The paper is important and timely. The practical application of analytical methods for durability design and the agreement of models that can be built into codes of practice are urgently required for important infrastructure. A key point made in the latest BRE Durability by Intent Workshop Report 16 is that prescriptive approaches to durability design may be adequate for the generality of structures, but that long service life structures in aggressive conditions need a more sophisticated approach. BRE noted that such approaches may involve modelling of deterioration mechanisms, a conscious approach to service life design and a probabilistic approach to the likelihood of deterioration in a given time. The latter is significant in that it is the first design code that opens the door to analytical and probabilistic methods of durability design.We have been interested in the corrosion model for reinforced concrete in a marine environment for almost 20 years in an attempt to improve on the relatively poor durability obtained from applying the prescriptive methods recommended by earlier codes and standards. Between us we have also been involved with the drafting of the latest BS 6349 and on the chloride sub-committee to the forthcoming Complementary British Standard to EN 206, so we can comment with some insight into their development. There are a number of points that arise from the paper which, with further explanation, may provide further insight to assist the development of a much needed consensus on general corrosion models for reinforced concrete in chloride environments.The authors note that the actual chloride diffusion coefficients found from an extensive literature review differ by orders of magnitude from the 'observed, actual values' (presumably obtained by best-fit analysis of the chloride profiles found in the Midlands Links bridge structures) listed in Table 1. We suggest that there may be two reasons for this.First, the values obtained from a literature review will likely cover a range of concrete qualities and cement types, whereas the structures of the Midlands Links are likely to have a more consistent and relatively uniform quality. Our recent investigations of chloride profiles in a number of bridge foundations on the M5 motorway have shown the concrete from a variety of exposure conditions with a range of different chloride profiles that were built around the same time, probably to similar specifications, have very similar apparent chloride diffusion coefficients. We note that the apparent diffusion coefficient of 220 mm 2 =year (or 7 3 10 À12 m 2 =s),obtained from the chloride profile data from two cross-beams and used in the Midlands Links model, would appear to be typical for a Portland cement concrete which had a water/ cement ratio of approximately 0·55 from comparison with values in the literature. 28 It is not clear from the paper, however, how this value relates to the proposed effective parameter values set out in Table 1 and the cement type has not been stated. Further explanation would...
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