On the construction temperature and cracks of conc

2022-10-15
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Discussion on the construction temperature and cracks of concrete details

concrete plays an important role in modern engineering construction. Today, concrete cracks are more common, and cracks are almost everywhere in bridge engineering. Although we take various measures and are cautious in the construction, cracks still occur from time to time. One of the reasons is that we do not pay enough attention to the change of concrete temperature stress

in mass concrete, temperature stress and temperature control are of great significance. This is mainly due to two reasons. Firstly, temperature cracks often appear in concrete during construction, which affects the integrity and durability of the structure. Secondly, during the operation, the temperature change has a significant influence on the stress state of the structure. We mainly encounter temperature cracks in construction, so this paper only discusses the causes and treatment measures of concrete cracks in construction

1 causes of cracks

there are many reasons for cracks in concrete, mainly including changes in temperature and humidity, brittleness and non-uniformity of concrete, unreasonable structure, unqualified raw materials (such as alkali aggregate reaction), formwork deformation, uneven settlement of foundation, etc

during the hardening of concrete, the cement releases a large amount of hydration heat, and the internal temperature continues to rise, causing tensile stress on the surface. In the later cooling process, due to the constraints of the foundation or old concrete, tensile stress will appear in the concrete. The decrease of temperature will also cause great tensile stress on the concrete surface. When these tensile stresses exceed the crack resistance of concrete, cracks will appear. The internal humidity of many concrete changes little or slowly, but the surface humidity may change greatly or violently. For example, if the curing is not complete, the surface drying shrinkage deformation is constrained by the internal concrete, which often leads to cracks. Concrete is a brittle material, and its tensile strength is about 1/10 of the compressive strength. The ultimate tensile deformation during short-term loading is only (0.6 ~ 1.0) × 104. The limit extension deformation during long-term loading is only (1.2 ~ 2.0) when it is used on the car with the change of loading × 104。 Due to uneven raw materials, unstable water cement ratio, and segregation during transportation and pouring, the tensile strength of the same concrete is uneven, and there are many weak parts with low tensile capacity and prone to cracks. In reinforced concrete, tensile stress is mainly borne by reinforcement, while concrete only bears compressive stress. If tensile stress occurs in the structure at the edge of plain concrete or reinforced concrete, it must be borne by the concrete itself. In general, no tensile stress or only a small tensile stress is required in the design. However, during construction, when the concrete is cooled from the highest temperature to the stable temperature during operation, it often causes considerable tensile stress in the concrete. Sometimes the temperature stress can exceed the stress caused by other external loads. Due to the influence of factors such as the sluggish downstream demand at the same time, it is very important to master the variation law of temperature stress for reasonable structural design and construction

2 analysis of temperature stress

according to the formation process of temperature stress, it can be divided into the following three stages:

(1) early stage: from the beginning of concrete pouring to the end of cement heat release, generally about 30 days. There are two characteristics of this stage, one is that the cement emits a large amount of hydration heat, and the other is the sharp change of elastic modulus on coagulation. Due to the change of elastic modulus, residual stress is formed in the concrete during this period

(2) medium term: from the end of the exothermic effect of cement to the time when the concrete cools to a stable temperature, during this period, the temperature stress is mainly caused by the cooling of concrete and the change of external temperature. These stresses are superimposed with the residual stress formed in the early stage, and the elastic modulus of concrete changes little during this period

(3) late stage: the operation period after the concrete is completely cooled. The temperature stress is mainly caused by the change of external temperature, and these stresses are superimposed with the first two residual stresses

according to the causes of temperature stress, it can be divided into two categories:

(1) autogenous stress: the structure without any constraint or completely static on the boundary. If the internal temperature is non-linear distribution, the temperature stress due to the mutual constraint of the structure itself is used to replace the metal parts. For example, the structural size of bridge pier shaft is relatively large. When the concrete is cooled, the surface temperature is low and the internal temperature is high. Tensile stress appears on the surface and compressive stress appears in the middle

(2) restraint stress: the stress caused by the fact that all or part of the boundary of the structure is restrained by the outside world and cannot deform freely. Such as box girder roof concrete and guardrail concrete

these two kinds of temperature stress often work together with the stress caused by the drying shrinkage of concrete

it is a complex work to accurately analyze the distribution and size of temperature stress according to the known temperature. In most cases, model tests or numerical calculations are needed. The creep of concrete causes considerable relaxation of temperature stress. When calculating temperature stress, the influence of creep must be considered. The specific calculation will not be described in detail here

3 temperature control and measures to prevent cracks

in order to prevent cracks and reduce temperature stress, we can start from two aspects: controlling temperature and improving constraints

the measures to control the temperature are as follows:

(1) measures such as improving aggregate gradation, using dry and hard concrete, mixing mixtures, adding air entraining agent or plasticizer, etc. are taken to reduce the amount of cement in concrete

(2) when mixing concrete, add water or cool the gravel with water to reduce the pouring temperature of concrete

(3) when pouring concrete in hot weather, reduce the pouring thickness and use the pouring layer to dissipate heat

