With ever more technological and economic solutions sought to deliver low carbon performance in construction, the options available and choices made in the use of concrete and reinforcement come under more scrutiny, including for tunnel linings, such precast segmental rings. The number of tunnel projects being developed internationally to have low carbon performance beginning to grow and prominent among their ranks, showing the way to delivering at scale, on large tunnel rings, is one of the latest lots on the Grand Paris Express (GPE) metro project.
The leap in low carbon performance on the GPE project is focused on Lot 1 section of Line 16 (Lot 16.1). Originally, the lot on the new metro line in the French capital was to have used precast concrete reinforcement with steel bar (rebar) cages – the traditional, long-term primary approach for strengthening structural concrete, and used in recent additions to the expanding metro network in Paris. But, for this element of GPE, studies and tests led to a strategically different choice, and benefits, by adopting only steel fibres to be the primary reinforcement for the structural concrete segments.
The step did not come out of the blue as the use of steel fibre reinforced concrete (SFRC) has been increasing, and accelerating, most especially over the last ten years. But not at the size and scale of GPE.
On Line 16.1, more than 14km of the segmental rings on the TBM-bored tunnels will be built with fibre-based segmental lining. More use of SFRC is anticipated on additional lots of Line 16.
On GPE, it is estimated that, on average, up to 10 000 tonnes of CO2 equivalent can be saved for every 10km of tunnel constructed with SFRC segmental lining compared to using traditional steel reinforcement, or bent rebars, assembled and formed into cages. Use of SFRC can also result in thinner segments compared to the rebar alternative, the segments being slimmer by approximately 20mm – 30mm, as the cover needs are less.
GRAND PARIS METRO: LINE 16.1
The strategic vision for expansion of the Paris rail and metro network was conceived in 2008 in a new transport masterplan for the French capita. Two years later, the Société du Grand Paris (SGP) was established by the state with involvement of local authorities to develop, build and operate the GPE project as the key to the vision.
The initial works for GPE involve constructing the first half of the expansion. This phase will bring a total of 100km of new, mostly underground metro and rail lines into service, including Line 16.1, in time for Paris to host the Olympic Games in 2024.
Afterward, the same total of network lines will be added again – again almost entirely underground – to bring the total of new lines to 200km by 2030. By then, it is expected that most of the inhabitants of Greater Paris will live within 2km of a rail or metro station served by the automated lines.
Four new metro lines will be built (Lines 15, 16, 17 & 18), and extensions added, north and south, to the existing Line 14. There will also be dozens of new stations along with associated neighbouring development works. While only a small part of the grand vision, Line 16.1 is notable itself as a large tunnel project and for its landmark environmental credentials.
Line 16.1 is located at the northeast corner of Paris, its twin tunnels branching from both Lines 14 and 15, and itself has Line 17 branching off. At 19.3km long, Line 16.1 is mostly underground and has an excavated length of 19km. The lot has stations at Saint-Denis Pleyel, La Courneuve Six-Routes, Le Bourget and Le Blanc Mesnil, plus ancillary structures.
The 9.5m o.d. tunnels on Line 16.1 have a 100-year design life. Eigis is designer for the SFRC precast lining. Construction is being led by Eiffage Génie Civil, which won bid for the lot in early 2018. With a build programme of almost 70 months, the lot is to be completed by the end of 2023.
The lot is being excavated with six Herrenknecht TBMs to enable construction of stretches of tunnel of three different internal diameters (6.70m i.d., 7.75m i.d. and 8.70m i.d.). Precasting of the segments is by Bonna Sabla using a concrete production plant in Conflans- Sainte-Honorine, which had already supported GPE’s Line 14 extension project.
POTENTIAL TO REDUCE CARBON
The low carbon performance of the SFRC concrete on the project, including the Dramix fibres, is calculated to eliminate, over a 10km length of tunnel, the need to produce 5000 tonnes of rebar steel and the associated emissions.
SGP Chair, Jean-Francois Monteils, has noted the use of SFRC on project Line 16.1 “is a first in France in underground work,” giving a notable saving on carbon emissions over rebar concrete. SGP is helping to lead change in infrastructure development towards more sustainable practices for public works in France. The leadership example of the GPE project in its approach to low carbon infrastructure, is being watched from other countries in Europe and beyond, even while SRFRC is not new. GPE has been demonstrating what can be achieved at scale.
The road to change in Paris began in earnest not so long before, in 2014, when potential for applications of steel fibre had opportunity to take on a greater role in tunnel segment design and production for metro construction in Doha, Qatar, where the environment was chemically aggressive. More use of SRFC has been taken up in the UK, Singapore, Malaysia, Australia and North America.
But it is in France the next leap forward has taken place – with the technique to be used entirely for reinforcement in casting the concrete segments required for erecting rings along 14km, or approximately two-thirds, of the 19km long underground section of Line 16.1. Four of the six TBMs are on the lot are erecting the SFRC segments, the first of which were installed in mid-2020, a little over half a year after casting began at the factory.
Manufacture of segments of Lot 16.1 took place in factory at Conflans Sainte Honorine previously set up casting concrete segments with rebar cage reinforcement, primarily, for the extension of Line 14.
