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Study: Cool Pavements Mitigate Heat, Emissions

Wednesday, August 25, 2021

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In a recent study, a team of current and former researchers from the Massachusetts Institute of Technology Concrete Sustainability Hub (MIT CSHub) looked at how surfaces that attribute to urban heat islands could have their intensities reduced.

Focusing on “cool pavements,” the researchers compared different pavement types for solar radiation reflectivity and heat emission. Traditionally, urban heat island effects are intensified when a densely built, impermeable surface like roadways absorb solar radiation and warm up their surroundings by re-emitting that radiation as heat.

Mostly occurring in cities, the phenomenon is reported to increase air temperatures by up as much as 7 degrees Fahrenheit. While places like Missoula, Montana and Phoenix have already conducted sizeable experiments with cool pavements, the technology has yet to be widely implemented.

Albedo Metrics, Radiation Impacts

To better describe the heat island phenomenon, climate scientists use a metric called “albedo” which measures a surface’s reflectivity. According to Hessam AzariJafari, the paper’s lead author and a postdoc at the MIT CSHub, “Surfaces with low albedo absorb more light and tend to be darker, while high-albedo surfaces are brighter and reflect more light.”

This metric is central to cool pavements. In general, typical pavements, such as conventional asphalt, will possess a low albedo and absorb more radiation and emit more heat, while cool pavements, comprised of brighter materials, can reflect more than three times as much radiation and, consequently, re-emit far less heat.

“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.”

For their research, the MIT CSHub team considered several pavement options and tested them in Boston and Phoenix. However, to better understand the environmental benefits of cool pavements, researchers also looked at radiative forcing.

MIT

In a recent study, a team of current and former researchers from the Massachusetts Institute of Technology Concrete Sustainability Hub (MIT CSHub) looked at how surfaces that attribute to urban heat islands could have their intensities reduced.

“By reflecting radiation back into the atmosphere, cool pavements exert a radiative forcing, meaning that they change the Earth’s energy balance by sending more energy out of the atmosphere—similar to the polar ice caps,” said AzariJafari.

In addition, cool pavements can also exert complex, indirect climate change impacts by altering energy use in adjacent buildings.

“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,” continued AzariJafari. “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.”

However, albedo effects are only a portion of the overall life cycle impacts of a cool pavement, as embodied impacts (impacts from construction and materials extraction) and a pavement’s use also effect the life cycle.

Pavement Report

In order to determine the ideal implementation of cool pavements in Boston and Phoenix, researchers looked at the life cycle impacts of shifting from conventional asphalt pavements to different variations of cool pavements: reflective asphalt, concrete and reflective concrete.

Through physical simulations to model buildings in thousands of hypothetical neighborhoods, the team trained a trained a neural network model to predict impacts based on building and neighborhood characteristics, looking at albedo effects, as well as the embodied impacts for all pavement types and the effect of pavement type on vehicle excess fuel consumption due to surface qualities, stiffness and deterioration rate.

After the lifecycle data was compiled, the team of researchers then calculated which material benefited each neighborhood most, finding that while cool pavements were advantageous in Boston and Phoenix overall, the ideal materials varied greatly within and between both cities.

For example, in Boston, cool pavements reduced energy demand as often as they increased it across all neighborhoods. In Phoenix, however, cool pavements had a negative impact on energy demand in most census tracts due to incident radiation.

“One benefit that was universal across neighborhood type and paving material, was the impact of radiative forcing,” noted AzariJafari. “This was particularly the case in areas with shorter, less-dense buildings, where the effect was most pronounced.”

It was only after researchers examined the embodied emissions and impacts on fuel consumption could they define an ideal pavement type for each neighborhood, ultimately determining that reflective concrete pavements had the best results, proving optimal in 53% and 73% of the neighborhoods in Boston and Phoenix, respectively.

Additional research conclusions in the study found that air temperatures in Boston and Phoenix could be lowered by up to 1.7 degrees Celsius (3 F) and 2.1 C (3.7 F), respectively. The change in pavement could also reduce greenhouse gas emissions, cutting total emissions by up to 3% in Boston over 50 years and 6% in Phoenix over the same time period, respectively.

“Though the climate change impacts we studied have proven numerous and often at odds with each other, our conclusions are unambiguous: Cool pavements could offer immense climate change mitigation benefits for both cities,” said Kirchain.

MIT reports that the study is “one of the most comprehensive studies of cool pavements to date—but there’s more to investigate.” Just as with pavements, building’s albedo could also be adjusted, which could result in changes to building energy demand. Other potential tweaks that could alter emissions generated by pavements include intensive grid decarbonization and the introduction of low-carbon concrete mixtures.

The MIT Concrete Sustainability Hub is a team of researchers from several departments across MIT working on concrete and infrastructure science, engineering, and economics. Its research is supported by the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation.

   

Tagged categories: Asphalt; Climate Control; Coating Materials; Coatings Technology; Colleges and Universities; concrete; Environmental Control; Massachusetts Institute of Technology; NA; North America; Quality Control; Research and development; Roads/Highways

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