How the life of a 1.2-mile-long viaduct in Colorado is being extended, turning a $800 million replacement project into a $21.6 million repair project 

by Troy Robinett, Technical Support, CTS Cement Manufacturing Corporation

When Interstate 70 was completed through Denver in 1964, its bridges were designed to last 30 years. The original construction of the interstate highway system was paid for with federal dollars with the states taking over responsibility for upkeep after completion.

ACC workers replace the reinforcing steel with galvanized wire mesh to add structural reinforcement to the concrete.

As a result, the highways were designed to meet only the minimum standard necessary for the projected traffic load to avoid having the federal government indirectly subsidize maintenance. Not only was the actual volume of traffic greater than anticipated, funding to replace worn out portions of the highways was not always available when needed and many sections, like this viaduct in Denver, are being used past their planned design lives.

This bridge between State Highway 265 (Brighton Boulevard) and State Highway 2 (Colorado Boulevard) is one of 46 bridges in the metro Denver area said to be in imminent need of repair by the Colorado Department of Transportation (CDOT). Its 60 expansion joints have been corroded by water and salt, resulting in deterioration of the reinforcing steel and the concrete surrounding that steel.

The 1.2-mile-long viaduct has a current sufficiency rating of only 44 out of 100, six points lower than the I-35W Bridge that collapsed in Minneapolis in 2007. CDOT considers the viaduct structurally defective, functionally obsolete and in need of replacement. Replacing the bridge, which is still used for an average of 130,000 to 150,000 vehicle trips per day, would cost $800 million the state does not have.

To extend the life of the bridge until funding for either replacing

The depth of removed concrete cannot be greater than 6 inches, but these sections can be as wide as 8 feet.

 it or rerouting the highway is obtained, CDOT hired the Denver office of American Civil Constructors (ACC) to repair the failed expansion joints. The goal of the repair project is to extend the life of the bridge for 10 to 15 years. ACC began its upgrade of the bridge’s substructure in September 2008. This structural repair work is part of a larger repair project for the bridge, which also includes replacing bridge railings, reconstructing the median, repairing the substructure of the bridge and improving drainage. The cost of the whole project is $21.6 million.

What the concrete repair work involves

ACC workers begin by inspecting the bridge’s substructure to determine where concrete is damaged and needs to be replaced. Then they jackhammer out the pieces of deteriorated concrete and rebar, continuing until they reach sound concrete. Concrete removal is the most time consuming part of the job. The exposed, undamaged concrete is then sandblasted to create a rough surface for good adhesion of the repair material. The depth of removed concrete cannot be greater than 6 inches, but these sections can be as wide as 8 feet.

“We replace the reinforcing steel and put in galvanized

Completing the job will take about 6,000 55-pound bags of product, which are being used at the rate of roughly 50 bags per night.

wire mesh to add structural reinforcement to the concrete, wet the surface, and then we start applying the repair material,” explained ACC Assistant Project Manager Jimmy Jones. CDOT requires that the rebar be encased in concrete within seven days after it is replaced. Two crews of 12 to 15 men are working to make the repairs using seven manlifts, four all-terrain scissor lifts, 20-25 jackhammers and other hand tools.

Due to the larger portion of the project involving temporary lane closures of I-70, all work is done at night and only a small section can be worked on at one time. One interstate lane is closed in each direction at 8 p.m., with a second lane closing at 10 p.m. In addition, either Westbound or Eastbound 46th Avenue, a city of Denver street under the bridge, remains closed at all times. At 5:30 a.m., the workers must be finished so traffic can resume in all interstate lanes or ACC is charged heavy lane rental fees.

To complete the work in the short overnight time frame, a fast-setting concrete repair material was needed. The first product selected could be applied no more than 3 inches deep, which meant two applications were required to repair each damaged area. This product cracked and was found to be unacceptable.