FR2762864A1 - SHEATHING ELEMENT FOR PRE-STRESS CABLE - Google Patents

Abstract

The sheath element (35) is designed to pass through a portion (9) of the prestressed structure in an upper part of a winding path followed by a prestressing cable. The sheath element comprises a vent channel (41) parallel to the tubular cavity (20) receiving the strands of the cable and of substantially smaller section than the tubular cavity. This vent channel is arranged so that after placing the sheath element (35) in said portion of the structure, the vent channel is positioned above the tubular cavity and communicates with the tubular cavity in a region of the sheath member located inside said portion of the structure, and with the exterior of the sheath in a region of the sheath member located outside said portion of the structure. the book. Use in particular in the field of removable external prestressing.

Description

SHEATHING ELEMENT FOR PRE-STRESS CABLE

  The present invention relates to the field of

  prestressing cables for construction works.

  It relates more particularly to cables consisting of a set of strands, the tensioning of which exerts prestressing, housed in a tubular sheath filled with a protective material. The sheath is usually made of plastic such as high density polyethylene (HDPE). The protective material

  is typically a cement grout.

  The prestressing cables of construction works frequently have a sinuous shape path comprising high points and low points, the prestressing force exerted by the cable being directed downwards

  in the vicinity of high points and vice versa.

  When injecting the sheaths of these cables with the cement slurry, the air present in the sheath, water which separates from the cement slurry by decantation or filtration, and the "poor" grout, this is ie whose decantation or filtration has extracted the heaviest constituents, are evacuated at the high points using devices called vents. These vents generally consist of a plastic shell: tightly attached to the sheath and a tube, integral or fixed to this shell, which allows to channel the grout, water and air out material

  constituting the structure (most often concrete).

  When the cable sheaths are completely embedded in the structure, this technique is called internal prestressing in concrete. In this case, it is easy to place vents at high points when pouring concrete and to equip them with sufficiently long tubes to ensure the evacuation of air, water and

poor grout.

  The present invention relates rather to the technique of external prestressing, in which the sheaths of the cables, in the rectilinear sections of

  their layout, are placed outside the structure.

  They nevertheless pass through the structure in solid masses in order to take up the forces exerted by the cable and to transmit them to the structure. The high and low points of

  cable are then located in deflecting spacers.

  In the particular case of removable external prestressing, the cable sheaths pass through these spacers passing through metal shuttering tubes, bent and embedded in the concrete. The cable can then be dismantled for replacement, by cutting it in accessible areas and releasing it

spacers and anchors.

  The space existing between the shuttering tube and the HDPE tube being small (typically 10 to 20 mm difference in diameter) and curved, it is impossible to place a conventional vent and thus ensure the correct filling of the sheath. It is therefore common to find voids at high points, due to the impossibility of correctly eliminating air and filtration water. In order to overcome the lack of high point vent, it is possible to reinject grout through orifices placed on either side of the high point. But such a technique is complex and delicate, and is not always entirely satisfactory. The installation of a vent from the top of the structure, through the concrete of the upper slab and the shuttering tube, would appear to be a complex and delicate operation to install a removable external prestress on a new structure under construction, and would be impossible to achieve in the case of strengthening a

preexisting work.

  An object of the present invention is to facilitate the removal of undesirable components from the sheath filling material at the high points of a

  external prestressing cable.

  To this end, the invention proposes a sheath element intended to cross a portion of the structure in an upper part of a sinuous path followed by the prestressing cable. According to the invention, this sheath element comprises a vent channel parallel to the tubular cavity receiving the strands and of section substantially smaller than said tubular cavity. This vent channel is arranged so that after the sheath element has been placed in said portion of the structure, the vent channel is positioned above the tubular cavity and communicates with the tubular cavity in a first region of the sheath element located inside said portion of the structure and with the outside of the sheath in a second region of the sheath element located outside

of said portion of the work.

  The poor air, water and / or grout can thus be captured in the first region, where the setting up of a conventional vent can be delicate, and evacuated

  in the second region, which is accessible from the outside.

  The highest point of the cable will normally be included in said first region, although this region could be somewhat offset from this highest point, especially downstream relatively to

the grout injection direction.

  An important advantage of the invention is that said first region, in which air, water and / or lean grout are extracted from the sheath during injection, can extend over a certain length of the duct element, instead of being punctual as in the case of conventional vents. This ensures more efficient evacuation. This length can be at least 50 cm or, preferably, at least 50 cm on either side of the highest point of said upper part of the

winding path of the cable.

  The invention is applicable in the field of non-removable external prestressing (the concrete of the structure being poured around the sheath over part of the length of the element). It then facilitates the installation of vents, especially when the construction of the structure is such that it is difficult to predict the

  passage of a tube towards the top of the structure.

