Using waste materials for road paving may not be unheard of, but a recent combination of minds has seen the use of waste plastic trialled for road pavements. By Richard Silcock.
A three-way partnership made up of Downer’s Road Science research team, Enviro NZ and the New Plymouth District Council, has come up with an idea for using waste plastic material as an integral part of asphalt.
Up until recently most plastic waste has been shipped to China for regeneration. However, last year this avenue of disposal was shut down following China’s decision not to accept any further waste plastic from other countries.
With a stockpile of plastic building up around New Zealand landfills one solution to the problem has seen shredded plastic being incorporated into the composition of road paving and laid in the same way as traditional asphalt.
“What we have developed is a combination of traditional asphalt and shredded plastic which we have called Plasmix,” says Road Science’s general manager, Murray Robertson.
While not wanting to divulge too much information around the analytic research that was carried out prior to testing, technical development manager at Road Science, Darcy Rogers says that putting bitumen and plastic together is relatively straightforward as they are both derived from crude oil.
“Normal asphalt is created by mixing aggregate, bitumen and a binder at a high temperature of around 160 degrees centigrade,” he says.
“By adding plastic, which essentially melts and blends with the bitumen, we get a product that is much higher performing and our initial tests to date have found that it lasts longer and there is no leaching even when broken up.
“Ninety-five percent of a road’s composition is aggregate and that is the primary component that is worn down by traffic over time,” he says.
“As the plastic is part of the bitumen mix there is very little opportunity for it to come into direct contact with vehicle tyres, so any release of the plastic particles is very unlikely.
We are essentially repurposing waste plastic without compromising the integrity of road pavement.
“In developing the product we put a lot of effort into identifying the optimal size of the shredded plastic, which is carried out by Enviro NZ.
“Another benefit we found during testing is that the performance of a road surface is enhanced by adding plastic types three to seven(1) particularly in relation to its resilience to water and rutting.”
Downer, as a part of its 10-year road maintenance contract with the New Plymouth District Council, is currently trialling Plasmix on a section of road near New Plymouth’s Pukekura Park and it is the first time here that plastic, as an asphalt component, has been used to surface a public road.
Some 500 kilograms of shredded hard plastic waste(2) was used over a 100-metre stretch of the road and, so far, it is showing excellent results.
It has not yet been tested on a main highway, where usually the aggregate is heavier and the asphalt layer thinner.
New Plymouth District Council infrastructure manager David Langford says in a media report that they are monitoring the trial section of road over a six month period for any abnormalities before they commit to using Plasmix on more of the city’s roads.
However, as Murray Robertson says: “By working together in collaboration to help solve local waste minimisation, we have developed an outcome that may be beneficial to road pavement construction in the future.
“We are essentially repurposing waste plastic without compromising the integrity of road pavement.”
Another potential game changer for structural asphalt is EME2(3). This is very high modulus stiff asphalt which was originally developed in France.
Darcey Rogers says it provides a much longer life than conventional structural asphalt and can be applied more thinly than a conventional mix.
“As a structural layer, it sits in between the base-course and the surface layers and uses a very hard grade binder which we manufacture at our plant in New Plymouth.
“Another advantage of EME2 is that the aggregate is not so important. With conventional asphalt it is important to get good aggregate interlocks to achieve strength. With EME2 it is more to do with the binder strength.
“Where aggregate may vary from quarry to quarry, we can create and supply a binder that provides good binding characteristics, does not vary and provides good pavement performance.
“We are getting a far higher modulus than that achieved with a conventional mix, in some cases up to 14,000–18,000 mega-pascals,” he says.
“In practice this means, due to the stiffness, far higher traffic loadings can be achieved, equating to around 10 times the fatigue rate over a conventional mix before the pavement will fail.
“It also allows the pavement thickness to be reduced by up to 30 percent to achieve the same fatigue life as a conventional mix, providing savings in terms of bitumen and construction time.”
EME2 has been trialled over a number of locations in New Zealand for a number of years on both high volume roads and heavy loading areas such as aircraft hardstands and taxiing areas and it has been found to outlast conventional pavements.
Darcey says EME2 is becoming the optimum solution for ‘special treatment’ pavements.
“We have the manufacturing capabilities to supply the design and materials for EME2 to the contractor and an additional beauty is their paving crews don’t need any special equipment as it is very easy to handle and use,” he says.
“We are committed to providing innovative, practical and cost effective solutions for the road construction sector in this country,” says Murray Robertson.
“In the case of Plasmix we are also helping to reduce solid waste at landfills, and with EME2 we are providing a cost effective solution and reducing the carbon footprint.”
• Note: Road Science’s work in pavements was recently recognised at the 2019 Workplace Health and Safety Awards, winning the ‘WorkSafe Best Initiative to Address a Work-Related Health Risk Award’, for its work in eliminating the risk of hydrogen sulphide being emitted during the manufacture and storage of bituminous products (Refer last month’s Contractor).
(1) Plastic types three to seven are the common hard plastics used for manufacturing food containers, ie, milk and juice bottles, butter containers etc, and polystyrene packaging.
(2) 500 kilograms of plastic waste is equivalent to what 1300 people would usually generate in one week.
(3) EME is an acronym for the French: Enrobe Module Eleve.