Post-Tensioning: How It Works

Post-tensioning (PT) is a method of reinforcing concrete by tensioning high-strength steel strands after the concrete has hardened.

Plain concrete is weak in tension. PT puts the concrete into compression before service loads apply — when load-induced tension arrives, it first has to overcome this compression before cracking can occur.

The basic sequence:

1. Ducts are placed within the formwork before concreting
2. Concrete is cast and cured to adequate strength (typically 30–40 MPa)
3. PT strands are threaded through the ducts
4. Strands are stressed using a hydraulic jack, elongating them
5. Force is locked off using an anchor plate and wedges
6. Ducts are grouted (bonded PT) or left with grease-filled sheath (unbonded PT)

Key parameters to record on site:

• Stressing force (in kN) — read from jack pressure gauge
• Elongation (in mm) — measured with a steel rule, checked against calculated value
• Slippage at anchor — recorded at lock-off

Bonded vs Unbonded: Bonded PT fills the duct with grout after stressing. More durable, used in bridges and segmental work. Unbonded PT uses individual strands in plastic sheath with grease — faster to install, used in slabs and building construction.