I. Overview of the Project
The case project is a high-rise residential community project with underground garage, supporting houses and other auxiliary facilities. The foundation forms are mostly pile-raft foundation and raft foundation, and involve independent foundation and strip foundation under the column. The floor, roof and concrete wall of the underground garage are provided with post-pouring belt. C35 concrete is selected for the raft, and the impermeability grade is P6. C35 concrete is selected for the roof, walls and columns, and the impermeability grade is P6. The width of post-cast strip is 800mm, and the non-shrinkage concrete mixed with expansion agent is selected, and the pouring height is higher than that on both sides.
Second, the design requirements of post-cast strip
1, requirements of post-cast strip
1. 1 Post-cast strip is a temporary construction joint set to overcome harmful cracks caused by temperature and shrinkage during the construction of cast-in-place reinforced concrete structure. Before pouring, joints should be reserved for a period of time according to the design requirements to connect the whole structure into a whole.
1.2 In the structure of post-cast strip, unless otherwise indicated in the drawing, the reinforcement of post-cast strip shall be reserved according to penetration.
1.3 The setting distance of post-cast strip should be calculated under the condition of effectively reducing temperature difference and shrinkage stress. Under normal construction conditions, refer to the relevant provisions of Code for Acceptance of Construction Quality of Concrete Structure Engineering.
1.4 After the settlement of post-cast strip is capped, the settlement record can be closed only after it is confirmed by the design unit; The shrinkage post-cast strip shall be closed after the concrete age on both sides reaches 45d.
The width of post-cast strip of 1.5 should consider simple construction to avoid stress concentration. Generally, its width is 70~ 100cm. According to the design requirements of this project, the width of post-cast strip is 800 mm, and the reinforcement of post-cast strip should be well protected.
1.6 protective measures must be taken to cover the post-cast strip in the basement, and all parts of the large cantilever formed by the lien of the post-cast strip should be supported, and the post-cast strip can only be removed when its concrete strength reaches 100%.
1.7 The post-cast strip must be thoroughly cleaned before filling. If there is water in the basement, it should be drained and cleaned, and then concrete can be poured after it is rinsed with water.
2. Design of post-pouring belt
2. 1 The formwork support system of post-cast strip should be an independent system bounded by its axes on both sides, and at the same time, it should be guaranteed to bear the same force as the overall support system, that is, when the adjacent structure is constructed, the support frame of post-cast strip will participate in the overall support system, but when the adjacent structure formwork and support frame are dismantled, the performance of the support frame and formwork of post-cast strip will remain unchanged, and the concrete structures on both sides of post-cast strip will bear the same force.
2.2 The formwork support system of the post-cast strip must be dismantled only after the concrete of the post-cast strip meets the design requirements of 100%. When the formwork of post-cast strip is supported, the steel mesh is considered on both sides, and its installation method is conventional.
2.3 In the construction of post-cast strip in basement, waterproofing of post-cast strip on bottom plate and external wall are two noteworthy issues. In order to prevent the sudden surge of groundwater, measures such as thickening the concrete cushion in the post-pouring zone of the bottom plate, improving the strength grade of concrete and local reinforcement have been taken (see the figure below). The additional reinforcement at ① in the figure is B 16@200.
