論混凝土的施工溫度與結(jié)構(gòu)裂縫

1、On the construction of concrete temperature and cracks1. the causes of the cracksCracks in concrete are a variety of reasons, mainly temperature and humiditychanges, brittleand uneven concrete, as well as the structure is irrational,failure of rawmaterials (such as alkali-aggregate reaction), templa

2、tedeformation, andso on the basis of differential settlement.The concrete to harden olarge number of cement during the hydration heat,theernal temperature rising on the surface caused by tensile stress. Latehe cooling pros, due to coagulation on the basis of or be bound by theold, will emergehe conc

3、rete tensile stress. Lower temperaturesheconcrete surface will be a lot of tensile stress. When the tensile stressthe capacity of concrete crack, the cracks will appear. A number of concretechangesheernal humidity or changes very little slow, butsible changesin surface humidity or more dramatic chan

4、ges. Such as conservation, failed tokeep dry when wet, surfahrinkage deformation of concrete subject toernalconstras, but also often leads to cracks. Short-term loading at the time ofultimate tensile deformation only (0.6 1.0) 104, the limit of long-termloading spawhen the only stretch deformation (

5、1.2 2.0) 104. Unevenlyas a result of raw materials, water-cement ratio of instability, andtransporion and pouringhe pros of segregation phenomena,he samepiece of concretehe tensile strength is uneven, there are many low tensilecapacity, easy to fracture in a weakition. In reinforced concrete, the te

6、nsilestress is mainly borne by the steel, concrete ibject to compressive stress.In plain concrete or reinforced coagulation onthe edge of the site if thestructure of the tensile stress appears to berelying on its own concretecommitment.2. thermal stressysisAccording to the formation of thermal stres

7、s can be dividedo the followingthree stages:(1)early: Since the beginning of pouring concrete to cementofexothermic basic general about 30 days. Two characteristics of this stage,to release a large number of cement hydration heat, and the other is the elasticmodulus of coagulation dramatic changes.

8、Modulus of elasticityas a result ofchanges withhis periodhe formation of residual stresshe concrete.(2) medium-term: from the basic role of cement heat untilof the stabletemperature of the concrete cooling time, during this period,the temperaturestress was mainly due to cooling and the outside concr

9、ete caused by temperaturechanges, stress and early formation of these residual stresposition phase,during which the coagulation of the elastic modulus changed little.(3) late: concrete compley cooling period after the operation. Thermalstress is mainly caused by external temperature changes, the str

10、ess and thetwo-phase suposition of residual stress.Under the thermal stress caused by the reasons can be dividedo twocategories:(1) self-stress:there is no constraon the border or compley sicstructure,if theernal temperature distribution is nonlinear, due tostructuralconstras arising from their co-t

11、emperature stress. For exle,the converlayerof its relatively larger size, concrete cooling surfacetemperature low, theernal temperatures are high, the surface tensile stress,compressive stress appearshe middle.(2) bound by stress: the structure of all or part of the boundary bound bythe outside worl

12、d can not be free-form deformation caused by stress. Roof box,such as concrete and concrete barrier.Both the temperature and the concrete stress is often caused by shrinkageof the combined effect of stress.Known to be in accordance win accurateysis of the temperature ofthe thermal stress distributio

13、n, size is a more complex task.ost cases, theneed to rely on mtest or numerical calculation. Creep of concrete sotthe temperature there is considerable stress relaxation, thermal stresscalculation, we must consider the effects of creep, is no longer calculated indetail here.3. temperature control an

14、d measures to prevent the cracksIn order to prevent cracking, reduce the thermal stress can control theconditions of temperature and to improve the binding of two aspects.Temperature control measures are as follows:(1) used to improve the aggregate gradation, dry hard concrete, mixed mixture,add air

15、-entraining agent or plasticizer, etc. measures to reduce the amount ofcement concrete;(2) mixing concrete by adding water or cooling water will be gravel and pouringconcrete to reduce the temperature;(3) pouringhot days to reduce the pouring of concrete thickness, the useof pouring heevels;(4) laid