  The invention is applicable even more advantageously in the field of removable external prestressing. The vent integral with the sheath element allows the cable to be dismantled after the grout has hardened. It can be used both in the case of new structures and in the case of reinforcements of existing structures. Other features and advantages of the

  present invention will appear in the description below

  after nonlimiting exemplary embodiments, with reference to the accompanying drawings in which: - Figure 1 is a block diagram illustrating an example of external prestressing; - Figure 2 is a diagram in longitudinal section of a sheath element according to the invention installed in a deflecting spacer; - Figures 3 and 4 are cross-sectional views of the sheath element of Figure 2 along the planes III-III and IV-IV indicated in Figure 2; - Figure 5 is a diagram similar to that of Figure 2 showing another sheath element according to the invention; and - Figure 6 is a cross-sectional view of this other element along the plane VI-VI indicated on the

figure 5.

  In the embodiment illustrated by FIG. 1, the prestressed structure 7 is a concrete bridge, the deck of which is supported on piers 8, 9. A prestressed cable 10, the strands of which are anchored at their ends by means of conventional anchoring devices 11, is used to exert a prestressing force on the deck 7. This cable 10 follows, between the anchoring devices 11 supported by the end stacks 8, a sinuous path in broken line whose straight portions are accessible from the outside. In the example considered, this winding path comprises a central high point, where the cable 10 crosses a diverter provided in a central stack 9, and two high end points, where the cable 10 crosses deviators

  respectively adjacent to the anchoring devices 11.

  Between the central stack 9 and each of the end stacks 8, the path of the cable 10 has a low point where it is guided in a deflecting spacer provided in a stack

intermediate 9a.

  The sheath of the cable 10 consists of a plastic tube, for example HDPE, which consists of successive elements which, in a conventional manner, are welded

  end to end or connected using heating sleeves.

  FIGS. 2 to 4 show a sheath element 15 according to the invention used in an upper part of the sinuous path of the cable, at the level of the central pile 9. A similar arrangement is provided in the upper parts of the cable located at the level of the piles end 8. At this level, the use of a sheath element according to the invention ensures good protection of the strands in the vicinity of the anchors, which can be sensitive points. In the case of an application with removable external prestressing, a curved steel formwork tube 16 has been embedded in the concrete of the pile 9. If the invention was implemented to reinforce a preexisting structure, the formwork tube 16, rather than being drowned directly in the concrete of pile 9, could be drowned in a concrete block attached to this pile 9 at the time of carrying out the reinforcement. The shuttering tube 16 has a diameter greater than that of the sheath of the cable 10. In the example considered, o the cable 10 comprises nineteen strands 17 of diameter 15.2 mm or 15.7 mm housed in a sheath of diameter outside 110 mm, one can for example use a shuttering tube 16 of inside diameter 128 mm. In the embodiment of Figures 2 to 4, the sheath element 15 has a circular section over its entire length. Inside this circular section are on the one hand a tubular cavity 20 occupying most of the section and intended to receive the strands 17, and on the other hand a vent channel 21. The section of the channel d vent 21 is significantly smaller than that of tubular cavity 20 (it is for example from 200 to

500 mm2).

  When the sheath element 15 is installed in the shuttering tube 16, the vent channel 21 is located above the tubular cavity 20, i.e. on the side

  outside of the curvature of the element 15.

  The vent channel 21 extends over only part of the length of the sheath element 15. At one of its ends, it is provided with an opening 22 (FIG. 4) which makes it communicate with the outside of the sheath. A threaded end piece 23 is welded around this opening 22 to allow the connection of a tube

threaded vent 24.

  The sheath element 15 is engaged in the shuttering tube 16 by its end opposite to the end piece 23, this end piece 23 remaining outside in the final position of the sheath element 15, as shown in FIG. 2. In this position, the vent channel 21 extends beyond the highest point of the passage of the cable through the stack 9 relative to the position of the end piece 23. In a region extending from both sides other from the highest point over a length L1, L2, the wall 26 separating the vent channel 21 from the tubular cavity 20 of the sheath element has orifices 25 which make the vent channel 21 communicate with the tubular cavity 20. These orifices, the section of which is for example of the order of 12 mm,

  are distributed over the length L1 + L2 of this region.

  This total length L1 + L2 is preferably at least 50 cm. More preferably, the length L1, L2 of this region on each side of the highest point is at least cm. The orifices 25 are for example staggered along region L1 + L2, so that some of them (shown diagrammatically by dashed lines in FIG. 3) are close to the junction between the separating wall 26 and the cylindrical wall of the cavity

tubular 20.

  After assembling the successive sheath elements, having threaded the strands 17 there and having tensioned and anchored these strands 17, the cement grout is injected into the sheath to fill the space

left free by the strands.

  In a first phase, the grout is injected through one of the ends of the sheath, or through one of the low points of its winding path. In this phase, the vent tube 24 is opened to let the air inside the duct escape. It could also be

  blocked, especially if a thixotropic grout is used.

  The settling of the grout and its filtration through the internal interstices of the strands 17 cause the formation of pockets of water and poor grout at the high points of the sheath. Once a time of the order of half an hour to two hours has elapsed after this first injection phase, two successive reinjections of grout are carried out by two orifices not shown, provided in the sheath on either side of the stack 9. During these two re-injections, the vent tube 24 is open. The first is carried out from the orifice located downstream of the stack 9 relative to the direction of the injection carried out in the first phase. During this first reinjection, the vent tube 24 rejects water, poor grout and possibly air which may have remained in the sheath element due to turbulence which may occur during the first injection downstream of the point the upper. During the second reinjection, carried out from the opposite side, the water and the poor grout that may remain in the sheath are in turn evacuated by the

channel 21 and the vent tube 24.

