Stress-relieving process for at least one prestressing reinforcement and civil engineering structure

Abstract

Method for releasing tension in at least one prestressing reinforcement bar and civil engineering structure. Method for releasing tension in a strand (11) anchored at its ends in respective anchorages (20), each comprising an anchor block (22) and at least one jaw (24) for retaining it in the anchor block. This method comprises the steps of: at the level of a first of said anchorages (20): re-anchoring the reinforcement, in particular the strand (11), behind the anchor block (22) of this first anchorage, in a tool anchor block (50), using a tool jaw; heating the tool jaw to a high temperature until the clamping force on the reinforcement, in particular the strand (11), is released by this tool jaw. Figure for the abbreviation: Fig. 2

Description

Title of the invention: Method for relieving stress in 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-bonded prestressing using reinforcement bars individually protected by coating with a flexible product and individually sheathed, such as "greased sheathed" strands. Prior art

[0002] Non-adherent prestressing is primarily used in applications requiring the ability to measure the tension of the prestressing cable, to retighten or loosen it, and, if necessary, to replace it completely or partially, particularly its reinforcements. In the case of "greased sheathed strand" reinforcements, the individual sheathing and coating allow for the individual tensioning and replacement of each reinforcement.

[0003] The cable generally consists of a bundle of reinforcement bars (typically strands) anchored at each end in an anchorage comprising an anchor plate, known as a "reinforcing plate," which ensures the convergence of the reinforcement bars and the distribution of forces within the concrete structure; a steel anchor block, perforated with conduits for the reinforcement bars, resting on the reinforcing plate; and jaws that individually anchor the reinforcement bars in the block. Each conical jaw gripping a reinforcement bar is housed inside a frustoconical hole at the end of the corresponding conduit in the anchor block. The radial clamping of the jaw, achieved by a wedge effect and increasing with the tensile force in the reinforcement, allows the reinforcement to be anchored. Each strand typically comprises 7 wires (6 wires helically wound around a central wire), generally being of type T15 (frg=1860MPa, Ai=150mm²).The jaw generally comprises three truncated conical wedges, each with an angular sector of 120°, held together by an open elastic annular ring.

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

[0005] Replacing the cable requires releasing the tension on the existing armatures prior to installing replacement armatures.

[0006] The extra length is used to partially or totally relax the whole of the reinforcements or each reinforcement individually using a jack.

[0007] In known structures where cable lengths remain limited (<50m for example), this extra length is chosen to be long enough to allow the tensile force in each reinforcement, linked to its elastic elongation, to be completely eliminated.

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

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

[0010] An alternative to using tensioning jacks consists of heating the end of the strand to be relaxed to a high temperature with an oxyacetylene torch until the anchoring jaw ceases to function and allows the strand to slide. This has the disadvantage of creating a risk of sudden and uncontrolled release of the wires from the strand or the entire strand, and also of damaging its individual sheath, typically made of HDPE. The individual sheath cannot be replaced, as it is generally embedded in a cementitious grout injected into the conduit or collective sheath of the prestressing cable, and must remain intact to allow for the replacement of the steel strand and to maintain the effective corrosion protection provided by this sheath. Description of the invention

[0011] There is therefore a need for a method of unstressing and replacing a non-adherent prestressing cable which reduces the bulk of the excess length and the protective cover, and remedies the above disadvantages. Summary of the invention

[0012] The invention aims to meet this need and relates to a method for releasing tension from at least one prestressing reinforcement anchored at its ends in respective anchorages, each comprising an anchor block and at least one jaw adapted to apply to the reinforcement to retain it in the anchor block, this method comprising the steps 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 on the armature is released by this tool jaw.

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

[0015] The stress-relieving operation by heating on a separate tool block ensures the integrity of each individual sheath. The invention thus allows for complete retention and easy replacement of each reinforcement element, particularly each strand of the bundle constituting a prestressing cable, and the replacement reinforcement elements, especially strands, can be threaded into the existing individual sheaths. The invention also allows for the replacement of reinforcement elements, especially strands, in a long cable with excess length at the ends and compact anchoring caps.

