Fiber-reinforced concrete: the new revolution in the tunnel boring machine market

Fiber-reinforced concrete became a real alternative to traditional reinforced concrete in certain sectors, particularly for tunnel segments. Alcimed, an innovation and new business consulting company, presents an overview of this technology with a promising future.

Metal fibers as structural reinforcement for concrete constructions appeared in the early 1980s with promises of time and financial savings compared to the traditional solution of steel reinforcement or welded wire mesh. But what about today? Have the fibers achieved their goal?

Fiber-reinforced concrete, an attractive alternative to reinforced concrete for tunnel segments

Historically, reinforcement steel has been incorporated into the concrete mass to improve its performance, particularly in terms of traction and shear. Such concrete is commonly referred to as reinforced concrete. Despite its widely acknowledged outstanding mechanical performance, it has certain disadvantages, particularly at the level of its implementation, which is perceived as tedious during the welding and installation of steel bars. In addition, this reinforcement with its so-called “continuous” design is known to promote the propagation of corrosion.

The incorporation of metal fibers is an attractive alternative to reinforced concrete, particularly for tunnel construction, because fiber-reinforced concrete gets around the limiting points of reinforced concrete: since the fibers are non-continuous, they provide no mechanism for propagation of corrosion and the manufacturing process is simple and can be automated.

Fiber-reinforced concrete, a technology that needed nearly 20 years to establish itself in the construction sector

Initially, the lack of technical data, the lack of feedback and the conservative spirit of the construction industry were the obstacles to the development of fiber-reinforced concrete. As a result, tt took 20 difficult years to show and convince the industry about the full potential of this technology.

In the early 2000s, the market took off, particularly in the Anglo-Saxon countries, with England and the USA leading the way. Today, there are about 180 projects running worldwide based on fiber-reinforced concrete tunnel segments, 80% of which are railway tunnels (train, metro) and water tunnels (supply, wastewater, sewers, etc.). The Anglo-Saxon countries, led by England and followed by the United States and Australia, have also adopted this technology most widely and represent about 40% of tunnel projects based on fiber-reinforced concrete segments. Among the major projects, it is worth mentioning the Brisbane Airport Tunnel, with an internal diameter of 11.4 m inaugurated in 2012.

Significant economic benefits

If fiber-reinforced concrete becomes the number one choice, it is mainly because of the associated economic benefits. In fact, significant savings of around 10% are often reported on tunnel projects based on fiber-reinforced concrete segments, mainly in the production and installation of the segments. For example, for the Hobson Bay Tunnel project in New Zealand, it was estimated that 50% of the time was saved on production and 10% was saved on the total project cost (NZ$118.6 million). These significant cost reductions on production are partly explained by the very low rejection rate of fiber-reinforced concrete segments. For example, for the Hobson Bay Tunnel project, out of the 15,000 segments produced, only 7 segments were rejected (0.05%) and 6 segments were damaged during production (0.04%), representing a total rejection rate of less than 0.1%.

Generally, once tested on one project, fiber-reinforced concrete becomes a standard in the country, as seen in England or Australia.

France is lagging behind…

France has remained on the sidelines in this area with the exception of a few projects of note: 38 fiber-reinforced concrete segments in the Blagis tunnel (a sewerage structure), a pilot on line 14 or the Paris metro and temporary fiber-reinforced concrete segments at the stations of line 2 of the Nice tramway. The lag is partly due to the lack of major infrastructure projects over the past 10-20 years. However, today, the issue is of great importance in the context of the Greater Paris (Grand Paris) project.

Evolving regulations

There are still regulatory barriers to overcome, but this situation is expected to change by 2020 with the revision of Eurocode 2 allowing for the implementation of a Europe-wide regulation. In this context, a dedicated working group has been set up whose objective is to study the integration of fibers into the future Eurocode 2. At present there are different national regulations or codes for the design of fiber-reinforced concrete elements and in particular segments (e. g. Spain, Italy, Germany…). A major step was taken when in 2012, the FIB Model code 2010 (a reference in structural design) integrated fibers as a structural element.

Heading towards synthetic fibers?

With claims of superior performance over metal fibers (weight savings, less abrasive, less risk to users, no rust…), synthetic fibers could be the innovation of the future. Nevertheless, this requires proactive behavior to acquire feedback and validate the technology. While synthetic fibers have been on the market for more than 20 years, only a few projects for segment tunnels made with synthetic fibers are beginning to appear, such as the tunnel at Malaga airport in Spain.

“France is undeniably lagging behind in terms of the use of fiber-reinforced concrete. The Greater Paris project is a great opportunity to get up to speed on this technology, but why not go further in terms of innovation and technological leadership by initiating a proactive approach to synthetic fibers? The Greater Paris project could be an opportunity to carry out pilot tests on segments made of synthetic fibers in order to prepare for the future”, says Vincent Pessey, project manager at Alcimed.