| Tendons | High-strength steel strands, usually in ducts or greased sheaths |
|---|---|
| Stressing | After concrete reaches the required transfer strength, using a jack at the anchorage |
| Bonded vs unbonded | Bonded: ducts grouted after stressing. Unbonded: greased strands that slide inside plastic sheaths (common in building slabs) |
| Benefits | Thinner slab, longer spans, less deflection, fewer cracks |
| Related code | IS 1343 (prestressed concrete) |
In a normal RCC slab the concrete cracks under load and the steel takes the tension. In a PT slab the tendons are pulled tight, so the concrete is pre-compressed and resists the tension from loads. It is widely used in flat slabs for car parks, offices and residential towers where long spans and low floor depth matter. If you are new to the topic, our guides on flat slabs and types of slabs are a good start.
How a post-tension slab is built
1. Shuttering and bottom steel are placed as usual.
2. Tendons are laid along a profile (high at supports, low at midspan) using chairs, with anchorages fixed at the slab edges.
3. Normal reinforcement is added where required and the concrete is poured.
4. When the concrete reaches the specified strength (checked by cubes), the tendons are stressed with a hydraulic jack.
5. Elongation and jacking force are recorded, the tendon ends are cut and the anchorage pockets are sealed. In bonded systems, the ducts are grouted.
Bonded vs unbonded tendons
| Point | Bonded | Unbonded |
|---|---|---|
| Tendon | Strands in a duct, later grouted | Greased strand inside a plastic sheath |
| Bond with concrete | Yes, after grouting | No; force is carried through the anchorages |
| Common use | Bridges, beams, heavy structures | Building slabs |
| Grouting needed | Yes | No |
| Cutting later | Needs great care | Needs great care; a cut tendon can lose its stress |
Post-tension slab vs normal RCC slab
| Point | Post-tension slab | RCC slab |
|---|---|---|
| Thickness | Thinner for the same span | Thicker |
| Span | Longer spans possible | Shorter economical span |
| Deflection and cracks | Lower | Higher |
| Skill and equipment | Specialist crew and jacks needed | Normal site labour |
| Future openings | Risky; locations must be planned | Easier to core or cut |
| Cost | Saves concrete and steel, adds PT cost | Lower specialist cost |
Safety points for the site
• Mark tendon locations clearly and keep the drawings. Never drill, core or cut a PT slab without scanning and approval.
• Stressing must be done by trained people; never stand behind the jack.
• Keep the tendon profile and cover as drawn.
• Do not stress before the required concrete strength is confirmed.
Always check the clause numbers and limits against the latest IS/IRC code and your structural designer before using them on a project.
What is a post-tension slab?
What is the difference between pre-tensioning and post-tensioning?
Why are post-tension slabs thinner?
Can I drill holes in a post-tension slab?
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