Gssi Highway Geo Fwd

8/20/2019 Gssi Highway Geo Fwd

1/9

INTEGRATION OF FWD AND GPR

Robert Parrillo and Roger Roberts Ph D,Geophysical Survey Systems Inc., Salem, NH

[email protected], [email protected]

Gary SanatiFoundation Mechanics, Inc., El Segundo, [email protected]

ABSTRACT

Falling weight deflectometers (FWD) are familiar and essential tools for road evaluation. FWD data isrequired by design engineers to establish proper rehabilitation strategies and achieve desired loadcapacities. Knowledge of existing asphalt thickness, commonly obtained using intermittent coring, isnecessary for subsequent data processing. Limitations of coring include data density (often 7 times lessfrequent than the FWD data), accuracy (the actual asphalt thickness under the FWD load plate isunknown), expense (coring crew costs approximately $2,000/day), safety (crews must operate in traffic),and data integration. FWD manufacturer Foundation Mechanics has integrated Geophysical SurveySystems RoadScan ground penetrating radar (GPR) system with their JILS truck-mounted FWD vehicle.GSSI’s 2 GHz air-launched horn antenna is used to determine asphalt thickness. Typical GPR asphaltthickness accuracy is within 5% and the system is FCC certified for use in the United States. Advantagesof the integrated FWD/GPR system include cost savings (coring costs are practically eliminated),accurate thickness information at each FWD location, dual-purpose vehicle (can perform high speed GPRsurveys independent of FWD), and data integration (FWD and asphalt thickness are stored in a singledatabase).

INTRODUCTION There are over 3.96 million public centerline road miles in the U.S. and of this, 2.50 million miles (or

about 63 percent) are paved (FHWA, 2002). Pavements are normally classified into two categories;flexible pavements and rigid pavements. Flexible pavements are asphalt pavements which may

incorporate underlying layers of stabilized or un-stabilized granular materials on a prepared sub-grade.These types of pavements are called "flexible" since the total pavement structure bends (or flexes) toaccommodate traffic loads. Rigid pavements are portland cement concrete (PCC) pavements, which mayincorporate underlying layers of stabilized or un-stabilized granular materials. Rigid pavements do notflex appreciably to accommodate traffic loads. Flexible or asphaltic pavements comprise about 82.2percent of U.S. paved roads while rigid pavements comprise about 6.5 percent [1].

Composite pavements are a combination of hot mix asphalt (HMA) and PCC pavements. Frequentlythey are the result of pavement rehabilitation (e.g., HMA overlay of PCC pavement) [1].

Pavement surface deflection measurements are the primary means of evaluating a flexible pavementstructure and rigid pavement load transfer. Deflection measurements can be used in back-calculationmethods to determine pavement structural layer stiffness and the sub-grade resilient modulus. Thegeneral principal is to apply a load of known magnitude to the pavement surface and analyze the shapeand magnitude of the deflection basin to assess the strength of the pavement structure [1].

The most common type of equipment for measuring pavement deflection is the falling weightdeflectometer (FWD). This device applies dynamic loads to a pavement surface, similar in magnitudeand duration to that of a single heavy moving wheel load. The FWD can either be mounted in a vehicle oron a trailer and is equipped with a weight and several velocity transducer sensors (see Figure 1). Toperform a test, the vehicle is stopped and the loading plate (weight) is positioned over the desiredlocation. The sensors are then lowered to the pavement surface and the weight is dropped. Multipletests can be performed on the same location using different weight drop heights [1].


2/9

Truck-mounted FWD Trailer-mounted FWD Figure 1 – Falling Weight Deflectometers

Deflection variation between test points within a section may be quite large; ranging from 15 percentto more than 60 percent. This variation reflects changes in layer thickness, material properties, moistureand temperature conditions, sub-grade support, and contact pressure under the load plate [2].

FWD data, combined with layer thickness information, may be used to obtain the "in-situ" resilient

elastic module of a pavement structure. Selecting the type of rehabilitation to be implemented on a givenpavement is of considerable economic significance and to reach that decision without an adequateknowledge of the structural condition of the pavement may have very costly consequences [1].

State Departments of Transportation estimate the cost to apply 1 inch of asphalt per lane mile at over$50,000; making an over-design rehabilitation strategy requiring excess asphalt clearly undesirable.Conversely, an under-design strategy using insufficient asphalt can result in premature pavement failureand the need to rehabilitate sooner than expected.One of the most critical elements in the accurate interpretation of deflection testing results is the use ofaccurate pavement layer thickness data in the process. The elastic layer model is extremely sensitive tosuch layer thickness. Moduli can vary by several hundred percent, based on assumed thickness ofpavement layers [4]. This is illustrated in Figure 2 by using a 4 inch asphalt thickness, varied by ± 50%,plotted against both the corresponding calculated composite stiffness, and the % error in compositestiffness.

