1. Preventing wall cracks caused by foundation (1) When uneven settlement of local foundation occurs, its manifestation is mostly cracks in the bottom wall, which can continue to extend upwards in severe cases.
1. Oblique cracks generally occur at both ends of the longitudinal wall of a building, or in the middle of the building and at the external corner of the building. The cause of oblique cracks is the local settlement of foundation, which makes the wall bear large shear force. When the tensile stress of masonry exceeds its tensile strength, it will break.
2. Most horizontal cracks occur on the wall between windows. When local uneven settlement occurs, the upper part of the settlement unit is resisted, which makes the wall between windows suffer greater horizontal shear force. When the mortar strength is not enough to resist this shear force, horizontal cracks will appear.
3. The vertical seam is mostly in the middle of the sill of a wider window. When uneven settlement occurs, the load on the window wall is large, and the load on the windowsill is small because the window is lying on it. Therefore, the windowsill wall is equivalent to the anti-beam and the window wall is equivalent to the anti-beam support. The windowsill wall is cracked due to excessive reverse deformation, and the upper width is wider than the lower width.
(2) Preventive measures 1. Strengthen basic investigation. When checking the trench, drill out to find out the local soft soil layer. According to the exploration report, the composition of soil layers can be distinguished to prevent some characteristic soils, such as expansive soil and collapsible loess, from being treated as general soil due to soilless sample analysis. The soft soil found should be treated before foundation construction.
2. Reasonable setting of settlement joint. When the foundation is relatively compressible, the building is long, the shape is complex or the foundation form in the same building is different, settlement joints should be set from the foundation.
3. Strengthen the rigidity of the superstructure and improve the shear and tensile strength of the wall. When the stiffness of the superstructure is large, the differential settlement can be adjusted appropriately.
(1) When the construction or material quality is poor, the cracks in the wall are irregular and unevenly distributed. However, when the construction sequence is unreasonable, more concentrated and obvious cracks can be produced.
(2) Control measures 1. Strictly control the quality of materials and resolutely do not use unqualified materials.
2. Construction in strict accordance with the specifications. Masonry should be staggered up and down, overlapping inside and outside, and horizontal mortar joint and vertical mortar joint should be full. It is forbidden to use mortar paving instead of caulking. Corners and interchanges should be built at the same time, and the utilization rate of half bricks should not exceed 5%. 3. Carefully analyze the building structure and reasonably arrange the construction process. Build the main body first, then the subsidiary, the high part first, and the low part later. For large-area cast-in-place slab, post-cast strip should be set.
4. Used for settlement joints, expansion joints, etc. Debris must be removed from the joint so that the joint can work normally.
5. There should be 2φ6 Rachel bars between the bearing or non-bearing structural column and the wall, with the spacing of eight skin bricks, and each end extends into the wall 1 m or to the hole.
6. When the precast lintel meets the constructional column, when the lap length is less than 250 mm, it should be cast-in-situ and extended into the constructional column.
(1) When the temperature difference changes too much and the house lacks effective resistance to the internal stress caused by the temperature difference, oblique and horizontal cracks often appear on the top floor of the house.
1. Oblique cracks mostly occur at both ends of the top longitudinal wall, and their width is generally large in the middle and small at both ends. When there are doors and windows at both ends of the outer longitudinal wall, the cracks crack obliquely along the windows.
2. Horizontal cracks mostly occur under the top ring beam, and both longitudinal and transverse walls can appear, especially at both ends of the house.
(2) Prevention and control measures 1. Adding insulation layer to the roof can effectively reduce the temperature difference between the roof and the wall. The insulating layer must have a certain thickness, and should be provided with gas barrier, protective layer and vent holes.
2. When the house is long and complex, expansion joints should be set reasonably.
3. When there is a big hole in the bay at both ends of the longitudinal wall, concrete constructional columns can be set on both sides of the hole to bind the upper and lower ring beams, which can not only strengthen the stiffness of the bay, but also prevent the development of cracks.
4. Improve the strength of the top masonry, so as to strengthen the thermal stress resistance of the wall.
5. Reasonably arrange the time and process of roof construction, try to avoid high temperature or cold season during construction, strengthen roof maintenance, and set up post-pouring belt when necessary to solve the internal stress problem in concrete construction.
Four, to prevent the wall cracks caused by design reasons (1) improper design.
1. If it is not handled properly, uneven settlement of local soft foundation may occur. When the rigidity of the superstructure is not enough to resist the internal stress caused by uneven settlement, cracks will occur.
2. The house is too long or too complicated, which is prone to uneven settlement or temperature difference cracks.
3. Due to the influence of adjacent building foundations, the foundations are prone to additional settlement.
4. There is no unfavorable load combination in the design, which leads to a big difference between the use load distribution and the design value.
5. Insufficient masonry strength design.
6. The ring beam design is too small or the strength is too low, and the lap length of the hole lintel is less than 250 mm, etc.
7. When the girder is placed on the masonry, the local bearing surface of the masonry is insufficient or small, resulting in cracking.
8. Deflection is due to the small rigidity of the main girder, and the displacement of the beam end embedded in the wall leads to the wall cracking.
(2) Preventive measures 1. The local soft foundation should be reinforced, and the rigidity of the superstructure should be strengthened, and the expansive soil and collapsible loess should be specially treated.
2. There should be a certain gap between the foundations of adjacent buildings, and at the same time, the settlement caused by the stress superposition of adjacent foundations should be calculated to make it the same as the settlement of the whole building.
3. When calculating, carefully combine unfavorable loads; In the design, indicate the use load value.
4. Carefully check the strength of masonry, and check the local pressure of masonry. When the local pressure is insufficient, concrete pads should be set.
5. The rigidity of each component shall conform to the specifications.
6. For a longhouse, the rigidity of the top floor room should be strengthened.
7. Do a good job in roof insulation design.
5. In order to prevent wall cracks caused by improper use of house decoration, the original designer should be consulted, and the load-bearing members should not be destroyed at will, and the load should not exceed the design value when decorating the floor; In use, the live load should not be too concentrated; When the house exceeds the reasonable service life of the structure, it shall entrust the relevant units to carry out appraisal. If necessary, reinforce the damaged parts and strengthen observation.