Ultrasonic Imaging of Concrete by Synthetic Aperture Focusing Technique Based on Hilbert-Huang Transform of Time Domain Data

摘要

For non-destructive evaluation (NDE) of concrete, efficient and accurate imaging techniques are needed for a reliable evaluation of safety and serviceability of structures. Imaging of concrete structures is considered a very challenging task since it is a non-homogeneous material. In this paper, the acoustic (stress wave) technique was applied by using Ultrasonics and Synthetic Aperture Focusing Technique (SAFT). In ultrasonic NDE, the presence of coarse aggregates requires that the ultrasonic testing be conducted at relatively low frequencies in order to avoid excessive attenuation. The Hilbert-Huang Transform (HHT), based on Ensemble Empirical Mode Decomposition (EEMD), was used as a signal processing tool for interpreting and analyzing the ultrasonic waveforms taken from a concrete specimen with an embedded steel rod. SAFT images for both processed and unprocessed signals were obtained and compared. Results showed that the reflections from the bottom surface as well as the steel rod were successfully imaged when the processed signal were used. Therefore, anomaly or defect detection in concrete structures through ultrasonic imaging was greatly improved by the combination of HHT and SAFT.