First, the reason analysis
(A) the reasons for the block material itself
1, concrete small hollow block is composed of concrete. Concrete is a composite material cemented by heterogeneous materials such as aggregate, cement stone, gas and moisture. Under the condition of temperature and humidity change, concrete gradually hardens, resulting in volume deformation and unevenness. The shrinkage of cement stone is large, but the shrinkage of aggregate is small; Generally speaking, aggregate and mortar have different thermal parameters, that is, their thermal expansion coefficients are different. The thermal expansion coefficient of aggregate is usually 0.7× 10-5/℃, and that of concrete is usually 1× 10-5/℃. The thermal conductivity of different types of aggregate concrete is also different. The deformation between them is not free, resulting in mutually constrained stress. When the thermal expansion coefficient of cement mortar is greater than that of aggregate, tensile stress will be generated at the interface, which will cause cracking and failure. The temperature change and shrinkage caused by hydration heat in concrete structure cause tensile stress at the interface. When the tensile stress reaches the interface bonding strength, a weak link on the interface will crack first, thus causing the cracking and failure of the material.
2. The evaporation of free water in concrete will cause the drying shrinkage of concrete, which will lead to the cracking of the block itself.
3. Under the action of CO2 in the atmosphere, the cementitious materials in concrete will carbonize and shrink, leading to cracking of concrete itself. After the block is on the wall, due to its own shrinkage, it will cause certain stress inside the wall. When this stress is greater than the tensile and shear strength of the wall, the wall will crack.
4. Block is a kind of hollow wall material made of concrete, which has the brittleness of concrete. In the process of production and transportation, micro cracks will occur due to vibration, and macro cracks will occur on the wall under the action of external factors after climbing the wall.
5. Due to the material of the block itself, the concrete block needs to be cured for 28 days. At this time, the deformation of the block is about 60%, and it takes 3-5 years for the block to be completely stable. However, in the process of production to construction, sometimes the blocks leave the factory before they reach the age, so it is difficult to check and control their age, which is also one of the reasons for wall cracking.
6, the strength of the block itself can not meet the requirements, the geometric size error is too big, and the lack of edges and corners is also one of the reasons for the wall cracking.
7. At present, the quality of equipment used by block production enterprises is mixed, and the quality of production equipment in many small factories is not up to standard. The produced block has low strength and compactness, and can't meet the quality requirements. This is the main reason for the poor quality of materials.
(B) the causes of temperature difference
The linear expansion coefficient of concrete block masonry is about 10× 10-6, which is twice that of solid clay brick masonry. Therefore, the block wall is more sensitive to temperature than the brick masonry, and it is easy to deform due to temperature changes, leading to wall cracking. Temperature crack is the main reason for early cracking of wall.
(C) the causes of foundation settlement
Due to the uneven settlement of buildings, the additional stress in the wall structure of buildings is caused, and the shear performance of block masonry is much lower than that of clay bricks, which is the main reason for the oblique cracking or vertical bending cracking of walls.
(IV) Design reasons Because designers don't know enough about the performance of block wall materials, traditional design methods are often adopted in the design process, and crack prevention measures are not taken in the structure, resulting in the phenomenon of "wearing new shoes and taking the old road", which will inevitably lead to cracks in the block wall.
(5) Construction reasons
1, artificial masonry of hollow block wall. Because of the height of hollow blocks and the existence of holes, the vertical mortar is not easy to be full, and the contact area of horizontal joints is small, which is not convenient for masonry, resulting in the fullness of horizontal and vertical mortar not meeting the requirements, thus weakening the shear, tensile and deformation resistance of the wall and causing the wall to crack.
2. In the construction process, the traditional bricklaying operation process is still adopted, and the traditional masonry mortar is used instead of the special masonry mortar, which leads to the weak bond between blocks, and the tensile and shear strength of the wall is reduced, which leads to the cracking of the wall.
3. There are no effective measures for stacking materials on site, and they are still stuck on the wall after being affected with damp, resulting in secondary shrinkage.
