Recognition of Rebar and Crack Based on Impact-Echo Phase Analysis
Abstract »The impact-echo test is often adopted to detect the defects or inclusions in concrete structures. Applying Fourier transform to the surface response of the target structure due to an impact, the depth of the interface can be determined by applying the impact-echo equation. Although the impact echo spectrum may disclose the existence of an interface in the structure, it cannot tell whether the interface comes from a crack or a rebar. Such information is crucial in the safety assessment of the structure. It is also necessary in the determination of interface depth, because the impact-echo equations for crack and rebar differ.
The objective of this study is to develop an index that can be used to differentiate crack echo from rebar echo. The idea is based on the phase change of the reflected wave. As a wave encounters a hard interface, it reflects with a phase change of pi, but as it encounters a soft interface, there is no phase change. Hence, it is proposed that both the magnitude and phase spectra are constructed in the Fourier analysis of the impact-echo signals. Determine the frequency of the interface echo using the magnitude spectrum. Then, use the phase at the echo frequency in the phase spectrum to determine whether the echo is from a crack or a rebar. Numerical and experimental tests were performed to verify the proposed method. As expected, the phases of crack echoes are close to 0, while the phases of rebar echoes are close to pi/2. The phase at the echo frequency in the phase spectrum appears to be an effective index in determining the type and depth of the interface in the concrete.