EP4276243B1 - Method for locally transferring the tension of a prestressing cable and system for bridging a prestressing cable using said method - Google Patents

Description

FIELD OF THE INVENTION

The invention belongs to the field of public works structures, more particularly engineering structures such as bridges integrating one or more prestressing cables.

More specifically, the invention aims at the maintenance of such structures, in particular when the prestressing cable(s) which they integrate have suffered damage, requiring correlative measures to be taken to safeguard the integrity of said structure.

PRIOR STATE OF THE TECHNIQUE

The implementation of prestressing cables external to concrete within engineering structures, and in particular bridges, is now widely used. Such prestressing cables undergo wear or corrosion phenomena over time, likely to cause their rupture and, as a corollary, that of the engineering structure within which they are integrated. This corrosion is generally not generalized over the entire cable, but concentrated at point defects, the vast majority either at connections of cable sheaths, or at so-called makeshift repairs dating from execution, or at the anchors.

Their monitoring has particularly increased in recent years, due in particular to the somewhat increasing number of bridge collapses, or more generally to the effects of aging being felt after thirty to forty years of service, since the external prestressing became widespread from the 1980s.

As part of this monitoring and the associated maintenance, it is most often appropriate to secure the corroded cable on either side of its fragile zone, then to consider its replacement. The elastic energy contained in a prestressing cable can greatly exceed 100 kilojoules. This energy can be released suddenly, particularly if the cable is injected with cement grout, which causes effects considerable dynamics, a risk for people near the cable, and possible damage to the structure beyond the simple loss of the cable.

For this purpose, it is known, for example from the document FR 3 112 561 , to position on either side of the fragile zone of the cable jaws, or collars, fixed in particular by pre-stressed bolting, and between which extend traction elements essentially made up of metal bars, and intended to take up the forces initially suffered by said cable.

This type of technique is defined as provisional, that is to say allows gentle relaxation of the cable without dynamic effect. It therefore allows the removal of the cable, generally with a view to its replacement, not its conservation. It is therefore common to replace a cable of more than 50 linear meters, while the corrosion defect on the same cable only exists over a length of the order of a meter.

In addition, this type of classic technique requires particular surface preparation to achieve the required coefficient of friction, which increases the exposure time of operators in the vicinity of potential rupture zones.

In addition, this type of tension recovery is achieved by the nailing force introduced into the bolts, and is therefore dependent on the perimeter available to carry out tightening. This is an active system, requiring monitoring or control measures to guarantee the conservation of the nailing force over the duration of the project.

Finally, regarding operations that can be carried out in an emergency context, the use of this type of solution requires the manufacture or provision of a jaw adapted to the precise type of cable encountered - residual force, external perimeter - which can lead to studies, development of methods and manufacturing, the delay of which can be penalizing.

The present invention aims to free itself from the installation of such jaws and therefore, as a corollary, to reduce the bulk necessary for their installation on the one hand, and on the other hand, to allow the conservation of the linear unbridged cable without requiring their removal.

STATEMENT OF THE INVENTION

• ▪ à positionner par calage au moins deux barres de précontrainte, lesdites barres de précontrainte s'étendant sensiblement parallèlement audit câble au niveau de la zone de reprise, et ce en amont et en aval de ladite zone ;
• ▪ à coffrer l'ensemble constitué par le câble et les barres de précontrainte en amont et/ou en aval de la zone de reprise, et sur partie au moins de la zone de reprise, définissant un volume sensiblement centré autour du câble ;
• ▪ à injecter, dans ledit volume, du béton fibré BFUP (béton fibré ultra haute performance) ou une résine de caractéristiques équivalentes, venant occuper l'intégralité dudit volume ;
• ▪ à confiner mécaniquement ledit volume alors rempli par le béton fibré BFUP ou la résine ;
• ▪ à laisser le béton ou la résine prendre en masse ;
• ▪ à mettre sous tension les barres de précontrainte.

