Benoit de Rivaz, Global Technical Manager, Bekaert Underground Solutions
As mentioned in the fib bulletin 83 on Fibre Reinforced Precast Segment: for sustainable use of structural concrete, environmental and mechanical performances of concrete structures must have the same importance. Through sufficiently high mechanical performances, the structural safety of a construction is ensured. Contemporarily, a low environmental impact guarantees a sustainable development, which is, in accordance with the Brundtland Commission of the United Nations (March 20, 1987), a “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”.
Mechanical excavated tunnels (tunnels excavated with a TBM – Tunnel Boring Machine) are more and more used in Civil Engineering. In these tunnels, the lining is made assembling precast segments used by the TBM as reacting elements in the excavation process. The use of Fibre-Reinforced Concrete (FRC) allows to reduce or eliminate the traditional reinforcement in the precast segment production. Over the last few years, the use of this technology has increased.
An aspect that is boosting the use of FRC in segmental linings is the introduction of guidelines for the design of FRC. In 2013, the fib presented the Model Code 2010 in which a specific part related to FRC is inserted. This document has sparked great interest in the tunnelling community and several documents consider Model Code 2010 as a reference.
Figure 1: FRC precast segment state of the artDesign approach
A minimum tensile strength > 1800MPA is recommended for final ling application considering the performance required and concrete class./80
The hooked ends ensure the desired fiber pull-out. This is the mechanism that actually generates the renowned concrete ductility and post-crack strength. Bekaert’s Dramix® 4D80/60BGP provide a net higher than 10000lm for 40kg/m3 and tensile strength > 2200MPa, using a hook specially designed for precast segment.
The tensile strength of a steel fiber has to increase in parallel with the strength of its anchorage. Only in this way can the fiber resist the forces acting upon it, otherwise it would snap, causing the concrete to become brittle. On the other hand, a stronger wire cannot be fully utilized with an ordinary anchor design. Therefore, the tensile strength of a fiber has to be perfectly aligned with its anchorage system and its diameter. Dramix® 4D80/60BGP is designed to capitalize on the wire strength to the maximum degree.
Model Code 2010 is the most comprehensive code on concrete structures. It covers their complete life cycle from conceptual design, dimensioning, construction and conservation through to dismantlement. It is edited by fib (fédération internationale du béton / international federation for structural concrete). fib Model Code 2010 was produced through the exceptional efforts of participants in 44 countries from five continents.
Figure 2 illustrates the design process involved from beam tests, classification, design values, to constitutive laws.
Figure 2: Design processThe compressive strength of the materials was measured by a testing cube with a side of 150 mm. For every cast made for the production of every single segment, three beams were produced. In agreement with EN 14651 [4], nominal strengths corresponding to four different crack mouth opening displacement (CMOD), namely 0.5, 1.5, 2.5 and 3.5 mm, were evaluated.
Figure 3 shows a typical result of the beam tests with significant strength values. FL is peak force, fR1 and fR3 are the stresses related to CMODs equal to 0.5 and 2.5 mm respectively. These values are the reference ones for final lining design performed according to the fib Model Code 2010 prescriptions.
Figure 3: Typical results of beam tests of FRC according to EN 14651 using 40kg/m3 Dramix® 4D80/60BGP. Performance type 5eTo dimension a steel fiber-reinforced concrete segment, a reference test methodology needs to be adopted for characterization of performance. In addition to the mechanical performance, various properties of the FRC can be specified. Since brittleness must be avoided in structural behavior, and fiber reinforcement can be used. If fibres are used as the only reinforcement for final lining, hardening post-crack behavior at section level (beam test) allow immediately the following:
- Cracking control at SLS
- Structural ductility (ULS)
- Fr1k > 5MPA
- fr3k/fr1K >1.1
Reduced Carbon Footprint
The use of steel and synthetic fibers to replace all or a part of conventional reinforcement has been demonstrated to lower the embodied CO2 of the segmental lining. Macro-synthetic fibers are shown to provide a lower embodied CO2 content than steel, however, as addressed in Section 5.1.1.2 Steel vs. Macro-Synthetic Fibers, are unable to meet all of the structural/mechanical performance requirements of a precast segment, with limited exceptions. Therefore, this section will focus on precast segment lining using SFRC to replace rebar/mesh. While it is possible to significantly reduce the embodied CO2 of a concrete mixture for segment production by replacing a portion of its cement content with alternative cementitious materials, there is little or no difference between the cementitious blends and contents required to produce fiber reinforced or conventionally reinforced concrete segments for tunnel linings.
Figure 4 is data from a paper titled, “Consultant’s View on Service Life Design” by Carola Edvardsen of COWI Denmark; it provides a comparison of embodied CO2 for different types of binder and steel reinforcement used for major infrastructure projects in Europe and the Middle East. On a per pound (kg) basis, the embodied CO2 of conventional rebar and steel fibers is assumed the same. This is a generalization assuming the wire rod that the fiber is produced from, and the rebar have similar percentage recycled material content and similar steel production methods. In a precast concrete segment design the percentage of reduction of steel is dependent on the structural and serviceability requirements of the segment, the fiber’s attributes, and performance in the specific concrete mix. Based on information from a leading steel fiber producer, a typical reduction percentage in steel when using steel fibers in a precast concrete segment is typically 60 to 65%. The actual reduction in embodied CO2 of the lining will depend on the contribution of the reinforcement plus the concrete.
Figure 4: “Consultant’s View on Service Life Design”, by Carola Edvardsen of COWI DenmarkThe Grand Paris Express Project
The Grand Paris Express project is an investment of 35.6 billion in 2012 numbers and will add more than 68 stations and 200 km of trackway to double the existing mass transit rail network for Paris. The 100 km currently in construction includes Line 16 which is scheduled to be in operation for the Paris Summer Olympic Games in 2024.
