Asphalt production evolves with mix design demand

Knowing the direction DOT and other agency customers were headed in specifying new, more challenging mix designs in order to achieve more durable and better performing pavements, Dunn made some modifications to its new East Thomas plant. Heavier motors were installed on the mixing drum to accommodate mix designs with a higher aggregate content. Also, the plant was equipped with a larger baghouse, the CFS-151, to accommodate the additional dust created by producing the new mix designs, and to improve airflow to reduce excess moisture content in the aggregate in the spring and allow for increased production during the summer.

"If you want to maintain the temperature required to produce some of these new, challenging mix designs, you have to have a system that maintains adequate airflow through the drum," Covington says.

"The system is designed to handle the additional moisture content they have in their aggregate during the spring and also allows them to increase production up to 350 tph during their summer peak paving season," says Charlie Bartell, Gencor regional sales manager who helped set up the plant.

The modifications have proved essential in maintaining the output and quality of mix the East Thomas plant turns out. The plant produces approximately 300,000 tons per year, with 85 percent of its production used to support two Dunn paving crews working on major interstate and state highway projects in and around the Birmingham area. With a typical OGFC mix, the plant efficiently operates at 275 tph, and Covington attributes the plant's efficiency to a larger baghouse that maintains good airflow through the drum, which maintains the mixing temperature required to produce mixes like OGFC. Since the OGFC mix is a primary mix design required by many Alabama DOT projects, maintaining the right temperature during mixing and not allowing the temperature to get too hot is crucial in producing a mix that's acceptable for DOT projects.

"With OGFC mixes there tends to be more heat generated during the mixing process, and the baghouse we have at this plant does a good job of moving air through the drum and preventing the burner from overheating the mix," Covington says. "At other plants where too much air can be a problem with the mix being produced, we just damper back the airflow through the drum. But at this plant, we want the extra airflow produced by a larger baghouse to prevent the mixes from overheating.

"We've been running this plant 48 weeks a year since we put it into service and I don't think we've ever had to change the bags in the baghouse," he adds. "It's such an efficient airflow system, that with the exception of our scheduled two-week shutdown over the Christmas holiday, we're generally producing asphalt at that plant unless the temperature drops below 35 degrees F. When the temperature drops that low, it's hard to get mix out to a project at the temperature required for compaction, even if you can still achieve the desired temperature at the plant."

Day-to-day reliability

Max Taylor, plant foreman at the East Thomas facility, says maintaining the plant's reliability has required little in the way of major overhauls or upgrades. Typically, Taylor will schedule maintenance work during the winter (usually at the end of January) to take care of major repairs and upgrades. This year, he installed a new set of scales and made repairs to the slat conveyor system, as well as replaced some flights in the drum.

"It's been a very reliable plant," Taylor notes. "We run about 3,000 tons a day out of the plant and that's with two storage silos and five cold feed bins. With the different mix designs we handle on a daily basis, an extra silo and a few more cold feed bins would be nice, but we're still able to maintain good productivity with what we have."

The biggest challenge Taylor faces on a day-to-day basis is maintaining a proper temperature when mixing some of the designs being specified.

"You have to keep and eye on each job you're producing, especially the tonnage your producing, because the temperature can fluctuate quite a bit," Taylor says. "It's not easy to control the heat with some of these mixes because they consist primarily of tar and rock, and not a lot fines. If the heat drops too much from time a load leaves the plant and arrives at the project, the state will reject the load. And if the temperature is too high during production, then you run the risk of segregation. So you have to constantly monitor the temperature. Our goal is to send the mix out at about 325 degrees in order to achieve the compaction desired on the project."