Prestressed Concrete Sleepers (Monobloc)

Prestressed Concrete Sleepers (Monobloc)

As a substitute to softwood some experimental work was carried out during the late 1930's on concrete sleepers. Origin was used but not found very satisfactory for a number of reasons. At that stage, concrete simply replaced timber, bullhead rails and cast iron chairs being used as in other conventional track.

After the Second World War prestressed concrete was developed and used extensively on new structures. The great advantage of prestressed concrete was that concrete is kept under compression under all conditions of flexure, both under load and after. This means that tension cracks do not occur which can allow the ingress of moisture and corrosion of embedded steel.

Development of prestressed sleepers took place about the same time as development of flat bottomed rail and direct fastenings.

At the time of writing the standard sleeper for main line railways in the UK is the F27(AS or BS) prestressed concrete sleeper manufactured by the pretensioned method. Variations of this standard sleeper are available with extra holes for supporting conductor rail insulators and with shallower depth where these conditions apply. In this method the prestressing tendons are tensioned prior to the concrete being placed and are only released once the concrete has reached sufficient compressive strength to resist the induced forces thus applied. This method is also sometimes referred to as the ?Long Line' system, as sleepers are cast five sleepers or more.

Some counties outside the UK adopt the post-tensioning method where tendons are placed in debonding sheaths and the stress is applied after the concrete has hardened by application of tensile force to the tendons by jacking and final anchoring. This method is slower but less capital intensive and lends itself to small-scale production and situations where demand is less.

Standard prestressed concrete sleepers used in the UK are normally 2515mm long by 264mm wide. The depth varies from 203mm at the rail seat to 165mm at the centre line giving a total weight of 285 kg. The prestress is provided by six No. 9.3mm strands for standard use increased to eight and strands for heavy duty. These sleepers are capable of sustaining an equivalent dynamic load of 24 tonnes at each rail seat. Allowing for impact, lurching, wheel flats, poor rail joints and etc this is equivalent to the effects of the passage of a static 25 tonnes axle.

Metros and light rail systems have extensively also adopted prestressed concrete sleepers. Where maximum axle loads are less than for main line as shown above, the sleeper dimensions may be reduced accordingly. However, great care must be taken in the design to ensure that adequate allowance is made for dynamic effects, particularly fo bending moments.

The main disadvantage of the concrete sleeper over its timber predecessor is that of weight. Timber sleepers were often manhandled into their final position and replacement of single def it is sometimes known) was also done by hand. With concrete sleepers some form of mechanisation is required for these operations.