WO2023052434A1

WO2023052434A1 - Fiber reinforced post-tensioned concrete slab with openings

Info

Publication number: WO2023052434A1
Application number: PCT/EP2022/076997
Authority: WO
Inventors: Hendrik Thooft; Carol HAYEK
Original assignee: Nv Bekaert Sa; Ccl Stressing International Ltd
Priority date: 2021-09-28
Filing date: 2022-09-28
Publication date: 2023-04-06
Also published as: AU2022354567A1

Abstract

The present invention concerns a concrete slab, the slab comprising conventional concrete and a combined reinforcement of both post-tension steel strands and fibers, said post-tension steel strands - having a diameter ranging from 5 mm to 20 mm,- having a tensile strength higher than 1700 MPa,said fibers being either steel fibers and being present in a dosage ranging from 5 kg/m³ to 90 kg/m³ or being macro-synthetic fibers and being present in a dosage ranging from 1.5 kg/m³ to 9.0 kg/m³, whereby the slab has upper as well as lower surfaces and sides, whereby further each strand or a group of strands has two ends, whereby further each strand or a group of strands is provided with end anchors on at least one of its ends, whereby the slab has at least one opening in its upper surface so that said anchor is accessible from said upper surface through said opening.

Description

Title: FIBER REINFORCED POST-TENSIONED CONCRETE SLAB WITH OPENINGS

Description

Technical Field

[0001] The invention relates to a concrete slab, the slab comprising conventional concrete and a combined reinforcement of both post-tension steel strands and fibers, whereby the slab has upper as well as lower surfaces and sides, whereby further each strand or a group of strands has two ends, whereby further each strand or a group of strand is provided with end anchors on at least one of its ends, whereby the slab has at least one opening in its upper surface so that said anchor is accessible from said upper surface through said opening.

Background Art

[0002] Post-tensioned concrete is a variant of pre-stressed concrete where the tendons, i.e. the post tension steel strands, are tensioned after the surrounding concrete structure has been cast and hardened. It is a practice known in the field of civil engineering since the middle of the twentieth century.

[0003] Steel fiber reinforced concrete is concrete where the reinforcement is provided by short pieces of steel wire that are spread in the concrete. US-A-1 ,633,219 disclosed the reinforcement of concrete pipes by means of pieces of steel wire. Other prior art publications US-A-3,429,094, US-A- 3,500,728 and US-A-3,808,085 reflect initial work done by the Batelle Development Corporation. The steel fibers were further improved and industrialized by NV Bekaert SA, amongst others by providing anchorage ends at both ends of the pieces of steel wire, see US-A-3,900,667. Another relevant improvement was disclosed in US-A-4,284,667 and related to the introduction of glued steel fibers in order to mitigate problems of mixability in concrete. Flattening the bent anchorage ends of steel fibers, as disclosed in EP-B1 -0 851 957, increased the anchorage of the steel fibers in concrete. The supply of steel fibers in a chain package was disclosed in EP-B1 -1 383 634.

[0004] Both reinforcement techniques, post-tensioned concrete and fiber reinforced concrete such as steel fiber reinforced concrete not only exist as such but also in combination. The purpose was to combine the advantages of both reinforcement types to obtain an efficient and reliable reinforced concrete slab.

[0005] Prior art concrete slabs with combined reinforcement of both post-tension strands and fibers suffer especially for example from an overdesign or from a complex design. In an attempt to stay on the very safe side and to meet the specifications, the dosage of steel fibers is often so high that problems such as ball forming occur during mixing of the steel fibers in the non-cured concrete, despite the existence of prior art solutions. Alternatively, or in addition to this, the distance between two neighbouring post-tension strands or between two neighbouring bundles of post-tension strands cannot exceed certain maximum spacing, causing a lot of labour when installing the post-tension strands, attaching end anchors and applying tension. In yet other prior art embodiments the composition of the concrete is such that shrinkage during curing is limited, i.e. for example a low shrinkage concrete or a shrinkage compensating concrete composition may be selected.

[0006] An example of a complex design of a concrete slab with reinforcement by both post-tension steel strands and steel fibers is disclosed in NZ-A- 220 693. This prior art concrete slab has an under and upper skin layer with steel fibers with a core layer in-between with post-tension tendons.

