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Cast-in-situ hollow floor technology is the use of the concept of prefabricated hollow floor slabs. Hollow circular tubes are embedded in concrete slabs, arranged in a certain direction, and cast in place to make the original solid concrete slabs become hollow slabs. In the hollow hole forming technology of cast-in-place hollow-core plate structure, the most widely used abroad is the metal spiral pipe. This product is made of high-quality low-carbon steel strips through rolling wave, roll forming, bite-cutting, cutting and other processes. In China, BDF pipe, GBF metal spiral pipe, glass steel pipe, pvc pipe, etc., or thin-walled boxes made by adding ore powder and cementing materials are generally used. The advantages of cast-in-situ concrete hollow beamless floor slab and ordinary beam slab structure are: the slab construction is adopted, the beam supporting moulding procedure is omitted, the construction progress is accelerated, the floor height is improved, and the free separation of indoor rooms is facilitated; The insulation and heat insulation effects are good, meeting the current energy-saving design requirements in the hot summer and cold winter regions; reducing the overall cost. The total amount of reinforced concrete for floor slabs is reduced, the weight is reduced, and the vertical structure and foundation load can be reduced accordingly, which can reduce the total construction cost by about 10%. Hollow tubes tend to float during the pouring process. If there are no effective fixing measures, the hollow tubes and the reinforcing steel mesh sheets can easily float up. Concrete pouring is difficult.
1 Project Example A primary school teaching building with a construction area of ​​4,920 m2 and a frame structure. A single classroom span of 7.7 m × 7.8 m was constructed using 250 mm thick DBF high-strength thin-walled tube cast-in-place concrete floor slabs. The construction difficulties of this project are as follows:
1) It is difficult to fix hollow tubes: Each hollow tube must be controlled at the left, right, and upright positions to ensure the design dimensions of the ribs and the position of the hollow tubes.
2) Rebar tying is more difficult: the middle rib width of the hollow pipe is only 50 mm, the iron on the inter-column is 2<8, the lower iron is 2<16, and the upper and lower irons have a row of tie hooks of <6 @200. Because the rebar tying and hollow tube arrangement must be performed alternately at the same time, and the cross-sectional dimensions are too small, cross-operation between the types of work is closely performed and the rebar tying is difficult.
3) Concrete pouring is difficult. Due to the tightness of the steel bars and the small gap between the beams of the intercostals, it is difficult to ensure the compaction of the concrete. The concrete under the core tube is difficult to vibrate and dense, and it is easy to form defects such as honeycomb, pits, holes and so on. When the concrete is poured, it is easy to cause the core tube to float and move the ribs, resulting in a large protective layer under the steel plate. Therefore, it is necessary to control the overall rise of the steel bar. However, its wall is thin and hollow, and necessary measures must be taken during construction to prevent it from floating and ensure the quality of the project.
2 Construction Procedures and Quality Control Points
2. 1 Hollow floor construction process sub-file elastic line → banding bottom iron → fixed lower steel bar → embedded water and electricity pipeline → placing hollow tube, ribs between bundled tubes and horse stools placed at the same time and cross-linking → binding iron → control tube Floating lead wire fixing → steel concealment inspection → concrete acceptance acceptance → pouring → vibrating → maintenance.
2. 2 Quality Control Points in Construction
2. 2. 1 Reinforced steel bars shall be controlled after the completion of iron banding. The plastic pads shall be cushioned with a spacing of 800 mm and shall be distributed in a plum shape. Two pieces shall be placed at the same position. In order to prevent the lower steel bar from floating during the pouring of the concrete, the lower bar is fixed to the top plate form by nails. The fixed point spacing is 800 mm, and it is adjacent to the position of the plastic pad. Each fixing point is nailed into the template keel with two 4 inch nails and the lower bar is fastened.
2. 2. 2 In the event of an electrical conduit running perpendicular to the direction of the pipe during pipelaying of a pre-embedded utility line, a short DBF pipe shall be used to leave no more than 100 mm of access to the conduit. For larger diameter and more concentrated water professional reserved holes, the hollow tubes can be rearranged through negotiation (see Figure 1, Figure 2).
2. 2. 3 Hollow tube anti-floating fixation, inter-tube rib tying and horse stool reinforcement The laying of the hollow tube and the horse stool ribs according to the discharge diagram of the tube, and the laying of the ribs and the ribs are performed at the same time, and they are laid and installed one by one. The left and right upper and lower control of the DBF high-strength thin-walled pipe is shown in Figure 3 and Figure 4.
