Learning Center · Slab Foundations
What Is a Post-Tension Slab (Texas Guide)
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 at the slab edge exists because of link two. Never cut a cable that is still pulling.
A post-tension slab is a concrete foundation reinforced by steel cables that get tightened after the concrete cures. The cables run through the slab in plastic sleeves, and once the pour hardens, a hydraulic ram stretches each one and locks it off, squeezing the whole footprint of concrete together. That squeeze is the point. Concrete is strong when you push on it and weak when you pull on it, and a slab riding Central Texas clay gets pulled every time the soil swells or shrinks. Compression lets the slab take that movement as one stiff plate instead of a patchwork of cracking panels.
If your house went up in a San Antonio subdivision after about 2000, the odds are good you are standing on one right now. Here is how the system works, how to confirm what you own, and the one rule that comes with it.
How the cables work
Before the pour, the crew lays steel cables across the slab area in both directions, each one riding inside a greased plastic sleeve. The grease and the sleeve matter, because the concrete must never bond to the cable. When the mix goes down, the cables end up buried but still free to slide, like a bike brake cable moving inside its housing. That freedom is what makes the later tightening possible.
A conventional slab works differently. Its rebar sits passive in the concrete and does nothing until the slab has already cracked and started to pull apart, at which point the bars hold the pieces together. A tendon works ahead of the damage by pre-loading the concrete so cracks have a harder time opening in the first place. On soil that changes volume with the weather, that head start is why Texas production builders switched. What the clay itself is doing under there is its own subject, covered in why Texas clay moves houses.
Live ends and dead ends
Every cable has two ends with different jobs. The dead end is an anchor cast straight into the concrete when the slab is poured, gripping the cable for good. The live end waits at the opposite edge in a small pocket, with a short tail of cable sticking out. Once the concrete has cured enough to take the load, a stressing crew comes back, grabs each tail with a hydraulic ram, stretches the cable to the tension the design calls for, and lets wedges in the anchor seat and hold the pull. The extra tail gets cut off and the pocket gets patched with grout. Those patches, small round scars spaced along the slab edge, are one of the tells an inspector still reads decades later.
The stamped warning on the edge
Builders press a warning into the concrete while it is still wet, most often in the garage floor near the overhead door, sometimes on the exposed slab edge. The wording varies, but it amounts to a do-not-cut notice, telling anyone with a saw that the slab is post-tensioned and that cutting or coring it is dangerous. Find that stamp and there is no mystery about what you own. If the stamp has been covered by epoxy floors or landscaping, the slab plans in your closing documents or a look at those patched stressing pockets settles it, and the build year is a strong hint on its own.
Why you never cut or drill one
A stressed tendon stores energy the way a drawn bow does, and it holds that pull for the life of the house. Nick one with a concrete saw or a core drill and the release can be violent. The cable can whip inside the slab, blow out through a stressing pocket at the edge, and injure whoever is standing near it, and the foundation permanently loses part of its designed strength along that line. Splicing and re-stressing a cut tendon is specialist work, not a patch job. So the rule never bends. Tendons get located from the surface with scanning equipment before any cut, and the opening gets planned around them. That applies to plumbers rerouting a drain line, remodelers moving an island, and anyone anchoring equipment to the floor.
None of this means the slab is fragile. Hairline surface cracks still show up on post-tension slabs and are usually cosmetic, because the cables carry the structure whether or not the surface is pretty. Reading which ones matter is its own skill, and the crack-by-crack walkthrough lives in cracks in a foundation slab.
Most newer San Antonio homes sit on one
Post-tensioning spread through Texas production building in the 1990s because it delivers a stiffer slab on expansive clay without the cost of a heavily reinforced conventional design, and by about 2000 it had become the default across most San Antonio subdivisions. If your neighborhood went in after that, assume post-tension until the paperwork says otherwise. Older homes are a mix, custom builds go both ways, and the stamp or the slab plans give the final answer. The reason to care day to day is renovation. Floor safes, sports-court anchors, plumbing reroutes through the floor, and saw-cut trench work all start with the same question, and asking it late is how tendons get cut.
How repair changes when the slab is post-tensioned
Start with the reassurance. If a post-tension slab settles, underpinning works the same way it works on any slab, because piers go beneath the grade beam at the perimeter and lift the foundation from below without cutting concrete. The tendons never come into play. We install steel and hybrid piers at $600 to $1,000 each, the piers carry a lifetime transferable warranty, and the whole sequence from elevation survey to lift is walked through in slab foundation repair.
The differences show up in two places. Interior piers need an opening broken through the floor, so on a post-tension slab the cable locations get mapped and marked first and the breakout lands between tendons. And when a plumbing repair calls for digging beneath the house, the tendons sit inside the concrete overhead rather than in the soil, so the dig passes under them. The care is in supporting the work and leaving the slab bottom undisturbed, which is exactly the trade covered in under-slab tunneling. After any structural repair we come back for a yearly performance follow-up inspection, post-tension or not.
What to do before you cut, core, or worry
If you are planning work that opens the floor, get the tendons located first and hand the map to whoever holds the saw. If the question is movement rather than remodeling, sticking doors, brick cracks, a floor that slopes where it did not, our ICC-certified inspectors measure the elevations and read the slab the same way on a post-tension foundation as on any other. The inspection is free, and about a third of the time it ends with no repair needed at all.