## 6 Conclusions  
This article aims to minimize the maintenance costs incurred at the project level of bridges. Based on the actual condition of bridge defects, the costs and benefits of different maintenance strategies were analyzed, and an optimization model for maintenance decision making was constructed. The applicability of the dynamic programming-policy iteration algorithm in solving the optimization problem of maintenance decision making at the project level of bridges was studied. The following conclusions were drawn:  
(1) In order to address the issues of unclear quantification and inaccurate estimations of costs and benefits in bridge maintenance decision analysis, this study simplifies the estimation process of maintenance costs. It improves the accuracy and reliability of estimations by considering the actual defect types and development of bridge defects, as well as the importance of future maintenance costs. The unit direct cost in the budget unit price method was used as the unit cost of maintenance measures, thereby aiming to improve the accuracy and reliability of estimations based on the actual problems of bridge maintenance. (2) Considering the limitation of the widely used Markov model in not accurately representing the time aspect, the Weibull distribution was introduced to model the duration of the elements in different technical conditions. The duration of bridge elements in each technical condition was determined, and it served as a decision basis through which to track the degradation of bridges in real time, to promptly detect and address issues, and to ensure the safety and reliability of bridges. (3) In the analysis of bridge maintenance decision making, where the decision results do not propose specific maintenance plans at the defect level, a sequential decision-making model was established and solved using the dynamic programming-policy iteration algorithm. The decision factors, such as the state distribution vector, cost matrix, and maintenance effectiveness matrix, were related to the actual bridge defects. The decision results can provide specific maintenance plans for elements with different service durations and different technical conditions, thereby achieving targeted decision making for the purpose of precise maintenance. The model can also adjust maintenance strategies based on the availability of maintenance funds to ensure the feasibility, economy, and sustainability of the decision results. (4) The sequential model constructed in this study uses a reward function based on bridge maintenance costs. However, in bridge maintenance decision making at the project level, other factors need to be considered, such as the importance weight of elements and the construction difficulty of maintenance plans. Therefore, future research will focus on considering more influencing factors in bridge project-level maintenance decision making. In addition, the use case of this research is a prestressed concrete box girder. The adaptability of other parts and other bridge types such as arch bridges, cable-stayed bridges, and suspension bridges remain to be proven. Conceptualization, G.X., Z.L. and G.L. ; Funding acquisition, G.X. and P.L. ; Methodology, Z.L. and P.L. ; Software, Y.H. and G.L. ; Validation, Y.H. and Y.Z. ; Visualization, Y.H., G.L. and Y.Z. ; Writing--original draft, Z.L., Y.H. and Y.Z. ; Writing--review and editing, G.X., Z.L., and P.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the Science and Technology Project of Shandong Department of Transportation (grant number: 2022B61). Not applicable. Data Availability Statement: Data are contained within the article. Conflicts of Interest: Author [PERSON] was employed by the company Shandong High-Speed Group Co. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.