lish the d e g re e o f risk like ly in b uild ing s w ith p re cast p re stre sse d co ncre te b e ams mad e w ith hig h alumina ce me nt. The re sults o f the BRE inve stig atio ns are summarise d b e lo w : 1 . Me asure me nts o f the d e g re e o f c o nve rsio n o f the c o nc re te use d in the b uild ing s ind icated that hig h alumina cement co ncrete reaches a hig h level o f co nversio n w ithin a few ye ars. Th e c o n c re te sp e c im e n s c u t fro m b e am s in d ic ate d th at so m e c o n c re te su ffe re d sub stantial lo ss o f stre ng th w he n co mp are d to o ne d ay stre ng th o n w hich the d e sig n w as e arlie r b ase d , (Fig . 2 .2 ). 2.

Lo ng te rm lab o rato ry te sts have sho w n that:

(a ) If c o nc re te w ith a fre e w ate r/ c e me nt ratio le ss than 0 .4 is sto re d in w ate r at 1 8 ° C thro ug ho ut its initial curing p e rio d and its sub se q ue nt life , a minimum stre ng th w ill

44

" Concrete Technology

b e re ac he d afte r ab o ut 5 ye ars and this minimum w ill no t b e ap p re c iab ly le ss than the stre ng th at o ne d ay. (b ) If co ncre te is sto re d in w ate r at 3 8 ° C, a fte r o n e d a y a t 1 8 ° C, it c o n ve rts rap id ly to h ig h lim it an d re ac h e s a minimum stre ng th in ab o ut 3 mo nths w hich is very substantially less than the stre ng th at o ne d ay. (c ) If c o nc re te is sto re d in w ate r at 1 8 ° C fo r a lo ng perio d (upto 8 1 / 2 years) and is im m e rse d in w ate r at 3 8 ° C it w ill rap id ly c o nve rt and lo se stre ng th to th e m in im u m le ve l, re a c h e d fo r co ntinuo us sto rag e at 3 8 ° C. (d ) Sin c e th e te m p e ra tu re a t 3 8 ° C re p re se nts an up p e r lim it o f w hat is Refractory concrete made with High Alumina like ly to b e re ac he d d uring c uring o f cement is used as refactory lining in furnaces and fire pits. the se se c tio ns o r in no rm ally he ate d b uilding , and the precise level is no t critical, it is reco mmended that desig n sho uld b e b ase d o n the minimum stre ng th at this te mp e rature . (e ) Hig hly co nve rte d hig h alumina ce me nt co ncre te is vulne rab le to che mical attack in the presence o f lo ng term w etness and a chemically ag g ressive ag ent, w hich may b e mo re se rio us risk fo r co ncre te s w ith g re ate r w ate r/ ce me nt ratio . O ne o f the mo st ad vantag e s o f hig h alumina ce me nt co ncre te is the ve ry hig h rate o f stre ng th d e ve lo p me nt. Ab o ut 2 0 p e r ce nt o f the ultimate stre ng th is achie ve d in o ne d ay. It also achie ve s a sub stantial stre ng th e ve n at 6 to 8 ho urs.

Refractory Concrete An impo rtant use o f hig h alumina cement is fo r making refracto ry co ncrete to w ithstand hig h te m p e rature s in c o njunc tio n w ith ag g re g ate having he at re sisting p ro p e rtie s. It is interesting to no te that hig h alumina cement co ncrete lo ses co nsid erab le streng th o nly w hen sub je c te d to hum id c o nd itio n and hig h te m p e rature . De sic c ate d hig h alum ina c e m e nt co ncrete o n subjecting to the hig h temperature w ill underg o a little amo unt o f co nversio n and w ill still have a satisfacto ry residual streng th. O n co mplete desiccatio n the resistance o f alumina ce me nt to d ry he at is so hig h that the co ncre te mad e w ith this ce me nt is co nsid e re d as o ne o f the refracto ry materials. At a very hig h temperature alumina cement co ncrete exhib its g o o d ce ramic b o nd inste ad o f hyd raulic b o nd as usual w ith o the r ce me nt co ncre te . Crushe d fire b rick is o ne o f the mo st co mmo nly use d ag g re g ate s fo r making re fracto ry co ncre te w ith hig h alumina ce me nt. Such co ncre te can w ithstand te mp e rature up to ab o ut 1350°C. Refracto ry co ncrete fo r w ithstand ing temperature upto 1600°C can b e pro d uced b y using ag g re g ate s suc h as silim anite , c arb o rund um , d e ad -b urnt m ag ne site . The re frac to ry co ncre te is use d fo r fo und atio ns o f furnace s, co ke o ve ns, b o ile r se tting s. It is also use d in fire p its, co nstructio n o f e le ctric furnace s, o rd inary furnace s and kilns. Hig h alumina ce me nt can b e use d fo r making re fracto ry mo rtars.

Types of Cement "

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Hig h alumina c e me nt is a slo w se tting b ut rap id hard e ning ce me nt. Its se ttin g tim e c a n b e re d u c e d co nsid erab ly b y mixing it w ith certain p ro p o rtio n s o f o rd in a ry Po rtla n d c e m e n t. In situ a tio n s su c h a s sto p p ing o f ing re ss o f w ate r o r fo r c o n stru c tio n b e tw e e n tid e s o r fo r re d u c in g p u m p in g tim e in so m e und e rw ate r co nstructio n a p articular mixture o f hig h alumina ce me nt and o rd inary Po rtland cement is ad o pted . Fig . 2 . 3 sh o w s se ttin g tim e o f m ixtu re s o f Po rtlan d an d alu m in a ce me nt. It can b e se e n fro m Fig . 2 .3 that w he n e ithe r ce me nt co nstitute s b e tw e e n 2 0 -8 0 p e r c e nt o f mixture , fla sh se t m a y o c c u r. Th e va lu e s sho w n in the g rap h is o nly ap p ro ximate . The actual p ro p o rtio ning and the re sultant se tting time are re q uire d to b e actually fo und o ut b y trial w he n such a co mb inatio n is p ractise d .

Very High Strength Cement (a) Macro-defect-free cements (MDF)2.4. The eng ineering o f a new class o f hig h streng th ce me nt calle d Macro -d e fe ct-fre e (MDF) ce me nts is an inno vatio n. MDF re fe rs to the ab se nce o f re lative ly larg e vo id s o r d e fe cts w hich are usually p re se nt in co nve ntio nal mixe d ce me nt p aste s b e cause o f e ntrap p e d air and inad e q uate d isp e rsio n. Such vo id s and d e fe cts limit the stre n g th . In th e MD F p ro c e ss 4 -7 % o f o n e o f se ve ral w ate r-so lu b le p o lym e rs (su c h as hyd ro xyp ro p ylme thyle ce llulo se , p o lyacrylamid e o f hyd ro lyse d p o lyvinylace tate ) is ad d e d as rhe o lo g ic al aid to p e rmit c e me nt to b e mixe d w ith ve ry small amo unt o f w ate r. Co ntro l o f p article size d istrib utio n w as also co nsid e re d imp o rtant fo r g e ne rating the stre ng th. At final p ro ce ssing stag e e ntrap p e d air is re mo ve d b y ap p lying a mo d e st p re ssure o f 5 MPa. With this pro ce ss a stre ng th o f 3 0 0 MPa fo r calcium aluminate syste m and 1 5 0 MPa fo r Po rtland ce me nt syste m can b e achie ve d .

(b) Densely Packed System (DSP). New materials termed DSP (Densified system co ntaining ho me g e ne o usly arrang e d ultre -fine p artic le s) is ye t ano the r inno vatio n in the fie ld o f hig h stre ng th c e me nt. No rmal Po rtland c e me nt and ultra-fine silic a fume are mixe d . The size o f cement particles may very fro m 0.5 to 100 µ and that o f silica fume varies fro m 0.005 to 0.5 µ. Silica fume is g enerally ad d ed fro m 5 to 25 % . A co mpressive streng th o f 270 MPa have b een achie ve d w ith silica fume sub stitute d p aste . The fo rmatio n o f typ ical DSP is sche matically re p re se nte d in Fig . 2 .4 .

(c) Pressure Densification and Warm Pressing. Fo r decades uncertainties existed reg arding the the o re tic al stre ng th o f hyd rate d c e m e nt p aste . Be fo re 1 9 7 0 , the p o te ntial stre ng th o f ce me nt p aste at the o re tical d e nsity (What T.C. Po w e rs calle d “intrinsic stre ng th”) had ne ve r b e e n ac hie ve d b e c ause o f c o nsid e rab le p o ro sity (2 0 to 3 0 % o r mo re ) alw ays re main o fte r co mpleting hydratio n o f cement. A new appro ach has b en develo ped fo r achieving very hig h stre ng th b y a me tho d calle d “Warm Pre ssing ” (applying he at and pre ssure simultane o usly) to ce me nt paste .

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Cement Silica Fume

Fig. 2.4. Packing of DSP (Silica fume) paste. So m e m o d e st in c re ase in stre n g th w as ac h ie ve d b y ap p lic atio n o f p re ssure alo n e . Co mp re ssive stre ng th as muc h as 6 5 0 MPa and te nsile stre ng th up to 6 8 MPa have b e e n o b taine d b y w arm pre ssing Po rtland and calcium aluminate ce me nts. Eno rmo us incre ase s in stre n g th re su lte d fro m th e re m o val o f m o st o f th e p o ro sity an d g e n e ratio n o f ve ry ho mo g e ne o us, fine micro -structure s w ith the po ro sitie s as lo w as 1 .7 % .

(d) High Early Strength Cement. Develo pment o f hig h early streng th beco mes an impo rtant facto r, so metimes, fo r repair and emerg ency w o rk. Research has been carried o ut in the recent p ast to d e ve lo p rap id se tting and hard e ning c e m e nt to g ive m ate rials o f ve ry hig h e arly stre ng th. Lithium salts have b een effectively used as accelerato rs in hig h alumina cement. This has re sulte d in ve ry hig h e arly stre ng th in c e m e nt and a m arg inal re d uc tio n in late r stre ng th. Stre ng th as hig h as 4 MPa has b e e n o b taine d w ithin 1 ho ur and 2 7 MPa has b e e n o b taine d w ithin 3 ho urs time and 4 9 MPa in o ne d ay.

(e) Pyrament Cement. So me ce me nt ind ustrie s in USA have d e ve lo p e d a sup e r hig h e arly stre ng th and d urab le c e m e nt c alle d b y trad e nam e “Pyram e nt Ce m e nt”. This p ro d uc t is a b le nd e d hyd raulic ce me nt. In this ce me nt no chlo rid e s are ad d e d d uring the manufacturing p ro ce ss. Pyrame nt ce me nt p ro d uce s a hig h and ve ry e arly stre ng th o f co ncre te and mo rtar w hich can b e use d fo r re p air o f Air Fie ld Run-w ays. In Ind ia Asso ciate d Ce me nt Co mp any in c o llab o ratio n w ith R & D Eng ine e rs, Dig hi, Pune have also p ro d uc e d hig h e arly stre ng th ce me nt fo r rap id re p air o f airfie ld s. The Pyrame nt ce me nt sho w e d the fo llo w ing stre ng th. Re fe r Tab le 2 .4 .

(f) Magnesium Phosphate Cement (MPC). Mag nesium Pho sphate Cement, an ad vanced cementing material, g iving very hig h early streng th mo rtar and co ncrete has b een d evelo ped b y Ce ntral Ro ad Re se arch Institute , Ne w De lhi. This ce me nt can b e use d fo r rap id re p air o f d am ag e d c o nc re te ro ad s and airfie ld p ave m e nts. This is an im p o rtant d e ve lo p m e nt fo r e me rg e ncy re p air o f airfie ld s, launching p ad s, hard stand ing and ro ad p ave me nts suffe ring d amag e d ue to e ne my b o mb ing and missile attack.

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Ta ble . 2 .4 . T ypic a l Prope r t ie s of Conc t e t e a nd M or t a r w it h Pyra m e nt Ce m e nt . Material

Co mpressive streng th MPa

Flexural Streng th MPa

Hard e ne d Co ncre te 4 ho urs

17

3 .4 5

1 d ay

34

5 .5 2

2 8 d ays

69

8 .2 7

Hard e ne d Mo rtar 2 ho urs

17

—

3 ho urs

24

4 .1

1 d ay

41

6 .9

7 d ays

69

1 0 .3

Th e MPC h as b e e n fo u n d to p o sse ss u n iq u e h yd rau lic p ro p e rtie s, in p artic u lar, a c o ntro lle d rap id se t and e arly stre ng th d e ve lo p me nt. MPC is a p re p ac ke d mixture o f d e ad b urnt mag nesite w ith fine ag g reg ate mixed w ith pho sphate. It sets rapid ly and yield s d urab le hig h stre ng th ce me nt mo rtar. This ne w ce me nt has a b rig ht future as an alte rnative to co stly synthe tic re sins curre ntly in use fo r e me rg e ncy re p air o f co ncre te p ave me nts. The fo llo w ing mate rials are use d fo r making MPC: Mag ne site (Mg CO 3 ) w he n c alc ine d at o r ab o ve 1 5 0 0 ° C g ive s d e ad b urnt m ag ne site (DBM). This mate rial is g ro und to a fine ne ss o f 3 0 0 -3 5 0 m 2 / kg (Blaine s). This is mixe d w ith co mmercially available crystalline Mo no Ammo nium Pho sphate after g rinding into fine po w der passing 6 0 0 µ se ive , and o the r ing re d ie nts like so d ium tri-p o lyp ho sp hate in the fo rm o f fine p o w d e r, d i-so d ium te trab o rate (Bo rax), fine ag g re g ate (crushe d d o lo mite sand ) and w ate r. The DBM and sand mixture is ad d e d into co ld p ho sp hate and b o rax so lutio n (1 2 -1 5 ° C) and mixe d fo r o ne minute . This mix is ap p lie d fo r the p urp o se o f re p air. It is air cure d and is re ad y fo r o pe ning traffic w ithin 4 -5 ho urs.

TESTING OF CEMENT Te sting o f ce me nt can b e b ro ug ht und e r tw o cate g o rie s: (a ) Fie ld te sting (b ) Lab o rato ry te sting .

Field Testing It is sufficie nt to sub je ct the ce me nt to fie ld te sts w he n it is use d fo r mino r w o rks. The fo llo w ing are the fie ld te sts: (a ) O p e n the b ag and take a g o o d lo o k at the ce me nt. The re sho uld no t b e any visib le lump s. The co lo ur o f the ce me nt sho uld no rmally b e g re e nish g re y. (b ) Thrust yo ur hand into the ce me nt b ag . It must g ive yo u a co o l fe e ling . The re sho uld no t b e any lump insid e . (c ) Take a p inch o f ce me nt and fe e l-b e tw e e n the fing e rs. It sho uld g ive a smo o th and no t a g ritty fe e ling . (d ) Take a hand ful o f ce me nt and thro w it o n a b ucke t full o f w ate r, the particle s sho uld flo at fo r so me time b e fo re the y sink.

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(e ) Take abo ut 100 g rams o f cement and a small q uantity o f w ater and make a stiff paste. Fro m the stiff paste , pat a cake w ith sharp e d g e s. Put it o n a g lass plate and slo w ly take it und er w ater in a b ucket. See that the shape o f the cake is no t d isturb ed w hile taking it d o w n to the b o tto m o f the b ucke t. Afte r 2 4 ho urs the cake sho uld re tain its o rig inal shap e and at the same time it sho uld also se t and attain so me stre ng th. If a sample o f cement satisfies the ab o ve field tests it may b e co nclud ed that the cement is no t b ad . The ab o ve tests d o no t really ind icate that the cement is really g o o d fo r impo rtant w o rks. Fo r using cement in impo rtant and majo r w o rks it is incumb ent o n the part o f the user to te st the c e m e nt in the lab o rato ry to c o nfirm the re q uire m e nts o f the Ind ian Stand ard sp e c ific atio n s w ith re sp e c t to its p h ysic al an d c h e m ic al p ro p e rtie s. No d o u b t, su c h co nfirmatio ns w ill have b een d o ne at the facto ry lab o rato ry b efo re the pro d uctio n co mes o ut fro m the facto ry. But the ce me nt may g o b ad d uring transpo rtatio n and sto rag e prio r to its use in w o rks. The fo llo w ing te sts are usually co nd ucte d in the lab o rato ry. (a ) Fine ne ss te st.

(b ) Se tting time te st.

(c ) Stre ng th te st.

(d ) So und ne ss te st.

(e ) He at o f hyd ratio n te st.

(f ) Che mical co mp o sitio n te st.

Fineness Test The fineness o f cement has an impo rtant b earing o n the rate o f hydratio n and hence o n the rate o f g ain o f stre ng th and also o n the rate o f e vo lutio n o f he at. Fine r ce me nt o ffe rs a g re ate r surface are a fo r hyd ratio n and he nce faste r the d e ve lo p me nt o f stre ng th, (Fig . 2 .5 ). The fine ne ss o f g rind ing has inc re ase d o ve r the ye ars. But no w it has g o t ne arly stabilised. Different cements are g ro und to d iffe re nt fine ne ss. The d isad vantag e s o f fine g rind ing is that it is susce ptib le to airse t an d e arly d e te rio ratio n . Maxim u m number o f particles in a sample o f cement sho uld have a size le ss than ab o ut 1 0 0 micro ns. The smallest particle may have a size o f abo ut 1.5 micro ns. By and larg e an ave rag e size o f the ce me nt p article s may b e taken as ab o ut 10 micro n. The particle size frac tio n b e lo w 3 m ic ro ns has b e e n fo und to have the pre d o minant e ffe ct o n the streng th at o ne day w hile 3-25 micro n fractio n has a majo r influe nce o n the 2 8 d ays stre n g th . In c re ase in fin e n e ss o f ce me nt is also fo und to incre ase the d rying shrinkag e o f co ncre te . In co mme rcial ce me nt it is sug g e ste d that the re sho uld b e ab o ut 2 5 -3 0 p e r ce nt o f p article s o f le ss than 7 micro n in size . Fine ne ss o f ce me nt is te ste d in tw o w ays : (a ) By se iving . (b ) By d eterminatio n o f specific surface (to tal surface area o f all the particles in o ne g ram o f ce me nt) b y air-p re me ab ility ap p artus. Exp re sse d as cm 2 / g m o r m 2 / kg . Ge ne rally Blaine Airp e rme ab ility ap p artus is use d .

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Sieve Test We ig h c o rre c tly 1 0 0 g ram s o f c e m e nt and take it o n a stand ard IS Sie ve No . 9 (9 0 mic ro ns). Bre ak d o w n the air-se t lump s in the samp le w ith fing e rs. Co ntinuo usly sie ve the sam p le g iving c irc ular and ve rtic al m o tio n fo r a p e rio d o f 1 5 m inute s. Me c hanic al sie ving d e vice s may also b e use d . We ig h the re sid ue le ft o n the sie ve . This w e ig ht shall no t e xce e d 1 0 % fo r o rd inary ce me nt. Sie ve te st is rare ly use d .

Air Permeability Method This m e tho d o f te st c o ve rs the p ro c e d ure fo r d e te rm ining the fine ne ss o f c e m e nt as re p re se nte d b y sp e cific surface e xp re sse d as to tal surface are a in sq . cm/ g m. o f ce me nt. It is also e xp re sse d in m 2 / kg . Le a and Nurse Air Pe rme ab ility Appartus is sho w n in Fig . 2 .6 . This appartus can b e used fo r measuring the specific surface o f cement. The principle is b ased o n the re latio n b e tw e e n the flo w o f air thro ug h the c e me nt b e d and the surfac e are a o f the p article s co mp rising the ce me nt b e d . Fro m this the surface are a p e r unit w e ig ht o f the b o d y mate rial can b e re late d to the p e rme ab ility o f a b e d o f a g ive n p o ro sity. The ce me nt b e d in the p e rme ab ility ce ll is 1 cm. hig h and 2 .5 cm. in d iame te r. Kno w ing the d e nsity o f ce me nt the w e ig ht re q uire d to m ake a c e m e nt b e d o f p o ro sity o f 0 .4 7 5 c an b e c alc ulate d . This q uantity o f ce me nt is place d in the pe rme ab ility ce ll in a stand ard manne r. Slo w ly pass o n air

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thro ug h the cement b ed at a co nstant velo city. Ad just the rate o f air flo w until the flo w meter sho w s a d iffe re nce in le ve l o f 3 0 -5 0 cm. Re ad the d iffe re nce in le ve l (h 1 ) o f the mano me te r and the d iffe re nc e in le ve l (h 2 ) o f the flo w me te r. Re p e at the se o b se rvatio ns to e nsure that steady co nditio ns have b een o b tained as sho w n b y a co nstant value o f h 1 / h 2 . Specific surface Sw is calculate d fro m the fo llo w ing fo rmula:

Sw = K h1 / h2 w he re ,

and

K=

14 d (1 − ! )

!3 A CL

ξ = Po ro sity, i.e. , 0 .4 7 5 A = Are a o f the ce me nt b e d L = Le ng th (cm) o f the ce me nt b e d d = De nsity o f ce me nt, and C = Flo w me te r co nstant.

The sp e cific surface fo r vario us ce me nts is sho w n in Tab le 2 .5 . Fine ne ss can also b e me asure d b y Blain Air Pe rme ab ility ap p rartus. This me tho d is mo re c o m m o nly e m p lo ye d in Ind ia. Fig . 2 .7 sho w s the ske tc h o f Blaine typ e Air Pe rm e ab ility ap p artus.

Standard Consistency Test Fo r find ing o ut initial se tting tim e , final se tting tim e and so und ne ss o f c e m e nt, and streng th a parameter kno w n as standard co nsistency has to be used. It is pertinent at this stag e to d escrib e the pro ced ure o f co nd ucting stand ard co nsistency test. The stand ard co nsistency o f a c e me nt p aste is d e fine d as that c o nsiste nc y w hic h w ill p e rmit a Vic at p lung e r having 1 0 mm d iame te r and 5 0 mm le ng th to p e ne trate to a d e p th o f 3 3 -3 5 mm fro m the to p o f the mo uld sho w n in Fig . 2 .8 . The appartus is calle d Vicat Appartus. This appartus is use d to find o ut the percentag e o f w ater req uired to pro duce a cement paste o f standard co nsistency. The stand ard co nsistency o f the cement paste is so me time called no rmal co nsistency (CPNC). The fo llo w ing pro ce d ure s is ad o pte d to find o ut stand ard co nsiste ncy. Take ab o ut 5 0 0 g ms o f ce me nt and p re p are a p aste w ith a w e ig he d q uantity o f w ate r (say 2 4 p e r ce nt b y w e ig ht o f ce me nt) fo r the first trial. The p aste must b e p re p are d in a stand ard manne r and fille d into the Vicat mo uld w ithin 3 -5 minute s. Afte r co mp le te ly filling the mo uld , shake the mo uld to expel air. A standard plung er, 10 mm diameter, 50 mm lo ng is attached and bro ug ht d o w n to to uch the surface o f the paste in the te st b lo ck and q uickly re le ase d allo w ing it to sink into the p aste b y its o w n w e ig ht. Take the re ad ing b y no ting the d e p th o f p e ne tratio n o f the p lung e r. Co nd uct a 2 nd trial (say w ith 2 5 p e r ce nt o f w ate r) and find o ut the d e p th o f penetratio n o f plung er. Similarly, co nduct trials w ith hig her and hig her w ater/ cement ratio s till such time the plung e r pe ne trate s fo r a d e pth o f 3 3 -3 5 mm fro m the to p. That particular percentag e o f w ater w hich allo w s the plung er to penetrate o nly to a depth o f 33-35 mm fro m the to p is kno w n as the percentag e o f w ater req uired to pro d uce a cement paste o f stand ard co nsiste ncy. This pe rce ntag e is usually d e no te d as ‘P’. The te st is re q uire d to b e co nd ucte d in a co nstant te mp e rature (2 7 ° + 2 ° C) and co nstant humid ity (9 0 % ).

Setting Time Test An arb itraty d ivisio n has b e e n mad e fo r the se tting time o f ce me nt as initial se tting time and final se tting time . It is d ifficult to d raw a rig id line b e tw e e n the se tw o arb itrary d ivisio ns.

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51

Fo r co nvenience, initial setting time is reg arded as the time elapsed betw een the mo ment that the w ater is added to the cement, to the time that the paste starts lo sing its plasticity. The final setting time is the time elapsed b etw een the mo ment the w ater is ad d ed to the cement, and the time w he n the paste has co mple te ly lo st its plasticity and has attaine d sufficie nt firmne ss to re sist ce rtain d e finite p re ssure . In ac tual c o nstruc tio n d e aling w ith c e m e nt p aste , m o rtar o r c o nc re te c e rtain tim e is req uired fo r mixing , transpo rting , placing , co mpacting and finishing . During this time cement p aste , m o rtar, o r c o nc re te sho uld b e in p lastic c o nd itio n. The tim e inte rval fo r w hic h the ce me nt p ro d ucts re main in p lastic co nd itio n is kno w n as the initial se tting time . No rmally a minimum o f 3 0 minute s is g ive n fo r mixing and hand ling o p e ratio ns. The co nstitue nts and fineness o f cement is maintained in such a w ay that the co ncrete remains in plastic co nd itio n fo r ce rtain minimum time . O nce the co ncre te is p lace d in the final p o sitio n, co mp acte d and finishe d , it sho uld lo se its p lasticity in the e arlie st p o ssib le time so that it is le ast vulne rab le to d amag e s fro m e xte rnal d e struc tive ag e nc ie s. This time sho uld no t b e mo re than 1 0 ho urs

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w hich is o fte n re fe rre d to as final se tting time . Tab le 2 .5 sho w s the se tting time fo r d iffe re nt ce me nts. The Vic at Ap p artus sho w n in Fig . 2 .8 is use d fo r se tting time te st also . The fo llo w ing pro cedure is ado pted. Take 500 g m. o f cement sample and g uag e it w ith 0.85 times the w ater req uired to pro duce cement paste o f standard co nsistency (0.85 P). The paste shall be g uag ed and fille d into the Vicat mo uld in sp e cifie d manne r w ithin 3 -5 minute s. Start the sto p w atch the mo me nt w ate r is ad d e d to the c e me nt. The te mp e rature o f w ate r and that o f the te st ro o m , a t th e tim e o f g au g in g sh all b e w ith in 2 7 ° C ± 2 ° C.

Initial Setting Time Lo w er the need le (C) g e n tly a n d b rin g it in c o ntac t w ith the surfac e o f th e te st b lo c k a n d q uickly re le ase . Allo w it to p e n e tra te in to th e te st b lo c k. In th e b e g in n in g , the needle w ill co mpletely p ie rc e th ro u g h th e te st b lo ck. But after so me time w h e n th e p a ste sta rts lo sin g its p la stic ity, th e

Vicat Apparatus and Accessories.

Automatic Vicat Apparatus.

Types of Cement "

53

ne e d ly m ay p e ne trate o nly to a d e p th o f 3 3 -3 5 m m fro m the to p . The p e rio d e lap sing b e tw e e n the time w he n w ate r is ad d e d to the c e me nt and the time at w hic h the ne e d le penetrates the test b lo ck to a d epth eq ual to 3 3 -3 5 mm fro m the to p is taken as initial setting time .

Final Setting Time Re p lace the ne e d le (C) o f the Vicat ap p artus b y a circular attachme nt (F) sho w n in the Fig 2 .8 . The ce me nt shall b e co nsid e re d as finally se t w he n, up o n, lo w e ring the attachme nt g e ntly co ve r the surface o f the te st b lo ck, the ce ntre ne e d le make s an imp re ssio n, w hile the circular cutting e d g e o f the attachme nt fails to d o so . In o the r w o rd s the p aste has attaine d such hard ne ss that the ce ntre ne e d le d o e s no t p ie rce thro ug h the p aste mo re than 0 .5 mm.

Strength Test The co mpressive streng th o f hardened cement is the mo st impo rtant o f all the pro perties. Therefo re, it is no t surprising that the cement is alw ays tested fo r its streng th at the lab o rato ry b e fo re the ce me nt is use d in imp o rtant w o rks. Stre ng th te sts are no t mad e o n ne at ce me nt p aste b e cause o f d ifficultie s o f e xce ssive shrinkag e and sub se q ue nt cracking o f ne at ce me nt. Stre ng th o f c e me nt is ind ire c tly fo und o n c e me nt sand mo rtar in sp e c ific p ro p o rtio ns. The standard sand is used fo r finding the streng th o f cement. It shall co nfo rm to IS 650-1991. Take 5 5 5 g ms o f stand ard sand (Enno re sand ), 1 8 5 g ms o f ce me nt (i.e. , ratio o f ce me nt to sand is 1 :3 ) in a no n-p o ro us e nam e l tray and mix the m w ith a tro w e l fo r o ne minute , the n ad d w ate r o f q uantity P + 3 . 0 p e r c e n t o f c o m b in e d 4 w e ig ht o f ce me nt and sand and mix the three ing redients tho ro ug hly until the mixture is o f unifo rm co lo ur. The tim e o f m ixin g sh o u ld n o t b e le ss th a n 3 m in u te s n o r m o re th a n 4 m inute s. Im m e d iate ly afte r m ixing , the mo rtar is filled into a cub e mo uld o f size 7 .0 6 cm. The are a o f the face o f the c ub e w ill b e e q ual to 5 0 sq c m . Co m p ac t th e m o rtar e ith e r b y h a n d c o m p a c tio n in a sta n d a rd specified manner o r o n the vib rating e q u ip m e n t (1 2 0 0 0 RPM) fo r 2 Moulding of 70.7 mm Mortar Cube Vibrating Machine. minute s.. Ke e p th e c o m p ac te d c u b e in the m o uld at a te m p e rature o f 2 7 ° C ± 2 ° C and at le ast 9 0 p e r c e nt re lative hum id ity fo r 2 4 ho urs. Whe re the facility o f stand ard te mpe rature and humid ity ro o m is no t availab le , the cub e may b e ke p t und e r w e t g unny b ag to simulate 9 0 p e r ce nt re lative humid ity. Afte r 2 4 ho urs the cub e s are re mo ve d fro m the mo uld and imme rse d in cle an fre sh w ate r until take n o ut fo r te sting . Thre e cub e s are te ste d fo r co mp re ssive stre ng th at the p e rio d s me ntio ne d in Tab le 2 .5 . The perio d s b eing recko ned fro m the co mpletio n o f vib ratio n. The co mpressive streng th shall b e the ave rag e o f the stre ng ths o f the thre e cub e s fo r e ach p e rio d re sp e ctive ly. The stre ng th re q uire me nts fo r vario us type s o f ce me nt is sho w n in Tab le 2 .5 .

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" Concrete Technology

Soundness Test It is ve ry im p o rtant that the c e m e nt afte r se tting shall no t und e rg o any ap p re c iab le c hang e o f vo lume . Ce rtain c e me nts have b e e n fo und to und e rg o a larg e e xp ansio n afte r setting causing d isruptio n o f the set and hard ened mass. This w ill cause serio us d ifficulties fo r the d urab ility o f structure s w he n such ce me nt is use d . The te sting o f so und ne ss o f ce me nt, to e nsure that the ce me nt d o e s no t sho w any ap p re ciab le sub se q ue nt e xp ansio n is o f p rime imp o rtance . The unso und ne ss in ce me nt is d ue to the p re se nce o f e xce ss o f lime than that co uld b e co mb ined w ith acid ic o xid e at the kiln. This is also d ue to inad eq uate b urning o r insufficiency in fine ne ss o f g rind ing o r tho ro ug h mixing o f raw mate rials. It is also like ly that to o hig h a p ro p o rtio n o f mag ne sium co nte nt o r calcium sulp hate co nte nt may cause unso und ne ss in ce me nt. Fo r this re aso n the mag ne sia co nte nt allo w e d in ce me nt is limite d to 6 p e r ce nt. It

c an b e re c alle d that, to p re ve nt flash se t, c alc ium sulp hate is ad d e d to the c linke r w hile g rind ing . The q uantity o f g ypsum ad d ed w ill vary fro m 3 to 5 p e r c e n t d e p e n d in g u p o n C 3 A c o n te n t. If th e additio n o f g ypsum is mo re than that co uld be co mbined w ith C3 A, e xce ss o f g ypsum w ill re main in the ce me nt in fre e state . This e xce ss o f g yp sum le ad s to an e xp ansio n and co nse q ue nt d isrup tio n o f the se t ce me nt p aste . Unso und ne ss in c e m e nt is d ue to e xc e ss o f lim e , e xce ss o f mag ne sia o r e xce ssive pro po rtio n o f sulphate s. Unso und ne ss in ce me nt d o e s no t co me to surface fo r a co nsid arab le p e rio d o f time . The re fo re , acce le rate d te sts are re q uire d to d e te ct it. The re are numb e r o f such te sts in c o m m o n use . The ap p artus is sho w n in Fig . 2 .9 . It co nsists o f a small sp lit cylind e r o f sp ring b rass o r o the r suitab le metal. It is 3 0 mm in d iameter and 3 0 mm hig h. O n either side o f the split are attached tw o indicato r arms 165 mm lo ng w ith po inted ends. Cement is g aug ed w ith 0 .7 8 time s the w ate r re q uire d fo r stand ard c o nsiste nc y (0 .7 8 P), in a stand ard manne r and fille d into the mo uld ke p t o n a g lass p late . The mo uld is co ve re d o n the to p

Autoclave.

Types of Cement "

55

w ith ano ther g lass plate. The w ho le assemb ly is immersed in w ater at a temperature o f 27°C – 3 2 ° C and ke p t the re fo r 2 4 ho urs. Measure the distance b etw een the indicato r po ints. Sub merg e the mo uld ag ain in w ater. He at the w ate r and b ring to b o iling p o int in ab o ut 2 5 -3 0 minute s and ke e p it b o iling fo r 3 ho urs. Remo ve the mo uld fro m the w ater, allo w it to co o l and measure the d istance b etw een th e in d ic ato r p o in ts. Th e d iffe re n c e b e tw e e n th e se tw o m e asu re m e n ts re p re se n ts th e e xp an sio n o f c e m e n t. Th is m ust n o t e xc e e d 1 0 m m fo r o rd inary, rap id hard e ning and lo w he at Po rtland ce me nts. If in c ase th e e xp an sio n is m o re th an 1 0 m m as te ste d ab o ve , the ce me nt is said to b e unso und . The Le Chate lie r te st d e te cts unso und ne ss d ue to fre e lim e o n ly. Th is m e th o d o f te stin g d o e s n o t in d ic ate th e p re se nce and afte r e ffe ct o f the e xce ss o f mag ne sia. Ind ian Stand ard Sp e c ific atio n stip ulate s that a c e m e nt having a mag ne sia co nte nt o f mo re than 3 p e r ce nt shall b e te ste d fo r so und ne ss b y Auto clave te st w hich is se nsitive to b o th fre e m ag n e sia an d fre e lim e . In th is te st a n e at c e m e n t sp e c im e n 2 5 × 2 5 m m is p lac e d in a stand ard auto c lave and the steam pressure inside the auto clave is raised in such a rate as to bring the g aug e pressure o f the steam to 21 kg / sq cm in 1 – 1 1 / 4 ho ur fro m the time the he at is turne d o n. This p re ssure is m aintaine d fo r 3 ho urs. The auto c lave is c o o le d an d th e le n g th m e asure d ag ain . Th e e xp an sio n p e rmitte d fo r all typ e s o f ce me nts is g ive n in Tab le 2 .5 . The h ig h ste am p re ssu re ac c e le rate s th e h yd ratio n o f b o th mag ne sia and lime . Automatic / Manual 5 litre Mortar Mixer.

N o sa tisfa c to ry te st is a va ila b le fo r d e d u c tio n o f unso und ne ss d ue to an e xce ss o f calcium sulp hate . But its co nte nt can b e e asily d e te rmine d b y che mical analysis.

Heat of Hydration The re actio n o f ce me nt w ith w ate r is e xo the rmic. The re actio n lib e rate s a co nsid e rab le q uantity o f he at. This can b e e asily o b se rve d if a ce me nt is g aug e d w ith w ater and placed in a thermo s flask. Much attentio n has b e e n p aid to the he at e vo lve d d uring the hyd ratio n o f c e m e n t in th e in te rio r o f m ass c o n c re te d am s. It is e stimate d that ab o ut 1 2 0 calo rie s o f he at is g e ne rate d in the hydratio n o f 1 g m. o f cement. Fro m this it can be asse sse d th e to tal q u an tu m o f h e at p ro d u c e d in a c o n se rvative syste m su c h as th e in te rio r o f a m ass c o nc re te d am . A te m p e rature rise o f ab o ut 5 0 ° C has b e e n o b se rve d . Th is u n d u ly h ig h te m p e ra tu re d e ve lo p e d at th e in te rio r o f a c o n c re te d am c ause s serio us expansio n o f the b o d y o f the d am and w ith the sub se q ue nt co o ling co nsid e rab le shrinkag e take s place re sulting in se rio us cracking o f co ncre te .

Heat of hydration Apparatus.

56

" Concrete Technology

The use o f le an mix, use o f po zzo lanic ce me nt, artificial co o ling o f co nstitue nt mate rials and inco rpo ratio n o f pipe system in the b o d y o f the d am as the co ncrete w o rk pro g resses fo r circulating co ld b rine so lutio n thro ug h the p ip e syste m to ab so rb the he at, are so me o f the metho d s ad o pted to o ffset the heat g eneratio n in the b o d y o f d ams d ue to heat o f hyd ratio n o f ce me nt. Te st fo r he at o f hyd ratio n is e sse ntially re q uire d to b e carrie d o ut fo r lo w he at ce me nt o nly. This test is carried o ut o ver a few days b y vaccum flask metho ds, o r o ver a lo ng er perio d in an ad iab atic calo rime te r. Whe n te ste d in a stand ard manne r the he at o f hyd ratio n o f lo w he at Po rtland ce me nt shall no t b e mo re than 6 5 cal/ g m. at 7 d ays and 7 5 cal/ g , at 2 8 d ays.

Chemical Composition Test A fairly d etailed d iscussio n has b een g iven earlier reg ard ing the chemical co mpo sitio n o f cement. Bo th o xide co mpo sitio n and co mpo und co mpo sitio n o f cement have been discussed. At this stag e it is sufficient to g ive the limits o f chemical req uirements. The Tab le 2.6 sho w s the vario us che mical co mpo sitio ns o f all type s o f ce me nts. Ratio o f p e rc e ntag e o f lim e to p e rc e ntag e o f silic a, alum ina and iro n o xid e , w he n calculate d b y the fo rmulae ,

CaO − 0.7 SO 3 2.8 SiO 2 + 1.2 Al 2 O 3 + 0.65 Fe 2 O 3 : No t g re ate r than 1 .0 2 and no t le ss than 0 .6 6 The ab o ve is calle d lime saturatio n facto r p e r ce nt. Tab le 2 .5 g ive s the co nso lid ate d p hysical re q uire me nts o f vario us typ e s o f ce me nt. Tab le 2 .6 g ive s the che mical re q uire me nts o f vario us typ e s o f ce me nt.

Test Certificate Every cement co mpany is co ntinuo usly testing the cement manufactured in their facto ry. Th e y ke e p a g o o d re c o rd o f b o th p h ysic a l a n d c h e m ic a l p ro p e rtie s o f th e c e m e n t manufacture d ap p lying a b atch numb e r. Batch numb e r ind icate s d ate , mo nth and ye ar. The y also issue te st ce rtificate . Eve ry p urchase r is e lig ib le to d e mand te st ce rtificate . A typical test certificate o f Birla super 5 3 g rad e cement fo r the w eek numb er 3 5 is g iven in Tab le 2 .7 fo r g e ne ral info rmatio n. So me cement co mpanies also w o rk o ut the standard deviatio n and co efficient o f variatio n fo r 3 mo nths o r 6 mo nths o r fo r o ne year perio d sub jecting the vario us parameters o b tained fro m the ir te st re sults. Tab le 2 .8 sho w s the typical stand ard d e viatio n fo r 3 d ays, 7 d ays and 2 8 d ays stre ng th in re sp e c t o f 5 3 g rad e c e m e nt Birla sup e r. Stand ard d e viatio n has b e e n w o rke d o ut fo r the w ho le ye ar fro m Jan. 9 9 to De c. 9 9 . The pro pe rtie s o f ce me nts, particularly the stre ng th pro pe rty sho w n in Tab le No . 2 .5 is te ste d as p e r the p ro ce d ure s g ive n b y BIS. In d iffe re nt co untrie s ce me nt is te ste d as p e r the ir o w n co untry’s co de o f practice. There are lo t o f variatio ns in the metho ds o f testing o f cement w ith re sp e c t to w / c ratio , size and shap e o f sp e c im e n, m ate rial p ro p o rtio n, c o m p ac ting me tho d s and te mp e rature . Stre ng th o f ce me nt as ind icate d b y o ne co untry may no t me an the sam e in ano the r c o untry. This w ill p re se nt a sm all p ro b le m w he n e xp o rt o r im p o rt o f cement fro m o ne co untry to ano ther co untry is co ncerned. Table No . 2.9. Sho w s the cements te sting p ro ce d ure and vario us g rad e s o f ce me nt manufacture d in so me co untrie s. The re is sug g e stio n that all the co untrie s sho uld fo llo w o ne me tho d re co mme nd e d b y Inte rnatio nal stand ard s o rg anisatio n fo r testing o f cement. If that system is ad o pted pro perties ind icated b y any o ne co untry w ill me an the same to any o the r co untry.

325 225 225 400

6 . Rap id Hard e ning (IS 8 0 4 1 -1 9 9 0 )

7 . Slag Ce me nt (IS 4 4 5 -1 9 8 9 )

8 . Hig h Alumina Ce me nt (IS 6 4 5 2 -1 9 8 9 ) 9 . Super Sulphated Cement (IS 6 9 0 9 -1 9 9 0 )

* N S – No t spe cifie d .

1 2 . IRS-T-4 0

370

*

300

5 . PPC (IS 1 4 8 9 -1 9 9 1 ) Part I

1 1 . Maso nry Ce me nt (IS 3 4 6 6 -1 9 8 8 )

225

4 . SRC (IS 1 2 3 3 0 -1 9 8 8 )

320

5

225

3 . 5 3 Grad e O PC (IS 1 2 2 6 9 -1 9 8 7 )

1 0 . Lo w He at Ce me nt (IS 1 2 6 0 0 -1 9 8 9 )

5

225

2 . 4 3 Grad e O PC (IS 8 1 1 2 -1 9 8 9 )

5

10

10

10

10

10

10

10

10

225

10

(m / kg ) Min. Le chatelier (mm) Max.

0 .8

1

0 .8

N S

N S

0 .8

0 .8

0 .8

0 .8

0 .8

0 .8

0 .8

Auto clave (% ) Max.

So und ness By

Cement 2

Fineness

Type o f

1 . 3 3 Grad e O PC (IS 2 6 9 -1 9 8 9 )

Sl.No .

60

90

60

30

30

30

30

30

30

30

30

30

600

1440

600

600

600

600

600

600

600

600

600

600

Initial Final (mts) min. (mts) max.

Setting Time

N S

N S

N S

N S

30

N S

16

N S

N S

N S

N S

N S

1 Day min. MPa

N S

N S

10

15

35

16

27

16

10

27

23

16

3 Days min. MPa

3 7 .5

2 .5

16

22

N S

22

N S

22

16

37

33

22

7 Days min. MPa

Co mpressive Streng th

Ta ble 2 .5 . Physic a l Cha ra c t e rist ic s of Va rious Type s of Ce m e nt .

N S

5

35

30

N S

33

N S

33

33

53

43

33

2 8 Days min. MPa

Types of Cement "

57

58

" Concrete Technology

Ta ble . 2 .6 . Che m ic a l Cha ra c t e rist ic s of Va rious Type s of Ce m e nt . Sr. No .

Typ e o f Ce me nt

Lime Saturatio n Facto r (% )

Alumina Iro n Ratio (% ) Min.

Inso lub le Re sid ue (% ) Max.

Mag ne sia (% ) Max.

Sulp huric Anhyd rid e

Lo ss o n Ig nitio n (% ) Max.

1

3 3 Grad e O PC (IS 2 6 9 -1 9 8 9 )

0 .6 6 Min. 1 .0 2 Max.

0 .6 6

4

6

2 .5 % Max. W he n C3 A is 5 o r le ss 3 % Max. W he n C3 A is g re ate r than 5

5

2

4 3 Grad e O PC (IS 8 1 1 2 -1 9 8 9 )

0 .6 6 Min. 1 .0 2 Max.

0 .6 6

2

6

2 .5 % Max. W he n C3 A is 5 o r le ss 3 % Max. W he n C3 A is g re ate r than 5

5

3

5 3 Grad e O PC (IS 1 2 2 6 9 -1 9 8 7 )

0 .8 Min. 1 .0 2 Max.

0 .6 6

2

6

2 .5 % Max. W he n C3 A is 5 o r le ss 3 % Max. W he n C3 A is g re ate r than 5

4

4

Sulp hate Re sisting Ce me nt (IS 1 2 3 3 0 -1 9 8 8 )

0 .6 6 Min. 1 .0 2 Max.

N S

4

6

2 .5 % Max.

5

5

Po rtland

N S

N S

6

3 % Max.

5

Po zzo lana Ce me nt (IS 1 4 8 9 -1 9 9 1 ) Part I

x+

4(100 − x ) 100

6

Rap id Hard e ning Ce me nt (IS 8 0 4 1 -1 9 9 0 )

0 .6 6 Min. 1 .0 2 Max.

0 .6 6

4

6

2 .5 % Max. W he n C3 A is 5 o r le ss 3 % Max. W he n C3 A is g re ate r than 5

5

7

Slag Ce me nt (IS 4 5 5 -1 9 8 9 )

N S

N S

4

8

3 % Max.

5

8

Hig h Alumina Ce me nt (IS 6 4 5 2 -1 9 8 9 )

N S

N S

N S

N S

N S

N S

9

Sup e r Sulp hate d -Ce me nt (IS 6 9 0 9 -1 9 9 0 )

N S

N S

4

10

6 % Min.

N S

10

Lo w He at Ce me nt (IS 1 2 6 0 0 -1 9 8 9 )

N S

0 .6 6

4

6

2 .5 % Max. W he n C3 A is 5 o r le ss 3 % Max. W he n C3 A is g re ate r than 5

5

11

IRS-T4 0

0 .8 Min. 1 .0 2 Max.

0 .6 6

2

5

3 .5 % Max.

4

x – De c lare d p e rc e ntag e o f flyash.

N S – No t sp e c ifie d .

Types of Cement "

59

Ta ble 2 .7 . Typic a l Te st Ce r t ific at e

5 3 Gra de Por t la nd Ce m e nt Birla Supe r We e k no. 3 5 Physic a l Ana lysis Fin e n e s s :

Re q u ire m e n ts o f I. S. 1 2 2 6 9 -1 9 8 7 3 0 3 m 2 / kg

Sho uld no t b e le ss than

2 2 5 m 2 / kg

a ) By Le chate lie r mo uld

0 .5 0 m.m.

Sho uld no t e xce e d

1 0 m.m.

b ) By Auto clave

0 .0 9 3 6 %

Sho uld no t e xce e d

0 .8 %

a ) Initial se t

1 3 0 mts.

Sho uld no t b e le ss than

3 0 mts.

b ) Final se t

1 9 5 mts.

Sho uld no t e xce e d

6 0 0 mts.

a ) 3 d ays

4 2 .3 MPa

Sho uld no t b e le ss than

2 7 M Pa

b ) 7 d ays

5 1 .6 MPa

Sho uld no t b e le ss than

3 7 M Pa

c ) 28 days (W e e k No . 3 2 )

7 1 .3 MPa

Sho uld no t b e le ss than

Te mp e rature d uring te sting

2 7 .0 °C

Sho uld b e

5 3 M Pa – 2°C 27°C +

Stand ard Co nsiste nce y

2 9 .7 %

Sp e cific Surface So u n d n e s s Exp ansio n o f unae rate d ce me nt

Se ttin g

Tim e :

Co m p re ssive

stre n g th :

Che m ic a l Ana lysis Pa rtic u la rs Lime Saturatio n Facto r (L.S.F.) no t Alumina Iro n Ratio

0 .9 2

Sho uld no t b e le ss than

1 .1 6

Sho uld no t b e le ss than

0 .8 0 and e xce e d 1 .0 2 0 .6 6

Lo ss o n Ig nitio n (LO I)

1 .2 9 %

Sho uld no t e xce e d

4%

Inso lub le Re sid ue (I.R.)

0 .8 4 %

Sho uld no t e xce e d

2%

Sulp huric Anhyd rid e (SO 3 )

2 .0 3 %

Sho uld no t e xce e d

3%

Mag ne sia (Mg O )

1 .1 6 %

Sho uld no t e xce e d

6%

Alkalie s

0 .4 6 %

Sho uld no t e xce e d

0 .6 %

Chlo rid e s

0 .0 1 6 2 %

Sho uld no t e xce e d

0 .0 5 %

Issue d to : Marke ting Divisio n sd / -O FFICER (Q C)

sd / -Sr. MANAGER (Q C)

Birla Supe r Ce m e nt - OPC 5 3 Gra de (I S 1 2 2 6 9 - 1 9 8 7 )

Ta ble 2 .8 . Consist e ncy Cur ve s for t he ye a r 1 9 9 9

60 " Concrete Technology

Types of Cement "

61

Ta ble 2 .9 . Brie f Sum m a r y of Ce m e nt Te st ing Proc e dure a nd gra de s of Ce m e nt in va rious c ont rie s. TESTING PRO CEDURE Co untry

Ind ia

Ge rmany

Grad e

Mate rial

33

1 :3 Mo rtar

7 0 .6 (5 0 cm 2 )

43

-

53

-

30

Mo rtar

U.S.S.R.

U.K.

1 d ay

3 d ays

7 d ays

2 8 d ays

Vib ratio n 1 2 0 0 0 / min Fo r 2 min

0 .3 9 to 0 .4 5

-

16

22

33

-

-

-

-

23

33

43

-

-

-

-

12

37

53

0 .5

-

12

-

30

35

-

-

-

-

15

-

35

40

-

-

-

-

20

-

40

45

-

-

-

-

25

-

45

50

-

-

-

25

-

-

50

-

-

55

-

-

-

25

275

1 :2 .5 Mo rtar

-

-

0 .4 4

-

-

16

28

325

-

-

-

-

-

12

19

33

425

-

-

-

-

-

16

25

43

525

-

-

-

-

-

21

32

53

625

-

-

-

-

-

27

41

63

725

-

-

-

-

-

36

-

73

400

1 :3 Mo rtar

Prism 40 × 40 × 160 **

-

0 .4

-

-

-

40

500

-

-

-

-

-

-

-

50

550

-

-

-

-

-

-

-

55

600

-

-

-

-

-

-

-

60

O PC

Mo rtar

Vib ratio n

1 :3

7 0 .6

12000 ± 4 0 0 fo r 2 min

0 .4

-

23

-

42

Co ncre te 1 :2 .5 :3 .5

1 0 1 .6

Tamp ing

0 .6

-

13

-

30

Mo rtar 1 :2 .7 5

50

Tamp ing

0 .4 8 5

-

13

20

29

-

U.S.A.

CO MPRESSIVE STRENGTH MPa W/ C ratio

Prism Vib ratio n 40 × 40 × 160 (2 5 c m 2 ) * *

55 China

Size o f Co mp actio n Cub e mm.

O PC Typ e 1

* P/ 4 + 3 % , w he re P is stand ard c o nsiste nc y. * * Afte r b e nd ing te st, o ne half o f the p rism is stre sse d alo ng the lo ng e r e d g e s w ith lo ad ing are a re stric te d to 2 5 c m 2 .

62

" Concrete Technology

Additional General Information on Cement and other Pozzolanic Materials

Com pa rison of Physic a l Cha ra c t e rist ic s of OPC Ite m

Fine ne ss, m 2 / kg IST, minute s FST, minute s

ASTM C-1 5 0 , Type

EN 1 9 7 -1 , Stre ng th Class

I

III

V

280

–@

280

45

3 2 .5

4 2 .5

5 2 .5

BIS, Stre ng th Grad e s 33

43

53

SRC

225 75

60

45

3 7 5 (Maximum)

30 6 0 0 (Maximum)

Co mp re ssive Streng th, Mpa (Minimum) at 1 d ay

–

12

–

–

– *

2 d ays

–

–

–

–/ 1 0

3 d ays

12

24

8

–

7 d ays

19

–

15

16

2 8 d ays

28

–

21

3 2 .5 5 2 .5

2 8 d ays (max)

–

10/ 20 –

– 20/ 30

–

–

–

–

–

–

–

–

16

23

27

10

–

22

33

37

16

4 2 .5

5 2 .5

33

43

53

33

6 2 .5

–

–

@ d e no te s no value sp e cifie d *

first value s fo r N (No rmal), ne xt fo r R (Rap id )

Com pa rison of Low he a t Ce m e nt s Ce me nt

Fine ne ss, m 2 / kg

He at o f hyd ratio n, Cal/ g m, at 7 d ays

2 8 d ays

Co mp re ssive stre ng th, Mpa, at 3 d ays 7 d ays

2 8 d ays

ASTM type IV

280

60

70

–@

7

17

IS : 1 2 6 0 0

320

65

75

10

16

35

@ – d e no te s no t sp e cifie d .

Types of Cement "

Com pa rison of Che m ic a l Cha ra c t e rist ic s of OPC Ite ms (mad value s)

I

ASTM C1 5 0 , Typ e s II III IV

EN 1 9 7 -1

33

IR

0 .7 5

3

5

4

3

3

4

LO I

3

-

3

2 .5

3

5

5

5

4

5

Mg O

6

6

6

6

6

5

6

6

6

6

Chlo rid e

–@

@

–

–

–

0 .1 0 *

0 .1 0 #

0 .1 0 #

0 .1 0 #

0 .1 0

Alkalis $

0 .6

0 .6

0 .6

0 .6

0 .6

–

0 .6

0 .6

0 .6

–

C3 A

–

8

15

7

5

–

–

–

–

5

2 C3 A + C4 AF

–

–

–

–

25

–

–

–

–

25

C3 S

–

–

–

35

–

–

–

–

–

–

C2 S (min)

–

–

–

40

–

–

–

–

–

–

V

0 .7 5 0 .7 5 0 .7 5

BIS O PC g rad e s 43 53

SRC

@ – d e no te s no value sp e cifie d *

– fo r p re -stre ssing ap p licatio ns, a lo w e r value may b e p re scrib e d

#

– 0 .0 5 % fo r p re stre sse d co ncre te

$

– limits o f alkali are o p tio nal, re co mme nd e d in case o f re active ag g re g ate s.

Com pa rison of Physic a l prope r t ie s of Por t la nd Pozzola na Ce m e nt s Ite m

ASTM C-1 5 0 , Typ e IP

EN 1 9 7 -1 , Stre ng th Classe s 3 2 .5 , 4 2 .5 , 5 2 .5

IS: 1 4 8 9 Part I

Fine ne ss, m 2 / kg

–@

All re q uire me nts are

300

IST, minute s

4 5 (Min.)

id e ntical to O PC as in

3 0 (Min.)

FST, minute s

4 2 0 (Max.)

Tab le 3

6 0 0 (Max.)

Co mp re ssive Streng th, Mpa (Minimum) at 3 d ays

13

16

7 d ays

20

22

2 8 d ays

25

33

63

64

" Concrete Technology

Com pa rison of Physic a l prope r t ie s of Por t la nd Sla g Ce m e nt s Ite m

ASTM C-1 5 0 , Typ e IP

Fine ne ss, m 2 / kg

EN 1 9 7 -1 , Stre ng th Classe s 3 2 .5 , 4 2 .5 , 5 2 .5

IS: 4 5 5

–@

All re q uire me nts are

225

IST, minute s

4 5 (Min.)

id e ntical to O PC as in

3 0 (Min.)

FST, minute s

4 2 0 (Max.)

Tab le 3

6 0 0 (Max.)

Co mp re ssive Streng th, Mpa (Minimum) at 3 d ays

13

16

7 d ays

20

22

2 8 d ays

25

33

@ - De no te s no value sp e cifie d

Com pa rison of Spe c ific at ions for Gra nulat e d Sla g S.No . 1.

Ite m

EN 1 9 7 -1

ASTM C-9 8 9

IS: 1 2 0 8 9

–@

–

1 .0

(C+M+1 / 3 A)/ (S+2 / 3 A) Or (C + M + A)/ S, min

–

–

1 .0

2.

(C + M + S), % mi n

67

–

–

3.

(C + M)/ S, min

1 .0

–

–

4.

Mg O , % max

–

–

17

5.

MnO , % max

–

–

5 .5

6.

Sulp hid e Sulp hur, % max

–

2 .5

2

7. 8.

Inso lub le Re sid ue , % max Glass Co nte nt, % min

– 67

– –

5 85

@ - d e no te s no t sp e cifie d . (C=CaO , M = Mg O , A = Al2 O 3 , S = SiO 2 )

Types of Cement "

65

Spe c ific at ions for Fly a sh in Ce m e nt a nd Conc re t e (values are %, unless other units are indicated) Ite m

ASTM C-6 1 8

Euro pe an Spe cificatio ns EN-4 5 0 EN-1 9 7 -1

BS 3 8 9 2 -I

SiO 2 , min Re active / so lub le SiO 2 , min. S + A + F, min.

25

IS : 3 8 1 2 Existing 1981

Pro p o se d

35

35

25

25

70

70

70

5

5

12

5

1 .5

1 .5

2 .7 5

2 .7 5

Mg O , max. LO I (1 ho ur) max.

6

To tal Alkalis, max.

1 .5

SO 3 , max

5 -7

5

7

3

Fre e CaO , max To tal/ re active CaO max. Fine ne ss, 4 5 micro n, max.

5 -7

34#

2

1

1

10

10

10

40@

12

Blaine ’s m 2 / kg min. Ce me nt activity, 2 8 d ays Lime reactivity, N/ mm

75*

75*

80**

34 320

320

80***

80***

4

4 .5

10

10

0 .8

0 .8

2

So undness, Lechatelier, mm

10

Auto clave, %

0 .8

10

10

No te i ) @ Pe rmitte d variatio n + 1 0 % o f ave rag e ii ) # Pe rmitte d variatio n + _ 5 % o f ave rag e iii) *

2 5 % fly ash

**

3 0 % fly ash

* * * 2 0 % fly ash iv)

Drying shrinkag e < 0 .1 5 in IS 3 8 1 2

R EF ER EN C ES 2.1.

Fast Setting Cement, Engineering News Records, Jan. 1956.

2.2

Product Literature of Birla White.

2.3

Information Supplied by CAI letter no. MISCEL/ENG/244 dated 7th Sept. 78.

2.4

Della, M. Roy, Advanced Cement System, including CBC, DSP, MDF, 9th International Congress on the Chemistry of Cement, New Delhi - 1992.

2.5

Comparison of BIS, ASTM and EN Cement Standards Compiled by Grasim Industries Ltd. (Cement Business) Mumbai.

66

! Concrete Technology

3

C H A P T E R For Production of well Shaped and well Graded Aggregates. Barmac Rock on Rock VSI Crusher

" General " Classification " Source " Aggregates from Igneous Rocks " Aggregates from Sedimentary Rocks " Aggregates from Metamorphic Rocks " Size " Shape " Texture " Strength " Aggregatte Crushing Value " Aggregate Impact Value " Aggregate Abrasion Value " Deval Attrition Test " Dorry Abrasion Test " Los Angeles Test " Modulus of Elasticity " Bulk Density " Specific Gravity " Absorption and Moisture Content " Bulking of Aggregates " Measurement of Moisture Content of Aggregates " Cleanliness " Soundness of Aggregate " Alkali Aggregate Reaction " Thermal Properties " Grading of Aggregates " Specific Surface and Surface Index " Standard Grading Curve " Crushed Sand " Gap grading " Test for Determination of Flakiness Index " Test for Determination of Elongation Index " Test for Determination of clay, fine silt and fine dust " Test for Determination of Organic Impurities " Test for Determination of Specific Gravity " Test for Determination of Bulk Density and Voids

Aggregates and Testing of Aggregates General

A

g g re g ate s are th e im p o rtan t c o n stitu e n ts in co ncrete. They g ive bo dy to the co ncrete, reduce shrinkag e and e ffe c t e c o no m y. Earlie r, ag g re g ate s w e re co nsid e rd as che mically ine rt mate rials b ut no w it has b e e n re co g nise d that so me o f the ag g re g ate s are chemically active and also that certain ag g reg ates e xhib it c he mic al b o nd at the inte rfac e o f ag g re g ate and p aste . The me re fact that the ag g re g ate s o ccup y 7 0 –8 0 p e r c e n t o f th e vo lu m e o f c o n c re te , th e ir im p ac t o n vario us c harac te ristic s and p ro p e rtie s o f co ncrete is und o ub ted ly co nsid erab le. To kno w mo re ab o ut the co ncrete it is very essential that o ne sho uld kno w m o re ab o ut the ag g re g ate s w hic h c o nstitute majo r vo lume in c o nc re te . W itho ut the stud y o f the ag g re g ate in d e p th an d ran g e , th e stu d y o f th e c o nc re te is inc o m p le te . Ce m e nt is the o nly fac to ry m a d e sta n d a rd c o m p o n e n t in c o n c re te . O th e r ing redients, namely, w ater and ag g reg ates are natural mate rials and can vary to any e xte nt in many o f the ir p ro p e rtie s. The d e p th and rang e o f stud ie s that are re q u ire d to b e m ad e in re sp e c t o f ag g re g ate s to

66

Aggregates and Testing of Aggregates !

67

understand their w idely varying effects and influence o n the pro perties o f co ncrete canno t b e und e rrate d . Co ncre te can b e co nsid e re d as tw o p hase mate rials fo r co nve nie nce ; p aste p hase and ag g reg ate phase. Having stud ied the paste phase o f co ncrete in the earlier chapters, w e shall no w stud y the ag g re g ate s and ag g re g ate p hase in c o nc re te in this c hap te r. The stud y o f ag g re g ate s can b e st b e d o ne und e r the fo llo w ing sub -he ad ing s: (a ) Classificatio n

(b ) So urc e

(c ) Size

(d ) Shap e

(e ) Texture

(f ) Stre ng th

(g ) Sp e cific g ravity and b ulk d e nsity

(h ) Mo isture co nte nt

(i ) Bulking facto r

( j ) Cle anline ss

(k ) So und ne ss

(l ) Che mical p ro p e rtie s

(m ) The rmal pro pe rtie s

(n ) Durab ility

(o ) Sie ve analysis

(p ) Grad ing

Classification Ag g re g ate s c an b e c lassifie d as ( i ) No rm al w e ig h t ag g re g ate s, ( ii ) Lig h t w e ig h t ag g re g ate s and (iii) He ary w e ig ht ag g re g ate s. Lig ht w e ig ht ag g re g ate and he avy w e ig ht ag g reg ate w ill b e discussed elsew here under appro priate to pics. In this chapter the pro perties o f no rmal w e ig ht ag g re g ate s w ill o nly b e d iscusse d . No rmal w e ig ht ag g re g ate s can b e furthe r classifie d as natural ag g re g ate s and artificial ag g re g ate s.

Natu ral Sand , Grave l, Crushe d

Artific ial Bro ke n Brick,

Ro ck such as Granite ,

Air-co o le d Slag .

Q uartzite , Basalt,

Sinte re d fly ash

Sand sto ne

Blo ate d clay

Ag g re g ate s c an also b e c lassifie d o n the b asis o f the size o f the ag g re g ate s as c o arse ag g re g ate and fine ag g re g ate .

Source Almo st all natural ag g reg ate materials o rig inate fro m b ed ro cks. There are three kind s o f ro cks, namely, ig neo us, sed imentary and metamo rphic. These classificatio ns are b ased o n the mo d e o f fo rmatio n o f ro cks. It may b e re calle d that ig ne o us ro cks are fo rme d b y the co o ling o f mo lten mag ma o r lava at the surface o f the crest (trap and basalt) o r deep beneath the crest (g ranite ). The se d ime ntary ro cks are fo rme d o rig inally b e lo w the se a b e d and sub se q ue ntly lifte d up . Me tam o rp hic ro c ks are o rig inally e ithe r ig ne o us o r se d im e ntary ro c ks w hic h are sub se q ue ntly m e tam o rp ho se d d ue to e xtre m e he at and p re ssure . The c o nc re te m aking pro pe rtie s o f ag g re g ate are influe nce d to so me e xte nt o n the b asis o f g e o lo g ical fo rmatio n o f the p are nt ro c ks to g e the r w ith the sub se q ue nt p ro c e sse s o f w e athe ring and alte ratio n. Within the main ro ck g ro up , say g ranite g ro up , the q uality o f ag g re g ate may vary to a ve ry g reat extent o w ing to chang es in the structure and texture o f the main parent ro ck fro m place to place .

68

! Concrete Technology

Aggregates from Igneous Rocks Mo st ig ne o us ro c ks m ake hig hly satisfac to ry c o nc re te ag g re g ate s b e c ause the y are no rmally hard, to ug h and dense. The ig neo us ro cks have massive structure, entirely crystalline o r w ho lly g lassy o r in co mb inatio n in b etw een, d epend ing upo n the rate at w hich they w ere co o led during fo rmatio n. They may be acidic o r basic depending upo n the percentag e o f silica co ntent. They may o ccur lig ht co lo ured o r d ark co lo ured . The ig neo us ro cks as a class are the mo st che mically active co ncre te ag g re g ate and sho w a te nd e ncy to re act w ith the alkalie s in cement. This aspect w ill b e d iscussed later. As the ig neo us ro ck is o ne o f the w id ely o ccurring type o f ro cks o n the face o f the e arth, b ulk o f the co ncre te ag g re g ate s, that are d e rive d , are o f ig ne o us o rig in

Aggregates from Sedimentary Rocks Ig ne o us ro cks o r me tamo rphic ro cks are sub je cte d to w e athe ring ag e ncie s such as sun, rain and w ind . The se w e athe ring ag e ncie s d e co mp o se , frag mantise , transp o rt and d e p o sit the particles o f ro ck, deep beneath the o cean bed w here they are cemented to g ether by so me o f the c e m e nting m ate rials. The c e m e nting m ate rials c o uld b e c arb o nac e o us, silic e o us o r arg illaceo us in nature. At the same time the d epo sited and cemented material g ets sub jected to static pre ssure o f w ate r and b e co me s co mpact se d ime ntary ro ck laye r. The d e p o sitio n, ce me ntatio n and co nso lid atio n take s p lace laye r b y laye r b e ne ath the o c e an b e d . The se se d im e ntary ro c k fo rm atio ns sub se q ue ntly g e t lifte d up and b e c o m e s c o ntine nt. The se d im e ntary ro c ks w ith the stratifie d struc ture are q uarrie d and c o nc re te ag g reg ates are d erived fro m it. The q uality o f ag g reg ates d erived fro m sed imentary ro cks w ill vary in q uality d e pe nd ing upo n the ce me nting mate rial and the pre ssure und e r w hich the se ro cks are o rig inally co mpacted . So me siliceo us sand sto nes have pro ved to b e g o o d co ncrete ag g re g ate . Similarly, the lime sto ne also can yie ld g o o d co ncre te ag g re g ate . The thic kne ss o f the stratific atio n o f se d im e ntary ro c ks m ay vary fro m a frac tio n o f a centimetre to many centimetres. If the stratificatio n thickness o f the parent ro ck is less, it is likely to sho w up e ve n in an ind ivid ual ag g re g ate and the re b y it may imp air the stre ng th o f the ag g re g ate . Such ro cks may also yie ld flaky ag g re g ate s. Se d ime ntary ro cks vary fro m so ft to h ard , p o ro u s to d e n se an d lig h t to h e avy. Th e d e g re e o f c o n so lid atio n , th e typ e o f c e m e n tatio n , th e th ic kn e ss o f laye rs an d c o n tam in atio n , are all im p o rtan t fac to rs in d e te rmining the suitab ility o f se d ime ntary ro ck fo r co ncre te ag g re g ate s.

Aggregates from Metamorphic Rocks Bo th ig ne o us ro cks and se d ime ntary ro cks may b e sub je cte d to hig h te mp e rature and p re ssure w hic h c ause s m e tam o rp hism w hic h c hang e s the struc ture and te xture o f ro c ks. Metamo rphic ro cks sho w fo liated structure. The thickness o f this fo liatio n may vary fro m a few ce ntime tre s to many me tre s. If the thickne ss o f this fo liatio n is le ss, the n ind ivid ual ag g re g ate m ay e xhib it fo liatio n w hic h is no t a d e sirab le c harac te ristic in ag g re g ate . Ho w e ve r, m any me tamo rp hic ro cks p articularly q uartizite and g ne iss have b e e n use d fo r p ro d uctio n o f g o o d co ncre te ag g re g ate s. It may b e me ntio ne d that many pro pe rtie s o f ag g re g ate s name ly, che mical and mine ral c o m p o sitio n, p e tro -g rap hic d e sc rip tio n, sp e c ific g ravity, hard ne ss, stre ng th, p hysic al and che mical stab ility, po re structure e tc. d e pe nd mo stly o n the q uality o f pare nt ro ck. But the re are so m e p ro p e rtie s p o sse sse d b y the ag g re g ate s w hic h are im p o rtant so far as c o nc re te making is co ncerned w hich have no relatio n w ith the parent ro ck, particularly, the shape and size. While it is to b e ad mitted that g o o d ag g reg ates fro m g o o d parent ro cks can make g o o d

Aggregates and Testing of Aggregates !

69

c o nc re te , it may b e w ro ng to c o nc lud e that g o o d c o nc re te c anno t b e mad e fro m slig htly infe rio r ag g re g ate s o b taine d fro m no t so g o o d p are nt ro c ks. Ag g re g ate s w hic h are no t so g o o d can b e used fo r making satisfacto ry co ncrete o w ing to the fact that a co ating o f cement p aste o n ag g re g ate s b rin g ab o u t im p ro ve m e n t in re sp e c t o f d u rab ility an d stre n g th characteristics. Therefo re, selectio n o f ag g reg ates is req uired to b e d o ne jud icio usly taking the e c o no mic fac to r into c o nsid e ratio n. Se ve ral fac to rs may b e c o nsid e re d in making the final selectio n o f ag g reg ates w here mo re than o ne so urce is availab le. The relative co st o f material in the se ve ral so urc e s is the mo st imp o rtant c o nsid e ratio n that sho uld w e ig h in making a cho ice . Re co rd s o f use o f ag g re g ate fro m a p articular so urce , and e xaminatio n o f co ncre te mad e w ith such ag g re g ate s, if such case s are the re , p ro vid e valuab le info rmatio n. The stud y w ill includ e ap p raisal o f lo catio n and the amo unt o f p ro ce ssing w hich e ach so urce may re q uire . The ag g re g ate w hich can b e d e live re d to the mixing p lant d ire ctly may no t b e the mo st eco no mical o ne. It may req uire a cement co ntent mo re than that o f ano ther so urce. Also very o ften the co st o f so me pro cessing , such as co rrectio n o f ag g reg ate, may b e fully re co ve re d , w he n the p ro ce ssing acco mp lishe s the re d uctio n in ce me nt co nte nt o f the co ncrete. In g eneral, that ag g reg ate w hich w ill bring abo ut the desired q uality in the co ncrete w ith le ast o ve rall e xp e nse , sho uld b e se le cte d .

Size The larg e st m axim um size o f ag g re g ate p rac tic ab le to hand le und e r a g ive n se t o f c o n d itio n s sh o u ld b e u se d . Pe rh ap s, 8 0 m m size is th e m axim u m size th at c o u ld b e co nveniently used fo r co ncrete making . Using the larg est po ssib le maximum size w ill result in (i) red uctio n o f the cement co ntent (ii) red uctio n in w ater req uirement (iii) red uctio n o f d rying shrinkag e. Ho w ever, the maximum size o f ag g reg ate that can b e used in any g iven co nd itio n may b e limite d b y the fo llo w ing co nd itio ns: (i ) Thickne ss o f se ctio n;

(ii ) Sp acing o f re info rce me nt;

(iii ) Cle ar co ve r;

(iv ) Mixing , hand ling and p lacing te chniq ue s.

Generally, the maximum size o f ag g reg ate sho uld b e as larg e as po ssib le w ithin the limits sp e c ifie d , b ut in any c ase no t g re ate r than o ne -fo urth o f the m inim um thic kne ss o f the me mb e r. Rub b le s 1 6 0 mm size o r up to any re aso nab le size may b e use d in p lain co ncre te . In such co ncre te , calle d p lum co ncre te , the q uantity o f rub b le up to a maximum limit o f 2 0 per cent b y vo lume o f the co ncrete, is used w hen specially permitted . The rub b les are placed o n ab o ut 6 0 cm thick p lastic co ncre te at ce rtain d istance ap art and the n the p lastic co ncre te is vib rate d b y p o w e rful inte rnal vib rato rs. The rub b le s sink into the co ncre te . This me tho d o f inco rpo rating larg e b o ulders in the co ncrete is also called displacement co ncrete. This metho d is ad o pte d in the co nstructio n o f Ko yna d am in Maharashtra. Fo r he avily re info rce d co ncre te me mb e r the no minal maximum size o f ag g re g ate sho uld usually b e re stricte d to 5 mm le ss than the minimum cle ar d istance b e tw e e n the main b ars o r 5 mm le ss than the minimum c o ve r to th e re in fo rc e m e n t, w h ic h e ve r is sm a lle r. Bu t fro m va rio u s o th e r p ra c tic a l co nsid e ratio ns, fo r re info rce d co ncre te w o rk, ag g re g ate s having a maximum size o f 2 0 mm are g e ne rally co nsid e re d satisfacto ry. Ag g re g ate s are d ivid e d into tw o c ate g o rie s fro m the c o nsid e ratio n o f size (i) Co arse ag g re g ate and (ii) Fine ag g re g ate . The size o f ag g re g ate b ig g e r than 4 .7 5 mm is co nsid e re d as c o arse ag g re g ate and ag g re g ate w ho se size is 4 .7 5 m m and le ss is c o nsid e re d as fine ag g re g ate .

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Shape The shap e o f ag g re g ate s is an imp o rtant characte ristic since it affe cts the w o rkab ility o f co ncre te . It is d ifficult to re ally me asure the shap e o f irre g ular b o d y like co ncre te ag g re g ate w hich are d e rive d fro m vario us ro cks. No t o nly the characte ristic o f the p are nt ro ck, b ut also the type o f crusher used w ill influence the shape o f ag g reg ates, e.g . , the ro cks available ro und ab o ut Pune re g io n are fo und to yie ld slig htly flaky ag g re g ate s, w he re as, g o o d g ranite ro ck as fo und in Bang lo re w ill yie ld cub ical ag g re g ate . The shap e o f the ag g re g ate is ve ry much influe nce d b y the typ e o f crushe r and the re d uctio n ratio i.e. , the ratio o f size o f mate rial fe d into c rushe r to the size o f the finishe d p ro d uc t. Many ro c ks c o ntain p lane s o f p arting o r jo inting w hic h is c harac te ristic o f its fo rm atio n. It also re fle c ts the inte rnal p e tro g rap hic structure. As a co nseq uence o f these tendencies, schists, slates and shales co mmo nly pro duce flaky fo rms, w he re as, g ranite , b asalt and q uartzite usually yie ld mo re o r le ss e q uid ime nsio nal p article s. Similarly, q uartizite w hich d o e s no t p o sse s cle avag e p lane s p ro d uce s cub ical shap e ag g re g ate s. Fro m the standpo int o f eco no my in cement req uirement fo r a g iven w ater/ cement ratio , ro und ed ag g reg ates are preferab le to ang ular ag g reg ates. O n the o ther hand , the ad d itio nal ce me nt re q uire d fo r ang ular ag g re g ate is o ffse t to so me e xte nt b y the hig he r stre ng ths and so metimes by g reater durability as a result o f the interlo cking texture o f the hardened co ncrete and hig he r b o nd characte ristic b e tw e e n ag g re g ate and ce me nt p aste . Flat particles in co ncrete ag g reg ates w ill have particularly o b jectio nab le influence o n the w o rkab ility, c e m e n t re q u ire m e n t, stre n g th an d d u rab ility. In g e n e ral, e xc e ssive ly flaky ag g re g ate make s ve ry po o r co ncre te . Classificatio n o f particle s o n the b asis o f shape o f the ag g re g ate is sho w n in Tab le 3 .1 . O n e o f th e m e th o d s o f e xp re ssin g th e an g ularity q ualitative ly is b y a fig ure c alle d Ang ularity Numb er, as sug g ested b y Sherg o ld 3.1 . This is b ased o n the percentag e vo ids in the ag g reg ate after co mpactio n in a specified manner. The test g ives a value termed the ang ularity numb e r. The me tho d o f d e te rminatio n is d e scrib e d in IS: 2 3 8 6 (Part I) 1 9 6 3 .

Ta ble 3 .1 Sha pe of Pa r t icle Classificatio n

Descriptio n

Examples

Ro und e d

Fully w ate r w o rn o r co mp le te ly shap e d b y attritio n

Rive r o r se asho re g rave ls; d esert, seasho re and w ind b lo w n sand s

Irre g ular o r Partly ro und e d

Naturally irre g ular o r p artly shap e d b y attritio n, having ro und e d e d g e s

Pit sand s and g rave ls; land o r d ug flints; cub o id ro ck

Ang ular

Po sse ssing w e ll-d e fine d e d g e s fo rme d at the inte rse ctio n o f ro ug hly p lanar face s

Crushe d ro cks o f all typ e s; talus; scre e s

Flaky

Mate rial, usually ang ular, o f w hich the thickne ss is small re lative to the w id th and / o r le ng th

Laminate d ro cks

Aggregates and Testing of Aggregates !

Round (spherical) concrete aggregate.

Flaky concrete aggregate.

71

Crushed concrete aggregate.

A q uantity o f sing le sized ag g reg ate is filled into metal cylind er o f three litre capacity. The ag g re g ate s are co mp acte d in a stand ard manne r and the p e rce ntag e o f vo id is fo und o ut. The vo id can b e fo und o ut b y kno w ing the sp e cific g ravity o f ag g re g ate and b ulk d e nsity o r b y p o uring w ate r to the cylind e r to b ring the le ve l o f w ate r up to the b rim. If the vo id is 3 3 p e r ce nt the ang ularity o f such ag g re g ate is co nsid e re d ze ro . If the vo id is 4 4 p e r ce nt the ang ularity num b e r o f suc h ag g re g ate is c o nsid e re d 1 1 . In o the r w o rd s, if the ang ularity numb e r is ze ro , the so lid vo lume o f the ag g re g ate is 6 7 p e r ce nt and if ang ularity numb e r is

Poorly shapped crushed aggregate. It will make poor concrete.

Good aggregate resulted from Barmac crusher.

Barmac crushed 20 mm cubical aggregate. It will make good concrete.

20 mm crushed angular aggregates not so good for concrete.

Courtesy : Durocrete Pune

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1 1 , the so lid vo lum e o f the ag g re g ate is 5 6 p e r c e nt. The no rm al ag g re g ate s w hic h are suitab le fo r m aking the c o nc re te m ay have ang ularity num b e r anything fro m ze ro to 1 1 . An g u larity n u m b e r ze ro re p re se n ts th e m o st p rac tic ab le ro u n d e d ag g re g ate s an d th e ang ularity num b e r 1 1 ind ic ate s the m o st ang ular ag g re g ate s that c o uld b e to le rate d fo r making co ncre te no t so und uly harsh and une co no mical. Murd o c k sug g e ste d a d iffe re nt m e tho d fo r e xp re ssing the shap e o f ag g re g ate b y a p arame te r calle d Ang ularity Ind e x ‘ fA’. 3 .2 Ang ularity Ind e x

fA =

3 fH + 1 .0 20

w he re fH is the Ang ularity numb e r.

Th e re h a s b e e n a lo t o f co ntro ve rsy o n the su b je c t w h e th e r th e a n g u la r a g g re g a te o r ro und e d ag g re g ate w ill m a ke b e tte r c o n c re te . W h ile d isc u ssin g th e shap e o f ag g re g ate , the te xture o f the ag g re g ate also enters the discussio n b e c a u se o f its c lo se a sso c ia tio n w ith th e sh a p e . G e n e ra lly, ro und e d ag g re g ate s are sm o o th te xtu re d a n d ang ular ag g re g ate s are ro u g h te xtu re d . So m e e n g in e e rs, p ro h ib it th e u se of ro u n d e d Shape and size of aggregates Courtesy : Ambuja Cement ag g re g ate o n th e p le a that it yields po o r co ncrete, due to lack o f bo nd betw een the smo o th surface o f the ag g reg ate and ce me nt p aste . The y sug g e st that if at all the ro und e d ag g re g ate is re q uire d to b e use d fo r e co no mical re aso n, it sho uld b e b ro ke n and the n use d . This co nce p t is no t fully justifie d fo r the re aso n that e ve n the so calle d , the smo o th surface o f ro und e d ag g re g ate s is ro ug h eno ug h fo r develo ping a reaso nably g o o d bo nd betw een the surface and the submicro sco pic c e m e n t g e l. But th e an g ular ag g re g ate s are sup e rio r to ro un d e d ag g re g ate s fro m th e fo llo w ing tw o po ints o f vie w : (a ) Ang ular ag g re g ate s e xhib it a b e tte r inte rlo cking e ffe ct in co ncre te , w hich p ro p e rty make s it sup e rio r in co ncre te use d fo r ro ad s and p ave me nts. (b ) The to tal surfac e are a o f ro ug h te xture d ang ular ag g re g ate is m o re than sm o o th ro unded ag g reg ate fo r the g iven vo lume. By having g reater surface area, the ang ular ag g re g ate may sho w hig he r b o nd stre ng th than ro und e d ag g re g ate s. The hig he r surface are a o f ang ular ag g re g ate w ith ro ug h te xture re q uire s mo re w ate r fo r a g iven w o rkab ility than ro unded ag g reg ates. This means that fo r a g iven set o f co nditio ns fro m th e p o in t o f vie w o f w ate r/ c e m e n t ratio an d th e c o n se q u e n t stre n g th , ro u n d e d ag g reg ate g ives hig her streng th. Superimpo sing plus and minus po ints in favo ur and ag ainst the se tw o kind s o f ag g re g ate s it can b e summe d up as fo llo w s:

Aggregates and Testing of Aggregates !

73

Fo r w ater/ cement ratio b elo w 0.4 the use o f crushed ag g reg ate has resulted in streng th up to 38 per cent hig her than the ro unded ag g reg ate. With an increase in w ater/ cement ratio the influence o f ro ug hness o f surface o f the ag g reg ate g ets reduced, presumab ly b ecause the stre ng th o f the p aste itse lf b e c o m e s p aram o unt, and at a w ate r/ c e m e nt ratio o f 0 .6 5 , no d iffe re nce in stre ng th o f co ncre te mad e w ith ang ular ag g re g ate o r ro und e d ag g re g ate has b e e n o b se rve d . The shap e o f the ag g re g ate s b e co me s all the mo re imp o rtant in case o f hig h stre ng th and hig h p e rfo rmance co ncre te w he re ve ry lo w w ate r/ ce me nt ratio is re q uire d to b e use d . In such cases cubical shaped ag g reg ates are req uired fo r better w o rkability. To pro duce mo stly c ub ic al shap e d ag g re g ate and re d uc e flaky ag g re g ate , im p ro ve d ve rsio ns o f c rushe rs are e m p lo ye d , suc h as Hyd ro c o ne c rushe rs, Barm ac ro c k o n ro c k VSI c rushe r e tc . So m e tim e s o rd inarily crushe d ag g re g ate s are furthe r p ro ce sse d to co nve rt the m to w e ll g rad e d cub ical ag g re g ate s. In th e ye ars to c o m e n atu ral san d w ill n o t b e availab le in larg e q u an tity fo r b ig infrastructural p ro je cts. O ne has to g o fo r manufacture d sand . Whe n ro ck is crushe d in the no rmal w ay it is like ly to yie ld flaky fine ag g re g ate . Imp ro ve d ve rsio n o f crushe rs are use d to pro d uce cub ical shaped w ell g rad ed fine ag g reg ate. This metho d o f pro d uctio n o f g o o d fine ag g re g ate is b e ing practise d fo r hig h rise b uild ing pro je cts at Mumb ai and fo r co nstructio n o f Mumb ai-Pune e xp re ss hig hw ay. O n re alising the imp o rtance o f shap e o f ag g re g ate s fo r p ro d uc ing hig h stre ng th c o nc re te the im p ro ve d ve rsio n o f c rushe rs are b e ing e xtinsive ly e mp lo ye d in Ind ia.

Texture Surface texture is the pro perty, the measure o f w hich d epend s upo n the re lative d eg ree to w hich particle surfaces are po lished o r d ull, smo o th o r ro ug h. Surface texture d epend s o n hard ne ss, g rain size , p o re struc ture , struc ture o f the ro c k, and the d e g re e to w hic h fo rc e s ac ting o n the p artic le surfac e have sm o o the d o r ro ug he nd it. Hard , d e nse , fine -g raine d materials w ill g enerally have smo o th fracture surfaces. Experience and lab o rato ry experiments have sho w n that the adhesio n b etw een cement paste and ag g reg ate is influenced b y several co mp le x facto rs in ad d itio n to the p hysical and me chanical p ro p e rtie s. As surface smo o thness increases, co ntact area decreases, hence a hig hly po lished particle w ill have le ss b o nd ing are a w ith the m atrix than a ro ug h p artic le o f the sam e vo lum e . A smo o th particle, ho w ever, w ill req uire a thinner layer o f paste to lub ricate its mo vements w ith re sp e c t to o the r ag g re g ate p artic le s. It w ill, the re fo re , p e rm it d e nse r p ac king fo r e q ual w o rkab ility and hence, w ill re q uire lo w er paste co ntent than ro ug h particles. It has b een also sho w n b y e xp e rim e nts that ro ug h te xture d ag g re g ate d e ve lo p s hig he r b o nd stre ng th in tensio n than smo o th textured ag g reg ate. The beneficial effects o f surface texture o f ag g reg ate o n fle xural stre ng th can b e se e n fro m Tab le 3 .2 .

Ta ble 3 .2 . I nflue nc e of Tex t ure on St re ngt h 3 .3 Per cent o f Particles Smo o th

Water/ Cement Ratio

Ro ug h

Streng th 2 8 d ays MPa Flexural

Co mpressive

100

0

0 .5 4

4 .3

3 4 .8

50

50

0 .5 7

4 .6

3 2 .1

0

100

0 .6 0

4 .8

2 9 .5

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Surface te xture characte ristics o f the ag g re g ate as classifie d in IS: 3 8 3 : 1 9 7 0 is sho w n b e lo w.

Ta ble 3 .3 . Surfa c e Cha ra c t e rist ic s of Aggre ga t e Gro up

Surface Texture

Examples

1.

Glassy

Black flint

2.

Smo o th

Che rt; slate ; marb le ; so me rhyo lite

3.

Granular

Sand sto ne ; o o lite s

4.

Crystalline

Fine : Basalt; trachyte ; me d ium : Do le rite ; g rano p hyre ; g ranulite ; micro g ranite ; so me lime sto ne s; many d o lo mite s. Co arse : Gab b ro ; g ne iss; g ranite ; g rano d io rite ; sye nite

5.

Ho ne yco mb e d and po ro us

Sco ria; Pumice , trass.

Measurement of Surface Texture A larg e num b e r o f p o ssib le m e tho d s are availab le and this may b e d ivid e d b ro ad ly into d ire c t and ind ire c t m e tho d s. Dire c t m e tho d s in c lud e ( i) m akin g a c ast o f th e surfac e an d m ag n ifyin g a se c tio n o f th is, ( ii) Trac in g th e irre g ularitie s b y d raw ing a fine p o int o ve r the su rfac e an d d raw in g a trac e m ag n ifie d b y m e c h an ic al, o p tic al, o r e le c tric al m e an s, ( iii) g e tting a se ctio n thro ug h the ag g re g ate s and examining a mag nified imag e. Indirect metho ds in c lu d e : ( i) m e asu re m e n t o f th e d e g re e o f d isp e rsio n o f lig h t fallin g o n th e surfac e , ( ii) d e te rm in in g th e w e ig h t o f a fin e p o w d e r req uired to fill up the interstices o f the surface to a truly sm o o th surfac e , (iii) the ro c k surfac e is h e ld a g a in st ru b b e r su rfa c e a t a sta n d a rd p re ssure and the re sistanc e to the flo w o f air b e tw e e n the tw o surface s is me asure d .

Strength

Aggregate Crushing Value Apparatus.

Whe n w e talk o f stre ng th w e d o no t imply the stre ng th o f the pare nt ro ck fro m w hich the ag g reg ates are pro duced, b ecause the streng th o f the ro ck do es no t exactly represent the stre ng th o f the ag g re g ate in co ncre te . Since co ncre te is an asse mb lag e o f ind ivid ual p ie ce s o f ag g reg ate b o und to g ether b y cementing material, its pro perties are b ased primarily o n the q uality o f the cement paste. This streng th is dependant also o n the bo nd betw een the cement p aste and the ag g re g ate . If e ithe r the stre ng th o f the p aste o r the b o nd b e tw e e n the p aste and ag g re g ate is lo w, a co ncre te o f p o o r q uality w ill b e o b taine d irre sp e ctive o f the stre ng th o f the ro ck o r ag g re g ate . But w he n ce me nt p aste o f g o o d q uality is p ro vid e d and its b o nd

Aggregates and Testing of Aggregates !

75

w ith the ag g reg ate is satisfacto ry, then the mechanical pro perties o f the ro ck o r ag g reg ate w ill influe nc e the stre ng th o f c o nc re te . Fro m the ab o ve it c an b e c o nc lud e d that w hile stro ng ag g reg ates canno t make stro ng co ncrete, fo r making stro ng co ncrete, stro ng ag g reg ates are an e sse ntial re q uire me nt. In o the r w o rd s, fro m a w e ak ro c k o r ag g re g ate stro ng c o nc re te canno t b e mad e . By and larg e naturally availab le mine ral ag g re g ate s are stro ng e no ug h fo r making no rmal stre ng th co ncre te . The te st fo r stre ng th o f ag g re g ate is re q uire d to b e mad e in the fo llo w ing situatio ns: (i ) Fo r p ro d uctio n o f hig h stre ng th and ultra hig h stre ng th co ncre te . (ii ) Whe n co nte mp lating to use ag g re g ate s manufacture d fro m w e athe re d ro cks. (iii ) Ag g re g ate manufacture d b y ind ustrial p ro ce ss.

Aggregatte Crushing Value Stre ng th o f ro ck is fo und o ut b y making a te st spe cime n o f cylind rical shape o f size 2 5 mm diameter and 25 mm heig ht. This cylinder is subjected to co mpressive stress. Different ro ck sample s are fo und to g ive d iffe re nt co mpre ssive stre ng th varying fro m a minimum o f ab o ut 4 5 MPa to a maximum o f 5 4 5 MPa. As said e arlie r, the co mpre ssive stre ng th o f pare nt ro ck d o e s no t e xactly ind icate the stre ng th o f ag g re g ate in co ncre te . Fo r this re aso n asse ssme nt o f stre ng th o f the ag g re g ate is mad e b y using a samp le o f b ulk ag g re g ate in a stand ard ise d manne r. This te st is kno w n as ag g re g ate crushing value te st. Ag g re g ate crushing value g ive s a re lative m e asure o f the re sistanc e o f an ag g re g ate sam p le to c rushing und e r g rad ually ap p lie d c o m p re ssive lo ad . Ge ne rally, this te st is m ad e o n sing le size d ag g re g ate p assing 1 2 .5 mm and re taine d o n 1 0 mm sie ve . The ag g re g ate is p lace d in a cylind rical mo uld and a lo ad o f 4 0 to n is ap p lie d thro ug h a p lung e r. The mate rial crushe d to fine r than 2 .3 6 mm is se p arate d and e xp re sse d as a p e rce ntag e o f the o rig inal w eig ht taken in the mo uld. This percentag e is re fe rre d a s a g g re g a te c ru sh in g va lu e . Th e crushing value o f ag g re g ate is re stricte d to 3 0 p e r c e nt fo r c o nc re te use d fo r ro ad s and p ave m e nts a n d 4 5 p e r c e n t m a y b e p e rm itte d fo r o th e r structure s. Th e c ru sh in g valu e o f ag g re g ate is rath e r inse nsitive to the variatio n in stre ng th o f w e ake r ag g reg ate. This is so b ecause having b een crushed b e fo re the ap p lic atio n o f the full lo ad o f 4 0 to ns, the w e ake r mate rials b e c o me c o mp ac te d , so that the amo unt o f crushing d uring late r stag e s o f the test is red uced . Fo r this reaso n a simple test kno w n as “10 per cent fines value” is intro duced. When the ag g reg ate crushing value beco me 30 o r hig her, the re sult is like ly to b e inac c urate , in w hic h c ase the ag g reg ate sho uld be subjected to “10 per cent fines value ” te st w hic h g ive s a b e tte r p ic ture ab o ut the stre ng th o f such ag g re g ate s. Th is te st is a lso d o n e o n a sin g le size d ag g re g ate as me ntio ne d ab o ve . Lo ad re q uire d to p ro d u c e 1 0 p e r c e n t fin e s (p artic le s fin e r th an 2 . 3 6 m m ) is fo u n d o u t b y o b se rvin g th e

Aggregate Impact Value Apparatus.

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p e ne tratio n o f p lung e r. The 1 0 p e r ce nt fine s value te st sho w s a g o o d co rre latio n w ith the stand ard crushing value te st fo r stro ng ag g re g ate s w hile fo r w e ake r ag g re g ate s this te st is mo re sensitive and g ives a truer picture o f the differences betw een mo re o r less w eak samples. It sho uld b e no te d that in the 1 0 pe r ce nt fine s value te st unlike the crushing value te st, a hig he r nume rical re sult d e no te s a hig he r stre ng th o f the ag g re g ate . The d e tail o f this te st is g ive n at the e nd o f this chap te r und e r te sting o f ag g re g ate .

Aggregate Impact Value With re sp e ct to co ncre te ag g re g ate s, to ug hne ss is usually co nsid e re d the re sistance o f the material to failure b y impact. Several attempts to d evelo p a metho d o f test fo r ag g reg ates impact value have b een mad e. The mo st successful is the o ne in w hich a sample o f stand ard ag g reg ate kept in a mo uld is sub jected to fifteen b lo w s o f a metal hammer o f w eig ht 14 Kg s. falling fro m a he ig ht o f 3 8 c ms. The q uantity o f fine r mate rial (p assing thro ug h 2 .3 6 mm) resulting fro m po und ing w ill ind icate the to ug hness o f the sample o f ag g reg ate. The ratio o f the w e ig ht o f the fine s (fine r than 2 .3 6 mm size ) fo rme d , to the w e ig ht o f the to tal samp le take n is e xp re sse d as a p e rc e ntag e . This is kno w n as ag g re g ate imp ac t value IS 2 8 3 -1 9 7 0 sp e cifie s that ag g re g ate imp act value shall no t e xce e d 4 5 p e r ce nt b y w e ig ht fo r ag g re g ate use d fo r co ncre te o the r than w e aring surface and 3 0 p e r ce nt b y w e ig ht, fo r co ncre te fo r w e aring surface s, such as run w ays, ro ad s and p ave me nts.

Aggregate Abrasion Value Apart fro m testing ag g reg ate w ith respect to its crushing value, impact resistance, testing the ag g re g ate w ith re spe ct to its re sistance to w e ar is an impo rtant te st fo r ag g re g ate to b e used fo r ro ad co nstructio ns, w are ho use flo o rs and pavement co nstructio n. Three tests are in c o m m o n use to te st ag g re g ate fo r its ab rasio n re sistanc e . (i) De val attritio n te st (ii) Do rry ab rasio n te st (iii) Lo s Ang e ls te st.

Deval Attrition Test In th e De val attritio n te st, p artic le s o f kn o w n w e ig h t are su b je c te d to w e ar in an iro n c ylin d e r ro tate d 1 0 0 0 0 time s at ce rtain sp e e d . The p ro p o rtio n o f material crushed finer than 1.7 mm size is expressed as a p e rc e ntag e o f the o rig inal m ate rial take n. This p e rc e n tag e is take n as th e attritio n valu e o f th e ag g reg ate. This test has b een co vered b y IS 2386 (Part IV) – 1963. But it is po inted o ut that w herever po ssib le Lo s Ang e le s te st sho uld b e use d .

Dorry Abrasion Test Th is te st is n o t c o ve re d b y In d ia n Sta n d a rd Sp e c ific a tio n . Th e te st in vo lve s in su b je c tin g a c ylin d ric al sp e c im e n o f 2 5 c m h e ig h t an d 2 5 c m d iam e te r to the ab rasio n ag ainst ro tating m e tal d isk sp rinkle d w ith q uartz sand . The lo ss in w e ig ht o f the c ylin d e r a fte r 1 0 0 0 re vo lu tio n s o f th e ta b le is d e te rm in e d . Th e h ard e n e ss o f th e ro c k sam p le is e xp re sse d in an e mp irical fo rmula

Los Angeles Abrasion Testing Machine.

Aggregates and Testing of Aggregates !

Hard ne ss = 2 0 –

77

Loss in Grams 3

Go o d ro ck sho uld sho w an ab rasio n value o f no t less than 1 7 . A ro ck sample w ith a value o f le ss than 1 4 w o uld b e co nsid e re d p o o r.

Los Angeles Test Lo s Ang e le s te st w as d e ve lo p e d to o ve rco me so me o f the d e fe cts fo und in De val te st. Lo s Ang e le s te st is characte rise d b y the q uickne ss w ith w hich a samp le o f ag g re g ate may b e te ste d . The ap p licab ility o f the me tho d to all typ e s o f co mmo nly use d ag g re g ate make s this metho d po pular. The test invo lves taking specified q uantity o f standard size material alo ng w ith specified number o f abrasive charg e in a standard cylinder and revo lving if fo r certain specified re vo lutio ns. The p artic le s sm alle r than 1 .7 m m size is se p arate d o ut. The lo ss in w e ig ht e xp re sse d as p e rc e n tag e o f th e o rig in al w e ig h t take n g ive s th e ab rasio n valu e o f th e ag g reg ate. The ab rasio n value sho uld no t b e mo re than 30 per cent fo r w earing surfaces and no t mo re than 5 0 p e r ce nt fo r co ncre te o the r than w e aring surface . Tab le 3 .4 g ive s ave rag e value s o f crushing stre ng th o f ro cks, ag g re g ate crushing value , ab rasio n value , imp act value and attritio n value fo r d iffe re nt ro ck g ro ups.

Modulus of Elasticity Mo dulus o f elasticity o f ag g reg ate depends o n its co mpo sitio n, texture and structure. The mo d ulus o f e lastic ity o f ag g re g ate w ill influe nc e the p ro p e rtie s o f c o nc re te w ith re sp e c t to shrinkag e and elastic behavio ur and to very small extent creep o f co ncrete. Many studies have b e e n c o nd uc te d to inve stig ate the influe nc e o f m o d ulus o f e lastic ity o f ag g re g ate o n the p ro p e rtie s o f co ncre te . O ne o f the stud ie s ind icate d that the ‘E’ o f ag g re g ate has a d e cid e d e ffe ct o n the e lastic p ro p e rty o f co ncre te and that the re latio n o f ‘ E’ o f ag g re g ate to that o f the c o nc re te is no t a line ar func tio n, b ut may b e e xp re sse d as an e q uatio n o f e xp o ne ntial typ e . 3 .4

Ta ble 3 .4 . Ave ra ge Te st Va lue s For Roc k s of Diffe re nt Groups Ro ck Gro up Crushing Streng th MPa

Ag g reg ate crushing value

Ab rasio n value

Impact value

Attritio n value

Specific g ravity

Dry

Wet

Basalt

207

12

1 7 .6

16

3 .3

5 .5

2 .8 5

Flint

214

17

1 9 .2

17

3 .1

2 .5

2 .5 5

Gab b ro

204

-

1 8 .7

19

2 .5

3 .2

2 .9 5

Granite

193

20

1 8 .7

13

2 .9

3 .2

2 .6 9

Gritsto ne

229

12

1 8 .1

15

3 .0

5 .3

2 .6 7

Ho rnfe ls

354

11

1 8 .8

17

2 .7

3 .8

2 .8 8

Lime sto ne

171

24

1 6 .5

9

4 .3

7 .8

2 .6 9

Po rphyry

239

12

1 9 .0

20

2 .6

2 .6

2 .6 6

Q uartizite

339

16

1 8 .9

16

2 .5

3 .0

2 .6 2

Schist

254

-

1 8 .7

13

3 .7

4 .3

2 .7 6

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! Concrete Technology

Bulk Density The b ulk d e nsity o r unit w e ig ht o f an ag g re g ate g ive s valuab le info rmatio ns re g ard ing the shape and g rad ing o f the ag g reg ate. Fo r a g iven specific g ravity the ang ular ag g reg ates sho w a lo w e r b ulk d e nsity. The b ulk d e nsity o f ag g re g ate is me asure d b y filling a co ntaine r o f kno w n vo lume in a standard manner and w eig hing it. Bulk density sho w s ho w densely the ag g re g ate is p ac ke d w he n fille d in a stand ard m anne r. The b ulk d e nsity d e p e nd s o n the p article size d istrib utio n and shap e o f the p article s. O ne o f the e arly me tho d s o f mix d e sig n make use o f this parameter b ulk density in pro po rtio ning o f co ncrete mix. The hig her the b ulk density, the lo w er is the vo id co ntent to be filled by sand and cement. The sample w hich g ives the minimum vo ids o r the o ne w hich g ives maximum b ulk density is taken as the rig ht sample o f ag g reg ate fo r making eco no mical mix. The metho d o f d etermining b ulk d ensity also g ives the me tho d fo r find ing o ut vo id co nte nt in the sample o f ag g re g ate . Fo r determinatio n o f bulk density the ag g reg ates are filled in the co ntainer and then they are c o mp ac te d in a stand ard manne r. The w e ig ht o f the ag g re g ate g ive s the b ulk d e nsity calculated in kg / litre o r kg / m 3 . Kno w ing the specific g ravity o f the ag g reg ate in saturated and surface -d ry co nd itio n, the vo id ratio can also b e calculate d . Pe rce ntag e vo id s = w he re Gs = sp e cific g ravity o f the ag g re g ate

Gs − !γ x 100 Gs and

γ = b ulk d e nsity in kg / litre .

Bulk d e nsity o f ag g re g ate is o f inte re st w he n w e d e al w ith lig ht w e ig ht ag g re g ate and he avy w e ig ht ag g re g ate . The p arame te r o f b ulk d e nsity is also use d in co ncre te mix d e sig n fo r co nve rting the p ro p o rtio ns b y w e ig ht into p ro p o rtio ns b y vo lume w he n w e ig h b atching e q uip me nts is no t availab le at the site .

Specific Gravity In c o n c re te te c h n o lo g y, sp e c ific g ravity o f ag g re g ate s is m ad e u se o f in d e sig n calculatio ns o f co ncrete mixes. With the specific g ravity o f each co nstituent kno w n, its w eig ht can b e co nverted into so lid vo lume and hence a theo retical yield o f co ncrete per unit vo lume can b e calculated . Specific g ravity o f ag g reg ate is also req uired in calculating the co mpacting facto r in co nnectio n w ith the w o rkability measurements. Similarly, specific g ravity o f ag g reg ate is re q uire d to b e c o nsid e re d w he n w e d e al w ith lig ht w e ig ht and he avy w e ig ht c o nc re te . Ave rag e sp e cific g ravity o f the ro cks vary fro m 2 .6 to 2 .8 .

Absorption and Moisture Content So me o f the ag g reg ates are po ro us and abso rptive. Po ro sity and abso rptio n o f ag g reg ate w ill affe c t the w ate r/ c e m e nt ratio and he nc e the w o rkab ility o f c o nc re te . The p o ro sity o f ag g reg ate w ill also affect the durab ility o f co ncrete w hen the co ncrete is sub jected to freezing and thaw ing and also w he n the co ncre te is sub je cte d to che mically ag g re ssive liq uid s. The w ate r ab so rp tio n o f ag g re g ate is d e te rmine d b y me asuring the incre ase in w e ig ht o f an o ve n d ry samp le w he n imme rse d in w ate r fo r 2 4 ho urs. The ratio o f the inc re ase in w e ig ht to the w e ig ht o f the d ry sample e xpre sse d as pe rce ntag e is kno w n as ab so rptio n o f ag g reg ate. But w hen w e d eal w ith ag g reg ates in co ncrete the 2 4 ho urs ab so rptio n may no t b e o f much sig nificance , o n the o the r hand , the p e rce ntag e o f w ate r ab so rp tio n d uring the time interval eq ual o f final set o f cement may b e o f mo re sig nificance. The ag g reg ate ab so rb s w ater in co ncrete and thus affects the w o rkab ility and final vo lume o f co ncrete. The rate and amo unt o f ab so rp tio n w ithin a time inte rval e q ual to the final se t o f the ce me nt w ill o nly b e

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a sig nific ant fac to r rathe r than the 2 4 ho urs ab so rp tio n o f the ag g re g ate . It may b e mo re re alistic to co nsid e r that ab so rptio n capacity o f the ag g re g ate s w hich is g o ing to b e still le ss o w ing to the se aling o f p o re s b y c o ating o f c e m e nt p artic le p artic ularly in ric h m ixe s. In allo w ing fo r e xtra w ate r to b e ad d e d to a co ncre te mix to co mpe nsate fo r the lo ss o f w ate r d ue to ab so rptio n, pro pe r appre ciatio n o f the ab so rptio n in particular time inte rval must b e mad e rathe r than e stimating o n the b asis o f 2 4 ho urs ab so rptio n. In p ro p o rtio ning the m ate rials fo r c o nc re te , it is alw ays take n fo r g rante d that the ag g reg ates are saturated and surface d ry. In mix d esig n calculatio n the relative w eig ht o f the ag g reg ates are b ased o n the co nditio n that the ag g reg ates are saturated and surface dry. But in p ractice , ag g re g ate s in such id e al co nd itio n is rare ly me t w ith. Ag g re g ate s are e ithe r d ry and ab so rp tive to vario us d e g re e s o r the y have surface mo isture . The ag g re g ate s may have b e e n e xp o se d to rain o r may have b e e n w ashe d in w hic h c ase the y may c o ntain surfac e mo isture o r the ag g reg ates may have b een expo sed to the sun fo r a lo ng time in w hich case they are abso rptive. Fine ag g reg ates dredg ed fro m river bed usually co ntains surface mo isture. Whe n stacke d in he ap the to p p o rtio n o f the he ap may b e co mp arative ly d ry, b ut the lo w e r po rtio n o f the heap usually co ntains certain amo unt o f free mo isture. It sho uld b e no ted that if the ag g re g ate s are d ry the y ab so rb w ate r fro m the mixing w ate r and the re b y affe c t the w o rkability and, o n the o ther hand, if the ag g reg ates co ntain surface mo isture they co ntribute e xtra w ate r to the mix and the re b y incre ase the w ate r/ ce me nt ratio . Bo th the se co nd itio ns are harmful fo r the q uality o f c o nc re te . In making q uality c o nc re te , it is ve ry e sse ntial that c o rre c tive me asure s sho uld b e take n b o th fo r ab so rp tio n and fo r fre e mo isture so that the w ate r/ ce me nt ratio is ke p t e xactly as p e r the d e sig n. Very o ften at the site o f co ncrete w o rk w e may meet dry co arse ag g reg ate and mo ist fine ag g re g ate . The ab so rptio n capacity o f the co arse ag g re g ate is o f the o rd e r o f ab o ut 0 .5 to 1 p e r ce nt b y w e ig ht o f ag g re g ate . A hig he r ab so rp tio n value may b e me t w ith ag g re g ate s derived fro m sand sto ne o r o ther so ft and po ro us ro cks. Recently it w as o bserved that the ro cks excavated in the cutting s o f Pune-Mumbai express hig hw ay, sho w ed abso rptio n o f aro und 4% unusualy hig h fo r ro c k o f the typ e De c c an trap . The hig h ab so rp tio n c harac te ristic has p re se nte d p le nty o f p ro b le ms fo r using such sto ne ag g re g ate fo r 4 0 MPa Pave me nt Q uality

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! Concrete Technology

Co ncre te (PQ C). The natural fine ag g re g ate s o fte n co ntain fre e mo isture anything fro m o ne to te n p e r c e n t o r m o re . Fig . 3 .1 sh o w s a d iag ram m atic re p re se n tatio n o f m o isture in ag g re g ate s. Free mo isture in bo th co arse ag g reg ate and fine ag g reg ate affects the q uality o f co ncrete in mo re than o ne w ay. In case o f w e ig h b atching , d e te rminatio n o f fre e mo isture co nte nt o f the ag g re g ate is ne ce ssary and the n co rre ctio n o f w ate r/ ce me nt ratio to b e e ffe cte d in this reg ard . But w hen vo lume b atching is ad o pted , the d eterminatio n o f mo isture co ntent o f fine ag g reg ate d o es no t b eco me necessary b ut the co nseq uent b ulking o f sand and co rrectio n o f vo lume o f sand to g ive allo w ance fo r b ulking b e co me s ne ce ssary.

Bulking of Aggregates Th e fre e m o isture c o n te n t in fin e ag g re g ate re sults in b ulkin g o f vo lum e . Bulkin g p he no me no n can b e e xp laine d as fo llo w s: Fre e m o isture fo rm s a film aro und e ac h p artic le . This film o f m o isture e xe rts w hat is kno w n as surface tensio n w hich keeps the neig hb o uring particles aw ay fro m it. Similarly, the fo rce exerted by surface tensio n keeps every particle aw ay fro m each o ther. Therefo re, no po int co ntact is p o ssib le b e tw e e n the p article s. This cause s b ulking o f the vo lume . The e xte nt o f surface te nsio n and co nse q ue ntly ho w far the ad jace nt p article s are ke p t aw ay w ill d e p e nd up o n the p e rc e ntag e o f m o isture c o nte nt and the p artic le size o f the fine ag g re g ate . It is inte re sting to no te that the b ulking inc re ase s w ith the inc re ase in mo isture c o nte nt up to a c e rtain lim it and b e yo nd that the furthe r inc re ase in the m o isture c o nte nt re sults in the d e c re ase in the vo lum e and at a m o isture c o nte nt re p re se nting saturatio n p o int, the fine ag g re g ate sho w s no b ulking . It c an b e se e n fro m Fig . 3 .2 that fine sand b ulks m o re and co arse sand bulks less. Fro m this it fo llo w s that the co arse ag g reg ate also bulks but the bulking is so little that it is alw ays ne g le cte d . Extre me ly fine sand and p articularly the manufacture d fine ag g re g ate b ulks as much as ab o ut 4 0 p e r ce nt.

Aggregates and Testing of Aggregates !

81

Due to the b ulking , fine ag g re g ate sho w s co mp le te ly unre alistic vo lume . The re fo re , it is abso lutely necessary that co nsideratio n must be g iven to the effect o f bulking in pro po rtio ning the co ncrete b y vo lume. If co g nisance is no t g iven to the effect o f b ulking , in case o f vo lume b atching , the re sulting co ncre te is like ly to b e und e rsand e d and harsh. It w ill also affe ct the yie ld o f co ncre te fo r a g ive n ce me nt co nte nt. The e xte nt o f b ulking c an b e e stim ate d b y a sim p le fie ld te st. A sam p le o f m o ist fine ag g re g ate is fille d into a me asuring cylind e r in the no rmal manne r. No te d o w n the le ve l, say h 1 . Po ur w ate r into the me asuring cylind e r and co mp le te ly inund ate the sand and shake it. Since the vo lume o f the saturated sand is the same as that o f the dry sand, the inundated sand co mp le te ly o ffse ts the b ulking e ffe ct. No te d o w n the le ve l o f the sand say, h 2 . The n h 1 – h 2 sho w s the b ulking o f the sample o f sand und e r te st. Pe rce ntag e o f b ulking =

h1 − h2 x 100 h2

In a similar w ay the b ulking facto r can b e fo und o ut b y filling the w e t sand in a w ate r tig ht me asuring b o x (farma) up to the to p and the n p o ur w ate r to inund ate the sand . The n me asure the sub sid e nce o f sand and e xp re ss it as a p e rce ntag e . This g ive s a mo re re alistic p icture o f the b ulking facto r. Th e fie ld te st to fin d o ut th e p e rc e n tag e o f b ulkin g is so sim p le th at th is c o uld b e co nd ucted in a very sho rt time interval and the percentag e o f b ulking so fo und o ut co uld b e e mp lo ye d fo r co rre cting the vo lume o f fine ag g re g ate to b e use d . This can b e co nsid e re d as o ne o f the im p o rtant m e tho d s o f fie ld c o ntro l to p ro d uc e q uality c o nc re te . Sinc e vo lum e b atc hing is no t ad o p te d fo r c o ntro lle d c o nc re te , the d e te rm inatio n o f the p e rc e ntag e o f mo isture co ntent is no t no rmally req uired . The q uantity o f w ater co uld b e co ntro lled b y visual examinatio n o f the mix and by experience. The percentag e o f free mo isture co ntent is req uired to b e d etermined and co rrectio n mad e o nly w hen w eig h b atching is ad o pted fo r pro d uctio n o f q uality co ncre te .

Measurement of Moisture Content of Aggregates De te rminatio n o f mo isture co nte nt in ag g re g ate is o f vital impo rtance in the co ntro l o f the q uality o f co ncrete particularly w ith respect to w o rkab ility and streng th. The measurement o f the mo isture co ntent o f ag g reg ates is basically a very simple o peratio n. But it is co mplicated b y se ve ral fac to rs. The ag g re g ate w ill ab so rb a c e rtain q uantity o f w ate r d e p e nd ing o n its p o ro sity. The w ate r co nte nt can b e e xp re sse d in te rms o f the w e ig ht o f the ag g re g ate w he n ab so lutely d ry, surface d ry o r w hen w et. Water co ntent means the free w ater, o r that held o n the surface o f the ag g re g ate o r the to tal w ate r co nte nt w hich includ e s the ab so rb e d w ate r p lus the fre e w ate r, o r the w ate r he ld in the inte rio r p o rtio n o f ag g re g ate p article s. The m e asure m e nt o f the m o isture c o nte nt o f ag g re g ate in the fie ld m ust b e q uic k, reaso nab ly accurate and must req uire o nly simple appartus w hich can b e easily hand led and use d in the fie ld . So me o f the me tho d s that are b e ing use d fo r d e te rminatio n o f mo isture co nte nt o f ag g re g ate are g ive n b e lo w : (i ) Drying Me tho d

(ii ) Disp lace me nt Me tho d

(iii ) Calcium Carb id e Me tho d

(iv ) Me asure me nt b y e le ctrical me te r.

(v ) Auto matic me asure me nt

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! Concrete Technology

Drying Method The applicatio n o f d rying metho d is fairly simple. Drying is carried o ut in a o ven and the lo ss in w e ig ht b e fo re and afte r d rying w ill g ive the mo isture co nte nt o f the ag g re g ate . If the d rying is do ne co mpletely at a hig h temperature fo r a lo ng time, the lo ss in w eig ht w ill include no t o nly the surfac e w ate r b ut also so me ab so rb e d w ate r. Ap p ro p riate c o rre c tio ns may b e mad e fo r the saturate d and surface d ry co nd itio n. The o ve n d rying me tho d is to o slo w fo r fie ld use . A fairly q uick re sult can b e o b taine d b y he ating the ag g re g ate q uickly in an o p e n p an. The p ro ce ss can also b e sp e e d e d up b y p o uring inflammab le liq uid such as me thylate d sp irit o r ace to ne o ve r the ag g re g ate and ig niting it.

Displacement Method In the lab o rato ry the mo isture c o nte nt o f ag g re g ate c an b e d e te rmine d b y me ans o f p ycno me te r o r b y using Sip ho n-Can Me tho d . The p rincip le mad e use o f is that the sp e cific g ravity o f no rm al ag g re g ate is hig he r than that o f w ate r and that a g ive n w e ig ht o f w e t ag g re g ate w ill o ccup y a g re ate r vo lume than the same w e ig ht o f the ag g re g ate w he n d ry. By kno w ing the specific g ravity o f the dry ag g reg ate, the specific g ravity o f the w et ag g reg ate c an b e c alc ulate d . Fro m the d iffe re nc e b e tw e e n the sp e c ific g ravitie s o f the d ry and w e t ag g re g ate s, the mo isture co nte nt o f the ag g re g ate can b e calculate d .

Calcium Carbide Method A q uic k and re aso nab ly ac c urate me tho d o f d e te rmining the mo isture c o nte nt o f fine ag g re g ate is to mix it w ith an e xce ss o f calcium carb id e in a stro ng air-tig ht ve sse l fitte d w ith p re ssure g aug e . Calcium carb id e re acts w ith surface mo isture in the ag g re g ate to p ro d uce ace tyle ne g as. The p re ssure o f ace tyle ne g as g e ne rate d d e p e nd s up o n the mo isture co nte nt o f th e ag g re g ate s. Th e p re ssu re g au g e is c alib rate d b y takin g a m e asu re d q u an tity o f ag g re g ate o f kno w n mo isture co nte nt and the n such a calib rate d p re ssure g aug e co uld b e used to read the mo isture co ntent o f ag g reg ate d irectly. This metho d is o fen used to find o ut the mo isture co nte nt o f fine ag g re g ate at the site o f w o rk. The e q uipme nt co nsists o f a small b alance, a standard sco o p and a co ntainer fixed w ith dial g aug e. The pro cedure is as fo llo w s: Weig h 6 g rams o f representative sample o f w et sand and po ur it into the co ntainer. Take o ne sc o o p full o f c alc ium c arb id e p o w d e r and p ut it into the c o ntaine r. Clo se the lid o f the co ntainer and shake it rig o ro usly. Calcium carb id e reacts w ith surface mo isture and pro d uces ace tyle ne g as, the p re ssure o f w hich d rive s the ind icato r ne e d le o n the p re ssure g aug e . The p re ssure g aug e is so calib rate d , that it g ive s d ire ctly p e rce ntag e o f mo isture . The w ho le jo b takes o nly less than 5 minutes and as such, this test can b e do ne at very clo se intervals o f time at the site o f w o rk.

Electrical Meter Method Recently electrical meters have b een d evelo ped to measure instantaneo us o r co ntinuo us re ad ing o f the m o isture c o nte nt o f the ag g re g ate . The p rinc ip le that the re sistanc e g e ts chang e d w ith the chang e in mo isture co nte nt o f the ag g re g ate has b e e n mad e use o f. In so me so phisticated b atching plant, electrical meters are used to find o ut the mo isture co ntent and also to re g ulate the q uantity o f w ate r to b e ad d e d to the co ntinuo us mixe r.

Automatic Measurement In mo d e rn b atching p lants surface mo isture in ag g re g ate s is auto matically re co rd e d b y means o f so me kind o f senso r arrang ement. The arrang ement is made in such a w ay that the q uantity o f free w ater g o ing w ith ag g reg ate is auto matically reco rded and simultaneo usly that

Aggregates and Testing of Aggregates !

83

much q uantity o f w ate r is re d uce d . This so phisticate d me tho d re sults in an accuracy o f ± 0 .2 to 0 .6 % .

Cleanliness The co ncrete ag g reg ates sho uld b e free fro m impurities and deletrio us sub stances w hich are likely to interfere w ith the pro cess o f hyd ratio n, preventio n o f effective b o nd b etw een the ag g re g ate s and matrix. The imp uritie s so me time s re d uce the d urab ility o f the ag g re g ate . Ge ne rally, the fine ag g re g ate o b taine d fro m natural so urce s is like ly to co ntain o rg anic imp uritie s in the fo rm o f silt and clay. The manufacture d fine ag g re g ate d o e s no t no rmally c o ntain o rg anic m ate rials. But it m ay c o ntain e xc e ss o f fine c rushe d sto ne d ust. Co arse ag g re g ate stacke d in the o p e n and unuse d fo r lo ng time may co ntain mo ss and mud in the lo w e r le ve l o f the stack. Sand is no rmally d re d g e d fro m rive r b e d s and stre ams in the d ry se aso n w he n the rive r b e d is d ry o r w he n the re is no t much flo w in the rive r. Und e r such situatio n alo ng w ith the sand, decayed veg etable matter, humus, o rg anic matter and o ther impurities are likely to settle d o w n. But if sand is d re d g e d w he n the re is a g o o d flo w o f w ate r fro m ve ry d e e p b e d , the o rg anic matte rs are like ly to g e t w ashe d aw ay at the time o f d re d g ing . The o rg anic matte rs w ill interfere w ith the setting actio n o f cement and also interfere w ith the b o nd characteristics w ith the ag g re g ate s. The p re se nce o f mo ss o r alg ae w ill also re sult in e ntrainme nt o f air in the co ncre te w hich re d uce s its stre ng th. To ascertain w hether a sample o f fine ag g reg ate co ntains permissib le q uantity o f o rg anic impurities o r no t, a simple test kno w n as co lo rimetric test is made. The sample o f sand is mixed w ith a liq uid c o ntaining 3 p e r c e nt so lutio n o f so d ium hyd ro xid e in w ate r. It is ke p t fo r 2 4 ho urs and the co lo ur d evelo ped is co mpared w ith a stand ard co lo ur card . If the co lo ur o f the sample is d arke r than the stand ard co lo ur card , it is infe rre d that the co nte nt o f the o rg anic impurities in the sand is mo re than the permissib le limit. In that case either the sand is rejected o r is use d afte r w ashing . So me time s e xce ssive silt and clay co ntaine d in the fine o r co arse ag g re g ate may re sult in increased shrinkag e o r increased permeab ility in ad d itio n to po o r b o nd characteristics. The e xce ssive silt and clay may also ne ce ssitate g re ate r w ate r re q uire me nts fo r g ive n w o rkab ility. The q uantity o f clay, fine silt and fine d ust are d e te rmine d b y se d ime ntatio n me tho d . In this m e tho d , a sam p le o f ag g re g ate is p o ure d into a g rad uate d m e asuring jar and the ag g re g ate is nic e ly ro d d e d to d islo d g e p artic le s o f c lay and silt ad he ring to the ag g re g ate p article s. The jar w ith the liq uid is co mp le te ly shake n so that all the clay and silt p article s g e t mixe d w ith w ate r and the n the w ho le jar is ke p t in an und isturb e d co nd itio n. Afte r a ce rtain tim e inte rval, the thic kne ss o f the laye r o f c lay and silt stand ing o ve r the fine ag g re g ate p artic le s w ill g ive a fair id e a o f the p e rc e ntag e o f c lay and silt c o nte nt in the sam p le o f ag g re g ate und e r te st. The limits o f d e le te rio us mate rials as g ive n in IS 3 8 3 -1 9 7 0 are sho w n in Tab le 3 .5 . Fine ag g re g ate fro m tid al rive r o r fro m p its ne ar se a sho re w ill g e ne rally co ntain so me pe rce ntag e o f salt. The co ntaminatio n o f ag g re g ate s b y salt w ill affe ct the se tting pro pe rtie s and ultimate stre ng th o f co ncre te . Salt b e ing hyg ro sco p ic, w ill also cause e fflo re sce nce and unsig htly ap p e arance . O p inio ns are d ivid e d o n the q ue stio n w he the r the salt co ntaine d in ag g reg ates w o uld cause co rro sio n o f reinfo rcement. But studies have indicated that the usual p e rce ntag e o f salt g e ne rally co ntaine d in the fine ag g re g ate w ill no t cause co rro sio n in any ap p re ciab le manne r. Ho w e ve r, it is a g o o d p ractice to w ash sand co ntaining salt mo re than 3 pe r ce nt.

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Ta ble 3 .5 . Lim it s of De le t e rious M a t e ria ls (I S: 3 8 3 -1 9 7 0 ) Sr. No .

Deleterio us sub stances

(1 )

(2 )

Metho d o f test (3 )

Fine ag g reg ate percentag e b y w eig ht , max uncrushed crushed (4 ) (5 )

Co arse ag g reg ate percentag e b y w eig ht, max uncrushed crushed (6 ) (7 )

(i )

Co al and lig nite

IS:2 3 8 6 (Part II)1963

1 .0 0

1 .0 0

1 .0 0

1 .0 0

(ii )

Clay lump s

IS:2 3 8 6 (Part II)1963

1 .0 0

1 .0 0

1 .0 0

1 .0 0

(iii)

Mate rials fine r than 7 5 -micro n IS Sie ve

IS:2 3 8 6 (Part I)1963

3 .0 0

1 5 .0 0

3 .0 0

3 .0 0

(iv )

So ft frag me nts

IS:2 3 8 6 (Part II)1963

-

-

3 .0 0

-

(v )

Shale

IS:2 3 8 6 (Part II)1963

1 .0 0

-

-

-

(vi)

To tal o f pe rce ntag e s o f all d e le te rio us mate rials (e xce p t mica) includ ing Sr. No . (i) to (v) fo r co l. 4 , 6 and 7 and Sr. No . (i) and (ii) fo r co l. 5 o nly

-

5 .0 0

2 .0 0

5 .0 0

5 .0 0

Notes: (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 in accordance with IS:2386 (Part II-1963) in sufficient quantities to effect adversely the strength or durability of concrete. A fine aggregate which fails in the test for 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)-1963 is not less than 95 per cent.

Ag g reg ates fro m so me so urce may co ntain iro n pyrites, clay no d ules, so ft shale particles and o ther impurities w hich are likely to sw ell w hen w etted . These particles also g et w o rn o ut w hen co ncrete is sub jected to ab rasio n and thereb y cause pitting in co ncrete. Such unso und particles cause d amag e to the co ncrete particularly, w hen sub jected to alternate freezing and thaw ing o r w etting and drying . A limitatio n to the q uantity o f such impurities is already sho w n in Tab le 3 .5 .

Aggregates and Testing of Aggregates !

