Chloride penetration by diffusion 
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 Profile of chloride content

The chloride content at various depths (profile) in concrete, after a certain time of contact, is shown on figure a. As a rule, it deals with chlorides dissolved in water.

When the concrete undergoes alternate wetting and drying, the liquid, which contains chlorides, can penetrate with a convection rate B This case mainly corresponds to constructions exposed to salt spray. In Fick's law :

where C is the content chloride with a depth x and at the moment t, and D the difusion coefficient , the flux dC/dt is simply decreased by B dC/dx.

In addition, when the relative humidity of the atmosphere around concrete fluctuates, the concrete moisture is neither homogeneous, or stable. So, the heart of material remains saturated with water, while at the same time its water content changes with thickness (gradient) and time. This fact influences the penetration of chlorides dissolved in water.

More precisely, during concrete drying, water evaporates and chlorides remain in material either in form adsorbed on the pore walls, or by forming salt crystals. So, if concrete is again wetted, these salts are again dissolved in entering water. The profile of chloride content can thus differ from that of a chloride penetration under permanent immersion (figure b).

Profiles of the content chloride [ Cl ], in a concrete in the atmosphere
a) under constant relative moisture b) under wetting - drying cycles

 Predicting penetration

The chloride penetration is a diffusion process, when the concrete is saturated with water and when cement does not react too much with these salts. Any reinforcement is in a concrete practically saturated with water, when its cover is rather thick and is not too dry (e.g. structures in marine or mountain sites).

So, the prediction of chloride penetration in concrete, uses diffusion laws (FICK) which needs the diffusivity D (or diffusion coefficient) to be known.

This diffusion coefficient D of chlorides in concrete varies, when this material ages. But, from the practical point of view, this parameter can be considered as constant, especially when the concrete is more than six months old.

For an existing structure, the coefficient D is determined by using the experimental content chloride profile (in parts far away from the concrete facing).

But, it is also possible to take concrete samples (cores) in such structures and, as for a structure to be built, the coefficient D is determined by laboratory tests. These are diffusion tests, under steady (figure a) or unsteady not (figure b) regime, or migration tests of chlorides under an electric field (figure c).

Measuring the diffusion coefficient D of chlorides in concrete
a) diffusion under a steady regime, D is given by the amount of chlorides which cross the concrete specimen, into the initially " pure " solution
b) diffusion in non-steady regime, D is given starting by the profile of chloride content in thick concrete
c) migration under electric field, D is given under a steady regime, as for a)

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