Numerical Study of Pore Solution Chemistry in Surface Crevices of Carbon Steel Rebar

Abstract

The variability and uncertainty associated with chloride thresholds can be partly explained by the surface conditions of carbon steel rebar, in particular, by the presence of crevices on the steel surface. It has been suggested in the literature that pore solution in the crevices on the steel surface may be different from that of the bulk pore solution, and this difference may create the necessary conditions for the breakdown of the passive film. To test this hypothesis, a numerical investigation was carried out using a non-linear transient finite element algorithm, which involved the solution of coupled extended Nernst-Planck and Poisson’s equations in a domain that represented typical surface crevices on carbon steel rebar. The numerical simulations showed that the chemistry of the pore solution, in particular pH and Cl-/OH-, within crevices provided more favourable conditions for depassivation than the bulk concrete pore solution. Local acidification and increase in Cl-/OH- within the crevice were observed in all simulations, albeit to different degrees. Simulations supported the hypothesis that the chemical composition of the pore solution within the crevices differs from that of the bulk solution through a process similar to the suggested mechanism of typical crevice corrosion.