Compressive strength of concreteafter early loading&1PeterClaisseMA,PhD,CEng,FICEProfessorofConstructionMaterials,FacultyofEngineeringandComputing,CoventryUniversity,Coventry,UK&2ChristopherDeanMScPostgraduateStudent,FacultyofEngineeringandComputing,CoventryUniversity,Coventry,UK1 2Inmodernconstructionprojectsit isnecessarytoworkwithconcreteat atimebeforeitsstrengthcanbefullydeveloped.Applyinglargeconstructionloadstoastructurecanleadtoareductioninstrengthiftheserviceabilitylimit has been exceeded. However, the effects of smaller compressive loading on concrete cubes, belowtheserviceability limit state, have been found to have a positive impact on the 28 day ultimate strength. The results fromthis study indicate the 28 day strength of wet cured concrete cubes increased on average by 6% when specimens wereloadedupto90%of their ultimatestrengthat 1, 3or 7daysafter casting. Concretespecimensunder thesameconditionsloadedpastthepointofmaximumstressatanearlyagedisplayedareductioninstrengthfrom5%orgreater, dependingupontheextent of theloading. This phenomenonof increasedstrengthafter loadingandsubsequent curinghas beenreportedintheliteraturefor manyyears, but theuseof moderncompressivetestapparatushasenabledthepresentauthorstoshowthatthefinalstrengthhasahighlevelofcorrelationwiththedisplacement during the initial early loading. The experiments carried out in the present study were to simulate highconstructionloadingat earlyagestobetter understandtheeffectsof earlyloadingandthechangesinultimatestrengthatlaterage.1. IntroductionConstructionloading is oftenthe most intensive stage of astructures life, withthe concrete still inits early stages ofstrength development it is essential to ensure it is notoverloaded. Premature removal of formwork, lifting or movingofprecastconcreteorlargeconstructionliveloadscancausecrackingor other damage(KaminetzkyandStivaros, 1994).Evenif astructuredoesnot collapseduringtheconstructionstage, itispossibletodamageitandreduceitsstrength. Theaimof this studywas toexaminetheeffects of compressiveloadingofconcreteatstagestypicalintheuseoftheshoring/reshoring technique, widely used in concrete floor constructioninmulti-storey buildings, where concrete canundergolargecompressivestrengthssoonaftercasting.In this study, 100mm concrete cubes were compressed to loadsof70,80and90%oftheirultimatestrengthtosimulateearlyoverloading at 1, 3 and 7 days after casting, which is typical ofearly stages in construction of concrete structures. Thesespecimens were reloaded at 28 days after being wet cured underlaboratory conditions. Comparing the results from earlyloaded specimens and control specimens, an increase instrengthwas recordedfor most, althoughnot all specimens.This study went on to further examine the relationship betweentheincreaseinstrengthandinitial loadingsofspecimensandfoundsome correlationbetweenthe initial displacements ofspecimensandthegaininstrength.Thisminorgaininstrengthcangiveconfidencetodesignersandcontractors, becauseitshowsthatearlyloadingisnotasdamagingastheymight haveexpected. Theauthorsarenot,however, suggesting that the observed additional strengthshould be relied upon in designs or that deliberate early loadingshouldbeusedtotrytoobtainit.Thisphenomenonwasalsofoundintheliteratureinsimilarstudies of sustainedbiaxial loading, long-termcreepandre-testingloadedspecimensoverdifferentperiods.TheresultsofConstructionMaterialsCompressivestrengthofconcreteafterearlyloadingClaisseandDeanProceedingsoftheInstitutionofCivilEngineershttp://dx.doi.org/10.1680/coma.11.00057Paper1100057Received12/10/2011 Accepted03/01/2012Keywords:concretestructures/concretetechnologyandmanufacture/strengthandtestingofmaterialsice|proceedings ICEPublishing:All rightsreserved1For citation information please see http://www.claisse.info/Publish.htm wet-cured specimens from a published