CROSS-REFERENCE TO RELATED APPLICATIONS/PRIORITY CLAIM

[0001 ] This application claims priority of United States provisional application 61/369, 140, filed July 30, 2010, the contents of which are hereby incorporated by reference.

FIELD

[0002] The improvements generally relate to the measurement of stress in pavements and more specifically in the surfacing layer thereof.

BACKGROUND

[0003] Road pavement typically includes a surfacing layer applied on a granular base. The surfacing layer is typically made of asphalt concrete or Portland-cement concrete and bound. The strain distribution in the surface layer is directly related to pavement performance. Horizontal strains at the bottom of the bound surface layer are considered to be directly related to the development of fatigue cracking initiated at the bottom of the layer. Tensile and compressive strains occurring near the surface of that layer are considered to be directly related to the initiation of fatigue cracking initiated at the surface of the layer and to the development of permanent deformation (rutting) in the layer.

[0004] Horizontal strains at the bottom of the pavement surfacing layer have been measured for several decades to support pavement mechanistic analysis and design. Two types of sensors have been used for that purpose. The first approach consist in placing an "H"-shaped gauge on top of the granular base prior to building of the surface layer (typically asphalt concrete or Portland cement concrete). The "H" shape of the proof- body allows proper anchoring of the gauge in the surfacing layer material for reliable measurements of horizontal strains. However, high mechanical and thermal stresses are imposed to the gauge by the compaction of the surface layer while the surfacing layer material is still hot. These stresses can result in gauge damage, displacement and misalignment. In any event, this solution is difficultly retro-fitted to an existing pavement and is limited to measuring the horizontal strains at the bottom of the surfacing layer.

[0005] The increasing need to investigate the mechanical response and performance of existing pavements has led to the development of another approach to strain measurements in existing pavement bound layers. The approach consist of coring existing pavements in order to reprocess a core of the surface layer which is glued back in its original position after a strain gauge is affixed to the bottom of the core. This approach also had limitations.

SUMMARY

[0006] There is provided a sensor for pavement stress analysis which can be used to instrument an existing pavement. The sensor has a narrow plate having at least one set of strain gauges longitudinally interspaced therealong. A correspondingly narrow slit is provided in the surfacing layer of the pavement, such as with a circular saw for instance, and the sensor plate can be engaged into the slit and adhered to the faces thereof. Preferably, the plate is of a material having mechanical behaviours similar to the material of the pavements (e.g. thermal expansion coefficient and elastic modulus) so as to be closely representative of the stress in the adjacent pavement surfacing layer material. The strain gauges can be interconnected to a single cable which can extend in a longitudinal channel defined along one face of the plate, for instance.

[0007] In accordance with another aspect, there is provided a sensor for measuring strains in a pavement, to be embedded in a slit provided in a surfacing layer of said pavement, the sensor comprising: a strain plate having a thickness dimension complementary to said slit width such that said strain plate substantially fills said slit, said strain plate being made of a material having an elastic modulus comparable to that of a material of said surfacing layer to be instrumented; and a set of sensors distributed longitudinally along said strain plate for measuring strains in said surfacing layer at multiple points along said strain plate.

[0008] In accordance with another aspect, there is provided a method of instrumenting a test pavement, the method comprising : machining a slit in a surfacing layer of the test pavement in a direction transversal to a traffic direction; and adhering a plate with sensors in the slit, with two faces of the plate adhered to corresponding faces of the slit.

[0009] In accordance with another aspect, there is provided a pavement stress analysis sensor comprising an elongated plate having a length, a height, and a narrow thickness, and at least one set of sensors bonded to the elongated plate in a regularly interspaced manner along its length, whereby the pavement sensor system can be used to instrument a test pavement by adhering the plate with the sensors in a narrow slit previously machined therein.

[0010] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.

[001 1 ] For the sake of convenience, it is stated here that in the context of this specification, the expressions height, horizontal, vertical, upper, lower are used to characterize the sensor and refer to when the sensor is in its intended use position, i.e. correctly inserted in the slit in the pavement, irrespective of the actual orientation of the sensor at any point in time.