2.4 Foundation slab post-cast strip support, as shown in the figure below.
2.7 slab post-cast strip
(1) The bottom formwork and supporting system of slab post-cast strip are separated from the cast-in-place slab formwork on both sides. Before concrete pouring in post-cast strip, the roof formwork and supporting system in post-cast strip shall not be removed. See the figure below for how to obtain the template:
2.8 The vertical construction joints on both sides of post-cast strip are reinforced with steel wire mesh formwork and steel pipe (reinforcement), and battens are set on both sides of the vertical construction joints. Ensure the smooth connection between new and old concrete. In order to prevent concrete and concrete slurry from flowing into the post-pouring zone due to the use of disposable single-layer steel mesh, it is not easy to clean up. In the construction, the following methods are adopted: precast (strip) blocks of cement sand with the same composition as concrete are used as the bottom reinforcement of beam and slab, and the bottom reinforcement is reserved to prevent concrete from pouring into the post-pouring zone from below the bottom reinforcement. Wall and deep beam (≥600mm) adopt double-layer net as one-time template, one layer of steel mesh and one layer of fine-mesh barbed wire, and the two layers of net can be tied and fixed together in advance. The double-layer net is supported by shaped steel bars and fixed by welding bottom bars and gluten. The specific method is shown in the following figure:
2.9 After 28 days of concrete pouring, the formwork and support below can be removed. In order to prevent workers from disassembling the support frame of post-cast strip by mistake, the frame and template are marked with yellow and black.
2. Micro-expansive concrete is used for post-pouring concrete of10 slab. The post-cast strip should be poured at one time, and no construction joints should be set.
2. 1 1 After the concrete of the roof post-cast strip is poured, it shall be cured in time, and the curing time shall not be less than 14 days. When the formwork of post-cast strip is removed, the cast-in-place beam slab structure on both sides of post-cast strip should return to the top.
3. Treatment of construction joints
3. 1 Measures to prevent garbage and other sundries from falling into the post-cast strip: After the construction joints are cleaned, two-skin brick walls should be built on the two sides of the post-cast strip, and brick masonry should be used as a waterproof barrier on the outside, and then the post-cast strip should be covered with multi-layer boards, with each side exceeding the brick wall by 20cm. Avoid polluting steel bars and construction waste falling into the post-pouring zone in the future construction process.
3.2 Position of beam head: Before binding reinforcement after formwork removal, the construction team's pay-off personnel will pop up the edge line of beam head position on the wall, and then the construction team will assign a special person to accurately determine the position of beam head with a grinding wheel cutter with a chisel, and it is required to chisel off the excess concrete, cement film, laitance and loose stones on the concrete surface and clean them up.
3.3 Before watering the post-cast strip, the reinforcement shall be derusted, straightened and straightened, and the concrete at the joint shall be chiseled. Loose stones and sundries shall be cleaned, kept moist and brushed with pure cement slurry.
3.4 When pouring concrete in the post-cast strip, the vibrating point of the vibrating rod should be vibrated on both sides first, then in the middle, and then lightly vibrated and compacted. After the post-cast strip is compacted, it should be leveled and the residual mortar should be removed.
Third, the protective measures of post-cast strip
1, temporary protection measures for floor post-cast strip 1. 1 During the period when the post-cast strip is vacant, the following sealing measures can be taken: build a brick strip with a width of 120mm and a height of 200mm on the upper side of the floor post-cast strip, and the upper part is fully enclosed with plywood or woven cloth.
2. Protective measures for post-cast strip of concrete wall
2. 1 protection of the inner side of the wall post-cast strip: the wall post-cast strip is fully enclosed with plywood to prevent sundries from falling in.
2.2 Protective measures for roof (floor) post-cast strip: Before the post-cast strip is closed, the roof formwork at the post-cast strip shall not be removed. Individual need to dismantle, need to dismantle in 28 days, and timely with steel pipe back to the top.
Fourth, the construction preparation before pouring
1, material preparation 1. 1 Before pouring the concrete of post-cast strip, contact the manufacturer of commercial concrete in advance, paying special attention to the concrete strength grade of the mixture ratio and the addition of micro-expansive agent.
2. Machine preparation
2. 1 The foundation, wall and beam of post-cast strip shall be vibrated with vibrating rods, and the site casting shall be vibrated with flat vibrator.
3. Technical preparation 3. 1 Pay attention to the concrete mixture ratio before construction, do a good job of disclosure by operators, do a good job of concealed inspection of steel bars alone, and pay attention to the lien of test blocks.
4.4. 1 The working condition meets the time specified in the design and construction drawing, and the settlement observation is approved by the design department, and the casting can only be carried out after the supervision is qualified.
