U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
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Office of International Programs
FHWA/US DOT (HPIP)
1200 New Jersey Ave., SE
Washington, DC 20590
The Federal Highway Administration provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.
Warm-mix asphalt (WMA) is a group of technologies that allow a reduction in the temperatures at which asphalt mixes are produced and placed. These technologies tend to reduce the viscosity of the asphalt and provide for the complete coating of aggregates at lower temperatures. WMA is produced at temperatures 20 to 55 °C (35 to 100 °F) lower than typical hot-mix asphalt (HMA). In 2007, a team of U.S. materials experts visited Belgium, France, Germany, and Norway to evaluate various WMA technologies through the Federal Highway Administration's International Technology Scanning Program.
The scan team learned that the benefits of WMA technologies include reduced fuel usage and emissions in support of sustainable development, improved field compaction, which can facilitate longer haul distances and cool weather pavement, and better working conditions. A range of technologies is available to produce WMA. European agencies expect WMA performance to be the same as or better than the performance of HMA. Although several areas need to be addressed as these technologies are adapted to U.S. materials and production practices, the scan team believes that the United States has no long-term barriers to WMA use. With additional research and trials, the team expects that highway agencies will allow WMA as an alternative to HMA.
Form DOT F 1700.7 (8-72)
Reproduction of completed page authorized
Prepared by the International Scanning Study Team:
John D'Angelo (cochair), FHWA
Eric Harm (cochair), Illinois DOT
John Bartoszek, Payne and Dolan, Inc.
Gaylon Baumgardner, Ergon, Inc.
Matthew Corrigan, FHWA
Jack Cowsert, North Carolina DOT
Thomas Harman, FHWA
Mostafa (Moe) Jamshidi, Nebraska Department of Roads
Wayne Jones, Asphalt Institute
Dr. Dave Newcomb, National Asphalt Pavement Association
Dr. Brian Prowell (report facilitator), Advanced Materials Services, LLC
Ron Sines, P.J. Keating Company
Bruce Yeaton, Maine DOT
Federal Highway Administration
U.S. Department of Transportation
American Association of State Highway and Transportation Officials
National Cooperative Highway Research Program
The International Technology Scanning Program, sponsored by the Federal Highway Administration (FHWA), the American Association of State Highway and Transportation Officials (AASHTO), and the National Cooperative Highway Research Program (NCHRP), evaluates innovative foreign technologies and practices that could significantly benefit U.S. highway transportation systems. This approach allows advanced technology to be adapted and put into practice much more efficiently without spending scarce research funds to re-create advances already developed by other countries.
FHWA and AASHTO, with recommendations from NCHRP, jointly determine priority topics for teams of U.S. experts to study. Teams in the specific areas being investigated are formed and sent to countries where significant advances and innovations have been made in technology, management practices, organizational structure, program delivery, and financing. Scan teams usually include representatives from FHWA, State departments of transportation, local governments, transportation trade and research groups, the private sector, and academia.
After a scan is completed, team members evaluate findings and develop comprehensive reports, including recommendations for further research and pilot projects to verify the value of adapting innovations for U.S. use. Scan reports, as well as the results of pilot programs and research, are circulated throughout the country to State and local transportation officials and the private sector. Since 1990, about 70 international scans have been organized on topics such as pavements, bridge construction and maintenance, contracting, intermodal transport, organizational management, winter road maintenance, safety, intelligent transportation systems, planning, and policy.
The International Technology Scanning Program has resulted in significant improvements and savings in road program technologies and practices throughout the United States. In some cases, scan studies have facilitated joint research and technology-sharing projects with international counterparts, further conserving resources and advancing the state of the art. Scan studies have also exposed transportation professionals to remarkable advancements and inspired implementation of hundreds of innovations. The result: large savings of research dollars and time, as well as significant improvements in the Nation's transportation system.
