by Wayne Jones, P.E.
Earlier this year, an FHWA- and AASHTO-sponsored Scan Team traveled to Europe to learn more about warm mix asphalt (WMA) technologies. They found an advanced technology that is routinely used wherever lower mix temperatures are needed (i.e. late season paving, longer haul-time, lower emissions, etc.).
Following the 2002 tour, four WMA technologies have been introduced in the United States. The 2007 Scan Team found at least eight newer technologies that have been developed in the last five years, some with production temperatures as low as 194° F.
With the research underway in Europe, the Scan Team’s impression is that there are more WMA processes on the way. The Scan Team found that the WMA technologies fall into two categories—wax-like additives and foaming action. A final report detailing the recent Scan Tour’s findings will be published early next year. Some of the early information coming out of the Scan’s findings is discussed below.
The wax-like additives work by reducing the viscosity of the binder. The choice of additive is critical. To avoid deformation or rutting under traffic, the softening point of the additive must be significantly higher than the in-service temperature of the binder. These additives, unlike typical paraffinic bitumen waxes, should not contribute to pavement cracking.
Water Based Foaming Action
The foaming action occurs when water turns to steam and its volume increases approximately 1,600 times. As the water expands, the binder viscosity is reduced. Water can be introduced into the mix by several different methods. A hydrophilic carrier may be used to introduce the water directly into the mix. Direct foaming can also be accomplished by using a nozzle similar to the ones used in HMA recycling, or through the use of emulsion based additives, or through in-line injection of diluted additives. While all of the water based foaming processes use a very small amount of water, all of these processes use an anti-stripping additive as a precaution.
Low Energy Asphalt
One of the newest technologies uses foaming action and is called the Low Energy Asphalt (LEA). This process dries the coarse aggregate at normal HMA temperatures. The dry, coarse aggregate is coated with asphalt and is mixed with cold, wet, fine aggregate (approximately 3 percent moisture). The water in the fine aggregate foams the hot asphalt, allowing the fine aggregate to also be coated and the mix temperature to stabilize just above the boiling point of water. This technology has been referred to as “half-warm asphalt” as it is produced in a temperature range half way between cold mix (ambient temperature) and HMA.