Method for relieving stress on at least one prestressing reinforcement and civil engineering structure

Abstract

Method for relieving stress on at least one prestressing reinforcement and civil engineering work Method for relieving stress on a strand (11) anchored at its ends in respective anchors (20) each comprising an anchoring block (22) and at least one jaw (24) to retain it in the anchoring block, this method comprising the steps consisting of: at a first of said anchors (20): re-anchoring the reinforcement, in particular the strand (11), behind the anchoring block (22) of this first anchor, in a tool anchoring block (50), using a tool jaw, heating the tool jaw to a high temperature until the tightening force of the reinforcement, in particular the strand (11), is released by this tool jaw. Figure for the abstract: Fig. 2

Description

Title of the invention: Method for relieving stress on at least one prestressing reinforcement and civil engineering structure Technical field

[0001] The present invention relates to the field of civil engineering prestressing and more particularly to replaceable non-adherent prestressing using reinforcements individually protected by coating with a flexible product and individually sheathed, such as "greased sheathed" type strands. Prior art

[0002] Non-adherent prestressing is mainly used in applications requiring the ability to measure the tension of the prestressing cable, to re-tension or relax it, and if necessary to be able to replace it completely or partially, in particular its reinforcements. In the case of reinforcements of the “greased sheathed strand” type, the individual sheathing and coating allow the individual tensioning and replacement of each reinforcement.

[0003] The cable is generally made up of a bundle of reinforcements (conventionally strands) anchored at each of their ends in an anchor composed of an anchor plate called a "tromplate", ensuring the convergence of the reinforcements and the diffusion of forces in the concrete of the structure, a steel anchor block pierced with conduits for the reinforcements, resting on the trombplate, and jaws ensuring the individual anchoring of the reinforcements in the block. Each conical jaw engaging a reinforcement is housed inside a truncated hole at the end of the corresponding conduit of the anchor block. The radial tightening of the jaw, obtained by wedge effect and increasing with the tension force prevailing in the reinforcement, allows the latter to be anchored. Each strand typically comprises 7 wires (6 wires helically around a central wire), generally being of the T15 type (frg=1860MPa, Ai=150mm2).The jaw generally comprises three truncated cone keys, each with an angular sector of 120°, held together by an open elastic annular ring.

[0004] The reinforcements generally have an excess length behind the anchor block, and the excess lengths are contained in a waterproof cover filled with a flexible protective product (generally grease or wax) to protect them from attack and in particular from corrosion.

[0005] Replacing the cable requires the existing reinforcements to be detensioned, prior to installing replacement reinforcements.

[0006] The excess length is used to partially or completely relax all of the reinforcements or each reinforcement individually using a jack.

[0007] In known works where the cable lengths remain limited (<50m for example), this excess length is chosen to be sufficiently long to allow the tensile force in each reinforcement, linked to the elastic elongation of the latter, to be completely cancelled.

[0008] The covers equipping cables covering large spans are for this reason necessarily long and consequently difficult to integrate from a practical or architectural point of view. A cable 200m long typically has an elastic elongation of 1.40m, i.e. a useful excess length of between 1.6m and 2m (i.e. 0.8m to 1.0m at each anchor).

[0009] Furthermore, the costs of supplying and installing these hoods are not negligible for construction.

[0010] An alternative solution to the use of stress relief jacks consists of heating the end of the strand to be relaxed at a high temperature with an oxyacetylene torch until the anchoring jaw ceases to perform its function and allows the strand to slide. This has the disadvantage of creating a risk of sudden and uncontrolled exit of the wires from the strand or the entire strand and also of damaging its individual sheathing, conventionally made of HDPE. The individual sheathing cannot be replaced, being generally fixed in a cement grout injected into the conduit or collective sheath of the prestressing cable, and must remain intact in order to allow the replacement of the steel strand and to maintain the good anti-corrosion protection that this sheathing provides. Presentation of the invention

[0011] There is therefore a need to benefit from a method of relieving tension and replacing a non-adherent prestressing cable making it possible to reduce the size of the excess length and the protective cover, and remedying the above drawbacks. Summary of the invention

[0012] The invention aims to meet this need and has as its subject a method for relieving tension of at least one prestressing reinforcement anchored at its ends in respective anchors each comprising an anchoring block and at least one jaw adapted to be applied to the reinforcement to retain it in the anchoring block, this method comprising the steps consisting of:

[0013] at the level of a first of said anchors: a. re-anchor the reinforcement, behind the anchor block of this anchor, in a tool anchor block, using a tool jaw, b. heat the tool jaw to a high temperature until the clamping force of the armature is released by this tool jaw.

