Structural & Sustainability Requirement for Precast Segment

Introduction

Scientists have shown that human generated emissions of greenhouse gases (primarily CO2) from fossil fueled power supplied to industrial processes and fossil fueled vehicles, are the primary contributors to the dramatic rise in the earth’s temperature since the 1950’s.

Although the consequences of climate change are difficult to predict, it is clear that the rapid increase in earth’s temperature will bring about detrimental impacts on future generations. It is increasingly evident that we must act to reduce man-made levels of CO2 emissions into our atmosphere.

For constructing a sustainable project, design engineers are required to make conscious efforts to reduce the carbon footprint as well as provide for a minimum service life of the structures they are designing.

Concrete is recognized as the second most widely consumed commodity on the planet after water. It also contributes approximately 8% of global carbon emissions; the main source of these emissions is the manufacture of Ordinary Portland Cement (CEM I).

In a tunnelling project, it is generally considered that 60% to 70% of embodied carbon is contained in the concrete linings of the shafts and tunnels. It is important therefore for the tunnelling industry to do its utmost to significantly reduce or eliminate its use of cement in all applications – segmental linings, in-situ linings, sprayed concrete and annulus grouts. This is the reason why a great challenge in the coming years will be to develop a solution for low carbon lining.

Recent projects have demonstrated that structural ductility, durability and sustainability go hand to hand - a combined approach that will clearly be a new booster for FRC tunnel lining

History

Steel fibre reinforced concrete (SFRC) was introduced in the European market in the second half of the 1970’s. No standards, nor recommendations were available at that time, which was a major obstacle for the acceptance of this new technology. In the meantime, SFRC has been applied ever since in many different construction applications, such as in tunnel linings, mining, floors on grade, floors on piles, prefabricated elements, etc. In the beginning, steel fibres were used to substitute a secondary reinforcement or for crack control in less critical constructions parts. Nowadays, steel fibres are widely used as the main and unique reinforcement for industrial floor slabs and prefabricated concrete products. Steel fibres are also considered for structural purposes helping to guarantee the construction’s ability and durability in:

• foundation piles reinforcement
• reinforcement of a slab on piles
• full replacement of the standard reinforcing cage for tunnel segments
• reinforcement of concrete cellars and slab foundations
• as shear reinforcement in pre-stressed construction elements.

This evolution into structural applications was mainly the result of the progress in SFRC technology, as well as the research done at different universities and technical institutes to understand and quantify the material properties. In the early 90’s, recommendations for design rules for steel fibre reinforced concrete started to be developed. Since October 2003, Rilem TC 162-TDF recommendations for design rules are available for steel fibre reinforced concrete. (Ref 1). Over the past years, the use of this technology has increased dramatically. Use of SFRC has several advantages over traditional steel mesh or steel bar reinforcement. These include:

• Can facilitate remote working from the face, and may remove the need for traditional reinforcement, to enhance the safety of the workers
• Provides cracking control and a small enhancement of the concrete’s tensile properties
• Less prone to carbonation and chloride attack
• Reduction in cost and time saving
• Less material usage (through minimizing the amount of steel and concrete cover required).

One of the aspects boosting use of FRC in segmental linings is the introduction of guidelines for the design of FRC. In 2013, the fib presented the Model Code 2010 in which a specific part related to FRC is inserted. This document has sparked great interest in the tunnelling community and several documents consider Model Code 2010 as a reference.

The fib bulletin 83 Precast tunnel segment in fibre reinforced concrete published in 2017 based on MC 2010 aims to support designer, contractor and clients with guidance for the use of steel fibre reinforced concrete, known as FRC, in precast segment lining tunnel constructed using tunnel boring machine (TBM). The document is intended to complement fib MC 2010.

Sustainability & Structural Requirement

For sustainable use of structural concrete, environmental and mechanical performances of concrete structures must have the same importance. By means of sufficiently high mechanical performances, the structural safety of construction is ensured. Contemporarily, a low environmental impact guarantees a sustainable development, which is, in accordance with the Brundtland Commission of the United, the "development that meets the needs of the present without compromising the ability of future generations to meet their own needs".

FRC acts on the tensile behavior of cracked concrete and imparts ductility to a fragile material. FRC's excellent properties which overcome cracking as well as its improved durability over reinforced concrete are why we continue to develop that material and also explain its economic success. The life cycle assessment (LCA) is a set of standardized data. It quantifies a material's impact on the environment over its entire existence, from extraction of the raw materials required for its production up to its end of life.