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

-50 -40 -30 -20 -10 0 10 20 30 40 50

% error in a sphalt thickness

c o m p o s i t e s t i f f n e s s ( p s i )

0

25

50

75

100

125

150

175

200

-50 -40 -30 -20 -10 0 10 20 30 40 50

%error in asphalt thickness

% e

r r o r i n

c o m

p o s i t e

s t i f f n e s s

Figure 2 – Asphalt thickness error vs. composite stiffness

Traditionally, determination of layer thickness has relied on either construction records or ondestructive (coring) calibration. Either method has now been shown to be capable of significant error.Construction records have been shown to vary from actual conditions by an order of magnitude or more,and actual pavement thicknesses have been shown to vary by an order of magnitude within only 100meters (328 ft) ; a rate of change not detectable by coring procedures [4].


3/9

Other limitations of coring include:• Data density (cores are taken about 7 times less frequent then the FWD data);• Accuracy (the actual asphalt thickness under the FWD load plate is unknown and is assumed to be

constant between cores);• Expense (coring crews cost approximately $2,000/day);• Safety (coring crews must operate in traffic);•

Data integration (the pavement thickness information is obtained by performing a manualmeasurement of the core then manually recording this data for subsequent data entry); An automated method of measuring and storing pavement thickness at each FWD test location would

offer financial advantages as well as much improved accuracy and safety.

GROUND PENETRATING RADAR FOR PAVEMENT THICKNESSGround Penetrating Radar (GPR) is an accepted electromagnetic evaluation technique used for the

transportation infrastructure and a variety of other applications including concrete inspection, utilitydetection, geology and archeology. The use of GPR for measuring pavement thickness is a wellestablished and verified technique.

GPR systems yield accurate data in a form ready for management consideration. They surveypavements quickly, cost-effectively, and with minimal traffic disruption and safety risks. The StrategicHighway Research Program (SHRP), the Federal Highway Administration (FHWA), and several statesand other agencies have carried out studies of GPR that demonstrate the advantages of this automatedsurveying system. Denmark, Finland, and the United Kingdom are already using GPR in their pavementevaluation programs, as are several states [3].

GPR systems used for pavement thickness measurement utilize either ground-coupled or air-launched horn antennas (see Figure 3). As the name suggests, a ground-coupled antenna needs toremain in contact with ground (or suspended very slightly above the ground) to properly couple theelectromagnetic energy to and from the antenna. This presents some obvious limitations in using theground-coupled antenna for high speed pavement surveys on roads in less than perfect condition. Moreimportantly, to calibrate the ground-coupled system it is necessary to obtain cores from the pavement andphysically measure the actual pavement thickness. The measured thickness value must be entered intothe GPR analysis program so that the appropriate velocity of the radar signal through the pavementlavers may be derived to determine the pavement thickness. Since the composition of the pavementchanges, it is necessary to obtain cores at regular intervals (1 core per km is one GPR manufacturer’srecommendation) to derive accurate pavement thicknesses. Even when the cores are acquired at regular

intervals, the composition of the pavement is assumed to be constant between cores. Therefore, anychange in pavement composition affecting radar signal velocity between cores is a source of error whenusing ground-coupled antennas.

Ground-coupled Antenna Air-launched Horn AntennaFigure 3 – Ground-coupled and air-launched antennas


4/9

The air-launched horn antenna is attached to the front or rear of a vehicle and suspended with thebottom of the antenna approximately 18 inches (45.7 cm) from the pavement surface. Consequently, theair-launched antenna is routinely used at highway speeds and is not physically affected by rough roadconditions. But most importantly, it is not necessary to obtain cores to calibrate the air-launched hornantenna system. The system is calibrated by placing a metal plate under the antenna and collecting aGPR data file. The calibration file data collection process includes recording metal plate reflections at thedifferent heights that the antenna may experience during data collection over pavement. This metal platefile is later processed to produce a horn antenna calibration file that is used with subsequent data files tocalculate the velocity of radar signal through the pavement. (A detailed description of the technique usedfor horn antenna calibration and data processing is available in Geophysical Survey Systems Handbookfor GPR Inspection of Road Structures.) Thus, when using an air-launched horn antenna with the metalplate calibration technique the velocity through the pavement, and the corresponding thickness, iscalculated for each individual GPR scan acquired. Typically 8 or 10 scans per foot are acquired for filescollected for the FWD application. In addition, since the metal plate calibration file is applied to eachscan, changes in the composition of the pavement are accommodated as they occur so that the accuracyof the system is not dependant on the last core location.

