| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 103 === This thesis aims at developing a technique based on the dispersion characteristics of surface R-wave for detecting the cracks around rebars in concrete structures. The study uses the time-frequency spectral analysis to obtain the energy distribution of surface R-wave in both the time and frequency domains and to investigate how the existence of cracks around rebars affects the shape of energy distribution. The location and length of cracks surrounding rebars were considered in the study to analyze the wave propagation behavior of surface R-wave. The time-frequency spectral analysis was performed to observe the energy distribution of R-wave in both time and frequency domains. A specific frequency in the range of high energy distribution was selected to locate the arrival time of the maximum energy associated with this specific frequency and then its corresponding wave velocity can be determined. The velocity of surface wave (C_R) varies with frequencies due to the presence of rebars and the surrounding cracks and this is called dispersion. It is preferred to observe the change in wave propagation characteristic for the R-wave with relatively low frequency (long wave length) that has the sufficient energy to propagate through rebars and this can serve as a basis for detection of cracks around the rebars.
Frist, the research uses numerical analysis to study the characteristics of the wave propagation in concrete structures containing rebar without/with surrounding cracks. The numerical study is of great help in understanding the dispersion phenomenon. Subsequently, experimental study was carried out to verify the numerical results. Both numerical and experimental results show that the velocity of R-wave propagating through the crack surrounding the rebar decreases. As a result, the dispersion phenomenon showing a late arrival of R-wave with long wavelength can be observed in the time-frequency spectrum with high energy distribution from upper-left to lower-right. In contrast, when the R-wave propagates through rebar without cracks, the velocity of R-wave with low frequency increases. In such a case, the dispersion phenomenon shows an early arrival of R-wave with long wavelength and the associated time-frequency spectrum exhibits high energy distribution from lower-left to upper-right. The detection of cracks around rebar can be achieved based on the different dispersion behaviors of R-wave propagating through various materials and those dispersion phenomena can be found in the recorded displacement waveforms and their corresponding time-frequency spectra. In addition, the location and length of crack around rebar can be evaluated by the dispersion curve and the energy amplitude distribution along time axis.
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