NDT&E International 60 (2013) 40–51

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Determination of the compaction of hot mix asphalt usinghigh-frequency electromagnetic methods

Cyrille Fauchard a,n, Bo Li b, Laurent Laguerre c, Bernard Héritier d,Nabil Benjelloun e, Moncef Kadi e

a Centre d’Études Techniques de l’Équipement - Normandie Centre, Université de Rouen, 10, chemin de la Pourdrière, BP 241, 76121 Le Grand Quevilly, Franceb Kuang-Chi Institute of Advanced Technology, 518000 Guangdong, PR Chinac Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux, Université de Nantes, CS4 44344 Bouguenais, Franced Eiffage TP, 93330 Neuilly-sur-Marne, Francee Esigelec, Université de Rouen, 76800 Saint-Étienne-Du-Rouvray, France

a r t i c l e i n f o

Article history:Received 4 October 2012Received in revised form28 June 2013Accepted 9 July 2013Available online 22 July 2013

Keywords:CompactionPermittivityHot mix asphaltCylindrical cavitiesStep-frequency radar

95/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.ndteint.2013.07.004

esponding author. Tel.: +33 235689295; fax:ail address: cyrille.fauchard@developpement-

a b s t r a c t

The main objective of this work is to assess the capability of electromagnetic (EM) methods based on wavepropagation to determine the compaction of hot mix asphalt (HMA) specimens. It is to be recalled that rocksare the main component of HMA. We begin by performing a dielectric characterization of rocks withcylindrical resonant cavities. This experiment shows that the rocks, and consequently HMA, may beconsidered as low-loss materials. We then use the same electromagnetic devices to assess the complexpermittivity of cylindrical HMA samples implemented in the laboratory and controlled with standard tests.The level of compaction is estimated according to a complex refractive index (CRI) model that takes intoaccount all HMA components in addition to the measured permittivity. During a final stage, we conduct anexperiment on HMA slabs, whose permittivity is measured with a step-frequency radar (SFR) along with anultra-wideband antenna. These experiments reveal that the electromagnetic approach is capable of assessingcompaction to within a few percent of the standard test compaction value and with a similar standarddeviation. The main outcome of this work is the possibility it raises of replacing the standard nuclear gaugescurrently used in the laboratory and in the field by EM-based systems.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Hot mix asphalt (HMA) is a mixture composed of both coarseand fine aggregates, asphalt binder and air. It is implemented asbonded layers on pavement. HMA layer compaction is consideredas one of the most important parameters to be controlled whenpaving new roads because this compaction step directly influencespavement life cycle. Compaction, a road design reference para-meter in France, is generally expressed in percent and defined bythe ratio of the bulk specific gravity ρ (in g/cm3) of the mix to thespecific gravity of the mix ρr (in g/cm3)

C ¼ ρ

ρrð1Þ

The specific gravity (or simply density) ρr is given by the mixdesign. The bulk density ρ is the reference parameter to becontrolled in the United States for this same purpose and typicallyexpressed in lb/ft3 (1 lb/ft3¼0.0160 g/cm3). The bulk density ρ ofHMA specimens is classically obtained by standard laboratorytesting methods (in accordance with [1,2] for non-absorbing

ll rights reserved.

+33 235688188.durable.gouv.fr (C. Fauchard).

mixtures) on cores extracted from HMA layers. Compaction canbe computed from Eq. (1). This time-consuming and destructivetesting approach constitutes the reference method for HMAdensity and compaction estimation. A second family of methodsfocuses on using nuclear probes in the laboratory [3], whichprovides an estimation of HMA core or slab compaction in con-junction with sampling at the centimeter scale. This method isbased on the absorption of gamma rays in the medium. The sameprinciple can also be applied onsite ([4] or [5] for a single boreholeand [6] in two boreholes); it requires drilling boreholes into thepavement in order to insert a nuclear source (for example, 137Cs)and to measure at the ground surface (or in a borehole) using ascintillator (or Geiger–Muller counter) the gamma rays travellingacross the pavement. Another non-destructive nuclear approach[7] consists of measuring the gamma rays backscattered by themedium, in which case both the source and detector are locatedin the same plane either at the HMA surface or just above it.The main drawbacks of such nuclear probes lie in the highmaintenance cost caused by use of the nuclear source, plus itstransportation, storage and the stringent use requirements. Repla-cing nuclear methods has therefore become a major issue.

Since their introduction to market at the end of 1990s, agrowing interest in the use of commercial electromagnetic

