TECHNICAL | LININGS - FRC
BEYOND TRADITIONAL FRC LININGS
Wider opportunities are arising across the needs of underground space development to apply FRC, says Benoit de Rivaz, Global technical Manager, Bekaert BP Underground Solutions.
As tunnelling projects come under increasing pressure to reduce embodied carbon, fibre reinforced concrete (FRC) is moving from a specialist option to becoming more of a mainstream design strategy. Concrete linings for tunnels and shafts are generally
estimated to account for 60% to 70% of a tunnel project’s embodied carbon, mainly because of the cement and steel they contain. Reducing this carbon impact requires the underground construction industry to look beyond conventional steel reinforcement, as bars or mesh, and consider pursuing a combination of using high- performance steel fibres, optimised structural design and supplementary cementitious materials. When these levers are applied together, the embodied carbon of segmental tunnel linings and permanent sprayed concrete linings (SCL) can be reduced by almost 50%. The use of FRC in tunnel linings has grown
significantly over the past two decades. This development has been supported by dedicated design guidance, including fib Model Code 2010 and fib Model Code 2020, which provide a recognised basis for the structural design of FRC. As a result, FRC is now widely accepted for final tunnel linings, offering practical benefits to owners, designers and contractors. One of the most established applications is the use of
FRC in precast tunnel segments. Mechanically excavated tunnels, particularly those built with tunnel boring machines (TBMs), rely on such segmental linings that act both as the reaction element for the TBM to push against during excavation and is also the permanent support.
PRECAST SEGMENTAL LININGS In precast segment production, FRC can reduce or even eliminate the use of traditional steel bar (rebar) or mesh reinforcement. Compared with such conventional reinforcement, the principal advantages of FRC, as identified in fib Bulletin 83 and other leading recommendations, include: ● Improved crack control during construction phases; ● Higher impact resistance; ● Enhanced long-term durability; ● Reduced costs; ● Improved sustainability performance; and, ● More efficient production processes.
Designers, contractors and clients get practical guidance
for steel fibre reinforced concrete (SFRC) in precast segmental tunnel linings, constructed with TBMs, from fib Bulletin 83. It complements fib Model Code 2010, which remains a key reference for FRC segmental lining design. In many tunnel projects, a Performance Class such as
C50/60 5d, according to Model Code 2010 5, is considered a minimum requirement. This level of performance can be achieved through an optimised concrete mix design and high-performance steel fibres, such as Dramix® 4D 80/60BGP. The typical performance criteria, referring to the classification of characteristic values of FRC residual strengths, are fRk,1> 5.0MPa and fRk,3> 5.5MPa, according to EN 14 651 and characteristic value determined following fib bulletin 83.
PERMANENT SCL At the same time, demand is growing for more advanced use of sprayed concrete. Current practice often relies on conservative structural designs using cast-in-place (CIP) concrete or additional lining systems. These approaches can significantly increase cost, the volume of excavated rock, construction time and CO₂ emissions. Replacing a temporary sprayed concrete liner (SCL) and
a permanent CIP liner with a permanent SCL, combined with an improved mix design, can reduce CO₂ emissions by up to 75%. Permanent SCL also allows the preliminary lining to
Dramix 4D 80/60BGP. All images courtesy of Bekaert except where noted.
16 | October 2026
be considered as part of the permanent load-bearing system. Depending on the project, the preliminary sprayed FRC lining may work together with a cast final lining, collaborate with a second sprayed layer, or act as the final lining itself. Recent publications, including fib Bulletin 116 and AFTES recommendations, provide clear guidance
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