Combined with practical engineering, this paper introduces the construction technology of replacing traditional beam-slab structure with cast-in-place concrete hollow beamless floor, and expounds that this technology is simple and fast in construction, which effectively reduces the cost and improves the space utilization rate.

Keywords: hollow floor, GBF hollow pipe, construction method

1 project overview

The underground garage of experimental middle school, with a construction area of 24680m2, is located in the northeast of the campus, with a playground at the upper part and a garage at the lower part, with a length of 220m from north to south and a width of 130m from east to west, and a basement (local second floor) as a stand. Adopt frame structure system. Cast-in-place reinforced concrete beamless hollow floor slab is used underground, and cast-in-place reinforced concrete floor slab is used on the ground. The foundation is cast-in-place independent column or strip foundation. The top floor is covered with soil about 1m thick and 4.9m high, with 652 designed parking spaces. According to the post-cast strip, there are GBF hollow pipes with 15 slab, hollow floor thickness of 400mm, embedded length of 1000mm and diameter of 300 mm.. According to the arrangement of end spacing of 60mm and pipe wall spacing of 100mm, C30 concrete pouring is adopted, and anti-floating limit measures are taken to prevent internal formwork from floating and steel bar displacement.

2 "Cast-in-place concrete hollow floor" brief introduction

Cast-in-place concrete hollow floor is a floor structure composed of hidden beams and hollow floor without core pulling. According to the design requirements of column network and slab span load, the parameters such as the diameter of thin-walled hollow pipe, the total thickness of hollow floor, the width of web rib and hidden beam between hollow holes in floor section, the thickness of pipe top and bottom wing, and the reinforcement of beam and slab are determined through structural design. GBF hollow pipe is a kind of high-strength composite thin-walled pipe, which is made of fiber cement and closed at both ends.

The hollow floor slab of this project adopts GBF thin-walled hollow pipe with a diameter of 300mm and a wall thickness of 5mm, which is used for the 400mm thick floor slab. The length of the internal mold of the hollow tube is 1000mm, and three specifications of 600mm, 300mm and 200mm are customized according to actual needs for end splicing.

3 construction process flow

Erecting scaffolding to install formwork, marking, positioning, binding, placing steel bars at the bottom of the slab, embedding thin-walled hollow pipes with water and electricity, binding steel bars between ribs, installing anti-floating steel bars, binding upper steel bars and pouring concrete.

4 Construction method

4. 1 Scaffolding Template The roof of underground garage can be divided into 15 plates with equal area according to the post-pouring zone, which are used as 15 construction units, numbered from east to west, and numbered from north and south sides to the middle, followed by assembly line. Set up a full-house steel pipe scaffold, lay the square timber as the formwork support rib, then lay the bamboo glue formwork as the hollow floor bottom formwork, fix the bottom formwork with the supporting square timber with nails, strictly control the firmness of the support, and copy it flat to ensure that the flatness of the bottom formwork meets the requirements of construction specifications, and seal the gap of the formwork with tape paper to prevent slurry leakage during concrete pouring.

4.2 After the cross-positioning formwork is laid flat and fixed, the formwork shall be released with elastic thread, and the arrangement scheme of thin-walled pipes, the pipeline arrangement of water heating electrical equipment and the plane positioning arrangement of vertical embedded sleeves shall be determined in consideration of the reserved embedment of water heating electrical equipment, so as to prevent excessive cross-positioning of water heating electrical pipelines from affecting concrete pouring construction, rationally arrange various pipelines, ensure the mechanical properties of thin-walled pipes unchanged and avoid stress concentration. Communicate with the design unit in time for the weak points, take reasonable measures to mark the positions of hidden beams, reserved holes, embedded pipelines and GBF thin-walled hollow pipes, and mark the discharging sequence of hollow pipes, and then pop up the position line of steel bars at the bottom of hollow floor. The clear distance between the hollow pipe and the edge of the reserved hole should not be less than 50 mm, the clear distance between the edge of the hidden beam and the bottom steel bar should be greater than 30 mm, and the clear distance between the longitudinal steel bar along the pipeline direction and the thin-walled hollow pipe wall should not be less than10 mm. According to the arrangement and requirements of the hollow pipe, the bottom steel bar and the thin-walled hollow pipe should be installed after the gap is marked and positioned.

4.3 Binding dark beam and slab bottom reinforcement According to the position marked by dark beam, the dark beam reinforcement is put in place, and then according to the played bottom reinforcement position line, short reinforcement is laid first, then long reinforcement is laid, and reinforcement is bound with splayed buckle. After the bottom steel bar binding is completed, install and weld φ 10 steel bar with a height of 200mm to fix the position of the hollow pipe and avoid rolling displacement. Both sides and ends of each hollow pipe are required to be welded and fixed with anti-displacement steel bars. When welding, pad a thin iron plate on the template to prevent the template from burning out and causing safety accidents.

4.4 The hollow pipes must be placed in the order of marking and arrangement, and the specifications of the hollow pipes must be installed according to the design drawings and sample drawings. The spacing between the hollow pipes is 50 mm, and the spacing between the ends is 100 mm. When laying pipes, wooden partitions should be laid on the hollow pipes for protection after they are in place. It is absolutely forbidden to step on the thin-walled hollow pipe directly to ensure the integrity of the hollow pipe. If it is damaged, it is forbidden to put it in. When placing hollow pipes, add columns, walls and corner stiffeners.

