METHOD FOR EXAMINING A DEVICE FOR ANCHORING A STRUCTURAL CABLE

Abstract

Method for monitoring an anchoring device for a structural cable of a construction work, the anchoring device including a cover (24) closing a volume containing anchored ends of reinforcements (14) of the structural cable and a filling material (40), the method comprising: modifying the temperature of at least a portion of the cover (24); measuring the temperature on an external surface (31A) of the cover (24); and analyzing measured temperature variations to detect filling defects in the cover (24). Abstract figure: Figure 7

Description

Title of the invention: METHOD FOR EXAMINING A DEVICE FOR ANCHORING A STRUCTURAL CABLE Technical field

[0001] The present disclosure relates to the field of methods for monitoring a device for anchoring a structural cable, such as, for example, a prestressing cable for a construction work or an anchor rod for a wall or reinforced mass. Prior art

[0002] In civil engineering structures, prestressing cables are used to permanently subject the structure to stresses, generally compression, making the structure more resistant to the stresses it faces during its useful life.

[0003] Conventionally, a prestressing cable comprises one or more reinforcements (often strands) stretched between two anchoring zones. The ends of each reinforcement are anchored in the anchoring zones using plates or blocks crossed by holes where the reinforcements are individually blocked, for example by means of truncated conical jaws. Similar devices can be used in the case of anchor rods.

[0004] In an anchoring zone, the ends of the reinforcements are generally capped by a cover placed on or around the anchoring block. A filling material having good anti-corrosion properties, such as a wax, a grease, a polymer, a resin or even a cement grout, is used to fill the volume delimited by the cover and the anchoring block. The filling material is introduced into this volume through an injection orifice, for example located on an end face of the cover. This orifice is closed by a plug at the end of filling.

[0005] At least one vent is provided at a high point, for example on a side wall of the hood, to purge air during injection. Filling is considered complete when filling material begins to overflow through the vent, confirming good filling of the hood and therefore good protection of the reinforcements against corrosion.

[0006] In service, it is sometimes required to check that the cover is properly filled with the filling material, in order to identify potential filling defects, such as filling voids that may remain during installation of the cover or appear during its useful life. Such voids increase the risk of corrosion of the reinforcements at the anchoring areas.

[0007] Currently, the filling level of the hood can be assessed in two ways. One option involves dismantling the hood, which allows for a complete view of the inside of the hood. Sometimes, the filler material adheres quite firmly to the inner surface of the hood, so that it can break during an attempt to dismantle it. This is often the case with cement grouts. Great care must therefore be taken, so that dismantling takes a long time, making inspection very long and expensive, especially for construction or civil engineering works with many prestressing cables. It may also be impossible to dismantle the hood without breaking it. Other types of filler material may remain somewhat fluid, so inspection methods by dismantling the hoods are unsatisfactory and require re-grouting under conditions that are difficult to control.

[0008] The second possibility involves removing only the plugs closing the filler holes in the bonnet, limiting the risk of breaking the bonnet. This possibility is quicker to implement than completely dismantling the bonnet, but the filling level can only be checked very partially.

[0009] It is desirable to have an inspection technique less affected by the aforementioned drawbacks. Summary

[0010] A method for monitoring an anchoring device for a structural cable of a construction work is proposed, the anchoring device including a cover closing a volume containing anchored ends of reinforcements of the structural cable and a filling material, the method comprising: modifying the temperature of at least a portion of the cover; measuring the temperature on an external surface of the cover; and analyzing measured temperature variations to detect filling defects in the cover.

[0011] When the hood includes a filling defect, such as a filling void, temperature variations on the external surface of the hood are detected between different points of this external surface. The method therefore makes it possible to detect the presence of filling voids in the hood using a non-destructive, rapid method which does not require disassembly of the hood.

[0012] Furthermore, the method makes it possible to investigate the entire surface of the hood simultaneously and thus to obtain an overall view of the filling quality of the entire hood, that is to say, a control of the filling quality of the hood not limited to the areas visible and accessible from the plug closing the filling orifice of the hood. It is therefore possible to obtain an exhaustive diagnosis on the presence or absence of filling defects, such as filling voids or filling heterogeneities, inside the hood.

[0013] According to one aspect, the method further comprises obtaining an evolution temporal variation of temperature during a given measurement time.

[0014] According to one aspect, the method further comprises a conversion into a frequency domain of the temporal evolution of the temperature variations.

[0015] According to one aspect, the conversion into the frequency domain of the temporal evolution of the temperature variations comprises obtaining an evolution of an amplitude and / or a phase of the temperature variations as a function of a frequency.

[0016] According to one aspect, the method further comprises obtaining a temporal evolution of the temperature at different points of the at least one portion of the hood.

[0017] According to one aspect, the method further comprises converting into a frequency domain the temporal evolution of the temperature at the different points of the at least one portion of the hood.

