But one consequence of the legislation was that in order for Caltrans to take advantage of the newfound benefits of warm mix asphalt technology, engineers would have to determine whether asphalt rubber could be used successfully with them.
On Sept. 15, 2009, Caltrans used warm mix asphalt technology in a rubberized hot mix asphalt-gap graded mix on a traveled way, U.S. 101 south of Eureka. “This use of warm mix asphalt technology was introduced for a one-day trial to evaluate RHMA-G production and placement at lower [ambient] temperatures,” wrote Caltrans District 1’s Wesley Johnson and Michael Stapleton in the March 2010 bulletin, Caltrans News.
Success with this technology would mean that many other projects using rubberized hot mix asphalt could have success in such cooler environments in the future. Typically RWMA-G costs Caltrans 15 to 20 percent more than conventional HMA, but the longevity is double, on average, so it more than pays for itself in reduced maintenance.
“Placement of rubberized HMA on the North Coast has always been challenging, due to the low ambient air temperatures and wet, misty conditions,” they wrote. “In addition, air quality has always been a sensitive issue in this pristine environment, and previous rubberized asphalt projects have met opposition due to odors emitted from the rubber product during production, transport and placement.”
On the other hand, warm mix asphalt technology used with rubberized hot mix production process allows the RHMA-G to be produced at lower temperatures, provides an overall energy savings, and lowers hydrocarbon emissions and off-gassing of noxious fumes, Johnson and Stapleton said.
“The use of rubberized hot mix is a ‘win-win’ for the environment and maximizes scarce road-maintenance dollars due using reduced required pavement thicknesses for pavement rehabilitation,” they said.
Performance of rubberized WMA
By 2010, the performance of rubberized WMA in California had become evident. For example, the benefits of combining asphalt rubber and warm mix asphalt technologies were articulated by R. Gary Hicks, Ph.D., P.E., DingXin Cheng, Ph.D., P.E., and Tyson Teesdale, all of the California Pavement Preservation Center at California State University-Chico in a presentation at the January 2011 Transportation Research Board meeting, Assessment of Warm Mix Technologies for Use with Asphalt Rubber Paving Applications
“The potential savings in energy and reduction in greenhouse gas emissions could be great,” they said. “The overall field testing and research results demonstrate that there are a wide range of benefits which can be attained by using warm mix technology in rubberized asphalt mixes. These may include reduced fuel usage and emissions, compaction aid, longer paving season, night paving, long hauling distances, and improved working conditions.”
Hicks, Cheng and Teesdale found:
- Warm mix technologies can be used with asphalt rubber mixes, allowing the mixes to be placed at night and in cooler climates
- Warm mix technologies can increase the workability of asphalt rubber mixes, extending the paving season and allowing their use where asphalt rubber could not be used before
- Warm mix technologies can improve workers’ working conditions, reducing undesired asphalt rubber odor and blue smoke coming associated with conventional asphalt rubber placements
- Warm mix technologies can reduce fuel usage because they reduce production temperatures by 30 to 80 deg F (19 to 45 deg C), with energy-saving benefits for asphalt rubber mixes
- Warm mix technologies reduce emissions at both production and paving sites, with reduced carbon footprint and greenhouse gases emitted
“To date, the initial performance of warm mixes asphalt rubber placed in California is good,” they said in January 2011. “More agencies should consider the use of warm mixes with asphalt rubber for night time construction and for cool climates.”