Keywords: basement, floor cracks, concrete mix ratio
In recent years, with the rapid development of the construction industry, the basement has become more and more important in the building. Due to the use of cast-in-place basement floor, the integrity, durability, applicability and structural safety of the house have been greatly guaranteed, but there are also some cracks in the basement floor, which makes many owners worry that these cracks will affect the safety of the building and complain to the relevant departments.
1 project overview
An office building in Guangzhou, located in the center of Guangzhou, is a commercial office building that has entered the e era. The basement has four floors, with a construction area of 70,000 square meters and a total investment of 200 million yuan. The average thickness of basement floor in this project is 0.8rfl, and the thickness of general foundation connecting plate is 1.5rfl, of which the thickest in elevator shaft is 4.25rfl below the floor, the concrete grade is C35, and the elevation of the floor is 15.5rfl ... The basement floor was found when the structure was capped.
Now there are irregular cracks, and the structure has been capped.
2 crack analysis
1) According to the field measurement, the maximum crack length is about 20 m (linear distance) and the maximum crack width is 0.27 rnm. After nearly a month's on-site continuous monitoring, no further development and new cracks were found.
According to the requirements of the owner, in order to confirm the strength of concrete, 24 parts were taken for rebound experiment, and the strength was corrected by ultrasonic wave and core sample. The experimental results meet the design strength requirements. From the raw material quality certificate, re-inspection report, concrete strength test report and raw material field sampling analysis results provided by the construction unit, various unqualified factors of raw materials can be ruled out.
2) Technological process: All the concrete used in this project is commercial concrete. Pumped directly from the pump to the working face, the concrete is vibrated mechanically. Through the field test, the commercial concrete in the mixing station meets the requirements of concrete mixture ratio error, and the concrete slump is actually controlled at about12 2 ciyl. From the appearance inspection of concrete, there is no honeycomb pitting surface phenomenon, and it is compacted by vibrating.
3) Concrete mix proportion: In order to improve the crack resistance and pumpability of concrete and reduce the drying shrinkage of concrete, 10% fly ash is added to the concrete mix proportion used in basement construction. In order to meet the pumpability and slow down the early hydration heat of cement, it is planned to add retarding superplasticizer and pumping agent. All the above three materials have different degrees of early strength. From the analysis of early cracks in concrete, the comprehensive application of the above three materials may be one of the reasons for early cracks in concrete.
4) Construction technology: After the basement construction, due to the lack of soil on site, it was not backfilled, and the basement was exposed for a long time, which was greatly affected by environmental changes, especially temperature changes.
5) Investigation of meteorological conditions: during the basement floor construction, it was 65438+ 10 in autumn in this area, with weather temperature of about 23℃, dry climate and relative humidity of 30%-40%. When the maximum wind speed of Et is 7 m/s, during the construction period, water conservation and maintenance measures such as ribbon enclosure under beams and slabs and regular watering and maintenance are adopted. , but no windproof measures were taken.
6) investigation of other factors: the building was under construction at that time, and its overall settlement was less than 3 ITtnl, with uniform settlement; After 10 d construction, the load on the upper part of this layer of concrete gradually increased, and no other vibration occurred during construction. Therefore, the influence of other factors (such as foundation settlement and external force) on the basement floor can be basically ruled out.
3 Cause analysis
1) Under the same construction conditions, the only difference is the climate change during construction, from the point of view that cracks only appear in the basement floor and there are no cracks in the cast-in-place slabs of other floors. As mentioned above, the cast-in-place slab construction of this floor is the driest period in autumn in this area, with the highest temperature of only 23℃, the maximum wind speed of 7 m/s at that time, and the humidity of only 30% ~ 40%, especially after the construction is completed at 2 1: 00 every day, the concrete is in the initial setting period, the strength has not developed greatly, and the working face has no windproof measures, which leads to the rapid water loss of concrete and roots.
According to relevant data, when the wind speed is 7 mrs, the evaporation rate of water is twice that of no wind. When the relative humidity is 30%, the evaporation rate is more than three times that of 90%. If the influence of wind speed and humidity during construction is superimposed, it can be calculated that the drying speed of concrete at this time is more than 6 times that under normal conditions. In addition, from the phenomenon that cracks are mostly concentrated on the floor with thin components and the largest contact area with the outside world, it can be confirmed that cracks have little to do with the materials used, but are greatly affected by meteorological conditions. The reason why the wall limb near the basement floor does not crack is because there are templates on both sides of the wall limb, which is not directly affected by the atmosphere. From this, it can be basically concluded that weather is the main factor leading to dry shrinkage cracks of cast-in-place concrete slabs. Because of its large volume, the basement floor is exposed to the environment with large temperature and humidity changes for a long time, especially in 1O month, and the temperature difference between morning and evening is large, which leads to concrete temperature shrinkage and cracks.
