Fibre-reinforced concrete (FRC) consists of a cementitious, hydrated paste into which reinforcement fibres, usually small, steel filaments about the size of a paperclip, are mixed.

The fibres redistribute the forces within the concrete, restraining the mechanism of formation and extension of cracks. The result is a more ductile concrete that is able to maintain a residual capacity in the postcracking phase. Steel fibres within the concrete literally stitch the sides of a forming crack together.

Micropolymer fibres, which are thinner than a human hair, are often used in conjunction with the steel structural 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 principal benefits of FRC are reduced shrinkage cracking, increased impact and fire resistance and a reduced need for conventional steel bar reinforcement.

In the UK, FRC has found favour mainly for the industrial floor slabs arena where its improved impact resistance characteristics are especially beneficial in applications where high or aggressive traffic loadings are expected.

In Europe, FRC, with both steel and polymer fibre, has a far more enthusiastic following with applications across a wide range of civil engineering applications.

Barcelona Metro
In Spain, construction of the 43km extension to the Barcelona Metro has made extensive use of precast tunnel lining segments incorporating steel reinforcement fibres.

When completed it will be the longest and one of the deepest lines in Europe and the longest metro line in the world of entirely new construction. It will also be the most expensive enterprise the Catalan government has ever undertaken. The final projected costs are believed to be close to EUR 6.5bn (USD 8.15bn) in 2012.

Three joint venture contractor groups, UTE Gorg, UTE Linea and UTE Aeroport, used precast FRC segments for lining their three sections of the 12m diameter tunnel totalling 11.7km in length.

At the 3.8km Sagrera TAV-Gorg section, construction work began in 2003. An EPB TBM was used to excavate the tunnel, with the precast lining segment rings erected mechanically behind.

During the tunnelling process the hydraulic thrust jacks on the TBM work off the precast concrete segments. Overall thrusts of up to 140MN can be reached during the jacking process, with a normal working range of between 90 and 120 MN.

The excavated diameter of the tunnel is 12.1m with an overall lining thickness of 400mm, including the precast concrete lining.

Segment fabrication
FRC tunnel ring segments were cast off site with rings comprising seven segments of 4.56m length (48 degree length of arc) plus a 24 degree keystone. Each ring has width of 1.8m and is 350mm thick.

Segments were cast in curved steel moulds with vibration applied to consolidate the concrete mix, and then heat cured at 40C to 50C for four to six hours, before de-moulding and stacking in an open yard.

Correct forming of the concrete in open curved moulds required a mix of stiff consistency (hence low workability). In turn, this led to the need for higher amplitude vibration. The top surfaces of the segments were finished manually.

The original design for precast segments required 120kg of traditional steel reinforcement in the form of a prefabricated cage, to provide the required structural strength. No fibre reinforcement was considered at the time.

An initial proposal of 30kg/m3 of Maccaferri Wirand FF1 steel reinforcement fibres was made in an attempt to reduce the amount of steel bar within the segments. Testing resulted in refinement of the fibre reinforcement and a new fibre, Wirand FF3 with a higher length/diameter value of 67 was developed, offering improved performance.

Eventually, only 25kg/m3 of Wirand FF3 was found to achieve the same performance as 30kg/m3 of the FF1 fibres. More testing was implemented and the amount of steel rebar was gradually reduced. A final optimised combination of 25kg of Wirand FF3 fibres and 54kg/m3 of steel rebar gave the required structural performance (28-day compressive strength of 40MPa).

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 of at least 40Mpa (2900psi) was also required to ensure sufficient crack resistance during the de-moulding and stacking phase, so curing accelerator additives were used in the mix.

Reinforcement fibres are added to theconcrete mix via purpose-made dosing equipment with a design specially modified by the supplier to ensure controlled introduction and consistent dispersion of the fibres. Five feeder machines were installed in the batching plant producing segments for all three lengths of tunnel.

To minimise seepage of water into the tunnel, crack widths were limited to 0.2mm, requiring a minimum 4-point loading flexural strength of 2.9MPa. Steel fibres, with high-strength/low-strain characteristics offer this performance; 25kg/m3 of Wirand FF3 offered a flexural strength of 3.5MPa.

The inclusion of reinforcement fibres also helped reduce the flexural stresses experienced during de-moulding andstacking, and controlled shrinkage cracking and thermal cracking caused by sudden ges when segments were moved from the curing chamber to storage yard.

Similarly, the lining segments were shown to have good resistance to accidental impact damage as well as the highly concentrated loads imposed during segment placement and the application of the jacking forces from the TBM; often a critical phase for precast lining segments.

Fibre-only reinforcement
Months into the tunnel construction programme, contractors proposed an alternative method of casting lining segments, this time without the inclusion of 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 were the principal motivations behind the proposal. The proposal was a revolutionary step, as this had not been considered for the final lining of tunnels.

In 2003/04 laboratory trials were carried out in conjunction with Maccaferri at the University of Bergamo in Italy to ascertain the viability fibre-only reinforcement. Fibre content was increased from 30kg/m3 to 60kg/m3 to replace the steel cage, yet maintain the required structural performance.

A series of measurements was implemented to evaluate the performance of the revised units, including:
• Finite element analysis to calculate the stresses incurred during stocking, handling and installation
• Characterisation of the flexo-traction resistance of the fibre-reinforced concrete with laboratory tests to prove actual material resistance
• Full scale in situ testing, installing approximately 20m of tunnel permanent final lining using segments reinforced with steel fibre only.

The results of the laboratory and site trials were presented at the 6th Annual RILEM Conference in 2004, and it was concluded that the segments reinforced with 60kg/m3 steel fibre could satisfy the project requirements without needing the inclusion of conventional steel bar cage reinforcement.

Despite the evident success of the 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/m3 to 54kg/m3 through the use of fibres.

Fire protection legislation
Recently introduced Spanish legislation concerning fire protection in tunnels has obliged contractors to incorporate polymer fibres into precast lining segments. Along with its steel materials, Maccaferri is also supplying Fibromac FR polymer fibres to the project.

At the conclusion of the works in 2012, the company will have supplied approximately 20,000t of steel and polymer reinforcement fibres to the Barcelona Metro construction project.

Conclusion
The implications of the Barcelona Metro trails may influence 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.

A paper describing the author’s work on the development of fibre reinforced concrete for tunnel linings at the Barcelona Metro, was presented at the BPCF Conference ‘Concrete 2010’ in May at Leicester.
www.concrete2010.org


Barcelona’s Metro is making use of precast tunnel lining segments with steel reinforcement fibres