Specification - Concrete Mix Design, Quality Control and Specification - page 124

Faced with absolute performance limits suppliers have tended to signifi-
cantly overdesign their concrete mixtures to help ensure compliance, which
increases production cost with unknown benefit in terms of durability
enhancement. The use of additional cementitious material to achieve
certain performance limits at early ages may sometimes have a detrimental
effect on fresh and hardened properties and increase the environmental
impact of the concrete.
An unexpected consequence of the increase in performance specification
has been the submission of inappropriate concrete mixtures just because
the necessary test data is available so that the producer did not have to
conduct additional trial mixes and long/expensive testing. This would
be analogous to requiring a range of complex tests such as creep on all
concrete mixes. The concrete industry came to accept that compressive
strength was a key parameter and easy to measure, and this provided an
indicator of other mechanical and deformation properties. Based on the
work by Ho and colleagues, the Australian standard has tried to use com-
pressive strength as an indicator of durability as well but that has been a
bridge too far.
What we need is more real field data on the actual performance of
concrete in aggressive environments related to their early age proper-
ties as well as further work on simple and inexpensive early-age tests
for chloride penetrability and water transport. There have been many
test procedures proposed for this purpose. We would suggest that the
best contenders would be absorption for physical salt attack, desorption
for water transport, and chloride migration for chloride penetration
with a simple resistivity test providing the continued compliance test.
The STADIUM program from North America has done a good job of
relating field performance to early age properties producing arguably the
most comprehensive service life prediction model available. The early-age
testing involves permeable voids, desorption, and chloride migration cou-
pled with petrographic analysis to confirm chemical properties. However,
there are still assumptions regarding the expected exposure conditions
and improvement of penetrability with time, but it is a step in the right
direction. Not all projects are going to conduct a detailed assessment
of service life but simple/cheap compliance tests based on resistivity and
desorptivity could easily be added to compressive strength to provide
much more information on the concrete's potential durability. When tests
are cheap and simple, accumulating statistical data is easy and producers
would be encouraged to get to understand how to optimise their mixes
rather than the current situation of sticking to a mix because it has a
compliant diffusion coefficient.
An appropriate performance-based specification for durability in a
chloride environment could be based on chloride migration testing over
a period up to 90 days to estimate the chloride penetration resistance and