Q.: From time to time I see references to "half cell" readings as a measure of corrosion activity in concrete. Often there is computerized equipment for recording and analyzing the data, and it looks like a "black box" to me. How does this system really work?

A.: To understand these half cell devices, it's important to remember that the corrosion of reinforcing steel in concrete is an electrochemical action. Both chemical processes and a flow of electricity are involved. The difference in electrical potential at various points on the rebar generates a flow of current from one point to another, forming an electric cell, also called a galvanic cell.

Detecting and measuring this current flow helps investigators assess the degree of unseen corrosion. Locations for readings are marked on the concrete, usually in a grid pattern. Then the negative terminal of a direct current voltmeter is electrically connected to the reinforcing steel mat. The operator moves a copper-copper sulfate half cell connected to the positive terminal of the voltmeter from point to point on the grid. The voltmeter readings at the grid points are recorded in millivolts.

For readings of 200 to 300 millivolts or higher, depending on the type of structure, investigators are concerned about hidden corrosion activity and conduct other tests to confirm the diagnosis. ASTM C 876, Standard Test Method for Half Cell Potentials of Reinforcing Steel in Concrete, sets rules for testing and indicates probability of corrosion for different levels of voltage readings.