The use of many fully bonded tendons allows for the provision of a larger prestressing force, with better stress distribution, in a given area of concrete. Consequently it is possible to produce sleepers with a greater moment of resistance than for the post-tensioned types. This has the sub sequential effect of allowing a greater spacing (70 centimeters) between the sleepers than is advisable with the post-tensioned monolithic or two block types.
ECONOMY IN STEEL
Drawn high tensile steel is required in the IHP sleeper. Only in post-tensioned designs in addition to purpose fabrication required for the steel tendons end anchorages are also required. In some types special alloy steels are used and these are not always easily obtainable. The tendons employed on the long line IHP process are common to the Prestressed concrete industry of the world, and in consequence are readily obtainable.
NO GROUTING
The IHP pre-tensioned system employs fully bonded tendons. The separate and unsatisfactory operation of grouting the tendons after stressing is thus completely eliminated.
STRESSING
The long-line process of manufacture dictates large extensions of the tendons required at stressing. Any error or inaccuracy in measuring this extension has, therefore, only a minimal effect on the final prestressing force in the sleeper. With post-tensioned types, the tendons are short and any small error can have a profound and disastrous effect on the final stress in the sleeper.
This statement leads to two conclusions :
(a) The IHP process produces a more uniform and accurate sleeper.
(b) The high-quality sleeper produced by the IHP system is the result of well proven production techniques rather than the individual skill of the personnel involved in the manufacturing process. It is sometimes claimed as an advantage that the post-tensioned concrete sleeper can be made shorter than the pretension sleeper because of the need to develop full bond in the latter. However some railways are lengthening their post tensioned sleeper because the additional length provides better ballast support to the track.
SLEEPER QUALITY
Every pretensioned sleeper is proven on transfer of stress; concrete and wire are immediately and obviously in balance any failures are detected at this stage and not in the track.
NO STOCK PILES
IHP sleepers can be taken straight from the production lines and loaded directly for delivery. The post-tensioned designs require a large stock piling area prior to dispatch.
PRODUCTION
The production operations are simple and do not require a sophisticated labour force as with the more complicated methods employed by the post-tensioned sleeper manufacturer.