Neil Young once sang the phrase
rust never sleeps. Any one who has ever owned a car in the
northern regions of our county is familiar with the insomnia of
rust and the damage it can cause. It starts out small. A little
nick or chip to the car's paint lays bare the shiny metal giving
rust all it needs to begin its destructive cycle.
metal is exposed to the elements the real damage begins.
Moisture seeps under the paint causing it to bubble and flake
off. The cycle has begun. As more paint flakes away more metal
is exposed and more moisture seeps in. And of course the exposed
metal rusts. Once rust has a toehold it spreads, turning the
entire car into a rust bucket.
Now imagine that the flaking
rusty car is a bridge. Not a pleasant sight, and aesthetics are
the least of your worries especially if you have to drive across
that bridge. There is nothing like a bridge failure to ruin your
The Lead Legacy
But how do you prevent a bridge
from turning into a rust bucket? In the past lead-based paints
were relied upon to provide the protection. Stephen Dickey,
director of marketing, Ameron International Protective Coatings
Group, says "Red lead and lead chromate were the primary
pigments from perhaps the beginnings of painting up to about
There is a reason why it has been used for such a
long time. Lead is a good anti-corrosive and it did a good job
protecting a bridge from the elements; however, in these current
environmentally conscious days it is no longer an intelligent
While there is no law against the use of lead, the
many environmental, health and safety concerns complicate its
Dickey explains, "Lead, lead chromates and chromates
are not illegal, but there are so many health, safety or
environmental problems associated with them that it is not
prudent to use them. You can use lead in the U.S. if you want,
but you have long-term liability, and you have worker health and
safety issues, both at the manufacturer and user levels. Sooner
or later the regulations catch up to you."
Because of this
added liability many will not bother using lead-based paints on
bridges. Ray Weaver, technical advisor, SSPC: The Society for
Protective Coatings (formerly Steel Structures Painting Council)
notes, "Bridge builders have been moving away from lead since
Dickey adds, "No one has supplied lead since
1980, and lead chromate since 1987."
Lead has been so good
at doing its job that it has left a lasting legacy. There are
two ways to approach this legacy when rehabilitating an older,
lead-coated bridge--the existing old lead coating can be
encapsulated by another coating system or it can be cleaned off
down to the base metal.
"There are two different
philosophies on lead. You can bite the financial bullet now and
take it off, or paint over it, and get 10 more years out of it
but the problem will reappear," states Weaver.
"Bridge authorities are now doing one of two things. They either
contain the bridge and take it back to bare metal, or they
encapsulate it to get another 10 or 15 years before having to do
the full clean up."
Encapsulate or Contain
have good points and bad to them. Encapsulation, which involves
painting over the existing old coat so the lead is not exposed
to the environment, is the simpler of the two options.
Encapsulation also can postpone the removal of lead until the
end of the bridge's life span.
"A paint over is estimated to
get another 15-20 years of protection," states Mike Grillo,
technical representative, Wasser.
However, encapsulation may
only prolong the problem and pass it on to the next generation.
And encapsulation may become damaged thus exposing the old lead
Stripping a bridge down to bare metal involves
blasting off the lead paint, which results in lead dust. Lead
dust poses a much more serious health problem because it can
pass into the water table.
Because of the danger associated
with airborne lead dust, a blasting project must be contained,
which is a major challenge facing bridge painters.
ongoing challenge to the industry is containing the lead dust
when stripping an old coating," states Weaver.
explains, "Many bridges require full removal of the existing
coating system, so you are forced to blast. That means that the
bridge will be cocooned in plastic, with negative air on the
inside so absolutely no dust can escape into the atmosphere
while the blasting is going on. The air from the cocoon will be
sucked out and passed through a filtration system."
the drawbacks to blasting is the added cost to a bridge coating
project. "Containment requirements for blasting are so stringent
that is drives the price of getting a structure painted up by
probably 30 percent-50 percent," says Grillo.
But there are
alternatives to blasting. High-pressure water jetting involves
cleaning the paint off with a jet of water at 10,000 to 25,000
psi. There also is ultra high-pressure water jetting, which is
cleaning performed at pressures above 25,000 psi.
explains, "Ultra high-pressure water jetting uses a water jet at
25,000 psi or greater to remove paint from a structure down to
the steel, without the use of dry abrasives. This helps lessen
the need for containment. However it leaves the steel wet, which
may result in some slight surface rust before you can get some
paint on it.
"But what the system does is not create any
dust. All the paint chips just fall down with the water. You
don't need as much containment as is necessary with a dry blast.
You only need containment underneath to catch the water and
filter the paint chips out of it."
Another way to avoid
containment during a spot preparation of the bridge's surface is
to use the power tool prep method.
"Power tool prep involves
the use of needle guns, or a grinder with a wire wheel or
grinding disc on it. These tools will have a vacuum attachment
on them that sucks up the dust and debris as the worker moves
along. This is used for spot preparation to remove rust. But
before this you always pressure wash to remove any loose paint
and contaiminates," says Grillo.
Another solution to
avoiding for preparing the surface is chemical stripping. Using
this method helps keep the dust to a minimum. is mentioned by
Weaver. "Use coatings that are tolerant to minimum surface
preparations so that you don't have to go in and blast. This is
especially useful around high residential areas. If you can get
a coating that can perform well over a marginally clean surface
then that eliminates the need for blast cleaning."
two technologies that complement the alternative solutions to
dry blasting. One is polyslioxane.
"Polyslioxane is a
coating designed to do two things. First it is more tolerate of
poorly blasted or poorly prepared surfaces. Second, it also
gives good weatherability in one coat as opposed to two,"
The second useful technology is moisture
cure urethanes, because it functions well in high humidity
environments and can be applied to surfaces that are slightly
damp. This second trait is useful when used with the water jet
blasting technique. And there are other benefits.
cure urethanes allow for a longer work season and a longer work
day. You don't have as much down time because of humidity and
you can keep on working even when the humidity gets up to 98
percent," adds Grillo.