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1、 Based upon analysis of the reasons caused the cracks of concrete the temperature stress of concrete is studied.The control measures of temperature stress as well as preventive measures of cracks are elaborated in detail in order to avoid production of cracks in construction process to ensure constr

2、uction quality.Key words: concrete ,temperature stress, crack, constructionIn the mass concrete, temperature stress and temperature control system has great significance. There are two reasons:1）In the construction of concrete it often appear temperature cracks, which affects the integrity and durab

3、ility of the structure;2） In the operation process, the temperature changes have influence on stress state of structure which cannot be ignored. Since temperature crack caused in the construction is mainly met , the causes of cracks and Processing measures in the construction is explored in this pap

4、er. 1.the causes of the cracks Cracks in concrete are a variety of reasons, mainly temperature and humidity changes, brittle and uneven concrete, as well as the structure is irrational, failure of raw materials （such as alkali-aggregate reaction）, template deformation, and so on the basis of differe

5、ntial settlement.The concrete to harden off a large number of cement during the hydration heat, the internal temperature rising on the surface caused by tensile stress. Late in the cooling process, due to coagulation on the basis of or be bound by the old, will emerge in the concrete tensile stress.

6、 Lower temperatures in the concrete surface will be a lot of tensile stress. When the tensile stress beyond the capacity of concrete crack, the cracks will appear. A number of concrete changes in the internal humidity or changes very little slow, but possible changes in surface humidity or more dram

7、atic changes. Such as conservation, failed to keep dry when wet, surface shrinkage deformation of concrete subject to internal constraints, but also often leads to cracks. Short-term loading at the time of ultimate tensile deformation only （0.6 1.0） 104, the limit of long-term loading spaces when th

8、e only stretch deformation （1.2 2.0） 104. Unevenly as a result of raw materials, water-cement ratio of instability, and transportation and pouring in the process of segregation phenomena, in the same piece of concrete in the tensile strength is uneven, there are many low tensile capacity, easy to fr

9、acture in a weak position. In reinforced concrete, the tensile stress is mainly borne by the steel, concrete is subject to compressive stress. In plain concrete or reinforced coagulation on the edge of the site if the structure of the tensile stress appears to be relying on its own concrete commitme

10、nt.2. thermal stress analysisAccording to the formation of thermal stress can be divided into the following three stages:（1） early: Since the beginning of pouring concrete to cement the end of exothermic basic general about 30 days. Two characteristics of this stage, first, to release a large number

11、 of cement hydration heat, and the other is the elastic modulus of coagulation dramatic changes. Modulus of elasticity as a result of changes within this period in the formation of residual stress in the concrete.（2） medium-term: from the basic role of cement heat until the end of the stable tempera

12、ture of the concrete cooling time, during this period, the temperature stress was mainly due to cooling and the outside concrete caused by temperature changes, stress and early formation of these residual stress superposition phase, during which the coagulation of the elastic modulus changed little.

13、（3） late: concrete completely cooling period after the operation. Thermal stress is mainly caused by external temperature changes, the stress and the first two-phase superposition of residual stress.Under the thermal stress caused by the reasons can be divided into two categories:（1） self-stress: th

14、ere is no constraint on the border or completely static structure, if the internal temperature distribution is nonlinear, due to structural constraints arising from their co-temperature stress. For example, the conversion layer of its relatively larger size, concrete cooling surface temperature low,

15、 the internal temperatures are high, the surface tensile stress, compressive stress appears in the middle.（2） bound by stress: the structure of all or part of the boundary bound by the outside world can not be free-form deformation caused by stress. Roof box, such as concrete and concrete barrier.Bo

16、th the temperature and the concrete stress is often caused by shrinkage of the combined effect of stress.Known to be in accordance with an accurate analysis of the temperature of the thermal stress distribution, size is a more complex task. In most cases, the need to rely on model test or numerical

