The Impact-Echo (IE) method, which is a kind of non-destructive testing method for concrete structures, has been applied to detect the flaws and the thickness of concrete structures. FFT based frequency domain analysis is generally used in IE test for thin structures. 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, and each components appear dominantly at different times. In the FFT based frequency domain analysis, sometimes, it is difficult and not straightforward to extract the information on the target echo. Usually surface wave component has very large amplitude and is frequently the main cause of error.
Time-frequency analysis is more advanced signal processing method than FFT analysis method. Time-frequency analysis can show time and frequency information of a signal simultaneously while FFT based frequency domain analysis looses time information of each frequency components. Time-frequency analysis can provide the temporal variation in frequency components of the signal and it can be used for eliminate surface wave or noise component. So this analysis method has advantages in the analysis of IE signal.
In this study, 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 slabs in which various types of defects or boundaries were included at known locations. Test results obtained by time-frequency analysis are compared to the conventional IE results and the advantages of using the proposed method are verified. Parameters affecting IE results also have been evaluated. And finally, proposed analysis method was applied in field structures and applicability was also verified.
