Post-tensioned slab
A reinforced-concrete slab in which, after casting, steel cables are tensioned to compress it. The prestress «closes the cracks in advance» and counters deflection, allowing thinner slabs and wider spans, with fewer columns. It is the technique of large decks — car parks, malls, open-plan offices — where reducing depth and supports matters.
Technical section of the system, from inside (left) to outside (right).
A reinforced-concrete slab in which, after casting, steel cables are tensioned to compress it. The prestress «closes the cracks in advance» and counters deflection, allowing thinner slabs and wider spans, with fewer columns. It is the technique of large decks — car parks, malls, open-plan offices — where reducing depth and supports matters.
A post-tensioned slab is a reinforced-concrete slab made prestressed on site: steel cables in ducts are set in the slab and, after the concrete has set, they are tensioned with jacks and anchored at the ends. The pulled cables compress the concrete, putting it «in tension» before the loads even arrive.
Concrete is strong in compression and weak in tension: prestress keeps it all in compression, so under load the bending zone never reaches tension and does not crack. And the cables, draped in a curve, «lift» the slab, countering deflection. The result is thinner slabs, wider spans and less deformation.
The cables can be grouted in the duct (bonded) or greased and sheathed (unbonded): the first protect the steel better and redistribute the forces, the second are simpler and lighter. In both cases the anchorage heads are highly stressed points, with bursting reinforcement, to be detailed and protected with care.
Post-tensioning imposes a precise sequence: pour, cure, stress in stages, grout if bonded, cut the cables. Durability depends on protecting the cables from corrosion — the number-one enemy of a highly tensioned steel. And in fire the prestressing steel loses strength before ordinary steel: cover and protection must be sized accordingly.
Why it works
Prestress that closes the cracksConcrete is generous in compression and almost useless in tension — which is why an ordinary slab cracks on its underside, where bending puts the bottom fibres into tension. Post-tensioning turns this around before the building is even loaded. Steel cables, laid in ducts and tensioned with jacks once the concrete has hardened, squeeze the whole slab into compression and are anchored at the edges. Now, under load, the bending simply reduces a compression that was already there: the bottom fibre never reaches tension, so it does not crack. Better still, the cables are draped in a gentle curve, low at mid-span and high over the supports, so their pull actively «lifts» the slab and cancels much of its deflection. The pay-off is thinner slabs over wider spans with fewer columns — the open, generous floors of car parks and offices. The price is rigour: a precise stressing sequence, well-detailed anchorages, and above all protecting that highly-stressed steel from corrosion and, in fire, from heat — because a prestressing cable loses its strength sooner than ordinary rebar.
Span for depth (slenderness)
Comparison · insulantsNodal details
Critical junctions · sectionsThe tendon is not straight: it is draped in a gentle curve, low at mid-span and high over the supports, held to that exact profile by bar chairs. The shape is the whole point — its upward pull «lifts» the slab against the load and counters the deflection. Ordinary reinforcement still runs top and bottom.
- Slab
- Cable in duct
- High point (support)
- Low point (mid-span)
- Bar chair
- Ordinary reinforcement
At the edge the cable is locked to a steel bearing plate by wedges that grip the strands. The plate concentrates a huge force into the concrete, so a spiral or links of bursting reinforcement wrap the zone to stop it splitting. The pocket left for the jack is then cut and grouted, protecting the head from corrosion.
- Slab (edge)
- Cable (strands)
- Anchorage plate
- Wedges
- Bursting reinforcement
- Pocket to grout
Installation controls
Specification · checklist01 · Ducts & tendons
02 · Concrete & cure
03 · Stressing
04 · Anchorages & grout
05 · Slab & fire
Recurring defects
Diagnostics · siteComponent materials
The network · materialsReference regulations
2 norms- D.P.R. 380/2001Consolidated Building Act (Testo Unico Edilizia)In force
- D.M. 16/02/2007Fire-resistance classification of construction products and elementsIn force
Informational links to the regulatory framework. Always verify the current text on the official source.