HIGH PERFORMANCECONCRETES ANDAPPLICATIONSEdited byS PShahWalterPMurphyProfessorofCivil Engineering,andDirectorofNSFCenter forScienceandTechnologyofAdvancedCementBasedMaterialsNorthWesternUniversity,Evanston,IL,USAS H AhmadProfessorofCivilEngineeringNorthCarolina StateUniversityRaleigh, NC,USAEdward ArnoldA member of theHodder Headline GroupLONDONMELB OU RNEAU CK LAND1994 P Shah and S H AhmadFirstpublished in GreatB r itain 1994British LibraryCataloguing in Publication DataHigh Perf ormance ConcretesandApplicationsI. Shah, Surendra P.II. Ahmad, S. H.691ISB N 0-340-58922-1All rights reserved.No part of this publication may be reproducedortransmitted in any f or m or by any means, electronically ormechanically, including photocopying, recording or any inf or mationstorage or retrieval system, without either prior permission in wr itingf r omthe publisher or a licencepermitting restricted copying. In theU nited K ingdom such licencesare issuedby the CopyrightLicensingAgency: 90 TottenhamCourtRoad, LondonWlP9HETypeset in 10/12 Times by Wearset, B oldon, Tyne andWear.Pr inted in Great B r itain f or Edward Arnol d, a division ofHodderHeadline PLC, Mill Road, DuntonGreen, Sevenoaks, K entTN13 2YA by St Edmundsbur y Press Ltd, B ur y St Edmunds,Suf f ol k,and bound by Hartnolls Ltd, B odmin, Cornwall.PrefaceHighper f or manceconcretes(HPC)representsar atherrecentdevelop-mentinconcretematerialstechnology.HPCisnotacommoditybutarange of products, eachspecificallydesignedtosatisf yin themostef f ectiveway theperf ormancerequirementsf or theintendedapplication.Concretehaslargenumberof propertiesorattr ibutes.These attr ibutescanbegroupedintothreegeneralcategories:(1)attr ibuteswhichbenef ittheconstructionprocess;(2)enhancedmechanicalproperties;(3)en-hancednon-mechanicalpropertiessuchasdur abil ityetc.Forhardenedconcrete,strengthand dur abil ity are thetwo mostimpor tantattributes.Inthelastthreeorf o u ryears,severalnational-scaleresearchprogramshavebeenestablishedtostudyvariousaspectsofhighperf ormanceconcretes.TheseincludethetwointheU S:CenterforScienceandTechnologyf orAdvancedCement-B asedMaterials(ACB M),StrategicHighwayResearchProgram(SHRP);TheCanadianNetwor kofCentersofExcellence(NCE)ProgramonHighPerf ormanceConcrete; theRoyalNorwegianCouncilf orScientificandIndustrialResearchProgram;theSwedishNationalProgramonHPC;theFrenchNational Programcalled'New Ways f or Concrete' and the JapaneseNew ConcreteProgram. As theresultsf romtheseprogramsstarttobedisseminatedanddigestedintheconcreteindustr y,theconcretetechnologywillexperienceasignif icantadvancement.Historically, moreattention has beengiven to the strength attr ibute andconcreteperf ormancehasbeenspecifiedandevaluatedintermsofcompressivestrength the higher the compressive strength, the bettertheexpectedperf ormance.However,experiencehasshownthatdur abil ityconsiderationsbecomemoreimpor tantf orstructuresexposedtohostileenvironments(e.g.marinestructuresandsanitarystructures)andf orstructuressuchasbridgesandpavementswhicharedesignedf orlongerservicel if e.TheSHRPprogramonHighPerf ormanceConcretehasdef inedHPCf orhighwayapplicationsintermsofstr ength anddur abil ityattr ibutesandwater-cementiousmaterialsratio.HPCisdef inedascon-cretethatmeetsthef ol l owingcriteria:It shall have one of the f ollowingstrength characteristics:4-hour strength^25OO psi(17.5MPa)24-hour strength^50OO psi(35 MPa)28-day str ength^10,0OO psi(70 MPa)It shall havea durabil ity f actor>80%af ter300 cycles of f reezingandthawingIt shall have a water-cementitious materialsratio^0.35Dur ingthelastdecade,developmentsinmineralandchemical admix-tureshavemadeitpossibletoproduceconcreteswithrelativelymuchhigherstrengthsthanwasthoughtpossible.Presentlyconcreteswithstrengthsof14,000to16,000 psi(98 to112 MPa)arebeing commerciallyproducedandusedintheconstructionindustr y inU SA.OthercountriessuchasEngl and,Canada,Norway,Sweden,France,Ital y,Japan,HongK ongandSouthK oreaareaggressivelyemployingthehighstrengthconcretetechnology in their constructionpractice.Theaimofthisbookistosummarizethedevelopmentsofthelastdecade in theareaof materialsdevelopmentfor producing higher strengthconcrete,productionmethods,mechanicalpropertieseval uation,nonmechanicalpropertiessuchasdur abil ity, andtheimplicationofmaterialpropertiesonthestructuraldesignandperf ormance.Theuseofhigherstrengthconcretesintheconstructionindustr yhassteadilyincreaseddur ingthelastdecade,andtheref oretwo chaptershavebeendevotedtosummarizetheapplicationsofhigherstrengthconcretes.Expertsf r omU SA,Canada,France,Nor way,SpainandJapanhavecontributedindi-vidual chaptersso as to give thebooka broadperspectiveof the prevailingstate-of -the-artin dif f er entparts of the world. Thebookis intended fortheacademics, engineers,consultants, contractorsandresearchers.Theelevenchaptersin thebookarearrangedso thatthereadercan beselective. Chapter1 providesthe background for the selectionof materialsand proportions.This chapteralso providesinf ormation on qual ity controlaspectsf or concreteswith higher strengths.Chapter2addressestheshorttermmechanicalpropertiessuchascompressivestr ength,modul us of elasticity, tensilestrengthetc.Thelongtermmechanicalpropertiessuchascreep,shrinkageandtemper atur eef f ectsarediscussedinChapter3.Chapter4 providestheinf or mation onthe bond and f atiguecharacteristics. The impor tant aspectof the dur abil ityanditsimplicationf ortheperf ormanceofconcretearediscussedinChapter s.Thef r actur e mechanicsapproachtotheunder standing of the str uctur alresponseis outlined in Chapter 6. Thebehavior of thestr uctur al memberssuchasbeam,columnsandslabsisdetailedinChapter ?.Theductilityissues of thestr uctur al members andthestr uctur al ductil ity is presentedinChapter 8.Chapter9 addressesstr uctur al designconsiderationsandthestr uctur alapplications withspecial emphasis on high-rise buil dings andbridges.Thischapter also summarizes the special construction considerations neededf ortheseconcretes.Chapter10isdedicatedtohighstr engthlightweightaggregate concreteandits applications.Thelast chapteris devotedtotheapplications of HPCin JapanandSouth EastAsia.List of ContributorsP AckerHead,Division: 'B etons et Ciments pour Ouvrages d'Ar t',LaboratoireCentral des Fonts et Chaussees,Paris,FranceS HAhmadProfessor, Departmentof Civil Engineering, North Carolina StateU niversity, Raleigh, NC, U SAP NBalaguruProfessor, Civil Engineering Department,Rutgers U niversity, Piscataway,N J, U S ATW BremnerProfessor of Civil Engineering, U niversity of New B runswick, Fredericton,CanadaFdeLarrardSenior Scientist, Division: 'B etons et Ciments pour Ouvrages d'Ar t',LaboratoireCentral des Fonts et Chaussees,Paris,FranceAS EzeldinAssistant Professor,Departmentof Civil, Environmental andCoastalEngineering, Stevens Institute of Technology, Hoboken,NJ,U SARGettuSenior Researcher,Technical U niversity of Catal unya, B arcelona, SpainS K GhoshDirector, Engineered Structures and Codes, Portland CementAssociation, Skokie, IL, U SAOEGj0rvProfessor, Division of B uilding Materials, Norwegian Institute ofTechnology - NTH, Trondheim - NTH, NorwayTAHolmVice Presidentof Engineering, Solite Corporation,PO B ox 27211,Richmond, VA, U SAR Le RoyResearchEngineer,Division: 'B etons et Ciments pour Ouvrages d'Art',LaboratoireCentral des Ponts et Chaussees,Paris,FranceSMindessProfessor,Departmentof Civil Engineering, U niversity of B ritishColumbia, Vancouver, CanadaSNagatakiProfessor of Civil Engineering, Tokyo Institute of Technology,O-okayama, Meguru-ku, Tokyo152,JapanA H Nilson(Professor)162 Round Pound Road, HC-60, B ox 162, Medomak, Maine, U SA(f ormerly of Cornell U niversity)HG RussellVice President, ConstructionTechnology LaboratoriesInc., 5420 OldOrchardRoad,Skokie, IL, U SAMSaatciogluAssociateProfessorof Civil Engineering, U niversity of Ottawa, CanadaE SakaiManager, Special Cement Additives Division, Denki K agaku K ogyo Co.Ltd, Yuraku-cho, Chiyoda-ku, Tokyo100,JapanSP ShahWalter P Murphy Professor of Civil Engineering; Director,NSF Center forScience and Technology of Advanced Cement-B asedMaterials; andDirector,Centerfor Concrete and Geomaterials,NorthwesternU niversity, Evanston, IL, U SAThis page has been reformatted by Knovel to provide easier navigation. v Contents Preface .................................................................................... ix List of contributors ................................................................... xi 1.Materials Selection, Proportioning and Quality Control ............................................................................. 1 1.1Introduction......................................................................... 1 1.2Selection of Materials ......................................................... 2 1.3Mix Proportions for High Strength Concrete....................... 13 1.4Quality Control and Testing................................................ 17 1.5Conclusions........................................................................ 23 References .................................................................................. 24 2.Short Term Mechanical Properties ................................ 27 2.1Introduction......................................................................... 27 2.2Strength.............................................................................. 28 2.3Deformation........................................................................ 50 2.4Strain Capacity ................................................................... 55 2.5Poisson's Ratio................................................................... 59 References .................................................................................. 60 3.Shrinkage Creep and Thermal Properties..................... 65 3.1Introduction......................................................................... 65 3.2Shrinkage ........................................................................... 66 3.3Creep ................................................................................. 80 3.4Drying Shrinkage and Drying Creep .................................. 86 3.5Thermal Properties............................................................. 97 3.6Structural Effects: Case Studies......................................... 102 This page has been reformatted by Knovel to provide easier navigation.vi Contents 3.7Summary and Conclusions ................................................ 108 References .................................................................................. 110 4.Fatigue and Bond Properties ......................................... 115 4.1Introduction......................................................................... 115 4.2Mechanism of Fatigue........................................................ 120 4.3Cyclic Compression ........................................................... 121 4.4Cyclic Tension .................................................................... 124 4.5Reversed Loading .............................................................. 126 4.6Effect of Loading Rate........................................................ 127 4.7Effect of Stress Gradient .................................................... 