## 2 Research Methodology  
In this study, Life Cycle Assessment (LCA) methodology was applied, in order to evaluate the environmental impact of the OSS, the FPS, and the VDS. According to ISO 14044 [43], LCA is a practical tool to assess and evaluate the environmental impacts of a product, process, or service throughout its life cycle. Three methods are mainly adopted, namely process-based analysis, economic input-output analysis, and a hybrid approach that would be utilised for environmental performance evaluation over the life cycle [44; 45; 46; 47].  
Process-based analysis is a bottom-up approach to evaluate environmental performance in consideration of all the processes and activities that are involved in producing a product or service that is in compliance with ISO 14044 [48] and ISO 21930 [49]. This method would identify all materials and energy used during the production to evaluate the environmental impacts. In addition, a process-based method would require high quality data to produce a more conclusive assessment of environmental impacts when economic input-output data are unavailable.  
Economic input-output analysis is a top-down method that considers both direct and indirect environmental impacts during the life cycle of a product or service. This approach evaluates the direct environmental impacts of a project or service, as well as the indirect ones associated with the supply chain of the product or service. [PERSON] et al. [50] applied an input-output analysis for the analysis of both direct and indirect energy use and carbon emissions during the production phase of buildings. Their study pointed out the difference between top-down and bottom-up analysis of the input-output approach, which showed difference in transport and the service sector of the construction of buildings. For economic input-output analysis, it is time consuming to accumulate extensive data from all of the relevant areas to enhance the accuracy of the results. This approach is prevalent in studies from the USA and Japan because of the accuracy of the data [11; 14; 45].  
Hybrid analysis is a combination of above-mentioned approaches to calculate the environmental impacts. It would make it possible to enhance the accuracy of the environmental performance evaluation complementary to insufficient data from either process-based or economic input-output analysis. There are two types of hybrid analysis: process-based hybrid analysis, and economic input-output-based hybrid analysis, depending upon which method is applied, and the basis [44; 46; 51; 52]. Process-based hybrid analysis assesses the environmental performance based on the process-based analysis procedure, and input-output analysis would supplement insufficient data [35; 44; 46; 51]. Likewise, economic input-output hybrid analysis would perform LCA by mainly applying economic input-output analysis, while process-based analysis would fill the lack of data.  
In this research, process-based analysis of LCA is applied to assess and evaluate the environmental impacts of three slab systems during the construction phase for an apartment building in South Korea. The procedure of this study was composed of goal and scope definition, life cycle inventory analysis, and impact assessment in accordance with ISO 14044 (see Figure 1) [48].