RC 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 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.
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.
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.
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.
|Lines 9 and 10 of Barcelona Metro|
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.