FR3098909A1 - Method for measuring the residual prestressing in a reinforcement - Google Patents

Abstract

Method for measuring the residual prestress in a reinforcement. Method for measuring the residual tension (T) in a prestressing reinforcement (40), the reinforcement (40) having its ends (45) anchored in a structure (30) and comprising a free portion (42) not adhering to this structure (30), the method comprising the steps of: Measure the displacement (d) of the armature in response to a tensile force (F) exerted on its free portion (42) in a direction perpendicular thereto, the traction (F) being exerted using a traction device resting on the structure, calculating from the knowledge of this force and the corresponding displacement the residual tension in the armature (40). Figure for abstract: Fig. 2

Description

Method for measuring the residual prestress in a reinforcement

The present invention relates to measurement methods and devices used in the field of civil engineering. The invention aims more particularly at measuring a residual stress in a shape memory reinforcement.

In the state of the art, it has been proposed to reinforce a structure using a shape-memory alloy reinforcement, anchored at both ends to the structure.

Shape memory materials have the particularity of returning to an initial shape when they are heated and then cooled, the initial shape having been modified by cold deformation.

Such materials can be used to produce prestressing reinforcements when, for example, a linear realization of this material in strip, lamella, ribbon, wire or bar, previously cold drawn while retaining a residual elongation, is fixed by its ends to a structure. and that, in this locked position, it is heated to a temperature activating its memory effect but that the corresponding shortening is prevented by the fixings, a force then appearing and, by equilibrium, inducing an equal and opposite force in the structure, thus achieving its prestress.

This technique produces an effect identical to that provided by tensioning and locking a conventional prestressing armature and a comparable benefit.

However, the residual force in the reinforcement resulting from the prevented shrinkage is not directly known after its cooling. It is considered that the final stress value in the material and therefore said force, is related to the maximum temperature reached.

In general, it is the temperature control that is the indication of the residual voltage. However, it is an indirect and deferred result, which prejudges performance but does not establish it. In reality, it is possible that part of the shrinkage is not hindered, if for example there is an anchoring play or if the framework presents a gap when it is fixed before heating. In this case, the final force results from a lower hindered shrinkage for the same temperature range, and therefore the performance provided to the structure is insufficient although the target temperature could have been respected.

On the other hand, there are devices for measuring tense reinforcement, wire, tape, strap, known under the term of crossbow and which allow by analyzing the stiffness of the transverse deformation of said reinforcement, to determine the effort of tension. These devices and their mode of implementation are well known to those skilled in the art and are the subject of numerous recommendations and publications, particularly in the field of activity of concrete prestressing. Ifsttar's Interactive Notebooks FicheC4-3-Guide_Auscultation_Ouvrage_Art-Cahier_Interactif are an example of such a publication. Known measuring devices are designed to deviate only a defined part of the tensile reinforcement, relying on two points of the latter.

There is a need to be able to measure the residual voltage in a shape memory armature, in a way that is precise and relatively easy to implement.

The invention aims to meet this need, and it achieves this by proposing a method for measuring the residual tension in a prestressing reinforcement, the reinforcement having its ends anchored in a structure and comprising a free portion not adhering to this structure, the method comprising the steps of:

• Measure the displacement of the reinforcement in response to a tensile force exerted on its free portion in a direction perpendicular to it, the traction being exerted using a traction device resting on the structure,
• calculate, from knowledge of this force and the corresponding displacement, the residual tension in the armature.

The invention allows the measurement of the tensile force in the tensioned reinforcement after activation, and to verify compliance with the objective. The measurement is then possible in the same way at any time in the life of the structure, and thus authorizes its monitoring.

The invention performs the direct measurement of the tension in the tensioned reinforcement by analyzing the balance of an additional force introduced and its length between anchorages, most often without any other need for shape or inertia characteristics.

Preferably, the prestressing reinforcement is a shape-memory reinforcement, made of a material that can pass from an initial configuration to a stretched configuration, then resume the initial configuration by being brought from room temperature to a sufficient temperature, from preferably between 160 and 300°C.

Preferably, the tensile force which is exerted and measured is a transverse force in the middle of the reinforcement, this force being seen by the tensioned reinforcement as a point load causing a bending deformation between the fixing points, the deflection, that is to say the displacement of the armature, related to the force, directly becomes an indicator of the force in tension.

