Learning Center · Slab Foundations
What Is a Post-Tension Slab? How It Works and How to Spot One
THE SLAB WORKS AS A THREE-PART SYSTEM
THE CABLES · CAST IN SLEEVES
Greased steel cables run through plastic sleeves in both directions before the pour, so the hardened concrete never grips them.
THE PULL · STRETCHED AND LOCKED
Once the concrete cures, a hydraulic ram stretches each cable from its live end and wedges lock the tension in for good.
THE SQUEEZE · ONE STIFF PLATE
The locked cables compress the whole slab, so it rides swelling and shrinking clay as a single plate instead of pulling apart.
The stamped warning in the garage floor exists because of part two. Never cut a cable that is still pulling.
A post-tension slab is a concrete foundation with steel cables running through it that are pulled tight after the concrete hardens. Each cable, called a tendon, is stretched by a hydraulic jack and locked off at the slab edge, and that tension squeezes the whole slab together. Concrete handles squeezing well and pulling badly, so the squeeze keeps cracks closed and lets the slab ride swelling and shrinking clay as one stiff plate. On Texas clay these foundations are designed to a Post-Tensioning Institute standard written for expansive soil, and they sit under a large share of newer houses around San Antonio. The quick way to confirm you have one is a stamped warning in the garage floor or a row of patched pockets along the slab edge.
One rule comes with it. Nobody cuts, cores or drills into a post-tension slab until the tendons have been located. The rest of this guide covers how the slab is built, how to tell whether you own one, what goes wrong, and who fixes which problem. We repair foundations with steel and hybrid piers. We do not stress or repair tendons, and we say so where it matters.
How a post-tension slab works
Concrete is strong when you push on it and weak when you pull on it. A slab sitting on Central Texas clay gets pulled in every direction as the soil swells after rain and shrinks in a drought, and that pull is what opens cracks. Ordinary rebar does not prevent it. The bars sit passive in the concrete and only start working after a crack has opened, holding the pieces together. A post-tension tendon works ahead of time. It puts the concrete under compression before the soil ever moves, so the slab has to overcome that squeeze before a crack can open.
In a house the tendon is usually a half-inch strand of seven high-strength steel wires twisted together, made to the ASTM A416 standard at Grade 270, which means a minimum tensile strength of 270,000 pounds per square inch. The stressing crew pulls each strand to about 33,000 pounds, which is 80 percent of its minimum breaking strength, and a 50-foot strand stretches roughly 4 inches as the jack pulls. The tendons run in both directions across the slab, a few feet apart, in the layout the engineer drew for that house. People call them cables, rods or tendons, and on a house they all mean the same strand. Threaded high-strength steel bars are also used for post-tensioning, mostly in structures much heavier than a house slab.
Two slab shapes are common. A uniform-thickness slab is one flat, thick plate. A ribbed slab adds deeper concrete beams that run in a grid under the floor. Both carry tendons, and both are designed to the same standard.
How a post-tension slab is built
The site work looks like any slab. The pad is graded, the perimeter and interior beams are dug, the under-slab plumbing goes in, and the forms go up. Then the tendon crew lays strand across the slab area in both directions, setting it on small supports called chairs so it runs through the middle of the slab's thickness, as the engineer's plan shows.
Every tendon has two ends with different jobs. The dead end has its anchor already fitted and gets cast into the concrete at one edge. The live end waits at the opposite edge with an anchor set in a pocket at the slab face and a tail of strand sticking out past the form. Then comes the pour. Because each strand is greased and sealed in its plastic sheath, the concrete never grips the steel. The strand stays free to slide inside its sheath, the way a bike brake cable slides in its housing.
Stressing waits until the concrete is strong enough to take the load at the anchors, which is usually a few days after the pour, at a strength the engineer specifies. Stressing too early can crush the concrete behind an anchor. When the slab is ready, a stressing crew clamps a hydraulic jack onto each tail, pulls the strand to its design force, and releases. Steel wedges grab the strand and seat in the anchor, locking the tension in for the life of the slab. The crew measures how far each tail stretched and checks it against the number the engineer calculated, which is how anyone knows the force actually went in.
Last, the tails get cut off close to the anchors and the pockets get filled with grout, which protects the anchor and wedges from rust. Those patches, small round or square scars in a row along the slab edge, are one of the tells an inspector still reads decades later.
