Abstract: A precast concrete slab is formed by positioning open-ended access units at selected locations at the bottom of a mold and providing an arrangement of parallel channels over the interior ends of the access units. Concrete then is poured into the mold to surround the sides of the access units and the exposed areas of the channel arrangement. After the concrete has cured, the casting is removed from the mold and is inverted. A floor assembly is produced by supporting the ends of at least two slabs in spaced relationship with their respective channels axially oriented. Connectors are positioned in axial alignment with the channels to interconnect them, and a header having at least one channel therein is positioned to overlie the connectors such that the header is located in the space between the ends of the slabs and extends in a direction substantially normal to the axial orientation of the channels in the slabs.