Historically, the productivity of the construction industry has been lower than that of other industries, such as manufacturing. However, a newly emerging argument suggests that embracing digital transformation may lead to productivity improvements similar to those observed in other industries [1,2,3]^[1][2][3]. Achieving such a digital transformation within the construction industry requires a method for systematizing construction information and converting it into digital data [4,5]^[4][5]. Furthermore, establishing approaches for its production, storage, delivery, and utilization is equally important. The extent and diversity of construction information make it challenging to precisely specify its scope. Hence, an appropriate construction information classification system must be developed to effectively store essential information in a digital format. However, the existing research on construction information classification systems primarily focuses on completed facilities, concentrating solely on storing and retrieving facility-related information. These existing systems are inadequate for comprehensively managing task-based information throughout the construction project lifecycle.

In particular, infrastructure construction projects involve long-term endeavors that span planning through operation. Therefore, they require the continuous securing of digitally transformed construction information throughout the project lifecycle. Ensuring the continuity of construction information throughout the infrastructure construction process facilitates the easier acquisition, analysis, and sharing of the required data at each stage. Thus, it enables prompt decision making and optimizes project outcomes.

2.. Traditional Construction Information Classification System

The construction information classification system originated in the United States in 1920 with the American Institute of Architects (AIA); it initially classified building materials and later expanded to include building types. In 1963, the Construction Specification Institute (CSI) introduced a standardized format for building specifications, which rapidly became the industry standard. Subsequently, in 1966, the AIA, CSI, and other stakeholders collaborated to develop this system into a unified framework for construction specifications, data filtering, and cost accounting, facilitating the organization and exchange of technical information across the construction industry [7]^[6]. These collaborative efforts led to the revision and renaming of the system as MASTER-FORMAT in 1978 [8]^[7]. In the United States, the classification system primarily focuses on cost management, specification organization, project management, and data consolidation [9,10]^[8][9]. In 1945, the Ministry of Construction Library in the United Kingdom extracted only the construction elements from the International Decimal Classification, a general book classification method, and presented the Universal Decimal Classification extraction method. The British Standards Association officially announced this method in 1948 [11,12]^[10][11]. In 1947, construction companies centered around the Swedish Institute of Architects collaborated to develop Samarbetkommitten for Byggadsfragor (SfB) [13]^[12], a system that classified a construction information classification system into three facets: functional elements, construction types, and materials. The concept behind SfB significantly influenced many subsequent classification systems, leading to the creation of various forms of classification system [14,15]^[13][14]. The first classification system designed for computer processing, known as the Coordinated Building Communication System, was developed in 1963. It enabled the generation of computerized itemized statements for construction purposes. Later, in 1968, the Royal Institute of Architects in England introduced the Construction Index/Samarbetskommitten for Byggnadsfragor (CI/SfB) classification system [16]^[15]. This system expanded the classification into five facets (facilities, parts, construction types, materials, and other) by applying an analytic–synthetic classification system [17]^[16]. Major construction companies in South Korea began adopting the CI/SfB system in the 1980s. The Technical Subcommittee TC59/SC13 of the International Organization for Standardization (ISO) has overseen construction information classification systems since 1988. However, individual countries have the flexibility to propose their own classification systems based on specific needs. In the United Kingdom, the NBS introduced UniClass2015 as an integrated classification system for the construction industry, whereas the CSI in the United States presented OmniClass [18]^[17]. These classification systems establish a standardized foundation for classifying construction information throughout the entire project lifecycle. Furthermore, they are actively being developed for the digital transformation of and integration with building information modeling (BIM) in the construction industry. Previous studies focused on classification methods for information related to completed buildings within the construction domain. Therefore, these methods have limited applicability to infrastructure facilities, which often exhibit atypical features, such as varied topography. Figure 21 presents an analysis of all the items in UniClass [19]^[18] and OmniClass [20]^[19], indicating that information elements directly associated with infrastructure construction represent 17% of OmniClass and 13% of UniClass. Uniclass also considers the infrastructure sector [21]^[20]. However, its utilization in infrastructure is low because it primarily focuses on buildings [22]^[21]. Omniclass makes similar assessments [23,24,25]^[22][23][24]. In addition, various national and international classification systems are also in place, such as CoClass [26]^[25] and Byggandets Samordning AB [27]^[26] in Sweden, CCS [28]^[27] in Denmark [25]^[24], and Talo [29]^[28] in Finland. However, no case has been optimized for infrastructure. In the international standards ISO 12006-2 and 12006-3, which pertain to construction information classification systems, business information, construction object information, and construction process information are closely interconnected [30,31]^[29][30]. However, practical construction projects might encounter difficulties in effectively utilizing the system owing to the potential impact on information associated with individual construction objects when linked activity information undergoes changes.

