Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 69

danger because the modifications may result in an unforeseen incompatibility
in the concrete. One of the reasons for ongoing modifications of admixtures
without changing the product name has been to avoid the requirement for
additional standard or compliance testing. The test requirements to comply
with ASTM C494, for example, involve extensive testing of up to one year,
especially if freeze-thaw testing is required. Recently AS 1478 removed
the requirement for one-year compressive strength testing, but the range
of test is still extensive and testing drying shrinkage is expensive and time
consuming. The standards also suffer from nominated test mixtures that
may be profoundly different from the actual application. It is no wonder
that suppliers would want to avoid unnecessarily conducting such tests,
but there is the danger of a significant problem occurring because of an
unforeseen incompatibility or negative reaction. If it is accepted that trial
mixes may be inaccurate and that other user's production results may not
be applicable, the only remaining practical selection basis is an extended
parallel trial. This may be simply a matter of using the admixture on trial
in one or two trucks per day and always testing these trucks. Over a period
it will be accurately seen whether there is any significant advantage from
using the new admixture. It may be considered necessary, for a short initial
period, to supply the special trucks to a noncritical location or for a use for
which a lower grade has been specified. There are many cubic metres of
blinding concrete for different projects on which we have worked that have
much higher quality control testing and performance than anticipated in
the specification. There is nothing like building up data on delivered con-
crete for noncritical locations to give one confidence that the performance
is adequate.
The problem of optimising and adjusting mixes is further complicated
by requirements for the approval of mix designs by statutory bodies and
specifiers. Theoretically any change in mix proportions or ingredients may
require additional trial mixes and testing. Sometimes this includes chloride
diffusion limits, which can take months to measure. All parties involved
hope that the proposed mix passes or the whole process will need to be
repeated. Accordingly, the current system, which attempts to keep control
over deviation, actually prevents appropriate modification of aggregate
proportions to maintain the grading curve as discussed in the quality
control section (Chapter 10) and the use of newer admixture technology
that may have significant advantages to avoid the painful testing protocol.
In Australia, which has a proud heritage in the concrete industry, there
are many examples of obsolete admixtures and inefficient mix designs
being used to avoid going through the bureaucracy of getting more suitable
admixtures and mixes approved.
In presenting the theme report on production of HSC/HPC at BHP 96, the
Paris symposium, Day (1996) remarked that of the more than 20 submitted
papers included in his report only 1 specifically dealt with a superplasticiser,