85

Soundness of Aggregate So und ne ss re fe rs to the ab ility o f ag g re g ate to re sist e xce ssive chang e s in vo lume as a re sult o f chang e s in p hysical co nd itio ns. The se p hysical co nd itio ns that affe ct the so und ne ss o f ag g re g ate are the fre e zing the thaw ing , variatio n in te mp e rature , alte rnate w e tting and d rying und e r no rmal co nd itio ns and w e tting and d rying in salt w ate r. Ag g re g ate s w hich are po ro us, w eak and co ntaining any und esirab le extraneo us matters und erg o excessive vo lume chang e w hen sub jected to the ab o ve co nd itio ns. Ag g reg ates w hich und erg o mo re than the spe cifie d amo unt o f vo lume chang e is said to b e unso und ag g re g ate s. If co ncre te is liab le to b e e xpo se d to the actio n o f fro st, the co arse and fine ag g re g ate w hich are g o ing to b e use d sho uld b e sub je cte d to so und ne ss te st. The so undness test co nsists o f alternative immersio n o f carefully g raded and w eig hed test samp le in a so lutio n o f so d ium o r mag ne sium sulp hate and o ve n d rying it und e r sp e cifie d co nditio ns. The accumulatio n and g ro w th o f salt crystals in the po res o f the particles is tho ug ht to pro d uce d isruptive internal fo rces similar to the actio n o f freezing o f w ater o r crystallisatio n o f salt. Lo ss in w eig ht, is measured fo r a specified number o f cycles. So undness test is specified in IS 2 3 8 6 (Part V). As a g e ne ral g uid e , it can b e take n that the ave rag e lo ss o f w e ig ht afte r 1 0 cycles sho uld no t exceed 1 2 per cent and 1 8 per cent w hen tested w ith so d ium sulphate and mag ne sium sulp hate re sp e ctive ly. It m ay b e p o in te d o u t th at th e su lp h ate so u n d n e ss te st m ig h t b e u se d to ac c e p t ag g re g ate s b u t n o t to re je c t th e m , th e assu m p tio n b e in g th at ag g re g ate s w h ic h w ill satisfacto rily w ithstand the te st are g o o d w hile tho se w hich b re akd o w n may o r may no t b e b ad . Unfo rtunate ly, the te st is no t re liab le . Ce rtain ag g re g ate s w ith e xtre m e ly fine p o re struc ture sho w alm o st no lo ss o f w e ig ht. Co nve rse ly, c e rtain ag g re g ate s that d isinte g rate re ad ily in the sulp hate te st b ut p ro d uce co ncre te o f hig h re sistance to fre e zing and thaw ing . A lo w lo ss o f w e ig ht usually. b ut no t alw ays, an e vid e nce o f g o o d d urab ility, w he re as a hig h lo ss o f w e ig ht p lace s the ag g re g ate in q ue stio nab le cate g o ry.

Alkali Aggregate Reaction Fo r a lo n g tim e ag g re g ate s h ave b e e n c o n sid e re d as in e rt m ate rials b ut late r o n , particularly, after 1940’s it w as clearly bro ug ht o ut that the ag g reg ates are no t fully inert. So me o f the ag g re g ate s c o ntain re ac tive silic a, w hic h re ac ts w ith alkalie s p re se nt in c e me nt i.e . , so d ium o xid e and p o tassium o xid e . In th e Un ite d State s o f Ame ric a it w as fo und fo r the first time that many failure s o f c o n c re te stru c tu re s like pavement, piers and sea w alls c o u ld b e a ttrib u te d to th e a lka li-a g g re g a te re a c tio n . Since the n a syste matic stud y has b e e n mad e in this re g ard and no w it is p ro ve d b e yo nd d o u b t th at c e rtain typ e s o f re a c tive a g g re g a te s a re re sp o n sib le fo r p ro m o tin g Typical Alkali - Aggregate reaction. Alkali silicate gels of unlimited swelling type are formed under favourable conditions. alkali-ag g re g ate re actio n.

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The types o f ro cks w hich co ntain reactive co nstituents includ e traps, and esites, rhyo lites, silice o us lime sto ne s and ce rtain typ e s o f sand sto ne s. The re active co nstitue nts may b e in the fo rm o f o pals, che rts, chalce d o ny, vo lcanic g lass, ze o lite s e tc. The re actio n starts w ith attack o n the reactive siliceo us minerals in the ag g reg ate b y the alkaline hyd ro xid e d erived fro m the alkalie s in ce me nt. As a re sult, the alkali silicate g e ls o f unlimite d sw e lling typ e are fo rme d . When the co nd itio ns are co ng enial, pro g ressive manifestatio n b y sw elling takes place, w hich re sults in d isruptio n o f co ncre te w ith the spre ad ing o f patte rn cracks and e ve ntual failure o f c o nc re te struc ture s. The rate o f d e te rio ratio n m ay b e slo w o r fast d e p e nd ing up o n the co nditio ns. There w ere cases w here co ncrete has beco me unserviceable in abo ut a year’s time. In Ind ia, the b asalt ro c ks o c c urring in the De c c an p late au, Mad hya Prad e sh, Kathiaw ar, Hyd e rab ad , Punc hal Hill (Jam m u and Kashm ir), Be ng al and Bihar sho uld b e vie w e d w ith cautio n. 3 .6 Similarly, lime sto ne s and d o lo mite s co ntaining che rt no d ule s w o uld b e hig hly re active . Ind ian lim e sto ne s o f Bijaw ar se rie s are kno w n to b e hig hly c he rty. Re g io ns o f o c c urre nc e includ e Mad hya Prad e sh, Rajasthan, Punjab and Assam. Sandsto nes co ntaining silica minerals like chalcedo ny, crypto to micro crystalline q uartz o r o pal are fo und to b e reactive. Reg io ns o f o ccurrence includ e Mad hya Prad esh, Beng al, Bihar and De lhi. So me o f the samp le s o b taine d fro m Mad hya Prad e sh, We st Be ng al and Kashmir w e re fo un d to b e c o n tain in g re ac tive c o n stitue n ts w h ic h c o uld b e id e n tifie d b y visual e xam in atio n . Th e se c o n tain su b stan tial q u a n titie s o f m in e ra ls like o p a ls, c h a lc e d o n y a n d a m o rp h o u s silic a . Q uartzite sam p le s o f ro c k o b taine d fro m Kash m ir w e re also fo u n d to b e h ig h ly re active . G e o g ra p h ic a lly In d ia h a s a ve ry e xte nsive d e p o sit o f vo lc anic ro c ks. The Deccan traps co vering the w estern part o f Mah arash tra an d Mad h ya Prad e sh , th e do lo mites o f Madhya Pradesh, Punjab and Ra ja sth a n , lim e sto n e s o f Ja m m u a n d Kashm ir w o uld fo rm e xte nsive so urc e o f Typical example of the alkali-aggregate reaction ag g re g ate fo r co ncre te co nstructio n. The product (swellable gel). ag g re g ate s fro m th e se ro c ks sh o uld b e stud ie d c autio usly to se e ho w far re ac tive are the y. It is inte re sting to no te that o nly suc h ag g reg ates w hich co ntain reactive silica in particular pro po rtio n and in particular fineness are fo und to exhib it tend encies fo r alkali-ag g reg ates reactio n. It is po ssib le to red uce its tend ency b y alte ring e ithe r the p ro p o rtio n o f re active silica o r its fine ne ss.

Factors Promoting the Alkali-Aggreate Reaction (i ) Re active type o f ag g re g ate ;

(ii ) Hig h alkali co nte nt in ce me nt;

(iii ) Availab ility o f mo isture ;

(iv ) O p timum te mp e rature co nd itio ns.

It is no t e asy to d e te rmine the p o te ntial re activity o f the ag g re g ate s. The case histo ry o f ag g reg ates may be o f value in judg ing w hether a particular so urce o f ag g reg ate is deleterio us o r harmle ss. The p e tro g rap hic e xaminatio n o f thin ro ck se ctio ns may also imme nse ly he lp to asses the po tential reactivity o f the ag g reg ate. This test o ften re q uires to b e supplemented b y o the r te sts.

Aggregates and Testing of Aggregates !

87

Mo rtar Bar Exp ansio n Te st d e vise d b y Stanto n has p ro ve d to b e a ve ry re liab le te st in asse ssing the re activity o r o the rw ise o f the ag g re g ate . A sp e cime n o f size 2 5 mm x 2 5 mm and 2 5 0 mm le ng th is cast, cure d and sto re d in a stand ard manne r as sp e cifie d in IS : 2 3 8 6 (Part VII 1 9 6 3 ). Me asure the le ng th o f the sp e cime n p e rio d ically, at the ag e s o f 1 , 2 , 3 , 6 , 9 , and 1 2 mo nths. Find o ut the d iffe re nce in the le ng th o f the sp e cime n to the ne are st 0 .0 0 1 p e r ce nt and re co rd the e xp ansio n o f the sp e cime n. The ag g re g ate und e r te st is co nsid e re d harmful if it expand s mo re than 0 .0 5 per cent after 3 mo nths o r mo re than 0 .1 per cent after six mo nths. The p o te ntial re activity o f ag g re g ate can also b e fo und o ut b y che mical me tho d . In this me tho d the p o te ntial re activity o f an ag g re g ate w ith alkalie s in Po rtland ce me nt is ind icate d b y the amo unt o f re actio n taking p lace d uring 2 4 ho urs at 8 0 ° C b e tw e e n so d ium hyd ro xid e so lutio n and the ag g re g ate that has b e e n crushe d and sie ve d to p ass a 3 0 0 micro n IS Sie ve and retained o n 150 micro n IS Sieve. The so lutio n after 24 ho urs is analysed fo r silica disso lved and re d uc tio n in alkalinity, b o th e xp re sse d as millimo le s p e r litre . The value s are p lo tte d as sho w n in Fig 3.3 repro duced fro m IS : 2386 (Part VII 1963). Generally, a po tentially deleterio us re actio n is ind icate d if the plo tte d te st re sult falls to the rig ht o f the b o und ary line o f Fig . 3 .3 and if plo tted result falls to the left side o f the bo undary line, the ag g reg ate may be co nsidered as inno cuo us. The ab o ve chemical test may also b e emplo yed fo r finding o ut the effectiveness o f adding a particular pro po rtio n o f po zzo lanic material to o ffset the alkali-ag g reg ate reactio n. Tab le 3 .6 sho w s d isso lve d silica and re d uctio n in alkalinity o f so me Ind ian ag g re g ate s.

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High Alkali Content in Cement The hig h alkali co ntent in cement is o ne o f the mo st impo rtant facto rs co ntributing to the alkali-ag g re g ate re actio n. Since the time o f re co g nitio n to the imp o rtance o f alkali-ag g re g ate re actio n p he no me na, a se rio us vie w has b e e n take n o n the alkali co nte nt o f ce me nt. Many specificatio ns restrict the alkali co ntent to less than 0.6 per cent. Their to tal amo unt, expressed as Na 2 O e q uivale nt (Na 2 O + 0 .6 5 8 K2 O ). A ce me nt, me e ting this sp e cificatio n is d e sig nate d as a lo w alkali ce me nt. Fie ld e xp e rie nce has ne ve r d e te cte d se rio us d e te rio ratio n o f co ncre te thro ug h the pro cess o f alkali-ag g reg ate reactio n w hen cement co ntained alkalies less than 0.6 p e r ce nt. In e xce p tio nal case s, ho w e ve r, ce me nt w ith e ve n lo w e r alkali co nte nt have cause d o b je ctio nab le e xp ansio n. Ge ne rally, Ind ian ce me nts d o no t co ntain hig h alkalie s as in U.S.A. and U.K. The result o f investig atio ns do ne to find o ut the alkali co ntent in the sample o f Indian cement is sho w n in Tab le 3.7. Tab le 3.7 sho w s that 11 o ut o f 26 Indian cement samples have to tal alkali co ntent hig her than 0.6 per cent. This is the statistics o f cement manufactured prio r to 1 9 7 5 . The pre se nt d ay ce me nt manufacture d b y mo d e rn so phisticate d me tho d s w ill have lo w e r alkali co nte nt than w hat is sho w n in Tab le 3 .7 .

Ta ble 3 .6 . Dissolve d silic a a nd re duc t ion in a lk a linit y of som e I ndia n a ggre gat e s 3 .5 Sl. No .

Descriptio n o f Ag g reg ate

Red uctio n in Alkalinity millimo les per litre — Ra

Disso lved Silica Millimo les Per litre Si

Ratio Si/ Ra

16

42

2 .6 3

1.

Banihal Trap

2.

De ccan Trap (Nag p ur)

320

177

0 .5 5

3.

Rajmahal Trap (We st Be ng al)

252

210

0 .8 3

4.

Banihal Q uartizite

16

57

3 .5 6

5.

Lime sto ne

24

Nil

Nil

6.

Lime sto ne (Balaso re )

96

Nil

Nil

7.

Lime sto ne (Bansw ara)

32

Nil

Nil

8.

Enno re Sand

4

Nil

Nil

9.

Gre ve lly Sand No . 1 (Banihal)

68

Nil

Nil

10.

Grave lly Sand No . 2 (Banihal)

100

Nil

Nil

11.

Pyre x Glass

160

926

5 .7 9

12.

O p al

120

721

6 .0 1

Ta ble 3 .7 . Alk a li Cont e nt in I ndia n Ce m e nt No . o f samples

Alkali co ntent per cent

Percentag e o f numb er o f sample to the to tal

8

Be lo w – 0 .4 0

3 0 .8

7

0 .4 0 – 0 .6 0

2 6 .9

5

0 .6 0 – 0 .8 0

1 9 .2

0 .8 0 – 1 .0 0

2 3 .1

6 Nil

Ab o ve – 1 .0 0

Nil

Aggregates and Testing of Aggregates !

89

Availability of Moisture Pro g ress o f chemical reactio ns invo lving alkali-ag g reg ate reactio n in co ncrete req uires the p re se nc e o f w ate r. It has b e e n se e n in the fie ld and lab o rato ry that lac k o f w ate r g re atly re d uce s this kind o f d e te rio ratio n. The re fo re , it is p e rtine nt to no te that d e te rio ratio n d ue to alkali-ag g reg ate reactio n w ill no t o ccur in the interio r o f mass co ncrete. The d eterio ratio n w ill b e mo re o n the surface. It is sug g ested that red uctio n in d eterio ratio n d ue to alkali-ag g reg ate re ac tio n c an b e ac hie ve d b y the ap p lic atio n o f w ate rp ro o fing ag e nts to the surfac e o f the co ncre te w ith a vie w to p re ve nting ad d itio nal p e ne tratio n o f w ate r into the structure .

Temperature Condition The id e al te mp e rature fo r the p ro mo tio n o f alkali-ag g re g ate re actio n is in the rang e o f 1 0 to 3 8 ° C. If the te mp e rature s c o nd itio n is mo re than o r le ss than the ab o ve , it may no t p ro vid e an id e al situatio n fo r the alkali-ag g re g ate re actio n.

Mechanism of Deterioration of Concrete Through the Alkali-Aggregate Reaction The mechanism o f alkali ag g reg ate reactio n has no t been perfectly understo o d. Ho w ever, fro m the kno w n info rmatio n, the me chanism o f d e te rio ratio n is e xplaine d as fo llo w s: The mixing w ate r turns to b e a stro ng ly caustic so lutio n d ue to so lub ility o f alkalie s fro m the cement. This caustic liq uid attacks reactive silica to fo rm alkali-silica g el o f unlimited sw elling type. The reactio n pro ceed s mo re rapid ly fo r hig hly reactive sub stances. If co ntinuo us supply o f w ater and co rrect temperature is availab le, the fo rmatio n o f silica g el co ntinues unab ated . This silic a g e ls g ro w in size . The c o ntinuo us g ro w th o f silic a g e l e xe rts o smo tic p re ssure to cause pattern cracking particularly in thinner sectio ns o f co ncrete like pavements. Co nspicuo us e ffe ct may no t b e se e n in mass co ncre te se ctio ns. The fo rmatio n o f p atte rn c rac ks d ue to the stre ss ind uc e d b y the g ro w th o f silic a g e l re sults in sub se q ue nt lo ss in stre ng th and e lasticity. Alkali-ag g re g ate re actio n also acce le rate s o ther pro cess o f deterio ratio n o f co ncrete due to the fo rmatio n o f cracks. So lutio n o f disso lved carb o n d io xid e , co nve rts calcium hyd ro xid e to calcium carb o nate w ith co nse q ue nt incre ase in vo lume . Many d e struc tive fo rc e s b e c o me o p e rative o n the c o nc re te d isrup te d b y alkaliag g re g ate re actio n w hich w ill furthe r haste n the to tal d isinte g ratio n o f co ncre te .

Control of Alkali-Aggregate Reaction Fro m th e fo re g o in g d isc ussio n it is ap p are n t th at alkali-ag g re g ate re ac tio n c an b e co ntro lle d b y the fo llo w ing me tho d s: (i )

Se le ctio n o f no n-re active ag g re g ate s;

(ii ) By the use o f lo w alkali ce me nt; (iii ) By the use o f co rre ctive ad mixture s such as p o zzo lanas; (iv ) By co ntro lling the vo id sp ace in co ncre te ; (v ) By co ntro lling mo isture co nd itio n and te mp e rature . It has b e e n d isc usse d that it is p o ssib le to id e ntify p o te ntially re ac tive ag g re g ate b y p e tro g rap hic e xaminatio n, mo rtar b ar te st o r b y che mical me tho d . Avo id ing the use o f the re ac tive ag g re g ate is o ne o f the sure m e tho d s to inhib it the alkali-ag g re g ate re ac tio n in co ncre te . In c ase avo id anc e o f susp ic io us re ac tive ag g re g ate is no t p o ssib le d ue to e c o no m ic re aso ns, the p o ssib ility o f alkali-ag g re g ate re actio n can b e avo id e d b y the use o f lo w alkali ce me nt. re stricting the alkali co nte nt in ce me nt to le ss than 0 .6 pe r ce nt o r po ssib ly le ss than 0 .4 pe r ce nt, is ano the r g o o d ste p.

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In the co nstructio n o f nucle ar po w e r pro je ct at Kaig a in Karnataka, initially, the y d id no t make p ro p e r inve stig atio n ab o ut the c o arse ag g re g ate the y are like ly to use in the p o w e r p ro je c t. W he n the y inve stig ate d , the y fo und that the c o arse ag g re g ate w as sho w ing a te nd e nc y fo r alkali-ag g re g ate re ac tio n. The y c o uld no t c hang e the so urc e fo r e c o no m ic al reaso n. Therefo re, they have g o ne fo r using , special lo w -alkali cement. w ith alkali co ntent less than 0 .4 pe r ce nt. It has b een po inted o ut earlier that g enerally the ag g reg ate is fo und to b e reactive w hen it co ntains silica in a p articular p ro p o rtio n and in p articular fine ne ss. It has b e e n se e n in the lab o rato ry that if this o ptimium co nd itio n o f silica b e ing in particular pro po rtio n and fine ne ss is d isturb e d , the ag g re g ate w ill turn to b e inno cuo us. This d isturb ance o f o p timum co nte nt and fineness o f silica can b e disturb ed b y the additio n o f po zzo lanic materials such as crushed sto ne d ust, d iato mac e o us e arth, fly ash o r surkhi. The use o f p o zzo lanic mixture has b e e n fo und to b e o ne o f practical so lutio ns fo r inhib iting alkali-ag g re g ate re actio n. It has b e e n said that d e ve lo p m e nt o f o sm o tic p re ssure o n the se t-c e m e nt g e l b y the sub se q ue ntly fo rme d alkali-silica g e l is re sp o nsib le fo r the d isrup tio n o f co ncre te . If a syste m is intro d uc e d to ab so rb this o sm o tic p re ssure , it is p ro b ab le that the d isrup tio n c o uld b e re d uce d . The use o f air-e ntraining ag e nt has fre q ue ntly b e e n re co mme nd e d as a me ans o f ab so rb ing the o smo tic pressure and co ntro lling expansio n d ue to alkali-ag g reg ate reactio n in mo rtar and co ncre te . Fo r the g ro w th o f silica g el a co ntinuo us availab ility o f w ater is o ne o f the re q uirements. If such co ntinuo us sup p ly is no t mad e availab le , the g ro w th o f silica g e l is re d uce d . Similarly, if the co rre ct rang e o f te mp e rature is no t p ro vid e d , the e xte nt o f e xp ansio n is also re d uce d .

Thermal Properties Ro c k an d ag g re g ate p o sse sse s th re e th e rm al p ro p e rtie s w h ic h are sig n ific an t in e stab lishing the q uality o f ag g re g ate fo r co ncre te co nstructio ns. The y are : (i ) Co e fficie nt o f e xpansio n;

(ii ) Sp e cific he at;

(iii ) The rmal co nd uctivity.

O ut o f the se , sp e c ific he at and c o nd uc tivity are fo und to b e im p o rtant o nly in m ass co ncrete co nstructio n w here rig o ro us co ntro l o f temperature is necessary. Also these pro perties are o f co nse q ue nce in case o f lig ht w e ig ht co ncre te use d fo r insulatio n p urp o se . Whe n w e are d e aling w ith the ag g re g ate in g e ne ral it w ill b e sufficie nt at this stag e to d e al w ith o nly the co efficient o f expansio n o f the ag g reg ate, since it interacts w ith the co efficient o f thermal e xp ansio n o f ce me nt p aste in the b o d y o f the se t-co ncre te . An averag e value o f the linear thermal co efficient o f expansio n o f co ncrete may b e taken as 9 .9 x 1 0 –6 pe r ° C, b ut the rang e may b e fro m ab o ut 5 .8 x 1 0 –6 pe r ° C to 1 4 x 1 0 –6 p e r ° C d e p e nd ing up o n the typ e and q uantitie s o f the ag g re g ate s, the mix p ro p o rtio ns and o the r facto rs. The rang e o f co e fficie nt o f the rmal e xp ansio n fo r hyd rate d ce me nt p aste may vary fro m 1 0 .8 x 1 0 –6 Pe r ° C to 1 6 .2 x 1 0 –6 p e r ° C. Sim ilarly, fo r m o rtar it m ay ran g e fro m 7 .9 x 1 0 –6 pe r ° C to 1 2 .6 x 1 0 –6 per ° C. Th e lin e ar th e rm al c o e ffic ie n t o f e xp an sio n o f c o m m o n ro c ks ran g e s fro m ab o u t 0 .9 x 1 0 –6 p e r ° C to 1 6 x 1 0 –6 p e r ° C. Fro m the ab o ve it co uld b e se e n that w hile the re is thermal co mpatib ility b etw een the ag g reg ate and co ncrete o r ag g reg ate and paste at hig her rang e, there exists thermal inco mpatibility betw een ag g reg ate and co ncrete o r ag g reg ate and p aste at the lo w e r rang e . This the rmal inco mp atib ility b e tw e e n the ag g re g ate and co ncre te at the lo w e r rang e cause s se ve re stre ss w hich has g o t d amag ing e ffe ct o n the d urab ility and inte g rity o f co ncre te structure s.

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Man y re se arc h w o rke rs h ave stu d ie d th e in te rac tio n o f ag g re g ate s w ith d iffe re n t co efficient o f thermal expansio n w ith that o f co ncrete. The result o f the experiments d o es no t p re se nt a ve ry cle ar cut p icture o f the e ffe cts that may b e e xp e cte d , and so me asp e cts o f the p ro b le m are co ntro ve rsial. Ho w e ve r, the re se e ms to b e a fairly g e ne ral ag re e me nt that the the rmal e xp ansio n o f the ag g re g ate has an e ffe ct o n the d urab ility o f co ncre te , p articularly und e r se ve re e xp o sure co nd itio ns o r und e r rap id te mp e rature chang e s. Ge ne rally, it can b e take n that, w he re the d iffe re nce b e tw e e n co e fficie nt o f e xp ansio n o f co arse ag g re g ate and m o rtar is larg e r, the d urab ility o f the c o nc re te m ay b e c o nsid e rab ly lo w e r than w o uld b e pred icted fro m the results o f the usual acceptance tests. Where the d ifference b etw een these c o e ffic ie n ts e xc e e d s 5 .4 x 1 0 –6 p e r ° C c au tio n sh o u ld b e take n in th e se le c tio n o f th e ag g re g ate fo r hig hly d urab le co ncre te . If a p articular co ncre te is sub je cte d to no rmal variatio n o f atmo sp he ric te mp e rature , the the rmal inco mp atib ility b e tw e e n the ag g re g ate s and p aste o r b e tw e e n the ag g re g ate and matrix may no t intro d uce se rio us d iffe re ntial mo ve me nt and b re ak the b o nd at the inte rface o f ag g reg ate and paste o r ag g reg ate and matrix. But if a co ncrete is sub jected to hig h rang e o f temperature d ifference the ad verse effect w ill b eco me acute. If q uartz is used as ag g reg ate fo r co ncrete that is g o ing to be subjected to hig h temperature the co ncrete is sure to underg o d isrup tio n as q uartz chang e s state and sud d e nly e xp and s 0 .8 5 p e r ce nt at a te mp e rature o f 572.7°C. It is also necessary to take care o f the peculiar aniso tro pic b ehavio r i.e. , the pro perty o f e xpand ing mo re in o ne d ire ctio n o r paralle l to o ne crystallo g raphic axis than ano the r. The m o st n o tab le e xam p le is c alc ite w h ic h h as a lin e ar th e rm al c o e ffic ie n t e xp an sio n o f 2 5 .8 x 1 0 –6 per °C parallel to its axis and – 4.7 x 10 –6 per °C perpend icular to this d irectio n 3.7 . Po tash fe ld spars are ano the r g ro up o f mine rals e xhib iting aniso tro pic b e havio ur. The re fo re , in e stimating the cub ical e xp ansio n o f co ncre te , care must b e take n to this aspect o f aniso tro pic b ehavio ur o f so me o f the ag g reg ates. The study o f co efficient o f thermal e xp ansio n o f ag g re g ate is also imp o rtant, in d e aling w ith the fire re sistance o f co ncre te .

Grading of Aggregates Ag g reg ate co mprises ab o ut 5 5 per cent o f the vo lume o f mo rtar and ab o ut 8 5 per cent vo lume o f mass co ncrete. Mo rtar co ntains ag g reg ate o f size o f 4.75 mm and co ncrete co ntains ag g re g ate upto a maximum size o f 1 5 0 mm. Thus it is no t surp rising that the w ay p artic le s o f ag g re g ate fit to g e the r in the mix, as influe nc e d b y the g rad atio n, shap e , and surfac e te xture , has an im p o rtant e ffe c t o n the w o rkab ility and finishing characte ristic o f fre sh co ncre te , co nse q ue ntly o n the p ro p e rtie s o f hard e ne d co ncre te . Vo lume s have b e e n w ritte n o n the e ffe cts o f the ag g re g ate g rad ing o n the p ro p e rtie s o f co ncre te and many so calle d “id e al” g rad ing curve s have b e e n p ro p o se d . In spite o f this extensive stud y, w e still d o no t have a clear picture o f the influence o f d ifferent type s o f ag g re g ate s o n the plastic pro pe rtie s o f co ncre te . It has b e e n this much und e rsto o d that there is no thing like “ideal” ag g reg ate g rading , because satisfacto ry co ncrete can be made w ith vario us ag g re g ate g rad ing s w ithin ce rtain limits. It is w e ll kno w n that the stre ng th o f c o nc re te is d e p e nd e nt up o n w ate r/ c e me nt ratio p ro vid e d the c o nc re te is w o rkab le . In this state m e nt, the q ualifying c lause “p ro vid e d the c o n c re te is w o rkab le ” assu m e s fu ll im p o rtan c e . O n e o f th e m o st im p o rtan t fac to rs fo r p ro d ucing w o rkab le co ncre te is g o o d g rad atio n o f ag g re g ate s. Go o d g rad ing imp lie s that a sample o f ag g reg ates co ntains all standard fractio ns o f ag g reg ate in req uired pro po rtio n such that the sample co ntains minimum vo id s. A sample o f the w e ll g rad e d ag g re g ate co ntaining minimum vo id s w ill re q uire minimum p aste to fill up the vo id s in the ag g re g ate s. Minimum

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p aste w ill me an le ss q uantity o f ce me nt and le ss q uantity o f w ate r, w hich w ill furthe r me an incre ase d e co no my, hig he r stre ng th, lo w e r-shrinkag e and g re ate r d urab ility. The ad vantag e s d ue to g o o d g rad ing o f ag g re g ate s can also b e vie w e d fro m ano the r ang le . If co ncre te is vie w e d as a tw o phase mate rial, paste phase and ag g re g ate phase , it is the p aste p hase w hic h is vulne rab le to all ills o f c o nc re te . Paste is w e ake r than ave rag e ag g re g ate in no rmal co ncre te w ith rare e xce p tio ns w he n ve ry so ft ag g re g ate s are use d . The paste is mo re permeable than many o f the mineral ag g reg ates. It is the paste that is susceptible to d eterio ratio n b y the attack o f ag g ressive chemicals. In sho rt, it is the paste w hich is a w eak link in a mass o f co ncrete. The lesser the q uantity o f such w eak material, the b etter w ill b e the c o nc re te . This o b je c tive c an b e ac hie ve d b y having w e ll g rad e d ag g re g ate s. He nc e the imp o rtance o f g o o d g rad ing . Many re se arch w o rke rs in the fie ld o f co ncre te te chno lo g y, having fully und e rsto o d the imp o rtanc e o f g o o d g rad ing in making q uality c o nc re te in c o nsiste nt w ith e c o no my, have d ire cte d the ir stud ie s to achie ve g o o d g rad ing o f ag g re g ate at the co nstructio n site . Fulle r and Tho mp so n 3 .8 co nclud e d that g rad ing fo r maximum d e nsity g ive s the hig he st stre ng th, and that the g rad ing curve o f the b e st mixture re se mb le s a p arab o la. Talb o t and Richart fro m the ir w o rks fo und that ag g re g ate g rad e d to p ro d uce maximum d e nsity g ave a harsh mixture that is ve ry d ifficult to p lace in o rd inary co ncre ting o p e ratio ns. Ed w ard s and Yo ung p ro p o se d a me tho d o f p ro p o rtio ning b ase d o n the surface are a o f ag g re g ate to b e w e tte d . O the r thing s b e ing e q ual, it w as co nclud e d that the co ncre te mad e fro m ag g re g ate g rad ing having le ast surfac e are a w ill re q uire le ast w ate r w hic h w ill c o nse q ue ntly b e the stro ng e st. Ab rams and o thers in co urse o f their investig atio ns have also fo und that the surface area o f th e ag g re g ate m ay vary w id e ly w itho ut c ausing m uc h ap p re c iab le d iffe re nc e in the co ncrete streng th, and that w ater req uired to pro duce a g iven co nsistency is dependent mo re o n o the r characte ristics o f ag g re g ate than o n surface are a. The re fo re , Ab rams intro d uce d a p arame te r kno w n as “fine ne ss mo d ulus” fo r arriving at satisfacto ry g rad ing s. He fo und that any sieve analysis curve o f ag g reg ate that w ill g ive the same fineness mo dulus w ill req uire the same q uantity o f w ater to pro duce a mix o f the same plasticity and g ives co ncrete o f the same streng th, so lo ng as it is no t to o co arse fo r the q uantity o f cement used. The fineness mo dulus is an ind e x o f the c o arse ne ss o r fine ne ss o f an ag g re g ate sam p le , b ut, b e c ause d iffe re nt g rad ing can g ive the same fine ne ss mo d ulus, it d o e s no t d e fine the g rad ing . W aym o u th in tro d u c e d h is th e o ry o f satisfac to ry g rad in g o n th e b asis o f “p artic le interference” co nsideratio ns3.9 . He fo und o ut the vo lume relatio nships betw een successive size g ro up s o f p artic le s b ase d o n the assum p tio n that p artic le s o f e ac h g ro up are d istrib ute d thro ug ho ut the co ncre te mass in such a w ay that the d istance b e tw e e n the m is e q ual to the mean d iameter o f the particles o f the next smaller size g ro up plus the thickness o f the cement film betw een them. He stated that particle interference o ccurred betw een tw o successive sizes w he n the d istanc e b e tw e e n p artic le s is no t suffic ie nt to allo w fre e p assag e o f the sm alle r particles. The determinatio n o f g rading by Waymo uth metho d usually results in finer g rading s. Many o ther metho ds have b een sug g ested fo r arriving at an o ptimum g rading . All these pro ce d ure s, me tho d s and fo rmulae po int to the fact that no ne is satisfacto ry and re liab le fo r fie ld applicatio n. At the site , a re liab le satisfacto ry g rad ing can o nly b e d e cid e d b y actual trial and erro r, w hich takes into co nsid eratio n the characteristics o f the lo cal materials w ith respect to size frac tio n, shap e , surfac e te xture , flakine ss ind e x and e lo ng atio n ind e x. The w id e ly varying p e culiaritie s o f co arse and fine ag g re g ate s canno t b e b ro ug ht und e r fo rmulae and se t pro ce d ure fo r practical applicatio n.

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O ne o f the practical metho ds o f arriving at the practical g rading by trial and erro r metho d is to mix ag g reg ates o f d ifferent size fractio ns in d ifferent percentag es and to cho o se the o ne sam p le w hic h g ive s m axim um w e ig ht o r m inim um vo id s p e r unit vo lum e , o ut o f all the alternative samples. Fractio ns w hich are actually available in the field, o r w hich co uld be made availab le in the fie ld includ ing that o f the fine ag g re g ate w ill b e use d in making samp le s.

Sieve Analysis This is the name g ive n to the o p e ratio n o f d ivid ing a samp le o f ag g re g ate into vario us frac tio ns e ac h c o nsisting o f p artic le s o f the sam e size . The sie ve analysis is c o nd uc te d to d e te rmine the p article size d istrib utio n in a samp le o f ag g re g ate , w hich w e call g rad atio n. A c o n ve n ie n t syste m o f e xp re ssin g th e g ra d a tio n o f a g g re g a te is o n e w h ic h th e co nsecutive sieve o pening s are co nstantly d o ub led , such as 1 0 mm, 2 0 mm, 4 0 mm e tc. Und e r such a syste m, e mplo ying a lo g arithmic scale , line s can b e spaced at eq ual intervals to represent the successive size s. The ag g re g ate s use d fo r making co ncre te are no rmally o f the maximum size 8 0 mm, 4 0 mm, 2 0 mm, 10 mm, 4.75 mm, 2.36 mm, 600 micro n, 300 m ic ro n and 1 5 0 m ic ro n. The ag g re g ate frac tio n fro m 8 0 m m to 4 .7 5 m m are te rm e d as c o arse ag g reg ate and tho se fractio n fro m 4.75 mm to 150 micro n are te rme d as fine ag g re g ate . The size 4 .7 5 mm is a co mmo n fractio n appearing b o th in co arse ag g re g ate and fine ag g re g ate (C.A. and F.A.). Grad ing p atte rn o f a samp le o f C.A. o r F.A. is assessed by sieving a sample successively thro ug h all the sie ve s mo unte d o ne o ve r the o the r in o rd e r o f size , w ith larg e r sie ve o n th e to p . Th e m ate rial Set of Sieves assembled for retained o n each sieve after shaking , represents the conducting Sieve analysis. frac tio n o f ag g re g ate c o arse r th an th e sie ve in q uestio n and finer than the sieve abo ve. Sieving can be do ne either manually o r mechanically. In the manual o peratio n the sieve is shaken g iving mo vements in all po ssib le d irectio n to g ive

Set of Sieves.

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chance to all particle s fo r passing thro ug h the sie ve . O pe ratio n sho uld b e co ntinue d till such time that almo st no p article is p assing thro ug h. Me chanical d e vice s are actually d e sig ne d to g ive mo tio n in all po ssible directio n, and as such, it is mo re systematic and efficient than handsieving . Fo r assessing the g rad atio n b y sieve analysis, the q uantity o f materials to b e taken o n the sie ve is g ive n Tab le 3 .8 .

Ta ble 3 .8 . M inim um w e ight of sa m ple for Sieve Ana lysis (I S: 2 3 8 6 (Pa r t I ) – 1 9 6 3 ) Maximum size present in sub stantial pro po rtio ns

Minimum w eig ht o f sample to b e taken fo r sieving

mm

kg

63

50

50

35

4 0 o r 3 1 .5

15

25

5

20 o r 16

2

1 2 .5

1

10

0 .5

6 .3

0 .2

4 .7 5

0 .2

2 .3 6

0 .1

Fro m the sieve analysis the particle size distributio n in a sample o f ag g reg ate is fo und o ut. In this co nne ctio n a te rm kno w n as “Fine ne ss Mo d ulus” (F.M.) is b e ing use d . F.M. is a re ad y ind e x o f c o arse ne ss o r fine ne ss o f the m ate rial. Fine ne ss m o d ulus is an e m p iric al fac to r o b tain e d b y ad d in g th e c um ulative p e rc e n tag e s o f ag g re g ate re tain e d o n e ac h o f th e stand ard sie ve s rang ing fro m 8 0 mm to 1 5 0 mic ro n and d ivid ing this sum b y an arb itrary numb er 100. The larg er the fig ure, the co arser is the material. Tab le No . 3.9 sho w s the typical example o f the sieve analysis, co nducted o n a sample o f co arse ag g reg ate and fine ag g reg ate to find o ut the fine ne ss mo d ulus. Many a tim e , fine ag g re g ate s are d e sig nate d as c o arse sand , m e d ium sand and fine sand . The se classificatio ns d o no t g ive any p re cise me aning . What the sup p lie r te rms as fine sand may b e re ally me d ium o r e ve n co arse sand . To avo id this amb ig uity fine ne ss mo d ulus co uld b e use d as a yard stick to ind icate the fine ne ss o f sand . The fo llo w ing limits may b e take n as g uid ance : Fine sand

:

Fine ne ss Mo d ulus

:

2 .2 - 2 .6

Me d ium sand

:

F.M.

:

2 .6 - 2 .9

Co arse sand

:

F.M.

:

2 .9 - 3 .2

A san d h avin g a fin e n e ss m o d u lu s m o re th an 3 .2 w ill b e u n su itab le fo r m akin g satisfacto ry co ncre te .

Combining Aggregates to Obtain Specified Gradings So me time s ag g re g ate s availab le at site s may no t b e o f spe cifie d o r d e sirab le g rad ing . In such cases tw o o r mo re ag g reg ates fro m different so urces may be co mbined to g et the desired

-

-

-

-

-

1 5 kg

1 .1 8 mm

6 0 0 micro n

3 0 0 micro n

1 5 0 micro n

lo w e r than 1 5 0 micro n

To tal

5

1 0 mm

-

6

2 0 mm

2 .3 6 mm

0

4 0 mm

4 .0

0

8 0 mm

4 .7 5 mm

We ig ht re taine d w e ig ht kg

IS Sie ve Size

F.M. =

7 1 3 .3

-

100

100

100

100

100

100

7 3 .3

40

0

0

Cumulative p e rce ntag e re taine d

713.3 = 7 .1 3 3 100

-

-

-

-

-

-

1 5 .0 0

11

6

0

0

Cumulative w e ig ht re taine d kg

Co arse Ag g re g ate

00

00

00

00

00

00

00

2 6 .7

60

100

100

Cumulative p e rce ntag e p assing

500 g m

35

85

175

95

50

50

10

0

-

-

-

We ig ht re taine d gm

F.M. =

246 = 2 .4 6 100

—

500

465

380

205

110

60

10

0

-

-

-

Cumulative w e ig ht re taine d g m

246

-

93

76

41

22

12

2

0

-

-

-

Cumulative p e rce ntag e w e ig ht re taine d

Fine Ag g re g ate

Ta ble 3 .9 . T he t ypic a l Ex a m ple of t he Sieve Ana lysis

-

7

24

59

78

88

98

100

-

-

-

Co mulative p e rce ntag e p assing

Aggregates and Testing of Aggregates !

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g rad in g . O fte n , m ixin g o f availab le fin e ag g re g ate w ith availab le c o arse ag g re g ate in ap p ro p riate p e rc e ntag e s m ay p ro d uc e d e sirab le g rad ing s. But so m e tim e s tw o o r m o re fractio ns o f co arse ag g reg ate is mixed first and then the co mb ined co arse ag g reg ate is mixed w ith fin e ag g re g ate to o b tain th e d e sire d g rad in g s. Kn o w in g th e g rad in g o f availab le ag g re g ate s, p ro p o rtio ns o f m ixing d iffe re nt size s c an b e c alc ulate d , e ithe r g rap hic ally o r arithme tically. This asp e ct w ill b e d e alt in mo re d e tail und e r the chap te r Mix De sig n. At this stag e a simp le trial and e rro r arithme tical me tho d o f co mb ining co arse and fine ag g re g ate is illustrate d . Tab le 3 .1 0 sho w s the g rad ing p atte rn o f the availab le co arse and fine ag g re g ate at site . This tab le also sho w s the sp e cifie d co mb ine d g rad ing . Tab le 3 .1 1 sho w s the g rad ing o f d ifferent co mb inatio n o f fine and co arse ag g reg ate fo r first trial and seco nd trial. The co mbined g rading o f first trial and seco nd trial is co mpared w ith the specified co mbined g rading . Whichever trial g ives the co mbined ag g reg ate g rading eq ual o r ne arly e q ual to the sp e cifie d g rad ing is ad o p te d .

Specific Surface and Surface Index The imp o rtance o f a g o o d g rad ing o f the co arse and fine ag g re g ate has alre ad y b e e n d iscusse d . The q uantity o f w ate r re q uire d to p ro d uce a g ive n w o rkab ility d e p e nd s to a larg e e xte nt o n the surface are a o f the ag g re g ate .

Ta ble 3 .1 0 . Show s t he gra ding pa t t e r n of t he ava ila ble c oa rse a nd fine a ggre ga t e a nd spe c ifie d c om bine d gra ding I.S. Sieve size

Percentag e passing C.A.

F.A.

Specified co mb ined ag g reg ate (Fro m Tab le No . 3 .1 1 )

40

100

100

100

20

96

100

98

10

35

100

61

4 .7 5

6

92

42

2 .3 6

0

85

35

1 .1 8

0

75

28

600

0

60

22

300

0

10

5

150

0

0

0

The surface are a p e r unit w e ig ht o f the mate rial is te rme d as sp e cific surface . This is an indirect measure o f the ag g reg ate g rading . Specific surface increases w ith the reductio n in the size o f ag g re g ate p article so that fine ag g re g ate co ntrib ute s ve ry much mo re to the surface area than do es the co arse ag g reg ate. Greater surface area req uires mo re w ater fo r lub ricating the mix to g ive w o rkab ility. The w o rkab ility o f a mix is, the re fo re , influe nc e d mo re b y fine r fractio n than the co arse r particle s in a sample o f ag g re g ate s. Th e fo re g o in g p arag rap h g ive s th e im p re ssio n th at sm alle r p artic le s o f ag g re g ate c o ntrib ute m o re surfac e are a and he nc e re q uire m o re w ate r fo r w e tting the surfac e o f ag g re g ate s; and fo r a g ive n q uantity o f w ate r, the p re se nce o f smalle r p article s re d uce s the w o rkab ility. This imp re ssio n is co rre ct up to a ce rtain e xte nt o f the fine r fractio n. This w ill no t ho ld g o o d fo r very fine particles in F.A. The every fine particles in F.A. i.e. , 300 micro n and 150 micro n particles, being so fine, co ntribute mo re to w ards w o rkability. Their o ver-riding influence

0

150

0

10

60

75

85

92

100

100

100

3

F.A.

0

5

22

28

35

42

61

98

100

4

co mb ined g rad ing

Specified

-

-

-

-

-

4 .2

2 4 .5

6 7 .2

70

5

70% C.A.

-

3 .0 0

1 8 .0 0

2 2 .5

2 5 .5

2 7 .6

30

30

30

6

30% F.A.

1 st Trial

-

3 .0 0

1 8 .0 0

2 2 .5

2 5 .5

3 1 .8

5 4 .5

9 7 .2

100

7

Co mb ined g rad ing

-

-

-

-

3 .6

2 1 .0

5 7 .6

60

60

8

60% C.A.

-

4 .0

2 4 .0

3 0 .0

3 4 .0

3 6 .8

40

40

40

9

40% F.A.

2 nd Trial

-

4 .0

2 4 .0

3 0 .0

3 4 .0

4 0 .6

6 1 .0

9 7 .6

100

10

Co mb ined g rad ing

It is se e n that the c o mb ine d g rad ing o b taine d b y the mixture o f C.A. and F.A. in the ratio o f 6 0 :4 0 c lo se ly c o nfo rm the sp e c ifie d g rad ing sho w n in c o lumn 4 .

0

300

0

1 .1 8

0

0

2 .3 6

35

10

6

96

20

4 .7 5

100

40

2

C.A.

600

1

Sieve

I.S.

Pe rc e ntag e p assing

Ta ble 3 .1 1 . Show ing t he Gra ding of Diffe re nt Com binat ion of Fine a nd Coa rse Aggre gat e for Diffe re nt Tria ls

Aggregates and Testing of Aggregates !

97

98

! Concrete Technology

in c o ntrib uting to the b e tte r w o rkab ility b y ac ting like b all b e aring s to re d uc e the inte rnal fric tio n b e tw e e n c o arse p artic le s, far o ut-w e ig h the re d uc tio n in w o rkab ility o w ing to the co nsump tio n o f mixing w ate r fo r w e tting g re ate r surface are a. Co nsid eratio n o f specific surface g ives a so mew hat mislead ing picture o f the w o rkab ility to b e expected . To o verco me this d ifficulty Murd o ck has sug g ested the use o f “Surface Ind ex” w h ic h is an e m p iric al n um b e r re late d to th e sp e c ific surfac e o f th e p artic le w ith m o re w eig htag e g iven to the finer fractio ns. The empirical numb ers representing the surface ind ex o f ag g re g ate particle s w ithin the se t o f sie ve size are g ive n in Tab le 3 .1 2 . The to tal surfac e ind e x (fx) o f a mixture o f ag g re g ate is c alc ulate d b y multip lying the p e rce ntag e o f mate rial re taine d o n its sie ve b y the co rre sp o nd ing surface ind e x and to the ir sum is ad d ed a co nstant o f 3 3 0 and the result is d ivid ed b y 1 0 0 0 . The metho d o f co mputing the to tal surface fo r any g ive n g rad ing is sho w n in Tab le 3 .1 3

Ta ble 3 .1 2 . Surfa c e I ndex of Aggre gat e Pa r t icle s 3 .2 Sieve size w ithin w hich particles lie

Surface Ind ex fo r Particles w ithin Sieve Size ind icated

8 0 –4 0 mm

– 2 .5

4 0 –2 0 mm

–2

2 0 –1 0 mm

–1

1 0 –4 .7 5 mm

+ 1

4 .7 5 –2 .3 6 mm

4

2 .3 6 –1 .1 8 mm

7

1 .1 8 –6 0 0 micro n

9

6 0 0 –3 0 0 micro n

9

3 0 0 –1 5 0 micro n

7

Smalle r than 1 5 0 micro n

2

Ta ble 3 .1 3 . Surfa c e I ndex of Com bine d Gra ding Sieve size w ithin w hich Particles lie

Percentag e o f particles w ithin sieve size

Surface Ind ex fo r particles w ithin sieve size

Surface Ind ex (fx)

2 0 — 1 0 mm

55

–1

–5 5

1 0 — 4 .7 5 mm

15

1

15

4 .7 5 — 2 .3 6 mm

7

4

28

2 .3 6 — 1 .1 8 mm

7

7

49

1 .1 8 — 6 0 0 micro n

7

9

63

6 0 0 — 3 0 0 micro n

7

9

63

3 0 0 — 1 5 0 micro n

2

7

14

To tal

177

Ad d co nstant

330 507

Aggregates and Testing of Aggregates !

Surface Ind e x (fx) =

99

507 = 0 .5 0 7 1000

Similarly, surface ind e x can b e calculate d fo r stand ard g rad ing curve , and this value o f surface ind e x can b e take n as the d e sirab le surface ind e x o f the co mb ine d ag g re g ate . This parameter o f surface index can be made use o f fo r finding o ut the pro po rtio n o f fine ag g re g ate to co arse ag g re g ate availab le in the fie ld to o b tain sp e cifie d o r d e sirab le surface ind e x in the fo llo w ing w ay. Let x = surface ind e x o f fine ag g re g ate

y = surface ind e x o f co arse ag g re g ate z = surface ind e x o f co mb ine d ag g re g ate a = p ro p o rtio n o f fine to co arse ag g re g ate The n a =

(z − y) ( x − z)

The fo llo w ing e xam p le w ill sho w ho w to c o m b ine the availab le fine ag g re g ate w ith availab le co arse ag g re g ate w ho se g rad ing p atte rns are kno w n to g e t the d e sirab le surface ind e x o f the co mb ine d ag g re g ate . The d e sirab le surface ind e x o f the co mb ine d ag g re g ate co uld b e calculate d fro m the g rad ing p atte rn o f the stand ard g rad ing curve .

Sieve size w ithin w hich particles lie

Percentag e o f particles w ithin sieve size

Surface ind ex fo r partcles fo r sieve size

Surface Ind ex (fx)

Co arse Ag g re g ate 2 0 mm — 1 0 mm

65

–1

1 0 mm — 4 .7 5

35

1

–6 5 35 To tal = –3 0 Ad d co nstant = 3 3 0 = 300

Surface Ind e x o f Co rase Ag g re g ate = Fine Ag g re g ate 4 .7 5 mm — 2 .3 6 2 .3 6 mm — 1 .1 8 1 .1 8 mm — 6 0 0 micro n 6 0 0 micro n — 3 0 0 micro n 3 0 0 micro n — 1 5 0 micro n

10 20 20 30 15

300 = 0 .3 0 1000 4 7 9 9 7

40 140 180 270 105 To tal = 7 3 5 Ad d co nstant = 3 3 0 = 1065

Surface Ind e x o f fine Ag g re g ate =

1065 = 1 .0 6 5 1000

100

! Concrete Technology

Le t the surface ind e x o f co mb ine d ag g re g ate re q uire d = 0 .6 .

x = surface ind e x o f F.A.

= 1 .0 6 5

y = surface ind e x o f C.A.

= 0 .3 0

z = surface ind e x o f co mb ine d ag g re g ate = 0 .6 0 If a = pro po rtio n o f fine to co arse ag g re g ate ,

a=

(z − y) (0.60 − 0.30) 1 = = 155 . ( x − z) (1065 . − 0.60)

The re fo re , F.A. : C.A. = 1 : 1 .5 5

Standard Grading Curve The g rad ing patterns o f ag g reg ate can b e sho w n in tab les o r charts. Expressing g rad ing limits b y me ans o f a chart g ive s a g o o d p icto rial vie w. The co mp ariso n o f g rad ing patte rn o f a number o f samples can be made at o ne g lance. Fo r this reaso n, o ften g rading o f ag g reg ates is sho w n b y me ans o f g rad ing curve s. O ne o f the mo st co mmo nly re fe rre d practical g rad ing c urve s are tho se p ro d uc e d b y Ro ad Re se arc h Lab o rato ry (U.K.). 3 .1 0 O n the b asis o f larg e number o f experiments in co nnectio n w ith bring ing o ut mix desig n pro cedure, Ro ad Research

Lab o rato ry has pre pare d a se t o f type g rad ing curve fo r all-in ag g re g ate s g rad e d d o w n fro m 2 0 mm and 4 0 mm. The y are sho w n in fig ure 3 .4 and Fig 3 .5 re sp e ctive ly. Similar curve s fo r ag g re g ate w ith maximum size o f 1 0 mm and d o w nw ard have b e e n p re p are d b y McInto sh and Ernto ry. It is sho w n in Fig . 3 .6 . Fig . 3 .7 sho w s the d e sirab le g rad ing limit fo r 8 0 mm ag g re g ate . Fo ur curve s are sho w n fo r e ach maximum size o f ag g re g ate e xce p t 8 0 mm size . Fro m value s o f p e rc e ntag e p assing it c an b e se e n that the lo w e st c urve i.e . , c urve No . 1 is the co arsest g rading and curve No . 4 at the to p represents the finest g rading . Betw een the curves No . 1 to 4 the re are thre e zo ne s: A, B, C. In p rac tic e the c o arse and fine ag g re g ate s are sup p lie d se p arate ly. Kno w ing the ir g rad atio n it w ill b e p o ssib le to mix the m up to g e t typ e g rad ing co nfo rming to any o ne o f the fo ur g rad ing curve s.

Aggregates and Testing of Aggregates !

101

In practice , it is d ifficult to g e t the ag g re g ate to co nfo rm to any o ne particular stand ard curve exactly. If the user insists o n a particular pattern o f g rading , the supplier may q uo te very hig h rate s. At the same time the use r also canno t acce pt ab so lute ly po o r g rad ing patte rn o f ag g re g ate s. As a via me d ia, g rad ing limits are laid d o w n in vario us sp e cificatio ns rathe r than to co nfo rm exactly to a particular g rading curve. Table 3.14 sho w s the g rading limits o f co arse ag g re g ate s. Tab le 3 .1 5 sho w s the g rad ing limits o f fine ag g re g ate s. Tab le 3 .1 6 sho w s the g rad ing limits o f all-in-ag g re g ate . It sho uld b e no ted that fo r crushed sto ne sand s, the permissib le limit o n 1 5 0 micro n I.S. Sie ve is incre ase d to 2 0 p e r ce nt. Fig s. 3 .8 a , b , c and d sho w the g rad ing limits o f F.A. Fine ag g re g ate co mp lying w ith the re q uire me nts o f any g rad ing zo ne in Tab le 3 .1 5 is suitab le fo r c o nc re te b ut the q uality o f c o nc re te p ro d uc e d w ill d e p e nd up o n a numb e r o f facto rs includ ing pro po rtio ns.

102

! Concrete Technology

Aggregates and Testing of Aggregates !

103

W h e re c o n c re te o f h ig h stre n g th an d g o o d d u rab ility is re q u ire d , fin e ag g re g ate co nfo rming to any o ne o f the fo ur g rad ing zo ne s may b e use d , b ut the co ncre te mix sho uld b e pro perly d esig ned . As the fine ag g reg ate g rad ing b eco mes pro g ressively finer, that is fro m G rad in g Zo n e s I to IV, th e ratio o f th e fin e ag g re g ate to c o arse ag g re g ate sh o u ld b e pro g ressively red uced . The mo st suitab le fine to co arse ratio to b e used fo r any particular mix w ill, ho w e ve r, d e p e nd up o n the actual g rad ing , p article shap e and surface te xture o f b o th fine and co arse ag g re g ate s. It is re co mme nd e d that fine ag g re g ate co nfo rming to Grad ing Zo ne IV sho uld no t b e used in reinfo rced co ncrete unless tests have been made to ascertain the suitability o f pro po sed mix pro po rtio ns.

100

8 5 –1 0 0

0 –3 0

0 –5

–

–

–

–

–

6 3 mm

4 0 mm

2 0 mm

1 6 mm

1 2 .5 mm

1 0 mm

4 .7 5 mm

2 .3 6 mm

6 3 mm

8 0 mm

IS Sieve Desig natio n

–

–

0 –5

–

–

0 –2 0

8 5 –1 0 0

100

–

4 0 mm

–

0 –5

0 –2 0

–

–

8 5 –1 0 0

100

–

–

2 0 mm

–

0 –5

0 –3 0

–

8 5 –1 0 0

100

–

–

–

1 6 mm

–

0 –1 0

0 –4 5

8 5 –1 0 0

100

–

–

–

–

1 2 .5 mm

Percentag e passing fo r sing le-sized ag g reg ate no minal size (b y w eig ht)

0 –5

0 –2 0

8 5 –1 0 0

100

–

–

–

–

–

1 0 mm

–

0 –5

1 0 –3 5

–

–

3 0 –7 0

9 5 –1 0 0

–

100

4 0 mm

–

0 –1 0

2 5 –5 5

–

–

9 5 –1 0 0

100

–

–

2 0 mm

–

0 –1 0

3 0 –7 0

–

9 0 –1 0 0

100

–

–

–

1 6 mm

–

0 –1 0

4 0 –8 5

9 0 –1 0 0

–

100

–

–

–

1 2 .5 mm

Percentag e passing fo r Grad ed ag g reg ate o f no minal size (b y w eig ht)

Ta ble 3 .1 4 . Gra ding Lim it s for Coa rse Aggre ga t e I S: 3 8 3 -1 9 7 0

104 ! Concrete Technology

Aggregates and Testing of Aggregates !

105

Ta ble 3 .1 5 . Gra ding lim it s of fine a ggre ga t e s I S: 3 8 3 -1 9 7 0 Percentag e passing b y w eig ht fo r I.S. Sieve Desig natio n 1 0 mm

Grad ing Zo ne I

Grad ing Zo ne II

Grad ing Zo ne III

Grad ing Zo ne IV

100

100

100

100

4 .7 5 mm

9 0 –1 0 0

9 0 –1 0 0

9 0 –1 0 0

9 5 –1 0 0

2 .3 6 mm

6 0 –9 5

7 5 –1 0 0

8 5 –1 0 0

9 5 –1 0 0

1 .1 8 mm

3 0 –7 0

5 5 –9 0

7 5 –1 0 0

9 0 –1 0 0

6 0 0 micro n

1 5 –3 4

3 5 –5 9

6 0 –7 9

8 0 –1 0 0

3 0 0 micro n

5 –2 0

8 –3 0

1 2 –4 0

1 5 –5 0

1 5 0 micro n

0 –1 0

0 –1 0

0 –1 0

0 –1 5

Ta ble 3 .1 6 . Gra ding lim it s of a ll-in-a ggre ga t e s I.S. Sieve Desig natio n

Percentag e b y w eig hts passing fo r all in-ag g rag rate o f 4 0 mm No minal size

2 0 mm No minal size

8 0 mm

100

–

4 0 mm

9 5 –1 0 0

100

2 0 mm

4 5 –7 5

9 5 –1 0 0

4 .7 5 mm

2 5 –4 5

3 0 –5 0

6 0 0 micro n

8 –3 0

1 0 –3 5

1 5 0 micro n

0 –6

0 –6

It must b e re me mb e re d that the g rad ing o f fine ag g re g ate s has much g re ate r e ffe ct o n w o rkability o f co ncrete than do es the g rading o f co arse ag g reg ate. Experience has sho w n that usually ve ry co arse sand o r ve ry fine sand is unsatisfacto ry fo r co ncre te making . The co arse sand results in harshness bleeding and seg reg atio n, and the fine sand req uires a co mparatively g re ate r am o unt o f w ate r to p ro d uc e the ne c e ssary fluid ity. Fo r fine ag g re g ate s, a to tal d e p arture o f 5 p e r ce nt fro m zo ne limits may b e allo w e d . But this re laxatio n is no t p e rmitte d b e yo nd the co arse r limit o f zo ne I o r the fine r limit o f zo ne IV.

Crushed Sand All a lo n g in In d ia , w e h a ve b e e n u sin g n a tu ra l sa n d . Th e vo lu m e o f c o n c re te manufactured in India has no t been much, w hen co mpared to so me advanced co untries. The infrastructure d e ve lo p me nt such as e xp re ss hig hw ay p ro je cts, p o w e r p ro je cts and ind ustrial d e ve lo p me nts have starte d no w. Availab ility o f natural sand is g e tting d e p le te d and also it is b e co ming co stly. Co ncre te ind ustry no w w ill have to g o fo r crushe d sand o r w hat is calle d manufacture d sand . Ad vantag e s o f natural sand is that the p artic le s are c ub ic al o r ro und e d w ith sm o o th surface te xture . The g rad ing o f natural F.A. is no t alw ays id e al. It d e pe nd s o n place to place . Be ing cub ical, ro und e d and smo o th te xture d it g ive s g o o d w o rkab ility. So far, c rushe d sand has no t b e e n use d m uc h in Ind ia fo r the re aso n that o rd inarily crushe d sand is flaky, b ad ly g rad e d ro ug h te xture d and he nce the y re sult in p ro d uctio n o f harsh co ncrete fo r the g iven desig n parameters. We have b een also no t using superplasticizer

106

! Concrete Technology

w idely in o ur co ncreting o peratio ns to impro ve the w o rkability o f harsh mix. Fo r the last abo ut 4 –5 ye ars the o ld me tho d s o f manufacturing o rd inary crushe d sand have b e e n re p lace d b y mo d e rn crushe rs sp e cially d e sig ne d fo r p ro d ucing , cub ical, co mp arative ly smo o th te xture d , w e ll g rad e d sand , g o o d e no ug h to re p lace natural sand . Many p ate nte d e q uip me nts are se t up in Ind ia to p ro d uce crushe d sand o f acce p tab le q uality at pro ject site. Pune-Mumbai express hig hw ay is o ne o f the big g est pro jects undertaken in Ind ia re ce ntly. Eno ug h q uantitie s o f natural sand is no t availab le in this re g io n. The to tal q uantity o f co ncre te invo lve d is mo re than 2 0 ,0 0 0 ,0 0 m 3 o f co ncre te . The autho ritie s have d e cid e d to use crushe d sand . A co mpany by name Svedala is o ne o f the co ncrete ag g reg ate manufacturers w ho have b e e n in the fo re fro nt fo r sup p lying c rush e r e q uip m e n ts b y trad e n am e Jaw master crusher, o r Barmac Ro ck o n Ro ck VSI c ru sh e rs in c o rp o ra tin g ro c k-o n -ro c k c ru sh in g te c h n o lo g y th a t has re vo lutio nise d the art o f making c o nc re te ag g re g ate s. This impo rte d te chno lo g y has b e e n use d fo r p ro d uc ing c o arse and fine ag g re g ate s o f d e sire d q uality in te rm s o f shap e , te xture and g rad ing . D u st is a n u isan c e an d te c h n ic ally und e sirab le in b o th co arse ag g re g ate and m o re so in fin e a g g re g a te . Ma xim u m permissible particles o f size finer than 75 µ is 1 5 % in fine ag g re g ate and 3 % in c o arse ag g re g ate . The re are pro visio n availab le in the se e q uip me nts to c o ntro l and se al the Barmac Rock-On-Rock VSI Crusher. d ust. In o ne o f the hig h rise building sites in w e ste rn sub urb o f Mumb ai, M 6 0 co ncre te w as sp e cifie d . The re q uire d slump co uld no t b e ac hie ve d b y natural sand w ith the g ive n p aram e te r o f m ix d e sig n. But w ith the use o f manufacture d sand w ith p ro p e r shap e , surface te xture , d e sirab le g rad ing to minimise vo id co nte nt, a hig hly w o rkab le mix w ith the g ive n p arame te r o f mix d e sig n, w as achie ve d . The fo llo w ing is the g rad ing p atte rn o f a samp le co lle cte d fro m a sand crushing p lant o n a p articular d ate and time at Pune -Mumb ai Ro ad Pro je ct:

Ta ble 3 .1 7 . Gra ding Pat t e r n of Crushe d Sa nd (Typic a l) I.S. Sieve mm

Percentag e passing As per

10 4 .7 5 2 .3 6 1 .1 8 600 300 150 75

Remarks

IS Req uirements fo r

actual test

Zo ne I

Zo ne II

100 9 7 .5 8 8 2 .3 6 5 5 .2 7 4 0 .5 6 2 9 .3 3 1 8 .7 8 1 0 .0 9

100 9 0 –1 0 0 6 0 –9 5 3 0 –7 0 1 5 –3 4 5 –2 0 0 –2 0 Max 1 5

100 9 0 –1 0 0 7 5 –1 0 0 5 5 –9 0 3 5 –5 9 8 –3 0 0 –2 0 Max 1 5

Falling in Zo ne II

Aggregates and Testing of Aggregates !

107

Th e in tro d u c tio n o f m o d e rn scientifically o perated crushers w hich are o perating all o ver the w o rld , w ill g o a lo n g w ay fo r m akin g q u ality a g g re g a te s in a ll c itie s in In d ia . O rd inary c rushe rs c anno t g ive the d e sire d sh ap e , su rfac e te xtu re o r g ra d in g o f b o th c o a rse a n d fin e ag g re g ate .

Gap grading Cone Crushers. So far w e discussed the g rading p atte rn o f ag g re g ate s in w hic h all particle size are present in certain pro po rtio n in a sample o f ag g reg ate. Such pattern o f particle size d istrib utio n is also re fe rre d to as co ntinuo us g rad ing .

O rig inally in the theo ry o f co ntinuo us g rad ing , it w as assumed that the vo id s present in the hig her size o f the ag g reg ate are filled up by the next lo w er size o f ag g reg ate, and similarly, vo id s cre ate d b y the lo w e r size are fille d up b y o ne size lo w e r than tho se p article and so o n. It w as re alise d late r th at th e vo id s c re ate d b y a p artic u lar frac tio n are to o sm all to acco mmo d ate the ve ry ne xt lo w e r size . The ne xt lo w e r size b e ing itse lf b ig g e r than the size o f the vo ids, it w ill create w hat is kno w n as “particle size interference”, w hich prevents the larg e ag g re g ate s co mpacting to the ir maximum d e nsity. It has b e e n se e n that the size o f vo id s e xisting b e tw e e n a p articular size o f ag g re g ate is o f the o rd e r o f 2 o r 3 size lo w e r than that frac tio n. In o the r w o rd s, the vo id size e xisting b e tw e e n 4 0 mm ag g re g ate is o f the size e q ual to 1 0 mm o r po ssib ly 4 .7 5 mm o r the size o f vo id s o ccurring w he n 2 0 mm ag g re g ate is use d w ill b e in the o rd e r o f say 1 .1 8 mm o r so . The re fo re , alo ng w ith 2 0 mm ag g re g ate , o nly w he n 1 .1 8 mm ag g re g ate size is use d , the samp le w ill co ntain le ast vo id s and co ncre te re q uire s le ast matrix. The fo llo w ing ad vantag e s are claime d fo r g ap g rad e d co ncre te : (i )

Sand re q uired w ill b e o f the o rd er o f ab o ut 2 6 per cent as ag ainst ab o ut 4 0 per cent in the case o f co ntinuo us g rad ing .

(ii ) Sp e c ific surfac e are a o f the g ap g rad e d ag g re g ate w ill b e lo w, b e c ause o f hig h pe rce ntag e o f C.A. and lo w pe rce ntag e o f F.A. (iii ) Re q uire s le ss ce me nt and lo w e r w ate r/ ce me nt ratio . (iv ) Because o f po int co ntact b etw een C.A. to C.A. and also o n acco unt o f lo w er cement and matrix co nte nt, the d rying shrinkag e is re d uce d . It w as also o b se rve d that g ap g rad e d c o nc re te ne e d s c lo se sup e rvisio n, as it sho w s g reater pro neness to seg reg atio n and chang e in the anticipated w o rkab ility. In spite o f many c laims o f the sup e rio r p ro p e rtie s o f g ap g rad e d c o nc re te , this me tho d o f g rad ing has no t b e co me mo re p o p ular than co nve ntio nal co ntinuo us g rad ing .

TESTING OF AGGREGATES Test for Determination of Flakiness Index The flakiness index o f ag g reg ate is the percentag e b y w eig ht o f particles in it w ho se least dimensio n (thickness) is less than three-fifths o f their mean dimensio n. The test is no t applicable to size s smalle r than 6 .3 mm.

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This test is co nducted b y using a metal thickness g aug e, o f the descriptio n sho w n in Fig . 3 .9 . A sufficie nt q uantity o f ag g re g ate is take n such that a minimum numb e r o f 2 0 0 p ie ce s o f any fractio n can be tested. Each fractio n is g aug ed in turn fo r thickness o n the metal g aug e. The to tal am o unt p assing in the g uag e is w e ig he d to an ac c urac y o f 0 .1 p e r c e nt o f the w e ig ht o f the samp le s take n. The flakine ss ind e x is take n as the to tal w e ig ht o f the mate rial p assing the vario us thic kne ss g aug e s e xp re sse d as a p e rc e ntag e o f the to tal w e ig ht o f the samp le take n. Tab le 3 .1 8 sho w s the stand ard d ime nsio ns o f thickne ss and le ng th g aug e s.

Ta ble 3 .1 8 . Show s Dim e nsions of T hick ne ss a nd Le ngt h Ga uge s (I S: 2 3 8 6 (Pa r t I ) – 1 9 6 3 ) Size o f Ag g reg ate Thickness Passing thro ug h IS Sieve

Retained o n IS Sieve

Leng th o f Gaug e * mm

Gaug e † mm

6 3 mm

5 0 mm

3 3 .9 0

–

5 0 mm

4 0 mm

2 7 .0 0

8 1 .0

4 0 mm

2 5 mm

1 9 .5 0

5 8 .5

3 1 .5 mm

2 5 mm

1 6 .9 5

–

2 5 mm

2 0 mm

1 3 .5 0

4 0 .5

2 0 mm

1 6 mm

1 0 .8 0

3 2 .4

1 6 mm

1 2 .5 mm

8 .5 5

2 5 .6

1 2 .5 mm

1 0 .0 mm

6 .7 5

2 0 .2

1 0 .0 mm

6 .3 mm

4 .8 9

1 4 .7

* This dimension is equal to 0.6 times the mean Sieve size. † This dimension is equal to 1.8 times the mean Sieve size.

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109

Test for Determination of Elongation Index The e lo ng atio n ind e x o n an ag g re g ate is the p e rce ntag e b y w e ig ht o f p article s w ho se g re ate st d ime nsio n (le ng th) is g re ate r than 1 .8 time s the ir me an d ime nsio n. The e lo ng atio n ind e x is no t applicab le to size s smalle r than 6 .3 mm. This test is co nducted by using metal leng th g uag e o f the descriptio n sho w n in Fig . 3.10. A sufficient q uantity o f ag g reg ate is taken to pro vide a minimum numb er o f 200 pieces o f any fractio n to be tested. Each fractio n shall be g aug ed individually fo r leng th o n the metal g uag e. The g uag e leng th used shall b e that specified in co lumn o f 4 o f Tab le 3.18 fo r the appro priate size o f m ate rial. The to tal am o unt re taine d b y the g uag e le ng th shall b e w e ig he d to an accuracy o f at least 0.1 per cent o f the w eig ht o f the test samples taken. The elo ng atio n index is the to tal w e ig ht o f the m ate rial re taine d o n the vario us le ng th g aug e s e xp re sse d as a percentag e o f the to tal w eig ht o f the sample g aug ed . The presence o f elo ng ated particles in excess o f 1 0 to 1 5 per cent is g enerally co nsid ered und esirab le, b ut no reco g anised limits are laid d o w n.

Length Gauge.

Ind ian stand ard e xp lain o nly the m e tho d o f c alc ulating b o th Flakine ss Ind e x and Elo ng atio n Ind e x. But the spe cificatio ns d o no t spe cify the limits. British Stand ard BS 8 8 2 o f 1 9 9 2 limits th e fla kin e ss in d e x o f th e c o a rse ag g reg ate to 5 0 fo r natural g ravel and to 4 0 fo r c rushe d c o rase ag g re g ate . Ho w ever, fo r w earing surfaces a lo w er value s o f flakine ss ind e x are re q uire d .

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Test for Determination of clay, fine silt and fine dust This is a g ravimetric metho d fo r determining the clay, fine silt and fine dust w hich includes particle s upto 2 0 micro ns. The sample fo r test is prepared fro m the main sample, taking particular care that the test samp le co ntains a co rre ct p ro p o rtio n o f the fine r mate rial. The amo unt o f samp le take n fo r the te st is in acco rd ance w ith Tab le 3 .1 9 .

Ta ble 3 .1 9 . We ight of Sa m ple for De t e r m ina t ion of Cla y, Fine Silt a nd Fine Dust Maximum size present in sub stantial pro po rtio ns mm

Appro ximate w eig ht o f sample fo r Test kg

6 3 to 2 5

6

2 0 to 1 2 .5

1

1 0 to 6 .3

0 .5

4 .7 5 o r smalle r

0 .3

Sed imentatio n pipette o f the d escriptio n sho w n in Fig . 3 .1 1 is used fo r d eterminatio n o f clay and silt co nte nt. In the case o f fine ag g re g ate , appro ximate ly 3 0 0 g m. o f sample s in the air-d ry co nd itio n, p assing the 4 .7 5 mm IS Sie ve , is w e ig he d and p lace d in the scre w to p p e d g lass jar, to g ether w ith 300 ml o f diluted so dium o xalate so lutio n. The rubber w asher and cap are fixe d . Care is take n to e nsure w ate r tig htne ss. The jar is the n ro tate d ab o ut its lo ng axis, w ith this axis ho rizo ntal, at a sp e e d o f 8 0 ± 2 0 re vo lutio ns p e r m inute fo r a p e rio d o f 1 5 minutes. At the end o f 15 minutes the suspensio n is po ured into 1000 ml measuring cylind er and the re sid ue w ashe d b y g e ntle sw irling and d e cantatio n o f succe ssive 1 5 0 ml po rtio ns o f so d ium o xalate so lutio n, the w ashing s b e ing ad d e d to the cylind e r until the vo lume is mad e up to 1 0 0 0 ml. In the case o f co arse ag g re g ate the w e ig he d samp le is p lace d in a suitab le co ntaine r, c o ve re d w ith a m e asure d vo lum e o f so d ium o xalate so lutio n (0 .8 g m p e r litre ), ag itate d vig o ro usly to re mo ve all fine mate rial ad he re d and the liq uid susp e nsio n transfe rre d to the 1 0 0 0 ml measuring cylind er. This pro cess is repeated till all clay material has b een transferred to the cylind e r. The vo lume is mad e up to 1 0 0 0 ml w ith so d ium o xalate so lutio n. The susp e nsio n in the m e asuring c ylind e r is tho ro ug hly m ixe d . The p ip e tte A is the n g ently lo w ered until the pipette to uches the surface o f the liq uid , and then lo w ered a further 1 0 cm into the liq uid . Thre e minute s afte r placing the tub e in po sitio n, the pipe tte A and the b o re o f tap B is fille d b y o pe ning B and applying g e ntle suctio n at C. A small surplus may b e d raw n up into the b ulb b e tw e e n tap B and tub e C, b ut this is allo w e d to run aw ay and any so lid matte r is w ashe d o ut w ith d istille d w ate r fro m E. The pipe tte is the n re mo ve d fro m the m e asuring c ylind e r and its c o nte nts run into a w e ig he d c o ntaine r. The c o nte nts o f the co ntaine r is d rie d at 1 0 0 ° C to 1 1 0 ° C to co nstant w e ig ht, co o le d and w e ig he d . The p e rce ntag e o f the fine slit and clay o r fine d ust is calculate d fro m the fo rmula.

100  1000 W2  − 0.8  W1  V  w he re

W 1 = w e ig ht in g m o f the o rig inal samp le .

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111

W 2 = w e ig ht in g m o f the d rie d re sid ue V = vo lume in ml o f the pipe tte and 0 .8 = w e ig ht in g m o f so d ium o xalate in o ne litre o f d ilute d so lutio n.

Test for Determination of Organic Impurities This te st is an ap p ro xim ate m e tho d fo r e stim ating w he the r o rg anic c o m p o und s are p re se nt in the natural sand in an o b je ctio nab le q uantity o r w ithin the p e rmissib le limit. The sand fro m the natural so urce is te ste d as d e live re d and w itho ut d rying . A 3 5 0 ml g rad uate d cle ar g lass b o ttle is fille d to the 7 5 ml mark w ith 3 p e r ce nt so lutio n o f so d ium hyd ro xid e in w ate r. The sand is ad d e d g rad ually until the vo lume me asure d b y the sand laye r is 1 2 5 ml. The vo lume is then made up to 200 ml b y adding mo re so lutio n. The b o ttle is then sto ppere d and shaken vig o ro usly. Ro ding also may be permitted to dislo dg e any o rg anic matter adhering to the natural sand b y using g lass ro d . The liq uid is the n allo w e d to stand fo r 2 4 ho urs. The co lo ur o f this liq uid afte r 2 4 ho urs is co mp are d w ith a stand ard so lutio n fre shly p re p are d , as fo llo w s: Ad d 2 .5 ml o f 2 pe r ce nt so lutio n o f tannic acid in 1 0 pe r ce nt alco ho l, to 9 7 .5 ml o f a 3 per cent so dium hydro xide so lutio n. Place in a 350 ml. b o ttle, sto pper, shake vig o ro usly and allo w to stand fo r 2 4 ho urs b e fo re co mp ariso n w ith the so lutio n ab o ve and d e scrib e d in the p re ce d ing p arag rap h. Alte rnative ly, an instrume nt o r co lo ure d ace tate she e ts fo r making the co mp ariso n can b e o b taine d , b ut it is d e sirab le that the se sho uld b e ve rifie d o n re ce ip t b y co mp ariso n w ith the stand ard so lutio n.

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Test for Determination of Specific Gravity Ind ian Stand ard Spe cificatio n IS : 2 3 8 6 (Part III) o f 1 9 6 3 g ive s vario us pro ce d ure s to find o ut the spe cific g ravity o f d iffe re nt size s o f ag g re g ate s. The fo llo w ing pro ce d ure is applicab le to ag g re g ate size larg e r than 1 0 mm. A sample o f ag g reg ate no t less than 2 kg is taken. It is tho ro ug hly w ashed to remo ve the fine r p artic le s and d ust ad he ring to the ag g re g ate . It is the n p lac e d in a w ire b aske t and im m e rse d in d istille d w ate r at a te m p e ratu re b e tw e e n 2 2 ° to 3 2 ° C. Im m e d iate ly afte r imme rsio n, the e ntrap p e d air is re mo ve d fro m the samp le b y lifting the b aske t co ntaining it 2 5 mm ab o ve the b ase o f the tank and allo w ing it to d ro p 2 5 time s at the rate o f ab o ut o ne d ro p p e r se c . During the o p e ratio n, c are is take n that the b aske t and ag g re g ate re m ain c o m p le te ly im m e rse d in w ate r. Th e y are ke p t in w ate r fo r a p e rio d o f 2 4 ± 1 / 2 h o u rs afte rw ard s. The b aske t and ag g re g ate are the n jo lte d and w e ig he d (w e ig ht A1 ) in w ate r at a temperature 22° to 32° C. The b asket and the ag g reg ate are then remo ved fro m w ater and allo w e d to d rain fo r a fe w minute s and the n the ag g re g ate is take n o ut fro m the b aske t and placed o n dry clo th and the surface is g ently dried w ith the clo th. The ag g reg ate is transferred to the seco nd dry clo th and further dried. The empty basket is ag ain immersed in w ater, jo lted 2 5 time s and w e ig he d in w ate r (w e ig ht A2 ). The ag g re g ate is e xp o se d to atmo sp he re aw ay fro m d irect sunlig ht fo r no t less than 1 0 minutes until it appears co mpletely surface d ry. Then the ag g re g ate is w e ig he d in air (w e ig ht B). The n the ag g re g ate is ke p t in the o ve n at a temperature o f 100 to 110°C and maintained at this temperature fo r 24 ± 1/ 2 ho urs. It is then co o le d in the air-tig ht co ntaine r, and w e ig he d (w e ig ht C). Sp e cific Gravity =

C ; B− A

Wate r ab so rptio n =

Appare nt Sp. Gravity =

C C− A

100 ( B − C) C

A= the w e ig ht in g m o f the saturate d ag g re g ate in w ate r (A1 – A2 ),

Whe re ,

B = the w e ig ht in g m o f the saturate d surface -d ry ag g re g ate in air, and C = the w e ig ht in g m o f o ve n-d rie d ag g re g ate in air.

Test for Determination of Bulk Density and Voids Bulk d e nsity is the w e ig ht o f mate rial in a g ive n vo lume . It is no rmally e xp re sse d in kg per litre. A cylindrical measure preferab ly machined to accurate internal dimensio ns is used fo r measuring bulk density. The size o f the co ntainer fo r measuring bulk density is sho w n in Table, 3 .2 0 .

Ta ble 3 .2 0 . Size of Cont a ine r for Bulk De nsit y Te st Size o f Larg est Particles

No minal Capacity

Insid e Diameter

Insid e Heig ht

litre

cm

cm

mm

3

15

17

3 .1 5

to 4 0 mm

15

25

30

4 .0 0

O ve r 4 0 mm

30

35

31

5 .0 0

4 .7 5 mm and und e r

Thickness o f Metal

O ve r 4 .7 5 mm

Aggregates and Testing of Aggregates !

113

The cylind rical me asure is fille d ab o ut 1 / 3 e ach time w ith tho ro ug hly mixe d ag g re g ate and tamped w ith 25 stro kes by a bullet ended tamping ro d, 16 mm diameter and 60 cm lo ng . The measure is carefully struck o ff level using tamping ro d as a straig ht ed g e. The net w eig ht o f the ag g re g ate in the me asure is d e te rmine d and the b ulk d e nsity is calculate d in kg / litre . Bulk d insity = w he re ,

net weight of the aggregate in kg ; capacity of the container in litre

G s = spe cific g ravity o f ag g re g ate

Pe rce ntag e o f vo id s = and

Gs − !γ × 100 Gs

γ = b ulk d insity in kg / litre .

Mechanical Properties of Aggregates IS: 2386 Part IV – 1963 Test for determination of aggregate crushing value The “ag g reg ate crushing value” g ives a relative measure o f the resistance o f an ag g reg ate to c rushing und e r a g rad ually ap p lie d c o m p re ssive lo ad . W ith ag g re g ate s o f ‘ag g re g ate crushing value’ 30 o r hig her, the result may be ano malo us and in such cases the “ten per cent fine s value ” sho uld b e d e te rmine d and use d inste ad . The standard ag g reg ate crushing test is made o n ag g reg ate passing a 12.5 mm I.S. Sieve and retained o n 10 mm I.S. Sieve. If req uired, o r if the standard size is no t available, o ther sizes up to 2 5 mm may b e te ste d . But o w ing to th e n o n h o m o g e n e ity o f ag g re g ate s th e re sults w ill no t b e c o m p arab le w ith tho se o b taine d in the stand ard te st. Ab o u t 6 .5 kg m ate rial c o n sistin g o f ag g re g ate s p assing 1 2 .5 mm and re taine d o n 1 0 mm sie ve is take n. The ag g re g ate in a su rfac e d ry c o n d itio n is fille d in to th e stand ard cylind rical me asure in thre e laye rs appro ximate ly o f e q ual d e pth. Each laye r is tamped 25 times w ith the tamping o rd and finally le ve lle d o ff using the tamp ing ro d as straig ht e d g e . The w e ig ht o f the sam p le co ntaine d in the cylind e r me asure is take n (A). The same w eig ht o f the sample is taken fo r the sub se q ue nt re pe at te st. The cylind e r o f the te st ap p artus w ith ag g reg ate filled in a standard manner is put in p o sitio n o n th e b a se -p la te a n d th e a g g re g a te is c a re fu lly le ve lle d a n d th e p lu n g e r in se rte d h o rizo n ta lly o n th is surface . The p lung e r sho uld no t jam in the cylind e r.

Aggregate Crushing Value Apparatus.

The appartus, w ith the test sample and plung er in po sitio n, is placed o n the co mpressio n testing machine and is lo ad ed unifo rmly upto a to tal lo ad o f 4 0 to ns in 1 0 minutes time. The lo ad is then released and the w ho le o f the material remo ved fro m the cylind er and sieved o n a 2 .3 6 mm I.S. Sie ve . The fractio n p assing the sie ve is w e ig he d (B),

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The ag g re g ate crushing value = w here,

B × 100 A

B = w e ig ht o f fractio n p assing 2 .3 6 mm sie ve , A = w e ig ht o f surface -d ry samp le take n in mo uld .

The ag g reg ate crushing value sho uld no t b e mo re than 4 5 per cent fo r ag g reg ate used fo r co ncre te o the r than fo r w e aring surface s, and 3 0 p e r ce nt fo r co ncre te use d fo r w e aring surface s such a runw ays, ro ad s and air fie ld p ave me nts.

Test for determination of ‘ten per cent fines value’ The sample o f ag g reg ate fo r this test is the same as that o f the sample used fo r ag g reg ate c rushing value te st. The te st samp le is p re p are d in the same w ay as d e sc rib e d e arlie r. The cylinder o f the test appartus is placed in po sitio n o n the b ase plate and the test sample added in th ird s, e ac h th ird b e in g sub je c te d to 2 5 stro ke s b y tam p in g ro d . Th e surfac e o f th e ag g re g ate is c are fully le ve lle d and the p lung e r inse rte d so that it re sts ho rizo ntally o n this surface . The appartus, w ith the test sample and plung er in po sitio n is placed in the co mpressio n testing machine. The lo ad is applied at a unifo rm rate so as to cause a to tal penetratio n o f the p lung e r in 1 0 minute s o f ab o ut: 15.00 mm fo r ro und ed o r partially ro und ed ag g reg ates (fo r example uncrushed g ravels) 2 0 .0 mm fo r no rmal crushe d ag g re g ate s, and 2 4 .0 mm fo r ho ne yco mb e d ag g re g ate s (fo r e xamp le , e xp and e d shale s and slag s). The se fig ure may b e varie d acco rd ing to the e xte nt o f the ro und ing o r ho ne yco mb ing . Afte r re aching the re q uire d maximum p e ne tratio n, the lo ad is re le ase d and the w ho le o f the m ate rial re m o ve d fro m the c ylind e r and sie ve d o n a 2 .3 6 m m I.S. Sie ve . The fine s p assing the sie ve is w e ig he d and the w e ig ht is e xp re sse d as a p e rce ntag e o f the w e ig ht o f the te st samp le . This p e rce ntag e w o uld fall w ithin the rang e 7 .5 to 1 2 .6 , b ut if it d o e s no t, a furthe r te st shall b e m ad e at a lo ad ad juste d as se e m s ap p ro p riate to bring the percentag e fines w ith the rang e o f 7.5 to 12.5 per cent. Repeat test is mad e and the lo ad is fo und o ut w hich g ive s a p e rce ntag e o f fine s w ithin the rang e o f 7 .5 to 1 2 .5 Lo ad re q uire d fo r 1 0 p e r ce nt fine s = w he re , ce nt fine s.

14 × X Y +4

X = lo ad in to ns, causing 7.5 to 12.5 per

Y = m e an p e rc e n tag e fin e s fro m tw o te sts at X to ns lo ad .

Test for determination of aggregate impact value The ag g reg ate impact value g ives relative measure o f the re sistanc e o f an ag g re g ate to sud d e n sho c k o r im p ac t. W h ic h in so m e ag g re g ate s d iffe rs fro m its re sistance to a slo w co mpre ssive lo ad .

Aggregate Impact Value Apparatus.

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115

The test sample co nsists o f ag g reg ate passing thro ug h 12.5 mm and retained o n 10 mm I.S. Sieve. The ag g reg ate shall b e d ried in an o ven fo r a perio d o f fo ur ho urs at a temperature o f 1 0 0 ° C to 1 1 0 ° C and co o le d . The ag g re g ate is fille d ab o ut o ne -third full and tamp e d w ith 2 5 stro ke s b y the tamp ing ro d . A furthe r similar q uantity o f ag g re g ate is ad d e d and tamp e d in the standard manner. The measure is filled to o ver-flo w ing and then struck o ff level. The net w e ig h t o f th e ag g re g ate in th e m e asu re is d e te rm in e d (w e ig h t A) an d th is w e ig h t o f ag g re g ate shall b e use d fo r the d uplicate te st o n the same mate rial. The w ho le sam p le is fille d into a c ylind ric al ste e l c up firm ly fixe d o n the b ase o f the machine. A hammer w eig hing abo ut 14 kg s. is raised to a heig ht o f 380 mm abo ve the upper surfac e o f the ag g re g ate in the c up and allo w e d to fall fre e ly o n the ag g re g ate . The te st samp le shall b e sub je cte d to a to tal 1 5 such b lo w s e ach b e ing d e live re d at an inte rval o f no t le ss than o ne se co nd . The crushe d ag g re g ate is re mo ve d fro m the cup and the w ho le o f it is sie ve d o n 2 .3 6 mm I.S. Sie ve . The fractio n p assing the sie ve is w e ig he d to an accuracy o f 0 .1 g m. (w e ig ht B). The fractio n re taine d o n the sie ve is also w e ig he d (w e ig ht C). If the to tal w e ig ht (B + C) is le ss than the initial w e ig ht A b y m o re than o ne g m the re sult shall b e d iscard e d and a fre sh te st mad e . Tw o te sts are mad e . Th e ratio o f th e w e ig h t o f fin e s fo rm e d to th e to tal sam p le w e ig h t in e ac h te st is e xp re sse d as p e rce ntag e . The re fo re , Ag g re g ate Imp act Value = w here,

B × 100 A

B = w e ig ht o f fractio n passing 2 .3 6 mm I.S. Sie ve . A = w e ig ht o f o ve n-d rie d samp le .

Th e ag g re g ate im p ac t valu e sh o u ld n o t b e m o re th an 4 5 p e r c e n t b y w e ig h t fo r ag g re g ate s use d fo r c o nc re te o the r than w e aring surfac e s and 3 0 p e r c e nt b y w e ig ht fo r co ncre te to b e use d as w e aring surface s, such as runw ays, ro ad s and p ave me nts.

Test for determination of aggregate abrasion value Ind ian Stand ard 2386 (Part IV) o f 1963 co vers tw o metho d s fo r find ing o ut the ab rasio n value o f co arse ag g reg ates: namely, b y the use o f Deval ab rasio n testing machine and b y the use o f Lo s Ang eles abrasio n testing machine. Ho w ever, the use o f Lo s Ang eles abrasio n testing machine g ives a better realistic picture o f the abrasio n resistance o f the ag g reg ate. This metho d is o nly d e scrib e d he re in. Tab le 3 .2 1 g ives the d etail o f ab rasive charg e w hich co nsists o f cast iro n spheres o r steel sp he re s ap p ro ximate ly 4 8 mm in d iame te r and e ach w e ig hing b e tw e e n 3 9 0 to 4 4 5 g m.

Ta ble 3 .2 1 . Spe c ifie d Abra sive Cha rge Grad ing

Numb er o f spheres

Weig ht o f charg e (g m )

A

12

5000 ± 25

B

11

4584 ± 25

C

8

3330 ± 20

D

6

2500 ± 15

E

12

5000 ± 25

F

12

5000 ± 25

G

12

5000 ± 25

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The te st sample co nsist o f cle an ag g re g ate w hich has b e n d rie d in an o ve n at 1 0 5 ° C to 1 1 0 ° C and it sho uld co nfo rm to o ne o f the g rad ing s sho w n in Tab le 3 .2 2 .

Ta ble 3 .2 2 . Gra dings of Te st Sa m ple s Sieve Size Passing

Retained o n

mm

mm

80

63

Weig ht in g m. o f Test Sample Fo r Grad e A

B

C

D

E

F

G

-

-

-

-

2500

-

-

63

50

-

-

-

-

2500

-

-

50

40

-

-

-

-

5000

5000

-

40

25

1250

-

-

-

-

5000

5000

25

20

1250

-

-

-

-

-

5000

20

1 2 .5

1250

2500

-

-

-

-

-

1 2 .5

10

1250

2500

-

-

-

-

-

10

6 .3

-

-

2500

-

-

-

-

6 .3

4 .7 5

-

-

2500

-

-

-

-

4 .7 5

2 .3 6

-

-

-

5000

-

-

-

Test sample and ab rasive charg e are placed in the Lo s Ang eles Ab rasio n testing machine and the m ac hine is ro tate d at a sp e e d o f 2 0 to 3 3 re v/ m in . Fo r g ra d in g s A, B, C a n d D , th e m ac h in e is ro tate d fo r 5 0 0 re vo lu tio n s. Fo r g ra d in g s E, F a n d G , it is ro ta te d 1 0 0 0 re vo lu tio n s. At th e c o m p le tio n o f th e ab o ve numb e r o f re vo lutio n, the mate rial is d ischarg e d fro m the machine and a preliminary separatio n o f the sample made o n a sieve co arser than 1.7 mm IS Sieve. Finer po rtio n is then sieved o n a 1.7 mm IS Sie ve . The m ate rial c o arse r than 1 .7 m m IS Sie ve d is w ashe d , d rie d in an o ve n at 1 0 5 ° to 1 1 0 ° C to a sub stantially c o nstant w e ig ht and accurate ly w e ig he d to the ne are st g ram. The d ifference b etw een the o rig inal w eig ht a n d th e fin a l w e ig h t o f th e te st sa m p le is expressed as a percentag e o f the o rig inal w eig ht o f the te st samp le . This value is re p o rte d as the p e rc e n tag e o f w e ar. Th e p e rc e n tag e o f w e ar sh o u ld n o t b e m o re th a n 1 6 p e r c e n t fo r co ncre te ag g re g ate s. Typ ic al p ro p e rtie s o f so m e o f th e In d ian ag g re g ate sample are sho w n in tab le 3 .2 3 . Los Angeles Abrasion Testing Machine.

Kirke e Ute rlai Rajasthan Bhatind a Jammu

Bhuj Nasik Ranchi

Co chin

We lling to n

Pre mnag ar (De hrad un) Sulur Co imb ato re Trivand rum

Muzzafarp ur Be lg aum

1. 2.

5. 6. 7.

8.

9.

10.

11.

13. 14

12.

3. 4.

Name o f Place (2 )

Sr. No . (1 )

2 2 .4 4 9 .3 8 1 4 .0 2 0 .2 0 3 1 .8 0 1 5 .7 0 2 4 .0 0

1 6 .8 2 2 .8 2 8 .0 2 0 .8 9 4 0 .1 3 3 5 .0 6 2 5 .2 2 7 .8 1 3 .2 8 2 3 .6 0 1 4 .0 2 3 .0 3 0 .0 1 4 .0 3 9 .0 3 6 .3 0 8 .0

Flakiness Ind ex % (3 )

2 5 .4 2 1 5 .7 4 1 8 .0 3 8 .8 0 2 9 .2 0 3 8 .4 0 2 9 .3 0 .

2 0 .8 2 3 .9 2 5 .2 1 7 .5 0 3 8 .9 0 3 7 .6 8 1 4 .2 3 1 .3 2 5 .2 8 2 1 .0 2 0 .0 1 1 .0 1 9 .0 2 9 .0 2 5 .0 0 2 5 .7 0 9 .0 2 .7 2 2 .7 1 2 .6 9 2 .9 4 2 .9 8 3 .0 0 3 .0 0

2 .8 4 2 .7 9 2 .7 6 2 .5 2 .7 3 2 .7 1 2 .9 0 2 .6 7 2 .6 9 2 .6 6 2 .8 5 2 .8 4 2 .8 7 2 .8 5 2 .6 2 2 .6 0 2 .7 0

Elo ng atio n Specific Ind ex % g ravity (4 ) (5 )

0 .2 5 0 .5 0 0 .2 0 0 .6 5 0 .4 7 1 .3 1 1 .0 0

1 .2 0 0 .8 0 0 .6 0 1 .0 1 0 .7 5 0 .5 0 0 .9 0 0 .7 5 0 .5 0 0 .5 0 0 .2 0 .2 0 .4 4 0 .5 0 1 .2 0 1 .2 5 0 .5 0

Water Ab so rptio n (6 )

Charac te ristic s

2 2 .7 0 2 0 .2 8 2 3 .2 2 0 .8 0 2 2 .3 0 2 1 .8 0 1 2 .4 0

1 6 .9 7 — 2 3 .0 2 2 .6 2 1 7 .8 1 8 .3 2 1 8 .8 0 2 4 .8 3 2 7 .4 7 3 3 .6 8 2 7 .0 2 8 .0 2 6 .0 2 7 .6 2 7 .3 0 2 9 .9 0 2 6 .0

Crushing Value % (7 )

1 7 .0 5 1 5 .2 3 2 2 .9 1 7 .2 0 1 4 .8 0 1 0 .1 5 1 3 .1 0

1 8 .6 5 1 7 .8 0 2 5 .4 7 3 1 .5 4 1 7 .6 2 0 .4 1 1 3 .2 5 — 2 9 .5 8 — 2 3 .0 2 7 .0 2 1 .0 1 9 .0 2 7 .4 0 2 5 .0 0 3 3 .0

Impact Value % (8 )

1 5 .2 2 4 .1 1 7 .8 0 8 .9 0 1 0 .1 0 1 0 .3 5 9 .9 5

1 8 .5 4 2 1 .0 1 9 .7 2 9 .1 2 6 .0 2 5 .1 1 1 .2 2 1 .0 0 3 8 .9 1 8 .8 2 0 .0 3 2 .0 1 9 .0 3 1 .3 3 1 .9 6 2 0 .0 0 3 5 .7

Ab rasio n Value % (9 ) 2 0 mm ag g re g ate 4 0 mm ag g re g ate 2 0 mm ag g re g ate 4 0 mm ag g re g ate 4 0 mm ag g re g ate 2 0 mm ag g re g ate 2 0 mm ag g re g ate 4 0 mm ag g re g ate Unre active 2 0 mm 4 0 mm ag g re g ate 4 0 mm ag g re g ate 2 0 mm ag g re g ate 4 0 mm ag g re g ate 2 0 mm ag g re g ate 2 0 mm ag g re g ate 1 2 .5 mm ag g re g ate 2 0 mm ag g re g ate

(1 1 )

Remarks

1 .5 0 2 0 mm ag g re g ate 1 .3 0 4 0 mm ag g re g ate 3 .9 0 2 0 mm ag g re g ate 0 .8 5 4 0 mm Type I ag g re g ate 0 .8 7 4 0 mm Type II ag g re g ate 0 .6 6 2 0 mm ,, I ag g re g ate 0 .6 3 2 0 mm Type II ,,

4 .2 2 .4 3 .1 1 0 .0 2 .4 2 .8 4 1 0 .0 4 .0 0 3 .0 1 .0 2 .0 8 .0 6 .0 6 .0 3 .5 4 .0 2 .0

So und ness % (1 0 )

Ta ble 3 .2 3 . Typic a l Prope r t ie s of Som e of t he I ndia n Aggre ga t e s 3 .1 1

Aggregates and Testing of Aggregates !

117

118

! Concrete Technology

R EFER EN C ES 3.1

Shergold FA, The Percentage Voids in Compacted Gravel as a Measure of its Angularity, Magazine of Concrete Research, Aug. 1953.

3.2

Murdock L.J., The Workability of Concrete, Magazine of concrete Research, Nov. 1960.

3.3

Bryant Mather, Shape, surface texture, and coatings, concrete and concrete-making materials ASTM, STP 169 – A.

3.4

Woolf D.O., Toughness, Hardness, Abrasion, Strength and Elastic Properties, ASTM STP 169 A.

3.5

Investigation work carried out by SEMT of College of Military Engineering (CME), Pune.

3.6

Jagus P.J., Alkali Aggregate Reaction in Concrete Construction, Road Research Paper No. 11 of 1958.

3.7

Cook H.K., Thermal properties, ASTM, STP 169 A.

3.8

Fuller W.B. and Thompson S.E., The Laws of Proportioning of Concrete, Transactions, American Society of Civil Engineers Vol. LIX, 1907.

3.9

Weymouth CAG, Designing Workable Concrete, Engineering News Record, Dec. 1938.

3.10. Road Research Laboratory, Design of Concrete Mixers, DSIR Road, Note No. 4, HMSO, London 1950 3.11. Investigation work carried out by SEMT of college of Military Engineering (CME) Pune.

4

C H A P T E R

Water requires to be tested to find out its suitability for large projects.

Water ! Qualities of Water ! Use of Sea Water for Mixing Concrete

ater is an impo rtant ing red ient o f co ncrete as it ac tive ly p artic ip ate s in the c he m ic al re ac tio n w ith cement. Since it helps to fo rm the streng th g iving c e m e n t g e l, th e q u an tity an d q u ality o f w ate r is re q uire d to b e lo o ke d into ve ry care fully. It has b e e n d iscusse d e no ug h in chap te r 1 ab o ut the q uantity o f mixing w ate r b ut so far the q uality o f w ate r has no t b e e n d isc usse d . In p rac tic e , ve ry o fte n g re at c o ntro l o n p ro p e rtie s o f ce me nt and ag g re g ate is e xe rcise d , b u t th e c o n tro l o n th e q u a lity o f w a te r is o fte n neg lected . Since q uality o f w ater affects the streng th, it is ne ce ssary fo r us to g o into the p urity and q uality o f w ate r.

W

Qualities of Water A po pular yard -stick to the suitab ility o f w ater fo r mixing co ncre te is that, if w ate r is fit fo r d rinking it is fit fo r making co ncre te . This d o e s no t ap p e ar to b e a tru e sta te m e n t fo r a ll c o n d itio n s. So m e w a te rs co ntaining a small amo unt o f sug ar w o uld be suitable fo r d rin kin g b u t n o t fo r m ixin g c o n c re te a n d c o nve rse ly w ate r suitab le fo r m aking c o nc re te m ay no t necessarily b e fit fo r d rinking . So me specificatio ns

119

120

" Concrete Technology

re q uire that if the w ater is no t o b tained fro m so urce that has pro ved satisfacto ry, the streng th o f c o nc re te o r m o rtar m ad e w ith q ue stio nab le w ate r sho uld b e c o m p are d w ith sim ilar co ncre te o r mo rtar mad e w ith pure w ate r. So me spe cificatio n also acce pt w ate r fo r making co ncre te if the p H value o f w ate r lie s b e tw e e n 6 and 8 and the w ate r is fre e fro m o rg anic matter. Instead o f depending upo n pH value and o ther chemical co mpo sitio n, the best co urse to find o ut w he the r a p articular so urce o f w ate r is suitab le fo r co ncre te making o r no t, is to make co ncrete w ith this w ater and co mpare its 7 days’ and 28 days’ streng th w ith co mpanio n cub e s mad e w ith d istille d w ate r. If the co mp re ssive stre ng th is up to 9 0 p e r ce nt, the so urce o f w ate r may b e acce p te d . This crite ria may b e safe ly ad o p te d in p lace s like co astal are a o f marshy area o r in o ther places w here the availab le w ater is b rackish in nature and o f do ub tful q uality. Ho w e ve r, it is lo g ical to kno w w hat harm the imp uritie s in w ate r d o to the co ncre te and w hat d e g re e o f imp urity is p e rmissib le is mixing co ncre te and curing co ncre te .

Underground water is sometime found unsuitable for mixing or even for curing concrete. The quality of underground water is to be checked.

Carbo nates and bi-carbo nates o f so dium and po tassium effect the setting time o f cement. While so d ium carb o nate may cause q uick se tting , the b i-carb o nate s may e ithe r acce le rate o r re tard the se tting . The o the r hig he r c o nc e ntratio ns o f the se salts w ill mate rially re d uc e the co ncrete streng th. If so me o f these salts exceeds 1,000 ppm, tests fo r setting time and 28 days stre ng th sho uld b e carrie d o ut. In lo w e r co nce ntratio ns the y may b e acce p te d . Brackish w ater co ntains chlo rides and sulphates. When chlo ride do es no t exceed 10,000 p p m and sulp hate d o e s no t e xce e d 3 ,0 0 0 p p m the w ate r is harmle ss, b ut w ate r w ith e ve n hig he r salt co nte nt has b e e n use d satisfacto rily. Salts o f Mang ane se , Tin, Zinc, Co p p e r and Le ad cause a marke d re d uctio n in stre ng th o f co ncrete. So dium io date, so dium pho sphate, and so dium b o rate reduce the initial streng th o f co ncre te to an e xtra-o rd inarily hig h d e g re e . Ano the r salt that is d e trime ntal to co ncre te is so d ium sulp hid e and e ve n a sulp hid e co nte nt o f 1 0 0 p p m w arrants te sting . Silts and suspended particles are undesirable as they interfere w ith setting , hardening and b o nd characteristics. A turb id ity limit o f 2 ,0 0 0 ppm has b een sug g ested . Tab le 4 .1 sho w s the to le rab le co nce ntratio n o f so me imp uritie s in mixing w ate r.

Water "

121

The initial se tting time o f the te st b lo ck mad e w ith a ce me nt and the w ate r pro po se d to b e use d shall no t d iffe r b y ±3 0 minute s fro m the initial se tting time o f the te st b lo ck mad e w ith same ce me nt and d istille d w ate r.

Ta ble 4 .1 . Tole ra ble Conc e nt ra t ions of Som e I m purit ie s in M ix ing Wa t e r Imp urity

To le rab le Co nce ntratio n

So d ium and p o tassium : carb o nate s and b i-carb o nate s

1 ,0 0 0 p p m (to tal). If this is e xce e d e d , it is ad visab le to m ake te sts b o th fo r se ttin g tim e an d 2 8 d ays stre ng th

Chlo rid e s

:

1 0 ,0 0 0 ppm.

Sulp huric anhyd rid e

:

3 ,0 0 0 p p m

Calcium chlo rid e

:

2 p e r ce nt b y w e ig ht o f ce me nt in no n-p re stre sse d co ncre te

So d ium io d ate , so d ium sulp hate , so d ium

:

ve ry lo w

So d ium sulp hid e

:

Eve n 1 0 0 p p m w arrants te sting

So d ium hyd ro xid e

:

0 .5 p e r ce nt b y w e ig ht o f ce me nt, p ro vid e d q uick

arse nate , so d ium b o rate

se t is no t ind uce d . Salt and susp e nd e d p article s :

2 ,0 0 0 p p m. Mixing w ate r w ith a hig h co nte nt o f susp e nd e d so lid s sho uld b e allo w e d to stand in a s e ttling b asin b e fo re use .

To tal d isso lve d salts

:

1 5 ,0 0 0 ppm.

O rg anic mate rial

:

3 ,0 0 0 p p m. Wate r co ntaining humic acid o r such o rg anic acid s may ad ve rse ly affe ct the hard e ning o f co ncre te ; 7 8 0 ppm. o f humic acid are re po rte d to have se rio usly imp aire d the stre ng th o f co ncre te . In the case o f such w ate rs the re - fo re , furthe r te sting is ne ce ssary.

pH

:

shall no t b e le ss than 6

The fo llo w ing g uid e line s sho uld also b e take n into co nsid e ratio n re g ard ing the q uality o f w ate r. (a ) To ne utralize 1 0 0 ml samp le o f w ate r using p he no p lhaline as an ind icato r, it sho uld no t re q uire mo re than 5 ml o f 0 .0 2 no rmal NaO H. (b ) To ne utralise 1 0 0 ml o f samp le o f w ate r, using mixe d ind icato r, it sho uld no t re q uire mo re than 2 5 ml o f 0 .0 2 no rmal H2 So 4 . (c ) Pe rmissib le limits fo r so lid s is as g ive n b e lo w in tab le 4 .2 .

122

" Concrete Technology

Ta ble 4 .2 . Pe r m issible lim it for solids a s pe r I S 4 5 6 of 2 0 0 0 Mate rial

Te ste d as p e r

O rg anic

IS 3 0 2 5 (p t 1 8 )

2 0 0 mg / l

Ino rg anic

IS 3 0 2 5 (p t 1 8 )

3 0 0 0 mg / l

Sulp hate s

IS 3 0 2 5 (p t 2 4 )

4 0 0 mg / l

Pe rmissib le limit Max.

(as So 3 ) Chlo rid e s

IS 3 0 2 5 (p t 3 2 )

(as Cl)

2 0 0 0 mg / l fo r co ncre te w o rk no t co ntaining e mb e d d e d ste e l and 5 0 0 mg / l fo r re info rce d co ncre te w o rk

Susp e nd e d

IS 3 0 2 5 (p t 1 7 )

2 0 0 0 mg / l

Alg ae in mixing w ate r may cause a marke d re d uctio n in stre ng th o f co ncre te e ithe r b y co mb ining w ith ce me nt to re d uce the b o nd o r b y causing larg e amo unt o f air e ntrainme nt in co ncre te . Alg ae w hich are p re se nt o n the surface o f the ag g re g ate have the same e ffe ct as in that o f mixing w ate r.

Use of Sea Water for Mixing Concrete Sea w ater has a salinity o f ab o ut 3.5 per cent. In that ab o ut 78% is so d ium chlo rid e and 15% is chlo ride and sulphate o f mag nesium. Sea w ater also co ntain small q uantities o f so dium and p o tassium salts. This can re act w ith re active ag g re g ate s in the same manne r as alkalie s in ce me nt. The re fo re se a w ate r sho uld no t b e use d e ve n fo r PCC if ag g re g ate s are kno w n to b e p o te ntially alkali re active . It is re p o rte d that the use o f se a w ate r fo r mixing co ncre te d o e s n o t ap p re c iab ly re d u c e th e stre n g th o f c o n c re te alth o u g h it m ay le ad to c o rro sio n o f re info rc e me nt in c e rtain c ase s. Re se arc h w o rke rs are unanimo us in the ir o p inio n, that se a w ate r can b e use d in un-re info rce d co ncre te o r mass co ncre te . Se a w ate r slig htly acce le rate s the e arly stre ng th o f co ncre te . But it re d uce s the 2 8 d ays stre ng th o f co ncre te b y ab o ut 1 0 to 15 per cent. Ho w ever, this lo ss o f streng th co uld be made up by redesig ning the mix. Water co ntaining larg e q uantitie s o f chlo rid e s in se a w ate r may cause e fflo re sce nce and p e rsiste nt dampness. When the appearance o f co ncrete is impo rtant sea w ater may be avo ided. The use o f se a w ate r is also no t ad visab le fo r p laste ring p urp o se w hich is sub se q ue ntly g o ing to b e painte d . Diverg ent o pinio n exists o n the q uestio n o f co rro sio n o f reinfo rcement d ue to the use o f se a w ate r. So m e re se arc h w o rke rs c autio ne d ab o ut the risk o f c o rro sio n o f re info rc e m e nt p articularly in tro p ical climatic re g io ns, w he re as so me re se arch w o rke rs d id no t find the risk o f c o rro sio n d u e to th e u se o f se a w ate r. Exp e rim e n ts h ave sh o w n th at c o rro sio n o f re info rc e me nt o c c urre d w he n c o nc re te w as mad e w ith p ure w ate r and imme rse d in p ure w ater w hen the co ncrete w as co mparatively po ro us, w hereas, no co rro sio n o f reinfo rcement w as fo und w hen sea w ater w as used fo r mixing and the specimen w as immersed in salt w ater w he n the co ncre te w as d e nse and e no ug h co ve r to the re info rce me nt w as g ive n. Fro m this it co uld b e infe rre d that the facto r fo r co rro sio n is no t the use o f se a w ate r o r the q uality o f w ater w here the co ncrete is placed . The facto rs effecting co rro sio n is permeab ility o f co ncrete and lack o f co ver. Ho w ever, since these facto rs canno t b e ad eq uately taken care o f alw ays at the site o f w o rk, it may b e w ise that sea w ater b e avo ided fo r making reinfo rced co ncrete. Fo r

Water "

123

Sea water is not to be used for prestressed concrete or for reinforced concrete. If unavoidable, it could be used for plain cement concrete (PCC).

e co no mical o r o the r p assing re aso ns, if se a w ate r canno t b e avo id e d fo r making re info rce d co ncre te , p articular p re cautio ns sho uld b e take n to make the co ncre te d e nse b y using lo w w ater/ cement ratio co upled w ith vib ratio n and to g ive an ad eq uate co ver o f at least 7 .5 cm. The use o f sea w ater must be avo ided in prestressed co ncrete w o rk because o f stress co rro sio n and und ue lo ss o f cro ss se ctio n o f small d iame te r w ire s. The late st Ind ian stand ard IS 4 5 6 o f 2 0 0 0 p ro h ib its th e use o f Se a Wate r fo r m ixin g an d c urin g o f re in fo rc e d c o n c re te an d p re stre sse d c o nc re te w o rk. This sp e c ific atio n p e rm its the use o f Se a Wate r fo r m ixing and curing o f p lain ce me nt co ncre te (PCC) und e r unavo id ab le situatio n.. It is p e rtine nt at this p o int to c o nsid e r the suitab ility o f w ate r fo r c uring . Wate r that c o ntains im p uritie s w hic h c ause d staining , is o b je c tio nab le fo r c uring c o nc re te m e m b e rs w ho se lo o k is impo rtant. The mo st co mmo n cause o f staining is usually hig h co nce ntratio n o f iro n o r o rg anic matte r in the w ate r. Wate r that co ntains mo re than 0 .0 8 ppm. o f iro n may be avo ided fo r curing if the appearance o f co ncrete is impo rtant. Similarly the use o f sea w ater may also b e avo id e d in such case s. In o the r case s, the w ate r, no rmally fit fo r mixing can also b e use d fo r curing .

124

! Concrete Technology

5

C H A P T E R Such a flow is the result of the use of Superplasticizer " GeneralAdmixtures " Construction " Chemicals Plasticizers (Water Reducers) " Factors Ef fecting the Workability " Slump Loss " Steps for " Ef fect of Reducing Slump Loss Superplasticizers on the Properties of Hardened Concrete " New Generation Superplasticizers " Carboxylated Acrylic " Retarders Ester (CAE) " Retarding Plasticizers " Accelerators Accelerating Plasticizers " Air-entraining Admixture " Pozzolanic or Mineral Admixtures " High Volume Fly Ash " Silica Fume Concrete (HVFA) " Rice Husk Ash " Surkhi " Metakaolin " Ground Granulated Blast Furnace Slag (GGBS) " Dampproofing and Waterproofing Admixture " Gas Forming Agents " Air-detraining agents " Alkali-aggregate expansion " inhibitors Workability Agents " Grouting Agents " Corrosion Inhibiting " Agents Bonding Admixture " Fungicidal, Germicidal and Insecticidal " Colouring Agents Admixtures " Miscellaneous Admixtures " Construction Chemicals " Membrane Forming Curing Compounds " Drying Behaviour " Types of Curing Compounds " Application Procedure " Polymer Bonding Agents " Mould Releasing " Installation Aids Agents " Floor Hardeners and Dust Proofers " Non-Shrink High Strength Grout " Surface Retarders " Bond Aid for Plastering " Ready to Use Plaster " Construction Chemicals for Waterproofing " Concrete Repair System

Admixtures and Construction Chemicals General

A

d m ixture is d e fine d as a m ate rial, o the r than ce me nt, w ate r and ag g re g ate s, that is use d as an ing red ient o f co ncrete and is ad d ed to the b atch im m e d iate ly b e fo re o r d uring m ixing . Ad d itive is a m ate rial w h ic h is ad d e d at th e tim e o f g rin d in g ce me nt clinke r at the ce me nt facto ry.

Th e se d ays c o n c re te is b e in g u se d fo r w id e varie tie s o f p urp o se s to make it suitab le in d iffe re nt c o nd itio ns. In the se c o nd itio ns o rd inary c o nc re te may fail to e xhib it the re q uire d q uality p e rfo rmance o r d urab ility. In suc h c ase s, ad m ixture is use d to mo d ify the p ro p e rtie s o f o rd inary co ncre te so as to make it mo re suitab le fo r any situatio n. Un til ab o u t 1 9 3 0 ad d itive s an d ad m ixtu re s tho ug h use d , w e re no t c o nsid e re d an im p o rtant p art o f c o nc re te te c hno lo g y. Sinc e the n, the re has b e e n an incre ase in the use o f ad mixture s. Tho ug h the use o f admixtures and additives is being fro w ned up o n o r sc o rne d b y so m e te c hno lo g ists, the re are m any o n the c o ntrary, w ho hig hly c o m m e nd and fo ste r the use and d e ve lo p me nt o f ad mixture s as it

124

Admixtures and Construction Chemicals !

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imparts many d esirab le characteristics and effect eco no my in co ncrete co nstructio n. It sho uld b e re me mb e re d , ho w e ve r, that ad mixture s are no sub stitute fo r g o o d co ncre ting p ractice s. The histo ry o f ad mixtures is as o ld as the histo ry o f co ncrete. It emb races a very vast field as sho w n in tab le 5 .2 2 . But a fe w typ e o f ad mixture s calle d Wate r Re d uce rs o r Hig h Rang e Wate r Re d uce rs, g e ne rally re fe rre d as p lasticize rs and sup e rp lasticize rs, are o f re ce nt inte re st. They are specifically develo ped in Japan and Germany aro und 1970. Later o n they w ere made p o p ular in USA and Euro p e e ve n in Mid d le East and Far East. Unfo rtunate ly, the use o f p lasticize rs and sup e rp lasticize rs have no t b e co me p o p ular in Ind ia till re ce ntly (1 9 8 5 ). The re are many reaso ns fo r no n acceptance fo r w id er use o f plasticizers in Ind ia: Ninety per cent o f co ncre ting activitie s are in the hand s o f co mmo n b uild e rs o r Go ve rnme nt d e p artme nts w ho do no t g enerally accept so mething new. Plasticizers w ere no t manufactured in India and they w e re to b e im p o rte d , and he nc e c o stly. Lac k o f e d uc atio n and aw are ne ss o f the b e ne fits ac c rue d b y the use o f p lastic ize rs, and w e w e re use d to m aking g e ne rally lo w stre ng th co ncre te o f the typ e M1 5 to M3 0 , w hich d o no t re ally ne e d the use o f p lasticize rs. No w, since e arly 1 9 8 0 ’s, so me inte rnatio nally re no w ne d co mp anie s co llab o rate d w ith Ind ian co mpanie s and have starte d manufacturing che mical ad mixture s in Ind ia. As a part o f marke ting the y starte d e d uc ating c o nsultants, arc hite c ts, struc tural e ng ine e rs and b uild e rs ab o ut the b e ne fits o f using ad mixture s. We , in Ind ia have also starte d using hig he r stre ng th co ncre te fo r hig h rise b uild ing s and b rid g e s. Use o f Re ad y mix co ncre te has re ally p ro mo te d the use o f ad mixture s in Ind ia, in re ce nt time s. It w ill b e slig htly difficult to predict the effect and the results o f using admixtures b ecause, many a time , the chang e in the b rand o f ce me nt, ag g re g ate g rad ing , mix p ro p o rtio ns and richness o f mix alter the pro perties o f co ncrete. So metimes many ad mixtures affect mo re than o ne pro pe rty o f co ncre te . At time s, the y affe ct the d e sirab le pro pe rtie s ad ve rse ly. So me time s, mo re than o ne ad mixture is use d in the same mix. The e ffe ct o f mo re than o ne ad mixture is d iffic ult to p re d ic t. The re fo re , o ne m ust b e c autio us in the se le c tio n o f ad m ixture s and in p re d icting the e ffe ct o f the same in co ncre te . As p e r the re p o rt o f the ACI Co m m itte e 2 1 2 , ad m ixture s have b e e n c lassifie d into 1 5 g ro ups acco rding to type o f materials co nstituting the admixtures, o r characteristic affect o f the use. When ACI Co mmittee 212 submitted the repo rt in 1954, plasticizers and superplasticizers, as w e kn o w th e m to d ay, w e re n o t e xistin g . Th e re fo re , in th is g ro up in g o f ad m ixture s, plasticizers and superplasticizers and a few variatio ns in them have no w b een includ ed und er ad mixture s. Classificatio n o f admixtures as g iven by M.R. Rixo m (slig htly mo dified to include a few new mate rials) is g ive n in tab le 5 .2 2 . In this chap te r, the fo llo w ing ad mixture s and co nstructio n che micals are d e alt w ith.

Admixtures "

Plasticize rs

"

Sup e rp lasticize rs

"

Re tard e rs and Re tard ing Plasticize rs

"

Acce le rato rs and Acce le rating Plasticize rs

"

Air-e ntraining Ad mixture s

"

Po zzo lanic o r Mine ral Ad mixture s

"

Damp -p ro o fing and Wate rp ro o fing Ad mixture s

"

Gas fo rming Ad mixture s

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Air-d e training Ad mixture s

"

Alkali-ag g re g ate Exp ansio n Inhib iting Ad mixture s

"

Wo rkab ility Ad mixture s

"

Gro uting Ad mixture s

"

Co rro sio n Inhib iting Ad mixture s

"

Bo nd ing Ad mixture s

"

Fung icid al, Ge rmicid al, Inse cticid al Ad mixture s

"

Co lo uring Ad mixture s

Construction Chemicals "

Co ncre te Curing Co mp o und s

"

Po lyme r Bo nd ing Ag e nts

"

Po lyme r Mo d ifie d Mo rtar fo r Re p air and Mainte nance

"

Mo uld Re le asing Ag e nts

"

Pro te ctive and De co rative Co ating s

"

Installatio n Aid s

"

Flo o r Hard e ne rs and Dust-pro o fe rs

"

No n-shrink Hig h Stre ng th Gro ut

"

Surface Re tard e rs

"

Bo nd -aid fo r Plaste ring

"

Re ad y to use Plaste r

"

Guniting Aid

"

Co nstructio n Che micals fo r Wate r-p ro o fing 1 . Inte g ral Wate r-p ro o fing Co mp o und s 2 . Me mb rane Fo rming Co ating s 3 . Po lyme r Mo d ifie d Mine ral Slurry Co ating s 4 . Pro te ctive and De co rative Co ating s 5 . Che mical DPC 6 . Silico n Base d Wate r-re pe lle nt Mate rial 7 . Wate rp ro o fing Ad he sive fo r Tile s, Marb le and Granite 8 . Inje ctio n Gro ut fo r Cracks 9 . Jo int Se alants

Plasticizers (Water Reducers) Re q uire me nt o f rig ht w o rkab ility is the e sse nce o f g o o d co ncre te . Co ncre te in d iffe re nt situatio ns re q uire d iffe re nt d e g re e o f w o rkab ility. A hig h d e g re e o f w o rkab ility is re q uire d in situatio ns like d e e p b e ams, thin w alls o f w ate r re taining structure s w ith hig h p e rce ntag e o f steel reinfo rcement, co lumn and beam junctio ns, tremie co ncreting , pumping o f co ncrete, ho t w eather co ncreting , fo r co ncrete to be co nveyed fo r co nsiderable distance and in ready mixed co ncre te ind ustrie s. The co nve ntio nal me tho d s fo llo w e d fo r o b taining hig h w o rkab ility is b y impro ving the g radatio n, o r b y the use o f relatively hig her percentag e o f fine ag g reg ate o r b y increasing the cement co ntent. There are d ifficulties and limitatio ns to o b tain hig h w o rkab ility in the field fo r a g iven set o f co nditio ns. The easy metho d g enerally fo llo w ed at the site in mo st o f the co nd itio ns is to use e xtra w ate r unmind ful o f the harm it can d o to the stre ng th and d urab ility o f co ncre te . It has b e e n stre sse d time and ag ain in almo st all the chap te rs o f this

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b o o k to the harmful e ffe ct o f using e xtra w ate r than ne ce ssary. It is an ab use , a criminal act, and uneng ineering to use to o much w ater than necessary in co ncrete. At the same time, o ne m ust ad m it that g e tting re q uire d w o rkab ility fo r the jo b in hand w ith se t c o nd itio ns and available materials is essential and is o ften difficult. Therefo re, eng ineers at the site are g enerally placed in co nflicting situatio ns. O ften he fo llo w s the easiest path and that is adding extra w ater to fluid ise the mix. This ad d itio n o f e xtra w ate r to satisfy the ne e d fo r w o rkab le c o nc re te is amo unting to so w ing the se e d o f cance r in co ncre te . To d ay w e h ave p lastic ize rs an d su p e rp lastic ize rs to h e lp an e n g in e e r p lac e d in intrig uing situatio ns. The se p lasticize rs can he lp the d ifficult co nd itio ns fo r o b taining hig he r w o rkab ility w itho ut using e xc e ss o f w ate r. O ne m ust re m e m b e r that ad d itio n o f e xc e ss w ater, w ill o nly impro ve the fluidity o r the co nsistency but no t the w o rkability o f co ncrete. The e xc e ss w ate r w ill n o t im p ro ve th e in h e re n t g o o d q u alitie s su c h as h o m o g e n e ity an d co hesiveness o f the mix w hich red uces the tend ency fo r seg reg atio n and b leed ing . Whereas the p lastic ize d c o nc re te w ill imp ro ve the d e sirab le q ualitie s d e mand e d o f p lastic c o nc re te . The p ractice all o ve r the w o rld no w is to use p lasticize r o r sup e rp lasticize r fo r almo st all the reinfo rced co ncrete and even fo r mass co ncrete to red uce the w ater req uirement fo r making co ncre te o f hig he r w o rkab ility o r flo w ing co ncre te . The use o f sup e rp lasticize r has b e co me

Fig. 5.1. Effect of surface-active agents on deflocculating of cement grains.

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almo st an unive rsal p rac tic e to re d uc e w ate r/ c e me nt ratio fo r the g ive n w o rkab ility, w hic h naturally increases the streng th. Mo reo ver, the red uctio n in w ater/ cement ratio impro ves the d urab ility o f c o nc re te . So me time s the use o f p lastic ize rs is e mp lo ye d to re d uc e the c e me nt c o nte nt and he at o f hyd ratio n in mass c o nc re te . The o rg anic sub stanc e s o r c o mb inatio ns o f o rg anic and ino rg anic sub stanc e s, w hic h allo w a reductio n in w ater co ntent fo r the g iven w o rkability, o r g ive a hig her w o rkability at the same w ate r co nte nt, are te rme d as p lasticizing ad mixture s. The ad vantag e s are co nsid e rab le in b o th cases : in the fo rmer, co ncretes are stro ng er, and in the latter they are mo re w o rkab le. The b asic p ro d ucts co nstituting p lasticize rs are as fo llo w s: (i )

An io n ic su rfac tan ts su c h as lig n o su lp h o n ate s an d th e ir m o d ific atio n s an d d e rivative s, salts o f sulp ho nate s hyd ro carb o ns.

(ii ) No nio nic surfactants, such as p o lyg lyco l e ste rs, acid o f hyd ro xylate d carb o xylic acid s and the ir mo d ificatio ns and d e rivative s. (iii ) O the r p ro d ucts, such as carb o hyd rate s e tc. Amo ng the se , c alc ium, so d ium and ammo nium lig no sulp ho nate s are the mo st use d . Plasticize rs are use d in the amo unt o f 0 .1 % to 0 .4 % b y w e ig ht o f ce me nt. At the se d o se s, at co nstant w o rkab ility the re d uctio n in mixing w ate r is e xp e cte d to b e o f the o rd e r o f 5 % to 1 5 % . This naturally increases the streng th. The increase in w o rkab ility that can b e expected , at the same w / c ratio , may b e anything fro m 3 0 mm to 1 5 0 mm slump , d e p e nd ing o n the d o sag e , initial slump o f co ncre te , ce me nt co nte nt and typ e . A g o o d plasticizer fluid izes the mo rtar o r co ncrete in a d ifferent manner than that o f the air-entraining ag ents. So me o f the plasticizers, w hile impro ving the w o rkability, entrains air also . As the e ntrainme nt o f air re d uce s the me chanical stre ng th, a g o o d p lasticize r is o ne w hich d o e s no t cause air-e ntrainme nt in co ncre te mo re than 1 o r 2 % . O ne o f the co mmo n che micals g e ne rally use d , as me ntio ne d ab o ve is Lig no sulp ho nic acid in the fo rm o f e ithe r its calcium o r so d ium salt. This mate rial is a natural pro d uct d e rive d fro m w o o d pro ce ssing ind ustrie s. Ad mixture s b ase d o n lig no sulpho nate are fo rmulate d fro m p urifie d p ro d uc t fro m w hic h the b ulk o f the sug ars and o the r inte rfe ring im p uritie s are remo ved to lo w levels. Such a pro d uct w o uld allo w ad so rptio n into cement particles w itho ut any sig nificant inte rfe re nce s w ith the hyd ratio n p ro ce ss o r hyd rate d p ro d ucts. No rmal w ate r re d ucing ad mixture s may also b e fo rmulate d fro m w ho lly synthe tic raw mate rials. It is also o b se rve d that at a re c o m m e nd e d d o se , it d o e s no t affe c t the se tting tim e sig nific antly. Ho w e ve r, at hig he r d o sag e s than p re scrib e d , it may cause e xce ssive re tard atio n. It must b e no te d that if unre fine d and no t p ro p e rly p ro ce sse d lig no sulp ho nate is use d as raw mate rial, the b e havio ur o f p lastic ize r w o uld b e unp re d ic tab le . It is so me time s se e n that this typ e o f ad m ixture has re sulte d in so m e inc re ase in air-e ntrainm e nt. It is ad vise d that use rs sho uld fo llo w the instruc tio ns o f w e ll e stab lishe d stand ard manufac ture rs o f p lastic ize rs re g ard ing d o sag e .

