A quick snapshot of Denver's street maintenance operation illustrates just how efficient the road agency is on providing a smooth ride for the taxpaying 600,000+ residents it serves.
With an annual budget of approximately $16 million for street resurfacing and reconstruction, pothole repair, an alley paving and grading, the department operates it own state-of-the-art hot mix asphalt (HMA) plant that churns out over 250,000 tons of asphalt each year containing 20 percent reclaimed asphalt pavement (RAP), mills 115,000 tons of old asphalt annually in its mill and overlay program, along with completing over 1,150,000 square yards of hot in-place work over the past five years, which has saved taxpayers over $5 million in comparison to traditional mill and overlay reconstruction. The department has also implemented an aggressive alley resurfacing program to improve over 1,100 deteriorated alleys, and also promotes a substantial chip seal program to further extend the life of many residential roadways.
While the resurfacing and reconstruction budget is expected to be lower this year, due to some agency reorganization, an intensive full-depth patching program and some assistance the street maintenance department is providing the Denver Police Department, the street department still has a substantial number of paving and preservation projects scheduled.
"Our network is in good condition and we'll continue to make improvements where needed with either mill and overlay or hot in-place, preserve good roads with chip seal application, and continue to work on improving our alleys," says Dan Roberts, P.E., director of the street maintenance division.
And while the division contracts its hot in-place and chip seal work, the cost-savings achieved by producing its own mix for mill and overlay projects, as well as the mix used for hot in-place projects generates close to $1 million annually over purchasing from an outside vendor. The same mix, which is designated as a Metropolitan Government Pavement Engineers Council (MGPEC) standard design (similar to a ½-inch stone Superpave PG 64-22) is used for both mill and overlay projects, and hot in-place projects.
"We purchased our Astec plant seven years ago and we're saving $4.50 per ton on the mix we produce versus purchasing from an independent facility," notes Pat Kennedy, P.E., a senior engineer for the department. "That savings has more than paid for the investment the city made in the new plant. As long as we continue to maintain the plant, it will continue to generate savings."
HMA production serves a variety of needs
While a bulk of the HMA produced at the department's asphalt plant supports the mill and overlay crews, which are outfitted with two Cat 565-B mills and two Cat AP-1000B pavers, the flexibility and control the plant has in serving the city's needs bodes well in supporting a variety of street maintenance programs.
With some of this year's organizational changes, Kennedy projects that the department will place 105,000 tons of HMA on mill and overlay projects, 50,000 tons on full-depth patch projects and over 50,000 tons on hot in-place and alley projects.
"We're reconstructing approximately 100 alleys each year and when we began the program several years ago, we identified 1,100 alleys that were in need of repair," Kennedy says. "So we have plenty of alley work to keep us busy in the years ahead."
To handle the alley workload, the department purchased additional milling and paving equipment for crews designated to work specifically on the narrow back streets.
Crews use a 48-inch-wide Wirtgen W-1200 milling machine and a Cat AP-800 paver to navigate the narrow 14- to 16-foot-wide backyard passages, many which have not been tended to in over 20 years and fallen into major disrepair. Prior to paving the alleys, repair efforts were limited to patching potholes, which cost over $1,500 per alley and only lasted for a season.
With most alley projects, 6 to 8 inches of existing material is milled out before 4 to 6 inches of HMA is put down.