What is an aramid fibre? Properties, types and application in asphalt

Aramid fibres are among the strongest synthetic fibres in existence, and have been used for decades in a wide range of high-performance applications, from bulletproof vests to aircraft components. In road construction they are increasingly used to reinforce asphalt, as an alternative to polymer modified bitumen (PMB) or reinforcement mesh. In this article we explain what an aramid fibre actually is, which types exist and how they work in asphalt mixes.

What is an aramid fibre?

Aramid (in full: aromatic polyamide) is a synthetic fibre that stands out for a unique combination of properties:

• Very high tensile strength: considerably stronger than steel per kilogram of weight.

• High heat resistance, allowing the fibre to withstand the high temperatures during mixing and processing of asphalt.

• Chemical inertness: aramid does not react with bitumen or other constituents of the asphalt mix.

• No corrosion, unlike steel fibres or steel reinforcement mesh.

• Low elongation, so the fibre takes up force at small deformations rather than only after stretching.

This combination makes aramid particularly suitable for slowing crack formation in asphalt and holding together the stone skeleton of open mixes (such as porous asphalt).

How does an aramid fibre work in asphalt?

During asphalt production, the aramid fibre — often together with a small proportion of cellulose fibre to prevent binder drainage — is added dry: the fibres are mixed with the aggregate before the bitumen is added. Once the fibres are well distributed through the mix, they act at micro level as three-dimensional reinforcement:

• Crack bridging: where a microcrack forms, the fibre spans the crack surface and so prevents the crack from progressing further.

• Load distribution: loading from passing traffic is distributed more evenly across the mix via the fibres, rather than concentrated on a single weak point.

• Stabilisation of the mortar: in porous asphalt, the fibre holds the mortar bridge between the stones of the stone skeleton more firmly, which counteracts ravelling (stones coming loose).

The result is an asphalt mix that, with a standard penetration bitumen (instead of the more expensive and harder-to-recycle PMB), still achieves comparable or better resistance to cracking and ravelling.

Why aramid fibres instead of PMB or reinforcement mesh?

Traditionally, asphalt is reinforced with polymer modified bitumen or with reinforcement mesh. Both options have a downside: PMB is not always the most sustainable choice and is harder to recycle, while reinforcement mesh requires a separate, time-consuming work step. With aramid fibres, the environment is less burdened, a work step is saved, and the asphalt becomes stronger at the same time.

Where is aramid fibre-reinforced asphalt used?

• National and provincial roads with high traffic intensity (such as the practical trial section on the A73)

• N-roads (regional roads)

• Heavily loaded construction roads and temporary access routes

• Business parks in the distribution and transport sector

• Roundabouts

• Racetracks

In conclusion

Aramid fibres combine very high tensile strength with chemical stability and heat resistance — precisely the properties needed to make asphalt last longer without compromising on workability or sustainability. For anyone looking for an alternative to PMB or reinforcement mesh, aramid fibre reinforcement is by now a proven and well-documented solution.