127 4.8Effect of Rest Periods......................................................... 127 4.9Effect of Loading Waveform ............................................... 128 4.10Effect of Minimum Stress: Comparison of Normal and High Strength Concrete...................................................... 128 4.11Effect of Concrete Mixture Properties and Curing.............. 128 4.12Biaxial State ....................................................................... 129 4.13Bond Properties ................................................................. 130 4.14Summary............................................................................ 134 References .................................................................................. 135 5.Durability ......................................................................... 139 5.1Introduction......................................................................... 139 5.2Permeability ....................................................................... 139 5.3Corrosion Resistance ......................................................... 143 5.4Frost Resistance ................................................................ 148 5.5Chemical Resistance ......................................................... 149 5.6Fire Resistance .................................................................. 150 5.7Abrasion-Erosion Resistance ............................................. 153 5.8Concluding Remarks.......................................................... 156 References .................................................................................. 156 This page has been reformatted by Knovel to provide easier navigation.Contents vii 6.Fracture Mechanics ........................................................ 161 6.1Introduction......................................................................... 161 6.2Linear Elastic Fracture Mechanics ..................................... 162 6.3The Fracture Process Zone ............................................... 166 6.4Notch Sensitivity and Size Effects...................................... 169 6.5Fracture Energy from Work-of-Fracture ............................. 172 6.6Nonlinear Fracture Mechanics of Concrete........................ 175 6.7Material Characterization ................................................... 187 6.8Other Aspects of Fracture in Concrete............................... 194 6.9Applications ........................................................................ 196 References .................................................................................. 200 7.Structural Members ........................................................ 213 7.1Introduction......................................................................... 213 7.2Axially Loaded Columns..................................................... 214 7.3Flexure in Beams ............................................................... 219 7.4Beam Deflections ............................................................... 222 7.5Shear in Beams.................................................................. 224 7.6Bond and Anchorage ......................................................... 227 7.7Flexural and Shear Cracking.............................................. 228 7.8Prestressed Concrete Beams ............................................ 229 7.9Slabs .................................................................................. 229 7.10Eccentrically Loaded Columns........................................... 230 7.11Summary and Conclusions ................................................ 233 References .................................................................................. 233 8.Ductility and Seismic Behaviour.................................... 237 8.1Introduction......................................................................... 237 8.2Deformability of High-Strength Concrete Beams ............... 239 8.3Deformability of High-Strength Concrete Columns ............ 274 This page has been reformatted by Knovel to provide easier navigation.viii Contents 8.4Deformability of High-Strength Concrete Beam-Column Joints .................................................................... 290 8.5Application of High-Strength Concrete in Regions of High Seismicity................................................................... 306 8.6Summary............................................................................ 310 References .................................................................................. 310 9.Structural Design Considerations and Applications..................................................................... 313 9.1Introduction......................................................................... 313 9.2Structural Design Considerations....................................... 314 9.3Construction Considerations .............................................. 317 9.4Quality Control.................................................................... 320 9.5High Rise Buildings ............................................................ 322 9.6Bridges ............................................................................... 334 9.7Special Applications ........................................................... 337 References .................................................................................. 338 10.High Strength Lightweight Aggregate Concrete .......... 341 10.1Introduction......................................................................... 341 10.2Materials for High Strength Lightweight Aggregate Concrete............................................................................. 349 10.3High Strength Lightweight Concrete Laboratory Testing Programs............................................................... 351 10.4Physical Properties of High Strength Lightweight Aggregate Concrete ........................................................... 352 10.5Constructability of High Strength Lightweight Aggregate Concretes ......................................................... 363 10.6Applications of High Strength Lightweight Aggregate Concrete............................................................................. 366 References .................................................................................. 371 This page has been reformatted by Knovel to provide easier navigation.Contents ix 11.Applications in Japan and South East Asia.................. 375 11.1Introduction......................................................................... 375 11.2Methods of Strength Development..................................... 376 11.3Applications ........................................................................ 379 11.4Summary and Conclusions ................................................ 393 References .................................................................................. 396 Index ....................................................................................... 399 1Materials selection,proportioningand qualitycontrolSMindess1.1IntroductionHighperformanceconcretes(HPC)areconcreteswithpropertiesorattributes which satisfytheperformancecriteria.Generally concretes withhigherstrengthsandattributessuperiortoconventionalconcretesaredesirablein theconstruction industry. Forthe purposesof this book,HPCis definedin terms of strengthanddurability. TheresearchersofStrategicHighway ResearchProgram SHRP-C-205 on High Performance Concrete1definedthe high performance concretesfor pavement applications in termsofstrength,durability attributes andwater-cementitious materials ratioasfollows:It shall have one of the following strength characteristics:4-hour compressivestrength^25OO psi (17.5MPa) termedas veryearly strengthconcrete(VES), or24-hourcompressivestrength^50OO psi(35 MPa)termedas highearly strengthconcrete(HES), or28-day compressivestrength^10,0OO psi (70 MPa)termedas veryhighstrengthconcrete(VHS).Itshallhaveadurability factorgreaterthan80%after300 cycles offreezingand thawing.It shall have a water-cementitious materials ratio=$0.35.Highstrength concrete(HSC)couldbeconsideredas high performanceifotherattributesaresatisfactoryintermsofitsintendedapplication.Generallyconcreteswithhigherstrengthsexhibitsuperiorityofotherattributes.InNorthAmericanpractice,highstrengthconcreteisusuallyconsideredtobea concretewitha 28-daycompressivestrength of atleast6000 psi(42MPa).InarecentCEB-FIPState-of-the-ArtReportonHighStrengthConcrete2itisdefinedasconcretehavingaminimum28-daycompressivestrengthof8700 psi(6OMPa).Clearlythen,thedefinitionof'high strength concrete' is relative; it dependsupon boththe periodof timein question,and thelocation.Theproportioning(ormix design)of normalstrengthconcretesis basedprimarily onthew/c ratio'law' first proposedby Abramsin1918.Atleastforconcreteswithstrengthsupto6000 psi(42MPa),itisimplicitlyassumedthatalmostanynormal-weightaggregateswillbestrongerthanthehardenedcementpaste.Thereisthusnoexplicitconsiderationofaggregatestrength(orelasticmodulus)inthecommonlyusedmix designprocedures,suchas those proposedby theAmericanConcreteInstitute.3Similarly,theinterfacialregions(orthecement-aggregatebond)arealsonotexplicitlyaddressed.Rather,itisassumedthatthestrengthofthehardenedcementpastewillbethelimiting factorcontrolling theconcretestrength.Forhighstrengthconcretes,however,allofthecomponentsoftheconcretemixture are pushedto their critical limits. High strengthconcretesmaybemodelledasthree-phasecompositematerials,thethreephasesbeing(i) thehardenedcementpaste(hep);(ii) theaggregate;and(iii)theinterfacialzonebetweenthehardenedcementpasteandtheaggregate.These threephasesmustall beoptimized,which meansthateachmustbeconsideredexplicitly in thedesign process. In addition, as has beenpointedoutby Mindessand Young,4'itisnecessarytopaycarefulattentiontoallaspectsofconcreteproduction(i.e.selectionofmaterials,mixdesign,handlingandplacing).Itcannotbeemphasizedtoostrongly thatquality controlisanessentialpartoftheproductionofhigh-strengthconcreteandrequiresfullcooperationamongthematerialsorready-mixedsup-plier,the engineer,and the contractor'.Inessencethen,theproportioningofhighstrengthconcretemixturesconsistsof threeinterrelatedsteps:(1) selectionof suitableingredients-cement,supplementarycementingmaterials,aggregates,waterandche-micaladmixtures,(2)determinationoftherelativequantitiesofthesematerials in ordertoproduce,as economicallyas possible,a concretethathasthedesiredrheologicalproperties,strengthanddurability, (3)carefulquality controlof every phaseof theconcrete-making process.1.2SelectionofmaterialsAs indicatedabove, it is necessaryto get themaximum performance out ofallofthematerialsinvolvedinproducinghighstrengthconcrete.Forconvenience, the various materials are discussed separately below.Howev-er, it must be rememberedthat prediction with any certainty as to how theywillbehavewhen combinedinaconcretemixture is notfeasible. Particu-larlywhenattemptingtomakehighstrengthconcrete,anymaterialincompatibilitieswillbehighly detrimentaltothe finished product.Thus,theculmination of anymix design process must betheextensive testing oftrial