POST-TENSIONED CONCRETE SLAB WITH FIBRES

Abstract

A concrete slab (20) comprises conventional concrete and a combined reinforcement of both post-tension steel strands (22, 26) and fibres (29). The post-tension steel strands (22, 26): —have a diameter ranging from 5 mm to 20 mm, —have a tensile strength higher than 1700 MPa. The fibres (29) are present in a dosage ranging from 10 kg/m.sup.3 to 40 kg/m.sup.3 in case of steel fibres are in a dosage ranging from 1.5 kg/m.sup.3 to 9 kg/m.sup.3 in case of macro-synthetic fibres.

Claims

1-14. (canceled)

15. A concrete slab comprising conventional concrete and a combined reinforcement of both post-tension steel strands and fibres, said post-tension steel strands having a diameter ranging from 5 mm to 20 mm, having a tensile strength higher than 1700 MPa, said fibres being either steel fibres and being present in a dosage ranging from 10 kg/m.sup.3 to 40 kg/m.sup.3 or being macro-synthetic fibres and being present in a dosage ranging from 1.5 kg/m.sup.3 to 9.0 kg/m.sup.3.

16. The concrete slab according to claim 15, wherein said conventional concrete has a characteristic compressive cube strength of 30 N/mm.sup.2 or higher.

17. The concrete slab according to claim 15, wherein said fibres are steel fibres and/or wherein the steel fibres are present in a dosage ranging from 15 kg/m.sup.3 to 35 kg/m.sup.3, preferably from 20 kg/m.sup.3 to 30 kg/m.sup.3, preferably from 10 kg/m.sup.3 to <30 kg/m.sup.3 or further preferred from 10 kg/m.sup.3 to 27 kg/m.sup.3 and/or wherein the amount of steel fibers used is below or equal to 1.2 times, preferably 1.0 time, further preferred between >0 and 1.1 times, the amount of steel recommended and used for the steel bars or rebars to be replaced and/or the amount of steel fibers is below or equal 1.2 times, preferably 1 time, further preferred between >0 and 1.1 times, the amount recommend as rebar or steel bar replacement.

18. The concrete slab according to claim 17, wherein said steel fibres comprise a straight middle portion that have a tensile strength above 1400 MPa, preferably higher than 1500 MPa, preferably higher than 1600 MPa, preferably higher than 1700 MPa, further preferred higher than 2000 MPa, even further preferred higher than 2200 MPa, preferably between 1400 MPa and 3500 MPa.

19. The concrete slab according to claim 17, wherein said steel fibres comprise anchorage ends at both ends, said anchorage ends each comprise three or four bent sections.

20. The concrete slab according to claim 15, said concrete slab having a maximum crack width of 0.5 mm after hardening.

21. The concrete slab according to claim 15, said concrete slab having a length L greater than 100 m, said concrete slab having joints, said joints having a distance between neighbouring joints higher than 40 m.

22. The concrete slab according to claim 15, wherein said concrete slab is a concrete slab on ground and/or wherein said concrete slab has a uniform average density and/or wherein said concrete slab is cast in one day and/or in one go and/or be fully casted and/or wherein said concrete slab contains only the fibres and the post-tension steel strands as reinforcement elements and/or wherein conventional concrete has normal shrinkage and/or does not encompass low shrinkage concrete and/or wherein said concrete slab is free of a vapor barrier.

23. The concrete slab according to claim 15, wherein said post-tension steel strands have a distance between neighbouring post-tension steel strands or between neighbouring bundles of post-tension steel strands higher than 0.80 m and/or wherein tension is applied to the post-tension steel strands only after the concrete has been cast and the post-tension steel strands remain in place also once the concrete is completely cured/hardened and/or wherein the post-tension steel strands have a tensile strength higher 1800 MPa, preferably higher than 1900 MPa, preferably higher than 2000 MPa, further preferred between 1800 MPa and 4000 MPa and/or wherein the post-tension steel strands have a maximum breaking load of higher than 190 kN, preferably higher than 195 kN, preferably higher than 200 kN, preferably higher than 220 kN, further preferred between 195 kN and 350 kN and/or wherein the post-tension steel strands comprise anchor systems and/or ducts or sheathing.