paperwererecalculatedtofindasimilarresulttotheonefoundhere.2. LiteraturereviewConcrete under uniaxial loading will develop cracks parallel tothe direction loaded. Under uniaxial loading from30%toaround 70% of maximum stress, the concrete will undergo slowcrackpropagation. Atbetween70and90%,crackswill beginto increase noticeably (Santiago and Hilsdorf, 1973). Finecracks that have been created in fractured concrete are capableofcompleterecoveryundermoistconditions; thisisaidedbythe formation of insoluble calcium carbonate from the calciumhydroxideinhydratedcement(Neville, 1994). Theprocessofcracked concrete undergoing self-repair is known as auto-geneoushealing.Fromearlierstudies,thephenomenonofautogeneoushealingwas found to heal cracked specimens to strengths almost equalto unloaded specimens, provided the specimens were not badlyshattered and were subject to continuous moist curing (Gilkey,1926). Autogeneous healing was first recognised by Abrams in1913(Whitlam, 1954)wherecracksdisappearedinahighwaybridge 3 years after their appearance. To quantify autogeneoushealing, reloading times have ranged from 3 days to 10 years inarangeofdifferentstudiessinceitsdiscovery.In1950astudyof 10?5 year old concrete specimens under sustained creepconditionswerereloadedtoexaminetheeffectsof long-termcreep (Washa and Fluck, 1950). Reloaded specimens displayedabout 5%higher [compressive strength] for hand-roddedconcrete,thanthatofcompanionunloadedcylinders.Abdel-JawadandHaddad(1992)carriedouttestswhichweresimilar to those reported here and concludedthat loadingconcrete,beyond8hofcasting,upto90%ofitscompressivestrength[at timeof loading] hasnoeffect onthestrengthofconcrete at a later age. Loading concrete past maximum stress(i.e. tofailure)resultedinastrengthlossoffrom10to50%,dependingontheageattimeofloading,theageattimeofre-testingandthecuringconditions.Uponre-examinationbythepresent authors of theresearchdone by Abdel-Jawad and Haddad (1992) in which 900specimens wereloaded8to72hafter casting, andreloadedbetween 7 and 90 days, it was found that under certainconditions the crushedspecimens displayedgreater strengthdevelopment against specimens not previously loaded. Theexperiment focused on re-testing concrete with different water/cement (w/c) ratios at 7, 28 and 90 days under both wet and dryconditions.Figure1istakenfromtheirdatacollectedforwetcuredspecimenswithaw/cratioof0?7showingthestrengthratio of specimens previously loaded against controlspecimens.Undertheconditionsof: (1)initial loadingtakingplaceafter8h of casting; (2) the load being less than 100%; and (3) the ageofre-testingbeing28days, itwasfoundthatthemajorityofspecimens displayeda substantial increase instrengthaver-aging5?7%abovethatofthecontrolstrength(Figure2).Thisdoes not conformto typical theoretical models regardingautogeneoushealingof specimens, wherespecimens areonlyexpected to regain up to their original strength (Neville, 1994).Coutinho(1977) suggestedthis is due toafactor of creep;pressure, like temperature, influences chemical reactions,particularly the reaction of the cement components with water.The pressure to which cement components are submittedincreasestheir solubilityinthewaterwithwhichtheyareincontact, thusincreasingthehydrationofcement. Ithasalsobeensuggestedthat this increaseinstrengthis greater whenapplied to younger age concrete for longer durations ofloading, although it has been noted by Coutinho that theincreaseincompressivestrengthdoesnotexceed15%.A study from 2002 (Liu et al., 2002) in which crack restorationunder sustainedbiaxialcompression(30% ofultimatestrengthover period of 14 days) also found this phenomenon ofincreased strength for specimens subject to compressiveloading. Itwasconcludedthat, Concretecreepdeformationswill not definitely cause damage of the material. On thecontrary, thesustainedcompressionloadatanearlyagecanincrease