Horses for courses - which fibre reinforcement for which application

Recently my LinkedIn feed has been awash with the promotion of Synthetic Fibre for use in concrete floors and pavements, and from time to time I get asked about them by clients - so I thought it was a good time for a quick post about fibre reinforced concrete. In this post I outline some basic ideas about why we use fibre reinforced concrete, the different types of fibres on the market, and the different properties they have. This leads us to some recommendations about what type of fibres to use in concrete floors.

Fibre Reinforced Concrete - not a new idea

Fibre reinforcement has been used in concrete for millennia. Yes millennia. Thousands of years ago the Romans used fibre reinforcement in their concrete - in the form of horse hair.

Ready to reinforce your concrete!

Fibres are used in concrete to create a composite material. Concrete is a very useful material on its own - in fact it's the second most used material on earth after water. But like most things in life concrete has both strengths and weaknesses. By combining concrete with fibre reinforcement we can create a composite that helps to mitigate some of the weakness.

The main reasons we use fibre reinforcement in concrete floors and pavements are:

1. As structural reinforcement - concrete is very strong in compression but relatively weak and brittle in tension. By creating a composite material of concrete and the right choice of fibre we increase ductility and tensile capacity and end up being able to carry more load. This is the same reason we use conventional reinforcing bars in concrete except that the way we determine the constitutive properties of fibre reinforced concrete is a bit more complicated.
2. To resist drying shrinkage and help control cracking - it's not a secret that concrete cracks. A lot of the cracking that you see in concrete - particularly concrete floors - is the result of drying shrinkage. The right choice of fibre reinforcement can help to mitigate drying shrinkage cracks and to control serviceability cracking. I have to stress that just adding fibre to a concrete floor isn't a magic bullet that will stop random cracking. But there is a lot of empirical evidence that high dosages of steel fibre combined with careful detailing can produce crack free floors.
3. To mitigate plastic shrinkage cracking - another type of cracking in concrete that generally occurs when the surface of young concrete dries out to quickly. There are a lot of ways to protect against this - like controlling aspects of the concrete mix design (bleed rate, set times) and environmental conditions (evaporation rate - protecting against wind, sun, using anti-evaporative spray or misting etc). But the right choice of fibre can be super effective at mitigating plastic shrinkage cracking when normal measures aren't going to work.

In this photo - Steel Fibre is the primary reinforcement for this pile supported (i.e. suspended) floor in Malaysia - with a little bit of supplementary traditional steel fabric for the edge spans.

These days we tend to use steel and synthetic fibres

As far as I know not many people are using horse hair fibres to reinforce concrete anymore. If you are - kudos to you and please drop me a note! The majority of fibres used in concrete these days are classified into three groups:

1. Steel Fibres - typically 30 - 70 mm long and 0.5 - 1 mm diameter
2. Macro Synthetic Fibres - typically 30 - 70 mm long and 0.5 - 1 mm diameter
3. Micro Synthetic Fibres (or cellulose) - typically 5 - 20 mm long and 0.03 mm or less diameter

Each of these types of materials have different properties and will have different effects in concrete. My area of expertise is concrete flooring and pavements so I am going to focus on the properties that matter in these applications - this means I'm not going to talk about fire resistance or anti-spalling properties in a fire for example.

How do these different types of fibres stack up?

How do these different types of fibres perform when it we look at our goals of increasing load carrying capacity, helping to control drying shrinkage cracking, or helping with plastic cracking?

When we look at the main goals we have in mind when we use fibre reinforcement in concrete - to provide post-crack strength, and to help control cracking - we can see why Steel Fibre is the work horse option - particularly in floor/pavement applications. Steel Fibre Reinforced Concrete has a robust and rigorous history in the research literature and in the real world. It can be used as structural reinforcement, and has positive impacts on drying shrinkage/crack control.

We can also see that if we really need some help controlling plastic shrinkage cracking then Micro-synthetic fibres are a great friend to call. This application is well supported by academic research and by practitioners - in fact any time I have a slab to build on a windy wharf I tend to add micro synthetic fibres to give me some comfort that I won't get plastic shrinkage cracks. Note that micro-synthetic fibres don't contribute to post-crack strength in concrete because the fibres are too small.

But what about Macro-Synthetic fibres? We know that they have been used in applications like shotcrete in mines and in ground supported slabs. There is no doubt that they can help increase the post-crack capacity of concrete. But why the question mark when it comes to "structural reinforcement", and why do they not contribute to crack control in the same way that Steel Fibre does?

We need to talk about Young's Modulus

I want to focus your attention on a key material property - Young's Modulus.

