1

NDT for Airport Pavements: Advances in Ground

Penetrating Radar Technology

Imad L. Al-Qadi

Background• Ground Penetrating Radar (GPR) is a

special kind of RADAR• Purpose of using GPR:

– Detect targets buried in a dielectric medium

– Estimate their depths• GPR applications: geophysics,

archeology, law enforcement, evaluation of civil structures (buildings, bridges, pavements)

Use of GPR for Pavement Evaluation• Flexible pavements:

• Detect changes in pavement structures (layers)• Estimate layer thicknesses (can be used as input

for FWD)• Detect subsurface distresses• Detect moisture presence

• Rigid pavements and bridge decks:• Measure concrete slab thickness• Detect voids or loss of support under slabs!• Detect rebar locations and estimate cover depth

2

EM Properties of MaterialsInteraction of a Material with Applied Electric (E) and Magnetic (H) Fields:

– Polarization– Conductivity– Magnetic Permeability – Permittivity/ Dielectric Constant

"'rrr jεεε −=

Dielectric Constant Values

3-5Dry Sand

5-40Clays5-30Silts20-30Saturated Sand

4-6Granite5-9Limestone

3-10HMA3-18Concrete81Water1Air

Dielectric ConstantMaterial

GPR TypesThree main types of GPR systems:• Frequency modulated GPR• Synthetic pulse GPR• Pulsed (or impulse) GPR

– Most common type of GPR systems– Principle: Transmit a short pulse (1 ns

or less) and record the reflected pulses from layers

1ns

3

GPR Antennas• Ground-coupled: the antenna is in

contact with the ground surface• Air-coupled: the antenna is 1 to 2 ft

above surface• Monostatic: one antenna used as Tx and

Rx• Bistatic: one antenna is used for Tx and

another one for Rx

Courtesy of GSSI

Layer 1

Layer 2

DMI

Air-Coupled Antenna

Transceiver

Ground-Coupled Antenna

Ground Penetrating Radar System

Control Unit

HMA

Base

Subgrade

t1

t2

A0

A1

A2

Driving Direction

Typical GPR Response (scan)

4

Data Collection

Base

Subgrade

Surface

Base

Subgrade

Surface

GPR System DescriptionAir-Couple Antenna

Ground-Couple Antenna

Camera

Air-Couple Antenna

Ground-Coupled Data, CRCP, Smart Road

Locating Reinforcement

Copper Plate under Slab

Transversal Reinforcement

Concrete

Asphalt OGDL Longitudinal Reinforcement

5

Without geocompositeWith geocomposite

HMA

21B Aggregate

Copper Plates Subgrade

Detecting Moisture

MoistureNo Moisture

QA for Reinforcing Mesh

Section With Reinforcing Mesh

Section Without Reinforcing Mesh

Ground-Coupled Data, Section L, Smart Road

Typical Problems with GPR Data Interpretation

• GPR doesn’t provide “real” images of the subsurface• Dielectric properties unknown

– Dielectric properties change with depth/ moisture presence• Sufficient contrast between layer dielectric constants

should exist • Material “loss” reduces GPR detectability of bottom layer

interfaces • Thin pavement layers are difficult to resolve• Extensive amount of data is collected• GPR results are operator dependent

6

Layer Thickness Estimation

• Calculate the thickness di of the ithlayer.

c is the speed of light in free space (0.3m/ns)

ti is the reflection amplitude from interface

εr,i is the relative permittivity or dielectric constant of layer i

ir

ii

ctd,ε2

=

PreprocessingEnhance signal quality:• Noise filtering• Coupling pulse removal• Depth resolution enhancement

-8000-6000-4000-2000

02000400060008000

1000012000

0 5 10 15 20

Time (ns)

Am

plitu

de

Pavement Surface

Noise

Coupling

Preprocessing

Noise due to CB radio

Noise due to cell phone towers

Raw GPR Data

Filtering & Coupling Removal

GPR (UWB 1GHz) susceptible to noise: CB radios, cell phones, cell phone towers, etc.

