Cracks in brick masonry have many effects on buildings, which not only affect the beauty of buildings and give people a sense of insecurity, but also destroy the integrity of masonry and reduce the stiffness, strength and stability of the structure. Under the influence of external conditions such as wind, rain and temperature change, cracks accelerate the destruction of masonry materials. In the analysis of cracks in brick-concrete structure, we should find out the reasons, analyze its adverse effects and determine the treatment methods. The main causes of cracks are settlement cracks, temperature cracks, load cracks and vibration cracks. , also does not rule out improper design, construction is not in place or materials do not meet the design requirements and other factors. From a practical point of view, cracks caused by uneven settlement and deformation of structural members caused by temperature changes around the structure are the most common.
First of all, the causes, treatment and avoidance measures of settlement cracks are analyzed as follows:
1, cracking reason: settlement cracks, as the name implies, are mostly caused by foundation settlement. The uneven settlement of foundation changes the distribution of supporting reaction force under brick masonry and produces new additional stress in masonry. Brick masonry has high compressive strength, but low tensile and shear strength, so cracks usually appear under tensile stress or shear stress. The cracks in brick masonry caused by uneven settlement of foundation are also strength failure in essence, but they are quite different from the strength cracks caused by load. The position of strength crack caused by load corresponds to the stress completely; The generation and development of cracks caused by foundation settlement depend on the form of foundation settlement line and the integrity of brick masonry.
(1) When the settlement line of the local foundation is horizontal, the settlement is relatively uniform, and the settlement value of each point is the same, regardless of the settlement value, no additional internal force is generated, so the brick masonry will not produce cracks. ⑵ When the local foundation settlement line is an inclined straight line, the bottom of brick masonry remains the original plane. When the inclination is not large, the brick masonry only produces the whole inclination, and does not produce cracks within the allowable range of brick masonry strength. (3) When the settlement line of local foundation bends, the bottom surface of brick masonry will no longer keep the original plane, and additional internal force will be generated in brick masonry. When the additional internal force increases beyond the strength of brick masonry, cracks will occur. Cracks in brick masonry caused by foundation settlement include oblique cracks, vertical cracks and horizontal cracks. Oblique cracks are caused by shear internal force, and the inclination angle of cracks is mostly 45. Cracks start from the position where the settlement line of brick masonry can not be kept straight, and rise to the side with large settlement. Vertical cracks appear in the form of tensile internal force. The first vertical crack is just below the highest point of the foundation settlement line, that is, the place with the smallest settlement, and the settlement line inclines to both sides. New vertical cracks also appeared on both sides of the first vertical crack, and the crack area developed to both sides. When there are windows in the wall, vertical cracks mostly occur on the wall between vertical windows, and there is less settlement below vertical cracks. Horizontal cracks are formed by shear internal forces. When the middle of the foundation settlement line is depressed, cracks appear in the middle of the wall above the depression below the settlement line, which mostly occur in the wall on soft soil foundation. Moreover, this kind of crack has a large building area and a large foundation settlement area.
The reasons for uneven settlement of foundation are: (1) The soil properties and compressibility in different parts of foundation are different. ⑵ The loads in different parts of the building vary greatly. (3) Improper handling of brick masonry structure. Due to the above three reasons, cracks in brick masonry caused by uneven settlement of foundation often appear in the following situations. The compressibility of (1) foundation soil is obviously different, especially in local soft foundation. ① Blind design and construction without exploration. (2) Although exploration has been carried out, there are few points and improper points, and no weak layer is found. (3) The design is wrong, and the soft soil is not treated. If the difference of foundation soil is large, use a larger same foundation. (4) Improper treatment of foundation soil during construction. Although soft soil has been dug up, the backfilled plain soil is not compacted enough, and the soil quality is still weaker than other places. (2) The foundation settlement of buildings built by stages is inconsistent, which leads to wall cracking. (3) When the height difference and load difference of different parts of the building are large, it is generally difficult to achieve uniform settlement of different parts in the case of soft soil foundation with large compression value. If there is no settlement joint in the design, brick masonry is easy to crack, and there is settlement joint in the design. When the settlement values are quite different, cracks will also appear in brick masonry. (4) Where the building structure or foundation type is different, uneven settlement will easily lead to brick wall cracking when the soft soil foundation settlement is large. 5] At the corner of the building plane, especially at the external corner, due to stress superposition, the settlement is large. In case of soft soil foundation, it is easy to cause cracking of brick masonry. [6] Improper use of buildings, such as stacking a large amount of materials on the ground, sometimes causes uneven settlement of the foundation, leading to cracking of brick masonry. Once a large amount of surface water invades the foundation, it causes local subsidence of the foundation, and the strength of fine sand, silt and various cohesive soils decreases and the compressibility increases after soaking, resulting in leakage and erosion, which leads to local subsidence of the foundation and cracking of brick masonry. Because of the underground water flow, the foundation will settle unevenly, which will lead to the cracking of brick masonry. Levies frost heaving of foundation soil can also cause cracking of brick masonry.
2. Treatment of cracks: When there are settlement cracks in brick masonry due to foundation reasons, the cracks in brick masonry should be observed and monitored and treated according to the development situation. When the foundation deformation has stabilized or tends to stabilize, it is generally not necessary to reinforce the foundation, but only to repair the damaged and cracked upper part. When the uneven settlement of the foundation is not stable, it can be repaired again after the settlement is stable according to the situation to avoid repeated repair of the damaged part on the upper part. If the waiting time is too long, temporarily increasing the load or controlling the foundation soaking can be used to accelerate the settlement stability. This can shorten the time required to wait for the settlement to stabilize, and can also prevent the development of settlement by reducing the load of the superstructure, strengthening the structure, increasing the area of the bottom of the foundation and strengthening the foundation.
