Civil Engineering Reference
In-Depth Information
anode to the cathode. The overall effect of reactions and migration is that
hydroxyl and sodium ions accumulate at the cathode and become depleted
at the anode.
Diffusion
The sodium and hydroxyl gradients due to depletion and accumulation at
the electrodes will cause diffusion of these species from elsewhere in the
concrete towards the anode (and from the cathode to elsewhere). Diffusion
is driven by a concentration gradient and not by electrical potential gradient.
As a first approach, electrical potential differences are also assumed not to
influence diffusion properties of the species involved. Consequently, the
net acid production is the sum of the hydroxyl consumption mitigated by
migration and diffusion of hydroxyl ions to the anode. The diffusion flux
can be estimated from the diffusion coefficients of the ions in concrete and
the concentration gradient near the anode. The NaOH diffusion flux, with a
diffusion coefficient in the range of 1 to 10 * 10
-12
m
2
/s and a concentration
gradient of about 0.1 mole/l over a few centimetres (e.g. the cover depth),
is about 10
-8
mole/s.m
2
. For normal CP current densities (1-10 mA/m
2
),
diffusion has the same order of magnitude as ionic migration and the net
acid production is close to zero. For significantly higher current densities the
relative contribution of diffusion is smaller, so the net acid production will
be higher.
The reduction of the OH consumption by migration and diffusion can
be characterised by a net acid degradation factor, NADF, which is the
ratio between the theoretical oxidation due to electrolysis at the anode
J
(acid,electrolysis) and the net oxidation reduced by migration and diffusion
J
(acid,net); the net acid production flux becomes:
J
(acid,net) =
J
(acid,electrolysis) * NADF
(6.8)
Coupled migration and diffusion
Recently, new insights into transport of ionic species during CP have become
available from numerical modelling based on coupling migration, diffusion
and electrode reactions (Peelen et al., 2008). It appears that in the early
life of the anode, steady state concentrations of all ionic species in the bulk
of the concrete are obtained. This suggests that only transport of Ca and
OH ions affects the acidification rate at the anode. Sodium and all other
species not electrochemically active are merely re-distributed in the concrete
until their gradients are such that migration and diffusion are in perfect
balance (causing zero net transport). Further evaluation and validation of
these numerical results are needed before quantitative results can be used
with confidence.
