RIASSUNTO
ABSTRACT
The traditional approach to corrosion modeling is relatively straightforward. Corrosion is expressed as a rate of penetration per year and failure is defined when a critical loss of section is predicted to have occurred. Such an approach is well suited to applications involving ships and pipelines where a small local leak can lead to more widespread problems. However, if this approach is applied to civil structures then it generally proves to be overly conservative. Civil structures are usually not affected by global failure due to a local event. This paper describes the development of an alternative approach that takes into account the overall variability in the environment and exposure conditions during the corrosion process. This generates a basic data set that can be employed to represent the natural variability in extent and severity of corrosion occurring on a structure. This was collated into cumulative frequency curves for several common metallic systems and a Weibull distribution fitted to the data. The model has been verified against several different types of structures. The results confirm that compared to the traditional approach, the predictions generated by this method are less conservative, more realistic and greatly improve confidence in the prediction of service lives.
INTRODUCTION
The life time of structures is governed by active and passive deterioration mechanisms. Active deterioration can be expressed as the actual loss of structural capacity. In the case of reinforced concrete structures loss of structural capacity occurs when the reinforcement corrodes losing cross-sectional area. This time dependent process can be described with the rate of corrosion of the affected steel element. Passive deterioration mechanisms lead to the degradation of the protective layer surrounding the steel. Carbonation of the cover concrete and the penetration of chlorides are the two main mechanisms causing the depassivation of the reinforcement. Steel structures also have such a protective layer which comprises coatings, galvanization and concrete encasings. Such layers will also break down with time due to ageing, UV and / or mechanical effects. Corrosion is a complex electrochemical process involving the formation of local anodes and cathodes on the surface a metal. Metal loss only occurs at the anode. Carbonation induced corrosion progresses evenly whereas for chloride induced corrosion the typical ratio of anode to cathode area based on practical experience is approximately 1:10. Therefore in a sample of heavily chloride contaminated concrete only 10% of the steel would be actively losing metal. Once the passive deterioration mechanism has been assessed with the determination of the amount of steel likely to corrode it is possible to assess the active deterioration process. To work out how long something will last you need to predict a corrosion rate. Historically the approach taken was for a university or government to take a large number of small plates, leave them outside for a period of time and then bring them in and measure them. There are two things that can be measured; the mass of metal remaining, or the thickness of metal remaining