To this end, the invention aims first of all at a method for the local recovery of the tension of a prestressing cable of a structure and in particular of a work of art, with a view to its relaxation. This process consists of:

• ▪ positioning at least two prestressing bars by wedging, said prestressing bars extending substantially parallel to said cable at the recovery zone, upstream and downstream of said zone;
• ▪ to form the assembly consisting of the cable and the prestressing bars upstream and/or downstream of the recovery zone, and over at least part of the recovery zone, defining a volume substantially centered around the cable;
• ▪ to inject, into said volume, UHPC fiber-reinforced concrete (ultra high performance fiber-reinforced concrete) or a resin of equivalent characteristics, occupying the entirety of said volume;
• ▪ to mechanically confine said volume then filled with UHPC fiber-reinforced concrete or resin;
• ▪ to let the concrete or resin set;
• ▪ to put tension on the prestressing bars.

In other words, the invention consists of ensuring the transfer by adhesion of the forces initially undergone by the cable, over a suitable covering length between the existing reinforcement(s) or cables and the bridging profiles or support bars. prestressed, thus making it possible to benefit from the confinement effect inherent to the formwork.

This formwork is preferably typically made of semi-cylindrical metal shells, or even made of composite material incorporating glass fibers or carbon fibers for example, particularly if one wishes to avoid leaving metal elements permanently in an aggressive atmosphere. .

According to one embodiment of the invention, if the geometry of the environment allows it, it is possible to have containment reinforcements typically in the form of metal hoops covered in HA steel directly in a temporary tubular formwork, this which allows, after stripping, to have an external concrete skin, and in the type of ultra high performance fiber-reinforced concrete (UHPFRC), with very advantageous durability and minimal maintenance.

Due to the integration of reinforcements or prestressing bars within a material presenting excellent characteristics of resistance to environmental attacks, the system thus installed can remain within the structure without requiring any cutting and/or evacuation of the entire cable in question.

Anchoring by adhesion, and not by nailing, provides system flexibility for different types of cables. Indeed, transfer by adhesion in a material only requires adaptation of the injected volume (length and section) to ensure the effective transfer of force between the cable and the bar bridging system.

Furthermore, the composition of the confinement system is significantly simplified compared to a clamping jaw of the prior art, also taking up the axial forces of prestressing bars, since the mechanical operation of the confinement tube is intrinsically different and a fortiori less penalizing.

Finally, this containment system does not need to be geometrically adjusted to the type of cable encountered. It is possible to vary the dimensions of the outer envelope, typically by several centimeters for example, to adapt to the diameters of commercial tube products, or quite simply to the geometry constraints of the work, for example the distance to the facing closest to the cable.

Concerning more specifically the bridging of such a cable at the level of an area close to an anchorage, and therefore requiring the installation of bars around the anchorage on one side of the bridged area, this latitude in terms of The adjustment makes it possible to position the prestressing bars in the rare eligible spaces in an environment generally strongly constrained by the construction provisions in place, which can induce strong constraints on the number of bars (for example two maximum), and the spacing between the different elements, taking into account the reinforcement in place.

• ▪ au moins deux armatures ou barres de précontrainte destinées à venir se positionner parallèlement audit câble au niveau d'une zone de reprise, et s'étendant en amont et/ou en aval de ladite zone de reprise ;
• ▪ un coffrage apte à définir un volume entourant le câble et les barres de précontrainte, non seulement au niveau de partie au moins de la zone de reprise mais de part et/ou d'autre de celle-ci, et apte à remplir une fonction de confinement mécanique dudit volume ;
• ▪ des moyens de positionnement relativement à l'ouvrage d'art, respectivement des barres de précontrainte et du coffrage, par rapport au câble,
• ▪ des moyens de mise en tension des barres de précontrainte.

The invention also relates to a system for bridging a prestressing cable of a structure and in particular of a structure. This system includes:

• ▪ at least two reinforcements or prestressing bars intended to be positioned parallel to said cable at the level of a recovery zone, and extending upstream and/or downstream of said recovery zone;
• ▪ formwork capable of defining a volume surrounding the cable and the prestressing bars, not only at the level of at least part of the recovery zone but on either side and/or other of it, and capable of fulfilling a function mechanical confinement of said volume;
• ▪ means of positioning relative to the structure, respectively the prestressing bars and the formwork, in relation to the cable,
• ▪ means of tensioning the prestressing bars.