3. Avoidance measures: In order to avoid building settlement cracks caused by foundation settlement, the foundation should be measured before design, and the location of measuring points should be appropriate and reasonable. In the design, local soft soil must be treated to avoid or reduce shape differences and load differences as much as possible, and settlement joints should be designed appropriately. The settlement at the corner of the building plane, especially at the shady corner, is large, so measures should be taken in the design of soft soil foundation. During construction, the backfill soil must be tamped and compacted, and the foundation soil treatment meets the design requirements. When the original foundation is found to be inconsistent with the design, the design should be changed in time to avoid the phased construction of the whole building. When the load of different parts of the building is quite different, it is necessary to build a deep foundation first, then a shallow foundation, or take other effective measures. The buildings in use and the heavy objects piled around them should be suitable, and attention should be paid to the settlement of the foundation. In order to avoid a large amount of surface water soaking into the foundation, attention should be paid to the treatment of underground water flow in design and construction, and the influence of the change of underground water flow on buildings should be paid attention to in use. The foundation of the building should be buried deep enough to avoid the uneven settlement of the building caused by the frost heave of the foundation soil.
Second, the causes of temperature cracks and control measures:
The mixed structure house composed of reinforced concrete floor and brick masonry roof is actually a box-shaped space structure. When the natural temperature changes or shrinks, each part of the house will have its own different deformation, which will inevitably produce mutual constraints and stresses. These two materials (concrete and brick masonry) are heterogeneous materials with weak tensile strength, so when the tensile stress generated in the component exceeds its ultimate tensile strength, different forms of cracks will appear.
1. Analysis of types and causes of cracks: (1) cracks in the top wall caused by deformation of reinforced concrete roof and wall (1) horizontal cracks and corner cracks in the outer wall under the roof panel. For houses without parapets, corner cracks and longitudinal horizontal cracks sometimes appear on the external walls. The cracks are located at the bottom of the roof panel or near the bottom of the top ring beam, and the depth of the cracks sometimes runs through the thickness of the wall. ② The splayed joints between the internal and external longitudinal walls and the transverse walls are mostly distributed at both ends of the building wall, or at the upper and lower corners of doors and windows. The main reason is that after the temperature rises, the elongation of the roof panel along the length direction is greater than that of the brick wall, which makes the top brick wall produce tension and shear. Generally, the distribution of tensile stress and shear stress is zero in the middle and maximum at both ends, so the splayed joints mostly occur near the two ends of the wall. The longer the house, the greater the relative deformation between the roof panel and the wall, and the more obvious the cracks. The cracks on the inner longitudinal wall are more obvious than those on the outer longitudinal wall. This is because the inner longitudinal wall is indoors, and the temperature difference between it and the roof panel is greater than that of the outer longitudinal wall. ③ cracks in parapet. At the junction of the parapet root and the flat roof, the masonry protrudes outward or the daughter leans outward, causing the parapet to crack. Some cracks are at the foot of the parapet, some at the top of the parapet, and some form a circle of cracks around the house. The main reason for this kind of crack is that after the temperature rises, the elongation of concrete roof slab and cement mortar surface layer in the length direction is greater than that of brick wall, which relatively prevents this elongation, thus forming an external thrust on the wall. (2) Cracks caused by relative deformation of reinforced concrete floor and wall. (1), when the building has staggered floors, the staggered-floor wall is prone to local vertical cracks. The reason for this kind of crack is that the deformation of reinforced concrete floor slab due to shrinkage and cooling is much larger than that of brick wall. Staggered walls prevent the shortening of the floor slab, so they have a greater tensile stress on the wall and cause the wall to crack. (2) Cast-in-place reinforced concrete buildings (houses) sometimes have cracks running through the floor (houses) and vertical cracks running through the walls. This kind of crack is also caused by the shrinkage of reinforced concrete floor (house) and the cooling of atmosphere.
2. Control measures of temperature cracks: the analysis shows that the causes of wall cracks are: excessive temperature difference → temperature stress caused by inconsistent deformation of reinforced concrete and brick masonry → tensile stress of members exceeds their tensile strength → cracks occur. Important control measures: (1) temperature control. ① Change the flat roof into a sloping roof. (2) Set the overhead insulation board roof. ⑵ Coordinated deformation control. ① Appropriately increase roof expansion joints. (2) Cantilever slab and cast-in-place slab are not long beams, and the length of cast-in-place cantilever slab and cast-in-place slab should not be greater than15m. (3) The reinforced concrete ring beam on the roof layer is changed into a brick ring beam. In order to strengthen the integrity, the roof panel adopts cast-in-place slab. (4) The temperature expansion joint shall be strictly controlled within the scope permitted by the specification. ⑶ Reinforcement measures: (1) The top unit bay is changed to reinforced brick wall masonry. (2) Appropriate reinforced concrete constructional columns are added at the top. ⑷ Quality control: ① The construction supervision department should strictly supervise and supervise the top floor, especially the insulation layer. (2) The quality requirements for roof insulation materials are strict. (3) The quality requirements of brick masonry are strict.