In this paper, masonry cracks in building engineering are taken as the main analysis object, and two kinds of cracks caused by temperature deformation and shrinkage deformation are expounded respectively. Through in-depth analysis, we can truly understand the causes of various cracks, so as to take effective preventive measures in the future construction process.
After the block building is completed, various cracks may appear in the wall for various reasons during the use. Generally speaking, however, wall cracks can be basically divided into two categories: stressed cracks and non-stressed cracks. The corresponding cracks in the wall under the direct action of various loads are called stress cracks. Cracks caused by masonry shrinkage, temperature and humidity changes and uneven settlement of foundation belong to non-stress cracks, also known as deformation cracks. In this regard, in order to let us know more about non-stress cracks, this paper will focus on two types of wall cracks caused by temperature and shrinkage deformation.
1 Causes of cracks in non-stressed walls
1. 1 cracks caused by temperature deformation
The top wall of multi-storey block houses is most prone to temperature cracks, just like brick masonry houses. Although there is no difference between the linear expansion coefficient of concrete masonry wall and the linear expansion coefficient of concrete slab with roof, there is still a certain temperature difference between them under the sunshine in summer. In summer, under the sunshine, the maximum temperature of the roof surface can reach 40℃ ~ 50℃, while the average maximum temperature of the top external wall is about 30℃ ~ 35℃. There is a temperature difference of 10℃ ~ 15℃ between the roof and the top external wall. In cold areas, gas barrier, thermal insulation layer, leveling layer and waterproof layer are sequentially arranged on the structural layer of the roof. The roof structure is protected by insulation layer, so the temperature difference between it and the external wall should be reduced. However, the insulation layer may not be thick enough, or the waterproof layer leaks and the insulation layer is soaked, which reduces the insulation effect. At this point, the temperature difference between the two may still lead to wall cracking.
In practical engineering, we found that only the expansion and contraction deformation of the cement mortar leveling layer (20mm thick, often super thick in actual construction) on the insulation layer can also promote the cracking of the external wall. Because according to the existing architectural structure shape node diagram, the mortar leveling layer has been spread to the root of the parapet, which not only keeps the opening without gaps, but also thickens the edge and piles it into a triangle (convenient for flooding). Although the leveling layer is thin, it still has considerable stiffness in the plane, and the coiled material waterproof layer above has no heat insulation effect. In summer, under the direct sunlight, the leveling layer expands and contracts, which leads to the cracking of the wall. There is a certain temperature difference between the top cover and the outer wall, which leads to uncoordinated temperature deformation and cracks in the wall. When the outside temperature rises, the deformation of the concrete roof is relatively large, and the deformation of the wall is relatively small, which makes the roof compressed and the wall tensile and shear. The two ends of the top floor of the building are most stressed, and they often show splayed cracks along the diagonal direction of the window, and horizontal cracks will also appear in the wall at the elevation of the top cover (the top cover pushes against the outer wall). When there is a parapet, the parapet will crack or tilt outward.
This kind of temperature crack has obvious regularity: the two ends are heavier than the middle, the top layer is lighter than the bottom layer, and the negative side is emphasized. Due to the temperature expansion and contraction of the top cover, the top transverse wall connected with the external longitudinal wall will also crack, which is generally located below the greenhouse and close to the external wall. The crack shape of the top wall is obviously related to the setting method of ring beam, but the setting of ring beam alone can not prevent the wall from cracking. When the roof slab is directly laid on the top ring beam, when the roof slab is well combined with the ring beam, oblique cracks may still appear under the ring beam. If the combination is not good, horizontal cracks may appear.
Cracks caused by shrinkage deformation 1.2
Clay bricks are sintered, and the shrinkage of finished products is very small, so the shrinkage of brick masonry houses can generally be ignored.
Small hollow block is made of concrete mixture by pouring and vibrating. In the process of hardening, concrete gradually loses water and dries up, and its drying shrinkage varies with materials and molding quality, and gradually decreases with time. Taking ordinary concrete block as an example, under the condition of natural curing, the shrinkage tends to be stable after 28 days of molding, and its dry shrinkage rate is 0.03% ~ 0.035%, and the water content is about 50% ~ 60%. After the completion of masonry, under normal use conditions, the water content continues to decrease, reaching about 10%, and its drying shrinkage rate is 0.065438+.
