It lurks below — Techcorp Field Notes

A two-lane optical-turnstile install in a downtown high-rise stopped before the first hole when we found the floor had already been x-rayed and marked. The slab was post-tensioned — and the people telling us to drill didn't know what that meant. The story is about the rule that kept us off that floor, and what's actually stored in the concrete under your boots.

Plan view of a terrazzo office floor with blue painter's tape marking three post-tension cable runs, a taped X over a confirmed crossing, and a taped note reading "do not drill."

AI-generated illustration (SVG). A scanned, marked-up post-tension floor — the tendons run under the tape.

The job

A downtown high-rise office floor, fall of last year. The scope was small: a couple of lanes of Alvarado optical turnstiles going in at a lobby entrance — the kind of gear that lives or dies on how well it's bolted to the slab. We rolled in with a crew of three. Two of us could have done it. It was not a complicated job.

It stopped being simple the moment we looked at the floor.

What we found

Somebody had been there ahead of us. The slab was marked up — lines and notes where a scanning crew had x-rayed the floor and mapped what was running through it. We didn't have to guess what we were looking at. The floor was post-tensioned, and the marks were telling us where the cables ran.

That's the whole job, right there. We don't drill post-tension floors. That's not a judgment call I make fresh on each site — it's a flat rule. You do not free-hand an anchor into a post-tensioned slab, and you don't drill one on someone's say-so, period.

So I told my contact on site the truth: this part is beyond us. Here's the path forward — you get the holes drilled by whoever owns that risk. We'll set the equipment in place first, mark exactly where the anchors need to land, and come back in the morning to bolt everything down once the holes are in.

The standoff

That did not land the way I'd have liked.

It kicked off a back-and-forth between the client and the office I was working through. Neither of them really knew what a post-tension cable was. They saw a floor and a drill and a schedule, and they wanted the holes drilled now. It took some explaining — and then it took pulling in somebody higher up the chain, who heard it out and said exactly what I'd said: no, we will not drill that floor.

And it's not improvised. The outfit I sub through runs a site survey before a job like this — sometimes it's a formal walk, sometimes it's a few questions over email with the client filling in the blanks — and one of those questions exists for exactly this reason: are there cables in the floor? You ask it before anyone's standing on the slab with a hammer drill and an argument.

A quick sidebar before we get back to it — what's actually in a floor like this.

What's actually in the floor

Post-tensioning is how a building gets a thinner slab to span farther with less concrete and fewer columns. After the slab is poured, steel strands — tendons — are stretched with hydraulic jacks and locked off, so the concrete is permanently squeezed into compression. It's everywhere modern concrete goes up: high-rise floors, parking garages, podium decks over retail, hospitals, condos.

The numbers are the reason you respect it. A standard tendon is a half-inch, seven-wire, 270-ksi strand — jacked to roughly 33,000 pounds and settling to about 27,000 to 28,000 pounds of locked-in force for the life of the building. In the most common ("unbonded") system, the strand floats free inside a greased plastic sheath, so that load isn't anchored to the concrete along the way — it's stored down the entire length of the cable like a drawn bow.

Cut into one and that energy comes out in milliseconds. The strand whips through the cover concrete, can blow an anchor casting clean out the edge of the slab, and takes the capacity it was providing with it. People who've heard it describe the sound as a gunshot; the plainest analogy I've seen is a loaded tow-truck cable letting go. The trade doesn't talk about strikes as a freak event — it talks about them as a matter of when, not if. The clearest human cost on the public record isn't even a drill strike: in 1999, OSHA documented a worker killed during a controlled de-tensioning when the concrete behind the anchor fractured and the jack "jackknifed up and struck him in the head" — hard hat and all. That's the energy you're handling when everything is done by the book. And the repair is the cheap part: a single clean tendon replacement starts in the low four figures, then climbs into five and six once a building is occupied, a floor needs temporary shoring, or the engineer of record has to sign off on the fix.

And here's the cruel geometry of it: tendons bunch up along the column lines — which is exactly where you most want to drop a structural anchor for a screen, a railing, a turnstile base. The best place to put your bolt is the worst place to go looking for a cable.

