For the past several years, the Colorado Department of Transportation (CDOT) has been evaluating the use, primarily in the southwestern part of the state, of a thin asphalt leveling course followed by a single-lift asphalt overlay for its road rehabilitation projects.
The aim of such a configuration would be to institute an economic method for rapidly and successfully adding years of life to degrading arteries, notably those that skirt or traverse the San Juan National Forest.
A lush, picturesque 1.8 million acres that touches more counties and communities than could reasonably be named with brevity, the San Juan National Forest is the keystone of southwestern Colorado, and the maintenance and health of its roads is of crucial importance to CDOT and forest officials alike.
The leveling course mix design in question requires 2-3% air voids, allowing for easier compaction, providing an adequate mix to fill in any surface ruts or other irregularities, as well as an oil-rich barrier against reflective cracking that may propagate over time. According to Mike Coggins, a CDOT Region 5 resident engineer stationed in Durango, “The strategy has resulted in a noticeable decrease in thermal and reflective cracking and improved performance. Some roads exhibit no cracks even after five or six years of service life.”
Consequently, the U.S. 160 Bayfield to Yellow Jacket Pass project, which describes a path though a portion of the national forest that sidles up near the Southern Ute Indian Reservation, was considered to be an ideal candidate for CDOT’s continued experiment.
The project was bid in January 2015, and Four Corners Materials (FCM), a division of Oldcastle Materials Southwest Group, was the successful low bidder at $5.2 million.
FCM came into the project on a tide of experience paving thin-lift overlays for CDOT, having successfully completed several projects of similar scope in recent years, notably the U.S. 160 Durango to Hesperus project in 2014, which was the recipient of a “Best in Colorado” Asphalt Pavement Smoothness Award and honorable mention for Best Rural Highway Resurfacing. This past project was constructed with the then-recently developed CDOT ST mix (3⁄8-in. nominal maximum) on the bottom lift for 1-in. paving thickness, followed by a 1.5-in. CDOT SX surface mix on top. Prior to construction on this project, the existing surface of the road was in fair condition with thermal cracking affecting ride quality. The placement of the thin lift on the bottom served to level and fill surface irregularities and fill cracks, and it was a perfect match by the CDOT design staff for the existing condition of the roadway. Thus the Bayfield to Yellow Jacket Pass project would bring FCM crews back to U.S. 160 and present a challenge it was equipped to meet.
Beginning in April 2015, the 11-mile Bayfield to Yellow Jacket Pass project consisted of a leveling course plus a 1.5-in. overlay, full guardrail replacement, shouldering, signing and striping, rumble strips, milling and two deep patches. Mike McBreen, FCM construction manager, stated that a game plan was developed shortly after the project was let.
“We began making plans and developing goals for a successful project. The highway asphalt crew always has a stated goal to achieve a finished roadway with zero surface grinds. Additionally, another common goal was to deliver maximum longitudinal and mat density quality,” said McBreen.
Asphalt plant personnel established a goal to deliver maximum quality in both the ST and SX mix types and to eliminate plant breakdowns, which can impact materials quality and project goals. Prior to construction, a meeting at the asphalt plant was held with construction staff, asphalt plant personnel, mechanics crews and management, the purpose of which was to have fresh sets of eyes perform an inspection of the plant and to identify items that could potentially create material stoppages during production. As a result of the meeting, several items were addressed and spare parts ordered, which would, over the course of the project, translate to minimal material stoppages due to plant breakdowns—a crucial point, as the asphalt plant averaged 1,900 tons of material per day over the life of the project.
A long haul
One logistical challenge identified early on by construction crew members was the 30-mile distance from the Gencor asphalt plant to the worksite. In order to achieve the contractor’s goal of zero surface grinds and maximum materials quality, the hauling of the asphalt had to be closely managed. Given the relatively rural setting of the project site, as well as the intervening landscape, maintaining sufficient trucks to haul asphalt was a major challenge. The FCM trucking manager worked closely with local trucking owner/operators to ensure a continuous supply of asphalt to the crew to mitigate lay-down machine and roller stops. Another challenge created by the long haul was that a large number of trucks were utilized to ensure a continuous material supply to the crew. While this allowed for non-stop paving, it also created a great deal of risk, with large amounts of unlaid asphalt on the road at any given time. McBreen explained that the weather in southwest Colorado can change drastically, such that a large quantity of hot-mix asphalt (HMA) could be lost to rain if the weather was not closely monitored.