[0018] According to one aspect, the conversion into the frequency domain of the temporal evolution of the temperature comprises obtaining an evolution of an amplitude and / or a phase of the absolute temperatures at the different points of the at least one portion of the cover.

[0019] According to one aspect, the modification of the temperature of the at least one portion of the cover is obtained by heating said at least one portion of the cover.

[0020] According to one aspect, the heating is carried out from at least one coil arranged around the external surface of the cover.

[0021] According to one aspect, the at least one portion of the cover is heated by induction, preferably at a power of between 100 W and 15000 W.

[0022] According to one aspect, the at least one portion of the cover is heated by pulse, by a modulated signal or by a pseudo-random binary signal.

[0023] According to one aspect, the heating of the at least one portion of the cover comprises an increase in temperature of said portion of between 1°C and 50°C relative to an initial temperature of said portion.

[0024] According to one aspect, the at least one portion of the cover is heated for a time interval of between 1 second and 600 seconds.

[0025] According to one aspect, the modification of the temperature of the at least one portion of the cover is obtained by cooling said at least one portion of the cover.

[0026] According to one aspect, the measurement of the temperature of the external surface of the hood is carried out by infrared thermography, for example from a thermal camera.

[0027] According to one aspect, the temperature on the external surface of the cover is measured for a measurement time of between 1 second and 3600 seconds.

[0028] According to one aspect, the construction work comprises several structural cables each having at least one anchoring device, each anchoring device including a respective cover closing a respective volume containing anchored ends of reinforcements and a filling material, in which the steps of modifying the temperature are repeated from one hood to another, and in which the temperature variations analyzed include variations between temperatures measured from one hood to another. Brief description of the drawings

[0029] Other characteristics, details and advantages will appear on reading the detailed description below, and on analyzing the attached drawings, in which:

[0030] [Fig-1] shows a schematic longitudinal sectional view of one end of a prestressing cable with its anchoring device.

[0031] [Fig.2] shows a schematic longitudinal sectional view of the front part of the anchoring device of [Fig.l], with a cover and by a heating device used for implementing the method according to the present disclosure.

[0032] [Fig.3] shows a schematic cross-sectional view of the part of the anchoring device covered by the hood according to a first example.

[0033] [Fig.4] shows an example of a thermogram of the surface of the hood of [Fig.3] subjected to the method according to the present disclosure.

[0034] [Fig.5A] shows a diagram illustrating a temporal evolution of the surface temperature in two elementary points of the hood of [Fig.3] subjected to the method according to the present disclosure.

[0035] [Fig.5B] shows a diagram illustrating a temporal evolution of the temperature difference between the two elementary points analyzed in [Fig.5A].

[0036] [Fig.6A] shows a diagram illustrating an evolution of the amplitude of the absolute temperatures in the two elementary points analyzed in [Fig.5A] as a function of the frequency.

[0037] [Fig.6B] shows a diagram illustrating an evolution of the phase of the absolute temperatures in the two elementary points analyzed in [Fig.5A] as a function of the frequency.

[0038] [Fig.7] shows a schematic cross-sectional view of the part of the anchoring device of [Fig.l] covered by the hood according to a second example.

[0039] [Fig.8] shows an example of a thermogram of the surface of the hood of [Fig.7] subjected to the method according to the present disclosure.

[0040] [Fig.9A] shows a diagram illustrating a temporal evolution of the temperature in two elementary points of the hood of [Fig.7] subjected to the method according to the present disclosure.

[0041] [Fig.9B] shows a diagram illustrating a temporal evolution of the temperature difference between the two elementary points analyzed in [Fig.9A].

[0042] [Fig.10A] shows a diagram illustrating an evolution of the amplitude of the absolute temperatures in the two elementary points analyzed in [Fig.9A] in function of frequency.

[0043] [Fig.10B] shows a diagram illustrating an evolution of a phase of the absolute temperatures in the two elementary points analyzed in [Fig.9A] as a function of the frequency.

[0044] [Fig. 11 A] shows a diagram illustrating two curves of evolution of the tem temperature as a function of time.

[0045] [Fig. 1 IB] shows a diagram illustrating two curves representative of an evolution of a temporal evolution of the temperature difference in elementary points of a hood comprising filling voids at different depths

[0046] [Fig. 12] shows a graph representing a maximum depth of detection of a filling void as a function of a size of this filling void depending on whether the method according to the present disclosure is implemented with a time analysis or a frequency analysis. Description of the embodiments

[0047] The invention is illustrated below in the non-limiting case of a structural cable consisting of a prestressing cable. [Fig. 1] shows one end of such a prestressing cable 10 and an anchoring device 12 for this cable 10.

