1. New Technologies for NDT of Concrete Pavement Structures
John S. Popovics
Department of Civil & Environmental Engineering
CEAT Seminar Series September 8, 2005

2. Outline Motivation & background Current NDE techniques/applications
New Research Directions (UIUC)

3. Motivation A need for structural/pavement NDE
US infrastructure is deteriorating: 2005 ASCE Report
card for American infrastructure gave an overall grade
of “D+” – estimated $1.3 trillion investment needed
for improvements
Increased use of performance-based specifications
require accurate in-place estimates of new
pavement thickness and strength
A need for structural/pavement NDE

4. Current NDE Techniques
Concrete structures and pavements
Impact-echo, GPR (RADAR), thermography
sounding/tapping, UPV and velocity
tomography, electro-chemical techniques,
radiography, modal analysis, acoustic
emission, impulse-response, etc.

5. Impact-Echo (ASTM C1383) Propagating P-waves generated by impact
event. Multiply-reflected
waves are detected by
surface sensor.
Reflected waves set
up a resonance
condition having a
characteristic frequency
FFT
Analogous to a bell’s tone

6. Impact-echo Analysis The resonant frequency (at the
peak) is related to
distance to reflector
(d or d*) and
wave velocity (VL):
f = VL/(2 d)
Thus,
d = VL/(2 f)
Reflection from slab bottom
is a correction factor
for the shape of the element.
 = 0.96 for slabs
Reflection from delamination

7. GPR (ASTM D4748) (ground penetrating RADAR) Wave pulses are reflected
at interfaces having
a difference in
electrical properties (r )
antenna
air: r = 1
concrete: r = 6 to 11
Reflected pulses (time
and amplitude) are
monitored in the
time domain signal
soil: r = 2 to 10
(water: r = 80; metal r = infinite)

8. Infra-red Thermography
Monitoring heat flow by surface temperature
Sub-surface defects disrupt heat flow. If defect near near surface,
surface temperature is affected.
Temp 2
cooler zone
warmer zone
Air-filled void
where T2 < T1
heat flow
(conduction)
Temp 1
Driven by thermal gradient
Heat flow must be established, but direction of flow does not matter

9. Thermography Results: FRP Bond
25.5°C
25.5°C
23.2°C
23.2°C
Thermography Results: FRP Bond
concrete
25 mm 1 2 3 4 5 6 7 8
114 mm
127 mm
38 mm
51 mm
bonded FRP sheet
disbonds 24 25
Heat flux sensor
Surface
thermocouple C
Flaw #5
Flaw #6
Flaw #8
Thermograph
(disbonds are
hot spots)

10. New Research Directions (UIUC)
New pavement Q/A assurance work
Accurate thickness and in situ strength
estimation for new concrete pavements
Contact-less (air-coupled) pavement
inspection using stress waves
Seismic time domain approach
Surface waves

11. In-situ Pavement Thickness
Motivation:
* Accurate (5mm) and non-destructive thickness estimates
needed for new pavement QC and pay factor application
* Best available method (standard impact-echo) does not
provide needed accuracy
Approaches:
Time domain
Frequency domain
sensors x1 h x2
pavement
wave source
Develop seismic approach
Improve impact-echo

12. pulses used to back-compute wave velocity and slab thickness
Seismic Approach for Slab Thickness
Impact h ax
P-wave
S-wave θp θs
(1-a)x
Receiver
Arrivals of mode-converted reflections (P-S and PP-SS) of short duration
pulses used to back-compute wave velocity and slab thickness
P-S arrival time

13. Field Testing Set-up Field testing setup comprised of sensed
Accelerometers
bb-gun Impactor
Impact positions
Impact sensor
Field testing setup
comprised of sensed
BB-gun and multiple
accelerometer set
Direct P-wave
tps
tppss
Time - microseconds
Surface wave
Arrivals of mode-
converted waves
determined in each
signal; velocity
and thickness
then computed

14. Impact-Echo Targets for improvement 1980s in NIST and Cornell
Effective in determining thickness of slabs and depth of flaws in plate structures
Does not work on beams & columns
Targets for improvement

15. Lamb Wave Basis for Impact-Echo
Guided waves in free plates
Symmetric modes
Anti-symmetric modes
Solutions: dispersion curve
S1-Cg=0
S1-k=0
A1-k=0
Resonance conditions
represented at
zero wave number or
zero group velocity
locations
Impact-echo?