“There were many decisions made by our paving team to shut down early because of the risk of rain,” McBreen said. “These decisions were always made with the quality of the finished product in mind; and at the end of the project, despite the haul distance we avoided laying any asphalt in inclement weather.”
All in the details
This project was not your standard milling and relaying situation; rather, it was a complete overlay.
The asphalt was delivered to the site via belly dump trailers, where it was picked up by a Weiler 650B elevator and eventually laid down by a Vögele Vision 5200-2 with a Carlson Easy Screed. The paving’s two-layer design followed a strict format, beginning with the CDOT ST mix, paved at a 1-in. lift, which consisted of a 3/8-in. aggregate (21%), crusher fine aggregate (21%) and screened crusher fine aggregate (31%) from the Cugnini, Colo., pit, and a ¼-in. washed crushed aggregate (16%) from the Bayfield, Colo., pit. In addition, 10% RAP and 1% hydrated lime were added to the mix, along with Western Refining PG 58-28 asphalt cement at a rate of 6.6%. Before moving on to the second and final layer, two Hamm HD140 tandem drum steel wheel rollers provided a full vibratory breakdown pass and an intermediate compactive pass, respectively, followed by a Hamm GRW280 rubber tired roller in the finish position, again for a single pass.
Layer two was a 1.5-in. lift of CDOT-specified SX, which consisted of a 5⁄8-in aggregate (27%), 3⁄8-in. aggregate (17%), crusher fine aggregate (10%) and screened crusher fine aggregate (35%; from the Cugnini, Colo., pit), and a 1⁄4-in. washed crushed aggregate (10%; from the Bayfield, Colo., pit). In addition, 1% hydrated lime and 5.7% Western Refining PG 58-28 asphalt cement were introduced into the mix. During the SX overlay portion, a Hamm HD140 was used for breakdown (two full vibratory high-frequency passes), a GRW280 rubber roller acted as the intermediate roller (two passes) and compaction was completed with a single static pass of the Hamm HD140.
A focus on QC
The FCM quality control staff set up a program to ensure that all materials and density characteristics exceeded project specifications. Its main laboratory is AASHTO Materials Reference Laboratory-accredited and contains the latest in materials-testing equipment, along with a fully trained and certified staff for in-house testing. Material samples on the project were obtained every 500 tons and tested for asphalt content, volumetric properties and gradation. Additionally, a technician was stationed on the project at all times to continuously monitor in-place density trends and to make slight adjustments to roller patterns to ensure maximum density quality. The specification results are shown in Table 1. (When complete, the project materials quality elements were all above a pay factor of 100% with the lowest being 102% and the highest pay factor achieving the maximum incentive of 105%. The in-place densities pay factor was 102% for mat and 105% for joint densities. FCM was awarded a materials quality incentive payment of $81,000 for its materials quality efforts.)
The job smoothness goal set by the crew of zero “must grinds” along the entire 11 miles of roadway was itself a challenge. While laying a thinner lift of HMA can certainly enhance roadway smoothness, if placed incorrectly it can leave chatter, and reflective issues from below can surface and decrease ride quality. Therefore, best practices associated with smooth paving were closely followed. Each transverse takeoff was closely inspected with straightedges. If a bump, dip or surface irregularity was revealed and couldn’t be corrected, the paving was stopped. The defect was removed, the surface was cleaned and re-tacked, and the takeoff was repeated. This process occurred at several areas multiple times. Once complete, there were no paving-related surface grinds over the whole 11 miles, and FCM was rewarded with a quality incentive payment of $300,000 which equated to a better than 90% capture of the available smoothness incentive for the project.
The next challenge
The partnership between CDOT Region 5 and FCM will continue into 2016.
“They have been working in the area for 30 years and they do it right,” CDOT’s Coggins added. “We look at every project from a team perspective. CDOT performs the design and then communicates with the contractor regarding specific expectations, intentions and possible problems and solutions in an effort to maximize quality. FCM is aware of the specification requirements and is constantly trying to improve its operational processes.”
A simple, moderately diverted traffic management plan in action.
Further projects on CDOT’s horizon include Wolf Creek Pass, also along U.S. 160, which will come to fruition in fall 2016, and some work on SH 145.
After many years and many miles of smooth paving, with the recent successful use of the leveling course plus thin overlay strategy, and with the teamwork concept working well, it appears that reliable methodology of highway improvement success has been found in southwestern Colorado. R&B