For ordinary concrete block masonry with stable drying shrinkage, if it is soaked again, it will dry again, which is usually called secondary drying shrinkage. The second drying shrinkage of ordinary concrete blocks after being saturated with water, its stability time is shorter than that of the first drying shrinkage in the process of forming and hardening, which is generally about 15d. The shrinkage rate of the second shrinkage is about 80% of the first shrinkage. The drying shrinkage of the blocks on the wall leads to the drying shrinkage of the masonry, and a certain shrinkage stress is generated inside the masonry. When the tensile and shear strength of masonry is not enough to resist shrinkage stress, cracks will occur. Wall cracks caused by dry shrinkage of blocks are very common in small block houses. It may appear on the interior and exterior walls and all the floors of the house. There are several types of shrinkage cracks, one is the stepped cracks in the middle of the wall, the other is the cracks in the mortar joint around the ring block, the third is the vertical uniform cracks in the outer wall, and the fourth is the vertical and horizontal cracks caused by shrinkage in large walls such as gables. Shrinkage cracks are generally manifested in low floors, which is due to the constraint of foundation and transverse wall on wall shrinkage and deformation. Some block houses have vertical cracks extending from the bottom to the third and fourth floors in the middle of the gable.
Because the strength grade of masonry mortar is not high and the mortar joint is not full, the cracks caused by drying shrinkage are often filiform and scattered in the mortar joint, which is not easy to find when plastering the drywall, but will be revealed when plastering. The crack width caused by drying shrinkage is not large, but relatively uniform. When the block is on the wall, the moisture content is relatively high. After a period of time, the moisture content of masonry decreases, and drying shrinkage cracks may appear. Even if there is no shrinkage crack in the completed masonry, when the block is soaked in water again for some reason, secondary shrinkage occurs, and cracks may still appear in the masonry.
2 Stress-free crack prevention measures
The generation of temperature shrinkage cracks in block houses involves block production, architectural design, construction quality and many other aspects. Therefore, measures should be taken from various aspects and links to prevent cracks. In terms of architectural design, in addition to following the main measures to prevent wall cracks in the Technical Specification for Small Concrete Hollow Block Building (JJ/T12004), the following measures can be taken to prevent the deformation and cracks of block houses according to the actual situation:
In view of the fact that the linear expansion coefficient of concrete block masonry is twice that of brick masonry, the maximum distance between temperature expansion joints of brick masonry buildings should be smaller than that of brick masonry buildings. The revision group of "Code for Design of Masonry Structures" drew up a revision plan, that is, the maximum spacing of expansion joints in the code multiplied by 0.75, which was adopted. For example, the spacing between brick masonry buildings is 50m, while the spacing between expansion joints of brick masonry buildings is about 37m. In some areas, it is planned to change it to 35m, because it is exactly equivalent to the length of two units in a residential building, which is more convenient to handle.
The cracks caused by temperature difference are mainly at the top floor of the building. The previous temperature stress calculation shows that even if the mortar strength is high, it is difficult to resist the tensile stress and shear stress caused by temperature difference. Therefore, it is best to consider measures to reduce the temperature difference and adopt the strategy of combining "resistance" with "release". It is necessary to increase the thermal insulation performance of the roof to prevent roof leakage, which can also achieve the purpose of reducing the temperature difference between layers of the roof structure. Increase the plane layout density of the ring beam at the top floor, and strengthen the resistance around the door and window openings at the inner and outer longitudinal walls of the top floor (the door and window openings are provided with reinforced concrete core columns and reinforced concrete windowsill beams). Bottom line: Use steel bars to resist temperature stress. It is an effective measure to set sliding bearings in the proper position of the roof support plate. However, the allowable range of seismic structures should be considered. For example, make a structure that allows micro-motion without slipping, and the sliding supports are criss-crossed, or only set a sliding house with roof panels at both ends, weakening the connection between roof panels and ring beams, and so on.
Change the design method of roof structure, disconnect the mortar leveling layer from the surrounding parapet, leave a chute and fill it with soft waterproof material. The leveling layer itself should be divided into 4m×6m blocks. This measure does not affect the use function of the house, but at least it can reduce the harm of temperature deformation of the top floor.
As the top masonry wall, it is best to set up local wall control joints with the spacing of two or three bays. At this time, the components of the top cover and the ring beam can be connected. Although it is not as good as setting joints in every bay abroad, it will greatly alleviate the temperature difference.
Cracks caused by shrinkage of block walls are mainly manifested in the bottom floor 1 and the second floor, because the foundation is strongly constrained. The shrinkage stress of block wall is equivalent to the temperature stress with a temperature difference of about 30℃, so the development of shrinkage cracks is more serious than the temperature difference deformation. In addition to strengthening the strength of the bottom mortar, strengthening the edge of the hole with core columns, matching the horizontal steel mesh under the window with the wall, and filling the hole of the block, it is also appropriate to consider setting the control joint of the wall.
Concluding remarks
In short, as long as we can clearly grasp the causes of various cracks and master the correct construction methods and masonry techniques, the cracks in building masonry can be completely avoided and prevented to a certain extent. Through the in-depth analysis of cracks, we can clearly understand that the quality of process control will directly affect the emergence and development of masonry cracks. Therefore, it is of great significance to do well the process control and quality control in the construction process to prevent cracks.
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