SUSTAINABILITY & CIRCULAR ECONOMY | INSIGHT
PROJECT ADVANCES Recent projects have demonstrated that structural ductility, durability, and sustainability are going hand-in-hand. This holistic approach will be clearly a new boost for FRC tunnel lining on underground infrastructure projects. It is today difficult to understand why we still continue
to consider steel rebar as a competent solution. Major underground projects that are helping to
take forward the use of steel FRC for precast tunnel lining on TBM drives include two large rail/metro transport projects in Europe - Grand Paris, in France, and Fortezza–Ponte Gardena, in Italy. The Fortezza–Ponte Gardena project marks a
significant leap, with 80% of the work involving mechanised excavation and the adoption of steel FRC - the first Italian adoption, for a big project, as described in a recent paper6
. This is
a groundbreaking step forward for mechanised tunneling in Italy. Details of the project experience with steel FRC
on the Italian project includes, for example, the type of Dramix fibres utilised in the concrete mix for the segmental structural design; and, a robust testing campaign, from beam tests to full-scale segment tests under bending and concentrated loads. A detail briefing is available on crack performance with description at different load levels, underling the FRC benefits6 Minimum performance required hardening post-
.
crack behaviour at section level, according to EN 14 651 three point bending test3
, allowing immediate
crack control at Service Level (SLS) and structural ductility, at Ultimate Level (ULS). A recommended performance class 5d, according , could be guaranteed with the use high
to MC 20101
performance steel fibres with some minimum requirement as to a maximum diameter, and tensile strength of more than 2200 MPa, and configured with an optimised hook shape. The tensile strength of a steel fibre has to increase in parallel with the strength of its anchorage. Only in this way can the fibre resist the forces acting upon it. An efficient and controlled feeding of the fibres
into the dosing unit was developed for this purpose. A buffer was therefore set up upstream, allowing a pre-feeding in a single movement of eight big bags at the beginning of the day (about 9 tonnes of fibres). This device was supplemented by a triple weighing
system of the quantity of fibres introduced into the mixer. The three elements of the weighing system are: doser; fibre reception belt; and, mixer feeding belt.
DURABILITY AND CRACK WIDTHS As regards durability, the requirement for conventional steel reinforcement cages was 100 years. However, comparative checks on the precast concrete segments installed have shown that the steel fibre reinforced segments have a better crack control behaviour. The use of fibres is perfectly suited to this type of
EVOLUTION IN QUALITY CONTROL There has been a new development in quality control to move from a piece of steel to peace of mind in use of steel fibres in precast concrete tunnel segments. Until now, the quality control of steel FRC is
performed in terms of the fibre content and post- cracking residual strength of the concrete. The former is usually assessed according to the standard EN 14721:2006 (CEN, 2006), which consists of washing out and weighing the fibres present in a certain volume of fresh concrete. This procedure takes around 45 minutes per test
and requires the use of many litres of water. It may also be performed in hardened samples, thus requiring to completely crush a concrete sample, and afterwards to extract and weigh the steel fibres present in that sample. Consequently, it is a very expensive, time
demanding and an environment-unfriendly destructive technique. All these disadvantages limit the number of tests that may be performed per day, thus compromising both the statistical representativeness of the results and limiting the effectiveness of the quality control system. To overcome these disadvantages, the research
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geometry, especially since the cracking process generates finer cracks than the cracking process of a beam on two supports. In the case of tunnel lining segments, the final coating constitutes a hyperstatic mechanical system. This is a situation in which the fibres work perfectly. Indeed, as only micro-cracks (<=0.2mm) are observed and the segments work in compression when the ring is formed, they close up automatically. When the cracks are very fine, i.e., with crack
openings not exceeding 0.5mm, the fibres are much more efficient than steel reinforcement bars in acting on this cracking. This is simply because the diameter of the fibres is mechanically better suited to these cracks than the diameter of concrete reinforcement bars. It is a problem of coherence of scale, as Pierre Rossi reminds us (international expert on fibre concretes: Les Betons de Fibres; Martialis Edition) In effect, the majority of steel fibre concretes
are mechanically efficient up to crack openings not exceeding about 2mm. Crack openings of between 1mm and 2mm correspond, for the vast majority of cases, to the ultimate behaviour of steel fibre concrete structures. Therefore, studying the durability of steel fibre concretes for crack openings around 1mm can be considered meaningless and would seem unnecessary in practice. Also noteworthy is the excellent corrosion
behaviour of the fibre reinforced segments, linked to the small diameter of the fibres and their distribution. This corrosion performance can be valuable when
high external water pressures are combined with saline conditions (high chloride concentrations), which can lead to severe corrosion scaling in reinforced concrete segments.
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