_A post-tensioned slab holds a large amount of stored energy in a small number of steel strands. Knowing whether you are standing on one changes how the next cut gets planned — and it is a question worth answering before the saw comes off the truck._

What post-tensioning actually is

In a post-tensioned slab, steel strands run through the concrete inside sheathing or ducts. After the concrete cures, those strands are pulled to a high tension with a hydraulic jack and locked off at anchors, putting the slab into compression. Concrete is strong in compression and weak in tension, so pre-compressing it lets a thinner slab span further and resist the cracking that soil movement would otherwise cause.

The design standard for residential slabs on ground is published by the Post-Tensioning Institute. The 2024 International Residential Code added Section R506.2, requiring that post-tensioned concrete slab-on-ground floors placed on expansive or stable soils be designed in accordance with PTI DC10.5. 1Jump to source 12Jump to source 2

That matters locally. Post-tensioned slabs on ground are a common answer to soils that move, which is why they show up throughout the Southwest in residential subdivisions built from the 1990s onward.

The number that explains the caution

The strand used in most of this work is 0.5-inch diameter, seven-wire, Grade 270 prestressing strand made to ASTM A416. Its cross-sectional area is 0.153 square inches, so its minimum ultimate tensile strength is 270 ksi multiplied by 0.153 square inches, or about 41.3 kips. 3Jump to source 34Jump to source 4

Codes limit jacking force to 80 percent of that ultimate strength, which puts a typical strand at roughly 33 kips — about 33,000 pounds of force — held in a piece of steel half an inch across. 3Jump to source 3

That force is why a severed tendon is not a routine repair. Releasing it removes that strand’s contribution to the slab’s force balance, and the release is sudden. Cutting into an anchorage zone can fracture the anchorage concrete, displace the bearing plate, or cut the tail of the tendon just past the anchor — any of which releases the prestress in that strand. 5Jump to source 5

Proportion: This is not a reason to avoid ever cutting a post-tensioned slab. Coring and sawing in PT slabs is done routinely and deliberately. It is a reason to know where the tendons are first, and to have a structural engineer involved when the cut is anywhere near one.

Signs you can check yourself

The stamp

The most direct indicator is a warning stamped into the concrete. Guidance from the Phoenix Residential Post-Tension Round Table Committee recommends that a warning stamp reading to the effect of “post-tension slab, do not cut or drill” be placed in a conspicuous location in the garage. 6Jump to source 6

In practice, on homes built after the mid-1990s, that stamp is often set in the garage floor near where the garage door comes down — though that location is not universal, and some buildings carry a plastic or paper placard fastened to a wall instead. 7Jump to source 7

The slab edge

Post-tensioning leaves anchors along the perimeter. After stressing, the strand tails are cut and the anchor pockets are patched, so you are often looking for a row of small, regularly spaced circular or oval patches in the edge of the slab, sometimes visible below the siding or stucco line. Regular spacing along an edge is the tell; random patching is usually just repair.

The paperwork

Original construction documents, the structural sheets, the foundation plan, or a soils report will normally say plainly whether the foundation is post-tensioned. For a homeowner, the builder, the subdivision, or the county or city permit record is often the fastest route to a definite answer.

What not to assume

  • No stamp does not mean no tendons. Stamping became common practice, not a universal legal requirement, and older slabs predate the convention entirely. 7Jump to source 7
  • A stamp in one bay does not describe the whole structure. Additions, patios, and later pours may be conventionally reinforced even when the main slab is post-tensioned, and the reverse also happens.
  • Tendons are not laid out on a neat grid you can extrapolate from one corner. In slab-on-ground work they are commonly draped and banded, and they shift around plumbing penetrations, beam lines and re-entrant corners.
  • A previous successful core is not a map. It tells you one location was clear on one day.

Where scanning fits

Ground-penetrating radar is the usual tool for mapping tendons, reinforcing steel and embedded conduit before a cut, and it is the method generally recommended for establishing the tendon layout before saw or core work begins in a post-tensioned slab. 5Jump to source 58Jump to source 8

What a scan produces is an interpretation of the radar response at the surface, marked out on the slab. It shows where the data supports a target and where it does not. It does not expose the tendon, and it does not certify an area. Depth of cover, congestion of reinforcement, moisture and slab condition all affect what the data can support — the limits are covered in more detail in our guide to what radar can and cannot see. 9Jump to source 9

The practical output for a contractor is usually a marked layout plus a conversation about the cut: whether the planned penetration can be shifted a few inches into a clear zone, whether the depth can be limited, and where the crew should stop and reassess. On a post-tensioned slab, that conversation is worth having before mobilizing the saw.

If something goes wrong: If a strand is struck, stop work and get a structural engineer and a qualified post-tensioning repair contractor involved. Do not keep cutting to “see how bad it is.”

View 9 numbered sources

Sources

Every numbered claim above traces to one of these. Links go to the publisher of record.

  1. Post-Tensioned Slab-on-Ground — code references and requirements — UpCodes — https://up.codes/s/post-tensioned-slab-on-ground
  2. PT Applications — Slab-on-Ground — Post-Tensioning Institute — https://www.post-tensioning.org/education/ptapplications/slab-on-ground.aspx
  3. Material Properties of Post-Tension Strands — AMSYSCO Post-Tensioning — https://www.amsyscoinc.com/2010/01/29/material-properties-of-post-tension-strands/ Source of the 0.153 sq in area, 41.3 kip MUTS and 33 kip jacking force figures for 0.5-inch Grade 270 strand.
  4. ASTM A416 — Standard Specification for Uncoated Seven-Wire Strand for Prestressed Concrete — PC Wire — https://pcwire.org/standards/a416-standard-specification-for-uncoated-seven-wire-strand-for-prestressed-concrete.html
  5. How to Safely Saw or Drill into Concrete in Post-Tensioned Slabs — GPRS — https://www.gp-radar.com/article/how-to-safely-saw-or-drill-into-concrete-in-post-tensioned-slabs
  6. Recommendations from the Phoenix Residential Post-Tension Round Table Committee — Maricopa Association of Governments, Building Codes Committee — https://azmag.gov/Portals/0/Documents-Ext/BCC/BCAS-8-Recommendations-from-the-Phoenix-Residential-Post-Tension-Round-Table-Committee.pdf Source of the garage warning-stamp recommendation.
  7. Post-Tension Basics — How Post-Tensioned Slabs Are Built — Concrete Network — https://www.concretenetwork.com/post-tension/basics.html
  8. Post-Tension Slab Safety: Drilling and Lifting Best Practices — Alchatek — https://info.alchatek.com/blog/how-to-avoid-serious-hazards-when-lifting-post-tension-slabs
  9. ASTM D6432-19 — Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation — ASTM International — https://www.astm.org/Standards/D6432.htm

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This guide is original writing by High Desert Scan & Locate, LLC. It cites third-party statutes, standards, codes and publications, all of which remain the property of their respective owners. We describe them in our own words and do not reproduce their text. Standards published by bodies such as ASTM, ACI, ASCE/UESI/CI and PTI are available from those publishers; where one governs your work, obtain the current edition and read it. The published source controls; our paraphrase does not, and an edition cited here may since have been revised or superseded.

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Private utility locating supplements, and does not replace, public one-call (811) notification. Utility designation is Quality Level B per ASCE/UESI/CI 38-22 unless otherwise stated. GPR interprets subsurface conditions; it does not expose them. Results are interpretations of conditions present within the requested area on the service date, and depend on target material, depth and site conditions.