This is the third and final article in the History of Sleepers series, authored by Mr D.C. Robertson and published in Railways Africa Magazine in 1957.
We have seen how railway sleepers developed from cast iron to wood, and
then to steel. The steel sleepers of the 19th Century were not strong enough for the increasing loads. The weakness was remedied by developing a heavier sleeper, using thicker plate and reinforced with ribs.
But for high speed work, steel sleepers do not produce as smooth a running top as wood sleepers. The ballast must be crushed smaller, which makes it more expensive. Unless insulated, it is useless for track circuiting.
Recently, however, a 4 mm Neoprene pad has been developed to insulate the rail from the sleeper. If this is successful, steel sleepers can be used for Centralized Train Control, and the various forms of electric signalling. This insulation may well help to bring steel sleepers back to modern tracks.
Concrete Sleepers
Concrete sleepers can be manufactured locally. Here lies their main advantage. The raw materials are cement, sand, stone, steel and water. Concrete is extremely durable and so far, no limit has been set on the life of a concrete sleeper as against the statistical life of 18 years for a Jarrah sleeper and 35 years for a steel sleeper.
The main disadvantage of concrete is its weight. For 3 ft. 6 in. gauge, a concrete sleeper weighs about 375 lbs. as against 175 lbs. for timber (Jarrah) and 135 lbs. for steel. Four men must handle a concrete sleeper, but once bedded down it is more stable. Being more stable makes concrete sleepers better for long rails, which tend to kick the roadbed out.
Concrete is a good conductor of electricity, and hence suffers from the same disadvantages as the steel sleeper in this respect. There is thus the same pressing need to find an insulating pad to make concrete sleepers good for track circuited sections.
Concrete Blocks
The concrete pad or block was introduced in South Africa in 1943. It is heavily reinforced and merely supports one rail. It carries a malthoid pad under the soleplate and the bolts may either be set in the concrete or coach screws screwed into prepared holes. The gauge is held by using transverse sleepers, one to every three pairs of blocks.
Block sleepers can be used extensively in all sidings and unimportant roads and save the timber sleepers for the main lines.
In France, M. Laval, District Engineer Western Region, French National Railways, developed track, which was supported on large longitudinal concrete blocks, each weighing about 700 lbs.
Rubber pads 3/16 inch thick were used between rail and concrete, more as a shock abscrber than for insulation. The gauge was held by steel cross pieces attached to the rails.
R.S. Sleepers
A further development occurred in France when the R.S. sleeper was developer on the National Railways. This consists of two blocks of high-grade concrete connected by a length of steel T-section. There is a rubber pad under the rail seat.
A feature of the R.S. sleeper is the flexible holding down clip. The flexible clip holds the rail, but still allows it to tilt in a vertical plane as the characteristic wave of settlement in the track passes over when a moving load is applied.
This flexible clip is essential for all concrete sleepers. There is a stretch of these R.S. sleepers between Johannesburg and Pretoria in the main line.
Transverse reinforced concrete Sleepers
In 1940, transverse reinforced concrete sleepers were produced by the Stanton Iron Works Company to a design supplied by the Ministry of Transport of the United Kingdom.
These were not, of course, prestressed sleepers. They were 7 ft. 6 in. long and had a section of about 10 in. by 5 in. Reinforcement consisted of eight 3/8 inch diameter rods, four in the top and four in the bottom, running right through.
B.S.S. 986 for 1941 demanded a moment of resistance of 40,000 lb. inches under the rail seat and 28,000 Ib. inches at the centre. An analysis of the design reveals that the sleeper achieves these requirements.
In South Africa, in 1944, the author produced enforced concrete transverse concrete sleepers, using only four 3/8 inch rods as reinforcement. These proved remarkably successful in sidings and, although cracked, continued to give good service for ten years, when they were removed on account of track alterations.
The designer of a reinforced concrete sleeper must put up with two things
- high shear stress, causing high principal tensile stresses.
- corrosion due to the hair cracks which must develop.
In order to overcome both these difficulties, we turn to prestressed concrete.
Prestressed Concrete Sleepers
Before 1939 the practice of prestressing concrete was already being carried out.
Prestressing requires a knowledge of the following:
- The stress strain characteristics of the high tensile steel wire used. This wire must have an ultimate stress of about 100 tons/sq. inch;
- the stress strain characteristics of the concrete;
- the shrinkage of the concrete.
The principle of prestressing may be simply described as follows.
In our well-proved beam bending theories there are equal tensile and compressive stresses on opposite fibres in a geometrically symmetrical section such as a wood sleeper. For a 10 in. x 5 in. section with a section modulus of 41.7 in3. units subject to a Bending Moment of 66,000 lb. in., a compressive and a tensile stress of about 1,500 1b./sq. in. is produced.
