Improved performance, better impact resistance and reduction in shrinkage cracking are all advantages claimed for Fibre Reinforced Concrete [FRC]. David Crowther of Maccaferri takes a look at the history of this composite material and its real world benefits to the construction industry.
There’s a saying that small things can make a big difference and this is as true in mixing concrete as it is in baking a cake. That tiny drop of vanilla essence which transforms a bland sponge into a culinary delight has an unlikely counterpart in this ubiquitous construction material.
Not that tiny steel fibres, no bigger than a paper clip, claim to improve the flavour of concrete. But they can and do, improve the strength, durability and long term performance of the material in a wide range of common applications.
Geotechnical specialists Maccaferri, are better known for their retaining wall products but for over 30 years they have, through their Italian parent company, built up a significant depth of knowledge in fibre reinforced concrete [FRC] technology.
David Crowther, Technical Manager for Maccaferri explains the background to concrete fibre reinforcement and shows how small things can make the world of difference to a material our industry almost takes for granted.
A modern take on an ancient recipe
The history of composite materials started over 2000 years ago with mud bricks, reinforced with straw fibres in Ancient Egypt. Fast-forward to more recent times and it goes through the applications of asbestos fibre cement – widely used during the 20th Century – and continued with the highly sophisticated carbon-fibre type materials of the aerospace and specialist automotive industries.
In the 1960s, studies by Italian industrial scientists Romualdi, Mandel and others, established the theoretical and experimental foundations for the development of steel fibre as a medium to reinforced concrete.
In 1973, James Romualdi, with Battelle Development Corporation of Columbus, Ohio (USA), patented the principles of steel fibre-reinforced concrete, creating what was essentially a completely new material for civil engineers.
Battelle DC lodged patents all over the world and also registered WIRAND as a trademark for wire, concrete reinforcement fibres. Through one of its subsidiary Companies, Maccaferri Group became licensee of the patents and acquired the right to use WIRAND trademark.
Research and development within the Italian cement industry and the University of Bologna, lead to improvements in concrete performance, using fibres with an improved shape which gave better mechanical strength and/or workability. Later, in the 1980s, an automated process was developed in which allowed the sprayed –on application of premixed and site-mixed FRC. Alongside this, the use of fibres in the manufacture of pre-cast concrete products progressed, particularly for components such as tunnel lining segments.
What is Fibre Reinforced Concrete [FRC]?
FRC is a compound consisting of a cementitious hydrated paste into which reinforcement fibres – in this case small steel filaments about the size of a paperclip, are mixed.
What do the fibres actually do?
The steel fibres redistribute the forces within the concrete, restraining the mechanism of formation and extension of cracks. The result is a more ductile, concrete which is able to maintain a residual capacity in the post cracking phase.
What are the real-world advantages of FRC?
Reduced shrinkage cracking, increased impact and fire resistance and a reduced need for conventional steel bar reinforcement [Rebar] are all advantages of the use of FRC. On-site storage and cutting/placement of rebar is reduced with consequent cost savings and improvement in site safety.
Increased joint spacing is also possible due to better control of shrinkage during curing and concrete thickness can be reduced because there is no need for the mesh cover layer.
Typical concrete improvements by fibres:
• Initial strength ↑ 50 %
• Compressive strength ↑ 20 %
• Direct tensile strength ↑ 30 %
• Flexural tensile strength ↑ 100 %
• Tensile splitting strength ↑ 65 %
• Impact strength ↑ 2000 %
• Shrinkage behaviour ↑ +++
• Fire resistance ↑ +++
What are the typical applications of FRC?
Fibre reinforced concrete is now widely used throughout the world on major infrastructure works, tunnels, underground railways, large reinforcement structures. In the UK, its use is most widespread in large area industrial floors.
The improved impact resistance characteristics are especially beneficial in applications where high or aggressive traffic loadings are expected. Heavy fork lift trucks can damage the edges and surfaces of traditionally reinforced concrete slabs. Steel fibre reinforcement can reduce the effects of these impacts.
Application field Concrete improvement by fibres
Strength Cracking Wear
Closure systems ü
Tunnels ü ü
Sole plates / cellar walls ü ü
Sewer systems / pipes ü ü
Precast elements ü ü
Toppings / wear layers ü ü
Roads / airfield paving ü ü
Industrial floors ü ü ü
How is Fibre reinforced concrete installed?
On large projects, the fibres are mixed into the concrete during the batching process to ensure an even distribution throughout the mix. Dosage machines are often used to deliver the correct amount of fibres into the batching plant.
Typical dosages of steel fibres for a cast-in-place industrial floor, where reduction of concrete slab depth and “jointless” construction are benefits, would be 30-45kg/m3. The dosage of fibres for precast elements e.g. tunnel segments, is dependent on the performance required, with 25-40kg/m3 being typical dosages.
For smaller projects, fibres are supplied in boxes for hand distribution within the concrete mixing process and are generally added just prior to the pour operation. Mixing thoroughly after the fibres have been added helps to distribute the fibres throughout the concrete mix, limiting clumping of the fibres.
For fibre reinforced sprayed concrete, the fibres are smaller than for precast and industrial flooring applications, because the longer fibres could clog the spray nozzle. Typical applications for fibre reinforced, sprayed concrete are first spray linings for tunnels, mine galleries and slope protection.
Conclusion
Although fibre reinforced concrete has been used widely throughout Western Europe and the rest of the world, its application in the UK is so far, relatively limited with industrial floors being the most common application. The lack of a formally accepted design standard may be an influence on this situation and the Maccaferri is supportive of the call for clear industry guidelines.
Amongst recent independent FRC research is a study undertaken by the Concrete Society. Its Technical Report TR63, “Guidance for the Design of Steel Fibre Reinforced Concrete” published in 2007 addresses many of the issues and points the way forward for future design standardisation.
