Concrete Sleepers
The need for concrete sleepers
has been felt mainly due to economic considerations coupled with changing
traffic patterns. In the early days of Indian Railways, wood was the only
material used for making sleepers in Europe. Even in those days, the occasional
shortage of wooden sleepers and their increasing price posed certain problems
and this gave a fillip to the quest for an alternative material for sleepers.
With the development of concrete technology in the nineteenth century, cement
concrete had established its place as a versatile building material and could
be adopted suitably to meet the requirements of a railway sleeper. In the year
1877, Mr Monnier, a French gardener and inventor of reinforced concrete,
suggested that cement concrete could be used for making sleepers for railway
tracks. Monnier in fact designed a concrete sleeper and obtained a patent for
it, but his design did not work successfully. The design was further developed
and the railways of Austria and Italy produced the first concrete sleepers with
a promising design around the turn of the nineteenth century. This was closely
followed by other European railways, where large-scale trials of concrete
sleepers were done mostly due to economic considerations.
However, not much progress could
not be achieved till the second world war, when wooden sleepers practically
disappeared from the European market and their prices shot up. Almost at the
same time, as a result of extensive research carried out by French Railways and
other European railways, the modern track was born. Heavier rail sections and
long welded rails came into existence. The necessity of a heavier and better
type of sleeper that could fit the modern track was felt. These conditions gave
a spurt to the development of concrete sleepers and countries such as France,
Germany, and Britain went a long way in developing concrete sleepers to
perfection.
Development
The development of concrete
sleepers that took place on various railway systems was mainly based on the
following concepts of design.
(a) RCC or
prestressed sleepers similar in shape and size to wooden sleepers
(b) Block-type
RCC sleepers connected by a steel tie bar
(c) Prestressed
concrete blocks and a steel or an articulated concrete tie bar
(d) Prestressed
(pre-tensioned or post-tensioned) type of concrete sleepers These four concepts
of design are the basis of the development of present-day
concrete sleepers.
Advantages and disadvantages
Concrete sleepers have the following advantages and disadvantages.
Advantages
(a) Concrete
sleepers, being heavy, lend more strength and stability to the track and are
specially suited to LWR due to their great resistance to buckling of the track.
(b) Concrete
sleepers with elastic fastenings allow a track to maintain better gauge, cross
level, and alignment. They also retain packing very well.
(c) Concrete
sleepers, because of their flat bottom, are best suited for modern methods of
track maintenance such as MSP and mechanical maintenance, which have their own
advantages.
(d) Concrete
sleepers can be used in track-circuited areas, as they are poor conductors of
electricity.
(e) Concrete
sleepers are neither inflammable nor subjected to damage by pests or corrosion
under normal circumstances.
(f) Concrete
sleepers have a very long lifespan, probably 40-50 years. As such rail and
sleeper renewals can be matched, which is a major economic advantage.
(g) Concrete
sleepers can generally be mass produced using local resources.
Disadvantages
(a) Handling
and laying concrete sleepers is difficult due to their large weights.
Mechanical methods, which involve considerable initial expenditure, have to be
adopted for handling them.
(b) Concrete
sleepers are heavily damaged at the time of derailment.
(c) Concrete
sleepers have no scrap value.
(d) Concrete
sleepers are not suitable for beater packing.
(e) Concrete
sleepers should preferably be maintained by heavy 'on track' tampers.
Design considerations
Two different concepts are being adopted by German and French
Engineers in designing the section of a concrete sleeper. The Germans, having
adopted a beam type sleeper, consider the sleeper as a rigid, stiff, and
continuous beam supported on a firm and unyielding bed. The French engineers
however, consider the sleeper as two separate blocks connected by a tie bar and
resting on a resilient ballast bed. The former design is based on static
loading, while the latter theory caters for a slightly differential settlement
of ballast support. As the calculations based on the latter theory are quite
complicated and difficult, the sleeper design based on this concept has been
evolved mostly on an empirical basis.
The forces and factors considered in the design of
concrete sleepers are the following.
(a) Forces
acting on a sleeper
(b) Effects
of the geometric form including shape, size, and weight
(c) Effect of
the characteristics of fastenings used
(d) Provision
of failure against derailments
Need for concrete sleepers in India
In India there has been a chronic
shortage of wooden sleepers over the last few decades. Wooden sleepers of
various species in India have a short life-span of about 15-20 years. In view
of this drawback of wooden sleepers, cast iron and steel trough sleepers have
been used extensively. The consumption of these metal sleepers at present is
quite high and Indian Railways consumes about 40% of the entire pig iron
production in the country. There is a need to reduce pig iron consumption by
the Railways so that the iron can be made available in large quantities for
defence purposes and other heavy engineering industries. In addition, higher
speeds, welding of rails, and installation of long welded rails have recently
been introduced in Indian Railways. A sleeper for a long welded track has to be
heavy and sturdy and should be capable of offering adequate lateral resistance
to the track. Wooden and steel sleepers were found to be totally lacking in
these requirements. Both these considerations led to investigations for
selecting a suitable concrete sleeper for use on Indian Railways.
Loading conditions adopted by Indian
Railways
Concrete sleepers have been designed by the Research Design
and Standard Organization (RDSO) wing of Indian Railways for the following different
loading conditions.
BG sleeper
(a) 15 t
vertical loads at the rail seat.
(b) Vertical
load of 15 t at rail seats plus a reaction at the centre of the sleeper equal
to half of the load under the rail seat.
(c) A
vertical load of 13 t and a lateral load of 7 t directed towards the outside of
one rail only.
The
sleeper is designed to resist a bending moment of 1.33 t m at the rail seat and
0.52 t m at the centre of the sleeper.
MG sleeper
(a) Vertical
loads of 10 t at the rail seats plus a reaction at the centre of sleeper equal
to half of that under the rail seat.
(b) Vertical
loads of 8 t at the rail seats with 4.5 t lateral force directed towards the
outside of one rail only.
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