7

Noise FilteringElliptic low-pass filter: most efficient filter• Lowest order fast to run on data• Lowest transition bandwidth high performance

-10000

-5000

0

5000

10000

15000

0 2 4 6 8 10 12 14Time (ns)

Am

plitu

de

Original Signal

Filtered Signal

Coupling Pulse RemovalTx Rx

Et

EcEr

Tx/Rx

Et Er

Eer

Et: TransmittedEr: Reflected Ec: CouplingEer: End Reflection

-8000-6000-4000-2000

02000400060008000

1000012000

0 5 10 15Time (ns)

Am

plitu

tde

Coupling pulse

Surface reflection

Subsurface reflections

Air

Coupling Pulse Removal

-0.6-0.4-0.2

00.20.40.60.8

1

-60 -40 -20 0 20 40 60Lag

Nor

mal

ized

Am

plitu

de

lm

Cross-correlation

-8000-6000-4000-2000

02000400060008000

10000

0 100 200Time (samples)

Am

plitu

de

yc(t)yr(t)

Original signals

-8000-6000-4000-2000

02000400060008000

10000

0 50 100 150 200 250Time (samples)

Am

plitu

de

yc(t)yr(t)

Shifted signals

-8000-6000-4000-2000

02000400060008000

10000

0 50 100 150 200 250Time (samples)

Am

plitu

de

Removed coupling pulse

Coupling pulse measurement

8

Dielectric Constant Estimation Methods

– Amplitude of reflected signal

– Common Midpoint

– Modified Common Midpoint

– Iteration (most applicable to airport pavements)

– New Modified Common Midpoint

Dielectric Constant Using Amplitude2

0

0

1,

1

1

⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛

−

+=

p

pr

AAAA

ε

2

12

1

2

0

12

1

2

0

1,,

1

1

⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜

⎝

⎛

−+⎟⎟⎠

⎞⎜⎜⎝

⎛−

++⎟⎟⎠

⎞⎜⎜⎝

⎛−

=−

−

=

−−

=

∑

∑

p

nn

i p

ii

p

p

nn

i p

ii

pn-rnr

AA

AAγ

AA

AA

AAγ

AA

εε

1,,

1,,

+

+

+

−=

irir

iriri εε

εεγ

HMA

Base

Subgrade

t1

t2

A0

A1

A2

Driving Direction

Thickness Accuracy

-30

-25

-20

-15

-10

-5

00 0.5 1 1.5 2 2.5 3 3.5

Dielectric constant error Δεr

Rel

ativ

e th

ickn

ess

erro

r (%

)

εr = 4εr = 6εr = 8εr = 9

Relative Layer Thickness Error versus Error in Dielectric Constant Estimation for Different Values of εr

9

Application for Quality Assurance

21-B aggregate base

IM-19.0 (1) w/ PG 64-22

BM-25.0 w/ PG 64-22

SM-12.5 w/ PG 76-22

IM-19.0 (2) w/ PG 76-22

150mm (6in)

300mm (12in)

100mm (4in)

75mm (3in)

65mm (2.6in)

50mm (2in)

Lime stabilized subgrade

• GPR surveys were performed on each layer after its construction

• Three surveys were performed per lane and per layer; in total, 36 surveys were performed

• DMI set at 10 scans/m• For HMA layers, static

measurements were taken near core locations

9.8m 9m 8.2m 6.2m 5.4m 4.6m 2.6m 1.8m 1m

HMA Base CoresHMA Intermediate 1 CoresHMA Intermediate 2 Cores

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

S1S2

QA/QC1

QA/QC2

QA/QC3

QA/QC4

QA/QC5

QA/QC6

Station 189 + 80

Station 186 + 10

Survey Direction

Offset

Right Lane Center Lane Left Lane

GPR Data Collection

Typical GPR Response

-8000-6000-4000-2000

02000400060008000

1000012000

0 1 2 3 4 5 6 7 8 9 10 11 12

Time (ns)

Am

plitu

de

21B Coupling Surface

Air

-8000-6000-4000-2000

02000400060008000

1000012000

0 1 2 3 4 5 6 7 8 9 10 11 12

Time (ns)

Am

plitu

de 21BHMA

GPR Scan over 21-B Layer

GPR Scan over HMA Base

10

Typical GPR Response

GPR Scan over HMA Intermediate 1

GPR Scan over HMA Intermediate 2

-8000-6000-4000-2000

02000400060008000

1000012000

0 1 2 3 4 5 6 7 8 9 10 11 12

Time (ns)