[0016] The invention notably allows, compared to the prior art, the installation of prestressing with mid-length caps offering limited space requirements and less expensive anchoring arrangements. For limited space behind the anchor, the extra length can be less than or equal to 80 cm, preferably 70 cm, or even 60 cm, being, for example, on the order of 50 cm at both ends of the cable, while allowing for partial unstressing of each reinforcement, in particular a strand, at one end, followed by release of the residual tensile force by high-temperature heating of the tool anchor jaws at the other end, as detailed later.

[0017] In one embodiment 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 beyond the tool anchor 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, with a small excess length.

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

[0019] The method may include the installation of a spacer (also called a "chair") intended to support the tool anchor block in order to provide minimal clearance from the permanent anchor block. This clearance allows each anchor jaw of a reinforcement, particularly a strand, to be extracted from the permanent block and prevents damage to the permanent anchor block during heating.

[0020] Preferably, before step b), the reinforcement, in particular the strand, is coupled to a tool wire. This tool wire allows tension to be applied to the reinforcement, in particular the strand, after its release, for the purpose of its replacement.

[0021] When the armature 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 in which only the central wire and the tool wire are introduced.

[0022] The process may include, before step b), the protection of neighboring reinforcements, in particular of neighboring strands, if any, by means of a 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 include the steps of: - At the level of a second of said permanent anchorages, in the case where the reinforcement, in particular the strand, has an excess length, and before implementing step b), or better yet, before implementing step a): • Use a jack to readjust the tension of the reinforcement, particularly the strand, to free the jaw of the permanent anchor block of this second anchor, and • After freeing the bit, partially relax the armature, especially the strand, by reducing the excess length.

[0024] This allows the detensioning to be carried out in step b) on an armature, in particular a strand, which is already partially detensioned.

[0025] A replacement reinforcement, in particular a strand, can be coupled, after complete release of the reinforcement, in particular the strand, to the end thereof located on the side of the second anchorage.

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

[0027] A flexible product such as grease is advantageously injected with the replacement reinforcement when threading the latter in place of the existing reinforcement, and when the replacement reinforcement is a strand protected by a sheath, the sheath can be continuously removed during the introduction of the strand.

[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 enabling the benefit of the stress relief process according to the invention.

[0030] This civil engineering structure comprises at least one greased, sheathed, non-adherent, multi-strand cable anchored at its ends in anchorages, each anchorage having a swaged plate on which rests an anchor block. This block has truncated conical holes through which the strands of the cable pass and in which strand anchor jaws are engaged. Each strand has an excess length W extending behind the anchor block, the excess length being less than or equal to (D / 2)+A, where D denotes the elastic elongation of the cable, and A is a minimum anchorage length of a single-strand coupler.

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

[0032] The invention will be better understood upon reading the detailed description that follows, the non-limiting examples of its implementation, and upon examination of the accompanying drawing, on which:

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

[0034] [Fig.1B] [Fig.1B] is a view analogous to [Fig.1A] of a variant embodiment of the tension release system,

[0035] [Fig.2] [Fig.2] schematically and partially represents the second side of the structure, also equipped with a prestressing cable anchorage, with another part of the tensioning system,

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

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

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

[0039] [Fig. A] [Fig. A] illustrates the preparation of the strands to be replaced in view of connecting the replacement strands,

[0040] [Fig. B] [Fig. B] 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 application of traction to 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 an anchorage and a prestressing cable anchored using such an anchorage,

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

[0047] [Fig. 13] [Fig. 13] represents a side view of an anchoring jaw. Description of the implementation methods

[0048] Figures IA and 2 show sides A and B of a civil engineering structure C comprising at least one non-adherent prestressing cable (not shown in full) anchored at its ends in the structure by means of anchorages 20 each comprising, in a manner known per se, a bolt 21, and a metal anchor block 22 bearing on the bolt 21.

[0049] The anchor block 22 comprises, as illustrated in particular in figures 11 to 13, truncated conical holes 23 in which are engaged anchor 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 anchor block 22 and held therein by means of a jaw 24.

[0051] The following description preferably relates to a prestressing reinforcement made 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 extra length W, as illustrated in [Fig.1A], which extends outside the anchor block 22, away from the bolt 21. The extra 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 50cm.

[0056] To proceed with the replacement of each strand 11, one can start by removing the caps and the wax covering strands 11 and anchor block 22 at each end.

