AU2004291708B2

AU2004291708B2 - Method for production of a block of concrete and reinforcing cage for a block produced thus

Info

Publication number: AU2004291708B2
Application number: AU2004291708A
Authority: AU
Inventors: Marcel Matiere
Original assignee: Soc Civ De Brevets Matiere; Societe Civile de Brevets Matiere
Current assignee: Societe Civile de Brevets Matiere
Priority date: 2003-11-18
Filing date: 2004-11-18
Publication date: 2010-05-27
Anticipated expiration: 2024-11-18
Also published as: FR2862323B1; ATE452255T1; MA28165A1; DK1689951T3; DE602004024707D1; AU2004291708A1; JP2007511689A; PT1689951E; JP4709767B2; TNSN06144A1; WO2005049934A1; KR20060129218A; EP2141300B1; FR2862323A1; ES2338235T3; EP1689951B1; EP1689951A1; EP2141300A2; EP2141300A3

Abstract

The method involves casting a cement mass (2) in a casing after setting a reinforcing housing (3) in the mass, to fabricate a slab (1). The housing has lower longitudinal frameworks (31) and upper longitudinal frameworks (31) connected with each other. Each framework (31) has locking parts (43, 43) supported in the mass at respective locking zones (B, B) formed inside the slab, in direction opposite to absorbed tensile stress. An independent claim is also included for a reinforced concrete piece comprising a reinforcing housing.

Description

METHOD FOR PRODUCTION OF A BLOCK OF CONCRETE AND REINFORCING CAGE FOR A BLOCK PRODUCED THUS The invention relates to a manufacturing method of a concrete piece and 5 concerns more particularly the use of a reinforcement of new type exhibiting multiple advantages and enabling, in particular, to increase considerably the threshold of stress from which detrimental cracks risk developing. It is known that the principle of reinforced concrete is based upon the association of two materials having complementary properties: the concrete io which exhibits high compression strength, but very low tensile strength; and the steel which exhibits excellent tensile strength and is shielded from air corrosion when it is bedded in concrete. Moreover, both materials having, neighbouring expansion coefficients, their association enables to realise composite pieces having the durability qualities of concrete but being able to resist to bending 15 moments or loads. Indeed, when subjected to an applied stress, such a piece includes two sections situated on both sides of a neutral axis, respectively a compressed section subjected to compressive stresses absorbed mainly by the concrete and a stretched section subjected to tensile stresses absorbed mainly by at least one stretched longitudinal reinforcement of the reinforcing cage 20 embedded in the concrete. To remain shielded from air and to avoid corrosion, the reinforcement should be situated at a minimum distance from an outer face of the piece, called "covering distance". However, under the effect of stresses, it is impossible to avoid deformation of the piece with an elongation of the stretched section which 25 causes the development of cracks in the covering concrete. These cracks, inevitable in practice, can be admitted as long as their width is rather small, for example smaller than 3/10 millimetre, to prevent a penetration of air and water so that they contact the reinforcement. As long as the applied loads do not exceed a certain threshold, a 30 reinforced concrete piece behaves like a composite piece being deformed globally with a transfer of the stresses between the two components. To do so, one aims usually to increase the transfer link between the reinforcement and the concrete, for example by using so-called high-adhesion bars, which are notched over their whole length. On the other hand, the ends of these bars are 35 conventionally curved to form anchoring crossheads which increase the length of reinforcement buried in the concrete and, consequently, the transfer length of the inner stresses between the reinforcement and the concrete.

2 However, if the stress exceeds a certain threshold, the cracks wide and the small length free part of the stretched reinforcement extending between the opposite faces of a crack supports the elongation corresponding to the thickness of this crack, whereas the neighbouring sections are locked in the concrete. 5 Therefore, the elongation tendency of the stretched face, under the effect of the applied stresses, is focussed on the free parts of the reinforcement and, due to their small length, the applied elongation can cause to exceed the elastic limit of the metal, which causes the striction of the reinforcement and the destruction of the works. 10 The invention relates to a new manufacturing method of concrete moulded pieces enabling to solve such problems thanks to the use of a new type of reinforcement, enabling, in particular, to reduce the risk of cracking under equal stresses. Additionally, the method enables to enhance considerably the resistance to extreme stresses, the concrete pieces thus realised being very 15 flexible and benefiting from a high safety coefficient between the occurrence of the first cracks and the complete destruction of the works. Such an advantage is particularly interesting for the erection of civil engineering works or buildings in areas subjected to seismic risk. The invention applies therefore, generally, to the realisation of a concrete 20 piece provided with a reinforcing cage entirely bedded in the concrete during the casting thereof and including, when it is subjected to a stress, two sections situated on both sides of a neutral axis, respectively a compressed section subjected to compressive stresses absorbed mainly by the concrete and a stretched section subjected to tensile stresses absorbed mainly by at least one 25 stretched longitudinal reinforcement of the reinforcing cage extending from a small covering distance of a stretched longitudinal face of the piece. According to the invention, at least each stretched longitudinal reinforcement is formed by at least one flat band with a rectangular section, with a wide face and a narrow face, and is shaped in order to provide, inside the 30 piece, at least two clamping zones spaced apart from one another, wherein a part of said stretched reinforcement rests on the concrete, in the opposite direction to the absorbed tensile stress, and at least one slippage zone comprised between the clamping zones, wherein a corresponding part of the stretched reinforcement is free to be elongated over its whole length, under the effect of the absorbed 35 stresses. As it will be seen in detail below, whereas the technique of the reinforced concrete enables generally to interconnect each reinforcement over its whole 3 length with the concrete with which it is covered, in order to provide a composite piece being deformed globally under the effect of the loads, conversely the idea of the invention is to realise spaced apart clamping zones wherein the reinforcement, because it is formed of a flat band, rests in the concrete by a wide 5 face without any risks of exceeding the admissible compression limit of the concrete and, beyond a certain threshold of stress, to leave the part of the reinforcement comprised between two zones of compression, free to slide slightly with respect to the concrete with which it is covered, after separation from the concrete, so that the elongation effect of the reinforcement resulting from the io tensile stresses applied is always distributed over the whole length comprised between the two clamping zones, which enables to avoid the concentrations of stresses liable to break the reinforcement in case of extreme stresses. Particularly advantageously, each clamping zone is realised by a deviation of the corresponding part of the stretched reinforcement having the form of a 15 band, with a gradual and continuous variation of the orientation of its wide face with respect to the stretched longitudinal face of the piece. In a first embodiment, each clamping zone is realised by twisting the band over a certain length, with a gradual rotation of its wide face around its longitudinal axis, over at least one quarter of a turn. But the twisted section of the 20 band can also rotate by a complete turn or more, so as to rest on the concrete over a greater length. In another embodiment, each clamping zone is realised by curving the flat band around a transversal axis, so as to form an anchoring crosshead resting on the concrete by a wide face. 25 But it is also possible to realise a clamping zone by fixing on the flat band, at the requested location, at least one rigid bar section extending transversally so as to rest in the concrete on both sides of the flat band. This transversal bar may be formed of a transversal distribution bar. Indeed, classically, the reinforcing cage usually includes several 30 longitudinal sectors connected together by transversal distribution bars formed of bars perpendicular to the longitudinal main bars, the assembly constituting a reinforcing layer. These transversal distribution bars can be welded to the longitudinal bars, whose form of flat band enables to realise particularly resistant weldings. 35 Each welded junction can constitute a clamping zone, the distribution bars resting in the concrete on both sides of the longitudinal bar.

