Introduction

Steel Fibre Reinforced Concrete (SFRC) was introduced to the market in the second half of the 1970s. Neither standards nor recommendations were available at that time, which was a major obstacle for the acceptance of the new technology. Since then, SFRC has been applied in numerous different construction applications, such as for tunnel linings, mining, floors at grade, floors on piles and prefabricated elements.

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 a main reinforcement method for industrial floor slabs, prefabricated concrete products and in tunnelling applications.

Steel fibres are now considered for structural purposes, helping to guarantee the structure’s ability and durability in tunnelling and mining. This is for:

  • Precast segments
  • Cast in place final lining
  • Permanent spray concrete lining
  • Shaft lining

This evolution into structural applications was mainly the result of the progress within SFRC technology, as well as the research done at different universities and technical institutes in order to understand and quantify the material properties. In the early nineties, recommendations for the design rules for SFRC started to be developed.

Since October 2003, the Rilem TC 162-TDF recommendations for design rules have been available for SFRC. One of the aspects that is boosting the use of fibre reinforced concrete (FRC) in segmental linings has been the introduction of guidelines for the design of FRC. In 2013, the fib presented the Model Code 2010 in which a specific section related to FRC was included. This document sparked great interest in the tunnelling community and several documents consider Model Code 2010 as a reference.

For this reason, fib Task Group 1.4 “Tunnels” decided to create Working Party 1.4.1 on “Tunnels in Fibre- Reinforced Concrete”. The Working Party prepared the current bulletin with the aim to support designer, clients and construction companies in introducing FRC in segmental lining tunnels referring to the indication of Model Code 2010.

Around the millennium, suppliers of micro synthetic fibres started to offer macro synthetic fibres. Micro synthetic fibres are typically 6 to 12mm long, have a diameter of 16 to 35μm, and are widely used to reduce plastic shrinkage cracks, as well as to reduce concrete spalling during a fire.

As the Young’s modulus of polyolefin is typically around 3,000 to 10,000MPa, it is generally understood that the reinforcing effect of these fibres is gone after a couple of hours of hardening of the concrete, as hardened concrete typically shows a Young modulus of around 30,000MPa. Macro synthetic fibres typically have dimensions equal to steel fibres, with length varying from 15 to 60mm, and diameter from 0.4 to 1.5mm.

Macro synthetic fibres are to be considered a specific construction material but are often marketed as being equal to steel fibres. But is this true?

1. Material properties of Steel and Polymer Fibres

Fibre Reinforced Concrete (FRC) is a composite material characterised by a cement matrix and discrete fibres (discontinuous). The matrix is either made of concrete or mortar. Fibres can be made of steel, polymers, carbon, glass or natural materials.

The properties of the composite depend on the characteristics of the constituting materials as well as on their dosage.

Other factors such as the geometry, the volume fraction and the mechanical properties of the fibres, the bond between fibre and concrete matrix, as well as the mechanical properties of the matrix, significantly affect the FRC properties.

The behaviour of FRC is more than a simple superposition of the characteristics of the concrete matrix and the fibres; to analyse the behaviour of this composite material, also the interaction between both has to be taken into account, i.e. the transfer of loads from the concrete matrix to the fibre system.

Therefore, for efficient load transfer, the following three conditions must be satisfied:

1. Sufficient exchange surface (number, length, diameter of fibres).
2. The nature of the fibre-matrix interface allows for proper load transfer.
3. The intrinsic mechanical properties (Young’s modulus, anchorage and tensile strength) of the fibre allows the forces to be absorbed without breaking or excessively elongating the fibre.

In fact, in a hyper static mechanical system, the better the cracking is “controlled” as soon as it arises (small openings), the better will be the multicracking process and thus the more the structure tends to show ductile behaviour.

1.1 The Young’s modulus of the fibres

The reinforcing ability of a fibre depends on the anchorage of the fibre into the concrete, the tensile strength and the Young’s modulus. The Young’s modulus of concrete is typically 30,000MPa, of steel fibre typically 210.000 MPa, and of polyolefin fibre typically 3,000 to 5,000MPa. For well anchored fibres, and equal solicitation of the fibre, the elongation of the polymer fibre, and subsequently the corresponding crack width in concrete, might be considerably higher compared to steel fibres. This might have an impact on the durability of the concrete, especially in combination with traditional reinforcement.

If we look at the Young’s modulus of different types of fibre we get a very important insight – steel fibres are significantly stiffer than concrete (a higher Young’s Modulus), whereas synthetic fibres are less stiff than concrete (a lower Young’s modulus).

1.2 Tensile strength of the fibres

The tensile strength of steel wire is typically 1,000-2,000 MPa, versus 300-600 MPa for most macro synthetic fibres.

1.3 Specific density of the fibres

The specific density of steel fibres is typically 7,850kg/m3, versus 910kg/m3 for polymer fibres, and 1,000kg/m3 for water. Polymer fibres are light, which is favourable for health and safety, but they are lighter than water: the polymer fibres float on water, with potential risks for fibres at the surface in, for instance, flooring applications.