This approach, combined with research into a low-carbon solution, will give new momentum to FRC. Bekaert's global sustainable development strategy is based on four major pillars: our responsibility in the workplace, in the market, and towards the environment and society in which we operate. For example, Bekaert is providing its expertise and support to the innovative Cargo Sous Terrain (CST) underground cargo logistics system in Switzerland. In reality, CST is just reinforcing and systematizing the aspects of sustainability which are already inherent in the system. Apart from the fact that the CST provides a zero-emissions delivery route, which is therefore climate-neutral, a work group is also already making preparations for construction in accordance with recognized sustainability standards.

Basic FRC Behavior

A minimum tensile strength > 1800MPA is recommended for final ling application considering the performance required and concrete class.

The hooked ends ensure the desired fiber pull-out. This is the mechanism that actually generates the renowned concrete ductility and post-crack strength. Bekaert’s Dramix® 4D steel fibers utilize the same principle, which translates into improved anchorage and ductile behavior.

The tensile strength of steel fiber has to increase in parallel with the strength of its anchorage. Only in this way can the fiber resist the forces acting upon it. Otherwise, it would snap, causing the concrete to become brittle. On the other hand, a stronger wire cannot be fully utilized with an ordinary anchor design. Therefore, the tensile strength of a fiber has to be perfectly aligned with its anchorage system and its diameter. Dramix® 4D is designed to capitalize on the wire strength to the maximum degree.

Wire ductility and concrete ductility are two different aspects. Dramix® 3D and 4D steel fibers create concrete ductility by the slow deformation of the hook during the pull-out process, and not by the ductility of the wire itself.

EPD Certificate

An EPD is a document that transparently communicates the key environmental performance indicators of a product over its lifetime.

A third-party verification ensures that data relating to environmental aspects of Dramix® has been validated by an external organization

This declaration is the Type III Environmental Product Declaration (EPD) based on EN 15804:2012+A1 and verified according to ISO 14025 by an external auditor. It contains the information on the impacts of the declared construction materials on the environment. Their aspects were verified by the independent body according to ISO 14025. Basically, a comparison or evaluation of EPD data is possible only if all the compared data were created according to EN 15804:2012+A1 (ref 4).

The environmental impact of Dramix® product (cradle to gate with options) is largely dependent on the energy intensive production of steel (half product) on which the manufacturer has only a limited influence. The carbon impact of steel production (Wire Rods) in the product stage A1 is as high as 85%. The impact of the production line largely depends on the amount of electricity consumed by manufacturing plant (0.34 kWh/kg of product). There are no significant emissions or environmental impacts in the A3 production processes alone (partly gas combustion). The production process itself does not have significant environmental impacts in the life cycle.

Interrogation of the LCA results show that the cradle-to-gate carbon (Global Warming Potential) impact of 1 kg of fibre production is 0.88 kg CO2eq. For comparison, ton of steel produced worldwide in 2019 emitted on average 1.85 tons of carbon dioxide.

The LCA results show that the cradle-to gate primary energy demand of fossil fuel is equal 9.4 MJ. This is due to the production of nuclear energy by the Czech Republic. The transport of raw materials from considerable distances is optimized and not significant (0.007 kg CO2/kg).

Design Principle

Model Code 2010 is the most comprehensive code on concrete structures. It covers their complete life cycle from conceptual design, dimensioning, construction and conservation through to dismantlement. It is edited by fib (fédération internationale du béton / international federation for structural concrete). Fib Model Code 2010 was produced through the exceptional efforts of participants in 44 countries from five continents.

Figure 1 illustrates the design process involved, from beam tests, classification, and design values, to constitutive laws.

The tensile behavior of the materials was characterized by performing bending tests on a notched beam. The tests were performed according to the EN 14651 European code, which is the reference standard for the CE label of steel and for ISO certification.

The compressive strength of the materials was measured by a testing cube with a side of 150 mm. For every cast made for the production of every single segment, three beams were produced. In agreement with EN 14651 [4], nominal strengths corresponding to four different crack mouth opening displacement (CMOD), namely 0.5, 1.5, 2.5 and 3.5 mm, were evaluated.

Figure 2 shows a typical result of the beam tests with significant strength values. FL is peak force, fR1 and fR3 are the stresses related to CMODs equal to 0.5 and 2.5 mm respectively. These values are the reference ones for final lining design performed according to the fib Model Code 2010 prescriptions.