Action of Plasticizers The ac tio n o f p lastic ize rs is m ainly to fluid ify the m ix and im p ro ve the w o rkab ility o f c o nc re te , m o rtar o r g ro ut. The m e c hanism s that are invo lve d c o uld b e e xp laine d in the fo llo w ing w ay:

Disp e rsio n. Po rtland c e m e nt, b e ing in fine state o f d ivisio n, w ill have a te nd e nc y to flo cculate in w et co ncrete. These flo cculatio n entraps certain amo unt o f w ater used in the mix and the re b y all the w ate r is no t fre e ly availab le to fluid ify the mix.

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Whe n p lasticize rs are use d , the y g e t ad so rb e d o n the ce me nt p article s. The ad so rp tio n o f charg ed po lymer o n the particles o f cement creates particle-to -particle repulsive fo rces w hich o verco me the attractive fo rces. This repulsive fo rce is called Zeta Po tential, w hich d epend s o n the b ase , so lid c o nte nt, q uantity o f p lastic ize r use d . The o ve rall re sult is that the c e m e nt particles are deflo cculated and dispersed. When cement particles are deflo cculated, the w ater trap p e d insid e the flo cs g e ts re le ase d and no w availab le to fluid ify the mix. Fig . 5 .1 e xp lains the me chanism. When cement particles g et flo cculated there w ill be interparticles frictio n betw een particle to particle and flo c to flo c. But in the dispersed co nditio n there is w ater in betw een the cement p article and he nce the inte rp article frictio n is re d uce d .

Retard ing Effect. It is me ntio ne d e arlie r that p lasticize r g e ts ad so rb e d o n the surface o f cement particles and fo rm a thin sheath. This thin sheath inhibits the surface hydratio n reactio n b e tw e e n w ate r and c e m e nt as lo ng as suffic ie nt p lastic ize r m o le c ule s are availab le at the p article / so lutio n inte rface . The q uantity o f availab le p lasticize rs w ill p ro g re ssive ly d e cre ase as the p o lyme rs b e co me e ntrap p e d in hyd ratio n p ro d ucts. Many re se arch w o rke rs e xp laine d that o ne o r mo re o f the fo llo w ing me chanisms may take p lace simultane o usly: "

Re d uctio n in the surface te nsio n o f w ate r.

"

Ind uce d e le ctro static re p ulsio n b e tw e e n p article s o f ce me nt.

"

Lub ricating film b e tw e e n ce me nt p article s.

"

Disp e rsio n o f ce me nt g rains, re le asing w ate r trap p e d w ithin ce me nt flo cs.

Inhib itio n o f the surfac e hyd ratio n re ac tio n o f the c e me nt p artic le s, le aving mo re w ate r to fluid ify the mix. " "

Chang e in the mo rp ho lo g y o f the hyd ratio n p ro d ucts.

"

Ind uce d ste ric hind rance p re ve nting p article -to -p article co ntact.

It may b e no ted that all plasticizer are to so me extent set retard ers, d epend ing upo n the b ase o f p lasticize rs, co nce ntratio n and d o sag e use d .

Superplasticizers (High Range Water Reducers) Superplasticizers co nstitute a relatively new categ o ry and impro ved versio n o f plasticizer, the use o f w hich w as d evelo ped in Japan and Germany d uring 1 9 6 0 and 1 9 7 0 respectively. The y are c he m ic ally d iffe re nt fro m no rm al p lastic isze rs. Use o f sup e rp lastic ize rs p e rm it the red uctio n o f w ater to the extent upto 3 0 per cent w itho ut red ucing w o rkab ility in co ntrast to the po ssib le re d uctio n up to 1 5 pe r ce nt in case o f plasticize rs. The use o f sup e rp lastic ize r is p rac tic e d fo r p ro d uc tio n o f flo w ing , se lf le ve lling , se lf co mp acting and fo r the p ro d uctio n o f hig h stre ng th and hig h p e rfo rmance co ncre te . The me chanism o f actio n o f sup e rp lasticize rs are mo re o r le ss same as e xp laine d e arlie r in c ase o f o rd inary p lastic ize r. O nly thing is that the sup e rp lastic ize rs are mo re p o w e rful as dispersing ag ents and they are hig h rang e w ater reducers. They are called Hig h Rang e Water Re d uce rs in Ame rican lite rature . It is the use o f supe rplasticize r w hich has mad e it po ssib le to use w / c as lo w as 0 .2 5 o r e ve n lo w e r and ye t to make flo w ing co ncre te to o b tain stre ng th o f the o rd e r 1 2 0 Mpa o r mo re . It is the use o f supe rplasticize r w hich has mad e it po ssib le to use fly ash, slag and p articularly silica fume to make hig h p e rfo rmance co ncre te . The use o f sup e rp lastic ize r in c o nc re te is an imp o rtant mile sto ne in the ad vanc e me nt o f co ncre te te chno lo g y. Since the ir intro d uctio n in the e arly 1 9 6 0 in Jap an and in the e arly 1 9 7 0 in Ge rmany, it is w id e ly use d all o ve r the w o rld . Ind ia is catching up w ith the use o f

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superplasticizer in the co nstructio n o f hig h rise b uild ing s, lo ng span b rid g es and the recently b e c o m e p o p u lar Re ad y Mixe d Co n c re te In d u stry. Co m m o n b u ild e rs an d G o ve rn m e n t d e p artme nts are ye t to take up the use o f this use ful mate rial. Sup e rp lasticize rs can p ro d uce : "

at the same w / c ratio much mo re w o rkab le co ncre te than the plain o ne s,

"

fo r the same w o rkab ility, it pe rmits the use o f lo w e r w / c ratio ,

as a c o nse q ue nc e o f inc re ase d stre ng th w ith lo w e r w / c ratio , it also p e rm its a re d uctio n o f ce me nt co nte nt. "

The superplasticizers also pro duce a ho mo g eneo us, co hesive co ncrete g enerally w itho ut any te nd e ncy fo r se g re g atio n and b le e d ing .

Classification of Superplasticizer. Fo llo w ing are a few po lymers w hich are co mmo nly use d as b ase fo r sup e rp lasticize rs. "

Sulp ho nate d malanie -fo rmald e hyd e co nd e nsate s (SMF)

Su lp h o n a te d n a p h th a le n e -fo rm a ld e h yd e co nd e nsate s (SNF) " "

Mo d ifie d lig no sulp ho nate s (MLS)

"

O the r typ e s

In ad d itio n to the ab o ve , in o the r c o untrie s the fo llo w in g n e w g e n e ratio n sup e rp lastic ize rs are also use d . "

Acrylic p o lyme r b ase d (AP)

" Co p o lym e r o f c arb o xylic ac rylic ac id w ith acrylic e ste r (CAE) "

Cro ss linke d acrylic p o lyme r (CLAP)

"

Po lycarb o xylate e ste r (PC)

"

Multicarb o xylate the rs (MCE)

"

Co mb inatio ns o f ab o ve .

O ut o f the ab o ve new g eneratio n superplasticizers b a se d o n c a rb o xylic a c rylic e ste r (CAE) a n d multicarb o xylate the r (MCE) are d iscusse d late r. As far as o u r c o u n try is c o n c e rn e d , at p re se n t (2000 AD), w e manufacture and use the first fo ur types of su p e rp la stic ize rs. Th e new g e n e ra tio n supe rplasticize rs have b e e n trie d in re ce nt pro je cts, b ut it w as no t fo und feasib le fo r g eneral usag e o n acco unt o f hig h co st. The first fo ur cate g o rie s o f p ro d ucts d iffe r fro m o ne ano the r b e cause o f the b ase co mp o ne nt o r o n a c c o u n t o f d iffe re n t m o le c u la r w e ig h t. As a c o n se q u e n c e e a c h c o m m e rc ia l p ro d u c t w ill h a ve d iffe re n t ac tio n o n c e m e n ts. W h ilst th e d o sag e o f c o n ve n tio n al p lastic ize rs d o n o t e xc e e d 0 . 2 5 % b y w eig ht o f cement in case o f lig no sulpho nates, o r 0 .1 % in case o f carb o xylic acid s, the pro d ucts o f type SMF o r NSF are u se d c o n sid e rab ly h ig h d o sag e s (0 . 5 % to 3 .0 0 % ), sin c e th e y d o n o t e n train air. Th e m o d ifie d

N 1 to N 5 , infra-red spectrograph of a naphthalene superplasticizer; M 6 , lignosulfonate superplasticizer and a mixed superplasticizer.

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lig no sulp ho nate (LS) b ase d ad m ixture s, w hic h have an e ffe c tive fluid izing ac tio n, b ut at the re lative ly hig h d o sag e s, the y can p ro d uce und e sirab le e ffe cts, such as acce le ratio ns o r d e lay in se tting time s. Mo re o ve r, the y inc re ase the air-e ntrainme nt in c o nc re te . 5 .1 Plastic ize rs and sup e rp lastic ize rs are w ate r b ase d . The so lid c o nte nts c an vary to any e xte nt in the p ro d uc ts m anufac ture d b y d iffe re nt c o m p anie s. Co st sho uld b e b ase d o n e fficie ncie s and so lid co nte nt, b ut no t o n vo lume o r w e ig ht b asis. Ge ne rally in p ro je cts co st o f sup e rp lastic ize rs sho uld b e w o rke d fo r o ne c ub ic me te r o f c o nc re te . Co nsiste nc y in the q uality o f sup e rp lastic ize rs sup p lie d o ve r a p e rio d o f time c an b e te ste d and c o mp are d b y “Infrare d Sp e ctro me try”.

Effects of Superplasticizers on Fresh Concrete It is to b e no te d that d ram atic im p ro ve m e nt in w o rkab ility is no t sho w ing up w he n plasticizers o r superplasticizers are ad d ed to very stiff o r w hat is called zero slump co ncrete at no minal d o sag e s. A mix w ith an initial slump o f ab o ut 2 to 3 c m c an o nly b e fluid ise d b y p lasticize rs o r sup e rp lasticize rs at no minal d o sag e s. A hig h d o sag e is re q uire d to fluid ify no slump co ncre te . An imp ro ve me nt in slump value can b e o b taine d to the e xte nt o f 2 5 cm o r mo re depending upo n the initial slump o f the mix, the do sag e and cement co ntent. It is o ften no tice d that slump incre ase s w ith incre ase in d o sag e . But the re is no ap p re ciab le incre ase in slump beyo nd certain limit o f do sag e. As a matter o f fact, the o verdo sag e may so metime harm the co ncre te . A typical curve , sho w ing the slump and d o sag e is sho w n in Fig . 5 .2 .

Compatibility of Superplasticizers and Cement It h as b e e n n o tic e d th at all su p e rp lastic ize rs are n o t sh o w in g th e sam e e xte n t o f imp ro ve me nt in fluid ity w ith all typ e s o f c e me nts. So me sup e rp lastic ize rs may sho w hig he r fluid izing e ffe c t o n so me typ e o f c e me nt than o the r c e me nt. The re is no thing w ro ng w ith e ithe r the sup e rp lasticize r o r that o f ce me nt. The fact is that the y are just no t co mp atib le to sho w maximum fluid izing effect. O ptimum fluid izing effect at lo w est d o sag e is an eco no mical co nsid eratio n. Giving maximum fluid izing effect fo r a particular superplasticizer and a cement is ve ry co mp le x invo lving many facto rs like co mp o sitio n o f ce me nt, fine ne ss o f ce me nt e tc. Altho ug h co mpatib ility pro b lem lo o ks to b e very co mplex, it co uld b e mo re o r less so lved b y sim p le ro ug h and re ad y fie ld m e tho d . Inc id e ntally this sim p le fie ld te st sho w s also the o p timum d o se o f the sup e rp lasticize r to the ce me nt. Fo llo w ing me tho d s co uld b e ad o p te d . "

Marsh c o ne te st

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"

Mini slum p te st

"

Flo w tab le te st.

O ut o f the ab o ve , Marsh co ne te st g ive s b e tte r re sults. In the Marsh co ne te st, ce me nt slurry is mad e and its flo w ab ility is fo und o ut. In co ncre te , re ally co me to think o f it, it is the ce me nt paste that influe nce s flo w ab ility. Altho ug h, the q uantity o f ag g re g ate s, its shape and te xture e tc . w ill have so m e influe nc e , it is the p aste that w ill have g re ate r influe nc e . The presence o f ag g reg ate w ill make the test mo re co mplex and o ften erratic. Whereas the using o f g ro ut alo ne w ill make the te st simple , co nsiste nt and ind icative o f the fluid ifying e ffe ct o f sup e rp lastic ize r w ith a c e m e nt. The fo llo w ing p ro c e d ure is ad o p te d in Marsh c o ne te st. Marsh c o n e is a c o n ic al b ra ss ve sse l, w h ic h h a s a smo o th ape rture at the b o tto m o f d iam e te r 5 m m . The p ro file o f th e ap p aratu s is sh o w n in Fig . 5 .3 . Ta ke 2 kg c e m e n t, p ro p o se d to b e u se d a t th e p ro je ct. Take o ne litre o f w ate r (w / c = 0 .5 ) an d say 0 .1 % o f plasticizer. Mix them tho ro ug hly in a me c hanic al mixe r (Ho b art m ixe r is p re fe ra b le ) fo r tw o minutes. Hand mixing do es no t g ive c o n siste n t re su lts b e c ause o f unavo id ab le lum p fo rm a tio n w h ic h b lo c ks th e

Fig. 5.3. Marsh Cone Test.

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133

ap e rture . If hand mixing is d o ne , the slurry sho uld b e sie ve d thro ug h 1 .1 8 sie ve to e xclud e lum p s. Take o ne litre slurry and p o ur it into m arsh c o ne d uly c lo sing the ap e rture w ith a fing e r. Start a sto p w atch and simultane o usly re mo ve the fing e r. Find o ut the time take n in se co nd s, fo r co mp le te flo w o ut o f the slurry. The time in se co nd s is calle d the “Marsh Co ne Time ”. Re p e at the te st w ith d iffe re nt d o sag e s o f p lastic ize r. Plo t a g rap h c o nne c ting Marsh c o n e tim e in se c o n d s an d d o sag e s o f p lastic ize r o r su p e rp lastic ize r. A typ ic al g rap h is sho w n in Fig . 5 .4 . The d o se at w hich the Marsh co ne time is lo w e st is calle d the saturatio n p o in t. Th e d o se is th e o p tim u m d o se fo r th a t b ra n d o f c e m e n t a n d p la stic ize r o r sup e rp lastic ize r fo r that w / c ratio .

Fig. 5.5. Marsh cone viscometer with attachments, 6 mm, 8 mm and 11 mm.

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Mixing by mechanical mixer is important for consistent results.

Th e te st c o uld b e c arrie d o ut fo r vario us b ran d s o f c e m e n t an d vario us b ran d s o f sup e rp lastic ize r at d iffe re nt w / c ratio . Alte rnative ly w / c ratio c o uld b e take n as fixe d b y Co ncrete Mix Desig n and this co uld b e a fixed parameter and o ther tw o namely the b rand o f c e me nt and typ e o f sup e rp lastic ize r c o uld b e varie d to find o ut the o p timum re sult. Lo t o f useful data co uld b e co llected fro m simple Marsh co ne test. Each test takes hardly 15 minutes. The typ ic al te st re sults as o b taine d at Mc Bauc he mie lab o rato ry is sho w n in Tab le 5 .1 . It is no te d that the re sult o f the te st is co nsiste nt. It is e xp e rie nce d that Marsh co ne w ith ap e rture o f 5 mm is no t useful fo r finding the Marsh co ne time o f thick slurry o r finding retarding effect o f sup e rp lasticize r o r the fluid izing e ffe ct o f mo rtar samp le . Fo r this p urp o se , the re are o the r attachme nts having b ig g e r ap e rture o f d iame te r 6 mm, 8 mm and 1 1 mm availab le w hich co uld b e use d . Fig . 5 .5 sho w s so me fo rms o f o the r attachme nts. Visc o sity o f c e m e nt p aste o r m o rtar w ith sup e rp lastic ize r c an also b e m e asure d b y Bro o kfie ld Visc o m e te r, o r Ro to n Visc o m e te r.

Flow time as a function of the supeplasticizer dosage

Admixtures and Construction Chemicals !

Represents the case of a fully compatible combination of cement and superplasticizer

135

Represents the case of incompatibility

Ta ble . 5 .1 . Com pa t ibilit y St udy of Pla st ic ize rs a nd Supe r pla st ic ize rs w it h Diffe re nt Ce m e nt s Ce m e nt = 2 k g. Sr. No.

Dosage % by wt of cement

Dosage Quantity in ml.

W/C = 0 .4 5

Type of Plasticizer

Marsh Cone Time for Cement Slurry in Seconds Cement Cement Cement Brand I Brand II Brand III

1

0 .1

2

Plasticize r A

105

110

120

2

0 .1

2

Plasticize r B

72

75

103

3

0 .1

2

Plasticize r C

86

88

105

4

0 .5

10

Sup e rp lasticize r A

69

72

72

5

0 .5

10

Supe rplasticize r B

59

62

65

6

0 .5

10

Sup e rp lasticize r C

165

170

202

1

0 .2

4

Plasticize r A

75

80

82

2

0 .2

4

Plasticize r B

64

69

70

3

0 .2

4

Plasticize r C

69

75

76

4

0 .7

14

Sup e rp lasticize r A

63

68

66

5

0 .7

14

Supe rplasticize r B

57

60

62

6

0 .7

14

Sup e rp lasticize r C

152

156

176

1

0 .3

6

Plasticize r A

75

69

70

2

0 .3

6

Plasticize r B

64

65

62

3

0 .3

6

Plasticize r C

69

68

65

4

0 .9

18

Sup e rp lasticize r A

63

65

62

5

0 .9

18

Supe rplasticize r B

57

55

60

6

0 .9

18

Sup e rp lasticize r C

127

132

134

1

1 .1

22

Sup e rp lasticize r A

58

60

60

2

1.1

22

Su p e rp lastic ize r B

45

50

46

3

1.1

22

Su p e rp lastic ize r C

75

83

90

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Fro m the ab o ve tab le it can b e se e n that ce me nt b rand I is sho w ing g o o d co mpatib ility than the o the r tw o b rand s o f ce me nts w ith all the p lasticize rs and sup e rp lasticize rs. If o ne is to cho o se the plasticizer and superplasticizers, o ne sho uld g o fo r plasticizer B o r superplasticizer B.Yo u also no tice that sup e rp lasticize r B co nsistantly g ive s ve ry g o o d p lasticising e ffe ct.

Factors Effecting the Workability " " " " " " "

Typ e o f sup e rp lasticize rs Do sag e Mix co mpo sitio n Variab ility in ce me nt co mpo sitio n and pro pe rtie s Mixing p ro ce d ure Eq uip me nts O the rs

Type of Superplasticizers It is a w e ll e stab lishe d fac t that the ave rag e m o le c ular w e ig ht o f the p lastic ize r is o f primary impo rtance fo r its efficiency as plasticizer in co ncrete. The hig her the mo lecular w eig ht, the hig he r is the e ffic ie nc y. Ho w e ve r, it sho uld b e no te d that the re is a maximum value o f mo le cular w e ig ht b e yo nd w hich e fficie ncy is e xp e cte d to d e cre ase . It may b e furthe r no te d th at se ve ral in trin sic p ro p e rtie s o f th e sup e rp lastic ize rs m ay in flue n c e th e p e rfo rm an c e . The re fo re , it is d iffic ult to c o m p are the e ffic ie nc y o f o ne p lastic ize r fro m the o the r in the ab se nce o f numb e r o f re late d p ro p e rtie s o f sup e rp lasticize rs.

Dosage. It has b e e n alre ad y e xp laine d w hile d e scrib ing the Marsh co ne te st that the d o sag e o f sup e rp lastic ize r influe nc e s the visc o sity o f g ro ut and he nc e the w o rkab ility o f

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137

co ncrete. The o ptimum d o sag e can b e ascertained fro m Marsh co ne test if b rand o f cement, plasticizer and w / c ratio is alread y fixed . Simple Marsh co ne test can g ive realistic d o sag e than manufacture rs instructio ns w hich is g e ne ral in nature . In o ur co untry g e ne rally lo w d o sag e is ad o pted fo r no rmal co ncreting o peratio ns. A d o sag e mo re than 2.5% b y w eig ht o f cement is rare ly use d . But in o the r co untrie s much hig he r d o sag e s up to 4 to 5 % are use d in sp e cial situatio ns. It has b e e n re p o rte d in lite rature s that up to a d o sag e o f ab o ut 3 % the re are no harmful effect o n the hardening pro perties o f co ncrete. Hig her do sag e is said to have affected the shrinkag e and cre e p p ro p e rtie s.

Mix Composition. The m ix c o m p o sitio n p artic ularly the ag g re g ate / c e m e nt ratio o r richness o f the mix, w / c ratio , and use o f o ther supplementary cementing materials like fly ash o r silica fume affe cts the w o rkab ility. We tte r the mix b e tte r is the d isp e rsio n o f ce me nt g rains and he nce b e tte r w o rkab ility. The size and shap e o f ag g re g ate , sand g rad ing w ill also have influe nce o n the fluid ifying e ffe ct. Variability in Cement Composition. Th e variab ility in c e m e n t w ith re sp e c t to co mp o und co mp o sitio n, in p articular C3 A co nte nt, C3 S/ C2 S ratio , fine ne ss o f ce me nt, alkali co ntent and g ypsum co ntent are respo nsible fo r the lack o f co mpatibility w ith a particular type o f superplasticizer and their perfo rmance in co ncrete. O ut o f the ab o ve C3 A co ntent w ill have o ver-rid ing influence o n the perfo rmance o f superplasticizer. Fig . 5 .6 sho w s the effect o f C3 A co nte nt. Mixing Procedure Plastic ize r m ust b e p ro p e rly and intim ate ly m ixe d in c o nc re te to b ring ab o ut p ro p e r d isp e rsio n w ith ce me nt p article s. The re fo re , hand mixing is o ut o f q ue stio n. Whe n yo u use co ncrete mixer, g enerally ab o ut 80% o f the to tal w ater is ad d ed to the empty d rum and then mate rials are lo ad e d into the d rum b y ho p p e r. Whe n yo u use sup e rp lasticize r, it is b e tte r to ad d all the w ate r to the d rum ke e p ing ab o ut o ne litre o f w ate r in sp are . The e xact q uantity o f sup e rp lasticize r is d ilute d w ith that o ne litre o f w ate r and thro w n into the d rum in tw o o r three installments o ver the w ell mixed co ncrete so that pro per dispersio n o f plasticizer actually take s p lace in the d rum. Having ad d e d the p lasticize r, the co ncre te must b e mixe d fo r ab o ut o ne mo re minute b e fo re d ischarg ing . The p ractice o f ad d ing sup e rp lasticize r alo ng w ith the b ulk m ixin g w ate r is n o t g ivin g g o o d re su lts. Exp e rim e n ta l re su lt sh o w e d th a t a d d in g p la stic ize r a fte r th re e m in u te s o f m ixin g h a s yie ld e d b e tte r re sults. Fig . 5 . 7 sh o w s th e e ffe c t o f ad d itio n o f w ate r th re e minute s afte r mixing w ith the w ate r. It h a s b e e n fo u n d th a t a ll o ve r In d ia , e le ctrically o p e rate d small lab o rato ry m ixe r is u se d fo r co nd ucting lab o rato ry tria ls. Th e se la b o ra to ry

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mixe rs are ine fficie nt and the y d o no t mix the co ncre te ing re d ie nts tho ro ug hly, le ave ap art the e fficie nt mixing o f sup e rp lasticize rs. The re sults o b taine d fro m the trials, using lab o rato ry mixe r, is far fro m re alistic. In such situatio ns the fo llo w ing p ro ce d ure g ive s b e tte r co nsiste nt re su lts. Ad d all the c alc ulate d q uantity o f w ate r into the d rum . The n ad d all the q uantity o f c e m e nt and sand . Mix the se ing re d ie nts ve ry w e ll. W he n the y are w e ll m ixe d , ad d the calculate d q uantity o f p lasticize r and tho ro ug hly mix the m to g e the r. Yo u w ill no tice the full actio n o f plasticizer in fluid ifying the mix. Then yo u ad d the co urse ag g reg ates and mix them fo r ano the r o ne minute s. Whe n the mixe r is no t e fficie nt as in the case o f lab o rato ry mixe r, the ab o ve p ro ce d ure o f mixing w ill g ive g o o d re sults.

Equipment. It has b e e n d iscusse d ab o ve that ine fficie nt sp o t mixe r at site o r lab o rato ry that is small mixe r d o e s no t e xplo it the actio n o f supe rplasticize r fully, o fte n the y d o no t e ve n mix the co ncrete unifo rmly and pro perly. The fab ricatio n o f co ncrete mixer is no t a simple jo b . The shape o f the drum, its bo tto m diameter and shape, number o f blades, the ang le o f blades, le ng th and d e p th o f b lad e s, the sp ac e b e tw e e n d rum and b lad e and sp ac e b e tw e e n the b lad e s w ill have lo t to d o w ith the mixing e fficie ncy. The manufacture rs o fte n ne g le ct the se details. Generally pan mixer sho w b etter efficiency particularly in case o f small scale lab o rato ry mixers. Such small capacity pan mixers are no t g enerally available fo r trial. So me manufacturers o f co ncre te mixe rs have no w starte d fab ricatio n and sup p ly o f p an mixe rs. The mixe rs in the b atching p lant are o f cap acity half a cub ic me te r and ab o ve . The y are g enerally o f pan type. They are w ell d esig ned and fab ricated and as such every efficient. The mixing time is aro und 2 0 seco nd s. Within this sho rt spell o f 2 0 seco nd s, very intimate mixing is do ne. It is o b served that fo r identical parameters co ncrete mixed in the b atching plant g ives ab o ut 20 to 30 mm mo re slump than trial mix carried o ut in lab o rato ry using small, inefficient mixe rs.

Others. The slump value o f a superplasticized co ncrete may also b e affected o n acco unt o f o the r ad mixture s use d co ncurre ntly in co ncre te such as air-e ntraining ag e nt, fly ash, slag o r silica fume . The te mp e rature and re lative humid ity also affe ct the re sult. Site Problems in the use of Superplasticizers So me o f the p ractical site p ro b le ms in the use o f sup e rp lasticize rs are liste d b e lo w : "

Slump o f re fe re nce mix. (i.e. , co ncre te w itho ut plasticize r)

"

Ine fficie nt lab o rato ry mixe r fo r trial.

"

Se q ue nce o f ad d itio n o f p lasticize r.

"

Pro b le m w ith crushe r d ust.

"

Pro b le m w ith crushe d sand .

"

Imp o rtance o f shap e and g rad ing o f co arse ag g re g ate .

"

Co mpatib ility w ith ce me nt.

"

Se le ctio n o f p lasticize r and sup e rp lasticize r.

"

De te rminatio n o f d o sag e .

"

Slump lo ss.

"

Ho w to re d uce slump lo ss.

"

Casting o f cub e s.

"

Co mpactio n at site .

"

Se g re g atio n and b le e d ing .

"

Finishing .

"

Re mo val o f fo rm w o rk.

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139

It is n o t in te n d e d to d isc u ss e ac h o f th e ab o ve p o in ts se p arate ly as so m e o f th e p ro b le m s are se lf e xp lanato ry. The p o ints m e ntio ne d ab o ve are d isc usse d in g e ne ral and w he re ap p lic ab le , so lutio n to the p ro b le ms are also ind ic ate d , fo r g e ne ral g uid anc e . It is m e n tio n e d e arlie r th at ve ry stiff ze ro slu m p c o n c re te c an n o t b e p e rc e p tib ly imp ro ve d at no minal d o sag e . Altho ug h the re is imp ro ve me nt in rhe o lo g y o f matrix w ith the use o f sup e rp lastic ize rs, it d o e s no t b e c o m e p e rc e p tib le and m e asurab le b y slum p te st. If the initial c o nc re te mix is d e sig ne d in suc h a w ay as to have ab o ut 2 to 3 c m slump , the n o nly the slump co uld b e enhanced to a hig h level. Hig h d o sag e o f plasticizer may g ive g o o d slum p in c ase o f a stiff m ix. But it is une c o no m ic al. Initial slum p in the re fe re nc e c o nc re te is an imp o rtant c o nsid e ratio n. Ge ne rally availab le lab o rato ry m ixe rs are ine ffic ie nt e sp e c ially w he n sm all q uantity o f p lasticize rs are use d fo r trial mix. Use o f p an mixe r w ill g ive b e tte r re sults. The se q ue nce and me tho d o f ad d itio n o f sup e rp lasticize r is d e scrib e d e arlie r fo r g o o d re sults. In the ye ars to co me fo r larg e p ro je cts o ne w ill have to g o fo r crushe d sand . In sp ite o f the mo d e rn w e ll d e sig ne d c rushe rs, hig he r q uantity o f d ust in g e ne rally p re se nt. This d ust interferes w ith plasticizing pro perties o f mix and hence anticipated results are no t o b tainab le. Fo r the co nstructio n o f Mumb ai-Pune express hig hw ay, they specified o nly the crushed sand . The initial trials p re se nte d lo t o f p ro b le ms w ith the p re se nce o f e xce ss o f crushe r d ust w he n reaso nab le d o ses o f superplasticizer w ere used . In so me secto rs, so me co ntracto rs had to g o fo r c o m b inatio n o f natural sand and c rushe d sand . W he re as so m e o the r c o ntrac ting firm m anag e d to use o nly c rushe d sand . W hate ve r it is, the d ust in c rushe d sand affe c ts the p e rfo rmance o f p lasticize rs. Incid e ntally, e xce ss o f crushe r d ust incre ase s d rying shrinkag e . Fo r no rm al stre ng th c o nc re te up to 3 0 o r 4 0 MPa, the shap e o f ag g re g ate is no t o f p rimary imp o rtance . Fo r p ro d uctio n o f hig h stre ng th co ncre te o f the o rd e r o f 5 0 MPa o r 6 0 MPa, the w / c ratio beco me so lo w that shape o f ag g reg ates beco mes very impo rtant and also the use o f supe rplasticize rs b e co me s e sse ntial fo r the re q uire me nt o f w o rkab ility, particularly w he n co ncre te is to b e transp o rte d o ve r lo ng d istance and p ump e d . In o ne situatio n in the co nstructio n o f hig h rise b uild ing at Mumb ai w here 60 MPa co ncrete w as used , w ell g rad ed , c u b ic a l sh a p e d a g g re g a te , sp e c ially m an u fac tu re d , c o u ld so lve the p ro b le m. In o ne o f the p ro je c t site s a t D e lh i, w h e re ag g re g ate s flakin e ss in d e x w as ve ry hig h, p artic ularly in 1 0 mm ag g re g ate , th e ac h ie ve m e n t o f h ig h slu m p w a s fo u n d to b e d iffic u lt in sp ite o f u sin g h ig h d o sag e o f sup e rp lasticize rs. In m any site s, c o m p atib ility p ro b le m w ith c e m e n t a n d p la stic ize r b e c o m e s p rim a ry co nsideratio ns. This can be so lved b y simple Marsh co ne test. Marsh c o n e te st a lso in d ic a te s th e e co no mical d o sag e .

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Sup e rp lastic ise rs are c o stly and the y o p e rate at hig he r d o sag e s. If the slum p value at a po int o f b atching and the slump value at a po int o f placing is kno w n, b y co nd ucting a few fie ld trials, it is p o ssib le to arrive at a d e cisio n w he the r p lasticize r w o uld b e sufficie nt o r o ne sho uld g o fo r sup e rp lasticize r. Site trials are also re q uire d to find o ut the d o sag e , the slump value and p ro b ab le slump lo ss.

Slump Loss. O ne

o f the mo st imp o rtant nag g ing site p ro b le m is the lo ss o f slump .

Slump at mixing po int is no t o f much impo rtance, b ut the slump at placing po int is o f primary imp o rtance . O fte n the re is d e lay b e tw e e n mixing and p lacing . Achie ving hig h slump at the mixer, o nly to be lo st w ith time, befo re placing is a bad eco no my. Lo ss o f slump is natural even w ith unp lastic ize d c o nc re te , b ut rate o f lo ss slump is little mo re in c ase o f sup e rp lastic ize d co ncre te . Fig . 5 .8 ind icate s the slump lo ss w ith time . Many use rs d e mand the slump value at mixing o r b atc hing p lant and also sp e c ify the slum p value afte r a d e lay o f 1 o r 2 o r 3 ho urs p e rio d at p lac ing p o int. It is no t a c o rre c t sp e cificatio n. Use r sho uld o nly sp e cify the slump value at p lacing p o int afte r a d e lay o f 1 o r 2 o f 3 ho urs. It sho uld b e le ft to the sup e rp lastic ize r manufac ture rs o r c o nc re te sup p lie r to sup p ly co ncre te o f slump value as d e mand e d b y use r at the time o f p lacing o f co ncre te .

Steps for Reducing Slump Loss The slump lo ss can b e manag e d b y taking any o ne o r mo re o f the fo llo w ing actio ns: "

Initial hig h slump .

"

Using re tard e rs.

"

Using re tard ing p lasticize r o r sup e rp lasticize r.

"

By re p e titive d o se .

"

By d o sing at final p o int.

"

By ke e p ing te mp e rature lo w.

"

By using co mp atib le sup e rp lasticize r w ith ce me nt.

When very hig h slump is manag ed at the mixing po int, even if lo ss o f slump takes place, still the resid ual slump w ill b e g o o d eno ug h fo r satisfacto ry placing o f co ncrete. Altho ug h this me tho d is no t a g o o d and e co no mical me tho d so me time this me tho d is ad o pte d . Fig . 5 .9 . Pure re tard e rs are use d at the tim e o f m ixing . This w ill ke e p the c o nc re te in a p lastic c o n d itio n o ve r a lo n g tim e . Ju st b e fo re ad d in g an ap p ro p riate d o se o f p lastic ize r o r sup e rp lastic ize r w hic h w ill g ive d e sira b le slump value fo r placing re q u ire m e n ts. Th is is p o ssib le o n ly w h e n c o n c re te is c o n ve ye d b y transit mixers. So me tim e in ste ad o f u sin g p u re re ta rd e rs a n d p lasticize r se p arate ly, a re tard ing p lastic ize r, o r r e t a r d i n g sup e rp lasticize r is use d in an appro priate d o se in the initial stag e itself. Th e re ta rd in g

Admixtures and Construction Chemicals !

141

p lastic ize r o r sup e rp lastic ize rs re tains the slump fo r lo ng e r p e rio d s w hic h may b e suffic ie nt fo r p lacing . O ne o f the c o m m o n m e tho d s to c o m b at the slum p lo ss is to g ive re p e titive d o se s at inte rvals and the re b y b o o sting the slump so that re q uire d slump is maintaine d fo r lo ng time . Fig ure 5 .1 0 sho w s the typ ical re p e titive me tho d o f using p lasticize r. The time inte rval sho uld b e cho se n in such a w ay that the co ncre te w ill have such a re sid ual slump value w hich can b e b o o ste d up . So me time s a small d o se o f sup e rp lasticize r is ad d e d at the b e g inning and the slump is b o o ste d up . W he n the co ncre te arrive s at the p o uring p o int, it w ill still have so me re sid ual slump b ut no t g o o d e no ug h fo r p lacing b y p ump o r b y tre mie . Fo r p ump ing co ncre te yo u ne e d a slump o f aro und 1 0 0 mm and tre mie p lacing the d e sirab le slump is 1 5 0 mm. At this po int an appro priate do se o f superplasticizer is added to b o o st up the slump to req uired level.

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It is a c o m m o n kno w le d g e that hyd ratio n pro cess can be retarded by ke e p ing the te m p e rature o f th e c o n c re te lo w . At lo w te m p e ra tu re th e slump lo ss is also slo w. Use o f ic e fla ke s in ste a d o f w ater is reso rted to reduce the slum p lo ss. O fte n the u se o f ic e fla ke s is a n ad d itio nal ste p to re d uc e the slump lo ss. Use of h ig h ly co mpatible admixture w ith the g ive n c e me nt o r vic e ve rsa w ill also re d uc e the p ro b le m o f slum p lo ss. A c e m e n t w ith lo w C 3 A c o n te n t w ill b e o f use in th is re g ard . In o n e o f th e lim ite d trial c o n d u c te d , 4 3 g rad e c e m e n t h as sh o w n b e tte r co mp atib ility and p e rfo rmance than 3 3 o r 5 3 g rad e ce me nt. It w ill b e sho w n late r that the ne w g e ne ratio n sup e rp lasticize rs are an e ffe ctive answ e r fo r this se ve re p ro b le m o f slump lo ss. Re fe r Fig . 5 .1 4 .

Other Potential Problems So metimes, it is po ssib le that a stro ng retard atio n and excessive air-entrainment may take p lac e w he n lig no sulp ho nate s are use d in larg e d o se , p artic ularly w he n the und e sirab le c o m p o ne nts in c o m m e rc ial lig no sulp ho nate s are no t re m o ve d . In so m e sup e rp lastic ize rs, pro b lems like lo w fluidificatio n, rapid slump lo ss, severe seg reg atio n, have b een repo rted. The pro b le m o f inco mpatib ility se e ms to b e o ne o f the co mmo n pro b le ms g e ne rally me t w ith in the field . The practical appro ach to so lve these pro b lems is to cro ss test w ith o ther plasticizers o r o the r ce me nt samp le s, and p ractical so lutio ns arrive d . In a o ve rd o se d mix, ce me nt p aste may b eco me to o fluid and no lo ng er retain the co arse o r even fine ag g reg ates in suspensio n, c ausing se ve re se g re g atio n. In suc h c ase s e ithe r the d o se c o uld b e re d uc e d o r ag g re g ate co nte nt, p articularly the sand co nte nt may b e incre ase d . W h e n c o n c re te p u m p an d p lac e r b o o m are u se d fo r p lac in g c o n c re te , th e slu m p re q uire m e nt is aro und 1 0 0 m m . Sup p o se 1 0 0 m m slum p c o nc re te is use d fo r a ro o f slab casting , such a hig h slump w hich is und e sirab le fo r ro o f casting , cause s pro b le ms b y w ay o f se g re g atio n and b le e d ing p articularly in the hand s o f ine xp e rie nce d w o rke rs. Such co ncre te w ill have to b e hand le d w ith care and und e rstand ing . Sim ilarly, w h ile c astin g c u b e s u sin g h ig h ly p lastic ize d c o n c re te , sp e c ial c are an d und e rstand ing o f co ncre te is re q uire d . Co mp actio n o f cub e s can no t b e d o ne in the usual m e tho d o f vib rating o r e ve n tam p ing . If the c asting o f the c ub e is d o ne b lind ly w itho ut understanding the b ehavio ur o f such plastic co ncrete, serio us seg reg atio n o ccurs in the cub e m o u ld . To p h alf o f th e c u b e m o u ld c o n sists o n ly o f m o rtar an d is d e vo id o f c o arse ag g re g ate s, w ith the re sult, that such se g re g ate d co ncre te cub e s sho w ve ry lo w stre ng th.

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Th e y b lam e th e p lastic ize rs o r c e m e n t fo r su c h lo w stre n g th . O fte n su c h an o m alo u s situatio ns have c o me to the no tic e e ve n w ith majo r c o ntrac to rs.

Effect of Superplasticizers on the Properties of Hardened Concrete Plastic ize rs o r su p e rp lastic ize rs d o n o t p artic ip ate in an y c h e m ic al re ac tio n s w ith c e m e nt o r b le nd ing m ate rial use d in c o nc re te . The ir ac tio ns are o nly p hysic al in fluid izing the mix, mad e e ve n w ith lo w w ate r co nte nt. The ir flud ifying actio n lasts o nly as lo ng as the m ix is in p lastic c o nd itio n. O nc e the e ffe c t o f ad so rb e d laye r is lo st, the hyd ratio n p ro c e ss co ntinue s no rmally. It can b e cate g o rically said that the use o f rig ht q uality o f p lasticize rs o r sup e rp lasticize rs w he n use d in usual small d o se (say up to 3 % b y w e ig ht o f ce me nt) the re is n o b a d e ffe c t o n th e p ro p e rtie s o f h a rd e n e d c o n c re te . O n ly in c a se o f b a d q u a lity lig no sulp ho nate b ase d p lasticize r is use d , it may re sult in air-e ntrainme nt, w hich re d uce s the stre n g th o f c o n c re te . Sin c e p lastic ize rs an d su p e rp lastic ize rs im p ro ve th e w o rkab ility, c o m p ac tab ility and fac ilitate re d uc tio n in w / c ratio , and the re b y inc re ase the stre ng th o f co ncre te , it co ntrib ute s to the alro und imp ro ve me nt in the p ro p e rtie s o f hard e ne d co ncre te . As a m atte r o f fac t, it is the use o f sup e rp lastic ize rs, w hic h is a p rag m atic ste p to im p ro ve alro u n d p ro p e rtie s o f h ard e n e d c o n c re te . Th e u se o f su p e rp lastic ize r h as b e c o m e an unavo id ab le mate rial in the mo d e rn Hig h Pe rfo rmance Co ncre te (HPC).

It has b e e n m e ntio ne d e arlie r that all p lastic ize rs and sup e rp lastic ize rs e xhib it c e rtain re tard ing p ro p e rtie s. The se re tard ing p ro p e rtie s d o no t make sig nificant d iffe re nce w he n the d o sag e is no rmal (say upto 3 % ). The streng th parameter is no t red uced b eyo nd o ne d ay. But w he n p lasticize rs are use d in hig he r d o se , the stre ng th d e ve lo p me nt w ill b e g re atly affe cte d in respect o f o ne day and even three days streng th. Ho w ever, seven day streng th and beyo nd,

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th e re w ill n o t b e a n y re d u c tio n in stre n g th . Th e typ ic a l stre n g th d e ve lo p m e n t o f lig no sulp ho nate typ e w ate r re d ucing ad mixture is sho w n in Fig . 5 .1 1 . At the same w / c ratio , nap hthale ne b ase d o r me lamine b ase d sup e rp lasticize rs d o no t co nsiderably mo dify the drying shrinkag e o f co ncrete. At the same co nsistency, they so metime re d uce d rying shrinkag e ap p re ciab ly. The to tal cre e p is hig he r w he n co ncre te co ntains nap hthale ne sulp ho nate s, at hig h w / c ratio (0.64). O n the co ntrary, w hen w / c ratio is lo w, the difference in creep betw een samples w ith and w itho ut p lasticize rs are insig nificant. Impermeab ility plays a primary ro le o n the d urab ility o f co ncrete and since this d epend s o n w / c ratio , superplasticizers sho uld exert a favo urab le effect. Superplasticizers, o w ing to the reductio n in w / c ratio , reduce the penetratio n o f chlo rides and sulphate into the co ncrete and, the re fo re , im p ro ve the ir re sistanc e to the d e -ic ing e ffe c t o f salt o r se a w ate r. Fo r the sam e re aso n, the re sistance to sulp hate attack is also imp ro ve d . Suffice it to say that the use o f p lasticize r o r sup e rp lasticize r, co uld le ad to the re d uctio n in w / c ratio , w ith o ut affe c tin g th e w o rkab ility an d th e re b y c o n c re te b e c o m e s stro n g e r. The re fo re , it w ill co ntrib ute to the alro und imp ro ve me nt o f hard e ne d p ro p e rtie s o f co ncre te .

New Generation Superplasticizers It h as b e e n am p ly b ro u g h t o u t th at su p e rp lastic ize rs are u se d , ( a ) to in c re ase th e w o rkab ility w itho ut chang ing the mixture co mp o sitio n, (b ) to re d uce the amo unt o f mixing w ater, in o rder to reduce the w / c ratio w hich results in increase o f streng th and durability, and (c ) to re d uce b o th w ate r and ce me nt in o rd e r to cut co st and incid e ntally to re d uce cre e p , shrinkag e , and he at o f hyd ratio n. O ne o f the mo st impo rtant d raw b acks o f trad itio nal superplasticizers such as SMF o r SNF o r MLS, is the slump lo ss. Slump lo ss w ith time presents a serio us limitatio n o n the advantag es o f superplasticizers. Mo re recently in Euro pe and Japan, new g eneratio n superplasticizers – all b ase d o n fam ily o f ac rylic p o lym e rs (AP) h ave b e e n in ve stig ate d . Th e n e w g e n e ratio n p lasticize rs have b e e n liste d o n p ag e 1 0 9 und e r classificatio n

Carboxylated Acrylic Ester (CAE) O u t o f th e se , tw o typ e s n am e ly c arb o xylate d ac rylic e ste r (CAE) c o p o lym e r an d multicarb o xylate the r (MCE) are o f p articular inte re st. Th e c arb o xylate d Ac rylic Este r c o n tain s c arb o xylic (CO O –) inste ad o f sulp ho nic (SO 3 –) g ro u p s p re se n t in th e SMF o r SN F. It w a s tho ug ht, as explained earlier, that the dispersio n o f c e m e nt g rain is c ause d b y the e le c tro static re p u lsio n , in c ase o f SMF an d SNF. Bu t th e re ce nt e xp e rime nts co nd ucte d b y M. Co llp ard i et al and Y.O . Tanaka et al d id no t co nfirm the ab o ve me chanism fo r the p lasticizing actio n o f the acrylic po lymers. Tab le 5.2 indicates that AP b ased superplasticizers pro d uce neg lig ib le Zeta po tential chang e (0.3 to 5.0 mV), in co ntrast to SNF – b ase d sup e rp lastic ize rs (2 3 –2 8 m V), in aq ue o us susp e nsio ns o f ce me nt p article s. 5 .4

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Ta ble 5 .2 . Z e t a pot e nt ia l of c e m e nt pa r t icle s in suspe nsion w it h supe r pla st ic ize rs 5 .4 Su p e rp lastic ize rs

Main co mpo nent

Ze ta -p o te n tia l (–m V)

A

AP*

5.0

B

AP

0.3

C

AP

1.0

D

AP

4.0

E

AP

4.0

F

AP

2.0

G

SN F

23.0

H

SN F

28.0

*AP = Polycarboxylate type Fig ure 5 .1 2 and Fig . 5 .1 3 sho w the ad so rp tio n o n c e m e nt p artic le s and Ze tap o te n tia l m e a su re m e n ts o f CAE in co mpariso n w ith SNF. In particular it is seen that adso rptio n o f CAE is abo ut 85% w hen co mpare d to ad so rptio n o f 7 5 % in case o f SN F. Fig . 5 . 1 3 in d ic a te th a t th e Z e ta po tential o f cement particle mixed w ith CAE ap p e are d to b e m uc h lo w e r than that o f SNF. It co uld b e infe rre d that in case o f APb ase d ad mixture s, the incre ase in fluid ity is n o t b e c a u se o f e le c tro sta tic re p u lsio n asso c iate d w ith Ze ta p o te ntial b ut w o uld se e m to b e the hig he r p o lyme r ad so rp tio n and ste ric hind ranc e e ffe c t. The ne w family o f sup e rp lastic ize rs b ase d o n ac rylic p o lyme rs, sho w the fo llo w ing characte ristics: (a ) Flo w ing co ncre te can b e pro d uce d at lo w e r w / c ratio . (b ) The effectiveness do es no t depend o n the additio n pro cedure (immediate o r delayed). (c ) The slump lo ss is much re d uce d than the trad itio nal sulp ho nate d sup e rp lasticize rs.

Ta ble 5 .3 . Effe c t of m e t hod of a ddit ion of N SF or CAE supe r pla st ic ize r on t he slum p of c onc re t e m ix 5 .5 W/ C

Slump

Type

Do sag e* (% )

Ad mixture Metho d o f Ad d itio n* *

Ratio

(mm)

NSF NSF CAE CAE

0 .4 8 0 .4 8 0 .3 0 0 .3 0

imme d iate d e laye d imme d iate d e laye d

0 .4 0 0 .4 0 0 .3 9 0 .3 9

100 230 230 235

*As dry polymer by weight of cement. ** Immediate: admixture with mixing water. Delayed: admixture after 1 min of mixing.

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The p ro p e rtie s o f CAE is sho w n in Tab le 5 .3 and in Fig . 5 .1 4 and Fig . 5 .1 5 w h ic h are se lf e xp lan ato ry ab o u t th e su p e rio rity o f CAE w ith re sp e c t to flu id ifyin g e ffe c t, lo ss o f slu m p a n d c o m p re ssive stre ng th. Fig . 5 .1 6 sho w s th e typ ic al slu m p lo ss fo r vario u s w / c ratio . It is p o inte d ag ain that inc re ase in co mpressive streng th is o n acco unt o f the ab ility o f CAE to re d uc e a hig he r w ate r co nte nt fo r the same w o rkab ility.

Mechanism of action of acrylic based new generation superplasticizers works on both electrostatic repulsion and steric hindrance (Courtesy : MBT)

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Sup e rp lastic ize rs are so m e tim e use d fo r hig h e arly stre ng th. The hig h e arly stre ng th is g e ne rally o b taine d b e c ause o f its ab ility to re d uc e the w ate r c o nte nt and , the re fo re w / c w hic h d e ve lo p s hig h e arly stre ng th and se c o nd ly suc h sup e rp lastic ize rs w ill have suc h a b ase that w ill no t cause much retard ing effect o n the co ncrete. Fig . 5.17 sho w s the influence o f sup e rp lastic ize rs o n the e arly stre ng th o f c o nc re te .

Multicarboxylatether The ne w g e ne ratio n o f sup e rp lasticize rs w hich are b ase d o n p o ly- carb o xylate the r w ith the g eneric name o f multicarb o xylatether (MCE) is fo und mo re suitab le fo r pro ductio n o f Hig h Pe rfo rmance Co ncre te . The p ro p e rtie s o f the se sup e rp lasticize rs are : " " " " " "

Exce lle nt flo w ab ility at lo w w / c ratio Hig h re d uctio n o f w ate r Lo w e r slump lo ss w ith time Sho rte r re tard atio n time Ve ry hig h e arly stre ng th It w o rks at lo w d o sag e s

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Retarders A re tard e r is an ad mixture that slo w s d o w n the c he mic al p ro c e ss o f hyd ratio n so that co ncre te re mains p lastic and w o rkab le fo r a lo ng e r time than co ncre te w itho ut the re tard e r. Re tard e rs are use d to o ve rc o m e th e ac c e le ratin g e ffe c t o f h ig h te m p e rature o n se ttin g p ro p e rtie s o f c o nc re te in ho t w e athe r c o nc re ting . The re tard e rs are use d in c asting and co nso lid ating larg e numb er o f po urs w itho ut the fo rmatio n o f co ld jo ints. They are also used in g ro uting o il w e lls. O il w e lls are so me time s take n up to a d e p th o f ab o ut 6 0 0 0 me te r d e e p w here the temperature may be abo ut 200°C. The annular spacing betw een the steel tube and the w all o f the w e ll w ill have to b e se ale d w ith c e m e nt g ro ut. So m e tim e s at that d e p th stratified o r po ro us ro ckstrata may also req uire to b e g ro uted to prevent the entry o f g as o r o il into so me o ther strata. Fo r all these w o rks cement g ro ut is req uired to b e in mo b ile co nd itio n fo r ab o ut 3 to 4 ho urs, e ve n at that hig h te mpe rature w itho ut g e tting se t. Use o f re tard ing ag e nt is o fte n use d fo r such re q uire me nts. So me time s co ncre te may have to b e p lace d in d ifficult co nd itio ns and d e lay may o ccur in transp o rting and p lacing . In re ad y mixe d co ncre te p ractice s, co ncre te is manufacture d in ce ntral b atching p lant and transp o rte d o ve r a lo ng d istance to the jo b site s w hich may take co nsid erab le time. In the ab o ve cases the setting o f co ncrete w ill have to b e retard ed , so that co ncre te w he n finally p lace d and co mp acte d is in p e rfe ct p lastic state . Retarding admixtures are so metimes used to o b tain expo sed ag g reg ate lo o k in co ncrete. The retard er sprayed to the surface o f the fo rmw o rk, prevents the hard ening o f matrix at the inte rfac e o f c o nc re te and fo rm w o rk, w he re as the re st o f the c o nc re te g e ts hard e ne d . O n re mo ving the fo rmw o rk afte r o ne d ay o r so , the unhard e ne d matrix can b e just w ashe d o ff b y a je t o f w ate r w hic h w ill e xp o se the ag g re g ate s. The ab o ve are so m e o f the instanc e s w he re a re tard ing ag e nt is use d . Pe rhap s the m o st c o m m o nly kno w n re tard e r is c alc ium sulp hate . It is inte rg ro und to re tard th e se ttin g o f c e m e n t. Th e ap p ro p riate am o u n t o f g yp su m to b e u se d m u st b e determined carefully fo r the g iven jo b. Use o f g ypsum fo r the purpo se o f retarding setting time is o nly reco mmend ed w hen ad eq uate inspectio n and co ntro l is availab le, o therw ise, ad d itio n o f e xc e ss am o unt m ay c ause und e sirab le e xp ansio n and ind e finite d e lay in the se tting o f co ncre te . In ad d itio n to g yp sum the re are numb e r o f o the r mate rials fo und to b e suitab le fo r this purpo se. They are: starches, cellulo se pro ducts, sug ars, acids o r salts o f acids. These chemicals may have variable actio n o n different types o f cement w hen used in different quantities. Unless e xp e rie nc e has b e e n had w ith a re tard e r, its use as an ad mixture sho uld no t b e atte mp te d w itho ut technical advice. Any mistake made in this respect may have disastro us co nseq uences. Co mmo n sug ar is o ne o f the mo st e ffe ctive re tard ing ag e nts use d as an ad mixture fo r d e laying the se tting tim e o f c o nc re te w itho ut d e trim e ntal e ffe c t o n the ultim ate stre ng th. Ad d itio n o f e xce ssive amo unts w ill cause ind e finite d e lay in se tting . At no rmal te mp e rature s ad d itio n o f sug ar 0 .0 5 to 0 .1 0 p e r ce nt have little e ffe ct o n the rate o f hyd ratio n, b ut if the q uantity is incre ase d to 0 .2 p e r ce nt, hyd ratio n can b e re tard e d to such an e xte nt that final se t may no t take p lace fo r 7 2 ho urs o r mo re . Skimme d milk p o w d e r (case in) has a re tard ing e ffe ct mainly d ue to sug ar co nte nt. O the r ad m ixture s w hic h have b e e n suc c e ssfully use d as re tard ing ag e nts are Lig no sulpho nic acid s and their salts, hyd ro xylated carb o xylic acid s and their salts w hich in ad d itio n to the re tard ing e ffe ct also re d uce the q uantity o f w ate r re q uire me nt fo r a g ive n w o rkab ility. This also inc re ase s 2 8 d ays c o mp re ssive stre ng th b y 1 0 to 2 0 p e r c e nt. Mate rials like muc ic

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149

acid , calcium ace tate and a co mme rcial p ro d ucts b y name “Ray lig b ind e r” are use d fo r se t re tard ing p urp o se s. The se d ays ad mixture s are manufacture d to co mb ine se t re tard ing and w ate r re d ucing p ro p e rtie s. The y are usually mixture s o f co nve ntio nal w ate r re d ucing ag e nts p lus sug ars o r hyd ro xylate d carb o xylic acid s o r the ir salts. Bo th the se tting time and the rate o f stre ng th b uild up are e ffe cte d b y the se mate rials. This is sho w n in Tab le 5 .4 .

Ta ble . 5 .4 . Effe c t of re t a rding/w at e r-re duc ing a dm ix t ure s on se t t ing t im e a nd st re ngt h build up Ad mixture ad d itio n litres/ 5 0 kg s.

Setting time hrs. Initial

W : C ratio

Final 9

0 .6 8

Co mpressive Streng th MPa 3 d ays

7 d ays

2 8 d ays

20

28

37

0

4 .5

0 .1 4

8 .0

13

0 .6 1

28

36

47

0 .2 1

1 1 .5

16

0 .5 8

30

40

50

0 .2 8

1 6 .0

21

0 .5 8

30

42

54

Retarding Plasticizers It is me ntio ne d e arlie r that all the p lasticize rs and sup e rp lasticize rs b y the mse lve s sho w ce rtain e xte nt o f re tard atio n. Many a time this e xte nt o f re tard atio n o f se tting time o ffe re d b y ad mixture s w ill no t b e sufficie nt. Inste ad o f ad d ing re tard e rs se p arate ly, re tard e rs are mixe d w ith p lasticize rs o r sup e rp lasticize rs at the time o f co mme rcial p ro d uctio n. Such co mme rcial brand is kno w n as retarding plasticizers o r retarding superplasticizers. ASTM type D is retarding p lasticize rs and ASTM typ e G is re tard ing sup e rp lasticize r. In the co mme rcial fo rmulatio n w e have also re tard ing and slump re taining ve rsio n. Retarding plasticizers o r superplasticizers are impo rtant categ o ry o f admixtures o ften used in the Ready mixed co ncrete industry fo r the purpo ses o f retaining the slump lo ss, during hig h te mp e rature , lo ng transp o rtatio n, to avo id co nstructio n o r co ld jo ints, slip fo rm co nstructio n and re g ulatio n o f he at o f hyd ratio n. O ne m ust b e c are ful in the se le c tio n o f suc h re ad y m ad e re tard ing ad m ixture s. O n ac c o unt o f he te ro g e ne o us nature and d iffe re nt m o le c ular w e ig ht o f re tard e rs use d w ith p lastic ize rs, the y te nd to se p arate o ut. It hap p e ns w he n sug ar so lutio n is use d as c he ap retard ers. When retard ers like g luco nate is used such separatio n o r settlement o f retard ers d o no t happen. It is cautio ned that such retarding plasticizers sho uld alw ays be shaken tho ro ug hly o r w e ll stirre d b e fo re u se . Th e re are c ase s th at se ttle m e n t o f re tard e rs fro m re st o f th e ing re d ie nts causing e xce ssive re tard atio n and failure o f structure s.

Accelerators Ac c e le rating ad m ixture s are ad d e d to c o nc re te to inc re ase the rate o f e arly stre ng th d e ve lo p me nt in co ncre te to " pe rmit e arlie r re mo val o f fo rmw o rk; " re d uce the re q uire d p e rio d o f curing ; " ad vance the time that a structure can b e p lace d in se rvice ; " partially co mpensate fo r the retarding effect o f lo w temperature during co ld w eather co ncre ting ; " in the e me rg e ncy re p air w o rk.

Wate r c o nte nt, p e rc e nt o f c o ntro l samp le , Max

Slump

Time o f se tting , allo w ab le d e viatio n fro m c o ntro l samp le ho urs: Intial Max Min Final Max Min

Co mp re ssive stre ng th, p e rc e nt o f c o ntro l samp le , Min 1 d ay 3 d ays 7 d ays 2 8 d ays 6 mo nths 1 ye ar

ii.

iii.

iv.

(2 )

(1 )

i.

Re q uire me nts

Sl No .

— 90 90 90 90 90

+ 3 + 1

–2 –1

— 125 100 100 90 90

+ 3 + 1

—

—

(4 )

Re tard ing Ad mixture

–3 –1

—

—

(3 )

Acce le rating Ad mixture

— 110 110 110 100 100

± 1 —

± 1 —

—

95

(5 )

Wate r Re d ucing Ad mixture

— 90 90 90 90 90

— —

— —

—

—

(6 )

Air-Entraining Ad mixture

80

(8 )

Re tard ing Typ e

140 125 125 115 100 100

± 1 .5 —

— + 1 .5

— 125 125 115 100 100

± 3 —

+ 4 + 1

No t mo re than 1 5 mm b e lo w that o f the co ntro l mix co ncre te

80

(7 )

No rmal

Sup e rp lastisticizing Ad mixture (fo r Wate rRe d uce d Co ncre te Mix)

Ta ble 5 .5 . Physic a l Re quire m e nt s Ac c ording t o I S 9 1 0 3 : 1 9 9 9

8 .2 .1

7 .2 .3

7 .2 .1

7 .2 .5

(9 )

Te st Re f. to IS co d e Clause

150 ! Concrete Technology

(2 )

—

ix. Air c o nte nt (% ) Max, o ve r c o ntro l

5

—

—

5

0 .0 1 0 0 .0 1 0 0 .0 1 0

0 .0 1 0 0 .0 1 0 0 .0 1 0

—

Ble e d ing , p e rc e nt inc re ase o ve r c o ntro l samp le , Max

90 90 90

(4 )

110 100 90

(3 )

viii. Lo ss o f w o rkab ility

vii.

v. Fle xural stre ng th, p e rc e nt o f c o ntro l samp le , Min 3 d ays 7 d ays 2 8 d ays vi. Le ng th c hang e , p e rc e nt inc re ase o ve r co ntro l samp le , Max 2 8 d ays 6 mo nths 1 ye ar

(1 )

—

—

5

0 .0 1 0 0 .0 1 0 0 .0 1 0

100 100 100

(5 )

Ta ble 5 .5 (Cont inue d)

—

—

5

0 .0 1 0 0 .0 1 0 0 .0 1 0

90 90 90

(6 )

1 .5

At 4 5 min the slump shall b e no t le ss than that o f c o ntro l mix c o nc re te at 1 5 min

5

0 .0 1 0 0 .0 1 0 0 .0 1 0

110 100 100

(7 )

7 .2 .4

8 .2 .3

8 .2 .2

(9 )

1 .5

At 2 h, the 7 .2 .1 .2 slump shall b e no t le ss than that o f c o ntro l mix c o nc re te at 1 5 min

5

0 .0 1 0 0 .0 1 0 0 .0 1 0

110 100 100

(8 )

Admixtures and Construction Chemicals !

151

a

100 100

2 8 d ays

6 mo nths

1 ye ar

100 100 100

3 d ays

7 d ays

2 8 d ays

Fle xural stre ng th, min p e rc e nt c o ntro l

110 110

7 d ays

— 110

3 d ays

— 6 0 e arlie r no r 9 0 late r

— 6 0 e arlie r no r 9 0 late r

95

Typ e A, w ate r re d ucing

1 d ay

Co mp re ssive stre ng th, min p e rce nt o f co ntro l a

Final: at le ast no t mo re than

Time o f se tting , allo w ab le d e viatio n fro m c o ntro l, min initial: at le ast no t mo re than

Wate r c o nte nt, max p e rc e nt o f c o ntro l

Pro p e rty

90

90

90

90

90

90

90

90

—

— 2 1 0 late r

6 0 late r 2 1 0 late r

—

Typ e B re tard ing

90

100

110

90

90

100

100

125

—

6 0 e arlie r —

6 0 e arlie r 2 1 0 e arlie r

—

Typ e C, acce le rating

100

100

100

100

100

110

110

110

—

— 2 1 0 late r

6 0 late r 2 1 0 late r

95

Typ e D, w ate r re d ucing and re tard ing

100

100

110

100

100

110

110

125

—

6 0 e arlie r —

6 0 e arlie r 2 1 0 e arlie r

95

Typ e E, w ate r re d ucing and acce le rating

100

100

110

100

100

110

115

125

140

— 6 0 e arlie r no r 9 0 late r

— 6 0 e arlie r no r 9 0 late r

88

Typ e F, w ate r re d ucing , hig h rang e

Ta ble 5 .6 . Spe c ific at ion for va rious t ype s of a dm ix t ure s a c c ording t o AST M 4 9 4 -8 2

100

100

110

100

100

110

115

125

125

— 2 1 0 late r

6 0 late r 2 1 0 late r

88

Typ e G, w ate r re d ucing hig h rang e and re tard ing

152 ! Concrete Technology

c

80

0 .0 1 0

135

80

0 .0 1 0

135

80

0 .0 1 0

135

80

0 .0 1 0

135

80

0 .0 1 0

135

This re q uire me nt is ap p lic ab le o nly w he n the ad mixture is to b e use d in air-e ntraine d c o nc re te w hic h may b e e xp o se d to fre e zing and thaw ing

c

w hile w e t.

Alternative req uirements, percent o f co ntro l limit applies w hen leng th chang e o f co ntro l is 0.030 percent o r g reater; increase o ver co ntro l limit applies w he n le ng th c hang e o f c o ntro l is le ss than 0 .0 3 0 p e rc e nt.

that attaine d at any p re vio us te st ag e . The o b je ctive o f this limit is to re q uire that the co mp re ssive and fle xural stre ng th o f the co ncre te co ntaining the ad mixture und e r te st shall no t d e c re ase w ith ag e .

The c o mp re ssive and fle xural stre ng th o f the c o nc re te c o ntaining the ad mixture und e r te st at any te st ag e shall b e no t le ss than 9 0 p e r c e nt o f

80

0 .0 1 0

135

b

a

80

0 .0 1 0

Incre ase o ve r co ntro l

Re lative d urab ility fac to r, min

135

Pe rce nt o f co ntro l

(alte rnative re q uire me nts) b

Le ng th Chang e , max shrinkag e

Ta ble 5 .6 (Cont inue d)

Admixtures and Construction Chemicals !

153

Atle ast 0 .0 3 ab o ve c o ntro l mix

—

*

Atle ast 1 hr lo ng e r than c o ntro l mix

—

Mo re than 1 hr.

—

W ithin 1 hr o f co ntro l mix

—

—

At le ast 1 hr lo ng e r than co ntro l mix

—

Mo re than 1 hr.

—

—

Atle ast 1 hr le ss than c o ntro l mix

—

W ithin 1 hr o f co ntro l mix

—

—

—

—

At le ast 1 hr le ss than c o ntro l mix

Time fro m co mp le ctio n o f mixing to re ach a re sistance to p e ne trate * o f: 0 .5 MPa (7 0 p si) 3 .5 MPa (5 0 0 p si)

110 110

90 90

125 110

125 90

110 110

90 90

—

90 95

—

125 95

p e rce nt o f co ntro l mix (minimum)

7 d ays 2 8 d ays

7 d ays 2 8 d ays

2 4 hrs 2 8 d ays

2 4 hrs 2 8 d ays

7 d ays 2 8 d ays

7 d ays 2 8 d ays

—

7 d ays 2 8 d ays

—

2 4 hr 2 8 d ays

Ag e

CO MPRESSIVE STRENGTH

The p e ne tratio n is d e te rmine d b y a sp e c ial b rass ro d o f 6 .1 7 5 mm in d iame te r. The air co nte nt w ith no w ate r re d uctio n shall no t b e mo re than 2 pe r ce nt hig he r than that o f the co ntro l mix, and no t mo re than to tal o f 3 p e r ce nt.

No t mo re than 0 .0 2 b e lo w c o ntro l mix

No t mo re than 0 .0 2 b e lo w co ntro l mix

8

8

At le ast 0 .0 3 ab o ve c o ntro l mix

—

Atle ast 0 .0 3 ab o ve c o ntro l mix

—

8

—

No t mo re than 0 .0 2 b e lo w co ntro l mix

—

Re tard ing w ate r-re d uc ing

—

8

No t mo re than 0 .0 2 b e lo w c o ntro l mix

No t mo re than 0 .0 2 b e lo w c o ntro l mix

—

8

Co mp actio n facto r

Wate r re d uctio n p e rce nt

w ate r-re d ucing

Ac c e le rating

No rmal-re d uc ing

Re tard ing

Ac c e le rating

Typ e o f Ad mixture

STIFFENING TIME

Ta ble 5 .7 . Spe c ific at ion for va rious t ype s of a dm ix t ure s a c c ording t o BS 5 0 7 5 : Pa r t 1 :1 9 8 2 .

154 ! Concrete Technology

—

Re tard ing sup e rp lasticizing

*

Lo ss o f slump

Slump : no t mo re than 1 5 mm b e lo w that o f co ntro l mix

Flo w tab le : 5 1 0 to 6 2 0 mm

Slump : no t mo re than 1 5 mm b e lo w that o f co ntro l mix

—

At 4 h no t le ss than that o f co ntro l mix at 1 0 to 1 5 min.

—

At 4 h no t mo re than that o f co ntro l mix at 1 0 to 1 5 min.

Flo w tab le : At 4 5 min no t le ss 5 1 0 to 6 2 0 mm than that o f co ntro l mix at 1 0 to 1 5 min.

W o rkab ility

1 to 4 ho ur lo ng e r than c o ntro l mix

—

W ithin 1 ho ur o f c o ntro l mix

—

—

—

W ithin 1 ho ur o f c o ntro l mix

—

—

Time fro m co mp le ctio n o f mixing to re ach a re sistance to p e ne trate * o f: 0 .5 MPa (7 0 p si) 3 .5 MPa (5 0 0 p si)

125 115

90 90

140 125 115

90

90

Pe rce ntag e o f co ntro l mix (minimum)

7 d ays 2 8 d ays

7 d ays 2 8 d ays

24 h 7 d ays 2 8 d ays

2 8 d ays

7 d ays

Ag e

CO MPRESSIVE STRENGTH

The p e ne tratio n is d e te rmine d b y a sp e c ial b rass ro d o f 6 .1 7 5 mm in d iame te r. The air co nte nt w ith no w ate r re d uctio n shall no t b e mo re than 2 pe r ce nt hig he r than that o f the co ntro l mix, and no t mo re than to tal o f 3 p e r ce nt.

16

16

Sup e rp lastticizing

Re tard ing sup e rp lastic iziing

—

W ate r re d uctio n p e r ce nt

Sup e rp lastic iizing

Typ e o f Ad mixture

STIFFENING TIME

Ta ble 5 .8 . Spe c ific at ion for supe r pla st ic izing a dm ix t ure a c c ording t o BS 5 0 7 5 : Pa r t 3 :1 9 8 5 .

Admixtures and Construction Chemicals !

155

2.

1.

Sl.No .

Sup e rp lasticize r b ase d o n me lamine fo rmald e hyd e Re tard ing supe rplasticize r

Sup e rp lasticize r

(f ) Ce ntrip last FF 9 0 (g ) Ze ntrame nt T5 BV

(h ) Murap last FK 6 1

(b ) Co nplast P 5 0 9 (c ) Co np last 3 3 7 (d ) Co nplast 4 3 0

Hafe e za Chamb e r, 2 nd Flo o r

1 1 1 / 7 4 , K.H. Ro ad

Bang alo re -5 6 0 0 2 7

(f ) Co np last NC

(e ) Co nplast RP 2 6 4

(a ) Co np last 2 1 1

Fo sro c Che micals (Ind ) Ltd .

- do -

Plasticize r

- do -

Sup e rp lasticize r

- do -

Wate r re d ucing p lasticize r

-d o -

Sup e rp lasticize r

(e ) Ze ntrame nt F BV

(i ) MC-Erstarrung sb re mse K T3

Sup e rp lasticize r

(d ) Ze ntrame nt Sup e r BV

Acce le rate s initial se tting time

Re tard s se tting time

- do -

Give s hig h w o rkab ility

Hig h pe rfo rmance plasticize r

Incre ase s w o rkab ility

Unive rsal re tard ing p lasticize r

Go o d p lasticizing e ffe ct

Hig h pe ro frmance re tard ing sup e rp lasticize r–it maintains slump fo r lo ng e r time

Exce lle nt co mp atib ility w ith all ce me nts

Pro d uce s hig h e arly ste rng th– make s the mix flo w ab le and p ump ab le

Pro d uce s flo w ing p ump ab le co ncre te

-d o -

Wate r re d ucing and re tard ing p lasticize r

(c ) Emce Plast RP

Se cto r-1 7 , Vashi Navi Mumb ai-4 0 0 7 0 3

Incre ase s w o rkab ilitty at lo w d o sag e

Wate r re d ucing p lasticize r

(b ) Emce Plast 4 BV

Functio n

(a ) Emce Plast BV

De scrip tio n

2 0 1 , Vard haman Chamb e rs

Brand Name

Mc-Bauche mie (Ind .) Pvt. Ltd .

Name and Ad d re ss

Ta ble 5 .9 . List of som e of t he c om m e rc ia l pla st ic ize rs a nd supe r pla st ic ize rs m a nufa c t ure d in I ndia .

156 ! Concrete Technology

4.

3.

Plasticize r Sup e rp lasticize r - do - do -

(a ) Ro ff Plast 3 3 0 (b ) Ro ff Sup e r Plast 3 2 1 (c ) Ro ff Supe r Plast 8 2 0 (d ) Ro ff Sup e r Plast 8 4 0

Ltd ., 1 2 C, Vikas Ce ntre

S.V. Ro ad , Santacruz (W)

Mumb ai-5 4

Sup e rp lasticize r

Ro ffe Co nstn. Che micals Pvt.

(c ) Sikame nt FF

Calcuta-1 6

- do -

- do -

(b ) Sikame nt 3 0 0 , 3 5 0 , 4 0 0

2 4 B, Park Stre e t

Plasticize r

(d ) Sikame nt 6 0 0

(a ) Plastime nt BV 4 0

Sika Q ualcre te Pvt. Ltd .

Ta ble 5 .9 . (Cont inue d)

Hig h p e rfo rmance re tard e r

- do -

Give s hig he r e arly stre ng th

Wate r re d uce r

Se tt re tard ing ag e nts

Hig h rang e w ate r re d uce r

- do -

Wate r re d ucing p lasticize r

Admixtures and Construction Chemicals !

157

158

! Concrete Technology

In th e p ast o n e o f th e c o m m o n ly u se d m ate rials as an ac c e le rato r w as c alc iu m c hlo rid e . But, no w a d ays it is no t use d . Inste ad , so m e o f the so lub le c arb o nate s, silic ate s flu o silic ate s an d so m e o f th e o rg an ic c o m p o u n d s su c h as trie th e n o lam in e are u se d . Ac c e le rato rs suc h as fluo silic ate s and trie the no lam ine are c o m p arative ly e xp e nsive . The re ce nt stud ie s have sho w n that calcium chlo rid e is harmful fo r re info rce d co ncre te and p re stre sse d c o nc re te . It may b e use d fo r p lain c e me nt c o nc re te in c o mp arative ly hig h d o se . The limits o f chlo rid e co nte nt in co ncre te is g ive n in chapte r o n Durab ility o f Co ncre te . So me o f the accelerato rs pro d uced these d ays are so po w erful that it is po ssib le to make the cement set into sto ne hard in a matter o f five minutes are less. With the availab ility o f such po w erful accelerato r, the und er w ater co ncreting has b eco me easy. Similarly, the repair w o rk that w o uld b e c arrie d o ut to the w ate rfro nt struc ture s in the re g io n o f tid al variatio ns has b e c o m e e asy. Th e u se o f su c h p o w e rfu l ac c e le rato rs h ave fac ilitate d , th e b ase m e n t w ate rp ro o fing o p e ratio ns. In the fie ld o f p re fab ric atio n also it has b e c o m e an invaluab le material. As these materials co uld be used up to 10°C, they find an unq uestio nable use in co ld w e athe r co ncre ting . So me o f the mo d e rn co mme rcial acce le rating mate rials are Mc-Schne ll O C, Mc-Schne ll SDS, Mc-To rkrethilfe BE, manufactured b y Mc-Bauchemic (Ind ) Pvt. Ltd . MC-To rkrethilfe BE is a material specially fo rmulated to meet the demand fo r efficient and multifo ld pro perties desired fo r sprayed co ncrete and sho tcreting o peratio ns. A field trial is essential to determine the do se fo r a g ive n jo b and te mp e rature co nd itio ns w he n the ab o ve mate rials are use d .

Accelerating Plasticizers Ce rtain ing re d ie nts are ad d e d to ac c e le rate the stre ng th d e ve lo p m e nt o f c o nc re te to plasticize rs o r supe rplasticize rs. Such acce le rating supe rplasticize rs, w he n ad d e d to co ncre te re sult in faste r d e ve lo p me nt o f stre ng th. The ac c e le rating mate rials ad d e d to p lastic ize rs o r superplasticizers are trietheno lamine chlo rides, calcium nutrite, nitrates and flo usilicates etc. The ac c e le ratin g p lastic ize rs o r ac c e le ratin g su p e rp lastic ize rs m an u fac tu re d b y w e ll kn o w n co mp anie s are chlo rid e fre e . Tab le 5 .5 , Tab le 5 .6 , Tab le 5 .7 and 5 .8 sho w s the specificatio n limits o f IS 9 1 0 3 o f 1 9 9 9 , ASTM 494 o f 1982, BS 5075 part I o f 1982 and BS part 3 o f 1985 respectively. Tab le 5.9 g ives the list o f so me o f the co mme rcial p lasticize rs and sup e rp lasticize rs manufacture d in Ind ia.

Air-entraining Admixture Pe rhap s o ne o f the im p o rtant ad vanc e m e nts m ad e in c o nc re te te c hno lo g y w as the d isco ve ry o f air e ntraine d co ncre te . Since 1 9 3 0 the re has b e e n an e ve r incre asing use o f air entrained co ncrete all o ver the w o rld especially, in the United States and Canad a. Due to the re c o g n itio n o f th e m e rits o f a ir e n tra in e d c o n c re te , a b o u t 8 5 p e r c e n t o f c o n c re te manufactured in America co ntains o ne o r the o ther type o f air entraining ag ent. So much so that air entraining ag ents have almo st co me to b e co nsid ered a necessary ‘fifth ing red ient’ in co ncre te making . Air e ntraine d co ncre te is mad e b y mixing a small q uantity o f air e ntraining ag e nt o r b y using air entraining cement. These air entraining ag ents inco rpo rate millio ns o f no n-co alescing air b ub b le s, w hic h w ill ac t as fle xib le b all b e aring s and w ill mo d ify the p ro p e rtie s o f p lastic co ncrete reg ard ing w o rkab ility, seg reg atio n, b leed ing and finishing q uality o f co ncrete. It also m o d ifie s the p ro p e rtie s o f hard e ne d c o nc re te re g ard ing its re sistanc e to fro st ac tio n and p e rme ab ility. The air vo id s p re se nt in co ncre te can b e b ro ug ht und e r tw o g ro up s: (a ) Entraine d air

(b ) Entrap p e d air.

Admixtures and Construction Chemicals !

159

Entraine d air is inte ntio nally inc o rp o rate d , m inute sp he ric al b ub b le s o f size rang ing fro m 5 m ic ro n s to 8 0 m ic ro n s d istrib u te d e ve n ly in th e e n tire m ass o f c o n c re te . Th e e ntrap p e d air is the vo id s p re se nt in the c o nc re te d ue to insuffic ie nt c o m p ac tio n. The se e n trap p e d air vo id s m ay b e o f an y sh ap e an d size n o rm ally e m b rac in g th e c o n to ur o f ag g reg ate surfaces. Their size may rang e fro m 1 0 to 1 0 0 0 micro ns o r mo re and they are no t unifo rmly d istrib ute d thro ug ho ut the co ncre te mass.

Air entraining agents The fo llo w ing types o f air entraining ag ents are used fo r making air entrained co ncrete. ( a ) Natural w o o d re sins ( b ) Animal and ve g e tab le fats and o ils, such as tallo w , o live o il and the ir fatty acid s suc h as ste aric and o le ic ac id s. ( c ) Vario u s w e ttin g ag e n ts su c h as alkali salts o r su lp h ate d an d su lp h o n ate d o rg anic co mpo und s. ( d

) W ate r so lub le so ap s o f re sin ac id s, and anim al and ve g e tab le fatty ac id s.

( e ) Misc e llane o us mate rials suc h as the so d ium salts o f p e tro le um sulp ho nic ac id s, hyd ro g e n p e ro xid e and aluminium p o w d e r, e tc. The re are a numb e r o f air e ntraining ag e nts availab le in the marke t. The c o mmo n air e ntraining ag e nts in Unite d State s are Vinso l re sin, Dare x, N Tair, Airalo n, O rvus, Te e p o l, Pe tro san and Che e co l. O ut o f the se the mo st imp o rtant air e ntraining ag e nts w hich at o ne time e njo ye d w o rld -w id e marke t are Vinso l re sin and Dare x. In Ind ia, larg e scale use o f air entrained co ncrete is no t b eing practised , primarily d ue to the fact that fro st scaling o f co ncrete is no t a serio us pro b lem in o ur co untry so far. Ho w ever, the ad vantag e s o f the use o f air e ntraine d co ncre te have b e e n re alise d fo r the co nstructio n o f multi-p urp o se d ams. Air e ntraine d co ncre te has b e e n use d in the co nstructio n o f Hirakud dam, Ko yna dam, Rihand dam etc. In these dams, to start w ith, American air entraining ag ents such as Vinso l resin, Darex etc. w ere used. Later o n in 1950’s certain indig eno us air entraining ag ents w ere develo ped. They are Aero sin— HRS., Rihand A.E.A., Ko ynaea, Ritha po w der, Hico , e tc . No w m o d e rn ad m ixture m anufac turing c o m p anie s are m anufac turing a num b e r o f co mmercial air entraining ag ents. MC-Mischo el LP, MC-Micho el AEA, Co mplast AE 215, Ro ff AEA 3 3 0 are so me o f the co mme rcial b rand s availab le in Ind ia.

Factors affecting amount of air entrainment The manufacture o f air entrained co ncrete is co mplicated b y the fact that the amo unt o f air e ntrainme nt in a mix is affe cte d b y many facto rs; the imp o rtant o ne s are : (a ) The typ e and q uantity o f air e ntraining ag e nt use d . (b ) Wate r/ ce me nt ratio o f the mix. (c ) Typ e and g rad ing o f ag g re g ate . (d ) Mixing time . (e ) The te mp e rature . (f ) Type o f ce me nt. (g ) Influe nce o f co mpactio n. (h ) Ad mixture s o the r than air e ntraining ag e nt use d .. Different air entraining ag ents pro duce different amo unts o f air entrainment, depending up o n the e lasticity o f the film o f the b ub b le p ro d uce d , and the e xte nt to w hich the surface tensio n is red uced . Similarly, d ifferent q uantities o f air entraining ag ents w ill result in d ifferent

160

! Concrete Technology

amo unts o f air e ntrainme nt. Wate r/ ce me nt ratio is o ne o f the impo rtant facto rs affe cting the q uantity o f air. At ve ry lo w w ate r/ ce me nt ratio , w ate r films o n the ce me nt w ill b e insufficie nt to p ro d uc e ad e q uate fo am ing ac tio n. At inte rm e d iate w ate r/ c e m e nt ratio (viz. 0 .4 to 0 .6 ) ab und ant air b ub b les w ill b e pro d uced . But at a hig her w ater/ cement ratio altho ug h to start w ith, a larg e amo unt o f air entrainment is pro duced, a larg e pro po rtio n o f the b ub b les w ill b e lo st p ro g re ssive ly w ith tim e . The g rad ing o f ag g re g ate has sho w n g o o d influe nc e o n the quantity o f air entrainment. It w as established that the quantity o f air increased fro m the lo w est fine ne ss mo d ulus o f sand to a p e ak at ab o ut F.M. o f 2 .5 , and , the re afte r, d e cre ase d sharp ly. The sand fractio n o f 3 0 0 and 1 5 0 micro ns sho w e d a sig nificant e ffe ct o n the q uantity o f air e ntrainme nt. The hig he r q uantity o f the se fractio ns re sulte d in mo re air e ntrainme nt. The amo unt o f air e ntrainme nt is fo und to incre ase w ith the mixing time up to a ce rtain time and thereafter w ith pro lo ng ed mixing the air entrainment g ets reduced. The temperature o f co ncre te at the time o f mixing w as fo und to have a sig nificant e ffe ct o n the amo unt o f air e n train m e n t. Th e am o un t o f air e n train m e n t d e c re ase s as th e te m p e rature o f c o n c re te increases. The co nstituents o f the cement especially the alkali co ntent plays an impo rtant part in the e ntrainme nt o f air in co ncre te . Similarly, the fine ne ss o f ce me nt is also a facto r. Air co nte nt is also re d uce d b y the pro ce ss o f co mpactio n, o n acco unt o f the mo ve me nt o f air b ub b le s to the surface and d e structio n. It is e stimate d that as much as 5 0 p e r ce nt o f the entrained air may be lo st after vibratio n fo r 2 1/ 2 minutes and as much as 80 per cent may b e lo st b y vib ratio n fo r 9 minutes. The experiments co nd ucted at Hirakud d am ind icated that an air co nte nt o f 1 0 .5 pe r ce nt afte r 3 0 se c o f vib ratio n came d o w n to 6 pe r ce nt afte r 1 8 0 sec o f vib ratio n. The o ther ad mixtures used in co njunctio n w ith air entraining ag ents w ill also sig nificantly affe ct the amo unt o f air e ntraine d . The use o f fly ash in co ncre te w ill re d uce the amo unt o f air entrained. Similarly, the use o f calcium chlo ride also has the tendency to reduce and limit air e ntrainme nt.

The Effect of Air Entrainment on the Properties of Concrete Air e ntrainme nt w ill e ffe ct d ire ctly the fo llo w ing thre e p ro p e rtie s o f co ncre te : (a ) Incre ase d re sistance to fre e zing and thaw ing . (b ) Imp ro ve me nt in w o rkab ility. (c ) Re d uctio n in stre ng th. Inc id e ntally air e ntrainme nt w ill also e ffe c t the p ro p e rtie s o f c o nc re te in the fo llo w ing w ays: (a ) Re d uce s the te nd e ncie s o f se g re g atio n. (b ) Re d uce s the b le e d ing and laitance . (c ) De cre ase s the p e rme ab ility. (d ) Incre ase s the re sistance to che mical attack. (e ) Pe rmits re d uctio n in sand co nte nt. (f ) Imp ro ve s p lace ab ility, and e arly finishing . (g ) Re d uce s the ce me nt co nte nt, co st, and he at o f hyd ratio n. (h ) Re d uce s the unit w e ig ht. (i )

Pe rmits re d uctio n in w ate r co nte nt.

( j ) Re d uce s the alkali-ag g re g ate re actio n. (k) Re d uce s the mo d ulus o f e lasticity.

Admixtures and Construction Chemicals !

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Resistance to Freezing and Thawing The g re ate st ad vantag e d e rive d fro m the use o f air e ntraine d c o nc re te is the hig h resistance o f hardened co ncrete to scaling due to freezing and thaw ing . It is fo und that w hen o rd inary co ncrete is sub jected to a temperature b elo w freezing po int, the w ater co ntained in the po re o f the co ncre te fre e ze s. It is w e ll kno w n that the vo lume o f ice is ab o ut 1 0 pe r ce nt hig he r than the c o rre sp o nd ing vo lum e o f w ate r. He nc e , the ic e fo rm e d in the p o re s o f h ard e n e d c o n c re te e xe rts p re ssu re . Th e c u m u lative e ffe c t o f th is p re ssu re b e c o m e s c o nsid e rab le , w ith the re sult that surfac e sc aling and d isrup tio n o f c o nc re te at the w e ake r se ctio n take s place . Similarly, surface scaling and d isruptio n also take s place in plain co ncre te w hen subjected to the actio n o f salt used fo r deicing purpo se. Similar pattern o f failure o f plain co ncre te is also no tice d in co ncre te structure s at the tid al zo ne and sp ray zo ne . It has b e e n

firm ly e stab lish e d th at air e n train m e n t in c o n c re te in c re ase s th e re sistan c e b y ab o ut thre e to se ve n time s in such situatio ns. Mo d ific a tio n o f p o re stru c tu re is b e lie ve d to b e re sp o n sib le fo r th e m arke d im p ro ve m e nt in re sistanc e to fro st attac k. In o rd in ary c o n c re te , th e re m ay e xist b ig g e r vo id s in te r-c o n n e c te d b y c ap illarie s, latte r b e ing larg e ly fo rme d b y the b le e d ing . But in the air entrained co ncrete tho ug h the to tal air vo id s are mo re , the vo id s are in the fo rm o f m in u te , d isc re te b u b b le s o f c o m p arative ly unifo rm size and reg ular spherical shape. This air vo id syste m re d uce s the te nd e ncy fo r the fo rm a tio n o f la rg e c rysta ls o f ic e in th e c o nc re te . Se c o nd ly, the inte r-c o nne c te d air vo ids system acts as buffer space to relieve the

Air Entrainment increses the durability of concrete in snowbound region.

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inte rnal p re ssure . Fig . 5 .1 8 sho w s the re lative d urab ility o f p lain and air e ntraine d co ncre te . The re sistance o f co ncre te to fre e zing and thaw ing w as me asure d b y Blanks b y me ans o f d urab ility fac to r w hic h he d e fine d as the num b e r o f c yc le s o f fre e zing and thaw ing to p ro d uc e failure d ivid e d b y 1 0 0 . The c urve s g ive n b y Blanks are re p ro d uc e d in Fig . 5 .1 9 to sho w the re latio nship s b e tw e e n the d urab ility and the air co nte nt fo r g o o d q uality co ncre te and p o o r q uality co ncre te . It can b e se e n that e xce lle nt q uality co ncre te w ith 4 p e r ce nt air e ntrainm e nt w ithsto o d up to 2 0 0 0 c yc le s o f fre e zing and thaw ing b e fo re d isinte g ratio n, w he re as, p o o r q uality co ncre te ne e d e d ab o ut 1 4 p e r ce nt air co nte nt and it d isinte g rate d at ab o ut 2 0 0 cycle s. In India, co ncrete is very rarely subjected to extreme freezing temperature. Ho w ever, w ith the d e ve lo pme nt o f co mmunicatio n in the No rthe rn Re g io ns, mo re and mo re co ncre te like ly to b e sub je cte d to fre e zing actio n is use d . The kno w le d g e o f air e ntrainme nt o f co ncre te is re q uire d to imp ro ve the d urab ility o f co ncre te structure s use d in marine co nd itio ns. Hithe rto the use o f air e ntrainme nt has b e e n p ractise d o nly in the case o f multi-p urp o se d ams fo r the purpo se o f w o rkab ility. And , therefo re, it is necessary that Ind ian eng ineers must b e ed ucated reg ard ing the use o f air entraining ag ents fo r air entrained co ncrete. The use o f air-entraining ag e nt, fo r imp ro ve me nt in w o rkab ility in g e ne ral co ncre te co nstructio n is also re q uire d to b e practised mo re and mo re. Air entrained mo rtar g ives much b etter perfo rmance fo r plastering w o rks.

Effect on workability The e ntrainm e nt o f air in fre sh c o nc re te b y m e ans o f air e ntraining ag e nt im p ro ve s w o rkab ility. It w as se e n that the place ab ility o f air e ntraine d co ncre te having 7 .5 cm slump is sup e rio r to that o f no n-air e ntraine d co ncre te having 1 2 .5 cm slump . This e asie r p lace ab ility o f a lo w e r slump sho uld b e re co g nise d b y the pe o ple co nce rne d w ith co ncre te co nstructio n in difficult situatio ns. Better placeability o f air entrained co ncrete results in mo re ho mo g eneo us c o n c re te w ith le ss se g re g atio n , b le e d in g an d h o n e yc o m b in g . Th e c o n c re te c o n tain in g entrained air is mo re plastic and ‘fatty’ and can be mo re easily handled than o rdinary co ncrete. The p ump ab ility o f the mix also incre ase s e no rmo usly. In fact, all the ab o ve q ualities mentio ned are clo sely related to w o rkab ility and as such let us co nsid e r the asp e ct o f w o rkab ility.

Admixtures and Construction Chemicals !

163

Fo r ad eq uate w o rkab ility o f c o n c re te , a g g re g a te p artic le s m u st b e sp ac e d so that the y c an mo ve p ast o ne a n o th e r w ith c o m p a ra tive e a se d u rin g m ixin g a n d p lac ing . In no n-air e ntraine d c o nc re te , w o rkab ility c an b e a c h ie ve d by in c lu d in g su ffic ie n t fin e san d , c e m e n t a n d w a te r to se p a ra te th e p a rtic le s of c o a rse r a g g re g a te a n d su p p ly o f m atrix, in w h ic h m o ve m e n t c a n o c c u r w ith m in im u m inte rfe re nce . By such me ans, sp acing o f the so lid s is incre ase d and the d ilatancy ne ce ssary fo r th e m an ip ulatio n o f fre sh c o n c re te is re d uc e d , w ith c o n se q ue n t re d uc tio n o f w o rk re q uire d . An imp ro ve me nt in w o rkab ility cause d b y air e ntrainme nt can b e vie w e d fro m ano the r ang le as fo llo w s: Pro p o rtio n in g o f c o n c re te m ixe s in vo lve s c o m p ro m ise b e tw e e n re q u ire m e n ts fo r w o rkab ility and re q uire me nts fo r stre ng th, d urab ility, vo lume stab ility and o the r p ro p e rtie s o f hardened co ncrete. Wo rkability req uires that the inter particle vo ids in the ag g reg ates be mo re than fille d b y ce me nt p aste , w he re as g o o d q uality o f hard e ne d co ncre te re q uire s that the se vo id s b e just fille d . It is the ro le o f e ntraine d air to so lve this c o nflic t. Firstly, e ntraine d air incre ase s the e ffe ct vo lume o f ce me nt p aste d uring mixing and p lacing , the re b y e liminating the ne e d fo r e xtra p aste to ind uce the w o rkab ility. Having fulfilled the re q uirements o f w o rkab ility fo r placing and co mpactio n the extra air w ill e sc ap e o r m ay b e m ad e to e sc ap e to ac h ie ve th e d e sire d d e n sity in th e h ard e n e d co ncrete. As a result o f co mpactio n 1/ 2 to 2/ 3 o f the air co ntent o f the fresh co ncrete may b e d rive n o ut d e p e nd ing up o n the d uratio n o f vib ratio n and w ate r/ ce me nt ratio . Th e m arke d im p ro ve m e n t in th e w o rkab ility o f air e n train e d c o n c re te c an also b e attrib ute d to the lub ricating e ffe ct o f the micro sco p ic b ub b le s b e tw e e n the fine ag g re g ate s and p ro vid ing a c ushio ning e ffe c t b e tw e e n the g rains o f sand , the re b y re d uc ing p artic le inte rfe re nc e to a m inim um . In fac t, the y se e m to intro d uc e b all b e aring up o n w hic h the p article s may slid e . The e ffe c t o f the p e rc e ntag e o f air e ntrainme nt o n the c o mp ac ting fac to r fo r d iffe re nt mixe s and w ate r/ ce me nt ratio s is illustrate d in Fig . 5 .2 0 w hich is p re p are d fro m info rmatio n g ive n b y Wrig ht. It is se e n that 5 p e r ce nt air may incre ase the co mp acting facto r b y 0 .0 7 . A co rre sp o nd ing incre ase o f the slump w o uld b e fro m 1 2 mm to 5 0 mm. The increase in the w o rkab ility is rather g reater fo r w etter mixes than fo r d rier o nes, and fo r the leaner mixes than fo r the richer o nes, and further it has b een sho w n that the increase is g re ate r fo r ang ular ag g re g ate s than fo r ro und e d o ne s.

Effect on strength It c an b e g e ne rally state d that air e ntrainm e nt in c o nc re te re d uc e s the c o m p re ssive stre ng th o f c o nc re te . But w he n the p ro c e ss is ap p lie d p ro p e rly, taking ad vantag e o f the

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b e n e fits ac c ru e d o n ac c o u n t o f air-e ntrainme nt, little o r no lo ss o f stre ng th sho uld take p lac e and it is e ve n p o ssib le th a t u n d e r c e rtain c irc u m stan c e s a g ain o f stre n g th m a y b e p o ssib le . It is true that at a g ive n w ate r/ ce me nt ratio , an in c re ase in air c o n te n t re sults in lo ss o f stre ng th, b ut the air e ntrainme nt e nab le s re d uctio n o f w ate r/ c e m e n t ratio an d san d co nte nt, fo r the g ive n w o rkab ility, the re b y re g aining m o st if no t all the lo st stre ng th. Th e re su lt o f te st o n th e c o m p re ssive stre n g th o f a ir e ntraine d c o nc re te c arrie d o ut at the Ro ad Re se arc h Lab o rato ry, U.K. as re p o rte d b y W rig ht5 .7 is sho w n in Fig . 5 .2 1 and in Fig . 5 .2 2 . In the se te sts fo ur m ixe s w e re inve stig ate d and the air c o nte nt w as inc re ase d w itho ut making any o the r ad justme nts to the mix p ro p o rtio ns. Fig . 5 .2 1 sho w s the ac tual stre ng th o b taine d . It w ill b e se e n that the stre ng th d e cre ase s in pro po rtio n to the amo unt o f air. In Fig . 5 .2 2 the same re sults are sho w n e xp re sse d as a p e rc e ntag e o f the stre ng ths o f c o n c re te s c o n tain in g n o air. It w ill b e se e n th at a sin g le straig h t lin e is o b tain e d . Th is re p re se nts a d e cre ase in stre ng th o f 5 .5 p e r ce nt fo r e ach p e r ce nt o f air e ntraine d . A curve is also p lo tte d sh o w in g th e stre n g th o f c o n c re te c o n tain in g air vo id s re su ltin g fro m inco mp le te co mp actio n i.e. , e ntrap p e d air. The g rap h sho w s that the e ntrap p e d air e ffe cts

Admixtures and Construction Chemicals !

165

the stre ng th slig htly mo re than the e ntraine d air. But it must b e ap p re ciate d , ho w e ve r, that air vo ids caused b y inco mplete co mpactio n do no t pro vide the o ther advantag es o f entrained air suc h as inc re ase d d urab ility and b e tte r w o rkab ility e tc . The e ffe c t o f air e ntrainm e nt o n stre ng th is d e p e nd e nt o n thre e fac to rs i.e ., o n the amo unt o f air entrained, the richness o f the mix, and o n the type o f air entraining ag ent used. The first tw o facto rs o n the streng th o f co ncrete b o th in co mpressio n and b end ing w ere investig ated b y Klieg er5.8 and the summary o f the results is repro duced in Tab le 5.10. It can b e

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no ticed that the w ater co ntent w as reduced to maintain the slump co nstant as the air co ntent w as incre ase d and this re d uctio n in the w ate r co nte nt so me time s mo re than co mp e nsate d fo r the re d uc tio n in stre ng th d ue to the inc re ase in air c o nte nt. Thus, in c ase s w he n the re d uc tio n in w ate r c o nte nt w as g re ate st suc h as the le an 2 1 MPa m ixe s w ith the sm alle r m axim u m size d ag g re g ate s, th e e n train m e n t o f air ac tu ally in c re ase d th e stre n g th o f co ncre te b o th in co mp re ssio n and in b e nd ing . O n the o the r hand , in case s w he re g re ate r re d uc tio n in w ate r c o nte nt c o uld no t b e e ffe c te d , w he n the mix is ric h and the maximum ag g re g ate size w as larg e , the re w as re d uc tio n in b o th the fle xural and the c o m p re ssive stre n g th s. Be tw e e n th e se tw o e xtre m itie s th e stre n g th va rie d fro m a n in c re a se o f ap p ro ximate ly 1 0 p e r ce nt in b e nd ing and ap p ro ximate ly 1 7 .5 p e r ce nt in co mp re ssio n to a re d uctio n o f ap p ro ximate ly 2 4 p e r ce nt in b e nd ing and 4 6 p e r ce nt in co mp re ssio n 5 .8 . The e ffe c t o f air-e ntrainm e nt o n fle xural stre ng th is, the re fo re , no t so g re at as the e ffe c t o n c o m p re ssive stre ng th. The ab o ve facts have also b e e n co nfirme d b y Blanks and Co rd o n. The Fig . 5 .2 3 sho w s the re sult o f the e xp e rime nts co nd ucte d b y Co rd o n to stud y the e ffe ct o f air e ntrainme nt o n the co mp re ssive stre ng th o f co ncre te . It can b e se e n that the stre ng th o f co ncre te d e cre ase s unifo rmly w ith the incre ase in air co nte nt o f fre sh co ncre te . At co nstant w ate r/ ce me nt ratio , the air entrainment resulted in the decrease o f streng th o f 1.4 MPa. (abo ut 5 per cent averag e) fo r each per cent air entrained . O n the co ntrary, the experiment carried o ut at the Co lleg e o f Military Eng ineering , Pune using Ritha Po w der as an air entraining ag ent sho w ed an averag e lo ss o f stre ng th o f ab o ut 0 .5 5 MPa. fo r e ach p e r ce nt air e ntraine d . (Fig . 5 .2 4 )5 .9

Effect on Segregation, Bleeding and Laitance Seg reg atio n and bleeding o f co ncrete are different manifestatio ns o f lo ss o f ho mo g eneity. Se g re g atio n usually im p lie s se p aratio n o f c o arse r ag g re g ate fro m m o rtar o r se p aratio n o f ce me nt p aste fro m ag g re g ate s. Ble e d ing is the auto g e no us flo w o f mixing w ate r w ithin, o r its emerg ence to the surface fro m freshly placed co ncrete, usually, as a result o f sedimentatio n o f the so lids due to co mpactio n and self w eig ht o f the so lids. Bleeding results in the fo rmatio n o f a se rie s o f w ate r channe ls so me o f w hich w ill e xte nd to the surface . A laye r o f w ate r w ill e me rg e at the surface o f the co ncre te , o fte n b ring ing so me q uantity o f ce me nt w ith it. The fo rmatio n o f this laye r o f ne at ce me nt p article s is calle d laitance . Se g re g atio n, b le e d ing and co nse q ue nt fo rmatio n o f laitance are re d uce d g re atly b y air entrainment. These actio ns pro bably result fro m physical pheno mena due to the inco rpo ratio n o f a system o f air bubbles. Firstly, the bubbles buo y up the ag g reg ates and cement and hence re d uce the rate at w hich se d ime ntatio n o ccurs in the fre shly p lace d co ncre te . Se co nd ly, the b ub b le s d e cre ase the e ffe ctive are a thro ug h w hich the d iffe re ntial mo ve me nt o f w ate r may o c c ur. Third ly, the b ub b le s inc re ase the mutual ad he sio n b e tw e e n c e me nt and ag g re g ate . Lastly, the surface are a o f vo id s in the p lastic co ncre te is sufficie ntly larg e to re tard the rate at w hich w ate r se p arate s fro m the p aste b y d rainag e . All re se arch w o rke rs are unanimo us in the ir o p inio n ab o ut the ad vantag e s o f e ntraine d air reg ard ing red uctio n in b leed ing b ut, o pinio ns are d ivid ed reg ard ing the ro le played b y air entrainment in red ucing seg reg atio n. Tests carried o ut at the Ro ad Research Lab o rato ry, U.K., using slo ping chute have indicated that, in the case o f a w ell desig ned mix having o nly a slig ht te nd e nc y to se g re g ate , the e ntrainm e nt o f air yie ld e d no im p ro ve m e nt, b ut a m ix w hic h se g re g ate s co nsid e rab ly w as imp ro ve d b y air e ntrainme nt. The larg e re d uc tio n in b le e d ing d e ve lo p e d b y the c o nc re te c o ntaining air e ntraining ad m ixture s is e vid e nt fro m the Tab le 5 .1 1 w hic h is re p ro d uc e d fro m Charle s E W ue rp e l’s artic le s o n lab o rato ry stud ie s o f c o nc re te c o ntaining air e ntraining ad m ixture s.

370

290

210

Cement co ntent in kg / cu.m

–1 .6

–1 .5

10

4 .7 5

–1 .4

–3 .5

40

20

–8 .6

72

+1 .0

–0 .9

10

4 .7 5

–2 .9

–3 .8

40

20

–3 .8

72

0

+4 .3

10

4 .7 5

+2 .2

0

40

20

–1 .0

1

–1 .6

–1 .5

–1 .6

–3 .2

–7 .2

+1 .0

–0 .9

–2 .6

–3 .8

–3 .4

0

+3 .6

+1 .7

–0 .5

–1 .0

2

–1 .6

–1 .5

–1 .9

–3 .5

–6 .0

+0 .3

–1 .1

–2 .7

–4 .0

–3 .6

0

+3 .4

+1 .5

–1 .0

–1 .3

3

–1 .5

–1 .7

–2 .1

–3 .7

–5 .0

–0 .1

–1 .3

–2 .6

–4 .0

–3 .6

0

+2 .6

+1 .1

–1 .2

–1 .6

4

In b end ing

–

–1 .9

–2 .2

–

–4 .1

–0 .4

–1 .5

–2 .6

–4 .0

–3 .7

0

+2 .0

+0 .4

–1 .7

–1 .9

5

–

–1 .9

–2 .4

–

–

–0 .9

–1 .7

–2 .7

–4 .0

–3 .8

0

+1 .7

0

–2 .1

–2 .1

6

–5 .1

–3 .0

–2 .8

–1 .4

–4 .0

+3 .6

0

–3 .3

–5 .0

–7 .5

0

+9 .6

+5 .0

+3 .7

0

1

–3 .7

–2 .8

–3 .4

–2 .8

–4 .3

+3 .6

0

–3 .3

–5 .0

–7 .5

0

+7 .5

+3 .4

+1 .5

–1 .2

2

–2 .7

–2 .7

–3 .3

–4 .1

–4 .9

+2 .4

–0 .4

–3 .6

–5 .3

–7 .5

0

+5 .9

+2 .2

–0 .4

–1 .9

3

In co mpressio n

–1 .8

–2 .6

–3 .9

–5 .0

–5 .3

+1 .8

–0 .8

–3 .8

–5 .5

–7 .5

0

+4 .4

+1 .7

–1 .8

–2 .6

4

–

–2 .8

–4 .2

–

–5 .7

+1 .1

–1 .3

–3 .8

–5 .8

–7 .6

0

+3 .5

+1 .0

–3 .2

–3 .2

5

Averag e percentag e chang e in streng th fo r each 1 per cent o f entrained air fo r to tal amo unts o f entrained air sho w n in per cent.

72

Maximum size o f aggregate mm

Age a t t e st , 2 8 da ys of m oist c uring, slum p 5 0 t o 7 5 m m

Ta ble 5 .1 0 . Effe c t of Ent ra ine d a ir on Conc re t e St re ngt hs 5 .8

–

2 .7

–4 .5

–

–

+0 .3

–2 .0

–3 .9

–6 .0

–7 .7

0

+2 .1

0

–4 .1

–3 .8

6

Admixtures and Construction Chemicals !

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! Concrete Technology

Ta ble 5 .1 1 . Effe c t of Adm ix t ure s on Ble e ding Air e ntraining ad mixtures

Ce m e nt fac to r = 2 4 0 kg / cu. m

Ce m e nt fac to r = 3 2 0 kg / cu. m

Cement A Bleed ing

Cement C Bleed ing

Cement A Bleed ing

%

%

%

%

100

100

100

100

Q

52

49

42

34

R

59

56

49

49

U

25

25

19

15

V

42

33

36

30

Z

55

57

25

20

Plain Co ncre te

Cement C Bleed ing

It w as fo und that the Ritha p o w d e r p ro ve d to b e an e ffic ie nt air e ntraining ag e nt in reducing the b leeding in cement mo rtar o r co ncrete. The results o f tests co nducted at Co lleg e o f Military Eng ine e ring , Pune using Ritha p o w d e r and Vinso l re sin are sho w n in Fig . 5 .2 5 . It can be seen that w hile the plain mo rtar bled 15 per cent o f the mixing w ater in abo ut 3 ho urs times, the mo rtar co ntaining Ritha po w d er b led o nly 7 1 / 2 per cent o f the mixing w ater and the mo rtar co ntaining Vinso l re sin b le d 1 1 p e r ce nt o f the mixing w ate r. Sinc e b le e d ing and fo rm atio n o f laitanc e are inte r-re late d , c o nsid e rab le re d uc tio n in b le e d ing w ill also auto m atic ally re d uc e the fo rm atio n o f laitanc e w hic h is o f c o nsid e rab le impo rtance. In co ncrete the red uced b leed ing permits early finishing o f the surface red ucing the w aiting perio d fo r the co mmencement o f tro w elling . Reductio n in b leeding also impro ves the w e aring q uality o f co ncre te .

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169

Effect on Permeability Th e e n train m e n t o f air d o e s ap p e ar to h ave m u c h e ffe c t o n th e p e rm e ab ility o f co ncre te . Gre ate r unifo rmity o f co ncre te w ith e ntraine d air d ue to its incre ase d w o rkab ility, m o d ifie d p o re -struc ture o f the air e ntraine d c o nc re te , re d uc tio n o f w ate r c hanne l d ue to re d uctio n in b le e d ing , are so me o f the re aso ns fo r impro ving the pe rme ab ility characte ristics o f air e ntraine d c o nc re te . Ce m e nt sto re d in silo s b uilt o f air e ntraine d c o nc re te , has b e e n fo und to sho w no c aking o f c e m e nt, w he re as, c e m e nt sto re d in silo s m ad e o f o rd inary c o n c re te re ve ale d c akin g alo n g th e p e rip h e ry o f th e silo . Th e m in ute d isc o n n e c te d air b ub b le s o ffe r a b e tte r b arrie r to the p assag e o f w ate r. The re d uce d w ate r/ ce me nt ratio also is o ne o f the fac to rs fo r re d uc e d p e rm e ab ility.

Effect on Chemical Resistance In vie w o f lo w e r p e rm e ab ility and ab so rp tio n, the air e ntraine d c o nc re te w ill have g re ate r re sistanc e fo r c he m ic al attac k than that o f no rm al c o nc re te . In the Ro ad Re se arc h Lab o rato ry, U.K., sp e cime ns o f co mp arab le mix o f o rd inary and air e ntraine d co ncre te have b een immersed in 5 per cent so lutio n o f mag nesium sulphate and the d eterio ratio n in q uality has b e e n asse sse d b y me asuring the d e cre ase in the ve lo city o f a ultraso nic w ave thro ug h th e sp e c im e n . It w as fo un d th at air e n train e d c o n c re te sh o w e d le ss d e te rio ratio n th an o rd inary c o nc re te .

Effect on sand, water and cement content The minute sp he ro id al air b ub b le s act as fine ag g re g ate s and e nab le the re d uctio n o f fine ag g re g ate s. The re d uc tio n o f fine ag g re g ate furthe r e nab le s the re d uc tio n o f w ate r re q uire me nt w itho ut imp airing the w o rkab ility and slump . This w ill have to b e co nsid e re d in

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d e sig ning an air e ntraine d mix. O n the b asis o f a larg e numb e r o f e xp e rime nts it is re p o rte d that sand co ntent b y w eig ht o f to tal ag g reg ate may b e red uced b y o ne per cent fo r each per ce nt incre ase in air e ntrainme nt up to ab o ut 8 p e r ce nt, w itho ut any ap p re ciab le chang e in w o rkab ility o r slump . The w ater req uirement o f an averag e co ncrete mix is reduced appro ximately 3.5 kg / cu.m w ith ro und e d ag g re g ate and 4 .8 kg / c u.m w ith ang ular ag g re g ate s fo r e ac h p e r c e nt air e n train in g . Fig . 5 .2 6 sh o w s th e re d uc tio n o f w ate r fo r n atural ag g re g ate s as g ive n b y Co rd o n. 5 .1 0 The re d uctio n in w ate r/ ce me nt ratio naturally e ffe cts the b asic incre ase in stre ng th and d urab ility d ue to the no n-availab ility o f e xce ss w ate r fo r the fo rmatio n o f b le e d ing channe ls thro ug h the matrix o f co ncrete. Tab le 5.12 sho w s the ad vantag es accrued b y air entrainment in co ncre te re g ard ing the re d uctio n o f sand and re d uctio n o f w ate r re q uire me nts. Entrainment o f air is particularly useful in the case o f lean co ncrete, even fro m the po int o f vie w o f stre ng th. Many a time incre ase s the stre ng th o f le an mixe s. In such case s it w ill b e p o ssib le fo r re d ucing the ce me nt co nte nt fo r the g ive n stre ng th. Blanks and Co rd o n fo und that co ncrete made w ith 160 mm maximum size ag g reg ate g ave satisfacto ry streng th fo r mass co ncre te w o rk w ith o nly 1 0 6 kg ce me nt co nte nt p e r cu.m. p ro vid e d air w as e ntraine d . The re d uctio n in ce me nt co nte nt re sults in a lo w e r he at o f hyd ratio n in mass co ncre te and lo w e r te mpe rature rise . The d e cre ase in te mpe rature rise re sults in re d uce d cracking o r und e sirab le inte rnal stre sse s.

Unit Weight The use ful facto r w hich sho uld no t b e o ve rlo o ke d , is the re d uctio n in d e nsity o f the air entrained co ncrete. Co mparing tw o mixes, o ne o rdinary co ncrete and the o ther air entrained, w hich have the same w o rkab ility and stre ng th, the air e ntraine d co ncre te w ill co ntain 5 p e r cent less o f so lid material, and hence w ill b e lo w er in w eig ht. Incid entally, this w ill result in an e c o n o m y o f ab o ut 5 p e r c e n t in th e c o st o f c e m e n t and ag g re g ate , le ss the c o st o f air e ntraining ag e nt and co st o f e xtra sup e rvisio n.

Alkali-Aggregate Reaction There are evidences that air entrainment reduces the alkali-ag g reg ate reactio n. Use o f air entraining ag ent has freq uently been reco mmended as a means fo r co ntro lling expansio n due to alklai-ag g re g ate re actio n in mo rtar and co ncre te .

Modulus of Elasticity Availab le d ata ind ic ate that the mo d ulus o f e lastic ity o f c o nc re te mix having the same w ate r/ ce me nt ratio and the same ag g re g ate is re d uce d b y 2 to 3 p e r ce nt fo r e ach p e r ce nt o f air e ntrainme nt.

Abrasion Resistance Co ncre te co ntaining le ss than 6 pe r ce nt air e ntrainme nt has ab o ut the same re sistance to ab rasio n as no rm al c o nc re te , w he n c e m e nt c o nte nts o f the c o m p arab le c o nc re te are co nstant. Ho w ever, there is a pro g ressive decrease in ab rasio n resistance w ith further increase in air c o nte nt. W he n the air e ntrainm e nt is o f the o rd e r o f ab o ut 1 0 p e r c e nt, ab rasio n resistance is markedly lo w. Since co ncrete used in pavements is g enerally specified to have no t mo re than 3 to 6 p e r ce nt o f e ntraine d air, the ab rasio n re sistance sho uld b e satisfacto ry.

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Optimum Air Content in Concrete The re c o m m e nd e d air c o nte nt in a g ive n c o nc re te is a func tio n o f (a ) the p urp o se fo r w hic h the c o nc re te is use d and its lo c atio n and c lim atic c o nd itio n (b ) the m axim um size o f ag g re g ate (c ) the ric hne ss o f the m ix. Usually, the d e sirab le air c o nte nt is rang ing fro m 3 to 6 pe r ce nt. Lo w e r air c o nte nt is no rm ally sp e c ifie d fo r c o nc re te flo o rs, in a b uild ing e ve n in c o ld c o untrie s, b e c ause the y are no t sub je c te d to se ve re w e athe r c o nd itio ns. An air c o nte nt o f abo ut 4 per cent may pro bably be sufficient fo r the required w o rkability and reduced bleeding . Fo r re info rce d co ncre te o f re lative ly hig h ce me nt co nte nt, a limit o f p ro b ab ly 3 to 4 p e r cent is ad eq uate fro m the w o rkab ility and b leed ing po int o f view. The streng th w ill b e und uly lo w e re d if the air co nte nt is incre ase d . Ag ain, the larg e r the ag g re g ate , the le ss is the amo unt o f air sufficie nt to g ive d e sire d results. Fo r mass co ncrete w ith 160 mm maximum size ag g reg ate, an air entrainment o f abo ut 2 .5 to 3 p e r c e n t w o uld b e suffic ie n t. But in th e m ass c o n c re te if th e m axim um size o f ag g re g ate is smalle r, a hig he r p e rce ntag e o f air e ntrainme nt is d e sirab le . De sp ite the variatio ns, the o ve rall limits are fro m 3 to 6 p e r ce nt. This rang e co nstitute s reaso nab le specificatio n limits co vering all co nd itio ns. There is no ad vantag e in increasing the air co nte nt ab o ve 6 p e r ce nt e ve n fro m the d urab ility p o int o f vie w. The o p timum d urab ility as me asure d fro m the re sistanc e to fre e zing and thaw ing fo r g o o d q uality c o nc re te , g o o d resistance is achieved w ithin 6 per cent o f entrained air as can b e seen fro m Fig . 5.19. O n the o the r hand , air e ntrainme nt b e lo w ab o ut 3 p e r ce nt may no t e xte nd e nvisag e d ad vantag e s o f an air e ntraine d co ncre te .

Measurement of Air Content in Air Entrained Concrete The exact air co ntent in co ncrete is extremely impo rtant as it affects the vario us pro perties o f co ncrete as explained earlier. If the amo unt o f air entrained in a mix d iffers w id ely fro m the d e sig n value , the p ro p e rtie s o f the co ncre te may b e se rio usly affe cte d . To o little air re sults in insufficie nt w o rkab ility and to o much air w ill re sult in lo w stre ng th. It is, the re fo re , ne ce ssary that the air co ntent sho uld b e maintained at the stipulated value. In view o f the many facto rs affecting the air co ntent, measurements must b e d o ne freq uently thro ug ho ut the pro g ress o f the w o rk. If the air co nte nt is fo und to b e varying b e yo nd the sp e cifie d limit, ad justme nt is mad e b y alte ring the amo unt o f air e ntraining ag e nt. The re are mainly thre e me tho d s fo r me asuring air co nte nt o f fre sh co ncre te : (a ) Gravime tric Me tho d ;

(b ) Vo lume tric Me tho d ;

(c ) Pre ssure Me tho d .

Gravimetric Method Gravimetric metho d w as the first to be used and it did no t req uire any special eq uipment. The pro ced ure is principally o ne o f d etermining the d ensity o f fresh co ncrete co mpacted in a stand ard m anne r. This is the n c o m p are d w ith the the o re tic al d e nsity o f air-fre e c o nc re te , calculated fro m the mix pro po rtio ns and specific g ravities o f the co nstituent materials making the co ncre te . Thus if the air-fre e d e nsity is 2 3 8 0 kg / cu.m. and the me asure d d e nsity is 2 2 2 0 kg / cu.m., then o ne cu.m. o f co ncrete w ill co ntain 2220/ 2380 cu.m. o f so lid and liq uid matter and the re st b e ing air. The re fo re , the air co nte nt the n is 1 –

2220 = 0 .0 7 o r 7 % . 2380

The g ravime tric me tho d is satisfacto ry fo r use in the lab o rato ry b ut it is no t w e ll-suite d fo r fie ld use . It ne c e ssitate s a skille d o p e rato r and an ac c urate b alanc e . It also re q uire s the

37 34 31 26

25

40

50

80

160

45 41 38 35 30

25

40

50

80

160

55

41

20

50

46

1 2 .5

20

51

(mm)

1 2 .5

to tal ag g reg ate

(ab so lute vo lume)

co arse ag g reg ate

Sand % o f

Max. size o f

Plain co ncrete

3 ± 1

3 .5 ± 1

4 ± 1

4 ± 1

4 .5 ± 1

5 ± 1

6 ± 1

(% )

air co ntent

Reco mmend ed

112

125

132

139

147

152

164

3 ± 1

3 .5 ± 1

4 ± 1

4 ± 1

4 .5 ± 1

5 ± 1

6 ± 1

B. Manufac ture d Ang ular Co arse Ag g re g ate

100

114

120

127

136

141

152

(kg .)

co ntent per cu.m

Net w ater

Air entrained co ncrete

A. N at ura l or Rounde d Coa rse Aggre ga t e

27

32

35

38

42

46

51

24

28

31

34

37

42

47

(ab so lute vo l.)

to tal ag g reg ate

Sand % o f

98

110

118

120

127

132

141

89

100

107

112

118

122

132

(kg )

co ntent per cu.m

Net w ater

Ta ble 5 .1 2 . Approx im at e sa nd a nd w a t e r c ont e nt pe r c ubic m e t re of pla in a nd a ir-e nt ra ine d c onc re t e 5 .1 0

172 ! Concrete Technology

Admixtures and Construction Chemicals !

173

kno w le d g e o f mix pro po rtio ns o f co ncre te . Eve n if w e ig h b atching is d o ne , lo ss o f mate rials, se g re g atio n and o the r fac to rs m ay c ause a sam p le o f c o nc re te take n in the c ylind e r fo r experiment to be different fro m the ideal mix. The accurate specific g ravity o f the materials also must b e kno w n. Witho ut the se the re sult w ill no t b e co rre ct.

Volumetric Method The vo lume tric me tho d o r d isp lace me nt me tho d aims at me asuring d ire ctly the vo lume o f air in the samp le o f fre sh co ncre te . The re are a numb e r o f mo d ificatio ns, b ut the p rincip le is to take a samp le o f co ncre te o f kno w n vo lume , re mo ve all the air and the n d e te rmine the amo unt o f w ate r re q uire d to re sto re the o rig inal vo lume . A ve sse l is p artly fille d w ith c o nc re te , the am o unt o f c o nc re te b e ing d e te rm ine d b y w eig hing , o r b y using a vessel mad e in tw o parts, the lo w er o ne b eing filled co mpletely and struck o ff le ve l so that a fixe d vo lume o f co ncre te is o b taine d . Wate r is ad d e d to make up a g ive n vo lume as ind icate d b y a mark o n the narro w ne ck o f the se co nd p art. The air is the n remo ved b y ag itatio n o f the mixture, either b y ro lling o r b y stirring . When the remo val o f the air is co mplete, the w ater level is resto red to its o rig inal po sitio n, by a further additio n o f w ater. The ad d itio nal w ate r re q uire d to make up the o rig inal le ve l ind ic ate the air c o nte nt in the co ncre te . The air co nte nt can also b e calculate d b y the w e ig ht o f e xtra w ate r. In so me variatio ns o f this me tho d , w e ig hing is e liminate d e ntire ly. This me tho d re q uire s g re at care and skille d o p e ratio n. It is a te d io us jo b to re mo ve all the air, and it is d ifficult to kno w the n the re mo val o f the air is co mple te . The tro ub le cause d b y the fo rmatio n o f fo am o n the surfac e is o fte n re d uc e d b y using so me alc o ho l. Co nsid e rab le time is c o nsume d in ro lling and stirring the samp le o f c o nc re te , and g e ne rally the p ro c e ss must b e re p e ate d to make sure that all the air has been remo ved. The chief advantag e o f the ro lling metho d is that it c an b e use d w ith all typ e s o f ag g re g ate s and it is sp e c ially re c o m m e nd e d fo r c o nc re te co ntaining lig ht w e ig ht ag g re g ate .

Pressure Method This is perhaps the b est metho d fo r finding the a ir c o n te n t o f fre sh c o n c re te b e c a u se o f its sup e rio rity and e ase o f o p e ratio n. The re are tw o d ifferent metho d s o f measuring air b y the pressure . O ne utilise s a p re ssure me te r that is kno w n as the w ate r typ e . The o the r is the Washing to n air me te r. Bo th o perate o n the principle o f Bo yle’s law, namely that the vo lum e o f g as at a g ive n te m p e rature is inve rse ly pro po rtio nal to the pre ssure to w hich it is sub je cte d .

The Water Type Meter The ve sse l is fille d w ith c o nc re te , c o m p ac te d in a stand ard manne r and struck o ff le ve l. A co ve r is the n c lam p e d in p o sitio n. W ate r is ad d e d until the le ve l re ache s “0 ” mark o n the tub e o f the co ve r and the n p re ssure is ap p lie d b y me ans o f a b icycle p u m p . Th e p re ssu re is tra n sm itte d to th e a ir e n tra in e d in th e c o n c re te , w h ic h c o n tra c ts acco rd ing ly. Then the w ater level falls. The pressure

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is inc re ase d to a p re d e te rm ine d value as ind ic ate d b y a sm all p re ssure g aug e m o unte d o n the c o ve r. Th e g la ss g a u g e tu b e is so c a lib ra te d th a t th e p e rc e ntag e o f air b y vo lum e is ind ic ate d d ire c tly. The instrum e nts are g e ne rally d e sig ne d to e m p lo y a w o rking pressure o f the o rd er o f 1 kg / sq . cm. The co rrectio n may b e mad e fo r the air co ntained in the ag g re g ate . The ap p aratus is sho w n in Fig ure 5 .2 7 .

Pozzolanic or Mineral Admixtures Th e u se o f p o zzo lan ic m ate rials is as o ld as th at o f th e art o f c o n c re te c o n stru c tio n . It w as re c o g n ise d lo n g tim e a g o , th a t th e su ita b le p o zzo lan s u se d in ap p ro p riate am o u n t, m o d ify c e rtain p ro p e rtie s o f fre sh and hard e ne d m o rtars and c o nc re te s. Anc ie nt G re e ks and Ro m ans use d Another Model of Air Entrainment Meter. c e rta in fin e ly d ivid e d silic e o u s m a te ria ls w h ic h w he n mixe d w ith lime p ro d uce d stro ng ce me nting m ate rial having hyd raulic p ro p e rtie s and suc h c e m e nting m ate rials w e re e m p lo ye d in the co nstructio n o f acq uad ucts, arch, b rid g e s e tc. O ne such mate rial w as co nso lid ate d vo lcanic ash o r tu ff fo u n d n e ar Po zzu o li (Italy) n e ar Ve su viu s. Th is c am e to b e d e sig n ate d as Po zzuo lana, a g e ne ral te rm c o ve ring sim ilar m ate rials o f vo lc anic o rig in fo und in o the r d e p o sits in Italy, Franc e and Sp ain. Late r, the te rm p o zzo lan w as e m p lo ye d thro ug ho ut Eu ro p e to d e sig n ate an y m ate rials irre sp e c tive o f its o rig in w h ic h p o sse sse d sim ilar p ro p e rtie s. Sp e c im e ns o f c o nc re te m ad e b y lim e and vo lc anic ash fro m Mo unt Ve suvius w e re use d in the c o nstruc tio n o f Calig ula W harf b uilt in the tim e o f Julius Cae sar ne arly 2 0 0 0 ye ars ag o is no w e xisting in a fairly g o o d co nd itio n. A numb e r o f structure s stand to d ay as e vid e nc e o f the sup e rio rity o f p o zzo lanic c e m e nt o ve r lim e . The y also atte st the fac t that Gre e ks and Ro m ans m ad e re al ad vanc e in the d e ve lo p m e nt o f c e m e ntitio us m ate rials. Afte r the d e ve lo p me nt o f natural ce me nt d uring the latte r p art o f the 1 8 th ce ntury, the Po rtland ce me nt in the e arly 1 9 th ce ntury, the p ractice o f using p o zzo lans d e cline d , b ut in mo re re ce nt time s, Po zzo lans have b e e n e xte nsive ly use d in Euro p e , USA and Jap an, as an ing re d ie nt o f Po rtland ce me nt co ncre te p articularly fo r marine and hyd raulic structure s. It has b e e n amply d e mo nstrate d that the b e st po zzo lans in o ptimum pro po rtio ns mixe d w ith Po rtland ce me nt imp ro ve s many q ualitie s o f co ncre te , such as: (a ) Lo w e r the he at o f hyd ratio n and the rmal shrinkag e ; (b ) Incre ase the w ate rtig htne ss; (c ) Re d uce the alkali-ag g re g ate re actio n; (d ) Imp ro ve re sistance to attack b y sulp hate so ils and se a w ate r; (e ) Imp ro ve e xte nsib ility; (f ) Lo w e r susce p tib ility to d isso lutio n and le aching ; (g ) Imp ro ve w o rkab ility; (h ) Lo w e r co sts. In additio n to these advantag es, co ntrary to the g eneral o pinio n, g o o d po zzo lans w ill no t und uly incre ase w ate r re q uire me nt o r d rying shrinkag e .