1 Both designations stepped-frequency radar and step-frequency radar can befound in the literature.

C. Fauchard et al. / NDT&E International 60 (2013) 40–51 41

capacitive devices has been shown as alternative to nucleardensity gauge for in place pavement density estimation. Numerousstudies have been performed, involving various teams fromAmerican transportation organizations for onsite quality control/quality assurance [8,9]. Data analysis specifically focussed on thestudy of correlation between EM capacitive gauge results andexisting methods (as coring and/or nuclear density gauge) con-sidered as the references in this field. Early studies from Romeroand Kuhnow [9] stated that densities obtained with non-nuclearcapacitive devices should be viewed with caution. Hausman andButtlar [10] reached a similar conclusion and pointed out the needto consider the influence of the environmental conditions. InEurope, De Backer and Glorie [11] found correlation coefficientsof 0.87–0.97 and of 0.41–0.53 between nuclear methods andstandard core weighing tests, and between capacitive methodsand nuclear methods respectively. They mentioned that capacitivedevice was less accurate than the nuclear gauge for thick roadlayer and that no information about its investigation depth wasgiven. They also confirmed that moisture strongly influencedpavement density estimation and concluded that non-nuclearcapacitive methods were not a viable alternative. During the2000s, studies have led to the improvement both in the deviceperformance from manufacturers as well as in the device calibra-tion by end-users. For instance, specific fitting functions have beenproposed to better correlate non-nuclear gauge results to existingmethods results by calibrating them on the concerned mix design[12]. Besides, multi-linear regression analysis was also proposed touse the pavement moisture index (as displayed by the commercialgauge) to correct the pavement density estimation from themoisture influence [8]. Differently from the above mentioned,Megali et al. [13] recently proposed a physical based approachthrough finite element modeling of the capacitive device in orderto gain insight into interpretation of non-nuclear capacitivemeasurements HMA density. The device impedance was modeledas a function of its geometrical characteristics and related to thecapacitive gauge readings. The authors were able to explain thepoor correlation between capacitive-derived and core densitymeasurements by the influence of the moisture content. Amongothers, they clearly showed that very different impedancescould give the same estimated density. However for moisturelower than 10%, they concluded that impedance could provide arelative measure of density as long the moisture content remainsconstant.

Even if EM capacitive commercial devices are devoted topavement density estimation, they cannot be regarded as theonly potential EM method to do so. For instance, ground pene-trating radar which has been extensively used for non-destructivenear subsurface applications was considered for the first time forquality control of new pavement via the estimation of air voidcontent in Saarenketo and Scullion [14]. Measurements of thedielectric properties of asphalt pavement materials were alsoconducted from either laboratory or in place experiments[15,16]. Jaselskis et al. [15] illustrated both the influence of thetemperature on the complex permittivity of asphalt pavement(from 100 Hz to 12 GHz) for three density ranges (low, medium,high) and two mixture designs and the increase of permittivitywith density over the same frequency range. More recently,dielectric modeling of dry HMA mixture was proposed by usingEM mixing theory to rely HMA density to its permittivity.Chang et al. [17] and Leng et al. [18] focussed on the permittivityreal part, Mardeni et al. [19] focussed on the permittivityimaginary part. Chang et al. [17] performed laboratory measure-ment of the complex permittivity of HMA cylindrical sample with19 mm-aggregate size between 200 MHz and 12 GHz for variousdensities and binder content. We can observe that the real part ofthe predicted permittivity from a two-phase or a four-phase HMA

mixture model respectively underestimates the measured per-mittivity at 7 GHz. A bias is also observed between the predictedand lab-measured densities leading to an increasing underesti-mation as the density decreases. Measurements using an smallopen ended coaxial probe (2b¼19 mm, 2a¼3.5 mm as used in[17]) is not suitable for permittivity measurements of coarse-grainmaterial as mentioned in Otto and Chew [20] and Adous et al.[21]. Only a hemispheric volume of the inner diameter dimension(i.e., 4 mm in this case) is probed from the surface sample [22].The 10 measurements performed at the top sample surface arenot enough to reach representative average permittivity as shownin Bois et al. [23] for a 9.5 mm aggregate size hydraulic concrete atX band. Consequently, the probed signal is so mainly too muchdominated by the aggregate permittivity (which is confirmed bygood agreement at high densities, i.e., less than 1% void content)leading to lower predicted values at low densities (at 10% voidcontent).

The aim herein is to present a complementary approach toearlier works based on controlled laboratory experiments to assessHMA compaction in the microwave range (1.4 GHz–10 GHz). To doso, the HMA mixture permittivity will be derived as a four-constituent dielectric mixing model of air, asphalt binder, rockaggregate and their respective density, mass fraction and permit-tivity. In addition to the combination of two dielectric measure-ment techniques, compaction will be measured using paraffin-coated samples (PCS, [1] or [2]) tests or with the gamma-raytransmission technique. In the first part, a special attention isgiven to the determination of the complex permittivity of a widerange of rock aggregate constituents as they represent the largestproportion and permittivity of the HMA mixture. The dielectriccharacterization will be performed with the resonant cavitytechnique. In a second part, a first validation of the four-phaseCRI model is conducted on core samples by comparing theestimated compaction of a quartzite aggregate HMA mixture toreference PCS test showing differences lower than 3% for thecompaction range from 0.91 to 0.94. The third part will be devotedto the extension of the proposed method to the non-destructiveestimation of the compaction of slab samples with a monostaticstep-frequency radar (a Vivaldi antenna and a Vector NetworkAnalyzer). Permittivity estimated compaction maps were obtainedby moving the radar along transect lines above HMA slabs at alaboratory scale. They will be systematically compared to gamma-ray [3] estimated compaction maps. Two procedures will be usedto extract the permittivity from SFR1 measurement: one procedureusing the surface reflection coefficient and the other using theradar wave two-way travel time through the thickness slab.The existence of a rutting effects resulting from laboratory HMAslab preparation will highlight the robustness of the “thicknessapproach” over the ”surface approach” to derive compaction fromSFR permittivity.

