Case studies of ultrasonic full waveform inversion for inspecting concrete structures

The non-destructive evaluation of reinforced pre-stressed concrete structures is critical to prevent catastrophic failures of aging infrastructure. Conventional non-destructive inspection methods often rely on manual and subjective interpretation of the complex data. Full waveform inversion is a high-resolution imaging technique that exploits accurate numerical simulations of wave propagation in a digital twin of the investigated specimen to iteratively refine the fit between simulated and measured data. The method yields quantitative maps of the distribution of elastic parameters. The simulations thereby account for the physics of elastic wave propagation with all its complexities, such as multiple scattering, heterogeneity, different wave types, and anelastic effects, and turns the raw data into easily interpretable images. We use a fully automated calibration procedure to determine the velocity of the initial model of full waveform inversion and the source pulse of the ultrasonic inspection device, enabling the application of full waveform inversion with minimal user input and no prior information. Furthermore, our massively parallel GPU implementation of 3D elastic full waveform inversion enables the rapid computation of detailed inspection images within minutes. Here, we report on two case studies of applying full waveform inversion to concrete non-destructive testing data recorded with a commercially available ultrasonic inspection device (Screening Eagle Pundit PD-8050): (1) The evaluation of tendon duct grouting quality in a reinforced test block, showing that full waveform inversion enables one to distinguish grouted from ungrouted tendon ducts in reinforced concrete. (2) The imaging of a retaining wall where location of counterforts and rock anchor heads were unknown. The full waveform inversion images clearly show the location of reinforcement rebar that are difficult to detect with conventional techniques.