Connecting to Concrete: Wireless Monitoring of Chloride ions in Concrete structures

For the first time, chloride ions are measured wirelessly in concrete. The half-cell potential of a silver/silver chloride (Ag/AgCl) electrode, which corresponds to the concentration of chloride ions, is measured wirelessly. Wireless communication is achieved by an inductive coupling (reflected-impedance) between the coils of the sensor and the readout. The resonance frequency of the sensor coil changes due to the change in the capacitance of a varactor element connected to the Ag/AgCl electrode. The capacitance of the varactor increases with the increase in chloride ion concentration (20 pF/decade). Furthermore, the capacitance can be reliability measured up to a distance of 35 mm between sensor and readout coil. No battery is needed to power the sensor terminal, which makes it feasible to embed it in concrete structures. (C) 2015 The Authors. Published by Elsevier Ltd.