土木工程英文论文

混凝土的施工温度与裂缝Concreteinmodernconstructionoccupiesanimportantposition.Today,concretecracksaremorecommonly,incracksinthebridgeprojectisalmostubiquitous.Althoughweintheconstructiontotakevariousmeasures,careful,butthecracksstillappearfromtimetotime.Thereason,westressontheconcretetemperaturechangesarenotenoughattentiontooneofthem.

Inmassconcrete,thethermalstressandtemperaturecontrolisimportant.Thisismainlybecauseoftworeasons.Firstofall,intheconstructionofconcretecracksusuallyoccurtemperature,affectthestructuralintegrityanddurability.Secondly,intheoperationprocess,thetemperaturechangeofthestressstateofthestructurehassignificantimpactcannotbeignored.Wehaveencounteredmajorconstructionarethetemperatureofthecracks,sothisarticleonlyfortheconstructionofconcretecracksinthecausesandtreatmentmeasurestoexploretheonetodo.

1cracks

Cracksinconcretethereisavarietyofreasons,mainlytemperatureandhumiditychanges,brittleandunevenconcrete,aswellasthestructureisirrational,unqualifiedrawmaterials(suchasalkali-aggregatereaction),templatedeformation,suchasunevensettlementoffoundation.

Theconcretetohardenoffalargenumberofcementhydrationduringthehot,theinternaltemperaturerisingatthesurfacecausedbytensilestress.Lateinthecoolingprocess,duetocoagulationonthebasisoforbeboundbytheold,willemergeintheconcretetensilestress.Lowertemperatureattheconcretesurfacewillbealotoftensilestress.Whenthetensilestressbeyondthecapacityoftheconcretecrack,thecrackswillappear.Lotofconcretechangesintheinternalhumidityorchangesverylittleslower,buthumiditymaychangethesurfaceofthelargerordramaticchangeshappen.Suchasconservation,failedtokeeptimewhendowetsurfaceshrinkagedeformationofconcretesubjecttointernalconstraints,butalsooftenleadstocracks.Concreteisabrittlematerial,tensilestrengthisthecompressivestrengthof1/10orso,short-termloadingattheultimatetensiledeformationonly(0.6~1.0)×104,whenthelimitoflong-termloadingspacesstretchingdeformationonly(1.2~2.0)×104.unevenlybecauseofrawmaterials,water-cementratioofinstabilityandtransportandpouringintheprocessofsegregationphenomenaatthesamepieceofconcreteinitstensilestrengthisuneven,thereisalotoftensilecapacitylow,easytofractureinaweakposition.Inreinforcedconcrete,thetensilestressismainlybornebythereinforcementofconcreteisonlysubjecttocompressivestress.Inplainconcreteorreinforcedcoagulationontheedgeofthesiteifthestructureofthetensilestressappearstobeconcretetorelyontheirowntobear.Generaldesignrequirementsinboththetensilestressdoesnotappearoronlyverysmalltensilestress.However,intheconstructionofconcretefromthemaximumtemperatureofcoolingtotheoperatingperiodofstabletemperatureandoftenintheconcretecausingconsiderableinternaltensilestress.Temperaturestressandsometimesmorethanotherthingscausedbyloadstress,temperaturestress,thereforegraspthechangesofthestructurefortherationaldesignandconstructionofparamountimportance.

2thermalstressanalysis

Accordingtotheformationofthermalstresscanbedividedintothefollowingthreestages:

(1)early:frompouringconcretetostarttocementbasicEndingexothermic,generallyabout30days.Twocharacteristicsofthisphase,first,toreleasemanyofcementhydrationheat,andtheotheriselasticmodulusofcoagulationonthedramaticchanges.Becauseofchangesinelasticmodulusduringthisperiodintheformationofresidualstressinconcrete.

(2)medium-term:fromthebasicroleofcementexothermicconcreteuntiltheendofcoolingtothestabletemperatureonly,thisperiodwasmainlyduetothermalstressofconcretecoolingandtemperaturechangecausedbytheoutsideworld,thesestressandearlyformationofresidualstresssuperpositionphase,duringwhichcoagulationontheelasticmoduluschangedlittle.

(3)late:concretecompletelycoolingperiodaftertheoperation.Thermalstressismainlycausedbyexternaltemperaturechanges,thestressandthefirsttwo-phasesuperpositionofresidualstress.

