Detecting Rebars and Tubes Inside Concrete Slabs Using Continuous Wavelet Transform of Elastic Waves

A typical problem of elastic wave methods, such as the impact echo method, is due to peak detection based solely on amplitude spectrum. Current study aims to improve the feature identification of impact-echo signals obtained from buried objects in concrete slabs. Steel rebar, steel tubes, and PVC tubes embedded in a concrete slab are tested. Numerical simulations are carried out based on models constructed using the finite element method. The received signals, both experimental and simulated, are analyzed using both fast Fourier transform and continuous wavelet transform (CWT). The amplitude spectra can only provide global information and lose some important local effects of frequency components. This can be resolved by continuous wavelet transform for preserving the transient effects in the frequency domain. Localized spectral contents are analyzed and thus better understanding is achieved for the impulse responses due to different objects below the surface of the concrete slab. Features related to steel rebar, PVC and steel tubes are readily identified in the coefficient plot of wavelet coefficients. Multiple reflections and vibration modes related to various characteristics of wave propagation in the concrete slab can now be decomposed into distinctive frequency bands with different time durations. The result of CWT provides more information and is easier to interpret than that of the spectral analysis. The same peak frequency found in the amplitude spectrum is now distinguishable between PVC and steel tubes at a resolution of 0.1kHz or better. Such findings provide a more effective way to pick up true rebar signals using the impact-echo method.