Post-tensioned (PT) concrete pre-compresses the slab using high-strength steel tendons stressed after the concrete has cured. The result is longer spans, thinner slabs, and faster construction — at a specification and supervisor premium.
How it works
Tendons are placed in ducts within the slab. After concrete cures (typically 5 days), hydraulic jacks tension the tendons, anchoring them at slab edges. The compressed concrete resists tensile stresses much more efficiently than reinforced concrete.
Where PT wins
- Long-span flat slabs (10m+ between columns)
- Car parks and office floors with column-free spaces
- Bridges
- Heavy industrial floors
- Where slab depth must be minimised (height-restricted developments)
Slab thickness comparison
For a 9m × 9m column grid, typical office slab:
- Reinforced concrete flat slab: 280–320mm
- Post-tensioned slab: 200–240mm
30% thickness reduction means 30% less concrete, lower self-weight, and potentially lower column and foundation costs.
Cost comparison
- RC flat slab supply and install: R3,600–R5,200 per m²
- PT slab supply and install: R4,200–R6,000 per m²
- Premium typically 10–18%, offset by savings in concrete, steel, foundations, and possibly building height
Programme
PT slabs are faster overall:
- Pour to stress: 5 days typical
- Form removal earlier than RC
- Floors can be loaded sooner
Limitations
- Specialist contractor required
- Future penetrations must be planned — cutting through tendons is catastrophic
- Coring or saw-cutting requires tendon mapping
- End anchorages take floor space at slab edges
Bonded vs unbonded
- Bonded: Tendons grouted after stressing. Better fire performance, smaller anchorages. Standard for buildings.
- Unbonded: Tendons remain in plastic sleeves. Cheaper, easier to repair, less robust if a tendon fails.
Long-term considerations
- Tendon corrosion is the main long-term risk
- Modern specifications include corrosion protection (grouting, sheathing)
- Periodic inspection of anchorages and external indicators
MCFAR designs PT solutions where they bring commercial advantage.
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MCFAR GROUP has been delivering structural engineering, building, and plumbing services since 1998. Talk to our team about your build, retrofit, or renovation.
Request a QuoteFrequently Asked Questions
Can I cut openings in a PT slab later?
Only with tendon mapping and engineering input. Coring through a tendon can cause unzipping failure.
Are PT slabs more expensive than RC?
Per m² yes, but they typically save in concrete volume, steel, columns, and foundations. Net cost often comparable or lower.