一. The treatment method of ordinary concrete crack
1. Surface repair
Commonly used methods include compaction and smoothing, application of epoxy adhesive, spraying of cement mortar or fine stone concrete, pressing and application of epoxy mastic, epoxy resin pasting off-duty silk cloth, increasing the overall surface layer, and suturing of steel anchor bolts. Surface smearing and surface patching method The scope of application of surface smearing is thin and shallow cracks that are difficult to pour into with grout, hairline cracks whose depth does not reach the surface of the steel bar, cracks that do not leak, cracks that do not stretch and cracks that are no longer active. The surface patch (geomembrane or other waterproof sheet) method is suitable for seepage prevention and plugging of large-scale water leakage (honeycomb pockmarked surface, etc. or it is difficult to determine the specific leakage location and deformation joint).
2. Partial repair method:
Commonly used methods include filling method, prestressing method, partial chisel removal and re-pouring of concrete, etc.
Fill the cracks directly with repairing materials, generally used to repair wider cracks, the operation is simple and the cost is low. For cracks with a width less than 0.3mm and a shallow depth, or cracks with fillers, cracks that are difficult to achieve with grouting, and small-scale cracks, simple treatment can be done by opening V-shaped grooves and then filling them.
3. Cement pressure grouting method
It is suitable for stitching stable cracks with a width ≥ 0.5mm.
This method has a wide range of applications, from small cracks to large cracks, and the treatment effect is good. Use the pressure feeding equipment (pressure 0.2~0.4Mpa) to inject the joint filling slurry into the concrete crack to achieve the purpose of occlusion. This method is a traditional method and the effect is very good. You can also use the elastic joint sealer to inject the joint glue into the cracks without electricity, which is very convenient and the effect is ideal.
4. Chemical grouting
It can be poured into cracks with a crack width ≥ 0.05mm.
5. Reduce the internal force of the structure
Commonly used methods include unloading or controlling loads, setting up unloading structures, and adding fulcrums or supports. Change simply supported beams to continuous beams, etc.
6. Structural reinforcement
Commonly used methods include adding steel bars, thickening slabs, outsourcing reinforced concrete, outsourcing steel, pasting steel plates, prestressed reinforcement systems, etc.
The structural reinforcement method can be adopted for the cracks caused by overload, the reduction of concrete durability caused by the cracks not being treated for a long time, and the cracks caused by fire, which affect the structural strength. Including section reinforcement method, anchor reinforcement method, prestressing method, etc. The inspection of concrete crack treatment effect includes repair material test; core sampling test; water pressure test; air pressure test, etc.
7. Change the structural scheme and strengthen the overall rigidity
For example: cracks in the frame are dealt with by adding partitions and deep beams.
8. Concrete replacement method
Concrete replacement is an effective method of dealing with severely damaged concrete by first removing the damaged concrete and then replacing it with new concrete or other materials. Commonly used replacement materials are: ordinary concrete or cement mortar, polymer or modified polymer concrete or mortar.
9. Electrochemical protection method
Electrochemical anti-corrosion is to use the electrochemical action of the applied electric field in the medium to change the environmental state of concrete or reinforced concrete and passivate the steel bars to achieve the purpose of anti-corrosion. Cathodic protection, chlorine salt extraction, and alkaline recovery are three commonly used and effective methods in chemical protection. The advantage of this method is that the protection method is less affected by environmental factors, and it is suitable for long-term anti-corrosion of steel bars and concrete, and can be used for both cracked structures and new structures.
10. Bionic self-healing method
The bionic self-healing method is a new crack treatment method, which imitates the function of biological tissue to automatically secrete certain substances to the wounded part, so that the wounded part can be healed, and some special components are added to the traditional components of concrete ( Such as liquid core fibers or capsules containing binders), an intelligent bionic self-healing neural network system is formed inside the concrete, and when cracks appear in the concrete, part of the liquid core fibers is secreted to make the cracks heal again.
11. Other methods
Commonly used methods include dismantling and redoing, improving the service conditions of the structure, passing tests or analysis and demonstrating without treatment, etc.
