Tale of Two Materials: Grout Segregation and Tendon Corrosion

When routine inspections of a major causeway on the Florida Gulf Coast revealed a ruptured external post-tensioned tendon, the contractor and department of transportation scrambled to determine the cause. The discovery was startling. The 8-year-old tendon consisting of 22 seven-wire steel strands encased in grout, and once carrying over 967,000 pounds of tension, laid limply within the segmental bridge sections. Was the problem with workmanship and installation? Were the materials faulty? Was the design adequate? As the investigation progressed, another tendon failed and amplified the level of concern.

Engineers from Pivot partnered with Lewis Engineering to investigate the tendon ruptures. Field investigation and laboratory autopsies determined the tendons failed due to severe corrosion near the high points of the tendon. Pivot personnel observed voids and soft, spongy grout at the high points of corroded tendons, unlike the solid, hardened grout at the lower tendon elevations with no corrosion. A mockup of the grouting process revealed that the proprietary grout material used at the bridge tended to segregate after placement: bleed water migrated to tendon high points, transporting components of the grout with it. The mock-up results were consistent with the failed tendon observations. This segregation process essentially created two materials from the same product. Pivot personnel and Lewis recreated the dissimilar grout conditions in a laboratory and demonstrated that an aggressive corrosion cell develops when steel bridges the two materials. In addition to identifying the cause of the corrosion, engineers from Pivot and Lewis developed a possible remediation solution for the remaining tendons, including a mock-up with positive test results.
Project Services:
Failure Investigation
Structural Repair Design
Corrosion Testing
Construction Litigation Support

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