FHWA and its partners continue to advance warm-mix asphalt, which can cut production costs and emissions and still provide long-lasting pavements.

Production and placement of hot-mix asphalt (HMA) pavements in the United States has evolved over the last 130 years, from hand mixing and application with rakes and shovels to computerized facilities feeding highly automated remixing, placement, and compaction equipment. During this time, engineers have learned that temperature control is crucial to aggregate coating, mixture stability during production and transport, ease of placement, compaction, density, and ultimately a pavement's long-term performance. During construction, the temperature must be high enough to ensure the coating and workability of the mix, but low enough that asphalt binder draindown (when the liquid binder flows down through the mixture and pools at the bottom), aging, and hardening do not occur.

Modern performance requirements often dictate use of polymer-modified asphalt binders, strong angular aggregate, and greater in-place density of the HMA. Engineers typically use polymer modification as insurance against permanent deformation of a pavement at high surface temperatures on high-volume roads. But mixes made with polymer binders can be more difficult to work with than mixes with unmodified binders.

For higher-traffic-volume pavements and surface courses in medium-traffic-volume pavements, specifications often mandate greater aggregate angularity. Such aggregate increases the internal friction of the material and makes it more durable, but greater angularity also increases the force required to mix and place the aggregate, especially in coarse gradations. Engineers often respond to this stiffness or lack of workability by raising temperatures for production, placement, and compaction temperatures to reduce the viscosity of the binder and improve the mixture flow.

Density specifications also affect the workability of HMA. Engineers use density as a measure of pavement quality: The greater a pavement's density, the lower its permeability to air and water and the better its long-term performance. If a mixture shows resistance to compression during compaction and therefore requires more effort to achieve the specified density, engineers typically respond by raising its temperature.

However, increasing production temperature is often expedient but not necessarily the most effective solution. The simple act of increasing the temperature can overheat the mixture and lead to accelerated aging in the short term and, ultimately, affect performance in the longer term. The adjustment also can result in greater fuel consumption, emissions, and odors at both the production plant and the paving site.

The Federal Highway Administration (FHWA), in cooperation with the HMA industry, researchers, and academia, is continually exploring technological improvements that will enhance the performance, construction efficiency, resource conservation, and environmental stewardship of asphalt mixtures. One approach to achieving all these goals is to reduce HMA production temperatures—sometimes by as much as 100 °Fahrenheit, °F (38 °Celsius, °C)—and to this end engineers are exploring the concept of warm-mix asphalt (WMA). WMA processes and products use various mechanical and chemical means to either reduce binder viscosity at lower temperatures or reduce the shear resistance of the mixture at construction temperatures while maintaining or improving pavement performance.

"We see the use of warm-mix asphalt technologies as a tremendous opportunity to improve construction quality, extend the construction season, and minimize negative impacts to the environment," says Peter Stephanos, P.E., director of FHWA's Office of Pavement Technology. "The collective effort of highway agencies and industry partners to advance warm-mix asphalt technologies as a standard practice has been tremendous."

Warm-Mix Asphalt

The various asphalt mixtures are distinguished by the temperature ranges at which they are produced, and by the strength and durability of the final product. Manufacturers produce cold asphalt mixtures at ambient temperatures, in the 68-122 °F (20-50 °C) range. They produce HMA in the 284-338 °F (140-170 °C) range. HMA has higher stability and durability than cold-mix asphalt, which is why cold mix is used in the lower pavement layers of low-volume roadways. Manufacturers typically produce WMA in the 220-275 °F (104-135 °C) range.

Relative to HMA, the immediate benefit of producing WMA is its lower energy consumption. HMA requires high heat to enable the asphalt binder to become fluid enough to coat the aggregate completely, have workability during laying and compaction, and retain durability during traffic exposure. With WMA's lower production temperatures comes the additional benefit of reduced emissions from burning fossil fuels, and decreased fumes and odors, at the plant and paving sites.

