Post-cast strips refer to temporary strip joints in cast-in-place overall reinforced concrete structures that are only retained during the construction period to absorb settlement, shrinkage and deformation. They are retained for a certain period of time according to project needs. After that, concrete is poured and densely packed into a continuous and integral structure. Based on the author's experience, the construction method of post-cast belts in cast-in-place concrete structures will be introduced.

1. Scope of application

This method is suitable for the construction of post-cast strips in cast-in-place integral reinforced concrete structures of high-rise and low-rise residential buildings, public buildings and extra-long structures. , the construction of post-cast strips in other structures with special requirements can refer to this method.

2. Construction technology

1. When post-casting tape needs to be set up due to construction reasons, it should be determined according to the specific structural shape of the project, and the location should be approved by the design institute.

2. Retention time of post-cast tape. It should be determined according to the design requirements. When there is no design requirement, it should be no less than 40 days; if it does not affect the construction progress, it should be retained for 60 days.

3. Protection of post-casting tape. After the post-casting tape of the foundation cap is left in place, protective measures should be taken to prevent garbage and debris from falling in. As a protective measure, a wooden cover can be used to cover the upper steel bars of the cap platform. Both sides of the cover should be at least 500 mm wider than the post-cast strip. The vertical back pouring strip of the basement exterior wall can be protected by brickwork. The beam bottom formwork and beam plate support frames on both sides of the post-cast belt of the floor panel shall not be removed.

4. Sealing of post-casting belt. When pouring structural concrete, a layer of steel mesh should be installed on the formwork of the post-pouring belt. During the construction of the post-pouring belt, the steel mesh does not need to be removed. No matter what form of post-pouring belt is used, the laitance on the entire concrete surface must be carefully removed before sealing, and the surface must be chiseled into a rough surface. The garbage and debris in the post-pouring belt must be thoroughly removed, and the water must be watered overnight to moisten it. Lay the cement slurry to ensure that the later poured concrete is well connected with the first poured concrete. The water-stop treatment of the basement floor and exterior wall back-casting tape shall be carried out in accordance with the design requirements and corresponding construction acceptance specifications. The sealing material of the post-poured belt should be poured and vibrated using micro-expansion concrete with a design strength level higher than that of the structural concrete poured first (micro-expansion agent UEA can be added to ordinary concrete at a dosage of 12%-15%). Dense, and keep warm and moisturizing for no less than 14 days.

3. Construction Points

1. The types of micro-expansion agents and admixtures used in post-cast concrete should be selected according to the nature of the project and on-site construction conditions, and determined through tests in advance Incorporation amount.

2. All micro-expansion agents and admixtures must have factory certificates and product technical data, and comply with corresponding technical standards and design requirements.

3. The amount of micro-expanding agent directly affects the quality of concrete. Therefore, its weighing should be carried out by a dedicated person, and the allowable error is generally ±2% of the amount.

4 , Concrete should be stirred evenly, otherwise excessive or small local expansion will occur, affecting the quality of the concrete. Therefore, the mixing time of concrete mixed with micro-expansion agent should be appropriately extended.

5. The post-poured concrete should be dense, and the connection with the first-poured concrete should be firm, and no cracks should appear after being stressed.

6. In prestressed structures, the non-prestressed tendons in the post-cast strip must leave necessary space for the anchoring and tensioning of the prestressed tendons.

7. There are non-prestressed tendons, prestressed tendons, anchors, various pipelines, etc. in the post-cast belt in the prestressed structure. When pouring concrete in the post-cast belt here, great care should be taken. Its density.

8. The construction joint in the post-pouring belt in the basement floor should be set in the middle of the thickness of the floor in a "U" shape.

9. After the post-poured concrete is poured, it should be cured under the conditions of heat preservation and moisturizing with formwork. It should be stipulated in the specification. Generally, the watering and curing time should not be less than 7 days. Admixtures should be added or resistant. The concrete required for penetration shall not be less than 14 days.

10. If the concrete in the poured belt has anti-permeability requirements after pouring, anti-permeability test blocks should be made according to the specifications.

IV. Quality Requirements

During the construction of the post-cast belt, the formwork should be supported and installed firmly, the steel bars should be cleaned and shaped, and the construction quality should meet the requirements of the reinforced concrete design and construction acceptance specifications to ensure The concrete is dense and has no cracks.

5. Benefit Analysis

By setting up post-pouring belts, large volumes of concrete can be constructed in blocks, speeding up the construction progress and shortening the construction period. Since there are no permanent settlement joints, the building structure design is simplified, the integrity of the building is improved, and the factors of water leakage are also reduced.

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