Precast prestressed concrete beams are widely used in bridge construction. As precast construction takes less time on site, these beams are especially useful when bridging over roads, railways and waterways, where interruptions to traffic must be minimised. The standardisation of beams in the UK began during a period of major road construction in the 1950s. A set of inverted tee beams, of various depths, was introduced for the span range 7 -15 m. The beams, which have holes at regular centres through the bottom of the web, are placed on the supports side by side at 508 mm centres. Ordinary reinforcing bars are threaded through the holes, and in-situ concrete is added between and over the tops of the beams to form a composite solid slab.