Admixtures and Construction Chemicals !

175

Pozzolanic Materials Po zzo lan ic m ate rials are silic e o u s o r silic e o u s an d alu m in o u s m ate rials, w h ic h in the mse lve s p o sse ss little o r no ce me ntitio us value , b ut w ill, in fine ly d ivid e d fo rm and in the p re se nc e o f m o isture , c he m ic ally re ac t w ith c alc ium hyd ro xid e lib e rate d o n hyd ratio n, at o rd inary te mp e rature , to fo rm co mp o und s, p o sse ssing ce me ntitio us p ro p e rtie s. It has b e e n sho w n in Chap te r I that o n hyd ratio n o f tri-c alc ium silic ate and d i-c alc ium silicate, calcium hyd ro xid e is fo rmed as o ne o f the pro d ucts o f hyd ratio n. This co mpo und has no ce me ntitio us value and it is so lub le in w ate r and may b e le ache d o ut b y the p e rco lating w ate r. The silice o us o r alumino us co mp o und in a fine ly d ivid e d fo rm re act w ith the calcium hyd ro xid e to fo rm hig hly stab le c e me ntitio us sub stanc e s o f c o mp le x c o mp o sitio n invo lving w ate r, c alc ium and silic a. Ge ne rally, amo rp ho us silic ate re ac ts muc h mo re rap id ly than the crystalline fo rm. It is po inte d o ut that calcium hyd ro xid e , o the rw ise , a w ate r so lub le mate rial is co nve rte d into inso lub le ce me ntitio us mate rial b y the re actio n o f p o zzo lanic mate rials. The re actio n can b e sho w n as Po zzo lan + Calcium Hyd ro xid e + Wate r → C – S – H (Ge l) This reactio n is called po zzo lanic reactio n. The characteristic feature o f po zzo lanic reactio n is firstly slo w , w ith th e re su lt th at h e at o f h yd ratio n an d stre n g th d e ve lo p m e n t w ill b e acco rd ing ly slo w. The re actio n invo lve s the co nsump tio n o f Ca(O H)2 and no t p ro d uctio n o f Ca(O H)2 . The re d uc tio n o f Ca(O H)2 imp ro ve s the d urab ility o f c e me nt p aste b y making the p aste d e nse and imp e rvio us. Po zzo lanic m ate rials c an b e d ivid e d into tw o g ro up s: natural p o zzo lana and artific ial p o zzo lana.

Natural Pozzolans "

Clay and Shale s

"

O p alinc Che rts

"

Diato mace o us Earth

"

Vo lcanic Tuffs and Pumicite s.

Artificial Pozzolans "

Fly ash

"

Blast Furnace Slag

"

Silica Fume

"

Rice Husk ash

"

Me takao line

"

Surkhi.

O the r m ine ral ad m ixture s, like fine ly g ro und m arb le , q uartz, g ranite p o w d e r are also use d . The y ne ithe r e xhib it the p o zzo lanic p ro p e rty no r the ce me ntitio us p ro p e rtie s. The y just act as ine rt fille r. Natural po zzo lans such as d iato maceo us earth, clay and shale, pumicites, o paline cherts etc., needs further g riding and so metimes needs calcining to activate them to sho w po zzo lanic activitie s. In Hirakud d am co nstructio n in O rissa, naturally o ccurring clay kno w n as Talab ara clay has b e e n use d as p o zzo lanic mate rials. The natural p o zzo lans have lo st the ir p o p ularity in vie w o f the availab ility o f mo re active artificial p o zzo lans.

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Artificial Pozzolans Fly Ash : Fly ash is fine ly d ivid e d re sid ue re sulting fro m the c o m b ustio n o f p o w d e re d c o al and transp o rte d b y the flue g ase s and c o lle c te d b y e le c tro static p re c ip itato r. In U.K. it is re fe rre d a s p u lve rise d fu e l a sh (PFA). Fly a sh is th e m o st w id e ly u se d p o zzo la n ic mate rial all o ve r the w o rld . Fly ash w as first u se d in larg e sc ale in th e c o n stru c tio n o f Hu n g ry Ho rse d am in America in the appro ximate amo unt o f 30 per cent b y w eig ht o f cement. Later o n it w as used in Can yo n an d Fe rry d am s e tc . In In d ia, Fly ash w as use d in Rih an d d am c o n struc tio n re placing ce me nt upto ab o ut 1 5 pe r ce nt.

A Plerosphere

Spherical and Glassy particles

Scanning electron micrograph of Class F, Fly Ash.

In the re ce nt time , the imp o rtance and use o f fly ash in co ncre te has g ro w n so much that it has alm o st b e c o m e a c o m m o n ing re d ie nt in c o nc re te , p artic ularly fo r m aking hig h stre ng th and hig h p e rfo rm anc e c o nc re te . Exte nsive re se arc h has b e e n d o ne all o ve r the w o rld o n the b e ne fits that co uld b e accrue d in the utilisatio n o f fly ash as a sup p le me ntary c e me ntitio us mate rial. Hig h vo lume fly ash c o nc re te is a sub je c t o f c urre nt inte re st all o ve r the w o rld. The use o f fly ash as co ncre te ad mixture no t o nly e xte nd s te chnical ad vantag e s to the p ro p e rtie s o f c o nc re te b ut also c o ntrib ute s to the e nviro nme ntal p o llutio n c o ntro l. In Ind ia alo ne , w e p ro d uc e ab o ut 7 5 m illio n to ns o f fly ash p e r ye ar, the d isp o sal o f w hic h has b e c o m e a se rio us e nviro nm e ntal p ro b le m . The e ffe c tive utilisatio n o f fly ash in c o nc re te m akin g is, th e re fo re , attrac tin g se rio u s c o n sid e ratio n s o f c o n c re te te c h n o lo g ists an d g o ve rnme nt d e p artme nts. Se c o n d ly, c e m e n t is th e b ac kb o n e fo r g lo b al in frastru c tu ral d e ve lo p m e n t. It w as e stimate d that g lo b al p ro d uctio n o f ce me nt is ab o ut 1 .3 b illio n to ns in 1 9 9 6 . Pro d uctio n o f e ve ry to ne o f c e me nt e mits c arb o n d io xid e to the tune o f ab o ut 0 .8 7 to n. Exp re ssing it in ano the r w ay, it can b e said that 7 % o f the w o rld ’s carb o n d io xid e e missio n is attrib utab le to Po rtland c e m e nt ind ustry. Be c ause o f the sig nific ant c o ntrib utio n to the e nviro nm e ntal p o llutio n and to the hig h co nsump tio n o f natural re so urce s like lime sto ne e tc., w e can no t g o o n p ro d ucing mo re and mo re ce me nt. The re is a ne e d to e co no mise the use o f ce me nt. O ne o f the practical so lutio ns to eco no mise cement is to replace cement w ith supplementary c e m e ntitio us m ate rials like fly ash and slag . In Ind ia, the to tal p ro d uctio n o f fly ash is ne arly as much as that o f ce me nt (7 5 millio n to ns). But o ur utilisatio n o f fly ash is o nly ab o ut 5 % o f the p ro d uc tio n. The re fo re , the use o f fly ash m ust b e p o p ularise d fo r m o re than o ne re aso ns.

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Rihand Irrigation Project, Uttar Pradesh - 1962

In India, fly ash was used for the first time in the construction of Rihand Irrigation Project, Uttar Pradesh in 1962, replacing cement upto about 15 per cent.

Th e re are tw o w ays th at th e fly ash c an b e use d : o n e w ay is to in te rg rin d c e rtain p e rc e ntag e o f fly ash w ith c e m e nt c linke r at the fac to ry to p ro d uc e Po rtland p o zzo lana c e m e n t (PPC) an d th e se c o n d w ay is to u se th e fly ash as an ad m ixtu re at th e tim e o f making c o nc re te at the site o f w o rk. The latte r me tho d g ive s fre e d o m and fle xib ility to the use r re g ard ing the p e rc e ntag e ad d itio n o f fly ash. The re are ab o ut 7 5 the rmal p o w e r p lants in Ind ia. The q uality o f fly ash g e ne rate d in d iffe re nt plants vary fro m o ne ano the r to a larg e e xte nt and he nce the y are no t in a re ad y to use co nd itio n. To make fly ash o f co nsiste nt q uality, make it suitab le fo r use in co ncre te , the fly ash is re q uired to b e further pro cessed . Such pro cessing arrang ements are no t availab le in Ind ia. The Tab le 5 .1 3 ind icate s the variab ility o f Ind ian fly ash fro m d iffe re nt so urce s. 5 .1 1 The q uality o f fly ash is g o verned b y IS 3 8 1 2 - part I - 2 0 0 3 . The BIS specificatio n limit fo r che mical re q uire me nt and p hysical re q uire me nt are g ive n in Tab le s 5 .1 4 and 5 .1 5 (IS 3 8 1 2 – 2 0 0 3 ). Hig h fine ne ss, lo w carb o n co nte nt, g o o d re activity are the e sse nce o f g o o d fly ash. Since fly ash is p ro d uce d b y rap id co o ling and so lid ificatio n o f mo lte n ash, a larg e p o rtio n o f c o m p o n e n ts c o m p risin g fly a sh p a rtic le s a re in a m o rp h o u s sta te . Th e a m o rp h o u s characteristics g reatly co ntrib ute to the po zzo lanic reactio n b etw een cement and fly ash. O ne o f the impo rtant characte ristics o f fly ash is the sphe rical fo rm o f the particle s. This shape o f particle impro ves the flo w ability and reduces the w ater demand. The suitability o f fly ash co uld b e d e cid e d b y find ing the d ry d e nsity o f fully co mp acte d samp le . ASTM b ro ad ly classify fly ash into tw o classe s. Class F: Fly ash no rmally pro duced by burning anthracite o r bitumino us co al, usually has le ss than 5 % CaO . Class F fly ash has p o zzo lanic p ro p e rtie s o nly. Class C: Fly ash no rm ally p ro d uc e d b y b urning lig nite o r sub -b itum ino us c o al. So m e c la ss C fly a sh m a y h a ve Ca O c o n te n t in e xc e ss o f 1 0 % . In a d d itio n to p o zzo la n ic p ro p e rtie s, c lass C fly ash also p o sse sse s c e m e n titio us p ro p e rtie s. Fly ash, w he n te ste d in ac c o rd anc e w ith the m e tho d s o f te st sp e c ifie d in IS: 1 7 2 7 – 1 9 6 7 * , shall c o nfo rm to the c he mic al re q uire me nts g ive n in Tab le 5 .1 4 .

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Ta ble 5 .1 3 . I llust ra t ive Prope r t ie s of Fly Ash from Diffe re nt Sourc e s 5 .1 1 Pro perty/ So urce Sp e cific g ravity We t sie ve analysis (Pe rce ntag e

A

B

C

D

E*

1 .9 1

2 .1 2

2 .1 0

2 .2 5

2.146 to 2.429

1 6 .0 7

5 4 .6 5

1 5 .6 0

5 .0 0

5 1 .0 0 (Dry)

re taine d o n No . 3 2 5 BS sie ve ) Sp e cific surface (cm 2 / g Blaine s) 2 7 5 9 Lime re activity (kg / sq .cm)

1325

8 6 .8

2175

5 6 .0

4 0 .3

4016

2 8 0 0 to 3 2 5 0

7 9 .3

56.25 to 70.31

Che mical Analysis Lo ss o n ig nitio n p e rce ntag e

5 .0 2

1 1 .3 3

1 .5 4

4 .9 0

SiO 2

5 0 .4 1

5 0 .0 3

6 3 .7 5

6 0 .1 0

SO 3

1 .7 1

–

–

–

Trace s to 2 .5

P2 O 5

0 .3 1

–

–

–

–

Fe 2 O 3

3 .3 4

1 0 .2 0

6 .4 0

0 .6 –4 .0

Al2 O 3

3 0 .6 6

1 8 .2 0

3 0 .9 2 –

1 –2 4 5 –5 9

1 8 .6 0

2 3 .3 3

Ti2

0 .8 4

–

–

–

0 .5 –1 .5

Mn 2 O 3

0 .3 1

–

–

–

–

CaO

3 .0 4

6 .4 3

2 .3 5

6 .3

5 –1 6

Mg O

0 .9 3

3 .2 0

0 .9 5

3 .6 0

1 .5 –5

Na 2 O

3 .0 7

–

–

–

–

Glass content: Highly variable within and between the samples but generally below 35%. *Lignite-based

Ta ble 5 .1 4 . Che m ic a l Re quire m e nt s (I S : 3 8 1 2 – Pa r t -1 : 2 0 0 3 ) Sl. No . ( 1 )

Ch arac te ristic

Re q uire m e nt

( 2 )

( 3 )

( i) Silic o n d io xid e (SiO 2 ) p lu s a lu m in iu m o xid e (Al2 O 3 ) p lu s iro n

7 0 .0

o xid e (Fe 2 O 3 ) p e r c e n t b y m a ss, Min ( i i ) Silic o n d io xid e (SiO 2 ), p e r c e n t b y m a ss, Min

35.0

( i i i ) Re ac tive silic a in p e re c e n t b y m ass, Min

20.0

( i v ) Mag n e siu m o xid e (Mg O ), p e r c e n t b y m ass, Max

5.0

( v ) To tal su lp h u r as su lp h u r trio xid e (SO 3 , p e r c e n t b y m ass, Max 3.0 ( v i ) Availab le alkalis, as so d iu m o xid e (Na 2 O , p e r c e n t b y m ass, Max 1.5 ( v i i ) To tal c hlo rid e in p re se nt b y m ass, Max

0.05

( v i i i )Lo ss o n ig nitio n, p e r c e nt b y m ass, Max

5.0

Note 1. Applicable only when reactive aggregates are used in concrete and are specially requested by the purchaser.

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179

Note 2. For determination of available alkalis, IS: 4032–1968 ‘Method of chemical analysis of hydraulic cement’ shall be referred to. Lim its re g ard in g m o istu re c o n te n t o f fly ash sh all b e as ag re e d to b e tw e e n th e p urc hase r and the sup p lie r. All te sts fo r the p ro p e rtie s sp e c ifie d shall, ho w e ve r, b e c arrie d o ut o n o ve n d ry samp le s.

Ta ble 5 .1 5 . Physic a l Re quire m e nt s (I S : 3 8 1 2 – Pa r t -1 : 2 0 0 3 ) Sl. No .

Ch arac te ristic

Re q uire m e nt Grad e o f Fly ash I

(1)

(2)

( i ) Fin e n e ss — Sp e c ific surfac e in m 2 / kg b y Blain e ’s

(3)

II

(4)

320

250

4.5

3.0

p e rme ab ility me tho d , Min ( i i ) Lim e re ac tivity — Ave rag e c o m p re ssive stre ng th in N/ mm , Min 2

( i i i ) Co m p re ssive stre ng th at 2 8 d ays in N/ m m 2 , Min

No t le ss than 8 0 p e rce nt o f the stre ng th o f co rre sp o nd ing p lain c e m e nt m o rtar c ub e s 0.8 ( i v ) So und ne ss b y auto c lave te st e xp ansio n o f sp e c im e ns, 0.8 p e r c e nt, Max

Effect of Fly Ash on Fresh Concrete G o o d fly ash w ith h ig h fine ne ss, lo w c arb o n c o nte nt, h ig h ly re ac tive fo rm s o n ly a sm all frac tio n o f to tal fly ash c o lle c te d . Th e ESP fly a sh c o lle c te d in c ham b e rs I and II are g e ne rally ve ry co arse , no n sp h e ric a l p a rtic le s sh o w in g larg e ig n itio n lo ss. Th e y c an b e c alle d c o al ash rathe r than fly a sh . Su c h fly a sh (c o a l ash ) are n o t su itab le fo r u se as p o zzo lan an d th e y d o n o t re d uce the w ate r d e mand . Use o f rig h t q u a lity fly a sh , re su lts in re d u c tio n o f w a te r d e m a n d fo r d e sire d slum p . W ith th e re d uc tio n o f un it w ate r c o n te n t, b le e d in g and d rying shrinkag e w ill also b e re d u c e d . Sin c e fly a sh is

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no t hig hly reactive, the heat o f hyd ratio n can b e red uced thro ug h replacement o f part o f the ce me nt w ith fly ash. Fig . 5 .2 8 sho w s the re d uctio n o f te mp e rature rise fo r 3 0 % sub stitutio n o f fly ash. 5 .1 2

Effects of Fly Ash on Hardened Concrete Fly ash , w h e n use d in c o n c re te , c o n trib ute s to th e stre n g th o f c o n c re te d ue to its p o zzo lanic re ac tivity. Ho w e ve r, sinc e the p o zzo lanic re ac tio n p ro c e e d s slo w ly, the initial stre ng th o f fly ash co ncre te te nd s to b e lo w e r than that o f co ncre te w itho ut fly ash. Due to co ntinue d p o zzo lanic re activity co ncre te d e ve lo p s g re ate r stre ng th at late r ag e , w hich may exceed that o f the co ncrete w itho ut fly ash. The po zzo lanic reactio n also co ntributes to making the texture o f co ncrete dense, resulting in decrease o f w ater permeability and g as permeability. It sho uld b e no te d that since po zzo lanic re actio n can o nly pro ce e d in the pre se nce o f w ate r e no ug h m o isture sho uld b e availab le fo r lo ng tim e . The re fo re , fly ash c o nc re te sho uld b e cured fo r lo ng er perio d . In this sense, fly ash co ncrete used in und er w ater structures such as d ams w ill d e rive full b e ne fits o f attaining imp ro ve d lo ng te rm stre ng th and w ate r-tig htne ss.

Durability of Concrete Sufficiently cured co ncrete co ntaining g o o d q uality fly ash sho w s d ense structure w hich o ffe rs hig h re sistivity to the infiltratio n o f d e le te rio us sub stance s. A p o int fo r co nsid e ratio n is that the p o zzo lanic re activity re d uce s the calcium hyd ro xid e co ntent, w hich results in reductio n o f passivity to the steel reinfo rcement and at the same time the ad d itio nal seco nd ary cementitio us material fo rmed makes the paste structure d ense, and the re b y g ive s m o re re sistanc e to the c o rro sio n o f re info rc e m e nt. W hic h o ne w ill have an o ve rrid in g e ffe c t o n th e c o rro sio n o f re in fo rc e m e n t w ill b e a p o in t in q u e stio n . Pub lishe d d ata re po rts that co ncre te w ith fly ash sh o w s sim ilar d e p th o f c arb o n atio n as th at o f c o n c re te w ith o u t fly a sh , a s lo n g a s th e co mp re ssive stre ng th le ve l is same . It is also re c o g nise d that the ad d itio n o f fly ash co ntrib utes to the reductio n o f the expansio n d u e to alkali-ag g re g ate re ac tio n . Th e d ilu tio n e ffe c t o f a lka li a n d re d u c tio n o f th e w a te r permeab ility d ue to d ense texture may b e o ne o f th e fac to rs fo r re d u c tio n o f alkali-ag g re g ate re actio n. In co nclusio n it may be said that altho ug h fly ash is an in d u strial w aste , its u se in c o n c re te sig nificantly imp ro ve the lo ng te rm stre ng th and d urab ility and re d uce he at o f hyd ratio n. In o the r w o rd s g o o d fly ash w ill b e an in d isp e n sab le mineral admixture fo r hig h perfo rmance co ncrete.

High Volume Fly Ash Concrete (HVFA) In India, the g eneratio n o f fly ash is g o ing to have a q uantum jump in the co ming decade. It is tentatively estimated that currently (2000 AD), w e

High volume Fly Ash has been used in the Barker Hall Project, University of California at Berkeley for the construction of shearwalls.

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181

p ro d uc e ab o ut 1 0 0 m illio n to ns o f fly ash and o ut o f w hic h o nly ab o ut 5 % is utilise d , in making b lend ed cements and in a few cases as mineral ad mixture. The d ispo sal o f remaining fly ash has b e co me a se rio us p ro b le m. The re w ill also b e g re ate r ne e d to e co no mise and to co nse rve the ce me nt fo r mo re than o ne re aso ns. O ne o f the practical metho ds fo r co nserving and eco no mising cement and also to reduce the d isp o sal p ro b le m o f fly ash is to p o p ularise the hig h vo lume fly ash co ncre te syste m. Hig h vo lume fly ash co ncre te is a co ncre te w he re in 5 0 to 6 0 % fly ash is inco rp o rate d . It w as first d e ve lo p e d fo r m ass c o nc re te ap p lic atio n w he re lo w he at o f hyd ratio n w as o f primary co nsid eratio n. Sub seq uent w o rk has d emo nstrated that this type o f co ncrete sho w ed e xc e lle n t m e c h an ic al an d d u rab ility p ro p e rtie s re q u ire d fo r stru c tu ral ap p lic atio n s an d p ave me nt co nstructio ns. So me inve stig atio ns have also sho w n the p o te ntial use o f the hig h vo lume fly ash syste m fo r sho tcre ting , lig ht w e ig ht co ncre te and ro lle r co mp acte d co ncre te . In Can ad a, c o n sid e rab le w o rk is g o in g o n th e d e ve lo p m e n t o f b le n d e d c e m e n t inco rp o rating hig h vo lume fly ash. The use o f this typ e o f ce me nt p e rmits to o ve rco me the p ro b le m o f ad d itio n al q uality c o n tro l an d sto rag e fac ilitie s at th e re ad y-m ixe d c o n c re te b atching p lants. Due to very lo w w ater co ntent o f hig h vo lume fly ash co ncrete, the use o f superplasticizer b e co me s ne ce ssary fo r o b taining w o rkab le co ncre te . Use o f air-e ntraining ad mixture s is also co ncurre ntly use d . Mo st inve stig atio ns o n hig h-vo lume fly ash co ncre te w e re carrie d o ut at Canad a Ce nte r fo r Mineral and Energ y Techno lo g y (CANMET). The typical mix pro po rtio n used and o ptimised o n the b asis o f inve stig atio ns are sho w n b e lo w 5 .1 3

Ta ble 5 .1 6 . T ypic a l (H V FA) M ix Propor t ions 5 .1 3 Lo w streng th

Med ium streng th

Hig h streng th

Wate r

1 1 5 kg / m 3

1 2 0 kg / m 3

1 1 0 kg / m 3

ASTM Typ e I ce me nt

1 2 5 kg / m 3

1 5 5 kg / m 3

1 8 0 kg / m 3

Class F fly ash

1 6 5 kg / m 3

2 1 5 kg / m 3

2 2 0 kg / m 3

C.A.

1 1 7 0 kg / m 3

1 1 9 5 kg / m 3

1 1 1 0 kg / m 3

F.A.

8 0 0 kg / m 3

6 4 5 kg / m 3

7 6 0 kg / m 3

Air-e ntraining Ad mixture

2 0 0 ml/ m 3

2 0 0 ml/ m 3

2 8 0 ml/ m 3

Sup e rp lasticize r

3 .0 l/ m 3

4 .5 l/ m 3

5 .5 l/ m 3

Pro p e rtie s o f (HVFA) Fre sh Co nc re te . Mo st o f the investig atio ns at CANMET have b een p e rfo rm e d w ith flo w ing c o nc re te , i.e ., c o nc re te w ith a slum p o f ab o ut 1 8 0 to 2 2 0 m m . Do sag e o f sup e rp lasticize r may vary co nsid e rab ly. The y have also use d ze ro slump co ncre te w itho ut sup e rp lasticize r fo r ro lle r-co mp acte d co ncre te ap p licatio ns. Ble e d in g an d Se ttin g Tim e . As the w ate r c o nte nt is lo w in hig h vo lume fly ash, the b leeding is very lo w and o ften neg lig ib le. Setting time is little lo ng er than that o f co nventio nal co ncre te . This is b e cause o f lo w ce me nt co nte nt, lo w rate o f re actio n and hig h co nte nt o f sup e rp lastic ize r. O ne w ill have to b e c are ful in c o ld w e athe r c o nc re ting in strip p ing the fo rmw o rk.

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He at o f Hyd ratio n. O n acco unt o f lo w cement co ntent, the heat o f hydratio n g enerated is rathe r lo w. CANMET inve stig atio ns have sho w n that the he at o f hyd ratio n o f HVFA w as ab o u t 1 5 to 2 5 ° C le ss th an th at o f re fe re n c e c o n c re te w ith o u t fly ash . In o n e o f th e e xp e rim e nts, in c ase o f a c o nc re te b lo c k o f size 3 .0 5 x 3 .0 5 x 3 .0 5 m e te r, the m axim um te m p e rature re ac he d w as 5 4 ° C (inc re ase o f 3 5 ° C) as ag ainst a he at o f hyd ratio n o f 8 3 ° C (incre ase o f 6 5 ° C) in a b lo ck o f same size mad e o ut o f co ncre te using ASTM Type I Ce me nt o nly. In b o th the case s the w e ig ht o f ce me ntitio us mate rials use d is same .

Curing of (HVFA) Concrete Hig h Vo lum e fly ash c o nc re te is to b e c ure d e ffe c tive ly and fo r lo ng e r d uratio n than o rd inary co ncre te and also no rmal fly ash co ncre te to o b tain co ntinue d p o zzo lanic re actio n so that HVFA d e ve lo p s d e sirab le me chanical p ro p e rtie s. HVFA co ncre te sho uld b e p ro p e rly p ro te cte d fro m p re mature d rying b y p ro p e rly co ve ring the surface .

Mechanical Properties of (HVFA) Concrete The pro perties o f HVFA co ncrete are larg ely d epend ent o n characteristics o f cement and fly ash. Ge ne rally the me c hanic al p ro p e rtie s are g o o d in vie w o f lo w w ate r c o nte nt, lo w e r w ate r to c e me ntitio us ratio and d e nse mic ro struc ture . The typ ic al me c hanic al p ro p e rtie s o f hig h-vo lume fly ash co ncre te as p e r CANMET inve stig atio n is g ive n b e lo w in Tab le 5 .1 7 .

Ta ble 5 .1 7 . T ypic a l M e cha nic a l Prope r t ie s of H a rde ne d H igh Volum e Fly Ash Conc re t e (M e dium St re ngt h) M a de at CAN M ET w it h AST M Type I Ce m e nt . 5 .1 3 Co mp re ssive Stre ng th 1 d ay

8 ± 2 MPa

7 d ays

2 0 ± 4 MPa

2 8 d ays

3 5 ± 5 MPa

9 1 d ays

4 3 ± 5 MPa

3 6 5 d ays

5 5 ± 5 MPa

Fle xural Stre ng th 1 4 d ays

4 .5 ± 0 .5 MPa

9 1 d ays

6 .0 ± 0 .5 MPa

Sp litting Te nsile Stre ng th 2 8 d ays

3 .5 ± 0 .5 MPa

Yo ung ’s Mo d ulus o f Elastic ity 2 8 d ays

3 5 ± 2 GPa

9 1 d ays

3 8 ± 2 GPa

Drying shrinkag e strain at 4 4 8 d ays

5 0 0 ± 5 0 x 1 0 –6

Sp e cific cre e p strain at 3 6 5 d ays p e r MPa o f stre ss

2 8 ± 4 x 1 0 –6

Durability of (HVFA) Concrete Se ve ral lab o rato ry an d fie ld in ve stig atio n c o n d u c te d in Can ad a an d U.S.A. h ave d e m o nstrate d e xc e lle nt d urab ility o f hig h vo lum e fly ash c o nc re te . It w as te ste d fo r w ate r permeability, resistance to freezing and thaw ing , resistance to the penetratio n o f chlo ride io ns,

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c o rro sio n to ste e l re info rc e me nt, re sistanc e to sulp hate attac k, c o ntro lling alkali-ag g re g ate expansio n, carbo natio n and durability in marine enviro nment. The results have sho w n superio r q uality o f hig h vo lume fly ash co ncre te 5 .1 3 .

Use of High Volume Fly Ash All alo ng co nve ntio nal fly ash co ncre te has b e e n in use in many p arts o f the w o rld fo r several decades. Vario us standards and co des have g enerally limited the additio n o f class F fly ash to 10 to 25 per cent. Lab o rato ry and field d emo nstratio n pro jects d uring last 10–12 years have sho w n that co ncre te co ntaining 5 5 to 6 0 p e r ce nt fly ash has e xce lle nt structural and durability characteristics, w hen mixed w ith lo w w ater to cementitio us ratio and superplasticizer. Since 1985 yet ano ther new eco no mical, useful co nstructio n material HVFA has appeared o n the co nstructio n sce nario . Since the n this hig h vo lume fly ash has b e e n use d in many hig hrise building s, industrial structures, w ater fro nt structures, co ncrete ro ads and Ro ller co mpacted co ncre te d ams. The re is hig h p o te ntial fo r this mate rial o n acco unt o f so und e co no my and use fulne ss to ab so rb larg e q uantity o f und e r utilise d o the rw ise harmful w aste mate rial.

Silica Fume Silica fume , also re fe rre d to as micro silica o r co nd e nse d silica fume , is ano the r mate rial that is used as an artificial po zzo lanic admixture. It is a pro duct resulting fro m reductio n o f hig h p urity q uartz w ith co al in an e le ctric arc furnace in the manufacture o f silico n o r fe rro silico n allo y. Silica fume rises as an o xidised vapo ur. It co o ls, co ndenses and is co llected in clo th b ag s. It is further pro cessed to remo ve impurities and to co ntro l particle size. Co nd ensed silica fume is e sse ntially silic o n d io xid e (mo re than 9 0 % ) in no nc rystalline fo rm. Sinc e it is an airb o rne m ate rial like fly ash, it has sp he ric al shap e . It is e xtre m e ly fine w ith p artic le size le ss than 1 micro n and w ith an ave rag e d iame te r o f ab o ut 0 .1 micro n, ab o ut 1 0 0 time s smalle r than ave rag e c e m e nt p artic le s. Silic a fum e has sp e c ific surfac e are a o f ab o ut 2 0 ,0 0 0 m 2 / kg , as ag ainst 2 3 0 to 3 0 0 m 2 / kg . Silic a fum e as an ad m ixture in c o n c re te h as o p e n e d up o n e m o re c h ap te r o n th e a d va n c e m e n t in c o n c re te te c h n o lo g y. Th e u se o f silic a fu m e in c o n ju n c tio n w ith superplasticizer has b een the b ackb o ne o f mo d ern Hig h perfo rmance co ncrete. In o ne article p ub lishe d in 1 9 9 8 issue o f ‘Co ncre te Inte rnatio nal’ b y Michae l Shyd lo w ski, Pre sid e nt, Maste r Build e r, Inc state s “Tw e nty five ye ars ag o no o ne in the co ncre te co nstructio n ind ustry co uld e ve n imag ine cre ating and p lacing co ncre te mixe s that w o uld achie ve in p lace co mp re ssive stre ng ths as hig h as 1 2 0 MPa . . . . . The structure s such as Ke y To w e r in Cle ave land w ith a d esig n streng th o f 85 MPa, and Wacker To w er in Chicag o w ith specified co ncrete streng th o f 85 MPa, and tw o Unio n Sq uare in Seattle w ith co ncrete that achieved 130 MPa streng th – are te stame nts to the b e ne fits o f silica fume te chno lo g y in co ncre te co nstructio n”. It sh o u ld b e re alise d th at silic a fu m e b y itse lf, d o n o t c o n trib u te to th e stre n g th dramatically, altho ug h it do es co ntribute to the streng th pro perty by being very fine po zzo lanic material and also creating dense packing and po re filler o f cement paste. Refer Fig . 5.29. Really sp e akin g , th e h ig h stre n g th s o f h ig h p e rfo rm an c e c o n c re te c o n tain in g silic a fu m e are attrib utab le , to a larg e d e g re e , to the re d uctio n in w ate r co nte nt w hich b e co me s p o ssib le in the p re se nce o f hig h d o se o f sup e rp lasticize r and d e nse p acking o f ce me nt p aste . Pie rre -Claud e Aitcin and Ad am Ne ville in o ne the ir p ap e rs “Hig h-Pe rfo rmance Co ncre te Demystified” states “Streng ths in the rang e o f 60 to 80 MPa w ere o btained w itho ut use o f silica fum e . Eve n hig he r stre ng ths up to 1 0 0 MPa have b e e n ac hie ve d , b ut o nly rare ly. In o ur o p inio n the re is no virtue in avo id ing silica fume if it is availab le and e co no mical, as its use

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sim p lifie s th e p ro d uc tio n o f h ig h p e rfo rm an c e c o n c re te an d m ake s it e asie r to ac h ie ve co mp re ssive stre ng ths in the rang e o f 6 0 to ab o ut 9 0 MPa. Fo r hig he r stre ng ths, the use o f silica fume is e sse ntial.”

Indian Scenario Silica fume has b e co me o ne o f the ne ce ssary ing re d ie nts fo r making hig h stre ng th and hig h p e rfo rmanc e c o nc re te . In Ind ia, silic a fume has b e e n use d ve ry rare ly. Nuc le ar Po w e r Co rp o ratio n w as o ne o f the first to use silica fume co ncre te in the ir Kaig a and Ko ta nucle ar p o w e r p ro je cts. Silica fume w as also use d fo r o ne o f the flyo ve rs at Mumb ai w he re , fo r the first time in Ind ia 75 MPa co ncrete w as used (1999). Silica fume is also no w specified fo r the co nstructio n o f p ro p o se d Band ra-Wo rli se a link p ro je ct at Mumb ai. At p re se nt, Ind ia is no t p ro d ucing silica fume o f rig ht q uality. Re ce ntly, Ste e l Autho rity o f Ind ia has p ro vid e d ne ce ssary facilitie s to p ro d uce annually ab o ut 3 0 0 0 to ns o f silica fume at their Bhad ravathi Co mplex. It appears that the q uality o f silica fume pro d uced b y them need s up g rad atio n. In Ind ia, ho w ever, the silica fume o f internatio nal q uality is marketed b y Elkem Metallug y (P) Ltd ., 6 6 / 6 7 , Mahavir Ce ntre , Se cto r 1 7 , Vashi, Navi Mumb ai-4 0 0 7 0 3 . Sinc e silic a fume o r mic ro silic a is an imp o rtant ne w mate rial, le t us se e this mate rial in so me d e tail. "

micro silica is initially p ro d uce d as an ultrafine und e nsifie d p o w d e r

"

at le ast 8 5 % SiO 2 co nte nt

"

me an p article size b e tw e e n 0 .1 and 0 .2 micro n

"

minimum sp e cific surface are a is 1 5 ,0 0 0 m 2 / kg

"

sp he rical p article shap e .

Available forms Micro silica is availab le in the fo llo w ing fo rms: "

Und e nsifie d fo rms w ith b ulk d e nsity o f 2 0 0 –3 0 0 kg / m 3

"

De nsifie d fo rms w ith b ulk d e nsity o f 5 0 0 –6 0 0 kg / m 3

"

Micro -p e lle tise d fo rms w ith b ulk d e nsity o f 6 0 0 –8 0 0 kg / m 3

"

Slurry fo rms w ith d e nsity 1 4 0 0 kg / m 3 .

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185

"

Slurry is p ro d uc e d b y m ixing und e nsifie d m ic ro silic a p o w d e r and w ate r in e q ual p ro p o rtio ns b y w e ig ht. Slurry is the e asie st and m o st p rac tic al w ay to intro d uc e m ic ro silic a into the c o nc re te m ix

"

Surfac e are a 1 5 –2 0 m 2 / g

"

Stan d ard g rad e slurry p H value 4 .7 , sp e c ific g ravity 1 .3 to 1 .4 , d ry c o n te n t o f micro silica 4 8 to 5 2 % .

Pozzolanic Action Mic ro silic a is m uc h m o re re ac tive than fly ash o r any o the r natural p o zzo lana. The reactivity o f a po zzo lana can be quantified by measuring the amo unt o f Ca(O H)2 in the cement p a ste at different times. In o ne case, 15% o f micro silica reduced the Ca(O H)2 o f tw o samples o f cement fro m 24% to 12% at 90 days and fro m 25% to 11% in 180 days. Mo st research w o rkers ag ree that the C – S – H fo rmed by the reactio n betw een micro silica and Ca(O H)2 appears dense and amo rpho us. 5 .1 4

Influence on Fresh Concrete Water d emand increases in pro po rtio n to the amo unt o f micro silica ad d ed . The increase in w ater d emand o f co ncrete co ntaining micro silica w ill b e ab o ut 1% fo r every 1% o f cement sub stituted. Therefo re, 20 mm maximum size ag g reg ate co ncrete, co ntaining 10% micro silica, w ill have an incre ase d w ate r co nte nt o f ab o ut 2 0 litre s/ m 3 . Me asure s can b e take n to avo id this incre ase b y ad justing the ag g re g ate g rad ing and using supe rplasticize rs. The ad d itio n o f mic ro silic a w ill le ad to lo w e r slump b ut mo re c o he sive mix. The mic ro silic a make the fre sh co ncrete sticky in nature and hard to hand le. It w as also fo und that there w as larg e red uctio n in b le e d in g an d c o n c re te w ith m ic ro silic a c o u ld b e h an d le d an d tran sp o rte d w ith o u t se g re g atio n. It is re p o rte d that c o nc re te c o ntaining m ic ro silic a is vulne rab le to p lastic shrinkag e c rac king and , the re fo re , she e t o r m at c uring sho uld b e c o nsid e re d . Mic ro silic a c o nc re te

Microsilica slurry

Microfiller effect

Courtesy : MC Bauchemie (India) Pvt. Ltd.

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p ro d u c e s m o re h e at o f h yd ratio n at th e in itial stag e o f h yd ratio n . Ho w e ve r, th e to tal g e ne ratio n o f he at w ill b e le ss than that o f re fe re nce co ncre te .

Influence on Hardened Concrete Co ncre te co ntaining micro silica sho w e d o utstand ing characte ristics in the d e ve lo p me nt o f stre ng th. Fig . 5 .3 0 sho w s that 6 0 to 8 0 MPa can b e o b taine d re lative ly e asily. It has b e e n also fo und o ut that mo d ulus o f e lasticity o f micro silica co ncre te is le ss than that o f co ncre te w itho ut micro silica at the same le ve l o f co mp re ssive stre ng th. As re g ard s, th e im p ro ve m e n t in d urab ility asp e c ts m an y p ub lish e d re p o rts, o f th is investig atio n carried o ut, indicate impro vement in durability o f co ncrete w ith micro silica. There are so m e inve stig atio ns ind ic ating c o ntrad ic tio n, p artic ularly w ith re fe re nc e to re sistanc e ag ainst fro st d amag e . With reg ard to w hether o r no t, silica fume is effective fo r alkali-ag g reg ate reactio n, so me re se arch w o rke rs re p o rt that it is e ffe ctive , o the rs co nclud e that w hile it is e ffe ctive , ad d itio n o f silica fume in small q uantitie s actually incre ase s the e xp ansio n.

Mixing By far the mo st po pular applicatio n o f micro silica is in the 50 : 50 slurry fo rm; as it is easy to sto re and d isp e nse . The re are c o nflic ting vie w s o n w he the r m ic ro silic a is b e st ad d e d in p o w d e r o r slurry fo rm . The w o rk b y Ho o to n am o ng o the rs sho w e d that, fo r e q uivale nt micro silica additio ns, slurry pro duced sig nificantly hig her co mpressive and tensile streng ths. 5 .1 5 The slurry ne e d s to b e ke p t ag itate d fo r a fe w ho urs in a d ay to avo id g e lling and se d ime ntatio n. Pre se ntly in Ind ia, Mc-Bauche mie (Ind ) Pvt. Ltd ., sup p ly the silica fume slurry und e r the trad e name “Ce ntrilit Fume s”.

Curing Curing is p ro b ab ly the m o st im p o rtant asp e c t o f m ic ro silic a c o nc re te as the m ate rial underg o es virtually zero b leeding . If the rate o f evapo ratio n fro m the surface is faster than the rate o f mig ratio n o f w ate r fro m inte rio r to the surfac e , p lastic shrinkag e take s p lac e . In the ab se nce o f b le e d ing and slo w mo ve me nt o f w ate r fro m inte rio r to the surface , e arly curing b y w ay o f me mb rane curing is e sse ntial.

Rice Husk Ash Rice husk ash, is o b taine d b y b urning rice husk in a co ntro lle d manne r w itho ut causing

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187

e nviro nme ntal p o llutio n. Whe n p ro p e rly b urnt it has hig h SiO 2 co nte nt and can b e use d as a co ncre te ad mixture . Rice husk ash e xhib its hig h p o zzo lanic characte ristics and co ntrib ute s to hig h stre ng th and hig h imp e rme ab ility o f co ncre te . Ric e h u sk a sh (RHA) e sse n tia lly c o n sist o f a m o rp h o u s silic a (9 0 % SiO 2 ), 5 % c arb o n, and 2 % K2 O . The sp e c ific surfac e o f RHA is b e tw e e n 4 0 – 1 0 0 m 2 / g . Ind ia pro d uces ab o ut 122 millio n to n o f pad d y every year. Each to n o f pad d y pro d ucers ab o ut 4 0 kg o f RHA. The re is a g o o d po te ntial to make use o f RHA as a valuab le po zzo lanic mate rial to g ive almo st the same p ro p e rtie s as that o f micro silica. In U.S.A., hig hly p o zzo lanic rice husk ash is p ate nte d und e r trad e name Ag ro silica and is marketed. Ag ro silica exhib it superpo zzo lanic pro perty w hen used in small q uantity i.e. , 10% b y w eig ht o f cement and it g reatly enhances the w o rkab ility and impermeab ility o f co ncrete. It is a mate rial o f future as co ncre te ad mixture s.

Surkhi Surkhi, w as the co mmo ne st p o zzo lanic mate rials use d in Ind ia. It has b e e n use d alo ng w ith lime in many o f o ur o ld structure s, b e fo re mo d e rn Po rtland ce me nt has take n its ro o ts in Ind ia. Eve n afte r Po rtland ce me nt mad e its ap p e arance in the fie ld o f co nstructio n, surkhi w as use d as an ad mixture to re me d y so me o f the sho rtco ming s o f ce me nt co ncre te . Surkhi w as o ne o f the main co nstitue nts in w ate rpro o fing tre atme nts in co njunctio n w ith lime and so metimes even w ith cement fo r extend ing valuab le po zzo lanic actio n to make the treatment imp e rvio us. Surkhi is an artificial p o zzo lana mad e b y p o w d e ring b ricks o r b urnt clay b alls. In so me majo r w o rks, fo r larg e scale pro ductio n o f surkhi, clay b alls are specially b urnt fo r this purpo se and then po w d ered . By its nature, it is a very co mplex material d iffering w id ely in its q ualities and p e rfo rmanc e s. Be ing d e rive d fro m so il, its c harac te ristic s are g re atly influe nc e d b y the

BHAKRA NANGAL DAM

In Bhakra Nangal Dam scientifically made surkhi (burnt clay Pozzolana) was used about 100 tons per day at the rate of 20% Cement replacement.

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co nstituent mineral co mpo sitio n o f so il, d eg ree o f b urning and fineness o f g rind ing . Because o f the c o mp le xity o f p ro b le m the re has b e e n muc h c o nfusio n o n ac c o unt o f c o ntrad ic to ry re sults o b taine d b y vario us re se arch w o rke rs. In the p ast, the te rm surkhi w as use d fo r a w id e ly varying m ate rial w ith re sp e c t to co mpo sitio n, temperature o f burning , fineness o f g rinding etc. No w the termino lo g y “calcined clay Po zzo lana” is use d inste ad o f the w o rd surkhi, g iving sp e cific p ro p e rty and co mp o sitio n to th is c o n struc tio n m ate rial. IS 1 3 4 4 o f 1 9 8 1 c o ve rs th e sp e c ific atio n fo r c alc in e d c lay p o zzo lana fo r use in m o rtar o r c o nc re te . IS 1 7 2 7 o f 1 9 6 7 c o ve rs the m e tho d s o f te st fo r p o zzo lanic mate rials. Surkh i h as b e e n use d as an ad m ixture in th e c o n struc tio n o f Van ivilas Sag ar d am , Krish n araja Sag ar d am , Hira Bh askar Sag ar d am , Nizam sag ar, Me ttu r, Lo w Bh avan i, Tung ab had ra, Chamb al, Kakrap ara, Bhakra, and in Rana Pratap Sag ar d am. In Bhakra Nang al Pro ject, illitic arg illacio us clay w as calcined in an o il fired ro tary kiln and the g rind ing o p e ratio n w as carrie d o ut thro ug h multichamb e r b all mill. Such a scie ntifically mad e surkhi w as use d 1 0 0 to ns p e r d ay at the rate o f 2 0 % ce me nt re p lace me nt. In Ind ia, the re are a larg e numb e r o f p o zzo lanic c lay d e p o sits o f straine d and imp ure kao lins, ferrug ino us o r o chreo us earths, altered laterites, b auxites and shales etc., availab le in different parts o f the co untry, w hich w ill yield hig hly reactive po zzo lanic materials. Central Ro ad Research Institute, New Delhi, have co nd ucted an all Ind ia survey o f po zzo lanic clay d epo sits. During late 1 9 7 0 ’s and e arly 8 0 ’s, w he n the re w as an acute sho rtag e o f ce me nt in the co untry, the ce me nt manufacture rs use d all kind s o f calcine d clay p o zzo lanic mate rials, that are no t strictly co nfo rming to the spe cificatio n limits in the manufacture o f PPC. This has le d to the b ad im p re ssio n ab o ut the q uality o f PPC in the m ind s o f c o m m o n b uild e rs in the c o untry. The q ualitie s o f PPC as manufac ture d in Ind ia to d ay, sp e c ially b y tho se c o mp anie s w ho g e ne rate and use fly ash in the ir o w n p lant, is o f hig h q uality. O fte n PPC c o uld b e co nsid e re d b e tte r than O PC. Insp ite o f this, the use rs at larg e , as saying g o e s, “o nce b itte n, tw ice shy”, have no t yet o verco me their bad experience o f 1980’s in respect o f q ualities o f PPC. Pre se ntly, in vie w o f the larg e sc ale availab ility o f fly ash and b le nd e d c e me nt the o ld p ractice o f using surkhi and the mo d e rn calcine d clay p o zzo lana has lo st its imp o rtance .

Metakaolin Co n sid e rab le re se arc h h as b e e n d o n e o n n atural p o zzo lan s, n am e ly o n th e rm ally activate d o rd inary clay and kao linitic clay. The se unp urifie d mate rials have o fte n b e e n calle d “Metakao lin”. Altho ug h it sho w ed certain amo unt o f po zzo lanic pro perties, they are no t hig hly re ac tive . Hig h ly re ac tive m e takao lin is m ad e b y w ate r p ro c e ssin g to re m o ve un re ac tive impurities to make 100% reactive po zzo lan. Such a pro duct, w hite o r cream in co lo ur, purified, the rmally activate d is calle d Hig h Re active Me takao lin (HRM). Hig h reactive metakao lin sho w s hig h po zzo lanic reactivity and reductio n in Ca(O H)2 even as early as o ne d ay. It is also o b served that the cement paste und erg o es d istinct d ensificatio n. The imp ro ve me nt o ffe re d b y this d e nsificatio n includ e s an incre ase in stre ng th and d e cre ase in p e rme ab ility. The hig h re active me takao lin is having the p o te ntial to co mp e te w ith silica fume . Hig h reactive metakao lin by trade name “Metacem” is being manufactured and marketed in Ind ia b y speciality Minerals Divisio n, Head o ffice at Arund eep Co mplex, Race Co urse, So uth Baro d a 3 9 0 0 0 7 .

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189

Ground Granulated Blast Furnace Slag (GGBS) Gro und g ranulate d b last-furnace slag is a no nme tallic p ro d uct co nsisting e sse ntially o f silic ate s and alum inate s o f c alc ium and o the r b ase s. The m o lte n slag is rap id ly c hille d b y q ue nching in w ate r to fo rm a g lassy sand like g ranulate d mate rial. The g ranulate d mate rial w hen further g ro und to less than 45 micro n w ill have specific surface o f abo ut 400 to 600 m 2 / kg (Blaine ). The che mical co mpo sitio n o f Blast Furnace Slag (BFS) is similar to that o f ce me nt clinke r. Tab le 5 .1 8 sho w s the appro ximate chemical co mpo sitio n o f cement clinker, b last-furnace slag (BFS) and fly ash.

Ta ble 5 .1 8 . Approx im at e Ox ide Com posit ion of Ce m e nt Clinke r, BFS a nd Fly Ash Sl. No .

Co nstitue nts

Pe rce ntag e co nte nts

Cement clinker

Blast furnace slag

Fly ash

1

CaO

6 0 –6 7

3 0 –4 5

1 .0 –3 .0

2

SiO 2

1 7 –2 5

3 0 –3 8

3 5 –6 0

3

Al2 O 3

3 .0 – 8 .0

1 5 –2 5

1 0 –3 0

4

Fe 2 O 3

0 .5 – 6 .0

5

Mg O

0 .1 – 4 .0

6

MnO 2

7

Glass

8

Sp e cific g ravity

0 .5 –2 .0

4 –1 0

4 .0 –1 7 .0

0 .2 –5 .0

1 .0 –5 .0 8 5 –9 8 3 .1 5

2 0 –3 0

2 .9

2 .1 –2 .6

The perfo rmance o f slag larg ely depends o n the chemical co mpo sitio n, g lass co ntent and fineness o f g rind ing . The q uality o f slag is g o verned b y IS 12089 o f 1987. The fo llo w ing tab le sho w s so me o f the imp o rtant sp e cificatio n limits.

Ta ble 5 .1 9 . Spe c ific at ions of BFS a s pe r I S 1 2 0 8 9 of 1 9 8 7 (1 )

Mang ane se o xid e %

(2 )

Mag ne sium o xid e %

(3 )

Sulp hid e sulp hur %

(4 )

Glass co nte nt %

(5 )

CaO+ MgO+ 1/ 3 Al 2 O 3 SiO 2 + 2 / 3 Al 2 O 3

♦

5 .5 Max 1 7 .0 Max 2 .0 Max 8 5 .0 Min

> _

1 .0

> _

1 .0

> _

1 .5

OR (6 )

CaO + MgO + Al 2 O 3 SiO 2 w he re MnO in slag is mo re than 2 .5 %

(7 )

CaO + C 2 S+ 1 / 2 MgO + Al 2 O 3 SiO 2 + MnO

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! Concrete Technology

Ta ble 5 .2 0 . Com posit ions of som e of t he I ndia n Bla st Fur na c e Sla gs 5 .1 6 Tisc o 7 8

Tisc o 9 0

Durg apur

Ro u rke la

Bo karo

Bh ilai

SiO 2

3 1 .6 6

3 1 .5 0

3 3 .0 0

3 3 .5 0

3 0 .5 0

3 2 .5 0

CaO

3 2 .2 5

3 1 .5 0

3 6 .0 0

2 8 .5 0

2 9 .5 0

3 3 .5 0

Al2 O 3

2 4 .0 0

2 2 .5 0

2 4 .0 0

2 4 .5 0

2 5 .0 0

2 2 .5 0

Mg O

5 .9 2

1 0 .0 0

4 .0 0

8 .0 0

8 .5 0

8 .0 0

MnO

1 .2 5

1 .2 5

1 .4 0

3 .0 0

1 .0 0

1 .0 0

Fe O

0 .8 0

0 .8 5

1 .0 0

1 .0 0

1 .0 0

0 .8 0

S Liq uid

0 .8 0

0 .8 5

0 .6 0

0 .7 0

0 .9 0

0 .8 0

1430°C

1482°C

1457°C

1431°C

1489°C

1430°C

Tisco 7 8

Tisco 9 0

Durg apur

Ro urkela

Bo karo

Bhilai

1 .0 2

1 .0 0

1 .0 9

0 .8 5

0 .9 7

1 .0 3

0 .6 8

0 .7 7

0 .7 0

0 .6 5

0 .6 8

0 .7 5

1 .9 6

2 .0 3

1 .9 4

1 .8 2

2 .0 6

1 .9 7

Te mp e rature

Hyd raulic Ind ices P B H IH I F No te :

P B H IH I F

= = = = = =

1 .7 0

1 .8 4

1 .6 7

1 .5 9

1 .8 2

1 .7 6

1 5 .8 9

1 6 .0 0

1 6 .0 0

1 0 .0 0

1 7 .7 5

1 5 .0 0

1 .8 1

1 .8 2

1 .8 7

1 .5 7

1 .8 8

1 .8 0

CaO/SiO2 (CaO + MgO)/(SiO2 + Al2O3) (CaO + MgO + Al2O3)/SiO2 (CaO + 0.56 Al2O3 + 1.40 MgO) SiO2 20 + CaO + Al2O3 + 0.5 MgO – 2 SiO2 (CaO + 0.5S + Al2O3 + 0.5 MgO)/(SiO2 + MnO)

In Ind ia, w e p ro d uce ab o ut 7 .8 millio n to ns o f b last furnace slag . All the b last furnace slag s are g ranulate d b y q ue nching the mo lte n slag b y hig h p o w e r w ate r je t, making 1 0 0 % g lassy slag g ranules o f 0 .4 mm size. Ind ian b last furnace slag has b een recently evaluated b y Bane rje e A.K. and the summary o f the same has b e e n re p ro d uce d in Tab le 5 .2 0 . The b last furnace slag is mainly use d in Ind ia fo r manufacturing slag ce me nt. The re are tw o me tho d s fo r making Blast Furnace Slag Ce me nt. In the first me tho d b last furnace slag is interg ro und w ith cement clinker alo ng w ith g ypsum. In the seco nd metho d b last furnace slag is se p arate ly g ro und and the n mixe d w ith the ce me nt. Clinke r is hyd raulically mo re active than slag . It fo llo w s the n that slag sho uld b e g ro und finer than clinker, in o rd er to fully d evelo p its hyd raulic po tential. Ho w ever, since slag is much hard e r and d iffic ult to g rind c o m p are d to c linke r, it is g ro und re lative ly c o arse r d uring the p ro ce ss o f inte r-g rind ing . This le ad s to w aste o f hyd raulic p o te ntial o f slag . No t o nly that the inte r-g rind ing se rio usly re stricts the fle xib ility to o p timise slag le ve l fo r d iffe re nt use s. The hyd raulic po te ntial o f b o th the co nstitue nts – clinke r and slag can b e fully e xplo ite d if the y are g ro und se p arate ly. The le ve l o f fine ne ss can b e co ntro lle d w ith re sp e ct to activity, w hich w ill result in energ y saving . The present trend is to w ard s separate g rind ing o f slag and clinker to d ifferent levels. The clinker and g ypsum are g enerally g ro und to the fineness o f less than 3 0 0 0 cm 2 / g (Blaine ) and slag is g ro und to the le ve l o f 3 0 0 0 –4 0 0 0 cm 2 / g (Blaine ) and sto re d se p arate ly. The y are b le nd e d afte r w e ig h b atching , using p ad d le w he e l b le nd e rs, o r p n e um atic b le n d e rs. Pn e um atic b le n d e rs g ive b e tte r h o m o g e n e ity w h e n c o m p are d to me chanical b le nd e rs.

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Just as fly ash is use d as an ad m ixture in m aking c o nc re te Gro und Granulate d Blastfurnac e Slag p o p ularly c alle d GGBS is use d as an ad m ixture in m aking c o nc re te . In o the r co untries its use as an ad mixture is mo re co mmo n than its use as slag cement. No w in Ind ia, sinc e it is availab le se p arate ly as g ro und g ranulate d b last-furnac e slag (GGBS), its use as ad m ixture sh o uld b e c o m e m o re c o m m o n . Re c e n tly fo r m arin e o utfall w o rk at Ban d ra, Mum b ai, G G BS h as b e e n use d as an ad m ixture to re p lac e c e m e n t to th e tun e o f 7 0 % . Pre se n tly in In d ia, w ith th e g ro w in g p o p u larity o f RMC, th e sc o p e fo r u sin g G G BS fo r custo me r’s tailo r mad e re q uire me nts sho uld also b e co me p o p ular.

Performance of GGBS in Concrete Fr e sh Co n c rre e te : The re p lac e m e nt o f c e m e nt w ith GGBS w ill re d uc e the unit w ate r Fre co ntent necessary to o btain the same slump. This reductio n o f unit w ater co ntent w ill be mo re pro no unced w ith increase in slag co ntent and also o n the fineness o f slag . This is b ecause o f the surface co nfig uratio n and p article shap e o f slag b e ing d iffe re nt than ce me nt p article . In ad d itio n, w ate r use d fo r m ixing is no t im m e d iate ly lo st, as the surfac e hyd ratio n o f slag is slig htly slo w er than that o f cement. Fig . 5.31 and Fig . 5.32 sho w s the reductio n in unit w ater co nte nt. Red uctio n o f b leed ing is no t sig nificant w ith slag o f 4 0 0 0 cm 2 / g fineness. But sig nificant b e ne ficial e ffe ct is o b se rve d w ith slag fine ne ss o f 6 0 0 0 cm 2 / g and ab o ve . Har d e n e d Co n c rre e te : Exclusive research w o rks have sho w n that the use o f slag lead s to Hard the e nhance me nt o f intrinsic p ro p e rtie s o f co ncre te in b o th fre sh and hard e ne d co nd itio ns. The majo r ad vantag e s re co g nise d are "

Re d uce d he at o f hyd ratio n

"

Re fine me nt o f po re structure s

"

Re d uce d p e rme ab ilitie s to the e xte rnal ag e ncie s

"

Incre ase d re sistance to che mical attack.

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Ta ble 5 .2 1 Effe c t of Fine ne ss a nd Re pla c e m e nt (pe r c e nt ) of BSF on va rious Prope r t ie s of Conc re t e 5 .1 9 Fin e n e ss

2750 – 5500

5500 – 7500

Re p lace me nt (% )

30

50

70

30

50

70

30

50 70

Wo rkab ility Ble e d ing Hyd ratio n co ntro l Ad iab atic te mp . rise Hyd ratio n-he at co ntro l Early ag e stre ng th 2 8 d ays stre ng th Lo ng te rm stre ng th Hig h stre ng th B Drying shrinkag e Fre e ze -thaw re sistance Carb o natio n co ntro l Wate rtig htne ss Chlo rid e io n re sistance Ab ility o f se aw ate r Che mical re sistance He at re sistance Availab ility o f acce le rate curing Co ntro l fo r AAR e xp ansio n

B B A – B B B B C B B – B B B B B B B

B B A – A C C A C B B – A A A B B B A

B C A A A C C A A B B C A A A A B B A

A A A – B B A B B B B – A A B B B B B

A A A – B C B A B B B – A A A B B B A

A A A A A C C A A B B C A A A A B B A

A A A – B B A A A B B – A A B B B B B

A A A – B B A A B B B – A A A B B B A

Notes:

(c m 2 / g )

7500 –

A A A A A C A B B B C A A A A B B A

A = superior to OPC (ordinary portland cement) B = slightly better than or as well as OPC C = inferior to OPC – = unknown

Th e ab o ve b e n e fic ial e ffe c t o f slag w ill c o n trib u te to th e m an y fac e ts o f d e sirab le pro perties o f co ncrete. Instead o f d ealing separately the impro vement o f vario us pro perties o f co ncre te , it is g ive n in a co nso lid ate d manne r as p e r the re p o rt o n co ncre te using GGBS b y the Archite ctural Institute o f Jap an (1 9 9 2 ). 5 .1 9 Tab le 5 .2 1 sho w s the e ffe c t o f Fine ne ss and re p lac e me nt (p e r c e nt) o f BFS o n vario us p ro p e rtie s o f co ncre te . Blast furnac e slag , altho ug h is an ind ustrial b y-p ro d uc t, e xhib its g o o d c e m e ntitio us p ro p e rtie s w ith little furthe r p ro c e ssing . It p e rm its ve ry hig h re p lac e m e nt o f c e m e nt and e xte nd s many ad vantag e s o ve r co nve ntio nal ce me nt co ncre te . At p re se nt in Ind ia, it is use d fo r b lend ed cement, rather than as cement ad mixture. In larg e pro jects w ith central b atching p lant and in RMC this ce me nt sub stitute mate rial co uld b e use d as use ful mine ral ad mixture and save ce me nt to the e xte nt o f 6 0 to 8 0 p e r ce nt.

Damp-proofing and Waterproofing Admixture In p rac tic e o n e o f th e m o st im p o rtan t re q u ire m e n ts o f c o n c re te is th at it m u st b e impervio us to w ater under tw o co nditio ns, firstly, w hen subjected to pressure o f w ater o n o ne sid e , se co nd ly, to the ab so rptio n o f surface w ate r b y capillary actio n. Many inve stig ato rs are o f the o pinio n that the co ncrete, carefully d esig ned , efficiently executed w ith so und materials

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w ill b e im p e rm e ab le to w ate r. Ho w e ve r, sinc e the usual d e sig n, p lac ing , c uring and in g e ne ral the vario us o p e ratio ns invo lve d at the site o f w o rk le ave m uc h to b e d e sire d , it is ac c e p te d that a use o f a w e ll c ho se n ad m ixture m ay p ro ve to b e o f so m e ad vantag e in re d uc ing the p e rme ab ility. It is to be no ted that the use o f admixture sho uld in no case be co nsidered as a substitute fo r b ad mate rials, b ad d e sig n o r w o rkmanship . In no case can an ad mixture b e e xp e cte d to co mp e nsate fo r cracks o r larg e vo id s in co ncre te causing p e rme ab ility. Wate rp ro o fing ad mixture s may b e o b taine d in p o w d e r, p aste o r liq uid fo rm and may co nsist o f p o re filling o r w ate r re p e lle nt mate rials. The chie f mate rials in the p o re filling class are silicate o f so da, aluminium and zinc sulphates and aluminium and calcium chlo ride. These are che mically active p o re fille rs. In ad d itio n the y also acce le rate the se tting time o f co ncre te and thus render the co ncrete mo re impervio us at early ag e. The chemically inactive po re filling mate rials are chalk, fulle rs e arth and talc and the se are usually ve ry fine ly g ro und . The ir chie f actio n is to impro ve the w o rkability and to facilitate the reductio n o f w ater fo r g iven w o rkability and to make d e nse co ncre te w hich is b asically imp e rvio us. So me mate rials like so d a, po tash so aps, calcium so aps, re sin, ve g e tab le o ils, fats, w axe s and co al tar re sid ue s are ad d e d as w ate r re p e lling mate rials in this g ro up o f ad mixture s. In so m e kind o f w ate rp ro o fing ad m ixture s ino rg anic salts o f fatty ac id s, usually c alc ium o r am m o nium ste arate o r o le ate is ad d e d alo ng w ith lim e and c alc ium c hlo rid e . Calc ium o r ammo nium ste arate o r o le ate w ill mainly act as w ate r re p e lling mate rial, lime as p o re filling material and calcium chlo ride accelerates the early streng th develo pment and helps in efficient curing o f co ncre te all o f w hich co ntrib ute to w ard s making imp e rvio us co ncre te . So me type o f w aterpro o fing ad mixtures may co ntain b utyl stearate, the actio n o f w hich is similar to so ap s, b ut it d o e s no t g ive fro thing ac tio n. Butyl ste arate is sup e rio r to so ap as w ate r re p e lle nt mate rial in co ncre te . He avy m ine ral o il fre e fro m fatty o r ve g e tab le o il has b e e n p ro ve d to b e e ffe c tive in re nd e ring the c o nc re te w ate rp ro o f. The use o f Asp halt Cut-b ac k o ils have b e e n trie d in q uantitie s o f 2 12 , 5 and 1 0 p e r c e nt b y w e ig ht o f c e me nt. Stre ng th and w o rkab ility o f the co ncre te w as no t se rio usly affe cte d . Pro d uc tio n o f c o nc re te o f lo w p e rm e ab ility d e p e nd s to a g re at e xte nt o n suc c e ssful unifo rm p lac ing o f the mate rial. An ag e nt w hic h imp ro ve s the p lastic ity o f a g ive n mixture w itho ut causing d eleterio us effects o r w hich limits b leed ing and thereb y red uces the numb er o f larg e vo id s, mig ht also b e c lassifie d as a p e rme ab ility re d uc ing ad mixture . Air e ntraining ag e nts may also b e c o nsid e re d und e r this, sinc e the y inc re ase w o rkab ility and p lastic ity o f co ncrete and help to reduce w ater co ntent and bleeding . An air entrained co ncrete has lo w er ab so rp tio n and cap illarity till such time the air co nte nt d o no t e xce e d ab o ut 6 p e r ce nt. The aspe ct o f d amp-pro o fing and w ate rpro o fing o f co ncre te is a ve ry co mple x to pic. It embraces the fundamentals o f co ncrete techno lo g y. Amo ng many o ther aspects, the w / c ratio use d in the co ncre te , the co mp actio n, curing o f co ncre te , the ad mixture use d to re d uce the w / c ratio , the he at o f hyd ratio n, the m ic ro -c rac king o f c o nc re te and m any o the r fac e ts influence the structure o f hardened cement paste and co ncrete, w hich w ill have direct bearing o n p e rme ab ility, d amp -p ro o fing and w ate rp ro o fing . This asp e c t is d e alt in little mo re d e tail und e r co nstructio n che micals late r.

Gas Forming Agents A g as fo rming ag e nt is a che mical ad mixture such as aluminium po w d e r. It re acts w ith the hyd ro xid e pro d uced in the hyd ratio n o f cement to pro d uce minute b ub b les o f hyd ro g en

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g as thro ug ho ut the matrix. The extent o f fo am o r g as pro d uced is d epend ent upo n the type a n d a m o u n t o f a lu m in iu m p o w d e r, fin e n e ss a n d c h e m ic a l c o m p o sitio n o f c e m e n t, te mp e rature and mix p ro p o rtio ns. Usually unp o lishe d aluminium p o w d e r is p re fe rre d . The amo unt ad d e d are usually 0 .0 0 5 to 0 .0 2 p e r ce nt b y w e ig ht o f ce me nt w hich is ab o ut o ne te asp o o nful to a b ag o f ce me nt. Larg e r amo unts are b e ing use d fo r the p ro d uctio n o f lig ht w e ig ht co ncre te . The actio n o f aluminium po w d er, w hen pro perly co ntro lled causes a slig ht expansio n in plastic co ncrete o r mo rtar and this red uces o r eliminates the settlement and may, acco rd ing ly, incre ase the b o nd to re info rcing b ars and imp ro ve the e ffe ctive ne ss o f g ro ut, in filling jo ints. It is particularly useful fo r g ro uting under machine bases. The effect o n streng th depends upo n w he the r o r no t the c o nc re te is re straine d fro m e xp and ing . If it is re straine d , the e ffe c t o n stre ng th is ne g lig ib le , and if no t, the lo ss o f stre ng th may b e c o nsid e rab le . It is, the re fo re , imp o rtant that the fo rms b e tig ht and the g ro ut is co mp le te ly co nfine d . In ho t w e athe r, the actio n o f aluminium p o w d e r may o ccur to o q uickly and b e ne ficial ac tio n m ay b e lo st. In c o ld w e athe r, the ac tio n w ill b e slo w e r and m ay no t p ro g re ss fast eno ug h to pro d uce the d esired effect b efo re the co ncrete has set. At no rmal temperature the re ac tio n starts at the time o f mixing and may c o ntinue fo r 1 12 to 4 ho urs. At te mp e rature s abo ve 38°C, the reactio n may be co mpleted in 30 minutes. At abo ut 4°C the reactio n may no t b e effective fo r several ho urs. Appro ximately tw ice as much aluminium po w d er is re q uired at 4 ° C as at 2 1 ° C to p ro d uce the same amo unt o f e xp ansio n. Be cause ve ry small q uantity o f aluminium p o w d e r is use d and as it has a te nd e ncy to flo at o n the w ate r, the p o w d e r is g e ne rally p re -mixe d w ith fine sand and the n this mixture is ad d e d to the mixe r. Alum in ium p o w d e r is also use d as an ad m ixture in th e p ro d uc tio n o f lig h t w e ig h t c o nc re te . Larg e r q uantitie s o f ab o ut 1 0 0 g m s p e r b ag o f c e m e nt is use d fo r this p urp o se . So d ium hyd ro xid e o r triso d ium p ho sp hate is so m e tim e s ad d e d to ac c e le rate the re ac tio n. So metimes an emulsifying ag ent may be added to stabilise the mix. By varying the pro po rtio ns o f alum inium p o w d e r d e p e nd ing up o n the te m p e rature and c are fully c o ntro lling the g as fo rm atio n , lig h t w e ig h t c o n c re te m ay b e p ro d u c e d in a w id e ran g e o f d e n sity. Zin c , mag ne sium p o w d e rs and hyd ro g e n p e ro xid e are also use d as g as fo rming ag e nts.

Air-detraining agents There have b een cases w here ag g reg ates have released g as into o r caused excessive air e ntrainm e nt, in p lastic c o nc re te w hic h m ad e it ne c e ssary to use an ad m ixture c ap ab le o f d issip ating the e xc e ss o f air o r o the r g as. Also it m ay b e re q uire d to re m o ve a p art o f the e ntraine d air fro m co ncre te mixture . Co mpo und s such as trib utyl pho sphate , w ate r-inso lub le alco ho ls and silico nes have been pro po sed fo r this purpo se. Ho w ever, tributyl pho sphate is the mo st w id e ly use d air-d e training ag e nt.

Alkali-aggregate expansion inhibitors We have already dealt w ith the alkali-ag g reg ate expansio n in Chapter 3. It has been seen that alkali-ag g re g ate re actio n can b e re d uce d b y the use o f p o zzo lanic ad mixture . We have alre ad y d e alt ab o ut the use o f p o zzo lanic m ate rial e arly in this c hap te r. The re are so m e evidences that air entraining admixture reduces the alkali-ag g reg ate reactio n slig htly. The o ther ad mixture s that may b e use d to re d uce the alkali-ag g re g ate re actio n are aluminium p o w d e r and lithium salts.

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Workability Agents Wo rkab ility is o ne o f the mo st impo rtant characte ristics o f co ncre te , spe cially und e r the fo llo w ing circumstance s: (a ) If the co ncrete is to b e placed aro und clo sely placed reinfo rcement, deep b eams, thin se ctio ns e tc. (b ) Whe re sp e cial me ans o f p lace me nt are re q uire d such as tre mie , chute o r p ump ing me tho d s. (c ) If the co ncre te is harsh b e cause o f p o o r ag g re g ate characte ristics o r g rad ing . (d ) Fo r making hig h stre ng th co ncre te w he n w / c ratio is ve ry lo w. In the ab o ve circumstance s e ve n the co st o f achie ving the w o rkab ility may have to b e o ve rlo o ke d . So me ad mixture s can b e use d to imp ro ve w o rkab ility. The mate rials use d as w o rkab ility ag e nts are : (a ) fine ly d ivid e d mate rial, (b ) p lasticize rs and sup e rp lasticize rs, (c ) air-e ntraining ag e nts The use o f finely d ivid ed ad mixture in appro priate q uantity impro ves w o rkab ility, red uces rate and amo unt o f b le e d ing , incre ase s the stre ng th o f le an co ncre te and may no t incre ase w ater req uirement and d rying shrinkag e. Co mmo n materials ad d ed as w o rkab ility ag ents are b e nto nite clay, d iato mace o us e arth, fly ash, fine ly d ivid e d silica, hyd rate d lime and talc. Use o f plasticizers and superplasticizers are o ne o f the mo st co mmo nly ado pted metho ds fo r imp ro ve me nt o f w o rkab ility in almo st all the situatio ns in co ncre te making p ractice s. We have se e n in g o o d d e tail ab o ut the use o f p lasticize rs and sup e rp lasticize rs. Tho ug h the c hie f use o f air-e ntraining ag e nt is to inc re ase re sistanc e to fre e zing and th aw in g , in o u r c o u n try, air-e n train m e n t in c o n c re te is m ain ly p rac tise d fo r im p ro vin g w o rkab ility. Air entraining ad mixtures are used as mo rtar and co ncrete plasticizers. This aspect has alre ad y b e e n d e alt w ith.

Grouting Agents Gro uting und er d ifferent co nd itio ns o r fo r d ifferent purpo ses w o uld necessitate d ifferent q ualitie s o f g ro ut-m ixture . So m e tim e s g ro ut m ixture s w ill b e re q uire d to se t q uic kly and so me time s g ro ut mixture s w ill have to b e in fluid fo rm o ve r a lo ng p e rio d so that the y may flo w into all cavitie s and fissure s. So me time s in g ro ut mixture s, a little w ate r is to b e use d b ut at the same time it sho uld exhib it g o o d w o rkab ility to flo w into the cracks and fissures. There are many admixtures w hich w ill satisfy the req uirements o f g ro ut mixture. Admixtures used fo r g ro uting are : (a ) Acce le rato rs

(b ) Re tard e rs

(c ) Gas fo rming ag e nts

(d ) Wo rkab ility ag e nts

(e ) Plasticize rs. Accelerating ag ents may be used in g ro ut to hasten the set in situatio n w here a plug g ing e ffe ct is d e sire d . In such a case calcium chlo rid e o r trie thano lamine can b e use d . Retard ers and d ispersing ag ents may b e used in a g ro ut to aid pumpab ility and to effect the p e ne tratio n o f g ro ut into fine cracks o r se ams. The y includ e mucic acid , g yp sum and a co mme rcial b rand kno w n as RDA (Ray Lig Blind e r) e tc.

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Gas fo rming ad mixtures can b e used w hile g ro uting in co mpletely co nfined areas, such as und e r m ac hine b ase s. Alum inium p o w d e r is the m o st c o m m o nly use d ag e nt, w hic h chemically reacts and fo rms small b ub b les o f hyd ro g en and pro d uces expansio n o f the g ro ut. This e xp ansio n e liminate s se ttle me nt and shrinkag e . Plasticize rs and sup e rp lasticize rs in p o w d e r fo rm is alw ays o ne o f the ing re d ie nts o f the g ro ut mixture fo r e ffe ctive flo w ab ility and o b taining hig h stre ng th.

Corrosion Inhibiting Agents The pro b lem o f co rro sio n o f reinfo rcing steel in co ncrete is universal. But it is mo re acute in c o nc re te e xp o se d to saline o r b rac kish w ate r o r c o nc re te e xp o se d to ind ustrial c o rro sive fume s. A p ate nte d p ro c e ss b y Do ug ill w as use d fo r the No rth Thame s Gas Bo ard in UK, in w hic h so d ium b e nzo ate w as use d as c o rro sio n inhib iting ad mixture to p ro te c t the ste e l in re info rce d co ncre te . In this p ro ce ss 2 p e r ce nt so d ium b e nzo ate is use d in the mixing w ate r o r a 1 0 p e r ce nt b e nzo ate ce me nt slurry is use d to p aint the re info rce me nt o r b o th. So d ium b e nzo ate is also an acce le rato r o f co mp re ssive stre ng th. It is fo u n d th at c alc iu m lig n o su lp h o n ate d e c re ase d th e rate o f c o rro sio n o f ste e l e m b e d d e d in th e c o n c re te , w h e n th e ste e l re in fo rc e m e n t in c o n c re te is su b je c te d to alte rnating o r d ire ct curre nt. So d ium nitrate and calcium nitrite have b een fo und to b e efficient inhib ito rs o f co rro sio n o f ste e l in auto clave d p ro d ucts. Tw o o r thre e p e r ce nt so d ium nitrate b y w e ig ht o f ce me nt is said to serve the purpo se. There are number o f co mmercial admixtures available no w to inhibit co rro sio n. Mc-Co rro d ur is o ne such ad mixture manufacture d b y Mc-Bauchimie (Ind ) Pvt. Ltd . They also manufacture a tw o co mpo nent co rro sio n inhib iting co ating fo r reinfo rcement. This co ating is use d in re p air syste m. Mo re ab o ut c o rro sio n o f re info rc e me nt w ill b e d e alt und e r Chap te r 9 o n d urab ility o f co ncre te .

Bonding Admixture Bo nding admixtures are w ater emulsio ns o f several o rg anic materials that are mixed w ith ce me nt o r mo rtar g ro ut fo r applicatio n to an o ld co ncre te surface just prio r to patching w ith mo rtar o r co ncre te . So me time s the y are mixe d w ith the to p p ing o r p atching mate rial. The ir functio n is to increase the b o nd streng th b etw een the o ld and new co ncrete. This pro ced ure is used in patching o f ero ded o r spalled co ncrete o r to add relatively thin layers o f resurfacing . The co mmo nly used b o nding admixtures are made fro m natural rub b er, synthetic rub b er o r fro m any o rg anic po lymers. The po lymers includ e po lyvinyl chlo rid e, po lyvinyl acetate etc. Bo nd ing ad mixture s fall into tw o g e ne ral cate g o rie s, name ly, re -e mulsifiab le typ e s and no n-re -e mulsifiab le typ e s. The latte r is b e tte r suite d fo r e xte rnal ap p licatio n since it is re sistant to w ate r. The se e mulsio ns are g e ne rally ad d e d to the mixture in p ro p o rtio ns o f 5 to 2 0 p e r ce nt b y w e ig ht o f c e m e nt. Bo nd ing ad m ixture s usually c ause e ntrainm e nt o f air and a stic ky co nsiste ncy in a g ro ut mixture s. The y are e ffe ctive o nly o n cle an and so und surface s.

Fungicidal, Germicidal and Insecticidal Admixtures It has b e e n sug g e ste d that ce rtain mate rials may e ithe r b e g ro und into the ce me nt o r ad d e d as ad mixture s to imp art fung icid al, g e rmicid al o r inse cticid al p ro p e rtie s to hard e ne d c e m e nt p aste s, m o rtars o r c o nc re te s. The se m ate rials inc lud e p o lyhalo g e nate d p he no ls, d ie ld re n e mulsio n o r co p p e r co mp o und s.

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Colouring Agents Pig me nts are o fte n ad d e d to pro d uce co lo ur in the finishe d co ncre te . The re q uire me nts o f suitab le ad m ixture s inc lud e (a ) c o lo ur fastne ss w he n e xp o se d to sunlig ht (b ) c he m ic al stability in the presence o f alkalinity pro duced in the set cement (c ) no adverse effect o n setting time o r stre ng th d e ve lo p me nt. Vario us me tallic o xid e s and mine ral p ig me nts are use d . Pig me nts sho uld p re fe rab ly b e tho ro ug hly mixe d o r inte rg ro und w ith the d ry ce me nt. The y can also b e mixe d w ith d ry co ncre te mixture s b e fo re the ad d itio n o f mixing w ate r. RMC (Ind ia) Ltd ., o ne o f the Re ad y Mixe d Co ncre te supplie r marke ts re ad y mixe d co lo ur co ncre te fo r d e co rative p ave me nts. So me time s the y make this co lo ur co ncre te inco rp o rating p o lyp ro p e lyne fib re s to arre st p o ssib le cracks and crazine ss in the co ncre te flo o r.

Miscellaneous Admixtures The re are hund re d s o f co mme rcial ad mixture s availab le in Ind ia. The y e ffe ct mo re than o ne p ro p e rty o f co ncre te . So me time s the y are ine ffe ctive and d o no t fulfil the claims o f the m an ufac ture rs. It is n o t in te n d e d to d e al in d e tail ab o ut th e se c o m m e rc ial ad m ixture s. Ho w ever, a few o f the mo re impo rtant admixtures are b riefly describ ed and so me o f them are just name d . All these co mmercial admixtures can b e ro ug hly b ro ug ht under tw o categ o ries (a ) Damp pro o fers (b ) Surface hardeners, tho ug h there are o ther ag ents w hich w ill mo dify the pro perties like stre ng th, se tting time , w o rkab ility e tc.

Damp Proofers o o f: It is a w hite po w der to be mixed w ith co ncrete at the rate o f 1 kg per bag (a ) Ac c o p rro o f ce me nt fo r the p urp o se o f incre asing imp e rme ab ility o f co ncre te structure s. ate rPr o o fe r: As the name indicates, it is a w aterpro o fing admixture (b ) Natso n’s Ce m e nt W Wate rPro to b e ad mixe d at the rate o f 1 .5 kg p e r b ag o f ce me nt. (c ) Trip -L-Se a l: It is a w h ite p o w d e r, th e ad d itio n o f w h ic h is c laim e d to d e c re ase p e rme ab ility o f co ncre te and mo rtars and p ro d uce rap id hard e ning e ffe ct. (d ) Cic o : It is a c o lo urle ss liq uid w hic h w he n ad m ixe d w ith c o nc re te , p o sse sse s the p ro p e rtie s o f c o ntro lling se tting time , p ro mo ting rap id hard e ning , inc re asing stre ng th and re nd e ring the co ncre te w ate rp ro o f. (e ) Fe b -Mix-Ad m ix: It is a lig ht ye llo w co lo ure d liq uid claime d to imp art w ate rp ro o fing q uality to co ncre te and incre ase w o rkab ility and b o nd . (f ) Ce m e t: It is a w ate rp ro o fing ad m ixture . The re c o m m e nd e d d o se is 3 p e r c e nt b y w e ig ht o f c e m e nt. It is also c laim e d that its use in c o nc re te w ill p re ve nt e fflo re sc e nc e and g ro w th o f fung i. In ad d itio n to th e ab o ve th e fo llo w in g are so m e o f th e c o m m e rc ial w ate rp ro o fin g ad mixture s: (a ) Arzo k

(b ) Bo nd e x

(c ) Imp e rmo

(d ) Luna-Ns-1

(e ) Sig me t

(f ) Arco nate No . 2

(g ) Sw ad co No . 1

(h ) Rela

(i )

We t se al

(k) Sco tt No . 1 (m ) O mso n’s “Watse ”

( j ) Wate r lo ck (l )

Hyd ro fug e

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Surface Hardeners e te : Me tal c re te is a m e tallic ag g re g ate w hic h is to ug h, d uc tile , sp e c ially (a ) Me tal Cr Cre pro cessed, size g raded iro n particles w ith o r w itho ut cement dispersing ag ent. It is claimed that it g ive s g re ate r w e ar re sistance , co rro sio n re sistance , no n-d usting and no n-slip p ing co ncre te surface . o c rre e te No . 1 : It is a surface hard e ne r and make s the co ncre te surface co mp act, (b ) Fe rr rro d e nse and ho mo g e ne o us. e te Ste e l Patc h: It is a surface hardener. When added 20 per cent b y w eig ht (c ) Me tal Cr Cre o f ce me nt, it is sup p o se d to incre ase the co mp re ssive stre ng th and ab rasio n re sistance . (d ) Ar Arcc o n ate No . 1 : It is a b lack p o w d e r co mp o se d o f iro n filing s. It is use d as surface hard e ne r in co ncre te . In ad d itio n to the ab o ve , the o the r ad mixture s use d as surface hard e ne rs are : (i ) Iro nite ;

(ii ) Me rco nite ;

(iii ) Me ta Ro ck;

(iv ) Pure lite .

An o th e r im p o rtan t ad m ixtu re w h ic h h as b e e n ve ry p o p u lar is “Lissp o l N”. It is a p o lye the o xy surfac e ac tive ag e nt w hic h im p ro ve s w o rkab ility, stre ng th and m any o the r impo rtant pro pe rtie s o f co ncre te w he n use d in a ve ry small d o se o f 12 o z pe r b ag o f ce me nt. The co mme rcial ad mixture s are no t d e p e nd ab le . It has b e e n co mmo n e xp e rie nce that m an y a tim e w h e n th e se ad m ixtu re s are te ste d in a lab o rato ry th e m an u fac tu re r’s o r distributo r’s claims are no t fulfilled. So it w ill be w ro ng to have much faiths in these co mmercial ad mixture s tho ug h so me o f the m g ive so me e nco urag ing re sults. The classificatio n o f ad mixture s and the vario us mate rials use d are sho w n in Tab le 5 .2 2 .

Construction Chemicals So far in this c hap te r w e have d isc usse d the m ate rials that are use d as ad m ixture s to mo d ify the pro perties o f co ncrete. There are o ther chemicals no t used as ad mixtures b ut used to e nhance the p e rfo rmance o f co ncre te , o r use d in co ncre te re late d activitie s in the fie ld o f c o nstruc tio n. Suc h c he m ic als are c alle d c o nstruc tio n c he m ic als o r b uild ing c he m ic als. The fo llo w ing is the list o f so me o f the co nstructio n che micals co mmo nly use d . "

Co ncre te Curing Co mp o und s

"

Po lyme r Bo nd ing Ag e nts

"

Po lyme r Mo d ifie d Mo rtar fo r Re p air and Mainte nance

"

Mo uld Re le asing Ag e nts

"

Installatio n Aid s

"

Flo o r Hard ne rs and Dustp ro o fe rs

"

No n-Shrink Hig h Stre ng th Gro ut

"

Surface Re tard e rs

"

Bo nd -aid fo r p laste ring

"

Re ad y to use Plaste r

"

Guniting Aid

"

Co nstructio n Che micals fo r Wate rp ro o fing 1 . Inte g ral Wate rp ro o fing Co mp o und s 2 . Acrylic Base d Po lyme r Co ating s

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3 . Mine ral b ase d p o lyme r mo d ifie d co ating s 4 . Pro te ctive and De co rative co ating s 5 . Che mical DPC 6 . Wate rp ro o fing Ad he sive fo r Tile s, Marb le and Granite 7 . Silico n Base d Wate r Re p e lle nt Mate rial 8 . Inje ctio n Gro ut fo r Cracks 9 . Jo int Se alants

Membrane Forming Curing Compounds In vie w o f in su ffic ie n t c u rin g g e n e rally c arrie d o u t at site o f w o rk, th e in c re asin g

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im p o rtanc e o f c uring fo r alro und g o o d q ualitie s o f c o nc re te , in p artic ular, stre ng th and d urab ility, the need fo r co nservatio n o f w ater and co mmo n availab ility o f curing co mpo und s in the co untry, it is felt that detail info rmatio n is req uired o n this vital to pic – curing o f co ncrete b y me mb rane fo rming curing co mp o und s. Availab ility o f e no ug h mo isture in c o nc re te is the e sse nc e fo r uninte rrup te d hyd ratio n p ro c e ss. In fre sh c o nc re te , the m o isture le ve l in c o nc re te is m uc h hig he r than the re lative hum id ity o f atm o sp he re . The re fo re , e vap o ratio n o f w ate r take s p lac e fro m the surfac e o f c o n c re te . To re c o up th e lo ss o f w ate r fro m th e surfac e o f c o n c re te an d to p re ve n t th e m ig ratio n o f w ate r fro m the inte rio r o f c o nc re te to surfac e o f c o nc re te , that is to re tain ad e q uate mo isture in the co ncre te , ce rtain me asure s are ad o p te d . Such me asure s take n are g e ne rally calle d curing o f co ncre te .

Drying Behaviour Drying b e havio ur o f c o nc re te d e p e nd s up o n air te m p e rature , re lative hum id ity, fre sh co ncrete temperature and w ind velo city. Fig ure 5 .3 3 sho w s d rying b ehavio ur as per Learch’s inve stig atio n. The ske tch is se lf e xp lanato ry.

Types of Curing Compounds Liq uid me mb rane fo rming curing co mp o und s are use d to re tard the lo ss o f w ate r fro m co ncrete d uring the early perio d o f setting and hard ening . They are used no t o nly fo r curing fresh co ncrete, but also fo r further curing o f co ncrete after remo val o f fo rm w o rk o r after initial w ate r curing fo r o ne o r tw o d ays. In the case o f w hite p ig me nte d curing co mp o und it also re d uce s the te mp e rature rise in co ncre te e xp o se d to rad iatio n fro m sun. Curing co mp o und s are mad e w ith the fo llo w ing b ase s. "

Synthe tic re sin

"

Wax

"

Acrylic

"

Chlo rinate d rub b e r.

Resin and w ax b ased curing co mpo und s seals the co ncrete surface effectively. With time the ir e fficie ncy w ill g e t re d uce d and at ab o ut 2 8 d ays the y g e t d isinte g rate d and p e e ls o ff. Plaste ring c an b e d o ne afte r ab o ut 2 8 d ays. If p laste ring is re q uire d to b e d o ne e arlie r, the surface can b e w ashed o ff w ith ho t w ater. As per o ne set o f experiments it has b een revealed that the typical curing efficiency w as 96% fo r 24 ho urs, 84% fo r 72 ho urs 74% fo r 7 days and 65% fo r 14 days and the averag e efficiency o f resin and w ax based membrane fo rming curing co mp o und can b e take n as ab o ut 8 0 % . Ac rylic b ase d m e m b rane fo rm ing c uring c o m p o und has the ad d itio nal ad vantag e o f having b e tte r ad he sio n o f sub se q ue nt p laste r. The me mb rane d o e s no t g e t crumb le d d o w n o r it ne e d no t b e w ashe d w ith ho t w ate r. In fac t o n ac c o unt o f inhe re nt c harac te ristic s o f acrylic e mulsio n the b o nd ing fo r the p laste r is b e tte r. Chlo rinated rubber curing co mpo unds no t o nly fo rm a thin film that pro tects the co ncrete fro m d rying o ut b ut also fill the minute p o re s in the surface o f co ncre te . The surface film w ill w e ar o ut e ve ntually.

Application Procedure The curing co mp o und is ap p lie d b y b rush o r b y sp raying w hile the co ncre te is w e t. In c ase o f c o lum ns and b e am s the ap p lic atio n is d o ne afte r re m o val o f fo rm w o rk. O n the ho rizo ntal surface , the curing co mp o und is ap p lie d up o n the co mp le te d isap p e arance o f all

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b le e d ing w ate r. Inc ase o f ro ad and Air fie ld p ave m e nts w he re te xturing is re q uire d , the c uring c o mp o und is ap p lie d afte r te xturing . Inc ase o f Pune -Mumb ai e xp re ss hig hw ay, the p ave m e nt is c ast b y slip fo rm p ave r. In this p ro c e ss c o nc re te is finishe d , te xturing is d o ne and curing co mp o und is sp raye d all b y me chanical me ans. The yo ung co ncre te is co ve re d b y te nts to p ro te c t g re e n c o nc re te fro m ho t sun and d rying w ind s. In the ab o ve e xp re ss hig hw ay it is sp e cifie d that the co ncre te is also w ate r cure d afte r o ne d ay using w e t he ssain c lo th. W ate r c uring o ve r me mb rane c uring is se e ming ly sup e rfluo us, b ut it may b e he lp ful in ke e p ing the te mp e rature d o w n. Inc ase the c o nc re te surfac e has d rie d , the surfac e sho uld b e sp raye d w ith w ate r and tho ro ug hly w e tte d and mad e fully d amp b e fo re curing co mp o und is ap p lie d . The co ntaine r o f curing co mp o und sho uld b e w e ll stirre d b e fo re use . At present w e d o no t have Bureau o f Ind ian Stand ard Specificatio n and Co d e o f Practice fo r me mb rane fo rming curing co mpo und s. It is und e r pre paratio n. Since curing co mpo und s are used very co mmo nly in o ur co untry in many o f the majo r pro jects, such as Sard ar Saro var d am pro je cts, e xpre ss hig hw ay pro je cts, e tc., a b rie f d e scriptio n in re spe ct o f ASTM: C 3 0 9 o f 8 1 , fo r “Liq uid Me mb rane -fo rming Co mp o und s fo r Curing co ncre te ” and ASTM C 1 5 6 o f 8 0 a fo r “Wate r Re te ntio n b y co ncre te Curing Mate rials” is g ive n b e lo w fo r info rmatio n o f use rs. Sc o p e : The sp e c ific atio n c o ve rs liq uid m e m b rane fo rm ing c o m p o und s suitab le fo r re tard ing the lo ss o f w ate r d uring the e arly p e rio d o f hard e ning o f c o nc re te . The w hite p ig m e nte d c uring c o m p o und also re d uc e s the te m p e rature rise in c o nc re te e xp o se d to rad iatio n fro m sun. The fo llo w ing typ e s o f co mp o und s are includ e d : "

cle ar o r transluce nt w itho ut d ye

"

cle ar o r transluce nt w ith fug itive d ye

"

w hite p ig me nte d .

Base : The me mb rane fo rming curing co mp o und may b e "

Re sin b ase d

"

Acrylic b ase d .

General Characteristics The clear o r translucent co mpo unds shall be co lo urless o r lig ht in co lo ur. If the co mpo und co ntains fug itive d ye , it shall b e re ad ily d isting uishab le o n the co ncre te surface fo r at le ast 4 ho urs afte r ap p lic atio n, b ut shall b e c o m e inc o nsp ic uo us w ithin 7 d ays afte r ap p lic atio n, if e xp o se d to sun lig ht. The w hite-pig mented co mpo und shall co nsist o f finely divided w hite pig ment and vehicle re ad y m ixe d fo r im m e d iate u se as it is. Th e c o m p o u n d sh all p re se n t in u n ifo rm w h ite ap p e arance w he re ap p lie d at the sp e cifie d rate . The liq uid me mb rane fo rming c o mp o und s shall b e o f suc h c o nsiste nc y that it c an b e re ad ily ap p lie d b y sp raying , b rushing o r ro lling at te mp e rature ab o ve 4 ° C. The liq uid memb rane-fo rming co mpo unds are g enerally applied in tw o co ats. If need b e mo re than tw o co ats may be applied so that the surface is effectively sealed. The first co at shall b e ap p lie d afte r the b le e d ing w ate r, if any, is fully d rie d up , b ut the co ncre te surface is q uite damp. Incase o f fo rmed surfaces such as co lumns and beams etc., the curing co mpo und shall b e ap p lie d imme d iate ly o n re mo val o f fo rmw o rk.

Water Retention Test Sco pe : This me tho d c o ve rs lab o rato ry d e te rminatio n o f e ffic ie nc y o f liq uid me mb rane

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fo rm ing c o m p o und s as m e asure d b y its ab ility to p re ve nt m o isture lo ss d uring the e arly perio d o f hard ening .

Apparatus Mo uld s: Mo uld s shall b e m ad e o f m e tal o r p lastic and shall b e w ate r tig ht. The size shall b e 1 5 0 mm x 3 0 0 mm at the to p , 1 4 5 mm x 2 9 5 mm at the b o tto m and 5 0 mm in heig ht. Curing Cab inet: A cab inet fo r curing the specimens at a temperature o f 3 7 ° C ± 1 ° C and a re lative humid ity o f 3 0 ± 2 % . Balances: The b alance fo r w e ig hing the mo uld and co nte nt shall have a cap acity o f 1 0 kg , se n sitive to 1 g ram o r le ss. Th e b alan c e use d fo r w e ig h in g th e m e m b ran e fo rm in g co mp o und shall have a cap acity o f 1 kg and shall b e se nsitive to 0 .1 g ram o r le ss.

Proportioning and Mixing Mortar Make ce me nt mo rtar o f sufficie nt q uantity re q uire d to fill the mo uld . The pro po rtio n o f cement to standard sand is fo und o ut by making mo rtar o f flo w value o f 35 ± 5 w ith w / c ratio o f 0 .4 0 . The m o uld is ke p t o n g lass p late and the m o rtar is fille d in tw o laye rs and is fully co mp acte d . The sp e cime n is struck o ff le ve l w ith a straig ht e d g e . The mo uld to g e the r w ith g lass plate is cleaned b y means o f w et clean clo th. Seal the b o tto m junctio n b etw een mo uld and g lass p late w ith p araffin w ax o r any o the r suitab le mate rial to p re ve nt o o zing o f w ate r fro m the junctio n.

Number of Specimen A set o f three o r mo re test specimens fo r reference test and three o r mo re fo r applicatio n o f curing co mp o und is d o ne to co nstitute a te st o f a g ive n curing co mp o und .

Storage of Specimen Immediately after mo ulding cleaning and sealing , place all the specimen in curing cabinet maintaine d at te mp e rature 3 7 ° C ± 1 ° C and re lative humid ity o f 3 0 ± 2 % . The sp e cime ns are place d in the cab ine t le aving a space o f ab o ut 5 0 to 2 0 0 mm. Within this limit, the spacing shall be same fo r all the specimens. Keep all the specimens in the sto rag e cabinet till such time, the bleeding w ater disappears. Apply tw o co ats o f curing co mpo unds by means o f brush. Take care to se e that curing co mpo und is applie d o nly to the to p surface and also e ffe ctive ly se al the junctio n b e tw e e n the mo rtar and mo uld b y curing co mp o und . Weig h the co ated specimens nearest to 1 g m. Find o ut the difference in w eig ht betw een the co ate d and unco ate d sp e cime n. This g ive s the w e ig ht o f the liq uid me mb rane fo rming curing mate rial. The w e ig ht o f the curing mate rial shall also b e fo und o ut se p arate ly. The e ntire o p e ratio n o f w e ig hing , co ating and re w e ig hing shall no t take mo re than 3 0 m inute s. All the sp e c im e ns are im m e d iate ly p lac e d in the sto rag e c ab ine t and stip ulate d te mp e rature and humid ity is maintaine d .

Duration of Test The specimens are kept in the curing cab inet fo r 72 ho urs after the applicatio n o f curing co mp o und s.

Corrections for Loss of Weight of Curing Compound Take a metal pan o r plate w ith ed g es raised 3 mm having an area eq ual to the to p area o f te st sp e cime n. Take the same q uantity o f curing mate rial as co ate d to the sp e cime n and co at the metal pan. Place the co ated metal pan in the curing cabinet alo ng w ith the specimen

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and w eig h this pan at the end o f the test. Use the lo ss in w eig ht o f the curing co mpo und as a co rre ctio n facto r in calculating the curing co mp o und ad d e d .

Calculation of Loss in Weight At the e nd o f the sp e c ifie d c uring p e rio d (7 2 ho urs) w e ig h the m o uld and sp e c im e n o f the unco ate d and co ate d samp le s. Find o ut the ave rag e lo ss in w e ig ht o f the unco ate d and co ate d sp e cime n and e xp re ss in kg / sq uare me te r o f surface . This value co uld b e use d as an ind icato r o f e fficie ncy o f liq uid me mb rane fo rming curing co mp o und .

Test results (Water Retention) The liq uid me mb rane fo rming co mp o und , w he n te ste d as sp e cifie d ab o ve shall re strict the lo ss o f w ate r to no t mo re than. 0 .5 5 kg / m 2 o f surface are a in 7 2 ho urs. The re are o the r te sts fo r Re fle ctance and Drying time (no t d e scrib e d ).

Reflectance The w hite-pig mented co mpo und, w hen tested, shall exhib it a daylig ht reflectance o f no t le ss than 6 0 % o f that o f mag ne sium o xid e .

Drying Time The liq uid membrane-fo rming co mpo und, w hen tested shall be dry to to uch in no t mo re than 4 ho urs. After 12 ho urs the co mpo und shall no t be tacky and o ne sho uld be able to w alk o n the co ating w itho ut any fo o t imp re ssio n le ft o n the surface co ate d . In vie w o f the re q uire me nt o f larg e q uantity o f w ate r fo r curing co ncre te and in vie w o f c o ntinuo us, uninte rrup te d c uring is no t d o ne at the site , p artic ularly o n ve rtic al surfac e s, slo p ing surfac e s and d iffic ult inac c e ssib le p lac e s, m e m b rane fo rm ing c uring c o m p o und , tho ug h less efficient than w ater curing , sho uld be made po pular in o ur co nstructio n practices. Emco ril and Emco ril AC are the re sin b ase d and acrylic b ase d liq uid me mb rane fo rming curing co mp o und s manufacture d b y Mc Bauche mie (Ind ) Pvt. Ltd .

Polymer Bonding Agents It is o ne o f the w ell kno w n fact that there w ill no t b e perfect b o nd b etw een o ld co ncrete and new co ncrete. Q uite o ften new co ncrete o r mo rtar is req uired to b e laid o n o ld co ncrete surface. Fo r example, fo r pro viding an o verlay o n an existing pavement, in pro viding a screed o ve r ro o f fo r w ate rp ro o fing o r re p air w o rk e tc . The b o nd ing c harac te ristic s c an b e g re atly imp ro ve d b y p ro vid ing a b o nd c o at b e tw e e n o ld and ne w c o nc re te surfac e o r mixing the bo nding ag ent w ith the new co ncrete o r mo rtar. The use o f bo nding ag ent distinctly impro ves the ad he sio n o f ne w co ncre te o r mo rtar to o ld surface . The mixing o f b o nd ing ag e nts w ith c o nc re te o r mo rtar imp ro ve s the w o rkab ility also at lo w e r w ate r c e me nt ratio and the re b y red uces the shrinkag e characteristic. It also helps in w ater retentio n in co ncrete to red uce the risk o f e arly d rying . It furthe r im p ro ve s the w ate r- p ro o fing q uality o f the tre ate d surfac e . Nafufull and Nafufill BB2 , Nito b o nd EP, Nito b o nd PVA, Sikad ur 32, Sikad ur 41, Ro ff Bo nd ERB, Ro ff Bo nd Sup e r are so me o f the co mme rcial p ro d ucts availab le as b o nd ing ag e nts.

Polymer Modified Mortar for Repair and Maintenance So m e tim e c o nc re te surfac e s re q uire re p air. The e d g e o f a c o nc re te c o lum n m ay g e t chipped o ff; o r ceiling o f co ncrete ro o f may g et peeled o ff, o r a co ncrete flo o r may g et pitted in co urse o f time. Hyd raulic structures o ften req uire repairing . Prefab ricated memb ers such as p ip e s, p o le s, p o sts and ro o fing e le me nts o fte n g e ts c hip p e d o ff w hile strip p ing fo rmw o rk, hand ling and transp o rtatio n. In the p ast ce me nt mo rtar w as use d fo r any kind o f re p air and as an universal repair materials. Cement mo rtar is no t the rig ht kind o f material fo r repair. No w the re are m any kind s o f re p air m ate rials, m o stly p o lym e r m o d ifie d , availab le fo r e ffe c tive

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re p air. The y ad he re ve ry firmly to the o ld co ncre te surface o n acco unt o f g re atly imp ro ve d b o nd characte ristics. The se mate rials are o fte n stro ng e r than the p are nt mate rials. The y are also ad m ixe d w ith so m e o the r m ate rials w hic h m ake the m se t and hard e n ve ry rap id ly. So m e tim e th e m ate rial ad d e d e lim in ate s th e re q u ire m e n t fo r c u rin g . Ze n trifix F 8 2 , Nafuq uic k, Ze ntrifix AS are so m e o f the m ate rials m anufac ture d b y Mc -Bauc he m ie .

Mould Releasing Agents Wo o d e n p lanks, o rd inary p lyw o o d , shutte ring p lyw o o d , ste e l p late s e tc ., are use d as shutte ring mate rials. Co ncre te w he n se t and hard e n ad he re to the surface o f the fo rmw o rk and make it d ifficult to d e mo uld . This affe cts the life and q uality o f shutte ring mate rials and that o f co ncrete. At times w hen extra fo rce is used to d emo uld fro m the fo rm w o rk, co ncrete g ets damag ed. So metime mo uld surface co uld be cement plastered surface, in w hich case the d e mo uld ing o r strip p ing o f c o nc re te me mb e r b e c o me s all the mo re d iffic ult. In the p ast to re d uc e the b o nd b e tw e e n fo rm w o rk and c o nc re te , so m e kind o f m ate rials suc h as b urnt eng ine o il, crude o il, co w dung w ash, po lythylene sheet etc. w ere used. All the abo ve are used o n ac c o un t o f n o n availab ility o f sp e c ially m ad e suitab le an d e ffe c tive m o uld re le asin g materials. No w w e have specially fo rmulated mo uld releasing ag ents, separately fo r abso rptive surfaces like timb er and plyw o o d and fo r no n ab so rb ent surface like steel sheet are availab le. Nafup lan K and Nafup lan UST are the m ate rials m anufac ture d b y Mc -Bauc he m ie . Re e b o l Fo rmco te, Reebo l spl, Reebo l Emulsio n are the materials supplied by Fo sro c. Separo l Sika Fo rm o il are the mate rials fro m Sika Q ualcre te .

Installation Aids Many a time w e leave ho les o r make ho les in w alls, staircases, g ate pillars etc., fo r fixing w ash b asin, lamp shad es, hand rails o r g ates etc. Invariab ly, the ho les mad e o r kept, is larg er than re q uire d . The e xtra sp ace is re q uire d to b e p lug g e d sub se q ue ntly. Mate rial use d in the past is cement mo rtar. Cement mo rtar takes a lo ng time to set and hard en, remain vulnerab le fo r d am ag e and it also shrinks. We have no w sp e c ially m anufac ture d m ate rials w hic h w ill harden to take lo ad in a matter o f 10-15 minutes and w o rk as an ideal material fro m all po ints o f vie w fo r the purpo se o f fixing such installatio ns. Fig . 5 .3 4 sho w s a fe w situatio ns w he re fast curing installatio n aid co uld b e used . They can also b e used fo r fitting o f antennae, fixing o f p ip e s and sanitary ap p liance s e tc. Emfix is the name o f the mate rial manufacture d b y McBauche mie .

Emfix

Fig. 5.34. Typical Application of Installation Aids.

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W ate r tan ks, d e e p p u m p h o u se s, b ase m e n ts, p ip e s c arryin g w ate r o r se w ag e , so m e tim e s d e ve lo p c rac ks and le aks. Suc h le akag e s c an b e p lug g e d b y using a m ate rial c a lle d Mc -Fix ST m a n u fa c tu re d b y Mc -Ba u c h e m ie (In d ia ) Pvt. Ltd . Mc -Fix ST is a po lymer mo dified, ready to use mo rtar fo r q uick and reliable sealing and plug g ing o f any kind o f le aks. The mo rtar p lug d e ve lo p s ve ry hig h stre ng th and is sto ne hard w ithin ab o ut 5 to 7 m inute s. Mc -Fix ST is m ixe d w ith a sm all q uantity o f w ate r and kne ad e d into stiff m o rtar. This stiff m o rtar is ke p t p re sse d ag ainst the c rac k fo r 5 to 7 m inute s.

Leakage

Cut joint

Apply MC-Fix-ST

Fig. 5.35. Methods of using Mc-Fix ST.

Floor Hardeners and Dust Proofers Flo o r is o ne o f the parts o f any building , particularly the industrial building s, co ntinuo usly sub jected to w ear and tear. The facto ry flo o r, o n acco unt o f mo vement o f materials, iro n tyred tro llies, vib ratio ns caused b y running machines is likely to suffer d amag es. Wear resistant and che mical re sistant flo o r must b e p ro vid e d in the b e g inning itse lf. Re p lacing and re p airing o f o ld flo o r w ill inte rfe re w ith the p ro d uctivity and p ro ve to b e co stly. In the p ast, the re w e re so me mate rials such as Iro nite , Hard o nate , Me taro ck and o the r liq uid flo o r hard e ne rs w e re use d to g ive b e tte r p e rfo rm anc e . But p e rfo rm anc e s o f the se mate rials w e re no t fo und to b e satisfacto ry. No w w e have mo d e rn flo o r to p p ing s mate rials co mpo se d o f co rb o rand um o r e me ry po w d e rs, syste matically g rad e d , mixe d w ith pro ce sse d and m o d ifie d c e m e nt. This m ixture w he n sp rinkle d o ve r w e t c o nc re te flo o r o f suffic ie nt stre ng th and d e pth is fo und to g ive an e ffe ctive w e ar re sistant, d ust fre e , no n slip flo o r. The q uantity to b e sp rinkle d is d e p e nd ing up o n the d e g re e o f w e ar re sistance re q uire d . O ne d ifficulty is experienced in the applicatio n o f w ear resistant hard to p material o n the w et base co ncrete. If the sprinkling o f this material is do ne w hen the base co ncrete is to o w et, the finishing o p e ratio n w ill make the se hard w e aring to p p ing mate rial sink, thus making the p ro c e ss ine ffe c tive . O n the o the r hand if the sp rinkling is d e laye d , the b ase c o nc re te w ill have se t and hard e ne d to such an e xte nt that the hard to p mate rial w ill no t b e co me inte g ral p art o f the flo o r. The hard to p p ing mate rial sho uld b e sp rinkle d at the ap p ro p riate time fo r o p timum re sult. Recently vacuum dew atering metho d is freq uently ado pted fo r casting facto ry flo o r, ro ad, airfie ld p ave me nts and co ncre te hard stand ing . In Ind ia, Tre mix Syste m o r Jamshe d ji Vaccum dew atering system is po pular. Emplo yment o f vacuum dew atering o f co ncrete fo r facto ry flo o r b y itse lf w ill g ive im p ro ve d p e rfo rm anc e . In ad d itio n, vac uum d e w ate ring o ffe rs an id e al co nd itio n fo r b ro ad casting the flo o r to pping o n the to p o f the co ncre te flo o r slab . The hard w e arin g , size d an d g rad e d ag g re g ate fo rm s th e to p surfac e o f c o n c re te flo o r, to o ffe r tre me nd o us ab rasio n re sistance . Dre ito p FH is the b rand name o f the p ro d uct manufacture d b y Mc-Bauche mie : Nito flo r Hard to p is the b rand name o f the pro d uct manufactured b y Fo sro c. These pro d ucts can also b e use d fo r e xisting flo o rs w ith the p ro visio n o f a co mp e nsating laye r und e rscre e d 2 5 to 3 0

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mm thick. Use o f p o lyme r b o nd ing ag e nt to imp ro ve the b o nd b e tw e e n e xisting flo o r and co mp e nsating laye r w ill imp ro ve the p e rfo rmance o f the flo o r as a w ho le . Ab rasio n and che mical re sistance o f ind ustrial flo o r can also b e imp ro ve d b y tre ating it w ith se lf-le ve lling e p o xy co ating o r b y scre e d ing the flo o r to p w ith e p o xy mo rtar scre e d ing . Nito flo r FC 1 4 0 , Nito flo r TF 3 0 0 0 , Nito flo r TF 5 0 0 0 are so me o f the mate rials manufacture d b y Fo sro c, Sikaflo o r 9 1 , Sikaflo o r 9 3 , Sikaflo o r 8 1 e tc., are the pro d ucts fro m Sika and Ro fflo r Co at ERS, Ro flo o r to p EFL, Ro flo o r to p EFH are so me o f the Ro ff pro d ucts. In ad d itio n to the ab o ve w e have a numb er o f epo xy flo o ring pro d ucts such as MC-DUR 1500, MC DUR 1100, MC DUR g ro ut e tc. manufacture d b y MC-Bauche mie (Ind ia) Pvt. Ltd . The re are also ce rtain mate rials w hich w he n ap p lie d o n the co ncre te flo o r, co nve rt the lim e ric h c e m e nt c o m p o und s into silic ifie d p ro d uc ts w hic h g ive s e xtre m e c he m ic al and me chanical re sistance and also d ustp ro o fing q ualitie s.

Non-Shrink High Strength Grout G ro u tin g asp e c ts h ave b e e n to u c h e d e arlie r in th is c h ap te r w h ile d e alin g w ith w a te rp ro o fin g o f b a se m e n t sla b a n d o th e r c o n c re te stru c tu re sh o w in g e xc e ss o f p e rm e ab ility. Ap art fro m th e ab o ve , g ro u tin g h as b e c o m e o n e o f th e m o st im p o rtan t o p e ratio n s in c ivil e n g in e e rin g c o n stru c tio n . G ro u tin g b e lo w b ase p late o r m ac h in e fo undatio ns, g ro uting o f fo undatio n b o lt ho les in industrial structures, g ro uting o f prestressed c o nc re te d uc t, g ro uting in anc ho ring and ro c k b o lting syste m s, g ro uting o f c urtain w alls, g ro uting o f fissured ro cks belo w dam fo undatio n, g ro uting the bo dy o f the new ly co nstructed d am itse lf, g ro uting o f d e te rio rate d c o nc re te o r fire affe c te d struc ture s to stre ng the n and re hab ilitate , g ro uting o f o il w e lls are so me o f the fe w situatio ns w he re g ro uting is re so rte d to . The g ro ut mate rial sho uld have hig h e arly and ultimate stre ng th, fre e flo w ing at lo w w ate r c o nte nt, sho uld d e ve lo p g o o d b o nd w ith se t c o nc re te , e sse ntially it sho uld b e no nshrink in nature . Th e g ro u tin g m ate rials c an b e b ro ad ly c lassifie d into tw o c ate g o rie s. O ne is fre e flo w g ro ut fo r use in machine fo und atio ns, fo und atio n b o lts and fixing crane rails e tc . Th e se c o n d c ate g o ry o f g ro u t is m e an t fo r in je c tio n g ro u tin g to fill u p th e sm all c rac ks w h ic h is no rmally d o ne und e r p re ssure . Em c e kre te and Ce ntic re te are the p ro d uc ts o f Mc Ba u c h e m ie . Co n b e xtra G P, Co n b e xtra EP a re th e p ro d uc ts o f Fo sro c .

Surface Retarders Exp o se d a g g re g a te fin ish is o n e kin d o f a rc h ite c tu ra l c o n c re te . A fe w ye a rs b a c k su c h a n arc h ite c tu ral c o n c re te fin ish w as ac h ie ve d b y b u sh hammering metho d, o r by w ire brushing and w ater spray me tho d s. The ab o ve o ld me tho d s are no t g iving a g o o d finish. No w w ith the availab ility o f surface retard ers, b o th fo r “face up ” o r “face d o w n” ap p licatio n, a ve ry p le asing Exposed aggregate architectural e xp o se d a g g re g a te fin ish c a n b e o b ta in e d . O fte n concrete by making use of Surface e xp o se d ag g re g ate finish can b e g ive n fo r p re fab ricate d Retarders.

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pane ls o r fo r in-situ co ncre te . The b e auty o f e xpo se d ag g re g ate can b e furthe r e nhance d b y using d iffe re nt c o lo ure d ag g re g ate s. In the fac e d o w n ap p lic atio n surfac e re tard e rs is b rushe d o n the surfac e o f m o uld s. Th is is g e n e rally d o n e o n p re fab p an e ls. Afte r a d ay o r so w h e n th e c o n c re te is stro n g eno ug h the panel is turned o ver. The co ncrete in the entire cro ss sectio n w ill have hard ened e xce p t the skin p aste in to uch w ith the mo uld . The unhard e ne d p aste is lig htly b rushe d and w ash e d o ff g e n tly. Th e c o arse ag g re g ate s b e c o m e s c le an an d fu lly e xp o se d g ivin g a p le asing arc hite c tural e ffe c t. Inc ase o f fac e up , the surfac e re tard e r is d ire c tly sp raye d o r b rushe d o n the c o nc re te surface b e fo re hyd ratio n p ro ce ss b e g ins. The ce me nt mo rtar o n the surface d o e s no t g e t se t w he re as the mo rtar g e t se t b e lo w ce rtain d e p th o f the surface w he re the co arse ag g re g ate g e ts fully e mb e d d e d in the hard e ne d matrix. At an ap p ro p riate time the unhard e ne d matrix and paste at the surface can b e nicely b rushed and w ashed , expo sing the co arse ag g reg ate. So me time such e xp o se d ag g re g ate finish is g ive n to the fo o t p aths and w alk w ays o n e ithe r sid e o f ro ad s so that the surface w ill b e co me no n slip p e ry. This kind o f tre atme nts are also g ive n in the auto mo b ile se rvice statio n and p arking g arag e s. Diffe re nt surface re tard e rs are availab le fo r different sizes o f co arse ag g reg ates. The ab o ve expo sed ag g reg ate techniq ue is also use d as me chanical ke y fo r ad he re nce o f p laste ring . Exp o se d ag g re g ate finish can b e ad o p te d fo r “w hisp e r co ncre te ” surface in e xp re ss hig hw ays. A kind o f e xp o se d ag g re g ate finish is g ive n to hund re d s o f b uild ing s at Asiad Villag e Co mp le x, Ne w De lhi.

Bond Aid for Plastering In the co nventio nal system o f co nstructio n, o n remo ving the fo rmw o rk, hacking is d o ne o n the surface o f co lumns and b eams and also o n the ceiling o f ro o f, to fo rm a key b etw een the structure and p laste r. Hacking g e ne rally g ive rs fo llo w ing p ro b le ms: "

Unifo rm hacking is d ifficult to achie ve .

"

If there is delay, the structural co ncrete beco mes so hard that hacking beco me difficult.

"

Manual hacking is time co nsuming p articularly at ce iling .

"

Slender members particularly cantilever chajjas, lo uvers, sunbreakers develo p structural cracks d ue to inco nsid e rate he avy hamme r b lo w s o n yo ung co ncre te .

To o b viate the ab o ve pro b lems liq uid po lymer b o nd aid in ready-to -use fo rm is made use o f. The surface sho uld b e cle an, fre e fro m o il and g re ase . If w ashe d it sho uld b e allo w e d to d ry. Bo nd aid fo r p laste ring sho uld b e ap p lie d in o ne c o at b y b rushing o r sp raying . The plastering sho uld b e d o ne o n the principal o f “w et-in-w et”. That is to say that b o nd aid liq uid sho uld no t d ry w he n yo u ap p ly p laste r. A w aiting p e rio d o f ab o ut 6 0 to 9 0 minute s w o uld b e e no ug h b e fo re p laste ring is ap p lie d .

Ready to Use Plaster O ne o f the co mmo n d e fe cts in b uild ing s is cracking o f plaste r. A lo t o f care is ne ce ssary w ith re sp e ct to q uality o f sand , surface p re p aratio n, p ro p e r p ro p o rtio ning , co nsiste ncy and b o nd ing o f plaste r to b ase mate rials. In the ab se nce o f such pre cautio ns, the plaste r cracks and p e e ls o ff. In India, recently Ready Mixed Plaster has b een intro duced b y a few industries. Ro o fit mix is o ne such b rand name .

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Re ad y m ixe d p laste r is b asic ally a p re -m ixe d m ate rials in d ry fo rm c o nsisting o f g o o d sand and c e m e nt in d iffe re nt p ro p o rtio ns fo r vario us use ag e . The m ix also inc lud e s, suc h as b o nd ing ag e nts, w ate r re te ntio n and w o rkab ility ag e nts like hyd rate d lime , air-e ntraining ag e nts, fly ash and o the r suitab le ad m ixture s to e nhanc e the p e rfo rm anc e o f a p laste r m ate rial. It is claime d that re ad y to use p laste r w ill sho w the fo llo w ing b e ne fits. "

Co nsiste ncy in q uality and finish

"

Le ss sto rag e and mixing are a

"

Lo w e r mate rial co nsumptio n

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Crack-fre e p laste r

"

No curing

"

Be tte r ad he sio n and w o rkab ility

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Minimal w astag e .

Ro o fit Ready to use Plaster is available in 40 kg bag and in many different g rades fo r using in internal o r external plaster, mo rtar fo r b rick o r b lo ck w o rk, and as a screed material fo r tiling w o rks. G un itin g Aid . Guniting and sho rtcre ting have b e co me p o p ular me tho d s o f ap p licatio n o f mo rtar o r co ncre te in ne w co nstructio ns o r re pair te chniq ue s. To o ve rco me the pro b le ms asso ciate d w ith sp raye d co ncre te , w e have no w g uniting aid in p o w d e r fo rm o r liq uid fo rm to acce le rate the se tting and hard e ning , to se al w ate r se e p ag e in tunne lling o p e ratio ns, to imp ro ve b o nd ing , to re d uce w astag e o f mate rial b y re b o und ing and to o b tain many mo re advantag e in sprayed co ncrete. Mc-To rkrethifle B.E. is the pro duct o f Mc-Bauchemie (India) Pvt. Ltd . and Co nplast Spray Se t is the Fo sro c pro d uct fo r the ab o ve purpo se .

Construction Chemicals for Waterproofing Insp ite o f m any fo ld ad vanc e m e nt m ad e in Co nc re te Te c hno lo g y and the ab ility to p ro d uce hig h q uality co ncre te , it has no t b e e n p o ssib le to re ally make w ate rp ro o f structure s. Th e p ro b le m o f w ate rp ro o fin g o f ro o fs, w alls, b ath ro o m s, to ile ts, kitc h e n s, b ase m e n ts, sw imming p o o ls, and w ate r tanks e tc. have no t b e e n much re d uce d . The re are numb e r o f materials and metho ds availab le in the co untry fo r w aterpro o fing purpo ses. But mo st o f them fail d ue to o ne o r the o the r re aso ns. Wate rp ro o fing has re maine d as an unso lve d co mp le x p ro b le m . A suc c e ssful w ate rp ro o fing no t o nly d e p e nd s up o n the q uality and d urab ility o f m ate rial b ut also the w o rkm anship , e nviro nm e nt and typ e o f struc ture s. Le aving all o the r asp e cts, the mate rial p art is o nly d iscusse d b e lo w. It sho uld b e re m e m b e re d that the use o f p lastic ize rs, sup e rp lastic ize rs, air-e ntraining ag e nts, p uzo lanic mate rials and o the r w o rkab ility ag e nts, he lp in re d ucing the p e rme ab ility o f c o n c re te b y re d uc in g th e re q uire m e n t o f m ixin g w ate r an d h e n c e th e y c an also b e re g ard e d as w ate rp ro o f m ate rial. In ad d itio n , th e re are o th e r m ate rials an d c h e m ic als availab le fo r w ate rp ro o fing c o nc re te struc ture s. The se mate rials can b e g ro up e d as fo llo w s "

Inte g ral w ate rp ro o fing co mp o und s

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Acrylic Base d Po lyme r Co ating s

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Mine ral Base d Po lyme r Mo d ifie d Co ating s

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Che mical DPC fo r Rising Dampne ss

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Wate rp ro o fing Ad he sive fo r tile s, Marb le and Granite

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Silico n Base d Wate r Re p e lle nt mate rial

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Inje c tio n g ro ut fo r c rac ks

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Pro te ctive and De co rative Co ating s

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Jo int Se alants

Integral Waterproofing Compounds This to p ic has b e e n p artly c o ve re d o n p ag e 1 6 6 . The integ ral w aterpro o fing co mpo unds have been in use fo r the last 4 – 5 decades. They w e re use d as ad mixture s to make c o nc re te w ate rp ro o f. The se c o nve ntio nal w ate rp ro o fing ad mixture s are e ithe r p o re fille rs, o r w o rkab ility ag e nts o r w ate r re p e lle nts, and as such the y are use ful to a lim ite d e xte nt. Fo r e xam p le , ro o t slab s und e rg o the rm al e xp ansio n and sub se q ue nt co ntractio n. With the re sult co ncre te slab s d e ve lo p minute cracks in the b o d y o f co ncrete. Co ncrete slab also develo p minute cracks o n acco unt o f lo ng term drying shrinkag e. In b o th the ab o ve case s, inte g ral w ate rpro o fing co mpo und w ill no t b e o f much use . O nly in situatio ns w here co ncrete is co ntinuo usly in w et o r in damp co nditio n, integ ral w ater pro o fing w ill b e o f so m e u se . Th e c lassic al in te g ral w ate rp ro o fin g c o m p o u n d s are Cic o , Pu d lo , Impe rmo , Acco pro o f e tc. The re are ne w b rand s o f inte g ral w ate rp ro o fing c o m p o und s suc h as Mc -sp e c ial DM, Dichtament DM, Putz-Dichtament fro m MC Bauchemie and co nplast pro lapin 421 IC, co nplast pro lapin I – P e tc. fro m Fo sro c che micals are use ful in making co ncre te mo re w o rkab le and h o m o g e n e o u s. Th e y also h e lp in re d u c in g w / c ratio , w h ic h p ro p e rtie s e xte n d b e tte r w aterpro ffing q uality. The mo dern integ ral w aterpro o fing co mpo unds are a shade better than the o ld pro d ucts. The perfo rmance re q uirements o f integ ral w aterpro o fing o f co mpo und are co ve re d in IS 2 6 4 5 o f 1 9 7 5 .

Acrylic Based Polymer Coatings O ne o f the im p o rtant re aso ns w hy a ro o f slab le aks, e ve n if yo u take all the c are in m aking g o o d c o nc re te , w e ll c o m p ac te d and w e ll c ure d is that the ro o f is sub je c te d to variatio n s o f te m p e ratu re b e tw e e n d ay an d n ig h t o r se aso n to se aso n . Variatio n o f temperature causes micro cracks in co ncrete and these micro cracks pro pag ate w ith time and make the cracks g ro w w id e r and w id e r w hich le ad s to le aking o f ro o f. The incre ase in lo ng te rm d rying shrinkag e w ith ag e , is also ano the r facto r co ntrib uting to the le akag e o f ro o f o r o the r co ncre te me mb e rs. Structural inad e q uacy, o r failure to ad he re to the p ro p e r d e tailing o f re info rce me nt, o r the une q ual se ttle m e nt e tc are so m e o f the ad d itio nal re aso ns fo r d e ve lo p m e nt o f c rac ks in c o nc re te m e m b e rs. In su c h situ atio n s a m e m b ran e fo rm in g w ate rp ro o fin g m ate rials are id e al. Th e m e m b ran e sh o u ld b e to u g h , w ate r re sistan t, so lar re fle c tive , e lastic , e lasto m e ric an d d urab le . The y allo w the mo ve me nt o f the co ncre te me mb e rs, b ut ke e p the q ualitie s o f the me mb rane intact. O n e suc h m ate rial availab le to d ay is Ro o fe x 2 0 0 0 , m ate rial m an ufac ture d b y MCBauche mie (Ind ) Pvt. Ltd . The surface is cle ane d , a p riming co at and d ust b ind e r is ap p lie d o ve r w hich Ro o fe x 2 0 0 0 is applie d b y me ans o f b rush o r spray in tw o co ats, rig ht ang le s to e ach o the r. In ap p lying this mate rial manufacture rs instructio ns sho uld b e strictly fo llo w e d .

