A paving material that promises excellent performance, environmental benefits, and reduced cost, warm-mix asphalt (WMA) is generating enthusiasm among transportation managers. Though still largely confined to trial and demonstration projects in the United States, prospects seem favorable for it to take over much, if not all, of the asphalt pavement market within 10 years.
‘SMALLER' ENERGY FOOTPRINT
Asphalt, technically “asphalt concrete,” is a compactable material that consists of a bituminous binder and aggregates.
Cold-mix asphalt, which is produced at ambient temperatures of about 68° to 122° F, is used for lower pavement layers on low-volume roadways. It lacks the durability and stability of hot-mix asphalt (HMA), which is produced and placed at temperatures of about 285° to 340° F, and accounts for virtually all asphalt surface pavements and high-volume roadways in the nation.
Warm mixes are produced at lower temperatures of 220° to 275° F, but are designed to provide the strength and durability of hot mixes.
The development of warm-mix technology began in Europe during the late 1990s, in part as a way to reduce greenhouse gas emissions at asphalt manufacturing plants in keeping with the Kyoto agreement. Avariety of methods have been explored, but the technologies can be divided into two major categories: wax-like additives, and processes that use water to generate foam. The goal in both cases is to keep the asphalt binder more fluid at lower production temperatures.
Reducing asphalt production temperatures offers a broad range of benefits. It reduces the amount and costs of energy consumed in heating. It reduces stack emissions from manufacturing plants, making it possible to operate plants in locations that currently are restricted. It eliminates smoke, fumes, and odor from the plant and pavement site, improving working conditions for employees and paving crews.
Because it's not just heat but also the additives that keep warm mixes workable, WMAextends the paving season into cooler months. It also increases the time and distance material can be transported between the plant and pavement site. The increased workability also may allow the addition of more recycled asphalt pavement into a given mix.
STUDYING EUROPE'S EXPERIENCE
U.S. interest in and activity around warm-mix asphalt has grown rapidly since its European introduction.
In 2002, leaders of the National Asphalt Pavement Association (NAPA) toured Europe to learn more about the technologies that were proliferating there. In 2003, a WMA presentation was featured at NAPA's annual convention, and a research program was initiated at the National Center for Asphalt Technology in Auburn, Ala.
The first U.S. field trials were constructed in 2004. In 2005, NAPA and the Federal Highway Administration (FHWA) formed the Warm-Mix Asphalt Technical Working Group to evaluate and validate technologies, share information among transportation officials and the asphalt industry, and implement effective policies and practices. In 2006, FHWA and Ohio DOT sponsored load tests on warm-mix pavements at Ohio University's Accelerated Pavement Load Facility in Athens. That same year saw a significant expansion of field trials around the country.
Last year, representatives from NAPA, the American Association of State Highway and Transportation Officials (AASHTO), FHWA, the Asphalt Institute, contractors, and consultants visited warm-mix projects, plants, and laboratories on another European tour. After examining pavements constructed as early as 1999, they agreed that WMA is viable, and that U.S. agencies and the asphalt industry should continue working cooperatively to implement it. One of the team's goals “is that WMA should be an acceptable alternative to HMA at the contractor's discretion.”
THE ROAD TO ACCEPTANCE
Based on European experiences as evidenced by last year's tour, as well as the U.S. field trials and laboratory research performed to date, there doesn't appear to be any serious challenges associated with warm-mix asphalt. Still, there are good reasons for continuing study and discussion.
“The industry doesn't change very fast, and the conservatism of the pavement engineers is understandable,” says Margaret Blain Cervarich, NAPA's vice president for marketing and public affairs. “You don't want to claim that a particular pavement will last 10 or 20 years, if none has been around for that long.”
Besides the issue of long-term durability, there are others yet to be resolved. NAPA Vice President for Research and Technology David Newcomb enumerates several in his paper titled “An Introduction to Warm-Mix Asphalt,” including:
What affect does the treatment of asphalt binder in the warm-mix process have on the pavement's susceptibility to moisture damage?
- What changes might be needed in mix designs for production at lower temperatures?
- What new guidelines might be needed for quality control and quality assurance of the mixing process?
- Is there a difference in cure time that would prevent warm-mix pavements from being opened to traffic at least as quickly as conventional hot-mix pavements?
- Can the reductions in fuel consumption and emissions be quantified to ascertain the cost benefits of warm-mix asphalt?
- Is warm-mix asphalt suitable for the high production rates in U.S. plants?
Ongoing research at Ohio University and continuing field trials around the country should answer some of those questions, while two projects being funded by the National Cooperative Highway Research Program (NCHRP) will provide valuable information. NCHRP is administered by the Transportation Research Board and sponsored by AASHTO member departments in cooperation with FHWA.
The objective of Project 09-43, “Mix Design Practices for Warm Mix Asphalt Technologies,” is to develop a performance-based mix design procedure in the form of a manual of practice. The project began last March and is scheduled for completion in March 2010. The California, Minnesota, North Carolina, Ohio, and Pennsylvania transportation departments are on the oversight panel.
Project 09-47, “Engineering Properties, Emissions, and Field Performance of Warm-Mix Asphalt Technologies,” will compare production and installation costs between warm- and hot-mix pavements, and provide relative emissions measurements for warm-mix versus conventional hot-mix technologies. It's due to begin later this year and will run for about 3½ years. Its oversight panel includes representatives from the Arizona, California, Florida, Rhode Island, Texas, and Washington transportation departments.
— Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.
Web extra: For more information on the Warm-Mix Asphalt Technical Working Group, jointly sponsored by FHWA and NAPA, visit the “article links” page under “resources” at www.pwmag.com.
Full speed ahead
Not bothering to wait for the results of field trials, managers move ahead with warm-mix paving projects.
Ohio and Illinois are just two states that are incorporating warm-mix asphalt (WMA) into their paving programs.
The material worked well in two trial projects constructed last year for the Illinois DOT.
In the first, 1,000 tons of WMA were placed as a 6-inch stabilized subbase under acontinuously reinforced concrete surface course on a section of the Dan Ryan Expressway in Chicago. In the second, 2,000 tons of warm surface mix were placed on a downstate township road west of Lawrenceville.
In both cases, the material was substantially cooler when placed and compacted than standard hot-mix asphalt, and didn't require additional equipment or new processes to place.
The agency doesn't plan to create new pavement specifications to accommodate warm mixes.
"That's part of the beauty of the material," says Hot Mix Asphalt (HMA) Operations Engineer Jim Trepanier. "If a contractor proposed substituting warm-mix on a project, we'd be willing to accept it as long as it represented the bid price."
In September 2006, Ohio built one of the early U.S. field trial pavements as an overlay on an existing road in Guernsey County. The trial included four test sections, each with a 3?4-inch layer of HMA and a 11?4-inch top layer. The top layers consisted of three types of warm mixes and a control section of a hot mix. The test pavements are considered successful demonstrations of warm-mix technology.
When James Beasley was hired as director of the Ohio DOT (ODOT) early last year, he was impressed enough to include WMA in the agency's 2008-2009 business plan.
"Our goal is to preserve Ohio's roadways with the least possible harmful impact on the environment," says ODOT Communications Director Scott Varner. "Especially in these tough financial times, the potential cost savings also is an important benefit."
The department will use WMA on six pavement projects slated for this summer. Each project will be a two-lane primary or secondary state road, and each will be done half with warm-mix and half with conventional hot-mix. The work will be contracted out, not performed by department crews.
Like Illinois, Ohio hasn't modified specifications to accommodate warm mixes, and plans to allow it as a substitution if the contractor can prove it would reduce the cost.