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Ž .Journal of Applied Geophysics 43 2000 119–138www.elsevier.nlrlocaterjappgeo

Road evaluation with ground penetrating radar

Timo Saarenketo a,), Tom Scullion b,1

a Roadscanners Oy, P.O.Box 2219, FIN-96201 RoÕaniemi, Finlandb Texas Transportation Institute, Texas A&M UniÕersity System, College Station, TX, 77843-3135, USA

Received 9 March 1999; received in revised form 8 June 1999; accepted 10 June 1999

Abstract

Ž .This paper provides a status report of the Ground Penetrating Radar GPR highway applications based on studiesconducted in both Scandinavia and the USA. After several years of research local transportation agencies are now beginningto implement GPR technology for both network and project level surveys. This paper summarizes the principles of operationof both ground-coupled and air-launched GPR systems together with a discussion of both signal processing and datainterpretation techniques. In the area of subgrade soil evaluation GPR techniques have been used to nondestructively identifysoil type, to estimate the thickness of overburden and to evaluate the compressibility and frost susceptibility of subgrade soil.In road structure surveys, GPR has been used to measure layer thickness, to detect subsurface defects and to evaluate basecourse quality. In quality control surveys, GPR techniques have been used for thickness measurements, to estimate air voidcontent of asphalt surfaces and to detect mix segregation. Future developments are described where the technique has greatpotential in assisting pavement engineers with their new pavement designs and in determining the optimal repair strategiesfor deteriorated roadways. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Ground penetrating radar; Road structure; Subgrade; Dielectric value

1. Introduction

In Scandinavia, the first Ground PenetratingŽ .Radar GPR tests with ground-coupled anten-

nae were performed in early 1980s in DenmarkŽ . Ž .Berg, 1984 and in Sweden Johansson, 1987 ,but the method did not gain general acceptanceat that time. In Finland the first tests were made

Ž .in 1986 Saarenketo, 1992 and after the RoadDistrict of Lapland of the Finnish National Road

Ž .Administration Finnra purchased its own unit

) Corresponding author. E-mail:[email protected]

1 E-mail: [email protected].

in 1988, the method has been used as routinesurvey tool in various road design and rehabili-

Žtation projects in Finland Saarenketo, 1992;Saarenketo and Maijala, 1994; Saarenketo and

.Scullion, 1994 . Most of the research and devel-opment works in highway applications in Fin-land has been performed with low frequencyŽ .100–500 MHz ground-coupled antenna in or-der to evaluate subgrade soils and their interlay-ers, probe the depth of overburden and surveyroad structural layers. GPR technique was also

Žapplied in aggregate prospecting Saarenketo.and Maijala, 1994 . In early- and mid-1990s

tests with high frequency 1.0–1.5 GHz air-cou-pled and ground-coupled antennae were started

0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 99 00052-X

( )T. Saarenketo, T. ScullionrJournal of Applied Geophysics 43 2000 119–138120

Žin bridge deck surveys Saarenketo and Sode-¨.rqvist, 1993; Maijala et al., 1994 , and in pave-Žment design and quality control Saarenketo and

Roimela, 1998; Scullion and Saarenketo, 1998;.Saarenketo, 1999 .

The history of GPR tests on road surveys inthe USA relates to the mid-1970s, when accord-

Ž .ing to Morey 1998 , Federal Highway Admin-istration tested the feasibility of radar in tunnelapplications and later on bridge decks. The firstvehicle mounted GPR system for highways wasdeveloped under a FHWA contract in 1985Ž .Morey, 1998 . Since then, most applicationshave been focused on pavement thickness mea-

Ž .surements Maser, 1994 , detecting voids underŽ .concrete slabs Scullion et al., 1994 and detect-

Žing deteriorated areas in bridge decks Alongi et.al., 1992 . These surveys have mainly been

Ž .performed with high frequency 1.0 GHz air-Ž .launched antennas see Scullion et al., 1992 .

A good description of the current practices ofGPR applications in highway agencies in North

Ž .America is given by Morey 1998 . The resultof the questionnaire sent to 50 states, PuertoRico, the District of Columbia and 11 Canadiantransportation agencies showed that 33 agenciesof the 51 responses had experience with GPR.The most common GPR applications reported

Ž .were pavement layer thickness 24 agencies ,Ž .void detection 22 agencies and bridge delami-

Ž .nation 16 agencies ; followed by delaminationŽ . Ždetection 11 agencies , depth to steel dowels 8

. Ž .agencies , buried objects 8 agencies , depth toŽ . Žbedrock 8 agencies , asphalt stripping 7 agen-

. Žcies , and scour around bridge support 6 agen-

.cies . Of the various applications GPR seemedto be the most successful for pavement layerthickness measurements, while agencies reportless satisfactory results with void detection andquestionable results locating areas of asphalt

Ž .stripping Morey, 1998 .This paper gives a state-of-the-art review of

GPR applications in road surveys in Scandi-navia and in the USA. The testing of bridgeswith GPR will not be addressed in this reviewbecause this is a large area which needs a

separate article. In other parts of the world GPRtechniques have been used for road monitoringin more than 20 countries and according to theknowledge of the authors, GPR surveys on roadsare widely used in Canada, France, Italy,Switzerland and the UK.

