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Application of the Time-of-Flight Reflection Technique to the Thickness Measurement of Concrete Elements
The objective of this thesis is to develop new nondestructive techniques for determining the thickness of plate-like concrete structure. Unlike the method proposed in ASTM C1383 using spectral analysis, the techniques proposed in this thesis makes use of stress waves and signal analysis in time domain to get insight into the behavior of reflected P-wave, which travels into the structure and reflected from the bottom boundary. The thesis performs both numerical and experimental studies. Results obtained from the studies show that as the distance between the receiver and the impact point is longer than 2.43 times the plate thickness, the arrival of the P-wave reflected from the plate bottom surface can be identified. The first proposed technique is to combine the arrival time of the reflected P-wave with the thickness frequency obtained from the impact-echo test to measure the thickness of plate-like structures. In addition, if the distance between the impactor and the receiver is short, the stress waves propagating along surface such as P-wave, S-wave and R-wave arrive at the transducer earlier than the P-wave reflected from the bottom surface. When the R-wave arrives at the transducer it causes a relatively large downward displacement. However, the downward displacement caused by the arrival of the reflected P-wave is significant to identify in the waveform. Thus, combining the arrival time of the reflected P-wave for the case of short distance between the impactor and the receiver with that obtained from the far distance case offers the second proposed technique for determining the thickness of concrete structures. Results obtained from the experimental studies show that two proposed alternative techniques are feasible for measuring the thickness of plate-like concrete structures. The first proposed technique can be used to determine the internal P-wave speed and the accurate thickness of the concrete plate in the case of dissimilar P-wave speeds between the concrete surface and inside concrete. For non plate-like elements, using the second proposed technique based on time-domain analysis can eliminate the effect on the frequency response caused by close side boundaries of the concrete elements.
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