Fibre-reinforced asphalt and noise reduction

Noise-reducing asphalt such as ZOAB (porous asphalt) and thin noise-reducing surface layers are now indispensable parts of the Dutch infrastructure. A frequently asked question is whether fibre reinforcement affects the noise performance of these mixes. This article answers that question based on practical research.

How does noise reduction work in asphalt?

Noise-reducing asphalt, such as ZOAB, owes its effect mainly to its open structure: a high percentage of void space in the mix absorbs part of the tyre noise and prevents air-pumping noise between tyre and road surface. Noise performance is therefore closely tied to the pore structure of the mix — a structure that must be maintained, even after years of traffic loading.

Does fibre reinforcement affect the pore structure?

This is exactly why it is a relevant question: if fibres were to clog the void space in the mix, this would come at the expense of both drainage and noise reduction. Practical research shows that this is not the case — fibre-reinforced mixes retain a comparable open structure and air void content to regular ZOAB mixes.

Practical results: CPX measurements

On the A73, the noise performance of four different mixes (reference with PMB, reference with cellulose fibre, PAN fibre and aramid fibre) was measured with the Close Proximity (CPX) method, twelve weeks after construction. The results (CPXP for light vehicles in the right-hand lane):

• Reference with PMB: 92.8 dB(A)

• Reference with cellulose fibre: 92.9 dB(A)

• Aramid fibre: 92.5 dB(A)

The differences between the mixes are negligible — in practice, fibre reinforcement has no negative effect on the noise reduction of the asphalt mix. The right-hand lane (carrying more heavy traffic) did consistently produce around 1.2 dB(A) more noise than the left-hand lane, which is linked to faster wear of the bitumen skin from more intensive use — an effect unrelated to the reinforcement type chosen.

Why is this relevant to the service life of noise performance?

Noise-reducing asphalt typically loses noise performance gradually as the pores clog with dirt, wear and ageing of the mortar. Because fibre reinforcement keeps the mortar in the stone skeleton flexible and intact for longer (as described in our article on ZOAB and ravelling), fibre-reinforced mixes are expected to retain their noise performance better over a longer period than mixes without this protection — although this is an effect that can only be fully measured over the longer term.

What does this mean for projects with noise requirements?

For road authorities dealing with noise production ceilings (for example alongside residential areas), it is relevant to know that choosing fibre reinforcement instead of PMB involves no compromise on noise performance. This makes fibre reinforcement a fully-fledged alternative for projects where both sustainability and noise reduction are important design criteria.

In conclusion

Fibre reinforcement and noise reduction are not mutually exclusive: practical measurements show that fibre-reinforced ZOAB mixes deliver noise performance comparable to mixes with polymer modified bitumen, with the possible additional benefit of retaining that performance for longer thanks to the delayed ageing of the mortar.