[0048] The cable 10 comprises metal reinforcements 14 parallel to each other, collectively housed in a sheath 16 of the prestressing cable. The space between the sheath 16 and each reinforcement 14 may be filled with a substance protecting against corrosion, for example a petroleum wax, a grease or a cement grout. The reinforcements 14 consist for example of metal strands which may each be contained in an individual sheath made of a polymer material, such as high density polyethylene (HDPE), or a metal sheath. They may also consist of bare strands or other type of metal reinforcement.

[0049] As shown in [Fig.l], one end 18 of each reinforcement 14 protrudes from the sheath 16. This end 18 is received and anchored in the anchoring device 12, as will be detailed. If the reinforcement is a sheathed strand, its individual sheath is removed near the anchor to allow the reinforcement to be anchored at its bare end 18.

[0050] The anchoring device 12 comprises an anchoring trumpet 20, an anchoring plate 22 and a cover 24. In the non-limiting example of [Fig. 1], the trumpet 20 and the plate 22 can form a single part called a trumpet.

[0051] The anchoring plate 22 comprises cavities 26, each cavity 26 being shaped to partially receive the bare end 18 of a respective reinforcement 14. A wedging member 28, such as a frustoconical jaw (also called a key), can be placed radially around each reinforcement 14. The frustoconical jaw 28 makes it possible to hold each reinforcement 14 in position in the respective cavity 26, so that each armature 14 is tensioned to a desired stress value.

[0052] The cover 24 is generally made of a metallic material, such as steel, with ferromagnetic properties.

[0053] The cover 24 has a substantially cylindrical shape comprising a side wall 30, a bottom wall 32 and an open end 34 located opposite the bottom wall 32. The bottom wall 32 advantageously comprises an orifice 33 which is closed by a removable plug 35. The cover 24 has a thickness e between an external surface 31A and an internal surface 31B of the cover 24.

[0054] As can be seen from [Fig. 1], the cover 24 covers the plate 22 in the longitudinal extension of the trumplate. In particular, the open end 34 of the cover 24 is connected, directly or indirectly, to one end of the trumplate, for example by screws 36.

[0055] The cover 24 closes a volume 38 comprising a filling material 40 which makes it possible to protect the bare end 18 of each reinforcement 14 from corrosion. The volume 38 is in particular delimited by the internal surface 31B against which the filling material 40 comes into contact.

[0056] The filling material 40 may be introduced into the cover 24 through the orifice 33 before closing it with the plug 35. The filling material 40 may be a wax, a grease, a polymer, a resin or a cement grout, among others.

[0057] The cover 24 may also comprise at least one vent 42. This vent 42 allows for the bleed of air, or even of a small volume of filling material 40, in order to be able to validate proper filling of the cover 24 during its installation.

[0058] Now a method for auscultating an anchoring device 12 similar or identical to that described previously will be described. As will be detailed in the following, this method makes it possible to detect filling defects, in particular filling voids, in the anchoring device 12. By “filling void” is meant a portion of the volume 38 closed by the cover 24 in which there is a lack of filling material 40. Even if in the following reference is mainly made to filling voids, other defects, which the method according to the invention can detect, include for example the presence of filling heterogeneities.

[0059] According to a first example of implementation of the method, the latter comprises a step of modifying the temperature of at least one portion of the cover 24 of an anchoring device 12. In the example described below, this portion of the cover 24 is heated, which is relatively easy to achieve, in particular by induction, but alternatively it could be cooled. Also, in what follows, all the terms “heat”, “heated”, and their equivalents, could be replaced by “cool”, “cooled”, etc. According to a non-limiting example, up to 4 cm of thickness of the cover can be heated.

[0060] The heated portion of the cover 24 comprises a certain number of elementary points. Each elementary point corresponds to a specific position on the cover 24, in particular in its side wall 30 or its bottom wall 32. If all the elementary points of the cover 24 are grouped together, the complete shape of the cover 24 is obtained.

[0061] To heat the hood 24, a heating device 44 may be arranged outside the hood 24, around its portion to be heated. According to the non-limiting example illustrated schematically in [Fig. 2], the heating device 44 covers the entire hood 24, in particular its side walls 30 and bottom walls 32.

[0062] In the following, it is considered that the heated portion of the cover 24 corresponds to its entire external surface 31A. However, the entire description is applicable to the case where this external surface 31A is partially heated (that is to say, the heated portion of the cover 24 does not include the entire external surface 31A).

[0063] The heating device 44 is for example an inductor making it possible to heat the cover 24 by induction. Induction heating is created by an alternating current of given frequency and makes it possible to obtain a rapid, efficient, homogeneous and reproducible rise in temperature of the heated portion. In addition, induction heating makes it possible to heat a precise area of the cover 24. It does not damage the cover.

[0064] Preferably, the inductor 44 comprises a housing shaped to receive the cover 24 and is adapted to each type of cover. The inductor 44 comprises a coil 46 arranged around the external surface 31A of the cover 24 when the latter is received in the housing of the inductor. The coil 46 and the external surface 31A are advantageously separated from each other. To heat the external surface 31A, an alternating current creating an electromagnetic field circulates in the coil 46. The energy created by this electromagnetic field can then create, by Joule effect, a thermal excitation which dissipates in the material of the cover 24 in the form of heat. Also, induction heating also has the advantage of being flameless heating, which increases the safety of the process.

