In ongoing research to advance bridge and highway building technologies, the Texas Department of Transportation (TxDOT) replaced a 50-ft.-long 32-ft.-wide,
In ongoing research to advance bridge and highway building technologies, the Texas Department of Transportation (TxDOT) replaced a 50-ft.-long _ 32-ft.-wide, single-span girder bridge in Refugio County with a new drainage-ditch crossing (FM-1684) using fiber-reinforced polymer (FRP) composite bridge beams. Customized 50-ft.-long _ 30-in.-deep, FRP flanged U-shaped beams implemented with concrete deck construction produced a hybrid structure Û the Refugio County Bridge, 35 miles from Corpus Christi. The project constitutes the state’s second FRP hybrid-bridge endeavor, following successful construction of the San Patricio County Bridge nearly three years ago.
TxDOT’s specification of FRP beams was dictated by the county’s humid, subtropical climate resulting in widespread corrosive salts and brackish water. Although more costly upfront, that decision would further the agency’s investigation of the long-term corrosion and structural performance benefits of FRP materials versus steel or concrete beams. Experience gained from the previous bridge project would provide a basis for refining customization and production processes to optimize performance and cost variables.
Once FRP beams were specified by TxDOT, general contractor Haas-Anderson Construction (Corpus Christi) enlisted concrete form manufacturer Molded Fiber Glass Construction Products (MFG) to fabricate eight customized flanged U-shaped beams whose depth and composite structure would provide optimal deflection under load. The completed beams, weighing approximately 5,000 lb. each, would sit on abutments, providing support for a reinforced concrete deck. Initiated in late 2006, bridge construction was slated for summer 2007 completion.
To fabricate the beams, MFG at its Texas location employed a vacuum infusion process (VIP) in contrast to the hand lay-up technique of the previous bridge project. Aiming to optimize the beam’s physical properties and expedite production, the manufacturer selected the vacuum infusion process for its benefits, including consistent fiber-to-resin ratio, less wasted resin, unlimited set-up time, and lower emissions. Accordingly, a U-shaped male mold was assembled to the beam design, after which dry sheets of stitched glass fabric and chopped strand mat were laid over it. With multiple repetitions of the process, a series of layers was created to achieve the appropriate 1.5-in. beam thickness. Subsequently, a plastic film was applied on top to form a complete vacuum in preparation for introduction of liquid resin into the laminate via carefully placed tubing. The resin was thus drawn through the fibers, filling all the voids and eliminating any remaining air along the flow-front. Notes Rich LaFountain, MFG Business Unit Leader/Open Molding, The trick is to get the film to draw down correctly so that wrinkles don’t develop in the individual layers of fabric, which could affect the ultimate strength of the composite.
Once formed, the beams were cured, trimmed and assembled with shear transfer members (brace bars) comprising flange plates/tubes placed at 16-in. intervals along each 50-ft. length. Holes drilled into the vertical sides allowed insertion of the 2-in.-diameter brace bars at the top of webs (lips) on either side.
Upon inspecting the beams at MFG Texas with his staff, TxDOT Materials Branch Manager/Construction Division’s Robert Sarcinella observed, Production proceeded as if it were on steroids. MFG took lessons learned from the first project and fabricated the beams more quickly and with better quality than before via its vacuum process.
In April 2007, prior to installation, Refugio beams #1 and #2 were subjected by Guillermo Ramirez, Ph.D., of the University of Texas at Arlington and Paul Ziehl, Ph.D., of the University of South Carolina to the Acoustic Emission Evaluation Test. Emission was monitored during the background check prior to loading, during load holds, and during the background check after completion of loading using type R15I sensors (resonant in the range of 150 kHz) manufactured by Physical Acoustics (PAC). Broadband sensors were used for supplemental evaluation. Activity from the R15I sensors was monitored and recorded with a 24-channel Transportation Instrument, also supplied by PAC. The test threshold was 40 dB, and the evaluation threshold was 48 dB.
According to Dr. Ramirez, testing verified the beams’ performance under load criteria established by project specifications. Not only did the beams meet all major criteria selected for the Acoustic Emission test, he emphasized, their demonstrated stiffness proved better than expected, substantiating their ability to sustain service loads.
Installation of the beams entailed 4-ft. center-to-center spacing to support the concrete deck, which was tied to the beams (close to their top) using horizontal, 2.6-in.-deep _ 2.3-in.-wide pipe. For optimal strength in joining beams to the deck, concrete was poured sufficiently deep to engage the brace pipe. Thus, builders achieved their goal of composite action to create an inflecture-solid connection between the deck and beams.
Assessing the short-term benefits of FRP hybrid-bridge construction, Haas-Anderson Superintendent Roy Tijerina affirmed, They delivered all the FRP beams in one truck; and, handling and installation using a small crane or large track hoe were far easier. Minimal equipment and people required equals built-in time and cost efficiencies on the project.
Besides rapid on-site deployment afforded by the lightweight FRP beams, he added, their material strength over time will reduce bridge maintenance costs. In a post-construction assessment, TxDOT/Federal Highway Administration Division Bridge Engineer Peter Chang noted, Funding was allocated by TxDOT as a research initiative to promote the new fiberglass girder technology. With load testing calculations and installation complete, the beams have demonstrated greater strength than we anticipated, thus proving the research productive.