How to prevent cracks in brick-concrete structure walls
How to prevent cracks in brick-concrete structure walls
Introduction: Brick-concrete structure refers to the vertical load-bearing structure in a building where the walls are made of bricks or blocks, with structural columns and horizontal The load-bearing beams, floor slabs, roof panels, etc. are made of reinforced concrete structures. In other words, the brick-concrete structure is a load-bearing structure with a small part of reinforced concrete and most of the brick walls. Brick-concrete structure is a type of hybrid structure. It is a hybrid structural system composed of brick walls to bear load and reinforced concrete beams, columns, slabs and other components. It is suitable for buildings with small bay depth, small room area, multi-story or low-rise buildings. The load-bearing wall cannot be modified, while the frame structure can modify most of the wall.
Currently, the brick-concrete structure residential and office buildings widely used in our country have achieved good economic benefits due to their low cost and fast construction speed. With the construction of a large number of brick-concrete houses, cracks in the walls and floors that are most likely to occur have gradually attracted people's attention. The reason for this is that the brick-concrete structure is a rigid structure and has strong resistance to vertical loads, but has poor horizontal pulling ability and coordination. Weak deformation ability, especially the weakness of resisting the out-of-plane bending moment difference of the wall, causes cracks in the wall, which we must pay great attention to.
1 Causes of cracks
1.1 Cracks caused by uneven settlement of the foundation
Most house wall cracks are caused by uneven settlement of the foundation. This depends on the form of the foundation settlement curve and the integrity of the masonry. There are three types of masonry cracks caused by foundation settlement: oblique cracks, vertical cracks and horizontal cracks. When the foundation settles unevenly, additional tension and shear forces will occur on the masonry. When this additional tension and shear force exceeds the bearing capacity of the masonry, the wall will crack.
1.1.1 The main reasons for uneven settlement
The foundation acts on soil layers of different properties, or the heights and loads of various parts of the house vary greatly; the foundation is uneven, There are mixed fills, pits, hidden pits, etc. of varying depths under the buildings. Some foundations act on weak foundations such as silt, silt soil, miscellaneous fill soil and other highly compressible soils. The foundation soil will deform greatly, and a small load difference will cause wall cracks; the moisture content of the foundation will change abnormally.
1.1.2 Locations where cracks caused by uneven settlement often occur
Places with large differences in compressibility of foundation soil; parts with large differences in building height or load; buildings Plane corner parts; construction of building junctions in batches.
1.2 Wall cracks caused by temperature changes
1.2.1 The mechanism and form of temperature cracks
Since the linear expansion coefficient of reinforced concrete is 1.0?10 -5, and the linear expansion coefficient of brick masonry is 0.5?10-5. At the same temperature, the deformation of concrete components is twice as large as that of brick walls. Therefore, when the temperature changes, the deformation of each component will be inconsistent. This causes additional tensile stress in the brickwork and causes cracks in the wall. The forms of temperature difference cracks include straight splayed cracks, inverted splayed cracks, horizontal cracks, oblique cracks, etc.
1.2.2 Causes of temperature cracks
The building is very long and no expansion joints are installed; the masonry is heated unevenly and has large temperature differences (such as canteen chimneys); the roof is not insulated Or the roof insulation construction was not done in time when the structure was capped.
1.3 Cracks caused by frost heaving in the foundation
Due to the cold climate in winter, when the temperature drops below 0°C, frozen crystals form in the soil, causing the soil to expand and bulge upward. The self-weight can no longer resist the force of the uplift of the soil, and the buildings are partially or completely lifted up. When the degree of frost heave in various parts is inconsistent, cracks will occur in the building. The main reasons for the occurrence of frost heave cracks are that the foundation depth does not meet the frost heave requirements, or the house is not heated in time when winter comes, or a 500mm thick slag cushion is not installed under the building's exterior wall floor beams or a 100mm thick benzene board is used instead.
In addition to the cracks caused by the above reasons, cracks can also occur under load due to insufficient strength of the masonry or improper handling of the support structure. Although this type of crack is not common, after the crack occurs, the load It continues to exist, causing the structure to undergo continuous reactions, so sufficient attention must be paid.
