# Half-Cell Potential Testing for Rebar Corrosion Assessment in Concrete


> A practical guide to ASTM C876 half-cell potential surveys for locating active corrosion in reinforced concrete before it becomes visible.


*Structural Diagnostics — July 1, 2026 — 8 min read*

By the time rust staining, cracking, or spalling is visible on a concrete surface, corrosion of the embedded reinforcement has usually been active for years. Half-cell potential testing, standardized under ASTM C876, gives inspectors a way to detect and map active corrosion activity while it is still hidden — turning a purely reactive maintenance program into one that can target repairs before deterioration becomes structural.

## Electrochemical Principle

Corroding steel in concrete behaves like a battery: at anodic sites, iron oxidizes and releases electrons; at cathodic sites elsewhere on the same rebar, oxygen is reduced, consuming those electrons. This sets up a small but measurable electrical potential difference between the reinforcement and the concrete surface directly above it.

A half-cell — most commonly a copper/copper-sulfate electrode (CSE) — placed in contact with a wetted concrete surface and connected to the rebar via a high-impedance voltmeter measures this potential. More negative readings indicate a higher statistical probability of active corrosion at that location; more positive (less negative) readings indicate a lower probability. The measurement is a probability indicator, not a direct corrosion-rate reading — that distinction matters when interpreting results.

![Bundled rusty steel rebar for construction](https://images.unsplash.com/photo-1763771420303-0f11ccf613d1?q=80&w=1200&auto=format&fit=crop)

*Corrosion of reinforcing steel — the electrochemical process half-cell potential testing detects. — Photo: [Zoshua Colah](https://unsplash.com/@zoshuacolah)*

## Equipment & Field Procedure

A half-cell survey needs three things: the reference electrode itself, a high-impedance voltmeter (to avoid drawing current that would disturb the measurement), and electrical continuity to the reinforcement, established by exposing and connecting to a rebar at one accessible point.

The concrete surface is pre-wetted to ensure adequate electrolytic contact — dry concrete gives unreliable, erratic readings — and survey points are marked out on a grid, typically 2 ft (0.6 m) spacing for routine bridge deck surveys per ASTM C876, tighter where higher resolution is needed. At each grid point the electrode is placed on the wetted surface and the potential difference to the rebar connection is logged, building up a potential map across the full surveyed area.

## Interpreting the Potential Map

ASTM C876 provides probability-of-corrosion thresholds against the copper/copper-sulfate electrode scale that most agencies use as a starting interpretation framework, understanding these are statistical guidelines rather than certainties for any single point.

| Potential vs. CSE | Corrosion Probability |
| --- | --- |
| More positive than −200 mV | Low (>90% probability no corrosion) |
| −200 mV to −350 mV | Uncertain / intermediate |
| More negative than −350 mV | High (>90% probability active corrosion) |

*Thresholds per ASTM C876 using a saturated copper/copper-sulfate reference electrode; different electrode types require converted thresholds.*

What makes half-cell testing valuable in practice is less the absolute reading at any one point and more the pattern across the surveyed area: contiguous zones of strongly negative readings — an "anodic zone" — reliably flag where active corrosion is concentrated, even on decks that show no surface distress yet. Equipotential contour maps built from the grid data are the standard deliverable, letting an owner see corrosion extent at a glance and prioritize where to open up concrete for confirmation and repair.

**Corrosion Assessment Method — Relative Suitability for Routine Bridge Deck Surveys**

- Half-Cell Potential (ASTM C876): 90%
- Concrete Resistivity: 75%
- Visual Inspection Alone: 45%
- Cover Meter (Rebar Depth/Layout): 60%

## Half-Cell in Context: Complementary Methods

Half-cell potential is rarely used alone in a thorough condition assessment. Concrete resistivity testing measures how easily corrosion current can flow through the concrete — low resistivity (often from high moisture or chloride content) accelerates corrosion rate even where potential readings are borderline — so the two techniques together distinguish "corrosion is occurring" (potential) from "how fast it is likely progressing" (resistivity). Chloride content testing on extracted powder samples, meanwhile, confirms whether the chloride threshold for corrosion initiation has actually been exceeded at the rebar depth, closing the loop between cause (chloride ingress) and effect (measured potential).

## Limitations

Half-cell potential surveys have well-documented limitations that a competent inspection program accounts for. Surface coatings, membranes, or very dry concrete can produce unreliable or unreadable results. Epoxy-coated reinforcement disrupts the electrochemical continuity the method depends on, requiring modified interpretation. And because the technique reports probability rather than corrosion rate, it cannot by itself predict remaining service life — it identifies where to look, not how much time remains.

**Key Takeaways**

- Half-cell potential testing (ASTM C876) detects the electrochemical signature of active steel corrosion before rust staining or cracking becomes visible at the surface.
- Readings are a statistical probability of corrosion, not a direct corrosion rate — more negative versus a copper/copper-sulfate reference indicates higher probability of active corrosion.
- The standard deliverable is an equipotential contour map built from a grid survey, which visually identifies anodic (high-corrosion-probability) zones across a deck or structure.
- Half-cell results are most reliable when paired with concrete resistivity and chloride-content testing, which together explain both whether corrosion is active and how quickly it is likely progressing.
- Surface coatings, epoxy-coated rebar, and dry concrete are known limitations that require the survey protocol to be adapted or the results interpreted with added caution.

**Q: Does a negative half-cell reading always mean the rebar is corroding right now?**
No — ASTM C876 explicitly frames the readings as statistical probabilities across a population of measurements, not certainties for any individual point. Very negative readings correlate strongly with active corrosion but should be confirmed with a second method or physical inspection before major repair decisions.

**Q: Can half-cell testing be used on epoxy-coated rebar?**
It can, but the coating disrupts normal electrochemical behavior at the bar surface, so readings need to be interpreted with modified criteria and typically alongside other diagnostic methods rather than relied on alone.

**Q: Why does the concrete need to be wetted before testing?**
The half-cell circuit depends on ionic conduction through the concrete pore structure. Dry concrete has much higher electrical resistance, which produces unstable, unreliable potential readings — pre-wetting establishes consistent electrolytic contact across the survey area.

1. ASTM C876-15, Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete, ASTM International.
2. ACI 222R, Protection of Metals in Concrete Against Corrosion, American Concrete Institute.
3. FHWA-RD-99-147, Corrosion Evaluation of Epoxy-Coated Rebar in Virginia Bridge Decks, Federal Highway Administration.


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**Author:** Retrofit Engineering Editorial Team — Structural Inspection & NDT Division


Source: https://retrofit-engineering.com/blog/half-cell-potential-corrosion-testing