Clarifying the effects of volume proportion and surface area of aggregate on chloride diffusivity of concrete through microstructural design

Increasing the aggregate proportion usually reduce the chloride diffusivity of concrete, but may also increase the chloride diffusivity as more interfacial transition zone (ITZ) is introduced with increased surface area of aggregate. Since both packing density and surface area of aggregate varied simultaneously with its particle size distribution (PSD), effects of volume proportion and surface area of aggregate on the chloride diffusivity of concrete has not been clarified separately yet. In the present study, we established a database containing over 500,000 aggregate PSDs and their corresponding packing density and specific surface area using the Compressible Packing Model (CPM) and photogrammetric 3D reconstruction. By individually controlling target volume proportion or surface area of aggregate, effects of aggregate volume proportion and surface area on the mechanical properties and chloride diffusivity of concrete were evaluated. Then an empirical equation, in consideration of aggregate size-dependent ITZ thickness the tortuosity, was proposed to describe the relationship between chloride diffusion coefficient and aggregate proportion and surface area, and a strategy for aggregate PSD optimization was provided to meet chloride diffusivity requirement, which offer an approach for enhancing the chloride resistance of concrete.