Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 71

that most concrete producers should be seeking the ideal admixture supplier
rather than the ideal admixture because the correct advice may be more
important than the best admixture.
4.1 SPECIFYING ADMIXTURE USAGE
Concrete users should avoid specifying the use of particular admix-
tures unless absolutely essential for a particular purpose. If they do so, it
should only be after the premix supplier has satisfied itself that the other
performance properties are not affected, otherwise the responsibility of
the concrete supplier for the performance of the concrete will be substan-
tially reduced and any and every problem encountered will in some way be
blamed on the specified admixture. As far as possible the concrete supplier
must be left to formulate its concrete and this should include the use of its
choice of admixtures. Where a particular admixture is considered essential,
this should be discussed with the concrete supplier and an attempt made
to have him use it of his own volition. If it became normal to impose the
concrete user's choice of admixture on the concrete producer, this would
sabotage his entire control system, as results could not be grouped together
for analysis.
As with other aspects of mix design, the purchaser should be entitled to
know what is being used in his concrete and to have the right of objecting to
unsatisfactory proposals. In general, this right should not be used lightly. The
purchaser should certainly refuse permission to use admixtures containing
any significant amount of chloride in concrete to contain reinforcement
or water resisting admixtures or durability enhancing admixtures with no
history. This is because unsatisfactory long-term performance may result.
Where resistance to freezing and thawing or salt scaling is required, the
purchaser should certainly specify that air entrainment be provided. It may
also be reasonable to object to an air entrainer that produces too large a
bubble size and has an unsatisfactory spacing factor. This is because it is the
spacing of the air bubbles that matters for frost resistance, whereas the total
volume is what is measured by all typical tests and what affects the strength
of the concrete. Until recently the spacing could only be determined by
microscopic examination of a cut-and-polished face of hardened concrete.
Now the air void analyser (AVA) enables air void parameters to be measured
in 25 minutes or less on fresh concrete.
The AVA is a great piece of equipment but not all sites will have one
and therefore air content will be the most common method used. Research
by U.S. Federal Highway Administration (FHWA) shows that there can
be significant differences in freeze-thaw resistance depending on the
air-entraining admixture used.