4 Moreover, conventionally, each sector of the reinforcing cage includes at least two longitudinal bars respectively compressed and stretched, connected together by connection stirrups. Preferably, the compressed reinforcements are also formed of flat bands. 5 Similarly, according to another advantageous arrangement, each connection stirrups can be formed of at least one flat band interconnected with the reinforcements by welding beads or bonded sections extending over a length equal to the width of the band, and hence, particularly resistant. Additionally, each band constituting a stirrup is tilted with respect to the io direction of the stretched bar and the assembly of the welded junction between both these flat bands of different orientation forms a kind of wedge resting on the concrete. Consequently, each clamping zone between the stretched reinforcement and the concrete may be formed by an interconnection junction between this 15 stretched reinforcement and a connection stirrup in the form of a band. But the use of flat bands for realising at least the stretched reinforcements provides still other possibilities. Thus, to form a clamping zone, the stretched reinforcement in the form of a band can be slit axially over a certain length, both thus formed band partitions 20 being spaced apart from one another to form an opening for the transversal insertion of at least one rigid bar section capable of resting on the concrete on both sides of the longitudinal band, in the opposite direction to the tensile stress applied to the longitudinal reinforcement. This rigid bar may be a distribution bar running through the longitudinal 25 band in the plane thereof, or a single bar length running through the longitudinal band perpendicular to its plane. Besides, the use of flat bands enables multiple possibilities for implementing the method according to the invention. In particular, each longitudinal reinforcement can include at least two 30 jointed bands extending longitudinally, substantially in the alignment of one another and subjected to tensile stress in opposite directions, both jointed bands overlapping each other over a certain length and being ended respectively, on both sides of the overlapped section, by two crossheads oriented towards one another, which have thus a tendency to come closer to one another under the 35 effect of the opposite tensile stress applied to both corresponding bands, while inducing the compression of a core of concrete comprised between both crossheads, over said overlapping length.

5 In the conventional case, when the reinforcing cage is formed of parallel sectors connected by distribution bars, the latter elements may run within crossheads of two neighbouring sectors, so as to transmit, to the section of concrete comprised between said sectors, the compression stresses induced in 5 each sector, by the tendency of the opposite crossheads to come closer to one another. According to a particular arrangement, both jointed bands forming the longitudinal reinforcement of each sector of the cage can constitute two strands of a single band ended by opposite crossheads and forming a loop between two io spaced apart levels, so as to surround a core of concrete compressed by the clamping of said loop under the effect of the tensile stresses applied in opposite directions on both strands of the longitudinal reinforcement. Particularly advantageously, in each sector, the longitudinal reinforcement may be formed of a plurality of consecutive bands each having two curved ends 15 in the form of a crosshead, the adjacent crossheads of two consecutive bands being placed beside one another while overlapping partially so as to delimit a space, in the transversal direction, for the insertion of at least one bar length forming a connection pin between both consecutive bands. The longitudinal reinforcement then behaves like a stretched chain formed of a plurality of links 20 each formed of a band and connected two by two by pairs of adjacent crossheads keyed to one another. In another also advantageous embodiment, in each sector, the longitudinal reinforcement includes a stack of flat bands, provided on at least two superimposed levels, respectively a first level, closest to a stretched longitudinal 25 face of the piece including a band, with two ends in the form of a crossheads oriented inwardly of the piece and at least one second level including at least two flat bands substantially parallel provided side by side with, each, two ends in the form of crossheads, respectively inner and outer crossheads, and said bands of the second level overlapping one another over a certain length, so that their inner 30 ends in the form of crossheads are oriented towards one another. The piece thus includes at least one section forming a core comprised between the internal crossheads of both bands and compressed by the tendency of said internal crossheads to come closer to one another under the effect of the tensile stresses applied in opposite directions on the corresponding bands, and two end sections 35 each comprised between the external crossheads of the bands of the first level and of the second level and compressed by the tendency of each external crosshead of the band of the first level to come closer inwardly under the effect of 6 the tensile stress supported by said band. It is thus possible to compress the whole concrete of the piece. But the invention enables other arrangements and covers numerous advantageous features which will appear in the following description of certain 5 embodiments given by way of non-limitative examples illustrated by the appended drawings. Figure 1 is a longitudinal sectional schematic view of a concrete piece manufactured according to the invention. Figure 2 is a cross-sectional partial view according to line 11-11 of Figure 1. 10 Figure 3 is a perspective view of a twisted part of the reinforcement. Figure 4 is a sectional view, according to a transversal middle plane, of the twisted part. Figure 5 is a partial view, in perspective, of a part in the form of a crosshead. 15 Figure 6 is a partial side view of a double crosshead. Figure 7 is a longitudinal sectional view of a part manufactured according to another embodiment of the invention. Figure 8 shows, in perspective, a twisted part applied to the embodiment of Figure 7. 20 Figure 9 shows, in longitudinal section, a piece manufactured according to still another embodiment of the invention. Figures 10 and 11 show, in longitudinal section, other embodiments of the invention, with several reinforcing layers. Figure 12 shows, in longitudinal section, a piece of the type represented on 25 Figure 10, with a closed-loop reinforcement. Figure 13 is a perspective view of a closed-loop reinforcement. Figure 14 shows, in longitudinal section, another embodiment of the invention, with a reinforcement in the form of a chain. Figure 15 shows, in perspective, the junction between two elements of the 30 reinforcement of the type represented on Figure 14. Figures 16, 17, 18 show, in perspective, other embodiments of a clamping zone. Figure 19 shows, in perspective, a clamping zone in the form of a crosshead. 35 Figure 20 shows, in perspective, another embodiment of one end of the reinforcement in the form of a crosshead.

