Rate of depassivation activation

We have treated the problem as one dimensional so far, considering the time to depassivation at one particular location. Carbonation depths, chloride profiles and rebar depths are not uniform so the spatial distribution of depassivation or initiation must be included in the calculation unless the ranges are small or the time from depassivation to damage is large. We know that all the concrete cover will not spall off at once so there must be a distribution of depassivation times and of time from depassivation to spalling. We must have realistic estimates of the time from the first spall to end of functional service life. 

By looking at the distribution of diffusion coefficients and the cover distribution it should be possible to calculate Tq for the first 1, 10, 20%, etc. of the structure. It may also be important to differentiate between different locations due to variations in exposure. This will include moving up a column from the sea level, areas of salt water run off on substructures, zones facing salt spray, etc. 

The diffusion models work reasonably well for predicting the initiation time. The chloride profile and the carbonation depth can be measured in the field or from cores in the laboratory. However, it is far more difficult to look at the next step in our model. Corrosion rate measurements are now being taken in the field with linear polarization instruments and empirical estimates have been made with different instruments for the time to spalling. 

The following broad criteria for corrosion have been developed from field and laboratory investigations with the sensor controlled guard ring device (Feliu et al., 1995)  

Passive condition Low to moderate corrosion Moderate to high corrosion High corrosion rate /  

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