24. The concrete slab according to claim 15, said concrete slab having a width b and a thickness h, said post-tension steel strands exercising a compression force that is 0% to 200% greater than a force according to following formula
μ.sub.o×γ.sub.c×b×h×L/2 where μ.sub.o is the coefficient of friction between the concrete slab and a subbase; and where γc is the specific weight of concrete and/or wherein thickness h of a slab according to the invention may thereby preferably be between 4 cm and 75 cm, preferably 5 cm and 65 cm, further preferred 10 cm and 55 cm.

25. A construction comprising the concrete slab according to claim 22, said construction further comprising a plastic slip sheet between said concrete slab and said ground.

26. A construction comprising the concrete slab according to claim 22, a plastic slip sheet not being present between said concrete slab and said ground.

27. The concrete slab according to claim 15, wherein said concrete slab is a slab on concrete piles or gravel columns.

28. The concrete slab according to claim 27, said concrete piles or gravel columns being arranged in a regular rectangular pattern or quadrilateral shape where each set of four concrete piles or gravel columns forms a rectangle, said concrete slab comprising straight zones connecting in the two directions, i.e. lengthwise and widthwise, the shortest distance between those areas of the concrete slab above the concrete piles or gravel columns, post-tension steel strands being present in said straight zones in bundles, the distance between neighbouring post-tension steel strands within a bundle being smaller than 0.80 m.

29. The concrete slab according to claim 28, post-tension steel strands being present outside said straight zones, the shortest distance between post-tension steel strands outside said straight zones and the post-tension steel strands in said straight zones being larger than 0.80 m.

30. A construction comprising a concrete slab according to claim 23, said construction further comprising a plastic slip sheet between said concrete slab and said ground.

31. A construction comprising a concrete slab according to claim 24, said construction further comprising a plastic slip sheet between said concrete slab and said ground.

32. A construction comprising a concrete slab according to claim 23, a plastic slip sheet not being present between said concrete slab and said ground.

33. A construction comprising a concrete slab according to claim 24, a plastic slip sheet not being present between said concrete slab and said ground.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0042] FIGS. 1a, 1 b, 1c, 1d, 1e, 1f, 1g, 1h, 1i all schematically represent various loading configurations of a concrete slab;

[0043] FIG. 2 is a schematic presentation of a concrete slab according to the invention;

[0044] FIG. 3 is a cross-section of a post-tension steel strand to be used in the present invention;

[0045] FIG. 4 shows a steel fibre that can be used in the present invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Explanation of the Principle Behind the Invention

[0046] Concrete is a very brittle material that is hardly resistant to tensile tensions, the purpose is to avoid or at least to reduce the presence of tensile stresses. In FIGS. 1a, 1 b, 1c, 1d, 1e, 1f, 1g, 1h, 1i the ⊕ symbol, a plus sign in a circle, points to compressive stresses, while the ⊖ symbol, a minus sign in a circle, points to tensile stresses.

[0047] FIG. 1a shows a concrete slab 10 reinforced by means of a post-tension steel strand 12 that is located in the middle of the slab 10. No external loads are present here. The post-tension steel strand 12 creates compressive stresses over the whole thickness of the slab 10.

[0048] FIG. 1b relates to a situation where the slab 10 has no post-tension steel strand but where a load F represented by arrow 14 is exercised on the slab 10. The load F creates a bending moment M, represented by arrow 16. As a result of bending moment M, compressive stresses are present at the top of the slab 10 and tensile stresses at the bottom of the slab 10.

[0049] FIG. 1c shows the situation where a load F is exercised on a slab 10 that is reinforced by means of a post-tension steel strand 12. As schematically shown in FIG. 1c, the tensile stresses at the bottom of slab 10 are compensated by the action of the post-tension steel strand 12. However, in case the load F is a little bit higher, tensile stresses will be present at the bottom of slab 10 and cracks may originate.

[0050] FIG. 1d shows a concrete slab 10 reinforced by means of a post-tension steel strand 12 that is located in the upper part of the slab 10. No external loads are present here. The post-tension steel strand 12 creates compressive stresses in the upper part of slab 10 and tensile stresses in the lower part of slab 10.

