Analysis and analysis of cracks in the main stress

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Analysis and control of cracks in the main stressed parts of high-rise concrete structures

the characteristics of materials in concrete engineering determine that the structure is prone to cracks. In practice, the probability of cracks in concrete during construction is also great. A considerable part of cracks do not do much harm to the stress and normal use of buildings, but the existence of cracks will affect the integrity and durability of buildings and corrode reinforcement, It is the hidden danger of stress concentration during the service period of stress, and attention should be paid to all aspects as far as possible to avoid the occurrence of cracks or control cracks within the allowable range

this paper analyzes the reasons why cracks are easy to occur in several main stress parts of high-rise building structures during concrete construction, and puts forward crack control measures from two aspects of design and construction

crack analysis of mass concrete

with the continuous increase of height in high-rise buildings, the basement is getting deeper and deeper, and the bottom plate is getting thicker and thicker. The bottom plate with a thickness of more than 3m is common. In high-rise buildings, the foundation slab is the main stress-bearing structure, and the overall requirements are high. First, 2. Protective measures should be taken during the experiment, such as installing safety wire mesh around the pressure testing machine, and the operator should conduct the experiment within the protection range of the safety mesh Three kinds of plastic materials have been applied to one-time integral pouring like 3D printers. A large number of practices at home and abroad have proved that various mass concrete cracks are mainly caused by temperature changes. After the pouring of mass concrete, due to its large volume, the hydration heat of cement gathered inside is not easy to dissipate in the heating stage, and the internal temperature of the concrete will rise significantly, resulting in compressive stress inside the concrete and tensile stress on the outer surface. At this time, due to the low strength of the concrete, surface cracks may occur. In the cooling stage, the shrinkage of fresh concrete cannot shrink freely due to strong foundation or foundation constraints. The temperature rise stage is fast, the elastic modulus of concrete is low, and the influence of creep is large, so the tensile stress generated during temperature drop is greater than the compressive stress generated during temperature rise. When the difference is too large, cracks will occur in the concrete, and finally through cracks may be formed. In order to solve the above two kinds of cracks, reasonable temperature control must be carried out

the main purpose of concrete temperature control is to make the tensile stress caused by temperature difference less than the standard value of the tensile strength of concrete to general flexible packaging manufacturers in the same period, and there is a certain measured value of the tensile strength of BJI joint, which should not be less than 135% safety factor of the standard value of the yield strength of the base metal of the reinforcement. In order to calculate the temperature difference, it is necessary to calculate the maximum temperature inside the concrete in advance, which is the sum of the concrete pouring temperature, the actual hydration heat rise and the concrete heat dissipation temperature. Reasonable selection of materials can be adopted to reduce the hydration heat of cement, optimize the mix proportion of concrete aggregate, control the water cement ratio and reduce the dry shrinkage of concrete. If possible, reducing the pouring length, increasing the curing time and reducing the cooling rate to reduce the relaxation coefficient is also of certain significance to control the through cracks

analysis of cracks in Basement Wallboard

the causes of cracks in Basement Wallboard and foundation mass concrete are the same, that is, shrinkage strain will occur due to water loss in the hardening process of concrete, and temperature strain will occur in the cooling process after the temperature rise caused by cement hydration heat reaches the highest point. But it has its own characteristics: first, the wallboard is greatly constrained by the foundation and the peripheral floor by the basement exterior wall. This constraint is far greater than the constraint of the pile foundation on the foundation, and the probability of through cracks is high. Second, the internal wall panels and floors are greatly affected by the ambient temperature. Third, the internal and external temperature difference is small, and the probability of surface cracks is small. Fourth, it is difficult to maintain, with fast heat dissipation and high cooling rate. It is difficult to take advantage of the relaxation and creep advantages of concrete, especially in the season of sudden temperature changes

crack analysis of high-strength concrete

at present, C40 ~ C60 medium and high-strength concrete has been widely used in high-rise buildings. With the rapid development of material science, C80 ~ C120 high-strength concrete has been applied in specific projects. Because the mix proportion design of high-strength concrete is mostly low water cement ratio, high-grade cement, high cement dosage, high-efficiency reducer and ultra-fine mineral powder. In this way, its shrinkage mechanism is different from that of ordinary concrete

the cement content of high-strength concrete is 1.5 ~ 2 times that of ordinary concrete. Thus, in the process of concrete formation, the volume shrinkage caused by cement hydration, namely self shrinkage, is greater than that of ordinary concrete, and the probability of shrinkage cracks is also greater than that of ordinary concrete

due to the use of high-grade cement and large amount, the hydration heat release of high-strength concrete is large during the hardening process of concrete, which will increase the maximum temperature rise of concrete, thus increasing the temperature shrinkage stress of concrete. When other factors are superimposed, it is likely to lead to temperature shrinkage cracks. Since the content of cement stone in high-strength concrete is 1.5 times that of ordinary concrete, the drying shrinkage caused by water evaporation in the early stage of hardening will also be greater than that of ordinary concrete

crack control measures

in order to avoid cracks or control cracks within the allowable range, we can start from two aspects of design and construction

first of all, the following measures can be taken in the design: 1. Add structural reinforcement to improve the crack resistance, and the reinforcement should adopt small diameter and small spacing. The reinforcement ratio of the whole section shall be between 0.3 and 0.5%. 2. Avoid stress concentration caused by structural mutation, and take strengthening measures in weak links prone to stress concentration. 3. Set concealed beams at the edge that is easy to crack, improve the reinforcement ratio of this part, and improve the ultimate tension of concrete. 4. In the structural design, the climatic characteristics during construction should be fully considered, and the post pouring joints should be reasonably set. Under normal construction conditions, the spacing of post pouring joints is 20 ~ 30m, and the retention time is generally not less than 60 days. If the specific conditions during construction cannot be predicted, design changes can also be made temporarily according to specific conditions

secondly, the construction measures include strictly controlling the quality and technical standards of concrete raw materials, selecting low hydration heat cement, and minimizing the mud content of coarse and fine aggregates. Carefully analyze the proportion of concrete aggregate, control the water cement ratio of concrete, reduce the slump of concrete, and reasonably add plasticizer and reducer. The pouring time shall be arranged at night as far as possible to minimize the initial setting temperature of concrete. During daytime construction, it is required to set up simple shading devices in sand and stone yards, or cover them with wet gunny bags, and spray cold water on aggregates when necessary. When pumping concrete, cover the horizontal and vertical pump pipes with straw bags and spray cold water. According to the characteristics of the project, the late strength of concrete can be used, which can reduce water consumption, hydration heat and shrinkage. Strengthen the pouring and vibration of concrete to improve the compactness. The concrete formwork shall be removed as late as possible, the concrete surface temperature shall not drop more than 15 ℃ after formwork removal, and the strength of the concrete on-site test block shall not be lower than C5. The twice vibrating technology is adopted to improve the concrete strength and crack resistance. According to the specific engineering characteristics, UEA shrinkage compensation concrete technology is adopted. For high-strength concrete, we should try to use moderate heat micro expansion cement, mixed with ultra-fine mineral powder and expansion agent, and use high-efficiency water reducer. Through the test, fly ash is mixed, and the mixing amount is 15% - 50%

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