1.8 Appendix
1.8.5 Appendix V. Polarization of the labour market (The
..1 Glass fibres
Glass fibres cannot be used as a permanent material because, after some time, they will become brittle and be destroyed by the basic part of the concrete matrix. Therefore, they have to be avoided in all types of concrete, sprayed concrete and cement based mortars.
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.. Polymer fibres
Normal short polymer fibres are resistant and durable in the concrete environment. Yet, their mechanical properties are similar to those of concrete and therefore cannot improve them or make concrete more viscous. This makes this type of polymer fibre unsuitable for the use in rock support. However, for applications where only reinforcement against shrinkage, and in particular plastic shrinkage, is asked for, as in sprayed concrete repair, polymer fibres are well suited: They are very efficient at distributing microcracks during the plastic phase of hardening and they also help reduce rebound in wet-mix spraying. In addition, polymer fibres have a positive influence in cases of fire as they reduce spalling of concrete.
Recently, a new type of polymer fibre has been developed. These structural polymer fibres (SP fibres) are more similar to steel fibres in terms of shape and length. They are produced from high quality materi-als and typically delivered in lengths of 30 to 50 mm. Numerous test results and applications in Australia, Canada, South Africa, Norway and other European countries have shown that this type of fibre can reach adequate toughness if dosed in the range of 5–10 kg / m3. Tests with this dosage range show that SP fibres can reach approximately 900–1 200 Joules according to the EFNARC plate test. This result is more or less equal to the result achieved with 30–40 kg/m3 of high quality steel fibres.
The new SP fibres are already widely used in the mining industry around the world and are now increasingly applied for sprayed concrete linings in civil tunnelling.
The advantages of the new SP fibres over steel fibres in sprayed con-crete are:
< Higher level of load bearing capacity (SP fibres absorb more
defor-mation)
< Reduced fibre rebound rates
< Easier logistics
< Lower wear and tear on spraying parts (wear plate, pump cylinders,
inside of pump, hoses and nozzle)
< No corrosion
< More economical
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UGC International promotes its SP fibres under the name of MEYCO® FIB SP530 / SP540 / SP550 (lengths 30 mm, 40 mm, and 50 mm, respectively; nominal cross section area 0.75 mm2; melting point 170 °C; tensile strength 250 MPa, elongation yield 24.4 %). The author believes that these new SP fibres will be the future of rein- forced sprayed concrete.
A combination of normal / short (fibrin) polymer fibres with the new SP fibres is probably the ideal reinforcement for sprayed concrete in terms of mechanical performances (reduction of plastic shrinkage, improved bond, reduced spalling in case of fire, improved ductility / load bear-ing capacity) and economical advantages (less cost per m3 applied, improved durability).
Table 9: Results from a comparison test made with Novotex 0730 steel fibres (dosage 65 kg/m3) and MEYCO® FIB SP550 fibres (dosage 9 kg / m3). Identical mix design, w / c ratio, spraying equipment, nozzle-man and accelerator type used for all three mixes (Mix 1 using MEYCO® SA160, Mixes 2 and 3 using MEYCO® SA170). All panels stored under identical curing conditions.
Novotex 0730 steel fibres (65 kg / m3), MEYCO® SA170
The most common argument against the use of SP fibres in civil tunnel linings is the concern about the magnitude of creep related deflections
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likely to occur in the long term under gravity loading. The creep behav-iour of SP fibres and of steel fibres (also steel fibres are subject to creep-ing!) has been studied by Dr Stefan Bernhard in a long-term investigation programme. The test is described in his paper (see References), where he concludes that «SP fibres are designed and have been shown to provide similar resistance to load deformation as steel fibres. Creep only becomes an issue if and when the sprayed concrete cracks. If a crack develops in steel fibre reinforced concrete, it creates the opportunity for corrosion and loss of performance in ductility. If the load continues, the load energy will continue to be absorbed, but there is a risk of sudden rupture. If a crack develops in SP fibre reinforced concrete there is no corrosion risk. If the load continues, the fibres will stretch rather than suddenly fail, and in doing so will continue to absorb the load energy.
The cracked sprayed concrete can be observed and action taken to remedy the cause of the problem before this capability is exhausted».
Table 10: Results from a comparison test made with Enduro SP fibres in three different dosages: 5, 7, and 9 kg / m3. Identical mix design, accelerator (MEYCO® SA162) and spraying equipment (manipula-tor) used for all three mixes. Toughness test performed according to EFNARC plate test.
Fibre
dosage Energy absorption Fibre
content Compressible strength
* = Norwegian Concrete Association, Sprayed Concrete Publication, no. 7
.. Carbon fibres
From a technical point of view the mechanical properties of carbon fibres should be ideal for rock support, but in practice they are not used because of their high price.
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.. Steel fibres
Steel fibres are the most commonly used fibres in sprayed concrete.
There are several types and qualities available on the market, but only a few types meet the requirements set for fibre reinforced sprayed concrete.
Critical and important parameters of the steel fibres are:
< Geometry
< Length
< Length/thickness ratio (L / D)
< Steel quality
In practice, we are looking for a thin and long fibre with high steel quality (same or higher than ordinary reinforcement). Most of the steel fibres available on the market have an insufficient steel quality.
Typical fibres that can meet the requirements for steel fibre reinforced sprayed concrete are Dramix 30 / 50 and 40 / 50 as well as Novotex 0730 (0.7 x 30 mm).