Warm mix project yields valuable research

As warm mix continues to develop as a viable option for asphalt paving projects, Knife River, an MDU Resources Company, spearheaded three test projects in 2007 to gain a better understanding of the technology and prove to road agency customers that it meets the same performance specifications of hot mix in a more environmentally-friendly manner.

One project in particular, Nebraska Department of Roads' Highway 12 between Verdel and Monowi in Knox County, also demonstrated that a long haul from plant to paver posed no challenge in achieving density compaction.

In early summer of last year, Knife River Midwest and Jebro Inc., another MDU Resources Company which supplies liquid asphalt binder, proposed a research project using the warm mix technology.

The asphalt producer/paving contractor proposed using one of three additives ? Evotherm, Evotherm DAT or Sasobit ? to produce the warm mix, with the Nebraska DOR finally specifying the Sasol wax brand, which Jebro blended into two different percentages ? 1.5 percent and 2.5 percent ? for test purposes that would allow Knife River to evaluate the benefits of using an extra 1 percent of the additive.

"We've been interested in exploring warm mix and we thought this would be a good project for our tests. We wanted to evaluate the fuel savings, increased workability, rolling patterns, increased haul distance, emissions (blue smoke), decreased aging of the binder, and the properties of the binder when Sasobit is added," notes Mike Collins, Knife River Midwest AMA Manager.

The 6.9-mile-long, two-lane project was located on Highway 12 approximately 85 miles northwest of Sioux City, IA. It was a typical mill and fill, with 1.5 inches of the oxidized and rutted surface course removed and replaced with a new 2-inch SP-4 Special (Superpave) overlay rated at 10 million ESAL mix design with gyratory level of 117 gyrations.

The binder for the project was a PG 64-28. Total mix required to complete the project was 11,570 tons, with the first 6,000 tons consisting of conventional hot mix asphalt and the remaining 5,570 tons of warm mix (half of the warm mix was produced with 1.5-percent Sasobit in a mix design containing 6-percent liquid asphalt and the other half with 2.5 percent).

Since the warm mix project was relatively small, Jebro blended the Sasobit with the binder at its terminal in Sioux Falls rather than Knife River installing equipment to blow in the additive at its production facility. All the mix required for the project was produced with a portable Cedar Rapids 400-tph parallel-flow plant that was set up in Tabor, SD, approximately 60 miles northeast of the project.

Determining the temperature drop during the long haul from the plant to the project was somewhat of a concern when planning the project. With hot mix, Collins said it was determined that the mix would have to leave the plant at around 325 degrees F in order to maintain a temperature of 290 degrees behind the paver screed.

"It's a typical temperature for us when working with a polymer modified hot mix," Collins says.

Mix for the project was constructed of quartzite and locally crushed gravel, with both materials presenting low absorption rates. Start-up temperatures on the conventional hot mix ran as hot as 340 degrees, but eventually settled in at 315 to 320 degrees. Because of the higher temperatures and long haul, blue smoke was an issue.

"We monitored the load (speed) on the hot leg conveyor and the baghouse temperature (265 degrees when producing the hot mix) to help gauge changes during production of the warm mix," Collins says. "We also monitored temperatures at the plant and the paver. Monitoring those temperatures were important in determining the rolling pattern required to achieve density for both the hot mix and the warm mix. With the hot mix, the temperature generally dropped 10 to 15 degrees during the haul and another 20 to 25 degrees from its windrow placement to the screed. But with the hot mix we were still able to achieve an average density of 94 percent (specifications required a 92.5+ percent density)."