[0065] The inductor 44 may be configured to heat the external surface 31A by pulse, by a modulated signal or by a pseudo-random binary signal. To select the type of heating, a control unit of the inductor 44 may be provided.

[0066] In the case of pulse heating (also known as the "flash method"), the electromagnetic field circulating in the coil 46 is applied for a certain time during which electromagnetic waves at multiple frequencies are generated. The excitation frequency of the electromagnetic field circulating in the coil 46 changes during the application of the electromagnetic field, for example in the form of a square wave, a triangle, etc. The shape of the induction excitation generated during heating therefore has a time and an excitation signal shape that can generate a wide band of excitation frequencies that makes it possible to detect any type of defect at the inside of the hood 24, and not only the voids.

[0067] In the case of modulated signal heating, the electromagnetic field circulating in the coil 46 is generated by a signal that varies sinusoidally over several cycles. This sinusoidal signal generates a single excitation frequency, which makes it possible to detect a single type of defect inside the cover 24 at a targeted depth. In this case, the excitation frequency of the sinusoidal signal has a value making it possible to detect voids in the cover 24.

[0068] In the case of heating by pseudo-random binary signal, the electromagnetic field circulating in the coil 46 is applied at low powers during time slots of random duration. A limited increase in the temperature of the external surface 31A is thus caused while generating wide frequency bands. It is therefore possible to detect any type of defect inside the cover 24, and not only voids.

[0069] The inductor 44 can be activated so as to cause a rise in temperature of the heated portion of the cover 24 of between 1°C and 50°C relative to its initial temperature, preferably between 10°C and 30°C. According to a non-limiting example, the increase in temperature of the external surface 31A is substantially equal to 20°C.

[0070] If the duration and / or intensity of the heating produced by the inductor 44 is varied, the maximum depth of detection of defects or of the surface of the heated portion can be modified.

[0071] According to a non-limiting example, the inductor 44 heats the cover 24 for a time interval of between 1 second and 600 seconds, preferably between 10 seconds and 120 seconds. According to a non-limiting example, the cover 24 is heated by the inductor for 60 seconds. The increase in temperature is therefore obtained quickly.

[0072] Furthermore, the inductor 44 can operate at a power of between 100 W and 15000 W. According to a non-limiting example, the power of the heating source is substantially equal to 3500 W. When the heating is done by pulse, this power can be between 1000 W and 16000 W. When the heating is done by modulated signal, the heating power can be between 1000 W and 8000 W. When the heating is obtained by pseudo-random binary signal, the heating power can be between 100 W and 1000 W. Preferably, the power of the source of the inductor 44 is constant throughout the step of modifying the temperature of the surface 31A of the cover 24.

[0073] It is noted that to obtain the same temperature rise value, the necessary heating time will be shorter the higher the operating power of the inductor 44.

[0074] When the cover 24 is heated, the heat propagates in its thickness e, mainly by conduction and reaches its internal surface 31B. Then, this heat dissipates in the filling material 40. The presence of one or more voids inside the cover 24 causes heterogeneity in the diffusion of heat in the filling material 24. In particular, the heat diffuses by conduction more easily in the filling material than in the voids. This results in a local accumulation of heat on the thickness of the cover of the zones of the external surface 31A aligned with the void(s), that is to say, which are in the extension of the voids. Also, as will be detailed later, when the interior of the cover 24 comprises voids, the temperature distribution on the external surface 31A is heterogeneous, the zones of this surface 31A aligned with the void(s) having a temperature higher than the rest of the external surface 31A.

[0075] After having modified the temperature of the external surface 31A of the cover 24, the method continues by measuring and / or recording by radiometric video, for a given measurement time, the surface temperatures on the set of elementary points of the heated portion. For this step, the heating device 44 is removed from the cover 24. Also, the temperature of the cover 24 gradually decreases during the measurement until reaching equilibrium with the external environment. The recording is therefore done in the cooling phase of the cover.

[0076] This measurement is for example made by infrared thermography. For this purpose, at least one thermal camera capable of obtaining thermal images of the heated surface of the cover 24 is used. According to a non-limiting example, four infrared cameras can be distributed around the external surface 31A to take thermal images of the latter.

[0077] Advantageously, each camera is placed at a distance from the cover 24, for example at a distance of between 50 cm and 150 cm.

[0078] The measurement time may be between 1 second and 3600 seconds, preferably between 180 seconds and 1500 seconds. According to a non-limiting example, the measurement time is 300 seconds.