2. Theoretical background of HMA compaction andpermittivity

The HMA (see Fig. 1) is a four-phase mix composed ofaggregates of rock of various geological origins (quartzite, sand-stone, granite, diorite, etc.), as well as asphalt binder, limestonefiller and air [24]. The compaction C of the mix is defined by thesum of the volume fractions of each solid constituent (i.e., aggre-gate, binder and filler)

C ¼ Ca þ Cb þ Cf ¼ 1�Cair ð2Þ

C. Fauchard et al. / NDT&E International 60 (2013) 40–5142

where the indices a; b, and f refer respectively to the aggregates,asphalt binder and filler; Ci is the volume fraction of the element i.The common volume fraction ranges of each HMA constituent aregiven in Fig. 1. The dielectric behavior of a mix containing fourhomogeneous materials, such as our HMA specimens, can bedescribed by the Lichtenecker-Rother [25] equation

ϵαHMA ¼ Caϵαa þ Cbϵ

αb þ Cf ϵ

αf þ Cairϵ

αair ð3Þ

where α is an empirical parameter that can theoretically vary from�1 to 1. In this study, α is fixed to 0.5, in which case Eq. (3) is theso-called complex refractive index (CRI) model [26]. This modelwas employed in [17,18,27] to study the bulk dielectric propertiesand porosity of asphalt mixtures, and for a more general case in[28] to describe the behavior of various types of dry and wet soils.Other models, based on an effective medium approximation, wereproposed for the description of wet HMA specimens [29].

When a new HMA layer is to be installed, a mix design isprovided to the road builders that specifies: the quarry locationand density of aggregates ρa (g/cm3), the filler density ρf (g/cm3)and the asphalt binder density ρb (g/cm3). Road builders arerequired to follow the technical indications of the given mixdesign, especially the mass fraction of constituents as defined byTi ¼mi=ðma þmb þmf Þ where index i can be either a, b or f andrefers to the constituent under consideration. By virtue of thislatter definition and both Eqs. (1) and (2), HMA compaction can bedefined as a function of the mass fraction, density and permittivityof each HMA constituent, according to the following:

C ¼ ðϵαHMA�1ÞZ

ϵαa þρaTb

ρbTaϵαb þ

ρaTf

ρf Taϵαf �Z

ð4Þ

where Z ¼ 1þ ðρaTb=ρbTaÞ þ ðρaTf =ρf TaÞ is a constant and Ta, Tf, Tbare the mass fractions of the aggregates, filler and asphalt binder

Fig. 1. Picture of HMA surface with typical scale values of constituents.

Table 1First column: the rocks number (x-axis in Fig. 3), the nature and origin of the rocks incolumns: number of samples, mean value of complex permittivity measured at 0.57 GHztan δ and porosity Φ.

Rock number – nature (quarry) N ϵ′r

1 – Sandstone (Challoué) 4 4.5270.052 – Quartzite (Vignats) 14 4.5670.133 – Quartzite (Barenton) 3 4.570.14 – Sandstone Quartzite (Montebourg) 6 4.7170.065 – Sandstone (Minville le Bingard) 6 5.1470.046 – Weathered granite (La Ferrières) 5 5.4170.077 – Gneiss (Tinchebray) 8 5.570.098 – Rhyolite (Averton) 6 5.6970.099 – Quartz arenite (La Roche Blain) 3 5.9170.0410 – Hornfels (St Honorine-Plafond) 4 5.7270.1511 – Amphibolite (Arvieu) 11 6.5770.2512 – Gneissic Sandstone (Vaubadon) 7 6.6870.0913 – Limestone (North region) 6 7.4770.1114 – Basalt (Rhone-Alpes region) 5 7.7170.18

respectively. A similar form to that expressed in Eq. (4), yet withfiller content being neglected, was presented in [30]. Anothersimilar expression is provided in [18,31], whose authors alsostudied compaction models based on various permittivity models.As shown in Eq. (4), ϵHMA is the relative measured permittivity bythe EM method, and the only unknowns are the dielectricpermittivities of limestone fillers, asphalt binder and aggregates.As it will be demonstrated below in this work (Section 4, Table 2),the dielectric permittivities of limestone and asphalt binder areknown; such is not the case however for the aggregates, whosepermittivity varies according to their density and chemical com-position relative to their geological origin. For our study, thequarries providing aggregate for road builders in the Normandyregion (France) have been selected on the basis of both theirproximity to the road works and the mechanical properties ofaggregates extracted from the rocks quarried (gneiss, granite,quartzite, etc.). Moreover, the volume fraction of aggregate inHMA ranges from 88% to 94%. Knowing the dielectric permittivityof the rocks composing the HMA sample is therefore highlyrelevant in describing the dielectric behavior of HMA.