mixes.Highstrengthconcretewillnormally containnotonly portlandcement,aggregateand water, butalso superplasticizersand supplementary cement-ingmaterials.Itispossibletoachievecompressivestrengthsofupto14,000 psi (98 MPa)using fly ash or ground granulated blast furnaceslag asthesupplementarycementingmaterial.However,toachievestrengths inexcess of14,000 psi (100 MPa),theuseof silica fumehasbeenfoundtobeessential,anditisfrequently usedforconcretesinthestrengthrangeof9000-14,000 psi (63-98 MPa)as well.Portland cementTherearetwodifferentrequirementsthatanycementmustmeet:(i)itmustdeveloptheappropriatestrength;and(ii)itmustexhibittheappropriaterheologicalbehaviour.Highstrengthconcretes havebeenproducedsuccessfully usingcementsmeetingtheASTMStandardSpecificationC150forTypesI,IIandIIIportlandcements.Unfortunately,ASTMC150isveryimpreciseinitschemicalandphysicalrequirements,andsocementswhichmeettheseratherloosespecificationscanvaryquitewidelyintheirfinenessandchemical composition.Consequently,cementsof nominally thesametypewillhavequitedifferentrheologicalandstrengthcharacteristics,particu-larlywhenusedincombinationwithchemicaladmixturesandsup-plementarycementingmaterials.Therefore,whenchoosingportlandce-mentsforusein high strengthconcrete,itis necessarytolookcarefullyatthecement fineness and chemistry.FinenessIncreasingthefinenessoftheportlandcementwill,ontheonehand,increase theearly strength of theconcrete,sincethehighersurfacearea incontactwith water will leadto a morerapid hydration. Ontheother hand,toohighafinenessmayleadtorheologicalproblems,asthegreateramountofreactionatearlyages,inparticulartheformation ofettringite,will leadto a higher rate of slump loss. Early work by Perenchio5 indicatedthat fine cementsproducedhigherearlyage concretestrengths,thoughatlateragesdifferencesinfinenesswerenotsignificant.MostcementsnowusedtoproducehighstrengthconcretehaveElaine finenesses thatare intherangeof1467to1957ft2/lb(300to400 m2/kg),though when TypeIII(highearlystrength)cementsareused,thefinenessesareintherange of2201 ft2/lb(450 m2/kg).Chemical composition of the cementThe previously citedwork of Perenchio5 indicates thatcements with higherC3Acontentsleadstohigherstrengths.However,subsequentwork6hasshownthathighC3Acontentsgenerallyleadstorapidlossof flow inthefreshconcrete,andasaresulthighC3Acontentsshouldbeavoidedincementsusedforhighstrengthconcrete.Aitcin7hasshownthattheC3Ashouldbeprimarilyinitscubic,ratherthanitsorthorhombic,form.Further,Aitcin7suggeststhatattentionmust be paidnotonly tothetotalamountofSO3 inthecement,butalsototheamountofsolublesulfates.Thus, thedegreeof sulfurization of theclinker is animportantparameter.InadditiontocommerciallyavailablecementsconformingtoASTMTypesI, IIand III, a number of cementshave been formulated specificallyforhighstrengthconcrete.Forinstance,in Norway, NorcemCementhasdevelopedtwospecialcementsforhighstrengthconcrete,inadditiontotheirordinaryportlandcement.ThecharacteristicsofthesecementsaregiveninTable1.1.sNotethatforthetwospecialcements(SP30-4AandSP30-4AMOD),theC3A contentswereheldto 5.5%.Table 1.1Composition of special cements for high strength concrete (developedby Norcem Cement8)* Ordinary portland cement, for comparisonIm2/kg=4.89ft2/lbSupplementary cementing materialsAsindicatedabove,mostmodernhighstrengthconcretescontainatleastone supplementary cementing material: fly ash, blast-furnace slag, or silicafume.Very often, the fly ash or slag is usedin conjunctionwith silica fume.These materials areall specified in theCanadianCSA Standard A23.5.9 IntheUnitedStates,flyashis specified inASTMC618,10andblast furnaceslaginASTMC98911;thereis,asyet,noU.S.standardforsilicafume.ThesematerialsaredescribedindetailinSupplementaryCementingMaterials forConcrete.12Usingasomewhatdifferentapproach,ahighsilicamodulusportlandC2S (%)C3S (%)C3A (%)C4AF (%)MgO (%)S03(%)Na2O equivalent (%)Elaine fineness (m2/kg)heat of hydration (kcal/kg)setting time (min): initialfinalSP30*18558933.31.130071120180SP30-4A28505.591.5-2.02-30.631056140200SP30-4AMOD28505.591.5-2.02-30.640070120170Table 1.2Bogue compositionandotherpropertiesofHTScement(afterAitcin et al.13)cement(referredtoasHTS,orHauteTeneurenSilica,orhighsilicacontent)was developed,13 with thecompositionshown in Table1.2.Notethat,comparedtomoreconventionalcements(suchastheSP-30ofTable1.1),thereisaveryhightotalsilicatecontent(84%),andC3Acontentofonly3.6%.Thecementisrathercoarselyground(Elainefinenessof1565ft2/lb(320 m2/kg)).Itis madefroma clinker composedofsmallaliteandbelitecrystals,andminuteC3Acrystals.Itiscapableofproducingconcreteswithexcellent28-daycompressivestrengths,as indi-catedin Table1.3,when usedin conjunction with 10% silicafume.Table 1.328 day compressive strengths of concretemade with HTS cementand10% silicafume13Silica fumeItispossibletomakehighstrengthconcretewithoutsilicafume,atcompressivestrengthsofuptoabout14,000psi(98MPa).Beyondthatstrengthlevel,however,silicafumebecomesessential,andevenatlowerstrengths 9000-14,000 psi (63-98 MPa), it is easier to make HSC with silicafumethanwithoutit.Thus,whenitisavailableatareasonableprice,itshould generally bea componentof theHSC mix.Silicafume14isawasteby-productoftheproductionofsiliconandsilicon alloys, and is thus nota very well-definedmaterial.Consequently, itis importanttocharacterizeanynewsourceof silica fume,by determiningthespecific surfaceareaby nitrogenadsorption,andthesilica,alkali andcarboncontents.Inaddition,itisdesirabletominimizethecontentofC2S(%)C3S (%)C3A (%)C4AF(%)Na2O equivalent (%)lime saturation factorsilica modulusElaine fineness, m2/kgIm2/kg =4.89ft2/lb22623.66.90.3892.74.8320vv/c0.310.230.200.171 ksi =6.89 MPalMPa = 0.145ksif c(MPd)74106115124Table1.4SomeCanadianspecifications forsilicafume(taken fromCSAStandard A23.59)crystallinematerial.Theacceptancelimitsforsilicafume,takenfromCSA-A23.59 aregiven in Table1.4.Silicafumeis available in severalforms.Inits bulk form, its unit weightis in therangeof118 to147.5 pcf(200-250 kg/m3), which makes handlingdifficult.Morecommonly now,silicafumeis available inadensifiedform,inwhichthebulkdensitiesareabouttwiceasgreatasthoseofthebulkform(i.e.400-500kg/m3).Ingeneral,thismakesiteasiertohandle.Inaddition,silicafumeis availableinslurry form(ofteninconjunction withsuperplasticizersintheliquid phase),with asolidscontentofabout50%.Thisformofsilicafumerequiresspecialequipmentforitsuse.Finally,silicafumeisavailablealreadyblendedwith portlandcement(atpercen-tages of thetotal mass of cementitious material in therange of 6.7 to9.3%)inCanada,FranceandIceland.Inspiteofthisapparentlywideselection,however, in any onelocationthechoiceof silica fumeswill be very limited,andonemust use whatis locally available.Fly ashFlyashhas,ofcourse,beenusedveryextensivelyinconcreteformanyyears.Flyashesare,unfortunately, muchmorevariablethansilicafumesin both their physical and chemical characteristics. Any fly ash which workswellinordinaryconcretemixesislikelytoworkwellinhighstrengthconcreteaswell.However,mostflyasheswillresultinstrengthsofnotmuch more than10,000 psi (70 MPa), though there have beena few reportsofhighstrengthconcreteswithstrengthsofupto14,000 psi(98MPa)inwhich fly ash has beenused.Forhigher strengths,silica fumemust beusedin conjunction with the fly ash,thoughthis practicehas notbeencommonin thepast.Chemical requirementsSi 02, mi n(%)SO3, max(%)Loss in ignition, max (%)Physical requirementsAcceleratedpozzolanic activity index, min, (%) of controlFineness, max, (%) retainedon 45 jxm sieveSoundness -autoclave expansion or contraction (%)Relative density, max variation fromaverage(%)Fineness, max variation fromaverage(%)Optional physical requirementsIncreaseof drying shrinkage, max (%) of controlReactivity with cementalkalis: min reduction(%)851.06.085100.2550.0380Ingeneral,forhighstrengthconcreteapplications,flyashisusedatdosageratesofabout15%ofthecementcontent.Becauseofthevariabilityoftheflyashproducedevenfromasingleplant,however,qualitycontrolisparticularlyimportant.Thisinvolvesdeterminations oftheElainespecific surfacearea,aswellasthechemicalcomposition(inparticularthecontentsofSiO2,Al2O3,Fe2O3,CaO,alkali,carbonandsulfates).And,aswith silicafume,itis importanttocheckthedegreeofcrystallinity; themoreglassy the fly ash, thebetter.Blast furnace slagInNorthAmerica,slag is notas widely available as in Europe,andhencethereisnotmuchinformationavailableastoitsperformanceinhighstrengthconcrete.However,theindications arethat,as with fly ash,slagsthat performwell in ordinary concretearesuitable for use in high strengthconcrete,atdosageratesbetween15% and30%.Thelowerdosageratesshould be used in the winter, so that theconcretedevelopsstrength rapidlyenoughforefficientformremoval.Forveryhighstrengths,inexcessof14,000psi(98 MPa),itwill likelybenecessarytousetheslag in conjunc-tion with silicafume.Thechemicalcompositionofslagsdoesnotgenerally vary very much.Therefore,routinequalitycontrolis generallyconfined toElaine specificsurfaceareatests,andX-raydiffractionstudiestocheckonthedegreeofcrystallinity(which shouldbelow).Limitation on the use of silica fume, fly ash or slagThereappeartobenoparticulardeleteriouseffectswhensilicafumeisusedinconcrete.However,theuseof fly ashandslag mayleadtosomeproblems:(i)TheearlystrengthdevelopmentofmixesinwhichsomeofthePortland cement has beenreplacedby slag or fly ash is less rapid thanthatwhenonlyportlandcementisused,andthismayadverselyaffectthetime at which theforms can be stripped,particularly at lowtemperatures.Onewayofdealingwiththisproblemisbyfurtherreductionsin the w/c ratio, through theuse of even moresuperplasti-cizer.Clearly,thisisnoteconomicallyveryattractive;ifhighearlystrengthis needed,it may well benecessarytoreducethe fly ashorslagcontent.(ii)Theexistingtestdataareratherambiguouswithregardtothefree-thawdurabilityofhighstrengthconcretemadewithsup-plementarycementitiousmaterials.Thisistruebothforair-entrainedandnon-air-entrainedmixes.Therefore,until moredataareavailable,designersshouldbe cautious when using high strengthconcrete in an environment in which the concretewill be subjected tomany freeze-thaw cycles in a saturatedstate.