Warm mix asphalt: proving Its benefits

 By Daniel C. Brown

“I’ve never seen anything take off as quickly as this warm mix asphalt process,” says Don Brock, chairman of Astec Industries. In recent months his company has sold more than 70 attachments that equip an Astec Double Barrel plant to run warm mix. From coast to coast, dozens of road agencies and contractors are doing field trial projects with warm mix asphalt. And late last year, two new warm mix technologies entered the market – Rediset™ WMX, and REVIX™.

Warm mix typically uses an additive or process that permits mix production at temperatures of 50 to 100 degrees lower than standard hot mix temperatures of 300 to 350 Fahrenheit. Gone are the blue smoke, fumes and odor 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.

What’s more, producers may be able to increase the amount of RAP in mixes. Haul distances and the paving season can both be extended. And because the mixes are produced at lower temperatures, the binder ages less in the production process. That factor should reduce thermal cracking and improve long-term durability.

Rapid Progress
“Warm mix has so many benefits that eventually it will become standard practice,” says Randy West, Director of the National Center for Asphalt Technology (NCAT). “And the key to getting there quickly is to realize the benefits without driving up the cost significantly.” Currently at least seven suppliers offer warm mix additives. They typically cost between $2.50 and $3 per ton of mix. That’s a drawback in these days of high binder costs. “The ones that survive will be the lowest cost alternatives that also perform well,” says West. “Low cost by itself will not win the day.”

  • Warm mix additives and suppliers in the U.S. include: Sasol Wax makes Sasobit, a wax-based additive
  • The Hubbard Group markets Aspha-min, a zeolite additive that releases small amounts of water into the mix
  • PQ Corporation markets Advera WMA, another zeolite additive
  • MeadWestvaco, which last year introduced Evotherm DAT, a delivery method that replaces its asphalt emulsion delivery with a chemical solution that is injected inline into the asphalt at the mixing plant
  • LEA-CO, a French firm, has signed with a distributor, McConnaughay Technologies, Cortland, New York, to distribute a warm mix additive and technology in New York state and eventually to other states. In Europe some 50,000 tons of LEA warm mix has been placed.
  • Akzo Nobel markets Rediset™ WMX, a warm mix additive that also functions as an anti-strip agent
  • REVIX technology, which was developed in the United States by Mathy Technology and Engineering Services Inc. and Paragon Technical Services Inc.

Brock promotes his process as the only one with no added cost to the mix. His system injects small quantities of water into the liquid asphalt stream of an Astec Double Barrel plant. The result is a foamed asphalt that collapses around the aggregates; the water evaporates. The benefits: lower fuel consumption, increased production and greater possible percentages of RAP in the mix. “Our system costs about $60,000, but it’s a one-time cost,” says Brock.

Running 50 Percent RAP
Astec and Southeastern Materials, a division of Talley Construction Company, collaborated last June to perform a 4,000-ton warm mix demonstration project for the city of Chattanooga, Tennessee. “The city milled out 2 inches of pavement. We fractionated it back to its original sizes and ran it at 50 percent recycle,” says Brock. “We ran at 270 degrees and the mix had no smoke and no odor.”

Running at lower temperatures enables Astec’s foamed asphalt technology to burn 11 percent less fuel – which permits the plant to increase its production rate at full burner. “If you use 11 percent less fuel, you can run 11 percent more tonnage. It’s as simple as that,” says Brock.

Foamed asphalt permits the liquid to disperse and coat the aggregate at temperatures below 285 degrees. That way the production process does not boil off the light oils present in the binder. “You avoid hardening of the new asphalt and that helps you to go to higher percentages of RAP,” Brock says. “Plus, if we drop the temperature, it cuts the oxidation of the asphalt dramatically.”

As well, he says the steam generated from drying the RAP keeps the virgin asphalt softer. All of those factors combine to permit running more RAP, which normally has a stiff binder that needs to be softened with the addition of softer binder.

“And you can do all that for no additional cost,” Brock says. In fact, the increased percentage of RAP saves money. He says a number of states are excited about Astec’s new foamed asphalt process. The state of Ohio, for example, has approved the use of Astec’s warm mix process.

Some industry leaders say that contractors can simply begin to use warm mix on their own—agencies will not need to specify it. Federal and regional air quality regulations already limit emissions in so-called nonattainment areas. “Warm mix gives contractors in these (nonattainment) areas opportunities to produce asphalt and avoid problems with emissions regulations,” says West.

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.
Good Performance
Long-term performance has not been proven in this country, but there are new studies under way to do that. 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.)

Test results so far indicate that warm mix will perform well. At the NCAT test track in Opelika, Alabama, two 200-foot-long sections of track were built with a warm mix containing Evotherm®, an emulsion-based technology. The lift thicknesses in both sections were one inch, and the two sections have been subjected to more than 5.6 million equivalent single-axle loadings.

