Cracks are common phenomena in brick-concrete structures. This paper analyzes the causes of cracks in an office building and puts forward the treatment measures. Keywords: causes of cracks in office buildings 1. Introduction Buildings, especially brick-concrete structure projects, will have different forms and degrees of cracks during construction and use, which will not only affect the appearance of buildings, but also endanger the normal use of buildings and the durability of structures. Therefore, the problem of cracks has attracted much attention. There are many reasons for building cracks, which should be analyzed according to the actual situation. The following is an analysis of cracks in an office building. Two. Project Overview An office building is a five-story brick-concrete structure, and the floor and roof are cast-in-place reinforced concrete slabs. The first floor lobby is 4.0mm high, and the second, third, fourth and fifth floors are 3.6m high. The total height of this building is 20.. 1m, and the building area is 1262m2. See the figure below for the layout. The project was completed and put into use in July 2002. Schematic diagram of the second floor layout III. Crack 1. There are diagonal cracks in the four corners of each floor of the house, which make an angle of 450 with the vertical and horizontal walls. 2. There is a transverse crack parallel to axis ⑤ in the lobby of axis ⑤ ~ ⑤ at the bottom of the building, which runs through it, and the floor tile has cracked. 3. On the second floor of the building, there is a transverse crack parallel to axis ⑦ in the finance room and storage room of axis ⑦ ~ ⑦. 4. There is a transverse crack parallel to axis 6 near the middle section of the roof panel, which runs through it. 5. There are regular splayed cracks at both ends of the external longitudinal wall on the top floor of the building, and there are 45-degree oblique cracks on the top surface of the external transverse walls on both sides. Four. 1 Cause analysis of cracks. Diagonal cracks in cast-in-place concrete floor slab of office building belong to cracks caused by concrete shrinkage, which is caused by the decrease of cement slurry humidity. Within 4 hours to 15 hours after concrete pouring, the hydration reaction of cement is fierce, bleeding and rapid evaporation occur, causing plastic shrinkage. When concrete is dehydrated and carbonized, concrete begins to shrink further. If shrinkage is limited and tensile stress increases, cracks will appear. When pouring concrete floor slab, in order to improve its fluidity, the slump is large, the water-cement ratio is high, the water content is high, and the admixture is mixed, which leads to the large shrinkage of concrete. In the four corners of the room, that is, the corners of the board, this contraction is constrained by the vertical and horizontal directions of the upper and lower walls. Due to the vertical and horizontal constraints, the dry shrinkage tensile stress increases. When the tensile stress exceeds the ultimate tensile strength of concrete, the slab will produce tangential cracks perpendicular to the principal tensile stress, and the cracks will run through the slab. 2. According to the geotechnical investigation report of the building, the foundation is uneven, and the soil within 6m at both ends of the building is soft soil with extremely low bearing capacity (fk=80kpa). Although the foundation has been treated, it can be seen from the construction log that the construction period of the building is much shorter than normal due to the requirements of Party A. Because the construction period is too short, the project was put into use before the foundation settled. At the same time, the building is located in a low-lying place with poor drainage. Because the foundation soil is softened by flood, the bearing capacity is reduced and the foundation settlement is uneven. These two factors lead to cracks on the ground floor and the second floor, which leads to transverse cracks parallel to the horizontal axis in the middle hall of the ground floor, and transverse cracks in the finance room and the finance room on the second floor. Such cracks should be closely monitored and observed. 3. The role of temperature stress is often one of the causes of roof cracking. As we all know, thermal expansion and contraction are common phenomena of objects. In order to prevent thermal expansion and contraction cracking, roads, bridges and houses will take some structural measures such as expansion joints. The top floor of the house is particularly affected by sunshine and weather changes. Under the scorching sun in summer, the temperature is as high as 40~600c, and the deformation of reinforced concrete roof is large after heating, and its temperature stress is correspondingly large. Under the action of temperature stress, concrete roof panels will expand and contract around. Because this project is 265,438+0m long and 65,438+065,438+0m wide, and it extends longitudinally at the east and west ends of the house, the roof panel cracks near the longitudinal middle of the house. In