University of California Pavement Research Center

Warm mix asphalt (WMA) is a relatively new technology. It was developed in response to needs for reduced energy consumption and stack emissions during the production of asphalt concrete, improved workability and compaction after long hauls and when using lower placement temperatures, and better working conditions for plant and paving crews. Studies in the United States and Europe indicate that significant reductions in production and placement temperatures, and, potentially, in related emissions are possible. However, concerns exist about how these lower production and placement temperatures could influence asphalt binder aging and, consequently, short- and longterm performance, specifically rutting. The overall objective of this warm mix asphalt study was to determine whether the use of technologies that reduce the production and construction temperatures of asphalt concrete mixes influences performance of the mix. The testing completed in this warm mix asphalt study provided no results to suggest that warm mix technologies should not be used in conventional, gap-graded asphalt rubber, and open-graded friction course mixes in California, provided that standard specified mix design, construction, and performance limits for hot mix asphalt are met. The use of warm mix asphalt has clear benefits when compared to hot mixes. These include significant reductions in, or even elimination of, smoke and odors, lower emissions, improved workability, better working conditions, and better performance on projects with long hauls or where mixes are placed under cool conditions. The slightly higher costs of using warm mix technologies are outweighed by these benefits. Based on the findings of this study, the use of warm mix asphalt technologies in asphalt mixes is encouraged, especially on asphalt rubber projects, projects in urban areas, and on projects with long hauls and/or where mixes are placed under cool conditions. Given that warm mix asphalt may be produced at significantly lower temperatures than hot mix asphalt (with associated lower aggregate heating temperatures), moisture sensitivity, especially on water-based warm mix asphalt technologies, should be closely monitored in mix design and quality control/quality assurance testing.