In July 2011 the workability and environmental benefits of warm mix additive were paired with the performance and environmental benefits of rubberized asphalt binder to create a high-performance pavement on scenic California 1 along the northern California coast south of Fort Bragg.
There, Evotherm DAT warm mix additive was used to ensure workability of a asphalt-rubber, gap-graded (RHMA-G) mix following a 2 1/2-hour drive across California’s Coast Ranges. Without the warm mix additive the length of the haul – in addition to the mix’s placement at night along the cool coastline – would have cooled to the point of losing workability.
As it eyes the potential benefits of warm mix asphalt (WMA), California has been studying WMA’s attributes. But there’s a catch: as the California DOT (Caltrans) is working under a legislated mandate calling for the use of increasing amounts of reclaimed rubber in asphalt pavements in future years, Caltrans had to determine whether – and which – warm mix additives would be compatible with asphalt-rubber binders.
Now, Caltrans materials engineers – working with stakeholders in the private sector – have determined that WMA will work with rubberized binders, and the California 1 section of rubberized warm mix asphalt utilizing Evotherm DAT additive placed in late July 2011 is proof.
Caltrans: WMA has benefits
After six years of study and field tests among the 12 differing climatic zones of the Golden State, warm mix asphalt technology has received strong endorsements from Caltrans.
“Hot mix asphalt production has many adverse consequences, including high energy consumption to maintain workable temperatures, and hazardous asphalt fumes that endanger workers at the plant and during construction,” said the Innovation Team (I-Team) at the Caltrans Division of Research and Innovation in an October 2010 brief, Warm Mix Asphalt.
“Asphalt mix producers seek energy-efficient, environmentally friendly, and worker-friendly methods,” the I-Team said. “The Caltrans I-Team strongly recommends the use of warm mix asphalt on paving projects throughout California because of its many proven improvements over hot mix asphalt.”
Benefits of WMA include, Caltrans’ I-Team said, improved mix compaction, a potentially longer construction season, and longer haul distances for asphalt mixes without loss of workability.
And in April 2011, WMA’s position was further bolstered by the issuance of an approval process for new warm mix asphalt technologies.
“Warm mix asphalt technology allows the mixing and placement of asphalt mix temperatures significantly lower than those used with conventional hot mix asphalt,” said Cathrina B. Barros, P.E., chief, Independent Assurance and Reference Sample Program, Caltrans’ Division of Engineering Services, in releasing the approval process.
“Lower mixing and placement temperatures provide the following benefits: fuel conservation, lower emissions, reduced worker exposure to asphalt fumes, and [improved] workability of asphalt at lower temperatures,” Barros said in April 2011.
Asphalt rubber and WMA
While WMA in California is relatively new, use of rubber-modified asphalt is decades old both in California and elsewhere.
Caltrans began using asphalt rubber in chip seals in the 1970s and in hot mixes in the 1980s, reported Larry Santucci, P.E., pavement specialist, University of California Pavement Research Center, and California LTAP field engineer, Technology Transfer Program, Institute of Transportation Studies at University of California-Berkeley, in the September 2009 of the institute’s Pavement Technology Update newsletter.
Use of rubber in asphalt now is required by law in the state. “In October 2005, California Assembly Bill 338 was signed into law,” Santucci said. This bill originally called for the use of increasing amounts of crumb rubber from waste tires in HMA: 20 percent of total statewide HMA tonnage in 2007, 25 percent in 2010, and 35 percent in 2013. The bill later was revised to allow a variety of types of rubberized asphalt such as terminal blends.