Description

Technical Field

[0001 ] The invention relates to a concrete slab comprising expanding concrete and a combined reinforcement of both post-tension steel strands and fibers.

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 posttension strands and fibers suffer 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 that 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 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-220693. 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] Expanding concrete, as such, may thereby also already be known in the prior art. However, the use of expanding concrete can lead to crack formation due to expansion and/or local or general overexpansion [0008] The present invention may thereby allow to simplify installation of the slabs, to reduce the risk of crack formation, to reduce the size or the opening of joints, to reduce the effects of shrinkage and/or cooling, especially during curing, to achieve load bearing capacity and/or to contribute to resist to bending stresses, to simplify the slab design, especially for example to reduce the number of post-tension steel strands. In addition, the present invention may improve the span of the slabs and/or reduce the thickness of the slab and/or the present invention may contribute to reduce the amount of concrete for a given slab thickness or a given span. Furthermore, the present invention may allow for easier and/or faster installation. In addition, the present invention may allow for the slabs to be structural slabs that can for example contribute to structural integrity of a building. 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. The present invention especially allows to combine for example improved shear or punching shear resistance with improved flexural capacity.

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 and/or simplify installation.

[0011] It is another aspect of the invention to provide a combination reinforcement of both post-tension strands and fibers to reinforce concrete slabs of expanding concrete efficiently and effectively. It is still another aspect of the invention to provide a combination reinforcement of both post-tension strands and fibers slabs of expanding concrete. The tendons or post-tension steel strands are thereby posttensioned which means that tension is applied to them only after the concrete has been cast and/or that the tendons or post-tension steel strands may for example remain in place also once the concrete is completely cured/hardened. The tendons or post-tension steel strands may thus be installed on-site and/or may be installed before or after casting. The tendons or post-tension steel strands may comprise anchor systems, that may especially attach the tendons or post-tension steel strands to the cast concrete of the slab according to the invention, and/or ducts or sheathing. This may especially contributes for example to allow to achieve bigger slabs, to help with continuity, to help with safety, to help with camber, to minimize pre-stress losses, especially due to creep, to increase the freedom regarding possible shapes and to facilitate a draped configuration of the tendons or post-tension steel strands. In contrast, pre-tensioning is used mostly for pre-cast elements casted offsite with tendons fixed to a form and being tensioned before any concrete is cast. The resulting pre-cast elements obtained by pre-tensioning are thus consequently of quite limited size due to the very need to use forms or moulds, so that flooring may usually require multiple pre-cast elements.

[0012] According to the invention, there is provided a concrete slab, the slab comprising 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 10 kg/m ³ to 75 kg/m ³ or being macro-synthetic fibers and being present in a dosage ranging from 1 ,5 kg/m ³ to 9,0 kg/m ³ , whereby the concrete is expanding concrete. The present invention may further concern a method to obtain a slab according to the invention.

[0013] The tendons or post-tension steel strands having 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, e.g. from 13 mm to < 18,0 mm. The post-tension steel strands have a tensile strength higher than 1700 MPa, e.g. higher than 1800 MPa, e.g. higher than 1900 MPa, e.g. higher than 2000 MPa, preferably between 1800 MPa and 4000 MPa. The post-tension steel strands may also for example have a maximum breaking load of higher than 190 kN, e.g. higher than 195 kN, e.g. higher than 200 kN, e.g. higher than 220 kN, preferably between 195 kN and 350 kN. According to the present invention, a tensile stress of between 5 and 15 % of the final stress may be applied in the post-tension steel strands in the first 24 hours after casting the slab. The concrete being expanding concrete and a tensile stress of between 5 and 15 % of the final stress being applied in the posttension steel strands in the first 24 hours after casting the slab may thereby for example allow to compensate for shrinkage and/or curing, especially during curing. It may particularly allow that the expansion of the concrete increase the compressive stress and thereby especially for example reduces the risk of crack formation and/or simplifies installation in that a (first) post-tensioning operation for example 2 to 5 days after casting, by a crew that may have to return on site, may not be necessary anymore. Instead, according to the invention a tensile stress of between 5 and 15 % of the final stress may be applied in the post-tension steel strands in the first 24 hours after casting the slab by the crew already on site for the casting. The fibers may thereby contribute to resist especially for example to bending stresses already right after the casting. This may clearly simplify installation. In an embodiment of the invention, the slab may thereby especially between 50 and 250 m long, preferably between 75 and 200 m long. The present invention may thereby be particularly useful for long slabs and/or jointless floors, as it may improve their dimensional stability and/or contribute to joints between such long slabs open up less, especially during curing and/or after more than 28 days. This may be due to the applied compressive stress counter acting the expansion of the concrete and thus contributing to the increased dimensional stability of the slab. [0014] 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. The present invention may thereby particularly for example allow to reduce the amount of post tension steel strands.

