Time frequency domain analysis on shotcrete bonding state using IE-SASW method

Impact Echo combine with Spectral Analysis of Surface wave (IE-SASW) method is a combination of two nondestructive testing method that has been widely used nowadays. This method is used to evaluate the integrity of concrete structure as well as for the construction quality control with a good accuracy without any structural damages. Impact Echo (IE) was used for detailed nondestructive evaluation whereas Spectral Analysis of Surface Wave (SASW) method was employed for the measurement of the representative P-wave velocity and for the evaluation of concrete quality. In this study, IE-SASW signals were applied to the shotcrete which has been considered the curing time for weathered soil in Malaysia. In experimental study using a slab model, effects of different bonding condition were examined to study the advantage in determining the variation of P-wave velocity for the whole thickness. The IE-SASW nondestructive test was applied to the shotcrete to investigate the bonding state with an advanced signal-processing technique known as Short-time Fourier Transform (STFT). The effect of the bonding state on IE-STFT signal characteristics was obtained through mathematical and experimental studies. This paper represents the analysis of time-frequency domain signals from laboratory studies through simulation of void,debonded and fully bonded conditions applied on the shotcrete. All conditions are considering the curing time and size of impact signals to compare the different thickness of void with the different bonding condition. The effect of bonding state in void condition is that the stress wave is reflected more from the interface between the shotcrete and soil layers due to the gradual increase in the impedance mismatch. Consequently, more entrapped air appear in a certain period in the shotcrete layer, implying that the main frequency components become higher and that the resonance duration becomes smaller. In conclusion, the combination of this technique provide more reliable data interpretation.