Core drilling in post-tension slabs

Much of South Florida’s elevated concrete — condo towers, office decks, parking structures — is post-tensioned. Core drilling it is routine work with the right workflow and genuinely dangerous without it. This guide is the workflow.

What post-tensioning is, in one paragraph

Instead of relying only on passive rebar, a PT slab contains steel tendons — strands in plastic sheathing — stressed to tens of thousands of pounds after the concrete cures and anchored at the slab edges. The tension lets the slab span farther with less depth. The consequence for anyone drilling: those strands are loaded springs running invisibly through the slab, typically draped in profiles that change height along the span.

Why cutting a tendon is a serious event

Sever a stressed tendon and three bad things can happen at once: the strand can burst out at the anchorage or through the slab surface with real injury potential; the slab locally loses design capacity; and the repair — exposing, splicing or replacing, and re-stressing the tendon under an engineer’s direction — costs orders of magnitude more than the hole that caused it. Repairs also leave a documented structural modification on a building’s record.

That asymmetry drives the entire discipline: the cost of prevention is a scan and a layout adjustment; the cost of failure is an engineered repair.

How to know if your slab is PT

• Drawings — structural sheets will show tendon layouts and note post-tensioning explicitly. This is the definitive source.
• Building era and type — elevated residential and office decks from the 1970s onward, and virtually all modern South Florida towers, should be assumed PT until drawings prove otherwise.
• Visual tells — round grout plugs at slab edges (anchor pockets) and warning stamps sometimes embossed near edges. Absence of tells proves nothing.
• Slab-on-grade — ground slabs in this region are rarely PT (unlike some other states), but verification beats assumption on engineered ground slabs.

When in doubt, the answer is not guessing — it is scanning.

The PT drilling workflow

1. Scan first. GPR scanning maps tendons, rebar, and embedded conduit at each penetration location, marked directly on the slab.
2. Adjust layout. Cores shift to pass between tendons. A couple of inches of adjustment resolves the vast majority of conflicts — which is why scanning happens before pipe is hung or sleeves are set, not after.
3. Resolve true conflicts. Where a required location sits on a tendon, the engineer of record decides: move the penetration, or (rarely) design around a relocated tendon. The drilling crew does not make that call.
4. Drill with respect. Drilling proceeds normally between mapped tendons; conservative crews treat marks as no-go stripes with a safety margin, not as precision targets to shave.

What this means for your schedule and budget

• Scanning adds a step and a line item — see the cost guide for how it fits the pricing model. On multi-core scopes it is a small fraction of the job.
• Layout flexibility is worth designing in: penetration schedules that allow ±3 in of tolerance move faster through PT decks than ones pinned to the quarter inch.
• Buildings know their own history: condo associations and facility managers often hold the structural drawings — asking for them early shortens everything.

The one non-negotiable: no blind drilling on elevated decks. Any provider willing to skip scanning on a probable-PT slab to win a bid is offering you their insurance deductible as a discount.

Ready to scope PT work? Send the request with the floor, building age, and whether drawings exist — the rest of the workflow follows.