We’re on the cusp of having warm-mix asphalt allowed for specialty applications," says Brian Prowell, assistant director of the National Center for Asphalt Technology. "It could be a contractor’s option to improve workability and achieve density in cooler weather, or to reduce emissions in urban areas that don’t meet national air quality standards, or to permit longer haul distances."

Warm mix is a hot topic these days. At the World of Asphalt Conference, held in March in Orlando, Florida, a series of warm-mix presentations drew an interested audience numbering in the hundreds. Over the past couple of years or so, the Federal Highway Administration, the National Asphalt Pavement Association, and NCAT have been working with material suppliers to evaluate a group of warm-mix technologies, and the results are promising (see Better Roads, February 2006).

Simply stated, a warm-mix process typically uses an additive to the mix or to the binder to create the asphalt mixture at a temperature range, according to NAPA, of 250 to 275 degrees F. Warm-mix processes have helped achieve compaction in stiffer, polymer-modified mixes. As Prowell notes, warm-mix technologies can help extend the paving season into cooler fall temperatures. For example, Aspha-min zeolite was used to improve workability in cool ambient temperatures when placing open-graded friction course on the Florida Turnpike and on a trial section in New Hampshire, which was placed in November 2005.

And at lower temperatures, warm mix gives off noticeably fewer gaseous emissions and odors than hot mix. Preliminary tests confirm that. As a result, environmentalists are stepping up pressure to bring the new technology to market — especially in urban areas that do not attain EPA’s air quality standards.

Will costs go down?

But what are the economics of warm mix? Initial costs of these technologies are higher, because the materials are not in high-volume production yet, say warm-mix proponents.

For Aspha-min zeolite, a warm-mix additive, the cost is about $3.60 per ton of mix. "That’s probably a little high, because it’s experimental," says Jeffrey L. Pochily, quality control manager for Pike Industries Inc., Belmont, New Hampshire. Late last year, Pike placed about 500 tons of warm mix at its asphalt plant in Hookset, New Hampshire. Zeolite costs about $0.60 a pound and is added at the rate of 0.3% by weight of the mix. Offered by Eurovia/Hubbard Construction, zeolite comes in powder form. It’s about a 50-mesh material.

On the other hand, because warm mix doesn’t require 300-degree mix temperatures, it saves fuel. No large production runs have been made in the United States to indicate how much fuel warm mix can save. But European experience indicates that the savings can run about 30%.

"We really need to run a large-scale test in the United States for four or five days, to really see how much fuel reduction and emissions reduction we can accomplish," says Larry Michael, an asphalt consultant who represents warm-mix additive manufacturer Sasol Wax and is a retired regional materials engineer from Maryland. "For emissions, we need to run good controlled stack gas tests." He points out that some 6-million tons of Sasobit warm mix has been placed in Europe and China with zero failures.

It costs Gallagher Asphalt, Thornton, Illinois, three therms of natural gas to make a ton of asphalt mix. The gas retails for about $2.40, or $0.80 per therm. At that price, a one-third savings would reach $0.80 per ton of mix.

What are fuel savings with zeolite? "We estimate about a 15 to 20% reduction in fuel for producing mix at 245 degrees," says Pochily. "The plant uses 1.8 gallons of diesel fuel per ton, so the savings is about $0.55 per ton."

"The additive cost of $4 per ton of mix is because warm mix is new," says Eric Harm, an assistant chief engineer for the Illinois DOT. "More volume will bring down the costs." In his presentation at the World of Asphalt, Harm said the industry should not worry about the cost of warm mix. And, he says it should be performed at the contractor’s option — not mandated.

Asked how contractors would spend extra money for an additive of choice, Harm replied that certain circumstances — such as late season paving against a deadline with financial penalties — could attract a contractor to warm mix. States set up temperature specifications that prohibit paving below a certain temperature. "A contractor’s cost would then be less than the penalty he’ll pay if he doesn’t finish by a deadline," Harm told Better Roads
Another circumstance might be cool-weather paving when density is difficult to achieve with conventional hot mix. That’s significant, because density is often a paid specification. "During the late season, a mix can cool down so quickly that you might want to use warm mix to make sure you get proper compaction," Harm said. "Density can be a challenge in cool weather and certain mixes are more difficult to compact."

The cost of Sasobit, an import made by Sasol Wax Americas Inc., depends on the location and size of the project, said Michael. Sasobit comes in the form of BB-like pellets, or in flakes, and can be added to the binder or injected into the mix with a modified fiber feeder or a pneumatic feeder made for the material.

An additive supplier, Carlos Usuda of Hi-Tech Solutions, Mechanicsville, Virginia, said in one case that a modified fiber feeder for Sasobit cost about $40,000 to install. But, he hastened to add, Hi-Tech Solutions would be willing to work out a creative financing package with a contractor who could use high volumes of Sasobit.
Emissions lowered

On a project using warm mix produced by Milestone Contractors LP, Triangle Asphalt paved 660 tons of asphalt mix on county road 900E near Indianapolis last year. Milestone employed the Evotherm warm-mix process, which delivers a chemical package in an emulsion. The water in the emulsion is released from Evotherm in the form of steam when it is mixed with the hot aggregate. This is the only American warm-mix technology to date, and it is made by MeadWestvaco Corp., Charleston, South Carolina.

