States’ interest is running high; demonstrations dot the nation. 

Reviews are pouring in from dozens of warm-mix asphalt demonstrations across the country. And the consensus is that while warm mix is not a panacea that will solve every challenge in asphalt paving, it shows promise to become one way to improve upon conventional hot mix in many respects and situations.


In this demonstration project by Astec, the warm mix (right) emits no smoke, but the hot mix at left gives off some blue smoke.

Warm mix typically uses an additive or process that permits mix production at temperatures of 50 to 100 degrees F below standard hot-mix temperatures of 300 to 350 degrees F. Blue smoke, odor, fumes, and stack gas emissions are virtually eliminated. Fuel consumption drops by 11 to 30%. Compaction is easier. You may be able to increase the amount of reclaimed asphalt pavement in the mix. Haul distances and the paving season both can be extended. And because the mixtures are produced at lower temperatures, the binder ages less in the production process.

A crew paves with warm mix at night on Interstate 70 near the Eisenhower Memorial Tunnel.

“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,” says 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.”

Michael says demonstrations of Sasobit, the additive he represents, show that contractors can accomplish those things to a certain extent. But there are trade-offs. “If you have a very stiff RAP you can’t reduce the temperatures quite as much,” says Michael. “None of these [warm-mix] technologies is a cure-all for everything. 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.”

News of warm mix

Warm-mix technologies are making rapid progress. Here are some recent developments:

- Many state DOTs, and even some counties, are hosting demonstration projects. Sasol Wax has participated in more than 20 such projects in the past 18 months, Michael says. And the warm-mix additive Evotherm has been used in an estimated 35 projects in the recent past, says Jonathan MacIver, business development manager for MeadWestvaco Asphalt Innovations, which sells Evotherm.

- Astec, the hot-mix plant manufacturer, has come up with a new warm-mix technology that injects small quantities of water and air into the liquid asphalt stream of a Double Barrel drum plant. The result is 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.

- A new process, offered by the French firm LEA-CO, is being used to produce warm-mix asphalt in production quantities in New York state. By September, the company had expected to place up to 70,000 tons of warm mix on roads near Cortland, New York. McConnaughay Technologies is the U.S. distributor.  

- Aspha-min, a zeolite warm-mix additive, is marketed in the U.S. by the Hubbard Group, Orlando, Florida. Hubbard’s parent company, Eurovia, has placed over 200,000 tons of warm mix with Aspha-min in Europe.

- A company called PQ Corp. is now marketing Advera WMA, another zeolite additive. 

- Stack gas emission tests are showing dramatic results. Tests from an Ohio demonstration project show that Sasobit and Aspha-Min reduced sulfur dioxide emissions by 83% compared to the control hot mix. What’s more, the two additives slashed NOx, carbon monoxide, and volatile organic compounds by a range of 21 to 63% compared to hot mix.

- At the test track run by the National Center for Asphalt Technology, one section using the Evotherm warm-mix additive has turned in an excellent performance. After 4.2-million equivalent single axle loadings, rutting in the Evotherm section only amounted to 0.0625 inch.

- Meanwhile, Evotherm reports that it has introduced a new delivery system for its warm-mix technology. Prior to this year, Evotherm has been delivered as an asphalt emulsion. In 2007, MeadWestvaco launched Evotherm DAT, a delivery method that replaces the asphalt emulsion with a chemical solution that is injected inline into the asphalt at the mixing plant. The new delivery system reduces cost and requires only minor plant modifications, says MacIver.

Running 50% 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, reports Astec chairman Don Brock. “The city milled out 2 inches of pavement. We fractionated it back to its original sizes and ran it at 50% recycle,” says Brock. “We ran at 270 degrees and the mix had no smoke and no odor.”

This warm mix for a Nebraska demonstration is about 215 degrees F.

Running at lower temperatures enables Astec’s foamed asphalt technology to burn 11% less fuel — which permits the plant to increase its production rate at full burner. “If you use 11% less fuel, you can run 11% 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. Demonstration projects are planned or under consideration for North Carolina, South Carolina, and Tennessee.

The LEA process

LEA-CO, a French firm, has signed with a distributor, McConnaughay Technologies, Cortland, New York, to distribute their warm-mix additive and technology in New York state and eventually to other states, says Andre Loustau, the U.S. representative for LEA-CO. McConnaughay operates a plant that had produced 20,000 to 25,000 tons of warm mix through July of this year.

