Tackling Milling Challenges at the NCAT Test Track

Buzz Powell talks about the history of the 1.7 mile NCAT test loop, now in its ninth cycle of being replaced, and how this year's work was different from year's past.

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Astec Inc.

One of the most important places concerning the future of asphalt is in Lee County, Alabama, at the National Center for Asphalt Technology (NCAT) test track. It’s a 1.7 mile loop of asphalt road that is specially designed and maintained through a partnership between the National Asphalt Pavement Association (NAPA) and Auburn University.

According to the NCAT website, "[It] was created to ensure this industry's ability to provide pavements that are durable, sustainable, quiet, safe and economical. We work with state highway agencies, the Federal Highway Administration and the highway construction industry to develop and evaluate new products, design technologies and construction methods that quickly lead to pavement improvements. 

The test track is broken up into forty-six sections which can be individually or group sponsored by various transportation industry collectives or by the industry’s major manufacturers like Astec Inc. Since its completion in 2000, it’s been essential in testing out experimental pavement designs in a safe research environment, and no one has had more direct involvement than Buzz Powell who began managing the test track since its construction 1999, and currently serves as the Technical Director for the Asphalt Pavement Alliance (APA).

"I retired last October, but NCAT hired me back part-time to support the transition to the young-bucks that are taking it over. I'm working a lot of hours, yeah," Powell explained. "I do have a full-time job. I'm the I'm the Technical Director of the APA. When I accepted that position they actually want me to continue with NCAT, so that I can be a liaison between NCAT and APA. I'm not very good at being retired."

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Powell Continued, “We've been in operation and finished the first track in 2000. The whole objective is to build test pavements and then run a fleet of trucks over it and tear the pavements up so we can help the Department of Transportations (DOTs), make better decisions, better designs, and better materials."

NCAT also partnered with the Minnesota Department of Transportation's (MnDOT) MnROAD facility to focus on two important issues that have an effect on all DOTs nationwide: the preservation of pavements and quantifying the impact of both recycled (RAP) and premium mix additives. Long term data on these area are deeply important when it comes to the future decisions made by our industry.

Quick Facts About The Test Track:

  • 46 Sections On the Track
  • 11,138,634 Total Miles Driven
  • 5 Trucks Simultaneously Driven
  • 156,995 lb Average Truck Weight in our Fleet
  • 1.7 Miles Test Track Length
  • 10,052,142 ESALs Applied in Current Research Cycle

The Circle Is Now Complete

Every three years the track is rebuilt, and this year marks the beginning of the ninth research cycle. Just like on public roadways, the milling work is phased into sections so that others may stay open for use.

“We usually rebuild about a third of it and then do traffic continuation on the other two thirds,” explained Powell. “This cycle, however, it was a little bit more than a third so it was quite a bit of work. Usually, the thickness of the milling varies between an inch and a half, down to as shallow as three quarters of an inch.”

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But this year the milling work went far beyond those typical ranges, digging down to layers as deep as five feet, as well as milling twenty-one of the tracks forty-six sections.

“They removed a tremendous amount of material,” said Powell, referring to the team at Astec Inc. and their RX-700. “What was different about this milling work this time, was that we asked [Astsec] if they would mill it to an average depth of an inch and a half, but mill it as smooth as possible. That would help us to build smoother test sections when we when we start paving.”

The Astec milling team knew just how to give Powell and the NCAT test track exactly what they wanted.

“Not every test track section was getting milled at the same depth, but they were all focused on rideability,” said Tom Lenger, Product Manager, Road Recycling at Astec. “In order to achieve that, we decided to use the Dual Averaging Sensors (DAS).”

While traditional milling end-gates only monitor about an eight foot length of work, the DAS can expand that monitoring window up to thirty feet long. The grading system then averages the grade depth over that much longer span, generating an incredibly smooth foundation for the pavements above it.

“Normally, if you hit a bump in the road, your mill is going to immediately correct because you're within that eight foot window, whereas with this system you're looking at the front of the machine, the middle of the machine, and the rear of the machine all at once,” explained Lenger. “You're not going to have as drastic changes in elevation on your road.”

The DAS system averages the cutting depth over the entire length of the ski, which translates into fewer and less abrupt changes in the mat. Any profile corrections made in this manner are done so no only with more precision, but at a more gradual rate of change. This, in turn, makes the eventual work of the paving team much easier.

“The grade control system and the milling quality were both outstanding,” said Powell. “It was just good work all the way around. It was a step up this If you can mill smooth, it’s easier to pave smooth.”

Keeping Track of The Data

1715863369120Astec Inc.

The performance data is something the APA and NCAT is starting to track and measure. And employing technology like the Dual Averaging Sensor system can have a direct impact on the final result.

“Up until now, the data for environmental product declarations (EPDs) has stopped at the gate, but we're now working to push that out to the job site,” said Powell. “When you run a milling machine and hit your target on the very first pass, because of the precision control systems, instead of having to make multiple passes to correct deficiencies or do extra work to correct imperfect grade, the less diesel fuel it's going to take. So, you end up with a lower embodied carbon roadway.”

However, there were some sections that needed to go much deeper, getting down far enough into the subgrade material, so that NCAT could run it out with a bulldozer for a special new experiment.

“The purpose was, that we have kind of our standard subgrade that we build on top of at the test track. And it's a good quality subgrade and we like to use it if the experiment accommodates it, because it won’t confound the research that you're trying to do on the surface of the track,” said Powell. “In the case of the deep test sections that we milled this time, it was necessary, because we were pulling out our good subgrade and putting back in a bad subgrade.”

Why would the NCAT test track facility want to pull out the good material and replace it with sub-standard sub grade? The state of Mississippi wanted to run a comparison test on the type of subgrade, versus a new material modified with Portland cement.

“We have a new technology that we're evaluating at the track that allows you to slash the cement content and stabilize layers through the use of an environmentally sustainable enzyme,” said Powell. “It’s a low embodied carbon approach to doing stabilization. That was the reason for milling the deep section.”

Down Deeper

1715863386431Astec Inc.

Even at depths in excess of thirty inches, the RX-700 took care of business. For these depths, the DAS ski system was removed, and the team removed material in three different ten inch lifts. It wasn't all asphalt either, there was also sub layer taken out, as well.

The whole job took about a week to complete, with many different working conditions, the likes of which normal contractors in normal circumstances might only come across occasionally. Out on the test track, it was business as usual.

“It was a really unique experience,” said Lenger. “In our industry, there's academia, there's manufacturers, there's producers and customers, and we really value what NCAT does for the industry. That's why we wanted to be involved in helping them rebuild their test track.”

“it was just good work all the way around,” concluded Powell.

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