While suitable for most timber sleepers, concrete under this tensile stress would crack like the proverbial carrot. If we can produce a compressive stress of 1,500 lb./sq. inch before the Bending Moment is put on, then the result of the tensile stress is zero.
Moreover, we apply the compressive stress at the edge of the core area, which, for a rectangular section, is the middle third. The total compressive stress is thus not increased.
This is demonstrated in Fig. 7. At the same time, the principal tensile stresses due to shear are very much reduced by the combination of compressive stress with shear stress.
The action of prestressing has thus eliminated the two important drawbacks in the plain reinforced sleeper. To apply a prestress of 1,500 lb./ sq. in. in a 10 in. x 5 in. or 8 1/2 in. x 6 in. section requires a force at the middle third of (1550×50) ÷ 2 lb. or about 17 long tons.
There are certain losses to be accounted for due to elastic contraction of steel and concrete, creep, shrinkage, and release slip. Hence, a load of about 20 tons is needed initially.
Generally, in sleepers this is applied by stretching high tensile steel wires and casting the sleeper round the wires as in the Hoyer system. The technique is to use 600 feet casting bed, doing about 70 sleepers in line. The sleepers are cast round the wires, and when the concrete has has acquired sufficient strength, the wires are cut. The tension in the wires is then passed to the sleepers as a compressive force by virtue of the bond between the concrete and the wire. Prestressing wire is made in three diameters, 7 mm, 5 mm, and 3 mm.
The wires are first tensioned to 70 tons/sq. in. or a pull of 1,700 lb. on a 3 mm diameter wire. To produce a force of 20 tons initially, some 26 wires are needed. When the concrete has attained a strength of 4,500 lb./sq. inch, the wires may be cut.
Anchorage at the ends is obtained by bond and by swelling of the wire after cutting. There are several types of prestressed sleepers on the market.
On the Continent are several German types, the Freyssinet type, and others, while in England we have, among others, the Dow Mac and Stent sleepers. It will be noticed that most prestressed concrete sleepers have flexible rail fastenings.
Negative Bending Moments
Whereas a timber sleeper must be of uniform section throughout, a concrete sleeper can be shaped to save weight and to minimize bending moments at the centre (Fig. 9).
By reducing the thickness of the sleeper at the central portion, the prestressing force can be made to act centrally.
A sleeper 8 in. x 6 in. under the rail seats can be made 8 in. x 4 in. at the centre. In this way, it will get its prestress in the middle and will be able to stand bending moments, both positive and negative. These are induced by centre packing and when handling. If a central B.M. of 20,000 lb. in. is expected, then the prestress required is about 1,000 lb./sq. in. applied centrally, or a force of 32,000 lb. on a section 8 in. by 4 in. This corresponds to the 17-ton force required at the middle third of the 8 in. by 4 in. section.
Quality of Concrete
Prestressed sleepers have to be of good quality concrete. 6,000 lb./sq. in. should be attained in 28 days. This calls for strict concrete mix control and usually the use of vibrators.
Using the data given by Mr. F. S. Fulton, Director of the Concrete Association, the following mix proportions are specified:
- Cement by volume 1 part Coarse
- Sand by volume 1.2 parts Stone by volume 1.8 parts
- Water/cement ratio by weight 0.40.
- This requires 10 cu. ft. of cement per cubic yard.
Cost of Prestressed Concrete Sleepers
Each sleeper will contain 2.4 cu. ft. of concrete and about 81 Ib. of prestressing wire. At current rates, the cost of these materials is 16s. 6d. Labour cost will be in accordance with local conditions.
Under average conditions, the prestressed sleeper holds its own financially with wood or steel sleepers. No account has been taken of its longer life, though this must give it a decided financial advantage.
Conclusion
The process has made a full circle. Instead of cutting his sleepers from the neighbour-ing forest, the platelayer can now get his sleepers made from local materials at a nearby depot. We can, moreover, control the quality and design of his new sleepers, thus using just the right material for a particular job, which, in the end, is the basis of engineering.
References listed in the article:
- B.S.S. No. 986 ‘Concrete Railway Sleepers’ 1941-1944.
- General Managers’ Reports, S.A. Railways 1930-1956.
- Walley: ‘Prestressed Concrete’.
- ‘Concrete and Constructional Engineering Jan. 1941, Feb. 1950.
- Railway Gazette, Aug. 29, 1952, March 26, 1954.
- D. C. Robertson: ‘Railway Sleep- er Problem in South Africa’.
- Erady: ‘Crisis in Britain’.
- Timoshenko & Lessels: ‘Applied Elasticity’.
