One is the crack caused by the change of external load (including static load and dynamic load).
The second is cracks caused by deformation (mainly cracks caused by stress caused by temperature change, uneven settlement or expansion). In civil buildings with masonry structure, most cracks in masonry are caused by deformation, and temperature change is the main factor causing cracks in walls. Because of the linear expansion coefficient of brick masonry and reinforced concrete, when the temperature changes, there is a deformation difference between them. In addition, because most of the components in the building belong to statically indeterminate bars with multiple constraints, the deformation caused by temperature change will be limited, thus generating temperature stress in the components. The deformation difference between wall and concrete will inevitably produce great tension and shear force in masonry. When these forces exceed a certain limit, the masonry will produce dislocation cracks, and temperature cracks are the main reason for the early cracking of the wall. Cracks caused by temperature stress and deformation have the characteristics and laws of "the top layer is more important than the bottom layer", "the two ends are more important than the middle" and "the front is more important than the back". The types and causes of cracks can be specifically divided into the following five types:
L, an open crack.
It mainly appears at both ends of the transverse wall and the longitudinal wall. This kind of crack is a splayed thermal expansion crack, which changes with the rise and fall of temperature. The reason is that the thermal resistance of roof insulation layer decreases or even fails due to the defects in design and construction, which leads to the temperature deformation of roof panel greater than that of masonry. When a certain temperature stress occurs, the thrust of the roof panel is transferred to the wall, and the additional stress of the wall temperature is greater at both ends of the house. When the strength of masonry mortar is low, the main tensile stress caused by shear force is easy to appear.
2. Inverted splayed cracks.
It is a cold shrinkage crack, which mainly appears in the holes at both ends of the vertical and horizontal walls, especially at both ends of the top floor. Because the additional stress of cold shrinkage of the wall is large at both ends of the wall, when the shrinkage deformation of the house is greater than that of the wall, the stress is relatively concentrated at the entrance of the door and window, which leads to the formation of inverted splay cracks and cracks in the wall.
3. Horizontal cracks.
More common in top horizontal walls, vertical walls, "parapets" and gables. When the thermal insulation of the roof is poor, the thermal expansion of the roof panel produces horizontal thrust on the wall. Because the shrinkage of the end wall is greater than that of the middle wall, the shear capacity of the masonry is low, and horizontal cracks occur at the contact surface between the vertical and horizontal walls and the roof.
4. Vertical cracks.
Mainly in the windowsill wall, lintel end, staggered floor. This kind of crack is mainly due to the temperature change, the wall is subjected to the tensile stress of the floor, resulting in the stress concentration effect at the entrance of doors and windows, or the vertical crack of the wall is caused by the cold shrinkage deformation of the upper beam end of reinforced concrete which is different from the wall paint and the staggered floor deformation.
5.x-shaped cracks.
Most cracks along the mortar joint of masonry are mainly caused by the repeated action of thermal expansion and cold contraction of buildings, while X-shaped cracks in the bottom wall are caused by uneven or uneven settlement of foundation.