the strength. Similarly to work done by Abdel-Jawadand Haddad (1992) the increase in strength was mostprominent around 4 weeks after casting, becoming lessprominentatlaterages.Resultsfromtheirstudyalsoindicatethatsustainedbiaxialloadingcausedgreaterstrengthincreasethanuniaxialloadingalone(Table1).3. ExperimentalprocedureCement class CEMII/ALL32.5RtoBSEN197(BSI, 2000)was usedtocreate three different mix designs with10mmuncrushed stone and fine crushed aggregate 50%passingthroughthe600 mmsieve(Table2).The 100mmconcrete cubes were made to BS 8500-1:2006(BSI, 2006) and tested to BS EN 12390-3-2009 (BSI, 2009) at 1,3 and 7 days up to their designated loads of 90, 80 and 70% oftheultimateload. Anaverageoftwocontrol specimenswereused for each variable to find the relevant percentage ofultimate loading for the test. Two replicate specimens pervariablewereloadeduptotheirrelevantvaluesatauniformrateof0?1N/mm2persandimmediatelyreleased.In addition to the standard test a linear displacementtransducer was used to record the displacement during testing.The apparatus recordedthe loadanddisplacement at 0?1sintervals. A correction factor derived from calibration tests wasConstructionMaterials CompressivestrengthofconcreteafterearlyloadingClaisseandDean2used on the displacement readings to account for machinestiffness.Thespecimensloadedtofailurewereloadeduntil adropof50kNstrengthwasrecorded; thiswastoaccommodatelocalfailures in specimens. The hydraulic pressure in the testmachine was applied with a very sensitive computer-controlledpumpwhichwas able torelease the loadexactly whenthe50kNdecreaseoccurred.These specimens were storedfor later reloading along withspecimensloadedtotheirdesignatedfractionof theultimatestrength.All specimens were stored in a curing tank after 24h of setting(20 2C)untilthe28daytests.Atreloading, all specimenswerecompressivelyloadedonthesameaxisuptofailure.Theresultswerecomparedwiththreeormorecontrolspecimenstogiveaproportionofthecontrolstrength/reload strength for each mix design, intensity of initialloadandageofspecimens.4. ResultsSpecimensloadedto90%orbelowshowedlittleornovisiblecrackingtothenakedeye. Loadingspecimensbeyondthistothe point of failure resulted in fast crack propagation and withcontinuedloadingspecimensweredestroyedbeyondthepointofre-test.Whenspecimenswereinitiallyloadedbetween70and90%oftheirultimateloadtheresultsindicateanincreaseinstrengthaveragingbetween0?9and7?8%per mixdesign(Figure2).This was most prominent in mix 3 resulting in some specimensreaching an increase of 14?7%, although this increase wasvisibleinalmostallspecimensusedinthisexperiment.Specimens loaded to beyond the point of failure showedprominentsignsofcrackingandamarkedreductioninre-teststrengthat28days.Mix design with 07 w/c ratio1211511105095Reload strength/control strength090850860% 75%7 DaysPercentage of maximum stress at initial loading,Age of reloading28 Days 90 Days75% 90% 90% 90%8162472Age of inital loading (h)60%10Figure1.Reloadstrengthratioofwetcuredconcretespecimenwith0?7w/cratiofromAbdel-JawadandHaddad(1992)12110908070605Reload strength/control strengthReload strength of samples against control strength10100% 90% 80% 70% 90%Mix 2 Mix 3Mix design and percentage of intial strength against ultimate loadMix 4InitialloadingDay 1Day 3Day 780% 70% 100% 90% 80% 70% 100%Figure 2. The reload strength of specimens loaded to a percentageoftheultimatestrengthattimeofloadingConstructionMaterials CompressivestrengthofconcreteafterearlyloadingClaisseandDean3Theextentoftheinitial loading, exceptforspecimensloadedup to failure, did not appear to have any relation to theincrease in strengths between 70 and 90%. It was found that therelationshipbetweentheincreasesinstrengthdependedmoreupontheextentofdeformationofspecimens(Figure3).Figure3shows theincreasein28dayreloadstrengthtobemost