Originally built to accommodate streetcars and rising automobile traffic six decades ago, the historic Rankin Bridge has long served as a critical access point to Pittsburgh’s large steel industry.
Today, the four-lane, 505-ft-long cantilever bridge—a steel superstructure resting 75 ft above the Monongahela River on concrete piers—carries approximately 26,400 cars every day and serves as the main access point to Kennywood Amusement Park and the expansive, 260-acre Waterfront entertainment complex.
In recent years, government officials and engineers deemed the structure to be structurally deficient. However, considering the importance of the bridge to area commuters, officials had little choice but fund a $47.8 million overhaul that included a full rehabilitation of concrete piers; a new, wider deck; painting of the superstructure; and access lane improvements.
Allegheny County officials entrusted the massive job to Pittsburgh-based Mosites Construction Co., which specializes in roads, bridges and unique concrete structures. To help make the repairs, the company selected QUIKRETE Shotcrete MS – Fiber Reinforced from a Pennsylvania Department of Transportation list of preapproved repair materials.
QUIKRETE Shotcrete MS – Fiber Reinforced is a portland cement-based, dry-process shotcrete material containing micro silica and AR fiberglass fibers designed for high strength, improved sulphate resistance, high adhesion, low permeability, low rebound and low sag.
In order to minimize the effects of corrosion over time, an integral corrosion inhibitor was also added during the manufacturing process.
“Originally, the pier work was specified form and pump,” said Dennis Bittner, a QUIKRETE regional sales representative. “However, through an on-site field evaluation, the contractor was able to demonstrate the viability of shotcrete as an alternative method of repair.”
Before applying the shotcrete, crews removed all loose and unsound material and pressure-washed the areas to remove any fractured concrete and expose the sound concrete below. Workers then sandblasted existing rebar and replaced deficient rebar that could not be reused.
For the application of the shotcrete, Mosites Construction opted for a dry process due to the stop-and-start nature of the work.
ACI-certified nozzle men predampened the mix to reduce dust and applied the Shotcrete MS in depths up to 18 in. Crews then shaved the excess material and shaped the shotcrete to match the contour of the existing piers. Immediately after the shaping was completed, the piers were sprayed with a state-approved, VOC-compliant curing compound.
The experience of Mosites and the shotcrete crew, in addition to the speed of application afforded by the shotcrete, allowed the project to stay on schedule. In total, crews used more than 6,000 cu ft of shotcrete on the project, which began in August 2008 and was completed in November 2010.
“The Rankin Bridge is a perfect example of how versatile QUIKRETE MS, micro silica enhanced shotcrete is as a structural and aesthetic repair material,” Bittner said. “The original shape of massive piers was perfectly replicated.”