Nonlinear resonant ultrasound spectroscopy for profiling thermal damage gradients and monitoring post-fire recovery in concrete

Prolonged exposure to elevated temperatures induces depth-dependent microstructural changes in concrete. This article investigates the application of nonlinear resonant ultrasound spectroscopy (NRUS) to assess depth-dependent thermal damage and post-fire recovery in concrete and mortar. Specimens were exposed to two heating scenarios and sliced into 2-cm thick sections to evaluate the nonlinear hysteretic parameter α as a function of depth. Measurements were performed under various moisture conditions and water curing periods, including ambient, dry and after 36 days of curing. Results of the nonlinear hysteretic parameter α and resonance frequency f₀ reveal a depth-dependent damage gradient, with the most pronounced degradation occurring within the first 2 cm beneath the exposed surface, as confirmed by scanning electron microscopy. Post-fire curing lead to an increase in f₀, consistent with the post-fire curing recovery process, but also to a counter-intuitive increase in α, which we explain possibly by the fact that the recovery mechanisms fill the microcracks but do not restore the contacts. Overall, NRUS allows to distinguish between exposed and non-exposed materials even after post-fire recovery, and to evidence persistent damage. These findings highlight NRUS as a sensitive, semi-destructive method for tracking thermal damage gradients and recovery in cement-based materials.