[0014] The reinforcement is typically a strand of a prestressing cable, the invention however not being limited to a strand.

[0015] The operation of detensioning by heating on an attached tool block makes it possible to guarantee the integrity of each individual sheathing. The invention thus allows complete detention and easy replacement of each reinforcement, in particular of each strand of the bundle constituting a prestressing cable, and the replacement reinforcements, in particular strands, can be threaded into the existing individual sheathings. The invention also allows the replacement of the reinforcements, in particular strands, of a very long cable with excess lengths at the ends and anchoring covers of limited size.

[0016] The invention allows in particular, compared to the state of the art, the installation of the prestressing with mid-length hoods offering limited space requirements and less expensive anchoring arrangements. For limited space requirements behind the anchor, the excess length may be less than or equal to 80cm, better still 70cm, or even 60cm, being for example of the order of 50cm at both ends of the cable, while allowing partial detensioning of each reinforcement, in particular a strand, at one end, followed by release of the residual tension force by heating the tool anchoring jaws at the other end to a high temperature, as detailed below.

[0017] In an exemplary implementation of the invention, prior to step a) a coupler, in particular a single-strand coupler, is installed on one end of the reinforcement, in particular the strand, which extends outside the tool anchoring block to couple it to a tool strand on which a jack can engage. This makes it possible to work on a reinforcement, in particular a strand, having a small excess length.

[0018] The method may include the installation of a stress relief support, also called a stress relief chair, on which the jack rests.

[0019] The method may include the installation of a spacer (also called a "chair") intended to serve as a support for the tool anchoring block to provide a minimum clearance relative to the permanent anchoring block. This clearance makes it possible to extract each anchoring jaw of a reinforcement, in particular a strand, to be relaxed from the permanent block and makes it possible to avoid damaging the permanent anchoring block during heating.

[0020] Preferably, before step b), the reinforcement, in particular the strand, is coupled to a tool wire. This tool wire makes it possible to exert traction on the reinforcement, in particular the strand, after its detensioning, with a view to its replacement.

[0021] When the reinforcement is made up of a strand, this strand may have a central wire, in which case the strand is advantageously coupled to the tool wire using a coupler into which only the central wire and the tool wire are introduced.

[0022] The method may comprise, before step b), the protection of neighboring reinforcements, in particular neighboring strands, if any, using thermal protection arranged around the additional jaw to be heated, this thermal protection being in the form, for example, of a temperature-resistant cylinder.

[0023] The method according to the invention may comprise the steps consisting of: - At the level of a second of said permanent anchors, in the case where the reinforcement, in particular the strand, has an excess length, and before implementing step b), better before implementing step a): • use a jack to regain tension in the reinforcement, particularly the strand, to release the jaw of the permanent anchor block from this second anchor, and • after releasing the jaw, partially relax the reinforcement, particularly the strand, by reducing the excess length.

[0024] This makes it possible to carry out the stress relief in step b) on a reinforcement, in particular a strand, which is already partially relaxed.

[0025] A replacement reinforcement, in particular a strand, may be coupled, after complete stress relief of the reinforcement, in particular the strand, to the end thereof located on the side of the second anchor.

[0026] The reinforcement to be replaced can be winched from the side of the first anchor, the replacement reinforcement preferably being pushed from the side of the second anchor.

[0027] A flexible product such as grease is advantageously injected with the replacement reinforcement when the latter is threaded to replace the existing reinforcement, and when the replacement reinforcement is a strand protected by a sheath, the sheath can be continuously removed when the strand is introduced.

[0028] The process can be repeated for several reinforcements, in particular strands of the prestressing cable.