Rut depths at the track, as of December 17, 2007, average less than 6 millimeters for both sections. Laboratory rutting results based on Asphalt Pavement Analyzer testing yielded an average rut depth of 7.85 millimeters. The control mix, a hot mix asphalt containing the same binder, yielded an average rut depth of 7.56 millimeters. The Asphalt Pavement Analyzer test consisted of 8,000 loaded wheel cycles in an air chamber.

In Europe, the performance of warm mix asphalt has generally proved equal to that of hot mix, says Brian Prowell, PhD, P.E., principal engineer at Advanced Materials Services LLC, Auburn, Alabama. Germany, for example, has a well-developed material approval process that includes extensive laboratory tests and field tests under heavy traffic. Based on seven sections of warm mix placed on Germany’s autobahns, warm mix performs as well or better than hot mix, Prowell said.

The warm mix additives tested in Germany were Sasobit, Asphaltan B, Subit, and Aspha-min (zeolite). Sasobit and Asphaltan B are both wax-based additives, and Subit is a fatty acid amide.
Minimal Concerns to Date
Concern has arisen over the moisture sensitivity of some warm mixes, as indicated by low tensile strength ratio (TSR) tests. However, all of the cores taken from warm mix asphalt roads to date show no evidence of stripping after durations ranging from four months to three years, says Prowell. Those cores were taken from projects in Florida, North Carolina, Wisconsin, Missouri, and the NCAT test track. “We are taught that internal moisture can be a concern,” says Prowell. “We’re not sure how much, if any internal moisture we’re leaving in the aggregates, but the projects are not that old yet, to truly assess their long-term performance.”

Another concern has developed over the discovery of unburned fuel oil in some warm mix pavement sections. In two such discoveries, the plants were burning recycled motor oil. “They may have needed to preheat the oil to a higher temperature,” says Prowell, “Or perhaps a burner manufacturer should adjust the burner for warm mix. The burner manufacturers say there should be no problem with operating their burners at lower settings than are used for hot mix.”

The newest technologies to enter the market are Rediset WMX and REVIX. REVIX was co-developed in the United States by Gerald Reinke of Mathy Technology and Engineering Services Inc. and Gaylon Baumgardner of Paragon Technical Services Inc. REVIX technology is an approach for producing reduced temperature paving mixtures that does not rely on the principles of foaming or viscosity reduction. A variety of surfactants and other materials may be used successfully at reduced mix production temperatures, says Gerald Reinke, president of Mathy Technology and Engineering, which is the research and technology arm of Mathy Construction, Onalaska, Wisconsin.

“Our system is not so much a product as a technology,” says Reinke. “We’ve discovered that you can add these surfactants without having to add water, or without having to reduce the viscosity of the asphalt in any other way. We found that it’s not necessary to add water or emulsions to the asphalt to produce warm mixes. In fact, we prefer to call this hot mix at a cooler temperature.”

With REVIX says Reinke, the appropriate additives can be incorporated at the liquid asphalt terminal. Treated binder is delivered to the hot mix plant in ready-to-use form. It is, however, possible to inject the REVIX additives into the binder at the hot mix plant. In that case the producer will need a pump and metering system to deliver the additives into the liquid asphalt line as the binder is being delivered to the storage tank or as the binder is being pumped into the drum mixer or pug mill.

But Reinke emphasizes that no modifications to the hot mix plant are needed. The mixes are typically produced at 60 to 80 degrees Fahrenheit less than the same mix without the additives. The mixes can also be compacted at 60 to 80 degrees below normal compaction temperatures. “This process does not add any additional water, which is important from a moisture sensitivity standpoint,” says Reinke, “And because of the ability to blend with the warm mix binder at the asphalt terminal, it should be competitive with the least expensive technologies on the market.”

Rediset WMX, the other newcomer to the market, is a blend of surfactants that include anti-stripping agents, says Prowell. It comes in a pellet form, and is added to the binder at the rate of 1.5 to 2.5 percent by weight of the binder.

Last November, Baldwin Contracting Company Inc. ran a trial project with Rediset WMX at its Chico, California, hot mix plant. Some 230 tons of warm mix were produced at 240 and 270 degrees Fahrenheit—and the producer also ran 240 tons of hot mix at 305 degrees as a control. They paved a haul road to the plant.

“The warm mix achieved about two percent better density at reduced temperatures than the hot mix control did,” says Prowell. “Some observers said the warm mix trial could not have gone any better.” Rediset WMX was introduced last November by Akzo Nobel.

Summing up, warm mix technologies are making rapid progress. Most problems, if any, have occurred in the construction process and can be solved. Says Prowell, “We have made great strides in the implementation of warm mix asphalt in the United States and are basically on a par with Europe now.”

Dan Brown is the principal of Technicomm.