With the more and more extensive application of concrete structure in building engineering, a large number of concrete cracks appear in construction, which seriously affects the safety of buildings. This paper analyzes the causes of concrete cracks in building engineering and discusses the preventive measures.
Concrete is a kind of heterogeneous brittle material, which is composed of sand and gravel aggregate, cement, water and other additional materials. Due to a series of problems such as concrete construction, deformation and constraint, there are many micropores, bubbles and micro-cracks in hardened concrete. When mass concrete hardens, it will release a lot of hydration heat, resulting in high internal temperature of concrete and frequent cracks. Micro-cracks do not damage the load-bearing and anti-seepage functions of concrete, but after concrete is subjected to load and temperature difference, micro-cracks will continue to expand and penetrate, eventually forming cracks visible to the naked eye. When the concrete crack width exceeds the specified limit, it will affect the applicability and durability of buildings and components, not only damage the appearance image, but also cause the exposure and corrosion of steel bars, reduce the bearing capacity of building structures and reduce the integrity and stiffness of building structures.
There are many forms and types of cracks. In order to control and solve the problem of concrete cracks, it is necessary to start with the causes of concrete cracks. Correctly judging and analyzing the causes of concrete cracks is the most effective way to effectively control and reduce concrete cracks. Therefore, it is very beneficial to control the construction quality to discuss the causes and control methods of concrete cracks.
1 Cause analysis of concrete cracks in building construction
1. 1 reasons for architectural structure design
Unreasonable architectural design leads to cracks in concrete structures, mainly in the following aspects: (1) stress concentration caused by sudden change of cross section in the structure leads to cracks in components; Improper prestress of components causes cracking of components; Structural reinforcement configuration is too small or too thick, resulting in cracking of components; The shrinkage deformation of concrete members is not fully considered; The high grade of concrete used leads to excessive ash consumption, which is not conducive to shrinkage. During the hardening process of concrete, water evaporates, and the volume gradually shrinks, resulting in shrinkage. However, due to the constraint of the support, the periphery of the plate cannot extend freely. When the constraint stress of slab caused by concrete shrinkage exceeds a certain degree, cracks will inevitably appear in cast-in-place slab.
1.2 concrete material factors
First of all, the drying shrinkage of mortar mixed with different types and contents of cement is very different. The shrinkage of slag portland cement is larger than that of ordinary portland cement, while that of fly ash cement is smaller and that of quick-hardening cement is larger. Generally speaking, when the water-cement ratio is constant, the more cement is used, the greater the shrinkage of concrete, because the drying shrinkage of concrete is mainly caused by the drying shrinkage of cement slurry, and the less cement slurry, the more obvious the restriction of aggregate in concrete on drying shrinkage.
Secondly, the evaporation of water in concrete leads to the shrinkage of concrete. The greater the water-cement ratio, the thinner the cement slurry, the greater the shrinkage rate and the greater the possibility of cracking. At the same time, reducing water consumption and cement consumption is more effective to improve the drying shrinkage and crack resistance of concrete, but to use it under the guidance of correct methods, the design strength requirements of concrete must be guaranteed.
Thirdly, excessive mud content in coarse and fine aggregates and poor aggregate particle grading will cause concrete shrinkage and cracks. High aggregate density, good gradation, high elastic modulus and large aggregate particle size can reduce the shrinkage of concrete. Admixtures and admixtures will affect the hardening speed, water consumption, shrinkage and creep of concrete, thus affecting the cracking of concrete. The dry shrinkage value of concrete mixed with admixture is large, especially the initial dry shrinkage value is large. The effective method to prevent cracks is to use expansion agent, which can effectively control the occurrence of cracks because of the early expansion of concrete and the small dry shrinkage value in the later period.
Finally, the more steel bars are used in concrete, the stronger the gripping force is, which restricts the deformation of concrete, reduces shrinkage and prevents cracks. Welding steel mesh, reasonable arrangement of longitudinal bars and stirrups, and thin and dense steel bars can effectively prevent cracks.