[0007] Moreover, the corresponding slab designs of the prior art requires a long narrow side strip at the edge of the slab or even at the edge of the floor, which is required to access the post tensioning strands and complete post tensioning, especially the stressing operation, before filling the side strip by casting of concrete or grout. A slab may thereby be part of a floor, which may in turn comprise one or preferably more slabs. This however may create problems with the timing of casting this side strip and/or the time delay required to do so and/or the equipment required that may damage the rest of the floor. There may also be problems regarding easy access to the strands and/or the side strip being a different colour and/or being hard to polish. Another problem with having a long narrow side strip at the edge of the slab or even at the edge of the floor to access the post tensioning strands and complete post tensioning, especially the stressing operation, may also be for example that this side strip by its nature may be, once casted, prone to the formation of a lot of cracks, especially perpendicular to the long side. This may lead to structural problems as well as be detrimental to the visual appearance. In addition, having a long narrow side strip at the edge of the slab or even at the edge of the floor that needs to be left open until the last stressing operation is completed (sometimes for example about 28 days) can also lead to restrictions with respect to bringing in or placing equipment onto the construction site. Finally, the same open long narrow side strip at the edge of the slab or even at the edge of the floor may be a security risk for construction workers and/or forklift operators.

[0008] The present invention may thereby especially allow for easier and/or faster installation. The present invention may also allow for a nicer or more homogeneous surface finish, especially polish or colour. The present invention may also contribute to reduce the risk of damage during installation or construction. The present invention may further eliminate the need for a side strip, whereby openings of the present invention may for example even be hidden under fixtures. The present invention may further contribute to increase the structural capacity for flexure, deflection, shear, punching shear, structural integrity, temperature resistance and/or resistance to shrinkage. Furthermore, the present invention advantageously allows for example that post tensioning strands can remain unstressed, even without partial stressing, without the need for shrinkage reinforcement.

Disclosure of Invention

[0009] It is a general aspect of the invention to avoid the disadvantages of the prior art.

[0010] It is a further general aspect of the invention to avoid overdesign.

[0011 ] It is another aspect of the invention to provide a combination reinforcement of both post-tension strands and fibers to reinforce concrete slabs that is particularly quick and/or easy to install and/or extends span, especially for example the length or width, of the slab efficiently and effectively. The present invention may also allow for a nicer or more homogeneous surface finish, especially polish or color. The present invention may also contribute to reduce the risk of damage during installation or construction. A quick installation may thereby be facilitated as a whole edge of a slab or even of a floor may not be required to be kept free to access the post tensioning strands. There is thus now need anymore to further cast long and narrow strips at the edge(s) once the post tensioning, especially the stressing operation, is completed to finish the slab and/or floor. Once the slab have been casted and post tensioning has been completed, openings can indeed be very quickly filled mostly at any convenient moment to finish the slab and/or floor. In contrast, casting a long narrow side strips may take longer and timing to do so would also be more critical. In addition, the present invention may also make for example installation easier, especially as it may be easier to access post tensioning strands through the opening then via a narrow side strips, especially for example at a strip next to a wall. Indeed, the dimension and/or form of the opening may contribute to easy access. The present invention may also contribute to a nicer or more homogeneous surface finish, especially colour. Indeed, any differences surface appearance for example by crack formation or by differences in colour over the slab may be limited to only the areas of the openings instead to of a whole side strip. It may also be easier to obtain a nicer or more homogeneous polish of the surface with the openings, especially since a narrow side strip at the edge of a slab and/or floor (and thus likely near a wall) may be particularly hard to polish. The present invention may also contribute to reduce the risk of damage during installation or construction, especially as the light equipment needed to fill the openings is unlikely to cause damage. In addition, the present invention may also contribute to make it easier to bring in or place equipment on the construction site. Finally, safety for construction personnel may be improved.

[0012] It is still another aspect of the invention to provide a combination reinforcement of both post-tension strands and fibers for conventional concrete slabs.

