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 f 0 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 f 0 , 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. • NRUS on core slices enables depth-resolved measurement of hysteretic nonlinearity α . • Depth profiles of α correlate with microcrack density observed by SEM. • Moisture and post-fire curing significantly affect the nonlinear response. • Curing increases α , indicating incomplete recovery of nonlinear elasticity. • NRUS remains sensitive to fire damage even after prolonged curing.