Concrete crack is a common phenomenon in concrete structure. Its appearance will not only reduce the anti seepage ability of the building, affect the use function of the building, but also cause corrosion of steel bar, carbonization of concrete, reduce the durability of the material and influence the bearing capacity of the building, so it is necessary to enter the concrete crack. Conduct careful study and treat differently to ensure the safety of buildings and components.

Plastic shrinkage crack

Plastic shrinkage refers to the shrinkage of concrete due to rapid loss of water before condensation. Plastic shrinkage cracks in the new pouring and exposed to the surface of the air in the surface of the structure, the shape is very irregular in the middle and wide, the two ends are thin and different, incoherent state, generally long 20-750px, long cracks can reach 2-3m, wide 1-5mm, similar to dry mud surface. Most of them are in dry hot or windy weather. The temperature of concrete itself is different from that of the outside. The temperature is too high for a long time and the climate is very dry.

Major preventive measures：

• When the concrete is prepared, the water cement ratio and the amount of cement should be strictly controlled. The good graded sand should be selected to reduce the void ratio and sand rate. At the same time, the compaction and compaction should be made to reduce the shrinkage and improve the crack resistance of concrete.

• Before the concrete is prepared, the water of the base and the template is soaked to avoid water absorption in the concrete. After the concrete is poured, the exposed surface should be covered with wet materials in time.

• When the temperature is high, the temperature is low or the wind speed is large, the water spray should be maintained early after the concrete is poured, so that the concrete should be kept moist. The large area concrete should be poured out for a section. In hot season, surface pressure and maintenance should be strengthened.

• Concrete curing can use curing agent, or cover wet grass bags, plastic cloth and other methods. When surface cracks are found, they should be pressed in time and covered with maintenance.

• Use the mixing water that meets the requirements and use clean river sand as much as possible.

• After cracks appear, if the concrete remains plastic, it can be removed in a timely manner by pressing or re vibrating, and then strengthening maintenance. If the concrete is hardened, dry cement powder can be filled into the cracks or treated with thin cement mortar on the surface. For prefabricated components, epoxy resin or epoxy glass cloth can be used to seal the surface of cracks, so as to prevent reinforcement corrosion.

Dry shrinkage crack

The dry shrinkage cracks often occur in a period of time after the end of concrete curing or after the completion of concrete pouring, which is mainly due to the different results of different deformation of water evaporation inside and outside the concrete. The cracks are superficial parallel lines or net thin cracks. The width is mostly between 0.05-0.2mm. For thin beams, plate members often distribute along short direction, and the overall structure occurs mostly at the variable cross-section of the structure. The plane crack extends to the variable section or the block edge. The mass concrete is more common in the plane part, and the thinner Liang Banzhong mostly distributes along its short direction.

Major preventive measures：

• The amount of concrete cement, water cement ratio and sand rate should not be too large. Strictly control the sand content and avoid using excessive silt.

• The concrete should be vibrated and compacted, and the surface of the concrete should be pressed. After the initial setting of concrete, two times of pressure can be applied to improve the tensile strength of concrete, reduce the shrinkage, and set appropriate contraction joints in the concrete structure.

• Strengthening early maintenance of concrete and appropriately prolonging curing time. For a long time, the prefabricated component can cover the curtain, the straw bag, avoid the exposure, and regularly sprinkle water, keep wet, and prolong the heat preservation time of concrete in winter.

Temperature crack

The temperature cracks often occur in the concrete structure of the mass concrete surface or the area where the temperature difference is bigger. The trend of temperature crack is usually not regular, the crack of large area structure often crosscutting, the structure of the length size of beam plate is larger, the cracks are parallel to the short side, the deep and penetrating temperature cracks are parallel or close parallel to the short side direction, the cracks appear along the long edge, and the middle is more dense. The temperature cracks usually vary in width and size, which are more obviously influenced by the temperature change. In winter, the width of the cracks is wider and the summer is narrower. The General intermediate ends are fine caused by the high temperature expansion, but the change of the thickness of the cold shrinkage cracks is not obvious.

Major preventive measures：

• Reasonable selection of raw materials and mix proportion, with good grading of stone, the mud content of sand and gravel is controlled in a lower range, and the mix ratio design is optimized to reduce the amount of cement and reduce the water cement ratio.

• The layers are poured or compacted or mixed with anti cracking and anti seepage agents to improve the tensile strength of concrete, to strengthen the curing and heat preservation of concrete, and to reserve temperature shrinkage joints.

• It is necessary to prolong the curing time in summer and prolong the curing time in order to improve the anti cracking energy in summer. In the winter, the heat preservation and release time should be extended to slow down slowly, in order to prevent the temperature from changing too much, and to avoid the hot weather to pour the large volume concrete.

• The cement should reduce the early hydration rate and the hydration heat, specifically to reduce C3A, alkali content, control cement fineness and particle gradation, mix the mixture reasonably, reduce the temperature of the cement and control the stability of cement, so as to reduce the cement dosage and reduce the hydration heat.

• Temperature cracks affect the corrosion of steel bar, carbonization, freeze thaw resistance and fatigue resistance. Therefore, measures should be taken to control them. For surface cracks, two times of epoxy adhesive or epoxy glass cloth, and smear, cement mortar and other methods are used to seal the surface, and the structure with integral waterproof and impervious requirements should be repaired according to the degree of fracture irrigating, using irrigation mud or chemical slurry to repair cracks, or the grouting and surface closure. It is adopted at the same time.

Settlement crack

Most of these cracks are deep or penetrating cracks, and their direction is related to the subsidence, usually along the ground perpendicular to the ground or in the direction of 30 degrees -45 degrees. The larger subsidence cracks often have a certain dislocation, and the width of the cracks is often proportional to the settlement. The crack width is less affected by the temperature change, and the settlement cracks tend to be stable after the foundation deformation is stable.

Major preventive measures：

• For loose soft soil, the filling foundation should be reinforced and reinforced before construction of the superstructure.

• Ensure that the template has sufficient strength and stiffness, and strong support, and make the foundation uniform

• To prevent the foundation from being soaked in water in the process of concrete irrigation, the time of dismantling the template can not be too early, and the order of dismantling should be paid attention to, and some precautions should be taken when the template is set up on the frozen soil.