The analysis of the relationship between the tensile force exerted and the deformation of the reinforcement, knowing the length between anchorages, is generally sufficient to calculate the tensile force at the time of measurement, and therefore the residual tension.

Contrary to the devices of the state of the art which deflect only a small part of the frame, corresponding to the wheelbase of the device, the invention makes it possible, because the traction device rests on the structure, the deflection of the reinforcement over a relatively large length, in particular equal to the length between its anchorages. This makes it possible to reduce the measurement uncertainty and to reduce the influence of bending stiffness, in particular for rebars with lengths of less than 50m, thus improving the accuracy of the measurement.

The residual tension is preferably determined by calculating the slope ΔF/ Δd representative of the variation ΔF of the tensile force exerted, as a function of the variation Δd of the corresponding displacement observed.

As indicated above, the traction is preferably exerted at mid-length of the free portion.

The residual voltage T is then given by the formula T = (ΔF/Δ d – a)* b, a and b being constants depending on the deflected length L' of the armature. This length L′ preferably corresponds to the length L of the free portion of the reinforcement, that is to say the length of reinforcement between anchorages.

The traction device comprises any system making it possible to exert a controlled force on the reinforcement, and for example a screw jack, hydraulic, pneumatic or electric, preferably a hydraulic or screw jack.

Preferably, the traction device rests on the structure on either side of the reinforcement. For this purpose, the device may comprise a frame comprising a base resting on the structure.

Preferably, the longitudinal axis of the reinforcement extends in a median plane of the frame of the traction device.

This gives increased stability to the device, and makes it possible to distribute the forces exerted by the device on the structure. It is possible to exert traction on the reinforcement by lifting it punctually, by taking it from below and/or on the sides, the surface of the device which comes to rest on the reinforcement preferably having a shape adapted to that framework, to minimize the risk of damage to the latter and to facilitate placement and removal of the device.

In an exemplary implementation of the invention, more particularly suitable for a flat frame, the traction is exerted by means of a claw placed under the frame. The frame being flat, the claw can be slid under the frame with the help of a mallet, for example.

The claw can be secured to one end of a frame, comprising at least one articulated upright, pivotally mounted between a raised position for inserting the claw under the frame and a folded position cooperating with the claw to transmit part of the tensile forces exerted on the frame by the traction device. Thus, when the frame is closed, the tensile forces are transmitted to the claw at its two ends located on either side of the frame.

In an implementation variant, the traction device comprises two claws forming a clamp which is closed on the armature when the traction must be exerted. Such a variant is especially suitable when the reinforcement has a round cross-section.

The cylinder is preferably removably connected to the frame or to the clamp. This can facilitate the mounting of the clamp or the frame without being hindered by the presence of the jack.

Preferably, the traction force is measured using a sensor integrated into the device. Any suitable sensor is suitable, the sensor chosen being able in particular to depend on the type of actuator used and the precision sought.

Preferably, the displacement of the armature is measured using a displacement sensor integrated into the device. For example, using the displacement sensor, we measure the displacement of the claw or the gripper relative to a frame supporting the cylinder and resting on the structure.

Another subject of the invention is a method for prestressing a structure, the latter comprising a prestressing reinforcement with shape memory, made of a material which can pass from an initial configuration to a stretched configuration, then resume the configuration initial by being brought from room temperature to a sufficient temperature, preferably between 160 and 300°C, the method comprising anchoring the ends of the reinforcement in the structure by providing a free portion not adhering to this structure, the tensioning of the armature by heating the latter, then measurement after cooling of the residual tension in the armature by implementing the measurement method according to the invention, as defined above.

The invention also relates to a method for monitoring the residual tension of a prestressing reinforcement, in particular with shape memory, made of a material which can pass from an initial configuration to a stretched configuration, then resume the initial configuration in being brought from ambient temperature to a sufficient temperature, preferably between 160 and 300°C, the method comprising the disengagement of a free portion of the armature then the measurement of the residual tension in the armature by placing implementation of the measurement method according to the invention, as defined above.

