Many applications of prestressed concrete involve the combination of precast prestressed concrete beams and in situ reinforced concrete slabs. Some examples of such composite construction. An in situ infill between precast beams is shown in Fig. 10.1(a) while an in situ topping is shown in Fig. 10.1(b). The former type of construction is often used in bridges, while the latter is common in building construction. The beams are designed to act alone under their own weight plus the weight of the wet concrete of the slab. Once the concrete in the slab has hardened and provided that there is adequate horizontal shear connection between them, the slab and beam behave as a composite section under design load. The beams act as permanent formwork for the slab, which provides the compression flange of the composite section. The section size of the beam can thus be kept to a minimum, since a compression flange is only required at the soffit at transfer. This leads to the use of inverted T-, or 'top-hat', sections.