Exodermic deck aids traffic flow during rehabilitation

The 3,200-foot U.S. 421 bridge, which spans the Ohio River between Milton, Ky., and Madison, Ind., is a crucial link between the two states, with the next closest crossing located 25 miles upriver. Two years ago, when the Kentucky Department of Highways (KDOH) decided to refurbish the aging battalion structure, officials were concerned about the effect on traffic.

Louisville, Ky.-based Hazelet & Erdal/Dames & Moore, the consulting firm for KDOH, recommended the department install a precast exodermic deck. Doing so would keep the bridge open to traffic during replacement of the bridge deck, reduce the load on the structure during repairs and ultimately increase the bridge’s weight capacity.

Exodermic deck panels feature a reinforced concrete slab on top and composite with an unfilled steel grid. The panels can be precast before they are placed on a bridge, and the steel grid acts as a form so concrete can be poured to attach the panels to existing stringers.

Work began in September 1996. Throughout the project, vehicles shared a reversible single lane around the work zone, with occasional brief stoppages. (Workers had only an 11-foot lane to work in, with traffic constantly passing in the adjacent lane.) The contractor removed and replaced the old deck in one-lane, 360-foot increments, each taking about a week.

For each stage of construction, one crew used saws and large forklift trucks to remove 360 feet of the deck. Those workers were followed by a second crew that cleaned the tops of the stringers and placed adhesive-backed foam strips on top of them. After that, two fork lift trucks moved the 11-by-7-foot exodermic deck panels — each of which included precast curb and railing anchorages — into place.

The fork lifts were driven over the previously laid panels to put other panels into place, thus minimizing traffic disruption and eliminating the need for a crane. Built-in leveling bolts were used to adjust the positions of the panels and to ensure that they were level. Shear studs were then welded on with a stud gun, and the panels were secured in place by concrete poured into 6-inch gaps running along the tops of the stringers.

After the 73,000 square feet of bridge deck was replaced, the deck was milled 1/4 inch and overlaid with 11/2 inches of latex-modified concrete. In addition to replacing the deck, workers painted the entire bridge, strengthened and replaced truss members and performed welding repairs. Much of the work was done at night when traffic was light.

The project was completed in June 1997. Using exodermic deck panels rather than installing a conventional cast-in-place concrete deck and doing much of the work at night hastened the project’s completion by at least six months, according to Project Superintendent John Crace of Lexington, Ky.-based Intech, the general contractor.