The invention also relates to a device for measuring the residual tension in a prestressing reinforcement extending in a structure, in particular for the implementation of the method according to the invention as defined above, this device comprising:

• a frame comprising a base allowing it to rest on the structure,
• at least one claw configured to come into engagement with the armature,
• at least one cylinder carried by the frame, to move the claw relative to the frame and exert traction on the frame,
• at least one traction force measurement sensor, and
• at least one claw displacement sensor relative to the chassis.

The device may include all or some of the characteristics presented above in relation to the measurement method, and in particular:

• the claw can be configured to be placed under the reinforcement, in particular when the latter is flat,
• the claw can be secured to one end of a frame, comprising at least one articulated upright, pivotally mounted between a raised position for inserting the claw under the frame and a folded down position cooperating with the claw to transmit part of the tensile forces exerted on the frame by the traction device,
• the traction device may comprise two claws forming a clamp which is closed on the reinforcement when the traction must be exerted,
• the cylinder is removably connected to the frame or to the clamp,
• the device comprises an integrated traction force sensor,
• the device comprises an integrated displacement sensor, to measure the displacement of the armature, in particular to measure the displacement of the claw or the gripper relative to a frame supporting the cylinder and resting on the structure.

The base of the frame may be substantially U-shaped. This may comprise two uprights connected to each other by a central bar.

The two uprights can be separated by a wheelbase (measured between the axes of the feet) of between 10 and 50cm, better still between 15 and 35cm, for example of the order of 25cm. The uprights can be equipped with height-adjustable feet, for example feet screwed into the uprights.

The uprights can each be substantially perpendicular to the central bar.

The device may comprise a frame on which is mounted, in particular in a removable manner, the base of the frame, said frame being capable of resting on the frame.

The frame can also be able to rest on the structure.

The invention also relates to a method for monitoring the residual tension of a prestressing reinforcement, in particular a shape-memory reinforcement, made of a material which can pass from an initial configuration to a stretched configuration, then resume the configuration initial temperature by being brought from room temperature to a sufficient temperature, preferably between 160 and 300° C., the method comprising the steps consisting in:

- release a free portion of the reinforcement,

- placing the frame of the device according to the invention on the free portion of the frame, so as to delimit a zone of the frame to be deviated,

- measure the displacement of the armature in response to a tensile force exerted on its free portion in a direction perpendicular to it, the traction being exerted using the device,

- calculate from knowledge of this force and the corresponding displacement the residual tension in the armature.

The invention may be better understood on reading the detailed description which follows, non-limiting examples of implementation thereof, and on examining the appended drawing, in which:

FIG. 1 represents a schematic and partial section of a prestressing reinforcement anchored in a structure,

Figure 2 is a perspective view, schematic and partial, of the frame of Figure 1 on which is installed a traction device according to the invention,

Figure 3 shows a schematic and partial section of the reinforcement subjected to a transverse tensile force,

Figure 4a is an example of a traction device,

Figure 4b illustrates the traction device of Figure 4a when actuated,

FIG. 5a is an example of a variant of a traction device,

Figure 5b illustrates the traction device of Figure 5a when actuated,

FIG. 6a illustrates the location of a force sensor according to the invention,

FIG. 6b illustrates the location of a force sensor according to the invention,

FIG. 6c illustrates the location of a force sensor according to a variant of the invention,

FIG. 6d illustrates the location of a force sensor according to a variant of the invention,

Figure 7a illustrates the installation of the traction device on a flat frame,

Figure 7b illustrates the installation of the traction device on the flat frame,

Figure 7c illustrates the installation of the traction device on the frame,

figure 7d illustrates an example of measurement and display of the value of the traction and the corresponding displacement,

Figure 8a illustrates the installation of a variant of the traction device on the frame,

Figure 8b illustrates the installation of a variant of the traction device on the frame,

figure 9 is a diagram and graph showing schematically examples of the evolution of the tensile force applied to the free portion of the armature, as a function of the transverse displacement of the armature,

figure 10 is a diagram and graph showing schematically examples of the evolution of the tensile force applied to the free portion of the armature, as a function of the transverse displacement of the armature,

FIG. 11a is a diagram and graph showing schematically examples of the evolution of the tensile force applied to the free portion of the armature, as a function of the transverse displacement of the armature,

FIG. 11b is a diagram and graph showing schematically examples of the evolution of the tensile force applied to the free portion of the reinforcement, as a function of the transverse displacement of the reinforcement, and

Figure 12 is a perspective view, schematic and partial, of the frame on which is installed a traction device according to an alternative embodiment.

detailed description

An example of a method for measuring the residual tension T in a prestressing reinforcement 40 will be described with reference to FIGS. 1 to 3. Preferably, the prestressing reinforcement is a shape memory reinforcement.

