Using Fick's Second Law and Nernst–Planck Approach in Prediction of Chloride Ingress in Concrete Materials

Abstract The service life of many concrete structures depends upon their resistance to chloride ingress. Service life models estimate the time required for chloride ions to reach the reinforcing steel, to build up a critical concentration, and to initiate corrosion. Fick’s second law and the Nernst–Planck equation are two of the more popular methods that are used to estimate chloride ingress. While chloride ions are usually the primary consideration, in general they are not present by themselves. The co-present cations and anions can influence the rate of chloride ingress. This paper discusses how the apparent chloride diffusion coefficient, based on Fick’s second law, is dependent on the chemical composition and concentration of the ponding solutions. This study examines the influence of the chemical composition and concentration of solutions on the chloride binding capacity, on the consequential microstructural changes as determined with scanning electron microscopy using energy dispersive X-ray spectra (SEM-EDS), on the surface charges of the pore walls, and on the overall chloride ingress of the concrete materials. Chloride ingress predictions based on the Nernst–Planck equation were also compared with the experimental chloride profiles. The Nernst–Planck approach provided good predictions at low salt concentrations (less than 1.0 mol/L NaCl) using a single porosity, tortuosity, and binding approach. At higher concentrations, the binding and change of microstructure was under-predicted, and thus the chloride ingress was over-estimated.