Chloride ingress rates initiation

For chloride ingress life is more complex as we are dealing with atmospherically exposed structures that are exposed to variable chloride contents. Also, we are not dealing with a front that moves through the concrete but a chloride profile that builds up in the concrete. 

The usual form of the diffusion equation used is Pick s second law  

In atmospherically exposed concrete there is no easy initial or surface concentration as the chlorides at the surface can vary from zero to 100% depending upon wetting, drying, evaporation, wash off, etc. It is therefore common practice to discard the first 5-10 mm of a chloride profile sample and take the next increment, around the 10 mm depth, as a constant initial, pseudo surface concentration. If this is done then diffusion calculations must use the depth from the sampling depth, not from the surface. 

A simplified method of calculating the initiation time for chloride attack is to look at the progress of the chloride threshold through the concrete. By taking samples with depth it is possible to fit a parabolic curve to the chloride concentration (or more simply to fit a straight line to a plot of depth vs. the square root of chloride concentration) and to find the depth 

However, an additional complication comes from the observation that the diffusion constant changes with time. It is therefore important to only take measurements on mature concrete as the diffusion rate may be higher in the first few years. 

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