At the Indiana project, Evotherm achieved some remarkable reductions, compared to the hot-mix control, in nitrous oxides, total particulate matter, total organic matter, and benzene soluble matter, said Everett Crews, a technical manager for MeadWestvaco, speaking at the World of Asphalt. The plant produced warm mix at 300 degrees F in the drum and 210 degrees F at discharge to the trucks. Added moisture in the mix caused no issues in the baghouse, which is sometimes a question with warm mix.

At an Evotherm project in Ramara, Ontario, Canada, stack gas emissions were all reduced by 40 to 60%, compared to a conventional hot-mix control. Because of lower mix temperatures, Evotherm permitted reductions in nitrous oxides, sulfur dioxide, carbon monoxide, and carbon dioxide — which Crews calls the four bad actors in stack gas emissions.

Crews cited a list of 10 or 12 Evotherm projects around the world. He said the technology has been successfully used with wide ranges of aggregates, binders, production equipment, and laydown equipment. Mixes have been made with both Superpave and Marshall designs. Lifts have been placed from 0.75- to 3-plus-inches thick. And on the project in Ramara, fuel usage was reduced by more than 50%, Crews told attendees at the World of Asphalt.

TSR tests

One remaining question with warm mix concerns the fact that its tensile strength ratio declines at lower mixing temperatures. That indicates moisture susceptibility, which can lead to stripping of the asphalt from the aggregate. "If you don’t dry out the aggregate completely, there are always concerns about moisture susceptibility," says Prowell.

Solutions, Prowell says, may include a suggestion from Don Brock that the 20% finer portion of the aggregate blend could be added in the RAP collar of a plant. (The RAP collar is the portion of the drum used to enter the recycled asphalt pavement.) That would leave the full force of the burner to heat the 80% of coarser aggregate, and the finer aggregate would be dried by contact with the coarse. (Don Brock is the chairman of Astec Industries, a hot-mix plant manufacturer.)

The jury is still out on these TSR results, however, because field cores taken of the same mixes show no stripping damage. The difference needs to be researched, says Prowell.

Meanwhile, he says NCAT tests show that anti-stripping agents can solve the problem. "Lime works with zeolite," says Prowell. "With Sasobit, a liquid anti-strip agent called Magnabond works. And certain anti-strip agents are part of the Evotherm technology."

"Sasobit is the only technology that doesn’t involve water, and I feel that’s a benefit," says Michael. "Is the TSR a concern? Yes. Is it a solvable problem? Yes it is."

Michael notes the success of a warm-mix project using Sasobit last year on the Washington Beltway. "We had an open house and placed a total of 1,500 tons of mix on two nights," he says. "We dropped the mix temperature from 300 to 250 degrees F and our intelligent compaction roller indicated that the stiffness of the warm mix was the same as the control (hot) mix. And yes, it’s holding up well to traffic. We used a 19-millimeter stone matrix asphalt with a modified PG 76-22 binder."

Field rut tests to date indicate excellent results for warm mix. Prowell says MeadWestvaco paid to have Evotherm tested at the 1.7-mile NCAT test track near Auburn, Alabama. "After 500,000 ESALS, we had only 1 mm of rutting in the two Evotherm test sections and an HMA control section," says Prowell. (An ESAL is an equivalent single axle loading, exerted at NCAT by trucks.) The mix was placed in late October-early November, but Alabama temperatures still ran with daytime highs in the 70s and 80s. Temperatures lowered somewhat after that.

"It’s an excellent result. The traffic was returned within an hour after construction — and few sites in the U.S. receive as high a loading rate as the NCAT test track, but just because the sections gave an excellent performance in that weather doesn’t guarantee that they would perform as well in the heat of the summer," says Prowell.

In a presentation at the World of Asphalt, Prowell made some warm mix recommendation

Optimum asphalt content be determined without the warm-mix additive. Lab tests would indicate that warm-mix additives might permit a reduction in asphalt cement content. "But we don’t see why asphalt content should be lower, so we’re recommending keeping it the same," he says. "We’re not sure your ability to obtain density would be as good if you reduced the AC content."
If the mixing temperature is below 250 degrees F, consider using a stiffer binder grade. Lower mixing temperatures don’t age the binder as much as hot-mix temperatures do. And less aging on the binder permits the mix to rut more in laboratory tests with the Asphalt Pavement Analyzer. "That’s why it might be better to start with a stiffer binder," says Prowell.
Conduct tensile strength ratio tests at anticipated production temperatures. Doing so better simulates what happens in field production.
So what’s next for warm mix? Larger trial projects, says Prowell. As of late March, plans were under way for a large trial in Missouri. And in Texas, Ohio and Wisconsin, officials are talking about large trial projects.