1.3 concrete mixture ratio
Poor grading of aggregate particles or improper discontinuous grading will easily lead to concrete shrinkage and cracks. Excessive water-cement ratio of concrete or excessive use of silt can also cause floor cracking. When the same cement and the same strength grade are used, the strength grade of concrete mainly depends on the water-cement ratio. When cement is hydrated, excess water remains in concrete, and bubbles or pores are formed after evaporation, which reduces the actual effective cross section of concrete to resist load. Under the load, stress concentration may occur around the pores, which may lead to cracks on the floor surface. However, the concrete made of silt with large silt content has large shrinkage and low tensile strength, and is prone to cracks due to plasticity. Improper mix design directly affects the tensile strength of concrete and is the cause of concrete cracking. Improper mixture ratio refers to excessive cement dosage, high water-cement ratio, improper sand content, poor aggregate type and improper admixture selection.
1.4 construction technology and maintenance reasons
First of all, uneven concrete mixing, too long mixing time, too long transportation time, changing the mixture ratio during transportation and pumping, unreasonable pouring sequence and too fast construction will change the quality of concrete, reduce the performance of concrete, and cause cracks in concrete structures or components after pouring. When vibrating concrete on site, improper vibration or insertion, vibration leakage, excessive vibration or too fast vibration withdrawal will affect the compactness and uniformity of concrete and induce cracks.
Secondly, the curing of concrete can change the hydration reaction speed of concrete and affect the strength of concrete. The higher the humidity, the lower the temperature and the longer the curing time. The smaller the shrinkage of concrete. In the process of curing, the hydration heat of concrete must be strictly controlled, and the mixed concrete should be precooled and cooled to minimize the maximum temperature and temperature gradient of concrete after pouring and the external constraints on concrete. The purpose of concrete curing is to ensure the normal setting and hardening of concrete. If the curing time is too short and the humidity is too low, the concrete will shrink and crack.
Finally, in the construction, the vibrating rod is directly placed on the steel bar to vibrate, and the steel bar is disturbed. At the same time, the poured concrete vibrates prematurely, which affects the bond between the steel bar and the concrete and also affects the uniformity and compactness of the concrete. The insufficient thickness of the protective layer of steel bars reduces the bond between steel bars and concrete, and weakens the constraint on the deformation and cracking of concrete. When the wind speed is too high or the sun is exposed, the shrinkage value of concrete is large. After mass concrete members are poured, plastering times and heat preservation work are not in place, and surface shrinkage cracks are easy to occur.
2. Precautionary measures for concrete cracks in building construction
2. 1 Do a good job in design and preparation before construction.
In the design of building structure, we should design the structure under the constraint condition. When the structure does not have enough deformation space, the structural steel bars should be reasonably arranged in the structural design to prevent cracks. In the design, stress concentration caused by sudden change of structural section should be avoided as far as possible. If it is not timely due to structural or modeling reasons, reinforcement measures such as structural reinforcement should be fully considered. Actively adopt shrinkage compensating concrete technology.
Arrange the crossover operation time of each type of work as reasonably and scientifically as possible. After the steel bar at the bottom of the plate is bound, the pipeline should be embedded in time, and it should be completed before the mold is sealed, so as to effectively reduce the number of workers after the steel bar is bound. When pouring, concrete workers should lay temporary movable springboard in the parts where cracks and negative bending moment steel bars are easy to occur, so as to expand the contact surface, disperse the stress and try to avoid the upper steel bars from being trampled again.
2.2 Pay attention to the selection and proportion of concrete raw materials.
First of all, if aggregate with high water absorption is used in concrete, the drying shrinkage of concrete will increase when the drying shrinkage is large and the aggregate contains more mud. When the aggregate size is large and the gradation is good, the drying shrinkage of concrete is small, because it can reduce the amount of cement slurry in concrete. Adding fly ash can reduce cement consumption and hydration heat, water consumption and cement consumption of concrete, and reduce the volume shrinkage of concrete itself. At the same time, adding fly ash or superplasticizer into concrete can not only make concrete have good workability, pumpability, impermeability and segregation resistance, but also reduce bleeding, which is beneficial to concrete surface treatment.
Secondly, the mixture ratio designer should go deep into the construction site, reasonably choose the design slump of concrete according to the pouring technology, operation level and component section of the construction site, adjust the construction mixture ratio in time according to the quality of sand and gravel raw materials on site, and assist the site to do a good job in component maintenance. Improve aggregate gradation, add fly ash or superplasticizer to reduce cement consumption and hydration heat; Selecting low-alkali cement and low-alkali or alkali-free additives; Actively adopt appropriate additives and concrete admixtures to inhibit alkali-aggregate reaction; Correctly grasp the application method of concrete shrinkage compensation technology.
