A methodology for coupling limit states in service life design of reinforced concrete structures

A current approach to the service life design of reinforced concrete structures will typically consider the limit state of reinforcement corrosion initiation as the defining durability limit state. Rarely are other limit states considered such as corrosion propagation or corrosion induced cracking and spalling of the concrete cover. This might stem from the lack of models or confidence in the models used for the propagation phase or it is simply caused by the greater uncertainties and the difficulties that come along when combining two individual service life calculations with different limit states. Currently, most models being used only capture the process of concrete carbonation or chloride ingress into the uncracked concrete that is at the initial phase of degradation. Therefore, these models disregard the actual damage, i.e. the corrosion of the reinforcing steel, subsequent crack formation and spalling of concrete cover, which may lead to a substantially shorter estimated service life of the structures. This paper presents a holistic approach which combines two models used for determining the time to depassivation (initiation phase) and the time to cover cracking as a result of reinforcement corrosion (propagation phase). A methodology is presented which merges the probabilistic determination of two limit states (corrosion initiation and corrosion induced cracking) in a single analysis, thus removing the need for one of the limit states. Consequently, this procedure contributes to the development of reliability-based approaches for the durability design and service life prediction of reinforced concrete structures. An example is provided that shows the developed methodology for a structural element which has been designed for the serviceability limit state of concrete cover cracking.