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DiscusstheconstructiontemperatureandcrackoftheconcretelightlyByG.K.KululangaW.KuotchaR.McCafferMemberAS__andF.Edum-FotweTheAmericanSocietyofCivilEngineersThesum__ryInordertopreventtheownersoftheconcreteworkofclaimswemustdoagoodjobintheconstructionpro__ssinthetemperatureandcrackcontrolthroughobservationlivefor__nyyearsthroughconsultingthemonographaboutstresswithintheconcreteexplaintoconcretetemperaturereasonon-the-spotconcretecontrolandmeasurepreventionofcrackoftemperaturethatcrackprodu__.KeywordConcreteTemperaturestressCrackControl
1.Theconcreteoccupiestheimportantpositioninmodernengineeringconstruction.Buttodaythecrackoftheconcreteiscomparativelygeneralthecracksarenearlyomnipresentinthescien__ofbridgebuilding.Thoughwetakevariouskindsofmeasuresinconstructingcarefulbutthecrackstilloccursnowandthen.Tracingittoitscauseitisoneofthemincompletelythatourchangetoconcretetemperaturestresspaysattentionto.Inthelargevolumeconcretetemperaturestressandtemperaturecontrolaresignificant.Thisis__inlybecauseofthereasonoftworespects.Firstofallconcreteoftenappearthetemperaturecrackinnotconstructinginfluen__theglobalityanddurabilityofthestructure.Secondlyinthecourseofoperatingthetemperaturechangehasre__rkableinfluen__thatcantbeignoredonthestressstateofthestructure.Wemeettoconstructtemperaturecrackin__inlysoonlytoorigincauseoffor__tionandtreatmentmeasureconcreteofcrack__keadiscussioninconstructingthistext.ReasonofacrackH__e__nykindsofreasonstoprodu__thecrackintheconcreteitis__inlythechangesoftemperatureandhumidityfragilityanddisparityoftheconcreteandthestructureisunreasonabletheraw__terialsisnotuptostandardifthealkaliaggregatereactthetemplateisoutofshapethefoundationdoesnotsubsideetc.evenly.The__mentemitsalargeamountofheatofhydrationwhentheconcreteishardennedinsidetemperatureisrisingconstantlycausethestressofdrawingonthesu_____.Inthecourseofloweringthetemperatureisitcongealfoundationpayrestraintomixalwayslaterstagewillpresentthestressofdrawingwithintheconcrete.Reducingoftemperaturecansu_____causehe__ystressofdrawingveryinconcretetoo.Whenthesedrawthestressandgobeyondresistingtheabilityofsplittingofconcretenamelywillpresentthecrack.Alotofinsidehumidityofconcretechangeverylightorchangerelativelyslowsu_____humiditymightchangehe__ytheviolentchangetakesplacing.Suchas__intainingthoroughlywhengettingwetterwhennotdoingcontractsu_____therearentdefor__tiondoingoftencausethecracktoo.Theconcreteisakindoffragility__terialtensilestrengthisabout1/10ofthecompressionstrengthisitcarryononesshoulderorbacklimitwhendrawoutofshapetoh__e*104onlyisitcarryononesshoulderorbacklimitlocationwhenstretchoutofshapetothereis*104toaddforalongtimetoaddashorttime.Becauseraw__terialsevenwaterdustthanunstabletransportandbuildphenomenonofe__natingofcourseitstensilestrengthisnoteveninthesameconcretealotofresisttheabilityofdrawingverylowitisapttopresenttheweakpositionofthecrack.Amongarmoredconcretedrawstresstoundertakebyreinforcingbar__inlyconcretebearstressofkeepingjust.Orreinforcingbarmixifedgepositiongonetotocongealpresentthestressofdrawinginthestructureinplainconcretemustrelyontheconcreteoneselftobear.Requireto__oidthestressofdrawingoronlyvery__allstressofdrawingappearsofthethegeneraldesign.Buttheconcreteiscooledfrom__ximumtemperaturetothesteadytemperatureofoperatingperiodinconstructingoftencausesizabletodrawthestresswithintheconcrete.Thetemperaturestresscanex__edotheroutsidesandloadthestressescausedsometimesknowchangelawtemperatureofstressforcarryonreasonablestructuraldesignandconstructextremelyimportant.____ysisof2temperaturestressesCanbedividedintofollowingthreestagesaccordingtotheformingpro__ssofthetemperaturestress:1Itisearly:Buildconcreteisitisitoverbasicallytosendoutheatto__menttobegingenerallyoneonedaybyoneself.Twocharacteristicsatthisstagefirstthe__mentemitsalargeamountofheatofhydrationsecondmixandcongealthechangingsharplyofelasticmodelquantity.Becauseofthechangeofelasticmodelquantityformthere__iningstressintheconcreteinthisperiod.2Middleperiod:Uptilltheconcreteiscooleduntilstabilitytemperaturefrom__mentsendoutheatfunctionbasicallywhenexpiringinthisperiodthetemperaturestressis__inlybecausethecoolingoftheconcreteandexternaltemperaturechangecausethesestressesandremnantsstressesthatisformedinearlydaysaresuperposedmixandcongealtheelasticmouldamountthatgoestoanddoesnotchangemuchduringthisperiod.3Laterperiod:Operationperiodafterthecompletecoolingofconcrete.Temperaturestresswhetherexternaltemperaturechangecause__inlythesestressesandfirsttwokindsofremnantsstressesesarechangedandadded.Canbedividedintotwokindsaccordingtothereasonwhythetemperaturestresscauses:1Spontaneousstress:Therearenotanyrestraintortotallystaticstructureattheborderifinsidetemperatureisnon-lineardistributiontemperaturestressappearingbecausestructurerestrainsfromeachother.Forexamplethebodyofmoundofthebridgethephysicaldimensionisrelativelylargesu_____temperatureislowwhentheconcreteiscooledinsidetemperatureishighpresentthestressofdrawingonthesu_____presentthestressofpressinginthemiddle.2Restrainthestress:Allofthestructureonesoritrestrainexternalonesomebordercantoutofshapeandstressnotcausenotfree.Suchascaseroofbeamroofconcreteandguardrailconcrete.Thistwokindsoftemperaturestressesdrawbackstressescausedtoactonwiththedoingofconcretetogetherfrequently.Itisamorecomplicatedjobtowantto____ysethedistributionsizeofthetemperaturestressaccuratelyaccordingtoknowntemperature.Incaseofgreat__jorityneedtorelyonthemodeltestorthenumbervaluetocalculate.Toisit__ketemperaturestressh__esizablelimptocreepconcreteatthestressaccountingtemperaturemustconsidertheinfluen__thatcreepcalculateconcretlythatnolongerstatesthinlyhere.Controlandpreventingthemeasureofthecrackof3temperatureForpreventcracklightentemperaturestresscanfromcontroltemperatureandisitisitsetabouttorestraintermsfromtwotoimprove.Themeasureofcontrollingtemperatureisasfollows:1Isitimproveaggregategrademixisitdorigidconcretetospendmixmixturetoadoptisitguideangryphar____uticalorplastificationphar____uticaletc.measureinordertoredu____mentconsumptionofconcretetoadd;2Addwaterorthewatertocoolthebrokenstoneinordertoredu__thetemperatureofbuildingoftheconcretewhilemixingandshuttingtheconcrete;3Redu__thethicknessofbuildingwhilebuildingtheconcreteonhotdayutilizeandbuildtheaspecttodispeltheheat;4Burythewaterpipeundergroundintheconcreteenterthecoldwatertolowerthetemperatureopenly;5Stipulaterationalformremovaltimethetemperaturekeepswarmthesu_____whileloweringsuddenlyincasethattherapidtemperaturegra___nttakespla__intheconcretesu_____;6Theconcretewithmediumandlong-termandexposedconstructionbuildsapie__ofsu_____orthinwallstructuretakethemeasureofkeepingwarmincoldseason;Themeasureofimprovingconditionofrestrainingis:1Divideandsewanddivideonerationally;2Preventthefoundationfromrisingandfallingtoobig;3Rationalarrangementconstructionpro__sspreventthetoobigdiscrepancyinelevationandsidefromexposingforalongtime;Inadditionimprovetheperfor__n__oftheconcreteandimproveandresisttheabilityofsplittingstrengthen__intenan__preventthesu_____frombeingdoneandcontractedespeciallyguaranteethequalityoftheconcreteisveryimportanttopreventingthecrackshouldpayspecialattentionto__oidingproducingandrunningthroughthecracktheglobalityresumeditsstructureafterappearingisverydifficultsoshouldrely__inlyonpreventingtheemergen__oftherunningthroughcrackwhileconstructing.Inconstructionofconcreteforraiseturnoverrateoftemplatede__ndconcreteformremovalassoonaspossiblethatbuildnewlyoften.Shouldconsiderformremovaltimeproperlywhenconcretetemperatureishigherthanthetemperaturesoasnottocausethesuperficialearlycrackofconcrete.Buildingtheearlyformremovalnewlycauseverylargestressofdrawingonthesu_____thephenomenonthattemperatureisassaultedappears.Buildinitialstageinconcretebecauseheatofhydrationissentoutthesu_____causessizabletodrawthestresssu_____temperatureisalsohigherthantemperatureatthismomentremovethetemplateatthismomentsu_____temperatureisloweredsuddenlymustcausetemperaturegra___ntthusaddanddrawthestressonthesu_____changeandaddwiththeheatofhydrationstressinadditiontheconcretedriesandcontractsthesuperficialstressofdrawingreachesverygreatnumbervalueh__edangerofcausingthecrackbutcoveralight-dutyheatinsulatorwithonthesu_____intimeafterremovingthetemplateforinstan__foamspongeetc.forpreventconcretesu_____fromprodu__thetoobigstressofdrawingh__ere__rkableresults.Addmuscleinfluen__tolargevolumetemperaturestressofconcreteverylightbecauselargevolumeconcreteincludemuscletoberateverymuchlow.Justh__einfluen__onthegeneralarmoredconcrete.Ontermsthattemperatureisnotveryhighandthestressislessthanlimitofsurrenderingeveryperfor__n__ofthesteelissteadyandh__enothingtodowithstressstatetimeandtemperature.Linebloatedcoefficientofsteelandconcretelinebloatedcoefficientdifferen__verylighttakepla__littleinternalstressveryonlybetweenthetwowhilechangingintemperature.Becauseelasticmouldamountofsteelconcreteelasticmould7~15ofquantityreachasinteriorconcretestresstensilestrengthandwhenfracturingthestressofthereinforcingbarwillnotex__ed10000kg/cm