3. Dielectric characterization of rocks

Our approach consists of measuring the permittivity of rocksextracted from quarries in a given region (Normandy, France) andthen used for road construction. The recorded permittivity willconstitute a priori information for further in situ testing, such asthickness control or compaction control of newly implementedHMA layers. The dielectric characterization of rocks has beenundertaken for a long time in the domain of ground penetratingradar applications in civil engineering and geophysical surveying;a major contribution can be found in [32]. In our study, thedielectric permittivity of the rocks listed in Table 1 was measuredinside two different cylindrical resonant cavities [33]. Cores fromeach studied rock were drilled (Fig. 2a) and sawed in cylindricalsamples (Φ¼ 5 cm;h¼ 2:5 cm) for dielectric characterization incylindrical cavities (Fig. 2b). All rock samples were dried beforemeasurements in the laboratory.

The three resonant frequencies of each cylindrical cavity, in trans-verse magnetic modes are respectively [0.57 GHz, 1 GHz, 1.7 GHz]and [1.15 GHz, 2.63 GHz, 4.13 GHz], thus allowing for a dielectriccharacterization in the frequency band used in GPR applications.The whole experiment has already been described in [30]. We haveobserved that the permittivity of dried rocks is constant at the

the Normandy Region (except for 11, from Arvieu, Aveyron, France); subsequent, 1 GHz, 1.7 GHz, 1.15 GHz, 2.63 GHz and 4.13 GHz in cylindrical cavities, loss tangent

ϵ″r tan δ Φð%Þ

0.0370.02 0.00670.005 0.40.0370.024 0.00870.005 0.30.0470.03 0.0170.008 0.50.0870.06 0.01870.014 0.20.2570.11 0.0570.02 0.350.0870.06 0.01670.011 0.60.0670.05 0.0170.009 0.30.1470.1 0.02570.019 0.30.2170.17 0.0370.1 0.30.0970.06 0.01770.01 0.250.0770.05 0.0170.008 0.160.1870.14 0.0370.02 0.20.1170.09 0.0270.01 0.60.1370.05 0.01770.05 0.5

C. Fauchard et al. / NDT&E International 60 (2013) 40–51 43

considered frequencies. Then, the present permititvites recorded inthis paper are the average of the permittivities measured at the sixfrequencies. These measurements provide the complex dielectricpermittivity defined by ϵ*r¼ϵ′r�jϵ″r . The real part of complex permit-tivity reflects the polarization effects due to bound charges displace-ment when a time-varying EM field is applied, while the imaginarypart of complex permittivity reflects both the real part of electricalconductivity due to free charge displacement and the relaxationpolarization effects due to bipolar moments. The results of measure-ments on the rocks studied herein are given in Fig. 3. The real part ofpermittivity ϵ′r (Fig. 3a) ranges between 4 and 8. Similar results ondry aggregates were presented by [34]; the imaginary part ϵ″r (Fig. 3b)is such that the loss tangent tan δ¼ ϵ″r=ϵ

′r is less than 0.03 for most of

the rocks. Only the Minville-le-Bingard sandstone exhibits the high-est loss tangent of 0.05. Nevertheless, the measured values of theimaginary part of permittivity in cylindrical cavities must be carefullyconsidered: the standard deviation and mean value are of the sameorder of magnitude. In Table 1, the porosity of rocks was estimatedin the laboratory. The porosity is less than 0.6%. This low value isin accordance with the literature data [35,36].The influence ofvoid content can be neglected. These results lead to the followingassumption: in considering the HMA volumetric concentration ofeach component and taking into account the dielectric values ofasphalt binder and filler given in Section 4, Table 2, HMA can beviewed as a low-loss dielectric material within the studied frequencyband under dry laboratory conditions.

4. Dielectric characterization of HMA specimens in cylindricalcavities

The objective in this section is to evaluate how the EMapproach could be compared with standard tests in order to

Fig. 2. (a) Quartzite rock extracted from the quarry of Challoué (France, 14) and core and

1 2 3 4 5 6 7 8 9 10 11 12 13 144

4.5

5

5.5

6

6.5

7

7.5

8

Rock number (see Table 1, col.1)

Fig. 3. (a) Real and (b) imaginary dielectric permittivities of the studied rocks. The x-a

estimate HMA compaction. The permittivity of cylindrical HMAsamples is obtained in the cylindrical cavities previously presentedfor rock characterization. The compaction of cylindrical HMAsamples can then be estimated according to Eq. (4). Results arecompared with the estimated compaction levels obtained by astandard test [1,2] based on PCS.

The cylindrical HMA samples are composed of quartzite aggre-gates from the Vignats quarry (France, Orne and Table 1, line 2). Theirdimensions are 2.52 cm (height) by 5 cm (diameter) (Fig. 4d). Wehave chosen five expected compaction levels of: 0.88, 0.90, 0.92, 0.94,and 0.96. Three samples per expected compaction were extractedfrom cores generated using a Superpave Gyratory Compactor ([37] or[38], Fig. 4a). According to the standard, the compaction of our HMAsamples is obtained by applying a given number of girations for anexpected compaction: an expected compaction of 88–96% required atleast 10–80 girations respectively, for an 0/10 HMA gradation. In all,we obtained 15 cylindrical HMA samples.

