Modeling the effects of corrosion on the lifetime of extended reinforced concrete structures

A modeling approach is presented for quantitative projections of corrosion damage in the substructure of marine bridges, taking into account the compounded variability of concrete cover, chloride diffusivity, and chloride surface concentration. The projections reduce to a multiple integral when the mechanisms of both corrosion initiation and propagation are such that distress is always manifested first in elements with the lowest clear concrete cover. The projected damage functions reflected the dispersion of the assumed controlling model variables. Two examples of model application are presented. In the first case, the deterioration of a dual bridge built with high-quality concrete but low-reinforcing steel cover was considered. For that case, projected deterioration on much of the substructure was dominated by the corrosion initiation stage. The second case addressed the development of corrosion of epoxy-coated rebar on a family of bridges where chloride diffusivity was high and there was much variability in concrete cover. In this case, the effect of concrete cover variability on the extent of the corrosion propagation period was introduced as well. Damage progression was found to be dominated by the development of the corrosion propagation stage. Application of the models to these two cases produced output consistent with observed behavior. However, absolute model forecasts are affected by uncertainty in inputs and basic assumptions. Modeling approaches such as those presented here are expected to be most useful when assisting in comparing design or rehabilitation alternatives, or as an aid in elucidating corrosion mechanisms. Areas for future improvement of this methodology include accounting for effective diffusivity and surface concentration variations with time, the effect of chloride ion binding on diffusion, the effect of rebar potential on corrosion threshold, and a more precise evaluation of the length of the corrosion propagation stage.