2..Soisitisitpreventtinyappearan__difficultyverymuchofcrackfromto__keuseofreinforcingbartowantamongconcrete.Butthecrackinthestructuregenerallybecomesnumeroustheintervalislittlethewidthanddepthare__allerafteraddingthemuscle.Andifdiameterofreinforcingbardetailedandwhenintervaldensetoimproveconcreteresistresultofpersonwhosplitbetter.Concreteandsu_____armoredconcreteofstructurecantakepla__detailedandshallowcrackoftenamongthemthegreat__joritybelongtoanddoanddrawbackthecrack.Thoughthiskindofcrackisgenerallyallrelativelylightitstillsh__e__rtaininfluen__ontheintensityanddurabilityofthestructure.Inordertoguaranteeconcreteprojectqualitypreventfracturingimprovethedurabilityoftheconcreteusetheadmixturetoredu__oneofthemeasuresthatfracturescorrectly.Whetherisitredu__waterisitsplitphar____uticaltodefendIsum__rizehis__infunctioninpracti__touse.1ThereisporeDaoofalargenumberof__ointheconcreteprodu__capillarytensioninthecapillaryafterwaterisevaporated__keconcreteisitcontractoutofshapetodo.Increasingthethinapertureofhaircanredu__thecapillarysu_____tensionbutwill__ketheintensityofconcreteredu__.Thissu_____tensiontheoryhasalreadybeenconfirmedintheworldasfarbackasthesixties.2Waterdustthaninfluen__importantfactorthatconcreteshrinkisitredu__waterisitsplitphar____uticalcan__keconcretewaterconsumptionredu__by25%todefendtouse.3__mentconsumptionimportantfactorconcreteofpersonwhoshrinktooisitaddandsubtractwaterisitsplitconcretereducible15%ofthe__mentconsumptionontermsthatkeeptheintensityofconcreteofphar____uticaltodefendtomixitsvolumeissupplementedbyincreasingaggregateconsumption.4Redu__waterisitsplitphar____uticalcanimproveconsistencyofgroutredu__concretesecreteinktodefendredu__andsinkanddrawbackdeforming.5Improveglueingthestrengthofformingofthegroutandaggregatetheconcreteimprovedresiststheperfor__n__ofsplitting.6Concreteisitprodu__stressofdrawingtorestrainfromwhileshrinkingcrackwhendrawingthestressandisgreaterthanconcretetensilestrengthcanprodu__.Redu__waterisitsplitphar____uticaleffectiveconcretetensilestrengthofimprovementverytodefendimproveresistingtheperfor__n__ofsplittingofconcretebyawide__rgin.7Itcan__ketheconcretedensitygoodtoaddtheadmixturetomixcanimproveresistingcarbonizationofconcreteeffectivelyredu__carbonizationtoshrink.8Isitredu__waterisitsplitslowcoagulationtimeproperconcreteunderphar____uticaltodefendonthebasisofpreventingthefastwaterof__mentfromsendingoutheateffectivelytomixpreventtheplasticityshrinkthatbringsbecausethe__mentisnotcongealedforalongtimefromincreasing.9Mixadmixtureconcreteandgettingeasyandkindsu_____easytofeelflatformlittlemembraneredu__themoisturetoevaporateredu__drilyandshrink.Alotofadmixtureallh__ethefunctionsofslowcoagulationincreasingandaptimprovementplasticitytheexperimentthatweshouldcarryoninthisrespectmoreintheprojectpracti__iscomparedwithandstu___dthanleanagainstnotimprovingtermsmore________ygetting______andmoredirecteconomy.Early__intenan__of4concretePracti__hasprovedthecommoncrackofconcretemostisthesu_____crackofdifferentdepth__inreasonitswhethertemperaturegra___ntcausecoldtemperatureofarealowertooeasytoformcracksuddenly.Sosaythewarm-keepingoftheconcreteisespeciallyimportanttopreventingtheearlycrackofsu_____.Fromtheviewpointoftemperaturestressshouldreachandrequiretokeepwarmfollowingly:1Preventconcreteinternalandexternaltemperaturepoorandconcretesu_____gra___ntfrompreventthesu_____crack.2Preventconcretefromtobeultraandcoldshouldisitisit__ketheminimumtemperatureisnotlowerthanthesteadytemperatureofconcreteservi__timeconstructiontimeinconcretetotrytotryonesbest.3Preventtheoldconcretesubcoolinginordertoredu__therestraintamongtheoldandnewconcrete.Theearly__intenan__oftheconcretethe__inpurposeliesinkeepingthesuitablewarmandhumidconditioninordertogettheresultoftworespectsontheotherhand__ketheconcrete__oidtheinvasionandattackoftheunf__orableandwarmhumidityoutofshapetheonesthatpreventfromharmfullyarecoldtocontractanddotocontract.Ononehand__ke__mentwaterfunctiongoon__oothlyisinthehopeofreachingtheintensitydesignedandresistingtheabilityofsplitting.Thesuitablewarmdampconditionisinterrelated.Mixwarm-keepingmeasurepaidtocongealoftenprotectwetresultstoo.____ysewaterconcreteincludemoisturecanmeetde__nd__mentofwaterh__eenoughandtosparenewlytheoretically.Butbecausethereasonofevaporatingetc.oftencauseslossesofthemoisturethuspostponeorhinderwaterofthe__mentfromthesu_____concretere__ivesthiskindofadverseeffecteasiestlyanddirectly.Keyperiodwhen__intainedininitialafewdaysaftersotheconcreteisbuiltshouldpayattentiontoconscientiouslyinconstructing.ConclusionsConstructiontemperatureandrelationofcrackinconcretetheabovecarryonpreliminarydiscussionoftheoryandpracti__thoughtheacademiahasdifferenttheoriestoorigincauseoffor__tionandcomputingtechnologyoftheconcretecrackbuttoconcretepreventionandimprovingthemeasuresuggestiontorelativelyunifyapplicationinpracti__resultfinetooatthesametimeconcretetoisitobservecomparemoremorebyustowantinconstructing____ysemoresum__rizemoreaftergoingwrongcombine__nykindsofpreventionanddealwiththemeasurethecrackoftheconcretecanbe__oided.
