Treatment of Wooden Sleepers
Untreated railway sleepers are prone to attack by decay and vermin. The life of untreated wooden sleepers is thus very less. The life of untreated sleepers can be prolonged considerably b treatment. An extra life of 30-50% is estimated for treated railway sleepers over untreated railway sleepers.
The fibers of wood contain millions of minute cells containing juices. When these juices ferment, they lead to decay of timber. In the treatment process these juices are removed as much as cells are filled with some preserving solution. The preserving solutions may be oil or some salt solution.
The treatment processes can be categories:
- Treatment by creosote oil
- Treatment by salt solutions
- Treatment heating under pressure
- Painting
Treatment by Creosote Oil
It is also known as creosoting. In this process the railway sleepers are placed in a cylinder of about 90" length and 6" diameter and are heated upto 175oF. A vacuum juices, afterwards, creosote oil can be applied in two ways:
- Full cell process In this process without destroying the vacuum, creosote oil is pumped into the cylinder at a pressure of 75-150psi. all the cells are thus filled with the oil thus giving the name full cell process to the treatment. About 10-15 lbs of oil per cubic foot is required in this process.
- Empty cell process in this process, air is first forced into the cells followed by oil under pressure. Less oil is used in this process. When the pressure is released some oil is forced out by the entrapped air. In this method sleeper does not ooze oil. The railway sleepers treated with this method are widely used in Pakistan. It resists the vermin and decay attack throughout its life and the life is considerably increased.
Treatment by Salt Solution
In this process not pressure is employed but a sleeper is soaked in the salt solution over a long period two types of salts are generally used i.e. Zinc Chloride. The process is called burnetising process if the former salt solution is used and is called kyanisizing process if the later salt solution is used.
The salt solution treated railway sleepers are cheaper and does not require expensive plants but the solutions are likely to be washed away from sleepers by rain water. Moreover the salts are poisonous and have greater danger towards labor handling them.
Heating under Pressure
In this process sleepers are subjected to high temperatures and pressures. The natural juices are rendered harmless by this method.
Painting
Sometimes untreated sleepers are painted. It pressures timber by preventing the entrance of moisture into the wood as long as the paint is intact. The ends should not be painted as it leads to decay known as dry rot. It is not an efficient process of treating sleepers.