DESCRIPTION OF THE FIGURES

[0012] In the figures,

[0013] Fig. 1 is a schematic view showing an example of a pavement stress analysis sensor in use on a test pavement;

[0014] Fig. 2 is a perspective view, enlarged, of the sensor of Fig. 1 , having three sets of strain gauges;

[0015] Fig. 3 a graph showing an example strain signal as measured in time by one strain gauge of the sensor;

[0016] Fig. 4 is a graph showing an example strain signal as measured in time by upper strain gauges (shown above) and corresponding lower strain gauges (shown below) both outside the tire (shown on the left) and under the tire (shown on the right);

[0017] Fig. 5 comprises four graphs showing example maximum horizontal strain measurement results obtained from the upper strain gauge set (shown above) and the lower strain gauge set (shown below) recorded during several passes of a single wide base tire (shown on the right) and of dual tires (shown on the left);

[0018] Fig. 6 comprises four graphs showing example strain measurement results obtained from strain gauges of the lower strain gauge set at various longitudinal positions, more particularly, from left to right @ X = 200 mm, X = 215 mm, X = 245 mm and X = 275 mm; and

[0019] Fig. 7 comprises three graphs showing example strain measurement results obtained from strain gauges of the upper strain gauge set at various longitudinal positions, more particularly, from left to right @ X = 185 mm, X = 235 mm and X = 245 mm.

DETAILED DESCRIPTION

[0020] Fig. 1 illustrates a sensor 100 adapted for strain measurements in a pavement surfacing layer 220 at multiple points along its length. The sensor 100 is designed to be mounted and bounded in a slit, or saw cut, performed in the surfacing layer 220 of an existing pavement with its length oriented transversally to the traffic direction 240 on the pavement.

[0021 ] Accordingly, when a tire rolls on the sensor 100 and the adjacent pavement portions, the weight of the vehicle applies a downward force onto the adjacent pavement portions which causes a stress field, which, given the elastic deformation of the materials, imparts elastic mechanical deformation, or strain, in the pavement. The bond between the strain plate 26 and the adjacent pavement portions contribute to making the strain in the strain plate correspond closely to the strain in the adjacent pavement portions. Given the fact that the sensor 100 can have at least one set of strain gauges located at interspaced positions along the width of the tire and beyond, the sensor 100 can provide an indication of the strain at more than one position both below the tire and adjacent the tire.

[0022] For installation of the sensor 100 in the pavement for measuring strains, a slit is first provided in a surfacing layer of the pavement along a transversal axis which is transversal to a traffic direction on the pavement. This slit can be performed using a circular saw adapted to saw concrete or similar materials, for instance. The slit is made such that it matches the dimensions of the sensor 100 and more specifically of the strain plate 26 as described hereinabove. The sensor 100 is mounted in the saw slit such that the two opposite surfaces of the strain plate 26 are bound to two complementary walls of the saw slit for stresses in the surfacing layer to be transmitted to the strain plate 26. Care is taken for the top edge 28 of strain plate 26 to be aligned with the external surface of surfacing layer and for the bottom edge 30 of the strain plate 26 to be aligned with the bottom surface of the surfacing layer. Also, the slit should be positioned in the pavement substantially under the wheel tracks of the pavement lane in order to make relevant measurements.

[0023] More particularly, in this example, the thickness of the strain plate 26 is selected such for the strain plate 26 to be snugly complementary to width of the slit which is cut in the surfacing layer 220, such that the strain plate 26 essentially fills the slit and for its faces to be easily bondable to the corresponding faces, or walls, of the slit. This feature can affect the precision of the measurements obtained. For purposes of illustration, the thickness of the strain plate 26 can be of 5 mm in this particular example.