prominent whenspecimens were initially deformedtoaround0?3mm,manyspecimensstilldisplayedanincreaseinstrengthtypicallyupto1mm. However, afteradeformationpast the thresholdof 0?3mmsome specimens lost strengththrough excessivecrackingbeyondwhichautogeneoushealingcould not take place. Past the threshold of 0?3mm a large dropinstrengthwas seeninsomespecimens showingadisparitybetweenconcretestrengthswherefailurehastakenplace.Whenre-testingspecimens previouslyloadedpast maximumstresswitha50kNrecordeddropinloadafterfailure,itwasseen that most specimens were subject to a noticeable reductioninstrength, insomecasesthiswasseentobeupto50%. Tostudy this in greater detail, additional tests were carried out onmix3specimens byloadingthemat 1dayafter castingtovarying degrees past ultimate failure. The reduction of strengthwas determinedtohave a relationshiptothe extent that aspecimenwasloadedpastfailure(Figure4).5. DiscussionSpecimens loaded past the point of failure underwent fastcrackpropagationclosetotheultimateload,ataround90to100%. Thecrackwidthsafteroverloadingweregenerallytoogreat for autogeneous healing totake place. The extent towhich specimens were loaded past the point of maximum stresshadasignificant impact onthestrengthdevelopment of thespecimens. As expected, once aspecimenreachedmaximumstress, anyfurtherloadingresultedinagreaterlossinreloadstrength.When specimenswereloaded upto 90%of maximumstressatan early age, an increase in strength relative to controlspecimenswasrecorded.Thefindingsinthisstudyfoundthatinitial loadingsbetween70and90%increasedthestrengthofspecimens by between 0?9 and 7?8%. This phenomenon isreinforcedbyworkbyCoutinho(1977)andAbdel-JawadandHaddad (1992) and where concrete loaded under similarconditions of 28 day re-testing and wet curing conditionsshowedsignsofincreasedstrength.WorkbyLiuetal.(2002)also emphasises that the strength of concrete is increasedthrough both biaxial and uniaxial compressive creep, providedthe strength is below 30% whereas sustained loading above thismay lead to slowcrack propagation, leading to excessivecrackingandfailureovertime.The strength increase found in this paper reinforces thetheoriesput forwardbyCoutinho(1977) wherehydrationofcement components is increased due to their solubility in waterwithwhichtheyareincontact. Theincreaseinstrengthfromthe experiments rarely exceeded 15%, as concluded byCoutinhoasthemaximumgaininstrengthfromcompressiveCreeptype,stresslevels:%ofultimatestrengthLoadage:daysLoadduration:daysStrengthofcreepspecimens:MPaStrength of non-creepspecimens:MPaStrengthofcreepspecimens/non-creepspecimens:%Biaxial 30% 14 45 8?2 Biaxial 30% 60 90 14?4 11?3 27?4Biaxial 30% 30 28 10?8 5?5 96?4Biaxial 30% 60 30 10?7 7?0 52?9Uniaxial 30% 60 12 19?7 18?3 7?7Uniaxial 60% 90 18 19?8 20?9 25?3Table1.Biaxial anduniaxiallyloadedconcreteonre-testingatvariedages(Liuetal.2002)Mixno. Cement:kg/m3Water:kg/m3w/cratio Coarseaggregate:kg/m3Fineaggregate:kg/m3Slump:mm1 450 225 0?5 863 863 1502 456 205 0?45 616 963 1103 585 205 0?35 815 815 110Table2.MixproportionsusedinexperimentsConstructionMaterials CompressivestrengthofconcreteafterearlyloadingClaisseandDean4loading. However, the results found by Liu et al. (2002) suggestthat concrete strengthcanbe increasedagreat deal furtherthan by increased hydration alone, where sustained biaxialloadingat30%increasedthereloadstrengthby96%.The increase in ultimate strength in concrete after creepcompared to concrete that has not been submitted topermanent loads is well documented. Short-termloadingofearly age concretes at higher stresses can also lead to anincreaseinstrengthupto15%showingthatconcretestrengthdevelopmentcanbepossiblewithoutlong-termcreep.The increase in strength was found in the present study to havesome correlation with the extent to which specimens weredisplaced when