Analysis on causes of cracks in brick-concrete structure floors and walls and prevention methods. Nowadays, some engineering floors have been changed from prefabricated to cast-in-place, and phenomena such as window sill cracks, floor cracks and top wall cracks have appeared. Common quality problems are discussed below. 1. Analysis of the causes of oblique cracks under the window sill and cracks on the cast-in-place concrete floor The causes of oblique cracks in the wall under the window sill and cracks on the cast-in-place concrete floor can be attributed to the following aspects. ⑴Cracks caused by the shrinkage of the concrete itself; because the volume of concrete shrinks when it solidifies and hardens in the air, and when its surroundings are consolidated and cannot shrink freely, tensile stress will occur and cause cracks. ⑵ Since the residence was changed from prefabricated to a fully cast-in-place floor, the integrity of the project was greatly enhanced, and the stiffness of some walls was relatively increased. However, the thickness of the cast-in-place floor was still between 8-10cm. Although the strength met the requirements, the floor The stiffness of the surface relative to the wall is reduced, so cracks often occur first in some weak locations, such as section mutations, construction joints, and wiring pipes. ⑶ Temperature stress cracks: At the connection between the cast-in-place concrete floor and the wall, the elongation of the floor due to temperature rise produces a horizontal thrust on the wall, causing cracks in the wall. The distribution of cracks is closely related to the stress characteristics of the wall. Cracks are more likely to appear at door and window openings and plane transitions due to stress concentration. ⑷ Since the current structural forms of residential buildings are basically the same and have not changed much, designers often only focus on strength and ignore deformation. They do not install expansion joints where expansion joints should be installed, and the foundation treatment is not meticulous, which often causes excessive settlement and deformation of residential projects. Large, causing cracks in walls and floors. ⑸Because the construction period is too short, the project is delivered for use before the foundation and main project are solidified. During use, the foundation and main project continue to deform. As a result, some projects have no cracks when they are delivered, but crack after a period of use. Just appeared. This phenomenon often occurs in projects where the foundation has been artificially treated. ⑹ In order to rush the construction period, some construction units remove the formwork before the concrete reaches the required strength. The construction load is also added too early and too large, causing internal cracks in the concrete during the hardening process. This is also one of the causes of floor cracks that cannot be ignored. one. ⒉ Preventive measures ⑴ Designers should consider foundation treatment and superstructure treatment together when deciding on foundation reinforcement and treatment plans. Methods for dealing with the upper structure include: improving the shape of the building, simplifying the building plane, rationally setting settlement joints, strengthening the overall rigidity of the house, etc., to avoid cracking of the upper body caused by uneven foundation settlement as much as possible. ⑵ Consider adding a waist beam under the window sill of the exterior wall. Depending on the foundation conditions, the waist beams are placed under the windowsills on the first floor or on other floors. ⑶ For brick-concrete structures with more than 5 floors (including storage rooms with no more than 6 floors), the appearance design of exterior walls and floors below 2 floors should minimize sudden changes, and appropriately strengthen the sudden changes. ⑷The spacing between expansion joints can be appropriately shortened. The specification stipulates that an expansion joint should be set at ≥50m. It is recommended to change it to 40-45m. ⑸ Structural columns and ring beams should be set up strictly in accordance with specifications. Wall width, wall height-to-thickness ratio, and wall width between windows should be calculated and determined in accordance with specification requirements. ⑹ The construction joints of the cast-in-place floor should be left on the wall of the house, and should not be set in the middle 1/3 of the room span. ⑺ Arrange the construction period reasonably and do not blindly pursue speed. Especially for foundations that have been manually treated, the construction progress must be strictly controlled to prevent cracks caused by uneven settlement of the foundation after the upper main structure is completed. ⑻ The construction of the main project should be carefully organized and reasonably arranged. Especially for the cast-in-place floor, the mixed soil must reach the specified strength during construction before the formwork can be removed; the upper construction load should not be added too early, too large or too concentrated, and hoisting is strictly prohibited. Impact of objects; when pouring mixed soil, the pipeline should be embedded at an appropriate depth. When the plate thickness is ≤80mm and the pipe diameter is ≥25mm, a layer of steel wire mesh with a width of 500mm and the same length as the pipe can be added to the bottom and surface of the plate. . 2. Top wall cracks ⒈Cause analysis Top wall cracks are more obvious at the top end of the building. The occurrence of cracks is caused by many factors, which can be attributed to the following aspects. ⑴ Temperature stress: Because the building is in a natural environment, it is greatly affected by natural conditions such as temperature, season, wind and rain, and the top floor has many exposed surfaces, and the temperature difference between indoor and outdoor is very different, causing the expansion of the walls and cast-in-place concrete floors. The contraction amplitude is large and the frequency is high, and the expansion and contraction coefficients of the two are quite different, and the temperature stress caused by mutual restraint is also large, which is the main reason for wall cracking. ⑵ Improper design: Design units often consider that the structure will bear more stress and less deformation in the design. Although wall cracks are caused by many factors, the failure to take corresponding measures in the design is also one of the reasons; such as brick-concrete structural walls Strict control of the height-to-thickness ratio, insufficient thickness of the roof retaining layer, no overhead layer, and excessive openings in the longitudinal walls of the end rooms are all causes of wall cracking. ⑶Improper construction: Poor construction quality of brick masonry is also an important factor in wall cracking. The strength of the masonry mortar does not meet the design requirements, the mortar of the mortar joints is not full, the mortar is not left correctly, and the dry bricks are installed on the wall, etc., which will cause the wall to crack. Cracks appear in the body. ⒉Preventive and control measures From a design perspective, the design control measures for temperature stress are relatively small. It is recommended to take the following measures. ⑴The minimum thickness of the roof insulation layer should be increased compared with the current practice. Try not to use slag insulation. The quality and moisture content of the insulation materials must be strictly controlled. If necessary, add an overhead insulation layer to the roof on top of the insulation layer.
⑵The thickness of the exterior wall should be 370mm, and the section of the structural column at the corner of the exterior wall can be 370mm.