GPR concrete scanning before core drilling

GPR Concrete Scanning Before Core Drilling: A Checklist

27 October 2026

Core drilling and saw cutting are routine tasks in renovations, tenant fit-outs, MEP installations and structural retrofits. They are also among the riskiest. A single core drilled through a post-tensioned tendon, an electrical conduit or a main reinforcing bar can cause injury, power outages, costly structural repairs and project delays.

Scanning concrete with ground penetrating radar (GPR) before drilling has become standard practice on well-run sites. This checklist explains what GPR can find in concrete, how to prepare for a scan, and how to use the results safely.

What GPR Detects in Concrete

Reinforcing steel (rebar) and wire mesh
Post-tension tendons and their ducts
Electrical and data conduits, both metallic and plastic
Embedded pipes such as radiant heating lines
Voids, honeycombing and changes in slab thickness
Beams, joists and other structural elements within slabs

Concrete GPR systems typically use high-frequency antennas, often in the range of about 1.5 to 2.7 GHz, which give fine resolution near the surface. Effective depth in typical slabs is commonly around 30 to 60 centimeters, depending on the antenna, concrete condition and moisture.

GPR vs X-Ray at a Glance

Factor	GPR Scanning	X-Ray (Radiography)
Access needed	One side of the slab or wall	Both sides
Safety exclusion zone	None	Required because of radiation
Results	Immediate on site	Film or digital images after exposure
Detail	Good location and approximate depth	Very high image detail
Limitations	Dense rebar can mask deeper objects, very wet concrete reduces depth	Thickness limits, slower, more disruptive
Typical use	Routine pre-drilling and cutting	Complex or critical locations

The Pre-Drilling Checklist

Before the Scan

Mark every proposed core and cut location clearly, with some tolerance for adjustment.
Gather drawings showing structural systems, especially post-tensioned slabs.
Clear the surface of debris, equipment and loose coverings. Carpet and thin tiles can stay in some cases, but check with the provider.
Arrange access to both sides if X-ray may be needed for critical locations.
Tell the scanning provider the core diameter and depth.

During the Scan

Scan a grid around each location, not a single line, so objects running in any direction are captured.
Mark findings directly on the surface with color-coded lines: commonly one color for rebar, another for conduits, another for tendons.
Record approximate depth for each target, noting that depth estimates rely on calibrated signal velocity.
Identify a clear zone for each core and confirm it with the drilling contractor.

After the Scan

Request a short written report with photos of the markings.
Do not move a core outside the scanned area without rescanning.
For post-tensioned slabs, follow the structural engineer’s requirements, which may include minimum clearances from tendons.
Stop drilling immediately if unexpected material, such as a sheathed tendon or a conduit, appears in the core.

Project teams deciding between methods will find a clear comparison of GPR concrete scanning and radiography in the guidance published by MAYA Global Group, which explains when each method is the safer and more efficient choice.

Common Mistakes

Scanning only the exact core point. Small layout changes then fall into unscanned concrete.
Ignoring the bottom mat of rebar. Dense top reinforcement can hide what lies below. Ask the technician about confidence at depth.
Assuming drawings are accurate. Conduits are frequently relocated during construction without updated records.
Losing the markings. Foot traffic and cleaning remove paint and tape. Photograph everything immediately.

Industry Guidance

The American Concrete Institute’s report ACI 228.2R describes nondestructive test methods for evaluating concrete in structures, including radar. Many owners and general contractors also have internal policies requiring scanning before any penetration of a structural slab or wall.

Frequently Asked Questions

Can GPR tell the difference between rebar and a conduit?

Often yes, from the pattern, spacing, depth and direction of the reflection, combined with experience and drawings. Where doubt remains, the target should be avoided.

How long does a concrete scan take?

A handful of core locations can often be scanned in an hour or two. Large areas or complex slabs take longer.

Is GPR safe to use in occupied buildings?

Yes. GPR emits low-power radio signals and requires no exclusion zone, unlike X-ray.

Conclusion

A few hours of GPR scanning is a small price for avoiding a cut tendon, a severed conduit or a damaged beam. With clear preparation, a thorough grid scan, careful marking and disciplined follow-through, core drilling becomes a predictable, low-risk activity on any site.

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