Typically and according to a first embodiment of the invention, the formwork is made up of two half-shells capable of being assembled to one another and of defining a substantially cylindrical volume, centered in relation to the cable in question.

According to another embodiment of the invention, the formwork, also cylindrical, is temporary, and is therefore intended to be removed after the UHPFRC or resin has set. As a corollary, metal hoops made of High Adherence (HA) steel of reinforced concrete are integrated into the volume defined by the formwork prior to the injection of the UHPC concrete or resin, and arranged around the cable and the prestressing bars provided with their sheath, these hoops being in the form of overlapping semi-circles mechanically overlapping at their two ends.

BRIEF DESCRIPTION OF THE FIGURES

• La figure 1 est une représentation schématique en section longitudinale partielle d'un pont, illustrant la mise en oeuvre d'un câble de précontrainte.
• La figure 2 est une vue en section transversale de la figure 1.
• La figure 3 est une vue schématique en section illustrant un exemple de fixation du système de confinement de l'invention.
• La figure 4 est une représentation schématique en section longitudinale d'une première forme de réalisation de l'invention.
• La figure 5 est une vue schématique en section transversale selon la ligne V-V de la figure 4.
• La figure 6 est une vue de détail de la figure 5.
• La figure 7 est une représentation schématique en section longitudinale d'une deuxième forme de réalisation de l'invention.
• La figure 8 est une vue schématique en section transversale selon la ligne VIII-VIII de la figure 7.
• La figure 9 est une vue schématique de détail de la figure 8.
• Les figures 10 à 15 illustrent schématiquement les différentes étapes de mise en oeuvre du système de l'invention, et plus particulièrement du mode de réalisation illustré en lien avec la figure 7.
• La figure 16 est une représentation schématique en section longitudinale d'une troisième forme de réalisation de l'invention.
• La figure 17 est une vue schématique en section transversale selon la ligne XVII - XVII de la figure 16.
• La figure 18 est une vue schématique de détail de la figure 17.

The manner in which the invention can be carried out and the advantages which result from it will become clearer from the example of embodiment which follows, given for informational and non-limiting purposes, in support of the appended figures.

• There figure 1 is a schematic representation in partial longitudinal section of a bridge, illustrating the implementation of a prestressing cable.
• There figure 2 is a cross-sectional view of the figure 1 .
• There Figure 3 is a schematic sectional view illustrating an example of fixing the confinement system of the invention.
• There Figure 4 is a schematic representation in longitudinal section of a first embodiment of the invention.
• There figure 5 is a schematic view in cross section along line VV of the figure 4 .
• There Figure 6 is a detailed view of the figure 5 .
• There Figure 7 is a schematic representation in longitudinal section of a second embodiment of the invention.
• There figure 8 is a schematic view in cross section along line VIII-VIII of the figure 7 .
• There Figure 9 is a detailed schematic view of the figure 8 .
• THE figures 10 to 15 schematically illustrate the different stages of implementation of the system of the invention, and more particularly of the embodiment illustrated in connection with the figure 7 .
• There Figure 16 is a schematic representation in longitudinal section of a third embodiment of the invention.
• There Figure 17 is a schematic view in cross section along line XVII - XVII of the Figure 16 .
• There Figure 18 is a detailed schematic view of the Figure 17 .

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the figure 1 is a schematic representation of a work of art, in this case a box bridge partially represented. Within this, we have shown on the left of this figure, by the reference (1), one of the sides of said bridge and more specifically an anchoring zone of a prestressing cable (2), this the latter further extending in a certain number of boxes attached to each other defined by the upper (3) and lower (4) walls, in a known manner.

We represented, on the figure 2 , a schematic cross-sectional view of such a box showing the prestressing cable (2), it being understood that, traditionally, such structures comprise several prestressing cables of this nature.

We represented in relation to the Figure 4 , a first embodiment of the invention, more particularly intended to allow the recovery in tension of such a prestressing cable (2) having a fragile zone, the system of the invention being more specifically intended to be implemented work only at one end of the damaged prestressing cable.

