A half-cell potential survey is one of the cheapest and most revealing tests you can run on a reinforced concrete bridge. You don't break out concrete, you don't wait weeks for laboratory results, and a two-person team can map a deck in a day. What you do need is a trustworthy connection to the steel, a sound reference electrode, and the discipline to take readings on a grid rather than wherever the concrete happens to look worst.
What the measurement actually tells you
The test records the electrical potential difference between the reinforcement and a reference electrode resting on the concrete surface. Damp concrete conducts ions well enough for a high-impedance voltmeter to sense the electrochemical condition of the steel beneath. Areas where the steel is actively corroding behave anodically and read more negative than areas where it remains passive.
The crucial caveat: half-cell potentials indicate where corrosion is likely to be active, not how much section has been lost. A bridge with visible rust staining and delamination can read passive once the concrete has dried out, and a freshly repaired patch will read quite differently from the surrounding parent concrete. Treat the survey as a screening tool that tells you where to drill, break out and test further.
Preparation, safety and continuity checks
Start with the drawings. You want the reinforcement layout, cover, previous repair locations, waterproofing membranes, coatings, any cathodic protection system, and whether prestressing tendons or post-tensioning ducts sit nearby. Where a cathodic protection system is installed, it must be isolated before you take a single reading.
Safety comes next: traffic management and permits, working at height, a suitable mobile platform with harness points, and a firm rule about not drilling into ducts or tensioned steel. Exposing a bar to make the connection is the highest-risk activity of the day, so plan it properly.
Then confirm electrical continuity, which is the single most common reason a survey produces nonsense. The meter reads the whole connected steel network, so a discontinuous cage makes the results meaningless. Expose a bar at one location, clean it back to bright metal, and measure the resistance to a second exposed bar some distance away. You want a fraction of an ohm. If continuity fails, divide the structure into zones and treat each one separately.
Typical kit for the day:
- Copper/copper sulphate reference electrode, plus a spare; silver/silver chloride is often preferred for saturated or chloride-rich environments
- High-impedance voltmeter, with an input impedance of at least 10 MΩ
- Leads on reels, crocodile clips, and a file or wire brush for cleaning the bar
- Angle grinder or hammer drill, dust extraction and eye protection
- Spray bottle or damp sponge, and clean water for wetting the surface
- Cover meter, chalk or spray paint, tape measure, thermometer and a notebook or data logger
Setting out the grid and taking readings
Grid spacing drives both cost and resolution. Around 1 m is usually adequate for general mapping of decks and beams; tighten to 0.5 m or less over suspect zones, around movement joints, at kerb lines and where the cover meter shows low cover. Align the grid with prominent features so the same points can be found next time.
Wet the concrete before reading. Dry concrete has a high surface resistance that slows the meter and can push readings in a falsely noble direction. A light spray or a pass with a damp sponge is enough; you are not trying to flood the surface.
- Confirm continuity, and record the connection point and its condition.
- Dampen the surface at the first grid point and press the electrode firmly onto clean concrete, avoiding coatings, membranes, aggregate or loose material.
- Hold until the reading stabilises, usually a few seconds, then record the value and its sign to the nearest 5 or 10 mV.
- Note the surface temperature, the moisture condition, and any point where the reading drifts or refuses to settle.
- Move to the next point, re-wetting as required. Return to a fixed datum point every half hour or so to catch a drifting connection or a failing electrode.
- On large decks, a wheeled electrode array with a data logger speeds things up considerably, but stop and repeat any anomaly with a stationary electrode before you trust it.
Reading the numbers
The classic interpretation criteria, applied to uncoated carbon steel in atmospherically exposed concrete and measured against a copper/copper sulphate electrode, are as follows:
- More positive than −200 mV: a high probability, conventionally quoted as around 90%, that the steel in that area is passive
- −200 mV to −350 mV: uncertain; corrosion may or may not be active
- More negative than −350 mV: a high probability that corrosion is active
Thresholds are a starting point, not a verdict. Look at gradients as much as absolute values: a shift of 100 to 150 mV between adjacent points often marks a localised anode, which is exactly what you want to find. Very negative values, well below −500 mV, can indicate oxygen starvation under a membrane or in saturated concrete rather than rapid corrosion. Plot the results as a contour map over the reinforcement layout; the pattern will tell you far more than a list of numbers.
Pitfalls that skew results
- Skipping the continuity check, or losing the connection mid-survey without noticing
- Reading over repair patches, membranes, asphalt or coatings, where thresholds simply do not apply
- Working on dry concrete and recording falsely positive potentials
- Applying carbon steel thresholds to epoxy-coated, galvanised or stainless bars, or to prestressing steel
- Comparing surveys taken in different seasons without noting temperature and moisture
- Confusing corrosion activity with section loss, which only breaking out can confirm
Reporting and following up
A useful report presents the contour map with the grid, test locations, the reference electrode used, thresholds applied, and the conditions on the day. Pair the potentials with cover depth, carbonation depth, chloride profiles and a sounding survey for delamination; together these distinguish chloride-driven corrosion from carbonation-induced corrosion, and they point to the right repair strategy. Where active areas appear, target them with drilled samples or local breakout rather than treating the whole structure uniformly.
Making the survey count
The real value comes from repetition. A single set of readings is a snapshot; a second survey a year or two later, using the same grid and similar conditions, shows whether anodes are growing or stabilising. Mark your grid points durably, photograph the connection locations, and keep the raw data. Interpretation carries real consequences for repair budgets and programme, so have the results reviewed by an engineer experienced in concrete durability, and take specialist advice before designing a repair based on potentials alone.
Photo: Hans / Pixabay
