The 3800 CR then placed the recycled, stabilized material to design slope with an attached screed with working width up to 14 feet. Compaction of the reclaimed, stabilized base followed immediately behind the screed using a high-frequency, double-drum vibratory roller.

Because the recycled pavement was placed immediately behind the recycler, no windrow elevator or pickup machine – or additional paver – was required, saving labor, fuel and equipment costs, in addition to reducing the carbon footprint of the project.

On Mason County Highway 15, the average working speed was 28 fpm, including base recycling and stabilization, with resulting material paved to slope, pre-compacted, at a rate of approximately 410 tons per hour with minimal disruption to traffic.

HIR Boosts Pavement Management

Hillsborough County, Fla., is maximizing the value of its pavement management dollars – while enhancing its program’s environmental sustainability – by using hot-in-place pavement recycling topped with a virgin lift of asphalt in one pass.

The county, which covers the heart of metro Tampa, is responsible for 3,250 centerline miles, with less than 5 miles of unpaved road. “It’s a good-sized inventory for a county of our size,” says W. Roger Cox, P.E., senior professional engineer, Engineering Division, Hillsborough County Department of Public Works.

The county uses MicroPAVER pavement management software to inventory streets, track condition and plan maintenance. “We transitioned from a previous system, using an overall condition index on a 1 to 10 scale,” Cox says. “This took into consideration seven defects, and evaluated the ride indexes.

“MicroPAVER is different,” Cox continues. “It allows us to break our system into segments, within which you perform samples. We then do an in-depth evaluation of those samples within the section. It’s a visual inspection system, with 19 defects available. We developed our own pavement inspection truck.”

Using its new system, Hillsborough County has achieved an audited pavement condition index (PCI) of 76. “Our goals are established by the county board of commissioners, and our goal is to be at a 55 or higher system-wide,” Cox says.

The county’s pavement management program tracks streets suited for resurfacing, although resurfacing of pavements is considered a capital expense, as opposed to a maintenance expense. “The resurfacing budget is not contained in the transportation maintenance division, but is more of a planned expense,” Cox states. These resurfacing capital pavement projects include mill-and-fill, hot-mix asphalt overlays and HIR repaving.

“You have to have the right treatment on the right road at the right time,” Cox says. “It’s the absolute key to a cost-effective program. If you put the wrong treatment out there, it will not have the survival rate that you need. Our hot-in-place repaving is aimed at our arterials, for the most part.”

For its growing hot-in-place recycling program, Hillsborough County uses a process from Cutler Repaving, of Lawrence, Kan. With HIR repaving, the existing pavement is heated to 300 degrees F. When in the resulting softened, pliant condition, the pavement is scarified to a depth of 1 inch, and in the mobile repaving unit, a recycling agent that restores the viscosity of the aged asphalt is mixed into the scarified, reclaimed asphalt.

This reclaimed material then is reapplied and distributed with a screed as a 1-inch leveling course. While that material remains at a minimum 225 degrees F, a virgin hot-mix asphalt overlay is placed over the recycled leveling course. Cutler’s repaving machine scarifies, applies a recycling agent, places the leveling course, and applies the new overlay simultaneously in one pass. That benefits road users because there is no delay between the time the pavement is recycled and the time a riding or friction course is placed, resulting in a safer work zone for road users and for contractor personnel.

Because the hot virgin mix is placed over the heated, recycled leveling course, the process achieves a thermal bond between the recycled layer and the new layer.

“It’s an accepted methodology,” Cox says. “The thing I like about it is that the recycled mat and placement of the virgin lift on top takes place simultaneously. The bond between the two lifts is homogeneous, laid flat-in. Also, the top lift is made to our asphalt spec; we know exactly what it is, so I know what my customers are driving on. And with the bond between the lifts as strong as it is, we’ve never had a delamination with that treatment. We’ve put it on roads with tremendous volumes of traffic, and we’ve not had a failure.”