2. GPR hardware and software for roads

2.1. Hardware

Impulse Radar techniques are based on themeasurement of travel time and reflection am-plitude of a short electromagnetic pulse trans-mitted through a pavement and then partly re-flected from electrical interfaces within thestructure. Electrical interfaces occur at layerinterfaces when the GPR wave encounters dif-ferent materials or changes in either moisturecontent or density. GPR systems typically have

Ž .the following three components: 1 a pulsegenerator which generates a single pulse of a

Ž .given center frequency and power, 2 an an-tenna which transmits the pulses into themedium and captures the reflected signal, andŽ .3 a samplerrrecorder which samples the re-flected signals and converts them into a formfor computer storage. The radar antennae incommon use fall into two broad categories:air-launched horn antennae and ground-coupleddipole antennae. A ground-coupled 500 MHzantenna mounted in front of a van is seen inFig. 1.

Ž .Fig. 1. Finnish National Road Administration FinnraGPR survey van with GSSI 500 MHz ground-coupledantenna.

( )T. Saarenketo, T. ScullionrJournal of Applied Geophysics 43 2000 119–138 121

Ground-coupled antennae operate in a widerange of central frequencies from 80 to 1500MHz. The clear advantage of ground-coupledsystems is their depth of penetration comparedwith air-launched systems, while the surfacecoupling and antenna ringing present problems,which make it difficult to obtain any quantita-tive information from the near surface withoutsignal processing. Data collection speed withground-coupled systems is normally 5–15kmrh. The leading manufacturer of ground-coupled systems is GSSI of New Hampshire,USA, other manufacturers include Sensors and

˚Software from Canada and MALA from Swe-den.

Air-launched systems operate in the range of500 MHz–2.5 GHz, the most common centralfrequency being 1.0 GHz. Their depth penetra-tion is typically 0.5–0.9 m. During the dataacquisition these antennae are suspended 0.3–0.5 m above the pavement surface. The datacollection speed can be up to 100 scansrswhich allows GPR survey velocities up to 100kmrh. Currently three vendors from the USAmanufacture and sell air-launched systems: GSSIof New Hampshire, Penetradar of New York,and Pulse Radar of Texas. Fig. 2 presents TTIPenetradar air-launched horn antenna.

In order to promote efforts for better GPRŽ .hardware systems, Scullion et al. 1996 have

proposed test protocol and performance specifi-cations for the air-launched antennae and

Ž .Fig. 2. Texas Transportation Institute TTI GPR surveyvan with Pulse Radar 1.0 GHz horn antenna.

ground-coupled antennae with higher frequen-cies than 500 MHz.

In the future, the road GPR hardware devel-opment will go towards smaller non-contactantennae and multichannel data collection sys-tems, which allow tomographic images from theroad. One example of these systems is the 64-channel HERMES bridge inspector under devel-opment by the Lawrence Livermore National

Ž .Laboratory Davidson and Chase, 1998 .

2.2. Software

The GPR software available for road surveysŽ .can be classified into four groups: 1 GPR data

Ž .acquisition software, 2 GPR data processingŽ .software, 3 interpretation and visualization

Ž .software, and 4 software for integrated roadanalysis and design.

Most of the GPR data acquisition softwarehas been developed by the GPR systems manu-facturers. However, custom built data collectionand quality control software packages are nowbeing developed. Improved packages are neededwhen air-launched horn antenna systems areused for pavement quality control purposes,where measurements have to be repeatable andwhere the GPR results will be used determinebonuses and penalties on newly completed pro-jects.

A very important feature in data acquisitionsoftware is the linkage to the positioning sys-tems, such as GPS, because modern pavement

Ž .management systems PMS pavement designsoftware need information to be attached tox, y, z coordinates. Currently, most systems areusing a distance based data acquisition control.In the next few years exact positioning andlinking of GPR data to other pavement surveydata is one key research area.

GPR manufacturers also provide GPR dataprocessing systems, but most of the softwarehas been written in order to process ground-cou-pled data collected for geological surveys. Air-launched GPR systems produce relatively cleanand repeatable signals, therefore the processing


requires only basic signal filtering and back-ground removal algorithms. A challenge in thefuture in data processing software is to infersome quantitative information on electricalproperties of the pavement layers and subgrade.

ŽSpagnolini Spagnolini, 1996; Spagnolini, 1997;.Agosti et al., 1998 has approached this problem

by investigating promising techniques to resolvesome of the essential problems in road structureevaluation, i.e., inversion technique to gain in-formation about vertical distribution of dielec-tric value in road structures and layer strippingtechnique to define the exact location of thereflections when overlapping each other.

GPR data interpretation and visualizationsoftware for roads are used for detecting layerinterfaces and individual objects from the GPRdata and transforming the GPR data time scaleinto depth scale. Many efforts, including the useof neural networks, have been made in order todevelop automatic interpretation software forroads and bridges. However, the results of thesedevelopment projects have not been encourag-ing and have caused confusion among highwayengineers. The reason for automatic interpreta-tion software packages will most likely neverwork is that the roads are mostly historicalstructures with ‘‘accumulation and deterioratedstructures’’ and discontinuities in longitudinal,vertical, and transverse directions. That is whysemiautomatic interpretation software used bywell trained and experienced interpreting stafftogether with limited coring or other referencesurvey results has proved to be the only work-ing solution in road surveys. The user must alsoensure that the echoes are from real interfacesand not the result of multiple reflections be-tween strong reflectors by comparing the echoprofiles of the interface in question with thosenearby. The only cases where automatic inter-pretation seems to calculate right thickness anddielectric values are surveys on new and defectfree pavements. More discussion on this will begiven in Section 5.

