Cause analysis of wall cracks in building 1
We carefully analyze the cracks in buildings, we can find that the formation of cracks in buildings is mainly due to design, materials or construction, but it boils down to various situations, which are nothing more than the following. Here we analyze the reasons one by one:
1. 1 temperature and shrinkage crack.
The cracks in the wall caused by temperature stress are mainly due to the uncoordinated temperature deformation caused by the temperature difference in different parts of the building, which leads to the cracks in the wall. This kind of crack mainly occurs in brick-concrete buildings with reinforced concrete flat roofs. The forms of cracks are "eight-shaped" cracks, 45-degree inclined cracks, horizontal cracks and vertical cracks. The difference of temperature cracks in brick-concrete structure is mainly caused by two reasons: first, the initial temperature difference between brick masonry and concrete floor: during the hardening process of concrete floor after pouring, the floor temperature rises due to hydration heat, while the masonry temperature remains unchanged, resulting in the initial temperature difference between brick masonry and reinforced concrete floor. Second, the temperature difference caused by sunlight: the temperature of the building rises due to the influence of sunlight during use, and the reinforced concrete floor slab is usually exposed to sunlight for a long time, which leads to the temperature difference between the floor slab and the brick masonry. Under the influence of two temperature differences, and the thermal expansion coefficients of reinforced concrete floor and brick masonry are also quite different (10× 10-6 for reinforced concrete and 5× 10-6 for brick masonry), which leads to temperature stress, shear stress and tensile stress in brick masonry. When the shear stress and tensile stress exceed the allowable stress of brick masonry, it will
1.2 wall cracks caused by uneven settlement of foundation
(1) Oblique cracks mainly occur on soft soil foundation. Due to the uneven settlement of foundation, the wall bears large shear force. When the structural stiffness is poor, the construction quality and material strength can not meet the requirements, cracks appear in the wall.
(2) The reason why the horizontal cracks appear in the window wall is that the upper part of the settlement unit is stressed, which makes the window wall suffer a large horizontal shear force, resulting in horizontal cracks in the upper and lower positions.
(3) The vertical cracks under the window sill of low-rise buildings are due to the fact that the window sill wall acts as an anti-beam after bearing the load, especially when a wide window or window sill wall bears a large concentrated load (such as auditorium, factory building and other projects), the window sill wall cracks due to excessive reverse deformation, which will crush the window in serious cases and affect the opening of the window sash. In addition, if the foundation is built on frozen soil, the window sill will also be cracked due to frost heaving.
The unreasonable engineering design of 1.3 leads to wall cracking.
The anti-crack design was not carried out seriously according to the requirements of the specification. Although many projects have anti-crack measures in design, they do not fully meet the requirements of the code, which leads to the failure to effectively ensure the anti-crack of walls or greatly shorten the quality guarantee period. Another important aspect is improper design, such as low strength of wall masonry materials, mixed masonry of different masonry, too large difference between masonry strength and masonry mortar strength, or too large difference between exterior wall mortar strength and wall strength, which will lead to wall cracking.
1.4 wall construction quality control does not meet the specification requirements, resulting in wall cracking.
(1) The strength of masonry is low. During the construction, the quality control of materials was not carefully done, and the strength of brick masonry materials was lower than the design requirements, or the compressive strength met the requirements. However, due to the long length of masonry, after the completion of masonry construction, the masonry broke from the middle part.
(2) In the construction process of mixed masonry with different strengths, different masonry materials are used as supporting blocks, and the wall cracks due to the different strengths of masonry materials, thermal expansion and contraction, water absorption and other reasons.
(3) The strength of masonry mortar is low (high). In the process of mortar mixing, uneven mortar mixing leads to some high mortar strength, some low strength, and some even have extremely low strength because of too little bonding material. In terms of ingredients, the strength of mortar is low when sand is added and high when cement is added; Too much water and low consistency of mortar affect mortar strength, mortar shrinkage increases, resulting in mortar joint cracking.
