Resonant Sensors for Detecting Corrosion in Concrete Bridges

A passive sensor platform was developed to detect the onset of corrosion in concrete bridge decks. The term “passive” is used to describe the sensors because they do not include onboard processing capabilities or sources of power. The sensors are powered and interrogated in a wireless manner by measuring the impedance through an external reader coil, which is magnetically coupled to resonant circuits within the sensor. The sensors are designed to be embedded in a concrete bridge during construction and interrogated periodically over its service life. Reinforced concrete slabs were subjected to long-term exposure tests to demonstrate the reliability of the prototype sensors. The middle region of each slab was exposed to alternating moisture cycles with saltwater. Sensors in this region indicated that threshold amounts of corrosion occurred, whereas sensors located in regions that were not exposed to saltwater indicated the absence of corrosion. At the conclusion of the tests, the sensor readings were confirmed by removing the concrete cover and examining the condition of the embedded reinforcement. One flaw in the initial design of the sensors, however, was that the transducing element (a corroding steel wire) passed through the epoxy potting that protected the circuit components. This configuration provided a pathway for contaminants to infiltrate the sensors, causing corrosion. To improve the durability of the passive sensors, a new configuration is being developed in which the corroding element is physically isolated from the resonant circuits within the sensor.