3 Causes of Common Concrete Cracks

(3 Causes of Common Concrete Cracks)

Cracks caused by load

Cracks produced in concrete under conventional static and dynamic loads and secondary stress are called load cracks. In summary, there are two main types: direct stress cracks and secondary stress cracks. Direct stress cracks refer to cracks caused by direct stress caused by external loads, and secondary stress cracks refer to cracks caused by secondary stress caused by external loads.

The characteristics of load cracks vary depending on the load, showing different characteristics. Such cracks often appear in tension, shear, or severe vibration areas. However, it must be pointed out that if peeling occurs in the pressure area or there are short cracks along the pressure direction, it is often a sign that the structure has reached the bearing capacity limit and is a precursor to structural damage. The reason is often that the cross-sectional size needs to be bigger.

 

Temperature-induced cracks

Concrete has the property of thermal expansion and contraction. When the external environment or the internal temperature of the structure changes, the concrete will deform. If the deformation is restrained, stress will be generated within the structure. When the stress exceeds the tensile strength of the concrete, temperature cracks will occur. Temperature stresses can reach or even exceed live load stresses in some long-span bridges. The main characteristic distinguishing temperature cracks from others is that they will expand or close with temperature changes.

 

Cracks caused by shrinkage

In actual engineering, cracks caused by concrete shrinkage are the most common. Among the types of concrete shrinkage, plastic shrinkage and shrinkage (drying shrinkage) are the main reasons for the volume deformation of concrete, as well as spontaneous shrinkage and carbonization shrinkage.

Plastic shrinkage occurs during construction and about 4 to 5 hours after the concrete is poured. At this time, the cement hydration reaction is intense; molecular chains are gradually formed, bleeding and rapid evaporation of water occur, the concrete shrinks due to water loss, and at the same time, the aggregate drops due to its weight. Sinking: the concrete has not yet hardened; this is called plastic shrinkage. Plastic shrinkage is very large, up to about 1%. If the aggregate is blocked by steel bars during the sinking process, cracks along the direction of the steel bars will form. At the vertical cross-sections of components such as T-beams and box beams where the webs and roof and bottom plates meet, surface cracks along the web direction will occur due to uneven sinking before hardening. To reduce the plastic shrinkage of concrete, the water-cement ratio should be controlled during construction, avoid mixing for too long, the material should not be cut too fast, the vibration should be dense, and the vertical cross-section should be poured in layers.

 

Shrinkage (dry shrinkage). After the concrete hardens, as the surface water gradually evaporates, the humidity gradually decreases, and the volume of the concrete decreases. This is called shrinkage shrinkage (dry shrinkage). Because the surface moisture loss of concrete is fast and the internal loss is slow, uneven shrinkage occurs with large surface shrinkage and small internal shrinkage. The surface shrinkage deformation is constrained by the internal concrete, causing the surface concrete to bear tensile force. When the surface concrete bears tensile force, it exceeds its tensile strength. When shrinkage cracks occur, the main shrinkage of concrete after hardening is shrinkage. For components with a large reinforcement ratio (more than 3%), the steel bars constrain the shrinkage of the concrete more obviously, and cracks are prone to appear on the concrete surface.

Autogenic shrinkage. Autogenic shrinkage is the hydration reaction between cement and water during the hardening process of concrete. This shrinkage has nothing to do with external humidity and can be positive (i.e., shrinkage, such as ordinary Portland cement concrete) or negative. (i.e., expansion, such as slag cement concrete and fly ash cement concrete).

 

Carbonization shrinkage is a shrinkage deformation caused by the chemical reaction between carbon dioxide in the atmosphere and cement hydrates. Carbonization shrinkage can only occur when the humidity is about 50% and accelerates as the concentration of carbon dioxide increases. Carbonization shrinkage is generally not calculated.

The characteristics of concrete shrinkage cracks are that most are surface cracks, the width of the cracks is relatively thin, and they are crisscrossed into cracks with no regular shape.

Supplier

TRUNNANO is a supplier of concrete crack reducing admixture(which is one of concrete additives) with over 12 years experience in nano-building energy conservation and nanotechnology development. It accepts payment via Credit Card, T/T, West Union and Paypal. Trunnano will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. If you are looking for high quality concrete crack reducing admixture, please feel free to contact us and send an inquiry. (sales@cabr-concrete.com).

 

(3 Causes of Common Concrete Cracks)