Precast prestressed hollow-core floor

A fully precast floor made of prestressed concrete planks lightened by continuous longitudinal voids. High-tensile steel strands, tensioned before casting, compress the section and let it span large distances with a slim depth and no ordinary rebar. The planks arrive finished, are laid side by side with a crane and — once the joints are grouted and any topping cast — form an immediately load-bearing deck, with no props or formwork.

The hollow-core slab is the most widespread precast floor element in industrial, commercial and residential construction: a concrete plank, usually 1.20 m wide, run through its whole length by tubular voids that cut the weight without losing structural depth. It is prestressed with strands, produced in the factory on long casting beds and cut to length: it reaches the site as a finished component, ready to carry.

Before casting, the high-tensile strands are tensioned and anchored to the ends of the bed; once the concrete has set they are released and, through bond, impose a permanent compression on the section. Under service loads this initial compression «cancels» the tensions that would crack the concrete: the slab stays sound, stiffer and able to span far more than an ordinary reinforced floor of the same depth.

The continuous voids remove concrete where, at the centre of the section, it contributes little to strength: this cuts the self-weight — and with it the stresses and the transport and lifting costs — while keeping the depth that gives stiffness. The cores can also house small services and slightly improve insulation. The solid webs between one core and the next carry the shear.

On site the slabs bear a few centimetres onto the beams and are set side by side; the key-shaped longitudinal joints are grouted to make them act together and spread concentrated loads. Where diaphragm behaviour is needed (seismic actions) or for longer spans, a collaborating reinforced topping is added. The delicate point is the bearing: minimum length, bearing pads and tying reinforcement must be detailed to avoid slip and ensure continuity with the structure.