7 Figures 1 and 2 represent, respectively in longitudinal section and in transversal section, a concrete piece realised according to the invention. As usual, this concrete piece 1 which forms, in the example represented on the drawings, a slab having two plane faces, respectively lower 20 and upper 5 21. The piece is moulded by casting a mass of concrete 2 in a casing after a reinforcing cage 3 have been laid therein. The reinforcing cage includes two reinforcing sets, respectively lower 30 and upper 30', substantially parallel to the two faces 20, 21 of the slab 1. The reinforcing cage 3 is formed, normally, from several sectors S1, S2... io respectively centred in planes P1, P2... parallel to a longitudinal axis of the piece 1, i.e. to the plane of Figure 1. Each sector includes a lower longitudinal reinforcement 31 and an upper longitudinal reinforcement 31' which are connected together, in the plane P1 (P2) of the sector S1 (S2) by transversal reinforcing elements, called stirrups, which are not represented on Figures 1 and 15 2. Moreover, the different sectors S1, S2... of the cage are connected together by transversal rods 32, 32' perpendicular to the plane of Figure 1. As known, under the effect, for example, of a vertical load M applied on its upper face 20', the concrete piece 1 may be divided into two sections on both 20 sides of a neutral axis 10 respectively a compressed section 11' comprised between the neutral axis 10 and the face 21 whereon the loads are applied, and a stretched section 11 extending up to the face 20 opposite to the loads. All these arrangements are conventional and do not require a detailed description. 25 However, the reinforcing cage 3 defers from the conventional cages in that, according to a lay out described in the patent application FR-A-2 814 480 of the same inventor, at least the longitudinal reinforcements 31 situated in the stretched section 11 of the piece and extending at a short covering distance from the lower face 20 are made from flat metal bars 4 with a rectangular section 30 having a wide face 41 and a narrow face 42, each flat bar being thus formed of a metal band having a width I substantially greater than its thickness e. Preferably, the compressed reinforcements 31' are also formed of flat bars but, for economic purposes, conventional rounded bars can be used. As indicated in the previous patent FR-A-2,814,480 of the same inventor, 35 the cross-section (I x e) of the band 4 is classically computed relative to the tensile stresses to be supported taking into account the loads applied to the piece 1. Indeed, under the effect of the load M, the piece 1 has a tendency to warp 8 slightly, each longitudinal reinforcement 31 of the lower set 30 being subjected to a tensile stress which tends to elongate it. In the conventional technique of reinforced concrete, notched reinforcement bars are generally used in order to increase the transfer link 5 between the reinforcement and the concrete. A composite piece is thus formed the rigidity of which is sought classically to be enhanced to avoid the occurrence of detrimental cracks. However, a slight deformation of the piece cannot be avoided, leading to an elongation of the stretched reinforcements as well as of the concrete with which the latter is covered, concrete the tensile strength of io which is small. Therefore, it is the occurrence of cracks beyond a certain load. Contrary to the usual prior art where the reinforcement is connected with the concrete over its whole length, according to the invention there are provided two clamping zones B, B' in the concrete 2, along each stretched longitudinal reinforcement 31. The two clamping zones B, B' are spaced apart on both sides 15 of the transversal middle plane Q of the piece and are separated by a central zone C wherein the reinforcement 31 is free to be elongated under the effect of the applied loads, by sliding with respect to the concrete which covered it. Since flat bands are used as longitudinal reinforcements, each clamping zone B, B' may be simply realised by a deviation of the corresponding part of the 20 stretched reinforcement 31, the orientation of the wide face of which is gradually and continuously varied with respect to the stretched longitudinal face 20 of the piece so that, in this zone, the band 4 forming the reinforcement 31 rests in the concrete, mainly by its wide side 41, in the opposite direction to the absorbed tensile stress. 25 In the embodiment represented on Figures 1 to 4, each clamping zone B, B' is realised by twisting the band 4 over a certain length L1, with a gradual rotation of its wide face 41 around its longitudinal axes 40. In the case represented on Figure 3, the band is twisted over a complete turn. Considering a horizontal band 4 subjected to a tensile stress T oriented to 30 the right and having a part 43 twisted over a complete turn and located on both sides of a middle plane P, the tensile stress T applied to the band 4 determines, as with a corkscrew, a resting point of this band 4 in the concrete by both its faces 41, 41'. Since the band is flat and rests in the concrete by two wide faces, the concrete cannot be shorn, the compressive stress being small. Besides, the 35 width I of the band 4 can be determined relative to the applied stresses, so that the compressive stress applied to the concrete remains smaller than an admissible limit.