A new generation among GPR software arethe integrated road analysis and design software

packages, which are designed for the combinedanalysis of GPR data with other road surveydata, with the ability to calculate parameterswhich describe the condition of the old road,and parameters needed for a new road structure

Ž .or rehabilitation design Saarenketo, 1999 . Inroad surveys GPR data output are presented innumeric format or they are visualized in longi-tudinal profiles or GIS maps. In many cases theactual GPR data is not presented, only the re-sults are visualized. When combined with otherdata types such as longitudinal profile, the GPRis used to identify the potential cause of surfacedefects.

3. Road subgrade surveys

3.1. General

Subgrade surveys and site investigations withGPR has been classified into the following three

Ž . Ž .categories Saarenketo and Scullion, 1994 : 1Ž .new road alignment and site investigations, 2

strengthening and widening of an existing road,Ž .and 3 using the existing road as an informa-

tion source for the design of a new roadwayalongside the existing road. In each case thebasic problem is similar, but the way the GPRtechniques are applied varies.

In subgrade surveys for a new road align-ment, GPR can be used to determine the soiltypes and their boundaries, estimating the depthof bedrock, and evaluating the ground water

Ž .level and frost susceptibility Saarenketo, 1992 .GPR data can be used also for guidance ofconventional site investigations such as drillingand, in places which cover the variety of geo-logical conditions, to optimize the use of thesetraditional methods. Low frequency ground-coupled systems are used for this application.

In highway bridge site investigations, GPRhas been used to monitor the bottom topographyof rivers and lakes to map the quality of under-water sediments. In the USA and Finland, GPR


techniques have been used also for detectingŽ .scours around bridge piers Haeni et al., 1992 .

In road rehabilitation projects and when de-signing a new road alignment alongside the oldroadway, most of the previously mentioned in-formation is collected from the subgrade underthe existing road. These subgrade data, com-bined with other GPR data collected from theexisting road layers, provide valuable informa-tion that can be used for predicting the perfor-mance of the new road.

3.2. Subgrade soil eÕaluation

It is often quite easy to identify coarse grainedgravel, sand and glacial till soils from GPR dataand GPR works well with organic peat soilsŽUlriksen, 1982; Doolittle and Rebertus, 1988;Ground Penetrating Radar, 1992; Saarenketo et

.al., 1992 . GPR signals have relatively goodsignal penetration in most silty soils, but prob-lems arise when surveys are carried out in claysoils. In Scandinavia, GPR signal penetration inclay soil areas is normally about 2 m, which isadequate for the cable and pipeline surveys butnot for the highway design purposes. In suchcases other geophysical methods, such as elec-trical resistivity profiling methods, have beenapplied.

In the USA, the penetration depth depends onthe mineralogy and clay content of clay soils. Apenetration depth of 5 m has been achieved inareas of Site Oxidic soils, while radar signalspenetrate only 0.15 m in Vaiden type Montmo-

Ž .rillonitic soils Doolittle and Rebertus, 1988 .Ž .According to Doolittle 1987 the best areas in

the USA for radar in soil surveys are easternand western states, while the greatest problemsoccur in the Mississippi–Missouri drainagebasin area in the central states.

In many cases the soil type can be deter-mined from the GPR data, because every soilhas its own geological structure, dielectric, and

Želectrical conductivity properties see Saaren-.keto, 1998a ; these properties cause a special

‘‘finger print texture’’ in a GPR profile. In

Finland, for instance, soils that have dielectricdispersion tend to have a special ‘‘ringing’’GPR reflection pattern whereas soils with nodispersive fine grained materials do not havethis phenomenon. This knowledge has been ap-plied especially when doing sand and gravel

Ž .aggregate prospecting Saarenketo, 1992 . Soiltype evaluations always need some ground truthdata to confirm the GPR interpretation. A veryhelpful technique for interpreters is to attachdrill core information to the GPR profile, whichhelps to select the right reflection interfaces forthe final interpretation, as shown in the casestudy conducted in TH 28 Burtrum in Min-

Ž . Ž .nesota Saarenketo, 1998b Fig. 3 . In thisfigure strong reflectors between 300–370 m and410–480 m present old road surface levels thathave settled due to highly compressive peatsubgrade. Excellent supporting information canalso be gained from the Falling Weight Deflec-

Ž .tometer FWD survey data.

3.3. Depth to bedrock and bedrock quality

In road surveys GPR information of the depthof overburden and location of the bedrock isused in the design of grade lines, or in designagainst uneven frost heave and other specifictechnical problems such as in backcalculation ofpavement layer moduli. GPR allows also obser-vation of bedrock stratification and major frac-ture zones when evaluating the stability of high-

Ž .way cutting walls Saarenketo, 1992 . Similarinformation can also be obtained in highwaytunnel surveys where both ground-coupled and

Ždrill-hole antennae have been used Westerdahl.et al., 1992 .

Identification of bedrock is often difficult,and it is impossible if the radar signal does notpenetrate the overburden soil. However, if evenweak reflections can be recorded, there are sev-eral interpretation techniques that can be used toconfirm the bedrock interface. If the dielectricproperties of bedrock and subgrade soil roadstructure are very close, the reflected signalamplitude from the interface will be weak. Also,


Fig. 3. 400 MHz ground-coupled antenna survey result from TH 28, Burtrum, Minnesota presented with interpretation andŽ .reference drill data Saarenketo, 1998b .

if the bedrock has been blasted under the roadto a certain level, the transition zone frombedrock to road structures may be wide and noclear discrete interface reflection can be ob-

Žserved in the radar trace Saarenketo and Scul-.lion, 1994 . That is also the case in areas of

weathering crust over bedrock. In all of thesecases, the estimation of bedrock interface has tobe done by identification of the zone in theradar profile where the bedrock structure starts.

