Thinlay Success in the Sooner State
When Division One of the Oklahoma Department of Transportation (ODOT) experienced some cracking issues, Division Engineer Chris Wallace looked to the asphalt industry for solutions. Specifically, he looked to Larry Patrick, Executive Director of the Oklahoma Asphalt Paving Association.
“We had some cracking issues on our older pavements, specifically overlays on our concrete pavements,” Wallace said. To rectify the issues, the division performed 2-inch mill-and-fills on a couple of the projects. “We used fabric before paving the asphalt, but placing fabric is very labor-intensive.”
When Patrick was invited to look at some of the projects in question to propose possible solutions, he recalled the success of using a rich intermediate layer that he’d seen on the National Center for Asphalt Technology Test Track. The method was also successful on a section of I-40 near Hinton, Oklahoma, and had been used successfully in Texas and in Oklahoma City.
“After [those] successes, I thought using a rich intermediate layer along with a thinlay would help preserve and maintain roads that were needing some attention, for which funding may not be in place for reconstructing and rehabilitation,” Patrick said. “This is a self healing mix that can be used, within reason, on roadways with cracking and even on concrete roads with success.”
A rich intermediate layer is a 9.5mm mix using highly modified asphalt (HiMA) technology and higher AC content to achieve lab molded density of 97 percent, which equates to about 0.5 to 1 percent higher AC content than Oklahoma’s normal 9.5mm mixes. Since a rich intermediate layer can be placed by a standard asphalt paver, it removed the manual labor required to place fabric, in addition to its crack mitigation properties.
Although ODOT has performed numerous ultrathin bonded wearing course projects over the years, the concept of rich intermediate layers was new to the agency. Wallace worked with the asphalt branch of ODOT’s materials group to come up with a combination of milling, placing the rich intermediate layer, then putting a thin lift of S6 on top.
“Patrick saw what we were trying to do to make our pavements last longer and slow down reflective cracking, and he was able to share his knowledge with us to come up with better solutions,” Wallace said. “The asphalt industry has a great representative in Larry Patrick.”
The Proof is in the Projects
Division One’s first project using this method was performed January 2019 on a badly cracked section of US 69 south of Muskogee. A series of freezing rain events had caused the pavement to deteriorate more rapidly than expected. “We were doing so much patching, we were having to close entire lanes,” Wallace said.
Despite less than ideal paving conditions in the middle of winter, something had to be done. The contractor, APAC, was able to get out there and perform the work on some of the warmer winter days.
Since then, Division One has performed four more thinlay projects incorporating a rich intermediate layer around the state during the regular paving season with “really good results,” Wallace said. Three of the jobs were also performed by APAC, and the fourth, by Rosscon LLC, Tulsa, Oklahoma.
Although the materials for the rich intermediate layer and thinlay process cost more–there are two lifts, after all, and one of the lifts is a specialized asphalt–Wallace is impressed by the results. Because the rich intermediate layer can be placed with a paver, it removes the manual labor of putting down the fabric. Although the two are comparable, Wallace said the added benefits of a rich intermediate layer make it a more attractive option.
“There was some concern with the skid numbers on these pavements,” Wallace said. However, ODOT performed a skid test on two of the thinlay projects and compared those numbers against the results of a skid test on a recently paved open-graded friction course. All three pavements tested were in the high 30s to mid 40s. “We discovered that the data was comparable, and it is possible to get the skid numbers we need with this type of treatment.”
Simultaneously, Wallace was impressed by the smoothness of pavement. “The pavements have held up well and we have not seen cracking in the new sections,” he said. Even the job APAC paved in the heart of winter has held up better than expected. Wallace hopes to see between five and 10 years of service on these projects.
Wallace has shared the program’s success stories with his counterparts around the state, so it’s possible the process may be adopted by ODOT’s seven other divisions.
Wallace advised agencies interested in this process to reach out to the asphalt industry in their state. “Work with asphalt industry representatives like Larry to see what technology is out there,” he suggested. “Reach out to contractors to see what experiences they’ve had with different treatments.”
One challenge with the rich intermediate layer, Wallace said, is that the polymerized AC it requires is time- and temperature-sensitive. It’s also imperative that the asphalt beneath be very clean. “When you’re paving a ¾-inch lift, any type of debris can create an issue,” he added.
Patrick stressed the importance of doing one’s due diligence to ensure a roadway is a good candidate for this process.
Considering this year’s downturn in DOT funding, Patrick has one more suggestion–for state asphalt pavement associations: “Assure them that the asphalt industry has technology and answers to the DOT’s needs.”