Time dependency of Cl diffusion coefficients in concretes with varied phase compositions and pore structures under different environmental conditions

Abstract

There have been many reports showing measured diffusion coefficients have a time dependency, but very few studies considering the mechanisms of that time dependency. The aim of this study is to investigate the variation of time dependency of diffusivity due to the type of cement and environmental condition using four parameters: the linear chloride binding capacity, unit cement content, porosity and pore structure factors. Concrete specimens with different phase compositions and pore structures were prepared using various cement-based materials and different mix proportions, and their apparent and effective diffusion coefficients Da and De were measured to know how they are influenced by the phase composition, pore structure and environmental conditions such as exposure temperature and marine tidal or splash zone environment. This paper verifies the applicability of analytical solutions of diffusion equations to the evaluation of Da with variations in surface chloride ion (Cl-) concentration depending on the immersion time. Over-time changes of Da and De are then discussed from the viewpoint of variations in influential factors depending on the mix proportions and environmental conditions. For the present study, specific focus is placed on the factors influencing Da including linear binding of Cl-, cement content and pore structure factors as well as tortuosity and constrictivity as typical pore structure parameters. © 2012 Japan Concrete Institute.