Analysis of cracks in block wall and preventive measures?
First, the cracking phenomenon of the top wall of the house and its preventive measures
This situation generally appears in the top 2-3 floors of the building, which is manifested as: horizontal cracks appear at the bottom of the beam; Vertical cracks or splayed cracks appear at the edge or middle of the infilled wall column; Cracks are not obvious in the morning, but become obvious in the sunny afternoon; There are more external walls than internal walls.
In maintenance, we have adopted two methods: one is to paste the non-woven fabric with a width of 100 mm at the crack with white latex on the plastering base, and then scrape putty to restore the surface layer; The second is to peel off the 200 mm wide plastering layer along the crack, and then plaster the surface layer after installing the steel wire mesh. However, after a period of time, new cracks often appear on the edge of steel mesh or non-woven fabric. Based on the analysis of the above phenomena, it can be concluded that the temperature difference between the upper and lower roof panels increases after the roof frame structure is exposed in the afternoon, and the frame beams and columns are deformed due to the temperature difference, while the infilled wall is a rigid structure, which can not coordinate with the frame structure and produce horizontal cracks; On the other hand, due to the different expansion temperature linear coefficients of reinforced concrete structure and masonry structure, when the temperature changes, poor deformation and vertical cracks will appear. It is unrealistic to prevent or reduce the temperature difference deformation of reinforced concrete structures for completed projects. The key to solve the problem is to make the infilled wall and frame structure form a whole, and have certain adaptability. The specific operation is as follows:
A) Cut the infilled wall with a hand-held cutting machine in both vertical and horizontal directions, with a depth of 20mm (to the surface of the masonry), a width of 20mm, and a groove spacing of 400-600mm (depending on the crack size of the wall) in a net shape. Vertical grooves extend from the bottom of the floor to the ground, and horizontal grooves pass through the wall and cover the surfaces of columns on both sides. B) Remove the dust from the oil tank and keep it dry. C) After mixing the commercially available epoxy resin and curing agent according to the instructions, evenly brush the resin in the groove with a brush, and at the same time evenly brush the derusted φ6 steel bar, then press the full-length φ6 steel bar into the groove, and at the same time press the premixed 1: 1 hard cement mortar into the groove, so as to keep the φ6 steel bar from moving, and the amount of steel bar is less than 15655 during construction. D) After the mortar is dry, tap it with a small hammer to check whether it is empty, and then restore the wall decoration layer. For the external wall, waterproof measures should be taken with cement-based waterproof coating.
This method, using the bonding effect of epoxy resin, on the one hand, makes the infilled wall become a reinforcement with certain strain capacity and improves the crack resistance. On the other hand, the frame and the infilled wall are connected into a whole through the steel mesh, and the deformation difference is evenly distributed throughout the wall, which increases the ability of joint deformation, thus avoiding or reducing the occurrence of cracks. In addition, this method also has the advantages of less damage to the wall, fast construction period and easy repair of the decorative layer.
In view of the universality of this kind of crack, certain measures must be taken from the design and construction stages to solve it:
A) Pay attention to and do a good job in roof insulation to reduce the temperature difference between the upper and lower roof panels. B) Because the roof panel (i.e. external frame beam) and parapet are all exposed, it is difficult to complete the heat preservation measures. Structural measures should be taken to increase constructional columns in the side span and reduce the span between columns (less than 3m), so as to reduce the deformation of the side beam caused by the temperature difference between the upper and lower parts and reduce the occurrence of horizontal cracks in the wall. C) The design shall minimize the exposed part of the roof structure. D) Pull Rachel bars on both sides of the infilled wall to form reinforced masonry, so as to improve the cracks caused by the deformation difference between the two materials. E) Wire mesh should be hung on the wall, and then plastered. Steel mesh and frame beams and columns should be reliably tied (such as bonded with epoxy resin), so that the wall and frame structure form a whole, and the common deformation ability is enhanced, thus reducing cracks.
Second, the autoclaved aerated concrete block wall cracking phenomenon and prevention.
Aerated concrete blocks account for the highest proportion of cracks in the wall, which is characterized by vertical or splayed cracks on the column side and the middle of the wall. There are two main reasons: first, the masonry material shrinks greatly; Secondly, due to the different linear expansion coefficients, there is a difference in temperature deformation between the wall and the concrete frame structure. In the maintenance, we used the method of pasting non-woven fabric or plastering with steel mesh, but the effect was not ideal. After analysis, there are the following reasons: first, the shrinkage period of cement products is long, and the drying shrinkage will not be basically completed until the third age; Second, aerated concrete block has developed pores and strong capillary action, which is greatly influenced by air humidity. In this regard, we also adopt the method of bonding steel bars with epoxy resin. According to the above method, steel bars are bound along the horizontal direction at the crack, with a distance of 200 mm and a length of more than 500 mm from each side of the crack. Practice has proved that this repair method has the advantages of high success rate, small wall damage and short construction period.
