Since the reform and opening up, China's infrastructure construction has developed rapidly, and a large number of reinforced concrete buildings and structures have been built. In these reinforced concrete structures, structural cracks are very common quality problems, which seriously affect the use and safety of the project. It is a technical problem that needs to be discussed and solved. In recent years, although the crack problem has attracted the attention of the industry, it has not been well solved. 1 1. 1 Analysis of the causes of cracks Cracked concrete is mainly composed of cement, sand, aggregate, mixing water and additives. Unqualified materials used to prepare concrete may cause cracks in the structure. Mainly manifested in the following points: unqualified cement stability, insufficient strength, wet cement or expired cement, resulting in insufficient strength of concrete, which leads to concrete cracking; Too small sand and gravel particle size, poor gradation and high porosity will lead to an increase in cement and water consumption, which will affect the strength of concrete and increase its shrinkage. When the content of impurities such as chloride in mixed water or admixture is high, it has a great influence on the corrosion of steel bars. 1.2 Cracks caused by steel bar corrosion Due to poor concrete quality or insufficient protective layer thickness, the protective layer of concrete is corroded and carbonized to the surface of steel bar by carbon dioxide, which reduces the alkalinity of concrete around steel bar, or the content of chloride ions around steel bar is high, which may cause the oxide film on the surface of steel bar to be destroyed. Iron ions in steel bar react with oxygen and moisture invading concrete, and the volume of corrosive substance ferric hydroxide is increased by about 2-4 times, thus generating expansion stress on the surrounding concrete, leading to the cracking of protective layer concrete. The cracked concrete caused by the temperature change of 1.3 has the characteristics of thermal expansion and cold contraction. When the external environment or the internal temperature of the structure changes, the concrete will be deformed. If deformation is restrained, stress will occur in the structure, and when the stress exceeds the tensile strength of concrete, temperature cracks will occur. The characteristics of temperature cracks are that the surface cracks are generally irregular, and the deep cracks or through cracks are generally parallel or nearly parallel to the main reinforcement; Cracks vary in width, and are hot and thin and cold due to temperature changes. Surface temperature cracks often appear between cast-in-place concrete 1~2d, and deep temperature cracks and through temperature cracks often begin to appear after 21d. The main causes of temperature change are: (1) Surface temperature cracks are mostly caused by large temperature difference. For example, after mass concrete (thickness more than 2m) is poured, the internal temperature is very high, and the temperature difference between inside and outside is too large, resulting in cracks on the surface. In winter construction, premature removal of insulation layer or cold wave attack will lead to low early strength of concrete and cracks. In addition, when the prefabricated members are cured by steam, the concrete surface shrinks and cracks appear due to rapid cooling or the members are eager to leave the pool and expose the cover. (2) Deep penetration cracks are mostly caused by large structural temperature difference and external constraints. For example, cast-in-place abutment concrete or mass rigid expanded foundation is poured on a hard foundation, and measures such as isolation are not taken to relax constraints or the contraction joint spacing is too large. When pouring concrete, the temperature is very high, and the hydration heat temperature of cement rises greatly, making the temperature higher. When concrete cools and shrinks, the tensile stress is large, which leads to cooling shrinkage cracks; This kind of crack is sometimes through. Cracks caused by artistry 1.4 In bridge construction, a considerable number of cracks in reinforced concrete bridges are caused by construction reasons. In the process of manufacture, transportation and installation of concrete structural members, the construction technology is unreasonable and the construction quality is low, which is easy to produce various forms of cracks. The specific points are as follows. (1) concrete mixing: the mixing equipment is a 500-type forced mixer, and each mixing time is about 3 minutes. The mixing time is moderate, the slump is 9~ 13cm, and there is unevenness in the mixing process. There is a problem that the water cement ratio is too large. When the water-cement ratio is too large, the drying shrinkage of concrete will increase, and drying shrinkage cracks will easily occur. (2) Concrete pouring: plug-in vibrators are mainly used in the pouring process, supplemented by attached vibrators (mainly used for 10m beam ends). Over-shock occurred in the pouring process, which led to the segregation of coarse and fine aggregates on the concrete surface and the concentration of fine aggregates near the formwork surface. (3) Concrete curing: In summer, as the daytime temperature mostly exceeds 30℃, and the highest temperature reaches 35℃, it is difficult for the lateral surface of the beam to absorb moisture. Excessive temperature accelerates the evaporation of moisture in the beam, resulting in dry shrinkage cracks on the surface. From the above analysis, it can be seen that horizontal structural reinforcement, construction raw materials, concrete stress, formwork defects, beam pedestal and construction technology are the causes of concrete shrinkage cracks. The superposition of various unfavorable factors leads to shrinkage cracks in the beam. 2 Suggestions on the treatment of concrete cracks 2. 