AU709765B2 - A cable sheath having a multilayer structure, a method of manufacturing such a sheath, and a machine for implementing the method - Google Patents

Description

2 Another solution for overcoming that drawback is to make sheaths of plastic. Unfortunately, because of the minimum thickness required for the wall of a sheath (about 2 millimeters) and because of the shape memory of the suitable plastics materials, it is necessary, in order to use the above-mentioned method of manufacturing metal sheaths, to heat the entire strip of plastic, to wind it, to weld together the adjacent edges, and to cool the sheath rapidly, all of this taking a long time, and involving a high cost in energy. That method of manufacturing is therefore too costly and cannot be performed in situ, i.e. on the building site. It is therefore necessary to manufacture such plastics sheaths, e.g. by extrusion or molding, in complicated and costly machines, and to transport them to the building site from the place of manufacture, which is also very costly, in particular since they are very voluminous. In addition, if the sheaths have to be transported, it is impossible to make long straight lengths of sheath.

Furthermore, such plastics sheaths generally include bands serving as projecting relief over their outside surfaces. Such bands generally do not enable sheath couplings to be screwed on. To overcome that drawback, threaded portions are generally provided at the ends of the sheaths, which portions are generally formed by molding. In addition to increasing the manufacturing costs, such threaded end portions prevent the sheaths from being cut to any desired length on site.

Alternatively, special sleeves in the form of half shells are provided which are assembled after the sheaths have been laid. That also increases manufacturing costs.

Machines exist for manufacturing by extrusion plastics sheaths provided with spiral threads over their outside surfaces. Unfortunately, such machines are very complicated and very costly and they cannot be used in situ. The above-mentioned drawbacks are therefore not eliminated.

3 Another drawback with plastics sheaths is that it is difficult to find a plastics material that satisfies all of the above-mentioned requirements simultaneously.

An advantage of the present invention is that it provides a cable sheath satisfying the above-mentioned requirements, while eliminating the above-mentioned drawbacks.

An advantage of the present invention is thus to provide an improved cable sheath which is watertight and strong, which has at least a minimum thickness of wall, which offers low friction on the inside, and which can transfer the prestress forces to the concrete. A particular advantage of the present invention is to provide an improved cable sheath which has a structure such that each of the above-mentioned requirements is satisfied as well as possible.

A further advantage of the present invention is to provide such an improved cable sheath that is simple, quick, and cheap to manufacture.

A further advantage of the present invention is to provide such an improved cable sheath that can be manufactured in situ to any desired length.

S"A further advantage of the present invention is to provide such an improved cable sheath that is essentially made of plastics material.

According to one aspect of the present invention there is provided a prestress or stay cable sheath, characterized in that it comprises a multilayer structure made up of at least two superposed continuous layers of plastics material, each layer being constituted by a strip disposed helically and whose side edges are touching and sealed together, said at least two layers of plastics material being welded together at at least one point.

This multilayer structure offers numerous advantages.

It is thus easy to vary the thickness of the wall of the sheath by changing the number of layers, each of which may be very thin, which eliminates, to a large extent, the difficulties of winding due to the shape memory of plastics materials. In addition, it is possible to incorporate layers of different materials, each layer then having particular properties.

The multilayer structure may include more than two layers of plastics material. In particular, this embodiment of the invention provides a cable sheath that is made entirely of plastic.

Preferably, said at least two layers of plastics material are welded together at at least one point, advantageously along a line, e.g. by heat sealing. This characteristic makes it possible to perform irreversible bonding between the layers, and guarantees repeatability and uniformity for the sheath.

Preferably, the cable sheath is provided with at least one continuous helical thread over its outside surface. The presence of the continuous helical or spiral thread makes it possible for sheath couplings to be fixed merely by being screwed on. Furthermore, the fact that the thread extends continuously over the entire length of the sheath makes it possible for the sheath to be cut to any desired length. In addition, depending on the pitch of said spiral thread, the rigidity of the sheath may be modified.

According to another embodiment of the invention, an insert such as a metal wire may be disposed in the .multilayer structure of the sheath, in particular inside at least one longitudinal rib of at least one 25 strip. The insert may act to reinforce the rigidity of oe the sheath, or optionally as a heater insert.

A particularly suitable use of the sheath of these examples of the invention is as a prestress sheath.