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Waterproofing by Mineral Based Polymer Modified Coatings.

Any c rac ks in the p laste r o f p arap e t w all o r ve rtic al surfac e c an b e tre ate d w ith this material. Generally this material is availab le in w hite co lo ur b ut it can b e made to o rder in any o the r co lo ur fo r ae sthe tic re q uire me nts. Th e In d ian stan d ard is b e in g fo rm u late d fo r th e u se o f su c h m e m b ran e fo rm in g w ate rpro o f co ating s.

Mineral Based Polymer Modified Coatings Waterpro o fing o f co ncrete, b rick maso nry and cement b o und surfaces can b e achieved b y a sp e cially mad e slurry co ating s. Slurry co nsists o f sp e cially p ro ce sse d hyd raulically se tting p o w d e r co mp o ne nt and a liq uid p o lyme r co mp o ne nt. The se tw o mate rials w he n mixe d in a sp e cifie d manne r fo rms a b rushab le slurry. Tw o co ats o f this slurry w he n ap p lie d o n ro o f surfac e o r o n an y o th e r ve rtic al surfac e in b ase m e n t, w ate r tan k o r sun ke n p o rtio n o f bathro o m etc. fo rms a lo ng lasting w aterpro o fing co at. This co ating req uires curing fo r a w eek o r so . The co ating so fo rme d is e lastic and ab rasio n re sistant to so me e xte nt. To make it lo ng lasting the c o ating s may b e p ro te c te d b y mo rtar sc re e d ing o r tile s. The trad e name o f the ab o ve material is Dichtament DS, manufactured b y MC-Bauchemic (Ind ia) Pvt. Ltd . The Brush Bo nd o f Fo sro c Co y and ano the r mate rial calle d xype x are also availab le in the marke t. The materials described abo ve altho ug h exhibit g o o d w aterpro o fing q ualities the co ating is no t very elastic. Its perfo rmance in sunken po rtio n o f b athro o m and such o ther areas w here the co ncre te is no t sub je cte d variatio n in te mp e rature , w ill b e g o o d . But it may no t p e rfo rm w e ll o n ro o f slab fo r no t b e ing fle xib le to the re q uire d e xte nt, to co p e up w ith the the rmal mo ve me nt o f ro o f slab . There is a mo dified versio n o f Dichtament DS called Dichtament DS – flex. It is fo rmulated in such a w ay that hig her amo unt o f po lymer co mpo nent is ad d ed to make it flexib le to take care o f po ssib le small cracks in ro o f slab o r such o the r situatio ns. A furthe r mo d ifie d ve rsio n o f the ab o ve has b e e n mad e to g ive a b e tte r w ate rp ro o fing and ab rasio n re sistance to the tre atme nt. The mo d ifie d ve rsio n w ill make the co ating to ug h, m o re e lastic and b e tte r w ate rp ro o fing . This m o d ifie d ve rsio n o f w ate rp ro o fing syste m is spe cially applicab le to te rrace g ard e ns, parking place s, b ase me nts, sw imming po o ls, sanitary areas etc. This co ating also g ives pro tectio n to chlo rid es, sulphates and carb o natio n attack o n

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b rid g e s an d also p ro te c t u n d e rg ro u n d stru c tu re s. Th e trad e n am e is Ze n trifix Elastic , manufacture d b y MC-Bauche mie (Ind ia) Pvt. ltd . The ab o ve is o ne o f the b e st w ate rpro o fing tre atme nts w he n applicatio n is d o ne strictly as pe r manufacture rs instructio ns. Be ing fle xib le and having g o o d crack b rid g ing q uality, it is an id e al mate rial fo r p re fab ricate d ro o f co nstructio n. Be fo re ap p lying the surface sho uld b e mad e d amp b ut no t w e t. It can b e applie d b y tro w e l o r b rush in tw o co ats. A g ap o f ab o ut 3 –4 hrs are g ive n b e tw e e n suc c e ssive c o ats. Tho ug h a stand ard thic kne ss o f 2 –3 mm are ac hie ve d in tw o c o ats, in e xc e p tio nal situatio n a m axim um thic kne ss o f 4 m m c o uld b e allo w e d and in suc h a c ase the ap p lic atio n sho uld b e d o ne in thre e c o ats. A thre e c o at tre atme nt co uld b e g ive n to the e xte rnal face o f maso nry w all in b ase me nt co nstructio n. Fo r th e Min e ral Base d Po lym e r Mo d ifie d Co atin g s th e BIS sp e c ific atio n is u n d e r p re p aratio n.

Protective and Decorative Coatings It w as a po pular belief that co ncrete structures do no t req uire pro tectio n and the co ncrete is a n atu rally d u rab le m ate rial. Late ly it is re alise d th at c o n c re te n e e d s p ro te c tio n an d m ainte nanc e to inc re ase its d urab ility in ho stile c o nd itio ns. This asp e c t w ill b e c o ve re d in g re ate r d e tail und e r c hap te r 9 o n d urab ility o f c o nc re te . Ho w e ve r, und e r the ab o ve to p ic co nsid e ratio n is g ive n o nly tto the w ate rp ro o fing q uality. This RCC me mb e rs such as sunb re ake rs, lo uve rs, facia, facad e s, sun shad e s and chajjas, crack and sp all o ff w ithin a matte r o f a fe w ye ars, p articularly w he n the co ve r p ro vid e d to the se thin and d e lic ate m e m b e rs are inad e q uate . W ate r se e p s into the se m e m b e rs and c o rro d e s the re info rc e m e nt in no tim e . Co rro sio n is also ac c e le rate d b y c arb o natio n. To e nhance the d urab ility o f such thin me mb e rs and to make the m w ate rp ro o f, acrylic b ase d w ate rp ro o f, c arb o natio n re sistant c o ating is g ive n. Inc id e ntally it w ill p re se nt ae sthe tic and d e co rative lo o k. A numb e r o f such pro te ctive , w ate rpro o f d e co rative paints, b ase d o n acrylic po lyme r and se le cte d mine ral fille r are availab le in marke t. Emce co lo ur-Fle x is o ne such paint

Water proofing for Risingdampness — Intrucing new DPC

m an u fac tu re d b y MC Bau c h e m ie an d De kg au rd S is th e p ro d u c t o f Fo sro c c h e m ic als. Ge ne rally the y are w hite b ut co uld b e p ro d uce d in any co lo ur in the facto ry.

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Chemical DPC O fte n o ld b uild ing s are no t p ro vid e d w ith d am p -p ro o f c o urse . The w ate r fro m the g ro un d rise s b y c ap illary ac tio n . Th is risin g w ate r b rin g s w ith it th e d isso lve d salts an d c he m ic als w hic h re sult in p e e ling o f p laste r affe c ting the d urab ility o f struc ture , and also m ake b uild ing s unhyg e nic . Atte m p ts w e re m ad e to c ut the w all thic kne ss in stag e s and in tro d uc e n e w DPC, b ut th is m e th o d w as fo un d to b e n o t o n ly c um b e rso m e b ut also ine ffe ctive . No w w e have mate rials that can b e inje cte d into the w all at ap p ro p riate le ve l to se al the cap illarie s and the re b y to sto p the up w ard mo ve me nt o f w ate r. The syste m invo lve s a tw o c o m p o n e n t m ate rial c alle d Sam afit VK1 an d Sam afit VK2 m an u fac tu re d b y MC Bauc he m ie (Ind ) Pvt. Ltd . Ab o ve the g ro und le ve l and b e lo w the p linth le ve l, ho le s are d rille d in a p articular syste m. Samafit VK1 is inje cte d into this ho le till ab so rptio n sto ps. Afte r ano the r 1 / 2 to 1 ho ur’s time the o the r fluid name ly Samafit VK2 is similarly intro d uc e d . The se tw o liq uid s re act w ith e ach o the r to fo rm a kin d o f je lly like sub stan c e w h ic h b lo c k the c ap illary c avitie s in the b ric kw all and sto ps the capillary rise o f w ate r. In this w ay rising d am p ne ss in b uild ing s, w he re d amp p ro o f co urse is no t p ro vid e d e arlie r, can b e sto p p e d .

Waterproofing Adhesives for Tiles, Marble and Granite The no rmal practice fo llo w ed fo r fixing g laze d tile s in b athro o m, lavato ry, kitche n, an d o th e r p lac e s is th e u se o f stiff n e at c e m e n t p a ste . Th e e xistin g p ra c tic e , Waterproofing Adhesives for Tiles, Marble and tho ug h so mew hat satisfacto ry in the ind o o r Granite. c o n d itio n s fro m th e p o in t o f fixity, su c h p ractice is unsatisfacto ry w he n use d in o utd o o r co nd itio ns and also fro m the p o int o f vie w o f w ate rp ro o fing q uality. The c e m e nt p aste ap p lie d at the b ac k o f tile s d o no t o fte n flo w to w ard s the ed g es o f the tiles and as such w ater enter at the ed g es, particularly w hen w hite c e m e nt ap p lie d as jo int fille r b e c o m e ine ffe c tive . In larg e num b e r o f c ase s it is se e n that p ainting s and p laste r g e ts affe cte d b e hind the se g laze d tile s sup p o se d ly ap p lie d to p re ve nt mo isture mo ve me nt fro m w e t are as. Ce m e nt p aste is no t the rig ht m ate rial fo r fixing the g laze d tile s. The re are , p o lym e r b ase d , hyd raulically se tting , re ad y to use , w ate rp ro o f tile ad he sive availab le in the marke t. The y o ffe r many ad vantag e s o ve r the co nve ntio nal me tho d o f tile fixing such as b e tte r b o nd and ad he sio n, stre ng ths, faste r w o rk, g o o d w ate rpro o fing q uality to the w all. The y are also suitab le fo r e xte rio r and o ve rhe ad surfac e s. No c uring o f tile surfac e b e c o m e s ne c e ssary. If the w all and p laste re d surfac e is d o ne to g o o d p lum b , a sc re e d ing o f o nly 1 – 2 m m thickne ss o f this mo d e rn mate rial w ill b e sufficie nt to fix the tile s in w hich case , the ad o p tio n o f th is m ate rial w ill also b e c o m e e c o n o m ic al. Th e m o d e rn tile ad h e sive m ate rial o ffe rs sp e cial ad vantag e s fo r fixing g laze d tile s in sw imming p o o ls b o th o n flo o r and at sid e w alls. It p ro vid e s o ne mo re b arrie r fo r the p urp o se o f w ate rp ro o fing . Many a time , the g laze d tile s fixe d o n the kitche n p latfo rm o r b athro o m flo o r g e ts d irty o r d am ag e d . It re q uire s to b e re p lac e d . No rm al p rac tic e is to c hip o ff the o ld tile , sc re e d

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c e m e nt p aste o r m o rtar and the n lay the ne w tile s. W ith m o d e rn tile ad he sive , it is no t ne ce ssary to re mo ve the o ld tile . Tile ad he sive can b e scre e d e d o n the e xisting tile s and ne w tile s are laid o ve r the o ld tile s. The b o nd ing q uality is such that g o o d ad he re nce take s p lace tile o ve r tile . This save s co nsid e rab le co st and time and the o p e ratio n b e co me s simp le . Marb le and g ranite are incre asing ly use d fo r clad d ing w all surface s b o th inte rnally and e xte rn ally. Marb le an d g ran ite h ave b e c o m e th e m o st c o m m o n tre atm e n t fo r e xte rn al c lad d ing o f p re stig io us b uild ing s. The y are use d in the fo rm o f tile s o r larg e p ane ls. In the p ast fo r fixing thin marb le and g ranite tile s ce me nt p aste w as use d and fo r fixing larg e slab s and p ane ls, e p o xy and d o w e l p ins w e re use d . No w the re are sp e cially mad e re ad y to use hig h stre ng th po lyme r b o nd ing mate rials availab le w hich can b e use d w ith co nfid e nce b o th fo r inte rnal and e xte rnal use . Re q uire m e nt o f d o w e ls are e lim inate d in m o st o f the c ase s e xc e p t fo r c lad d ing o f larg e p ane ls at ve ry hig h le ve l fo r e xtra safe ty. Marb le and g ranite can e ve n b e fixe d o n b o ard s, incline d surface und e rsid e o f b e ams and in ce iling s b y the use o f this ne w p o w e rful ad he sive s. Ze n trival PL fo r fixin g g laze d tile s an d c e ram ic tile s an d Ze n trival HS fo r m a rb le , g ra n ite a n d sto n e s a re th e m a te ria ls m a n u fa c tu re d b y Mc Bau c h e m ie (In d ia) Pvt. Ltd ., Nito b o n d EP, Nito b o nd PVA, Nito tile SP are so m e o f the p ro d ucts manufacture d b y Fo sro c.

Silicon Based Water Repellant Materials So me time s, in b uild ing s b rick w o rks are no t p laste re d . Bric ks are e xp o se d as they are. If g o o d q uality, w ell b urnt b ricks are no t use d in suc h c o nstruc tio ns, the ab so rp tive b ric ks p e rm its the m o ve m e nt Waterproofing by Silicon Based Water Repellant o f mo isture insid e. O ld heritag e b uild ing s Material. b uilt in sto ne m aso nry m ay suffe r fro m minute cracks in mo rtar jo ints o r p laste re d surface may d e ve lo p crazine ss. In such situatio ns o ne canno t use any o ther w aterpro o fing treatment w hich w ill spo il the intended architectural b e auty o f the structure s. O ne w ill have to g o fo r transp are nt w ate rp ro o fing tre atme nt. Fo r this p urp o se silico n b ase d w ate r re p e llant mate rials are use d b y sp raying o r b rushing . This silic o n b ase d m ate rial fo rm s a thin w ate r re p e llant transp are nt film o n the surfac e . The manufac ture rs slig htly mo d ify this mate rial to make it little fle xib le to ac c o mmo d ate mino r b uild ing mo ve me nts d ue to the rmal e ffe ct. The ap p licatio n must b e d o ne in o ne lib e ral co at so that all the cracks and cre vice s are e ffe c tive ly se ale d . Bric k surfac e ab so rb s this m ate rial m aking the surfac e w ate r re p e llant. So me time s b ric ks o r b lo c ks are imme rse d in suc h mate rials b e fo re using fo r g re ate r w ate r re p e llant q ualitie s. This type o f w aterpro o fing materials are used in many mo numental sto ne b uild ing s and o ld p alac e s so that o rig inal lo o k o f the sto ne m aso nry is m aintaine d , w hile m aking the maso nry w ate rpro o f. The tre atm e nt tho ug h e ffe c tive , is no t fo und to b e lo ng lasting o n ac c o unt o f the mo vement o f building co mpo nents and the lack o f required flexibility o f the film. The treatment

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m ay have to b e re p e ate d at c lo se r inte rvals, say o nc e in 3 –4 ye ars. As it is no t a c o stly mate rial, o ne can affo rd to re p e at the tre atme nt. This m ate rial is c o ve re d in IS 1 2 0 2 7 o f 1 9 8 7 . NISIW A SH is the b rand nam e o f o ne su c h m ate rial m an u fac tu re d b y MC-Bau c h e m ie (In d ) Pvt. Ltd .

Injection Grout for Cracks Inje c tio n g ro uting is o ne o f the p o w e rful m e tho d s c o m m o nly ad o p te d fo r sto p p ing le akag e s in d ams, b ase me nts, sw imming p o o ls, co nstructio n jo ints and e ve n in the le aking ro o fs. A fe w ye ars b ac k, c e m e nt w as use d fo r g ro uting p urp o se s. Ce m e nt is no t an id e al m ate rial fo r g ro uting , as it shrinks w hile se tting and hard e ning . No n-shrink o r e xp ansive c e me nting mate rial is the ap p ro p riate mate rial. W e have q uite a fe w mate rials availab le in the marke t fo r filling up cracks and cre vice s in co ncre te structure s to make the m w ate rp ro o f o r fo r re p air and re hab ilitatio n o f struc ture s. The g ro uts are p ro d uc e d w ith se le c te d w ate r re p e llant, silic ifying c he m ic al c o m p o und s and ine rt fille rs to ac hie ve varie d c harac te ristic s like w ate r imp e rme ab ility, no n shrinkag e , fre e flo w ab ility e tc . The y are suitab le fo r g ravity g ro uting as w e ll as p re ssure g ro uting . Gro uting o f co ncre te structure is o ne o f the p o w e rful metho d s fo r streng thening and w aterpro o fing o f unhealthy structures. Centicrete is the trad e name o f o ne o f the mate rials manufacture d b y MC-Bauche mie . Co nb e x 1 0 0 is the mate rial m arke te d b y Fo sro c c he m ic als.

Joint Sealants Jo ints in b uild ing s, b rid g e s, ro ad s and airfie ld p ave m e nts are ine sc ap ab le . The y m ay b e e xp ansio n jo ints, c o nstruc tio n jo ints o r d um m y jo ints. Suc h jo ints m ust b e e ffe c tive ly sealed to facilitate mo vement o f structure, to pro vid e w aterpro o fing q uality o r to impro ve the rid ing q ualitie s. While p ro vid ing larg e o p e ning s and w ind o w s in b uild ing s the re e xists g ap b e tw e e n w all and w ind o w fram e s, thro ug h w hic h w ate r flo w s insid e . Suc h g ap s in the

STAGES OF REPAIR WORK

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w ind o w sho uld also b e e ffe c tive ly se ale d . The g ap s re sulting in installatio n o f sanitary ap p lian c e s are also re q uire d to b e se ale d . Th e re w e re n o e ffe c tive m ate rials in In d ian marke t hithe rto . No w w e have mo d e rn mate rials like Po lysulphid e se alants and g un applie d Silic o n e Ru b b e r se a la n ts, Sa n ita ry se a la n t a n d Ac rylic se a la n ts. Nito se a l 2 1 5 (1 ) o f Fo sro c , Sikalastic , Sika-SII A, Sikac ryl G P o f Sika Q iu alc re te an d san i se al o f Ro ff are so me o f the mate rials availab le in the marke t fo r the p urp o se o f se aling the jo ints.

Concrete Repair System It w as o nce tho ug ht that co ncre te structure s are d urab le and lasts almo st fo re ve r. But no w it is re alise d that c o nc re te is no t as d urab le as it w as tho ug ht to b e . It w as also the e arlie r b e lie f that co ncre te ne e d s no p ro te ctio n. It w as d iscusse d e arlie r that co ncre te ne e d s to b e maintaine d and p ro te c te d . Ano the r w ro ng no tio n that p re vaile d w as that c o nc re te canno t b e re p aire d . No w the re are mate rials and me tho d s fo r e ffe ctive re p air o f d amag e d c o nc re te struc ture s w hic h is d isc usse d b e lo w . Co n c re te is c o n stan tly un d e r attac k o f e n viro n m e n tal p o llutio n , m o isture in g re ss, p e ne tratio n o f c hlo rid e s and sulp hate s and o the r d e le te rio us c he m ic als. The d urab ility o f co ncre te is the n affe cte d . O f all fo rce s o f d e g rad atio n, carb o natio n is b e lie ve d to b e o ne o f the p o te nt cause s o f d e te rio ratio n o f co ncre te . This asp e ct is g o ing to b e d iscusse d in d e tail und e r chap te r 9 – d urab ility o f co ncre te . Co ncre te re p air has b e co me a majo r sub je ct all o ve r the w o rld . In Ind ia, a fe w ne w ly co nstructed majo r bridg es have co me fo r repair. In places like Mumbai, innumerable building s re q uire re p air. Many g o ve rnme nt d e p artme nts have co nstitute d the ir o w n se p arate “Re p air Bo ard s” to d e al o nly w ith re p air. Wate r tanks are o ne typ e o f structure s o fte n co me to re p air p re mature ly. In the past, there w as no effective metho d o f repairing cracked, spalled and deterio rated co ncrete. They w ere left as such fo r eventual failure. In the recent past, g uniting w as practised fo r re pair o f co ncre te . Guniting has no t pro ve d to b e an e ffe ctive me tho d o f re pair. But no w ve ry e ffe c tive c o nc re te re p air syste m is availab le . The re p air syste m c an take c are o f the c o nc re te c anc e r and inc re ase the lo ng e vity o f the struc ture . The re p air m ate rial use d are stro ng er than the parent material. The efficient b o nd co at, effective carb o natio n resistant fine mo rtar, co rro sio n inhib iting primer, pro tective co ating make the system very effective. Where re info rce me nt is co rro d e d mo re than 5 0 % , e xtra b ars may b e pro vid e d b e fo re re pair mo rtar is applied. The w ho le repair pro cess b eco mes a b it co stly b ut o ften repair is inevitab le and the hig he r co st has to b e e nd ure d Mc -Bauc he m ie (Ind ia) Pvt. Ltd . have a se rie s o f re p air m ate rials and w e ll d e sig ne d re p air syste m . The Fig ure sho w s the re p air p ro c e ss w hic h is se lf e xp lanato ry.

R EFER EN C ES 5.1

Murata, et.al., Development in the use of superplasticizers, ACI Publication, SP 68-1981.

5.2 Massaza F. and Testalin M., Latest development in the use of Admixtures for Cement and Concrete, Cemento, 77 No. 2-1980. 5.3

Collepardi M., The World of Chemical Admixtures in Concrete, Processing of the Congress, Our World in Concrete and Structures, Singapore 1993.

5.4 Sakai E and Daiman M, Dispersion Mechanism of Alite Stabilised by Superplasticizers Containing Polyethylene oxide Graft Chains - 1997.

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5.5. Collepardi M et.al., Zero Slump Loss Superplasticized Concrete, Proceedings of the Congress, Our World in Concrete and Structures, Singapore, Singapore - 1993. 5.6

Fakuda M et.al., Slump Control and Properties of Concrete with a new superplasticizer, Proceedings of International RILEM Symposium on Admixtures for Concrete - Omprovement of Properties, Barcelona, Spain - May 1990.

5.7

Wright PJF, Entrained Air in Concrete, Proceedings of Institution of Civil Engineers, London - May 1953.

5.8

Klieger P, Studies on the Effect of Entrained Air on the Strength and Durability of Concrete made with Various Maximum size of Aggregates, Proceedings of the Highway Research Board, Vol 31 - 1952.

5.9

Shetty M.S., Use of Ritha Powder as an Air-Entraining Agent, Indian Concrete Journal, March 1972.

5.10

Cordon W.A., Entrained Air—A Factor in the Design of Concrete Mixes, ACI Journal, Jan 1948.

5.11

Chatterjee A.K., Availability and use of Pozzolonic and Cementitious Solid Wastes in India, Concrete Technology for Sustainable Development in the Twenty-first Century, organised by CANMET/ACI, Hyderabad, Feb. 1999.

5.12 Efrent R.J., Bureau of Reclamation, Experiences with Fly ash and other Pozzolans in Concrete, Proceedings 3rd International Ash Utilisation Symposium - 1973. 5.13

Malhotra and A. Bilodean, High Volume Fly-ash System, The Concrete for Sustainable Development, Concrete Technology for Sustainable Development in the Twenty-First Century, Organised by CANMET/ACI, Hyderabad, Feb. 1999.

5.14 Micro Silica in Concrete, Technical Report Ni. 41, Report of a Concrete Society Working Party - 1993. 5.15 Hooton R.G., Some Aspects of Durability with Condensed Silica Fume in Pastes, Mortars and Concretes, Proceedings of an International Workshop on Condensed Silica Fume in Concrete, Montreal, May 1987. 5.16 Banerjee A.K., Evaluation of Indian Blast Furnace Salg for Cement, Iron and Steel Review, July 1997. 5.17

Takano et.al., Slump Loss of Concrete Made with GGBS, Symposium on the Use of GGBS, Japan Socciety of Civil Engineers - 1987.

5.18

Endo H. et.al., Effects of the Use of GGBS on Mix Proportion and Strength of Concrete, Symposium on the use of GGBS, Japan Society of Civil Engineers - 1987.

5.19

Architectural Institute of Japan, State-of-the-Art Report on Concrete Using GGBS - 1992.

5.20 Rixom H.R., Concrete Admixtures—Use and Applications, Cement Admixtures Association - 1977.

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6

C H A P T E R Concrete Pump and Placing Boom at Work in a Major Construction Site.

" Workability " Segregation

Fresh Concrete

" Bleeding " Setting Time of Concrete " Process of Manufacture of Concrete " Choosing the Correct Pump " General Points on Using Vibrators " Further Instructions on use of Vibrators " Curing of Concrete " Finishing

F

re sh c o n c re te o r p lastic c o n c re te is a fre sh ly mixe d mate rial w hich can b e mo uld e d into any shape. The relative q uantities o f cement, ag g reg ates and w ate r mixe d to g e the r, co ntro l the p ro p e rtie s o f co ncrete in the w et state as w ell as in the hard ened state. It is w o rthw hile lo o king b ack at w hat w e have d iscusse d in Chap te rs I and III re g ard ing q uantity o f w ate r b e fo re w e d iscuss its ro le in fre sh co ncre te in this chapte r. In Ch ap te r I, w e h ave d isc usse d th e ro le o f w a te r a n d th e q u a n tity o f w a te r re q u ire d fo r che mical co mb inatio n w ith ce me nt and to o ccup y the g e l p o re s. We have se e n that the the o re tic al w ater/ cement ratio req uired fo r these tw o purpo ses is ab o ut 0 .3 8 . Use o f w ater/ cement ratio mo re than this, w ill result in capillary cavities; and less than this, w ill re sult in inco mp le te hyd ratio n and also lack o f space in the syste m fo r the d e ve lo pme nt o f g e l. In Chap te r III, w e have d isc usse d that w hile making mo rtar fo r c o nc re te , the q uantity o f w ate r u se d w ill g e t a lte re d a t site e ith e r d u e to th e p re se nce o f fre e surface mo isture in the ag g re g ate s o r d ue to the ab so rp tio n characte ristics o f d ry and

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p o ro us ag g re g ate s. The w ate r/ ce me nt ratio to b e actually ad o p te d at site is re q uire d to b e ad juste d ke e p ing the ab o ve in mind . In this chap te r o ne mo re asp e ct fo r d e cid ing the w ate r/ ce me nt ratio w ill b e intro d uce d i.e., the w ate r/ ce me nt ratio re q uire d fro m the p o int o f vie w o f w o rkab ility o f co ncre te .

Workability A the o re tical w ate r/ ce me nt ratio calculate d fro m the co nsid e ratio ns d iscusse d ab o ve is no t g o ing to g ive an id e al situatio n fo r maximum stre ng th. Hund re d pe r ce nt co mpactio n o f c o nc re te is an im p o rtant p aram e te r fo r c o ntrib uting to the m axim um stre ng th. Lac k o f co mpactio n w ill result in air vo ids w ho se demag ing effect o n streng th and durab ility is eq ually o r mo re p re d o minant than the p re se nce o f cap illary cavitie s. Harsh concrete unworkable

Medium workability generally workable

Highly workable concrete

Degree of workability

To enable the co ncrete to be fully co mpacted w ith g iven effo rts, no rmally a hig her w ater/ cement ratio than that calculated b y theo retical co nsid eratio ns may b e req uired . That is to say the functio n o f w ater is also to lub ricate the co ncrete so that the co ncrete can b e co mpacted w ith sp e c ifie d e ffo rt fo rthc o ming at the site o f w o rk. The lub ric atio n re q uire d fo r hand ling co ncre te w itho ut se g re g atio n, fo r placing w itho ut lo ss o f ho mo g e ne ity, fo r co mpacting w ith the amo unt o f effo rts fo rth-co ming and to finish it sufficiently easily, the presence o f a certain q uantity o f w ate r is o f vital impo rtance . The q uality o f co ncrete satisfying the abo ve req uirements is termed as w o rkable co ncrete. The w o rd “w o rkab ility” o r w o rkab le co ncrete sig nifies much w id er and d eeper meaning than the o ther termino lo g y “co nsistency” o ften used lo o sely fo r w o rkability. Co nsistency is a g eneral te rm to ind icate the d e g re e o f fluid ity o r the d e g re e o f mo b ility. A co ncre te w hich has hig h co nsiste ncy and w hich is mo re mo b ile , ne e d no t b e o f rig ht w o rkab ility fo r a p articular jo b . Eve ry jo b re q uire s a particular w o rkab ility. A co ncre te w hich is co nsid e re d w o rkab le fo r mass co ncre te fo und atio n is no t w o rkab le fo r co ncre te to b e use d in ro o f co nstructio n, o r e ve n in ro o f co nstructio n, co ncrete co nsidered w o rkab le w hen vib rato r is used, is no t w o rkab le w hen co ncre te is to b e co mp acte d b y hand . Similarly a co ncre te co nsid e re d w o rkab le w he n use d in thick sectio n is no t w o rkable w hen req uired to be used in thin sectio ns. Therefo re, the w o rd

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w o rkab ility assum e s full sig nific anc e o f the typ e o f w o rk, thic kne ss o f se c tio n, e xte nt o f re info rce me nt and mo d e o f co mpactio n. Fo r a co ncre te te chno lo g ist, a co mp re he nsive kno w le d g e o f w o rkab ility is re q uire d to desig n a mix. Wo rkability is a parameter, a mix desig ner is req uired to specify in the mix desig n p ro c e ss, w ith full und e rstand ing o f the typ e o f w o rk, d istanc e o f transp o rt, lo ss o f slum p , metho d o f placing , and many o ther parameters invo lved. Assumptio n o f rig ht w o rkability w ith p ro p e r und e rstand ing b acke d b y e xp e rie nce w ill make the co ncre ting o p e ratio n e co no mical and d urab le . Many research w o rkers tried to define the w o rd w o rkability. But as it sig nifies much w ider p ro p e rtie s and q ualitie s o f c o nc re te , and d o e s no t p ro je c t any o ne p artic ular m e aning , it eludes all precise definitio ns. Ro ad Research labo rato ry, U.K., w ho have extensively studied the fie ld o f co mp actio n and w o rkab ility, d e fine d w o rkab ility as “the p ro p e rty o f co ncre te w hich determines the amo unt o f useful internal w o rk necessary to pro duce full co mpactio n.” Ano ther d e finitio n w hic h e nve lo p e s a w id e r m e aning is that, it is d e fine d as the “e ase w ith w hic h c o nc re te c an b e c o mp ac te d hund re d p e r c e nt having re g ard to mo d e o f c o mp ac tio n and place o f depo sitio n.” Witho ut dw elling much o n the merits and demerits o f vario us definitio ns o f w o rkab ility, having explained the impo rtance and full meaning o f the term w o rkab ility, w e shall se e the facto rs affe cting w o rkab ility.

Factors Affecting Workability Wo rkab le co ncre te is the o ne w hich e xhib its ve ry little inte rnal frictio n b e tw e e n p article and p article o r w hich o ve rco me s the frictio nal re sistance o ffe re d b y the fo rmw o rk surface o r re in fo rc e m e n t c o n tain e d in th e c o n c re te w ith ju st th e am o u n t o f c o m p ac tin g e ffo rts fo rthco ming . The facto rs he lp ing co ncre te to have mo re lub ricating e ffe ct to re d uce inte rnal frictio n fo r he lping e asy co mpactio n are g ive n b e lo w : (a ) Wate r Co nte nt

(b ) Mix Pro po rtio ns

(c ) Size o f Ag g re g ate s

(d ) Shape o f Ag g re g ate s

(e ) Surface Te xture o f Ag g re g ate

(f )

Grad ing o f Ag g re g ate

(g ) Use o f Ad mixture s. (a ) W Wate ate r Co n te n t: Wate r co nte nt in a g ive n vo lume o f co ncre te , w ill have sig nificant influe nce s o n the w o rkab ility. The hig he r the w ate r co nte nt p e r cub ic me te r o f co ncre te , the h ig h e r w ill b e th e flu id ity o f c o n c re te , w h ic h is o n e o f th e im p o rtan t fac to rs affe c tin g w o rkab ility. At the w o rk site, superviso rs w ho are no t w ell versed w ith the practice o f making g o o d co ncre te , re so rt to ad d ing mo re w ate r fo r incre asing w o rkab ility. This practice is o fte n re so rte d to b e cause this is o ne o f the e asie st co rre ctive me asure s that can b e take n at site . It sho uld b e no te d that fro m the d e sirab ility po int o f vie w, incre ase o f w ate r co nte nt is the last reco urse to b e taken fo r impro ving the w o rkab ility even in the case o f unco ntro lled co ncrete. Fo r co ntro lle d co ncre te o ne canno t arb itrarily incre ase the w ate r co nte nt. In case , all o the r ste p s to im p ro ve w o rkab ility fail, o nly as last re c o urse the ad d itio n o f m o re w ate r c an b e co nsidered. Mo re w ater can be added, pro vided a co rrespo nding ly hig her q uantity o f cement is also added to keep the w ater/ cement ratio co nstant, so that the streng th remains the same. (b ) Mix Pr o p o rtio ns: Ag g reg ate/ cement ratio is an impo rtant facto r influencing w o rkability. Pro The hig he r the ag g re g ate / c e m e nt ratio , the le ane r is the c o nc re te . In le an c o nc re te , le ss q uantity o f paste is availab le fo r pro vid ing lub ricatio n, per unit surface area o f ag g reg ate and hence the mo bility o f ag g reg ate is restrained. O n the o ther hand, in case o f rich co ncrete w ith lo w e r ag g re g ate / ce me nt ratio , mo re p aste is availab le to make the mix co he sive and fatty to g ive b e tte r w o rkab ility.

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(c ) Size o f Ag g rre e g ate : The b ig g er the size o f the ag g reg ate, the less is the surface area and he nce le ss amo unt o f w ate r is re q uire d fo r w e tting the surface and le ss matrix o r p aste is req uired fo r lub ricating the surface to red uce internal frictio n. Fo r a g iven q uantity o f w ater and paste, b ig g er size o f ag g reg ates w ill g ive hig her w o rkab ility. The ab o ve, o f co urse w ill b e true w ithin ce rtain limits. ( d ) Sh ap e o f Ag g rre e g ate s: The shap e o f ag g re g ate s influe nc e s w o rkab ility in g o o d m e asure . An g ular, e lo n g ate d o r flaky ag g re g ate m ake s th e c o n c re te ve ry h arsh w h e n c o m p are d to ro und e d ag g re g ate s o r c ub ic al shap e d ag g re g ate s. Co ntrib utio n to b e tte r w o rkab ility o f ro unded ag g reg ate w ill co me fro m the fact that fo r the g iven vo lume o r w eig ht it w ill have le ss surface are a and le ss vo id s than ang ular o r flaky ag g re g ate . No t o nly that, b eing ro und in shape, the frictio nal resistance is also g reatly red uced . This explains the reaso n w hy rive r sand and g rave l p ro vid e g re ate r w o rkab ility to c o nc re te than c rushe d sand and ag g re g ate . The imp o rtanc e o f shap e o f the ag g re g ate w ill b e o f g re at sig nific anc e in the c ase o f present d ay hig h streng th and hig h perfo rmance co ncrete w hen w e use very lo w w / c in the o rd e r o f ab o ut 0 .2 5 . We have alre ad y talke d ab o ut that in the ye ars to co me natural sand w ill b e exhausted o r co stly. O ne has to g o fo r manufactured sand . Shape o f crushed sand as availab le to d ay is unsuitab le b ut the mo d e rn crushe rs are d e sig ne d to yie ld w e ll shap e d and w e ll g rad e d ag g re g ate s. (e ) Su rfac e T e xtu rre e : The influe nce o f surface te xture o n w o rkab ility is ag ain d ue to the Te fact that the to tal surface are a o f ro ug h te xture d ag g re g ate is mo re than the surface are a o f smo o th ro und e d ag g re g ate o f same vo lume . Fro m the e arlie r d iscussio ns it can b e infe rre d that ro ug h te xture d ag g re g ate w ill sho w p o o r w o rkab ility and sm o o th o r g lassy te xture d ag g reg ate w ill g ive b etter w o rkab ility. A red uctio n o f inter particle frictio nal resistance o ffered b y smo o th ag g re g ate s also co ntrib ute s to hig he r w o rkab ility. ( f ) Grad ing o f Ag g re g ate s: This is o ne o f the facto rs w hich w ill have maximum influence o n w o rkability. A w ell g raded ag g reg ate is the o ne w hich has least amo unt o f vo ids in a g iven vo lume . O the r facto rs b e ing co nstant, w he n the to tal vo id s are le ss, e xce ss p aste is availab le to g ive b e tte r lub ricating e ffe ct. With e xce ss amo unt o f p aste , the mixture b e co me s co he sive and fatty w hich prevents seg reg atio n o f particles. Ag g reg ate particles w ill slide past each o ther w ith the le ast am o unt o f c o m p ac ting e ffo rts. The b e tte r the g rad ing , the le ss is the vo id co nte nt and hig he r the w o rkab ility. The ab o ve is true fo r the g ive n amo unt o f paste vo lume . ( g ) Use o f Ad m ixture s: O f all the facto rs mentio ned abo ve, the mo st impo rt facto r w hich affe cts the w o rkab ility is the use o f ad mixture s. In Chap te r 5 , it is amp ly d e scrib e d that the p lasticize rs and sup e rp lasticize rs g re atly imp ro ve the w o rkab ility many fo ld s. It is to b e no te d that initial slump o f co ncrete mix o r w hat is called the slump o f reference mix sho uld b e ab o ut 2 to 3 cm to enhance the slump many fo ld at a minimum do ze. O ne sho uld manupulate o ther facto rs to o b tain initial slump o f 2 to 3 cm in the re fe re nce mix. Witho ut initial slump o f 2 – 3 cm, the w o rkab ility can b e incre ase d to hig he r le ve l b ut it re q uire s hig he r d o sag e – he nce une co no mical. Use o f air-entraining ag ent being surface-active, reduces the internal frictio n betw een the particle s. The y also act as artificial fine ag g re g ate s o f ve ry smo o th surface . It can b e vie w e d that air b ub b les act as a so rt o f b all b earing b etw een the particles to slide past each o ther and g ive e asy m o b ility to the p artic le s. Sim ilarly, the fine g lassy p o zzo lanic m ate rials, insp ite o f incre asing the surface are a, o ffe r b e tte r lub ricating e ffe cts fo r g iving b e tte r w o rkab ility.

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Measurement of Workability It is d iscusse d e arlie r that w o rkab ility o f co ncre te is a co mple x pro pe rty. Just as it e lud e s all p re c ise d e finitio n, it also e lud e s p re c ise m e asure m e nts. Num e ro us atte m p ts have b e e n mad e b y many re se arch w o rke rs to q uantitative ly me asure this imp o rtant and vital p ro p e rty o f co ncrete. But no ne o f these metho d s are satisfacto ry fo r precisely measuring o r expressing this p ro p e rty to b ring o ut its full me aning . So me o f the te sts, me asure the p arame te rs ve ry c lo se to w o rkab ility an d p ro vid e use ful in fo rm atio n . Th e fo llo w in g te sts are c o m m o n ly e mplo ye d to me asure w o rkab ility. (a ) Slump Te st

(b ) Co mpacting Facto r Te st

(c ) Flo w Test

(d ) Ke lly Ball Te st

(e ) Ve e Be e Co nsisto me te r Te st.

Slump Test Slump te st is the mo st c o mmo nly use d me tho d o f me asuring c o nsiste nc y o f c o nc re te w hich can b e e mp lo ye d e ithe r in lab o rato ry o r at site o f w o rk. It is no t a suitab le me tho d fo r ve ry w e t o r ve ry d ry co ncre te . It d o e s no t me asure all facto rs co ntrib uting to w o rkab ility, no r is it alw ays re p re se ntative o f the p lacab ility o f the co ncre te . Ho w e ve r, it is use d co nve nie ntly as a co ntro l te st and g ive s an ind icatio n o f the unifo rmity o f co ncre te fro m b atch to b atch. Re p e ate d b atc he s o f the same mix, b ro ug ht to the same slump , w ill have the same w ate r co ntent and w ater cement ratio , pro vid ed the w eig hts o f ag g reg ate, cement and ad mixtures are unifo rm and ag g re g ate g rad ing is w ithin ac c e p tab le lim its. Ad d itio nal info rm atio n o n w o rkab ility and q uality o f c o nc re te c an b e o b taine d b y o b se rving the m anne r in w hic h co ncre te slump s. Q uality o f co ncre te can also b e furthe r asse sse d b y g iving a fe w tap p ing s o r b lo w s b y tam p ing ro d to the b ase p late . The d e fo rm atio n sho w s the c harac te ristic s o f co ncre te w ith re sp e ct to te nd e ncy fo r se g re g atio n. The appartus fo r co nd ucting the slump test essentially co nsists o f a metallic mo uld in the fo rm o f a frustum o f a co ne having the inte rnal d ime nsio ns as und e r: Bo tto m d iame te r

:

2 0 cm

To p d iame te r

:

1 0 cm

He ig ht

:

3 0 cm

The thickness o f the metallic sheet fo r the m o uld sho uld no t b e thinne r than 1 .6 m m . So metimes the mo uld is pro vided w ith suitable g uid es fo r lifting vertically up. Fo r tamping the c o n c re te , a ste e l ta m p in g ro d 16 mm d ia, 0.6 meter alo ng w ith b ullet end is use d . Fig . 6 .1 , sho w s the d e tails o f the slump c o n e ap p artu s. Th e in te rn al su rfac e o f th e mo uld is tho ro ug hly c le ane d and fre e d fro m sup e rfluo us m o isture and ad he re nc e o f any o ld se t co ncre te b e fo re co mme ncing the te st. The mo uld is place d o n a smo o th, ho rizo ntal, rig id and no n-ab so rb ant surface The mo uld is then filled in fo ur layers, each appro ximately 1/ 4 o f the he ig ht o f the m o uld . Eac h laye r is tam p e d 2 5 tim e s b y the tam p ing ro d taking

Slump Test Apparatus

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care to distribute the stro kes evenly o ver the cro ss sectio n. After the to p layer has been ro dded, the co ncrete is struck o ff level w ith a tro w el and tamping ro d. The mo uld is remo ved fro m the co ncre te imme d iate ly b y raising it slo w ly and care fully in a ve rtical d ire ctio n. This allo w s the co ncre te to sub sid e . This sub sid e nce is re fe rre d as SLUMP o f co ncre te . The d iffe re nce in le ve l b e tw e e n the he ig ht o f the mo uld and that o f the hig he st p o int o f the sub sid e d co ncre te is measured . This d ifference in heig ht in mm. is taken as Slump o f Co ncrete. ASTM measure the ce ntre o f the slump e d co ncre te as the d iffe re nce in he ig ht. ASTM also sp e cifie s 3 laye rs. The p atte rn o f slump is sho w n in Fig . 6 .2 . It ind icate s the characte ristic o f co ncre te in ad d itio n to the slump value . If the co ncre te slump s e ve nly it is calle d true slamp . If o ne half o f the co ne slid e s d o w n, it is calle d she ar slump . In case o f a she ar slump , the slump value is me asure d as the d iffe re nce in he ig ht b e tw e e n the he ig ht o f the mo uld and the ave rag e value o f the sub sid e nc e . She ar slump also ind ic ate s that the c o nc re te is no n-c o he sive and sho w s the characte ristic o f se g re g atio n. It is seen that the slump test g ives fairly g o o d co nsistent results fo r a plastic-mix. This test is no t se nsitive fo r a stiff-mix. In case o f d ry-mix, no variatio n can b e d e te cte d b e tw e e n mixe s o f d iffe re nt w o rkab ility. In the case o f rich mixe s, the value is o fte n satisfacto ry, the ir slump b e ing se nsitive to variatio ns in w o rkab ility. IS 4 5 6 o f 2 0 0 0 sug g e sts that in the “ve ry lo w ” c ate g o ry o f w o rkab ility w he re stric t c o ntro l is ne c e ssary, fo r e xam p le , p ave m e nt q uality co ncre te , (PQ C) me asure me nt o f w o rkab ility b y d e te rminatio n o f co mp acting facto r w ill b e mo re appro priate than slump and a value o f 0 .7 5 to 0 .8 0 co mpacting facto r is sug g e ste d .

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The ab o ve IS also sug g e sts that in the “ve ry hig h” cate g o ry o f w o rkab ility, me asure me nt o f w o rkab ility b y d e te rminatio n o f “flo w ” b y flo w te st w ill b e mo re appro priate . Ho w e ve r, in a le an-mix w ith a te nd e ncy o f harshne ss a true slump can e asily chang e to she ar slump . In such case , the te sts sho uld b e re pe ate d . Despite many limitatio ns, the slump test is very useful o n site to check day-to -day o r ho urto -ho ur variatio n in the q uality o f mix. An incre ase in slump , may me an fo r instance that the mo isture co ntent o f the ag g reg ate has sud d enly increased o r there has b een sud d en chang e in the g rad ing o f ag g re g ate . The slump te st g ive s w arning to co rre ct the cause s fo r chang e o f slump value . The simplicity o f this te st is ye t ano the r re aso n, w hy this te st is still po pular in spite o f the fact that many o the r w o rkab ility te sts are in vo g ue . Tab le 6 .1 sho w s the no minal slump value fo r d iffe re nt d e g re e s o f w o rkab ility. The Bure au o f Ind ian stand ard s, in the p ast, g e ne rally ad o p te d co mp acting facto r te st value s fo r d e no ting w o rkab ility. Eve n in the IS 1 0 2 6 2 o f 1 9 8 2 d e aling w ith Re co mme nd e d Guid e Line fo r Co ncrete Mix Desig n, ad o pted co mpacting facto r fo r d eno ting w o rkab ility. But no w in the re visio n o f IS 4 5 6 o f 2 0 0 0 the co d e has re ve rte d b ack to slump value to d e no te the w o rkability rather than co mpacting facto r. It sho w s that slump test has mo re practical utility than the o the r te sts fo r w o rkab ility.

K-Slump Tester Ve ry recently a new appartus called “K-Slump Tester” has b een d evised . 6 .1 It can b e used to me asure the slump d ire ctly in o ne minute afte r the te ste r is inse rte d in the fre sh co ncre te to the level o f the flo ater d isc. This tester can also b e used to measure the relative w o rkab ility.

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The ap p artus c o mp rise s o f the fo llo w ing fo ur p rincip al p arts:1.

A c hro m e p late d ste e l tub e w ith e xte rnal and inte rnal d iame te rs o f 1 .9 and 1 .6 c m re spe ctive ly. The tub e is 2 5 cm lo ng and its lo w e r p art is use d to m ake th e te st. Th e le n g th o f th is p a rt is 1 5 . 5 c m w h ic h in c lu d e s th e so lid c o n e th a t fa c ilita te s inse rting the tub e into the c o nc re te . Tw o types o f o pening s are pro vid ed in this part: 4 rectang ular slo ts 5.1 cm lo ng and 0.8 cm w id e a n d 2 2 ro u n d h o le s 0 . 6 4 c m in d iameter; all these o pening s are d istrib uted un ifo rm ly in th e lo w e r p art as sh o w n in Fig ure 6 .3 .

K-Slump Tester

Ta ble 6 .1 . Wor k a bilit y, Slum p a nd Com pa c t ing Fa c t or of Conc re t e s w it h 2 0 m m or 4 0 m m M a x im um Size of Aggre ga t e Deg ree o f w o rkab ility

Ve ry Lo w

Slump mm

Co mpacting facto r Small Larg e appartus appartus

–

0 .7 8

0 .8 0

Ro ad s vib rate d b y p o w e r-o p e rate d machines. At the mo re w o rkab le end o f this g ro up, co ncrete may be co mpacted in ce rtain case s w ith hand -o p e rate d machine s.

2 5 –7 5

0 .8 5

0 .8 7

Ro ad s vib rate d b y hand -o p e rate d machines. At the mo re w o rkab le end o f this g ro up , co ncre te may b e manually co mpacted in ro ad s using ag g reg ate o f ro und e d o r irre g ular shap e . Mass co ncre te fo und atio ns w itho ut vib ratio n o r lig htly re info rce d se ctio ns w ith vib ratio n.

5 0 –1 0 0

0 .9 2

0 .9 3 5

At the le ss w o rkab le e nd o f this g ro up, manually co mp acte d flat slab s using crushe d ag g re g ate s. No rmal re info rce d co ncre te manually co mp acte d and he avily re info rce d se ctio ns w ith vib ratio n

1 0 0 –1 5 0

0 .9 5

0 .9 6

Fo r se ctio ns w ith co ng e ste d re info rce ment. No t no rmally suitab le fo r vib ratio n. Fo r p ump ing and tre mie p lacing

–

–

–

co mpacting facto r is suitab le Lo w

Me d ium

Hig h

Ve ry Hig h

Use fo r w hich co ncrete is suitab le

Flo w tab le te st is mo re suitab le .

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2.

A d isc flo ate r 6 cm in d iame te r and 0 .2 4 cm in thickne ss w hich d ivid e s the tub e into tw o p arts: the up p e r p art se rve s as a hand le and the lo w e r o ne is fo r te sting as alre ad y me ntio ne d . The d isc se rve s also to p re ve nt the te ste r fro m sinking into the co ncre te b e yo nd the p re se le cte d le ve l.

3.

A ho llo w plastic ro d 1.3 cm in d iameter and 25 cm lo ng w hich co ntains a g rad uated sc ale in c e ntime te rs. This ro d c an mo ve fre e ly insid e the tub e and c an b e use d to me asure the he ig ht o f mo rtar that flo w s into the tub e and stays the re . The ro d is plug g ed at each end w ith a plastic cap to prevent co ncrete o r any o ther material fro m se e p ing insid e .

4.

An aluminium c ap 3 c m d iame te r and 2 .2 5 c m lo ng w hic h has a little ho le and a scre w that can b e use d to se t and ad just the re fe re nce ze ro o f the ap p aratus. The re is also in th e up p e r p art o f th e tub e , a sm all p in w h ic h is use d to sup p o rt th e measuring ro d at the beg inning o f the test. The to tal w eig ht o f the appartus is 226 g .

The fo llo w ing p ro ce d ure is use d : (a ) We t the te ste r w ith w ate r and shake o ff the e xce ss. (b ) Raise the measuring ro d , tilt slig htly and let it rest o n the pin lo cated insid e the tester. (c ) Insert the tester o n the levelled surface o f co ncrete vertically do w n until the disc flo ater re sts at the surfac e o f the c o nc re te . Do no t ro tate w hile inse rting o r re mo ving the te ste r. (d ) Afte r 6 0 se co nd s, lo w e r the me asuring ro d slo w ly until it re sts o n the surface o f the co ncrete that has entered the tub e and read the K-Slump d irectly o n the scale o f the me asuring ro d . (e ) Raise the me asuring ro d ag ain and le t it re st o n its p in.

Compacting Factor Apparatus

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Remo ve the tester fro m the co ncrete vertically up and ag ain lo w er the measuring ro d slo w ly till it to uc h e s th e surfac e o f th e c o n c re te re tain e d in th e tub e an d re ad w o rkab ility (W) d ire ctly o n the scale o f the me asuring ro d .

Remarks In the co ncre te ind ustry, the slump te st is still the mo st w id e ly use d te st to co ntro l the co nsistency o f co ncrete mixtures, even tho ug h there are so me q uestio ns ab o ut its sig nificance and its effectiveness. Many ag ree that the test is aw kw ard and is no t in keeping w ith the strides that the ind ustry has mad e since 1 9 1 3 w he n the slump co ne w as first intro d uce d . Se ve ral apparatus have b een pro po sed to replace o r supplement the slump co ne, b ut in g eneral they have p ro ve d to b e ric h in the o ry and p o o r in p rac tic e . The ir use is still lim ite d m ainly to re se arch w o rk in lab o rato rie s. The K-slump apparatus is very simple, practical, and eco no mical to use, b o th in the field and the lab o rato ry. It has pro ven, w ith o ver 4 5 0 tests, that it has a g o o d co rrelatio n w ith the slump co ne . The K-slum p te ste r c an b e use d to m e asure slum p in o ne m inute in c ylind e rs, p ails, buckets, w heel-barro w s, slabs o r any o ther desired lo catio n w here the fresh co ncrete is placed. A w o rkab ility ind e x can b e d e te rmine d b y the te ste r.

Compacting Factor Test The co mp acting facto r te st is d e sig ne d p rimarily fo r use in the lab o rato ry b ut it can also be used in the field. It is mo re precise and sensitive than the slump test and is particularly useful fo r c o nc re te m ixe s o f ve ry lo w w o rkab ility as are no rm ally use d w he n c o nc re te is to b e co mp acte d b y vib ratio n. Such d ry co ncre te are inse nsitive to slump te st. The d iag ram o f the ap p aratus is sho w n in Fig ure 6 .4 . The e sse ntial d ime nsio ns o f the ho p p e rs and mo uld and the d istance b e tw e e n the m are sho w n in Tab le 6 .2 . The co mp acting facto r te st has b e e n d e ve lo p e d at the Ro ad Re se arch Lab o rato ry U.K. and it is c laim e d that it is o ne o f the m o st e ffic ie nt te sts fo r m e asuring the w o rkab ility o f co ncrete. This test w o rks o n the principle o f d etermining the d eg ree o f co mpactio n achieved b y a stand ard am o unt o f w o rk d o ne b y allo w ing the c o nc re te to fall thro ug h a stand ard he ig ht. The d e g re e o f co mp actio n, calle d the co mp acting facto r is me asure d b y the d e nsity ratio i.e., the ratio o f the d ensity actually achieved in the test to d ensity o f same co ncrete fully co mp acte d .

Ta ble 6 .2 . Esse nt ia l Dim e nsion of t he Com pa c t ing Fa c t or Appa r t us for use w it h Aggre gat e not ex c e e ding 4 0 m m N om ina l M a x . Size Up p e r Ho p p e r, A

Dime nsio n cm

To p inte rnal d iame te r

2 5 .4

Bo tto m inte rnal d iame te r

1 2 .7

Inte rnal he ig ht

2 7 .9

Lo w e r ho p p e r, B To p inte rnal d iame te r

2 2 .9

Bo tto m inte rnal d iame te r

1 2 .7

Inte rnal he ig ht

2 2 .9

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Cylind e r, C Inte rnal d iame te r

1 5 .2

Inte rnal he ig ht

3 0 .5

Distance b e tw e e n b o tto m o f up p e r ho p p e r and to p o f lo w e r ho ppe r

2 0 .3

Distance b e tw e e n b o tto m o f lo w e r ho p p e r and to p o f cylind e r

2 0 .3

The sample o f co ncre te to b e te ste d is place d in the uppe r ho ppe r up to the b rim. The trap-do o r is o pened so that the co ncrete falls into the lo w er ho pper. Then the trap-do o r o f the lo w e r ho p p e r is o p e ne d and the co ncre te is allo w e d to fall into the cylind e r. In the case o f a d ry-mix, it is like ly that the co ncre te may no t fall o n o p e ning the trap -d o o r. In such a case , a slig ht p o king b y a ro d may b e re q uire d to se t the co ncre te in mo tio n. The e xce ss co ncre te re maining ab o ve the to p le ve l o f the c ylind e r is the n c ut o ff w ith the he lp o f p lane b lad e s supplie d w ith the apparatus. The o utsid e o f the cylind e r is w ipe d cle an. The co ncre te is fille d up exactly upto the to p level o f the cylinder. It is w eig hed to the nearest 10 g rams. This w eig ht is kno w n as “Weig ht o f partially co mpacted co ncrete”. The cylinder is emptied and then refilled w ith the co ncre te fro m the same samp le in laye rs ap p ro ximate ly 5 cm d e e p . The laye rs are heavily rammed o r preferab ly vib rated so as to o b tain full co mpactio n. The to p surface o f the fully c o mp ac te d c o nc re te is the n c are fully struc k o ff le ve l w ith the to p o f the c ylind e r and w eig hed to the nearest 10 g m. This w eig ht is kno w n as “Weig ht o f fully co mpacted co ncrete”. The Co mpacting Facto r =

Weight of partially compacted concrete Weight of fully compacted concrete

Th e w e ig h t o f fu lly c o m p ac te d c o n c re te c an also b e c alc u late d b y kn o w in g th e p ro p o rtio n o f mate rials, the ir re sp e ctive sp e cific g ravitie s, and the vo lume o f the cylind e r. It is seen fro m experience, that it makes very little d ifference in co mpacting facto r value, w hether the w e ig ht o f fully co mp acte d co ncre te is calculate d the o re tically o r fo und o ut actually afte r 1 0 0 pe r ce nt co mpactio n. It can b e re alise d that the co mp acting facto r te st me asure s the inhe re nt characte ristics o f the co ncre te w hich re late s ve ry clo se to the w o rkab ility re q uire me nts o f co ncre te and as such it is o ne o f the g o o d te sts to d e p ict the w o rkab ility o f co ncre te .

Flow Test This is a lab o rato ry test, w hich g ives an ind icatio n o f the q uality o f co ncrete w ith respect to co nsiste ncy, co he sive ne ss and the p ro ne ne ss to se g re g atio n. In this te st, a stand ard mass o f co ncrete is sub jected to jo lting . The spread o r the flo w o f the co ncrete is measured and this flo w is re late d to w o rkab ility. Fig . 6 .5 sho w s the d e tails o f apparatus use d . It can b e se e n that the apparatus co nsists o f flo w tab le , ab o ut 7 6 cm. in d iame te r o ve r w hich co nce ntric circle s are marke d . A mo uld made fro m smo o th metal casting in the fo rm o f a frustum o f a co ne is used w ith the fo llo w ing inte rnal d ime nsio ns. The b ase is 2 5 cm. in d iame te r, up p e r surface 1 7 cm. in d iame te r, and he ig ht o f the co ne is 1 2 cm. The tab le to p is c le ane d o f all g ritty mate rial and is w e tte d . The mo uld is ke p t o n the ce ntre o f the tab le , firmly he ld and is fille d in tw o laye rs. Each laye r is ro d d e d 2 5 time s w ith a tamping ro d 1 .6 cm in d iame te r and 6 1 cm lo ng ro und e d at the lo w e r tamping e nd . Afte r

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the to p laye r is ro d d e d e ve nly, the e xc e ss o f c o nc re te w hic h has o ve rflo w e d the mo uld is re mo ve d . The mo uld is lifte d ve rtically up w ard and the co ncre te stand s o n its o w n w itho ut sup p o rt. The tab le is the n raise d and d ro p p e d 1 2 .5 mm 1 5 time s in ab o ut 1 5 se co nd s. The d iame te r o f the sp re ad co ncre te is me asure d in ab o ut 6 d ire ctio ns to the ne are st 5 mm and the ave rag e sp re ad is no te d . The flo w o f co ncre te is the p e rce ntag e incre ase in the ave rag e d iame te r o f the sp re ad co ncre te o ve r the b ase d iame te r o f the mo uld Flo w, pe r ce nt =

Spread diameter in cm − 25 x 100 25

The value co uld rang e anything fro m 0 to 1 5 0 p e r ce nt. A clo se lo o k at the patte rn o f spre ad o f co ncre te can also g ive a g o o d ind icatio n o f the characte ristics o f co ncre te such as te nd e ncy fo r se g re g atio n.

Flow Table Apparatus The BIS has re c e ntly intro d uc e d ano the r ne w e q uip me nt fo r me asuring flo w value o f co ncrete. This new flo w tab le test is in the line w ith BS 1881 part 105 o f 1984 and DIN 1048 p art I. The ap p aratus and me tho d o f te sting is d e scrib e d b e lo w.

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The flo w tab le ap p aratus is to b e co nstructe d in acco rd ance w ith Fig . 6 .6 . (a) and (b ) Flo w tab le to p is co nstructe d fro m a flat me tal o f minimum thickne ss 1 .5 mm. The to p is in p lan 7 0 0 mm x 7 0 0 mm. The ce ntre o f the tab le is marke d w ith a cro ss, the line s w hich run p aralle d to and o ut to the e d g e s o f the p late , and w ith a ce ntral circle 2 0 0 mm in d iame te r. The fro nt o f the flo w tab le to p is p ro vid e d w ith a lifting hand le as sho w n in Fig . 6 .6 (b ) The to tal mass o f the flo w tab le to p is ab o ut 1 6 ± 1 kg . The flo w tab le to p is hing e d to a b ase frame using e xte rnally mo unte d hing e s in such a w ay that no ag g reg ate can b eco me trapped easily b etw een the hing es o r hing ed surfaces. The fro nt o f the b ase frame shall e xte nd a minimum 1 2 0 mm b e yo nd the flo w tab le to p in o rd e r to p ro vid e a to p b o ard . An up p e r sto p similar to that sho w n in Fig . 6 .6 . (a) is p ro vid e d o n e ach sid e o f the tab le so that the lo w e r fro nt e d g e o f the tab le can o nly b e lifte d 4 0 ± 1 mm. The lo w e r fro nt e d g e o f the flo w tab le to p is pro vid e d w ith tw o hard rig id sto ps w hich transfe r the lo ad to the b ase frame . The b ase frame is so co nstructe d that this lo ad is the n transfe rre d d ire ctly to the surface o n w hich the flo w tab le is p lace d so that the re is minimal te nd e ncy fo r the flo w tab le to p to b o unce w he n allo w e d to fall.

Accessory Apparatus Mo uld: The mo uld is made o f metal readily no t attacked b y cement paste o r liab le to rust and o f m inim um thic kne ss 1 .5 m m . The inte rio r o f the m o uld is sm o o th and fre e fro m p ro je ctio ns, such as p ro trud ing rive ts, and is fre e fro m d e nts. The mo uld shall b e in the fo rm o f a ho llo w frustum o f a co ne having the internal d imensio ns as sho w n in Fig . 6 .7 . The b ase and the to p is o pen and parallel to each o ther and at rig ht ang les to the axis o f the co ne. The mo uld is p ro vid e d w ith tw o me tal fo o t p ie ce s at the b o tto m and tw o hand le s ab o ve the m. Tam p in g Bar: The tamping b ar is mad e o f a suitab le hard w o o d and having d imensio ns as sho w n in Fig . 6 .8 . Sam p lin g : The samp le o f fre shly mixe d co ncre te is o b taine d . Pr o c e d ur e : The table is made level and pro perly suppo rted. Befo re co mmencing the test, Pro ure the tab le-to p and inner surface o f the m o uld is w ip e d w ith a d am p c lo th. The slump co ne is placed centrally o n the table. The slump co ne is filled w ith c o n c re te in tw o e q u al laye rs, e ac h layer tamped lig htly 10 times w ith the w o o d e n tamping b ar. Afte r filling the mo uld , the co ncre te is struck o ff flush w ith th e u p p e r e d g e o f th e slu m p c o ne and the fre e are a o f the tab le to p cle ane d o ff. Half a m in ute afte r strikin g o ff the co ncrete, the co ne is slo w ly raised vertically by the handles. After this, the tab le -to p raise d b y th e h an d le an d allo w ed to fall 15 times in 15 seco nds. The c o nc re te sp re ad s itse lf o ut. The d iame te r o f the co ncre te sp re ad shall

Flow Table Apparatus

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231

the n b e me asure d in tw o d ire ctio ns, p aralle l to the tab le e d g e s. The arithme tic me an o f the tw o d iame te rs shall b e the me asure me nt o f flo w in millime te rs.

Kelly Ball Test This is a sim p le fie ld te st c o nsisting o f the m e asure m e nt o f the ind e ntatio n m ad e b y 1 5 cm d iame te r me tal he misp he re w e ig hing 1 3 .6 kg . w he n fre e ly p lace d o n fre sh co ncre te . The te st has b e e n d e vise d b y Ke lly and he nc e kno w n as Ke lly Ball Te st. This has no t b e e n c o ve re d b y In d ian Stan d ard s Sp e c ific atio n . Th e ad van tag e s o f th is te st is th at it c an b e perfo rmed o n the co ncrete placed in site and it is claimed that this test can be perfo rmed faster w ith a g reater precisio n than slump test. The d isad vantag es are that it req uires a larg e sample o f co ncrete and it canno t b e used w hen the co ncrete is placed in thin sectio n. The minimum

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d e p th o f c o nc re te must b e at le ast 2 0 c m and the minimum d istance fro m the ce ntre o f the b all to ne are st e d g e o f the co ncre te 2 3 cm. The surface o f the co ncrete is struck o ff le ve l, avo id ing e xc e ss w o rking , the b all is lo w e re d g rad ually o n th e surfac e o f th e co ncre te . The d e p th o f p e ne tratio n is re ad im m e d iate ly o n th e ste m to th e n e are st 6 mm. The te st can b e p e rfo rme d in ab o ut 1 5 se c o n d s a n d it g ive s m u c h m o re co nsiste nt re sults than Slump Te st. Fig . 6 .9 . sho w s the Ke lly Ball apparatus.

Vee Bee Consistometer Test Th is is a g o o d la b o ra to ry te st to m e a su re in d ire c tly th e w o rka b ility o f c o nc re te . This te st c o nsists o f a vib rating tab le , a m e tal p o t, a she e t m e tal c o ne , a stand ard iro n ro d . The apparatus is sho w n in Fig ure . 6 .1 0 . Slu m p te st a s d e sc rib e d e a rlie r is p e rfo rme d , p lacing the slump co ne insid e th e sh e e t m e ta l c ylin d ric a l p o t o f th e c o nsisto m e te r. The g lass d isc attac he d to the sw ivel arm is turned and placed o n the to p o f the co ncrete in the po t. The electrical vib ra to r is th e n sw itc h e d o n a n d sim ultane o usly a sto p w atc h starte d . The vib ratio n is co ntinued till such a time as the c o nic al shap e o f the c o nc re te d isap p e ars an d th e c o n c re te assu m e s a c ylin d ric al shape. This can be judg ed by o bserving the g lass disc fro m the to p fo r disappearance o f tra n sp a re n c y. Im m e d ia te ly w h e n th e co ncre te fully assume s a cylind rical shap e , the sto p w atc h is sw itc he d o ff. The tim e re q u ire d fo r th e sh a p e o f c o n c re te to c h a n g e fro m slu m p c o n e sh a p e to c ylind ric al shap e in se c o nd s is kno w n as Ve e Be e D e g re e . Th is m e th o d is ve ry suitab le fo r very d ry co ncrete w ho se slump value canno t b e me asure d b y Slump Te st, b u t th e vib ra tio n is to o vig o ro u s fo r co ncre te w ith a slump g re ate r than ab o ut 5 0 mm.

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Segregation Seg reg atio n can b e defined as th e se p aratio n o f th e c o n stitue n t m a te ria ls o f c o n c re te . A g o o d c o n c re te is o n e in w h ic h all th e ing re d ie nts are p ro p e rly d istrib ute d to make a ho mo g eneo us mixture. If a sa m p le o f c o n c re te e xh ib its a te n d e n c y fo r se p a ra tio n o f sa y, c o arse ag g re g ate fro m the re st o f the ing redients, then, that sample is said to b e sho w ing the te nd e nc y fo r se g re g atio n. Suc h c o nc re te is no t o nly g o ing to b e w e ak; lack o f h o m o g e n e ity is a lso g o in g to ind uce all und e sirab le p ro p e rtie s in the hard e ne d co ncre te . Th e re a re c o n sid e ra b le d iffe re nce s in the size s and sp e cific g ra vitie s o f th e c o n stitu e n t ing re d ie nts o f co ncre te . The re fo re , it is natural that the mate rials sho w a te n d e n c y to fa ll a p a rt. Se g re g atio n may b e o f thre e typ e s — firstly, th e c o a rse a g g re g a te se p aratin g o u t o r se ttlin g d o w n fro m th e re st o f th e m a trix, se c o n d ly, th e p a ste o r m a trix se p arating aw ay fro m co arse ag g re g ate and third ly, w ate r se p arating o ut fro m the re st o f the m ate rial b e ing a mate rial o f lo w e st sp e cific g ravity. A w ell made co ncrete, taking into co nsideratio n vario us parameters such as g rad ing , size, shape and surface te xture o f ag g re g ate w ith o p timum q uantity o f w ate rs make s a co he sive mix. Such co ncre te w ill n o t e xh ib it a n y te n d e n c y fo r se g re g a tio n . Th e c o he sive and fatty c harac te ristic s o f m atrix d o no t allo w the ag g re g ate to fall apart, at the same time , th e m atrix itse lf is su ffic ie n tly c o n tain e d b y th e ag g re g ate . Similarly, w ate r also d o e s no t find it e asy to mo ve o ut fre e ly fro m the re st o f the ing re d ie nts.

Vee-Bee Consistometer

The co nd itio ns favo urab le fo r se g re g atio n are , as c an b e se e n fro m th e ab o ve p ara, th e b ad ly pro po rtio ned mix w here sufficient matrix is no t there to b ind and c o ntain the ag g re g ate s. Insuffic ie ntly mixe d co ncre te w ith e xce ss w ate r co nte nt sho w s a h ig h e r te n d e n c y fo r se g re g a tio n . D ro p p in g o f

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co ncre te fro m he ig hts as in the case o f p lacing co ncre te in co lumn co ncre ting w ill re sult in seg reg atio n. When co ncrete is d ischarg ed fro m a b ad ly d esig ned mixer, o r fro m a mixer w ith w o rn o ut b lad e s, c o nc re te sho w s a te nd e nc y fo r se g re g atio n. Co nve yanc e o f c o nc re te b y co nve yo r b e lts, w he e l b arro w, lo ng d istance haul b y d ump e r, lo ng lift b y skip and ho ist are the o the r situatio ns p ro mo ting se g re g atio n o f co ncre te . Vib ratio n o f c o nc re te is o ne o f the im p o rtant m e tho d s o f c o m p ac tio n. It sho uld b e remembered that o nly co mparatively dry mix sho uld be vibrated. It to o w et a mix is excessively vib rate d , it is like ly that the c o nc re te g e ts se g re g ate d . It sho uld also b e re m e m b e re d that vib ratio n is co ntinue d just fo r re q uire d time fo r o p timim re sults. If the vib ratio n is co ntinue d fo r a lo ng time , p articularly, in to o w e t a mix, it is like ly to re sult in se g re g atio n o f co ncre te d ue to se ttle me nt o f co arse ag g re g ate in matrix. In the recent time w e use co ncrete w ith very hig h slump particularly in RMC. The slump value re q uire d at the b atc hing p o int may b e in the o rd e r o f 1 5 0 mm and at the p ump ing po int the slump may be aro und 100 mm. At bo th these po ints cubes are cast. O ne has to take c are to c o m p ac t the c ub e m o uld w ith the se hig h slum p c o nc re te . If suffic ie nt c are and u n d e rstan d in g o f c o n c re te is n o t e xe rc ise d , th e c o n c re te in th e c u b e m o u ld m ay g e t se g re g ate d and sho w lo w stre ng th. Similarly care must b e take n in the co mp actio n o f such co ncre te in actual structure s to avo id se g re g atio n. While finishing co ncre te flo o rs o r p ave me nt, w ith a vie w to achie ve a smo o th surface , maso ns are like ly to w o rk to o muc h w ith the tro w e l, flo at o r tamp ing rule imme d iate ly o n p lac ing c o nc re te . This im m e d iate w o rking o n the c o nc re te o n p lac ing , w itho ut any tim e interval, is likely to press the co arse ag g reg ate do w n, w hich results in the mo vement o f excess o f m atrix o r p aste to th e su rfac e . Se g rag atio n c au se d o n th is ac c o u n t, im p airs th e ho m o g e ne ity and se rvic e ab ility o f c o nc re te . The e xc e ss m o rtar at the to p c ause s p lastic shrinkag e cracks. Fro m the fo reg o ing discussio n, it can b e g athered that the tendency fo r seg reg atio n can b e re me d ie d b y co rre ctly p ro p o rtio ning the mix, b y p ro p e r hand ling , transp o rting , p lacing , co mp acting and finishing . At any stag e , if se g re g atio n is o b se rve d , re mixing fo r a sho rt time w o uld make the co ncre te ag ain ho mo g e ne o us. As me ntio ne d e arlie r, a co he sive mix w o uld re d uce the te nd e ncy fo r se g re g atio n. Fo r this re aso n, use o f ce rtain w o rkab ility ag e nts and p o zzo lanic mate rials g re atly he lp in re d uc ing se g re g atio n. The use o f air-e ntraining ag e nt ap p re ciab ly re d uce s se g re g atio n. Seg reg atio n is difficult to measure q uantitatively, b ut it can b e easily o b served at the time o f co ncre ting o pe ratio n. The patte rn o f sub sid e nce o f co ncre te in slump te st o r the patte rn o f spread in the flo w test g ives a fair idea o f the quality o f co ncrete w ith respect to seg reg atio n.

Bleeding Ble e d ing is so me time s re fe rre d as w ate r g ain. It is a p artic ular fo rm o f se g re g atio n, in w hich so me o f the w ate r fro m the co ncre te co me s o ut to the surface o f the co ncre te , b e ing o f the lo w est specific g ravity amo ng all the ing redients o f co ncrete. Bleeding is predo minantly o b se rve d in a hig hly w e t mix, b ad ly p ro p o rtio ne d and insufficie ntly mixe d co ncre te . In thin me mb e rs like ro o f slab o r ro ad slab s and w he n co ncre te is p lace d in sunny w e athe r sho w e xce ssive b le e d ing . Due to bleeding , w ater co mes up and accumulates at the surface. So metimes, alo ng w ith this w ater, certain q uantity o f cement also co mes to the surface. When the surface is w o rked up w ith the tro w e l and flo ats, the ag g re g ate g o e s d o w n and the ce me nt and w ate r co me up to the to p surface . This fo rmatio n o f ce me nt paste at the surface is kno w n as “Laitance ”.

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In such a case , the to p surface o f slab s and p ave me nts w ill no t have g o o d w e aring q uality. This laitance fo rme d o n ro ad s p ro d uce s d ust in summe r and mud in rainy se aso n. O w ing to the fact that the to p surface has a hig he r co nte nt o f w ate r and is also d e vo id o f ag g re g ate matter; it also develo ps hig her shrinkag e cracks. If laitance is fo rmed o n a particular lift, a plane o f w eakness w o uld fo rm and the bo nd w ith the next lift w o uld be po o r. This co uld be avo ided b y re mo ving the laitance fully b e fo re the ne xt lift is p o ure d .

Example of external bleeding

Wate r w hile trave rsing fro m b o tto m to to p , make s c o ntinuo us c hanne ls. If the w ate r c e m e nt ratio use d is m o re than 0 .7 , the b le e d ing c hanne ls w ill re m ain c o ntinuo us and unse g m e nte d b y the d e ve lo p m e nt o f g e l. This c o ntinuo us b le e d ing c hanne ls are o fte n re sp o nsib le fo r causing p e rme ab ility o f the co ncre te structure s. W h ile th e m ixin g w ate r is in th e p ro c e ss o f c o m in g u p , it m ay b e in te rc e p te d b y a g g re g a te s. Th e b le e d in g w a te r is like ly to a c c u m u la te b e lo w th e a g g re g a te . Th is accumulatio n o f w ate r cre ate s w ate r vo id s and re d uce s the b o nd b e tw e e n the ag g re g ate s and the paste. The ab o ve aspect is mo re pro no unced in the case o f flaky ag g reg ate. Similarly, the w ate r that accumulate s b e lo w the re info rcing b ars, p articularly b e lo w the cranke d b ars, red uces the b o nd b etw een the reinfo rcement and the co ncrete. The po o r b o nd b etw een the ag g reg ate and the paste o r the reinfo rcement and the paste due to bleeding can be remedied b y re vib ratio n o f co ncre te . The fo rmatio n o f laitance and the co nse q ue nt b ad e ffe ct can b e re d uce d b y d e laye d finishing o p e ratio ns. Ble e d ing rate inc re ase s w ith time up to ab o ut o ne ho ur o r so and the re afte r the rate d e cre ase s b ut co ntinue s mo re o r le ss till the final se tting time o f ce me nt. Ble e d ing is an inhe re nt p he no me no n in c o nc re te . All the same , it c an b e re d uc e d b y p ro p e r p ro p o rtio ning and unifo rm and c o m p le te m ixing . Use o f fine ly d ivid e d p o zzo lanic mate rials re d uce s b le e d ing b y cre ating a lo ng e r p ath fo r the w ate r to trave rse . It has b e e n already discussed that the use o f air-entraining ag ent is very effective in reducing the bleeding . It is also repo rted that the bleeding can be reduced by the use o f finer cement o r cement w ith lo w alkali co nte nt. Rich mixe s are le ss susce p tib le to b le e d ing than le an mixe s. The b le e d ing is no t c o m p le te ly harm ful if the rate o f e vap o ratio n o f w ate r fro m the surfac e is e q ual to the rate o f b le e d ing . Re m o val o f w ate r, afte r it had p laye d its ro le in p ro vid ing w o rkab ility, fro m the b o d y o f c o nc re te b y w ay o f b le e d ing w ill d o g o o d to the

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co ncre te . Early b le e d ing w he n the co ncre te mass is fully p lastic, may no t cause much harm, b e c ause c o nc re te b e ing in a fully p lastic c o nd itio n at that stag e , w ill g e t sub sid e d and co mp acte d . It is the d e laye d b le e d ing , w he n the co ncre te has lo st its p lasticity, that cause s und ue harm to the co ncre te . Co ntro lle d re vib ratio n may b e ad o p te d to o ve rco me the b ad e ffe ct o f b le e d ing . Ble e d ing pre se nts a ve ry se rio us pro b le m w he n Slip Fo rm Pave r is use d fo r co nstructio n o f c o n c re te p ave m e n ts. If tw o m uc h o f b le e d in g w ate r ac c um ulate s o n th e surfac e o f p ave m e nt slab , the b le e d ing w ate r flo w s o ut o ve r the unsup p o rte d sid e s w hic h c ause s co llap sing o f sid e s. Ble e d ing b e co me s a majo r co nsid e ratio n in such situatio ns. In the pavement co nstructio n finishing is do ne b y texturing o r b ro o ming . Bleeding w ater d e lays the te xturing and ap p licatio n o f curing co mp o und s.

Method of Test for Bleeding of Concrete This metho d co vers determinatio n o f relative q uantity o f mixing w ater that w ill bleed fro m a samp le o f fre shly mixe d co ncre te . A cylind rical co ntaine r o f ap p ro ximate ly 0 .0 1 m 3 cap acity, having an insid e d iame te r o f 250 mm and inside heig ht o f 280 mm is used. A tamping bar similar to the o ne used fo r slump test is used. A pepette fo r draw ing o ff free w ater fro m the surface, a g raduated jar o f 100 cm 3 capacity is re q uire d fo r te st. A samp le o f fre shly mixe d co ncre te is o b taine d . The co ncre te is fille d in 5 0 mm laye r fo r a d e p th o f 2 5 0 ± 3 mm (5 laye rs) and e ach laye r is tamp e d b y g iving stro ke s, and the to p surface is mad e smo o th b y tro w e lling . The test specimen is w eig hed and the w eig ht o f the co ncrete is no ted. Kno w ing the to tal w ate r c o nte nt in 1 m 3 o f c o nc re te q uan tity o f w ate r in th e c ylin d ric al c o ntaine r is also calculate d . The cylind rical co ntaine r is ke pt in a le ve l surface fre e fro m vib ratio n at a te mpe rature o f 2 7 ° C ± 2 ° C. it is c o ve re d w ith a lid . Wate r ac c um ulate d at the to p is d raw n b y m e ans o f pipette at 10 minutes interval fo r the first 40 minutes and at 30 minutes interval sub seq uently till bleeding ceases. To facilitate co llectio n o f bleeding w ater the co ntainer may be slig htly tilted. All the b le e d ing w ate r co lle cte d in a jar. Ble e d ing w ate r p e rce ntag e =

Total quantity of bleeding water Total quantity of water in the sample of concrete x 1 0 0

Setting Time of Concrete We have discussed abo ut the setting time o f cement in Chapter 2. Setting time o f cement is fo und o ut b y a stand ard vicat apparatus in lab o rato ry co nd itio ns. Se tting time , b o th initial and final ind icate the q uality o f ce me nt. Se tting tim e o f c o nc re te d iffe rs w id e ly fro m se tting tim e o f c e m e nt. Se tting tim e o f co ncrete d o es no t co incid e w ith the setting time o f cement w ith w hich the co ncrete is mad e. The se tting time o f c o nc re te d e p e nd s up o n the w / c ratio , te mp e rature c o nd itio ns, typ e o f c e me nt, use o f mine ral ad mixture , use o f p lastic ize rs–in p artic ular re tard ing p lastic ize r. The se tting p arame te r o f co ncre te is mo re o f p ractical sig nificance fo r site e ng ine e rs than se tting time o f cement. When retarding plasticizers are used, the increase in setting time, the duratio n upto w hich co ncre te re mains in plastic co nd itio n is o f spe cial inte re st.

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The setting time o f co ncrete is fo und b y pentro meter test. This metho d o f test is co vere d b y IS 8 1 4 2 o f 1 9 7 6 and ASTM C – 4 0 3 . The p ro ce d ure g ive n b e lo w may also b e ap p lie d to p re p are d mo rtar and g ro uts. The ap p aratus co nsist o f a co ntaine r w hich sho uld have minimum late ral d ime nsio n o f 1 5 0 mm and minimum d e p th o f 1 5 0 mm. The re are six p e ne tratio n ne e d le s w ith b e aring are as o f 6 4 5 , 3 2 3 , 1 6 1 , 6 5 , 3 2 and 16 mm 2 . Each needle stem is scribed circumferentially at a distance o f 25 mm fro m the bearing are a. A d e vice is p ro vid e d to me asure the fo rce re q uire d to cause p e ne tratio n o f the ne e d le . The test pro cedure invo lves the co llectio n o f representative sample o f co ncrete in sufficient q uantity and sieve it thro ug h 4.75 mm sieve and the resulting mo rtar is filled in the co ntainer. Co mpact the mo rtar b y ro d d ing , tapping , ro cking o r b y vib rating . Level the surface and keep it co vered to prevent the lo ss o f mo isture. Remo ve b leeding w ater, if any, b y means o f pipette. Insert a needle o f appro priate size, depending upo n the deg ree o f setting o f the mo rtar in the fo llo w ing manne r.

Bring the b e aring surface o f ne e d le in co ntact w ith the mo rtar surface . Grad ually and unifo rmly ap p ly a ve rtical fo rce d o w nw ard s o n the ap p aratus until the ne e d le p e ne trate s to a d epth o f 25 ± 1.5 mm, as ind icated b y the scrib e mark. The time taken to penetrate 25 mm d epth co uld b e ab o ut 1 0 seco nd s. Reco rd the fo rce re q uired to pro d uce 2 5 mm penetratio n and the time o f inse rting fro m the time w ate r is ad d e d to ce me nt. Calculate the p e ne tratio n re sistanc e b y d ivid ing the re c o rd e d fo rc e b y the b e aring are a o f the ne e d le . This is the p e ne tratio n re sistance . Fo r the sub se q ue nt p e ne tratio n avo id the are a w he re the mo rtar has

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been disturbed. The clear distance sho uld be tw o time s the d iame te r o f the b e aring are a. Needle is inserted at least 25 mm aw ay fro m the w all o f co ntaine r. Plo t a g rap h o f p e ne tratio n re sistanc e as o rd inate and e lap se d tim e as ab sc issa. No t le ss th an six p e n e tratio n re sistan c e d e te rm inatio n is m ad e . Co ntinue the te sts until o ne p e ne tratio n re sistanc e o f at le ast 2 7 .6 MPa is re ac he d . Co nne c t the vario us po int b y a smo o th curve . Needle with different bearing area Fro m p e ne tratio n re sistanc e e q ual to 3.5 MPa, draw a ho rizo ntal line. The po int o f intersectio n o f this w ith the smo o th curve, is read o n the x-axis w hich g ive s the initial se tting time . Similarly a ho rizo ntal line is d raw n fro m the p e ne tratio n re sistance o f 2 7 .6 MPa and p o int it cuts the smo o th curve is re ad o n the x-axis w hich g ive s the final se t.

A typical g raph is sho w n in Fig . 6 .1 1

Process of Manufacture of Concrete Pro d uc tio n o f q uality c o nc re te re q uire s m e tic ulo us c are e xe rc ise d at e ve ry stag e o f manufacture o f co ncre te . It is inte re sting to no te that the ing re d ie nts o f g o o d co ncre te and b ad c o nc re te are the sam e . If m e tic ulo us c are is no t e xe rc ise d , and g o o d rule s are no t o bserved, the resultant co ncrete is g o ing to be o f bad q uality. With the same material if intense care is take n to e xe rcise co ntro l at e ve ry stag e , it w ill re sult in g o o d co ncre te . The re fo re , it is necessary fo r us to kno w w hat are the g o o d rules to be fo llo w ed in each stag e o f manufacture o f c o nc re te fo r p ro d uc ing g o o d q uality c o nc re te . The vario us stag e s o f m anufac ture o f co ncre te are : (a ) Batching

(b ) Mixing

(c ) Transp o rting

(d ) Placing

(e ) Co mpacting

( f ) Curing

(g ) Finishing .

(a) Batching The measurement o f materials fo r making co ncrete is kno w n as b atching . There are tw o me tho d s o f b atching : (i) Vo lume b atching

(ii ) We ig h b atching

( i) V o lu m e b atc h in g : Vo lum e b atc hing is no t a g o o d m e tho d fo r p ro p o rtio ning the Vo mate rial b e cause o f the d ifficulty it o ffe rs to me asure g ranular mate rial in te rms o f vo lume . Vo lume o f mo ist sand in a lo o se co nd itio n w e ig hs much le ss than the same vo lume o f d ry c o m p ac te d sand . The am o unt o f so lid g ranular m ate rial in a c ub ic m e tre is an ind e finite q uantity. Be cause o f this, fo r q uality co ncre te mate rial have to b e me asure d b y w e ig ht o nly. Ho w ever, fo r unimpo rtant co ncrete o r fo r any small jo b , co ncrete may b e b atched b y vo lume. Ce me nt is alw ays me asure d b y w e ig ht. It is ne ve r me asure d in vo lume . Ge ne rally, fo r e ach b atch mix, o ne b ag o f ce me nt is use d . The vo lume o f o ne b ag o f ce me nt is take n as thirty five (35) litres. Gaug e b o xes are used fo r measuring the fine and co arse ag g reg ates. The typ ical ske tch o f a g uag e b o x is sho w n in Fig ure 6 .1 2 . The vo lume o f the b o x is mad e e q ual to the vo lume o f o ne b ag o f ce me nt i.e., 3 5 litre s o r multip le the re o f. The g aug e b o xe s are

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mad e co mp arative ly d e e p e r w ith n a rro w su rfa c e ra th e r th a n sh a llo w w ith w id e r su rfa c e to fac ilitate e asy e stim atio n o f to p le ve l. So m e tim e s b o tto m le ss g a u g e -b o xe s a re u se d . Th is sho uld b e avo id e d . Co rre ctio n to th e e ffe c t o f b ulkin g sh o uld b e mad e to cate r fo r b ulking o f fine a g g re g a te , w h e n th e fin e ag g re g ate is m o ist an d vo lum e b atching is ad o p te d . Gaug e b o xe s are g e ne rally calle d farmas. The y can b e mad e o f timb er o r steel plates. O ften in Ind ia vo lume b atching is ad o pted even fo r larg e co ncreting o p e ratio ns. In a majo r site it is re co mme nd e d to have the fo llo w ing g aug e b o xe s at site to cate r fo r chang e in Mix De sig n o r b ulking o f sand . The vo lume o f e ach g aug e b o x is cle arly marke d w ith p aint o n the e xte rnal surface .

Ta ble 6 .3 . Volum e of Va rious ga uge boxe s Item

Wid th cm

Heig ht cm

Depth cm

Vo lume litres

Q uantity numb er

A

3 3 .3

30

20

20

1

B

3 3 .3

30

25

25

2

C

3 3 .3

30

30

30

2

D

3 3 .3

30

35

35

2

E

3 3 .3

30

40

40

2

F

3 3 .3

30

45

45

2

G

3 3 .3

30

50

50

1

The b atch vo lume fo r so me o f the co mmo nly use d mixe s is sho w n in Tab le 6 .4 .

Ta ble 6 .4 Bat ch volum e of m a t e ria ls for va rious m ixe s Cement kg .

Sand , litres

1 : 1 : 2 (M 2 0 0 )

50

35

1 : 1 1 / 2 : 3 (M 2 0 0 )

50

5 2 .5

Co arse ag g reg ate, litres 70 105

1 : 2 : 3

50

70

105

1 : 2 : 4 (M 1 5 0 )

50

70

140

1 : 2 1/ 2 : 5

50

8 7 .5

175

1 : 3 : 6 (M 1 0 0 )

50

105

210

Water is measured either in kg . o r litres as may b e co nvenient. In this case, the tw o units are sam e , as the d e nsity o f w ate r is o ne kg . p e r litre . The q uantity o f w ate r re q uire d is a pro d uct o f w ater/ cement ratio and the w eig ht o f cement; fo r a example, if the w ater/ cement

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ratio o f 0 . 5 is sp e c ifie d , th e q u an tity o f m ixin g w ate r re q u ire d p e r b ag o f c e m e n t is 0 .5 x 5 0 .0 0 = 2 5 kg . o r 2 5 litre s. The q uantity is, o f co arse , inclusive o f any surface mo isture pre se nt in the ag g re g ate . The fo llo w ing tab le g ive s the appro ximate surface mo isture carrie d b y ag g re g ate s

Ta ble 6 .5 . Approx im a t e Surfa c e m oist ure in a ggre ga t e –I .S. 4 5 6 -2 0 0 0 Ag g reg ates

Appro ximate Q uantity o f surface w ater Percent b y Mass

Litre per m 3

(1 ) Ve ry w e t sand

(2 ) 7 .5

(3 ) 120

Mo d e rate ly w e t sand

5 .0

80

Mo ist sand

2 .5

40

1 .2 5 – 2 .5

20 – 40

Mo ist g rave l o r crushe d ro ck

(ii) W e ig h Batc hing : Strictly speaking , w eig h batching is the co rrect metho d o f measuring We the materials. Fo r impo rtant co ncrete, invariab ly, w eig h b atching system sho uld b e ad o pted . Use o f w e ig ht syste m in b atching , facilitate s accuracy, fle xib ility and simplicity. Diffe re nt type s o f w e ig h b atche rs are availab le , The p articular typ e to b e use d , d e p e nd s up o n the nature o f the jo b . Larg e w e ig h b atching p lants have auto matic w e ig hing e q uip me nt. The use o f this au to m atic e q u ip m e n t fo r b atc h in g is o n e o f so p h istic atio n an d re q u ire s q u alifie d an d experienced eng ineers. In this, further co mplicatio n w ill co me to adjust w ater co ntent to cater fo r the m o isture c o nte nt in the ag g re g ate . In sm alle r w o rks, the w e ig hing arrang e m e nt co nsists o f tw o w eig hing buckets, each co nnected thro ug h a system o f levers to spring -lo aded d ia ls w h ic h in d ic a te th e lo a d . Th e w e ig h in g b u c ke ts are m o u n te d o n a central spind le ab o ut w hich they ro tate. Thus o ne can be lo aded w hile the o ther is b e ing d ischarg e d into the mixe r skip . A simple spring b alance o r the co mmo n p latfo rm w e ig hing m ac hine s also c an b e use d fo r small jo b s. O n larg e w o rk site s, th e w e ig h bucket type o f w eig hing equipments are use d . This fe d fro m a larg e o ve rhe ad sto rag e h o p p e r an d it d isc h arg e s b y g ra vity, stra ig h t in to th e m ixe r. Th e w e ig hing is d o ne thro ug h a le ve r-arm syste m and tw o inte rlinke d b e ams and jo ckey w eig hts. The req uired q uantity o f sa y, c o a rse a g g re g a te is w e ig h e d , Weigh Batcher h a vin g o n ly th e lo w e r b e a m in o peratio n. After balancing , by turning the smaller lever, to the left o f the beam, the tw o beams are inte rlinke d and the fine ag g re g ate is ad d e d until the y b o th b alance . The final b alance is indicated by the po inter o n the scale to the rig ht o f the beams. Discharg e is thro ug h the sw ivel g ate at the b o tto m. Auto matic b atching plants are availab le in small o r larg e capacity. In this, the o pe rato r has o nly to p re ss o ne o r tw o b utto ns to p ut into m o tio n the w e ig hing o f all the d iffe re nt

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mate rials, the flo w o f e ach b e ing cut o ff w he n the co rre ct w e ig ht is re ache d . In the ir mo st ad vance d fo rms, auto matic p lants are e le ctrically o p e rate d o n a p unche d card syste m. This type o f plant is particularly o nly suitab le fo r the pro d uctio n o f re ad y-mixe d co ncre te in w hich ve ry fre q ue nt chang e s in mix p ro p o rtio n have to b e mad e to me e t the varying re q uire me nts o f d iffe re nt custo me rs. In so me o f the recent auto matic w eig h b atching eq uipments, reco rd ers are fitted w hich re co rd g rap hically the w e ig ht o f e ach mate rial, d e live re d to e ach b atch. The y are me ant to re co rd , and che ck the actual and d e sig ne d p ro p o rtio ns. Ag g re g ate w e ig hing m ac hine s re q uire re g ular atte ntio n if the y are to m aintain the ir accuracy. Che ck calib ratio ns sho uld alw ays b e mad e b y ad d ing w e ig hts in the ho ppe r e q ual to the full w eig ht o f the ag g reg ate in the batch. The erro r fo und is adjusted fro m time to time. In small jo b s, ce me nt is o fte n no t w e ig he d ; it is ad d e d in b ag s assuming the w e ig ht o f the b ag as 5 0 kg . In re ality, tho ug h the ce me nt b ag is mad e o f 5 0 kg . at the facto ry, d ue to transp o ratio n, hand ling at a numb e r o f p lace s, it lo se s so me ce me nt, p articularly, w he n jute b ag s are used . In fact, the w eig ht o f a cement b ag at the site is co nsid erab ly less. So metimes, the lo ss o f w e ig ht b e c o me s mo re than 5 kg . This is o ne o f the so urc e s o f e rro r in vo lume b atc hing and also in w e ig h b atc hing , w he n the c e m e nt is no t ac tually w e ig he d . But in impo rtant majo r co ncre ting jo b s, ce me nt is also actually w e ig he d and the e xact pro po rtio n as d e sig ne d is maintaine d .

Measurement of Water: When w eig h b atching is ado pted, the measurement o f w ater must b e d o ne accurate ly. Ad d itio n o f w ate r b y g rad uate d b ucke t in te rms o f litre s w ill no t b e accurate eno ug h fo r the reaso n o f spillag e o f w ater etc. It is usual to have the w ater measured in a ho rizo ntal tank o r ve rtic al tank fitte d to the mixer. These tanks a re fille d u p a fte r e ve ry b a tc h . Th e filling is so d e sig ne d to h ave a c o n tro l to a d m it a n y d e sire d q u a n tity o f w a te r. So m e tim e s, w a te rm e te rs a re fitte d in th e m a in w a te r su p p ly to th e m ixe r Cans for measuring water fro m w hich the e xact q uantity o f w ate r can b e le t into the mixe r. In m o d e rn b atc hing p lants so p histic ate d auto m atic m ic ro p ro c e sso r c o ntro lle d w e ig h b atching arrang e me nts, no t o nly accurate ly me asure s the co nstitue nt mate rials, b ut also the mo isture co ntent o f ag g reg ates. Mo isture co ntent is auto matically measured b y senso r pro b es and co rrective actio n is taken to d ed uct that much q uantity o f w ater co ntained in sand fro m the to tal q uantity o f w ate r. A numb e r o f such so phisticate d b atching plants are w o rking in o ur co untry. fo r the last 4 – 5 ye ars.

Mixing Tho ro ug h mixing o f the materials is essential fo r the pro ductio n o f unifo rm co ncrete. The m ixin g sh o u ld e n su re th at th e m ass b e c o m e s h o m o g e n e o u s, u n ifo rm in c o lo u r an d co nsiste ncy. The re are tw o me tho d s ad o p te d fo r mixing co ncre te :

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(i ) Hand mixing

(ii)Machine mixing

Han d Mixin g : Hand mixing is practise d fo r small scale unimpo rtant co ncre te w o rks. As the mixing canno t b e tho ro ug h and e fficie nt, it is d e sirab le to ad d 1 0 p e r ce nt mo re ce me nt to cate r fo r the infe rio r co ncre te p ro d uce d b y this me tho d . Hand mixing sho uld b e d o ne o ve r an imp e rvio us co ncre te o r b rick flo o r o f sufficie ntly larg e size to take o ne b ag o f cement. Spread o ut the measured q uantity o f co arse ag g reg ate and fine ag g re g ate in alte rnate laye rs. Po ur the ce me nt o n the to p o f it, and mix the m d ry b y sho vel, turning the mixture o ver and o ver ag ain until unifo rmity o f co lo ur is achieved . This unifo rm mixture is spread o ut in thickness o f ab o ut 20 cm. Water is taken in a w ater-can fitted w ith a ro se -h e ad an d sp rin kle d o ve r th e m ixture an d sim ultan e o usly turn e d o ve r. Th is o p e ratio n is co ntinue d till such time a g o o d unifo rm, ho mo g e ne o us co ncre te is o b taine d . It is o f particular impo rtance to se e that the w ate r is no t po ure d b ut it is o nly sprinkle d . Wate r in small quantity sho uld be added to w ards the e n d o f th e m ixin g to g e t th e ju st re q u ire d c o n siste n c y. At th a t sta g e , e ve n a sm a ll q uantity o f w ate r make s d iffe re nce . Mac h in e Mixin g : Mixing o f c o nc re te is almo st invariab ly carrie d o ut b y machine , fo r re info rce d co ncre te w o rk and fo r me d ium o r larg e sc ale m ass c o n c re te w o rk. Mac h in e m ixin g is n o t o n ly e ffic ie n t, b u t a lso eco no mical, w hen the q uantity o f co ncrete to b e p ro d uce d is larg e . Man y typ e s o f m ixe rs are availab le fo r m ixin g c o n c re te . Th e y c an b e c lassifie d as batch-mixers and co ntinuo us mixers. Batch mixers pro duce co ncrete, batch by batch w ith time interval, w hereas co ntinuo us mixers pro duce co ncrete co ntinuo usly w itho ut sto ppag e till such time the p lant is w o rking . In this, mate rials are fe d c o ntinuo usly b y sc re w fe e d e rs and the mate rials are co ntinuo usly mixe d and co ntinuo usly d ischarg e d . This typ e o f mixe rs are use d in larg e w o rks such as d ams. In no rmal co ncre te w o rk, it is the b atch mixe rs that are use d . Batch mixe r may b e o f p an typ e o r d rum typ e . The d rum typ e may b e furthe r classifie d as tilting , no n-tilting , re ve rsing o r fo rce d actio n typ e . Laboratory tilting drum mixer

Ve ry little is kno w n ab o ut the re lative mixing e fficie ncie s o f the vario us typ e s o f mixe rs, b ut so me e vid e nce s are the re to sug g e st that p an mixe rs w ith a re vo lving star o f b lad e s are mo re efficient. They are specially suitable fo r stiff and lean mixes, w hich present difficulties w ith mo st o the r typ e s o f mixe rs, mainly d ue to sticking o f mo rtar in the d rum. The shap e o f the d rum, the ang le and size o f b lad es, the ang le at w hich the d rum is held , affect the efficiency o f mixe r. It is se e n that tilting d rum to so me e xte nt is mo re e fficie nt than no n-tilting d rum. In no n-tilting d rum fo r d ischarg ing co ncre te , a chute is intro d uce d into the d rum b y o p e rating a lever. The co ncrete w hich is b eing mixed in the d rum, falls into the inclined chute and g ets discharg ed o ut. It is seen that a little mo re o f seg reg atio n takes place, w hen a no n-tilting mixer is use d . It is o b se rve d in p ractice that, g e ne rally, in any typ e o f mixe r, e ve n afte r tho ro ug h mixing in the d rum, w hile it is d ischarg e d , mo re o f co arse ag g re g ate co me s o ut first and at the e nd matrix g e ts d ischarg e d . It is ne ce ssary that a little b it o f re -mixing is e sse ntial, afte r d isc harg e d fro m m ixe r, o n the p latfo rm to o ff-se t the e ffe c t o f se g re g atio n c ause d w hile co ncre te is d ischarg e d fro m the mixe r.

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As p e r I.S. 1 7 9 1 –1 9 8 5 , c o nc re te m ixe rs are d e sig nate d b y a num b e r re p re se nting its no minal mixed b atch capacity in litres. The fo llo w ing are the standardized sizes o f three types:

a . Tilting : 8 5 T, 1 0 0 T, 1 4 0 T, 2 0 0 T b . No n-Tilting : 2 0 0 NT, 2 8 0 NT, 3 7 5 NT, 5 0 0 NT, 1 0 0 0 NT c . Re ve rsing : 2 0 0 R, 2 8 0 R, 3 7 5 R, 5 0 0 R and 1 0 0 0 R The letters T, NT, R deno te tilting , no n-tilting and reversing respectively. Fig 6.13 illustrates d iag rammatically the type o f mixe rs. No rm ally, a b atc h o f c o nc re te is mad e w ith ing re d ie nts co rre sp o nd ing to 5 0 kg c e me nt. If o ne has a c ho ic e fo r ind e nting a mixe r, o ne sho uld ask fo r suc h a c ap ac ity mixe r that sho uld ho ld all the m ate rials fo r o ne b ag o f c e m e nt. This o f c o urse , d e p e nd s o n th e p ro p o rtio n o f th e m ix. Fo r e xam p le , fo r 1 : 2 : 4 m ix, the id e al mixer is o f 2 0 0 litres capacity, w hereas if the ratio is 1 : 3 : 6 , the re q uire me nt Concrete mixer with Pan / paddle mixer w ill be o f 280 litres capacity to facilitate hydraulic hopper 10/7 o n e b a g m ix. Mixe r o f 2 0 0 litre s c ap ac ity is in su ffic ie n t fo r 1 : 3 : 6 m ix an d also m ixe r o f 2 8 0 litre s is to o b ig , h e n c e une co no mical fo r 1 : 2 : 4 co ncre te . To g e t b e tte r e fficie ncy, the se q ue nce o f charg ing the lo ad ing skip is as und e r: Firstly, ab o ut half the q uantity o f co arse ag g reg ate is placed in the skip o ver w hich ab o ut half the q uantity o f fine ag g reg ate is po ured. O n that, the full q uantity o f cement i.e. , o ne bag is p o ure d o ve r w hic h the re m aining p o rtio n o f c o arse ag g re g ate and fine ag g re g ate is d epo sited in seq uence. This prevents spilling o f cement, w hile d ischarg ing into the d rum and also this p re ve nts the b lo w ing aw ay o f ce me nt in w ind y w e athe r. Be fo re th e lo a d e d skip is d isc h a rg e d to th e d ru m , a b o u t 2 5 pe r ce nt o f the to tal q uantity o f w ate r re q uire d fo r mixing , is intro duced into the mixer drum to w et the d rum and to p re ve nt any c e m e nt sticking to the blades o r at the bo tto m o f the d rum. Imme d iate ly, o n d isc harg ing th e d ry m a te ria l in to th e d ru m , th e remaining 75 per cent o f w ater is added to th e d ru m . If th e m ixe r h as g o t an arrang ement fo r independent feeding o f w ate r, it is d e sirab le that the re maining 7 5 p e r c e n t o f w a te r is a d m itte d sim u ltan e o u sly alo n g w ith th e o th e r mate rials. The time is co unte d fro m the mo ment all the materials, particularly, the c o m p le te q uantity o f w ate r is fe d into the d rum. Whe n p lasticize r o r sup e rp lasticize r

Reversible drum concrete mixer / mini batching plant

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is use d , the usual p ro ce d ure co uld b e ad o p te d except that abo ut o ne litre o f w ater is held back. Ca lc u la te d q u a n tity o f p la stic ize r o r sup e rp lastic ize r is m ixe d w ith that o ne litre o f w ater and the same is added to the mixer drum afte r ab o ut o ne minute o f mixing . It is d e sirab le th at c o n c re te is m ixe d little lo n g e r (say 1 / 2 m inute m o re ) so that the p lastic izing e ffe c t is fully achie ve d b y p ro p e r d isp e rsio n.

Concrete high-speed mixer in a batching plant.

W he n p lasticize rs are use d , g e ne rally o ne has to d o numb e r o f trials in the lab o rato ry fo r arriving at p ro p e r d o sag e and re q uire d slump . Small scale lab o rato ry mixe rs are ine fficie nt and do no t mix the ing redients pro perly. Plasticizer in sm all q uantity d o no t g e t p ro p e rly d isp e rse d w ith cement particles. To impro ve the situatio ns, the fo llo w ing se q ue nce may b e ad o p te d .

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Firstly, ad d all the w ater except ab o ut half a litre. Ad d cement and then ad d sand . Make an intimate mo rtar mix. Dilute calculated q uantity o f plasticizer w ith the remaining half a litre o f w ate r and p o ur it into the d rum . Ro tate the d rum fo r ano the r half a m inute , so that p lasticize r g e ts w e ll mixe d w ith ce me nt mo rtar and the n ad d b o th the fractio ns (2 0 mm and 1 0 mm) o f co arse ag g re g ate . This p ro ce d ure is fo und to g ive b e tte r and co nsiste nt re sults. Mixin g T im e : Co nc re te m ixe rs are g e ne rally d e sig ne d to run at a sp e e d o f 1 5 to 2 0 Tim re vo lutio ns p e r m inute . Fo r p ro p e r m ixing , it is se e n that ab o ut 2 5 to 3 0 re vo lutio ns are req uired in a w ell d esig ned mixer. In the site, the no rmal tend ency is to speed up the o utturn o f co ncrete b y reducing the mixing time. This results in po o r q uality o f co ncrete. O n the o ther

hand , if the co ncre te is mixe d fo r a co mp arative ly lo ng e r time , it is une co no mical fro m the p o int o f vie w o f rate o f p ro d uc tio n o f c o nc re te and fue l c o nsum p tio n. The re fo re , it is o f impo rtance to mix the co ncre te fo r such a d uratio n w hich w ill accrue o ptimum b e ne fit. It is se e n fro m the e xp e rim e nts that the q uality o f c o nc re te in te rm s o f c o m p re ssive streng th w ill increase w ith the increase in the time o f mixing , b ut fo r mixing time b eyo nd tw o minutes, the impro vement in co mpressive streng th is no t very sig nificant. Fig . 6.14. sho w s the e ffe ct o f mixing time o n stre ng th o f co ncre te . Co ncrete mixer is no t a simple apparatus. Lo t o f co nsideratio ns have g o ne as input in the desig n o f the mixer drum. The shape o f drum, the number o f blades, inclinatio n o f blades w ith re sp e ct to d rum surface , the le ng th o f b lad e s, the d e p th o f b lad e s, the sp ace b e tw e e n the drum and the b lades, the space b etw een metal strips o f b lades and speed o f ro tatio n etc., are imp o rtant to g ive unifo rm mixing q uality and o p timum time o f mixing . Generally mixing time is related to the capacity o f mixer. The mixing time varies b etw een 1½ to 2½ minutes. Big g er the capacity o f the drum mo re is the mixing time. Ho w ever, mo dern hig h sp e e d p an mixe r use d in RMC, mixe s the co ncre te in ab o ut 1 5 to 3 0 se cs. O ne cub ic meter capacity hig h speed Pan Mixer takes o nly abo ut 2 minutes fo r batching and mixing . The b atching plant take s ab o ut 1 2 minute s to lo ad a transit mixe r o f 6 m 3 capacity. So m e tim e s, at a site o f w o rk c o n c re te m ay n o t b e d isc h arg e d fro m th e d rum an d co ncre te may b e ke p t ro tating in the d rum fo r lo ng time , as fo r instance w he n so me q uarre l

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o r d isp ute take s p lac e w ith the w o rke rs, o r w he n unantic ip ate d re p air o r m o d ific atio n is re q uire d to b e d o ne o n the fo rmw o rk and re info rce me nt. Lo ng -time mixing o f co ncre te w ill g e ne rally re sult in incre ase o f co mp re ssive stre ng th o f co ncre te w ithin limits. Due to mixing o ver lo ng perio d s, the effective w ater/ cement ratio g ets red uced , o w ing to the ab so rptio n o f w ate r b y ag g re g ate and e vap o ratio n. It is also p o ssib le that the incre ase in stre ng th may b e d ue to the im p ro ve m e nt in w o rkab ility o n ac c o unt o f e xc e ss o f fine s, re sulting fro m the ab rasio n and attritio n o f c o arse ag g re g ate in the m ix, and fro m the c o arse ag g re g ate s the mse lve s b e co ming ro und e d . The ab o ve may no t b e true in all co nd itio ns and in all case s. So metimes, the evapo ratio n o f w ater and fo rmatio n o f excess fines may reduce the w o rkability and he nc e b ring ab o ut re d uc tio n in stre ng th. The e xc e ss o f fine m ay also c ause g re ate r shrinkag e .

Modern ready mixed concrete plant.

In c ase o f lo ng haul invo lve d in d e live ring re ad y-m ixe d c o nc re te to the site o f w o rk, co ncre te is mixe d inte rmitte ntly to re d uce the b ad e ffe ct o f co ntinuo us mixing . A p e rtine nt po int to no te in this co nne ctio n is that w he n the co ncre te is mixe d o r ag itate d fro m time to time w ith a sho rt inte rval, the no rmal rule o f initial se tting time is no t b e co ming ap p licab le . The co ncrete that is kept in ag itatio n, do es no t exactly fo llo w the setting time rule as applicable to co ncre te ke p t in an unag itate d and q uie sce nt co nd itio n.

Retempering of Concrete O ften lo ng hauls are invo lved in the fo llo w ing situatio n-d elivery o f co ncrete fro m central m ixing p lant, in ro ad c o nstruc tio n, in c o nstruc ting le ng thy tunne ls, in transp o rtatio n o f co ncrete b y manual lab o ur in hilly terrain. Lo ss o f w o rkab ility and undue stiffening o f co ncrete may take place at the time o f placing o n actual w o rk site. Eng ineers at site, many a time, reject the co ncre te p artially se t and und uly stiffe ne d d ue to the time e lap se d b e tw e e n mixing and p lacing . Mixe d co ncre te is a co stly mate rial and it can no t b e w aste d w itho ut any re g ard to co st. It is re q uire d to se e w he the r such a stiffe ne d co ncre te co uld b e use d o n w o rk w itho ut

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und ue harm . The p ro c e ss o f re m ixing o f c o nc re te , if ne c e ssary, w ith ad d itio n o f just the req uired q uantity o f w ater is kno w n as “Retempering o f Co ncrete”. So metimes, a small q uantity o f e xtra c e m e n t is also ad d e d w h ile re te m p e rin g . Man y sp e c ific atio n s d o n o t p e rm it re te mp e ring . I.S. 4 5 7 – 1 9 5 7 d id no t p e rmit re te mp e ring o f p artially hard e ne d co ncre te o r mo rtar re q uiring re ne w e d mixing , w ith o r w itho ut ad d itio n o f ce me nt, ag g re g ate o r w ate r. Ho w ever, many research w o rkers are o f the view that retempering w ith the additio n o f a small q uantity o f w ater may be permitted to o btain the desired slump pro vided the desig ned w ater/ cement ratio is no t exceeded. They cautio n that the pro ductio n o f co ncrete o f excessive slump o r ad d ing w ate r in e xc e ss o f d e sig ne d w ate r c e m e nt ratio to c o m p e nsate fo r slum p lo ss re su ltin g fro m d e lays in d e live ry o r p lac in g sh o u ld b e p ro h ib ite d . It is se e n fro m th e investig atio ns, retempering o f co ncrete w hich is to o w et a mix, at a d elay o f ab o ut o ne ho ur o r so sho w ed an increase in co mpressive streng th o f 2 to 15 per cent. Retempering at further d e lay re sulte d in lo ss o f stre ng th. Ho w e ve r, this lo ss o f stre ng th is sm alle r than w o uld b e expected fro m the co nsid eratio n o f the to tal w ater/ cement ratio i.e., the initial w ater cement ratio p lus w ate r ad d e d fo r re te m p e ring to b ring the m ix b ac k into the initial d e g re e o f w o rkab ility.

Maintenance of Mixer Co nc re te m ixe rs are o fte n use d c o ntinuo usly w itho ut sto p p ing fo r se ve ral ho urs fo r c o ntinuo us mixing and p lac ing . It is o f utmo st imp o rtanc e that a mixe r sho uld no t sto p in b etw een co ncreting o peratio n. Fo r this reaso n, co ncrete mixer must b e kept w ell maintained . Mixer is placed at the site o n a firm and levelled platfo rm. The d rum and b lad es must b e kept ab so lute ly c le an at the e nd o f c o nc re ting o p e ratio n. The d rum must b e ke p t in the tilting po sitio n o r ke pt co ve re d w he n no t in use to pre ve nt the co lle ctio n o f rain w ate r. The skip is o p e rate d care fully and it must re st o n p ro p e r cushio n such as sand b ag s.

Transporting Concrete Co ncre te can b e transp o rte d b y a varie ty o f me tho d s and e q uip me nts. The p re cautio n to be taken w hile transpo rting co ncrete is that the ho mo g eneity o btained at the time o f mixing sho uld b e maintaine d w hile b e ing transp o rte d to the final p lace o f d e p o sitio n. The me tho d s ad o pte d fo r transpo rtatio n o f co ncre te are : (a ) Mo rtar Pan (b ) Whe e l Barro w, Hand Cart (c ) Crane , Bucke t and Ro p e w ay (d ) Truck Mixe r and Dump e rs (e ) Be lt Co nve yo rs ( f ) Chute (g ) Skip and Ho ist (h ) Tansit Mixe r (i ) Pump and Pip e Line ( j ) He lico p to r. Mo rtar Pan: Use o f mo rtar pan fo r transp o ratio n o f co ncre te is o ne o f the c o m m o n m e th o d s a d o p te d in th is c o u n try. It is lab o u r in te n sive . In th is c a se , c o n c re te is c a rrie d in sm a ll q uantitie s. W hile this m e tho d nullifie s th e se g re g a tio n to so m e e xte n t, p artic ularly in thic k me mb e rs, it suffe rs fro m the d isad vantag e that this metho d expo ses g reater surface area o f co ncrete fo r d ryin g c o n d itio n s. Th is re su lts in Tough Rider for transporting concrete.

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Truck mixer and dumper for transporting stiff concrete

g reater lo ss o f w ater, particularly, in ho t w eather co ncreting and under co nd itio ns o f lo w humid ity. It is to b e n o te d th at th e m o rtar p an s must b e w etted to start w ith and it m u st b e ke p t c le a n d u rin g th e e n tire o p e ratio n o f c o n c re tin g . Mo rtar pan metho d o f co nveyance o f c o n c re te c an b e ad o p te d fo r c o n c re tin g at th e g ro u n d le ve l, b e lo w o r ab o ve the g ro und le ve l w itho ut much d ifficultie s.

W he e l Barr o w : Wheel b arro w s are no rmally used fo r transpo rting co ncrete to b e placed Barro at g ro und le ve l. This m e tho d is e m p lo ye d fo r hauling c o nc re te fo r c o m p arative ly lo ng e r distance as in the case o f co ncrete ro ad co nstructio n. If co ncrete is co nveyed by w heel barro w o ve r a lo ng d istance , o n ro ug h g ro und , it is like ly that the co ncre te g e ts se g re g ate d d ue to vib ratio n. The c o arse ag g re g ate s se ttle d o w n to the b o tto m and m atrix m o ve s to the to p surface. To avo id this situatio n, so metimes, w heel barro w s are pro vided w ith pneumatic w heel to re d uc e vib ratio n. A w o o d e n p lank ro ad is also p ro vid e d to re d uc e vib ratio n and he nc e se g re g atio n. Cran e , Buc ke t an d Ro p e W ay: A crane and b ucke t is o ne o f the rig ht e q uip me nt fo r transpo rting co ncrete abo ve g ro und level. Crane can handle co ncrete in hig h rise co nstructio n p ro je cts and are b e co ming a familiar site s in b ig citie s. Crane s are fast and ve rsatile to mo ve c o nc re te ho rizo ntally as w e ll as ve rtic ally alo ng the b o o m and allo w s the p lac e m e nt o f c o nc re te at the e xac t p o int. Crane s c arry skip s o r b uc ke ts c o ntaining c o nc re te . Skip s have d ischarg e d o o r at the b o tto m, w he re as b ucke ts are tilte d fo r e mp tying . Fo r a me d ium scale jo b the b ucke t cap acity may b e 0 .5 m 3 . Ro p e w ay and b uc ke t o f vario us size s are use d fo r transp o rting c o nc re te to a p lac e , w he re simp le me tho d o f transp o rting co ncre te is fo und no t fe asib le . Fo r the co ncre te w o rks in a valley o r the co nstructio n w o rk o f a pier in the river o r fo r d am co nstructio n, this metho d o f transp o rting b y ro p e w ay and b ucke t is ad o p te d . The mixing o f co ncre te is d o ne o n the b ank o r ab utment at a co nvenient place and the b ucket is b ro ug ht b y a pulley o r so me o ther arrang e me nt. It is fille d up and the n take n aw ay to any p o int that is re q uire d . The ve rtic al mo vement o f the b ucket is also co ntro lled b y ano ther set o f pullies. So metimes, cab le and car arrang e m e nt is also m ad e fo r c o ntro lling the m o ve m e nt o f the b uc ke t. This is o ne o f the me tho d s g e ne rally ad o p te d fo r co ncre ting d am w o rk o r b rid g e w o rk. Since the size o f the b ucke t is co nsid e rab ly larg e and co ncre te is no t e xp o se d to sun and w ind the re w o uld no t b e much chang e in the state o f co ncre te o r w o rkab ility. Fo r d isc harg ing the c o nc re te , the b uc ke t may b e tilte d o r so me time s, the c o nc re te is mad e to d isc harg e w ith the he lp o f a hing e d b o tto m. Disc harg e o f c o nc re te may also b e thro ug h a g ate system o perated b y co mpressed air. The o peratio n o f co ntro lling the g ate may b e do ne manually o r mechanically. It sho uld b e practised that co ncrete is discharg ed fro m the smalle st he ig ht p o ssib le and sho uld no t b e mad e to fre e ly fall fro m g re at he ig ht. Truc k Mixe r and Dum p e rs: Fo r larg e co ncrete w o rks particularly fo r co ncrete to be placed at g ro und level, trucks and d umpers o r o rd inary o pen steel-b o d y tipping lo rries can b e used . As the y c an trave l to any p art o f the w o rk, the y have m uc h ad vantag e o ve r the jub ile e w ag o ns, w hich req uire rail tracks. Dumpers are o f usually 2 to 3 cubic metre capacity, w hereas

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the cap acity o f truck may b e 4 cub ic me tre o r mo re . Be fo re lo ad ing w ith the co ncre te , the insid e o f the b o d y sho uld b e just w e tte d w ith w ate r. Tarp aulins o r o the r c o ve rs m ay b e p ro vid e d to co ve r the w e t co ncre te d uring transit to p re ve nt e vap o ratio n. Whe n the haul is lo ng , it is ad visab le to use ag itato rs w hich p re ve nt se g re g atio n and stiffe ning . The ag itato rs he lp the mixing pro ce ss at a slo w spe e d . Fo r ro ad co nstructio n using Slip Fo rm Pave r larg e q uantity o f co ncre te is re q uire d to b e sup p lie d c o ntinuo usly. A num b e r o f d um p e rs o f 6 m 3 c ap ac ity are e m p lo ye d to sup p ly co ncre te . Small d ump e r calle d To ug h Rid e rs are use d fo r facto ry flo o r co nstructio n. Be lt Co n ve yo rs: Be lt co nve yo rs have ve ry limite d ap p licatio ns in co ncre te co nstructio n. The p rinc ip al o b je c tio n is the te nd e nc y o f the c o nc re te to se g re g ate o n ste e p inc line s, at transfer po ints o r chang e o f directio n, and at the po ints w here the b elt passes o ver the ro llers. Ano ther disadvantag e is that the co ncrete is expo sed o ver lo ng stretches w hich causes drying and stiffe ning p articularly, in ho t, d ry and w ind y w e athe r. Se g re g atio n also take s p lace d ue to the vib ratio n o f rub b er b elt. It is necessary that the co ncrete sho uld b e remixed at the end o f d e live ry b e fo re p lacing o n the final p o sitio n. Mo d ern Belt Co nveyo rs can have ad justab le reach, travelling d iverter and variab le speed b o th fo rw ard and re ve rse . Co nve yo rs c an p lac e larg e vo lum e s o f c o nc re te q uic kly w he re acce ss is limite d . The re are p o rtab le b e lt co nve yo rs use d fo r sho rt d istance s o r lifts. The e nd d ischarg e arrang e me nts must b e such as to p re ve nt se g re g atio n and re mo ve all the mo rtar o n the return o f belt. In adverse w eather co nditio ns (ho t and w indy) lo ng reaches o f belt must b e co ve re d . C h u te : Ch u te s a re g e n e ra lly p ro vid e d fo r transpo rting co ncrete fro m g ro und level to a lo w er level. The se ctio ns o f chute sho uld b e mad e o f o r line d w ith m e tal and all runs shall have ap p ro xim ate ly the sam e slo pe, no t flatter than 1 vertical to 2 1 / 2 ho rizo ntal. The lay-o ut is made in such a w ay that the co ncrete w ill slide e ve nly in a c o m p ac t m ass w itho ut any se p aratio n o r se g re g atio n. The re q uire d c o nsiste nc y o f the c o nc re te sho uld no t b e chang e d in o rd e r to facilitate chuting . If it b e c o m e s ne c e ssary to c hang e the c o nsiste nc y the co ncre te mix w ill b e co mp le te ly re d e sig ne d . Th is is n o t a g o o d m e th o d o f tran sp o rtin g c o n c re te . Ho w e ve r, it is ad o p te d , w h e n m o ve m e n t o f lab o u r canno t b e allo w e d d ue to lack o f sp ace o r fo r fe a r o f d istu rb a n c e to re in fo rc e m e n t o r o th e r arran g e m e n ts alread y inco rpo rated . (Electrical co nd uits o r sw itch b o ard s e tc.,). Skip an d Ho ist: This is o ne o f the w id e ly a d o p te d m e th o d s fo r transp o rting c o nc re te ve rtic ally up fo r m u ltisto re y b u ild in g c o n stru c tio n . Emp lo ying mo rtar p an w ith the stag ing a n d h u m a n la d d e r fo r tra n sp o rtin g

Transporting and placing concrete by chute.

Tower Hoist and Winch, for lifting concrete to higher level.

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co ncre te is no t no rmally p o ssib le fo r mo re than 3 o r 4 sto re ye d b uild ing co nstructio ns. Fo r laying co ncre te in talle r structure s, chain ho ist o r p latfo rm ho ist o r skip ho ist is ad o p te d . At th e g ro u n d le ve l, m ixe r d ire c tly fe e d s the skip and the skip trave ls up o ve r ra ils u p to th e le ve l w h e re c o n c re te is req uired . At that po int, the skip d ischarg es the c o nc re te auto matic ally o r o n manual o p e ratio n. The q uality o f co ncre te i.e. the fre e d o m fro m se g re g atio n w ill d e p e n d up o n the e xte nt o f trave l and ro lling o ve r th e rails. If th e c o n c re te h as trave lle d a c o nsid e rab le he ig ht, it is ne c e ssary that c o nc re te o n d isc harg e is re q uire d to b e turne d o ve r b e fo re b e ing p lace d finally.

Transit Mixer

Transit Mixer, a popular mathod of transporting concrete over a long distance.

Tra n sit m ixe r is o n e o f th e m o st po pular eq uipments fo r transpo rting co ncrete o ver a lo ng distance particularly in Ready Mixed Co ncrete plant (RMC). In India, to day (2000 AD) there are abo ut 35 RMC plants and a number o f central batching plants are w o rking . It is a fair estimate that there are o ver 600 transit mixers in o pe ratio n in Ind ia. The y are truck mo unte d having a capacity o f 4 to 7 m 3 . The re are tw o variatio ns. In o ne, mixed co ncrete is transpo rted to the site b y keeping it ag itated all alo ng at a sp e e d varying b e tw e e n 2 to 6 re vo lutio ns p e r minute . In the o the r cate g o ry, the co ncre te is b atched at the central b atching plant and mixing is d o ne in the truck mixer either in transit o r immediately prio r to discharg ing the co ncrete at site. Transit-mixing permits lo ng er haul and is le ss vulne rab le in case o f d e lay. The truck mixe r the sp e e d o f ro tating o f d rum is b e tw e e n 4 –1 6 re vo lutio n p e r minute . A limit o f 3 0 0 re vo lutio ns fo r b o th ag itating and mixing is laid d o w n b y ASTM C 9 4 o r alte rnative ly, the co ncre te s must b e p lace d w ithin 1 12 o f mixing . In c ase o f transit m ixing , w ate r ne e d no t b e ad d e d till suc h tim e the m ixing is c o m m e nc e d . BS 5328 – 1991, restrict the time o f 2 ho urs during w hich, cement and mo ist sand are allo w ed to re main in co ntact. But the ab o ve re strictio ns are to b e o n the safe sid e . Exce e d ing the se limit is no t g o ing to b e harmful if the mix re mains sufficie ntly w o rkab le fo r full co mpactio n. With the d e ve lo p me nt o f tw in fin p ro ce ss mixe r, the transit mixe rs have b e co me mo re e fficie nt in mixing . In the se mixe rs, in ad d itio n to the o ute r sp irals, have tw o o p p o se d inne r spirals. The o uter spirals co nvey the mix materials to w ard s the b o tto m o f the d rum, w hile the o ppo sed mixing spirals push the mix to w ards the feed o pening . The repeated co unter current m ixin g p ro c e ss is takin g p lac e w ith in th e m ixe r d rum. So m e tim e s a sm all c o n c re te p u m p is also mo unte d o n the truc k c arrying transit mixe r. This pump, pumps the co ncrete discharg ed fro m transit mixe r. Curre ntly w e have place r b o o m also as part o f the truc k c arrying transit m ixe r and c o nc re te pump and w ith the ir he lp co ncre te is transpo rte d , p u m p e d a n d p la c e d in to th e fo rm w o rk o f a structure e asily. Pumping arrangements

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As p e r e stimate mad e b y CM Do o rd i, the c o st o f transp o rtatio n o f c o nc re te b y transit mixe r varie s b e tw e e n Rs 1 6 0 to 1 8 0 p e r cub ic me tre . 6 .2

Pumps and Pipeline Pump ing o f c o nc re te is unive rsally ac c e p te d as o ne o f the main me tho d s o f c o nc re te transp o rtatio n and p lac ing . Ad o p tio n o f p um p ing is inc re asing thro ug ho ut the w o rld as p um p s b e c o m e m o re re liab le and also the c o nc re te m ixe s that e nab le the c o nc re te to b e p ump e d are also b e tte r und e rsto o d . De ve lo p m e nt o f Co nc rre e te Pum p : The first patent fo r a co ncrete pump w as taken in USA in the year 1 9 1 3 6 .3 . By ab o ut 1 9 3 0 several co untries d evelo ped and manufactured co ncrete pump w ith sliding plate valves. By ab o ut 1950s and 1960s co ncrete pumping b ecame w idely used metho d in Germany. Fo rty per cent o f their co ncrete w as placed b y pumping . The keen rivalry b e tw e e n the le ad ing Ge rman manufacture rs, name ly, Schw ing , Putzme iste r and Elb a, has b o o ste d the d e ve lo p me nt o f co ncre te p ump and in p articular the valve d e sig n w hich is

the mo st impo rtant part o f the w ho le syste m. Co n c re te Pu m p s: The mo d e rn c o n c re te p u m p is a so p h istic ate d , re liab le and ro b ust m ac hine . In the p ast a simp le tw o -stro ke me c hanic al p u m p c o n siste d o f a re c e ivin g ho p p e r, an inle t and an o utle t valve , a p isto n an d a c ylin d e r. Th e p um p w as po w ered b y a diesel eng ine. The

Pump and pipeline

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pumping actio n starts w ith the suctio n stro ke draw ing co ncrete into the cylinder as the pisto n mo ves b ackw ards. During this o peratio n the o utlet value is clo sed. O n the fo rw ard stro ke, the inlet valve clo ses and the o utlet valve o pens to allo w co ncrete to b e pushed into the d elivery pipe . Fig . 6 .1 5 illustrate s the principle . Th e m o d e rn c o n c re te p u m p still o p e rate s o n th e sam e p rin c ip le s b u t w ith lo t o f imp ro ve me nts and re fine me nts in the w ho le o p e ratio ns. During 1 9 6 3 , sq ue e ze typ e p ump w as develo ped in U.S.A. In this co ncrete placed in a co llecting ho pper is fed by ro tating blades into a fle xib le p ip e co nne cte d to the p ump ing chamb e r, w hich is und e r a vacuum o f ab o ut 6 0 0 mm o f me rcury. The vacuum e nsure s that, e xce p t w he n b e ing sq ue e ze d b y ro lle r, the p ip e shap e re mains cylind rical and thus p e rmits a co ntinuo us flo w o f co ncre te . Tw o ro tating ro lle rs p ro g re ssive ly sq ue e ze the fle xib le p ip e s and thus mo ve the co ncre te into the d e live ry pipe . Fig . 6 .1 6 . sho w s the actio n o f sq ue e ze pump. The hydraulic pisto n pump is the mo st w idely used mo dern pump. Specificatio n differ but co nce p t o f w o rking o f mo d e rn p ump is the same as it w as fo r o rig inal me chanically d rive n p ump s. A p ump c o nsists o f thre e p arts, a c o nc re te re c e iving hap p e r, a valve syste m and a po w e r transmissio n syste m. There are three main types o f co ncrete pump. They are mo bile, trailo r o r static and screed o r mo rtar pump.

