‘Cool pavements’ could cool cities and mitigate climate change

Photo: Pixabay/RyanMcGuire

Cities around the world continue to warm and it isn’t just because of a changing climate. Massive volumes of concrete built into large blocks of buildings trap the heat generated by the sun, to which heavy traffic on roads adds yet more heat.

This phenomenon is known as the urban heat island effect and is of a growing concern as air temperatures carry on rising, which can make life a lot less pleasant in cities during warmer months.

One solution aimed at mitigating the island heat effect involves replacing traditional pavements with “cool pavements.” The benefits of doing so could be considerable, a team of experts at the Massachusetts Institute of Technology’s Concrete Sustainability Hub (CSHub) has found.

Cool pavements reflect solar radiation, thereby cooling their surroundings by anywhere from 1.7 degrees Celsius to 2.1 degrees C, based on measurements conducted in Boston and Phoenix where experiments with cool pavements have already been done, according to the researchers at MIT.

The technology takes advantage of a phenomenon known as albedo whereby darker surfaces get hotter in the sun than lighter ones. Commonly used paving materials such as asphalt have a low albedo, which means they absorb more radiation and emit more heat. Cool pavements, on the other hand, are constructed with brighter materials that reflect more than three times as much radiation and emit far less heat in the process.

Through their analysis in the two US cities, the scientists at MIT found that a marked advantage resulted when concrete, reflective asphalt, and reflective concrete were used instead of conventional asphalt in pavements.

“We can build cool pavements in many different ways,” explains Randolph Kirchain, a researcher in the Materials Science Laboratory and co-director of the Concrete Sustainability Hub. “Brighter materials like concrete and lighter-colored aggregates offer higher albedo, while existing asphalt pavements can be made ‘cool’ through reflective coatings.”

If applied on a large enough scale, cool pavements could lead to considerable benefits because pavements cover as much as 40 percent of cities in the United States while conventional asphalt accounts for more than 95% of the material used in pavements worldwide.

As an added benefit, these surfaces could help mitigate climate change by reducing greenhouse gas emissions from cities, according to the researchers.

“On the one hand, by lowering temperatures, cool pavements can reduce some need for AC [air conditioning] in the summer while increasing heating demand in the winter,” says Hessam AzariJafari, a researcher at MIT’s CSHub. “Conversely, by reflecting light — called incident radiation — onto nearby buildings, cool pavements can warm structures up, which can increase AC usage in the summer and lower heating demand in the winter,” he adds.

“What’s more, albedo effects are only a portion of the overall life cycle impacts of a cool pavement. In fact, impacts from construction and materials extraction (referred to together as embodied impacts) and the use of the pavement both dominate the life cycle,” the scientists write.

“The primary use phase impact of a pavement — apart from albedo effects — is excess fuel consumption: Pavements with smooth surfaces and stiff structures cause less excess fuel consumption in the vehicles that drive on them,” they add.