## 3 Results and Discussion  
### Total Carbon Dioxide Emissions of Each Slab System  
Table 5 summarises the estimated carbon dioxide emissions of alternative slab systems over the ordinary reinforced concrete slab during the construction phase, as well as the share of each life cycle (i.e., the manufacturing of building materials, transportation of building materials, and carbon dioxide emissions of construction equipment) in the total carbon dioxide emissions. The total carbon dioxide emissions during the construction phase of the ordinary reinforced concrete slab, flat plate slab, and the voided slab systems are 3275, 3157, and 2943 tons CO\({}_{2}\) eq. (equivalent to 242, 234, and 218 kg CO\({}_{2}\) eq./m\({}^{2}\)), respectively. A reduction of 0.8 tons per 100 m\({}^{2}\) was achieved in the building with flat plate slab, which is approximately 3.6% less than the ordinary reinforced concrete slab. Likewise, 2.4 tons per 100 m\({}^{2}\) of carbon dioxide was reduced in the voided slab system, about 10.1% less CO\({}_{2}\) compared with the ordinary reinforced concrete slab. The reduction of carbon dioxide emissions from the voided slab system indicated the highest reduction ratio amongst all three cases. The high reduction potential from the voided slab system might be attributed to the reduction of building materials, even though steel decking and void formers were additionally utilised in the voided slab system. In particular, high reduction of concrete and reinforcing bars would make it possible for the voided slab system to emit less carbon dioxide compared with the two alternatives of this study. In addition, the flat plate slab is frequently suggested as an alternative  
Figure 3: Description of the voided slab system: (**a**) Schematic of the voided slab system, (**b**) Anchoring mechanism of the voided slab system. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Emissions (kg CO\({}_{2}\) eq.)**} & \multicolumn{3}{c}{**Ratio (\%)**} \\ \cline{2-7}
**Source** & **Ordinary Reinforced** & **Flat Plate** & **Voided slab** & **Ordinary Reinforced** & **Flat Plate** & **Voided slab** \\  & **Concrete Slab (ORC)** & **Slab (FPS)** & **System (VDS)** & **Concrete Slab (ORC)** & **Slab (FPS)** & **System (VDS)** \\ \hline \(E_{1}\) & 3,230,945 & 3,117,203 & 2,905,564 & 98.63 & 98.73 & 98.70 \\ \(E_{2}\) & 24,381 & 23,428 & 22,657 & 0.74 & 0.74 & 0.77 \\ \(E_{3}\) & 20,386 & 16,628 & 15,474 & 0.62 & 0.53 & 0.53 \\
**Total** & 3,275,712 & 3,157,260 & 2,943,695 & 100 & 100 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Total carbon dioxide emissions of each slab system. structural system for residential or commercial building in South Korea, since it would enhance the workability for construction workers and serviceability for occupants. In South Korea, it has been taboooed to apply a flat plate slab to buildings since the collapse of a department store building in 1995 [51; 52]. However, flat plate slabs have been increasingly applied to commercial and residential buildings in recent years, because of the system's advantages, such as simple formworks during the construction phase, and extended span for enhanced serviceability. As indicated in Table 6, the carbon dioxide emissions of flat plate slab were reduced by about 3.5% over the ordinary reinforced concrete slab. Based on these results, when the environmental impact is considered to select the structural system in buildings, both flat plate slab and the voided slab system would be an alternative method to the ordinary reinforced concrete slab. Table 6 summarises the reduced amount of carbon dioxide emissions and the reduction ratio of the flat plate slab and voided slab system over the ordinary reinforced concrete slab. Columns 4 and 5 in Table 5 present the reduction ratio of each emissions source to the total carbon dioxide emissions reduction brought by the application of flat plate slab and the voided slab system.