GPR scanning before core drilling

Ground-penetrating radar is the cheapest insurance in concrete work: a scan takes minutes per location and removes the three worst surprises a core bit can find. Here is what it does, when you need it, and when you can honestly skip it.

What GPR sees

A concrete GPR antenna is passed over the drilling location in a grid; reflections map what is inside the slab, marked in the field directly on the concrete:

• Post-tension tendons — the critical target. See why cutting one is a serious event.
• Rebar layout — spacing and depth of mats, letting holes thread between bars where the reinforcement matters.
• Embedded electrical conduit — the classic accident. Slabs carry live conduit runs from decades of work; severing one means outage, repair, and sometimes a shock hazard.
• Slab thickness and voids — useful context, especially in older buildings where drawings lie.

What GPR does not reliably see: objects below the slab (it maps the slab, not the dirt), fine detail at depth in very thick or wet elements, and non-metallic targets with poor contrast. It is a mapping tool with skilled interpretation, not X-ray vision.

GPR vs concrete X-ray

Radiography produces beautiful images but needs access to both faces of the slab, radiation safety clearance of the area, and more time and money. GPR works from one face, in occupied buildings, immediately. For routine penetration scanning, GPR has effectively replaced X-ray; radiography survives for special cases (dense congestion near critical embeds, forensic work).

When scanning is essential

• Elevated decks in South Florida — assume post-tension until drawings prove otherwise. Non-negotiable.
• Slabs with electrical history — office floors, hospitals, labs, anywhere conduit was ever run in or under the slab.
• Structural elements — beams, columns, shear walls, where main steel placement is a design matter.
• Anything with “unknown” in the description — unknown thickness, unknown construction, no drawings. The scan replaces the missing knowledge.

When it is honestly optional

• Residential slab-on-grade with drawings and no plausible in-slab utilities at the location — a judgment call the drilling contractor can own.
• Non-structural toppings and pads where the worst case is a broken bit.
• Locations already scanned on a current project — scans don’t expire, but layouts move; confirm the marks still match the final hole positions.

How it shows up in your project

Practically, scanning is either a line item from the drilling provider (common, convenient, one mobilization) or a separate scanning firm ahead of the crew (common on large GC-run projects). The deliverable is field markings — tendon and conduit paths crayoned on the slab with clear/no-go zones — plus, on documented projects, annotated photos or a short report.

On the request form, the GPR field has three honest options: Completed (attach the markings if you have photos), Required (scope it with the drilling), or Not sure (the estimator will make the call from your building details). Cost-wise, scanning scales with location count and is a small fraction of any multi-hole scope — context in the cost guide.

The rule that summarizes this whole page: scan when the slab can contain anything you’d regret cutting. In Miami’s elevated inventory, that is nearly always.