A post-tensioned slab is a concrete slab compressed by high-strength steel tendons that are tensioned after the concrete has hardened. The system is common in car parks, offices, apartments and mall podiums because it can span farther with a relatively thin slab. This article explains how it works, when prestressed concrete is more efficient, and what building owners must know before renovating.
How prestressed concrete works
Concrete is strong in compression but weak in tension. In ordinary reinforced concrete, the steel bars only start working once the concrete on the tension side begins to crack. Prestressed concrete takes a different approach: the concrete is compressed in advance, so part of the tension caused by later loads is cancelled out.
There are two ways to apply prestress:
- Pre-tensioning: strands are tensioned before the concrete is cast, usually in a precast factory.
- Post-tensioning: tendons are placed in ducts or sheathing, the concrete is cast, and the tendons are tensioned once the concrete reaches its specified strength.
In a post-tensioned slab, tendons are usually draped: low at midspan and high over the columns. When tensioned, this curved profile pushes upward and offsets part of the slab's dead load, a principle called load balancing. Post-Tensioning Institute (PTI) seminar material suggests balancing about 60–70% of dead load as a starting point for slabs, but the final value always comes from calculation.
In Indonesia, structural concrete design for buildings, including prestressed concrete, follows SNI 2847:2019, the structural concrete requirements for buildings.
Bonded vs unbonded post-tensioned slabs
| Aspect | Bonded | Unbonded |
|---|---|---|
| Tendon protection | Duct grouted after stressing | Greased strand in plastic sheathing |
| Link to the concrete | Bonded along the full length | Free to move inside the sheathing |
| Typical use | Larger members such as bridges and car parks | Widely used in building slabs |
| If a tendon is cut | Force is lost mainly near the cut | Force in that tendon can be lost along its length |
That last row is why cutting an unbonded tendon is so risky.
When are post-tensioned slabs more efficient?
Post-tensioned slabs are not always the better choice. They tend to win when:
- Column spans are long. For parking, open-plan offices or ballrooms, tendons allow longer spans than an ordinary reinforced slab of the same thickness.
- Floor height is limited. A thinner slab without deep beams can save floor-to-floor height or leave more room for MEP services.
- Deflection and cracking must be controlled. Because compression acts from the start, prestress helps stop cracks forming and reduces deflection.
- The building is tall with repetitive floors. Repetition makes tendon placement and stressing easier to schedule.
On the other hand, the system offers less when spans are short, the plan is very irregular, large openings are scattered across the slab, or experienced crews are hard to find. Stressing must be done by trained workers under an engineer's supervision.
Tip: Compare slab systems at the concept stage. A fair comparison covers slab thickness, floor-to-floor height, schedule and services, not just concrete and steel quantities.
The construction sequence on site
To help owners understand the schedule, here is the usual sequence for a post-tensioned slab pour:
- Formwork goes up, then ordinary reinforcement, tendons and anchorages are placed to the design profile.
- Tendon positions and heights are checked before the pour, because the profile determines the upward force.
- The concrete is cast and cured until it reaches the strength specified for stressing, confirmed by test specimens.
- Tendons are stressed with hydraulic jacks, and the measured elongation is recorded and compared with the design value.
- Tendon tails are cut and the anchorage pockets are grouted to protect them from corrosion.
Formwork and shoring should not be removed until stressing is complete and approved by the engineer.
What to watch for when renovating a building with post-tensioned slabs
This is the most important section for owners and facility managers. With post-tensioned slabs, never drill, chase or cut the slab without a structural engineer's review. A cut tendon is very hard to repair, and in unbonded systems the prestress can be lost along that tendon.
Safe steps before making a new opening, for example for a stair, shaft or pipe:
- Collect the structural drawings and the as-built tendon layout.
- Ask a structural engineer to check whether the opening location is acceptable.
- Locate the tendons on site before drilling, especially near columns and anchorages.
- If a tendon really must be cut, the procedure involves de-tensioning, casting new anchorages, re-stressing and reinforcing the opening edges where needed.
Also avoid adding excessive conduits or penetrations near anchorages and columns; PTI material lists this as a common problem.
If you notice unusual cracking in a post-tensioned slab, start by identifying the crack type with our guide to hairline vs structural cracks. If you plan to add load or a storey, read 6 things to check before adding a floor.
Conclusion
Post-tensioned slabs are efficient for long spans, limited floor heights, and control of deflection and cracking. But they demand disciplined design, construction and maintenance, especially during renovation. Whether a post-tensioned slab suits your building can only be answered by calculation from a competent structural engineer.
Comparing slab systems for a new project? Talk to Stadin's structural design team.
This article provides general information. Every project still needs to be analysed and calculated by a competent structural or geotechnical engineer, in line with the applicable SNI codes and regulations.