To dimension a steel fiber-reinforced concrete segment, a reference test methodology needs to be adopted for the characterization of performance. In addition to the mechanical performance, various properties of the FRC can be specified.

Since brittleness must be avoided in structural behavior, fiber reinforcement can be used as substitution (even partially) of conventional reinforcement (at ULS), only if both the following relationships are fulfilled:

fR1k/ fLk > 0.4

fR3k / fR1k > 0.5

Where fLk is the characteristic value of the nominal strength, corresponding to the peak load (or the highest load value in the interval 0 – 0.05 mm), determined from the EN 14651 beam test.

It is recommended to realize 12 beams per dosage and concrete mix formula.

If fibres are used as the only reinforcement for final lining, hardening post-crack behavior at section level (beam test) allow immediately:

• Cracking control at SLS
• Structural ductility (ULS)

Precast Segment

The use of steel to replace all or a part of conventional reinforcement has been demonstrated to lower the embodied CO2 of the segmental lining. While it is possible to significantly reduce the embodied CO2 of a concrete mixture for segment production by replacing a portion of its cement content with alternative cementitious materials, there is little or no difference between the cementitious blends and contents required for the production of fiber reinforced or conventionally reinforced concrete segments for tunnel linings.

Figure 3 shows an example of reduction in CO2 emissions on a project made possible by modification of the concrete and further reduction by being able to replace the rebar with steel fibers in a dosage that satisfied all of the design requirements. On a per pound (kg) basis, the embodied CO2 of conventional rebar and steel fibers is assumed the same. This is a generalization assuming the wire rod that the fiber is produced from and the rebar has similar % recycled material content and similar steel production methods. In a precast segment the reduction in carbon footprint is due to the steel fibers being more efficient in reinforcing the element. In this example, the elimination, the combination of the right binder and steels fibre could conduct to a reduction 70% .


The recent project for the Grand Paris linea 16.1 has shown the following:

• From the saving in the ratio of fibers compared with steel reinforcement bars, leading to a significant reduction in CO2 emissions during transportation. If we compare 85kg/m3 for steel reinforcement bars with the 40kg/m3 for fibers, we get a saving on materials of more than 50%.
• By the benefit of better optimized loading for the fibers. 22 big bags of 1,100/kg per truck = 24.2 tons per load for the delivery of the fibers in comparison with 60 equivalent segments per truck = 17.85 T for the delivery of the concrete reinforcement bars.
• From the small diameter of the fibers which helps to further limit toxic emissions from the primary steel industry, due to primary coils which do not exceed 1 mm of wire diameter. The drawing technology is low emission.
• Fewer trucks on the road and optimized waste management in a large city like Paris is an important element to take into account. From an ecological point of view, the carbon balance is therefore very positive. In this respect, Bekaert has recently obtained its EPD (Environmental Product Declaration) Type III ITB certificate number 215/2021.

In terms of concrete, there will be a before and after Grand Paris Express. Until now, to design the segments, we used reinforced concrete, that is to say concrete poured around cages of massive metal reinforcements. Since 2020 on line 16, approximately 4 km of tunnel have been designed in fiber-reinforced concrete and a much larger deployment is now planned given the latest contracts awarded. This is a rare scale of deployment in the transport infrastructure sector in France.

Benefits At All Levels

Fiber-reinforced concrete, which consumes less steel, saves resources. As its name suggests, reinforced concrete is reinforced by a steel frame which represents large quantities of steel, around 100 kg of steel for 1 m3 of concrete", deciphers Alex Moubé, head of the low carbon mission at the Greater Paris Society. The FRC option consumes twice less steel for the same performance. It takes 40 kg of steel fibre for 1 m3 of concrete for linea 16.1 thanks to Dramix® 3D 80/60BGP. Steel consumption is half and 5,000 tonnes of steel are saved for 10 km of tunnel allowing, at the same time, to achieve substantial cost savings.

Still in terms of resources, fiber-reinforced concrete can reduce the quantities of concrete by 2 to 3 cm in segment thickness.

In addition to the quantities of steel and concrete saved, fiber-reinforced concrete also makes it possible to reduce CO2 expenditure, both in cement plants and in steelworks: 10,000 tonnes of CO2 are saved on average for 10 km of tunnels compared to concrete armed.


That's not all, fiber-reinforced concrete also improves the technical performance of the structures built. Thanks to the presence of fibres, the segments have better behavior in the face of cracks. Not only are they less important than for reinforced concrete, but they also close over time. Similarly, the segments are more resistant to corrosion. Indeed, steel, in contact with air and water, corrodes. As the fibers are interspersed in the cementitious matrices, if a fiber is corroded, it will not spread its corrosion in the other fibers. In time, we will therefore be in the presence of a much more durable material.

A paper by Carola Edvardsen of COWI Denmark entitled ‘The consultant’s view on service life design’ [5] provides an example of reduction of concrete and further reduction by being able to replace the rebar with steel fibres in a dosage that satisfied all of the design requirements.

The Montreal Metro Blue Line Extension Project consists of construction of 6 kilometers of tunnel, as well as five new underground stations. This represents a good example as well shows that durability and sustainability go hand to hand. Indeed, the reduction in segment thickness achieved with fibers can be primarily attributed to the concrete cover requirements of 60-75 mm on both intrados and extrados rebar to ensure the durability against corrosion when designing according to Canadian code CSA A23.1:19 (2019). In contrast, when subjected to chloride exposure, corrosion in steel fiber reinforced concrete is limited to just a few millimeters from the surface, and nonetheless, does not lead to spalling cracks and is not regarded as a durability issue.

CO2 savings in the segments is realized by replacing rebar with steel fibers as the quantity of steel required is 50% less per m3 of concrete with fibers (40 kg/m3 vs 80 kg/m3). Additionally, the CO2 equivalent factor for rebar is reported to be 1.85 vs 0.88 for fibers. The fiber reinforced segments can be reduced in thickness due to no requirement for cover like rebar. This quantity of concrete savings also lowers the carbon footprint.

The owners’ design engineer, AECOM (ref 7), as part of a commitment to integrate sustainability best practices, performed a study utilizing the Envision framework to evaluate alternatives to achieve a most sustainable infrastructure project. Based on the results of this study, the TBM bored tunnel sections will be lined with steel fiber reinforced precast concrete segments using low-carbon Supplementary Cementitious Materials (SCM) concrete.

In the TBM tunnel sections lined with pre-cast concrete segments, high performance Dramix steel fiber 4D80/60BGP with a dosage of 40 kg/m3 is designed as standalone reinforcement.

The table 1 summarizes the results of the evaluation showing a reduction in total CO2 equivalent by nearly 50% using SFRC with an optimized SCM concrete mix design.

Conclusion

There has been a trend in the last years that concrete tunnel linings have increased material consumption, cost and environmental loads. Nowadays, to develop and/or improve tunnel construction methodolgy, one must choose the optimal tunnel lining, including the environmental footprint and cost-effectiveness. One also need to create awareness and the required knowledge to produce the final lining to meet the new large infrastructure projects with modern demands for functionality including 100-years of service life and the environmental impact.

The use of steel fibre reinforced concrete has the potential to meet low carbon lining by lowering concrete consumption and steel reinforcement saving. If ductility and durability have been the key words in the last 40 years, sustainability will be the key driver for further FRC lining development in the coming years.

Indeed, the new generation of binder combined with FRC allows new achievements:

• Excellent long-term durability performa- nce exceeding that of Portland cement-based concretes.
• Extremely low embodied carbon footprint compared to conventional concretes on Portland cements.
• Compared to reinforced concrete, fiber-reinforced concrete notably represents savings of around 5,000 tonnes of steel for 10 kilometers of tunnels (Typical Metro Tunnel)

References

• RILEM “Recommendations for design rules are available for steel fibre reinforced concrete”. TC 162-TDF
• FIB “Model Code 2010 – First complete Draft”. 2010, Bulletin 55-56
• Fib bulletin 83 FRC precast segment
• ITB “EPD – Environmental Product Declaration Type III ». 2015, No. 215
• EN 14651 « Test Method for metallic fibre concrete. Measuring the flexural tensile strength”. 2005
• Edvardsen, C. “The consultant’s view on service life design”, WTC Congress, COWI A/S, Parallelvej 2, DK-2800 Kongens Lyngby, Denmark WTC Congress
• Verya Nasri, Medhi Bakshi “Design and Construction of FRC Tunnel Segments in North America with Fiber-Enabled Carbon Footprint Reduction” fib ACI 2023