Concrete Sleepers

PRESTRESSED CONCRETE RAILWAY SLEEPERS

IHP introduced the Prestressed Concrete Monoblock Sleepers for railways in 1970. Prestressed Concrete Sleepers are made by the company by long line system initially in technical association with Dow Mac Concrete Ltd., of England, the then world leaders in sleepers. About 2.75 million IHP Sleepers made by the company are in the prestigious routes of Indian Railways.

The Company successfully completed a contract for operating a sleeper plant for Indian Railway Construction Company at Samawa in the Republic of Iraq in 1983 and Operated one of the biggest sleeper plants at Abu Ghraib - Republic of Iraq in 1982-85.

ADVANTAGES OVER TIMBER SLEEPERS

DURABILITY

The life of an IHP sleeper in track is estimated at not less than 50 years. This exceeds that of any type of timber sleeper, even in the most favorable of climate. In certain areas of the world extremes in temperature, humidity, fungus or insect attack, not to mention timber quality, reduces timber sleeper life to as little as 15 years.

ECONOMY + ENVIRONMENTAL BENEFIT

In some countries of the world, the economy of initial supply is now with the concrete sleeper. As suitable timber becomes scarce and increased production requirements reduce the price of the concrete sleeper, the comparison will become even more favorable to the concrete sleeper. Apart from this, use of concrete sleepers encourages the preservation of precious forest wealth.

LOW MAINTENANCE COSTS

Maintenance costs associated with concrete sleepers can show a significant saving over that for timber sleepers. The built-in nature of the fastening avoids the frequent attention which has to be given to the normal base-plate fastened to the timber sleeper.

STABILITY

The heavier IHP sleeper produces a higher resistance to lateral movement and also reduces the vertical movement under traffic of imperfectly packed sleepers; important factors in relation to continuous welded rail. In condition of excessive heat, a concrete sleeper track is less prone to vertical and horizontal distortion, and this has important safety as well as maintenance implications.

MAINTENANCE OF GAUGE

Due to the rigidity of the cast-in portion of the rail fitting, a concrete sleeper maintains the rail gauge much better than a timber sleeper, where the increase of gauge over the years is an accepted phenomenon. This applies especially to curved track.

ADVANTAGES OVER POST TENSIONED MONOLITHIC TYPES

ECONOMY THROUGH STRENGTH

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 :

• The IHP process produces a more uniform and accurate sleeper.
• 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.

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.

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.

ADVANTAGES OVER TWO BLOCK SLEEPERS

STABILITY

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.

• Dead weight of sleeper.
• TUniformity and holding characteristics of rail to sleeper fitting.
• Ballast consolidation around sleeper.
• 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.

LONGITUDINAL BALLAST RESISTANCE

CThe Projected transverse area of the IHP sleeper, compared with the two block sleeper, is greater by the ratio of 1.6 to 1.

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.

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.