[0015] 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 _p o,i to the tensile strength R _m is higher than 0,75. The final stress may thereby be for example a tensile stress between 1000 MPa and 3000 MPa, preferably between 1500 MPa and 2500 MPa.

[0016] Low relation post-tension steel strands may have relaxation losses of not more than 2.5 % when initially loaded to 70 % of specified minimum breaking strength or not more than 3.5 % when loaded to 80 % of specified minimum breaking strength of the post-tension steel strand after 1000 hours.

[0017] The fibers can be steel fibers and may be present in a dosage ranging for example from 10 kg/m ³ to 45 kg/m ³ , preferably from 10 kg/m3 to 40 kg/m3, alternatively from > 25 kg/m ³ to 75 kg/m ³ , preferably from > 40 kg/m ³ to 60 or 65 kg/m ³ , further preferred from 15 kg/m ³ to 40 kg/m ³ ,, further preferred from > 20 kg/m ³ to < 40 kg/m ³ , preferably from 15 kg/m ³ to 35 kg/m ³ , preferably from 20 kg/m ³ to 30 kg/m ³ or from 10 kg/m ³ to < 30 kg/m ³ or further preferred from 10 kg/m ³ to 27 kg/m ³ . 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. Lower to intermediate dosages may thereby be particularly preferred to improve the homogeneous distribution of the fibers and/or to reduce and/or delay crack formation and/or to reduce the risk of forming fiber balls (i.e. by entanglement of fibers, especially at high dosages) that may lead to defects, especially surface defects. In the present invention, lower dosages of fibers may thus be preferred especially to reduce and/or delay crack formation in demanding conditions, especially for example such as in the present invention where tensions arise not only from shrinking but rather both from expanding and shrinking during curing.

[0018] The fibers can be macro-synthetic fibers and are present in a dosage ranging from 1 ,5 kg/m ³ to 9 kg/m ³ , e.g. from 2,5 kg/m ³ to 7 kg/m ³ , e.g. from 3,5 kg/m ³ to 5,0 kg/m ³ .

[0019] 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. This may especially allow to contribute for example to improving punching shear. This may mean that a slab according to the invention does therefore especially not comprise regions or parts of lower density, especially no aggregated and/or aerated parts and/or no polymer based insulating material, further more preferred no aggregated and/or aerated blocks and/or no polymer based insulating material, which has/have a lower density, especially compared to cast concrete. In an embodiment, the slab may preferably be cast in one or multiple steps, preferably in one step. A concrete slab in the sense of the present invention may thereby further for example also preferably be cast in one day and/or in one go and/or be fully casted, whereby especially for example no use of or assembly of blocks or other concrete parts is involved. A concrete slab in the sense of the present invention may further for example contain only the fibers and the post-tension steel strands as reinforcement elements, which especially for example may mean that the slab may be free of any other reinforcement elements, especially other metal or steel reinforcement elements besides the fibers and the post-tension steel strands, especially free of rebars or steel bars, steel mesh, steel rods or the like. A concrete slab in the sense of the present invention there comprises both fibers and post-tension steel strands. A concrete slab in the sense of the present invention may comprise a slip-sheet, especially for example a perforated slip-sheet. On the other hand, a concrete slab in the sense of the present invention may thereby further for example be free of a vapor barrier, especially at the basis of the concrete slab, so that said slab does preferably not comprise a vapor barrier.

