New Approach for SFRC Final Support and Lining

Achieving decarbonisation in tunnel construction is not only possible but crucial from a sustainable perspective.

Currently, nearly 70% of embodied carbon in tunnels is attributed to concrete linings. To bring this down, we need to better consider the sprayed concrete lining as part of the permanent lining.

The excessive structural design using cast-in-place (CIP) concrete or other lining structures, used in the current practice, have a significant impact on cost, excavated volume of rock mass, construction time and also CO2 emissions.

Changing the combination of temporary sprayed concrete liner and permanent CIP liner to, instead, only a permanent shotcrete liner and having improved mix design could reduce CO2 emissions from the materials production and lining construction by 75%.

Historically, tunnels constructed using sprayed concrete have been based on:

1. Temporary sprayed concrete lining is placed to stabilise the opening after excavation and to contain short to medium-term loads;
2. A waterproof membrane is then placed upon the temporary lining (once that initial lining is fully stabilised), and therefore will be located between this layer and the secondary, permanent linings yet to be placed; and,
3. A permanent CIP concrete lining is installed to contain long-term loads and provide durability and watertightness.

Over the last 30 years, modern sprayed concrete technology and quality has evolved and now equips the tunnelling industry with a more economical tunnel lining system as a Permanent Sprayed Concrete Lining (PSCL), which replaces the CIP final lining. This is also known as the Single Shell Method.

The use of high-performance steel fibres in the sprayed concrete eliminates the traditional steel bar reinforcement in the final lining, providing a reduction in the total quantity of reinforcing steel required, a reduction in lining thickness and also in the quantity of concrete required.

Also, with less labour being required plus savings in both construction time and cost, this new method provides a more durable and sustainable structure while reducing the carbon footprint of the project.

Note, the term ‘single shell’ does not refer to the placing of a single sprayed concrete layer but to the interaction of several layers as a single shell (initial support), rock bolts (if needed), water proofing membrane (if needed), and the shotcrete final layer.

The possibility of using permanent sprayed concrete allows this preliminary lining to be considered for carrying the permanent load. In this case, the possible solutions are to consider:

1. the preliminary sprayed, fibre reinforce concrete (FRC) lining collaborating with the final lining;
2. the preliminary sprayed FRC collaborating with a further sprayed lining in a second stage; or
3. only the preliminary FRC lining as the final stage.

The use of sprayed concrete to ensure the double functions of temporary support and final lining is a relevant approach to the eco-design of underground structures. This approach can minimise the thickness of concrete to be used, thus reducing the volume of material to be excavated or, alternatively, saving space within the cross-section, in order to facilitate the longterm management of the structure.

Specific technical strengths and weakness of the different fibres available on the market are often less well-known, and this can lead to confusion. Fibres for concrete exist in all colours, shapes, sizes and materials.

High performance Dramix steel fibres are suitable reinforcement material for concrete in that: the thermal expansion coefficient is equal to that of concrete; the Young’s Modulus is at least five times higher than that of concrete; and, the creep of regular carbon steel fibres can only occur above 370°C. This performance (high tensile strength > 1800 MPa, optimised anchorage, and high l/d ratio to guarantee the minimum network required) allows a hardening post crack behaviour at section level, allowing crack control at Service Limit State and structural ductility at Ultimate Limit State.

Consequently, justifying the mechanical behaviour of fibre reinforced sprayed concrete for permanent or long-term temporary structures is a major issue for the development of this technical possibility.

Testing to determine the residual strengths for sprayed fibre reinforced concrete can be difficult because specimens for the beam tests must be cut from sprayed panels. With the increased use of steel fibre reinforced for sprayed concrete lining (SCL) a new standard was developed that allows the residual strength parameters to be determined by performing a three-point bending test on the standard EN type square panel.

The advantages of this new three-point bending test on a notched panel method (EN 14488-3 Method b) in steel fibre reinforced SCL (SFRSCL) applications include:

• The geometry and dimensions of the specimens, as well as the spray method adopted will ensure distribution of the fibres in the matrix, which is close as possible to that encountered in the real structure;
• The dimensions of the test specimen will be acceptable for handling (no excessive weights or dimensions);
• The test will be compatible with use in many normally equipped laboratories (no unnecessary sophistication);
• The geometry is the same as in the plate test for Energy Absorption;
• The plate can be sprayed on the job site – Eliminates the need to saw a beam out of a panel;
• The scatter will be lower than the current standardised beam tests because of the larger specimen;
• The notch will provide a controlled cracking process, thereby reducing the risk of a sudden fall; and,
• The test provides the required residual flexural strength values needed in structural designs according to Model Code and existing standards.

