Since 1999, various tram networks around Europe have been built or upgraded using the Rheda City ballastless track system from German slab-track experts Rail One. The new Edinburgh tram uses Rheda track.
The key component of this track system is a bi-block sleeper. Two concrete sleeper pads are separated and located by an integral steel lattice-girder construction which is embedded into a poured concrete slab, resulting in a stable and permanent trackbed.
Rheda City was developed from the earlier Rheda 2000 system, used for many years on main line and high speed railways.
Due to the light loading on tram tracks, the system is installed in Germany in one layer and without longitudinal reinforcement in the concrete slab. However, in some other countries this is not permitted by local regulations which insist on reinforcement.
Traditional steel bars can cause interference problems with signalling systems, so they are not a good solution.
Another form of reinforcement was needed, so Rail One worked with engineering consultants Rosenberg Engineering Offices to look at using non-metallic fibres.
The aim was to produce a product that met the requirements of international standards for reinforced concrete, but used synthetic fibres in place of the conventional metal rods. The insulating properties of these fibres would remove any chance of them interacting with signalling.
A test programme was undertaken to develop a fibre / concrete mix that would be affordable and also comply with the technical specifications.
This testing determined an optimum concrete mix which met compressive strength and environmental exposure requirements. Compressive and tensile strength were only marginally increased when compared to concrete without the addition of fibres.
Fracture surfaces and the post-cracking strength of bending-tensile beams gave very good results.
The fracture surface likewise showed an effective interlocking structure, which significantly enhanced the transfer of lateral forces and which assured the effective distribution of loads from the concrete supporting layer to the subgrade. The new system also gave significantly longer service life.
The next step was to try out the new system in practice. On 20 October 2010, a 30 metre length of Rheda City track was installed in the Berlin BVG tram network, as part of a twin-track section on a bridge in the borough of Lichtenberg.
The test setup featured the use of the newly developed synthetic-fibre concrete on one track and a conventional track-supporting layer of concrete on the other track.
The test section, with the new synthetic-fibre concrete, attempted to simulate the most stringent requirements that can be required of the system. For this reason, the track was separated from the existing bridge structure by an elastic pad.
The use of the elastic pad under the track structure leads to increased vertical deflection of the concrete track-supporting layer. This deformation means that the fibre concrete must both accept and transfer greater bending-tensile forces than normal.
During construction, use of the new fibre concrete caused no complications. The concrete was poured and processed as for the conventional system.
The synthetic fibres were added at the concrete plant, although this can also be done on-site. Financially, there was no difference between the conventional construction and the new fibre-reinforced concrete
In February of 2011, after several weeks of tram operation, a detailed inspection was made. No cracks were discovered in the track with the synthetic-fibre concrete, while lateral cracks had developed in the conventional track, distributed over the entire length of the bridge structure.
These cracks resulted from the increase in forces and movement brought about by the use of the elastic support.
The test results showed that the fibre concrete demonstrated improved material properties over conventional concrete in the track-supporting layer.
Analysing the results of reinforcing the concrete track-supporting layer under the Rheda City track with synthetic fibres showed several advantages.
There is no steel reinforcement, so the cost of that is saved. In addition, track construction is quicker as there is no need to install that reinforcement layer and, if track is being replaced, this reduces possession times needed.
Of course, the original requirement of this project is also met as the new fibre concrete does not interfere with signalling systems. Electromagnetic compatibility is no longer an issue, even if one considers the future introduction of ever more complex technology.
Substitution of longitudinal reinforcement by synthetic fibres also removes the need to divert stray current from the track. Consequently, the expensive installation of earthing measures, and the associated connection cables, is no longer required.
So a project to remove signalling interference caused by the concrete trackbed has resulted in a simpler and more cost-effective construction technique. Already, engineers at NAMA Consulting Engineers and Planners SA have picked up on this new technology, and are using it as part of the upgrade of Line 1 of the Athens Metro in Greece.
Information for this article kindly supplied by Hans-Christian Rossmann – Rail One GmbH, Torsten Rosenberg – Rosenberg Engineering Offices and Wulf Heineking-Fürstenau – Berlin Public Transport Authority (BVG).