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.

Introduction

In recent years, concrete-filled steel tube (CFST) columns have been widely employed in high-rise buildings and longspan bridges due to their advanced mechanical behaviors such as higher load-carrying capacity and more ductility under dynamic excitation, compared with normal reinforced concrete (RC) structures. However, various types of defects, such as cracks, air-void, honeycomb and debonding between concrete reinforcement in concrete structures and CFST members impose a huge threat to the structural safety [1]. For example, the interface debonding between concrete core and steel tube of CFST member affects the mechanical behavior of CFST members due to its negative effect on the confinement of steel tube on concrete core and finally decrease the load-carrying capacity and ductility. Therefore, the detection for concrete core defect and interface debonding between concrete core and steel tube of CFST is an emergent concern in civil engineering and represents a more challenging task due to the inaccessibility of concrete core covered by steel tube and the electromagnetic shielding effect of steel tube. Ultrasonic technology has been widely used in nondestructive testing (NDT) and evaluation of concrete structures in civil engineering owing to its high resolution [1,2]. In addition to achievement in ultrasonic testing hardware development, significant progresses have been made in ultrasonic data processing and interpretation for defect detection [4]. Traditional Ascan analysis methods, such as time-frequency analysis [5,6], wavelet analysis [7,8], time-of-flight (TOF) analysis [9] and etc., can be used to analyze signal characteristics and qualitatively detect defects in concrete structures. Xu et al. proposed a piezoceramic lead zirconate titanate (PZT) based active interface debonding detection approach for CFST members and verified the performance of the approach experimentally, but it is still a challenging task to detect the location and dimension of the interface debonding [7,10]. Most of these A-scan data analysis methods can detect the existence of defects in concrete structures but the quantitative evaluation is still a challenging task. From B-scan or C-scan data, ultrasonic imaging techniques can characterize the inaccessible defects embedded in concrete structures.