Comparisons of instantaneous chloride diffusion coefficients determined by RCM method and chloride natural diffusion test

Abstract

Chloride diffusion coefficients, which include the chloride diffusion coefficient DRCM measured from the rapid chloride migration (RCM) test, the instantaneous chloride diffusion coefficient Dins and the apparent chloride diffusion coefficient Da determined by the chloride natural diffusion test, are the important indexes for the chloride permeability of concrete. Da denotes the average diffusion coefficient over the whole exposure period. Both DRCM and Dins essentially represent the instantaneous diffusion coefficient, which reflect the influence of real-time changing environmental factors on the chloride permeability of concrete. In this paper, the method for determining the time-dependent model of Dins was firstly studied based on Fick’s second law. Subsequently, the time-dependent models of Dins and DRCM were respectively established through the chloride natural diffusion test under artificial simulation marine environment and the RCM test. According to the comparative analysis of Dins and DRCM, the RCM modified factor f(t) was proposed to quantify the relationship between DRCM(t) and Dins(t), and the modified model of DRCM was established. Finally, the suggested modified model of DRCM was applied to predict the chloride concentrations in concrete, which was verified by experimental results. Results showed that the values of Dins and DRCM were identical at the initial exposure time, while DRCM gradually came to be larger than Dins with the increasing of exposure time. The main reason for the difference between Dins and DRCM is that the process of chloride diffusion in concrete is influenced by convection and chloride binding during the chloride natural diffusion test, which can be ignored in the RCM test.