Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings (2013)

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358 a p p e N D I x V Broken tiles on the interior of a tunnel (especially on the roof) are hazardous to vehicles passing through the tunnel at 55 mph. Routine tunnel maintenance measures include examination of tiles and detection and repair of loose tiles. The current state of the practice is visual inspection and hammer tapping of the tiles. Before this field investigation, the Chesapeake Bay Bridge-Tunnel (CBBT) owners had employed one engineer for 1 month to evaluate the bond- ing of roof tiles in the Chesapeake Channel Tunnel with the hammer sounding method. Broken and loose tiles were found and marked as such. During the first round of field evaluation using the SPACETEC scanner in April 2011, a thermal anomaly (an isolated hot spot) around Station 483 that did not correspond to any known or marked loose tiles was detected. Manual measurements using impact echo (IE) and ultrasonic echo were carried out to investigate the bonding of tiles at the loca- tion of the thermal anomaly. Both IE and ultrasonic echo tests were conducted on the selected eight-tile by eight-tile grid covering the location of the detected anomaly. IE was carried out on an adjacent four-tile by four-tile grid. Measurements were taken on individual tiles and repeated three times. Data analysis was done in time domains, examining the time histories of the signal recorded on each tile. Frequency spectra as well as the short-time Fourier transform- based spectrograms were calculated and examined. The envel- opes of the US signals depicting their attenuation rates were also calculated. To showcase the data corresponding to the areas of good bonding and possible debonding, two rows of the eight-tile by eight-tile grid were selected for presentation here. Figures V.1 to V.3 provide the IE test results corresponding to the tiles on the sixth row (from the top) of the eight-tile by eight-tile grid, illustrating the time histories, spectra, and short-time Fourier transform (STFT) spectrograms. The time signals in Fig- ure V.1 attenuate rapidly (the impact energy propagates in the lining); and the frequency spectra in Figure V.2 are broadband with spectral energy centered on 50 kHz. Four records (one on the top left and three at the bottom left) depict additional fre- quency peaks of lower frequencies as well. Time and frequency features can be seen simultaneously in the spectrograms of Figure V.3. These characteristics in time and frequency domains are indications of good bonding between the tested tiles and the underlying lining. Figures V.4 through V.6 provide the IE test results corre- sponding to the tiles on the third row (from the top) of the eight-tile by eight-tile grid, including the time histories, spectra, and spectrograms. In contrast to those shown in Fig- ure V.1, the time signals of Figure V.4 show little or no atten- uation. The frequency spectra in Figure V.5 contain multiple equally spaced frequency peaks. Both measured time and frequency features are expected for loose tiles, as the debond- ing from the tunnel lining leads to multiple reflections of the acoustic energy between the tile and the underlying lining. The individual records obtained on each tile were analyzed in both time and frequency domains, and their assessed bonding conditions were color coded and superimposed on the thermal image in Figure V.7. In this figure, green indicates well-bonded tiles, while loose tiles are marked with orange- to-red spots. A comparison of the obtained results reveals that the tiles at the thermal anomaly detected by SPACETEC as a noticeable warm spot were diagnosed as debonded by IE measurements. Ultrasonic echo measurements were taken on the same eight-tile by eight-tile grid (the eight-tile by eight-tile grid only) as described for IE testing. The ultrasonic echo time histories and spectra obtained on the sixth row of the tiles (from top) are shown in Figures V.8 and V.9, respectively. The ultrasonic echo time histories and spectra obtained on the third row of the tiles (from top) are shown in Figures V.10 and V.11, respectively. Analysis of SPACETEC Data

359 Figure V.1. Time histories of IE signals along the sixth row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected. Figure V.2. Frequency spectra of IE signals along the sixth row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected.

360 Figure V.3. STFT spectrograms of IE signals along the sixth row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected. Figure V.4. Time histories of IE signals along the third row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected.

361 Figure V.5. Frequency spectra of IE signals along the third row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected. Figure V.6. STFT spectrograms of IE signals along the third row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected.

362 Figure V.7. Visual image (left) and thermal image (right) at about Station 483 of the Chesapeake Channel Tunnel. Bonding conditions of tiles around the location of the thermal anomaly (warm spot) evaluated on the basis of IE measurements are color coded and superimposed on the thermal image. Joint Joint IE Results loose tiles fully bonded tiles Figure V.8. Ultrasonic echo time histories along the sixth row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected.

363 Figure V.9. Frequency spectra of ultrasonic echo signals along the sixth row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected. Figure V.10. Ultrasonic echo time histories along the third row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected.

