Evaluating changes in fundamental cross-sectional mode of vibrations using a modified time domain for impact echo data

The impact echo (IE) method is a truly non-destructive test and can be very useful for structural health monitoring of concrete structures. Results from the present study provide an improved understanding the data from this method. Experiments were carried out using a cement mortar block which was subjected to uniform compression and the changes in the IE response in terms of the fundamental frequency of cross-sectional mode of vibrations were studied at different levels of applied load. A combination of the continuous wavelet transform (CWT) and fast Fourier transform (FFT) was used to study changes in the fundamental frequency. The results obtained from the modified method are compared with the results obtained from the traditional analysis, where only the FFT is applied using the entire waveform. It is found that the former has a clear potential of being more useful for a better understanding of the peak load that a structural element has been subjected to (in the past) compared to the latter. From the limited experiments carried out, it was found that in the former case, a change in the dominant frequency was observed at a much lower level of applied load (about 30% of failure) whereas in the latter such changes were seen at a much higher (about 70–80% of failure) load level.