What is a post-tensioned slab and does it move on Oklahoma clay?
In short: A post-tensioned (PT) slab is a concrete slab foundation reinforced with steel cables, called tendons, that are pulled tight after the concrete is placed. PT slabs are one of the foundation types used on expansive soil. In Oklahoma, newer PT homes are identified by a label on the electrical panel door, and PT slabs still move when clay under them swells or shrinks unevenly.
What makes a slab post-tensioned?
Trade terms used on this page:
- Tendon: a high-strength steel strand inside a plastic sheath, laid in the slab before the pour.
- Stressing: pulling the tendons tight with a hydraulic jack after the concrete has gained strength, then locking them at the slab edge.
- Anchor pocket: the recess at the slab edge where a tendon is locked off, later filled with grout or patching.
A conventionally reinforced slab uses rebar or wire mesh instead of stressed tendons.
Why are PT slabs used on clay?
Clay-rich soils can swell 10% or more by volume when wetted, and volume increases of 3% or more are generally considered damaging enough that engineers call for specially designed foundations 6. The residential code sends foundations on expansive soil to the building code's expansive-soil design section (IBC 1808.6) 3. A soil counts as expansive under that framework if, among other tests, its plasticity index is 15 or more or its expansion index exceeds 20 3. Engineering guidance for swelling soils lists post-tensioned slabs and structural floors as options under certain conditions, while ordinary slabs on grade were not recommended on swelling soils in that guidance 2.
How do you identify a PT slab in Oklahoma?
- Panel label: Oklahoma's adoption of the 2024 International Residential Code, effective September 14, 2026, requires post-tensioned slabs on grade to carry a label on the electrical panel door 1.
- Anchor pockets: patched circles or squares spaced along the slab edge or garage stem.
- Paperwork: builder plans, engineering letters or permit records.
Homes built before the 2024 code took effect may not have a panel label 1, so older PT slabs are often identified by their anchor pockets or plans.
How do PT slabs still move?
The tendons do not stop the soil from moving. On a uniformly loaded rectangular slab on swelling soil, the corners press down with about one-fourth the pressure of the center, so corners tend to lift relative to the middle, and moisture differences at the slab edge make this worse 4. Seasonal moisture change in clay can reach from 3 to 40 ft deep 4.
Research modeling a leaking water line under a slab-on-ground foundation on expansive soil compared three conditions: edge swelling after a wet climate, center lift after a long dry period, and edge shrinkage after a dry climate, and examined how a leak changed the slab's differential movement and bending 5. These patterns correspond to the following movement types:
| Pattern | What drives it | Typical sign |
|---|---|---|
| Perimeter drying settlement | Clay at the edge dries and shrinks 5 | One side or corner low, often the whole edge tilted |
| Perimeter wetting heave | Edge soil wets and swells 45 | Edges and corners high |
| Center heave | Edges dry and shrink while the middle stays wetter, leaving the center high 5 | Interior hump, interior door problems |
| Leak-induced heave | Water line leak under the slab 5 | Local hump near plumbing |
An elevation survey maps which of these shapes a slab has taken.
What should be considered when repairing a PT slab?
PT slabs use the same repair families as other slabs: perimeter piers, drainage, moisture control and leak repair. Work that cuts or cores through the slab, such as interior piers, passes through the zone where tendons run. A broken or corroded tendon is its own mechanism, covered under post-tension tendon failure.
Related
- Slab on Grade
- Post-Tension Tendon Failure
- Interior Piers (Tunnel or Core)
- Construction Era by Decade
- Working with Engineers
Common questions
What is a post-tensioned slab?
It is a concrete slab foundation reinforced with steel tendons that are tightened after the concrete is placed, rather than relying only on rebar or wire mesh.
How can I tell if my Oklahoma house has a post-tensioned slab?
Under Oklahoma's adoption of the 2024 International Residential Code, effective September 14, 2026, post-tensioned slabs on grade must be identified by a label on the electrical panel door. Older homes may show patched tendon anchor pockets along the slab edge or have builder plans that say so.
Can a post-tensioned slab still move on clay?
Yes. Expansive clay can still lift the edges or center of any slab when moisture changes, and research models show leaks under a slab changing how it bends.
Why are post-tensioned slabs used on expansive soil?
Building codes require foundations on expansive soil to be designed for it, and engineering guidance lists post-tensioned slabs among the options for swelling soils under certain conditions.
Sources
- Adoption of International Residential Code 2024 (IRC 2024) Oklahoma Uniform Building Code Commission
- Home Construction on Shrinking and Swelling Soils (Special Publication 11) Colorado Geological Survey (Holtz and Hart, 1978)
- Foundations on Expansive Soils – Residential (IHB TB 10-01) Texas Department of Licensing and Regulation
- Damage to Foundations from Expansive Soils Missouri University of Science and Technology (J. David Rogers, Robert Olshansky, Robert B. Rogers)
- Investigation of a leaking water line beneath residential foundation on expansive soils American Society of Civil Engineers, Geotechnical Special Publication 147 (Michigan Technological University Digital Commons)
- Expansive Soil Colorado Geological Survey
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