In-plant foamed asphalt processes grow in popularity as producers, contractors and agencies embrace warm mix asphalt

By Tom Kuennen, Contributing Editor

Warm mix asphalt (WMA) is created by mixing a solid chemical compound additive with hot mix asphalt in the plant. Except when a liquid additive is used to make WMA.

But ask a contractor in different parts of the United States if he produces warm mix asphalt, and he may say, “Yes, we foam our asphalt.” That’s because in-plant foamed asphalt using plant-mounted equipment also is known as warm mix asphalt. This dedicated equipment foams the liquid asphalt in the drum during mixing.

But foamed asphalt also is created in a cold mix plant, and even in situ in the field, using as much as 100-percent reclaimed asphalt pavement (RAP). That technology antedates in-plant foamed processes, but also is called foamed asphalt or foamed bitumen.

And some asphalt producers are creating warm mixes by using liquid additive combined with dedicated in-plant foaming equipment.

Welcome to the sometimes confusing new world of what might be better termed low-energy mixes; that is, asphalt mixes produced at significantly lower temperatures, with substantial benefits to the producer and consumer in terms of energy costs and environmental impact.

The confusion is exacerbated by the fact that few state DOTs have specifications for warm mix asphalts, leaving their composition up to the contractor, so long as their use is permitted by the contract, and the contractor stands behind his or her work.

The most recent survey, Synthesis of Warm Mix Asphalt Paving Strategies for Use in Montana Highway Construction, was published in November 2009 and notes that just 12 states had reported WMA specs. They are Alabama, California, Florida, Idaho, Indiana, Iowa, Maine, Ohio, Pennsylvania, Texas, Virginia, and Washington State.

A review of the specs reprinted in the survey shows tremendous variation in the types of WMA permitted, the circumstances under which they can be used, and in the detail or length of specifications provided by each state. This essential publication may be downloaded at http://www.mdt.mt.gov/research/docs/research_proj/warmmix/final_report.pdf.

A template exists for road agencies to start developing a WMA specification. A Warm Mix Asphalt (WMA) Guide Specification for Highway Construction was articulated by the Warm Mix Asphalt Technical Working Group in November 2008, but it’s just a specification framework into which states may insert their own requirements. This guide spec framework may be downloaded at http://www.warmmixasphalt.com/submissions/93_20081209_WMA Guide Specification version 1.07 Final_WMATWG.pdf.

Experts from the National Asphalt Pavement Association (NAPA), state departments of transportation (DOTs), Federal Highway Administration (FHWA), National Center for Asphalt Technology (NCAT), American Association of State Highway and Transportation Officials (AASHTO), the National Institute for Occupational Safety and Health (NIOSH), and many others, comprise the WMA technical working group. Additional information is available at http://www.warmmixasphalt.com/.

What is Warm Mix Asphalt?

As is obvious, warm mix asphalt is not a single product, but a variety of technologies that reduce the temperatures at which asphalt mixes are produced and placed. WMA processes generally reduce the viscosity of the liquid asphalt through a variety of means, and enable the complete coating of aggregates at temperatures 35 to 100 degrees Fahrenheit lower than conventional hot mix asphalt (HMA).