[0079] During this time of measuring and recording the evolution of the temperature, several thermal images are obtained. In particular, the image acquisition frequency of the camera is for example between 0.1 Hz and 100 Hz, preferably between 9 Hz and 60 Hz.

[0080] Infrared thermography therefore makes it possible to visualize the spatial and temporal mapping of the temperatures on the heated surface of the cover 24 using thermograms obtained from the thermal images taken by each camera or from video-radiometric (thermal film). These thermograms represent the thermal signature of the heated surface of the cover 24.

[0081] Figures 4 and 8 show examples of thermograms represented in grayscale, in which the darker areas correspond to areas of higher temperature, and the lighter areas correspond to areas of lower temperature.

[0082] [Fig. 4] shows an example of a thermogram obtained for a cover 24 which is heated over its entire external surface 31A and which is completely filled with filling material 40, like that of [Fig. 3]. A homogeneous distribution of temperatures is then observed over the entire cover 24. A homogeneous distribution of temperatures over the entire external surface 31A would also be observed if the cover 24 were completely empty but at equal heating power and equal excitation time and form higher temperatures would be measured in the latter case.

[0083] [Fig. 8] shows an example of a thermogram obtained for a cover 24 which is heated over its entire external surface 31A and which is half-filled with filling material 40 ([Fig. 7]). In particular, the part included in the dotted curve corresponds to the area not filled with filling material 40. A heterogeneous distribution of temperatures is then observed over the entire cover 24, with higher temperatures on the part of the cover which is directly in contact, or closer, to the area comprising filling voids.

[0084] Furthermore, by measuring the evolution of the temperature on the set of elementary points of the heated portion, it is possible to obtain graphs representing the curves of evolution of the absolute temperature T as a function of time t on one or more elementary points chosen from the heated surface of the cover 24. Preferably, the elementary points chosen correspond to points of the heated surface of the cover 24 which are located at similar distances from the center of the cover 24.

[0085] [Fig.5A] represents the temperature evolution curves T as a function of time t for a hood which is completely filled, like that of [Fig.3]. In particular, this [Fig.5A] comprises a first curve Cl-1 and a second curve C2-1 representing the evolution of the temperature T as a function of time t in, respectively, a first elementary point PI and a second elementary point P2 of the heated surface of the hood 24 of [Fig.3]. Since the points PI and P2 are located on areas of the hood which have similar temperatures according to the thermogram of [Fig.4], the evolutions of the temperatures on the points PI and P2 are also similar.

[0086] [Fig.9A] represents the temperature evolution curves T as a function of time t for a hood which is only half filled, like that of [Fig.7]. In particular, this [Fig.9A] comprises a first curve D1-1 and a second curve D2-1 representing the evolution of the temperature T as a function of time t in, respectively, a first elementary point PI' and a second elementary point P2' of the heated surface of the hood 24 of [Fig.7]. As the points PI' and P2' are located on areas of the hood which have very different temperatures according to the thermogram in [Fig.8], the temperature changes at points PI' and P2' are also dissimilar.

[0087] The method then comprises the analysis of the temperature variations AT (also called in the following temperature difference AT) measured to detect filling defects in the cover 24, in particular filling voids. In particular, this step comprises the detection of a temperature difference AT existing during said measurement time between the first elementary point PI, PI' and the second elementary point P2, P2' of the cover 24. For this purpose, curves representing a temporal evolution of the temperature difference AT between the elementary points chosen during the measurement time can be obtained.

[0088] [Fig.5B] represents an evolution as a function of time t of the temperature difference AT between the points PI and P2 of the cover of [Fig.3]. As clearly visible in this [Fig.5B], the temperature difference between the two elementary points PI and P2 is small, with absolute values of difference AT between 0°C and 1°C. This confirms that the interior of the cover 24 is filled homogeneously with the filling material 40. It is noted that similar temperature differences would be obtained if the cover 24 were completely empty.

[0089] [Fig.9B] represents an evolution as a function of time t of the temperature difference AT between the points PI' and P2' of the cover of [Fig.7]. As clearly visible in this [Fig.9B], the temperature difference between the two elementary points PI' and P2' is large, with absolute values of difference AT which can exceed 1°C. This confirms that the filling of the cover 24 by the filling material is heterogeneous, with empty zones.

[0090] More generally, when a filling void is present in the cover 24, the evolution of the temperature at two points can have the appearance presented on the curves of [Fig. 1 1A]. The curve of evolution of the temperature difference AT as a function of time t corresponding to the curves of [Fig. 11A] is presented on [Fig. 1 1B] with the curve EL The appearance of the curve has an increasing part, followed by a peak, then a decreasing part. The deeper the filling void is inside the cover 24, the lower the peak of the curve of evolution of the temperature difference AT as a function of time t is in the ordinate axis and shifted to the right in the abscissa axis. Indeed, the deeper the void, the greater the heat dissipated in the filling material 40, and the longer it takes for the thermal waves created during the heating of the cover 24 to reach this void. In this case, the curve El of [Fig.1 IB] is representative of a more shallow void filling than curve E2, since the peak of curve E1 is higher in the y-axis and further left in the x-axis than the peak of curve E2. Also, . only from the curve representing the temperature difference AT as a function of time t it is possible to obtain information on the depth of the filling void.