[0020] Dosages of fibers of 10 kg/m ³ to 40 kg/m ³ in case of steel fibers and 1 ,5 kg/m ³ to 9 kg/m ³ in case of macro-synthetic fibers are low to moderate in comparison with prior art dosages of more than 40 kg/m ³ or more than 9 kg/m ³ . 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. In an embodiment of the present invention, the fibers may for example have a length of 10 mm to 100 mm, further preferred between > 10 mm and 70 mm, further preferred > 11 mm and < 65 mm. This may contribute for example also to a good anchorage of the fibers in concrete and/or to limit crack sizes and/or to allow for self-healing. This may further help to the fibers to be particularly useful for example in structural applications, where they can contribute to the slab strength, especially for example to resist flexural stresses and/or shear forces. [0021 ] The concrete or expanding concrete may preferably have a characteristic compressive cube strength or comparable cylinder strength 25 N/mm ² or higher, preferably 28 N/mm ² or higher, further preferred 30 N/mm ² or higher. Expanding concrete in the sense of the present invention may thereby also be for example shrinkage compensating concrete, which may expand while curing by about the same amount than the concrete shrink while curing. However, expanding concrete in the sense of the present invention may however preferably be concrete that expands, also called expansive or self-expansive concrete that contains an agents that expands the concrete while curing by more than the concrete shrinks while curing, so that (overall) the dimensions of the concrete expand by at least > 0 % as the concrete cures. This may allow a concrete slab according to the invention to somewhat self-tension due to its expansion, so as for example to reduce the risk of crack formation and/or to simplifiy installation in that a (first) post-tensioning operation for example 2 to 5 days after casting, by a crew that may have to return on site, may not be necessary anymore. More preferably, the concrete may be conventional concrete, whereby the 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. 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 rebars or steel nets or steel mesh beside steel fibers and post-tensioning steel strands, especially there may no rebars neither at the top nor at the bottom, further preferred there may also be for example even no rebars at any supports. It is thereby especially advantageous that the slabs according the present invention can act as structural slabs, especially for example to contribute to the structural integrity of a building. A concrete slab according to the invention may thereby especially have a thickness for example between 4 cm and 75 cm, preferably between 5 cm and 65 cm, further preferred between 10 cm and 55 cm, further preferred between > 10 cm and < 40 cm and/or have a width higher than the thickness and/or have a width higher than the thickness and a length higher than the thickness. In an embodiment, a concrete slab according to the invention may especially for example have the outline of a rectangular cuboid. In an embodiment, a concrete slab according to the invention may especially for example have the outline of a cuboid or of a rectangular cuboid, whereby preferably the gross sectional modulus of inertia may be according to the formula b.h ³ /12 with ’b’ being the width of the slab and ‘h’ being the thickness of the slab. In an embodiment, the slab according to the present invention may be a slab wherein the compression stress at first stressing is for example between 0.5 N/mm ² and 7 N/mm ² , further preferred between 1 N/mm ² and 5 N/mm ² or between 5 N/mm ² and 100 N/mm ² preferably 10 N/mm ² and 75 N/mm ² , further preferred 15 N/mm ² and 50 N/mm ²

[0022] In an embodiment of the invention, expanding concrete may comprise one or more additives selected from: CaO, MgO, CaSC or any other additive that may lead to the expansion of concrete during the curing of said concrete. Such additive(s) may also be called expansive admixture or shrinkage reducing agent. In an embodiment of the invention, expanding concrete may comprise one or more additives selected from: CaO, MgO, CaSO4 or any other additive that may lead to the expansion of concrete during the curing of said concrete in an amount of between 5 and 35 kg/m ³ of concrete or expanding concrete, preferably for example between 10 and 30 kg/m ³ of concrete or expanding concrete, further preferred between 15 and 25 kg/m ³ of concrete or expanding concrete. Expanding concrete may be obtained for examble using limestone and clay on one hand as well as limestone, calcium sulphate and bauxite on the other hand, whereby especially for example sulfoaluminate may be formed, which can expand in volume when exposed to water and the concrete cures. Expanded concreted in the sense of the invention is thus preferably not concrete that merely contains expanded materials, especially for example expanded clay, but may for example especially not expand overall by at least > 0% as the concrete cures. This may allow a concrete slab according to the invention to somewhat self-tension due to its expansion, so as for example to reduce the risk of crack formation and/or to simplifiy installation in that a (first) post-tensioning operation for example 2 to 5 days after casting, by a crew that may have to return on site, may not be necessary anymore.