9 The reinforcement band 4 thus interconnected with the concrete by both its twisted part 43, 43' which constitute clamping zones B and B' can conversely be freely elongated in its central part 44 comprised between both twisted parts 43, 43'. 5 Even if there exists an adherence link between the concrete and the reinforcement over its whole length after concrete casting, said adherence need not be improved for example by means of notches, unlike with the usual technique. Indeed, in the invention, each band 4 constituting a longitudinal io reinforcement 31 is interconnected with the concrete only in both clamping zones B, B' which are spaced apart from one another and the tendency to elongation of the reinforcement under the effect of the stress applied can be distributed uniformly over the whole length L 2 of the central part 44 comprised between both twisted part 43, 43', this central part 44 being able to be separated from the 15 concrete beyond a certain threshold of stress. Concrete 2 will not be deformed in the same way as the reinforcement and the occurrence of cracks is therefore inevitable. But the risk of cracking is distributed uniformly over the whole length L 2 of the central part 44 and the more numerous cracks can have an enough small width so as not to be detrimental. 20 Consequently, it is not any longer necessary to increase the rigidity of the concrete piece in order to reduce its deformation as in the prior art, and, conversely, concrete pieces which are relatively flexible can be manufactured. When the applied stresses remain small, the piece behaves as usual as a composite piece, the concrete being deformed in the same way as the 25 reinforcement. Beyond a certain threshold of stress, the reinforcement remains interconnected with the concrete only in its locking parts 43, 43' and its central part 44 can, conversely, separate from the concrete which it is covered with and slide slightly relative thereto. The tension and, consequently, the tendency to 30 elongation of the reinforcement are therefore distributed substantially uniformly over the whole length L 2 of the central part 44. Any concentration of the tensile stress is avoided over a small length of the reinforcement, thereby suppressing the risk of breaking the reinforcement close to a crack, under the effect of an excessive stress. 35 By way of example, slabs realised according to the invention have been subjected to bending tests with a gradual increase of the load applied and it could 10 be observed that a slab thus realised could, surprisingly, accept quite a significant deflection before breaking and with relatively narrow cracks. Additionally, it should be noted that, as indicated in the patent application FR-A-2 814 480, the use of flat bands as reinforcements enables to globally 5 reduce the thickness of the slab, since the stirrups which are also formed from a flat band, possibly twisted, can be welded to the internal face of each band, the external face of which must only be separated from the facing side 20 of the slab by the minimum covering distance imposed by the regulation. Additionally, in a reinforced concrete piece realised classically, the 10 stretched longitudinal bars should be curved in the form of a crosshead at their ends in order to increase the transfer length between the reinforcement and the concrete. But, taking into account the hardness of the steel used, the curve which can be imparted to a bar is necessarily limited. Moreover the regulation imposes to confer to the crosshead a diameter of at least 10 times the diameter of the bar. 15 For this only reason, a concrete piece must have a minimum thickness of twelve times the diameter of the stretched bars, minimum thickness to which twice the minimum covering thickness must be added. When using flat bars as in the invention, said bars can also be provided at their ends with crossheads 5, as represented in perspective on Figure 5. But 20 these crossheads 5 can have an inner diameter D smaller than in the case of one or several rounded bars, since the thickness e of a flat band with a rectangular section is lower than the diameter of one or even several rounded bars of the same equivalent section. Additionally, even with high elastic-strength steels, the use of flat bands 25 enables to reduce further the diameter D of the curved part, since the latter can be bended more easily around an axis 50 parallel to the plane of the wide face of the band. Moreover, as shown on Figure 20, it is possible to rotate the crosshead around a vertical axis in order to reduce the thickness of the slab 1 still further. 30 For this reason, it is possible with an equal weight of reinforcement material, to realise thinner pieces than the piece realised classically in reinforced concrete. Besides, each crosshead 5 also constitutes a clamping zone B1 in the concrete 2. 35 Since, according to the invention, the longitudinal reinforcement is, in a way, connected to concrete in the clamping zones 43, 5 whereas the central part 44 is free to be elongated, the stresses applied by the reinforcement to the 11 concrete are greater than that in the case of conventional reinforcement where the point is to provide interconnection over the whole length of the reinforcement. Obviously in the clamping zones, this means a greater compressive stress in the concrete but, precisely, the concrete exhibits excellent compressive strength. 5 Moreover, there are no risks as in the case of a rounded bar, of a shearing effect of the concrete, since the width I of each flat bar can be determined relative to the loads applied so that the compressive stress of the concrete does not exceed a given limit. Since band 4 is subjected to a great tensile stress T which is absorbed by 10 the clamping zone B1 situated inside the crosshead 5, the latter can have a tendency to unwind if the adherence between bar 4 and the concrete is not sufficient enough. To avoid such a phenomenon, it is possible to add to the crosshead 5 an auxiliary crosshead 51 oriented in the opposite direction and having a slightly 15 smaller diameter to avoid this unwinding phenomenon. It should be noted, moreover, that in the preferred embodiment represented on Figure 1, each stretched bar 31 formed of a flat band 4 is oriented so that its wide face 41 is substantially parallel to the stretched face 20 of the piece. Such an arrangement increases the flexibility of the slab since bar 4 thus 20 oriented only exhibits a small bending strength and the deflection of the piece simply correspond to an elongation of the reinforcement. However, in order to increase the bending strength, it is possible to use bands 4 oriented as indicated on Figure 7. Indeed, in such a case each reinforcement band 4 is twisted but only over a quarter turn in two parts 43, 43' 25 spaced apart from each another so that, in the central part 44' and over the whole length thereof, the wide side 41 of band 4 is situated in a plane orthogonal to the stretched face 20 of the piece. In such a case, band 4 is clamped in the concrete substantially at the level of zones B1, B'1, inside the crossheads 5, 5' provided at the ends of band 4, the 30 clamping effect of the corkscrewed parts 43, 43' being reduced. Conversely, each reinforcement band 4, oriented perpendicular to the stretched face 20 in its central part 44', brings a certain bending strength which enables to increase the rigidity of the piece and reduce the deflection resulting from the applied loads. 35 As usual, the longitudinal bars 31, 31' of both sets, respectively lower 30 and upper 30', may be connected by a stirrups which, as described in document FR-A-2 814 480, are advantageously formed of flat corrugated bands 13 I4 interconnected with the longitudinal reinforcements by welding or bonding at the apexes of the corrugations. However, as shown on Figure 9, for implementing the method according to the invention, it is preferable that the weldings between the corrugated bands 13 5 and the bands 4 constituting the stretched reinforcements 31 are only carried out close to both ends of piece 1 so that the central part 44 of each band 4 can be elongated freely. Conversely, the stirrups can be welded or bonded to the compressed bars 31' at the upper part of each corrugation. To do so, it is preferable that each 10 compressed bar 31' is also formed by at least one flat band in order to provide a welding bead over the whole width of the corrugated band 13. It should be noted that each welded or bonded junction 39 between a bar 4 and the corresponding part of stirrup 13 which forms an angle with this bar 4 constitutes a clamping zone B by a wedging effect. In such a case, there is hence 15 no point in fitting band 4 with twisted parts. Conversely, it is advantageous to curve the ends in the form of crossheads 5 so as to provide, as previously, clamping zones B1, B'1 at both ends of reinforcement 31. But the invention may be subjected to other variations, in particular while making full use of the bearing loads exerted in the concrete by the clamping 20 zones of the longitudinal reinforcements, for example as represented schematically on Figures 10 to 20 which illustrate diverse variations. To make it simple compressed reinforcements 31' have not been represented on these figures. In a first variation represented on Figure 10, the lower set 30 of the 25 reinforcing cage 3 includes two superimposed levels. At the lowermost level, closest to the lower stretched face 20 of the piece, each sector of the reinforcing cage includes a longitudinal reinforcement 31 formed of a flat band which extends over the whole length of piece 1 and whose ends 5, 5' are curved in the form of a crosshead as indicated previously. 30 However these longitudinal bars 31 are associated with a second level of longitudinal bars offset upwardly and including, in each sector of the cage, two bars 33, 34 each formed of a flat band and laid side by side. Each of these bars 33, 34 is provided with curved ends in the form of crossheads 51, 51', 52, 52'. Additionally, both Jointed bands 33, 34 of the same sector are offset 35 longitudinally with respect to one another so that the two crossheads 51, 51' (52, 52') arranged at the ends of each band 33 (34) are placed at different distances from the transversal middle plane Q of piece 1, on each side thereof.

