Located below a major financial institution's office, a 150,000-square-foot parking structure was in need of repair to increase its service life. Constructed in 1968, the parking garage is a cast-in-place post-tensioned concrete structure with four supported levels. Having worked successfully with Structural Preservation Systems (SPS) in the past, the property manager of the facility and the consulting engineer contacted SPS to assist with the repair.
A thorough inspection of the facility revealed corrosion of the reinforcing steel and the post-tensioning system. This structure was built using a buttonhead post-tensioning system. This type of system typically consists of seven to twelve ¼-inch-diameter, high-strength 240-ksi wires wrapped in spiral kraft paper that are passed through steel anchorage assemblies. Following stressing of the wires, the ends are "buttonheaded" using cold forming in the field to keep the wires from sliding back through the anchorage and releasing the tension. Waterproofing this type of system is very difficult, and thus corrosion is very common, as was the case with this structure.
This corrosion resulted in a reduction of post-tensioning capacity and caused cracking to occur in the supported slabs, followed by resultant concrete spalling and delamination due to corrosion of the embedded steel. The SPS repair team determined the locations in need of repair through visual examination, mechanical soundings and areas identified on construction documents generated by the engineer.
Although post-tensioning is commonly employed on parking garage projects, the type of post-tensioning used has evolved since this project was originally constructed. Today, an encapsulated monostrand system is more commonly used. Because of safety risks associated with stressing buttonhead systems, as well as the difficulty in providing a watertight repair, where applicable, SPS opted to convert sections of the buttonhead system that were in need of repair to a monostrand system.
At one such location, a drive lane was strengthened by removing the existing damaged buttonhead post-tensioning system. It was replaced with a monostrand post-tensioning system. To install the new system, SPS cut trenches into the drive lane and cast new sections of monostrand tendon into the slab. Following stressing of the anchorage system and casting of concrete, the trenches were covered with a urethane traffic membrane to provide additional waterproofing.
When post-tensioning repairs were not feasible, other strengthening methods were utilized. One such method added conventional reinforcing steel and increased the slab thickness. At a drive lane where the existing post-tensioning system was substantially damaged, the team bonded an engineered reinforced concrete overlay to the topside of the slab. At a second location, the engineer elected to have SPS roughen the underside of the slab; after which, conventional reinforcing steel was added, and gunite was applied overhead to increase the slab thickness.
It was critically important to the owner that repairs were made with as little disruption as possible to the occupants. Crews worked at night to keep the noise to a minimum during business hours. Further, the repair team could only occupy a bare minimum number of spaces during the repair process to ensure that traffic flow was maintained. Adding to the challenge of this project were the discrepancies between the original drawings and actual field conditions. SPS' relationship with the engineer enabled the team to work together very well to modify the scope as necessary to minimize costs and project delays.
The project would not have been such a success without tremendous flexibility and teamwork between the owner, engineer and contractor. In many cases, the information required for the engineer's structural analysis was not available until deteriorated concrete was removed around reinforcing strands. After the strands were exposed, deterioration was inspected and testing was performed. This presented many unforeseen challenges, and the team experienced a continuously evolving scope of work. Throughout these challenges, the project team was able to successfully collaborate, finding repair solutions that met everyone's requirements, including results that effectively addressed safety, cost and schedule.
This project represents some of the many different strengthening solutions available to an owner for a project. SPS evaluated several repair strategies and incorporated different strengthening methods for specific elements of the project, including section enlargement, post-tensioning replacement and repairs, as well as conventional reinforced slab replacement.
Work occurred from August 2006 to November 2007, and the owner was so pleased with the results that they are considering SPS for other repairs at the site.