[0057] Next, we can proceed with the following action:

[0058] Side A: - Partially relax the strand 11 using a single-strand jack 43, for example of type SC2 offered by Freyssinet, whose nose rests on the anchor block 22 as illustrated in [Fig. 1A]. The strand 11 cannot be completely relaxed because the total elastic elongation (7 mm / m) is greater than the available excess length.

[0059] Alternatively, if the available excess length is insufficient, the following actions can be carried out, as illustrated in [Fig.1B]: - Couple the end of the strand 11 to be relaxed with a tool strand 40, for example of approximately 2m in length, by means of a single-strand coupler 41, such a coupler being known in itself; - Install a tensioning chair 42 to support the single-strand hollow cylinder 43, - partially loosen strand 11 using 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 continuation 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, inserting a spacer (also called a "chair") 51 sufficiently long to allow the removal of the anchor jaws 24 and to offset this tool block 50 as far as possible from the existing block 22, taking into account the required grip length of a single-strand cylinder 53 used subsequently. The tool anchor block may have frustoconical holes 23 identical to those of the anchor block 22, for example, being identical to it. 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 cylinder 53, take up the tension force in the strand 11 and re-anchor it in the added 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 tensioning 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 from side A and side B of the relevant strands 11 by cutting the peripheral wires 63 to expose 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 solder joint 61 can be made.

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

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

[0066] The strand to be replaced and a replacement strand 91 can then be connected by ball soldering of the central wire or as illustrated in [Fig.6B] with a connector 90 with central wire (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 at the tool jaw 24 is heated to a high temperature with a blowtorch, as illustrated [Fig. 8], until sliding of the strand in the tool jaw, allowing the force to be released; previously, the neighboring strands located near the strand to be released may have been protected with a cylinder 120 arranged around the tool anchoring jaw 24 to be heated, on the tool anchoring block 50. The strand 11 slides and drives the coupler 70 and the tool wire 80 through the permanent anchorage 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 have been relaxed, the jaws, the tool block 50 and the support spacer 51 can be disassembled from side B if desired.

[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, winch, roller pusher / driver or equivalent means, as illustrated in [Fig. 9]. When the traction force to be exerted is too high, the operation can be completed by means of a single-strand jack.

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

[0071] The operation can be stopped when the required overlength for anchoring is reached.

[0072] After confirming the good surface condition of the conical holes 23, one can put in Place the 24 jaws at both ends and stretch the 91 strand.

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

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

[0075] The extra lengths W can be relatively small, and the axial bulk of the hoods as well, which facilitates their architectural integration.

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

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

Claims

Demands

1. Method for releasing tension from at least one prestressing reinforcement, in particular a strand (11) of a prestressing cable (10), anchored at its ends in respective anchorages (20) each comprising an anchorage block (22) and at least one jaw (24) adapted to apply itself to the reinforcement, in particular the strand (11), to retain it in the anchorage block, this method comprising the steps of: - at the level of a first of said anchorages (20): a. re-anchoring the reinforcement, in particular the strand (11), behind the anchorage block (22) of this first anchorage, in a tool anchorage 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. 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 armature, 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 tension support (56) on which the cylinder (53) rests.

4. A 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 anchor block (50) to keep it away from the anchor block (22).

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

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

7. A method according to any one of the preceding claims, comprising, before step b), the protection of adjacent reinforcement bars, in particular of neighboring strands, possibly using thermal protection arranged around the jaw of the tool to be heated.

8. A method according to any one of the preceding claims, comprising the steps of: - At the level of a second of said anchorages (20), in the case where the reinforcement, in particular the cable, has an excess length (W), and before carrying out step b), or better, before carrying out step a): • using a jack (43) to take up the tension of the reinforcement, in particular the strand, to unjam the jaw (24) of the anchorage block (22) of this second anchorage (20), and • after unjamming the jaw, partially relax the reinforcement, in particular the strand (11), by reducing the excess length.

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

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

11. A method according to any one of claims 9 and 10, a flexible product such as grease being injected with the replacement reinforcement when threading the latter in place of the existing reinforcement, a method wherein, when the replacement reinforcement is a strand protected by a sheath, the sheath is continuously removed during the introduction of the strand.

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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