Causes of cracking of plain concrete strip foundation

Brick-concrete structure building wall has low tensile and shear resistance, and it is easy to produce cracks locally, which destroys the integrity and use function of the building and even endangers safety. Cracks in brick-concrete structure wall mainly include uneven settlement cracks of foundation, temperature difference cracks and structural cracks. 1. Uneven settlement crack of foundation (1) The uneven settlement crack first occurs at the bottom of the building, and may develop seriously in more than two floors, accompanied by ground cracking and house inclination. The wall creates a vertical seam with a wide bottom and a narrow top. Oblique joints, splayed joints and horizontal corner joints are produced obliquely at both ends and doors and windows. Once cracks appear, with the development of uneven settlement of foundation, cracks gradually widen and extend. (II) Cause of crack 1. The foundation soil is not firm and uniform. The local soil quality of building foundation is uneven, and it will inevitably produce excessive uneven settlement after compression. 2. Improper foundation treatment and unreasonable foundation design. Building load will inevitably produce greater additional stress on the foundation, and the soft foundation with low bearing capacity and large deformation should be strengthened to improve the bearing capacity of the foundation. The foundation design should consider shallow foundation or pile foundation according to the upper load and foundation soil quality. The layout of this building is unreasonable. In order to pursue architectural modeling, the starting point of architectural plane is complex and the turning point is changeable; The house is too long, and the length and height are relatively large; The building elevation fluctuates and the load changes greatly; Daimon Masaru's windows were opened and the walls were weakened. 4. Abnormal change of foundation water content. Due to the change of some surrounding conditions, the groundwater level in the construction site rises, or the upper and lower pipes leak, and the surface water penetrates into the foundation of the building, and the soil softens or even is washed away for a long time, resulting in uneven settlement. 5. Improper use of buildings. With the change of house use and the increase of load, the live load exceeding the design requirements accumulates on the indoor ground, which makes the additional stress of foundation increase sharply, leading to uneven settlement of buildings and cracking of walls. (iii) Preventive measures 1. Housing construction projects should be investigated before design. Before architectural design, the engineering geology should be investigated in detail to find out the hydrogeological conditions such as foundation soil, distribution range, bearing capacity and groundwater level. Then, according to the requirements of safety, reliability, economy, advanced technology and convenient construction, the advantages and disadvantages are comprehensively analyzed and weighed, and the reasonable building layout and structural type are determined, so that the superstructure and foundation can interact and work together. This is especially true for soft foundation and uneven foundation. 2. Reduce the weight of the building structure. The compression deformation of foundation is proportional to the upper load. Therefore, reducing the self-weight of the structure is an effective measure to reduce the additional stress and settlement of the base. For the foundation, you can choose the foundation form with light weight and less soil cover, such as wide foundation shallow burial, hollow foundation, thin shell foundation and even box foundation; Or set up a basement semi-basement, and use a raised floor instead of indoor filling. For the superstructure, prestressed, light steel structure and light wall material with small bulk density can be selected to reduce the pressure on the foundation and the settlement of the foundation. 3. Reasonable arrangement of architectural modeling. The plane modeling of the building should be as simple as possible, and the vertical walls should be pulled through to avoid changeable turning points and complex bumps. Building facade should try to avoid uneven, large load difference, or opening large doors and windows to weaken the wall. Make the center of mass and stiffness of the building basically the same, and improve the ability of the building to resist uneven settlement. 4. Enhance the overall stiffness of the building. (1) Enhance the longitudinal stiffness of the building. Uneven settlement causes the house to bend longitudinally, so the longitudinal wall should try to avoid turning, interrupting and opening too large doors and windows, the distance between transverse walls should not be too large, and it should be firmly connected with the longitudinal wall. (2) Control the building height. The smaller the aspect ratio, the greater the building stiffness. For buildings with soft foundation and more than three floors, the length-width ratio should not be greater than 2.5. (3) Setting settlement joints. In the proper position of long buildings, where the plane turns, the height is uneven, the load difference is large, and the foundation or foundation type changes, settlement joints or connecting profiles are set to disconnect the building from the roof to the foundation, and the building is divided into several units with large stiffness and small length-height ratio to form their own settlement systems. (4) Set plane closed reinforced concrete ring beam (or reinforced brick belt) at the top of the wall under the foundation and floor slab, or adopt cast-in-place floor slab to enhance the integrity of the building. (5) Strengthen the rigidity of foundation. For buildings on soft and uneven compressive foundation, or according to the load of superstructure, the foundation type with higher rigidity can be adopted, such as reinforced concrete cross-bar foundation, raft foundation and even box foundation. 5. Adjust the load distribution of each part. For buildings with large difference in height (or load), heavy and high loads should be arranged reasonably; The mixed bearing form of vertical and horizontal walls and different foundation widths are adopted to reasonably adjust the uneven settlement of various parts of the building. For buildings with strict requirements for uneven settlement, if necessary, smaller base stress can be selected to increase the base area for design. 6. For single-storey warehouses and houses stacked in a large area, statically determinate structures should be adopted to prevent wall cracking. 7. A certain distance should be kept between new and old buildings or adjacent buildings to avoid new additional stress and stress superposition on the foundation and uneven settlement. 