Post-Tensioned Concrete
Concrete reinforced with steel cables tensioned after it hardens.
Quick Answer
Post-tensioned concrete is concrete reinforced with high-strength steel tendons that are stressed with jacks after the concrete has hardened, compressing it. This lets slabs and beams span farther with less thickness and cracking. It is common in parking structures, residential towers, and long-span floors, and requires specialist installation and careful safety around tendons.
The Full Picture
Concrete is strong in compression and weak in tension. Post-tensioning (PT) addresses this by placing steel tendons, usually strands inside protective sheathing or ducts, in the formwork before the pour. After the concrete reaches specified strength, the tendons are stressed with hydraulic jacks and anchored at the slab edges, putting the concrete into compression.
The compression counteracts the tensile stresses from loads, which reduces cracking and deflection and allows thinner slabs or longer spans than conventionally reinforced concrete. Thinner floors can lower floor-to-floor heights, reduce building weight, and cut concrete and rebar quantities, which is why PT is widely used in parking garages and multifamily buildings.
Tendons may be bonded, grouted into ducts, or unbonded, greased and sheathed so they can slip within the concrete. Most building slabs use unbonded tendons, while bridges and some transfer girders use bonded systems. The Post-Tensioning Institute publishes recommendations, and ACI 318 contains the governing structural provisions.
PT introduces particular coordination needs. Tendon layouts must avoid penetrations and later drilling or coring, so scanning before cutting is essential. Stressing is a safety-critical operation, and the PT supplier usually provides shop drawings and a stressing sequence. Estimators price PT as a specialty subcontract, often alongside the cast-in-place concrete contractor.
Real Examples
Common Misconceptions
People assume: You can drill or core a PT slab like any other slab.
Actually: Cutting a stressed tendon can release large energy and compromise the slab. Tendons must be located by scanning or documents, and openings need engineering approval.
People assume: Post-tensioning and prestressed precast are the same thing.
Actually: Both compress the concrete, but PT tensions the tendons after casting, typically in place, while pretensioned precast stresses strands before the concrete is cast, in a plant.
Frequently Asked Questions
What is the difference between bonded and unbonded post-tensioning?
Bonded tendons are grouted inside ducts so they bond to the concrete. Unbonded tendons are greased and sheathed so they can move relative to the concrete. Building slabs mostly use unbonded tendons.
Why use post-tensioned slabs?
They allow longer spans and thinner slabs, reduce cracking and deflection, and can lower material quantities and floor-to-floor height, though they need specialty contractors and coordination.
Can you cut or core a PT slab?
Only after tendons are located and the engineer approves. Scanning, such as with ground-penetrating radar, is standard practice, and cutting a tendon without proper procedure is a serious safety hazard.
Who designs the post-tensioning?
The engineer of record sets the structural criteria, and a PT specialist or the engineer typically develops tendon layouts and shop drawings, which the engineer reviews.