9 However, the sheath of the invention may be used as 9*° 30 a stay sheath. In which case, the advantageous presence •of a continuous spiral thread over the outside surface :9 of the sheath offers aerodynamic advantages with respect ~to wind, and to rainwater which can run off more easily by being channelled, and the multilayer structure makes it possible to make an outer layer having any desired practical or aesthetic characteristic.

According to another aspect of the present invention there is provided a method of manufacturing a cable sheath, the method including the following steps: supplying at least two strips of plastics material; assembling them helically and in superposed manner by means of assembly apparatus such that the respective edges of each of said strips are touching so as to form a sleeve having a multilayer structure made up of at least two layers of plastic; and welding together said at least two superposed layers of plastic at at least one point.

Advantageously, the welding is performed by heat sealing along a continuous helical line, which makes the sheath more watertight.

Preferably, prior to the assembly step, the method involves forming at least one longitudinal rib in each strip.

The invention further provides a machine for :implementing the method, said machine including at least S. two drums for receiving rolls of strip so as to supply 20 said at least two strips, and assembly apparatus for "assembling the strips helically such that their respective side edges are touching.

In a first variant of the invention, the assembly apparatus includes a stationary cylindrical former which may be provided with at least one continuous spiral thread over its outside surface, on which thread a respective longitudinal rib of each strip is wound, the rib being formed previously by means of a corresponding rib-forming module, the winding being advantageously performed by means of drive and/or presser wheels. This variant makes it possible to make sheaths of substantially circular cross-section.

In a second variant of the invention, the assembly apparatus includes two rotary cylindrical formers around which the strips are helically wound, the set formed by said two rotary formers itself being rotary. The winding is also advantageously performed by means of drive and/or 7V 1

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presser wheels. This variant makes it possible to make sheaths of oblong cross-section.

Other characteristics and advantages appear from the following detailed description given by way of nonlimiting example and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic longitudinal section view through the wall of a sheath having a multilayer structure of one embodiment of the invention; Figure 2 shows a portion of the assembly apparatus of one embodiment of the invention, and diagrammatically shows how the sheath having a multilayer structure of one example of the invention is assembled; Figure 3 is a diagrammatic view of a machine for implementing the method of manufacturing a sheath having a multilayer structure of one example of the invention, said machine incorporating a first variant of the assembly apparatus; and Figure 4 is a view similar to Figure 3, the machine incorporating a second variant of the assembly apparatus As shown in Figure i, the sheath 1 of this embodiment of the invention comprises a multilayer structure that may, for example, comprise three superposed layers i!!i 10b, 10c. Naturally, the number of layers 10, of which there are at least two, is not limited and the embodiment of the sheath comprising three layers as shown in the figures merely represents an advantageous embodiment. The three-layer structure is advantageous when the sheath is used as a prestress sheath because, to obtain the required 30 minimum thickness of walls, i.e. about 2 millimeters prior S.to tensioning the cables, it is then possible to use layers of material of standard thickness of about 0.7 millimeters. Naturally, if the cable sheath of the *invention is not to be used as a prestress sheath, the thickness of the wall may be smaller or larger, and the number of layers may therefore be different.

According to an aspect of the invention, the sheath having a multilayer structure includes at least one layer of plastics material. Preferably, said sheath includes at least two superposed layers of plastics material, thereby enabling them to be intimately bonded together as explained below in the description of the method of manufacturing the sheath.

The detailed description of this embodiment of the invention given with reference to the drawings concerns more particularly a sheath having a multilayer structure comprising three layers 10a, o10b, 10c of plastics material, but clearly the sheath may include a layer made of another material, e.g. such as glass fiber, or metal.

A particularly advantageous characteristic of the sheath having a multilayer structure of this example of the present invention lies in the fact that it is possible to choose a particular material having particular intrinsic characteristics for each layer so that each layer of the multilayer structure performs a particular function. For example, the inner layer 10a of the sheath may be made of a material having low friction during tensioning of the prestress cables, e.g. Teflon or graphite-containing high-density polyethylene (HDPE). The second layer o10b, i.e. the layer that is superposed directly on said inner layer 10a, may, for example, be made of a material having relatively high rigidity. The outer layer 10c may be made of a material having good resistance to ultra-violet radiation. This is advantageous :particularly when the sheaths are stored on building sites eeoc situated in hot countries. Softening and distortion of the sheath caused by solar radiation can thus be limited. This multilayer structure also makes it possible for the cable sheath of the invention to be adapted to uses other than as a prestress sheath. Such a sheath may be used as a stay 0** sheath. In which case, the characteristics required for the inner layer 10a and the outer layer 10c of the sheath are not necessarily the same as those of a prestress sheath. In a stay sheath, the outer layer 10c may, for example, be made of a colored material to satisfy aesthetic requirements, said colored material also having good resistance to ultra-violet radiation so as to guarantee that the colour does not fade.

