This chapter discusses rational methods for mix design specific to RAC, accounting for the unique properties of recycled aggregates. Various approaches are reviewed, including empirical methods, compressible packing models (CPM), particle packing, and rational proportioning based on adhered mortar volume. The study compares the performance of fresh and hardened RAC mixtures using 44 different mix designs. Mechanical properties are correlated with mix variables, and predictive models are proposed. The findings suggest that with proper design adjustments, RAC can achieve strength and durability levels comparable to conventional concrete.