[0029] The invention also relates to a civil engineering work making it possible to benefit from the stress relief method according to the invention.

[0030] This civil engineering work comprises at least one greased sheathed multi-strand cable, with non-adherent prestressing, anchored at its ends in anchors each comprising a trumplate on which rests an anchoring block crossed by truncated holes crossed by the strands of the cable and in which anchoring jaws of the strands are engaged, each strand having an excess length W extending behind the anchoring block, the excess length being less than or equal to (D / 2)+A, D designating the elastic elongation of the cable, and A a minimum anchoring length of a single-strand coupler

[0031] The length L of the cable may in particular be greater than or equal to 100m, or even 150m or even 200m. The excess length W may be less than or equal to L*0.007 / 2+A m. For a single-strand coupler, A=0.100m may be considered. Brief description of the drawings

[0032] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the attached drawing, in which:

[0033] [Fig. 1 A] [Fig. 1 A] schematically and partially represents a first side of a civil engineering structure comprising a non-adherent prestressing cable anchor, with part of the stress relief system,

[0034] [Fig.lB] [Fig.lB] is a view similar to [Fig.lA] of an alternative embodiment of the stress relief system,

[0035] [Fig.2] [Fig.2] schematically and partially represents the second side of the structure, also equipped with an anchor for the prestressing cable, with another part of the stress relief system,

[0036] [Fig.3] [Fig.3] schematically and partially represents a variant of the stress relief system of [Fig.2],

[0037] [Fig.4] [Fig.4] schematically and partially illustrates the preparation of the strand for coupling it using a coupler engaging the central wire,

[0038] [Fig.5] [Fig.5] schematically and partially represents an example of a coupler for a central wire,

[0039] [Fig.ôA] [Fig.ôA] illustrates the preparation of the strands to be replaced with a view to connecting the replacement strands,

[0040] [Fig.ôB] [Fig.ôB] illustrates the connection of the replacement strands to the strands to be replaced,

[0041] [Fig.7] [Fig.7] illustrates the connection of the strands to be replaced to tool wires,

[0042] [Fig.8] [Fig.8] illustrates the heating of an anchoring jaw of the tool block,

[0043] [Fig.9] [Fig.9] illustrates the threading of the replacement strand,

[0044] [Fig. 10] [Fig. 10] illustrates the exertion of traction on the tool wire to extract the strand to be replaced,

[0045] [Fig. 11] [Fig. 11] represents in isolation, in perspective and schematically, an example of anchoring and prestressing cable anchored using such an anchor,

[0046] [Fig. 12] [Fig. 12] shows in cross-section and in exploded view, schematically, the keys of an anchoring jaw and the strand on which they are intended to be applied, and

[0047] [Fig. 13] [Fig. 13] shows a side view of an anchoring bit. Description of the embodiments

[0048] Figures 1A and 2 show the sides A and B of a civil engineering structure C comprising at least one non-adherent prestressing cable (not shown in its entirety) anchored at its ends in the structure using anchors 20 each comprising, in a manner known per se, a trumplate 21, and a metal anchor block 22 resting on the trumplate 21.

[0049] The anchoring block 22 comprises, as illustrated in particular in figures 11 to 13, frustoconical holes 23 in which are engaged anchoring jaws 24 comprising toothed keys 25, joined by an open annular ring 26 received in a groove 27.

[0050] The cable comprises a bundle of strands 11 each engaged in a hole 23 of the anchoring block 22 and held therein using a jaw 24.

[0051] The following description preferably relates to a prestressing reinforcement consisting of a strand of prestressing cable, but the invention applies to prestressing reinforcements other than strands.

[0052] Each strand 11 has at each end an excess length W, as illustrated in [Fig.1A], which extends outside the anchoring block 22, away from the trumplate 21. The excess lengths may differ on each side.

[0053] A protective cover (not shown) protects the excess lengths of the strands.

[0054] The strands 11 are preferably of the 7-wire T15 steel type, and comprise a straight central wire and six helical wires wound around the central wire.

[0055] The excess length W can be relatively short thanks to the invention, and it is advantageously less than or equal to 50 cm.

