Cast-in-place hollow floor technology is to use the concept of prefabricated hollow floor, bury hollow round pipes in concrete slab, arrange them in a certain direction, and pour them on site, so that the original solid concrete slab becomes hollow slab. In the hollow hole-forming technology of cast-in-place hollow slab structure, metal spiral pipe is widely used abroad. This product is made of high-quality low-carbon steel strip, which is made by rolling wave, rolling pipe, biting and cutting. BDF pipes, GBF metal spiral pipes, glass steel pipes, PVC pipes or thin-walled boxes filled with mineral powder and cementing materials are generally used in China. Compared with the ordinary beam-slab structure, the cast-in-place concrete hollow beamless floor has the following advantages: the slab construction saves the beam formwork support process and speeds up the construction progress; The increase of floor clear height is beneficial to the free separation of indoor rooms; Sound insulation, heat preservation and heat insulation are good, which meets the current energy-saving design requirements in hot summer and cold winter areas; Reduce the comprehensive cost. The total amount of reinforced concrete in the floor slab is reduced, the self-weight is reduced, the load of vertical structure and foundation can be reduced accordingly, and the total cost can be reduced by about 10%. Hollow pipes are easy to float during pouring, and if there are no effective fixing measures, it is easy to cause hollow pipes and steel mesh to float as a whole; Concrete pouring is difficult.
1 project example
A primary school teaching building, with a building area of 4 920 m2, has a frame structure. The span of a single classroom is 8. 7 m ×7。 8 m, the cast-in-place concrete floor adopts 250 mm thick DBF high-strength thin-walled pipe. The construction difficulties of this project are as follows:
1) difficulty in fixing hollow pipes: each hollow pipe should be controlled from left to right and up and down to ensure the design size of ribs and the position of hollow pipes.
2) steel binding is difficult: the width of the middle rib of the hollow pipe is only 50 mm, and the iron between ribs is 2.
3) Concrete pouring is difficult. Due to the dense steel bars and small gaps between ribs, it is difficult to guarantee the compactness of concrete. The concrete under the core pipe is not easy to be vibrated and compacted, and it is easy to form defects such as honeycomb, pits and holes. When pouring concrete, it is easy to cause the core tube to float, which will drive the steel bar in the slab to move, resulting in the protective layer of the steel bar under the slab being too large. Therefore, it is necessary to control the overall floating of steel bars. However, its wall is thin and hollow, so necessary measures should be taken to prevent floating during construction to ensure the engineering quality.
2 construction procedures and quality control points
2. 1 construction technology of hollow floor
Elastic line layer by layer → tie the bottom iron → fix the lower steel bar → embed the water and electricity pipeline → place the hollow pipe, the steel bar between the tied pipes and the horse stool at the same time → tie the iron → fix the lead wire to control the floating of the pipe → conduct hidden inspection of the steel bar → concrete entry acceptance → pouring → vibrating → curing.
2.2 Key points of quality control in construction
2.2. 1 integral floating control of reinforcement
After the tie of the lower iron is completed, put plastic pads with a spacing of 800 mm and plum blossom distribution, and the two pieces must be placed in the same position. In order to prevent the lower reinforcement from floating in the process of pouring concrete, the lower reinforcement is fixed with the roof formwork with nails. The distance between the fixed points is 800 mm, which is adjacent to the plastic pad. Each fixed point is nailed into the formwork keel with two 4-inch nails, and the lower reinforcement is fastened.
2.2.2 Embedded water and electricity pipelines
When laying pipelines, when the electric conduit is perpendicular to the pipeline direction, short DBF pipes should be selected, leaving no more than 100 mm for the electric conduit. For large-diameter and centrally reserved water holes, the hollow pipes can be rearranged through negotiation (see Figure 1, Figure 2).
2.2.3 Anti-floating fixation of hollow pipes, binding reinforcement between pipes, and placing horse stool reinforcement.
According to the pipeline layout, lay hollow pipes and horse stool steel bars, lay pipes and tie steel bars at the same time, and lay and install them in turn from one side. The left, right, up and down control of DBF high-strength thin-walled pipe is shown in Figure 3 and Figure 4.
1) Left-right fixation of hollow pipe: the tractor should be close to the pipe wall to control the left-right position of thin-walled pipe. In order to prevent the tractor from rotating, a short steel bar is added on one side, and it is firmly bound with the upper steel mesh with binding wire.
2) The hollow pipe is fixed up and down: the horse stool is adopted.
2.2.4 Concrete pouring
Concrete pouring requires the slump of concrete to be 170mm+ 10mm, the maximum particle size of concrete stones to be 16 mm, the concrete pouring direction of hollow slab is basically symmetrical, and the materials are evenly discharged from both ends of the slab, and the distribution should follow the gap between pipes. Concrete is poured in two layers. When the first layer is poured, it is poured into the middle and upper part of DBF pipe, and vibrated with φ 30 vibrating rod to make the concrete flow to the bottom of hollow pipe. Hollow pipes float to the designated position under the buoyancy of concrete, and individual pipes can be adjusted manually if they do not float in place. After all the pipes are in place, the upper concrete shall be poured in a large area, which must be evenly distributed to prevent the roof formwork from overloading and sinking. The concrete on the second floor of the roof is vibrated by a flat vibrator, which can be used vertically and horizontally. The blank holder is 3 cm~5 cm, and the surface is horizontally collected. When vibrating, it is required to connect one board after another, and vibration shall not be missed. The plate vibrator should move at a uniform speed and vibrate for not less than two times. The interval of joint pouring shall not exceed 2 h, so that the joint is dense, and pouring and vibrating shall be carried out in sequence. In addition, attention should be paid to the uniformity and basic symmetry of concrete blanking to avoid the displacement of steel bars being squeezed aside.
2.2.5 Concrete curing
After the concrete is poured and the surface is closed, cover it with plastic film for 12 h, and then water it for curing to keep the concrete moist for at least 7 d. When the concrete strength reaches 100% of the design strength, formwork removal is allowed.
3 Anti-floating nail care
After the form removal of the roof, the nails that originally fixed the bottom reinforcement are exposed outside the roof, so it is necessary to break the nails flush with the roof and apply antirust paint for anticorrosion treatment.
4 conclusion
Cast-in-place hollow floor slab is widely used in long-span structural systems. In the process of hollow slab concrete pouring, hollow pipes and steel mesh are easy to float up, and the concrete pouring is not dense. Only by taking effective technical measures can we ensure the project quality, put an end to hidden dangers and ensure the safety of life and property of project users.