The following is the relevant content brought to you by Zhongda Consulting on the setting and construction technology of post-cast strips in reinforced concrete structures for your reference.
With the development of high-rise buildings in the direction of larger volumes and diversified structural functions, long-length, large-area high-rise panel buildings with basements and other buildings, as well as independent underground buildings, are based on Design specifications require the installation of settlement or telescopic post-casting strips. This article briefly introduces the reasonable setting and main construction technical measures of the post-cast belt of the building, and analyzes the existing problems to ultimately ensure the construction quality of the post-cast belt.
0Introduction
The technology of setting post-cast belts in reinforced concrete structures is suitable for cast-in-place integral reinforced concrete structures of high-rise residential buildings, public buildings and over-long structures of high and low structures. In the construction of post-pouring belts, large volumes of concrete can be constructed in blocks by setting up post-pouring belts, speeding up the construction progress and shortening the construction period. Since there are no permanent settlement joints, the building structure design is simplified and the integrity of the building is improved: it also reduces the factors of water leakage.
1 The main functions of post-casting tapes
1.1 Post-casting tapes can effectively solve the problem of settlement differences. The structure and foundation of high-rise buildings and podiums are designed as a whole, but post-casting tapes are used during construction. The belt temporarily disconnects the two parts. After the construction of the main structure is completed and most of the settlement (above 50%) has been completed, the concrete of 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.
In addition, the following adjustment measures can also be taken at the same time
: 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.
②Adjust 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.
③Adjust the height difference. After settlement calculation, the elevation of the main building was set slightly higher and the elevation of the podium was set slightly lower, and the settlement difference between the two was reserved to make the final actual elevations of the two consistent.
1.2 Post-casting tape can reduce the impact 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 and 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.
2 Requirements for the setting of post-casting belts
2.1 The setting of post-casting belts should follow the design principle of “having both resistance to discharge and focusing on discharge”. 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.
2.2 In the structural design, the post-cast strips designed due to settlement reasons should be set strictly according to the design drawings during construction; when the post-cast strips need to be set up due to construction reasons, they should be set according to the specific conditions of the project. The location shall be approved by the design unit.
2.3 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. Generally The appropriate width is 700~1000mm.
2.4 The stress-bearing steel bars of the beam and plate at the post-casting zone must be connected through and are not allowed to 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.
2.5 Before pouring concrete, part of the formwork and pillars of the post-pouring belt cannot be removed, otherwise it will cause the beam plate to form a cantilever and cause deformation; the position of the post-pouring belt should be selected in a part of the structure that bears less stress. , generally near the reverse bending point of the beam or plate, where the bending moment is not large and the shear force is not large; it can also be selected in the middle of the beam or plate, where the bending moment is large but the shear force is small.
2.6 The cross-sectional form of the post-cast belt should be considered to ensure a firm connection after the concrete is poured, and straight joints should generally be avoided. For slabs, diagonal joints can be left; for beams and foundations, tongue-and-groove joints can be left, and tongue-and-groove joints come in many forms, which can be determined according to the structural cross-section.
3 Construction technology of post-pouring belt
3.1 Formwork support The formwork (steel mesh formwork) shall be supported in the concrete pouring construction sections divided according to the block diagram, and the construction shall be strictly carried out in accordance with program requirements.
3.2 Structural concrete pouring. The thickness of concrete pouring should be carried out strictly in accordance with the specifications and construction plans to avoid dimensional deviation caused by the increase in lateral pressure of the steel mesh formwork due to the thicker pouring. Steel wire mesh should be used. For the vertical construction joints of the 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. To ensure the density of the concrete, vertical construction joints should be tamped with steel drills.
3.3 Treatment of vertical construction joints after pouring structural concrete For vertical construction joints using steel mesh formwork, when the concrete reaches the initial setting (fingerprints can appear on the surface of the concrete when pressed by hand), flush it with pressure water (water It should be in the form of mist), remove the laitance and debris and expose the aggregate in the flushing area, and rinse the steel wire mesh clean at the same time. After the concrete is finally set, remove the steel wire mesh and immediately flush the surface of the construction joints again with high-pressure water; high-pressure water can be used to flush the vertical construction joints at the wooden formwork; the formwork can also be removed as early as possible according to the site conditions and specifications. Chiseling; For hardened concrete surfaces, use a chisel machine; Serious honeycombs or holes should be repaired; Use a spray gun (water and air) to clean the surface before pouring concrete in the post-cast zone.
