The method suits older buildings, coastal structures, car parks and structures showing rust stains or spalled concrete, helping to locate high-risk areas before the damage spreads. It is one type of non-destructive structural evaluation (NDE). This article explains the measuring principle, the site procedure, how to interpret readings against the ASTM C876 criteria, and the limitations you should know before using the results to make repair decisions.

How Does Half-Cell Potential Work?

Half-Cell Potential measures the potential difference between the reinforcing steel and a reference electrode placed on the concrete surface. When the reinforcement corrodes, an electrochemical reaction takes place, so the potential of the steel in the corroding area (anode) differs from that of the sound area (cathode). The measurement uses a reference electrode, such as copper/copper sulfate (Cu/CuSO₄), placed on a pre-wetted concrete surface, with the other lead connected to the reinforcement.

A voltmeter reads the potential in millivolts (mV). Strongly negative values indicate a high tendency for corrosion in that area. Readings from many points are then plotted as a potential map to show the distribution of high-risk areas.

Half-Cell Potential measuring principle per ASTM C876: a voltmeter is connected between the reinforcement and a Cu/CuSO4 reference electrode on the concrete surface The left image is a cross-section of a concrete slab with longitudinal reinforcement. The concrete is opened at one point on the right end to connect a lead to the reinforcement. Another lead runs to a copper/copper sulfate reference electrode touching the concrete surface through a damp sponge, with a high-impedance voltmeter between the two leads. The middle-left part of the bar is a corrosion spot, or anode, surrounded by rings of equipotential lines. With the reference electrode above the corrosion spot the reading is minus 350 millivolts, while above sound steel it is about minus 150 millivolts. The right image details the reference electrode: a copper rod immersed in saturated copper sulfate solution with excess crystals remaining, with a porous plug and damp sponge at the bottom end. The bottom bar shows the steps: connect the lead to the reinforcement, wet the surface, and move the reference electrode across a grid to produce a potential map. Half-Cell Potential principle (ASTM C876) Not to scale Corrosion (anode) Sound steel −350 mV V Lead to rebar Ref. electrode Cu/CuSO₄ Damp sponge Moved here ≈ −150 mV Red dashes = equipotential lines around corrosion More negative = higher chance steel below is corroding Ref. electrode detail Copper rod CuSO₄ solution saturated Crystals left = still saturated Porous plug Damp sponge 1 Lead to rebar Open one spot 2 Wet surface first Moisture sensitive 3 Move probe on grid Potential map
Measuring principle: a voltmeter measures the potential difference between the reinforcement and a Cu/CuSO₄ reference electrode placed on the concrete surface — areas where the steel is corroding give more negative potentials than sound areas.

In practice, readings are taken on a grid covering the suspect area. The images below show the site measurement as a 2D model (side section) and a 3D model — from the lead connection to the reinforcement and the reference electrode moved point by point, to the relationship between the readings and the area where the steel is corroding.

