Post-Tensioned Slabs and the Tendon You Must Not Cut
Why this matters
A post-tensioned slab looks exactly like any other concrete floor, and inside it are steel cables stretched to tens of thousands of pounds each. Cut one with a core bit, a saw or a deep anchor hole and two things happen at once. The stored tension lets go in an instant, and the strand can burst out of the slab edge or up through the floor with enough force to injure anyone near it. And the slab loses the squeeze that was holding it together, which is a structural failure even if nobody gets hurt. There is no field repair for either. The rule is simple and absolute: no cut, core or deep anchor into a slab until you know whether it is post-tensioned, and if it is, not without the tendons located and an engineer's agreement.
What post-tensioning is
Concrete is strong in compression and weak in tension. Ordinary reinforced slabs carry tension in steel bars that sit passively in the concrete. A post-tensioned slab instead uses tendons: in residential slabs-on-ground, usually single 1/2 in seven-wire steel strands, each greased and sealed in a plastic sheath so it can slide. The slab is poured around them, and once the concrete gains strength a crew pulls each strand tight with a hydraulic jack and locks it off at an anchor cast into the slab edge.
Each strand is stressed to roughly 33,000 lb, about 80 percent of its breaking strength, and keeps most of that for the life of the building. The anchors at each end push that force into the concrete as compression across the whole slab. That compression is what lets a thinner slab ride out shrinkage and the swelling and shrinking of expansive clay soils without cracking apart.
That is why the system is common in regions with expansive soils and in large tract developments: Texas, California, Arizona, Nevada and Florida among them. A design standard for these slabs is published by the Post-Tensioning Institute (PTI DC10.5), and building codes reference it for slabs on expansive soil in many jurisdictions. It is far from the only place you will meet one, so do not rule it out by region.
Why a cut tendon is worse than a cut bar
A reinforcing bar cut by a core bit is a local weakening; the concrete around it stays where it was. A tendon is different in three ways:
- Stored energy. The strand is a stretched spring tens of feet long. When a bit or blade cuts most of the way through, the remaining wires break in a moment and the strand recoils toward its anchors. Strands have burst out through the slab edge and up through the floor surface.
- The whole slab depends on it. The compression from that tendon is lost along its entire length, not just at the cut. Cracks and movement can follow across the slab.
- It cannot be fixed by the trade that cut it. Repair means a specialty post-tensioning contractor exposing the strand, splicing or replacing it, and restressing it to the engineer's direction.
Elevated post-tensioned slabs, such as parking decks and multi-story floors, carry their own weight on the tendons. A cut there is a collapse risk, not just a cracking risk, and nothing in this card makes drilling one acceptable.
How you know you are standing on one
No single sign proves a slab is or is not post-tensioned. Collect what you can.
- A stamp or plaque. Many builders, and some local rules, mark the slab with a stamped warning in the garage floor or a plaque near the entry or the electrical panel, often along the lines of "post-tensioned slab, do not cut or core". Its presence settles the question; its absence settles nothing, because it is often covered by flooring or was never placed.
- Anchor pockets at the slab edge. Where a tendon was stressed, a pocket in the slab edge was filled with grout over the anchor. Along the exposed edge of the slab outside, look for small evenly spaced grout patches.
- The builder's documents. The permit set, the structural drawings or the homeowner's closing papers often include a tendon layout plan. The local building department may hold the permit drawings.
- The house. A slab-on-ground house in a tract development, in an expansive-soil region, built from the 1970s on, is a candidate until shown otherwise.
Slab edge seen from outside, at grade
house wall
================================================
| o o o o o |
================================================
^ grout patches over tendon anchors
Plan view: each pocket starts one tendon, which
runs across the slab to a pocket on the far edge
N o------------------------------------o S
N o------------------------------------o S
(a first estimate only: tendons bundle,
sag, and bend around plumbing and openings)
The pocket spacing tells you roughly how far apart tendons run, and a line drawn between a pocket and its partner on the far edge tells you roughly where one is. Roughly is the key word: tendons are often bundled in bands in one direction, are shifted around plumbing and openings during the pour, and sit at a height that varies across the slab. Pocket lines are for planning a scan, not for choosing where to cut.
How the cut usually happens
It is rarely a big saw cut on a known post-tensioned slab. It is the small job nobody thought of as cutting:
- An anchor for something heavy. A garage lift, a storage rack, a safe or a water heater restraint, anchored with holes several inches deep, by someone who never looked for a stamp because the job was "just drilling".
