Eastbound traffic exits the work zone on the Sixth Avenue Viaduct, where concrete deck repairs are to be completed this month.

Denver’s Sixth Avenue Viaduct got a facelift for its 50th birthday, and taxpayers got a lesson in cost-effectiveness.

It’s a lesson based on Ben Franklin’s maxim that “an ounce of prevention is worth a pound of cure.”

Replacing support piers and bearings under the westbound bridge last year and rehabbing the concrete decking atop the eastbound structure this year for a total cost of $6 million will add 25 years to the useful life of the “twin” bridges.

Without it, Denver Public Works’ chief infrastructure engineer Jim Barwick estimated the aging structures eventually would have cost $30 million to replace.

“If we hadn’t done this, we would have had to replace it in 10 years,” Barwick said.

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The repair work involved the two bridges that make up the viaduct’s eastbound and westbound lanes from Kalamath to Osage streets.

What that means is that, by incurring a cost that averages $240,000 for each year of life – or $400,000 a year for the 15 net new years of life it bought, Denver is saving drivers up to $260,000 a year by not having to replace the bridges.

While many bridges will have to be replaced at some future point, timely projects to upgrade and maintain them can extend that useful lifetime.

The two bridges are “fraternal” twins rather than identical. While they look similar from above, underneath they are two different structures. The westbound bridge uses single-column reinforced concrete support piers; the eastbound one uses steel-framed supports.

View a slideshow of the viaduct’s substructure and repairs:

To expand to full screen and read the captions, first start the show. Then click on the expand box that will appear at the lower right corner of the screen. When the full-screen version begins, click on “Show Info” at the menu bar on the top right.

Motorists who used the Sixth Avenue Freeway to get in and out of central Denver up through the mid-1990s remember well the roller coaster ride they used to get from driving over the hubcap-jarring expansion joints on the viaduct. They were placed every 75 feet to allow for contraction and expansion of the 1,250-foot structures. In 1998, the joints were taken out and the replaced at either end.

But the results weren’t all good. While the ride was smoothed out, the now-continuous superstructure of the bridge experienced more contraction and expansion with weather extremes and it failed to slide properly along the new bearing pads. During the winter of 1999-2000, this resulted in visible damage underneath. The city commissioned a study to gauge the viaduct’s safety on the short term and the impact on its long-term lifespan. While measurements of pier movement showed it was safe, the city needed to make changes to ensure the viaduct’s long-term health.

“The bridge became fixed in the middle but it could expand and contract toward the ends, where we put the new large expansion joints,” Barwick said. “But the problem we found was that when it moved, it didn’t slide over the columns. It was dragging them.”

In the center of the span, this wasn’t a big problem. But the effect was more pronounced at the two ends.

“There was one concrete pier at the end where when we took the load off that sucker, it moved three and a half inches to the east,” Barwick recalled. “It had been pushed to the west. We literally saw it move back.”

New reinforced concrete support piers replaced the ones that moved with the deck expansion on the westbound viaduct.

The city put together additional savings in the project by applying some innovative techniques and choices, such as using temporary piers to shore up the westbound bridge while reinforced concrete piers were torn out and replaced with new ones. As a result, the city didn’t need extensive traffic closures and saved about $150,000 in traffic control costs.

Instead, it used short 10 to 20-minute stoppages during off-peak traffic times when each of the 18 piers was taken offline one at a time and 18 more when the viaduct was rested down on each new one. The viaduct carries a combined 60,000 vehicles a day and is a key commuting route to and from Interstate 25, Lakewood and Golden. Lengthy closures would require difficult detours.

URS Corp. was the engineering firm that headed up the project. Barwick said the city had several bottom-line requirements that URS had to work with, including safe removal of lead-based paint from the underneath structure.

URS addressed all of the points, and found a way to include some of the temporary shoring pilings into the foundations for the new reinforced concrete piers, saving some more money, Barwick said.

Denver also saved several hundred thousand dollars when project inspector Bill Cusack suggested removing the lead-based paint with a system that is used to contain residue of contaminated paint in nuclear power plants. It captures virtually all the material and can be operated by workers without heavy protective hazmat suits. The hazardous material goes directly into a barrel for shipment to a disposal facility.

After some research, the city bought the system and its net cost was lower even after taking the purchase into account.

Edward Kraemer and Sons won the contract and work started in February 2008 on the westbound structure, where the piers were replaced. That was finished early this year, and the rehabilitation of poor concrete area on the eastbound structure started soon after and will be completed by the end of this month.

In the midst of all the modern technology used on design today, the eastbound bridge’s repair spots were determined in a decidedly old-fashioned way.

Denver hired engineering interns to get up on lifts and sound the underside of the eastbound bridge concrete deck for hollow spots that were to be repaired.

He compared it to dental assistants probing for cavities in teeth. Using lifts, the workers got up to the underside of the concrete and started hammering on it. Hollow spots sound different. They were marked, and then the locations were mapped on the topside.

Barwick said the other old fashioned way to “sound” the deck would have been to shut down all traffic, mill off the asphalt and drag chains across the top surface. But the hammering could go on while traffic was still moving on top.

“The interns got some practical real-world engineering work and we got the locations we needed.”

Traffic was reduced to two lanes, first on the north side and then on the south, so Kraemer’s workers could drill out the bad concrete from around the steel rebar – again, like dental work preparing to fill a cavity. It left a hole in the deck except for the steel grid.

“You have to get all the concrete out without affecting the reinforcing steel,” Barwick said.

New concrete was then placed into the opening.

In the official Colorado state highway map of 1946, there is no Valley Highway, and no Sixth Avenue east of Federal. Traffic entered town via the Eighth Avenue Viaduct.

The two bridges making up the viaduct were part of Denver’s first big freeway building boom in the 1950s. The Valley Highway, now Interstate 25, had pushed south to Zuni Street near Eighth Avenue. There, a rickety old viaduct carried east-west traffic over the Denver & Rio Grande Western’s Burnham Yard to Federal Boulevard. Across Federal, motorists could swing southwest along the hillside and connect with Sixth Avenue, which was going to be developed into a freeway in part to serve the Denver Federal Center.

The Sixth Avenue Viaduct was conceived as a straight-on connector from that future freeway into town, and the interchange with the Valley

The 1960 state highway map showed the new Valley Highway, labeled as 185 rather than I-25, and the Sixth Avenue connector. But Eighth was reduced to a tiny line.

It is a vital high-speed traffic corridor on the west side of the metro area.