System and method for biaxial semi-prefabricated lightweight concrete slab

Abstract

The present invention solves the existing problem of obtaining a self-carrying biaxial homogeneous lightweight concrete slab. The present invention consists of a system and method comprising semi prefabricated elements and special stringer structures, designed in such a way, that the finished flat slab structure appears homogeneous and can be achieved without temporary supports during the execution. The present invention solves the problem in a simple and economical manner, increasing building speed, and providing an enhanced range of applicability.

Claims

1. A biaxial lightweight concrete slab system, comprising semi-prefabricated elements, characterized in that said elements (140) are self-carrying, each incorporating a bottom (10) functioning as a slab formwork and further incorporating semi-prefabricated stringers (40) integrated in the bottom of said elements (140) and comprising a high strength composite zone (60) of reinforced concrete with a steel arrangement (50) protruding from the concrete surface of said stringers (40) towards a bottom reinforcement (30), and an open zone (80) allowing the installation of post-tension tendons (90) enabling an optimized effect of post-tension, which is applied after concreting of the elements (140), the post-tension tendons (90) being positioned in said elements (140) such that the stringers (40) and the concrete bottom (10) provide full carrying capacity in one direction over the main span for the final dead load, and in that said slab system comprises a final concrete slab (170) acting as a biaxial homogeneous plate with carrying capacity according to the design load on the slab, and in that the system comprises lightweight members (120) as hollow spheres placed in a geometrical grid.

2. The biaxial lightweight concrete slab system according to claim 1, characterized in that the stringers (40) incorporate the steel arrangement (50) in the lengthwise direction of the stringer (40), and where a part of a steel arrangement (100) is exposed, in the direction opposite to the protruding steel arrangement (50) relative to the concrete, and prepared for future connections at the top, enabling a top reinforcement (130) to be welded or otherwise connected to said steel arrangement (100).

3. The biaxial lightweight concrete slab system according to claim 1 or 2, characterized in that openings (110) perpendicular to lengthwise direction of the stringer (40) are integrated in the stringer (40).

4. The biaxial lightweight concrete slab system according to claim 1 or 2, characterized in that the self-carrying semi-prefabricated elements (140) are made partly with a material other than concrete.

5. The biaxial lightweight concrete slab system according to claim 1 or 2, characterized in that a supporting element (180) with a similar stringer (40) provides spanning between permanent vertical structural supports as columns or walls, and supports the end of the series of the elements (140), and after final concreting of the system acts as an integrated part of a functional and geometrical unity with the elements (140) creating a biaxial homogeneous slab (170) obtained with no temporary supports.

6. The biaxial lightweight concrete slab system according to claim 1 or 2, characterized in that the stringer or a part of the stringer (40) in the elements (140) protrudes out from said elements (140) such that a protruding part (190) of said elements (140) lands on a bottom flange (200) of the supporting element (180) designed such that the bottom surface of said elements (140) is on the same level as the bottom surface of the supporting element (180), thus creating a completely flat plate slab with uniform bottom level, and which, after placing joint splice bars across the bottom reinforcement (210) and the top reinforcement (220), and after a final concreting (160) of the system, creates the biaxial homogeneous flat plate slab (170) obtained with no temporary supports.

7. The biaxial lightweight concrete slab system according to claim 1 or 2, characterized in that the tendons (90) are placed with varying vertical positions within the supporting element (180).

Description

(1) FIG. 1 illustrates a cross section cut in a traditional semi-precast element, where a thin concrete bottom plate (10) is given a certain carrying capacity by implementing steel lattice girders (20), which is placed on the bottom reinforcement (30) and integrated in the concrete bottom. These lattice girders enable the semi-precast element to be transported, lifted and to span 1-2 meters between lines of temporary supports. The concrete bottom (10) constitutes a bed for later supplementary final concreting.

(2) FIG. 2-11 illustrate construction principles and construction method of the present application.

(3) FIG. 2-3 describes the principle in the special stringer (40) structures which substitutes normal steel lattice girders (20). The semi-prefabricated stringers (40) are carried out as a composite construction comprising a) a steel arrangement (50), sufficient to transfer proper forces between a future concrete plate (10) and stringer (40), and b) a part (60) with a special composite mix of high strength concrete and reinforcement in order to obtain maximum compression forces, and c) a part with standard concrete (70), and d) an open part (80) prepared for post tensioning tendons (90) to secure necessary tension forces.

