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Ground-Penetrating Radar Applications for theAssessment of Pavements
Lulu EdwardsResearch Engineer
andDon R. Alexander
Chief, Airfields and Pavements Branch
Airfields and Pavements Branch (APB)Geotechnical and Structures Laboratory
US Army Engineer Research and Development CenterVicksburg, MS
2005 Tri-Service Infrastructure Systems Conference and Exhibition“Re-Energizing Engineering Excellence”
August 2-5, 2005St. Louis, MO
US Army Engineer Research and Development Center
Ground-Penetrating Radar (GPR)
Capabilities of GPR• Layer thicknesses• Void location• Stripping in asphalt layers• Presence of moisture• Detection/locate subsurface anomalies
GPR contributes to the structural assessment of pavements• Predict pavement performance• Determine upgrade requirements• Prevent unforeseen pavement failures
GPR is nondestructive• Quicker results• Fewer delays and interference
US Army Engineer Research and Development Center
ERDC GPR Applications
� Airfield evaluations• Current pavement condition• Layer thicknesses can be used with falling weight
deflectometer (FWD) data to backcalculate layer moduli� Road structures
• Maintenance and repair• Future design
� Test sections at ERDC• Quality assurance tool
US Army Engineer Research and Development Center
Pulse Radar System
� Developed under Small Business Innovative Research(SBIR) with Pulse Radar (Houston, TX)
� Multi-Antenna• 1 GHz (1 meter) • 250 MHz (3 meters)• 500 MHz (2 meters) • 100 MHz (5-10 meters)
� Operates at highway speeds
US Army Engineer Research and Development Center
Short Pulse GPR
RadarAntenna
A0
A1A2A3
Surface
Base
Subgrade Time
Volts
Dt1 Dt2
EndReflection
A0
SurfaceEcho
A1
FirstInterface
ReturnA2
SecondInterface
ReturnA3
Note: Requires dielectric discontinuity at layer interfaces
US Army Engineer Research and Development Center
� Layer thickness*
� Dielectric values*• Use assumed value (typically 6.0 for asphalt, 8.0 for concrete)• Backcalculate dielectric from core• Use equations
*Scullion et al, 1994
GPR Equations
ε×=2t∆ch
h = layer thicknessc = speed of light∆t = two way travel timeε = dielectric
2
m
0m
0
a
AA1
AA1
−
+=ε
−
−
+
−
ε=ε
m
12
m
0
m
12
m
0
ab
AA
AA1
AA
AA1
εa = dielectric of first layerεb = dielectric of base layerAm = metal reflection amplitudeA0 = surface reflection amplitudeA1 = base reflection amplitude
US Army Engineer Research and Development Center
GPR Data Analysis
� Layer interfaces (signal peaks) are found using a cross-correlation technique
� Layer thickness are calculated using the locations of thesignal peaks and the previous equations
� Layer thickness measurements improve whencalibrated/corrected with a thickness value from a singlecore (“ground truth”)
RA
DA
R, V
OLT
S
TIME, NSEC
-0.5-0.4-0.3-0.2-0.1
00.10.20.30.40.5
0 2 4 6 8 10 12 14 16 18
STATION 81AC BASE6"2"
DNA8.DRWTime, nanoseconds
Rad
ar, v
olts
RA
DA
R, V
OLT
S
TIME, NSEC
-0.5-0.4-0.3-0.2-0.1
00.10.20.30.40.5
0 2 4 6 8 10 12 14 16 18
STATION 81AC BASE6"2"
DNA8.DRWTime, nanoseconds
Rad
ar, v
olts
US Army Engineer Research and Development Center
GPR Display
B scan
A scan
Layer change
US Army Engineer Research and Development Center
Detection of Subsurface Utilities
Pipe
Utility
US Army Engineer Research and Development Center
Verification – Flexible PavementsLayer 1 original and corrected thicknesses as determined from the
1 GHz antenna on the ERDC asphalt test pavement
2
3
4
5
6
7
8
9
0 50 100 150 200 250 300 350
Distance, ft
Thic
knes
s, in
Layer 1 Layer 1, corrected with thickness of core at 50 ft Cores
Section 1 Section 2 Section 3
US Army Engineer Research and Development Center
Asphalt test pavement – Asphalt, layer 1 (1 GHz)
Differences/Errors
0.30
1.15
0.15
0.00
0.45
0.15
0
0.5
1
1.5
2
2.5
3
50 195 305
Station, feet
Diff
eren
ce, i
nche
s original GPR layer 1
GPR layer 1corrected withthickness of core at50 ft
1.08
1.67 1.
