Polymer bandages may give old bridges new life
Long polymer "bandages," designed so that troops could repair or reinforce bridges to bear the weight of 113-ton military tank transport vehicles, now could quickly and inexpensively strengthen aging rural bridges and concrete culverts around the country.
With initial funding from the Army Corps of Engineers, Civil and Environmental Engineering Professor Lawrence Bank and his then-student, Anthony Lamanna, perfected these bandages, or fiber-reinforced polymer (FRP) strips, which they patented 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, Wisconsin, 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 runoff poured through the cracks. "You cannot use a technique where you glue 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.
After the successful installation, Bank shared the technology with colleagues around the country, including those at the Center for Infrastructure Engineering Studies (CIES) at the University of Missouri, Rolla. "We see great possibilities in the off-system bridge market," says Antonio Nanni, professor of civil engineering and CIES director. "These are structures owned by local communities, always strapped for maintenance funds and with small crews. The proposed application method allows such crews to install the system without any sophisticated training nor heavy equipment."
Working with the Meramec Regional Planning Commission, the CIES researchers received a grant to implement bridge improvements in four Missouri counties.
As part of the project, says CIES research engineer Nestore Galati, three bridges in Phelps County were strengthened using Bank and Lamanna's technology. The concrete was too hard and powder-actuated fasteners would have excessively damaged the surface, so the Missouri group mechanically fastened the fiber-reinforced strips to the bridge using wedge anchors.
Co-inventor Lamanna now is an assistant professor of civil and environmental engineering at Tulane University. There, his research group is conducting laboratory experiments to examine long-term fatigue behavior of beams strengthened using the method he and Bank developed. The researchers also are testing a variety of mechanical fasteners to determine which fasteners are most effective under repeat loading. In the FRP strips, the group is studying strain distribution around the fasteners to determine ways in which the strip might fail.
In addition, Lamanna is working with the Louisiana Department of Transportation to strengthen a span of a bridge over the White Bayou on State Route 19. "We will strengthen three spans, each using a different strengthening method," he says. "For the span where we will be using this method, we will use concrete screws, half of which will be epoxy coated to help prevent galvanic corrosion where the steel contacts the carbon fibers in the FRP strip."
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.
Concrete culverts are structures that look like bridges and abound on country roads, traversing small creeks or just dips in the terrain. They're too short, however, for the Wisconsin DOT to classify them as actual bridges. So even though they look like stumpy bridges, they're not eligible for federal rehabilitation or replacement funding and would be good candidates for Bank and Lamanna's strengthening strips, says Murphy.
Strengthening culverts and bridges will allow communities to open bridges to more traffic and facilitate the movement of freight, farm equipment and products, and commuter traffic, says Missouri's Galati. In the case of the three Missouri bridges, the technique increased the bridges' weight-bearing capacity. "Strengthening removed load postings or significantly increased postings so that bridges are now open to more traffic," he says. "These bridges will allow for more access from county roads to major routes running through the area, directly impacting the economic development potential of the region."