Where exposed to an aggressive environment, concrete structures will develop reinforcement corrosion at some point in their life. Chloride ions from de-icing salt or sea water induce pitting corrosion. Local cross section reduction may occur relatively quickly and expansion due to corrosion products precipitating may crack and spall concrete cover and reduce steel-concrete bond. With the aging of the existing stock, questions arise about the effects of corrosion on their structural behaviour, safety and reliability.TNOhas started a four-year programme to investigate this subject. One project concerns modelling of pitting corrosion from electrochemical principles, aimed at prediction of cross section loss with time and, in a next stage, corrosion product formation. Existing models for pitting corrosion are highly simplified and essentially two-dimensional, while pitting is clearly a three-dimensional phenomenon. The difference may have serious consequences for structural safety. New approaches are needed, including new conceptual models and addressing uncertainties on several levels. The paper presents some conceptual models for pitting corrosion and reports preliminary results of numerical (finite element) modelling of pitting corrosion, based on solving potential and current distributions and resulting in cross section loss as a function of time. Further work including experimental validation is foreseen.

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Chloride induced reinforcement corrosion is the predominant degradation mechanism affecting reinforced concrete structures. Chlorides (Cl^-) contained in sea water or de-icing salts penetrate through concrete pores by diffusion and/or convection. Reinforcement corrosion initiates when the Cl concentration at the reinforcing steel surface equals or exceeds a specific concentration. This concentration is known as the critical chloride content (C_crit). This study presents an experimental method proposed by the RILEM Committee 235-CTC "corrosion initiating chloride threshold concentrations". Two series of reinforced concrete specimens were fabricated: one with ordinary Portland cement (CEM I) and another with ground granulated blast furnace slag-GGBS- (CEM III/B) cement, both commercially available in The Netherlands. Subsequently, the specimens were partially submerged in a chloride-rich solution (3.3 wt. % NaCl) for 6 months. During this period, continuous monitoring of the open-circuit potential (OCP) of the steel reinforcement was used to determine the initiation of reinforcement corrosion. The concentration of Cl could be determined by acid digestion and subsequent titration of powder samples collected from individual layers in the concrete cover. Results show that after the exposure period, the C_crit could be determined in PC specimens whereas in GGBS concrete specimens the higher resistance to chloride ingress prevented from obtaining corrosion initiation.

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Chloride induced reinforcement corrosion is the most common degradation mechanisms for reinforced concrete structures. The service life of concrete structures is normally predicted by estimating the rate of chloride ingress and the necessary time to initiate reinforcement corrosion. Normally, chloride ingress is modeled as a diffusive process in which concrete is considered as a semi-infinite continuous medium. This modelling approach disregards the influence of cracks on the rate of chloride ingress in concrete. However, experimental studies have shown that the influence of cracks on chloride ingress is significant and cannot be neglected. In practice, cracks in concrete may originate due to different mechanisms. Recommendations on crack control in flexural members consider cracks in the range between 0.15 mm (0.006 in.) and 0.3 mm (0.011 in.) to be permissible in deicing and/or seawater exposure; with the same limit for both exposure classes in Europe. The influence of cracks on service life prediction remains to be clarified. This paper presents describes a conceptual approach for incorporating the effect of flexural cracks on the calculation of the time-tocorrosion initiation of steel reinforcement due to chloride-ingress. The proposed approach consists of applying a correction factor to the chloride diffusion coefficient, which is dependent on the surface crack width. @en