Scan reports can be obtained through FHWA free of charge by e-mailing firstname.lastname@example.org. Scan reports are also available electronically and can be accessed on the FHWA Office of International Programs Web Site at www.international.fhwa.dot.gov.
All Publications are Available on the Internet at www.international.fhwa.dot.gov.
A number of new technologies have been developed to lower the production and placement temperatures of hot-mix asphalt (HMA). Generically, these technologies are referred to as warm-mix asphalt (WMA). WMA has been used in all types of asphalt concrete, including dense-graded, stone matrix, porous, and mastic asphalt. It has also been used in a range of layer thicknesses, and sections have been constructed on roadways with a wide variety of traffic levels
A team of 13 materials experts from the United States visited four European countries-Belgium, France, Germany, and Norway-in May 2007 to assess and evaluate various WMA technologies. The team learned about a wide range of technologies, discussed with various agencies how and why they were implementing these technologies, visited construction sites, and viewed in-service pavements. A number of factors were consistently identified as driving WMA development in Europe:
A number of potential benefits from the use of WMA were discussed:
Reduced emissions: Data indicate plant emissions are significantly reduced. Typical expected reductions are 30 to 40 percent for CO2 and sulfur dioxide (SO2), 50 percent for volatile organic compounds (VOC), 10 to 30 percent for carbon monoxide (CO), 60 to 70 percent for nitrous oxides (NOx), and 20 to 25 percent for dust. Actual reductions vary based on a number of factors. Technologies that result in greater temperature reductions are expected to have greater emission reductions.
Reduced fuel usage: Burner fuel savings with WMA typically range from 11 to 35 percent. Fuel savings could be higher (possibly 50 percent of more) with processes such as low-energy asphalt concrete (LEAB) and low-energy asphalt (LEA) in which the aggregates (or a portion of the aggregates) are not heated above the boiling point of water.
Paving benefits: Paving-related benefits discussed included the ability to pave in cooler temperatures and still obtain density, the ability to haul the mix longer distances and still have workability to place and compact, the ability to compact mixture with less effort, and the ability to incorporate higher percentages of reclaimed asphalt paving (RAP) at reduced temperatures.
Reduced worker exposure: Tests for asphalt aerosols/ fumes and polycyclic aromatic hydrocarbons (PAHs) indicated significant reductions compared to HMA, with results showing a 30 to 50 percent reduction. It should be noted that all of the exposure data for conventional HMA were below the current acceptable exposure limits.
Before the scanning study, the team members believed that they were aware of all of the processes for producing WMA. However, new processes were identified and more are under development in the United States and abroad. A number of methods are used to classify these technologies. One is classifying the technologies by the degree of temperature reduction. Warm asphalt mixes are separated from half-warm asphalt mixtures by the resulting mix temperature. There is a wide range of production temperatures within warm mix asphalt, from mixes that are 30 to 50 C° (55 to 85 F°) below HMA to temperatures slightly above 100 °C (212 °F).
Another way to classify the technologies is those that use water and those that use some form of organic additive or wax to affect the temperature reduction (this classification method allows for a more descriptive discussion of the processes).
Processes that introduce small amounts of water to hot asphalt, either via a foaming nozzle or a hydrophilic material such as zeolite, or damp aggregate, rely on the fact that when a given volume of water turns to steam at atmospheric pressure, it is dispersed in hot asphalt, which results in an expansion of the binder and a corresponding reduction in the mix viscosity. The amount of expansion depends on a number of factors, including the amount of water added and the temperature of the binder.
The processes that use organic additives (waxes or amides) show a decrease in viscosity above the melting point of the wax. The type of wax must be selected carefully so that the melting point of the wax is higher than expected in-service temperatures (otherwise permanent deformation may occur) and to minimize embrittlement of the asphalt at low temperatures.