[0024] The strain plate 26 can thus be adapted to be mounted in the saw slit performed in the pavement by binding the two opposite surfaces of the strain plate 26 to two complementary inside walls of the slit typically using epoxy glue other an other adhesive material, such as cement glue or the like, such that stresses in the surfacing layer are transmitted to the strain plate 26. This allows measurement of horizontal and vertical strains in the surfacing layer with minimal disruption of the stress fields induced in the surfacing layer by vehicles moving on the pavement. The glue used to bind the strain plate 26 is selected such that it also has an elasticity modulus similar to that of the surface layer, or more rigid than the surface layer, in order to ensure the transmission of stress and strain across the bonds. It is also noted that the glue can more easily cover all the surface of the walls of the slit and all the surface ruggedness if it is selected to have low viscosity. For illustrative purposes, in one embodiment, the glue can be an epoxy glue such as Sikadur ^® -52 commercialized by SIKA. This glue is at least slightly stiffer than the materials it bond in order to assure full transmission of stress between the surfacing layer and the sensor 100.

[0025] The strain plate 26 can be made of a material having mechanical properties matching the mechanical properties of the surfacing layer 220. More specifically, the elastic modulus of the material of the strain plate 26 can be selected to be similar to that of the material of the surfacing layer to be instrumented, for both materials to mechanically behave in the same manner and allow transmission of the stress fields to be characterized by the sensor. Also, the material of the strain plate 26 is typically selected such that the coefficient of thermal expansion of the strain plate 26 and that of the surfacing layer correspond in the temperature range of the measurements to be performed.

[0026] To this end, the strain plate 26 can be made of a composite material or a polymer. In one embodiment where the surfacing layer is made of asphalt concrete, the strain plate 26 is made of high-density polyethylene. In another embodiment, the strain plate 26 is made of polyphenylene sulfide.

[0027] The height of the strain plate 26 can be selected to match the depth of the slit, which can correspond to the thickness of the surfacing layer 220 for instance. In this manner, the strain plate 26 can be mounted in the surfacing layer with the top edge 28 of the strain plate 26 is aligned with the external surface of surfacing layer 220 and the bottom edge 30 of the strain plate 26 is aligned with the bottom surface of the surfacing layer 220. For purposes of illustration, the thickness of the surfacing layer 220 is of about 100 mm and so is the height of the strain plate 26 in this particular example. Accordingly, the slit is performed so that its depth corresponds to the thickness of the surfacing layer 220.

[0028] Fig. 1 also shows how the pavement 200 can have a base layer 210 and a surfacing layer 220. It shows that the height of the strain plate 26 can be selected to match the thickness of the surfacing layer 220. As shown in Fig. 1 , the sensor 100 can be used in the context of a pavement stress analysis system which can include a processor to receive and treat the data received, as well as auxiliary sensors, such as, for example, a multi-depth water content transducer 250, a water content sensor (road base) 260, an asphalt temperature sensor 270. A positioning system 280 can also be used in combination with the stress analysis sensor 100 for the processor to match signals received from strain gauges in a given set with a position of a vehicle wheel 230, and thereby provide a relative strain indication, which may be more instructive than the absolute strain indications since over several passages, the wheel will likely not always pass on the position.

[0029] Turning now to Fig. 2, it can be seen that in this particular example, the sensor 100 comprises a strain plate 26 with three sets of strain gauges : a first set 1 -8 of upper horizontal strain gauges, a second set 9-16 of upper vertical strain gauges and a third set 17-24 of lower horizontal strain gauges. Each set is distributed in an equally interspaced manner along the length of the strain plate 26 in a manner that each strain gauge is positioned at a longitudinal position corresponding to the position of corresponding strain gauges of the other sets. In alternate embodiments, either one, or both, of the upper strain gauge sets can be omitted for instance, if they are not required in view of the specific experiment at hand. In still alternate embodiments, the strain gauges of the sets can be longitudinally misaligned if desired, keeping in mind that aligning the strain gauges with those of the other sets can allow specific analyses. Still further, in alternate embodiments, the sensor can have strain gauge sets positioned at an intermediary height along the plate, rather than upper or lower, and the sensors of any set can be oriented obliquely rather than horizontally or vertically, for instance.

[0030] Further, in this particular example, the strain gauges 1 -24 can be fiber optic strain gauges and more specifically Fizeau interferometers. It is however noted that other fiber optic strain gauges, such as fiber Bragg grating sensors, can alternately be used in other embodiments, and that other types of strain gauges, such as electrical sensors, can also be used in alternate embodiments.