  The sheath element 15 has the advantage that the capture of the components to be evacuated is not punctual,

  but distributed over the length L1 + L2.

  As shown in FIG. 2, the wall 26 separating the vent channel 21 from the tubular cavity 20 is gently connected, with a grazing incidence, to the cylindrical wall of the tubular cavity 20 at the two ends of the vent channel 21. This prevents the ends of the channel 21 from forming obstacles to the threading of the strands 17

  inside the tubular cavity 20.

  In the embodiment illustrated in FIGS. 2 to 4, the tubular cavity 20 of the sheath element has a circular section which includes the section of the vent channel 21. To make such a sheath element, one possibility is to weld together two pieces of generally tubular shapes obtained from two plastic tubes. One of these parts 32 constitutes the major part of the sheath element 15. It corresponds to a tube having a longitudinal notch produced by cutting a first strip 27 which

  will constitute the upper wall of the vent channel 21.

  The opening 22 is pierced at one end of this strip 27, and the end piece 23 is welded, at 28, around this

  opening 22 on the outer face of the strip 27.

  The other plastic tube is cut to release a second strip which will constitute the bottom wall 26 of the vent channel 21, separating it from the tubular cavity 20. The orifices 25 are drilled in this second strip 26, the edges of which longitudinal are then welded at 30, on the inner face of the first strip 27. The first strip 27 thus completed by the formation of the vent channel 21 is finally

  rewelded, at 31, onto the tube from which it comes.

  As is usual in plastic boilermaking, the welds 28, 30, 31 can be carried out by depositing a rod of melted HDPE using a hot air gun. The sheath element 35 shown in FIGS. 5 and 6 differs from that previously described by the constitution of its vent channel 41. This vent channel 41 is produced from a tubular piece of HDPE 47 whose cross section is generally circular in shape. This part 47, which has the orifices 25 distributed on the inner side of its curvature, is welded to another part 52 obtained as previously by making a longitudinal cut on a plastic tube. The part 47 forming the vent channel 21 is fixed, using two diametrically opposite weld beads 51 on the edges of the notch of the part 52. Thus, as shown in FIG. 6, the section of the channel vent 41 exceeds some

  little of the circular section of the tubular cavity 20.

  As the cross section of the vent channel 41 is reduced, it can be ensured that this excess does not hinder the introduction of the sheath element 35 into the tube

formwork 16.

  On the side of its free end 44, provided with closure means not shown, the vent channel 41 tangentially separates from the wall of the tubular cavity 20, as shown in the right part of the

figure 5.

Claims (8)

R E V E N D I C A T IONS   1. Sheath element for a prestressing cable (10) to be installed along a winding path on a construction work (7), the cable comprising, inside the sheath, strands (17) stretched to prestress the structure and a sheath filling and strand protection material, said sheath element (15; 35) being provided to pass through a portion (8, 9) of the structure in an upper part of said winding path, characterized in that that it comprises a vent channel (21; 41) parallel to the tubular cavity (20) receiving the strands and of section substantially smaller than said tubular cavity, and in that the vent channel is arranged so that '' after placement of the sheath element in said portion of the structure, the vent channel is positioned above the tubular cavity and communicates with the tubular cavity in a first region of the sheath element located inside said portion of the structure and with l of the sheath in a second region of the sheath element located outside of said portion of the book.   2. Sheath element according to claim 1, in which said first region of the sheath element, o the vent channel (21; 41) communicates with said tubular cavity (20), extends over a length (Ll + L2) of minus 50 cm.   3. Sheath element according to claim 2, in which the length (L1, L2) of said first region is at least 50 cm on either side of the highest point   of said upper part of the sinuous path of the cable.   4. Sheath element according to any one of   claims 1 to 3, wherein the vent channel (21;   41) is separated from said tubular cavity by a wall (26) provided with orifices (25) distributed along said   first region of the sheath element (15; 35).   5. Sheath element according to any one of   claims 1 to 4, produced by first and second welding   second plastic parts of generally tubular shapes, the first part (26, 27; 47) corresponding to the wall of the vent channel (21; 41) and the second part (32; 52) corresponding to the wall of said cavity tubular (20) with a longitudinal notch at the edges   from which the first part is welded.   6. Sheath element according to claim 5, wherein said first part (47) has a section substantially circular.   7. Sheath element according to any one of   claims 1 to 4, wherein the tubular cavity (20)   has a circular section which includes the channel section vent (21).   8. Sheath element according to any one of   claims 1 to 7, wherein the vent channel (21;   41) is separated from said tubular cavity (20) by a wall (26) which, at its two ends, gently connects to the wall of the tubular cavity so as to   do not obstruct the threading of the strands (17).