V. Construction Process and Method of Post-poured Strip
1, construction process of demolition of post-cast strip, cleaning up sundries of post-cast strip → derusting, shaping, binding or repair welding of steel bars → formwork support → supervision and acceptance → pouring concrete.
2. Construction method of post-cast strip 2. 1 Demolition and cleaning of sundries in post-cast strip:
(1) In site construction, in order to pursue the accuracy and impression of the retained position of post-cast strip, there are steel wire mesh and wood square on both sides of post-cast strip. At present, all the wood squares should be removed during construction, and the redundant steel wire mesh should also be removed.
(2) The cement slurry and excess concrete in the beam-slab post-pouring zone shall be chiseled clean and straight, and all the original concrete wastes shall be cleaned before entering the next working procedure.
2.2 steel derusting and straightening:
(1) rebar derusting includes derusting all rebar, and derusting rebar in foundation with long wire brush;
(2) After derusting, rinse the steel bars in the post-cast strip with water for corrosion;
(3) After derusting, straighten the bending of steel bars, lap weld the stressed steel bars in the beam and slab according to the design requirements, and supplement the steel bars that need to be supplemented;
(4) After the acceptance of the supervision unit, the foundation shall be cast immediately after derusting and cleaning, and the floor post-cast strip shall be washed before concrete pouring to prevent the secondary corrosion of steel bars.
2.3 templates are used for:
(1) After the post-pouring belt of basement floor is cleaned, concrete can be poured directly without installing formwork.
(2) The post-cast strip in beam slab, external wall and other parts can be poured only after the formwork is installed.
(3) The formwork support should be the same as the original design beam (wall) plate, and the lower support must be tightened. The basement exterior wall should be reinforced with water stop screws.
(4) The formwork of floor post-cast strip shall be poured in layers from bottom to top without interruption.
2.4 concrete pouring of post-cast strip:
(1) Before concrete pouring, the sundries in the post-cast strip should be cleaned, rinsed with water, and then brushed with pure cement slurry.
(2) pouring micro-expansive concrete with a strength grade higher than the design, and vibrating and compacting.
(3) When pouring concrete in the post-cast strip, the vibrating point of the vibrating rod should be vibrated on both sides first, then in the middle, with light vibration and more tamping. After the post-cast strip is compacted, it should be leveled and the residual mortar should be removed.
(4) In order to ensure the strength of the concrete in the post-pouring zone to meet the design requirements within the specified age and control the early shrinkage cracks of the concrete, it is necessary to do a good job of curing. The curing time after concrete pouring shall not be less than 12h, and the horizontal beam and slab shall be watered and cured, and the sack shall be covered to ensure that the concrete body is in a wet state.
(5) When pouring concrete, specially-assigned personnel shall check whether the formwork at the bottom of the joint is loose, suddenly cracked and deformed. If yes, stop pouring and repair the formwork before pouring.
Six, the problems that should be paid attention to in the construction of post-cast strip
The post-cast strip shall be constructed in strict accordance with the construction specifications and design requirements, and the following issues shall be attached great importance to during the construction:
1, after the completion of pouring concrete first, the post-cast strip should be protected, partially covered, and surrounded by temporary railings to prevent the steel bars from being polluted during construction and ensure that the steel bars are not trampled. Resolutely avoid the phenomenon that the post-pouring belt is unprotected, the steel bar is bent, the steel bar is messy, and there is a lot of construction waste, which is difficult to clean up.
2. Before pouring concrete in post-cast strip, the whole section must be treated according to the requirements of construction joints, and the surface impurities, cement film, loose sand particles and weak concrete layer should be removed, and the concrete on both sides should be chiseled and washed with water to keep the concrete on both sides fully wet, generally not less than 24 hours. After the surface is coated with cement slurry or concrete interface treatment agent, concrete shall be poured in time. If the concrete on both sides is not cut and poured, it is easy to ensure the bond strength of new and old concrete. If it is not handled well, two hollow cracks will appear on both sides of the post-cast strip, which is easy to seep.
