Influence of Embedded Sensor Structural Design on Stability of Ultrasonic Signal in Mortar Hydration Monitoring

Signal interpretation in embedded ultrasonic monitoring depends strongly on the internal transmission path of the sensing element, yet this design factor has received limited systematic attention. This work compares two triangular quartz sensors that differ only in the configuration of their epoxy region: an epoxy-notched triangular quartz (ENTQ) device and an epoxy-solid triangular quartz (ESTQ) device. Their responses were examined during 24 h mortar hydration, controlled heating and cooling, and loading-induced debonding. The notch in ENTQ exposes part of the direct-wave route to changing external media, whereas the continuous epoxy region in ESTQ provides a more uniform internal route. Across the investigated conditions, ESTQ retained a more consistent direct-wave waveform and dominant-frequency pattern, while its later-arriving components remained responsive to changes in the surrounding mortar. Energy and time-frequency features also tracked the evolution of mortars prepared at two mix proportions. The comparison indicates that separation of a stable internal reference path from an environmentally sensitive reflected-wave path can improve the interpretability of embedded ultrasonic measurements. The results support structural-path design as a useful strategy for sensor development.