Asphalt is the sustainable material for constructing pavements. From the production of the paving material, to the placement of the pavement on the road, to rehabilitation, through recycling, asphalt pavements minimize impact on the environment. Low consumption of energy for production and application, low emission of greenhouse gases, and conservation of natural resources help to make asphalt the environmental pavement of choice.
The environmental sustainability movement is obviously here to stay. But, just what is it?
Pavement designers and specifiers are already familiar with the “sustainability” part. We sometimes call it Perpetual Pavements — building a strong, flexible foundation so that the pavement requires only an occasional overlay. Frequently Perpetual Pavements are built that average a single overlay every 13 years, with others lasting 35 years or more between overlays.
The “environmental” portion is added to ensure that the impact on natural resources is minimized. Sustainable pavements are high quality, long-lasting pavements that have low impact on the environment, both when they are placed, and when they are manufactured.
In the environmentally sustainable world, economics sometime take a less prominent role. For example, look at the popularity of more expensive organic milk and produce. People often are willing to pay a premium for an environmentally friendly product. To be competitive in the American marketplace, superior products will be environmentally sustainable. And this is occurring daily in the construction field; in the way that structures are designed and built, and in the systems used to run those structures. Look at the importance of Energy Star ratings on windows and HVAC systems, the incorporation of (real) green rooftops, the reliance on LEED certification for building materials, and the use of porous asphalt pavements to manage stormwater runoff from parking lots. The environmental sustainability movement is being driven from a number of different directions: government regulations, grass roots, consensus-based organizations, and political affiliations.
In May 2006, more than 200 mayors signed the U.S. Mayors’ Climate Protection Agreement, pledging to cut greenhouse gases, such as CO2 emissions, while promoting “greener” land-use policies. Environmental sustainability is affecting pavement selection and design now, and will continue for years to come.
So, just what makes asphalt pavement the environmentally sustainable pavement? We’ve identified four key environmentally sustainable areas that affect pavement selection and design:
• Energy and recycling
• Improved water quality
• Clean air and cool cities
Energy and Recycling
The amount of energy consumed in building and maintaining pavements is of utmost importance. This includes the amount of energy required to obtain and process all raw materials used in the construction of the pavement as well as the energy required to place or apply the pavement material.
Sometimes it is difficult to get an accurate picture of the true energy required as this entails a total life-cycle analysis. Plus increasing energy costs makes this analysis slightly more difficult. But it is fairly straight-forward; energy is measured in joule-equivalent units, like BTUs.
The amount of energy consumed by the traveling public is another important parameter in understanding the total energy picture. Traffic congestion leads to consumption of fuel and production of pollutants. Reducing congestion by constructing pavements faster, just makes sense.
Green equals recycled and recyclable. To be environmentally sustainable, a pavement should be made up of a large amount of recycled material, and at the end of its useful life, should be equally and easily recyclable.
According to an EPA/FHWA study, the asphalt industry recycles more than 80 million tons (72.6 million t) of its own product every year, making the industry America’s No 1 recycler. This saves precious natural resources, utilizes less energy, costs less, and allows for continuing cycles of reclamation. Asphalt recycling saves taxpayers approximately $2 billion per year.
Environmental sustainability means performance. When materials last longer, they are more sustainable. When appropriately designed and constructed, asphalt pavement foundations are perpetual. Removing and recycling the top asphalt layer can be done quickly; it saves money and requires less overall energy as well. The newly overlaid road surface (which also may contain recycled or reclaimed material) is a good-as-new pavement. Total removal and reconstruction of the road is not needed; a truly sustainable construction process has begun.
The performance part of environmental sustainability isn’t just about longer-lasting Perpetual Pavements; it’s also about noise reduction and public safety. Quieter pavements are a quality of life issue, the human environment. People must be able to live comfortably around roads without competing with the roar of traffic.
Open-graded asphalt pavements are known to significantly reduce road noise. These pavements also affect public safety by allowing rainwater to drain through the surface, reducing hydroplaning and subsequent overspray kicked-up by vehicles. In fact, many states require their interstate highways to be repaved using open-graded asphalt.
Smooth roads also can have a positive environmental impact. Tests at a pavement test track in Nevada has shown that driving on smoother surfaces can reduce fuel consumption in the neighborhood of 4.5 percent. Smoothness also can affect the life of the pavement. When trucks are driven on rough surfaces, the tires bounce and deliver heavy, punishing impacts to the pavement. Some experts estimated that a 25 percent increase in smoothness can result in a 9 to 10 percent increase in the life of pavements. Longer lasting roads means less energy consumed in rebuilding the roads and less congestion while the roads are being constructed.
Improved Water Quality
Cleaner water is environmentally friendly. Porous asphalt pavements have been used to successfully manage stormwater runoff by turning runoff into infiltration; restoring, or even improving the hydrology of a site, including water quality. This practice also eliminates the need for detention basins.
Once constructed, asphalt pavements have minimal impact on the environment. Studies show that asphalt pavements do not leach into the environment. Also, stockpiles of reclaimed asphalt pavement have been shown not to leach.
In fact, drinking water reservoirs are often lined with asphalt. And asphalt cement also is used to line water pipes that supply potable water to our communities. Oregon and Washington State Fish and Wildlife agencies use asphalt pavement to line their fish rearing ponds.
Clean Air and Cool Cities
Asphalt plants are environmentally sound. Emissions from asphalt plants, including greenhouse gases, are very low and well-controlled. Since 1970, total asphalt emissions have decreased by 97 percent while production has increased by 250 percent. Emissions from asphalt plants, including CO2, are so low, that EPA considers asphalt plants as only minor sources of industrial air emissions.
The ability of the built environment (buildings, pavement, land mass) to retain heat from the sun, in highly urbanized areas, is known as the urban heat island effect. It is not a black and white issue. Complex conditions occur which artificially raise an urban area’s surface temperature. Porous asphalt pavements, with their ability to dissipate heat, have been shown to lower nighttime pavement surface temperatures.
When cars and trucks are mired in congestion, they consume fuel and produce pollutants. This is one issue at the heart of greenhouse gas reduction. Asphalt’s speed of construction allows planners and managers a way to fix congestion hot spots and bottlenecks, quickly and cost-effectively, thereby reducing congestion and the impact of idling engines on the roadways, keeping the air cleaner.
• Cleaner air.
• Improving water quality.
• Long lasting, high quality pavements.
• Energy savings through recycling.
These characteristics mark asphalt as the sustainable pavement. By minimizing its impact on the environment and maximizing its positive value, asphalt pavement technology is clearly positioned to provide the public with the roads for the future.
(Howard Marks is director of Regulatory Affairs of the National Asphalt Pavement Association).
(Reprinted by permission of National Asphalt Pavement Association from its “HMAT” magazine, January/February 2007.)