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Concrete paving, Colorado style

Concrete paving, Colorado style

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    State Highway 66 runs through the foothills of the Colorado Rockies between Lyons and Longmont. CDOT repaired 27 lane-miles (244,000 square yards) of the roadway with 6-inch thin whitetopping in 2009. After two years in service, the pavement shows no signs of distress. Photo: Bill Palmer

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  • SIX-INCH THIN WHITETOPPING VS. FOUR-INCH HOT-MIX ASPHALT OVERLAYS: A COST ANALYSIS

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    CDOT has developed a cost comparison model, based on material costs for a sample project: a 1-mile road, 44-feet wide (a 12-foot lane and 10-foot shoulder in each direction), with a total area of 25,800 square yards. Based on initial construction costs, asphalt is $95,000 less expensive. Based on material costs over a 40-year period, thin whitetopping saves $160,000.

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    Thin whitetopping requires closely-spaced construction joints — a deterrent to some municipalities that favor quicker repair techniques. This 6-inch thin whitetopping project has longitudinal and transverse joints every 9 feet. Photo: Bill Palmer

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    In 1999, CDOT placed a 5-inch thin whitetopping overlay on Parker Road, a high-volume urban corridor on the southeast side of Denver. The $2.9 million project covers 12.5 lane-miles; in 12 years, repairs have been limited to minor transverse cracks. Photo: CDOT

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    In urban areas, curbs and gutters pose a challenge to thin whitetopping design. CDOT has two approaches: covering the curb face with an angled, mountable profile (Figure 1); or removing the curb face (Figure 2) to reduce the vertical profile by 4 inches before placing the overlay. Courtesy of CDOT

Utility maintenance. Because concrete overlays typically aren't replaced for more than a decade, CDOT coordinates with appropriate agencies to schedule utility maintenance before thin whitetopping is placed. If emergency repairs must be made, the department advises local crews on how to remove and replace pavement sections — a more involved process than patching asphalt. “Crews are getting used to cutting out sections when full-depth repair is necessary,” says Goldbaum. In the Denver area, local agencies specify a flowable or flash fill material prior to replacing the concrete pavement to avoid settlement or material incompatibilities.

Local materials. A word of caution: know your aggregates. In areas where aggregates are highly expansive, thin whitetopping overlays may require pressure relief joints similar to those used on bridges. After several days of high temperatures, the concrete can expand too much. “It can blow up on you — just pop up at a random joint. We've seen it a couple of times,” says Goldbaum. Although there's no way to entirely predict or prevent the problem, it can be mitigated with a pressure relief joint.

Education. Working with thin white-topping requires a paradigm shift for maintenance crews who have placed and patched only asphalt. The American Concrete Pavement Association (ACPA) conducts classes for state agencies to help bridge the gap to concrete. Goldbaum also recommends the association's Concrete Pavement Restoration reference binder as a resource.

Is thin whitetopping right for you?

Goldbaum offers a list of parameters that can help determine if thin whitetopping is the right solution for a project:

  • Evaluate the existing pavement — if there is not enough hot-mix asphalt base leftover after grinding, full-depth replacement may be needed.
  • Evaluate the subgrade soil — is the subgrade stable, or does it require reconditioning before an overlay is added?
  • Estimate future traffic loads —in areas with heavy truck traffic, a conventional whitetopping or full-depth replacement may last longer.
  • Existing curb and gutter — whitetopping is easier in rural areas, or on highways with shoulders instead of curbs and gutters that require an overlay.
  • Overhead clearance — pay attention to overpasses, light poles, power lines, or anything that could be an issue with vertical clearance for paving equipment (For CDOT, the minimum is 16 feet).
  • Constructability issues — can you shut down the lane and reroute traffic without excessive impact to the traveling public?
  • The ACPA provides an online Bonded Concrete Overlay on Asphalt Thickness Designer that calculates the amount of concrete needed, based on specific project parameters.


WEB EXTRA

American Concrete Pavement Association (ACPA) Tech Summary: Design of Concrete Overlays Using Existing Methodologies (PDF)

Use ACPA's online Bonded Concrete Overlay on Asphalt Thickness Designer to calculate the amount of concrete needed for a specific project.

Sample joint detail for a 4-inch thin whitetopping project (Courtesy of American Concrete Pavement Association)

List of Colorado whitetopping projects since 1990 (Courtesy of Colorado Department of Transportation)

  • Related articles:
  • Avoid full reconstruction with ultra-thin whitetopping (Public Works, April 2011)
    Mill-and-overlay alternative adds a decade of service life to industrial park streets and saves a Chicago suburb almost $11 million.
  • Trial run (Public Works, October 2006)
    Inquiring minds want to know: asphalt or concrete? The Colorado DOT is finding some answers to that question via a 15-year test