Aggregate-controlled stiffness in high-performance recycled concrete: mechanisms, data synthesis, and modeling perspectives

Abstract

The elastic modulus of recycled aggregate concrete (RAC) remains significantly lower than that of natural aggregate concrete (NAC), even when compressive strength is fully recovered through high-performance mix design. This stiffness deficit critically limits the structural application of sustainable high-performance concrete (S-HPC), particularly under serviceability-controlled conditions. This review synthesizes experimental, reconstructed, and physics-informed data published between 2019 and 2025 to elucidate the governing mechanisms of elastic modulus degradation in high-performance RAC (HP-RAC). A five-phase microstructural framework is adopted to explain compliance amplification induced by adhered old mortar and double interfacial transition zones (ITZs). To overcome experimental data fragmentation and variability, a data-augmented methodology combining literature reconstruction (n ≈ 186), probabilistic uncertainty quantification, and Physics-Informed Machine Learning (PIML) is employed. Results demonstrate that elastic modulus in HP-RAC is predominantly controlled by recycled aggregate compliance rather than matrix strength, exhibiting a stiffness saturation phenomenon beyond a matrix modulus of approximately 45 GPa. Carbonation treatment, nano-engineering of ITZs, and hybrid fiber reinforcement are critically evaluated as stiffness recovery routes, with carbonation curing showing a favorable balance between stiffness enhancement and carbon efficiency. The review suggests the need to move beyond empirical strength-based formulations toward micromechanical and physics-informed approaches, while acknowledging the limitations of reconstructed data and heterogeneous literature sources.