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Voided suspended slabs can be constructed by incorporating hollow plastic spheres between the upper and lower reinforcement layers. The effect of the spheres is to displace concrete from areas in the span in which it has no structural benefit; a solid slab may be required over the columns to prevent punching shear.
The spheres can reduce the dead weight of the slab by up to 35% compared to a solid slab of the same load capacity, while still maintaining its structural capacity. The plastic spheres are brought to site preassembled into mats, held in position by welded steel fabric on the top and bottom. (There are limitations on the plan dimensions of the mats imposed by the need to transport them to site.) On site, the mats are simply craned into position, to rest on the bottom layers of reinforcement previously installed on the soffit formwork. Any additional top reinforcement is installed and the concrete cast.
Currently two systems are available, namely Bubbledeck and Cobiax.