InTransition Magazine
Article URL:
InTransition Magazine : Transportation Planning, Practice & Progress

Archive Edition

Archived editions: 

Technical Toolbox

Green is the New Blacktop

Companies, Cities Experiment with Eco-Friendly Surface Materials

By Chris Gaetano

About Technical Toolbox

  • Technical Toolbox highlights innovations and emerging technologies making an impact in the transportation sector.

They’re the roads less traveled, although perhaps not for long. Environmentally friendly pavements, while not widely used in the U.S. today, have been the subject of increased attention over the last few years, and their success is beginning to make believers—and buyers—in cities around the world.

Some of these materials, such as porous pavement, have been used in public and private sector projects in the U.S. with great success. Others, like warm-mix asphalt, are relatively new, but are on their way to widespread use as a practical and environmentally friendly substitute for conventional pavement. The exhaust-absorbing Noxer block is at the fore of green paving technology and is still being piloted in Europe and Asia.

Large paving projects often draw criticisms from concerned citizens and conservationists over drainage problems and negative impacts on air quality. As awareness and use of green paving materials spreads, however, some of these classic confrontations may be smoothed over like freshly paved roads.

Material Cleans Emissions

One material designed to actually reduce pollution from auto emissions is Noxer, a type of paving stone manufactured by Japan’s Mitsubishi Materials Corporation. Noxer uses light-reactive materials to absorb nitrogen oxide (NOx), a component in car exhaust that contributes to smog and acid rain. The component in Noxer that pulls everything together is titanium dioxide (TiO2), the same chemical found in some white paints and sunblocks. Noxer is a block of concrete coated with a layer of TiO2 in order to produce its pollution-absorbing effects.

Noxer blocks work to reduce the pollution casued by car exhaust.

When sunlight hits a Noxer block, oxygen is produced along its surface. When NOx contacts these blocks, it combines with the oxygen to form a nitrate (NO3) that can then be washed away by the next rain or neutralized due to the block’s alkaline nature. NO3 is highly water soluble and often used in fertilizers, although it can be harmful if mixed in high concentrations with drinking water or marine environments. Mitsubishi says that a 3-kilometer stretch of Noxer with a 5-meter width will remove about 10 percent more NOx than 800 ginkgo trees because trees lose their leaves in the winter while Noxer works as long as there’s light.

A spokesperson for Mitsubishi Materials said demand for Noxer has been growing worldwide, with the company expecting to sell between 3,000 to 10,000 square meters of the material per year. So far, she said, its most popular use has been for sidewalks, though it can also be used for public squares, sound barriers and roads.

Noxer has been utilized in various cities in Japan, as well as European locations like Milan, London and, most recently, Madrid, where a pilot program involving the use of Noxer was launched in August. According to José Baena, a Spanish engineer working on the project, the material has been used on a street called Martín de los Heros near Moncloa Metro Station, a major commuter hub in the capital. Baena said the material was given to Madrid for free by the manufacturer in order to test it for future use in the city. The trial will last for one to two years, and after that, the city will decide if it will use it again.

“For the moment, it is only an experiment to check the behavior of this product,” Baena said. “If it works, we will see how to implement it on a large scale.”

He noted that the project garnered little opposition from the public, as “in Madrid, these kinds of sustainable solutions are always appreciated.”

According to Mitsubishi, Noxer needs to be periodically cleaned to maintain its effectiveness. It costs about 50 percent more than similar, non-reactive paving stones.

At the moment, Noxer is being used in both Asia and Europe, but has received little attention in the United States.

Porous Pavement

Porous pavement, which has grown steadily in use over the past 25 years, absorbs water and filters it into the ground below. While basins have traditionally been used to control runoff caused by standard impervious pavement, porous pavement allows water to be managed better, according to Wisconsin-based Presto Geosystems, a manufacturer of the material.

Presto spokesperson Patricia Stelter said demand has been on the rise as governments at the federal, state and local levels enact stronger rules regarding the management of water and runoff. Interest in porous paving technologies has grown to the point where it is now a feature in the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) system, which rates projects on energy efficiency and environmental friendliness.

An up-close look at FilterPave, a recycled glass pavement

manufactured by Presto Geosystems. Notice the gaps through

which stormwater can drain.