Bonded vs unbonded post-tension tendons
House slabs use unbonded tendons. Unbonded means the strand is coated in grease and sealed in a plastic sheath, so it never bonds to the concrete and can slide while it is stressed. The grease also guards the steel against rust. That is the system used in residential slabs on the ground, and it is what the PTI design standard for slabs on expansive soil is written around.
Bonded tendons work the other way. The strand runs through a metal or plastic duct, and after stressing the duct is pumped full of grout, which bonds the steel to the concrete along its whole length. Bonded systems are used mostly in bridges and large structures. That settles a question that turns up on construction quizzes. In a bonded post-tension slab the tendons are not coated with grease. They sit in ducts that get grouted. Grease and a plastic sheath are the mark of an unbonded tendon.
Why Texas builders use post-tension slabs on expansive clay
Expansive clay changes volume with its moisture, and it rarely does it evenly under a house. When the clay around the perimeter gets wet and swells while the middle stays put, the slab edge rises. Engineers call that edge lift. When the perimeter dries and shrinks in a drought while the covered middle stays damp, the slab ends up resting on a hump in the center, which is center lift. A slab on this soil has to span over that uneven support without breaking apart, and a stiff slab that is already squeezed together is what post-tensioning delivers.
The building code writes this in. The International Residential Code sends houses on expansive soil to the foundation section of the International Building Code, and that section requires post-tensioned slabs on expansive soil to be designed to PTI DC10.5, the Post-Tensioning Institute standard for shallow post-tensioned foundations on expansive soils. PTI first published the design method behind it in 1980, and its own technical note says it has been used on millions of residential foundations since. The engineer works from the soil report for the lot to size the slab, the beams and the tendon force for that specific clay.
A post-tension slab does not stop clay from moving. It spreads the movement across a stiffer plate, so the house sees less of it. Water still decides how hard the clay pushes, which is why drainage, gutters and steady moisture around the perimeter matter as much on a post-tension slab as on any other, and why Texas clay moves houses covers the soil side. When a post-tension slab does move too far, it gets measured and repaired like any other slab.
How to tell if your house has a post-tension slab
Start in the garage. Builders often press a warning into the wet concrete there, and the wording varies from builder to builder. Common versions read along the lines of “post-tensioned slab, do not cut or core,” and some builders set a small metal marker with the same warning instead of a stamp. Check along the walls and under whatever is stored against them, because shelving, floor coatings and rugs hide a lot of stamps.
Then walk the outside. Where the slab edge shows above the soil, look for a row of small patches in the concrete, round or square, spaced evenly along the edge. Those are the grouted stressing pockets, one for each tendon that ends on that side. On some houses the patches have weathered to the same color as the slab and only show in low light. A bit of cable or a plastic cap showing in the slab edge tells you the same thing, and it also deserves a closer look, which we get to under problems below.
Paperwork settles it. The foundation plan in your closing documents will show the tendon layout and say post-tensioned on the sheet. Age is a hint too. A newer production-built house is a good bet for post-tension, and an older one may sit on a conventionally reinforced slab or on piers and beams. Until a stamp, a plan or a scan says otherwise, treat the slab as post-tensioned.
Post-tension slab vs conventional rebar slab
Both are concrete slabs on the ground with steel inside. The difference is what the steel is doing, and that changes how you drill, cut and repair them.
For an owner, two differences matter day to day. Drilling and cutting take more care on a post-tension slab, and a damaged tendon needs a specialist. Foundation repair for settlement works the same way on both.
Can you drill or cut into a post-tension slab?
Not until the tendons are located. A stressed tendon was pulled to about 33,000 pounds, and it keeps most of that pull for the life of the house. Nick one with a core drill or a saw and the strand can break and release that force at once. It can whip inside its sheath, blow out through the slab edge or the floor, and hurt whoever is standing near it. Even when nobody gets hurt, the slab has lost part of its designed strength along that line, and the repair is specialist work.
The way to do it right is to have the slab scanned first. A concrete scanning company uses ground-penetrating radar to find each tendon and mark its path and depth on the floor, and the hole or cut gets laid out between the marks. That applies to every job that goes into the slab, from a plumber's floor cut to anchoring a safe or a storm shelter. Ask for the scan before the work is scheduled, not when the drill is already in hand. The foundation plan helps the scanner, but the scan is what you trust, because the plan shows where the tendons were meant to go.
If someone does hit a tendon, stop work and keep people away from the slab edge where that tendon ends. Call the builder if the house is still under its warranty, and a post-tension repair contractor either way. Patching over a cut tendon and hoping is how a small mistake turns into a bigger one.