According to this embodiment of the invention, each layer 10 of the sheath is continuous, i.e. each layer of the sheath forms a closed sleeve 10, the multi-layer structure therefore being constituted by a set of superposed individual sleeves 10a, 10b, 10c. As shown in the figures, each layer 10 of the sheath is made from a strip or sheet of material 11 which is assembled such that its side edges 12, 13 are adjacent and touching so as -o form said sleeve. According to a characteristic of this example of the invention, said side edges 12a, 13a; 12b, 13b; 12c, 13c of each strip 1la, llb, llc of plastics material, which are adjacent and touching after they are assembled, are welded together by heat sealing to form a homogeneous and watertight plastics sheath layer.

Preferably, the heat-sealing zones 17 in which the side edges of each strip of plastics material are welded together are offset from one layer to another axially along the sheath 1. This offers a further guarantee that the multilayer structure of the sheath is properly watertight, since the zones 17 of the layers of the sheath where the watertightness is relatively weak are not situated facing one another.

According to an example of application of the invention, the sheath 1 is provided with at least one continuous helical or spiral thread 15 over its outside surface. The spiral thread 15 acts in particular to 30 satisfy the above-mentioned requirement of transferring S° forces to the concrete, after the cement slurry has set.

Preferably, the continuous outer spiral thread 15 of the P sheath is formed by one or more longitudinal ribs 14a,

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*14b, 14c provided on each strip lla, lib, lic making up said multilayer structure. Said longitudinal ribs 14a, 14b, 14c of the various strips are preferably equidistant so that they are superposed on one another on assembling said sheath. Thus, in the example shown in Figure i, each of the strips lla, lib, lic respectively constituting the layers 10a, 10b, 10c of the sheath 1 is provided with three longitudinal ribs of identical shape which are superposed on one another on assembling said strips spirally, as described below with reference to the method. In this embodiment, the sheath 1 is thus provided with three continuous helical threads 15 on its outside surface. This characteristic is particularly advantageous when the sheath is of circular crosssection, because it enables sheath couplings to be fixed merely by being screwed onto said spiral threads 15, at the ends of the sheath i.

The width of each of the various strips lla, llb, llc forming the multilayer structure and the number of and/or the spacing between the longitudinal ribs 14a, 14b, 14c formed in each of said strips may be varied, thereby advantageously enabling the rigidity of said sheath 1 to be modulated.

According to another advantageous aspect of the invention, an insert 19 is disposed in the multilayer structure of the sheath. The insert 19, which may, for example, be a metal wire, may serve to reinforce the rigidity of said sheath, but it may also act as a heater insert by passing an electric current. In which case, said insert 19 acts to heat the sheath when the temperature is extremely low, thereby enabling the cement slurry to be injected even in cold weather. This insert or metal wire is advantageously disposed inside one or more of said longitudinal ribs 14a, 14b, 14c of one or more of said strips lla, llb, llc. In which case, it extends over the entire length of the sheath in spiral manner.

As mentioned above, the sheath of the invention may have a circular cross-section, but it may have a different cross-section, in particular an oblong crosssection as shown in Figure 4. In certain applications, this provides better distribution of the prestress forces, in particular in thin structures.

The invention also concerns a method of manufacturing a sheath having a multi-layer structure as described above. The method essentially involves spirally assembling said strips in superposed manner by means of assembly apparatus 20. Assembling is performed such that the respective side edges of each strip touch so as to form continuous layers of sheath. As mentioned above, at least one of said strips Ila, llb, llc is made of a plastics material, and, according to a preferred aspect of the method of manufacturing, at least two of said strips Ila, llb, llc are made of plastics material so as to form at least two superposed plastics layers in the multilayer structure of the sheath.

