The stronger bridges of Madison County

Concrete deck reinforced with a fibre-reinforced polymer grid system could replace conventional epoxy-coated reinforcing bars inside future bridges.

When the Highway 151 bypass around Fond du Lac, Wisconsin, USA opens later this year, vehicles travelling northbound will cross DeNeveu Creek on a bridge like no other in the country.

Externally, the bridge looks identical to its adjacent twin. However, internally, the concrete deck is reinforced with a fibre-reinforced polymer (FRP) grid system that could replace conventional epoxy-coated reinforcing bars (rebars) inside future bridges.

University of Wisconsin-Madison Professor of Civil and Environmental Engineering Larry Bank, Associate Professor Mike Oliva and graduate students David Jacoboson and Mack Conachen developed the system, which is said to have several advantages over the steel rebars bridge-builders currently use.

“The steel reinforcing bar in the concrete deck corrodes,” explained Bank. “That corrosion causes cracking of the concrete, the concrete then chips off and you get potholes and all of those problems that people don’t like on bridges.”

Because it is non-metallic, the fibre-reinforced polymer material won’t corrode, giving it the durability to last at least 75 years, said Bank. “Most bridge decks will only last somewhere between 30 and 40 years before they have to be replaced,” he said.

Traditionally, both steel and FRP reinforcing elements have been long bar shapes, which workers then have to secure together. But working with Strongwell, a Chatfield, Minnesota manufacturer, Bank’s group designed prefabricated, three-dimensional FRP grids that cranes can rapidly lay into place, eliminating weeks of labour-intensive work and allowing workers to pour concrete more quickly, speeding up bridge construction or deck replacement.

“The idea is, if you have to do a deck replacement on a busy road, you want to do it as quickly as possible so you don’t have bypasses that are dangerous, or detours, which are even worse,” said Bank.

The FRP grid system is the first of its kind installed anywhere, due to funding from a US Federal Highway Administration program called Innovative Bridge Research and Construction, and co-operation from partners at the Wisconsin Department of Transportation (DOT).

“Wisconsin DOT is looking at this FRP research as a promising potential investment for the future for the design of reinforced concrete that is exposed to chlorides,” said Gerald Anderson, DOT chief of structures. “Wisconsin DOT may eventually be able to design concrete reinforced with a non-corrosive FRP material that by far outperforms the conventional epoxy-coated rebar decks.

“There will be a savings in construction to offset the FRP materials cost, as well as savings in both routine deck maintenance longer service life before deck replacement is required.”

Although the cost of building this first FRP grid bridge was higher than that of a traditional bridge, Bank said that mass-produced, rather than custom-designed FRP grids, as well as their extended durability, will make the system cost-competitive.

“The FRP reinforcement system costs around $20 a square foot, which given the time and labour savings obtained in construction, is competitive with a conventional steel rebar-reinforced bridge,” he added.