The hottest topic in asphalt paving to day is really not all that hot. And that's the point. Whereas hot mix asphalt is normally mixed and placed at temperatures in excess of 300 degrees Fahrenheit, warm mix asphalt is mixed and placed at temperatures around 250 degrees Fahrenheit. That difference, though it comes at a cost, can potentially mean substantial savings in energy and the ability to lay asphalt in colder ambient temperatures, thus possibly extending the paving season in regions — such as most of Mountain America — where cold weather limits the season.
Like other paving technologies adopted in this country in recent years (stone matrix asphalt, for example), this one traces its roots to Europe, where it has been successfully utilized for several years. Three different technologies have been developed and employed in European countries to produce warm mix asphalt (WMA):
- The addition of a synthetic zeolite called Advera® (and previously known as Aspha-Min®) during mixing to create a foaming effect in the binder.
- The use of organic additives such as Sasobit®, a Fischer-Tropsch process paraffin wax, and Asphaltan B®, a low molecular weight esterfied wax.
- A two-component binder system called WAM-Foam® which introduces a soft binder and hard foamed binder at different stages during plant production.
Additionally, a fourth technology has been developed and used in the United States to product WMA:
- Plant production with an asphalt emulsion product called Evotherm™, which uses a chemical additive technology and a "dispersed asphalt technology" delivery system.
All four of these technologies allow the production of WMA by reducing the viscosity of the asphalt binder at a given temperature. This reduced viscosity allows the aggregate to be fully coated at a lower temperature than traditionally required in hot mix asphalt production.
This summer, the Federal Highway Administration's Western Federal Lands Highway Division (FLH) utilized two of the WMA technologies — Advera and Sasobit — along with a control section of HMA on an ongoing project to reconstruct the east entrance road in Yellowstone National Park.
The final phase of the Yellowstone east entrance road reconstruction, segment C, covers 6.93 miles of the highway just inside the east entrance on US-14/16/20 and up the east side of Sylvan Pass to its summit. Previous projects had widened and reconstructed the remainder of the two-lane road over the pass. The segment C project was awarded to HK Contractors of Idaho Falls, Idaho, on May 26, 2005, and was originally scheduled for completion by October 5, 2006, but a massive landslide during the first year of work caused the loss of a full construction season and delayed completion by a year, to October 2007. With the widening, drainage improvement and rock retaining wall work completed earlier this year, paving was finally scheduled for August and September, with a total of 28,430 metric tons of asphalt to be placed.
FLH negotiated a change order with HK Contractors to conduct the WMA demonstration on this project. The 4 inches of asphalt pavement on the seven-mile stretch of highway were to be placed in two 2-inch lifts. A control section of traditional Superpave HMA was to be placed first, followed by a section of Advera WMA and a section of Sasobit WMA, the three segments to be roughly equal in length.
Because the National Park Service had terminated the contractor's use of a convenient source of aggregate at the summit of Sylvan Pass, and because the asphalt plant could not be set up within the boundaries of Yellowstone National Park, nor in the popular recreation area along the Shoshone River east of the park, the only practical source of aggregate was a long-used pit on the west edge of Cody, 60 miles from the park. The pit, however, provided an ideal location for the asphalt plant. But trucking the HMA/WMA 60 miles to the paving site over a two-lane highway heavily traveled by tourists presented another challenge, requiring a sizable fleet of belly-dumps and a patient group of drivers. Adding to the difficulty of delivering the asphalt to the paver in a steady flow was the necessity of closing one side of the two-lane highway as the other side was paved, thus restricting traffic to one direction at a time and trapping both loaded and empty asphalt trucks in the string of waiting vehicles. This situation, unfortunately, could not be avoided and meant the paver was forced to make frequent stops during the paving process.
Even though this was one of the first WMA demonstrations in the West and the largest such project yet in the United States (the Colorado Department of Transportation conducted a much smaller, 1,000-ton test of Advera on the I-70 west approach to Eisenhower Tunnel this summer), the volume of the Advera and Sasobit additives involved was not great enough to warrant the use of bulk material stored in a dedicated silo, so a small pneumatic delivery device was employed for the Advera that allowed large bags of the additive to be fed to HK's older Barber-Greene drum-mix plant at the rate of approximately 5 pounds of Advera per 1,000 pounds of WMA mix. The delivery hose was connected to the end of the drum-mixer just below the liquid asphalt feed line, so the hot oil fell through the additive, creating a thorough distribution before the oil began coating the aggregate (to which, in standard FHWA practice, lime had been added).
Additionally, the small volume of warm mix did not warrant making changes to HK's drum-mix plant, which has large burner jets designed to produce temperatures in excess of 350 degrees Fahrenheit. Consequently, when producing the warm mix, the plant would be started in the morning and initially produce the mix at temperatures higher than necessary, gradually cooling down during the day until the mix was being produced at temperatures as low as 245 degrees Fahrenheit.
The mix itself handles no differently than a standard Superpave hot mix, and HK used a Caterpillar AP-1055B paver fitted with a Barber-Greene BG-650 windrow elevator to pick up and lay the WMA. Breakdown compaction was done by a pair of Ingersoll Rand DD-130HF double-drum vibratory rollers, with secondary compaction by an Ingersoll Rand SD-105 single-drum vibratory compactor.
Placement of the hot mix control segment began on Aug. 21 and was completed on Aug. 23, allowing the Advera WMA placement to begin on Aug. 27. The photos here were taken during that work, which was completed by Aug.30, with placement of the Sasobit WMA undertaken during the week of Sept. 4.
FHWA and the Western Federal Lands Highway Division will closely monitor the three pavements over their lifetime to determine the viability of the two warm mix options.