Typ e s o f valve : The mo st imp o rtant p art o f any co ncre te p ump is the valve syste m. The main types o f valve are peristaltic o r sq ueeze type valves, sliding g ate o r ro tating value, flapper valve s, and ho llo w transfe r tub e valve s. Ho llo w transfer tub e valves are mo st co mmo nly used type o f valve. Ano ther type w hich is use d e xte nsive ly is the Ro ck Valve . The S valve use d b y Putzme iste r is ano the r e xamp le o f a transfe r tub e value . Pip e line s and c o up ling s: It is no t eno ug h to have an efficient pump. It is equally impo rtant to have c o rre c t d iam e te r o f p ip e line w ith ad e q uate w all thic kne ss fo r a g ive n o p e rating p re ssure and w e ll d e sig ne d co up ling syste m fo r tro ub le fre e o p e ratio n. A p o o r p ip e line can e asily c ause b lo c kag e s arising fro m le akag e o f g ro ut. Pushing o f ab rasive mate rial at hig h p re ssure , thro ug h p ip e line ine vitab ly c re ate s a g re at d e al o f w e ar. Co ntinuo us hand ling , fre q ue n t se c urin g an d re le asin g o f c o up lin g s c re ate s w e ar at jo in ts. All th e se m ust b e maintaine d w e ll fo r tro ub le fre e functio n and safe ty.

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It is impo rtant to cho o se the co rrect diameter and w all thickness o f the pipeline to match the p um p and re q uire d p lac ing rate . Ge ne rally alm o st all p um p e d c o nc re te is c o nve ye d thro ug h 1 2 5 mm pipeline. There are exceptio ns. Fo r lo ng , ho rizo ntal d istance invo lving hig h pumping pressures, a larg e diameter pipe w o uld be mo re suitable o n acco unt o f less resistance to flo w. Fo r pumping co ncrete to heig hts, o n acco unt o f the fact that g ravity and the w eig ht o f co ncre te in the line , a smalle st p o ssib le d iame te r o f p ip e line s sho uld b e use d . As a g uid e , a p um p w ith an o utp ut o f 3 0 m 3 / h and w ith no t m o re than 2 0 0 m o f pipeline o ne may sug g est 100 mm diameter, b ut fo r leng th in excess o f 500 meter, a 150 mm d iame te r co uld b e co nsid e re d . Diame te r o f p ip e line has also b e aring o n the size o f ag g re g ate . Ge ne ral rule is that the p ip e d iame te r sho uld b e b e tw e e n 3 to 4 time s the larg e st size o f ag g re g ate . Fo r e xamp le if maximum size o f ag g re g ate in co ncre te is 4 0 mm, the d iame te r o f p ip e co uld b e b e tw e e n 1 2 0 mm to 1 6 0 mm. But use o f 1 2 5 mm p ip e can b e co nsid e re d suitab le . The ind ivid ual p ip e se ctio ns w ith le ng ths o f 1 m, 2 m o r 3 m are co nne cte d b y me ans o f vario us types o f q uick-lo cking co upling s. Fo r chang e in pipe line d irectio ns b end s o f d ifferent d e g re e s (9 0 d e g ., 6 0 d e g ., 4 5 d e g ., 3 0 d e g . and 1 5 d e g .) are availab le . The b e nd s have a rad ius o f 1 m. But b e nd s w ith rad ius o f r = 2 5 0 mm are use d in p lacing b o o ms. Layin g th e Pip e lin e : A carefully laid pipeline is the prereq uisite fo r tro ub le free pumping o peratio n. Time, mo ney and tro ub le are saved at sites if the installatio n o f co ncrete pump and the laying o f p ip e line s are tho ro ug hly p lanne d and carrie d o ut w ith care . Le aky p ip e s and c o up ling p o ints o fte n re sults in p lug s and imp e d e the p ushing o f c o nc re te o n ac c o unt o f e scap e o f air o r w ate r. Pip e line s must b e w e ll ancho re d w he n b e nd s are intro d uce d . Particular care must be taken w hen laying vertical line. It is difficult to dismantle individual p ip e . The re fo re , install o nly such p ip e s w hich are in g o o d co nd itio n. Pump s sho uld no t b e kept very clo se to the vertical pipe. There must b e so me starting distance. This co uld b e ab o ut 1 0 to 1 5 % o f the ve rtical d istance . Cap ab ilitie s o f Co nc rre e te Pum p : Co ncrete has been pumped to a heig ht o ver 400 m and a ho rizo ntal d istance o f o ve r 2 0 0 0 m. This re q uire s se le cte d hig h p re ssure p ump and sp e cial atte ntio n to co ncre te mix d e sig n. It is re p o rte d that in Fe b ruary, 1 9 8 5 , a re co rd fo r ve rtical co ncre te p ump ing o f 4 3 2 m w as achie ve d at the Estang e nto salle nte p o w e r statio n in the Sp anish Pyre ne e s. A Putzme iste r statio nary hig h p re ssure p ump w ith an S-transfe r tub e valve w as use d . This p ump had a the o re tical o utp ut o f 1 2 0 m 3 / h, 1 8 0 mm d e live ry cylind e r and an effective co ncrete pressure o f o ver 2 0 0 b ar, 6 3 0 meter o f 1 2 5 mm d iameter hig h pressure p ip e line w as use d .

Well pumpable concrete

Badly pumpable concrete

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Fo r the ab o ve w o rk, co ncre te mix co nsiste d o f 5 0 6 kg 1 2 – 2 5 mm g ranite ag g re g ate , 3 6 2 kg 5 – 1 2 mm g ranite ag g re g ate , 6 5 5 kg 0 – 5 mm g ranite sand , 0 – 3 mm rive r sand , 2 1 1 kg ce me nt, 9 0 kg fly ash and 1 8 3 litre w ate r.

Pumpable Concrete : A c o nc re te w hic h c an b e p ushe d thro ug h a p ip e line is c alle d a p um p ab le c o nc re te . It is m ad e in suc h a m anne r that its fric tio n at the inne r w all o f the p ip e line d o e s no t b e co me ve ry hig h and that it d o e s no t w e d g e w hile flo w ing thro ug h the p ip e line . A c le ar und e rstand ing o f w hat hap p e ns to c o nc re te w he n it is p ump e d thro ug h pipeline is fundamental to any study o f co ncrete pumping . Pumpable co ncrete emerg ing fro m a p ip e line flo w s in the fo rm o f a p lug w hic h is se p arate d fro m the p ip e w all b y a thin lub ricating layer co nsisting o f cement paste. The w ater in the paste is hydraulically linked w ith the inte rp article w ate r laye r in the p lug . Fig . 6 .1 7 sho w s the co ncre te flo w und e r p re ssure . Fo r co ntinuo us plug mo vement, the pressure g enerated b y the flo w resistance must no t b e g reater than the pump pressure rating . Ho w ever, if the co ncrete is to o saturated at hig her w / c ratio , the co ncrete at certain pump pressures may b e such that w ater is fo rced o ut o f the

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mix, creating an increase in flo w resistance and a po ssib le b lo ckag e. Fig . 6 .1 8 illustrates such a co nd itio n. In o the r w o rd s, a ve ry stiff co ncre te is no t p ump ab le and also a co ncre te w ith hig h w / c ratio is also no t pumpab le . It is inte re sting to no te that if a co ncre te is pumpab le , it is implie d that it is a g o o d co ncre te .

Design Considerations for Pumpable Concrete : The mix is pro po rtio ne d in such a w ay that it is ab le to b ind all the co nstituent materials to g ether under pressure fro m the pump and thereb y avo id ing seg reg atio n and b leed ing . The mix must also facilitate the rad ial mo vement o f sufficient g ro ut to maintain the lub ricating film initially placed o n the pipeline w all. The mix sho uld also b e ab le to d e fo rm w hile flo w ing thro ug h b e nd s. To achie ve this, the p ro po rtio n o f fines i.e. , cement and fine particles belo w 0.25 mm size (particles belo w 300 micro ns Appx.) is o f p rim e im p o rtan c e . Th e q uan titie s o f fin e p artic le s b e tw e e n 3 5 0 to 4 0 0 kg / m 3 are c o nsid e re d ne c e ssary fo r p um p ab le c o nc re te . The ab o ve q uantitie s are no t o nly fo und ne ce ssary fo r maintaining the lub ricating film, b ut it is imp o rtant fo r q uality and w o rkab ility and to co ve r ind ivid ual g rains. There are tw o main reaso ns w hy b lo ckag es o ccur and that the plug o f co ncrete w ill no t mo ve : "

Water is being fo rced o ut o f the mix creating bleeding and blo ckag e by jamming , o r

"

There is to o much frictio nal resistance due to the nature o f the ing redients o f the mix.

Fig . 6 .1 9 . sho w s the relatio nship b etw een cement co ntent and ag g reg ate vo id co ntent and e xce ssive frictio nal re sistance o n se g re g atio n and b le e d ing .

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W hile it is imp o rtant to maintain g o o d g rad ing and lo w vo id co nte nt, it is no t alw ays p o ssib le to d e sig n p ump ab le mix aro und id e al ag g re g ate . Naturally o ccurring ag g re g ate as w e ll as crushe d ag g re g ate s are suitab le fo r pumpab le mix, b ut it is e sse ntial to b e aw are o f g rad ing , vo id co nte nt and unifo rmity. The slump o f p ump ab le co ncre te is ke p t at 7 5 mm to co llap se rang e and the d iame te r o f the p ip e line is at le ast 3 – 4 time s the maximum size o f ag g re g ate . Mix De sig n p ro c e ss o f p um p ab le c o nc re te w ill b e furthe r d e alt in Chap te r 1 1 und e r “Co ncre te Mix De sig n.”

Choosing the Correct Pump Fo r cho o sing the co rre ct pump o ne must kno w the fo llo w ing facto rs "

Le ng th o f ho rizo ntal p ip e

"

Le ng th o f ve rtical p ip e

"

Numb e r o f b e nd s

"

Diame te r o f p ip e line

"

Le ng th o f fle xib le ho se

"

Chang e s in line d iame te r

"

Slump o f Co ncre te .

Fig . 6 .2 0 h o w s th e lin e p re ssure an d p um p in g rate as fun c tio n s o f lin e d iam e te r, pumping d istance and slump. Making use o f this no mo g raph o ne can find the rated capacity o f the p ump . This rate d cap acity sho uld b e mo d ifie d to actual cap acity re q uire d .

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257

Pre ssure in the p ip e can b e e stimate d using the fo llo w ing g uid e line s. "

Start up p re ssure re q uire d b y p ump

= 2 0 b ars

"

Eve ry 2 0 m ho rizo ntal p ip e line

= 1 .0 b ar

"

Eve ry 4 m Ve rtical pipe line

= 1 .0 b ar

"

Eve ry 9 0 ° b e nd

= 1 .0 b ar

"

Eve ry 4 5 ° b e nd

= 0 .5 b ar

"

Eve ry p ip e co up ling

= 0 .1 b ar

"

Eve ry 5 m e nd ho se

= 2 .0 b ar

"

Safe ty facto r

= 1 0 % e xtra

Exam p le No . 1 . If a trailer mo unted pump kept 4 0 m aw ay fro m the b uild ing and if it is re q uire d to p ump co ncre te 1 0 0 m ve rtically, calculate p re ssure in the p ip e line . "

Start up pre ssure

"

Ve rtical le ng th o f p ip e line

= 100 m

∴ Pre ssure

=

Ho rizo ntal le ng th

= 40 m

∴ Pre ssure

=

"

"

2 5 b ars

40 20

2 b ars

= 6 0 x 0 .1

6 b ars

= 2 x1

2 b ars

= 1 x2

2 b ars –— –— 5 7 b ars

9 0 ° b e nd s 2 no s Pre ssure

"

100 4

Co upling s 6 0 No s Pre ssure

"

2 0 b ars

End Ho se 5 m Pre ssure To tal Pre ssure

Ad d 1 0 % as safe ty facto r

6 –— –— 6 3 b ars –— –—

∴ To tal Pre ssure

Exam p le No . 2 . A co ncre te p ump is p lace d 4 5 m fro m a b uild ing o f he ig ht 5 0 m. The p lacing b o o m p ro je cts 4 m e xtra he ig ht o ve r the b uild ing and it can re ach a ve rtical he ig ht o f ano the r 2 5 m w ith fo ur 9 0 ° b e nd s and thre e 3 0 ° b e nd s. The ave rag e o ut p ut re q uire d is 3 0 m 3 / h. The d iame te r o f pipe line is 1 2 5 mm. The slump o f co ncre te is 7 0 mm. First find o ut the the o re tical le ng th o f p ip e line The le ng th o f pipe line = 4 5 m + 5 0 m = 9 5 m Th e re a re fo u r 9 0 ° b e n d s 4 x 90 + 3 x 30 = 360° + 90° = 450°.

and

th re e

30°

bends

m a kin g

a

to ta l o f

Assuming that the b end s have a rad ius o f 1 m, 3 0 ° is eq uivalent to 1 m, and , therefo re , 4 5 0 ° is e q uivale nt to

450 = 15 m 30

The ve rtical re ach o f p lacing is 2 5 m, and the b e nd s in the p lacing b o o m are assume d to b e e q uivale nt to 1 0 m. The re fo re , the the o re tical le ng th o f p ip e line is 9 5 + 1 5 + 2 5 + 1 0 = 1 4 5 m.

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The he ig ht to w hich the co ncre te has to b e pumpe d is 5 0 + 4

= 54 m

The static p re ssure d ue to ve rtical p ump ing is, the re fo re , 5 4 x 0 .2 5

= 1 4 b ar

Using the ab o ve d ata i.e ., c o rre c te d o utp ut, p ip e line d iam e te r, the o re tic al le ng th o f pipe line and slump, it is po ssib le to arrive at the line pre ssure using no mo g raph Fig . 6 .2 1 . O n th e n o m o g rap h Fig 6 .2 1 lo c ate 4 0 m 3 / h o u tp u t. (3 0 m 3 x 4 / 3 = th e o re tic al o utput = 4 0 m 3 / h). Mo ve acro ss to the rig ht to cut p ip e line d iame te r (1 2 5 mm). The n mo ve d o w nw ard s to meet the theo retical leng th o f pipeline (1 4 5 m). No w mo ve to left to intersect the slump line (7 0 mm). The n mo ve ve rtic ally up to me e t the p ump ing p re ssure line . The read ing at this po int is sho w n as 35 b ar (Appno x). To this sho uld b e ad d ed the static pressure o f 14 bar, g iving a to tal o f 49 bar. The pump cho sen, therefo re, sho uld have a rated maximum p re ssure o f a fig ure in e xce ss o f 4 9 b ar. The manufacture rs w ill p ro vid e the ir re co mme nd e d p e rc e ntag e to b e ad d e d w hic h is no rm ally b e tw e e n 2 0 and 3 0 % . in this c ase , the p um p re q uire d w o uld , the re fo re , have a line p re ssure cap acity o f b e tw e e n 6 0 and 7 0 b ars. Co m m o n Pr o b le m s in Pu m p in g Co n c rre e te : The mo st c o mmo n p ro b le m in p ump ing Pro co ncrete is blo ckag e. If co ncrete fails to emerg e at the end o f pipeline, if pump is mechanically so und , it w o uld mean that there is b lo ckag e so mew here in the system. This w ill b e ind icated by an increase in the pressure sho w n o n the pressure g aug e. Mo st blo ckag es o ccur at tapered se ctio ns at the p ump e nd . Blo ckag es take place g enerally due to the unsuitab ility o f co ncrete mix, pipeline and jo int d e ficie ncie s and o p e rato r’s e rro r o r care le ss use o f ho se e nd . It has b een alread y d iscussed reg ard ing the q uality o f pumpab le co ncrete. A co ncrete o f rig ht co nsistency w hich fo rms a co ncrete plug surro und ed b y lub ricating slurry fo rmed insid e

Fresh Concrete !

259

the w all o f p ip e line w ith rig ht amo unt o f w ate r, w e ll p ro p o rtio ne d , ho mo g e ne o usly mixe d c o nc re te c an o nly b e p um p e d . It c an b e rig htly said that a p um p ab le c o nc re te is a g o o d co ncre te . So metimes, hig h temperature, use o f admixtures, particularly, accelerating admixtures and use o f hig h g rad e cement may cause b lo ckag es. Chances o f b lo ckag e are mo re if co ntinuo us pumping is no t d o ne . A p ip e line w hich is no t w e ll cle ane d afte r the p re vio us o p e ratio n, uncle ane d , w o rn-o ut ho se s, to o m any and to o sharp b e nd s, use o f w o rn o ut jo ints are also o the r re aso ns fo r b lo ckag e s. O p e rato rs m ust re alise and use suffic ie nt q uantity o f lub ric ating g ro ut to c o ve r the co mp le te le ng th o f p ip e line b e fo re p ump ing o f co ncre te . The ho se must b e w e ll lub ricate d . Extreme care sho uld b e taken in hand ling the flexib le rub b er end ho se. Careless b end ing can cause b lo ckag e s. Cle aring Blo c kag e s: A mino r blo ckag e may be cleared by fo rw ard and reverse pumping . Exce ss p re ssure sho uld no t b e b lind ly e xe rte d . If may make the p ro b le m w o rse . So me time sho rte ning the p ip e line w ill re d uce p re ssure and o n re starting p ump ing the b lo ckag e g e ts cle are d o ff. Tap p ing the p ip e line w ith ham m e r and o b se rving the so und o ne c an o fte n lo c ate a b lo ckag e . Blo ckag e co uld b e cle are d b y ro d d ing o r b y using sp o ng e b all p ushe d b y co mp re sse d air o r w ate r at hig h p re ssure .

Placing Concrete It is n o t e n o u g h th a t a c o nc re te m ix c o rre c tly d e sig ne d , batched, mixed and transpo rted, it is o f utm o st im p o rtanc e that the c o n c re te m u st b e p la c e d in syste m a tic m a n n e r to yie ld o p timum re sults. The p re c autio ns to b e taken and metho d s ad o pted w h ile p la c in g c o n c re te in th e und e r-m e ntio ne d situatio ns, w ill b e d iscusse d . Paving concrete by slip-forming to get sinusoidal profile for (a ) Plac in g c o n c re te w ith in linking with the adjacent slab. e arth mo uld . Courtesy : Wirtgen (e xa m p le : Fo u n d a tio n co ncre te fo r a w all o r co lumn).

(b ) Placing co ncre te w ithin larg e e arth mo uld o r timb e r p lank fo rmw o rk. (e xamp le : Ro ad slab and Airfie ld slab ). (c ) Placing co ncre te in laye rs w ithin timb e r o r ste e l shutte rs. (e xample : Mass co ncre te in d am co nstructio n o r co nstructio n o f co ncre te ab utme nt o r p ie r). (d ) Placing co ncre te w ithin usual fro m w o rk. (e xample : Co lumns, b e ams and flo o rs). (e ) Placing co ncre te und e r w ate r.

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! Concrete Technology

Co n c re te is in va ria b ly la id a s fo undatio n bed belo w the w alls o r co lumns. Be fo re p la c in g th e c o n c re te in th e fo u n d atio n , all th e lo o se e arth m u st b e re m o ve d fro m the b e d . Any ro o t o f tre e s p assing thro ug h the fo und atio n m ust b e cut, charre d o r tarre d e ffe ctive ly to p re ve nt its further g ro w th and piercing the co ncrete at a late r d ate . The surfac e o f the e arth, if d ry, m ust b e just m ad e d am p so that the earth d o es no t ab so rb w ater fro m co ncrete. O n the o the r hand if the fo und atio n b e d is to o w e t an d rain -so ake d , th e w ate r an d slu sh m u st b e re m o ve d c o m p le te ly to e xpo se firm b e d b e fo re placing co ncre te . If the re is any se e p ag e o f w ate r taking p lace into the fo undatio n trench, effective metho d fo r d ive rtin g th e flo w o f w ate r m u st b e ad o p te d b e fo re c o nc re te is p lac e d in the tre nch o r pit.

Mould with floating suspension for simultaneous

castig of parapetwall. Fo r th e c o n stru c tio n o f ro ad slab s, a irfie ld sla b s a n d g ro u n d flo o r sla b s in building s, co ncrete is placed in bays. The g ro und surface o n w hich the co ncrete is placed must b e free fro m lo o se earth, po o l o f w ater and o ther o rg anic matters like g rass, ro o ts, leaves etc. The earth must b e pro perly co mpacted and made sufficiently damp to prevent the ab so rptio n o f w ater fro m co ncrete. If this is no t d o ne, the b o tto m po rtio n o f co ncrete is likely to b eco me w e ak. So me time s, to p re ve nt ab so rp tio n o f mo isture fro m co ncre te , b y the larg e surface o f e arth, in case o f thin ro ad slab s, use o f p o lye thyle ne film is use d in b e tw e e n co ncre te and g ro und . Co ncrete is laid in alternative b ays g iving eno ug h sco pe fo r the co ncrete to und erg o sufficie nt shrinkag e . Pro visio ns fo r co ntractio n jo ints and d ummy jo ints are g ive n. It must b e rememb ered that the co ncrete must b e d umped and no t po ured . It is also to b e ensured that co ncrete must b e placed in just req uired thickness. The practice o f placing co ncrete in a heap at o ne p lace and the n d rag g ing it sho uld b e avo id e d .

W he n co ncre te is laid in g re at thickne ss, as in the case o f co ncre te raft fo r a hig h rise b uild ing o r in the co nstructio n o f co ncre te p ie r o r ab utme nt o r in the co nstructio n o f mass co ncre te d am, co ncre te is p lace d in laye rs. The thickne ss o f laye rs d e p e nd s up o n the mo d e o f co mpactio n. In reinfo rced co ncrete, it is a g o o d practice to place co ncrete in layers o f abo ut 1 5 to 3 0 cm thick and in mass co ncre te , the thickne ss o f laye r may vary anything b e tw e e n 3 5 to 4 5 cm. Se ve ral such laye rs may b e p lace d in succe ssio n to fo rm o ne lift, p ro vid e d the y fo llo w o ne ano ther q uickly eno ug h to avo id co ld jo ints. The thickness o f layer is limited b y the me tho d o f co mp actio n and size and fre q ue ncy o f vib rato r use d . Be fo re p lacing the co ncre te , the surface o f the p re vio us lift is cle ane d tho ro ug hly w ith w ate r je t and scrub b ing b y w ire b rush. In case o f d am, e ve n sand b lasting is also ad o p te d . The o ld surface is so metimes hacked and mad e ro ug h b y remo ving all the laitance and lo o se mate rial. The surface is w e tte d . So me time s, a ne at ce me nt slurry o r a ve ry thin laye r o f rich mo rtar w ith fine sand is dashed ag ainst the o ld surface, and then the fresh co ncrete is placed.

Fresh Concrete !

261

The w ho le o p e ratio n must b e p ro g re sse d and arrang e d in such a w ay that, co ld jo ints are avo id e d as far as po ssib le . Whe n co ncre te is laid in laye rs, it is b e tte r to le ave the to p o f the laye r ro ug h, so that the suc c e e d ing laye r c an have a g o o d b o nd w ith the p re vio us laye r. Where the co ncrete is sub jected to ho rizo ntal thrust, b o nd b ars, b o nd rails o r b o nd sto nes are pro vided to o btain a g o o d bo nd betw een the successive layers. O f co urse, such arrang ements are re q uire d fo r p lacing mass co ncre te in laye rs, b ut no t fo r re info rce d co ncre te . Ce rtain g o o d rule s sho uld b e o b se rve d w hile p lacing co ncre te w ithin the fo rmw o rk, as in the case o f b eams and co lumns. Firstly, it must b e c he c ke d that the re info rc e m e nt is c o rre c tly tied, placed and is having appro priate co ver. The jo ints b e tw e e n planks, plyw o o d s o r she e ts must b e p ro p e rly an d e ffe c tive ly p lu g g e d so th at m atrix w ill n o t e sc ap e w h e n th e c o n c re te is vib rate d . The insid e o f the fo rmw o rk sho uld b e ap p lie d w ith m o uld re le asing ag e nts fo r e asy strip p ing . Suc h p urp o se mad e mo uld re le asing ag ents are separately availab le fo r steel o r timb er shutte ring . The re info rc e m e nt sho uld b e c le an and free fro m o il. Where reinfo rcement is placed in a c o ng e ste d m anne r, the c o nc re te m ust b e p lace d ve ry care fully, in small q uantity at a time so that it d o es no t b lo ck the entry o f sub seq uent co ncrete. The abo ve situatio n o ften takes place in he avily re info rc e d c o nc re te c o lumns w ith c lo se late ral tie s, at the junctio n o f co lumn and b e am and in d e e p b e am s. G e ne rally, d iffic ultie s are experienced fo r placing co ncrete in the co lumn. O fte n co ncre te is re q uire d to b e p o ure d fro m a g re ate r h e ig h t. W h e n th e c o n c re te is p o ure d fro m a he ig ht, ag ainst re info rce me nt and late ral Placing concrete by pump and placing tie s, it is like ly to se g re g ate o r b lo ck the space to boom. p re ve nt furthe r e ntry o f co ncre te . To avo id this, co ncrete is directed b y tremie, dro p chute o r b y any o ther means to direct the co ncrete w ithin the re info rce me nt and tie s. So me time s, w he n the fo rmw o rk is to o narro w, o r re info rce me nt is to o co ng ested to allo w the use o f tremie o r d ro p chute, a small o pening in o ne o f the sid es is mad e and the co ncre te is intro d uce d fro m this o pe ning inste ad o f po uring fro m the to p. It is advisable that care must be taken at the stag e o f detailing o f reinfo rcement fo r the difficulty in p o uring co ncre te . In lo ng sp an b rid g e s the d e p th o f p re stre sse d co ncre te g ird e rs may b e o f the o rd e r o f e ve n 4 – 5 m e te rs invo lving c o ng e ste d re info rc e m e nt. In suc h situatio ns planning fo r placing co ncrete in o ne o peratio n re q uires serio us co nsid eratio ns o n the part o f d e sig ne r. Fo rm w o rk: Fo rm w o rk shall b e d e sig ne d and co nstructe d so as to re main sufficie ntly rig id d uring p lacing and co mp actio n o f co ncre te . The jo ints are p lug g e d to p re ve nt the lo ss o f slurry fro m co ncre te . Strip p in g T Tim im e : Fo rmw o rk sho uld no t b e re mo ve d until the co ncre te has d e ve lo p e d a streng th o f at least tw ice the stress to w hich co ncrete may b e sub jected at the time o f remo val o f fo rmw o rk. In special circumstances the streng th d evelo pment o f co ncrete can b e assessed

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! Concrete Technology

by placing co mpanio n cubes near the structure and curing the same in the manner simulating curing co nditio ns o f structures. In no rmal circumstances, w here ambient temperature do es no t fall b e lo w 1 5 ° C and w he re o rd inary Po rtland ce me nt is use d and ad e q uate curing is d o ne , fo llo w ing striking p e rio d can b e co nsid e re d sufficie nt as p e r IS 4 5 6 o f 2 0 0 0 .

Ta ble 6 .6 . St ripping T im e of For m w ork Sr. No .

Type o f Fo rmw o rk

1.

Ve rtical fo rmw o rk to co lumns w alls and b e ams So ffit fo rmw o rk to slab s (p ro p s to b e re fixe d imme d iate ly afte r re mo val o f fo rmw o rk) So ffit fo rmw o rk to b e ams (Pro p s to b e re fixe d imme d iate ly afte r re mo val o f fo rmw o rk)

2.

3.

4.

Minimum perio d b efo re striking fo rmw o rk

Pro ps to slab spanning up to 4 .5 m sp anning o ve r 4 .5 m

5.

Pro ps to b e am and arche s Sp anning up to 6 m Sp anning o ve r 6 m

1 6 – 2 4 ho urs 3 d ays

7 d ays

7 d ays 1 4 d ays 1 4 d ays 2 1 d ays

No te : Fo r o ther cements and lo w er temperature, the stripping time reco mmended abo ve may b e suitab ly mo d ifie d .

Underwater Concreting Co n c re te is o fte n re q uire d to b e p lac e d un d e rw ate r o r in a tre n c h fille d w ith th e b e nto nite slurry. In such case s, use o f b o tto m d ump b ucke t o r tre mie p ip e is mad e use o f. In the b o tto m d ump b ucke t co ncre te is take n thro ug h the w ate r in a w ate r-tig ht b o x o r b ucke t and o n reaching the final place o f depo sitio n the bo tto m is made to o pen by so me mechanism and the w ho le co ncre te is d ump e d slo w ly. This me tho d w ill no t g ive a satisfacto ry re sult as ce rtain amo unt o f w ashing aw ay o f ce me nt is b o und to o ccur. In so me situatio ns, dry o r semi-dry mixture o f cement, fine and co arse ag g reg ate are filled in c e me nt b ag s and suc h b ag g e d c o nc re te is d e p o site d o n the b e d b e lo w the w ate r. This me tho d also d o e s no t g ive satisfac to ry c o nc re te , as the c o nc re te mass w ill b e full o f vo id s inte rsp e rse d w ith the p utric ib le g unny b ag s. The satisfac to ry m e tho d o f p lac ing c o nc re te und e r w ate r is b y the use o f tre mie p ip e . The w o rd “tre mie ” is d e rive d fro m the fre nch w o rd ho p p e r. A tre mie pipe is a pipe having a d iame te r o f ab o ut 2 0 cm capab le o f e asy co upling fo r increase o r decrease o f leng th. A funnel is fitted to the to p end to facilitate po uring o f co ncrete. The b o tto m e nd is clo se d w ith a p lug o r thick p o lye thyle ne she e t o r such o the r mate rial and take n b e lo w the w ate r and m ad e to re st at the p o int w he re the c o nc re te is g o ing to b e p lace d . Since the e nd is b lo cke d , no w ate r w ill have e nte re d the p ip e . The co ncre te having a ve ry hig h slump o f ab o ut 1 5 to 2 0 cm is p o ure d into the funne l. Whe n the w ho le le ng th o f p ip e is fille d up w ith the co ncre te , the tre mie p ip e is lifte d up and a slig ht je rk is g ive n b y

Fresh Concrete !

263

a w inch and pully arrang ement. When the pipe is raised and g iven a jerk, d ue to the w e ig ht o f co ncre te , the b o tto m p lu g fa lls a n d th e c o n c re te g e ts d isc h arg e d . Partic u lar c are m u st b e take n at this stag e to se e that the e nd o f the tre m ie p ip e re m ains insid e the co ncre te , so that no w ate r e nte rs into th e p ip e fro m th e b o tto m . In o th e r w o rd s, th e tre m ie p ip e re m a in s plug g ed at the lo w er end b y co ncrete. Ag a in c o n c re te is p o u re d o ve r th e funne l and w he n the w ho le le ng th o f the tre mie p ip e is fille d w ith co ncre te , th e p ip e is ag ain slig h tly lifte d an d g ive n slig ht je rk. Care is take n all the tim e to ke e p th e lo w e r e n d o f th e tre mie p ip e w e ll e mb e d d e d in the w e t c o nc re te . The c o nc re te in the tre m ie p ip e g e ts d isc h a rg e d . In th is w a y, c o nc re te w o rk is p ro g re sse d w itho ut sto p p ing till the c o nc re te le ve l c o m e s ab o ve the w ate r le ve l. Fig . 6 .2 2 sho w s the und e rw ate r co ncre ting b y tre mie . This me tho d if e xe cute d p ro p e rly, has the ad vantag e that the c o nc re te d o e s no t g e t affe cte d b y w ate r e xce p t the to p layer. The to p layer is scrub b ed Fig. 6.22. Under Water Concreting by Tremie Method. o r c u t o ff to re m o ve th e a ffe c te d co ncre te at the e nd o f the w ho le o pe ratio n. D u rin g th e c o u rse o f c o n c re tin g , n o p u m p in g o f w ate r sh o u ld b e p e rm itte d . If simultane o us p ump ing is d o ne , it may suc k the c e me nt p artic le s. Und e r w ate r c o nc re ting need no t be co mpacted, as co ncrete g ets auto matically co mpacted by the hydro static pressure o f w ate r. Se c o nd ly, the c o nc re te is o f suc h c o nsiste nc y that it d o e s no t no rm ally re q uire co mpactio n. O ne o f the disadvantag es o f under w ater co ncreting in this metho d is that a hig h w ate r/ ce me nt ratio is re q uire d fo r hig h co nsiste ncy w hich re d uce s the stre ng th o f co ncre te . But at pre se nt, w ith the use o f supe rplasticize r, it is no t a co nstraint. A co ncre te w ith as lo w a w / c ratio as 0 .3 o r e ve n le ss can b e p lace d b y tre mie me tho d . Ano the r m e tho d , no t so c o m m o nly e m p lo ye d to p lac e c o nc re te b e lo w w ate r is the g ro utin g p ro c e ss o f p re p ac ke d ag g re g ate . Co arse ag g re g ate is d um p e d to assum e full dimensio n o f the co ncrete mass. Cement mo rtar g ro ut is injected thro ug h pipes, w hich extend up to the b o tto m o f the ag g re g ate b e d . The p ip e s are slo w ly w ithd raw n, as the g ro uting pro g re sse s. The g ro ut fo rce s the w ate r o ut fro m the inte rstice s and o ccupie s the space . Fo r p lug g ing the w e ll fo und atio n this me tho d is o fte n ad o p te d . Co ncre te also can b e place d und e r w ate r b y the use o f pipe s and co ncre te pumps. The p ip e line is p lug g e d at o ne e nd and lo w e re d until it re sts at the b o tto m . Pum p ing is the n

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starte d . Whe n the p ip e is co mp le te ly fille d , the p lug is fo rce d o ut, the co ncre te surro und ing the lo w er end o f the pipe seals the pipe. The pumping is do ne ag ainst the pressure o f the plug at the lo w e r e nd . Whe n the p ump ing e ffo rt re q uire d is to o g re at to o ve rco me the p re ssure , the p ip e is w ithd raw n and the o p e ratio n is re p e ate d . This p ro ce ss is re p e ate d until co ncre te re ache s the le ve l ab o ve w ate r.

Slip-Form Technique The re are sp e c ial m e tho d s o f p lac e m e nt o f c o nc re te using slip -fo rm te c hniq ue . Slip fo rming can b e d o ne b o th fo r ve rtical co nstructio n o r ho rizo ntal co nstructio n. Slip-fo rming o f vertical co nstructio n is a pro ven metho d o f co ncrete co nstructio n g enerally ad o p te d fo r tall structure s. In this me tho d , co ncre te is co ntinuo usly p lace d , co mp acte d and fo rmw o rk is pulled up by number o f hydraulic Jacks, g iving reactio n, ag ainst jack ro ds o r main reinfo rcements. The rate o f slipping the fo rmw o rk w ill vary d epend ing upo n the temperature and streng th develo pment o f co ncrete to w ithstand w itho ut the suppo rt o f fo rmw o rk. In India numb e r o f tall structure s like chimne ys and silo s have b e e n b uilt b y this te chniq ue . Altho ug h this me tho d o f co nstructio n is suitab le fo r unifo rm shap p e d structure s it w as ad o p te d fo r the co re co nstructio n o f sto ck exchang e b uild ing at Bo mb ay having irreg ular shape and numb er o f o pening s. The co re o f 380 feet tall structure w as co mpleted in abo ut 38 days. The fo rmw o rk w as slippe d at the rate o f ab o ut 1 2 .5 cm pe r ho ur. The ho rizo ntal slip-fo rm co nstructio n is rather a new techniq ue in Ind ia. It is ad o pted fo r ro ad p ave me nt co nstructio n. Fo r the first time the slip -fo rm p aving me tho d w as ad o p te d in De lhi-Mathura co ncre te Ro ad co nstructio n d uring mid 1 9 9 0 ’s. The slip-fo rm pavers w ere used by many co ntracting firms in the co nstructio n o f MumbaiPune six lane express hig hw ay. The state-o f the art metho d o f slip fo rm pavement co nstructio n has co me to Ind ia in a b ig w ay. Slip-fo rm paver is a majo r eq uipment, capable o f spreading the co ncrete dumped in fro nt o f the machine b y tippers o r dumpers, co mpacting the co ncrete thro ug h numb er o f po w erful internal needle vibrato rs and do uble beam surface vibrato rs. The paver carries o ut the smo o th finishing o p e ratio n to the hig he st ac c urac y and the n te xture the surfac e w ith nylo n b rush o perating acro ss the lane. The eq uipment also d ro ps the tie b ar at the pred etermined interval and p ush the m thro ug h and p lac e s the m at the p re d e te rmine d d e p th and re c o mp ac t the co ncrete to co ver up the g ap that are created b y the do w el b ars. Generally no b leeding takes place b ecause o f the stiff co nsistency o f the co ncrete (2 cm slump) that is desig ned fo r placing by slip-fo rm paver. If at all any little bleeding w ater is there, upo n its disappearance, membrane fo rming curing co mp o und is sp raye d o n to the te xture d surface o f co ncre te . All the ab o ve o p e ratio ns are c o ntinuo usly c arrie d o ut and the slip -fo rm p ave r c raw ls co ntinuo usly o n tracked w heel, g uid ed b y laser co ntro l. Pro per alig nment to cater fo r straig ht line , o r c urve o f any d e g re e w ith c alc ulate d sup e r e le vatio n, o r up w ard o r d o w nw ard g rad ie nts are co ntro lle d b y lase r ap p licatio n. Co mp ute rise d lase r co ntro l is the b ackb o ne o f this state -o f- the art slip -fo rm p ave r e q uip me nt. The sp e e d o f co nstructio n i.e., the sp e e d o f c o ntinuo us m o ve m e nt o f p ave r is aro und 1 m e te r p e r m inute and in a d ay o f 1 6 ho urs w o rking , this e q uipme nt can co mple te ab o ut o ne km o f o ne lane ro ad o f w id th 3 .7 5 m and d e p th 3 5 cm. In the Mumb ai-Pune express hig hw ay co nstructio n, they have used tw o types o f paving e q uip me nts name ly w irtg e n SP 5 0 0 and CMI. They are used fo r lane b y lane co nstructio n. Whereas in Euro pe and the o ther advanced co untries, slip-fo rm pavers capable o f co mpleting tw o o r three lanes in o ne o peratio n are used.

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Placing high quality concrete by slip-form technique for a width of 8.5 m.

To fe e d such a pave r, larg e q uantity o f co ncre te o f unifo rm q uality is re q uire d . In Ind ia to d ay, the capacity o f batching is a limitatio n. In Euro pe co ntinuo us batching plants w hich can supply co nsiste nt q uality o f co ncre te at a rate o f 1 5 0 to 2 5 0 m 3 / hr are availab le . This rate w ill make it p o ssib le to sup p ly e xtra w id e slip -fo rm p ave r. So p histicatio n in ro ad co nstructio n has just starte d in Ind ia. With the e xp e rie nce g aine d , w e w ill b e ab le to p ro d uce larg e q uantitie s o f manufacture d fine and co arse ag g re g ate o f rig ht q uality ne e d e d fo r hig h rate o f p ro d uctio n o f co ncre te to me e t the re q uire me nt o f multi lane slip -fo rm p ave r.

Compaction of Concrete Co mpactio n o f co ncrete is the pro cess ad o pted fo r expelling the entrapped air fro m the c o nc re te . In the p ro c e ss o f mixing , transp o rting and p lac ing o f c o nc re te air is like ly to g e t e ntrappe d in the co ncre te . The lo w e r the w o rkab ility, hig he r is the amo unt o f air e ntrappe d . In o ther w o rds, stiff co ncrete mix has hig h percentag e o f entrapped air and, therefo re , w o uld ne e d hig he r co mp acting e ffo rts than hig h w o rkab le mixe s. If this air is no t re mo ve d fully, the co ncre te lo se s stre ng th co nsid e rab ly. Fig . 6 .2 3 sho w s the re latio nship b e tw e e n lo ss o f stre ng th and air vo id s le ft d ue to lack o f co mpactio n. It can b e se e n fro m th e fig ure th at 5 p e r c e n t vo id s re d uc e th e stre n g th o f c o c re te b y ab o ut 3 0 p e r ce nt and 1 0 p e r ce nt vo id s re d d uce the stre ng th b y o ve r 5 0 p e r ce nt. The re fo re , it is impe rative that 1 0 0 pe r ce nt co mpactio n o f co ncre te is o ne o f the mo st impo rtant aim to b e ke p t in mind in g o o d co ncre te -making p ractice s.

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It must be bo rne in mind that 100 p e r ce nt co mp actio n is imp o rtant no t o nly fro m the po int o f view o f streng th, b ut also fro m the po int o f d urab ility. In re c e nt time , d urab ility b e c o me s mo re imp o rtant than stre ng th. Insufficie nt co mp actio n incre ase s the p e rme ab ility o f c o nc re te re sulting in e asy e ntry fo r ag g re ssive che micals in so lutin, w hic h attac k c o nc re te and re info rce me nt to re d uce the d urab ility o f c o nc re te . The re fo re , 1 0 0 p e r c e nt c o m p a c tio n o f c o n c re te is o f paramo unt impo rtance . In o rd e r to a c h ie ve fu ll c o m p ac tio n an d m axim u m d e n sity, w ith re aso n ab le c o m p ac tin g e ffo rts availab le at site, it is necessary to use a mix w ith adeq uate w o rkability. It is also o f c o m m o n kno w le d g e that the m ix sh o u ld n o t b e to o w e t fo r e a sy c o m p ac tio n w h ic h also re d uc e s th e stre n g th o f c o n c re te . Fo r m axim u m stre n g th , d rie st p o ssib le c o n c re te sho uld b e c o m p ac te d 1 0 0 p e r c e nt. Th e o ve rall e c o n o m y d e m an d s 1 0 0 p e r ce nt co mp actio n w ith a re aso nab le co mp acting e ffo rts availab le in the fie ld . The fo llo w ing me tho d s are ad o p te d fo r co mp acting the co ncre te : (a) Hand Co mpactio n (i ) Ro d d ing (ii ) Ramming (iii ) Tamp ing (b ) Co mpactio n b y Vib ratio n (i ) Inte rnal vib rato r (Ne e d le vib rato r) (ii ) Fo rmw o rk vib rato r (Exte rnal vib rato r) (iii ) Tab le vib rato r (iv ) Platfo rm vib rato r (v ) Surface vib rato r (Scre e d vib rato r) (vi ) Vib rato ry Ro lle r. (c ) Co mp actio n b y Pre ssure and Jo lting (d ) Co mpactio n b y Spinning . Han d Co m p ac tio n : Hand co mp actio n o f co ncre te is ad o p te d in case o f unimp o rtant co ncre te w o rk o f small mag nitud e . So me time s, this me tho d is also applie d in such situatio n, w he re a larg e q uantity o f re info rce me nt is use d , w hich canno t b e no rmally co mp acte d b y mechanical means. Hand co mpactio n co nsists o f ro d d ing , ramming o r tamping . When hand c o m p ac tio n is ad o p te d , the c o nsiste nc y o f c o nc re te is m aintaine d at a hig he r le ve l. The thic kne ss o f the laye r o f c o nc re te is limite d to ab o ut 1 5 to 2 0 c m. Ro d d ing is no thing b ut p o king the c o nc re te w ith ab o ut 2 m e tre lo ng , 1 6 m m d iam e te r ro d to p ac k the c o nc re te

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b etw een the reinfo rcement and sharp co rners and edg es. Ro dding is do ne co ntinuo usly o ver the co mplete area to effectively pack the co ncrete and d rive aw ay entrapped air. So metimes, inste ad o f iro n ro d , b amb o o s o r cane is also use d fo r ro d d ing p urp o se . Ramming sho uld b e d o ne w ith care . Lig ht ramming can b e p e rmitte d in unre info rce d fo und atio n co ncre te o r in g ro und flo o r co nstructio n. Ramming sho uld no t b e p e rmitte d in case o f re info rce d co ncre te o r in the up p e r flo o r co nstructio n, w he re co ncre te is p lace d in the fo rmw o rk sup p o rte d o n struts. If ramming is ad o p te d in the ab o ve case the p o sitio n o f the re info rce me nt may b e d isturb e d o r the fo rmw o rk may fail, p articularly, if ste e l ramme r is use d . Tamp ing is o ne o f the usual me tho d s ad o p te d in co mp acting ro o f o r flo o r slab o r ro ad pavements w here the thickness o f co ncrete is co mparatively less and the surface to be finished sm o o th and le ve l. Tam p ing c o nsists o f b e ating the to p surfac e b y w o o d e n c ro ss b e am o f sectio n ab o ut 1 0 x 1 0 cm. Since the tamping b ar is sufficiently lo ng it no t o nly co mpacts, b ut also le ve ls the to p surface acro ss the e ntire w id th. Co m p ac tio n b y V ib ratio n : It is p o inte d o ut that the co mp actio n b y hand , if p ro p e rly Vib carrie d o ut o n co ncre te w ith sufficie nt w o rkab ility, g ive s satisfacto ry re sults, b ut the stre ng th o f the hand co mpacted co ncrete w ill be necessarily lo w because o f hig her w ater cement ratio req uired fo r full co mpactio n. Where hig h streng th is req uired, it is necessary that stiff co ncrete,

Plate Vibrator

Table Vibrator

Screed Board Vibrator

Needle Vibrator Electric

Needle Vibrator Petrol

w ith lo w w ate r/ c e m e nt ratio b e use d . To c o m p ac t suc h c o nc re te , m e c hanic ally o p e rate d vib rato ry e q uip me nt, must b e use d . The vib rate d co ncre te w ith lo w w ate r/ ce me nt ratio w ill have many ad vantag e s o ve r the hand co mp acte d co ncre te w ith hig he r w ate r/ ce me nt ratio . The mo d e rn hig h fre q ue ncy vib rato rs make it p o ssib le to p lace e co no mically co ncre te w hich is imp racticab le to p lace b y hand . A co ncre te w ith ab o ut 4 cm slump can b e p lace d an d c o m p ac te d fully in a c lo se ly sp ac e d re in fo rc e d c o n c re te w o rk, w h e re as, fo r h an d co mp actio n, much hig he r co nsiste ncy say ab o ut 1 2 cm slump may b e re q uire d . The actio n o f vib ratio n is to se t the p article s o f fre sh co ncre te in mo tio n, re d ucing the frictio n b e tw e e n the m and affe cting a te mp o rary liq ue factio n o f co ncre te w hich e nab le s e asy se ttle me nt. While vib ratio n itse lf d o e s no t affe ct the stre ng th o f co ncre te w hich is co ntro lle d b y the w ate r/ ce me nt ratio , it p e rmits the use o f le ss w ate r. Co ncre te o f hig he r stre ng th and b e tte r q uality can, therefo re, be made w ith a g iven cement facto r w ith less mixing w ater. Where o nly

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Double Beam Screed Board Vibrator

a g ive n stre ng th is re q uire d , it can b e o b taine d w ith le ane r mixe s than p o ssib le w ith hand co mpactio n, making the pro cess eco no mical. Vibratio n, therefo re, permits impro vement in the q uality o f co ncre te and in e co no my. Co mpactio n o f co ncre te b y vib ratio n has almo st co mple te ly re vo lutio nise d the co nce pt o f co ncrete techno lo g y, making po ssible the use o f lo w slump stiff mixes fo r pro ductio n o f hig h q uality c o nc re te w ith re q uire d stre ng th and im p e rm e ab ility. The use o f vib ratio n m ay b e e sse ntial fo r the p ro d uctio n o f g o o d co ncre te w he re the co ng e stio n o f the re info rce me nt o r the inaccessib ility o f the co ncrete in the fo rmw o rk is such that hand co mpactio n metho d s are no t practicab le . Vib ratio n may also b e ne ce ssary if the availab le ag g re g ate s are o f such po o r shape and texture w hich w o uld pro d uce a co ncrete o f po o r w o rkab ility unless larg e amo unt o f w ater and cement is used . In no rmal circumstances, vib ratio n is o ften ad o pted to impro ve the co mp actio n and co nse q ue ntly imp ro ve the d urab ility o f structure s. In this w ay, vib ratio n can, und e r suitab le co nd itio ns, p ro d uce b e tte r q uality co ncre te than b y hand co mp actio n. Lo w e r ce me nt co nte nt and lo w e r w ate r-ce me nt ratio can p ro d uce e q ually stro ng co ncre te mo re e co no mically than b y hand co mp actio n. Altho ug h vib ratio n p ro p e rly ap p lie d is a g re at ste p fo rw ard in the p ro d uctio n o f q uality co ncre te , it is mo re o fte n e mp lo ye d as a me tho d o f p lacing o rd inary co ncre te e asily than as a m e th o d fo r o b tain in g h ig h g rad e c o n c re te at an e c o n o m ic al c o st. All th e p o te n tial ad vantag e s o f vib ratio n can b e fully re alise d o nly if p ro p e r co ntro l is e xe rcise d in the d e sig n and m anufac ture o f c o nc re te and c e rtain rule s are o b se rve d re g ard ing the p ro p e r use o f d iffe re nt type s o f vib rato rs. In te rn al Vib rato r: O f all the vib rato rs, the internal vib rato r is mo st co mmo nly used . This is also c alle d , “Ne e d le Vib rato r”, “Im m e rsio n Vib rato r”, o r “Po ke r Vib rato r”. This e sse ntially c o nsists o f a p o w e r unit, a fle xib le shaft and a ne e d le . The p o w e r unit may b e e le c tric ally d riven o r o perated b y petro l eng ine o r air co mpresso r. The vib ratio ns are caused b y eccentric w e ig h ts a tta c h e d to th e sh a ft o r th e m o to r o r to th e ro to r o f a vib ra tin g e le m e n t. Ele ctro mag ne t, p ulsating e q uip me nt is also availab le . The fre q ue ncy o f vib ratio n varie s up to 12,000 cycles o f vib ratio n per minute. The needle diameter varies fro m 20 mm to 75 mm and its le ng th varie s fro m 2 5 cm to 9 0 cm. The b ig g e r ne e d le is use d in the co nstructio n o f mass co ncre te d am. So me time s, arrang e me nts are availab le such that the ne e d le can b e re p lace d b y a b lad e o f ap p ro xim ate ly the sam e le ng th. This b lad e fac ilitate s vib ratio n o f m e m b e rs, w he re , d ue to the co ng e ste d re info rce me nt, the ne e d le w o uld no t g o in, b ut this b lad e can effectively vib rate. They are po rtab le and can b e shifted fro m place to place very easily d uring co ncre ting o p e ratio n. The y can also b e use d in d ifficult p o sitio ns and situatio ns. Fo rm w o rk V ib rato r (Exte rn al V ib rato r): Fo rmw o rk vib rato rs are use d fo r c o nc re ting Vib Vib c o lum ns, thin w alls o r in the c asting o f p re c ast units. The m ac hine is c lam p e d o n to the e xte rnal w all surfac e o f the fo rm w o rk. The vib ratio n is g ive n to the fo rm w o rk so that the c o nc re te in the vic inity o f the shutte r g e ts vib rate d . This m e tho d o f vib rating c o nc re te is p artic ularly use ful and ad o p te d w he re re info rc e me nt, late ral tie s and sp ac e rs inte rfe re to o

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much w ith the internal vibrato r. Use o f fo rmw o rk vibrato r w ill p ro d uc e a g o o d finish to the c o nc re te surfac e . Sinc e the vib ratio n is g ive n to th e c o n c re te in d ire c tly th ro ug h th e fo rmw o rk, the y co nsume mo re p o w e r and the e fficie ncy o f e xte rn al vib rato r is lo w e r th an th e e ffic ie n c y o f in te rn al vib rato r. Tab le Vib rato r: This is the sp e c ial c ase o f fo rm w o rk vib rato r, w here the vib rato r is clamped to the tab le. o r tab le is mo unte d o n sp ring s w hic h are vib rate d transfe rring the vib ratio n to the tab le. They are co mmo nly used fo r vib rating co ncre te cub e s. Any article ke p t o n the tab le g e ts vib rate d . Vibrating Table This is ad o p te d m o stly in the lab o rato rie s and in m aking small b ut p re cise p re fab ricate d R.C.C. me mb e rs. Platfo rm Vib rato r: Platfo rm vib rato r is no thing b ut a tab le vib rato r, b u t it is larg e r in size . Th is is u se d in th e manufacture o f larg e p re fab ricate d co ncre te e le me nts such as e le c tric p o le s, railw ay sle e p e rs, p re fab ric ate d ro o fin g e le m e n ts e tc . So m e tim e s, th e p latfo rm vib rato r is also co upled w ith jerking o r sho ck g iving arrang ements such that a tho rug h co mp actio n is g ive n to the co ncre te . Surfac e V ib rato r: Surface vibrato rs are so metimes kno w s Vib as, “Scre e d Bo ard Vib rato rs”. A small vib rato r p lace d o n the scre e d b o ard g ive s an e ffe ctive me tho d o f co mp acting and le ve lling o f thin co ncre te me mb e rs, such as flo o r slab s, ro o f Vibrating Table slab s and ro ad surface. Mo stly, flo o r slab s and ro o f slab s are so thin that internal vib rato r o r any o ther type o f vib rato r canno t b e easily emplo yed . In such cases, the surface vibrato r can be effectively used. In g eneral, surface vibrato rs are no t effective b e yo n d ab o u t 1 5 c m . In th e m o d e rn c o n stru c tio n p rac tic e s like vac c u m d e w ate rin g te c hniq ue , o r slip -fo rm p aving te c hniq ue , the use o f sc re e d b o ard vib rato r are c o m m o n fe ature . In the ab o ve situatio ns d o ub le b e am scre e d b o ard vib rato rs are o fte n use d . Co m p ac tio n b y Pr e ssur e and Jo lting : This is o ne o f the effective metho ds o f co mpacting Pre ssure very d ry co ncrete. This metho d is o ften used fo r co mpacting ho llo w b lo cks, cavity b lo cks and so lid c o nc re te b lo c ks. The stiff c o nc re te is vib rate d , p re sse d and also g ive n jo lts. W ith the co mb ine d actio n o f the jo lts vib ratio ns and pre ssure , the stiff co ncre te g e ts co mpacte d to a d e nse fo rm to g ive g o o d stre ng th and vo lum e stab ility. By e m p lo ying g re at p re ssure , a co ncre te o f ve ry lo w w ate r ce me nt ratio can b e co mp acte d to yie ld ve ry hig h stre ng th. Co m p ac tio n b y Sp in n in g : Sp inning is o ne o f the re c e nt m e tho d s o f c o m p ac tio n o f co ncre te . This me tho d o f co mp actio n is ad o p te d fo r the fab ricatio n o f co ncre te p ip e s. The p lastic co ncre te w he n sp un at a ve ry hig h sp e e d , g e ts w e ll co mp acte d b y ce ntrifug al fo rce . Pate nte d p ro d ucts such a “Hume Pip e s”, “sp un p ip e s” are co mp acte d b y sp inning p ro ce ss. Vib rato rry y Ro lle r: O ne o f the re c e nt d e ve lo p m e nts o f c o m p ac ting ve ry d ry and le an co ncrete is the use o f Vib rato ry Ro ller. Such co ncrete is kno w n as Ro ller Co mpacted Co ncrete. This me tho d o f co ncre te co nstructio n o rig inate d fro m Jap an and sp re ad to USA and o the r c o untrie s mainly fo r the c o nstruc tio n o f d ams and p ave me nts. He avy ro lle r w hic h vib rate s w hile ro lling is used fo r the co mpactio n o f d ry lean co ncrete. Such ro ller co mpacted co ncrete o f g rad e M 1 0 has b een successfully used as b ase co urse, 1 5 cm thick, fo r the Delhi-Mathura hig hw ay and Mumb ai-Pune e xp re ss hig hw ays.

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General Points on Using Vibrators Vib rato rs may b e p o w e re d b y any o f the fo llo w ing units: (a ) Electric mo to rs either driving the vibrato r thro ug h flexible shaft o r situated in the head o f the vib rato r. (b ) Inte rnal co mb ustio n e ng ine d riving the vib rato r ne e d le thro ug h fle xib le shaft, and (c ) Co mp re sse d -air mo to r situate d ne ar the he ad o f the vib rato r. Whe re re liab le supplie s o f e le ctricity is availab le the e le ctric mo to r is g e ne rally the mo st satisfac to ry and e c o no m ic al p o w e r unit. The sp e e d is re lative ly c o nstant, and the c ab le s sup p lying curre nt are lig ht and e asily hand le d . Small p o rtab le p e tro l e ng ine s are so me time s use d fo r vib rating co ncre te . The y are mo re easily put o ut o f actio n by site co nditio ns. They are no t so reliable as the electric o r co mpressedair mo to rs. They sho uld b e lo cated co nveniently near the w o rk to b e vib rated and sho uld b e pro pe rly se cure d to the ir b ase . Co mpressed-air mo to rs are g enerally quite suitable but pneumatic vibrato rs are so metimes d ifficult to manipulate w here the co mpresso r canno t b e placed ad jacent to the w o rk such as o n hig h scaffo ld ing s o r at d e p ths b e lo w g ro und le ve l d ue to the he avy w e ig ht o f air ho se s. Co mp re sse d -air vib rato rs g ive tro ub le e sp e cially in co ld w e athe r, b y fre e zing at e xhaust unle ss alco ho l is trickle d into the air line o r d ry air is use d . Glyco l typ e antifre e ze ag e nts te nd to cause g umming o f the vib rato r valve s. The re is also a te nd e ncy fo r mo isture to co lle ct in the mo to r, he nce care sho uld b e take n to re mo ve the p o ssib le d amag e . The sp e e d o f b o th the p e tro l and c o m p re sse d -air m o to rs te nd to vary g iving rise to variatio n in the co mpacting e ffe ct o f the vib rato r.

Further Instructions on use of Vibrators Care shall b e taken that the vib rating head do es no t co me into co ntact w ith hard o b jects like hard ened co ncrete, steel and w o o d , as o therw ise the impact may d amag e the b earing s. The p rime mo ve r sho uld as far as p o ssib le , b e starte d o nly w he n he ad is raise d o r re sting o n so ft sup p o rt. Sim ilar p re c autio ns shall b e o b se rve d w hile intro d uc ing o r w ithd raw ing the vib rato r in the co ncre te to b e co nso lid ate d . Whe n the sp ace fo r intro d uctio n is narro w, the vib rato r sho uld b e sw itc he d o n o nly afte r the vib rato r he ad has b e e n intro d uc e d into the co ncre te . Unne ce ssary sharp b e nd s in the fle xib le shaft d rive shall b e avo id e d . Vib rato rs co nfo rming to the req uirements o f IS 2505-1963 (i.e., Specificatio n fo r co ncrete vib rato rs, imme rsio n typ e ) shall b e use d . The size and characte ristics o f the vib rato r suitab le fo r a p artic ular jo b vary w ith the c o nc re te mix d e sig n, q uality and w o rkab ility o f c o nc re te , p lac ing c o nd itio ns, size and shap e o f the me mb e r and shall b e se le c te d d e p e nd ing up o n vario us re q uire me nts. Guid ance re g ard ing se le ctio n o f a suitab le vib rato r may b e o b taine d fro m Tab le 6 .7 . Co rre ct d e sig n o f co ncre te mix and an e ffe ctive co ntro l in the manufacture o f co ncre te , rig ht fro m the selectio n o f co nstituent materials thro ug h its co rrect pro po rtio ning to its placing , are e sse ntial to o b tain maximum b e ne fits o f vib ratio n. Fo r b e st re sults, the c o nc re te to b e vib rate d shall b e o f the stiffe st po ssib le co nsiste ncy, g e ne rally w ithin a rang e o f 0 .7 5 to 0 .8 5 c o m p ac tin g fac to r, p ro vid e d th e fin e m o rtar in c o n c re te sh o w s at le ast a g re asy w e t appearance w hen the vib rato r is slo w ly w ithd raw n fro m the co ncrete and the material clo ses o ve r the sp ace o ccup ie d b y the vib rato r ne e d le le aving no p ro no unce d ho le . The vib ratio n o f co ncrete o f very hig h w o rkab ility w ill no t increase its streng th; it may o n the co ntrary, cause se g re g atio n. Fo rmatio n o f a w ate ry g ro ut o n the surface o f the co ncre te d ue to vib ratio n is an indicatio n that the co ncrete is to o so ftly made and unsuitable fo r vibratio n; a clo se textured laye r o f visco us g ro ut may, ho w e ve r, b e allo w e d .

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Fo r vib rate d c o n c re te , th e fo rm w o rk sh all b e stro n g e r th an is n e c e ssary fo r h an d co mpacted co ncrete and g reater care is exercised in its assemb ly. It must b e d esig ned to take up increased pressure o f co ncrete and pressure variatio ns caused in the neig hbo urho o d o f the vib rating he ad w hic h may re sult in the e xc e ssive lo c al stre ss o n the fo rmw o rk. Mo re e xac t d e tails o n the p o ssib le p re ssure s are no t availab le and m uc h d e p e nd s up o n e xp e rie nc e , ju d g e m e n t an d th e c h arac te r o f w o rk. Th e jo in ts o f th e fo rm w o rk sh all b e m ad e an d maintained tig ht and clo se eno ug h to prevent the sq ueezing o ut o f g ro ut o r sucking in o f air d uring vib ratio n. Ab se nc e o f this p re c autio n m ay c ause ho ne y-c o m b ing in the surfac e o f co ncre te , imp airing the ap p e arance and so me time s w e ake ning the structure . The amo unt o f mo rtar le akag e o r the p e rmissib le g ap b e tw e e n she athing b o ard s w ill d e p e nd o n the d e sire d final ap p e arance o f the w o rk b ut no rmally g ap s larg e r than 1 .5 mm b e tw e e n the b o ard s sho uld no t b e p e rm itte d . So m e tim e s e ve n narro w e r jo ints m ay b e o b je c tio nab le fro m the p o int o f vie w o f the ir e ffe c t o n the surfac e ap p e aranc e o f c e rtain structure s. The numb e r o f jo ints sho uld b e mad e as fe w as p o ssib le b y making the shutte r se c tio ns larg e . Ap p lic atio ns o f m o uld re le asing ag e nts o n the fo rm w o rk, to p re ve nt the adhesio n o n co ncrete sho uld b e very thin as o therw ise they may mix w ith the co ncrete under the e ffe ct o f vib ratio n, and cause air e ntrainme nt and b lo w ho le s o n the co ncre te surface . The vib rato r m ay b e use d ve rtic ally, ho rizo ntally o r at an ang le d e p e nd ing up o n the nature o f the jo b . But needle vib rato rs sho uld b e immersed in b eams and o ther thick sectio ns, ve rtically at re g ular inte rvals. The co ncre te to b e vib rate d shall b e p lace d in p o sitio n in le ve l laye rs o f suitab le thickne ss no t g re ate r than the e ffe ctive le ng th o f the vib rato r ne e d le . The c o nc re te at the surfac e m ust b e d istrib ute d as ho rizo ntally as p o ssib le , sinc e the c o n c re te flo w s in slo p e s w h ile b e in g vib rate d an d m ay se g re g ate . Th e vib ratio n sh all, the re fo re , no t b e d o ne in the ne ig hb o urho o d o f slo p e s. The inte rnal vib rato r sho uld no t b e use d to sp re ad the c o nc re te fro m the filling as this c an c ause c o nsid e rab le se g re g atio n o f c o nc re te . It is ad visab le to d e p o sit c o nc re te w e ll in ad vanc e o f the p o int o f vib ratio n. This p re ve nts the c o nc re te fro m sub sid ing no n-unifo rm ly and thus p re ve nts the fo rm atio n o f inc ip ie nt p lastic c rac ks. W he n the c o nc re te is b e ing c o ntinuo usly d e p o site d to an unifo rm depth alo ng a memb er, vib rato r shall no t b e o perated to o near the free end o f the advancing co ncre te , usually no t w ithin 1 2 0 cm o f it. Eve ry e ffo rt must b e mad e to ke e p the surface o f the previo usly placed layer o f co ncrete alive so that the succeed ing layer can b e b o nd ed w ith it b y the vib ratio n p ro c e ss. Ho w e ve r, if d ue to unfo re se e n c irc umstanc e s the c o nc re te has hardened in the underlying layer to such an extent that it canno t be penetrated by the vibrato r b ut is still fre sh (just afte r initial se t) unimp o se d b o nd can b e achie ve d b e tw e e n the to p and und e rlying laye rs b y syste matically and tho ro ug hly vib rating the ne w co ncre te into co ntact w ith o ld .

Height of Concrete Layer Co ncre te is p lace d in thin laye rs co nsiste nt w ith the me tho d b e ing use d to p lace and vib rate the co ncrete. Usually co ncrete shall b e placed in a thickness no t mo re than 60 cm and o n initial placing in thickness no t mo re than 15 cm. The suprimpo sed lo ad increasing w ith the he ig ht o f the laye r w ill favo ur the actio n o f the vib rato r, b ut as it is also the path o f air fo rce d upw ards, it may trap air rising up by vibratio n. Very deep layers (say mo re than 60 cm) sho uld, therefo re, be avo ided altho ug h the heig ht o f layer can also be o ne metre pro vided the vibrato r use d is sufficie ntly po w e rful, as in d ams.

Depth of Immersion of Vibrator To b e fully e ffe ctive , the active p art o f the vib rato r shall b e co mp le te ly imme rse d in the c o nc re te . Its c o mp ac ting ac tio n c an b e usually assiste d b y maintaining a he ad o f c o nc re te

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ab o ve the active part o f the vib rato r, the primary o b je ct o f w hich is to pre ss d o w n upo n and co nfine the co ncrete in the zo ne o f influence o f the vibrato r. The vibrato r head shall be dipped thro ug h the filling w hich is to b e co nso lid ated to a further d epth o f 1 0 to 2 0 cm in the lo w er laye r w hic h has alre ad y b e e n c o nso lid ate d so that the re is a g o o d c o mb inatio n o f vario us laye rs and the g ro ut in the lo w e r laye r is d istrib ute d in the ne w filling .

Spacing and Number of Insertion Positions The p o ints o f inse rtio n o f the vib rato r in the co ncre te shall b e so sp ace d that the rang e o f ac tio n o ve rlap to so m e e xte nt and the fre shly fille d c o nc re te is suffic ie ntly c o m p ac te d e ve ryw he re . The rang e o f ac tio n varie s w ith the c harac te ristic s o f the vib rato r and the co mpo sitio n and w o rkab ility o f co ncre te . The rang e o f actio n and the d e g re e o f co mpactio n can b e reco g nized fro m the rising air b ub b les and the fo rmatio n o f a thin shining film aro und the vibrating head. With co ncrete o f w o rkability o f 0.78 to 0.85 co mpacting facto r, the vibrato r shall g e ne rally b e o p e rate d at p o ints 3 5 to 9 0 cm ap art. The sp e cifie d sp acing b e tw e e n the d ip p ing p o sitio ns shall b e maintaine d unifo rmly thro ug ho ut the surface o f co ncre te so that the co ncre te is unifo rmly vib rate d .

Speed of Insertion and Withdrawal of the Vibrating Head The vib rating he ad shall b e re g ularly and unifo rmly inse rte d in the c o nc re te so that it penetrates o f its o w n acco rd and shall b e w ithd raw n q uite slo w ly w hilst still running so as to allo w re d istrib utio n o f co ncre te in its w ake and allo w the co ncre te to flo w b ack into the ho le behind the vibrato r. The rate o f w ithdraw al is determined by the rate at w hich the co mpactio n in the ac tive zo ne is c o m p le te d . Usually a sp e e d o f 3 c m / s g ive s suffic ie nt c o nso lid atio n w ith o u t u n d u e strain o n th e o p e rato r. Fu rth e r c o n c re te is ad d e d as th e vib rato rs are

Correct

Incorrect

Correct

Incorrect

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w ith d raw n so as to m ain tain th e h e ad o f th e c o n c re te u n til th e lift o f th e c o n c re te is co mp le te d .

Duration of Vibration New filling shall b e vib rated w hile the co ncrete is plastic, preferab ly w ithin o ne ho ur. The d uratio n o f vib ratio n in e ach p o sitio n o f inse rtio n is d e p e nd e nt up o n the he ig ht o f the laye r, the size and characte ristics o f the vib rato r and the w o rkab ility o f the co ncre te mix. It is b e tte r to insert the vibrating head at a number o f places than to leave it fo r a lo ng time in o ne place, as in the latter case, there is a tendency fo r fo rmatio n o f mo rtar po cket at the po int o f insertio n o f the vib rato r. The vib rato r he ad shall b e ke p t in o ne p o sitio n till the c o nc re te w ithin its influe nc e is co mpletely co nso lidated w hich w ill be indicated by fo rmatio n o f circular shaped cement g ro ut o n the surface o f co ncre te , ap p e arance o f flatte ne d g liste ning surface and ce ssatio n o f the rise o f e ntrap p e d air. Vib ratio n shall b e co ntinue d until the co arse ag g re g ate p article s have b le nd e d into the surface b ut have no t d isap p e are d . The time re q uire d to e ffe ct co mp le te co nso lid atio n is re ad ily jud g e d b y the e xp e rie nce d vib rato r o p e rato r thro ug h the fe e l o f the vib rato r, re sump tio n o f fre q ue ncy o f vib ratio n afte r the sho rt perio d o f d ro pping o ff o f freq uency w hen the vib rato r is first inserted . Do ub t ab o ut the ad e q uacy o f vib ratio n sho uld alw ays b e re so lve d b y furthe r vib ratio n; w e ll p ro p o rtio ne d co ncre te o f the co rre ct co nsiste ncy is no t re ad ily susce p tib le to o ve r-vib ratio n.

Vibrating Concrete at Junctions with Hardened Concrete In case s w he re co ncre te has to b e jo ine d w ith ro ck o r hard e ne d co ncre te , d e fe cts can o ccur o w ing to the laye rs ne are st to the hard e ne d co ncre te no t b e ing sufficie ntly vib rate d . In such case s the p ro ce d ure g ive n b e lo w sho uld b e ad o p te d : The hard e ne d c o nc re te surfac e sho uld b e p re p are d b y hac king o r ro ug he ning and re mo ving laitance , g re asy matte r and lo o se p article s. The cle ane d surface shall b e w e tte d . A ce me nt sand g ro ut o f pro po rtio n 1 :1 and o f cre amy co nsiste ncy is the n applie d to the w e t surface o f the o ld co ncre te , and the fre sh co ncre te vib rate d ag ainst it.

Vibrating the Reinforced Concrete The re info rc e m e nt sho uld b e d e sig ne d to le ave suffic ie nt sp ac e fo r the vib rating head. Where po ssib le, the reinfo rcement may b e g ro uped so that the w idth o f g ro ups o f b ars do es no t exceed 25 cm and a space o f 7.5 cm exists b etw een the g ro ups o f b ars to allo w the vibrato r to pass freely; the space betw een the bars in any g ro up may be reduced to tw o -third s o f the no minal size o f co arse ag g re g ate . Whe n the re info rce me nts lie ve ry clo se to e ach o the r, g re ate r care is take n in vib rating so that no po ckets o r co llectio ns o f g ro ut are fo rmed . Except w here so me o f the co ncrete has alre ad y se t and p ro vid e d that the re info rce me nt is ad e q uate ly sup p o rte d and se cure d , the vib rato r may b e p re sse d ag ainst the re info rce me nt.

Vibrating near the Formwork Fo r o b taining a sm o o th c lo se te xture d e xte rnal surfac e , the c o nc re te sho uld have a sufficie nt co nte nt o f matrix. The vib rato r he ad shall no t b e b ro ug ht ve ry ne ar the fo rmw o rk as this may cause fo rmatio n o f w ater w hirls (stag natio ns), especially if the co ncrete co ntaining to o little o f fine ag g reg ate. O n the o ther hand , a clo se textured surface may no t b e o b tained , if the po sitio ns o f inse rtio n are to o far aw ay fro m the fo rmw o rk. The mo st suitab le d istance o f the vibrato r fro m the fo rmw o rk is 10 to 20 cm. With the vibratio n do ne at the co rrect depth

(2 )

up to 3 5 0

2 5 0 to 5 0 0

2 5 0 to 7 0 0

(i)

(ii)

(iii)

mm

Leng th o f the Vib rating Need le

(1 )

Sr. No .

up to 7 5

O ve r 6 0

up to 6 0

O ve r 3 5

up to 3 5

(3 )

mm

Diameter o f the Vib rating Need le

7000

9000

9000

(4 )

Reco mmend ed freq uency o f vib ratio n und er no Lo ad State, Min Vib ratio n VPM

Characteristics o f Vib rato r

up to 7 5

O ve r 6 0

up to 6 0

O ve r 3 0

3 0 to 5 0

(5 )

g+

* Reco mmend ed vib ratio n Acceleratio n (o peratio n in Air), Min

ad jace nt to fo rms mass co ncre te and p ave me nts.

flo o rs, b rid g e d e ck and ro o f slab s. Auxiliary vib ratio n

such as w alls, co lumns. b e ams, pre cast pile s, he avy

Plastic, w o rkab le co ncre te in g e ne ral co nstructio n,

co nstructio n jo ints.

b e ams, pre cast pile s, lig ht b rid g e d e cks, and alo ng

Plastic, w o rkab le co ncre te in thin w alls, co lumns,

cause co ng e stio n in the fo rms.

in p re stre sse d w o rk, w he re many cab le s and d ucts

spe cime ns. Suitab le as an auxiliary to larg e r vib rato rs

co nfine d place s and fo r fab ricatio n o f lab o rato ry te st

Plastic, w o rkab le co ncre te in ve ry thin me mb e rs and

(6 )

Applicatio n

Ta ble 6 . 7 . Cha ra c t e rist ic s a nd Applic at ions of I m m e rsion V ibrat ors

274 ! Concrete Technology

3 0 0 to 4 5 0

2 0 0 to 4 7 5

(iv)

(v )

O ve r 9 0

up to 9 0

O ve r 7 5

(3 )

6000

7000

(4 )

O ve r 9 0

up to 9 0

O ve r 7 5

(5 )

Mass co ncre te co ntaining 1 5 cm. ag g re g ate d e p o site d in incre me nts up to 8 m 3 , in g ravity d ams, larg e p ie rs, massive w alls, e tc. Tw o o r mo re vib rato rs w ill b e req uired to o perate simultaneo usly to melt do w n and co nso lidate increments o f co ncrete o f 4 m 3 o r g reater vo lume d epo sited at o ne time in the fo rms.

cing ste e l.

ne ar fo rms and aro und e mb e d d e d ite ms and re info r-

tio ns and fo r auxiliary vib ratio n in d am co nstructio n

fo und atio ns and fo r auxiliary vib ratio n in fo und a-

fo rms, in p o w e r ho use s, he avy b rid g e p ie rs and

up to 2 m 3 in he avy co nstructio n in re lative ly o p e n

Mass and structural co ncre te d e p o site d in incre me nts

(6 )

* Value of acceleration measured in concrete should not be less then 75 per cent of the values given above. † Acceleration due to gravity.

(2 )

(1 )

Ta ble 6 .7 (Cont d.)

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and w ith sufficie nt g ro ut rising up at the fo rmw o rk, the o utsid e surface w ill g e ne rally have a clo se textured appearance. In the po sitio ns o f fo rmw o rk difficult to reach and in co ncrete w alls less than 30 cm thick it is preferab le to use vib rato rs o f small size w hich can b e b ro ug ht to the re q uire d p lace and w hich w ill no t e xce ssive ly strain the fo rmw o rk.

Vibrating High Walls and Columns While d e sig ning the fo rmw o rk, re info rce me nt, as w e ll as the d ivisio n o f laye rs fo r hig h w alls and co lumns, it sho uld b e kept in mind that w ith the usual d riving shaft leng ths it is no t po ssib le to penetrate the vib rating head mo re than three metres in the fo rmw o rk. In the case o f hig her w alls and co lumns it is reco mmended to intro duce the shaft driven vib rating needle thro ug h a sid e o pening into the fo rmw o rk. Fo r use w ith hig h w alls and co lumns, the flexib le d riving shaft can b e b ro ug ht to a leng th o f six to eig ht metres o r even mo re b y using ad o pter pieces. The mo to r-in-head type vibrato rs are mo re useful fo r the purpo se in cases w here a very lo ng curre nt cab le can b e use d fo r sinking the vib rato r to a g re ate r d e p th.

Over-Vibration The re is a p o ssib ility o f o ve r-vib ratio n w hile trying to achie ve tho ro ug h vib ratio n, b ut it is e xc e e d ing ly unlike ly in w e ll p ro p o rtio ne d m ixe s c o ntaining no rm al w e ig ht ag g re g ate s. Ge ne rally, w ith p ro p e rly d e sig ne d mixe s, e xte nd e d vib ratio n w ill b e o nly a w aste o f e ffo rt w itho ut any p articular harm to the co ncre te . Ho w ever, w here the co ncrete is to o w o rkab le fo r the co nditio ns o f placing , o r w here the q uantity o f mo rtar is e xc e ss o f the vo lume o f vo id s in the c o arse ag g re g ate , o r w he re the g rad ing o f the ag g reg ate is unsatisfacto ry, o ver-vib ratio n w ill enco urag e seg reg atio n, causing mig ratio n o f the lig hter and smaller co nstituents o f the mix to the surface, thereb y pro d ucing layer o f mo rtar o r laitance o n the surface, and leakag e o f mo rtar thro ug h the d efective jo ints in the fo rmw o rk. This may pro d uce co ncre te w ith po o r re sistance to ab rasio n and attack b y vario us ag e ncie s, such as fro st, o r may re sult in plane s o f w e akne ss w he re succe ssive lifts are being placed. If o ver vibratio n o ccurs, it w ill be immediately evident to an experienced vibrato r o p e rato r o r sup e rviso r b y a fro thy ap p e aranc e d ue to the ac c umulatio n o f many small air b ub b les and the settlement o f co arse ag g reg ates b eneath the surface. These results are mo re liab le to o ccur w he n the co ncre te is to o w e t and the pro pe r co rre ctio n w ill b e to re d uce the w o rkab ility (no t the vib ratio n), until the e vid e nc e o f o ve r-vib ratio n d isap p e ars d uring the amo unt o f vib ratio n jud g ed necessary to co nso lid ate the co ncrete and to eliminate air-b ub b le b le mishe s.

Output of Immersion Vibrator O utput o f co mpacted co ncrete may b e taken as 3 to 5 cub ic metre per ho ur d epend ing upo n the co nsistency o f the mix fo r a lig ht type o f vib rato r, having a centrifug al fo rce o f ab o ut 2 0 0 kg . The o ut-turn w ill b e as much as 1 2 to 2 5 cub ic me tre p e r ho ur fo r a he avy typ e o f vib rato r having a ce ntrifug al fo rce o f 4 5 0 kg .

Re-vibration Re -vib ratio n is d e laye d vib ratio n o f c o n c re te th at h as alre ad y b e e n p lac e d an d co mp acte d . It may o ccur w hile p lacing succe ssive laye rs o f co ncre te , w he n vib ratio ns in the up p e r laye r o f fre sh c o nc re te are transm itte d to the und e rlaying laye r w hic h has p artially hard e ne d o r may b e d o ne inte ntio nally to achie ve ce rtain ad vantag e s. Except in the case o f expo sed co ncrete and pro vided the co ncrete beco mes plastic under vib ratio n, re-vib ratio n is no t harmful and may b e b eneficial. By repeated vib ratio n o ver a lo ng

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p e rio d (re p e titio n o f vib ratio n e arlie st afte r o ne ho ur fro m the time o f initial vib ratio n), the q uality o f c o nc re te c an b e im p ro ve d b e c ause it re arrang e s the ag g re g ate p artic le s and e lim in ate s e n trap p e d w ate r fro m u n d e r th e ag g rag ae an d re in fo rc in g ste e l, w ith th e c o nse q ue nc e o f full c o ntac t b e tw e e n mo rtar and c o arse ag g re g ate o r b e tw e e n ste e l and mo rtar and thus p ro d uce s stro ng e r and w ate rtig ht co ncre te . Plastic shrinkag e cracks as w e ll as o the r d isturb ance s like ho llo w sp ace b e lo w the re info rce me nt b ars and b e lo w the co arse ag g reg ate, can thereb y b e clo sed ag ain pro vid ed the co ncrete b eco mes so ft ag ain w hen the vib rato r he ad in intro d uc e d . Re -vib ratio n o f c o nc re te re sults in imp ro ve d c o mp re ssive and b o n d stre n g th , re d u c tio n o f h o n e y-c o m b , re le ase o f w ate r trap p e d u n d e r h o rizo n tal re info rcing b ars and re mo val o f air and w ate r p o cke ts. Re -vib ratio n is m o st e ffe c tive at the lap se o f m axim um tim e afte r the initial vib ratio n, pro vid ed the co ncrete is sufficiently plastic to allo w the vib rato r to sink o f its o w n w eig ht into the co ncre te and make it mo me ntarily plastic.

Vibration of Lightweight Concrete In g eneral, principles and reco mmended practices fo r co nso lidatio n o f co ncrete o f no rmal w e ig h t h o ld g o o d fo r c o n c re te m ad e w ith lig h t w e ig h t ag g re g ate , p ro vid e d c e rtain p re cautio ns are o b se rve d . The re is alw ays a te nd e ncy fo r lig ht w e ig ht p ie ce s o f ag g re g ate to rise to the surface o f fre sh co ncre te , p articularly und e r the actio n o f o ve r-vib ratio n; and a fairly stiff mix, w ith the minimum amo unt o f vib ratio n ne c e ssary to c o nso lid ate the c o nc re te in the fo rms w itho ut ho ne y-c o mb is the b e st insuranc e ag ainst und e sirab le se g re g atio n. The rise o f lig htw e ig ht co arse ag g re g ate p article s to the surface , cause d b y o ve r-vib ratio n re sulting fro m to o w e t a mix make s finishing d ifficult if no t imp o ssib le .

Curing of Concrete We have d isc usse d in Chap te r I the hyd ratio n asp e c t o f c e m e nt. Co nc re te d e rive s its stre ng th b y the hyd ratio n o f ce me nt p article s. The hyd ratio n o f ce me nt is no t a mo me ntary actio n b ut a pro ce ss co ntinuing fo r lo ng time . O f co urce , the rate o f hyd ratio n is fast to start w ith, b ut co untinue s o ve r a ve ry lo ng time at a d e cre asing rate . The q uantity o f the p ro d uct o f hyd ratio n and c o nse q ue ntly the am o unt o f g e l fo rm e d d e p e nd s up o n the e xte nt o f h yd ra tio n . It h a s b e e n mentio ned earlier that cement re q uire s a w ate r/ ce me nt ratio ab o ut 0 .2 3 fo r hyd ratio n and a w ate r/ c e me nt ratio o f 0 .1 5 fo r filling the vo id s in the g e l p o re s. In o th e r w o rd s, a w ate r/ c e m e nt ratio o f ab o ut 0 . 3 8 w o u ld b e re q u ire d to h yd ra te a ll th e p a rtic le s o f c e m e n t an d also to o c c u p y th e sp ac e in th e g e l p o re s. Th e o re tic ally, fo r a c o n c re te m a d e a n d c o n ta in e d in a se a le d c o n ta in e r a w a te r Fig.6.24. Cracks on concrete surface due to inadequate curing. c e m e n t ratio o f 0 .3 8 w o u ld sa tisfy th e re q u ire m e n t o f

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w ater fo r hydratio n and at the same time no capillary vavities w o uld be left. Ho w ever, it is seen that practically a w ater/ cement ratio o f 0.5 w ill b e req uired fo r co mplete hydratio n in a sealed co ntaine r fo r ke e p ing up the d e sirab le re lative humid ity le ve l. In the field and in actual w o rk, it is a different sto ry. Even tho ug h a hig her w ater/ cement ratio is use d , sinc e the c o nc re te is o p e n to atm o sp he re , the w ate r use d in the c o nc re te evapo rates and the w ater availab le in the co ncrete w ill no t b e sufficient fo r effective hydratio n to take place particularly in the to p layer. Fig . 5.33 o n pag e 173, Chapter 5, sho w s the drying b e havio ur o f co ncre te . If the hyd ratio n is to co ntinue unb ate d , e xtra w ate r must b e ad d e d to re ple nish the lo ss o f w ate r o n acco unt o f ab so rptio n and e vapo ratio n. Alte rnative ly, so me me asure s must b e take n b y w ay o f pro visio n o f impe rvio us co ve ring o r applicatio n o f curing co mpo unds to prevent the lo ss o f w ater fro m the surface o f the co ncrete. Therefo re, the curing c an b e c o nsid e re d as c re atio n o f a favo urab le e nviro nm e nt d uring the e arly p e rio d fo r uninte rrup te d hyd ratio n. The d e sirab le c o nd itio ns are , a suitab le te m p e rature and am p le mo isture . Curing can also b e d e scrib e d as ke e p ing the co ncre te mo ist and w arm e no ug h so that the hyd ratio n o f ce me nt can co ntinue . Mo re e lab o rate ly, it can b e d e scrib e d as the p ro ce ss

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o f m aintaining a satisfac to ry m o isture c o nte nt and a favo urab le te m p e rature in c o nc re te d urin g th e p e rio d im m e d iate ly fo llo w in g p lac e m e n t, so th at h yd ratio n o f c e m e n t m ay c o n tin ue un til th e d e sire d p ro p e rtie s are d e ve lo p e d to a suffic ie n t d e g re e to m e e t th e re q uire me nt o f se rvice . Curing is b e ing g ive n a p lace o f incre asing imp o rtance as the d e mand fo r hig h q uality co ncre te is incre asing . It has b e e n re co g nize d that the q uality o f co ncre te sho w s all ro und imp ro ve me nt w ith e fficie nt uninte rrup te d curing . If curing is ne g le cte d in the e arly p e rio d o f hyd ratio n, the q uality o f co ncrete w ill experience a so rt o f irreparab le lo ss. An efficient curing in the e arly p e rio d o f hyd ratio n can b e co mp are d to a g o o d and w ho le so me fe e d ing g ive n to a ne w b o rn b ab y. A co ncre te laid in the afte rno o n o f a ho t summe r d ay in a d ry climatic re g io n, is apt to d ry o ut q uic kly. The surfac e laye r o f c o nc re te e xp o se d to ac ute d rying c o nd itio n, w ith the co mbined effect o f ho t sun and drying w ind is likely to be made up o f po o rly hydrated cement w ith inferio r g el structure w hich do es no t g ive the desirable bo nd and streng th characteristics. In ad d itio n, the to p surface , p articularly that o f ro ad o r flo o r p ave me nt is also sub je cte d to a larg e mag nitude o f plastic shrinkag e stresses. The dried co ncrete naturally being w eak, canno t w ithstand the se stre sse s w ith the re sult that innum e rab le c rac ks d e ve lo p at the surfac e Fig . 6 .2 4 , sh o w s p lastic sh rin kag e c rac ks o n c o n c re te surfac e d ue to q uic k d ryin g an d inad e q uate e arly curing . The to p surface o f such hard e ne d co ncre te o n acco unt o f p o o r g e l structure, suffers fro m lack o f w earing q uality and abrasio n resistance. Therefo re, such surfaces cre ate mud in the rainy se aso n and d ust in summe r. The q uick surface drying o f co ncrete results in the mo vement o f mo isture fro m the interio r to the surfac e . This ste e p m o isture g rad ie nt c ause hig h inte rnal stre sse s w hic h are also re sp o nsib le fo r inte rnal micro cracks in the se mi-p lastic co ncre te . Co ncrete, w hile hyd rating , releases hig h heat o f hyd ratio n. This heat is harmful fro m the po int o f view o f vo lume stability. If the heat g enerated is remo ved by so me means, the adverse e ffe ct d ue to the g e ne ratio n o f he at can b e re d uce d . This can b e d o ne b y a tho ro ug h w ate r curing . Fig . 6 .2 5 , sho w s the influe nce o f curing b y p o nd ing and w e t co ve ring . 6 .4

Curing Methods Curing me tho d s may b e d ivid e d b ro ad ly into fo ur cate g o rie s: (a ) Wate r curing

(b ) Me mb rane curing

(c ) Applicatio n o f he at

(d ) Misce llane o us

Water Curing This is b y far the b e st m e tho d o f c uring as it satisfie s all the re q uire m e nts o f c uring , nam e ly, p ro m o tio n o f hyd ratio n, e lim inatio n o f shrinkag e and ab so rp tio n o f the he at o f hyd ratio n. It is po inte d o ut that e ve n if the me mb rane me tho d is ad o pte d , it is d e sirab le that a certain extent o f w ater curing is do ne befo re the co ncrete is co vered w ith membranes. Water curing can b e d o ne in the fo llo w ing w ays: (a ) Imme rsio n

(b ) Po nd ing

(c ) Spraying o r Fo g g ing

(d ) We t co ve ring

The precast co ncrete items are no rmally immersed in curing tanks fo r a certain d uratio n. Pave m e nt slab s, ro o f slab e tc . are c o ve re d und e r w ate r b y m aking sm all p o nd s. Ve rtic al re taining w all o r p laste re d surface s o r co ncre te co lumns e tc. are cure d b y sp raying w ate r. In so me case s, w e t co ve ring s such as w e t g unny b ag s, he ssian clo th, jute matting , straw e tc., are w rapped to vertical surface fo r keeping the co ncrete w et. Fo r ho rizo ntal surfaces saw dust,

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earth o r sand are used as w et co vering to keep the co ncrete in w et co nditio n fo r a lo ng er time so that the co ncre te is no t und uly d rie d to p re ve nt hyd ratio n.

Membrane Curing So me time s, co ncre te w o rks are carrie d o ut in p lace s w he re the re is acute sho rtag e o f w ater. The lavish applicatio n o f w ater fo r w ater curing is no t po ssib le fo r reaso ns o f eco no my. It h a s b e e n p o in te d o u t e arlie r that c uring d o e s no t m e a n o n ly a p p lic a tio n o f w ate r, it me ans also cre atio n o f c o nd itio ns fo r p ro m o tio n of u n in te rru p te d and p ro g re ssive h yd ratio n . It is a lso p o in te d o u t th a t th e q uantity o f w ate r, no rm ally mixed fo r making co ncrete is m o re th a n su ffic ie n t to h yd ra te th e c e m e n t, p ro vid e d th is w ate r is n o t allo w e d to g o o ut fro m the b o d y o f c o n c re te . Fo r th is re aso n , c o n c re te c o u ld b e Membrane curing by spraying. c o ve re d w ith m e m b ra n e w hic h w ill e ffe c tive ly se al o ff the e vap o ratio n o f w ate r fro m c o nc re te . It is fo und that the applicatio n o f me mb rane o r a se aling co mpo und , afte r a sho rt spe ll o f w ate r curing fo r o ne o r tw o d ays is so me time s b e ne ficial. So metimes, co ncrete is placed in so me inaccessib le, d ifficult o r far o ff places. The curing o f such co ncre te canno t b e p ro p e rly sup e rvise d . The curing is e ntire ly le ft to the w o rkme n, w ho d o no t q uite und erstand the impo rtance o f reg ular uninterrupted curing . In such cases, it is much safe r to ad o p t me mb rane curing rathe r than to le ave the re sp o nsib ility o f curing to w o rke rs. Larg e numb e r o f se aling co mp o und s have b e e n d e ve lo p e d in re ce nt ye ars. The id e a is to o b tain a co ntinuo us se al o ve r the co ncre te surface b y me ans o f a firm imp e rvio us film to prevent mo isture in co ncrete fro m escaping b y evapo ratio n. So metimes, such films have b een use d at the inte rface o f the g ro und and co ncre te to p re ve nt the ab so rp tio n o f w ate r b y the g ro und fro m the c o nc re te . So m e o f the m ate rials, that c an b e use d fo r this p urp o se are b itumino us co mpo unds, po lyethylene o r po lyester film, w aterpro o f paper, rub b er co mpo unds e tc. Bitumino us co mpo und b eing b lack in co lo ur, ab so rb s heat w hen it is applied o n the to p surface o f the co ncre te . This re sults in the incre ase o f te mp e rature in the b o d y o f co ncre te w hich is und esirab le. Fo r this purpo se, o ther mo d ified materials w hich are no t b lack in co lo ur are in use. Such co mpo unds are kno w n as “Clear Co mpo unds”. It is also sug g ested that a lime w ash may b e g ive n o ve r the b lack co ating to p re ve nt he at ab so rp tio n. Memb rane curing is a g o o d metho d o f maintaining a satisfacto ry state o f w etness in the b o d y o f co ncre te to p ro mo te co ntinuo us hyd ratio n w he n o rig inal w ate r/ ce me nt ratio use d is no t le ss than 0 .5 . To achie ve b e st re sults, me mb rane is ap p lie d afte r o ne o r tw o d ays’ o f ac tual w e t c uring . Sinc e no re p le nishing o f w ate r is d o ne afte r the m e m b rane has b e e n

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a p p lie d it sh o u ld b e e n su re d th a t th e me mb rane is o f g o o d q uality and it is ap p lie d effectively. Tw o o r three co ats may be req uired fo r e ffe ctive se aling o f the surface to p re ve nt the e vapo ratio n o f w ate r. Eno ug h has b e e n w ritte n in Chap te r 5 o n the m o d e rn c uring c o m p o und s that are a va ila b le to d a y. In c re a se in vo lu m e o f co nstructio n, sho rtag e o f w ate r and ne e d fo r c o n se rvatio n o f w ate r, in c re ase in c o st o f lab o u r an d availab ility o f e ffe c tive c u rin g Curing vertical surface by wet covering. c o m p o u n d s h ave e n c o u rag e d th e u se o f curing co mpo unds in co ncrete co nstructio n. Curing co mpo und is an o bvio us cho ice fo r curing canal lining , slo p ing ro o fs and te xture d surface o f co ncre te p ave me nts. It is se e n th at th e re are so m e fe ar an d ap p re h e n sio n in th e m in d o f b uild e rs an d co ntracto rs reg arding the use o f membrane fo rming curing co mpo unds. No do ubt that curing c o m p o u n d s are n o t as e ffic ie n t an d as id e al as w ate r c u rin g . Th e e ffic ie n c y o f c u rin g co mpo unds can be at best be 80% o f w ater curing . But this 80% curing is do ne in a fo o lpro o f manne r. Altho ug h w ate r curing is id e al in the o ry, it is o fte n d o ne inte rmitte ntly and he nce , in re ality the e nvisag e d ad vantag e is no t the re , in w hic h c ase me mb rane c uring may g ive b e tte r re sults. Fo r furthe r d e tails re fe r Chap te r 5 w he re mo re info rmatio n ab o ut curing co mp o und s. Me tho d fo r d e te rmining the e fficie ncy o f curing co mp o und s e tc., are g ive n. Whe n w ate rp ro o fing p ap e r o r p o lye thyle ne film are use d as me mb rane , care must b e take n to se e that the se are no t p uncture d anyw he re and also se e w he the r ad e q uate lap ing is g ive n at the junctio n and this lap is e ffe ctive ly se ale d .

Application of heat The d e ve lo pme nt o f stre ng th o f co ncre te is a functio n o f no t o nly time b ut also that o f te mp e rature . Whe n co ncre te is sub je cte d to hig he r te mp e rature it acce le rate s the hyd ratio n pro cess resulting in faster develo pment o f streng th. Co ncrete canno t b e sub jected to dry heat to acce le rate the hyd ratio n pro ce ss as the pre se nce o f mo isture is also an e sse ntial re q uisite . The re fo re , sub je c ting the c o nc re te to hig he r te m p e rature and m aintaining the re q uire d w e tne ss can b e achie ve d b y sub je cting the co ncre te to ste am curing . A faste r attainm e nt o f stre ng th w ill c o ntrib ute to m any o the r ad vantag e s m e ntio ne d b e lo w. (a ) Co ncre te is vulne rab le to d amag e o nly fo r sho rt time . (b ) Co ncre te me mb e r can b e hand le d ve ry q uickly. (c ) Le ss sp ace w ill b e sufficie nt in the casting ye rd . (d ) A smalle r curing tank w ill b e sufficie nt. (e ) A hig he r o utturn is po ssib le fo r a g ive n capital o utlay. (f ) The w o rk can b e p ut o n to se rvice at a much e arly time , (g ) A few er numb er o f fo rmw o rk w ill b e sufficient o r alternatively w ith the g iven numb er o f fo rmw o rk mo re o utturn w ill b e achie ve d . (h ) Pre stre ssing b e d can b e re le ase d e arly fo r furthe r casting .

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Fro m the ab o ve me ntio ne d ad vantag e s it can b e se e n that ste am curing w ill g ive no t o nly eco no mical ad vantag es, b ut also technical ad vantag es in the matter o f prefab ricatio n o f co ncre te e le me nts. The e xp o sure o f co ncre te to hig he r te mp e rature is d o ne in the fo llo w ing manne r: (a ) Ste am curing at o rd inary p re ssure . (b ) Ste am curing at hig h p re ssure . (c ) Curing b y Infra-re d rad iatio n. (d ) Ele ctrical curing .

Steam curing at ordinary pressure This me tho d o f curing is o fte n ad o p te d fo r p e fab ric ate d c o nc re te e le m e n ts. Ap p lic a tio n o f ste a m curing to in situ co nstructio n w ill b e a little d iffic u lt task. Ho w e ve r, at so me p lace s it has b e e n trie d fo r in situ c o n stru c tio n b y fo rm in g a ste a m ja c ke t w ith th e h e lp o f ta rp a u lin o r th ic k p o lye th yle n e sh e e ts. Bu t th is m e th o d o f applicatio n o f steam fo r in situ w o rk is fo u n d to b e w aste fu l an d th e in te n d e d rate o f d e ve lo p m e n t o f stre n g th an d b e n e fit is n o t re ally achie ve d .

Beam under steam curing.

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Ste am curing at o rd inary p re ssure is ap p lie d mo stly o n p re fab ricate d e le me nts sto re d in a chamber. The chamber sho uld be big eno ug h to ho ld a day’s pro ductio n. The do o r is clo sed and ste am is ap p lie d . The ste am m ay b e ap p lie d e ithe r c o ntinuo usly o r inte rm itte ntly. An accelerated hyd ratio n takes place at this hig her temperature and the co ncrete pro d ucts attain the 2 8 d ays stre ng th o f no rmal co ncre te in ab o ut 3 d ays. In larg e p re fab ric ate d fac to rie s the y have tunne l c uring arrang e me nts. The tunne l o f su ffic ie n t le n g th an d size is m ain tain e d at d iffe re n t te m p e ratu re startin g fro m a lo w temperature in the b eg inning o f the tunnel to a maximum temperature o f ab o ut 9 0 ° C at the e nd o f the tunne l. The c o nc re te p ro d uc ts m o unte d o n tro llie s m o ve in a ve ry slo w sp e e d subjecting the co ncrete pro ducts pro g ressively to hig her and hig her temperature. Alternatively, the tro llie s are ke p t statio narily at d iffe re nt zo ne s fo r so m e p e rio d and finally c o m e o ut o f tunne l. The influe nce o f curing te mp e rature o n stre ng th o f co ncre te is sho w n in Fig . 6 .2 6 and 6 .2 8 . 6 .5 It is interesting to no te that co ncrete sub jected to hig her temperature at the early perio d o f hyd ratio n is fo und to lo se so me o f the streng th g ained at a later ag e. Such co ncrete is said to und e rg o “Re tro g re ssio n o f Stre ng th”. Fig ure 6 .2 9 sho w s the e ffe c t o f te m p e rature o n stre ng th o f co ncre te . It can b e se e n fro m Fig ure 6 .2 9 that the co ncre te sub je cte d to hig he r te mp e rature at e arly ag e , no d o ub t attains hig he r stre ng th at a sho rte r d uratio n, b ut suffe rs c o nsid e rab le re tro g re ssio n o f stre ng th. Fig . 6 .2 9 . O n the c o ntrary, c o nc re te c ure d at a c o m p arative ly lo w e r te m p e rature take s lo ng e r tim e to d e ve lo p stre ng th b ut the stre ng th attaine d w ill no t b e lo st at late r ag e s. The phe no me no n o f re tro g re ssio n o f stre ng th e xplains that faste r hyd ratio n w ill re sult in the fo rm atio n o f p o o r q uality g e ls w ith p o ro us o p e n structure, w hereas the g el fo rmed slo w ly but steadily at lo w er temperature are o f g o o d q uality w hich are co mpact and dense in nature. This aspect can be co mpared to the g ro w th o f w o o d

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ce lls. It is co mmo n kno w le d g e that a tre e w hich g ro w s faste r, w ill yie ld timb e r o f p o o r and no n-d urab le q uality, w he re as a tre e , w hic h g ro w s slo w ly w ill yie ld g o o d d urab le tim b e r. Similarly, co ncre te sub je cte d to hig he r te mp e rature in the e arly p e rio d o f hyd ratio n w ill yie ld p o o r q uality g e ls and co ncre te w hich is sub je cte d to rathe r lo w te mp e rature (say ab o ut 1 3 d e g re e Ce ntig rad e ) w ill yie ld the b e st q uality g e l, and he nce g o o d co ncre te . It has b e e n e mp hasize d that a ve ry yo ung co ncre te sho uld no t b e sub je cte d sud d e nly to hig h te mp e rature . Ce rtain amo unt o f d e lay p e rio d o n casting the co ncre te is d e sirab le . It has b e e n fo und that if 4 9 ° C is re ac he d in a p e rio d sho rte r than 2 to 3 ho urs o r 9 9 ° C is re ache d in le ss than 6 to 7 ho urs fro m the time o f mixing , the g ain o f stre ng th b e yo nd the first fe w ho urs is e ffe cte d ad ve rse ly. The stre ng th o f such rapid ly he ate d co ncre te falls in the zo ne B and the streng th o f g rad ually heated co ncrete falls w ithin the zo ne A in Fig ure. 6 .3 0 . Co nc re te sub je c te d to ste am c uring e xhib its a slig htly hig he r d rying shrinkag e and mo isture mo ve me nt. Sub je cting the co ncre te to hig he r te mp e rature may also slig htly e ffe ct the ag g reg ate q uality in case o f so me artificial ag g reg ate. Steam curing o f co ncrete made w ith rapid hardening cement w ill g enerate a much hig her heat o f hydratio n. Similarly, richer mixes may have mo re ad ve rse e ffe ct than that o f le an mixe s.

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In India, steam curing is o ften ado pted fo r precast elements, specially prestressed co ncrete sle e p e rs. Co nc re te sle e p e rs are b e ing intro d uc e d o n the e ntire Ind ian Railw ay. Fo r rap id develo pment o f streng th, they use special type o f cement namely IRST 40 and also sub ject the sle e p e rs to ste am curing . Larg e numb e r o f b rid g e s are b e ing b uilt fo r infrastructural d e ve lo p me nt in Ind ia. The re are re q uire m e nts fo r c asting o f innum e rab le p re c ast p re stre sse d g ird e rs. The se g ird e rs are steam cured fo r faster develo pment o f streng th w hich has many o ther asso ciated advantag es. A ste am-curving cycle co nsists o f: "

an initial d e lay prio r to ste aming ,

"

a p e rio d fo r incre asing the te mp e rature ,

"

a p e rio d fo r re taining the te mp e rature ,

"

a p e rio d fo r d e cre asing the te mp e rature .

A typ ical ste am curing cycle at o rd inary p re ssure is sho w n Fig . 6 .3 1 and typ ical stre ng th o f ste am cure d co ncre te at d iffe re nt te mpe rature in sho w n in Fig . 6 .3 2 .

High Pressure Steam Curing In the ste am curing at atmo sp he ric p re ssure , the te mp e rature o f the ste am is naturally b e lo w 1 0 0 ° C. The ste am w ill g e t co nve rte d into w ate r, thus it can b e calle d in a w ay, as ho t w ate r curing . This is d o ne in an o p e n atmo sp he re . The hig h pressure steam curing is so mething different fro m o rdinary steam curing , in that the curing is carrie d o ut in a clo se d chamb e r. The sup e rhe ate d ste am at hig h p re ssure and hig h te mp e rature is ap p lie d o n the c o nc re te . This p ro c e ss is also c alle d “Auto c laving ”. The auto claving pro cess is practised in curing precast co ncrete pro ducts in the facto ry, particularly, fo r the lig htw eig ht co ncrete pro d ucts. In Ind ia, this hig h pressure steam curing is practised in the manufacture o f ce llular co ncre te p ro d ucts, such as Sip o re x, Ce lcre te e tc. The fo llo w ing ad vantag e s are d e rive d fro m hig h p re ssure ste am curing p ro ce ss: (a ) Hig h pressure steam cured co ncrete develo ps in o ne day, o r less the streng th as much as the 28 days’ streng th o f no rmally cured co ncrete. The streng th develo ped do es no t sho w re tro g re ssio n. (b ) Hig h p re ssure ste am c ure d c o nc re te e xhib its hig he r re sistanc e to sulp hate attac k, fre e zing and thaw ing actio n and che mical actio n. It also sho w s le ss e fflo re sce nce . (c ) Hig h p re ssure ste am cure d co ncre te e xhib its lo w e r d rying shrinkag e , and mo isture mo ve me nt. In hig h pressure steam curing , co ncrete is subjected to a maximum temperature o f abo ut 1 7 5 ° C w hich co rre spo nd s to a ste am pre ssure o f ab o ut 8 .5 kg / sq .cm. When the co ncrete is to b e sub jected to hig h pressure steam curing , it is invariab ly made b y admixing w ith 20 to 30 per cent o f po zzo lanic material such as crushed sto ne dust. In case o f no rmal c uring , the lib e ratio n o f Ca(O H)2 is a slo w p ro c e ss. The re fo re , w he n p o zzo lanic mate rials are ad d e d , the p o zzo lanic re activity also w ill b e a slo w p ro ce ss. But in case o f hig h p re ssure ste am curing a g o o d amo unt o f Ca(O H)2 w ill b e lib e rate d in a ve ry sho rt time and re actio n b e tw e e n Ca(O H)2 and p o zzo lanic mate rial take s p lace in an acce le rate d manne r. A g o o d amo unt o f te chnical ad vantag e is achie ve d b y ad mixing the co ncre te w ith p o zzo lanic mate rial. Hig h pressure steam curing exhibits hig her streng th and durability particularly in the case o f cement co ntaining a pro po rtio nately hig her amo unt o f C3 S. A sample o f cement co ntaining

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hig her pro po rtio n o f C2 S is no t b enefited to the same extent, as it pro d uces lo w er amo unt o f Ca(O H)2 It is also o b se rve d that imp ro ve me nt in d urab ility is mo re fo r the c o nc re te mad e w ith hig he r w ate r/ ce me nt ratio , than fo r the co ncre te mad e w ith lo w w ate r/ ce me nt ratio . O w ing to the co mbinatio n o f Ca(O H)2 w ith siliceo us material w ithin a matter o f 24 ho urs in the case o f hig h ste am curing , co ncre te b e co me s impe rvio us and he nce d urab le . The fact is that the c o nc re te in the ab se nc e o f fre e Calc ium Hyd ro xid e b e c o m e s d e nse and le ss p e rme ab le , and also acco unts fo r hig he r che mical re sistance and hig he r stre ng th. The hig he r rate o f d e ve lo p me nt o f stre ng th is attrib ute d to the hig he r te mp e rature to w hich a co ncrete is sub jected . Earlier it is b ro ug ht o ut that if the co ncrete is sub jected to very hig h te mp e rature , p articularly in the e arly p e rio d o f hyd ratio n, mo st o f the stre ng th g aine d w ill b e lo st b e cause o f the fo rmatio n o f p o o r q uality g e l. The ab o ve is true fo r ste am cure d co ncre te at atmo sp he ric p re ssure . The hig h p re ssure ste am cure d co ncre te d o e s no t e xhib it re tro g re ssio n o f stre ng th. The po ssib le e xplanatio n is that in the case o f hig h pre ssure ste am curing , the q uality and unifo rmity o f p o re structure fo rme d is d iffe re nt. At hig h te mp e rature the amo rp ho us calcium silicate s are p ro b ab ly co nve rte d to crystalline fo rms. Pro b ab ly d ue to hig h pressure the frame w o rk o f the g el w ill b eco me mo re co mpact and d ense. This perhaps e xp lains w hy the re tro g re ssio n o f stre ng th d o e s no t take p lace in the case o f hig h p re ssure ste am curing . In o rd inarily cure d co ncre te , the sp e cific surface o f the g e l is e stimate d to b e ab o ut tw o millio n sq cm per g ram o f cement, w hereas in the case o f hig h pressure steam cured co ncrete, the specific surface o f g el is in the o rder o f seventy tho usand sq cm per g ram. In o ther w o rds, the g els are ab o ut 2 0 times co arser than o rd inarily cured co ncrete. It is co mmo n kno w led g e, that fine r mate rial shrinks mo re than co arse r mate rial. The re fo re , o rd inary co ncre te mad e up o f fine r g e ls shrinks mo re than hig h p re ssure ste am cure d co ncre te mad e up o f co arse r g e l. In q uantitative terms, the hig h pressure steam cured co ncrete und erg o es shrinkag e o f 1 / 3 to 1 / 6 o f that o f co ncre te cure d at no rmal te mp e rature . Whe n p o zzo lanic mate rial is ad d e d to the mix, the shrinkag e is fo und to b e hig her, b ut still it shrinks o nly ab o ut 1/ 2 o f the shrinkag e o f no rmally cure d co ncre te . Due to the ab se nce o f fre e calcium hyd ro xid e no e fflo re sce nce is se e n in case o f hig h p re ssure ste am cure d co ncre te . Due to the fo rmatio n o f co arser g el, the b o nd streng th o f co ncrete to the reinfo rcement is re d uce d b y ab o ut 3 0 p e r ce nt to 5 0 p e r ce nt w he n co mp are d w ith o rd inary mo ist-cure d co ncre te . Hig h p re ssure ste am cure d co ncre te is rathe r b rittle and w hitish in co lo ur. O n the w ho le , hig h p re ssure ste am curing p ro d uce s g o o d q uality d e nse and d urab le co ncre te : The co ncrete pro d ucts as mo uld ed w ith o nly a co uple o f ho urs d elay perio d is sub jected to maximum temperature o ver a perio d o f 3 to 5 ho urs. This is fo llo w ed by abo ut 5 to 8 ho urs at this te m p e rature . Pre ssure and te m p e rature is re ale ase d in ab o ut o ne ho ur. The d e tail steaming cycle d epend s o n the plant, q uality o f material thickness o f memb er etc. The leng th o f d elay perio d b efo re sub jecting to hig h pressure steam curing d o es no t materially affect the q uality o f hig h p re ssure ste am cure d co ncre te .

Curing by Infra-red Radiation Curing o f co ncrete by Infra-red Radiatio n has been practised in very co ld climatic reg io ns in Russia. It is claimed that much mo re rapid g ain o f streng th can be o btained than w ith steam curing and that rap id initial te mp e rature d o e s no t cause a d e cre ase in the ultimate stre ng th

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as in the case o f ste am curing at o rd inary p re ssure . The syste m is ve ry o fte n ad o p te d fo r the curing o f ho llo w co ncrete pro d ucts. The no rmal o perative temperature is kept at ab o ut 90°C.

Electrical Curing Ano the r m e tho d o f c uring c o nc re te , w hic h is ap p lic ab le m o stly to ve ry c o ld c lim atic re g io ns is the use o f e le ctricity. This me tho d is no t like ly to find much applicatio n in o rd inary climate o w ing to e co no mic re aso ns. Co ncre te can b e cure d e le ctrically b y p assing an alte rnating curre nt (Ele ctro lysis tro ub le w ill b e e n c o un te re d if d ire c t c urre n t is use d ) th ro ug h th e c o n c re te itse lf b e tw e e n tw o e le ctro d e s e ithe r b urie d in o r ap p lie d to the surface o f the co ncre te . Care must b e take n to p re ve nt the mo isture fro m g o ing o ut le aving the co ncre te co mp le te ly d ry. As this me tho d is no t like ly to b e ad o pte d in this co untry, fo r a lo ng time to co me , this aspe ct is no t d iscusse d in d e tail.

Miscellaneous Methods of Curing Calcium chlo rid e is used either as a surface co ating o r as an ad mixture. It has b een used satisfac to rily as a c uring m e d ium . Bo th the se m e tho d s are b ase d o n the fac t that c alc ium c hlo rid e b e ing a salt, sho w s affinity fo r mo isture . The salt, no t o nly ab so rb s mo isture fro m atmo sp he re b ut also re tains it at the surface . This mo isture he ld at the surface p re ve nts the mixing w ater fro m evapo ratio n and thereby keeps the co ncrete w et fo r a lo ng time to pro mo te hyd ratio n. Fo rmw o rk pre ve nts e scaping o f mo isture fro m the co ncre te , particularly, in the case o f beams and co lumns. Keeping the fo rmw o rk intact and sealing the jo int w ith w ax o r any o ther se aling co mp o und p re ve nts the e vap o ratio n o f mo isture fro m the co ncre te . This p ro ce d ure o f p ro mo ting hyd ratio n, can b e co nsid e re d as o ne o f the misce llane o us me tho d s o f curing .

When to Start Curing and how Long to Cure Many a time an e ng ine e r at site w o nd e rs, ho w e arly he sho uld start curing b y w ay o f applicatio n o f w ater. This pro b lem arises, particularly, in case o f ho t w eather co ncreting . In an arid reg io n, co ncrete placed as a ro ad slab o r ro o f slab g ets d ried up in a very sho rt time, say w ithin 2 ho urs. O fte n q ue stio ns are aske d w he the r w ate r c an b e p o ure d o ve r the ab o ve co ncrete w ithin tw o ho urs to prevent the drying . The asso ciated pro blem is, if w ater is applied w ithin say tw o ho urs, w hether it w ill interfere w ith the w ater/ cement ratio and cause harmful e ffe cts. In o the r w o rd s, q ue stio n is ho w e arly w ate r can b e ap p lie d o ve r co ncre te surface so that uninterrupted and co ntinued hydratio n takes place, w itho ut causing interference w ith the w ate r/ ce me nt ratio . The answ e r is that first o f all, co ncre te sho uld no t b e allo w e d to d ry fast in any situatio n. Co ncre te that are liab le to q uick d rying is re q uire d to b e co ve re d w ith w e t g unny b ag o r w e t he ssian clo th p ro p e rly sq ue e ze d , so that the w ate r d o e s no t d rip and at the same time , d o e s no t allo w the co ncre te to d ry. This co nd itio n sho uld b e maintaine d fo r 24 ho urs o r at least till the final setting time o f cement at w hich duratio n the co ncrete w ill have assume d the final vo lume . Eve n if w ate r is p o ure d , afte r this time , it is no t g o ing to inte rfe re w ith the w ater/ cement ratio . Ho w ever, the best practice is to keep the co ncrete under the w et g unny b ag fo r 2 4 ho urs and the n co mme nce w ate r curing b y w ay o f p o nd ing o r sp raying . O f co urse, w hen curing co mpo und is used immed iately after b leed ing w ater, if any, d ries up, the q ue stio n o f w he n to start w ate r curing d o e s no t arise at all. The re is a w ro ng no tio n w ith co mmo n b uild e rs that co mme nce me nt o f curing sho uld b e d o n e o n ly o n th e fo llo w in g d ay afte r c o n c re tin g . Eve n o n th e n e xt d ay th e y m ake arrang e me nts and b uild b und s w ith mud o r le an mo rtar to re tain w ate r. This furthe r d e lays

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the curing . Such practice is fo llo w ed fo r co ncrete ro ad co nstructio n b y municipal co rpo ratio ns also . It is a b ad practice. It is d ifficult to set time frame ho w early w ater curing can b e started . It d e p e nd s o n, p re vailing te mp e rature , humid ity, w ind ve lo city, typ e o f ce me nt, fine ne ss o f ce me nt, w / c use d and size o f me mb e r e tc. The p o int to o b se rve is that, the to p surface o f c o n c re te sh o uld n o t b e allo w e d to d ry. En o ug h m o isture m ust b e p re se n t to p ro m o te hydratio n. To satisfy the abo ve co nditio ns any practical steps can be undertaken, including the ap p licatio n o f fine sp ray o r fo g g ing w itho ut d isturb ing surface finish. Such me asure s may b e take n as e arly as tw o ho urs afte r casting . It is p o inte d o ut that e arly curing is imp o rtant fo r 5 3 g rad e ce me nt. Incid entally, it is seen that test cub es cast at site are allo w ed to d ry w itho ut co vering the to p w ith w e t co ve ring . The y are allo w e d to d ry in the ho t sun. Such cub e s d e ve lo p cracks and sho w lo w stre ng th w he n crushe d . It is usual that the y co mp lain ab o ut p o o r q uality o f ce me nt o r co ncre te . Re g ard ing ho w lo ng to c ure , it is ag ain d iffic ult to se t a lim it. Sinc e all the d e sirab le p ro p e rtie s o f c o nc re te are im p ro ve d b y c uring , the c uring p e rio d sho uld b e as lo ng as p ractical. Fo r g e ne ral g uid ance , co ncre te must b e cure d till it attains ab o ut 7 0 % o f sp e cifie d stre ng th. At lo w e r te mp e rature curing p e rio d must b e incre ase d . Since the rate o f hydratio n is influenced b y cement co mpo sitio n and fineness, the curing p e rio d sh o u ld b e p ro lo n g e d fo r c o n c re te s m ad e w ith c e m e n ts o f slo w stre n g th g ain characte ristics. Po zzo lanic ce me nt o r co ncre te ad mixe d w ith p o zzo lanic mate rial is re q uire d to be cured fo r lo ng er duratio n. Mass co ncrete, heavy fo o ting s, larg e piers, abutments, sho uld b e cure d fo r at le ast 2 w e e ks.

Finishing of road pavement

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To asse rtain the pe rio d o f curing o r stripping o f fo rmw o rk, cub e s o r b e ams are cast and ke p t ad jace nt to the structure the y re p re se nt and cure d b y the same me tho d . The stre ng th o f these cubes o r beams at different intervals o f time w o uld g ive better idea abo ut the streng th d e ve lo p m e n t o f stru c tu re s. Th e ab o ve m e th o d d o e s n o t tru ly in d ic ate th e stre n g th d evelo pment o f massive g ird er sub jected to steam curing b ecause o f size d ifference o f cub es and g ird e rs.

Finishing Finishing o peratio n is the last o peratio n in making co ncrete. Finishing in real sence d o es no t apply to all co ncrete o peratio ns. Fo r a b eam co ncreting , finishing may no t b e applicab le, w he re as fo r the co ncre te ro ad pave me nt, airfie ld pave me nt o r fo r the flo o ring o f a d o me stic b uilding , careful finishing is o f g reat impo rtance. Co ncrete is o ften dub b ed as a drab material, inc ap ab le o f o ffe ring p le asant arc hite c tural ap p e aranc e and finish. This sho rtc o m ing o f c o nc re te is b e ing re c tifie d and c o nc re te s the se d ays are m ad e to e xhib it p le asant surfac e finishes. Particularly, many types o f prefabricated co ncrete panels used as flo o r slab o r w all unit are mad e in such a w ay as to g ive very attractive architectural affect. Even co ncrete clad d ing s are mad e to g ive attractive lo o k. In re ce nt ye ars the re has b e e n a g ro w ing te nd e ncy to d e ve lo p and use vario us surface tre atme nts w hich p e rmit co ncre te structure s to p ro ud ly p ro c laim its nature inste ad o f c o ve ring itse lf w ith an expensive veneer. The pro perty o f co ncrete to repro duce fo rm m arkin g s su c h as b o ard m ark fin ish e s, u se o f lining s o r special types o f fo rmw o rks, special techniq ues fo r th e a p p lic a tio n o f a p p lie d fin ish e s h a ve b e e n enco urag ed. Surface finishes may be g ro uped as under: (a ) Fo rmw o rk Finishe s (c ) Ap p lie d Finishe s.

(b ) Surface Tre atme nt

Formwork Finishes Co n c re te o b e ys th e sh a p e o f fo rm w o rk i. e . , centering w o rk. By judicio usly assemb ling the fo rmw o rk either in plane surface o r in undulated fashio n o r having th e jo in ts in a p artic u lar “V” sh ap e d m an n e r to g e t re g ular fins o r g ro ve s, a p le asing surface finish can b e g ive n to c o nc re te . The arc hite c t’s imag inatio ns c an b e fu lly e xp lo ite d to g ive m an y varie tie s o f lo o k to th e c o nc re te surfac e . The use o f sm all b atte ns c an g ive a g o o d lo o k to the co ncre te surface . A p re -fa b ric a te d w a ll u n it c a st b e tw e e n ste e l fo rm w o rk h avin g ve ry sm o o th su rfac e u sin g rig h t p ro p o rtio ning o f mate rials can g ive such a nice surface w h ic h c an n e ve r b e o b tain e d b y th e b e st m aso n s. Similarly, the p re fab ricate d flo o r units can have such a fine finish at the c e iling w hic h c anno t b e o b taine d b y the b e st maso ns w ith the b e st e ffo rts. The se d ays w ith Mechanical trowel for finishing factory floor. th e c o st o f lab o u r g o in g u p , atte n tio n is n atu rally Sometimes surface hardener is d ire c te d to the se lf-finishing o f the c o nc re te surfac e , sprinkled and finished.

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p artic ularly, fo r flo o r slab s, b y the use o f g o o d fo rm w o rk m ate rial suc h as ste e l she e ts o r shutte ring typ e p lyw o o d .

Surface Treatment This is o ne o f the w idely used metho ds fo r surface finishing . The co ncrete pavement slab is re q uire d to b e p lane b ut ro ug h to e xhib it skid re sistance , so is the air-fie ld p ave me nts and ro ad slab s. Co nc re te having b e e n b ro ug ht to the p lane le ve l surfac e , is rake d lig htly o r b ro o me d o r te xture d o r scratche d to make the surface ro ug h. A d o m e stic flo o r slab is re q uire d to b e sm o o th , w e ar re sistin g an d c rac k-fre e . Th e te c h n iq u e o f fin ish in g th e c o n c re te flo o r re q u ire s ve ry c a re fu l c o n sid e ra tio n s. Th e p ro p o rtio ning o f the mix must b e ap p ro p riate w itho ut e xce ss o r d e ficie nt o f matrix. Wate r/ cement ratio sho uld be such that it pro vides the just req uired co nsistency to facilitate spreading and g o o d levelling , yet to g ive no b leed ing . Surface must b e finished at the same rate as the placing o f co ncre te . Particular care must b e take n to the e xte nt and time o f tro w e lling . Use o f w o o d e n flo at is b e tte r to start w ith b ut at the e nd ste e l tro w e l m ay b e use d . In all the o pe ratio n, care must b e take n to se e that no laitance is fo rme d and no e xce ssive mo rtar o r w ater accumulates o n the surface o f the flo o r, w hich red uces the w ear resistance o f the flo o r. The e xce ss o f mo rtar at the surface cause s crazine ss d ue to incre ase d shrinkag e . Achie ving a g o o d surface finish to a co ncrete flo o r re q uires co nsid erab le experience and d evo tio n o n the part o f the maso n. A hurrie d co mple tio n o f surface o pe ratio n w ill make a po o r surface . O fte n co ncre te is p lace d at a much faste r rate than the sp e e d o f finish b y the maso ns w ith the re sult that co ncre te d rie s up and maso n is no t ab le to b ring the co ncre te to a g o o d le ve l. He re so rts to ap p lying e xtra rich mo rtar to b ring the flo o r surface to g o o d le ve l. This practice o f applying rich mo rtar spcecially mad e to the surface is no t d esirab le. This practice, firstly re d uc e s th e b o n d . Se c o n d ly, re d uc e s th e stre ng th and w e ar re sistanc e than the h o m o g e n e o us c o n c re te . Th ird ly, m o rtar sh rin ks m o re th an th at o f c o n c re te . Th e re fo re , applicatio n o f a thick laye r o f mo rtar o ve r se t co ncre te is o b je ctio nab le . It is a g o o d p rac tic e to the finish the flo o r w ith the m atrix that c o m e s to the to p o f the c o nc re te d ue to the c o m p a c tio n o f c o n c re te a n d b y w o rking w ith maso n’s tamping rule . In case the ab o ve is no t po ssib le, use o f e xtra mo rtar may b e pe rmitte d to avo id ve ry p o o r surface finish. But it is necessary to o b serve the fo llo w ing p re cautio ns: (a ) The m o rtar c o m p o sitio n b e th e sa m e a s th a t o f co ncre te . (b ) It sho uld b e ap p lie d as thin Exposed aggregate finish a layer as po ssible befo re the b ase co ncre te is hard e ne d , and rub b e d smo o th. (c ) Sp rinkling o f d ry ce me nt in g o o d q uantity is no t a g o o d p ractice , ho w e ve r a small q uantity may b e p e rmitte d to re d uce the b ad e ffe ct o f b le e d ing , taking care to se e that it d o e s no t make the to p laye r to o rich.

