SFRC segments take a leap forward in France

Direct by Design

A leap forward for SFRC segmental linings in France TunnelTECH

Shani Wallis, TunnelTalk

T

April 2022

he advantages of steel fibre reinforced precast concrete (SFRC) segmental linings have been engaged by some countries now for many years.

Other countries have taken a different route towards researching, testing, developing and approving the steel fibre alternative to traditional steel rebar cage reinforcement for segmental linings. In France the effort has been spearheaded by construction company Eiffage for its €1.84 billion contract on the Grand Paris Express expansion of the Paris Metro.

Jean-Luc Bischoff, Directeur Opérationnel of Eiffage Underground Projects, explains the genesis and journey of the adoption of the technology in France.

“There have been trials and studies in the past and installation of SFRC segment rings in nonpermanent, nonpublic areas of metro projects,” said Jean-Luc Bischoff of Eiffage, “but not until now has a thorough investigation of steel fibre reinforced segments been A 40kg/m3 dose of steel fibre into the concrete mix for precast segmental progressed to convince designers, clients and linings reduces by more than 50% the required comparative 85kg/m3 of contractors to pursue the large scale application traditional rebar segment reinforcement and provides a fibre network of of the technology for permanent full life cycle 10,000km/m3 structures in France. We are now into the fourth year of working with the SFRC precast segmental objectives by using steel fibre as an alternative to rebar linings on our €1.84 billion Grand Paris Express Line 16 Lot reinforcement. The expectation was to reduce the amount 1 contract and the experience is confirmation that SFRC of steel required per cubic metre of precast lining and for segmental linings is here in France to stay. Through capture the benefits that steel fibre creates less carbon our research and development programme, 100% SFRC is pollution in its manufacture and transportation.” the reinforcement of the segmental lining for the 12km of 9.5m o.d. TBM drives for the Line 16 and Line 15 sections of The development of low carbon concrete is an urgent and the contract. This is one of the largest diameter segmental associated process, as it is well established that concrete, linings with seven large 2m wide segments per ring ever as the second most used construction material after to use steel fibre alone as the reinforcement.” water, has an exceptionally high carbon footprint in the production of cement and in the application of concrete. As It was soon after award of initial contracts for the Grand a first phase, the potential to reduce the carbon footprint Paris Express that Société du Grand Paris, the client of of concrete by replacing steel rebar reinforcement with the mega-project public infrastructure investment, began steel fibre reinforced concrete (SFRC) was positive but the to explore the use of steel fibre reinforcement for the technical assurances for use of SFRC segments for the segmental linings to replace the rebar reinforcement. “One Line 16 Lot 1 metro drives were missing. It has required of the principal objectives for Société du Grand Paris was to a comprehensive programme of research to meet the optimise the sustainability credentials of the project,” said Société du Grand Paris performance and minimum 100 Bischoff, “to maximise respect for the urban environment year design-life specifications. and reduce as much as possible the carbon footprint of the construction. In the production of the segments for our Lot The Line 16 Lot 1 contract was awarded to Eiffage in 1 of Line 16 there was significant potential to meet these February 2018 and comprises 19.3km of underground 1

Fig 1. The Grand Paris Express project is an investment of €35.6 billion in 2012 figures which will add more than 68 stations and 200km of track-way to double the existing mass transit rail network for Paris. The 100km of the masterplan currently in construction, including Line 16, is to be in operation for when Paris hosts the Summer Olympic Games in 2024 with the additional 100km of the next phase planned to be operational by 2030.

route linking five new underground stations (Fig 2). TBM excavation started in July 2019 with segments to the project design specifications using rebar reinforcement fabricated by casting subcontractor Bonna Sabla.

“The technical and financial benefits of SFRC segments have been promoted by fibre producers and clients and contractors in other countries for some time,” said Bischoff. “In particular, the UK was an early adopter of the technology in the 1990s. But we had some suspected limitations that we wanted to investigate including control of homogeneity of fibre in the mix, localised cracking in likely areas with fewer fibres and susceptibility to localised stresses” (Tables 1 and 2).

Fig 2. Eiffage is leading construction of Lot 1 of the new Line 16 project, the largest contract of the current phase of works at €1.84 billion and comprising 19.3km of tunnel excavated in eight TBM drives using six EPBMs of different diameters to integrate Line 16 with existing lines of the network through the new major interchange station at Saint-Denis Pleyel • TBM 1 has completed an 8.9m o.d. single-tube double-track drive of 1.6km for a connection with Line 14 • TBM 4 completed twin drives of 1km long x 7.7m o.d. for the Line 17 link • TBMs 2, 3, 5 and 6 have completed 14.7km of 9.5m o.d. excavation for the new Line 16 and a section for the new Line 15

the concrete mix was carried out by the Eiffage site management supported by the Eiffage innovation department and the CERIB concrete research institution of France, and the Tor Vergata University of Rome conducted the insert pull-out and 1:1 scale loading tests. The design criteria for the research was based on the 2017 Bulletin 83 and 2010 Model Code of fib, (the International Federation for Structural Concrete) as well as on the recommendations for the design of SFRC published by AFTES (the French Association of Tunnels and Underground Space), in 2013.

