Dude, according to you, your floor is a serious quality problem of the main structure. Cheng, assistant engineer of Hefu Fengqing Housing Inspection Company, gave you the conclusion that the safety and durability of the floor must be greatly reduced, and it must be repaired and reinforced in time. The following is your analysis:
1. Cheng Summary, Assistant Engineer of Hefu Fengqing Housing Inspection Company?
The types and causes of concrete cracks are as follows:
1. Dry shrinkage crack?
Shrinkage cracks usually occur some time after concrete curing or about a week after concrete pouring. The evaporation of water in cement mortar will cause drying shrinkage, which is irreversible. Shrinkage cracks are mainly caused by different deformation caused by different evaporation degrees of water inside and outside concrete. Shrinkage cracks are mostly surface parallel lines or reticulated shallow fine cracks, and the width is mostly between 0.05 and 0.2 mm, which are common in the plane part of mass concrete and distributed in the short direction in the thin beam and slab. The drying shrinkage of concrete is mainly related to the water-cement ratio of concrete, the composition of cement, the dosage of cement, the nature and dosage of aggregate and the dosage of admixture. ?
2. Plastic shrinkage cracks?
Plastic shrinkage refers to the shrinkage of concrete surface due to rapid water loss before setting. Plastic shrinkage cracks generally appear in dry heat or windy weather, and the cracks are mostly in a state of wide middle, thin at both ends, different lengths and discontinuity. The main reasons are: the concrete has little or no strength before the final setting, or the concrete has little strength just after the final setting. Due to the influence of high temperature or strong wind, the surface of concrete loses water too quickly, which leads to a large negative pressure in the capillary and makes the volume of concrete shrink sharply. At this time, the strength of concrete cannot resist its own shrinkage, so cracks appear. The main factors affecting plastic shrinkage cracking of concrete are water cement ratio, concrete setting time, ambient temperature, wind speed and relative humidity. ?
3. Subsidence cracks?
Settlement cracks are caused by uneven and soft soil of structural foundation, or uneven settlement caused by false backfill or soaking. Or because of insufficient formwork stiffness, too large formwork support spacing or loose support bottom, especially in winter, formwork is supported on frozen soil, and uneven settlement occurs after frozen soil is thawed, resulting in cracks in concrete structure. Most of these cracks are deep, large or penetrating cracks, and their trend is related to settlement. Generally, it develops along the direction perpendicular to the ground or 30 ~ 45. Larger settlement cracks often have some dislocation, and the width of cracks is often proportional to settlement. The crack width is less affected by temperature change. After the foundation deformation is stable, the settlement cracks tend to be stable. ?
4. Temperature crack?
Temperature cracks often appear on the surface of mass concrete or concrete structures in areas with large temperature difference. The trend of temperature cracks is usually irregular, and large-area structural cracks are often criss-crossed; For the structure with long beam and slab length, the cracks are mostly parallel to the short side; Deep and penetrating temperature cracks are generally parallel to the short side direction, and cracks appear in sections along the long side, with dense middle. The crack width is different, which is obviously affected by the temperature change, wider in winter and narrower in summer. The temperature cracks of concrete caused by high temperature expansion are usually thick in the middle and thin at both ends, while the thickness of cold shrinkage cracks changes little. The appearance of such cracks will cause the corrosion of steel bars and carbonation of concrete, and reduce the frost resistance, fatigue resistance and impermeability of concrete. ?
5. Cracks caused by chemical reactions?
After concrete is mixed, it will produce some alkaline ions, which will react with some active aggregates and absorb the water in the surrounding environment, thus increasing the volume and causing concrete to be brittle, swollen and cracked. This kind of crack usually appears in the use of concrete structures, and once it appears, it is difficult to remedy it, so effective measures should be taken to prevent it in construction. Main preventive measures: First, choose sand aggregate with low alkali activity. Second, choose low-alkali cement and low-alkali or alkali-free additives. Thirdly, choose appropriate additives to inhibit alkali-aggregate reaction. ?
Second, assistant engineer Cheng of Hefu Fengqing House Inspection Company summarized the preventive measures for concrete cracks as follows:
(1) cheng, assistant engineer of Hefei Fengqing house inspection company-concrete temperature rise control?
1. Choose cement with low hydration heat. Hydration heat is the heat released by hydration of cement clinker. In order to reduce the temperature rise of concrete, on the premise of meeting the design strength requirements, we can reduce the dosage of cement and try to choose medium and low heat cement. General engineering can choose slag cement or fly ash cement. ?
2. Use the late strength of concrete. According to the test data, the cement dosage per cubic meter of concrete is increased or decreased by 65438±00kg, and the concrete temperature is correspondingly increased or decreased by 65438 0℃ due to hydration heat. Therefore, according to the actual situation of the structure, the stiffness and strength of the structure are recalculated, and after the approval of the design and quality inspection department, f45, f60 or f90 can be used instead of f28 as the design strength of concrete, so that the cement consumption per cubic meter of concrete will be reduced by 40 ~ 70kg/ cubic meter. The corresponding hydration heat temperature rise is also reduced by 4℃ ~ 7℃. ? The later strength of concrete is mainly based on the mix design, and the test proves that the strength of concrete can continue to improve after 28 days. The design strength can be reached or exceeded by the expected time. ?
3. Add water reducing agent and micro-expansive agent. Adding a certain amount of water reducer or retarder can reduce the amount of cement, improve workability and postpone the peak period of hydration heat. Adding a proper amount of micro-expansive agent or expansive cement can also reduce the temperature stress of concrete. ?