Post-tension slab problems
When something goes wrong with a post-tension slab, it is usually one of four things, and they call for different people.
A broken or blown-out tendon
A tendon that lets go can do it with a loud bang, and a length of strand can push out through the slab edge, the pocket patch or the garage floor. It can also fail quietly. Then the first signs are a cable end or a rust stain at the edge, a pocket patch that has cracked or popped out, or a spall where concrete broke away around an anchor. The usual causes are a drill or saw, and rust. Rust starts where water reaches the steel, most often at an anchor whose pocket was never sealed well or where the sheathing was damaged, and soil kept wet against the slab edge by a sprinkler or a downspout feeds it. A broken tendon is a structural concrete repair, not foundation settlement, and it belongs to a post-tension repair contractor. If we see signs of a tendon problem during an inspection, we tell you and point you to one.
Cracks in a post-tension slab
Post-tension slabs still crack. Concrete shrinks as it cures, and hairline cracks can form in the days between the pour and the stressing. Once the slab is squeezed, those cracks stay tight. A hairline in a garage floor is usually cosmetic. A crack that keeps widening, has one side higher than the other, or shows up alongside sticking doors and stair-step cracks in the brick points to movement under the slab, and that gets measured rather than guessed at. Our guide to whether cracks in a slab foundation are bad walks through the patterns.
Settlement and heave
A post-tension slab still sits on the clay and moves with it. If the soil under one side dries and shrinks, the slab settles there. If a leak swells the clay under the middle, the slab rises there. Because the slab is stiff, that movement often shows less as big cracks in the concrete and more as a floor that tilts, doors that stick, and cracks in the drywall and brick above. A broken drain line is a common source of the wet clay, and leak heave is easy to mistake for settlement, which is why a hydrostatic plumbing test often comes before any foundation decision.
What is and is not a foundation problem
A broken tendon is a slab problem for a post-tension specialist. A rust stain or a loose patch at a pocket is maintenance, and it should be looked at before it becomes a broken tendon. Hairline cracks are normal. Settlement and heave are foundation problems, and they are the part we measure and repair.
Post-tension slab repair
Repair depends on which of those problems you have, and on a post-tension house more than one trade can be involved.
Repairing a broken tendon
A post-tension repair contractor exposes the tendon and finds the damage. Depending on what failed, the crew repairs the anchor, splices in new strand, or replaces the tendon, then stresses it again to the design force and patches the concrete. The Post-Tensioning Institute publishes a guide for evaluating and repairing unbonded post-tensioned structures, DC80.3, and the work should follow an engineer's direction. It is not a job for a handyman or a general concrete crew, and it is not one we do.
Repairing settlement on a post-tension slab
Settlement gets repaired with piers, the same way it does on any slab. The piers go under the perimeter beam from outside the house and the lift happens from below, so no tendon is cut. We install steel and hybrid piers at $600 to $1,000 each, the piers carry a lifetime transferable warranty, and we come back once a year for a performance follow-up inspection. The full sequence from survey to lift is in slab foundation repair, and the two pier systems are explained on the steel pier and hybrid pier pages. One step changes on a post-tension house. When a pier has to go under an interior beam, the floor is scanned and the tendons are marked before any concrete is broken out, and the opening lands between them.
Plumbing repairs under a post-tension slab
When a drain line under a post-tension slab fails, the usual way to reach it without cutting concrete is from below. A tunnel is dug under the foundation to the broken pipe, the repair is made down there, and the slab and its tendons stay intact. The plumbing itself is done by a licensed master plumber. Our part is digging the tunnel and backfilling it in compacted layers when the plumber is finished, and under-slab tunneling explains how that works. Breaking the floor from above is still possible on a post-tension slab, but only after a scan.
Repairing cracks in a post-tension slab
Once movement is ruled out, a crack in a post-tension slab gets sealed the way it would in any slab, and retail crack sealers handle hairlines that have stopped moving. Any crack repair that drills into the slab gets the same scan first.
What we check on a post-tension slab
On an inspection, a post-tension slab gets the same work as any other. Our ICC-certified inspectors measure floor elevations across the house, map the cracks inside and out, and walk the slab edge, where we also look at the stressing pockets for rust, cracked patches or exposed strand. Then you get a plain answer about whether the house has moved and what, if anything, should be done about it. About a third of our inspections end with no repair needed, and the foundation inspection is free across our Central Texas service area.
