# Corrosion-Fatigue Evaluation of Uncoated Weathering Steel Bridges
## 7 Conclusions  
The uncoated weathering steel (UWS) bridges become increasingly popular due to their advantages in both the lifetime cost and environment protection. However, the C-F issue is of particular concern in UWS bridges, which is further escalated by the designated corrosion process. On this end, a solid evaluation method is urgently required for the C-F process in UWS bridges to ensure the lifetime serviceability and, more important, safety. In this paper, an innovative model was established through combining the electrochemistry theory and fracture mechanics. The proposed model for C-F crack initiation was then validated using the experimental data in existing literatures. Based on the verified model, the case study was carried out on two typical types of structural steel, i.e., HPS 70W and 14 MnNbq steel. Besides, investigations were also made on the effect of crucial C-F parameters, including the stress range, stress ratio, corrosive environment and average daily truck traffic (ADTT). Based on the above analysis, the following conclusions could be drawn:  
(1) Analysis was made to find out a suitable index to reflect the degree of corrosion, based on the experiment data collected at various places with different corrosion categories in China. As a result, a clear relation could be observed between the degree of corrosion and the pitting current, which were  
Figure 16: Crack development under different average daily truck traffic (ADTT): (**a**) HPS 70W and (**b**) 14 MnNbq. Figure 17: Impact of stress ADTT on C-F life: (**a**) Initiation life and (**b**) propagation life. only related to the corrosion environment. Thus, the pitting current was utilized to reflect the degree of pitting corrosion in this study. (2) An innovative model was established to simulate the C-F process in UWS bridges. The two stages in the C-F process were independently treated in series, i.e., the C-F crack initiation stage and the C-F crack propagation stage. Meanwhile, the critical pitting depth could serve as an indicator of the fatigue effect, which competed against the corrosion effect. At the C-F crack initiation stage, the pit could be regarded as an equivalent initial flaw that only grew under the corrosion. Once the flaw reached the critical size, the fatigue effect joined as a parallel driving force in crack propagation. The two effects then competed with each other, and the C-F process was dominated by the one with the higher growth rate. As a result, the fatigue effect gradually replaced the corrosion as the major driving force, which was the symbol of the C-F crack propagation stage, and the crack then grew under the cyclic stress until the final failure. In the study, the pitting corrosion process was considered through Faraday's law, while fracture mechanics was employed to describe the propagation of fatigue cracks. (3) The model proposed for the initiation stage and the whole C-F process was validated using the experimental data in the existing literatures. The result shows that the prediction curve matched well with the test data, indicating the effectiveness of the proposed method in predicting the C-F life. (4) A case study was carried out on the bridge respectively constructed of two brands of steel, i.e., the normal steel 14 MnNbq and the high-performance weathering steel HPS 70W. The result shows that when considering the coupled C-F process, the bridge with the HPS 70W had a remarkably higher service life than the one with the 14 MnNbq. Besides, it also suggests that the C-F crack initiation stage accounted for the largest part in the whole C-F life, i.e., around 61.6%-80.3%. (5) A sensitive analysis was conducted on various C-F parameters, including the stress range, stress ratio, corrosive environment and ADTT, on the C-F life of normal steel 14 MnNbq and HPS 70W. The investigation was performed with respect to the influence on both the initiation life and propagation life. The result shows that stress range and stress ratio both had dramatic impacts on C-F life.