Evidence of Transient Thermo-Hygro-Mechanical Stress Effects in Ultrasonic Measurements from Embedded Piezoelectric Sensors in Concrete

Long-term ultrasonic monitoring of concrete requires separating damage-related variations from envi ronmental effects. This issue is investigated within the ODOB-US collaboration, which aims to monitor Alkali-Silica Reaction using embedded piezoelectric transducers. Measurements performed at different depths in a metric-scale plain concrete block (1.80H x 1.85L x 1.00W m3) reveal ultrasonic time-of-flight variations that respond to surface temperature changes much faster than expected from thermal diffu sion. Strain measurements exhibit a similarly rapid response, suggesting a thermo-mechanical coupling involving transient stress redistribution and acoustoelastic effects. A complementary experiment on a 300 mm concrete cube in a climatic chamber confirmed a strong sensitivity of ultrasonic measurements to temperature variations. However, the response differs from that observed in the metric-scale block. These results highlight the need to characterize environmental and ultrasonic measurements coupling to develop correction approaches able to isolate damage-related ultrasonic signatures.