3. Object-Oriented Information Classification System

Studies focused on constructing an effective construction information classification system continue to be conducted separately from international standardization efforts. However, all classification systems are designed for the integrated management of substantial construction information, surpassing a simple arrangement of construction data [32,33]^[31][32]. Although the developed classification systems are designed to conform to existing systems and enhance usability through careful analyses of requirements, design and construction work remain disconnected. Therefore, an information classification system that is widely used throughout construction projects is necessary. Some cite the lack of publicity and voluntary participation as reasons for low utilization in the early stages [34]^[33]. Nevertheless, the fundamental cause lies in the lack of integrated development for construction information classification systems. Cerezo-Narváez et al. [35]^[34] proposed an integrated approach, combining the cost breakdown structure and work breakdown structure (WBS) to facilitate budget management and quality control in construction projects. Their method involved breaking down project information into smaller cost-calculating scales and defining unit tasks, allowing its integration with BIM. This integration proved significant as it enabled the representation of construction information in a visual format, centered around the subdivision of information units based on construction costs. However, a challenge arises when directly linking construction cost units to tasks, making it difficult to handle information processing in scenarios involving changes in the project scope or construction methods during the construction phase. Reevaluating and reestablishing the construction information classification system is necessary to overcome this challenge and enable effective information processing, particularly concerning the interconnection of various stakeholders during construction. Research has been conducted from various perspectives on using BIM to digitize the construction industry. Specifically, in the context of architectural structures, studies focusing on the concepts and structure of object classification systems within BIM revealed that shape information for defined components could be systematically managed. This was achieved through the integration of the object breakdown structure, WBS, and project numbering system, as demonstrated in the construction of timber structures [36]^[35]. This approach is particularly suitable for buildings with independent spaces for completed objects. Developing specialized software to implement this approach can further enhance its effectiveness. Research on the requirements of information classification systems based on BIM information frameworks in infrastructure construction in South Korea yielded two key findings. First, it is necessary to implement three-dimensional (3D) integrated information, preferably linked to existing classification systems. Second, a comprehensive perspective calls for an integrated standards system rather than fragmented standards tailored to individual purposes [37]^[36]. Revising the construction information classification system is necessary to effectively utilize BIM, as it allows for the connection between BIM objects and items within the construction information classification system. Additionally, it should incorporate geometric and attribute information, representing it in 3D models, and enable the linking of attribute information to define individual objects [38]^[37]. Research was conducted to expand the model breakdown structure under the component classification system of the construction information classification system, which is based on the WBS used in road construction projects in South Korea. The primary aim was to develop an information classification system incorporating the existing facet-based and object-oriented classification systems [39,40]^[38][39]. It is feasible to develop a BIM that aligns with widely used construction information classification systems and organizes construction information based on an object-oriented approach. However, further research is necessary to address the flexible handling of construction information during its generation and adaptation to meet specific requirements at individual construction sites.

4. Construction Information Classification System for Construction Lifecycle Information

4.1. Object-Oriented Construction Information Classification System

The existing construction information primarily serves as a tool for delivering concepts and terminology to experts. However, digitally transformed road construction information is represented as a 3D construction information model using digital data, enabling the conveyance of additional information through spatial representation. Therefore, to ensure efficient utilization, construction information should be classified based on the construction objects represented in 3D models. Road facilities can be designated as a single construction object or divided into segments. Additionally, the individual facility elements constituting the road can be designated as separate construction objects. The designation of construction objects can take various forms, depending on the most suitable form for delivering the required construction information. The construction objects that must be represented in three dimensions are defined by their shape, location, and attribute information, as shown in Figure 32. Linking information concerning the individual works performed during the design and construction phases of a road project to individually defined 3D construction objects enables the classification of information. This classification allows users to intuitively understand and convey the information.

