|
Chloride
ingress into concrete is generally held to follow Fick's
second law of diffusion, forming a chloride profile
with depth into the concrete.
where
[Cl- ] is the chloride concentration, at depth x and
time t and Dc is the diffusion coefficient (usually
of the order of 10-8 cm2.s-1). The solution to the differential
equation for chlorides diffusing in from a surface is:
where
Cmax is the surface or near surface concentration Cx,t
is the chloride concentration at depth x at time t Cmin
is the background chloride concentration erf is the
error function
Cmax
must be constant which is why a near surface measurement
is used, to avoid fluctuations in surface levels on
wetted and dried surfaces.
An
alternative to the error function calculation is to
calculate the progress of a threshold chloride concentration
as a parabolic function of time (8, 9), using a similar
function to that used for the carbonation front progression.
Carbonation
rates generally follow parabolic kinetics:
where
d = carbonation depth, t = time and A is a constant,
generally of the order 0.25 to 1.0 mm.year-½
Carbonation
rates are a function of the environment where indoor
concrete will carbonate faster than outdoor concrete
in North American and Northern European environments,
but wet/dry cycling in warm conditions can accelerate
carbonation in more southerly latitudes. As corrosion
will not proceed in the absence of water, carbonation
rates are usually unimportant inside buildings except
in bathrooms, kitchens and other situations where there
is wet/dry cycling and sufficient moisture to cause
corrosion after carbonation to rebar depth.
The
actual corrosion rate can be measured using the techniques
discussed in Corrosion Rate Measurement or in Corrosion
Monitoring.
There is far more detail on these and other techniques in the latest edition of my book. Details for ordering are on the home page of the website.
What
we can do for you
- Review the condition
of your structure
- Recommend repair
options
- Provide designs
and performance specifications
- Provide lists of
qualified and experienced contractors
- Provide corrosion
monitoring after treatment
|