[0023] In an embodiment of the invention, the slab or the dimensions of the slab may expand for example by between -5 % and 5 % during the first 168 hours of curing, preferably by between 0 % or > 0% and 5 % during the first 168 hours of curing, further preferred by between -2.5 % and 2.5 % during the first 168 hours of curing, further preferred by between 0 % or > 0 % and 2.5 % during the first 168 hours of curing, further preferred by between -1 .5 % to 1.5 % during the first 168 hours of curing, further preferred by between 0 % or > 0 % and 1 .5 % during the first 168 hours of curing, further preferred by between -0.5 % to 0.5 % during the first 168 hours of curing, further preferred by between 0 % or > 0 % and 0.5 % during the first 168 hours of curing, further preferred by between -0.1 % or 0 % or > 0 % to 0.1 % during the first 168 hours of curing, further preferred by between -0.07 % to 0.07 % during the first 168 hours of curing, further preferred by between 0 % or > 0 % and 0.07 % during the first 168 hours of curing, further preferred by between -0.05 % to 0.05 % during the first 168 hours of curing, further preferred by between 0 % or > 0% and 0.05 % during the first 168 hours of curing, further preferred by between > -0.05 % to < 0.05 % during the first 168 hours of curing, further preferred by between -0.04 % to 0.04 % during the first 168 hours of curing, further preferred by between > -0.04 % or > 0 % to < 0.04 % during the first 168 hours of curing. In one embodiment, the tensile stress in the strands of the slab may be for example between 50 MPa and 900 MPa during the first 168 hours of curing, preferably between 100 to 650 MPa during the first 168 hours of curing. The tensile stress mentioned in the above is thereby preferably for example the total tensile stress that may correspond to the initial tensile stress and the additional tensile stress related to expansion elongating the strands. Especially for example when the expension of the dimension of the slab is > 0 %, this may further contribute to allow a concrete slab according to the invention to somewhat self-tension due to its expansion, so as for example to reduce the risk of crack formation and/or to simplifiy installation in that a (first) post-tensioning operation for example 2 to 5 days after casting, by a crew that may have to return on site, may not be necessary anymore..

[0024] In a preferable embodiment of the invention, the fibers are steel fibers and have a straight middle portion and anchorage ends at both ends. Steel fibers may thereby especially contribute to allow for example for good dispersion in concrete and/or good compatibility with concrete. The use of steel fibers, alone or especially also in combination with posttensioning that may exert compression, may for example also help to limit crack sizes and/or allow for self-healing. Furthermore, the use of steel fibers may also for example contribute to the formation of irregular cracks that, delay moisture propagation and thus help to improve the durability of the slab. Steel fibers may further have a high tensile strength and/or a high E-modulus and/or a high shear resistance, which may make them particularly useful for example in structural applications, where they can contribute to the slab strength, especially for example to resist flexural stresses and/or shear forces.

[0025] Most preferably the tensile strength of the middle portion is above 1400 MPa, preferably above 1500 MPa, preferably above 1600 MPa, preferably above 1700 MPa, further preferred above 1900 MPa, even further preferred above 2000 MPa, even further preferred higher than 2200 MPa, preferably between 1400 MPa and 3500 MPa.

[0026] 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. These may be particularly useful in view of their good dosage/performance ratio, especially in combination with post tensioning as in the present invention, so that they may contribute to achieve good performance, especially regarding for example crack control, at relatively moderate dosages.

[0027] In an embodiment of the invention, the slab may rest on ground or on at least two support. In an embodiment of the invention, the supports may be part of a foundation, preferably located underneath the slab and/or away from the foundation, or preferably, the supports may not be part of a foundation. In case the supports are part of a foundation of a building, they may be preferably in contact with the soil or ground. On the other hand, in the case the supports are not part of a foundation of a building, the slab may preferably be a so-called elevated slab, they may especially be part of a multi-story building above or below the ground level. Elevated slabs and/or their supports may thereby preferably not be contact with the soil or ground, preferably elevated slabs (in contrast to slabs laid on the ground) may thereby also not be uniformly supported along the slab but rather punctually supported at the supports. It is thereby especially advantageous that the slabs according the present invention can act as or be structural slabs, especially for example to contribute to the structural integrity and structural resistance of a building. In contrast, slabs laid on the ground do for example not act as structural slabs. Slabs according to the present invention can thereby preferably be for example elevated slabs that are structural slabs.

[0028] In an embodiment of the invention, the supports may be concrete supports, masonry supports, steel supports or supports combining concrete, masonry and/or steel.

[0029] In an embodiment of the invention, the supports may comprise columns, walls, piles or beams or any combination thereof or any other elements acting as vertical support, whereby further such supports can especially be point supports, linear supports or area supports.

[0030] 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 above the supports they are positioned in the upper half of the concrete slab and in-between the supports they are positioned in the lower half of the concrete slab.

