If you speed up the drum and have the proper flights in the drum, you prevent the aggregate from dropping prematurely. This creates a better aggregate veil, which allows it to be heated better by the dryer. The technique translates directly to the roadway.
“Everything looks good,” said Starling. “That was one of the easier FC-2 jobs I’ve been on. Once they started running, they never had any problems with the mix.”
Change is scary. But so are car accidents.
Along a stretch of U.S. 264 in central North Carolina, drivers had an extremely high risk for hydroplane accidents. To address the most dangerous areas, the North Carolina Department of Transportation (NCDOT) patched sections of the road with open-graded friction course (OGFC). But after several years, the state agency decided an entirely new layer of OGFC would best protect the traveling public.
In January 2012, NCDOT awarded a contract to S.T. Wooten Corp. to mill, resurface and add the new layer of OGFC to the entire troubled area. The job covered 20.5 miles of roadway from the Wake County border through Nash County and Wilson County, intersecting with U.S. 301.
The choice to apply OGFC was easy. But S.T. Wooten’s proposal to lower the mix temperature—and still achieve a quality outcome—was untested in North Carolina.
When worlds collide
Warm-mix asphalt was already in regular use by both S.T. Wooten and NCDOT; so was OGFC. But the two technologies had never been tried together in a state project.
But S.T. Wooten had good reasons to combine them for this project.
“Of course warm mix would reduce emissions, but we also considered it a safety issue,” said Reade Dawson, general manager at S.T. Wooten. “Normally, when you pave with OGFC, you heat it to 335°F. So when it’s applied on the road, it produces a huge amount of steam. That’s a potential hazard for passing motorists, especially as the road would remain open during the paving process.”
But that was not the only change proposed for the OGFC FC-2 Modified mix.
The original specifications called for a nonstandard coarse aggregate size. That would mean extra work for the quarry—and higher charges for NCDOT.
Obviously, the permeability of the OGFC was non-negotiable, but S.T. Wooten Division Manager Chris Croom, who oversees asphalt quality control, believed that a standard No. 78M stone would do the job just as well as the special aggregate size.
“Using standard aggregate is not only a hard cost savings but a time savings,” said Croom. “With a nonstandard size, we’d need to have it specially made for the project and possibly have a longer haul to the plant site.”
The idea of using standard aggregate was run through the Carolina Asphalt Pavement Association’s Technical Committee. Todd Whittington, asphalt design engineer at the NCDOT, also reviewed the proposal. The result? NCDOT agreed to change the project specs to utilize standard-size aggregate.
Paving a point
Dawson noted that sustainability is an important part of the company philosophy at S.T. Wooten. “We’re always trying to be as environmentally conscious as we can. For this design, we wanted to use reclaimed asphalt pavement [RAP] and recycled asphalt shingles [RAS]. But we also wanted to take it a step further and lower the mix temperature, so we’d have even fewer emissions. Even though this was untried with OGFC in North Carolina, we believed we had the pieces we needed to succeed.”
But using warm mix and OGFC together? There were a lot of potential risks. If the temperature was too low, the mix would get stiff and would be hard to work.
NCDOT Division 4 Quality Assurance Supervisor Gary Starling explained the proposal “was really the first time we had considered running warm mix with friction course. The liquid can be so tacky and hard to work with.”
S.T. Wooten had done several projects with NCDOT Division 4, including the Sheldon G. Hayes Award-winning I-795 project in 2001. So the company had a trusted reputation—but they would still need to prove the idea’s viability.
So S.T. Wooten set up a test pave.
On a sunny afternoon, NCDOT’s Division 4 team, including Starling and Division Engineer Wendy Johnson, arrived at one of S.T. Wooten’s plants to watch the company slowly lower the mix temperature as they paved.
“We discovered that 300°F was about as low as we could go and still have quality mix,” said Croom. The team elected to pave the U.S. 264 project at 310°F, just to be safe.
A key element of S.T. Wooten’s solution was EvoTherm 3G warm-mix additive, which also acts as an antistrip agent. It keeps the mix workable despite a lower temperature.
And Starling’s concerns about tackiness? “It was never a problem after that,” Starling said.
Putting it all together
Consistency and quality of the mix were key to getting a road that not only met but exceeded expectations.
Croom explained that when the company first began using warm mix, it “didn’t look as good.” Then company CEO Seth Wooten Jr. had an idea.
“Variable-speed drums in combination with V-flights are designed to help you run more efficiently by keeping the baghouse temperature more consistent,” said Croom. “So we decided to use this system to make a better quality OGFC mix.”
Normally, when aggregates used for a friction course mix enter the drum, the flights may drop the material before it gets to the top of the drum. But if you speed up the drum and have the proper flights in the drum, you prevent the aggregate from dropping prematurely. This creates a better aggregate veil, which allows it to be heated better by the dryer.
“If the exhaust gas from the drum is too hot, you can damage the baghouse. If it’s too cold, the baghouse may not work as well in collecting dust because mud can form on the bags as well as in other areas of the plant,” Croom said.
The technique translates directly to the roadway, where the goal is to reduce thermal segregation. “If you didn’t have good mixing at the plant, you could get spots of binder or AC and areas of bare aggregate,” said Croom. “So the variable-frequency drivers and V-flight system help us achieve better mixing between the binder and aggregate, as well as produce a mix with a uniform temperature.”
The project also utilized a material-transfer vehicle, an NCDOT requirement, to help mix the material on the roadway, reducing the potential for segregation and ensuring continuous, smooth paving.
After milling was completed, but before the friction course could be applied, the road’s hydroplane risks became all too evident.
“The first time it rained after we replaced the base, there were five or six accidents in the first three hours,” said Starling. “I saw two accidents right in one spot. I stopped to check on one vehicle and another lady spun out right in front of me.”
After the new friction course was applied, however, “we didn’t see any more accidents,” he said.
“Everything looks good,” said Starling. “That was one of the easier FC-2 jobs I’ve ever been on. Once they started running, they never had any problems with the mix or with the liquid going up and down. It was really pretty consistent the whole project.”
Densities were not a concern. “Once the roller hits it once or twice, that’s it,” Starling said.
Traffic control also ran smoothly, with the public remaining safe during the paving process.
“It reduced emissions, it protected the public and it resulted in a roadway that’s preventing injuries and saving lives,” said Dawson. “All for the price of 100 tons of demo paving. You can’t expect owners to take your word for it. You have to show them it works.” AT