Coring a Post-Tensioned Slab for a Roof Drain
On a 7-in. bonded post-tensioned elevated slab, 0.6-in. strands are spaced at 4 ft on center with about 1-1/4 in. cover. The mechanical contractor lays out a 6-in. roof-drain core 18 in. from a column line and tells the ironworker crew to "stay 12 in. off the column and you'll miss the tendons." No tendon-location scan has been done. On a similar job, a core hit a strand, and the repair required strand replacement and re-stressing. The structural engineer of record has not seen the penetration. The coring rig is on the deck, the concrete saw is fueled, and the schedule requires the drain by Friday. The superintendent says verbal approval is enough because "the engineer approved the same thing on another building."
1. Should the core proceed based on the 12-in. rule and the superintendent's verbal approval?
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No. The "stay 12 in. off the column" rule is not an engineering basis for locating a penetration in a post-tensioned slab. Tendons are placed in profiles, can be bundled or displaced during placement, and are often concentrated near columns and anchorages. A 6-in. core with only about 1-1/4 in. of cover can intersect a strand even when the layout appears clear. Severing a bonded tendon damages the prestressing system, can crack the slab, and may require strand replacement and re-stressing, as the earlier project showed.
Verbal approval from another building is not approval for this penetration. The engineer of record must review the specific location, slab, tendon layout, and opening size, and the approval must be in writing before any drilling or coring. The schedule pressure does not change the structural risk. The crew should stop, notify the superintendent that the work is on hold, and request the tendon-location information and a written review.
2. What is the correct sequence for locating and approving the penetration?
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Start with the post-tensioning placement drawings and the as-built information, then use ground-penetrating radar or X-ray by a qualified technician to locate the tendons, non-prestressed reinforcing, conduits, and any embedded items. Mark the tendon lines on the slab surface and keep the marks visible during layout. The proposed 6-in. core should be checked against those marks with a clear zone, not simply placed between assumed tendon lines.
The engineer of record then reviews the marked location and either approves it, requires relocation, or designs reinforcing or a blockout. Only after written approval should coring proceed, using equipment and methods that do not impact-damage the slab. Slurry should be controlled, and the cut should be inspected. If a strand, conduit, or reinforcing bar is encountered, stop immediately, do not cut it, and notify the engineer. Any repair must follow an engineer-approved procedure. The approval, scan results, and final location belong in the project record.
3. The scan cannot be scheduled before Friday. What should the crew do?
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The crew should not core on Friday just to meet the schedule. Without a scan and written engineering approval, the drain location is unverified, and the risk of severing a tendon is unacceptable. The superintendent should be told that the penetration is on hold and why. Options include relocating the drain to a location the engineer can approve from the placement drawings, using a surface-mounted drainage route, or arranging an expedited scan. Any of these is preferable to drilling blind.
If a schedule exception is requested, it must come from the engineer of record, in writing, after reviewing the available tendon information, not from the superintendent or the mechanical contractor. The decision and its basis should be documented. If the drain must be in that area, a planned blockout coordinated before the slab was placed would have been the correct approach; at this stage, the safe path is verification and approval, not speed.