(iii)Atthesubstitutionlevelsused(15-30%),flyashorslagwillhaveverylittleeffectonthemaximumtemperature developmentinmassconcretepours.SuperplasticizersInmodernconcretepractice,itisessentiallyimpossibletomakehighstrengthconcreteatadequateworkabilityinthefieldwithouttheuse ofSuperplasticizers.Unfortunately,differentSuperplasticizerswillbehavequitedifferentlywithdifferentcements(evencementsofnominallythesame type). This is due in part to the variability in the minor components ofthecement(which arenotgenerally specified), andin parttothefactthattheacceptancestandardsforSuperplasticizersthemselvesarenotverytightlywritten.Thus,somecementswillsimply befound tobeincompati-ble with certain Superplasticizers.Thereare,basically,threeprincipaltypesofsuperplasticizer:(i)ligno-sulfonate-based\(ii)polycondensateofformaldehydeandmelaminesul-fonate(oftenreferredtosimplyas melaminesulfonate;and(iii)polycon-densateof formaldehydeandnaphthalenesulfonate,(oftenreferredtoasnaphthalenesulfonate).Inaddition,avarietyofothermoleculesmightbemixedinwiththesebasicformulations.ItmaythusbeverydifficulttodeterminetheprecisechemicalcompositionofmostSuperplasticizers;certainlymanufacturerstrytokeep theirformulations as closelyguardedsecrets.ItshouldbenotedthatmuchofwhatweknowaboutSuperplasticizerscomes fromtests carriedout on normal strength concretes, at relatively lowsuperplasticizercontents.Thisdoesnotnecessarilyreflecttheirperform-anceat very low w/c ratiosandvery high superplasticizeradditionrates.Lignosulfonate-based SuperplasticizersInhighstrengthconcrete,lignosulfonateSuperplasticizersaregenerallyused in conjunction with eithermelamine or naphthaleneSuperplasticizers.Theytendnottobeefficientenoughfortheeconomicproductionof veryhighstrengthconcreteson theirown.Sometimes,lignosulfonates areusedforinitialslumpcontrol,withthemelaminesornaphthalenesusedsubsequently forslump controlin thefield.Melamine sulfonate SuperplasticizersUntilrecently,onlyonemelaminesuperplastizerwasavailable(trade-nameMelment),butnowothermelamine-basedSuperplasticizersarelikely tobecomecommerciallyavailable.MelamineSuperplasticizersareclearliquids, containing about22%solidparticles;theyaregenerallyintheformoftheirsodiumsalt.Thesesuperplasticizers have been used for many years now with goodresults, andso they remain popularwith high strengthconcreteproducers.Naphthalene sulfonate superplasticizersNaphthelenesuperplasticizershavebeeninuselongerthananyoftheothers, and are available under a greaternumber of brand names. They areavailableasbothapowderandabrownliquid;intheliquidformtheytypicallyhavea solidscontentofabout40%.Theyaregenerally availableaseithercalciumsalts,ormorecommonly,sodiumsalts.(Calciumsaltsshould be used in case where a potentially alkali-reactive aggregate is tobeused.)Theparticularadvantagesofnaphthalenesuperplasticizers,apartfromtheirbeing slightly lessexpensivethantheothertypes,appearstobe thattheymakeiteasiertocontroltherheologicalpropertiesofhigh strengthconcrete, becauseof theirslight retardingaction.Superplasticizer dosageThereisnoaprioriwayofdeterminingtherequiredSuperplasticizerdosage;itmustbedetermined,in theend,by somesortof trialanderrorprocedure.Basically,if strengthis theprimarycriterion,thenoneshouldwork with the lowest w/c ratio possible, and thus the highest Superplasticiz-erdosagerate.However,if therheologicalproperties of thehigh strengthconcretearevery important,thenthehighest w/c ratioconsistent with therequiredstrengthshouldbeused,withtheSuperplasticizerdosagethenadjustedtogetthedesiredworkability.Ingeneral,ofcourse,someintermediatepositionmustbefound,so thatthecombinationof strengthandrheologicalpropertiescanbeoptimized.TypicalSuperplasticizerdosagesforanumberofhigh strengthconcretemixesaregiven below, inTables1.5 to1.10.Table 1.5Mix proportions for InterfirstPlaza, Dallas (adaptedfromCook15)water (kg/m3)cement, Type I (kg/m3)fly ash, Class C (kg/m3)coarseaggregate (kg/m3)fine aggregate (kg/m3)water reducer L/m3Superplasticizer L/m3w/cementitious ratiofc28-day (MPa)- moist curedfc91-day (MPa)- moist cured1 lb/yd3 =0.59 kg/m3or1 in.=25. 4 mmor/ cm maxsizeaggregate16636015010526831.012.540.3379.589.0lkg/m3 =1.69pcf1 in.=0.0393 mm25 cm maxsizeaggregate14835714911836041.012.520.2985.892.4Table 1.6Highstrengthconcretemix design guidelines(afterPetermanandCarrasquillo16)Table 1.7Mix proportionsfor high strength concreteat Pacific FirstCenter,Seattle(adaptedfromRandall and Foot17)Table 1.8Five examplesof commercially producedhigh strength concretemixdesigns(afterAitcin, Shirlaw and Fines18)water (kg/m3)cement(kg/m3)fly ash (kg/m3)coarseagg./fineagg.ratiosuperplasticizerw/cementitious ratiofc'56-day(MPa)H-H-OO1955582.00.3466H-H-Ol1434742.0yes*0.3072H-H-IO1733911672.00.3169H-H-Il1343351442.0yes*0.2776* Use highest dosageof superplasticizerwhich will notleadto segregationorexcessiveretardation.1 lb/yd3 =0.59 kg/m3or1 kg/m3 =1.69 pcfl i n.=25. 4 mmor1 in.=0.0393 mmwater (kg/m3)cement -Type II (kg/m3)flyash -Type F (kg/m3)silica fume(kg/m3)coarseaggregate -1 cm max. size(kg/m3)fineaggregate - F. M.=3.2 (kg/m3)water reducerI(LIm3)water reducerII (L/m)w/cementitiousratiofc'56-day(MPa)1 lb/yd3 =0.59 kg/m3or1 kg/m3 =1.69 pcf1 in.=25.4 mmor1 in.=0.0393 mm131534594010696231.777.390.21124water (kg/m3)cement (kg/m3)fly ash (kg/m3)slag (kg/m3)silica fume(kg/m3 )coarseaggregate(kg/m3)fine aggregate(kg/m3)water reducer(L/m3 )retarder(L/m3)superplasticizer(L/m3 )w/cementitiousratiofc'28-day(MPa)fc'91-day(MPa)1955056010306300.9750.3564.878.6165451103074511.250.3779.887.01355003011107004.5140.2742.5106.51453151373611307450.91.85.90.3183.493.413051343108068515.70.251191451 lb/yd3 = 0.59kg/m3or1 kg/m3 =1.69 pcf1 in.=25.4 mmor1 in.=0.0393 mmTable 1.9Highstrength mixtures in theChicagoarea(adaptedfromBurg andOst19)Table 1.10Mix design for a high strength concretedesignedfor a low heat ofhydration (adaptedfromBurg and Ost)RetardersAtonetime retarderswererecommendedfor somehigh strengthconcreteapplications,tominimize theproblemof overrapidslump loss.However,itisdifficulttomaintainacompatibilitybetweentheretarderandthesuperplasticizer,i.e.tominimizeslumplosswithoutexcessivelyreducingearlystrengthgain.Inmodernpractice,retardersarerecommendedonlyasalastresort;therheologyisbettercontrolledbytheuseoftheappropriatesupplementarycementingmaterialsdescribedabove.AggregatesTheaggregatepropertiesthataremostimportantwithregardtohighstrength concreteare:particleshape,particlesize distribution,mechanicalpropertiesoftheaggregateparticles,andpossiblechemicalreactionsbetweentheaggregateandthepastewhichmayaffectthebond.Unlikewater (kg/m3 )cement(kg/m3)fly ash (kg/m3)silica fume(kg/m3 )coarseaggregate,SSD 12 mmmax sizefine aggregate,SSD (kg/m3)superplasticizer -Type F (L/m3)retarder - Type D (L/m3)w/cementitious ratiofc28-day (MPa) -moistcuredfc'56-day (MPa) -moistcuredfc91-day (MPa) -moistcuredMixnumber1158564106864711.611.120.28178.681.486.521604755924106865911.611.040.28788.597.3100.43155487471068:67611.220.970.29191.994.296.0414456489L06859320.121.470.220118.9121.2131.8515147510474106859316.451.510.231107.0112.0119.3water(kg/m3 )cement -Type I (kg/m3 )flyash -Type F (kg/m3)silica fume(kg/m3 )coarseaggregate -25 mm max. size (kg/m3)fine aggregate (kg/m3 )superplasticizer,ASTMTypeF (L/m3)superplasticizer, ASTM TypeG (L/m)water/cementitious ratiofc28-day (MPa) -moistcuredfc91-day(MPa) -moistcured14132787271217426.313.250.323.188.6their use in ordinary concrete, where we rarely considerthestrength oftheaggregates,inhigh strengthconcretetheaggregatesmay well becomethestrengthlimitingfactor.Also,sinceitisnecessarytomaintainalow w/cratiotoachievehighstrength,theaggregategrading mustbevery tightlycontrolled.Coarse aggregateIt goes without saying that, for high strength concrete, the coarseaggregateparticles themselves must be strong.Anumber of differentrock types havebeenusedtomakehighstrengthconcrete;theseincludelimestone,dolomite, granite, andesite,diabase,and so on.It has beensuggested1 thatinmostcasestheaggregatestrengthitself is notusually thelimiting factorinhighstrength concrete; rather,it is thestrength of thecement-aggregatebond.Aswithordinaryconcretes,however,aggregatesthatmaybesusceptibletoalkali-aggregatereaction,ortoD-cracking,shouldbeavoidedif atall possible,eventhoughthelow w/c ratiosusedwilltendtoreducetheseverity of thesetypesof reaction.Frombothstrength and rheological considerations,the coarseaggregateparticlesshouldberoughlyequi-dimensional;eithercrushedrockornatural gravels, particularly if they areof glacial origin, are suitable. Flatorelongatedparticlesmust beavoidedat all costs. Theyare inherently weak,andleadtoharshmixes.Inaddition,itisimportanttoensurethattheaggregateisclean,sincealayerofsiltorclaywillreducethecement-aggregatebondstrength,inadditiontoincreasingthewaterdemand.Finally,theaggregatesshouldnotbehighly polished(as is sometimesthecasewithriver-rungravels),becausethistoowillreducethecement-aggregatebond.Notmuchworkhasbeencarriedoutontheeffectsofaggregatemineralogyonthepropertiesofhighstrengthconcrete.However,adetailedstudybyAitcinandMehta,20involvingfourapparentlyhardstrongaggregates(diabase,limestone,granite,naturalsiliceousgravel)revealedthatthegraniteandthegravelyieldedmuchlowerstrengthsandE-valuesthantheothertwoaggregates.Theseeffectsappearedtoberelatedbothtoaggregatestrengthandtothestrengthofthecement-aggregatetransitionzone.Cook15hasalsopointedouttheeffectofthemodulusofelasticityoftheaggregateonthatoftheconcrete.However,muchwork remains tobedonetorelatethemechanicaland mineralogicalpropertiesof theaggregate to thoseof theresulting high strengthconcrete.It is commonly assumedthat a smaller maximum size of coarseaggregatewillleadtohigherstrengths,1'2'5'6'21largelybecausesmallersizeswillimprovetheworkability oftheconcrete.However,thisis notnecessarilythecase.WhileMehtaandAitcin6recommendamaximumsizeof10-12 mm,they reportthat 20-25 mm maximum size may be usedfor highstrengthconcrete.Ontheotherhand,usingSouthAfricanmaterials,Addis22foundthatthestrengthofhis highstrengthconcreteincreasedasthe maximum size of aggregate increasedfrom13.2 to 26.5 mm. This, then,is anotherareawhich requiresfurtherstudy.Fine aggregateThe fineaggregateshouldconsistof smoothroundedparticles,2toreducethe water demand.Normally, the fine aggregategrading should conform tothelimitsestablishedbytheAmericanConcreteInstitute3fornormalstrength concrete.However,it is recommendedthat the gradings should lieonthecoarsersideoftheselimits;afinenessmodulusof3.0orgreaterisrecommended,1'6 bothto decrease the waterrequirements and to improvetheworkability of thesepaste-richmixes.Of course,thesandtoomust befreeofsilt orclay particles.1.3Mix proportions for high strength concreteOnlyafewformalmixdesignmethodsforhighstrengthconcretehavebeendevelopedtodate.7'22'23Mostcommonly,purelyempiricalproce-duresbasedontrialmixturesareused.Forinstance,accordingtotheCanadianPortlandCementAssociation,'the trial mix approachis bestforselectingproportionsforhigh-strengthconcrete'.24Inothercases,mixdesign 'recipes' areprovidedfor differentclassesof high strength concrete;anexampleof thisapproachis given by PetermanandCarrasquillo.16Inthissection,itisnottheintentiontoprovideacanonicalmixproportioningmethod.Muchworkremainstobedonebeforeanymixproportioningmethodforhighstrengthconcretebecomesas universallyaccepted,atleastinNorthAmerica,as hastheACIStandard 211J3 fornormalstrengthconcretes.Rather,theprinciplesonwhichsuchamixdesignmethodshouldbebasedwillbediscussed,andsomegeneralguidelines(andanumberofempiricallyderivedmixesdrawnfromtheliterature)willbepresented.Proportions of materialsWater/cementitious ratioFor normal strength concretes,mix proportioningis basedto a large extenton the w/c ratio'law'.For theseconcretes,in which theaggregate strengthis generally much greaterthan the pastestrength, the w/c ratio doesindeeddeterminethestrengthoftheconcreteforanygivensetofrawmaterials.Forhigh strengthconcretes,however,in which theaggregatestrength,orthestrengthofthecement-aggregatebond,areoftenthestrength-controllingfactors,theroleofthew/c ratiois lessclear.Tobesure,it isnecessary to use very low w/c ratios to manufacture high strength concrete.However, the relationship between w/c ratio and concretestrength is not asstraightforwardas it is fornormalstrengthconcretes.w/cratioFig.1.1Compressivestrengthversus w/c materialratio:(1) afterAitcin7;(2) after Fiorato25;(3) afterCook15; (4) normalstrength concretefromCPCA24Figure1.1showsaseriesofw/cementitiousmaterialvscompressivestrengthcurves forhigh strengthconcrete.Thesetsof curves numbered1,2and3showthestrengthrangethatmightbeexpectedforagivenw/cementitiousratio.