364 Figure V.11. Frequency spectra of ultrasonic echo signals along the third row of the eight-tile by eight-tile grid at about Station 483 of the Chesapeake Channel Tunnel, where the thermal anomaly in SPACETEC data was detected. The characteristics of the ultrasonic echo signals were sim- ilar to those of the IE signals: when the tiles were loose, the time signals were less attenuated. The spectral energy in the frequency spectra on bonded tiles were centered on 50 kHz (which is about the center frequency of the transducer). The spectra obtained on presumably debonded tiles were broader, showing multiple peaks. Ultrasonic echo amplitudes were generally higher on debonded tiles. However, given the vari- ability of the pressing pressure during hand measurements, no reliable correlation between the ultrasonic echo ampli- tude and debonding condition could be concluded. The individual ultrasonic echo signals were analyzed, and their bonding conditions were color coded and super- imposed on the thermal image of Figure V.12. Similar to the IE results, the manual ultrasonic echo measurements indi- cated the presence of loose tiles where a thermal anomaly (warm spot) by SPACETEC was registered. It appears that loose tiles can be detected as thermal anomalies in SPACETEC thermal images. To further investigate this hypothesis, the thermal and visual images obtained from the SPACETEC scanner along one direction were compared with the manual hammer sounding maps provided to the research team by CBBT own- ers. An example of such comparisons obtained at about Sta- tion 475 is given in Figure V.13. Thermal anomalies (marked green) seemed to correspond well to the tiles deemed as loose (or debonded) during the hammer sounding survey. A statistical analysis was performed to establish the sensi- tivity of thermal and visual imaging to the debonding of tiles on the tunnel ceiling, as detected by hammer sounding. Both thermal and visual images were used to find anomalies in SPACETEC survey results. Data collected along one direction

365 Figure V.12. Visual image (left) and thermal image (right) at about Station 483 of the Chesapeake Channel Tunnel. Bonding conditions of tiles around the location of the thermal anomaly (warm spot) evaluated based on ultrasonic echo (US) measurements are color coded and superimposed on the thermal image. US Figure V.13. Comparison of anomalies detected in SPACETEC visual and thermal images (left) against results of manual hammer sounding survey conducted by tunnel owners (right) at about Station 475. Thermal anomalies are superimposed on the hammer sounding map as green-colored tiles, while broken tiles seen on visual images are shown as red-hatched areas. The hammer sounding data and the SPACETEC data were collected in opposite directions; to compare the data sets, the plan sheet images had to be reversed.

366 Figure V.14. Sensitivity of SPACETEC thermal and visual imaging to debonded tiles as detected by manual hammer sounding. Sensitivity is calculated for defect areas of various sizes (tile count). (southbound) was used for this analysis. The sensitivity was calculated according to Equation V.1: ( )= +Sensitivity (V.1)TP TP FN where TP and FN stand for true positives and false negatives, respectively. Given the nature of the available data, only the sensitivity could be estimated here. The hammer sounding results were assumed to give the true number and location of debonded tiles. That means anomalies detected by SPACETEC, where no delaminated tiles were marked, were considered false alarms. Sensitivity was calculated separately for defect groups of various sizes (tile counts) as shown in Figure V.14. The overall sensitivity (independent of the defect size) was obtained as 0.71, or 71%. Ninety-seven percent of areas including more than 50 tiles could be detected, compared with 55% for areas covering less than 50 tiles. A visual comparison of thermal and visual anomalies versus the delaminated tiles is provided in Figures V.15 through V.37. In these figures, thermal anomalies are superimposed on hammer sounding maps at various loca- tions along the tunnel. The hammer sounding maps were flipped because the hammer sounding data and the thermal anomaly data were collected in opposite directions. An additional analysis was performed to investigate why some of the debonded areas were not detected in SPACETEC data. Very small debonded areas covering less than 20 tiles seem not to be always detectable in thermal images obtained during this particular survey. Reflection of light from the sur- face of tiles (at certain scanning angles) and the interference with the temperature gradient in front of the air vents were found to be the top two factors why larger debonded areas were not detected. This analysis suggests that a combination of thermal and visual imaging offers a reliable alternative to the tedious practice of hammer sounding on individual tiles. The great advantage of such scanning operations becomes obvious considering the speed of the SPACETEC survey (about 1 h at 1.5 km/h, or 1 mph) compared with that of hammer sounding (one man-month).

367 Figure V.15. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 47000 and 47223. Figure V.16. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 47223 and 47457.

368 Figure V.17. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 47457 and 47707. Figure V.18. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 47707 and 47928.

369 Figure V.20. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 48163 and 48401. Figure V.19. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 47928 and 48163.

370 Figure V.21. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 48401 and 48628. Figure V.22. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 48628 and 48853.

371 Figure V.24. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 49075 and 49285. Figure V.23. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 48853 and 49075.

372 Figure V.25. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 49285 and 49523. Figure V.26. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 49523 and 49764.

373 Figure V.27. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 49764 and 49982. Figure V.28. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 49982 and 50220.

374 Figure V.29. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 50220 and 50460. Figure V.30. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 50460 and 50686.

375 Figure V.31. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 50686 and 50921. Figure V.32. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 50921 and 51157.

376 Figure V.33. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 51157 and 51390. Figure V.34. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 51390 and 51624.

377 Figure V.35. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 51624 and 51849. Figure V.36. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 51849 and 52084.

378 Figure V.37. Visual comparison of thermal anomalies and delaminated tiles (as detected by hammer sounding) between Stations 52084 and 52248.5.