Concrete sleepers / ties

Since wooden sleepers are made from an expensive raw material, experiments were conducted to see what other materials were suitable for the substructure of the track. A search was made for a building material that was easy to bring to a suitable shape. Concrete was found to be such a material.
Due to the ease with which fresh concrete can be processed it offers a number of design possibilities.
The first concrete sleepers were introduced around 1890.

Production of concrete sleepers:

A reinforcement is placed in the sleeper so that it can withstand the tensile forces (which lead to crack formation). With the early stripping of the concrete from the mould elongated cavities are inserted in the sleeper, so that later the steel reinforcement bars can be fed into them and clamped. After the casting and hardening of the sleeper, the bonding takes place.

The bonding of concrete at a higher temperature occurs faster, and generates a chemical, exothermic process. By supplying steam (heat) the bonding is sped up, in order to achieve the required strength quicker.

In winter when it is extremely cold heat has to be supplied to the concrete from outside. The pre-stressing rods are introduced into the channels. The tractive force is applied and then the cavity between the steel and the concrete is filled with cement sludge.
In the late stripping process the formwork is used as a tensioning bed for the reinforcement. For prestressing in direct bonding, the prestressing reinforcement is tensioned before concreting (against the formwork) and then enveloped in the tensioned state by fresh concrete. The concrete remains in the formwork (late stripping) until it is cured.

The concrete sleeper is first cast for the preload and then must be bonded in order to achieve the compressive strength. This is called concrete production with subsequent bonding. Subsequently, the stressing steel is threaded in and prestressed against the concrete.
The compressive strength after the completion of the sleeper should be 60 N/mm². After about a day the new concrete sleeper has reached its outer form, but the final strength of the concrete takes approximately 28 days to develop and therefore much longer.

Benefits:

• resistant to animal and vegetable parasites
• restoring with plastic dowels is feasible
• high weight, thereby improving geometrical security
• good gauge retention
• low price
• long service life

Disadvantages:

• heavy weight when installing
• if there is a derailment there is severe damage to the sleeper and a replacement will be necessary
• no repair if there is cracking