For the above reasons, if we don't pay attention to all links, it is inevitable that the block wall will crack.
Second, the crack control
Crack control of block wall is a complex system engineering. For a long time, people have been looking for practical methods to control cracks in masonry structures, and put forward some measures to prevent cracks according to the nature and influencing factors of cracks. From the concept of preventing cracks, the idea of combining prevention, release and resistance is vividly derived. Some of these ideas and measures have been applied to engineering practice and achieved good results.
Design measures
China's new Code for Design of Masonry Structures (GB50003-200 1) has greatly strengthened the anti-crack measures of masonry buildings, especially the anti-crack and anti-crack structural measures of new wall masonry structures, from 2 in the old code to 9. In order to prevent or reduce wall cracking, the following measures should be taken according to the specifications and actual conditions:
1, in order to prevent or reduce the vertical cracks in the wall caused by temperature difference and masonry shrinkage under normal use conditions, the wall should be provided with expansion joints. Expansion joints should be located where temperature and shrinkage deformation may cause stress concentration, and masonry is the most prone to cracks. According to the specification, the spacing of expansion joints can be adopted according to table 1.
2, in order to prevent or reduce the cracks in the building wall, the following structural measures can be taken in the design according to the situation:
(1) Increase the rigidity of foundation ring beam; 2 When the foundation is uneven, φ4 steel bar spot welding net or φ 2 6 steel bar should be set in the 2nd and 4th mortar joints of the bottom window sill wall, and the distance should not be less than 600mm on the wall between the windows on both sides; To control the occurrence of vertical cracks. (3) The reinforced concrete window sill plate shall be adopted, and the window sill plate shall be embedded in the wall between windows not less than 600mm；; (4) At the corner of the wall and the junction of vertical and horizontal walls, Rachel bars should be set every 400-500mm along the vertical direction, and the number should be no less than 1φ6 or every 120mm wall thickness, and the buried length should be no less than 600mm. 5] From the corner or junction of the wall. Should be in the bottom door, window lintel above the horizontal mortar joint and windowsill first and second horizontal mortar joint set welding steel mesh or 2 Φ 6 reinforcement, welding steel mesh or reinforcement should be extended into the wall between the two sides of the window is not less than 600 mm.. [6] Set mortar joint reinforcement. Once in the building wall set steel belt.
The above-mentioned anti-cracking measures can also be comprehensively adopted according to the specific conditions of buildings, such as site soil and seismic fortification intensity, layout type of infrastructure, plane and shape of buildings, etc.
(2) Block production measures
Management should be strengthened in block production. At present, there are some problems in the production field, such as poor quality of production equipment and imperfect quality system, which lead to a series of problems, such as poor density, poor geometric size, lack of edges and corners, high moisture content, no moisture-proof packaging, and poor age. The quality of the block itself has a great relationship with the wall cracking. Therefore, in view of the production process, the following measures should be taken:
1, introduce high-quality production equipment, eliminate those manual workshop-style production processes, and ensure the quality of block production.
2, the good material factory, the age of the block must reach more than 28d, block specifications, strength grade, moisture content, etc. Should undergo rigorous testing. Meet the requirements before entering the construction site for use. Block transportation and stacking should pay attention to rain, keep the yard clean and tidy, without water, and it is forbidden to dump it at will during transportation.
3, block to moistureproof packaging.
(3) Construction measures
1, strictly control the test block for 28d before leaving the factory and building on the wall to ensure the curing period of the concrete test block. Shrinkage cracks in concrete block buildings have great influence on buildings. One of the most important links is to control the original moisture content of concrete blocks. In addition to improving the inherent quality of the block itself, including controlling the maximum water absorption, it is very important to ensure that the concrete block will be on the wall at the age of 28 days. From the practical point of view, it is better to ensure that the block age is more than one month.
2, concrete block masonry should use special mortar masonry block, masonry mortar must use good workability, strong adhesion, consistency control under 50mm mixed mortar, cement mortar masonry is strictly prohibited;
3. The horizontal mortar joint and vertical joint of the wall must be full. The plumpness of horizontal joint mortar should reach 90%, and that of vertical joint mortar should reach 80%. Blind joints and transparent joints are strictly prohibited during masonry.
