Once the 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 day.
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 1990."
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 option.
While there is no law against the use of lead, the many environmental, health and safety concerns complicate its use.
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 the 1970s."
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.
Dickey adds, "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
Both choices 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 coating.
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.
"An ongoing challenge to the industry is containing the lead dust when stripping an old coating," states Weaver.
Grillo 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."
One of 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.
Grillo 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."
There are 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," explains Dickey.
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.
"Moisture 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.
