Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
205 a p p e N D I x O Introduction This appendix describes the progress of a particular non- destructive testing (NDT) technique known as acoustic sounding and outlines how this system will work within the framework of the second Strategic Highway Research Pro- gram (SHRP 2) Renewal Project R06G. This system requires further development to be efficiently implemented for tile debonding in tunnel linings. But research thus far has shown it to be a promising technique capable of quickly determining the stage of tile debonding in tunnel linings. This appendix discusses how the system will be used in inspection procedures and provides an idea of what the end product will be. Evaluations of public tunnels and test specimens have been conducted and the preliminary results are given. acoustic Sounding technique When debonding occurs on tiled surfaces, hammer sounding by ear or by microphone can readily differentiate bonded from debonded tile. Debonded areas have a characteristic lower-frequency pinging relative to fully bonded tiles. The goal here is to devise a less subjective method for inspectors to quickly and efficiently characterize the condition of tile bonding. Technical Needs In general, tile debonding can occur for two reasons: improper installation or external influences. Improper installation com- monly includes the following: ⢠Improper use of bonding agent (e.g., the wrong mixing ratios or the wrong type of agent); ⢠Improper tile spacing; ⢠Excessive open time; and ⢠A low standard of workmanship (e.g., not âback butteringâ the tile). External influences can include environmental conditions (e.g., thermal expansion) and/or excessive tunnel lining forces (e.g., damage from voids, cracks, delamination, or debonding). In either case, debonding of the tile does occur and can pose a danger to the public. This SHRP 2 project uses many NDT techniques to identify the onset of damage behind the tiled wall lining before debonding occurs and to quickly and efficiently identify regions that need immediate attention after debonding occurs. Research Approach The system under development is used with a laptop com- puter capable of recording audio signals and installed with a version of MATLAB (developed by MathWorks, http://www .mathworks.com/products/matlab/), along with an impact source (preferably a ball-peen hammer). As the operator lightly taps the center of each tile with the hammer, the laptopâs internal microphone records the audio signal. MATLAB software performs a fast Fourier transform (FFT) to the data set and uses pattern recognition techniques to monitor the fundamental frequencies of flexural vibration for each individual tile. The modes of vibration frequencies in a voided tile can be predicted with acoustic theory for a rectangular plate with simply supported edges (Rossing and Fletcher 2003): 0.453 1 12 2 = +ï£«ï£ ï£¶ï£¸ + + ï£ï£¬       f c h m L n L mn L x y where cL is the longitudinal wave speed, h is the thickness of the tile, m and n are the integers describing the current mode of excitation (m = n = 0 for the fundamental frequency of Evaluation of Tiled Tunnel Linings by Using Acoustic Sounding
206 flexural vibration), and Lx and Ly are the respective side lengths of the tile. The vibration frequencies increase as the voided section of tile decreases (Liu et al. 2011). Therefore, it is theoretically possible to relate the fundamental frequency to the approximate area of debonding. This technique can be incorporated into a program that assigns a color scale to the frequency spectrum of a tile wall under inspection. The research team envisions that the final program will be able to operate in two modes. The first is for near-real-time inspection. In this mode, a threshold fre- quency from an expected frequency band representing sound concrete is established and used to make a pass-fail decision, telling the user whether a tile is most likely bonded or debonded. The second mode is intended for mapping a large region of tile, and the final result is a map of the tiles showing the levels of expected bond. As in the first mode, the user selects a section of tile representing a fully bonded state for the program to determine the fundamental frequencies asso- ciated with bonded sections. The user then taps each tile in a predetermined order. For instance, the section might consist of an area 13 tiles high by 40 tiles wide. The program prompts the user to select the layout desired, and after the user taps each tile in the given order, the program will output a plot showing the frequency spectrum. Field application in the Washburn tunnel A rudimentary version of this technique was used for a proof- of-concept test in the Washburn Tunnel in Houston, Texas. The Washburn Tunnel (Figure O.1) is the only underwater vehicle tunnel in operation in Texas and was completed in 1950. It car- ries a federal road beneath the Houston Ship Channel, joining two Houston suburbs. The tunnel was constructed via the immersed tube method, with sections joined together in a prepared trench, 26 m (85 ft) below the water line. The entire inner wall is tiled with 110-mm by 110-mm (4.3-in. by 4.3-in.) ceramic tiles. Like many underwater tunnels with tiled walls, this one is experi- encing debonding of tile in various areas. Three sections of tile that contained debonded regions (as determined by an inspector performing hammer sounding by ear) were cho- sen. The regions, shown on the left side of Figure O.2, show the area under consideration outlined with blue painterâs tape. The debonded section (determined by human ear) is indicated with a blue painterâs tape âxâ on the debonded section. On the right side of Figure O.2, scans made via ultrasonic tomography (UST) are shown for each of the three regions. (The ultrasonic tomography technique and its specific application to the Washburn Tunnel can be read in Appendix M) The depths of the C-scans (plan views) in Figure O.2 range from 16 mm to 103 mm (0.63 in. to 4.1 in.). One of the areas investigated (Figure O.2, top) was evaluated using a rudimentary version of the acoustic sounding technique and is shown in Figure O.3. This exam- ple demonstrates a strong correlation between hammer sounding by ear and the automated acoustic sounding technique. In Figure O.3, the bottom left plot depicts the tiles color coded in grayscale, with the higher frequencies (predicting a fully bonded state) as white and the lower frequencies (pre- dicting a debonded state) as black. As previously discussed, the lower frequencies observed should theoretically corre- spond to larger voided areas behind the tile. The bottom right plot in Figure O.3 shows the output with a pass-fail algorithm denoting tiles that fall below the expected fully bonded state (red is the expected debonded state, green is the expected fully bonded state). Figure O.1. Washburn Tunnel: exterior view (left) and interior view (right).
207 Figure O.2. Debonded regions of tile (left) paired with the associated UST C-scans (right).
208 Conclusion This automated sounding technique is still under development. Many factors influence the peak frequencies observed in the fre- quency spectrum from a single tile tap, including the size of the void, whether or not the hammer tap is directly in the center of the tile, and multiple mode interference. Preliminary results indicate that this technique, although basic in its approach, will offer the tunnel inspector a quick, efficient, inexpensive, and objective technique that provides sufficient information for repair procedures or further investigation. references Liu, S., F. Tong, B. Luk, and K. Liu. 2011. Fuzzy Pattern Recognition of Impact Acoustic Signals for Nondestructive Evaluation. Sensors and Actuators: A. Physical, Vol. 167, No. 2, pp. 588â593. Rossing, T., and N. Fletcher. 2003. Principles of Vibration and Sound, 2nd ed. Springer, New York. 2 2 4 6 8 10 12 4 6 8 10 12 1500 2000 2500 3000 Figure O.3. Debonded regions of tile (top) paired with the acoustic sounding results (bottom).
