## 5 Conclusions  
In this paper, the validity of an overall assessment of the mechanical behavior of self-compacting concrete (SCC) with recycled aggregate (RA) on the basis of ultrasonic pulse velocity (UPV) readings was analyzed. To do so, the statistical relationships between four mechanical properties (compressive strength, modulus of elasticity, splitting tensile strength, and flexural strength) and the UPV of nine different SCC mixes made with coarse, fine and/or powdery RA were evaluated. The statistical tools used in this analysis were correlations, simple regression, and multiple regression, which led to the following conclusions:  
* UPV was strongly correlated with all the mechanical properties, although the mechanical properties related to compressive behavior (compressive strength and modulus of elasticity) presented a higher correlation. The dependence of UPV on mechanical properties was fundamentally monotonic.
* In line with the correlation analysis, the best-fitting simple-regression model for all mechanical properties was an inverse square-root model (potential model). However, the estimation accuracy obtained for splitting tensile strength and flexural strength was lower than those for compressive strength and modulus of elasticity.
* The mechanical properties related to bending-tensile behavior (splitting tensile strength and flexural strength) had practically no percentage influence on the UPV value when developing linear-combination multiple-regression models through standardized properties. Furthermore, their forced introduction reduced the estimation accuracy. Therefore, the development of multiple-regression models that linked UPV readings with splitting tensile strength and flexural strength was not adequate.
* Relating UPV readings, compressive strength, and modulus of elasticity values of SCC containing RA was feasible through multiple regression, either through individual treatment of each mechanical property (Equation (6)) or as the mean value of compressive behavior (Equation (7)). Thus, both mechanical properties can be estimated from the UPV readings when one of them is known.  
The test results reported in this paper have shown the utility of UPV readings for estimating compressive strength and modulus of elasticity values of SCC containing RA. Thus, it has been shown that this non-destructive measurement can be useful, not only in the evaluation of conventional vibrated concrete, but also for the evaluation of sustainable concretes containing RA.  
Figure 8: Validation of Equation (6) through other studies in the literature regarding SCC containing RA [56; 57].  
**Author Contributions:** Conceptualization, A.B.E., V.R.-C., M.S. and V.O.-L.; methodology, A.B.E. and V.R.-C.; software, V.R.-C.; validation, M.S., F.F. and V.O.-L.; formal analysis, A.B.E. and V.R.-C.; investigation, A.B.E., M.S. and V.R.-C.; resources, F.F. and V.O.-L.; data curation, A.B.E. and V.R.-C.; writing--original draft preparation, A.B.E. and V.R.-C.; writing--review and editing, M.S., F.F. and V.O.-L.; visualization, M.S., F.F. and V.O.-L.; supervision, M.S., F.F. and V.O.-L.; project administration, M.S., F.F. and V.O.-L.; funding acquisition, M.S., F.F. and V.O.-L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Spanish Ministry of Universities, MICIN, AEI, EU, ERDF, and NextGenerationEU/PRTR, grant numbers PID2020-113837 RB-I00, 10.13039/501100011033, and TED2021-129715B-I00; the Junta de Castilla y Leon (Regional Government) and ERDF, grant number UIC-231; and the University of Burgos, grant numbers SUCONS, Y135.GI.  
**Institutional Review Board Statement:** Not applicable.  
**Infrared Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. All the data used in this study are shown in the paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.