[0056] To replace each strand 11, one can start by removing the covers and the wax covering the strands 11 and anchor block 22 at each end.

[0057] Then, we can proceed to the following action:

[0058] Side A: - Partially relax the strand 11 using a single-strand jack 43, for example type SC2 offered by the Freyssinet company, the nose of which rests on the anchor block 22 as illustrated in [Fig.lA]. The strand 11 cannot be completely relaxed because the total elastic elongation (7mm / m) is greater than the available excess length.

[0059] Alternatively, if the available excess length is not sufficient, the following actions can be carried out, as illustrated in [Fig.lB]: - Couple the end of the strand 11 to be relaxed with a tool strand 40, for example approximately 2m long, by means of a single-strand coupler 41, such a coupler being known in itself; - Install a stress relief chair 42 to support the hollow single-strand jack 43, - partially relax the strand 11 using the single-strand jack 43.

[0060] The area behind the anchor can then be secured to prevent any risk associated with a sudden projection of the strand 11 to the rear of the anchor, during the rest of the process.

[0061] The following actions can be performed on side B, as illustrated [Fig.2]: - Attach a tool anchor block 50 behind the existing anchor block 22, by inserting a spacer (also called a “chair”) 51 long enough to allow the anchor jaws 24 to be removed and to move this tool block 50 as far as possible from the existing block 22, taking into account the required grip length of a single-strand jack 53 used subsequently. The tool anchor block may have frustoconical holes 23 identical to those of the anchor block 22, being for example identical to the latter. The tool jaws 24 used with the tool block 50 may be identical to those used with the anchor block 22. - Using the single-strand jack 53, take up the tension force in the strand 11 and re-anchor it in the attached tool block 50 using a tool anchoring jaw 24; extract and dismantle the anchoring jaw from the permanent anchoring block 22.

[0062] If the strand 11 is too short, a single-strand coupler 54, a tool strand 55 and a stress relief chair 56 can be used, in the same way as for side A, as illustrated in [Fig.3].

[0063] The central wire 60 can then be extracted on side A and side B from the strands 11 concerned by cutting the peripheral wires 63 to reveal the central wire, over a suitable length, typically 100 mm, as illustrated in [Fig.4]. A bevel 62 can be made at the end of the peripheral wires 63 and a brazing 61 can be carried out.

[0064] On side B, the central wire 60 of the strand 11 can be coupled by means of a central wire coupler 70, called a “phi5” coupler, shown in [Fig. 5], known in itself and proposed by the company Freyssinet, to a tool wire 80, a few m long, preferably 2 to 5 m, as illustrated in [Fig. 7]. Alternatively, the coupling is achieved differently, for example by a ball weld.

[0065] This operation of releasing the central wire is carried out similarly on side A for the existing strand 11, as illustrated [Fig.6A].

[0066] It is then possible to connect by ball welding of the central wire or as illustrated in [Fig.6B] with a central wire connector 90 (for example identical to that illustrated in [Fig.5]), the strand to be replaced and a replacement strand 91, the central wire of the latter having been previously extracted from the other wires as for strand 11.

[0067] To release the tension on side B, the strand to be relaxed 11 is heated at high temperature with a blowtorch at the level of the tool jaw 24, as illustrated [Fig.8], until sliding of the strand in the tool jaw, allowing the force to be released; beforehand, the neighboring strands located near the strand to be relaxed may have been protected with a cylinder 120 placed around the tool anchor jaw 24 to be heated, on the tool anchor block 50. The strand 11 slides and drives the coupler 70 and the tool wire 80 through the permanent anchor 22 located on side B.

[0068] The above operations can be repeated for each of the strands 11 to be relaxed. Once all these strands 11 are relaxed, it is possible, if desired, to dismantle the jaws, the tool block 50 and the support spacer 51 on side B.

[0069] On side B, as illustrated in [Fig. 10], the existing strand 11 can be pulled with any suitable traction means 100, for example a winch, and the replacement strand can be pushed with a winch, tow bar, roller pusher / driver or equivalent means, as illustrated in [Fig.9]. When the tractive force to be exerted is too high, the operation can be completed by means of a single-strand jack.

