早龄期混凝土ppt课件

.,第十章早龄期混凝土,10.1定义和意义10.2配料、拌合与运输10.3浇灌、捣实和抹面10.4养护和脱模10.5工作性10.6坍落度损失10.7离析和泌水,.,第十章早龄期混凝土,10.8早期体积变化10.9凝结时间10.10混凝土的温度10.11质量控制,.,第十章早龄期混凝土,10.1定义和意义大量的工程实践告诉我们：从配料、搅拌、运输、浇灌、捣实、表面修整及养护等工序所采取的措施正确与否直接关系到混凝土的质量。尽管这一时期和混凝土的使用寿命相比微不足道。所谓早龄期？开始于凝结硬化前，结束于1-2天的硬化混凝土。,.,第十章早龄期混凝土,10.2配料、拌合与运输配料：计量过程质量法体积法采用体积法计量时，应该注意砂子的容胀问题！坍落度损失：如何改善？,.,Figure10-1Centrallymixedconcreteinaready-mixedconcreteplant,.,第十章早龄期混凝土,10.3浇灌、捣实与抹面,.,第十章早龄期混凝土,10.3浇灌、捣实与抹面,.,第十章早龄期混凝土,10.3浇灌、捣实与抹面,.,第十章早龄期混凝土,10.3浇灌、捣实与抹面捣实：使混凝土充填模板，排出气体并使之密实的过程。常用振捣器。插入式附着式抹面=找平+抹平,.,第十章早龄期混凝土,10.4养护和脱模养护的目的：防止水分蒸发；控制混凝土的温度。（书上译得有误，P232）Thetwoobjectivesofcuringaretopreventthelossofmoistureandtocontrolthetemperatureofconcreteforaperiodsufficienttoachieveadesiredstrengthlevel.,.,第十章早龄期混凝土,10.4养护和脱模脱模早晚关系到模板的周转速度，影响到所需模板的数量，进而关系到工程的成本。何时脱模？能够承受自重及施工时候所施加的荷载。,.,第十章早龄期混凝土,10.5工作性10.5.1定义和意义ASTMC-125定义：工作性是混凝土所具有的一种性质，它决定了在保证最低限度均匀性的前提下，操作一定数量新拌混凝土所需要的功。是混凝土的一项综合性能，至少应该包含以下二个方面的性质：稠度和稳定性。,.,第十章早龄期混凝土,10.5工作性稠度：是简单表征新拌混凝土流动能力或流动性的指标。常用坍落度试验或维勃稠度试验测得。稳定性：反映了混凝土防止其组分相互分离能力大小的一个指标。即泌水和离析越小，稳定性越好。通常用粘聚性或内聚性来定性地衡量泌水和离析。,.,第十章早龄期混凝土,10.5工作性谈论工作性的好坏与否，必须与施工条件联系起来。,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法有多种，但目前尚无理想的表征工作性的方法。常用的有：坍落度法维勃稠度密实系数,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：所用设备非常简单，只包括一根捣棒和一个高300mm,上口直径100mm，下口直径200mm的截口圆锥筒。先将混凝土填满圆锥筒，然后慢慢将圆锥筒提起。圆锥状的混凝土因重力作用而坍落。因坍落而减小的高度就是坍落度。,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：1.StandonthetwofootpiecesofconetoholdinfirmlyintheplaceduringSteps1though4.Fillconemold1/3fullbyvolume2-5/8“(67mm)highwiththeconcretesampleandroditwith25strokesusingaround,straightsteelrodof5/8(16mm)diameter24“(600mm)longwithahemisphericaltipend.Uniformlydistributestrokesoverthecrosssectionofeachlayer.Forthebottomlayer,thiswillnecessitateincliningtherodslightlyandmakingapproximatelyhalfthestrokesneartheperimeter(outedge),thenprogressingwithverticalstrokesspirallytowardthecenter.,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：2.Fillcone2/3fullbyvolume(halftheheight)andagainrod25timeswithrodjustpenetratinginto,butnotthrough,thefirstlayer.DistributestrokesevenlyasdescribedinStep1.,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：3.Fillconetooverflowingandagainrod25timeswithrodjustpenetratinginto,butnotthrough,thesecondlayer.Againdistributestrokesevenly.,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：4.Strikesoffexcessconcreteformtopofconewiththesteelrodsothattheconeisexactlylevelfull.Cleantheoverflowawayfromthebaseoftheconemold.,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：5.ImmediatelyaftercompletionofStep4,theoperationofraisingthemoldshallbeperformedin52sec.byasteadyupwardliftwithnolateralortorsionalmotionbeingimpartedtotheconcrete.Theentireoperationfromthestartofthefillingthroughremovalofthemoldshallbecarriedoutwithoutinterruptionandshallbecompletedwithinanelaspedtimeof2-1/2min.,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：6.Placethesteelrodhorizontallyacrosstheinvertedmoldsothattherodextendsovertheslumpedconcrete.Immediatelymeasurethedistancefrombottomofthesteelrodtothedisplacedoriginalcenterofthespecimen.Thisdistance,tothenearest1/4in(6mm),istheslumpoftheconcrete.Ifadecidedfallingawayorshearingoffconcretefromonesideorportionofthemassoccurs,disregardthetestandmakeanewtestonanotherportionofthesample.,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法坍落度法：此法不适用流动度过大或过小的混凝土。