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12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK

Step frequency radar applied for asphalt thickness measure-ments with various interface conditions

C.Fauchard1, F.Rejiba2, X.Dérobert3, Ph. Côte3, and F.Sagnard1

1Équipe de Recherche Associée n°23, Centre d'Etude Technique de l'Equipement-Normandie Centre, 76121 Le Grand Quevilly Cedex, France

[email protected]é Mixte de Recherche Sisyphe, Université Pierre et Marie Curie,

75005 Paris, FranceLaboratoire Central des Ponts et Chaussées,

44341 Bouguenais Cedex, France

In submitting this paper for EuroGPR2008 I hereby assign the copyright in it to the University of Birmingham and confirm that I have had the permission of any third party for the inclusion of their copyright material in the paper. The University of Birmingham will license EuroGPR to use this paper for non-commercial purposes. This will be the sole use of this ma-terial.

Abstract This work deals with three cases of study of step frequency radar used for the thickness determination of an asphalt layer.

The used system is composed of a portable network ana-lyzer (PNA) and of two exponential tapered slot antennas (ETSA). For the first case on the Viaduct of Millau (France), the used system has been validated in a particular context: the contractors have requested very precise results. The as-phalt layer on the metallic deck was unknown during mea-surements and treatments, and the results should be given with a millimetre precision. In the second case on the motor-way A9 in France, the asphalt layer structure was approxi-mately known (2 centimeters), but the milled interface with the concrete sub-layer depreciated the time picking. The used of specific antennas (centered a 7.2 GHz) and signal treatment (Wiener deconvolution) gave very interesting re-sults for the thin asphalt layer estimation of the order of cen-timeter. In the third case, we study a road sample made of two layers with a notched interface, reproducing the milled interface. This study is on course and a better understanding of the waves phenomena in such cases is discussed.

Keywords - Step frequency radar, ETSA, network analyser, bituminous layer, thickness, Viaduct of Millau, highway.

I. INTRODUCTIONDetermining thickness of road layers is a recurrent prob-lem in civil engineering. The use of GPR is the efficient non-destructive technique for this purpose. Many GPR commercial systems are composed of impulse generator with couple of horn or bowtie antennas. These time-do-main systems are high output methods and usually give accurate thickness determination for thin asphalt layer of about 4 cm.Despite of some announced minimal detectable thickness, experiments on road or in laboratory rarely reach 2 cm

and contractors currently demand this limit. Three solu-tions could be attempted: an improvement of signal pro-cessing (deconvolution, adopted here in the second case of study, or high resolution algorithm, [6]), an improvement of GPR system (higher central frequency of ultra wide band (UWB) antenna, adapted pulse generator system) or the used of step frequency radar with adapted antennas.In the first study, the demand was the determination with millimeter accuracy of the implemented thickness of the asphalt layer on the metallic deck of the Viaduct of Mil-lau. In the second study, the demand was the evaluation of the thickness of a thin asphalt layer of about 2 cm imple-mented on the milled surface of a concrete layer (Motor-way A9, France). For both cases, results are presented and discussed. Complementary laboratory measurements have also been realized for a better evaluation of step frequency radar, and a better comprehension of electromagnetic phe-nomena on milled interface.

II. THE STEP FREQUENCY RADARThe basic principle (Figure 1) has been widely presented in [8,9]. The Step frequency radar is used since many years now for geophysical [5] and civil engineering appli-cations [2]. The used step frequency radar is composed of a PNA (Agilent E8362B), and a pair of ETSA, belonging to the "Vivaldi" family, placed behind a vehicle, at a given height from road surface. The PNA could generate step by step a given number of monochromatic waves be-tween 10 MHz and 20 GHz. It transmits them via an an-tenna toward middle under test (here, the road). When the electromagnetic waves encounter a dielectric contrast, for instance at the interface between two pavement layers, a part of the energy is reflected to the receiver antenna and is measured by the PNA via the S21 parameter. The In-verse Fourier Transform (IFT) gives the reflected ampli-tude in function of time propagation. The technical char-

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acteristics of the ETSA have been presented in [4]. The main advantage of these antennas is the large available band pass, and the polarization of the emitted electric field (TE mode). Two pairs of antennas have been used. The pair called ETSA-A3, respectively ETSA-A5, has a S11 modulus lower than –10dB from 430 MHZ to 6 GHz, re-spectively from 1,4 GHz upper 20 GHz.