13 Thus, the external crosshead 51 of bar 33 placed on the left on Figure 10 is further away from the middle plane Q than the external crosshead 51' of the same band 31, placed on the right, and the disposition is reversed for the second longitudinal band 34. On the other hand, the lengths of the longitudinal bars 33, 5 34 are determined so that the different crossheads 5, 51, 52 placed on the same side of the transversal middle plane Q are distributed over a certain length of piece 1 from its end 11. Piece 1 is thus divided into several adjacent zones: - a central zone B3 comprised between the internal crosshead 51' of the 10 longitudinal reinforcement 33 placed on the right of the transversal middle plane Q and the internal crosshead 52 of the reinforcement 34 placed on the left of said plane; - two lateral zones B2, B'2 placed respectively on the left and on the right of plane Q, the left-hand zone B2 being comprised between the 15 external crosshead 51 of band 33 and the internal crosshead of bar 34, and the right-hand zone B'2 between the external crosshead 52' of band 34 and the internal crosshead 51' of band 33; - two end zones B1 and B'1 comprised respectively on the left and on the right, between the crossheads 5, 5' of the lower band 31 and the 20 external crossheads 51, 52' of the upper bands 33, 34. When piece 1 is subjected to a load having a tendency to make it sag downwards, the longitudinal reinforcements are subjected to tensile stresses and bear upon the concrete by their ends in the form of a crossheads which are hence strained inwardly. 25 The longitudinal reinforcements 33, 34, which overlap each other in the central part of the piece, are stretched in different directions and the central clamping zone B3 is therefore compressed by the tendency of the internal crossheads 51', 52 of both reinforcements 33, 34 to come closer to one another. Similarly, the external crossheads 51, 52' have a tendency to come closer 30 to plane Q under the effect of the tensile stresses applied to bars 33, 34 and the lateral zones B2, B'2 are thereby compressed. For the same reasons, the end zones B1, B'1 are also compressed by the crossheads 5, 5' which counter-act the tensile stresses applied to reinforcement 31. 35 Thus, piece 1 is compressed practically over its whole length by the tendency of the crossheads of the various reinforcements to come inwardly closer to one another and this compression phenomenon is exerted over the whole 14 thickness of the concrete which is limited by the crossheads and not only, as conventionally, above the neutral axis 10. The cracking risk of the piece, even in the vicinity of the stretched face 20, is hence considerably reduced and loading tests conducted on a piece thus 5 realised have shown that such a piece could sustain, before breaking, quite a significant deflection, which is rather unusual for an reinforced concrete piece. In the embodiment which has just been described, the compression of the central zone 63 of the piece, between the internal crossheads 52, 51' of both jointed bands 33, 34, results from the overlapping of the jointed bands 33, 34 io which are stretched in opposite directions on both sides of the transversal middle plane Q. However, as shown on Figure 11, the stretched reinforcement of cage 3 can also be formed of a simple stack of longitudinal bands of different lengths, respectively a first band 35 extending over a length 11 slightly shorter than that of 15 the piece and a second band 36 extending over a length 12 shorter than 11, each band being provided with ends in the form of crossheads oriented towards the inside of the piece. In such a case, also, the concrete is compressed by the crossheads 5, 5', 51, 51' which absorb the tensile stresses applied on the bands 35, 36 by reason 20 of the deflection of the piece. In this view, it may be advantageous to realise the bands in metals having different mechanical features so as to modulate the compressive stresses applied by the crossheads relative to the distribution of the loads applied to the piece. It should be noted that the use of flat bands as reinforcement bars enables 25 rather easily to vary their modulus of elasticity since the flat bands can be made by slitting of metal sheets, and metal sheets with different features are available on the market whereas the possible choices for concrete rounded bars are less numerous. Besides, contrary to the conventional technique where the point is to 30 interconnect the reinforcement with the concrete with which it is covered, it is possible to position the two stacked bands 31, 35 against one another, the stacked bands then behave as a kind of leaf spring. Figures 12 and 13 show a variation of the embodiment of Figure 10 where the two adjacent bands 33, 34 constitute two strands of a single band forming a 35 closed loop, both opposite crossheads 51', 52 being connected by a part 37 of the band situated at the upper level 30' of cage 3.

15 Each longitudinal reinforcement is thus formed by a single band 33, 37, 34 surrounding a core of concrete B3 which is compressed by tightening the loop thus formed under the effect of the tensile stresses applied on both strands 33, 34 of the longitudinal reinforcement in opposite directions when piece 1 is 5 subjected to a vertical load. It should be noted that, in such a case, the upper part 37 of each longitudinal reinforcement forming a loop can constitute a part of the upper set 30' of the reinforcing cage 3. On the other hand in the case of Figure 10 as that of Figure 12, the io opposite crossheads 51', 52 which tend to compress a core of concrete B3, are not necessarily spaced apart symmetrically on both sides of the transversal middle plane Q of piece 1. Indeed, according to the loading mode of the piece, it is possible to determine certain sections of piece 1 where the tensile stresses induced in the concrete by the loads applied are maximum and to arrange the 15 reinforcements and their crossheads so that the tensile stresses resulting from the loads are compensated, at least partially, by the compression, in the same zones, of a core of concrete located between two crossheads stressed towards one another by the tensile stresses applied to the reinforcement. Generally, each of the figures described previously shows a sector of the 20 reinforcing stand centred in a vertical plane parallel to the longitudinal axis and the crossheads arranged on the reinforcements of two neighbouring sectors are located substantially at the same level. So, through the aligned clamping zones of sectors S1, S 2 , S 3 , transversal tie rods 32 can be which are also stressed inwardly. They distribute the compression stress applied by the crossheads 5, 51, 25 52 over the concrete located between two neighbouring sectors. These distribution transversal bars can have a rounded or rectangular section, as shown on the Figures. However, the use of flat bars enables to facilitate the carrying out of a welding bead between the longitudinal and transversal bars. In another embodiment represented schematically on Figure 14, each 30 stretched longitudinal reinforcement 6 of the cage 3 is formed of a plurality of consecutive bands 6a, 6b... each having two curved ends in the form of crossheads 5a, 5'a, 5b, 5'b... and these bars are positioned so that the pairs of adjacent crossheads 5'a, 5b of two successive bands 6a, 6b are situated substantially at the same level, while overlapping slightly, so as to surround a 35 space E wherein a bar 38 is inserted which can constitute a transversal distribution bar.