In a comprehensive programme of investigation, more than 64 tonne of materials were used to conduct 90 different mix tests, 850 beam bending tests, 150 tests To investigate these concerns, Eiffage initiated a to determine K coefficients of fibre distribution and €1.2 million research programme. The research for orientation within the concrete and 100 tests to confirm suitability of SFRC specifically for Lot Table 1. Expected benefits of the SFRC alternative 1 applications. Mixes tested two steel fibre geometries – single and double Reduction in risk of spalling hook, multiple tensile strengths Better control of shrink cracking of steel and two fibre diameters – Micro-cracking in SFRC versus expansion cracking of rebar reinforced concrete 0.75mm and 0.9mm. Mix design also optimised inclusion of poly-fibres Better coating of the concrete mix around the fibres to provide the necessary fire load Less prone than rebar reinforcement to corrosion resistance of the segmental lining.

Multi-directional strength The choice for producing 40 full Greater SFRC ductility versus rebar reinforced concrete scale segments to test TBM thrust Environmental savings in steel fibre production and fibre transportation ram loads and complete more than 1,000 bending tests on beans cut from 1:1 scale segments, was Table 2. Anticipated limitations of SFRC to be investigated Dramix 3D 80/60BGP glued 0.75mm diameter fibres of 60mm long, with Localised cracking in areas with fewer fibres a 1,800MPa tensile strength and at Susceptibility to localised stresses a dosage of 40kg/m3 that equates to Susceptibility to TBM thrust ram stresses • 4,584 fibres/kg Control of homogeneity of fibres through the concrete matrix • a total number of 183,360 fibres/m3 2

•

a total length of fibres end-toend of more than 10,000km/m3

To achieve the recommended 10,000km/m3 network with the larger 0.9mm diameter fibre would have required a dosage of about 55kg/m3.

When the programme of validation tests were completed in December 2019, the new SFRC segments were approved for installation for the 12km x 9.5m o.d. drives for Line 16 and Line 15. Fabrication was at the same Bonna Sabla factory casting the VIDEO: A leap forward for SFRC segmental linings for the Grand Paris Express projects rebar reinforced segments using segments at 2m wide and 400mm thick and there are 40kg/m3 of Dramix 3D 80/60BGP fibres for the 2m wide the same seven segments in the 9.5m o.d. rings. There is rings. This is compared with 85kg/m3 of steel rebar cages the potential to reduce the thickness of the lining, based for the same 2m wide x 9.5m o.d. segments. on, among other parameters, withstanding the potential The production used the same moulds, steam curing and maximum loads of the TBM thrust rams and the size of concrete mixing equipment as for the rebar reinforced the TBM rams and their pads that distribute the horizontal segments. “Fibre dosing equipment was added,” said Bischoff, “and they adjusted the 3 concrete mixing and mix design for the optimum of steel + poly fibre and chemical admixtures, 3 and the methods for placing into the moulds the cast-in load into the installed rings of lining. Collaboration with segment inserts. Otherwise the casting factory took well TBM manufacturers is needed to research the potential to to producing the new SFRC segments. The switch from reduce the thickness of segmental linings and the volumes conventional steel cage technology to fibre technology of concrete involved. also provided a world first for using mixed anchored cast-in gaskets for the SFRC segments. Cast-in gaskets Performance comparisons is a technology that is used conventionally in rebar In practice, Lot 1 by Eiffage provided the best environment reinforced concrete in France. ” for comparing the whole process of producing SFRC The steel fibre is produced at the Bekaert manufacturing segments and monitoring their installed performance plant at Petrovice in the Czech Republic and was trucked compared with their rebar reinforced counterparts. The the 1,500km journey to the Bonna Sabla casting factory project started with rebar reinforced segments and near Paris. Through associated studies and existing carbon introduced SFRC segments on the contract. Both rebar and footprint statistics, it is calculated that the production of the steel fibre reinforced segments are installed in the same fibre and its transportation provides a 300% environmental ground, by the same contractor and crews using the same benefit over the same criteria for the production and TBMs, produced by the same casting factory and stacked, transportation of steel rebar. stored, handled and transported using the same equipment and in exactly the same way. As Benoit de Rivaz of Bekaert By using the same moulds, the SFRC segments are confirmed: “This is one of the most comprehensive trials the same width and thickness as the rebar reinforced

