Going Deep with Cold

By Dan Brown, Contributing Editor

How do you cold-recycle the top 6 inches of a 9-mile stretch of rural asphalt roadway for just $5 million? In several stages, says the Utah DOT. And they did just that this spring on US 191 near Bluff.

A milling machine leads off the cold-in-place recycling train, followed by a mixing trailer, which is followed by an emulsion tanker and a paver.

 The first stage involved milling 3 inches of asphalt and stockpiling the reclaimed asphalt pavement (RAP) at a central plant location. In the second stage, Coughlin Co, St. George, Utah, cold-recycled – in place – the second 3-inch lift and added lime slurry and emulsion in the process. The third stage entailed rejuvenating the stockpiled RAP with emulsion and lime at the central cold plant, then paving it back. A double chip seal completed the process.

By comparison, just overlaying the pavement with 6 inches of hot mix would cost $5 million, says Kirk Thornock, asset management engineer for UDOT’s Region 4. That would not include any milling or trucking.

The thought process

Block cracking on this section of US 191 was severe. Over the years, cracks had grown wider and deeper. Some of them reached up to 10 to 12 inches wide and extended to the full depth of the 12-inch-thick asphalt, says Thornock. Fortunately for traffic, most of the cracks were longitudinal. “It was a very rough road for the traveling public,” says Thornock.

Block cracking on this section of US 191 in Utah was severe.

A few years ago, the Utah DOT named a budget figure of $5 million to fix the road. Several companies had tried to fill the cracks with some type of mastic or cement. “Every one of them failed,” says Thornock. “Plus, the prices for milling and filling those areas with mastic or some type of asphalt got out of hand, even for that simple type of work.”

Meanwhile, Utah has had success with cold-in-place recycling (CIR). “So I started thinking about cold recycling to help bridge these cracks, knowing this would not be an end-all solution, but a very good alternative,” says Thornock. “I noticed that the Nevada DOT had incorporated central (cold) plant recycling into their spec, and the Utah DOT had not tried that yet.”

Thornock wanted to rehabilitate the pavement as deeply as possible – a minimum of 6 inches. Limited funds prevented milling 6 inches deep, then filling back with a hot-mix overlay. And cold-in-place recycling could only go 4 inches deep in one step.

Bergkamp’s Mobile Stockpile eliminates the need to locate off-site material stockpiles

Initially UDOT considered milling 2 inches off and then doing a 4-inch CIR. But UDOT officials discussed that with contractors and other team members. They decided that to mill off 3 inches, then cold-recycle 3 inches would be a superior solution. “One, we would get better compaction on 3 inches,” says Thornock. “And, secondly, the 3-inch lift would help to get the moisture out of the cold-in-place material faster.”

Frehner Construction won the bid to handle the CIR and repaving process. Frehner selected Coughlin for the cold recycling and opted to self-perform the paving work. Coughlin used two Roadtec RX-900 milling machines for the initial 3-inch milling. Once that was complete, the CIR could begin, says Darren Coughlin, owner of the company.

How the train works

Two milling machines led off the CIR train. The first was a Caterpillar PR-450 that upcut a pass 7 feet wide and left RAP in a windrow for the second milling machine, a Roadtec RX-900 that cut a pass 12.5 feet wide. By overlapping a bit, the two machines could cut an 18-foot-wide pass. Quicklime slurry, at the rate of 1.5 percent, was added in the cutting chamber of the Roadtec mill.