In its continuing research of new structural highway construction technologies, the Texas Department of Transportation (TxDOT) has expanded its implementation of custom fibre-reinforced polymer (FRP) composite bridge beams for a drainage ditch bridge in Refugio County, TX. The bridge is a replacement for a single-span girder-bridge (50 feet long x 32 feet wide) that utilizes customized FRP flanged U-Shaped Beams (50 feet long x 30 inches deep) and a concrete deck construction. The county has a humid subtropical climate (averages thirty-seven inches of rain annually) which results in corrosive salt and brackish water. Therefore, although more costly up front, TxDOT specified the FRP beams to advance the research of the long-term corrosion and structural performance benefits of FRP materials versus traditional steel or concrete beam solutions.

With the FRP beams specified by TxDOT, General Contractor Haas-Anderson Construction enlisted Molded Fiber Glass Construction Products, who had produced the previous FRP beams at San Patricio, to manufacture eight customized flanged U-Shaped beams whose depth and composite structure would provide optimal deflection under load. Once completed, the beams would weigh approximately 5,000 pounds each and sit on abutments where the concrete deck would be poured onto it.

Vacuum infusion process selected

MFG fabricated the beams in its Texas location utilizing a Vacuum Infusion Process (VIP) versus the previous project’s hand lay-up to optimize the physical properties of the beam and facilitate production. This process was selected because vacuum infusion provided a number of benefits including; consistent fibre-to-resin ratio, less wasted resin, unlimited set-up time and much lower emissions. VIP utilizes a vacuum bag to de-bulk or compact the parts’ complete laminate ply schedule of reinforcements and or core materials that are laid onto the mold.

For the Refugio beams, a male mold was produced to the beam design and then dry sheets of stitched glass fabric and chopped strand mat were laid over the U-Shaped mold. This process was applied in a series of layers to achieve the appropriate 1.5-inch beam thickness, and then a plastic film was laid on top to serve as a vacuum bag. Once a complete vacuum was achieved, liquid resin was then introduced into the laminate via carefully placed tubing. The vacuum then draws the resin through the fibres, filling all the voids and eliminating any remaining air along the flow-front.

According to Rich LaFountain, MFG Business Unit Leader/ Open Molding, “The trick is to get the bag 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.”

Beams placed easily

Once completed, the beams were cured, trimmed and assembled with sheer transfer members (brace bars) that included flange plates/tubes across every 16 inches in a 50-foot beam. Holes were drilled into the vertical sides and brace bars (two-inch diameter) were inserted through the beam at the top of the webs (lips) on either side. The beams were placed at four-foot centre-to-centre spacing with the concrete reinforced deck placed on top. The deck was then tied to the beams with horizontal pipe (2.6 inches deep x 2.3 inches wide) close to the top of beams. The concrete deck pour was deep enough to engage the brace pipe for optimal strength to tie the beams to the deck. The goal was to achieve composite action between the beams and the deck; creating an inflecture-solid connection between the deck and beams.

Acoustic testing prior to installation indicated the beams performed as planned. According to The University of Texas at Arlington’s Dr. Guillermo Ramirez, “The test verifies the performance of the beams under the load criteria set forth by the project specifications. The beams performed well during load testing— passing the major criteria selected for the Acoustic Emission test. In fact, the beams’ stiffness tested better than expected substantiating their ability to sustain in service loads.” Ramirez added, “The beams looked very nice, with no visible flaws. The method of fabrication resulted in a very good product.” Roy Tijerina, Superintendent for Haas-Anderson Construction, assessed the short term benefits of the FRP beams stating, “They delivered all the FRP beams in one truck and handling and installation were easier; using a small crane or large track hoe vs. multiple cranes with steel or concrete options. This means minimal equipment and people are required; which equates to built-in time and cost efficiencies on the project.” Tijerina concluded that, “In addition to the lightweight FRP beams allowing for rapid onsite deployment, its material strength over time will reduce maintenance costs on the overall construction of the bridge.” A post-construction assessment by TxDOT/Federal Highway Administration Division Bridge Engineer Peter Chang noted, “The funding to promote the new fiberglass girder technology was allocated by TxDOT as a research project. With the load testing calculated and installation complete, the beams are actually stronger than we anticipated, thus proving the research positive.”