16 Each bar 38 inserted through a space E compressed by the two opposite crossheads 5'a, 5b constitutes a connection pin between two consecutive bands 6a, 6b. Each longitudinal reinforcement 6 formed of a plurality of bands 6a, 6b, 6c... thus behave like a chain whose links 6a, 6b, 6c... are stretched under the 5 effect of the loads applied to piece 1. But the invention is not limited to the details of the diverse embodiments which have just been described, and could also be the subject of other variations, without departing from the scope of the invention. For example, each clamping zone could be obtained simply by using a 10 transversal bar inserted through an opening provided in the longitudinal reinforcement bar. Indeed, as represented on Figure 17, the use of a flat band 4 as a longitudinal reinforcement bar enables to provide it with a middle slot 45 extending over a certain length and delimiting two lateral portions 46, 47 which 15 can be spaced apart from one another so as to open an opening 45' for a transversal bar 48. When the band 4 is subjected to a tensile stress, for example in the direction of the arrow indicated on Figure 16, it rests on the transversal bar 48 which, in turn, rests in the concrete, thereby clamping band 4. In the case of Figure 16, both sections 46, 47 of the band are stretched 20 while resting on the plane thereof, the transversal bar 48 being thus orthogonal to this plane. But it is possible also, as represented on Figure 17, to space apart both portions 46, 47 perpendicularly to the plane S of the band, for the insertion of a transversal bar 48 which, in this case, is parallel to the plane of the band. This bar 25 could therefore constitute a transversal distribution bar 32 between several sectors of the reinforcement stand. In the case where the band is twisted for forming a clamping zone, this twisted part 43 could also be provided with a middle slot enabling to space apart the two portions of the band from one another so as to provide an opening 45' for 30 a transversal distribution bar 32, as represented on Figure 18. Besides, as represented on Figure 19 the clamping effect of a crosshead 5 can be improved by providing orifices 45' on both parts thereof for the insertion of a transversal bar 52 which, additionally, would prevent the crosshead from unwinding under the effect of the tensile stress applied to band 4. 35 It should be noted, besides, that the use of flat bands 4 as longitudinal reinforcement bars enables to vary the orientation of the crosshead provided at the end of such a bar. Indeed, generally, the flat band 4 may be easily curved 17 around an axis parallel to its plane. It would hence be possible, as shown on Figure 20, to twist the end of band 4 over a quarter turn so that the crosshead 5 revolve around a vertical axis 50. The same clamping effect would then be obtained by giving to the crosshead a height limited to the width e of band 4. 5 Such an arrangement could be advantageous for manufacturing particularly slim pieces or, for example, for narrowing the ends of a slab. In particular, the use of a flat band 4 enables, if required, to vary the orientation thereof, as shown on Figure 20, for example, for narrowing certain sections of the piece. On the other hand, as already indicated, it is advantageous to make the io compressed reinforcements 31' in the form of flat bands but, for economic purposes, conventional bars could also be used. It should be noted, however, that the advantages provided by the use of flat bars compensate more than enough the possible extra costs of the reinforcements. 15 In this view, it could even be advantageous to realise bars in stainless steel not only because of the corrosion strength which would enable to reduce the covering thickness, but also of a greater ductility which increases the fatigue strength and the energy absorption capacity. Such advantages are particularly interesting for the realisation of civil engineering works since they improve the 20 strength to differential settlement and, in certain cases, to seismic shocks. Besides, for simplifying them, the Figures represent slabs or flat beams but it should be observed that the use of flat bands as reinforcement bars exhibits numerous advantages for the realisation of curved pieces or even warped pieces. Indeed, a flat band having a wide side parallel to the facing side of the piece may 25 easily match the curved profile thereof. In particular, it is thus possible to manufacture a reinforcing cage matching a curved piece down flat, such a cage can then adopt quite naturally the profile of the bottom of the mould when it is laid therein. The flat bands constituting the longitudinal and transversal reinforcements 30 may even be curved in opposite directions so as to match a warped profile in the transversal direction, such as a ruled surface having non parallel transversal generating lines. It is the case, for example, of the covering of a bridge tilted laterally with a variable angle from one end to another and whose the reinforcement is difficult to realise by conventional processes. 35 On the other hand, as it enables to realise particularly flexible concrete pieces which may accept rather significant deflection, the invention is especially suited to the realisation of crossing works buried below an embankment and 18 having a curved profile enabling them to be deformed slightly under the applied load for resting by their sides on the lateral embankments. Additionally, as indicated above, the clamping zones between which the concrete is compressed may be placed in locations, determined by computation, 5 wherein the concrete is subjected to a maximum tensile stress and where the risk of cracking is maximum. The invention enables therefore, starting from a typical model, to adapt the reinforcement of each piece to the predictable distribution of the stresses, taking the loads applied into account. For example, it is well known that within a slab or a beam loaded vertically 10 uniformly, a central zone may be provided wherein the tensile stress applied to the lower face is maximum, two lateral zones wherein the slab is subjected to a shearing stress, and two bearing ends. However, as indicated previously, the disposition of the compression zones need not be symmetrical with respect to the middle plane of the piece. 15 Thanks to the invention it will be possible, without complication of the reinforcement, to adapt said reinforcement, in each section of the piece, to the main loads to be absorbed, by varying judiciously the orientation of the flat bands constituting the reinforcements so as to realise clamping zones in the most stretched sections, in order to compensate this tension by a compression effect of 20 the concrete. Moreover, as indicated above, it is possible to orient vertically the stretched bands, as represented on Figure 8, to increase the bending strength, or to tilt the bands, as indicated on Figure 20, to resist optimally the shearing stress, or to realise horizontal crossheads, as shown on Figure 20, to reduce the 25 thickness of the piece at the bearing points, these different variations being able to be combined. Additionally, as indicated previously, the arrangement of the compression zones need not be symmetrical with respect to the middle plane of the piece. The arrangements according to the invention enable therefore to vary the 30 shape of the reinforcing cage so as to modulate the action of the reinforcements relative to the loads applied on the piece in operation.