[0091] The curves of the evolution of the temperature T and of the temperature difference AT as a function of time, such as those of figures 5A, 5B, 9A and 9B are derived from image and signal processing in the time domain. In certain cases, these evolutions of the temperatures and of the temperature difference as a function of time can be the subject of image and signal processing in the frequency domain. This processing in the frequency domain makes it possible to confirm the conclusions concerning the presence or absence of filling voids reached during the processing in the time domain.

[0092] In frequency, from the series of thermograms recorded previously, the Fourier transform of the evolution of the temperature as a function of time can be calculated for each elementary point chosen. In practice, the calculation can be carried out by the Fast Fourier Transform algorithm. The result of the transformation is a complex spectrum which is composed of the amplitude and the phase of the signal, both functions of the frequency. The amplitude corresponds in particular to the power P of heat dissipated at each elementary point. The lower the power P, the more the cover 24 is filled with filling material 40, that is to say, the fewer filling voids it has.

[0093] [Fig.6A] comprises a curve Cl-2 and a curve C2-2. Curve Cl-2 shows the evolution of the power P for the first point PI of the completely filled cover 24 of [Fig.3] as a function of the frequency of the absolute temperatures in this first point PI, while curve C2-2 shows the evolution of the power P for the second point P2 of this cover 24 as a function of the frequency of the absolute temperatures in this second point P2.

[0094] [Fig.6B] comprises a curve Cl-3 and a curve C2-3 showing the evolution of phase 0 for, respectively, point PI and point P2 of the completely filled cover 24 of [Fig.3] as a function of the frequency of the absolute temperatures in the corresponding point PI or point P2.

[0095] Curves Cl-2 and Cl-3 are obtained by applying the Fourier transform to the temperature evolution as a function of time at point PI (i.e., by converting the temperature evolution over time in point PI into the frequency domain). Curves C2-2 and C2-3 are obtained by applying the Fourier transform to the temperature evolution as a function of time at point P2 (i.e., by converting the temperature evolution over time in point P2 into the frequency domain).

[0096] The curves Cl-2 and C2-2 of [Fig.6A] being almost identical to each other, and the curves C1-3 and C2-3 of [Fig.6B] also being almost identical to each other, it is possible to confirm that the cover 24 of [Fig.3] is completely filled with filling material 40. By "identical" we mean here that the temperature difference and the phase shift are negative. (The amplitude would be higher for a completely empty cover - we would have a zero difference and a zero phase shift)

[0097] [Fig.10A] comprises a curve D1-2 and a curve D2-2. Curve D1-2 shows the evolution of the power P or DSP (Power Spectral Density) for the first point PI' of the half-filled cover 24 of [Fig.7] as a function of the frequency of the absolute temperatures in this first point PI', while curve D2-2 shows the evolution of the power P or DSP (Power Spectral Density) for the second point P2' of this cover 24 as a function of the frequency of the absolute temperatures in this second point P2'.

[0098] [Fig.10B] comprises a curve D1-3 and a curve D2-3 showing the evolution of the phases 0 of the temperatures for, respectively, the point PI' and the point P2' of the cover 24 of [Fig.7] as a function of the frequency of the absolute temperatures in the point PI' or the corresponding point P2'.

[0099] Curves Dl-2 and Dl-3 are obtained by applying the Fourier transform to the temperature evolution as a function of time at point PI' (i.e., by converting the temperature evolution over time in point PI' into the frequency domain). Curves D2-2 and D2-3 are obtained by applying the Fourier transform to the temperature evolution as a function of time at point P2' (i.e., by converting the temperature evolution over time in point P2' into the frequency domain).

[0100] Curves Dl-2 and D2-2 of [Fig.lOA], and curves Dl-3 and D2-3 of [Fig.lOB] are clearly different from each other, which makes it possible to deduce the existence of at least one filling void. In particular, the power P or DSP (Power Spectral Density) associated with point PI' is lower than that associated with point P2' ([Fig.lOA]). As indicated previously, the lower the power P or DSP (Power Spectral Density), the more the cover 24 is filled with filling material 40. In this case, as is apparent from [Fig.7], point PI' corresponds to an elementary point of the cover 24 which is closer to the part of the cover 24 filled with filling material 40, than to the area of the empty cover 24, which is closer to point P2'. The result concerning the presence of voids in the cover 24 obtained in the time domain is therefore confirmed.