The reinforcement 40 is anchored at its ends 45 in a concrete structure 30 34, as shown in Figure 1. In the example shown, the reinforcement 40 has a free portion 42 not adherent to the structure 30.

The measurement of the residual tension T is obtained by analyzing the balance of an additional transverse force introduced on its free portion of length L.

This force corresponds to a tensile force F exerted on the free portion 42 in a direction perpendicular (or transverse) thereto. The traction is exerted using a traction device 1, as illustrated in FIG. 2. In the example illustrated, the traction force F is a transverse force applied to the middle of the armature 40. This force is seen by the armature 40 as a point load causing a bending deformation between the anchors 45, as shown in Figure 3.

Preferably, the traction device 1 rests on the structure 30 on either side of the frame 40, as illustrated in FIG. 2. In the example illustrated, the traction device 1 comprises a frame 10 equipped with a base 11 allowing it to rest on the structure 30.

The base 11 of the frame 10 can be U-shaped. This can comprise two uprights 19 connected to each other by a central bar 18.

The two uprights 19 may be separated by a wheelbase Δ (measured between the axes of the feet) of between 10 and 50cm, better still between 15 and 35cm, for example of the order of 25cm , as illustrated in FIG . The uprights 19 can be equipped with height-adjustable feet 29, for example feet screwed into the uprights, as shown in Figure 12.

In order to exert the traction force F, the traction device 1 can comprise a screw jack, as illustrated in FIGS. 4a and 4b. In this example, the screw jack consists of a screw 16 moved by a crank 17.

As a variant, the device 1 comprises a hydraulic cylinder 15 which can be actuated by hydraulic pressure, as illustrated in FIGS. 5a and 5b.

In one embodiment, illustrated in Figures 7a to 7d, more particularly suitable for a flat frame 40, the traction F can be exerted by means of a claw 13 placed under the frame 40, and connected to the cylinder by a rod 14.

The claw 13 can be secured to one end of a frame 50, comprising two articulated uprights 55. The uprights can pivot between a raised position for inserting the claw 13 under the frame 40 and a folded position cooperating with the claw 13 to transmit part of the tensile forces exerted on the frame by the traction device 1. Thus, when the frame is closed, the tensile force F is transmitted to the claw 13 at its two ends 13a and 13b located on either side on the other side of the armature 40.

In an implementation variant, the traction device comprises two claws 13 forming a clamp 20 which is closed on the armature when the traction F is exerted. Such a variant is particularly suitable when the reinforcement is of round cross-section, as illustrated in FIGS. 8a and 8b.

The jack 15 is preferably removably connected to the frame 50 or to the clamp 20, as illustrated in FIGS. 7c and 8b.

Preferably, the tensile force F is measured using a sensor 25 integrated into the device 1. Any suitable sensor is suitable, the sensor 25 chosen being able in particular to depend on the type of jack 15 used and the precision sought.

The sensor 25 can thus be a traction sensor, placed for example on the rod 14 above the claw 13.

The sensor can also be a compression sensor, placed for example under the rotation nut of the rod 14, as illustrated in FIG. 6b.

As a variant, the sensor is a bending sensor, for example placed under the reaction support of the rod 14.

In another variant, the sensor 15 relies on a calibrated measurement of the pressure in the jack 15, when the latter is a hydraulic jack.

The traction exerted by the traction device 1 on the armature causes its transverse deviation from a “neutral” position illustrated in FIG. 1 to a “loaded” position illustrated in FIG. 3, by a displacement d .

The analysis of the relationship between the tensile force F and the displacement d of the reinforcement 40, knowing the length L between anchorages 45, makes it possible to calculate the residual tension T in the reinforcement 40.

Preferably, the displacement of the armature is measured using a displacement sensor integrated into the device 1. For example, the displacement of the claw 13 or of the clamp 20 is measured using the displacement sensor. relative to chassis 10.