In highway engineering the bedrock is de-fined as a stiff material that cannot be excavatedwithout blasting. Because GPR gives structuralinformation, the survey data always needs somesupporting information and reference data, suchas drilling, excavation pits or seismic surveydata. A very promising and economical tech-nique in road surveys has been the analysis ofthe GPR data together with the FWD dataŽSaarenketo, 1999; Scullion and Saarenketo,

.1999 . If the bedrock is close to the surface itcan be seen in the FWD deflection bowl data.The depth can be confirmed by using FWDbackcalculation algorithms. Once the depth to

the stiff layer has been calculated from theFWD measurement point, the surface of thebedrock between the FWD measurement pointscan be defined from the GPR data as done in Rd

Ž .17469 in Kuortane, Western Finland Fig. 4 . IfGPR surveys are performed in the wintertime,the areas of the bedrock closer to the surfacethan the frost level are easy to identify becausethe frost line reflection in the bedrock cannot be

Ž .seen from the radar profile Fig. 4 .

3.4. Soil moisture and frost susceptibility

Moisture content has a great effect on thestrength and deformation properties of the roadstructure and subgrade soils. Information aboutthe subgrade soil moisture content is needed inestimating stability and compressibility of thesubgrade soils, when designing highways onareas with expansive clays and when evaluatingfrost susceptibility of soils.

Both positive and negative pore water pres-sure in soil have major effect on shear strengthand volume change of the soil. In unsaturated


Fig. 4. 500 MHz ground-coupled data collected from Rd 17469, Kuortane, Western Finland. Measurements were performedin May 1998 when the subgrade was still partly frozen. GPR data is presented together with FWD data. Clear reflections insection 2140–2240 at depth of 1.4 and 2.0 m present upper and lower interface of the frost table.

soils and unbound road structures the term‘‘suction’’ is used for the thermodynamic quan-tity, Gibbs free energy, which generates tensionin the pore water between soil particles. Thetotal suction is composed of two components:Ž . Ž .1 matric suction and 2 osmotic suctionŽ .Fredlund and Rahardjo, 1993 . The magnitudeof the matric suction is mostly affected by theamount of fines in soils, while cation exchange

Ž .capacity CEC and salt content affect the os-motic suction.

In low moisture contents, suction can in-crease the stiffness of soils and unbound aggre-gates and they have high modulus values, butwhen moisture content increases, suction de-

Ž .creases Fredlund and Rahardjo, 1993 . At highmoisture content, positive pore water pressurecan decrease the material resistance to perma-nent deformation.

In the case of widening andror strengtheningan existing highway, or when constructing newlanes, the best information about the changes incompressibility of the subgrade soil can be ob-tained directly by surveying the existing roadwith GPR. This technique is especially usefulwhen estimating the amount of settlements innew road and when designing preloading em-bankments over clay, silt or peat subgradeŽ .Saarenketo et al., 1992 . A good example illus-trating the information obtained from an oldroad is the case study conducted on HW 170

Ž .near Helsinki, Finland Fig. 5 .Frost action, which appears in areas where

frost susceptible soils have a great effect uponthe fatigue of the road, can be seen as perma-nent deformation of road structures taking placeduring the spring frost thawing period. Anothertypical frost damage is uneven frost heave re-


¨Fig. 5. GPR data from HW 170 Ostersundom near Helsinki, Finland. The section of settled road can be seen between 4440and 4620 m, where the old concrete road constructed in 1930s lies at the depth of 1.5 m. The bridge presents the level wherethe road was originally constructed.

lated to subgrade soil and moisture transitionareas, or to the presence of bedrock or boulders.Structural elements in the road body, such asculverts, have also caused surface damage if thetransition wedges are not properly constructed.

Frost susceptibility is also closely related tothe moisture content and drainage character-istics of the subgrade, which can be estimatedwith GPR and other dielectric measurement de-vices, such as the Percometer technique. Saaren-

Ž .keto 1995 has proposed a frost susceptibility

and compressibility classification for the sub-grade soils in Finland, which is based on in situmeasurements of dielectric value and electricalconductivity.

A tentative quality assessment of the strengthand deformation properties of soils and unboundroad materials based on the dielectric properties

Žis presented in Table 1 see also Hanninen and¨.Sutinen, 1994; Hanninen, 1997 .¨

The evaluation of potential frost action areasand presence of segregation ice using GPR can

Table 1Quality assessment of soils and unbound road materials according to their dielectric properties

Dielectric value Interpretation

Ž .4–9 Dry and non-frost susceptible soil, in most cases good bearing capacity excluding some sands9–16 Moist and slightly frost susceptible soil, reduced but in most cases adequate bearing capacity16–28 Highly frost susceptible and water susceptible soil, low bearing capacity, under repeated dynamic

load positive pore water pressure causes permanent deformation28– Plastic and unstable soil


also be performed in winter time when frost haspenetrated into the subgrade soil and when thedielectric value of frozen soils is closer to the

Žrelative dielectric value of frozen water 3.6–.4.0 . When analyzing the GPR data the inter-

preter has to pay attention to the followingŽ .phenomena: 1 the appearance of the

Ž .frozenrnon-frozen soil interface reflection, 2the depth of frost table and clarity of the reflec-

Ž .tion, and 3 the effect of the frost action uponŽthe road structures Saarenketo, 1995, 1999;

.Saarenketo and Scullion, 1994 .If the frost level cannot be identified from the

radar data, then the subgrade soils have a lowdielectric value and thus are non-frost suscepti-ble. If the frost level reflection is very clear inthe GPR data, then the frost has penetrated thesubgrade without forming segregation ice lensesthat cause frost heave. High and uneven frostheave can be found in areas where the frostlevel comes close to the surface. Here the segre-

gation ice containing unfrozen water can beseen in the GPR data above the frost level as inthe case from HW 322 Skalstugavagen in West-¨

Ž .ern Central Sweden Fig. 6 .