The permittivity of both limestone filler and the asphalt binderwas estimated in the laboratory as a part of a previous work [30].These two elements systematically contribute to the production ofthe studied HMA specimens presented in this work. The mixdesign of these HMA samples is given in Table 2. The expectedcompaction ranges from 0.88 to 0.96. Compaction levels controlledby the PCS method vary between 0.91 and 0.94. These differencescan be explained by implementation with the SGC device, whichyields cylindrical cores 30 cm high and 10 cm in diameter (Fig. 4b).The cylindrical samples used to perform characterization in thecylindrical cavities were subsequently extracted by coring andsawing of the SGC samples (Fig. 4c). We have assumed that theoverall expected compaction of SGC cores samples according tothe standard test differs from local compaction in the center ofthe SGC samples, i.e., where the cylindrical samples for dielectriccharacterization are obtained.

(b) rock sample sawed from core for dielectric characterization in cylindrical cavity.

1 2 3 4 5 6 7 8 9 10 11 12 13 140

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Rock number (see Table 1, col.1)

xis number corresponds to the rock number listed in the first column of Table 1.

0.9

0.92

0.94

0.96

0.98

Com

pact

ion

from

ε′ r

,Cav

ity

C. Fauchard et al. / NDT&E International 60 (2013) 40–5144

Compaction is calculated by considering the real part ofconstituent permittivity (low-loss materials). Compaction Cϵ′r ;Cavity

estimated according to the CRI model (Eq. (4) is shown in they-axis in Fig. 5. The y-error bars indicate the standard deviation ofmeasurements on each sample at the six resonant frequenciespreviously discussed in the dielectric characterization of rocks. Theestimates stemming from the PCS test for sample compaction CPCSare shown in the x-axis. The x-error bars indicate the estimatedstandard deviation of the standard test method (1%): this techni-que is considered as the reference for controlling the compactionof cylindrical HMA samples. The difference between these twoapproaches is less than 2% for the higher compaction rates (around0.94) and between 1% and 3% for the compaction rates between0.91 and 0.935. The linear regression is expressed as: Cϵ′r ;Cavity

¼1:18nCPCS�0:172. The correlation coefficient is 0.859. Additionalcorrelation tests based on various rock compositions and gradationhave to be performed in the future.

Both approaches are time consuming. For one part, the EMapproach in cylindrical cavity needs a long preparation for

Table 2Mix design of cylindrical quartzite HMA samples.

Aggregates(0/10) T (%) ρ (g/cm3) ϵnr

6/10 31.2 2.628 (4.5670.13)� j(0.0370.02)4/6 15.1 2.628 (4.5670.13)� j(0.0370.02)0/4 47.3 2.628 (4.5670.13)� j(0.0370.02)

Filler (limestone) 0.9 2.635 (7.270.02)� j(0.0170.005)

Asphalt binder 5.5 1.03 (2.5070.02)� j(0.00370.002)

Fig. 4. Cylindrical HMA samples generated with: (a) a Superpave Gyratory Compacto(c) sawed into smaller cylinders (d) of 2.52 cm height for the dielectric characterization

precisely sawing the cylindrical samples at the correct height(2.52 cm). For another part, PCS method requires to coat sampleswith paraffin. Nevertheless, it is obvious that both methods exhibitstrong correlations, hence the EM approach for estimating thecompaction of quartzite HMA specimens could indeed replace the

r (SGC). The SGC samples were (b) drilled into cylindrical cores of ϕ¼5 cm; andin cylindrical cavities.

0.86 0.88 0.9 0.92 0.94 0.96 0.980.86

0.88

PCS compaction (NF EN 12697−6 or ASTM D118−07)

y = 1.18*x − 0.172; Cor=0.859

Fig. 5. Comparison of the estimated compaction rates using the CRI model(y-axis) and measured by the PCS test (x-axis). The dashed line defines the idealcase Cϵ′r ;Cavity

¼CPCS. The red line is the linear fitting between both methods.(For interpretation of the references to color in this figure caption, the reader isreferred to the web version of this article.)

C. Fauchard et al. / NDT&E International 60 (2013) 40–51 45

PCS method. That way, the next step consists of implementing anon-destructive EM approach available both in the laboratory andin the field.

Fig. 6. SFR system in the laboratory: a motorized bench sweeps the ETSA above theHMA slab. The EM signal is emitted and recorded by the VNA. A laptop thencontrols both the SFR and the motorized bench.

5. Dielectric characterization of HMA slabs with a step-frequency radar (SFR)

In order to determine whether the electromagnetic approachwould be as effective in the laboratory as on newly constructedHMA layers, the HMA slabs were designed in the laboratory andtested with a SFR system providing similar results to the GPR.

5.1. HMA slabs implementation

Several HMA slabs (40 cmn60 cmn8 cm) with various compac-tion ranges have been designed in the laboratory with a two-wheel roller compactor (RC) according to the standard test [39].The slabs were designed with two different types of aggregates:limestone and basalt (see Table 1, rock numbers 13 and 14). Twosets of four limestone and basalt HMA slabs have been respectivelyproduced at four RC-designed compaction rates. Maximum aggre-gate size was 10 mm (0/10). The dielectric properties of the rocksand mix designs of both types of slabs are summarized in Table 3.

