Recognition of rebars and cracks based on impact-echo phase analysis

•The phase spectrum of the impact echo test can be used to differentiate crack echo from rebar echo.•The phases of crack echoes are nearly 0, while the phases of rebar echoes are nearly π/2.•Echo phase = π/4 can be used as a decision line to judge the type of inclusion.

The impact-echo test is often used to detect 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 through the impact-echo equation. Although the impact echo spectrum may reveal the existence of an interface in the structure, it cannot tell whether the interface results from a crack or rebar. Such information is crucial in the structural safety assessment and to determine interface depth because the impact-echo equations for cracks and rebar differ.This research proposes using the phase spectrum to differentiate crack echoes from rebar echoes. Both the amplitude and slope of the phase spectrum at the echo frequency are studied. Fourier analysis of the simplified signals shows that the phases of crack echoes are nearly 0, while those of rebar echoes are nearly π/2. Numerical and model tests were conducted to verify the results. As expected, the phases can be successfully divided into two groups. The phases of crack echoes are less than π/4 and cluster around 0, while the phases of rebar echoes exceed π/4 and cluster around π/2. Unfortunately, the slope of the phase spectrum at the echo frequency cannot serve as an index for the classification of inclusion type.Hence, it is suggested that both the magnitude and phase spectra be constructed in the Fourier analysis of the impact echo signals. Use the magnitude spectrum to determine the echo frequency and use the phase spectrum to determine the phase at the echo frequency. Then, use π/4 as a threshold to judge the type of inclusion. As such, one can obtain the correct type and depth of the inclusion.