91
0.00
0.58
0.49
0
0.5
1
1.5
2
2.5
3
50 195 305
Station, feet
Diff
eren
ce, i
nche
s original GPR layer 2
GPR layer 2 correctedwith thickness of coreat 50 ft
Asphalt test pavement – Base, layer 2 (1 GHz)
US Army Engineer Research and Development Center
Verification – Rigid Pavement
0
2
4
6
8
10
12
14
16
18
20
0 1000 2000 3000 4000 5000
Distance, ft
Thic
knes
s, in
Layer 1 Layer 1, corrected with thickness of core at 2000 ft Cores
Core at 800 ft(14 inches)
Core at 5100 ft(15.25 inches)
Layer 1 original and corrected thicknesses as determined from the 1 GHzantenna on the Portland Cement Concrete (PCC) airfield pavement
US Army Engineer Research and Development Center
Differences/Errors
1.44
0.81
1.79
0.21
1.10
0.08
0.05
0.00 0.08
0.05
0
0.5
1
1.5
2
2.5
3
200 800 2000 3200 5100
Station, feet
Diff
eren
ce, i
nche
s original GPR layer 1data
GPR layer 1 datacorrected withthickness of core at2000 ft
2.38
1.38
2.27 2.
47
2.02
0.66
0.09
0.00 0.
25
0.28
0
0.5
1
1.5
2
2.5
3
200 800 2000 3200 5100
Station, feet
Diff
eren
ce, i
nche
s original GPR layer 1data
GPR layer 1 datacorrected withthickness of core at2000 ft
PCC airfield pavement – layer 1 (1 GHz)
PCC airfield pavement – layer 1 (500 MHz)
US Army Engineer Research and Development Center
Cement-Stabilized Soil
Cement-Stabilized PavementGround Penetrating Radar Results
10
10.5
11
11.5
12
12.5
13
13.5
14
0 500 1000 1500 2000 2500 3000 3500 4000
Distance (feet)
Thic
knes
s (in
ches
)
Layer 1
Average
Average + 1 Stdev
Average - 1 Stdev
US Army Engineer Research and Development Center
Summary and Conclusions
� Continuous thickness measurements along the entire lengthof the pavement ensures that changes in layer structure willbe detected
� Combination of the 1GHz and 500 MHz antenna appears toprovide both the resolution and penetration necessary forsampling most typical pavement structures
� At least one core is required to calibrate GPR thicknesses• Flexible pavement - error is reduced from an average of 1.04 inches to
0.42 inches• Rigid pavement – error is reduced from an average of 1.59 inches to
0.19 inches
US Army Engineer Research and Development Center
Summary and Conclusions
� Measuring layer thicknesses with GPR has the potential tominimize time required for pavement evaluations byoptimizing coring and DCP testing
� GPR is useful for detecting utilities
� Using layer thicknesses from GPR along with FWD dataresults in more accurate backcalculated moduli, andtherefore, more reliable predictions of structural capacity
US Army Engineer Research and Development Center
Questions?
Don AlexanderAirfields and Pavements Branch
CEERD-GM-A(601) 634-2731
Lulu EdwardsAirfields and Pavements Branch
CEERD-GM-A(601) 634-3644