4, it is forbidden to masonry wall after the rain and soaking, be affected with damp be affected with damp block masonry on the wall;
5, to ensure the quality of wall masonry materials, at the same time to ensure the strength and fullness of masonry mortar, increase the contact surface of masonry mortar joint, so as to ensure the stiffness of the wall;
6, in order to avoid the new masonry compression deformation is too large, strictly control the daily build by laying bricks or stones high, external wall daily build by laying bricks or stones high about 2m advisable;
7. Ensure that the strength of masonry mortar at the top floor or the second and third floors of the top floor is not lower than M7.5, increase the shear and tensile capacity of the wall, and ensure the overall stiffness of the wall;
8. When there are concealed pipes and wires on the inner side of the external wall, special-shaped auxiliary blocks with longitudinal grooves or transverse grooves of the same material should be adopted. During construction, it is necessary to cooperate closely with hydropower construction personnel. When building a wall, make sure that the reserved pipes and trunking positions are correct, and it is forbidden to gouge or make holes after building a wall. In addition, external wall masonry should not hang heavy objects, and air conditioning equipment such as springboard and balcony should be considered in design;
9, can be added under the masonry windowsill reinforcement or masonry arch, resist the reaction of foundation;
10. The connection between the wall and the concrete structure adopts "horse teeth rub" connector, and tie bars are added. Because the hollow block wall is thin and the horizontal mortar joint contact surface is small, mortar with good plasticity should be selected to ensure the design strength. When laying, the bottom of the block is upward, the mortar is full, and the vertical mortar joint should be filled, squeezed and overlapped reasonably;
1 1. In strict accordance with the masonry method, attention should be paid to the horizontal connection of upper and lower staggered joints to increase the strength and stiffness of the structure;
12, strictly control the handling and stacking of blocks. Block handling must be handled with care to prevent rough handling. To prevent the stress that can't be released at the moment due to the internal injury of the block, it is required to pile up neatly, affix waterproof items, and it is forbidden to get wet;
13, block sorting scheme must be done well before construction, and strictly implemented during construction;
14, do a good job in construction personnel training, improve the quality of masonry;
(4) Anti-cracking measures in plastering process
For block wall, plastering according to ordinary wall is easy to cause wall cracking. Therefore, the traditional plastering method must be changed and plastering should be carried out according to the principle of "step by step and elastic crack resistance". Different from the traditional plastering method, this method adopts a step-by-step flexible anti-crack technical route in structural design.
1, its basic principle is that all structural layers meet the principle of unity of allowable deformation and limited deformation, the properties of materials of each layer meet the requirements of dispersing and dissolving deformation stress at any time, the change indexes of elastic modulus of each layer match and gradually change, and the flexible deformation of the outer layer is higher than that of the inner layer; According to this principle, the flexible progressive anti-crack system can effectively absorb stress and deformation, and can solve the technical problem that harmful cracks are easy to appear on the exterior wall surface.
2. The surface layer should be made of anti-crack flexible waterproof putty, and the bottom layer should be made of polymer emulsion elastic coating with good air permeability. The combination of flexible waterproof putty and elastic primer coating not only meets the deformation requirements of the surface layer, but also has good waterproof, breathable, freeze-thaw resistance and decorative effects.
3. Exterior wall plastering should be carried out after the building structure is capped 15 days, so that the wall has a stable drying shrinkage process to avoid painting cracking in the future; Top plastering should be carried out after the construction of roof thermal insulation overhead slab to reduce the influence of temperature difference; Exterior wall plastering should start from the second top floor and finally be plastered at the top floor, which is quite effective in preventing shrinkage cracks. Practice has proved that this plastering technology has a good effect on preventing wall cracking.
In a word, the treatment of cracks in concrete small hollow block wall is a systematic project. Only when the departments of construction, design, construction, scientific research, production and government management work together can the problem of wall cracks be completely solved.
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