1、Construction cracks in concrete, causes and prevention Construction cracks of concrete reason for the external and internal. The initial mainly refers to the internal defects of concrete in early concrete, its internal hardening process will exist numerous micro-pores, air-pocket and micro cracks, E
2、xternal refers to the load reason: temperature crack, dry shrinkage effect etc. In these, can produce various forms of concrete cracks. To grasp the construction process of raw materials, in pouring process configuration of progress and thickness, grasp timely maintenance work well, which can avoid
3、concrete cracks. A, The cause of the tear 1.The initial defects of concrete Concrete is composed of cement, sand aggregate, mixed with water and other materials and formation of heterogeneity brittle materials. In the beginning, because hardened concrete construction and deformation, a series of pro
4、blems, such as constraint in the concrete shape sclerosis will exist numerous micro-pores, air-pocket and micro cracks of concrete that usually called initial defects. Micro cracks is usually a cracks in the concrete, bearing, and some other function not harm. But under load, temperature of concrete
5、, shrinkage and function, micro cracks will be expanded and connectivity, and finally form the visible cracks of concrete, macroscopic crack, said it was the use of concrete structure, properties and durability of adverse effects. 2.The load factor Due to the tensile strength of concrete structure o
6、r structures, small as a part of the external effects of stress than concrete tensile strength, will crack. Therefore, the concrete in the load and the load, can produce a crack. To load the crack, can produce by reasonable design and construction to prevent. For the load of crack, due to reasons, i
7、t is hard to handle and prevent. The more common than load factors have temperature, dry shrinkage, settling contraction deformation, plastic shrinkage deformation, etc, in these function, can make concrete inside tensile stress. 3.Big volume concrete crack formation reasons Massive concrete crackin
8、g reasons can be divided into two types: one is the structural cracks, is caused by external load, 2 it is material of crack is not caused by the changes of deformation and temperature stress and is mainly composed of concretes shrinkage. This article mainly discussed type materials. One specific re
9、asons are as follows: The temperature stress caused by fracture (temperature crack) temperature cracks in the main reason is caused by temperature difference. Temperature can be divided into three types: early concrete pouring, produce large amounts of the hydration heat, due to the concrete is a po
10、or conductor of heat accumulation in concrete, the hydration heat to send out, often make internal temperatures rise, concrete and reinforced concrete surface temperature for outdoor environment temperature, it is formed, the temperature and temperature inside of condensate in early concrete tensile
11、 stress when more than concrete compressive strength, will cause cracking in concrete, In addition, in the dismantling formwork, surface temperature and reduce the temperature caused by fast, and will cause cracking. When the concrete temperature, maximum heat radiates to the minimum temperature, te
12、mperature, or use them with the highest temperature difference is the internal temperature, These three temperature will produce temperature crack. In these three temperature, is mainly caused by hydration heat and the temperature. Shrinkage crack contraction there are many causes, including drying
13、shrinkage, plastic shrinkage, shrinkage, carbonation contraction, etc. Here mainly introduces drying shrinkage and plastic shrinkage. After drying shrinkage of concrete sclerosis, in dry environment, the water concrete to the loss caused by inside extroversion, concrete the shrinkage deformation cra
14、cks. The plastic shrinkage of cement and concrete in the activity of higher temperature, or in low water-cement condition exacerbate cause craze. When the concrete surface water obviously decreased, lactation evaporation of water cannot get the timely supplements, then concrete is still in a state o
15、f plastic, by a concrete surface tension, uneven distribution will appear the cracks of concrete cracks, and further intensify the moisture inside body, and further expansion crack Second, the measures to prevent cracks From the above analysis, the crack is mainly caused by temperature difference an
16、d contraction, so in order to prevent the cracks of, will maximize reduce temperature and reduce the shrinkage of concrete, specific measures as follows. 1. Optimizing raw materials (1)Cement. The cement should be chosen according to the engineering properties, construction condition, climate and ot
17、her technical requirements, comprehensive consideration, to meet the strength and durability of concrete design requirements. When the choice should be labeled with cement concrete design match the label. For full strength of cement, cement, cement save from the label for concrete label 1.5 to 2 tim
18、es as appropriate. (2) aggregate. (a) coarse aggregate. The coarse aggregate concrete rubble is close-grained, must have the enough strength, hard to expand the coarse aggregate size. To ensure the concrete pouring dao close-grained, coarse aggregate maximum diameter less than the minimum size of th
19、e structural sections 1/4, also not greater than the minimum distance between steel net for thickness 3/4, in 10 centimeters and concrete, allowing adopts maximum diameter 1 1/2 of the aggregate number, but it shall not exceed 25%. (b) fine aggregate. Sand particle gradation of technical properties
20、of concrete influence, requests the porosity and total surface area small, small need total surface on the surface of the gravel package will reduce quantity of cement slurry, can reduce the dosage of cement hydration heat, reduce to prevent cracks. On the other hand is to control the hydraulic sand
21、, the content of 5% in concrete, the contraction deformation, the hydraulic and larger crack, fine aggregate to use clean in the sand. (3) admixtures. (a) water-reducing agent. Jianshuiji belongs to the surface active substances, is a kind of cement dispersant. Its main function of improving concret
22、e workability, reduce the consumption of concrete mixture, reduce water-cement ratio increases, and improve the liquidity of concrete strength of concrete or keep in concrete cement strength must decrease, while the water-cement ratio, the dosage of cement to prevent cracking is reduced. (b) retarde
23、r. Retarder is delaying the role of concrete exothermic peak time, 2 it is to improve workability, and reduce the loss of collapsbillity. (c) air-entraining agent. In the application of concrete air-entraining agent for improving the concrete workability, can pump, improving the concrete durability is very favorable. In a certain extent increase anti-cracking performance of concrete. (4) mass concrete chooses raw materials should pay attention to the following: The coarse aggregate gradation, appropriate USES continuous fine aggregate ZhongSha
