Coherent and incoherent Rayleigh wave attenuation for discriminating microstructural effects of thermal damage from moisture conditions in concrete

Thermal damage results in a depth-dependent variation in concrete mechanical properties, which is commonly investigated by invasive sampling and destructive testing. Ultrasonic Rayleigh wave (R-wave) testing offers a non-destructive alternative for mapping mild to moderate thermal damage in cementitious materials. However, the coupled influence of damage and moisture gradients on the ultrasonic measurements is not known, while it is known that rehydration causes partial recovery of mechanical properties. In this study, we prepare concrete and mortar blocks subjected to two distinct heating scenarios: a short-duration high-temperature exposure following the ISO fire standards, and a lower-intensity longer heating using radiant panels. We then use P-wave refraction, R-wave phase velocity and attenuation and diffuse field analysis to test the exposed blocks first at ambient conditions and later after water saturation, in order to decouple the influence of water saturation from that of thermal damage. The ultrasonic results are interpreted in relation to other non-destructive measurements: resistivity and capacitance. We find that although R-wave phase velocity is indeed sensitive to thermal damage, it is also significantly affected by the effect of moisture as it can hardly distinguish between exposed and intact surfaces after post-fire re-curing. In contrast, coherent R-wave attenuation effectively discriminates a damaged surface. The distinct analysis of coherent and incoherent attenuation enables to separate the impacts of thermal damage and moisture content. Overall, this article highlights the potential of attenuation for assessing damage under unknown moisture conditions.