2D model (side section) of Half-Cell Potential measurement on a grid over a concrete slab Section through a reinforced concrete slab. A lead is connected to the reinforcement at a point where the concrete has been broken out; another lead connects to a copper/copper sulfate reference electrode touching the pre-wetted concrete surface, via a voltmeter. The reference electrode is moved point by point along the grid, and each reading is shown as a label above its point. Readings are most negative above the area where the steel is corroding beneath a crack, through which water and chlorides reach the steel, and gradually become less negative further away. Label colors follow the ASTM C876 criteria. Side section (2D) Not to scale Cl⁻ Cl⁻ Cl⁻ −120 −150 −170 −210 −260 −330 −390 −410 −320 −230 −180 −440 mV Rebar connection point Local concrete break-out Cu/CuSO₄ ref. electrode Moved grid point by point Voltmeter Reads in millivolts Wet surface first Sound (cathode) Less negative Corroding steel (anode) Strongly negative Crack: water & chlorides reach the steel Cover ASTM C876 criteria (vs CSE): Above −200 mV · low −200 to −350 · uncertain Below −350 mV · high
2D model (side section) — connect a lead to the reinforcement at one point, then move the Cu/CuSO₄ reference electrode point by point across the grid on the pre-wetted surface. The most negative readings lie above the corroding steel, with a steep change (gradient) around that area. Colors follow the ASTM C876 criteria. (Values are illustrative examples, not actual test results · Image: SPN Soil Engineering)
3D model of Half-Cell Potential measurement on a grid over a concrete slab, with a cutaway showing the reinforcement A concrete slab with grid lines on its surface; each grid intersection shows its potential as a color according to the ASTM C876 criteria. The front-right corner of the slab is cut away to reveal the reinforcement mesh. The steel beneath a crack is rusted and lies under a cluster of red, strongly negative points. A copper/copper sulfate reference electrode stands on a pre-wetted grid point, wired to a voltmeter whose other lead connects to the reinforcement through a spot where the concrete has been broken out. Detail box D shows the internal construction of the reference electrode. Not to scale −410 Detail D — Cu/CuSO₄ ref. electrode Copper rod (Cu) Saturated CuSO₄ solution Excess CuSO₄ crystals Porous plug Damp sponge on face Voltmeter Point readings (mV) Rebar connection point Local concrete break-out Cu/CuSO₄ ref. electrode On a wetted grid point Surface crack Water & chloride path Most negative over the corroding zone Cutaway: rusted steel under red points Rebar mesh Sound area Grid colors (ASTM C876): Above −200 mV −200 to −350 mV Below −350 mV
3D model — potentials are measured point by point on a grid over the concrete surface. Red points cluster above the corroding steel beneath the crack (the cutaway corner reveals the rebar mesh). Box D shows the Cu/CuSO₄ reference electrode, with a copper rod immersed in saturated copper sulfate solution. (Values are illustrative · Image: SPN Soil Engineering)

What Are the Site Steps for Half-Cell Potential Measurement?

Site measurement involves six steps. The steps that most affect reliability are connecting the lead to the reinforcement and wetting the surface.

  1. Locate the reinforcement — use a Cover Meter / Rebar Scanner to find the bar positions and cover, so you can choose the break-out point and lay out the grid
  2. Break out the concrete at one point and connect the lead to the reinforcement — clean the bar to bright metal before clamping the lead, and check that the steel in the test area is electrically continuous
  3. Mark the measurement grid on the surface — e.g. every 0.5–1.0 m, depending on member size and the resolution required
  4. Wet the concrete surface — to complete the measuring circuit and obtain stable readings
  5. Place the reference electrode at each grid point and record the mV value — wait for the reading to stabilize before recording each point
  6. Produce the potential map and interpret it — divide it into color bands by the criteria, then choose points to open up and confirm the actual condition of the steel

How Do You Interpret Half-Cell Potential Readings?

Potentials are interpreted as ranges of corrosion probability. ASTM C876 gives the following reference criteria when a copper/copper sulfate reference electrode (Cu/CuSO₄, or CSE) is used:

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Potential (mV, vs CSE)Probability of corrosion
More positive than −200Low probability of corrosion (less than 10%)
−200 to −350Uncertain
More negative than −350High probability of corrosion (greater than 90%)

The key point is that Half-Cell Potential indicates the "probability" that corrosion is occurring; it does not indicate the "corrosion rate" or the amount of steel lost. Interpretation should also consider the distribution pattern of readings (gradient) rather than relying on a single number, and if another type of reference electrode is used, the readings must be converted to a CSE basis before comparing them with the criteria in the table.

💡 Look at the distribution, not a single number

Areas where the potential changes steeply (high gradient) usually indicate the extent of corrosion better than any single reading. Measurements should therefore be taken on a grid and plotted as a potential map to assess the overall picture.

