Concrete sleepers are produced in two basic shapes, Monoblock (that run transversely and support both rails) and Twin Block (one block under each rail with a steel tie-bar between). Monoblocks may be pre-tensioned or post-tensioned while Twin Blocks are reinforced. Pre-tensioned monoblocks are widely used on railways in the UK, having first been introduced as a replacement for the traditional wooden sleepers in the 1940s.
Pre-tensioned sleepers are manufactured using the long line method, with between 25 and 75 moulds in the line. Originally all pre-tensioned sleepers were made with wire but strand is now more commonly used. Typically the concrete used has a final strength in the region of 70 N/mm2. The necessary fixings are cast into the sleepers for fixing the rails.
Post-tensioned sleepers are cast with a duct into which the post-tensioning tendon is inserted after demoulding. When the concrete has gained sufficient strength, the unit is stressed, the tendon anchored and the duct grouted. This form of construction is more expensive than using long line pre-tensioning, but is suitable for small volume production.
Twin block sleepers consist of two blocks of reinforced concrete to support the rail and containing the fastening system, held to the correct gauge by a steel cross bar. They are generally made with a dry low workability concrete.