Corrosion of reinforcing steel is an electrochemical process, and the behaviour of the steel can be characterised by measuring its half cell potential. The greater the potential, the higher the risk that corrosion is taking place.
An electrode forms one-half of the cell and the reinforcing steel in the concrete the other.
CSI test to ASTM standard C876-09: Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete.