Precast concrete slabs, beams and other sections are commonly used as permanent formwork, often in composite construction. Generally the surface of the precast concrete element will be prepared (e.g. the surface will have been roughened) so that new concrete bonds adequately to it. Depending on the expected loading the precast elements may be supplied with some projecting reinforcement that will be cast into the in situ concrete.
A common application is for floors, with precast concrete units forming the soffit. The units will contain some, or in the case of lattice girder slabs a large proportion, of the reinforcement required for the finished floor slab. (See also Precast/buildings/lattice girder slabs and Precast/buildings/beam & block slabs.) Lattice girders have also been used to form the inclined soffits of staircases cast in situ.
Precast units can be used to form the exposed face of a wall or column. High quality concrete (for example using white cement or pigments in the mix) can be used to give the required appearance with lower quality concrete for the in-fill material. The use of precast concrete permanent formwork may be an effective option when construction is close to a hazzard, such as a live railway line, that would cause problems for erection and removal of normal formwork.