Paving the Way

How asphalt breakthrough technologies can lower greenhouse gas emissions. By Mike Acott, President of the National Asphalt Pavement Association

Asphalt pavement infrastructure is vast in scale. Of the 2.6 million miles of paved roads in the United States, over 94 percent are surfaced with asphalt. Approximately 85 percent of the nation’s airfield pavements and parking lots are also surfaced with this material. Because of its extensive use, small changes in technology can make a big difference in greenhouse gas emissions.

If you look at a cross section of asphalt pavement, you can see crushed stone particles of various sizes. These particles are held together by a mortar of asphalt cement (glue) and fine particles. These materials are combined in a manufacturing facility. The aggregate is dried and mixed with the asphalt cement binder. Typically, the composition of the mix is about five percent asphalt cement and 95 percent stone, sand, and gravel by weight. How the material is manufactured and the ingredients used in the mix are key to understanding the opportunities for reducing greenhouse gases.

The asphalt pavement industry is on the verge of several major advances in asphalt pavement technology that have the potential to transform the environmental impact of the paving industry. The breakthrough technologies are warm-mix asphalt, reuse/recycling, Perpetual Pavement, and porous asphalt.

Warm-mix asphalt
Warm-mix technologies allow for production and placement of asphalt pavement material at lower temperatures than conventional technologies. Running warm mix can reduce energy consumption during the manufacturing of the asphalt pavement mixture by an average of 20 percent. This equates to a one million ton reduction in greenhouse gases. Since 2004, implementation has proceeded with virtually no complications. The state departments of transportation (DOTs) and Federal Highway Administration (FHWA) have been very receptive to the use of warm mix. Agencies such as the Environmental Protection Agency (EPA), the National Institute for Occupational Safety and Health (NIOSH), and the labor unions are fully engaged with us.

Warm mix also makes it possible to increase the rates of reuse/recycling even further. Research on this topic could be helpful in speeding the rate of acceptance. Research funding will also be needed both for emissions studies and for monitoring of long-term pavement performance.

Reuse and recycling
The use of reclaimed asphalt pavement (RAP) has been widespread for about 30 years, and today asphalt pavement is America’s most reused/recycled material. Every year, more than 100 million tons of asphalt pavement material is reclaimed. Virtually all of it is reused or recycled into new pavements. A singular quality of asphalt cement in old pavement is that it can be completely rejuvenated in the recycling process. It becomes an integral part of the binder. This is referred to as the highest and best use. No other pavement material has this unique quality.

In view of the high reuse/recycling rate in many states and evidence that the quality of asphalt pavements incorporating RAP is equal to or better than pavements using all virgin materials, there is a great opportunity to double the quantity of recycled asphalt pavement used within five years.

For asphalt pavement, it is possible to reduce greenhouse gas emissions simply by incorporating recycled asphalt in new pavement. Currently, the percentage of recycled material in a ton of mix is 12 percent nationally. Doubling that to about 25 percent recycled asphalt pavement reduces greenhouse gas emissions by 2 million tons annually.

We estimate that we have 18 billion tons of asphalt pavement already in place on America’s roads and highways. Because of the ability to reuse and recycle this material indefinitely, our existing highways are a resource for future generations.

Perpetual Pavement
Perpetual Pavement is the name given to an asphalt pavement that is designed not to fail. Construction is in layers whose properties serve a combination of different functions; they all add up to an extraordinarily long-lasting pavement. Surface distresses may occur eventually, but they do not penetrate deep into the pavement’s structure. Routine maintenance involves infrequent milling of the top layer for recycling, then placing a smooth, quiet, durable, safe new overlay. A Perpetual Pavement never needs to be completely removed and replaced. In the world of pavements, this is the ultimate in economic and environmental sustainability.

Perpetual Pavements can mitigate climate change by reducing greenhouse gas emissions, both now and for generations to come. Perpetual Pavements reduce greenhouse gas production several ways. Since only the surface is renewed, the base structure stays in place, thereby significantly reducing greenhouse gases associated with acquisition of virgin raw materials, production of pavement material, and placement of the pavement on the road or highway. Greenhouse gas emissions associated with complete removal and replacement of pavements that have reached the end of their useful life is avoided. Taking a long view – 100 years or more – building Perpetual Pavements is a highly sustainable practice.

Porous and open-graded asphalt pavements
Porous and open-graded asphalt pavements have been shown to have a dramatic beneficial effect on water quality. These pavements have been used widely for over 30 years with an excellent record of success. Open-graded pavement is made with same-size rocks, creating a web of interlocking pores that allow water to flow through the surface.

Open-graded pavements are used mainly in two types of applications. First, open-graded friction courses are widely used for surfacing roads and highways. The pavement layer directly beneath this is impermeable. During a rainstorm, instead of pooling on the surface or bouncing off it, rain drains through the surface and out to the sides. Splash and spray are greatly reduced, enhancing safety. Second, porous pavement systems are stormwater management tools with an open-graded surface over a stone recharge bed. The system is designed and constructed to collect stormwater, which then infiltrates into the ground. Porous pavement systems are used mostly for parking lots, but they have also been used successfully for roads in communities like Pringle Creek in Salem, Oregon.

Both applications can be used to improve water quality. Porous asphalt surfaces allow roads and highways to function as linear stormwater management systems. Porous parking lots store stormwater, reduce runoff, promote infiltration and groundwater recharge, allow evaporative cooling of the atmosphere, diminish erosion on stream banks, reduce particulates in stream water after storms, and improve water quality.

Asphalt for sustainability
The asphalt pavement industry has worked for the past three decades to create and deploy these sustainable technologies. Three major environmental goals – conservation of natural resources, reduction of greenhouse gas emissions, and improvements in water quality – are not only achievable, they are already being achieved by the thousands of asphalt companies across the country. Many, if not most, of the 300,000 workers in the industry may be called green workers.

The potential benefits to the environment and the economy are clear, and the challenge to the industry and its customers now is to ramp up the implementation of these green pavement technologies. NAPA is not only working actively with its partners in government agencies and academia to advance asphalt’s green agenda, we recently testified before Congress to ask for a major research program to help push deployment out onto our streets, roads, highways, runways, and parking lots. With or without the funding from Congress, the industry will continue to pave the way to a sustainable future.