When you're dealing with post-tension slabs, the absolute worst thing you can do is hit a tendon with a core drill. I've seen it happen twice in my career and the sound alone is enough to give you nightmares, not to mention the cost of the repair. Before you even think about bringing out the heavy equipment, you need to get your layout markings down perfectly. Don't just rely on the original shop drawings because things shift during the pour. I always tell the new guys to use a high-contrast spray paint system for the diferent components. Mark your banded cables in one color and your distributed tendons in another, and never forget those stud rails around the columns. If you aren't 100% sure where a cable is running, get the GPR out and verify it before anyone touches a trigger. It takes an extra hour but it saves you a week of headache and a massive structural bill. The trick is to be consistent with your symbols across the whole floor. If you're using red for the main cables and black for the stud rail boundaries, stick to it so the MEP guys acutally know what they're looking at when they come in later. Most of the time they're just looking for a clear spot to drop a pipe, and clear markings are the only thing standing between them and a snapped cable.
Tips for marking PT layouts that nobody talks about
Sep 16, 2026
Last reply: 8 hours ago
5 Replies
I learned the hard way that even the best markings won't save you if the scan is off. On a parking garage project last year, we were using a Hilti PS 1000 to map out a 7-inch slab. The GPR guy was rushed beacuse the MEP crew was breathing down his neck to start thier core holes. He missed a slight sweep in the distributed tendons by about three inches. When the drill hit that half-inch cable, it sounded like a gunshot. The snap was so violent it actualy chipped the surface concrete five feet away from the hole.
Now, I always insist on a double-check if the cable layout looks too "perfect" on the floor. Those shop drawings are more like suggestions once the concrete starts flowing and guys are walking all over the chairs. I also started using fluorescent orange for any "no-drill zones" around the column heads where the shear rails are bunched up. It’s alot cheaper to spend the morning recalibrating the scanner than it is to call in a specialist to splice a tendon and re-tension the whole run.
Man, reading this makes me so glad I'm just getting started! That gunshot sound you mentioned sounds absolutely terrifying. I’m still learning the ropes on my first big commercial site and I definately don't want to be the guy who snaps a cable. Since you guys are talking about marking everthing, does anyone have a favorite brand of high-vis paint that acutally stays put if it rains right after the pour? We used some cheap stuff last week and it was a total smeary mess. Also, random question, but do you guys wear special boots when walking on all those chairs and cables before the pour?
Regarding the paint, I’ve had the best luck with the Rust-Oleum Precision Line. It’s got a high solids content so it doesn't just wash away the second a pump truck leaks some water. What really saved me on the last deck was actually taking a quick photo of every layout area with a tape measure in the frame before the pour. When the GPR guy came in later and his readings looked funky, I pulled up the photos on my phone and we could see exactly how much that tendon had bowed out of place. It’s a five-second step that’s way more reliable than just trusting a drawing.
While photographic evidence is certainly a prudent secondary verification method, one must also account for the inevitable vertical deviation of the tendons within the chair heights, as even a perfectly horizontal layout on the deck can be compromised by a tendon that has 'floated' toward the upper third of the slab during a particularly vigorous vibration pass. I have observed that many technicians fail to calibrate their dielectric constant settings for the specific moisture content of green concrete, which frequently leads to depth perception errors that can be just as catastrophic as a lateral miscalculation of a few inches.
Update: We just wrapped up the coring on the west wing. That tip about taking photos with a tape measure in the frame was a lifesaver when the paint started fading under the slurry. We also had to account for some tendon float on the deeper sections, just like you guys warned. everythign stayed intact and we didn't have any nightmare snaps. Thanks for the advice!
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