What is GPR concrete scanning and when do you need it

GPR concrete scanning is the use of ground penetrating radar to image the inside of a concrete structure without cutting, coring, or otherwise damaging it. If you have a question about what is inside a slab, wall, beam, or column, where the rebar is, whether there are post-tension cables, where the conduits run, whether there is a void, GPR concrete scanning is usually the first tool that should come out of the van.

What it actually does

A handheld antenna is rolled across the surface of the concrete in a controlled grid. As it moves, it sends short electromagnetic pulses into the structure. Anything with different electrical properties from the surrounding concrete, steel reinforcement, water, plastic conduit, voids of air, reflects part of the signal back. The system records the time it takes for each reflection to return, and software builds a depth-indicative map of detectable reflectors.

In practice, this means the surveyor walks away with a layered picture of the slab: top reinforcement, bottom reinforcement, embedded conduits, likely post-tension cable positions where detectable, and any anomalies that don’t match the design. They mark the targets directly on the slab with paint or chalk, and the data is later turned into an annotated PDF report and a CAD-ready DXF or DWG plan.

When you need it

The short answer is: any time you are about to put a hole through structural concrete and you do not have a complete and reliable as-built record of what is in there. The longer answer covers a number of common situations.

Before drilling, coring, or cutting. Pre-drill scanning is the most common reason GPR is called in. You cannot rely on original drawings, they are often missing, incomplete, or out of date. A struck cable, a damaged conduit, or a severed post-tension tendon can cost far more than the survey that would have prevented it.

Before structural alterations. When a wall is being formed in a slab, or a beam is being trimmed, or a slab is being cut for a stairwell or shaft, the engineer needs to know what is inside the structure they are about to alter. GPR provides layout and indicative depth data for the engineer to work from.

When post-tension cables may be present. Post-tension construction is common in modern UK commercial and residential buildings. Damaging a tendon is one of the most expensive mistakes you can make on a slab. GPR is the standard non-destructive way to identify likely PT tendon positions where detectable and mark them before any penetration.

When drawings are missing. Older buildings, refurbishments, and unusual structures often arrive at the design table without a usable set of drawings. GPR gives you a record of the reinforcement and embedded items it can detect.

When you need a void or slab investigation. GPR can locate voids, honeycombing, and changes in slab thickness. If a defect is suspected, GPR is a quick way to map it without drilling exploratory cores.

What it does not do

GPR is not magic. It does not give you the chemistry of the concrete (NDT testing does that), it does not give you a definitive answer on bar diameter (ferro scanning is more useful for that), and it does not see through thick reinforcement easily, once you get below a dense top mat, signal can attenuate.

A good GPR programme uses the right tool for the question. On dense reinforcement, GPR is paired with ferro scanning. Where strength is in question, GPR is paired with NDT. Where deeper subsurface features are involved, lower-frequency GPR systems do the work.

What good looks like

A defensible GPR concrete scanning deliverable includes:

• A method statement and risk assessment supplied before attendance.
• Calibration record for the equipment used on the day.
• On-slab markings in survey paint or chalk for every target identified.
• An annotated PDF report with depths and layout.
• A DXF or DWG file you can open in your design software.
• The name and signature of the surveyor who did the work, on the report.

If the deliverable lacks any of these, ask for it. None of them are optional on engineering-grade work.

How to get a survey

For most jobs, a single email or phone call with the location, the access window, and the question being asked is enough to get a same-day call-back. Larger or more complex jobs may need a brief site visit before quotation. UK-wide, expect a Corvus surveyor on site within days of the booking, and the written report within 5 working days.

What this means for your job

See GPR concrete scanning for what we scan and how.