[0101] As indicated, to obtain the curves 6A, 6B, 10A, 10B, a conversion is made in the frequency domain of the temporal evolution of the temperature in the first elementary point PI, PI', and of the temporal evolution of the temperature in the second elementary point P2, P2'. Alternatively or complementary, it It is possible to convert the time evolution of the temperature difference into the frequency domain. In such a case, an evolution of a difference in amplitude and / or phase is obtained from the temperature difference as a function of a frequency. Applying frequency analysis to the time evolution of the temperature difference between two elementary points instead of the time evolution of the temperature in each chosen elementary point makes it possible to improve the thermal contrast and the phase shift of the results obtained since in the case of the time evolution of the temperature difference the scale used is more discretized.

[0102] The processing of images and signals in the time domain or in the frequency domain as described above can be implemented for example by a computer. Image processing software and signal processing software can be used to process the images and signals in the time domain and / or in the frequency domain.

[0103] It is noted that in general, when frequency processing is applied, the results obtained for the power P or DSP (Power Spectral Density) are more sensitive to the emissivity and the uniformity of the heating. The phase shift results have the advantage of being less affected by the non-uniformity of the heating and the variations in surface emissivity encountered in the case of an on-site application.

[0104] The larger the filling void in the injected volume, the greater the depth at which it can be detected. In [Fig. 12], the points included in the straight line referenced 50 correspond to the maximum depth at which a filling void can be detected by analysis in the time domain as a function of the size of this void. The points included in the straight line referenced 60 correspond to the maximum depth at which a filling void can be detected by analysis in the frequency domain as a function of the size of this void. It can be deduced from this that, in addition to the possibility of confirming the results obtained by the temporal processing of the images and signals, the frequency processing makes it possible to improve the detection of filling voids in depth than when only the temporal processing is used.All points above line 60 correspond to depths at which the void is not detectable by either time-domain analysis or time-domain analysis.

[0105] The following definition makes it possible to estimate the depth z at which a filling void detected in the cover 24 is located:

[0106] in which, C corresponds to the specific heat of the filling material 40, a corresponds to the diffusivity of this material 40 and fb corresponds to the frequency of vacuum detection.

[0107] Alternatively, to determine the depth of the defect it is possible to introduce an endoscope through the plug 35 or one of the vents.

[0108] This first example of implementation of the method disclosed in the present text therefore makes it possible to detect the filling voids present in a cover 24 from the application of the different steps described above to the cover 24 of a single anchoring device 12.

[0109] In a second example of implementation of this method, the cover of a first anchoring device and the cover of a second anchoring device are involved. The first and second anchoring devices are similar or identical to the anchoring device 12 described previously.

[0110] For the sake of brevity, in the following only the differences of this second exemplary embodiment compared to the first example described above are detailed. The rest of the characteristics of the method explained for the first exemplary embodiment are applicable to this second exemplary embodiment.

[0111] In this second exemplary embodiment, the method comprises modifying the temperature of a portion of the covers 24 of each of the first and second anchoring devices 12. In the following, it is considered that the two covers 24 are structurally (i.e., in geometry and material) identical to each other, but as will be detailed later they could be structurally different.

[0112] As explained previously, the modification of the temperature of each cover 24 can be obtained by heating, for example as detailed above, or by cooling. Advantageously, an identical stress making it possible to modify the temperature is applied to the covers 24 of the first and second anchoring devices. Also, the evolution of the temperatures on the heated portion is identical between the two covers 24 only if the filling of the two covers 24 is identical. When the filling between the two covers 24 is different, the evolution of the temperatures on the heated portion differs between the two covers 24. In particular, as indicated previously, the zones of the heated portion of each cover 24 aligned with the void(s) have a temperature higher than the rest of the heated portion of the respective cover.

[0113] After having modified the temperature of the cover 24 of the first and second anchoring devices 12, the change in temperature is measured over a given measurement time on the set of elementary points of the heated portion of each cover 24. This measurement can be carried out in the manner described previously for the embodiment where a single cover is involved in the method. The thermograms representing the thermal signature of each of the covers 24 are thus obtained.

[0114] If the temperature distribution differs between the thermograms of the two hoods 24, filling voids exist in at least one of them. By “differs” is meant that the temperature of one cover deviates from the temperature of the other cover by more than 1°C. Furthermore, if the temperature distribution is heterogeneous on the same cover 24, filling voids exist on this cover, as explained above with reference to the first exemplary embodiment.

[0115] From the measurement of the evolution of the temperatures on each of the covers, it is possible to obtain graphs representing the curves of evolution of the absolute temperature T as a function of time t on a first elementary point located on one of the covers 24 and a second elementary point located on the other cover 24. Advantageously, the first point and the second elementary point are located on an identical position of their respective covers, without this being limiting. Subsequently, it is possible to apply to these selected elementary points of the two covers 24 the time and / or frequency processing as explained above with reference to the first example embodiment with a single cover.

