What is the main cause of wall cracking?

A, the main reason of cement mortar plastering wall cracks:

Shrinkage of cement mortar is one of the most common factors causing wall cracks, which mainly includes chemical shrinkage, drying shrinkage, autogenous shrinkage, temperature shrinkage and plastic shrinkage. Each shrinkage has its own characteristics, and its performance is different when it causes cracks in plastered walls.

(1) chemical shrinkage, also known as hydration shrinkage. Cement hydration will produce hydration heat, which will increase the solid volume, but the absolute volume of cement-water system will decrease. All cementitious materials have this shrinkage reduction effect after hydration. The volume reduction of most silicate cement pastes after complete hydration is 7%-9%. Before hardening, the solid volume increased by the hydration of plastering mortar fills the space originally occupied by water, making the cement paste compact and the macro volume reduced. The macro volume of hardened plastering mortar remains unchanged, but many capillary cracks are formed after the cement-water system shrinks, which affects the performance of plastering mortar.

(2) Dry shrinkage refers to the irreversible shrinkage of plastering mortar after it stops curing and loses the water absorbed by internal capillary holes and gel holes in unsaturated air;

(3) Self-shrinkage means that after the initial setting of plastering mortar, the cement continues to hydrate, and in the absence of external water supplement, the macro volume of plastering mortar decreases due to negative pressure generated by self-drying. Self-contraction starts from initial setting, mainly in the early stage;

(4) The temperature shrinkage of plastering mortar is also called cold shrinkage, which is caused by the increase of hydration temperature of cement inside plastering mortar and then cooling to ambient temperature. The temperature shrinkage is related to the thermal expansion coefficient, the highest internal temperature of plastering mortar and the cooling rate.

(5) The plastic shrinkage of plastering mortar refers to the plastic dry shrinkage caused by the fact that the evaporation rate of surface water is greater than the internal top-down bleeding rate before the plastering mortar hardens. The plastic drying shrinkage of plastering mortar surface is affected by time, temperature, relative humidity and the bleeding characteristics of plastering mortar itself. Once the plastering mortar has a certain strength, it cannot adapt to plastic shrinkage through plastic flow, and plastic shrinkage cracking will occur at this time. Whether the plastic shrinkage joint of plastering mortar is visible or not will affect the durability of plastering mortar. Due to these shrinkage of cement mortar, there is a contradiction between a short strength growth period (the main strength is completed above 10 hour) and a long volume shrinkage period (several months or even hundreds of days, the shrinkage rate is 8%- 10%), which will cause tensile stress in plastered walls. When the tensile stress exceeds the tensile strength of cement mortar, cracks will appear.

Second, the causes of cracks in hollow brick wall and preventive measures:

At present, there are two kinds of wall hollow masonry commonly used in buildings: sintered hollow brick (clay sintered red brick) and cement mortar hollow brick (crushed stone mixed with cement steamed brick). The main advantages of using these two kinds of thin-walled macroporous blocks as filling wall materials are saving land resources and reducing wall load.

However, through years of observation, it shows that the problem of cracks in porous brick walls is more prominent, especially after painting is completed, and even found in the return visit of engineering quality after handover and acceptance.

Three, a variety of different crack treatment methods:

1, temperature crack-the most common building crack, which has little influence on the structural safety of buildings. This kind of crack only affects the indoor beauty of the building and will not affect the safety of the building. Some remedial measures can be taken appropriately: stick non-woven fabric, PVC grid cloth or mortar to block the cracks, and then paint and repair them.

2. Settlement cracks —— When the settlement develops slowly and tends to weaken after the settlement cracks occur, they should be repaired after the cracks are stabilized. The foundation of new houses will be gradually stabilized in 2-5 years. After the cracks are repaired, the wall may still crack due to the settlement of the foundation, so don't worry too much. If the cracks have little effect on the overall beauty of the house, they can be repaired after the first repair. Repairs are generally filled with cement mortar, polymer mortar or elastic putty and gypsum, and glued with caulking tape.

3, structural cracks-ordinary decoration methods can not be repaired, we must find professional building structural personnel according to the actual situation, and take corresponding measures.

4. There are cracks in the surface latex paint wallpaper: If there are only cracks in the surface paint film, polish the cracks with fine sandpaper and repaint them. If the wallpaper cracks at the joint, moisten it with warm water, and then paste it with wallpaper glue.

5. Cracks appear in the putty leveling layer: If cracks have penetrated into the putty layer, first use sharp tools to expand the cracks to the size of a screwdriver, fill in caulking gypsum, grind it flat, paste grid cloth or kraft paper, then level it with putty, and finally paint or paste wallpaper according to normal procedures.

6. Cracks in the plastering layer of cement mortar: If there is a large area of hollowing in the plastering layer, the original plastering layer should be knocked off and a new plastering layer of cement mortar should be made. After leveling, paste paint or wallpaper according to normal procedures. However, after this treatment, the cost will increase relatively, and the owner can choose the repair method according to his own economic ability. If it only cracks in a large area and there is no sand turning, hollowing and falling off, you can paste dacron cloth all over the wall, and then paint or wallpaper it according to the normal process. The relatively low-cost way is to directly nail a layer of gypsum board on the original wall, which requires environmental protection white latex and steel nails to be used at the same time.

7. Cracks at the joints: It is recommended to repair after the deformation of the whole building is stable, and the repair in a short time is easy to occur repeatedly.