A quick method to construct 3-D velocity contour map for assessing anomalies in concrete.

• A new procedure is proposed to construct a 3-D contour of A0 modal group velocity. • Test line is performed with a tap of steel ball and a sensor away from the source. • The speed decreases at a wavelength about 2 times the defect's depth. • Defects exist in the low-velocity area but may not be the exact positions. In this study, a quick way to assess the location of anomalies in concrete is presented. The test of one test line is done with one tap of the steel ball and the receiver far from the signal source. The spectrogram of the recorded waveform is obtained by Short-Time Fourier transform (STFT) and reassigned method. Then the dispersion curve of A0 modal group velocity is extracted from the spectrogram. This research proposes a fast and systematic procedure, which starts from setting the layout of the test line to constructing the 3-D map of the A0 modal group velocity. Observing the dispersion curve with test line passing only single defect, the loss of wave speed usually begins at wavelengths approximately 2 times the defect depth. For delaminated crack, the larger and shallower the defect, the more reduction of the wave velocity is found under the described wavelength. Since the grid lines may pass through multiple defects, the low-velocity area in the 3-D velocity image may indicate possible defects in its vicinity instead of precise depth and location. Because the current method reduces the consumption of manpower and time, resources can be placed on potential defect areas to conduct detailed investigations with conventional C-scan methods. [ABSTRACT FROM AUTHOR]