Analysis on the setting and construction technology of the post-casting belt in the basement of construction projects
Paper keywords: Construction technology of post-casting belt in high-rise buildings
Paper abstract This article is based on engineering examples, aiming at The characteristics and functions of the post-casting belt on the floor and roof of the basement of Huangling Mining Group Company's single apartment building and staff canteen were analyzed and summarized on the construction technology and quality control measures of the project to ensure the construction quality of the post-casting belt of the entire project.
1. Foreword
Setting up the post-pouring belt in the basement project of a high-rise building is a key link in the project. It can be seen from the construction process of the building that for high-rise building basement post-pouring The construction of the belt must be based on the design, specifications and the specific actual conditions of the project, and the position of the post-cast belt must be reasonably set. Therefore, the construction of the basement post-pouring belt must be handled in a planned manner, so as to reduce the difficulty of post-pouring belt construction, solve the problems and problems that have long troubled construction personnel, and ensure the construction quality control of the post-pouring belt.
2. Project Overview
The single apartment building and staff canteen project of Huangling Mining Group Company has a total construction area of ??27715.3m2, 25132.7m2 above ground, 2582.6m2 underground, and the building base area 2582.6m2. Among them, the single apartment has 18 floors above ground and one underground floor, with a height of 66.9m. It has a beam and raft foundation and the foundation depth is -6.5m. The staff canteen has 2 floors underground and 1 floor underground, with a height of 9.3m. It has an independent foundation and the foundation depth is -6.5m. .
The basement floor of this project is made of 60cm thick C35 and P6 impermeable concrete, the wall panels are 30cm thick C40 and P6 impermeable concrete, and the basement roof is made of 45cm thick C35 and P6 impermeable concrete. The plane shape is "□". The floor, side walls and roof of the basement are all equipped with two vertical and horizontal post-pouring belts. The width of the post-pouring belt is 1000mm. They are arranged in a cross shape on the 1st axis and the 5-6 axis. The total length of the post-pouring belt is about 700m and the thickness is the same. Beam and plate thickness. The steel bars on the bottom plate and top plate are double-layered and bidirectional, with a steel mesh in the middle. The steel bars at the post-casting zone are densely packed. The overlap length of the steel bars at this location is 950mm and there is a 4mm thick water-stop steel plate. Therefore, the processing of the post-casting zone is more complicated.
3. The main function of the post-casting belt
3.1 Solve the settlement difference
The structure and foundation design of high-rise buildings and podiums are integrated, but during construction Use post-pouring tape to temporarily disconnect the two parts. After the construction of the main structure is completed and most of the settlement (more than 50%) has been completed, the concrete in the connecting part is poured to connect the upper and lower floors as a whole. When designing, the different stress states of the foundation in the two stages should be considered and the strength should be checked separately. The calculation after being connected as a whole should consider the additional internal force caused by the later settlement difference. This approach requires good foundation soil and the settlement of the house can be basically completed during the construction period. At the same time, the following adjustment measures can also be taken:
1) Adjust the pressure difference. The main building has a large load, so the overall foundation is used to reduce the earth pressure and increase the burial depth to reduce additional pressure; the lower floors use shallower cross-beam foundations to increase the earth pressure and make the settlements of the upper and lower floors close to each other.
2) Adjust the time difference. The main building is constructed first, and after it is basically completed and the settlement is basically stable, the podium is then constructed so that the subsequent settlement will be basically similar.
3.2 Reduce the influence of temperature shrinkage
Newly poured concrete will shrink during the hardening process. The built structure will expand when heated and shrink when cooled. Most of the hardening shrinkage of concrete will be completed within 1 to 2 months after construction, while temperature changes have a constant effect on the structure. When its deformation is restrained, temperature stress will be generated inside the structure, and in severe cases, cracks will appear in the components. After leaving the post-casting tape, the concrete can shrink freely during the construction process, thus greatly reducing shrinkage stress. Most of the tensile strength of concrete can be used to resist temperature stress and improve the structure's ability to resist temperature changes.
4. Setting of post-casting tape
4.1 Setting requirements for post-casting tape
The setting of post-casting tape should follow the principle of "having both resistance to erosion and "Master" design principle. Because ordinary concrete has cracking problems, the purpose of setting up post-cast joints is to release most of the restraint stress, and then fill the joints with expanded concrete to counteract the residual stress. When post-cast strips need to be installed due to construction reasons, it should be determined according to the specific conditions of the project, and the location should be approved by the design unit. The spacing between post-casting strips should be reasonable. The spacing between post-casting strips for rectangular structures can generally be set to 30 to 40m. The width of the post-casting strips should be considered to facilitate construction operations and be determined according to the structural requirements. The width of the post-casting strips for this project is set to 1000mm. .
