Impermeable pavement made of concrete or asphalt covers more than 30 percent of most urban areas and can exceed 140 degrees Fahrenheit in the summertime.
It heats the air, posing human health risks, and surface runoff, threatening aquatic life.In cities with 1 million or more people, average air temperatures can be 1.8 to 5.4 degrees Fahrenheit higher than in less densely populated areas. The difference can be up to 22 degrees at night. The heat can increase peak demand for energy in the summertime, air conditioning costs, air pollution, and greenhouse gas emissions, heat-related illness and deaths, and water pollution, according to the US Environmental Protection Agency.
The researchers developed designs for permeable concrete that is highly effective in handling heat. Permeable pavement contains large connected pores, allowing water to drain through and reducing pavement temperature. Water in pores will also evaporate, reducing pavement surface temperature. Moreover, permeable concrete pavement does a better job reflecting heat than asphalt pavement.
The study in the Journal of Cleaner Production finds that permeable concrete pavement gives off slightly more heat on sunny days compared with conventional concrete pavement, but 25 to 30 percent less heat on days after rainfall. The engineers improved the design of permeable concrete with high thermal conductivity—meaning it can transfer heat more quickly to the ground—further reducing heat output by 2.5 percent to 5.2 percent.