Demystifying science | Clearer roads

Every week, our journalist answers scientific questions from readers.

We will probably have to wait a good decade to get there.

Materials that reflect more sunlight – and therefore its heat – are more expensive. And other avenues for reducing heat absorption by roads are still at the stage of laboratory projects.

“In my opinion, it will be quite long,” says Alan Carter, professor of civil engineering at the École de Technologie Supérieure. We’ve been able to make coatings for years [terme technique désignant l’asphalte ou le bitume] clearer. But it costs more. »

Hessam Azarijafari, a civil engineer at the Massachusetts Institute of Technology who has been working on this issue since his doctorate at the University of Sherbrooke, believes it is possible, in some cases, to build roads with materials that absorb less heat: when the necessary stones are available nearby.

One solution currently being considered is to paint the pavement lighter. But the painting must be redone every two or three years, or even more often in Quebec with snow removal, specifies Mr. Carter.

Another avenue: the concrete road, a less dark material than asphalt. “It costs more initially, but it is more sustainable,” underlines Mr. Azarijafari.

Mr. Carter, however, notes two problems: concrete becomes darker over the years, and in cities, streets often have to be opened for the maintenance of sewers, aqueducts and other conduits, which is more expensive with concrete.

“As asphalt becomes lighter over the years, after six or seven years the color is similar to concrete,” he says.

The repair problem can be solved by laying a thin layer of cement on the asphalt, Azarijafari maintains.

Albédo

Alan Carter is working on other strategies to reduce heat absorption by roads. To have less dark roads, we must increase the albedo, that is to say the proportion of the sun’s rays which are reflected, sent back towards the sky. Snow, for example, has a high albedo. And therefore the sun’s heat spreads less deeply on snow-covered areas.

It’s a bit like an insulator, explains Mr. Carter. In his laboratory, he tested construction waste such as brick, ceramic and glass as an insulating base under a thin layer of bitumen. He also tested a one-centimeter layer of recycled glass placed on top of the bitumen, which absorbs heat without transmitting it to the bitumen.

“We did an exterior test with the glass layer, and the temperature was on average six degrees lower,” he says. Traffic did not affect these insulating properties and durability. »

For this approach to be adopted, there would need to be a public will to reduce the absorption of heat by roads, according to Mr. Carter: either as part of the fight against climate change, or as part of the fight against heat islands. heat. The professor is working to establish the cost of the two isolating approaches he has developed.

Plants

As for parking lots, we can reduce their heat storage using a vegetated approach, by planting trees between the rows, or by using “grass grids”, i.e. slabs pierced with holes through which the grass pushes. This latter approach, however, is hardly compatible with current snow removal practices.

“In theory, metal scrapers should not come into contact with the roadway, to avoid damaging it,” says Mr. Carter. There should be small pieces of plastic under the scraper. But often they break and are not replaced. If the scrapes come into contact with the grass grids, they can tear them off. It’s a shame [parce que] this is an approach that works well in Scandinavia, where we clear snow, like [au Québec]. »