###### Abstract  
This study intends to evaluate high and low-strength concrete mixes made with high volume of fly ash (FA) and recycled concrete aggregates (RCA) from both a mechanical and economic point of view. For this purpose, the mechanical characteristics of concrete, namely compressive strength (\(f_{\text{cm}}\)), splitting tensile strength (\(f_{\text{cm}}\)), and modulus of elasticity (E\({}_{\text{cm}}\)) were correlated with the cost of 1 m\({}^{3}\) of concrete mixes, taking into account the most common scenarios (e.g., cost of the raw materials, transportation between supplier and concrete plant, and mixing procedure) in the centre of Portugal. The results show that the incorporation of FA and RCA are detrimental to the mechanical properties of concrete. E\({}_{\text{cm}}\) is predominantly influenced by RCA, and \"\(f_{\text{cm}}\)\" and \"\(f_{\text{ctm}}\)\" are mainly controlled by FA incorporation. However, after a given age, the rate of the strength development (\(f_{\text{cm}}\),\(f_{\text{ctm}}\) and E\({}_{\text{cm}}\)) of RCA concrete containing FA significantly accelerates over time relative to the reference concrete (without FA and RCA) and to the mixes made with either RCA or FA. Furthermore, the cost of concrete does not significantly change by incorporating RCA. The use of superplasticizer (SP) significantly increases the cost of concrete. However, the higher cost of concrete due to the use of SP can be offset by replacing cement with FA. Regarding the optimization process, concrete mixes with the lowest cost may not necessarily be the optimum choice regarding cost efficiency. In fact, the mechanical properties of concrete also need to be considered to aid the decision on the optimal concrete mix. Finally, the results show that the optimum mixes in terms of cost and mechanical characteristics are mostly the ones made with simultaneous incorporation of RCA, FA, and SP, rather than with their individual incorporation. concrete; compressive strength; splitting tensile strength; modulus of elasticity; economy; optimization; fly ash; recycled concrete aggregates +
Footnote †: journal: Applied  
specific price value for all countries. In this study, the most common scenario for the centre of Portugal is considered to assess the cost of concrete mixes. Furthermore, the ranking of the best concrete mixes in terms of economy does not necessarily select the one with the lowest cost. In fact, the strength of concrete is also another important factor to aid the selection of the optimal concrete mix. For example, lower-strength concrete mixes require a higher cross-section of the structural elements, relative to higher strength concrete, and that directly affects the total amount of concrete consumed to build the structure and the total cost as a result. In fact, a study by [PERSON] et al. [2] for recycled concrete aggregates (RCA) concrete concluded that, for a simply supported beam (rectangular cross-section), the strength of concrete affects the cross-section of the beam (Equation (1)). Similar facts can be true for cost and durability of the concrete mixes in which the thickness of the concrete cover is affected by its durability characteristics [3]. \[M_{Rd}=0.8095\cdot b\cdot x\cdot f_{ck}\cdot\gamma_{c}\cdot z \tag{1}\]  
where \(M_{Rd}\) is cross-section's design resisting moment, \(b\) is the cross-section's width (m), \(x\) is the concrete compression zone's height (m), \(f_{ck}\) is the characteristic compressive strength (MPa), \(\gamma_{c}\) is the concrete's partial safety factor for ultimate limit state (equal to 1.50), and \(z\) is the distance between the pressure centre of the concrete compression zone and the centre of the bottom steel reinforcement (m), which is equal to Equation (2). The value of \(x\) was solved by considering \(M_{Rd}\) equal to the beam's mid-span design bending moment. \[z=d-0.416\cdot x \tag{2}\]  
where depth (\(d\)) is the distance between the rectangular cross-section's top and the centre of the bottom steel reinforcement.