## 2 Experimental Program  
This experimental study used recycled concrete aggregates (RCAs) produced from two different sources in South Korea. The 'RA' aggregate and 'RB' aggregate were crushed with a maximum size of 20 mm RCA from the same source of old concrete at 'I' recycling plant in South Korea. Meanwhile, the 'RR' aggregate was produced with a maximum size of 20 mm RCA from old railway concrete sleepers. The specific gravity [31], absorption ratio [31], LA abrasion coefficient [32], and residual mortar content (RMC) of the RCAs properties were tested [33; 34] and are presented in Table 1. Test results showed that the average absorption ratio of 'RA', 'RB', and 'RR' was 3.82%, 6.61%, and 4.53%, respectively. It should be noted that all RCAs did not meet the specified Korean standards (KS) for concrete with respect to 2.5 as the specific gravity and 3.0% as water absorption, except for the 'RA' aggregate, which marginally satisfied the specific gravity standard with a value of 2.52 [35; 36; 37; 38]. The RMC values for three RCAs were determined by the thermal treatment suggested by [PERSON] and [PERSON]. [33]. Recycled aggregate samples were prepared and dried in a muffle furnace (DF-5 model made from Daeheung Science in South Korea) at 500 C for two hours. The sample was then immersed in cold water. Extra mortar that still remained may be removed by the sudden cooling. Figure 1: Schematic diagrams of different mix designs; TM: Total Mortar; TCA: Total Coarse Aggregate; NA: Natural Aggregate; NM: New Mortar; RM: Residual Mortar; OVA: Original Virgin Aggregate; RCA: Recycled Concrete Aggregate. In addition, for the 'RR' aggregate, the RMC value was evaluated by the chemical treatment method recommended by [PERSON] et al. [34]. Recycled specimens are prepared in a 2 L beaker and a 3 M H\({}_{2}\)SO\({}_{4}\) solution was added, where the volume was five times higher than the RCA sample volume. Finally, the samples were washed with a 4-mm sieve to detach the tangled mortar, and the washed and oven-dried RCA sample weights were weighed to evaluate RMC contents. The RMC was evaluated by using the following equation [8]:  
\[\text{RMC }(\%)=(W_{\text{RCA}}-W_{\text{OVA}})/W_{\text{RCA}} \tag{1}\]  
where \(W_{\text{RCA}}\) is the first oven-dried RCA sample weight and \(W_{\text{OVA}}\) is the final oven-dried OVA weight after removal of the RM. It is surprising from the test results of the 'RR' sample in Table 1 that the average RMC value of 39.9% acquired from the thermal treatment method was very close to the RMC value of 40.1% obtained from the chemical treatment method. Natural river sand was incorporated as fine aggregate. Aggregate test results showed specific gravity with a value of 2.60 and absorption ratio with a value of 0.95%. Figure 2a shows that the particle size of the river sand is well distributed along the midpoint of the lower and upper limit of the gradation test requirement in Korean standards. Crushed granite was used as natural coarse aggregate (NA) with the specific gravity of 2.71 and the water absorption ratio of 0.37. Table 2 tabulates the material properties of natural aggregates. Figure 2b shows the aggregate gradation results, satisfying Korean standards. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Items** & **RAC** & **RB** & **RR** & **KS Specification [38]** \\ \hline Specific gravity & 2.52 & 2.34 & 2.48 & \textgreater{}2.5 \\ Absorption ratio (\%) & 3.82 & 6.61 & 4.53 & \textless{}3.0 \\ LA abrasion coef. (\%) 1 & - & - & 32.2 & \textless{}25(paving), 40(others) \\ RMC 2,3 & 25.0 2 & 46.8 2 & 39.9 2 (40.1 3) & - \\ \hline \hline \end{tabular}  
* tested by reference [32], 2 RMC test results from thermal treatment of reference [33], 3 RMC test results from chemical treatment of reference [34].  
\end{table} 
Table 1: RCA material properties; RA: Type A recycled aggregate; RB: Type B recycled aggregate; RR: Recycled aggregate derived from Railway concrete sleepers; KS: Korean standards.