3.5. Other subgrade related surÕeys

GPR techniques have also been used whenŽlocating sinkholes under the road Saarenketo

.and Scullion, 1994; Casas et al., 1996 . AdamsŽ .et al. 1998 have reported surveys results where

GPR has been used to detect the washout ofsandy material beneath the pavement in IS 44,Springfield, MO. After locating the area ofvoids, GPR has also been used to monitor the

Žgrout injection into the voids see also Ballard,.1992 . Another GPR application related to the

subgrade surveys is the location of undergroundŽutilities under the existing road Bae et al.,

.1996 . Numerous authors have also had successwith detecting buried tanks close to the edge of

Fig. 6. 500 MHz ground-coupled data collected in March 1998 from HW322 Skalstugavagen, Sweden presented together¨Ž .with IRI International Roughness Index data. Clear reflection at the level of 1.4–2.0 m present frost line. The GPR data at

the road section with severe damages between 5720 and 5820 m, shows strong reflection above frost line indicating thepresence of segregation ice. Uneven section at 5875 m is caused by settlement in transition zone from till subgrade to peatsubgrade.


the existing pavement structure prior to widen-ing of the highway.

4. Road structure evaluation

4.1. General

In highway engineering a potential majorgrowth area for GPR applications will be in theevaluation of existing road structures. This isbecause the basic road network in the industrial-ized world is now almost complete and theactivities of national road authorities will beincreasingly focused on maintaining and im-proving the service standards of their currentroad network. At the same time national roadadministrations have faced the fact that publicfunding has decreased markedly. That is whynew maintenance strategies and rehabilitationtechniques are needed to optimally allocate theshrinking available funds. GPR can certainlyplay an important role in this area, it is the onlytechnology that can be used at highway speed toprovide subsurface information about the pave-ment structure. Combining GPR with othernon-destructive road survey techniques providesa powerful tool for diagnosing current pavementproblems and selecting the optimum repair tech-

Ž .nique Saarenketo, 1999 .In pavement structure evaluation, both

ground-coupled and air-coupled antennae sys-tems are used. Most of the ground-coupled an-tennae surveys are conducted using 400–500MHz antennae in order to get information fromthe whole pavement structure up to 3 m, buthigh frequency 1.5 GHz ground-coupled anten-nae have been used to locate stripping and other

Ž .defects in pavement Saarenketo, 1998c .The air-launched GPR systems are increas-

ingly being used for evaluation of the upper partof the pavement structure. They produce rela-tively clean signals and can operate at close tohighway speed. Furthermore, on defect freepavements the signals can be processed to com-pute both layer thicknesses and layer dielectrics.

This is demonstrated in the following, withreference to Fig. 7 which shows a GPR reflec-tion from a homogeneous newly constructedflexible pavement. The reflections A , A and1 2

A are reflections from the surface, the top of3

granular base and the top of subgrade, respec-tively. The principles of using GPR reflectionsto compute layer properties has been given by

Ž .Maser and Scullion 1991 . By automaticallymonitoring the amplitudes and time delays be-tween peaks, it is possible to calculate bothlayer dielectrics and thicknesses. These equa-tions are summarized below:

21qA rA1 m´ s , 1Ž .a 1yA rA1 m

where, ´ s the dielectric value of the asphalta

surfacing layer; A s the amplitude of the re-1Žflection from the surface in volts peak A in1

.Fig. 7 ; A s the amplitude of the reflectionmŽfrom a large metal plate in volts this represents

.the 100% reflection case .

c=D t1h s , 2Ž .1

´( a

where, h s the thickness of top layer; cs the1

speed of the GPR wave in air as measured by

Fig. 7. GPR trace from a new pavement measured with a1.0 GHz horn antenna and after background removal.Peaks A , A , and A are reflections from road surface,1 2 3

base and subgrade.


the system; D t s the time delay between peaks1

A and A of Fig. 71 2

2A A1 21y q

A Am m´ s ´ , 3( ( Ž .b a A A1 2

1y qA Am m

where, ´ s the dielectric of the base layer;b

A s the amplitude of reflection from the top of2

the base layer in volts.These simple equations have been proven to

work well for flexible pavements over granularbases. They assume that no attenuation of theGPR signal occurs in the surface layer. Thisassumption appears to be reasonable for asphaltpavements and also gives reasonable dielectricvalues for base layer, if the asphalt is thickerthan 60 mm and there are no also thin layers onthe top of the base. However, the computationsbecome less reliable for concrete pavements andfor computing subgrade dielectrics beneath

granular base layers. Current research activitiesare aimed at incorporating signal attenuationinto the calculation process.

