ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/306198
MAINTENANCE, REPAIR, AND REHABILITATION of concrete bridges To restore the shear deficiencies at the end of the AASHTO girders on the 5.5-mile-long Sunshine Skyway Bridge in Tampa Bay, Fla., designers used a bidirectional carbon- fiber fabric wrapped in a specific sequence to limit the number of plies while supplying additional strength. ASPIRE , Winter 2010 | 43 Sunshine Skyway Bridge: CARBON FIBER REPAIRS GIRDERS With its signature bright-yellow stay cables, the Sunshine Skyway Bridge in Tampa Bay, Fla., is one of the most recognizable structures in the United States. The bridge comprises the main spans, the high-level approach spans, and the low-level trestle spans. The concrete girders of the low level trestle spans recently underwent repairs to provide significant new service life. The work on the project stood out to the extent that it won an Award of Excellence from the International Concrete Repair Institute. Repairing the girders represents a monumental project. The bridge is 5.5 miles long with a main span vertical clearance of more than 190 ft. The trestle spans use 1300 precast, prestressed concrete girders. AASHTO Type IV girders are used for a majority of the 100-ft-long spans. The Florida Department of Transportation (FDOT) in Tampa, Fla., worked with SDR Engineering Consultants in Tallahassee, Fla., on the project. Intron Technologies in Jacksonville, Fla., served as repair contractor. Shear cracks had been observed during routine inspections of the trestle span girders, leading to the repair plan. Inclined shear cracking was much more prevalent in the exterior girders than the interior girders, and numerous pier caps also showed cracks large enough to exhibit signs of water penetration and damage. The damage impacted both the flexural and shear capacity and required several repair procedures. Shear cracks with a width exceeding 0.012 in. were epoxy-injected. Spalls were patched with a cementitious repair mortar, uneven surfaces were filled with a leveling mortar, and small cavities were repaired with an epoxy paste. A clear protective sealer was applied to protect the concrete after the work was completed. Each fiberglass jacket was installed in four sections. Crew members pumped grout comprising portland cement and sand from the bottom of the jacket to the top from alternate pumping ports to create a single monolithic fill approximately 2 in. thick. When the grout fill cured, lead wires from the reinforcing steel in the pile were connected with the zinc anode in the junction box. The existing concrete and the grout fill in the jacket formed a common electrolyte. Finally, the repair contractor bolted two 48-lb bulk zinc anodes to the bottom of each jacketed pile and connected them to the pile's CP system to add protection to the pile's submerged portion and prevent current dump-off from the jacket's lower region. The repair system, designed to protect the piles for more than 25 years, will self-adjust to meet changes in temperature, humidity, concrete resistivity, and other factors. For piles with minor damage, including hairline cracks smaller than 1 / 16 in., repairs were made by routing out the cracks and packing them with cementitious material. Any spalls on the piles were also patched with cementitious material. In addition, a cementitious overlay coating was applied to the rectangular concrete caps, which run horizontal across each set of three piles. The road rests atop these caps. The three-pronged system and careful analysis allowed repairs to be closely targeted and achieve the twin goals of being long lasting and easy to maintain. ___________________ This article is an abridged version of an article published in the November/December 2008 issue of Concrete Repair Bulletin and is published with permission of the International Concrete Repair Institute. For more information on the organization, visit www.icri.org. Carbon-Fiber Fabric Wraps Members To increase shear strength at the end of the AASHTO girders, a bidirectional carbon- fiber fabric from Sika Corp. in Lyndhurst, N.J. was used to wrap the members. The carbon fiber was applied in a specific way to achieve the strength necessary and meet the design live load requirement. A 24-in.-wide strip was placed vertically down the girder web, around the bottom flange and up the other face to create a U-wrap similar to a stirrup. Then a strip was wrapped around the bottom flange of the girder to strengthen the flange. Finally, another strip was placed longitudinally along and under the top flange adjacent to the soffit of the bridge deck. The bidirectional fabric allowed the members to achieve supplemental strength in multiple directions without having to install additional plies. Most of the repairs took place over the water, from where it was difficult to access the bridge's underside to perform the repairs. The contractor worked from a barge using man lifts, because the vertical clearance from the barge to the underside of the girders on the trestle span was about 15 ft. Waves and tides also were major concerns, as was the threat of hurricanes. Protecting the Environment Working in a marine environment, the contractor had to ensure no degradation of water quality occurred due to construction, and staging was not permitted in any environmentally sensitive habitats or wetlands. Also, the bridge was located in a Manatee Watch Area, so a lookout was required to watch for the protected mammals when boats were moved. Despite difficult working conditions, the project proved a success. The repairs were able to be made underneath the bridge without having to restrict even one lane of traffic during the entire process. By close scheduling of the construction sequence, the work was completed one month ahead of schedule, making not only the contractor happy, but FDOT as well. ___________________ This article is an abridged version of an article published in the November/ December 2008 issue of Concrete Repair Bulletin and is published with permission of the International Concrete Repair Institute. For more information on the organization, visit www.icri.org. ASPIRE_Winter10.indb 43 12/18/09 2:06:42 PM