Another important advantage of the horn antenna is the ability to measure thin pavement layers.Since the antenna is suspended above the pavement surface, the direct-coupling (the portion of thetransmitted energy radiated from the transmit antenna directly to the receive antenna) occurs at theantenna and not at the pavement surface where the ground-coupling occurs. With the ground-coupled

antenna the direct-coupling and ground-coupling occur together, creating near-field interference that limitsthe minimum detectable pavement thickness. The 2 GHz air-launched antenna can reliably resolve layerthicknesses of 1 inch while a 1.5 GHz ground-coupled antenna is normally able to reliable resolve firstlayer thicknesses greater than approximately 3 inches.

Geophysical Survey Systems RoadScan product is a complete GPR system for measuring pavementthickness at highway speeds and assessing the road structure beneath the pavement (base and sub-base). The RoadScan system includes GSSI’s SIR-20 dual-channel data collection system withintegrated Panasonic Toughbook computer, air-launched horn antenna (1 GHz or 2 GHz), wheel mountedDistance Measuring Instrument (DMI), universal vehicle antenna mount, and RADAN software with theRoad Structure Assessment Module. The system is FCC certified for use in the United States. TheRoadScan system output includes both a graphic representation of the pavement structure with layerdepths and a corresponding generic ASCII file containing data that is easily shared with other applications(See Figure 7).

FWD AND GPR INTEGRATIONEl Segundo California based Foundation Mechanics manufacturers the JILS truck-mounted FWD

system. In the truck mounted design, the FWD system is integrated into the bed of a truck rather than ona separate trailer towed behind a vehicle.

In 2004 the Arizona Department of Transportation, an owner of a JILS truck mounted FWD system,expressed their desire to implement a method of collecting and storing pavement thickness informationduring the FWD data collection process.

Several years earlier Foundation Mechanics had experimented with integrating GPR with FWD for theCalifornia Department of Transportation. However, these efforts were performed using ground-coupledantennas (See Figure 4). The ground-coupled antenna needed to be raised and lowered. GPR datacollection could only occur at very slow speeds, which meant that high speed GPR surveys, independentof FWD, could not be performed. Additionally, it was not possible to measure thin overlays and it was still

necessary to obtain cores for GPR calibration.


5/9

Figure 4 – Earlier efforts integrating FWD & GPR using ground-coupled antennas

In a collaborative effort between Foundation Mechanics and Geophysical Survey Systems, theRoadScan GPR system was integrated with the truck mounted FWD system. The result is a singlevehicle that collects both FWD and pavement thickness data (see Figure 5). The 2 GHz air-launchedhorn antenna is attached to the front of the vehicle. The antenna is easily installed or removed asrequired so that it is not necessary for the antenna to remain on the vehicle before or after data collection.The GPR and FWD systems share the same laptop computer for a single control station. The pavementthickness directly under the FWD load plate is determined and stored with the FWD data in a singledatabase.

Figure 5 – Foundation Mechanics JILS Truck-Mounted FWD with integrated GPR

The combined FWD/GPR data collection process is similar to normal FWD operation. Approximately50 feet before the vehicle reaches the desired FWD test location the GPR data collection process isinitiated from the same laptop computer that operates the FWD system. A short GPR file is collectedwhile the vehicle slows and reaches the FWD test location (see Figure 6).


6/9

Figure 6 - Short run GPR file (10 scans per foot)

The raw GPR file may either be processed immediately after collection or at later time. Processing

the GPR file consists of first applying a stored metal plate calibration file and then by identifying thedesired layers. Identifying the layers is performed in an interactive interpretation mode using a simpleinnovative point and click technique called EZ Tracker. By creating a simple processing macro in GSSI’sRADAN software, the GPR processing may be fully automated. The output consists of both a graphicdisplay of the road structure and an ASCII output file (see Figure 7).

Figure 7 – Processed GPR file with corresponding ASCII output

The JTEST FWD software program imports the GPR ASCII file to obtain the layer thicknessinformation. This may be accomplished either before or after FWD data collection.

The JTEST software provides the operator with a simple method for importing processed GPR dataprior to FWD data collection (see Figure 8). Once imported, the JEST software not only displays thepavement layer thickness under the FWD load plate but also provides supporting information includinggraphs of the pavement layer thickness, radar velocities, statistical confidence indicators, and aninteractive spreadsheet view of the GPR data where advanced users may experiment with and specifydata threshold values.