[0051] FIG. 1e relates to a situation where the slab 10 has no post-tension steel strand but where a load F, represented by arrow 14, is exercised on the slab 10. The load F creates a bending moment M, represented by arrow 16. As a result of bending moment M, compressive stresses are present at the top of the slab 10 and tensile stresses at the bottom of the slab 10.

[0052] FIG. 1f shows the situation where a load F is exercised on a slab 10 that is reinforced by means of the post-tension steel strand 12 of FIG. 1d. As schematically shown in FIG. 1f, the tensile stresses at the bottom of slab 10 are not compensated by the action of the post-tension steel strand 12, on the contrary.

[0053] FIG. 1g shows a concrete slab 10 reinforced by means of a post-tension steel strand 12 that is located in the lower part of the slab 10. No external loads are present here. The post-tension steel strand 12 creates compressive stresses in the lower part of slab 10 and tensile stresses in the upper part of slab 10.

[0054] FIG. 1h relates to a situation where the slab 10 has no post-tension steel strand but where a load F, represented by arrow 14, is exercised on the slab 10. The load F creates a bending moment M, represented by arrow 16. As a result of bending moment M, compressive stresses are present at the top of the slab 10 and tensile stresses at the bottom of the slab 10.

[0055] FIG. 1i shows the situation where a load F is exercised on a slab 10 that is reinforced by means of the post-tension steel strand 12 of FIG. 1g. As a result of bending moment M, the tensile stresses present at the top of the slab 10 are compensated and the compressive stresses present at the bottom of slab 10 are compensated as well.

[0056] Although it is best for a concrete slab on ground to position the post-tension steel strand in the middle of the slab, no position can guarantee absence of tensile stresses. Within the context of the present invention, post-tension steel strands are therefore designed to take up and compensate the tensile stresses that may originate during hardening and shrinkage of a concrete in addition to applied loads. The post-tension steel strands are of a sufficiently high tensile strength, i.e. above 1700 MPa or even above 1800 MPa, so that conventional concrete can be used and ingredients to compensate shrinkage can be avoided.

[0057] The fibres are mixed in the concrete as homogeneously as possible so that they are present over the whole volume and able to take tensile stresses caused by various loads.

[0058] In a first embodiment of the invention, the concrete is poured on a slip sheet that is put on a substantially flat or flattened underground. Alternatively, the slip sheet can be avoided and the concrete is poured directly on the subbase.

[0059] In a second embodiment of the invention, a concrete slab is formed on piles or columns. A slip sheet may be or may not be present between the ground or subbase, the piles and the slab.

[0060] FIG. 2 schematically illustrates a concrete slab 20 on ground according to the invention. The slab 20 has a width W and a length L. A first series of post-tension steel strands 22 bridge the width W and are anchored at one side with fixed end anchors 23 and at the other side with stressing end anchors 24. A second series of post-tension steel strands 26 bridge the length L and are anchored at one side with fixed end anchors 27 and at the other side with stressing end anchors 28. Alternatively, stressing may also occur on both sides, i.e. stressing end anchors are present at both sides.

[0061] As the length L is greater than the width W, the post-tension steel strands 26 are positioned closer to each other than the post-tension steel strands 22, but the distance between neighbouring post-tension steel strands 26 can be kept greater than 0.80 m independent of the length L.

[0062] Steel fibres 29 are spread over the whole volume of the slab 20.

Post-Tension Steel Strand

[0063] FIG. 3 shows a cross-section of a typical post-tension steel strand 30.

[0064] Post-tension steel strand 30 has a 1+6 construction with a core steel wire 32 and six layer steel wires 34 twisted around the core steel wire 32. In the embodiment of FIG. 3, the post-tension steel strand 30 is in a non-compacted form.

[0065] In an alternative preferable embodiment, the post-tension steel strand may be in a compacted form. In this compacted form, the six layer steel wires no longer have a circular cross-section but a cross-section in the form of a trapezium with rounded edges. A compacted post-tension steel strand has less voids and more steel per cross-sectional area.

[0066] As mentioned, the post-tension steel strand has a high yield point, i.e. the yield force at 0.1% elongation is high. The ratio yield force F.sub.p0.1 to breaking force F.sub.m is higher than 75%, preferably higher than 80%, e.g. higher than 85%.

