Paving Contractors Offer Warm Welcome to Warm Mix

Wed August 27, 2008 - National Edition
Daniel C. Brown




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 to do field demonstrations and 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 F 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. Haul distances and the paving season may both 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 intensified ever since.

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

“Warm-mix is progressing very rapidly,” said 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 said, 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 and the State Asphalt Pavement Associations, NCAT undertook the first U.S. research on warm-mix. Since then, NCAT has published reports on a number of 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, a research engineer at NCAT. After a half-million equivalent single axle loads (ESALs), the average rut depth for the Evotherm sections was 0.035 in. (0.09 cm); for the Evotherm-plus-3 percent-latex section the average rut depth also was 0.035 in.; and the hot-mix control section had an average rut depth of 0.043 in. (0.11 mm). 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, 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, Fla.; an Evotherm section at the NCAT test track; Hall Street in St. Louis, where Aspha-Min, Sasobit and Evotherm were placed; and Ryan Road in Milwaukee, where Sasobit and Evotherm were used.

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

Wharton, N.J. — Mt. Hope Facility

Contractor Tilcon New York paved a quarry road with 350 tons (318 t) 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 F, 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,” said 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, Texas — the APWA Convention

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.”

Wyoming — Yellowstone National Park

“Our clients are the National Park Service and the National Forest Service, and should this warm-mix technology prove itself, the nature of our work would be enhanced, because the areas we work in are environmentally sensitive,” said Brad Neitzke, a materials engineer for the Federal Lands Highway, a division of FHWA. “Certainly, warm-mix improved working conditions at the paving site. The crew’s first reaction was to say, ’There’s no smoke!’”

At the eastern entrance to the park and the road to Sylvan Pass, the demo used 9,920 tons (9,000 t) 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, Wyo. 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 F. 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 this writing.

“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, Tenn. — Hillsboro Park

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 (635 and 1,089 t) 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, Calif. — Graniterock’s Quarry

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-ft.-long (45.7 m) sections that used additives Advera WMA, Sasobit and Evotherm DAT, said 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 F.

The HVS is a framework approximately 80 ft. (24.3 m) 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 said 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 the experience 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. For one, compaction is easier to achieve. He said compaction could have been achieved at even lower temperatures than Graniterock used, which was 240 to 250 F. Second, you can increase production at the plant.

“You can put out more mix per hour at lower temperatures,” said Cook.

Nearly 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,” said 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.”

This article was reprinted from HMAT magazine, January/February 2008 edition, Volume 13, Number 1.