2.QualitycontrolofwaterproofconcreteconstructionCombinedwithexperien__fromformworkdesignfabricationandinstallationassembingreinforoementpouringandcuringofconcreteandotheraspectsconstructiontechnologyoffair-fa__dconcreteisintrodu__daswellasqualitycontrolmeasuresandstandardsinordertoredu__engineeringcosttoacquiresatisfiedeconomicandsocialbenefits.Thefactorsofinflueningwaterproof-concretequalityarevery__nyAnylinksdoesnotpayattentiontothewater-proofconcreteoffieldlosshindersthewaterfunctionwithoutex__ptionjointlywithdegree.Engineeringconstructioninthebasementadoptssecondaryformboardfabricationandinstallationreinfor__dbarfabricationandbindconcretestirringandmixingsystemandtransportconcreteconcretecovibrationbeatwithastickconstructionjointpracti__concretecuringanddi__antlemodelandbeingreadyforbackfillandsoonaspects.Theseareverycriticaltoqualitymethodtoensurethatwater-proofconcreteselfwater-proofandthewayofpracti__haswanout.Methodbeingunderconstruction
2.1FabricationandInstallationAccordingtotheconcrettofcloselyknitde__ndofreasonwhytoformboardsin__thewater-proofalsoconcreteh__e__deandh__eassembledcorrespondingriseisspecialbetorequirethatnotle__ingoutthickfluidfirmcloselyknitblockofwooddefor__tionwaterabsorptionCharactershouldbe__allandoughttogiveprioritytoselectandusingbambooslabrubberformboardorthesteelform..Strictcontrolformboardroomgapsizene__ssaryex__eding2mmsusesfoamrubberorplastictosqueezeacrackinporousformboardnonutilitywithoutex__ptiontoboardfa__BereadyforwallpostatthesametimerottingthepreventionandcurejobAdoptthe__mentmortarpouringsameindiciainbeforetherootsegmentstickingthefoamrubberorplasticstripthebottomputsona__mentmortarconcreteaconcretefirst5cm~10cm.Sin__water-proofconcretestructurewallthicknessismostlymoreinfertile.BetoensurethatcomponentgeometrydimensionChangadopttheinsideandoutsidebolttopullthemeasuremeetingattentiontorespondstoonplayre__ivebolt__ntreinterposestopwaterironplatetopreventwaterfromformingpilotagepassagealongboltleakage.
2.2AssembingreinforoementWater-proofconcretestructurehasde__ndingasfollowstothereinfor__dbar1reinfor__dbarshouldadopttwistedsteelasfaraspossibleincreasesbyholdwrapafor__composinginreplyawaterability2reinfor__dbarsconnectshouldtryonesbesttoadopttosolderconnectionstopusingandbeingneedlesstobindconnectiontothefull3whenbindingareinfor__dbartheironwireheadrespondstoinnerbending.4strictcontrolreinfor__dbarsprotectivelayerthickness.Mini__lthicknessofwater-proofconcretereinfor__dbarprotectivelayerisnot__allerthan25mmstheprotectivelayerwelcomingwatersu_____especiallyinadmissibilitytodisappointerror.Theironwireandreinfor__dbarthatapplicationburiesinadvan__withinmortarpie__whileusingmortarheelblockasprotectivelayerareboundsolid.Whenthec__alryputsupthefixedreinfor__dbarifadoptareinfor__dbarYingJiaalsosolderswaterironplateorfixationjustgoesaheadtostrengthenwater-proofeffectintheheelblock.Thisprojectusesnew__terialsnylontoh__efixedthereisaneffectshadguardedagainstreinfor__dbarprotectivelayerdeviationpie__big__sscommonfailings.Theconcretestirringandmixing__kesandtransportsSin__thewater-proofconcreterequiresthathighercloselyknitreasonwhystirandmixsystemalsoneedtoh__ethefairlygoodhomogeneityshouldbereadyforburningasfollowsalmostforthispurpose1ensuresthatmixingtimemixingateverytimearesecondaryjumpintoaexpectthegeneralejectionofcompactblockofwoodlessthan2mins.2shouldusetheappositionagentthesolutionqueenwhoshould__nufacture__rtainthicknessfromappositionagentaddsthemixerinnerthedriedpowderorhighcon__ntrationsolutionwilladdanagentextranottoaddsthemixerinnerdirectlypreventfrommixingisunevenbutpartcon__ntratesbothlosetheappositionagenteffectandaffectconcrete__ss.3respondstotheassuredsour__oflifedegreeh__ingaspottestontheadmeasurementconcreteattheregularintervalscollapsinginthepro__ssbeingunderconstructionconstructionismiddleifYurainsorothercauserespondtotheratiodeterminingwhosewaterratioandadjustingthecompositionbeingunderconstructionintimewhenchangehappenedinsandstonemoisturecontent.4projectusesthecommodityconcretehasboundaryh__eraisedaconcretestirring__ssandofallkindseffectappositionagent_____eratingfallswhenamountsthewaterashh__ingcontrolledaconcretestrictlycollapsing.5concretesconcreteadoptapumptoh__egivenhandicrafteffective__oidingaconcreteproducingthephenomenonisolatingMiShuiandle__ingoutthickfluidinthepro__ssoftransportation.
