Specific concrete characteristics are obtained by using different fibre types, or mixtures of different fibres, depending on what the performance requirements are. Longer high E-modulus fibres are used to achieve high energy absorption, whilst longer low E-modulus fibres are used for increased ductility and crack reduction.
Specific concrete characteristics are obtained by using different fibre types, or mixtures of different fibres, depending on what the performance requirements are. Longer high E-modulus fibres are used to achieve high energy absorption, whilst longer low E-modulus fibres are used for increased ductility and crack reduction.
With smaller fibres, those with a low E-modulus also improve the reduction of cracking, whilst those with a low melting point provide increased fire resistance. As such, different combinations and quantities of these fibres can be used across a wide range of applications and project requirements. Here are some of the most commonly sought-after requirements in more detail:
Structural Behaviour
Concrete is generally strong in compression but weak in tension. If concrete fractures because of high bending stress and no extra reinforcement has been used, the system collapses without warning. As with conventional steel reinforcement, high forces can be transferred and distributed within the concrete using suitable fibres.
Crack-bridging fibres not only improve post-cracking behaviour but also help to keep bigger cracks from spreading. The fibres that cross the crack and are anchored in the matrix on both sides, effectively “sew” its two sides together and prevent it from widening. As a result, fibre-reinforced concrete has increased ductility and is capable of absorbing higher energy in the area, under load versus deflection.
Crack Distribution
When cement-based binders start to harden, shrinking stresses can often lead to the concrete cracking, which can be seen visually as damage. By adding fibres to the mix, these stresses are split and distributed. Bigger cracks are not able to form because the shrinkage volume is compensated for by the formation of smaller cracks instead, which will not significantly reduce the concrete’s strength.
The surface aspect is improved and the concrete is able to essentially heal itself. As such, the concrete is a lot more durable with the addition of fibres.
Fire Protection
Traditional concrete can be problematic when exposed to fire, because the sharp rise in temperature makes the concrete’s physically and chemically-bound water evaporate very quickly. When the water turns to vapour, its volume increases a thousand-fold. The denser the matrix and higher the moisture content of the concrete, the higher the developing vapour pressure will become. If this pressure is not relieved quickly enough, explosive concrete spalling can happen after only a few minutes. Any extra reinforcement is then exposed to damage from the fire, leading to extensive and deep-reaching damage to the overall structure.
Mechanical Resistance
The impact and shock resistance, notched bar impact strength and edge strength can all be increased significantly with synthetic fibres and most steel fibres.
By adding polypropylene fibres to the concrete, the risk of explosive spalling in a fire is reduced considerably, if not completely. These fibres have a relatively low melting point of 160°C, which means they will progressively melt and create a capillary system through which the evaporating water can escape, without any significant destructive pressure build-up.
Impact strength can be improved by adding steel or polypropylene fibres in quantities of only 0.1% by volume. This strength improves considerably when a higher quantity of fibres is used, and adding a combination of fibres with a high and low E-modulus and high elongation at break has proved beneficial too.