Young's Modulus is a measure of the stiffness of a material and this is super important in engineering applications. Another way of explaining Young's modulus is as the relationship between stress (loading) and strain (deformation). If we look at the Young's Modulus of different types of fibre and of concrete we get a very important insight - Steel Fibres are significantly stiffer than concrete (higher Young's Modulus), whereas Synthetic Fibres are less stiff than concrete (lower Young's Modulus)

What this means in practice is that for synthetic fibre to take up the same load as steel fibre:

1. You could get a much bigger deformation.
2. You could therefore get a much bigger crack opening in your concrete element (such as your floor)

As a general rule when it comes to designing concrete floors - we don't like cracks, and we particularly don't like wide cracks. We also have to consider that if we build saw cut slabs we don't want our saw cuts to open to be too wide - because then we will lose aggregate interlock and could have a major headache with our saw cuts pumping and failing. For this reason some highly regarded sources (UK Concrete Society TR34) don't recommend constructing fibre reinforced floors with saw cuts at all.

I think in the context of this discussion I would suggest being very very careful if you are constructing saw cut floors with macro synthetic fibres and if you are going to give it a try keep the joint spacings conservative (very close together) as a starting point.

But what about Creep

Creep is the tendency of a material to deform under long term load - even if the load is well below a level that would cause the material to yield. Creep is another reason that we have a question mark when it comes to using Macro Synthetic fibre. Generally speaking these fibres are "visco-elastic" during normal operating temperature ranges (above -20 degrees celcius and up to a melting point of about 160 degrees celcius). This means that Macro Synthetic fibres will exhibit time and temperature dependent stress/strain behaviour and it raises a pretty big issue for a designer wanting to use these fibres in a load bearing role.

Simply put - if you have a Synthetic Fibre under load for a prolonged period of time (maybe in a warehouse floor?) there is a possibility that the fibres will get weaker and weaker over time....

It's not a surprise that the design approach for fibre reinforced concrete is based on steel fibre

The existing framework for designing structures with fibre reinforced concrete is based on Steel Fibre - which is an elastic material, not a visco-elastic material like Synthetic fibre. The design properties we use for steel fibre reinforced concrete are typically derived from flexural beam tests that take place over a short period of time (15 minutes) and without significant temperature variation.

If we use the same design framework but apply it to using Macro Synthetic fibre - which has much different material properties -we could be walking into a big problem with creep. This has been verified by some of the academic research looking at longer term performance of Macro Synthetic Fibre reinforced concrete - which suggests that over time the load carrying capacity can reduce and even cause collapse. For the boffins out there who are interested in the research on this topic I suggest starting with a paper by Kusterle ? "CCC 2009 ? Viscous Material Behaviour of Solids ? Creep of Polymer Fibre Reinforced Concrete’.

Imagine you design a structure based on a given value for post-crack flexural strength... but you have no certainty about those values over time, and over a broad range of temperatures. To me this seems risky - and I'm not alone in making that assessment. Synthetic fibres are excluded from all of the international design standards for Fibre Reinforced Concrete that I am aware of; NZS3101, AS 5100, DIN1045 and FIB Model Code, to name a few. As an example, the FIB model Code states in the scope:

fibre materials with a Youngs modulus which is significantly affected by time and or thermo-hygrometrical phenomena are not covered by this Model Code.

Stop horsing around with Macro Synthetic fibres

When we put this all together I think we should be cautious and very selective with using Macro Synthetic fibres. I bet there are plenty of anecdotal success stories out there for slab on grade applications - but a slab on grade is hyper static and will tend to have a lot more capacity than most of the design models predict if they have good ground support.

My take is that you should limit the application of Macro Synthetic fibres to nominally loaded ground slabs like footpaths and cycle ways or lightly loaded driveways - that way you mitigate a lot of the downside risks. But given that these are applications that work perfectly well as unreinforced/plain concrete designs - it does raise a question as to whether you get "bang for your buck" and should bother with Macro Synthetic fibres at all. Remember that with the low Young's Modulus of synthetic fibre (relative to concrete) you should not be relying on them for crack control, and with the potential risk of creep you should not be relying on them to sustain loading over time.

Do make the most of steel fibre and micro-synthetic fibre

In contrast to my take on Macro Synthetic fibres I woud encourage designers and contractors to explore the use of steel fibre and micro-synthetic fibres in floors. Steel fibre reinforced concrete has an excellent track record as the primary reinforcement for industrial concrete floors (ground supported or pile supported), and there is a robust framework for design that traces back over 30 years.

Micro-Synthetic (or Cellulose fibres performing the same role) are also a great tool to have in the tool box. If you are dealing with an environment where plastic shrinkage cracking is a serious concern they are great value for money. I've also had feedback that they can have a positive impact on the appearance of decorative concrete.