FRC is a compound consisting 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 ofcracks.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.

 

Polymer 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 in 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 – both steel and polymer fibre - has a far more enthusiastic following with applications across a wide range of civil engineering applications.

 

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.

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 thought to be close to €6.5 billion, when the project is completed in 2012.

Here, Joint Venture Construction Consortia, UTE Gorg, UTE Linea and UTE Aeroport, used precast FRC segments for the lining to three individual sections of the 12.0m diameter tunnel, [], totalling some 11.7 km in length.

At the 3.8km long, Sagrera TAV-Gorg section, construction work began in 2003. An earth pressure balance, tunnel boring machine [TBM] was used to

excavate the tunnel, with the precast lining segments placed ring by ring behind the machine, using a robotic arm.

 

During the boring/construction process, hydraulic jacks on the TBM push against the previously placed precast concrete segments as the machine cuts into the rock and soil ahead of it. Overall pressure values of up to 140MN can be reached during the jacking process, with a normal working range of between 90 and 120 NM. The excavated diameter of the tunnel is 12.1m and an overall lining thickness of 400mm, including the precast concrete final lining.

 

FRC tunnel ring segments were cast off-site and comprised 7 segments of 4.56m length [48degree length of arc] plus a 24 degree keystone, per ring. Each ring has going-length of 1.80m and is 350mm thick.

Segments were cast in curved steel formers with vibration applied to consolidate the concrete mix and heat cured at between 40-50deg C for 4-6 hours, before de-moulding and stacking them in an open yard.

 

Consolidating the concrete in open curved moulds required a mix of stiff consistency [low workability]. In turn, this led to the need for higher amplitude vibration. Top surfaces of the segments are manually finished.

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

 

An initial proposal of 30kg/cum of Maccaferri Wirand FF1 steel reinforcement fibres was made in an attempt to reduce the amount of steel bar within the segments. Ongoing testing refined the fibre reinforcement and a new fibre, Wirand FF3 with a higher L/D [length/diameter] value of 67 was developed, which offered improved performance.

 

Eventually, only 25kg/cum of Wirand FF3 was found to achieve the same performance as 30kg of the FF1 fibres. Ongoing 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 [5800psi]).

 

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 – hence the use of accelerated curing agents within the mix.

 

Reinforcement fibres are added to the concrete mix via purpose made dosing equipment, of a design specially modified by the supplier to ensure controlled introduction and consistent dispersion of the fibres. 5 feeder machines have been installed in the batching plant producing segments for all three tunnel section.

  

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 [4200psi]. Macro steel fibres, with high-strength / low-strain characteristics offer this performance; 25kg/cum 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 and stacking, and controlled shrinkage cracking and thermal cracking caused by sudden temperature changes 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.

 

Some 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 being the principal motivations behind the proposal. The proposal was a revolutionary step, as this had not been considered within the final-linings 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 content was increased from 30kg/cum to 60kg/cum 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 elements 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 test, placing approximately 20lin.m of tunnel permanent final lining using segments reinforced with steel fibre only.

The results of the laboratory and site trials were presented at the 6^th Annual RILEM Conference in 2004 [Authors: M di Prisco, R Felicetti, GA Plizzari.] and it was concluded that the segments reinforced with 60kg/cum steel fibre could satisfy the requirements of the project without necessitating 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/cum to 54kg/cum through the use of fibres.

  

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 is also supplying Fibromac FR polymer fibres to the project.

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

The implications or the Barcelona Metro trails 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 in 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?

 

Bernard Berge is Product Specialist Engineer, Fibre Division, for Maccaferri.       A paper describing his work on the development of fibre reinforced concrete for tunnel linings at the Barcelona Metro, will be presented at the BPCF Conference Concrete 2010, 11^th May at the Athena, Leicester. www.concrete2010.org