(4) bury water pipes in the concrete and pour cold water to cool down

(5) specify a reasonable formwork removal time, and conduct surface insulation when the temperature drops sharply to avoid sharp temperature gradient on the concrete surface

(6) for the concrete pouring block surface or thin-walled structure exposed for a long time in the construction, thermal insulation measures shall be taken in the cold season

the measures to improve the constraint conditions are:

(1) reasonably dividing seams and blocks

(2) avoid excessive fluctuation of foundation

(3) reasonably arrange the construction process to avoid excessive height difference and long-term exposure of the side

in addition, it is very important to improve the performance of concrete, improve the crack resistance, strengthen curing, prevent surface drying shrinkage, especially to ensure the quality of concrete to prevent cracks. Special attention should be paid to avoid through cracks. It is very difficult to restore the integrity of its structure after they occur, so the prevention of through cracks should be given priority in construction

in the construction of concrete, in order to improve the turnover rate of formwork, it is often required that the newly poured concrete be removed as soon as possible. When the concrete temperature is higher than the temperature, the formwork removal time shall be properly considered to avoid early cracks on the concrete surface. The early formwork removal of new pouring causes great tensile stress on the surface, resulting in the phenomenon of "temperature impact". At the initial stage of concrete pouring, due to the dissipation of hydration heat, the surface causes considerable tensile stress. At this time, the surface temperature is also higher than the air temperature. At this time, when the formwork is removed, the surface temperature will drop sharply, which will inevitably cause a temperature gradient. Thus, a tensile stress is added to the surface, which is superimposed with the hydration heat stress, plus the drying shrinkage of concrete. When the tensile stress on the surface reaches a large value, there is a risk of cracks, However, if a light insulation material, such as foam sponge, is covered on the surface in time after the formwork is removed, it has a significant effect on preventing excessive tensile stress on the concrete surface

reinforcement has little effect on the temperature stress of mass concrete, because the reinforcement ratio of mass concrete is very low. It only affects ordinary reinforced concrete. When the temperature is not too high and the stress is lower than the yield limit, the properties of the steel are stable, and have nothing to do with the stress state, time and temperature. The difference between the linear expansion coefficient of steel and that of concrete is very small, and there is only a small internal stress between them when the temperature changes. Because the elastic modulus of steel is 7~15 times that of concrete, when the internal concrete stress reaches the tensile strength and cracks, the stress of reinforcement will not exceed 100~200kg/cm2... Therefore, it is difficult to prevent the occurrence of small cracks by using the characteristic reinforcement of steel pressure blasting tester in concrete. However, after reinforcement, the cracks in the structure generally become more numerous, smaller spacing, and smaller width and depth. Moreover, if the diameter of reinforcement is thin and the spacing is dense, the effect of improving the crack resistance of 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 such cracks are generally shallow, they still have a certain impact 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 correct use of admixtures is also one of the measures to reduce cracking. For example, using water reducing and anti cracking agent, the author summarizes its main functions in practice as follows:

(1) there are a large number of capillary channels in the concrete, and capillary tension is generated in the capillary after water evaporation, which makes the concrete dry shrinkage and deformation. Increasing the capillary pore size can reduce the capillary surface tension, but it will reduce the strength of concrete. This surface tension theory has been recognized internationally as early as the 1960s

(2) the water cement ratio is an important factor affecting the shrinkage of concrete. The use of water reducing and crack preventing agents can reduce the water consumption of concrete by 25%

(3) the amount of cement is also an important factor in the shrinkage rate of concrete. The concrete mixed with reduced water cracking agent can reduce the amount of cement by 15% under the condition of maintaining the strength of concrete, and its volume can be supplemented by increasing the amount of aggregate

(4) water reducing and anti cracking agent can improve the consistency of cement slurry, reduce concrete bleeding and reduce shrinkage deformation

(5) improve the adhesion between cement slurry and aggregate and improve the crack resistance of concrete

(6) concrete is restrained during shrinkage to produce tensile stress, and cracks will occur when the tensile stress is greater than the tensile strength of concrete. Water reducing and crack prevention agent can effectively improve the tensile strength of concrete and greatly improve the crack resistance of concrete

(7) adding admixtures can improve the compactness of concrete, effectively improve the carbonation resistance of concrete and reduce carbonation shrinkage

(8) the retarding time of concrete after adding water crack prevention agent is appropriate. On the basis of effectively preventing the rapid hydration and heat release of cement, the increase of plastic shrinkage caused by long-term non setting of cement is avoided

(9) the concrete mixed with admixtures has good workability, the surface is easy to touch flat, forming a micro film, reducing water evaporation and drying shrinkage

many admixtures have the functions of retarding, increasing workability and improving plasticity. We should carry out more experimental comparison and research in this aspect in engineering practice, which may be simpler and more economical than simply improving external conditions

4 early curing of concrete

practice has proved that most of the common cracks in concrete are surface cracks of different depths. The main reason is that the sudden drop of temperature in cold areas caused by temperature gradient is also easy to form cracks. Therefore, the insulation of concrete is particularly important to prevent early cracks on the surface

from the point of view of temperature stress, the insulation should meet the following requirements:

1) prevent the temperature difference inside and outside the concrete and the gradient of the concrete surface

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