Warm Mix Asphalt (WMA)

Warm mix asphalt (WMA) is a general term for a a group of technologies that allow a reduction in the temperatures at which asphalt mixes are produced and placed. Conventional HMA is typically produced at temperatures from 280°F to 340°F, while WMA is typically produced from 215°F to 275°F. The required JMF is currently the same for HMA and WMA. These technologies tend to provide complete aggregate coating at lower temperatures and act as compaction aides. The mechanisms which allow better coating and compaction vary from one technology to another.

Where Did Warm Mix Asphalt Come From?

In the fall of 1996, the German Ministry of Labor and Social Affairs was considering exposure limits for workers exposed to asphalt fumes.  At the same time, other European countries were faced with the greenhouse gas reduction targets as a result of the 1997 Kyoto treaty on climate change. Their solution was to develop technologies that would result in lowering the temperature at which the asphalt mixtures were produced (i.e. WMA). Before these efforts, several much older efforts (e.g., 1960s and 1970s) that effectively produced what is now known as WMA have come and gone.

Warm Mix Asphalt Technologies

There are now over 30 different warm mix technologies, all of which are commonly classified into three groups:

1. Chemical additives (surfactants). Chemical additives, mostly surfactants, are added in the manufacturing process (either separately or as part of the asphalt cement) to assist the asphalt binder in binding to the aggregate during the mixing process. This chemistry reduces the amount of heat needed in mixing to ensure adequate aggreagate coating by the asphalt cement. These same additives also produce a lubricating effect during placement and compaction.
2. Foaming. Foaming introduces a small amount of water to hot asphalt binder as it is injected into the mixing drum or pugmill. When the water contacts the hot asphalt it flashes to steam, which results in a rapid expansion of the asphalt binder (approximately 5 to 10 times). This foaming action allows the asphalt binder to adequately coat the aggregate particles at lower mixing temperatures.
3. Organic additives (waxes).Organic additives (usually waxes) melt at the hot mixing temperatures of the drum mixer or pugmill and result in a lubricating effect asphalt binder to adequately coat the aggregate particles at lower mixing temperatures. The resulting mix is more workable at placement time. Later, when the waxes solidify as the new pavement cools down, they form long organic molecules that incrase the mixes resistance to deformation at high in-service temperatures.

Benefits of Warm Mix Asphalt

• Reduced Emissions. Because the asphalt binder is at a lower temperature fewer volatiles leave and end up as emissions and fumes. The exact compounds that are reduced and by how much depends on the particular WMA technology, amount of temperature reduction, and the molecular makeup of the asphalt binder. Also, for a given reduction in temperature the reduction in emissions is usually much larger.
• Reduced Fuel Usage: Lower production temperatures result in reduced fuel use by the HMA plant. Reductions in fuel use vary but have generally been reported to range between 10 to 30 percent.
• Improved Working Conditions: The lower temperature of WMA reduces the fumes at the project site and results in a cooler work environment. It has been suggested that these improvements result in a more productive worker.
• Paving Benefit: Lower production temperatures allow subsequent pavement layers to be placed sooner. The pavement also cools faster and allows traffic on the road earlier than with an HMA application.
• Compaction Aid: WMA technologies reduce the viscosity and/or the surface tension of the asphalt binder, which reduces friction that resists compaction efforts.
• Higher Use of Reclaimed Asphalt Pavement (RAP): WMA technologies can be beneficial to mixes containing a high percent of RAP. The decreased aging of the asphalt binder resulting from lower production temperatures seems to help rejuvenate the RAP binder.
• Longer Paving Life: The lower production temperature reduces the aging of the asphalt binder, which may result in a longer pavement life and a reduction in the potential for cracking.
• Cold Weather Paving: WMA technologies offer the potential to extend the paving season in colder climates. WMA is more compactable at lower temperatures than HMA. The rate of cooling is driven by the difference in between the asphalt mix and the ambient air; the smaller the difference the slower the mix cools.
• Longer Haul Distances: Longer haul distances are also facilitated by the slower rate of cooling of WMA and improved compaction at lower temperatures.

Performance

WMA has been used in the field in Europe since about 1995 and in the U.S. since about 2004. Given the typical HMA pavement surface life (in excess of 10 years) even the first field trials in the U.S. still have not observed the entire surface course life (in Europe results are good with generally equal performance). In the U.S., long-term performance results are available from several pavement testing facilities that use accelerated testing to simulate years of performance in a matter of months. Resuts from these tests (e.g., Jones et al., 2009; Prowell et al., 2005) show WMA to perform well (equal to or better than HMA).

Where is Warm Mix Asphalt Used?

Between 2004 and about 2008 WMA was used in the U.S. primarily in field trial situations as the industry was testing and becoming comfortable with its characteristics and performance. Since then, the use of WMA has been growing rapidly and has incrased each year since. The Hansen and Newcomb (2011) survey showed U.S. WMA use rose from 19.2 million tons in 2009 to 47.2 million tons in 2010, a 148% increase. As of 2012, most states have a WMA specification and allow its use in some fashion. Usually the specification is "permissive" meaning that WMA is allowed to be used as a substitute for HMA if the contractor wishes. Usually there is a requirement to show that the particular WMA technology the contractor wishes to use has been used successfully and performs as well as HMA.

In Hawaii, WMA specifications are in the review process (as of 2012) for incorporation to HDOT standard specifications. In the meantime, WMA has been used on several projects:

• In 2006 a section of Farrington Highway, near the old Cornet store was paved with Sasobit WMA.
• In ____, the two right turn lanes on Fort Weaver Road that turn onto the H-1 freeway going east bound (Double Green Barrel foaming technology).
• In 2012 a portion of Ala Moana Boulevard fronting Ala Moana Shopping Center was paved using WMA from an Astec Industries Double Barrel Green plant that uses foaming technology.

In Hawai'i, WMA can theoretically be produced from any current plant (there are a number of additive technologies that allow this), however, the price and type of WMA is dependant on the particular plant, availability of the WMA technology and any modifications required to the plant. Several plants in the state are alrady fully equipped to produce WMA.

Cost of WMA

Depending on the technology used, the cost may either be more or less. For example, once the cost of the equipment needed for the foaming process is recovered, there is a minimal cost associated with this technology as it involves the addition of a small amount of water to the asphalt binder; the cost would be about the same or less as HMA. For other technologies, the savings in fuel costs from producing warm mix at lower temperatures may not offset the material cost of the warm mix technology. In addition, the cost savings from reduced emissions and improvements in working conditions are in the process of being quantified.