8. Enhance the strength of doors and windows. Set reinforced concrete door frames on both sides of the door and window openings; In order to prevent the reverse bending deformation and cracking of the bottom window sill wall, an appropriate amount of steel bars can be set in the masonry above the window sill wall. 9. Reasonable arrangement of construction sequence. Houses with high and low opposites and large load changes should be organized in stages and sections. Generally, high-rise buildings with large load are built first, and then light-weight low-rise buildings are built; Build deep foundation first, then shallow foundation to avoid adding new additional stress. 10. Use the house correctly according to the design requirements. After the house is completed, it is not appropriate to change the use function of the house at will, increase the use load or arbitrarily increase the thickness of the ground to prevent surface water from infiltrating into the foundation. Second, the temperature difference crack (I) The location and shape of the crack The temperature difference crack generally occurs on one or both vertical and horizontal walls at the top of the house, and a circle of horizontal cracks is formed on the wall around the bottom of the top beam. However, oblique seams and splayed seams will occur at the corners of doors and windows. This crack is wide at the top and narrow at the bottom. The remarkable feature of temperature difference crack is that the crack width opens and closes with the random rise and fall of temperature. (II) Cause of crack 1. Performance difference between concrete and clay brick masonry. Because the linear expansion coefficient of concrete and brick masonry is different (the linear expansion coefficient of concrete is 10× 10-6), while that of brick masonry is 5× 10-6. Under the influence of temperature difference, the deformation caused by the difference of concrete houses is relatively large, while the deformation of brick walls is much smaller, and there is a trend of relative displacement between them, resulting in houses. 2. Temperature difference is the main factor that causes cracks, and the size of temperature difference cracks is directly proportional to the environmental temperature difference. Cracks with large temperature difference are larger, and cracks with small temperature difference are smaller. The cracks in the roof with good thermal insulation effect are small, and the cracks with poor thermal insulation effect are large. 3. It is related to the opening size of the wall doors and windows. Masonry has high strength, good construction quality, strong tensile and shear resistance and slight cracks. 4. It is related to masonry strength and construction quality. Masonry has high strength, good construction quality, strong tensile and shear resistance and slight cracks. (3) Preventive measures for temperature difference cracks 1. Reduce the telescopic distance of the roof. The expansion joint spacing of roof ring beam should not exceed 30m. For cast-in-place integral reinforced concrete roof slab, a flexible frame joint is generally set every 6m or so. Corresponding expansion joints are reserved between the plate end, the plate edge ring beam and the prefabricated roof panel wall, so that the concrete members can freely expand and contract. 2. Design the roof overhang. (1) Internal gutter drainage is given priority in the design; (2) setting an insulation layer on the surface of the reinforced concrete overhangs; (3) Cast-in-place overhangs are provided with an expansion joint every 10m or so (the slab is constantly broken); (4) Cast-in-place overhangs shall be prefabricated. 3. Use 1:3 lime mortar to set horizontal movable joints between roofs and wall tops at both ends of the house. 4. Set the overhead insulation layer on the reinforced concrete roof. 5. Improve the strength grade of the top masonry. (1) Improve the strength grade of the top masonry; (2) In the horizontal mortar joint of one or two bay masonry at the top, set an appropriate amount of long steel bars; (3) In the non-seismic fortification area, reinforced concrete structural columns are set at the corner of the external wall, and Rachel with the wall is strengthened; (4) Reduce the width of six windows on the top wall and increase the width of the wall between the top window and the side chimney. Three. Structural cracks (1) Comminuted cracks generally occur in windows, brick columns and other parts. At the bottom floor, as well as the girder and roof truss support under concentrated load, the load is large and the section size is small. Due to excessive load or low bearing capacity of masonry and insufficient local pressure, masonry is damaged by compression, resulting in vertical comminuted cracks. The joint is wide at the top and narrow at the bottom, and there are irregular broken cracks in the upper section. (2) Structural cracks 1. Structural design error. Due to the omission of structural load calculation, wrong design, unreasonable structure and excessive load, the bearing capacity of masonry is weakened. 2. All kinds of pipelines are buried through the wall in the masonry, which destroys the integrity of the masonry, reduces the cross-sectional area of the masonry and weakens the nonconformity of the masonry. Brick column adopts core-cladding method, and there are defects such as "through seam" in masonry, which reduces the bearing capacity of masonry. 3. The quality of masonry engineering is poor. Due to the low strength grade of masonry brick and mortar, the masonry mortar is not full, and the masonry does not meet the requirements. Brick mortar strength masonry method has some defects such as "straight seam", which reduces the bearing capacity of masonry. 4. Improper use. Because of changing the use of the house, increasing the use load and increasing the vibration force, the wall is damaged. (3) Crack prevention measures 1. Correct structural calculation and design. When the load is large and the section size of the member is limited, the strength grade of brick and mortar should be improved, or reinforced masonry should be used to improve the strength of masonry; Reinforced concrete pads are designed at the supports of beams and roof trusses. 2. uninstall. For the wall with cracks due to excessive load and low masonry strength, the self-weight and service load of the upper structure can be reduced, or reinforced concrete beams can be added to the top masonry to bear the load from the upper part. Brick walls and columns are set under the original beam to share part of the upper load and protect the cracked masonry. 3. Structural reinforcement. For walls with cracks due to large load, small masonry section size and insufficient bearing capacity, the section size can be increased, such as blocking all or part of doors and windows with bricks, adding pilaster, pouring reinforced concrete plywood outside the listed masonry, and changing cracked brick walls into reinforced concrete walls and columns to improve their bearing capacity. 4. Cracks in brick-concrete structure walls may be caused by the single factor mentioned above or by many factors. When cracks appear, correct judgment should be made through careful analysis, and effective measures should be taken to control the development of cracks, ensure structural safety and prolong service life.