This floating fibre could also create a safety issue blocking the pump in the presence of water, for tunnelling and mining. Floating fibres could also pose some problems for dewatering. Blocking the pump is a critical issue that you have to solve in all areas of your gallery for obvious safety reasons. Low density combined with high rebound using with spray concrete process could create critical environmental issues.

1.4 Fire resistance of the fibres

Polypropylene fibres typically melt at temperatures around 160°C. Therefore, micro polypropylene fibres are proven to be suitable to improve the fire resistance. The exact reason is not yet fully understood, but it is generally accepted that fine micro fibres start to melt in extreme fire conditions, thereby leaving small channels through which the pressurised vapour can escape. Consequently, less damage, less spalling of the concrete is to be expected.

Macro synthetic fibres do melt at equal temperature but are not fine enough to provide the concrete under fire with the necessary network of channels. Moreover, since the fibres melt, they are less suitable in those building constructions, where the reinforcing effect of the fibres is important.

1.5 Resistance against oxidation

Polymer fibres don’t rust, even if the fibres are sticking out at the surface. Bright steel fibres can show some staining if the fibres are at the surface, but never cause spalling of the concrete. If for aesthetic reasons, staining is not allowed, as in some prefabricated structures, galvanised steel fibres can be applied.

1.6 Mixability of the fibres

Some macro synthetic fibres tend to fibrillate during mixing. This fibrillation process goes on in the truck-mixer, until all fibres are completely destroyed. Quality degradation during mixing does not occur for steel fibres.

1.7 Properties of steel and macro synthetic fibre concrete

Fibre concrete is well known for its ductility. The effect of fibres is a combination of reinforcement and networking. Steel fibres in particular mainly change the behaviour of the concrete: steel fibres transform a brittle concrete into a ductile material which is able to withstand fairly large deformations without losing its bearing capacity. Ductility means load redistribution and a higher bearing capacity of the structure with the mechanical properties of the basic concrete material unchanged.

1.8 Reinforcing effect measured in beam tests

In general, most macro synthetic fibres perform rather moderately in a bending test. The pure reinforcing effect is rather poor due to the low modulus of Young, and the rather low tensile strength.

Most macro synthetic fibres start working at much larger crack widths than steel fibres; steel fibres with anchorage, depending on the specific fibre type, typically work optimally at crack widths 0.5 to 2.5mm, whereas macro synthetic fibres work optimally after some 3mm of crack width.

2. Durability

2.1 Creep of steel fibres and macro synthetic concrete

Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and generally increases as the melting point is neared. Experience from 14 years of creep testing of steel and polymer fibre reinforced concrete conclude ( ref FRC-Creep 2016 | Rilem TC 261-CCF) concludes:

  • The majority of tested Polymer FRC specimens fail under sustained loading while all steel FRC specimens remain intact.
  • Load-crack mouth opening displacement (CMOD) increases exponentially by a delta temperature of 10°C

2.2 Aggressive environment

The main factor that determines the durability of a concrete structure is achieving a low permeability which reduces the ingress of potentially deleterious substances. Low permeability is achieved by using the right sprayed concrete mix design with reduced shrinkage. Control of micro-cracking is also an important parameter. Fibres have been used successfully in permanent sprayed concrete tunnel projects to reduce cracking widths to 0.2mm.

Fibres have the advantage over conventional anti-crack reinforcement to be randomly distributed through the entire tunnel lining structure. The homogeneous reinforcement allows a redistribution of the tensile stresses resulting in a greater quantity of uniformly distributed micro-cracks of limited width and depth. To obtain durable sprayed concrete and to ensure the material properties satisfy the requirements of the design, the application process should conform following criteria:

  • To provide a high performance sprayed concrete with minimal variance in quality;
  • Thoroughly mixed homogeneous concrete, including fibres;
  • Reduce the risk of human influences affecting negatively the quality of the sprayed concrete; robotic spraying mobiles should be used where possible, allowing a good quality sprayed concrete to be applied by a certified operator in safer and more comfortable conditions with a minimum of rebound;
  • In case of loose ground and running ground water, the system should be adjustable to provide sprayed concrete with immediate setting characteristics.

“The durability of SFRC and in particular corrosion of steel fibres has been the pivot of numerous research projects for the past decades. The existing literature on durability of SFRC is vast and covers a broad field, including different deterioration mechanisms and exposure conditions… It can be concluded, that SFRC presents an overall improved durability to corrosion compared to conventional reinforcement.” Fib bulletin 83

We should keep in mind:

  • When the crack does not exceed 300μm of opening in fibre-reinforced concrete, it presents a very tortuous and, at times, discontinuous path, which makes the circulation of aggressive agents more difficult.
  • When the crack opening does not exceed 300μm, self-healing mechanisms can occur and the corrosion products (in the case of metal fibres) can be deposited in the interior of the cracks. These two physical mechanisms consequently obstruct the cracks and therefore prevent the circulation of aggressive ions.