[0031 ] Fiber optic strain gauges are typically responsive to strains along their sensing axis. The strain gauges 1 -8 of the first set are disposed with their sensing axis horizontal and are disposed close to the top 28 of the strain plate 26, such as at 20 mm from the top for instance in the case of an exemplary strain plate having a height of 100mm, in order to measure horizontal strains near the top surface of the surfacing layer. The strain gauges 1 -8 of the first set are uniformly spaced along the longitudinal axis of the strain plate 26, such as by a spacing of 50 mm from one another for instance, in the case of an exemplary strain plate having a length of 500mm and 8 strain gauges in each set, for instance. In this configuration, the horizontally positioned strain gauges of the first set allow measurement of top surface horizontal strains at multiple distributed points on the pavement across the vehicle movement line.

[0032] The strain gauges 9-16 of the second set are disposed with their sensing axis vertical and are also disposed close to the top 28 of the strain plate 26, such as 25 mm in the same example, in order to measure vertical strains near the top surface of the surfacing layer. The strain gauges 9-16 of the second set are also uniformly spaced along the longitudinal axis of the strain plate 26, and allow measurement of top surface vertical strains at multiple distributed points which correspond to the multiple distributed points of the strain gauges of the first set. The strain gauges 9-17 are respectively disposed closed to strain gauges 1 -8, and defining an x-axis as corresponding to the longitudinal axis of the strain plate 26, each of strain gauges 9-16 correspond in x with one of strain gauges 1 -8.

[0033] The strain gauges 17-24 of the third set are disposed with their sensing axis horizontal and are disposed close to the bottom 30 of the strain plate 26, such as at 5 mm therefrom following the same example, in order to measure horizontal strains near the bottom surface of the surfacing layer. The strain gauges 17-24 of the second set are also uniformly spaced along the longitudinal axis of the strain plate 26, to allow measurement of bottom surface horizontal strains at multiple distributed points. Each of strain gauges 17-24 also correspond in x with one of strain gauges 1 -8.

[0034] The strain gauges 1 -24 can be received in using cavities or grooves 32, 34, 36 defined in a face of the strain plate 26 and connected to a cable which can be received in a channel 38 defined longitudinally along the length of the strain plate 26. The strain plate 26 has two flat opposite faces. The main cable receiving channel 38 can extend on one of the faces, longitudinally to the strain plate 26 at an intermediary height thereof for instance. Each cavity can have groove engraved or otherwise formed on the surface of the strain plate 26 which reaches the channel 38. In this particular embodiment, a first set of grooves 32 is provided to receive the strain gauges 1 -8 of the first set of strain gauges, a second set of grooves 34 is provided to receive the strain gauges 9-16 of the second set of strain gauges, and a third set of grooves 36 is provided to receive the strain gauges 17-24 of the third set of strain gauges. Each groove 32, 34, 36 connects to the main cable receiving channel 38.

[0035] Each of the grooves 32, 34, 36 is dimensioned to receive a strain gauge 1 -24 as well its corresponding connection optical fiber 44, or connection line. It has a fiber optic strain gauge receiving portion 40 with dimensions complementary to that of a fiber optic strain gauges 1 -24 and a connection fiber receiving portion 42 with dimension complementary to that of the connection optical fiber 44 of the fiber optic strain gauges. The strain gauge receiving portion 40 is aligned either vertically or horizontally, for the sensing axis of its corresponding strain gauge to be properly oriented as described hereinabove. The strain gauge receiving portion 40 receives a strain gauge 1 -24 and connects with the connection fiber receiving portion 42 which receives the connection optical fiber 44 and which it turn connects with the main channel 38 to guide the connection optical fiber 44 thereto. All connection optical fibers 44 meet in the main channel 38 which carries the connection optical fibers 44 into a main cable, to a connection output of the strain plate 26. At this output, the connection optical fibers 44 are all combined into the single optical fiber cable 46 which exits the strain plate 26 and connects to an external control and data acquisition unit (not shown). The strain gauges 1 -24 and connection optical fibers 44 are bonded in their corresponding grooves 32, 34, 36. This can be achieved using epoxy glue or the like, for instance.

