Exploring the potential of null subtraction imaging for detecting sub-rebar regions in reinforced concrete structures at nuclear power plants

Abstract

While tomography-based non-destructive testing (NDT) techniques have advanced in identifying inclusions in reinforced concrete (RC) structures, no existing method fully visualises sub-rebar objects with high accuracy. This study introduces the null subtraction imaging (NSI) technique, a novel nonlinear ultrasound beamforming algorithm, to identify sub-rebar objects located beneath rebar configurations. To evaluate the effectiveness of NSI, ultrasonic imaging was performed on an RC block that simulates a radioactive waste storage unit in a nuclear power plant (NPP). Compared to the traditional total focusing method (TFM), NSI improved the contrast-to-noise ratio (CNR) by approximately 48%, significantly suppressing artefact and background noise in the lateral direction. This capability enabled the clear identification of sub-rebar objects (drums) and precise differentiation of the interface between rebar layers and drums. The proposed NSI method also demonstrated high compatibility with other imaging modalities, such as ground penetrating radar (GPR), providing significantly enhanced image contrast and resolution of the internal structure of the RC block. These results highlight the feasibility of the NSI technique for assessing the health conditions of RC structures, particularly in managing critical components like hazardous chemical or radioactive waste storage units in NPPs.