16、 inconcrete watipes, acs to cold water to cool;(5) the provis of reasonable time, the temperature drop when the surfaceinsulation, concrete surface in order to avoid a sharp temperature gradient;(6) Construction of long-term exure to block the surface of the concretepouring, or thin-walled structure

17、s, thermal insulationhe cold season to takemeasures;Measures to improve the constras are:(1) a reasonable sub-block jos;(2) basis to avoid too much ups and downs;(3) reasonable arrangements for the construction pros, to avoid exsiveside elevation and long-term exure;In addition, to improve the perfo

18、rmance of concrete to improve theracking ability, to strengthen the conservation, to prevent surface drying,in particular, is to ensure the quality of concrete is very important to preventcracks, should pay spel attention to avoid cross-cracks appears to restoretheegrity of its structure it is very

19、difficult, so the construction shouldbe to prevent the occurrence of cross-cutting the main crack.In concrete construction, in order to increase the turnover rate of thetemplate, often require new pouring concrete as soon assible. When theconcrete temperature is highern the temperature should be app

20、ropriate toconsider time, so as to avoid the early crackshe concrete surface. New pouringearly, a lot of the surface tensile stress, a temperature shock phenomenon.Concrete pouringhe early heat of hydration as a result of the disseminationof the surface caused by a large tensile stress, when the sur

21、face temperatureis highern the temperature at this time to remove the template, a sharp fallin surface temperature is bound to lead to temperature gradient, so an additionalone on the surface tensile stress, thermal stresposition and hydration,coupled with the shrinkage of concrete,Tensile stress to

22、 reach the surface of great value, there is the dangeroflead to cracks, but if in a timely manner after the removal of the templateonthe surface coverage of a light insulating material, such as foam sponge andsoon, have a concrete surface to prevent exsive tensile stress, with significantresults.Rei

23、nforced concrete on the large volume of thermal stress has little effectbecause the large volume of concrete reinforced wivery low rate. Only onteral impact of reinforced concrete. Not too highemperature and stressbelow the yield limit under the condition of the properties of steel is stable,and wit

24、h the stress, time and temperature has nothing to do. Linear expancoefficient of steel and concreinear expancoefficient difference betnthe small changesemperature betn the two with only a very small stress.Since the modulus of elasticity of steel to concrete elastic modulus of 7 15-fold,when the str

25、ess reached the tensile strength of concrete and cracking, the steelstress will not exceed 100 200kg/cm2 . So, Want to use in reinforced concrete to prevent the emergence of small cracksis difficult. However, after the reinforced structure of the number of cracksin general hase moren a small distanc

26、e, a smaller widnd depth. Andif the diameter of steel thin and close spacing, the crackof concreteto enhance the effect of better. Concrete and reinforced concrete structure ofthe surface often occur crackshin and shallow, most of them belonging toshrinkage cracks. While this is generally shallow cr

27、acks, but its strengnddurability of the structure is still a certain impact.In order to ensure the quality of concrete to prevent cracking and improvethe durability of concrete, the proper use of admixtures is also one of themeasures to reduce cracking. Such as the use ofracking agent and water,in p

28、ractice, its major role in concluding as follows:(1) the existence of a large number of poreshe concretet, afterevaporation of water generated by capillary capillary ten, deformation ofthe concrete shrinkage. Pores increased capillary diameter can reduce the surfaceten, but would reduce the strength

29、 of concrete.(2) the impact of water-cement ratio is an important factor for concreteshrinkage, the use ofracking agent can reduce water consumption reducedby 25% concrete.(3) the amount of cement concrete is also an important factor in shrinkage,water addition and subtraction mixed concrete crackin

30、g agentaaining thestrength of concrete under the condition of 15 percent can reduce the amount ofcement, aggregate size, by increasing the amount to supplement.(4)racking agent and water can improve the consistency of cement slurryto reduce the bleeding of concrete, reduhrinkage deformation Shen.(5)