2 Preventive measures to avoid cracks in walls
2.1 Preventive measures to prevent uneven settlement cracks
2.1.1 In the selection of design plans, strict Control uneven settlement of structures. For example, choose a foundation form (pile foundation) with small settlement to control it within a reasonable range.
2.1.2 Set up settlement joints reasonably. For example, in places with large load differences, different foundation forms, complex plane shapes, and between two structural forms, settlement joints must be set and ensure sufficient width.
2.1.3 Strengthen the rigidity of the superstructure and improve the shear strength of the wall. Stronger rigidity can adjust a small amount of uneven settlement of local foundations.
2.1.4 Add structural columns, ring beams, and cast-in-place strips in areas prone to deformation, increase the marking of red brick mortar, and other structural measures to improve the integrity of the structure and local crack resistance. Based on my many years of design experience, adding structural columns and ring beams (such as structural columns at the intersections of interior walls and exterior walls, interior walls and interior walls, and ring beams on each wall on each floor) is like a steel hoop, which can effectively Prevent the development of wall cracks.
2.1.5 Strengthen the trenching work of the foundation, and strengthen the weak parts before foundation construction can be carried out.
2.2 Preventive measures for temperature change cracks
2.2.1 Reasonably arrange the construction of the roof insulation layer. Roof construction should try to avoid high-temperature seasons to improve the thermal insulation performance of the roof. Using a pitched roof and opening holes in the roof to set up dormer windows can effectively control roof cracks.
2.2.2 Increase the cross-section size and steel bar diameter of the top ring beam, and increase the reinforcement of the corner structural columns. When the length of the roof panel is greater than 30m, draw-through steel bars should be installed vertically and horizontally, and the reinforcement should be increased appropriately. Top masonry mortar markings.
2.2.3 According to the specification requirements, temperature expansion joints should be reasonably set when the building is too long. The author suggests that when the brick-concrete length does not exceed 10% of the length allowed by the specification, post-pouring tape and expansion tape can be used instead of expansion joints. If it exceeds 10%, expansion joints must be installed.
2.2.4 Exposed reinforced concrete overhangs should be provided with telescopic joints every 12m. If the roof parapet is partly a brick wall and partly a concrete wall, a temperature joint must be set between the two.
3 Reinforcement measures for cracks
For walls with vertical and horizontal cracks, the most effective method is to reinforce one or both sides of the wall with steel mesh cement mortar. In addition to meeting the reinforcement specifications, some structural requirements should also be met:
3.1 Reinforcement mesh steel bars should be laid in 6 to 8 square grids, and the spacing is generally 100m to 150mm. The steel mesh needs to be fixed to the wall with 4 or 6 through-wall ?S? bars. The spacing between ?S? bars is generally 1m. To reinforce the wall on one side, 6U ribs can be nailed into the wall and fixed to the wall.
3.2 The thickness of the cement mortar surface layer is generally 30mm~40mm, the thickness of the outer protective layer of the steel bars is not less than 10mm, and the cement mortar label should be M7.5 or above.
3.3 Before applying the cement mortar surface layer, water should be applied along the wall to moisten it. Wait until the wall is slightly dry before plastering. The cement mortar should be applied in layers according to the construction regulations to the designed thickness. The thickness of each layer Not larger than 15mm, requires compaction and firm adhesion.
3.4 After the cement mortar is finally set, the surface layer of the wall should be watered 3 to 5 times a day to prevent the surface from drying out.
Alternatively, you can chisel out the cracks along the wall, nail them into the wall with 4-inch nails, and then apply high-grade cement mortar.
4 Conclusion
Cracks in buildings are characterized by multiple occurrence, diversity, and complexity. Preventing and limiting cracks in buildings involves many factors such as the quality of project management personnel, quality of raw materials, construction methods, construction environment and operating level. This is also a long-term task for engineering and technical personnel. Only with high ideological attention, strict management requirements, and meticulous operations can cracks in the building be eliminated or limited to a certain range.
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