[0013] According to the invention, there is provided a concrete slab, the slab comprising conventional concrete and a combined reinforcement of both post-tension steel strands and fibers, said post-tension steel strands

- having a diameter ranging from 5 mm to 20 mm,

- having a tensile strength higher than 1700 MPa, said fibers being either steel fibers and being present in a dosage ranging from 5 kg/m³ to 90 kg/m³ or being macro-synthetic fibers and being present in a dosage ranging from 1.5 kg/m³ to 9.0 kg/m³, whereby the slab has upper as well as lower surfaces and sides, whereby further each strand or a group of strands has two ends, whereby further each strand or a group of strands is provided with an end anchor on at least one of its ends, whereby the slab has at least one opening in its upper surface so that said anchor is accessible from said upper surface through said opening.

[0014] An anchor end in the sense of the present invention may thereby be preferably for example a live anchor, especially a live anchor end, where tension maybe be applied to the strand and/or tension on the strand may be adjusted. In an embodiment of the invention, both ends of a strand are foreseen with live anchor ends. Alternatively, one end of a strand may be foreseen with a live anchor end, while the other end of the strand is a dead anchor end. A dead anchor end may thereby just be an end where the strand is fixed and/or attached. A concrete slab according to the invention thus comprises at least one opening arranged in a way to allow access to an anchor of at least one post-tensioning strand or group of strands. The concrete slabs according to the invention are post-tensioned concrete slabs, which are particularly large and/or long and/or for example suitable for jointless floors, so that they can preferably not be made, or at least not effectively, by precast and/or pre-tensioning techniques.

[0015] In an embodiment of the present invention, each opening may correspond for example to between 0.005 to 10 %, preferably 0.01 to 5 %, further preferred 0.05 to 2.5 %, even further preferred 0.1 to 1.5 %, further preferred 0.2 to < 1.5 %, further preferred 0.25 to 1 .25 % of the volume of the slab and/or the openings allow to apply tension to the post-tension steel strand and/or release tension applied to the post-tension steel stands.

[0016] In an embodiment of the present invention, the outline of an opening or of each opening may be a polygon, preferably for example a polygon comprising angles of 100° or less. The outline of an opening or of each opening may be especially for example a rectangle or square. Outline in the sense of the present invention may thereby preferably refer to an outline in a top down view. In such, case some reinforcement through for example rebars or steel mesh may be foreseen, especially at the corners/angles to contribute to limit possible crack formation or propagation. In an embodiment of the present invention, the outline of an opening or of each opening may be a polygon, especially a polygon comprising only 2 angles being < 90°, a polygon comprising only 2 angles being < 90° and arranged as angles at or closest to the outside border of the concrete slab, a polygon comprising only 2 angles being < 90° and arranged as the angles that are the farthest away from the center of the upper surface of the slab, further preferred a polygon comprising only 2 angles being < 90° and at least 2 angles being > 90°, further preferred a polygon comprising only 2 angles being < 90° and at least 4 angles being > 90°. Such outlines, may especially contribute to reduce and/or limit possible tensions in the angles of the opening, especially so that this may for example further contribute to avoiding reinforcement, especially for example by rebars, at or near the comers. This may be particularly important for post-tensioned slabs according to the invention, while crack formation is less likely for precast elements due to the potential mitigating effect of the mold and/or pre-tensioning against crack formation, especially for example early on. This may further contribute especially to improve the homogeneity of the surface finish.

[0017] In an embodiment of the present invention, the outline of an opening or of each opening maybe an ellipse, especially for example a circle or an oval. In an embodiment of the present invention, the outline of an opening or of each opening may be partially elliptic, especially semi-circular (in the form of a half circle) or partially oval. Outline in the sense of the present invention may thereby preferably refer to an outline in a top down view. Such outlines, may especially contribute to reduce and/or limit possible tensions in the angles of the opening, especially so that this may for example further contribute to avoiding reinforcement, especially for example by rebars, at or near the comers. This may again be particularly important for post-tensioned slabs according to the invention, while crack formation is less likely for precast elements due to the potential mitigating effect of the mold and/or pre-tensioning against crack formation, especially for example early on. This may further contribute especially to improve the homogeneity of the surface finish.

