Combined with engineering practice, three kinds of cracks are analyzed, and some views on the prevention and treatment of cracks are put forward.

Keywords: masonry structure; Cracks; cause

Masonry structure is widely used in small and medium-sized cities and rural areas in China because of its low cost. However, due to the characteristics of masonry structure, cracks will inevitably exist. The most common causes of cracks are temperature deformation and uneven settlement of foundation, but there are also cracks caused by excessive load or too small cross section, which is very harmful. Based on many years' experience in design and engineering practice, this paper analyzes three kinds of cracks and puts forward its own opinions on the prevention and treatment of cracks.

1 temperature deformation

Temperature deformation is caused by the temperature difference between the inside and outside of the building, and the linear expansion coefficient of concrete slab and masonry is quite different (1× 10-5/c for concrete and 5× 10-6/c for sintered clay brick masonry). The deformation of concrete slab will burn to the masonry block, which will produce tensile stress in the masonry wall and cause cracks in the masonry wall. Cracks often appear near the top of the house, especially at both ends; Cracks may appear in both longitudinal and transverse walls, and cold shrinkage cracks may appear in the lower part of unheated houses in cold areas. Vertical cracks located near the middle of the house length may also belong to this kind of cracks. The most common form of crack is oblique crack, which is wide at one end, thin at the other end and thin at both ends in the middle. Followed by horizontal cracks, mostly broken and flocculent, wide in the middle and thin at both ends. The cracks at the joint between the workshop and the living room are related to the shape of the roof, which is nearly horizontal, and the cracks are generally continuous, with little change in joint width. The third type is vertical cracks, which are mostly caused by longitudinal shrinkage, and the width of cracks does not change much. Cracks often appear after summer or winter. The development of cracks changes with the change of air temperature or environmental temperature. When the temperature is the highest or lowest, the width and length of cracks are the largest and the number is the largest, but they will not expand and worsen indefinitely.

In order to prevent or reduce temperature cracks, necessary measures should be taken in the design process.

(1) Expansion joints are set on the wall to release the stress caused by the deformation of the concrete slab and reduce the occurrence of cracks. The spacing of expansion joints can be adopted according to specifications.

(2) The roof is provided with insulation layer to reduce the temperature difference between indoor and outdoor.

(3) A sliding layer is arranged at the contact part between the roof and the wall.

(4) Set concrete ring beam under the roof to pull through the internal and external walls.

(5) Lightweight roof with less rigidity shall be adopted.

(6) increase the tensile strength of the wall:

A, arranging horizontal steel bars in the wall;

B, the top wall is encrypted with constructional columns;

C strengthen the weak parts of the wall, such as setting horizontal steel bars at the openings of doors and windows. The above methods can take corresponding measures according to the engineering situation.

2 uneven settlement of foundation

The uneven settlement of foundation is caused by the uneven load and stiffness of superstructure and the uneven distribution of foundation bearing capacity. Most cracks appear in the lower part of the house; A few can develop to 2~3 floors; For long buildings with the same height, cracks often appear near both ends; For houses with other shapes, the cracks are all near the place where the settlement changes dramatically; It usually appears on vertical walls, but it is rare on horizontal walls. When the foundation property changes suddenly, cracks may also appear at the top of the house and extend downward, which may run through the whole height of the house in severe cases. Oblique cracks are more common in the shape characteristics of cracks, and the cracks at the entrance of doors and windows are wider; Secondly, vertical cracks, whether in the upper part of the house, under the window sill or throughout the whole height of the house, are generally wide at the top and thin at the bottom; There are few horizontal cracks, some of which appear at the corner of the window, and the cracks near the end of the window are wider; Some horizontal cracks are caused by local collapse of foundation, and the width of cracks is often large. Most of the joint width appeared shortly after the completion of the building, and a few projects showed obvious cracks during the construction period, which was too serious to be completed. The development and change of cracks increase with the increase of foundation deformation and time. After the general foundation is deformed and stabilized, the cracks no longer change, and a few foundations produce shear failure, and the development of cracks leads to the collapse of buildings.

The following measures can be taken to prevent or reduce cracks caused by uneven foundation:

(1) Increase the stiffness of the foundation ring beam.

(2) The horizontal welded steel mesh shall be set in the mortar joint of the bottom wall, and both ends of the steel mesh shall be reliably anchored.

(3) The reinforced concrete window sill plate is adopted, and the plate end is embedded in the wall.

3 insufficient carrying capacity

Due to the stress concentration caused by the sudden change of structural cross section, cracks occur locally in the wall. Cracks mostly appear in the parts of masonry with high stress. In multi-storey buildings, the ground floor is more common, but other floors may also occur. Cracks in axially compressed columns often appear near the height of the lower part of the column 1/3, and rarely appear at the upper and lower ends of the column. Most cracks in masonry under beam or beam pad are caused by insufficient local bearing strength. The morphological characteristics of cracks are that the direction of cracks in compression members is consistent with the stress, and the middle of cracks is wide and the two ends are thin; Tensile cracks are perpendicular to the stress direction, and cracks along mortar joints are common; Bending cracks are wide at the outer edge of the tensile zone, but not obvious in the compressive zone, and most of them develop along the mortar joint. Brick flat arch may produce oblique cracks under the combined action of bending moment and shear force. The shear crack is in the same direction as the shear force. Most cracks occur when the load suddenly increases, such as the support of the main beam is removed; Pools, silos, etc.

According to the root cause of this kind of crack, the following points should be paid attention to in design:

(1) The change of structural section is easy to slow down, so as to avoid stress concentration caused by sudden change, and measures should be taken to strengthen the inevitable sudden change parts.

(2) bear concentrated force and add rigid pad to disperse stress.

(3) For masonry subjected to bending moment and tension, reinforced brick masonry or composite brick masonry can be considered to give full play to the compressive performance of masonry and the tensile performance of steel bars.

In reality, cracks in masonry buildings can be seen everywhere, which has a certain impact on our lives. Designers should take certain measures to reduce or prevent cracks in masonry structures in the design process.

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