Designing a sustainable road with PRAS and a Safety Edge

When the North Carolina DOT (NCDOT) asked Boggs Paving Inc., a highway/heavy contractor out of Monroe, NC, to identify a few projects they could try a mix design containing post-consumer reclaimed asphalt shingles (PRAS), Rock Hill Church Road in Union County, NC, surfaced as a possibility.

 "We had been processing shingles for use in asphalt mixes for several months and preparing our mix designs for submittal to NCDOT," says Greg Tucker, project manager with Boggs Paving. "Once we got the design approvals, we worked with NCDOT to select a good candidate road to utilize the PRAS mixes, and this one was chosen."

Both Boggs Paving and NCDOT wanted a project that had less traffic so the mixes could be tested and the contractor could take its time. "This road was chosen because it's primarily low traffic volume road and a good candidate to try new technology," says Tucker. In addition to a mix containing PRAS, the project was also to be a test the Federal Highway Administration's Safety Edge technique.

The project

Beginning on October 4, 2010, the project on Rock Hill Church Road consisted of patching and widening (two feet) the existing road from NC 218 to Lawyers Road in Union County. The 2.5-inch strengthening course used 5,350 tons of RI 19.0C mix, while the 1.5-inch surface course used 2,900 tons of RS 9.5B mix.

The 1.5-inch surface course contained 5% PRAS while the strengthening course consisted of 4% PRAS. Paving was performed at approximately 35 fpm for both courses.  All mixes used on the project also contained percentages of reclaimed asphalt pavement (RAP) for the hot mix designs.

The original plan for Rock Hill Church Road project was to use PRAS in all three layers of asphalt. However, PRAS was not used in the base course, as Boggs decided to start the project prior to the mix design being approved.

Equipment used included a Roadtec 185-8 paver, DD112 and DD90 Ingersoll Rand steel drums, a PT125 Ingersoll Rand rubber-tire drum, and an Etnyre distributor. The project was completed December 6, 2010.

This was the first state road project that Boggs used PRAS in the mix design, says Tucker. "We have not had prior experience on any NCDOT projects with PRAS," he says. "We have done some smaller parking lots and smaller roads to get testing data, but this was the first state road we did. We have manufactured the mix to assist in mix designs and utilized the produced mix at our facility in Monroe for stockpile areas as well."

Tear-off shingles were used in the mixes on Rock Hill Church Road.  "Manufacture-waste products were not accessible as there is no shingle manufacturing plant in our region," says Tucker. "We have a permit that allows us to be an intake and processing center for tear-off shingles from residential construction." (See box, "Roof to Roads Program.")

Mix design challenges

According to Tucker, Boggs Paving didn't experience any challenges during the actual paving on the project. "Our quality control personnel did a great job of managing the design process and working with NCDOT for any necessary changes or additional information they required."

However, Tucker notes that from a mix design standpoint, there were several challenges Boggs Paving faced. "We had to do a fair amount of R&D in a limited time frame with limited expertise in the area," he explains. "Some issues we had included calculating the correct time to add shingles in the mix design process, the appropriate temperature to add the shingles to the mix, and at what temperatures to compact those samples. Another issue was determining exactly how much liquid was being recovered from the shingles and what our equivalent binder content actually was."

Despite these challenges, Boggs Paving is heavily involved in green initiatives, says Tucker, and sees many benefits to utilizing PRAS in its mixes. "Our company has been a leader in the 'green' movement by increasing the amounts of reclaimed asphalt pavements in all mixes, piloting the largest warm mix project in North Carolina – which involved more than 90,000 tons of warm mix asphalt (WMA) on NC 218 in Union and Anson Counties – and now introducing PRAS into our portfolio."