Tuesday | January 23, 2018

Racetrack Asphalt Mixes Require Perfection

Check out Austin Bridge & Road’s good paving practices with chalklines marking out their paving pattern. With a high profile project with high standards of specifications, crew members remember atte... [Full View]

For the intermediate and surface courses, MTVs fed pavers working in echelon.

Richard Mills is the vice president of operations for Austin Bridge & Road, a division of Austin Industries, and he credits the project’s success to the high level of collaboration

Austin Bridge & Road team members watched every element of quality control.

Richard Mills credits the company’s competitor in the market for supplying great mix for good collaboration on the project.

Just as not all paving projects are the same, not all auto racing is the same. The most popular racing series in America, NASCAR, has very little in common with the most popular racing series in the world, Formula 1. NASCAR tracks are oval, while F1 circuits are road courses featuring a combination of twists, fast turns, and long straights followed by heavy braking zones. F1 circuits include purpose built racetracks and converted street courses.

Now that the United States has joined the F1 world with the new Circuit of the Americas (COTA) purpose-built track outside of Austin, it’s time to take a look at the paving and the mix involved there. As expected, the paving of a racetrack presented an opportunity for asphalt professional Austin Bridge & Road, an Austin Industries Company, based in Dallas. The racetrack building industry is notoriously secretive with its mix designs, but we’ll reveal this one here. Let’s look at how it all came together.

The Fédération Internationale de l’Automobile (FIA) oversees the Grand Prix series of races and has exacting specifications for the design and construction of any new tracks, all with an eye on driver safety. Nothing is perhaps more critical than the asphalt-paved surface. Imagine achieving speeds of 200 mph and heading into a 90-degree turn or tighter. The road surface has to work for the driver.

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“We were awarded the construction contract for the COTA track, specifically the construction of the top four layers of the track, which includes the flex base, an asphalt base course, an intermediate course and the wearing course,” Richard Mills said. He’s the vice president of operations for Austin Bridge & Road. “We also completed subgrade preparation, flex base and asphalt for the side roads, parking lots, the pit lanes and the service roads inside the track.”

The approximately $33 million project for Austin Bridge & Road required a total of 380,000 tons of flex base and asphalt paving throughout the 900-acre site. The racetrack itself required 90,000 tons of asphalt.

“The amount of stress this pavement will undergo is tremendous,” Mills said. “It’s not downward stress, it’s shear stress. As a result the specifications for paving are extremely stringent. These include very tight gradation requirements on the aggregate and extremely high performance requirements for the bitumen or liquid asphalt and we had very tight control factors relative to temperature and consistency of the material itself—it was a big challenge to accomplish all of that.”

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Austin Bridge & Road focused on three critical areas of the overall paving specifications:

  1. Consistency and quality of the material
  2. Consistent paving practices
  3. Smoothness and compaction requirements

“We needed consistent practices throughout,” Mills said. “We needed to deliver the asphalt to the pavers at the exact temperature and without interrupting them. It was critical that we continuously progress the pavers, making sure the paver consistently placed a thickness that did not exceed 1/8 of an inch tolerance. That was a big challenge.”

To help maintain the material’s required minimum temperature consistency of 302 degrees F and to help ensure that a continuous flow of asphalt fed to the three high density pavers, Austin Bridge & Road used two Roadtec SB-2500D Shuttle Buggy® MTVs (material transfer vehicles).

For the intermediate and surface courses, MTVs fed pavers working in echelon.

For the intermediate and surface courses, MTVs fed pavers working in echelon.

The Roadtec Shuttle Buggy MTV is designed to deliver the asphalt from the haul truck to the paver. The Shuttle Buggy features a patented anti-segregation auger that remixes materials to eliminate aggregate segregation and temperature differences.

The Shuttle Buggy MTV also supports non-stop, continuous paving, which was critical for Austin Bridge & Road’s ability to meet the pavement specifications.

“There was no wiggle room in the specs,” Mills said. “We needed the paved surface to be smoother than smooth throughout the race course.”

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The 25-ton (22.7 metric ton) capacity of the larger Roadtec SB-2500D Shuttle Buggy, compared to the smaller units in the company’s MTV product line, gave Austin Bridge & Road the ability to deliver an incredibly smooth pavement.