(4) Firstly, the steel arrangements (50, 100) are placed in in a formwork. The steel bars (100) must be placed in a specific way, in order to allow practical fabrication without difficult and expensive formwork, and also to enable flexibility in future onsite connection of top reinforcement (130). Only a specific execution where steel extrudes partly from the concrete part (60) fulfils these demands. One specific method is to place steel in longitudinal groves in the formwork, where only part of the steels cross section is embedded herein. Another specific method is placing a steel profile with one plane face directly above the formwork, so this face will be visible after concreting. Traditional ways of letting steel extent out from the concrete beam do not achieve this, as the steel extending outwards from the concrete is not continuously present along the beam. And this is required, as the position of the steel to be placed later on in the process is not known at this stage.

(5) Secondly, a steel arrangement (50), sufficient to transfer proper forces between a concrete plate (10) and stringer (40), is placed inside the formwork. The vertical part of the steel arrangement (50) which protrudes into an open part (80) can either be made as closed cages, or open upwards, thereby providing extra freedom throughout the following production processes.

(6) Thirdly, a layer (60) of approximately 20% of final stringer height is concreted around a special high strength steel core and using (ultra) high strength concrete, and leaving partly exposed steel bars (100) from the bottom arrangement prepared for future steel connections at slab top. The basic high strength core (60) will form the top of the stringer when turned and implemented in a semi-precast element. The core has increased compression strength of up to 8 time's normal concrete strength and can individually obtain the compression forces of the slab moment.

(7) Fourthly, if the first pouring of concrete (60) leaves space, standard concrete (70) is poured to reach the final pre-cast height (H) minus app 90% of the thickness of bottom plate (10) and so leaving an open space (80) inside the remaining steel arrangement (50) for later implementing of high strength steel as tendons (90). To this pouring can be used standard concrete as an option to save money, as high strength concrete is not needed in this section, but with the actual small volumes it is acceptable and maybe even preferable to concrete fully in strong concrete and save one operation.

(8) Openings or voids (110), perpendicular to lengthwise direction of stringer (40) structure, can be integrated in this part of the stringer (40). The preferably circular openings (110) can be incorporated in order to obtain weight saving and thereby ease for handling and to allow for installations and possibly on site crossing reinforcement. Further the openings will secure stronger integration between on-site concrete and stringer. Additional openings/penetrations can be implemented.

(9) After the concrete is hardened, the stringer (40) can be stored for later use.

(10) The system is practical and flexible as the stringers (40) can be made in a separate standard production and the concrete can achieve 100% strength while storing, which means that the stringers at any time and with immediate full concrete strength and applied with, but not limited to, relevant post-tension tendons (90), can be directly implemented in a semi-precast element bottom by simply being concreted together with the bottom plate (10). The execution can be done either in factory or next to the building site. After hardening, necessary post tension can be applied and the semi-prefab element is ready for use.

(11) FIG. 3 illustrates the optimal position of tendons. Tendons (90) can be placed either within the concrete (60, 70) in the stringers (40), or within a closed steel arrangement (50) protruding from the stringer (40), or between an open steel arrangement (50) protruding from the stringer (40) and a bottom reinforcement (30), where the design of the steel arrangement (50) is essential as it must allow for a proper transfer of forces between stringer (40) and the concrete bottom (10) of the element. The chosen version will depend on practical factors, but the most efficient is to place the tendons (90) as close to the bottom reinforcement (30) as possible and directly below the stringers (40) in order to optimize the effect. Vertical position of tendons can vary along the stringer for optimized effect of post-tensioning.

(12) FIGS. 4 and 5 show the fabrication of the semi-precast elements. Bottom reinforcement (30) is placed on spacers on a traditional formwork. Stringers (40) are then placed bottom side up with the high strength core (60) turning upwards and steel arrangement (50) for the tendons (90) turned downwards. The stringers can be placed either on spacers, or preferable directly on the bottom reinforcement (30). The tendons (90) are preferable straight but the end parts can be placed with a slight angle to ease the practical work, and increase the effect. Then, lightweight members (120) as, but not limited to, hollow spheres can be placed above the bottom reinforcement (30), in order to obtain maximum reduction of concrete. If lightweight members are placed at this stage, a thin mesh of top reinforcement (130) can be placed in order to fix and maintain the position of lightweight members. The top reinforcement (130) can be attached or welded to the steel (100) extruding from the stringer (40). Fixing or welding the top reinforcement (130) to the top of the stringers (40) is an effective mean for holding the lightweight members (120) in the prescribed position even during concreting to prevent floating due to uplift Next, a layer of concrete (10) is gently and skilfully distributed thus covering bottom reinforcement (30) and the open part of the steel arrangement (50) with tendons (90), extending downwards from the stringer (40) structure, thereby composing a semi-prefabricated element (140) structure shaped as a turned T, or a number of Ts. Alternatively, bottom reinforcement (30), tendons (90) and stringers (40), and if chosen also lightweight members (120) and top reinforcement (130), can be lowered into an already poured layer of concrete (10). The succession of procedure is flexible and can be adjusted to the circumstances. After hardening, the element (140) is ready for storing or direct use.

