RCD is a testing method that helps determine the susceptibility of uncoated steel reinforcement to corrosion. The service estimates the electrical potential at intervals in continuous reinforcing steel, and therefore helps quantify the rate at which steel can be expected to oxidise (rust).
A client may require RCD when they need to know the susceptibility of reinforcement to corrosion. Any load-bearing element needs to be in good condition to fulfil its role in keeping a structure supported and safe.