New Technologies for NDT of Concrete Pavement Structures

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

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

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. 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) (groundpenetratingRADAR) 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 warmer zone cooler 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. 25.5°C 25.5°C 23.2°C 23.2°C 114 mm 127 mm 127 mm 127 mm 114 mm 25 mm 38 mm 25 mm 1 2 3 4 Surface Surface 38 mm thermocouple thermocouple 51 mm 38 mm 25 25 ° 25 C 25 mm 7 8 5 6 Flaw #5 Flaw #8 Flaw #6 38 mm ° 24 24 24 C 25 mm 25 mm 25 mm 25 mm 25 mm 25 mm Surface Surface Heat flux sensor Heat flux sensor thermocouple thermocouple Thermography Results: FRP Bond concrete bonded FRP sheet disbonds 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. x1 x2 wave source sensors h pavement 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 Improve impact-echo Develop seismic approach

12. Impact Receiver (1-a)x ax P-wave P-wave h S-wave θs θs θp S-wave θp Seismic Approach for Slab Thickness Arrivals ofmode-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. bb-gun Impactor Accelerometers Impact sensor Impact positions tppss tps Direct P-wave Surface wave Time - microseconds 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. 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. Lamb Wave Basis for Impact-Echo S1-Cg=0 S1-k=0 A1-k=0 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?

16. 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

17. In-situ Strength Estimation Motivation: * Accurate and non-destructivein 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. 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. V13 V42 “T” d23 = V12 V43 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 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. Wave Source Blind zone Air-coupled Sensing • Challenge: Large acoustic impedance mismatch between air and concrete 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 180-210mm • Concrete R wave velocity VR=2300m/s • Microphone height h = 66cm • Shadow zone size: 15cm

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