Construction temperature and crack of concrete
      Concrete plays an important role in the construction of modern engineering.Today, cracks in concrete are common, and cracks are almost ubiquitous in bridge engineering.Although we have taken various measures in the construction, careful and cautious, the cracks still appear.The reason is that the change of temperature stress of concrete is not one of them.
      In bulk concrete, temperature stress and temperature control are of great significance.This is mainly due to two reasons.First of all, concrete in construction often appears temperature crack, which affects the integrity and durability of the structure.Secondly, during the operation, the temperature change has significant influence on the stress state of the structure.The main problems we encounter are temperature cracks in construction, so this paper only discusses the causes and treatment of concrete cracks in construction.
1. reasons for cracks
      Cracks in concrete has a variety of reasons, mainly is the change of temperature and humidity, the brittleness of concrete and the non-uniformity, and the structure is unreasonable, unqualified raw materials (such as alkali aggregate reaction), template deformation, foundation uneven settlement, etc.
      During hardening of concrete, the cement releases a lot of hydration heat, and the internal temperature rises constantly, causing tensile stress on the surface.Later in the cooling process, due to the constraints of the base or old mixing, the tensile stress will occur within the concrete.The decrease of temperature will also cause a large tensile stress on the concrete surface.Cracks occur when these tensile stresses exceed the ability of the concrete to crack.The internal humidity of many concrete is small or slow, but the surface humidity may vary greatly or dramatically.If the curing is not week, the time is wet, the surface dry shrinkage deformation is restrained by the internal concrete, also often leads to the crack.Concrete is a brittle material, tensile strength is about 1/10 of the compressive strength, the limit of the short-term load tensile deformation only (0.6 ~ 1.0) by 104, when the load over a long period of time limit of a stretching deformation only (1.2 ~ 2.0) x 104. Because of the uneven raw materials, water cement ratio is not stable, and segregation phenomenon in the process of transporting and pouring, in the same piece of concrete in its tensile strength is uneven, there is a lot of tensile capacity low, easy to crack the weak positions.In reinforced concrete, the tensile stress is mainly borne by the steel bar, and the concrete is simply subjected to compressive stress.If tensile stress occurs in the edge part of the concrete or the concrete, it must be borne by the concrete itself.In general design, there is no tensile stress or only small tensile stress.However, during the construction, the concrete is cooled to the stable temperature during the operation period, which usually causes considerable tensile stress inside the concrete.Sometimes temperature stress can exceed the stress caused by other external loads, so it is very important to master the changing rules of temperature stress for the rational structure design and construction.
2. analysis of temperature stress
      According to the forming process of temperature stress can be divided into three stages:
      (1) early days: generally about 30 days, from concrete start to cement release.The two characteristics of this stage are that the cement releases a large amount of hydration heat, and the second is a drastic change in the elastic modulus of the coagulation.Due to the change of elastic modulus, residual stress is formed during this period in concrete.
      (2) the middle: since cement at the end of the exothermic effect basic to concrete cooling when stable temperature in this period, the temperature stress is mainly caused by the cooling and outside temperature change of concrete, the stress and the early formation of residual stress superposition, during that time, the elastic modulus on the coagulation.
      (3) late stage: the period after the concrete has been completely cooled.The temperature stress is mainly caused by external temperature change, which is superimposed on the residual stress of the first two.
      According to the causes of temperature stress, the causes can be divided into two categories:
      (1) self-generating stress: there is no constraining or completely static structure on the boundary, and if the internal temperature is nonlinear, the temperature stress will occur due to the constraints of the structure itself.For example, the bridge pier is relatively large in size, and the surface temperature is low when the concrete is cooled, the internal temperature is high, and the surface appears tensile stress, and the compressive stress occurs in the middle.
(2) constraint stress: all or part of the structure of the structure is constrained by the outside world and cannot be free to deform.Such as box girder roof concrete and guardrail concrete.
      These two temperature stresses are often combined with the stress caused by dry shrinkage of concrete.
      It is a complicated task to analyze the distribution and size of temperature stress according to the known temperature.In most cases, model tests or numerical calculations are required.The creep of concrete causes the temperature stress to be quite loose. When calculating the temperature stress, the influence of creep must be considered, and the calculation is not discussed in detail.
3. control of temperature and measures to prevent cracks
      In order to prevent cracks, the temperature stress can be reduced from two aspects: controlling temperature and improving constraint condition.
      The measures to control temperature are as follows:
      (1) adopt measures to improve aggregate grading, use hard and hard concrete, mix materials, and add air or plasticizers to reduce the amount of cement in concrete;
      (2) adding water or water for mixing concrete to reduce the concrete pouring temperature;
      (3) when pouring concrete in hot weather, it can reduce the thickness of pouring and heat dissipation of the pouring surface.
      (4) bury water pipes in concrete and cool them in cold water;
      (5) it provides reasonable time for the removal of mold, and the surface insulation is carried out when the temperature drops sharply, so as to avoid a sharp temperature gradient on the concrete surface.
      (6) concrete pouring surface or thin-wall structure exposed in the medium to long term, taking heat preservation measures in cold season;
      The measures to improve the constraints are:
      (1) to divide and divide reasonably;
      (2) avoid excessive foundation fluctuations;
      (3) reasonable arrangement of construction procedures to avoid excessive high and lateral long-term exposure;
      In addition, to improve the performance of the concrete, improve anti-cracking ability, strengthen the maintenance, to prevent surface shrinkage, in particular, guarantee the quality of concrete to prevent cracks is very important, should pay special attention to avoid to produce cracks, appeared to restore the integrity of the structure are very difficult, so should give priority to in order to prevent the happening of the perfoliate crack during construction.
