1.Load-induced cracks
Cracks generated in concrete under conventional static and dynamic loads and secondary stresses are called load cracks, which can be summed up as direct stress cracks and secondary stress cracks. Direct stress cracks refer to cracks generated by direct stress caused by external loads, and secondary stress cracks refer to cracks generated by secondary stress caused by external loads. The characteristics of load cracks show different characteristics depending on the load. Such cracks mostly appear in tension zone, shear zone 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 bearing capacity and is a precursor to structural failure. The reason is often that the section size is too small.
2. 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 deform. If the deformation is restrained, stress will be generated in the structure. When the stress exceeds the tensile strength of the concrete, temperature cracks will occur. In some long-span bridges, the thermal stress can reach or even exceed the live load stress. The main feature of temperature fractures that distinguish them from other fractures is that they will expand or close with temperature changes.
3. Cracks caused by shrinkage
In practical engineering, cracks caused by concrete shrinkage are the most common. Among the types of concrete shrinkage, plastic shrinkage and shrinkage shrinkage (drying 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 the concrete is poured. At this time, the cement hydration reaction is intense, the molecular chain is gradually formed, the bleeding and the rapid evaporation of water occur, the concrete loses water and shrinks, and the aggregate sinks due to its own weight. When the concrete has not yet hardened, it is called plastic shrinkage. The magnitude of plastic shrinkage is very large, up to about 1%. In the process of aggregate sinking, if it is blocked by steel bars, cracks along the direction of the steel bars will be formed. At the vertical section of the member, such as the junction of the web of T beam and box girder with the roof and floor, the surface cracks along the web direction will occur due to uneven settlement before hardening. In order to reduce the plastic shrinkage of concrete, the water-cement ratio should be controlled during construction, to avoid excessively long stirring, the material should not be cut too fast, the vibration should be compact, and the vertical variable section should be poured in layers.
- Shrinkage shrinkage (drying shrinkage). After the concrete hardens, as the surface moisture gradually evaporates, the humidity gradually decreases, and the volume of the concrete decreases, which is called shrinkage (drying shrinkage). Because the water loss of the concrete surface is fast and the internal loss is slow, the uneven shrinkage of large surface shrinkage and small internal shrinkage occurs. The surface shrinkage and deformation are constrained by the internal concrete, causing the surface concrete to bear the tensile force. When the surface concrete bears the tensile force beyond its tensile strength , shrinkage cracks occur. The main shrinkage of concrete after hardening is shrinkage shrinkage. For components with a large reinforcement ratio (more than 3%), the restraint of the reinforcement on the shrinkage of the concrete is obvious, and the concrete surface is prone to cracks.
- 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 external humidity and can be positive (ie shrinkage, such as ordinary Portland cement concrete) or negative (ie expansion , such as slag cement concrete and fly ash cement concrete).
- Charcoal shrinkage. The shrinkage deformation caused by the chemical reaction between carbon dioxide in the atmosphere and the hydrate of cement. Carbonization shrinkage can only occur at about 50% humidity, and it accelerates with the increase of carbon dioxide concentration. Carbonization shrinkage is generally not calculated.
The characteristics of concrete shrinkage cracks are that most of them are surface cracks, the width of the cracks is relatively small, and the cracks are crisscrossed in a crack-like shape without any regularity in shape.
4. Cracks caused by ground foundation deformation
Due to the uneven vertical settlement or horizontal displacement of the foundation, additional stress is generated in the structure, which exceeds the tensile capacity of the concrete structure, resulting in structural cracking.
5. Cracks caused by corrosion of steel bars
Due to poor quality of concrete or insufficient thickness of the protective layer, the protective layer of concrete is eroded and carbonized to the surface of the steel bar by carbon dioxide, which reduces the alkalinity of the concrete around the steel bar, or the chloride ion content around the steel bar is high due to the intervention of chloride, which can cause the surface of the steel bar to oxidize. The membrane is damaged, and the iron ions in the steel bar react with the oxygen and moisture intruding into the concrete, and the volume of the rust iron hydroxide increases by about 2 to 4 times than the original, which will cause expansion stress to the surrounding concrete, resulting in cracking and peeling of the protective layer concrete. , Click here to download the construction technical data for free. Cracks occur along the longitudinal direction of 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 grip between the steel bar and the concrete is weakened, the bearing capacity of the structure is reduced, and other forms of cracks will be induced, which will aggravate the corrosion of the steel bar and lead to structural damage.
In order to prevent the corrosion of steel bars, the crack width should be controlled according to the requirements of the specification, and sufficient protective layer thickness should be used (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 be increased when subjected to stress); 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 chloride salts, especially in coastal areas or other areas with strong corrosive air and groundwater.
6.Cracks caused by frost heave
When the atmospheric temperature is lower than zero, the water-saturated concrete freezes, the free water turns into ice, and the volume expands by 9%, so the concrete generates expansion stress; at the same time, the supercooled water in the concrete gel pores (the freezing temperature is below -78 degrees) ) migration and redistribution in the microstructure cause osmotic pressure, which increases the expansion force in the concrete, reduces the strength of the concrete, and leads to the appearance of cracks. In particular, the concrete is most severely frozen when it is initially set, and the strength loss of concrete after ageing can reach 30% to 50%. During construction in winter, frost heave cracks along the pipeline direction may occur if thermal insulation measures are not taken after grouting the prestressed tunnel.
7. Cracks caused by the quality of construction materials
Concrete is mainly composed of cement, sand, aggregate, mixing water and admixtures. The material used to configure the concrete is of substandard quality and may result in cracks in the structure.
8. Cracks caused by the quality of construction workmanship
In the process of concrete structure pouring, component production, formwork, transportation, stacking, assembly and hoisting, if the construction process is unreasonable and the construction quality is poor, it is easy to produce vertical, horizontal, oblique, vertical, horizontal, Surface, deep and penetrating cracks, especially slender and thin-walled structures, are more likely to appear. The location and direction of cracks, and the width of cracks vary depending on the cause.
