Half-cell potential mapping measures the electrochemical potential of embedded reinforcement against a reference electrode placed on the concrete surface. Steel passivated by the alkaline pore solution sits at a relatively noble potential; once chloride or carbonation breaks that passive film the corroding steel becomes anodic and the potential moves negative. The measurement is a voltmeter reading between the reference electrode and a direct connection to the reinforcement, so electrical continuity of the bar network has to be proved first — an isolated bar or an epoxy-coated mat makes the survey meaningless.

The familiar thresholds come from ASTM C876 and are stated against a copper/copper sulphate electrode: more positive than −200 mV means a greater than 90% probability of no corrosion, more negative than −350 mV means a greater than 90% probability of corrosion, and the band between is uncertain. Any other reference electrode shifts the whole scale by the difference in its standard potential, so a reading taken with a silver/silver chloride electrode must be shifted by −119 mV before those numbers apply. Working the other way, −350 mV CSE corresponds to about −231 mV on a saturated Ag/AgCl electrode and −273 mV on a saturated calomel electrode.

Absolute potentials are only half the story, and the current edition of ASTM C876 emphasises the point: potentials depend on moisture content, oxygen availability, cover depth, concrete resistivity and temperature, and a uniformly wet, oxygen-starved element can read very negative without active corrosion. What survives all those influences is the gradient. Map on a grid, contour the results, and look for steep local gradients and negative-going hot spots — those mark the anodes worth breaking out. A whole element sitting at a uniform −400 mV is a different finding from a −150 mV background with a −450 mV spot in it.

Treat the result as a probability of corrosion activity, never as a corrosion rate or a section loss. Corroborate with cover depth measurement, chloride and carbonation profiles, resistivity, and — for rate — linear polarisation resistance. Breaking out at the most negative gradient to look at the bar remains the only direct confirmation.

Worked example

Measured potential (mV)	-320
Reference electrode used	agagcl_sat
Offset to the CSE scale (mV)	-119
Potential vs Cu/CuSO4 (mV)	-439
ASTM C876 zone	3
-350 mV CSE threshold in this electrode (mV)	-231
-200 mV CSE threshold in this electrode (mV)	-81

A saturated Ag/AgCl electrode sits at +0.199 V vs SHE against +0.318 V for Cu/CuSO4, so readings shift by (0.199 − 0.318) x 1000 = −119 mV. A reading of −320 mV becomes −439 mV vs CSE, below the −350 mV threshold, so greater than 90% probability of corrosion. On the Ag/AgCl scale the two ASTM thresholds sit at −231 mV and −81 mV.

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