The I-85 bridge was planned as a four-span concrete structure with eight columns per bent. Prefabricated elements were used for the substructure’s columns, pier caps and deck beams. The bridge components were cast off-site and shipped to the site on conventional semi-trailers. Each component was carefully cast to within a 0.25-inch tolerance so connections made in the field would fit precisely.

“We’re doing some innovative things, using precast, prestressed columns and caps on the bridge in order to expedite the work,” said then-Georgia DOT District 3 engineer Thomas Howell. “It’s a first in this district. The pieces were actually cast at a yard and brought out, instead of forming and pouring them on-site.”

Safety data sets were collected before, during and after construction to ensure that the innovations did not increase risks. With PBES, no worker injuries were reported. A single motorist incident involved minor vehicle damage with no personal injury. The cost savings with PBES were equally compelling, saving nearly $2 million, or 45 percent, of what the interchange would have cost if it had been built conventionally.

FRP Composites Refined

Even as concrete and steel bridge construction is accelerated via new technologies and techniques, fiber-reinforced polymer (FRP) bridge materials continue to make inroads as their engineering is refined.

Three new FRP technologies were in the spotlight in 2011, with rigidified FRP tube arches, hybrid composite beams and reinforced thermoplastics technology this year being named as focus areas by the AASHTO Subcommittee on Bridges and Structures’ Technical Committee T-6: Fiber Reinforced Polymer Composites.

Concept of the superstructure placement for the I-93 ‘Fast 14’ Rapid Bridge Replacement Project in Medford, Mass. completed in the summer of 2011.

Maine DOT has volunteered to be the lead state, taking on the next step in the implementation process, which will include conducting a market analysis and developing a marketing plan for implementation. Other state DOTs represented on the team include Massachusetts, Michigan, Missouri and New York, along with the Maine Composites Alliance and the University of Maine.

“For nearly 30 years, FHWA has supported research and development technology transfer, deployment and standardization of FRPs as a promising solution for bridge construction and rehabilitation,” Louis N. Triandafilou, P.E., FHWA Office of Infrastructure team leader, Bridge & Foundation Engineering Team, said this summer.

“After a long history of worldwide research, use of FRP composites in seismic retrofits and bonded repairs has become almost commonplace,” Triandafilou said. “Also, highway agencies are applying this technology to a growing number of projects involving bridge deck panels and reinforcing bar and prestressing applications. However, despite widespread government and industry support, there has been little self-sustaining, competitive deployment of this technology.”

Nonetheless, several emerging FRP composite technologies could play an important role in future rehabilitation and replacement, Triandafilou said. “Some promising emerging approaches are focused field applications of rigidified FRP tube arches, hybrid composite beams and reinforced thermoplastics.”

FRP is a general term for polymer-matrix composites reinforced with cloth, matting, strands or other fibers, Triandafilou said. FRP composites consist of thermoset resins, which, once cured, cannot be returned to an uncured state. Reinforced thermoplastic resin composites, on the other hand, can be softened repeatedly by heating or hardened by cooling. In the softened state, workers can reshape these composites by means of molding or extrusion. “FRP and reinforced thermoplastic composites have the potential to create cost-effective, durable and long-lasting bridge structures,” Triandafilou said.

* Rigidified FRP tube arches are derived from a kit consisting of three main components: carbon- and glass-FRP composite tube arches, a self-consolidating concrete (SCC) mix design, and corrugated fiberglass panels, Triandafilou reports. “Once on site, workers inflate the 12- to 15-inch- diameter diam tubes and bend them around arch forms,” he said. “The crew then uses a vacuum-assisted transfer molding process to infuse the tubes with resin. The tubes, which cure in a matter of hours, function as stay-in-place forms for the SCC, eliminating the need for temporary formwork, and provide structural reinforcement for the concrete in the longitudinal direction, in shear, and as confinement, eliminating the need to install rebar.”