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Cracking increases the vulnerability of concrete structures to aggressive substances in environments. In this study, water absorption of mechanical cracked cement mortar has been monitored using acoustic emission (AE). The AE waves from the cracking and the wetting were both recorded and analysed using wavelet transform. Two analytical methods, ie wavelet energy and empirical mode decomposition (EMD), were employed to determine the entropy value of the acoustic signals. The results show that the signal of water absorption denoised by a 6-level Daubechies wavelet of order 5 (db5) exhibits lower amplitude and attenuates in a much longer duration compared to that of cracking. The time-frequency distribution, using Continuous Wavelet Transform (CWT), indicates that the maximum magnitude of the waves from water absorption is distributed in frequency of 20 ~ 70 kHz as compared to 80~160 kHz for cracking . The power spectral density of AE waves from water absorption is much less than that of cracking due to the different nature of AE sources. The entropy values by empirical mode decomposition (EMD) and wavelet for water absorption are about 0.51 ± 0.05 and 1.44 ± 0.11 of that of cracking, respectively. The distinct characteristics of the waves could be taken to identify the acoustic sources.
Chen et al. (Mon,) studied this question.