Proving Ground

By Barry Gantenbein, Editor, Western Builder | September 28, 2010

Except for the 75 or so yellow-vested visitors to the site, it seemed like a typical asphalt paving project for the crew from Payne & Dolan, Inc., Waukesha, Wis.

There was one other difference on the job on Ryan Road in Oak Creek, Wis. The material being placed was warm mix asphalt, not hot mix.

As the name implies, warm mix asphalt is produced and placed at a lower temperature than hot mix asphalt, which reduces emissions and energy consumption.

Two test sections were placed in Oak Creek on June 19–20 as part of a Warm Mix Open House hosted by the Wisconsin Asphalt Pavement Association (WAPA), the Asphalt Pavement Alliance (APA), the Wisconsin Department of Transportation (WisDOT), and Payne & Dolan.

The driving force behind the development of warm mix asphalt is a reduction in energy usage and emissions in the production and placement of asphalt.

If production and placement temperatures are lowered, emissions are reduced and energy costs are cut, said Brian Prowell of the National Center for Asphalt Technology (NCAT).

The use of warm mix asphalt may also extend the paving season to colder parts of the year, as well as increase the distance that asphalt mix can be transported for placement.

Improved workability at lower temperatures is another reason to use warm mix asphalt.

There is also some evidence that the use of warm mix asphalt will prevent imperfections in pavements being overlaid from working their way to the surface, Prowell said.

"There are lots of different reasons for doing this, and lots of potential benefits we're seeing with warm mix technology," said Prowell.

One of the test sections on Ryan Road was paved with a warm mix that included the additive Sasobit, while the other test section included the additive Evotherm.

Sasobit is a synthetic wax added directly to the asphalt binder or blown into the mixing chamber at the plant.

Shaped like pellets, Sasobit is a high-molecular-mass synthetic aliphatic hydrocarbon produced by Fischer-Tropsch synthesis from coal or natural gas.

Sasobit decreases both viscosity and stiffness in the binder, said Larry Michael, an asphalt consultant from Maryland.

"The key is viscosity reduction. With viscosity reduction, we get improved workability, and we get improved compaction and density. My feeling is if we improve density, we're automatically going to improve quality," Michael said.

Another Process

Evotherm is produced using a chemical process that includes additives to improve coating and workability, adhesion promoters and emulsification agents.

In a low energy asphalt process, course aggregate is coated with asphalt. Wet, fine aggregate is then added. Water from the wet, fine aggregate and chemicals is used to create a foaming process to allow coating at a lower temperature.

Evotherm is delivered in the form of a high-residue emulsion, with approximately 70-percent binder residue.

"With 70-percent binder residue, there's 30-percent water. There's going to be some steam liberated. That's part of the process. That water will flash off when it hits the hot aggregate,"Prowell said.

The steam that is produced isn't an environmental concern.

For the Oak Creek test section, Sasobit was blended with liquid asphalt at Payne & Dolan's Muskego, Wis. plant.

"It melts in there really nice, and it worked out real well for us," said Rick Schmidt of Payne & Dolan.

The Evotherm was pumped directly into the asphalt plant from a tanker.

"We ended up re-doing some piping at the plant," Schmidt said.

With both Sasobit and Evotherm, Payne & Dolan crews used the same asphalt mix as on the rest of the project — a Superpave 12.5-millimeter E-3 mix with 14-percent RAP and 4.6-percent AC. Binder on the job is PG 64-28.

Lifts were placed 1.75-inches thick.

"All the equipment that we're using is the same as for standard mixes," said Schmidt. "We're doing everything the same as we do with standard mixes. We're using the same equipment and the same rolling patterns, except we're doing it at a lower temperature."

Payne & Dolan crews will typically place hot mix asphalt at 310–320 degrees Fahrenheit, but wanted to place the warm mix with Sasobit at approximately 250 degrees Fahrenheit and the section containing Evotherm at about 230 degrees Fahrenheit, Schmidt said.

Crews found the warm mix more pliable and workable at lower temperatures than regular hot mix asphalt.