The differential between a production temperature established for an original HMA design and an alternative design using WMA technology only partially determines the reductions that are achieved. The final production temperature depends on more than a simple decision and turning down a dial. Other factors include the production plant capabilities; the tuning of the burner that heats the mix (to provide complete fuel combustion at the reduced production temperature), binder, and mixture design; and the specific WMA technology used.

History of WMA

Development of WMA technologies began in Europe in the late 1990s, prompted by the German Bitumen Forum, a broad coalition of industry, labor, and government to address asphalt material issues. In 1997, the European Union was in the process of adopting the Kyoto Protocol on climate change. Growing environmental awareness and activism by the public along with increased regulatory pressure to reduce greenhouse gas emissions prompted the forum to develop strategies to reduce air emissions.

The forum's Temperature Reduction Working Group evaluated technologies that save energy, reduce carbon dioxide production, and lower emissions by targeting temperature reductions of Gussasphalt (an asphalt rich in bitumen and limestone), which was traditionally produced at temperatures greater than 450 °F (232 °C)—a temperature much higher than those used in the United States for HMA. Similar initiatives to reduce temperatures soon began in France, the Netherlands, Norway, South Africa, and elsewhere.

In the United States, the National Asphalt Pavement Association (NAPA) soon became interested in these technologies and began communicating with the European asphalt pavement industry about WMA. For the 2003 World of Asphalt Trade Show and Conference, NAPA invited representatives from European companies to make presentations on WMA. The following year, NAPA and the Association of Equipment Manufacturers conducted a WMA demonstration project for the 2004 World of Asphalt Trade Show and Conference. That same year, FHWA, NAPA, and three warm-mix technology suppliers initiated a research project at Auburn University's National Center for Asphalt Technology (NCAT) on methods for reducing asphalt mixture production and placement temperatures.

"By 2005 it was obvious that WMA needed to be fully understood and adopted in the United States through a coordinated effort of asphalt pavement industry partners," says King W. Gee, associate administrator of the FHWA Office of Infrastructure.

FHWA and NAPA responded by forming the Warm Mix Asphalt Technical Working Group and tasked it with proactively providing national guidance in investigating and implementing WMA technologies. The group included multiple sectors of the asphalt pavement industry, such as State highway agencies, academia, and contractors. The group's longstanding goal is to provide technical WMA guidance that will lead to a product with quality, cost-effectiveness, and performance at least equal to conventional HMA.

The working group has developed recommendations on WMA technologies and statements on nationally significant research needs. The group's products include a Web site (warmmixasphalt.com), a guide specification for WMA highway construction, materials and emissions testing recommendations, a best practices document on WMA construction, and organization of the first International Conference on WMA.

"Warm-mix asphalt is the future of flexible pavements in the United States," says NAPA President Mike Acott. "It represents the next evolution of the asphalt industry's quest for continuous improvement. We have worked diligently with our partners to improve pavement performance through materials selection, mix design, and pavement design. Lowering our production and paving temperatures promises improved energy consumption, operations, and quality."

