1 Overview
Concrete is a non-homogeneous material composed of cement stone, sand (fine aggregate) and stone (coarse aggregate). Since air pockets, micropores and microcracks exist during the hardening process of cement stone, from a microscopic perspective, concrete is a multi-phase combination of cement stone, sand, stone and micropores and microcracks filled with air and water. Microcracks themselves do not do much harm to the load-bearing, anti-seepage and other functional functions of concrete. However, when concrete is subjected to large loads and temperature differences, micro-cracks will continue to expand and connect, eventually forming cracks visible to our naked eyes, which are often referred to as cracks in concrete engineering. From the perspective of practical application, a considerable number of cracks will not cause much harm to the stress and normal use of the building. However, the existence of cracks will affect the integrity and durability of the building, cause corrosion to the steel bars, and produce lifetime stress. Concentration phenomenon, therefore attention should be paid to all aspects to avoid the occurrence of cracks or to control cracks within the allowable range.
2 Causes of cracks
The reasons for cracks in reinforced concrete are relatively complex, mainly including material and climate factors, improper construction, improper design and construction, changing the use function or unreasonable use, etc. It can be summarized into the following aspects:
2.1 Design reasons
(1) The sudden change of the structural section during the design leads to the concentration of component stress and the occurrence of component cracks.
(2) Improper prestressing is applied to components during design, causing cracks in components (eccentricity, excessive stress, etc.). cracks (eccentricity, excessive stress, etc.).
(3) The configuration of structural steel bars in the design is too small or too thick, causing component cracks.
(4) The shrinkage deformation of concrete members was not fully considered in the design.
(5) When the house is long, expansion joints are not installed, causing shrinkage cracks in weak links. (According to the American Concrete Institute, concrete has two types: dry shrinkage and temperature deformation. The dry shrinkage deformation shrinks about 19 mm every 30.48 m. The deformation caused by temperature change is about 19 mm shrinkage or extension every 30.48 m at a temperature change of 37°C. Domestic Some people believe that the longitudinal shrinkage of a 40 m long floor slab due to hardening and solidification is 8 to 20 mm)
2.2 Material reasons
( 1) The mud content of coarse and fine aggregate is too large, causing the shrinkage of concrete to increase. Poor aggregate particle gradation or inappropriate discontinuous gradation can easily cause the concrete to shrink and lead to cracks.
(2) Improper selection of concrete admixtures and admixtures can seriously increase concrete shrinkage.
(3) Due to the cement type, the shrinkage of slag Portland cement is larger than that of ordinary Portland cement, the shrinkage value of fly ash and alumina cement is smaller, and the shrinkage of fast hardening cement is larger.
(4) Reasons for cement grade and concrete strength grade: The higher the cement grade, the finer the fineness, and the higher the early strength, the greater the impact on concrete cracking. The higher the concrete design strength level, the more brittle the concrete and the easier it is to crack.
2.3 Reasons for concrete mix ratio
(1) Improper selection of cement grade or variety in design.
(2) The water-cement ratio in the mix ratio is too large.
(3) Improper selection of sand ratio and water-cement ratio in mix design will cause workability deviation of concrete, resulting in concrete separation, bleeding, poor water retention, and increased shrinkage value.
(4) The greater the amount of cement and water used in unilateral concrete, the greater the volume of cement slurry, the greater the slump, the greater the shrinkage, and the easier it is to produce cracks.
(5) The amount of concrete expansion agent in the mix design is improperly selected.
2.4 Reasons for construction and on-site maintenance
(1) When pouring concrete on site, the vibrator is vibrated or inserted improperly, causing leakage of vibration, over-vibration, or excessive withdrawal speed of the vibrator. Soon, it will affect the compactness and uniformity of concrete, leading to the occurrence of cracks.
(2) For large-volume concrete projects, the lack of two plasters can easily cause surface shrinkage cracks.
(3) During large-volume concrete pouring, the calculation of the heat of hydration is inaccurate, and the on-site concrete cooling and insulation work is not in place, resulting in an excessive temperature difference between the inside and outside of the concrete, and the concrete is prone to temperature cracks.
(4) On-site maintenance measures are not in place, and early dehydration of concrete causes shrinkage cracks.
2.5 Use-related reasons
The cause of cracks in the use of buildings and industrial plant cracks is the change of use functions. For example, some industrial plants were originally designed as light industrial plants, but after they were completed and put into operation, they were converted into equipment plants. The original design load was light, but the introduction of a large number of production equipment produced a weight that was double or even several times the original load. These are the main causes of cracks in factory buildings.
