First, the construction technology
1. When it is necessary to set up post-cast strip for construction reasons, it shall be determined according to the specific structural shape of the project, and the location shall be approved by the design institute.
2. Retention time of post-cast strip. Should be determined according to the design requirements, when there is no design requirement, it should be no less than 40 days; Under the condition of not affecting the construction progress, it should be kept for 60 days.
3. Protection of post-cast strip. After the post-cast strip of foundation pile cap is reserved, protective measures should be taken to prevent garbage and sundries from falling into it. Protective measures can be covered with wooden cover plates on the skin steel bars on the bearing platform, and both sides of the cover plate should be more than 500 mm wider than the post-cast strip. The vertical post-pouring zone of basement exterior wall can be protected by bricklaying. The beam bottom formwork and beam slab support frame on both sides of the floor post-cast strip shall not be removed.
4. Close the post-pouring belt. When pouring structural concrete, a layer of steel wire mesh should be set on the template of post-cast strip, and it is not necessary to dismantle the steel wire mesh during the construction of post-cast strip. No matter what form the post-cast strip is set up, the laitance on the whole concrete surface must be carefully chiseled before closing, and the rough surface must be chiseled to thoroughly remove the garbage and sundries in the post-cast strip, and water it overnight to wet it and lay cement slurry to ensure a good connection between the post-cast strip concrete and the initial pouring concrete. The sealing material of the post-cast strip should be micro-expansive concrete (ordinary concrete can be mixed with micro-expansive agent LTEA, with the dosage of 12%- 15%) which is one level higher than the design strength grade of the first-poured structural concrete, and it should be poured and vibrated, and the curing time should be no less than 14 days.
Second, the construction points
1. The varieties of micro-expansive agent and admixture used in post-cast strip concrete should be selected according to the engineering nature and site construction conditions, and the dosage should be determined by experiments in advance.
2. All micro-expansive agents and admixtures must have the factory certificate and product technical data, and meet the corresponding technical standards and design requirements.
3. The content of micro-expansive agent directly affects the quality of concrete, therefore, its weighing should be carried out by special personnel, and the allowable error is generally +2% of the content.
4. The concrete should be mixed evenly, otherwise it will cause local expansion to be too large or too small, which will affect the quality of concrete. Therefore, the mixing time of concrete mixed with micro-expansive agent should be appropriately extended.
5. The concrete of post-cast strip should be dense, and the connection with the initial concrete should be firm, and there should be no cracks after stress.
6. In the prestressed structure, the non-prestressed reinforcement in the post-pouring belt must leave the necessary space for the anchorage and tension of prestressed reinforcement.
7. There are non-prestressed tendons, prestressed tendons, anchorages and various pipes in the post-pouring belt in the prestressed structure. When pouring concrete in post-cast strip, attention should be paid to its compactness.
8. The construction joints of the post-pouring belt of the basement floor should be set in the middle of the thickness of the floor and be U-shaped.
9. After the concrete pouring of post-cast strip is completed, it should be maintained under the condition of thermal insulation and moisture preservation with formwork. According to the regulations, the water curing time of concrete shall not be less than 7 days, and the concrete with admixture or impermeability requirements shall not be less than 14 days.
10. When there is impermeability requirement for pouring concrete in post-cast strip, impermeability test blocks shall be made according to the specification requirements.
Three, post-cast strip and template support setting requirements
(A) requirements for the setting of post-cast strip
1. The width of the post-cast strip is 700- 1000 mm, and the spacing is generally 20-30 m, which runs through the cross section of the whole structure and divides the structure into several independent sections, but it does not necessarily go straight through a bay to avoid overlapping of 100% steel bars. The setting of post-pouring belt should consider effectively reducing the temperature difference and shrinkage stress, and determine its lien position and distance through calculation. The lap length of slab wall reinforcement with post-pouring belt is 45d, and the main reinforcement of beam can be continuously opened to keep connection.
2. If there are clear requirements for the design of the above parts, the construction shall be carried out in strict accordance with the drawings.
3. There are four forms of post-cast strip: straight joint, stepped joint, convex joint and concave joint. If there is no clear requirement in the design, the specific form depends on the specific situation, in which the basement exterior wall generally adopts straight seam and steel plate water stop is installed.
4. During the construction of foundation cushion, the foundation cushion of post-cast strip should be reduced by 50- 100 mm, so as to deal with construction joints, remove garbage and eliminate accumulated water. During the construction in the rain period, a sump shall be set every 50m in the post-pouring zone, so as to discharge rainwater and keep water in time.
