## 2 Results and Discussion
### Response Surface Model
#### 2.4.1 Cost to Carbon Emission Correlation  
Data for cost and carbon emission correlation were developed using Design-Expert software. The 3D model graph for four main cases which are: fly ash based concrete, and blast furnace based concrete used in single story and double story residential houses. The data were collected from 20 single story houses constructed in the Perak state, Malaysia. Similarly, other data on 20 double story houses were obtained from a housing project in Pulau Pinang, Malaysia. The 3D graphs in Figures 11 and 12 show the correlation between cost, carbon emission, and eco-efficiency of single story residential houses made with two types of concrete using fly ash or BFS. Similarly, Figures 13 and 14 are the 3D plots for double story residential houses. For a single story house, the correlation of eco-efficiency significantly decreases with an increasing amount of carbon emissions, because high carbon emission  
Figure 10: Predicted vs. actual for double story housing based on two types of concrete: (a) fly ash, (b) blast furnace slag. harms the environmental score. In terms of cost correlation with eco-efficiency, it shows less cost on the element lead toward lower eco-efficiency values. The cost material might be lower due to low standard quality material use, which produces high carbon emission that causes eco-efficiency value to decrease. Eco-efficiency for fly ash shows that the material is not efficient for a reinforced concrete house, since it shows a negative value. The negative result from the amount of equivalent carbon emission for fly ash is higher than normal concrete (Ordinary Portland Cement). In addition to that, eco-efficiency significantly decreases with an increasing amount of carbon emissions, because high carbon emission harms the environmental score. In terms of cost correlation with eco-efficiency, it shows less cost on the element lead toward lower eco-efficiency values. The cost material might be lower, due to low standard quality material use, which produces high carbon emission, which causes the eco-efficiency value to decrease. Eco-efficiency for BFS shows that the material is efficient for a reinforced concrete house, since the value for eco-efficiency is positive. Figure 11: Response surface graph of fly ash based concrete used for single story houses. Figure 12: Response surface graph of blast furnace slag based concrete used for single story houses. According to ANOVA analysis, the 3D model graph was plotted in cubic form for a double story house, as shown in Figures 13 and 14. High carbon emission with the lowest cost of material results in a positive value of eco-efficiency. However, low carbon emission and high cost of material result in a negative value of eco-efficiency, because there is a relationship between the carbon emission, mean score, and eco-efficiency that represent an environmental, economic score, and eco-efficiency, respectively. The value of carbon emissions produced might be too low to overcome the extremely high-cost material used. The data plotted was mainly located at a negative value of eco-efficiency, hence, proving that fly ash is not an excellent alternative material for the reinforced concrete house. Furthermore, high carbon emissions with the lowest cost of material and low carbon emission with low cost results in a positive value of eco-efficiency. However, low carbon emissions and the high cost of material result in a negative value of eco-efficiency, because there is a relationship between the carbon emission, economic score, and eco-efficiency, that are represented in terms of the environmental, economic score, and eco-efficiency, respectively. The value of carbon emission produced might be too low to overcome the extremely high-cost material used. The data plotted was mainly located at a negative value of eco-efficiency, which is not an excellent alternative material for the reinforced concrete house. Figure 14: Response surface graph of blast furnace slag concrete used for double story houses.