Composite lintels are stressed by a wire or wires at the centre of their depth and are designed to be used with the brickwork they support which acts as a composite part of the lintel in supporting loads. These comparatively thin precast lintels are built in over openings and brickwork is built over them. Prestressed lintels over openings more than 1200 mm wide should be supported to avoid deflection, until the mortar in the brickwork has set. When used to support blockwork the composite strength of these lintels is considerably less than when used with brickwork.
Non-composite prestressed lintels are made for use where there is insufficient brickwork over to act compositely with the lintel and also where there are heavy loads.
These lintels are made to suit brick and block wall thicknesses, as illustrated in Fig. 92. They are mostly used for internal openings, the inner skin of cavity walls and the outer skin where it is covered externally.
Precast, or prestressed lintels may be used over openings in both internal and external solid walls. In external walls prestressed lintels are used where the wall is to be covered with rendering externally and for the inner leaf of cavity walls where the lintel will be covered with plaster.
Precast reinforced concrete lintels may be exposed on the external face of both solid and cavity walling where the appearance of a concrete surface is acceptable.