J-M. Simonin, V. Baltazart, P. Hornych, J-P. Kerzrého, X. Dérobert, S. Trichet, O. Durand, J. Alexandre & A. Joubert
LUNAM Université, IFSTTAR, Bouguenais, France
ABSTRACT: This paper presents an experiment conducted on the pavement fatigue carrousel of IFSTTAR. Several defects (debonded areas or transverse vertical cracks) were intentionally incorporated during construc- tion. Before the start of the loading planned in March 2012, different non-destructive test techniques were used to detect and locate these artificial defects. Two different electromagnetic wave techniques were tested: Ground Penetrating Radar (GPR) with horn or coupled antennas and a higher frequency system (step frequency radar). Deflection basin measurements with a Falling Weight Deflectometer (FWD) were also taken. Another technique based on Frequency Response Function (FRF) in intermediate frequency domain (100–10,000 Hz) to detect and locate the defects showed interesting results. Further measurements will be made with these different techniques during the experiment to assess the evolution of the defects. It will also allow comparison of the ability of each technique to detect such damage, to precisely quantify their limits of application, and to optimize the survey process of roads.
1 INTRODUCTION
The French road network mostly completed more than 30 years ago, and consists mainly of old bituminous pavements. Some of them have also been maintained several times with thin overlays (less than 8 cm thick). On these pavements, considerable damage such as potholes and alligator cracking has been observed in recent years, in particular after periods of heavy rain or freeze/thaw. Frequently, this type of damage is assumed to be associated with moisture effects linked to interface debonding between the overlays and the old pavement. These debonding mechanisms have a large influence on the residual life of the pavement, and thus their early detection is a very important issue for pavement maintenance (Savuth, 2006).
To detect such interface damage, selected non- destructive techniques (NDT), such as electro- magnetic techniques (ground penetrating radar, step-frequency radar, or infra-red) or mechanical techniques (from static deflection and radius of cur- vature measurements to seismic wave propagation methods), appear as promising approaches. They could also be efficient to detect and survey internal cracks.
This paper compares measurements with differ- ent NDT to detect and locate debonding and internal cracks, performed before the start of an experiment carried out on the large pavement fatigue carrousel of IFSTTAR in Nantes. These techniques will be used to survey the roadway during the planned loading phase.
2 DESCRIPTION OF THE FULL SCALE
EXPERIMENT
2.1 Description of the accelerated testing device
The pavement fatigue carrousel of IFSTTAR (Fig- ure 1) is a large scale circular outdoor test facility, unique in Europe by its size (120 m long) and load- ing capabilities (maximum loading speed of 100 km/h equating to a loading rate of one million cycles per month). Different to most ALT equipment, it is able to test pavements to failure in a few weeks. The machine comprises a central motor unit and four arms that can be equipped with different wheel configurations. The circular test track can be divided into several differ- ent test sections, loaded simultaneously. The width of the test track (6 m) allows for the application of traffic loads on the same track at two different radii.
2.2 Description of the test site
This study is part of a full scale experiment started on the test track in March 2011, investigating low traffic pavements with two different thicknesses of asphalt layer, namely 8 cm and 14 cm. A quarter of the fatigue test track has been dedicated to this study (Sector B). This 25 m long pavement section, consists of two bituminous layers (8 cm thick base layer, and 6 cm thick wearing course), over a granular subbase. Several types of defects were intentionally incorpo- rated into the base layer or at the interface between the
Figure 1. The pavement fatigue carrousel at IFSTTAR. two asphalt layers. A vertical crack was also included in a hydraulic pavement on another sector. The verti- cal crack was made by sawing the hydraulic base layer, and then covering it with a new 4 cm thick bituminous overlay.
The experiment will be performed in two parts. The first part consists of loading the outer radius (19 m), during 1.2 million load cycles (Hornych et al., 2012). This first part of the experiment is outside the scope of this paper, and concerns mainly the testing of pave- ments with geogrid reinforcement. The second part will start in the spring of 2012, and will consist of loading the inner radius (16 m), where the section with defects is located. Before the start of the loading on the inner radius, different non destructive techniques were performed to evaluate the initial state of the pave- ment, and to evaluate the capacity of the equipment to detect the defects. The results of these initial tests are presented in this paper. Similar tests will be performed during the accelerated test, to characterize the state of the different defects after different numbers of loads, until the failure of the pavement.
2.2.1 Material characteristics and pavement construction
The pavement structures were built on the existing sub- grade of the test track, which is sand with 10% fines, and sensitive to water. The modulus of this subgrade is approximately 70 MPa.
The structure built on this subgrade includes the following layers:
– A granular subbase consisting of 30 cm of 0/31.5 mm unbound granular material (UGM). After construction, this base was covered with a spray seal.
