The existing pavement surface on St. Louis County C-SAH 7 was severely cracked with some distortion. This roadway is classified as a major collector and serves as a connector between Duluth, Minn., and the Iron Range. The 7 miles of roadway needed to be upgraded to 9-ton in order to meet the needs of the traveling public.
“We entertained a few options and did some cost comparisons,” said Ross Benedict, engineering technician principle with the St. Louis County Public Works Department.
Option 1: Reclaim and overlay. This option would be the least costly, but the thickness of the bituminous courses necessary to meet the required GE would increase the height of the roadway too much, reducing the width of the shoulders to less than satisfactory.
Option 2: Do a complete reconstruction. This option was not economically feasible. Current reconstruction costs are between $1 million and $1.25 million per mile for a project such as this, and the $7 million to $8.75 million total cost would consume two-thirds of the total state-aid funds available for projects. Total reconstruction also would require road closure for an extended period of time, resulting in a 21-mile detour route.
Option 3: Reclaim and overlay with Geogrid. This option involves reclaiming the in-place surface, removing and salvaging the uppermost 8 in. of the resulting aggregate base, placing a layer of Geogrid, placing and compacting the salvaged aggregate base and placing the bituminous courses.
The additional cost of this option is approximately $5.25 per sq yd, a total of $565,000 for a project of this size. This option also required a complete road closure, although the time frame would be much shorter than the reconstruction option. Also, the contractor would have to construct the roadway half at a time, stockpiling the salvaged aggregate on the opposite side of the roadway. Because of the narrow width of the roadway, there would be a risk of losing the aggregate over the edge.
Option 4: Reclaim, stabilize and strengthen the aggregate base and overlay. This option involves reclaiming the in-place surface, treating the resulting aggregate base with a chemical additive to stabilize and strengthen and placing the bituminous courses. The estimated additional cost for this option was $1.30 per sq yd, for a total additional cost of $140,000. All construction required for this option could be completed with lane closures, eliminating the need for a detour.
St. Louis County chose Option 4. To minimize the height increase, approximately 2 in. of the in-place surface was milled off prior to reclamation. It was then reclaimed to a depth of 8 in.; the aggregate base was prepared by the motor grader with a rear-mounted scarifier to a depth of approximately 2 in. to loosen up the aggregate base and provide striations to prevent the chemical additive from running off.
The base was then treated with Team Laboratory Chemical Corp.’s BASE ONE Aggregate Base Stabilizer . It was applied with a 5,000-gal tanker truck with a computerized metering system. The mixture consisted of 165 gal of the base stabilizer concentrate and 5,000 gal of water. It was applied at a rate of 0.60 gal per sq yd in order to provide an effective rate of 0.005 gal per sq yd per in. of depth.
This tanker was normally used for the application of a calcium-chloride solution. If the surface started to dry out prior to mixing, a second tanker truck applied additional water. A standard road reclaimer was used to mix in the base stabilizer mixture to a depth of 4 in. A pads-foot roller followed immediately behind the reclaimer, followed by a motor grader and smooth-drum roller.
This method was chosen because it was the least expensive way to meet the required 9-ton loading and offered the least disruption to the traveling public. It also was a good opportunity to try the stabilizer with minimal risk, as the proposed typical section theoretically provided the required GE without the additional strength provided by the stabilizer.
The cost of milling 2 in. of bituminous pavement would be offset by the reduced cost of bituminous mixtures due to the availability of RAP in an area that did not have existing stockpiles of the material. The estimated cost of $140,000 for stabilization could have provided approximately 2.5 in. of aggregate base, Class 5, over the entire project, but the resulting height increase would be unacceptable.
The cost of the stabilization could also have provided for an additional 0.5 in. of bituminous mixture over the entire project, but, based on the data provided by Team Lab and the experiences in other counties, the additional GE provided by the additional bituminous mixture would not equal the strength obtained by utilizing the stabilizer.
Bituminous pavement milling took place Aug. 31, 2009, through Sept. 4, 2009. The bituminous pavement reclamation occurred Sept. 18, 2009, through Sept. 28, 2009, and was immediately followed by aggregate base stabilization through Oct. 1, 2009. Bituminous paving started on Oct. 26, 2009, and all paving was complete on Nov. 10, 2009.
During the 25 days between the completion of base stabilization and the start of paving, no maintenance grading was required until the 22nd day, even though rain or snow occurred on 12 of the 22 days.
The stabilizing chemical improved the stability and strength of the aggregate base, as demonstrated by the ability of the aggregate to resist rutting and potholing for the better part of three weeks. Fine grading of the treated base could not be accomplished unless there was sufficient moisture to temporarily “unlock” the aggregate base.
The operator of the motor grader reported that it certainly was more difficult to cut into the aggregate base treated with the stabilizer than an untreated reclaimed aggregate base. Field observations indicate that there was less dust caused by traffic than the typical gravel road, which demonstrated the ability of the treatment to lock in the fines.
Throughout all 7 miles of the project, there were only two locations that showed any pavement distress in the non-wear course and required some additional work prior to placing the wear course. The pavement surface will be observed for signs of pavement distress or displacement over the next few years, but, based on field observations during paving operations, it is unlikely that there will be any widespread failures.