3.4 Protection measures for the post-poured belt. For the post-poured belt of the base plate, temporary water-retaining brick walls should be added to the walls at both ends of the post-poured belt. The height should be higher than the height of the base plate. Apply waterproofing on both sides of the wall. Mortar: In order to prevent the construction water around the base plate from flowing into the back pouring belt, use mortar to make water retaining belts of 5cm wide and 510cm high at the 50cm wide areas on both sides of the back pouring belt. After the construction joints of the back pouring belt are processed and cleaned , the top is covered with wooden formwork or iron sheet, and a water retaining belt is made with mortar, and temporary railings are set up around it to prevent contamination of steel bars and accumulation of garbage during the construction process; after the post-casting belt of the foundation cap is left, protective measures should be taken , to prevent garbage and debris from falling into the post-pouring belt. For protection measures, wooden cover plates can be used to cover the upper steel bars of the cap platform. Both sides of the cover plates should be more than 500mm wider than the post-pouring strips. The protection measures for the vertical back-pouring strips of the basement exterior walls can be protected by bricklaying.
3.5 Concrete pouring with post-poured belts. Different types of post-poured belt concrete have different pouring times: telescopic post-poured belts depend on the completion of shrinkage of the first-poured concrete, generally 60 days after construction; settlement post-poured belts It should be carried out after the building has basically completed settlement. In some projects, the design unit has special requirements for the retention time of the post-poured tape. Before pouring the post-poured tape concrete, the construction joints should be flushed with water, kept moist for 24 hours, and the accumulated water on the concrete surface should be eliminated; Before pouring, it is advisable to lay a layer of cement mortar with the same composition as the mortar in the concrete at the construction joint; the concrete for the post-pouring zone must be non-shrinkage concrete, which can be prepared with expansion cement, or with the addition of admixtures with expansion effects and ordinary cement. The strength of the concrete should be increased by one level, and its mix ratio should be determined through testing. It is advisable to add early-strength water-reducing admixture, and it should be carefully prepared and vibrated carefully.
4 Quality Control Measures
4.1 Formwork Support For post-pouring zones with large volumes of concrete such as thick floor slabs and girders in basements, special formwork and supports must be set up on both sides to prevent concrete leakage. The post-pouring belt cannot be broken due to the slurry. If the basement has waterproof and anti-seepage requirements, a water stop belt or tongue-and-groove formwork should be left to prevent water seepage in the post-pouring belt.
The supports retained in the post-cast belt should be retained until the concrete in the post-cast belt is poured and the strength reaches the design requirements, and then they can be removed layer by layer.
4.2 Technical measures before pouring the post-cast belt. After the concrete in the post-cast belt is poured, dry shrinkage and deformation will occur, resulting in poor combination of new and old concrete. The following measures should be taken to prevent cracks in the post-cast belt.
Use quick and easy closing mesh, steel mesh or plugging plates on both sides of the post-casting belt as side molds. The plugging plates should be grooved up and down according to the spacing of the steel bars. The form of the post-casting belt should be based on the design requirements. If the design When there is no clear requirement, the form to be used will depend on the specific situation. Among them, the exterior walls of basements generally adopt straight joints and install steel plate waterstops. Before the construction of post-cast concrete, the rust layer on the surface of the steel bars should be removed and the surface of the concrete should be chiseled. If quick and easy closing mesh or steel wire mesh is used, the mesh does not need to be dismantled and there is no need to chisel. Clean the debris on the concrete surface and rinse it with pressure water.
5 Conclusion
For the construction of the post-pouring strip of the building, the position of the post-casting strip must be reasonably set according to the design, specifications and specific conditions of the project. At the same time, corresponding construction quality control techniques should be adopted from several aspects such as the structural concrete pouring on both sides of the post-cast belt, the treatment of vertical construction joints in the post-cast belt, the protection before concrete pouring in the post-cast belt, and the pouring construction of the post-poured belt to ensure that Construction quality of post-cast strip.
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