[0018] In an embodiment of the present invention, the depth of the opening(s) may for example especially correspond to at least 50 %, preferably at least 55 %, further preferred at least 60 % of the thickness of the slab or the depth of the opening(s) may for example correspond to the whole thickness of the slab and/or the opening(s) may for example have a width/length ratio < 1 and/or a length between 2 cm and 100 cm, preferably between 5 cm and 50 cm, preferably between 6 cm and 20 cm..

[0019] In an embodiment of the present invention, the opening(s) may be filled with grout or concrete, especially conventional concrete.

[0020] In an embodiment of the present invention, the length over width ratio of the slab may be for example between > 1 .5 and 35, preferably between > 2.0 and 30, further preferred > 2.5 and 25. The present invention may thereby limit the need for reinforcement and/or limit the crack formation and/or crack propagation, especially for particularly long slabs.

[0021] The tendons or post-tension steel strands may have a diameter ranging from 5 mm to 20 mm, e.g. from 6 mm to 20 mm, e.g. from 6.5 mm to 18.0 mm. The post-tension steel strands may especially for example have a tensile strength between 1700 MPa and 3500 MPa, preferably between higher than 1700 MPa and 3000 MPa, further preferred higher than 1800 MPa, even further preferred higher than 1900 MPa or higher than 2000 MPa.

[0022] The tendons or post-tension steel strands may be bonded or unbonded. In addition, the steel strands may preferably for example be present in bundles.

[0023] Particularly with a view to be used as post-tension steel strand, the steel strand preferably has a low relaxation behaviour, i.e. a high yield point at 0,1 % elongation. The yield point at 0,1 % can be considered as the maximum elastic limit. Below the yield point, the post-tension strand will remain in elastic mode. Above the yield point, the post-tension strand may start to elongate in plastic mode, i.e. an elongation that is not reversible. Preferably, the ratio of the yield strength R_po,i to the tensile strength R_m is higher than 0,75. [0024] Low relaxation post-tension steel strands may have relaxation losses of not more than 2.5 %, preferably between > 0 and 2.0%, when initially loaded to 70 % of specified minimum breaking strength or not more than 3.5 %, preferably between > 0 and 3.0%, when loaded to 80 % of specified minimum breaking strength of the post-tension steel strand after 1000 hours

[0025] The fibers can be steel fibers and are present in a dosage ranging from 5 kg/m³ to 90 kg/m³, preferably whereby steel fibers are present in the slab in a dosage ranging from 7 kg/m³ to 75 kg/m³, preferably from > 7 kg/m³ to < 65 kg/m³, preferably from > 10 kg/m³ to 60 kg/m³, preferably 15 kg/m³ to 50 kg/m³, further preferred 20 kg/m³ to 45 kg/m³ or alternatively > 45 kg/m³ to 60 or < 65 kg/m³, further preferred from 15 kg/m³ to 40 kg/m³,, further preferred from > 20 kg/m³ to < 40 kg/m³, preferably from 15 kg/m³ to 35 kg/m³, preferably from 20 kg/m³ to 30 kg/m³ or from 10 kg/m³ to < 30 kg/m³ or further preferred from 10 kg/m³ to 27 kg/m³. In an embodiment, 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 equall ,2 times, preferably 1 time, further preferred between > 0 and 1 ,1 times, the amount or level recommend as rebar or steel bar replacement. Higher dosages of steel fibers may therefor example contribute to increased fatigue resistance and/or to increase load cycles, especially at high stresses. Nonetheless, in the present invention lower dosages may be perfectly suitable or even particularly preferred for structural applications, especially for example as homogeneity of the distribution of the fibers is improved and/or likelihood of fiber ball formation (i.e. by fiber entanglement) can be reduced for lower dosages. Moreover, such lower dosages can thus limit the risk of defects, especially surface defects (such as from fiber balls or entangled fibers, while the fibers can further limit and/or delay crack formation effectively. [0026] The fibers can be other reinforcing fibers and are present in a dosage ranging from 1 .5 kg/m³ to 9.0 kg/m³, e.g. from 2.5 kg/m³ to 7.0 kg/m³, e.g. from 3.5 kg/m³ to 5.0 kg/m³.

