What is ferro scanning in construction

Ferro scanning is the use of electromagnetic induction to detect steel reinforcement inside concrete. It is one of the workhorses of non-destructive surveying on UK construction. If you have ever seen someone walking a small handheld device methodically over a concrete wall and writing depth values in chalk, you have seen a ferro scan.

How it works

Ferro scanning relies on electromagnetic induction. The probe contains a coil that generates an alternating magnetic field. When that field interacts with a piece of ferrous metal, a reinforcement bar, it induces a small response in the bar that the probe can detect. The closer the bar, the stronger the response.

By moving the probe in a controlled pattern across the surface, the system maps the position of bars within its working range. Modern equipment also estimates cover depth and, with calibration, gives a useful indication of bar diameter.

The depth range is shorter than ground penetrating radar, up to about 200 mm for locating bars with the Hilti PS 300, with accuracy falling as depth increases, but within that range, ferro scanning is fast and high-resolution. Results are subject to site conditions including bar spacing, multiple layers and cover depth.

What it tells you

A ferro scan answers three questions, well:

1. Where are the bars? A bar position map at the density at which the surveyor scanned.
2. How deep are they? Cover-depth measurement, within the instrument’s stated tolerance.
3. What size are they? Estimated bar diameter, where conditions allow.

What a ferro scan does not tell you is whether anything non-ferrous is in the concrete. It does not see plastic conduit or voids, and it cannot tell post-tension cables apart from ordinary bars, which often sit deeper than its range anyway. For those, GPR is the right tool. In practice, ferro is often run alongside GPR on the same job, GPR for the depth and the non-ferrous targets, ferro for the cover and diameter detail.

When you need it

• Cover depth checks. Codes and specifications require minimum cover for fire, durability, and corrosion protection. Ferro scanning is the usual way to check cover non-destructively.
• Bar diameter estimation. When drawings are unclear or unavailable, ferro scanning gives a working estimate of bar diameter without coring.
• Reinforcement mapping. For walls, slabs, and beams where the bars are within the working depth, ferro produces a dense map quickly.
• Defect investigation. When a defect is observed and an engineer wants to know what reinforcement is involved, ferro is often the quickest way to characterise the steel near the defect.
• Small-job efficiency. Ferro can be quicker to set up and scan with than GPR. On a small or simple job, it is often the more efficient option.

What good ferro scanning looks like

A defensible ferro scanning deliverable includes:

• Calibration record from the start of the session.
• Bar position plan with depths annotated.
• Cover depth statistics, minimum, mean, percentage below code minimum.
• Bar diameter estimate (where the equipment supports it).
• Photographs of the marked-up element.
• The surveyor’s name and signature on the report.

For engineering work, for example, structural assessment of an existing element, a defensible ferro report should also tabulate the cover-depth measurements and let the engineer see the distribution, not just a summary number.

Where it falls short

Ferro scanning has limits. The biggest are:

• Depth. Beyond about 200 mm the equipment runs out of range. For bottom mats on thick slabs, GPR is usually the practical option.
• Dense top mat. A dense top mat shadows the lower steel from the ferro probe. The probe sees the upper bars and reads them, but the lower mat is effectively invisible.
• Non-ferrous targets. Anything not made of steel is not detectable. Plastic conduits and voids will not show, and post-tension tendons cannot be told apart from ordinary bars.
• Edge effects. Close to corners and edges, the probe’s response changes. Surveyors handle this with technique and calibration, but it is a real limitation.

How it fits into a survey programme

On a typical engagement Corvus runs ferro alongside GPR. GPR covers the layered map, likely post-tension tendon positions, and the void check. Ferro adds cover and estimated diameter data on the same areas. The combined deliverable is one report and one CAD plan for the engineer.

For pure pre-drill work, GPR is usually enough on its own. For structural assessment, retrofit design, or compliance checks, the cover and diameter detail from ferro scanning is often the deciding factor in whether the assessment converges. Used together, they take much of the guesswork out of in-situ reinforced-concrete work.

What this means for your job

For cover, bar spacing and bar size on site, see ferro scanning.