Evaluating Warm-Mix Asphalt Field Performance
Warm-mix asphalt (WMA) is gaining acceptance across the U.S., with at least 45 states either actively using WMA technology or having constructed a trial project. A number of states, including Alabama, California, Florida, Illinois, New York, North Carolina, Ohio, Pennsylvania, Texas, Virginia, Washington and Wisconsin, have adopted permissive specifications allowing the use of WMA on many highway projects.
Potential economic and environmental benefits, including less fuel usage and lower emissions, have spurred interest in using WMA technology to reduce production temperatures to 275°F or less. Early performance of WMA projects has been very good—comparable to that of conventional hot-mix asphalt (HMA).
A warm-mix asphalt (WMA) placement job in Indiana
Early WMA performance is favorable
Rutting and moisture susceptibility have been a concern for some WMA mixes. Lower temperatures could result in incomplete drying of the aggregate, compromising the bond between asphalt and aggregate. With reduced mixing temperatures there is also less binder aging, which can result in lower tensile strengths and/or increases in rutting in lab tests.
However, field evaluations have not shown any practical difference in rutting between WMA and control HMA sections. One case study is a WMA project in Missouri, which has no appreciable rutting in any of the WMA or HMA sections after two years of heavy traffic. The warm-mix technologies used in the Missouri trial were Aspha-min, Sasobit and Evotherm ET. In addition, no moisture damage was observed in the WMA sections evaluated and tensile strength of the WMA was shown to increase over time so that tensile strength was similar to the HMA after two years. A field trial in Ohio also revealed that WMA sections with Aspha-min, Sasobit and Evotherm performed as well as or better than HMA control mixes in laboratory testing for rutting and moisture susceptibility (Asphalt Pavement Analyzer, Hamburg rut testing and tensile strength ratio).
NCAT is working on NCHRP 9-47A, a project that will document field performance of WMA technologies across the U.S., compare engineering properties of WMA to HMA, and evaluate energy savings and emissions reductions for WMA production. Researchers will monitor short-term WMA performance over a two-year period and will use the Mechanistic-Empirical Pavement Design Guide (MEPDG) to predict long-term performance.
NCAT and Advanced Materials Services (AMS) are currently documenting 14 regionally diverse WMA projects and testing materials from the eight projects that are new construction (see Table 1). A variety of WMA technologies will be included. Documentation at several of the projects will include an asphalt plant energy audit and stack emissions testing, as well as workspace respiration monitoring to measure asphalt fume exposure levels for paving crew members. Six existing sites, as shown in Table 2, are being monitored for performance. Both new and existing sites have control HMA sections for comparison.