[0027] The fibers are present in all parts of the concrete slab, i.e. the concrete slab is preferably a monolithic slab and the fibers are substantially homogeneously or homogeneously distributed in the concrete slab. Substantially homogeneously may thereby mean for example except for a very thin (preferably below 10 mm, further preferred below 6 mm) upper skin layer that is applied to provide a flat and wear resistant surface to the slab and to avoid fibers from protruding. In an embodiment, the slab may preferably be cast in one or multiple steps, preferably in one step.

[0028] Dosages of fibers of 5.0 kg/m³ to 40 kg/m³ in case of steel fibers and 1 .5 kg/m³ to 9.0kg/m³ in case of other reinforcing fibers are low to moderate in comparison with prior art dosages of more than 40 kg/m³ or more than 9 kg/m³. Such low to moderate dosages may for example further allow integrating the fibers in a more homogeneous way in the concrete and facilitate the mixing of the fibers in the concrete.

[0029] Conventional concrete may thereby preferably have a characteristic compressive cube strength or comparable cylinder strength 25 N/mm² or higher, preferably 28 N/mm² or higher, further preferred 30 N/mm² or higher . More preferably, conventional concrete has a strength equal to or higher than the strength of concrete of the C20/25 strength classes as defined in EN206 or equivalent national code requirements and smaller than or equal to the strength of concrete of the C50/60 strength classes as defined in EN206. These types of concrete are widely available and avoid adding ingredients that reduce the shrinkage during hardening. For the avoidance of doubt, self-compacting concrete is considered as conventional concrete. In a preferred embodiment, the slab does not contain any further reinforcement elements, such as for example rebars or steel nets or steel mesh beside steel fibers and post-tensioning steel strands within the body of the slab, especially there may no rebars neither at the top nor at the bottom within the body of the slab. However, dowels that may be provided or foreseen at the end of the slab and/or reinforcement that may be provided or foreseen at the end anchors of the post-tension steel strands may preferably not be considered, in the sense of the present invention, not considered further reinforcement elements within the body of the slab in the sense of the present invention,.

[0030] In a preferable embodiment of the invention, the fibers are steel fibers and have a straight middle portion and anchorage ends at both ends.

[0031] Most preferably the tensile strength of the middle portion is between 1400 MPa and 3500 MPa, preferably above 1400 MPa, preferably above 1500 MPa, preferably above 1700 MPa, further preferred above 1900 MPa, even further preferred above 2000 MPa.

[0032] The anchorage ends preferably each comprise three or four bent sections. Examples of such steel fibers are disclosed in EP-B1-2 652 221 and in EP-B1-2 652 222.

[0033] In an embodiment of the invention, the steel fibers have for example an elongation capacity of between 2.5 and 12 %, preferably at least 2.5%, preferably at least 3.5%, further preferred at least 4.5%, even more preferred a least 5.5 %. Elongation capacity in a certain range in the sense of the present invention may thereby especially for example refer to an elongation at maximum load in said range. In another preferable embodiment of the invention, the middle portion of the steel fibers may for example have an elongation at maximum load higher than 4%, e.g. higher than 5%, e.g. higher than 5.5%. By the elongation at maximum load is understood the total elastic and plastic elongation of a straight steel fibre sample at maximum load during the tensile testing test. This means that structural elongation for example by straightening may preferably not be taken into account when considering elongation at maximum load.

[0034] In the present invention, the post-tension steel strands may be draped i.e. they are positioned for example to take away as much as possible the tensile stresses in the concrete, so that they may arranged in a sinusoidal way when looking at a longitudinal section, especially whereby they may for example be positioned in the upper half of the concrete slab in a portion of the slab and along of the length of the slab go down to be positioned in the lower half of the concrete slab, go up again and so forth. In an embodiment of the invention, the post tensioning strands may be for example draped and/or straight and/or arranged in the middle or the higher third or the lower third of the slab. In an embodiment of the invention the ends of the strands may be straight or pointing upwards or pointing downwards. Having the ends of a strand pointing upwards or downwards may thereby especially for example contribute to counteract any curling of the strands.