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Exp o se d Ag g rre e g ate -Finish: This is o ne o f the metho ds o f g iving g o o d lo o k to the co ncrete surface . The b e auty can b e furthe r e nhance d b y the use o f co lo ure d p e b b le s o r q uartz. O ne o r tw o days after casting , the matrix is remo ved by w ashing the surface w ith w ater o r by slig ht b rushing and w ashing . O ne face o f the ag g reg ate particles w ill ad here to the matrix and the o ther face g ets expo sed . This expo sed surface w ill g ive a pleasing lo o k. So metimes, retard ing ag ent is applied to the fo rmw o rk surface. The matrix at the surfac e b e ing in c o ntac t w ith the re tard ing ag e nt, d o e s no t g e t hard e ne d , w he re as re st o f the p o rtio n g e ts h a rd e n e d . O n w a sh in g o r lig h t b rushing , the unhard e ne d matrix g e ts w ashe d o ut, e xpo sing the ag g re g ate .

Applied finish work done at CME

So m e tim e s use o f h yd ro c h lo ric ac id so lutio n mad e up o f o ne part o f acid to six parts o f w ate r is used fo r w ashing the co ncrete surface to expo se the ag g reg ate. The acid attacks the cement and enab les it to b e b rushe d o ff. Care m ust b e take n that the w o rkm a n sh o u ld u se ru b b e r-g lo ve s a n d o n c o m p le tio n o f w ash in g b y th e ac id , th e su rfac e sho uld b e treated w ith alkaline so lutio n to neutralise an y re m ain in g ac id . Th is m e th o d o f u sin g ac id sh o u ld n o t b e ap p lie d fo r c o n c re te m ad e w ith lime sto ne ag g re g ate .

Bush Ham m e rin g : A Bush Hammer is a to o l w ith a series o f pyramid al teeth o n its face. They may b e hand o perated o r pneumatically o r electrically o perated . Hand to o ls are suitab le fo r small jo b s b ut p o w e r o p e rate d e q uip me nt is use d fo r larg e surface . Bush Hammer g ives rapid b lo w s to the co ncrete surface and no t o nly remo ves the o uter ce me nt film b ut also b re aks so me o f the e xp o se d ag g re g ate g iving a b rig ht, co lo urful and attrac tive su rfac e . Ve ry p le asan t e ffe c ts m ay b e o b tain e d b y c are fu lly arran g in g larg e ag g re g ate s at the surface and late r re mo ving the matrix b y b ush hamme r. Co nc re te sho uld b e at le ast thre e w e aks o ld , b e fo re it is b ush hamme re d . O the rw ise , the re is a d ang e r o f w ho le p ie c e s o f ag g re g ate s b e ing d islo d g e d . The q uality o f c o nc re te w hic h is to b e tre ate d this w ay b y b ush ham m e ring m ust b e o f hig h q uality and g o o d w o rkmanship . Ap p lie d Finish: The term applied finish is used to d eno te the applicatio n o f rend ering to the e xte rio rs o f co ncre te structure s. The co ncre te surface is cle ane d and ro ug hne d and ke p t w e t fo r sufficie ntly lo ng time . O ve r this a mo rtar o f p ro p o rtio n o f ab o ut 1 :3 is ap p lie d . This mo rtar re nd e ring can b e g ive n any re q uire d p le asant finish, such as ce me nt stip p ling e ithe r fine o r co arse, c o m b e d fin ish ish, keying , rend ering s etc. So me time s this re nd e ring ap p lie d o n w all is p re sse d w ith sp o ng e . The sp o ng e ab so rb s ce me nt and w ate r e xp o se s sand p article s. The sp o ng e is w ashe d and ag ain rub b e d ag ainst San d the surfac e . W ith the re p e titio n o f this p ro c e ss, the surfac e g e ts a finish, kno w n as “San Fac ing ”. A w et plastic mix o f three parts o f cement, , o ne part o f lime, six parts o f sand and 4 parts o f ab o ut 5 mm size peag ravel ag g reg ate is thro w n ag ainst w all surface b y means o f a sco o p Ro ug h Cast Fin ish o r plasterer’s tro w el. This finish is kno w n as “Ro ish”.

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Upo n a 10 mm thick co at o f o ne part o f cement, o ne part o f lime and five parts o f sand , w hile it is still p lastic, is thro w n ab o ut 6 mm size se le cte d w e ll-w ashe d p e b b le s. This kind o f Pe b b le Dash Dash”. finish is kno w n as “Pe e te The latest metho d o f finish g iven to the co ncrete surface is kno w n as “Fair Cre te” finish. Fair Cr This re nd e ring can b e g ive n to the co ncre te in situ o r b etter still to the co ncrete panels. Such p ane ls are use d as clad d ing to the co ncre te structure s. Fair cre te is no thing b ut a hig hly air-e ntraine d mo rtar (air-e ntrainme nt is to b e e xte nt o f 2 5 p e r ce nt) mixe d w ith cho p p e d jute fib re . This mo rtar is sp re ad and p re sse d b y a mo uld having d iffe re nt d e sig ns. The im p re ssio n o f the m o uld is translate d into the m o rtar. Air e ntraine d mo rtar b e ing fo amy in nature take s the impre ssio n o f the mo uld . A w id e varie ty o f d e sig ns and m urals c an b e translate d to the air e ntraine d m o rtar surfac e . The jute fib re incre ase s the te nsile stre ng th o f the fair cre te . Misc e llane o us Finishe s: No n-slip Finish: Surface o f ramps, railw ay platfo rms, surro unding s o f sw imming po o ls etc., are req uired to po sses a hig hly no nslip texture. To o b tain this q uality, an ab rasive g rit is sp rinkle d o ve r the surfac e d uring the flo ating o p e ratio ns. The surfac e is lig htly flo ate d just to e mb e d the ab rasive g rit at the surface . So me time s, e p o xy scre e d is also g ive n to the surface o ve r w hich silica sand is sp rinkle d w hile the e p o xy is still w e t.

Co lo ured Finish : Princip al mate rials use d fo r co lo uring co ncre te are : (a ) Pig me nt ad mixture s (c ) Paints

(b ) Che mical stains (d ) White ce me nt

(e ) Co lo ure d co ncre te .

Pig me nt ad mixture s may b e ad d e d inte g rally to the to p p ing mix, b le nd e d w ith the d ry cement, o r pig ments may b e d usted o n to the to pping immed iately o n applicatio n o f screed . O f all the m e tho d s, m ixing inte g rally w ith the m o rtar is the b e st m e tho d , ne xt to using c o lo ure d c e m e nts. So m e tim e s, c e rtain c he m ic als are use d to g ive d e sirab le c o lo ur to the co ncre te surface . Similarly, ce me nt b ase d paints o r o the r co lo ur paints are also use d . White ce me nt is use d in d iffe re nt w ays to g ive d iffe re nt lo o k to the co ncre te . White and co lo ure d ce me nts have b e e n use d as to p p ing s in facto ry flo o r finish. Recently RMC (Ind ia), a Read y Mix Co ncrete supplying co mpany, have started supplying Re ad y Mixe d c o lo ure d c o nc re te in vario us c o lo urs. The y inc o rp o rate c e rtain p e rc e ntag e o f fib res in this co ncrete to take care o f shrinkag e cracks and to impart o ther d esirab le pro perties to the c o nc re te .. Suc h c o lo ure d c o nc re te c an b e use d ind o o rs o r o utd o o r ap p lic atio n as a sub stitute to o rd inary co ncre te . W e ar Re sistan t Flo o r Fin ish : A w e ar re sisting q uality o f a co ncre te flo o r surface can b e impro ved b y using so lutio ns o f certain chemicals kno w n as “Liq uid Hard erners”. They includ e, fluo silicates o f mag nesium and zinc, so d ium silicate, g ums and w axes. When the co mpo und s p e ne trate the p o re s in the to p p ing , the y fo rm crystalline o r g ummy d e p o sits and thus te nd to make the flo o r less pervio us and reduce dusting either by acting as plastic binders o r making the surface hard e r. So me time s, iro n filing and iro n chips are mixe d w ith the to pping s and the flo o r is mad e in a no rmal manner. The rusting o f the iro n filing s and chips increases in vo lume and thereb y make s the co ncre te d e nse g iving the flo o r b e tte r w e ar re sistance . The y are kno w n as “Iro nite flo o r to p p ing s”. Fib re re info rc e d c o nc re te also has d e m o nstrate d a b e tte r w e ar-re sistanc e q uality in case o f ro ad and airfield slab s. We have already discussed ab o ut Wear Resistant flo o r finish in Chap te r 5 und e r c o nstruc tio n c he m ic als. Atte ntio n is d raw n to the p re se nt d ay availab ility and applicatio n Epo xy Paint, Epo xy mo rtar, se lf le ve lling Epo xy scre e d e tc., fo r the

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use o f We ar Re sistant flo o r and d e c o rative flo o r finish. The y are w id e ly use d in m o d e rn co nstructio n. Re q u ir e m e n t o f a g o o d fin ish : A g o o d co ncre te flo o r sho uld have a surface w hich is ire d urab le, no n-ab so rptive, suitab le texture, free fro m cracks, crazing and o ther d efects. In o ther w o rd s, the flo o r sho uld satisfac to rily w ithstand w e ar fro m traffic . It sho uld b e suffic ie ntly impervio us to passag e o f w ater, o ils o r o ther liq uids. It sho uld po ssess a texture in keeping w ith the re q uire d ap p e aranc e , sho uld b e e asy to c le an and b e safe ag ainst slip p ing . It sho uld structurally b e so und and must act in uniso n w ith sub -flo o r. G rin d in g an d Po lish in g : Flo o rs w he n p ro p e rly c o nstruc te d using m ate rials o f g o o d q uality, are d ustle ss, d e nse , e asily c le ane d and attrac tive in ap p e aranc e . W he n g rind ing is sp e c ifie d , it sh o u ld b e starte d afte r th e su rfac e h as h ard e n e d su ffic ie n tly to p re ve n t dislo dg ement o f ag g reg ate particles and sho uld b e co ntinued until the co arse ag g reg ates are e xp o se d . The m ac hine use d sho uld b e o f ap p ro ve d typ e w ith sto ne s that c ut fre e ly and rap id ly. The flo o r is ke p t w e t d uring the g rind ing p ro ce ss and the cutting s are re mo ve d b y sp raying and flushing w ith w ate r. Afte r the surfac e is g ro und , air ho le s, p its and the o the r b lemishes are filled w ith a thin g ro ut co mpo sed o f o ne part o f fine carb o rundum g rit and o ne part o f po rtland cement. This g ro ut is spread o ver the flo o r and w o rked into the pits w ith the straig ht edg e after w hich it is rubbed into the flo o r w ith a g rinding machine. When the filling s are hard e ne d fo r se ve n d ays, the flo o r is g ive n final g rind ing . Crazin e ss: While w e are d iscussing ab o ut the surface finish it w ill b e pertinent to d iscuss the crazine ss i.e. , the d e ve lo pme nt o f fine shallo w hair cracks o n co ncre te surface . The surface appe arance o f co ncre te is o fte n spo ilt b y a fairly clo se patte rn o f hair cracks w hich may appe ar w ithin the first ye ar, o ccasio nally afte r lo ng e r pe rio d s. The cracks d o no t penetrate d eep into the co ncrete and d o n o t in d ic ate an y stru c tu ral w e akn e ss. The y are mo st o b vio us imme d iate ly afte r th e su rfac e o f th e c o n c re te h as d rie d w he n w e tte d the y b e co me p ro mine nt. It is no t p o ssib le to state any p re c autio ns w hich w ill definitely prevent craziness b ut its o c c u rre n c e c a n b e m in im ise d . Crazine ss is d ue to d rying shrinkag e o r c a rb o n a tio n o r d u e to d iffe re n tia l sh rin kag e b e tw e e n th e su rfac e o f th e c o n c re te a n d th e m a in b o d y o f th e c o n c re te . Th is d iffe re n tial sh rin kag e is acce ntuate d if the skin is riche r than the p are nt c o nc re te . It is kno w n that d rying Craziness in the surface of concrete. shrinkag e is g re ate st w he n the co ncre te d rie s u p fa st a fte r c a stin g . Th e first precautio n to take, therefo re, is very careful curing so that the initial drying perio d is extended o ve r as lo ng a time as po ssib le so that the shrinkag e o f the o ute r skin is ke pt in co nfo rmity w ith the shrinkag e o f the main b o d y o f the co ncre te . Ste e p mo isture g rad ie nts b e tw e e n the surface and the inte rio r o f the co ncre te must b e avo id e d if p o ssib le . Cracking w ill no t o ccur if the co ncre te is sufficie ntly stro ng to re sist the te nsile fo rce s cause d b y d iffe re ntial shrinkag e b ut it d o e s no t ap p e ar p o ssib le to p re ve nt crazing b y making a ve ry stro ng co ncre te .

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The o b je ct must the re fo re b e to minimise shrinkag e o f the surface skin and this is b e st achie ve d b y ad e q uate curing and b y taking me asure s to p re ve nt shrinkag e b y avo id ing to o rich surface . The fo llo w ing p re cautio ns w ill he lp g re atly: (a) Tro w elling the surface as little as po ssib le and in particular avo id ing the use o f a steel flo at. (b ) Avo id ing the use o f rich facing mixe s, say, no t riche r than 1 :3 . (c ) Use o f as lo w a w ater-cement ratio as po ssib le co nsistent w ith ad eq uate co mpactio n. (d ) Avo id ing g ro uting p ro ce sse s o r rub b ing the surface w ith ne at ce me nt p aste . (e ) O ver vib ratio n w hich results in b ring ing to o much slurry to the to p o r sid e. (ad jacent to fo rmw o rk). Crazing may also b e d ue to carb o natio n and the rmal e ffe cts. A ce me nt-rich skin is liab le to expand and co ntract mo re w ith difference in temperature than the interio r o f the co ncrete. The w e tting and d rying pro ce ss is, ho w e ve r, a far mo re po te nt facto r fo r causing crazine ss. The mo st imp o rtant cause s o f crazing are the rmal stre sse s and lo ng te rm d rying shrinkag e .

Whisper Concrete Finish : O ne o f the disadvantag es o f Co ncrete Ro ads is that they pro duce lo t o f no ise w he n ve hicle s trave l at hig h sp e e d , d ue to frictio n b e tw e e n tyre s and hard ro ad surface . In Euro p e the no ise le ve l has b e co me into le rab le to the p e o p le living b y the sid e o f ro ads w here abo ut a lakh o f vehicles mo ve at a speed o f abo ut 120 km per ho ur. Belg ium w as the first co untry to take me asure to re d uce no ise po llutio n. It may be recalled that, texturing o r bro o ming is do ne as a surface finish fo r the new ro ad pavement co nstructio n to pro vid e skid resistance. O ver the time, the texturing g ets w o rn o ut and the surnace b e co me s smo o th. Whe n it rains, the po o l o f w ate r o n the smo o th co ncre te surfac e c ause s a p he no m e no n c alle d “Hyd ro p laning ” w he n ve hic le m o ve at hig h sp e e d , w hich re sults in lo ss o f co ntro l and skid d ing . Co ncre te ro ad s ne e d s ro ug he ning and re surfacing afte r so me ye ars o f use . This is d o ne b y re g ro o ving . Re g ro o ving is no thing b ut cutting and cre ating g ro o ve s ab o ut 2 mm d e e p ac ro ss the ve hic ular m o ve m e nt. This is a c o stly and lab o rio us p rac tic e . Inste ad , Be lg ium autho rities tried expo sed ag g reg ate finish. O n the smo o thened ro ad surface, they o verlaid 40– 50 mm o f co ncrete, having a maximum size o f 6–8 mm co arse ag g reg ate. The surface o f new c o nc re te , w hile still g re e n, w as sp raye d w ith a re tard e r c o nsisting o f g luc o se , w ate r and alc o ho l. It w as im m e d iate ly c o ve re d b y p o lye thyle ne she e t. Afte r ab o ut 8 –3 6 , ho urs, the po lyethylene sheet is remo ved and the ro ad surface w as sw ept and w ashed w ith stiff ro tating b ristle b rushe s. The to p unse t ce me nt mo rtar to a d e p th o f ab o ut 1 .5 to 2 mm is re mo ve d e xp o sing the ag g re g ate , making the surfac e ro ug h e no ug h fo r safe hig h sp e e d ve hic ular mo ve me nt. Whe n ve hicle s mo ve d at hig h sp e e d o n such e xp o se d ag g re g ate surface it w as fo und to e ve ry o ne s surp rise that no ise le ve l w as much re d uce d than no rmal co ncre te surface . In fact, it w as fo und that no ise le ve l w as lo w e r than the case o f b lack-to ppe d ro ad pave me nt. Furthe r trials w e re co nd ucte d and it w as co nfirme d that e xp o se d ag g re g ate finish p ro vid e s no t o nly skid resistance but also reduces the no ise. The Belg ian autho rities called it as “w hisp e r” co ncre te . Mo stly in m any c o ntine ntal c o untrie s w he re c o nc re te p ave m e nts are p o p ular, the y pro vid e 4 0 –5 0 mm layer o f w hisper co ncrete. They fo und that there is very little d ifference in co st b e tw e e n re g ro o ving and p ro vid ing w hisp e r co ncre te . It w as also se e n that p ro vid ing a

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w hite to pping , that is, pro vid ing co ncre te pave me nt o ve r b itunino us pave me nt, ad o ptio n o f w hisp e r co ncre te g ave g o o d e co no my. In U.K., they to o k up the use o f w hisper co ncrete pavement during 1995, and has g iven g o o d g uid e line s fo r ad o p tio n o f w hisp e r co ncre te . So me o f the imp o rtant g uid e line s are : " Und e r stand ard hig hw ay c o nd itio ns, a c o nc re te ro ad sho uld c o nsist o f c e m e nt b o und sub -b ase , b e tw e e n 1 5 0 –2 0 0 mm thick. O n to p o f this, the re sho uld b e 2 0 0 mm o f co ntinuo usly re info rce d co ncre te p ave me nt (CRCP) fo llo w e d b y 5 0 mm o f w hisp e r co ncre te surfacing . " No rmally 8 mm size co arse ag g reg ate sho uld be used fo r w hisper co ncrete layer. No t mo re than 3 % o f the se sho uld b e o ve rsize and 1 0 % und e rsize . "

The flakine ss ind e x sho uld b e le ss than 2 5 % .

Co arse ag g re g ate sho uld fo rm aro und 6 0 % o f w hisp e r c o nc re te . Sand sho uld b e ve ry fine . "

" Spray retard er co nsisting o f g luco se, w ater and alco ho l. They co ver the surface w ith p o lye thyle ne she e t. " Afte r 8 to 3 6 ho urs, re m o ve the p o lye thyle ne she e t and b rush the surfac e w ith me chanically ro tating stiff b ristle to re mo ve ce me nt mo rtar fro m the to p 1 .5 mm.

As far as Ind ia is co nce rne d , w hisp e r co ncre te is no t g o ing to b e a ne ce ssity fo r so me ye ars to co me .

R EFER EN C ES 6.1

Nasser KW, New and simple tester for slump of concrete ACI Journal, October 1976.

6.2

Dordi C.M., Equipment for Transporting and Placing Eoncrete, Seminar on Concrete Plant and Equipment, Ahmedabad, Nov. 1995.

6.3 cooke T.H., Concrete Pumping and Spraying, A Practical Guide, Thomas Telford, London. 6.4

Klieger P, Early High Strength Concrete for Prestressing, world conference on prestressed concrete July 1957.

6.5

Price W.H., Factors Influencing Concrete strength, ACI Journal Feb. 1951.

6.6

Verbick et.al., Structure and Physical Properties of Cement Pastes, Proceedings, Fifth International Symposium on the Chemistry of Cement, The Cement Association of Japan, Tokyo, 1968.

6.7

Klier P, Effect of Mixing and Curing Temperature on concrete strength ACI Journal June 1958.

6.8 U.S. Bureau of Reclamation, Concrete Manual, 8th Edition, Denver, Colarado, 1975.

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7

C H A P T E R Testing Equipments for Finding Strength of Concrete

" General

Strength of Concrete

" Water/Cement Ratio " Gel/Space Ratio " Gain of Strength with Age " Accelerated Curing test " Maturity Concept of Concrete " Effect of Maximum size of Aggregate on Strength " Relation Between Compressive and Tensile Strength " Central point loading and third points loading " Bond Strength " High Strength Concrete " Ultra High Strength Concrete " High-Performance concrete

General

T

he c o m p re ssive stre ng th o f c o nc re te is o ne o f th e m o st im p o rtan t an d use ful p ro p e rtie s o f co ncre te . In mo st structural applicatio ns co ncre te is emplo yed primarily to resist co mpressive stresses. In tho se cases w here streng th in tensio n o r in shear is o f primary impo rtance , the co mpre ssive stre ng th is fre q ue ntly use d as a m e asure o f the se p ro p e rtie s. Therefo re, the co ncrete making pro perties o f vario us ing re d ie nts o f mix are usually me asure d in te rms o f the c o mp re ssive stre ng th. Co mp re ssive stre ng th is a lso u se d a s a q u a lita tive m e a su re fo r o th e r p ro p e rtie s o f h a rd e n e d c o n c re te . N o e xa c t q u an titative re latio n sh ip b e tw e e n c o m p re ssive stre n g th an d fle xu ral stre n g th , te n sile stre n g th , mo dulus o f elasticity, w ear resistance, fire resistance, o r p e rme ab ility have b e e n e stab lishe d no r are the y like ly to b e . Ho w e ve r, ap p ro xim ate o r statistic al re latio nships, in so me case s, have b e e n e stab lishe d a n d th e se g ive m u c h u se fu l in fo rm a tio n to e n g in e e rs. It sh o u ld b e e m p h a sise d th a t c o m p re ssive stre ng th g ive s o nly an ap p ro xim ate

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value o f th e se p ro p e rtie s an d th at o th e r te sts sp e c ific ally d e sig n e d to d e te rm in e th e se p ro p e rtie s sho uld b e use ful if mo re p re c ise re sults are re q uire d . Fo r instanc e , the ind ic ate d c o m p re ssive stre ng th inc re ase s as the sp e c im e n size d e c re ase s, w he re as the m o d ulus o f e lastic ity d e c re ase s. The mo d ulus o f e lastic ity in this c ase d o e s no t fo llo w the c o mp re ssive streng th. The o ther case w here the co mpressive streng th do es no indicate the useful pro perty o f co ncre te is w he n the co ncre te is sub je cte d to fre e zing and thaw ing . Co ncre te co ntaining ab o ut 6 p e r ce nt o f e ntraine d air w hich is re lative ly w e ake r in stre ng th is fo und to b e mo re d urab le than d e nse and stro ng co ncre te . Th e c o m p re ssive stre n g th o f c o n c re te is g e n e rally d e te rm in e d b y te stin g c ub e s o r cylind e rs mad e in lab o rato ry o r fie ld o r co re s d rille d fro m hard e ne d co ncre te at site o r fro m the no n-d e struc tive te sting o f the sp e c im e n o r ac tual struc ture s. The te sting o f hard e ne d co ncre te is d iscusse d in the sub se q ue nt chap te r. Streng th o f co ncrete is its resistance to rupture. It may be measured in a number o f w ays, such as, streng th in co mpressio n, in tensio n, in shear o r in flexure. All these ind icate streng th w ith reference to a particular metho d o f testing . When co ncrete fails under a co mpressive lo ad the failure is e sse ntially a mixture o f crushing and she ar failure . The me chanics o f failure is a co mplex pheno mena. It can b e assumed that the co ncrete in resisting failure, g enerates b o th co he sio n and inte rnal frictio n. The co he sio n and inte rnal frictio n d e ve lo p e d b y co ncre te in re sisting failure is re late d to mo re o r le ss a sing le p arame te r i.e., w / c ratio . The mo d e rn ve rsio n o f o rig inal w ate r/ ce me nt ratio rule can b e g ive n as fo llo w s: Fo r a g iven cement and acceptab le ag g reg ates, the streng th that may b e d evelo ped b y w o rkab le, pro perly placed mixture o f cement, ag g reg ate and w ater (und er the same mixing , curing and te sting co nd itio ns) is influe nce d b y: (a ) Ratio o f ce me nt to mixing w ate r; (b ) Ratio o f ce me nt to ag g re g ate ; (c ) Grad ing , surface te xture , shap e , stre ng th and stiffne ss o f ag g re g ate p article s; (d ) Maximum size o f ag g re g ate . In the ab o ve it c an b e furthe r infe rre d that w ate r/ c e m e nt ratio p rim arily affe c ts the streng th, w hereas o ther facto rs indirectly affect the streng th o f co ncrete by affecting the w ater/ ce me nt ratio .

Water/Cement Ratio Stre ng th o f co ncre te p rimarily d e p e nd s up o n the stre ng th o f ce me nt p aste . It has b e e n sho w n in Chap te r I that the stre ng th o f ce me nt p aste d e p e nd s up o n the d ilutio n o f p aste o r in o the r w o rd s, the stre ng th o f p aste incre ase s w ith ce me nt co nte nt and d e cre ase s w ith air and w ate r co nte nt. In 1 9 1 8 Ab rams p re se nte d his classic law in the fo rm:

S=

A Bx

w he re x =w ate r/ ce me nt ratio b y vo lume and fo r 2 8 d ays re sults the co nstants A and B are 1 4 ,0 0 0 lb s/ sq . in. and 7 re sp e ctive ly. 7 .1 Ab rams w ate r/ ce me nt ratio law state s that the stre ng th o f co ncre te is o nly d e p e nd e nt upo n w ater/ cement ratio pro vid ed the mix is w o rkab le. In the past many theo ries have b een p ro p o und e d b y many re se arc h w o rke rs. So me o f the m he ld valid fo r so me time and the n und e rw e nt so m e c hang e s w hile o the rs d id no t stand the te st o f tim e and he nc e slo w ly disappeared. But Ab rams’ w ater/ cement ratio law sto o d the test o f time and is held valid even

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to day as a fundamental truth in co ncrete-making practices. No do ubt so me mo dificatio ns have b e e n sug g e ste d b ut the truth o f the state me nt co uld no t b e challe ng e d . Strictly speaking , it w as Feret w ho fo rmulated in as early as 1897, a g eneral rule defining th e stre n g th o f th e c o n c re te p aste an d c o n c re te in te rm s o f vo lu m e frac tio n s o f th e co nstitue nts b y the e q uatio n:

 c  S=K   c + e + a w he re c , e and

2

S = Stre ng th o f co ncre te a = vo lume o f ce me nt, w ate r and air re sp e ctive ly and K = a co nstant.

Strength of Concrete !

301

In this e xp re ssio n the vo lume o f air is also inc lud e d b e c ause it is no t o nly the w ate r/ cement ratio but also the deg ree o f co mpactio n, w hich indirectly means the vo lume o f air filled vo id s in the co ncrete is taken into acco unt in estimating the streng th o f co ncrete. The relatio n b etw een the w ater/ cement ratio and streng th o f co ncrete is sho w n in Fig . 7.1. It can b e seen that lo w er w ater/ cement ratio co uld b e used w hen the co ncrete is vib rated to achieve hig her streng th, w hereas co mparatively hig her w ater/ cement ratio is required w hen co ncrete is handc o m p ac te d . In b o th c ase s w he n the w ate r/ c e m e nt ratio is b e lo w the p rac tic al lim it the stre ng th o f the co ncre te falls rapid ly d ue to intro d uctio n o f air vo id s. The g rap h sho w ing the re latio nship b e tw e e n the stre ng th and w ate r/ c e me nt ratio is ap p ro ximate ly hyp e rb o lic in shap e . So me time s it is d iffic ult to inte rp o late the inte rme d iate value . Fro m g e o me try it can b e d e d uce d that if the g rap hs is d raw n b e tw e e n the stre ng th and the ce me nt/ w ate r ratio an ap p ro ximate ly line ar re latio nship w ill b e o b taine d . This line ar re latio nship is m o re c o nve nie nt to use than w ate r/ c e m e nt ratio c urve fo r inte rp o latio n. Fig . 7 .2 sho w s the re latio nship b e tw e e n co mpre ssive stre ng th and ce me nt/ w ate r ratio .

Gel/Space Ratio Man y re se arc h w o rke rs c o m m e n te d o n th e valid ity o f w ate r/ c e m e n t ratio law as p ro p o und e d b y Duff Ab rams. The y have fo rw ard e d a fe w o f the limitatio ns o f the w ate r/ ce me nt ratio law and arg ue d that Ab rams w ate r/ ce me nt ratio law can o nly b e calle d a rule and no t a law b ecause Ab rams’ statement d o es no t includ e many q ualificatio ns necessary fo r its valid ity to call it a law. So me o f the limitatio ns are that the stre ng th at any w ate r/ ce me nt ratio d epend s o n the d eg ree o f hyd ratio n o f cement and its chemical and physical pro perties, the te mp e rature at w hich the hyd ratio n take s p lace , the air co nte nt in case o f air e ntraine d co ncre te , the chang e in the e ffe ctive w ate r/ ce me nt ratio and the fo rmatio n o f fissure s and cracks d ue to b le e d ing o r shrinkag e . Instead o f relating the streng th to w ater/ cement ratio , the streng th can be mo re co rrectly re late d to the so lid p ro d ucts o f hyd ratio n o f ce me nt to the sp ace availab le fo r fo rmatio n o f this p ro d uc t. Po w e rs and Bro w nyard have e stab lishe d the re latio nship b e tw e e n the stre ng th and g e l/ sp ace ratio . 7 .2 This ratio is d e fine d as the ratio o f the vo lum e o f the h yd ra te d c e m e n t p a ste to th e su m o f vo lumes o f the hyd rated cement and o f the capillary po re s. Po w e r’s e xp e rim e nt sho w e d that the stre n g th o f c o n c re te b e a rs a sp e c ific re latio n sh ip w ith th e g e l/ sp ac e ratio . He fo und the re latio nship to b e 2 4 0 x 3 , w here x is the g e l/ space ratio and 2 4 0 re pre se nts the intrinsic stre ng th o f the g e l in MPa fo r the typ e o f c e me nt and sp e c ime n use d . 7 .3 Th e stre n g th c a lc u la te d b y Po w e r’s e xp re ssio n ho ld s g o o d fo r an id e al c ase . Fig . 7 .3 sh o w s th e re latio n sh ip b e tw e e n stre ng th and g e l/ sp ac e ratio . It is p o inte d o u t th a t th e re la tio n sh ip b e tw e e n th e stre ng th and w ate r/ c e me nt ratio w ill ho ld

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g o o d p rimarily fo r 2 8 d ays stre ng th fo r fully co mp acte d co ncre te , w he re as, the re latio nship b e tw e e n the stre ng th and g e l/ sp ac e ratio is ind e p e nd e nt o f ag e . Ge l/ sp ac e ratio c an b e calculate d at any ag e and fo r any fractio n o f hyd ratio n o f ce me nt. The fo llo w ing e xamp le s sho w ho w to calculate the g e l/ space ratio . Calc ulatio n o f g e l/ sp ac e ratio fo r c o mp le te hyd ratio n Le t

C = w e ig ht o f ce me nt in g m. VC = sp e cific vo lume o f ce me nt = 0 .3 1 9 ml/ g m. W O = vo lume o f mixing w ate r in ml.

Assuming that 1 ml. o f ce me nt o n hyd ratio n w ill pro d uce 2 .0 6 ml o f g e l, Vo lume o f g e l = C x 0 .3 1 9 x 2 .0 6 Space availab le = C x 0 .3 1 9 + W O

∴

Ge l/ Sp ace ratio = x =

Volume of gel 0.657 C = Space available 0.319 C + WO

Calculatio n o f g e l/ sp ace ratio fo r p artial hyd ratio n Let

α = Fractio n o f ce me nt that has hyd rate d

Vo lume o f g e l = C x α x 0 .3 1 9 x 2 .0 6 To tal space availab le C VC α + W O

∴

Ge l/ space ratio = x =

2.06 × 0.319 × Cα . 0.319 Cα + WO

Exa m p le : Calc ulate the g e l/ sp ac e ratio and the the o re tic al stre ng th o f a sam p le o f co ncre te mad e w ith 5 0 0 g m. o f ce me nt w ith 0 .5 w ate r/ ce me nt ratio , o n full hyd ratio n and at 6 0 p e r ce nt hyd ratio n. Ge l/ space ratio o n full hyd ratio n

=

0.657 C = 0 .8 0 2 say 0 .8 0.319 C + WO

∴ The o re tical stre ng th o f co ncre te

= 2 4 0 x (0 .8 )3 = 1 2 3 MPa Ge l/ sp ace ratio fo r 6 0 p e rce nt hyd ratio n.

x=

0.657 C 0.657 × 500 × 0.6 1971 . = = = 0 .5 7 0.319 Cα + WO 0.319 × 500 × 0.6 + 250 345.7

The o re tical stre ng th o f co ncre te at 6 0 p e r ce nt hyd ratio n = 2 4 0 x (0 .5 7 )3 = 4 4 .4 MPa The re is a lo t o f d iffe re nc e b e tw e e n the the o re tic al stre ng th o f c o nc re te and ac tual stre ng th o f co ncre te . Actual stre ng th o f co ncre te is much lo w e r than the the o re tical stre ng th e stimate d o n the b asis o f mo le cular co he sio n and surface e ne rg y o f a so lid assume d to b e perfectly ho mo g eneo us and flaw less. The actual red uctio n o f streng th is d ue to the presence o f flaw s. Griffith po stulate d his the o ry o n the flaw s in co ncre te . He e xplains that the flaw s in co ncre te le ad to a hig h stre ss co nce ntratio ns in the mate rial und e r lo ad , so that a ve ry hig h stress is reached in and aro und the flaw s w ith the result that the material g ets fractured aro und this flaw w hile the ave rag e stre ss o n the mate rial, taking the cro ss se ctio n o f the mate rial as a w ho le, remains co mparatively lo w. The flaw s vary in size. The hig h stress co ncentratio n takes place aro und a few o f the larg er flaw s. This situatio n lead s to failure o f the material at a much lo w e r stre ss inte nsity co nsid e ring the w ho le p ro ce ss. Pre se nce o f b ig g e r flaw s b ring s d o w n the actual stre ng th to a much lo w e r value than the the o re tical stre ng th.

Strength of Concrete !

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Cement paste in co ncrete co ntains many d isco ntinuities such as vo id s, fissures, b leed ing channe ls, rupture o f b o nd d ue to d rying shrinkag e and te mpe rature stre sse s e tc. It has b e e n d iffic ult to e xp lain ho w e xac tly the se vario us flaw s c o ntrib ute to the re d uc tio n in ac tual streng th o f co ncrete. Ho w ever, Griffith’s theo ry w hich explains the failure o f co ncrete has been acce p te d to satisfacto rily e xp lain the failure o f b rittle mate rials such as co ncre te .

Gain of Strength with Age The co ncre te d e ve lo p s stre ng th w ith co ntinue d hyd ratio n. The rate o f g ain o f stre ng th is faster to start w ith and the rate g ets reduced w ith ag e. It is custo mary to assume the 28 days streng th as the full streng th o f co ncrete. Actually co ncrete d evelo ps streng th b eyo nd 2 8 d ays also . Earlie r co d e s have no t b e e n p e rmitting to co nsid e r this incre ase o f stre ng th b e yo nd 2 8 days fo r desig n purpo ses. The increase in streng th beyo nd 28 days used to g et immersed w ith the fac to r o f safe ty. W ith b e tte r und e rstand ing o f the mate rial, p ro g re ssive d e sig ne rs have b e e n trying to re d uce the facto r o f safe ty and make the structure mo re e co no mical. In this d ire c tio n, the inc re ase in stre ng th b e yo nd 2 8 d ays is take n into c o nsid e ratio n in d e sig n o f structure s. So me o f the mo re pro g re ssive co d e s have b e e n pe rmitting this practice . Tab le 7 .1 g ive s the ag e facto rs fo r p e rmissib le co mp re ssive stre ss in co ncre te , as p e r British Co d e .

Ta ble 7 .1 . Age Fa c t ors for Pe r m issible Com pre ssive St re ss in Conc re t e a s pe r Brit ish Code Minimum ag e o f memb er w hen full d esig n lo ad is applied , mo nths

Ag e facto r fo r lo w -streng th co ncrete

Ad d itio nal streng th fo r hig h streng th co ncrete MPa

1

1 .0 0

0

2

1 .1 0

4 .2

3

1 .1 6

5 .5

6

1 .2 0

7 .7

12

1 .2 4

1 0 .2

Earlie r IS co d e 4 5 6 o f 1 9 7 8 co nsid e re d ag e facto r and allo w e d the incre ase in d e sig n stre ss in the lo w e r c o lum ns in m ultisto re y b uild ing s. Earlie r o nly o ne typ e o f c e m e nt i.e ., c e me nt g o ve rne d b y IS- 2 6 9 o f 1 9 7 6 w as use d in w hic h c ase th e re w as ap p re c iab le inc re ase in stre ng th afte r 2 8 d ays. Afte r g rad atio n o f O PC th e p re se n t d a y c e m e n ts p a rtic u la rly 53 g ra d e ce me nts, b e ing g ro und fine r, the incre ase in stre ng th afte r 2 8 d ays is no m inal. Mo st o f the streng th d evelo pments in respect o f w ell cured co ncrete w ill have take n p lac e b y 2 8 d ays. Therefo re, allo w ing ag e fac to r is no t g e ne rally fo und Bandra Worli Sea Link Project - an artist’s impression. necessary. Therefo re, in IS 456 In this 8 lane bridge 60 MPa concrete is going to be used. o f 2000, the clause is revised . Courtesy : Hindustan Construction Company.

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The c lause state s “The re is no rm ally a g ain o f stre ng th b e yo nd 2 8 d ays. The q uantum o f increase d epend s upo n the g rad e and type o f cement, curing and enviro nmental co nd itio ns e tc. The d e sig n sho uld b e b ase d o n 2 8 d ays characte ristic stre ng th o f co ncre te unle ss the re is an e vid e nce to justify a hig he r stre ng th fo r a p articular structure d ue to ag e ” The tab le numb e r 7 .2 g ive s the g rad e s o f co ncre te as p e r IS-4 5 6 o f 2 0 0 0

Ta ble 7 .2 . Gra de s of Conc re t e a s pe r I S - 4 5 6 of 2 0 0 0 Gro up

Grad e Desig natio n

Specified characterstic co mpressive streng th o f 1 5 0 mm cub e at 2 8 d ays in N/ mm 2

O rd in ary

M 10

10

Co n c re te

M 15

15

M 20

20

Stan d ard

M 25

25

Co n c re te

M 30

30

M 35

35

M 40

40

M 45

45

M 50

50

M 55

55

Hig h

M 60

60

Stre n g th

M 65

65

Co n c re te

M 70

70

M 75

75

M 80

80

Pe rmissib le stre sse s in co ncre te is g ive n in Tab le . 7 .3 and 7 .4 . (IS 4 5 6 o f 2 0 0 0 )

Ta ble 7 .3 . Pe r m issible st re sse s in Conc re t e All va lue s in N /m m 2 (I S 4 5 6 of 2 0 0 0 ) Grad e o f co ncrete

Permissib le stress in co mpressio n Bend ing

Direct

Permissib le stress in Bo nd (averag e) fo r Plain Bars in tensio n

M

10

3 .0

2 .5

—

M

15

5 .0

4 .0

0 .6

M

20

7 .0

5 .0

0 .8

M

25

8 .5

6 .0

0 .9

M

30

1 0 .0

8 .0

1 .0

M

35

1 1 .5

9 .0

1 .1

M

40

1 3 .0

1 0 .0

1 .2

M

45

1 4 .5

1 1 .0

1 .3

M

50

1 6 .0

1 2 .0

1 .4

Note: The bond stress may be increased by 25 per cent for bars in compression.

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305

Ta ble 7 .4 . Pe r m issible she a r st re ss in c onc re t e a s pe r I S 4 5 6 of 2 0 0 0 Permissib le shear stress in co ncrete N/ mm 2 Grad es o f co ncrete

100 × As bd M 15 (2 )

M 20 (3 )

M 25 (4 )

M 30 (5 )

M 3 5 M 4 0 and ab o ve (6 ) (7 )

≤ 0 .1 5

0 .1 8

0 .1 8

0 .1 9

0 .2 0

0 .2 0

0 .2 0

0 .2 5

0 .2 2

0 .2 2

0 .2 3

0 .2 3

0 .2 3

0 .2 3

0 .5 0

0 .2 9

0 .3 0

0 .3 1

0 .3 1

0 .3 1

0 .3 2

0 .7 5

0 .3 4

0 .3 5

0 .3 6

0 .3 7

0 .3 7

0 .3 8

1 .0 0

0 .3 7

0 .3 9

0 .4 0

0 .4 1

0 .4 2

0 .4 2

1 .2 5

0 .4 0

0 .4 2

0 .4 4

0 .4 5

0 .4 5

0 .4 6

1 .5 0

0 .4 2

0 .4 5

0 .4 6

0 .4 8

0 .4 9

0 .4 9

1 .7 5

0 .4 4

0 .4 7

0 .4 9

0 .5 0

0 .5 2

0 .5 2

2 .0 0

0 .4 4

0 .4 9

0 .5 1

0 .5 3

0 .5 4

0 .5 5

2 .2 5

0 .4 4

0 .5 1

0 .5 3

0 .5 5

0 .5 6

0 .5 7

2 .5 0

0 .4 4

0 .5 1

0 .5 5

0 .5 7

0 .5 8

0 .6 0

2 .7 5

0 .4 4

0 .5 1

0 .5 6

0 .5 8

0 .6 0

0 .6 2

3 .0 0

0 .4 4

0 .5 1

0 .5 7

0 .6 0

0 .6 2

0 .6 3

(1 )

and Above Many a time it may b e ne ce ssary to e stimate the stre ng th o f co ncre te at an e arly ag e . O ne may no t b e ab le to w ait fo r 28 d ays. Many research w o rkers have attempted to estimate the streng th o f co ncrete at 1, 3 o r 7 days and co rrelate it to 28 days streng th. The relatio nship b e tw e e n the stre ng th o f co ncre te at a lo w e r ag e and 2 8 d ays d e p e nd s up o n many facto rs such as co mp o und co mp o sitio n o f ce me nt, fine ne ss o f g rind ing and te mp e rature o f curing e tc . Fu rth e rm o re m ixe s w ith lo w w ate r/ c e m e nt ratio g ains stre n g th , e xp re sse d a s a p e rc e n ta g e o f lo n g te rm stre n g th , m o re rap id ly th an th at o f c o n c re te w ith h ig h e r w a te r/ c e m e n t ra tio . Th is is p re su m a b ly b e c a u se th e c e me nt p artic le s are he ld at a c lo se r in te rval in c ase o f lo w w ate r/ c e m e nt ratio than that o f hig he r w ate r/ c e me nt ratio , in w hich case the re is a much b e tte r p o ssib ility fo r th e fo rm a tio n o f c o n tin u o u s system o f g el w hich g ives mo re stre n g th . Ma n y re se a rc h Bandra Worli Sea Link Project under construction with high w o rke rs h a ve fo rw a rd e d performance concrete. Courtesy : Hindustan Construction Company. certain relatio nships betw een 7

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d ays stre ng th and 2 8 d ays stre ng th. In Ge rmany the re latio n b e tw e e n 2 8 d ays stre ng th, σ28 and the 7 d ays stre ng th, σ7 is take n to lie b e tw e e n,

σ28 = 1 .4 σ7 + 1 5 0 and σ b e ing e xp re sse d in p o und s/ sq . inch.

σ2 8 = 1 .7 σ7 + 8 5 0

Ano the r re latio n sug g e ste d is o f the typ e

f28 = K2 (f7 )K1 w here, f7 and f28 are the stre ng ths at 7 and 2 8 d ays re sp e ctive ly and K1 and K2 are the co e fficie nts, w hich is d iffe re nt fo r d iffe re nt ce me nts and curing co nd itio ns. The value o f K1 rang e s fro m 0 .3 to 0 .8 and that o f K2 fro m 3 to 6 . The stre ng th o f co ncre te is g e ne rally e stimate d at 2 8 d ays b y crushing fie ld te st cub e s o r c ylin d e rs m ad e fro m th e re p re se n tative c o n c re te u se d fo r th e stru c tu re . O fte n it is q ue stio ne d ab o ut the utility o f asc e rtaining 2 8 d ays stre ng th b y w hic h tim e c o nsid e rab le amo unt o f c o nc re te w ill have b e e n p lac e d and the w o rks may have p ro g re sse d . It is the n rathe r to o late fo r re me d ial me asure s, if the re sult o f the te st cub e at 2 8 d ays is to o lo w. O n the o the r hand , the structure w ill b e une co no mical if the re sult o f the te st cub e is to o hig h. It is, the re fo re , o f tre me nd o us ad vantag e to b e ab le to p re d ict 2 8 d ays stre ng th w ithin a fe w ho urs o f c asting the c o nc re te so that w e have a g o o d id e a ab o ut the stre ng th o f co ncre te , so that satisfacto ry re me d ial me asure s co uld b e take n imme d iate ly b e fo re it is to o late . The re are many me tho d s fo r p re d icting the 2 8 d ays stre ng th, w ithin a sho rt p e rio d o f c astin g . O ut o f th e se th e m e th o d sug g e ste d b y Pro f. Kin g is fo un d to h ave g o o d fie ld co rre latio ns.

Accelerated Curing test In the acclerated curing test the standard cub es are cast, they are co vered w ith to p plate and the jo ints are se ale d w ith spe cial g re ase to pre ve nt d rying . Within 3 0 minute s o f ad d ing w ate r, the c ub e s having se ale d e ffe c tive ly, are p lac e d in an air-tig ht o ve n w hic h is the n sw itche d o n. The o ve n te mpe rature is b ro ug ht to 9 3 ° C in ab o ut o ne ho ur time . It is ke pt at this te mp e rature fo r 5 ho urs. At the e nd o f this p e rio d the c ub e s are re mo ve d fro m o ve n, strippe d , co o le d , and te ste d . The time allo w e d fo r this o pe ratio n is 3 0 minute s. The stre ng th o f c o nc re te is d e te rm ine d w ithin 7 ho urs o f c asting and this ac c le rate d stre ng th sho w s g o o d re latio nship w ith 7 and 2 8 d ays stre ng ths o f no rmally cure d co ncre te . Fig . 7 .4 sho w s re latio nship b e tw e e n ac c le re ate d stre ng th and no rm ally c ure d c o nc re te stre ng th at 7 and 2 8 d ays. O ne o f the main facto rs that affects the rate o f g ain o f streng th is the fineness o f cement. It has b e e n e stim ate d that p artic le s o f c e m e nt o ve r 4 0 m ic ro n in size c o ntrib ute to the co mp re ssive stre ng th o f co ncre te o nly o ve r lo ng p e rio d s, w hile tho se p article s smalle r than 2 5 to 3 0 mic ro n c o ntrib ute to the 2 8 d ays stre ng th, tho se p artic le s smalle r than 2 0 to 2 5 micro n co ntrib ute to the 7 d ays stre ng th, and particle s smalle r than 5 to 7 micro n co ntrib ute to the 1 o r 2 d ays stre ng th. Re lative g ain o f stre ng th w ith the time o f co ncre te s mad e w ith d iffe re nt w ate r/ ce me nt ratio using o rd inary Po rtland ce me nt is sho w n in Fig . 7 .5 .

Maturity Concept of Concrete While d e aling w ith curing and stre ng th d e ve lo p me nt, w e have so far co nsid e re d o nly the tim e asp e c t. It has b e e n p o inte d o ut e arlie r that it is no t o nly the tim e b ut also the te mp e rature d uring the e arly p e rio d o f hyd ratio n that influe nce the rate o f g ain o f stre ng th

Strength of Concrete !

307

o f c o n c re te . Sin c e th e stre n g th d e ve lo p m e n t o f c o n c re te d e p e n d s o n b o th tim e an d te mp e rature it can b e said that stre ng th is a functio n o f summatio n o f p ro d uct o f time and te mp e rature . This summatio n is calle d maturity o f co ncre te . Maturity = Σ (time x te mp e rature ) The te m p e rature is re c ko ne d fro m an o rig in lying b e tw e e n –1 2 and –1 0 ° C. It w as experimentally fo und that the hydratio n o f co ncrete co ntinues to take place upto abo ut –11°C. The re fo re , –1 1 ° C is take n as a d atum line fo r co mp uting maturity. Maturity is me asure d in d e g re e ce ntig rad e ho urs (° C hrs) o r d e g re e ce ntig rad e d ays (° C d ays). Fig . 7 .6 . sho w s that the stre ng th p lo tte d ag ainst the lo g arithm o f m aturity g ive s a straig ht line . A samp le o f co ncre te cure d at 1 8 ° C fo r 2 8 d ays is take n as fully mature d co ncre te . Its maturity w o uld b e e q ual to 2 8 x 2 4 x [1 8 – (–1 1 )] = 1 9 4 8 8 ° C h. Ho w e ve r, in stan d ard c alc ulatio n s th e m aturity o f fully c ure d c o n c re te is take n as 1 9 ,8 0 0 ° Ch.. (The d iscre p ancy is b e cause o f the o rig in o r the d atum is no t e xactly b e ing –1 1 C as use d in calculatio n). If the p e rio d is b ro ke n into sm alle r inte rvals and if the c o rre sp o nd ing te m p e rature is reco rded fo r each interval o f time, the summatio n o f the pro duct o f time and temperature w ill

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Strength of Concrete !

309

g ive an ac c u rate p ic tu re o f th e m atu rity o f c o n c re te . In th e ab se n c e o f su c h d e taile d te mpe rature histo ry w ith re spe ct to the time inte rval, the maturity fig ure can b e arrive d at b y multiplying d uratio n in ho urs b y the ave rag e te mpe rature at w hich the co ncre te is cure d . O f co urse , the maturity calculate d as ab o ve w ill b e le ss accurate . Maturity co nce pt is use ful fo r e stimating the stre ng th o f co ncre te at any o the r maturity as a pe rce ntag e o f stre ng th o f co ncre te o f kno w n maturity. In o the r w o rd s, if w e kno w the stre ng th o f co ncre te at full maturity (1 9 ,8 0 0 ° Ch), w e can calculate the p e rce ntag e stre ng th o f identical co ncrete at any o ther maturity by using the fo llo w ing eq uatio n g iven by Plo w man. Stre ng th at any maturity as a pe rce ntag e o f stre ng th at maturity o f 1 9 ,8 0 0 ° Ch. = A + B lo g 10

(maturity) 10 3

The values o f co efficients, A and B d epend o n the streng th level o f co ncrete. The values are g ive n in Tab le 7 .5

Ta ble 7 .5 . Plow m a n’s Coe ffic ie nt s for M a t urit y Equat ion 7 .5 Streng th after 2 8 d ays at 1 8 ° C (Maturity o f 1 9 ,8 0 0 ° Ch): MPa

Co efficient A

B

10

68

1 7 .5 – 3 5 .0

21

61

3 5 .0 – 5 2 .5

32

54

5 2 .5 – 7 0 .0

42

4 6 .5

Le ss than 1 7 .5

Th e valu e s o f A an d B are p lo tte d ag ain st th e c u b e stre n g th at th e m atu rity o f 1 9 ,8 0 0 ° Ch. A straig ht line re latio nship w ill b e o b taine d ind ic ating that the y are d ire c tly p ro p o rtio nal to the stre ng th. Plo w man d ivid e d the stre ng th into 4 zo ne s as sho w n in Tab le 7 .5 and has assig ne d the value s o f A and B fo r e ach zo ne . It is to b e no te d that the maturity e q uatio n ho ld s g o o d fo r the initial te mp e rature o f c o nc re te le ss than ab o ut 3 8 ° C. Fig . 7 .7 g ive s the value o f co nstant A and B w he n stre ng th and te mp e rature are e xp re sse d in lb s/ sq inch and ° F re sp e ctive ly. The fo llo w ing e xamp le s illustrate the the o ry: Exam p le 1 : The streng th o f a sample o f fully matured co ncrete is fo und to b e 40.00 MPa find the stre ng th o f id e ntic al c o nc re te at the ag e o f 7 d ays w he n c ure d at an ave rag e te mp e rature d uring d ay time at 2 0 ° C, and nig ht time at 1 0 ° C. Maturity o f co ncre te at the ag e o f 7 d ays = Σ (time x Te mp e rature ) = 7 x 1 2 x [2 0 – (–1 1 )] + 7 x 1 2 x [1 0 – (– 1 1 )] = 7 x 12 x 31 + 7 x 12 x 21 = 4 3 6 8 ° Ch. The stre ng th rang e o f this co ncre te falls in Zo ne III fo r w hich the co nstant A is 3 2 and B is 5 4 .

∴ the p e rce ntag e stre ng th o f co ncre te at maturity o f 4 3 6 8 ° Ch. = A + B lo g = 3 2 + 5 4 x lo g 10 (4 .3 6 8 ) = 3 2 + 5 4 x 0 .6 4 0 3 = 6 6 .5

10

(4368) 1000

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∴ The stre ng th at 7 d ays = 4 0 .0 x

66.5 = 2 6 .5 MPa 100

Exam p le 2 : Lab o rato ry e xp e rime nts co nd ucte d at (Pune ) o n a p articular mix sho w e d a streng th o f 32.5 MPa fo r fully matured co ncrete. Find w hether fo rmw o rk can b e remo ved fo r an id e ntical co ncre te p lace d at Srinag ar at the ag e 1 5 d ays w he n the ave rag e te mp e rature is 5 ° C if the co ncre te is like ly to b e sub je cte d to a stripping stre ss o f 2 5 .0 MPa. Streng th o f fully matured co ncrete = 3 2 .5 MPa. Maturity o f id entical co ncrete at 1 5 d ays w he n cure d at a te mp e rature o f 5 ° C = 1 5 x 2 4 x [5 – (– 1 1 )] = 1 5 x 2 4 x 1 6 = 5 7 6 0 ° Ch. This co ncre te falls in Zo ne No . II fo r w hich the value o f co nstants are A = 21

and

B = 61.

Pe rce ntag e o f stre ng th = A + B lo g 10

 (5760)  (Maturity)  = 2 1 + 6 1 lo g 10   1000  1000

= 2 1 + 6 1 x Lo g 10 5 .7 6 0 = 2 1 + 6 1 x 0 .7 6 0 4 = 6 7 .3 8

∴ The stre ng th o f co ncre te at 1 5 d ays = 3 2 .5 x

67.38 = 2 1 .9 MPa. 100

Since the stre ng th o f co ncre te is le ss than the stre ss to w hich it is like ly to b e sub je cte d w hile strip p ing the fo rm w o rk, the c o nc re te m ay fail. The re fo re , the fo rm w o rk c anno t b e re mo ve d at 1 5 d ays time .

If the stre ng th at a g ive n maturity is kno w n, the n the numb e r o f d ays re q uire d to re ach the same stre ng th at any o the r te mp e rature can also b e calculate d fro m,

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M 24[t − ( −11)] w he re , M = Maturity fo r the g ive n stre ng th, and t the alte rnative te mp e rature in ce ntig rad e . In the ab o ve e xamp le fo r re aching the same stre ng th, numb e r o f d ays re q uire d ,

=

M 19800 19800 = = = 5 2 d ays. 24[t − ( −11)] 24[5 − (11)] 24 × 16

This is to say that the co ncre te cure d at 5 ° C w o uld take ab o ut 5 2 d ays to achie ve full maturity.

Effect of Maximum size of Aggregate on Strength At o ne time it w as tho ug ht that the use o f larg er size ag g reg ate leads to hig her streng th. This w as d ue to the fact that the larg er the ag g reg ate the lo w er is the to tal surface area and , the re fo re , the lo w e r is the re q uire me nt o f w ate r fo r the g ive n w o rkab ility. Fo r this re aso n, a lo w e r w ate r/ ce me nt ratio can b e use d w hich w ill re sult in hig he r stre ng th. Ho w e ve r, late r it w as fo und that the use o f larg e r size ag g re g ate d id no t co ntrib ute to hig her streng th as expected fro m the theo retical co nsid eratio ns d ue to the fo llo w ing reaso ns. The larg e r maximum size ag g re g ate g ive s lo w e r surface are a fo r d e ve lo p me nts o f g e l b o nds w hich is respo nsib le fo r the lo w er streng th o f the co ncrete. Seco ndly b ig g er ag g reg ate size cause s a mo re he te ro g e ne ity in the co ncre te w hich w ill p re ve nt the unifo rm d istrib utio n o f lo ad w he n stre sse d . W he n larg e size ag g re g ate is use d , d ue to inte rnal b le e d ing , the transitio n zo ne w ill b e c o m e m u c h w e ake r d u e to th e d e ve lo p m e n t o f m ic ro c rac ks w h ic h re su lt in lo w e r co mp re ssive stre ng th. Generally, hig h streng th co ncrete o r rich co ncrete is adversely affected by the use o f larg e size ag g re g ate . But in le an mixe s o r w e ake r co ncre te the influe nce o f size o f the ag g re g ate g ets reduced. It is interesting to no te that in lean mixes larg er ag g reg ate g ives hig hest streng th w hile in rich mixe s it is the smalle r ag g re g ate w hich yie ld s hig he r stre ng th. Fig . 7 .8 . sho w s the influence o f maximum size o f ag g reg ate o n co mpressive streng th o f co ncrete. 7 .6 Fig . 7 .9 . depicts the influence o f size o f ag g reg ate o n co mpressive streng th o f co ncrete fo r different w / c ratio .

Relation Between Compressive and Tensile Strength In re info rce d co ncre te co nstructio n the stre ng th o f the co ncre te in co mpre ssio n is o nly take n in to c o n sid e ratio n . Th e te n sile stre n g th o f c o n c re te is g e n e rally n o t take n in to c o nsid e ratio n. But the d e sig n o f c o nc re te p ave m e nt slab s is o fte n b ase d o n the fle xural streng th o f co ncrete. Therefo re, it is necessary to assess the flexural streng th o f co ncrete either fro m the co mp re ssive stre ng th o r ind e p e nd e ntly. As m e asure m e nts and c o ntro l o f c o m p re ssive stre ng th in fie ld are e asie r and m o re co nvenient, it has been custo mary to find o ut the co mpressive streng th fo r different co nditio ns and to co rrelate this co mpressive streng th to flexural streng th. Having established a satisfacto ry re latio nship b e tw e e n fle xural and c o mp re ssive stre ng th, p ave me nt, c an b e d e sig ne d fo r a sp e c ifie d fle xural stre ng th value , o r this value c o uld b e use d in any o the r situatio ns w he n re q uire d . It is se e n that stre ng th o f co ncre te in co mp re ssio n and te nsio n (b o th d ire ct te nsio n and fle xu ral te n sio n ) are c lo se ly re late d , b u t th e re latio n sh ip is n o t o f th e typ e o f d ire c t

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Strength of Concrete !

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p ro p o rtio n ality. Th e ratio o f th e tw o stre n g th s d e p e n d s o n g e n e ral le ve l o f stre n g th o f c o nc re te . In o the r w o rd s, fo r hig he r c o m p re ssive stre ng th c o nc re te sho w s hig he r te nsile stre ng th, b ut the rate o f incre ase o f te nsile stre ng th is o f d e cre asing o rd e r. The typ e o f c o arse ag g re g ate influe nc e s this re latio nship . Crushe d ag g re g ate g ive s relatively hig her flexural streng th than co mpressive streng th. This is attrib uted to the impro ved b o n d stre n g th b e tw e e n c e m e n t p aste an d ag g re g ate p artic le s. Th e te n sile stre n g th o f co ncre te , as co mpare d to its co mpre ssive stre ng th, is mo re se nsitive to impro pe r curing . This may b e due to the inferio r q uality o f g el fo rmatio n as a result o f impro per curing and also due to the fact that impro pe rly cure d co ncre te may suffe r fro m mo re shrinkag e cracks. The use o f p o zzo lanic mate rial incre ase s the te nsile stre ng th o f co ncre te . Fro m the e xte nsive stud y, c arrie d o ut at Ce ntral Ro ad Re se arc h Lab o rato ry (CRRI) the fo llo w ing statistic al re latio nship b e tw e e n te nsile stre ng th and c o m p re ssive stre ng th w e re e stab lishe d . (i ) y = 1 5 .3 x – 9 .0 0 fo r 2 0 mm maximum size ag g re g ate . (ii) y = 1 4 .1 x – 1 0 .4 fo r 2 0 mm maximum size natural g rave l. (iii) y = 9 .9 x – 0 .5 5 fo r 4 0 mm maximum size crushe d ag g re g ate . (iv ) y = 9.8 x – 2.52 fo r 40 mm maximum size natural g ravel. Where y is the co mpressive stre ng th o f co ncre te MPa and x is the fle xural stre ng th o f co ncre te MPa.

Sub jecting all the data to statistical treatment the fo llo w ing g eneral relatio nship has b een e stab lishe d at CRRI b e tw e e n fle xural and co mp re ssive stre ng th o f co ncre te :

y = 1 1 x – 3 .4 Fig . 7.10 sho w s the relatio nships b etw een co mpressive streng th and flexural streng th o f co ncre te fo r vario us ag g re g ate s.

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The fle xural stre ng th o f co ncre te w as fo und to b e 8 to 1 1 pe r ce nt o f the co mpre ssive ste ng th o f the co ncre te fo r hig he r rang e s o f co ncre te stre ng th (g re ate r than 2 5 MPa) and 9 to 1 2 .8 pe r ce nt fo r lo w e r rang e s o f co ncre te stre ng th (le ss than 2 5 MPa) as sho w n in Tab le 7 .6 . The flexural to co mpressive streng th ratio w as hig her w ith ag g reg ate o f 40 mm maximum size than w ith tho se o f 2 0 mm maximum size . In g e ne ral the ratio w as fo und to b e slig htly hig he r in the case o f natural g rave l as co mp are d to crushe d sto ne . Fle xural stre ng th o f c o nc re te is usually fo und b y te sting p lain c o nc re te b e am s. Tw o me tho d s o f lo ad ing o f the b e am sp e cime n fo r find ing o ut fle xural stre ng th are p ractise d :

Ta ble 7 .6 . Flex ura l St re ngt h Ex pre sse d a s Pe rc e nt a ge s of Com pre ssive St re ngt h of Conc re t e using Grave l a nd Cr ushe d St one Aggre gat e 7 .7 Co mpressive Streng th MPa

Flexural Streng th as Percentag e o f Co mpressive Streng th per cent Gravel Ag g reg ate w ith Maximum size 2 0 mm

4 0 mm

Crushed Sto ne Ag g reg ate w ith Maximum Size 2 0 mm

4 0 mm

49

8 .7

-

7 .7

-

42

9 .0

1 0 .8

7 .9

1 0 .2

35

9 .3

1 0 .9

8 .2

1 0 .3

28

9 .9

1 1 .1

8 .6

1 0 .2

21

1 0 .8

1 1 .3

9 .3

1 0 .3

14

1 2 .5

1 2 .0

1 0 .8

1 0 .5

Ave rag e

1 0 .0

1 1 .2

8 .8

1 0 .3

Central point loading and third points loading Exp e rie nce sho w s that the variab ility o f re sults is le ss in third -p o int lo ad ing . The re sults o f the fle xural stre ng th te ste d und e r ce ntral and third -p o ints lo ad ing w ith co nstant sp an to d e p th ratio s o f 4 w e re analyze d statistic ally and the fo llo w ing g e ne ral re latio nship w as o b taine d at Ce ntral Ro ad Re se arch Lab o rato ry.

x1 = x2 + 0 .7 2 w he re ,

x1 = flaxural stre ng th (MPa) o f co ncre te und e r ce ntral p o int lo ad ing and x2 = fle xural stre ng th (MPa) o f co ncre te und e r third p o int lo ad ing .

In all the cases the central lo ading g ave hig her averag e value than the third-po int lo ading irre sp e c tive o f the size o f the sam p le . The hig he r stre ng th o b taine d in the c ase o f c e ntral lo ad ing may b e attrib uted to the fact that the b eam is b eing sub jected to the maximum stress at a pre-d etermined lo catio n no t necessarily the w eakest. In the stand ard metho d s fo r find ing the fle xural stre ng th o f co ncre te , the span to d e pth ratio o f the spe cime n is ke pt at 4 . If the sp an to d e p th ratio is inc re ase d o r d e c re ase d , the fle xural stre ng th w as fo und to alte r. A chang e in this ratio b y o ne ind uce d 3 p e r ce nt and 2 .5 p e r ce nt chang e in stre ng th w he n tested b y third-po int and central po int lo ading respectively. With the increase in span to depth ratio the fle xural stre ng th d e cre ase d . The rate o f stre ss ap p licatio n w as fo und to influe nce the fle xural stre ng th o f co ncre te to a ve ry sig nific ant e xte nt. The stre ng th inc re ase d up to ab o ut 2 5 p e r c e nt w ith inc re ase in

Strength of Concrete !

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stre ssing rate co mpare d to the stand ard rate o f 0 .7 MPa pe r minute . The incre ase w as fo und mo re w ith the le ane r mixe s. There are numb er o f empirical relatio nships co nnecting tensile streng th and co mpressive stre ng th o f co ncre te . O ne o f the co mmo n re latio nship s is sho w n b e lo w. Te nsile Stre ng th = K (Co mp re ssive Stre ng th)n w he re , value o f K varie s fro m 6 .2 fo r g rave ls to 1 0 .4 fo r crushe d ro ck (ave rag e value is 8 .3 ) and value o f n may vary fro m 1 / 2 to 3 / 4 Furthe r d ata o b taine d at the Lab o rato rie s o f Po rtland Ce m e nt Asso c iatio n g iving the re latio nship b e tw e e n co mp re ssive and te nsile stre ng th o f co ncre te is sho w n in Tab le 7 .7 .

Ta ble 7 .7 . Re lat ion Be t w e e n Com pre ssive a nd Te nsile St re ngt h of Conc re t e 7 .8 Co mpressive Streng th o f Cylind ers

*

Streng th Ratio

MPa

Mo d ulus o f repture* to co mpressive stre ng th

Direct tensile Direct tensile streng th to co mpressive streng th to stre ng th mo dulus o f rupture

7

0 .2 3

0 .1 1

0 .4 8

14

0 .1 9

0 .1 0

0 .5 3

21

0 .1 6

0 .0 9

0 .5 7

28

0 .1 5

0 .0 9

0 .5 9

35

0 .1 4

0 .0 8

0 .5 9

42

0 .1 3

0 .0 8

0 .6 0

49

0 .1 2

0 .0 7

0 .6 1

56

0 .1 2

0 .0 7

0 .6 2

63

0 .1 1

0 .0 7

0 .6 3

De te rmine d und e r third -p o int lo ad ing .

The Ind ian Stand ard IS = 4 5 6 o f 2 0 0 0 g ive s the fo llo w ing re latio nship b e tw e e n the co mp re ssive stre ng th and fle xural stre ng th Fle xural Stre ng th = 0 .7

f ck

w he re fck is the characte ristic co mp re ssive stre ng th o f co ncre te in N/ mm 2

Bond Strength We can co nsid er the b o nd streng th fro m tw o d ifferent ang les; o ne is the b o nd streng th betw een paste and steel reinfo rcement and the o ther is the bo nd streng th betw een paste and ag g re g ate . Firstly, le t us co nsid e r the b o nd stre ng th b e tw e e n p aste and ste e l re info rce me nt. Bo nd stre ng th b e tw e e n p aste and ste e l re info rce me nt is o f co nsid e rab le imp o rtance . A p e rfe ct b o nd , e xisting b e tw e e n co ncre te and ste e l re info rce me nt is o ne o f the fund ame ntal assum p tio ns o f re info rc e d c o nc re te . Bo nd stre ng th arise s p rim arily fro m the fric tio n and ad he sio n b e tw e e n co ncre te and ste e l. The ro ug hne ss o f the ste e l surface is also o ne o f the facto rs affe cting b o nd stre ng th. The b o nd stre ng th o f co ncre te is a functio n o f co mp re ssive stre ng th and is appro ximate ly pro po rtio nal to the co mpre ssive stre ng th upto ab o ut 2 0 MPa. Fo r hig her streng th, increase in b o nd streng th b eco mes pro g ressively smaller. Tab le 7 .3 g ives

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the value o f b o nd stre ng th co rre sp o nd ing to the co mp re ssive stre ng th. The b o nd stre ng th, is also a functio n o f spe cific surface o f g e l. Ce me nt w hich co nsists o f a hig he r pe rce ntag e o f C2 S w ill g ive hig her specific surface o f g el, thereb y g iving hig her b o nd streng th. O n the o ther hand , co ncre te co ntaining mo re C3 S o r the co ncre te cure d at hig he r te mp e rature re sults in smaller specific surface o f g el w hich g ives a lo w er b o nd streng th. It has b een alread y po inted o ut that hig h p re ssure ste am c ure d c o nc re te p ro d uc e s g e l w ho se sp e c ific surfac e is ab o ut 1 / 2 0 o f the sp e cific surface o f the g e l p ro d uce d b y no rmal curing . The re fo re , b o nd stre ng th o f hig h p re ssure ste am cure d co ncre te is co rre sp o nd ing ly lo w e r.

Aggregate-Cement Bond Strengths Co ncrete can b e reg arded as a chain in w hich ag g reg ates are the links b o nded to g ether b y ce me nt p aste . Just as the stre ng th o f a chain as a w ho le is d e p e nd ing upo n the stre ng th o f w eld ing o f the ind ivid ual links, the streng th o f co ncrete as a w ho le is d epend ing upo n the stre ng th (b o nd stre ng th) o f the hyd rate d hard e ne d c e m e nt p aste (hc p ). By and larg e the streng th o f hcp is depending upo n w / c ratio w hich determines the q uality, co ntinuity, density, po ro sity o f the pro ducts o f hydratio n in particular the C-S-H g el. Stro ng er the g el bo nd stro ng er is the c o nc re te . Ag g re g ate s g e ne rally b e ing m uc h stro ng e r than the p aste (g e l b o nd ), its stre ng th is no t o f co nse q ue nce in no rmal stre ng th co ncre te . The stre ng th o f ag g re g ate is o f co nsid e ratio n in hig h stre ng th co ncre te and lig ht w e ig ht co ncre te . The explanatio n that the streng th o f Co ncrete is limited by streng th o f the paste, w ill ho ld g o o d w he n w e c o nsid e r c o nc re te as tw o p hase mate rial. If w e take a c lo se r lo o k into the structure o f the co ncre te , a third phase co me s into co nsid e ratio n i.e., inte r-face b e tw e e n the p aste and ag g re g ate kno w n as Transitio n Zo ne . In the ultimate analysis it is the inte g rity o f the transitio n zo ne that influe nce s the stre ng th o f co ncre te . As w e h a ve se e n e a rlie r, b le e d in g ta ke s p la c e in fre sh co ncrete. The b leed ing w ater in the p ro c e ss o f c o m in g u p g e ts in te rc e p te d by a g g re g a te s, p a rtic u la rly la rg e size fla ky a n d e lo n g a te d a g g re g a te a n d g e ts a c c u m u la te d a t th e in te r-fa c e b etw een paste and ag g reg ates. The e xtra w ate r re m aining at the inte rface , re sults in p o o r p aste structure and p o o r g e l b o nd at the transitio n zo ne . Th e p a ste sh rin ks w h ile h a rd e n in g . Th e m a g n itu d e o f sh rin ka g e is h ig h e r w ith h ig h e r w ate r c o n te n t, in w h ic h c ase , a hig he r shrinkag e take s p lace at the tra n sitio n zo n e w h ic h re su lts in g re a te r sh rin ka g e c ra c ks a t th e transitio n zo ne . In c a se o f sh rin ka g e ta kin g p la c e o n a c c o u n t o f h e a t o f

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hyd ratio n, the w e ak g e l struc ture at the transitio n zo ne also suffe rs a hig he r d e g re e o f shrinkag e. The same situatio n w ill take place if the co ncrete is subjected to heat o r co ld during the se rvice life . It can b e deduced that there are co nsiderab le micro cracks o r w hat yo u call “faults”, exists in the transitio n zo ne e ve n b e fo re the co ncre te structure s are sub je cte d to any lo ad o r stre ss. Whe n sub je cte d to so me stre ss, the e xisting micro cracks in transitio n zo ne pro pag ate much faste r w ith tiny jum p s and d e ve lo p b ig g e r c rac ks than re st o f the b o d y o f c o nc re te and structure fails much earlier than the g eneral streng th o f co ncrete. Therefo re, the transitio n zo ne is the w e ake st link o f the chain. It is the stre ng th limiting p hase in co ncre te . Po int to no te is that w e have to co me b ack to the b asics. It is the w / c ratio that ag ain influences the q uality o f transitio n zo ne in lo w and med ium streng th co ncrete. The w / c ratio is no t exerting the same influence o n hig h streng th co ncrete ie., fo r very lo w w / c ratio . It has b een seen that fo r w / c less than 0.3, dispro po rtio nately hig h increase in co mpressive streng th can b e achie ve d fo r ve ry small re d uctio n in w / c. This p he no me no n is attrib ute d mainly to a sig nificant imp ro ve me nt in the stre ng th o f transitio n zo ne at ve ry lo w w / c ratio . Ag g re g ate characte ristics o the r than stre ng th, such as size , shap e , surface te xture and g rad ing are kno w n to affe ct the stre ng th o f co ncre te . The incre ase in stre ng th is g e ne rally attrib u te d to in d ire c t c h an g e in w / c ratio . Re c e n t stu d ie s h ave sh o w n th at th e ab o ve c harac te ristic s o f ag g re g ate s have ind e p e nd e nt influe nc e o n the stre ng th p ro p e rtie s o f co ncre te o the r than thro ug h w / c ratio b y imp ro ving the q uality o f transitio n zo ne .

J.J Flyover at Mumbai where high strength, high performance concrete 75 MPa was used for the first time in India (2002). Courtesy : Gammon India

There are numb er o f pub lished literatures w hich indicate that und er id entical co nd itio ns, calcare o us ag g re g ate s g ive hig he r stre ng th than silice o us ag g re g ate s. The re sult o f stud ie s co nd ucte d at unive rsity o f Califo rnia is sho w n in Fig . 7 .1 1 . The stre ng th o f co ncre te e mb race s so many asp e cts that it is d ifficult to d e scrib e all the facto rs that influe nce s the stre ng th o f co ncre te . The e ntire b o o k o n co ncre te te chno lo g y, in a w ay is d ealing w ith the streng th pro perties o f co ncrete. In this b o o k, the vario us aspects o n stre ng th p ro p e rtie s o f co ncre te is d e scrib e d in vario us chap te rs.

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High Strength Concrete Co n c re te is g e n e rally c lassifie d as No rm al Stre n g th Co n c re te (NSC), Hig h Stre n g th Co ncre te (HSC) and Ultra Hig h Stre ng th Co ncre te (UHSC). The re are no cle ar cut b o und ary fo r the ab o ve classificatio n. Ind ian Stand ard Re co mme nd e d Me tho d s o f Mix De sig n d e no te s the bo undary at 35 MPa betw een NSC and HSC. They did no t talk abo ut UHSC. But elsew here in the inte rnatio nal fo rum , ab o ut thirty ye ars ag o , the hig h stre ng th lab le w as ap p lie d to co ncre te having stre ng th ab o ve 4 0 MPa. Mo re re ce ntly, the thre sho ld ro se to 5 0 to 6 0 MPa. In the w o rld sce nario , ho w e ve r, in the last 1 5 ye ars, co ncre te o f ve ry hig h stre ng th e nte re d the fie ld o f c o nstruc tio n, in p artic ular c o nstruc tio n o f hig h-rise b uild ing s and lo ng sp an b ridg es. Co ncrete streng ths o f 90 to 120 MPa are o ccasio nally used. Tab le 7.8 sho w s the kind o f hig h stre ng th pro d uce d in RMC plant. The advent o f Prestressed Co ncrete Techno lo g y Techniques has g iven impetus fo r making c o nc re te o f hig he r stre ng th. In Ind ia, the re are c ase s o f using hig h stre ng th c o nc re te fo r p re stre sse d co ncre te b rid g e s. The first p re stre sse d co ncre te b rid g e w as b uilt in 1 9 4 9 fo r the Assam Rail Link at Silig uri. In fifty’s a n u m b e r o f p re tre sse d c o n c re te structure s w e re b uilt using co ncre te o f stre ng th fro m 3 5 MPa to 4 5 MPa. But stre ng th o f co ncre te mo re than 3 5 MPa w as no t co mmo nly use d in g e n e ra l c o n stru c tio n p ra c tic e s. Pro b ab ly c o nc re te o f stre ng th mo re than 35 MPa w as used in larg e scale in Ko n kan Railw ay p ro je c t d u rin g e a rly 9 0 ’s a n d c o n c re tisa tio n o f Mu m b a i Mu n c ip a l Co rp o ra tio n Ro ad s. It is o nly d uring 9 0 ’s use o f hig h stre ng th co ncre te has take n its Vidya Sagar Setu at Kolkata where longest cable stayed d u e p lac e in In d ian c o n stru c tio n bridge (in India) was built using high strength concrete. scenario . O f late co ncrete o f streng th varying fro m 4 5 MPa to 6 0 MPa has b e e n use d in hig h rise b uild ing s at Mumb ai, De lhi and o the r Me tro p o litan c itie s. Sim ilarly hig h stre ng th c o nc re te w as e m p lo ye d in b rid g e s and flyo vers. Presently (year 2 0 0 0 ) in Ind ia, co ncrete o f streng th 7 5 MPa is b eing used fo r the first time in o ne o f the flyo vers at Mumb ai. O ther no tab le example o f using hig h streng th co ncrete in Ind ia is in the co nstructio n o f co ntainme nt Do me at Kaig a Po w e r Pro je c t. The y have use d Hig h p e rfo rmance co ncre te o f stre ng th 6 0 MPa w ith silica fume as o ne o f the co nstitue nts. Ready Mixed Co ncrete has taken its ro o ts in India no w. The manufacture o f hig h streng th co ncre te w ill g ro w to find its d ue p lace in co ncre te co nstructio n fo r all the o b vio us b e ne fits. In the mo d e rn b atching p lants hig h stre ng th co ncre te is p ro d uce d in a me chanical manne r. O f c o urse , o ne has to take c are ab o ut m ix p ro p o rtio ning , shap e o f ag g re g ate s, use o f sup p le m e ntary c e m e ntitio us m ate rials, silic a fum e and sup e rp lastic ize rs. W ith the m o d e rn e q uip m e nts, und e rstand ing o f the ro le o f the c o nstitue nt m ate rials, p ro d uc tio n o f hig h stre ng th co ncre te has b e co me a ro utine matte r. The re are sp e cial me tho d s o f making hig h stre ng th co ncre te . The y are g ive n b e lo w. (a ) Se e d ing

(b ) Re vib ratio n

(c ) Hig h sp e e d slurry mixing ;

(d ) Use o f ad mixture s

(e ) Inhib itio n o f cracks

(f ) Sulp hur imp re g natio n;

Strength of Concrete !

319

(g ) Use o f ce me ntitio us ag g re g ate s.

Seeding: This invo lve s ad d ing a sm all p e rc e ntag e o f fine ly g ro und , fully hyd rate d Po rtland ce me nt to the fre sh co ncre te mix. The me chanism b y w hich this is sup p o se d to aid stre ng th d e ve lo p me nt is d ifficult to e xp lain. This me tho d may no t ho ld much p ro mise . Revibration: Co nc re te und e rg o e s p lastic shrinkag e . Mixing w ate r c re ate s c o ntinuo us capillary channels, b leeding , and w ater accumulates at so me selected places. All these reduce the stre ng th o f co ncre te . Co ntro lle d re vib ratio n re mo ve s all the se d e fe cts and incre ase s the stre ng th o f co ncre te . High Speed slurry mixing:This p ro ce ss invo lve s the ad vance p re p aratio n o f ce me ntw ater mixture w hich is then blended w ith ag g reg ate to pro duce co ncrete. Hig her co mpressive stre ng th o b taine d is attrib ute d to m o re e ffic ie nt hyd ratio n o f c e m e nt p artic le s and w ate r achie ve d in the vig o ro us b le nd ing o f ce me nt p aste . Use of Admixtures: Use o f w ate r re d uc ing ag e nts are kno w n to p ro d uc e inc re ase d co mpre ssive stre ng ths.

Inhibition of cracks: Co ncre te fails b y the fo rmatio n and pro pag atio n o f cracks. If the p ro p ag atio n o f cracks is inhib ite d , the stre ng th w ill b e hig he r. Re p lace me nt o f 2 – 3 % o f fine ag g reg ate by po lythene o r po lystyrene “lenticules” 0.025 mm thick and 3 to 4 mm in diameter re sults in hig he r stre ng th. The y ap p e ar to act as crack arre ste rs w itho ut ne ce ssitating e xtra w ater fo r w o rkab ility. Co ncrete cub es mad e in this w ay have yield ed streng th upto 1 0 5 MPa.

Sulphur Impregnation: Satisfac to ry hig h stre ng th c o nc re te have b e e n p ro d uc e d b y impre g nating lo w stre ng th po ro us co ncre te b y sulphur. The pro ce ss co nsists o f mo ist curing the fresh co ncrete specimens fo r 24 ho urs, drying them at 120°C fo r 24 ho urs, immersing the sp e cime n in mo lte n sulp hur und e r vacuum fo r 2 ho urs and the n re le asing the vacuum and so aking them fo r an additio nal ½ ho ur fo r further infiltratio n o f sulphur. The sulphur-infiltrated co ncre te has g ive n stre ng th upto 5 8 MPa. Use of Cementitious aggregates: It h as b e e n fo u n d th at u se o f c e m e n titio u s ag g reg ates has yielded hig h streng th. Cement fo ndu is kind o f clinker. This g lassy clinker w hen finely g ro und results in a kind o f cement. When co arsely crushed, it makes a kind o f ag g reg ate kno w n as ALAG. Using Alag as ag g re g ate , stre ng th upto 1 2 5 MPa has b e e n o b taine d w ith w ate r/ ce me nt ratio 0 .3 2 .

Ultra High Strength Concrete As te chno lo g y ad vance s, it is b ut natural that co ncre te te chno lo g ists are d ire cting the ir atte ntio n b e yo nd hig h stre ng th c o nc re te to ultra hig h stre ng th c o nc re te . The fo llo w ing te chniq ue s are use d fo r p ro d ucing ultra hig h stre ng th co ncre te . (a ) Co mpactio n b y pre ssure

(b ) He lical b ind ing ;

(c ) Po lyme risatio n in co ncre te

(d ) Re active p o w d e r co ncre te .

Compaction by Pressure: It has b e e n p o inte d o ut e arlie r that ce me nt p aste d e rive s stre ng th d ue to the co mb ine d e ffe ct o f frictio n and b o nd . In ce ramic mate rial, g rain size and po ro sity w o uld b e the mo st impo rtant parame te rs affe cting frictio n and b o nd and he nce the stre ng th. It has b e e n atte m p te d to re d uc e g rain size and p o ro sity b y the ap p lic atio n o f tre me nd o us p re ssure at ro o m te mp e rature and also at hig he r te mp e rature . Unusually hig h stre ng th have b e e n g e ne rate d in mate rials b y e mp lo ying “ho t p re ssing ” te c hniq ue s and inte rm e d iate rang e s o f stre ng ths have b e e n ac hie ve d b y ap p lying hig h p re ssure at ro o m te m p e rature to Po rtland c e m e nt p aste s. Stre ng ths as hig h as 6 8 0 MPa

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(co mp re ssive ), 6 6 MPa (ind ire ct te nsile ) have b e e n o b taine d b y sub je cting ce me nt p aste s to 3 5 7 MPa p re ssure und e r a te mp e rature o f 2 5 0 ° C. The w ate r/ ce me nt ratio use d w as 0 .0 9 3 . It w as also se e n that ho t p re sse d mate rials are vo lume stab le . The mic ro struc ture o f suc h m ate rials are ve ry c o m p ac t, c o n sistin g o f in te rg ro w th o f d e n se h yd rate d c e m e n t g e l surro und ing re sid ual unhyd rate d c e me nt g rain c o re s. The lo w e st p o ro sity o f the mate rials me asure d w as ap p ro ximate ly 1 .8 % .

Ta ble 7 .8 . Com posit ion of Ex pe rim e nt a l Conc re t e s Produc e d in a Re a dy-m ix Pla nt 7 .9 (I n U.S.A.) Co ncrete type

Reference

Silica fume

Fly ash

0 .3 0

0 .3 0

0 .3 0

0 .3

0 .2 5

127

128

129

131

128

W/ (c + m) Wate r kg / m

3

Slag + silica fume

Ce me nt ASTM Typ e II

kg / m 3

450

425

365

228

168

Silica fume

kg / m 3

-

45

-

45

54

Fly ash

kg / m 3

-

-

95

-

-

Slag

kg / m 3

-

-

-

183

320

1100

1110

1115

110

1100

815

810

810

800

730

1 5 .3

14

13

12

13

110

180

170

220

210

Do lo mite lime sto ne Co arse ag g re g ate

kg / m 3

Fine ag g re g ate

kg / m 3

Sup e rp lasticize r*

L/ m

3

Slump afte r 4 5 minute s (mm) Stre ng th at 2 8 d ay

(MPa)

99

110

90

105

114

Stre ng th at 9 1 d ay

(MPa)

109

118

111

121

126

Stre ng th at 1 ye ar

(MPa)

119

127

125

127

137

* So d ium salt o f a nap hthalin sulp ho nate

Helical Binding: This is an ind irect metho d o f achieving ultra hig h streng th in co ncrete. Hig h te nsile ste e l w ire b ind ing e xte rnally o ve r the co ncre te cylind e r re sults in g o o d stre ng th. Polymer Concrete: Impreg natio n o f mo no mer into the po res o f hardened co ncrete and th e n g e ttin g it p o lym e rise d b y irrad iatio n o r th e rm al c atalytic p ro c e ss, re su lts in th e develo pment o f very hig h streng th. This metho d o f making ultra hig h streng th co ncrete ho lds much pro mise. This aspect has b een d iscussed in d etail in Chapter 12 und er special co ncrete. Reactive Powder Concrete: Hig h streng th Co ncrete w ith streng th o f 1 0 0 – 1 2 0 MPa have b e e n use d fo r the co nstructio n o f structural me mb e rs. Co ncre te w ith 2 5 0 to 3 0 0 MPa are also use d fo r no n-struc tural ap p lic atio ns suc h as flo o ring , safe s and sto rag e o f nuc le ar w aste s. Fo r structural use s, hig h d uctility is re q uire d alo ng w ith hig h-stre ng th. Re active p o w d e r co ncre te (RPC) has b e e n d e ve lo pe d to have a stre ng th fro m 2 0 0 to 8 0 0 MPa w ith re q uire d d uctility. Co ncrete is a hetero g eneo us material and streng th o b tained b y cement paste is no t fully re taine d w he n sand and ag g re g ate s are ad d e d . The Re ac tive Po w e r c o nc re te is mad e b y replacing the co nvential sand and ag g reg ate by g ro und q uartz less than 300 micro n size, silica

321

Strength of Concrete !

fum e , synthe size d p re c ip itate d silic a, ste e l fib re s ab o ut 1 c m in le ng th and 1 8 0 m ic ro n in d iame te r. The typical co mpo sitio n and me chanical pro pe rty o f the RPC o f 2 0 0 MPa stre ng th and 8 0 0 MPa stre ng th are sho w n in Tab le . 7 .9 . and Tab le . 7 .1 0 re sp e ctive ly.

Ta ble N o.7 .9 . Typic a l Com posit ion of Re a c t ive Pow de r Conc re t e 2 0 0 1

Po rtland Ce me nt-Typ e V

9 5 5 kg / m 3

2

Fine Sand (1 5 0 -4 0 0 micro n)

1 0 5 1 kg / m 3

2

3

Silica Fume (1 8 m / g ram)

2 2 9 kg / m 3

4

Pre cipitate d Silica (3 5 m 2 / g )

1 0 kg / m 3

5

Sup e rp lasticize r (Po lyacrylate )

1 3 kg / m 3

6

Ste e l fib re s

1 9 1 kg / m 3

7

To tal w ate r

1 5 3 kg / m 3

8

Co mp re ssive Stre ng th (Cylind e r)

1 7 0 - 2 3 0 MPa

9

Fle xural stre ng th

2 5 –6 0 MPa

10

Yo ung ’s Mo d ulus

5 4 - 6 0 GPa

Ta ble 7 .1 0 . Typic a l Com posit ion of Re a c t ive Pow de r Conc re t e 8 0 0

7 .1 0

1 0 0 0 kg / m 3

1

Po rtland Ce me nt-Typ e V

2

Fine Sand (1 5 0 - 4 0 0 micro ns)

5 0 0 kg / m 3

3

Gro und q uartz (4 micro ns)

3 9 0 kg / m 3

2

7 .1 0

2 3 0 kg / m 3

4

Silica fume (1 8 m / g ram)

5

Sup e rp lasticize r (Po lyacrylate )

6

Ste e l fib re s (le ng th 3 mm and d ia. 1 8 0 µ)

6 3 0 kg / m 3

7

To tal w ate r

1 8 0 kg / m 3

8

Co mp re ssive Stre ng th (cylind e r)

4 9 0 - 6 8 0 MPa

9

Fle xural Stre ng th

4 5 - 1 0 2 MPa

10

Yo ung ’s Mo d ulus

6 5 - 7 5 GPa

1 8 kg / m 3

High-Performance concrete Re ce ntly a ne w te rm “Hig h p e rfo rmance co ncre te ” is use d fo r co ncre te mixture w hich p o sse ss h ig h w o rkab ility, h ig h stre n g th , h ig h m o d u lu s o f e lastic ity, h ig h d e n sity, h ig h d ime nsio nal stab ility, lo w pe rme ab ility and re sistance to che mical attack. The re is a little c o ntro ve rsy b e tw e e n the te rm s hig h-stre ng th and hig h-p e rfo rm anc e co ncrete. Hig h-perfo rmance co ncrete is also , a hig h-streng th co ncrete b ut it has a few mo re attrib utes specifically d esig ned as mentio ned ab o ve. It is, therefo re, lo g ical to d escrib e b y the mo re w id e ly e mb racing te rm “Hig h Pe rfo rmance Co ncre te ” (HPC). It may b e re calle d that in no rmal co ncre te , re lative ly lo w stre ng th and e lastic mo d ulus are the result o f hig h hetero g eneo us nature o f structure o f the material, particularly the po ro us an d w e ak tran sitio n zo n e , w h ic h e xists at th e c e m e n t p aste -ag g re g ate in te rfac e . By d e nsific atio n and stre ng the ning o f the transitio n zo ne , m any d e sirab le p ro p e rtie s c an b e impro ved many fo ld . This aspect has b een alread y d iscussed in d etail. A sub stantial red uctio n

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o f q uantity o f mixing w ate r is the fund ame ntal ste p fo r making HPC. Re d uctio n o f w / c ratio w ill re sult in hig h stre ng th co ncre te . But re d uctio n in w / c ratio to le ss than 0 .3 w ill g re atly imp ro ve the q ualitie s o f transitio n zo ne to g ive inhe re nt q ualitie s e xp e cte d in HPC. To impro ve the qualities o f transitio n zo ne, use o f silica fume is also fo und to be necessary. Silica fume s b e co me s a ne ce ssary ing re d ie nt fo r stre ng th ab o ve to 8 0 MPa. The b e st q uality fly ash and GGBS m ay b e use d fo r o the r no m inal b e ne fits. Insp ite o f the fac t that the se p o zzo la n ic m a te ria ls in c re a se th e w a te r d e m a n d , th e ir b e n e fits w ill o u t w e ig h th e d isad vantag e s. The crux o f w ho le p ro b le m lie s in using ve ry lo w w / c ratio , co nsistant w ith hig h w o rkab ility at the time o f placing and co mpacting . Ne ville o pine s that the lo w e st w / c ratio that co uld b e use d is 0 .2 2 . 7 .9 Ad o pting w / c ratio in the rang e o f 0.25 to 0.3 and g etting a hig h slump is po ssib le o nly w ith the use o f superplasticizer. Therefo re, use o f appro priate superplasticizer is a key material in making HPC. The asso ciated pro b lem is the selectio n o f superplasticizer and that o f cement so that the y are co mpatib le and re tain the slump and rhe o lo g ical pro pe rtie s fo r a sufficie ntly lo ng time till co ncre te is p lace d and co mp acte d .

Aggregates for HPC In no rmal stre ng th co ncre te , the stre ng ths o f ag g re g ate b y itse lf plays a mino r ro le . Any ag g re g ate availab le at the site c o uld b e use d w ith little mo d ific atio n o f the ir g rad ing . The situatio n is rathe r d iffe re nt w ith HPC, w he re the b o nd b e tw e e n ag g re g ate and hyd rate d cement paste is so stro ng that it results in sig nificant transfer o f stress acro ss the transitio n zo ne. At the same time , the stre ng th o f the ce me nt paste phase , o n acco unt ve ry lo w w / c ratio is

Example of some well-known Structures where HPC was used. Strength of concrete is shown on the top of each building.

Strength of Concrete !

323

so hig h that so me time s it is hig he r than the stre ng th o f ag g re g ate p article s. O b se rvatio n o f fractured surface in HPC has sho w n that they pass thro ug h the co arse ag g reg ate particles as o fte n as, if no t mo re o fte n than, thro ug h the ce me nt p aste itse lf. Ind e e d in many instance s, the streng th o f ag g reg ate particles has b een fo und to b e the facto r that limits the co mpressive stre ng th o f HPC. O n the b asis o f p ractical e xp e rie nce it is se e n that fo r co ncre te stre ng th up to 1 0 0 MPa, maximum size o f 2 0 mm ag g re g ate co uld b e use d . Ho w e ve r, fo r co ncre te in e xce ss o f 1 0 0 MPa, the maximum size o f co arse ag g re g ate sho uld b e limite d to 1 0 to 1 2 mm.

Ta ble 7 .1 1 . Typic a l H PC m ix t ure s use d in som e im por t a nt buildings in U SA a nd ot he r c ount rie s. Mixture Numb er

1

2

3

4

5

Wate r

kg / m 3

195

165

135

145

130

Ce me nt

kg / m 3

505

451

500

315

513

Fly ash

kg / m 3

60

-

-

-

-

Slag

kg / m 3

-

-

-

137

-

Silica fume

kg / m 3

-

-

30

36

43

Co arse ag g re g ate s

kg / m 3

1030

1030

1100

1130

1080

Fine ag g re g ate

kg / m 3

630

745

700

745

685

3

-

Wate r re d uce r

ml/ m

975

-

-

900

Re tard e r

L/ m 3

-

4 .5

1 .8

-

-

Supe rplasticize r

L/ m 3

-

1 1 .2 5

14

5 .9

1 5 .7 0 .2 5

W/ (c + m)

0 .3 5

0 .3 7

0 .2 7

0 .3 1

Stre ng th at 2 8 d ay

(MPa)

65

80

93

83

119

Stre ng th at 9 1 d ay

(MPa)

79

87

107

93

145

1- Water Tower Place, Chicago 1975 2- Joigny Bridge, France 1989 3- La Laurentienne Building, Montreal (1984) 4- Scotia Plaza, Toronto (1987) 5- Two Union square, Seattle (1988) Reg arding the shape o f the ag g reg ate, crushed ag g reg ate can b e used, b ut utmo st care sho uld b e taken to see that ag g reg ates are cub ic in shape, w ith minimum amo unt o f flaky o r elo ng ated particles. The latter w o uld effect no t o nly the streng th b ut also ad versely affect the w o rkability. In o ne site in Mumbai even fo r 60 MPa co ncrete they had to g o fo r w ell pro cessed and w e ll g rad e d cub ic shap e d , co arse ag g re g ate fro m the p o int o f vie w o f w o rkab ility. Fo r HPC shape and size o f the ag g reg ate b eco mes an impo rtant parameter. Tab le No . 7.11. g ives the co mpo sitio n and ste ng th o f HPC that are use d in so me impo rtant b uild ing s in USA and o the r co untrie s. In Ind ia, it is re p o rte d , that HPC o f the stre ng th 6 0 MPa w as use d fo r the first time fo r the co nstructio n o f co ntainme nt d o me at Kaig a and Rajasthan Ato mic Po w e r Pro je cts.

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R EFER EN C ES 7.1

Kenneth Newman, The structural and properties of concrete –an Introducting review, International concrete on the structure of concrete, Sept. 1965.

7.2

T.C. Powers and T.L. Brownyard, Studies of the Physical properties of hardened Portland Cement paste, Journal of Americal Concrete Institute, Oct 1946 to April 1947. (Nine parts).

7.3 Powers T.C. The physical structure and Engineering properties of concrete, Portland Cement Association Research department Bulletin, July 1958. 7.4

King JUH, Further Notes on the Acclereated Test for the concrete, chartered civil Engineer, May 1957.

7.5 Plowman J.M., Maturity and strength of concrete, Magazene of concrete Research, March 1956. 7.6 Cordon W.A. et al, Variables in concrete aggregates and portland cement paste which influence the strength of concrete, ACI Journal, Aug. 1963. 7.7

Ghosh, R.K., et al, Flexural strength of Concrete–its variations, relationship with compressive strength and use in concrete mix design, Proc. IRC Road Research Bulletin, No. 16 of 1972.

7.8

Price W.H., Factors Influencing concrete strength, ACI Journal, Feb. 1951.

7.9

Aitcin P.C. and Neville A., High Performance Concrete Demgstified, Concrete International, Jan. 1993.

7.10

Richard P. and Chegrezy M.H., Reactive Powder Concretes with High Ductility and 200 – 800 MPa Compressive Strengths, Proceedings of V.M. Malhotra Symposium, Sp-144, ACI, 1994.

Elasticity, Creep and Shrinkage !

325

8

C H A P T E R Length Comparator with specimen in place

" Elastic Properties of Concrete " Creep " Microscopic Rheological Approach " Shrinkage

Elasticity, Creep and Shrinkage Elastic Properties of Concrete

I

n the the o ry o f re info rce d co ncre te , it is assume d that co ncrete is elastic, iso tro pic, ho mo g eno us and that it c o nfo rm s to Ho o ke ’s law. Ac tually no ne o f the se assum p tio ns are stric tly true and c o nc re te is no t a p re fe c tly e lastic m ate rial. Co nc re te d e fo rm s w he n lo ad is ap p lie d b ut this d e fo rmatio n d o e s no t fo llo w any simple set rule. The defo rmatio n depends up o n the mag nitud e o f the lo ad , the rate at w hich the lo ad is applied and the elapsed time after w hich th e o b se rva tio n is m a d e . In o th e r w o rd s, th e rhe o lo g ical b e havio ur o f co ncre te i.e. , the re sp o nse o f co ncre te to applie d lo ad is q uite co mple x. Th e kn o w le d g e o f rh e o lo g ic al p ro p e rtie s o f c o n c re te is n e c e ssary to c alc u late d e fle c tio n o f struc ture s, and d e sig n o f c o nc re te m e m b e rs w ith re sp e ct to the ir se ctio n, q uantity o f ste e l and stre ss analysis. W he n re info rc e d c o nc re te is d e sig ne d b y elastic theo ry it is assumed that a perfect b o nd exists b etw een co ncrete and steel. The stress in steel is “m ” tim e s the stre ss in c o nc re te w he re “m ” is the ratio

325

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betw een mo dulus o f elasticity o f steel an d c o n c re te , kn o w n as m o d u lar ratio . Th e ac c u rac y o f d e sig n w ill n atu rally b e d e p e n d e n t u p o n th e value o f the mo d ulus o f e lasticity o f c o n c re te , b e c ause th e m o d ulus o f e lastic ity o f ste e l is m o re o r le ss a d e finite q uantity. It is to b e fu rth e r n o te d th at c o n c re te e xh ib its ve ry p e c u lia r rhe o lo g ical b e havio ur b e cause o f its b e ing a he te ro g e ne o us, multi-p hase m a te ria l w h o se b e h a vio u r is influe nc e d b y the e lastic p ro p e rtie s and m o rp ho lo g y o f g e l struc ture s. The mo d ulus o f e lasticity o f co ncre te b e ing so imp o rtant and at the same time so co mplicated, w e shall see this aspe ct in little mo re d e tail. Th e m o d u lu s o f e la stic ity is d e te rmine d b y sub je cting a cub e o r c ylin d e r sp e c im e n to u n ia xia l c o m p re ssio n a n d m e a su rin g th e d e fo rm atio ns b y m e ans o f d ial g aug e s fixe d b e tw e e n c e rtain g aug e le ng th. Dial g aug e reading divided by g aug e leng th w ill g ive the strain and lo ad applied divided by area o f cro ssse ctio n w ill g ive the stre ss. A se rie s o f re ad ing s are take n and the stre ss-strain re latio nship is e stab lishe d . The mo d ulus o f elasticity can also b e d e te rm in e d b y su b je c tin g a c o nc re te b e am to b e nd ing and the n using the fo rmulae fo r d e fle ctio n and su b stitu tin g o th e r p aram e te rs. Th e mo dulus o f elasticity so fo und o ut fro m actual lo ad ing is calle d static mo d ulus o f e lasticity. It is se e n that e ve n und e r sho rt te rm lo ad ing co ncre te d o e s no t b e h a ve a s a n e la stic m a te ria l. Ho w e ve r, up to ab o ut 1 0 -1 5 % o f the u ltim a te stre n g th o f c o n c re te , th e stre ss-strain g rap h is n o t ve ry m uc h c u rve d a n d h e n c e c a n g ive m o re accurate value. Fo r hig her stresses the stre ss-strain re latio nship w ill b e g re atly c u rve d a n d a s su c h it w ill b e in ac c urate . Fig ure 8 .1 sh o w s stre ssstrain re latio nship fo r vario us co ncre te mixe s.

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In vie w o f the p e culiar and co mp le x b e havio ur o f stre ss-strain re latio nship , the mo d ulus o f e lasticity o f co ncre te is d e fine d in so me w hat arb itrary manne r. The mo d ulus o f e lasticity o f co ncrete is desig nated in vario us w ays and they have been illustrated o n the stress-strain curve in Fig ure 8.2. The term Yo ung ’s mo dulus o f elasticity can strictly b e applied o nly to the straig ht part o f stre ss-strain curve . In the case o f co ncre te , since no part o f the g raph is straig ht, the mo d ulus o f e lasticity is fo und o ut w ith re fe re nce to the tang e nt d raw n to the curve at the o rig in. The mo d ulus fo und fro m this tang e nt is re fe rre d as initial tang e nt mo d ulus. This g ive s satisfacto ry results o nly at lo w stress value. Fo r hig her stress value it g ives a misleading picture. Tang e nt can also b e d raw n at any o the r p o int o n the stre ss-strain curve . The mo d ulus o f e lastic ity c alc ulate d w ith re fe re nc e to this tang e nt is the n c alle d tang e nt m o d ulus. The tang ent mo d ulus also d o es no t g ive a realistic value o f mo d ulus o f elasticity fo r the stress level much ab o ve o r much b e lo w the po int at w hich the tang e nt is d raw n. The value o f mo d ulus o f e lasticity w ill b e satisfacto ry o nly fo r stre ss le ve l in the vicinity o f the p o int co nsid e re d . A line can b e d raw n co nnecting a specified po int o n the stress-strain curve to the o rig in o f the curve . If the mo d ulus o f e lasticity is calculate d w ith re fe re nce to the slo p e o f this line , the mo dulus o f elasticity is referred as secant mo dulus. If the mo dulus o f elasticity is fo und o ut w ith re fe re nce to the cho rd d raw n b e tw e e n tw o sp e cifie d p o ints o n the stre ss-strain curve the n such value o f the mo d ulus o f e lasticity is kno w n as cho rd mo d ulus. The mo d ulus o f elasticity mo st co mmo nly used in practice is secant mo d ulus. There is no stand ard me tho d o f d e te rmining the se c ant mo d ulus. So me time it is me asure d at stre sse s rang ing fro m 3 to 1 4 MPa and so me time the se cant is d raw n to po int re pre se nting a stre ss le ve l o f 1 5 , 2 5 , 3 3 , o r 5 0 p e r ce nt o f ultimate stre ng th. Since the value o f se cant mo d ulus d ecreases w ith increase in stress, the stress at w hich the secant mo d ulus has b een fo und o ut sho uld alw ays b e state d .

1 2

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Mo d ulus o f e lasticity may b e me asure d in te nsio n, co mp re ssio n o r she ar. The mo d ulus in te nsio n is usually e q ual to the mo d ulus in co mp re ssio n. It is interesting to no te that the stress-strain relatio nship o f ag g reg ate alo ne sho w s a fairly g o o d straig ht line. Similarly, stress-strain relatio nship o f cement paste alo ne also sho w s a fairly g o o d straig ht line . But the stre ss-strain re latio nship o f c o nc re te w hic h is c o m b inatio n o f ag g re g ate an d p aste to g e th e r sh o w s a c urve d re latio n sh ip . Pe rh ap s th is is d ue to th e develo pment o f micro cracks at the interface o f the ag g reg ate and paste. Because o f the failure o f bo nd at the interface increases at a faster rate than that o f the applied stress, the stress-strain c urve c o ntinue s to b e nd faste r than inc re ase o f stre ss. Fig ure 8 .3 sho w s the stre ss-strain re latio nship fo r ce me nt p aste , ag g re g ate and co ncre te .

Relation between Modulus of Elasticity and Strength Fig ure 8 .4 sho w s the strain in co ncre te o f d iffe re nt stre ng ths p lo tte d ag ainst the stre ssstrain ratio . At the sam e stre ss-stre ng th ratio , stro ng e r c o nc re te has hig he r strain. O n the co ntrary, stro ng e r the co ncre te hig he r the mo d ulus o f e lasticity. This can b e e xp laine d that stro ng e r the c o nc re te the stro ng e r is the g e l and he nc e le ss is the strain fo r a g ive n lo ad . Be cause o f lo w e r strain, hig he r is the mo d ulus o f e lasticity. The Tab le 8 .1 g ive s the value s o f mo d ulus o f e lasticity fo r vario us stre ng ths o f co ncre te . Mo d ulus o f e lastic ity o f c o nc re te inc re ase s ap p ro xim ate ly w ith the sq uare ro o t o f the stre ng th. The IS 4 5 6 o f 2 0 0 0 g ive s the Mo d ulus o f e lasticity as EC = 5 0 0 0 w he re EC is the sho rt te rm static mo d ulus o f e lasticity in N/ mm 2 .

f ck

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Ta ble 8 .1 . M odulus of Ela st ic it y of Conc re t e of Diffe re nt st re ngt hs Averag e co mpressive streng th o f w o rks cub es MPa

Mo d ulus o f Elasticity GPa

21

2 1 .4

28

2 8 .5

35

3 2 .1

42

3 5 .7

56

4 2 .9

70

4 6 .4

Actual me asure d value s may d iffe r b y ± 2 0 p e r ce nt fro m the value s o b taine d fro m the ab o ve e xp re ssio n.

Factors Affecting Modulus of Elasticity As e xp laine d e arlie r, o ne o f the imp o rtant facto rs affe cting the mo d ulus o f e lasticity o f c o nc re te is the stre ng th o f c o nc re te . This c an b e re p re se nte d in m any w ays suc h as the re latio nship b e tw e e n ratio o f m ix o r w ate r/ c e m e nt ratio . The m o d ulus o f e lastic ity also d e p e nd s up o n the state o f w e tne ss o f co ncre te w he n o the r co nd itio ns b e ing the same . Wet co ncre te w ill sho w hig he r mo d ulus o f e lasticity than d ry co ncre te . This is in co ntrast to the streng th pro perty that dry co ncrete has hig her streng th than w et co ncrete. The po ssible reaso n is that w et co ncrete b eing saturated w ith w ater, experiences less strain fo r a g iven stress and , the re fo re , g ive s hig he r mo d ulus o f e lastic ity, w he re as d ry c o nc re te sho w s hig he r strain fo r g ive n stre ss o n acco unt o f le ss g e l w ate r and inte r-crystal ad so rb e d w ate r. Fig ure 8 .5 sho w s the influe nce o f mo isture co nte nt o n the mo d ulus o f e lasticity. Fig ure 8.5 also sho w s the relatio nship b etw een the mo dulus o f elasticity, mix pro po rtio ns and ag e o f c o nc re te . It c an b e se e n that ric he r m ixe s sho w hig he r m o d ulus o f e lastic ity. Similarly o ld er the co ncrete w hich ag ain is suppo sed to have b eco me stro ng er sho w s hig her

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mo d ulus o f elasticity, thereb y co nfirming that the stro ng er the co ncrete hig her is the mo d ulus o f e lasticity. The q uality and q uantity o f ag g re g ate w ill have a sig nificant e ffe ct o n the mo d ulus o f elasticity. It is to b e rememb ered that the streng th o f ag g reg ate w ill no t have sig nificant effect o n the stre ng th o f co ncre te , w he re as, the mo d ulus o f e lasticity o f ag g re g ate influe nce s the mo d ulus o f elasticity o f co ncrete. Fig ure 8 .6 sho w s the mo d ulus o f elasticity o f co ncrete w ith g rave l ag g re g ate s and e xp and e d c lay ag g re g ate s. It has b e e n se e n that if the mo d ulus o f elasticity o f ag g reg ate is Ea and that o f the paste Ep then the mo d ulus o f elasticity o f co ncrete E is fo und o ut to b e

1 V p Va = + E E p Ea w he re Vp and Va are vo lume o f p aste and ag g re g ate re sp e ctive ly in the co ncre te . The mo d ulus o f e lasticity o f lig ht w e ig ht co ncre te is usually b e tw e e n 4 0 to 8 0 p e r ce nt o f the m o d ulus o f e lastic ity o f o rd inary c o nc re te o f the sam e stre ng th. Sinc e the re is little d iffe re nce b e tw e e n the mo d ulus o f e lasticitie s o f p aste and lig ht w e ig ht ag g re g ate the mix p ro p o rtio ns w ill have ve ry little e ffe ct o n the mo d ulus o f e lasticity o f lig ht w e ig ht co ncre te . 8 .1 The re latio n b e tw e e n the m o d ulus o f e lastic ity and stre ng th is no t m uc h e ffe c te d b y te m p e ratu re u p to ab o u t 2 3 0 ° C sin c e b o th th e p ro p e rtie s vary w ith te m p e ratu re in appro ximately the same manner. Steam-curved co ncrete sho w s a slig htly lo w er mo d ulus than w ate r-curve d co ncre te o f the same stre ng th.

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Exp e rime nts have sho w n that the mo d ulus in te nsio n d o e s no t ap p e ar to d iffe r much fro m m o d ulus in c o m p re ssio n. As the e xp e rim e ntal se t-up p re se nts so m e d iffic ultie s, o nly limite d w o rk has b e e n d o ne to d e te rmine the mo d ulus o f e lasticity in te nsio n. Since the principal use o f reinfo rced co ncrete is in flexural members, co nsiderable amo unt o f w o rk has b e e n co nd ucte d to find o ut the mo d ulus o f e lasticity in fle xure o n sp e cime ns o f b eam. The appro ach w as to lo ad the b eam, measure d eflectio n caused b y kno w n lo ad s and to calculate the mo d ulus o f e lasticity fro m w e ll-kno w n b e am d e fle ctio n fo rmulae . It has b e e n se e n that the stre ss-strain curve s in fle xure ag re e d w e ll w ith the stre ss-strain curve o b taine d in co mp anio n cylind e rs co nce ntrically lo ad e d in co mp re ssio n.

Dynamic Modulus of Elasticity It h as b e e n e xp lain e d e arlie r th at th e stre ss-strain re latio n sh ip o f c o n c re te e xh ib its co mplexity particularly due to the peculiar behavio ur o f g el structure and the manner in w hich the w ate r is he ld in hard e ne d c o nc re te . The value o f E is fo und o ut b y ac tual lo ad ing o f co ncre te i.e., the static mo d ulus o f e lasticity d o e s no t truly re p re se nt the e lastic b e havio ur o f co ncre te d ue to the p he no me no n o f cre e p . The e lastic mo d ulus o f e lasticity w ill g e t affe cte d mo re se rio usly at hig he r stre sse s w he n the e ffe ct o f cre e p is mo re p ro no unce d . Atte mp ts have b e e n mad e to find o ut the mo d ulus o f e lasticity fro m the d ata o b taine d b y no n-d e struc tive te sting o f c o nc re te . The m o d ulus o f e lastic ity c an b e d e te rm ine d b y sub je c ting the c o nc re te m e m b e r to lo ng itud inal vib ratio n at the ir natural fre q ue nc y. This m e tho d invo lve s the d e te rm inatio n o f e ithe r re so nant fre q ue nc y thro ug h a sp e c im e n o f co ncrete o r pulse velo city travelling thro ug h the co ncrete. (Mo re d etail o n this aspect is g iven und er the chapter (‘Testing o f co ncrete’). By making use o f the ab o ve parameters mo d ulus o f e lasticity can b e calculate d fro m the fo llo w ing re latio nship .

Ed = Kn 2 L2 ρ w he re

Ed is the d ynamic mo d ulus o f e lasticity; K is a co nstant, n is the re so nant fre q ue ncy; L is the le ng th o f sp e cime n; and ρ is the d e nsity o f co ncre te .

If L is me asure d in millime tre s and ρ in kg / m 3 the n

Ed = 4 x 1 0 –1 5 n 2 L2 ρ GPa

The value o f E fo und o ut in this me tho d b y the ve lo city o f so und o r fre q u e n c y o f so u n d is re fe rre d as d yn am ic m o d u lu s o f e lastic ity, in co ntrast to the value o f E fo und o ut b y ac tual lo ad ing o f the sp e c im e n an d fro m stre ss-strain re latio n sh ip w hich is kno w n as static mo d ulus o f e lasticity. The value o f d ynamic mo d ulus o f elasticity co mputed fro m ultraso nic p ulse ve lo c ity m e tho d is so m e w hat h ig h e r th an th o se d e te rm in e d b y static m e th o d . Th is is b e c ause th e mo d ulus o f e lastic ity as d e te rmine d by dynamic mo dulus is unaffected by c re e p . Th e c re e p a lso d o e s n o t

Ultra Sonic Pulse Velocity Equipment is used for finding dynamic modulus of elasticity.

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sig nificantly e ffe ct the initial tang e nt mo d ulus in the static me tho d . The re fo re , the value o f dynamic mo dulus and the value o f initial tang ent mo dulus are fo und to b e mo re o r less ag ree w ith e ach o the r. Ap p ro ximate re latio nship b e tw e e n the tw o mo d ulai e xp re sse d in GN/ m 2 is g ive n b y

Ec = 1 .2 5 Ed – 1 9 w he re Ec and Ed are the static and d ynamic mo d ulus o f e lasticity. The re latio nship d o e s no t ap p ly to lig ht w e ig ht co ncre te o r fo r ve ry rich co ncre te w ith ce me nt co nte nt mo re than 5 0 0 kg / m 3 . Fo r lig ht w e ig ht co ncre te the re latio nship can b e as fo llo w s

Ec = 1 .0 4 Ed – 4 .1

Poisson’s Ratio So m e tim e s in d e sig n and analysis o f struc ture s, the kno w le d g e o f p o isso n’s ratio is re q uire d . Po isso n’s ratio is the ratio b e tw e e n late ral strain to the lo ng itud inal strain. It is g e ne rally d e no te d b y the le tte r µ. Fo r no rmal co ncre te the value o f po isso n’s ratio lie s in the rang e o f 0 .1 5 to 0 .2 0 w he n actually d e te rmine d fro m strain me asure me nts. As an alternative metho d, po isso n’s ratio can be determined fro m ultraso nic pulse velo city me tho d and b y find ing o ut the fund ame ntal re so nant fre q ue ncy o f lo ng itud inal vib ratio n o f co ncre te b e am. The p o isso n’s ratio µ can b e calculate d fro m the fo llo w ing e q uatio n.

 V2     2 nL  w he re

2

=

1− µ (1 + µ )(1 − 2 µ )

V is the p ulse ve lo city (mm/ s), n is the re so nant fre q ue ncy (Hz) and L is the le ng th o f the b e am (in mm). The value o f the p o isso n’s ratio fo und o ut d ynamically is little hig he r than the value o f static me tho d . The value rang e s fro m 0 .2 to 0 .2 4 .

Dynamic mo d ulus o f e lasticity can also b e fo und o ut fro m the fo llo w ing e q uatio n.

Ed = ρV w he re

2

(1 + µ)(1 − 2µ) (1 − µ)

V is the p ulse ve lo city

ρ is the d e nsity and µ is the Po isso n’s ratio

Creep Creep can b e d efined as “the time-d epend ent” part o f the strain resulting fro m stress. We have d isc usse d e arlie r that the stre ss-strain re latio nship o f c o nc re te is no t a straig ht line relatio nship but a curved o ne. The deg ree o f curvature o f the stress-strain relatio nship depends upo n many facto rs amo ng st w hich the intensity o f stress and time fo r w hich the lo ad is acting are o f sig nificant interest. Therefo re, it clearly sho w s that the relatio n b etw een stress and strain fo r co ncrete is a functio n o f time. The g radual increase in strain, w itho ut increase in stress, w ith the time is d ue to cre e p . Fro m this e xp lanatio n cre e p can also b e d e fine d as the incre ase in strain und e r sustaine d stre ss. All mate rials und e rg o c re e p und e r so me c o nd itio ns o f lo ad ing to a g re ate r o r smalle r extent. But co ncrete creeps sig nificantly at all stresses and fo r a lo ng time. Furthermo re, creep

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o f co ncrete is appro ximately linear functio n o f stress upto 30 to 40 per cent o f its streng th. The o rder o f mag nitude o f creep o f co ncrete is much g reater than that o f o ther crystalline material e xce p t fo r me tals in the final stag e o f yie ld ing p rio r to failure . The re fo re , cre e p in co ncre te is c o nsid e re d to b e an iso late d rhe o lo g ic al p he no m e no n and this is asso c iate d w ith the g e l structure o f cement paste. Cement paste plays a do minant ro le in the defo rmatio n o f co ncrete. Th e ag g re g ate s, d e p e n d in g u p o n th e typ e an d p ro p o rtio n s m o d ify th e d e fo rm atio n c harac te ristic s to a g re ate r o r le sse r e xte nt. The re fo re , it is lo g ic al initially to e xam ine the structure o f ce me nt p aste and ho w it influe nce s cre e p b e havio ur and the n to co nsid e r ho w the p re se nce o f ag g re g ate mo d ifie s the cre e p b e havio ur. Ce m e nt p aste e sse ntially c o nsists o f unhyd rate d c e m e nt g rains surro und e d b y the p ro d uct o f hyd ratio n mo stly in the fo rm o f g e l. The se g e ls are inte rp e ne trate d b y g e l p o re s and inte rsp e rse d b y cap illary cavitie s. The p ro ce ss o f hyd ratio n g e ne rate s mo re and mo re o f g e l and sub se q ue ntly the re w ill b e re d uctio n o f unhyd rate d ce me nt and cap illary cavitie s. In yo ung co ncre te , g e l p o re s are fille d w ith g e l w ate r and cap illary cavitie s may o r may no t b e fille d w ith w ate r. The mo ve me nt o f w ate r he ld in g e l and p aste structure take s p lace und e r the influence o f internal and external w ater vapo ur pressure. The mo vement o f w ater may also take p lace d ue to the sustaine d lo ad o n co ncre te . The fo rmatio n o f g el and the state o f existence o f w ater are the sig nificant facto rs o n the d efo rmative characteristics o f co ncrete. The g el pro vid es the rig id ity b o th b y the fo rmatio n o f chemical b o nds and b y the surface fo rce o f attractio n w hile the w ater can b e existing in thre e cate g o rie s name ly co mb ine d w ate r, g e l w ate r and cap illary w ate r. It is inte re sting to find ho w such a co ng lo me ratio n o f ve ry fine co llo id al p article s w ith enclo sed w ater-filled vio ds behave under the actio n o f external fo rces. O ne o f the explanatio ns g iven to the mechanics o f creeps is based o n the theo ry that the co llo idal particles slide ag ainst e ac h o the r to re -ad just the ir p o sitio n d isp lac ing the w ate r he ld in g e l p o re s and c ap illary cavities. This flo w o f g el and the co nseq uent displacement o f w ater is respo nsib le fo r co mplex d e fo rmatio n b e havio ur and cre e p o f co ncre te . Cre e p take s p lac e o nly und e r stre ss. Und e r sustaine d stre ss, w ith tim e , the g e l, the ad so rb e d w ate r laye r, the w ate r he ld in the g e l p o re s and cap illary p o re s yie ld s, flo w s and re ad just the mse lve s, w hich b e havio ur is te rme d as cre e p in co ncre te .

Rheological Representation of Creep Analysis o f the me chanical b e havio ur o f a mate rial like hard e ne d ce me nt p aste w hich e xhib its b o th e lastic and ine lastic co mp o ne nts o f d e fo rmatio n und e r lo ad , can b e e xp re sse d in rheo lo g ical terms. The rheo lo g ical appro ach illustrates the mechanical behavio ur o f an ideal e lastic, visco us and p lastic co mp o ne nts.

Macroscopic Rheological Approach At th e m ac ro sc o p ic le ve l, th e stru c tu re o f c e m e n t p aste c an b e re p re se n te d as a co ntinuo us so lid p hase co ntaining saturate d vo id s having a w id e rang e s o f size s. Fig ure 8 .7 (a) sho w s macro sco pic representatio n o f d efo rmatio nal b ehavio ur o f hard ened cement paste. This mo d e l can sho w the time -d e p e nd e nt vo lume chang e s, as lo ng as the iso tro p ic stre sse s are ap p lie d thro ug h the so lid p hase and the d rainag e o f the liq uid can take p lace . 8 .2 The co rrespo nd ing rheo lo g ical mo d el co nsists o f a spring d evice representing the elastic mass aro und a central visco us d ash-po t representing the co nfined liq uid . Refer Fig ure 8.7 (b ). With the he lp o f this mo d e l it is p o ssib le to have an id e a ab o ut the d e fo rmatio nal b e havio ur o f ce me nt paste .

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Microscopic Rheological Approach At th e m ic ro sc o p ic le ve l, th e struc ture o f c e m e n t o f g e l c an b e re p re se n te d as an aniso tro pic crystal clusters rand o mly o riented in a so lid matrix (Fig ure 8 .8 ). The applicatio n o f a m ac ro sc o p ic she ar stre ss to the aniso tro p ic syste m re sults in an irre c o ve rab le vo lum e tric co ntractio n o f the sp ace s in so me o f the cluste rs [Fig ure 8 .8 (a )] and a se p aratio n in o the r clusters [Fig ure 8.8 (b )]. O nly. a fractio n o f the elements is sub jected to pure shear [Fig ure 8.8 (c )]. O n re mo val o f the lo ad the re is a visco -e lastic re co ve ry, b ut d ue to so me d e viato ry stre ss c o mp o ne nt, c e rtain lo c al irre c o ve rab le vo lume c hang e s w ill re main. Fig ure 8 .9 sho w s the furthe r sub m ic ro sc o p ic m o d e ls. Th e y re p re se n te d m e ta sta b le c rystalline g e l c o nsisting o f tw o she e t like crystals se p arate d b y a laye r o f w ate r. Th re e b asic ally d iffe re n t m e c h a n ism s of d e fo rm atio n are p o ssib le . The y are c o mp re ssive stre sse s no rmal to c o ntac t laye r [Fig ure 8 .9 (a )] te n sile stre sse s n o rm al to th e c o n ta c t la ye r [Fig u re 8 . 9 ( b )] sh e e r stre sse s p a ra lle l to th e co ntant laye r [Fig ure 8 .9 (c )]

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In m e c h a n ism ( a ), th e liq u id is c o m p re sse d a n d sq u e e ze d o u t la te ra lly. Th is is acco mpanied b y a red uctio n o f the intercrystalline space. The rate o f liq uid mo vement is slo w an d w ill d e c re ase w ith n arro w in g o f sp ac e w h ic h te n d s to w ard s a lim it e q u al to a mo no mo le cular co mp re sse d w ate r laye r (ab o ut 3 Å). This sq ue e zing aw ay o f liq uid ag ainst stro ng frictio nal fo rce s is the principal cause o f the time d e pe nd e nt, irre co ve rab le chang e s in the ce me nt g e l. In me chanism (b ), visco -e lastic e lo ng atio n may b e e xp e cte d , at a faste r rate than in the case o f co mpressio n. This elo ng atio n is restrained , ho w ever, b y the so lid matrix and d elayed , altho ug h co mp le te re co ve ry may b e e xp e cte d lo ng afte r unlo ad ing . In me chanism (c ), the she ar stre ss re sults in the w ate r laye rs. 8 .3 Und e r the co mp le x syste ms o f ap p lie d lo ad ing , b e lo w the e lastic limit o f the mate rial, vario us co mb inatio ns o f the se b asic me chanisms o f d e fo rmatio n may b e e xp e cte d . O n the b asis o f th e availab le e xp e rim e n tal e vid e n c e , it m ay b e assu m e d th at th e lo n g te rm d e fo rm atio n m e c hanism in c e m e nt g e l is that invo lving narro w ing o f the inte rc rystalline sp ace s. This is re fle cte d in the slo w and d e cre asing rate o f time -d e p e nd e nt o f d e fo rmatio n, as w e ll as in the irre co ve rab le co mp o ne nt o f the d e fo rmatio ns w hich incre ase w ith lo ad ing time . The time d epend ent d efo rmatio n b ehavio ur o f lo ad ed and unlo ad ed hard ened cement paste sho w s a distinct similarity betw een creep (and its reco very) and shrinkag e (and sw elling ). All these pro cesses are g o verned by mo vement o r mig ratio n o f the vario us types o f w ater held. It can b e furthe r e xp laine d as fo llo w s: Applicatio n o f uniaxial co mpressio n w hich is the mo st usual type o f lo ad ing , results in as instantaneo us elastic respo nse o f b o th so lid and liq uid systems. The external lo ad is distrib uted betw een these tw o phases. Under sustained lo ad, the co mpressed liq uid beg ins to diffuse and mig rate fro m hig h to lo w e r stre sse d are as. Und e r unifo rm p re ssure , mig ratio n take s p lac e o utw ards fro m the b o dy. This mechanism is acco mpanied b y a transfer o f lo ad fro m the liq uid p hase to the surro und ing so lid , so that stre ss acting o n the so lid matrix incre ase s g rad ually, re sulting in an incre ase d e lastic d e fo rmatio n. The re is re aso n to b e lie ve that, afte r se ve ral d ays und e r sustaine d lo ad , the p re ssure o n the cap illary w ate r g rad ually d isap p e ars, b e ing transfe rre d to the surro und ing g e l. Similarly, the p re ssure o n the g e l p o re w ate r d isap p e ars afte r so me w e e ks. The p re ssure o n the inte r and intrac rystalline ad so rb e d w ate r c o ntinue s to ac t d uring the e ntire p e rio d o f lo ad ing , altho ug h the mag nitud e d e cre ase s g rad ually. It can b e said that the ultimate d e fo rmatio n o f the hard e ne d ce me nt p aste , in fact, is the e lastic re sp o nse o f its so lid matrix, w hich b e have s as if the sp ace s w ithin it (w hich are fille d w ith unstab le g e l) w e re q uite e mp ty.