The part that makes it dangerous is the part that makes it invisible. An unbonded slab has no surface tell — no stamp, no paint, no different feel under the bit. It looks exactly like every other eight-inch slab you've ever drilled. You cannot tell by looking. The only thing between a routine anchor hole and a very bad afternoon is information: a real scan, or drawings you actually trust. It lurks below, and it's engineered to be invisible.

The morning after

We came back the next day. The holes were in — the client had arranged to get them drilled overnight.

When I looked at where they'd landed, the holes sat closer to the marked lines than I'd have wanted to drill myself — they usually do, when the anchor pattern is fixed and the cables aren't. But they'd laid it out against the marks, the bits cleared the cables, and it held. I was just glad it wasn't my hand on the trigger making that call.

But nothing got hit. The anchors went down, everything torqued up clean, the lanes commissioned, and we were out. A good outcome — and the part worth sitting with is whose outcome it was. The people who owned the floor and the schedule are the ones who took on the risk I wouldn't. That was theirs to take. They own the rules on their floor; they own what happens when they drill it.

Who owns it

There's a version of this where I'm the agreeable guy, drill the holes myself to keep the schedule, and it goes fine ninety-nine times out of a hundred. The hundredth time, close isn't clear, and the name on the drill is mine.

The lesson isn't "be afraid of concrete." It's that the risk belongs to whoever owns the rules and the specs — and your job, when the floor in front of you is one you shouldn't touch, is to put that risk back where it belongs and get it in writing. Ask the question before you mobilize: is there anything in this floor? Make the client or the GC answer it. If the answer is post-tension and nobody will own the drilling, then you stage, you mark, you hand it back — or you walk.

It points at something bigger than one floor, too. The as-built drawing is the only record that travels with a building for its whole life, and our trade treats it like a nice-to-have. On any drill, anywhere, the first question is show me the as-builts — and the second is who confirmed they still match the slab.

You don't have to win the argument about what's under the slab. You just have to not be the one holding the drill when it lurks below.

Two reference sidebars to close — what it costs to do it right, and what the rulebook actually says.

What it costs to see it (and the people who do it for a living)

Here's the part installers need to hear without panicking, so let me put real numbers on it.

You cannot clear one of these floors with the rebar finder off the rental counter. Telling a tendon from rebar takes ground-penetrating radar and someone who can read it. The price ladder, current as of writing:

A basic handheld concrete scanner — a GSSI StructureScan Mini (~$10,600) or a Proceq GP8000 (~$12,200) — with a tablet makes a complete rig around $19,000.
Imaging-grade handhelds that picture multiple layers run higher: the Leica C-thrue lands roughly $16,000–$25,000, and Hilti's PS 1000 X-Scan is $51,280 straight from the manufacturer.
(The $14,000–$100,000 wheeled rigs you'll see quoted are utility-locating units — the wrong tool for in-slab work. Don't let anyone talk you into pricing the job off those.)

Bar chart of handheld concrete-scanning GPR purchase prices: GSSI StructureScan Mini $10,600; Proceq GP8000 $12,200; complete rig with tablet about $19,000; Leica C-thrue $16,000–$24,810; Hilti PS 1000 X-Scan $51,280 — plus a certified operator or a hired scanning firm.

Buy or hire, at a glance — purchase prices, service rates, and why scanning is the cheapest line on the job. (AI-generated SVG chart.)

So the honest spread is about eleven grand at the cheap end to past fifty at the good end — and the box is useless without the hands. A real scan is run by a certified operator; even the entry-level non-destructive-testing certification is eight hours of classroom plus sixty hours of field practice, and the level you actually want signing a clearance is a step past that. What you're paying for is a marked-up slab: tendons drawn, safe zones boxed, a depth the operator will stand behind — with their name, cert level, and equipment on the report. There's a cheaper tier out there too: somebody with an eight-hour class and a rented box who leaves you a sharpie mark and a "you're good." On a post-tension floor, that's the scan that gets someone hurt.

And here's the part that should make you feel better, not worse — there is an entire industry that exists for nothing but this. National concrete-scanning firms will roll in, map your slab, paint a "green box" you're cleared to cut inside, and back it with a guarantee. One of the big ones publishes a 99.8%-plus accuracy rate across hundreds of thousands of scans and will pay the material-damage cost if their box is wrong. Think about what that means: companies have built whole businesses on standing between a drill and a live tendon, and they're confident enough to put money on it. And it's cheap relative to the stakes — a quick scan-and-mark runs about $200 an hour with a two-hour minimum, a full marked-up day with a report $1,000 to $3,000 — set against a tendon repair at several thousand dollars apiece, or an average subsurface strike that runs around $56,000. The scan is the cheapest line on the job.

