FIBRES CAN ADD SIGNIFICANT STRENGTH TO CONCRETE

By Jan de Beer on behalf of the Cement & Concrete South Africa (CCSA).

Including stiff natural or synthetic fibres in the concrete mix can significantly augment the strength of concrete, says Bryan Perrie, CEO of Cement & Concrete South Africa (CCSA).

Perrie says concrete made with Portland cement is relatively strong in compression but weak in tension which can be overcome not only by the usual insertion of conventional rod reinforcement but also by the inclusion of enough stiff fibres in the mix. The fibres alter the behaviour of the fibre-matrix composite after it has cracked, thereby improving its toughness.

Perrie says for the effective use of fibres in hardened concrete:

• The fibres must be significantly stiffer than the matrix
  • The fibre content must be adequate
  • There must be a good fibre-matrix bond; and
  • The fibres must have a high aspect ratio, i.e. their length must be in the correct relation to their diameter

There are natural and synthetic types of fibre with steel and glass versions the most commonly used:

STEEL

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 through 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 with a crescent-shaped cross section.

“Typically, steel fibres have equivalent diameters (based on cross sectional area) of from 0.15mm to 2mm and lengths from 7mm to 75mm,” Perrie states. “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 to facilitate handling and mixing. Steel fibres have high tensile strength (0.5 – 2GPa) and modulus of elasticity (200GPa), 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 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 10mm.”

Steel fibre-reinforced concrete containing up to 1.5% fibre by volume has been pumped successfully using pipelines of 125 to150mm diameter. Steel fibre contents up to 2% by volume have been used in shotcrete applications using both the wet and dry processes.

“Steel fibre contents of up to 25% by volume have been obtained in slurry-infiltrated fibre concrete. Concretes containing steel fibre have been shown to substantially improve resistance to impact and show greater ductility of failure in compression, flexure and torsion. The elastic modulus in compression and modulus of rigidity in torsion are no different before cracking when compared with plain concrete tested under similar conditions. Steel fibre-reinforced concrete could, because of its improved ductility, find applications where impact resistance is important. Fatigue resistance of the concrete is reported to be increased by up to 70%. “The inclusion of steel fibre as supplementary reinforcement in concrete could assist also in the reduction of spalling due to thermal shock and thermal gradients. However, the lack of corrosion resistance of normal steel fibres could be a disadvantage in exposed concrete situations where spalling and surface staining are likely to occur,” Perrie adds.

GLASS

In the form first used, glass fibres were found to be alkali reactive and products in which they were used deteriorated rapidly. However, alkali-resistant glass containing zirconia was successfully formulated in the 1960s and was soon in commercial production. 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 are used in premix applications. GRC products are used extensively in agriculture; for architectural cladding and components; and for small containers,” Perrie explains.