Biaxial slabs, also known as voided biaxial slabs, are concrete construction elements reinforced with metal frames and interspersed with pockets of air sealed in plastic containers. These voids reduce the density and weight of the ultimate material by up to 50% compared to solid concrete, making them more affordable, less damaging to the environment, more resistant to seismic activity, and better suited for certain aspects of construction, like horizontal surfaces.
Voided slabs have been in use since the 1950s, featuring diverse designs from simple hollow cores to waffle shapes. Biaxial slabs are so named because their internal structure features beams of concrete along two axes. Proprietary variations have included different materials and shapes for the air pockets, in some cases replacing the air itself with blocks of polystyrene.
All biaxial slabs feature some internal reinforcement, though the shapes and components can vary broadly. Although it can take more time to construct biaxial slabs onsite, that approach can ultimately be cheaper than using prefabricated elements.