That cost is the whole reason the scan is the first line item somebody tries to value-engineer off a job. It's also exactly why, when a floor's already been scanned and marked and the answer is the cables are right there, you don't get clever. You hand the drilling back to whoever's willing to own it — or you call one of those firms and let them own the green box.

What the standards actually say

There's less hard law here than you'd think, which is itself worth knowing.

Government. The one federal standard — OSHA 29 CFR 1926.701(c) — covers the original stressing operation: barriers and signs around the jacking zone, nobody behind the jack while it's under load. It protects the crew tensioning a fresh slab. It says nothing about a trade drilling into that slab years later. On the exact thing that nearly happened on this floor, there is no federal clearance number.

Industry. This is where the real guidance lives. The Post-Tensioning Institute (PTI) publishes technical notes and repair guidance for existing PT structures; ACI 318 and ACI 423 carry the structural code behind it. The most explicit written procedure in English is the Australian PTIA Guidance Note GN06, "Post Cut Holes through Post-Tensioned Slabs" — its rule is that the only low-risk penetrations are small holes that thread the gap between tendons without cutting one, and anything in that zone gets an engineer's check first. Whoever runs the scan should hold an ASNT SNT-TC-1A certification.

The do's and don'ts the scanning firms actually publish:

Do scan with GPR before any cut, core, or drill — and get the tendons and safe zones marked on the slab.
Do stay an inch or two clear of any marked line before you cut.
Do escalate to X-ray if the scan is inconclusive — never "drill slow and see what happens."
Don't free-hand an unscanned slab. You will eventually find a cable.
Don't cut a tendon: recoil, concrete spall, anchorage blowout, injury or worse.

Put together, the rulebook says exactly what I told the client on day one — somebody qualified scans it, somebody licensed owns the drilling, and you stay well off the lines. None of it hands that job to the integrator with the anchor gun. Neither did I.

A note on what this isn't

This is my view as a field integrator with decades on the trade — not engineering or legal advice. Whether and how you drill near a post-tensioned slab depends on the structure, the drawings, a qualified scan, and the people licensed to sign off on it. If you're facing a floor you're not sure about, stop and get a certified concrete-scanning operator and the engineer of record involved before anyone drills. The cost figures and standards below are current as of writing and meant to show the shape of doing it right — not to substitute for a real scan on your actual floor.

References

Ground Penetrating Radar Systems (GPRS). (n.d.). Ground penetrating radar: Should you rent, buy, or hire? https://www.gp-radar.com/article/ground-penetrating-radar-should-you-rent-buy-or-hire

Hilti. (n.d.). PS 1000 X-Scan concrete scanner. https://www.hilti.com/c/CLS_MEA_TOOL_INSERT_7127/CLS_CONCRETE_SCANNERS_SENSORS_7127/r6436760

Leica Geosystems. (n.d.). C-thrue concrete scanner. https://shop.leica-geosystems.com/utility-detection/concrete-scanner

Occupational Safety and Health Administration. (n.d.). 29 CFR 1926.701(c) — Post-tensioning operations. U.S. Department of Labor. https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.701

Post-Tensioning Institute. (n.d.). Technical notes and field guidance for existing post-tensioned structures. https://www.post-tensioning.org/

Post-Tensioning Institute of Australia. (2014). Post cut holes through post-tensioned slabs (Guidance Note GN06 v.2). https://www.ptia.org.au/wp-content/uploads/PTIA_GN06V2_PostCutHoles-through-Post-Tensioned-Slabs_Dec14.pdf

American Society for Nondestructive Testing. (n.d.). Recommended practice no. SNT-TC-1A: Personnel qualification and certification in nondestructive testing. https://www.asnt.org/

Occupational Safety and Health Administration. (n.d.). Accident report 126008770 — Employee is killed when struck by post-tensioning device. U.S. Department of Labor. https://www.osha.gov/ords/imis/accidentsearch.accident_detail?id=126008770

Structure Magazine. (2018). Post-tensioned concrete construction. https://www.structuremag.org/article/post-tensioned-concrete-construction/