i. To hold the rails to correct gauge (exact in straight and flat curves, loose in sharp curves and tight in diamond crossings).

ii. To hold the rails in proper level transverse tilt i.e., level in turnouts, crossovers, etc., and at 1 in 20 tilt in straight tracks, so as to provide a firm and even support to rails.

iii. To act as elastic medium in between the ballast and rails.

iv. To distribute the load from the rails to the index area of ballast underlying it or to the girders in case of bridges.

v. To support the rails at a proper level in straight tracks and at proper super elevation on curves.

vi. Sleepers also add to the longitudinal and lateral stability of the permanent track on the whole.

vii. They also provide means to rectify track geometry during service life.

For good performance of sleepers to fulfill the above functions or objectives an ideal sleeper should possess the following characteristics:

i. The sleepers to be used should be economical, i.e., they should have minimum possible initial and maintenance costs.

ii. The fittings of the sleepers should be such that they can be easily adjusted during maintenance operations such as easy lifting, packing, removal and replacement.

iii. The weight of sleepers should not be too heavy or excessively light, i.e., they should have moderate weight, for ease of handling.

iv. The design of sleepers should be such that the gauge, alignment of track and levels of the rails can be easily adjusted and maintained.

v. The bearing area of sleepers below the rail seat and over the ballast should be enough to resist the crushing due to rail seat and crushing of the ballast underneath the sleeper.

vi. The sleeper design should be such as to facilitate easy removal and replacement of ballast.

vii. The sleepers should be capable of resisting shocks and vibrations due to passage of heavy loads of high-speed limits.

viii. The design of the sleepers should be such that they are not damaged during packing processes.

ix. The insulation of rails should be possible for track circuiting, if required, through sleepers.

Prestressed concrete is the concrete in which permanent internal stresses are deliberately introduce, usually by tensioned steel, to counteract to the designed degree, the stresses caused in the member in service.

Pre-tensioning is the method of prestressing concrete in which the tenons are tensioned before concreting. Whereas post-tensioning is the method of prestressing concrete in which prestressing steel is tensioned against the hardened concrete. Initial tension is the maximum stress induced in the prestressing tenon at the time of the stressing operation and initial prestress is the stress in the concrete at transfer. Although the first experiments were made in USA and the first patent for prestressed concrete was issued in 1988 for P.H. Jackson of Sanfransisco, it was developed in Europe. French, German & Danish engineers tried various ways of prestressing concrete, but none was successful till 1930 when in France, the high strength steel wire was used for prestressing concrete. After 1950s, actually, the prestressed concrete has found its major uses in fabricated constructions where it can be shop controlled & tested. Lift slab constructions were precast or cast on-site. Today it can be well seen that prestressed concrete is finding its varied uses in various types of constructions.

Prestressed concrete sleepers are now being increasingly used by most of the railways throughout the world in preference to timber ones because of scarcity of timber and the inherent technical advantages of concrete sleepers for use with long welded tracks carrying heavier axle loads and designed for greater speeds. The superior structural properties of concrete sleepers add considerably to the overall stability and better performance of the total track-structure. Concrete sleepers with their elastic fastenings also provide better safeguard for important track parameters, such as, gauge, cross levels form of twist, alignment etc. as compared to other types of sleepers. Of the several types of concrete sleepers in use, survey indicates that a majority of them, as much as 70% are monoblock prestressed concrete sleepers.

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