Mobile Asphalt Lab Testing Since 2006, FHWA's Mobile Asphalt Lab has traveled to six WMA projects to conduct material sampling and testing, and mixture performance testing, with the asphalt mixture performance tester and Hamburg Wheel Track Device. The projects include the following: A Missouri Department of Transportation (MoDOT) WMA demonstration project in St. Louis included construction of asphalt pavements using two additives and a water foaming product. The lab fabricated a large number of Superpave volumetric and asphalt mixture performance specimens for testing, in addition to loose mix sampled for later testing to investigate any effects due to reheating and residual moisture. "MoDOT first experimented with WMA in 2006," says Dale A. Williams, P.E., a materials engineer at MoDOT. "Since then the use of WMA has grown steadily with a spurt in 2009 when several contractors purchased foaming equipment. In 2009 we placed 500,000 tons [453,600 metric tons] of WMA, which accounts for approximately 12.5 percent of all mix produced, and expect the use of WMA to continue to grow." A Colorado Department of Transportation WMA project on I-70 near Dillon, CO, included construction of pavements using two additives and a water foaming product. The project location was on the uphill, eastbound lanes as the elevation climbs from 8,800 feet (2,682 meters) to 11,100 feet (3,383 meters) at the Eisenhower/Johnson Memorial Tunnel. The project included cooler, late season paving performed at night and at a high elevation, with a relatively soft performance graded binder, on an interstate facility that carries a high volume of truck traffic. An FHWA Western Federal Lands Highway Division project in Yellowstone National Park in Wyoming placed a total of 30,000 tons (27,216 metric tons) of asphalt mixture, split between a traditional HMA control section and a WMA section using an additive and a water foaming product. The asphalt mixture haul distance was 50-55 miles (80-86 kilometers) from the mixture production plant. Longer haul distances provide an opportunity for HMA to encounter too much cooling of the mixture during transportation, which can make it difficult to place and compact. The WMA mixtures start at a cooler temperature and can be placed and compacted at lower temperatures than traditional HMA. A Texas Department of Transportation project near Jasper, TX, was one of the first projects to combine a wax additive in addition to a chemical antistrip in one product. Two Pennsylvania Department of Transportation projects near Centre Hall and Spring Mills, PA, utilized two HMA control sections and four WMA technologies—one additive and three water foaming technologies. A MoDOT project on I-55 near Sikeston, MO, used a polymer-modified binder in conjunction with a water foaming technology. Hamburg testing on the WMA project in Colorado showed that the rutting results were equivalent for the HMA control (left photo) and one of the WMA technologies (right photo). They both exhibited similar rut depths when tested in accordance with Colorado Procedure - Laboratory 5112 Standard Method of Test for Hamburg Wheel-Track Testing of Compacted Bituminous Mixtures at a water bath temperature of 113 °F (45 °C) for the PG58-28 binder to 10,000 cycles. (Note: This was a 75 gyration mix design that CDOT typically does not subject to Hamburg testing requirements.) In general, the Mobile Asphalt Lab's testing results have been able to distinguish the WMA technologies from the HMA control sections. The asphalt mixture performance tester's dynamic modulus master curves and flow number results and the Hamburg rut testing results show that the reduced production temperatures of WMA pavements cause less aging of the mixtures during production. The results suggest that WMA mixtures initially are less stiff when compared with the HMA control sections and are dependent on the production temperature and WMA technology used. As a result, the testing reveals lower flow number test values and deeper rut depths than the researchers normally would expect from traditional HMA pavements. Agency and university researchers across the United States have used data from these projects to support multiple NCHRP projects and develop performance prediction models for WMA pavements. (Left) A work crew is using WMA to pave this section of the East Entrance Road inside Yellowstone National Park, WY, between the East Entrance Gate and Sylvan Pass in fall 2007. (Right) The completed road. Photos: Brad Neitzke, Western Federal Lands Highway Division

Other Avenues

The American Association of State Highway and Transportation Officials (AASHTO) also has been active in implementation of WMA. Since 2006, AASHTO has approved five research needs statements and funded them through three National Cooperative Highway Research Program (NCHRP) projects: Project 09-43 Mix Design Practices for Warm Mix Asphalt; Project 09-47 Engineering Properties, Emissions, and Field Performance of Warm Mix Asphalt Technologies; and Project 09-49 Performance of WMA Technologies. In addition to continuing to develop research needs statements for future funding, the technical working group is focused on gathering detailed information on State WMA pavement projects and specification changes made to accommodate WMA technologies.

Although the original WMA technologies came out of Europe, documented performance data were limited. In 2007, FHWA conducted an International Technology Scanning Program tour in cooperation with AASHTO and NCHRP. A team of 13 asphalt pavement materials experts assembled to assess and evaluate European WMA experiences and pavement performance. FHWA published the results of the scan as Warm Mix Asphalt: European Practice (FHWA-PL-08-007), which concludes: "The consensus among the scan team members was that WMA is a viable technology and that U.S. highway agencies and the HMA industry need to cooperatively pursue this path."