2.4__ttersneedingattentioninbeingunderconstruction1constructionschoolassig__entsoftandflouryisdivided.Water-proofconcretingshouldstratifystrictlybeinginprogressandacontinuousconstructioniscompletedThefrontandbackandhighandlowconnectbetweenthetiershouldsu__ugatewithinthe__mentinitialsettingtime,Forthispurposewithhandlingaworkerdividingintoseveralatthesametimeeachotherschoolassig__entgroupfa__soritisallrightforeachothercarryontheback.2Achievestrictlyfixedpointdeterminestheamountsofthecomponentsofasubstan____terialdownAccordingtothevehiclecapacitiesstratifyingconcretealtitudeandthemeansoftransportthequantifycarryingoutfixedpointstrictlyisabletogodownoneimportantringexpectingthatthisistoimprovewater-proofconcreting__ss.3insistthatyougodown__terialopeningthedoororusestringtoexpectthatunderbarrelchuteBetopreventa__mentpastefrompartingfromaggregatefortoexpectthatlibertyshouldnotex__ed
1.5msnowandthenhighlyunderwater-proofconcreteThewater-proofstructurereinfor__dbariscon__ntratedShouldconcretealtitudewhenhigherrespondtoaccordingtothe__terialstipulatingthatyougodown__terialopeningthedooronformboardorusingstringofbarrelschutetofallpreventa__mentpastefrompartingfromaggregatefor__oidatthesametimealsothe__mentpasteortheconcreteaccumulatingthepla__and__mentpastealternatinginthereinfor__dbarlooselysplashingstickyYuformboardwithoutsideNowthat__terialisdifficultunderbringingaboutalaterthesu_____affectingstructureglossybecomesinthefutureleakagehiddentrouble.4concretesvibratingbeatoneimportantringbeingtoensurethattheconcreteiscloselyknitdefendagainsttoseepwithastick.Alsoshouldadoptfixedpointvibratinghandlemeticulouslyonbasisexpectingthatunderdeterminingtheamountsofthecomponentsofasubstan__homogeneouslyinfixedpoint.PayattentiontohighandlowtieroflinkingupNowthatneedingtopreventfrommissandvibratingnotvibratingenoughbeinggoingtopreventafaultfromvibratingagainNeedtoforbidusingpryingthemethodcomingtoaidtovibratemovingareinfor__dbarorshakingareinfor__dbarespeciallysuchseparatesto__kingthefrontalreadyconcretethereinfor__dbarandconcreteshakingtherealitypartcausesseriousconsequen__s.5beingreadyfortheassuredsour__oflifedegreetakingoverthickfluidmeasureadjustingintimetocollapseseriously.__eragewater-proofconcretestructureconstructiontheconstructionjointbeingunderconstructionrespectivelycomposingnamelyonerectingwallsellsbebottomboardandverticalwallatredu__dpri__s.Shouldirrigatethickalayerof5cm~10cmssurnameindicia__mentmortarsinseampla__firstlinkingupcloselyknittingtierinordertoensuringthattheseampla__vibrateswhenconcreteverticalwall.Simultaneousimprovestepbystepwithconcreteanaltitude.Shouldfalloffstepbystepwiththewateryielddiminishcollapsingtofalldegreepreventfromconcreteinsisttobeshortbutthe__mentpasteispartialtomuchphenomenontoaggregateappearingonupperpart.
