Save Time and Money with Two-Coat Painting Systems for Steel Bridges
Paint steel bridges faster
and more cost-effectively using fast-dry two-coat systems. Traditionally,
zinc-rich epoxy/polyurethane systems that require three coats have been
considered the best coating systems for protecting steel bridges from
corrosion. However, applying three coats, including the spraying and drying
time for each coat, is time consuming. The new two-coat system consists of a
zinc-rich primer with a topcoat of fast-dry polyaspartics, polyurethane, and
polysiloxane. "This is an attractive coating system for protecting steel
bridges since the intermediate layer is eliminated," says Shuang-Ling Chong of
the Federal Highway Administration (FHWA). Using the two-coat system saves
time, decreases overall labor and material costs, and minimizes the impact of
lane closures on the traveling public.
FHWA compared the performance of the two-coat systems
with the conventional three-coat systems using both a 5,000-hour accelerated
laboratory test at its Turner-Fairbank Highway Research Center in McLean,
Virginia, and a 2-year test of outdoor exposure in a marine environment in New
Jersey. "The test results showed comparable good performance of four recently
formulated two-coat zinc-rich systems on a steel surface compared to three
three-coat zinc-rich systems," says Chong. No surface failures were seen for
either system. A small amount of rust creepage was exhibited where an
intentional scribe or scratch had been made on the test panels. The scribe
represents the worst scenario of defects in paint films and at the edges and
corners of steel structure members. Both the two-coat and three-coat systems
exhibited similar low levels of rust creepage, demonstrating their comparable
performance. "The excellent performance of the two-coat systems suggests that
they can be used to protect steel bridges as well as three-coat systems," says
Chong. Based on the study, interchanging use of different vendor's products is
not recommended for two-coat painting, due to the risk of surface failures.
For new steel bridges, FHWA recommends that the two-coat
system be applied in a fabrication shop. The controlled conditions of shop
painting will eliminate salt contamination and aid in proper curing and drying
of each coat, ensuring good durability. When repainting existing bridges, use
of a rapid deployment strategy is recommended. Rapid deployment is primarily
designed for use on highway overpasses where the structural steel is easily accessible
from the roadway below using a mobile work platform. This mobile work unit
includes a containment device, dust collector, and blast equipment. Using the
two-coat systems, painting jobs on overpasses can often be completed overnight.
This compares to a minimum of 24 hours for traditional painting.
| "The excellent performance of the two-coat systems suggests that they can be used to protect steel bridges as well as three-coat systems." |
The Connecticut Department of Transportation (ConnDOT)
has used the rapid deployment two-coat system to paint five steel bridges to
date. These structures have primarily been overpasses and bridges on limited
access highways. At a bridge on I-84 in Danbury, ConnDOT painted one side of
the bridge using a two-coat system and the other side using a three-coat system
to compare the impact on the contractor's time and cost. "It is cheaper to use
the two-coat system but the biggest difference was the minimal interference
with traffic," says Brian Castler of ConnDOT. "Ninety-nine percent of the work
is done at night and the two-coat system cuts significantly the length of time
a lane needs to be closed, greatly lessening the total impact on the traveling
public." ConnDOT's contractor has suggested refinements to the rapid deployment
system, including coming back and applying the second coat of paint separately,
sometimes on the same night and sometimes on the next night, to save time on
having to switch equipment. While time savings vary by project, Castler
estimates that on average the length of a project can be cut by 25 to 30
percent using rapid deployment. Several bridges that ConnDOT has painted using
the two-coat rapid deployment system have now weathered two winters and are
performing satisfactorily.
 |
| The finished bridge over I-84 in Danbury, CT, after
the Connecticut Department of Transportation
repainted it using the two-coat rapid deployment
system. |
Other States that have used rapid deployment for steel bridge painting include Kentucky, Ohio, Maryland, New York, and Pennsylvania.
For more information on two-coat painting systems for steel bridges, contact Shuang-Ling Chong at FHWA, 202-493-3081 (email:
shuang-ling.chong@fhwa.dot.gov). For more information on ConnDOT's use of the
rapid deployment system, contact Brian Castler at ConnDOT, 860-594-2201 (email:
l.brian.castler@po.state.ct.us).]
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