WMA Technologies

Since 2006 NCAT has documented more than 140 WMA projects in 43 States and the District of Columbia. In the same period, providers have introduced 21 named WMA technologies into the U.S. market. The technologies generally fall into three categories: materials processing, mixture and binder additives (chemicals and waxes), and water foaming technologies. Some technologies also include such additives as surfactants or chemical antistripping agents. All of these technologies, introduced at some point or other in the production process, make it easier for manufacturers and crews to make and place WMA pavements, and facilitate the pavements' performance and long lives.

FHWA is involved in evaluating and implementing these and other WMA technologies. The agency is working closely with State and industry partners to develop and monitor demonstration projects and research, and to advance the knowledge and state of practice of WMA materials and technologies.

State agencies have requested the services of FHWA's Mobile Asphalt Pavement Mixture Laboratory (Mobile Asphalt Lab) to support further research and validation through material sampling and performance testing on WMA projects. Experienced technicians and engineers travel with the mobile lab to pavement construction sites across the country to help transportation partners resolve national issues related to implementation of new pavement technologies.

Although the primary purpose of WMA has been to lower production temperatures, providers also market some WMA technologies as compaction aids. Crews use the technologies at traditional HMA temperatures to improve field compaction, provide more consistent pavement density across an entire pavement, or increase the final in-place density of the pavement. A less variable, better compacted asphalt pavement should have improved performance overall. As documented in Volumetric Requirements for Superpave Mix Design (NCHRP Report 567), better compacted asphalt pavements often have superior fatigue and rutting performance. Greater pavement density also can decrease the permeability of asphalt mixtures, which would decrease the amount of field aging in the mixture and improve performance in terms of cracking and moisture susceptibility.

Lower Temperature, Increased Moisture Susceptibility?

Reducing the production temperature of HMA without the additional implementation of materials handling and production best practices might lead to incomplete drying of the aggregate, which could have negative implications for pavement performance. There is concern that HMA pavements might be more susceptible to moisture if the aggregate is not completely dry, and early rutting could occur due to reduced production aging of the binder. There also needs to be a way to ensure the effectiveness and long-term performance of WMA technologies introduced into the marketplace.

To be sure, moisture damage is also possible in HMA mixtures, but it could be exacerbated in WMA due to production issues. Inadequately dried aggregates at lower production temperatures, plus the possible introduction of additional moisture (although small in amount, typically less than 1.5 percent of the weight of the binder) to the WMA from the various WMA foaming or emulsion technologies, creates a concern that moisture could displace asphalt in coating certain kinds of aggregate. This displacement could affect the asphalt binder-to-aggregate adhesion, increase asphalt stripping and moisture susceptibility, and generally reduce mixture performance.

The extent to which WMA technologies and additives affect moisture sensitivity will depend on a number of conditions, both regional (climate, aggregate type, and asphalt binder source) and pavement-specific (traffic loading, density, permeability, and general pavement integrity). This problem is not unique to mixes produced at lower temperatures, however, and in the past engineers have treated HMA effectively to resist stripping (when the liquid asphalt binder strips away from the aggregate it is coating due to the presence of moisture).

Reduced binder aging could reduce the cracking of pavement later in its life, although recent evaluations of in-place WMA pavements show they reach a similar aged condition as HMA pavements after 2 or 3 years in service. Another concern is that the reduced aging of WMA in the early stages of a pavement's life could contribute to loss of stability in hot weather and increase susceptibility to rutting. However, approaches to materials and mix type selection may provide effective solutions.

Mixture design/selection strategies, such as increasing the high-temperature asphalt binder grade or selecting rut-resistant mixtures like stone matrix asphalt, are a couple of strategies being explored. Use of more angular aggregate will provide greater internal friction to the mix, which in turn increases its shear strength without only relying on the cohesion of the binder. Using a higher high-temperature, performance-graded binder—essentially grade bumping—can counteract the effects of reduced plant oxidation during mixing if it is needed.

Test data on moisture damage and rutting performance often show contradictory results between the laboratory testing versus the field. The Mobile Asphalt Lab, State agencies, and other researchers have found that in the lab WMA mixtures often fail to meet the Hamburg Wheel Tracking Device test criteria for maximum allowable rut depth when immersed in a conditioning water bath, yet moisture damage and rutting have not commonly been witnessed in the field.

