Ground penetrating radar (GPR) has become the go-to tool for scanning concrete before cutting, coring and drilling, and for investigating structures where drawings are missing. It's fast, non-destructive and very effective. But it isn't X-ray vision, and knowing what it can and can't show is the key to getting a reliable result.

How GPR works

A GPR scanner sends short pulses of high-frequency radio waves into the concrete. When a pulse meets a change in material, such as steel, plastic, air or water, part of the signal is reflected back to the scanner. By measuring the strength of each reflection and how long it takes to return, the system builds an image of what lies beneath the surface.

A single pass produces a cross-section of the concrete along that line. By scanning a grid, 3D GPR joins these sections together into a three-dimensional picture that can be viewed as depth slices, making layouts much easier to understand and mark out accurately on site.

What GPR can see

In the right conditions, GPR can reliably locate:

• Steel reinforcement, including its layout, spacing and approximate depth
• Post-tension tendons and ducts, so they can be avoided before cutting or coring
• Electrical conduits and embedded services, both metallic and, in many cases, plastic
• Voids and areas of poor compaction, within slabs and beneath them
• Slab thickness, where there's a clear reflection from the underside
• Embedded steelwork, such as steel beams encased in concrete
• Structural form, such as block and beam floors, ribbed slabs and changes in construction

What GPR can't do (or can't do well)

GPR has limits, and a good survey allows for them:

• Bar diameter. GPR is very good at finding bars but poor at sizing them. We use an electromagnetic cover meter alongside GPR where bar sizes are needed.
• Seeing past dense reinforcement. A tight top mesh can shadow the signal, making deeper bars, tendons or services harder to see. Scanning from both faces, where access allows, helps.
• Wet or chloride-contaminated concrete. Moisture and salts absorb the radar signal and reduce how deep GPR can see.
• Seeing through metal. Steel decking in composite slabs, metal sheeting or foil-backed finishes block the signal completely. Scanning from the soffit is impossible through a metal deck.
• Exact depths without calibration. Depth readings depend on the properties of the concrete, so for accurate cover and depth figures the scan should be calibrated against a known depth or a cover meter reading.
• Condition of the steel. GPR shows where reinforcement is, not whether it is corroding. For that, a corrosion survey with half-cell potential mapping, carbonation and chloride testing is needed.
• Whether a service is live. GPR can locate a conduit or cable, but it can't tell whether it's live. Always treat services as live.

Getting the most from a GPR survey

The best results come from pairing the right technique with the right question. GPR is ideal for locating reinforcement, tendons, services and voids quickly across large areas. Combined with cover meter testing for bar sizes, and with targeted break-outs where absolute certainty is needed, it gives engineers and contractors reliable information without unnecessary damage.

A good surveyor will also be clear about uncertainty, marking areas where the data is less reliable rather than presenting every scan as definitive.

GPR concrete scanning in London and across the UK

ACUO Structural Insights provides GPR and 3D concrete scanning, post-tension cable detection and void detection for contractors, engineers and property owners across London, Kent, Essex and the South East.

For full details on what the service covers, visit our GPR concrete scanning service page. If you're specifically planning to core or cut a post-tensioned slab, see our article on scanning before coring post-tensioned slabs. For utility mapping beneath ground, see our PAS 128 utility survey service.

Need a slab scanned before cutting or coring, or a structure investigated? Call 07340 549892 or email [email protected].