Polymer bandages may give new life to old bridges

Long polymer “bandages,” designed so that troops could quickly repair or reinforce bridges to bear the weight of 113-ton military tank transport vehicles, now could be used to quickly and inexpensively strengthen aging rural bridges and concrete culverts around the country.

With initial funding from the Army Corps of Engineers, Lawrence Bank, professor of civil and environmental engineering at UW–Madison, and his then-student Anthony Lamanna, perfected these bandages, or fiber-reinforced polymer (FRP) strips. They then patented the strips through the Wisconsin Alumni Research Foundation.

In wartime, the strips could be key to keeping important transportation routes available, says James Ray, a structural engineer for the U.S. Army Engineer Research and Development Center. “The main thing these strips would be used for is if the bridges don’t have sufficient capacity to start with,” he says. “The military loadings are very heavy compared to what bridges are normally designed for.”

Using fiber-reinforced composite strips to bolster concrete structures isn’t a new idea. Crews have been gluing them in place for more than a decade.

But transforming the crumbly, cracked and pockmarked underside of a decades-old concrete bridge into a surface suitable for glue takes good weather, a lot of time and more than a little labor.

“You have to sandblast; you have to repair with a mortar,” says Bank. “Typically on bridges, you’re doing things overhead, which is also unpleasant.”

Fastening the strips to the bridge with a tool akin to a power nailer seemed like an obvious alternative. The problem, however, was that existing strips, which contain only longitudinal fibers, wouldn’t hold up when the fasteners punctured them. They split, much like a dry board might crack when a nail hits the wrong place.

“When you attach with fasteners, you have to have different properties in the strip,” says Bank. “You have to have high bearing strength – which is that you could press on the strip with these fasteners and it’s not going to crack and split.”

Sort of like duct tape without the stick, Bank and Lamanna’s reinforcing strips combine carbon fibers, glass fibers and glass mats. The mats, which are woven in tight crisscrosses, are key to the new strips’ success.

“If you make a hole in the strip and you push on the hole, the weave allows it to carry that load,” says Bank. “If you just have these longitudinal fibers, if you make a hole and you push on it, it’s going to slide.”

Bank’s strips, which are stiff but not rigid, act like super-strong bandages that workers can quickly and inexpensively attach to the underside of a bridge with powder-actuated concrete fasteners.

To test the strips, county workers installed them on the decaying 1930s Stoughton Road bridge in Edgerton, Wis., in 2002. “It was really bad,” says Tom Hartzell, Edgerton public works director. “There were some big cracks that went all the way through.”

During the installation, which took three workers less than a day, a thunderstorm whipped up. The bridge was in such poor condition that rainwater and run-off poured through the cracks. “You cannot use a technique where you bond on strips in that environment,” says Bank.

Total cost for strengthening the bridge was about $8,000; eventually, Edgerton replaced it at a cost of $196,800, including plan development, state review, old bridge removal and new bridge construction.

In Wisconsin, the state Department of Transportation (DOT) evaluates all of the state’s bridges every two years and assigns them a sufficiency rating. If a bridge’s rating is below 50, it probably is on the docket for partial federal funding for replacement, says DOT bridge maintenance and inspection engineer Matt Murphy, who monitors the structures in Wisconsin’s 10 southwestern counties.

Of the 1,800 small bridges – structures greater than 20 feet long – in those counties, as many as three dozen might have sufficiency ratings below 50. In that case, they’re probably load-posted, which means that they’re not safe for heavier vehicles like tractors or milk trucks to cross. “It’s an inconvenience to the traveling public and the locals,” he says.