## 1 Introduction  
Infrastructure assets, including roads, bridges, pipelines, etc., are essential foundations for any country. They provide services that are necessary for everyone's daily life. However, they are highly capital intensive and sufficient investment is not always made [1, 2].  
Infrastructure asset management (IAM), also named asset management, is proposed to efficiently manage the infrastructure assets. Because of its broad coverage, IAM has diverse definitions with different focuses [3]. For example, New Zealand Asset Management Support (NAMS) defines IAM as \"systematic and coordinated activities and practices of an organization to optimally and sustainably deliver on its objectives through the cost-effective life cycle management of assets\" [4].  
The Federal Highway Administration (FHWA) in the USA defines IAM as a business process that \"incorporates the economic assessment of trade-offs among alternative investment options\" and therefore makes \"cost-effective investment decisions\" [5]. Austroads, from Australia, defines IAM as \"a systematic process of effectively maintaining, upgrading and operating assets\" considering engineering principles, business, and economy in order to deliver optimal community benefits [6]. In general, it arranges management interventions e.g., maintenance and rehabilitation (M&R), update, etc., to an infrastructure project or an infrastructure network so that the project or the entire network is able to provide the required level of service, subject to the management requirements, e.g., maintenance budget.  
Decision making (DM) is an essential part of IAM, which determines the management plan of interventions for the infrastructure asset project or the network. When generating different management plans, i.e., implementing different interventions to different network segments at different points of time, the management outcomes, e.g., management cost and infrastructure asset condition, are different. DM is required to find an appropriate management plan so that its outcomes satisfy the goals and requirements of IAM.  
However, in modern society, DM is not easy as it faces a wide range of challenges, including large infrastructure asset networks, limited resources, various outcomes, conflicting goals, and uncertainties [7; 8]. Hence, optimization is applied, and in more recent years, has become popular in DM. A large body of articles applies a broad range of optimization methods to a variety of DM problems. Firstly, the majority of these articles focus on the applications and improvements of individual methods in the context of a certain type of DM and a comprehensive knowledge, given the broad range of optimization methods, is hardly discussed. Secondly, with a growth of publications, it is valuable to analyze and graphically present the existing knowledge on this subject.  
This paper aims at enhancing the knowledge of optimization methods and their applications when making management decision in IAM. Its main focuses are below as follows:  
* Clarifying optimization methods and their abilities when dealing with DM problems in IAM;
* Summarizing and discussing the previous applications of optimization in DM in IAM, based on a total of 337 articles;
* Specifying the popular optimization methods applied in DM in IAM and making recommendations on their applications.