[0070] The replacement strand 91 may be guided using one or more rollers 130 positioned near the corresponding hole 23 of the anchor block 22, as illustrated in [Fig.9]. The stripping of this strand 91 to remove a protective sheath may be carried out by making two diametrically opposed longitudinal cuts, as the strand 91 is threaded into the existing sheathing. In addition, the replacement strand 91 may be coated with a flexible product such as grease, continuously at the hole in the anchor block and the sheathing, in order to compensate for the grease extracted with the strand 11 to be replaced.

[0071] The operation can be stopped when the required excess length at the anchor is reached.

[0072] After having confirmed the good surface condition of the conical holes 23, it is possible to put into place the jaws 24 at both ends and tighten the strand 91.

[0073] These operations can be repeated for all the strands of the cable requiring replacement.

[0074] The covers can then be replaced and their volume reinjected with a flexible anti-corrosion protection product, such as grease or wax.

[0075] The excess lengths W can be relatively small, and the axial size of the hoods also, which facilitates their architectural integration.

[0076] Of course, the invention is not limited to the examples which have just been given.

[0077] For example, the invention can be applied to other types of reinforcements than 7-wire strands.

Claims

Claims

1. Method for relieving tension in at least one prestressing reinforcement, in particular a strand (11) of a prestressing cable (10), anchored at its ends in respective anchors (20) each comprising an anchoring block (22) and at least one jaw (24) adapted to be applied to the reinforcement, in particular the strand (11), to retain it in the anchoring block, this method comprising the steps consisting of: - at a first of said anchors (20): a. re-anchoring the reinforcement, in particular the strand (11), behind the anchoring block (22) of this first anchor, in a tool anchoring block (50), using a tool jaw, b. heating the tool jaw to a high temperature until the clamping force of the reinforcement, in particular the strand (11), is released by this tool jaw.

2. A method according to claim 1, wherein prior to step a) a coupler, in particular a single-strand coupler (54), is installed on one end of the reinforcement, in particular of the strand (11), which extends out of the tool anchor block (50) to couple it to a tool strand (55) on which a jack (53) can engage.

3. Method according to claim 2, comprising the installation of a stress relief support (56) on which the jack (53) comes to bear.

4. Method according to any one of the preceding claims, comprising the installation of a spacer (51) intended to serve as a support for the tool anchoring block (50) to keep it away from the anchoring block (22).

5. Method according to any one of the preceding claims, in which before step b), the reinforcement, in particular the strand (11), is coupled to a tool wire (80).

6. Method according to claim 5, the reinforcement being constituted by a strand (11) having a central wire (60), the strand being coupled to the tool wire (80) using a coupler (70) in which only the central wire and the tool wire are introduced.

7. Method according to any one of the preceding claims, comprising before step b) the protection of neighboring reinforcements, in particular neighboring strands, possibly using thermal protection placed around the tool jaw to be heated.

8. Method according to any one of the preceding claims, comprising the steps of: - At a second of said anchors (20), in the case where the reinforcement, in particular the cable, has an excess length (W), and before implementing step b), better before implementing step a): • taking up the tension of the reinforcement, in particular the strand, using a jack (43), to release the jaw (24) from the anchoring block (22) of this second anchor (20), and • after releasing the jaw, partially relaxing the reinforcement, in particular the strand (11), by reducing the excess length.

9. Method according to claim 1, a replacement reinforcement, in particular a replacement strand (91), being coupled after complete detensioning of the reinforcement, in particular of the strand (11), to the end thereof located on the side of the second anchor.

10. Method according to claim 9, the reinforcement to be replaced being winched from the side of the first anchor, the replacement reinforcement preferably being pushed from the side of the second anchor.

11. Method according to one of claims 9 and 10, a flexible product such as grease being injected with the replacement reinforcement when the latter is threaded to replace the existing reinforcement, method in which, when the replacement reinforcement is a strand protected by a sheath, the sheath is continuously removed when the strand is introduced.

12. Method according to any one of the preceding claims, being repeated for several reinforcements, in particular several strands (11) of the prestressing cable (10).

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