Ground-Penetrating Radar Applications for the Assessment ... · Ground-Penetrating Radar Applications for the Assessment of Pavements Lulu Edwards Research Engineer and ... a = di

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


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


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


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


� 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


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


GPR Display

B scan

A scan

Layer change


Detection of Subsurface Utilities

Pipe

Utility


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


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)


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


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)


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


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


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


Questions?

Don AlexanderAirfields and Pavements Branch

CEERD-GM-A(601) 634-2731

[email protected]

Lulu EdwardsAirfields and Pavements Branch

CEERD-GM-A(601) 634-3644

[email protected]

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Ground-Penetrating Radar Applications for the Assessment ... · Ground-Penetrating Radar Applications for the Assessment of Pavements Lulu Edwards Research Engineer and ... a = di