(13) Depending on needed strength, the elements (140) can be carried out with any combination of bottom reinforcement (30) and tendons (90). The element, comprising plate bottom (10) and stringers (40), is post-tensioned by applying tension stress in the tendons (90) already incorporated in the concrete. After hardening and post-tensioning, is obtained a semi-prefabricated element (140) with sufficient strength to act as self-carrying scaffolding for full concrete slab load at a span at least 30 times slab thickness.

(14) FIGS. 6 and 7 illustrates the effect of the high strength composite head. FIGS. 6 and 7 are an identity, where FIG. 7 shows the H-effect and actual execution if standard concrete profile should have been used, as the stringer core has 8 times normal strength. With the current design, a practical, extreme flexible and time-saving solution is obtained with extended space for implementing light materials saving 50% of the concrete.

(15) FIG. 8 shows the basic semi-prefabricated element (140) with filling of arbitrary light material (150) and/or light weight members (120) as hollow spheres. The light weight members can be arranged in layers if more practical. After placing the light weight material (150) the top reinforcement (130) can be installed, either on factory or on site, and fastened to the partly exposed steel rods (100) in the top of stringers (40).

(16) FIGS. 9 to 10 show cross sections of semi-prefabricated lightweight elements (140), equipped with lightweight members (120) placed in a geometrical cell structure between the stringers (40), and embedded in a final layer of concrete (160), thus obtaining a final concreted slab (170). If using lightweight members (120), these can be placed either before or after concreting the bottom (10) depending on the desired design, but preferable before. If using hollow volumes as spheres, with space for concrete between them, is obtained a homogeneous (geometric porous) concrete mass in the full slab thickness resulting in a light massive slab as full massive strength like a solid slab is maintained.

(17) Using maximum lightweight elements is essential in order to achieve long spans without temporary supports. The present invention constitutes the absolutely lightest biaxial floorand without loss of strength.

(18) Concreting can be done in one or more steps depending on slab thickness.

(19) FIG. 11 shows a longitudinal cut in a fully concreted semi-precast element/slab (170). The semi-prefabricated elements (140) can, before final concreting, be installed in the construction side by side, supported at their ends on any form of support, but preferably on a semi-prefabricated component (180) of same composition as semi-prefabricated element (140) acting as a supporting component, placed and spanning between permanent vertical structural supports as columns and/or walls.

(20) A part of the stringers (40) in the individual element (140) protrudes out from the full semi-prefabricated element (140) so this protruding part (190), can land on the bottom flange (200) of the supporting component (180), designed so the bottom surface of the elements (140) levels the bottom surface of the supporting component (180), thus creating a completely flat plate slab with uniform bottom level.

(21) These supporting components (180) are designed so bottom connection reinforcement bars (210) of sufficient length can be placed on the bottom (10) through opening in the stringer (40) of the supporting component (180) between two neighbouring elements (140).

(22) After placing connection reinforcement bars (220) at the top across the elements (140), the full configuration can be finally concreted and a fully biaxial lightweight homogeneous flat plate slab is obtained without the use of any temporary supports.

REFERENCE LIST FOR DRAWINGS

(23) 10. Flat concrete bottom 20. Steel girder 30. Bottom reinforcement 40. Semi-prefabricated stringer 50. Reinforcement arrangement 60. Zone with high strength composite concrete 70. Zone with standard concrete 80. Open volume 90. Tendons 100. Protruding steel 110. Voids in stringer 120. Lightweight filling members 130. Top reinforcement 140. Semi-prefabricated lightweight element 150. Arbitrary lightweight fill 160. Final concrete fill 170. Completed lightweight slab 180. Supporting component 190. Protruding part of stringer 200. Protruding bottom flange of supporting component 210. Connection reinforcement in bottom 220. Connection reinforcement in top