[0035] In an embodiment of the invention, the post-tension steel strands may be in a banded-banded steel strands configuration or in a banded-distributed steel strands configuration or in a distributed-distributed steel strands configuration or in a configuration resulting from any combination thereof, and/or the post tension steel strands can be arranged in any configuration, preferably without any maximum and/or minimum spacing requirements and/or the post-tension steel strand may be used for bonded or unbonded post-tensioning and/or the end anchors for the post-tension steel strands may be designed as described for example in patent application US 63/052,283 and/or wherein the fibers are substantially homogenously or homogeneously distributed in the slab. A banded or banded-banded configuration of steel strands may thereby allow to keep the slab freer from steel strands, so as to allow for example for more design freedom or safe drilling through the slabs. Bonded post-tensioning may thereby use bonded strands that may be bonded to the concrete of the slabs for example using grout, so that even in case of a problem an anchor structural integrity is preserved through the bonding. On the other hand, unbonded post-tensioning strand may be provided with a plastic sheeting and may not be bonded to the concrete of the slabs.

[0036] In an embodiment, the amount concrete can be reduced for a given thickness or a given span over a slab but without fibers and post-tension steel strands by between 5 and 50 %, preferably between 10 or 40 % or between 15 and 35 %, further preferred at least 5 %, 15 %, 20 %, 25 % or 30 %.

[0037] The present invention thereby also concerns a method of making a concrete slab according to claim 1 , said method comprising the following steps:

- positioning post-tensioning steel strands including end anchors at their ends

- positioning opening formers to allow the formation of at least one opening in the upper surface of the slab, so as to permit access to the strands and/or the anchors from the upper surface of the slab

- pouring concrete to form a concrete slab according to the invention. Opening formers may thereby for example be made out of wood and/or plastic and/or steel. Opening formers thereby allow the slab to be poured out of concrete but avoid the concrete used to pour a slab from going into the space reserved for openings so that the openings are thereby formed. It may thus be that no slab is cast at one or two of the end edges of the slabs. In one embodiment, the opening formers may thereby for example either remain in place when the openings are filed with concrete or grout or be removed before the opening is filled with concrete or grout. In an embodiment, the method according to the invention comprises filling the openings with grout or concrete, especially conventional concrete. Mode(s) for Carrying Out the Invention

[0038] In some embodiments, a post-tension steel strand may also be arranged in the middle of the slab.

[0039] However, no position can guarantee the total absence of tensile stresses. Within the context of the present invention, post-tension steel strands may therefore be designed especially for example 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.

[0040] The fibers are mixed in the concrete as homogeneously as possible so that may preferably be present over the whole volume of the slab and able to take tensile stresses caused by various loads.

Post-Tension Steel Strand

[0041 ] A typical post-tension steel strand may have for example a 1 +6 construction with a core steel wire and six layer steel wires twisted around the core steel wire. In an embodiment, the post-tension steel strand may be in a non-compacted form.

[0042] 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.

[0043] As mentioned, the post-tension steel strand may have a high yield point, i.e. the yield force at 0,1 % elongation is high. The ratio yield force F_po,i to breaking force F_m is higher than 75%, preferably higher than 80%, e.g. higher than 85%.

[0044] 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%.

[0045] All steel wires may be provided with a metallic coating, such as zinc or a zinc aluminium alloy. A zinc aluminium coating has a better overall corrosion resistance than zinc. In contrast with zinc, the zinc aluminium coating is temperature resistant. Still in contrast with zinc, there is no flaking with the zinc aluminium alloy when exposed to high temperatures.

[0046] A zinc aluminium coating may have an aluminium content ranging from 2 per cent by weight to 12 per cent by weight, e.g. ranging from 3 % to 11 %.

[0047] 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.

[0048] Another preferable composition contains about 10% aluminium. This increased amount of aluminium provides a better corrosion protection then the eutectoid composition with about 5% of aluminium.

