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Credit: Photo: National Asphalt Pavement Association

After three years, the 250 tons of warm-mix asphalt that Charlotte, N.C., placed on this section of Old Statesville Road showed no appreciable rutting or other deterioration. Core comparisons with a hot-mix section show that hot and warm mixes densify at about the same rate.
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Credit: Photo: National Asphalt Pavement Association

Produced and placed at lower temperatures than hot-mix asphalt, warm mixes eliminate fumes and increase worker comfort during paving operations.

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.