Mesoscale modelling of recycled aggregate concrete: A parametric analysis of properties of mesoscopic phases of RAC

This paper analyses the relative effects of ten parameters of recycled aggregate concrete (RAC) mix design on its mechanical properties. Research on the effect of the source concrete and of the quality of the interfacial transition zones (ITZ) on the properties of RAC is scarce and additional knowledge would guide RAC mix design towards efficiency and understanding of the effect of recycled aggregates (RA) on concrete. Therefore, this paper presents the outcomes of parametric numerical analyses on this topic. Within the range of parameters commonly found in conventional concrete mix design, it is found that: (1) the quality of both the old and new mortar substantially influences the mechanical behaviour of RAC. High-quality RAC can be produced with RA sourced from, at least, medium quality concrete (C40); (2) a poor-quality ITZ results in a significant decrease in concrete properties, particularly decreasing the tensile strength up to 58%, since micro-cracks that initiate in weak ITZ can readily develop into mortar; (3) the most relevant factors that determine the strength of RAC are the quality of mesoscopic phases, with the relative effects that range from 8% to 53% for compressive strength and from 22% to 59% for tensile strength; (4) the concrete mix design was less relevant, with a maximum influence of 23%; (5) the previous two observations imply that the most efficient way to improve the properties of RAC is to use RA of better quality; (6) a high proportion of coarse aggregates and a lower replacement ratio of natural aggregate with RA can effectively improve the stiffness of RAC by 16% and 15%. This paper contributes to understanding the influence of mix design and raw materials on the properties of RAC and determines the limiting factors that prevent a concrete mix from reaching its intended performance.