[0067] A typical steel composition of a post-tension steel strand is a minimum carbon content of 0.65%, a manganese content ranging from 0.20% to 0.80%, a silicon content ranging from 0.10% to 0.40%, a maximum sulfur content of 0.03%, a maximum phosphorus content of 0.30%, the remainder being iron, all percentages being percentages by weight. Most preferably, the carbon content is higher than 0.75%, e.g. higher than 0.80%. Other elements as copper or chromium may be present in amounts not greater than 0.40%.

[0068] All steel wires 32, 34 may be provided with a metallic coating 36, such as zinc or a zinc aluminium alloy. A zinc aluminum coating has a better overall corrosion resistance than zinc. In contrast with zinc, the zinc aluminum coating is temperature resistant. Still in contrast with zinc, there is no flaking with the zinc aluminum alloy when exposed to high temperatures.

[0069] A zinc aluminum coating may have an aluminum content ranging from 2 per cent by weight to 12 per cent by weight, e.g. ranging from 3% to 11%.

[0070] A preferable composition lies around the eutectoid position: Al about 5 per cent. The zinc alloy coating may further have a wetting agent such as lanthanum or cerium in an amount less than 0.1 per cent of the zinc alloy. The remainder of the coating is zinc and unavoidable impurities.

[0071] Another preferable composition contains about 10% aluminum. This increased amount of aluminum provides a better corrosion protection then the eutectoid composition with about 5% of aluminum.

[0072] Other elements such as silicon (Si) and magnesium (Mg) may be added to the zinc aluminum coating. With a view to optimizing the corrosion resistance, a particular good alloy comprises 2% to 10% aluminum and 0.2% to 3,0% magnesium, the remainder being zinc. An example is 5% Al, 0.5% Mg and the rest being Zn.

[0073] An example of a post-tension steel strand is as follows: [0074] diameter 15.2 mm; [0075] steel section 166 mm.sup.2; [0076] E-modulus: 196000 MPa; [0077] breaking load F.sub.m: 338000 N; [0078] yield force F.sub.p0.1: 299021 N; [0079] tensile strength R.sub.m 2033 MPa.

Steel Fibre

[0080] Steel fibres adapted to be used in the present invention typically have a middle portion with a diameter D ranging from 0.30 mm to 1.30 mm, e.g. ranging from 0.50 mm to 1.1 mm. The steel fibres have a length so that the length-to-diameter ratio /D ranges from 40 to 100.

[0081] Preferably the steel fibres have ends to improve the anchorage in concrete. These ends may be in the form of bent sections, flattenings, undulations or thickened parts. Most preferably, the ends are in the form of three or more bent sections.

[0082] FIG. 4 illustrates a preferable embodiment of a steel fibre 40. The steel fibre 40 has a straight middle portion 42. At one end of the middle portion 42, there are three bent sections 44, 46, 48. At the other end of the middle portion 42 there are also three bent sections 44′, 46′ and 48′. Bent sections 44, 44′ make an angle a with respect to a line forming an extension to the middle portion 42. Bent sections 46, 46′ make an angle b with respect to a line forming an extension to bent sections 44, 44′. Bent sections 48, 48′ make an angle c with respect to bent sections 46, 46′.

[0083] The length of the steel fibre 40 may range between 50 mm and 75 mm and is typically 60 mm. The diameter of the steel fibre may range between 0.80 mm and 1.20 mm. Typical values are 0.90 mm or 1.05 mm.

[0084] The length of the bent sections 44, 44′, 46, 46′, 48 and 48′ may range between 2.0 mm and 5.0 mm. Typical values are 3.2 mm, 3.4 mm or 3.7 mm.

[0085] The angles a, b and c may range between 20° and 50°, e.g. between 24° and 47°.

[0086] The steel fibres may or may not be provided with a corrosion resistant coating such as zinc or a zinc aluminium alloy.

[0087] In a particular preferable embodiment of the steel fibre, there are four bent sections at each end of the middle portion.