17、calculation. Creep of concrete so that the temperature there is considerable stress relaxation, thermal stress calculation, we must consider the effects of creep, is no longer calculated in detail here.3. temperature control and measures to prevent the cracksIn order to prevent cracking, reduce the

18、thermal stress can control the conditions 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-entraining agent or plasticizer, etc. measures to reduce the amount of

19、 cement concrete;（2） mixing concrete by adding water or cooling water will be gravel and pouring concrete to reduce the temperature;（3） pouring hot days to reduce the pouring of concrete thickness, the use of pouring heat levels;（4） laid in concrete water pipes, access to cold water to cool;（5） the

20、provisions of reasonable time, the temperature drop when the surface insulation, concrete surface in order to avoid a sharp temperature gradient;（6） Construction of long-term exposure to block the surface of the concrete pouring, or thin-walled structures, thermal insulation in the cold season to ta

21、ke measures;Measures to improve the constraints are:（1） a reasonable sub-block joints;（2） basis to avoid too much ups and downs;（3） reasonable arrangements for the construction process, to avoid excessive side elevation and long-term exposure;In addition, to improve the performance of concrete to im

22、prove the anti-cracking ability, to strengthen the conservation, to prevent surface drying, in particular, is to ensure the quality of concrete is very important to prevent cracks, should pay special attention to avoid cross-cracks appears to restore the integrity of its structure it is very difficu

23、lt, so the construction should be to prevent the occurrence of cross-cutting the main crack.In concrete construction, in order to increase the turnover rate of the template, often require new pouring concrete as soon as possible. When the concrete temperature is higher than the temperature should be

24、 appropriate to consider time, so as to avoid the early cracks in the concrete surface. New pouring early, a lot of the surface tensile stress, a temperature shock phenomenon. Concrete pouring in the early heat of hydration as a result of the dissemination of the surface caused by a large tensile st

25、ress, when the surface temperature is higher than the temperature at this time to remove the template, a sharp fall in surface temperature is bound to lead to temperature gradient, so an additional one on the surface tensile stress, thermal stress superposition and hydration, coupled with the shrink

26、age of concrete,Tensile stress to reach the surface of great value, there is the danger of lead to cracks, but if in a timely manner after the removal of the template on the surface coverage of a light insulating material, such as foam sponge and so on, have a concrete surface to prevent excessive t

27、ensile stress, with significant results.Reinforced concrete on the large volume of thermal stress has little effect because the large volume of concrete reinforced with a very low rate. Only on the general impact of reinforced concrete. Not too high in temperature and stress below the yield limit un

28、der the condition of the properties of steel is stable, and with the stress, time and temperature has nothing to do. Linear expansion coefficient of steel and concrete linear expansion coefficient difference between the small changes in temperature between the two with only a very small stress. Sinc

29、e the modulus of elasticity of steel to concrete elastic modulus of 7 15-fold, when the stress reached the tensile strength of concrete and cracking, the steel stress will not exceed 100 200kg/cm2 . So, Want to use in reinforced concrete to prevent the emergence of small cracks is difficult. However

30、, after the reinforced structure of the number of cracks in general has become more than a small distance, a smaller width and depth. And if the diameter of steel thin and close spacing, the crack resistance of concrete to enhance the effect of better. Concrete and reinforced concrete structure of t

31、he surface often occur cracks in thin and shallow, most of them belonging to shrinkage cracks. While this is generally shallow cracks, but its strength and durability of the structure is still a certain impact.In order to ensure the quality of concrete to prevent cracking and improve the durability

32、of concrete, the proper use of admixtures is also one of the measures to reduce cracking. Such as the use of anti-cracking agent and water, in practice, its major role in concluding as follows:（1） the existence of a large number of pores in the concrete that, after evaporation of water generated by capillary capillary tension, deformation of the concrete shrinkage. Pores increased capillary diameter can reduce the surface tension, but would reduce the strength of concrete.（2） the impact of water-cement rat