A waveform generated by a small impact hammer and recorded by single transducer is used to assess different kinds of anomalies in concrete. Using Short-Time Fourier Transform and the reassignment technique, the group slowness spectrogram can be calculated from the waveform. The surface wave response in the diagram of velocity vs. wavelength corresponding to the reassigned spectrogram is used for defect evaluation. Experiments were conduct on a concrete plate specimen containing multiple delaminated cracks, and a honeycomb. The group velocity profiles for test lines passing through different kinds of defects were obtained and compared. It was found the surface wave velocity begins to decrease with the increase of wavelength when the wavelength is about two times of the depth of the defect.
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