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Figure 4.5: Reliability index versus concrete cover for environmental class XS3 (w/c=0,45) a) for CEM I with fly ash

The results of chapters 3.2 and 3.3 have shown that lower values for the nominal concrete cover result in a significant reduction of the reliability index, leading to a decrease of the safety against reinforcement corrosion. It can be concluded that the nominal concrete cover should not be decreased as a consequence of the reduction of allowance in design for tolerance.

4.4 Conclusions

The calculations have shown the dependence of the rate of deterioration on the service life, the thickness of the concrete cover and the environmental class. It is shown that the provisions as given in EC-2 recognize the most important influencing factors. Further influences, which have not yet been regarded in detail in EN 1992-1-1 are the type of cement and the temperature.

The comparisons show that, theoretically, the prescribed reliability indexes are not always met. It should be realized, however, that there are many uncertainties in the input values of the calculations. Further to the influence of the type of cement and the temperature there is the variation in climatic conditions (wet - dry cycles, local differences due to different orientation with regard to solar radiation and wind).

The recommendations in the code are the result of theoretical considerations and engineering experience. In some respects the deterioration models give valuable information. It is shown for instance that prolonging the service life of a structure from 50 to 100 years requires globally an increase of the concrete cover between 8 and 12 mm. The advised increase of 10mm is therefore a good average value, regarding the many unknown factors. Moreover, research is necessary to close the gap between scientific models and practical observations.

Reference

[1] DuraCrete - Probabilistic Performance Based Durability Design of Concrete Structures: Statistical Quantification of the Variables in the Limit State Functions. Report No.: BE 95-1347, pp. 62-63, 2000.

[2] FIB Bullettin n. 34 Model Code for Service Life Design, June 2006, ISBN 978-2-88394-074-1

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