Concrete fills up on fibre

Materials World magazine
,
1 Feb 2010

A novel bi-component fibre using two different polymers may pave the way for a concrete that has the required impact resistance and flexural strength but without the need for steel fibre reinforcement.

While steel fibres increase the tensile loads that concrete can withstand and inhibit crack
propagation, they corrode, are heavy and their high stiffness can be a health hazard if the steel fails and protrudes.

Scientists at EMPA, the Swiss Federal Laboratories for Materials Testing and Research, claim to have overcome the challenges of substituting steel with lighter polymeric reinforcement. The new fibre, produced by co-extrusion and called Concrix, comprises an ‘economic’ polypropylene core with a more costly thin fibre sheath made from a proprietary polymer.

Researcher Dr Josef Kaufmann explains, ‘Fibre materials such as polyvinyl alcohol or Kevlar would overcome [the] drawbacks [of steel], but their high costs prevent wide application in concrete. Polyolefin-based fibres [such as polypropylene], on the other hand, are relatively cheap and possess relatively high strengths [but] one of the biggest problems is the bad bond to the cementitious matrix so that the strength cannot be transmitted to the concrete in a sufficient way. This is partly due to the bad wetting properties and lack of chemical bonding, but also originates from the relatively low elastic properties’.

He says, bi-component fibres present a solution by allowing ‘independent optimisation of
surface and core material. Furthermore, expensive components may be used at a reduced volume, either in the sheath or in the core only’.

Pushing the parameters

 

To optimise Concrix, the selected sheath polymer has to be less viscous than the core polymer to inhibit the latter’s fibrillation and poor bonding with the cement matrix under mechanical loading caused by high degrees of stretching during production.

Furthermore, the researchers say that mechanical embossing, chemical modification and incorporating nanoparticles into the sheath’s surface promote good dispersion in the cement mix and ensure durability and strength.

‘Finally, the problem of the surface-structured fibres’ entanglement when mixing them with concrete was solved,’ explains Kaufmann. ‘Mixing properties were improved by adding
additives to the sheath polymer, acting on the wetting angle. The best way to improve the
dispersion properties was the “power packet” – packing 6,000 individual fibres into bundles wrapped with a water-soluble foil.’ When the foil comes into contact with the wet cement mix, it dissolves, releasing the aligned fibres, whose orientation prevents entanglement as they become dispersed with the cement grains.

Trials in concrete plants with two metres cubed batches have revealed good dispersion without negatively influencing the concrete’s properties. A shotcrete test application in a tunnel
segment has confirmed this.

Fibre diameters of 0.35-0.55mm and fibre lengths of 40-60mm obtain the required workability, at up to two volume per cent for standard concrete with a grain size of 32mm. Reduced grain sizes enable high fibre dosage for ultra-high performance concrete.

Manufacturer Brugg Contec, based in Romanshorn, Switzerland, is gearing up for commercial Concrix production this April. Kaufmann says, ‘Five kilogramme of these fibres can replace 30kg of steel fibres. Besides the other positive aspects (anti-corrosion, lower weight in transport and dosage, less abrasion of machines and mixers, improved pumpability), the material application costs will be at least 10% less than for steel fibres. In shotcrete, the lower rebound (<15%) will lead to further cost reductions’.

Concrete and cement science specialist Dr Charles Fentiman of Fentiman Consulting, based in Horsham, UK, says, ‘It looks like an interesting development. The novelty is that a polypropylene fibre can [now] behave like a steel fibre’.

He adds that a novel application for the material might be in pre-cast elements, such as roof tiles. ‘Concrete roof tiles are made without any fibres but there is interest in producing thin larger formats. To use such fibres for improved ductility and strength could be interesting. Steel fibres would be a no go for this application due to possible damage to the tooling and the safety issues if the tiles get damaged.’