4. Add fly ash admixture. Adding a small amount of ground fly ash to concrete instead of part of cement can not only reduce the hydration heat, but also improve the plasticity of concrete. ?
5. Selection of aggregate. The concrete prepared with continuously graded coarse aggregate has good workability, less water and cement consumption and high compressive strength. In addition, the silt content of sand and stone should be strictly controlled. The silt content of sand is less than 2%, and that of stone is less than 1%.
6. Reduce the outgoing temperature and pouring temperature of concrete. First, reduce the concrete mixing temperature. The most effective way to reduce the temperature of concrete is to reduce the temperature of stone. When the temperature is high, it is necessary to avoid direct sunlight on the aggregate, spray water mist on the aggregate if necessary or rinse the aggregate with cold water before use. In addition, the temperature of concrete will also be affected during loading, unloading, transportation and pouring. Therefore, in the hot summer, it is necessary to minimize the time from the mixing station to the mold. ?
(2) Cheng, assistant engineer of Hefu Fengqing Housing Inspection Company-using heat preservation or moisture preservation to slow down the cooling rate of concrete?
In order to reduce the temperature difference between inside and outside after concrete pouring, wet curing should be adopted in summer and heat preservation curing should be adopted in winter. After the final setting of mass concrete structure, water with a certain depth is stored on its surface, which plays a certain role in heat preservation, reduces the temperature difference between inside and outside the concrete, and thus controls the development of cracks. However, after the formwork removal of mass concrete structure in foundation engineering, it is advisable to backfill soil as soon as possible to avoid sudden temperature change, and also to slow down the cooling rate and avoid cracks. ?
(3) Cheng Hefu, assistant engineer of Fengqing House Inspection Company-improve the construction technology and improve the crack resistance of concrete?
1. The method of pouring concrete in layers and sections is beneficial to the loss of digestion heat of concrete and reduces the temperature difference between inside and outside. ?
2. Improve reinforcement, avoid stress concentration, and enhance the ability to resist temperature stress. Stress concentration will occur around the hole, at the corner of variable cross section and at the corner. Therefore, adding inclined steel bars and steel mesh pieces around the hole, partially treating the variable cross section, making the cross section gradually transition, and adding anti-crack steel bars can prevent cracks. It is worth noting that the steel bars should be as small in diameter and spacing as possible, and symmetrically arranged in the whole section. ?
3. Set the post-pouring belt. For mass concrete with large plane size, post-pouring belt should be set to reduce external binding force and temperature stress; At the same time, it is also beneficial to heat dissipation and reduce the internal temperature of concrete. ?
4. Do a good job in temperature monitoring, reflect the temperature difference in time, guide the maintenance at any time, and control the temperature difference between the inside and outside of concrete not to exceed 25 degrees Celsius. ?
3. How to deal with concrete cracks of assistant engineer Cheng of Hefei Fengqing Housing Inspection Company?
(1) Cheng, assistant engineer of Hefu Fengqing Housing Inspection Company, confirmed through investigation and analysis that surface repair method, filling method and grouting method were adopted without reducing the bearing capacity of cracks.
1. Surface repair method. This method is suitable for narrow joints to restore the surface beauty and improve the durability of components. Usually, film materials are laid along the surface of concrete cracks, and epoxy resin or resin impregnated glass cloth can be generally used. ?
2. Filling method. When the cracks are wide, V-shaped or U-shaped grooves can be chiseled along the cracked concrete surface, which can be filled with resin mortar materials or cement mortar or asphalt. ?
3. Injection method. When the crack width is small and deep, the repair method of injecting repair material into concrete can be adopted. Firstly, an injection tube is arranged at the crack, and other parts are sealed by surface treatment. Inject the material with low viscosity epoxy resin, and inject it with electric pump or manual pump for repair. ?
(2) Cheng, assistant engineer of Hefu Fengqing Housing Inspection Company, thinks that if cracks affect the structural safety, structural reinforcement methods such as enclosure reinforcement, steel hoop reinforcement and paste reinforcement can be adopted. This method belongs to structural reinforcement and must be approved by design checking calculation. ?
1. Enclosure reinforcement method. When the surrounding size allows, the reinforced concrete enclosure is wrapped on one or more sides of the structure to increase the reinforcement and section and improve its bearing capacity; For members with serious cracks, if they are not broken or broken on one side, the protective layer of steel bars at the crack part should be chiseled away and covered with a layer of steel wire mesh; The foundation of large equipment is generally treated by adding steel hoop to increase the circumferential tensile strength. ?
2. Reinforcement method with steel hoop. U-shaped bolts or steel hoops are added around the cracks in the structure to fasten the components, so as to prevent the cracks from spreading and improve the rigidity and bearing capacity of the structure. During reinforcement, the steel hoop should be in close contact with the concrete surface to ensure that * * * works together. ?
3. Paste reinforcement method. The steel plate or section steel with modified epoxy resin and adhesive is bonded to the surface of the concrete crack part of the member, so that the steel plate or section steel and concrete become a whole and work together. Before bonding, the steel surface shall be sandblasted, derusted, brushed with concrete and dried. The thickness of the adhesive layer is 65438 0 ~ 4 mm ...
Cheng, assistant engineer of Hefu Fengqing Housing Inspection Company, concluded:
To sum up, concrete cracks should be based on the causes, implement the principle of prevention first, improve design and strengthen construction management, so as to avoid cracks in the structure or minimize the number and width of cracks to ensure structural safety.