Each strip Ila, llb, lic, is paid out from a roll or a drum 50a, 50b, 50c, and it advantageously penetrates firstly into a rib-forming first module 40 in which at least one longitudinal rib 14a, 14b, 14c is formed in said strip Ila, llb, 1lc. The modules 40 are organized to form identical ribs 14a, 14b, 14c in the various strips making up the multilayer structure, and they advantageously include heater means enabling the material to be deformed to form said ribs Ila, llb, llc in the respective strips. Said strips Ila, llb, llc making up the multilayer structure are then assembled, preferably simultaneously as shown in Figures 3 and 4, by means of assembly apparatus 20. Assembling is preferably performed by spirally winding said strips so that the their longitudinal ribs Ila, llb, llc are mutually superposed. Said strips thus form the various layers 10b, 10c of the multilayer structure of the sheath.

In a first variant of the invention, said assembly apparatus 20 includes a stationary cylindrical former 21 which, in this example, is provided with at least one continuous helical or spiral thread 22 over its outside surface. In this case, the assembly apparatus 20 is organized to assemble a sheath having a multilayer structure that has a substantially circular crosssection. As shown in Figure 2, the former 21 is provided with a continuous spiral thread 22 for each longitudinal rib 14a, 14b, 14c of the various strips llAa, llb, llc.

Thus, in the example shown, each of the strips is provided with three longitudinal ribs and the former is provided with three threads. Thus, each of said longitudinal ribs 14a, 14b, 14c of each of said strips is engaged on a respective spiral thread 22 of the former 21, during assembly. Assembling is advantageously performed by helically winding said strips about said former 21. As shown in Figure 3, the winding is preferably performed by means of drive wheels 25 which are advantageously disposed sloping so as to follow said continuous spiral threads 22 of the former 21, and which are rotated by suitable drive apparatus 26. The wheels act in particular to guide the strips so that their longitudinal ribs 14a, 14b, 14c engage on said spiral threads 22 of the former 21. Each strip Ila, llb, llc is thus wound around said former 21 to form a respective layer 10a, 10b, 10C of the multilayer structure of the sheath 1, the winding being performed such that the side edges 12a, 13a; 12b, 13b; 12c, 13c of each strip Ila, llb, lc are adjacent to each other and touching after the strip is wound around said former 21. This can be seen in particular in Figure 2 which also shows how the three strips Ila, llb, llc are wound such that they are mutually superposed with their respective side edges offset axially. Preferably, after the longitudinal ribs 14a, 14b, 14c have been formed by means of the respective forming modules 40, and prior to winding by means of said assembly apparatus 20, the method advantageously makes provision to dispose a heater module 30 for each strip of plastics material serving in particular to heat its side edges 12a, 13a; 12b, 13b; 12c, 13c. In this way, during winding around the former 21, the heated adjacent edges of each strip of plastics material are welded together by heat sealing by means of said drive wheels 25 which, in this case, also act as presser wheels. This characteristic makes it possible to obtain a watertight and homogeneous layer of plastic with each strip of plastics material.

Preferably, each of said heater modules 30 also serves to heat "fusing" zones 18 locally in each strip so that, during winding, the respective heated zones 18 of two strips which are superposed during said winding are pressed together by means of said presser wheels 25 so that said superposed layers formed by said two strips are intimately bonded in said fusing zones 18. Irreversible bonds are thus obtained between the various layers of the multilayer structure, thereby guaranteeing that the sheath 1 is properly uniform.

In certain applications in which the sheath is not embedded in concrete, the spiral ribs no longer serve to transfer forces to the concrete. They may therefore be omitted. It should however be noted that the ribs contribute to imparting good rigidity to the sheath.

Figure 4 shows a second variant of the abovedescribed assembly apparatus 20. In this variant, instead of having one stationary former, the assembly apparatus 20 has two rotary cylindrical formers 23a, 23b, each of which may be provided with one or more continuous spiral threads (not shown) over its outside surface. The set made up of said two rotary formers 23a and 23b is itself mounted to rotate about an axis. Thus, the strips are wound around said two formers 23a, 23b to form a multilayer structure having an oblong cross-section.

Once the various strips have been assembled together to form the multilayer structure, said structure advantageously passes through a cooling module 60 to set the shape of said sheath 1. Since the heater modules act only locally, i.e. on the side edges and in particular fusing zones of each strip, cooling is not essential, and in any event does not need to be very considerable.

Then, the multilayer structure preferably passes through a cutting module 70, in which said sheath is cut to any desired length.

