Damaged post-tensioned tendon

In post-tensioned concrete construction, reinforcing bars and post-tensioning tendons are required to support self-weight and intended service loads. As these structures age, many time-related problems occur, such as corrosion of the tendons or strands. For owners of a post-tensioned structure, the responsibility of ensuring the safety of tenants or those using the structure is paramount.

Methods of repairing post-tensioning tendons depend upon the type of problem the system is experiencing, as well as the type of post-tension system utilized in the original construction. The options to be considered are:

• Full-strand replacement - Feeding new strand from one anchorage location to another existing anchorage location. The type of sheathing used in original construction may affect the use of full-strand replacement. For example, if a paper wrap system was used in the original construction, it will be difficult to push the strand the full length required for the repair.
• Partial-strand replacement utilizing a variety of splicing techniques:
  - Using an existing anchorage, splicing and stressing coupler
  - Installing a new anchorage and splicing coupler
    (stressing at the new anchorage location)
  - Installing a new stressing coupler while utilizing a dead-end anchor location
• Replacement of broken strands with external strengthening systems, such as new post-tensioning, CFRP, steel or concrete.

The following case studies provide insight into the types of post-tensioning repair and recommended procedures to ensure long-term success.

Tendon splicing

Case Study #1

Located near Washington, D.C., a hotel was purchased by a new owner who wanted to transform it into a high-end facility. During the renovation process, several structural issues were discovered, and the owner contacted Structural Preservation Systems (SPS) to address the post-tensioning structural concerns in the parking garage.

Preliminary surveys indicated that a number of post-tensioned cables were broken. The engineer-of-record for the project recommended that the post-tensioning cables be repaired so that the structural integrity of the slab was not compromised. SPS inspected all post-tensioning anchors around the three sides of the building and identified corroded anchors in need of replacement. The remaining anchors were found to be in good condition and were sandblasted, painted and waterproofed. Next, almost 5,000 linear feet of post-tensioned cables were replaced in the garage. Splices had to be used because of constraints with accessing the cable on the outside of the building. Nearly 200 cables were spliced and re-stressed. SPS then performed concrete spall repair work on both ramps of the parking garage and applied a waterproof penetrating sealer. 

Case Study #2

A 600,000-square-foot parking structure in Washington, D.C. had extensive deterioration to its existing post-tensioning system and mild-reinforcing steel (caused by water penetration along the joints). SPS was called in to provide a comprehensive repair solution. The team began by detensioning the system tendons and removing the corroded steel. New corrosion-resistant monostrand with tightly extruded, high-density polyethylene sheathing was then spliced in. All splicing hardware was encapsulated and sealed against water intrusion, while the tendons were stressed at center stressing anchors.

A separate program was developed to address concrete and post-tensioning deterioration on the ramp connecting the second and third levels of the parking structure. Closing the ramp to replace it with a new, conventionally reinforced concrete deck was not an option, as it would have strictly limited parking for customers. Instead, the plan called for a system of external post-tensioned tendons for the ramp's beams and new internal post-tensioned tendons in the slab that would allow half of the ramp to be open while the other half was repaired and strengthened.

The external post-tensioning system consisted of pairs of external tendons placed on both sides of the beams. The stressing anchors were located in heavily reinforced concrete column collars that provided anchorage for the tendons and shear reinforcement for the columns. To strengthen the slab, eight 8-inch-wide trenches, containing a bundle of six monostrand tendons, were installed. The new slab tendons were anchored with fully encapsulated dead-end anchorages and were stressed from pockets in the slab using center-stressing anchors. In addition, the beams were strengthened with external post-tensioning stirrups, while the slabs received an application of two strips of 12-inch-wide CFRP sheets in each bay.

To protect the repairs, the entire structure was prepared by shotblast and abrasive blast, then coated with a 52-mil urethane deck coating system. All construction joints and cracks were prepared and sealed with a polyurethane sealant. To ensure adequate drainage at the ramp landings, new drains were added.

Full strand replacement

Next Steps...

While the aforementioned techniques are some of the many ways to repair a post-tensioned structure, it is important to note that it's impossible to describe all potential post-tension repair scenarios and solutions. So, when choosing a repair contractor for a post-tensioned structure, it is crucial for that contractor to have significant experience with post-tension repairs, as well as knowledge of what is possible when installing post-tension systems in original construction. The latter is extremely important due to the flexibility of post-tension construction - the contractor must have the ability to make assumptions of how the post-tension system might have been installed in original construction (even with original as-built drawings) in order to determine the correct repair strategy and techniques.

It is also important for any repair contractor to keep up-to-date on new technologies and advances in post-tension repair systems. Utilizing these new technologies and the most up-to-date post-tension systems and components will add to a longer lasting repair.