initially loaded. After a displacement of0?3mm, someconcretespecimenswereseentoshowalargereduction in strength as cracks propagated parallel to theloading. However, some specimens continued to show anincreaseinstrengthupto1?0mmdisplacement.Theauthorsmuststressthattheincreaseinstrengthfoundinthis paper and similar work occurred under laboratoryconditions. Although this may provide some comfort todesigners andcontractors it shouldinnowaybetakenintoaccount whendesigningorconstructingstructures. All speci-mens in this experiment were wet cured and this paper does notmake a correlationbetweenwet-curedanddry-curedspeci-mens, whichmayhavedifferent, possiblynegative, effectsinpractice.6. ConclusionOver 100 concrete cubes were tested to study the effect of staticuniaxial compressiveloadingonthestrengthdevelopment at28days.Fromtheresultsthefollowingconclusionshavebeendrawn.(a) Loadingspecimenstofailureatearlyagesreducesthestrengthat28days.(b) The reduction in strength of specimens is dependent upon12Initial displacement versus reload/control strength28 day reloaded strength/control strength100806040202 04 06 08Initial displacement: mm10 12 14 16 18 20MixMix 1Mix 2Mix 3design00Figure3.Reloadstrengthofspecimensagainstdisplacementatinitial loadingatearlyage12Initial day 1 displacement versus day 28reload strength of overloaded specimens100806Reload/control strength06 08 10Initial displacement: mm12 14 16 18 20ProportionRecordedload drop10 kN50 kN100 kN90%of ultimate load initiallyloaded0404020200Figure 4. Drop in reload strength at 28 days of specimens of mix 3loadedpastfailure1dayaftercastingConstructionMaterials CompressivestrengthofconcreteafterearlyloadingClaisseandDean5theextenttowhichthespecimenhasbeenloadedafterfailure.(c) Specimensloadedbetween70and90%oftheirultimateloadbetween1dayand7daysexhibitedanincreaseinstrengthat28daysaftercasting.REFERENCESAbdel-JawadYandHaddadR(1992)CementandConcreteResearch.EffectofEarlyOverloadingofConcreteonStrengthatLaterAges.PergamonPress,Oxford,UK.BSI(2000)BSEN1971:2000:Cement.Composition,specifications and conformity criteria for common cements.BSI,London,UK.BSI(2006)BS85001:2006:Concrete.ComplementaryBritishStandardtoBSEN206-1.Methodofspecifyingandguidanceforthespecifier.BSI,London,UK.BSI (2009)BSEN12390-3:2009Testinghardenedconcrete.Compressivestrengthoftestspecimens. BSI, London,UK.CoutinhoA(1977)Acontributiontothemechanismofconcretecreep. MaterialsandStructures10(1): 316.GilkeyHJ(1926)TheAutogeneousHealingofConcretesandMortars.ASTM,WestConshohocken,PA,USA.KaminetzkyDandStivarosPC(1994)Early-ageconcrete:constructionloads,behaviour,andfailures.ConcreteInternational16(1):5863.LiuGT,GaoHandChenFQ(2002)MicrostudyonCreepofConcreteatEarlyAgeunderBiaxialCompression.CementandConcreteResearch,Beijing,ChinaNevilleAM(1994)PropertiesofConcrete,4thedn.LongmanGroupLimited,Harlow,Essex,UK.SantiagoSDandHilsdorfHK(1973)Fracturemechanismsofconcreteundercompressiveloads.CementConcreteResearch3(4):363388.WashaGWandFluckPG(1950)Effectofsustainedloadingoncompressivestrength.ACIJournal46(5):693700.WhitlamEF(1954)Autogeneoushealingofconcreteincompression.TheStructuralEngineer32:235243.WHATDOYOUTHINK?To discuss this paper, please email up to 500 words to theeditor at journals@ice.org.uk. Your contributionwill beforwardedtotheauthor(s)forareplyand,ifconsideredappropriatebytheeditorial panel, will bepublishedasdiscussioninafutureissueofthejournal.Proceedings journals rely entirely on contributions sent inby civil engineering professionals, academics and stu-dents. Papersshouldbe20005000wordslong(briefingpapersshouldbe10002000wordslong),withadequateillustrationsandreferences. Youcansubmityourpaperonline via www.icevirtuallibrary.com/content/journals,whereyouwillalsofinddetailedauthorguidelines.ConstructionMaterials CompressivestrengthofconcreteafterearlyloadingClaisseandDean6