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New Technologies for NDT of Concrete Pavement Structures

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Presentation on theme: "New Technologies for NDT of Concrete Pavement Structures"— Presentation transcript:

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

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