Figure 1. Basic principle of step frequency radar on a two layers media

III. MEASUREMENTS ON THE VIADUCT OF MIL-LAU

III.1 Site description and settingsThe Viaduct of Millau is a cable-stayed road bridge, cross-ing the Tarn River. It has been inaugurated on the 14 th De-cember of 2004. It is composed of seven concrete pylons, ranged in height from 77 m to 245 m. Seven masts of 87 m have been constructed on the top of each pylon. Each mast is hold with eleven pair of stays. The metallic deck is 2460 m long and 32 m wide, on which a waterproofing course of about 0.5 cm and a specific asphalt layer of un-known thickness have been implemented.The demand of the contractor was the determination of the exact thickness of the implemented layer of bituminous layer on the four lanes of the motorway. The used system was the step frequency radar equipped of the PNA and a pair ETSA-A3 antennas (Figure 3). 128 frequencies comprised between 430 MHz and 6 GHz were used. For the ETSA-A3, the result of a measurement in time domain of the emitted waves is a pulse centered at 1.7 GHz (Figure 4), similar to the pulse of GPR system us-ing horn antennas. Actually this configuration is equiva-lent to a classical GPR system, and offers the ability of an analysis in phase and amplitude in function of frequency.

In counter part, the output of this method is lower than im-pulse radar. It's used here for a validation of the method.The settings have been chosen for allowed the recording of time scan at 5 km/h with a space step of 0.5 m. Per-sonal Computer under a LabWindows CVI interface man-ages all the system. A survey wheel trigs the PNA for starting the sweep frequency and the PC records the S21 parameter in time domain at the end of each sweep.

Figure 2. Picture of the Viaduct of Millau and the seven stayed-masts.

Figure 3. Step frequency radar system in vehicle


III.2 Measurements and thickness determinationFour profiles of about 2460 m have been done, with a step measurement of 0.5 m. The time domain response is ob-tained by IFT transform of the S21 parameter. A Hanning window in frequency domain is used for filtering the sig-nal. Each scan of a given profile are aligned for position-ing the zero time reference at the first negative event, cor-responding to the road surface. The thickness is classically [1,7] obtained by pointing the events in time domain and the amplitudes associated with the reflection at each interface. The difference dt (Figure 5) in time propagation between the road surface and the metallic deck is obtained by averaging the zero time and the times of the negative and positive peaks of each inter-face. The antennas are 0.97 m height from the road sur-face and distant of 0.195 cm. All measurements are cor-rected by tacking account of this bistatique configuration: an algorithm [3] calculates the angles of reflection and re-fraction in the unknown layer by minimizing the travel time in function of Snell Descartes law (see section n VII). The total reflection on flat metallic plate (fmp) is used for normalization of the reflection coefficient (see section VII). Figure 5 shows one of calibration measurements made on each lane of the motorway, at fixed points on the bridge. For each lane, the real dielectric permittivity has been es-timated by a classical procedure using the fmp. On this figure, the reflected amplitudes of road surface, metallic deck and second order reflection on metallic deck are clearly detected, as on Figure 6, where a step frequency radar profile of 60 m is represented. As expected, the thickness of waterproofing course is too thin to be de-tected for the used frequencies. The second picked time (at 2.6 ns, in blue on Figure 5), is mainly caused by metal -lic deck.

Figure 4. ETSA-A5 pulse generated by a sweep between 1.4 GHz and 20 GHz. The central frequency is 1,7 GHz

Figure 5. Measurements with ETSA-A3 antennas with and without a fmp on the road structure of the Viaduct of Mil -lau.

Along the profile of Figure 6, one can see thickness varia-tions due to the structure of the metallic deck, made of different bridge coverings of about 22 m. For the four pro-files, the real relative dielectric permittivity has been esti-mated at 6.1 +/-0.1, tacking account the amplitude varia-tion on surface along profiles. With the error due to esti-mated travel times the total relative error on calculated thickness is about 3%. For the four profiles, the calculated average thickness was 75 mm +/-3mm. The average mea-sured thickness was 75 mm +/- 5 mm, with a minimum at 56 mm and a maximum at 93 mm. On Figure 7, the calcu-lated thickness of the profile of figure 6 is shown. On the table 1, the total results of the measurements in function of lanes are summarized. The real implemented thickness (70 mm + 5 mm of waterproofing course) was unknown and was given after these measurements.

