Abstract: Cracks in building walls are not only various in types and shapes, but also common. Therefore, it is very necessary and urgent to correctly analyze the causes and carry out effective prevention. This paper summarizes and analyzes the causes of cracks in building wall and the principles of crack control, and puts forward some suggestions on the concrete structure and construction measures of crack control in wall structure. Cracks; Precautions 1 Foreword Wall cracks are common problems in buildings, which are caused by many factors, including pile foundation, temperature, shrinkage and other factors, as well as design negligence, construction quality, unqualified materials and lack of experience. According to engineering practice and statistical data, this kind of crack accounts for almost 80% of all cracks that can be encountered, but there are also cracks caused by excessive load or too small cross section. Cracks in the building wall not only affect its appearance, but also cause external wall leakage, which is particularly prominent in the humid and rainy Wuhan area. How to solve the phenomenon of cracks in building walls is very important. This paper analyzes the causes of wall cracks and puts forward corresponding preventive measures. 2. Cause analysis of wall cracks 2. 1 Analysis of wall cracks caused by uneven settlement of foundation 1. When the soil layer of building foundation is unevenly distributed and the soil quality is quite different, obvious uneven settlement often occurs at the junction of different soil layers or at different thicknesses of the same soil layer, resulting in cracks in the wall, which are large or small and tilt in the direction of soft soil or thick soil layer. 2. In the case of large building height difference or large load difference, when there is no settlement joint, it is easy to produce large uneven settlement cracks at the junction of high and low weights. At this time, the crack is located in the part with less layers and light load, and inclines upward to the part with more layers and heavy load. 3. When the compressibility of the soil at both ends of the house is large and the middle part is small, the settlement distribution curve will be convex. At this time, in addition to the outward inclined cracks at both ends of the longitudinal wall, vertical cracks often appear at the top of the longitudinal wall. 4. In a multi-storey building, when the bottom windowsill is too wide, the uneven settlement of the foundation is often caused by the concentrated transfer of load by the wall between windows, which makes the windowsill bend reversely under the action of the foundation reaction, resulting in vertical cracks in the middle of the windowsill. 5. If the foundation of the new building is in size: on the basis of the original building, the ratio of the height difference H of the bottom surface of the new and old foundation to the clear distance L should be less than 0.5-L. Otherwise, the foundation settlement caused by the load of the new house will cause cracks in the original house and wall. Similarly, in the construction of adjacent high-rise and low-rise buildings, the construction should be organized according to the principle of high before heavy, low before light; Otherwise, if the low-rise building is built first and then the high-rise building, it will also cause cracks in the low-rise wall. From the above analysis, we can see that the distribution of cracks is closely related to the height-width ratio of the wall. Houses with large ratio of length to height have poor rigidity and poor deformation resistance, and are prone to cracks. Because the length-height ratio of the longitudinal wall is greater than that of the transverse wall, most cracks occur on the longitudinal wall. The distribution of cracks is closely related to the distribution curve of foundation settlement. When the settlement distribution curve is concave, cracks mostly occur in the lower part of the house, and the width of cracks is large or small. When the settlement distribution curve is convex, cracks often appear in the upper part of the house, and the width of cracks is large or small. The distribution of cracks is closely related to the mechanical properties of the wall. Due to stress concentration, cracks often occur at doors and windows, plane turning points and height changes. Because the wall was destroyed by shear, its principal tensile stress was 45. Therefore, the cracks are also inclined at 45. 2.2 Temperature stress leads to wall cracking. Generally speaking, materials have the property of thermal expansion and cold contraction, and the deformation of building structures caused by the change of environmental temperature is called temperature deformation. If the structure is free from any constraints and can deform freely when the temperature changes, then no additional stress will be generated in the structure. If the structure is constrained and cannot deform freely, additional stress or temperature stress will be generated in the structure. Structural expansion value caused by temperature stress. Because the linear expansion coefficient of reinforced concrete is a = 1 .08x1c, while that of ordinary brick masonry is 0.5X 1O/C, the elongation of reinforced concrete structure is about1times larger than that of brick masonry at the same temperature difference. Therefore, in the mixed structure, when the temperature changes, the expansion and contraction of reinforced concrete columns, beams and brick walls are different, which will inevitably contain each other and produce temperature stress, which will cause the building structure to crack and destroy. 2.3 Construction factors ① The construction speed is too fast. At this time, the masonry strength has not yet reached the design strength, the foundation is rapidly deformed, and the adjustment of soil stress lags behind, resulting in premature uneven settlement of foundation soil. Therefore, excessive initial stress and strain are generated in masonry, which forms a potential cracking factor. After the main body is renovated, the residents are further stressed and cracked, which leads to the cracking of the wall; (2) Mortar strength does not meet the requirements, such as large silt content in sand, uneven, not strictly measured, the mixture ratio is not allowed, and even Shi Ding field materials are not used for trial matching, and the mixture ratio is determined by the laboratory. Reference mix proportion provided only based on some data; (3) Insufficient mortar mixing, poor workability, end of operation, insufficient plumpness and uneven thickness of horizontal mortar joint, which leads to the decrease of masonry strength; (4) The building brick lacks swimming water, absorbs water prematurely, and the cement hydration reaction is insufficient; ⑤ The construction technology is wrong, and the masonry construction joints are kept straight or even wiped. Masonry joints, mortar joints are not full, moisture content is not properly mastered, feet, hands and eyes are not properly set, and masonry is not properly set.