An example of experience gained from a subsea tunnel:

  • The Norwegian Public Road Association has recently updated (November 2015) Process Code 1 – Standard description for road contracts, Manual R761. In this document it made changes to the requirements for fibre in shotcrete for rock support. The requirement in process 33.4, securing with shotcrete now is that fibre should be acc. EN 14889-1 Fibre for concrete, Part 1 steel fibres. This means that only steel fibres are an option for shotcrete that will be used for rock support. Macro synthetic inclusion is currently an active research and development programme. Durable structures have had an action within the durability of shotcrete, where extensive investigations in several tunnels have recently been completed. It is then made a compilation of these and previous studies. The results indicate that long-term durability of shotcrete with steel fibres can be addressed by stricter requirements for durability class in areas with saline (M40) in combination with increased shotcrete thickness for given rock mass classes. Implicit in this is also stricter requirements for the identification of corrosive environments during the geological mapping.

As of November 2015, macro polypropylene fibre is prohibited in Norwegian road tunnels and steel fibre is currently the only alternative to sprayed concrete. The polymer fibre has environmental considerations (refer to the ban of polymer in FRSC in Novatian Road Tunnels – spray concrete symposium 2018 publication – Synove Myren).

  • For the past few decades, the construction industry has boomed in major parts of the Middle East. One of the rising markets within the construction industry has been bored tunnels with segmental lining for various usage, e.g. metro, sewage and stormwater to mention a few examples. The harsh environmental exposure conditions in the Middle East, which includes a very high content of chlorides and/or sulphates in the soil and groundwater, which challenges the durability of the reinforced concrete structures in order to meet the, often seen, extended requirements to design life, 80 – 120 years (refer to Consultant’s view of durable and sustainable concrete tunnel constructions in the Middle E Cowi C. Edvadrsen). Steel fibre has been considering as the best solution available to deal with most aggressive condition (soil ground water) in the world (for example the Strategic Tunnel Enhancement Programme (STEP)). This featured:
  • 12% chlorides 4-5 times that of seawater
  • 5,000mg/l sulphates
  • Possible attack (MIC) inside the tunnel due to sulfuric acid.

2.3 Biodeterioration of marine fibre-reinforced concrete

Recently, synthetic fibres have been used as an alternative to nominal reinforcing bars in a concrete marine structure at the Fylde coast in northwest England. This structure provides an excellent platform for studying the effects of a marine environment on the long-term mechanical performance of synthetic fibres and the durability of the surrounding concrete.

It’s well-known that green algae and other species can cause significant deterioration of concrete through bio solubilisation. Peter Hugues’s study (November 2012, Concrete International) indicates that biological activity can lead to weakening of the bond between macro synthetic fibres and the concrete matrix.

The mechanism described is detrimental to the long-term performance of the polymer fibres and has a significant effect on the durability of the concrete surface. This research has generated new insights into algal colonization and opens the prospect of more detailed studies on the mechanical bio deterioration of macro synthetic fibre reinforced marine concrete.

3. Design rules for Steel and macro synthetic fibres

Since October 2003, Rilem TC 162-TDF design guidelines1 are available for SFRC. No such guideline is available yet for macro synthetic fibre concrete.

Fibre materials with a Young’s modulus which is significantly affected by time and/or thermo-hydrometrical phenomena are not covered by the Model Code.

Steel fibres are suitable reinforcement material for concrete because they possess a thermal expansion coefficient equal to that of concrete, their Young’s Modulus is at least five times higher than that of concrete and the creep of regular carbon steel fibres only occurs above 370°C.

4. Quality control of Steel Versus Macro Synthetic Fibre Concrete

As part of the quality production control, wash-out tests are quite common in order to check the dosage of fibres in fresh concrete. This is always time consuming, but a lot easier when the fibres can be removed by a magnet, as is the case for steel fibres.

5. Conclusion

Specific technical strengths and weakness of the different fibres, are often less well-known, and lead to confusion.

Fibres for concrete come in all colours, shapes, sizes and materials. Today the majority of the fibres used in concrete can basically be classified into three families for underground applications:

1. Steel fibres: structural application, cracking control, durability, SLS and ULS design

2. Micro Synthetic fibres: fire protection

3. Macro synthetic fibres: non-structural applications when SLS is not important and when fire resistance is not important. Mainly for use for temporary structures and high deformation

There is no good or bad product but the right product for the right use. The main purpose of this presentation was to offer an insight into the technical performance of the different materials.

This paper should help to answer “which fibre to use/specify for which application and why based on a good understanding of the material property

Steel fibre concrete/shotcrete has proven over the years to be a reliable construction material. After 30 years of experience, the first Rilem design guidelines for steel fibre concrete were edited in October 20031 and model code in 2010. Fibre concretes, such as macro synthetic fibre concrete, are more and more understood and could be used in some specific appropriate technical context.

Creep data, shear resistance, crack control, durability, design methods, sustainability are lacking at the moment for macro synthetic fibre concrete, but the body of experience in this area will increase.