Claims

1. A manufacturing method of a concrete piece provided with a reinforcing cage bedded in concrete during the casting thereof, said piece being subjected to a load and including, under the effect thereof, two sections on both sides of a 5 neutral axis, respectively a compressed section subjected to compressive stress absorbed mainly by the concrete and a stretched section subjected to tensile stress absorbed mainly by at least one stretched longitudinal reinforcement of the reinforcing cage extending along a longitudinal direction of application of the tensile stress, at a short covering distance of a stretched longitudinal face of the 10 piece and having a cross-section determined relative to the stressed induced by the load, each stretched longitudinal reinforcement being formed by at least one flat band having a rectangular section, with a wide face and a narrow face, said flat band having at least a central part comprised between two clamping parts gripping in the concrete, wherein the clamping parts and the central part extend 15 along a same longitudinal direction parallel to the stretched longitudinal face of the piece, so that each clamping part rest in the concrete in the opposite direction to the absorbed tensile stress, thus providing inside the piece, at least two clamping zones between which extends at least a central slippage zone, wherein the central part of the stretched flat band is free to be elongated under the effect 20 of the absorbed stress, with a uniform distribution of tension over the whole length of said central part.

2. A method according to claim 1, wherein, from a certain threshold of stress, the central part of the stretched flat band comprised between the two clamping parts is free to separate from the concrete with which it is covered, by sliding 25 slightly relative thereto.

3. A method according to any of the claims 1 and 2, wherein the width and the thickness of a stretched longitudinal reinforcement having the form of a band are determined relative to a maximum tensile stress to be absorbed, on the one hand in order to provide a sufficient cross-section for the absorption of said 30 maximum stress while remaining in the resilient domain and, on the other hand, 20 so that, in each clamping zone, the part of the concrete where the wide face of the band rests is subjected to a compressive stress not exceeding an admissible threshold.

4. A method according to any of the previous claims for manufacturing a 5 concrete piece, wherein each clamping part is realised by a deviation of a corresponding part of the stretched flat band, with a gradual and continuous variation of the orientation of its wide face with respect to the stretched longitudinal face of the piece.

5. A method according to claim 4, wherein each clamping part of the band is 10 twisted over a certain length, with a gradual rotation of its wide face around its longitudinal axis over at least one quarter of a turn.

6. A method according to claim 5, wherein the twisted part of the band is twisted by a complete turn around its longitudinal axis.

7. A method according to any of the claims 1 to 4, wherein each clamping 15 zone is realised by curving one end of the flat band around a transversal axis to form an anchoring crosshead resting on the concrete by a wide face.

8. A method according to claim 7, wherein each clamping zone includes two successive crossheads, respectively a first crosshead having a concavity oriented in the direction of the tensile stress applied to the band and extended by a second 20 crosshead oriented in the opposite direction to counter-act an unwinding tendency of the first crosshead under the effect of the tensile stress applied to the band.

9. A method according to any of the previous claims, wherein the reinforcing cage includes two longitudinal reinforcement sets, respectively in the stretched 25 section and in the compressed section, and wherein the compressed longitudinal reinforcements are also formed of flat bars. 21

10. A method according to any of the previous claims, for manufacturing a concrete piece provided with a reinforcing cage including at least two longitudinal sectors connected together by transversal distribution bars and each including at least two longitudinal reinforcements, respectively a compressed reinforcement 5 and a stretched reinforcement in the form of a flat band, said compressed and stretched reinforcements being connected together by at least two connection stirrups, wherein the connection stirrups are in the form of a band and are interconnected with the stretched reinforcement by at least one junction realised close to each longitudinal end of the reinforcement so as to form a clamping zone 10 at each junction, the central part of the stretched reinforcement being free to be elongated.

11. A method according to claim 10, wherein the connection stirrups are interconnected at their upper section with the compressed reinforcements, in points distributed over the whole length thereof. 15

12. A method according to any of the previous claims, wherein, to form a clamping zone between a stretched longitudinal reinforcement and the concrete, said reinforcement having the form of a band is slit axially over a certain length and that the two thus formed partitions of the band are spaced apart from each another to form a opening for the transversal insertion of at least one rigid bar 20 section able to rest on the concrete in the opposite direction to the tensile stress applied to the band constituting the longitudinal reinforcement.

13. A method according to any of the claims 7 and 8, wherein each longitudinal reinforcement includes at least two jointed bands extending longitudinally, substantially in the alignment of one another, and subjected to 25 tensile stresses in opposite directions, said bands having internal ends overlapping one another over a certain length and are respectively terminated by two internal crossheads oriented towards one another and stressed towards one another under the effect of the opposite tensile stresses applied to both corresponding bands, while inducing the compression of a core of concrete 30 located between said internal crossheads over said covering length. 22

14. A method according to claim 13, wherein the core of concrete located between two opposite crossheads is situated in a part of the piece wherein the concrete is subjected to tensile stresses induced by the loads applied to the piece, said tensile stresses being compensated, at least partially, by the 5 compression of said core between two opposite crossheads.

15. A method according to any of the claims 13 and 14, wherein both jointed bands forming a longitudinal reinforcement constitute two strand of a single band forming a loop between two spaced apart levels, so as to surround a core of concrete compressed by clamping said loop under the effect of the tensile 10 stresses applied in opposite directions on both strands of the longitudinal reinforcement.

