For many years, producers have promoted concrete by touting benefits such as better nighttime visibility and cooler surfaces to home builders and developers. Now, researchers are providing even more evidence to the claim that concrete pavement has an environmentally positive impact.
Planners are seeking to reduce the “urban heat island,” a phenomenon in which a city area experiences higher temperatures than surrounding regions that are not developed. Since darker surfaces absorb the sun's rays, asphalt surfaces remain warmer longer. This temperature spike requires more energy to cool buildings.
Urban residential areas can contribute significantly to heat islands. So providing a product that doesn't retain heat lessens heat build-up. This presents a tremendous opportunity for paving streets, driveways, and sidewalks with concrete.
Recognizing this effect, Leadership in Energy and Environmental Design (LEED) for Homes projects can earn one point if 50% or more of the horizontal hardscape areas within 50 feet of a home have a solar reflectance index (SRI) of at least 0.29.
At the Greenbuild International Conference last fall, the Portland Cement Association (PCA) announced a new study on concrete's solar reflectance and its impact on limiting the heat island effect. The CTLGroup's research documents how architects can use concrete hardscaping to contribute to points for New Construction Sustainable Sites (LEED-NC SS) Credit 7.1 Heat Island Effect: Non-Roof criteria without additional testing.
“This study formally confirms that ordinary concrete, regardless of its ingredients, can help to limit the heat island effect,” says David Shepherd, PCA's director of sustainable development.
More importantly, this research was the first time concrete's solar reflectance has been examined for LEED points. Until now, producers have provided their reflectivity data for their individual products when helping building owners apply for LEED credits.
Using recycled materials
The research measured 45 sets of concrete samples that met the required LEED reflectivity criteria, regardless of the mix design. The mixes used several different components, including recycled material, such as fly ash and slag cement.
Any type of ordinary concrete meets this requirement. (Colored and stained concrete haven't yet been tested.) It seems that the solar reflectance of cement affected the concrete surface's solar reflectance more than any other constituent material. Supplementary cementitious material had the second greatest effect.
The results may have a greater impact for concrete in the near future. The LEED for Neighborhood Development rating system (scheduled for a 2009 launch) will consider the sustainability levels of entire neighborhoods, based on their design and location.