(4) Masonry mortar is not used as required. Mortar is mixed too much at one time and stored for too long, which leads to the initial setting of mortar before masonry, and the strength of mortar has been greatly reduced when it is used, which seriously affects the quality of wall and causes cracks.
2 wall crack control measures
2. 1 Measures to prevent temperature and shrinkage cracks
(1) The roof is provided with thermal insulation layer or insulation layer.
(2) The control joints should be set in the proper parts of the roof, with a spacing of 30 mm ..
(3) When cast-in-place concrete overhangs are used, the length is >; 12mm, it is advisable to set a separation seam with a width >; 20 mm.
(4) Reasonable setting of mortar joint steel bars, the requirements are as follows: ① Set steel bars at the upper and lower first and third mortar joints of the hole of the wall, and the length of steel bars extending into each side of the hole should be >; 600 mm. (2) In the second or third mortar joint above the floor elevation and below the floor elevation, near the top of the wall, steel bars are set. (3) The spacing of mortar joint reinforcement is 600 mm..⑤ The length of mortar joint reinforcement should be set. When it is inconvenient to set the length, overlapping is allowed. Overlap length >; 300mm. ⑥ Both ends of mortar joint reinforcement shall be anchored into the intersecting wall or corner wall, and the anchoring length is >; 300 mm. ⑦ Mortar joint reinforcement shall be buried in mortar, and the protective layer shall be ≥3mm, and the outside.
(5) Set a visor with a width of 40-50mm on the top ring beam to prevent direct sunlight on the reinforced concrete ring beam and reduce the stress caused by temperature difference.
(6) For masonry with temperature cracks, although cracks will not affect the structural safety of the building in general, the appearance of cracks will affect the beauty and use of the building, and will also affect the integrity and durability of the structure. Therefore, after the cracks are stabilized, measures should be taken in time: for a few wall cracks with small crack width, after removing dirt such as dust, lime, scum and loose layer on the crack surface, pressure grouting can be used to repair them; For a large number of wall cracks with large width, it is advisable to remove all plastering on the wall first, and remove the mortar with a depth of not less than 10mm in the mortar joint of the wall, clean the dust on the wall and water it wet, seal the cracks with cement slurry, hang bidirectional φ6@200 steel mesh pieces on both sides of the brick wall, hook the two steel mesh pieces with φ6 through-wall reinforcement, and then plaster the surface with high-strength mortar. 2.2 Measures to prevent cracks caused by foundation settlement
(1) Reasonable setting of settlement joint. All houses with different loads (houses with large height difference), houses with long plane shape and different foundation treatment methods of the same building and some basements should be divided into several parts and set settlement joints to make them settle separately to reduce or prevent cracks.
(2) Strengthen the rigidity of the superstructure and improve the shear strength of the wall. Ring beams can be set on the foundation (0.00) and the upper parts of doors and windows of each floor. In the process of masonry work, all rules and regulations should be strictly implemented. Such measures as watering and wetting bricks, improving the workability of mortar, improving the strength and fullness of mortar, increasing the bonding between brick layers and strictly prohibiting temporary interruptions in construction can greatly improve the shear strength of walls.
(3) Strengthen the exploration of foundation trench. For complex foundation, drilling should generally be carried out after foundation trench excavation, and foundation construction can only be carried out after weak parts are reinforced.
(4) The rotation of concrete beam or brick should be considered in the lower part of the large window to adapt to the deformation of the window sill and prevent vertical cracks at the window sill. In order to avoid cracks under the bottom windowsill of multi-storey residential buildings, in addition to strengthening the integrity of the foundation, reinforcement methods can also be adopted. In addition, don't use too many half bricks when building windowsills. 40mm thick reinforced concrete belt is added below the window opening, so that a U-shaped reinforced concrete belt is formed between the gable 1-2 room and the gable, which solves the problem of cracks in the lower corner of the window and improves the structural integrity.