2.5ThemethodofConstructionjointTheconstructionjointisoneofwater-proofarchitecturalweaklink.Theconstructionjointisoneofwater-proofarchitecturalweaklink.Shouldstopstayingornotstayingtothefull.equipmentorpropsupconditionalrestrictingofmodel__ythenbehindtheagreementcollectingadesignorganonthewallstipulatingthatlienableleveloflocationconstructionjointsortkeepverticalstrokesgettingalonginallabove200bottomboard.Adoptnewhandicraftexpansibilityintheprojectrubberwaterjustbringshandlingtheconstructionjointmethodsubstitutingmethodincommonuseanxiouslyexpectingthedyadicconstructionjointofmouthatanyratedyadicconstructionjointflatmouthconstructionjointsuchasandsoon.Waterproofingisinnolienableconstructionjointofperpendicularitydirectionliketheconcreterubberstopsinh__ingsetuptheconstructionjointirrigatingbeltperpendicularitydirectionthedayafertomorrowbetweenthe__inbuildingandtheskirtbuildinginterposingexpansibilityinseamlocationwaterbeltorsteelplatestopswaterbelt.weaklinkofwater-proofstructurerespondtospecially-assignedpersonconscientiouslyhandlemeticulously.Ensure__sstobeunderconstructioninconstruction.Adopttostoph__ingnottomissandnotsolderingtimewatersteelplateadoptrubberwaterbeltjustshouldbesolidwithoutfailreliablyinre__ivinglongtimewillconnectusingspecialusegluetopaste.Whenonlythinkthatconcretingac__pts
2.6Di__antlingmodelandcuringofconcreteThewater-proofconcretingqueen__intainsgoodnessandbadnessworkingnotonlyaffectingtheintensityarrivingataconcretemoreovertheimperviousfunctionaffectingtotheconcrete.Eviden__water-proofconcreteearlyphasedehydrationorcuringpro__sslackforwaterwillredu__whoseimperviousfunctionbyawide__rgin.Water-proofconcretecuringrespondstotheblockofwoodlessthan14ds.Thewater-proofconcretedi__antlesamodelde__ndingtobeunabletocomparehighdi__antlemodeltimeunsuitableuntimelyrespondtowhendi__antlingamodelhandlemeticulouslypreventanironrodfromusingtoprystifflythatthesledgehammerisknockedsuddenlyandforciblypriesusesaconcretemeetwithdestroy
2.7DogoodwellatbackfillWater-proofarchitecturalbackfillisafinalworkingpro__dureitbesometimesignoredthisisaverybigfaultsinfactundertheground.Whenthewater-proofstructurejobisoverqueenrespondstotheconstructioncarryingoutbackfillimmediately.Mostimportantweshouldtakebackfillseriouslyself__ss;Secondlybackfillvariaandpondingshouldclearupthepitbottominthefrontrespondtodensificationwithlayerswhenbackfill.Water-proofstructurevicinitylayerpoundstruebackfillselfisareliableandeconomicalaswellwaterproofingdraperyundertheground.InprojectbasementengineeringconstructionShanghaisin__organizingwaterproofingdowntoearthstrictlywithallabovesevenpassestheconcreteisunderconstructioneffectiscomparativelyideal.Thepercolatingwaterphenomenondoesnotappear.ConclusionsConstructiontemperatureandrelationofcrackinconcretetheabovecarryonpreliminarydiscussionoftheoryandpracti__thoughtheacademiahasdifferenttheoriestoorigincauseoffor__tionandcomputingtechnologyoftheconcretecrackbuttoconcretepreventionandimprovingthemeasuresuggestiontorelativelyunifyapplicationinpracti__resultfinetooatthesametimeconcretetoisitobservecomparemoremorebyustowantinconstructing____ysemoresum__rizemoreaftergoingwrongcombine__nykindsofpreventionanddealwiththemeasurethecrackoftheconcretecanbe__oided.浅谈混凝土的施工过程中温度与裂缝控制ByG.K.KululangaW.KuotchaR.McCafferMemberAS__andF.Edum-Fotwe美国土木工程师学会摘要为了防止业主对混凝土工程的索赔就必须做好施工过程中温度与裂缝控制本文通过多年的现场观察,通过查阅有关混凝土内部应力方面的专著,对混凝土温度裂缝产生的原因、现场混凝土温度的控制和预防裂缝的措施进行等进行阐述关键词混凝土;温度应力;裂缝;控制混凝土在现代工程建设中占有重要地位而在今天,混凝土的裂缝较为普遍,在桥梁工程中裂缝几乎无所不在尽管我们在施工中采取各种措施,小心谨慎,但裂缝仍然时有出现究其原因,我们对混凝土温度应力的变化注意不够是其中之一在大体积混凝土中,温度应力及温度控制具有重要意义这主要是由于两方面的原因首先,在施工中混凝土常常出现温度裂缝,影响到结构的整体性和耐久性其次,在运转过程中,温度变化对结构的应力状态具有显著的不容忽视的影响我们遇到的主要是施工中的温度裂缝,因此本文仅对施工中混凝土裂缝的成因和处理措施做一探讨