“We took advantage of the Shuttle Buggy’s ample storage capacity to smooth out our truck cycles and prevent paving disruptions,” Mills said. “The Roadtec MTVs helped give us confidence that we knew we always had enough material there to feed the pavers.”

The Roadtec Shuttle Buggy’s truck unloading capacity is rated at 1,000 tons per hour (907 metric tons/hour) and paver loading capacity at 600 tons per hour (544 metric tons/hour).

An aerial view of the COTA track shows its 3.4 mile twists and turns.

An aerial view of the COTA track shows its 3.4 mile twists and turns.

Once the mix got to the paver, the word that the engineering consulting firm used to describe it was “homogeneous.”  The engineers’ visual and testing evaluations specifically looked for the asphalt texture to be homogeneous and maintain temperature consistency in delivering the asphalt mix to the pavers at the right temperature. The expectation was for uninterrupted paving. Plus, the elevation of the paver screeds needed to maintain thickness tolerances that did not exceed 1/8-inch requirement.

Richard Mills credits the company’s competitor in the market for supplying great mix for good collaboration on the project.

Richard Mills credits the company’s competitor in the market for supplying great mix for good collaboration on the project.

For Austin Bridge & Road, the four-layer project started with a flex base layer lift placed over a bed of six inches of crushed concrete. The flex base was crushed limestone processed through a pug mill adding 8 percent moisture and was laid with a paver. The density was to be 97 percent of maximum density.  The company achieved a smoothness on the flex base layer of less than or equal to 4/10 of an inch.

“Typically we would lay flex base using blades and rollers but we were after tighter control,” Mills said. “We needed to control the thickness, density, consistency, smoothness and we achieved that by using the paver.”

The second layer placed on top of the flex base was a 3.1-inch course asphalt base—a standard HMA mix, which was placed by the paver.

For the binder and final wear lifts, the engineering firm spelled out all of the proprietary custom-mix physical properties of the material, the control requirements, and the finish results required for a high-speed precision racecourse.

Austin Bridge & Road team members watched every element of quality control.

Austin Bridge & Road team members watched every element of quality control.

“We’ve never worked on a paving project before that has this level of control and specificity,” Mills said. “It makes sense that they’re expecting near perfect conditions to optimize the performance of these elite F1 drivers. If you were to show these requirements to most paving companies, they’ll likely say the third course is near impossible to achieve, while the top, final course would be impossible. These paving factors are very, very stringent and just meeting the specifications is an accomplishment—we’re proud to have done so.”

Mills credits Brian Arent, quality control manager for Austin Bridge & Road, for developing a plan that requires precision execution and then taking responsibility for executing it.

“There are two keys to the success of the project,” Mills said. “The first was the unwavering commitment to succeed on the part of our people. Everyone wanted to make sure we met these stringent specs. And the second, was a very high level of collaboration among the technical people on how to execute the project, the materials people on meeting the specifications, our competitor in our market who we subcontracted the mixing portion, and the equipment suppliers to ensure we had the right machines, support and contingency plans should there be equipment failure.“

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Supplied by Closner Equipment, San Antonio, Texas, the Roadtec SB-2500D Shuttle Buggy MTVs played a significant role in Austin Bridge & Road’s success according to Arent.

“The Roadtec Shuttle Buggies were not a requirement in our specs but for us they were necessary on this project,” Arent said. “They gave us additional mixing activity to ensure the critical material consistency we needed and they gave us additional storage that minimized the possibility the pavers would stop during the paving process. It was clear to everyone that the pavers could not pause or stop if we had hoped to achieve the smoothness we wanted and needed. Considering the end-result specifications on this project, I would have felt less confident and extremely nervous about achieving the results if we did without the Roadtec Shuttle Buggies.”

Austin Bridge & Road had three pavers and two shuttle buggies effectively working side-by-side. They were staggered but functionally side-by-side. They completed the project within the 391 days allotted. The newly completed Circuit of the Americas F1 track is not only a welcome addition to the global FIA Formula One World Championship Race Calendar but locally as well.

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