Reverse time migration of acoustic waves for imaging based defects detection for concrete and CFST structures

Ultrasonic non-destructive testing (NDT) technology has been widely used for defect inspection of concrete structures in civil engineering. However, most of the current data processing methods can only provide qualitative information regarding the existence of concrete inner defects. In this study, an ultrasonic inner defects inspection approach with a high-resolution imaging method which combines travel time tomography (TTT) and reverse time migration (RTM) is proposed for concrete and concrete-filled steel tube (CFST) columns. TTT estimates a reasonable distribution of ultrasonic velocity over the cross-section of the concrete and CFST columns from the first arrival time of the ultrasonic transmission signal. The velocity distribution is used as an input of the initial model for RTM to image the defects inside the concrete and CFST column cross-sections with a high resolution. Numerical experiments demonstrate that the air cavity inside the concrete and CFST columns, and the debonding between the concrete core and the steel tube of the CFST column can be identified clearly, and that the location, size and shape of both defects can be determined accurately. It is concluded that the proposed defect detection approach with a high-resolution imaging method is efficient for the non-destructive inspection of concrete and CFST structures using ultrasonic waves.