5. Brick enclosure should be built around the post-cast strip, and the top should be covered to prevent garbage from polluting the joint surface and construction joints of steel bars during construction.
(2) Formwork support
1. According to the block diagram, divide the concrete pouring construction layers, set up templates, and construct in strict accordance with the construction plan.
2. The construction joint of post-cast strip generally adopts quick and easy convergent net, steel wire net and plug plate as side formwork, and the plug plate should be slotted up and down according to the spacing between steel bars.
Quick-closing net (permanent concrete formwork) is a kind of formwork which is stamped into one-way U-shaped multi-ribbed frame and one-way three-dimensional grid by thin hot-dip galvanized steel plate, and is especially suitable for concrete poured in sections (such as post-pouring zone in basement, construction joint and poor concrete strength).
The quick closing net is fixed as a consumable template. When pouring concrete behind the formwork, the inclined pieces on the mesh are embedded in the concrete and connected with the concrete, and the surface is corrugated, forming a mechanical wedge connected with the adjacent pouring blocks. It is mainly used to form the construction joint of concrete post-pouring belt, and the treated bonding and shear strength can be comparable to that of rough joints.
Four, concrete pouring and waterproof, prevent sundry fall into measures
(A) structural concrete pouring technical measures
1. The pouring thickness of concrete shall be carried out in strict accordance with the specifications and construction scheme, so as to avoid the protruding of steel mesh formwork due to the increase of lateral pressure caused by excessive pouring thickness, resulting in dimensional deviation.
2. Steel wire formwork shall be used for vertical construction joints. When pouring and vibrating concrete, special attention should be paid to the thickness of layered pouring and the distance between the vibrator and the steel mesh template. In order to prevent serious cement slurry loss during concrete vibrating, the distance between the vibrator and the formwork should be limited (not less than 4002 when using 50 vibrating rods and not less than 502 when using 70 vibrating rods).
3. In order to ensure the compactness of concrete, the vertical construction joints should be compacted with steel bars.
(2) Treatment measures for vertical construction joints of post-cast strip after concrete pouring.
1. For the vertical construction joints with steel wire mesh formwork, when the concrete reaches initial setting (fingerprints appear on the concrete surface by hand), the laitance is removed by flushing with pressure water, and the flushing parts are exposed to the aggregate, and the steel wire mesh is rinsed clean at the same time. After the final setting of concrete, remove the steel wire mesh, and immediately flush the surface of construction joint with high-pressure water again. Therefore, after concrete pouring, a special person should be appointed to master the initial setting and final setting time, so as to select the washing time.
2. For the vertical construction joints of wooden patterns, high-pressure water can be used to wash and nap; Can also according to the site conditions and specification requirements, dismantle the template as soon as possible, artificial chisel hair in time.
3. The hardened concrete surface should be treated with chisel.
4. Before pouring concrete in post-cast strip, the surface shall be cleaned by spray gun (water and air).
(3) Measures to prevent water from soaking into the bottom post-cast strip
1. Add temporary water retaining brick walls on both sides of post-cast strip, with the height of the water retaining brick wall higher than the bottom plate, and coat waterproof mortar on both sides of the wall.
2. In order to prevent groundwater from seeping out of the cushion, spread two felts and three oils on the cushion at the position of post-cast strip. When pouring the concrete of post-cast strip on the bottom plate, pour a layer of plain concrete with a thickness of 10cm on the surface of the bottom plate, and both of them will be poured at the same time. Additional plain concrete layers on both sides of post-cast strip +600mm wide.
3. In order to prevent the construction water around the bottom plate from flowing into the post-cast strip, in the post-cast strip, a water stop with a width of 50mm and a height of 50- 100 mm is made of mortar at 50cm wide on both sides of the post-cast strip.
(d) Measures to prevent sundries from falling into the post-cast strip
After the construction joint is cleaned, the top is closed with wooden formwork or iron sheet, mortar is used as water retaining belt, and temporary railings are set around to avoid polluting the steel bars in the post-cast strip and accumulating garbage during construction.
Verb (abbreviation of verb) quality requirements
During the construction of post-cast strip, the formwork should be supported and installed firmly, and the reinforcement should be cleaned and shaped. The construction quality shall meet the requirements of Code for Acceptance of Design and Construction of Reinforced Concrete, so as to ensure that the concrete is dense and crack-free.
Benefit analysis of intransitive verbs
By setting the post-pouring belt, mass concrete can be constructed in blocks, which speeds up the construction progress and shortens the construction period. Because there is no permanent settlement joint, the structural design of the building is simplified, the integrity of the building is improved and the factors of water leakage are reduced.