– A bituminous base layer, consisting of 8 cm of road base asphalt material (RBA) (0/14 mm grading). – A bituminous wearing course, consisting of 6 cm of
bituminous concrete (BC) (0/10 mm grading). The main characteristics of the bituminous mate- rials are given in Table 1. Complex modulus tests on trapezoidal specimens (NF EN 12 697-31 2008) were performed on the two bituminous mixes. The standard elastic moduli obtained for the two materials at 15◦C and 10 Hz are also given in Table 1.
Table 1. Composition and characteristics of the bituminous mixes.
Fractions BC (Class 2) RBA (Class 3)
0/2 (%) 29.3 32.47 2/6 (%) 25.52 19.1 6/10 (%) 38.75 9.55 10/14 (%) – 33.43 Filler (%) 0.95 0.95 Bitumen 30/50 (%) 5.5 4.49 E Modulus (MPA) 11,320 12,670
Table 2. Different defects and characteristics introduced into test track.
Dimensions, m Position, m,
Defect (length× width) R= 16 m
zones Type (m) along radius
I-1 sand 0.5× 2.0 2.5, 3.0
I-2 geotextile 0.5× 2.0 3.5, 4.0 I-3 tack coat free 0.5× 2.0 4.5, 5.0 I-4-I-9 geotextile 0.5× 0.5 6.5, 9.0 I-10 geotextile 3.0× 1.0 9.5, 12.5
I-11 sand 1.5× 2.0 13.5, 15.0
I-12 geotextile 1.5× 2.0 17.0, 18.5 I-13 tack coat free 1.5× 2.0 20.5, 22.0
2.2.2 Debonded interface
The construction of the pavements was carried out by a road construction company, using standard equip- ment. Rectangular debonded areas of different size and longitudinal or transversal direction were created artificially, using different techniques (sand, plastic film, or absence of tack coat) and are summarized in Table 2. They are located at the interface between the two bituminous layers. Figure 2 presents a photo- graph of the debonded areas before the wearing course construction.
The debonded areas I-1, I-2 and I-3 are 2 m wide and 0.5 m long, and centered on the radius of 16 m. I-4 to I-9 are small defects, 50× 50 cm, with a geotextile interface, located in and outside the wheel paths. I- 10 is a 3 m long by 0.5 m large defect, centered in the wheel path. I-11 to I-13 are bigger debonded areas, 2 m wide by 1.5 m long, with different types of interface (sand, geotextile or without tack coat).
Figure 3 shows a map of the experimental sec- tion, with the location of the different defects. These defects were centered on the radius R= 16 m, which corresponds to the centre of the wheelpath when the pavement is loaded. The total width of the wheelpath (with lateral wandering) will be approximately 1.0 m (between 15.5 m and 16.5 m).
2.2.3 Artificial cracks
A transverse vertical crack was introduced by saw- ing in another sector of the test track, where an
Figure 2. Interface defects before wearing course construc- tion.
older pavement structure was retained from a previ- ous experiment (Figure 4). The structure included the following layers:
– 20 cm of a granular subbase of 0/20 mm unbound granular material;
– Part of an old bituminous base layer, consisting of 4 cm of Road Base Asphalt Material (0/14 mm grading);
– A base layer consisting of 13 cm of a Fiber Rein- forced Cement Concrete (FRCC);
– A bituminous wearing course consisting of 4 cm of bituminous concrete (0/10 mm grading).
The wearing course was milled off in a square area of 2 m by 2 m. The FRCC base layer was then sawed
Figure 3. Map of the different debonded areas in sector B.
Figure 4. Sawing of a transverse vertical crack inside the fiber reinforced cement concrete layer.
with a 2.5 mm thick disk to a depth of 40 mm. Finally, a new wearing course was laid on this area to cover the internal crack. Other vertical cracks may be created during the experiment.
3 PAVEMENT INVESTIGATION
The main objective of the experiment is to compare dif- ferent NDT techniques (FWD, inclinometer, Colibri, radar) used to detect different geometrical characteris- tics of artificial defects. Other objectives are to follow the evolution of the defects during loading, and to eval- uate their effect on pavement performance. This will be helpful in optimizing pavement monitoring with the different NDT methods.
It is planned to:
– Characterize the initial state of the pavement (these results are presented in this paper).
– Repeat the NDT tests after different load levels: • After 10,000 loads, when the structure is consol-
idated;
• After 50,000, 100,000 and 150,000 loads to survey the structure;
• When visual surveys indicate the start of distress in the structure (e.g., cracking, rutting). – Analyze in detail the final state of the pavements,
using NDT tests, and also by coring and excava- tion of trenches, to compare the actual state of the pavement with the NDT results.
