# Feasibility of Developing Sustainable Concrete Using Environmentally Friendly Coarse Aggregate
## 4 Results and Discussion
### Mechanical Properties  
The compressive strength and dry density variation of combined RCA concrete between 7 and 90 days are shown in Figure 2. The observed 28-day compressive strength of combined RCA concrete made with the two different mixes are identical (Refer Table 4 and Figure 2). The strengths varied between 25.85-24.89 MPa for Combined RCA-25 and 41.10-43.41 MPa for Combined RCA-40. This shows that combined RCA concrete can repeatedly produce similar compressive strength in repeated mixes. Combined RCA-25 and RCA-40 concretes displayed a compressive strength development over time, ranging from 19.29 to 44.50 MPa and 36.35 to 60.70 MPa between 7 and 90 days, respectively. During this period, the combined RCA-25 and combined RCA-40 concretes achieved 130.7% and 81.8% strength development, respectively. In literature, the performance of most other concretes with RCA has shown a 20% to 30% reduction in compressive strength in comparison to corresponding natural aggregate concrete [33; 61]. However, the compressive strengths obtained from the combined RAC in this study demonstrated no significant difference in the strengths achieved. Furthermore, [PERSON] et al. [39] also observed no reduction in compressive strength when using a similar RAC produced using returned concrete, though this contained only 30% replacement of coarse aggregates. The compressive strength of combined RCA-25 and combined RCA-40 concretes are in well excess of the specified lowest strength noted Australian Standards, AS 3600 [62] for exposure category A1 (20 MPa), A2 (25 MPa), B1 (32 MPa), and B2 (40 MPa) at 28 days. Hence, combined RCA-25 concrete could be used for residential structures in non-aggressive soil and non-industrial near coastal structures while combined RCA-40 concrete can be applied in industrial buildings and coastal structures. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Concrete Type**} & \multirow{2}{*}{**Design Strength**} & \multirow{2}{*}{**Mix Notation**} & \multicolumn{3}{c}{**Compressive Strength (MPa)**} \\ \cline{3-5}  & & & & **7-day** & **28-day** \\ \hline \multirow{2}{*}{BCA concrete} & 25 MPa & BCA-25 & 19.15 \(\pm\) 2.06 & 26.47 \(\pm\) 1.27 \\  & 40 MPa & BCA-40 & 32.49 \(\pm\) 2.92 & 42.15 \(\pm\) 1.14 \\ \hline \multirow{2}{*}{RCA Concrete} & 25 MPa & RCA-25 & 15.07 \(\pm\) 0.76 & 19.50 \(\pm\) 1.83 \\  & 40 MPa & RCA-40 & 27.34 \(\pm\) 1.95 & 32.28 \(\pm\) 2.06 \\ \hline \multirow{2}{*}{Combined RCA} & 25 MPa & Combined RCA-25 & 18.40 \(\pm\) 0.86 & 25.85 \(\pm\) 0.45 \\ \cline{1-1}  & Concrete & 40 MPa & Combined RCA-40 & 34.07 \(\pm\) 1.30 & 41.10 \(\pm\) 1.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Compressive strength of concrete with aggregate grading. Figure 1: Aggregate grading curve for recycled coarse aggregate (RCA) and Basalt coarse aggregates. While RCA has higher water absorption and moisture content than Basalt aggregates, both were used in saturated surface dry condition. This prevented extra water loss from the concrete mix due to absorption by the coarse aggregates. In compressive strength testing, both Basalt aggregate and combined RCA concretes displayed cylindrical cone failure [3]. Considering the Basalt aggregate concrete, the fracture plane passed mainly around the aggregates, but in the combined RCA concrete, it passed mainly through the RCA aggregates. It is speculated that the cause of the fracture through the RCA aggregates is due to lower strength of the coarse aggregates compared with cement paste. It was observed that the crushing value of RCA-25 and RCA-40 coarse aggregates are 35.9% and 30.4%, respectively, which are significantly higher than Basalt coarse aggregate (21.2% crushing value). The higher the aggregate crushing value will lower the strength of coarse aggregate. Hence, the examined fracture behaviour proves that RCA's strength is the governing factor for the final compressive strength of combined RCA concrete.