While the masonry structure is widely used, there are also many quality accidents. Common quality problems in masonry engineering include the following four categories:
First, the masonry strength is insufficient.
1, the design section is too small and the bearing capacity is not enough;
2, water, electricity, heating, sanitation equipment to leave holes, slots weaken the wall section is too large;
3, the material quality is unqualified, such as masonry brick and mortar strength grade does not meet the design requirements, the use of unqualified cement and admixture, etc.
Second, the masonry dislocation and deformation
1, the height-thickness ratio of masonry wall is too large, which leads to unstable deformation in the use stage;
2. Construction quality problems, such as vertical deflection of the wall, increase in deformation or even dislocation after use;
3, improper construction sequence, such as vertical and horizontal walls bite if masonry at the same time, resulting in new masonry wall out-of-plane deformation and instability;
4. Improper construction technology such as lime-sand brick masonry leads to instability during masonry.
Third, local damage or collapse
1. The wall falls off corners, pierces holes or even partially collapses due to collision or impact during construction or use;
2. The wall was corroded by caustic soda during use, which made some walls seriously damaged;
3. Freezing method was adopted in winter, and no appropriate measures were taken during thawing period, which led to the collapse of masonry walls.
Four, masonry cracks
Masonry cracks are the most common quality accidents. Insufficient strength, deformation, instability and possible local collapse of masonry can also be analyzed and judged by the appearance of cracks. The types and causes of masonry cracks are summarized as follows:
1, temperature deformation
(1), due to sunshine and temperature changes, the deformation of different materials and different structural parts is inconsistent, and there are strong constraints. For example, the cracks in the top brick wall of flat-topped brick-concrete structure, due to the different temperature linear expansion coefficients of the two materials, the deformation of the roof and the brick wall is inconsistent, and most of them are located on the top wall at both ends.
(2) The temperature or environmental temperature difference is too large. If the length of the house is too long and no expansion joints are set, the vertical cracks running through the whole height of the house are often located in the middle of the longitudinal wall.
(3) The temperature deformation of brick wall is restricted by foundation. For example, there is no heating during the construction in the northern region, and the shrinkage of brick walls is constrained by the foundation, which leads to oblique or vertical cracks in the windowsill and the masonry below it.
2. Uneven settlement of foundation
(1), the foundation settlement difference is large. For example, in a brick-concrete building with a large ratio of length to height, there will be splayed cracks when the settlement of the middle foundation is greater than both ends; When the settlement at both ends of the foundation is greater than that in the middle, inverted splay cracks appear; Vertical cracks appear when the foundation suddenly changes and one end of the foundation has a large settlement.
(2) Local collapse of foundation. For example, the masonry located in air-raid shelters and ancient wells cracked due to local collapse of the foundation.
(3) Frost heave of foundation. For example, the building foundation in the northern region is not buried deep enough, and the foundation soil has frost heave, which leads to cracks in the masonry.
(4) The foundation is flooded. For example, the uneven settlement of filled soil foundation or collapsible loess foundation after local immersion causes longitudinal wall cracking.
(5), the groundwater level is reduced. For example, in soft soil foundation with high groundwater level, additional settlement caused by artificially lowering groundwater level leads to masonry cracking.
3. The structural load is too large or the masonry section is too small.
(1), the compressive strength, bending strength, shear strength and tensile strength are insufficient. For example, the vertical crack of the central compression brick injection; Insufficient bending strength of brick flat arch produces vertical or oblique cracks; Insufficient shear strength of retaining wall leads to horizontal cracks; Cracks appear in the brick pool wall along the mortar joint.
(2) The local bearing strength is insufficient. Such as oblique cracks or vertical cracks under a girder or beam pad.
4. Improper design and structure
(1), the settlement joint is not set properly. If the settlement joint position is not at the maximum settlement difference; Settlement joint is too narrow. After the settlement and deformation of high-rise buildings, the masonry of low-rise buildings will be squeezed and cracked.
(2) The integrity of the building structure is poor. For example, in a mixed structure building, cracks occur because the reinforced concrete ring beam of the brick wall in the stairwell is not closed.
(3) Leave a hole in the wall. If the chimney hole is left at the junction of the interior and exterior walls of the house, it will affect the connection of the interior and exterior walls. Cracking due to temperature change after use.
(4) Different structures are mixed and improper measures are taken. For example, excessive deflection of reinforced concrete wall beams will lead to cracks in the wall.
(5) Improper connection between old and new buildings. For example, when the original building is expanded, the foundation is separated, and the old and new brick walls are integrated, resulting in cracks in the wall at the junction.
5. Poor material quality
(1), the volume of mortar is unstable. If the safety of cement is unqualified, replacing sand with pyrite slag with excessive sulfur content will cause mortar cracking.
(2) The volume of brick is unstable. If the wall is made of lime-sand bricks which have been in the factory for a short time, cracks will easily occur due to inconsistent shrinkage.
6. Poor construction quality
(1), unreasonable assembling method and lack of structural reinforcement. If the inner and outer walls are not laid at the same time, no layer-by-layer summer-planting is left, or no steel bars are put in, long vertical cracks appear at the joints of the inner and outer walls.
(2) Masonry with broken bricks, with many seams on the wall and heavy seams. For example, the external wall of a single-storey factory building is cracked due to the concentrated use of broken bricks.
(3) Improper hole or groove. For example, an office building left feet and eyes on the wall between 500mm wide windows, which led to cracks in masonry.
To sum up, it is easier to observe and distinguish the cracks caused by improper design, poor materials, poor construction and earthquake and mechanical vibration. The most common causes of masonry cracks are temperature deformation and uneven settlement of foundation, but there are also cracks caused by excessive load or too small cross section, which are often serious.
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