The use of recycled concrete aggregates (RCA) is gaining increasing interest as it promotes more sustainable construction practices by reducing landfill waste and preserving natural resources. However, incorporating RCA affects key mechanical properties such as compressive strength and elastic modulus. This study proposes a semi-empirical approach to predict these properties by adapting existing models. Bolomey’s formula for compressive strength and the Swiss code SIA 262 expression for elastic modulus were modified to account for RCA incorporation. Additionally, the Maxwell model was applied by separating RCA into two phases: the original natural aggregate and the residual mortar attached. Laboratory experimentation on 33 concrete mixes with RCA contents ranging from 25% to 95% produced an internal dataset, which was supplemented with data from the literature to build a robust database for refining and validating these models. The modified Bolomey’s formula successfully predicted compressive strength with a coefficient of variation (COV) below 10% and an average experimental-to-estimated ratio of 1. Similarly, the modified SIA 262 expression for elastic modulus yielded a COV of approximately 5% and an average experimental-to-estimated ratio very close to 1. The Maxwell model enables a more detailed prediction of the elastic modulus. Despite some variability attributed to the assumptions regarding the residual mortar estimation and aggregate equivalencies, the model demonstrated good predictive accuracy, with a COV of less than 10% and an experimental-to-estimated ratio of 0.99. Overall, the results indicate that well calibrated semi-empirical models can effectively capture the complex behaviour of RCA across various replacement ratios and mix designs. Improving the reliability of these predictive tools facilitates the broader adoption of RCA in structural applications, thereby supporting material reuse and resource efficiency.
