中英论混凝土的施工温度与结构裂缝

Ontheconstructionofconcretetemperatureandcracks1,thecausesofthecracksCracksinconcreteareavarietyofreasons,mainlytemperatureandhumiditychanges,brittleandunevenconcrete,aswellasthestructureisirrational,failureofrawmaterials(suchasalkali-aggregatereaction),templatedeformation,andsoonthebasisofdifferentialsettlement.Theconcretetohardenoffalargenumberofcementduringthehydrationheat,theinternaltemperaturerisingonthesurfacecausedbytensilestress.Lateinthecoolingprocess,duetocoagulationonthebasisoforbeboundbytheold,willemergeintheconcretetensilestress.Lowertemperaturesintheconcretesurfacewillbealotoftensilestress.Whenthetensilestressbeyondthecapacityofconcretecrack,thecrackswillappear.Anumberofconcretechangesintheinternalhumidityorchangesverylittleslow,butpossiblechangesinsurfacehumidityormoredramaticchanges.Suchasconservation,failedtokeepdrywhenwet,surfaceshrinkagedeformationofconcretesubjecttointernalconstraints,butalsooftenleadstocracks.Short-termloadingatthetimeofultimatetensiledeformationonly(0.6~1.0)×104,thelimitoflong-termloadingspaceswhentheonlystretchdeformation(1.2~2.0)×104.Unevenlyasaresultofrawmaterials,water-cementratioofinstability,andtransportationandpouringintheprocessofsegregationphenomena,inthesamepieceofconcreteinthetensilestrengthisuneven,therearemanylowtensilecapacity,easytofractureinaweakposition.Inreinforcedconcrete,thetensilestressismainlybornebythesteel,concreteissubjecttocompressivestress.Inplainconcreteorreinforcedcoagulationontheedgeofthesiteifthestructureofthetensilestressappearstoberelyingonitsownconcretecommitment.2,thermalstressanalysisAccordingtotheformationofthermalstresscanbedividedintothefollowingthreestages:early:Sincethebeginningofpouringconcretetocementtheendofexothermicbasicgeneralabout30days.Twocharacteristicsofthisstage,first,toreleasealargenumberofcementhydrationheat,andtheotheristheelasticmodulusofcoagulationdramaticchanges.Modulusofelasticityasaresultofchangeswithinthisperiodintheformationofresidualstressintheconcrete.(2)medium-term:fromthebasicroleofcementheatuntiltheendofthestabletemperatureoftheconcretecoolingtime,duringthisperiod,thetemperaturestresswasmainlyduetocoolingandtheoutsideconcretecausedbytemperaturechanges,stressandearlyformationoftheseresidualstresssuperpositionphase,duringwhichthecoagulationoftheelasticmoduluschangedlittle.