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Figure 4.1 resistance, action, failure probability and reliability index

Figure 4.1 resistance, action, failure probability and reliability index

The numerical problems which are treated in the context of durability design are not as easy to calculate as this, because there are numerous variables which have to be statistically evaluated and they are often part of non-linear functions. Besides these aspects, variables are not always normally distributed. For this reason, computer programs are used to calculate these kinds of problems.

4.2. Definition of reliability levels

As already mentioned, the corrosion process can be divided into two time periods. For both periods, limit state conditions can be defined. Depassivation of the reinforcement (= end of the initiation period) is defined as a serviceability limit state (SLS), because after the end of the initiation period the corrosion process starts [1]. As the corrosion process in the propagation period can lead to severe consequences (loss of safety of people and structure), this situation is defined as ultimate limit state (ULS). Since the calculations performed in this study only concern the initiation period, the durability design presented here durability design is a SLS assessment.

Design limit states are often defined by means of the reliability index p. According to EN1990, the reliability index for SLS is determined to p = 1,5; in other National Standards the reliability index is even higher. In the case of durability design a risk oriented grading of the reliability index is proposed. Because of the fact that only the initiation period is considered, the corrosion process itself is so far not included in durability design. A possible way of taking account of the different corrosion risks is to adjust the reliability index to the environmental classes. As a consequence, with respect to the corrosion process a moderate humid environment (e.g. XD1) or environments with cyclic wetting and drying (XC4, XD3, XS3) should fulfil higher safety requirements than totally dry or wet environments (e.g.XC1, XC2, XC3, XS2, XD2) and therefore a higher reliability index is proposed. For the same reason in chloride environments higher reliability indices should be applied due to the risk of higher corrosion rates compared to carbonation induced corrosion. A proposal is given in Table 4.1.

Environmental class

Reliability index ßSLS,50

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