3. When pouring concrete in the post-cast strip, the concrete label is one grade higher than the original concrete. Adding anti-crack and anti-seepage agent to concrete and using micro-expansive cement can make it produce micro-expansive pressure, offset the tensile stress caused by concrete shrinkage and temperature difference, make concrete structure not crack, and improve anti-seepage ability.
4. The concrete of post-cast strip must be carefully vibrated and compacted, and water conservation should be paid attention to. Avoid the phenomenon that the amount of concrete in the post-pouring zone is small, and the concrete is manually mixed next to the post-pouring zone, which seriously affects the engineering quality.
5. The steel bars in the post-cast strip of beams and slabs shall not be cut off, and the steel bars in the beams and slabs in this part shall be set in strict accordance with the design drawings.
6. Before the concrete pouring of post-cast strip, the beam and slab within the span of post-cast strip are in cantilever stress state for a long time. During the construction period, the formwork and support within the span cannot be removed, and it can only be removed from top to bottom after the concrete strength of the post-cast strip reaches more than 75% of the design strength value. It is forbidden to dismantle the formwork and support in the span of post-cast strip in advance, which will cause the edge of the plate to crack and reduce the bearing capacity of the structure.
7, in the process of concrete pouring and vibrating, pay special attention to the distance between vibrator and template, to prevent serious loss of cement slurry when concrete vibrating. Generally, the distance between the vibrator and the template is controlled between 40 and 50 cm. In order to ensure the compactness of concrete, steel wire mesh and formwork, deep vertical construction joints should be tamped with artificial steel bars.
8, due to the post-cast strip concrete should be used to improve a strength grade, early strength, shrinkage compensation of concrete. Before construction, apply for concrete mixture ratio separately. During the construction, the inspection and test of cement, sand, stone, admixture and admixture should be done well in advance, and the concrete mixture ratio should be applied in time. When pouring, it should be carefully measured. When pouring concrete, standard specimens and specimens cured under the same conditions shall be retained as required to test and prove the strength of concrete in post-cast strip. Attention should be paid to the maintenance of concrete after pouring to improve the compactness and impermeability of concrete.
Seven, quality assurance measures of post-cast strip
1, personnel management In order to achieve the one-time qualified quality goal of post-cast strip construction, all post-cast strip cleaning, steel derusting, straightening, formwork support, concrete pouring, etc. It will be managed by a special person and will be tracked until the end of maintenance.
2. Maintenance measures for concrete of post-cast strip In order to ensure the strength of concrete of post-cast strip to meet the design requirements within the specified age and control the early shrinkage cracks of concrete, maintenance must be done well, and the maintenance should be carried out 12h after concrete pouring, and the maintenance time should not be less than 14d. The horizontal beam and slab should be watered and covered with sacks for curing to ensure that the concrete body is in a wet state.
Eight, post-cast strip safety and civilized construction matters needing attention
1. Operators must strictly abide by labor protection regulations and wear and use personal protective equipment correctly.
2, homework personnel must strictly implement the safety technical disclosure and shift leader speech requirements.
3, cross construction, should have reliable protective measures, shall not hurt others, also avoid being hurt by others.
4. Any operator shall not dismantle the scaffolding, protective facilities, safety signs and warning signs on the construction site without authorization. If it is necessary to dismantle them, it must be approved by the construction director.
5, homework personnel shall not be arbitrarily spilled construction waste and sewage and other human pollution of the environment.
6. In addition to the operation time limit, the operators should consciously reduce and eliminate noise during the operation.
7, homework personnel should adhere to the civilized construction, personal behavior to adapt to the requirements of CI image management.
8. When dismantling the roof formwork, dismantle the formwork and dismantle the support. It is forbidden to dismantle the support at one time. The top surface of the post-cast strip on the floor slab that has not been poured should be tightly covered to prevent objects from falling from the post-cast strip and hurting people.
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