Asphalt accounts for the surface of more than 90% of American roads. Recently, and worldwide, some of that asphalt contains plastic. If adding waste plastic to asphalt pavement becomes routine, it could sequester vast quantities of it. But are plastic roads eco-friendly? We need to consider the pros and cons of plastic roads carefully.
Typically, asphalt is 90-95% aggregate (gravel, sand, etc.) and 5-10% bitumen (the tarry substance that binds it together.) Melting plastic replaces up to 10% of the bitumen. In other words, plastic in asphalt roads still means that the surface is mostly aggregate and probably only about 1% plastic. Still, if plastic pavements become standard, they could take up as much as 40% of the plastics municipalities have the most trouble with.
Plastic roads in the US
The US has only recently begun to experiment with plastic in pavement. At least half a dozen states have started pilot programs to explore the viability of plastic roads. That is, they mix recycled plastic pellets with asphalt and pave a section of heavily traveled roadway. Then, they monitor it, both to see if it provides an adequate driving surface and to make sure that it does not have a bad environmental impact.
For example, Missouri’s and Hawaii’s tests use pellets from plastic bottles. California’s uses printer ink cartridges. Pennsylvania’s test uses a formula with 2% added plastic bags in two half-mile segments. That little bit of pavement represents at least 150,000 shopping bags.
By comparing the test pavement with traditional asphalt, transportation experts determine whether the plastic roads stand up to freezing temperatures in winter and scorching temperatures in summer. They look for any signs of cracks, potholes, ruts, or other safety concerns. They measure the test material’s cost-effectiveness. And, for environmental impact, they monitor ditches along the road to make sure that microplastics do not leach into waterways.
Plastic roads in Third World countries
India pioneered plastic roads two decades ago. A chemistry professor at the Thiagarajar College of Engineering in Madurai recognized that both bitumen and plastic come from oil. In 2001, he mixed shredded plastic, gravel, and bitumen and saw that it bonded well. His experiments led to exploration of using plastic in road building.
Since then, India has installed more than 60,000 miles of asphalt pavement. And by a law enacted in 2016, that pavement must include waste plastic.
Since 2018, Ghana has embarked on a program to reuse and recycle plastic equal to the waste they produce. Today, Ghanaians toss 5,000 tons of plastic every day, and barely 5% of it gets recycled. The goal is to achieve 100% by 2030.
Plastic recycling doesn’t work in the US. What chance does it have in a Third World country such as Ghana, which lacks our infrastructure?
Paving with plastic, among other creative uses for it, gives it some value. That, in turn, can reduce illegal dumping or burning plastic in open pits. Currently, only 23% of its roads are paved. Paving more roads could go a long way toward achieving Ghana’s goal of a circular economy for plastic waste.
Since only 9% of all waste plastic worldwide currently gets recycled, paving with plastic, in combination with other strategies, could dramatically increase that percentage.
Paving with plastic does not require mixing it with asphalt, however. A company in the Netherlands has built a short length of bicycle path using nothing more than hollow modules of waste plastic. A company in Ghana mixes sand and shreds waste plastic to make pavement blocks.
Possible environmental downsides of plastic roads
So far, testing of plastic roads has shown that they perform at least as well as traditional asphalt roads. They appear to last longer and resist water damage and other problems better. And a longer-lasting road surface means less frequent repaving, making it more cost-effective. Unfortunately, most plastic roads are so new that we have no data on how well they will perform over time.
I will describe only two of the problems critics of plastic roads have pointed out.
What happens to asphalt with plastic when it deteriorates?
The friction of tires and the weight of traffic break down all roads. What is in the resulting dust? Plastics comprise a family of chemicals, related but with individual properties. Each plastic itself is a mixture of chemicals. No one formula accounts for all plastic pavements. Therefore, the environmental behavior of one plastic pavement does not necessarily match another.
So far, no one has conducted studies to determine the long-term impact of any plastic pavement formula as the roads break down. One company published a leaching study based on a 24-hour lab experiment. It observed no leaching in that time. Does that mean plastic will not leach out under traffic conditions over the road’s useful life?
And, of course, any pavement formula will behave differently in a hot climate such as the Sahel compared to a frigid climate such as much of Canada. Most plastic pavement has been in place for less than a decade. Even if some third-party research team has begun a longitudinal study, it cannot yield usable results for several more years.
Not only do roads deteriorate over time, but so do tires. Distinguishing which microplastics come from the road and which from tires will present a considerable challenge. Will paving with plastic merely hide plastic wastes until the roads release it into the atmosphere? Humans have a long history of solving one problem at the expense of creating another that only appears later.
The safety of road workers presents another potential problem of plastic in asphalt. Heating plastics first liquefies them and then, at about 270º C, turns them into a gas. That gas is hazardous to anyone working near it and adds carbon dioxide to the atmosphere. However, some methods of making plastic asphalt only require heating it to 170º C. Without producing its own gases, every ton of plastic that replaces bitumen reduces carbon dioxide emissions as much as a ton.
Possible problems with makng and using plastic asphalt do not end with temperature, however. Many plastic products do not use a single plastic but a layered mixture. For example, PET, the kind of plastic used for bottles, is relatively safe, but the bottles may have a PVC label. And PVC when heated releases chlorine. Other chemicals in some plastics release carbon monoxide and/or various carcinogens.
The real problem with plastics
As with all methods of plastic recycling, plastic in asphalt fails to address the real issue. We make and use too much plastic and create too much waste.
The more successful a plastics recycling method seems, the harder it will be to persuade industry and the public to find ways to reduce plastic manufacturing. The goal of zero-waste requires reduction in demand for any single-use product.
Single-use paper products, for example, don’t present the same environmental problems as plastic. They present their own, equally serious problems.
We make and use so much plastic because it’s relatively cheap. We make and use so many single-use products because we’re addicted to convenience. Plastic roads at their best will not help us recover from that addiction.
How paving with plastic could make a dent in the global waste problem / Ann Parson, Yale Environment 360. February 11, 2021
“Plastic roads” are paved with good intention / Jenni Bergal, PEW Stateline. December 12, 2022
Plastic roads: not all they’re paved up to be / Katie Conlon, Portland State University Toulan School of Urban Studies. 2021