What Is a Post-Tensioned Slab-on-Grade Design?
A post-tensioned (PT) slab-on-grade is a residential or light-commercial foundation where steel tendons, tensioned after the concrete cures, compress the slab — resisting the cracking and bending from expansive-soil movement. The standard foundation across the expansive-soil Southwest: the PT slab floats on the swelling ground, its stiffness and prestress bridging the soil's differential movement. PTI's DC45 and DC10 methods govern the design.
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How PT slabs work
Tendons (1/2-inch strand in sheathing) drape through the slab — low at midspan, high at supports — and get tensioned to ~33,000 pounds each after the concrete reaches strength. The prestress compresses the concrete (preventing shrinkage cracks) and the draped profile resists the soil's uplift (the slab designed for the differential swell: center-lift and edge-lift conditions per PTI, from the geotechnical report's swell parameters). Thickened ribs (12-24 inches deep) stiffen the slab between tendons.
Design inputs
The geotechnical report provides the design parameters: the swell potential (the PTI design needs the soil's edge moisture variation distance and differential movement — not just 'expansive'), bearing values, and the Thornthwaite index or local equivalents. The structural engineer runs the PTI analysis (tendons sized and spaced for the moments, punching shear at concentrated loads, rib depths for stiffness). The drawings show every tendon profile — the field places them to the profile, not approximately.
Construction and risks
Tendon placement to the design profile (chairs and supports — a tendon 2 inches off profile loses significant effectiveness), concrete placement without displacing tendons, stressing only at the specified strength (early stressing cracks the slab; late stressing delays the schedule), and tendon-end anchorage protection (grout-filled pockets — exposed anchorages corrode). Cutting a PT slab later (plumbing repairs) can sever tendons explosively — locate tendons with GPR before any cutting. Homeowners must know: the slab contains tensioned steel.
The bottom line
PT slabs-on-grade resist expansive soils via tensioned tendons per PTI DC45/DC10 — design from the geotechnical swell parameters, place tendons to profile, and never cut without GPR location.
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