Am

plitu

de 21B HMA

-8000-6000-4000-2000

02000400060008000

1000012000

0 1 2 3 4 5 6 7 8 9 10 11 12

Time (ns)

Am

plitu

de 21BHMA

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120 140 160 180

Distance (m)

Thic

knes

s (m

m)

Offset 6.2mOffset 5.4mOffset 4.6mDesign

Offset values are with respect to left edge of left lane

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120 140 160 180

Distance (m)

Thic

knes

s (m

m)

Offset 6.2mOffset 5.4mOffset 4.6mDesign

21-B/ HMA Intermediate-2 Thicknesses

020406080

100120140160180200220

9.8 9 8.2 6.2 5.4 4.6 2.6 1.8 1Offset (m)

Offset values are with respect to left edge of left lane

Average Thickness, 21-B Layer

Average Thickness, HMA Base Layer

Right

02 04 06 08 01 00

1 2 01 4 01 6 01 8 02 002 2 0

2 4 02 6 02 8 03 003 2 0

9 . 8 9 8 .2 6 .2 5 .4 4 .6 2 .6 1 . 8 1O ffse t (m)

Thick. 0-184mThick. 184-370mStd. Dev. 0-184mStd. Dev. 184-370m

Center Left

0

20

40

60

80

100

120

140

160

9.8 9 8.2 6.2 5.4 4.6 2.6 1.8 1Offset (m)

Right Center Left

11

New Pavements (QC/QA )Classic GPR thickness estimation gives accurate results:

050

100150200250300350400450500

40 42.5 45 47.5 50 52.5 55 57.5 60Distance (m)

Dep

th (m

m)

HMA Base

HMA Design Base Design

GPR Accuracy: New Pavements

For exiting pavement, a different method is needed

because of changes in dielectric constant profile

12

Dielectric Constant Using CMPCommon midpoint (CMP) technique (or common-depth point, CDP) is used as follows:

T/RT R

x

t1

t2

P

HMAεr1

h

ν : EM velocity in the layer 2

21

22

2

21

22

222

1

22

2

xttc

ttxcv

xhvt

hvt

r

r

)(

)/(

−=

−==

+=

=

ε

ε

Modified CMP Technique Modified common midpoint technique:

h1

T/R

x0

t1

t2

P

PCCεr1

h0

airεr0=1

x1

T R

θi

θt

21

22

1

ttxv−

=

trir θsinεθsinε 10 =

01i0 θtan2 xxh =+

1

1

1

1t 2θtan

vtx

hx

==

Snell’s law of refraction:

Using the figure:

(1)

(2)

(3)

(4) 2

t

i1 θsin

θsin⎟⎟⎠

⎞⎜⎜⎝

⎛=rε

Modified CMP Technique Modified common midpoint algorithm:1. Measure the reflection times t1 and t2

2. Calculate the transmission angle θt using:3. Find the angle θi by solving numerically

4. Solve for εr1 using:

5. Compute HMA thickness using t1 and εr1

1

21

22

tθtanttt −

=

021

22

i

ti0 θsin

θsinθtan2 xttch =−+

2

t

i1 θsin

θsin⎟⎟⎠

⎞⎜⎜⎝

⎛=rε

111 2 rcth ε=Modified CMP Setup

13

050

100150200250300350400450500

0 250 500 750 1000 1250 1500Distance (m)

Dep

th (m

m)

Bridge

Modified CMP Technique Results: Old Unknown Pavements (I-81 N)

050

100150200250300350400450500

0 250 500 750 1000 1250 1500Distance (m)

Dep

th (m

m)

Full-depth repair

PCC

HMA

PCCPCCOverlay Thickness

HMA Layer Thickness

Dielectric Constant Using Modified CMP

Drawbacks• Cannot be used to estimate the dielectric

constant of thin layers or layers with high dielectric constants (because t1 ≈ t2)

• Results are sensitive to the magnitude of the time difference (t2-t1)

• It is difficult to accurately measure t1 from ground-coupled data

-6000-4000-2000

02000400060008000

1000012000

0 5 10 15Time (ns)

Am

plitu

de

Surface Reflection

OGDL/Base Reflection

WS/BM-25.0 Reflection

BM-25.0/OGDL Reflection

Base/Subgrade Reflection

Depth Resolution Enhancement

Measured Signal from: Thin layer interfaces

not visible because of reflection overlap

Synthesized Signal

-8000-6000-4000-2000

02000400060008000

1000012000

0 5 10 15Time (ns)