[0049] Other elements such as silicon (Si) and magnesium (Mg) may be added to the zinc aluminium coating. With a view to optimizing the corrosion resistance, a particular good alloy comprises 2 % to 10 % aluminium and 0.2 % to 3.0 % magnesium, the remainder being zinc. An example is 5% Al, 0.5 % Mg and the rest being Zn. [0050] An example of a post-tension steel strand is as follows:

- diameter 15.2 mm;

- steel section 166 mm²;

- E-modulus: 196000 MPa;

- breaking load F_m: 338000 N;

- yield force F_po.i: 299021 N;

- tensile strength R_m 2033 MPa.

Steel Fiber

[0051] Steel fibers 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 fibers have a length ^so that the length-to-diameter ratio ^/D ranges from 40 to 100.

[0052] Preferably, the steel fibers 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. In one embodiment, steel fibers may be glued.

[0053] Figure 1 shows a schematic top down view of a slab (1 ) according to the invention with openings having a semi-circular outlines (2) next to a wall (8) to allow access to post-tensioning strands (9).

[0054] Figure 2 illustrates a preferable embodiment of a steel fiber (3). The steel fiber (3) has a straight middle portion (4). At one end of the middle portion (4), there are three bent sections (5), (6) and (7). At the other end of the middle portion (4) there are also three bent sections (5’), (6’) and (7’). Bent sections (5), (5’) make an angle (a) with respect to a line forming an extension to the middle portion (4). Bent sections (6), (6’) make an angle (b) with respect to a line forming an extension to bent sections (5), (5’). Bent sections (7), (7’) make an angle (c) with respect to bent sections (6), (6’). [0055] The length

of the steel fiber (3) may range between 50 mm and 75 mm and is typically 60 mm.

The diameter of the steel fiber may range between 0.80 mm and 1 .20 mm. Typical values are 0.90 mm or 1 .05 mm.

The length of the bent sections (5), (5’), (6), (6’), (7) and (7’) may range between 2.0 mm and 5.0 mm. Typical values are 3.2 mm, 3.4 mm or 3.7 mm.

The angles (a), (b) and (c) may range between 20° and 50°, e.g. between 24° and 47°.

[0056] The steel fibers may or may not be provided with a corrosion resistant coating such as zinc or a zinc aluminium alloy.

[0057] In a particular preferable embodiment of the invention, there may be three or four bent sections at each end of the middle portion.

[0058] Figure 3 show a schematic top down view of a slab (1 ) according to the invention having a openings (2) with a square and a semi-circular outlines, whereby cracks (10) may appear especially in the corners of the opening with a square outline and a rebar (11 ) may thus be foreseen at or near such comers to contribute to counter crack formation.

Other reinforcing fibers

[0059] Examples of other reinforcing fibers may be selected from carbon fibers, glass fibers, basalt fibers or other non-steel based fibers, such as fibers based upon polyolefins like polypropylene or polyethylene or based upon other thermoplastics.

Claims

1 . A concrete slab, the slab comprising conventional concrete and a combined reinforcement of both post-tension steel strands and fibers, said post-tension steel strands

- having a diameter ranging from 5 mm to 20 mm,

- having a tensile strength higher than 1700 MPa, said fibers being either steel fibers and being present in a dosage ranging from 5 kg/m³ to 90 kg/m³ or being macro-synthetic fibers and being present in a dosage ranging from 1 .5 kg/m³ to 9.0 kg/m³, whereby the slab has upper as well as lower surfaces and sides, whereby further each strand or group of strands has two ends, whereby further each strand or group of strands is provided with an end anchor on at least one of its ends, whereby the slab has at least one opening in its upper surface so that said anchor is accessible from said upper surface through said opening.

2. Concrete slab according to claim 1 , whereby each opening corresponds to between 0.005 to 10 %, preferably 0.01 to 5 %, further preferred 0.05 to 2.5 %, even further preferred 0.1 to 1 .5 %, further preferred 0.2 to < 1 .5 %, further preferred 0.25 to 1 .25 % of the volume of the slab. and/or the openings allow to apply tension to the post-tension steel strand and/or release tension applied to the post-tension steel stands.

