Image based quantification and fracture modelling of fire induced damage in metakaolin modified recycled aggregate concrete

The rapid expansion of urban infrastructure has significantly increased concrete consumption, intensifying natural aggregate depletion and construction waste generation. Recycled aggregate concrete (RAC) offers a sustainable alternative; however, its broader structural application remains limited due to uncertainties regarding its performance under high temperatures and fire exposure. This study experimentally investigates the fire induced degradation mechanisms of metakaolin containing recycled aggregate concrete, with a particular focus on microstructural damage evolution and fracture related behavior rather than strength loss alone. Concrete mixtures incorporating recycled aggregate replacement levels of 0%, 25%, 50%, 75%, and 100% were developed with optimized metakaolin content and exposed to temperatures range of 20, 100, 200, 300, 400, 500, 600 and 800 °C. A novel SEM assisted quantitative image analysis framework was proposed to accurately characterize thermally induced microcrack evolution. Unlike conventional grayscale thresholding methods, the developed approach integrates Canny edge detection with morphological filtering and skeletonization to isolate true crack networks from non-crack interfaces, enabling reliable computation of damage indices and crack density. The results demonstrate that fire-induced deterioration in recycled aggregate concrete is governed by temperature-dependent microcrack evolution and interfacial degradation rather than monotonic strength reduction. Quantitative damage analysis revealed that the damage index ranged from 0.19 to 0.40 at 500 °C and increased sharply to 0.70–0.90 at 800 °C, confirming a nonlinear S-shaped damage evolution law with a critical transition temperature between 500 and 600 °C. SEM-based crack density measurements further indicated that accelerated degradation initiates when crack density exceeds approximately 30–35%, corresponding to this transition regime. At 800 °C, the strength retention of 100% RAC was limited to 0.11, whereas metakaolin containing RAC achieved 0.20, representing an 82% enhancement in fire resistance. Additionally, evidence of thermal adaptive mechanisms, including microstructural densification and localized sintering at 800 °C, suggests that metakaolin alters the high temperature degradation pathway. The findings provide the first direct microstructural evidence that metakaolin not only enhances ambient performance but actively alters the thermal degradation pathway of recycled aggregate concrete. This study establishes a robust experimental computational framework for fire damage assessment and offers new insights into the design of sustainable, fire resilient concrete incorporating recycled aggregates.