Different recent research and publication have confirmed that the Test standard EN14488-3 Method B is particularly useful for characterising fibre reinforced sprayed concrete and the Performance Class 3c could be achieved with Dramix steel fibre.

The paper published by Universitat Politecnica de Catalunya (UPC) during WTC 2025 presented results from an experimental programme aimed at developing a sprayed steel fibre reinforced concrete (S-SFRC) mix that consistently achieved the 3c strength class (Annex L, EC-2) by analysing the strength class obtained via Method B of EN 14488-3. Sprayed panels were produced to evaluate the mechanical performance of S-SFRC with 40kg/m3 Dramix steel fibre:

• Sprayed panels show lower variability (CoV 10%-15%) than cast beams (20%-29%) due to differences in cracked area, resulting in higher strength classes for the same S-SFRC with Method B;
• S-SFRC panels (Method B) reached strength class 3c according to Model Code with 40 kg/m³ of Dramix 4D 65/35BG steel fibres and the tested concrete mix, demonstrating sufficient potential to meet mechanical performance requirements for the target class; and,

In other paper presented by Catherine Larive, of CETU, during the WTC conference, underlined the:

• Achievability of mechanical performances of strength class 4c with very high performance Dramix fibre; and,
• Test standard EN14488-3 method B is particularly useful for characterising fibre-reinforced sprayed concrete, for several reasons: the larger specimens, used without sawing, are more representative of in-situ sprayed concrete; and, dispersions are greatly reduced, which makes it possible to obtain more interesting characteristic strengths that can be considered relevant for dimensioning.

Numerous improvements were made to the concrete mix design: increasing the cement dosage; reducing the water/cement ration (W/C); optimising the granular skeleton; adding silica fume; selecting a highperformance fibre (reducing diameter and increasing tensile strength).

Steel still is the most recycled material, and the Bekaert R&D team, together with universities, continues to investigate recyclability. Remelted or reused steel could be directly reused as steel fibres to reinforce tunnels and mines. From the increasing number of tunnel projects — where environmental authorities approved tunnel muck disposal in the sea or river — steel fibre spray concrete delivers a microplastic and pollution free solution, as is increasingly required by governments, and as seen with Norway where plastic fibres are banned.3

To provide additional peace of mind for dosage in situ on sprayed concrete linings, a new inductive test to determine the content and orientation of steel fibres in reinforced concrete has been developed in collaboration with UPC. The equipment allows to determine the content and orientation of the fibres present in the concrete from the variation produced by them in a magnetic field generated by the equipment. A clear view of how fibres are distributed helps to verify that design requirements are being met before commitment to large-scale pours of steel fibre reinforced concrete. The eyeD® Inspector helps to reduce risks, optimise performance, and save time.

The assessment of concrete linings requires the definition of both the Sustainability Index and Mechanical Index.

Designers, constructors and suppliers must work together to achieve reductions of CO2 equivalent (CO2e) emissions in design and construction, and this collaboration must be incentivised by the client.

The tunnelling supply chain’s objective needs to be to reduce the carbon footprint of tunnel construction and contribute to the prevention of climate change.

Every available lever needs to be used to reduce CO2e emissions, but first there needs to be focus on the areas the supply chain can make the biggest difference:

• Design Optimisation
• Reducing Portland Cement & Steel
• No microplastics pollution

We believe tunnels should use smart and sustainable construction materials. The future of tunnelling is choosing these materials today.

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REFERENCES

1. Carlesso, D. M., Leporace-Guimil, B., Aguado, A., De la Fuente, A. & De Rivaz, B. (2025) ‘Sprayed steel fibre reinforced concrete: strength classes obtained through EN 14488-3 Method B’. WTC 2025, Stockholm.
2. Larive, C., Chalencon, F., Leclere, N., De Rivaz, B., Tolka, N., Bonjour, T., Mestari, A., Zghondi, J. & Bouteille, S. (2025) ‘New experiments to improve sprayed concrete performances for final support and lining’. WTC 2025, Stockholm.
3. Myren, S. A., Hagelia, P. & Bjøntegaard, Ø. (2018) ‘The ban of polymer fibre in FRSC in Norwegian road tunnels’. 8th International Symposium on Sprayed Concrete – Modern Use of Wet Mix Sprayed Concrete for Underground Support. Trondheim, Norway, 11-14 June 2018.