4.2. An Information Classification System That Separates Construction Objects and Activities

As road facilities are created through design and construction, it becomes challenging to entirely segregate the associated activities. Therefore, most existing information classification systems define construction information by linking construction objects with activity details, as shown in Figure 43. While this approach maintains flexibility for partial changes in construction objects or activity information, it cannot adequately accommodate significant modifications in the scope of construction objects or activity content. To address this issue, scholars developed a construction information classification system for each information type. This allows for independent modifications of construction objects, such as splitting or merging, and the replacement of all the activities assigned to construction objects without modifying the other data (Figure 54). The connection between construction objects and activities is established by referencing the unique IDs assigned to each of them.

3.3. Information Classification System for Construction Objects

4.3. Information Classification System for Construction Objects

Construction objects are discrete 3D units within road facilities; they possess engineering significance. These objects can be identified independently, allowing for their classification as singular construction units for the entire road facility or as distinct portions. This enables a hierarchical classification of all construction objects. For example, an entire road facility can be designated as one object, while the portion constructed as a bridge can be designated as a bridge construction object. Furthermore, detailed elements, such as piers, copings, abutment concrete, and bridge supports, can also be identified as separate construction objects using the same method. These construction objects are classified within a hierarchical structure that falls under the infrastructure facilities category. In construction, facility objects represent the structure of a facility, either in its entirety or in part. They are implemented as 3D models corresponding to the final products after the construction process. However, it may not be appropriate to rely only on facility objects to represent all construction activities throughout the construction lifecycle. To address this limitation, different types of objects have been defined. Element objects refer to specific components, such as reinforcing bars and concrete. These elements are subdivided to a level where functional aspects, which are necessary to achieve the purpose of the facility, are not considered. Construction-related components, such as temporary facilities, are collectively designated as temporary objects. These temporary objects are then assigned as sub-elements under the main facility object, enabling hierarchical structuring. Intangible objects cover construction elements not explicitly assigned to 3D visible facility objects but rather associated with higher-level facility objects. Each construction object within the hierarchical structure carries distinct associated information that should be inheritable based on the hierarchy. If the information does not follow an inherited relationship, separate branches must be defined for construction objects.

3.4. Information Classification System for Activities

4.4. Information Classification System for Activities

Road construction projects involve various activities contributing to the design and construction of facilities. Figure 65 illustrates how these activities constitute a collection of unit works focused on constructing the components of the road. Each unit is further subdivided to achieve certain objectives, such as safety, quality, and process management, providing detailed information. Moreover, these activities specify the necessary resources and conditions for their execution. These activities are interconnected with the construction objects, presenting independent information that can be classified hierarchically and facet-wise. They can be grouped based on similarities or hierarchically arranged by dividing or merging unit activities. When an activity is linked to a construction object, it modifies the attribute information of the construction object, indicating the allocation of resources and ongoing on-site construction processes. Therefore, information regarding which activities are connected to specific objects becomes a critical aspect of the construction information model, providing insights into the execution of construction projects. Infrastructure is managed and constructed by national or public institutions, each with its own distinct method for overseeing construction objects and activities. Consequently, the volume of construction information to be produced and managed may differ significantly. It is unfeasible to implement a universal regulation system for construction information models across various facilities managed by different public institutions. Therefore, an information classification system that permits flexible adjustments to be made to the construction information model is necessary. In principle, construction objects must be associated with one or more activities. While some objects may not be directly linked to any activity, one or more intermediate objects between the top- and bottom-level objects must have assigned activities. The unit construction cost is assigned to activities classified as billable, while activities of the same type but applied differently based on conditions are grouped as billable activities. These defined activities comprise multiple smaller unit activities grouped together to facilitate process management and progressive billing. Various types of construction information, including the materials, equipment, and personnel, define these activities. Time information related to the duration of unit activities can also be included. If a specific unit activity requires special management, such as safety or quality control, it was classified as an elementary activity. General management activities that could not be specifically identified in the construction process were classified as intangible.

3.5. Unit Works Defined by Construction Objects and Activities

4.5. Unit Works Defined by Construction Objects and Activities

Road construction projects encompass various works during the design and construction phases. Notably, the specifications of the target facility may undergo changes, and the content of activities can change throughout the construction lifecycle owing to advancements in engineering technology, the development of new construction methods, and fluctuations in social conditions. Therefore, effectively managing construction information involves maintaining continuity between stages, ensuring that the changes occurring in each stage are recorded and traced back to the preceding stages. In cases where construction plans are modified, the hierarchical structure of the construction objects enables the easy selection and modification of objects that need revision. Simultaneously, associated activities can be modified concurrently or at a later stage. During the execution of the unit work, activities can be assigned or newly specified.