GPR (Ground Penetrating Radar) scanning is a non-destructive method for mapping what is inside a concrete slab before coring, cutting, or drilling. It is the standard pre-coring safety step for commercial construction and facilities work — and one of the most valuable tools available for reducing the risk of hitting reinforcement, post-tension cables, or embedded utilities.
What GPR scanning is
GPR scanning uses radar pulses transmitted into the concrete; when the signal encounters a change in material density — rebar, conduit, a void, or the bottom of the slab — it reflects back to the surface receiver and creates a subsurface image. A trained technician interprets these reflections, typically visible as hyperbolic “parabola” signatures, to map the location, depth, and orientation of embedded objects. The result is a marked slab surface or digital overlay showing where embedded objects are located — and where it is safe to core or cut.
What GPR can detect in a slab
GPR can locate: conventional rebar (mild steel reinforcement); post-tension (PT) cables and tendons — critical to avoid given the catastrophic structural consequences of severing a stressed PT cable; embedded conduit (electrical, data, mechanical); slab thickness variations; and anomalies consistent with sub-slab voids or delamination zones. The ability to detect any of these depends on slab conditions, moisture content, depth, and contrast between the object and the surrounding concrete. Experienced interpretation matters significantly — the same scan data can be read more or less accurately depending on the technician’s training.
When to use GPR (real scenarios)
Use GPR before any destructive work so you avoid hitting something critical and causing delays or safety incidents. The most common use cases in commercial construction and facilities work include: before coring for HVAC, plumbing, or electrical penetrations; before installing moisture testing probes (ASTM F2170) — drilling without scanning risks hitting PT cables or conduit; before anchor bolt installation; before saw-cutting control joints in existing slabs; and in forensic investigations where slab construction documents are unavailable or incomplete. Learn more about IFTI’s floor testing and inspection services and how GPR integrates with the broader Flooring Expert Solution.
Limitations and why interpretation matters
GPR is powerful but has defined limitations. Depth range is typically effective to 12–18 inches in concrete depending on slab density, moisture content, and aggregate size — saturated slabs attenuate the signal more than dry slabs. Dense reinforcement or closely spaced rebar grids can make interpretation of individual elements challenging. GPR can identify anomalies consistent with voiding but cannot definitively characterize a void’s structural significance without additional investigation. Reports should clearly state confidence levels and recommend confirmation methods for any ambiguous findings. This is not a limitation of GPR technology — it is an honest representation of what the data can support.
Deliverables clients should require
A useful GPR deliverable is a clear marked slab map or digital overlay that guides safe field work. It should include: findings mapped by zone with depth estimates for embedded objects, risk callouts for PT cables, conduit, and other critical items, recommended safe coring and cutting locations based on the scan, and any areas where signal quality or conditions limit interpretation confidence. Physical marking on the slab surface before coring is the most operationally useful format for field teams.
How GPR supports flooring risk management
Combining GPR scanning with moisture testing and FF/FL floor flatness testing before flooring installation creates a complete pre-installation picture of the slab’s condition. GPR confirms safe drilling locations for moisture probes. Moisture testing confirms RH conditions. FF/FL testing confirms surface tolerance. Together they reduce change orders, avoid safety incidents, and provide documented basis for installation decisions.
Frequently Asked Questions
Can GPR find post-tension cables?
Yes — GPR can typically locate PT cables and tendons, which is one of the most critical use cases given the catastrophic structural risk of severing a stressed tendon.
Can GPR detect voids under a slab?
GPR can identify anomalies consistent with sub-slab voiding, depending on slab conditions and depth. The report should state confidence level and recommend confirmation methods for significant findings.
Is GPR destructive?
No. GPR scanning requires no drilling or cutting — the equipment scans the surface and transmits the signal through the slab.
How accurate is it?
Accuracy depends on slab conditions and operator interpretation experience. Well-prepared, dry slabs with simple reinforcement patterns yield the clearest results. All GPR reports should include stated confidence levels and limitations.
Should I scan before drilling for moisture probes?
Yes — scanning before drilling for ASTM F2170 moisture probes is a best practice to confirm that probe drill locations are clear of PT cables, rebar, and embedded conduit.