Porous pavement has seen many changes over the years and now covers an extremely diverse set of technologies, all designed with stormwater control in mind. An early example, used at Walden Pond in Massachusetts, was produced much like standard asphalt––a mix of large and small stone particles bound together by a tar, except with less tar and fewer stones. This creates gaps that water can pass through. While many porous pavements still apply this same basic principle, there is great variation in the types of materials used. For example, Stelter said that Presto offers a FilterPave system composed of 100 percent recycled glass, material that would otherwise be landfilled.

One of the most enthusiastic adopters of porous pavement technology has been Portland, Ore., which first used it in 2001 for driveways before expanding to street paving around 2005. Brett Kesterson, senior engineer with the city’s Bureau of Transportation Engineering and Development, said porous pavement’s use was proposed in response to local regulations restricting the use of impervious pavement when reconstructing city streets.

One drawback to porous pavement, however, is that it costs slightly more than standard pavements. Kesterson said that the materials Portland used cost about $56 per square yard, versus $40 for impervious pavement. Although the cost of the material was about the same, he said, its installation required extra labor and training, as the layout process was different than that of conventional materials. Another drawback is maintenance, the engineer noted, because porous pavement needs vacuum sweepers, as opposed to mechanical street sweepers, to avoid clogs in the material that could limit its permeability. In addition, it’s possible for resilient weeds to grow through the small gaps in the material. Such maintenance issues have somewhat discouraged the city from expanding its use.

“The main concern is long-term plugging of the pavement surface,” Kesterson said. “For now, it does not appear to be an issue, but only time will tell.”

“Will the need to spray weeds twice a year for permeable pavers be tolerated by the public? So far it has, but Portland’s Maintenance Bureau does not like the idea of having to spray permeable paver streets twice a year,” he said.

It’s also not ideal for high-volume streets, as the gaps can make for a rough ride.

Overall, Kesterson said porous pavement has been working very well for a city concerned with how its paving practices impact the environment.

“I have never observed water along the gutters or going into the inlets on any of the streets,” Kesterson said.

A Cooler Asphalt

The majority of roads Americans drive today are made from hot-mix asphalt, a combination of aggregate and liquid asphalt cement, heated up to 300 degrees during the manufacturing process. While most drivers probably aren’t familiar with the technical details of this material, almost all have at one point or another seen it in use. A dark, powdery mixture is laid down and a steamroller flattens it into what will become a new lane or, perhaps, a refurbished old one. People observing these projects can see smoke rising from the newly paved road.

This smoke, and the acrid smell that can accompany it, is the result of the intense temperatures that hot-mix asphalt requires to function properly. From an environmental standpoint, this can be problematic. Hot-mix asphalt, both when it’s mixed and when it’s laid out, produces greenhouse gases, particulate matter and other pollutants, according to the Environmental Protection Agency. Even taking emissions out of the equation, this material requires large volumes of energy to properly heat.

Warm-mix asphalt, over the past decade, has been increasingly viewed as a viable solution to these issues and is being used for paving projects across the country. The methods for reducing hot asphalt’s temperature while still retaining its workability vary from manufacturer to manufacturer, though many rely on the use of chemicals such as Sasobit, a type of paraffin wax, or WAM-Foam, which binds the component materials at lower temperatures. The end result is an asphalt mix that is between 30 to 100 degrees cooler than typical hot-mix asphalt.

While it was used in Europe in the mid-90s, the first domestic trial of warm-mix asphalt took place at an exhibition in 2004 called the World of Asphalt, according to U.S. Department of Transportation spokesperson Nancy Singer. Since then, 25 states have done trial projects.

Andrea Kvasnak, an engineer with the National Center for Asphalt Technology, said that many states have welcomed warm-mix asphalt, especially Texas, which has expanded its use since a 2006 pilot project.

“Texas has really embraced the technology and have had several large tonnage projects this construction season,” Kvasnak said.

Warm-mix asphalt also holds the practical benefit of extending the paving season. The window for laying out hot-mix asphalt typically closes when the weather gets too cold. This window stays open much longer with warm-mix, which has a higher tolerance for lower temperatures. These practical considerations, combined with increased environmental sensitivities, are increasing warm-mix asphalt’s popularity throughout the country, Texas Department of Transportation spokesman Mark Cross said.

“[Its] better [densities], less oxidation of asphalt and less asphalt absorption ... generally produces a more durable pavement,” Cross said. As green pavements become more widely used, demand for these materials should spark further scientific development, and materials like porous pavement, warm-mix asphalt and Noxer may be just the beginning.

Return to this Issue