Usually, steel embedded in concrete is recognized to be invulnerable to corrosion because the high alkalinity of pore solution in concrete generates the thin protective oxide layer on the surface of steel. However, recent observations in field and experimental evidences show that even steel in concrete can be corroded. The primary reasons for the corrosion are carbonation and chloride attack. Existing researches on the durability of RC structures are mainly focused on the estimation of time for the corrosion initiation due to carbonation or chloride attack. Reasonable estimation of steel corrosion rate after the initial depassivation of steel on the basis of the properly devised analysis system is still insufficient. Moreover, most of the existing researches on the analysis of steel corrosion are performed independently from the analysis of carbonation or chloride attack. For the realistic estimation of steel corrosion in concrete, these aspects should be considered simultaneously.
The objective of this study is to develop a unified numerical analysis program which can investigate the steel corrosion behavior considering carbonation and chloride attack in the same time and space domain, so that the realistic behavior of steel corrosion in concrete can be reasonably estimated. To achieve this goal, three-dimensional finite element program which can performed the analysis of moisture transport, carbonation, and chloride attack simultaneously are developed first. In this program, the influence of convective flow on the chlorides penetration, and the change in binding property due to carbonation are considered, thereby the special cases, such as cyclic wetting and drying or combined deterioration (i.e. chloride attack accompanying carbonation), can also be analyzed reasonably. Developed program for penetration analysis is subsequently integrated with the analysis program for steel corrosion, so that the real-time behavior of steel corrosion in RC structures can be properly described. To consider the realistic variation of electrical resistance in concrete, a model equation for the resistance, which is a function of moisture content, chloride concentration, and temperature, is proposed. Additionally, the effective diffusivity of a concrete element having an ideal crack is derived to apply the smeared crack concept in the penetration analysis.
The applicability of developed program is verified by comparing the analysis results with the experimental data extracted from other researches for various aspects; single deterioration (i.e. carbonation or chloride attack only), combined deterioration, and the influence of crack on the penetration and that on the corrosion behavior. Verification results show that developed program can reasonably reproduce the real nature of each aspect.
Finally, some important features related to the estimation of service life or steel corrosion in concrete are numerically investigated by performing a number of case and parametric studies, which reveals the following interesting behaviors: 1) the increase in chloride penetration due to carbonation is dependent on the exposed environment; 2) the correction factor $S_{cr}$ is essential for the accurate description of concentration profile near crack when crack is modeled by using smeared crack elements; and 3) the resistance of concrete is the most important parameter which determines the acceleration of steel corrosion near crack.
