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Carbonation modelling for concrete structures

The deterioration of concrete structures by environmental actions has become a great concern for engineering design and maintenance.

In recent years the cases of unsatisfactory durability of concrete structures has increased at an alarming rate. In most cases, corrosion of the reinforcing steel in the concrete is the main cause of the deterioration affecting the durability performance of the structures. The alkaline environment of the concrete protects the reinforcing steel by passivating it, i.e. maintaining it in an unreactive state. However, the protection offered by the concrete can be compromised by aggressive agents that penetrate the cover concrete and de-passivate the steel. Apart from chlorides, the diffusion of carbon dioxide (CO2) into the concrete is one of the major environmental factors responsible for the depassivation of steel, which is usually followed by corrosion in the presence of oxygen and moisture. This has directly resulted in a need from both industry and in the field of research; not only to understand and diagnose the process of carbonation, but also to predict the initiation of corrosion of reinforced concrete structures. Most of the durability models (or service life prediction (SLP) models) currently available predict the corrosion initiation (e.g. carbonation-induced corrosion) in terms of depasivation of steel as the limit state. However, it has to be kept in mind that service life predictions based on such an approach are conservative, since the presence of moisture is necessary for actual corrosion to take place. Hence a carbonation model coupled with moisture model, which will not only predict the rate and extent of carbonation but also the propensity of corrosion to start in terms of moisture, needs to be developed. This is the basis of the proposed study.

The main aim of the study is to develop and (experimentally) validate a model for the initiation of reinforcement corrosion due to carbonation, with the onset of corrosion reaction as the limit state with moisture as a prerequisite. The development of the model will be based either on existing models and theory, making them more reliable and practical, or by developing a new model. The final output will be a combined model for SLP which will give a comprehensive solution in terms of prediction of corrosion initiation due to carbonation. Since the deterministic output of the models contrasts with the fact that concrete properties, moisture movement and corrosion processes are variable, the numerical model developed has to be converted to a probabilistic SLP model.