Warm Mix: Green Is Cool

Warm-mix asphalt is generating more avid interest than any other new or imported technology in this industry in recent memory. From coast to coast, dozens of road agencies and contractors are collaborating on field demonstrations to test warm-mix performance. Virtually all of the demonstrations have been successful thus far. What’s more, Europeans have used warm mix for some 10 years with acceptable results.

Warm mix typically uses an additive or process that permits mix production at temperatures of 50 to 100 degrees Fahrenheit below standard hot-mix temperatures of 300 to 350 F. Gone are the blue smoke, fumes and odor that may be associated with hot mix. Stack gas emissions are even further reduced. Fuel consumption drops by 11 to 30 percent. Workability is improved, so compaction is easier, even with reclaimed asphalt pavement (RAP) in the mix.

There’s more. You may be able to increase the amount of RAP in mixes. Both haul distances and the paving season may be extended. And because the mixes are produced at lower temperatures, the binder may age less in the production process. That factor would reduce thermal cracking and improve long-term durability. There has been some concern with moisture susceptibility, which has not shown up in any of the field trials.

Rapid Progress

Warm-mix asphalt first came to the U.S. as a result of a study tour to Europe organized by NAPA in 2002. On the tour, NAPA leaders looked at various technologies that were being developed in France, Germany and Norway. A seminar on the topic at NAPA’s 2003 Annual Convention drew a standing-room-only crowd, and interest in the subject has risen ever since.

"Warm mix has been tried on stone-matrix asphalt (SMA), open-graded friction courses and dense-graded mixes, all with success," says David Newcomb, NAPA vice president of research and technology.

"Warm mix is progressing very rapidly," says Newcomb. "The whole technology has taken off very quickly, and the implementation will be much wider than with SMA, for example."

Federal and regional air quality regulations already limit emissions in non-attainment areas. "Warm mix gives contractors in these (non-attainment) areas opportunities to produce asphalt and avoid problems with emissions regulations," said Randy West, director of the National Center for Asphalt Technology (NCAT).

Moreover, West says, warm mix offers reduced energy costs because less fuel is needed to produce a ton of asphalt. That will give contractors an incentive to use warm-mix additives or processes on their own.

Under sponsorship by NAPA, NCAT undertook the first U.S. research on warm mix. Since then, NCAT has published reports on a number of the warm-mix technologies. These research reports are available at www.ncat.us.

"People are looking at the lowered emissions and fuel savings as two of the benefits, but now contractors are looking at how they can use warm mix as part of the paving process," said Larry Michael, an asphalt consultant who represents warm-mix additive manufacturer Sasol Wax. "Contractors are looking for ways to improve workability with RAP or with polymer mixes, for a way to run more RAP, and ways to improve density."

Long-term performance has not been proven in this country, but there are new studies under way to do that. And as Newcomb points out, a mix design procedure needs to be developed that incorporates warm-mix temperatures. That too is happening under NCHRP 9-43. (NCHRP is the National Cooperative Highway Research Program.)

"None of the warm-mix technologies are a cure for everything," cautions Michael. "If you have a very stiff RAP, you can’t reduce the temperatures quite as much. We have to be careful not to oversell the technologies. You’re not going to take a stiff mix and improve density and drop the mix temperature by 100 degrees. All those things aren’t going to happen."

Test results so far, though, indicate that warm mix will perform well. Crews placed two sections of Evotherm warm mix and a control on the NCAT Test Track in fall 2005, said Andrea Kvasnak, Ph.D., a research engineer at NCAT. After a half-million equivalent single axle loads (ESALs), the average rut depth for the Evotherm sections was 0.9 millimeters; for the Evotherm-plus 3 percent-latex section the average rut depth was also 0.9 millimeters; and the hot-mix control section had an average rut depth of 1.1 millimeters. The rut depths were determined via a wire-line.

Questions have been raised about the tensile strength ratio (TSR) of warm mix. If the TSR ratings fall too low, that would indicate a tendency for moisture damage to be occurring. However, Brian Prowell, Ph.D., a researcher for Advanced Materials Services, said more than 20 cores taken from four locations show no indications of water damage after durations ranging from four months to a year after construction. The four locations were an Aspha-min stretch in Orlando, FL; an Evotherm section at the NCAT test track; Hall Street in St. Louis, MO, where Aspha-Min, Sasobit and Evotherm were placed; and Ryan Road in Milwaukee, WI, where Sasobit and Evotherm were used.

Demonstration Projects

Warm-mix trial pavement sections recently have been constructed in New Jersey; San Antonio, TX; Yellowstone National Park; Watsonville, CA; Nashville, TN; and Vancouver, British Columbia.

Wharton, NJ –

Contractor Tilcon New Jersey paved a quarry road with 350 tons of warm mix produced at its Mt. Hope facility in New Jersey. Tilcon, which is owned by Oldcastle Materials, held a two-day best practices seminar and paved using the warm mix additive Sasobit in a 30-percent RAP mix – on the second day. Attendees included Oldcastle employees, county and municipal engineers, as well as officials from the New Jersey Turnpike and the Port Authority of New York and New Jersey. Both Larry Michael and NCAT’s Randy West made presentations.

"We produced the first 100 tons at 265 to 270 degrees (Fahrenheit), then dropped the temperature to 240 to 250 based on the nuclear density readings," said Scott Laudone, Tilcon’s general manager of asphalt operations and quality control. "We found we got better compaction as the temperature dropped."

