New Technologies for NDT of Concrete Pavement Structures

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New Technologies for NDT of Concrete Pavement
Structures

• John S. Popovics
• Department of Civil Environmental Engineering

CEAT Seminar SeriesSeptember 8, 2005
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Outline

• Motivation background
• Current NDE techniques/applications
• New Research Directions (UIUC)

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Motivation

• 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
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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.
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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 bells tone
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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
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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)
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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
warmer zone
cooler zone
Air-filled void
where T2 lt T1
heat flow (conduction)
Temp 1
Driven by thermal gradient
Heat flow must be established, but direction of
flow does not matter
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25.5C
25.5C
23.2C
23.2C
Thermography Results FRP Bond
concrete
bonded FRP sheet
disbonds

Thermograph (disbonds are hot spots)
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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
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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
Frequency domain
Time domain
Develop seismic approach
Improve impact-echo
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Seismic Approach for Slab Thickness
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
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Field Testing Set-up
Field testing setup comprised of sensed BB-gun
and multiple accelerometer set
Arrivals of mode- converted waves determined in
each signal velocity and thickness then computed
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Impact-Echo

• 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
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Lamb Wave Basis for Impact-Echo
Guided waves in free plates
Anti-symmetric modes
Symmetric modes
Solutions dispersion curve
Resonance conditions represented at zero wave
number or zero group velocity locations

Impact-echo?
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Verification
Impact-echo frequency
FEM (ABAQUS) model verified by experiment
Analytical (Lamb) model verified by FEM
b 0.96
Impact echo frequency is S1 ZGV
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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
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Testing Set-up
Ultrasonic wave source (200 kHz)
Field testing set-up
Wave sensors (accelerometers)
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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
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Self-calibrating Wave Transmission

• In the frequency domain, we can represent wave
  signal sent by i and received by j (Vij) as
• V12A1d12S2,
• V13A1d12d23S3,
• V43A4d43S3 and
• V42A4d43d32S2,
• 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

1
2
3
4
surface waves
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Correlation to Concrete Strength
On-going work correlation to flexural strength
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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

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Air-coupled Sensing

• Challenge Large acoustic impedance mismatch
  between air and concrete

Use leaky R-waves?
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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

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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
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Air-Coupled MASW
MASW 2D spectrum image
Test performed on a concrete slab with thickness
200mm, CR2300m/s
Nils Ryden provided the MASW analysis program
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Imaging surface-opening cracks

• Top layer concrete thickness 180-210mm
• Concrete R wave velocity VR2300m/s
• Microphone height h 66cm
• Shadow zone size 15cm

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Imaging surface-opening cracks
Energy Ratio
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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
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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

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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