What a Post-Tension Slab Actually Is
A post-tension slab is a concrete slab that gets its strength from steel cables stretched after the concrete has cured, rather than from passive rebar cast into the pour.
During construction, high-strength seven-wire steel strands, called tendons, are laid across the slab area inside greased plastic sheathing that keeps them from bonding to the concrete. Once the slab cures to sufficient strength, a crew attaches a hydraulic ram to each tendon at the slab edge, stretches it to a design force in the range of roughly 25,000 to 33,000 psi, and locks it off at a wedge anchor. The stretched cables squeeze the slab from both directions, so the concrete lives its whole service life in compression.
That compression is the entire point. Concrete is strong in compression and weak in tension, and cracks form when something puts the slab into tension: a soft spot in the subgrade, a swelling lobe of clay, a dried-out perimeter. A slab already held in compression has to overcome that preload before tension can develop, so it bridges soft support and resists cracking far better than a conventionally reinforced slab of the same thickness. Builders in Texas, Arizona, Nevada, California, and much of Florida adopted post-tension construction for precisely that reason, and in many Sun Belt subdivisions built since the late 1980s it is the default rather than the exception.
The tradeoff arrives when something goes wrong. A conventional slab is a passive object you can drill almost anywhere. A post-tension slab is a loaded spring holding several tons of stored force, and every decision about penetrating it has to account for that.
Post-Tension vs. Conventionally Reinforced Slab
| Post-tension slab | Conventionally reinforced slab | |
|---|---|---|
| How it is built | Greased strands in plastic sheathing are stressed with a hydraulic ram after the concrete cures, then locked at edge anchors | A passive grid of rebar and sometimes welded wire mesh is cast into the pour and left unstressed |
| How the steel works | Active. Puts the slab in permanent compression from day one | Passive. Carries tension only after the concrete starts to crack |
| Crack behavior | Fewer and narrower cracks. Slab tends to move as one stiff plate and tilt rather than break apart | Cracks earlier and more visibly, but movement is more localized and easier to read |
| Typical thickness | Thinner sections carry the same span because of the preload | Thicker sections and heavier steel for the same performance |
| Repair approach | Same piers and brackets, but every location must be scanned, marked, and worked around. Lifting is done in smaller increments | Piers, brackets, and core holes placed wherever the elevation survey calls for them |
| Penetration risk | Severe. Cutting a live tendon can release stored force violently and locally de-stress the slab | Low. A cut rebar is a minor structural loss and no safety hazard |
| Plumbing access | Requires a scan before any trenching or coring, and often an engineer’s sign-off | Cut, repair, and patch with standard masonry tools |
| Typical era and region | Mid-1980s onward, concentrated in TX, AZ, NV, CA, FL | Universal, and still standard in most of the Midwest, Northeast, and Southeast |
If you are still working out which foundation type your house has at all, the differences between slab and pier and beam foundations are the first thing to settle, because the entire repair conversation branches from there.
How to Tell If You Have a Post-Tension Slab
There are four practical checks, in the order most homeowners can actually perform them.
Look for the warning notice. Most builders using post-tension construction cast or stamp a notice into the garage slab, sometimes on a plaque near the electrical panel or at the slab edge, warning that the slab contains post-tensioned cables and must not be cut, cored, or drilled. This is the single fastest confirmation. If someone has since covered the garage floor with epoxy or tile, the notice may be hidden rather than absent.
Check the slab edge for anchor pockets. Walk the exposed perimeter and look for small patched circles or rectangles roughly two inches across, spaced at regular intervals, often at the ends of the slab where the concrete is visible below the brick ledge or siding. Those are the pockets where each tendon was stressed and then grouted over. Their spacing tells you the approximate tendon layout.
Check the construction era and location. Post-tension residential slabs became common in the Sun Belt from roughly the mid-1980s onward and are dominant in newer subdivisions across the Texas metros, Phoenix, Las Vegas, and much of central and southern Florida. A 2004 house on Blackland Prairie clay outside Dallas is very likely post-tension. A 1968 ranch in Ohio almost certainly is not.
Pull the plans. Original structural drawings, the builder’s file, or the county permit record will show the tendon layout directly, including strand spacing and anchor positions. On any job involving significant coring, having that drawing in hand saves scanning time and money.
If those checks leave you uncertain and concrete is about to be cut, treat the slab as post-tension until a scan proves otherwise. Assuming conventional reinforcement and being wrong is the expensive error.
