Glassphalt trial: Surface layer consists of 35 percent window glass
Glass is usually something one wants to avoid on the road. But in Aarhus Municipality, the Technical and Environmental Administration has just laid new asphalt on a stretch of road in Højbjerg, which consists of 35 percent crushed window glass.
This is a new type of “sustainable asphalt” that the company NCC has developed for the pilot project. The glass replaces part of the imported granite chippings, which are normally used in the surface layers of asphalted road sections. And it is somewhat of a breakthrough, explains Bjarne Bo Lund-Jensen, product manager at NCC:
“I don’t know of any other examples of glass being used in such quantities for surface layers in Denmark before. We carried out some experiments in our own parking lot years ago, where we used up to 80 percent glass—and it’s still there. But, of course, that’s a different matter.”
He says that NCC was contacted by Aarhus Municipality, which wanted to do an experiment with sustainable asphalt. Since the surface layer must be able to withstand a lot of traffic, the materials used must not be too soft or the road could absorb water and crack in freezing weather. And since glass has the right properties, and NCC had previously toyed with the idea, they chose to use waste glass for the pilot project.
“We could have used far more glass if the municipality had not wanted the largest piece to be 8 millimetres. We had difficulty finding pieces of glass larger than 6 millimetres. Therefore, the asphalt contains 16 percent new materials in the form of granite chippings. The remaining 84 percent consists of recycled materials,” Bjarne Bo Lund-Jensen says.
Thus, the asphalt consists of approximately 30 percent crushed asphalt, 35 percent crushed glass, and the remaining fraction consists of mixed crushed construction waste. In comparison, conventional asphalt usually contains around 70 percent new materials.
Good waste glass is in short supply in Denmark
As WasteTech has previously revealed, Denmark does not dispose of waste glass suitable for recycling. In 2020, Ardagh Glass Holmegaard, the country’s only remaining packaging glassworks, had to import 13,000 tonnes of crushed glass from abroad for recycling in manufacturing. This is because remelting glass requires significantly less energy than producing it from new raw materials, which is why waste glass is in high demand.
Therefore, it may be surprising that NCC and Aarhus Municipality choose to use waste glass in asphalt. Anders Damgaard, senior researcher at DTU Environment, also finds this surprising. He thinks that it sounds impressive that NCC has developed a product with such a large degree of recycled materials. But it also gives rise to questions:
“Would it be possible to find something more suitable to use for the purpose than waste glass? After all, we have to balance the environmental savings against the risk of having to find new raw materials to produce glass from if we start displacing too much waste glass from the recycling market.”
Bjarne Bo Lund-Jensen from NCC says that he has already been “challenged” as to whether the glass used for the asphalt could have been recycled into new glass. However, he denies that this would be possible.
“The waste management company we bought the glass from, they say that it was not suitable for glass recycling. It contains the remains of crushed sanitation,” he says and continues:
“Remember that we import and ship granite chippings for asphalt from Norway and Sweden. It’s therefore a reasonably large parameter in the calculation of the climate footprint of our regular asphalt. It’s a significant factor that we avoid by using waste glass.”
In an earlier interview with the local newspaper Aarhus Stiftstidende, Bjarne Bo Lund-Jensen has stated that if the pilot project in Højbjerg is satisfactory, he does not rule out that “crushed recycled glass may form a significant component of the asphalt on Danish roads in the future.” However, he reiterates to WasteTech that this only applies to contaminated waste glass that is not suitable for ordinary glass recycling:
“We absolutely must not use waste glass that can be recycled into glass again. We will only use the glass that contains contaminants that make it unsuitable for new glass.”
WasteTech has asked RGS Nordic, the company that has supplied the waste glass for the project, to explain where the waste comes from and why it cannot be recycled. In an email reply, Charlotte Scott Larsen, downstream and logistics manager at RGS Nordic, explains that it is a glass fraction from demolitions that is “contaminated with ceramics, stone, and porcelain as well as other fractions” and which therefore could not be recycled by the glass industry.
“It also makes sense to use certain types of waste glass in asphalt, because it replaces virgin materials in the form of granite chippings from Norway and Sweden, which must be shipped to Denmark. Asphalt with glass can be used again and again in the production of new asphalt on an equal footing with traditional asphalt,” the email continues.
Too early to draw conclusions
Whether the experiment with the use of glass in asphalt in Aarhus can be called a success or not, we will only know in “a few years.” According to Bjarne Bo Lund-Jensen, a follow-up project has been created which will keep an eye on the road and evaluate wear and tear and durability:
“We don’t yet know how good this asphalt is, because it’s an experiment. What one could fear is that glass will not hold as well in our binder as stone does, but only time will tell. It should ideally have roughly the same lifespan as conventional asphalt, else it won’t be sustainable.”
Since the new type of asphalt is laid out on a road in front of a roundabout which contains conventional asphalt, there is a good basis for comparison of the two types of surfaces, as the level of traffic is more or less the same. In the long term, a more accurate life cycle analysis and an environmental data sheet must also be prepared, Bjarne Bo Lund-Jensen says.
Anders Damgaard from DTU Environment calls to remember the waste hierarchy in any further work as well. He believes that, from a holistic point of view, one should always aim for as high-quality recycling as possible before starting to use waste resources as a residual product that can be added to, for example, asphalt.
“It may well be that some waste glass can be used this way, but that must not become an excuse for not sorting glass for high-quality recycling,” he says and continues:
“NCC should be careful about creating a demand for something for which there should not be a market.”
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