Following inspection and structural load rating analysis, the historic Wisconsin Avenue Bridge located in the Georgetown area of Washington, D.C., was found to be inadequate in supporting vehicle loads at the minimum HS 20 rating required by the American Association of State Highway and Transportation Officials (AASHTO). The problem of accommodating heavy traffic in an intensely commercial area, as well as preserving the oldest historical bridge in the country, was the challenge faced by several public organizations.

A joint collaboration of transportation officials from the Federal Highway Administration, the Eastern Federal Lands Highway Division, the District Department of Transportation, and the National Park Service was formed to overcome this challenge.

Typically, when a masonry arch bridge is in need of repair or is determined to have less than the required AASHTO rating, it is demolished and replaced with a concrete or steel structure. Therefore, there are a diminishing number of such masonry structures remaining in the United States.

The repair solution
During the investigation and research process, the officials learned of a bridge strengthening system that would accomplish the objective of raising the load rating, as well as preserving the structure in its original condition. The system they discovered is provided by Cintec America, Inc., headquartered in Baltimore, Md. At the time, Cintec had installed its reinforcing system in approximately 70 bridges throughout the world.

The Cintec system allowed stainless steel structural reinforcement members to be imbedded entirely within the arch structure without altering the visible appearance of the bridge, and equally important, the installation work was performed in one lane at a time, allowing the bridge to remain open throughout the entire process.

During the design phase, a three-dimensional simulation model of the bridge was created using a computer-aided design program. Live loads were simulated and reinforcement configurations were matched to the requirements until an optimal, final design layout was achieved. Construction drawings were prepared and installation began in September 2004.

The installation proceeded as follows: Holes to accept the stainless steel anchors were drilled into the arch. The anchor bars for this project were Type 304 grade stainless steel, 1 inch in diameter, and the holes were 2 1/2 inches in diameter. (In all bridges, the hole size is approximately double the bar size.) The angles at which the holes were drilled were precisely calculated according to the geometry of the bridge and were drilled to a tolerance of +/- 0.10 degree. All drilling was non-percussive, slow-speed, high-torque, which eliminated any vibration that could potentially cause damage to the bridge.

The anchors, which are enclosed in a polyester sock material, were then inserted into the holes. Twenty-six reinforcing anchors were required for this project.

Cementitious grout (sympathetic to the bridge material) was then pumped into the sock, causing it to inflate and encase the steel in 5,800 pounds per square inch compression grout. The purpose of the sock is to control the grout flow and ensure that it stays within the drilled holes. Pumping uncontrolled grout or other materials into a masonry arch could cause damage to the bridge and the environment.

Over long periods of time, rainwater creates voids and spaces within the fill of all masonry bridges. On the other hand, the bonding strength of the anchor is calculated based on total contact between the grout and the substrate material. It is clear that the flexible sock is imperative to ensuring that the grout shapes itself to all irregularities within the arch, thereby ensuring maximum mechanical and chemical bonding strength of the embedded anchor.

The entire installation process took place from the surface of the bridge roadway and did not require any excavation or relocation of any utilities. The bridge contained water and gas lines, electrical power cables, sanitary sewers, and stormwater drains.

The Cintec system required substantially less construction time than other methods of bridge strengthening, and had minimal impact on local traffic. In fact, the Wisconsin Avenue project took a total construction time of approximately two weeks. The cost of the project was $350,000 excluding design costs. This price was a fraction of the cost of bridge replacement, even before considering the significant economic impact on the community if the bridge had to be closed.

After the installation was completed, the only visible evidence of the installation was a small amount of grout on the road surface. In essence, the Cintec system allowed the installation of a complete structural steel reinforcement system into an arch bridge with virtually no effect on the appearance of the bridge.

The Cintec system offered a cost-effective option for strengthening historic bridges and allowed the structural engineer to address historic preservation regulations while also satisfying bridge codes, safety, and environmental requirements.

Prior to the project, the Wisconsin Avenue Bridge’s load rating was posted at 25 tons. After completion, the rating was increased to HS 25 or 45 tons.