16. Verification FEM (ABAQUS) model verified by experiment
Impact-echo frequency
FEM (ABAQUS)
model verified
by experiment
Analytical (Lamb)
model verified
by FEM
b = 0.96
Impact echo frequency
is S1 ZGV

17. In-situ Strength Estimation
Motivation:
* Accurate and non-destructive in situ concrete strength
estimation needed for new pavement QC and pay factors
* Best available methods (rebound hammer, UPV, maturity)
do not provide needed accuracy, reliability, or applicability
Approach:
Use surface waves: one-sided method
Sensors
Wave source d
Measure surface wave velocity
and transmission (attenuation)
and correlate to in situ strength
Surface waves

18. Ultrasonic wave source
Testing Set-up
Ultrasonic wave source
(200 kHz)
Field testing set-up
Wave sensors
(accelerometers)

19. Surface Wave Measurements
Acceleration signals
far sensor
near sensor DA Dt
Surface wave velocity: arrival time of far sensor- near sensor
Surface wave transmission: amplitude ratio for far sensor/near sensor

20. Self-calibrating Wave Transmission
In the frequency domain, we can represent wave signal sent by i and received by j (Vij) as
V12=A1d12S2,
V13=A1d12d23S3,
V43=A4d43S3 and
V42=A4d43d32S2,
where Ai and Si are the sending and receiving response functions, dij is the transmission between points i and j.
We aim to isolate and measure d23
V12 V43
V13 V42
d23 =
“T” 1 2 3 4
surface waves

21. Correlation to Concrete Strength
On-going work: correlation to flexural strength

22. Contact-less (air-coupled) inspection
NDT imaging techniques provide
a direct approach for assessment
Stress-wave based NDT methods are
usually less efficient due to coupling
problem
Here we aim to develop effective
non-contact NDT techniques for pavements

23. Air-coupled Sensing Use leaky R-waves?
Challenge: Large acoustic impedance mismatch between air and concrete
Wave Source
Blind zone
Use leaky R-waves?

24. Air-Coupled SASW
SASW provides surface wave velocity profiles with depth (layered system)
SASW test performed on floor slab (thickness 200mm)
Signals show good coherence through 22kHz
Rayleigh wave velocity is about 2300m/s

25. Air-Coupled MASW Multichannel analysis of surface waves (MASW)
Compared to SASW, MASW can determine phase velocities precisely using whole waveform data
Avoids spatial aliasing
Distinguish fundamental mode from higher modes and body waves
Multi-source setup
Multi-sensor setup

26. Air-Coupled MASW MASW 2D spectrum image
Test performed on a concrete slab with thickness 200mm, CR=2300m/s
Nils Ryden provided the MASW analysis program

27. Imaging surface-opening cracks
Top layer concrete thickness mm
Concrete R wave velocity VR=2300m/s
Microphone height h = 66cm
Shadow zone size: 15cm

28. Imaging surface-opening cracks
1-D Y scan
1-D X scan
2-D scan
Energy Ratio

29. Air-Coupled Impact-Echo
Musical microphone: frequency response 20Hz-20kHz
Air-coupled sensor
Tested in ambient noise condition without any sound insulation
Good agreement with the contact impact-echo test result
The PCB microphone can determine thickness of shallow delaminations
PCB measurement microphone: 4Hz-80kHz

30. Air-Coupled Impact-Echo – Delamination
Delamination at depth 60mm
Flexual mode at 2.68kHz
strong and easy to detect
Impact-echo mode 33.2kHz for delaminations
Gives delamination depth 58mm
33.2kHz can be detected by the PCB microphone

31. Summary
Non-destructive test methods are needed for concrete pavements. Many existing NDT methods exist for concrete pavements
New research efforts focus on improving the capability of NDT for pavements, for example for in-place pavement thickness estimation
Surface wave measurements can be carried out on concrete. The self-compensating scheme allows measurement of surface wave signal transmission. Results show correlation to in-place strength.
Contact-free methods have potential for rapid and effective NDT for pavements