Concrete future - Concrete Mix Design, Quality Control and Specification - page 280

or combinations of these. The ASTM C1202 or Coulomb test has been a
commonly specified procedure in different parts of the world. This pro-
cedure is a measurement of saturated resistivity and has been correlated
to chloride diffusion. Although the standard includes a rough guideline
for the interpretation of the coulomb values obtained, specifications often
require more onerous performance limits, which appear more related to
risk aversion than technical performance. Faced with onerous absolute
performance limits suppliers have tended to significantly overdesign their
concrete mixtures to help ensure compliance, which reduces sustainability
and increases production cost with unknown benefit in terms of durability
enhancement. The use of additional cementitious material to achieve cer-
tain performance limits at early ages may have a detrimental effect on fresh
and hardened properties. The test result can have quite high variability so
that individual results should not be specified as a rejection criterion for the
sampled concrete, rather a characteristic value based on statistical analysis
of results should be established.
Chloride diffusion is perhaps the most relevant test, but it is expensive
and time consuming to test and therefore not well suited for compliance
testing. Chloride migration is a much faster and cheaper procedure that still
measures chloride penetration. As mentioned in Chapter 7, the improve-
ment in chloride resistance of the concrete with time has been a difficult
area in performance specification. We would suggest that the best proce-
dure would be to conduct verification trials to determine the correlation
between chloride migration and resistivity for the proposed mix and deter-
mine the improvement with time. For compliance testing, measure resistiv-
ity frequently and migration occasionally to confirm adequate performance
based on service life predictions modeling using a characteristic value for
assessment.
What is needed are more field data on the actual performance of concrete
in aggressive environments related to its early-age properties to provide a
better technical basis for performance requirements. There are good service
life models that relate long-term field performance to early-age properties,
but not all projects are going to conduct a detailed assessment of service life.
However, simple and cheap compliance tests based on resistivity (for aggres-
sive environments) and desorptivity (for water transport) could easily be
added to compressive strength to provide much more information on the con-
crete's potential durability. When tests are cheap and simple, accumulating
statistical data is easy and producers would be encouraged to get to under-
stand how to optimise their mixes rather than the current situation of stick-
ing to a mix because it has a diffusion coefficient or other expensive test data.
Specifications for temperature rise and differentials in massive pours
require attention. A default peak temperature of 70°C is prudent as it
would virtually eliminate the possible problem of delayed ettringite forma-
tion (DEF). Although DEF is uncommon, it can cause enormous damage.