Initiation time carbonation induced corrosion

The time for initiation of carbonation-induced corrosion is the time required for the carbonation front to reach a depth equal to the thickness of the concrete cover. It depends on all the factors mentioned above (that influence the carbonation rate) and on the thickness of the concrete cover. If the evolution of carbonation in the course of time and the thickness of the concrete cover are known, the initiation time can be evaluated. It should, however, be taken into consideration that the carbonation front may not be uniform across the concrete surface. 

For carbonation-induced corrosion, the service life (ti) is expressed as the sum of the initiation (h) and propagation (tp) periods up to the threshold at which deterioration becomes unacceptable = h + tp (Figure 4.1). The initiation time (h) may be calculated as a function of the properties of concrete, in particular the coefficient K of carbonation, the environment and the thickness of the concrete cover x), for example with models by Tuutti, Bakker, or Parrott (Chapter 5). The propagation time (tp) can be estimated if the corrosion rate is known, once the maximum acceptable penetration of corrosion has been fixed. A maximum penetration for corrosion attack that is often accepted in reinforced (but not prestressed) concrete elements is 100 tm. 

Provided that the concrete is not water-saturated, it may be reasonable to assume that the initiation phase is considerably longer than the propagation period and that the end of the initiation period alone is a useful indicator of service fife. Clifton and Pommersheim have reviewed simple models based on this approach. For chloride-induced rebar corrosion, one of these is the use of Fick s second law of diffusion and the concept of a critical chloride concentration. Limitations and simplifying assumptions of this approach have been discussed in previous sections. Actual chloride concentration profiles can be measured on structures, to estimate parameters such as the diffusion coefficient used in the model. For carbonation, it has been proposed that the depth of carbonation is proportional to the square root of the exposure time. Again, the measurement of actual carbonation depth with time can be used to estimate a proportionality constant for a specific structure. 

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