There are many reasons for cracks in building walls, including the following:
First of all, the forms of temperature difference cracks are straight splayed joints, inverted splayed joints and horizontal joints.
Taking the brick-concrete multi-storey building structure as an example, when the roof is reinforced concrete slab and the wall is brick wall, the wall is particularly prone to temperature difference cracks, especially at the top floor and the root of parapet.
Because the roof material is reinforced concrete (linear expansion coefficient is 10× 10-6) and the wall material is brick masonry (linear expansion coefficient is 5× 10-6), the linear expansion coefficient of the two is twice as different. Moreover, the average solar radiation heat received by the roof is about twice that of the wall, especially in summer. If the roof insulation treatment is improper, the roof will have a large temperature expansion deformation (cold contraction deformation will occur in winter), which will cause a large tensile stress and shear stress between the roof and the wall. When the shear stress and tensile stress are greater than the tensile shear stress of masonry, the wall will crack.
Straight splayed joints often appear at both ends of the top vertical wall (generally in the range of one to two bays), which can develop to the length of 1/3 of the house in severe cases, and sometimes may appear on the transverse wall. The crack width is generally large in the middle and small at both ends. When there are windows at both ends of the outer longitudinal wall, the cracks crack along the symmetrical direction of the windows. Cracks have the characteristics of "heavy at both ends, light in the middle, heavy in the sun and light at the back".
Horizontal cracks generally appear in the mortar joint position of 2-3 skin bricks of the top ring beam under the cornice of flat roof. Cracks generally continue to be distributed along the top of the external wall, and the middle of both ends is more serious. At the corner, the horizontal cracks of the vertical and horizontal pools intersect to form corner cracks.
Oblique crack is that when the expansion of one end of the wall is limited, the splayed crack becomes oblique crack, which mostly occurs at the gable, and the crack width is large or small.
In some houses, due to the cold shrinkage of the roof, there are inverted splayed seams on the top floor at both ends of the longitudinal wall.
In short, the severity of temperature difference cracks is related to indoor and outdoor temperatures. Construction quality, expansion joint spacing, roof insulation, window size, wall thickness and so on.
Although temperature difference cracks are related to many factors such as building shape, material properties and construction quality, the main reason is temperature difference change. In order to prevent the occurrence of temperature difference cracks, we have taken the following preventive measures in design and construction:
1, set expansion joints according to the standard to reduce the cumulative value of roof thermal expansion. The Code for Design of Brick-concrete Structures stipulates that the roof with insulation layer shall be provided with expansion joints every 60 meters, and the roof without insulation layer shall be provided with expansion joints every 40 meters. This provision is considered from the overall structure. When expansion joints are set according to regulations, the overall structure will not be abnormal, but temperature difference cracks will still appear on the roof.
2. In order to reduce the temperature difference between the roof and the wall, overhead insulation board can be added to the roof, which has obvious effect and is also the key to control temperature cracks.
3. The raw materials for roof insulation should meet the requirements, materials with excellent thermal insulation performance should be selected, and the thickness of roof insulation layer should be increased to effectively control the heating rate of roof panels.
4. Changing the roof practice, it is suggested to change the flat roof into a sloping roof, which can not only improve the use conditions of the roof, but also reduce the temperature difference cracks.
5. Generally, the roofing waterproof is to stick the pisolite on the linoleum coil as a protective layer or not to stick it on the SBS waterproof layer, so as to absorb more heat when exposed to the sun and improve the temperature of the roof panel. It is suggested to use silver powder coating instead of pisolite as protective layer. The silver powder coating on the surface layer has a strong reflection effect on sunlight and can effectively reduce the surface temperature of the coil.
6. Appropriately improve the mortar label of the top masonry, add some pull-through anchor bars in the horizontal joints of brick masonry (2φ6 horizontal bars should be set at the joints with frequent cracks), or appropriately increase the width of the window wall and the side crib at the end of the longitudinal wall.
7, to ensure the construction quality, masonry quality is the internal cause of cracks. Construction personnel should strictly implement the construction procedures and operation procedures, and the bricks should be carefully wetted. Don't dry the bricks on the wall. It is forbidden to go straight at the big corner. Rachel bars shall be placed in strict accordance with the specifications to improve the fullness of masonry mortar and ensure the design label. The field measurement must be accurate.