Two views of the same project show how warm mix, left, emits no smoke — compared to the hot-mix control at right.

 
With the LEA process, coarse aggregate gets dried in the drum dryer at a lower temperature than with hot-mix asphalt, says Loustau. Liquid asphalt and the LEA additive are added to the dried aggregate. The Cortland plant has a pugmill that follows the drum dryer. After the coarse aggregate is dried, the cold, wet, fine aggregate is injected between the dryer and the pugmill. When the wet fine aggregates contact the heated, coated coarse aggregates, the asphalt foams and coats the fine aggregate, Loustau says. The resulting mix is produced at 200 to 210 degrees F.

Some water is present in the mix even after mixing, and it acts to lubricate the mix and make it more workable. The LEA additive prevents stripping, helps to coat the aggregates, and aids in workability, asserts Loustau. “It’s a specifically formulated concoction that helps everything,” he says. 

What controls moisture? A moisture content probe is inserted into the fine aggregates supply stream, and if the moisture goes too high, Loustau says, “We compensate for it, for example, by heating the coarse aggregate more.” Conversely, the LEA process would call for adding a bit of water to the fine aggregates if necessary. 

The New York state DOT is conducting exhaustive research on the LEA process, Loustau says. In Europe, close to 50,000 tons of LEA warm mix has been placed. The process saves significant amounts of energy by not heating the fine aggregates. “Typically, we dry only 60% of the aggregates and we dry only the portion of the aggregates that contains little water,” Loustau explains.

Density at high altitudes

Seeking an improved way to achieve density at high altitudes on I-70 near the Eisenhower Memorial Tunnel, the state of Colorado recently performed a demonstration project featuring 1,000 tons each of warm mix made with Advera WMA, Sasobit, and Evotherm. The state DOT wants to pave at night on the Interstate, and at elevations of 10,000 feet or more, ambient temperatures get cold, even in the summer, says Bill Schiebel, a regional materials engineer with the DOT. The cold, coupled with long haul distances, can make it problematic to achieve density with conventional hot mix.

Note the lack of smoke or steam in this warm-mix demonstration by Astec.

All three warm mixes were delivered to the I-70 site at 250 to 275 degrees F, says Loren Eckles, assistant project engineer. Each of the three warm mixes was paved for about a mile in the center lane of the three-lane pavement.

“We need warm mix to get density for paving at night,” says Schiebel, “but we want to answer the question of how it performs in high-moisture situations. Up that high in the mountains, we get hundreds of inches of snow per year, and we have 40-odd days when trucks can use chains. We want to make sure that the warm mix doesn’t have moisture sensitivity problems. It does facilitate compaction; we’ve seen that in results from other states. We had no problem getting density with it.”

Colorado seeks to use warm mix to improve ease of compaction on this high-altitude portion of Interstate 70. 

About a year ago, Flexible Pavements of Ohio, the state’s asphalt pavement association, hosted a demonstration featuring Sasobit, Evotherm, and Aspha-min, says Bill Fair, director of engineering services for the association. Contractor Shelly and Sands placed 1,155 cubic yards each of Evotherm and Aspha-min mixes, along with 1,339 cubic yards of Sasobit mix and 1,602 cubic yards of a control hot mix.

Aside from a short learning curve with placing warm mix, the demonstration was very successful, Fair said. “They started out producing at unrealistically low temperatures, and they had to increase production temperatures,” he said. The average temperature for the hot mix was 309 degrees behind the paver, and the average temperature for warm mix was 245 degrees. Compaction? “We had no complaints,” said Fair. “The contractor said it was very much like hot mix. I looked at the job this spring and the four sections were indistinguishable. None of them showed any distress.”

According to the suppliers, Evotherm, Sasobit, and Advera WMA all cost about $3 per ton of mix, including the cost of the feeding equipment. “That price will come down with larger volumes,” says Annette Smith, project leader for the Warm Mix Asphalt Technology Group, PQ Corp. “Eventually, the price will probably arrive at $2.00 to $2.50 per ton of mix.”

Sasol’s Michael says contractors will grow to see warm mix as a tool to achieve savings. He believes some contractors right now are looking at warm mix as a break-even cost proposition. “If you talk to the contractors, one of them is thinking that he’ll increase his tons per hour production,” says Michael. “Another one thinks he can pull a roller out of the roller train. That’s how contractors will justify the cost — by the savings in their operation.

“We’re on the verge of using warm mix at production quantities,” says Michael.