The toughness and tension of concrete made with portland cement can be greatly improved by the inclusion of certain fibres in the mix. Here Bryan Perrie, MD of The Concrete Institute, discusses the role of glass and steel fibres in the mix, and the characteristics of such concrete:
Fibre‐reinforced concrete can sustain load at deflections or strains much greater than those at which cracking first appears in the matrix.
- Fibres should be significantly stiffer than the matrix, i.e. have a higher modulus of elasticity than the matrix.
- Fibre content by volume must be adequate.
- There must be a good fibre‐matrix bond.
- Fibre length must be sufficient.
- Fibres must have a high aspect ratio, i.e. they must be long relative to their diameter.
If the fibre content of the concrete is too low, the fibres will not have a significant effect on the strength or modulus of elasticity of the composite, so It is important to evaluate published test data and manufacturer’s claims carefully.
It must also be noted that high volume concentrations of certain fibres may make the plastic concrete unworkable.
Glass fibres
Glass fibres initially were found to be alkali-reactive and products in which they were used deteriorated rapidly. Alkali‐resistant glass containing 16% zirconia was successfully formulated in the 1960s and by 1971 was in commercial production in the UK. Other sources of alkali‐ resistant glass were developed during the 1970s and 1980s with higher zirconia contents. Alkali‐ resistant glass fibre is used in the manufacture of glass‐reinforced cement (GRC) products which have a wide range of applications.
Glass fibre is available in continuous or chopped lengths. Fibre lengths of up to 35mm are used in spray applications and 25mm lengths for premix applications.
Glass fibre has high tensile strength (2 – 4 GPa) and elastic modulus (70 – 80 GPa) but has brittle stress‐strain characteristics (2,5 – 4,8% elongation at break) and low creep at room temperature. Claims have been made that up to 5% glass fibre by volume has been used successfully in sand‐ cement mortar without the cement and water clumping together (“balling”).
Glass‐fibre products exposed to outdoor environment have shown a loss of strength and ductility – it is speculated that alkali attack or fibre embrittlement are possible causes. GRC has been confined to non‐structural uses where it has wide applications. It is suitable for use in direct spray techniques and premix processes and has been used as a replacement for asbestos fibre in flat sheet, pipes and a variety of precast products. GRC products are used extensively in agriculture, for architectural cladding and components, and for small containers.
Steel fibres Steel fibres have been used in concrete since the early 1900s. The early fibres were round and smooth and the wire was cut or chopped to the required lengths. The use of straight, smooth fibres has largely disappeared and modern fibres have either rough surfaces, hooked ends or are crimped or undulated throughout their length.
Modern commercially available steel fibres are manufactured from drawn steel wire, from slit sheet steel or by the melt‐extraction process which produces fibres that have a crescent‐shaped cross section. Typically, steel fibres have equivalent diameters (based on cross sectional area) of from 0,15 mm to 2 mm and lengths from 7 to 75 mm. Aspect ratios generally range from 20 to 100. (Aspect ratio is the ratio between fibre length and its equivalent diameter, which is the diameter of a circle with an area equal to the cross‐sectional area of the fibre).
Carbon steels are most commonly used to produce fibres but fibres made from corrosion‐resistant alloys are available. Stainless steel fibres have been used for high‐temperature applications. Some fibres are collated into bundles using water‐soluble glue for ease of handling and mixing. Steel fibres have high tensile strength (0,5 – 2 GPa) and modulus of elasticity (200 GPa), a ductile/plastic stress‐strain characteristic, and low creep.
Steel fibres have been used in conventional concrete mixes, shotcrete and slurry‐infiltrated fibre concrete. Typically, content of steel fibre ranges from 0.25% to 2% by volume. Fibre contents of over 2% by volume generally result in poor workability and fibre distribution, but can be used successfully where the paste content of the mix is increased and the size of coarse aggregate is not larger than about 10 mm. Steel‐fibre‐reinforced concrete containing up to 1,5% fibre by volume has been pumped successfully using pipelines of 125 to 150 mm diameter. Steel fibre contents up to 2% by volume have been used in shotcrete applications using both the wet and dry processes, and fibre contents of up to 25% by volume have been obtained in slurry‐infiltrated fibre concrete.
Concretes containing steel fibre have substantially improved resistance to impact and greater ductility of failure in compression, flexure and torsion. The elastic modulus in compression and modulus of rigidity in torsion also are no different before cracking when compared with plain concrete tested under similar conditions.
Steel‐fibre‐ reinforced concrete, because of its improved ductility, could find applications where impact resistance is important. Fatigue resistance of such concrete is reported to be increased by up to 70%.
It is thought that the inclusion of steel fibre as supplementary reinforcement in concrete could assist in the reduction of spalling due to thermal shock and thermal gradients. Corrosion of normal steel fibres could be a disadvantage in exposed concrete situations where spalling and surface staining are likely to occur.
(The role of other fibres, such as synthetic and natural fibres, will be discussed in the next issue of Concrete Trends).
