Post-tensioning
Reinforcement in which high-strength steel strands inside sheathing are tensioned after the concrete hardens, compressing the concrete and closing cracks.
What does post-tensioning mean?
Pre-compressing the concrete lets slabs span farther, stay thinner, and remain crack-tight, which is why post-tensioned foundations are standard on expansive soils in parts of the South and Southwest. The strands remain under enormous force for the life of the structure, so cutting or coring into a post-tensioned slab without locating tendons is genuinely dangerous. Anchorage zones and tendon corrosion protection are critical details.
In practice
Never core, saw, or drill into a slab marked as post-tensioned without scanning first, and the warning stamped at the slab edge is not decorative.