Why Cutting a Post-Tension Slab Is So Dangerous
A tendon at working stress stores an enormous amount of energy in a small cross section. When a saw or core bit severs one, that energy releases in a fraction of a second. The strand recoils inside its sheath, the anchor can blow out at the slab edge, and concrete fragments can be ejected from the cut. Injuries from severed tendons are well documented in the concrete trades, and they happen to professionals, not just to homeowners with rented equipment.
The structural consequence outlasts the moment. A cut tendon no longer compresses the strip of slab it was designed to serve, so that band loses the preload the engineer counted on and becomes a candidate for cracking under exactly the soil conditions the design was meant to survive. A slab that has quietly lost three tendons to a plumbing trench dug by someone who did not scan is a slab operating outside its design assumptions.
The safe process is not complicated, only non-negotiable:
- Scan the work area. Ground-penetrating radar or an electromagnetic cable locator maps the tendons, along with rebar, conduit, and embedded plumbing. Tendons are draped in curved profiles rather than laid flat, so their depth varies across the span, which is why a scan reads both position and depth.
- Mark everything on the floor. The crew paints cable runs and clear zones directly on the concrete, and photographs them before work begins.
- Core only inside cleared zones. Openings are located between tendons, with a working margin on each side rather than right at the paint line.
- If a tendon must be cut, de-tension it deliberately. The strand is exposed, the load released under controlled conditions, the cut made, and the tendon later spliced with a coupler and re-stressed to the specified force, then re-anchored and grouted. This is specialty work, designed and signed off by a structural engineer familiar with post-tension design.
Cable scanning of a work area typically costs a few hundred dollars. Set that against the cost of a severed cable repair and the calculation is not close.
How Post-Tension Slabs Fail
Post-tensioning changes crack behavior. It does not make the foundation immune to the things that damage any slab.
Soil settlement beneath the slab. The most common cause of movement, and the one least related to the cables. When subgrade shrinks during drought, erodes from a leak, or compresses because the original fill was poorly compacted, the slab loses support and drops. The tendons keep it from breaking apart, so the house tilts as a unit instead. Doors bind, brick cracks near corners, and floors slope, all while the slab itself may show almost no interior cracking. This is why homeowners in post-tension markets are sometimes surprised to be told they have foundation movement.
Tendon corrosion. Water reaching a strand through a damaged sheath, a poorly grouted anchor pocket, or a failed edge detail will corrode it. Coastal Florida, irrigated landscapes that keep the slab edge perpetually wet, and soils with aggressive chloride or sulfate content are the higher-risk settings. Corrosion at the anchor is more common than corrosion at midspan, because the anchor is closest to the exterior and the pocket grout is the weak point.
Broken or slipped tendons. A tendon can fail from corrosion, from an anchor wedge slipping, or from being struck during earlier work. The symptom is usually a localized crack pattern that does not match the overall movement of the house, sometimes with a strand end visible or a blown-out patch at the perimeter.
Anchor blowouts at the perimeter. The concrete around an anchor spalls and the anchor pushes outward, leaving a cone-shaped void at the slab edge. This is a repairable but genuinely structural failure, and it usually indicates either corrosion or insufficient edge concrete around the anchorage.
Plumbing work that damaged tendons. A recurring pattern: a plumber trenches or cores the slab to reach a leaking line, does not scan, and cuts one or more cables. The house may show no immediate change, then develops cracking in that band over the following seasons. Any time you inherit a house with a history of under-slab plumbing repair, ask whether the slab was scanned.
Under-slab plumbing leaks. A supply or drain leak saturates and erodes subgrade, creating voids. This causes movement in post-tension and conventional slabs alike, and it must be found and fixed before any structural repair is worth paying for.
How Repair Differs from Conventional Slab Repair
The structural remedies are largely the same. The procedure around them is not.
Piering is nearly identical in principle. Push piers driven with the home’s weight as reaction force, and helical piers screwed to a torque-verified capacity, both transfer load to competent bearing strata regardless of how the slab is reinforced. Bracket details and depths follow the same engineering. If your problem is settlement, the fix is the same family of solutions described in the general slab foundation repair guide and in foundation underpinning.
Every penetration adds a scan step. Interior pier locations, plumbing access, and even anchor bolts for a later remodel all require cable location first. Expect the engineered pier plan to be adjusted so bracket positions fall between tendons, and expect that adjustment to be documented on a marked-up slab drawing you receive before drilling starts.
