Quebec has rolled out 325,000 km of highways, boulevards, avenues and streets of all kinds on its territory, one-third of which are in urban areas. These black roads, in summer, cook under the rays of the sun, so much so that their temperature regularly reaches 70 degrees Celsius. Would white asphalt counteract heat islands and help cool cities, where thermometers often show several degrees higher than in the countryside?
A planetary reflection has arisen around this question. “Roads and parking lots are largely responsible for the heat island phenomenon,” explains Alan Carter, head of the roadways and bituminous materials laboratory at the École de technologie supérieure (ETS). Currently, few places in the world can afford to ignore this phenomenon. »
Qatar, for example, where the mercury can soar to 45 degrees Celsius, started as early as 2017 to paint some of its roads to counter heat absorption. Red, green, blue, purple: in this country where homosexuality remains illegal and punishable by 7 years in prison, the paths paradoxically become rainbows.
The logic behind the Qatari approach is based on science. The blackness of bituminous mixes increases their albedo, which is the measurement that determines the reflective or absorptive capacity of a surface. A very low albedo indicates a material that stores heat. Conversely, freshly fallen and still pristine snow has a very high albedo, which causes sunlight to “bounce” off its white carpet.
Because it would increase the albedo of roads, white pavement would definitely reduce heat, says Carter, using the same principle as white roofs. What is possible above our heads, however, is not necessarily possible under our tires and our feet. The advent of white asphalt is therefore not for tomorrow, firstly for technical reasons, but also for safety.
Technical constraints
Difficult, indeed, to counterbalance the black color induced by the “glue” which holds our roads together. “One of the components of asphalt mixes is bitumen, a product derived from petroleum. Even though it makes up only 5% of our mixes, it makes them very black, and there’s not much we can do about it,” says Carter. Making white asphalt would require a colossal amount of dye, emphasizes the researcher, and it would therefore be expensive.
It is possible to “dye” pavements by adding materials that act as colorants. “The easiest thing is probably to add a red tint,” says the ETS expert. If you add iron oxide — commonly called rust — it’s relatively easy to go from black to rather reddish. »
Other materials allow the roads to have a higher albedo, in particular those made of concrete, which have a paler color than that of traditional asphalt. These roads absorb less heat at the start of their life. On the other hand, these surfaces become dirty under the passage of cars and pass, in a few years, from gray to black.
“What happens,” says Professor Carter, “is that this pavement starts off with a high albedo and a lighter color, but we get dirty over time, so the concrete pavement gets darker and darker, and its albedo decreases a lot. Nobody wants, I think, to go and rub our asphalt to keep them pale. »
Security issues
It would theoretically be possible to paint the surface of our roads white to increase their albedo. This solution, however, is undesirable for several reasons. First of all, the action of the snowplows risks making life difficult for this painting. Then, a white coating would make the roadway blinding, since it would reflect the light as much as a first snowfall. Alan Carter decides: “We don’t want white, because we wouldn’t be able to drive. »
However, science is opening up other avenues to counter heat islands caused by asphalt. The laboratory headed by Mr. Carter, for example, is currently working to include recycled materials in the composition of asphalt, such as glass or crushed brick. The results are conclusive: these mixes are not only more economical, but also more effective.
The easiest way to reverse heat islands is to develop the canopy to shade the streets.
“That’s enough to tip the scales, that’s for sure,” says Professor Carter. We are able to go for five, six, even seven degrees easily, which makes a difference in the end. » A decrease in temperature which, for example, could reduce the creation of ruts, the furrows dug by the passage of heavy goods vehicles when the road surface, overheated by the summer sun, becomes more malleable.
Another solution exists: trees. “The easiest way to reverse heat islands is to develop the canopy to shade the streets,” concludes Alan Carter. If we planted greenery in the parking lots, that would solve a large part of the problem. »