In multi-storey masonry buildings, cracks often appear in the wall, and the time of cracks varies from building to building, some appear early, some appear late, but most of them appear in the new building 1-3 years; The width of the joint varies from 3 to 2, and a penetrating crack is formed in severe cases. The problem of cracks in masonry structure has become a universal problem, which not only affects the normal use of buildings, reduces building functions and shortens service life, but also is extremely unfavorable to earthquake resistance. Especially in the commercialization of housing today, this problem has aroused widespread concern of developers and residents. Therefore, how to control the cracks in masonry buildings is a new topic for engineers and technicians.
2 Causes and types of cracks
2. 1 splay crack. It mainly appears at both ends of the transverse wall and the longitudinal wall. This kind of crack is a splayed thermal expansion crack, which changes with the rise and fall of temperature. The reason is that the thermal resistance of roof insulation layer decreases or even fails due to the defects in design and construction, which leads to the temperature deformation of roof panel greater than that of masonry. When a certain temperature stress occurs, the thrust of the roof panel is transferred to the wall, and the additional stress of the wall temperature is greater at both ends of the house. When the strength of masonry mortar is low, the main tensile stress caused by shear force is easy to appear.
2.2 inverted splay crack. It is a cold shrinkage crack, which mainly appears in the holes at both ends of the vertical and horizontal walls, especially at both ends of the top floor. Because the additional stress of cold shrinkage of the wall is large at both ends of the wall, when the shrinkage deformation of the house is greater than that of the wall, the stress is relatively concentrated at the entrance of the door and window, which leads to the formation of inverted splay cracks and cracks in the wall.
2.3 horizontal cracks. More common in top horizontal walls, vertical walls, "parapets" and gables. When the thermal insulation of the roof is poor, the thermal expansion of the roof panel produces horizontal thrust on the wall. Because the shrinkage of the end wall is greater than that of the middle wall, the shear capacity of the masonry is low, and horizontal cracks occur at the contact surface between the vertical and horizontal walls and the roof.
2.4 Vertical cracks. Mainly in the windowsill wall, lintel end, staggered floor. This kind of crack is mainly due to the temperature change, the wall is subjected to the tensile stress of the floor, and the stress concentration effect occurs at the door and window opening 1: 1, which causes the vertical crack of the wall, or the deformation difference between the upper beam end of reinforced concrete and the floor is the largest due to the cold shrinkage deformation. 25X-shaped crack. Most cracks along the mortar joint of masonry are mainly caused by the repeated action of thermal expansion and cold contraction of buildings, while X-shaped cracks in the bottom wall are caused by uneven or uneven settlement of foundation.
3. Active control of masonry cracks in the design process.
Once cracks appear in masonry structure, it will reduce the use function of the building, and serious cracks will also affect the safety of the structure. At the same time, it is difficult to reinforce cracks. Therefore, it is very important to prevent and control cracks in masonry structures, especially in earthquake areas, otherwise it will have serious consequences.
3. 1 Strengthen the setting of reinforced alkali ring beam to improve the integrity of the wall. Ring beams must be set at different heights of each bay, staggered floor and roof on the top floor of the building; The top outer ring beam should be designed as a hidden ring beam, which can not leak out, so as to protect the outer ring beam from direct sunlight or atmospheric influence; No matter whether the parapet is high or low, reinforced concrete coping ring beams should be set up and integrated with the "constructional column" to resist cracks.
3.2 In addition to setting "constructional columns" according to the specification requirements, "constructional columns" must be set at the junction of vertical and horizontal walls with "L", "R" and "L" plane shapes, so as to improve the overall stiffness of the building and the ductility of the wall and limit the expansion of cracks in the wall.
3_3 Improve the overall quality of roof panels. It is best to use cast-in-place roof panels or add cast-in-place layers on prefabricated roof panels; Cast-in-place slats are arranged between prefabricated roof panels and external longitudinal walls, and cast-in-place plate beams are arranged between prefabricated roof panels, so that the roof is assembled into a whole.
3.4 The wall in the l-2 bay at the top of the building adopts reinforced masonry, that is, 2-6 steel bars are added to the horizontal mortar joint every 8 skin bricks, which are pulled in the L-2 bay and combined with the "constructional column" steel bars. Bricks for the top floor should not be lower than MU7.5, and the strength of masonry mortar should not be lower than MS, so as to improve the anti-cracking ability of the wall.
3.5 The "cornice" of the roof is an exposed structure, and the temperature changes greatly in one day, which is not only easy to crack, but also has a certain impact on the wall cracking. Therefore, the longitudinal reinforcement of overhanging eaves should be appropriately increased, and "deformation joint" or "post-pouring belt" should be added to reduce shrinkage. The practice of "post-pouring belt" is to reserve a "post-pouring belt" with a width of 300ram at an appropriate position in the middle where the longitudinal stress is small, and use steel bars to penetrate it, and then pour it after 40-60 days of construction, which plays a control role of putting it first and then resisting it.
3.6 Pay attention to roof insulation. When selecting the roof insulation layer, properly thicken or select materials with good insulation and productivity. Thermal calculation of roof insulation layer must be carried out in accordance with building energy-saving standards to further improve the insulation performance of roof insulation layer. Poor thermal insulation of the roof is the direct cause of the temperature stress of the roof slab, which will lead to cracking of the top wall or roof leakage in severe cases. The insulation layer should be made into "cornice" or gutter to prevent concrete structure from leaking. If possible, you must add and overhead.
4 Masonry crack reinforcement treatment
4. 1 When the roof insulation layer can't meet the thermal requirements and energy-saving standards, the roof insulation layer should be redone to stabilize the cracks, because it is useless to reinforce the temperature cracks only in general, and the temperature stress must be reduced. The thermal insulation materials used in the thermal insulation layer should meet the requirements of various technical indexes such as apparent density, particle size, thermal conductivity and moisture content, and should be constructed in strict accordance with the requirements of the design and current construction specifications to achieve the designed thermal insulation effect.
4.2 For masonry cracks caused by uneven settlement of foundation, the foundation should be strengthened first, and then the wall should be strengthened after the settlement reaches the stability standard (the average El settlement is within 0.02-0.03).
4.3 Cracks in external longitudinal wall, transverse wall and internal longitudinal wall shall be reinforced by plastering with steel wire mesh cement mortar. When the depth of mortar joint is 12cm, the anchor bolts will expand by @500, showing a plum blossom distribution. Must hang steel mesh, MIO cement mortar 40mln thick, 3 road can survive. After the completion of construction, attention should be paid to sprinkler maintenance to prevent empty drum.
4.4 For minor cracks, cement mortar and 107 glue can be used to repair them.
5 concluding remarks
Controlling the generation and expansion of cracks is an indispensable and important link in building engineering, which should be paid enough attention to, especially in the current situation that buildings are developing from ordinary to high-rise and large-scale, and it is urgent to formulate unified norms and technical standards. In order to control cracks, focusing on prevention, it is necessary to make joint efforts in design and construction, take targeted anti-crack measures, increase the intensity of active control, and improve the reliability of new residential quality. As long as the regulations are strictly implemented and the design and construction are closely coordinated, it is completely possible to control cracks. Practice has proved that many projects that took measures to control cracks in the past generally achieved good results and were rated as real high-quality projects.
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