In this case, we first remove the sheath on either side of the area chosen for the subsequent cutting of the cable (2). This sheath is typically made of PVC (polyvinyl chloride), HDPE (high density polyethylene), or steel, and traditionally surrounds the cable in question.

We then proceed to remove the filling of the material filling the space between the aforementioned sheath and the prestressing reinforcements constituting the cable (2), said material in principle ensuring the protection of the cable (2) against corrosion. This material to be removed may vary depending on the construction period and the prestressing technology in place; for works prior to the 2000s, it is frequently made of cement grout. From the 2000s, in some countries such as France, petroleum wax is frequently encountered. Finally, the cable (2) being generally composed of a bundle of smaller units, we can find in certain more qualitative situations individual protection of the wires or strands composing the cable (2) in addition to filling with grouting the general sheath, this arrangement thus providing a double protective barrier. As an example, we can cite the widespread technology of sheathed-greased monitorons. If applicable, this individual protection must also be removed.

In all cases, whatever the materials present between the sheath and the steel of the cable (2), the desired objective is to free the constituent reinforcements of the cable (2) in tension, in order to allow the adhesion of the future material, typically UHPC type or resin type, on these same reinforcements.

This removal or release of the filling material can be carried out by hydrodemolition, particularly in the frequently encountered case of structures injected with cement grout.

In doing so, the cable without the sheath and its filling is generally in the form of a certain number of wires or metal strands, depending on the prestressing process used.

Remember that the cable (2) is generally made up of a bundle of smaller units. In such a configuration, it is possible that the outermost wires or strands of the cable mask an interior volume containing non-accessible wires or strands, typically coated in cement grout; in this situation, it is appropriate to release as much of the peripheral strands as possible, then to adapt the length of the device of the invention to the adhesion surface actually available, via an approach similar to that of package anchoring length calculations. of bar for reinforced concrete steels, as specified in the Eurocode 2 corpus.

At least two prestressing bars (5, 6) are positioned on either side of said cable (2), said bars (5, 6) passing through the anchor head (1) previously drilled, typically by coring, for the purposes of to allow their insertion up to the level of the box within which the various prestressing cables pass, and in particular the weakened cable (2). The bars (5, 6) are held in place on the one hand at the anchoring level, and on the other hand by stakes, for example metallic, preceding the half-cylindrical shells (7) described below. These bars typically have a diameter of around 40 millimeters for common prestressing cables, they have significant rigidity and therefore only undergo very small deformations over several meters under their own weight, which facilitates their installation implemented.

These prestressing bars are initially provided with a PVC or HDPE sheath provided for in the technical advice of the prestressing supplier, which should be removed directly above the fragile zone of the damaged prestressing cable (2) of which we intend to carry out the bypass. This stripping of the prestressing bars aims to allow their subsequent anchoring by adhesion in the UHPC concrete or in the resin, as will be described later. They are held in position by means of fixing or connecting members, for example hangers or stakes fixed to the nearest facing, as for example illustrated on the figures 11 and 12 .

As a corollary, these connecting or fixing members are used to position, around the prestressing cable (2), not only directly above the cable damage zone, but also upstream and downstream of it. here, a cylindrical metal shell (7), whose axis of revolution is, in this case, coincident with the cable (2).

An interface allowing slight sliding when tensioning is implemented between the shell (7) itself and the connecting or fixing members. Such an interface is for example made up of a sheet of PTFE (polytetrafluoroethylene), or even a layer of grease.

We then proceed to caulking the two respective ends of the volume defined by the cylindrical shell, then we inject into said volume a fiber-reinforced concrete, advantageously of the UHPC type, in this case with ultra high performance. Such concrete is for example marketed under the reference “SMART Up” by the company VICAT, or under the reference “BSI” by the company Eiffage. This concrete can be replaced by a resin, for example marketed under the reference “Sikadur 52” by the company Sika.

This concrete or this resin will therefore occupy the entire volume defined previously, and come into contact respectively with the cable (2) and the prestressing bars (5, 6).

The characteristics of a concrete or resin of this type are such that it ensures extremely effective adhesion, on the one hand with the prestressing cable (2), and on the other hand with the prestressing bars (5, 6). He/she also makes it possible to protect the parts anchored by adhesion against corrosion or other attacks, and thus contributes to the long-term durability of the device.

After the concrete has solidified, the prestressing bars (5, 6) are then put under tension. This tensioning is preferably carried out on the basis of computer-assisted control, based on the theoretical force of the cable and displacement measurements, for example via extensometer type sensors placed between fixed points on the structure. on the one hand, typically concrete core or slabs, and several points of the cylindrical shell and tension elements, prestressing bars (5, 6) and the cable (2), on the other hand.

Advantageously, the tensioning under hydraulic control is carried out in such a way that one can have up to one hydraulic channel per bar. This dedicated control makes it possible to adapt the tensile force in each prestressing bar, and thus cover the effects of possible installation defects of said bars in relation to the cable. In the case of a two-bar system only, the connection system, illustrated by the figures 11 and 12 , contributes to recovering the effects of a foreseeable implantation fault which cannot be recovered satisfactorily by a simple difference in tension between the bars.

Finally, if necessary, the damaged section of cable is cut out.

The embodiment shown in relation to the figures 4 to 6 , is intended to be implemented only at one of the ends of the damaged prestressing cable.

Within the Figure 4 , zone (10) corresponds to the section of the damaged cable and zone (11) corresponds to the section of the preserved cable. In this case, the system of the invention is therefore positioned astride zones (10) and (11).

The prestressing bars (5, 6) pass through the wedging zone (1) and tension bolts (14) ensure the tensioning of the prestressing bars (5, 6) after the fiber concrete has set.

We have shown, by the reference (13), lower wedges, intended to be fixed against the lower wall (4) of a box, and by the reference (12), associated with a complementary wedge (15), the device for fixing the two half-shells intended to constitute the cylindrical metal shell (7), intended to define the volume within which said fiber-reinforced concrete is injected.

THE figures 7 to 9 represent another embodiment of the invention, in which the system of the invention is positioned on either side of the section (17) of the damaged cable, that is to say also partly at the level of the sections (16) where the cable is kept. In the example described, the system is suspended from the upper wall (3) of a box by means of hangers (18) fixed by any means, in particular by screwing.

The lower end of the hangers is connected to the cylindrical shells (7) intended to define the volume within which the fiber-reinforced concrete is injected.

In this example, four upper and lower prestressing bars (5, 6) are shown, regularly distributed in relation to the damaged prestressing cable (2).

It should be emphasized that in this configuration, as shown in the Figure 14 , the prestressing bars are anchored to the ends of the cylindrical shells, via a system of anchoring plates (22), commonly used in prestressing by bar. The dimensions of these anchoring plates can be reduced taking into account the performance of the confined material, advantageously UHPC concrete.

In this configuration, the prestressing bars retain their sheathing (23) over the length of the cylindrical shells, without anchoring by adhesion to the confined material (24), advantageously of the UHPC type, with the aim of allowing them to be put under tension . On the other hand, the cable (2) is always anchored by adhesion via this same material.

THE figures 10 to 15 schematically illustrate the different phases of setting up the system according to the invention, in accordance with the embodiments described in connection with the figures 4 to 9 .

THE figures 16 to 18 represent a variant of the embodiment of the invention previously described. According to this variant, a temporary formwork (29) is positioned, typically metallic, or even made of PVC according to the same location as that described in connection with the figures 7 to 9 , and this, using hangers (18). As a corollary, there are around the cable (2) and the prestressing bars (5, 6) provided with their sheath (23) hoops (27, 28) constituting half-elements mechanically overlapping at their two ends, these hoops being typically made of HA steel, that is to say steels with high adhesion properties. These hoops are more or less regularly distributed, at a rate of around ten per meter of formwork.

These hoops can for example be placed directly against the half-shells of the temporary formwork via covering wedges, a ligature on the tube, and longitudinal mounting steels, these arrangements avoiding a posteriori implementation of the creation of the formwork . The covering of the hoops, as shown in the Figure 18 without any longitudinal steel, is improved by the use of UHPC compared to a traditional reinforced concrete situation.

