## 3 Results and Discussion
### Optimization  
Generally, the optimization process of concrete depends on its application. Therefore, ranking concrete only according to economy or mechanical properties is not reasonable. This study thus considered a business as usual scenario (i.e., the least cost possible for a given durability and mechanical performance), and the optimum concrete may be one that has a moderate performance in both characteristics. For this reason, two zones (cost-efficient and cost-inefficient) were considered and separated by the line equivalent to the reference concrete (Figure 8). Mixes were optimized by comparing their MPR at longer ages (365 days) with their cost. The reason for which longer ages were considered is that at early ages, the full potential properties of the mixes containing FA widely differs from that of the mixes made with OPC only, due to the delayed hydration process of the former [36]. Apart from the fact that SP significantly increases the cost (Section 3.2), all the mixes (with and without FA and RCA) made with SP are in the suitable zone (cost-efficient) in terms of mechanical and cost characteristics, except those containing 100% of fine RCA (Figure 8). Therefore, the use of SP is highly cost-efficient. The reasons for decreased performance of SP with the use of high volume of fine RCA have been previously discussed in [PERSON] et al. [25]. Additionally, most of the optimum mixes (optimum line) are those made with SP (as explained in the next paragraphs). Figure 9a,b show the effect of incorporating FA and RCA, respectively, on the relationship between the relative cost and mechanical performance ratio of the mixes previously shown in Figure 8. The results show that the NA concrete containing 30% of FA has a slightly better mechanical performance and lower cost than those of the reference concrete. For mixes containing a high volume of FA, the mechanical performance decreased by up to 20%. However, according to the weights considered in this study, the mixes mentioned are suitable for some specific application (e.g., residential housing), because their cost decreased about the same percentage as their strength. Regarding mixes containing RCA, low incorporation FA (M5, M6, M14, M15, M5 sp and M14 sp in Figure 8) is advisable, since the corresponding mixes are all located in the suitable zone (cost-efficient) in terms of mechanical and cost characteristics. A high volume of FA can be also used with RCA concrete (M9 and M9 sp in Figure 8). However, most of the mixes are located near the line between the suitable and unsuitable  
Figure 8: Relationship between relative cost and mechanical performance ratio of concrete mixes made with (square signs) and without (other signs) SP, regardless of the binders and aggregates used. FA, F, C, and SP stand for fly ash, fine RCA, coarse RCA, and superplasticizer, respectively, and the values are their incorporation level in the mixes. zones (Figure 9a). In addition, the mentioned hypotheses concerning the lower cost of concrete mixes produced with both FA and RCA can only be confirmed when the application of non-traditional materials becomes more common in construction practice. Concerning the effect of RCA on the mechanical and cost characteristics of concrete, the results show that mixes only with coarse RCA are all located in the suitable zone (M10 and M10 sp in Figure 8). However, for fine RCA, it depends on its incorporation ratio in concrete. The incorporation of RCA can be considered as suitable up to 50% (M5 and M14 in Figure 8) (Figure 9b). Higher incorporation ratios of fine RCA are not so advisable, even if FA and SP are simultaneously incorporated in the mix. In Figure 8, the optimum mixes were selected based on their distance from the \"line equivalent to the reference mix\" in the cost-efficient zone, since this distance is proportional to their cost-efficiency.