Underthethermalstresscausedbythereasonscanbedividedintotwocategories:

(1)self-generatedstress:thereisnoconstraintontheborderorcompletelystaticstructure,iftheinternaltemperaturedistributionisnonlinear,becauseofthestructureofmutualconstraintsarisingfromthermalstress.Forexample,abridgepier,thestructureofrelativelylargesize,concretecoolingsurfacetemperaturelow,theinternaltemperaturesarehigh,atthesurfacetensilestress,compressivestressappearsinthemiddle.

(2)boundbystress:thestructureofallorpartoftheborderbytheexternalconstraints,shouldnotfree-formdeformationcausedbystress.Suchasroofofconcreteboxgirderandconcretebarrier.

Boththetemperatureandtheconcretestressisoftencausedbytheshrinkageofthecombinedeffectofstress.

Tothebasisofknownaccurateanalysisofthetemperatureofthethermalstressdistribution,sizeisamorecomplicatedjob.Inmostcases,necessarytorelyonmodeltestornumericalcalculation.Creepofconcretesothatthetemperaturethereisconsiderablestressrelaxation,thermalstresscalculation,wemustconsidertheeffectsofcreep,thespecificcalculationwillnotdetailhere.

3temperaturecontrolandmeasurestopreventthecracks

Inordertopreventcracking,reducethethermalstresscancontrolthetemperatureandimprovetheconditionoftwoaspects.

Temperaturecontrolmeasuresareasfollows:

(1)usedtoimprovetheaggregategradation,anddohardconcrete,dopedmixture,addair-entrainingagentorplasticizer,antisepticmeasurestoreducetheamountofcementconcrete;

(2)concretemixingwateroraddwatercoolingtoreducetorubbleconcretepouringtemperature;

(3)whenthehotdaysofpouringconcretepouringtoreducethethicknessdimensionoftheuseofpouringheat;

(4)laidinconcretewaterpipes,accesstocoldwatertocool;

(5)theprovisionsofreasonabletime,thetemperaturedroppedtothesurfacewhentheinsulation,theconcretesurfaceinordertoavoiddrastictemperaturegradient;

(6)Constructionoflong-termexposureofconcretepouringblocksurfaceorthin-walledstructures,thermalinsulationinthecoldseasontotakemeasures;

Measurestoimprovetheconstraintsare:

(1)reasonablehourssewingsub-block;

(2)toavoidtoomuchfoundationanddowns;

(3)reasonablearrangementsfortheconstructionprocess,toavoidexcessivesideelevationandlong-termexposure;

Inaddition,toimproveconcreteperformance,improveanti-crackingability,strengtheningtheconservation,topreventsurfaceshrinkage,inparticular,istoensurethequalityofconcretetopreventcracksisveryimportant,specialattentionshouldbepaidtoavoidcross-cracksappearstorestoretheintegrityofitsstructureitisverydifficult,soconstructionshouldbetopreventtheoccurrenceofcross-cuttingthemaincracks.

Atconcreteconstruction,inordertoimprovetheturnoverrateofthetemplate,oftenrequiringnewpouringconcreteassoonaspossible.Whentheconcretetemperatureishigherthanthetemperatureshouldbedueconsiderationtime,soastoavoidtheearlycracksintheconcretesurface.Newpouringearly,atthesurfaceofalotoftensilestress,a"temperatureshock"phenomenon.Concretepouringattheinitialstageofhydrationheatbecauseofthedisseminationofthesurfacecausedbyalargetensilestress,whenthesurfacetemperatureishigherthanthetemperature,whentheremovaloftemplate,asharpfallinsurfacetemperatureisboundtoleadtotemperaturegradient,whichatthesurfaceoftheadditional1tensilestress,thermalstresssuperpositionandhydration,coupledwiththeshrinkageofconcrete,thesurfacetensilestresstoreachalotofvalue,thereisthedangerofleadtocracks,butifinatimelymannerafterremovalofthetemplatesurfaceatonelightinsulationmaterials,suchasfoamspongeandsoon,haveaconcretesurfacetopreventexcessivetensilestress,asignificanteffect.