- A core for a single pipe. A plumber or an electrician coring a small hole for a floor drain, a conduit stub or a condensate line, on the theory that a 2 in hole is too small to matter. Bit diameter has nothing to do with it; one strand is enough.
- A trusted estimate. A cut placed on a pocket line, or on an old drawing that was not what got built, without a scan of the actual slab.
- A covered warning. The stamp sits under the epoxy floor coating, the tile, or the shelving the homeowner installed, so the tech never sees it.
In each case the tool was small and the decision felt routine. The route below exists because the slab gives no warning before the strand breaks.
The route before any core, cut or anchor
- Assume the slab is post-tensioned until the documents or a qualified person say otherwise.
- Get the drawings if they exist: the tendon layout plan shows the design intent.
- Have the tendons located by a concrete scanning firm with ground-penetrating radar, which marks them on the slab surface. Ask them to mark depth as well as position. Ordinary reinforcing bars and other embedded items will show too; telling a tendon from a bar is part of what you are paying for.
- Ask the engineer of record, or a structural engineer the owner engages, whether the work is acceptable at the scanned location, and at what size and depth. Walk in with the scan report, the location measured from two slab edges, the opening or hole size, and the reason for the work.
- Get the answer in writing, with the limits, before any tool touches the slab.
Shallow fasteners. Flooring installers and others sometimes ask whether shallow fastening, such as tack strip or a light anchor, is safe. Some builders publish a shallow depth they permit; get it from the builder or engineer for that house, in writing, rather than from a rule of thumb. Tendons are not always at mid-depth; where chairs collapsed during the pour, one can sit close to the surface.
Alternatives that avoid the slab. For plumbing under a post-tensioned slab, rerouting the line above the slab through walls and the attic, or tunneling under the slab from outside, often avoids cutting it at all. For equipment that needs anchoring, a design that bolts to the walls or spreads load on a base plate may avoid deep anchors.
Worked example: a slab leak under the kitchen
A plumbing shop is called to a 2004 house with a hot-water slab leak. Acoustic listening and a thermal camera place the leak under the kitchen floor, 9 ft from the north wall and 14 ft from the east wall. The normal repair would be a 24 by 24 in opening cut through the slab centered on the leak.
Is it post-tensioned? The garage floor has a stamped warning near the door to the house. Outside along the north edge, the tech finds grout patches roughly every 4 ft, one of them 14 ft from the northeast corner. Along the east edge, patches sit at about 3 ft, 7-1/2 ft and 12 ft from the same corner.
First estimate from the pockets. The north-edge pocket at 14 ft starts a tendon running north to south, and its line passes straight through the leak, which sits 14 ft from the east wall. The east-edge pocket at 7-1/2 ft starts a tendon running east to west, and its line passes 1-1/2 ft, or 18 in, north of the leak, which sits 9 ft from the north wall. A 24 in opening centered on the leak extends 12 in each way, so on the pocket lines alone, the north-south tendon is inside the opening and the east-west one clears its edge by 6 in.
Scan. The shop does not cut on that estimate. A scanning firm's radar finds the north-south tendon 13 ft 4 in from the east wall at the leak's position, 8 in east of the pocket line, so 8 in from the leak, and it confirms the east-west tendon 18 in from the leak.
The decision. The north-south tendon sits 8 in from the center of a 24 in opening, 4 in inside its edge. Shrinking the opening enough to clear it by any margin would leave a hole too small to repair the pipe in, and the engineer would still have to approve any cut. The shop recommends rerouting the hot line overhead instead: abandon the leaking branch under the slab and run a new line through the walls and attic to the kitchen. The customer agrees, and no tool touches the slab.
What changes the answer. Had the radar put both tendons well clear of a workable opening, the next step would still have been the engineer, with the scan in hand, deciding whether a cut at that size and location was acceptable. The scan makes a cut possible to consider; it does not make it permitted.
What this card does not settle
Whether a given cut is acceptable is the engineer's decision, and who pays if a tendon is cut is a question for the contract, the insurers and possibly a court. This is not legal advice. What a shop controls is whether it asked before cutting and has the stamp photo, the scan, and the engineer's written answer in the job record to show it.
References
- Post-Tensioning Institute, PTI DC10.5, design standard for shallow post-tensioned foundations, in the edition your jurisdiction references
- Concrete scanning firm reports (ground-penetrating radar) for the specific slab
- The builder's tendon layout plan and permit drawings
- See related: How to Core a Foundation Wall and Seal It
- See related: The Structural Cut Approval SOP