Out with the old, in with the new

Tim Gregorski / May 13, 2001

Bridge No. B-0171 on Five Mile Road in Hamilton County, Ohio, became the first bridge in the nation to receive an advanced fiber-reinforced polymer (FRP) composite deck on pre-cast concrete beams. The bridge, along with two others slated to receive FRP composite decks, is part of Project 100, a statewide initiative by the National Composite Center (NCC) to replace 100 conventional bridge decks across the state of Ohio with composite materials over the next six years.

Unlike the Westbrook Road bridge in Montgomery County, Ohio, which kicked off Project 100 last June when it exchanged its old concrete and steel bridge for a new composite deck, the Hamilton County bridge represents a new step for this application.

"The Westbrook project required a total bridge replacement," said Mark Murton, project manager for the NCC?s Project 100. "But the bridges in Hamilton County presented a different challenge. Design engineers were faced with creating a solution that would allow them to attach FRP deck panels to decades-old concrete beams."

Hamilton County bridge engineers took the first step by evaluating the condition of the existing pre-cast concrete beams on all three bridges. Engineers found that beams on two bridges were sound and could be reconditioned. Beams on the third needed to be replaced.

Replacing the bridges? old concrete decks with a lightweight FRP deck presented a special problem for the project?s consulting engineering firm Lockwood, Jones and Beals (LJB).

"Normally, bridges that use concrete beams rely on a cast-in-place deck to provide additional structural rigidity," said Mark Henderson, a principal engineer for LJB. "FRP decks are very strong, but they do not contribute to structural rigidity the way concrete decks do. We had to modify the beams to account for this difference to ensure the bridge structure performs properly with the new deck in place."

LJB?s solution was to design a thicker top flange on the concrete I-beams to increase stiffness. The engineering firm also worked closely with deck manufacturer Hardcore Composites to design the connection of the panels to the redesigned beams.

The second challenge tasked design engineers to marry new FRP deck technology with old concrete beams to create a structure with a lifespan of at least 50 years.

"The old concrete beams were made before today?s design standards were developed, so we had to calculate the remaining strength based on a detailed analysis of the beam structure which took into account the condition of the concrete and reinforcing steel," said Henderson. "The analysis showed we could reuse some of the beams and save money on the project."

A low-maintenance, long-life solution was created by rebuilding parts of the old beams, applying waterproofing sealants to the concrete and installing a new, impermeable FRP deck.

The University of Cincinnati agreed to perform load testing for Hamilton County on the existing concrete beams removed from Bridge B-0171. Engineers expect the tests to provide additional information on the actual capacity of the remaining beams.

The university also installed monitoring devices on the bridge to determine the behavior and load transfer of the FRP deck panels in relation to the concrete beams.

The Five Mile Road project included evaluation of existing structures, removal of the old concrete decks, fabrication or repair of pre-cast concrete beams, installation of new FRP deck panels and refurbishment of bridge abutments, guardrails and adjacent roadway. The project also is providing the NCC with valuable insight into ways to apply composite technology to the infrastructure industry.

Forever young?

Initially developed at Wright-Patterson AFB in Dayton, Ohio, for critical aircraft, composite material has proven its durability over the last 40 years in the aerospace, marine and offshore industries. The technology?s resistance to corrosion, strength, lightweight and design flexibility also is proving a natural fit for the infrastructure industry.

"The whole idea of using a composite material to replace old bridge decks is still very young, and we?re finding that the technology isn?t for every structure," said Murton.

According to Murton, the Five Mile Road project in Hamilton County and other jobs completed for Project 100 are helping NCC lay important ground-work in a brand new field.

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