Laboratory and field performance data were presented in three of the countries visited. Based on the laboratory and short-term (3 years or less) field performance data, WMA mixes appear to provide the same performance as or better performance than HMA. Poor performance was observed on limited sections in Norway; the poor performance was not directly attributed to WMA use. Both France and Germany have established procedures for evaluating new products such as WMA that combine laboratory testing and controlled field trials where performance is monitored.
WMA technologies have been used with all types of asphalt mixtures, including dense-graded asphalt, stone matrix asphalt (SMA), and porous asphalt. WMA has been used with polymer-modified binders and in mixes containing RAP. WMA has been placed on pavements with high truck traffic, up to 3,500 heavy vehicles per day, which over a 20-year design period would be expected to exceed 30 million 18-kip-equivalent single-axle loads. WMA has also been placed at bus stops, on airfields, and on port facilities.
The European agencies visited during the scan expect the same performance from WMA as HMA. One factor that may affect the willingness of European agencies to allow WMA is that short, 2- to 5-year workmanship warranties are included in most European paving contracts. Further, evaluation systems are in place in France and Germany to assess and approve new products. A similar process, combining laboratory performance tests and controlled field trials, should be developed in the United States for WMA as well as other innovative process. The evaluation process should be implemented on a national level.
Overall, the use of WMA in Europe was not as high as the scan team expected. It should be emphasized that WMA, in most cases, is allowed but not routinely used. Discussions with contractors and agencies suggested two reasons for this. First, many of the oldest WMA sections are just exceeding the period of the workmanship warranty. The contractors who developed these processes want to develop confidence in their long-term performance before using WMA widely. Second, in most cases WMA still costs more than HMA, even when fuel savings are considered. Representatives of the French Department of Eure-et-Loir noted that they were willing to pay more for WMA because they believed it would last longer.
During the scanning study, the team noticed several differences between typical European practices for the design, production, and placement of asphalt mixtures. In addition, the team also noticed differences between European and U.S. contractors. The water absorption of aggregates used to produce asphalt mixtures was generally less than 2 percent in the countries visited and less than 1 percent in France. The water absorption of aggregates used to produce asphalt mixtures in parts of the United States is higher, up to 5 percent. The Europeans experienced with the production of WMA repeatedly emphasized that the coarse aggregate must be dry. The higher the aggregate water absorption, the more difficult it may be to completely dry the aggregate at lower production temperatures.
The contractors in the countries visited routinely blend or modify binders. By comparison, the United States has invested heavily in the performance grade (PG) binder system with supplier certification. Several WMA processes modify the binder, and the grading of the binder may be affected in all cases because of reduced aging during production. Throughout Europe, performance tests play a broader role in the mix design process. Performance tests enable European agencies and contractors to better assess innovative products, such as WMA, before conducting field trials.
Based on the countries visited, batch plants and in some cases smaller drum plants appear to be more prevalent in Europe. Increased drying may result at reduced WMA production temperatures in a batch plant, compared to a drum plant at the same reduced temperature. This may be an advantage when producing WMA. Although some differences in placement practices were observed between Europe and the United States, placement practices did not differ between HMA and WMA, with the exception of lower compaction temperatures. Finally, European contractors appear to be better equipped in terms of research and development capabilities than U.S. contractors. This capability aids European contractors in developing and selecting innovative materials like WMA.
The scan team identified a number of challenges as the United States moves forward with the implementation of WMA. Challenges include verifying performance, evaluating and approving products, specifying changes, adapting to high production rates, ensuring dry coarse aggregate, and selecting technologies with the widest range of application by contractors. The team considers none of these challenges insurmountable.
The United States has already made great strides in evaluating WMA. A WMA Technical Working Group (TWG) has been established to oversee the implementation of WMA. A large number of trial sections and demonstration projects have been completed. In some cases, WMA has been used in production paving.
Based on the team's experience, the United States has no long-term barriers to WMA use. Many elements of WMA still need to be investigated. The consensus among the scan team members is that WMA is a viable technology and that U.S. highway agencies and the HMA industry need to cooperatively pursue this path. Key implementation goals include the following: