Polymer bandages may give old bridges new life
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From left: Dave Winke
and Frank Schneider from the Rock County, Wis., Public Works Department
and Andrew Kuether (MSCEE ‘03) use a powder-actuated driver
to mechanically fasten the strips to the underside of the Stoughton
Road Bridge in rural Edgerton, Wis.
(View larger image) |
 ong 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,
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.”
His 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.”
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 at the University of Missouri,
Rolla, who launched research projects of their own.
Co-inventor Lamanna now is an assistant professor
of civil and environmental engineering at Tulane University. There,
his research group is conducting lab experiments to examine long-term
fatigue behavior of beams strengthened using the method. 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. They’re not eligible for federal
rehabilitation or replacement funding and would be good candidates for
the strengthening strips, says Murphy.
Strengthening culverts and bridges will allow communities
to open bridges to more traffic—moves that benefit local travelers
and lighten cost burdens for the communities. “We see great possibilities
in the off-system bridge market,” says Antonio Nanni, professor
of civil engineering at the University of Missouri-Rolla. “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.”
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