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Prevention and treatment of concrete cracks

Abstract: The problem of concrete crack is a common and difficult engineering problem. In this paper, some common cracks in concrete engineering are discussed and analyzed, and some prevention and treatment measures are put forward according to the specific situation.

Keywords: prevention and treatment of concrete cracks

I. Introduction

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. It is precisely because these early concrete buildings and components usually work with cracks. Due to the existence and development of cracks, internal steel bars and other materials are usually corroded, which reduces the bearing capacity, durability and impermeability of reinforced concrete materials and affects the appearance and service life of buildings. In severe cases, there are many reasons for concrete cracks, including cracks caused by deformation. Cracks caused by external load; There are also cracks caused by improper environmental maintenance and chemical action. In practical engineering, we should treat it differently and solve the problem according to the actual situation.

Second, the common cracks in concrete engineering and prevention

1. Dry shrinkage crack and its prevention

Shrinkage cracks usually occur some time after concrete curing or about a week after concrete pouring. The evaporation of water in cement slurry will cause drying shrinkage, which is irreversible. Shrinkage cracks are mainly caused by different degrees of evaporation of water inside and outside the concrete, which leads to different deformation: under the influence of external conditions, the water on the concrete surface loses too quickly, resulting in large deformation, while the internal humidity changes little, resulting in small deformation, and the large surface shrinkage deformation is constrained by the internal concrete, resulting in large tensile stress and cracks. The lower the relative humidity, the greater the drying shrinkage of cement paste, and the easier it is to produce drying shrinkage cracks. Shrinkage cracks are mostly surface parallel lines or reticulated shallow fine cracks, and the width is mostly between 0.05 and 0.2 mm, which are common in the plane part of mass concrete and distributed in the short direction in the thin beam and slab. Shrinkage cracks usually affect the impermeability of concrete, cause steel corrosion and affect the durability of concrete, and hydraulic fracturing will affect the bearing capacity of concrete under water pressure. The drying shrinkage of concrete is mainly related to the water-cement ratio of concrete, the composition of cement, the dosage of cement, the nature and dosage of aggregate and the dosage of admixture.

Main preventive measures:

First, choose cement with small shrinkage, generally use medium and low heat cement and fly ash cement to reduce the amount of cement.

Secondly, the drying shrinkage of concrete is greatly influenced by the water-cement ratio. The greater the water-cement ratio, the greater the drying shrinkage. Therefore, the selection of water-cement ratio should be controlled as much as possible in the design of concrete mixture ratio, and an appropriate amount of water reducer should be added at the same time.

Third, strictly control the mix proportion of concrete mixing and construction, and the water consumption of concrete shall not be greater than the water consumption given by the mix proportion design.

Fourth, strengthen the early curing of concrete and appropriately extend the curing time of concrete. In winter construction, it is necessary to appropriately extend the thermal insulation and covering time of concrete, and use curing agent for curing.

Fifthly, proper contraction joints are set in concrete structures.

2. Plastic shrinkage crack and its prevention

Plastic shrinkage refers to the shrinkage of concrete surface due to rapid water loss before setting. Plastic shrinkage cracks generally appear in dry heat or windy weather, and the cracks are mostly in a state of wide middle, thin ends and different lengths, which are discontinuous. The shorter cracks are generally 20~30cm long, the longer cracks can reach 2~3m, and the width is 1~5mm. The main reasons are: the concrete has little or no strength before the final setting, or the concrete has little strength just after the final setting. Due to the influence of high temperature or strong wind, the surface of concrete loses water too quickly, which leads to a large negative pressure in the capillary and makes the volume of concrete shrink sharply. At this time, the strength of concrete can't resist its own shrinkage, so cracks occur. The main factors affecting plastic shrinkage cracking of concrete are water cement ratio, concrete setting time, ambient temperature, wind speed and relative humidity.

Main preventive measures: First, choose silicate or ordinary portland cement with small dry shrinkage value and high early strength. Second, strictly control the water-cement ratio, add superplasticizer to increase the slump and workability of concrete, and reduce the amount of cement and water. Third, before pouring concrete, water evenly to drench the foundation and formwork. Fourth, before the final set, cover the concrete surface with plastic film, wet straw mat and hemp chips in time to keep it moist, or spray curing agent on the concrete surface for curing. Fifth, in hot and windy weather, we should set up sunshade and windproof facilities and maintain them in time.

3. Settlement cracks and prevention

Settlement cracks are caused by uneven and soft soil of structural foundation, or uneven settlement caused by false backfill or soaking. Or because of insufficient formwork stiffness, too large formwork support spacing or loose support bottom, especially in winter, formwork is supported on frozen soil, and uneven settlement occurs after frozen soil is thawed, resulting in cracks in concrete structure. Most of these cracks are deep, large or penetrating cracks, and their trend is related to settlement. Generally, it develops along the direction perpendicular to the ground or 30 ~ 45. Larger settlement cracks often have some dislocation, and the width of cracks is often proportional to settlement. The crack width is less affected by temperature change. After the foundation deformation is stable, the settlement cracks tend to be stable.