(Curve1 isfromAitcin7;curve 2isfromFiorato25;curve 3isfromCook.15)Forcomparison,thew/c ratiovs strengthcurvefornormalstrengthconcreteisshownascurve 4.24Figure1.2showsasimilarseriesofw/cementitiousvsstrengthcurvesobtainedbyotherinvestigators.Curve 1 is fromAddisand Alexander,23 who used high earlystrengthcement.Curve 2isfromHattori.25Curves3and4arefromSuzuki27;curve 3isforordinaryportlandcement,andcurve 4forhighearly strengthcement.SeveralconclusionsmaybedrawnfromFigs.1.1and1.2.First,whilestrengthclearly increasesas thew/cementitiousratiodecreases,thereis aconsiderablescatteroftheresults,which mustbeduetovariationsinthematerials,usedinthedifferentinvestigations.Second,andmoreimpor-tant, the range of strengths for a given w/cementitious ratio increasesas thew/cementitious ratiodecrease.If onelooksatall of thecurves in Figs.1.1and1.2,ataw/cementitiousratioof0.45,therangeinstrengthisfrom5400 psi(37MPa)to9500 psi(66MPa);ataratioof0.26,therangeisfrom11,300 psi(78MPa)to17,400 psi(12OMPa).Therefore,the28-day compressive strength (MPa)w/c ratioFig.1.2Compressive strength versus w/c material ratio: (1) high early strength cement, afterAddis and Alexander23; (2) afterHattori26; (3) ordinary Portland cement, afterSuzuki27; (4)high early strength cement afterSuzuki24w/cementitiousratiobyitselfisnotaverygoodpredictorofcompressivestrength.Thew/cementitious vs strengthrelationshipmustthusbedeter-minedforany given setof rawmaterials.Cementitious materials contentFornormalstrengthconcretes,cementcontentsaretypically in therangeof590 to930 pcf (350 to550 kg/m3). Forhigh strength concretes,however,thecontentof cementitiousmaterials(cement, fly ash,slag,silica fume)ishigher,rangingfromabout845to1090pcf(500to650kg/m3).Thequantityofsupplementarycementingmaterialsmayvaryconsiderably,dependinguponworkability,economyandheatofhydrationconsidera-tions.Supplementary cementing materialsAsindicatedearlier,itis possibletomakehighstrengthconcretewithout28-day compressive strength (MPa)usingflyash,slagorsilicafume.Forhigherstrengths,however,sup-plementary cementing materialsare generally necessary.In particular,theuseofsilicafumeisrequiredforstrengthsmuchinexcessof14,000psi(98MPa).Inanyevent,theuseofsilicafume(whichisnowreadilyavailableinmostareas)makestheproductionofhighstrengthconcretemucheasier;itisgenerallyaddedatratesof5%to10%ofthetotalcementitious materials.SuperplasticizersWithverycarefulmixdesignandaggregategrading,itispossibletoachievestrengthsofabout14,000 psi(98MPa)withoutSuperplasticizers.However,astheyarereadilyavailabletheyarenowalmost universallyusedinhighstrengthconcrete,sincethey makeitmucheasiertoachieveadequate workability at very low w/cementitious ratios.Ratio of coarse to fine aggregateFornormalstrengthconcretes,theratioof coarseto fine aggregate(for a0.55 in.,14mmmaxsizeofaggregate)isintherangeof0.9to1.4.24However,forhighstrengthconcrete,thecoarse/fine ratiois much higher.Forinstance,PetermanandCarrasquillo16recommendacoarse/fineratioof 2.0. And,as seenin Tables1.5 to1.10, coarse/fine ratios used in practicevary in therangeof1.5 to1.8.Examples of high strength concrete mixesAsstatedearlier,thereisyetnogenerallyagreeduponmethodofmixproportioning.Mix designs for high strength concretehave, hitherto,beendevelopedempirically,dependingontherawmaterialavailableinanylocation.Inthissection,a numberof typicalmix designs,drawn fromtherecentliterature,willbepresented.Table1.5showsthemixproportionsforInterfirstPlaza,Dallas,15inwhichtheconcreteachievedcompressivestrengthofabout11,500 psi(8OMPa).Table1.6giveshighstrengthconcretemixdesignguidelinesoriginallydevelopedfortheTexasStateDepartmentofHighwaysandPublicTransportation.16Theexpected56-daystrengthsforthesefourmixes range from9500 to 11,000 psi (66 to 76 MPa). It should be notedthatthemix designs in Tables1.5 and1.6 donotinvolve theuse of silicafume.Table1.7showsthemixproportionsofPacificFirstCenter,Seattle17inwhichtheconcretereacheda56-daycompressivestrengthof18,000 psi(126MPa).Table1.8givesaseriesofmixdesignsforanumberofhighstrengthconcreteprojects,18 while Table1.9 describeshigh strengthconcretemixescommerciallyavailableinChicago.19InTables1.7,1.8 and1.9,itshouldbenotedthatthehigherstrengthmixesall containedsilicafume.Finally,Table 1.10 presents a mix designfor a high strength,low heat ofhydrationconcrete.19FromTables1.5 to1.10,itmaybeseenthatthemix designs,evenforconcretesofapproximatelythesamestrength,varyconsiderably.Thisreflectsthedifferencesin thequality of all of theraw materials available foreachspecificmix.So,whiletheseexamplesmayserveasageneralguidelinefortheproductionofhighstrengthconcrete,copyingamixdesignusedinonelocationisunlikelytoproducethesameconcretepropertiesin anotherarea.Intheend,aswithconventionalconcrete,mixdesignwillrequiretheproductionofanumberoftrialmixes,thoughtheexamplesgivenabovemay providereasonableguidance for the first trial batch.In particular, it isessentialfirsttoensurethattheavailablerawmaterialsarecapableofproducingthedesiredstrengths,andthattherearenoincompatibilitiesbetweenthecements,theadmixture(s)andthesupplementarycementingmaterials.Withmaterialsforwhichthereis notmuch field experience,itmaybenecessarytotrydifferentbrandsofcement,differentbrandsofsuperplasticizers,anddifferentsourcesofflyash,slag,orsilicafume,inorderto optimizeboth the materials and the concretemixture. This soundslikealotofwork,andingeneralitis.Atpresent,thereissimplynostraightforwardprocedureforproportioningahighstrengthconcretemixture withunfamiliarmaterials.1.4Quality control and testingConventional normal strengthconcreteis a relatively forgivingmaterial; itcantoleratesmallchangesinmaterials,mix proportionsorcuringcondi-tionswithoutlargechangesinitsmechanicalproperties.However,highstrength concrete, in which all of thecomponentsof themix are working attheir limits, is notatall a forgivingmaterial.Thus,toensurethequality ofhighstrengthconcrete,everyaspectoftheconcreteproductionmustbemonitored,fromtheuniformityoftherawmaterialstoproperbatchingand mixing procedures,to propertransportation,placement, vibration andcuring, through to propertestingof thehardenedconcrete.Thequalitycontrolprocedures,suchasthetypesoftestonboththefreshandhardenedconcretes,thefrequencyof testing,andinterpretationoftestresultsareessentiallythesameasthoseforordinaryconcrete.However,Cook15haspresenteddatawhichindicatethatforhishighstrengthconcrete,thecompressivestrengthresultswerenotnormallydistributed,andthestandarddeviationforagivenmix wasnotindepen-dentoftestageandstrengthlevel.Thisledhimtoconcludethatthe'qualitycontroltechniquesusedforlowtomoderatestrengthconcretesmaynotnecessarilybeappropriateforveryhighstrengthconcretes.'Tothisdate,however,separatequality control/qualityassuranceproceduresforhigh strength concretehave notbeendeveloped.Theremainderof thissectiondealsprimarily with thedeterminationofthecompressivestrength, /c', sincethis is thebasis on which high strengthconcreteis designedandspecified.Age at testTraditionally,theacceptancestandardsforconcreteinvolvestrengthdeterminationsatanage of 28 days.Althoughthereis,of course, nothingmagicalaboutthisparticulartestage,ithasbeenuseduniversally asthereference time at which concretestrengths are reported. However,for highstrengthconcretes,ithasbecomecommontodeterminecompressivestrengthsat56days,oreven90days.Thejustificationforthisisthatconcreteinstructureswillrarely,ifever,beloadedtoanythingapproachingitsdesignstrengthinlessthan3months,giventhepaceofconstruction. Theincreasein strength between28 and56 or 90 days can beconsiderable(10%to20%),andthiscanleadtoeconomiesinconstruc-tion.Itis thus perfectly reasonabletomeasurestrengths atlaterages,andtospecify theconcrete strengthin termsof these longercuring times.Thereare,however,twodrawbackstothisapproach.First,itcanbemisleadingtocomparethecompressivestrengthsofnormalandhighstrengthconcretes,ifthesearemeasuredatdifferenttimes.Ofmoreimportance, thereis a certain margin of safetywhen concretestrengthsaremeasuredat28days,sincetheconcretewillgenerallybesubstantiallystrongerwhen it finally has tocarry its design loads,perhapsattheage ofoneyear fora typical high-rise concretebuilding. If strengthsare specifiedatlaterages,this margin is reduced(byanunknown amount),andhencethere is an implicit reduction in thefactor of safety.And, of course, findinghigherstrengthsatlatertestagesdoesnotinanywayimplythattheconcretehas somehow become'better' than a concretewhose strength wasmeasuredin theconventional way at28 days.Curing conditionsIngeneral,thehighestconcretestrengths willbeobtainedwith specimenscontinuouslymoistcured(at100%relativehumidity)untilthetimeoftesting.Unfortunately,theavailabledataonthispointareambiguous.Carrasquillo,NilsonandSlate28 foundthathigh strengthconcrete,moist-curedfor7daysandthenallowedtodryat50%relativehumidity till 28days showed a strength loss of about10% when comparedto continuouslymoist-curedspecimens.However,insubsequentwork,CarrasquilloandCarrasquillo29 found that up toan age of15 days, specimens treatedwith acuring compoundand allowed to cure in the field under ambient conditionsyieldedslightlyhigherstrengthsthanmoist-curedspecimens.At28 days,moist-curedspecimensandfield-curedspecimens(with orwithout curingcompounds) yielded approximatelythesame results.Only at laterages (56and91 days) didthestrengths of themoist-curedspecimenssurpassthoseof the field-cured specimens treatedwith a curing compound. Similarly, forthemixes shown in Table1.9,Burg andOst19 foundthat, when specimensthathadbeenmoistcuredfor28 dayswerethensubjectedtoair curing,theirstrengthsat91daysexceededthoseofcontinuouslymoist-curedspecimens;however, by 426 days, thecontinuously moist-cured specimenswerefromabout3%to10% higher in strengththantheair-cured ones.Ontheotherhand, several investigators have reportedthat, as long as aweekorsoofmoistcuring is provided,subsequentcuring under ambientconditionsis notparticularlydetrimentaltostrengthdevelopment.Peter-manandCarrasquillo16havestatedthat'the28-daycompressivestrengthof high strength concretewhich has beencured under ideal conditions for 7days aftercasting is notseriously affectedby curing in hotordry conditionsfrom7 to28 days aftercasting.'Finally,contraryresults werereportedby Moreno30 who indicated thatair-curedspecimenswereabout10% strongerthan moist-cured specimensatall ages upto91 days.Type of mold for casting cylindrical specimensASTMC470:MoldsforFormingConcreteTestCylindersVertically,describestherequirementsforbothreusableandsingle-usemolds,andASTMC31:MakingandCuringConcreteTestSpecimensontheFieldpermitsbothtypesofmoldtobeused.However,ithaslong beenknownthatdifferentmoldsconformingtoASTMC470willresultinspecimenswithdifferentmeasuredstrengths.Thisistrueforbothnormalstrengthandhighstrengthconcretes.Ingeneral,moreflexiblemoldswillyieldlowerstrengthsthanveryrigidmolds,becausethedeformationoftheflexible molds during rodding or vibration leads to less efficientcompactionthanwhen using rigidmolds. Theexperimentaldatalargely bearthisout.Itshouldbenotedthat,whateverthemoldmaterials,themoldsmustbeproperlysealedtopreventleakageofthemixwater.Ifanysignificantleakage doesoccurs, theapparentstrength will generally increase,becauseofthelowereffectivew/cratio,andincreaseddensificationofthespecimens.Forthestandard6x12in.