Finally, according to the requirements of concrete strength grade and quality inspection and the workability of concrete, the mixture ratio is determined, and the water cement ratio and cement dosage are strictly controlled. Choose well-graded stone, control the particle size and content of sand, reduce the porosity to reduce the shrinkage of concrete and improve the crack resistance of concrete. 2.3 to strengthen the requirements of concrete pouring.
First of all, it is suggested that the vibrator vibrate vertically and be arranged in determinant, so that it can be inserted quickly and pulled slowly. According to the different slump of concrete, the vibrating time should be controlled correctly to avoid over-vibration or vibration leakage. Advocate secondary vibrating and secondary plastering technology to eliminate bleeding, water and bubbles in concrete. Because many pumped concrete flows freely, concrete is poured in layers. When vibrating the upper layer, the next layer should be inserted to eliminate the seam between the two layers. The naturally formed thickness of the upper concrete should not exceed 1.25 times of the vibrator length. The vibrating time of concrete should not be too long, usually 8 s ~ 10 s, to prevent uneven concrete structure caused by stone sinking. When pouring concrete on the surface, the surface should be leveled and compacted to eliminate bleeding, moisture and bubbles inside the concrete, so as to improve the compactness of the concrete.
Secondly, after cast-in-place molding, necessary water storage and heat preservation measures should be taken, and the surface should be covered with film and wet sacks for maintenance to prevent temperature cracks caused by excessive temperature difference between the inside and outside of concrete. Avoid pouring concrete in rain or strong wind; For underground structure concrete, backfill soil as soon as possible is beneficial to reduce cracks. When supporting the formwork, it is necessary to ensure that the formwork has sufficient strength and rigidity, the support is firm, and the foundation is uniformly stressed.
Finally, it is forbidden to sprinkle concrete on the parts that have not been poured, so as to avoid forming potential cold joints or weak points. When pouring concrete on the surface, the surface should be leveled and compacted to eliminate bleeding, moisture and bubbles inside the concrete, so as to improve the compactness of the concrete.
2.4 Take reasonable maintenance measures.
First of all, heat preservation and curing is the key link of concrete construction, and its main purpose is to reduce the temperature difference between the inside and outside of mass concrete pouring blocks, so as to reduce the self-restraint stress of concrete blocks. It can reduce the cooling speed of concrete pouring block, make full use of the tensile strength of concrete, improve the crack resistance of concrete block under the action of external binding force, and achieve the purpose of preventing or controlling temperature cracks.
Secondly, properly raising the curing environment temperature will help to reduce the temperature difference between inside and outside, slow down the cooling speed, thus reducing the temperature stress, and also help to increase the strength and stress relaxation of concrete, thus avoiding the plastic shrinkage of concrete caused by surface cracking. During the curing period, the difference between the surface temperature of concrete and its central temperature shall not exceed 25℃. In the process of concrete pouring, in case of stormy weather, protective rainbows should be erected to cover it, and at the same time, open ditches should be well drained around to prevent rainwater from flowing into the foundation pit, so as to ensure the continuity of concrete pouring and the construction quality.
Finally, strengthen the maintenance of cast-in-place slab. Concrete curing is an indispensable link in the whole construction process. Ignoring the curing of concrete will not only reduce the strength of concrete, but also lead to cracks because the water loss in the hardening process can not be compensated in time. Especially in the construction at high temperature, frequent watering can reduce the cracks caused by temperature, reduce the constraint stress caused by concrete shrinkage and effectively control the cracks.
3 Conclusion
Cracks are common phenomena in concrete structures. Their appearance will not only reduce the impermeability of buildings and affect the use function of buildings, but also cause corrosion of steel bars and carbonation of concrete, reduce the durability of materials and affect the bearing capacity of buildings. In the construction, we should grasp the concrete mix design, construction technology, time and depth of joint cutting, concrete maintenance and other links. By improving the quality of concrete and taking effective preventive measures after pouring, concrete cracks can be avoided.
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