Mechanically excavated tunnels (tunnels excavated with a Tunnel Boring Machine (TBM)) are more and more used in civil engineering. In these tunnels, the tunnel lining is made by insitu assembly of precast segments behind the TBM, acting both as tunnel support and reacting element to help enable the ongoing excavation process.
For such precast tunnel linings, segments manufactured as steel fibre reinforced concrete (FRC) are becoming more and more a standard solution. The use of steel FRC allows the elimination of traditional steel bar or mesh reinforcement in the production of precast concrete tunnel segments.
Over the last 20 years, the use of this technology has increased and allows for several advantages compared with mesh or rebar, according to fib bulletin 832 , and all main recommendations published, as providing:
- Higher impact resistance;
- Cracking control during construction phases;
- Durability advantages at final stage;
- Reduction of costs;
- Sustainability advantages; and,
- Boosting of the production process.
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 to MC 20101, could be guaranteed with the use high 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 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.
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 group led by Professor A. Aguado at the Polytechnic University of Catalonia (UPC-BarcelonaTech) developed the non-destructive magnetic induction test – the Inductive Method – (Juan 2011; Torrents et al. 2012). This allows assessment of the content and orientation of steel fibres in FRC specimens using the method, which reduces cost and can be undertaken rapidly – in a matter of less than two minutes.
The Inductive Method is based on the ferromagnetic properties of the steel fibres that are able to alter the magnetic field around them. It is one of the main contributions of the research group to the field of systems to control and characterise FRC.
The non-destructive magnetic induction test represents a step towards an enhanced methodology for FRC characterisation, given that it can be implemented in an easy and user-friendly way, allowing for fast, repetitive and reproducible measurements in a large number of concrete samples.
This test method provides an easier, more reliable and robust characterisation of the fibre content in FRC. It contributes to the optimisation of systematic quality control for FRC in terms of content and orientation of the steel fibres.
Dramix® Eyed Inspector
The Dramix® eyeD Inspector is a registered brand that encompasses a line of products for the assessment of material properties and structures.
A huge testing programme conduct by Roma University saw the extraction of seven sample cores from two precast concrete segments, and four specimens taken from each core – giving a total of 28 samples.
The Dramix® eyeD inspector device shows good sensitivity and can be considered a valid screening tool, but its accuracy strongly depends on the acceptance threshold adopted.
The device could be used with a lower margin than that indicated in Bulletin 83, and, if necessary, supported by destructive tests on specimens that exceed this threshold, in order to ensure compliance more reliably. In addition to the technical aspects, the use of the inductive method offers significant economic, time related advantages. The ability to perform checks without crushing the specimens allows for reduced costs and shorter verification times.
Sustainability Developments
Among various construction processes, tunnel construction results in a significant amount of CO2 emissions because almost all tunnels are lined with reinforced concrete and utilise various high energy consuming equipment for excavation. Embodied carbon and high energy consumption can be minimised through three distinctive ways.
Two main complementary approaches can be adopted to mitigate embodied carbon and reduce high energy consumption: the first method includes decreasing the overall quantity of reinforced concrete utilised through design optimisation; the second approach is lowering the embodied carbon within each unit volume of the reinforced concrete by reducing the usage of Portland cement and steel rebar. The latter can be established by two substitutions in the concrete mix – a low carbon binder instead of Portland cement, and steel fibres rather than steel rebar.
These approaches were discussed by speakers from Cowi/Technical University of Denmark during the STUVA congress. This topic shows an exciting mega trend5.
A further reduction can be achieved by combining steel fibre reinforcement with alkali-activated concrete (also known as a geopolymer). This combination can reduce CO2 emissions by more than 70% compared to Portland cement-based concrete that also contains conventional steel reinforcement.
The conference paper presents various scenarios and illustrates their impact on the sustainability performance of tunnel linings.
The combination of innovative binder and reinforcement concepts offers great potential for the realisation of sustainable and robust bored tunnel linings.
The following conclusion has been presented in fib congress7 based on testing realised with Dramix® 4D 80/60BGP:
- Deflection hardening: The tested SFR AAC shows a substantial deflection-hardening and post-cracking flexural tensile strength. The observed COV for the test data lies within reported results for beam bending tests of traditional SFR concrete;
- Structural feasibility: The presented results show that it is possible to produce a structural feasible Portland cement-free concrete in regard to three point beam bending tests. It is expected that a sufficient steel fibre class in accordance with EC2/fib MC is achievable. This enables the use in structural applications.
- Impact of mixing sequence on fibre distribution: The mixing sequence had a strong impact on the fibre distribution for the used fibres and mix design. Adding the activator solution after fibre addition led to a clear improvement of the fibre distribution. A significant impact on other measured properties was not observed.
As has been stated, literature suggests a better bond behaviour between AAC and steel fibres in comparison to Portland cement systems. Future research aims to investigate this for the presented mix design with high content of low-kaolinitic clay. Indeed, the total mass of CO2eq is what we want to minimise from environmental product declaration and by decreasing the total mass of material.
An Environmental Product Declaration (EPD) is a document that transparently communicates the key environmental performance indicators of a product over its lifetime.
A third-party verification ensures that data relating to environmental aspects of Dramix® has been validated by an external organisation.
This declaration is the Type III Environmental Product Declaration (EPD) based on EN 15804:2012+A1 and verified according to ISO 14025 by an external auditor. It contains the information on the impacts of the declared construction materials on the environment. Their aspects were verified by the independent body according to ISO 14025. Basically, a comparison or evaluation of EPD data is possible only if all the compared data were created according to EN 15804:2012+A1.
The environmental impact of Dramix® product (cradle to gate with options) is largely dependent on the energy intensive production of steel (half product) on which the manufacturer has only a limited influence.
The carbon impact of steel production (Wire Rods) in the product stage A1 is as high as 85%. The impact of the production line largely depends on the amount of electricity consumed by manufacturing plant (0.34 kWh/kg of product). There are no significant emissions or environmental impacts in the A3 production processes alone (partly gas combustion).
The production process itself does not have significant environmental impacts in the life cycle. The use of steel fibre reinforced concrete will help to meet the aims of producing low carbon lining by concrete consumption and steel reinforcement saving, respectively. If ductility and durability have been the key words the last 40 years, then sustainability will be the key driver for further FRC lining development in the coming years.
In the meantime the new level of quality control from product specification, FRC testing , introduction device and non-destructive method provide additional peace of mind.
We all believe that tunnels should use smart and sustainable construction materials. The future of tunnelling is choosing these materials today.
