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Experimentalstudyofmechanicalpropertiesofnormal-strengthconcreteexposedtohightemperaturesatanearlyageBingChen,ChunlingLi,LongzhuChenDepartmentofCivilEngineering,ShanghaiJiaotongUniversity,800DongchuanRoad,Shanghai200240,PRChinaarticleinfoArticlehistory:Received6June2008Receivedinrevisedform17June2009Accepted19June2009Keywords:CompressivestrengthConcreteCuringHightemperatureMechanicalpropertiesabstractInChina,accidentalfiresareknowntooccurduringconstruction,causingconcretetobeexposedtohightemperatureswhenitisatanearlyage(i.e.‘‘young’’).Inthispaper,compressiveandsplittingtensilestrengthsofconcretescuredfordifferentperiodsandexposedtohightemperatureswereobtained.Theeffectsofthedurationofcuring,maximumtemperatureandthetypeofcoolingonthestrengthsofconcretewereinvestigated.Experimentalresultsindicatethatafterexposuretohightemperaturesupto8001C,early-ageconcretethathasbeencuredforacertainperiodcanregain80%ofthecompressivestrengthofthecontrolsampleofconcrete.The3-day-curedearly-ageconcretewasobservedtorecoverthemoststrength.Thetypeofcoolingalsoaffectsthelevelofrecoveryofcompressiveandsplittingtensilestrength.Forearly-ageconcrete,therelativerecoveredstrengthsofspecimenscooledbysprayedwaterarehigherthanthoseofspecimenscooledinairwhenexposedtotemperaturesbelow8001C,whilethechangesfor28-dayconcretearetheconverse.Whenthemaximumtemperatureexceeds8001C,therelativestrengthvaluesofallspecimenscooledbywatersprayarelowerthanthoseofspecimenscooledinair.&2009ElsevierLtd.Allrightsreserved.1.IntroductionFirehasbecomeoneofthegreatestthreatstobuildings.Beingaprimaryconstructionmaterial,thepropertiesofconcreteafterexposuretohightemperatureshavegainedagreatdealofattentionsincethe1940s[1–3].Generalreviewsofconcretebehaviorathightemperaturecanbefoundintheliterature[4–6].Althoughconcreteisgenerallybelievedtobeafireproofmaterial,studieshaveshownextensivedamageandevencatastrophicfailureathightemperatures.Themechanicalpropertiesofconcreteathightemperaturedegrademainlybecauseoftworelevantmechanisms:mechanicalandphysico-chemicaldamage[7–14].Inthecaseofelevatedheatingconditions,whenthetemperaturereachesabout3001C,theinterlayercalciumsilicatehydrate(C–S–H)waterandsomeofthecombinedwaterfromC–S–Handsulfoaluminatehydrateswillevaporate.Calciumhydroxide(Ca(OH)2),whichisoneofthemostimportantcompoundsincementpaste,dissociatesataround5301C,resultingintheshrinkageofconcrete.Thechemicalchangesintheaggregatesoccurattemperaturesabove6001C.Themechan-icaldegradationofconcretetakesplaceduetotheformationofcracksandmicrocracks.Theyappearinconcretebecauseofself-equilibratingstresses(causedbyincompatiblethermalstrains)whenconcreteisrapidlyheateduporcooleddownandthetemperaturegradientsarehigh.Accordingtopriorresearch[15],thefactorsthatinfluencethestrengthofconcreteathightemperaturecanbedividedintotwogroups:materialpropertiesandenvironmentalfactors.Someresearchhasstudiedtheeffectofmaterialproperties,suchaspropertiesoftheaggregate,cementpaste,aggregate–cementpastebondandtheirthermalcompat-ibilitywitheachother,ontheresistanceofconcrete.Inaddition,heatresistanceofconcreteisaffectedbyenvironmentalfactorssuchasheatingrate,durationofexposuretomaximumtempera-ture,coolingrate,loadingconditionsandmoistureregime.InChina,largefireaccidentsoccureveryyearduringconstruction.Normally,inthesesituationstheconcreteinthefieldisatitsearlyage(i.e.‘‘young’’).Theinnerstructureandchemicalcompositionofearly-ageconcretearedifferentfromthatinserviceduetouncompletedhydrationatanearlyage.Therearefewornostudiesinthetechnicalliteraturetodeterminehowtheseearly-ageconcretesbehaveafterexposuretohightempera-turesandwhethertheyareabletorecovertothedesignstrengthaftercertaincuring.Thus,anexperimentalstudyonthestrengthpropertiesofearly-ageconcreteafterexposuretohightempera-turewascarriedoutinthispaper.Thecompressiveandsplittingtensilestrengthsofconcreteafterdifferentcuringages,wherethespecimenswerekeptinthecuringroomforanother28daysafterexposuretohightemperatures,wereexamined.TheeffectsofARTICLEINPRESSContentslistsavailableatScienceDirectjournalhomepage:firesafFireSafetyJournal0379-7112/$-seefrontmatter&2009ElsevierLtd.Allrightsreserved.doi:10.1016/j.firesaf.2009.06.007Correspondingauthor.Tel.:+862154705110.E-mailaddress:hntchen@sjtu.edu.cn(B.Chen).FireSafetyJournal44(2009)997–1002ARTICLEINPRESScuringage,typeofcoolingandthemaximumtemperatureontherelativerecoveredstrengthsofconcreteafterexposuretohightemperatureswereinvestigated.2.Experimentalstudy2.1.MaterialsThecementusedinthisexperimentwasordinaryPortlandcementprovidedbyShanghaiHailuoCementPlantwitha28-daycompressivestrengthof57.5MPa.ThechemicalandphysicalpropertiesofcementandtypeCflyash(FA)producedbytheShanghaiWujinPowerplantarepresentedinTable1.Naturalsiliceousriversand(finenessmodulusof2.8)andnaturegranitecrushedstone(maximumsizeof19mm)wereusedasaggregates.Inthispaper,thedesignedcompressivestrengthofconcreteis35MPaat28days,whichisatypicalvalueforconcreteformostcivilengineeringstructuresinChina.Thedesignations,proportionsandproper