For new or defect free pavements, the surfaceand base dielectrics together with the surfacethickness can be easily calculated. Current workin Texas and Finland is aimed at performingthese calculations in real-time primarily forquality control purposes. The results for a sec-tion of highway are often displayed as shown inFig. 8. It is the magnitude and variation of boththe surface and base dielectric which yieldssubstantial information about subsurface condi-tions. The factors which primarily impacts thesurface dielectric is the density of the asphaltlayer, the factor impacting the base dielectric isthe volumetric moisture content of the base. TheGPR reflections can also be used to judge thehomogeneity of the pavement layers. The reflec-tions shown in Fig. 7 are from a defect freehomogeneous pavement structure. As can beseen from this figure there are no reflections

Fig. 8. Basic processing of GPR data. The actual traces are shown in upper left box. The computed layer thickness anddielectrics are shown in other boxes.


between the reflections from the top of thesurface and base. If the layer contained a defectat middepth such as stripping or trapped mois-ture then additional reflections would occur be-tween peaks A and A .1 2

4.2. PaÕement thickness

The GPR pavement thickness data has beenŽ .collected for: a network level surveys where

thickness is required for PMS data basesŽ . Ž .Fernando et al., 1994 , b to supplement FWD

Ždata in calculation of layer moduli Briggs et. Ž .al., 1991 , c pavement design purposes, e.g.,

to check if the pavement is thick enough forŽ .recycling milling, and d for quality control

Žpurposes Saarenketo and Roimela, 1998; Scul-.lion and Saarenketo, 1998 .

Using the techniques described above for newpavements the accuracy of GPR thickness pre-dictions has been around 3–5%, without takinga validation core. For the base layer, the accura-cies have been reported to be close to 8–10%.For older pavement, validation cores are recom-

Žmended Maser and Scullion, 1991; see also.Morey, 1998 .

4.3. PaÕement defects

Even though a greatest part of the defectsobserved on asphalt pavements are related tomaterial problems of the layers beneath thepavement, there are also some pavement dam-ages caused by deterioration problems inside thepavement. The most common asphalt pavementdamage is stripping, which is a moisture relatedmechanism where the bond between asphalt andaggregate is broken leaving an unstable lowdensity layer in the asphalt. Stripped layersshould always be detected and removed beforeplacing a new overlay. That is why engineersresponsible for in-pavement design should knowif stripping is present or not, how deep and howwidespread it is.

GPR technique has been widely used to de-Žtect stripping with varying success see Morey,

.1998 . Stripping can be bee seen in most casesŽas an additional positive or negative reversed

.reflection polarity reflection in the pavement.However, similar reflection can be received froman internal asphalt layer with different electricalproperties, which is why reference data, such asdrill cores and FWD data should always be usedto confirm the interpretation. Fig. 9 presents acase from Willmar, Minnesota, where 1.5 GHzground-coupled and FWD data were used to

Ž .evaluate stripping Saarenketo, 1998c .Another major cause of surface distress is

moisture becoming trapped within the surfacelayer. This happens when impermeable fabricsor chip seals are placed between asphalt layersor when the existing surface is milled and re-placed with a less dense layer. GPR signals arehighly sensitive to variations in both moistureand density. In a recent study in Texas, fivetypes of reflections were identified from withinasphalt surfacing. These are shown schemati-cally in Fig. 10. The ‘‘ideal’’ trace with littlecontrast between the new and old Hot Mix

Ž .Asphalt HMA is defined as a Case 1 reflectionin Fig. 10, a small reflection B is proposed1

from the interface between the new and oldHMA layers.

GPR reflections from existing highways canbe complex particularly if the old HMA layercontains numerous thin layers constructed withdifferent aggregates compacted to different den-sities. Moisture barriers within the layer andcollecting data shortly after significant rain canalso complicate the analysis. Fig. 10 was devel-oped to aid in signal interpretation.

Other asphalt defects include crackingŽ .Saarenketo and Scullion, 1994 , thermal crack-ing and debonding, which takes place when thebonding between separate asphalt layers comesloose.

The major defects in concrete pavement havebeen reported as voids beneath the joints, crack-ing and delamination of the concrete pavement.The formation of voids beneath concrete pave-ments is a serious problem, which is particularlynoted on jointed concrete pavements built with


Fig. 9. 1.5 GHz ground-coupled antenna data collected from TH71 Willmar, Minnesota presented with FWD data. The roadsection with severe stripping on the bottom of the asphalt between 900 and 990 m can be seen both with GPR and FWDdata.

Ž .stabilized bases Saarenketo and Scullion, 1994 .Over time, bases erode and supporting materialis frequently pumped out under the action oftruck loads.

The problem when using GPR to detect thevoids beneath concrete slabs is the moisturecontent of the voids, i.e., depending on whetherthe voids are dry, semidry or saturated, in eachcase the GPR reflection pattern looks different.The size of the voids has to be big enough; GPRcan detect only voids bigger than 15 mmŽ .Saarenketo and Scullion, 1994 .