Preferably, the traction device 1 is equipped with a screen 60 enabling the value of the traction force and/or the displacement d of the armature 40 to be displayed, as illustrated in FIG. 7d.

Preferably, the measurement of the displacement of the armature 40 is carried out for a plurality of values of the tensile force F.

Thus, it is possible to trace with several measurement points, corresponding to pairs of values ( d , F), obtained on an ad hoc or continuous basis, depending on the means of collecting the measurements from the force and displacement sensors, a curve C associating the tensile force exerted F and the corresponding displacement d observed. An example of a curve C obtained according to the invention is given in FIGS. 9 and 10.

Figure 9 illustrates the curve C obtained in the case where the armature 40 is flat. This may have an initial portion I extending substantially along a very steep straight line D1, then a curved portion II, and finally a final portion III which extends substantially along a less steep straight line D2, as illustrated in figure 9. In this example, curve C is obtained for a flat frame with a width of approximately 100mm and a thickness of approximately 1.5mm.

The first part I can correspond to the adjustment of the movement of the armature already obtained by interposing the claw 13 between it and the structure 30 due to its thickness. As long as the effort deployed by the invention is not greater than that which corresponds to this movement, there is no or very little additional movement.

When this force is reached and then exceeded in portion II of curve C, the force becomes directly proportional to the movement as shown in portion III of curve C.

As a variant, in the case of a round reinforcement, the curve C can present a single straight line D3, as illustrated in figure 10. This curve C is calculated for a reinforcement with a diameter of approximately 6mm

At any time during portion III of curve C in FIG. 9 or portion IV of curve C in FIG. 10, the tensile force F deployed by the invention balances the reaction of the armature 40. -this can include two components:

A first component, illustrated in figure 11a, results from the tension of the armature; the calculation of the force F_T is accessible by simple observation of the balance of the forces, considering in particular that the displacementdis low relative to the length L between the anchors 45 of the reinforcement 40. The force F_T, when the tensile force F is exerted at mid-length, is given by the following formula:

F _T = 4*T*(d/L)

A second component F _F , illustrated in FIG. 11b, results from the bending stiffness of the armature 40, the calculation of which, when the tensile force F is exerted at mid-length (center) is obtained using the formula below:

F _F =192*EI*d/L ³ , E and I being the Young's modulus and the inertia in bending of the reinforcement, respectively.

The residual tension T can then be determined by calculating the slope ΔF/ Δd representative of the variation ΔF of the tensile force exerted as a function of the variation Δd of the corresponding displacement observed.

This is then given, when the tensile force is applied to the center of the free portion of the reinforcement, by the formula T = (ΔF/Δ d – 192EI/L ³ )* L/4.

Residual stress measurement in the prestressing reinforcement can also be carried out at any time during the life of the structure.

When monitoring this residual voltage, a portion of the armature is first released. The length of this exposed portion may not correspond to the length L between the anchors 45. In this case, the traction device 1 may comprise a frame 70 on which the base 11 of the chassis rests as illustrated in FIG. 12. More precisely , the feet of the frame rest on the long sides 75 of the frame. The central bar 18 of the frame 10 is oriented parallel to the short sides 73. In the example shown, the frame 70 has a rectangular shape, and rests on the cleared portion. By resting on the armature by its short sides 73, the frame 70 determines the length L′ of the deviation thereof, which depends on the length of the long sides 75. The measurement of the residual voltage T can thus be performed as described above, replacing the length L between the anchors 45 by the deviated length L′.

Of course, the invention is not limited to the examples which have just been described. For example, traction can be exerted using other types of jacks.

Claims (18)

Method for measuring the residual tension (T) in a prestressing reinforcement (40), the reinforcement (40) having its ends (45) anchored in a structure (30) and comprising a free portion (42) not adhering to this structure (30), the method comprising the steps of:

• Measure the displacement (d) of the armature in response to a tensile force (F) exerted on its free portion (42) in a direction perpendicular thereto, the traction (F) being exerted using a traction device (1) resting on the structure,
• calculating from the knowledge of this force and of the corresponding displacement the residual tension in the armature (40).

