## 2 Results and Discussion
### Eco-Efficiency Index (ECOeff) of Reinforced Concrete Houses  
The results shown in Table 2 are the estimated values of the ECOeff for twenty single story reinforced concrete houses in Malaysia. For the analysis of the impact of the concrete mix proportions, two types of concrete were considered. A concrete mix containing either fly ash or blast furnace slag as the cement replacement material. A higher value of the eco-efficiency index indicates a better eco-efficiency performance of the housing unit. For each type of concrete member, sizes were analyzed using different grades of concrete, as well as the quantity of steel, which was calculated for each design. The results of the fifth housing unit (where the concrete grades used in the foundation were 30 MPa, slabs 20 MPa, beams 30 MPa, and columns 30 MPa) showed the highest eco-efficiency index, ECOeff  
Figure 8: Surface Model Graph. Figure 7: Predicted vs Actual Relationship. In this unit, both types of concrete using fly ash and blast furnace slag as alternative materials resulted in the eco-efficient design. The design optimization using a different grade of concrete has led to controlling the eco-efficiency in the design of the structure. It has been observed that by increasing the grade of concrete to 5 Mpa, it has caused about a 3% increase in the equivalent carbon emission of concrete. For an eco-efficient structural design, the characteristic strength of concrete and steel reinforcement have caused significant impacts on the amount of equivalent carbon emission; the higher strength concrete produces a high amount of carbon emission. Therefore, using the optimum grade of concrete can control the economic and environmental factors in the structural design, which can facilitate achieving the desired eco-efficiency index of the reinforced concrete structure. In this analysis, it was further observed that the concrete contained blast furnace slag as cement replacement material resulted in an increase in the eco-efficiency of design, as compared to the designs based on fly ash based concrete, because of blast furnace slag, which caused higher positive impacts on the economic and environmental criteria. Based on the above discussion, some observations are made for controlling the desired level of eco-efficiency of a given structural design project:  
1. Choose a minimum specified value of the compressive strength of concrete for designing a structural element that meets the reinforced concrete design requirements for satisfying the ultimate limit state (ULS) design and serviceability limit state (SLS) design criteria. 2. An allowance of 5 MPa in the characteristic strength of concrete on the failure strength is suggested for achieving the desired level of eco-efficiency in the design. Otherwise, optimizing the dimensions of the structural elements would be an option for achieving the optimum cost of the structure. 3. For controlling the cost and eco-efficiency in the columns of a building structure, it has been observed that an increase of 5 MPa in the characteristic strength of concrete had a significant effect on the entire structural assessment. This is because the columns contributed about 42% of the total concrete volume used in the design. 4. Based on an analytical study for achieving an optimum value of eco-efficiency in the RC design for typical Malaysian housing units, the recommendation og 20 MPa for up to 150 mm thick slabs  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multicolumn{3}{c}{**Fly Ash**} & \multicolumn{3}{c}{**Blast Furnace Slag**} \\ \cline{2-7}  & **ENV\({}_{\text{s}}\)** & **ECO\({}_{\text{s}}\)** & **ECO\({}_{\text{eff}}\)** & **ENV\({}_{\text{s}}\)** & **ECO\({}_{\text{s}}\)** & **ECO\({}_{\text{eff}}\)** \\ \hline
1 & \(-\)0.0801 & 0.0092 &  
is suggested, as well as a 35 MPa characteristic strength of concrete for columns, which would control the amount of concrete volume and the weight of the steel reinforcement.