The document was conceptually agreed during the 2009 meeting of WG2 in Budapest and was promoted by the former animateurs Eric Leca of France, Chungsik Yoo of Korea, and the current animator Elena Chiriotti from France. An early draft document was presented in Vancouver in 2010. A revision was approved this year.
In the last two decades, the use of FRC progressed and was adopted for several tunnel projects. Among the benefi ts related to the use of fi bre reinforcement in cementitious composites, the most important are the noticeable increase of toughness – that is the post-cracking tensile residual strength, which enhances the resistance to crack development – and the possibility to optimise the reinforcement for controlling diffused stresses. FRC can be combined with reinforcing bars where high localised stresses occur, and the use of fi bres facilitates the production process.
The enhancement of the general structural behaviour together with the improvement of the industrialised production of precast tunnel segments are probably the two key factors in the continuous growth of using FRC in precast tunnel linings.
The aim of the document prepared by WG2 is to provide advances in the design of FRC tunnel lining in accordance with the objectives of the International Tunnelling and Underground Space Association (ITA) prescribed in Section II of its Statutes (ITA, 1976). Standards and recommendations related to the properties of FRC, to the design of general FRC elements (mainly slabs or beams), and some recommendations on FRC for tunnel lining (mainly limited to steel fi bre reinforced concrete) are already available (Figure 1). Such documents generally do not provide details on specifi c requirements and loading conditions applicable.
The document proposes a brief overview on current available applicable standards for the characterisation of FRC’s fracture properties and for the design of FRC elements. Among them, the new FIB Model Code 2010 is mentioned, which refers to EN 14651 for determining the main signifi cant residual post-cracking strengths. The well-known Model Code 2010 provides general design rules that can be easily applied for typical structures such as beams or slabs, but they need to be contextualised to the specifi c issues concerning tunnel lining elements. The lining segments are, for instance, characterised by a temporary loading condition during the excavation of the tunnel, where considerable localised stresses act on the lining due to high concentrated forces exerted by the TBM’s hydraulic jack (TBM thrust phase). The noticeable enhancement of post-cracking residual strengths due to the addition of fi bres can be exploited during this stage even if no specifi c recommendations are included in Model Code 2010.
The document takes advantage of 20 years of FRC practice in precast tunnel lining – including research and feedback from more than 70 real cases – to provide a guide to complete the existing standards and recommendations for the specifi c case of tunnel lining. More specifically, the scope includes:
¦ Supporting a performance-based design approach for FRC structural elements, which allows all fibres meeting requirements for long-term behaviour to be considered for structural applications;
¦ Detailing the results of recent research advancements in the field;
¦ Providing additional design principles to complete the existing standards and recommendations, in particular with regards to the loading conditions and design procedure;
¦ Giving specific advice on analytical and numerical procedures necessary and adequate to quantify, during the design process, the beneficial effects of fibres.
The quantification of toughness provided by fibre reinforcement is of paramount importance for the definition of rules and recommendations for the design of FRC structural elements, since the classical design, based on the elastic approach, is acceptable only for ordinary reinforced concrete structures. A proper design procedure that takes into account the significant residual tensile strength provided by fibres after cracking has to be adopted in case of FRC structural elements. In particular, the cracking phenomena of concrete matrices containing fibres can be accurately represented by using an approach based on non-linear fracture mechanics.
In the document special attention is devoted to some particular loading conditions that, based on experience, should be considered since they can be very severe for a tunnel segment reinforcement solution based on fibres only. As an example, the case of loads generated on the already installed lining segments during the TBM thrust phase should be analysed at both a local and a global scale. The local behaviour regards the analysis of the tensile transverse stresses (splitting or bursting stresses perpendicular to the loading direction). Specific experimental tests prove that FRC enables a stable propagation of cracks compared to plain concrete. However, appropriate design tools have been used in order to properly model and consider this aspect. Such tools include non-linear numerical analyses and experimental tests.
The global behaviour concerns considering the possible irregularities of contact among adjacent segments, which can be due to an eccentricity of the thrust shoes, to an uneven support, etc. FRC tunnel segments with fibres only are more vulnerable to irregular load conditions, since this generates increased localised stresses. At the design stage, specific attention has to be paid to the type and probable occurrence of such irregularities; to consider and properly model different boundary conditions for the segments.
The WG2 document proposes a step-by-step approach based on numerical analyses consisting of:
¦ Simplified 2D non-linear numerical models of local conditions on a single segment,
¦ 2D non-linear numerical models of a group of segments, including different boundary conditions for modelling the possible irregularities of contact,
¦ 3D advanced segment models.
It is recommended that the three steps are carried out systematically, and that the use of 3D models is subordinated to the previous understanding of the phenomena through simpler models.
In addition, experimental tests on small samples (for the local behaviour) or full-scale tunnel elements (for the local and global behaviour) are recommended as useful tools for proving the design approach. However, they should not substitute a robust design approach.
Since localised stresses are better resisted by localised reinforcement, such as traditional steel rebars, while diffused stresses are better resisted by spread reinforcement, such as fibres, depending on FRC properties, lining geometry and ground-conditions, the proposed design approach allows achieving and optimising design solutions that could be based on fibres only or fibres plus traditional rebar (hybrid solution) when needed.
The author would like to take advantage of this space to acknowledge the members of the WG2 who have prepared and reviewed the document, ITA Ex Co for leading the review process and ITAtech Support, Sub AG PFRCS for their useful feedback.
