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
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
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
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)