Impact of moisture gradients on ultrasonic evaluation of concrete and mortar blocks subjected to thermal exposures

Non-destructive evaluation (NDE) of concrete has significantly advanced, providing valuable insights into material properties. While the sensitivities of various NDE methods conducted in concrete to damages and environmental conditions are well-established, the interactions between these factors - such as thermal damage gradients and moisture gradients - remain areas of active research. This study investigates the ability of ultrasonic Rayleigh waves to detect thermal damage in concrete and mortar under varying moisture conditions. Two types of heating — intense and brief ISO-standard fire exposure, and moderate, prolonged heating using radiant panels — were applied to the materials. Ultrasonic measurements, including P-wave refraction, Rayleigh wave velocity, and attenuation, were conducted before and after water saturation, and compared to electromagnetic methods such as capacitance and resistivity. Results show that Rayleigh wave velocity is influenced by both damage and moisture, making it difficult to differentiate exposed from intact surfaces after saturation. In contrast, coherent Rayleigh wave attenuation remains sensitive to thermal damage, even after long-term re-curing. The separate analysis of coherent and incoherent components allows the dissociation of moisture effects from structural degradation. These findings demonstrate the potential of attenuation measurements for non-destructive evaluation of fire-damaged materials, even when moisture conditions are unknown.