When considering how cars and trucks generate such a large part of the world's greenhouse gas pollution, it's easy to overlook what lies beneath them. But under all that traffic, there are roads. And the paving material itself-the asphalt, concrete, and rock-and how it is placed, have an important impact on the atmosphere.

Energy goes into the construction of every highway and byway, whether it's made of asphalt, concrete, or even gravel; whether it's a narrow ribbon winding around mountains, an endless flat stretch across the prairie, or a congested freeway. Moreover, the way that roads are built and maintained has a significant impact on how much energy is burned by the vehicles that roll-or crawl-on the surface.

As governments weigh spending money to restore or expand existing roadways in the developed world and to construct entirely new ones in the developing world, they are asking questions about sustainability that were never considered in the early years of infrastructure build-out. The answers are not easy.

With public investment about $190 billion per year on highway, street, and bridge construction in the United States and Europe, researchers are working to develop a way for policymakers to make road construction choices that take into account the energy in every step of the pavement's long lifecycle. And the asphalt and concrete industries are introducing new technologies and mustering evidence on old ones, all to demonstrate their commitment to making paving greener.

Pavement Rocks

Of course, the building of roads is widely viewed as the antithesis of sustainable activity, as songwriter Joni Mitchell lamented when she wrote the memorable lyrics, "They paved paradise, and put up a parking lot," at the dawn of the modern environmental movement.

But a branch of environmental science today is devoted to looking more closely at the pavement itself. As long as society is relying on roads to move goods and people, these scientists argue that government officials should have the data to make design and construction decisions that minimize the impact on land and air. (They note that the same issues apply to railroads, canals, airports, and any other transportation infrastructure.)

When it comes to roads, the truth is difficult to sort out because the two competing industries that manufacture pavement materials-the makers of asphalt and concrete-each marshal plenty of evidence of their respective product's value as a sustainable choice. Asphalt, also known as bitumen, is a petroleum product, but the industry is quick to point out that its manufacture releases less in greenhouse gas emissions than the production of cement, the characteristic binder material of concrete, due to the carbon intensity of the manufacturing process in cement kilns. (Nearly a ton of carbon dioxide (CO2) is emitted for every ton of cement produced in the United States.)

Asphalt is certainly the more visible road-building material, accounting for the surface of more than 90 percent of the 2.5 million miles (4 million kilometers) of paved road in the United States and the 3.2 million miles (5.2 million kilometers) in Europe. But concrete frequently is in the road structure beneath, and public transportation officials who are looking for a long-lasting surface often choose concrete. That industry's advocates stress that concrete's durability means less energy for raw materials and rehabilitation in the long run.

But John Harvey, principal investigator at the University of California's Pavement Research Center, says that "asphalt versus concrete" is the wrong way of looking at the environmental questions around pavement. Often forgotten in the debate, he says, is that more than 90 percent of pavement is rock, no matter what kind of glue holds it together. "We need to keep remembering that rock is the primary ingredient, so there's tremendous energy that goes into the mining, crushing, and hauling of rock in the materials production phase" of a roadway, says Harvey, a professor of civil and environmental engineering. The UC center, housed at both Berkeley and Davis, is working on developing a framework and models to help government officials make better decisions about roadways, taking into account the entire lifecycle of these long-lived civil engineering projects.

For instance, he says such a lifecycle assessment (LCA) helps to underscore the benefits of using recycled material when rebuilding roadways. "Like oil, rock is a finite resource and a lot of our best rock is in the road right now," Harvey says.

Any driver who has been stuck behind a milling machine has seen pavement recycling in action. When roads are renovated or resurfaced, these milling machines break up, remove and crush the existing pavement surface to make the products known as Recycled Concrete Aggregate (RCA) or Reclaimed Asphalt Pavement (RAP). By using recycled aggregate for pavement, road builders save the energy of extracting and transporting virgin rock, Harvey says. The energy consumed by transporting both virgin rock and recycled materials should be considered in a lifecycle analysis, allowing road builders to consider the most energy-efficient choice, Harvey says.

Many virgin rock quarries and some sources of recycled materials are located far from population centers where road construction takes place, so recycling of materials in the existing pavement into the same stretch of road eliminates all of the energy of hauling those heavy loads. Where such onsite recycling is impossible, it has become increasingly popular to import both recycled and virgin materials into aggregate-hungry urban areas by low-energy barge, often from hundreds of miles away.

The concrete industry has long touted its pavements as "100 percent recyclable;" it says recycling dates back to the 1940s in Europe and the 1970s in the United States. The asphalt industry also boasts a strong record on recycling. The U.S. industry, which is now producing about 400 million tons of new asphalt paving annually, reclaimed about 73 million tons in 2010. Virtually all of that RAP was recycled back into pavement, which the industry notes is its highest and best use.

