In concrete structures, the problem of cracks is a common phenomenon that has always existed. Due to various problems in design, construction and other links, many concrete structures inevitably appear in different degrees and forms in the process of construction and use. crack. Therefore, under the scientific method, the cracks should be reasonably classified, the main causes of crack formation in concrete structures should be identified, and the design, materials, and related processes should be effectively controlled, and the harmful degree of cracks should be controlled within the allowable range. , thereby minimizing the impact.
1.Classification of cracks in concrete structures:
1.1 Cracks during construction
For the special building material of concrete structure, the causes and conditions of cracks are various and complicated. The strength of early concrete is not high. Once it is subjected to tensile force or impact, it is easy to cause cracks and cracks. In the process of construction, there are also various uncertain factors, which lead to the occurrence of cracks.
1.1.1 Temperature change
For concrete, the phenomenon of temperature change is one of the causes of cracks. During construction, the heat of hydration reaction produced by the concrete causes a change in its internal temperature. The heat of hydration reaction is an exothermic process. The heat release is large in the early stage and gradually decreases in the later stage, which makes the temperature inside the concrete increase rapidly in the early stage and slow in the later stage. At the same time, there is also heat exchange between the concrete structure and the outside world, which basically has a stable state from heating up to cooling down. The expansion and contraction of concrete makes its tensile stress change constantly. Once it exceeds the range that the concrete itself can bear, it will lead to the generation of cracks.
1.1.2 Construction Factors
In the construction process of concrete structures, a series of maintenance from the start of concrete pouring to the completion of concrete pouring and setting also greatly affects the crack problem. For example: ①The covering process is not carried out in time after the concrete is poured, which leads to the rapid evaporation of water on the concrete surface; ②During the pouring process, sufficient vibration is not carried out within a certain period of time, so that after the completion of the concrete aggregate distribution Segregation phenomenon occurs; ③The construction is not carried out strictly in accordance with the specifications, the curing time is insufficient, and the strength is insufficient; ④There is a high-strength turnover cycle in some projects. Stacking, these will lead to cracking of the floor and produce cracks.
1.2 Cracks due to design problems
In the concrete structural components, the crack problem will also be caused to a certain extent due to the design reasons. There are generally the following reasons: ① The shrinkage of the concrete structural components is not fully considered under certain conditions; ② The designer does not fully consider the shrinkage of the concrete structural components; Reasonable deflection design is carried out according to the crack resistance of the actual material; ③ For the concrete material, the designer does not compare the difference, but design according to the past experience; ④ When configuring the reinforcement, due to the negligence of the structural calculation personnel , The unreasonable configuration of too few or too thick steel bars will cause cracks in the components; ⑤ The load is not fully considered, and the design load is too small, resulting in the design force being less than the actual component force, resulting in cracking.
1.3 Cracks caused by material problems
Nowadays, the use of concrete in project construction generally comes from concrete mixing plants. From the start of mixing to transportation to the completion of hardening, the concrete itself is affected by the distance of transportation, the method of transportation, the mix ratio, the heat of hydration, etc. The reason, to a certain extent, will make it affected to a certain extent. At the same time, the variety and type of cement, the addition and use of admixtures, the strength grade of cement and the strength of concrete, the aggregate particle size, and the content of needle flakes will all bring certain material characteristics to the concrete itself. It is also one of the important causes of cracks in concrete structures.
2. Treatment measures for cracks in concrete structures:
2.1 Strictly control the construction temperature and the construction process
One of the important steps in the concrete construction process is temperature control. The unsatisfactory temperature control effect is also the cause of concrete cracks. Starting from the construction process can effectively reduce the impact. Do a good job in the insulation and cooling of concrete to ensure that the heat of hydration of concrete can be dissipated in time within a certain period of time, and do a good job of surface maintenance to ensure that the curing conditions of concrete are met.
At the same time, in the process of pouring the concrete, it should be vibrated sufficiently to ensure the compactness of the structural components, and it should be carried out in strict accordance with the requirements of the construction specification. to proceed to the next work.
2.2 Optimal structural design
When designing a concrete structure, a comprehensive analysis should be carried out from various factors. While choosing an efficient and safe structural form, the characteristics of the material should also be combined, and comprehensive consideration should be taken while designing, so as to improve the concrete structure. The crack resistance of the structure.
In the process of rebar configuration, the structural stress analysis should be carried out carefully, and the configuration should be carried out reasonably. Special attention should be paid to the selection of the diameter and quantity of the rebar.
For the load borne by the structural members, the designer should fully consider during the design process to avoid concentrated stress on the structure, and strengthen it appropriately if it cannot be satisfied in some aspects.
2.3 Concrete material control
As a kind of mixture, concrete is made of a variety of materials. For cracks in components, it can be dealt with from the following aspects.
①According to the actual situation of the project, the construction mix ratio of concrete is strictly designed, by improving the gradation of aggregates, and at the same time selecting suitable cement varieties; ②Ensure that the concrete meets the design conditions and does not violate the relevant engineering specifications. Under the premise, the cement with the lower grade should be selected as much as possible to prepare the concrete; ③ Under the premise of meeting the requirements of pumping concrete, the aggregate with good gradation and larger particle size should be selected as far as possible, so that the cement mortar and the aggregate can be separated. The contact area is greatly improved, and the bonding force between the two is enhanced; ④ The water-cement ratio is strictly controlled, and appropriate additives are selected and added.
2.4 Treatment of the generated cracks
2.4.1 Surface treatment method
Among the types of cracks in concrete structures, some cracks do not affect the overall stability and bearing capacity of the structure. For such cracks that exist in the surface or shallower layers, the general practice is to apply grout on the surface of the cracks. At the same time, in order to prevent the existing cracks from continuing to crack under the influence of other external factors, glass fiber cloth can generally be pasted on the surface.
2.4.2 Gluing method
When the existence of concrete cracks affects the overall stability and bearing capacity of the structure, as well as the impact on anti-seepage, the glue-filling method can generally be used to repair the cracked parts. Use concrete pressure equipment to grouting the cracks, so as to achieve the effect of repair and reinforcement.
2.4.3 Concrete replacement method
For concrete structural members with large cracks and serious damage, the concrete replacement method is a more commonly used method. The treatment method is to remove the damaged concrete with tools, re-form the formwork, and replace it with concrete with a higher strength, so that the concrete can be effectively repaired.
