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

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Presentation on theme: "1 New Technologies for NDT of Concrete Pavement Structures John S. Popovics Department of Civil & Environmental Engineering CEAT Seminar Series September."— Presentation transcript:

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

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

3 3 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

4 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 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 Analogous to a bell’s tone FFT

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

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

8 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 Temp 1 Air-filled void where T2 < T1 heat flow (conduction) Heat flow must be established, but direction of flow does not matter Driven by thermal gradient warmer zone cooler zone

9 9 23.2°C 25.5°C 23.2°C 25.5°C concrete bonded FRP sheet disbonds Thermograph (disbonds are hot spots) Thermography Results: FRP Bond

10 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 approachSurface waves

11 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: Frequency domain Time domain Develop seismic approach Improve impact-echo

12 12 Impact h ax P-wave S-wave θpθp θsθs (1-a)x θpθp θsθs P-wave S-wave Receiver 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

13 13 Accelerometers bb-gun Impactor Impact positions Impact sensor Direct P-wave t ps t ppss Time - microseconds Surface wave 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

14 14 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

15 15 Solutions: dispersion curve Symmetric modes Anti-symmetric modes Lamb Wave Basis for Impact-Echo S 1 -Cg=0 S 1 -k=0 A 1 -k=0 Guided waves in free plates Resonance conditions represented at zero wave number or zero group velocity locations Impact-echo?

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

17 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: Wave source Surface waves d Sensors Use surface waves: one-sided method Measure surface wave velocity and transmission (attenuation) and correlate to in situ strength

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

19 19 Surface Wave Measurements Acceleration signals tt AA Surface wave transmission: amplitude ratio for far sensor/near sensor Surface wave velocity: arrival time of far sensor- near sensor far sensor near sensor

20 20 In the frequency domain, we can represent wave signal sent by i and received by j (V ij ) as V 12 =A 1 d 12 S 2, V 13 =A 1 d 12 d 23 S 3, V 43 =A 4 d 43 S 3 and V 42 =A 4 d 43 d 32 S 2, where A i and S i are the sending and receiving response functions, d ij is the transmission between points i and j. We aim to isolate and measure d 23 Self-calibrating Wave Transmission surface waves V 12 V 43 V 13 V 42 d 23 = “T”

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

22 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 23 Wave Source Blind zone Air-coupled Sensing Challenge: Large acoustic impedance mismatch between air and concrete Use leaky R-waves?

24 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 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 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 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 28 Imaging surface-opening cracks 1-D Y scan 1-D X scan 2-D scan Energy Ratio

29 29 Air-Coupled Impact-Echo 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 Musical microphone: frequency response 20Hz-20kHz PCB measurement microphone: 4Hz-80kHz

30 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 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

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