40kg/m of 0.75mm diameter x 60mm long

fibres provides a steel network of 10,000km/m

The same SEALABLE (previously Dätwyler) cast-in mixed anchored M 38923 CVV gaskets, as used for the rebar cage segments, were used also in the moulds for the 100% fibre-reinforced segments

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Table 3. Comparison of SFRC and rebar reinforced segments and lining rings as applied on the 8.9m o.d. TBM1 drive for the Line 14 link and for the TBM2, 3, 5 and 6 drives for the 9.5m o.d. Lines 16 and 15 Number of rings inspected

Number of lining segments inspected

Number of lining segments showing cracking

Proportion of cracked lining segments FRC

FRC

Reinforced concrete

Total

FRC

Reinforced concrete

Total

FRC

0

855

855

0

5,985

5,985

0

TBM-2A

618

88

706

4,326

616

4,942

134

15

149

TBM-3

390

104

494

2,730

728

3,458

43

2

45

TBM-5

607

261

868

4,249

1,827

6,076

125

64

TBM-6

238

761

999

1,666

5,327

6,993

47

257

TBM-1

Reinforced concrete

Total

Reinforced concrete

Total

5.08%

5.08%

3.10%

2.44%

3.10%

1.58%

0.27%

1.30%

189

2.94%

3.50%

3.11%

304

2.82%

4.82%

4.35%

304

Performance data of the rebar reinforced segments for the TBM4 drives for the Line 17 link were not included in the comparison study

Fig 3. The percentage of cracked segments and their location in the 2m wide rings of 85kg/m3 rebar reinforced segments for the 8.9m o.d. drive of TBM 1 for the Line 14 connection

Cracks in SFRC segments were smaller, took longer to develop and closed sooner (left) and those that developed some seepage were mostly self-healing (right)

Figs 4 and 5. The percentage of cracked segments and their location in 14.7km x 9.5m o.d. drives of TBM 2A (left) and TBM 3 (right) for the new Lines 16 and 15 lined with 2m wide rings of 100% 40kg/m3 steel fibre reinforced segments with 85kg/m3 rebar reinforced segments installed at the launch portals and through curves of less than 300m radius

and comparisons of the SFRC segmental lining alternative conducted. It advances the documentation and study of SFRC segmental linings significantly.”

The observed performance of installing SFRC segments provided valuable results. “Initially, the TBM operators were cautious when knowing that the TBMs were pushing off SFRC segment rings,” said Bischoff. “It took a bit of time before operators were comfortable with the ability of the segments to withstand the TBM pushing forces and disregarded whether it was a rebar or SFRC reinforced segment ring.”

Performance data after ring build recorded more cracks in SFRC segments than for rebar segments (Figs 3, 4, 5) but that the cracks were of a smaller size, of less than 1mm and most less than 0.2mm, and that they closed up earlier and were largely self-healing. Any water seepage that developed over time through cracks in SFRC segments took longer to develop and was less in volume than for cracks in rebar segments. Water leaks that developed through cracks in rebar segments were generally larger and required repair with epoxy repair products or other methods. “We also expected more micro cracks in the 4

installed SFRC segments,” said Bischoff, “but we didn’t get them.”

Further observation confirmed that segments of both designs showed increased cracking in rings installed through curves as asymmetric loads were applied to pilot the TBM off the segmental lining around the curves. On the Lot 1 drives, the heavier 85kg/m3 rebar cage reinforced segments were used on the 9.5m o.d. SFRC segmentally lined drives through curves of less than 300m radius and were compared with the 100% SFRC segment rings of the rest of the drives. The SFRC lining was also compared with the TBM1 single-tube double-track drive using only rebar reinforced segment rings, which also exhibited more cracking through the curves (Fig 3). More research, using sophisticated in-situ monitoring processes, is needed to understand how the TBM thrust loads into the lining cause cracks in segmented linings through curves.

At the conclusion of its study, with 90% of the 14.7km of large single-tube double-track 2m wide rings x 400mm thick x 9.5m o.d. metro tunnels lined with seven 100% steel fibre reinforced segments/ring, Eiffage found that:

2012

IT CONTINUES WITH

1968 STARTING WITH

2020 TODAY WE ARE

The expertise remains, only the name has changed:

GRAND PARIS EXPRESS, FRANCE

TOGETHER WE B MORE THAN CO By 2025, at the end of project phase 1, Société du Grand Paris, 13 construction companies and 21 Herrenknecht state-of-the-art TBMs will connect the Parisian underground metro network with the suburbs. Grand Paris Express provides an exceptional opportunity to train the next generation of engineers and pushes innovation forward. herrenknecht.com/grandparis

