Fibre Reinforced Concrete

john-eustace

John Eustace, Product Manager for Concrete at Sika Limited explains….

Recent developments of fibre reinforced concrete (FRC) underline why every year within the concrete industry, more and more concrete containing fibres is being produced as a mode of aesthetic or structural reinforcement.

It’s long been known that concrete is very strong in compression and can very efficiently carry significant compressive loads. However, its tensile capacity is limited and relatively low tensile forces cause concrete to crack so without appropriate reinforcement to limit the crack(s), rapid failure would occur. By reinforcing the concrete a composite element is created, able to resist tensile and flexural forces. Typically this is achieved by the introduction of steel reinforcing bars or mesh, however fibre alternatives have been available for a significant time now and over the last 40 years have become more widely accepted.

Pre-1960, asbestos fibres were the main type used in concrete and in other materials using cementitious products as a binder. In the sixties, experimentation with the use of fibres in concrete stepped up a gear. Different material types were being explored (Steel/Glass/Polypropylene) and research was undertaken to further advance the current materials in use and to understand the possibilities of what other materials can bring continues today.

Polypropylene fibres have been regularly used since the 1980s and their use to modify a number of concrete properties is well documented with the most significant benefits being seen in plastic concrete. These fibres are generally used at a low dose rate per m3 although due to their small nominal size, millions of individual fibres are introduced to the concrete, reducing, for example, plastic shrinkage cracking. However it is important to recognise that these “micro fibres” do not provide any significant post-crack structural performance so to address this, continual developments have made ‘macro’ synthetic fibres (MSF) available to the market.

MSF have an appearance much akin to a typical steel fibre, however they are manufactured from Polypropylene. MSF are able to provide a level of structural performance as they give significantly greater post cracking strength than micro fibres enabling engineers to design out fabric reinforcement. Macro fibres can now provide post-crack performance of high tensile steel fibres or traditional steel reinforcement. For example high performance fibres such as Bonar’s Durus S400 macro fibre, are increasingly been used as a viable alternative to traditional steel mesh or steel fibres. Macro fibres can provide the desired performance and also provide many other benefits from health & safety to labour savings.

The Future

As well as advancements within the current materials, tests are being carried out worldwide with Basalt Fibres, Kevlar Fibres, recycled PET to name a few. The following extract demonstrates others being considered too:

Fibre type Diameter
(µm)
Tensile strength
(MPa)
Elastic modulus
(GPa)
Specific modulus
(GPa×cm3/g)
Elongation
at failure
(%)
E-glass <17 2000-3500 70-76 29 1.8-4.8
Alfa 35 22 25 5.8
Bamboo 25-40 140-800 11-32 25 2.5-3.7
Banana 12-30 500 12 9 1.5-9
Coir 10-460 95-230 2.8-6 4 15-51.4
Cotton 10-45 287-800 5.5-12.6 6 3-10
Flax 12-600 343-2000 27.6-103 45 1.2-3.3
Hemp 25-600 270-900 23.5-90 40 1-3.5

More and more infrastructure, industrial, commercial and residential projects are recognising the potential of macro fibres as an innovative solution. Fibre manufacturers can also offer engineered solutions tailored to specific projects, and with the majority being able to supply product indemnity insurance against their designs, the potential for FRC is being increasingly realised.

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