Main preventive measures: First, the soft soil and filled soil foundation should be tamped and strengthened before the superstructure construction. The second is to ensure that the formwork has sufficient strength and rigidity, firm support and uniform stress on the foundation. The third is to prevent the foundation from being soaked in water when concrete is poured. Fourth, the form removal time should not be too early, pay attention to the form removal order. Fifth, some preventive measures should be taken to set up formwork on frozen soil.

4. Temperature cracks and their prevention

Temperature cracks often appear on the surface of mass concrete or concrete structures in areas with large temperature difference. After concrete pouring, in the process of hardening, the hydration of cement produces a lot of hydration heat (when the cement dosage is 350 ~ 550 kg/m3, the heat of 17500 ~ 27500 kJ will be released per cubic meter of concrete, which will raise the internal temperature of concrete to about 70℃ or even higher). Due to the large volume of concrete, a large amount of hydration heat is accumulated in the concrete, which is not easy to dissipate, resulting in a sharp rise in internal temperature, while the concrete surface dissipates heat quickly, resulting in a large temperature difference between inside and outside, which leads to different degrees of thermal expansion and cold contraction inside and outside, resulting in a certain tensile stress on the concrete surface (practice has proved that when the temperature difference of concrete itself reaches 25℃~26℃, the tensile stress inside the concrete is about 10MPa. When the tensile stress exceeds the tensile strength limit of concrete, cracks will occur on the surface of concrete, which mostly occurs in the middle and late stage of concrete construction. In concrete construction, when the temperature difference changes greatly, or when concrete is attacked by cold wave, the surface temperature of concrete will drop sharply, which will lead to shrinkage. Concrete with surface shrinkage is constrained by internal concrete, which will produce great tensile stress and cracks, which usually only occur in the shallow range of concrete surface.

The trend of temperature cracks is usually irregular, and large-area structural cracks are often criss-crossed; For the structure with long beam and slab length, the cracks are mostly parallel to the short side; Deep and penetrating temperature cracks are generally parallel or nearly parallel to the short side direction, and cracks appear in sections along the long side with dense middle. The crack width is different, which is obviously affected by the temperature change, wider in winter and narrower in summer. The temperature cracks of concrete caused by high temperature expansion are usually thick in the middle and thin at both ends, while the thickness of cold shrinkage cracks changes little. The appearance of such cracks will cause the corrosion of steel bars and carbonation of concrete, and reduce the frost resistance, fatigue resistance and impermeability of concrete.

Main preventive measures: First, try to choose low-heat or medium-heat cement, such as slag cement and fly ash cement. The second is to reduce the dosage of cement, and try to control the dosage of cement below 450kg/m3. The third is to reduce the water-cement ratio, which is generally controlled below 0.6. Fourthly, improve aggregate gradation, add fly ash or superplasticizer, and reduce cement consumption and hydration heat. The fifth is to improve the mixing processing technology of concrete, and adopt a new technology of "secondary air cooling" on the basis of the traditional "three cooling technology" to reduce the pouring temperature of concrete. Sixth, add a certain amount of additives with water reducing, plasticizing and retarding functions. Adding concrete can improve the fluidity and water retention of concrete mixture, reduce the hydration heat and delay the appearance of hot peak. Seventh, when pouring in high temperature season, auxiliary measures such as setting sun visor can be used to control the temperature rise of concrete and reduce the temperature of pouring concrete. Eighth, the temperature stress of mass concrete is related to the structural size. The larger the structural size of concrete, the greater the temperature stress. Therefore, it is necessary to arrange the construction process reasonably and pour in layers and blocks to facilitate heat dissipation and reduce constraints. Nine, a cooling pipe is arranged inside the mass concrete, and cold water or cold air is introduced for cooling, so as to reduce the temperature difference between the inside and outside of the concrete. Ten is to strengthen the monitoring of concrete temperature and take timely cooling and protection measures. Eleven is the reserved temperature contraction joint. Twelve is to reduce constraints. Before pouring concrete, it is advisable to spread a sand cushion of about 5mm on bedrock and old concrete or paint it with materials such as asphalt. Thirteen is to strengthen concrete curing. After concrete pouring, cover it with wet straw curtain and hemp chips in time, and pay attention to sprinkler maintenance, and extend the maintenance time appropriately to ensure the slow cooling of concrete surface. In cold season, thermal insulation measures should be set on the concrete surface to prevent cold wave attack. Fourteen is to configure a small amount of steel bars or mix fiber materials in concrete to control the temperature cracks of concrete within a certain range.