(5) The core column of block structure usually adopts the method of "hiding the core column", so it is impossible to use mechanical vibration when pouring concrete, and the quality of the core column is difficult to guarantee. In order to overcome this shortcoming, the "hidden core column" is replaced by an open structural column of 240mmx240mm or 240mmx 190mm, and stirrups and tie bars are retained as required to improve the seismic capacity and facilitate quality inspection.
2.3 From the engineering design, effectively prevent wall cracks.
Strengthen the essentials and theories of wall anti-crack design, and design the wall in strict accordance with the requirements of the code to ensure the quality of the wall.
(1) A certain amount of fiber is added to the wall plastering mortar to enhance the crack resistance.
(2) All exterior wall decoration conditions are supplemented by steel wire mesh.
(3) When the masonry wall has a window sill, all concrete window sills shall be used.
(4) Try to use one material for wall masonry, and avoid the mixed use of multiple materials.
(5) The strength, water absorption, thermal expansion and cold contraction of the block, masonry mortar and plastering mortar used in the wall should be as consistent as possible.
(6) Steel mesh shall be added at the interface of different materials, and steel mesh shall be added at the embedded part of the pipeline.
2.4 Other measures to prevent wall construction cracks
(1) In the process of masonry construction, the quality of various raw materials should be strictly controlled, and mortar mixing should be carried out in strict accordance with the requirements. The strength grade of wall masonry mortar should be improved to increase the tensile strength of masonry.
(2) The daily masonry height of masonry engineering shall not exceed the specification requirements of1.8m.
(3) Do a good job in the construction scheme of wall decoration, and do a good job in the technical disclosure of leveling layer, surface layer and each sub-item.
(4) Batch swinging shall be carried out in layers as required. The plastering layer of cement mortar and cement mixed mortar can only be carried out after the previous layer condenses; Lime mortar plastering layer should be carried out only after the previous layer reaches 7 ~ 80% dry.
(5) It is forbidden to chisel away masonry, especially lightweight masonry. Strictly control the quality of masonry, the mortar is full, and the Rachel reinforcement is designed according to the specification requirements.
(6) Take effective measures to strengthen the quality management of grass-roots construction.
(7) Local walls are too thick to be reinforced with steel wires.
(8) When the plastering layer of the wall adopts steel wire mesh to resist cracking, effective measures should be taken to ensure that the steel wire mesh is in the middle position of the batch swing layer, so that the steel wire mesh can give full play to its anti-cracking effect.
(9) The reserved construction holes shall be designed and sealed as required.
(10) Before the concrete wall is poured, reliable construction platforms and walkways must be set up, and special personnel shall be assigned to take care of the reinforcement during the construction to ensure that the reinforcement position meets the construction specifications and design requirements; (1) has been pouring concrete must be maintained in accordance with the requirements of construction specifications. After concrete pouring (after final setting) 12h, it shall be covered with moisture curing; According to the climatic conditions, the number of watering should make the concrete wet. Maintenance water should be the same as mixing water; Cover and maintain with plastic sheets, the concrete should be completely covered tightly, and there should be condensed water in the plastic sheets; When the average temperature of the day is lower than 5℃, water spraying is not allowed; (2) The curing time of concrete shall be determined according to the variety of cement used. The curing time of concrete mixed with portland cement, ordinary portland cement or slag portland cement shall not be less than 7d; The curing time of concrete mixed with retarding admixture or with impermeability requirements shall not be less than14d; ; If it is not convenient to cover with water for curing, protective layer (such as film curing solution) should be adopted for curing to reduce the evaporation of water in concrete.
3 Conclusion
The causes of cracks in building walls are complex and diverse, with many influencing factors and great difficulty in control, but generally speaking, they are nothing more than the above. As long as the whole process control method is adopted, management is strengthened from design to material selection and construction, and relevant codes and operating procedures are strictly observed, the possibility of wall cracking can be greatly reduced, or the number of cracks can be controlled to a minimum, thus ensuring the quality of engineering construction and improving people's living standards.
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