Why do some slabs have cables running through them?

That said, hanging a TV doesn't require panic, just some care. The tendons ussually run in a grid in both directions, spaced a couple feet apart, and there are extra cables bunched up near the perimeter of the slab, so the edges are actually worse than the middle. But the cable height varies through the slab too, they drape up and down, so distance from the edge alone isn't a gaurantee. The real answer is he shold ask the builder or the city for the original structural drawings, which show the tendon layout. If those aren't available, a concrete scanning guy with ground penetrating radar can locate them in an hour or two for a few hundred bucks, which is cheap compared to a snapped cable. Or he can just mount the TV to the wall studs above the slab if it's a framed house, and keep anything fastened to the slab itself shallow, like tapcons for a floor rack. It's not that a homeowner can never touch a PT slab, it's that he should know where the cables are before the drill bit goes in.

The soil thing is basially the whole answer. I helped out on a job years back where the ground was gumbo clay and you coud watch the slab corners curl up over a dry summer. PT keeps the whole slab squeezed togehter so when the clay swells it lifts it as one piece instead of cracking it in half. Regular rebar just holds cracks shut after they form, the cables try to stop them from forming in the first place.

One thing for your buddy to check: look along the slab edge for little round plastic caps or grout plugs, those are the anchor pockets where the cables get jacked and they're the worst spot to hit. You can also sometimes spot faint herringbone cracks patterned over the slab that hint at the layout. We used to hear the cable whang like a guitar string when the crew cut the tails off at the anchors, that sound stuck with me.

The less concrete question is a yes, partly. PT lets them pour thinner slabs and skip the sawed control joints we use up here, so you get these big unbroken floor surfaces. Around my area every slab on grade has joints cut in every so often so the cracks go where we want them, and the PT approach kind of does away with that since the whole slab's being squeezed. First time I watched a tensioning crew work I couldn't get over how the jack stretches each cable before they wedge it off. It's basically pre-cracking prevention built into the pour, which makes way more sense once you've seen it done.

This thread is answering stuff I never even thought to ask. Up here I've only ever worked on full basements, so slabs on grade were never something I paid attention to until a cousin moved down to Arizona and his place came with a little card stapled in the electrical panel warning about tendons. That seemed like an odd place to put it. One thing I'm still fuzzy on though, if a cable does get nicked by a drill bit, does it just relax and lose some of the squeeze, or does it whip around like people say? My cousin's docs mention the ends are pocketed at the slab edge somewhere, so I wasn't sure if the stored energy is even a big deal by the time it's cured.