Deck Repairing

Structure Details: The deck is 8 in. thick, 2-way, post-tensioned, flat slab. The column-strip post-tensioning in the E-W direction was inadequate. A repair was developed using a heavily-reinforced gunite beam added to the underside of the slab column strip.

Problem: After the repair, de-lamination between few beams and slab was noticed. An NDT investigation (pulse velocity test), concluded that approximately 30 percent structure may have more than 40% de-lamination. The engineering firm then recommended more NDT inspection utilizing impact echo. Of the 133 beams tested, 105 have showed some de-lamination, and 19 had 33% or more de-laminated.

Repairing Methodology: The first phase of the repair included an in-depth and in-situ load testing of the existing slab under various conditions that existed and strengthened with FRP sheets. A load testing procedure was proposed to provide a non-destructive demonstration of the performance of structural components, utilizing hydraulic jacks to induce force equivalent to those resulting from distributed loads.

Test results indicated that the beam in the E-W direction with a minimum of 70% bond performed well. A span with no beam demonstrated poor performance. Results also showed CFRP strengthened slabs (after removing the Gunite beams) were satisfactory. Accordingly, 19 locations E-W beams would remove and the slab would be strengthened with CFRP sheet. Typical span in the N-S direction were found to be acceptable without additional upgrade. External steel-framing was installed reducing the clear span, and additional steel supports were installed under the cantilever slabs. At steel support points, "pancake" jacks were permanently installed to engage the steel frame.FRP Solution Restores Strength to Skyway Double Tee Beams. Engineers discovered the presence of numerous hairline cracks in the precast concrete guide-way beams that support the monorail track nearly a year after the last segment of the Skyway was completed. An investigation revealed that excessive un-bonding of the pre-stressing tendons had taken place and caused cracks in 63 of the 500 beams. While the condition was a cause for concern, it posed no immediate danger to the Skyway. However, temporary supports were installed while a determination was made on how to repair these cracks. Engineers had developed five different repair scenarios to address the problem. Repair strategies were evaluated based on a ranking system that considered durability, maintenance, aesthetics, and cost. After careful review, the repair strategy selected involved injection of the cracks and the application carbon fiber reinforced polymer (FRP).Repairing team had injected all of the cracks, grinded, and pressure washed the beams. The FRP system was externally bonded onto the ends of the deficient beams to restore the strength lost due to the un-bonded strands. Since ensuring the continued operation of the Skyway was important to the JTA, Structural Preservation Systems developed a strategy to complete the repairs and upgrades during the evenings and weekends.