5.2. Permittivity measurement with the SFR system

The permittivity and compaction of HMA can be estimatedonsite by examining the GPR signal reflected at the HMA surface40 This approach was presented by Leng et al. [18], i.e., GPRmeasurements are performed at a test site where various HMAspecimens were implemented and where HMA compaction wasobtained by measuring the reflected pulse on the HMA surfacewith several horn antennas placed above the pavement surface.Jaselskis et al. [41] used a similar system based on the signalanalysis of a reflected EM wave at 11.2 GHz. They studied thevariation in signal strength just before and after HMA implemen-tation. The present work is based on a similar principle but usinf aSFR inovative system installed on a motorized bench to performpermittivity mapping. SFR is used herein as a non-destructivemethod for assessing the HMA compaction of slab samples imple-mented for laboratory testing. The SFR system is composedof a vector network analyzer (VNA) and an exponential taperedslot antenna (ETSA), which is an ultra-wideband antenna ofthe “Vivaldi” family [42]. The analyzer generates a step-by-step

Table 3Mix design of the studied HMA slabs implemented using the RC test method.

Aggregates (gradation) Basalt (0/10) Limestone (0/10)

HMA slabsNumber of slabs 8 (2 setsn4 slabs) 8 (2 setsn4 slabs)Expected compaction 0.88, 0.90, 0.92, 0.95 0.88, 0.90, 0.92, 0.94

Aggregates characteristicsϵa (7.7170.18)� j(0.1370.05) (7.4770.11)� j(0.1170.09)Ta 0.93 0.94ρa (g/cm3) 2.89 2.75

Asphalt binder characteristics (similar to Table 2)ϵb (2.5070.02)� j(0.00370.002)Tb 0.052 0.056ρb (g/cm3) 1.03 1.03

Limestone filler characteristics (similar to Table 2)ϵf (7.270.02)� j(0.0170.005)Tf 0.009 0.017ρf (g/cm3) 2.635 2.635

sinusoidal EM signal (1601 points) over a selected frequencyrange [1.4–10 GHz] included in the bandpass of the antenna[1.4–20 GHz]. A data processing operation to measure both theamplitude and phase of the reflected signal on the HMA slabs iscarried out in the time domain by an inverse fast Fourier transform(IFFT). The complete system has already been described in [43]and applied to the measurements of thin HMA layers.

The system is displayed in Fig. 6. A motorized bench allows theETSA to move along the x-axis; the bottom of the antenna is placed19 cm above the HMA slab surface. The antenna is used as both atransmitter and receiver (monostatic configuration). Data proces-sing subsequent to the IFFT yields a result similar to the GPRmeasurements; an example of this measurement (the so-calledB-scan) along the x-direction is illustrated in Fig. 7a. The use ofhigh VNA frequencies and a wide bandpass however allows formore accurate resolution in the time domain of the dielectriccontrasts encountered in the observed structure.

5.3. Permittivity calculation on HMA slabs specimens

In the monostatic configuration used here, the slab permittivityis calculated using two approaches, one is based on the reflectioncoefficient at the slab surface and the second is based on the two-way travel time through the slab thickness [27]. In the firstapproach, the measured surface permittivity is expressed as

ϵ′SFR;Surface ¼1þ R01

1�R01

� �2

ð5Þ

with

R01 ¼AHMAðhref ÞAmetalðhref Þ

ð6Þ

where R01 is the reflection coefficient of EM wave at the air(medium 0) and HMA (medium 1) interface, AHMAðhref Þ andAmetalðhref Þ are the reflection amplitude moduli in the time domainrespectively from the HMA surface and a flat metal plate (totalreflection) recorded at the same antenna height href¼19 cm; andϵ′SFR;Surface is the real relative permittivity of the HMA slab. Thisassumption actually remains true as long as the following hypoth-eses are satisfied: (1) incident EM waves are considered as planewaves; (2) the wavelength relative to the frequencies used isgreater than the HMA aggregate size; (3) the HMA slab is regardedas an infinite medium in both the x and y directions. The first

Fig. 7. (a) B-scan of an HMA paraffin slab (c) obtained with the SFR system [1.4 GHz, 10 GHz] along the x-direction defined in Fig. 6 and (b) recorded modulus on a metalplate (black line) and on the HMA slab surface (dashed blue line) for calculating permittivity according to Eqs. (6) and (5), and then the two-way travel time fit betweenthe slab surface and bottom for calculating permittivity according to Eq. (7). (d) Example of a 3D permittivity map obtained on a paraffin slab used as calibration material.(For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

C. Fauchard et al. / NDT&E International 60 (2013) 40–5146

assumption according to the Fresnel zone definition depends onthe distance between antenna and slab specimen, the frequencyconsidered and the ETSA dimension and gain. Since a wideband isused with the SFR system, the plane wave assumption is barelysatisfied. Yet the experiments conducted on calibration materialsindicate that the permittivity calculated in the time domain at afixed distance (19 cm) according to Eqs. (6) and (5) is correct. Thesecond assumption strongly depends on the higher frequencyused (10 GHz in this experiment). If we were to consider amaximum aggregate size of 10 mmwith respect to the mix design,for an HMA specimen permittivity of between 4 and 7, thewavelength would range from 1.13 to 1.5 cm: we are just belowthe limit where diffraction effects on aggregates invalidate Eq. (6).The third assumption is true as long as the reflected signal islocated far enough from the slab edges. In the second approach,the measured slab permittivity is deduced from