2) C35 concrete is used for the basement floor. After autumn construction, fly ash, retarding superplasticizer and pumping agent are added to concrete, which have different degrees of early strength. The construction water consumption is relatively reduced, and the concrete strength is rapidly increased. At the same time, due to the dry weather, the evaporation of water and the development of concrete cracks are aggravated.
3) As can be seen from the structural plan of this project, the rigidity of the core tube and the wall limb is very large, which has a strong restraining effect on the cast-in-place slab, and the internal stress at the four corners of the core and at both ends of the wall limb is very concentrated. From the initial position of cracks in the cast-in-place slab, shrinkage cracks first appear at the four corners of the core tube, and then in the middle of the slab, which is the most concentrated, complicated and weakest part of the cast-in-place slab. Due to the strong constraint of the stiffness of the wall limb and the core tube, when the shrinkage stress of concrete is greater than its tensile strength, cracks appear and develop along this position.
4 treatment method
Through the above investigation and analysis, this layer structure is safe. The bearing capacity of beam and slab meets the design requirements. However, considering the importance of this project and the owner's attention to this problem, and in order to prevent the corrosion of steel bars from affecting the durability, based on the principle of prevention first, it is decided to repair according to the terms that need to be repaired.
4. 1 basement leakage construction technology 4.2 material selection
1) Permeable crystalline waterproof material: It can make the chemical substances in it react with calcium ions in cement to generate dendritic crystals, thus blocking the channels of capillary holes and micro-cracks, and playing a waterproof and seepage-proof role. 2) Modified epoxy resin, water-soluble cyanated one-component grouting material, quick-hardening micro-expansive cement and quick-sealing polymer elastic cement-based waterproof coating. Modified epoxy is used for those with low water seepage, and cyanidation is used for those with high water seepage pressure. Its effect is fast coagulation and hardening and high strength.
4.3 Key points of construction
1) Check the leak. 2) Base treatment: after inspection and cleaning, it is found that there is a crack in the bottom plate structure, and a ""groove with a width of 60 feet and a depth of 80 mm is opened along the crack position with a chainsaw. 3) Open the open water hole with electric hammer to the depth of 150ITRL't, and bury the grouting nozzle for grouting. 4) Remove sundries, flush the V-shaped groove with water, then embed the draft tube into the V-shaped groove, then smooth the V-shaped groove with quick-setting cement, and embed the grouting nozzle every 300 mm~500 ITtnl. 5) Prepare the grouting liquid, fully and evenly stir it according to the appropriate ratio of modified epoxy accelerated curing agent 10: 1, pay attention to cooling while stirring, and pay attention to the temperature.
Control it below 5℃ ~ 30℃ ~ 30℃. Add curing agent and accelerator to epoxy liquid slowly, and do not pour them all at once. 6) Adding the modified epoxy chemical grouting liquid into the grouting machine, and performing pressure grouting with a manual air pump. The concentration of modified epoxy chemical grouting liquid is 10%, the pressure is controlled at 0.6 MPa ~ 0.8 MPa, and the gelation time intensity is controlled at 0.3 MPa. Press it into all internal spaces until the original liquid overflows from the gap, and then close the grouting nozzle after complete penetration. 7) Brush the permeable crystalline waterproof material again. 8) Observe for one month after grouting. If there is no new crack leakage, the overall waterproof construction can be carried out after inspection and acceptance.
5 conclusion
In view of some cracks in the cast-in-place floor slab of the basement, after many times of analysis and research, the reasons are found out and the right medicine is given. Through practice, some prevention measures have been obtained and certain effects have been achieved. In order to completely eliminate cracks, it is still necessary to improve construction technology, accumulate construction experience and adopt more scientific solutions.
References:
Zhang sheng. Prevention and treatment technology of concrete cracks [J]. Shanxi Architecture, 2005 (5): 87.89.
[2] He Wenyao. On the prevention of concrete cracks [J]. Shanxi Architecture, 2005 (10): 87-89.