[0036] In this embodiment, all the strain gauges 1 -24 are mounted on a same face of the strain plate 100 but it is noted that in alternate embodiments, some strain gauges may be mounted on the other face along with a second cable receiving channel.

[0037] One will understand that the dimensions of the strain plate, the number of strain gauges, the number of sets of strain gauges, the spacing between strain gauges of a same set, etc. are provided for illustration purposes only, and are vary according to the specific needs of alternate embodiments. It is also noted that the arrangement of strain gauges may be modified to suit the needs such alternate embodiments.

[0038] Results of strain measurement tests performed at the Laval University Road Experimental Site (SERUL) are now presented. A first goal of this project was to measure the strain evolution at different depths within asphalt concrete layers and to determine the entire strain basin under the tires for two typical tire types.

[0039] INSTRUMENTATION

[0040] The tests took place at the Laval University Road Experimental Site located at the Montmorency Forest in the province of Quebec, Canada. This large scale pavement laboratory was built to study surfacing materials, pavement behaviour for various conditions (materials, drainage and climate) and heavy vehicles effects on experimental embankment materials. A section called HVE is 100-m long and was used for the present study. The pavement structure at this location consists of the following layers: 100-mm hot mix asphalt (HMA), a 200-mm granular base (MG-20), a 480-mm granular subbase (MG 1 12) and more than 1370-mm silty till (natural soil).

[0041 ] Referring to Figs. 1 and 2, in order to quantify the strains occurring at different depths within the bounded surface layer, the sensor 100 was used. For this specific application, the sensor was made with high-density polyethylene having an elastic modulus similar to asphalt concrete, allowing both materials to mechanically behave in a similar manner. The sensor was positioned inside an 8-mm width saw slit in the road and fixed with epoxy resin. The groove width is minimal in order to minimize the asphalt concrete layer disturbance. The sensor is instrumented with twenty-four optical fiber sensors located at various positions and levels. Eight sensors (sensor 1 -8) are placed horizontally at the top of the plate at 20 mm below the plate surface. Eight sensors (sensor 9-16) are positioned vertically at 5 mm under sensors 1 to 8. Finally, sensors 17- 24 are positioned horizontally at the bottom of the plate, i.e. 95 mm below the plate surface. In order to measure the strain in two directions, the plate was installed perpendicularly according to traffic direction. The sensors oriented in the X-direction (1 -8 and 17-24) measure the transversal strain while those (9-16) oriented in the Z-direction measure the vertical strain. Fizeau fiber-optic strain gauges are temperature independent, insensitive to transversal strain and their design is miniature. A fiber optic strain gauge is used at each 50 mm for each set. When designing the grooves along the face of the strain plate, into which the fiber optic strain gauges and their connecting optical fiber are received, care should be taken not to exceed the critical radius of curvature.

[0042] As complementary data to the strains measurements, a temperature/water content transducer 260 and a multi-depth water content transducer 250 were used to monitor these significant parameters. The temperature measurement will give the asphalt concrete temperature, which is important to take into account considering the temperature sensitivity of asphalt concrete modulus. To minimize thermal variations, thermal blankets were installed in the morning before each tests day in order to keep asphalt concrete temperature at 8°C. A variation of ± 2°C was tolerated. The blankets, which are connected to thermal baths, are removed from the pavement surface just before the truck passage and replace immediately after. This procedure allows keeping the asphalt concrete temperature quite constant. The temperature/water content transducers 260 are positioned near the strain place 100, but outside the wheel path to ensure that these transducers cause no disturbance on the results.

[0043] METHODOLOGY

[0044] The tests were conducted using a six-axle vehicle (steering axle, two-axle drive) and a trailer equipped with a three axles tridem. The truck was loaded with concrete blocks, which were positioned on the trailer to obtain an 80-kN standard load on the rear tridem axle.

