Reasons for ordinary concrete cracks:
1. Cracks caused by load
The cracks produced by concrete under conventional static and dynamic loads and secondary stresses are called load cracks, which can be summarized as direct stress cracks and secondary stress cracks. Direct stress cracks refer to cracks produced by direct stress caused by external loads, and secondary stress cracks refer to cracks produced by secondary stresses caused by external loads. The characteristics of cracks under load vary with different loads and present different characteristics. Such cracks mostly appear in tension areas, shear areas or severe vibration parts. 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 its bearing capacity and a precursor to structural failure. The reason is often that the cross-sectional size is too small.
2. Cracks caused by temperature:
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 in the structure. When the stress exceeds the concrete tensile strength, temperature cracks will occur. In some long-span bridges, the temperature stress can reach or even exceed the live load stress. The main feature of temperature cracks that distinguishes other cracks is that they will expand or close with temperature changes
3. Cracks caused by shrinkage:
In actual engineering, cracks caused by shrinkage of concrete are the most common. Among the types of concrete shrinkage, plastic shrinkage and shrinkage (dry shrinkage) are the main reasons for the volume deformation of concrete, and there are also autogenous shrinkage and carbonization shrinkage.
Plastic shrinkage occurs during the construction process and about 4 to 5 hours after concrete pouring. At this time, the cement hydration reaction is intense, molecular chains are gradually formed, bleeding and water evaporate rapidly, and the concrete shrinks due to loss of water. sink, so the concrete has not yet hardened, which is called plastic shrinkage. The magnitude of plastic shrinkage is very large, up to about 1%. If the aggregate is blocked by the steel bar during the sinking process, cracks along the direction of the steel bar will be formed. In the vertically variable section of the component, such as the junction of the web of the T beam and box girder and the top and bottom plates, cracks along the web direction of the surface will occur due to uneven sinking before hardening. In order to reduce the plastic shrinkage of concrete, the water-cement ratio should be controlled during construction to avoid too long agitation, the material should not be cut too fast, the vibration should be dense, and the vertically variable cross-section should be poured in layers.
Shrink to shrink (do and shrink), after the concrete is formed hard, as the top layer moisture evaporates progressively, the humidity reduces progressively, the volume of concrete is reduced, is called and shrunk to shrink (do to shrink). Due to the rapid loss of moisture on the surface of the concrete and the slow internal loss, uneven shrinkage occurs with large surface shrinkage and small internal shrinkage. The surface shrinkage deformation is restricted by the internal concrete, causing the surface concrete to bear tensile force. , shrinkage cracks occur. Shrinkage after hardening of concrete is mainly shrinkage. For example, for components with a large reinforcement ratio (more than 3%), the restraint of the reinforcement on the shrinkage of the concrete is more obvious, and cracks are prone to cracks on the concrete surface.
Autogenous shrinkage, autogenous shrinkage is the hydration reaction between cement and water during the hardening process of concrete. This shrinkage has nothing to do with the external humidity, and it can be positive (that is, shrinkage, such as ordinary portland cement concrete) or negative. (ie expansion, such as slag cement concrete and fly ash cement concrete).
Carbonization shrinkage is the shrinkage deformation caused by the chemical reaction between carbon dioxide in the atmosphere and cement hydrate. Carbonization shrinkage can only occur when the humidity is about 50%, and it will accelerate as the concentration of carbon dioxide increases. Carbonization shrinkage is generally not calculated.
Concrete shrinkage cracks are characterized by the fact that most of them are surface cracks, the crack width is relatively thin, and they are criss-crossed, cracked and irregular in shape.
4. Cracks caused by foundation deformation:
Due to uneven vertical settlement or horizontal displacement of the foundation, additional stress is generated in the structure, exceeding the tensile capacity of the concrete structure, resulting in cracking of the structure.
5. Cracks caused by steel corrosion:
Due to poor concrete quality or insufficient thickness of the protective layer, the concrete protective layer is eroded by carbon dioxide and carbonized to the surface of the steel bar, which reduces the alkalinity of the concrete around the steel bar, or due to the intervention of chlorides, the content of chloride ions around the steel bar is high, which can cause oxidation on the surface of the steel bar The membrane is destroyed, and the iron ions in the steel bar react with the oxygen and moisture intruded into the concrete, and the volume of the rusted iron hydroxide increases by about 2 to 4 times compared with the original, thus generating expansion stress on the surrounding concrete, resulting in cracking and peeling of the protective layer concrete , Cracks occur longitudinally along the steel bar, and rust seeps into the concrete surface. Due to corrosion, the effective cross-sectional area of the steel bar is reduced, the binding force between the steel bar and the concrete is weakened, the structural bearing capacity is reduced, and other forms of cracks will be induced, which will aggravate the corrosion of the steel bar and lead to structural damage. To prevent corrosion of steel bars, the width of cracks should be controlled according to the specification requirements during design, and sufficient thickness of the protective layer should be adopted (of course, the protective layer should not be too thick, otherwise the effective height of the component will be reduced, and the crack width will increase when the force is applied); Control the water-cement ratio of concrete, strengthen vibration, ensure the compactness of concrete, prevent oxygen intrusion, and strictly control the amount of admixture containing chlorine salt, especially in coastal areas or other areas with strong corrosive air and groundwater.
