Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 103

A better way to calculate ionic diffusion is the extended Nernst-Planck
model, which can be used to describe the flux of multiple ionic species
(Samson and Marchand, 1999).
5.3.8 Chloride migration
Chloride migration involves accelerating chloride ion movement by sub-
jecting the concrete specimen to a potential difference. Chloride migration
can be tested by both steady-state and non-steady-state methods similar
to diffusion. Some concrete technologists dismiss electrical acceleration
because it is unnatural. However, ionic movement is necessarily “electrical”
because ions are charged particles. Tang and Sorensen (2001) showed that
non-steady-state chloride migration (NT Build 492) was directly correlated
with the bulk diffusion test, provided the relative maturity was considered.
A fundamental tenet of this topic is concrete quality control and timely
response to variability. Any rapid and relatively inexpensive test that gives
a good indication of an important transport like chloride diffusion is
extremely valuable and should not be lightly dismissed.
5.3.9 Resistivity
Resistivity is the resistance of the concrete to the flow of electrical current.
It is the reciprocal of conductivity. Resistivity is dependent on the size and
tortuosity of the pore system as well as the conductivity of the pore solu-
tion. As corrosion of reinforcement is an electrochemical process, its rate
will be strongly influenced by the resistivity of the surrounding concrete.
Therefore, the resistivity of the concrete not only provides an indication of
the penetrability of the concrete but also the rate of corrosion after depas-
sivation has occurred. The most commonly used test for resistivity is the
ASTM C1202 or Coulomb test. This is often misleadingly called the rapid
chloride permeability test and is discussed in Chapter 7. This standard test
takes 6 hours to complete, but the real advantage of resistivity is that it can
be measured virtually instantly.
Because resistivity is influenced by both pore structure and the compo-
sition of the pore solution, it cannot be used to directly measure either.
Pozzolanic materials that react with calcium hydroxide will greatly
increase resistivity by both refining the pore structure and reducing the
conductivity of the pore solution, particularly highly reactive materials
such as silica fume and rice husk ash. Certain admixtures that contain
salts, such as calcium nitrite, reduce resistivity primarily by increasing free
ions in the pore solution. Hydrophobic admixtures and surface treatments
may profoundly increase in situ resistivity by reducing the internal moisture
content and the water filled pathways within the concrete. In summary,
resistivity is a very important parameter.