Treatment and prevention of structural cracks in t

2022-07-27
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Treatment and prevention of structural cracks in underground buildings

cause analysis:

(1) design factors: the general design pays attention to the reinforcement and strength of the vertical structure of the underground building wallboard engineering, and ignores the horizontal crack resistance reinforcement. Because the stress on each particle in the wall structure section is uneven, the resultant force of shrinkage stress and temperature difference stress reaches the ultimate tensile strength of concrete, causing plastic deformation and cracks

(2) improper materials are selected, such as large shrinkage of cement, poor grading of stones, and large silt content; Fine grain size of sand; The mixing lock body and the stopper (or lock catch) shall be able to bear 4440n transverse load at the semi locking position; In the fully locked position, it shall be able to bear the transverse load of 8890n, and the admixture that cannot be disengaged is of poor quality. Due to the high strength grade of commercial concrete, large cement consumption, large water consumption and high temperature caused by hydration heat, when the ambient temperature drops significantly, the linear expansion coefficient of newly poured concrete under the action of temperature difference! c=10 ×/℃, the tensile stress caused by shrinkage increases when the temperature is reduced. The surface area of concrete wall is large, and the dry shrinkage stress is greater than the internal binding force, forming the comprehensive stress of dry shrinkage, shrinkage and temperature difference, which is greater than the ultimate tensile strength of concrete and produces cracks

(3) illegal operation: first, there are no cooling measures for high-temperature construction in summer: the concrete mixture molding temperature is greater than 35 ℃. When the temperature drops at night, the internal and external temperature difference is large and cracks are easy to occur; Second, there is no thermal insulation measures during low-temperature construction in winter, that is, when the commercial concrete is put into the mold, the hydration heat of cement rises significantly and the ambient temperature is about 0 ℃, the outer side of the wallboard shrinks under low temperature, resulting in cracks. Third, the pouring method is improper. For example, pumping concrete is used instead of pouring in layers, but pouring from one part to the top, and then moving the paving pipe to pour another section; The joint of concrete mixture is vertical, with fast pouring speed and uneven vibration, resulting in uneven wall concrete and uneven stress on each particle, resulting in cracks at the stress. Fourth, the reinforcement installation is not standard. For example, the spacing is uneven, the horizontal reinforcement is not straightened, the joint binding is loose, or the protection control is not strict, resulting in the collision during concrete pouring, which makes the reinforcement skew, uneven stress, and easy to produce cracks. In particular, since GB, the technical code for waterproofing of underground engineering, has made a mandatory provision that the thickness of the protective layer of the reinforcement on the upstream surface of the underground waterproof concrete structure is not less than 50mm, the problem of structural cracks in the super long basement concrete that can set and display the height of the hammer in real time has become more prominent. Further improve the market competitiveness

treatment and prevention measures:

(1) vertical cracks in underground building walls: mainly cracks generated under the comprehensive stress of concrete shrinkage, dry shrinkage, cold shrinkage, etc., generally do not affect the bearing capacity, and can be treated by chemical grouting method. The purpose of the treatment is to close the gap, prevent water leakage, make the crack bond and restore the original function of the wall, and prevent the reinforcement from rusting due to the damage of the passive film by the crack

(2) at the outer side of the underground building (i.e. the upstream surface), comprehensively check the joint grouting quality. After the slurry solidifies, remove the knots on the wall surface, and paste the high polymer waterproof roll to the crack, with a width of about 250mm. First, polish and clean along the crack, brush the base treatment agent, select the qualified adhesive matching with the coiled material, and stick the coiled material firmly

(3) it is the first line of defense for waterproofing of underground buildings to do a good job in backfilling around underground buildings

it is expected to slow down the increasing of plastic and electronic waste (4) it is suggested that the design unit should add horizontal reinforcement with a diameter of not less than Ф 12. The spacing shall not be greater than 150mm. The joint shall be welded and straightened by cold drawing. It shall be kept straight after installation. The spacing and the position of the protective layer shall be accurate. Adding reinforced concrete ring beams and concealed columns to enhance the role of wall crack resistance, and adopting the method of "post pouring joints" or adding UEA expansion agent into the concrete for super long buildings are measures to effectively reduce the development of component cracks. The cement content of concrete can be reduced by using the late strength of concrete for 60 days

(5) control the quality of raw materials, timely grasp the changes of construction environment, and strictly follow the construction specifications

(6) the method of thin-layer continuous pouring is adopted to ensure the uniformity of concrete compactness. The curing method of heat storage and insulation is adopted to reduce the internal and external temperature difference of components. The cooling rate is strictly controlled to 1.5 ℃/d, creating stress relaxation conditions for concrete, which is one of the main measures to prevent shrinkage cracks of concrete components

(7) cooling measures shall be taken for high-temperature construction in summer, and the concrete molding temperature shall not be greater than 28 ℃; Sunshade measures shall be taken at the construction site to reduce the construction temperature

the winter construction shall be provided with specific anti freezing and thermal insulation measures. For example, if the molding temperature of concrete is above 5 ℃, the thermal insulation of components shall be done well to prevent the rapid heat dissipation from causing cold shrinkage cracks

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