What happens when a post-tension slab tendon is cut or fails?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: A post-tensioned slab is held in compression by steel cables called tendons. When a tendon is cut during a remodel or corrodes at its anchor, that part of the slab loses the squeeze that helps it resist cracking from soil movement. A stressed tendon also stores a large force, so cutting one is a safety hazard as well as a structural one.

What is a post-tensioned slab?

A post-tensioned (PT) slab is a concrete foundation with steel cables cast inside it. Each tendon is wrapped in plastic and greased so it does not bond to the concrete. A few days after the pour, a hydraulic ram stretches each tendon, which puts the slab and its footings into compression ³. A typical residential tendon is tensioned to about 33,000 pounds, stretching a half-inch cable roughly 4 inches over a 50-foot run ³. Standard tendons are seven-wire steel strands with an ultimate strength of 270 ksi ⁴.

In homes, tendons are typically spaced 3 to 4 feet apart, and the outermost tendons sit within 2 feet of the slab edge ³. Each tendon ends in a cast anchor that transfers its force into the concrete ⁴.

Why do builders use them on clay?

The Post-Tensioning Institute describes PT slabs as a residential foundation for shrink-swell soils: the compression offsets tension caused by soil movement, and the added stiffness helps the slab resist bending ¹. Residential PT slabs on expansive soil are designed under the PTI DC10.5 standard, and millions have been built in the U.S. ². Oklahoma State University researchers note that DC10.5 is widely recognized across many states ⁷. The 2024 International Residential Code added Section R506.2, which requires PT slabs to be designed to DC10.5 ¹.

That matters in Oklahoma because more than 75% of the state has bedrock that can produce expansive soils ⁸. For background, see shrink-swell and the active zone and post-tensioned slabs.

How do tendons fail?

Cutting during alterations

Tendons can be cut by other trades or during renovations ⁶. Plumbing and other openings are normally placed between tendons at their 3-to-4-foot spacing without changing them ⁵, and saw cuts are supposed to be placed so they do not damage tendons ⁵. When a strand is cut, its force is lost along that line.

Corrosion

Unbonded tendons can corrode, especially in aggressive environments ⁶. Water that gets inside the sheathing collects at low points, where corrosion is often worst ⁶. If an anchor exposed to water or chlorides is not properly protected, its wedges may corrode to the point that they can no longer grip the strand ⁶. During construction, the stressing pockets at the slab edge are grouted to protect the anchor, cable and wedges ³. Encapsulated tendons add a plastic boot over the anchor to isolate it from moisture ⁴.

Why is a cut tendon a hazard?

The stresses behind a stressed anchor are very high, and the engineering literature warns they can cause sudden, explosive concrete failure ⁶. Sudden loss of tension on a tendon can damage ceiling and floor finishes ⁶. Engineers treat safety as the first consideration in any tendon repair ⁶.

What can tendon failure look like?

The typical sign is a visible slab crack, often near where work was done or along the line of a tendon. Damage at an anchor may show as cracking or spalling at the slab edge. Because ordinary shrinkage and soil movement also crack slabs, the history of cuts, cores and remodels is a key part of the diagnosis. See visible slab cracks and heave vs settlement.

How is tendon damage repaired?

Repairs are essentially the same whether the strand was cut or corroded: the damaged length is spliced so it can be stressed again ⁶.

These repairs are engineered work. In severe cases, slab or foundation replacement may be considered, and a monitoring program can track whether cracks are changing. See working with engineers.

Related

• Post-tensioned slabs
• Visible slab cracks
• Slab or foundation replacement
• Monitoring program
• Codes and permits
• Working with engineers