A Wireless, Passive Embedded Sensor for Real-Time Monitoring of Water Content in Civil Engineering Materials

A wireless, passive embedded sensor was applied for real-time monitoring of water content in civil engineering materials such as sands, subgrade soils, and concrete materials. The sensor, which comprised of a planar inductor-capacitor (LC) circuit, was embedded in test samples so that the internal water content of the samples could be remotely measured with a loop antenna by tracking the changes in the sensor's resonant frequency. Since the dielectric constant of water was much higher compared with that of the test samples, the presence of water in the samples increased the capacitance of the LC circuit (capacitance of the capacitor was proportional to the dielectric constant of the medium between its electrodes), thus decreasing the sensor's resonant frequency. Using the described sensor, a study was conducted to investigate the drying rate of sand samples of different grain sizes. A study was also conducted to measure the curing rate of a portland cement concrete slab during casting, and its drying rate after it has been soaked in water. The described sensor technology can be applied for long-term monitoring of localized water content inside soils and sands to understand the environmental health in these media. In addition, this sensor will be useful for monitoring water content within concrete supports and road pavements. The measurement of water content is important for civil engineering infrastructure since excess water may hasten their degradation.