Half-cell potential testing and corrosion mapping (ASTM C876)

How the test works

We make a connection to the rebar and confirm the bars are electrically continuous. Then we take readings with a copper/copper sulfate reference electrode on a measured grid, with the spacing chosen for the element, often 1 to 2 ft. The surface is pre-wetted so the readings are stable.

The readings are plotted as a contour map over your drawings, so the areas at risk can be located on site and carried straight into repair documents.

Reading the results

ASTM C876 gives a numeric guide for readings against a copper/copper sulfate electrode:

The numbers are a guide, not a verdict. Moisture, oxygen, concrete cover and coatings all shift the readings, so we also read the map for gradients, where potentials drop sharply over a short distance, and confirm with chloride and carbonation results and a few exploratory openings.

Where half-cell mapping is used

• Pool shells, tested from inside the drained pool. See pool shell corrosion assessment.
• Balconies, slab edges and parking decks. See balcony and garage corrosion testing.
• Columns, beams and girders where cracks or rust stains have appeared.
• Cathodic protection projects, to set baseline potentials before a system is energized. See cathodic protection.

Limits worth knowing

Half-cell mapping shows the probability that corrosion is active, not how much steel has been lost or how fast it is corroding. It is less reliable on epoxy-coated or galvanized bars, through coatings and membranes, and on very dry or saturated concrete, and it needs continuous reinforcement. That is why it is one part of a corrosion condition assessment, not a stand-alone answer.