Cool Pavements = Cool Idea

Posted on September 01, 2022

Solar radiation management (SRM) is a discipline focused on reflectivity, mostly in urban areas. MIT’s Concrete Sustainability Hub (CS Hub) estimates that pavement covers about 40% of America’s cities.

Conventional dark pavements absorb 80–95% of solar radiation, emitting much of it into the air as heat. The resulting urban heat islands (UHIs) raise air temperatures as much as 7° F during the daytime and 5° F at night.

According to the United Nations, 55% of the world’s population lived in cities in 2018. One estimate is that urban dwellers will comprise 68% of earth's total population by 2050. It is clear that the importance of cool pavements will only grow.

Transition to High Albedo Surfaces

“Cool pavement” is a widely used term that can mean different things. For example, it might simply refer to the use of a light-colored material like concrete as opposed to a dark material like asphalt. Cool pavement may also refer to the use of additives to reflect more solar radiation back into the atmosphere. At night, more reflective pavements reduce lighting requirements, saving energy while reducing emissions.

Urban pavements

CS Hub says that better paving material selection could mitigate urban heat. It could also reduce greenhouse gas (GHG) emissions. High-albedo materials are an alternative to traditional asphalt. Albedo is a measure of the fraction of solar energy reflected by a given surface.

Compared to asphalt, concrete has a relatively high albedo. As the Global Cement and Concrete Association (GCCA) notes, traditional concrete has an albedo of 0.40. Grass is 0.25. White paint is 0.80. By contrast, asphalt’s albedo is 0.10.

More reflective pavement counters the urban heat island effect. LOne quick way to increase a city's overall albedo is to choose concrete pavements. It is also possible to coat asphalt surfaces with a light gray coating. Los Angeles and Chicago are cities experimenting with coating asphalt to increase reflectivity.

High albedo concrete extends the advantage even further. It is increasingly popular for parking lots and concrete overlays on roads. The inclusion of coated crushed glass makes pavements more reflective, for example. It is also a convenient way to recycle waste glass.

The addition of 3-5% titanium dioxide makes concrete edifices more reflective without impacting strength. As a photocatalyst, titanium dioxide also speeds certain chemical reactions, making it an ally in combating smog pollution.

Urban roofing

Again, the urban heat island effect is more pronounced because of the profusion of dark materials on horizontal surfaces. Now, there's a push to replace black roofing with white EPDM and other alternatives. White roof materials may consist of EPDM or TPO. Another option is an asphaltic membrane coated with white elastomeric paint.

In one study, researchers from Columbia University collaborated with NASA scientists. They found that peak temperatures on white roofs in New York City were 43° F cooler than dark roofs. ASU researchers calculated that cool roofs could reduce ambient temperatures by 1.8° C in Washington, D.C.

Rooftop grass and gardens are increasingly common as well. In general, expanding parklands, planting street trees and installing green roofs can reduce local temperatures by up to 4° F. Such efforts also increase the quality of life for residents and workers alike.

The National Center for Atmospheric Research is in Boulder, Colorado. Researchers at the Center calculated the impact of replacing heat absorbing roofing with more reflective alternatives. They calculated the impact on the urban heat island effect if the average reflectivity of the planet's urban roofs increased through the use of available products. The current average is 32%. The use of high-albedo roofing could increase overall reflectively to as high as 90%. Although only theoretical at this point, the researchers concluded that such a transformation would reduce the urban heat island effect by approximately 30%.

Reflectivity research

An Arizona State University study examined reflectivity of surface materials in urban environments. The study revealed that solar reflectivity is not the only variable impacting the UBI effect. Researchers constructed six 12 x 12 text slabs near ASU’s Tempe, AZ, campus. They included two asphalt slabs: hot mix asphalt (HMA) and porous hot mix asphalt (PHMA). The two concrete slabs were Portland cement concrete (PCC) and pervious Portland cement concrete (PPCC). The other test areas consisted of artificial turf and landscape gravel.

PCC had the highest albedo, followed by PPCC, gravel, and turf. HMA and PHMA had the lowest albedo. Since the slabs were small, air temperatures three feet above the slabs were similar.

Case Study - City of Phoenix

Phoenix, AZ, has also experimented with light coatings on dark pavements. The Cool Pavement Pilot Program is a collaborative effort of the City of Phoenix and ASU. Technicians deployed a water-based coating on existing asphalt pavements. The water-based coating includes an emulsifier, asphalt, mineral fillers, polymers and recycled materials.

In September 2021, researchers reported first-year results. Among their findings:

  • Cool pavement had a lower surface temperature than traditional asphalt at all times.

  • At noon, the average surface temperature of the cool pavement was 10.5-12 degrees F lower than asphalt.

  • Even at sunrise, surface temperatures averaged 2.4 degrees F lower.

  • Surface solar reflectivity did decline over a 10-month period. The average across eight neighborhoods declined from 33-38% percent to 19-30%. By comparison, the reflectivity of untreated asphalt is 12%.

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The Pennsylvania Aggregates and Concrete Association (PACA) reports on industry innovations and initiatives. Readers include PACA members as well as the general public.

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