Causes and prevention of cracks in the hottest rei

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Causes and prevention of cracks in reinforced concrete structures

cracks in building structures are a very common phenomenon. Theoretically, concrete structures, especially flexural members, always work with cracks. Under the condition of small service load, there is no crack gap or such structural crack gap is very subtle, which is not easy to be detected by the naked eye. However, in real buildings, there will be various cracks in concrete structures, among which the most common ones are reinforced concrete components and brick wall cracks. Here we mainly discuss that the causes of cracks in reinforced concrete beams are very complex, mainly including material or climate factors, improper construction, design and construction errors, changing the use function or unreasonable use, etc., which can generally be summarized as the following:

analysis of common cracks in reinforced concrete

1, material quality

cracks caused by material quality problems are more common because of poor quality of cement, sand, stone, etc, If such unqualified materials are used in the project, it will produce "tofu dregs project". Therefore, only when the quality of materials is well controlled, the project quality can be fundamentally guaranteed

2. Construction technology

construction technology involves a wide range of aspects, and it is impossible to describe them one by one. Generally, it is often used:

(1). Water evaporation and concrete shrinkage of cement stones are usually important reasons for concrete cracks

(2) concrete is an artificial mixed material, and an important sign of its quality is the uniformity and compactness of the concrete after molding. Therefore, any defects and omissions in the processes of concrete mixing, transportation, pouring and vibrating may be the direct or indirect cause of cracks

(3) improper formwork structure, water leakage, grout leakage, insufficient support stiffness, supported foundation subsidence, premature formwork removal, etc. may cause concrete cracking. During the construction process, the surface pollution of reinforcement, the too small or too large concrete guarantee layer, the collision of reinforcement during pouring and its displacement may cause cracks. The construction control is not strict, and the overload and surcharge may also cause cracks

(4) concrete curing, especially the early curing quality, is closely related to cracks. When the concrete is not completely hardened after several 10 years of canned packaging, shrinkage cracks will occur if it dries too fast, The following is the monthly import quantity and amount of plastic extruders since 2016 (unit: set usually occurs on the surface, with irregular cracks and small width. In addition, during the hydration and hardening process of cement, a large amount of heat is emitted, resulting in a temperature difference between the inside and outside of the concrete. When it exceeds a certain value, cracks will occur due to inconsistent shrinkage of the concrete.

(5) avoid construction under extreme weather conditions, which can reduce the cracking of the concrete structure

3. Foundation deformation

in reinforced concrete structures, the main cause of cracking is uneven settlement. The size, shape and direction of cracks depend on the deformation of the foundation. Because the stress caused by the deformation of the foundation is relatively large, the cracks are generally penetrating

4. Temperature deformation

concrete has the property of thermal expansion and cold contraction, and its linear expansion coefficient is generally 1 ×/0C. When the ambient temperature changes, temperature deformation will occur, resulting in additional stress. When this stress exceeds the tensile strength of concrete, cracks will occur. In engineering, such cracks are more common, such as cracks on cast-in-place roof panels, cracks in mass concrete, etc

5. Humidity deformation

when the concrete hardens in the air, the volume will gradually decrease, which is generally called dry shrinkage. Shrinkage cracks are common in cast-in-place wall slab structures, cast-in-place frame structures, etc., which are usually caused by poor maintenance. The shrinkage value of concrete is generally 0.2~0.4 ‰, and its development law is fast in the early stage and slow in the late stage. Therefore, for ultra long buildings or structures, it is usually to add micro expansion agent to the automobile front-end module, which can basically solve the early drying shrinkage problem of concrete

6. Structure under load

there are many factors that cause cracks after the structure is under load, and cracks may occur during construction and use. For example, cracks may occur due to early earthquake, premature formwork removal or improper method, improper placement of cushion blocks or lifting points during component stacking, transportation, hoisting, overload construction, excessive tensioning prestress value, etc. The most common reinforced concrete beams and slabs are flexural members, which often have different degrees of cracks under service loads. When ordinary reinforced concrete members bear 30 ~ 40% of the design load, cracks may appear, which is generally difficult to detect with the naked eye, and the ultimate failure load of the members is often more than 1.5 times the design load, so in general, reinforced concrete members are allowed to work with cracks. In the process of use, changing the original use function, such as changing the office into a warehouse, adding layers to the roof, improper use, increasing the load, etc., may cause cracks. In the reinforced concrete design code, the maximum width of cracks is specified to be 0.2 ~ 0.3mm under different conditions Cracks whose width exceeds the specification and cracks are not allowed should be considered harmful and should be carefully analyzed and handled

7. The design is not comprehensive

if the section is insufficient, the span of the beam is too large, the height is too small, or due to calculation errors, the section of the stressed reinforcement is too small or the plate is too thin, the reinforcement position is improper, the node is unreasonable, the section of the structural member is abrupt, or the stress concentration is caused by opening and grooving, and the structural treatment is improper, if there is no additional stirrup at the place where the cast-in-place main beam is placed on the secondary beam, Or additional hanging bars and improper setting of various structural joints can easily lead to concrete cracking, which conforms to the testing of bending, stretching, tightening, peeling and impact mechanical properties

8. Creep

examples of cracking or crack development caused by concrete creep are also very common in engineering. According to the literature, the compression and creep of concrete in the section of bending members can increase the deformation of members by 2-3 times, and the prestressed structure will produce large stress loss due to creep, which will reduce the crack resistance of the structure

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