High temperature fracture behavior and damage evolution of recycled aggregate concrete produced from metakaolin modified parent concrete

Abstract

The increasing use of recycled aggregate concrete (RAC) offers significant environmental benefits by reducing construction and demolition waste; however, its fracture performance and damage evolution under elevated temperatures remain insufficiently understood, particularly when recycled aggregates are derived from metakaolin modified parent concrete. Existing studies have primarily focused on residual mechanical strength, with limited attention given to the coupled fracture response and damage progression under thermal loading. To address this research gap, this study proposes a novel integrated analytical framework that combines fracture energy, characteristic length, brittleness index, thermal fracture sensitivity, energy dissipation index, and damage index within a unified phase space interpretation. Concrete mixes were prepared with recycled aggregate replacement levels of 25%–100% using recycled aggregates obtained from parent concrete containing 0%, 5%, 10%, and 20% metakaolin, and were exposed to temperatures ranging from 20 to 800°C. The results reveal a consistent three stage response across all fracture parameters, comprising an initial microstructural stability stage, an intermediate energy dissipative regime (~200–600°C), and a high temperature degradation stage. Metakaolin significantly improved fracture performance, with a 10% dosage producing up to an 80%–100% increase in fracture energy, while 20% metakaolin provided greater thermal stability at elevated temperatures. In contrast, higher recycled aggregate contents (≥75%) accelerated degradation due to increased porosity and weaker interfacial transition zones. The proposed phase space damage framework demonstrates that metakaolin modifies the fracture behavior of RAC from brittle, heterogeneity controlled failure toward a more stable, energy regulated damage mechanism. Furthermore, the observation of thermally re‐precipitated ettringite like phases near 600°C provides additional insight into chemo thermal reactivation phenomena. Overall, the study identifies recycled aggregate replacement levels of 25%–50% combined with 10%–20% metakaolin in the parent concrete as the optimum condition for achieving enhanced fracture resistance, thermal stability, and sustainability.