[0031 ] 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 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 anchors for the post-tension steel strands may be designed as described for example in patent application US 63/052,283 so as to reduce bursting behind the post-tensioning anchors during or after post-tensioning 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 connected to the concrete of the slabs.

[0032] The supports may be arranged in a regular rectangular pattern or quadrilateral shape where a set of four supports or a set of four groups of supports forms a quadrilateral shape. The concrete slab comprises straight zones at the supports that connect the supports in the two directions, i.e. in length direction and in width direction, the shortest distance between those areas of the concrete slab above the supports. The straight zones have a width that may vary between 0 % and 80 %, e.g. between 5 % and 50 % of the greatest cross-sectional dimension of the slab width direction between two supports. Post-tension steel strands are present in bundles in those straight zones. The presence of bundles of post-tension steel strands in the straight zones is often referred to as banded pattern. Post-tension steel strands may or may not be present outside the straight zones.

[0033] In an embodiment, the supports may be arranged to form a regular rectangular pattern or quadrilateral shape, the concrete slab comprising straight zones connecting the supports via the shortest distance in two directions, i.e. lengthwise and width-wise, post-tension steel strand bundles being present only in said straight zones in closely-spaced arrangement , where for example the maximum distance between bundles may not exceed 0.8m, in a so-called banded-banded configuration, and/or the supports may be arranged to form a regular rectangular pattern or quadrilateral shape, the concrete slab comprising straight zones connecting the supports via the shortest distance in two directions, i.e. lengthwise and width-wise, post-tension steel strand bundles in any or both directions being present inside and/or outside said straight zones in a largely-spaced arrangement, where for example the maximum distance between bundles may exceed 1.5m, in a so called distributed or banded-distributed configuration. A bundle may thereby be a closely spaced arrangement, where two or more individual strands that may be arranged in close proximity to each other to form a bundle, whereby preferably the maximum distance between individual strands of a bundle may be < 0.8 m, further preferred < 0.25 m. As individual strands may be rarely used, as such, but may be more frequently used as part of a bundle, strands and bundles can be used interchangeably (or as synonyms) herein. A banded-distributed configuration is thereby achieved by having steel strand bundles arranged in a closely spaced arrangement one way i.e. in one direction (for example widthwise) and arranged in a largely spaced arrangement the other way i.e. in the other direction (for example lengthwise). Strands or bundles of strands can thereby be arranged especially for example in an arrangement selected from the group of: a two way distributed arrangement, a one way banded and one way distributed arrangement, a one way banded and one way mixed arrangement, whereby a mixed arrangement comprises both strands or bundles both in banded and distributed arrangements, a two way banded arrangement, a one way banded and one way mixed arrangement, whereby a mixed arrangement comprises both strands or bundles both in banded and distributed arrangements, a two way mixed arrangement, whereby a mixed arrangement comprises both strands or bundles both in banded and distributed arrangements.

[0034] In an embodiment, the slab and any supports may be either permanently fully connected, so that the slab is not free to move from its supports, permanently fully disconnected, so that the slab is free to move, partially connected, so that the slab is partially free to move in certain directions or temporarily disconnected, so that the slab is free to move at least temporarily until a connection is put in place. A disconnection or partial connection may thereby allow for example to reduce shortening restraint forces that may appear upon shrinkage and may lead to large cracks. This may be particularly useful for example for very stiff or very long slabs that may be particularly susceptible to shortening restraint forces for example due to the shrinkage of concrete, due to elastic shortening related to post-tensioning, due to creep of concrete or due to temperature changes. On the other hand, a connection may help to support higher loads, especially for example seismic loads. [0035] In an embodiment, the span of the slab for a given thickness is increased 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 % over a slab with the same slab thickness but without fibers and post-tension steel strands and/or wherein the thickness of the slab for a given span between two supports is reduced 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 % over a slab with the same span but without fibers and post-tension steel strands.

[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 posttension 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] In an embodiment, the present invention, especially for example a combination of post-tensioned steel strands and fibers, may contribute to increases in the structural capacity for flexure, deflection, shear, punching shear, temperature resistance and/or resistance to shrinkage for example over a slab without steel fibers and/or steel strands. The present invention can thereby especially contribute to increase punching shear by for example 10 % to 100 %, preferably 20 % to 60 % compared to embodiments not according to the invention. Said combination can replace partially or totally any other form of steel reinforcement, and/or replace partially or totally over-thickening measures at supports such as for example drop cap or drop panel.

