Weathering steel is a high-strength, low-alloy steel that has been proven to provide significantly more corrosion resistance than regular carbon steel. This corrosion resistance is achieved by adding small amounts of certain alloys to the steel that promote the formation of a protective oxide layer when the steel is exposed to the environment. The main advantage of using weathering steel in bridge applications is that under normal conditions it may be left unpainted, leading to significantly reduced maintenance costs. This means periodical blast-cleaning and repainting of highway bridges could be avoided, which also has significant environmental benefits. Weathering steel has been a material of choice for highway structures for almost half a century in North America, Europe, and Japan, with a very large number of structures being constructed with it. Although its use in bridge applications has frequently been successful, a number of cases have been identified more recently where its corrosion performance has been worse than expected. In some cases, the reasons for this poor performance have been identified. In others, adjacent bridges constructed at the same time and subjected to similar environments have performed very differently, making the cause of the poor performance difficult to determine conclusively. Recently, the Ministry of Transportation of Ontario (MTO) has found that a number of their weathering steel highway bridges are corroding at higher-than-expected rates. This has led to concerns regarding the safety of these structures. Firstly, this corrosion is causing a reduction in the thicknesses of the bridge girder plates. This has structural implications that have yet to be examined in depth. Secondly, the corrosion product, in addition to being unsightly, has also been seen to spall off of these structures in pieces that are sufficiently large to pose a threat to traffic passing underneath. As a direct result of these concerns, this thesis project was initiated to examine the potential structural safety issues resulting from the corrosion problems observed in weathering steel highway bridges in Ontario and the possible mitigation of these problems through the application of specialized zinc-based coating systems.
In many of the “problem structures”, the regions of poor corrosion performance can be related to the “splash zones” caused by trucks passing underneath the weathering steel structures. Figure 1.1 shows an example of this. In this figure, the region where the corrosion is most severe is the underside of the box girder bottom flange, on the side of the girder that faces the oncoming traffic.