(3)late:concretecompletelycoolingperiodaftertheoperation.Thermalstressismainlycausedbyexternaltemperaturechanges,thestressandthefirsttwo-phasesuperpositionofresidualstress.Underthethermalstresscausedbythereasonscanbedividedintotwocategories:(1)self-stress:thereisnoconstraintontheborderorcompletelystaticstructure,iftheinternaltemperaturedistributionisnonlinear,duetostructuralconstraintsarisingfromtheirco-temperaturestress.Forexample,theconversionlayerofitsrelativelylargersize,concretecoolingsurfacetemperaturelow,theinternaltemperaturesarehigh,thesurfacetensilestress,compressivestressappearsinthemiddle.(2)boundbystress:thestructureofallorpartoftheboundaryboundbytheoutsideworldcannotbefree-formdeformationcausedbystress.Roofbox,suchasconcreteandconcretebarrier.Boththetemperatureandtheconcretestressisoftencausedbyshrinkageofthecombinedeffectofstress.Knowntobeinaccordancewithanaccurateanalysisofthetemperatureofthethermalstressdistribution,sizeisamorecomplextask.Inmostcases,theneedtorelyonmodeltestornumericalcalculation.Creepofconcretesothatthetemperaturethereisconsiderablestressrelaxation,thermalstresscalculation,wemustconsidertheeffectsofcreep,isnolongercalculatedindetailhere.3,temperaturecontrolandmeasurestopreventthecracksInordertopreventcracking,reducethethermalstresscancontroltheconditionsoftemperatureandtoimprovethebindingoftwoaspects.Temperaturecontrolmeasuresareasfollows:(1)usedtoimprovetheaggregategradation,dryhardconcrete,mixedmixture,addair-entrainingagentorplasticizer,etc.measurestoreducetheamountofcementconcrete;杨(恋2)m液ixin渡gco足ncre椒teb影yad脱ding瓜wat盈ero旱rco砌olin及gwa良ter安will毙be蛋grav旁ela陡ndp寻ouri彩ngc促oncr慌ete杏tor肉educ神eth砖ete肝mper富atur碗e;描(吼3)p逃ouri融ngh奇otd石ays愤tor滔educ怠eth尖epo焦urin剩gof声con抢cret油eth菜ickn俗ess,宰the毅use傅of恢pour豪ing盈heat拉lev读els;胶(壳4)l敬aid均inc崖oncr攀ete零wate予rpi佛pes,红acc骨ess表toc层old球wate环rto奴coo揉l;辟(雨5)t衫hep廊rovi歪sion甜sof骗rea引sona域ble疏time吸,th聪ete惭mper靠atur尸edr裳opw侨hen好the避surf浩ace晨insu琴lati吧on,妙conc储rete萄sur愈face绘in静orde端rto著avo唐ida成sha榨rpt雷empe述ratu湿reg煮radi独ent;所(桥6)C供onst码ruct攀ion辱ofl撕ong-趴term胖exp蒜osur静eto舅blo央ckt克hes色urfa崖ceo瑞fth钉eco裕ncre适tep宇ouri杀ng,缠ort拘hin-截wall府eds驳truc益ture载s,t轧herm药ali柿nsul基atio风nin刮the侨col竞dse壶ason舍to特take毛mea冲sure尽s;剑Meas构ures刚to扑impr冷ove懒the劫cons朋trai栏nts漠are:如(敲1)a领rea芒sona趋ble神sub-螺bloc不kjo嚷ints刃;橡(栽2)b卷asis偷to辆avoi瓜dto素omu虑chu屋psa喘ndd膛owns轨;购(江3)r定easo翁nabl愤ear涌rang遮emen盯tsf贫ort垮hec值onst碰ruct释ion放proc卸ess,哗to蓬avoi腰dex舍cess淡ive归side议ele线vati雷ona犬ndl南ong-互term忧exp厦osur势e;晴Ina皱ddit吊ion,瓶to博impr缸ove画the刺perf但orma妙nce超ofc趟oncr描ete尚toi扔mpro筋vet淘hea荐nti-垦crac茧king妥abi相lity泳,to奴str堵engt宴hen寺the胜cons垮erva死tion竞,to败pre阅vent捐sur论face趁dry胀ing,房in筑part遍icul费ar,滋ist邀oen沿sure怖the顷qua兵lity法of袭conc梢ret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喷混凝土中争产生裂缝弊有多种原崖因，主要殿是温度和填湿度的变艳化，混凝渐土的脆性辆和不均匀怨性，以及拖结构不合恢理，原材献料不合格迹（如碱骨狐料反应）俗，模板变变形，基础怨不均匀沉盯降等。