1) The left and right hollow tubes are fixed: The pull hooks should be tightly attached to the pipe wall to control the left and right position of the thin-wall pipe. To prevent the rotation of the hooks, a short steel bar is added on one side and the upper layer reinforcing mesh is tied tightly.
2) The hollow tube is fixed up and down: The horse stool adopts <16 reinforcement to control the upper and lower reinforcement meshes, and the hollow tube anti-float reinforcement 2 m long hollow tube is arranged with two horse stool reinforcements, and a horse is arranged above the 1 m long hollow tube. The stool ribs are provided with 30 thick pads to prevent the hollow tube from floating and effectively control the position of the hollow tube in the hollow floor.
2.2.4 Concrete placement Concrete placement requires a concrete slump of 170 mm ± 10 mm and a concrete aggregate with a maximum particle size of 16 mm. The pouring direction of the hollow slab concrete is basically uniform and symmetrically fed from both ends of the slab, and the fabric should follow the gap between the tubes. The pouring of concrete takes place in two layers. The first layer shall be poured first into the upper middle of the DBF pipe, and vibrated with a Φ30 vibrator to make the concrete flow to the bottom of the hollow pipe. The hollow pipe floats under the buoyancy of the concrete. Specified positions, individual tubes can be manually adjusted without floating. After all the pipes are in place, the upper part of the concrete shall be poured over a large area and must be evenly distributed to prevent the top plate formwork from sinking under load. The second floor of the top slab is casted with a flat plate vibrator to vibrate. The plate vibrator can be used horizontally and vertically, with an edge pressure of 3 cm to 5 cm, and the surface is preferably a horizontal slurry. When vibrating, it is required to be plate-to-plate and there must be no leakage. The flat plate vibrator must move at a uniform speed and vibrate not less than twice. The interval between the placement of joints shall not exceed 2 h, so that the joints are densely packed and the pouring vibrations are performed in sequence. In addition, attention should be paid to the fact that the concrete should be evenly and uniformly sifted, and the displacement of the steel pipe on one side should be avoided.
2. 2. 5 Concrete curing After the completion of concrete placement, it is covered with a plastic film for 12 hours, then watered and cured to keep the concrete wet for at least 7 days. Only when the concrete strength reaches 100% of the design strength is it allowed to remove the mould.
3 After the anti-float nails are removed from the top plate, the nails that originally fixed the bottom steel bar are exposed outside the top plate. The nail top plate needs to be broken, and antirust paint is applied for anti-corrosion treatment.
4 Conclusion Cast-in-place hollow floor slabs are widely used in large-span structural systems. In the hollow slab concrete pouring process, hollow tubes and steel mesh floats and concrete pouring are not easy to occur. Only effective technical measures can guarantee the situation. The quality of the project eliminates engineering risks and ensures the safety of life and property of project users.
1 Project Example A primary school teaching building with a construction area of ​​4,920 m2 and a frame structure. A single classroom span of 7.7 m × 7.8 m was constructed using 250 mm thick DBF high-strength thin-walled tube cast-in-place concrete floor slabs. The construction difficulties of this project are as follows:
1) It is difficult to fix hollow tubes: Each hollow tube must be controlled at the left, right, and upright positions to ensure the design dimensions of the ribs and the position of the hollow tubes.
2) Rebar tying is more difficult: the middle rib width of the hollow pipe is only 50 mm, the iron on the inter-column is 2<8, the lower iron is 2<16, and the upper and lower irons have a row of tie hooks of <6 @200. Because the rebar tying and hollow tube arrangement must be performed alternately at the same time, and the cross-sectional dimensions are too small, cross-operation between the types of work is closely performed and the rebar tying is difficult.
3) Concrete pouring is difficult. Due to the tightness of the steel bars and the small gap between the beams of the intercostals, it is difficult to ensure the compaction of the concrete. The concrete under the core tube is difficult to vibrate and dense, and it is easy to form defects such as honeycomb, pits, holes and so on. When the concrete is poured, it is easy to cause the core tube to float and move the ribs, resulting in a large protective layer under the steel plate. Therefore, it is necessary to control the overall rise of the steel bar. However, its wall is thin and hollow, and necessary measures must be taken during construction to prevent it from floating and ensure the quality of the project.
