The function of the concrete layer is very important, the shortcoming is the easy to appear the crack and the sag. If the concrete is not well preserved, these cracks may cause serious structural problems, even the collapse of the structure as a whole. But not all the cracks are dangerous, and some slight cracks do not affect the safety of the house.Crack repair is more impotant.

1. Unevenly descending crack

   
   

The uneven deformation of the building will cause the vertical and horizontal bending deformation of the building, poor stiffness, the foundation is not enough to adjust the stress due to differential settlement,will produce tensile stress and shear stress in some parts of the brick masonry, occur cracks when insufficient to resist deformation stress. 

There are more cracks caused by uneven settlement, such as, diagonal crack， horizontal cracks and vertical cracks in masonry cracks . Most of these cracks occur on the first floor, and a few will appear in other layers. 

2. Temperature cracks 

In brick concrete building structure as an example, when the roof is reinforced concrete slab and the wall is brick wall, the wall is very easy to produce temperature crack, especially the top wall and the parapet wall。

3. Other cracks

There are some other factors that cause the cracks in the masonry structure wall, except for the temperature difference cracks and the uneven settlement cracks. Some loads are directly caused by cracks. The vibration and horizontal force generated by non-destructive earthquakes will also cause cracks in some weak parts of brick masonry. And the quality of the reinforced material is very poor, which will also cause cracks in masonry.