Reactive transport modelling of concurrent chloride ingress and carbonation in concrete

The carbonation is a fundamental durability process for concrete structures exposed to nearly all environments. In air-borne chloride environments, the chloride ingress occurs simultaneously with the carbonation, constituting a multi-phase and multi-species transport-reaction problem. This paper regards concrete as a reactive porous medium partially saturated by pore solution and addresses this problem through a comprehensive transport-reaction model taking into account the dissociation and dissolution–precipitation equilibria in pore solution, transport of gas and aqueous phases and local electrical field. Especially the incongruent dissolution of C–S–H is addressed, a coating-reaction kinetics is assumed for the CH carbonation and new chloride adsorption laws are used upon carbonation of C–S–H. The established model is solved through finite volume method and validated by laboratory experiments of chloride ingress in carbonated OPC concretes. The parametric analysis for concurrent carbonation and chloride ingress under constant relative humidity shows that: (1) the concurrent carbonation will globally promote the chloride ingress and the involved corrosion risk for embedded steel; (2) the significant RH range for this carbonation impact will be on medium levels around 75%; (3) the concretes incorporating SCMs, especially in large quantities, will behave differently to the influence of carbonation. Accordingly, the concurrent carbonation should be considered in appropriate way in durability design against chloride ingress.