Kentucky Puts Low Gyration Mix to the Test
We all know that in our industry, what works for one climate may not work for another, and low gyration mix designs are no exception.
“We have a similar mix design at the National Center for Asphalt Technology (NCAT) test track in Alabama,” said Robert Semones, branch manager of the Kentucky Department of Transportation (KYTC) Division of Materials. “The environment of the NCAT test track does not experience the freeze thaw cycles Kentucky does, so the asphalt industry and KYTC wanted to test the low gyration mix in our region so we could research it within our regular freeze thaw cycles.”
So, in October 2017, KYTC decided to perform a pavement durability test section along U.S. 62 in Woodford County, between U.S. 60 and Old Frankfort Pike. The 3,660-ton, 4.3-lane-mile project consisted of a 1-inch lift of a low gyration (65 gyrations) mix with a higher asphalt content and a lower air void content than a typical mix design.
“The compactive effort of the roller is based upon the design gyrations of the mix,” Semones said. “With the increase in asphalt content along with the lower air voids, the compactive effort is much less, therefore less damage is inflicted upon the aggregate.”
The roller used on the asphalt mat was an Ingersoll Rand DD-110 HF with a low amplitude and high frequency. The static weight of the roller was 11.48 tons.
“There is concern that the design gyration (Ndes) levels used with the Superpave gyratory compactor (SGC) have not been optimized to maximize field performance,” Semones said. “This is our opportunity to place a hot mix asphalt to test that statement. While a high gyration design is beneficial for rutting resistance, it may inadvertently cause durability issues such as mixture cracking and raveling.”
“We’d been talking about trying out a low gyration mix in the state of Kentucky with Brian Wood, executive director of the Plantmix Asphalt Industry of Kentucky (PAIKY), but we couldn’t find a suitable project to try it on,” Semones said. Then, Quality Control Director of ATS Construction, Keith Bishop, came to KYTC with their U.S. 62 project. “And that job turned out to be a perfect fit.”
So, ATS’s QC/QA Manager, Howard Meade, began developing a low gyration mix design for the pilot project that was then approved by KYTC.
“As an association, it is our job to help our contractors and state transportation agency to work hand-in-hand,” said PAIKY Marketing Director Paul Del Rio. “It’s encouraging when one of our larger members, ATS Construction, approaches the transportation cabinet to suggest modifying a project section to help it obtain real-world results in a local application.”
What they landed on was a 65 gyration mix with two-tenths percent more asphalt content, as well as fewer air voids compared to traditional hot mix. KYTC’s target air voids for normal hot mix is 4 percent, whereas this mix is designed to have target air voids between 3 and 3.5 percent.
“Whenever you increase the asphalt content for added durability, you’re going to lower the air voids because there just isn’t as much space for air,” Semones said.
In the end, the CL3 0.38D surface mix contained 5.9 percent AC (PG58-28 virgin binder), 2.8 percent air voids, 20 percent RAP and 15.7 percent voids in mineral aggregate (VMA) with 55 percent material passing the #8 sieve.
“ATS sent us a portion of the mix and we checked it in our lab to make sure all the volumetrics met specification,” he added. They performed the tests at KYTC’s central lab in Frankfort, Kentucky. “The main criteria for us is acceptable volumetrics.”
After the mix design specifications were met, ATS delivered the mix to the job site.
“While we were on the job site, we noticed that the mix was performing really well under the heavy roller, too, with very little deformation,” Semones said. “That was one of the main things we observed in the field.”
However, the real proof will be the new mix’s performance over time. KYTC plans to inspect the roadway every three months for at least the next three years, if not longer.
“We’ll drive out there, walk the project, look at the mat and document any cracking and raveling of the mat over time,” Semones said. “Cracking is the main issue we’ll be looking for.”
Although the job was a great fit for the project, one downfall is that the roadway doesn’t have the same equivalent single axle loading (ESAL) counts as the NCAT test track.
“U.S. 62 just doesn’t see the excessive truck loads like NCAT provides,” Semones said. “The NCAT test track is set up to give you results over a short period of time, whereas it’s going to take us longer to study the mix’s performance on this roadway because we don’t have the truck loading that NCAT provides with their research.”
“If it looks like it works well, we’ll try to introduce it in other parts of the state—maybe even on higher volume roadways,” Semones said.
“We are pleased that our membership is invested in producing consistently better asphalt,” Del Rio said. “A longer lasting and safer product can mean saved lives while a decreased life-cycle cost means a better value for the taxpayer.”
Mix and Match the Joint
Another goal of the project was to take a closer look at the construction of the longitudinal joint.
“We’ve been having a lot of trouble with our longitudinal joints quickly deteriorating over the past few years,” Semones said.
They tested out a new method of constructing the joint.
On one section of the job, the joint was constructed in the traditional method of butting up the joint to the mat placed the previous day. On another section, they applied trackless tack to the joint before laying the second pass a few minutes later.
“Our theory behind that is that when the trackless tack cures, it’ll bond the joint between the two mats together and help keep the water out,” Semones said.