Embedded diffuse ultrasonic sensing for early-age concrete hydration monitoring in real bridge structures

Reliable in-situ monitoring of early-age concrete hydration is important for intelligent bridge construction and quality control. This study proposes a wavelet packet decomposition (WPD)-based multi-frequency diffuse ultrasonic method for monitoring of early-age concrete hydration. Embedded smart aggregate (SA) sensors are used to transmit and receive ultrasonic waves. The received signals are decomposed into multiple reconstructed sub-band signals, from which frequency-dependent transport parameters are estimated to characterize hydration evolution. The method is validated through continuous monitoring of a reinforced concrete (RC) bridge during the first 72 h after casting. The extracted transport parameters closely track hydration evolution and show higher sensitivity than conventional ultrasonic pulse velocity (UPV). The influence of hydration-induced temperature variation is also investigated, demonstrating the robustness of the proposed method under field conditions. The successful bridge implementation confirms the feasibility of combining embedded diffuse ultrasonic sensing with multi-frequency analysis for in-situ monitoring of early-age concrete and intelligent construction.