ϵ′SFR;Thickness ¼cΔt

2dði; jÞ

� �2

ð7Þ

where c¼3.108 (m/s2) is the speed of light in vacuum and d(i,j)refers to the slab thickness at a given location along (Ox) and (Oy)axis as explained below. The two-way travel time Δt is measuredbetween the slab surface and its bottom as shown in (Fig. 7b)

As shown in Fig. 7d, a (x; y) survey was performed on a paraffinslab to calibrate the SFR system. The permittivity map obtained inthe area defined by y¼[5, 10, 15, 20, 25, 30, 35 cm] and x¼[0 to60 cm by 1-cm increments] exhibits strong edge effects in they direction, just above the slab edges: the estimated permittivityvaries widely, revealing that Eqs. (6) and (5) are no longer valid.For the remainder of this study, a smaller area will be surveyed,

with dimensions of 40 cm in the x direction and 20 cm in the ydirection and placed exactly in the middle of the slab surface.

Once the slab compactions are deduced from Eq. (4) (knowingpermittivity from Eqs. (6) and (7)), they will be compared to theestimated compaction rate measured with the gamma-ray trans-mission laboratory bench [3]. The above-mentioned thicknessesd(i,j) were measured for each HMA slab during the gamma-raytransmission bench measurement.

5.4. Results of HMA slabs measurements and discussion

Figs. 8 and 9 display the maps of the compaction valuesobtained from the SFR system and the nuclear test bench for thelimestone and basalt aggregate respectively. Only one set of eachHMA slab type (i.e., 4 slabs) is presented, but each HMA slab set(i.e., 4n2 slabs) has been characterized. The compaction value fromthe SFR system is derived from the CRI model by estimating thepermittivity from (1) the reflection coefficient at the air/HMAinterface (Eq. (5) and first column referred as “SFR_Surface”) and(2) the time-of-flight in the thickness of the HMA slab (Eq. (7) andsecond column referred as “SFR_Thickness”). The third column inFig. 8 and 9 being referred as “gamma rays”.

Figs. 10 and 12 shows the compaction estimated from SFR_Sur-face and SFR_Thickness measurements respectively versus compac-tion estimated from the gamma-ray bench for both the limestoneand basalt aggregate. The x-error bars and y-error bars representthe standard deviation of the estimated compaction obtained withthe gamma-ray bench and the SFR system respectively.

From Fig. 10, it is obvious that estimated SFR_Surface com-pactions yield poor results (high standard deviation) in compar-ison with the estimated compaction with nuclear gauge values

Fig. 8. Compaction maps of the limestone slabs obtained with the SFR system (left and middle columns), and the standard nuclear test (right column) on one set oflimestone HMA slabs: (a) SFR_Surface, (b) SFR_Thickness and (c) Gamma rays.

C. Fauchard et al. / NDT&E International 60 (2013) 40–51 47

associated. By comparing results from Figs. 10 and 12, weobserve that the results from SFR_Thickness match better withthe gamma-ray than those from SFR_Surface with the gamma-ray (lower standard deviation and better correlation values).The linear fit between compaction obtained by SFR and nuclear

methods for the limestone and basalt HMA slabs are respectivelyCSFR;Limestone ¼ 1:18 Cnuclear�0.16 and CSFR;Basalt ¼ 0:84 Cnuclear+0.14with the mean differences between the average results of thesetwo methods being less than 2%. The correlation coefficients are0.888 and 0.863 respectively.

Fig. 9. Compaction maps of the basalt slabs obtained with the SFR system (left and middle columns), and standard nuclear test (right column) on one set of basaltHMA slabs: (a) SFR_Surface, (b) SFR_Thickness and (c) Gamma rays.

C. Fauchard et al. / NDT&E International 60 (2013) 40–5148

The rutting effects generated by the RC device locally depressedthe HMA slab surface (Fig. 11) are such that the calibrationprocedure with a metal plate is no longer valid for the estimationof the air/HMA interface reflection coefficient. Rutting effects

produce local antenna-to-HMA surface distance variations whichare not considered in the calibration procedure since it is generallyused for flat surfaces. As the metal plate is based on the surfacecrest, we obtain an over estimated reflected amplitude of the

0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.960.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

Gamma ray compaction (NF EN 12697−7)

Com

pact

ion

from

ε′ S

FR_S

urfa

cey = 1.33*x − 0.325; cor=0.828

0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.960.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

Gamma ray compaction (NF EN 12697−7)

Com

pact

ion

from

ε′ S

FR_S

urfa

ce

y = 3.15*x − 1.9; cor=0.8424

Fig. 10. Compaction obtained from the SFR permittivity measurements according to Eq. (5) with the reflected pulse on both the (a) limestone and (b) basalt HMA slabsurfaces. Results exhibit high standard deviations, and certain values lie outside the range of physical acceptability.

