Time-frequency analysis for impact Echo-SASW (IE-SASW) method

Abstract

IE-SASW method which combines two nondestructive testing techniques can be applied to detect the flaws and the thickness of concrete structures. In this method, IE method is used for the detailed nondestructive evaluation of concrete structure and Spectral Analysis of Surface Waves (SASW) method is employed for the measurement of the average P-wave velocity of whole thickness. The signal received by the transducer in the IE test consists in a sum of the direct surface wave and side echoes, the bottom or defect echo (target echo), and the background noise. In the FFT based frequency domain analysis, sometimes, it is difficult and not straightforward to extract the information on the target echo. The time-frequency analysis can provide the temporal variation in frequency components of the signal and thereby has advantages in the analysis of IE signal. In this paper, time-frequency analysis based on short-time Fourier transform is presented to improve the interpretation of IE signal. Experimental studies were performed in the concrete model slab in which poor concrete was included at known location. Test results obtained by time-frequency analysis are compared to the conventional IE results and the advantages of using the proposed method were verified.