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There are many situations in which a connection is required between a precast concrete element and in situ concrete. A typical example is the connection between precast concrete bridge beams and the in situ concrete deck slab (see also Civil engineering/bridges/precast bridge beams). So that the precast and in situ concretes act compositely, it is necessary to ensure an adequate transfer of load (mainly in shear) between the two. This is achieved by the surface of the precast concrete being roughened (generally as one stage of the production process) and having some reinforcing steel projecting through the interface. (The latter prevents separation of the surfaces as well as carrying some of the shear load.) Guidance on the surface preparation and the amount of reinforcement required for a particular application is given in the appropriate design standards.
Where it is only necessary to transfer a limited amount of shear per unit area of the interface between the precast and in situ concretes, such as a precast concrete floor slab with a structural topping, it will be sufficient to simply have a roughened surface on the precast unit; no projecting steel will be required.
When forming any connection, it is important that the surface of the precast concrete unit is clean. All loose material and any contaminants, such as oil, must be removed before the in situ concrete is cast.
See also related category ´Standards, codes & regulations/standards/design´.
Acknowledgement: The Concrete Society