8. Choosing a suitable construction temperature can reduce the temperature difference. After the roof structure layer is finished, the insulation layer should be done in time. In addition, it is suggested to increase the thickness of the non-bearing wall and reserve Rachel bars when lifting the structure to strengthen the ability of the inner wall to resist temperature difference. Temperature difference cracks generally belong to stability cracks, which can be stabilized in one winter and one summer. After the cracks are stabilized, they should be repaired in time. However, some wall cracks have regional characteristics, which should be comprehensively investigated and analyzed in conjunction with the design and construction departments, combined with local climate, environment, structural forms and construction methods, and then measures should be taken to solve them.
Second, uneven settlement cracks.
The uneven settlement of buildings will cause irregular bending and deformation of buildings in vertical and horizontal directions. When the overall stiffness of the building is poor and the foundation is not enough to adjust the stress caused by poor settlement, some parts of the brick masonry will produce tensile stress and shear stress, and cracks will occur when it is not enough to resist the deformation stress. Common masonry cracks caused by uneven settlement include oblique cracks, horizontal cracks and vertical cracks. Most of these cracks occur on the first floor, and a few will also appear on other floors. Usually, as the ground cracks, the house will tilt in severe cases.
The causes of uneven settlement of buildings can be roughly divided into:
1, the foundation is uneven, especially after the foundation trench is excavated, there are no dry wells, graves and underground ditches in the foundation trench, and the soil quality is poor, so it is unnecessary to drill holes. If proper measures are not taken, uneven settlement of buildings will inevitably occur. Due to uneven settlement, the wall bears large shear stress. When the structural rigidity is poor and the construction quality and material strength can't meet the requirements, the wall cracks. This kind of crack is mainly inclined crack.
2. The reason for the horizontal cracks in the window wall is that the resistance of the upper part of the settlement unit makes the window wall suffer a large horizontal shear force, which leads to horizontal cracks in the upper and lower positions.
3. The vertical cracks under the window sill of low-rise buildings are caused by the reaction of the window sill wall after the main partition wall bears the load. The windowsill wall is cracked due to excessive reverse deformation, which will crush the window seriously and affect the opening of the window sash. In addition, if the foundation is built on frozen soil, the window sill will be cracked due to frost heaving.
In order to prevent uneven settlement of wall cracks, the following preventive measures should be taken in design and construction:
1, setting settlement joint reasonably. All buildings and some basements with different loads, overlong, complex plane shapes and different foundation treatment methods for the same building should be provided with settlement joints to make them settle separately, so as to reduce or prevent cracks. Settlement joints should have sufficient width. During construction, the broken parts should be prevented from pouring together when pouring ring beams, or bricks, mortar and other sundries fall into the joints, so as to avoid cracks in the wall caused by the house's non-free settlement.
2. Strengthen the foundation trench work. On the basis of exploration, the foundation trench should be drilled and inspected together with the design and exploration departments, and the foundation construction can only be carried out after the weak parts are reinforced.
3. Strengthen the rigidity of the superstructure and improve the shear strength of the wall. Ring beams should be set at the top of the foundation and each floor to reduce the number of windows at the end of the building. Strictly implement various rules and regulations during construction, such as watering and wetting bricks, improving the workability of mortar, improving the fullness of mortar and the bonding between bricks.
Wall cracks caused by uneven settlement belong to unstable cracks, which generally have a trend of sustainable development, and often last for a long time from unstable to stable. In severe cases, it will continue to develop, making the building in a dangerous state or even destroyed. Once found, we must observe, analyze and take effective measures to control the development of cracks.
Three. Cracks caused by other reasons
In addition to temperature difference cracks and uneven settlement cracks, there are other factors that will cause cracks in brick-concrete structure walls. Some loads are directly borne by brick masonry and bows! Cracks; The vibration and horizontal force caused by non-destructive earthquake will also cause cracks in some weak parts of brick masonry; Some unqualified building materials can also cause cracks in masonry. For these cracks, corresponding measures should be taken in time to prevent and maintain them.
Fourth, synthesis.
From the above analysis, it can be seen that the causes of cracks in brick-concrete structure walls may be one of the above factors, several factors may work together, and there may be other unexpected factors. In order to prevent wall cracks, we should do the following three things:
1. Investigate and analyze the surrounding environment and geological conditions as well as the short-term and long-term planning of this area.
2, should consider the key measures to prevent wall cracking.
3. Ensure reasonable construction methods and quality.