[0116] This second embodiment proves to be particularly advantageous for determining whether a hood is completely filled with filling material 40 or completely empty. Indeed, when the method described here is applied to a single hood and the temperature is uniform over its entire surface, it is not possible to conclude that it is completely empty or completely filled with filling material unless it is compared to a control hood for which the filling level is known. If, for example, the control hood is completely filled, and the temperature distribution is the same as for the analyzed hood, it is possible to conclude that the analyzed hood is completely filled. On the contrary, if the temperature on the external surface 31A of the analyzed hood is higher than that of the completely filled control hood, it can be concluded that the analyzed hood is completely empty.

[0117] As indicated above, advantageously, the two covers are identical, in particular in shape, size, thickness, material, etc. This scenario is typically encountered when the same structure has been constructed with multiple prestressing cables equipped with anchoring devices, and covers, of the same model. It is then convenient to proceed by comparisons of thermograms obtained from one cover to another. However, it is also possible that there is some structural difference between the two covers. In such a case, to compare the temperature changes of the two covers, it is possible to establish relationships between the two covers which take these differences into account when exploiting the different curves of changes in temperature, temperature difference, power and / or phase shift associated with the chosen elementary points of each cover 24.

[0118] It is noted that the process described below also makes it possible to detach the material from the hood.

[0119] The embodiments described above are a simple illustration of the present invention. Various modifications may be made to them without departing from the scope of the invention which emerges from the appended claims. For example, in addition to or as an alternative to the thermal camera, the surface temperature measurements on the set of elementary points of the heated portion may be made by thermocouple temperature probes. These may be arranged on the external surface 31A of the cover 12, without this being limiting.

Claims

Claims

1. Method for monitoring an anchoring device (12) of a structural cable (10) of a construction work, the anchoring device (12) including a cover (24) closing a volume containing anchored ends (18) of reinforcements (14) of the structural cable (10) and a filling material (40), the method comprising: - modifying the temperature of at least a portion of the cover (24); - measuring the temperature (T) on an external surface (31 A) of the cover (24); and - analyzing measured temperature variations (AT) to detect filling defects in the cover (24).

2. Method according to claim 1, further comprising obtaining a temporal evolution of the temperature variations (AT) during a given measurement time.

3. Method according to claim 2, further comprising a conversion into a frequency domain of the time evolution of the temperature variations (AT).

4. Method according to claim 3, in which the conversion into the frequency domain of the temporal evolution of the temperature variations (AT) comprises obtaining an evolution of an amplitude (P) and / or of a phase (0) of the temperature variations (AT) as a function of a frequency (f).

5. Method according to one of the preceding claims, further comprising obtaining a temporal evolution of the temperature (T) at different points (PI, PI', P2, P2') of the at least one portion of the cover (24).

6. Method according to claim 5, further comprising converting into a frequency domain the temporal evolution of the temperature (T) at the different points of the at least one portion of the cover (24).

7. Method according to claim 6, in which the conversion into the frequency domain of the temporal evolution of the temperature (T) comprises obtaining an evolution of an amplitude (P) and / or of a phase (0) of the absolute temperatures at the different points of the at least one portion of the cover (24).

8. Method according to one of the preceding claims, in which the modification of the temperature (T) of the at least one portion of the cover (24) is obtained by heating said at least one portion of the cover (24).

9. A method according to claim 8, wherein the heating is carried out from at least one coil (46) arranged around the external surface (31 A) of the cover (24).

10. Method according to one of claims 8 or 9, in which the at least one portion of the cover (24) is heated by induction, preferably at a power of between 100 W and 15000 W.

11. Method according to one of claims 8 to 10, in which the at least one portion of the cover (24) is heated by pulse, by a modulated signal or by a pseudo-random binary signal.

12. Method according to one of claims 8 to 11, in which the heating of the at least one portion of the cover (24) comprises an increase in temperature of said portion of between 1°C and 50°C relative to an initial temperature of said portion.

13. Method according to one of claims 8 to 12, in which the at least one portion of the cover (24) is heated for a time interval of between 1 second and 600 seconds.

14. Method according to one of claims 1 to 7, in which the modification of the temperature (T) of the at least one portion of the cover (24) is obtained by cooling said at least one portion of the cover (24).

15. Method according to one of the preceding claims, in which the measurement of the temperature of the external surface of the cover (12) is carried out by infrared thermography, for example from a thermal camera.

16. Method according to one of the preceding claims, in which the temperature (T) on the external surface of the cover (24) is measured during a measurement time of between 1 second and 3600 seconds.

17. A method according to any one of the preceding claims, wherein the construction work comprises a plurality of structural cables (10) each having at least one anchoring device (10), each anchoring device (10) including a respective cover (24) closing a respective volume containing anchored ends (18) of reinforcements (14) and a filling material (40), wherein the steps of modifying the temperature (T) are repeated from one cover (24) to another, and wherein the temperature variations (AT) analyzed comprise variations between temperatures measured from one cover (24) to another.

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