31、 to improve and cement bonded aggregate, and enhance therackingperformance of concrete.(6) concrete boundhe contraction proded by tensilestress, when thetensile stress is greatern thetensile streth of concrewill be producedwhen the cracks. Water-reducing agent can be an effectiveracking of theconcre

32、te tensile strength to increase, a substantial increaseinrackingperformance of concrete.(7)pacting concrete admixtures can, and can effectively improvethe carbonationof concrete to reduce the carbonation shrinkage.(8)-doped water-reducing agent after the concrete cracking retardation timedue,he effe

33、ctive prevention of the rahydration heat of cement based oncement to avoid long-term result of non-condensable increase plastic shrinkage.(9)-doped workability of concrete admixtures, and easy to find flat surfato form a micro-membrane to reduce evaporation and reduce drying shrinkage.Many have a re

34、tarding admixture to increase the workability and improve thefunction of plasticity, we in engineering practicehis area should be morecontrast and research experimentsn simply relying on external conditions toimprove may be even more simple and economic.4. early concrete conservationPractice has pro

35、vedt the common concrete cracks, the majority of surfacecracks at different depths, mainly because of the temperature gradient causedby the cold temperature region is also vulnerable to a sharp fallhe formationof cracks. Insulation sot the concrete surface to prevent early cracking isteucngparticula

36、rly important.Thermal stress from the poof view, the insulation should meet thefollowing requirements:(1) to prevent the concrete inside and outside the concrete surfacetemperature difference and gradient, to prevent surface cracks.(2) to prevent the concrete super-cold, it should besible to make co

37、ncretethe construction period of not lessn the minimum temperature of concrete usedfor the stability of the temperature.(3) to prevent cold-old concrete in order to reduce the concrete betn theold and new constras.Early concrete conservation, the main objective is to maain theappropriate temperature

38、 and humidity conditions in order to achieve the effectof two aspects, on the one hand, the concrete from adverse temperature and humiditydeformation invato prevent the harmful and drying shrinkage. On the one hand,to enable the smooth progress of cement hydration, wiview to meet the designcapacity

39、of the strengnd crack.Appropriate conditions of temperatureand humidity areerrelated.Coagulation on the insulation often has theeffect of moisture.Theoreticalysis of fresh concrete containedhe water fully meetsthe requirements of cement hydration and moren. However, due to evaporationand other facto

40、rs often cause water loss, thereby delay or prevent the hydrationof cement, concrete surface and most likely to be directly affected by such anegative impact. Concrete pouring, therefore thefew days after the criticalperiod of conservationhe construction shouldpay attention to.5. concluding remarksA

41、bove the temperature of concrete and crackshe construction of therelation betn theory and practice of the preliminary study, although theacademic community about the causes of cracks in concrete and calculation methodsare different theories, but specific measures to prevent and improve the opinionit

42、 is quite uniform At the same time, the application in practice is also arelatively good effect, concrete construction and more depends on our observation,and more, when compared to multi-ieysis, and more sum up, combined witha wide range of prevention measures, the crackshe concrete is compleyavoid

43、ed.論混凝土的施工溫度與結(jié)構(gòu)裂縫1.裂縫的原因混凝土中產(chǎn)生裂縫有多種原因，主溫度和濕度的變化，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（如料反應(yīng)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，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

44、 110 左右，短期加荷時(shí)的極限拉伸變形只有（0.61.0）104，長期加荷時(shí)的極限位伸變形也只有（1.22.0）104.由于原材料不均勻，水灰比不穩(wěn)定，及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)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

45、應(yīng)力的變化規(guī)律對于進(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），模要是堿骨根據(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)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)要是外界氣溫變化所引起，這些應(yīng)力與前

46、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的結(jié)構(gòu)，如果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，轉(zhuǎn)換層其結(jié)構(gòu)尺寸相對較大，混凝土冷卻時(shí)表面溫度低，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恋氖箿囟葢?yīng)力有相當(dāng)大的松馳，計(jì)算溫度應(yīng)力時(shí)，必須考慮的影

47、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┤缦拢海?）采用改善骨料級配，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）施工中長期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；（

48、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)面長期；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，