1 Surface treatment method adopts surface treatment to repair, and resin protective film is coated on the concrete surface along the cracks. During construction, the attachments on the concrete surface shall be removed with a wire brush, then cleaned with clear water, dried, filled with putty-like resin, and then smoothed if necessary. Surface treatment methods include surface painting and surface repair. The scope of application of surface leveling is thin and shallow cracks that are difficult to be filled with cement slurry, capillary cracks whose depth is not up to the surface of steel bars, watertight cracks, non-telescopic cracks and inert cracks. Surface patch method is suitable for seepage prevention and plugging of large area water leakage. 2.2 Filling method directly fills cracks with repairing materials, which is generally used to repair wide cracks (0.3mm), with simple operation and low cost. The simple treatment of cracks with width less than 0-3mm, shallow depth and small scale can adopt V-groove post-filling treatment. For the cracks in the middle zone of the bridge deck, the upper part is treated by grouting construction method. Within the range of 7~8cm along the crack width, free lime and dust on the concrete surface shall be removed by grinder and wire brush, cleaned by cleaning agent, and then epoxy resin with permeability and adhesion shall be injected under pressure to fill the concrete cracks, so as to improve the waterproof performance of the bridge deck and prevent the corrosion of steel bars and the aging of concrete. 2.3 Cracks caused by overloading of structural reinforcement methods, deterioration of concrete durability caused by long-term untreated cracks, and cracks caused by fire will all affect the structural strength. Structural reinforcement method, anchorage reinforcement method, prestress method, etc. In addition, grouting method can also be used. At the same time, it is necessary to strengthen the inspection of the treatment effect of concrete cracks, including the test of repair materials; Core sampling test; Hydrostatic test; Air compression test, etc. 3 Prevention of cracks 3. 1 As far as raw materials are concerned, the hydration of cement releases heat during the hardening process of concrete, thus increasing the temperature shrinkage stress of concrete. Therefore, low hydration heat cement should be used as far as possible without affecting the strength and construction progress. In terms of cement dosage, the cement dosage can be reduced by 15% without affecting the strength of concrete, and coarse aggregate can be increased to supplement the reduction of cement dosage. According to the different strength grades of concrete, the specifications have different requirements on the silt content of sand, crushed stone, construction water and the amount of ash powder in crushed stone. In a word, all materials should be strictly controlled within the specification requirements. 3.2 concrete construction process control (1) concrete mixing: the mixing time of each plate of concrete should be controlled at about 2 ~ 3 minutes, and the mixing time should not be too long or too short. Too short will lead to uneven mixing, too long will destroy the material structure; In the process of concrete pouring, slump test should be carried out many times, and the construction water cement ratio should be strictly controlled. (2) Concrete pouring: plug-in vibrators are mainly used in the pouring process, supplemented by attached vibrators (mainly used for 10m beam ends). Attached vibrators should vibrate intermittently, and each opening time is about 30s, so as to avoid over-vibration during pouring and isolate concrete. The vibrating distance of the internal vibrator should not be greater than 1.5 times the vibrating radius of the internal vibrator, so as to avoid vibration leakage during pouring. (3) Concrete curing: When the construction temperature exceeds 30℃, the water in the beam evaporates quickly, and the side of the beam is not easy to absorb water. If the water on the beam surface is evaporated, the concrete on the beam surface will undergo the hydration and hardening reaction of cement, absorbing the water inside the beam, thus accelerating the drying shrinkage of concrete. In order to make the beam have enough water for the hydration and hardening reaction of cement, it is necessary to spray water on the prestressed members immediately. The specification requires that the interval between watering and curing should be at least 2h, and it can be reduced to 1.5h if necessary. In summer construction, if the temperature exceeds 30℃ after concrete pouring and before formwork removal, water can be sprayed on the outer layer of formwork to cool down after pouring 12h. In order to avoid the internal temperature is too high, the volume expansion is too large, and the volume shrinkage is too large after cooling to produce shrinkage cracks. Curing time should be kept until after prestressing. 3.3 Formwork design and pedestal side formwork: according to the beam structure design, under the premise of meeting the geometric dimensions, minimize the right-angle connection and stress concentration points; The surface of the template is polished smoothly to reduce friction. Beam-making pedestal: The surface of beam-making pedestal should be as smooth as possible with little friction. Smooth materials, such as terrazzo, steel plate and plywood, can be laid on the surface of the beam-making pedestal. Conclusion In engineering practice, cracks are very common in concrete bridges. For the parts prone to cracks, strictly control the construction process to avoid or reduce cracks as much as possible and reduce the length and width of cracks. Through proper treatment of cracks, cracks will not harm the structure and ensure the normal use of the structure. In a word, many cracks can be overcome and controlled as long as proper preventive measures are taken. It is hoped that the discussion in this paper can help bridge engineers and technicians to further strengthen their understanding of concrete bridge cracks, formulate corresponding quality prevention measures, and try to avoid harmful cracks in the project.
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