Reinforcedconcreteonthelargevolumeofthermalstresshaslittleeffect,becauseofmassconcreterateisalowglutencontent.Onlyonthegeneralimpactofreinforcedconcrete.Atnottoohightemperatureandstressbelowtheyieldlimitundertheconditionofthepropertiesofsteelarestable,withthestressstate,timeandtemperaturehasnothingtodo.Linearexpansioncoefficientofsteelandconcretelinearexpansioncoefficientdifferencebetweentheverysmallchangesintemperaturebetweenthetwohappenedonlyaverysmallinternalstress.Steelbecauseoftheelasticmodulusofconcreteelasticmodulusof7~15-fold,whenthestressreachedthetensilestrengthofconcreteandcracking,thesteelstresswillnotexceed100~200kg/cm2..So,youwanttouseinconcretereinforcedtopreventtheemergenceofsmallcracksisverydifficult.However,afterthereinforcedstructuregenerallybecomesthenumberofcracksinmany,smallpitch,widthanddepthofthesmaller.Andifthediameterofsteelthinanddensespacing,thecrackresistanceofconcretetoenhancetheeffectisgood.Concreteandreinforcedconcretestructureofthesurfacewillusuallyhappenthinandshallowcracks,mostofwhichbelongtoshrinkagecracks.Whilesuchcracksaregenerallyshallow,butitsstrengthanddurabilityofthestructuremuststillaffected.

Inordertoensurethequalityofconcreteworkstopreventcracking,improvethedurabilityofconcrete,thecorrectuseofadmixturesisalsooneofthemeasurestoreducecracking.Suchastheuseofanti-crackingagentminusthewater,theauthorconcludesthatinpracticeitsmainroleis:

(1)theexistenceofalargenumberofporesinconcreteRoad,waterevaporationafterthecapillarycapillarytensiongenerated,sothatshrinkagedeformationofconcrete.Largerdiameterporescanreducethecapillarysurfacetension,butwouldreducethestrengthofconcrete.Thetheoryofsurfacetensionintheearlysixtieshadbeenrecognizedbytheinternationalcommunity.

(2)theimpactofwater-cementratioconcreteshrinkageareimportantfactors,theuseofwater-reducingagentcancrackconcretewaterconsumptionreducedby25%.

(3)cementconcreteshrinkagerateisalsoanimportantfactorinadditionandsubtractionofwater-dopedconcretecrackingagentinmaintainingthestrengthofconcretecanbereducedundertheconditionsof15%ofcementconsumption,anditsvolumebyincreasingaggregateamounttosupplement.

(4)water-reducingagentcanimprovethecrackingoftheconsistencyofcementslurrytoreducethebleedingofconcrete,reduceshrinkagedeformationShen.

(5)improvewaterslurryandaggregatebondstrength,improveanti-crackingperformanceofconcrete.

(6)concreteboundatcontractiongeneratedtensilestress,whenthetensilestressisgreaterthanthetensilestrengthofconcretewhenthecrackswillbegenerated.Water-reducingagentcanbeaneffectiveanti-crackingofconcreteimprovetensilestrength,asubstantialincreaseinanti-crackingperformanceofconcrete.

(7)mixing-compactingconcreteadmixturescan,andcaneffectivelyimprovetheanti-carbonationofconcrete,reduceshrinkagecarbonation.

(8)-dopedwater-reducingagentaftertheconcretecrackingretardationtimedue,attheeffectivepreventionoftherapidhydrationofcementfoundationonexothermictoavoidlong-termnon-condensablecementwasbroughtaboutbyincreaseinplasticshrinkage.

(9)-dopedadmixturesConcreteworkability,andeasytofindflatsurfacestoformamicro-membranetoreduceevaporationandreducedryingshrinkage.

Retardingadmixtureshavemuchtoincreaseworkability,improvethefunctionofplasticity,weinengineeringpracticeshouldbecarriedoutmanyexperimentsinthisareacomparisonsandresearch,thansimplybyimprovingtheexternalconditionsmightbemoresimpleandeconomic.

Concrete4earlyconservation

Practicehasprovedthatthecommonconcretecracks,themajorityareatdifferentdepthsofsurfacecracks,themainreasonisthetemperaturegradientcausedbythecoldtemperatureregionisalsovulnerabletoasharpfallintheformationofcracks.Saythereforetopreventthethermalinsulationofconcretesurfacecracksearlyisespeciallyimportant.

Fromthermalstresspointofview,theinsulationshouldmeetthefollowingrequirements:

1)Topreventtheconcreteinsideandoutsidethebadandtheconcretesurfacetemperaturegradient,topreventsurfacecracks.

2)topreventtheconcretesuper-cold,itshouldbepossibletomakeconcretetheconstructionperiodofnotlessthantheminimumtemperatureofconcreteusedforthestabilityoftemperature.

3)topreventtheoldconcretetoocold,oldandnewconcreteinordertoreduceinter-bound.

Concreteearlyconservation,themainobjectiveistomaintainappropriatetemperatureandhumidityconditionsinordertoachievetheeffectoftwoaspects,ontheonehand,theconcre