BUILD ONNECTIONS

Client: Société du Grand Paris Contractors: BAM Bessac Bouygues Travaux Publics Demathieu Bard Eiffage GC Implenia NGE / NGE GC Pizzarotti Razel-Bec Soletanche Bachy France / Soletanche Bachy Tunnels Spie Batignolles Vinci Construction Webuild

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The life-cycle of SFRC is up to 300% more beneficial to the environment overall compared with rebar reinforced concrete (Table 4)

and that more research and study is needed:

Table 4. Environmental sustainability benefits of steel fibre compared with steel rebar 1kg of steel fibre production = 0.88kg of carbon dioxide

1 tonne of steel production 1.85 tonne of carbon dioxide 1kg of steel production = 1.85=tonne of carbon dioxide SFRC segments have 50% less steel than rebar reinforced segments saving more than 5,000 tonne of steel production carbon for 10km of tunnel One truck can transport 24.2 tonne of fibre per load compared with 17.85 tonne per truck load for reinforcing rebar The concrete chosen for the Line 16 Lot 1 fibre reinforced segments has a low carbon footprint of 170kg CO2 equivalent/m3 and reduces the carbon weight of the steel in the segments by 90kg equivalent CO2/m3 or nearly 11,000 tonne equivalent CO2 per 10km

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To ensure a consistent SFRC mix

•

To improve the vibration of SFRC into the moulds without disturbing the even distribution of fibres in the mix

•

•

To review the design and operation of the lids of the segment moulds to finish the extrados of smaller diameter SFRC segments. The shallower extrados of larger diameter segments and those with rebar cage reinforcement particularly, need no lids on the moulds to hold the casting shape To control the debate of what constitutes a proven design of good fibres in precast concrete segmental linings including the debate about the design and quality criteria of steel fibre, the orientation and concentration of the fibre within the precast segments and the investigation of any areas that might exhibit a lower concentration of fibres

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To streamline the validation testing processes required to verify the quality of the segments as produced and installed

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To develop nondestructive methods such as radiography and tomography for seeing inside the segments to ensure good distribution of fibres in the finished cast SFRC lining.

These last points are important as taking cores from cast or installed segments is difficult and costly, and the laboratory process of crushing a core to count the fibres and extrapolate fibre concentration and orientation within the precast SFRC lining is also difficult and of questionable value. Doubting the even distribution of fibres through the cast segments fuels doubt by sceptics about the long-term design-life durability of SFRC concrete. A nondestructive method of assuring fibre distribution through SFRC cast concrete segments would allay these doubts.

The Lot 1 contract studies revealed that there is very little documentation about the K coefficient distribution and orientation of fibres in cast concrete and that this needs addressing to avoid too conservative a fibre dosage design.

In the meantime, the experience of the 100% large diameter SFRC segments on Line 16 Lot 1 in Paris brings France into the group of countries in the world that have approved and accepted SFRC segments for large scale long-term public area metro construction. Starting with Line 16 Lot 1, the Société du Grand Paris has approved the use of SFRC segments for linings on other Grand Paris Express contracts and is ready to specify SFRC segments on future project contracts to secure the environmental, economic and sustainability benefits of the material while guaranteeing the flexural strength ductility and durability 8

needed to provide metro linings with a minimum 100-year design-life.

The experience confirms also that this is the start of the next phase of development for the technology, to: •

Develop low carbon concrete alternatives

•

Reduce the thickness and volume of concrete in segmental linings as part of increasing the sustainability credentials

•

Perfect the interaction of TBMs and SFRC linings to reduce thrust cracks, particularly around curves

•

Streamline and develop nondestructive validation testing for SFRC segments and linings

•

Increase the documentation of SFRC to support design standards and codes, particularly in reference to K coefficient distribution and orientation of fibres in SFRC elements and perhaps also reduce the fibre dosage in the mix to avoid conservative over design.

The study and development continues. n Details and the design parameters of the SFRC research and development campaign are provided in a conference paper by Jean-Luc Bischoff of Eiffage for the BTS Conference of October 2021 in London. Download a PDF copy of the Powerpoint conference presentation

TunnelTalk references • • • • • • • • • • •

Choosing right fibre for SFRC precast segments – August 2020 Video: Face to Face with Marc Vandewalle, pioneer of steel fibre reinforcement – May 2020 Management buyout of Datwyler gaskets business – May 2020 Integrated engineering for quality segmental linings – February 2020 Crack width control in FRC segments – February 2020 SFRC durability in a chlorinated environment – April 2016 Lee Tunnel wins prestigious concreting award – December 2016 Review of FRC brittle failure considerations – March 2015 Fibre-reinforced concrete for precast segments – May 2014 20 years of fibre concrete linings in the UK – October 2010 Video: Introduction of steel fibre concrete in the UK – August 2020