Figure 6. Example of step frequency radar profile obtained on the Viaduct of Millau


Figure 7. Calculated thickness (from figure 6) of bituminous layer on the metallic deck of the Viaduct of Millau

Table 1. Summary of calculated thickness on the Viaduct of MillauRadar Direction 1 Direction 2

Total(unit in mm) Lane 1 Lane 2 Lane 1 Lane 2

Average 75 72 74 78 75

Standard deviation 4 4 5 5 5

Min 60 55 52 60 52

Max 90 89 89 93 93

Nb scans 4480 4025 4587 4557 17649

IV. MEASUREMENTS ON MOTORWAY A9

IV.1 Site description and settingsThe motorway A9 is suited in south of France. Some works in 2006 have been realized and the thin asphalt layer was milled for implemented a new one on the ce-ment sub-layer. The demand was the determination of the new asphalt layer, about 2 cm, with as result as accurate as possible. The main difficulty relies on the fact that the milled inter-face (Figure 8) presents defaults of the order of the thick-ness to be measured. This point will be discussed in the section III.

Figure 8. Milled surface on concrete pavement structure

The same PNA and the same system configuration (PC, survey wheel) were used with a pair of ETSA-A5 anten-nas. For this configuration, a sweep of 128 frequencies from 1.4 GHz to 15 GHz was done for the time signal con-struction. The emitted pulse is centered at 7.2 GHz (Figure 9), narrower than those given by impulse radar. The asso-ciated vertical resolution vr could be in a first approxima-tion considered as the half dominant wavelength in as-phalt layer (considered here as a perfect dielectric):

[m]

With c=3.108 (m/s) the speed of light in vacuum, and r

the real relative dielectric constant. In such a case, for a classical real relative dielectric constant between 4 and 7, the vertical resolution is about 1 cm. This configuration is adapted for the thickness measurement of few centimeters.

Figure 9. ETSA-A5 pulse generated by a sweep between 1.4 GHz and 20 GHz. The central frequency is 7.2 GHz

IV.2 Measurements and thickness determinationThree profiles of 2100 m were done on lanes of the motor-way A9. With the used system, the measurement speed


was about 10 km with a time signal recorded every 1 m. As on the Viaduct of Millau, the frequency S21 parameter was filtered with a Hanning window and the time signal was constructed by IFT. The estimation of thickness fol-lows the same procedure (see section VII) than described in the previous section. A calibration measurement was done at given points with a fmp for recording the total re-flection. Then the fmp was removed and a measurement on road structure was done. By picking the travel time and the amplitudes at each interface, the real dielectric permit-tivity was calculated and was 5.3 +/-0.1. The permittivity of cement layer was estimated at 8.1 +/- 0.2. Then the thickness was calculated with this estimated per-mittivity on the whole profile. In this bistatique configura-tion, the antennas were 57 cm height, and distant of 17.5 cm. The reflection and refraction angle of the reflection between the asphalt layer and the cement were estimated at each point.The step frequency radar profile presented on Figure 11 shows the low contrast and the low resolution of the re-flection between asphalt layer and cement layer. For an ef-ficient travel time pointing procedure, a Wiener deconvo-lution is achieved by considering the source as the re-flected signal of the fmp measurement, after Hanning win-dowing. An example of this treatment is presented on Fig-ure 10. The Figure 12 shows the results of this treatment on the profile of the Figure 11. It gives a thinner signal as-sociated to the reflection on the milled interface and it al-lows a more precise time picking. Nevertheless, the re-flected amplitude stills low and could lead to uncertainties in time picking. The calculated thickness shows strong variations between two neighboring points. That's why the final results have been averaged each 5 m. For the section of 200 m presented in Figure 11 and in Figure 12, the mean thickness (Figure 13) is 26 mm +/-3, with a minimum of 18 mm at 190 m in axis and a maxi-mum of 34 mm at 42 m. For one part, it's important to no-tice that the given resolution relies on estimated errors in time picking and in permittivity determination. For an-other part, between the beginning and the end of a fre-quency stepping with the PNA, the vehicle covers a dis-tance of less than the space step (1m). So the estimated thickness is at least an average estimation of the real thickness between two space steps.

Figure 10. Example of a signal before and after treatment (Hanning window and Wiener deconvolutio)

Figure 11. Raw Step frequency radar profile on a milled in-terface.


Figure 12. Same profile as Figure 11 after Hanning filtering and Wiener deconvolution

Figure 13. Calculated thickness of Figure 11 after time pick-ing on Hanning filered and deconvoluted profile of Figure 12

V.– COMPLEMENTARY MEASUREMENTS ON ROAD SAMPLE

V.1 Road sample with notched interfaceA recent complementary study in laboratory deals with as-phalt layers of the order of 2 cm and with as-milled inter-faces. The goal is firstly to confirm the ability of the step frequency radar to detect very thin asphalt layer. Secondly we want to study the interaction between high frequency waves (around 7.2 GHz) with a crenellated interface with notches (1*1 cm) of the order of the wavelength in the as-phalt layer. A rectangular road sample (the rectangular base is 39cm*58cm) is presented in Figure 14. The sample is composed an asphalt layer (AL, granularity 0/6 mm) of 3 cm lied on a hydraulic concrete layer of 10 cm (CL, gran-ularity 0/6 mm).

