Macro synthetic fibres became commercially available in the late 1990’s. They are used to control cracking in concrete due to drying shrinkage, thermal movements or both, and to provide post-cracking energy absorption capacity or toughness.
Most macro synthetic fibres have dimensions broadly similar to steel fibres, and are from materials with a specific gravity in the order of 0.9. They maintain their mechanical properties in alkaline as well as in acidic environments. Typical ‘equivalent’ diameters of macro synthetic fibres range from 0.5 to 1 mm, with tensile strengths between 350 to 700 MPa. The modulus of elasticity of these fibres is typically around 3,000 to 10,000 M Pa.
Macro synthetic fibres are made from a wide variety of organic polymers. They are generally 40-60mm in length with aspect ratios ranging from 70-90. The shape of some fibres is cylindrical, and are often ‘crimped’ or ‘ribbed’, while others are thin and flat.
Macro synthetic fibres rely on sufficient bond to the cement paste (which can be improved with the addition of fly ash or silica fume). ‘Flat’ shaped fibres are designed in part to increase area by providing a larger surface area to volume ratio.
For optimum fibre efficiency, the elastic modulus of a fibre should closely match the elastic modulus of the hardened cement paste in which the fibre is embedded. This allows the fibres to transfer stresses across a crack after cracking has begun.
As the elastic modulus of macro synthetic fibres is much less than hardened concrete – a typical elastic modulus value of concrete used for slab on grade is around 23,000 MPa – they are generally designed to fail when the fibres break (whereas the failure mode of SFRC is when the fibres pull out of the cemented matrix). This should be taken into consideration in the intended application: macro synthetic fibres are generally used in shotcrete for ground support applications and in concrete for slabs on grade where wider cracks (i.e. cracks wider than about 0.4 or 0.5 mm) can be accommodated and/or closer joint spacing is provided. It should however be noted that NZS 3101 recommends maximum surface widths of cracks at the serviceability limit state above 0.4
mm only for benign exposure classifications when the load category is IV (i.e. permanent loads plus infrequent combinations of transient loads).
The key to efficient fibre performance is the bond between the fibre and the hardened cement paste. Some fibres have surface irregularities to strengthen the bond with the cement paste, whereas others rely on the physical bond between the fibre surface and the hardened cement paste.
Macro synthetic fibres are particularly beneficial when larger crack widths, say >0.5 mm, can be accommodated in the concrete as they need to elongate or ‘stretch’ before they are able to transfer significant amounts of stress across the cracks.