The structure will be investigated using the follow- ing methods:
– Three radar devices: two classical with a coupled 2.6 GHz antenna and a Horn 2.0 GHz antenna, and a step frequency radar which uses a network analyzer; – The Colibri apparatus, which is based on the fre- quency response function (FRF) presented below; – FWD tests, to estimate deflection basins under a
dynamic load;
– Inclinometer measurements, to estimate the radius of curvature of the deflection basin under a rolling load.
– Benkelman beam, to measure the deflection basin under a rolling load.
The project will also consider evaluating other NDT methods used in civil engineering such as impact echo, pulse echo, wave propagation, infra-red methods, if available technically feasible.
3.1 Radar probing techniques
Over the past few years, radar systems have emerged as a powerful non-destructive testing (NDT) technique for pavement surveys (Scullion, 1995; Saarenketo, 2000; Cardimonda et al., 2003; FHWA, 2010) includ- ing assessing defects such as segregation, stripping, and cracking (Forest, 2004). Radar systems have sev- eral major advantages, such as a high data acquisition rate and global monitoring through quasi-continuous measurements. Radar systems take advantage of the penetration capability of electromagnetic (EM) waves to image any dielectric contrast within the subsur- face. Within the scope of this paper, the capabilities of the two existing radar techniques (i.e., pulse and step-frequency radar) are compared on a qualitative basis to detect debonding embedded in the pavement section.
3.1.1 Radar measurement principle
As shown in Figure 5, the transmitting antenna (Tx) sends a radar wavelet into the medium. Scattered echoes are generated by any dielectric contrast within the medium and propagate back to the receiving antenna (Rx). The vertical structure of the pavement,
Figure 5. Radar principle and antenna configuration: The zero-offset mode is the conventional radar configuration for pavement survey, when Tx and Rx antennas are co-localized at x=0; CMP radar configuration consists in moving Tx and Rx apart at different x values (multioffset).
which provides a horizontally stratified medium, can then be measured by the radar data through echo detec- tion and amplitude analysis. At vertical incidence, defining t1and v1as the travel time and the wave
velocity, respectively, the thickness of a pavement layer is given by the following equation:
Radar techniques typically require some core sam- ples of the pavement structure to determine the wave speed v1, with respect to the light speed cO. How-
ever, further amplitude analysis of successive echoes enables the use of a coreless technique for which, the wave speed v1is retrieved from the echo amplitudes
A1 and A0, associated with the top surface echo ampli- tude and the amplitude of the Tx pulse, respectively, according to:
Radar data consists of vertical profiles of electro- magnetic wave amplitudes (or A-scan) recorded at each location along the track. The data vectors col- lected along the track are gathered into a data matrix which is called the B-scan image.
For detection purposes, data processing mostly aims at selecting the maximum echoes from the vertical pro- file (signal amplitude) and at performing accurate time delay estimation. Layer thickness estimation is based on the time delay of primary echoes.
3.1.2 Application to debonding
For the analysis of radar signals, a debonded interface is considered as a thin layer with different electromag- netic properties. A major limitation of radar techniques remains in the size and thickness of this interface with
respect to the wavelength as well as the time resolu- tion. When the thickness of the defect is greater than about half of the wavelength, (Derobert, 2004), two separate echoes are generated, and the thickness of the defect can be determined using Equation 1. When defects are thinner than this limit, as is the case in this experiment, the two echoes overlap each other, merging into an apparent single echo with longer time duration. According to (Gregoire, 2001), thickness up to a tenth of the wavelength of the GPR pulses can still be detected provided that accurate waveform analy- sis at high signal to noise ratios can be completed. The advanced signal processing techniques which have been used to perform UTAS (ultra thin asphalt surface) pavement survey in (LeBastard et. al., 2007) may over- come the latter limitation by an additional approximate factor of four.
Classical data processing used to date to detect debonding in pavements consists of performing an amplitude survey of the echo attached to the inter- face between the wearing course and the base layer (Simonin et al., 2012). Any spatial variation of that echo amplitude along the radar profile (Bscan) with respect to an intact zone would reveal debonded areas. As shown in Section 3.1.4, the shorter pulses provided by the step-frequency radar enhances the amplitude variations due to overlapping echoes.
3.1.3 Pulse and step-frequency radar technologies
Impulse radar was the first technology used in the GPR community. The pulse is usually a Ricker-type with a smooth spectrum and without any zero within the bandwidth, as shown in Figure 6 (bottom). In the 1990s, step-frequency technology enabled larger bandwidth and better measurement flexibility. Based on a network analyzer, the Tx antenna successively radiates monochromatic waves (i.e., one single fre- quency at a time) into the road pavement. The received signal is synthesized in the time domain by an inverse Fourier transform. Network analyzers have increased in speed and some commercial step-frequency radars are now able to collect data at traffic speeds. Moreover, step-frequency technology has moved a step further by providing surface probing capability thanks to array antenna technology and appropriate processing.
