May 29, 2008
‘Green’ Asphalt May Become Reality
A new way to make greener asphalt for U.S. highways and byways isn't new at all. They've been doing it for decades in Africa and India.
Asphalt is the sticky byproduct left after fuel and lubricating oil are extracted from crude petroleum. It's too thick to be laid on roads as is, so in the United States it's heated to as much as 300 degrees Fahrenheit to make it easy to pump and apply.
A new national research program called the Asphalt Research Consortium (ARC) aims to make asphalt cheaper and more environmentally friendly to produce. That means it might, for example, last longer, or use recycled material, or be manufactured using less energy.
There's good reason for the effort: America has more than 4 million miles of roads. The only place you can be 22 miles away from a road in the contiguous states is a spot in the southeast corner of Yellowstone National Park in Wyoming.
In other countries, so-called cold-mix asphalts save up to seven times the energy of the hot-mix counterparts, according to University of Wisconsin-Madison civil engineering professor Hussain Bahia.
"This is a no-brainer," said Bahia, who has studied asphalt for more than 20 years. "If any person involved in managing our infrastructure looks at the data, why would you spend more energy and money on something else? But the challenge will be to show through advanced design of these materials that the performance is equal."
Which is to say: There's much research yet to do.
As part of the ARC consortium, Bahia got $5 million to figure out how to make a cold mix that could work here.
In South Africa, asphalt is made workable by shearing it into fine particles, and then mixing it with water and soap-like chemicals that harden after the stuff is laid down.
Studies by Canada's Office of Energy Efficiency and others have found that paving with these cold mixes (also called emulsions) saves significant amounts of energy, especially when combined with recycling efforts. These asphalts also cut emissions of carbon dioxide and other gases.
Bahia is looking into other stumbling blocks to any possible changeover.
"At U.S. refineries today, there are very mature, established specifications for hot binders - our paving grade asphalts," he said in a statement this week. "But for emulsions, there is no clear agreement on how to define the quality. So, we have emulsions already, but we don't produce them as much because the specifications aren't as clear."
Bahia plans to experiment with adding polymers or plastics to the cold-mix process to make pavement more durable, safer and even quieter.
The big question: Why didn't the United States switch to cold mixes long ago? We haven't had to, Bahia said, because we've long had a wealth of resources that supported the less efficient hot-mix approach.
"In South Africa, they initially decided to go with the low-energy approach because it can save a lot of money," says Bahia. "Then as their economy grew, they had to build high-performance roads. But instead of switching to hot mixes, they improved their knowledge to build better cold mixes."