Optimisation of transducer positioning in air-coupled ultrasound inspection of concrete/asphalt structures

Air-coupled ultrasound (ACU) is the novel emerging technique in non-destructive testing that allows removal of the coupling requirement, which significantly increases inspection speed. However, specifics of ACU caused by high impedance mismatch between air/solid interfaces set a number of limitations one of them being a requirement of two transducers in pitch-catch mode instead of a traditional single transducer in echo-mode. The goal of this paper is optimization of an ACU system geometry configuration in a specific task of single-side inspection of concrete/asphalt road pavements on the presence of subsurface delaminations. The findings provide the baseline for specification the general system setup guidelines for ACU system development under FP7 RPB HealTec project. 3D finite element modelling (FEM) is used for investigation of the impact of transducer positioning on the system´s efficiency in detection of subsurface delaminations of varying depth and type. The simulation results indicated strong impact of transducer positioning on the measured response from test slabs with and without delaminations together with the significant impact of even small surface defects. Consequently, it was concluded that while ACU is sufficiently effective detection of various types of delaminations, it is highly sensitive to the surface condition and system setup.