2 Construction Procedures and Quality Control Points
2. 1 Hollow floor construction process sub-file elastic line → banding bottom iron → fixed lower steel bar → embedded water and electricity pipeline → placing hollow tube, ribs between bundled tubes and horse stools placed at the same time and cross-linking → binding iron → control tube Floating lead wire fixing → steel concealment inspection → concrete acceptance acceptance → pouring → vibrating → maintenance.
2. 2 Quality Control Points in Construction
2. 2. 1 Reinforced steel bars shall be controlled after the completion of iron banding. The plastic pads shall be cushioned with a spacing of 800 mm and shall be distributed in a plum shape. Two pieces shall be placed at the same position. In order to prevent the lower steel bar from floating during the pouring of the concrete, the lower bar is fixed to the top plate form by nails. The fixed point spacing is 800 mm, and it is adjacent to the position of the plastic pad. Each fixing point is nailed into the template keel with two 4 inch nails and the lower bar is fastened.
2. 2. 2 In the event of an electrical conduit running perpendicular to the direction of the pipe during pipelaying of a pre-embedded utility line, a short DBF pipe shall be used to leave no more than 100 mm of access to the conduit. For larger diameter and more concentrated water professional reserved holes, the hollow tubes can be rearranged through negotiation (see Figure 1, Figure 2).
2. 2. 3 Hollow tube anti-floating fixation, inter-tube rib tying and horse stool reinforcement The laying of the hollow tube and the horse stool ribs according to the discharge diagram of the tube, and the laying of the ribs and the ribs are performed at the same time, and they are laid and installed one by one. The left and right upper and lower control of the DBF high-strength thin-walled pipe is shown in Figure 3 and Figure 4.
1) The left and right hollow tubes are fixed: The pull hooks should be tightly attached to the pipe wall to control the left and right position of the thin-wall pipe. To prevent the rotation of the hooks, a short steel bar is added on one side and the upper layer reinforcing mesh is tied tightly.
2) The hollow tube is fixed up and down: The horse stool adopts <16 reinforcement to control the upper and lower reinforcement meshes, and the hollow tube anti-float reinforcement 2 m long hollow tube is arranged with two horse stool reinforcements, and a horse is arranged above the 1 m long hollow tube. The stool ribs are provided with 30 thick pads to prevent the hollow tube from floating and effectively control the position of the hollow tube in the hollow floor.
2.2.4 Concrete placement Concrete placement requires a concrete slump of 170 mm ± 10 mm and a concrete aggregate with a maximum particle size of 16 mm. The pouring direction of the hollow slab concrete is basically uniform and symmetrically fed from both ends of the slab, and the fabric should follow the gap between the tubes. The pouring of concrete takes place in two layers. The first layer shall be poured first into the upper middle of the DBF pipe, and vibrated with a Φ30 vibrator to make the concrete flow to the bottom of the hollow pipe. The hollow pipe floats under the buoyancy of the concrete. Specified positions, individual tubes can be manually adjusted without floating. After all the pipes are in place, the upper part of the concrete shall be poured over a large area and must be evenly distributed to prevent the top plate formwork from sinking under load. The second floor of the top slab is casted with a flat plate vibrator to vibrate. The plate vibrator can be used horizontally and vertically, with an edge pressure of 3 cm to 5 cm, and the surface is preferably a horizontal slurry. When vibrating, it is required to be plate-to-plate and there must be no leakage. The flat plate vibrator must move at a uniform speed and vibrate not less than twice. The interval between the placement of joints shall not exceed 2 h, so that the joints are densely packed and the pouring vibrations are performed in sequence. In addition, attention should be paid to the fact that the concrete should be evenly and uniformly sifted, and the displacement of the steel pipe on one side should be avoided.
2. 2. 5 Concrete curing After the completion of concrete placement, it is covered with a plastic film for 12 hours, then watered and cured to keep the concrete wet for at least 7 days. Only when the concrete strength reaches 100% of the design strength is it allowed to remove the mould.
3 After the anti-float nails are removed from the top plate, the nails that originally fixed the bottom steel bar are exposed outside the top plate. The nail top plate needs to be broken, and antirust paint is applied for anti-corrosion treatment.
4 Conclusion Cast-in-place hollow floor slabs are widely used in large-span structural systems. In the hollow slab concrete pouring process, hollow tubes and steel mesh floats and concrete pouring are not easy to occur. Only effective technical measures can guarantee the situation. The quality of the project eliminates engineering risks and ensures the safety of life and property of project users.
Guangzhou Jointair Co., Ltd. , https://www.nbjointair.com