(150x300mm)molds,CarrasquilloandCarrasquillo29 foundthatsteelmolds gave strengths about5%higher thanplastic molds, while Hester31 foundabouta 10% difference.Similar resultswerereportedbyHowardandLeatham.32PetermanandCarrasquillo16reportedthatsteelmoldsgavestrengthsabout10%higherthanthoseobtained with cardboardmolds,and Hester31 showed that steelmolds gavestrengths about6%higher than tinmolds.On theotherhand, Cook15 reportedthat'goodsuccess was experiencedontheuseofsingle-userigidplasticmolds',whileAitcin33reportsincreasinguseofrigid,reusableplasticmolds.Inaddition,CarrasquilloandCarrasquillo29havereportedthatforthesmaller4x8i n.(100 x 200 mm)molds,therewerenostrengthdifferencesbetweensteel,plastic or cardboardmolds.Inview of theaboveresults, it would be prudent touse rigid steel moldswhenever practicable, particularly for concretestrengths in excess of about14,000 psi (98 MPa),atleastuntil moretestdatabecomeavailable forthesmaller molds.Specimen sizeFormost materials, including concrete,it has generally beenobservedthatthesmallerthetestspecimen,thehigherthestrength.Forhigh strengthconcrete,however,thoughthiseffectis oftenobserved,therearecontra-dictory results reported in theliterature. Theresults of a number of studiesarecomparedinTable1.11.Itmaybeseenthattheobservedstrengthratiosof4x8i n.(100x200 mm)cylindersto6x12 in.(15Ox300 mm)cylinders range fromabout 1.1 to 0.93. These contradictory resultsmaybeduetodifferencesintestingproceduresamongstthevariousinvestigators.Itmustbenotedthatwhileforagivensetofmaterialsandtestprocedures,itmay bepossibletoincreasetheapparentconcretestrengthbydecreasingthespecimensize,thisdoesnotinanywaychangethestrength of the concretein the structure. One particular specimensize doesnotgive'truer'resultsthananyother.Thus,oneshouldbecarefultospecifya particular specimensize fora given project,rather than leaving itas a matter ofchoice.Specimen end conditionsAccordingtoASTMC39:CompressiveStrength ofCylindricalConcreteSpecimens,theendsofthetestspecimensmustbeplanewithin 0.002 in.(0.05 mm). This may be achieved eitherby capping the ends (usually with asulfurmortar)orbysawingorgrinding.Unfortunately,differentendTable 1.11Effectof specimen size on thecompressive strength of high strengthconcreteInvestigatorPeterman and Carrasquillo16Carrasquillo, Slate and Nilson34Howard and Leatham32Cook15Burg and Ost19Aitcin33Moreno3083 MPa concrete119 MPaconcreteCarrasquillo and Carrasquillo29fc(100 x 200 mm cylinder)fc'(150 x 300 mm cylinder)-1.1-1.1-1.08-1.05-1.01ambiguous results-1.0-0.93-0.93conditionscanleadtodifferentmeasuredstrengths,andsotheendpreparationfortestinghighstrengthconcretespecimensshouldbespe-cifiedexplicitly foranygiven project.Themostcommonmethodforpreparingtheendsofnormal strengthconcreteistousesulfurcaps;forhighstrengthconcrete,highstrengthsulfurmortarsarecommercially available.However,if thestrength ofthecap is lessthanthestrength of theconcrete,thecompressiveloadwillnotbetransmitteduniformlytothespecimenends,leading toinvalidresults.Thus,forhighstrengthconcrete,inadditiontohighstrengthcappingcompounds,anumberofotherendpreparationtechniquesarebeinginvestigated. These include grinding the specimen ends, or using unbondedsystems,consisting ofa padconstrainedinaconfiningring which fits overthespecimenends.Mostcompressivestrengthtestsonhighstrengthconcretearestillcarriedoutusingahighstrengthcappingcompound.Thematerialsavailable in North Americawill achieve compressive strengths of 12,000 psito13,000 psi(84MPato91 MPa)whentestedas2 in.(50mm)cubes.33PetermanandCarrasquillo21recommendtheuseofsuchcappingcom-pounds,sincetheygivehigherconcretestrengthsthanordinarycappingcompounds. Cook16 has used such compounds for concretestrengths up to10,000 psi(7OMPa),while Moreno30considersthemtobesatisfactory atstrengths up to17,000 psi (119MPa).Burg and Ost19 report that a high strength capping material may be usedwithconcretestrengthsofupto15,000 psi(105MPa);beyondthat,themodeof failureof thecylinders changed fromthenormal conefailureof acolumnar one. They recommend grinding of thecylinder ends for strengthsbeyond15,000 psi(105MPa).Similarly, Aitcin33hasreportedthataboveabout17,000psi(119MPa),thehighstrengthcapping materialis pulver-izedas thespecimensfail,which might well affectthemeasuredstrength.Hetoorecommendsgrinding ofthespecimenendsforvery high strengthconcretes.(Itmight benotedthatendgrinders forconcretecylinders arenowcommerciallyavailable.In1992,thecostofsuchamachinewasapproximately US$12,000.)Becauseof theuncertaintywith high strengthcapping compounds,andthecostsandtimeinvolvedinendgrinding,aconsiderableamountofresearchhas beencarriedout on unbondedcapping systems. These consistofmetalrestrainingcapsintowhichelastomericinsertsareplaced;theassemblies then fit over theends of thecylinder. As theelastomericinsertsdeterioratewithrepeateduse,they arereplacedfromtime to time.Richardson35usedasystemofneopreneinsertsinaluminium capsfortestingnormalstrengthconcretesintherangeof3000 psito6000 psi(21 MPa to 42 MPa). Hefoundthat below 4000 psi (28 MPa), theneoprenepadsgave somewhatlowerstrengthsthanconventionalsulfurcaps, whileabove4000 psi(28MPa)theygavesomewhathigherstrengths.Overall,however,themeancompressivestrengthswerenotsignificantlydifferentbetween thetwo systems.Carrasquillo andCarrasquillo29compareda high strengthsulfurcappingcompoundtoanunbondedsystemconsisting ofapolyurethanepadinanaluminiumrestrainingring.Theyfoundthatuptoabout10,000 psi(7OMPa),theunbondedsystemgavestrengthsthatwere97%ofthoseobtainedwiththecappingcompound.Beyond10,000 psi(7OMPa),however,theunbonded system gave much higher strengths;theyhypothe-sizedthatthismight beduetogreaterendrestraintofthecylinders withsuchasystem.Insubsequentwork,36theyfoundthatupto10,000 psi(7OMPa),polyurethanepadsinanaluminium capgaveresultswithin 5%ofthoseachievedwithhighstrengthsulfurcaps,whileupto11,000 psi(77MPa),neoprenepadsinsteelcapsgaveresultswithin3%ofthoseobtained with thesulfurend caps. However,they concluded that theuse ofeitherunbondedsystemwasquestionable;substantialdifferencesintestresultswereobtainedwhentwosetsofrestrainingcaps(fromthesamemanufacturer)wereused.Toimprovetheresultsobtainedwithunbondedsystems,Boulay37developedasysteminwhich,insteadofelastomericinserts,amixture ofdrysandandwax is used.Itwas found38thatthesandmixture gave resultswhich were intermediate betweenthoseobtainedwith ground ends or withsulfurmortarcaps.Insummary,then,belowabout14,000psi(98MPa),athin,highstrengthsulfurmortarcapmaybeusedsuccessfully. Beyondthatstrengthlevel, it would appearthat grinding specimenends is currently theonly waytoensurevalid testresults.Testing machinecharacteristicsIngeneral,fornormalstrengthconcrete,thecharacteristicsof thetestingmachineitselfareassumedtohavelittleornoeffectonthepeakload.However,forveryhighstrengthconcretesthemachinemaywellhavesomeeffectontheresponse of thespecimentoload.Froma review oftheliterature,Hester31concludedthatthelongitudinal stiffnessofthetestingmachinewillnotaffectthemaximum load,andthis view is sharedalsobyAitcin.33However,if themachineis notstiffenough,thespecimensmayfailexplosively,and,of course,a very stiffmachine (with servo-controls)isrequiredif one wishes to determinethe post-peakresponseof the concrete.Ontheotherhand,Hester31alsoreportsthatifthemachineisnotstiffenoughlaterally,compressivestrengthsmay beadversely affected.Onemustalsobeconcernedaboutthecapacityofthetestingmachinewhentesting very high strengthconcretes.Aitcin33 calculatedtherequiredmachinecapacitiesfordifferentstrengthlevelsandspecimensizes,usingthe commonassumptionthat thefailure loadshould not exceed2/3 of themachinecapacity.SomeofhisresultsarereproducedinTable 1.12.Relativelyfew commerciallaboratoriesareequippedtotesthighstrengthconcrete,sinceacommoncapacityofcommercialtestingmachineis292,500 lbs(1.3 MN).Totesta6x12 in.(15Ox 300 mm)cylinderofTable1.12Machine capacity required for high strength concrete3321,400psi(15OMPa)concreterequiresa900,000Ib(4.0 MN)testingmachine,andrelatively few machines of this sizeareavailable incommer-cial laboratories.Thisthen,is probably thedriving forcebehind themoveto thesmaller 4 x 8 in.(100 x 200 mm) cylinders.Effect of loadingplatensAgain,forordinary concrete,theeffectsofthespherically seatedbearingblocks(platens)arenotexplicitlyconsidered,aslongastheymeettherequirements of ASTMC39:Compressive Strength ofCylindricalConcreteSpecimens.However,recentworkat theConstructionTechnologyLabor-atoriesinSkokie,Illinois39hasshownthat,forhighstrengthconcrete,even this cannot be ignored. Sphericalbearing blocks which deform in suchaway thatthestressesarehigheraroundtheperipheryofthespecimenthanatthecentre,yieldhighercompressivestrengthsthanblocks whichdeformso thatthehigheststressesareatthecentreof thespecimen,andfallofftowardstheedges(i.e.a'concave'ratherthana'convex'stressdistribution).Measureddifferencescanbeashighas15%forconcreteswithcompressivestrengths greaterthan16,000 psi (112 MPa).1.5ConclusionsIn conclusion, then,it has beenshown that theproductionof high strengthconcreterequirescarefulattentiontodetails.Italsorequiresclosecooperationbetween the owner, the engineer,the suppliers andproducersoftheraw materials,thecontractor,andthetesting laboratory.32Perhapsmostimportant,wemustrememberthatthewell-known'laws'and'rules-of-thumb'thatapply to normal strength concretemay well notapplyto high strength concrete, which is a distinctly differentmaterial.Nonethe-less,wenowknowenoughabouthighstrengthconcretetobeabletoproduce it consistently, notonly in the laboratory,butalso in the field. It istobehopedthatcodesof practiceandtesting standardscatchup with thehighstrengthconcretetechnology,sothattheuseofthisexcitingnewmaterial can continue toincrease.Specimen size100 x 200 mm150 x 300 mmFailureloadfc'= 100 MPa0.785 MN1.76 MNNote: IMN=225,000 lbffc'=150 MPa1.18 MN2.65 MNMachinecapacityfc'= 100 MPa1.2MN2.65 MNfc'=150 MPa1.75 MN4.0 MNAcknowledgementsThisworkwas supportedby theCanadian Network ofCentres of Excell-ence on High-Performance Concrete.References1SHRP-C/FR-91-103(1991)Highperformanceconcretes,astateoftheartreport.StrategicHighwayResearchProgram,NationalResearchCouncil,Washington,DC.2FIP/CEB(1990)Highstrengthconcrete,stateoftheartreport.Bulletind'InformationNo.197.3ACIStandard 211.1 (1989) Recommendedpractice forselecting proportionsfornormalweight concrete.AmericanConcreteInstitute,Detroit.4Mindess,S.andYoung,J.F.(1981)Concrete.PrenticeHallInc.,EnglewoodCliffs.5Perenchio,W.F.(1973)Anevaluationofsomeofthefactorsinvolvedinproducingveryhigh-strengthconcrete.ResearchandDevelopmentBulletin,No.RD014-01T,PortlandCementAssociation,Skokie.6Mehta,P.K.andAitcin,P.-C.(1990)Microstructuralbasisofselectionofmaterialsandmixproportionsforhigh-strengthconcrete,inSecondInterna-tionalSymposiumonHigh-StrengthConcrete,SP-121.AmericanConcreteInstitute,Detroit,265-86.7Aitcin,P.-C.(1992)privatecommunication8Ronneburg,H.and Sandvik, M.