There are many factors involved in the formation of cracks in the wall, including foundation settlement, temperature change and shrinkage deformation, as well as design structure, materials, construction quality and project management. According to the causes, the most common cracks can be divided into four categories. First, temperature cracks; The second is drying shrinkage crack, referred to as drying shrinkage crack. And cracks caused by temperature and shrinkage; The third is the cracks caused by the design structure; The fourth is the cracks caused by construction quality.
1. Temperature crack: The temperature change caused by sunshine, temperature difference between day and night, indoor and outdoor temperature difference and seasonal temperature difference will cause thermal expansion and cold contraction of materials. When the temperature stress caused by temperature deformation under the constraint condition is large enough, the wall will produce temperature cracks, such as horizontal cracks along the top of the block under the frame beam, corner cracks at the edges of doors and windows, etc.
2. Dry shrinkage crack: For fly ash aerated concrete block, with the decrease of water content, the material will have great shrinkage deformation. Generally, the shrinkage rate is .3-.45 mm/m. The shrinkage deformation is characterized by rapid development in the early stage, and about 5% shrinkage deformation can be completed if the block is left for 28 days. This kind of deformation is widely distributed on the wall, with a large number and serious cracks. Such as vertical cracks in the wall, stepped cracks, oblique cracks on the windowsill edge, cracks between frame columns and infilled walls.
However, the above-mentioned cracks are often formed under the combined action of temperature stress deformation and drying shrinkage deformation.
3. The factors causing the cracks in the design structure are as follows:
(1) The non-load-bearing block wall is a post-filled enclosure structure. When the wall is too long and too high, no structural strengthening measures are taken.
(2) The four corners of the door and window openings and reserved openings are in the stress concentration area, and no reasonable connection structural measures are taken.
(3) Grooving and opening the wall to install pipelines, boxes and sockets, etc., and no detailed treatment requirements are put forward.
(4) The wall is deformed and cracked when heavy objects are hung on the wall.
(5) structural measures such as waterproof and drainage, flooding and dripping are not considered for the wall surface in contact with water, which causes cracking and leakage.
4. The factors causing cracks due to the quality of masonry construction are:
(1) Blocks are missing edges and corners or are randomly chiseled and laid for nonstandard blocks: different blocks are used for masonry: blocks with insufficient age are easy to crack.
(2) When the water content of the block is too high or the block is wet during the construction in rainy season, the wall will crack due to drying shrinkage.
(3) No matching special mortar is used.
(4) The arrangement of blocks is unreasonable, and it is not laid or jointed as required; Horizontal and vertical joints are uneven in thickness and mortar is not full; The workability and water retention of mortar are poor; Excessive daily masonry height is easy to cause wall cracking.
(5) The spread surface of mortar is too large, and the length of mortar spreading should not be greater than 75cm. When it is too long, mortar is easy to lose its plasticity, resulting in mortar joints, especially vertical joints, being not dense.
(6) There is no reinforcement between the masonry and the concrete column or the connection is not firm: when it is 3mm high from the bottom of the beam, the interval between the masonry is not enough and the top masonry is not dense.
(7) improper caulking and waterproof treatment between the door window frame and the wall will easily lead to cracking and leakage at the joint.
(8) Improper filling of wall slots, reserved holes and through-wall sleeves will cause local cracks.
5. The causes of cracks caused by wall plastering are:
(1) The plastering mortar does not use the matching special mortar.
(2) The physical and mechanical properties of ordinary plastering mortar are quite different from those of masonry. For example, the coefficient of linear expansion and linear contraction of the mortar is quite different, and the strength of the mortar is also quite different, which leads to cracking due to the shrinkage of the mortar itself.
(3) The grass roots are not cleaned. When the interface agent is not used in the basic treatment, the mortar cracks because the water retention performance of plastering mortar can not meet the water absorption requirements of block.
(4) Plastering survives once, or layered plastering has no proper interval, or the plastering layer is too thick and no strengthening measures are taken.
(5) No crack prevention measures were taken for the joints of different materials between frame columns, beams and masonry.
(6) The water loss is too fast after plastering in summer, and the temperature difference between day and night in winter makes the mortar lose its cohesive force.
To sum up, the crack states of non-load-bearing walls can be divided into four types, namely, horizontal cracks, vertical cracks and splayed cracks; Irregular stepped cracks, cross cracks, etc. There are many reasons for the above cracks. To take technical control measures from all aspects, we must first strengthen the management of block products and ensure the quality of materials. Special masonry mortar and plastering mortar matching with the block shall be adopted. At the same time, it is necessary to aim at all kinds of cracking reasons, carefully design, carefully construct and strictly manage, so as to effectively eradicate the common problems of wall cracking.