The impact elastic wave method (IEW) has been applied to evaluate the thickness and internal defects of the target structure based on the dominant frequency of the response wave that is formed by the repeated reflections in the thickness direction. However, it is difficult to evaluate the size and position of the defect by IEW if the size and depth are relatively small and deep, respectively, and further, it is known that the technique is inapplicable if the target is not a plate-like structures. Therefore, the authors propose a new technique that uses Difference value as a new evaluation index to overcome the limitations of the conventional methods. Difference value shows the change of the response waveform in the time domain; it is computed by using a response waveform of the structures in sound condition as a reference. In this paper, the practicality of the Difference value is investigated by performing experiments using concrete specimens. The results of the experiments demonstrate that Difference value changes by the influence of internal defects, and Difference value evaluates the location of the relatively small defect that is difficult to evaluate by the conventional technique.

In the soundness evaluation method based on waveform analysis in time domain, the response waveforms that are measured while the target is in sound condition is indispensable. Therefore, it is expected that this method is for the evaluation of precast concrete products because the state of the products is almost identical and it is able to measure the response waveforms before using the products. On the other hand, although there are difficulties to apply this method for the evaluation of the existing structures because the response waveform is not measured before using, it is expected that this method can be applied to the evaluation of the existing structures by making the response waveform on the basis of the numerical simulation. The computational conditions should be set properly based on the actual measurement conditions for the numerical simulation. Among the computational conditions, the input frequency is determined based on the diameter of the steel ball and contact time if the excitation is given by making an impact with the steel ball by using the theory proposed by Sansalone et al. However, Kubo et al. reported that the measured contact time is different from the theoretical value and the difference reaches maximum twice. The influence of the difference would not be negligible for the soundness evaluation and should be considered carefully to perform the soundness evaluation accurately. Therefore, in this study, the response waveforms are computed on a numerical model with excitations of various input frequencies, and the influence of the input frequency to the response waveform is discussed by comparing the computed response waveforms.