## 3 Results and Discussion
### Carbon Dioxide Emissions from the Manufacturing of Building Materials  
As the largest source of total carbon dioxide emissions amongst the three different types of slab system, the CO\({}_{2}\) emissions from each building material varied from its input amount. Tables 7 and 8 show the carbon dioxide emissions per square meter of each building material. The building materials used in the three cases were divided into permanent and temporary materials. Concrete, reinforcement rebars, steel decking, and void formers were categorised into permanent materials that were integrated with the slab. Temporary materials were auxiliary materials that were removed when the construction work was finished. In this study, forms were the only temporary materials, and were reused 7 times for concreting and moulding work. Figure 5: Reduction of CO\({}_{2}\) emissions of the flat plate slab and the voided slab systems. The carbon dioxide emissions of building materials generated approximately 238, 231, and 215 kg CO\({}_{2}\) eq./m\({}^{2}\) for the ordinary reinforced concrete slab, flat plate slab, and voided slab systems, respectively. In all cases, ready-mixed concrete is the largest contributor of carbon dioxide emissions during the manufacturing of building material stage. Specifically, the contribution percentage of each material in the ordinary reinforced concrete slab system is 50.7% for ready-mixed concrete, 46.9% for reinforcing bars, and 2.4% for forms. The flat plate slab system indicated similar proportions to the ordinary reinforced concrete slab system, and accounts for 53.9% of ready-mixed concrete, 44.7% of rebars, and 1.4% of forms, respectively. Additionally, the proportions of materials for the voided slab system are 47.6% for ready-mixed concrete, 42.1% for rebars, 1.0% for forms, 9.2% for steel decking, and 0.1% for void formers. In the voided slab system, the steel decking was the third largest carbon dioxide emissions source. Table 8 shows that the carbon dioxide reduction in the voided slab system was the largest amount of carbon dioxide reduction amongst all the three slab systems. A total of 113,741 and 325,381 kg CO\({}_{2}\) eq. in reduction of carbon dioxide emissions were contributed by the flat plate slab and the voided slab systems, compared to the ordinary reinforced concrete slab system (see Table 8). These amounts yielded approximately 3.5% and 10.1% in reduction compared to the ordinary reinforced concrete slab in this study. Comparisons of the three slab systems indicate that the reinforcing bars and forms were significant building materials to mitigate the emissions of carbon dioxide in the flat plate slab and the voided slab systems. In particular, reinforcing bars in the flat plate slab and voided slab systems were the main contributor to lowering the carbon dioxide emissions during the manufacturing of building materials stage. Totals of 119,290 and 280,072 kg CO\({}_{2}\) eq. for the flat plate slab and voided slab systems, respectively, were reduced in rebars amongst all the building materials. Moreover, a reduction of 249,656 kg CO\({}_{2}\) eq. in ready-mixed concrete was contributed by the voided slab system over the ordinary reinforced concrete slab. In the voided slab system, the hollow parts were filled with light-weight void formers, which were made by Expanded Polystyrene (EPS). Since the void formers were relatively lighter than the other materials, it would make it possible to not only reduce the carbon dioxide emissions, but also to lower the weight of the slab itself. On the other hand, a slight increase of 47,518 kg CO\({}_{2}\) eq. in the ready-mixed concrete was indicated in the flat plate slab system. It is considered that this might be caused by the additional structural members, such as drop panels