Abstract: A process for producing reinforced concrete building units such as concrete floor panels having coffer-like inner holes with smooth upper and lower surfaces, comprises providing a layer of reinforced concrete on the lower edges of a building unit comprised by an upper panel and joining ribs intersecting each other. A reinforcing steel mat is secured to the lower edges of the ribs which is then immersed in a layer of concrete resting on a plate. The assembly is vibrated to consolidate the concrete around the mat and after hardening, the plate is removed, leaving a smooth undersurface on the resulting hollow construction. The panel can be prestressed to a downwardly convex shape, in which case the plate is given a corresponding upwardly concave curvature.