What is known about the benefits and risks of using waste-derived plastics in highway construction?

W ith pressure mounting on the waste industry to increase the percentage of material it repurposes, the prospect of binding waste into highway pavements has moved up the agenda in recent years.

Demonstration projects have been seen across the UK, as some councils start to explore innovations that promise to divert waste from landfill and reduce the amount of virgin material needed to build new roads. The materials most commonly used in such trials are glass, rubber from tyres and plastic.

But is the drive to reuse more waste this way a positive step?

While examples have been plentiful, in most cases they achieve little more than confirming that it is possible to mix granulated waste with asphalt and create a material that can be laid to form a road pavement. Consultant Winter Associates director Mike Winter says that where the additive in question is waste-derived plastic, detail has been particularly lacking.

According to Winter, no attempt has been made to test or record the technical performance, longevity or stability of the material. Its potential impact on the environment either in service and at the end of its service life is also unknown.

Plastic content in asphalt has been a popular choice for reusing waste materials

The granulated material may be sourced from a range of plastic types in a variety of mixes, making it difficult to establish any comparative measures or even the factors that might influence its suitability.

Winter and Coopland Consultancy director Paula Coopland are the authors of the recently published report The use of waste-derived materials in road construction, commissioned by the Scottish Road Research Board (SRRB). They investigated whether the claims being made for these new materials were proven or credible and what issues they might create.

Winter and Coopland’s knowledge of waste regulation legislation enabled them to take a critical look at this rapidly developing sector. Winter is a specialist in geomaterials and in the reuse of materials in the highway sector, while Coopland was previously on the secretariat of Mineral Products Association Scotland’s asphalt, concrete and technical division.

The report suggests a framework for an assessment protocol which could be used by highway authorities to assess the benefits and drawbacks of a new material before they agree to use it.

S ome producers were claiming that their new materials were superior to those already on the market, even though they had not been fully tested

After assessing suitability, end users need to be sure that redirecting waste in this way is not just a way of pushing the disposal problem further down the line. The potential for nanoplastics to be released into the environment due to pavement wear or when the surface has to be replaced at the end of its life is a real concern.

Consultancy Solace Enterprises associate and former lead of the Scottish Roads Collaboration Programme Angus Bodie says that there was a real the need for the research. According to Brodie, these new waste derived materials were being promoted more and more by the waste recycling sector, sometimes with bold claims about their environmental or technical benefits.

“Some producers were claiming that their new materials were superior to those already on the market, even though they had not been fully tested. And there was no transparency around their qualities and characteristics,” Bodie says.

Approaches were being made directly to local authority officers who, while keen to open up to innovation, wanted to be sure they were doing it properly. The SRRB-funded research project was developed to support them in this.

“Quite frankly, the industry was starting to get frustrated with the ‘trashphalt’ label and the fact that these products were being passed off as ‘green’ roads,” Coopland reveals. “Asphalt suppliers were being asked for materials with waste incorporated into it, but they were very wary. There were no mix specifications for these type of materials.

“We didn’t know if they were going to be durable, sustainable or have end-of-life issues. They weren’t even necessarily cheaper. Longevity is very important in this type of use; you can’t have roadworks every couple of years because the pavement has failed,” she adds. “These materials might be fantastic, but at the moment we really don’t know.”

Winter adds that while there were dozens of demonstration projects across the country, no real performance data or evidence of reporting was available.

“The demonstration projects are all different, which is the problem really,” he explains. “They use different materials, in different thicknesses, with different ways of laying them and so on. They are not trials, they are used as an opportunity for politicians to say that they are doing something and tend to be driven by the material providers or the local authority waste division. There is nothing wrong with demonstration projects, as long as you are doing trials as well.”

The potential issues with such materials is highlighted by their use in housing developments. These may be built by private developers but the local authority tends to inherit responsibility for them.

T hese materials might be fantastic, but at the moment we really don’t know

Bodie explains: “There are roads with these materials in Scotland that are being offered up to local authorities for adoption and at the moment officers are saying no to that adoption.”

That could change in the longer term and councils could be left with a serious legacy, he says.

Winter and Coopland’s report offers a series of recommendations. Among these is that no viable technical, environmental or economic case has yet been made for the use of waste-derived plastic in asphalt and that it should not be used at all in unbound pavement layers. It proposes the introduction of a protocol that can be used to steer the appropriate use of these new pavement materials.

The report sets out what is currently known about them based on actual evidence and was well received, Bodie confirms.

He is now seeking a sponsor and funding to take forward the recommendations, particularly the development of the protocol and agreement of an evaluation and testing regime for new materials.

Bodie notes that there is a strong interest in the issue across the UK, with one of Adept’s recent Live Lab trials involving a series of test installations in Cumbria.

Although the protocol would be aimed at helping highways authorities to assess new materials, Winter points out that it would also serve materials suppliers. “Any bona fide company developing a new material should welcome this as it is essentially a route to market.

“If their material passes, they can go ahead – if it fails, it takes away the risk of them being pursued in the future for failures,” he says. 

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Tagged with: asphalt Recycling Roads

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