The cracks in the concrete structure of industrial buildings are very common, among which the reinforced concrete members are more. Practice has proved that concrete structure and a considerable part of cracks can be overcome through design means and construction measures. This paper summarizes the common problems easily ignored and causes of cracks, and puts forward corresponding preventive measures.
1. Analysis of common concrete cracks
Concrete is a kind of brittle material with very low tensile capacity, especially in the reconstruction and expansion projects of industrial buildings. In the process of construction and use, when there are changes in temperature and humidity, rolling mill vibration and uneven settlement of foundation, cracks are very easy to occur. In recent years, with the development of building technology, building materials, construction technology and the progress of test and detection means, we not only pay attention to its physical properties, but also its chemical properties. For example, "AAR" alkali aggregate reaction and chemical medium erosion will make concrete structure crack early and reduce concrete strength.
1.1 material quality
The common cause of cracks caused by material quality problems is poor quality of cement, sand and stone. Using these unqualified materials in bitter project will lead to "bean curd dregs project", so only when the quality of materials is well controlled can the project quality be fundamentally guaranteed.
1.2 foundation deformation
In reinforced concrete structure, the main cause of cracking is uneven settlement. The size, shape and direction of the cracks determine the deformation of the foundation. Because the stress of the deformation of the foundation is relatively large, the cracks are generally penetrating.
1.3 construction technology
The following points are emphasized in the construction process:
Water evaporation, cement stone and concrete shrinkage are usually the important reasons for concrete cracks.
Concrete is a kind of man-made mixed material. An important sign of its quality is the uniformity and compactness of concrete after forming. Therefore, any defects and omissions in concrete mixing, transportation, pouring and vibrating may cause cracks directly or indirectly.
Improper formwork construction, water leakage, grout leakage, insufficient support stiffness, foundation sinking of support, over removal of formwork, etc. may cause concrete cracking. In the process of construction, the surface of reinforcement is polluted, the concrete cover is too large or too small, and the collision of reinforcement during pouring may cause cracks.
Concrete maintenance, especially early maintenance quality and cracks are closely related. Early surface drying can make the internal and external temperature larger and easier to produce cracks.
1.4 structural loading
There are many factors that cause cracks after the structure is loaded, and cracks may appear in construction and use. For example, early earthquake, early formwork removal or improper method, component stacking, transportation, improper location of cushion blocks or lifting points during hoisting, construction overload, excessive tensile stress value, etc. may cause cracks. The most common is reinforced concrete beams, slabs and other flexural members, which often appear different degrees of cracks under the use of load. When the ordinary reinforced concrete members bear 30% - 40% of the design load, cracks may appear, which can not be detected by naked eyes, and the ultimate failure load of the members is often more than 1.5 times of the design load. So in general, reinforced concrete members are allowed to work with cracks. In the code for design of reinforced concrete, the maximum width of cracks is 0.2nun-0.3nun in different situations. For those cracks whose width exceeds the code, as well as the cracks on the components that are not allowed to crack, it should be considered as harmful, which should be carefully analyzed and handled.