By Paulette Salisbury, FCSI, CDT
How can the color of a pavement make it more sustainable? The answer lies in the materials chosen by design engineers and public works agencies. Long known for its durability and strength, concrete has withstood the test of time on millions of miles of highways and local streets and roads. Now another attribute of concrete pavements is emerging as a benefit to owners, engineers and citizens. Concrete’s light color improves light reflectivity and makes it a cooler material for pavements, roof tiles and building facades. Concrete has been recognized as a solution to the Heat Island Effect by the US Secretary of Energy, Dr. Steven Chu.
Urban areas are usually warmer than their rural surroundings, a phenomenon known as the “heat island effect.” Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality.
Portland cement concrete absorbs less energy from light so it stores less heat and thus has a cooler surface temperature than black pavements such as asphalt. Typical summertime pavement surface temperatures can range from 120-150 degrees. Dark pavements hold heat internally and re-release it at night. Hot pavements also heat stormwater and run-off that flows into local waterways decreasing the water quality and its ability to support plants, fish and wildlife.
In many U.S. cities, pavements represent the largest percentage of a community’s land cover, compared with roof and vegetated surfaces. As part of EPA’s Urban Heat Island Pilot Project Lawrence Berkeley National Laboratory is investigating the impact of utilizing light-colored paving materials including concrete to reduce temperatures and the green house gas (GHG) emissions associated with energy production.
While decision-makers generally choose paving materials based on the function they serve, many private and public owners are considering how the pavement can provide a dual role in reducing energy bills from excessive air conditioning as well as providing a smooth, stable long-lasting driving or parking surface. Studies show that parking lots typically make up a large portion of paved surfaces in urban areas and therefore most of the research has been done on them. Streets, roads and highways also have a significant impact on urban temperatures. Support is growing among public works agencies for using concrete overlays on asphalt roadways to improve the longevity of the pavement and increase reflectivity thus “cooling” the environment. Recent “thermographic” images show the ambient temperature an average of 15 to 20 degrees cooler on concrete surfaces than asphalt pavements.
The US EPA’s Cool Pavements Compendium indicates that concrete is one of the most readily available solutions for the heat island effect. They also explain that concrete’s high albedo or solar reflectance is 40% compared to black asphalt at 5%. Concrete’s albedo can be increased to 70% by using slag or white cement. (http://www.epa.gov/heatisld/resources/pdf/CoolPavesCompendium.pdf ).
Even when a concrete pavement is worn and dirty its solar reflectance is high (about 25%) while asphalt may get lighter in color as the petroleum-based matrix wears away or ages reaching a maximum reflectance of 15-20%. At this point the asphalt usually needs to be resurfaced in a perpetual maintenance cycle making it black again diminishing its solar reflectance.
Concrete’s light reflectivity makes it an ideal material to enhance night time visibility. An AASHTO standard illustrates that illumination demands are roughly 40 to 50 percent lower for concrete pavements than for asphalt pavements. Another report comparing the two pavement types suggested a cost savings of as much as 31 percent in initial energy and maintenance costs for lighting concrete pavements. This finding is important for municipalities where utility costs associated with street illumination are often a large budgetary item.
In order to create sustainable communities architects, engineers, public works agencies and all government officials need to embrace all options. Concrete as a building and paving material is clearly one of the most promising solutions.
"If you look at all the buildings and if you make the roofs white and if you make the pavement more of a concrete type of color rather than a black and if you do that uniformly, that would be the equivalent of... reducing the carbon emissions due to all the cars in the world by 11 years – just taking them off the road for 11 years.”
Stephen Chu – Energy Secretary
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