3 Crack control measures
3.1 Design measures
(1) Configure structural reinforcements according to specifications to improve crack resistance. Component reinforcements should be of small diameter, Small spacing. The reinforcement ratio of the full section should be between 0.3 and 0.5. When using small-diameter steel bars, appropriately increasing the reinforcement ratio can increase the ultimate tensile strain of concrete.
(2) Avoid stress concentration. Stress concentration will occur around holes, corners of variable sections, corners, etc. due to temperature changes and shrinkage of concrete, leading to cracks. To this end, diagonal steel bars and steel mesh can be added around the holes; to avoid sudden changes in the section at the changing section, local treatments can be made to make the section gradually transition, and anti-cracking steel bars can be added at the same time.
(3) Set up concealed beams at the edge areas where cracks are prone to occur to increase the reinforcement ratio in this area and improve the ultimate tensile properties of concrete.
(4) In the structural design, the climatic characteristics during construction should be fully considered, and the post-pouring belt should be reasonably set up. Under normal construction conditions, the post-pouring belt should be properly set up. The spacing between post-cast joints is 25 to 30 m, and the retention time is generally not less than 60 days. If the specific conditions during construction cannot be predicted, the design can be temporarily changed based on the specific conditions.
3.2 Construction Measures
(1) Strictly control the quality and technical standards of concrete raw materials, select low hydration heat cement, and the mud content of coarse and fine aggregates should be controlled within 1~ 1.5 or less.
(2) Carefully analyze the proportion of concrete aggregates, control the water-cement ratio of concrete, reduce the slump of concrete, and rationally add plasticizers and water-reducing agents.
(3) The pouring time should be arranged at night or when the temperature is low to minimize the initial setting temperature of concrete. When the temperature is high during construction, simple shading facilities should be set up in the sand and stone yard, and cold water should be sprayed on the aggregate when necessary. When pumping concrete, cover the horizontal and vertical pump pipes with straw bags and spray cold water.
(4) Strengthen the pouring and vibration of concrete to improve the density; use multiple vibration techniques to improve the strength of concrete and improve crack resistance.
(5) The concrete formwork removal time should be arranged as late as possible and the concrete surface temperature should not drop by more than 10℃ after the formwork is removed.
(6) According to the specific project characteristics, UEA shrinkage-compensating concrete technology is used.
(7) For high-strength concrete, medium-heat micro-expansion cement should be used as much as possible, mixed with ultra-fine mineral powder and expansion agent, and high-efficiency water reducing agent should be used.
4 Crack treatment
4.1 Surface repair method
The surface repair method is mainly suitable for the treatment of surface cracks that are stable and have little impact on the structural load-bearing capacity. The usual treatment method is to apply epoxy glue on the surface of the crack or paint, asphalt and other anti-corrosion materials on the concrete surface. While protecting, in order to prevent the concrete from continuing to crack under the influence of various effects, glass can also be pasted on the surface of the crack. fiber cloth.
4.2 Grouting method
The grouting method is mainly suitable for repairing concrete cracks that have an impact on the structural integrity or have anti-seepage requirements. It uses air pressure equipment to press cementing materials into In the cracks of concrete, the cementing material hardens and forms a whole with the concrete, thereby achieving the purpose of sealing and strengthening. Commonly used cementing materials include cement slurry, epoxy resin, methacrylate, polyurethane and other chemical materials.
4.3 Caulking and sealing method
Caulking method is the most commonly used method for crack sealing. It usually involves cutting grooves along the cracks and filling the grooves with plastic or Rigid water-stop material to seal cracks. Commonly used plastic materials include polyvinyl chloride mastic, plastic ointment, butyl rubber, etc.; commonly used rigid water-stopping materials are polymer cement mortar.
4.4 Structural reinforcement method
Structural reinforcement can be adopted for cracks caused by overload, reduced durability of concrete caused by cracks left untreated for a long time, cracks caused by fire, etc. Law. Structural reinforcement methods include section reinforcement method, anchor reinforcement method, prestressing method, etc.
5 Conclusion
Cracks in reinforced concrete structures are inevitable, but they can be minimized by taking control measures. This article analyzes the various causes of cracks in concrete structures from the aspects of design, construction, and construction material deformation, and proposes technical measures for crack control from the aspects of materials, design, and construction for discussion.
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