Aerated concrete blocks are more prone to cracking for the following reasons:
A) Because the cement block shrinks greatly within 28 days, it is clearly stipulated in the specification that the age of the block product under construction should not be less than 28 days. And many manufacturers ignore this rule. When production is tight, blocks often leave the factory in advance, and the construction site lacks detection means. In the case of a narrow construction site, the basic problem is how much is used, which directly causes the problem of large shrinkage of the wall after masonry.
B) The water content of the block is neglected during construction, resulting in water loss and increased shrinkage after completion of masonry. C) Due to the requirement of using cement mortar, wet working environment is inevitable. D) When plastering the wall, cracks in the masonry itself often exist or are developing. When plastering mortar dries up, cracks in masonry increase.
It is precisely because of the characteristics of aerated concrete blocks and the special requirements for the construction environment that aerated concrete blocks are more likely to crack. Therefore, certain measures must be taken in the design and construction stages to reduce and avoid the occurrence of such cracks. Specific measures are as follows:
A) The construction unit shall select qualified local manufacturers with licenses. When signing the contract, it is necessary to specify the time when the block will enter the site, and the manufacturer must guarantee the service life and bear the corresponding responsibilities. B) The construction unit shall strengthen the detection of the blocks entering the site. C) After the block comes into play, it will be transported into the paved construction floor as soon as possible, and will be scattered and piled up to the masonry position, and waterproof measures will be taken in advance to ensure that the maintenance water and rainwater of the main structure will not flow into the floor. In order to increase the age of the block as much as possible, masonry should be carried out at intervals of one week, and the moisture content of the block should be determined by electrothermal method. When the water content is lower than 15%, construction is allowed. D) According to the characteristics of aerated concrete block, before laying, it is not advisable to water and wet it in advance, so as to avoid the increase of water content in the block caused by uneven watering. But before laying mortar, water is poured on the surface of masonry. E) Strengthen the setting of ring beam and constructional column. When the wall length exceeds 4m, constructional columns should be set; If the wall is more than 3m high, ring beams should be set. When the length and height of the wall are large and there is a doorway, the setting of constructional columns should first ensure both sides of the entrance to avoid shrinkage cracks at the corner of the entrance. When the main structure is not reinforced, or the position deviates, rebar planting must be adopted. F) Because it is easily influenced by air humidity and has poor deformation with the frame structure, it is advisable to tie bars on both sides of the wall to improve crack resistance. G) Construction shall be carried out in strict accordance with operating procedures to ensure mortar strength and mortar joint fullness (especially vertical joints). H) After the completion of masonry, water should be kept to reduce the shrinkage of mortar. I) Before plastering the wall, the following steps shall be completed:
1) Ensure that the wall is completed for more than 28 days.
2) Carefully check whether there are cracks in the wall, and take measures according to the situation, such as slotting to repair or nailing steel mesh. For the pipeline part embedded after grooving, the groove must be filled with hard fine stone concrete and nailed with steel mesh with a width greater than 200 mm.
3) Water the wall properly to prepare 1: 1 cement mortar, in which the content of 108 glue should account for more than 30% of water (mortar consistency should be suitable for using rollers). Roll the mortar on the wall twice with a roller to close the pores of the masonry as the base of the plastering layer.
4) A 200mm wide steel mesh should be nailed to the joint between the wall and the frame (the diameter of the wire rod of the steel mesh should be greater than 1 15mm, and the mesh should be greater than 15mm). After the steel mesh is nailed, the nail mesh shall be leveled with cement mortar of 1: 1 to cover the mesh body and increase the mesh body.
5) After 7 days of curing in C and D, large-area plastering construction will be carried out.
6) In order to reduce the shrinkage of plastering layer, maintenance must be strengthened.
Third, the improvement of the top brick of the infilled wall
In the process of maintenance, we found that the infilled wall, especially the top brick of 200 mm thick wall, is prone to problems. The reason is that there is a lack of special top bricks in the market, and it is difficult to make them on site, so red bricks are often used to build 180 mm wall top bricks. In this regard, we think that the dry hard fine stone concrete caulking method can be used instead to solve the problem of wall top sealing.
A) The wall should be built to the bottom of the beam (plate) by 50 mm as a reserved joint. B) After 28 days of wall masonry, C20 hard fine stone concrete is used for caulking. The standard of hard concrete is that it can be kneaded into a ball by hand. C) Tickle joint shall be conducted in three times, once a day, and the concrete shall be plugged by hand. The last time should be compacted and leveled.
Four. conclusion
In view of the fact that there are many cracks in the infilled wall at present, we should not only do a good job in construction management in strict accordance with the specifications, but also take corresponding structural measures from the design and construction stages according to the characteristics of the structure and materials, and be willing to invest. The cost management department should also appropriately increase the corresponding construction costs, so as to truly solve the problem of wall cracking.
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