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DISCUSSTHECONSTRUCTIONTEMPERATUREANDCRACKOFTHECONCRETELIGHTLY淺談混凝土的施工過程中溫度與裂縫控制資料來源THEAMERICANSOCIETYOFCIVILENGINEERS設(shè)計(jì)題目長(zhǎng)春市外貿(mào)局辦公樓結(jié)構(gòu)、施工設(shè)計(jì)學(xué)院名稱土木工程學(xué)院專業(yè)名稱土木工程班級(jí)名稱土木1043完成時(shí)間2014年3月25日DISCUSSTHECONSTRUCTIONTEMPERATUREANDCRACKOFTHECONCRETELIGHTLYTHECONCRETEOCCUPIESTHEIMPORTANTPOSITIONINMODERNENGINEERINGCONSTRUCTIONBUTTODAY,THECRACKOFTHECONCRETEISCOMPARATIVELYGENERAL,THECRACKSARENEARLYOMNIPRESENTINTHESCIENCEOFBRIDGEBUILDINGTHOUGHWETAKEVARIOUSKINDSOFMEASURESINCONSTRUCTING,CAREFUL,BUTTHECRACKSTILLOCCURSNOWANDTHENTRACINGITTOITSCAUSE,ITISONEOFTHEMINCOMPLETELYTHATOURCHANGETOCONCRETETEMPERATURESTRESSPAYSATTENTIONTOINTHELARGEVOLUMECONCRETE,TEMPERATURESTRESSANDTEMPERATURECONTROLARESIGNIFICANTTHISISMAINLYBECAUSEOFTHEREASONOFTWORESPECTSFIRSTOFALL,CONCRETEOFTENAPPEARTHETEMPERATURECRACKINNOTCONSTRUCTING,INFLUENCETHEGLOBALITYANDDURABILITYOFTHESTRUCTURESECONDLY,INTHECOURSEOFOPERATING,THETEMPERATURECHANGEHASREMARKABLEINFLUENCETHATCANTBEIGNOREDONTHESTRESSSTATEOFTHESTRUCTUREWEMEETTOCONSTRUCTTEMPERATURECRACKINMAINLY,SOONLYTOORIGINCAUSEOFFORMATIONANDTREATMENTMEASURE,CONCRETEOFCRACKMAKEADISCUSSIONINCONSTRUCTINGTHISTEXT1REASONOFACRACKHAVEMANYKINDSOFREASONSTOPRODUCETHECRACKINTHECONCRETE,ITISMAINLYTHECHANGESOFTEMPERATUREANDHUMIDITY,FRAGILITYANDDISPARITYOFTHECONCRETE,ANDTHESTRUCTUREISUNREASONABLE,THERAWMATERIALSISNOTUPTOSTANDARDIFTHEALKALIAGGREGATEREACT,THETEMPLATEISOUTOFSHAPE,THEFOUNDATIONDOESNOTSUBSIDEETCEVENLYTHECEMENTEMITSALARGEAMOUNTOFHEATOFHYDRATIONWHENTHECONCRETEISHARDENNED,INSIDETEMPERATUREISRISINGCONSTANTLY,CAUSETHESTRESSOFDRAWINGONTHESURFACEINTHECOURSEOFLOWERINGTHETEMPERATURE,ISITCONGEALFOUNDATIONPAYRESTRAINTOMIXALWAYSLATERSTAGE,WILLPRESENTTHESTRESSOFDRAWINGWITHINTHECONCRETEREDUCINGOFTEMPERATURECANSURFACECAUSEHEAVYSTRESSOFDRAWINGVERYINCONCRETETOOWHENTHESEDRAWTHESTRESSANDGOBEYONDRESISTINGTHEABILITYOFSPLITTINGOFCONCRETE,NAMELYWILLPRESENTTHECRACKALOTOFINSIDEHUMIDITYOFCONCRETECHANGEVERYLIGHTORCHANGERELATIVELYSLOW,SURFACEHUMIDITYMIGHTCHANGEHEAVYTHEVIOLENTCHANGETAKESPLACINGSUCHASMAINTAININGTHOROUGHLY,WHENGETTINGWETTERWHENNOTDOING,CONTRACTSURFACETHEREARENTDEFORMATIONDOING,OFTENCAUSETHECRACKTOOTHECONCRETEISAKINDOFFRAGILITYMATERIAL,TENSILESTRENGTHISABOUT1/10OFTHECOMPRESSIONSTRENGTH,ISITCARRYONONESSHOULDERORBACKLIMITWHENDRAWOUTOFSHAPETOHAVE104ONLY,ISITCARRYONONESSHOULDERORBACKLIMITLOCATIONWHENSTRETCHOUTOFSHAPETOTHEREIS104TOADDFORALONGTIMETOADDASHORTTIMEBECAUSERAWMATERIALSEVEN,WATERDUSTTHANUNSTABLE,TRANSPORTANDBUILDPHENOMENONOFEMANATINGOFCOURSE,ITSTENSILESTRENGTHISNOTEVENINTHESAMECONCRETE,ALOTOFRESISTTHEABILITYOFDRAWINGVERYLOW,ITISAPTTOPRESENTTHEWEAKPOSITIONOFTHECRACKAMONGARMOREDCONCRETE,DRAWSTRESSTOUNDERTAKEBYREINFORCINGBARMAINLY,CONCRETEBEARSTRESSOFKEEPINGJUSTORREINFORCINGBARMIXIFEDGEPOSITIONGONETOTOCONGEALPRESENTTHESTRESSOFDRAWINGINTHESTRUCTUREINPLAINCONCRETE,MUSTRELYONTHECONCRETEONESELFTOBEARREQUIRETOAVOIDTHESTRESSOFDRAWINGORONLYVERYSMALLSTRESSOFDRAWINGAPPEARSOFTHETHEGENERALDESIGNBUTTHECONCRETEISCOOLEDFROMMAXIMUMTEMPERATURETOTHESTEADYTEMPERATUREOFOPERATINGPERIODINCONSTRUCTING,OFTENCAUSESIZABLETODRAWTHESTRESSWITHINTHECONCRETETHETEMPERATURESTRESSCANEXCEEDOTHEROUTSIDESANDLOADTHESTRESSESCAUSEDSOMETIMES,KNOWCHANGELAW,TEMPERATUREOFSTRESSFORCARRYONREASONABLESTRUCTURALDESIGNANDCONSTRUCTEXTREMELYIMPORTANT2ANALYSISOFTEMPERATURESTRESSESCANBEDIVIDEDINTOFOLLOWINGTHREESTAGESACCORDINGTOTHEFORMINGPROCESSOFTHETEMPERATURESTRESS(1)ITISEARLYBUILDCONCRETEISITISITOVERBASICALLYTOSENDOUTHEATTOCEMENTTOBEGIN,GENERALLYONEONEDAYBYONESELFTWOCHARACTERISTICSATTHISSTAGE,FIRST,THECEMENTEMITSALARGEAMOUNTOFHEATOFHYDRATION,SECOND,MIXANDCONGEALTHECHANGINGSHARPLYOFELASTICMODELQUANTITYBECAUSEOFTHECHANGEOFELASTICMODELQUANTITY,FORMTHEREMAININGSTRESSINTHECONCRETEINTHISPERIOD(2)MIDDLEPERIODUPTILLTHECONCRETEISCOOLEDUNTILSTABILITYTEMPERATUREFROMCEMENTSENDOUTHEATFUNCTIONBASICALLYWHENEXPIRING,INTHISPERIOD,THETEMPERATURESTRESSISMAINLYBECAUSETHECOOLINGOFTHECONCRETEANDEXTERNALTEMPERATURECHANGECAUSE,THESESTRESSESANDREMNANTSSTRESSESTHATISFORMEDINEARLYDAYSARESUPERPOSED,MIXANDCONGEALTHEELASTICMOULDAMOUNTTHATGOESTOANDDOESNOTCHANGEMUCHDURINGTHISPERIOD(3)LATERPERIODOPERATIONPERIODAFTERTHECOMPLETECOOLINGOFCONCRETETEMPERATURESTRESSWHETHEREXTERNALTEMPERATURECHANGECAUSEMAINLY,THESESTRESSESANDFIRSTTWOKINDSOFREMNANTSSTRESSESESARECHANGEDANDADDEDCANBEDIVIDEDINTOTWOKINDSACCORDINGTOTHEREASONWHYTHETEMPERATURESTRESSCAUSES(1)SPONTANEOUSSTRESSTHEREARENOTANYRESTRAINTORTOTALLYSTATICSTRUCTUREATTHEBORDER,IFINSIDETEMPERATUREISNONLINEARDISTRIBUTION,TEMPERATURESTRESSAPPEARINGBECAUSESTRUCTURERESTRAINSFROMEACHOTHERFOREXAMPLE,THEBODYOFMOUNDOFTHEBRIDGE,THEPHYSICALDIMENSIONISRELATIVELYLARGE,SURFACETEMPERATUREISLOWWHENTHECONCRETEISCOOLED,INSIDETEMPERATUREISHIGH,PRESENTTHESTRESSOFDRAWINGONTHESURFACE,PRESENTTHESTRESSOFPRESSINGINTHEMIDDLE(2)RESTRAINTHESTRESSALLOFTHESTRUCTUREONESORITRESTRAINEXTERNALONESOMEBORDER,CANTOUTOFSHAPEANDSTRESSNOTCAUSENOTFREESUCHASCASEROOFBEAMROOFCONCRETEANDGUARDRAILCONCRETETHISTWOKINDSOFTEMPERATURESTRESSESDRAWBACKSTRESSESCAUSEDTOACTO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混凝土常常出現(xiàn)溫度裂縫,影響到結(jié)構(gòu)的整體性和耐久性。其次,在運(yùn)轉(zhuǎn)過程中,溫度變化對(duì)結(jié)構(gòu)的應(yīng)力狀態(tài)具有顯著的不容忽視的影響。我們遇到的主要是施工中的溫度裂縫,因此本文僅對(duì)施工中混凝土裂縫的成因和處理措施做一探討。1裂縫的原因混凝土中產(chǎn)生裂縫有多種原因,主要是溫度和濕度的變化,混凝土的脆性和不均勻性,以及結(jié)構(gòu)不合理,原材料不合格(如堿骨料反應(yīng)),模板變形,基礎(chǔ)不均勻沉降等?;炷劣不陂g水泥放出大量水化熱,內(nèi)部溫度不斷上升,在表面引起拉應(yīng)力。后期在降溫過程中,由于受到基礎(chǔ)或老混凝上的約束,又會(huì)在混凝土內(nèi)部出現(xiàn)拉應(yīng)力。氣溫的降低也會(huì)在混凝土表面引起很大的拉應(yīng)力。當(dāng)這些拉應(yīng)力超出混凝土的抗裂能力時(shí),即會(huì)出現(xiàn)裂縫。許多混凝土的內(nèi)部濕度變化很小或變化較慢,但表面濕度可能變化較大或發(fā)生劇烈變化。如養(yǎng)護(hù)不周、時(shí)干時(shí)濕,表面干縮形變受到內(nèi)部混凝土的約束,也往往導(dǎo)致裂縫。混凝土是一種脆性材料,抗拉強(qiáng)度是抗壓強(qiáng)度的110左右,短期加荷時(shí)的極限拉伸變形只有(0610)104,長(zhǎng)期加荷時(shí)的極限位伸變形也只有(1220)104由于原材料不均勻,水灰比不穩(wěn)定,及運(yùn)輸和澆筑過程中的離析現(xiàn)象,在同一塊混凝土中其抗拉強(qiáng)度又是不均勻的,存在著許多抗拉能力很低,易于出現(xiàn)裂縫的薄弱部位。在鋼筋混凝土中,拉應(yīng)力主要是由鋼筋承擔(dān),混凝土只是承受壓應(yīng)力。在素混凝土內(nèi)或鋼筋混凝上的邊緣部位如果結(jié)構(gòu)內(nèi)出現(xiàn)了拉應(yīng)力,則須依靠混凝土自身承擔(dān)。一般設(shè)計(jì)中均要求不出現(xiàn)拉應(yīng)力或者只出現(xiàn)很小的拉應(yīng)力。但是在施工中混凝土由最高溫度冷卻到運(yùn)轉(zhuǎn)時(shí)期的穩(wěn)定溫度,往往在混凝土內(nèi)部引起相當(dāng)大的拉應(yīng)力。