Figure 14 Road sample with notched interface, reproducing

a milled interface

It has been constructed with a wood mould, notched with rectangular wood sticks of 1cm*1cm*39 cm spaced of 1.5 cm. We have implemented the concrete layer from the bottom to the top. After the drying of the concrete layer, we have turned over it, removed the wood mould and ob-tained a quiet regular notched shape (see Figure 15). A

thin metallic sheet (6*7cm) has been put at the interface between asphalt layer and concrete layer. Then the bitu-minous layer has been compacted.

Figure 15. Road sample with a thin asphalt layer of 3 cm and

a concrete layer of 10 cm. The interface is crenellated, as a milled interface

V.2 Measurements on road sampleSeveral profiles with the step frequency radar have been realized. The configuration of the system is quasi-similar to the measurements done on motorway A9, except for an-tenna position and space sampling. The two antennas ETSA-A5 are placed at 19 cm from the top of the sample, spaced of 7 cm, and the step sampling in distance is 2 mm. Here a step-by-step motor moves the antennas. All the system (PNA and step-by-step motor) is pc-controlled.

Figure 16. Step frequency radar profile of raod sample of

Figure 14.


The radar profile of the road sample is shown on Figure 16. One can clearly see the electromagnetic contrasts of the surface of the sample, of the interface between asphalt layer and concrete layer, reinforced by the presence of the metallic sheet, and of the notched interface. The dielectric permittivity of asphalt layer is estimated by two ways: with the classical procedure described before (

), and with the picked time be-tween surface and interface (and the known thickness of 3cm, dt=0.42 ns, ). The width of pulses associated to the reflection on surface and inter-face are about 0.3 ns, without deconvolution. The notched interface, similar to a milled one but with ex-aggerated defaults (squares of 1cm*1cm, separated by 1.5cm) appears like a two-thin layers, with some little dif-fraction hyperbolas at the bottom of notches. From an electromagnetic point of view, the main wavelength in the media is about 2 cm (at 7.2 GHz), and the reflection from the top and the bottom of notches can be distinguished (vr~1cm) but they are stacked at mid-time, and the associ-ated pulse appears like sinusoid of three periods of 0.1 ns. This phenomenon has not been observed on motorway A9, because the vehicle moves while the PNA measures (see section II ) and because the step between each scan is 1 m. In such conditions, the milled interface is seen as a global interface. In the laboratory case, the frequency stepping is done at a static position, and the space sampling (2mm) is five times less than the characteristic dimension of notches.

This study has recently begun and many improvements are on course. A treatment based on deconvolution should give a better understanding of the phenomena at the notched interface. Some simulations with the finite differ-ences in time domain has been done, and the comparison with the measurements are studied.

VI.– CONCLUSIONThis work deals with the abilities of the step frequency radar to measure very thin asphalt layers under various conditions of in-terface. The used radar system is pc-controlled and is mainly composed of a portable network analyzer and two exponential tapered slot antennas. In a first study, the system has been used for validation on the Viaduct of Millau, with a pair of ETSA an-tennas giving a width pulse centered at a similar frequency of the horn antennas of classical GPR systems. The measured thickness (75 mm +/- 3) on the metallic deck of the viaduct is quasi similar to the implemented one (70 mm of asphalt layer and 0.5 mm of waterproofing course). This study has shown equivalent performances with actual GPR systems, in term of measurements precisions. In the second study, the demand relies on the measurement of very thin asphalt layer (between 2 and 3 cm), with a milled interface between asphalt and concrete lay-ers. A different pair of ETSA antenna was used, giving a width pulse centered at 7.2 GHz. The measured thickness of the pre-sented example is comprised between 18 and 34 mm. The result

is an average thickness obtained each meter. Actually, the char-acteristic dimension of the milled defaults should be tacking ac-count. For a better understanding of the electromagnetic phe-nomena on a milled interface, a third study in laboratory has been started. A road sample reproducing a milled interface is composed of a notched interface, with known characteristics. Some step frequency radar measurements, with a pulse centered at 7.2GHz have been done and have shown that an interface composed of centimeter notches appears like two-thin layers, with little hyperbolas at the notches bottom. This last study stills on course. FDTD simulations and measurements will be com-pared for a better understanding of electromagnetic phenomena in such interface context.