Soon after the award of initial contracts for the Grand Paris Express that Société du Grand Paris (owner of the mega-project) began to explore the use of steel fiber reinforcement for the segmental linings to replace the rebar reinforcement. One of their primary objectives was to enhance the sustainability credentials of the project to promote respect for the environment and reduce as much as possible the carbon footprint. (Figure 5)
Grand Paris 16.1 Picture Courtesy: EiffageThe specifications for the 16-1 work package was provided for conventional rebar only – no fibers. Three months after awarding of the contract for Line 16-1 in 2018, Eiffage Génie Civil proposed to Société du Grand Paris to implement a 100% SFRC segments. The objectives of using steel fiber as an alternative to rebar reinforcement was to reduce the amount of steel required per cubic meter of precast lining and capture the benefits that steel fiber creates less carbon pollution in its manufacture and transportation. The use of SFRC plus formulating a low carbon concrete mix would optimize the carbon footprint of the project.
SFRC segments have 50% less steel than rebar reinforced segments saving more than 5,000 tonnes of steel production carbon for 10 km of tunnel.
One truck can transport 24.2 tonnes of fiber per load compared with 17.85 tonnes per truck load of rebar.
The concrete chosen for the Paris Metro Line 16, Lot 1, fiber reinforced segments had a low carbon footprint of 170 kg CO2 equivalent/m3 and reduces the carbon weight of the steel in the segments by 90 kg CO2 equivalent/m3 or nearly 11,000 tonnes equivalent CO2
Figure 5: Comparison Summary Grand Paris 16.1
The use of SFRC in a portion of the segments in the Paris Metro Line 16-1 has provided substantial environmental benefits and lowered carbon footprint derived from:
- The reduction of steel quantity required provided a corresponding reduction in transport-related CO2 emissions. Comparing the 40 kg/m3 Dramix® steel fibers against the 85 kg/m3 of rebar yielded a potential material saving of more than 50%.
- One truckload delivery of fibers to the manufacturing plant allows the production of nearly 185 segments, compared with 60 equivalent segments per truckload of conventional reinforcement.
- The smaller diameter of fibers enables toxic emissions from the primary steelmaking industry to be reduced, as the resultant transformation provides wire no thicker than 0.04 in (1mm) in diameter. Wire drawing technology is not a major source of emissions.
The project Forrestfield Airport Link Project (Perth, Western Australia) comprises the design, construction, and maintenance for 10 years of the Forrestfield-Airport Link, which will connect the eastern suburbs of Perth with the existing suburban rail network as well as the airport. The line is expected to generate 20,000 passenger trips on the network every day. It will also reduce road traffic and travelling times.
The precast concrete segmental lining was designed to meet the project’s concrete requirements of 120 years of service life with a minimum water/binder ratio of 0.35. Webuild and its joint-venture partner NRW Pty Ltd (80% of the joint-venture is held by Webuild) were the design build contractors for the project. The tunnel excavation diameter is 7.1 m (the segmental lining has an inside diameter of 6.1m (and thickness of segment is 300mm) with an average length of 1,600mm the ring is made by universal type of five segments plus a key. The segments have a double gasket which makes them state-of-the-art and quite unique. The configuration of rings provides a minimum tunnel radius of 300m.
The solution utilized for the manufacture of segmental linings for the project is based on a hybrid system using steel fiber-reinforced concrete and light steel reinforcement rebar cage. In addition, Micro-Polypropylene fibers are also incorporated within the concrete mix to comply with the fire resistance requirements of the tunnel lining. Mix-design was developed based on triple blend cementitious materials, an accurately graded fine and coarse aggregates with 20 mm max size as specified. This is to ensure that the segmental linings are of highest quality with very low permeability and porosity, can withstand any deterioration caused by external conditions and meet its design durability life span.
Design of segmental lining considered special type of high-performance steel fibers with a tensile strength of 1800 N/mm2. These fibers were designed for durable and liquid-tight structures and can perfectly combine with traditional steel reinforcement. The production of concrete segments started with the mix design that had steel fiber content of about 40 kg /m3; however, during the early stages of production, it was observed that the residual strength of concrete with 40kg/m3 of steel fiber provided much higher residual strength than that required by design and specification. On this basis, a reduction of 5 kg/m3 was implemented during the succeeding production. Several trials were performed with cast concrete beams according to standard BS EN 14651 (single trial was made with 9 concrete beams) with 35 kg/m3. The characteristic values of these results were more than the required serviceability state 5.08 MPa at CMOD1 and 5.28 MPa at CMOD3. The results have confirmed the assumptions for the quantity reduction and the mix with 35 kg/m3 of steel fibers content were adopted in project works.
A LCA (life cycle assessment) evaluation per cubic meter of tunnel lining concrete was performed to evaluate the hybrid reinforced segment versus a conventional reinforcement only alternative. Figure 6 shows the results of this evaluation.
Figure 6: LCA Assessment – Dramix® reinforcement VS Rebar DesignAs can clearly be seen the savings realized from the use of high-performance steel fibers to replace a portion of the conventional rebar along with optimized low carbon concrete provides large reductions in carbon footprint.
Conclusion
The use of steel fibre reinforced concrete will highly participate to meet low carbon lining by concrete consumption and steel reinforcement saving. The new generation of binder combined with FRC allows the following new achievements:
- Excellent long-term durability performance exceeding that of Portland cement-based concretes.
- Extremely low embodied carbon footprint compared to conventional concretes on Port-land cements.
- Compared to reinforced concrete, fiber-reinforced concrete notably represents savings of around 5,000 tons of steel for 10 kilometers of tunnels (Typical Metro Tunnel).