(1990)HighStrength Concretefor North SeaPlatforms,Concrete International,12,1, 29-349CSAStandardA23.5-M86(1986)Supplementarycementingmaterials.Cana-dianStandardsAssociation,Rexdale,Ontario.10ASTMC618Standardspecificationforflyashandraworcalcinednaturalpozzolanforuse as a mineral admixturein portlandcement concrete.AmericanSocietyfor TestingandMaterials,Philadelphia,PA.11ASTMC989 Standard specificationforgroundiron blast-furnaceslag foruse inconcreteandmortars.AmericanSocietyforTestingandMaterials,Phila-delphia,PA.12Malhotra,V.M.(ed)(1987)Supplementarycementingmaterials forconcrete.Ministerof Supply andServices,Canada.13Aitcin,P.-C.,Sarkar,S.L.,Ranc,R.andLevy,C.(1991)AHighSilicaModulusCementforHigh-PerformanceConcrete,inS.Mindess(ed.),Advancesincementitiousmaterials.CeramicTransactions16, TheAmericanCeramicSocietyInc.,102-21.14Malhotra,V.M.,Ramachandran,V.S.,Feldman,R.F.andAitcin,P.-C.(1987)Condensedsilicafumeinconcrete.CRCPressInc.,BocaRatan,Florida.15Cook,I.E.(1989)10,000 psi Concrete.Concrete International,11, 10, 67-75.16Peter man,M. B.andCarrasquillo,R. L.(1986)Productionofhighstrengthconcrete. NoyesPublications, ParkRidge.17Randall,V.R.andFoot,K.B.(1989)HighstrengthconcreteforPacificFirstCenter.Concrete International: Design and Construction, 11, 4,14-16.18Aitcin,P.-C.,Shirlaw,M.andFines,E.(1992)Highperformanceconcrete:removingthemyths,inConcrescere,NewsletteroftheHigh-PerformanceConcreteNetworkof Centresof Excellence(Canada),6,March.19Burg,R.G.andOst,B.W.(1992)Engineeringpropertiesofcommerciallyavailablehigh-strengthconcretes.ResearchandDevelopmentBulletinRD104T, PortlandCementAssociation,Skokie.20Aitcin,P.-C.andMehta,P.K.(1990)Effectofcoarseaggregatetypeormechanicalpropertiesofhighstrengthconcrete.ACIMaterialsJournal,AmericanConcreteInstitute, Detroit,87, 2,103-107.21ACICommittee363(1984)State-of-the-artreportonhighstrength concrete(ACI363R-84).AmericanConcreteInstitute,Detroit.22Addis,B.H.(1992)PropertiesofHighStrengthConcreteMadewithSouthAfricanMaterials,Ph.D.Thesis,University of theWitwatersrand,Johannes-burg,SouthAfrica.23Addis,BJ.andAlexander,M.G.(1990)Amethodofproportioningtrialmixesforhigh-strengthconcrete,inACISp-121,Highstrengthconcrete,SecondInternational Symposium,AmericanConcreteInstitute, Detroit,287-308.24CanadianPortlandCementAssociation(1991) Design and control ofconcrete.EditionCPCA,Ottawa.25Fiorato,A.E.(1989)PCAresearchonhigh-strengthconcrete.ConcreteInternational,11, 4,4450.26Hattori,K.(1979)Experienceswith mighty superplasticizerin Japan,inACISP-62,Superplasticizersinconcrete,AmericanConcreteInstitute,Detroit,37-66.27Suzuki,T.(1987)Experimentalstudiesonhigh-strengthsuperplasticizedconcrete,inUtilizationofhighstrengthconcrete,Symposiumproceedings.Stavanger, Norway: TapisPublishers, Trondheim, 53-4.28Carrasquillo,R.C.,Nilson,A.H.andSlate,P.O.(1981)Propertiesofhighstrengthconcretesubjecttoshort-termloads.JournalofAmericanConcreteInstitute, 78, 3, 171-8.29Carrasquillo,P.M. andCarrasquillo,R.L.(1988).Evaluationoftheuseofcurrentconcretepracticeintheproductionofhigh-strengthconcrete.ACIMaterialsJournal,85,1, 49-54.30Moreno,J.(1990)225 W.WackerDrive.Concrete International,12, 1,35-9.31Hester, W.T.(1980)Fieldtestinghigh-strength concretes:a critical review ofthestate-of-the-art.Concrete International, 2,12, 27-38.32Howard,N.L. andLeatham,D.M.(1989)Theproductionanddelivery ofhigh-strengthconcrete.Concrete International,11, 4, 26-30.33Aitcin,P.-C. (1989)Lesessaissuelesbetonsatreshautesperformances, inAnnalesde L'InstitutTechniqueduBatiment et desTravaux Publics,No. 473.Mars-Avril.Serie:Beton263, 167-9.34Carrasquillo,R.L., Slate,P.O.andNilson,A.H.(1981)Microcrackingandbehaviour of high strength concretesubjected to short term loading. AmericanConcrete Institute Journal,78, 3,179-86.35Richardson,D.N.(1990)Effectsoftestingvariablesonthecomparisonofneoprenepadandsulfurmortar-cappedconcretetestcylinders. ACIMaterialJournal,87, 5, 489-95.36Carrasquillo,P.M. andCarrasquillo,R.L.(1988)Effectofusingunbondedcappingsystemsonthecompressivestrengthofconcretecylinders.ACIMaterialsJournal,85, 3,141-7.37Boulay,C.(1989)Laboiteasable,pourbienecraserlesbetonsahautesperformances.BulletindeLiaisondesLaboratoiresdesPontsetChausses,Nov/Dec.38Boulay,C,,Belloc,A.,Torrenti,J.M.andDeLarrard,F.(1989)Miseaupointd'unnouveaumodeoperatoire d'essai decompression pourles betons ahaute performances.Internal report,LaboratoireCentraldes PontsetChaus-sees,Paris,December.39CTLReview(1992)ConstructionTechnologyLaboratories,Inc.,Skokie,Illinois,15, 2.2Short term mechanicalpropertiesS H Ahmad2.1IntroductionChapter1 discussedtheproductionofconcreteandtheeffectsofalargenumberof constituentmaterials-cement,water, fine aggregate,coarseaggregate(crushedstoneorgravel),airandotheradmixturesontheproductionprocess.Somequalitycontrolissueswerealsoaddressed.Inthe presentchapter,the mechanical propertiesof hardenedconcreteundershorttermconditions orloadings arediscussed.Concretemustbeproportionedandproducedtocarryimposedloads,resistdeteriorationandbedimensionally stable. Thequality of concreteischaracterizedbyitsmechanicalpropertiesandabilitytoresistdeteriora-tion.Themechanicalpropertiesofconcretecanbebroadlyclassifiedasshort-term(essentiallyinstantaneous)andlong-termproperties.Short-termpropertiesincludestrengthincompression,tension,modulusofelasticity and bondcharacteristics. The long-term propertiesinclude creep,shrinkage,behaviorunderfatigue,anddurabilitycharacteristicssuchasporosity, permeability, freeze-thaw resistanceand abrasionresistance.ThecreepandshrinkagecharacteristicsarediscussedinChapters,thebe-havior under fatigueand the bond characteristicsis addressedin Chapter4.Theimportantaspectof durability is presentedinChapter5.Whileinformationonhighperformanceconcretes(HPC)asdefinedinChapter1isscarce,thereisasubstantialbodyofinformationonthemechanical propertiesof high strengthconcreteand additional informationisbeingdevelopedrapidly.Oneclassofhighperformanceconcretesaretheearlystrengthconcretes.Themechanicalpropertiesofthesetypes ofhighperformanceconcretesarebeinginvestigatedundertheStrategicHighwayResearchProgramSHRPC-205whichisinprogressatNorthCarolina State University. Since high performanceconcretestypically havelowwater/cementitiousmaterials(w/c)ratiosandhighpastecontents,characteristicswillinmanycasesbesimilartothoseofhighstrengthconcrete.Much of thediscussion in this chapterwill thereforeconcentrateonhigh strengthconcretes.Asignificantdifferenceinbehaviorbetweentheearly strengthandthehighstrengthconcretesisintherelationshipofcompressivestrengthtomechanicalproperties.Strengthgainincompressionistypicallymuchfasterthanstrengthgaininaggregate-pastebond,forinstance.Thiswillleadtorelativedifferences in elastic modulusandtensilestrength of earlystrengthconcretesandhigh strengthconcretes,expressedas a functionofcompressive strength. The relationships of mechanical propertiesto 28-daycompressivestrengthdevelopedinotherstudiescannotnecessarilybeexpectedtoapply toearly strengthconcretes.Theinformationdevelopedunder theSHRP programwill beusefulto fill this knowledge gap.2.2StrengthThestrength of concreteis perhapsthemostimportantoverallmeasure ofquality,althoughothercharacteristicsmayalsobecritical.Strengthis animportantindicatorofqualitybecausestrengthisdirectlyrelatedtothestructureofhardenedcementpaste.Althoughstrengthisnotadirectmeasureofconcretedurabilityordimensionalstability,ithasastrongrelationshiptothew/cratiooftheconcrete.Thew/cratio,inturn,influencesdurability,dimensionalstabilityandotherpropertiesoftheconcretebycontrollingporosity.Concretecompressivestrength,inpar-ticular,iswidelyusedinspecifying,controllingandevaluatingconcretequality.Thestrengthofconcretedependsonanumberof factorsincluding thepropertiesandproportionsoftheconstituentmaterials,degreeof hydra-tion, rateof loading,methodof testing andspecimengeometry.The propertiesof theconstituentmaterialswhich affectthestrengtharethequalityoffineandcoarseaggregate,thecementpasteandthepaste-aggregatebondcharacteristics(propertiesoftheinterfacial,ortransition,zone).These,inturn,dependonthemacro-andmicroscopicstructuralfeaturesincludingtotalporosity,poresizeandshape,poredistributionandmorphologyofthehydrationproducts,plusthebondbetweenindividualsolidcomponents.Asimplifiedviewofthefactorsaffectingthestrengthof concreteis shown in Fig. 2.1.Testingconditionsincluding age, rateof loading, methodof testing,andspecimengeometrysignificantlyinfluencethemeasuredstrength.Thestrength of saturatedspecimenscan be15% to20%lower than thatof dryspecimens. Underimpact loading, strength may be as much as 25% to 35%higherthanunderanormalrateofloading(10to20microstrainspersecond).Cubespecimensgenerallyexhibit20%to25%higherstrengthsthancylindricalspecimens.Largerspecimensexhibitloweraveragestrengths.F ig.2.1Anoversimplifiedview of thefactorsinfluencing strength of plain concrete53Constituent materials and mix proportionsConcretecompositionlimits theultimate strengthwhich canbeobtainedandsignificantlyaffectsthelevelsofstrengthattainedatearlyages.Amorecompletediscussionoftheeffectsofconstituentmaterialsandmixproportionsis given in Chapter1.However,a review of thetwo dominantconstituentmaterialsonstrengthis usefulatthispoint.Coarseaggregateandpastecharacteristicsaretypicallyconsideredtocontrolmaximumconcretestrength.Coarse aggregateTheimportantparametersofcoarseaggregateareitsshape,textureandthe maximum size. Since the aggregateis generally stronger than the paste,itsstrengthis nota majorfactorfornormalstrengthconcrete,orinearlystrengthconcrete.However,theaggregatestrengthbecomesimportant inthecaseofhigher-strengthconcreteorlightweightaggregateconcrete.Surfacetextureandmineralogyaffectthebondbetweentheaggregatesandthepasteandthestresslevelatwhichmicrocrackingbegins.Thesurfacetexture,therefore,mayalsoaffectthemodulusofelasticity,theshapeofthestress-straincurveand,toalesserdegree,thecompressivestrengthofconcrete.Sincebondstrengthincreasesataslowerratethancompressivestrength,theseeffectswillbemorepronouncedinearlystrength concretes. Tensilestrengths may be very sensitive to differences inaggregatesurface textureandsurface areaperunit volume.C O N C R E T E S T R E N G T HSPE C IME NP A R A M E T E R SD im ensionsG e o m e tryM o is tu re s ta teS tre n g tho fth ecom ponent phasesLO ADIN GP A R A M E T E R SS tre s st y p eR a teo f s t r e s s a p p lic a tio nM AT R IXPO R O SIT YW a t e r / c e m e n t ra tioM in e ra l a d m ix tu r e sD egreeo f h y d ra tio ncu rin g tim e, te m p .,hum idityA ir c o n te n te n tra p p e d a irentrained airA g g r e g a t ep o ro s ityT R A N S IT IO NZ O N E P O R O S IT YW a t e r / c e m e n t ra tioM ineral a d m ix tu r e sBleeding c h a ra c te ris tic sa g g re g a te g ra d in g ,m a x .s iz e ,a n dg e o m e tryD egreeo f c o n s o lid a tio nD egreeo f h y d ra tio nc u rin gtim e ,te m p ., hum idityC hem ical in te ra c tio n b e tw e e na g g re g a te andcem ent pasteTheeffectof differenttypes of coarseaggregate on concrete strength hasbeenreportedinnumerousarticles.Arecentpaper12reportsresultsoffourdifferenttypesofcoarseaggregatesinavery highstrengthconcretemixture(w/c =0.27).Theresultsshowedthatthecompressivestrengthwassignificantlyinfluencedbythemineralogicalcharacteristicsoftheaggregates.Crushedaggregatesfromfine-graineddiabaseandlimestonegave thebestresults.Concretesmadefroma smoothriver gravel andfromcrushed granite that contained inclusions of a softmineral were found to berelatively weakerin strength.Theuseoflargermaximumsizeofaggregateaffectsthestrengthinseveralways.Sincelargeraggregateshavelessspecificsurfacearea,thebondstrengthbetweenaggregatesandpasteislower,thusreducingthecompressive strength. Largeraggregate results in a smaller volume of pastethereby providing morerestraint to volume changes of the paste. This mayinduceadditionalstressesinthepaste,creatingmicrocrackspriortoapplicationofload,whichmaybeacriticalfactorinveryhighstrengthconcretes.TheeffectofthecoarseaggregatesizeonconcretestrengthwasdiscussedbyCooketal.22Twosizesofaggregateswereinvestigated:a3/8 in.