And now, the use of recycled material in asphalt pavement can be greatly increased, due to an important development in greener paving: "warm mix."

Turning Down the Heat

Conventional hot mix asphalt must be heated to 300°F (150° C) or more so that it is workable during mixing, laying, and compacting. But by introducing water into the asphalt mix through foaming agents, or with the help of additives such as waxes, asphalt makers have found they can reduce the temperature of the mix by 50° to 100°F (30 to 60°C), while still providing a road-surface medium with the same properties as hot mix.

First introduced in Europe, and then in the United States less than a decade ago, warm mix asphalt has been increasingly popular. The U.S. National Asphalt Pavement Association (NAPA), an industry group, says the United States now leads the world in production, which grew from less than 100,000 tons in 2004 to about 47 million tons in 2010.

The industry organization expects warm mix will soon claim far more than its current 11 percent of the market. "Warm mix produces lower plant emissions, offers construction benefits, and provides more comfort for workers. All these things together have launched the technology," says Howard Marks, NAPA's director of environmental and regulatory affairs. "And now, with increasing fuel costs, contractors will see this as a competitive advantage for them because it does take substantially less fuel to use warm mix."

John Read, global bitumen technology manager for the oil company Shell, says that reducing the temperature of asphalt mix was long seen as a desirable goal in the industry, but the challenge was formidable.

Shell's engineers believed that the idea of an emulsion held promise for cooling off the hot mix-breaking the bitumen into small particles suspended in water with an emulsifying agent, or soap. The paving industry long has used asphalt emulsions for certain applications-sealing roads or driveways, or refreshing road surface. But the risk of actually making an asphalt mix with an emulsion was that the water would be trapped when the asphalt is compacted during paving. The road would be too soft and ruts would develop too easily.

Shell's researchers developed a two-stage mixing solution, called the WAM Foam process, in which a hard asphalt component is steamed into a soft asphalt component. The result is an intermediate grade mix that works just like conventional hot asphalt mix, but at a much lower temperature: about 230° F (110° C), a reduction of about 70°F (40°C). Shell says that energy consumption and carbon emissions of the mixing and paving process are cut about 35 percent.

But there are also much simpler foaming technologies that don't require any modifications to mixing plants beyond addition of a nozzle to introduce water into the mix. Admittedly, these can sometimes be very specialized; the Double Barrel Green system, developed by Astec Industries of Chattanooga, Tennessee, relies on a multi-nozzle foaming device that is controlled by computer to regulate speed and production.

Other manufacturers rely on additives to lower temperature. Specialty chemical maker PQ Corporation, based outside of Philadelphia, markets Advera, a synthetic zeolite-a mineral containing water. The water is time-released as the asphalt mixture is heated, creating a controlled foaming effect. Sasobit, produced by a division of the South African oil company, Sasol, uses a paraffin wax additive to reduce temperature. (It is a by-product of the process Sasol uses in South Africa to make diesel fuel from coal.) (Both of these technologies were used in a landmark application of 20,000 tons of warm mix in Yellowstone National Park in 2007, a project that helped build U.S. Government support for the cooler paving alternative.)

In all, there are more than 20 technologies and numerous brands available. NAPA says there is an added cost for some, but others now cost about the same as conventional hot mix.

The U.S. Federal Highway Administration says that across all technologies, fuel consumption is typically reduced 20 percent, and road workers are exposed to significantly fewer fumes and polycyclic aromatic hydrocarbons (PAHs.

And importantly, various properties of the warm mix-including its improved ability to coat the aggregate-allow a higher percentage of recycled asphalt to be incorporated into the pavement. This further cuts energy consumption and carbon emissions because the need for mining and hauling virgin rock is reduced. Field tests even have been conducted in Germany on warm mix asphalt that is 100 percent from reclaimed asphalt pavement.

Shell says the greatest demand for its WAM foam product has been in nations that have taken action on carbon emissions. The company had no WAM foam licensees in Switzerland until the nation adopted penalties for CO2 emissions; in a matter of months the company gained eight licensees, Read said.

"When we started out back in 1995, if we are genuinely honest, this wasn't about CO2," says Read. "This was about improving the worker experience. But a consequence is to reduce CO2."

Concrete Improvement

Meanwhile, the makers of concrete pavement also have taken steps to reduce their carbon footprint. The industry points out that U.S. cement manufacturers have improved their energy efficiency 33 percent since 1972, and have made a commitment to reducing their carbon dioxide emissions 10 percent from 1990 levels by 2020.