4.5 The arrangement of thin-walled hollow pipes should be considered first in the water and electricity embedment and casing embedment of water heating pipes bound with steel bars between ribs, and it is not allowed to directly pass through the pipe body; When there are many horizontal embedments of electrical pipelines, consider adding junction boxes to facilitate pipeline winding avoidance and avoid too many crossed hollow pipes, which will lead to the failure of concrete to cover the pipelines. At the same time, tie reinforcement between ribs, and pay attention to the correct and firm position of reinforcement, so as not to damage the hollow pipe wall. Iron parts do not need to be embedded separately.

4.6 Install and bind anti-floating steel bars, and lay prefabricated special-shaped cushion blocks with flat bottom and concave top. After the hollow pipes are padded to the design elevation, anti-floating fixation shall be made on the length of each hollow pipe from 1/4 ends, and the anti-floating reinforcement shall be fastened with 12# iron wire and connected with the supporting steel pipe or square timber through the formwork. Each hollow pipe span is provided with two φ.

4.7 After the binding of upper reinforcement and anti-floating reinforcement is completed, φ6@200 steel mesh shall be bound, and the effective height of steel mesh shall be ensured. Mesh reinforcement shall not be exposed outside the plate rib reinforcement, and shall be bound neatly.

4.8 After the completion of pouring concrete reinforcement and formwork, the acceptance and assembly should be carefully carried out, with emphasis on the thickness of protective layer, the height of slab reinforcement, the integrity and fixation of thin-walled hollow pipe. After acceptance, install concrete pumping pipe and pour concrete. When installing pumping pipes and pouring concrete, special attention should be paid to the following items:

4.8. 1 After the pump pipe is hoisted to the top of the plate, it is forbidden to pile up in bundles, and it should be evenly dispersed in the area without hollow pipes to ensure that the hollow pipes are not crushed;

4.8.2 Pump pipe erection shall ensure a certain distance between steel pipe rack and thin-walled hollow pipe, and the root of steel pipe shall be wrapped with soft objects to avoid rubbing thin-walled hollow pipe;

4.8.3 When pouring concrete, use φ30 small vibrator and flat vibrator to vibrate slowly. Don't touch the thin-walled hollow pipe and formwork when vibrating, and vibrate in strict accordance with the requirements of construction specifications;

4.8.4 When the thin-walled hollow pipe is damaged, temporary emergency remedial measures should be taken: a) When it is damaged in a small area, paste it with a wet cement bag to repair the leakage; B) If a large area is damaged, fill polystyrene particles with wet sacks and wrap them with cement bags; C) If the pipe end is damaged, the woven bag shall be fastened with 12# iron wire.

After pouring is completed, water and maintain it in time, and cover the concrete surface with film, sack or straw curtain.

5 Performance and characteristics

5. 1 Compared with the ordinary beam-slab structure, the cast-in-place concrete hollow beamless floor slab has the advantages of fast construction speed, eliminating the formwork supporting process of the beam and shortening the construction period by more than 10%; Save template cutting loss; Save the rental cost of machines and reusable materials.

5.2 Compared with solid beamless floor slab, cast-in-place concrete hollow beamless floor slab has light dead weight and low vertical structure cost; Long span (up to 15 m without prestress and 25m); Prestressed); Without column cap, the effective floor height is increased.

5.3 Material saving and cost reduction: Compared with the general floor system, the cost of reinforced concrete is reduced by 5%, the formwork loss is reduced by 50%, and the vertical water, electricity, heating and ventilation, interior and exterior decoration and other expenses are saved to varying degrees.

5.4 The floor is conducive to the flexible partition of the room and is suitable for the layout of large bay. For example, if you use a large-sized house, you only need to fix the bathroom and kitchen, and the rooms such as partition walls can be designed and arranged by the residents themselves.

5.5 There is no need for suspended ceiling in the room: because there is no beam or column cap under the floor, there is no need for suspended ceiling in the room, which can save the renovation cost of suspended ceiling.

5.6 Excellent use function: Compared with the general flat slab and flat slab, the hollow floor slab has no column cap, thus realizing a real flat slab with high rigidity, small deformation and good seismic performance. The closed cavity structure effectively reduces the transmission of heat and noise, and the heat insulation, heat preservation and sound insulation performance are obviously improved.

5.7 Because there is no structural beam protruding from the floor surface, the clearance height is increased, the space utilization efficiency is improved, the building height is saved by about 0.5m, and the horizontal installation of water and electricity pipelines and HVAC is simplified.

5.8 High-strength thin-walled pipes and reinforced concrete under cooperative stress can improve the mechanical properties of cast-in-place structures and effectively reduce the weight of floors and buildings.

5.9 The construction is simple and quick, which reduces the construction cost.

6 conclusion

With the joint efforts of designers, constructors and product developers, the cast-in-place hollow floor structure has made great progress in just a few years. The structural selection is more and more reasonable, and the construction method is more and more mature. Since the Ministry of Construction issued the Technical Specification for Cast-in-place Concrete Hollow Floor in 2005, it has developed from the early thin-walled hollow tube floor to the box-shaped hollow floor, and then to the through-hole box-shaped hollow floor which has been put into use with higher hollow rate and better material utilization.

Modern architecture tends to be lenient in space, flat in interior, and humanized in overall development. In view of the characteristics of hollow floor, such as long span, light weight, free interval and shortened construction period, it conforms to the development direction of modern architecture. I hope the landlord will adopt it. . .