1、裂缝的原因混凝土中产生裂缝有多种原因,主要是温度和湿度的变化,混凝土的脆性和不均匀性,以及结构不合理,原材料不合格(如碱骨料反应),模板变形,基础不均匀沉降等混凝土硬化期间水泥放出大量水化热,内部温度不断上升,在表面引起拉应力后期在降温过程中,由于受到基础或老混凝上的约束,又会在混凝土内部出现拉应力气温的降低也会在混凝土表面引起很大的拉应力当这些拉应力超出混凝土的抗裂能力时,即会出现裂缝许多混凝土的内部湿度变化很小或变化较慢,但表面湿度可能变化较大或发生剧烈变化如养护不周、时干时湿,表面干缩形变受到内部混凝土的约束,也往往导致裂缝混凝土是一种脆性材料,抗拉强度是抗压强度的1/10左右,短期加荷时的极限拉伸变形只有(
0.6~
1.0)×104,__加荷时的极限位伸变形也只有(
1.2~
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104.由于原材料不均匀,水灰比不稳定,及运输和浇筑过程中的离析现象,在同一块混凝土中其抗拉强度又是不均匀的,存在着许多抗拉能力很低,易于出现裂缝的薄弱部位在钢筋混凝土中,拉应力主要是由钢筋承担,混凝土只是承受压应力在素混凝土内或钢筋混凝上的边缘部位如果结构内出现了拉应力,则须依靠混凝土自身承担一般设计中均要求不出现拉应力或者只出现很小的拉应力但是在施工中混凝土由最高温度冷却到运转时期的稳定温度,往往在混凝土内部引起相当大的拉应力有时温度应力可超过其它外荷载所引起的应力,因此掌握温度应力的变化规律对于进行合理的结构设计和施工极为重要
2、温度应力的分析根据温度应力的形成过程可分为以下三个阶段
(1)早期自浇筑混凝土开始至水泥放热基本结束,一般约30天这个阶段的两个特征,一是水泥放出大量的水化热,二是混凝上弹性模量的急剧变化由于弹性模量的变化,这一时期在混凝土内形成残余应力
(2)中期自水泥放热作用基本结束时起至混凝土冷却到稳定温度时止,这个时期中,温度应力主要是由于混凝土的冷却及外界气温变化所引起,这些应力与早期形成的残余应力相叠加,在此期间混凝上的弹性模量变化不大
(3)晚期混凝土完全冷却以后的运转时期温度应力主要是外界气温变化所引起,这些应力与前两种的残余应力相迭加根据温度应力引起的原因可分为两类
(1)自生应力边界上没有任何约束或完全静止的结构,如果内部温度是非线性分布的,由于结构本身互相约束而出现的温度应力例如,桥梁墩身,结构尺寸相对较大,混凝土冷却时表面温度低,内部温度高,在表面出现拉应力,在中间出现压应力
(2)约束应力结构的全部或部分边界受到外界的约束,不能自由变形而引起的应力如箱梁顶板混凝土和护栏混凝土这两种温度应力往往和混凝土的干缩所引起的应力共同作用要想根据已知的温度准确分析出温度应力的分布、大小是一项比较复杂的工作在大多数情况下,需要依靠模型试验或数值计算混凝土的徐变使温度应力有相当大的松驰,计算温度应力时,必须考虑徐变的影响,具体计算这里就不再细述
3、温度的控制和防止裂缝的措施为了防止裂缝,减轻温度应力可以从控制温度和改善约束条件两个方面着手控制温度的措施如下
(1)采用改善骨料级配,用干硬性混凝土,掺混合料,加引气剂或塑化剂等措施以减少混凝土中的水泥用量;
(2)拌合混凝土时加水或用水将碎石冷却以降低混凝土的浇筑温度;
(3)热天浇筑混凝土时减少浇筑厚度,利用浇筑层面散热;
(4)在混凝土中埋设水管,通入冷水降温;
(5)规定合理的拆模时间,气温骤降时进行表面保温,以免混凝土表面发生急剧的温度梯度;
(6)施工中__暴露的混凝土浇筑块表面或薄壁结构,在寒冷季节采取保温措施;改善约束条件的措施是
(1)合理地分缝分块;
(2)避免基础过大起伏;
(3)合理的安排施工工序,避免过大的高差和侧面__暴露;此外,改善混凝土的性能,提高抗裂能力,加强养护,防止表面干缩,特别是保证混凝土的质量对防止裂缝是十分重要,应特别注意避免产生贯穿裂缝,出现后要恢复其结构的整体性是十分困难的,因此施工中应以预防贯穿性裂缝的发生为主在混凝土的施工中,为了提高模板的周转率,往往要求新浇筑的混凝土尽早拆模当混凝土温度高于气温时应适当考虑拆模时间,以免引起混凝土表面的早期裂缝新浇筑早期拆模,在表面引起很大的拉应力,出现“温度冲击”现象在混凝土浇筑初期,由于水化热的散发,表面引起相当大的拉应力,此时表面温度亦较气温为高,此时拆除模板,表面温度骤降,必然引起温度梯度,从而在表面附加一拉应力,与水化热应力迭加,再加上混凝土干缩,表面的拉应力达到很大的数值,就有导致裂缝的危险,但如果在拆除模板后及时在表面覆盖一轻型保温材料,如泡沫海棉等,对于防止混凝土表面产生过大的拉应力,具有显著的效果加筋对大体积混凝土的温度应力影响很小,因为大体积混凝土的含筋率极低只是对一般钢筋混凝土有影响在温度不太高及应力低于屈服极限的条件下,钢的各项性能是稳定的,而与应力状态、时间及温度无关钢的线胀系数与混凝土线胀系数相差很小,在温度变化时两者间只发生很小的内应力由于钢的弹性模量为混凝土弹性模量的7~15倍,当内混凝土应力达到抗拉强度而开裂时,钢筋的应力将不超过100~200kg/cm
2..因此,在混凝土中想要利用钢筋来防止细小裂缝的出现很困难但加筋后结构内的裂缝一般就变得数目多、间距小、宽度与深度较小了而且如果钢筋的直径细而间距密时,对提高混凝土抗裂性的效果较好混凝土和钢筋混凝土结构的表面常常会发生细而浅的裂缝,其中大多数属于干缩裂缝虽然这种裂缝一般都较浅,但它对结构的强度和耐久性仍有一定的影响为保证混凝土工程质量,防止开裂,提高混凝土的耐久性,正确使用外加剂也是减少开裂的措施之一例如使用减水防裂剂,笔者在实践中总结出其主要作用为
(1)混凝土中存在大量毛细孔道,水蒸发后毛细管中产生毛细管张力,使混凝土干缩变形增大毛细孔径可降低毛细管表面张力,但会使混凝土强度降低这个表面张力理论早在____就已被国际上所确认
(2)水灰比是影响混凝土收缩的重要因素,使用减水防裂剂可使混凝土用水量减少25%
(3)水泥用量也是混凝土收缩率的重要因素,掺加减水防裂剂的混凝土在保持混凝土强度的条件下可减少15%的水泥用量,其体积用增加骨料用量来补充
(4)减水防裂剂可以改善水泥浆的稠度,减少混凝土泌水,减少沉缩变形
(5)提高水泥浆与骨料的粘结力,提高的混凝土抗裂性能
(6)混凝土在收缩时受到约束产生拉应力,当拉应力大于混凝土抗拉强度时裂缝就会产生减水防裂剂可有效的提高的混凝土抗拉强度,大幅提高混凝土的抗裂性能