农混凝土硬案化期间水洁泥放出大带量水化热短，内部温锈度不断上鸡升，在表棚面引起拉候应力。后庆期在降温屋过程中，密由于受到肚基础或老皂混凝上的守约束，又匪会在混凝撇土内部出杰现拉应力鹅。气温的梳降低也会岗在混凝土团表面引起扇很大的拉却应力。当姻这些拉应粪力超出混战凝土的抗貌裂能力时逃，即会出谋现裂缝。膊许多混凝园土的内部痕湿度变化匙很小或变前化较慢，闻但表面湿拆度可能变穿化较大或存发生剧烈资变化。如毛养护不周钓、时干时和湿，表面事干缩形变洒受到内部烟混凝土的川约束，也姨往往导致蓬裂缝。混孕凝土是一瓣种脆性材诊料，抗拉胜强度是抗掠压强度悠的偷1汗／搏1攀0嫩左右，短挪期加荷时欧的极限拉则伸变形只燥有疮（叮0.炎6疗～傍1.氏0掉）殿×10衰4拔，长期加命荷时的极薯限位伸变碍形也只有魂（旦1.退2裁～榜2.岛0悟）渐×104某.幸由于原材成料不均匀懒，水灰比欧不稳定，捎及运输和万浇筑过程甚中的离析堤现象，在嘱同一块混询凝土中其构抗拉强度豆又是不均农匀的，存造在着许多防抗拉能力棒很低，易尝于出现裂页缝的薄弱搂部位。在粘钢筋混凝淘土中，拉渴应力主要脾是由钢筋辆承担，混打凝土只是随承受压应蓝力。在素括混凝土内崇或钢筋混泪凝上的边搭缘部位如缸果结构内劫出现了拉崭应力，则菊须依靠混哭凝土自身允承担。一忧般设计中羊均要求不柄出现拉应拳力或者只舍出现很小肉的拉应力纪。但是在弱施工中混敲凝土由最缺高温度冷漫却到运转蹦时期的稳哪定温度，抱往往在混什凝土内部吃引起相当燕大的拉应延力。有时习温度应力粒可超过其仅它外荷载席所引起的祸应力，因歪此掌握温甩度应力的岁变化规律民对于进行舱合理的结久构设计和会施工极为喂重要。退2军、温度应彻力的分析妨根据温度腔应力的形坊成过程可围分为以下士三个阶段萄：眼（吗1粘）早期：润自浇筑混惰凝土开始秆至水泥放图热基本结式束，一般涛约坚3屡0针天。这个糖阶段的两摇个特征，守一是水泥菊放出大量物的水化热暑，二是混芒凝上弹性童模量的急饿剧变化。调由于弹性辣模量的变密化，这一撑时期在混可凝土内形绞成残余应味力。衡（搁2流）中期：云自水泥放剧热作用基酷本结束时弟起至混凝乞土冷却到授稳定温度浮时止，这欠个时期中商，温度应听力主要是荐由于混凝拖土的冷却刺及外界气央温变化所东引起，这喷些应力与箭早期形成腔的残余应判力相叠加虑，在此期跪间混凝上肝的弹性模陷量变化不牺大。臂（乒3承）晚期：纵混凝土完催全冷却以写后的运转笔时期。温栽度应力主薯要是外界真气温变化膊所引起，取这些应力联与前两种傍的残余应亡力相迭加赞。根据温膛度应力引展起的原因底可分为两鬼类：呆（悦1白）自生应刘力：边界兵上没有任嘱何约束或谈完全静止燕的结构，祝如果内部渐温度是非医线性分布霉的，由于士结构本身梢互相约束头而出现的开温度应力宵。例如，俯转换层其停结构尺寸存相对较大惯，混凝土梦冷却时表辨面温度低终，内部温位度高，在茄表面出现谎拉应力，逢在中间出量现压应力尊。雨（皮2扭）约束应能力：结构妖的全部或用部分边界刘受到外界叫的约束，裹不能自由铅变形而引坚起的应力垂。如箱梁位顶板混凝樱土和护栏弓混凝土。饺这两种温地度应力往秃往和混凝柄土的干缩您所引起的肿应力共同客作用。捆要想根据锡已知的温馋度准确分蜜析出温度僵应力的分棕布、大小门是一项比宋较复杂的杏工作。在访大多数情糟况下，需墓要依靠模老型试验或怜数值计算齐。混凝土衫的徐变使漂温度应力参有相当大储的松驰，鞭计算温度汽应力时，我必须考虑牺徐变的影肃响，具体傲计算这里涉就不再细沾述。若3机、温度的鸽控制和防放止裂缝的战措施社为了防止情裂缝，减炉轻温度应梯力可以从机控制温度答和改善约牧束条件两残个方面着小手。控制榨温度的措得施如下：勤（扇1滔）采用改模善骨料级蔽配，用干肾硬性混凝探土，掺混干合料，加似引气剂或乌塑化剂等蔑措施以减窗少混凝土搜中的水泥驼用量；愈（壶2花）拌合混卷凝土时加快水或用水厉将碎石冷忌却以降低杆混凝土的劣浇筑温度途；溪（匹3轮）热天浇姑筑混凝土纳时减少浇拒筑厚度，阶利用浇筑沃层面散热斑；呜（旱4漏）在混凝遍土中埋设堪水管，通答入冷水降时温；池（据5标）规定合近理的拆模嗽时间，气烟温骤降时经进行表面从保温，以盈免混凝土扩表面发生荷急剧的温朗度梯度；灾（掩6融）施工中冬长期暴露峰的混凝土添浇筑块表捷面或薄壁猜结构，在顿寒冷季节歇采取保温基措施；改倦善约束条规件的措施慌是：牲（驰1思）合理地拼分缝分块虚；锅（正2桐）避免基谋础过大起饮伏；取（情3刮）合理的档安排施工婚工序，避骡免过大的忌高差和侧籍面长期暴温露；维此外，改墓善混凝土精的性能，判提高抗裂竹能力，加荷强养护，例防止表面界干缩，特欲别是保证鞠混凝土的圣质量对防爆止裂缝是粗十分重要驱，应特别分注意避免团产生贯穿佳裂缝，出蜻现后要恢思复其结构锅的整体性欠是十分困皂难的，因恰此施工中治应以预防橡贯穿性裂发缝的发生贫为主把。托答在混凝土址的施工中半，为了提矮高模板的向周转率，再往往要求辩新浇筑的幸混凝土尽而早拆模。救当混凝土木温度高于麻气温时应积适当考虑程拆模时间丘，以免引灿起混凝土欠表面的早其期裂缝。旧新浇筑早内期拆模，供在表面引利起很大的恒拉应力，笋出烛现件“枪温度冲拆击禁”付现象。在钻混凝土浇秩筑初期，较由于水化会热的散发萄，表面引讨起相当大赤的拉应力惊，此时表羡面温度亦奶较气温为本高，此时沉拆除模板鬼，表面温理度骤降，趴必然引起课温度梯度镇，从而在铸表面附加肆一拉应力畜，与水化赶热应力迭绿加，再加沉上混凝土雪干缩，表服面的拉应食力达到很拖大的数值押，就有导浆致裂缝的颤危险，但吴如果在拆丹除模板后笑及时在表席面覆盖一迫轻型保温泉材料，如闹泡沫海棉捉等，对于笋防止混凝寨土表面产膊生过大的倡拉应力，瓜具有显