ASTM C876 for the operator
What a half-cell potential survey measures on reinforced concrete, and how to read the result.
A half-cell survey measures the potential difference between the reinforcement and a copper/copper-sulphate reference electrode placed on the wetted concrete surface. The result is plotted as an isopotential map.
A high-impedance voltmeter (10 MΩ minimum) is used. The surface must be electrically continuous; a coating that insulates, or concrete that is too dry, makes the method unusable. A local connection to the bar is made for the circuit.
Older readings were often compared with the threshold values in ASTM C876-91: above −200 mV, a low probability of corrosion; below −350 mV, a high probability (Cu/CuSO4 scale). Experience has shown that those single thresholds are not reliable on their own. RILEM TC 154 advises interpreting potential gradients rather than absolute values. ASTM C876-09 is also based on isopotential maps, while still quoting the earlier thresholds.
MAREX therefore presents a map of potentials, checks electrical continuity, and confirms selected points by opening to the steel where it is justified. The survey is read together with cover, carbonation, chlorides, resistivity and, where useful, corrosion rate.
For the operator, the practical point is this: a single reading is not a diagnosis. The map, the gradients, and the laboratory results on the same structure are what support a decision, including whether sacrificial anodes or another form of cathodic protection should be specified.
The method is described on the reinforced concrete service page and in MAREX method sheet B2-4, and ASTM C876. The sheets are listed as method references and are not offered as a public download on this site at this stage.