Both Laudone and quality control manager Rebecca Guardino agreed that the Sasobit improved workability – even with 30-percent RAP. Tilcon added an antistrip additive to the mix because stripping has been a problem with some RAP mixes in that region, Laudone said.

"The obvious advantage is environmental – you get zero smoke at the paver," says Laudone. "I see warm mix as a win-win for paving inside of tunnels, because the smoke is gone. One of the reasons we went with Sasobit was that it can be added at the refinery."

San Antonio, TX –

The American Public Works Association’s street construction demo of warm mix drew some 250 people last September. "We’ve done about 5,000 tons of warm mix through various demos, so our plant people are very comfortable with the process," said Harry Bush of Vulcan Materials, which supplied the mix. "The temperature of the mat under the paver was about 100 degrees less than normal hot mix. And compaction went very smoothly." 

Yellowstone National Park, WY –

Contractor Tilcon New Jersey paved a quarry road with 350 tons of warm mix produced at its Mt. Hope facility in New Jersey. Tilcon, which is owned by Oldcastle Materials, held a two-day best practices seminar and paved using the warm mix additive Sasobit in a 30-percent RAP mix – on the second day. Attendees included Oldcastle employees, county and municipal engineers, as well as officials from the New Jersey Turnpike and the Port Authority of New York and New Jersey. Both Larry Michael and NCAT’s Randy West made presentations.

At the eastern entrance to the park and the road to Sylvan Pass, the demo used 9,000 metric tons of asphalt on each of three sections – a control section, a Sasobit section and a section of Advera WMA. Because of the remoteness of the park, the mixes had to be hauled about 90 minutes from a portable plant in Cody, WY. Still, paving crews achieved good densities: Advera WMA average – 93.9 percent of maximum theoretical density; Sasobit average – 93.4 percent; and the control – 93.2 percent. "Density was not difficult to achieve, even at much lower mixture temperatures," said Neitzke.

He said it did seem somewhat difficult to maintain lower mix temperatures; production temperatures had a tendency to jump from 250 to 260 degrees Fahrenheit. Tests showed that aggregates were adequately dried, even at the lower temperatures. Moisture contents ran below the maximum of 0.5 percent for both the warm mixes and control mix.

The FHWA posted its mobile asphalt testing lab at the Yellowstone demonstration, and ran a full suite of tests including dynamic modulus and flow number, said Matthew Corrigan, an FHWA asphalt paving engineer. Results were not available at time of publication.

"We want performance from warm mix that is equal to hot mix, and based on the information we have, the performance is there," said Neitzke. "Warm mix has done well in Europe for 10 years."

Franklin, TN –

Country music stars living on a section of Hillsboro Pike near Nashville received a demonstration of warm-mix asphalt, courtesy of the Tennessee DOT and LoJac Enterprises. Between 700 and 1,200 tons each of four different warm mixes – using Astec Double Barrel Green System and the additives Sasobit, Evotherm DAT and Advera WMA – were applied to the road last October. The Astec System employs two banks of nozzles to inject a small quantity of water (approximately 0.1 percent by weight of the total mix) and air on opposite sides of the stream of liquid asphalt. The water is converted to steam by the heat of the liquid asphalt, and foamed asphalt is created as it is introduced into the outer drum of the asphalt plant. The foam collapses around the aggregate and the water evaporates during the production and construction process.

"On non-attainment days, when the air quality is bad, we often get shut down and are unable to pave," said Brian Egan, assistant director of construction for TDOT. "But with warm mix, because the emissions are reduced, we might be able to pave even on days when the air quality is not the best."

Watsonville, CA –

Accelerated performance testing has been launched on three warm- mix sections and a control at Graniterock’s quarry in Aromas, near Watsonville. Working for Caltrans, the University of California–Davis has set up its Heavy Vehicle Simulator (HVS) over the four 150-foot-long sections that used additives Advera WMA, Sasobit and Evotherm DAT, says Mike Cook, Graniterock’s director of quality services.

The pavement section is enclosed where the HVS runs, and the temperature inside is elevated to 100 degrees Fahrenheit. The HVS is a framework approximately 80 feet long with a weighted tire that runs in one direction over the pavement, then is lifted, returns, and runs on the pavement again. "Our real emphasis is on trying to see how the in-place mixes perform under stress."

Cook says each warm-mix additive has a specific temperature range that best suits the mix for placement. The small quantities used for the California demo didn’t permit the contractor to learn exactly the best temperature and dosage for each warm-mix additive, Cook said. "If you could make some mixes over time, you could optimize the best temperature and dosage combination," he said.

Cook said warm mix has two advantages. First, compaction is easier to achieve. He said compaction could have been achieved at even lower temperatures than Graniterock used, which was 240 to 250 degrees Fahrenheit. Second, you can increase production at the plant. "You can put out more mix per hour at lower temperatures," said Cook.

Just about all traffic signals are green for warm mix, with a few caution lights tossed in. "As long as everybody adheres to best practices, controls moisture content, controls volumetric properties and other mix design parameters, then warm-mix technologies look good," says Corrigan of FHWA. "It’s too early to tell if the long-term performance is there. But we haven’t seen anything to date that would indicate any issues we have with warm mix."