Reliability analysis of corrosion onset: initiation limit state

Abstract

The relatively premature failure of structures by steel corrosion has increased interest in incorporating to the standards calculation of the time to corrosion. These models in general identify the end of service life with the starting of bar corrosion, either by chlorides or by carbonation. Additionally, from the publications of the project Duracrete, funded by the EU, probabilistic treatments have been applied to steel depassivation which is associated with a serviceability limit state (SLS). Thus, fib Model Code (MC2010) recommends a probability of failure of 10% for calculation of the time to corrosion. However, this recommendation leads to uncertainties because, on the one hand, the chloride threshold is not a fixed value and the carbonation front affects first the external face of the bar, developing progressive perimetral damage and, on the other hand, the consideration of the limit state as an SLS, with so low a fixed failure probability, introduces some degree of contradiction with the classical definition of the SLS. This is because, while the SLS is defined as a failure to fulfil the design prescriptions, any structural design performance changes just at the moment of steel depassivation. Additionally, no treatment on how to consider the damaged section or corrosion propagation model is usually given in the MC2010 or other standard and how to verify the classical SLS and ultimate limit state (ULS) is undefined. The present work introduces new perspectives on the subject from a probabilistic point of view by, (1) treating statistically the depassivation step, (2) by introducing a corrosion propagation model and quantifying the corrosion limit states and (3) by calculation of the depassivation probability which is dependent on the rate of the deterioration process itself. The whole exercise stresses the need to not consider a fixed value (10%) associated with a SLS as the probability of corrosion, but to consider it a limit state of “corrosion initiation” following ISO 2394. The value of the adequate probability of corrosion would be dependent on the consequences of the failure and on the rates of chloride ingress, of carbonation or of steel corrosion.