## 4 Conclusions  
It is recognised that construction projects emit substantial volumes of carbon dioxide emissions. Various construction methods and alternatives have been suggested to minimise the emissions of carbon dioxide during construction work. A structural system of a building is a significant factor to not only sustain the entire building from external forces, but also to determine the quantities of building materials. Previous studies regarding structural systems have mainly focused on the structural performance, such as the flexural or shear capabilities of structures, and optimal design for workability during the construction stage, rather than the evaluation of environmental impacts by different structural systems. The purpose of this study was to evaluate the carbon dioxide emissions of different slab systems as a means of mitigating the environmental impacts of a high-rise commercial building project in South Korea. The total carbon dioxide emissions of the three slab systems during the construction phase are comprised of the manufacturing of building materials, transporting of building materials to the construction site, and carbon dioxide emissions from the energy consumption of construction equipment. The results show the total CO\({}_{2}\) emissions of 3275, 3157, and 2943 tons CO\({}_{2}\) eq., which are equivalent to 242, 234, and 218 kg CO\({}_{2}\) eq., for the ordinary reinforced concrete slab, flat plate slab,and voided slab systems, respectively. The manufacturing of building materials is the main contributor of CO\({}_{2}\) emissions, which indicate 3,230,945, 3,117,203, and 2,905,564 kg CO\({}_{2}\) eq., respectively. Based on the results, approximately 3.7% and 10.1% reduction of carbon dioxide emissions would be achieved by replacing the ordinary reinforced concrete slab by the flat plate slab and the voided slab systems, respectively. In this context, the voided slab system would be able to achieve the most carbon dioxide reduction amongst three different slab system of this study. Comparing the building materials in the three slab systems, the reinforcing bars and forms were significant building materials to reduce the CO\({}_{2}\) emissions in the flat plate slab and voided slab systems. In this study, reinforcing bars were the main contributor to lowering the carbon dioxide emissions in the flat plate slab and voided slab systems. Moreover, the emissions of carbon dioxide from transporting building materials were relatively small, compared to the manufacturing of building materials. However, it is recommended that the selection of location of material providers would contribute to lowering the CO\({}_{2}\) emissions from this stage. Based on the results of this study, the reduction potential of the voided slab system is the greatest amongst all the three different slab systems. Moreover, replacing the ordinary reinforced concrete slab system by alternative methods would make it possible to reduce the carbon dioxide emissions during the construction phase in building projects. While this study was carried out on the evaluation and comparison of the carbon dioxide emissions from an actually designed building in South Korea, there is a limitation that should be addressed in future research. In order to verify the practicability and expand the applicability of the voided slab system in building projects, more cases need to be investigated in future studies. Moreover, life cycle assessment of the flat plate slab and voided slab systems during the operation and maintenance phases should be performed from the whole life cycle perspective of a building. I.P. administrated the overall of this study and calculated the quantities of building materials. S.N. carried out the structural analysis and the assessment of the carbon dioxide emissions from three slab systems. This research received no external funding. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government Ministry of Education (No. NRF-2017R1D1A1B03033452).