2. Reasons for mass concrete cracks:
In mass concrete structures, due to the large structural section and the large amount of cement used, the heat of hydration released by cement hydration will cause large temperature changes and shrinkage, and the resulting temperature shrinkage stress is the main cause of cracks in reinforced concrete. reason. There are two types of cracks: surface cracks and through cracks. The surface cracks are caused by the different heat dissipation conditions between the surface and the interior of the concrete. The temperature is low outside and high inside, forming a temperature gradient, which causes compressive stress inside the concrete and tensile stress on the surface. The tensile stress on the surface exceeds the tensile strength of the concrete.
The through-crack is due to the tensile stress caused by the deformation caused by the cooling of the concrete when the strength of the mass concrete reaches a certain level, plus the volume shrinkage and deformation caused by the loss of water in the concrete, and is constrained by the foundation and other structural boundary conditions. Cracks through the entire cross-section that may occur when the tensile strength of concrete is exceeded. These two kinds of cracks are all harmful cracks to varying degrees.
The early shrinkage of high-strength concrete is large. This is because 30%~60% mineral fine admixtures are used to replace cement in high-strength concrete. The ratio is 0.25~0.40, which improves the microstructure of concrete and brings many excellent properties to high-strength concrete, but the most prominent negative effect is the increase in the probability of concrete shrinkage cracks. The shrinkage of high-strength concrete is mainly drying shrinkage, temperature shrinkage, plastic shrinkage, chemical shrinkage and autogenous shrinkage.
The time of concrete cracks can be used as a reference to judge the cause of cracks: plastic shrinkage cracks appear about a few hours to ten hours after pouring; temperature shrinkage cracks appear about 2 to 10 days after pouring; autogenous shrinkage mainly occurs after concrete hardens From a few days to dozens of days; drying shrinkage cracks appear in a period close to 1 year old.
1. Drying shrinkage:
When the concrete loses the adsorbed water in the internal pores and gel pores in the unsaturated air, it will shrink. The porosity of high-performance concrete is lower than that of ordinary concrete, so the shrinkage rate is also low.
2. Plastic shrinkage:
Plastic shrinkage occurs during the plastic phase of concrete before it hardens. High-strength concrete has a low water-to-binder ratio, less free moisture, and fine mineral admixtures are more sensitive to water. High-strength concrete basically does not bleed, and the surface loses water faster, so plastic shrinkage of high-strength concrete is easier than ordinary concrete. .
3. Self-shrinking:
The relative humidity inside the closed concrete decreases with the progress of cement hydration, which is called self-drying. Self-drying causes the water in the capillary to be unsaturated and produces negative pressure, thus causing the self-shrinkage of the concrete. Due to the low water-binder ratio of high-strength concrete and the rapid development of early strength, free water will be consumed quickly, causing the relative humidity in the pore system to be lower than 80%. Self-shrinking.
In the total shrinkage of high-strength concrete, dry shrinkage and autogenous shrinkage are almost equal, and the lower the water-binder ratio, the greater the proportion of autogenous shrinkage. It is completely different from ordinary concrete. Ordinary concrete is mainly dry shrinkage, while high-strength concrete is mainly self-shrinking.
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4. Temperature shrinkage:
For concrete with high strength requirements, the amount of cement is relatively large, the heat of hydration is large, and the rate of temperature rise is also large, generally up to 35~40°C, and the maximum temperature can exceed 70~80°C when the initial temperature is added. Generally, the thermal expansion coefficient of concrete is 10×10-6/°C, and when the temperature drops 20~25°C, the cold shrinkage is 2~2.5×10-4, while the ultimate tensile value of concrete is only 1~1.5×10- 4. Therefore, cold shrinkage often causes concrete cracking.
5. Chemical shrinkage:
After the cement is hydrated, the volume of the solid phase increases, but the absolute volume of the cement-water system decreases, forming many capillary pores and cracks. The water-binder ratio of high-strength concrete is small, and the degree of hydration is restricted by the addition of fine mineral admixtures. The chemical shrinkage of high-strength concrete is less than that of ordinary concrete. When concrete shrinks and is constrained externally or internally, tensile stresses develop and can potentially cause cracking. Although high-strength concrete has high tensile strength, its elastic modulus is also high. Under the same shrinkage deformation, it will cause high tensile stress, and due to the low creep capacity of high-strength concrete, the stress relaxation is small, so Poor crack resistance.
