STABILITY
The resistance to longitudinal and transverse movement is mainly dependent on four criteria :
(a) Dead weight of sleeper ..
(b) Uniformity and holding characteristics of rail to sleeper fitting.
(c) Ballast consolidation around sleeper.
(d) Friction area between sleeper and ballast.
These are discribed below :
DEAD WEIGHT
Compared with the well known two block sleeper, the dead weight per unit length is in favor of the IHP sleeper in the ratio of 1.25 to 1. Resistance to differential longitudinal movement of rail to rail
(i.e. one rail moving ahead of the other) is in favor of the IHP sleeper in the ratio of 2 to 1.
RAIL FASTENING
The spring-clip fastening holds the rail firmly yet resiliently, and with uniform toe-load, automatically on installation. The fastening used with two block sleepers is capable of producing similar holding loads, but is wholly dependent on the continued efficiency of a bolted fastening in an exacting vibration situation.
LONGITUDINAL BALLAST RESISTANCE
The Projected transverse area of the IHP sleeper, compared with the two block sleeper, is greater by the ratio of 1.6 to 1.
BASE FRICTION
The Surface area of the base of sleeper is continuous and assured contact with the ballast creates frictional resistance to lateral movement. This area per unit length is in favor of the IHP sleeper in the ratio of 1.5 to 1.
MAINTENANCE OF GAUGE
The rigidity of the monolithic sleeper ensures the rails are located constantly at the correct gauge and are maintained in the correct relative inclination. For example, it is impossible for one end of the sleeper to rotate relative to the other end. These advantages become apparent during aligning and leveling operations.
EASE OF MAINTENANCE
Maintenance of cross level is more difficult with the two block type of sleeper, since twist is a weakness of the design which is hard to eliminate in the track.
DURABILITY
The steel tie bars used with the two block sleeper are subject to corrosion and attack from atmosphere conditions. The tie bars are particularly vulnerable on account of their relative slenderness. Spalling of the concrete blocks at the point of the metal tie bars is inevitable and progressive. The high quality concrete used in the IHP sleeper is more resistant to the effects of abrasion than that of the un-prestressed concrete in the two block type.
RESISTANCE TO DERAILMENT
In cases of derailment the tie bars of the two block sleepers are inevitably deformed, with consequential loss of track gauge and certain derailment of all following wheels.
ELIMINATION OF CRACKING
If, due to abnormal overload conditions, cracks appear in the prestressed concrete monolithic sleepers, these will automatically re-close upon return to normal loads. This is not the case with the un-prestressed concrete of the two block type. The cracks remain open for corrosive attacks upon the internal steel reinforcement.
SIGNALING
Route signaling depends upon the reliability of the track circuiting, which is achieved by the inclusion of insulation pieces of the rail fastening. In the case of the IHP sleeper a failure of these insulation pieces at each end of a single sleeper will have little noticeable effect on the overall insulation properties of a given length of track. However, the same is not true with the two block sleeper, as the rail fastening is in direct contact with the steel tie bar. A complete short circuit is effected should the insulator at both ends of a single sleeper fail. The consequence of such a failure is extremely serious, and, when using continuously welded track, the location of the faulty sleeper is a major operation, as only one sleeper may be defective in many thousands.