Early-age strength development monitoring using integrated sensing elements (ISE): Complementary insights from coda wave interferometry (CWI) and wavelet analysis

This study investigates an integrated sensing element (ISE) for in-situ strength development monitoring of cementitious materials. Ultrasonic signals are actively excited and received, with the direct wave serving for self-consistency evaluation of excitation and receiver, and the coda wave used as the primary monitoring part. Cement paste with a water-to-cement ratio of 0.4 was monitored from casting to 28d, encompassing exploratory monitoring of the relative changes and strength development. Coda wave interferometry (CWI) was employed to obtain the relative velocity change ( Δv/v ) and decorrelation, complemented by the received signal energy, frequency-domain, and wavelet analysis to characterize strength evolution. Independent P-wave ultrasonic pulse velocity (UPV) measurements were used for validation. Results show that the direct wave part maintains a stable single peak spectral signature throughout the monitoring and thus provides a reliable self-evaluation index for the ISE, whereas the coda wave part exhibits systematic phase and amplitude evolution driven by hydration-induced acoustic impedance changes. The Δv/v derived from CWI tracks strength gain consistently, while the wavelet scalogram identifies stage transitions via dominant frequency ridges and contours of wavelet power, delineating the evolution from the liquid to solid transition through subsequent development.