XCr/√t as an Indicator of the Resistance Against Bulk Chloride Diffusion

Abstract

Chloride bulk diffusion is generally analysed with apparent diffusion coefficients fitted on full chloride penetration profiles, which are resource intensive to obtain with the grinding and titration method. As part of a comprehensive project on chloride ingress in blended cement pastes, this paper employs a dataset including a wide range of systems with 6 different types of binders and 3 different water-to-binder ratios. More specifically, bulk diffusion experiments were performed on cement paste specimens exposed to a 0.5M NaCl solution. Chloride profiles were then obtained on cross-sections by micro-X-ray fluorescence after 6 months, 1 year and 4 years of exposure. Results indicate that, for the classification of chloride penetration resistance using bulk diffusion results, the apparent diffusion coefficient Dapp fitted over the whole profile could be replaced by XCr/√t (the penetration depth for a specific reference chloride content divided by the square root of time). For reference chloride contents (Cr) in the lower half of the measured profiles, √Dapp was highly correlated with XCr/√t (R2 = 0.93–0.99). Thus, measuring chloride penetration depths (e.g., using a silver nitrate spray) after different times of exposure to chloride concentration typical of seawater may be a simpler classification indicator than the conventional profile fitting of bulk diffusion results (which needs grinding and titration).