Direct Coupling at antenna

Top of pavement surface

Bottom of asphalt overlayBottom of pavement layers

Bottom of base layer

Time-based radar data

Depth-based layer data

ASCII output file


7/9

Figure 8 – GPR results loaded prior to FWD data collection

The other (and perhaps better) alternative is to collect all the GPR and FWD data then process andimport the GPR data into the FWD database sometime later. This approach results in a shorter overalldata collection time. Collecting the GPR data does not add any significant data collection time since theGPR data is collected as the vehicle approaches and stops at the FWD test location. After the GPR datahas been processed, the FWD database is opened in JTEST. The appropriate FWD test location isselected and the corresponding GPR results file is specified. The JTEST software then imports thespecified GPR data (see Figure 9).

Using either method, the result is a single FWD database file with pavement thickness informationstored at each FWD test location.

Figure 9 – GPR results loaded after FWD data collection

In an evaluation in Arizona, 24 individual cores and 24 individual GPR scans were performed on twoseparate highways. In 12 of the locations, the asphalt was overlaid with a thin friction course. The asphaltthickness obtained from the core data ranged from 2.88 to 7.13 inches. The average difference in asphalt


8/9

thickness between the measured cores and GPR calculated thickness was less than 4%. Althoughoccasional coring is still required to obtain material properties, the evaluation study confirmed that GPR isa reliable method for obtaining accurate asphalt thickness without coring.

An additional benefit of the FWD/GPR integration is the ability to use the same vehicle to performlong distance pavement studies at highway speeds. For this application, the RoadScan GPR system isused independently of the FWD system. The RoadScan system accommodates an optional GPS systemused to collect GPS data simultaneously with the GPR data. The results may then be plotted to producea graphic representation of the pavement thickness using contour mapping software such as Surfer® byGolden Software or DPlot by HydeSoft Computing (See Figure 10).

0-2.49"

2.5-3.49"

3.49-5"

Pavement Thickness

> 4

3.75

3.5

3.25

3

2.75

2.5

2.25

2

1.75

< 1.5

Figure 10 – GPS data integrated with GPR thickness data

CONCLUSIONSThe successful integration of GSSI’s RoadScan GPR system with the Foundation Mechanics truck-

mounted FWD resulted in a flexible vehicle satisfying the customer’s requirements to collect and storepavement thickness data with the FWD data.

The benefits of the integrated FWD and GPR system include:• Cost Savings – Coring costs are dramatically reduced. The dual-purpose vehicle provides

combined FWD and GPR data collection without significantly impacting FWD data collecting time;• Safety – Less time for personnel operating in dangerous traffic environments to obtain cores;• Data Integration – FWD and pavement thickness data are integrated into a single database;• Data Quality – Pavement thickness at each FWD location provides designers with the data they

need to make confident maintenance and rehabilitation decisions; and• Flexibility – The FWD vehicle may be used for independent project and network level GPR

surveys at highway speeds without the need or expense of traffic control.

Following the initial implementation at the Arizona DOT, the integrated technology has been adoptedthe City of Los Angeles and Montana DOT who have experienced similar success (See Figure 11).

Figure 11 – City of Los Angeles Montana DOT


9/9

REFERENCES[1] Washington State Department of Transportation Interactive Pavement Guide

www.wsdot.wa.gov/biz/mats/pavement/fwd.htm [2] LTPP Manual for Falling Weight Deflectometer Measurements Operational Field Guidelines

Version 3.1 August 2000[3] Ground Penetrating Radar For Measuring Pavement Thickness, U.S. Department of

Transportation, Federal Highway Administration, Publication Number: FHWA-HIF-00-015 FHWAPublication Number: FHWA-HIF-00-015

[4] Eckrose, R. A., “Ground Penetrating Radar Supplements Deflection Testing to Improve AirportPavement Evaluations” Nondestructive Testing of Pavements and Back-calculation of Moduli,

ASTM STP 1026. A. J. Bush III and G. Y. Baladi, Eds., American Society for Testing andMaterials. Philadelphia. 1989. pp. 563-573

RECOMMENDED FURTHER READING Noureldin, A.S., Zhu, K., Li, S., Harris, D. Network Pavement Evaluation Using Falling WeightDeflectometer and Ground Penetrating Radar, Transportation Research Board 82nd Annual Meeting,Washington, D.C.

Al-Qadi, I.L., Lahouar, S. Jiang, K., MeGhee, K. K., Mokarem, D., Validation of Ground Penetration

Radar Accuracy for Estimating Pavement Layer Thickness, Paper No. 05-2341, TransportationResearch Board, 84th Annual Meeting, January 9-13, 2005, Washington, D.C.

Gssi Highway Geo Fwd

Documents

Transcript of Gssi Highway Geo Fwd