Am

plitu

de

Reflection Overlap

Surface Reflection HMA/Base

Reflection

Base/Subgrade Reflection

Base

OGDL

WS

BM-25.0

14

Measured vs. Simulated Signal

Layer Thickness Estimation by Iteration

Raw GPR Data

Layer Interface Detection

Dielectric Properties Estimation

Layer Thicknesses

Preprocessing

Resolving Thin Layers (Deconvolution)

21A

OGDL

WS

BM-25.0

21B1

2

34

5

Depth

Pavement Surface

OGDL/21A Interface

Spurious interface reflection

1

2

34

5

Depth

Pavement Surface

OGDL/21A Interface

WS

BM-25.0

OGDL

Base

15

Detection Results

02468

10121416

20 30 40 50 60Distance (m)

Tim

e D

elay

(ns)

Detected Layer Interfaces

WS

BM-25.0

OGDL

Base

Copper plates

Resolving Thin Layers:Better Results

0

50

100

150

200

250

300

350

0 20 40 60 80 100Distance (m)

Dep

th (m

m)

Design

WS

BM-25.0

OGDL

0

50

100

150

200

250

300

350

0 20 40 60 80 100Distance (m)

Dep

th (m

m)

Total HMA Design

GPR Data Analysis Results0

50100150200250300350400

0 20 40 60 80 100Distance (m)

Dep

th (m

m)

GPR Design0

50100150200250300350400

0 20 40 60 80 100Distance (m)

Dep

th (m

m)

GPR Design

050

100150200250300350400

0 20 40 60 80 100Distance (m)

Dep

th (m

m) WS BM-25.0

OGDL Design

Overall Thickness Individual Thicknesses Modified CMP Thickness

16

GPR Thickness Accuracy

Overall Thickness Individual Thicknesses Modified CMP Thickness

150

200

250

300

350

400

450

150 250 350 450

Core Thickness (mm)

GPR

Thi

ckne

ss (m

m)

+12%

-12%

150

200

250

300

350

400

150 200 250 300 350 400

Core Thickness (mm)

GPR

Thi

ckne

ss (m

m)

+4%

-4%

150

200

250

300

350

400

150 200 250 300 350 400

Core Thickness (mm)

GPR

Thi

ckne

ss (m

m)

+3%

-3%

GPR Data Collection (18 Projects)

GPR Result of Dielectric Constant

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0 1 2 3 4 5 6

Layer Number

Coe

ffici

ent V

aria

nce

of D

iele

ctric

C

onst

ant

Site 01 Site 02 Site 03Site 04 Site 05 Site 06Site 07 Site 08 Site 09

17

GPR Result Versus Core Data(Distributed Thickness Error)

250

260

270

280

290

300

310

320

330

340

350

250 260 270 280 290 300 310 320 330 340 350

Core Thickness (mm)

GPR

Thi

ckne

ss (m

m)

+4.7%

-4.7%

GPR Error Vs. Pavement Age

4.4 4.6 4.9

5.8

0

1

2

3

4

5

6

7

0-5 10 -- 15 >20 (Surf.<10) >20 (Surf.>10)

Pavement Age (years)

Ave

rage

GPR

Thi

ckne

ss E

rror

(%)

GPR Result of Layer Thickness(Iteration Method)

0

100

200

300

400

500

600

0 100 200 300 400 500 600 700 800 900Distance (m)

Dep

th (m

m)

SM-2B+S-5

Base

S-5

Subgrade

Core

18

GPR Result of Layer Thickness

200

250

300

350

400

450

500

200 250 300 350 400 450 500

Core Thickness (mm)

GPR

Thi

ckne

ss (m

m)

Summary• Errors may result from estimating

dielectric constant from surface reflection

• A modified CMP technique can be used to estimate the bulk dielectric constant

• Detection stage is affected by the deconvolution procedures

• For accurate GPR thickness results the reflections from all the layer interfaces should be separated

SummaryAccuracy of GPR results depends on adopted data analysis technique– For pavements with multiple thin layers or

old pavements, use:• Deconvolution: individual thicknesses • Modified CMP technique: total thickness

– For new pavements: use classic thickness estimation technique

19

Thank You!