3. Concrete slabs according to claim 1 or 2, wherein said conventional concrete has a characteristic compressive cube strength of 25 N/mm²or higher, preferably 28 N/mm² or higher, further preferred 30 N/mm²or higher and/or wherein the slab does not contain any further reinforcement elements beside steel fibers and post-tensioning steel strands within the body of the slab, and/or wherein the slab is cast in one or multiple steps. Concrete slab according to any of the claims 1 to 3, wherein said fibers are steel fibers or wherein the fibers are glued or wherein macro-synthetic fibers may be selected from carbon fibers, glass fibers, basalt fibers or other non-steel based fibers, preferably polyolefin fibers, further preferred polypropylene fibers or polyethylene fibers. The concrete slab according to any of the preceding claims, whereby the outline of an opening or of each opening may be a polygon, preferably for example a polygon comprising angles of 100° or less, especially for example a rectangle or square and/or whereby the outline of an opening or of each opening maybe an ellipse, especially for example a circle or an oval and/or whereby the outline of an or of each opening may be partially elliptic, especially semi-circular or partially oval and/or whereby the outline of an or of each opening a polygon comprising only 2 angles being < 90°, a polygon comprising only 2 angles being < 90° and arranged as angles at or closest to the outside border of the concrete slab, a polygon comprising only 2 angles being < 90° and arranged as the angles that are the farthest away from the center of the upper surface of the slab, further preferred a polygon comprising only 2 angles being < 90° and at least 2 angles being > 90°, further preferred a polygon comprising only 2 angles being < 90° and at least 4 angles being > 90°. Concrete slab according to any of the preceding claims , whereby the depth of the opening(s) corresponds to at least 50%; preferably 55 %; further preferred 60 % of the thickness of the slab or the depth of the opening(s) may for example correspond to the thickness of the slab and/or whereby the opening(s) have a width/length ratio < 1 and/or a length between 2 cm and 100 cm, preferably between 5 cm and 50 cm, preferably between 6 cm and 20 cm.. Concrete slab according to any of the preceding claims, whereby the opening can be filled with grout or concrete, especially conventional concrete. 8. Concrete slab according to any of the preceding claims, wherein the post tensioning strands are draped and/or arranged in the middle of the slab and/or in the upper third of the slab and/or in the lower third of the slab and/or whereby the ends of the strand are straight or pointing upwards or pointing downwards.

9. Concrete slab according to any of the preceding claims, wherein the fibers are substantially homogenously or homogeneously distributed in the slab.

10. Concrete slab according to any of the preceding claims,

-wherein the post-tension steel strands are in a banded-banded steel strands configuration or in a banded-distributed steel strands configuration or distributed-distributed steel strands configuration or in a configuration resulting from any combination thereof, and/or

-wherein the post-tension steel strand are used for bonded or unbonded posttensioning.

11 . Concrete slab according to any of the preceding claims, whereby fibers are steel fibers and are present in a dosage ranging from 5 kg/m³ to 90 kg/m³, preferably whereby steel fibers are present in the slab in a dosage ranging from 7 kg/m³ to 75 kg/m³, preferably from > 7 kg/m³ to < 65 kg/m³, preferably from > 10 kg/m³ to 60 kg/m³, preferably 15 kg/m³ to 50 kg/m³, further preferred 20 kg/m³ to 45 kg/m³ or alternatively > 45 kg/m³ to 60 or < 65 kg/m³, further preferred from 15 kg/m³ to 40 kg/m³,, further preferred from > 20 kg/m³ to < 40 kg/m³, preferably from 15 kg/m³ to 35 kg/m³, preferably from 20 kg/m³ to 30 kg/m³ or from 10 kg/m³ to < 30 kg/m³ or further preferred from 10 kg/m³ to 27 kg/m³. In an embodiment, 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 21 or equall ,2 times, preferably 1 time, further preferred between > 0 and 1 ,1 times, the amount or level recommend as rebar or steel bar replacement. A method of making a concrete slab according to any of the claims 1 to 11 , said method comprising the following steps:

- positioning post-tensioning steel strands including end anchors at their ends

- positioning opening formers to allow the formation of at least one opening in the upper surface of the slab, so as to permit access to the strands and/or the end anchors from the upper surface of the slab

- pouring concrete to form a concrete slab according to claim 1. A method of making a concrete slab according to claim 12, said method comprising filling the openings with grout or concrete, especially conventional concrete.