For the new casting work, plant purchased included a new concrete mixing plant, moulds to suit Lot 16.1 segment dimensions, and a fibre mixing machine. A dosing unit for the steel fibres was developed along with an upstream buffer, to pre-feed up to 9 tonnes of fibres. A triple weighing system (at the doser, receiving belt and mixer feed belt) ensured the correct quantities of steel fibre were delivered for the mix design. There was also a press, for check of tensile strength of cast units, to EN 14 651 standard. In total, the new plant needed cost approximately Euro 2 million, in late 2019.
FIBRE STUDIES FOR CUTTING CARBON
The Line 16.1 stretch of tunnel using SFRC is constructed with 2m long rings, 8.70m i.d. and erected from 400mm thick segments (7, incl key). Concrete is C50/40 and the mix design uses Bekaert Underground Solution’s Dramix fibres. In total, approximately 200 000m3 of concrete will be cast for the tunnel segments on Line 16.1.
While the plan had been for Lot 16.1 to use rebar for the tunnel segments, a feasibility study involving the client, contractor and suppliers examined what could be possible by using steel fibres as an alternative for concrete reinforcement. There could be lower transport needs and consequently less emissions from savings in the ratio of steel fibre to rebar, with better comparative loading density per truckload for segment production. A result is that steel consumption is cut by half using fibres rather than rebar.
Steel fibres could help the segments meet the minimum performance values set by Model Code 2010, with supplemental rules for service life loading as set out in the International Federation for Structural Concrete’s (FIB) Bulletin 83 (2017) VP 1.4.1. A total of 10 load cases are possible to consider and of those the feasibility study looked at six – the in-tunnel operational cases of geology and TBM cylinder thrust, and the prior stages dealing with cases during manufacture and movement which are demoulding, storage, handling, transport. The feasibility study examined the largest segments (for the 8.70m i.d. tunnels) as a basis for assessing the cases.
A test programme followed the feasibility study and the performance of steel fibres from various manufacturers were examined in the concrete matrix, not least for distribution. There was particularly close attention paid to key parameters such as the cement quality, water/cement (w/c) ratio, grading curve, sand type, and sand/gravel ratio as a minimum of fines is of key importance for effective processing of the steel fibres, in different dosages, and their distribution throughout a concrete matrix.
Along with other manufacturers, Bekaert provided its Dramix 3D and 4D fibres for various tests in the concrete matrix. The tests, performed for Eiffage by Italy’s University of Tor Vergata, included large-scale destruction of full concrete segments. The results showed performance of the different concrete segments in shear strength (potentially loaded up to a maximum of 5200kN, from TBM hydraulic cylinders), and their breaking strength (under bending). A total of 10 concrete segments were put through the full-scale tests.
Fire tests were also performed, which led to polypropylene (PP) fibres to be included in the concrete matrix along with the steel fibres. The PP fibres provide passive fire resistance by melting under heat to leave a network of connected micro-voids that release vapour pressure and so help to reduce risk of explosive damage by spalling of concrete. Under load, three fibre-based concrete segments were tested to NF EN 1363-1 for two hours at 1000 °C, and conditions also included 20 minutes to the standard temperature explore of 800 °C.
The SFRC design was finalised at the end of 2019, enabling segment production to commence.
With Bekaert’s Dramix fibres selected, on a 40kg/m3 dosage, the fibre was produced at the Petrovice plant in the Czech Republic. Within two years, 5300 tonnes of fine fibre steel of 0.75mm diameter, and 60mm long, were produced and quality sampled throughout at different locations – the precasting plant, Bekaert’s facilities in Belgium, and the contractor’s site.
The fibres were made to a higher tensile strength – 1800N/mm2 – than the normal range of 1100N/mm2 – 1300N/mm2. Optimising a combination of the hook shape and length/diameter ratio (80) helped to maximise how the steel fibres are anchored in the CEM III concrete matrix on Lot 16.1, where aggregate size is up to 20mm. The design has 4584 fibre units/kg, and the fine steel, evenly distributed, has a total length of approximately 11km per cubic metre of concrete.
GETTING CARBON DOWN
Focusing on the steel fibres, the total shift away from use of rebar cages for segments on Lot 16.1 meant less tonnage of steel to be produced and transported, and therefore the emissions associated with both were reduced. The steel density in the segments went down from approximately 85 kg/m3 (rebar) to the 40kg/m3 level (Dramix).
At peak production, the daily truck shipments brought steel fibres at full load of approximately 24 tonnes (22 x 1100kg large bags), enough to cast 60 segments. Single truckloads of steel rebar carried less tonnage, so more transport would have been required to meet segment production demand in the factory.
As noted earlier, one outcome therefore of the approach of using only steel fibres for the structural reinforcement of the precast segments on Lot 16.1 is a saving of 5000 tonnes of steel (as rebar) that would otherwise have been required. In addition to the extra tonnage there would also have been more associated costs with transport to the casting factory, shaping, then transport after casting to the site and into the tunnel, every step also generating relatively more emissions. By using SFRC, instead, a saving of 9000 tonnes – 10 000 tonnes of CO2 equivalent are produced, on average, for every 10km of tunnel.
But it is not only the steel that goes in that needs to be considered. The type of steel also leads to more, or less, requirements in terms of segmental size. Rebar needs extra concrete all round solely as protective cover to the outer steelwork cage, which is unnecessary for segments with steel fibres, evenly distributed. Slimmer segments can then result.
Improvements in carbon performance in tunnel lining are cumulative, coming from many strands of activity, such as changing the cement, the concrete mix design, the type of reinforcement. The changes, and benefits, run through the project and product supply chains.
