## 4 Conclusions and Future Research  
This paper presents a comprehensive overview of the applications of optimization methods in DM in IAM. As a consequence of their intelligence, optimization methods are increasingly applied to assist DM and facilitate IAM. This paper, based on a review of 337 articles, introduces, summarizes, and discusses the achievements on this subject. According to the review, it concludes the following:  
* Optimization can greatly assist DM in IAM. It analyzes DM problems even for the large infrastructure networks and life-cycle analysis and attempts to generate the best solution(s) under the goals and requirements of IAM. Therefore, DM becomes easier and more informed and its result is more reasonable and more objective.
* Optimization methods have been applied in DM in IAM for a long time and became popular in the 2000s.
* IAM has the following two types of DM: Project- and network-level, where the former is often used for a specific project and the latter is often used to analyze an infrastructure network when deciding the M&R management.
* DM problems can be defined as SOO or MOO depending on the number of the main goals. Optimization methods can handle both and generate one solution or a set of solutions accordingly.
* There are a broad range of optimization methods that can be applied to assist DM in IAM. This paper summarizes the popular optimization methods within the classes of deterministic methods, heuristics, and other methods and software. These methods have their strengths and weaknesses. The recommendations and suggestions on their applications are also made.
* DM may involve various management outcomes which can be analyzed by optimization methods in terms of objectives and constraints. The common outcomes include financial expense and infrastructure asset condition, while other outcomes are also considered.  
Optimization is a useful tool for DM in IAM and great achievements are gained by the articles. Its significance is widely admitted by researchers and practitioners. However, with the development of IAM, DM becomes more complex. Firstly, an essential issue in IAM is to keep the infrastructure condition at an acceptable level. This requires an accurate model and prediction of the infrastructure deterioration, while the deterioration is affected by the changing environment and filled with uncertainties. Hence the prediction of infrastructure condition over the long-term is still a critical topic.  
Moreover, private actors play an important role in IAM. Public agencies and private investors have different intention when managing infrastructure assets. How to consider their roles and satisfy their requirements in IAM is another important topic.  
Additionally, each DM problem is unique and none of the optimization methods is a panacea. All of the algorithms have their strengths and limitations. Hence, the selection of a proper optimization algorithm depends on the addressed problem.  
While optimization is mathematics and it tries to correctly describe practical DM problems, it is worth noticing that there is always a gap between practical problems and mathematical models. Decision makers should carefully examine their problems and tailor the optimization results in order to make more practical decisions for the addressed DM problems.  
This paper is based on a total of 337 articles, while these articles do not cover all the published articles on this subject and some sound articles may be missed. This paper focuses on the optimization algorithms and other subjects are briefly discussed.  
This research was funded by the Fundamental Research Funds for the Central Universities (Grant No. 300102218101, 300102218401, 300102218302).  
The authors declare no conflict of interest.