(10 mm)and a1 in.(25 mm) limestone.A superplasticizer was usedinallthemixes.Ingeneral,thesmallestsizeofthecoarseaggregateproducesthe highest strength for a given w/c ratio, see Figs 2.2-2.6. It maybe notedthat compressivestrengths in excess of 10,000 psi (70 MPa) can beproducedusing a 1 in.(25 mm) maximum size aggregate when themixtureis properlyproportioned.Althoughthesestudies12'22 provideusefuldataandinsight, muchmoreresearchisneededontheeffectsofaggregatemineralpropertiesandT e s t age, d a y sF ig.2.2Effectof aggregate type on strength at differentages for a constant w/c materialsratio without superplasticizer22Compressive strength, psiw / c-0 .3 2W a t e r -c e m e n t it io u sra tioFig.2.3Effectof aggregate type on 56 day strength for concretefor differentw/c materialsratio22particleshapeonthestrengthanddurability of higher strength concrete.ThiswasrecognizedasoneoftheresearchneedsbytheACI363Committee.3PastecharacteristicsThe most important parameteraffectingconcretestrength is thew/c ratio,S u p e rp la s tic iz e rW a t e r -r e d u c e rW a te r-c e m e n titio u sra tioFig.2.4Relationship of w/c materials ratio with and without a high rangewater-reducingadmixture for coarseaggregate size not exceeding |in. (10 mm)22N osuperplasticizer56-day compression strength, psi56-day compression strength, psiF ly ashiin . lim e s to n eW a t e r -c e m e n t it io u sra tioF ig.2.5Relationship of w/c materials ratio with and without a high range water-reducingadmixture for coarse aggregate size not exceeding 1 in. (25.4 mm)22sometimes referredtoas thew/b (binder) ratio.Eventhough the strengthof concrete is dependentlargely on the capillary porosity or gel/space ratio,thesearenoteasyquantities tomeasureorpredict.Thecapillary porosityof a properly compacted concreteis determined by the w/c ratio anddegreeofhydration. Theeffectof w/c ratioonthecompressivestrength is shownin Fig. 2.7. The practical use of very low w/c ratio concretes has been madepossibleby use of both conventional andhigh range water reducers, whichpermit productionof workable concretewith very low water contents.Supplementarycementitiousmaterials(fly ash,slagandsilicafume)havebeeneffectiveadditionsin theproductionof high strengthconcrete.Although fly ashis probablythemostcommonmineraladmixture,onavolumebasis,silicafume(ultra-fineamorphoussilica,derivedfromtheproductionofsiliconorferrosilicaalloys)inparticular,usedincombina-SuperplasticizerW a te r -r e d u c e rW a te r-c e m e n titio u sra tioF ig.2.6Effectof aggregate type on strength at differentages for a constant w/c materialsratio, with superplasticizer22w i t hsuperplasticizer56-day compression strength, psi56-day compression strength, psiF lyash1inchlim e sto n eWater-cementitious materials ratioFig.2.7Summary of strengthdataas a function of w/c materials ratio29tionwithhigh-rangewaterreducers,hasincreasedachievablestrengthlevels dramatically (Fig. 2.7).10'51'52Theeffectofcondensedsilicafumeonthestrengthofconcretewasreportedin a very comprehensivestudy.28 Thebeneficial effectof using upto16% (by weight of cement) condensedsilica on the compressive strengthis shown in Fig. 2.8.Thedataindicatethattoachieve10,000 psi (70 MPa)28day4 x4 x4 i n.(100x 100x 100mm)cubestrength,thew/cratiosilicafu m eN otes: T estages2 8 to 105d a y s4x8 o r6x12 -in.(102x2 0 3o r152x3 0 5-m m ) cylin d e rsMoist curing atleast 1 day A llnon-air-entrained c o n c re te s1.O k s i =6.895M P a'z e r o 'slum p8%C SFHighp e rfo rm a n c ec o n c re te16%C SFR eferencec o n c re tew / cFig.2.828-day compressive strength versus w/c materialsratiofor concrete withdifferentcondensedsilica fume contents28Compressive strength, ksiCompressive strength, MPaR e f. 15R et. 13R e f.2R e f.3R e f.4R e f.9R e f. 16R e f .7R e f.5requiredisabout0.35ifnosilicafumeisused;however,with8%silicafume,thew/c neededis about0.50,andwith16% silica fumecontentthew/cratiorequirementincreasestoabout0.65.Thisindicatesthathighercompressivestrengthcanbeachievedveryeasilywithhighsilicafumecontentat relatively higher w/c ratios.Theefficiencyofsilicafumeinproducingconcreteofhigherstrengthdepends on water/cement + silica fumeratio, dosageof silica fume,age andcuring conditions.Yogenendramet al.S5 investigated theefficiencyof silicafumeatlower w/c ratio.Theirresultsindicatedthattheefficiencyis muchlower at w/c ratio of 0.28as comparedtotheefficiencyat w/c ratio of 0.48.Theperformanceof chemicaladmixturesis influencedby theparticularcementandothercementitiousmaterials.Combinationswhich havebeenshowntobeeffectivein many casesmay notwork in all situations,duetoadversecementandadmixtureinteraction(seeFig. 2.9).Substantialtestingshouldbeconductedwithanynewcombinationofcements,andmineral or chemicaladmixtures priorto largescaleuse.T est a g e -d a y sF ig.2.9Effectof varying dosage rates of normal retarding water-reducing admixtures onthestrength development of concrete22A S T MC -494 a d m ixtu reM ix no.84-61 13ozs. T ype F :3ozs.T ypeA]M ix no.84-60 13ozs.T ype F :6ozs.T ypeA]M ix no.84-59 13o zs.T ypeF :9o zs.T y p eAlM ix no. 84-5813o z s .T ype F :9ozs.T ypeD]Compressive strength, psiA g e ,d a y sF ig. 2.10Normalizedstrengthgain with age for limestoneconcretes moist-cureduntiltesting16Strength development and curing temperatureThestrengthdevelopmentwithtimeisafunctionoftheconstituentmaterialsandcuringtechniques.Anadequateamountofmoistureisnecessarytoensurethathydrationis sufficienttoreducetheporositytoalevelnecessarytoattainthedesiredstrength.Althoughcementpastewillnevercompletelyhydrateinpractice,theaimofcuringistoensuresufficienthydration.Inpasteswithlowerw/cratios,self-desiccationcanoccurduring hydrationandthus preventfurtherhydrationunless water issuppliedexternally.Thestrengthdevelopmentwith timeupto95 days fornormal,mediumand high strengthconcretesutilizing limestoneaggregatesand moistcureduntiltestingareshowninFig. 2.10.Theresultsindicateahigherrateofstrengthgainforhigherstrengthconcreteatearlyages.Atlateragesthedifferenceisnotsignificant.Thecompressivestrengthdevelopmentof9000 psi,11,000 psi,and14,000 psi(62MPa,76MPa,97MPa)concretesuptoa periodof 400 days is shown inFig. 2.11.Theresultsshown inthefigureareformixescontainingcementonlyorcementandflyash,withsomemixes using high range water-reducing agents. Thedata indicate thatformoist-curedspecimens, strengths at 56 days areabout10% greater than28 daystrengths.Strengthsat90 days areabout15%greaterthan28 daystrengths. While it is inappropriateto generalizefromsuch results, they doindicatethepotentialfor strengthgain at laterages.Inarecentstudy45atNorthCarolinaStateUniversity(NCSU),con-cretesutilizing anumberofdifferentaggregatesandmineraladmixtures,withstrengthsfrom7000 psito12,000 psi(48 MPato83 MPa)at28 daysandfrom10,000 psi toalmost18,000 psi(69 MPato124 MPa)atoneyearweretested.Onexamining theabsolutestrengthgainagainstthepercen-tagestrengthgain with time,it was concludedthatthereappearstobeno4 " x8" (10 2mmx2 0 3m m )cylinderHighs tre n g thM edium s tre n g thN orm als tre n g thCompressive strengthCompressive strength at 95 daysA g e , d a y sF ig. 2.11Compressive strengthdevelopment for concretes with and withouthigh rangewater reducers29single,constantfactorwhichcanbeusedtopredictlaterstrengthsaccuratelyfromearlystrengthsexceptinavery generalsense.Thisisnodoubtduetothecontributionsofnotonlytheultimatestrengthoftheaggregateandthemortar,buttothestrengthof thetransitionzone.Thetransitionzonestrength,orinterfacial bondstrengthof themortartotheaggregate,ofconcretesofhigherstrengths,istypicallyaffectedbythebindercompositionas well as theultimate strengthof themortar.Resultsforsplitting tensilestrengthandmodulus of rupturewere similar.Theeffectof condensedsilica fume(CSF)on concrete strengthdevelop-mentat20 0C generallytakes placefromabout3 to28 daysaftermixing.Johansen40 measuredstrengthup to 3 years and concludedthatthere waslittleeffectof CSF oneitherthestrengthgain between28 days and1 yearor between1 and 3 years for water-storedspecimens.Theeffectof cementtypesonthestrengthdevelopmentis presented inTable 2.1.Atordinarytemperatures,fordifferenttypesofportlandandTable 2.1Approximate relative strength of concreteas affectedby cement typeR e M I ( I4 1O O O p S i ) R e f . 1 0 ( 1 1 , 0 0 0 p s i )R e f . 10(9,0 0 0psi)R e f . 5(n o H R W R )R e f . 5 (H R W R )R e f . 9 , 1 0(9,0 0 0 psi; a irc u r e d a f te r 7d a ys)N o te sM o ist c u rin g unless n o te d1,0 0 0p si=6.895M P aPercent of 28-day strengthType of portland cementASTMIIIIIIIVVDescriptionNormal or general purposeModerate heat of hydrationand moderatesulf ateresistingHigh early strengthLow heat of hydrationSulfate resistingCompressive strength(percent of strength of Type I ornormal portland cement concrete)1 day1007519055657 days10085120657528 days10090110758590 days100100100100100A g eF ig.2.12Compressive strength development of concrete cured at 20 0C withdifferentdosages of condensed silicafume48blendedcements,thedegreeof hydrationat90 daysandaboveis usuallysimilar; therefore,theinfluenceof cementcompositionontheporosity ofthematrixandstrengthis primarilya concernatearlyages.Theeffectofcondensedsilica fumeonthestrengthdevelopmentof concreteswithfourdifferenttypesof cementwas investigated by MaageandHammer.48Thefourcementtypeswereordinaryportlandcement,10%and25%pulve-rizedfuelash(fly ash)blends,anda15%slagblend.Concretemixeswithout CSF and with 0%, 5%, and 10% CSF were made at 5 0C, 20 0C and35 0Candmaintainedatthesetemperaturesin waterforuptooneyear.The compressive strengths were measuredfrom16 hours up toa period ofone year. Mixes in three strength classes were made: 2000 psi, 3500 psi and6500 psi (15 MPa, 25 MPa and 45 MPa).Figure 2.12 shows thecompressivestrengthdevelopmentofconcretewater-curedat20 0C, with variousCSFdosagesandutilizingdifferentcementtypes.Inthefigureeachcurverepresentsameanvalueforfourcementtypes,andrelativecompressivestrengthof100%represents28 daystrength foreachmix type.Fromthefigure, it can be seen that at 20 0C curing, regardless of the cement type, theCSF had thesame influenceon thestrength-age relationship.Figures 2.13and 2.14 show relative strength developmentat 5 0C with and without 10%CSF for thefourcementtypes,and similar data for 35 0C curing are showninFigs.2.15and2.16.At50Ccuring,theblendedcementlagsbehindordinary portland cement concrete (OPC)up to 28 days; with 10% CSF thelagincreaseswhichindicatesthatthepozzolanicreactionshavenotcontributedmuchtothestrengthin the28 day perio