This embodiment of the invention makes it possible to have an external UHPC skin directly at these levels, not requiring repainting or regular maintenance, and also allowing the detection of possible cracks on the system in as part of the routine monitoring of the structure in general, typically during periodic detailed inspections as part of a road bridge.

It should be emphasized that the invention is applicable in the case of detensioning of the cable (2) with a view to its replacement. To this end, we first choose the cutting point of said cable according to the constraints of access and protection of personnel.

The consecutive operations are similar to the embodiments previously described. After cutting the section of bridged cable, the system of prestressing bars is gradually relaxed in order to release the tension gently, without dynamic effect.

Claims (7)

1. Method for locally taking-up the tension of a prestressing cable (2) of a structure and in particular of a civil engineering structure, with a view to its removal or its conservation, consisting in:

   ▪ positioning by wedging at least two prestressing bars (5, 6) on either side of the cable in question, said prestressing bars extending substantially parallel to said cable at the take-up area, that is to say the area of fragility of said cable, upstream and downstream of said area;

   ▪ forming the assembly consisting of the cable (2) and the prestressing bars (5, 6) upstream and/or downstream of the take-up area, and over at least part of the take-up area, defining a volume substantially centred around the cable;

   ▪ injecting, into said volume, fibred concrete BFUP (ultra-high performance fibred concrete) or a resin with equivalent characteristics, occupying the entirety of said volume;

   ▪ mechanically confining said volume then filled with the fiber concrete BFUP or resin;

   ▪ allowing the concrete or resin to set;

   ▪ tensioning the prestressing bars (5, 6).
2. System for bridging a prestressing cable (2) of a structure and in particular a civil engineering structure, with a view to its removal or preservation, comprising:

   ▪ at least two prestressing bars or frameworks (5, 6) intended to be positioned on either side of said cable at a take-up area, and extending upstream and/or downstream of said take-up area;

   ▪ a formwork (7) capable of defining a volume surrounding the cable and the prestressing bars, not only at at least a part of the take-up area but on either side thereof, and capable of fulfilling a function of mechanically confining said volume;

   ▪ positioning means relative to the civil engineering structure, respectively the prestressing bars and the formwork, relative to the cable,

   ▪ means for tensioning the prestressing bars (5, 6).
3. System for bridging a prestressing cable (2) of a structure and in particular of a civil engineering structure according to claim 2, wherein the formwork (7) consists of two half-shells capable of being assembled to each other and of defining a substantially cylindrical volume, centred with respect to the prestressing cable (2).
4. System for bridging a prestressing cable (2) of a structure and in particular of a civil engineering structure according to claim 3, wherein the semi-cylindrical shells are made of metal or of composite material incorporating glass fibres or carbon fibres.
5. System for bridging a prestressing cable (2) of a structure and in particular a civil engineering structure with a view to its removal or preservation, comprising:

   ▪ at least two prestressing bars or frameworks (5, 6) intended to be positioned on either side of said cable at a take-up area, and extending upstream and/or downstream of said take-up area;

   ▪ a temporary formwork (26) capable of defining a volume surrounding the cable and the prestressing bars, not only at at least a part of the take-up area but on either side thereof, and capable of fulfilling a function of mechanically confining said volume;

   ▪ means (27, 28) for mechanically confining said volume;

   ▪ positioning means relative to the civil engineering structure, respectively the prestressing bars and the formwork, relative to the cable,

   ▪ means for tensioning the prestressing bars (5, 6).
6. System for bridging a prestressing cable (2) of a structure and in particular a civil engineering structure according to claim 5, wherein the mechanical containment means consist of metal hoops (27, 28) integrated into the volume defined by the temporary formwork (26), and arranged around the cable (2) and the prestressing bars (5, 6) provided with their sheath (23).
7. System for bridging a prestressing cable (2) of a structure and in particular of a civil engineering structure according to claim 6, wherein the hoops (27, 28) consist of half-elements mechanically overlapping at their two ends.

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