The Wisconsin Avenue Bridge is a 174-year-old, single 54-foot-span stone masonry arch bridge over the Chesapeake and Ohio (C&O) Canal. Located within the Georgetown Historic District, it is the oldest bridge in Washington and is listed on the National Register. The Wisconsin Avenue Bridge is also a main thoroughfare through Georgetown and is the only road in Georgetown that provides adequate access for truck traffic between M Street and K Street. Built in 1831, it was clearly not designed to handle modern truck traffic with minimum live loads of 36 tons. Although not in immediate danger of failing, a study of the bridge indicated that it would eventually suffer irreversible damage with continued use by modern truckloads, which occasionally exceeded legal limits.

The District Department of Transportation (DDOT) brought in KCI Associates, a division of Hunt Valley, Maryland-based KCI Technologies, to investigate alternatives for strengthening the bridge to an AASHTO HS25 (45 tons) design load while maintaining its historic integrity and character. Besides being a main thoroughfare through Georgetown, a very affluent area of Washington, the bridge sits amidst thriving businesses, many of which are located in historic buildings. Therefore, any rehabilitation method would be required to keep traffic disruptions to a minimum and ensure that the nearby historic structures and businesses would not be affected by construction or design enhancements to the bridge. Consequently, more traditional methods were ruled out due to the historic design of the bridge, site considerations and multiple physical constraints.

The DDOT then decided to approach Cintec America about using the Archtec method to strengthen the bridge. Tested extensively and used throughout Europe, the Archtec method involves the accurate placement of stainless steel Cintec anchors in precisely cored holes within the masonry arch barrel. In cooperation with KCI and Cintec, Gifford and Partners, an engineering firm in the United Kingdom, was responsible for the arch analysis and strengthening design as part of the team. Gifford and Partners has extensive experience analyzing and designing reinforcement using Cintec anchors for many historic masonry arch bridges as well as buildings and castles throughout Europe.

A single operator, using a variable speed, electric 2-horsepower core drill that allows for wet or dry drilling, core drilled 26, 2.56-inch-diameter longitudinal holes across the existing 36-foot-wide clear roadway. The core barrels consist of 12-inch-long to 36-inch-long segments that screw together. The barrelhead is a segmented 4-inch bit composed of carbide and diamonds arranged in various compound combinations depending on the hardness of the substrate material. The drills and the core barrels are held in place with a customized drill rig and are advanced along a geared guide beam attached to the drill rig. Drilling at precise predetermined angles through the concrete roadway slab and the granite-like stones in the arch barrel, the length of the holes varied from 28 feet to 29 feet depending on the location on the arch.

Because of its historic significance, the National Park Service, the bridge's owner, required that the core material from each core-drilled hole be carefully preserved and identified. This precautionary step was taken in the unlikely event that the original core would require replacement if the strengthening process ever needs to be reversed. Once the holes were core drilled, a 1-inch-diameter stainless steel ribbed reinforcement anchor enclosed within a Cintec fabric sock was installed, followed by low-pressure injection of Cintec Presstec grout.

Because of the small size and minimal amount of equipment required for the job, the construction team was able to schedule and configure the project site so that only one bridge lane was temporarily closed at a time. The entire core drilling and anchor installation for the arch strengthening process was completed in two weeks.

In addition to strengthening the arch barrel, the existing, original wrought iron picket bridge railing was also restored. The original railing was anchored in large capstones of Aquia Creek Sandstone, a relatively weak stone that is no longer quarried. Over time, portions of the sandstone had broken away from the bridge along the embedded railing elements, and portions of the railing were no longer securely anchored. One of the primary reasons the capstones were fracturing was due to corrosion of the embedded pickets. KCI chose a restoration method that replaced several wrought iron pickets with high-strength stainless steel rods machined to match the existing pickets. Several salvaged existing wrought iron pickets were retrofitted with stainless steel stubs at the bases to prevent corrosion and then reinstalled. The remaining railing components were cleaned and painted.

Other improvements included replacing the existing concrete roadway slab and sidewalks over the bridge and on portions of the approaches; performing various cosmetic stone restoration repairs; and reconstructing isolated areas of stone masonry retaining walls.

Once under way the project went smoothly and quickly, considering the challenges of coordinating the work with several government agencies and the concerns of an influential community. The historic Wisconsin Avenue Bridge is now strong enough to handle modern traffic loads for years to come.