4.4. Base course and unbound road structures

Base course layer made of unbound aggre-gates which supports asphalt or concrete pave-ment is one of the most critical layer in roadstructure. A series of research projects have

been carried out in Finland and Texas where therelationship of dielectric properties of Finnishand Texas base materials to their water contentand to their strength and deformation properties

Žwere studied with a DCP Dynamic Cone Pene-.tration test and dynamic triaxial tests

ŽSaarenketo and Scullion, 1995, 1996; Saaren-.keto et al., 1998 . A special Tube Suction labo-

ratory test based on the dielectric measurementsof the material at its moisture equilibrium levelhas also been developed in order to test themoisture susceptibility of road materialsŽSaarenketo and Scullion, 1995; Scullion and

.Saarenketo, 1997 .Results of the surveys conducted have shown

that the dielectric value, which is a measure ofhow well the water molecules are arrangedaround and between the aggregate mineral sur-faces and how much free water or ‘‘loosely


Fig. 10. Classification and interpretation of different sub-surface reflections from asphalt layers containing a buried

Ž .moisture barrier chip seal or fabric .

bound water’’ exists in materials, is a muchbetter indicator of the strength and deformationproperties of road aggregates than the moisturecontent. Each aggregate type has a unique rela-tionship between material dielectric and mois-

Ž .ture content. As shown in Eq. 3 , the amplitudeof the GPR reflection from the top of the granu-lar layer is used to calculate the dielectric ofthat layer. Furthermore, high dielectric valuesŽ .)16 , calculated from the GPR data, for ex-ample, are good indicators for potential prob-

Ž .lems in the layer Saarenketo et al., 1998 .A tentative quality assessment scale for un-

stabilized flexible base is proposed in Table 2.

4.5. Project leÕel surÕeys and road analysis

One of the best fields for the applications ofGPR technology is in the project level testing

for pavement management purposes. Identifyingthe cause of existing problems and defining theoptimal rehabilitation strategy is an area of criti-cal importance. GPR alone can provide only apart of the answer, but it becomes more power-ful when combined with other geophysical andnon-destructive pavement testing methods. InTexas, GPR data has been analyzed togetherwith FWD structural data, pavement distressdata from the video image and drill core dataŽ .Scullion and Saarenketo, 1999 . Integratedanalysis techniques has also been applied inairports where GPR data has been analyzedtogether with infrared thermography, magne-tometer and pachometer measurement dataŽ .Weil, 1998 . Case studies of GPR in projectlevel surveys has been published by Hugen-

Ž . Ž .schmidt et al. 1996 and Wimsatt et al. 1998Ž .see also Morey, 1998 .

In Finland, a comprehensive Road Analysispackage has been developed by RoadscannersOy in cooperation with Finnra. It includes an

Ž .evaluation of: 1 overall pavement condition,Ž .2 condition assessment of the unbound pave-

Ž .ment structure, 3 subgrade damage related toŽ . Ž .frost fatigue, 4 drainage condition, and 5

local damage of the surveyed road. The analysisis based on measurements conducted with GPR,with the support of drill core samples, rough-ness and rutting measurements with 5 or 10 m

Ž .mean value 5 m-IRI , FWD measurements, andvisual observations of pavement surface condi-tion.

A key tool in Road Analysis is the RoadDoctor software developed by Roadscanners Oy,which is used for the integrated analysis of the

Table 2Quality criteria for unstabilized base material

Dielectric Interpretation

5–10 Good aggregate base at or belowoptimum moisture content

10–16 Warning signal, wet base)16 Plastic base susceptible to load

associated and environmental damage


pavement structure. In the first phase, RoadDoctor processes the ground-coupled and air-coupled GPR data and the thickness of theasphalt and other structural layers are calcu-lated. In the next phase, FWD rutting androughness survey data are imported to the RoadDoctor database together with other reference

Ž .data such as drill core information Fig. 11 . Ifx, y, z information of the road under survey isavailable, the GPR data can be viewed withtopography information and all the layer inter-face information can be transferred to a roadCAD systems together with the coordinates. Inthe Road Analysis, the road is divided intohomogenous sections and classified accordingto the subgrade soil quality and effect of thefreeze–thaw process to the road structures. Also

structural course thickness and material proper-ties are evaluated. As a result of a detailed RoadAnalysis of the road structure, the quality andextent of the damages in road structures arereported to the pavement engineer in order toprovide reliable information to support rehabili-tation design decisions.

5. Quality control of a new structure

5.1. General

Nondestructive pavement testing methods,such as GPR, have gained increasing popularityin quality control surveys of new road struc-tures. The greatest advantages of GPR methods

Fig. 11. Road Analysis profile over section 2200–2600 m of HW930 in Ylitornio, Finnish Lapland.


are that they are not destructive when comparedto the traditional drill core methods, they arelow cost methods and GPR surveys can also beperformed from a moving vehicle reducingsafety hazards to highway personnel. GPR al-lows for the possibility for continuous data col-lection and thus a 100% coverage can be ac-quired from the new road structure under in-spection. Drill core methods provide point spe-cific information and they cannot be reliablyused to find defect areas in new pavements.

The GPR applications in quality control sur-veys include thickness verification of the roadstructures. This technique has been discussedearlier in Section 4.2. GPR has also been usedto check if the soil replacement and transitionwedges are constructed according to instructionsand if dowel bars between concrete slabs havebeen installed properly. New GPR quality con-trol applications include measuring the air voidscontent of asphalt and detecting segregation inasphalt.

5.2. Asphalt air Õoid content

Asphalt air void content, i.e., the amount ofair incorporated into the material or its functionasphalt density, is one of the most importantfactors affecting the life span and deformationproperties of pavements. This parameter hasearlier been estimated by means of drilled sam-ple analysis or radioactive methods, but after 3years of testing the GPR surface reflection tech-nique, it has also been accepted as official

Žquality control method in Finland Saarenketo,1997; Saarenketo and Roimela, 1998; Roimela,

.1998, 1999 .Measuring voids content using its dielectric

value relies on the fact that the dielectric valueof the asphalt pavement is a function of volu-metric proportions of the dielectric values of itscomponents. Compaction of the asphalt reducesthe proportion of low-dielectric value air in theasphalt mixture and increases the volumetricproportions of bitumen and rock and thus resultsin higher dielectric values of asphalt. The rela-

tionship between dielectric value of asphalt andair voids content measured in the laboratory ispresented in Fig. 12.

