Yesterday's lesson covered prestressed concrete in general terms. Post-tensioning is the variant where the steel is tensioned after the concrete has cured — which flips the construction sequence and unlocks geometries that pre-tensioning can't touch.

In pre-tensioning, strands are stretched between bulkheads, concrete is poured around them, and once cured, the strands are released — they try to contract and compress the concrete through bond friction. This requires a precasting yard with massive abutments. Pre-tensioned members are made in factories and trucked to site.

In post-tensioning, the concrete is poured first with hollow ducts embedded along carefully designed curved paths. After the concrete reaches strength (typically 75% of f'c, around 7 days), high-strength strands are threaded through the ducts and stretched with hydraulic jacks against the hardened concrete itself. The strands are locked off with wedge anchors at the ends, transferring compression into the member.

The ducts are then either grouted (bonded system) for corrosion protection and load sharing, or left greased and sheathed (unbonded system) for future inspection and re-stressing.

Why post-tension instead of pre-tension?

• Cast-in-place feasibility — you can post-tension a 200-foot parking garage slab on site. You can't truck one in.
• Curved tendon profiles — strands follow a parabolic drape that mirrors the moment diagram, putting compression exactly where tension would otherwise occur.
• Long spans, thin slabs — typical PT flat slab is L/45 thickness vs. L/28 for conventional reinforced concrete. A 30-foot span needs only 8 inches.

Real-world example: Modern parking garages almost universally use post-tensioned slabs. A typical 60-ft × 60-ft bay uses ½-inch 270 ksi seven-wire strands stressed to about 33,000 lb each (75% of ultimate). One strand per ~12 inches of slab width gives roughly 250 psi average precompression — enough to keep the slab in net compression under live load and crack-free against de-icing salt intrusion.

Quick calculation: Required jacking force per strand:

P = 0.75 × f_pu × A_ps = 0.75 × 270,000 psi × 0.153 in² ≈ 31,000 lb

Account for losses — friction in the duct, anchor seating (~¼ inch slip), elastic shortening, plus long-term creep and shrinkage. Total losses typically eat 20–25% of initial jacking force, so design effective stress around 0.6 × f_pu.

Failure modes to respect: anchor blowout (massive local force at the end zones requires spiral reinforcement), corrosion of unbonded tendons (water entry through cracked sheathing has caused parking-garage collapses), and never drilling into a PT slab without locating tendons — a severed strand releases tens of thousands of pounds explosively.