The modeling and optimization of concrete compressive strength produced by replacing natural aggregate (NA) with recycled coarse aggregate (RCA) and recycled asphalt pavement (RAP) aggregates was investigated. The materials used were cement, water, fine aggregate, and coarse aggregate (NA, RCA, and RAP). The RCA and RAP replaced NA at 20, 40, 60, 80, and 100% separately, and in combination, with a total of two hundred and forty (240) cubes cast, while the concrete compressive strength was determined after curing by complete immersion in water at 3, 7, 14, 21, and 28 days. The modeling and optimization was done with the aid of design expert (version 13) software, while the Analysis of Variance (ANOVA) was done using SPSS (Statistical Package for Social Sciences) version 23 and adopting the Least Square Difference (LSD) method. Results from the findings showed that the optimum 28 days concrete compressive strength are concrete produced with 20%RAP, 40%(20%RAP +20%RCA), 60%(30%RAP + 30%RCA), and 80%(40%RAP + 40%RCA) replacements which are not significantly different from the control concrete, however, 40%(20%RAP +20%RCA) and 80%(40%RAP + 40%RCA) concrete had compressive strength higher than the control concrete. Also, the 3, 7, 21, and 28 days compressive strength can be predicted with quadratic model, while the 14 days strength can be predicted with 2-Factor Interaction (2FI) model. The optimization result showed that by replacing natural aggregate with 10.0% RAP and 50.0% RCA, the 28 days compressive strength of the concrete was 33.74N/mm2.

The compressive strength and optimization of concrete produced by replacing cement with CSA and GSA was conducted in this study. The materials used were cement, water, fine aggregate, coarse aggregate, CSA, and GSA. The CSA and GSA replaced cement at 5, 10, 15, 20, and 25% in combination, with a total of seventy two (72) cubes cast, while the concrete compressive strength was determined after curing by complete immersion in water at 7, 14, 21, and 28 days. The modeling and optimization was done with the aid of design expert (version 13) software, while the Analysis of Variance (ANOVA) was done using SPSS (Statistical Package for Social Sciences) version 23 and adopting the Least Square Difference (LSD) method. Results from the findings showed that the CSA used in the study is a good reactive pozzolana, and the GSA is not a reactive pozzolana. Also, the increase in CSA and GSA content in concrete reduces its workability, and concrete compressive strength. However, cement replaced with 5%CSA-0%GSA gave the optimum 28 days compressive strength which is not statistically significantly from the control concrete and can be used to produce concrete of compressive strength close to the control. Further findings from the regression model showed that CSA-GSA concrete compressive strength significantly fits a linear model and can be used to predict the 7, 14, 21, and 28 days compressive strength of CSA-GSA concrete, while the optimized result showed that CSA and GSA replacement of cement at 3.29% and 4.45% respectively yielded an optimum compressive strength of approximately 22.31N/mm2.