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The quieter road to recycling tyres

Rubber roads have long been presented as the smart way to dispose of waste tyres and reduce noise pollution. Yet, despite their popularity in the USA, China and more recently Malaysia, the trend has seen limited take-up in the UK.

That could change if the government and the tyre industry offered incentives to encourage manufacturers and suppliers of construction and building materials to reuse end-of-life tyres (ELT) in civil engineering applications, particularly roads.

Pavement engineers already have solutions to decrease the noise produced by road traffic. This comes from using ‘quiet pavements’ as surface courses. These technologies provide an improved level of safety, even in wet conditions and it is possible to manufacture them by reusing ELTs. Incorporating ELTs in asphalt not only makes the surface courses quieter and smoother, it also creates a more flexible pavement with a longer lifespan.

Recycled tyre rubber can be an alternative material on roads, given its resilience and ability to be reused in other products.

Our research shows that ‘rubberised asphalt,’ which consists of regular asphalt incorporating ‘crumb rubber,’ bears significantly improved engineering properties over asphalt mixes made with conventional paving grade bitumen. It also performs as well as asphalt mixes that incorporate synthetic polymers. In addition, when it is properly engineered, rubberised asphalt offers specific benefits that make the roads even better.

These are:

  • Reduced noise - Incorporating rubber crumbs into the formulation creates porous asphalt which doesn’t require the use of fibres. This will lead to an appreciable reduction in traffic noise due also to the fact that rubber crumbs dissipate vibrations
  • Roads remain black - Better long-term colour contrast in pavement markings enhance safety (carbon black in the rubber acts as a pigment that retains the roads’ dark colouring for longer)
  • Using the vast amount of ELTs to build roads, saves energy and natural resources and creates an end-of-life solution.

Despite the success of trials on rubberised asphalt, at present, there is little industrial development and use of recycled tyre rubber (RTR) in the UK. This can be attributed to a lack of information, insufficient training of staff and stakeholders and poor support of local policies.

The tyre industry recently created a forum for stakeholders to share their views and this was a great first step. If it was developed further, it can bring about some positive change. The lack of policies and product specifications have limited the widespread use of rubberised asphalt. However, reusing ELTs to build roads is a successful example of an end-of-life solution for the tyre industry. For this reason, the tyre industry should also act independently and recognise its responsibility in the process.

Our research has shown that the most successful technologies linked with incorporating recycled tyre rubber in asphalt mixes is based on modifying the bituminous binders with RTR.

These better performing binders are produced in a process that requires continuous agitation with special equipment to keep the RTR particles uniformly distributed. Phase separation during storage where the RTR sinks to the bottom is one issue that manufacturers have to contend with. In addition, rubberised asphalt needs to be handled by trained personnel who are familiar with the laying and compaction process. This particular method is not normally practised by contractors in the UK and EU, so the adoption of this technology would need up-front costs from contractors and specialised training for the workers. Moreover, rubberised asphalt mixtures cost more than conventional asphalt because more of this costly bitumen is needed to achieve the benefits.

NTEC’s research suggests several solutions, including modifying the binder by combining it with other polymers, reducing storage temperatures and using waxes to reduce operative temperatures, among other measures.

These solutions work in principle but are usually not economically viable in the short term. However, in the long term, they are a more economically and environmentally sustainable solution. Further collaboration between academia and industry is needed to enhance the transfer of knowledge to devise solutions for widespread application.

Crumb rubber can be successfully incorporated with asphalt to create innovative mix designs, and these are now being used in the EU. These technologies are known as the ‘dry’ or ‘semi-wet’ process and do not require adaptations to asphalt plants. However, they are still not considered a viable option when it comes to building roads. The only successful application of rubberised asphalt in pavements occur when governments supported by research centres, work to exploit these technologies.

California, for instance, has had a government mandate in place since 2005 that supports increasing the adoption of these technologies. From 2013, the use of rubberised asphalt pavements had to rise to at least 35% of the total weight of asphalt paving materials produced in the country. This level recently rose even further to 50% due mainly to predicted substantial savings in the long term.

Offering tax benefits to companies that recycle waste are a viable option. Another approach involves supporting collaborative academia and industry research to solve technical issues in order to make this application more economically and commercially viable. This should be led by policymakers and supported by the tyre industry to create initiatives to open this particular market.

Investing in research and supporting the industry are ways in which we can address environmental issues instead of resorting to burning and exporting our waste tyres. While these practices may be economically convenient, they most certainly contribute to higher global emissions and worse still, delay a viable and tangible solution.

Widespread use of rubberised asphalt technologies is a sustainable practice that should be supported in the UK. There is great evidence that rubberised asphalt works and if it can be introduced on a wider scale, it offers a readily available solution to reuse large amounts of waste tyres.

Dr Davide Lo Presti, principal research fellow, Nottingham Transportation Engineering Centre, University of Nottingham

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