## 3 Results and Discussion
### Cost Reduction of Construction Waste Disposal  
The additional costs arising from the generation of construction waste for different structural systems are demonstrated in this section. The estimated additional cost referred to in this research means the added material purchasing costs due to the loss of building materials rather than the disposal costs of construction waste on site. In this study, the major building materials were considered to assess the cost reduction during the construction of slab systems for the studied building. The major building materials were ready-mixed concrete, rebars, spacers, and forms for the solid slab and the flat plate slab. The building materials for the void slab system applied three additional items, which were void formers, steel decking, and anchoring metals. Calculation of the additional cost for purchasing the building materials was applied to the Korea Price Information Database [42] to establish the unit price of each building materials in this study. Table 5 indicates the lists of the unit price of the building materials of this study. The unit price of the building materials was converted from South Korean Won (KRW) to US dollars (USD), which would make it possible to easily comprehend and compare to foreign countries. The conversion of USD to KRW was calculated at the rate of KRW 1178 to USD 1. Moreover, the additional building materials purchasing cost was applied to Equation (3) as below:  
\[C_{m_{i}}=q_{i}\times P_{i} \tag{3}\]  
where \(C_{m}\) refers to the additional materials purchasing cost of building material \(i\), \(q_{i}\) represents the quantity of building materials \(i\) from the bill of quantities, and \(P_{i}\) indicates the unit cost of the building material \(i\). Table 6 summarises the additional costs of purchasing building materials from three different structural systems of this study. The additional construction costs generated from the additional building materials were USD 50,054.12, 76,941.50, 77,507.83, and 80,709.76 for the void slab system, the flat plate slab 180, the flat plate slab 210, and the solid slab, respectively. In this study, the plywood forms would determine the additional building material costs, which accounted for about 33.09%, 56.49%, 56.87%, and 57.14% of the void slab system, the flat plate slab 180, the flat plate slab 210, and the solid slab, respectively. Even though the plywood forms were reused approximately 3 times on site, the plywood forms would be difficult to reuse for other purposes or recycle because they were made to various forms to mould the concrete during the construction phase. In addition, regarding the cost of purchasing additional building materials, it was found that the cost of rebars accounts for a relatively high proportion of the additional costs, although the amount of construction waste generation was less than the waste of the ready-mixed concrete. Meanwhile, the additional purchasing costs from the building materials were found to be decreased by approximately USD 30,655.65 when the void slab system was applied as an alternative method to the solid slab system. Furthermore, the flat plate slab systems would be beneficial to reduce the construction costs arising by the purchasing of the building materials during the construction phase, even though its reduction ratio was smaller than for the void slab system (see Table 7). Based on the results shown in this study, the additional purchase of plywood forms were the dominant factors to increase the additional material costs, which would produce construction waste. In particular, it was difficult to reduce the amount of building materials and waste, since the beams and girders require a number of plywood forms to create the structural members. However, the void slab system minimises the beams and girders due to its reduction of self-weight, and it would be the optimal slab system to mitigate the generation of waste during the construction phase.