[0045] In order to study the tire type effect on the flexible pavement response, the conventional dual tires 1 1 R24.5 type was compared to wide base tires 455/55R22.5. The tire inflation pressure is maintained at a standard pressure of 100 psi for both types. The vehicle speed during the tests was 30 km/h. Preliminary tests were performed to evaluate the performance of the instrumentation and the influence of the tire type. During these preliminary tests, it was noticed that transversal strain variation is high near the edge of the tires. Therefore, in order to ensure the results quality, a visual positioning system 280 was installed and all the passages are recorded on video. The video is consulted image per image to precisely identify the position of the tire 230 according to the sensor 100. The axe of the visual positioning system 280 is positioned according to the axe of the sensor 100 to precisely measure the distance of the tire according to the sensors 1 -24. In order to obtain a precise quantification of the strain basin under the tires, several passages are performed with various tire offsets in the X-direction. The data acquisition is performed at 500 Hz using a RadSens™ control and data acquisition unit commercialized by Opsens, Quebec City, Canada. The steering axle tires and load remained unchanged during all the tests. The tire effect is evaluated by measuring the strain caused by the tridem equipped with the two different tire configurations.

[0046] Referring to Fig. 3, a vehicle passage results in strain (με) versus time (s) graphs showing the sensors response to loading. A data point is measured and recorded every 2 ms. Moving average is applied to the data and each of the six strain values of the truck axle is identified and analyzed. To obtain the strain, the difference between the maximum strain of the axle and the zero load value recorded between each axle group is calculated using a software was specially designed for this purpose.

[0047] PRESENTATION AND ANALYSIS OF THE RESULTS

[0048] In the following, the results obtained for the upper and lower horizontal strain gauge sets are presented (sensor 1 to 8 and 17 to 24). In order to analyze the tire type effect, the three tridem axles are isolated from the entire signal. Finally, to plot the strain basin, only the first axle of the tridem is used. [0049] Fig. 4 presents the results obtained for upper and lower transversal sensors. Depending on the sensor position according to the tire (center or edge), two characteristic signals can be obtained. Under the tire, upper transversal sensors response is negative which is associated with a compression zone, while lower sensors present a positive response which is associated with a traction zone. Therefore, a strain inversion takes place somewhere between the two instrumented levels within the asphalt concrete layer. Outside the tire, the same conclusion can be made since the upper sensors response is positive and the lower sensors response is negative. The typical signal shapes presented in Fig. 4 are similar for both tire types. However, the position of the sensor in X has the more pronounced influence on the strain amplitude. Indeed, the strain sign changes depending on the position under the tire. Near the tire's edge, there is a transition from traction to compression 20 mm below the surface, and from compression to traction at the bottom of the layer.

[0050] Different repeatability tests were performed and comparisons between sensors at the same position under the tire were done in order to evaluate the results confidence. Table 1 shows the results of several test series having the same tire offset according to the sensors. The difference, expressed as percentages and micro-strains between the axle strain amplitudes, is calculated for each sensor. Then the average difference and standard deviation are calculated in percent and με. The second analysis consists in a strain comparison between various sensors at equivalent tire offsets, repeating the calculation for different tire offsets. This explains the larger number of samples used for this comparison between the sensors.

[0051 ] Table 1 : Sensors repeatability and comparison

Sensor Average Average Standard Standard Number repeatability difference (%) difference (□□. deviation (%) deviation (□□. of value

Upper sensor 6.22% 2.49 3.76% 1 .64 28

Lower sensor 2.48% 3.38 1 .70% 2.24 25

Sensors Average Average Standard Standard Number comparison difference (%) difference (□□. deviation (%) deviation (□□. of value

Upper sensor 12.73% 6.47 1 1 .12% 6.35 76

Lower sensor 6.07% 7.87 4.70% 5.49 79 [0052] Since the lower sensors strain amplitude is higher than the upper sensors, the analysis expressed in percentages is more significant. Difference of 7% is typically found between two similar measurements on the same sensor. Thus, it can be concluded that the sensors signal is reliable. Nevertheless, some differences still exist between the upper and the lower sensors, which may be explained offset variability effect is less pronounced for the lower sensors. The upper sensors are more sensitive to tire offset. The differences found for the sensors comparison is explained by several factors. Indeed, the gain of all sensors is different and the position of the sensor within the layer can influence the signal. As a matter of fact, the layer heterogeneity has an impact on the signal. For example, the presence of an aggregate near the sensor can increase the local modulus and reduce the strain amplitude.

