Fibre Myths
MYTH ONE: YOU CAN'T FINISH FIBRE REINFORCED CONCRETE
Fibre reinforced concrete can be finished using all finishing techniques. Even high dosages of FORTA-FERRO® macro fibres can be finished with normal finishing techniques and a ‘bug’ roller can be used, particularly if the concrete needs to have a polished or steel floated finish.Regardless, experienced concrete finishers can ensure that there are very few, if any, exposed fibres by simply waiting 20 - 30 minutes longer before finishing compared to plain concrete. With synthetic fibres, any exposed fibres can be burnt off, or left to abrade or weather with traffic or exposure to ultraviolet light.
MYTH TWO: FIBRES ARE NOT SUITABLE FOR LARGE SLAB AREAS
This still seems to be a prevailing sentiment amongst those in the construction industry: that fibres placed in concrete will compromise the end result and ‘curling’ will eventuate.However, internationally certified fibres are being used every day in global projects with complete confidence in demanding warehouse construction for companies like IKEA, Lexus, Amazon and so forth. Massive warehouses the likes of which are not seen in New Zealand. Warehouses that cannot tolerate any cracking, movement between joints or curling.With the right fibre and the right concentration of that fibre performance is assured for the life of the concrete slab. FORTA-FERRO® has been proven across numerous extremely large scale warehouses and was used in a warehouse project in Chicago that was monitored for years. Test results showed that FORTA-FERRO® can be used to extend joint spacing and reduce or eliminate curl in the slab.
MYTH THREE: FIBRES CANNOT BE PUMPED
In general, a mix that can be pumped satisfactorily without fibres, will pump with fibres. In fact, fibres reduce the line pressure and make it easier to pump, especially over long distances or heights.Ensuring that there is a good enough distance from the ready-mix truck’s chute and the grate on the pump hopper should ensure a steady flow of concrete. Do not be tempted to remove the grate, however tempting it might be.Certainly, synthetic fibre pump better than steel fibres. Whatever the situation, here are some basic hints that will help with the pumping of the fibre infused concrete:
- Add the fibres with any other ingredients or at the end of the batch cycle
- If superplasticisers are used, add and mix the fibres beforehand
- Do not allow the fibre reinforced concrete to build up on the grate. Allow a constant flow so that the pump auger can deal with the volume of concrete
- A strong vibrator on the grate will also ensure a smooth delivery of the mix and reduce build up on the grate.
- With macro fibres, a handy hint is to place round plastic pipes over the top of the regular steel bars in the grate.
MYTH FOUR: FIBRES ARE ALL THE SAME
There are fibres, and there are fibres. Not all fibres are created equal.If the fibre doesn’t clearly state that it complies with the stringent European Standard EN 14889-2 (and carry the CE Marking) which is the internationally recognised standard for polypropylene, macro-synthetic (or EN 14889-1 for steel fibres) – then it should be avoided! You might be tempted to consider a cheaper, non-compliant fibre but don't – the integrity of the concrete design may be compromised, possibly resulting in costly repair work.As to the argument about steel fibres versus synthetic fibres. Sure, steel fibre has its place, but it can be difficult to handle, and it can cause injury during placing. If the fibre stands proud of the concrete surface, discolouration due to rusting will, more than likely, occur. New advances in synthetic fibre systems make them a very attractive alternative to steel fibres, or even steel mesh/rebar.And just how do polypropylene fibres stack up against cellulose fibres? While cellulose fibres do perform reasonably well across some performance areas (e.g. reduction of plastic shrinkage cracking and they offer a minimal increase in compressive strength) however only polypropylene fibres can offer true peace of mind across all performance measures.Also see CERTIFICATION
MYTH FIVE: FIBRES DO NOT PERFORM AS WELL AS MESH OR REBAR
Mesh and rebar have their place.Fibres, however, are much more versatile. In New Zealand compliance with the Building Code for residential builds means that steel reinforcement must be used when required for, say, structural work. However, complementing the steel mesh with fibres will result in a superior reinforcement that steel mesh on its own cannot achieve. For commercial and industrial work an engineer’s specifications allow a design with fibres.Micro fibres are ideal in situations which allow un-reinforced or light-gauge mesh. Fibres provide total peace of mind in respect of total reinforcement and can even replace the light-gauge mesh in some instances.FORTA-FERRO® macro fibres can replace steel mesh and rebar in slab on grade and in precast applications.Fibres are certainly better than steel in these situations primarily because they initially control early-age shrinkage which can develop into long-term drying shrinkage cracks. Steel reinforcing can only hold cracks together after they form – they cannot prevent cracking.FORTA-FERRO® macro fibre blended system not only controls early age cracking development but also holds cracks tightly together should they form. Concrete that is not cracked is stronger than cracked concrete with reinforcement holding it together. Furthermore, cracked concrete with steel reinforcement can rust and compromise the structure or weathertightness of the structure, whereas FRC fibres are rust-proof and completely stable in the alkaline concrete environment.