对稠度或者是流动性测量较实用。但不能表征混凝土的内聚性。,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法维勃稠度法：,.,第十章早龄期混凝土,10.5工作性10.5.2测量方法密实系数法：,.,第十章早龄期混凝土,10.5工作性10.5.3影响工作性的因素及控制用水量水泥用量骨料特征外加剂,.,第十章早龄期混凝土,10.6坍落度损失10.6.1定义：在实践中，如果说某一种混凝土有坍落度损失，则通常指一种很快发生且特别大的稠度损失。现象：稠度随时间流失而逐渐减小原因：水分失去？减水剂？,.,第十章早龄期混凝土,10.6坍落度损失10.6.2重要性：你能说明混凝土坍落度损失对混凝土施工的重要性吗？,.,第十章早龄期混凝土,10.6坍落度损失10.6.3原因和控制：你能说出混凝土坍落度损失的原因吗？水泥？作业因素吗？环境因素？,.,Chapter10concreteatearlyages,10.5.3FactorsAffectingWorkabilityandTheirControlWatercontent.foragivenmaximumsizeofcoarseaggregate,theslumportheconsistencyofconcreteisadirectfunctionofthewatercontent;i.e.,withinlimitsitisindependentofotherfactorssuchasaggregategradingandcementcontent.Concretemixtureswithveryhighconsistencytendtosegregateandbleed,therebyadverselyaffectingthefinishability;mixtureswithtoolowaconsistencymaybedifficulttoplaceandcompact,andthecoarseaggregatemaysegregateonplacement.,.,Chapter10concreteatearlyages,Cementcontent.Withconventionalportland-cementconcreteatagivenwatercontent,adrasticreductionofthecementcontentwouldproduceaharshmixturewithpoorfinishability.Concretemixturescontainingaveryhighcementcontentorhighproportionoffineparticlesshowexcellentcohesivenessbuttendtobesticky.,.,Chapter10concreteatearlyages,Aggregatecharacteristics.Theparticlesizeofcoarseaggregateinfluencesthewaterrequirementforagivenconsistency(Table9-2).Also,veryfinesandsorangularsandsrequiremorewaterforagivenconsistency.Alternatively,theywillproduceharshandunworkablemixturesatthewatercontentthatmighthavebeenadequatewithacoarseorawell-roundedsand.,.,Chapter10concreteatearlyages,Admixtures.whenthewatercontentofaconcretemixtureisheldconstant,theadditionofawater-reducingadmixturewillincreasetheconsistency.Entrainedairincreasesthepastevolumeandimprovestheconsistencyofconcreteforagivenwatercontent(Table9-2).Italsoincreasescohesivenessbyreducingbleedingandsegregation.Theimprovementinconsistencyandcohesivenessbyairentrainmentismorepronouncedinharshandunworkablemixturessuchasthoseusedinmassconcrete,whichhasalowcementcontent.Pozzolanicadmixturestendtoreducebleedingandimprovethecohesivenessofconcrete.Flyash,whenusedasapartialreplacementforfineaggregate,generallyincreasestheconsistencyatagivenwatercontent.,.,Chapter10concreteatearlyages,10.6.SlumpLoss10.6.1DefinitionsSlumplossisdefinedasthelossofconsistencyinfreshconcretewithelapsedtime.Undernormalconditions,thevolumeofhydrationproductsduringthefirst30minutesaftertheadditionofwatertocementissmallandtheslumplossisnegligible.Thereafter,concretestartslosingslumpataratedeterminedmainlybyelapsedtimeafterhydration,temperature,cementcomposition,andtheadmixturespresent.Undersomeconditions,aconcretemixtureexhibitinganunusuallylargelossofslumpduringthefirst30minutesor1hourmayhavetheeffectofmakingthemixing,convening,placing,compacting,andfinishingoperationsdifficultor,attimes,evenimpossible.,.,Chapter10concreteatearlyages,10.6.2SignificanceTheprematurestiffeningoffreshconcrete,dependingonwhentheproblemappears,maymeananincreaseinthemixerdrumtorque,requirementofextrawaterinthemixeroratjobsite,hang-upofconcretewithinthedrumofatruckmixer,difficultyinpumpingandplacingtheconcrete,extralaborforhandlingandfinishingoperations,andfinallylossofproductionandqualityofworkmanship,lossofstrength,durability,andotherpropertieswhentheretemperingwaterisexcessiveorisnotmixedthoroughly.,.,Chapter10concreteatearlyages,10.6.3CausesandControlTheprimarycausesofslump-lossproblemswithconcreteareasfollows:theuseofanabnormal-settingcement;unusuallylongtimeformixing,transporting,placement,compaction,orfinishingoperation;hightemperatureofconcreteduetoexcessiveheatofhydrationand/ortheuseofconcrete-makingmaterialsthatarestoredatahighambienttemperature.(seefig.10-2).,.,Chapter10concreteatearlyages,TABLE10-2,.,Chapter10concreteatearlyages,Fromfig.10-2,Thefollowingconclusionsweredrawnfromtheinvestigation:1Ingeneral,theamountofslumplosswasproportionaltotheinitialslump;thehighertheinitialslump,thehighertheslumploss.Forexample,inthecaseofCementA,atthecloseofthe2-hourtestat23(70),Concretemix1(initialslump180mmor71/2in)lost125mm(51/4in)slump,whereaswithConcreteMix3(initialslump125mmor5in)lost88mm(31/2in)slump,andconcreteMix5(initialslump85mmor3-5/8in.)lost57mm(21/4in.)slump.Regardlessoftheinitialslump,thefinalslumpvaluesafter2hoursofhydrationwereoftheorderof37to50mm(11/2to2in).Insuchacasethemethodofcompensatingfortheexpectedslumplossbydesigningforahigherinitialslumpisnotrecommendedbecausetheretemperingwaterrequiredatthejobsitemayhavetheeffectofpushingupthewater-cementratiooftheconcretemixturetoanundesirablelevel.,.,Chapter10concreteatearlyages,2Ingeneral,earlyslumplosstendstobedirectlyproportionaltothetemperatureofconcrete.Forexample,acomparisonofthe180mm(7-in)slumpconcretesmadewithCementAattwodifferenttemperaturesi.e.,23(70)(concretemix1)and30(85)(concretemix7)showedthatat30,60,and90minuteselapsedtimes,theformerlost13mm,28mm,and95mm(1/2in,17/8inand33/4in)slump,whilethelatterlost44mm,70mm,and114mm(13/4in.,23/4in.,and41/2in.),respectively.3Regardtotheeffectofcementcomposition,greaterslumplossrateswereobservedforalltestconditionsinthecaseofconcretesmadewiththecementcontaininghigherC3Aandhigh-alkalicontent(CementB).Forinstance,at23(70)and30,60,and90minelapsedtimes,concreteMix1lost13mm,28mmand95mm(1/2in.,17/8inand33/4in)slump,whilethelatterlost44mm,70mm,and114mm(13/4in.,23/4in.,and41/2in.)comparedto68mm,98mm,and143mm(22/3in,37/8in,and45/8in),respectively,forconcreteMix2.,.,Chapter10concreteatearlyages,Slump-lossproblemsoccurmostofteninhotweather.Thehigherthetemperatureatwhichaconcreteismixedandplacedthemorelikelyitisthatslumplossturnsouttobethecauseofanyoperatingproblem.Inconclusion,eliminationofeverypossibledelayinconcretehandlingoperations,keepingthetemperatureofconcreteasclosedtothe10to21rangeaspossible,andalaboratorycheckonthestiffeningandsettingcharacteristicsofthecement(withorwithouttheadmixturesselectedforuse)arethenecessarypreventivemeasurestocontrolslumplossproblems.,.,Chapter10concreteatearlyages,10.7.SegregationandBleeding10.7.1.DefinitionsandSignificanceSegregationisdefinedastheseparationofcomponentsofafreshconcretemixturesothattheyarenolongeruniformlydistributed.Therearetwokindsofsegregation.Thefirst,whichischaracteristicofdryconcretemixtures,consistsofseparationofmortarfromthebodyofconcrete.Bleeding,asexplainednext,isthesecondfromofsegregationwhichischaracteristicofwetconcretemixtures.,.,Chapter10concreteatearlyages,Bleedingisdefinedasaphenomenonwhoseexternalmanifestationistheappearanceofwateronthesurfaceafteraconcretemixturehasbeenplacedandcompactedbutbeforeithasset(i.e.,whensedimentationcannolongertakeplace).Bleedingisaformofsegregationbecausesolidsinsuspensiontendtomovedownwardundertheforceofgravity.Bleedingresultsfromtheinabilityoftheconstituentmaterialstoholdallthemixingwaterinadispersedstateastherelativelyheavysolidssettle.,.,Chapter10concreteatearlyages,Laitance,associatedwiththeexternalmanifestationofbleeding,iscausedbythetendencyofwaterrisingintheinternalchannelswithinconcrete,carryingwithitveryfineparticlesofcement,sand,andclay(presentasacontaminantinaggregate)anddepositingthemintheformofascumattheconcretesurface.,.,Chapter10concreteatearlyages,10.7.2MeasurementTherearenotestsformeasuringsegregation;visualobservationandinspectionofcoresofhardenedconcretearegenerallyadequatetodeterminewhethersegregationhasoccurred.Thereis,however,anASTMstandardtestforthemeasurementofrateofbleedingandthetotalbleedingcapacityofaconcretemixture.AccordingtoASTMC-232,asampleofconcreteisplacedandconsolidatedinacylindricalcontainer,250-mmdiameterand280-mmhigh.Thebleed-wateraccumulatedonthesurfaceiswithdrawnat10-minuteintervalsduringthefirst40minutes,andthereafterat30-minutesinterval.Bleedingisexpressedintermsoftheamountofaccumulatedwateraspercentageofthenetmixingwaterintheconcretesample.,.,Chapter10concreteatearlyages,10.7.3CausesandControlAcombinationofimproperconsistency,excessiveamountoflargeparticlesofcoarseaggregatewitheitheratoohighoratoolowdensity,presenceoflessfines(duetoalowcementcontent,alowsandcontent,orapoorlygradedsand),andinappropriateplacingandcompactingmethodsareamongthegeneralcausesforsegregationandbleedingproblemsinconcrete.,.,Chapter10concreteatearlyages,Itisinterestingtopointoutthathigh-C3Aandhighalkalicements,whichshowgreaterslumploss,tendtoreducebleedingasaresultofrapidformationofsulfoaluminatehydratessuchasettringite.,.,Chapter10concreteatearlyages,10.8.EarlyVolumeChanges10.8.1.DefinitionsandSignificanceAfterfreshconcretehasbeenplacedindeepforms,suchastheformsforatallcolumnorawall,afterafewhoursthetopsurfacewillhavesubsided.Thetendencytowardsubsidenceisalsoconfirmedbythepresenceofshorthorizontalcracks.Thisreductioninvolumeoffreshconcreteisknownbytermssuchasprehardening,presettingshrinkage,orplasticshrinkage,sincetheshrinkageoccurswhiletheconcreteisstillintheplasticstate.Asaresultofprehardeningshrinkage,cracksdevelopoverobstructionstouniformsettlement,i.e.,forinstance,reinforcingbarsandlargeaggregateparticles.,.,Chapter10concreteatearlyages,Figure10-7Plasticshrinkagecrackinginfreshlyplacedconcrete.,.,Chapter10concreteatearlyages,10.8.2.CausesandControlAvarietyofcausescontributetoplasticshrinkageinconcrete:bleedingorsedimentation,absorptionofwaterbysub-gradeorformsoraggregate,rapidwaterlossbyevaporation,reductioninthevolumeofthecement-watersystem,andbulgingorsettlementoftheformwork.Settlementcracksincolumnsandplasticshrinkagecracksinslabscanbeeliminatedbyrevibrationofconcretewhenitisstillintheplasticstate.Revibrationalsoimprovesthebondbetweenconcreteandreinforcingsteel,andenhancestheconcretestrengthbyrelievingtheplasticshrinkagestressesaroundthecoarseaggregateparticles.,.,Chapter10concreteatearlyages,10.9.SettingTime10.9.1DefinitionsandSignificanceThereactionsbetweencementandwateraretheprimarycauseofthesettingofconcrete.thesettingtimeofconcretedoesnotcoincidewiththesettingtimeofthecementwithwhichaconcretemixturehasbeenmade.thephenomenaofstiffening,setting,andhardeningarethephysicalmanifestationsofprogressivehydrationofcementwithtime.Also,theinitialandthefinalsettingtimesofcementarethepointsarbitrarilydefinedbythemethodoftest.Thesepointsindicatetherateofsolidificationofafreshlymixedcement-watersystem.Similarly,settingofconcreteisdefinedastheonsetofsolidificationinafreshconcretemixture.Boththeinitialandthefinalsettingtimesofconcretearearbitrarilydefinedbyatestmethodsuchasthepenetrationresistancemethod(ASTMC403),whichisdescribedbelow.,.,Chapter10concreteatearlyages,Theinitialsettingtimeandthefinalsettingtime,asmeasuredbypenetrationresistancemethods,donotmarkaspecificchangeinthephysical-chemicalcharacteristicsofthecementpaste;theyarepurelyfunctionalpointsinthesensethattheformerdefinesthelimitofhandlingandthelatterdefinesthebeginningofdevelopmentofmechanicalstrength.,.,Chapter10concreteatearlyages,Figure10-9Theprogressofsettingandhardeninginconcrete.,.,Chapter10concreteatearlyages,10.9.2MeasurementandControlForconcretemixtureswithgreaterthanzeroslump,ASTMC-403,TestforTimeofSettingofConcreteMixturesbyPenetrationResistance,providesastandardprocedureforthemeasurementofsettingtimebytestingthemortarsievedfromaconcretemixture.Briefly,thetestconsistsofremovingthemortarfractionfromconcrete,compactingitinastandardcontainer,andthenmeasuringtheforcerequiredtocauseaneedletopenetrate25mmintothemortar.Thetimesofsetaredeterminedfromtherateofsolidificationcurveobtainedfromalinearplotofdatawithelapsedtimeastheabscissaandpenetrationresistanceastheordinate.Initialandfinalsetaredefinedastimesatwhichthepenetrationresistancesare3.5MPa(500psi)and27.6MPa(4000psi),respectively.Thesearbitrarilychosenpointsdonotindicatethestrengthofconcrete;infact,at3.5MPa(500psi)penetrationresistancevaluetheconcretehasnocompressivestrength,whileat27.6MPa(4000psi)penetrationresistancevaluethecompressivestrengthmaybeonlyabout0.7MPa(100psi).,.,Chapter10concreteatearlyages,Theprincipalfactorscontrollingthesettingtimeofconcretearecementcomposition,water-cementratio,temperature,andadmixtures.Theeffectsofcementcomposition,temperature,andretardingadmixturesontypicalratesofsettingobtainedbyASTMC403testareshowninFig.10-10.,.,Chapter10concreteatearlyages,Figure10-10(a)Effectoftemperatureoninitialandfinalsettingtimesofconcrete(ASTMC403);(b)effectofaretardingadmixtureonsettingtimesofconcrete,.,Chapter10concreteatearlyages,10.TemperatureofConcrete10.1.SignificanceInhotweather,unprotectedconcreteissubjecttoplasticshrinkagecracking.Ontheotherhand,incoldweatherthelowtemperatureofconcretecuringmayseriouslyimpedetherateofstrengthdevelopment.Constructionengineersshouldhaveageneralunderstandingofthepossibleeffectsofbothlower-andhigher-than-normalcuringtemperaturesonpropertiesofconcreteatearlyages,andthemethodsofevaluatingandcontrollingthem.,.,Chapter10concreteatearlyages,10.10.2.Cold-WeatherConcretingIntheeventoflittlecementhydration,nostrengthgainoccurswhentheconcreteisfrozenandiskeptfrozenbelow-10.Therefore,freshconcretemustbeprotectedfromfreezinguntiladequatestrengthhasbeengained.Disruptiveexpansionisalsopreventedwhenthedegreeofsaturationofconcreteissufficientlyreducedbysomeprogressinthehydrationprocess.,.,Chapter10concreteatearlyages,ACICommittee306Rrecommendationforcold-weatherconcretingonplacementtemperaturesfornormal-weightconcreteareshowninTable10-3.,.,Chapter10concreteatearlyages,Controlofconcretetemperature.Dependingontheambienttemperatureandtransporttimeformtheproductionsitetothejobsite,thetemperatureofconcreteasmixedismaintainedatnotmorethan5.6(10)abovetheminimumrecommendedinTable10-3.Thetemperatureoffreshconcreteisusuallycontrolledbyadjustingthetemperaturesofmixingwaterandaggregates.,.,Chapter10concreteatearlyages,Concretetemperaturecanbemeasureddirectlybyamercurythermometerorabimetallicthermometer.Itcanalsobeestimatedusingtheexpression(10-1)whereTistemperatureofthefreshconcretein;Ta,Tc,Tw,andTwaaretemperaturesofaggregates,cement,mixingwater,andfreemoistureinaggregates,respectively;andWa,Wc,Ww,andWwaareweights(inpounds)ofaggregates,cement,mixingwater,andfreemoistureinaggregates,respectively.TheformularemainsthesameinSIunitsexceptthatischangedtoandpoundstokilograms.,.,Chapter10concreteatearlyages,Inadditiontotheincreaseinslumplossandplastic-shrinkagecracking,andthedecreaseofsettingtimeinfreshconcrete(alreadydescribed),hotweatherincreasesthemixingwaterrequirementforagivenconsistency(Fig.10-11)andcreatesdifficultyinholdingtheairinanair-entrainedconcretemixture.(tobecontinued).,.,Chapter10concreteatearlyages,(continued)Retemperingoffreshconcreteisfrequentlynecessaryinhotweather.Attimes,thiscausesadverseeffectsonstrength,durability,dimensionalstability,andappearanceofthehardenedconcrete.Also,concreteplacedandcuredathigherthanmoderateambienttemperaturesnormallydevelopshighearlystrengthbutat28daysandlateragesthestrengthisusuallylowerthanthesameconcreteplacedandcuredatarelativelyl