[0038] The present invention further comprises a method to obtain a concrete slab according to the invention, comprising: -casting a concrete slab, the slab comprising 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 10 kg/m ³ to 75 kg/m ³ or being macro-synthetic fibers and being present in a dosage ranging from 1 ,5 kg/m ³ to 9,0 kg/m ³ , whereby the concrete is expanding concrete and

-applying a tensile stress of between 5 and 15 % of the final stress in the post-tension steel strands in the first 24 hours after casting the slab.

Mode(s) for Carrying Out the Invention

Explanation of the Principle behind the Invention

[0039] 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.

[0040] Fig. 1 shows schematic longitudinal cross-section of a slab (1 ) with expanding concrete (2) and post tension steel strands (3) (fibers are thereby not shown in this schematic) according to the invention having a length (4) and a thickness (5), whereby compressive stress (6, arrows) is applied by the post tension steel strands (3) and the expanding concrete (2) expands as shown by the arrows (7).

[0041] In some embodiments, a post-tension steel strand may also be arranged in the middle of the slab.

[0042] 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 may be for example of a sufficiently high tensile strength, i.e. above 1700 MPa or even above 1800 MPa.

[0043] 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.

[0044] In a second embodiment of the invention, a concrete slab is formed on supports. A slip-sheet may be or may not be present between the supports and the slab.

Post-Tension Steel Strand

[0045] 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.

[0046] 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.

[0047] 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 _p o,i to breaking force F _m is higher than 75%, preferably higher than 80%, e.g. higher than 85%.

[0048] 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%.

[0049] 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.

[0050] 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 %.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] An example of a post-tension steel strand is as follows:

- diameter 15,2 mm;

- steel section 166 mm ² ;

- E-modulus: 196000 MPa;

- breaking load F _m : 338000 N;

- yield force F _p o.i: 299021 N; - tensile strength R _m 2033 MPa.

Steel Fiber

[0055] 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.

[0056] 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.

[0057] Figure 2 illustrates a preferable embodiment of a steel fiber (8). The steel fiber (8) has a straight middle portion (9). At one end of the middle portion (9), there are three bent sections (10), (11 ) and (12). At the other end of the middle portion (9) there are also three bent sections (1 O’), (11 ’) and (12’). Bent sections (10), (10’) make an angle (a) with respect to a line forming an extension to the middle portion (9). Bent sections (11 ), (1 T) make an angle (b) with respect to a line forming an extension to bent sections (10), (10’). Bent sections (12), (12’) make an angle (c) with respect to bent sections (11 ), (11 ’).

[0058] The length of the steel fiber (8) 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 (10), (1 O’), (11 ), (1 T), (12) and (12’) 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°. [0059] The steel fibers may or may not be provided with a corrosion resistant coating such as zinc or a zinc aluminium alloy.

[0060] In a particular preferable embodiment of the steel fiber, there are four bent sections at each end of the middle portion.

[0061 ] In another particular preferable embodiment of the steel fiber, the middle portion has an elongation at maximum load higher than 4%, e.g. higher than 5%, e.g. higher than 5,5%. Steel fibers 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 fiber

[0062] Examples of macro-synthetic 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.

[0063] EXAMPLE

Length of the slab or distance between two joints : 100m

Distance between two strands : 2 m

Slab thickness : 160 mm

Initial stressing applied within the first 24 hours:

10 % of the final stress of 1860 MPa = 186MPa

Strand size : 15,2 mm (Area 140 mm ² )

[0064] Tensile force applied on a strand : 186 N/mm ² *140 mm ² = 26.040 N Compressive stress applied on the concrete : tensile force applied on a strand= 26040/concrete section=2000 mm *140 mm = 0.09 N/mm ²

[0065] When the slab expands by 0.5 mm/m in 7 days or 168 hours and the length of the slab is 100m, the slab expands by 50 mm in 7 days.

[0066] The initial applied stress on the strand is 186 N/mm ² [0067] The additional stress due to the elongation of the steel results from a 50 mm expansion and is calculated as follows:

Additional stress=tensile strength of the steel strand (200000 N/mm ² )x expansion (50 mm)/length of the slab (100000 mm)=200000 N/mm ² *50 mm/100000 mm= 100 N/mm ²

The total stress after 7 days is thus the initial stress (186 MPa or N/mm ² ) + the additional stress (100 N/mm ² )= 186 +100= 286 N/mm ² of total tensile stress in the post-tension strands after 7 days