Warm mix project yields valuable research

From hot to warm
After three days of hot mix placement, the Knife River production and paving crews switched over to the warm mix portion of the project. The warm mix portion was divided in two, with the first half containing 2.5-percent Sasobit (the first project in the United States to use that percentage of the additive) and the second half with the conventional 1.5-percent Sasobit content. As Collins noted, European warm mix projects use 2.5-percent Sasobit because of the harder binders used in the mixes designed.

"We wanted to try a higher Sasobit content to determine if there were any added benefits," Collins says. Sasol told us to anticipate a temperature drop of 50 to 60 degrees with a 1.5-percent Sasobit content and by adding an extra 1 percent we were hoping for a temperature drop (during production) of 70 degrees."

With the recorded amperage pull on the hot leg motors during the hot mix production, Knife River's production crew started out producing the 2.5-percent warm mix at 70 degrees lower temperature than what they produced the hot mix. The amperage pull was similar to that during hot mix production, so the production crew maintained the temperature for the day's production to see what kind of density would be obtained out on the project.

The noticeable difference in placement of the warm mix was in how fast density was achieved in the rolling pattern. While there was virtually no difference during the breakdown phase of the rolling pattern, there was very little additional compaction achieved during the intermediate phase of the rolling pattern.

With a density reading of 94 to 95 percent, Collins production and paving crews decided to lower the production temperature from 255 degrees to 225 degrees on the second day of the warm mix application. They were still able to achieve 93 to 95 percent density, but the paving crew also learned the screed heater had to be left on during the day to prevent a 3-foot-wide "shadow" in the center of the mat, which was caused by a cold screed dragging across the cooler mix.

After placing the 2.5-percent mix, crews switched over to the 1.5-percent mix. The only adjustment made to compensate for lower Sasobit additive in the mix was to increase the production temperature to about 265 degrees.

"We could have lowered the temperature about 20 degrees and still maintained our density requirements, but we were concerned about the drop in the baghouse temperature," Collins says. "When your baghouse temperature is running around 265 degrees with a conventional hot mix, you become a little concerned about the temperature dropping to 200 degrees during warm mix production. When you drop below the boiling point of water (212 degrees), you run the risk of moisture build-up in your baghouse which could cause the bags to plug and reduce airflow."

Project benefits
The primary benefit for most asphalt producers/contractors in utilizing warm mix technology is the cost savings generated in the fuel required to produce the mix. While some reports project a fuel savings of 30 to 50 percent, Collins says on this particular project Knife River achieved a 15-percent reduction in overall fuel usage.

The average fuel usage for convention hot mix was from 1.3 to 1.7 gallons per ton of mix produced, 1.3 to 1.5 gallons per ton when producing the warm mix with 2.5-percent Sasobit content at temperature 70 degrees lower than the hot mix.

The fuel savings on this particular project basically offset the additional cost of the Sasobit additive. With hot mix costing approximately $30 a ton and warm mix $32 a ton (Sasobit for the project cost $1.12 per pound), it's easy to question the cost-savings advantages when comparing only the fuel savings. But Collins believes additional savings can be achieved by eliminating the intermediate roller in the rolling pattern.

But what's probably a more notable benefit of warm mix, according to Collins, is the reduction of blue smoke and other emissions.

"From an environmental perspective for customers, neighbors around your plant facility, and crews producing and placing the mix, warm mix eliminates a lot of those concerns while delivering the same density requirements and quality mix," Collins says. "So for us, the project did prove we can produce a quality mix at a lower temperature, eliminating emissions in the process; haul that product a long way and place the product (without the blue smoke) and achieve density and ride specifications. We also know it will extend our paving season, and the lower temperature required to produce warm mix will extend the life of the binder."