The combined city and county of Denver is saving time and money by using its own forces to cold-mill and resurface 1,100 alleys in a dedicated four-year program.
The result is a substantial reduction in the backlog of alley work, improved cash flow for other street maintenance projects and thousands of satisfied citizens whose crumbling alleys are transformed in days from a patchwork pavement to milled surface to fresh hot-mix asphalt (HMA) pavement.
“In Denver we have about 5,000 alleys total, of different lengths,” said Dan Roberts, director, Street Maintenance Division, Department of Public Works for the combined city and county of Denver. “Some are of different configurations—some L-shaped, some T-shaped—but all are about a block in length.”
Of those 5,000 alleys, half are concrete surfaced and in good shape. But about 1,400 of these alleys are a century old, or older, constructed of portland cement concrete and overlaid in the 1950s or 1960s with HMA.
“That program ran for 20 years, and work generally stopped in 1980,” Roberts said. “But by 2000 we were inundated with pothole complaints and we found we had about 1,100 alleys we needed to resurface after no work on them having been done for 20 years. We had a significant backlog.”
Roberts added the city was spending $1,500 to $2,000 in pothole patching per alley when it responded to complaints. “It would take a day or two for a two-man patch truck to patch the holes, and the patches would not last very long.”
So at the turn of the millennium, in the year 2000, the city undertook a pilot program to remove old alley asphalt and replace with new. “For that pilot program we rented a Wirtgen W 1200 F/T and did 29 alleys that first year,” said Roberts. “Productivity was fairly low. We did about three-quarters of an alley per day in removal, then another three-quarters of alley per day to overlay. It was marginally cost-effective compared to what we were spending on pothole repairs, but it was really done just to get our feet wet.”
The second year saw big steps forward, though. “We made big strides in our crew productivity,” Roberts said.
That second year crews renewed 71 alleys and more than doubled productivity. “That year we did about 13?4 alleys per day, a big jump,” Roberts said.
Three a day
The alleys present tight working quarters, generally 14 to 16 ft wide, with irregular boundaries, and Roberts found the smaller mills—with their 48-in.-wide cut and tracked propulsion—to be most productive for this application.
“We had milled alleys with a big cold milling machine with a 7- or 8-ft-wide drum, but we tore the alleys up,” Roberts said.
Denver is attempting to remove all asphalt from the concrete surface, not just plane off the top few inches. “We get most of it,” Roberts said. “Generally the guys make four passes in a 16-ft-wide alley,” he said. “They’ll get that mandrel down and run down the center of the alley, then turn around and come down the center to the left, followed by two outside passes. A good operator will be able to take the mandrel right down to the concrete.” After four passes a grader can be used if needed to clean up any remaining asphalt.
The reclaimed asphalt pavement (RAP) is loaded by conveyor into city-owned dump trucks and taken to stockpiles for reuse.
Following its purchase of the rented mill in late 2001, the city bought a second W 1200 F/T in 2002. Now, Denver has increased its alley program remarkably. From 29 alleys in 2000, to 71 in 2001, to 75 in 2002, the program did 325 alleys in 2003. “Our productivity jumped from 3?4 of an alley per day in 2000 to three alleys per day last year,” Roberts said.
Milling is followed in less than two weeks by an overlay of 1.5 in. (on the outside) of a modified Superpave mix, the city’s standard mix, with PG 64-22 binder with 3?4-in. top size stone.
“We’re thin on the edges and thick in the middle, because we’re paving an invert and it’s hard to break the screed opposite what would be the normal break,” Roberts said. “A lot of it is up to the roller operators to start compaction in the center, then roll towards the outside to get the thickness down. But after three years, our crews have gotten pretty good at doing this work.”
Helping those not as fortunate
RAP from this program never is wasted, but instead reused in a variety of ways. “In addition to the concrete and asphalt-over-concrete alleys, we have 1,100 old gravel or dirt alleys,” Roberts said. “For these unimproved alleys, in the spring and fall, we remove 6-8 in. of surface and replace with the cold millings. This turns them into a gravel alley and reduces dust and runoff components.”
Formerly the city would bring in trucks, a grader and a loader backhoe to dig the alleys out. Now the W 1200 F/T does the excavation work in one pass. “It was extremely effective,” Roberts said. “It used to take us three days to do one alley; now we do one alley in a day.”
Following 8 in. of excavation, the cold millings are brought in, graded and compacted. “It’s a really great use for this equipment when it’s too cold for mill and overlay work,” he added. “We’ve gone from doing about 10 unimproved alleys a year to about 50, and our goal is to get up to 200 per year.”
In the heat of the sun, the millings knit together to form a uniform surface. “They rebind in the sun,” Roberts said. “It creates a firm surface under residential traffic loads, with no truck traffic other than the refuse trucks once a week.”
And Denver is considering a pilot project to treat the millings in the yard with a rejuvenator, mixing it, then placing it on the jobsite with a paver. “It would be just like placing cold mix,” Roberts said.
The zero-clearance design of the W 1200 F/T permits Denver to mill directly against curbs for street drainage improvents via curb reveal.
The city also uses its two midsized cold mills to remove extensive areas of failed street pavement when placing long patches.
Good morning, good night
In 2004 the city expects to complete its alley mill-and-overlay program, with 550 alleys completed. But as each year elapses, Denver has found its cost per sq yd per in. of material has plummeted.
“Initial cold milling cost in 2000 was on the order of $1.50 to $1.60 per square yd/in.,” Roberts said. “We’ve been able to drop that down to about $1.35 to $1.40 in 2003. Even though we’ve seen inflation in operating costs, because of our productivity increases, we’ve been able to lower our costs.”
And this carries over into the total budget. “Right now it costs us about $6,000 to mill and overlay a single alley, compared to up to $2,000 to patch potholes, with only six months to a year of life for those patches,” Roberts said. “But we anticipate a 12- to 15-year life for the overlay, so the city/county comes out ahead.” The 2004 alley program is a $3.3 million program, which will mill and resurface 550,000 sq yd of alley.
Alleys typically are a forgotten pavement for municipalities, so citizen response to the alley renewal program has been exuberant.
“For a lot of the people who live in Denver,” Roberts said, “their day begins in an alley and ends in an alley. And we have eliminated the worst driving surface they have to deal with. They come out constantly and thank the crews for the work they’re doing.”
More information about Denver’s alley resurfacing program, including weekly programs, is available on the Internet at www.ci.denver.co.us/Street_Maintenance/36910384template1jump.asp.