[0053] Fig. 5 shows an example result of the distribution of maximum horizontal strains at the top and at the bottom of the asphalt bound layer recorded during several passes of single and dual tires. In order to determine the strain basin under the tires, measurements at progressively increasing tire offset (X position) were performed. The tires position and edges are also represented. In order to investigate on the tire type, the analysis is performed on the first tridem axle only. This approach allows eliminating the influence of the second and the third axle (dash line in Fig. 4).

[0054] When looking at the obtained results for the lower sensors, the transversal strain curve shows a maximum value of 155 με at tire's center for the wide base tire. For the dual tire, the strains reach two maximum values (143 με and 138 με) at the center of each tire. The strain caused by the dual tires reach a minimum value of -33 με at 260 mm of the outside tire's edge. The negative value means that, between the dual tires, the layer is under compression. The strain basins measured for the dual tires are also larger. As a matter of fact, the affected zone is more important due to the dual tires width. The tensile strain zone is 510 mm wide for the wide base tire. For the dual tires, this zone is 552 mm wide (285 mm and 267 mm). The maximum strain caused by wide base tires is higher to the ones induced by dual tires. However, it should be noticed that the strain variation is lower, which is explained by the fact that there is only two transitions from compression to traction for the wide-base tire. For standard dual tire, the bottom of the asphalt concrete layer is subjected to two additional sign inversions on a short distance between the tires (100 mm). [0055] A similar analysis is performed for the upper sensor. The strain basin also seems symmetrical for each tire type. A maximum compression strain value (-58 με) is reached for the wide base tire. For the dual tires, the curve shows the first maximum strain at -43 με and the second at -41 με. For both tire type, the signal remains negative under the load (compression). However, the spacing found between the two tires (dual tires) causes a slight local strain reduction. The compression zone width is 550 mm and 740 mm for the wide base tire and dual tires respectively. The data scattering is important to notice for offsets higher than 400 mm, which is explained by road borders proximity.

[0056] The higher surface contact of the dual tires creates a better load dispersion. Also, near the tire edges, the wide base tire strain basin slopes are less pronounced. In addition, for dual tires, the signal shape under the tires spacing is particular. During the load passage, the strains are not only negative and the material is submitted to a complex traction and compression solicitation.

[0057] Fig. 6 shows example strain measurement results obtained from lower sensors at various positions in X, i.e., from left to right, X = 200 mm, X = 215 mm, X = 245 mm and X = 275 mm. These results are obtained by selecting measurements at progressively increasing tire offsets. The transition from traction to compression under the dual tires spacing can then be precisely analyzed for the lower sensors. Near the edge of the tire, the signal shape dissents from Fig. 4. At 200 mm and 215 mm, the maximum strain values are not measured at the axle passage (corresponding to the axis of symmetry of the signal) but slightly before and slightly after. At increasing offsets, this phenomenon becomes more pronounced. At a specific tire offset, the applied load starts to impose a compression to the asphalt concrete, the 245-mm tire offset being a good example. During 0.08 second, a traction signal is recorded (maximum strain of 16.3 με). Afterwards, during 0.025 second, a compression signal is observed (maximum strain of -10.1 με) and finally, during 0.06 second, a tensile strain is measured again. The maximum compression strain value is reached at the middle of the spacing (275 mm). Between the X offsets of 275mm and 350mm, the compression effect decreases and the signal remains positive (traction). Therefore, the performed analysis suggests that, for standard dual tires loading, the bottom of the asphalt concrete layer is subjected to two kinds of mechanical solicitation (traction-compression-traction) during a very short load application time. [0058] Fig. 7 shows example strain measurement results obtained from upper horizontal sensors at various positions in X, i.e., from left to right, X = 185 mm, X = 235 mm and X = 245 mm. Near the layer surface, another phenomenon can be observed for the dual tires. The signal shape is reversed in comparison with the lower sensors. The signal remains in compression, but during the axle passage, the strain magnitude decreases before it increases again.

[0059] In this investigation, the sensor 100 was used to characterize the various compression and tensile strains occurring in asphalt concrete layers and to obtain the transversal strain basin for two tire types. Significant strain basin differences were measured between the two tire types.

[0060] As can be seen therefore, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.