Improved apparatus and methods for making, assembling and constructing a standardized wall and floor or ceiling panel made of a cementitious material, or similar material, for walls, ceilings, roofs and foundations. An embodiment of the invention implements both pre-cast and cast in place steel reinforced systems of interconnected panels to form a monolithic building structure. A panel unit is formed with longitudinal cavities and interconnected transverse cavities and when used in a structural system, a panel is connected to adjoining panels using an alignment plug. The interconnected panels thus provide passages or ducts for piping, wires and other conduit to run uninterrupted from panel to panel, wall to wall and wall to floor, which can be accessed through a closable access opening. A continuity alignment pipe or a shear transfer bar may be used in the structure. The panel may be customized by providing a door or window opening for applications calling for such embodiments.