Cracks caused by the shrinkage of concrete itself: As the volume of concrete shrinks when it condenses and hardens in the air, tensile stress will be generated when the consolidation around it cannot shrink freely, resulting in cracks.
Because the grace of the wall is relatively large, and the thickness of the cast-in-place surface of the floor is generally between 8- 10cm, although the strength meets the requirements, the stiffness of the floor to the wall is reduced, so some weak parts, such as sudden changes in section, construction joints, threading pipes, etc., often show up first, and cracks appear there.
The main reason for the inclined cracks in the wall under the windowsill is the uneven settlement of the foundation. Secondly, due to the large openings of doors and windows in the longitudinal wall (some projects did not lintel when the openings were more than 2M, and the foundation was not treated accordingly), the stiffness of this part was obviously weakened, resulting in stress concentration at the four corners of the window opening and wall cracking.
At present, the structural forms of residential buildings are basically the same, and there is not much change. Designers tend to focus only on strength and ignore deformation. There is no joint where expansion joints are set, and the foundation treatment is not careful, which often leads to excessive settlement and deformation of residential projects, resulting in cracks in walls and floors.
Because the construction period is too short, the project was put into use before the foundation and main structure settled. In the process of use, the deformation of foundation and ground continues to deform, resulting in some projects without cracks when delivered, but cracks appear after a period of use. This phenomenon often occurs in the engineering of artificial foundation treatment.
In order to catch up with the construction period, some construction units dismantle the formwork when the concrete fails to meet the strength requirements, and the construction load is too early, which leads to internal cracks in the hardening process of concrete, which is also one of the reasons that can not be ignored.
Preventive measures for cracks in high-rise buildings;
(1) Reduce the spacing of roof expansion joints and shorten the straight section length of concrete members; The plane of the roof overhang is arranged in a concave-convex zigzag shape to shorten the straight line length of the overhang.
(2) Improve the cantilever design. Internal drainage should be given priority in design; The surface of reinforced concrete overhangs is provided with an insulation layer; Cast-in-place overhangs shall be provided with an expansion joint every 10m or so; Change cast-in-place overhangs into prefabricated overhangs.
(3) Generally, the roof panel absorbs more heat by sunlight radiation, and the thickness of insulation layer should be appropriately thickened; Materials with low thermal conductivity and excellent thermal insulation performance are selected, and air insulation layer is added to effectively control the temperature rise of the roof panel and prevent the top wall from cracking.
(4) The insulation layer should be prepared in time according to the situation of the roof slab base; For houses that are not used for a long time after completion, attention should be paid to indoor ventilation to prevent the floor from expanding and the top wall from cracking due to excessive indoor temperature.
(5) Rooms that are not used for a long time after the completion of the house should be kept indoor ventilated to prevent the floor from thermal expansion due to excessive indoor temperature and the wall from generating excessive cracks. Simple temperature cracks belong to stability cracks. Once it appears, it can release energy and has little impact on structural safety. It usually stabilizes in a year. After the cracks are stabilized, they should be repaired.