有時(shí)溫度應(yīng)力可超過其它外荷載所引起的應(yīng)力,因此掌握溫度應(yīng)力的變化規(guī)律對(duì)于進(jìn)行合理的結(jié)構(gòu)設(shè)計(jì)和施工極為重要。2溫度應(yīng)力的分析根據(jù)溫度應(yīng)力的形成過程可分為以下三個(gè)階段(1)早期自澆筑混凝土開始至水泥放熱基本結(jié)束,一般約30天。這個(gè)階段的兩個(gè)特征,一是水泥放出大量的水化熱,二是混凝上彈性模量的急劇變化。由于彈性模量的變化,這一時(shí)期在混凝土內(nèi)形成殘余應(yīng)力。(2)中期自水泥放熱作用基本結(jié)束時(shí)起至混凝土冷卻到穩(wěn)定溫度時(shí)止,這個(gè)時(shí)期中,溫度應(yīng)力主要是由于混凝土的冷卻及外界氣溫變化所引起,這些應(yīng)力與早期形成的殘余應(yīng)力相疊加,在此期間混凝上的彈性模量變化不大。(3)晚期混凝土完全冷卻以后的運(yùn)轉(zhuǎn)時(shí)期。溫度應(yīng)力主要是外界氣溫變化所引起,這些應(yīng)力與前兩種的殘余應(yīng)力相迭加。根據(jù)溫度應(yīng)力引起的原因可分為兩類(1)自生應(yīng)力邊界上沒有任何約束或完全靜止的結(jié)構(gòu),如果內(nèi)部溫度是非線性分布的,由于結(jié)構(gòu)本身互相約束而出現(xiàn)的溫度應(yīng)力。例如,橋梁墩身,結(jié)構(gòu)尺寸相對(duì)較大,混凝土冷卻時(shí)表面溫度低,內(nèi)部溫度高,在表面出現(xiàn)拉應(yīng)力,在中間出現(xiàn)壓應(yīng)力。(2)約束應(yīng)力結(jié)構(gòu)的全部或部分邊界受到外界的約束,不能自由變形而引起的應(yīng)力。如箱梁頂板混凝土和護(hù)欄混凝土。這兩種溫度應(yīng)力往往和混凝土的干縮所引起的應(yīng)力共同作用。要想根據(jù)已知的溫度準(zhǔn)確分析出溫度應(yīng)力的分布、大小是一項(xiàng)比較復(fù)雜的工作。在大多數(shù)情況下,需要依靠模型試驗(yàn)或數(shù)值計(jì)算?;炷恋男熳兪箿囟葢?yīng)力有相當(dāng)大的松馳,計(jì)算溫度應(yīng)力時(shí),必須考慮徐變的影響,具體計(jì)算這里就不再細(xì)述。3溫度的控制和防止裂縫的措施為了防止裂縫,減輕溫度應(yīng)力可以從控制溫度和改善約束條件兩個(gè)方面著手??刂茰囟鹊拇胧┤缦拢?)采用改善骨料級(jí)配,用干硬性混凝土,摻混合料,加引氣劑或塑化劑等措施以減少混凝土中的水泥用量;(2)拌合混凝土?xí)r加水或用水將碎石冷卻以降低混凝土的澆筑溫度;(3)熱天澆筑混凝土?xí)r減少澆筑厚度,利用澆筑層面散熱;(4)在混凝土中埋設(shè)水管,通入冷水降溫;(5)規(guī)定合理的拆模時(shí)間,氣溫驟降時(shí)進(jìn)行表面保溫,以免混凝土表面發(fā)生急劇的溫度梯度;(6)施工中長(zhǎng)期暴露的混凝土澆筑塊表面或薄壁結(jié)構(gòu),在寒冷季節(jié)采取保溫措施;改善約束條件的措施是(1)合理地分縫分塊;(2)避免基礎(chǔ)過大起伏;(3)合理的安排施工工序,避免過大的高差和側(cè)面長(zhǎng)期暴露;此外,改善混凝土的性能,提高抗裂能力,加強(qiáng)養(yǎng)護(hù),防止表面干縮,特別是保證混凝土的質(zhì)量對(duì)防止裂縫是十分重要,應(yīng)特別注意避免產(chǎn)生貫穿裂縫,出現(xiàn)后要恢復(fù)其結(jié)構(gòu)的整體性是十分困難的,因此施工中應(yīng)以預(yù)防貫穿性裂縫的發(fā)生為主。在混凝土的施工中,為了提高模板的周轉(zhuǎn)率,往往要求新澆筑的混凝土盡早拆模。當(dāng)混凝土溫度高于氣溫時(shí)應(yīng)適當(dāng)考慮拆模時(shí)間,以免引起混凝土表面的早期裂縫。新澆筑早期拆模,在表面引起很大的拉應(yīng)力,出現(xiàn)“溫度沖擊”現(xiàn)象。在混凝土澆筑初期,由于水化熱的散發(fā),表面引起相當(dāng)大的拉應(yīng)力,此時(shí)表面溫度亦較氣溫為高,此時(shí)拆除模板,表面溫度驟降,必然引起溫度梯度,從而在表面附加一拉應(yīng)力,與水化熱應(yīng)力迭加,再加上混凝土干縮,表面的拉應(yīng)力達(dá)到很大的數(shù)值,就有導(dǎo)致裂縫的危險(xiǎn),但如果在拆除模板

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