[0088] In another particular preferable embodiment of the steel fibre, the middle portio0n has an elongation at maximum load higher than 4%, e.g. higher than 5%, e.g. higher than 5.5%. Steel fibres with such a high elongation at maximum load may be used in structural applications such as floors on piles, elevated systems and structural wall systems.

Macro-Synthetic Fibre

[0089] Examples of macro-synthetic fibres are fibres based upon polyolefins like polypropylene or polyethylene or based upon other thermoplastics.

Example of Replacing Steel Bars

[0090] In an embodiment of the invention, a slab according to the invention preferably may not comprise any further reinforcement or reinforcement elements besides the fibers and the post-tension steel strands, especially no steel bars.

[0091] For a slab with a thickness of 150 mm and steel bars with a diameter of 6 mm and a spacing 150 mm at the top as well as steelbars with a diameter of 6 mm and a spacing 150 mm at the bottom with a steel cover of 15% this represents 45 kg/m.sup.3 steel and a concrete cover of 30 mm (top and bottom) to achieve a resisting bending moment M.sub.Rd=11.44 (positive and negative moment capacity).

[0092] On the other hand, according to the present invention, an equivalent resisting bending moment M.sub.Rd=11.54 can be achieved with only 24 kg/m.sup.3 of steel fibre: DRAMIX® 4D 65/60/BG, i.e. a steel fibre with three bent sections according to FIG. 4 for the same slab with the same concrete. This means that according to the invention the amount or level of steel can be significantly reduced by using steel fibers compared to the amount of steel required and recommended using steel bars. Furthermore, the amount or level of steel fibers according to the invention may preferably be for example below or equal to 1.2 times, preferably 1 time, the amount or level recommended and determined as rebar replacement, especially for example at equivalent performance, preferably in terms of resisting bending moment (positive and negative moment capacity). Accordingly, the amount of steel fibers used according to the present invention may be for example preferably below or equal to 1.2 times, preferably 1.0 time, further preferred between >0 and 1.1 times, the amount or level of steel recommended and used for the steel bars or rebars to be replaced and/or the amount or level of steel fibers may be below or equal 1.2 times, preferably 1 time, further preferred between >0 and 1.1 times, the amount or level recommend as rebar replacement.

Example of a Slab on Ground

[0093] slab thickness h: 150 mm [0094] slab length L: 100 m [0095] slab width b: 1 m (to calculate forces of post-tension steel strands in one direction) [0096] type of steel fibre: DRAMIX® 4D 65/60/BG, i.e. a steel fibre with three bent sections according to FIG. 4 [0097] dosage of steel fibres: 25 kg/m.sup.3 [0098] γ.sub.c: 23.560.0 N/m.sup.3 [0099] μ.sub.o: 0.5 [0100] minimum force exercised by post-tension steel strands: 88.350.0 N/m [0101] tensile strength R.sub.m post-tension steel strand: 1860 MPa [0102] diameter post-tension steel strand: 15 mm [0103] spacing between two neighbouring post-tension steel strand be higher than 2.0 m, even higher than 2.5 m [0104] spacing between two neighbouring joints: 90 m, even larger than 120 m [0105] maximum crack width: 0.5 mm.

EXAMPLES OF A SLAB ON CONCRETE PILES

First Example

[0106] distance between neighbouring joints: 100 m [0107] thickness of concrete slab: 0.229 m [0108] upper load: 50 kN/m.sup.2 [0109] distance between neighbouring concrete piles: 4 m×4 m [0110] distance between post-tension steel strands within the straight zones: 0.25 m [0111] not necessary that there are post-tension steel strands outside the straight zones, but in case there are post-tension steel strands, the distance between post-tension steel strands outside the straight zones is greater than 0.90 m, preferably greater than 1.60 m

Second Example

[0112] distance between neighbouring joints: 100 m [0113] thickness of concrete slab: 0.495 m [0114] upper load: 50 kN/m.sup.2 [0115] distance between neighbouring concrete piles: 6 m×6 m [0116] distance between post-tension steel strands within the straight zones: 0.15 m [0117] not necessary that there are post-tension steel strands outside the straight zones, but in case there are post-tension steel strands, the distance between post-tension steel strands outside the straight zones is greater than 0.80 m, preferably greater than 1.50 m