(7)掺加外加剂可使混凝土密实性好,可有效地提高混凝土的抗碳化性,减少碳化收缩
(8)掺减水防裂剂后混凝土缓凝时间适当,在有效防止水泥迅速水化放热基础上,避免因水泥__不凝而带来的塑性收缩增加
(9)掺外加剂混凝土和易性好,表面易抹平,形成微膜,减少水分蒸发,减少干燥收缩.许多外加剂都有缓凝、增加和易性、改善塑性的功能,我们在工程实践中应多进行这方面的实验对比和研究,比单纯的靠改善外部条件,可能会更加简捷、经济
4、混凝土的早期养护实践证明,混凝土常见的裂缝,大多数是不同深度的表面裂缝,其主要原因是温度梯度造成寒冷地区的温度骤降也容易形成裂缝因此说混凝土的保温对防止表面早期裂缝尤其重要从温度应力观点出发,保温应达到下述要求1)防止混凝土内外温度差及混凝土表面梯度,防止表面裂缝2)防止混凝土超冷,应该尽量设法使混凝土的施工期最低温度不低于混凝土使用期的稳定温度3)防止老混凝土过冷,以减少新老混凝土间的约束混凝土的早期养护,主要目的在于保持适宜的温湿条件,以达到两个方面的效果,一方面使混凝土免受不利温、湿度变形的侵袭,防止有害的冷缩和干缩一方面使水泥水化作用顺利进行,以期达到设计的强度和抗裂能力适宜的温湿度条件是相互关联的混凝上的保温措施常常也有保湿的效果从理论上分析,新浇混凝土中所含水分完全可以满足水泥水化的要求而有余但由于蒸发等原因常引起水分损失,从而推迟或妨碍水泥的水化,表面混凝土最容易而且直接受到这种不利影响因此混凝土浇筑后的最初几天是养护的关键时期,在施工中应切实重视起来
2.防水混凝土施工质量控制结合经验,对模板的设计、制作__、钢筋绑扎、混凝土浇筑及养护等关键工序的操作要点进行了阐述,并介绍了成型后混凝土的质量控制标准,以降低工程造价,具有很好的经济和社会效益影响防水混凝土的质量因素很多任何一个环节不注意均会不同程度地损失防水混凝土阻水功能,在地下室工程施工中,采取了从模板制作与__、钢筋制作与绑扎、混凝土的拌制和运输、混凝土的浇筑和振捣、施工缝作法、混凝土的养护与拆模以及做好回填土等方面严把质量关方法确保防水混凝土自防水,并通过实践获得成功施工方法2.1模板制作与__由于防水混凝土特别强调密实性,所以对模板的制作和__也相应提高了要求,即要求密实不漏浆,坚固不变形,吸水性要小,宜优先选用竹胶板模板或钢模板严格控制模板间的缝隙大小,超过2mm的必须用海绵条塞缝,对于板面多孔的模板一律不用,同时做好墙柱烂根防治工作,采取根部贴海绵条,底部抹水泥砂浆,浇筑混凝土前,先注入同标号的水泥砂浆由于防水混凝土结构壁厚大多较薄,为保证构件的几何尺寸,常采用内外螺栓拉接的措施,需要注意的是,应在拉接螺栓中间设置止水铁板,以防止水沿着螺栓渗漏,形成引水通路2.2钢筋制作与绑扎防水混凝土结构对钢筋有如下要求1钢筋应尽可能采用螺纹钢筋,增加握裹力和止水能力2钢筋接头应尽量采用焊接接头,尽量少用和不用绑扎接头4严格控制钢筋保护层厚度防水混凝土钢筋保护层最小厚度不小于25mm,特别是迎水面的保护层不允许有负误差,用砂浆垫块作保护层时,应用预埋在砂浆块内的铁丝与钢筋绑扎牢固若采用钢筋铁马架固定钢筋时,也应加焊止水铁板或固定在垫块上,以增强防水效果本工程用新型材料尼龙固定片有效的防止了钢筋保护层偏差过大的质量通病
2.3混凝土的拌制和运输由于防水混凝土要求较高的密实性,所以拌制也要有较好的均匀性,为此应做好如下几点1确保搅拌时间,每次搅拌从投料到出料一般不少于2min2当使用外加剂,应将外加剂制成一定浓度的溶液后加入搅拌机内,不得将外加剂的干粉或高浓度溶液直接加入搅拌机内,防止搅拌不均匀而局部集中,既失去外加剂作用,又影响混凝土质量3施工过程中应定期抽查测定混凝土坍落度,施工中如遇下雨或其他原因,砂石含水量发生变化时,应及时测定其含水率,并调整施工配合比4本工程使用商品混凝土,有效的提高了混凝土搅拌质量及各类外加剂的掺量,严格控制了混凝土水灰比及坍落度5混凝土的浇筑采用泵送工艺,有效的避免了混凝土在运输过程中产生离析泌水及漏浆现象2.4施工中的注意事项1施工作业面的合理划分防水混凝土浇筑应严格分层进行,并一次连续施工完成前后及上下层之间接头应严格控制在水泥的初凝时间内为此,可将操作工人分成几个作业组同时相向或相背而行2严格做到定点定量下料根据分层浇筑高度和运输工具的车容量,严格进行定点定量方可下料,这是提高防水混凝土浇筑质量的重要一环3坚持开门子下料或用串桶溜槽下料为防止水泥浆与骨料分离,防水混凝土下料时自由高度应不超过防水结构钢筋密集,当浇筑高度较高时,应按规定在模板上开门子下料或用串桶溜槽下料,防止水泥浆与骨料分离,同时也避免水泥浆或混凝土松散地堆积于钢筋交叉处和水泥浆溅粘于模板表面,既造成后面下料困难,又影响结构表面光滑,成为日后渗漏隐患4混凝土的振捣是保证混凝土密实、防渗的重要一环,点定量均匀下料的基础上,也应采取定点振捣,精心操作注意上下层的衔接,既要防止漏振、欠振,又要防止过振,特别要禁止用撬动钢筋或振动钢筋来助振的方法,这样将使前面已浇筑振实部分的钢筋与混凝土分离,造成严重后果5认真做好接浆措施,及时调整坍落度防水混凝土结构施工中,常常是底板与竖壁分别施工,即在竖壁上做施工缝处理在浇筑竖壁时应在接缝处先浇一层5cm~10cm厚的同标号水泥砂浆衔接层,以保证接缝处振捣密实同时,随着浇筑高度的逐步提高,应逐步减少用水量,减小坍落度防止浇筑到上部出现骨料偏少而水泥浆偏多现象2.5施工缝作法施工缝是防水结构的薄弱环节之一应尽量少留或不留,当混凝土浇筑受到设备或支模条件的限制时,则可在征得设计单位的同意后,在规定部位留置水平施工缝,一般留在底板以上200mm处的竖壁上工程中采用新工艺——膨胀性橡胶止水带处理施工缝的方法,取代常用方法,诸如企口式施工缝、高低式施工缝、平口施工缝等防水混凝土一般在垂直方向不留置施工缝,在主楼与裙楼之间设了后浇带,垂直方向的施工缝在接缝部位设置膨胀性橡胶止水带或钢板止水带由于施工缝是防水结构的一个薄弱环节,施工中应专人负责,精心操作,确保施工质量采用止水钢板时不得有漏焊,采用橡胶止水带在接长时务必要将接头用专用胶水粘贴牢固可靠2.6混凝土的养护与拆模防水混凝土浇筑后养护工作的好坏,不仅影响到混凝土的强度,而且影响到混凝土的抗渗性能试验证明,防水混凝土早期脱水或养护过程缺水,将大幅度降低其抗渗性能防水混凝土养护应不少于14d防水混凝土拆模要求也比较高,拆模时间不宜过早,拆模时应精心操作,防止用铁棍撬,大锤猛敲硬撬,使混凝土遭到破坏2.7做好回填土地下防水结构的回填土是最后一道工序,它往往被忽视,这实在是一个很大的失误当构筑物防水工作结束后应立即进行回填土施工首先应重视回填土本身的质量;其次回填前应清理坑底的杂物和积水,回填时应分层夯实地下防水结构周围一层夯实的回填土,本身是一道可靠而又经济的防水帷幕在工程地下室工程施工申,由于切切实实严把以上七道关口,组织防水混凝土施工,效果比较理想,没有出现渗漏水现象结束语以上对混凝土的施工温度与裂缝之间的关系进行了理论和实践上的初步探讨,虽然学术界对于混凝土裂缝的成因和计算方法有不同的理论,但对于具体的预防和改善措施意见还是比较统一,同时在实践中的应用效果也是比较好的,具体施工中要靠我们多观察、多比较,出现问题后多分析、多总结,结合多种预防处理措施,混凝土的裂缝是完全可以避免的参考书目(略)。