ACKNOWLEDGMENTSThe author thanks Eiffage and Autoroutes du Sud de la France for the partial reproduced results of measurements respectively done on the viaduct of Millau and motorway A9. The author thanks also respectively Olivier Durand and Bruno Beaucamp for the development of operating system on these two sites, and in laboratory. A special thanks is expressed for the Laboratoire Central des Ponts et Chaussées (LCPC), and the Centre d'Etudes Techniques de l'Equipement of Normandie-Centre (CETE-NC) for giving well afford for conducting these studies.

REFERENCES[1] Al-Qadi I.L., Loulizi A., Lahouar S., "Dielectric char-

acterization of hot-mix asphalt at the smart road using GPR", GPR 2000, Gold Coast, Australia, pp. 176-181, 23-26 (May 2000)

[2] Dérobert, X., Fauchard, C., Côte Ph., Le Brusq, E., Guillanton, E., Dauvignac, J.Y., Pichot, Ch., Step-fre-quency radar applied on thin road layers, J. Appl. Geophys. 47, 317-325 (2001)

[3] Fauchard, C., Dérobert X., Cariou, J., Côte, Ph, GPR performances for thickness calibration on road test sites, NDT&E International. 36, 67-75 (2003)

[4] Guillanton, E., Dauvignac, J.Y., Pichot, Ch., Cash-man, J., A new design tapered slot antenna for ultraw-ide-band applications. Micr. Opt. Technol. Lett. 19, 286-289 (1998)

[5] Kong, F.-N., By, T.L., Performance of a GPR system which uses Step frequency signals. J. Appl. Geophys. 33, 15-26 (1995)

[6] Le Bastard C., Baltazart V., Wang Y. and Saillard J., Thin pavement thickness estimation with a GPR by high and super resolution methods, IEEE Trans. Geosc. Rem. Sensing, vol. 45, n 8, 2511-2519 (2007)

[7] Spagnolini U., "Permittivity measurements of multi-layered media with monostatic pulse radar", IEEE Transaction on Geoscience and Remote Sensing, Vol. 35, N° 2 (March 1997)


[8] Valle S., Zanzi L., Lentz H., Braun H.M., Very high resolution raar imaging with a stepped frequency radar, Proceedings of SPIE-Vol 4084, Eight Int. Conf. On Ground Penetrating Radar, 264-470 (April 2000)

[9] Weedon W.H., Chew W.C, Mayes P.E., A step-fre-quency radar imaging system fr microwave nonde-structive evaluation, Progress in Electromagnetic Re-search, PIER28, 121-146, (2000)

ANNEXE: BASIC BACKGROUND FOR THICKNESS DETERMINATION

For all the studied cases, the antennas of the step fre-quency radar are placed (see Figure 17) at a height e0 in air where the relative dielectric permittivity is 0=1. The antennas are separated by a distance x. We want to mea-sure the thickness of an asphalt layer, of unknown permit-tivity. The material is considered as a perfect dielectric.

Figure 17. Configuration of step frequency radar in TE mode on a two layers media.

The main polarity of emitted waves is in (Oy) direction. The successive waves are supposed to be plane and the propagation is along (Oz) axis. In such transverse electric (TE) mode, the measured electric field (V/m) at the receiver is given by:

(1)

where a0 is the amplitude (V/m). k1z is the propagation factor and, in perfect dielectric, is given by k1z=k0cos0l, with k0=/c, the pulsation (rad/s) and 0l the lth reflec-tion angle between the air and the asphalt layer. is the total reflection coefficient :

(2)

It takes account of the reflection on asphalt surface (first term), on interface between asphalt and the third layer (second term), and the second order reflection on this in-terface (third term, R12=1 in the case of the viaduct of Mil-lau). In TE mode, for perfect dielectric, with non-mag-netic materials, the propagation factor of a layer m is

, the reflection coefficient with the m+1 layer for the lth reflection is given by:

(3)

When a fmp is posed on the surface, we consider that the reflection is total, and the modulus of the measured field is the amplitude a0.

(4)

When the fmp is removed, the modulus of the measured field in time domain, corresponding to the air/asphalt re-flection is directly proportional to the reflection coeffi-cient :

(5)

,With the Snell-Descartes law :

(6)

The dielectric permittivity of asphalt layer is given by:

(7)

For the thickness of the asphalt layer e1, one have to found the reflection angle . The measure time t(s) at interface between asphalt and the third layer is:

(8)

And the distance antenna x (m) is given by:

(9)

With equations (6), (8) and (9), e1 is the only unknown and could be evaluated.

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