Fig. 11. Rutting effects due RC implementation on (a) limestone HMA slab surface and (b) basalt HMA slab surface.

C. Fauchard et al. / NDT&E International 60 (2013) 40–51 49

metal plate relatively to the reflected amplitude at the air/HMAslab surface (because of the antenna beam spreading loss effect).In this case, the reflection coefficient of the HMA surface (with anHMA surface locally lower than the metal sheet position) isunderestimated, as will be the estimated permittivity and com-paction values. Additionally, the derivation of the permittivity byconsidering a theoretical reflection coefficient for a smooth surfaceas performed here rather than a theoretical reflection coefficientfor a slightly rough surface [44] in the specular direction (whichwould be necessarily lower than for the smooth surface case)could lead to an overestimated permittivity. Even, if this latterconsideration is beyond the scope of this work, the abovequalitative arguments are not in favour of the compaction estima-tion from surface measurement in the presence of rutting effects.So, a procedure which is not directly dependent upon the waveamplitudes but which is based on the time-of-flight of wavepropagating into the medium is a good candidate for the estima-tion of the compaction from the estimated permittivity in the caseof rutting effect. An argument in this sense can be found in Pinelet al. [45] who showed that for a slightly rough pavement surface,the modulus of the air/ interface echo in the specular direction wasmuch more influenced than its phase, by the surface roughness.This is the first reason why the compactions derived from thethickness SFR measurement procedure better match the gammaray compactions than those derived from the surface SFR mea-surement. A second reason is that we considered the actual HMAthickness (measured every centimetre during the nuclear benchmeasurement test) instead of a global thickness to calculate thepermittivity (Eq. (7)) to take into account the slab thicknessvariations. A third reason is that, in the case of thickness

measurement, wave travels “deeper” into the material, and tendsto cancel a possible material spatial inhomogeneity to lower thevariability as observed in the lower y-error bar levels. This latterresult is very important since it suggests that the standard nucleartest in the laboratory could be replaced by an EM bench for thepurpose of slab compaction control. Major efforts must bedeployed in order to validate this approach for cylindrical cores,which are the main HMA specimens typically controlled in thelaboratory.

6. Conclusion and outlook

The objective of this work has been to study the capabilities ofEM methods based on wave propagation to assess the compactionof HMA specimens in the laboratory by measuring their permit-tivity. In considering a classical CRI model to describe the dielectricbehavior of HMA and in taking into account the classical definitionof compaction in road construction engineering, the compactionrate is expressed as a function of the measured HMA permittivity,as well as the mass fraction, permittivity and density of eachelement composing the HMA. The first stage has revealed theimportance of the dielectric characterization of rocks given thatthey represent the major component contribution to the HMA. Wehave also shown that the rocks can be assumed as low-lossmaterials within the frequency band used for GPR civil engineer-ing applications. The second key main result pertains to thepermittivity measurements performed using cylindrical cavitieson cylindrical HMA specimens and to the compaction ratededuced from the CRI model. The compaction obtained with the

0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.960.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

Gamma ray compaction (NF EN 12697−7)

Com

pact

ion

from

ε′ S

FR,T

hick

ness

0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.960.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

Gamma ray compaction (NF EN 12697−7)

Com

pact

ion

from

ε′ S

FR,T

hick

ness

Fig. 12. Compaction obtained from the SFR permittivity measurements according to Eq. (7) with the two-way travel time extracted from the B-scans of (a) limestone and(b) basalt HMA slabs. These results exhibit strong correlation with the standard nuclear test.

C. Fauchard et al. / NDT&E International 60 (2013) 40–5150

EM approach using a CRI model lies close, to within a few percent(less than 2% for the more highly compacted samples), to thatobtained with the standard PCS test. The third stage has presentedthe compaction results derived via permittivity measurementsperformed with an SFR system on HMA slab specimens. Ruttingeffects due to the size of HMA slabs and the implementationprocess with RC preclude the EM approach based on reflectedsignal measurements in the laboratory. Further studies mustnaturally need to be conducted in situ in order to validate themethod on newly-built roads with perfect surface conditions.Nevertheless, SFR measurements combined with accurate knowl-edge of the HMA slab thickness lead to an estimated compactionas accurate as that provided by the standard nuclear test. Thisstudy has therefore demonstrated that EM approaches based onwave propagation allow for an excellent estimation of HMAcompaction in the laboratory. Some complementary studiesshould nonetheless be performed on HMA with various types ofrocks, especially for those displaying higher dielectric losses. Thiswork has shown that the standard nuclear test used in thelaboratory for HMA slab compaction estimation could be replacedby a SFR system. A major outstanding issue is now to validate thisapproach for cylindrical cores typically extracted for HMA imple-mentation control.

Acknowledgment

This work was supported by IFSTTAR (French Institute ofScience and Technology for Transport, Development and Net-works) and USIRF (Union of French Road Industry Federations).We would also like to thank Eiffage Travaux Publics for providingthe HMA slabs and Cyril Ledun (with the Normandy-Center PublicWorks Research Center), who performed the sawing of cylindricalHMA and rock specimens for cavity measurements.

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