Thick-film resistors sintered on natural stone as embedded smart aggregates for strain monitoring

Ensuring impeccable compatibility between the sensor and the monitored medium is undeniably a crucial challenge in the exploration of embedded stress-strain monitoring sensor technology. The thick-film resistor sensor, renowned for its exceptional stability and high gauge factor, has been introduced into the field of civil engineering. However, a significant mismatch exists between the elastic modulus of alumina ceramic (the conventional substrate for thick-film resistors) and that of concrete (the primary monitored medium). Coarse aggregates, most commonly natural stone, which constitute 50–70 % of concrete, have an elastic modulus close to that of the concrete matrix. Based on this observation, this study proposes an innovative approach utilizing natural stones as substrates for thick-film resistor to fabricate smart aggregates, thereby reducing the strain mismatch between the sensor and concrete. Thick-film resistors were successfully fabricated on quartz sandstone and olivine gabbro stone substrates. Various key performance parameters including sheet resistivity, stability of resistance values, temperature coefficient of resistance, and gauge factor were systematically evaluated. The natural stone-based smart aggregates were embedded into mortar specimens to extensively evaluate their compatibility with practical applications. The experimental results indicate that thick-film resistors fabricated on sandstone substrates exhibit good stability in their resistance values. Compared with those on alumina ceramic substrates, resistors on quartz sandstone substrates demonstrate lower sheet resistivity (averaging 14.07 kΩ/□ versus 20.88 kΩ/□), a higher temperature coefficient of resistance (245.40 ppm/°C compared to 65.65 ppm/°C), and an increased gauge factor (13.2 versus 10.6). However, the thick-film resistors on olivine gabbro exhibited increased sheet resistivity (approximately 30–50 kΩ/□), which was attributed to cracking of the resistor layer caused by residual stress after sintering. Despite this degradation, they exhibited an unusually high strain coefficient (up to 60). Notably, when embedding the thick-film resistor on quartz sandstone substrate into mortar, it maintains a gauge factor of 11.8, markedly higher than the 0.503 observed for a thick-film resistor on an alumina substrate. This enhancement increases sensitivity to strain variations and minimizes interfacial stress by virtue of mechanical compatibility with concrete. Such characteristics render them particularly well-suited for practical applications in structural health monitoring—such as in bridges, tunnels, and load-bearing structures—thus contributing to the enhanced durability and safety of civil engineering systems.