Evaluation of crack depth in concrete using non-contact surface wave transmission measurement
The purpose of this study is to develop a non-contact air-coupled NDT method to identify and characterize surface-breaking cracks in concrete structures using surface wave transmission measurements. It has been found that the surface wave transmission (SWT) across a surface-breaking crack is related to the crack depth. However, inconsistence was noticed in surface wave transmission measurements. In this dissertation, the author first summarized limitations of the current SWT method for application to concrete structures, which include inconsistent sensor coupling, near-field effect of sensors, effects of crack width, external loading effect on surface wave transmission coefficient, and lack of a repeatable source. In this dissertation, the author attempts to find solutions to the aforementioned problems. First, non-contact air-coupled sensors were applied to the SWT method to reduce experimental errors caused by inconsistent coupling condition of conventional contact sensors. Air-coupled sensing enables reliable and consistent results, and significantly improves test-speed. Results from laboratory and field tests demonstrate effectiveness of air-coupled sensors. Second, appropriate sensor-to-source configurations are proposed to reduce undesirable effects: (i) the near-field effect of sensors around a crack, and (ii) contribution of multiple modes in a plate-like structure with a finite thickness. Near-scattering of surface waves interacting with a surface-breaking crack was investigated using numerical simulations (finite element method) and experimental studies over a wide range of the normalized crack depth (h/λ: crack depth normalized by wavelength of surface waves) and the normalized frequency-thickness ratio (f-H/CR: frequency-thick normalized by Rayleigh wave speed). Third, effects of external loadings on transmission coefficient of surface waves in concrete were investigated through a series of experimental studies. In the research, variation of the transmission coefficient is presented as a function of crack mouth opening displacement (CMOD). This provides a guideline on minimum CMOD to which the SWT method can be reasonably applied. In addition, the author experimentally demonstrates that using low-cost piezoceramic sensors is effective in generating consistent stress waves in concrete. Finally, the author demonstrates that the air-coupled SWT method developed in this study is effective for in-situ estimates of a surface-breaking crack in large concrete structures.