Method according to the preceding claim, the residual tension (T) being determined by calculating the slope ΔF/Δ d representative of the variation ΔF of the tensile force (F) exerted as a function of the variation Δ d of the corresponding displacement of the armature (40) observed. Method according to claim 2, the traction (F) being exerted at mid-length of the free portion (42), the residual tension (T) being given by the formula T = (ΔF/Δ d – a)* b, a and b being constants depending on the deflected length (L') of the armature, the length (L') preferably corresponding to the length (L) of the free portion (42). Method according to one of Claims 1 to 3, the prestressing reinforcement (40) being a shape-memory reinforcement, made of a material which can pass from an initial configuration to a stretched configuration, then return to the initial configuration by being brought from room temperature to a sufficient temperature, preferably between 160 and 300°C. Method according to one of Claims 1 to 4, the traction device (1) comprising a hydraulic, pneumatic or electric screw jack (16; 17), preferably a hydraulic or screw jack. Method according to any one of the preceding claims, the traction device (1) resting on the structure (30) on either side of the reinforcement (40). Method according to any one of the preceding claims, the traction (F) being exerted by means of a claw (13) placed under the armature (40). Method according to the preceding claim, the armature (40) being flat and the claw (13) being slid under the armature (40). Method according to the preceding claim, the claw (13) being secured to one end of a frame (50) comprising at least one articulated upright (55), pivotally mounted between a raised position of insertion of the claw (13) under the armature (40) and a folded position cooperating with the claw (13) to transmit part of the tensile forces (F) exerted on the frame (50) by the traction device (1). Method according to any one of claims 1 to 7, the traction device (1) comprising two claws forming a clamp (20) which is closed on the armature (40) when the traction (F) is to be exerted. Method according to Claim 4 and one of Claims 9 and 10, the cylinder (15) being detachably connected to the frame (50) or to the clamp (13). Method according to any one of the preceding claims, the tensile force (F) being measured using a sensor (25) integrated into the device (1). Method according to any one of the preceding claims, the displacement of the shape-memory armature being measured using a displacement sensor integrated in the device (1). Method of prestressing a structure, the latter comprising a prestressing reinforcement (40) with shape memory, made of a material which can pass from an initial configuration to a stretched configuration, then return to the initial configuration by being worn from the ambient temperature to a sufficient temperature, preferably between 160 and 300°C, the method comprising the anchoring of the ends of the reinforcement in the structure by providing a free portion (42) not adhering to this structure, the setting voltage of the armature by heating the latter, then the measurement after cooling of the residual voltage in the armature by implementing the method of measurement as defined in any one of the preceding claims. Method for monitoring the residual tension (T) of a prestressing reinforcement (40), in particular with shape memory, made of a material that can pass from an initial configuration to a stretched configuration, then return to the initial configuration while being worn from room temperature to a sufficient temperature, preferably between 160 and 300°C, the method comprising releasing a free portion (42) of the armature (40) then measuring the residual voltage (T) in the armature (40) by implementing the measurement method as defined in any one of claims 1 to 13. Device (1) for measuring the residual tension (T) in a prestressing reinforcement (40) extending in a structure (30), the device comprising:

• a frame (10) comprising a base (11) allowing it to rest on the structure (30),
• at least one claw (13) configured to come into engagement with the armature (40),
• at least one actuator (15) carried by the frame, to move the claw (13) relative to the frame (10) and exert traction (F) on the armature (40),
• at least one sensor (25) for measuring the tensile force (F), and
• at least one claw displacement sensor (13) relative to the frame (10).

Device according to the preceding claim, comprising a frame (70) on which is mounted, in particular in a removable manner, the base (11) of the frame (10), said frame (70) being capable of resting on the frame. Method for monitoring the residual tension (T) of a prestressing reinforcement (40), in particular a shape-memory reinforcement, made of a material which can pass from an initial configuration to a stretched configuration, then resume the initial configuration being brought from room temperature to a sufficient temperature, preferably between 160 and 300°C, the method comprising the steps consisting in:

• release a free portion of the reinforcement (40),
• placing the frame (70) of the device according to claim 17 on the free portion of the frame (40), so as to delimit a zone of the frame to be deflected,
• measuring the displacement ( d ) of the armature in response to a tensile force (F) exerted on its free portion in a direction perpendicular thereto, the traction (F) being exerted using the device (1) according to claim 17, and
• calculate from knowledge of this force (F) and the corresponding displacement (d) the residual tension (T) in the armature.