16. A reinforcing cage for a concrete piece subjected to a load and including, under the effect thereof, two sections on both sides of a neutral axis, respectively a compressed section subjected to compression stresses absorbed mainly by the 15 concrete and a stretched section subjected to tensile stresses absorbed mainly by at least one stretched longitudinal reinforcement of said reinforcing cage which extends, along a longitudinal direction of application of the tensile stresses, at a short covering distance of a stretched longitudinal face of the piece and exhibits a cross-section determined relative to the stresses induced by the load, each 20 stretched longitudinal reinforcement being formed by at least one flat band with a rectangular section, with a wide face and a narrow face, said flat band having at least one central part comprised between two clamping parts having a modified orientation for gripping in the concrete, wherein the said clamping part and central part of the flat band extend along a same longitudinal direction parallel to the 25 stretched longitudinal face of the piece, so as to provide, inside the concrete piece, two spaced apart clamping zones wherein the band rests in the concrete by at least one wide face in the opposite direction to the tensile stress to be absorbed, and at least one slippage zone comprised between said spaced apart clamping zones wherein the band is free to be elongated under the effect of the 30 tensile stress, over the whole length of its part comprised between said clamping parts. 23

17. A reinforcing cage according to claim 16, wherein each clamping zone is formed by a part of the flat band which is twisted over a certain length, with a gradual rotation of its wide face around its longitudinal axis over at least one quarter of a turn. 5

18. A reinforcing stand according to claim 17, wherein the twisted part of the band is twisted over a complete turn around its longitudinal axis.

19. A reinforcing cage according to claim 16, wherein each clamping zone is realised by curving one end of the flat band around a transversal axis so as to form at least one anchoring crosshead resting on the concrete by a wide face. 10

20. A reinforcing cage according to claim 19, wherein each end of a flat band is curved so as to form two successive crossheads, respectively a first crosshead having a concavity oriented in the direction of the tensile stress applied to the band and extended by a second crosshead oriented in the opposite direction, so as to counter-act an unwinding tendency of the first crosshead under the effect of 15 the tensile stress applied to the band.

21. A reinforcing cage according to any of the claims 16 to 20, including, in at least one sector, two longitudinal reinforcements, respectively stretched and compressed, wherein the compressed reinforcements are also formed of flat bands. 20

22. A reinforcing cage according to any of claims 20 and 21, wherein the longitudinal reinforcements, respectively stretched and compressed, are connected together, in each sector of the cage, by at least two connection stirrups, and each connection stirrup is formed of a flat band welded by one end on said reinforcements in the form of a flat band, at least close to the ends of the 25 cage, by forming an angle therewith, so that each welded junction between a stretched reinforcement and the stirrup forms, by a wedging effect, a clamping zone. 24

23. A reinforcing cage according to claim 22, wherein the connection stirrups between the stretched reinforcement and the compressed reinforcement are formed of a corrugated band and that said corrugated band is only welded with the stretched reinforcement close to both ends thereof, so as to provide two 5 clamping zones between which the central part of the stretched reinforcement is free to be elongated.

24. A reinforcing cage according to any of the claims 16 to 23, wherein, to form a clamping zone between a stretched longitudinal reinforcement and the concrete, said reinforcement in the form of a band is slit axially over a certain 10 length and that the two thus formed band partitions are spaced apart from one another to form an opening for the transversal insertion of at least one rigid bar section able to rest on the concrete in the opposite direction to the tensile stress applied to the band constituting the longitudinal reinforcement.

25. A reinforcing cage according to claim 24, wherein both partitions of the 15 longitudinal band arranged on both sides of an axial slit are spaced apart while staying in the plane of the band for the insertion of a section of a locking bar, orthogonally to said plane.

26. A reinforcing cage according to claim 24, wherein both partitions of the longitudinal reinforcement arranged on both sides of an axial slit are spaced apart 20 from one another perpendicular to the plane of the band, for the insertion, parallel to said plane, of a locking bar as a transversal distribution bar.

27. A reinforcing cage according to any of the claims 19 to 26, wherein each stretched longitudinal reinforcement includes a plurality of flat bars in the form of bands, provided on at least two stacked levels, respectively a first level, closest to 25 a stretched longitudinal face of the piece, wherein is arranged at least one stretched flat bar extending substantially over the whole length of the piece with two ends in the form of crossheads oriented inwardly, and at least one second level wherein is arranged at least one flat bar shorter than the bar of the lower level, with two ends in the form of crossheads oriented inwardly and spaced apart 30 longitudinally on both sides of a transversal middle plane, so as to delimit a 25 compressed zone between said crossheads and at least two lateral compressed zones by the crossheads of the bar of the lower level.

28. A reinforcing cage according to any of the claims 16 to 26, wherein each longitudinal reinforcement is formed of a stack of at least two flat bars applied 5 against each another along a wide side and having together a total cross-section determined relative to the applied stresses, said stack having the same tensile strength as a single bar of the same section, and resisting to deformation as a leaf spring.

29. A reinforcing cage according to any of the claims 27, 28, wherein the 10 stacked flat bars have different mechanical features.

30. A reinforcing cage according to any of the claims 16 to 29, wherein at least the stretched longitudinal reinforcements are made of stainless steel.

31. A reinforcing cage according to claim 19, wherein each longitudinal reinforcement is formed of a plurality of consecutive bands each having two 15 curved ends in the form of a crosshead, the adjacent crossheads of two consecutive bands being placed beside one another and overlapping partially so as to delimit a space in the transversal direction for the insertion of at least one rigid bar section forming a connection pin between said consecutive bands, said longitudinal reinforcement behaving like a stretched chain formed of a plurality of 20 links each formed of a band and connected in twos by a pair of adjacent crossheads pinned to one another.

32. A reinforcing cage according to any of the claims 19 to 31, including at least two sectors connected by transversal distribution bars, wherein the crossheads of the reinforcements of two neighbouring sectors are placed 25 substantially at the same level inside the piece and said transversal distribution bars are inserted through said crossheads so as to compress the whole concrete placed at that level. 26

33. A manufacturing method of a curved piece obtained by casting concrete into a mould or casing with a curved bottom, after laying into the mould a reinforcing cage according to any of the claims 16 to 32, wherein the reinforcing cage is manufactured down flat and exhibits, by reason of the use of flat bands, 5 sufficient flexibility to adopt, by its own weight, the curved shape of the bottom of the mould after laying the cage therein.

34. A concrete piece realised by the method according to any of the claims 1 to 15.

35. A reinforced concrete piece including a reinforcing stand according to any 10 of the claims 16 to 32. SOCIETE DIVILE DE BREVETS MATIERE WATERMARK PATENT & TRADE MARK ATTORNEYS P27150AUOO