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Prevention and treatment of cracks in concrete engineering?
What is the specific content of crack prevention in concrete engineering? Zhong Da Consulting will answer your questions below.

1 Common crack types in concrete engineering

1. 1 shrinkage crack

Shrinkage cracks usually appear on the surface of concrete structures within a period of time after concrete curing or about a week after concrete pouring. Before the cement paste hardens, if the water in it evaporates, shrinkage cracks will appear, which is irreversible. In the initial hardening process of concrete, a large amount of hydration heat will be released, and the internal temperature will continue to rise. At this time, a certain tensile stress will be formed on the surface of concrete structure. After the concrete hardens, the temperature will drop, and due to the action of surrounding concrete members, a certain tensile stress will be generated inside the concrete. When the internal and external tensile stress of concrete exceeds the tensile strength of concrete itself, cracks will occur in concrete. Shrinkage cracks generally appear as parallel lines or meshes. In mass concrete, shrinkage cracks generally appear in the plane, while in beam slab, shrinkage cracks generally distribute along the short direction of beam slab. Shrinkage cracks in concrete usually lead to the corrosion of steel bars in concrete structures, which affects the overall durability of concrete and the bearing capacity of concrete structures.

1.2 temperature crack

In the early hardening process of concrete, cement releases a lot of hydration heat, which leads to the increase of internal temperature and tensile stress on the surface. However, in the later cooling process, due to constraints, tensile stress will appear inside the concrete. The decrease of air temperature will also produce great tensile stress on the concrete surface. When the tensile stress exceeds the tensile strength limit of concrete, cracks will occur on the surface of concrete. The internal humidity of concrete changes little or slowly, but the surface humidity changes greatly, which often shrinks due to drastic changes. Concrete with surface shrinkage is constrained by internal concrete, which will produce great tensile stress and cracks. Concrete is a brittle material, and its tensile strength is about110 of its compressive strength. Although it is clearly required in the design that there is no tensile stress or only a small tensile stress, it is often unsatisfactory in construction. When concrete cools from the highest temperature to a stable temperature during operation, it often causes considerable tensile stress inside the concrete, and sometimes the temperature stress can exceed the stress caused by other external loads, so it is difficult to cause cracks.

1.3 subsidence crack

In concrete engineering, if the soil in the construction area is uneven, soft, the backfill soil is not solid, soaked, the formwork rigidity is insufficient, the formwork support spacing is too large, and the support is loose, it may lead to concrete settlement cracks. Especially when the temperature is low, when the formwork supports the frozen soil, the frozen soil will have uneven settlement after thawing, which will lead to settlement cracks in the concrete structure. The settlement cracks of concrete are usually extended cracks or through cracks, and their trend mainly depends on the settlement. After a large settlement crack occurs, the concrete structure often dislocates, and the width of the settlement crack is directly proportional to the settlement. At the same time, the change of temperature will also have a certain impact on the cracks. After the foundation deformation is stable, the settlement cracks of concrete will not expand basically.

2 concrete engineering crack prevention

In concrete engineering, in order to prevent cracks, when choosing cement, we should try to choose low-heat or medium-heat cement. Because the cracks in concrete are mainly caused by the large temperature difference between inside and outside when the cement is hydrated, choosing low-heat or medium-heat cement can effectively reduce the heat generated in the cement hydration project, thus reducing the temperature difference between the inside and outside structures of concrete. In addition, when mixing concrete, we should try to reduce the amount of cement and improve the aggregate gradation. The dosage of cement is one of the main factors causing concrete shrinkage. By adding fly ash and water-reducing anti-cracking agent, the amount of cement can be reduced, and the reduced volume is supplemented by aggregate, so that the strength of concrete will not decrease under the condition of reducing the amount of cement. Replacing cement with high-quality fly ash can play a lubricating role, improve the fluidity, cohesiveness and water retention of concrete mixture, reduce the heat generated in the hydration process of concrete, and effectively prevent shrinkage cracks. In addition, water reducer and anti-cracking agent can adjust the consistency of cement paste and reduce concrete bleeding and shrinkage deformation. Adding an appropriate amount of admixture into concrete slurry can make the workability of concrete good, form a film on the surface of concrete, reduce the evaporation of water in concrete and effectively reduce the occurrence of shrinkage cracks in concrete.

