Why do cracks occur in concrete?

There are many causes of concrete cracks, such as plastic shrinkage cracks, that is, before the concrete sets, it is affected by high temperature or wind, resulting in too fast water loss and rapid brush shrinkage, resulting in surface cracks; Settlement shrinkage cracks, soft and unsolid foundations or insufficient hardness of formwork lead to cracking of concrete structures; Temperature cracks, the temperature difference between the inner surface of concrete is large, forming thermal expansion and cold contraction, resulting in cracks on the surface of concrete.

There are chemical reactions and physical changes in the process of condensation and hardening of concrete, accompanied by heat release and water loss and evaporation, resulting in volume changes, and the main reasons for cracks are:

Volume expansion is caused by heating and heating, and stress and uneven changes are produced due to limited volume changes.

Volume shrinkage due to surface heat dissipation and water evaporation;

Inhomogeneous stress action.

Four common causes of cracks in concrete building structures.

Hello 01 Cracks caused by load Cracks in concrete under conventional static, dynamic load and secondary stress are called load cracks, which can be summarized into two 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 according to different loads, and present different characteristics.

This type of crack mostly appears in the tension zone, shear zone or severe vibration area. However, it must be pointed out that if there is peeling or short cracks along the compression direction in the compression zone, it is often a sign that the structure has reached the limit of bearing capacity and is a precursor to structural failure, and the reason is often that the cross-sectional size is too small. 02 Cracks caused by temperature changes Concrete has the properties of thermal expansion and contraction, when the external environment or the internal temperature of the structure changes, the concrete will be deformed, if the deformation is constrained, the stress will be generated in the structure, and when the stress exceeds the tensile strength of the concrete, temperature cracks will be generated.

In some long-span bridges, temperature stresses can reach or even exceed live-load stresses. The main characteristic of temperature cracks that distinguish other cracks is that they will expand or close with temperature changes. 03 Cracks caused by shrinkageIn actual engineering, cracks caused by shrinkage of concrete are the most common.

Among the types of concrete shrinkage, plastic shrinkage and shrinkage shrinkage (dry shrinkage) are the main causes of concrete volume deformation, as well as autogeneous shrinkage and carbonization shrinkage.

Causes of cracks in concrete:

1. Cracking, which may be related to insufficient maintenance, such as after pouring concrete, the surface has not had time to be covered, and it has been exposed to wind and sun, and the moisture on the surface shrinks rapidly, reducing the strength. Unable to resist this stress, cracking occurs.

2. There are also some cracks in the concrete that have been formed, but are affected by the temperature, mainly because of the cracks in the surface temperature, and the cracks caused by the large temperature. This kind of crack is called a temperature difference crack, generally speaking, there is no law on its direction, some are relatively large, and some are relatively small.

3. Cracks in concrete may also be caused by uneven settlement, mainly because of the settlement of the foundation, resulting in the foundation beam, including the ring beam, etc., which is too large, and cracks appear from the late cluster.

4. Cracks may also be related to inadequate construction, such as pouring too fast and low fluidity. In addition, if the vibration is not timely, tight and uneven in the later stage, there will be honeycomb or hemp surface.

How to avoid cracks in concrete.

1. Selection of concrete materials. We need to choose cement with low heat and low dry shrinkage. For example, Portland cement, ordinary Portland cement, or slag Portland cement. It should be noted that the content of tricalcium aluminate in the cement we choose should not be greater than 8%.

2. We also have to strictly control the mud content of coarse aggregate and fine aggregate and the particle size of coarse aggregate. At the same time, it is also necessary to strictly control the mix ratio of concrete, reduce the water-cement ratio and sand rate. Choose some concrete with a low amount of cement per unit.

3. Mix some suitable water reducers. What we need to pay attention to is to reduce the unit of water consumption. We can mix some fly ash with good water retention performance and fine particles, so as to improve the aggregate gradation and use low-flow concrete.

4. We should choose some concrete with small thermal expansion coefficient, good thermal conductivity, large specific heat, low elastic modulus, large virtual change, and small dry shrinkage rate of micro-expansion. These contents need to be paid attention to when purchasing concrete, and explain to the manufacturer in advance.

Four common causes of cracks in concrete building structures.