Example Half-Cell Potential map from grid measurements, with contour boundaries and a section showing the location of corroded steel The upper-left image is an example potential map on a concrete slab, measured on a grid of 8 columns by 4 rows, in millivolts versus a CSE reference electrode. Values around the edges are about minus 120 to minus 190, shown as green cells. Toward the middle of the slab the values become more negative, shown as yellow cells, and the middle of the third row has values of minus 360, minus 420 and minus 380, shown as red cells. An orange dashed line along the cell edges marks the minus 200 boundary, and a solid red line marks the minus 350 boundary. Areas where values change steeply, or high gradient, indicate the extent of corrosion. The lower-left image is section A–A through the third row: the reinforcement under the strongly negative cluster is rusted, with a crack from the surface, while the steel under the green cells is still sound. The right image shows the ASTM C876 criteria: more positive than minus 200, probability of corrosion less than 10 percent; minus 200 to minus 350, uncertain; more negative than minus 350, probability greater than 90 percent; and a reminder that the result is a probability, not a corrosion rate, and critical points must be confirmed by opening up. The bottom bar notes: measure on a grid, for example every 0.5 to 1.0 meters; wet the surface before measuring; and interpret together with chloride, carbonation and concrete electrical resistivity. Potential map: read pattern & gradient, not one number Not to scale Example potential map (mV vs CSE · illustrative) −120 −130 −140 −150 −170 −160 −140 −130 −130 −150 −180 −240 −290 −260 −180 −150 −140 −170 −230 −360 −420 −380 −230 −170 −130 −150 −190 −260 −310 −280 −190 −150 A A Section A–A Rusted steel under strongly negative cluster · sound under green −200 edge −350 edge High gradient = corrosion limit ASTM C876 criteria Above −200 mV Low corrosion risk (< 10%) −200 to −350 mV Uncertain Below −350 mV High corrosion risk (> 90%) Result is a “probability” not a corrosion rate or steel loss Moisture sensitive Coatings / paint interfere Critical spots → open up Confirm actual steel before repair decisions 1 Measure on a grid e.g. every 0.5–1.0 m 2 Wet surface first Standard method 3 Interpret with Chloride · carbonation · resistivity
Potential map from grid measurements — reading the distribution pattern and gradient locates the extent of corrosion better than relying on a single value. Contour lines on the map divide the readings according to the ASTM C876 criteria, and section A–A shows rusted steel beneath the strongly negative cluster.

When Is Half-Cell Potential Used?

Half-Cell Potential is used when the reinforcement is suspected to be rusting but the full extent of the damage cannot be seen from the surface. Common cases include:

  • Buildings or structures with rust stains running down the surface, or spalled concrete exposing the steel
  • Coastal structures or structures exposed to salt spray, which are at risk of chloride ingress
  • Car parks, roof decks, bridges and areas that are regularly wet and dry
  • Condition assessment of older buildings before repair or renovation

Which Tests Should Be Used Together with Half-Cell?

Half-Cell shows "where the risk is" but not the cause. A complete corrosion assessment therefore combines several methods, each answering a different question.

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TestWhat it answersTest type
Half-Cell PotentialWhere the steel is likely to be corrodingNon-destructive; surface opened only at the connection point
Carbonation TestHas the loss of alkalinity reached the steel?Semi-destructive; needs a freshly broken surface
Chloride Content TestHow much chloride has accumulated at the steel level?Semi-destructive; concrete powder collected for analysis
Cover MeterWhere is the steel, and how much cover remains?Non-destructive
Opening up (break-out inspection)Is the steel actually rusted, and how much section remains?Locally destructive; used to confirm results

In addition, concrete electrical resistivity may also be measured to help assess the severity of corrosion.

What Are the Limitations of Half-Cell Potential?

The main limitations are that readings are sensitive to the condition of the concrete, and the result only indicates probability — it does not confirm how much steel has been lost. Factors to watch for include:

  • Concrete moisture — very dry or high-resistivity concrete distorts the readings, so the surface must be wetted per the standard method
  • Coatings or paint — non-conductive coatings interfere with the measurement; the surface must be opened to the concrete at each measurement point
  • Electrically discontinuous reinforcement — readings will not represent the steel across the whole area; continuity must be checked before measuring
  • No rate or amount — it does not tell you how fast the steel is corroding or how much section has been lost

At critical points, the concrete should therefore be opened up (break-out inspection) to confirm the actual condition of the steel, and an engineer should assess the results together with other data.

⚠️ The result is a "probability", not a verdict

Half-Cell Potential is good at locating high-risk areas, but it does not tell you how much section the steel has lost. Confirming critical points by opening up, and an assessment by a licensed engineer, are therefore essential before making repair decisions.

SPN provides Half-Cell Potential measurement with potential maps and reports certified by a civil engineer licensed by the Council of Engineers Thailand (COE), complete with signature and license number.