INTRODUCTION
Chloride-induced rebar corrosion is a common degradation process for concrete infrastructure, which is a practical concern especially for cold-climate states and coastal areas. In this work, numeric models based on Finite Element Method (FEM) are utilized to study service life of concrete structures subject to chloride ingress. The stochastic nature of model inputs is taken into consideration. Specifically, the surface chloride concentrations and concrete cover depth follow the normal distribution; the diffusion coefficients obey the gamma distribution; and the actual chloride threshold features the triangular distribution. The nonlinear partial differential equations (PDEs) to characterize the spatial and temporal evolution of ionic species are numerically solved, the results of which are utilized to elucidate the influence of various factors on the service life of reinforced concrete, such as mix design, surface chloride concentrations, crack level, coarse aggregate and concrete cover depth.
Due to its high strength, flexibility in geometries, and variability in mechanical properties, reinforced concrete has been the most widely used construction materials for infrastructures across the world. Concrete pore solution can have pH values well above 12, which allows rebar surface to be covered with a thin, passive and impermeable oxide film1-2. In addition, the dense concrete cover forms a physical barrier to reduce the penetration of deleterious species. Under such protective environments within concrete, steel rebars do not corrode spontaneously, which prevents anodic reactions from occurring and makes steel inert for corrosion. Chloride-induced degradation in concrete structures is a common occurrence in the United States due to the use of deicers in cold-climate states and the saline environments in coastal areas. By capillary effect and diffusion, chloride ions can migrate into concrete. Although capillary movement is fast, it is less favored by the discontinuous pore system in concrete. Diffusion thus provides a practical manner for chloride ions to migrate across concrete cover and reach rebar surface, and the transport of chloride ions through concrete cover to rebar surface is a major cause of concrete degradation, the results of which lead to serious economic and safety implications. Chloride ions are responsible for the localized breakdown of passive film and the initiation of corrosion3-4. Reliable predictions for existing rebar performance are indispensible, which can take into account the extent of deterioration, the expected remaining service life, and the optimum repair strategy. A well-defined procedure allows sufficient information to be obtained for timely rehabilitation and replacement of reinforced concrete structures. Although research in the reinforcement corrosion in concrete has been extensively documented in the past decades5-8, the current state of knowledge in corrosion initiation remains unsatisfactory. In traditional service life modeling, the concerns are addressed with the stochastic technique, with only the diffusion of chlorides taken into consideration. Such treatments9-10 fail to adequately describe concrete degradation due to mutual interactions among multiple ionic species, thereby underestimating or overestimating the risk of chloride contamination.