5. Cracks caused by chemical reactions and their prevention

Cracks caused by alkali-aggregate reaction and steel corrosion are the most common chemical reactions in reinforced concrete structures. After concrete is mixed, it will produce some alkaline ions, which will react with some active aggregates and absorb the water in the surrounding environment, thus increasing the volume and causing concrete to be brittle, swollen and cracked. This kind of crack usually occurs in the use of concrete structures, and once it appears, it is difficult to remedy it, so effective measures should be taken to prevent it in construction. Main preventive measures: First, choose sand aggregate with low alkali activity. Second, choose low-alkali cement and low-alkali or alkali-free additives. Thirdly, choose appropriate additives to inhibit alkali-aggregate reaction. Due to poor vibration of concrete pouring or too thin protective layer of steel bars, harmful substances enter the concrete, which leads to corrosion of steel bars, volume expansion of corroded steel bars and cracking of concrete. These cracks are mostly longitudinal cracks, which appear along the reinforcement position. Third, crack treatment.

The appearance of cracks will not only affect the integrity and stiffness of the structure, but also cause the corrosion of steel bars, accelerate the carbonation of concrete and reduce the durability, fatigue resistance and impermeability of concrete. Therefore, according to the nature and specific conditions of cracks, we should treat them differently and deal with them in time to ensure that the repair measures of concrete cracks in buildings mainly include the following methods: surface repair, grouting, caulking, structural reinforcement, concrete replacement, electrochemical protection and bionic self-healing.

1. Surface repair method

Surface repair method is a simple and commonly used repair method, which is mainly suitable for the treatment of surface cracks and deep cracks that are stable and do not affect the bearing capacity of the structure. The usual treatment measures are to smear cement slurry, epoxy mastic or anticorrosive materials such as paint and asphalt on the surface of cracks. In order to prevent concrete from cracking due to various effects, measures such as sticking glass fiber cloth on the crack surface can usually be taken.

2. Grouting mosaic sealing method

Grouting method is mainly suitable for repairing concrete cracks that have influence on structural integrity or have anti-seepage requirements. It uses pressure equipment to press cementing material into concrete cracks, and the cementing material hardens and forms a whole with concrete, thus achieving the purpose of plugging and strengthening. Commonly used cementing materials include cement slurry, epoxy resin, methacrylate, polyurethane and other chemical materials. Sealing method is the most commonly used method in crack sealing. Usually, a groove is opened along the crack and filled with plastic or rigid water-stopping material to seal the crack. Commonly used plastic materials include PVC mastic, plastic ointment, butyl rubber, etc. The commonly used rigid waterproof material is polymer cement mortar.

3. Structural reinforcement methods

When cracks affect the performance of concrete structures, it is necessary to consider the reinforcement of concrete structures. The main methods commonly used in structural reinforcement include: increasing the cross-sectional area of concrete structure, wrapping steel in the corners of members, prestressing reinforcement, sticking steel plates, adding fulcrum reinforcement, shotcreting reinforcement and so on.

4. Concrete replacement method

Concrete replacement method is an effective method to deal with seriously damaged concrete. This method is to remove the damaged concrete first, and then replace it with new concrete or other materials. Commonly used alternative materials are: ordinary concrete or cement mortar, polymer or modified polymer concrete or mortar.

5. Electrochemical protection method

Electrochemical corrosion protection is to change the environmental state of concrete or reinforced concrete by using the electrochemical action of external electric field in the medium, so as to passivate the steel bars and achieve the purpose of corrosion protection. Cathodic protection, chloride extraction and alkali recovery are three common and effective methods in chemical protection.

6. Bionic self-healing method

Bionic self-healing method is a new type of crack treatment method, which imitates the function of biological tissue and automatically secretes some substances to the injured part to heal the injured part. Adding some special components (such as liquid core fiber or capsules containing binder) to the traditional components of concrete, an intelligent bionic self-healing neural network system is formed in concrete. When cracks appear in concrete, some liquid core fibers will be secreted to make the cracks heal again [4].

Fourth, conclusion.

Cracks are common phenomena in concrete structures. Their appearance will not only reduce the impermeability of buildings, 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. Therefore, concrete cracks should be carefully studied, treated differently, treated with reasonable methods, and various effective preventive measures should be taken during construction to prevent the occurrence and development of cracks and ensure the safe and stable work of buildings and components.

References:

Code for design of reinforced concrete structures. China Building Industry Press, 1999.2.

2 Ju Liyan. Research progress of concrete crack suppression measures. Concrete, May 2002.

3 Guo Shiwan, Xiao Xin and Zhao Heping. Crack control in concrete construction. Shanxi water conservancy science and technology, 2000.38+0.

4 Ju Liyan, Zhang Xiong. Two new methods to prevent concrete cracks. Building technology, July 2002.