Micromechanical Modelling of the Varying Interfacial Transition Zone (ITZ) in Concrete with Different Recycled Aggregates

Abstract

Sustainable use of recycled aggregates in concrete poses key challenges for characterizing the interfacial transition zone (ITZ). This study addresses a knowledge gap regarding how Recycled Solid Brick Aggregate (RSB) and Recycled Concrete Aggregate (RCA) affect ITZ behaviour relative to Natural Concrete Aggregate (NAC). Conventional models treat the ITZ as a uniform shell, overlooking its spatial variability. Extending established continuum‐micromechanics multiscale frameworks across scales from the nanometre to the decimetre, we represent the ITZ as a sequence of concentric layers whose porosity is taken directly from quantitative SEM‐BSE image analysis. Nanoindentation and SEM‐EDX characterization are integrated to assess how different recycled aggregate types influence ITZ properties. Results reveal significant variations in ITZ thickness, porosity, and stiffness across concrete regions, governed by aggregate type and its interaction with cement paste. The micromechanical model provides a refined representation of concrete's elastic behaviour and clarifies the microstructural mechanisms controlling performance with recycled aggregates, supporting more informed decisions in sustainable material design and more efficient use of recycled materials.