Fringing Field Impedance Sensor for Hydration Monitoring and Setting Time Determination of Concrete Material

This paper deals with the development of a simple, low-cost, fringing field impedance sensor suitable for early age hydration monitoring and setting time determination of hydraulic cement paste. An impedance technique using a double-sided copper-cladded printed circuit board substrate as a sensor is proposed. Different batches of sensors with different probe sizes and substrate thickness have been fabricated and tested for hydraulic cement paste. The cement samples having different water-to-cement ratios (w:c) by weight are considered to study the effectiveness of the sensor. The purpose of this study is to develop a simple yet effective measurement system to determine the change in impedance of the cement paste with hydration time and thus to correlate the impedance change to determine the critical initial and final setting times. Initial and final hydration times of different cement samples are compared with the results of the commercial Vicat apparatus (VA). The values of the setting times closely match the Vicat values (the average value at 100 kHz differs by 4 m). Therefore, the proposed sensor-based system can be used in place of conventional VA to determine the hydration condition. The output of the sensor can be electronically processed further to develop a prototype system.