1, temperature deformation
(1), due to sunshine and temperature changes, the deformation of different materials and different structural parts is inconsistent, and there are strong constraints. For example, the cracks in the top brick wall of flat-topped brick-concrete structure, due to the different temperature linear expansion coefficients of the two materials, the deformation of the roof and the brick wall is inconsistent, and most of them are located on the top wall at both ends.
(2) The temperature or environmental temperature difference is too large. If the length of the house is too long and no expansion joints are set, the vertical cracks running through the whole height of the house are often located in the middle of the longitudinal wall.
(3) The temperature deformation of brick wall is restricted by foundation. For example, there is no heating during the construction in the northern region, and the shrinkage of brick walls is constrained by the foundation, which leads to oblique or vertical cracks in the windowsill and the masonry below it.
(4) The shrinkage (temperature and drying shrinkage) of concrete in masonry is large. Such as inclined cracks on the walls at both ends of cast-in-place long awning beams.
2. Uneven settlement of foundation
(1), the foundation settlement difference is large. For example, in a brick-concrete building with a large ratio of length to height, there will be splayed cracks when the settlement of the middle foundation is greater than both ends; When the settlement at both ends of the foundation is greater than that in the middle, inverted splay cracks appear; Vertical cracks appear when the foundation suddenly changes and one end of the foundation has a large settlement.
(2) Local collapse of foundation. For example, the masonry located in air-raid shelters and ancient wells cracked due to local collapse of the foundation.
(3) Frost heave of foundation. For example, the building foundation in the northern region is not buried deep enough, and the foundation soil has frost heave, which leads to cracks in the masonry.
(4) The foundation is flooded. For example, the uneven settlement of filled soil foundation or collapsible loess foundation after local immersion causes longitudinal wall cracking.
(5), the groundwater level is reduced. For example, in soft soil foundation with high groundwater level, additional settlement caused by artificially lowering groundwater level leads to masonry cracking.
(6), the influence of adjacent buildings. If the new high-rise building near the original building causes additional settlement and cracks in the original building.
3. The structural load is too large or the masonry section is too small.
(1), the compressive strength, bending strength, shear strength and tensile strength are insufficient. For example, the vertical crack of the central compression brick injection; Insufficient bending strength of brick flat arch produces vertical or oblique cracks; Insufficient shear strength of retaining wall leads to horizontal cracks; Cracks appear in the brick pool wall along the mortar joint.
(2) The local bearing strength is insufficient. Such as oblique cracks or vertical cracks under a girder or beam pad.
4. Improper design and structure
(1), the settlement joint is not set properly. If the settlement joint position is not at the maximum settlement difference; Settlement joint is too narrow. After the settlement and deformation of high-rise buildings, the masonry of low-rise buildings will be squeezed and cracked.
(2) The integrity of the building structure is poor. For example, in a mixed structure building, cracks occur because the reinforced concrete ring beam of the brick wall in the stairwell is not closed.
(3) Leave a hole in the wall. If the chimney hole is left at the junction of the interior and exterior walls of the house, it will affect the connection of the interior and exterior walls. Cracking due to temperature change after use.
(4) Different structures are mixed and improper measures are taken. For example, excessive deflection of reinforced concrete wall beams will lead to cracks in the wall.
(5) Improper connection between old and new buildings. For example, when the original building is expanded, the foundation is separated, and the old and new brick walls are integrated, resulting in cracks in the wall at the junction.
(6) Improper wall structure and large hole. For example, under the wall of a large windowsill, the vertical cracks are wide and narrow.
5. Poor material quality
(1), the volume of mortar is unstable. If the safety of cement is unqualified, replacing sand with pyrite slag with excessive sulfur content will cause mortar cracking.
(2) The volume of brick is unstable. If the wall is made of lime-sand bricks which have been in the factory for a short time, cracks will easily occur due to inconsistent shrinkage.
6. Poor construction quality
(1), unreasonable assembling method and lack of structural reinforcement. If the inner and outer walls are not laid at the same time, no layer-by-layer summer-planting is left, or no steel bars are put in, long vertical cracks appear at the joints of the inner and outer walls.
(2) Masonry with broken bricks, with many seams on the wall and heavy seams. For example, the external wall of a single-storey factory building is cracked due to the concentrated use of broken bricks.
(3) Improper hole or groove. For example, an office building left feet and eyes on the wall between 500mm wide windows, which led to cracks in masonry.
7. Earthquake and engineering vibration
(1), earthquake. For example, inclined cracks or cross cracks appear in multi-storey brick-concrete dormitory under strong earthquake.
(2), no bottom chord herringbone wooden frame. For example, a herringbone roof truss with no bottom chord at the top will generate horizontal thrust during an earthquake, vertical horizontal cracks will appear in the top wall, and V-shaped cracks will appear at the corner of the top floor during an earthquake.
(3) Uneven seismic subsidence. If the floor has ring beams, there will be inclined cracks in the window wall with large earthquake subsidence on one side during the earthquake.
(4) Mechanical vibration. For example, cracks caused by blasting near a project.
To sum up, it is easier to observe and distinguish the cracks caused by improper design, poor materials, poor construction and earthquake and mechanical vibration. The most common causes of masonry cracks are temperature deformation and uneven settlement of foundation, but there are also cracks caused by excessive load or too small cross section, which are often serious.
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