Thickened-edge slabs make practical foundations where soils are stable. But building codes mandate special foundation designs in areas with expansive soils, because seasonal swelling and shrinking of the soil can cause significant cracking of both the foundation and the structure it supports. A proven solution for these conditions is a post-tensioned slab, which uses a grid of high-tensile steel cables — rather than conventional wire mesh or rebar — to provide strength and control cracking.

The reinforcing cables and their specialized end fittings are cast into the center of the slab before the pour. After the concrete has cured for a few days, each cable, known as a tendon, is stretched with a hydraulic ram, placing slab and footings under compression. The tendons are sheathed with a tough plastic that prevents them from bonding...

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