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Cool pavement is a road surface that uses additives to reflect solar radiation unlike conventional dark pavement. Conventional dark pavements contribute to urban heat islands as they absorb 80–95% of sunlight and warm the local air. ^[1] Cool pavements are made with different materials to increase albedo, thereby reflecting shortwave radiation out of the atmosphere. Increasing albedo reduces heat transfer to the surface and can hypothetically cause local cooling if the spatial scale of the albedo reduction is sufficiently large. The EPA reports "that if pavement reflectance throughout a city were increased from 10 to 35 percent, the air temperature could potentially be reduced by 1°F (0.6°C)." ^[2] Existing dark pavement can be altered to increase albedo through whitetopping or by adding reflective coats and seals. New pavement can be constructed to increase albedo by using modified mixes, permeable pavements, and vegetated pavements. ^[3]

Composition

The composition of cool pavements is engineered to enhance albedo through the use of reflective aggregates, advanced binders, and surface coatings. Reflective aggregates like quartz, limestone, or granite are chosen for their high reflectivity, improving the pavement's ability to reflect sunlight. ^[4] Advances in binder technology have introduced eco-friendly options such as tree resin, which lowers surface temperature by reflecting more sunlight and offers a sustainable alternative to petroleum-based bitumen.

In concrete pavements, the use of white cement or light-colored slag further increases reflectivity. White cement, lacking the iron and manganese oxides of traditional Portland cement, enhances sunlight reflection. Light-colored slag, a byproduct of iron and steel production, when integrated into concrete, improves reflectivity, structural integrity and environmental durability. ^[5]

These material choices in cool pavements significantly raise their albedo, contributing to cooler surfaces and mitigating urban heat islands, emphasizing sustainability in urban development.

Benefits

• Lower air temperatures.
  • Reduction in energy usage. Energy usage is reduced as local temperatures are cooled. Lower temperatures allow air conditioners to cool buildings with less energy. Temperature reductions attributed to increased pavement albedo in Los Angeles resulted in over $90 million per year in savings. ^[6]
  • Improvement in air quality. A reduction in energy usage would lower greenhouse emissions and air pollution (dependent on electric power fuel mix). ^[2] Lower temperatures would also slow chemical reactions that create smog. ^[1] In 2007, Surabi Menon and Hashem Akbari estimated that an increase of global pavement albedo of 35 to 39 percent could reduce carbon dioxide emissions worth about $400 billion. ^[7]
  • Improvement in pedestrian experience. A decrease in air temperature also leads to more pleasant conditions for pedestrians and a decrease in heat-related illnesses. ^[8]
• Lower surface temperatures. According to a study done by the Arizona State University’s Global Institute of Sustainability and Innovation, the surface temperatures of cool pavements range from 9°F to 16°F lower than the surface temperatures of asphalt concrete (during mid-day hours). ^[9]
  • Improved environmental impact. Lower surface temperatures lead to a decrease in the temperature of runoff water, which benefits the wildlife in the waters where the stormwater flows. ^[10]
• Increased permeability.
  • Improvement in safety. Increased permeability leads to a decrease in water runoff, a decrease in standing water on roads (which increases tire traction and safety)
  • Decrease in noise pollution. Increased permeability on the roads lead to a decrease in tire noise by up to eight decibels. ^[10]
• Increased reflectivity.
  • Lower energy usage. Increased visibility at night reduces the need for streetlights (and may negate it in some situations). ^[8]
  • Improvement in safety. Cool pavement also increases night time safety by increasing the visibility of headlights and other cars. ^[8]

Drawbacks

• Brightness. Cool pavement reflects much more light into the eyes of drivers, thus having the potential to temporarily blind them.
• Durability. Some cool pavements can degrade faster than traditional pavements due to the coating used being not as durable as asphalt or concrete. ^[11]
• High energy usage. A potential issue can be higher energy usage by nearby buildings, however this would depend on the material the buildings are made of. As glass buildings themselves reflect heat whilst concrete buildings absorb heat.
• Aesthetic considerations. Cool pavements would deviate from the traditional look of pavements found in urban environments.
• Cost. There is a higher initial cost in building cool pavements compared to traditional pavements which needs to be taken into consideration before widespread adoption. ^[12]
• Considerable variation in unit price. Cool pavement costs vary as a result of numerous factors: ^[1]
  • Geographical region
  • Local climate
  • Labor contractors
  • Time of year
  • Site accessibility
  • Underlying soils
  • Project size
  • Expected traffic
  • The desired life of the pavement

Cities

• Los Angeles, Phoenix, and Tokyo have experimented with cool pavements. Los Angeles, for example, has laid out 181 lane-miles of solar-reflective coating. ^[13]

Maintenance

• Prolonging life of cool pavements. When it comes to maintaining and prolonging the life of cool pavements, an efficient and accurate design and diligent construction methods are vital. In addition, cool pavements are built using pores or holed that are open which makes them permeable pavements. ^[10] Due to this reason, maintaining cool pavements requires sweeping and blowing to keep the sediments off the surface. ^[14] It is also important to have a proper drain system and water run-off sections to the side of the road so that when water carrying debris can properly drain away and not stagnate and damage the pavement. ^[14]

See also

• Whitetopping
• Permeable pavement
• Solar mirror

References

^[1]