Lifting is slower and more conservative. A post-tension slab acts as a single stiff plate. Jacking hard at one point transfers stress across the whole diaphragm and risks cracking the slab, stressing anchors, or shearing plumbing. Crews lift in smaller increments across more points, and reputable contractors will tell you honestly that the target is stabilization with partial recovery rather than a perfect return to grade.
Tendon repair itself is a separate specialty. Splicing a strand with a coupler, re-stressing it with a hydraulic ram, replacing a failed anchor, or grouting a corroded pocket is post-tension work, not general foundation work. Many excellent foundation companies subcontract it. There is nothing wrong with that, as long as the subcontractor is named and the engineer signs the repair detail.
The engineer’s role is larger. On a conventional slab, a competent contractor working from an elevation survey can often scope the job. On a post-tension slab, a structural engineer familiar with post-tension design should specify the repair, particularly any work that de-tensions, cuts, splices, or re-stresses a cable, or that removes concrete near an anchorage. That report also gives you an identical scope to bid out to multiple contractors.
Repair Methods and What They Cost
| Work item | Typical range | Notes |
|---|---|---|
| Cable location scan, work area | A few hundred dollars | Priced by area and access. Full-slab mapping costs more |
| Structural engineer report or repair design | $800-$2,500 | Higher for complex scope or multiple tendons |
| Single tendon repair (splice, re-stress, patch) | $1,000-$3,000 per tendon | Published contractor ranges for a single severed strand start near $1,800 |
| Multi-cable repair project | $5,000-$15,000+ | Rises quickly when concrete removal reveals more corrosion |
| Anchor pocket repair or blowout remediation | Varies widely | Depends on edge concrete condition and access |
| Piering / underpinning the slab | $4,000-$25,000 | Driven by pier count, depth, interior vs. perimeter placement |
| Under-slab plumbing repair | Separate trade | Must be scanned and cleared before trenching or coring |
Two planning notes. First, carry a contingency of roughly 20 to 30 percent on any tendon work, because opening concrete around one damaged anchor commonly exposes adjacent deterioration, and corrosion rarely stops at a single strand. Second, if the diagnosis is settlement rather than cable failure, your budget belongs in the piering column, not the tendon column. Full pier-by-pier pricing and regional variation are covered in the slab foundation repair cost breakdown, and soil conditions move those numbers more than almost anything else, as explained in the guide to how soil type affects repair cost.
Before Any Plumbing Repair or Slab Penetration
This is where most avoidable post-tension damage happens. A leak is found, a plumber is scheduled, and concrete gets cut the same week.
Do these five things first:
- Tell the plumber the slab is post-tension before they quote, not after they arrive. It changes their method and their price.
- Require a scan of the work area and a marked floor before any saw, jackhammer, or core bit is used. Get photos of the markings.
- Ask whether tunneling is an option. Tunneling from outside the house under the slab avoids interior penetrations entirely. It costs more and takes longer, but on a post-tension slab it removes the cable risk from the equation and protects finished floors.
- Confirm insurance and responsibility in writing. If a tendon is struck, who pays for the repair and the engineering? Settle that before work starts.
- Repair the leak before any structural work. Piers installed over soil that is still being saturated will hold their position while the surrounding soil keeps moving, and you will pay twice.
Questions to Ask a Contractor
- How will you locate the tendons, with what equipment, and will I receive a marked slab plan before drilling?
- Which structural engineer is designing or approving this repair, and can I see their report?
- Are any tendons expected to be cut? If so, what is the de-tensioning, splicing, and re-stressing procedure?
- Is tendon work self-performed or subcontracted, and who is the subcontractor?
- What is the lift goal: stabilization, partial recovery, or full recovery, and what are the risks of pushing further?
- How were pier locations adjusted to clear the cables, and does that change the pier count?
- Has under-slab plumbing been leak tested before this scope was written?
- What does the warranty cover on a post-tension slab specifically, and does it exclude cable failure?
- What happens to the price and schedule if additional corroded tendons are found once concrete is opened?
- Can you provide license, insurance, and local references from post-tension jobs, not just conventional slab jobs?
A contractor who cannot answer the first three questions clearly is not qualified for this work, no matter how strong their reputation on conventional slabs. The scanning step, the engineering, and the slower lift are what you are paying the premium for, and they are what protect the house.
Related Guides
- Slab Foundation Repair
- Slab Foundation Repair Cost
- Slab vs. Pier and Beam Foundation
- Foundation Underpinning
- How Soil Type Affects Foundation Repair Cost