Hollow-core Slabs
In the 1950s, hollow-core slabs were invented. These are prefabricated, one-way spanning, concrete elements with hollow cylinders. Due to the prefabrication, these are inexpensive, and reduce building time, but can be used only in one-way spanning constructions, and must be supported by beams and/or fixed walls.
Read more about this topic: Voided Biaxial Slab