The practical application of three different processing methods for impact-echo test data—the classical impact-echo method, the peak frequency map, and the 4D spectrum technique—is performed and analyzed in this study. For comparative evaluation, a concrete slab with simulated delaminations and voids was cast and employed. Two conventional methods, the impact-echo and peak frequency map, are used to determine the relative performance of the 4D spectrum technique, in which the impact-echo data are presented as 4D maps representing x, y, the frequency, and the frequency amplitude for the tested area. For verification, the 4D spectrum technique and the other two methods were applied to three artificial defects at the depth of rebar in the designated concrete slab. The results obtained from comparing the three processing methods show that although three methods are comparable, the 4D spectrum technique is more effective and accurate for the detection of shallow defects in the slab.
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