this project, the cracks in the middle section of the roof slab parallel to the horizontal axis belong to temperature stress. If this factor can be taken into account in the design, some structural steel bars that can resist temperature stress can be appropriately added to the roof panel, especially at the end sections at both ends, and such cracks can be completely avoided. The walls at the four corners of a building are directly exposed to sunlight and influenced by cold and hot environment, and their temperature stress is the largest, while the linear expansion coefficient of concrete (10× 10-6) is much larger than that of walls (5 × 10-6), and the expansion and contraction of roof panels are greater than that of brick masonry, so vertical and horizontal walls are always affected by roof panels. When the principal tensile stress [1] is greater than the shear strength of brick masonry, horizontal or splayed cracks will inevitably occur at the weak mortar joint of the four-corner wall, and the oblique cracks of the four-corner wall in this project are very obvious. If the ring beam under the roof plate is rigid enough, this kind of crack can be suppressed. V. Treatment plan
1. The most economical and simple method is to seal the surface, that is, to cut a U-shaped or V-shaped groove with a chainsaw or chisel, and then seal it with high-grade cement mortar after the surface is cleaned. In order to improve the sealing density and water resistance, epoxy structural adhesive is a better choice. Using epoxy resin as sealant can help to restore the tensile strength of cracks, effectively prevent harmful chemicals such as water vapor and carbon dioxide from entering, and the surface smoothness is also satisfactory. 2. Check the uneven settlement of foundation every year. That is, check whether there are radial oblique cracks around the foundation, whether the transverse cracks are enlarged or new cracks appear, take measures to deal with them in time if they are abnormal, and at the same time do a good job in drainage measures around the house to prevent rainwater from directly infiltrating into the foundation under the office building. 3. For the transverse through cracks on the roof panel, the pressure grouting method can be adopted, and chemical grouting materials (such as polyurethane, epoxy resin or cement liquid) are injected into the deep cracks through the pressure grouting equipment to restore the integrity, waterproofness and durability of the structure. ⑴ Process flow: gouging → grouting nozzle burying → joint sealing → hole sealing inspection → slurry preparation → grouting → hole sealing → grouting quality inspection. ⑵ Construction points: ① Grouting materials should be resin materials with strong adhesion and good groutability, generally epoxy resin; ② Cement materials can be used for extra-large cracks with a width greater than 2mm, and diluted epoxy resin or polyurethane should be used for active cracks; (3) chemical grouting pressure should be controlled in 0.2 ~ 0.4 MPa, and cement grouting pressure should be controlled in 0.4 ~ 0.8 MPa. Increasing the pressure will not improve the grouting speed, nor is it conducive to the grouting effect. (4) After grouting, the grouting nozzle (box, tube) can only be removed when the slurry is initially set and there is no extravasation. It can also be repaired and strengthened with epoxy resin slurry or epoxy resin mortar (when the cracks are wide) according to Appendix II "Crack Repair Methods" of Technical Specification for Strengthening Concrete Structures (CECS 25-90), and it can also be repaired and strengthened with new reinforcement adhesives such as Gulaote, Jincaotian and Kunlun Reinforcement [3]. 4. The wall cracks are reinforced with steel mesh cement mortar, and the original plastering is removed within 500 wide areas on both sides of the cracks, and then it is nailed with Ф 6 @120 bidirectional double-sided steel mesh, and the steel mesh on both sides of the wall is anchored with Ф 6 @ 500 quincunx-shaped Rachel bars, and then it is plastered with M 10 cement mortar with a thickness of 25-30, and it is maintained as usual. 6. Conclusion After the above technical treatment measures, the cracks in the office building have been basically controlled, but it is difficult to treat the secondary and tertiary cracks caused by foundation settlement, because it involves foundation treatment and should be closely monitored and observed. A large number of engineering practices show that cracks are a common phenomenon in building construction and use. As long as we take it seriously, find out the causes of cracks and take correct treatment measures, cracks can be controlled and prevented. References [1] Tang, Wan. Building renovation and disease treatment. Beijing: China Building Industry Press. 2000.[2] Wang Tiemeng. Crack control of engineering structure. Beijing: China Building Industry Press. 1997.[3] Sichuan Provincial Building Science Research Department. Technical specification for reinforcement of concrete structures. Beijing:.
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