**2. Literature Review**

#### *2.1. Theory of the Building LCA Assessment Method*

Life cycle assessment (LCA) is a method that includes record of a list of invested materials and products related to the system life cycle of a product, and which allows examination of the potential environmental impacts related to these. For this study, when the list-analysis results and impact-assessment results were interpreted, research was actively performed according to the main methodology. This methodology has been used for environmental performance assessments of buildings from the 1990s to present day. Overseas LCA studies have proceeded as government-directed projects since 1970, and the majority of the research results on creating a LCI DB basic unit depending on the life cycle stage are already complete [10]. Research is also actively underway in Korea both nationally and publically, on a regular basis. Life cycle assessment is a tool that quantitatively lists the resources and energy that are used and the pollutants that are released during the product system's life cycle. It also systematically assesses their potential adverse effects on the environment. LCA is also used to derive measures to minimize and improve these adverse effects under the supervision of the Korea Ministry of Environment. It is applied to a product's eco-friendly design and other environmental claims, process improvements, and public policy establishment through the ISO 14040 series. LCA is divided into the four stages shown in Figure 2.

The definition of purpose and scope is the basic stage that sets the study's purpose and the system's boundary, which must be defined first in the life cycle assessment. Record analysis is the stage that quantifies the materials that are used and the products related to the product system. During this stage, data that describes all energy and by-product raw materials that are used and calculated, are collected and used in the calculations. Data are collected through a repetitive process, and new data demands and restrictions can be expected in order to achieve the purpose of the study by acquiring more information for each system boundary. Impact assessment uses the list-analysis results, and its purpose is to assess the importance of potential environmental impacts [11,12]. During impact assessment, listed data are generally linked to a specific environmental impact category, and category score, in order to understand their impacts and to provide information in the interpretation stage. An assessment range is set according to the LCA methodology of this type of series, and is generally created according to ISO 21930 as shown in Figure 3.

**Figure 2.** LCA's stages according to ISO 14040.

**Figure 3.** Life cycle stages for the building assessment in ISO 21930.

This study was set on gate-to-gate life cycle assessment, and an analysis was performed on CO2 emission characteristics by dividing the construction process, from a building's life cycle assessment range, into transportation, construction, and demolition stages [13,14].

#### *2.2. Research on the Construction Process*

Preceding literature was reviewed regarding methods to estimate the amount of CO2 emissions from construction equipment and materials used during the apartment construction process in Table 1. Kim et al. presented an estimation study on the amount of energy consumed and CO2 released, for each work type during the apartment design stage. Chung et al. calculated the amount of energy consumed and CO2 released in the building establishment stage and construction stage. After considering the results in preceding literature on the calculation methods for CO2 emissions, it was found that most existing studies applied the LCA model based on input-output analysis [15,16]. The input-output analysis method enables easy abundant data calculation. It is an analysis method that describes the inter-industry relation table's connections in units of monetary value, and considers energy or resources according to the flow of monetary amounts and goods. However, because the LCA model based on input-output analysis calculates the amount of CO2 emissions only by using information on the cost of products or services, its disadvantage is that it can only be used as an assessment outline. It is also limited as it does not consider system boundaries, fuel extraction, processing, production process, waste disposal, and other various factors regarding used materials that should be considered in the construction process. The key preceding research and literature on important factors that should be considered in the construction process, including environment, materials, equipment, transportation, and managemen<sup>t</sup> were examined. However, there is very little research on CO2 emission assessment of construction processes because data analysis is difficult due to the short construction period, and data is difficult to secure. Hadjimitsis et al. analyzed the amount of energy used with regard to concrete in the transportation stage, Jo et al. analyzed the weight of the construction work and managing work type component category conclusion [17,18], and Murat et al. suggested that recycling of ferrous and non-ferrous metals, cardboard, plastic and glass maximize the environmental and economic savings. Also, Roh et al. analyzed the CO2 emission characteristics regarding concrete construction. After analyzing the previous research performed in Korea regarding construction processes, it was found that the research was focused only on analyzing some work types and on the amount of energy used in the construction process [19,20]. Moreover, limitations were seen in the amount of CO2 emissions that occurred in the transportation of construction materials, construction process, and process of disposing of construction waste, all of which are involved in the construction process, and which were generally not being considered. Although the use of construction equipment and materials, transportation, energy use on site, and other matters were defined in detail, there were still parts that were excluded from consideration. Research on the basis of calculation used in actual CO2 assessment was not being performed. Most research evaluated only a portion of construction work types, or was based on specific materials or equipment used at the construction site. As a result, research that includes construction materials, construction equipment, construction work types, transportation, construction process, construction waste disposal, and other factors should be conducted. This study looked into the construction process that was divided into transportation, construction, and demolition stages for a detailed assessment. Moreover, this work was focused on analyzing construction equipment, transportation equipment, and construction materials. In addition, an individual method using the Korea National LCI DB was used instead of using existing analyses of CO2 emissions based on monetary values, to perform an analysis on CO2 emissions characteristics for each work type during apartment construction. In prior studies regarding the assessment of CO2 emissions during the construction process, CO2 emission-impact factors were merely listed, or only extremely limited factors were applied to the building environmental impact assessment and analysis process [21,22].


#### **Table 1.** Review of existing literature.