Researchers report similar results (failures of WMA in the lab but not in the field) when using the AASHTO T 283 test procedure and comparing the tensile strengths—the greatest longitudinal stress a substance can bear without tearing apart—of WMA and HMA. In general, the WMA tensile strengths for both dry and conditioned specimens are lower than the HMA control specimens, and WMA specimens often do not pass the 80 percent tensile strength ratio typically required by most State agencies.

An evaluation of WMA projects by NCAT found that 16 of 27 ratios for WMA specimens that were compacted immediately without reheating did not meet the 80 percent minimum ratio. That is, the ratio of conditioned specimen strengths to dry specimen strengths was below 0.80, or the effects of being partially vacuum saturated, subjected to freezing, and soaked in warm water caused the tensile strengths to decrease by more than 20 percent. When NCAT compacted the specimens after reheating the mixture, 8 of the 27 did not meet or exceed the 80 percent minimum. Some of those constructed WMA pavements have shown raveling, which could indicate moisture damage.

These differences between laboratory test results and field performance suggest modifications to material preparation and/or test procedures might be required in the laboratory when evaluating WMA moisture damage and rutting susceptibility so that field conditions can be simulated properly. However, this issue raises some immediate questions. Should WMA mixture testing require different conditioning or test procedures than HMA mixtures when researchers expect that WMA should perform equal to or better than HMA? If researchers change testing protocols, are the modifications appropriate for countrywide adoption? Or should the researchers make regional modifications to reflect differences in climate, traffic, and virgin materials?

An Even Better Environment for WMA?

Current and pending regulations regarding greenhouse gas emissions are making the consideration of greater reductions in HMA production temperatures more attractive. Although production plant emissions have decreased significantly over the last 35 years due to pollution controls, further reductions in greenhouse gases will likely be mandated in the future.

Many environmental factors are driving development and implementation of WMA technologies globally. Nevertheless, for WMA to succeed in the United States, pavements must have equal or better performance compared to traditional HMA pavements. Engineers must be satisfied that WMA mixtures will be as strong and durable as current pavements.

So far, the future looks bright. "The warm-mix asphalt technologies have shown great promise as a new standard for asphalt production," FHWA's Stephanos says. "At FHWA, we are committed to working with our industry, State, and academic partners to continue to monitor and evaluate the long-term performance of this technology."

In addition, technicians and construction personnel need to acquaint themselves with the characteristics and behavior of the new materials. Further research is needed to measure the degree of environmental improvement, fundamental mix characteristics, and impact on performance of the new technologies.

"WMA technology provides an important tool to the pavement engineer," says Pete T. Grass, P.E., president of the Asphalt Institute. "With the widespread test sections around the country, designers and contractors alike now have a great opportunity to learn more about this promising practice that is revolutionizing the paving industry in North America."

Matthew Corrigan is an asphalt pavement engineer with FHWA's Office of Pavement Technology. He is FHWA's coordinator for investigation and implementation of WMA technologies, cochairman of the Warm Mix Asphalt Technical Working Group, and manager of the Mobile Asphalt Lab. Corrigan is a graduate of The Pennsylvania State University with a degree in civil engineering and is a licensed professional engineer in the Commonwealth of Virginia.

Dave Newcomb is the vice president for research and technology at NAPA. He is a licensed professional engineer in Minnesota.

Thomas Bennert is the senior research engineer at the Center for Advanced Infrastructure and Transportation at Rutgers, The State University of New Jersey. He is a member of the WMA working group and the Transportation Research Board's Committee AFK30: Characteristics of Nonbituminous Components of Bituminous Paving Mixtures.

For more information, contact Matthew Corrigan at 202-366-1549 or matthew.corrigan@dot.gov, Dave Newcomb at 301-731-4748, ext. 104, or dnewcomb@hotmix.org, or Thomas Bennert at 732-445-5376 or bennert@eden.rutgers.edu.