And concrete makers also have improved their environmental profile by using substitutes for cement as binder, including the fly ash that is produced when coal is burned at power plants, and slag, a residue of the steel industry. These waste products can be recycled into concrete, replacing as much as half of its cement content, and reducing the need for the carbon-intensive cement manufacturing. The use of fly ash also increases workability of the pavement, even while improving its strength and reduces water demand, according to the U.S. Federal Highway Administration. Dating as far back as 1974, the U.S. Government has encouraged states to substitute fly ash for cement wherever feasible. The California Department of Transportation requires 25 percent fly ash in most of its concrete used for pavements, says Harvey.

In Search of Smoothness

But Harvey argues that if road builders consider the energy impact over the full life cycle of a road, the fuel savings of warm mix and reductions of carbon dioxide emissions from cement manufacture and concrete will mean more on some roads than others. For highways that carry lots of traffic, the fuel consumed in pavement construction is minuscule compared to the oil burned by the cars, trucks, and buses that travel on the surface each day. And how the road is built has an important impact on the amount of fuel consumed by those vehicles.

"As your vehicle is rolling, if it is experiencing roughness, and the roughness is working the shock absorbers and tires, which are basically energy dissipaters," explains Harvey, "they actually will get hot, smoothing out the bumps."

Researchers continue to other explore aspects of pavement for their impact on vehicle fuel economy, including the concrete industry's argument that its more rigid surface reduces rolling resistance. But on smoothness, Harvey says, the scientific consensus is clear. Studies have shown that driving on a smooth road, as opposed to a rough road surface, can mean a 2 to 5 percent improvement in fuel economy. "Those are small percentages, but when we talk to people who are managing greenhouse gas strategies, we find that those are significant numbers," Harvey says. "One of the interesting things about improving the smoothness of pavement is that it affects every single vehicle on the road, and it happens immediately."

As for warm asphalt, Harvey thinks its greatest importance might not be in energy savings at the asphalt plant, but in helping keep roads smoother for a longer time, as data seem to indicate that it compacts more effectively than hot asphalt mix.

The concrete pavement industry, keenly aware of carbon-intensity comparisons that focus only on the manufacturing of the road surface, and not on its years of use, argues that its benefits will also become more apparent if policymakers take a longer view. The industry says most concrete pavements are designed for 20 years and often last 25 or more years without significant repair or rehabilitation. The asphalt industry says its surveys show that transportation departments repave asphalt surfaces after 15 years on average, although the underlying structure can be designed to last as long as concrete pavements.

"[The transportation community has] been focused almost solely on the construction and production phase of this infrastructure's lifecycle when we talk about sustainability," says Leif Wathne, a professional engineer and vice president of highways and federal affairs for the American Concrete Pavement Association. "And this is natural. Engineers focus on what they can control. But our contention is that we may inadvertently, as a transportation community, be ignoring some significant opportunities in the use phase of a pavement's life cycle."

But Harvey emphasizes that neither concrete nor asphalt comes out as the clear winner on smoothness. Both types of pavement can be "born rough," as he puts it, if attention is not paid to producing a smooth surface during paving. Later in life, a concrete pavement can produce a bumpy ride if the joints between the concrete slabs are uneven. Older asphalt is prone to developing ruts over time. And all materials expand in the heat and shrink in the cold, and are stressed by heavy trucks, contributing to cracks and every driver's nemesis, potholes.

Ironically, that's why Harvey and other pavement researchers argue that policymakers who are weighing how to reduce carbon emissions ought to consider investment in roads-at least, in renewing and maintaining the current infrastructure to smooth out the bumps that are causing cars and trucks to burn more fuel than they need to. Harvey and his fellow researchers would urge that the existing road network should be smoothed before new highway is built, requiring additional maintenance.

The pavement industries, unsurprisingly, also urge more road investment. Their pleas often are for expansion of the system, but they, like the pavement sustainability researchers who are not affiliated with industry, also argue for rehabilitation and preservation of the infrastructure already in place. In fact, governments are neither keeping up with road maintenance nor are they relieving congestion. When adjusted for inflation, highway spending per mile traveled in the United States has fallen by 50 percent since the 1950s, and estimates of the gap between spending and highway needs (pdf) range from $47 billion to $166 billion per year. Ironically, those costs grow as rehabilitation is delayed and repair jobs become more difficult and expensive.

On the need to pay more attention to roads, at least, the asphalt and concrete industries agree. "The costs of doing nothing are much greater than a meaningful and sustainable expansion of capacity," says Wathne of the concrete pavement association. Says Marks, of the asphalt association, "We've got to get the public to realize how important this is. This is how our economy runs. We can ship things only so far, and take them by rail only so far-they've got to get on the road."