Causes and treatment of cracks in the hottest conc

2022-08-13
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Causes and treatment methods of concrete bridge cracks

concrete has become the most widely used building material in the world today because of its wide range of materials, low price, high compressive strength, can be poured into various shapes, good fire resistance, not easy to weathering, and low maintenance cost. However, the main disadvantage of concrete is that it has poor anti positional ability and is easy to crack. Concrete cracks are inevitable, but their harmful degree can be controlled. Some cracks continue to produce and expand under the action of service load or external physical and chemical factors, causing concrete carbonation, peeling off of protective layer, corrosion of reinforcement, weakening the strength and stiffness of concrete, reducing durability, and endangering the normal use of the structure, which must be controlled

types and causes of cracks

the causes of cracks in concrete structures are complex and various, and sometimes many factors affect each other, but each crack has one or several main factors. The types of cracks in concrete bridges can be roughly divided into the following types according to their causes:

first, cracks caused by loads. The cracks of concrete bridge beams under static, dynamic loads and secondary stresses are called load cracks, which mainly include direct cracks and secondary stress cracks. Direct stress crack refers to the crack caused by direct stress caused by external load; Secondary stress crack refers to the crack caused by secondary stress caused by external load

second, cracks caused by temperature changes. Concrete has the property of thermal expansion and cold contraction. When the external environment or internal temperature changes, the concrete will deform. If the deformation is constrained, stress will be generated in the structure. When the stress exceeds the tensile strength of concrete compared with small cars, temperature cracks will occur. In some long-span bridges, the temperature stress can reach or even exceed the live load stress. The main difference between temperature cracks and other cracks is that they will expand or close with the change of temperature. The main factors causing temperature changes are: annual temperature difference, sunshine, sudden cooling, hydration heat, steam curing or improper winter construction measures

third, cracks caused by shrinkage. In practical engineering, cracks caused by concrete shrinkage are the most common. Among the types of concrete shrinkage, plastic shrinkage and shrinkage shrinkage (dry shrinkage) are the main reasons for the volume deformation of concrete, in addition to autogenous shrinkage and carbonization shrinkage. The research shows that the main factors affecting concrete shrinkage cracks are: cement variety, grade and dosage, aggregate variety, water cement ratio, admixture, curing method, external environment, vibration method and time

fourth, cracks caused by foundation deformation. Due to the uneven vertical settlement or horizontal displacement of the foundation, the additional stress in the structure will exceed the tensile capacity of the concrete structure, resulting in structural cracking. The main reasons for the uneven settlement of the foundation are: the accuracy of geological survey is not enough, and the test data is not accurate; The geological difference of foundation is too large; The structural load difference is too large; The difference of structural foundation types is too large; The ground is frost heaving; When the bridge foundation is based on unfavorable geology such as landslide, karst cave or active fault, uneven settlement may be caused

fifth, cracks caused by reinforcement corrosion. To prevent the reinforcement from rusting, the crack width should be controlled and sufficient protective layer thickness should be adopted in the design according to the specification requirements (of course, the protective layer should not be too thick, otherwise the effective height of the component will be reduced and the crack width will be increased when stressed); During construction, the water cement ratio of concrete shall be controlled, and the vibration shall be strengthened to ensure the compactness of concrete and prevent oxygen intrusion. At the same time, the dosage of additives containing chlorine salt shall be strictly controlled, especially in coastal areas or other areas with strong corrosive air and groundwater

six are cracks caused by frost heave. When the atmospheric temperature is below zero, the water saturated concrete freezes, the free water turns into ice, and the volume expands by 9%, so the concrete produces expansion stress; At the same time, supercooled water (freezing temperature below -78 ℃) in the concrete gel pores migrates and redistributes in the microstructure, causing osmotic pressure, increasing the expansion force in the concrete, reducing the strength of the concrete, and causing cracks. In particular, the concrete is most seriously frozen at the initial setting, and the strength loss of concrete after age can reach 30% - 50%. During winter construction, frost heave cracks along the pipeline may also occur if insulation measures are not taken after grouting the prestressed duct

seventh, cracks caused by the quality of construction materials. Concrete is mainly composed of cement, sand, aggregate, mixing water and additives. The quality of the materials used for concrete configuration is unqualified, which may lead to cracks in the structure. Such as cement, sand, stone aggregate, mixing water and additives

eight are cracks caused by the quality of construction technology. In the process of concrete structure pouring, component manufacturing, formwork lifting, transportation, stacking, assembly and hoisting, if the construction technology is unreasonable and the construction quality is poor, it is easy to produce longitudinal and transverse cracks, especially the slender thin-walled structure

suggestions on the treatment of concrete cracks

the first is the surface treatment method: including surface coating and surface patching method. The scope of application of surface coating is the fine and shallow cracks that are difficult to pour grout, hairline cracks that are not deep enough to the surface of reinforcement, water tight cracks, non telescopic cracks and inactive cracks. The surface sticking method (woodworking film or other waterproof sheet) is applicable to the seepage prevention and plugging of large-area water leakage (honeycomb, pitted surface, etc., or it is difficult to determine the specific water leakage position and deformation joint)

the second is the filling method: the cracks are directly filled with repair materials. Multinational enterprises increase investment in China. Generally, accurate impact testing machines are used to confirm the corresponding relationship between the quality of standard impact body and its impact energy; Select at least 3 energy values in the energy section of the impact testing machine to be verified to repair the wider crack (0.3mm), which is simple and low cost. The simple treatment of cracks with width less than 0.3mm and shallow depth, as well as small-scale

cracks, can be taken out of the V-shaped groove and then filled

the third is grouting method: this method has a wide range of applications, from fine cracks to large cracks, and has good treatment effect

fourth, structural reinforcement method: structural reinforcement method, anchoring reinforcement method, prestress method, etc. can be adopted for the cracks caused by overload, the reduction of concrete durability caused by long-term non treatment of cracks, and the cracks caused by fire, which affect the structural strength

fifth, the inspection of concrete crack treatment effect: including repair material test; Core drilling sampling test; Water pressure test; Air pressure test, etc

from the above, it can be seen that the concrete bridge used by PBT may have cracks due to careless design, poor construction and poor supervision. Therefore, the design, construction and supervision in strict accordance with relevant national specifications and technical standards are the premise and foundation to ensure the safety and durability of the structure. In operation management

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