A metallurgical approach to the durability of reinforced concrete

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Summary ***    The application of rock salt and, more recently, concentrated brines to our highways and bridges is instrumental in high deterioration levels of concrete and carbon steel reinforcement (rebar). The deterioration arises from the ingress of chlorides and breakdown of the very thin film of oxides on the steel surface in concrete which normally protect the steel from rusting. The result is that the highway structures and parking garages in the Windsor to Montreal corridor have the greatest accumulation of salt of anywhere in North America and correspondingly high deterioration rates. ***    At the same time, the Canadian Highway Bridge Code has increased the required service life of highway bridges to 75 years and is proposing to increase it further to 100 years. Consequently, transportation authorities are interested in using more corrosion resistant alloys as rebar to limit the need for frequent maintenance and the associated traffic delays. Despite the significantly lower lifetime costs of a structure reinforced with these alloys, the greatest obstacle to their use is the initial cost. To address this problem, steel companies are developing stainless steel grades with less expensive alloying components, but the authorities are reluctant to install untested alloys in structures intended to last for 75 years or more. A number of research groups in Europe and North America (including the applicant's) have been investigating the relative corrosion performance of the newer alloys in short-term laboratory tests. However, these tests use various methods to accelerate corrosion and typically consist of three to five years exposure to salts at most, but sometimes are as short as 3 - 4 days. Extrapolating the results of accelerated tests to real world exposure for 3/4 century leaves many questions. ***    Therefore, in order to better predict the long term behaviour of stainless and corrosion resistant reinforcing alloys, the goal of the proposed research program is to define their metallurgical and electrochemical state when they are embedded in concrete and, eventually, exposed to the de-icing salts. Determining the nature of the passive films in-situ in a high pH environment, with and without chlorides, will be challenging and require modification to traditional methods. Another major obstacle to this research will be to actually achieve corrosion in these highly resistant alloys without radically changing the conditions. These data are needed in order to determine their corrosion rates. Training the next generation of engineers who will implement these groundbreaking advances is one important outcome. Another is that the results of this study will permit infrastructure designers to specify the most cost-effective materials for particular applications depending on the severity of the local climate and the desired service life of the structure. ***    **