Causes of cracks in Concrete Structures

Many concrete buildings will have cracks of different forms and widths during their construction and use, especially when the building has a large size in a certain direction, it is more likely to produce cracks. According to statistics, among the cracks caused by direct load and the cracks caused by indirect action, the cracks mainly caused by indirect use (changes in ambient temperature, uneven settlement of the foundation, and shrinkage of coagulation) account for about 80%. The cracks caused by the action account for about 20%.Sodium lignosulfonate uses in the construction to change the concrete workability.

The mechanism of the structure due to the shrinkage of the concrete is: when the shrinkage deformation or temperature deformation of the concrete is restricted by the restraint effect and cannot be deformed freely, it will produce shrinkage stress or humidity stress in the structure. When the length of the structure is short, the stress value is small and can be ignored; but when the length of the structure gradually increases, the restraint effect increases along the length direction, so the increase in the stress value will cause cracks or even cracks in the concrete structure. The damage of structural components has become the main reason for the design.

Cracks caused by concrete shrinkage

In a concrete structure, the shrinkage deformation of concrete is restrained by the foundation, vertical members, and steel bars. When the tensile stress generated inside the structure exceeds the ultimate tensile strength of the concrete, cracks will occur. The shrinkage of concrete is a physical and chemical phenomenon of the material itself, regardless of the stress it receives. Under normal circumstances, the shrinkage of the concrete itself is mainly controlled by the gradation of coarse and fine aggregates, cement label, curing temperature, atmospheric water content and other material characteristics, construction process, and external environment.

During the hardening process of concrete in the air, its own composition has undergone a chemical change. Free water reacts with cement and turns into combined water, and the resulting shrinkage becomes auto-shrinkage.

The plastic shrinkage of concrete means that after 4h~15h of coagulation stops pouring, the hydration reaction molecular chain of cement is gradually formed, bleeding and water evaporates rapidly, causing water loss and shrinkage, and at the same time there is the production between the aggregate and the cement. Uneven shrinkage deformation.

Drying shrinkage is the shrinkage that occurs after the end of the curing period of concrete. The dry shrinkage of concrete is the shrinkage caused by the gradual loss of capillary water in the concrete, which accounts for the largest proportion of the total shrinkage of concrete.

The carbonization shrinkage of concrete mainly refers to the reaction of carbon dioxide in the air with the calcium hydroxide in the concrete cement stone to form calcium carbonate, which releases crystal water and causes the concrete to shrink. The carbonization shrinkage is negligible in the general environment.

In general, the shrinkage of concrete increases with time, and the development rate of early shrinkage of concrete is faster. The shrinkage develops faster in the early stage, and then gradually slows down.

Kingsun is a professional concrete admixtures manufacturer and supplier.