Connecting Pittsburgh to its southern hills, the Liberty Tunnels serve as one of the major arteries to the city, eliminating the need to travel over or around Mt. Washington. A century ago, before the tunnels were built, this trip to the city was a long, impractical, and dangerous drive.

But after 85 years of heavy traffic and wear and tear, the pair of 5899-foot-long, two-lane tunnels was in dire need of structural repair and restoration. The rehabilitation was necessary to keep this connecting throughway open for the more than 63,000 commuters who depend on the subterranean roadway every day. PennDOT, which oversees the tunnels, turned to Pittsburgh-based Mosites Construction Co., which focuses on specialized concrete structures.

The time constraints for completing the project were one of the biggest challenges. With the high volume of daily traffic, long-term closure was out of the question. This gave workers an eight-hour window, between 10 p.m. and 6 a.m., to move all equipment in, do the work, clean up, and move out before the morning commute. In addition, the crew needed a repair product that would leave little mess in the enclosed space.

Mosites opted to use a single-component dry-process shotcreting material containing silica fume (microsilica). Quikrete Shotcrete MS–AR Fiberglass Reinforced provides high compressive strength, improved sulphate resistance, high adhesion, low permeability, low rebound, and low sag characteristics ideal for this project.

To meet additional performance requirements, alkali-resistant fiberglass fibers were added to control shrinkage cracking, and a migrating corrosion inhibitor was added for corrosion protection of embedded steel reinforcement. Both were blended into the mix during the manufacturing process.

"The limited window of time and constricted working space in the Liberty Tunnels created difficult conditions," says Dennis Bittner, Quikrete regional sales representative. "The product was the ideal solution because it allowed for an extremely quick cure time between applications with little-to-no dust creation. Because the crew was able to spend more time working and less time cleaning, the project was completed ahead of schedule and under budget."

After prepping the area using hydrodemolition and cleaning exposed rebar, workers cleaned the substrate and moistened it to a saturated surface dry (SSD) condition. Reaching up to 26 inches thick in some sections of the project, the shotcrete was applied with a gunning rig with a 750-cubic-foot/minute compressor.

Crews added a temporary sealer, which was removed after the initial layer cured. Then a flash coat of shotcrete was applied at depths of 1/2 to 1 inch to give the top layer the smooth appearance of a gun finish and an appropriate texture to receive a waterproof topcoat.

PennDOT extended the project by 76,000 square feet of shotcrete flash coat based on the performance and appearance of the work.

Starting in September 2009, the total amount of shotcrete product used on the project exceeded 12,000 cubic feet by the time it reached completion in December 2010.

"It was great to see the tunnels returned to nearly new condition and ready to serve commuters for the next 85 years," Bittner says.

Learn more about The Quikrete Companies at