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CHARLOTTE, N.C. – Bridge experts have known the benefits of uncoated weathering steel for decades–and thanks to research by the University of Delaware’s Jennifer McConnell, PhD, we can now quantify those advantages.
McConnell, recipient of the 2023 T.R. Higgins Lectureship Award, closed out NASCC: The Steel Conference by presenting findings from more than a decade of research on weathering steel bridges–work that’s crucial as the nation focuses on infrastructure, because corrosion is the most widespread problem those systems face. In 2016, the National Association of Corrosion Engineers (which is now called the Association for Materials Protection and Performance) estimated that the worldwide economic impact of corrosion across all sectors is $2.5 trillion annually.
McConnell’s research focuses on weathering steel, an alloy that was first introduced in 1964 and has been used in more than 10,000 bridges. What makes it unique is that it’s intended to be used uncoated to allow its distinctive protective finish to develop. The lack of applied coatings provides cost savings both initially and throughout a structure’s service life as well as environmental benefits. However, it may not be an ideal choice for all environments.
“Weather is not just precipitation,” she said. “It’s the chemical elements that make up that precipitation”–and controlling exposure is the most important thing engineers can do to ensure that a structure reaches its target lifespan.
Humidity is a crucial consideration, because the chemical reactions that lead to corrosion can’t start without water. Chlorine–commonly found in the air above salt water and in deicing agents applied to roadways–act as a catalyst to speed up the process. And one of the main problems is that water and chlorides often coexist, creating the most corrosive environment that a typical steel structure can experience. McConnell set out to examine the impact of humidity and chlorides on UWS bridges over time.
Over the course of roughly a decade, McConnell’s team evaluated UWS bridges that had been in service for at least 20 years (in most cases) and up to 40 years in others in two general environments: coastal conditions, and those subjected to deicing. Some bridges in the study were subjected to both.
Her findings suggest that UWS is an ideal choice for environments that are neither extremely humid nor high in chlorides. As either of those environmental elements increase, thoughtful planning could be the key to success.
For instance, designers could consider including a sacrificial thickness of steel–that is, use thicker steel for things like bottom flange plates (where water and salt tend to collect) to mitigate the effects of highly corrosive environments. Under those conditions, UWS may lose ⅛” of thickness after 80 years; simply adding an additional ⅛” to the bottom flange plates over heavily salted traffic lanes, for example, would counteract that and allow owners to take advantage of UWS’s cost and sustainability benefits.
Designers should also consider drainage and the potential for joint leakage. McConnell pointed out that engineers could handle drainage by simply placing expansion joints behind the back wall with a drain pipe to discharge runoff away from the superstructure.
Her team reviewed 70 inspection reports from four different agencies (two in coastal areas and two in areas with regular use of deicing agents) and found a striking pattern: a third of the bridges they examined had worse performance below deck joints than in the remainder of the structure. These joints were leaking–a frequent cause of compromised corrosion resistance. Ideally, UWS bridges should have as few joints as possible, and careful detailing can make a big difference, too.
Although well-designed UWS requires little maintenance, some agencies have found that a surprisingly simple practice can extend the service life of a UWS bridge: washing it to clean off any accumulated chlorides after winter deicing. It’s common to wash trucks and loaders in the spring, McConnell noted, so why not wash bridges?
“Reaching [a structure’s] intended service life and optimizing the associated cost relies on design and maintenance actions,” she said. “Our goal is to better understand the specifics of steel corrosion to enable us to better design and maintain steel structures to mitigate and prevent corrosion problems.”
Recuperado el 18 de abril de 2023 de Decade of Research Provides Valuable Insights into Uncoated Weathering Steel Bridge Performance (aisc.org)