What is fibre-reinforced concrete and how can rail engineers benefit from its unique properties? Bernard Berge, product specialist engineer, fibre division for Maccaferri, explains.
Major infrastructure projects worldwide consume huge quantities of concrete, much of it reinforced. As consumption has grown over recent years, so has the use of steel fibres as an alternative to conventional rebar reinforcement. Growth has occurred both in the sheer volume of concrete consumed as well as the variety of applications in which fibre-reinforced concrete (FRC) is employed.
In the early years of its development, steel fibre-reinforced concrete was principally used in two major areas of application: reinforcement in temporary spray concrete tunnel linings and industrial paving in ports, airports and factory flooring. With more recent advancements in fibre reinforcement technology, the use of FRC has spread rapidly into new, innovative applications such as precast segmental tunnel linings and other structural concrete uses.
What is fibre-reinforced concrete?
FRC is a compound consisting of a cementitious concrete mix into which reinforcement fibres – in this case, small steel filaments about the size of a paperclip, are mixed.
What do the fibres actually do?
The multiple steel fibres redistribute the forces within the concrete, restraining the mechanism of formation and extension of cracks. The result is a more ductile, concrete which is able to maintain a residual capacity in the post cracking phase. The steel fibres within the concrete literally ‘stitch’ the sides of a forming crack together.
Fibre-reinforced composites – the origin of the species
The principles of fibre-reinforced composites are far from new and history shows the concept was established well over 2,000 years ago with mud bricks, reinforced with straw fibres in Ancient Egypt. Fast-forward to more recent times and it goes through the applications of asbestos fibre cement – widely used during the 20th Century – and continued with the highly sophisticated carbon-fibre type materials of the aerospace and specialist automotive industries.
In the 1960s, studies by American industrial scientists, Romualdi, Mandel and others, established the theoretical and experimental foundations for the development of steel fibre as a medium to reinforced concrete.
In 1973, Battelle Development Corporation of Columbus, Ohio (USA) patented the principles of steel fibre-reinforced concrete, creating what was essentially a completely new material for civil engineers.
Battelle lodged patents all over the world and also registered WIRAND as a trademark for steel-wire concrete reinforcement fibres. Through one of its subsidiary companies, the Maccaferri Group became licensee of the patents and acquired the right to use the WIRAND trademark.
Subsequent research within the Italian cement industry and the University of Bologna, lead to further improvements in concrete performance, using fibres with an improved shape which gave better mechanical strength and/or workability. Later, in the 1980s, an automated process was developed which allowed the sprayed-on application of premixed and site-mixed FRC. Alongside this, the use of fibres in the manufacture of pre-cast concrete products progressed, particularly for components such as tunnel lining segments.
Steel vs polymer fibre reinforcement
Polymer fibres which are thinner than a human hair are also suitable as a medium reinforcement for concrete and are often used in conjunction with steel fibres to provide greatly enhanced fire resistance. These polymer fibres melt when exposed to great heat, leaving multiple microscopic tubes within the concrete into which latent moisture can evaporate. This moisture would otherwise cause explosive spalling of the concrete as it would have nowhere to expand to within the concrete matrix.
The non-metallic composition of polymer fibre reinforced concrete also has benefits in applications where the use of ferrous materials would be inappropriate due to their electromagnetic properties.
FRC in the real world
In Spain, construction of the 43km long extension to the Barcelona Metro has made extensive use of precast tunnel lining-segments incorporating steel reinforcement fibres.
Here, Joint Venture Construction Consortia, UTE Gorg, UTE Linea and UTE Aeroport used an earth pressure balance, tunnel boring machine (TBM) to excavate tunnels, with the precast lining segments placed ring by ring behind the machine, using a robotic arm.
FRC tunnel ring segments were cast off-site with the original design for the precast units required 120kg of traditional, fabricated steel cage reinforcement per cubic metre of concrete, to provide the required structural strength. No fibre reinforcement was considered at this time.
Subsequently, an initial proposal of 30kg / cum of Maccaferri Wirand steel reinforcement fibres was made in an attempt to reduce the amount of steel bar within the segments. Through ongoing testing, the amount of steel rebar was gradually reduced and a final optimised combination of 25kg of Wirand fibres and 60kg / m³ of steel rebar gave the required structural performance.
This design specification gave the strength to provide adequate performance during the placement of the segments and during the early service life of the tunnel. An early age compressive strength was also required to ensure sufficient crack resistance during the de-moulding, stacking and onsite handling phases.
Reinforcement fibres were added to the concrete mix via purpose-made dosing equipment to ensure controlled introduction and consistent dispersion of the fibres.
Some months into the tunnel construction programme, contractors proposed an alternative method of casting lining segments, this time without the inclusion of any steel cage reinforcement and relying solely on steel fibre reinforcement for the structural integrity of the unit.
The high cost of steel cage reinforcement and reduced casting time / increased mould utilisation being the principal motivations. Despite successful trials, it was ultimately decided that the use of fibre-only reinforced concrete segments was a technological step too far for the project team, having already reduced rebar content from 120kg / cum to 60kg / cum through the use of fibres.
Fire protection legislation
Recently introduced Spanish legislation concerning fire protection in tunnels has obliged contractors to incorporate polymer reinforcement fibres into precast lining segments. Along with its steel materials, Maccaferri also supplied Fibromac FR polymer fibres to the project.
At the conclusion of the works it is anticipated that the company will have supplied approximately 20,000 tonnes of steel and polymer reinforcement fibres to the Barcelona Metro construction project.
The implications of the Barcelona Metro trials may be a portent to the future design and construction of tunnels. Through a willing project team comprising contractors, designers, material suppliers and research, cost savings and performance enhancements were made possible.
With London’s Crossrail, Europe’s next huge tunnelling infrastructure project on the immediate horizon, the question is: will the project team be as innovative as those in Spain and embrace an all-steel fibre reinforced concrete segment?