Micro Surfacing Pavement Maintenance Process Addresses Severe Concrete Road Surface Issues and Improves the Ride Smoothness of the Final Overlay

Major repairs of the C-470 bike trail will likely require weekly closures, with access around the work zones.

Castle Rock Construction Co. of Centennial, CO is the primary contractor for the project, with all repair work on C-470 scheduled to be completed by June 1, 2010.

Sequence of operation

According to Ron Buck, CDOT resident engineer involved with the project, the original 25-year-old concrete roadway, was in desperate need of maintenance due to the heavy traffic load it's experienced over the years.

"Traffic levels have increased threefold over what the original design specifications were intended to meet," Buck notes. "The sequence we're following in repairing and improving the overall structure is to remove and patch badly deteriorated slabs of old concrete, seal and repair cracks, fill ruts with micro surfacing material, cover the entire travel lane surface with micro surfacing material, and then overlay the structure with a 2-inch SMA surface course."

When the original estimate was put together, Buck says plans allowed for ¾-inches of micro surfacing covering the old concrete road (1-inch in rutted areas).

The Type III micro surfacing design being used on the C-470 contains aggregate sized from ¼-in. to 3/8-in., with application thickness ranging from 3/8-in. to ½-in.The Type III coarse aggregate mixture is designed to correct severe surface conditions and provide improved skid resistance under heavy traffic loads.

On this particular project, however, improving skid resistance was not an objective; whereas correcting surface conditions of the existing concrete roadway and improving the final ride quality of the SMA wearing course are.

"We designed the mix to fill ruts using a rut box and then come back to place an edge layer over the entire travel lane to serve as a bond breaker for the SMA overlay," explains Dan Quire, account manager for Road Science LLC, supplier of the emulsion mix design and technical service.

Having the budget to achieve further surface enhancements proved beneficial to CDOT's desire to do everything it could to upgrade this particular section of highway.

"With that built into the plan that was approved for funded, we had some leeway to cover additional micro surfacing applications to improve the overall ride quality of the road," Buck says. "We placed a tack coat before placing the micro surfacing lift and again before placing the SMA wearing course to ensure a good bond between multiple layers being placed."

Masoud Ghaeli, CDOT Region 6 materials engineer, agrees with the solution to structure and ride improvements

"After analyzing the existing road structure conditions and the traffic volume the road needed to support, we determined asphalt to be the best approach," Ghaeli says. "Using the micro surfacing paving approach, we're able to fill the ruts and provide a leveling course as well. Applying more than one coat of micro surfacing layer is helping to achieve a smoother ride with the final SMA overlay. And, we hope the extra micro surfacing will help retard reflective cracking from the concrete joints."

A-1's portion of the project represents approximately $4 million. When the project is completed, A-1 will place approximately a half million square yards of the Type III micro surfacing slurry over the 36 lane miles and adjacent shoulders.

"We've completed about 25% of the work we need to do and we'll be back on the project in the spring to finish the work," Vagher says. "It's our first micro surfacing project and we're pleased with the way it's turning out."

In all, Colorado will receive more than $400 million for transportation projects and $103 million for transit projects under the ARRA stimulus program. For more information about how Colorado is putting the recovery act to work, visit www.colorado.gov/recovery. For a list of CDOT's Recovery Act projects, visit www.dot.state.co.us/arra.