The stress-bearing steel bars of the beam and plate at the post-casting zone must be connected through and must not be disconnected. If the span of the beam or plate is not large, enough steel bars can be provided at one time; if the span is large, they can be disconnected according to regulations and welded before filling with concrete. Part of the formwork and pillars of the post-pouring belt cannot be removed before concrete is poured, otherwise it will cause the beam plate to form a cantilever and cause deformation.
4.2 The specific method of post-casting tape
The post-casting tape in this project mainly refers to the basement floor, exterior walls, floor beams, and slab post-casting tape. The specific methods are as follows:
1) The method of post-pouring the basement floor is shown in Figure 1.
5. Construction of post-pouring belt
5.1 Formwork support
According to the block diagram, the concrete pouring construction section support formwork (steel mesh formwork) and strictly follow the requirements of the construction plan.
5.2 Concrete pouring of basement roof
1) The thickness of structural concrete pouring on both sides of the post-pouring belt should be strictly in accordance with the specifications and construction plan to avoid the steel mesh formwork caused by the large pouring thickness. The side pressure increases and bulges outward, resulting in dimensional deviation.
2) For vertical construction joints using steel mesh formwork, during the concrete pouring and vibrating process, special attention should be paid to the layered pouring thickness and the distance between the vibrator and the steel mesh formwork. In order to prevent serious loss of cement slurry during concrete vibration, the distance between the vibrator and the formwork should be limited. When using a Φ50mm vibrator, the time interval is ≮40cm; when using a Φ70mm vibrator, the time interval is ≮50cm.
5.3 Treatment of vertical construction joints after pouring basement roof concrete
1) For vertical construction joints using steel mesh formwork, when the concrete reaches the initial setting (the surface of the concrete can be pressed by hand) If fingerprints appear), rinse with pressure water (the water should be mist-like) to remove laitance and debris and expose the aggregate in the rinsed area, and rinse the steel mesh clean at the same time. After the concrete is finally set, remove the steel mesh and immediately flush the surface of the construction joint again with high-pressure water.
2) The vertical construction joints at the wooden formwork can be flushed with high-pressure water, or the formwork can be removed as early as possible and artificially chiseled in time according to the site conditions and specification requirements.
3) For hardened concrete surfaces, use a chiseling machine.
4) Serious honeycombs or holes should be repaired.
5) Use a spray gun (water and air) to clean the surface before pouring concrete in the post-cast zone.
5.4 Protection measures for the post-poured belt of the basement floor
1) For the post-poured belt of the floor, add temporary water-retaining brick walls at both ends of the post-poured belt. The height is higher than the height of the base plate, and waterproof mortar is applied on both sides of the wall.
2) In order to prevent the construction water around the base plate from flowing into the post-pouring belt, use mortar to make water-retaining tapes 5cm wide and 5-10cm high at 50cm wide areas on both sides of the post-pouring belt.
3) After the construction joints of the post-casting belt are processed and cleaned, the top should be covered with wooden formwork or iron sheet, and a water retaining belt should be made with mortar, and temporary railings should be set up around it to avoid contamination during the construction process. Steel bars, piles of garbage.
4) After the back pouring belt of the foundation cap is left in place, protective measures should be taken to prevent garbage and debris from falling into the back pouring belt.
As a protective measure, wooden cover plates can be used to cover the epithelial steel bars of the cap platform. Both sides of the cover plates should be at least 500mm wider than the post-cast belt.
5) Bricklaying can be used to protect the vertical back-pouring belt on the basement exterior wall
The construction quality of the basement floor back-pouring belt directly affects the waterproofing effect of this project, especially It is more difficult to clean when the post-cast strip meets the ground beam. For this reason, we plan not to leak. After the concrete for the floor and floor beams is poured, seal it with nine plywood and send people to check to prevent debris from falling into it. After the concrete on both sides is poured for 60 days, use a higher-level micro-expansion agent mixed with 12UEA micro-expansion agent. The concrete must be poured densely. Before pouring, the steel bars should be derusted, laitance, gravel and other debris should be removed and rinsed.