Service Life Modelling for Chloride Ingress in Reinforced Concrete

Service life modelling for concrete structures exposed to severe environments has been developed and used for several years. The upcoming revision of the Eurocode 2 for concrete structures is incorporating service life design (SLD) based on performance based models as an alternative to the more common concept of deemed-to-satisfy material requirements. The performance based approach is generally recognised to have a more sustainable profile than the traditional prescriptive one. The article describes basic features of SLD regarding chloride ingress of concrete structures exposed to marine conditions and/or de-icing agents. Chloride induced corrosion is one of the most common and wide spread causes for degradation of concrete. The well-known error function solution to the Fick's second law of diffusion is applied. Furthermore, the paper provides an easy-to-use design diagram that takes into account the most important parameters of SLD. The diagram enables the designer to get a better overview of the material parameters and the exposure conditions influence on the minimum cover thickness and the design service life. The main purpose of this paper is to present SLD modelling of chloride induced corrosion in a manner that is easily adaptable and understood by the structural designers. If performance based design of concrete structures should be taken further there is a need to share the knowledge across structural and material engineers. As a case story for the SLD diagram, material data from the new Queensferry Crossing situated outside Edinburgh is used. The owner has required documentation for at least 120 years of service life for the structure. As a leading member of the design joint venture Ramboll has provided the documentation for the SLD related to the concrete foundations and the concrete structures exposed to marine environment. The paper also highlights some of the main topics that still need further research. This is especially the case for the diffusion coefficient and the ageing effect associated with this material property. At present the literature contains many models based on several test methods and there is a need for a robust and designer-friendly approach that is able to capture the material behaviour in a realistic - but yet simple - manner. For concretes containing supplementary cementitious materials this becomes even more pronounced in order to utilise the full potential of the concrete performance.