Roads and bridges are threatened by corrosion and degradation as they are continuously exposed to climatic elements and vibration from vehicle movements. The cost of corrosion mitigation to national economies around the world is estimated to be billions of dollars every year. The impact can be both economic through the cost of repairs and maintenance as well as a physical threat to workers, the public, and even other nearby structures. One of the two main ways to protect an asset from corrosion is to physically isolate a structure from the environment by applying a protective coating.
Protection of the various structural elements was integral to the design of Australia’s Darlington Upgrade Project. The project is another important stage in the delivery of Adelaide’s North-South Corridor and will deliver an upgrade of ~3.3 km of the existing Main South Rd. The $620 million project is jointly funded by Australian and state governments ($496 million federally funded and $124 million state funded) and is due to be completed in 2019.
Three of the eight bridges constructed for the project consisted of a steel boxgirder design with reinforced concrete decks. Half of the girder sections for these structures were constructed by Haywards Engineering in Tasmania, with McElligott’s then responsible for applying a protective corrosion-resistant coating to the girders. The 22 coated bridge girder sections weighed more than 1,300 tons in total. The other half of the girders were constructed and coated by Bowhill Engineering in South Australia.
According to Ivan Berry, general manager at McElligott’s, each girder was manufactured and delivered to the company’s workshop. “Our facility is a continuous covered line where structures to be coated are profiled, primed, and coated,” Berry says. Each girder section was abrasive blasted to blasting standard NACE No. 2/SSPC-SP 10/Sa 2 1/2.1 Profiling a substrate produces an even-textured surface that increases the adhesion of a coating. Once profiled, the girders moved down the line where they were primed and then had two layers of topcoat applied.
Jeremy Hawkes, managing director of Bowhill Engineering, concurred that moving the 48-m by 4-m high units of open top box girder within their facility was a challenge. Abrasive blasting and painting in the same facility produced its own difficulties. “For that size structure, we used a lot of abrasive garnet to profile the steel. The used garnet had to be collected and cleaned for reuse,” Hawkes says. “We began using shovels and brooms to collect the material but a short while into the project, we managed to acquire a surplus three-stage vacuum system that automatically separates the material and prepares it for reuse.”
Hawkes stated that the weather was also a challenge. “There was often only a small window of opportunity when the temperature was the correct level above the dew point, allowing us to work within the manufacturer’s specifications,” he says. Bowhill upgraded its workshops to install ducted gas-fired heating, which allowed them to get the girder sections to an overall even surface temperature. “It was important to heat the girder sections so that the coating materials adhered to the substrate and cured effectively,” Hawkes adds, “but the size made it difficult to evenly heat the girders.”
The duration of the project meant that some of the coating work took place during the Tasmanian winter, requiring McElligott’s to adjust its work schedule. “To meet overall deadlines, abrasive profiling was conducted overnight and painting took place during the day in order to reduce the amount of extra heating required,” Berry adds. The coating system was a zinc-rich epoxy, followed by an aluminum pigmented epoxy, and the final coat was Hardtop AS,† manufactured by Jotun. The total surface area coated by each coating company was approximately 6,500 m2 of exterior surface and 9,000 m2 of internal surfaces.
Rob Butcher, state manager—Protective Coatings (SA and Victoria) at Jotun, said his company’s challenge was to specify a material that could be used by two separate companies located in quite different climatic environments. “One company is 200 km inland in South Australia where it can get very hot and dry, whereas the other is on the northern coast of Tasmania where there can be days when the temperature is close to zero,” he says. The specification of the coating material had to accommodate its application in both places.
It was essential that the finished coatings from both applicators matched as much as practical. “We were able to use the Australian Standard 2312.1.2 This standard has been tried and tested in our industry for many years and is well accepted,” Butcher states. The Australian Corrosion Association has been a part of the research into the performance of coatings in Australia for many decades. It was involved in the report that led to removal of lead paints from Australia in the 1990s and continued this by contributing to international standards relating to coatings safety.
“When applying the coating, the big sections made it challenging to maintain a ‘wet edge,’” Berry says. “We had to continually adjust the hardeners and thinners in order to extend the drying time of each coating layer.” A “wet edge” refers to the process of avoiding obvious join lines between sections of coating. They also applied a “stripe coat” over welds and other joints. Berry adds that the massive girders—the heaviest weighed 86 tons—were the largest components that his company has been involved with. Scissor-lift platforms were required to allow the applicators to safely reach the highest areas on the outside and scaffolding for those on the inside.
“These girder sections were huge and comprised 20 individual sections each,” says Hawkes. “The Ayliffes Road bridge will be a total length of 390-m long, which is a pretty decent bridge in anyone’s terms—especially when you have to move it.”
The three steel box girder structures for the project were constructed using an innovative method whereby the structures were built off-site, transported, and precisely maneuvered into place using self-propelled modular transports (SPMTs). This method of bridge construction is commonplace throughout Europe and the Americas. However, this was the first time SPMTs have been used in Australia by the infrastructure sector to install a fully completed structure.
When a coating is properly applied, inspected, and qualified, it should easily provide 25 years or more of protection, although many projects today are even specifying 50 and 100-year life expectancy. New capital investment in some areas may be slowing down, but governments around the country have recently announced plans for large-scale road and rail projects that will provide many opportunities for corrosion control and prevention companies.
† Trade name.
References
1 NACE No. 2/SSPC-SP 10/Sa 2 1/2, “Near-White Metal” (Houston, TX: NACE International).
2 AS/NZS 2321.1, “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings—Paint coatings” (Sydney, Australia: Standards Australia).
Source: The Australasian Corrosion Association, www.corrosion.com.au. Contact Christine Filippis, Teraze Communications Pty Ltd.—tel: +61 3 8391 0701, email: info@teraze.com.au.