This study evaluates the mechanical properties of medium-strength concrete, 30 to 35 MPa, incorporating recycled concrete paver aggregates (RCPA) as partial replacements for fine aggregates (2%, 4%, and 6%). While the target compressive strength was achieved, even exceeding expectations in some mixes (up to 43.1 MPa at 4% RCPA) the elastic modulus (E) showed a significant reduction compared to ACI 318-19 predictions. Experimental results revealed that even the elastic modulus of conventional concrete was 38.1 % lower than the ACI empirical formula (E = 4700 √f’c) in some cases, indicating that the overestimation is not limited to recycled mixes but also applies to concrete made with aggregates found in Ecuador. The derived modulus equations, such as E = 3403 √f´c for 4% RCPA, indicate a substantial deviation from code-based models, suggesting that ACI’s assumptions may not apply to recycled aggregate concrete. Notably, the 4% RCPA mix exhibited optimal performance, balancing strength (42.93 MPa) and modulus (20,133 MPa). These findings highlight the need for revised empirical models or adjusted safety factors in structural concrete with recycled aggregates, as current standards overestimate stiffness. The study underscores the trade-off between sustainability (construction waste reduction) and mechanical performance and emphasizes the importance of using conservative design parameters based on experimental evidence, especially when using aggregates found in Ecuador.