# Corrosion-Fatigue Evaluation of Uncoated Weathering Steel Bridges
## 1 Introduction  
Weathering steel has been widely used in bridge engineering in the U.S., Europe and Japan since half a century ago. Due to the excellent corrosion resistance of weathering steel, it can be applied in the corrosive environment without coating. Various grades of weathering steel have already beenincorporated in the codes of practice, such as American grades A242, A588, A606 and A709, Japanese grades JIS SMA and the British S355J2G1W [1;2]. Since 2017, over 50% of highway steel bridges in the US were constructed with weathering steel. In Japan, 75% of the weathering steels were used in bridge engineering in 2008, and 80% of newly built steel bridges were constructed with weathering steel in the past 25 years [3;4]. In China, the first weathering steel bridge was constructed in 1991 [5], followed by the massive application in the 2010s. A great advantage of uncoated weathering steel (UWS) bridges is the reduction of cost. [PERSON] et al. [1] claimed that approximately 5% of the initial cost and far in excess of the 5% on capital costs could be saved. Moreover, [PERSON] et al. [6] indicated that weathering steels could reduce about 25% life cycle cost according to recently under construction bridges. In addition, [PERSON] also listed the environmental benefits of UWS bridges arising from avoiding painting [1]. It is also worth noting that the fire incident has now become a very critical issue in steel bridges since it can lead to extremely detrimental consequences [7]. [PERSON] et al. [8] claimed that the chemical element Mo could notably improve the fire-resistant of steel. Since the element Mo is generally included in making the high-performance weathering steel, a much better fire-resistant performance can be expected when compared with the common carbon steel. However, further investigations are still required to reveal the actual fire resistance of weathering steel. In steel bridges, the fatigue issue is of particular concern due to the persistent loading-and-unloading process by the vehicles. Similar to the degradation of steel bridges from the fire hazard [7], the corrosion effect should be considered into the design. Nevertheless, the fatigue issue of steel bridges will be further accelerated when exposed to the corrosive environment. The corrosion process will lead to geometric irregularities in components, escalating the local stress concentration, which serves the fatigue issue. In UWS bridges, the problem is even more serious: The fatigue crack is hard to be detected due to the rust layer used to proof corroducts. Thus, it is crucial to take into consideration the coupling effect of corrosion-fatigue (C-F) on the service life of UWS bridges. Figure 1 shows a typical fatigue crack in a UWS bridge, which is identified by the dye penetration check [9]. It is worth stating that, this type of crack is very difficult to be detected using common approaches, especially in its early stage. Due to the difficulty in detection, a considerable number of unrecognized fatigue cracks might exist in the aged UWS bridges, imposing high risks on the durability or even safety of the bridge consequently. Therefore, a well-established model is urgently required for the evaluation on the service life of UWS bridges considering the C-F process. The aim of the present study is to provide a deep insight into the corrosion-fatigue (C-F) performance of UWS bridges, which can serve as a useful guidance for design and maintenance of this type of bridges. On this end, an innovative deterioration model has been established to simulate the coupled C-F process. In the model, the whole C-F process has been divided into two interrelated stages, i.e., the corrosion-critical stage and fatigue-critical stage. In the study, the C-F process is assumed to be started with the pitting corrosion, in which the crack-like pitting flaw will initiate [10]. In modeling the pitting corrosion, the electrochemistry theory is applied, and the pitting current is utilized as the index reflecting the influence of different environments.