Impact-echo is a nondestructive testing method used to detect internal defects, primarily delamination, in concrete elements. The amplitude spectrum is commonly used to determine the thickness frequency that gives information of the element thickness or is an indication of the presence of a defect or anomaly. However, the amplitude spectrum is usually contaminated with multiple modes, and the localization of the thickness frequency is not straightforward. Furthermore, preprocessing is required to remove the unwanted wave components, and to eliminate or reduce noise. The phase spectrum of the impact echo signal contains information about the dispersion properties. Recently, these properties were used to determine the dominant response frequency, which corresponds to the zero-group velocity mode (S1-ZGV). This study examines the use of the phase spectrum of impact-echo signals, recorded from multiple air-coupled sensors, to locate internal defects in concrete elements. The phase spectra of these signals are stacked together to obtain a 2D frequency-sensor offset profile. This profile shows a constant phase at the thickness mode frequency. The performance of the method is successfully examined on decks with shallow and deep delamination, and without delamination. Furthermore, an algorithm is developed to automatically locate the frequencies corresponding to a constant phase, making the proposed method suitable for automated detection and characterization of delamination.

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