3 concrete crack control measures

3. 1 Start with design and preparation.

In the structural design of concrete engineering, the overall structure of the project should be handled well. When the whole structure of the project does not have enough room for deformation control, the structural reinforcement should be arranged reasonably in the structural design, which can effectively prevent cracks. In the design, the stress concentration caused by the sudden change of the end face of the structure should be avoided as much as possible. When this situation is unavoidable, measures such as structural reinforcement should be taken to control it to prevent the influence of concentrated stress on concrete structures. In addition, in the process of construction, it is necessary to reasonably arrange the crossover operation between various types of work. After the steel bar at the bottom of the plate is bound, the embedded work of the conduit should be interspersed with the sealing work of the template in time, leaving no tail, which can effectively reduce the number of workers after the steel bar is bound on the plate surface and reduce the weight borne by the structure. In concrete pouring construction, the deformation caused by workers' trampling should be avoided as far as possible in the parts that are prone to cracks.

3.2 Selection of raw materials and concrete mixture ratio

The large accumulation of hydration heat of cement leads to the early temperature rise and late temperature drop of concrete. The hydration heat of cement varies greatly due to the different mineral composition and additive content. In order to reduce the hydration heat of cement and the volume deformation of concrete, medium and low heat cement varieties should be selected for mass concrete, and slag cement with low hydration heat is generally selected. Compared with ordinary portland cement with the same grade, the hydration heat of low-heat slag cement can be reduced by about 1/4. In addition, on the premise of meeting the strength and durability of concrete, the amount of cement should be reduced as much as possible.

Selection of (1) additives

After adding a certain amount of fly ash into concrete, the workability of concrete can be significantly improved under the condition of constant water consumption because the fly ash particles are spherical and have a "ball effect". If the original fluidity of concrete mixture is kept unchanged, the unit water consumption can be reduced, thus improving the compactness and strength of concrete. It can be seen that adding a proper amount of fly ash into concrete can not only meet the workability of concrete, but also reduce the hydration heat.

(2) Addition of additives

In concrete engineering, a certain amount of admixture is usually added. Fly ash or calcium lignosulfonate water reducer can be added to general concrete. For example, adding 0.25% water reducing agent to concrete can reduce cement by about 10%. While reduce that cost of concrete, it also greatly improves the performance of concrete. On the one hand, because the admixture replaces the temperature difference of some cement, it is beneficial to crack prevention; On the other hand, the water reducer can retard setting, avoid cold cracks in concrete, improve workability and fluidity, and is beneficial to pumping.

3.3 Reasonable pouring and curing of concrete

(1) Reasonable pouring

In concrete pouring construction, comprehensive layering method, sectional layering method and inclined plane layering method should be selected according to the requirements of overall continuous pouring and the size of components. It is also necessary to follow the principle of "fixed points by sections, one slope, thin layer pouring, step by step, and reaching the top at one time" that has been formed in concrete construction. The pouring temperature should be controlled below 25 degrees. Therefore, it is necessary to arrange the construction time reasonably and try to avoid pouring concrete at high temperature. At the same time, try to speed up the construction, shorten the pouring time, reduce the pouring temperature of concrete, reduce the temperature difference between inside and outside the structure, and extend the initial setting time of concrete.

(2) Maintenance measures

After each concrete pouring, heat preservation and curing shall be carried out in time according to the requirements of temperature control technical measures. In different construction seasons, different concrete curing methods should be chosen. During construction in summer, cooling methods such as straw curtain covering, water storage, water spraying and water spraying should be adopted for maintenance; When the temperature is normal, it can be maintained by spraying health care liquid; In winter construction, sealing materials such as film curing liquid and plastic film can be used to keep concrete warm and moist, and wind-proof heat preservation shed can be set up in cold season.

4 abstract

In concrete engineering, cracks will do great harm to the stability of buildings and building structures and pose great threat to the safe use of buildings. Therefore, the prevention and control of cracks in concrete engineering is the focus of the whole concrete work. During construction, strict control should be taken, early maintenance and preventive measures should be taken to minimize the probability of concrete cracks, and remedial measures should be taken in time to ensure the stability of the overall structure of the project.

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