The relationship between chloride diffusion and migration coefficients in concrete

Chloride diffusion coefficient is an indicator of the chloride resistance of concrete and service life prediction of concrete structure subjected to chloride environment. Many methods have been developed to measure the chloride transport in concrete. Based on the driving force, these methods can be classified into diffusion test and migration test. In terms of the change in chloride concentration in concrete, they can be classified into steady-state and non-steady-state testing. However, different methods give different results. This makes the comparison of all these results difficult. In this paper, the relationship between non-steady-state migration test (D-nssm), steady-state migration (D-ssm) test and non-steady-state diffusion (D-nssd) test were investigated. D-nssm was determined by NT build 492, breakthrough time method and the methods proposed by Castellote, respectively. D-ssm was determined by upstream method proposed by Truc and downstream method (NT build 355), respectively. D-nssd was determined according to NT build 443. Four different concretes, containing various mineral admixtures and with water/binder ratio of 0.48, were used for study. A same chloride concentration of 1mol/l, was used for all the tests. The results indicated that all the non-steady-state testing methods gave the same order of the chloride resistance of concretes although the diffusion/migration coefficients were somewhat different. Both NT build 492 and method proposed by Castellote gave a good estimation of diffusion coefficient. Breakthrough time method underestimated chloride diffusion coefficient. Steady-state migration coefficient was one order of magnitude lower than non-steady-state migration coefficients obtained from other methods. Steady-state migration coefficient obtained from upstream method was higher than that obtained from NT build 355.