Testing - Concrete Mix Design, Quality Control and Specification - page 184

two orders of magnitude less than from a bulk diffusion test. This appears
to be due to the presence of other ions in seawater.
This field exposure trial included concretes with both low sorptivity
incorporating hydrophobic admixtures and low diffusivity incorporating
supplementary cementing materials. The results indicate that diffusivity is
the dominant transport property under conditions of frequent wetting and
limited drying.
7.6.7 Chloride migration
Because of the time to conduct a chloride diffusion test, there has been a great
deal of attention to electrically accelerating chloride penetration. Nordtest NT
Build 492 non-steady state chloride migration test adapts the ASTM C1202
resistivity test to rapidly measure chloride penetration. A 50 mm thick speci-
men is exposed to a potential difference depending on its resistivity. After a
prescribed period, generally only one day, the specimen is split and sprayed
with silver nitrate solution to determine the chloride penetration. Therefore it
is quick as well as avoids the laborious profile grinding and chemical analysis
necessary for the NT Build 443 or ASTM C1556 procedures.
Tang and Sorensen (2001) found that the chloride migration procedure has
a repeatability coefficient of variation in the range of 5% to approximately
9%, and reproducibility in the range of 12 to approximately 24%. They con-
clude that; 'This test is therefore a good alternative method due to its simplic-
ity, rapidity, good precision and fairly comparable results with the NT BUILD
443 test”. As the migration test can be done very rapidly, it will generally
give higher migration values than the diffusion test due to the greater aver-
age maturity of the test specimen in the latter. Tests conducted on concrete
in Australia after 28 days curing showed the migration coefficient (NT Build
492) was roughly 70% greater than the diffusion coefficient (NT Build 443).
Some concrete technologists are passionately against electrical acceler-
ation on the basis that it is totally artificial. However, ions are charged
particles and therefore their electrical interactions and necessary charge
balance is part of their movement. We would suggest that both diffusion
and migration are measured before construction to determine performance
and migration (or even resistivity) should be used for ongoing compliance.
Audenaert et al. (2010) measured chloride migration at 28, 56, and 90
days up to 5 years to determine time dependent effect. During verification
testing, the authors would suggest that chloride migration testing between
say 7 - 90 days might help eliminate the need for prescriptive cementitious
material limits.
7.6.8 Resistivity
Resistivity can be easily and quickly measured on concrete. It is strongly
influenced by moisture content and therefore the most popular resistivity