Hydration under Sustained Load Und e r sustaine d lo ad the ce me nt p aste co ntinue s to und e rg o cre e p d e fo rmatio n. If the m e m b e r is sub je c te d to a d ryin g c o n d itio n th is m e m b e r w ill also un d e rg o c o n tin uo us shrinkag e. The mig ratio n o f liq uid fro m the g el po re due to creep may pro mo te the shrinkag e to small e xte nt. It can b e vie w e d that the cre e p , the shrinkag e and the slip d e fo rmatio ns at the d isco ntinuitie s cause d e fo rmatio ns and micro cracks. It sho uld b e re me mb e re d that the pro cess o f hydratio n is also simultaneo usly pro g ressing due to w hich mo re g el is fo rmed w hich w ill naturally he al-up the micro cracks p ro d uce d b y the cre e p and shrinkag e . This he aling up m ic ro c rac ks b y th e d e laye d h yd ratio n p ro c e ss is also re sp o n sib le fo r in c re asin g th e irre co ve rab le co mp o ne nt o f the d e fo rmatio n.

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Inc re ase d rig id ity and stre ng th d e ve lo p m e nt w ith ag e are ad d itio nal c o ntrib utio n o f hydratio n to time dependent defo rmatio n. Mo reo ver, it is likely that, w ith co ntinuing hydratio n the g ro w th o f the so lid p hase at the e xp e nse o f the liq uid p hase g rad ually c hang e s the p arame te r g o ve rning the e xte nt and rate o f the to tal cre e p . Co ncre te structure s in p ractice are sub je cte d to lo ad ing and d rying . At the same time ce rtain amo unt o f d e laye d hyd ratio n also take s p lace . Und e r such a co mp le x situatio n, the struc ture c re e p s, und e rg o e s d rying shrinkag e , e xp e rie nc e s m ic ro c rac ks and also d ue to pro g re ssive hyd ratio n, he als up the micro cracks that are fo rme d d ue to any re aso n.

Measurement of Creep Creep is usually determined by measuring the chang e w ith time in the strain o f specimen sub je cte d to co nstant stre ss and sto re d und e r appro priate co nd itio n. A typical te sting d e vice is sho w n in Fig ure 8 .1 0 . The sp ring e nsure s that the lo ad is sensib ly co nstant in spite o f the fac t that the sp e c im e n c o ntrac ts w ith tim e . Und e r such co nd itio ns, cre e p co ntinue s fo r a very lo ng time, b ut the rate o f creep decreases w ith time . Un d e r c o m p re ssive stre ss, th e c re e p me asure me nt is asso c iate d w ith shrinkag e o f c o nc re te . It is ne c e ssary to ke e p c o m p anio n unlo ad e d sp e cime ns to e liminate the e ffe ct o f sh rin ka g e a n d o th e r a u to g e n o u s vo lu m e c hang e . W hile this c o rre c tio n is q ualitative ly co rrect and yields usable results, so me research w o rke rs maintaine d that shrinkag e and cre e p are no t ind e p e nd e nt and are o f the o p inio n th a t th e tw o e ffe c ts a re n o t a d d itive a s assume d in the te st. It is g e n e rally assu m e d th at th e c re e p co ntinue s to assume a limiting value afte r an infinite time und er lo ad . It is estimated that 2 6 p e r ce nt o f the 2 0 ye ar cre e p o ccurs in 2 w e e ks. 5 5 p e r ce nt o f 2 0 ye ar cre e p o ccurs in 3 mo nths and 7 6 p e r ce nt o f 2 0 ye ar cre e p o ccurs in o ne ye ar. If creep after o ne year is taken as unity, then the averag e value o f creep at later ag es are: 1 .1 4 afte r 2 ye ars 1 .2 0 afte r 5 ye ars 1 .2 6 afte r 1 0 ye ars 1 .3 3 afte r 2 0 ye ars and 1 .3 6 afte r 3 0 ye ars There are many expressio ns to g ive the mag nitude o f ultimate creep in co ncrete member. Ro ss sug g e ste d the re latio n b e tw e e n sp e cific cre e p (cre e p strain p e r unit stre ss) ‘ c ’ and time und e r lo ad ‘t’ in the fo rm 8 .4

c=

t a + bt

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w he re ‘a ’ and ‘ b ’ are co nstants. If a g raph is d raw n w ith t in the x-axis and t/ c in the y-axis it sho w s a straig ht line o f slo pe b and the inte rce p t o n the

t is e q ual to a . c

Then the co nstant can be easily fo und o ut. Refer Fig ure 8.11. The ultimate crep at infinite tim e w ill b e

c =

1 fro m the ab o ve e xp re ssio n. It is inte re sting to o b se rve that w he n t = a / b , b

1 b . i.e. , o ne half o f the ultimate cre e p is re alise d at time t = a / b . 2

As ind ic ate d e arlie r if a lo ad e d c o nc re te me mb e r is ke p t in atmo shp he re sub je c te d to shrinkag e , the m e m b e r w ill und e rg o d e fo rm atio n fro m 3 d iffe re nt c ause s: nam e ly e lastic defo rmatio n, drying shrinkag e and creep defo rmatio n. Fig ure 8.12 sho w s the time dependent d e fo rm a tio n in c o n c re te su b je c te d to sustaine d lo ad . In o rd e r to e stim ate the m a g n itu d e o f c re e p in a m e m b e r su b je c te d to d ryin g , a c o m p a n io n sp e c im e n is alw ays p lac e d at th e sam e te mp e rature and humid ity co nd itio n and the shrinkag e o f the unlo aded specimen is fo und and this mag nitud e o f d efo rmatio n is sub tracted fro m the to tal defo rmatio n o f th e lo a d e d m e m b e r. Kn o w in g th e in stan tan e o u s e lastic d e fo rm atio n , th e c re e p d e fo rm atio n c an b e c alc ulate d . In this, fo r sim p lic ity sake it is assum e d that the shrinkag e o f co ncre te d o e s no t e ffe ct the cre e p in ad d itio n to the lo ad . In fact it is to b e no ted that in ad d itio n to the lo ad ,

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the shrinkag e also w ill have so m e influe nc e o n the m ag nitud e o f c re e p , and c re e p o n shrinkag e . If a m e m b e r is lo ad e d and if this lo ad is sustaine d fo r so m e le ng th o f tim e and the n re m o ve d , th e sp e c im e n in stan tan e o u sly re c o ve rs th e e lastic strain . Th e m ag n itu d e o f instantaneo us reco very o f the elastic strain is so mething less than that o f the mag nitude o f the e lastic strain o n lo ad ing . W ith tim e , c e rtain am o unt o f c re e p strain is also re c o ve re d . It is e stimate d that ab o ut 1 5 p e r ce nt o f cre e p is o nly re co ve rab le . The me mb e r w ill have ce rtain amo unt o f re sid ual strain. This sho w s that the cre e p is no t a simp ly re ve rsib le p he no me no n. Fig ure 8 .1 3 sho w s the p atte rn o f strain o f a lo ad e d sp e cime n and the re co ve ry o f strain o n unlo ad ing afte r so me time .

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Factors Affecting Creep Influe nc e o f Ag g rre e g ate : Ag g reg ate underg o es very little creep. It is really the paste w hich is respo nsib le fo r the creep. Ho w ever, the ag g reg ate influences the creep o f co ncrete thro ug h a re straining e ffe c t o n the mag nitud e o f c re e p . The p aste w hic h is c re e p ing und e r lo ad is re straine d b y ag g re g ate w hic h d o no t c re e p . The stro ng e r the ag g re g ate the m o re is the re straining e ffe ct and he nce the le ss is the mag nitud e o f cre e p . Fig ure 8 .1 4 sho w s the e ffe ct o f the q uality o f ag g re g ate o n the mag nitud e o f cre e p. The g rading , the shape, the maximum size o f ag g reg ate have b een sug g ested as facto rs affe c tin g c re e p . Bu t it is late r sh o w n th at th e e ffe c t o f ag g re g ate an d th e ir p ro p e rtie s mentio ned ab o ve per se d o no t effect the creep , b ut ind irectly they affect the creep fro m the p o in t o f vie w o f to tal ag g re g ate c o n te n t in th e c o n c re te . Th e m o d u lu s o f e lastic ity o f ag g reg ate is o ne o f the impo rtant facto rs influencing creep. It can b e easily imag ined that the h ig h e r th e m o d u lu s o f e lastic ity th e le ss is th e c re e p . Lig h t w e ig h t ag g re g ate sh o w s sub stantially hig her creep than no rmal w eig ht ag g reg ate. Persuamb ly this is b ecause o f lo w er mo d ulus o f e lasticity. In flue n c e o f Mix Pro p o rtio n s: The amo unt o f paste co ntent and its q uality is o ne o f the mo st imp o rtant facto rs influe ncing cre e p . A p o o re r p aste structure und e rg o e s hig he r cre e p . The re fo re , it can b e said that cre e p incre ase s w ith incre ase in w ate r/ ce me nt ratio . In o the r w o rd s, it c an also b e said that c re e p is inve rse ly p ro p o rtio nal to the stre ng th o f c o nc re te . Bro ad ly speaking , all o ther facto rs w hich are affecting the w ater/ cement ratio is also affecting the cre e p . The fo llo w ing tab le sho w s the cre e p o f co ncre te s o f d iffe re nt stre ng th.

Ta ble 8 .2 . Cre e p of Conc re t e of Diffe re nt St re ngt h Co mpressive streng th at the time o f applicatio n o f lo ad MPa

Ultimate specific creep 1 0 -6 per MPa

Ultimate creep at stress-streng th ratio o f 3 0 per cent 1 0 -6

14

999

933

28

114

1067

42

78

1100

56

57

1067

Fig ure 8 .1 5 . ho w s the spe cific cre e p as a functio n o f w ate r/ ce me nt ratio .

In flue n c e o f Ag e : Ag e at w hich a co ncrete memb ers is lo ad ed w ill have a pred o minant effect o n the mag nitud e o f creep. This can b e easily und ersto o d fro m the fact that the q uality o f g e l imp ro ve s w ith time . Such g e l cre e p s le ss, w he re as a yo ung g e l und e r lo ad b e ing no t so stro ng e r cre e p s mo re . What is said ab o ve is no t a ve ry accurate state me nt b e cause o f the fact that the mo isture co ntent o f the co ncrete b eing d ifferent at d ifferent ag e, also influences the mag nitud e o f cre e p . Ef fe c ts o f Cr e e p : The mag nitude o f creep is dependent o n many facto rs, the main facto rs Effe Cre being time and leval o f stress. In reinfo rced co ncrete beams, creep increases the deflectio n w ith time and may b e a critical co nsid e ratio n in d e sig n. In re in fo rc e d c o n c re te c o lu m n s, c re e p p ro p e rty o f c o n c re te is u se fu l. Un d e r lo ad immed iately elastic d efo rmatio n takes place. Co ncrete creeps and d efo rms. It can no t d efo rm ind e pe nd e nt o f ste e l re info rce me nt. The re w ill b e g rad ual transfe r o f stre ss fro m co ncre te to steel. The extra lo ad in the steel is re q uired to b e shared b y co ncrete and this situatio n results

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in e m p lo ym e n t an d d e ve lo p m e n t o f fu ll stre n g th o f b o th th e m ate rials. Ho w e ve r, in e cce ntrically lo ad e d co lumns, cre e p incre ase s the d e fle ctio n and can lo ad to b uckling . In case o f statically ind eterminate structures and co lumn and b eam junctio ns creep may relieve the stress co ncentratio n ind uced b y shrinkag e, temperatures chang es o r mo vement o f sup p o rt. Cre e p p ro p e rty o f c o nc re te w ill b e use ful in all c o nc re te struc ture s to re d uc e the inte rnal stre sse s d ue to no n-unifo rm lo ad o r re straine d shrinkag e . In m ass c o n c rre e te stru c tu rre e s su c h as d am s, o n ac c o unt o f d iffe re ntial te m p e rature co nditio ns at the interio r and surface, creep is harmful and by itself may be a cause o f cracking in the interio r o f dams. Therefo re, all precautio ns and steps must b e taken to see that increase in te mp e rature d o e s no t take p lace in the inte rio r o f mass co ncre te structure . Lo ss o f prestress d ue to creep o f co ncrete in prestressed co ncrete structure is w ell kno w n and p ro visio n is mad e fo r the lo ss o f p re stre ss in the d e sig n o f such structure s.

Shrinkage It has b e e n ind ic ate d in the e arlie r c hap te r that c o nc re te is sub je c te d to c hang e s in vo lum e e ith e r auto g e n o us o r in d uc e d . Vo lum e c h an g e is o n e o f th e m o st d e trim e n tal p ro p e rtie s o f co ncre te , w hich affe cts the lo ng -te rm stre ng th and d urab ility. To the p ractical e ng ine e r, the asp e ct o f vo lume chang e in co ncre te is imp o rtant fro m the p o int o f vie w that it c ause s unsig htly c rac ks in c o nc re te . We have d isc usse d e lse w he re the e ffe c t o f vo lum e chang e due to thermal pro perties o f ag g reg ate and co ncrete, due to alkali/ ag g reg ate reactio n, d ue to sulp hate ac tio n e tc . Pre se ntly w e shall d isc uss the vo lum e c hang e o n ac c o unt o f inhe re ne t p ro p e rtie s o f co ncre te “shrinkag e ”. O ne o f the mo st o b je ctio nab le d e fe cts in co ncre te is the p re se nce o f cracks, p articularly in flo o rs and p ave me nts. O ne o f the imp o rtant facto rs that co ntrib ute to the cracks in flo o rs and pavements is that d ue to shrinkag e. It is d ifficult to make co ncrete w hich d o es no t shrink an d c rac k. It is o n ly a q ue stio n o f m ag n itud e . No w th e q ue stio n is h o w to re d uc e th e

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shrinkag e and shrinkag e cracks in co ncrete structures. As shrinkag e is an inherent pro perty o f c o nc re te it d e m and s g re ate r und e rstand ing o f the vario us p ro p e rtie s o f c o nc re te , w hic h influence its shrinkag e characteristics. It is o nly w hen the mechanism o f all kind s o f shrinkag e and the facto rs affecting the shrinkag e are understo o d, an eng ineer w ill be in a better po sitio n to co ntro l and limit the shrinkag e in the b o d y o f co ncre te . The term shrinkag e is lo o sely used to d escrib e the vario us aspects o f vo lume chang es in co ncrete due to lo ss o f mo isture at different stag es due to different reaso ns. To understand this asp e ct mo re clo se ly, shrinkag e can b e classifie d in the fo llo w ing w ay: (a ) Plastic Shrinkag e ;

(b ) Drying Shrinkag e ;

(c ) Auto g e ne o us Shrinkag e ;

(d ) Carb o natio n Shrinkag e .

Plastic Shrinkage Shrinkag e o f this type manifests itself so o n after the co ncrete is placed in the fo rms w hile the c o nc re te is still in the p lastic state . Lo ss o f w ate r b y e vap o ratio n fro m the surfac e o f c o nc re te o r b y the ab so rp tio n b y ag g re g ate o r sub g rad e , is b e lie ve d to b e the re aso ns o f plastic shrinkag e. The lo ss o f w ater results in the red uctio n o f vo lume. The ag g reg ate particles o r the reinfo rcement co mes in the w ay o f sub sid ence d ue to w hich cracks may appear at the surface o r inte rnally aro und the ag g re g ate o r re info rce me nt. In c ase o f flo o rs and p ave me nts w he re the surfac e are a e xp o se d to d rying is larg e as c o m p are d to d e p th, w he n this larg e surfac e is e xp o se d to ho t sun and d rying w ind , the surface o f co ncre te d rie s ve ry fast w hich re sults in p lastic shrinkag e . So me time s e ve n if the co ncre te is no t sub je cte d to se ve re d rying , b ut po o rly mad e w ith a hig h w ate r/ ce me nt ratio , larg e q uantity o f w ate r b le e d s and accumulate s at the surface .

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Whe n this w ate r at the surface d rie s o ut, th e su rfac e c o n c re te c o llap se s c au sin g cracks. Pla stic c o n c re te is so m e tim e s su b je c te d to u n in te n d e d vib ra tio n o r yielding o f fo rmw o rk suppo rt w hich ag ain c au se s p lastic sh rin kag e c rac ks as th e Typical Plastic Shrinkage cracks due to rapid co ncre te at this stag e has no t d e ve lo p e d evaporation of water from hot sun and drying wind. e no ug h stre ng th. Fro m the ab o ve it c an b e inferred that hig h w ater/ cement ratio , b ad ly pro po rtio ned co ncrete, rapid d rying , g reater b leed ing , unintend ed vib ratio n etc., are so me o f the reaso ns fo r plastic shrinkag e. It can also b e further ad d ed that richer co ncrete und erg o es g reater plastic shrinkag e. Fig ure 8.16 sho w s the influe nce o f ce me nt co nte nt o n p lastic shrinkag e . 8 .5 Plastic shrinkag e c an b e re d uc e d m ainly b y p re ve nting the rap id lo ss o f w ate r fro m surface. This can b e d o ne b y co vering the surface w ith po lyethylene sheeting immed iately o n finishing o peratio n; b y mo no mo lecular co ating s b y fo g spray that keeps the surface mo ist; o r b y w o rking at nig ht. An e ffe ctive me tho d o f re mo ving p lastic shrinkag e cracks is to re vib rate th e c o n c re te in a c o n tro lle d m an n e r. Use o f sm all q uan tity o f alum in ium p o w e r is also sug g e ste d to o ffse t the e ffe ct o f p lastic shrinkag e . Similarly, e xp ansive ce me nt o r shrinkag e

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c o m p e nsating c e m e nt also c an b e use d fo r c o ntro lling th e sh rin kag e d u rin g th e se ttin g o f c o n c re te . Th e p rin c ip al p ro p e rty o f su c h cement is th a t th e e xp an sio n in d u c e d in th e p lastic c o nc re te w ill alm o st o ffse t the no rmal shrinkag e d u e to lo ss o f m o istu re . Un d e r c o rre c t u sag e , th e d istance b e tw e e n the jo ints c an so m e tim e s b e trip le d w itho ut incre asing the le ve l o f sh rin ka g e c ra c kin g . Fu rth e r, u se o f u n n e e d e d hig h slum p c o nc re te , o ve r sa n d e d m ix, h ig h e r a ir e n tra in in g sh o u ld be d isc o u ra g e d in o rd e r to re d u c e th e h ig h e r p lastic shrinkag e .

Drying Shrinkage Just as the hydratio n o f ce me nt is an e ve r lasting p ro ce ss, the d rying shrinkag e is also an e ve r lasting p ro ce ss w he n

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co ncre te is sub je cte d to d rying co nd itio ns. The d rying shrinkag e o f co ncre te is analo g o us to the me chanism o f d rying o f timb e r sp e cime n. The lo ss o f fre e w ate r co ntaine d in hard e ne d co ncre te , d o e s no t re sult in any ap p re ciab le d ime nsio n chang e . It is the lo ss o f w ate r he ld in g el po res that causes the chang e in the vo lume. Fig ure 8.17 sho w s the relatio nship b etw een lo ss o f mo isture and shrinkag e. Under drying co nditio ns, the g el w ater is lo st pro g ressively o ver a lo ng time , as lo ng as the co ncre te is ke pt in d rying co nd itio ns. It is the o re tically e stimate d that the to tal line ar c hang e d ue to lo ng tim e d rying shrinkag e c o uld b e o f the o rd e r o f 10,000 x 10 –6 . But values upto 4,000 x 10 – 6 have b een actually o b served . Fig ure 8.18 sho w s the typ ical ap p aratus fo r me asuring shrinkag e . Cement paste shrinks mo re than mo rtar and mo rtar shrinks mo re than co ncrete. Co ncrete m ad e w ith sm alle r size ag g re g ate sh rin ks m o re th an c o n c re te m ad e w ith b ig g e r size ag g re g ate . The mag nitud e o f d rying shrinkag e is also a functio n o f the fine ne ss o f g e l. The fine r the g e l the mo re is the shrinkag e . It has b e e n p o inte d o ut e arlie r that the hig h p re ssure steam cured co ncrete w ith lo w specific surface o f g el, shrinks much less than that o f no rmally cure d ce me nt g e l.

Factors Affecting Shrinkage O ne o f the mo st imp o rtant fac to rs that affe c ts shrinkag e is the d rying c o nd itio n o r in o ther w o rds, the relative humidity o f the atmo shphere at w hich the co ncrete specimen is kept. If the co ncrete is placed in 100 per cent relative humidity fo r any leng th o f time, there w ill no t b e any shrinkag e , inste ad the re w ill b e a slig ht sw e lling . The typ ic al re latio nship b e tw e e n shrinkag e and time fo r w hic h c o nc re te is sto re d at d iffe re nt re lative humid itie s is sho w n in Fig ure 8 .1 9 . The g rap h sho w s that the mag nitud e o f shrinkag e incre ase s w ith time and also w ith the re d uctio n o f re lative humid ity. The rate o f shrinkag e d e cre ase s rap id ly w ith time . It is o b se rve d th at 1 4 to 3 4 p e r c e n t o f th e 2 0 ye ar sh rin kag e o c c u rs in 2 w e e ks, 4 0 to 80 per cent o f the 20 year shrinkag e o ccurs in 3 mo nths and 66 to 85 per cent o f the 20 year shrinkag e o ccurs in o ne ye ar. Ano ther impo rtant facto r w hich influences the mag nitud e o f shrinkag e is w ater/ cement ratio o f the co ncre te . As me ntio ne d e arlie r, the richne ss o f the co ncre te also has a sig nificant influence o n shrinkag e. Table 8.3 sho w s the typical values o f shrinkag e o f mo rtar and co ncrete sp e cime ns, fo r d iffe re nt ag g re g ate / ce me nt ratio , and w ate r/ ce me nt ratio .

Ta ble 8 .3 . Typic a l Va lue s of Shrink a ge of M or t a r a nd Conc re t e Spe c im e ns, 1 2 5 m m squa re in c ross-se c t ion; St ore d at a Re lat ive H um idit y of 5 0 pe r c e nt a nd 2 1 °C. 8 .8 Ag g reg ate/ cement ratio

Shrinkag e after six mo nths (1 0 -6 ) fo r w ater/ cement ratio o f 0 .4

0 .5

0 .6

0 .7

3

800

1200

–

–

4

550

850

1 ,0 5 0

5

400

600

750

850

6

300

400

550

650

7

200

300

400

500

Ag g reg ate plays an impo rtant ro le in the shrinkag e pro perties o f co ncrete. The q uantum o f an ag g re g ate , its size , and its m o d ulus o f e lastic ity influe nc e the m ag nitud e o f d rying

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shrinkag e. The g rad ing o f ag g reg ate b y itself may no t d irectly make any sig nificant influence. But since it affe cts the q uantum o f paste and w ate r/ ce me nt ratio , it d e finite ly influe nce s the d rying shrinkag e ind ire ctly. The ag g re g ate p article s re strain the shrinkag e o f the p aste . The hard er ag g reg ate d o es no t shrink in uniso n w ith the shrinking o f the paste w hereb y it results in hig he r shrinkag e stre sse s, b ut lo w mag nitud e o f to tal shrinkag e . But a so fte r ag g re g ate yie ld s to the shrinkag e stre sse s o f the p aste and the re b y e xp e rie nc e s lo w e r m ag nitud e o f shrinkag e stre sse s w ithin the b o d y, b ut g re ate r mag nitud e o f to tal shrinkag e .

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Fig ure 8 .2 0 sho w s the typ ical value s o f shrinkag e o f co ncre te mad e w ith d iffe re nt kind s o f ag g re g ate . It can b e se e n fro m the ske tch that a hard e r ag g re g ate w ith hig he r mo d ulus o f e lasticity like q uartz shrinks much le ss than so fte r ag g re g ate s such as sand sto ne . It is to b e also no ted that internal stress and the resultant micro cracks w ill also be mo re in case o f q uartz than that o f the sandsto ne o n acco unt o f shrinkag e stress. The lig ht-w eig ht ag g reg ate usually lead s to hig her shrinkag e, larg ely b ecause such ag g reg ate having lo w er mo d ulus o f elasticity o ffe rs le sse r re straint to the p o te ntial shrinkag e o f the ce me nt p aste . The vo lume fractio n o f ag g re g ate w ill have so me influe nce o n the to tal shrinkag e . The ratio o f shrinkag e o f c o nc re te Sc to shrinkag e o f ne at p aste Sp d e p e nd s o n the ag g re g ate co nte nt in the co ncre te , a . This can b e w ritte n as Sc = Sp (1 –a )n Exp e rim e n tal value s o f ‘ n ’ vary b e tw e e n 1 .2 an d 1 .7 . Fig ure 8 .2 1 also sh o w s th e influe nce o f w ate r/ ce me nt ratio and ag g re g ate co nte nt o n shrinkag e . It is to b e vie w e d that the d rying shrinkag e is o ne o f the mo st d e trime ntal pro pe rtie s o f co ncrete. Fro m the mechanism o f shrinkag e it can be seen that the lo ng term drying shrinkag e is an inhe re nt p ro p e rty o f co ncre te . At b e st, b y taking pro pe r pre cautio ns the mag nitud e o f shrinkag e can o nly b e re d uce d , b ut canno t b e eliminated . The restraining effect o f ag g reg ate and reinfo rcement causes hig h inte rnal stre sse s and ind uc e s inte rnal m ic ro c rac ks w hic h no t o nly im p airs the struc tural inte g rity and stre ng th b ut also re d uce s the d urab ility o f co ncre te . Ano the r aspe ct to b e se e n w ith re sp e c t to the d rying shrinkag e is that mo isture lo ss take s p lac e at the surfac e o f the member, w hich may no t be co mpensated in the same rate by the mo vement o f mo isture fro m inte rio r to the surfac e . As a re sult, mo isture g rad ie nt is se t up in a c o nc re te sp e c ime n. The mo isture g rad ie nt ind uce s d iffe re ntial stre sse s, w hich ag ain ind uce s cracks. As the d rying take s p lac e at the surfac e o f the c o nc re te , the mag nitud e o f shrinkag e varie s c o nsid e rab ly w ith the size and thic kne ss o f the sp e c im e n. Inve stig atio ns have b e e n carrie d o ut to find o ut the influe nce o f the size o f sp e cime n o n shrinkag e . It is o b se rve d that

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347

shrinkag e d e cre ase s w ith an incre ase in the size o f the spe cime n. But ab o ve so me value , the size e ffe ct is no lo ng e r appare nt. It is pe rtine nt at this po int to b ring o ut that the co ncre te o r ce me nt pro d uct und e rg o e s lo n g te rm d ryin g sh rin kag e in varyin g m ag n itud e d e p e n d in g up o n th e vario us fac to rs mentio ned in the pro ceeding parag raphs. The effect o f this shrinkag e is to cause cracks in the c o n c re te . O rd in ary Po rtlan d c e m e n t d o e s n o t sh o w g o o d e xte n sib ility (th e p ro p e rty to w ithstand g re ate r, vo lume chang e w itho ut b e ing cracke d ). In this re sp e ct lo w he at ce me nt o r Po rtland po zzo lana cement w ill have hig her extensibility. It may no t be o ut o f place to po int o ut that ad d itio n o f a certain q uantity o f lime w ill impro ve the extensib ility o f o rd inary cement co ncre te . The sup e rio rity o f lime mo rtar fo r inte rnal p laste r o ve r ce me nt mo rtar is fro m the po int o f view o f the superio r extensib ility o f lime mo rtar o ver cement mo rtar b y ab o ut 7 times. A co ntinuo us surface, like plaster o n the w all, und erg o es tremend o us chang e in vo lume, and as such ce me nt mo rtar having lo w e xte nsib ility, is no t ab le to w ithstand the vo lume chang e w itho ut cracking , w here lime mo rtar o r g aug ed mo rtar having hig her extensibility g ives better p e rfo rmance .

Moisture Movement Co nc re te shrinks w he n allo w e d to d ry in air at a lo w e r re lative humid ity and it sw e lls w hen kept at 100 per cent relative humidity o r w hen placed in w ater. Just as drying shrinkag e is an e ve r co ntinuing p ro ce ss, sw e lling , w he n co ntinuo usly p lace d in w ate r is also an e ve r c o ntinuing p ro c e ss. If a c o nc re te sam p le sub je c te d to d rying c o nd itio n, at so m e stag e , is sub je c te d to w e tting c o nd itio n, it starts sw e lling . It is inte re sting to no te that all the initial d rying shrinkag e is no t re co ve re d e ve n afte r p ro lo ng e d sto rag e in w ate r w hich sho w s that the p he no m e no n o f d rying shrinkag e is no t a fully re ve rsib le o ne . Fo r the usual rang e o f c o nc re te , the irre ve rsib le p art o f shrinkag e , re p re se nts b e tw e e n 0 .3 and 0 .6 o f the d rying shrinkag e , the lo w e r value b e ing mo re co mmo n. Just as the d rying shrinkag e is d ue to lo ss o f ad so rb e d w ate r aro und g e l p artic le s, sw e lling is d ue to the ad so rp tio n o f w ate r b y the c e me nt g e l. The w ate r mo le c ule s ac t ag ainst the c o he sive fo rc e and te nd to fo rc e the g e l particles further apart as a result o f w hich sw elling takes place. In additio n, the ing ress o f w ater d e cre ase s the surface te nsio n o f the g e l. The p ro p e rty o f sw e lling w he n p lace d in w e t co nd itio n, and shrinking w he n p lace d in d rying co nd itio n is referred as mo isture mo vement in co ncrete. Fig ure 8 .2 2 sho w s the typical mo isture mo ve me nt o f 1 :1 ce me nt mo rtar mix, sto re d alte rnative ly in w ate r and d rie d in air to 5 0 p e r c e nt re lative humid ity. The mo isture mo ve me nt in c o nc re te ind uc e s alte rnative ly co mp re ssive stre ss and te nsile stre ss w hich may cause fatig ue in co ncre te w hich re d uce s the d urab ility o f co ncre te o w ing to re ve rsal o f stre sse s.

Autogeneous Shrinkage In a c o nse rvative syste m i.e . w he re no m o isture m o ve m e nt to o r fro m the p aste is p e rmitte d , w he n te mp e rature is co nstant so me shrinkag e may o ccur. The shrinkag e o f such a co nse rvative syste m is kno w n as a auto g e ne o us shrinkag e . Auto g eneo us shrinkag e is o f mino r impo rtance and is no t applicab le in practice to many situatio ns e xce p t that o f mass o f co ncre te in the inte rio r o f a co ncre te d am. The mag nitud e o f auto g e ne o us shrinkag e is in the o rd e r o f ab o ut 1 0 0 x 1 0 –6 .

Carbonation Shrinkage Carb o natio n shrinkag e is a p he no m e no n ve ry re c e ntly re c o g nise d . Carb o n d io xid e p re se nt in the atmo shp he re re acts in the p re se nce o f w ate r w ith hyd rate d ce me nt. Calcium

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hyd ro xid e [Ca(O H) 2 ] g e ts c o nve rte d to c alc ium c arb o nate and also so m e o the r c e m e nt c o m p o und s are d e c o m p o se d . Suc h a c o m p le te d e c o m p o sitio n o f c alc ium c o m p o und in hyd rated cement is chemically po ssib le even at the lo w pressure o f carb o n d io xid e in no rmal atm o shp he re . Carb o natio n p e ne trate s b e yo nd the e xp o se d surfac e o f c o nc re te o nly ve ry slo w ly. The rate o f p e ne tratio n o f carb o n d io xid e d e p e nd s also o n the mo isture co nte nt o f the co ncre te and the re lative humid ity o f the amb ie nt me d ium. Carb o natio n is acco mp anie d b y an incre ase in w e ig ht o f the co ncre te and b y shrinkag e . Carb o natio n shrinkag e is p ro b ab ly caused by the disso lutio n o f crystals o f calcium hydro xide and depo sitio n o f calcium carbo nate in its p lace . As the ne w p ro d uct is le ss in vo lume than the p ro d uct re p lace d , shrinkag e take s p lace . Carb o natio n o f c o nc re te also re sults in inc re ase d stre ng th and re d uc e d p e rm e ab ility, p o ssib ly b e cause w ate r re le ase d b y carb o natio n p ro mo te s the p ro ce ss o f hyd ratio n and also c alc iu m c arb o n ate re d u c e s th e vo id s w ith in th e c e m e n t p aste . As th e m ag n itu d e o f carbo natio n shrinkag e is very small w hen co mpared to lo ng term drying shrinkag e, this aspect is no t o f much sig nificance . But carb o natio n re d uce s the alkalinity o f co ncre te w hich g ive s a pro tective co ating to the reinfo rcement ag ainst rusting . If d epth o f carb o natio n reaches upto ste e l re info rce me nts, the ste e l b e co me s liab le fo r co rro sio n.

R EFER EN C ES 8.1 Shideler J.J., Light weight concrete for structural use, ACI Journal, Oct 1957. 8.2

Ori Ishai, The Time—dependent Deformational Behaviour of Cement Paste, Mortar and Concrete, International conference on structure of concrete, Sept 1965.

8.3 Troxell G.C. etal, Long-time creep and shrinkage Tests of plain and Reinforced concrete Proceedings ASTM V 58, 1958. 8.4 Ross A.D., Concrete creep Data, The structural Engineer, 1937. 8.5

L’Hermite, Volume Changes of Concrete, Proceedings, 4th International Symposium on the Chemistry of Cement, Washington D.C. 1960.

8.6

Powers T.C., Causes and Control of Colume Change, Journal of Portland Cement Association, Research and Development Laboratories No. 1, Jan 1959.

8.7

Odman STA, Effects of Variation in Volume, Surface Area Exposed to Drying and Composition of Concrete on Shrinkage, RILEM/CEMBREAU, International Colloguium of the Shrinkage of Hydrulic Concretes, Madrid 1968.

8.8 Lea F.M., The Chemistry of Cement and Concrete, 1956.

9

C H A P T E R An architect’s rendering of the Hindu Temple built at Kauai Island, Hawaii. A massive concrete foundation was laid to last for at least one thousand years. They have used high volume fly ash concrete replacing OPC by 57%. Courtesy : P.K. Mehta


General

Durability of Concrete


Strength and Durability Relationship
Volume Change in Concrete
Permeability
Permeability of Cement Paste
Factors Contributing to Cracks in Concrete
Mass Concrete

General


Concrete Subjected to High Temperature

F


Freezing and Thawing
Moisture Movement
Transition Zone
Biological Process
Structural Design Difficiencies
Chemical Action
Sulphate Attack
Alkali-Aggregate Reaction
Acid Attack
Concrete in Sea Water
Carbonation
Chloride Attack
Crack Width
Deterioration of Concrete by Abrasion, Erosion and Cavitation
Effects of Some Materials on Durability
Surface Treatments of Concrete
Concluding Remarks on Durability

or a long time, concrete was considered to be very durable material requiring a little or no maintenance. The assumption is largely true, except when it is subjected to highly aggressive environments. We build concrete structures in highly polluted urban and industrial areas, aggressive marine environments, harmful sub-soil water in coastal area and many other hostile conditions where other materials of construction are found to be non-durable. Since the use of concrete in recent years, have spread to highly harsh and hostile conditions, the earlier impression that concrete is a very durable material is being threatened, particularly on account of premature failures of number of structures in the recent past. In the past, only strength of concrete was considered in the concrete mix design procedure assuming strength of concrete is an all pervading factor for all other desirable properties of concrete including durability. For the first time, this pious opinion was proved wrong in late 1930’s when 349

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they found that series of failures of concrete pavements have taken place due to frost attack. Although compressive strength is a measure of durability to a great extent it is not entirely true that the strong concrete is always a durable concrete. For example, while it is structurally possible to build a jetty pier in marine conditions with 20 MPa concrete, environmental condition can lead this structure to a disastrous consequences. In addition to strength of concrete another factor, environmental condition or what we generally call exposure condition has become an important consideration for durability. Concrete durability is a subject of major concern in many countries. Number of international seminars are held on concrete durability and numerous papers written on failures of concrete structures are discussed and state-of-the-art reports are Hoover Dam, USA (1931-36). A symbolic structure for Sustainable Development with written and disseminated , regularly. ever 1000 years of predicted life In the recent revision of IS 456 of 2000, one of the major points discussed, deliberated and revised is the durability aspects of concrete, in line with codes of practices of other countries, who have better experiences in dealing with durability of concrete structures. One of the main reasons for deterioration of concrete in the past, is that too much emphasis is placed on concrete compressive strength. As a matter of fact, advancement in concrete technology has been generally on the strength of concrete. It is now recognised that strength of concrete alone is not sufficient, the degree of harshness of the environmental condition to which concrete is exposed over its entire life is equally important. Therefore, both strength and durability have to be considered explicitly at the design stage. It is interesting to consider yet another view point regarding strength and durability relationship.

Strength and Durability Relationship In the previous paragraphs, we have been discussing all the time that although the strength of concrete has direct relationship with durability it does not hold gold in all situations. This aspect needs little more discussions. Generally, construction industry needs faster development of strength in concrete so that the projects can be completed in time or before time. This demand is catered by high early strength cement, use of very low W/C ratio through the use of increased cement content and reduced water content. The above steps result in higher thermal shrinkage, drying shrinkage, modulus of elasticity and lower creep coefficients. With higher quantity of cement content, the concrete exhibits greater cracking tendencies because of increased thermal and drying shrinkage. As the creep coefficient is low in such concrete, there will not be much scope for relaxation of stresses. Therefore, high early strength concretes are more prone to cracking than moderate or low strength concrete. Of course, the structural cracks in high strength concrete

Durability of Concrete 

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can be controlled by use of sufficient steel reinforcements. But this practice does not help the concrete durability, as provision of more steel reinforcement, will only results in conversion of the bigger cracks into smaller cracks. All the same even this smaller cracks are sufficient to allow oxygen, carbon dioxide, and moisture get into the concrete to affect the long term durability of concrete. Field experience have also corroborated that high early strength concrete are more cracks-prone. According to a recent report, the cracks in pier caps have been attributed to the use of high cement content in concrete. Contractors apparently thought that a higher than the desired strength would speed up the construction time, and therefore used high cement content. Similarly, report submitted by National Cooperative Highway Research Programme (NCHRP) of USA during 1995, based on their survey, showed that more than, 100000 concrete bridge decks in USA showed full depth transverse cracks even before structures were less than one month old. The reasons given are that combination of thermal shrinkage and drying shrinkage caused most of the cracks. It is to be noted that deck concrete is made of high strength concrete. These concretes have a high elastic modulus at an early age. Therefore, they develop high stresses for a given temperature change or amount of drying shrinkage. The most important point is that such concrete creeps little to relieve the stresses. A point for consideration is that, the high early strength concrete made with modern Portland cement which are finer in nature, containing higher sulphates and alkalis, when used 400 kg/m3 or more, are prone to cracking. Therefore if long-term service life is the goal, a proper balance between a too high and a too low cement content must be considered. This is where the use of mineral admixtures comes in handy. We discussed in the above paragraphs, that the present day practice is to use high early strength concrete for early completion of projects. We have also seen that high early strength concrete made by using very low W/C ratio of the order of 0.30 or less by using low water content and high cement content is prone to micro cracking which affects the long term durability. It is interesting to see that the above point of view is not fully convincing when seen from many other considerations. Firstly, the high early strength concrete has high cement content and low water content. On account of low water content, only surface hydration of cement particle would have taken place leaving considerable amount of unhydrated core of cement grains. This unhydrated core of cement grains has strength in reserve. When micro cracks have developed, the unhydrated core gets hydrated, getting moisture through micro cracks. The hydration products so generated seal the cracks and restore the integrity of concrete for long term durability. Secondly, as per Aiticin, the quality of products of hydration (gel) formed in the case of low W/C ratio is superior to the quality of gel formed in the case of high W/C ratio. 9.1 Again as per Aiticin, in low W/C ratio concrete (high early strength concrete) the weak transition zone between aggregate and hydrated cement paste does not exist at all. Unhydrated cement particles are also available in such low W/C ratio concrete for any eventual healing of micro cracks. Thirdly, the micro structure of concrete with very low W/C ratio, is much stronger and less permeable. The interconnected network of capillaries are so fine that water cannot flow any more through them. It is reported that when tested for chloride ion permeability, it showed 10-50 times slower penetration than low strength concrete.

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It is difficult to conclude whether the micro cracks developed in high early strength concrete reduces the long term durability or the delayed hydration of unhydrated core of cement grains would heal up the micro cracks and thereby improve long term durability along with the better quality of product of hydration, higher strength, reduced permeability, in case of low W/C ratio concrete. It is a subject for research.

Volume Change in Concrete It will not be wrong to attribute the lack of durability to the reason of volume change in concrete. Volume change in concrete is caused by many factors. As a matter of fact, probing into the factors causing volume change in concrete will lead to an interesting study of concrete technology. The various causes that are responsible for volume change, fully expose the various factors affecting durability which encompasses wide spectrum of concrete technology. If one takes a close look, one comes to know that, the entire hydration process is nothing but an internal volume change, the effect of heat of hydration, the pozzolamic action, the sulphate action, the carbonation, moisture movement, all types of shrinkages, the effect of chlorides, rusting of steel reinforcement and host of other aspects come under the preview of volume change in concrete. It can also be viewed that it is the permeability that leads to volume change. The volume change results in cracks. It is the cracks that promotes more permeability and thus it becomes a cyclic action, till such time that concrete undergoes deterioration, degradation, disruption and eventual failure. Definition of Durability The durability of cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration. Durable concrete will retain its original form, quality, and serviceability when exposed to its environment. Significance of Durability When designing a concrete mix or designing a concrete structure, the exposure condition at which the concrete is supposed to withstand is to be assessed in the beginning with good judgement. In case of foundations, the soil characteristics are also required to be investigated. The environmental pollution is increasing day by day particularly in urban areas and industrial atmospheres. It is reported that in industrially developed countries over 40 per cent of total resources of the building industries are spent on repairs and maintenance. In India, the money that is spent on repair of buildings is also considerable. Every government department and municipal bodies have their own “Repair Boards” to deal with repairs of buildings. It is a sad state of affairs that we do not give enough attention to durability aspects even when we carry out repairs. We carry out repairs job in a casual manner using only ordinary cement mortar practised decades back. Today, special repair materials and techniques are available. The use of such materials make the repair job more effective and durable. This aspects have been covered in chapter 5. Another point for consideration is that, presently, the use of concrete has been extended to more hostile environments, having already used up all good, favourable sites. Even the good materials such as aggregate, sand are becoming short supply. No doubt that the cement production is modernised, but sometimes the second grade raw materials such as limestones containing excess of chloride is being used for pressing economical reasons. Earlier

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specifications of portland cement permitted a maximum chloride content of 0.05 per cent. Recently, maximum permissible chloride content in cement has been increased to 0.1 per cent. This high permissible chloride content in cement demands much stricter durability considerations in other aspects of concrete making practices to keep the total chloride content in concrete within the permissible limits. In other words, considerations for durability of modern concrete constructions assume much more importance, than hitherto practised. Impact of W/C Ratio on Durability In the preceding pages we have discussed that volume change results in cracks and cracks are responsible for disintegration of concrete. We may add now that permeability is the contributory factor for volume change and higher W/C ratio is the fundamental cause of higher permeability. Therefore, use of higher W/C ratio — permeability — volume change — cracks — disintegration — failure of concrete is a cyclic process in concrete. Therefore, for a durable concrete, use of lowest possible W/C ratio is the fundamental requirement to produce dense and impermeable concrete. There is a tremendous change in the micro structure of concrete made with high W/C ratio and low W/C ratio. With low W/C ratio the permeability decreases to such a level that these concretes are impervious to water. This does not mean that they do not contain interconnected network of capillaries, but these capillaries are so fine that water cannot flow any more through them. When such concretes are tested for chloride ions permeability test, it is found that chloride ions diffuse such concretes at a rate 10 — 50 times slower than that of high W/C ratio concrete. It has been proved beyond doubt that low W/C ratio concrete are less sensitive to carbonation, external chemical attack and other detrimental effects that causes lack of durability of concrete. It has been reported that it become impossible to corrode unprotected steel reinforcement in accelerated corrosion test of a concrete with very low W/C ratio. From this it could be inferred that the best way to protect reinforcing steel against corrosion is to use low W/C and adequate cover, rather than using higher W/C ratio and then protecting the steel by epoxy coating.

Degree of Permeability

Cap and Initial Surface Absorption Test (ISAT)

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It is easy to preach on paper the virtues of using low W/C ratio for all-round durability of concrete. But in actual practice for many years it has been found almost impossible to reduce the W/C ratio below 0.4. This situation has changed for the last fifteen years in India with the practice of using superplasticizers. The advent and use of superplasticizers have revolutionised the art and science of making durable concrete by drastically reducing the W/ C ratio of concrete. The modern superplasticizers are so efficient that it is now possible to make flowing concrete with a W/C as low as 0.25 or even as low as 0.20. This technological breakthrough, in conjunction with the use of silica fume and other secondary cementitious materials, has made it possible to develop a new family of high-strength concrete which is generally referred as high-performance concrete—a concrete which is very durable. In most of these new low W/C ratio concretes, as explained earlier, there is not enough water available to fully hydrate all the cement particles. The water available can only hydrate the surface of cement particles and there exist plenty of unhydrated particles which can play an important role as they constitute strength in reserve. If for any reasons, structural or environmental, concrete gets cracked, the unhydrated cement particles begin hydrating as soon as water or moisture starts penetrating through cracks. This is to say that unhydrated cement particles offer self healing potential to improve durability of concrete.

Permeability We have discussed that W/C ratio is the fundamental point for concrete durability. Another important point for consideration is the permeability of concrete. When we talk about durability of concrete, generally we start discussion from the permeability of concrete as it has much wider and direct repercussion on durability than that of W/C ratio. For example, microcracks at transition zone is a consideration for permeability whereas W/C ratio may not get involved directly. It may be mentioned that microcracks in the initial stage are so small that they may not increase the permeability. But propagation of microcracks with time due to drying shrinkage, thermal shrinkage and externally applied load will increase the permeability of the system.

Permeability of Cement Paste The cement paste consists of C-S-H gel, Ca(OH)2 and water filled or empty capillary cavities. Although gel is porous to the extent of 28 per cent, the gel pores are so small that hardly any water can pass through under normal conditions. The permeability of gel pores is estimated to be about 7 x 10–16 m/s. That is approximately about 1/100 of that of paste.9.2 Therefore, the gel pores do not contribute to the permeability of cement paste. The extent and size of capillary cavities depend on the W/C ratio. It is one of the main factors contributing to the permeability of paste. At lower W/C ratio, not only the extent of capillary cavities is less but the diameter is also small. The capillary cavities resulting at low W/ C ratio, will get filled up within a few days by the hydration products of cement. Only unduly large cavities resulting from higher W/C ratio (say more than 0.7) will not get filled up by the products of hydration, and will remain as unsegmented cavities, which is responsible for the permeability of paste. Table 9.1 shows the permeability of cement paste at various ages

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Table 9.1. Reduction in Permeability of Cement Paste (W/C Ratio = 0.7) with Progress of Hydration. 9.3 Age days

Coefficient of Permeability Km/s

fresh 5 days 6 days 8 days 13 days 24 days ultimate

2 4 1 4 5 1 6

x x x x x x x

10–6 10–10 10–10 10–11 10–12 10–12 10–13 (calculated)

It is very interesting to see that the permeability of cement paste with very low W/C ratio can be compared to the permeability of dense rocks. Table 9.2 shows the comparison between permeabilities of rocks and cement paste. Table 9.2. Comparison Between Permeabilities of Rocks and Cement Pastes 9.2 Type of Rocks 1. 2. 3. 4. 5. 6. 7.

Dense trap Quartz cliorite Marble Marble Granite Sandstone Granite

Coefficient of Permeability m/s 2.47 8.24 2.39 5.77 5.35 1.23 1.56

x x x x x x x

10–14 10–14 10–13 10–12 10–11 10–10 10–10

Water/cement ratio of mature paste of the same permeability 0.38 0.42 0.48 0.66 0.70 0.71 0.71

From Table 9.2 it is seen that the cement paste even with high W/C ratio of 0.70 is quite impervious as that of granite with coefficient of permeability of 5.35 x 10–11 m/s. This value of coefficient of permeability is so small, that physically no water will permeate through in any perceptible manner. However in actual practice, it is noticed that mortar and concrete exhibit appreciable permeability much higher than the values shown in the table 9.2. This is definitely not because of the permeability of aggregates in mortar or concrete. The aggregate used in mortar or concrete is as impermeable as that of paste as can be seen in Table 9.2. The higher permeability of mortar or concrete in actual structures is due to the following reasons. (a ) Formation of microcracks developed due to long term drying shrinkage and thermal stresses. (b ) The large microcracks generated with time in the transition zones. (c ) Cracks generated through higher structural stresses. (d ) Due to volume change and cracks produced on account of various minor reasons. (e ) Existence of entrapped air due to insufficient compaction. Fig. 9.1 shows the relation between permeability and capillary porosity of cement paste

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Fig. 9.1. Relation between permeability and capillary porosity of cement paste9.2

Fig. 9.2. Relation between permeability and water/cement ratio for mature cement pastes (93 per cent of cement hydrated)9.3

Fig. 9.2 shows the relation between permeability and water/cement ratio for mature cement paste (93 per cent of cement hydrated) From Fig. 9.2 it can be seen that coefficient of permeability increases more than 100 times from W/C ratio of 0.4 to 0.7. Therefore, many code of practices fix the maximum W/C ratio at 0.4 so that the ingress of aggressive chemicals is restricted. The restriction of W/C ratio is also imposed in liquid retaining structures. Permeability of Concrete Theoretically, the introduction of aggregate of low permeability into cement paste, it is expected to reduce the permeability of the system because the aggregate particles intercept the channels of flows and make it take a circuitous route. Compared to neat cement paste, concrete with the same W/C ratio and degree of maturity, should give a lower coefficient of

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permeability. But in practice, it is seen from test data it is not the case. The introduction of aggregate, particularly larger size of aggregates increase the permeability considerably. The explanation lies in the development of microcracks that are produced in the transition zone. Opinion differs in this regard about the size of microcracks that are generated at the transition zone. However, the drying shrinkage, thermal shrinkage and externally applied load may cause cracks in weak transition zone at the young age. It is reported that the size of the cracks in transition zone is much bigger than most of the capillary cavities present in cement paste. Table 9.3 shows the typical observed values of permeability of concrete used in some of the In-situ Water Permeability of Concrete dams in the United States. The use of pozzolanic materials in optimum proportion reduces the permeability of concrete. This is evidently due to the conversion of calcium hydroxide, otherwise soluble and leachable, into cementitious product. Though air-entrainment, makes the concrete porous, when used up to 6%, makes the concrete more impervious, contrary to general belief. Table 9.3. Typical values of Permeability of concrete used in Dams Cement Content kg/m 3

Water/Cement Ratio

Permeability 10–12 m/s

156 151 138 223

0.69 0.74 0.75 0.46

8 24 35 28

High pressure steam cured concrete in conjunction with crushed silica decreases the permeability. This is due to the formation of coarser C-S-H gel, lower drying shrinkage and accelerated conversion of Ca(OH)2 into cementitious products. Interaction Between Permeability, Volume Change and Cracking In the preceding pages we have discussed about permeability, volume change and cracking of concrete are responsible for lack of durability of concrete and concrete structures. It is difficult to pin point which of these are primarily responsible for affecting durability. Permeability of concrete is often referred as the root cause for lack of durability. But it can be seen that volume change that takes place in an otherwise impervious concrete due to heat of hydration or internal manifestation can crack the concrete affecting durability. Microcracks in transition zone even in initially impermeable concrete, can start the cycle of deterioration process in concrete. Therefore, these three factors, one follows the other two, like day follows the night, are responsible for affecting durability of concrete and concrete structures.




Table 9.4. Types and Causes of Concrete Cracks9.4

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Table 9.5. Other Properties, Types and Causes of Concrete Cracking 9.4

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Table 9.6. Various Types and Causes of Cracks in Concrete

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In discussing durability of concrete, one can go through permeability route, or volume change route or cracks route. By following any one of the above routes, it is possible to discuss the entire aspects of durability of concrete. Therefore, we shall follow the “cracks in concrete” route to discuss and describe the various factors that affect the durability of concrete.

Factors Contributing to Cracks in Concrete Mercer L.B,9.4 in his paper on classification of concrete cracks has given two tables to explain the types and causes of concrete cracks Refer Table 9.4 and Table 9.5. But in our discussion, we shall follow the Table 9.6 step by step and describe the cracks in concrete which affect durability of concrete. Plastic Shrinkage Cracks Water from fresh concrete can be lost by evaporation, absorption by subgrade, formwork and in hydration process. When the loss of water from surface of concrete is faster than the migration of water from interior to the surface, the surface dries up. This creates moisture gradient which results in surface cracking while concrete is still in plastic condition. The magnitude of plastic shrinkage and plastic shrinkage cracks are depending upon ambient temperature, relative humidity and wind velocity. In other words, it depends upon the rate of evaporation of water from the surface of concrete. Rate of evaporation in excess of 1 kg/m2 per hour is considered critical (refer Fig. 5.33). In such a situation, the following measures could be taken to reduce or eliminate plastic shrinkage cracks.
Moisten the subgrade and formworks.
Erect temporary wind breakers to reduce the wind velocity over concrete.
Erect temporary roof to protect green concrete from hot sun.
Reduce the time between placing and finishing. If there is delay cover the concrete with polythylene sheets.
Minimise evaporation by covering concrete with burlap, fog spray and curing compound. Fig. 9.3 shows the typical plastic shrinkage cracks. It is seen that cracks are parallel to one another and are spaced 0.3 to 1.0 meter apart. They can be deep and the width varying from 0.1 to 3.0 mm.

Fig. 9.3. Typical plastic shrinkage cracks. 9.5

Plastic shrinkage cracks are very common in hot weather conditions in pavements floor and roof slab concrete.

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Once they are formed it is difficult to rectify. In case of prefabricated units, they can be healed by controlled revibration, if the concrete is in plastic condition. In roof and floor slab it is difficult to repair. However, sometimes, a thick slurry is poured over the cracks and well worked by trowel after striking each side of the cracks to seal the same. The best way is to take all precautions to prevent evaporation of water from the wet concrete, finish it fast, and cure it as early as feasible. In Mumbai - Pune express highway, the fresh concrete is protected by 100 meter long low tent erected on wheel to brake the wind and also to protect the green concrete from hot sun. In addition, curing compound is sprayed immediately after finishing operations. Plastic shrinkage cracks, if care is not taken, will affect the durability of concrete in many ways. Settlement Cracks Plastic concrete when vibrated or otherwise settles. If the concrete is free to settle uniformly, then there is no cracks. If there is any obstruction to uniform settlement by way of reinforcement or larger piece of aggregate, then it creates some voids or cracks. This is called settlement cracks. This generally happens in a deep beam. Concrete should be poured in layers and each layer should be properly compacted. Building up of large quantity of concrete over a beam should be avoided. Sometimes, the settlement cracks and voids are so severe it needs grouting operators to seal them off. Revibration, if possible is an effective step. Otherwise, they effect the structural integrity of the beam or any other member and badly affects, the durability. Bleeding Water being the lightest ingredient of all the other materials in concrete, bleeding, i.e., the upward movement of water when concrete settle downwards, is natural in concrete. The bleeding water, in certain situations emerge at the surface and in some other situations may not come up to the surface. But bleeding does take place. The bleeding water gets trapped by flat or flaky pieces of aggregates and also by reinforcement and gets accumulated below such aggregates and reinforcement. This is known as internal bleeding. In addition to internal bleeding, the water may further emerge out and accumulate on the top surface of concrete. Firstly the internal bleeding water trapped below flat pieces of aggregate and reinforcement affect the bond between hardened cement paste, (hcp) and aggregate or reinforcement on account of local higher W/C ratio. The interface is easily prone to microcracking due to shrinkage stresses caused on dissipation of heat of hydration and drying shrinkage. The interface becomes a weak link in concrete. On loading, the micro cracks propagate further, making the concrete susceptible to degradation by environmental agencies. The bleeding water, emerged at the top surface of concrete, when evaporates make the top surface porous, having very little abrasion resistances. Often, masons float the concrete when bleeding water is still standing on the surface. Too much working of the top surface presses the coarse aggregate down and brings up fine particles of cement and water. Such top surface made up of too fine materials with excess water develops cracks and craziness, affecting durability of concrete. Delayed Curing Fundamental requirement for good concrete is to maintain uninterrupted hydration,

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especially at early age, when the hydration process is faster. If young concrete dries up fast due to hot sun, drying winds and lower relative humidity, the top surface of concrete is devoid of enough water for continuous hydration process. This results in, as described earlier, formation of plastic shrinkage cracks, poorly formed hydration products and all other deformities in the structures of hydrated cement paste. Modern high grade cements, being finely ground, with higher C3S content needs early curing particularly in hot weather conditions. Structural members which are thin with large surface to volume ratio, such as sunbreakers, chajja etc., needs early curing . The common builders in India have wrong notion that curing is to be done only on the following day of concreting. They are insensitive to the cry of drying and thirsty concrete for water or keeping it in required state of wetness. Delayed curing or interruption in continuous curing or not curing for required period are common bad practices followed in most of the construction site in India. Delayed or interrupted curing or not curing for required period can be compared to an ill fed babies or poorly fed human beings during growing years. Such persons are vulnerable to all kinds of diseases and sure to die prematurely. Similarly, insufficient curing is one of the major causes for lack of strength and durability of concrete structures. Constructional Effects In many construction sites, properly designed standard formworks are not used. Formworks are made in an adhoc manner. Such formworks may fail to maintain their rigidity and firmness when wet concrete is placed and vibrated. Sinking, bending, settlement or lack of rigidity of formwork may cause cracks or deformation in plastic concrete, after compaction, which may go unnoticed. It is well known that excess vibration causes segregation which affects the uniformity of concrete mix. These days high consistency concrete is used either for pumping requirements or on account of using superplasticizers. Care must be taken to vibrate such high slump concrete, otherwise, segregation is sure to take place. Segregated concrete matrix, devoid of coarse aggregate, shrinks more than homogeneous concrete and exhibits high shrinkage cracks. Recently, in one of the major construction sites in Mumbai-Pune express highway, in a road over bridge prestressed concrete girder, 2 to 3 cm thick matrix emerged next to the shuttering plate on account of careless over vibration. On removal of formwork, due to delayed or inefficient curing visible cracks and craziness appeared in such places only. The contractor, much against advice, chipped off such cracked mortar and replastered. This action is sure to reduce the durability of such important prestressed concrete girder. Finishing becomes an important operation in situations where abrasion resistance is an important factor, such as roads and airfield pavements, factory floor, dock yard, warehouse floor etc. Ideally, cement paste must be contained by fine aggregate and matrix must be contained by coarse aggregates. Such a uniform mixture, devoid of excess paste on the surface will suffer from almost no shrinkage and exhibit good abrasion resistance. The stiffness of concrete at the time of trowelling, extent of trowelling and method of trowelling will all become important to improve the abrasion resistance and durability of concrete surface. Early Frost Damage At low temperature, the rate of hydration is slow. The hydration process stops at about –10°C. Till such low temperature hydration process though slow, continues. Freshly mixed concrete must not be exposed to freezing condition to protect the same from disruptive action

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of ice lens. Ice lens will assume 9 per cent more volume than the equivalent water volume. The cumulative effect of increased volume disrupts the integrity of fresh concrete. Once frozen, it is difficult to bring back the integrity of concrete subsequently. It is reported that significant ultimate strength reductions up to about 50 per cent, can occur if concrete is frozen within a few hours after placement or before it attains a compressive strength of 3.5 MPa. Pure water freezes at 0°C. The water in fresh concrete is not pure, but it is a solution of various salts and as such it does not freeze at 0°C, but at lower than 0°C. It must also be understood that, as long as the temperature is more than –10°C, the hydration process continues and concrete gets heated due to heat of hydration. The temperature inside concrete is also influenced by formwork material, reinforcements exposed to outside weather, and thickness of member. Therefore, it is difficult to forecast whether the concrete has undergone freezing or not. When concrete has attained a strength of 3.5 MPa, some quantity of water may have been consumed in the hydration process as bound water and certain amount of water may have been imbibed in gel pores. The gel pores are so fine that no ice could be formed in it. Partially filled capillary water, even if it is frozen no appreciable damage will have taken place to seriously disrupt the concrete. It is only saturated and fully filled capillary water, when no hydration has taken place, if frozen, will cause disruption of concrete and affect the longterm durability. Unsound Materials Cement or aggregate is considered unsound when they cause unacceptable extent of volume change in hardened concrete or mortar which causes cracks and affects durability. In cement, if the raw materials contain more lime that can combine with other acidic oxides, or if the raw materials are not properly burnt to the required temperature for the lime to get fully combined with other oxides, cement becomes unsound. Similarly, the presence of MgO which reacts with water in the similar manner as CaO, can also cause unsoundness. We all know that gypsum is added in appropriate quantity depending upon the C3A content to prevent flash setting. By chance if gypsum is added in excess quantity, it can cause unsoundness in cement by way of slow expansion in hardened concrete. Aggregates containing certain materials such as shale, clay lumps, coal, iron pyrites etc. show unsoundness later when concrete undergoes wetting and drying or freezing and thawing. Moisture absorption is often used as a rough index for unsoundness. But there are standard tests for unsoundness of coarse aggregates. Now a days, crushed sand is being used more often in large works and this practice will grow. Unless proper care is taken crushed sand is likely to contain considerable amount of dust. The excess dust (very fine particle less than 75 micron) is harmful from many points of view and more important being that it causes cracks in concrete. In many parts of our country, good natural fine aggregate is not available. Often they contain unacceptable amount of organic and inorganic fine particles referred as silt. Excess silt in sand interfere with setting time, shrinkage and bond strength. The ultimate effect is the reduction in tensile strength and shrinkage cracks. One of the contributory causes of cracks and craziness in plaster is the presence of excessive silt and mud in natural sand. Shrinkage Shrinkage of concrete is one of the important factors contributing to lack of durability of concrete. Shrinkage is mainly responsible for causing cracks of larger magnitude or minor

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microcracks. The aspect of cracking in concrete is very complex, involving many factors such as magnitude of shrinkage, degree of retraint, extensibility of concrete, extent of stress relaxation by creep and at what age the shrinkage is appearing etc. Fig. 9.4 shows the influence of shrinkage and creep relaxation on concrete cracking. From Fig. 9.4 it can be seen that the cracking does not take place at the predicted point, but the cracking is delayed because of stress relaxation due to creep. Cracks can be avoided only if the stress induced by shrinkage strain, after relaxation by creep, is at all time less than the tensile strength of concrete. The above situation is not happening in most of the cases and as such generally shrinkage causes cracks in concrete.

Fig. 9.4. Influence of shrinkage and creep on concrete cracking9.6

When we discuss about the shrinkage of concrete, there are mainly three aspects of shrinkage which are required to be considered. Firstly, the drying shrinkage, secondly the thermal shrinkage related to heat of hydration and subsequent cooling, and thirdly thermal shrinkage in connection with concrete subjected to variation of ambient temperature. First let us consider the long-term drying shrinkage, which is referred as drying shrinkage. Drying Shrinkage Drying shrinkage in concrete is an inherent property of concrete. This aspect has been dealt in greater detail in chapter 8. The shrinkage is one of the fundamental reasons for initial induction of micro cracks in concrete. The mechanism involved are too complex. Generally the pattern of shrinkage is a function of cement content, water content and W/C ratio. In practical terms at a constant W/C ratio, shrinkage increases with an increase in cement content. But at a given, workability, which approximately means a constant water content, shrinkage is unaffected by the increase in cement content, or shrinkage even decreases because of lower W/C ratio. At lower W/C ratio concrete is stronger to resist shrinkage. But it should not be forgotten that stronger concrete, creeps less, there is less stress relaxation and therefore more microcracks. Fig. 9.5 shows the pattern of shrinkage connecting cement content, W/C ratio and water content. The drying shrinkage takes place over long period if concrete is subjected to lower relative humidity. Fig. 9.6 shows the range of shrinkage vs time. It is seen that shrinkage increases with time but at a decreasing rate.

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Fig. 9.5. The pattern of shrinkage as a function of cement content, water content, and water/cement ratio; concrete moist-cured for 28 days, thereafter dried for 450 days 9.7

According to ACI 209 R–92 the development of shrinkage with time is given by the following equation : St = when

t x Sult. 35 + t

St = shrinkage after t days since the end of 7 days moist curing Sult. = ultimate shrinkage, and t = time in days since the end of moist curing. Although the above equation is subject to considerable variability, this equation can be used to estimate ultimate shrinkage of a wide range of moist cured concrete. It can be seen that 50 per cent of ultimate shrinkage is expected to occur after 35 days of drying. For steam

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cured concrete, the value of 35 in the denominator is replaced by 55, and time t is reckoned from the end of steam curing, generally 1 to 3 days. The IS 456 of 2000, makes the following statement: In the absence of test data, the approximate value of the total shrinkage strain for design may be taken as 0.0003. In addition to drying shrinkage, which takes place throughout the mass, may be in different magnitude, across the cross-section, there is another type of shrinkage known as carbonation shrinkage which occurs near surface zone of concrete where CO2 can react with

Fig. 9.7. Drying shrinkage and carbonation shrinkages of mortar at different relative humidities9.9

Ca(OH)2 to form CaCO3. Carbonation of concrete has serious repercussion on concrete durability which we shall deal separately. For the time being we shall deal with the total shrinkage due to drying and subsequent carbonation. Fig. 9.7 shows the drying shrinkage and carbonation shrinkage of mortar at different relative humidities. It can be noticed that carbonation shrinkage is nearly nil at less than 25 percent relative humidity because of lack of water in the pores to produce carbonic acid. The carbonation shrinkage is also nearly nil at 100 per cent relative humidity because the diffusion of CO2 is not taking place on account of pores filled with water. It can also be noted that at 100 per cent relative humidity even drying shrinkage is also not there. Possibly there is a slight swelling. It is mentioned earlier that 50 per cent of shrinkage will have taken place in about 35 days or considerable amount of shrinkage will have taken place near the surface even earlier. At such an early age, the concrete will not have attained good strength to resist shrinkage stress and therefore the concrete is much more vulnerable to cracking inspite of stress relaxation by high creep at low strength.

Thermal Shrinkage We shall now discuss the aspects of shrinkage associated with heat of hydration. Before we go into the aspect of thermal shrinkage it is necessary at this stage to go into the thermal

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properties of concrete to understand the behaviour of concrete to heating and cooling. The study of thermal properties of concrete is an important aspect while dealing with the durability of concrete. Concrete is a material used in all climatic regions for all kinds of structures. Knowledge of thermal expansion is required in long span bridge girders, high rise buildings subjected to variation of temperatures, in calculating thermal strains in chimneys, blast furnace and pressure vessels, in dealing with pavements and construction joints, in dealing with design of concrete dams and in host of other structures where concrete will be subjected to higher temperatures such as fire, subsequent cooling, resulting in cracks, loss of serviceability and durability. The important properties that will be discussed are:
Thermal conductivity
Thermal diffusivity
Specific heat
Coefficient of thermal expansion Thermal conductivity: This measures the ability of material to conduct heat. Thermal conductivity is measured in joules per second per square metre of area of body when the temperature deference is 1°C per metre thickness of the body. The conductivity of concrete depends on type of aggregate, moisture content, density, and temperature of concrete. When the concrete is saturated, the conductivity ranges generally between about 1.4 and 3.4 j/m2s °c/m. Typical values of conductivity of concrete made with different aggregates are listed in Table 9.7. Table 9.7. Typical Values of Thermal Conductivity of Concrete Made with Different Aggregates.9.11 Type of Aggregate

Wet density of concrete kg/m3

Conductivity J/m2 S°C/m

Quartzite

2440

3.5

Dolomite

2500

3.3

Limestone

2450

3.2

Sandstone

2400

2.9

Granite

2420

2.6

Basalt

2520

2.0

Baryte

3040

2.0

Expanded shale

1590

0.80

Table 9.8 shows the values of conductivity recommended by Loudon and Stacey

9.10

Thermal diffusivity: Diffusivity represents the rate at which temperature changes within the concrete mass. Diffusivity is simply related to the conductivity by the following equation. Diffusivity = where C is the specific heat, and P is the density of concrete. The range of diffusivity of concrete is between 0.002 to 0.006 m2/h

320 480 640 800 960 1120 1280 1440 1600 1760 1920 2080 2240 2400

kg/m3

Unit weight

0.109 0.145 0.203 0.260 0.315 0.389 0.476

Aerated concrete

0.087 0.116 0.159 0.203 0.260 0.315 0.389 0.462 0.549 0.649

Light weight concrete with foamed slag

0.130 0.173 0.230 0.303 0.376 0.462 0.562 0.678 0.794 0.952

Light weight concrete with expanded clay or sintered fly ash

For concrete protected from weather

0.706 0.838 1.056 1.315 1.696 2.267

Normal weight aggregate concrete

0.123 0.166 0.223 0.273 0.360 0.433 0.533

0.100 0.130 0.173 0.230 0.289 0.360 0.433

Light weight concrete with foamed slag

0.145 0.187 0.260 0.332 0.433 0.519 0.635 0.808 0.952 1.194 1.488 1.904 2.561

Light Normal weight weight concrete aggregate with concrete expanded clay or sintered fly ash

For concrete exposed to weather Aerated concrete

Conductivity, Jm/m2sºC/m

Values of Conductivity Recommended by Loudon and Stacey.9.10

Table 9.8

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Specific heat: It is defined as the quantity of heat required to raise the temperature of a unit mass of a material by one degree centigrade. The common range of values for concrete is between 840 and 1170 j/kg per °C Coefficient of thermal expansion Coefficient of thermal expansion is defined as the change in unit length per degree change of temperature. In concrete it depends upon the mix proportions. The coefficient of thermal expansion of hydrated cement paste varies between 11 x 10–6 and 20 x 10–6 per °C. Coefficient of thermal expansion of aggregates vary between 5 x 10–6 and 12 x 10–6 per °C. Limestones and Gabbros will have low values and Gravel and Quartzite will have high values of coefficient of thermal expansion. Therefore the kind of aggregate and content of aggregate influences the coefficient of thermal expansion of concrete. It has been discussed earlier while dealing with the properties of aggregate in chapter 3, that too much of thermal incompatibility between aggregate and paste, causes differential expansion and contraction resulting in rupture of bond at the interface of paste and aggregate. Coefficient of thermal expansion of 1:6 concrete made with different aggregates is given in Table 9.9 Table 9.9. Coefficient of Expansion of 1:6 Concretes Made with Different Aggregates 9.12 Linear coefficient of thermal expansion Type of aggregate

Air-cured concrete 10–6 per °C

Water-cured concrete 10–6 per °C

Air cured and wetted concrete 10–6 per °C

Gravel Granite Quartzite Dolerite Sandstone Limestone Portland stone Blast furnace slag Foamed slag

13.1 9.5 12.8 9.5 11.7 7.4 7.4 10.6 12.1

12.2 8.6 12.2 8.5 10.1 6.1 6.1 9.2 9.2

11.7 7.7 11.7 7.9 8.6 5.9 6.5 8.8 8.5

The values of the coefficient of thermal expansion of concrete, so far discussed applies to concrete subjected to a temperature less than about 65°C. It has been seen that the concrete subjected to higher temperatures show somewhat different values, presumably because of the lower moisture content in the concrete. The importance of the values of coefficient of thermal expansion becomes necessary at higher temperature when dealing with concrete subjected fire or higher temperatures. Table 9.10 shows the values of the coefficient of thermal expansion at conditions of higher temperatures. Having seen a few aspects of properties of concrete which have bearing on expansion and contraction on heating and cooling, let us revert back to thermal shrinkage associated with heat of hydration. A large quantity of heat, up to about 500 j/g (120 cal/g) could be liberated in the hydration of cement. Since the thermal conductivity of concrete is low, a very high

435 310 245

0.4 0.6 0.8

0.4 0.6 0.8

0.68 0.68

Moist

Air 50 per cent relative humidity

Moist air

355 3.55

435 310 245

content Kg/m3

cement ratio

condition

Cement

Water/

Curing

Expanded Shale

Calcareous Gravel

Calcareous Gravel

Aggregate

6.1 4.7

7.7 7.7 9.6

7.6 12.8 11.0

below 260°C 10– 6 per °C

7.5 9.7

18.9 21.1 20.7

20.3 20.5 21.1

Above 430°C 10– 6 per °C

28 days

— 5.0

12.2 8.8 11.7

6.5 8.4 16.7

below 260°C 10– 6 per °C

— 8.8

20.7 20.2 21.6

11.2 22.5 32.8

above 430°C 10– 6 per °C

90 days

Linear coefficient of thermal expansion at the age of

Coefficient of Thermal Expansion of Concrete at High Temperature9.13

Table 9.10

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temperature could be generated in the interior of a large mass of concrete. At the same time, the exterior of the concrete mass loses heat with the result a steep temperature gradient may get established. During subsequent cooling of the interior, serious cracking may take place. The rate of evolution of heat as well as total heat generated depend on the compound composition of cement. C3S and C3 A produces large amount of heat in a short time. The fineness of cement also influences the rate of heat development. The faster rate of heat development is more harmful than the total heat of hydration which develops slowly. Therefore for mass concrete and hydraulic structures they use cement with low C3S and C3 A. It is also advantages to use low cement content and blended cement. Blended cement with high pozzolanic material content gives out the heat rather slowly because of slow pozzolanic reaction, during which time certain quantity of heat gets dissipated, virtually reducing temperature difference between interior and exterior.

Mass Concrete Mass concrete is a concrete having considerable dimensions that may get affected by thermal behaviour of concrete. Concrete dam is one such example of mass concrete. In the design of dam, strength of concrete is of less importance. The primary considerations are given to the aspect of how to reduce the heat of hydration, or if certain amount of heat is generated, how to absorb such heat so that the heat inside the body of concrete is minimised so that it does not cause any detrimental effect by way of cracks in concrete. Now a days, there are many structural elements which are of sizeable dimensions, such as bridge piers, deep beams, massive columns, thick foundation concrete etc. The pouring of concrete in such massive sections need understanding of thermal behaviour mainly with respect to heat of hydration of concrete. Fig. 9.8 and Fig. 9.9 show the typical pattern of temperature change which causes external and internal cracking of large concrete mass. Fitz Gibbon from his research work has shown that temperature deference of more than 20°C between surface and interior causes cracks, assuming the coefficient of thermal expansion of

Fig. 9.8. An example of the pattern of temperature change which causes external cracking of large concrete mass. The critical 20åC temperature difference occurs during cooling. 9.14

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Fig. 9.9. An example of the pattern of temperature change which causes internal cracking of a large concrete mass. The critical 20°C temperature difference occurs during heating but the cracks open only when the interior has cooled through a greater temperature range than the exterior9.14

concrete as 10 x 10–6 per °C. A deference of 20°C, the differential strain would come to 200 x 10–6. This amount of strain is considered as a realistic tensile strain at cracking. Aitcin and Raid cites a case where a 1.1 m square column made of reinforced concrete with ASTM type I cement content of 500 kg/m3 and a silica fume content of 30 kg/m3, showed a rise in temperature of 45°C above the ambient temperature, after 30 hours of placing.9.15 Therefore, there is a need for controlling the heat of hydration in concrete and also not allowing the surface of the concrete to cool rapidly. If the surface is insulated, the difference in temperature between interior and exterior is reduced which improves the cracking behaviour. The retention of the formwork and its insulating properties may be made use of to reduce the difference in temperatures between interior and surface,. In reinforced mass concrete structures also cracking will develop, if the difference in temperature between the interior and the exterior is large. However, appropriate detailing of the reinforcement could be made to control the width and spacing of cracks. Fitz Gibbon estimated that the temperature rise under adiabatic condition is 12°C per 100 kg of cement per cubic metre of concrete, regardless of the type of cement used, for cement contents between 300 to 600 kg/m3. The above fact shows the importance of using blended cement in mass concrete and use of high volume fly ash in concrete constructions for crack free durable concrete. Thermal Expansion and Shrinkage Now let us see the effect of expansion and contraction of concrete subjected to ambient increase or decrease of temperature and their effect on concrete cracking. Earlier we have discussed about the increase and decrease of temperature in concrete due to heat of hydration. The thermal changes due to heat of hydration will only be important for first few days in normal structures but may last for longer time in large mass concrete.

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Under the above sub heading we are going to discuss about the concrete members, subjected to external temperatures, such as roof slabs, road or airfields pavements, bridge decks and other members. They are subjected to diurnal or seasonal changes of temperature. In India, in certain states or cities, the change of ambient temperature in day and night could be as high as 30°C and the change in temperature of actual concrete surface is much higher than 30°C. The seasonal variation could be as large as 40°C. In other countries, in the middle east it is of still higher ranges. The change in diurnal or seasonal temperatures mentioned above makes the concrete expand and contract. Since the structures are not free to expand and contract on account of restraint at support in case of roof slabs and sub grade reaction in case of pavements, a considerable tensile stress more than the tensile strength is generated resulting in cracks in concrete. It is usual that the diurnal variation of temperature in a place like Pune is 20°C or more, (In many other cities, in India too the variation could be as high as 30°C or more). We can have an idea of the tensile strain or the tensile stress that could develop in concrete, from the following calculations. Assume the characteristic compressive strength of concrete = 25 MPa Modulus of Elasticity = 5000 x

N/mm2

= 2.5 x 104 N/mm2 Flexural strength = 0.7 x

f ck N/mm2

= 3.5 N/mm2 Assume the coefficient of thermal expansion of concrete = 10 x 10–6 per °C Assume the diurnal variation of temperature is 20°C ∴ The thermal shrinkage strain = 20 x 10 x 10–6 = 200 x 10–6

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It has been seen from experimental work by Lowe that concrete cracks at a differential strain of 200 x 10–6. A strain which is more than 200 x 10–6 will cause a very high degree of microcracking in concrete. Further, Modulus of elasticity =

stress strain

2.5 x 104 = Tensile stress = 2.5 x 104 x 200 x 10–6 = 5.0 N/mm2 Tensile strength of concrete = 3.5 N/mm2. Therefore a tensile stress of 5.0 N/mm2 is sure to cause microcracks in concrete. The tensile stress will be much higher in case of stronger concrete with higher modulus of elasticity and higher degree of variation of temperature. No doubt concrete of higher compressive strength will have higher tensile strength to withstand the higher tensile stress. But due to lower value of creep of such concrete, a smaller extent of stress relaxation takes place and as such, the stronger concrete will crack more than weaker concrete, from this consideration. However, as written earlier, cracking of concrete is a complex matter. Extensibility

stress 200 × 10 −6

From the above examples and explanation the magnitude of the shrinkage strain is only one of the factors contributing to the cracking of concrete. The following are the other factors influencing the cracking of concrete.
Modulus of elasticity: The lower the modulus of elasticity, the lower will be the amount of induced elastic tensile stress for a given magnitude of shrinkage.
Creep: The higher the creep, the higher is the extent of stress relaxation and hence lower is the net tensile stress.
Tensile strength: Higher the compressive strength, the higher will be the tensile strength, and therefore the lower is the risk that tensile stress will exceed the tensile strength. The combination of factors that are desirable to reduce the advent of cracking in concrete can be described by a single term called extensibility. Concrete is said to have a higher degree of extensibility when it can be subjected to large deformations without cracking. Obviously, for a minimum cracking, the concrete should undergo not only less shrinkage but also should have high degree of extensibility (i.e., low elastic modulus, high creep, and high tensile strength). In general, as said earlier, high strength concretes may be more prone to cracking on account of greater thermal shrinkage (higher cement content) and lower stress relaxation (lower creep). On the other hand, low-strength concretes tend to crack less, because of lower thermal shrinkage (lower cement content) and higher stress relaxation (higher creep). Incidentally, lime mortar has 5 – 7 times more extensibility than cement mortar. Therefore, lime mortar used as plaster, cracks less than cement mortar plaster. In the foregoing paragraphs we have described about the cracks produced in concrete by virtue of variation in the ambient temperature. Sometimes it is possible that combined effect of shrinkage caused by heat of hydration, long-term drying shrinkage and shrinkage on account of variation in ambient temperature, may cause such a high total shrinkage, which

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may virtually cause considerable cracking of concrete members, particularly in roof slabs and concrete pavements. It is seen that roof slab may not leak for a few years. But due to long-term drying shrinkage increasing with age and cyclic expansion and contraction due to variation of external temperatures, the roof slab is likely to leak, after some years. The leakage causes corrosion of reinforcements and affect the durability. Therefore however good is the quality of concrete in roof slab water-proofing treatment is necessary to stop leakage and to increase the durability of structures. Similarly, to improve the functional efficiency and to increase the durability of concrete pavements well-planned and designed joints must be provided in the construction of road, airfield pavements and industrial floors. Joints in Concrete Industrial floors and concrete pavements are constructed generally in alternate bays to allow for the incidental early shrinkage of concrete. A time interval as much as practicable is given between the adjacent bays to provide scope for the maximum possible shrinkage. In case of roof slab of large dimension and in other special cases, expansion joints are provided to cater for the expansion and contraction. In pavements proper joints are provided to direct the possible cracks arising out of expansion and out of thermal expansion and contraction, due to variation in temperature and also due to long-term drying shrinkage. Joints can be broadly classified into four categories:
Construction joints
Expansion joints
Contraction joints
Isolation joints Construction Joints Construction joints are the temporary joints left between subsequent concreting operations. The position of the construction joints should be pre-planned before concreting is started. Till such position and location, concrete must be poured in one operation. The joints must be made at such places that the concrete is less vulnerable to maximum bending moment and maximum shear force. In walls and columns construction, joints should be horizontal and arranged at such a level to coincide with the general architectural features. In columns, the concrete should be filled to the level, preferably, a few inches below the junction of beams. Joints in beams and slabs should be formed at the point of minimum shear. It is also not desirable to have the construction joints at the point of maximum bending moment, therefore the joints may be made at the extreme position of the middle third. The procedure for joining the new concrete to the old concrete at the place of construction joint has been described under placing of concrete. Construction joint should be properly masked when finishing the structure. Badly made and unmasked construction joint will give an ugly appearance to the concrete construction. The groove may be incorporated at the joint to make a feature and to hide the joint. Refer Fig. 9.10. Expansion Joints Concrete is subjected to volume change due to many reasons. Provision must be made to cater for the volume change by way of joint to relieve the stresses produced. Expansion is a function of length. In case of a small building, the magnitude of expansion is not much and

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Fig. 9.10. Construction Joins

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therefore no joint is necessary either in the floor or in the roof. A long building experiences large expansion. It is estimated that for the worst conditions, a long building may undergo an expansion as much as 2.5 cm. Therefore, buildings longer than 45 metres are generally provided with one or more expansion joints. The roof is one of the building elements subjected to maximum temperature differences. The roof is subjected to expansion and contraction during day and night or from season to season and causes pushing or pulling to the supporting load bearing walls. Serious cracks have been experienced in the masonry walls supporting the slab. Attempts are made to create a condition for slab to slide over the wall to prevent pushing and pulling. In the past, expansion joints were provided at closer intervals in the floors and pavements. These days from experience, it is seen that concrete does not actually expand to the extent indicated by the simple analytical calculations, because of the frictional resistance offered by the subgrade. It is therefore, possible to provide expansion joints at a much farther interval than in the past. I.S. 456-2000 recommends as under: In view of the large number of factors involved in deciding the location, spacing and nature of expansion joints, the provision of expansion joint in reinforced cement concrete structures should be left to the discretion of the reinforced concrete designer. For purposes of general guidance, however, it is recommended that structures exceeding 45 m in length shall be divided by one or more expansion joints. The details as to the length of a structure where expansion joints have to be provided can be determined after taking into consideration various factors, such as temperature, exposure to weather, the time and season of the laying of the concrete, etc. Under no circumstances shall a structure of length 45 m or more be without an expansion joints. Refer Fig. 9.11.

Fig. 9.11. Expansion Joints

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Contraction Joints Concrete undergoes plastic shrinkage and drying shrinkage as a result of which concrete shrinks. When shrinkage is restrained, stresses are developed which result in the formation of cracks. To avoid these cracks, contraction joints are provided. Normally, the interval at which these joints are provided will vary from 5 to 10 metres. Contraction joints are sometimes called dummy joints or control joints. Contraction joints will not be necessary if sufficient reinforcement is provided to take up the shrinkage stresses. Contraction joints are generally provided in unreinforced floors and pavements. Contraction joints are made at the time of laying concrete by embedding a timber batton or plank of sufficient depth and required thickness. This is subsequently removed when the concrete is hardened. Sometimes, steel plates of sufficient thickness and width are beaten

Fig. 9.12. Control Joints (or Shrinkage Joints or Dummy Joints or Contraction Joints)

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down into the fresh concrete and then removed when the concrete is hardened. Thirdly, the contraction joint of stipulated width and depth is cut by employing a joint sawing machine. Sawing the joint is a very common practice in the recent time. Sawing is done as soon as the concrete is strong enough to resist tearing or other damage by the blade. Normally sawing is performed within about 24 hrs of finishing. Saw cut can also be done after seven days or more in which case depth of cut could be Concrete Joints Cutter Joints are sawn within 6 to 18 hours before the contraction phase of 1/3 the thickness of slab. Due the concrete commences. to the standard width of diamond blades manufactured world wide, a minimum width of 3 mm to 4 mm would be sufficient. Wider width cuts are unnecessary and will lead to higher cutting and sealing costs. It is necessary that the groove made or cut should be filled up with joint sealing compound to improve the riding quality, to protect the edges of the concrete and also to prevent water from being held and subsequent ingress of moisture. The depth of the joint should be about 1 the thickness of slab., Refer Fig. 9.12. 4

Fig. 9.13.

Fig. 9.14.

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Spacing of Contraction Joints Contraction joints are generally spaced 4.5 to 5.0 m intervals in unreinforced slabs. The spacing could be increased approximately upto 15 m in reinforced slabs depending upon the amount of reinforcement. The IRC stipulations on contraction joint spacing, with and without reinforcement, are given in table below: Slab thickness (cm) a. Unreinforced Slabs 10 15 20 b. Reinforced slabs 10 15 20 25 and above

Air-cured concrete Joints spacing (m)

Weight of reinforcement for reinforced slabs (kg/m2)

4.5 4.5 4.5

-

7.5 13.0 14.0 17.0

2.2 2.7 3.8 5.5

Spacing of Contraction Joints The spacing of expansion joints, has been a matter of discussion because of varied practices and ranges from 20 meters to a few hundred meters. Recommended expansion joint spacing for Indian temperature conditions are given in table below: Period of construction

Degree of roughness of sub grade / sub base

Maximum expansion joint spacing (m) Slab thickness 20 cm 30 cm 40 cm

Winter (Oct – March)

Smooth Rough

50 140

50 140

60 140

Summer (April – Sept)

Smooth Rough

90 140

90 140

120 140

In residential flooring the conventional contraction joint is omitted by casting the slab in alternate bays, to allow for the complete plastic shrinkage and also for maximum extent of drying shrinkage. It is usual to place glass-strip or aluminium strip in between the bays to create discontinuity between adjacent bays to prevent the development of continuous cracks. Isolation Joints: This joint, as the name indicates is provided where the concrete floor meets the permanent structural elements such as walls, columns, foundation blocks, machine foundations etc. Since the movements associated with these structural elements are different from those of the concrete floor, an isolation joints are provided between them. It is provided to full depth of the concrete floor. The width (the gap) of such joint is kept about 10 to 12 mm. To avoid ingress of moisture or other undesirable materials, these joints are filled with a resilient materials and topped with joint fillings compounds. Refer fig. 9.13. Typical joint layout for concrete floor on ground is shown in fig. 9.14

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Concrete Subjected to High Temperature Fire Resistance Concrete, though not a refractory material, is incombustible and has good fire-resistant properties. Fire resistance of concrete structure is determined by three main factors—the capacity of the concrete itself to withstand heat and the subsequent action of water without losing strength unduly, without cracking or spalling; the conductivity of the concrete to heat; and coefficient of thermal expansion of concrete. In the case of reinforced concrete, the fire resistance is not only dependent upon the type of concrete but also on the thickness of cover to reinforcement, The fire introduces high temperature gradients and as a result of it, the surface layers tend to separate and spall off from the cooler interior. The heating of reinforcement aggravates the expansion both laterally and longitudinally of the reinforcement bars resulting in loss of bond and loss of strength of reinforcement. The effect of increase in temperature on the strength of concrete is not much upto a temperature of about 250°C but above 300°C, definite loss of strength takes place. Hydrated hardened concrete contains a considerable proportion of free calcium hydroxide which loses its water above 400°C leaving calcium oxide. If this calcium oxide gets wetted or is exposed to moist air, rehydrates to calcium hydroxide accompanied by an expansion in volume. This expansion disrupts the concrete. Portland blast furnace slag cement is found to be more resistant to the action of fire in this regard. In mortar and concrete, the aggregates undergo a progressive expansion on heating while the hydrated products of the set cement, beyond the point of maximum expansion, shrinks. These two opposing actions progressively weaken and crack the concrete. The various aggregates used differ considerably in their behaviour on heating. Quartz, the principal mineral in sand, granites and gravels expands steadily upto about 573°C. At this temperature it undergoes a sudden expansion of 0.85%. This expansion has a disruptive action on the stability of concrete. The fire resisting properties of concrete is least, if quartz is the predominant mineral in the aggregate. The best fire resistant aggregates, amongst the igneous rocks are, the basalts and dolerites. Limestone expands steadily until temperature of about 900°C and then begins to

Fig. 9.15. Compressive strength of concrete after heating to high temperatures

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contract owing to decomposition with liberation of carbon dioxide. Since the decomposition takes place only at a very high temperature of 900°C, it has been found that dense limestone is considered as a good fire resistant aggregate. Perhaps the best fire resistant aggregate is blast furnace slag aggregate. Broken bricks also form a good aggregate in respect of fire resistance. The long series of tests indicated that even the best fire resistant concretes have been found to fail if concrete is exposed for a considerable period to a temperature exceeding 900°C, while serious reduction in strength occurs at a temperature of about 600°C. Concrete does not show appreciable loss of strength upto a temperature of about 300°C. The loss of strength may be about 50% or more at about 500°C. Figures 9.15 and 9.16 show the effect of different temperatures on the strength of concrete and Fig. 9.17 shows the influence of temperature on the relative modulus of elasticity.

Freezing and Thawing The lack of durability of concrete on account of freezing and thawing action of frost is not of great importance to Indian conditions. But it is of greatest considerations in most part of the world. However, certain regions in India, experience sub-zero temperatures in winter.

Fig. 9.16. Compressive strength of concrete after heating to different temperatures

Fig. 9.17. Influence of temperature on Modulus of Elasticity of concrete

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The concrete structures particularly, the one which are exposed to atmosphere are subjected to cycles of freezing and thawing and as such suffer from the damaging action of frost. The frost action is one of the most powerful weathering action on the durability of concrete. In the extreme conditions, the life span of concrete can be reduced to just a couple of years. The damage from freezing and thawing is most common and as such it is one of the extensively studied field on weathering of concrete in the United States of America, Russia and Northern European countries. Though the durability of concrete is affected by alternative wetting and drying, heating and cooling, penetration and deposition of salt and other aggressive chemicals, leaching of calcium hydroxide, action of certain acids, alkali-aggregate reaction, mechanical wear and tear, abrasion and cavitation, one of the very important factors affecting the durability of concrete in the cold countries, is the action of frost. Therefore the aspect of frost resistance is of much importance and has been studied for more than 70 years. It is very well known that fresh concrete should not be subjected to freezing temperature. Fresh concrete contains a considerable quantity of free water; if this free water is subjected to freezing temperature discrete ice lenses are formed. Water expands about 9% in volume during freezing. The formation of ice lenses formed in the body of fresh concrete disrupt the fresh concrete causing nearly permanent damage to concrete. The fresh concrete once subjected to forst action, will not recover the structural integrity, if later on allowed to harden at a temperature higher than the freezing temperature. Therefore, the fundamental point to note in dealing with cold weather concreting is that the temperature of the fresh concrete should be maintained above 0°C. The hardening concrete also should not be subjected to an extremely low temperature. It has been estimated that the freezing of water in the hardened concrete may exert a pressure of about Concrete structures subjected to alternate cycles of freezing and 14 MPa. The strength of thawing undergoes considerable loss of durability. concrete should be more than the stress to which it is subjected at any point of time to withstand the damaging action. The fully hardened concrete is also vulnerable to forst damage, particularly to the effect of alternate cycles of freezing and thawing. The severest conditions for frost action arise when concrete has more than one face exposed to the weather and is in such a position that it remains wet for a long period. Examples are road kerbs, parapets, concrete members in hydraulic structures just above water level etc. There are various explanations for frost damage. One of the theories attributes the damage directly to the empty space available being insufficient to accommodate the additional

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solid produced when the free water held in concrete freezes. The damage is related to the degree of saturation. Another theory attributes the failure to the production of pressure due to the growth of ice lenses parallel to the surface of the concrete owing to the migration of water from capillaries where the freezing point is depressed. This is similar to the theory of forst heaving in soils. Yet another theory explains the failure to generation of water pressure within the capillary cavities as the ice crystals grow. This hydraulic pressure can only be delivered by flow of water in other spaces, since the ice formed on the surface seals the exterior and the pressure generated forces the water through the fine capillaries. The local pressure so generated eventually exceeds the tensile strength of the concrete and causes breakdown. In all these theories, the permeability, rate of absorption and degree of saturation of the concrete are all important factors. Freezing starts at the surface in the largest cavities and gradually extends to smaller cavities. Water contained in the gel pores are too small to get frozen till the temperature goes below—78°C. In practice no ice is formed in the gel pores. The resistance of concrete to frost action depends on the strength of the paste, water/cement ratio, type of aggregate used, age of concrete, duration and extent to which the concrete is subjected to freezing action. More than all these one of the main factors is the degree of saturation of concrete.

Fig. 9.18. Increase in volume of concrete during prolonged freezing as a function of age when freezing starts9.16

Figure 9.18 indicates the increase in volume to the length of exposure at the different ages and figure 9.19 shows the increase in volume with the number of cycles of freezing at different ages. The fully dry concrete is totally unaffected by frost action, but this is a theoretical statement; because when freezing takes place, naturally, the concrete becomes wet subsequently and loses durability. Figure 9.20 illustrates the influence of water/cement ratio on the frost resistance of concrete. The fine air bubbles entrained in the body of the concrete will act as a buffer to relieve the pressure created while freezing. The part of the water while getting frozen, runs into neighbouring air voids which are partially or fully empty. This relieves the pressure. Figure 9.20 shows the superiority of air entrained concrete with respect to freezing action.

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Fig. 9.19. Increase in volume of concrete subjected to freezing and thawing as a function of age at age when first freezing starts

Fig. 9.20. Incluence of water/cement ratio on the resistance of concrete moist-cured for 28 days9.16

When the concrete is young, it contains more water and if such concrete is subjected to a low temperature greater quantity of water gets frozen and the total disruptive force is of a high order; whereas concrete at later ages contains less moisture and the freezing of such concrete will exert less total pressure. This is shown in Fig. 9.18 The frost damage can be assessed in several ways. Assessment of loss of weight of a sample of concrete subjected to a certain number of cycles of freezing and thawing is one of the methods. Measuring the change in the ultrasonic pulse-velocity or the change in the dynamic modulus of elasticity of specimen is another method. The resistance of the concrete to freezing and thawing is also measured by the durability factor. Blanks defined the durability factor as the “Number of cycles of freezing and thawing to produce failure divided by one hundred”. ASTM method of calculating the durability factor is to continue freezing and thawing for 300 cycles or until the dynamic modulus of elasticity is reduced to 60% of its original value, whichever occurs first.

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The durability factor = There are no established criteria for acceptance or rejection of concrete in terms of the durability factor. Its value is primarily for comparison of different concretes, preferably when only one variable is changed. However, some guidance in interpretation can be obtained from the following. A factor smaller than 40 indicates that the concrete is unsatisfactory with respect to resistance to freezing and thawing. 40 to 60 is the range for doubtful performance. Above 60, the concrete is probably satisfactory and value around 100 is considered satisfactory. Deicing Effect of Salts Deicing chemicals used for snow and ice clearance can cause and aggravate surface scaling. Studies have shown that formation of salt crystals in concrete may contribute to concrete scaling and deterioration layer by layer. In cold regions in the winter, sodium chloride or calcium chloride is used for de-icing snow clearance on concrete roads. The repeated use of salts causes surface scaling of concrete roads. This has been attributed to the physical action of salt and not the chemical action. The use of air-entrainment makes the concrete road more resistance to surface scaling on account of salt action.

Moisture Movement We discussed the topic moisture movement in chapter 8. We may recall that concrete shrinks when allowed to dry in air at low relative humidity and it swells when placed in water. Number of cycles at theConcrete end of t est × Percentage of original modulus members in outdoor conditions such as pavements, bridge decks, transmission poles, 300swimming pools etc are subjected to alternate wetting and drying conditions and water tanks, therefore undergoes expansion and shrinkage. The exposure of concrete to repetitive expansion and shrinkage or repetitive compressive stress and tensile stress which may cause fatigue in concrete and affect the durability of concrete. It is a common experience that swimming pools which are kept dry for some times for repairs or such other reasons, develops cracks and leaks.

Transition Zone We have dealt with the topic transition zone in some detail under the topic of strength in chapter 7. Now, when we are dealing with cracks and durability of concrete, it is necessary to touch upon this as it is of fundamental nature in inducing micro cracks due to very many reasons. Micro cracks in transition zone is a strength limiting factor. Concrete is a brittle material which develops microcracks even before any load is applied.

Transition zone between aggregate and hydrated cement paste.

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On account of the dissimilar material, lack of bond, higher W/C ratio, and bleeding water, the transition zone becomes the weakest link in concrete mass. Under load, microcracks propagate further starting from largest microcracks. At a stress level of 70 per cent of the ultimate strength, the mortar matrix develops small cracks. With increasing stress level, the matrix cracks gradually and spreads throughout the mass. The microcracks in the transition zone at the interface with steel reinforcement becomes more permeable and admits air and water to promote corrosion of steel reinforcement. Incidentally these microcracks increases the depth of carbonation also . Generally speaking, the microcracking at the transition zone is a general feature of concrete which is fundamentally responsible for reducing the long term durability of concrete.

Biological Process It is a common site that in many buildings plants grow and the roots slowly penetrate into concrete or small cracks in concrete and converts it into bigger cracks with further growth. Even small plants such as lichen, algae and mass growing on concrete surface attract moisture and encourage physical and chemical process to deteriorate the concrete. Besides, humic acid produced by micro-growth reacts with cement. In tropical countries, the concrete sewers carrying sewage, produce hydrogen sulphide (H2S) due to anerobic decomposition of sulphur compounds. Hydrogen sulphide gets oxidised by aerobic bacteria producing sulphuric acid. The sulphuric acid attack the concrete above the liquid level on the crown portion of concrete sewer. In this way progressive deterioration of concrete takes place. Marine borers and marine plants also contribute to the deterioration of concrete. Sometimes in tropical conditions algae, fungi and bacteria use atmospheric nitrogen to form nitric acid which attack concrete.

Structural Design Difficiencies Sometimes inadequate provision of main steel reinforcement, or inadequate provision for temperature reinforcement, or wrong spacing of bars, or absence of corner reinforcement may cause unacceptable cracks in concrete. One of the most common occurrence is the displacement of top bars in cantilever thin chajjas, by the movement of concreting gang, causes cracks at the junction point of cantilever chajja. Innumerable examples can be cited such as conjestion of reinforcement and difficulties in proper compacting concrete, particularly at the column and beam junctions, deep beams, the negative reinforcement over T and L beams, should be taken care. In the absence of such care concrete is sure to crack. In certain structures the ultimate creep deformation must be considered, otherwise more than the permissible deflection due to excess creep and unacceptable width of cracks will affect durability. The permissible width of crack depends upon the functions of the structural members and on the exposure conditions of the concrete. Reis et al suggest the following permissible crack widths.9.17 Interior members 0.35 mm Exterior members under normal exposure conditions 0.25 mm Exterior members exposed to aggressive environment 0.15 mm

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Chemical Action When we are dealing with the durability of concrete, chemical attack which results in volume change, cracking of concrete and the consequent deterioration of concrete becomes an important part of discussion. Under chemical attack, we shall discuss about sulphate attack, alkali-aggregate reaction, carbonation, deicing effect of salt, acid attack and effect of sea water.

Sulphate Attack Most soils contain some sulphate in the form of calcium, sodium, potassium and magnesium. They occur in soil or ground water. Because of solubility of calcium sulphate is low, ground waters contain more of other sulphates and less of calcium sulphate. Ammonium sulphate is frequently 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 the formation of H2S, which can be transformed into sulphuric acid by bacterial action. Water used in concrete cooling towers can also be a potential source of sulphate attack on concrete. Therefore sulphate attack is a common occurrence in natural or industrial situations. Solid sulphates do not attack the concrete severely but when the chemicals are in solution, they find entry into porous concrete and react with the hydrated cement products. Of all the sulphates, magnesium sulphate causes maximum damage to concrete. A characteristic whitish appearance is the indication of sulphate attack. The term sulphate attack denote an increase in the volume of cement paste in concrete or mortar due to the chemical action between the products of hydration of cement and solution containing sulphates. In the hardened concrete, calcium aluminate hydrate (C-A-H) can react with sulphate salt from outside. The product of reaction is calcium sulphoaluminate, forming within the framework of hydrated cement paste. Because of the increase in volume of the solid phase which can go up to 227 per cent, a gradual disintegration of concrete takes place. The reactions of the various sulphates with hardened cement paste is shown below Let us take the example of Sodium Sulphate attacking Ca(OH)2 Ca(OH)2 + Na2SO4 . 10H2O CaSO4 . 2H2O + 2NaOH + 8H2O. The reaction with calcium aluminate hydrate is as follows 2(3CaO . Al2O3 . 12H2O) + 3(Na2SO4 . 10H2O) 3CaO . Al2O3 . 3CaSO4 . 31H2O + 2Al(OH)3 + 6NaOH + 17 H2O Calcium sulphate attacks only calcium aluminate hydrate producing calcium sulpho aluminate (3CaO . Al2O3 . 3CaSO4 . 32H2O) known as ettringite. Molecules of water may be 32 or 31. On the other hand magnesium sulphate has a more far reaching action than other sulphates because it reacts not only with calcium hydroxide and hydrated calcium aluminates like other sulphates but also decomposes the hydrated calcium silicates completely and makes it a friable mass. The rate of sulphate attack increases with the increase in the strength of solution. A saturated solution of magnesium sulphate can cause serious damage to concrete with higher water cement ratio in a short time. However, if the concrete is made with low water cement ratio, the concrete can withstand the action of magnesium sulphate for 2 or 3 years. The concentration of sulphates is expressed as the number of parts by weight of SO3 per million

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parts. 1000 PPM is considered moderately severe and 2000 PPM is considered very severe, especially if MgSO4 is the predominant constituent. Another factor influencing the rate of attack is the speed in which the sulphate gone into the reaction is replenished. For this it can be seen that when the concrete is subjected to the pressure of sulphate bearing water on one side the rate of attack is highest. Similarly, alternate wetting and drying due to tidal variation or spraying leads to rapid attack. Methods of Controlling Sulphate Attack Having studied the mechanism of sulphate attack on concrete it will be easy for us to deal with the methods for controlling the sulphate attack. (a) Use of Sulphate Resisting Cement The most efficient method of resisting the sulphate attack is to use cement with the low C3 A content. This has been discussed in detail earlier in chapter I. In general, it has been found that a C3 A content of 7% gives a rough division between cements of good and poor performance in sulphate waters. (b) Quality Concrete A well designed, placed and compacted concrete which is dense and impermeable exhibits a higher resistance to sulphate attack. Similarly, a concrete with low water/cement ratio also demonstrates a higher resistance to sulphate attack. (c) Use of air-entrainment Use of air-entrainment to the extent of about 6% (six per cent) has beneficial effect on the sulphate resisting qualities of concrete. The beneficial effect is possibly due to reduction of segregation, improvement in workability, reduction in bleeding and in general better impermeability of concrete. (d) Use of pozzolana Incorporation of or replacing a part of cement by a pozzolanic material reduces the sulphate attack. Admixing of pozzolana converts the leachable calcium hydroxide into insoluble non-leachable cementitious product. This pozzolanic action is responsible for impermeability of concrete. Secondly, the removal of calcium hydroxide reduces the susceptibility of concrete to attack by magnesium sulphate. (e) High Pressure Steam Curing High pressure steam curing improve the resistance of concrete to sulphate attack. This improvement is due to the change of C3 AH6 into a less reactive phase and also to the removal or reduction of calcium hydroxide by the reaction of silica which is invariably mixed when high pressure steam curing method is adopted. (f) Use of High Alumina Cement The cause of great resistance shown by high alumina cement to the action of sulphate is still not fully understood. However, it is attributed in part to the absence of any free calcium hydroxide in the set cement, in contrast to Portland cement. High alumina cement contains approximately 40% alumina, a compound very susceptible to sulphate attack, when in normal portland cement. But this percentage of alumina present in high alumina cement behaves in a different way. The primary cause of resistance is attributed to formation of protective films which inhibit the penetration or diffusion of sulphate ions into the interior. It should be

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remembered that high alumina cement may not show higher resistance to sulphate attack at higher temperature. A comprehensive study of concrete exposed to natural sulphate soils and to pure sulphate solution in the laboratory for periods ranging upto 25 years, was reported by Miller and Manson. The conclusions derived from this extensive study is given below. (a ) There was a definite correlation between the sulphate resistance of Portland cement and the amount of tricalcium aluminate (C3 A) it contained. High resistance was found for Portland cements containing not more than 5.5 per cent C3 A. (b ) There was no indication that the finer grinding of the cements had any influence on sulphate resistance. (c ) The resistance of seven Portland-pozzolana cements varied over nearly as wide a range as was observed for the 122 Portland cements. (d ) Four calcium aluminate cements (Cement Fondu or similar-non Portland cement) consistently showed a very high resistance to the sulphate bearing water. There was however, some indication that these cements are not completely stable at temperatures above 21 to 38°C. (e ) Specimens cured in steam at temperatures of 100°C and especially at 176°C were highly resistant. The degree of improvement was greatest for those cements originally not highly resistant, that is, those with relatively high C3 A. (f ) Few of the 40 admixtures tried gave markedly improved resistance; many had no effect and some were deleterious. The most effective were linseed, soyabean, and tung oils. A large scale study of resistance of concrete to sulphate soils formed a part of the longtime study of cement performance in concrete. This was carried out by the Portland Cement Association (USA) under the general supervision of an Advisory committee. About 1000 concrete beams of size 15 x 13 x 86 cm were embedded horizontally to half their 15 cm depth in soils containing about 10 per cent soluble sulphates. For half of the specimens the sulphate was principally sodium sulphate. For the other half, 2/3 number of specimen it was sodium sulphate and 1/3 magnesium sulphate. The soil in each basin was alternately made wet and dry. The prevailing temperature was above 0°C. Twenty-seven different Portland cements including all five ASTM type, were used in three concrete mixtures containing cement 223, 307, 390 kg/m3. A report of results to 20 years was published in 1965. With respect to sulphate attack, the following conclusions were drawn. 9.18 (a ) The resistance of concrete to attack by solutions of sulphate salts increases with reduction of C3 A content in the cement. At 6 years, a C3 A content of 7% as calculated without correction for minor oxides provided a good separation between cements of good and poor sulphates resistance. After 20 years, it was concluded that a C3 A content of 5.5 per cent as corrected for minor oxides and about 3.5 per cent as determined by X-ray analysis, were fairly good values for separating superior and poor resistance in the richest mix. It is interesting to note that the 10 year report (1953) observed that beams in the soil containing MgSO4 as well Na2SO4 were less attacked than those in the soil containing mainly Na2SO4. This result is contrary to expectations based on some studies conducted with concretes and mortars continually immersed in sulphate solutions. It was tentatively ascribed to differences in the nature of the salt deposit on the beams resulting from evaporation.

Class

(2)

1

2

3

(1)

(i )

(ii )

(iii )

0.5 to 1.0

0.2 to 0.5

Traces (< 0.2)

(3)

Percent

Total SO3

1.9 to 3.1

1.0 to 1.9

Less than 1.0

(4)

g/l

In Soil SO3 in 2 : 1 Water: Soil Extract

1.2 to 2.5

0.3 to 1.2

Less than 0.3

(5)

g/l

In Ground Water

Concentration of Sulphates, Expressed as SO3

Ordinary Portland cement or Portland slag cement or Portland pozzolana cement Ordinary Portland cement or Portland slag cement or Portland pozzolana cement Supersulphated cement or sulphate resisting Portland cement Supersulphated cement or sulphate resisting Portland cement Portland pozzolana cement or Portland slag cement

(6)

Type of Cement

0.50 0.45

350

0.50

310 330

0.50

0.55

(8)

Maximum Free Water/ Cement Ratio

330

280

(7)

Minimum Cement Content kg/m 3

Dense, Fully compacted Concrete. Made with 20 mm Nominal Maximum Size Aggregates Complying with IS 383




Sl.

Table 9.11. Requirements for Concrete Exposed to Sulphate Attack. As per IS 456 : 2000

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5

(v)

1.0 to 2.0 More than 2.0

3.1 to 5.0 More than 5.0

2.5 to 5.0 Morethan 5.0

Supersulphated or sulphate resisting Portland cement Sulphate resisting Portland cement or supersulphated cement with protective coating.

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 contents near the upper limit of any class, cement contents above these minimum are advised.

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 C 3A 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.

4.

5.

6.

7.

0.40

0.45

3.

400

370

2.

Notes 1. Cement content given in this table is irrespective of grades of cement.

4

(iv)

Table 9.11. (Contd.)

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(b ) Air entrainment improved the performance of almost all of the specimens exposed to alternate drying and soaking in sulphate soils. (c ) Influence of cement content of concrete (and accompanying change in water-cement ratio) was highly significant. For the richest mix, attack was slow, at 6 years little difference in resistance between cements could be seen. For the intermediate and lean mixes, the attack was more rapid, and important differences related to cement composition, especially C3 A content, were clearly evident at that time. Where hardened Portland cement concrete is exposed to soil or ground water containing sulphate compounds, it is necessary to limit the permeability of the concrete. It is also recommended that with the higher sulphate concentration it is necessary to use cement with higher resistance to sulphates, higher cement content and lower water/cement ratio. IS 456 of 2000 gives the recommendations for the type of cement, maximum free W/C ratio and minimum cement content, which are required at different sulphate concentrations in nearneutral ground water having pH of 6 to 9. Table 9.11 shows the requirements for concrete exposed to sulphate attack. For very high sulphate concentrations class 5 in the table 9.11, some form of lining such as polyethylene or polychloroprene sheet, or surface coating based on asphalt, chlorinated rubber, expoxy, or polyurethene materials should be used to prevent access by the sulphate solution. IS 456 of 2000 also stipulates the sulphates in concrete in the following way. Sulphates are present in most cements and in some aggregates: excessive amount of water-soluble sulphate from these or other mix constituents can cause expansion and disruption of concrete. To prevent this, the total water-soluble sulphate content of the concrete mix, expressed as SO3, should not exceed 4 per cent 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 per cent limit does not apply to concrete made with supersulphated cement complying with IS 6909.

Alkali-Aggregate Reaction Detailed discussion has been done on alkali-aggregate reaction in chapter 3 under aggregate and testing of aggregates. Alkali-aggregate reaction (AAR) is basically a chemical reaction between the hydroxyl ions in the pore water within concrete and certain types of rock minerals which sometimes occur as part of aggregates. Since reactive silica in the aggregate is involved in this chemical reaction it is often called alkali-silica reaction (ASR). Since the first paper published by Stantan during 1940’s on this subject, a considerable studies have been made and now it is recognised as one of the major causes of cracking of concrete. Primarily the reaction produces what is called alkali-silica gel of unlimited swelling type under favourable conditions of moisture and temperature, Typical map cracking due to Alkali-Aggregate reaction

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in voids and cracks and further it causes disruption and pattern cracking. The crack width can range from 0.1 mm to as much as 10 mm. What was seen as a rare case in 1940’s have been recognised now as one of the general occurrence in present day concrete to a greater or smaller magnitude. Aggregates used in large concrete construction should be suitably tested to detect tendency for alkali-aggregate reaction. In the construction of nuclear power project at Kaiga initially they did not investigate the quality of aggregate. Later on they suspected the aggregate and as a remedial measure, they went in for low alkali cement having alkali content of less than 0.4. As the concrete technologists are now more conscious about AAR, the cement manufactures are more careful about alkali content (K2O and Na2O) or what is called soda equivalent. This is calculated as the actual Na2O content plus 0.658 times the K2O content of the clinker. It should be less than 0.6 per cent by mass of cement. Alkali content of 0.6 could be considered as a threshold point of high alkali cement. It is to be pointed out that alkali-silica reaction takes place only at high concentrations of OH–, that is at high pH value in the pore water. The pH of the pore water depends on the Dark reaction rim on aggregate border alkali content of cement. Heigh alkali cement may lead to a pH of about 13.5 to 13.9 and low alkali cement results in a pH of about 12.7 to 13.1. An increase in pH of 1.0 represents a ten fold increase in hydrogen ion concentration. Therefore low alkali cement which produces low pH value in the pore water is safe against potentially reactive aggregate. Alkalis not only comes from cement but also comes from sand containing sodium chloride, admixtures, mixing water, sea water penetration, fly ash, blast furnace slag and deicing salt getting into concrete. Alkalis from all these sources must be included in finding the total alkalis. British standard 5328 : part 1 : 1091 specifies a maximum of 3.0 kg of alkalis (expressed as soda equivalent) in 1 m3 of concrete in case of alkali reactive aggregates are used.

Acid Attack Concrete is not fully resistant to acids. Most acid solutions will slowly or rapidly disintegrate portland cement concrete depending upon the type and concentration of acid. Certain acids, such as oxalic acid and phosphoric acids are harmless. The most vulnerable part of the cement hydrate is Ca(OH)2, but C-S-H gel can also be attacked. Silicious aggregates are more resistant than calcareous aggregates. Concrete can be attacked by liquids with pH value less than 6.5. But the attack is severe only at a pH value below 5.5. At a pH value below 4.5, the attack is very severe. As the attack

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Relative acids resistance of concrete Courtesy : P.K. Mehta

proceeds, all the cement compounds are eventually broken down and leached away, together with any carbonate aggregate material. With the sulphuric acid attack, calcium sulphate formed can proceed to react with calcium aluminate phase in cement to form calcium sulphoaluminate, which on crystallisation can cause expansion and disruption of concrete. If acids or salt solutions are able to reach the reinforcing steel through cracks or porosity of concrete, corrosion can occur which will cause cracking.

Concrete in Sea Water Large number of concrete structures are exposed to sea water either directly or indirectly. For several reasons, effect of sea water on concrete deserves special attention. The coastal and offshore structures are exposed to simultaneous action of a number of physical and chemical deterioration process. The concrete in sea water is subjected to chloride induced corrosion of steel, freezing and thawing, salt weathering, abrasion by sand held in water and other floating bodies. Sea water generally contains 3.5 per cent of salt by weight. The ionic concentration of Na+ and Cl– are the highest, typically 11,000 and 20,000 mg/litre respectively. It also contains Mg2+ and SO42–, typically 1400 and 2700 mg/litre respectively. The PH of sea water varies between 7.5 and 8.4. The average value is 8.2. Sea water also contains some amount of CO2. We have already seen earlier in this chapter that magnesium sulphate reacts with free calcium hydroxide in set Portland cement to form calcium sulphate, at the same time precipitating magnesium hydroxide. MgSO4 also reacts with the hydrated calcium aluminate to form calcium sulpho aluminate. These have often been assumed to be the actions primarily responsible for the chemical attack of concrete by sea water. It is commonly observed that deterioration of concrete in sea water is often not characterised by the expansion found in concrete exposed to sulphate action, but takes more the form of erosion or loss of constituents from the parent mass without exhibiting undue expansion. It is inferred that the presence of chlorides in sea water may have retarded the swelling of concrete in sulphate solution. It is also found that concrete will have lost some part

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of lime content due to leaching. Both calcium hydroxide and calcium sulphate are considerably more soluble in sea water and this, will result in increased leaching action. To put it briefly, concrete undergoes several reactions concurrently when subjected to sea water. A concrete of not too massive dimensions exposed to sea water is more likely to show the effects of leaching than expansion, whereas massive structures Diagrammatic representation of deterioration of concrete exposed to like dock walls etc. may show seawater. the effects of expansion also. Courtesy : P.K. Mehta The rate of chemical attack is increased in temperate zones. Experience has shown that most severe attack of sea water on concrete occurs just above the level of high water. The portion between low and high water marks is less affected and the parts below the water level which are continuously remain immersed are least affected. The crystallisation of salt in the portion of concrete above high water level is responsible for disruption of concrete. In place of cold climatic region, the freezing of water in pores at the spray level of concrete is responsible for causing lack of durability in concrete. Freezing of water may also take place between the tidal variation level. It is to be admitted that concrete is not 100% impervious. The water that permeates into the concrete causes corrosion of steel. The product of corrosion being of higher volume than the material they replace, exert pressure which results in lack of durability to reinforced concrete. It is also seen that the lack off durability is more in case of reinforced concrete than the identical plain concrete. Sea water holds certain quantity of sand and silt particularly in the shallow end. The velocity of wave action causes abrasion of concrete. The impact and the mechanical force of wave action also contributes to the lack of durability of concrete. From the foregoing discussion it will be easy to formulate steps to improve the durability of concrete in sea water. Apart from the right type of cement with low C3 A content, the other factor to be considered is the use of rich concrete with low water/cement ratio. The rich concrete with low water/cement ratio mainly makes the concrete impervious to the attack of sea water, and also having very little capillary pores does not hold water, to cause expansion either by freezing or by crystallisation of salt. Provision of adequate cover is another desirable step for increasing durability of reinforced concrete. Use of pozzolanic material is yet another desirable step that could be taken to improve durability against sea water. A good compaction, well made construction joints etc. are other points helping the durability of concrete in sea water. Whenever possible, high pressure steam-cured prefabricated concrete elements should be used for better durability.

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Carbonation Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates. We have seen earlier that the conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a small shrinkage. Now we shall see another aspect of carbonation. CO2 by itself is not reactive. In the presence of moisture, CO2 changes into dilute carbonic acid which attacks the concrete and also reduces alkalinity of concrete. Air contains CO2. The concentration of CO2 in rural air may be about 0.03 per cent by volume. In large cities the content may go up to 0.3 per cent or exceptionally it may go up to even 1.0 per cent. In the tunnel, if not well ventilated the intensity may be much heigher. The pH value of pore water in the hardened concrete is generally between 12.5 to 13.5 depending upon the alkali content of cement. The high alkalinity forms a thin passivating layer around steel reinforcement and protect it from action of oxygen and water. As long as steel is placed in a highly alkaline condition, it is not going to corrode. Such condition is known as passivation. In actual practice CO 2 present in atmosphere in smaller or greater concentration, permeates into concrete and carbonates the concrete and reduces the alkalinity of concrete. The pH value of pore water in the hardened cement paste which was around 13 will be reduced to around 9.0. When all the Ca(OH)2 has become carbonated, the pH value will reduce upto about 8.3. 9.19 In such a low pH value, the protective layer gets destroyed and the steel is exposed to corrosion. The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel. Rate of Carbonation: The rate of carbonation depends on the following factors.
The level of pore water i.e., relative humidity.
Grade of concrete
Permeability of concrete
Whether the concrete is protected or not
depth of cover
Time

Fig. 9.21. Depth of carbonation with respect to strength (grade) of concrete

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It is interesting to know that if pore is filled with water the diffusion of CO2 is very slow. But whatever CO2 is diffused into the concrete, is readily formed into dilute carbonic acid reduces the alkalinity. On the other hand if the pores are rather dry, that is at low relative humidity the CO2 remains in gaseous form and does not react with hydrated cement. The moisture penetration from external source is necessary to carbonate the concrete.

Fig. 9.22. Depth of carbonation for protected and unprotected concrete

Table 9.12. Depth of carbonation with age and grade of concrete. Age-years

Depth of Carbonation (mm) M 20

M 40

2

5.0

0.5

5

8.0

1.0

10

12.0

2.0

50

25.0

4.0

The highest rate of carbonation occurs at a relative humidity of between 50 and 70 per cent The rate of carbonation depth will be slower in case of stronger concrete for the obvious reason that stronger concrete is much denser with lower W/C ratio. It again indicates that the permeability of the concrete, particularly that of skin concrete is much less at lower W/C and as such the diffusion of CO2 does not take place faster, as in the case of more permeable concrete with higher W/C ratio. Fig. 9.21 and table 9.12 show the depth of carbonation in various grades of concretes. It is now well recognised that concrete needs protection for longer durability. Protective coating is required to be given for long span bridge girders, flyovers, industrial structures and chimneys. The fig. 9.22 shows carbonation depth of protected and unprotected concrete. Depth of cover plays an important role in protecting the steel from carbonation. The table

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9.13 shows relationships between W/C, depth of cover and time in years for carbonation depth to reach the reinforcement. Table 9.13. Approximate relations between W/C, depth of cover and time in years for carbonation depth to reach the reinforcement. W/C

Depth of cover (mm)

ratio

15

20

25

30

0.45

100+

100+

100+

100+

0.50

56

99

100+

100+

0.55

27

49

76

100

0.60

16

29

45

65

0.65

13

23

36

52

0.70

11

19

30

43

Time in years for carbonation Measurement of depth of carbonation: A common and simple method for establishing the extent of carbonation is to treat the freshly broken surface of concrete with a solution of phenophthalein in diluted alcohol. If the Ca(OH) is unaffected by CO2 the colour turns out to be pink. If the concrete is carbonated it will remain uncloured. It should be noted that the pink colour indicates that enough Ca(OH)2 is present but it may have been carbonated to a lesser extent. The colour pink will show even up to a pH value of about 9.5.

Chloride Attack

Concrete is under continuous attack by aggressive environmental agencies. Good concrete and suffecient cover is the answer for durability.

Chloride attack is one of the most important aspects for consideration when we deal with the durability of concrete. Chloride attack is particularly important because it primarily causes corrosion of reinforcement. Statistics have indicated that over 40 per cent of failure of structures is due to corrosion of reinforcement. We have already discussed that due to the high alkality of concrete a protective oxide film is present on the surface of steel reinforcement. The protective passivity layer can be lost due to carbonation. This protective layer also can be lost due to the presence of chloride in the presence of water and oxygen. In reality the action of chloride in inducing corrosion of reinforcement is more serious than any other reasons. One may recall that sulphates attack the concrete whereas the chloride attacks steel reinforcements.

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Concrete is unaffected by CO 2

401

Concrete is carbonated

Pink colour indicates that Ca(OH) 2 is unaffected by carbonation. The uncoloured portion indicates that concrete is carbonated.

Chloride enters the concrete from cement, water, aggregate and sometimes from admixtures. The present day admixtures are generally contain negligible quantity of chloride or what they call chloride free. Chloride can enter the concrete by diffusion from environment. The Bureau of Indian Standard earlier specified the maximum chloride content in cement as 0.05 per cent. But it is now increased the allowable chloride content in cement to 0.1 per cent. IS 456 of 2000 limits the chloride content as (cl) in the concrete at the time of placing is shown in Table 9.14. Table 9.14. Limits of Chloride Content of Concrete (IS 456 of 2000) Sl. No

1. 2. 3.

Type or Use of Concrete

Maximum Total acid soluble chloride Content. Expressed as kg/m3 of concrete

Concrete containing metal and steam cured at elevated temperature and prestressed concrete Reinforced concrete or plain concrete containing embedded metal Concrete not containing embedded metal or any material requiring protection from chloride

0.4 0.6 3.0

The amount of chloride required for initiating corrosion is partly dependent on the pH value of the pore water in concrete. At a pH value less than 11.5 corrosion may occur without the presence of chloride. At pH value greater than 11.5 a good amount of chloride is required. Limiting values of chloride contents, above which corrosion may be imminent, for various values of pH are indicated in table 9.15. The total chloride in concrete is present partly as insoluble chlorialuminates and partly in soluble form. It is the soluble chloride, which is responsible for corrosion of reinforcement.

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Table 9.15. Limiting Chloride Content Corresponding to pH of concrete9.20 pH

Chloride content g/litre

ppm

13.5 13.0 12.5 12.0 11.5 11.0 10.0 9.02

6.7400 2.1300 0.6720 0.2130 0.0670 0.0213 0.0021 0.0002

6740 2130 672 213 67 21 2 0.2

Chloride Permeability Based on Charge Passed (As per ASTM C 1202) Chloride Permealility High Moderate Low Very Low Negligible

Charges passed (Coulombs)