Garry Martin, Director - Major Projects, Low & Bonar Construction Fibres, presents a new examination of the benefits of micro fibres in both the plastic and hardened state of concrete and their contribution to increased sustainability.
Many readers and regular users of micro fibres in concrete will think that after 30 or so years in the market, there is nothing new to know about the product and that surely it is just a matter of throwing a bag into the mixer and that it will satisfy the specifier, and that somehow or the other it benefits the concrete. They might even have the vague idea of it being related to plastic shrinkage cracking in some way, as opposed to putting in a layer of steel mesh in the top of slabs (yes, there are still people doing that!). But they will be asking themselves: “what has all of this got to do with making concrete more sustainable?”
The start point for my overview is that the most sustainable concrete is a concrete that reaches and (hopefully) surpasses its design life in a still useable state and hasn’t needed to be ripped out and replaced by yet more concrete. Micro fibres are the first step in ensuring that this is a possibility.
Very rarely do we actually see micro fibres used in concrete, since they come pre-bagged to a set weight per cubic metre of concrete, and manufacturers recommend that they be thrown into the mixer “bag and all” for optimum results, so it is easy to overlook just how many of them there can be. For example, a kilo of 18 micron, 6mm long fibres will contain approximately 720 million individual filaments, and it is this high number of fibres, which will allow concrete to fulfil its design potential over time.
If micro fibres are correctly manufactured for purpose, their effects on concrete should be barely noticeable at the mixing and placing stage. There should be no clumping, no balling, and definitely no increase in air content, due to either entrapment of air or due to a chemical reaction between the chemicals coating the fibres to aid dispersion and the chemicals used to manufacture the concrete, such as super plasticizers etc. In fact, the fibres should be barely noticeable at this stage. The same goes for when the concrete is placed, compacted, and finished. It is only then that the fibres presence should be felt.
What happens if fibres are not added to concrete
The important thing to remember about concrete at the earliest stage is that it is going through a metamorphosis – it is in transition from liquid to solid with a lot going on, and a lot of things that could go wrong, before the process is complete.
Some of the biggest factors to consider at this very early stage (post-mixing and initial placement) are external to the concrete itself: the weather and gravity, both of which play a large role in how non-fibrous concretes turn out. Starting with gravity – a non-fibrous concrete mix in the still liquid state will be subject to the pull of gravity and heavier elements will start to make their way through the liquid state concrete to the bottom, displacing water and lighter elements such as cement and sand particles before any chemical reaction has time to take place and for crystallization to start locking in all the elements.
A good recipe
Now, as any good cook knows, if you are going to make a success of your recipe, you need all the ingredients to be thoroughly mixed and for them to stay that way. However, if we are allowing our mix to segregate, there will be inherent weak spots in the hardened concrete, especially at the surface, where there will be a lot of cement and sand, which isn’t bonded to any aggregates, and which will be weak and prone to spalling and cracking.
Given that a lot of concrete is poured outside, weather at the time is also going to play its part, as the non-fibre concrete is gently engaging in its battle against time to harden before gravity has pulled all the aggregates to the bottom of the slab! Depending on where you are, the impact of the weather is going to be of greater or lesser consequence; but if we are experiencing a high degree of bleeding of our concrete, any amount of sun or breeze on a cloudy day, or adverse humidity, will further draw out and evaporate water from the slab, thus changing the volume of the concrete, meaning that there is less free water for hydration of the cement, which leads to weakness and stresses that cause concrete cracking.
The scenario is as simple as it gets
Small, almost microscopic plastic shrinkage cracks might not seem more than a small visual imperfection, but let’s not forget, these cracks propagate right down through the slab and if we are using a bottom structural mesh, then we are opening a door for corroding agents directly onto the steel. However, there is another way, and all you have to do is throw a bag of fibres into the mixer.
Once the bag has dispersed its content, and the fibres are fully mixed throughout the concrete, and the concrete has been placed and properly compacted, the fibres simply hang all the components in place and let time do its thing, whilst the cement hydration process takes place, all the while preventing displacement and water migration.
Signs that things are working
How do we know that this is a good thing and actually happens? Well, the first sign that things are working as they should is the absence of plastic shrinkage cracking on the surface. Now, most people would say at this point: yes, we are convinced, that is why we added the fibres in the first place. But wait, there is more going on… plastic shrinkage cracks are just the snotty nose that lets you know you have a cold, they let you know that there hasn’t been any excess loss of water in the concrete and that no excessive early stresses have developed that have started ripping the (as yet) immature concrete apart. However, what is even more impressive is what happens to the concrete in the hardened state!
Micro fibres and plastic shrinkage cracks are discussed endlessly, but the real magic in terms of durability and sustainability are increased impact and abrasion resistance and resistance to freeze-thaw cycles. All this from adding in micro fibres? Well, if the only difference between a plain concrete and the addition of micro fibres to obtain these types of results is all that has taken place, the answer must be yes, it is down to the micro fibres.
Let’s look at the evidence
The simple use of a rebound hammer test carried out in accordance with BS 1881 – part 202 – 1986, will give a plain concrete mix showing a surface harness of 32N/mm2, whilst the same mix with the addition of 910g of micro fibres shows an increase to 36N/mm2, which translates to a significant increase in a concrete’s ability to resist the forces generated by trailer legs being dropped and to stop surface spalling, which would lead to the opening up of ingress points for water molecules, as well as other particulates, which would be detrimental to any steel embedded within the concrete and thus lead to a loss of serviceability.
Similarly, when looking at freeze-thaw resistance of a fibre reinforced and plain concrete sample, as per BS5075, one particular blend of micro fibres was shown to have expanded by only 0.03%, whereas a control mix expanded by 0.11%, thus surpassing the standard requirement for maximum expansion of concrete subject to freeze-thaw of 0.06%. Again, the fibres closed out the pores during the plastic phase, which translated into less ingress points for water in the hardened state. If the water cannot enter the concrete, there is no expansive forces to cause spalling and thus there is greater serviceability of the fibre concrete over time, which does not have to be prematurely replaced.
Hopefully, this look at some of the lesser-known benefits of using micro fibres and how they are the building blocks for increased service life of concrete from the plastic, right through to the hardened state, thus generating a higher level of sustainability, will have given readers a new respect for what is often seen as a commodity product with little value.
Garry Martin has 20+ years’ experience of working in the field of fibre reinforced concrete, covering areas such as: explosive spalling of concrete in tunnels, sprayed concrete applications underground, precast concrete and industrial flooring.