      In the construction of concrete, in order to improve the turnover rate of the template, it is often required that the newly cast concrete should be removed as soon as possible.When the concrete temperature is higher than the temperature, the time should be taken into account to avoid the early cracks in the concrete surface.The new casting early disassembly, which causes a large tensile stress on the surface, shows the phenomenon of "temperature impact".At the beginning of the concrete       pouring, due to the hydration heat loss, caused considerable tensile stress in the surface, the surface temperature in the temperature is high, the demolition of the template, surface temperature, inevitably cause temperature gradient, thus additional tensile stress, the surface stress superposition and hydration heat, coupled with the drying shrinkage of concrete, the surface tensile stress to reach large number, there is the risk of fracture, but if on the surface after dismantling templates covering a lightweight thermal insulation material, such as foam sponge, to prevent excessive tensile stress in concrete surface, has a significant effect.
      The reinforcement has little effect on the temperature stress of large concrete, because the high volume of concrete has very low.It is only for general reinforced concrete.The properties of steel are stable and independent of stress state, time and temperature under conditions of not too high temperature and stress below the yield limit.The linear expansion coefficient of steel is very different from that of the concrete line, and it only has a small internal stress when the temperature changes.Because the elastic modulus of steel is 7 ~ 15 times that of concrete elastic modulus, the stress of reinforcement will not exceed 100 ~ 200kg/cm2 when the inner concrete stress reaches the tensile strength.Therefore, it is difficult to use reinforcement in concrete to prevent the occurrence of small cracks.However, the cracks in the reinforced posterior structure tend to be large, small, width and depth.Moreover, if the diameter of the steel bar is thin and the spacing is close, the effect on the crack resistance of the concrete is better.Thin and shallow cracks often occur on the surface of concrete and reinforced concrete structures, most of which belong to dry shrinkage cracks.Although the crack is generally shallow, it still has some influence on the strength and durability of the structure.
      In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the proper use of admixture is also one of the measures to reduce cracking.For example, the author summarized the main function of the use of water-reducing anti-cracking agent.
      (1) there is a large number of capillary channels in the concrete, and the capillary tension in the capillaries after the evaporation of water causes the concrete to shrink and deform.Increasing capillary diameter reduces capillary surface tension, but reduces the strength of concrete.This theory of surface tension was recognized internationally as early as the 1960s.
      (2) water-cement ratio is an important factor affecting the shrinkage of concrete, and the use of water-reducing anti-cracking agent can reduce the water consumption of concrete by 25%.
      (3) the cement dosage is also an important factor of concrete shrinkage, mixing and subtract water of concrete anti-cracking agent under the condition of keeping concrete strength can decrease the cement dosage of 15%, its volume by increasing aggregate amount to supplement.
      (4) reducing water anticracking agent can improve the consistency of cement slurry, reduce the concrete's bleeding and reduce shrinkage and deformation.
      (5) improve the bonding strength of cement slurry and aggregate, and improve the anti-cracking performance of concrete.
      (6) the concrete is constrained to produce tensile stress when it contracts, and cracks are created when the tensile stress is greater than the tensile strength of concrete.The anticracking agent can effectively improve the tensile strength of concrete and greatly improve the crack resistance of concrete.
      (7) adding admixtures can make the concrete compactly good, and can effectively improve the anti-carbonization of concrete and reduce the carbonization shrinkage.
      (8) the concrete retarded time after mixing with the anticracking agent is suitable, and the plastic shrinkage of the cement can be increased without the long-term non-coagulation of cement.
      (9) mixing with admixtures concrete and easy to be good, the surface easy to touch, forming microfilm, reducing water evaporation, reducing drying shrinkage.
      Many admixture have retarded, increase the workability and improve the function of the plastic, we should be more in the engineering practice of experiment contrast and research in this respect, than just by improving external conditions, could be more simple and economic.
4. early curing of concrete
      It is proved that the common cracks in concrete are mostly surface cracks of different depths, and the main reason is that the temperature gradient caused by the temperature gradient can also create cracks.Therefore, the thermal insulation of concrete is especially important to prevent early cracks in the surface.
      From the point of view of temperature stress, insulation should meet the following requirements:
      1) prevent internal and external temperature difference and surface gradient of concrete to prevent surface cracks.
      2) to prevent the concrete from being too cold, try to make the minimum temperature of the concrete construction period not lower than the stable temperature of the concrete.
      3) prevent old concrete from being too cold to reduce the constraint of new and old concrete.
      The early curing of concrete, the main purpose is to keep the suitable temperature humidity conditions, in order to achieve the effect of two aspects, on the one hand, the concrete from adverse temperature and humidity deformation, prevent harmful cold and dry shrinkage.On the one hand, the cement hydration is carried out smoothly with a view to achieving the design strength and crack resistance.
      The appropriate temperature and humidity conditions are interrelated.The thermal insulation of coagulation often also has the effect of moisturizing.
      Theoretically, the water contained in the newly poured concrete can satisfy the requirement of cement hydration.However, the water loss caused by evaporation is often caused by water loss, which can delay or prevent the hydration of cement, and the surface concrete is most easily and directly affected by this.Therefore, the first few days after concrete pouring are the key period of maintenance, and should be taken seriously in the construction.