Steel fibres – essentially short lengths of specialised steel wire with ‘hooked’ ends – are added to concrete used in tunnelling applications to provide much-needed strength, resistance to cracking when concrete dries, and importantly the possibility to use less concrete than would otherwise be necessary. When the steel fibres are introduced into the wet concrete, they naturally form a 3D grid, providing the strength needed.

London’s rapidly growing population means that the travel authority Transport for London (TFL)’s goal of keeping London working and growing is more challenging than ever. Currently, 8.4 million people live in the city. This is expected to become 10 million in the 2030s, so TFL must continue to support London’s infrastructure requirements if the city’s success is to continue. A key component of that is the GBP 14.8 billion Crossrail project, which includes 21 kilometres of tunnels, 42 metres below ground level.

To deliver the necessary steel reinforcement for the tunnels, our technical experts delivered detailed guidance throughout the project. This included the development of new steel fibre qualities to meet the demanding project specifications, optimising dosage rates, the supervision of performance tests, the installation of dosage equipment and ongoing on-site support throughout the project.

Two new high-performance steel fibres with exceptional tensile strength were developed by ArcelorMittal specifically for the Crossrail project. They used a new steel wire quality with an ultra-high tensile strength (above 1900 N/mm2), allowing contractors to reach demanding performance specifications and demonstrate how it is possible, with relatively low dosage rates, to reach new levels of fibre-reinforced concrete performance and residual flexural strengths. The two new grades of premium quality steel fibre are HE++ 90/60 and HE+ 55/35. These high-tensile fibres can also be used in mining, pre-cast production and other applications requiring high-strength concrete.

Scheduled to be completed in 2020/21, Crossrail’s trains will transport up to 72,000 passengers per hour through the new tunnels, increasing rail capacity and reducing journey times for everyone who crosses the capital. Crossrail, which is jointly funded by the Department for Transport and TFL, is a vital part of the UK Government’s commitment to invest record amounts in the rail network as part of its long-term economic plan.