A full semitruck puts its entire weight down on a thin, two-lane rural road in Mason County, Ill., as it rolls hard and fast to its delivery destination. It is all the glamour the movie industry can give Highway 4.
Every day, lines of 18-wheelers packed to capacity with popcorn kernels from a local plant peel off route after route. For a while, the pavement was taking on the life of the little golden snack nuggets. Every so often, the constant strain would cause it to pop unpredictably.
Potholes and reflective cracking had their way with Highway 4 without needing to put up much of a fight. It was time for more muscle.
For years Mason County milled off anywhere from ¾ to 1½ in. of damaged asphalt and replaced it with fresh material. But the cracks were more like the neighboring resilient weeds—their roots ran deep.
“The freeze-thaw cycle makes those cracks reflect right back up the new pavement,” Mike Pedigo, Mason County engineer, told Roads & Bridges.
After taking core samples, Pedigo’s crew discovered the cracks dug in well beyond the 1½-in. maximum milling depth. There also was another revelation: The 4½-mile stretch contained two different pavement designs. The far north and far south portions of the job were made up of pure asphalt, while the middle section contained a layer of concrete.
“The plan was to [mill] 3 in. [all the way through],” said Pedigo. “When we were out there coring we discovered we needed to potentially go down as far as 4 in. The old concrete is where we decided to recycle just 3 in. of pavement.”
From the word “recycle,” Mason County was venturing into something entirely new, but it was entirely necessary to keep the trucking economy pushing forward. There was one other pivotal requirement: the need to withstand up to 80,000 lb.
“We were spending a lot of money just patching the road year after year,” said Pedigo. “It definitely needed that 80,000-lb upgrade.”
Reassuring recycling
In searching for the right solution, cold in-place recycling was quick to enter in on the conversation. The procedure would allow traffic to carry out its daily tasks, but it still left an uneasy tickle in the pit of Pedigo’s stomach.
“Anything new I am spending the taxpayers’ money. I am always a little concerned if it does not go well because people are going to be looking at me asking, ‘Why did you try that?’ I was a little nervous about it.”
Lead designer Hutchison Engineering Inc. and SemMaterials L.P., which handled the mix design and quantity of material, were a calming influence to Pedigo and his crew, and on June 22 contractor Midstate Reclamation and Trucking stepped out to make its first cut with a Terex PR-1050 Roto-Mill Pavement Profiler. Chewing a 12½-ft-wide path, the profiler placed the material with a 1½-in. aggregate top size through a Nesbitt CRMX2 recycler. Any aggregate exceeding the 1½-in. maximum was recrushed, and when the size specifications were met, binder was added to the mix.
“One of the challenges of recycling is, although we have a screening deck in the train in order to control that top size, you really have to go with what the milling machine can accomplish on the road,” Chip Ray, general manager for SemMaterials L.P., told Roads & Bridges. “It is not like you are moving it off site to crush it, size it, screen it and get a uniform gradation. That is one of the challenges and why an engineered process gives you more predictable results than a nonengineered process, because you do a lot of testing up front to react and make changes in the field.”
SemMaterials L.P. used its Reflex Asphalt Emulsion for the job. The oil is custom-fit for the road at hand and had to reach a high level of performance in four categories: rattling, rutting resistance, thermal cracking resistance and moisture susceptibility.
“The issue you have with recycling is you are using existing materials in different locations,” said Ray. “Even in this project we had two different pavements that we were looking at, and although you might not have to change the formulation for your emulsion, you do have to design it to meet those performance criteria.”
The 4-in. sections on the north and south portions of the project contained 2.7-3.1% emulsion, while the 3-in. concrete portion received 2.2-2.5%.
The emulsion content also reflected mix designs for two different gradations: medium and coarse.
“We could change the emulsion content based on the gradation,” Mike Exeline, field engineer for SemMaterials L.P., told Roads & Bridges. “Overall there was good quality out there. The 3-in. section came out coarser. It was a little harder aggregate, therefore it had a little less emulsion in it.”
Once the right emulsion was added it was thrown back into a Cedarapids M51 Windrow Elevator and then a Cedarapids 561 asphalt paver. The machine laid material 12 ft wide, which fell over into the shoulder. Two rollers handled compaction duties on the recycled portion of the roadway. Serving as the breakdown roller was a Hamm HD130 tandem vibratory roller with 84-in.-wide drums. A 50,000-lb Hamm GRW18 rubber-tired roller followed. Appropriate density was determined by the number of passes. For the 4-in. portion, both the steel-drum and pneumatic rollers made three passes. The vibratory roller made just two passes on the 3-in. lift, while the pneumatic roller executed three. A nuclear gauge was used to check the density every half mile, and crews checked the smoothness with a 16-ft straight edge set at 3/16 in.
With compaction complete, crews gave the recycled layer some alone time. This allowed the surface to dry and reach a moisture content of 2%.
Prime contractor R.A. Cullinan & Son Inc. wrapped the project with a 3-in. layer of hot-mix asphalt (HMA). The HMA overlay consisted of a 1½-in. leveling binder and a 1½-in. surface course. Both mixes contained a PG 64-22 binder. The leveling mat contained Illinois 12.5 aggregate, while the surface held Illinois 9.5 or 12.5 aggregate.
A continuous-flow HMA plant located about 45 minutes away cranked out the material, which was then trucked to the site and dropped into the Cedarapids CR451 paver at 280-300°F. The paving path of the overlay was 11 ft wide. R.A. Cullinan used a tarp to cover the material for the ride to the site. Insulation was used in cooler weather.
The Hamm pneumatic roller was used first during compaction, followed by the Hamm vibratory roller. Nuclear gauges were again used to check density, which came in at 93.5-94%.
Smoothness was checked with the 16-ft straight edge set at 3/16 in.
Account for savings
“The trucks have been hitting that pavement hot and heavy,” observed Pedigo. “They are coming out of that popcorn plant making rough turns, and there is no sign of any fatigue in that pavement at all.”
The cold in-place recycling project did not tire out the county’s budget, either. Officials estimate that by avoiding traditional construction methods about $300,000 was saved. In addition, energy costs were about 10% of what was usually consumed.
“It also would have taken three times longer to lay it down the traditional way,” added Pedigo.
“The only thing new that you brought into the recycled portion was the oil. There was no new asphalt or rock, so you save a lot on trucking costs and material costs. That is big savings.”
Everything about the cold in-place process has gone over big in Mason County.
“I am glad we decided to do it this way, and I would definitely do it again,” said Pedigo.