The above-described method thus offers the following very important advantages: the sheath obtained comprises a multilayer structure, which offers the above-mentioned advantages, namely that, depending on the desired characteristics, the number of layers and/or the materials of the strips constituting the layers can be varied very simply; the machine for implementing the method is simple and compact, and can therefore be used on site, i.e. on a building site, to manufacture the sheath having a multilayer structure of the invention; the sheath obtained by implementing this method may be provided with one or more continuous spiral threads over its outside surface, thereby enabling sheath couplings to be made merely by screwing on suitable sleeves, regardless of the desired length of the sheath; the cutting module makes it possible to cut the sheath obtained to any desired length; the fact that the method can be implemented on the building site optionally makes it possible to make very long sheaths, which is impossible when the sheaths have to be transported from the factory to the building site; the costs in terms of energy of said method are low because heating is applied at localized points only; and merely by replacing the stationary former of the first variant of the assembly apparatus with a former having a slightly larger outside diameter, the machine for implementing the method of manufacturing the sheath of the invention also makes it possible to make said coupling sleeves on site, which sleeves are screwed onto the ends of said sheaths to couple them together.

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1. A prestress or stay cable sheath, characterized in that it comprises a multilayer structure made up of at least two superposed continuous layers of plastics material, each layer being constituted by a strip disposed helically and whose side edges are touching and sealed together, said at least two layers of plastics material being welded together at at least one point.

2. A cable sheath according to claim 1, provided with at least one continuous helical thread over its outside surface.

3. A cable sheath according to any preceding claim, in which an insert such as a metal wire is disposed in the multilayer structure of the sheath, in particular inside at least one helical thread.

0 b 20 4. A method of manufacturing a cable sheath, the method being characterized in that it includes the following steps: 0*S* supplying at least two strips of plastics material; ~assembling them helically and in superposed manner by *25 means of assembly apparatus such that the respective edges of each of said strips are touching so as to form a sleeve having a multilayer structure made up of at least two layers of plastic; and welding together said at least two superposed layers of plastic of the sheath at at least one point.

A method according to claim 4, in which, prior to assembly, fusing zones of the strips of plastic material where said strips are to be fused together are locally heated, the welding step involving pressing the respective heated fusing zones of the strips of plastics material that are superposed during assembly, so that

Claims (6)

1. 6. A method according to any one of claims 4 or 5, in which, prior to assembly, the side edges of each strip of plastics material are heated, the assembly step further involving pressing the heated side edges of each strip of plastics material when they are adjacent so as to weld them together by heat sealing to form a homogeneous and watertight layer of plastics material of the sheath.
2. 7. A method according to any one of claims 4 to 6, in which, prior to assembly, at least one longitudinal rib is formed in each strip, and the assembly step involves winding the strips helically such that the respective longitudinal ribs of the strips are superposed on one another to form at least one continuous helical thread over the outside surface of the sheath. 20 8. A machine for implementing the method according to any one of claims 4 to 7, said machine being characterized in that it includes at least two drums for receiving rolls of strip so as to supply said at least two strips, and assembly apparatus for assembling the strips helically such that their respective side edges are touching.
3. 9. A machine according to claim 8, in which the assembly apparatus includes a stationary cylindrical former around which the strips are helically wound. A machine according to claim 9, in which the assembly apparatus includes two rotary cylindrical formers around which the strips are helically wound, the set formed by said two rotary formers itself being rotary. -16-
4. 11. A machine according to claim 9, further including a rib-forming module for each strip, serving to form at least one longitudinal rib in each strip, prior to strip assembly, said stationary cylindrical former being provided with at least one continuous helical thread over its outside surface, on which thread a respective rib of each strip is wound.
5. 12. A cable sheath or a method of manufacturing a cable sheath substantially as herein described with reference to the accompanying drawings.
6. 13. A machine for manufacturing a cable sheath, said machine being substantially as herein described with reference to the accompanying drawings. Dated this 7 th day of July 1999 :FREYSSINET INTERNATIONAL STUP By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia ABSTRACT A CABLE SHEATH HAVING A MULTILAYER STRUCTURE, A METHOD OF MANUFACTURING SUCH A SHEATH, AND A MACHINE FOR IMPLEMENTING THE METHOD The invention concerns a cable sheath comprising a multilayer structure made up of at least two superposed continuous layers (10a, 10b) of plastics material, each layer being constituted by a strip (lla, lib) disposed helically and whose side edges (12a, 13a; 12b, 13b) are touching and sealed together. Translation of the title and the abstract as they were when originally filed by the Applicant. No account has been taken of any changes that may have been made subsequently by the PCT Authorities acting ex officio. e.g. under PCT Rules 37.2, 38.2. and/or 48.3.