The area of Barnes County in east-central North Dakota is about the same as Rhode Island: approximately 1,500 sq miles. But while on average about 700 people reside in each square mile of Rhode Island, the population density of Barnes County is about 10 per square mile.
Accordingly, Barnes County has fewer than 350 miles of county roads. So, instead of keeping a full-time civil engineer on staff, the county board contracts for civil engineering services with Kadrmas, Lee and Jackson (KLJ), a company with offices in Minnesota, the Dakotas, Montana and Wyoming.
Ross Eberle, a KLJ senior transportation engineer, and Barnes County Highway Superintendent Kerry Johnson began planning the rehabilitation of Barnes County Road 21 in 2001. County 21 is a picturesque National Scenic Byway that runs north-south along the Sheyenne River. The road had not had major rehabilitation since an overlay was placed in 1982. By 2005, the road had fairly severe transverse cracking, which is caused by expansion and contraction. Eberle and Johnson’s first choice might have been to reconstruct the pavement; but that was ruled out because the county’s budget had shrunk as a result of the escalating cost of oil. It was then that Eberle and Johnson recalled a presentation by Tom Johnson, president of Midstate Reclamation and Trucking, at a 2003 pavement engineering conference in Rapid City, S.C. Johnson had explained the advantages of cold in-place recycling (CIR), a process of milling and then reusing the top part of a pavement’s asphalt layer as a base for a new surface treatment.
“The cost of CIR along with a thin overlay is about the same as the cost of a thicker structural overlay,” said Eberle. The CIR strategy has an advantage over a thicker structural overlay. If little is done to rehabilitate the underlying existing pavement before placing an overlay, the transverse cracks in the old layer are inevitably reflected up into the new layer.
But CIR greatly reduces reflective cracking because the top of the old pavement is milled, mixed with new liquid asphalt emulsion, replaced and compacted—so whatever cracks there might have been are history. In the short term, it improves ride quality; over the long term, it reduces maintenance costs.
CIR has another advantage in terms of pavement width. The transverse cross section of any overlay is in the shape of a truncated pyramid. To maintain stability at the edges, the pavement must be progressively narrowed as more thickness is added. Unfortunately, the overlay leaves potentially dangerous drop-offs at the edges that can catch a tire and send a vehicle out of control. But CIR adds very little extra height to a pavement because it is a process of reconstituting and replacing the existing material.
“That was an issue with County 21,” said Eberle. “It was already narrow—a 24-ft asphalt traveling surface with 2-ft earth shoulders. For safety reasons, we wanted to at least maintain that 24-ft width. CIR allowed us to do that without widening the road.”
KLJ pulled about 40 cores from County 21 and discovered a 5.5-in. asphalt layer supported by a 6- to 8-in. layer of base material.
“That was another reason we thought CIR would be the right choice,” said Eberle.
“The CIR equipment is heavy, and industry experts told us we would need at least 6 to 8 in. of base to support that equipment.”
An offer they could not refuse
Because Eberle had no previous personal experience with CIR, he asked engineers in Minnesota, Montana and South Dakota who had used the process for their opinions of CIR. He received only positive assessments. Then he and the county officials contacted Johnson at Midstate Reclamation and Trucking, who made an unusual offer. Johnson said he would demonstrate CIR on a mile of County 21, and if the Barnes County officials were not happy with it, Johnson would not charge them. That sealed the deal.
Johnson, who is based in Lakeville, Minn., visited Barnes County along with Dan Wegman of the St. Paul office of asphalt producer SemMaterials. They put on a day-long seminar for Eberle and the county officials to further explain the CIR process. They also took delivery of Eberle’s 40 pavement cores and took them to American Engineering Testing (AET) in St. Paul. AET’s Dave Rettner assessed the residual asphalt content of the old pavement and produced a mix design for the CIR project. Rettner specified a Superpave 52-34 asphalt emulsion to be mixed with the milled material.
Johnson’s crew arrived at the site on June 1, 2007, but could not begin work for several days because of heavy rain that fell before and during the work period—a total of 11½ in. in two weeks. To avoid breaking through into the subgrade, it was decided that only the top 4 in. of the 5.5-in.-thick asphalt would be milled. After observing work on the first mile, the county officials approved the project and added mileage; in the end, the project length was 9.5 miles. Despite the rain, Johnson’s crew completed the project within the specified time limit.
That first mile of the road (just south of Valley City, the county seat) averages about 1,800 vehicles a day, whereas the other 8.5 miles see a maximum of 600 vehicles a day. Eberle originally planned to place a 3-in. overlay on the first mile and a 1.5-in. overlay on the remaining 8.5 miles. However, as the cost of a barrel of oil continued to increase throughout the planning and design phases, Eberle and the county officials were forced to consider a phased construction approach for the project. They deferred the overlays and instead redesigned the project to include a double chip seal as a temporary surface. However, given the higher traffic volume on the first mile, they also had a contingency plan to place an overlay on that part of the project if it began to exhibit performance problems.
“We did begin to see some rutting,” said Eberle, “so a 1½-in. overlay was placed on the first mile before doing the first chip seal. We’re also getting some reflective cracking on the project, but it’s much less than what we had before. The ride is not as good as if we had done an overlay, but it’s not bad.”
To further address the rutting, the current plan is to do microsurfacing in 2009 instead of the previously planned second chip seal. Barnes County also plans to do additional miles of CIR in 2009.
Overall, Eberle and the county officials are happy with the results. The cost for the first mile (4-in. CIR + 1½-in. overlay + single chip seal) was $180,000. The cost of the remaining 8.5 miles (4-in. CIR + single chip seal) was $80,000 per mile. These costs are somewhat lower than industry averages because the county provided traffic control during the CIR process and constructed the chip seal with its own crew. Considering that the problem of a continuously shrinking budget is common to most jurisdictions, it appears that CIR is a process whose time has come.