For the experiment, Tx and Rx antennas were set 20 cm apart and located 40 cm above the pavement surface. Both systems have roughly the same footprint on the pavement surface, i.e., the same spatial integra- tion on the surface. A 2.6 GHz ground-coupled GSSI antenna was also used for qualitative comparison. The ground-coupled configuration insures better signal to noise ratio and allows finer spatial resolution.
The first system uses GSSI impulse radar with 2 GHz air-coupled antennas. The antennas are fixed to a trolley which is manually moved. A fine spatial sampling is achieved (40 profiles per meter) and is controlled by a survey wheel. A B-scan profile was recorded along the radius 16 m (Figure 7a).
The second system is a step-frequency within a bandwidth of 0.7 to 7.4 GHz and uses Vivaldi Rx and
Figure 6. Measured radar pulses in both the time (top) and the frequency domains (bottom) for the two radar sys- tems, i.e., the 2 GHz impulse radar (black line) and the step-frequency system (thick gray line).
Tx antennas (Gibson, 1979; Langley et al., 1996). The ultra-wide band antennas have been especially designed for pavement survey application at the Elec- tronics, Antennas and Telecommunications Labora- tory (LEAT) at the Nice-Antipolis University. They use the “stripline” technology, display a small lateral dimension and yield a bandwidth of 800 MHz to 8 GHz in a bi-static air-coupled configuration. This frequency range induces a central frequency around 4 GHz and a resolution about twice that of the impulse technique. The antennas were mounted behind a car and the B- scans were recorded over a smaller range from the I-13 defect to the end of the I-11 defect along the same radius as before, i.e., 16 m. The B-scan profile in Fig- ure 8b was collected at very low speed. The spatial sampling only allowed four profiles per meter, due to the data acquisition capacity, but plans are in place to improve this sampling rate.
3.1.4 Experimental results on the carrousel
Figure 7a presents the radar Bscans which were collected with the 2 GHz impulse radar along the 16 m radius. Qualitative comparison with the 2.6 GHz ground-coupled radar is shown in Figure 7b.
Vertical white lines identify the different defect zones shown in Figure 3. Four zones are labeled A to D with the following defects: A – I-1 to I-3 (0.5 m long defects), B – I-4 to I-9 (small square geotextile defects), C - I10 (3 m long geotextile defects), D – I- 11 to I-13 (1.5 m large defects). Despite this marking, data interpretation required further investigation.
The horizontal trace corresponds to the first echo on the pavement surface. Vibrations of the trolley were removed using conventional post-processing. The sec- ond echo corresponds to the interface to detect distress between the wearing course and the base layer. The third echo corresponds to the bottom of the base layer. The narrowest defects in Regions A and B could hardly
Figure 7. 2 GHz impulse air-coupled and 2.6 GHz impulse ground-coupled B-scan radar images over the four defect zones, A to D.
be detected. At the end of the B-scan, some hyperbolas revealed embedded metal instrumentation within the pavement.
For debonding detection, attention was focused on the largest defects in Region D (I-11 to I-13 in Table 2). Stronger echoes than expected were observed for I- 13 and will be investigated further with cores that will be taken at the end of the experiments. Figure 8a shows a magnified image of the I-11 and I-12 defects to simplify the comparison with the step-frequency Bscan shown in Figure 8b. Vertical white dashed lines indicate the defect delimitations. The echo from the interfaces between the wearing course and the base layer are shown as white lines. This echo was auto- matically detected by selecting the maximum of the amplitude within an appropriate time window.
The amplitude variations of the echo along the dis- tance are used to detect debonding. Then, taking the echo on the zone with no distress between I-11 and I-12 as the reference signal, the two defects can be seen in Figure 8 using both a smaller delay shift and some con- trast in the signal amplitude. The variations of that echo
Figure 8. Bscan images obtained by the two radar technolo- gies over the “D” zone shown in Figure 7, including sand (I-11) and geotextile (I-12) interface defects; a) impulse radar (zoom on Figure 7a); b) step-frequency radar.
Figure 9. Debonding detection on the Bscans shown on Fig- ure 8: Variations along the longitudinal profile of the echo amplitude attached to the wearing course interface. along the longitudinal profile are shown in Figure 9. For the 2 GHz impulse radar in Figure 9a, the location of the defects is difficult because of both the noise and the limited contrast amplitude over the defects (with regards to the zone with no distress). In Figure 9b, the shorter pulses provided by the step-frequency radar enhance the amplitude variations over the debonded areas. The amplitude variation is about five times larger over the defects. Clearly, data interpretation of the step-frequency Bscans is more intuitive and readily reveals the location of the defects.
This first comparison of the different radar mea- surement techniques shows the better efficiency of