The GPR measurements in the field are per-formed using a 1.0 GHz horn antenna. Dielec-tric values of HMA surfacing are calculated byusing the surface reflection techniques describedearlier. Following the GPR field evaluation oneor two calibration cores are taken. These coresare returned to the laboratory for traditionalvoid content determination. Long term studiesin Finland have concluded that an exponentialrelationship exists between surface dielectric andair void content. For each type of aggregate andmix design, similarly shaped relationships havebeen developed. The calibration cores are usedto establish the link for each specific project.The correlation between air void content mea-sured with GPR technique and air void contentusing laboratory results is presented in Fig. 13.These results are obtained from a comprehen-sive series of field surveys conducted in Finland

Ž .in 1997 Saarenketo and Roimela, 1998 . TheseŽresults and additional survey results Roimela,

.1998, 1999 have convinced Finnra that GPR isa viable tool for monitoring the quality of newHMA surfacing. In 1999, GPR can be used as aquality control tool among other pavement den-sity measurement techniques on all new surfac-ing projects in Finland.

Fig. 12. Results of the Finnra laboratory tests 1996, dielec-tric values versus void content.


Fig. 13. Asphalt void content measured with GPR in thefield versus void content measured in the laboratory.

5.3. Segregation

One of the major functions of Hot Mix Sur-facing on highways is to provide waterproofingfor the underlying structural base layers. If ex-cessive moisture can enter the base layer, thenrapid load and environmental damage will oc-cur. The surfacing layer will only function asintended if it is compacted to optimum density,it does not contain any defects such as areas ofsegregation, and if the longitudinal constructionjoints are waterproof. Segregation manifests it-self as localized periodic small areas of lowdensity material in the compacted surfacinglayer. Upon close inspection in these small lo-calized areas, an excess of coarse aggregates isfound. The causes are often traced to improperhandling or construction techniques. Duringconstruction, the segregated areas are often im-possible to detect visually; they only becomeapparent after 1 or 2 months in service. How-ever, these areas often have a major impact onlong term pavement performance. The segre-gated areas are often sources for moisture toenter the lower pavement layers. The asphaltindustry is well aware of the segregation prob-lem and in the past decade, several new con-struction techniques have been implemented in-cluding the use of Materials Transfer Vehiclesas part of the paving train. Despite progress inthis area, segregation is still a major concern

and at the present time, there are no ways ofdetecting these problems during construction. Asecond problem area is that of the permeabilityof longitudinal joints.

GPR offers tremendous potential to assist inmonitoring the quality of the completed surfac-ing layer. The GPR parameter which can detectthese problem areas is the surface dielectric.GPR traces can be collected at closely spacedintervals, and with minimal data processing, aplot of surface dielectric versus distance can beproduced. The dielectric of the surface layer is afunction of the dielectric of the component ma-

Ž .terials aggregate, air and asphalt and theirvolumetric ratios. If an asphalt surface is uni-formly compacted, the surface dielectric shouldbe constant; however, if a low permeability areahas excessive air voids, this will be observed inthe surface dielectric plot as a decrease in mea-sured dielectric.

The first use of GPR technology to monitorthe quality control of pavements was reported

Ž .by Saarenketo 1997 . Large changes in thesurface dielectric were attributed to the use ofan experimental paver. Fig. 14 shows surfacedielectric plots taken over longitudinal construc-tion joints in Texas. The vehicle was travellingalong the highway and made three transitionsover the longitudinal joint between the lanes. Ascan be seen further away from the joint, thesurface dielectric is relatively constant at around

Fig. 14. Surface dielectrics from a longitudinal construc-tion joint on IH20 near Pecos, Texas. The drop in dielec-tric value indicates density problems over the joint.


5.5, whereas over the joint, the value is closer to4.5. Research studies are underway in bothTexas and Finland to relate the change in sur-face dielectric to changes in both air voids andmix permeability.

6. Future developments

Even though the history of the GPR applica-tions in roads and highways is relatively short,the method has proven to be a very useful toolto solve various kinds of highway engineeringproblems. The life cycle of the GPR applica-tions on roads are still on the introduction phasein most of the highway agencies. Awarenessand education are still needed to get diffusionand adoption for the method.

A very important factor in the future of GPRin road surveys is to establish the technique inroutine road analysis and pavement rehabilita-tion and design procedures. The status of radaras an interesting but obscure method for specificpavement survey projects should be changed toa routine road survey tool that is used amongother survey techniques. For that to occur, cer-tain standards and specifications for GPR equip-ment, data acquisition and interpretation tech-niques will be needed.

Even though this paper has described theGPR technique on roads and presents somesuccessful cases, there are unfortunately severalfailures that have been reported around theworld. These are usually attributed to over-sell-ing the technology by vendors who understandGPR but do not appreciate the complexity of thepavement systems. The best hope for the future

Ž .involves three critical steps: 1 developinguser-friendly software packages in order to con-vert the GPR data with other road survey datainto information which is meaningful to pave-

Ž .ment engineers, 2 gaining an understanding ofthe electrical properties of road materials andsubgrade soils and their relation to the moisture

Ž .and strength and deformation properties, and 3

provide training for highway agencies and othercustomers who are using the GPR data and forthe staff who are responsible for GPR surveys.When these steps have been accomplished, GPRtechniques and applications will have huge mar-kets in road design, construction and mainte-nance industry.

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