"At 200 degrees, our normal mix would start to get pretty stiff and unworkable. This allows you to do additional compaction and work the mix when it gets colder, even below 200 degrees," said Schmidt.

For the Sasobit test section, the hot roller worked on pavement that was 160–180 degrees Fahrenheit, and the cold roller compacted pavement in the range of 140–150 degrees Fahrenheit. All compaction was completed by the time the pavement was 140 degrees.

The operating window for Evotherm is approximately 160 to 250 degrees Fahrenheit, said Jonathan MacIver, who spoke about the product at the open house.

"We're not changing the aggregate, we're not changing gradation, we're not changing the binder. We're changing the temperature. Our rule of thumb is about 100-degrees cooler than HMA," said MacIver.

He added, "Improved compaction is related to the larger compaction window we have, and the fact that the mixes are compactable at lower temperatures."

An Ingersoll-Rand DD130 was the hot roller on the job, and a Hypac double roller served as the cold roller.

Time Will Tell

NCAT will be monitoring the Ryan Road test sections

At the start of this year, there were 10 warm mix test sections in the field in the United States, as well as warm mix testing being done at the NCAT test track.

NCAT wants to gain a better understanding of the proper asphalt content for warm mix asphalt mixes.

Some warm mix asphalt mixes have had lower VMA measurements than hot mix asphalt, indicating a tighter locking of the aggregate, Prowell said.

"One of the things we're trying to learn is do we need lower asphalt contents in warm mixes compared to hot mixes. In our laboratory testing, we didn't see why we should have that. We're not recommending that right now. We'd sooner see lower designed air voids, until we better understand that," said Prowell.

A better understanding of mix design for warm mix asphalt is an NCAT goal.

"Long term, we need some kind of specifications," Prowell said. "There's a real range of mixtures in use now."

NCAT currently recommends designing warm mix the same as hot mix, and then dropping the additives in.

Tests seem to indicate no changes in roller patterns are necessary when paving with warm mix.

"Generally speaking, we haven't seen tenderness issues in warm mix. We've been up tight to the paver, and not seen any tender mix issues," said Prowell.

Tests have shown an improvement in compaction using warm mix compared to hot mix.

"Looks like the warm mix additives are enabling a better orientation of the aggregates, so they get tighter packing," said Prowell.

With the lower temperatures associated with warm mix asphalt, moisture damage is a concern.

"Running at lower temperatures, there's the possibility you might not get all the water out of the aggregate. Is that going to cause you problems down the road? We need to take a look at that," Prowell said.

With warm mix asphalt's lower temperatures, there isn't necessarily as stiff of a binder as there is with hot mix asphalt.

"The laboratory shows that potential, but in the field we don't see as dramatic of a shift in the rutting numbers. The Sasobit and the Evotherm seem to mitigate the increase in rutting.

"Even though we get an increase with the reduction in temperature, it's not as severe as if we didn't have any additive. Part of that is because we get better compaction with the warm mix additives. With better compaction, we reduce some of that consolidation," said Prowell.

Warm mix with Sasobit will be as stiff or stiffer than a conventional binder, Prowell said.

"When you heat it up to the compaction temperature range, it shifts the viscosity down to the less-stiff material so it's easier to compact," he said.

Evotherm, Sasobit, and Aspha-Min are the three most commonly used additives for warm mix asphalt in the U.S.,Prowell said.

Aspha-Min is a zeolite, which is a material with a crystalline structure that holds water internally.

"When that material is heated by putting it in the hot mix asphalt, that water is released. Basically, that water acts the same as if you have a foaming-type nozzle to microscopically foam the asphalt, which increases workability," Prowell said.

The asphalt paving industry's goal is to get the fuel savings and environmental benefits of warm mix asphalt without a loss in performance.

Mixes, additives and processes will be tweaked as the testing of warm mix asphalt continues.

Another warm mix asphalt test section is planned for Milwaukee in 2007, according to WAPA Executive Vice President Pat Goss.