Specification - Concrete Mix Design, Quality Control and Specification - page 126

6.9 DEVELOPMENT OF STANDARD MIXES
Specifications have tended to assume that the concrete supplier will design
a special mix to comply with the specification. This may be necessary in
relatively rare cases, but it does have some disadvantages:
1. No history of previous satisfactory performance on actual projects.
2. No common pool of test results with same mix on other projects.
3. Truck drivers less familiar with mix, less able to judge workability
and detect abnormality.
4. Variability may be increased if every now and then the standard mix
is supplied in error.
It might be reasonable to provide a financial advantage to suppliers who
have satisfactory standard mixes in use, under routine control and with a
range of properties established. The form of encouragement could be to
allow a reduced testing frequency for such mixes and to require pretesting,
and a higher testing frequency for the first months, of new mixes.
The aforementioned points apply even for major projects, but their
importance is far greater for the many “ordinary” projects that probably
account for most of concrete produced. Small projects cannot economically
generate sufficient test data to maintain good control. This means that they
are essentially dependent upon the producer's quality assurance system. In
such circumstances it is counterproductive to specify nonstandard mixes
unless absolutely essential. It is possible that a very small project could
nevertheless derive great advantage from the use of 100+ MPa concrete in
a particular column or involve a single wall of exposed aggregate concrete
of super critical appearance. In such circumstances special mixes are obvi-
ously involved and control costs are of little importance. However, a refusal
to accept a standard mix for a 25 MPa internal floor slab would be justified
only if the standard mix were distinctly unsatisfactory.
The specifier should generally concentrate on obtaining full information,
both past and current, about standard mixes. The aim should be to check
that the supplier's control system is working well rather than to supplant it.
These remarks are relevant when only compressive strength is regarded as
important. This chapter deals with requirements other than strength, and
the importance of using standard mixes of established performance is much
greater in respect of such requirements.
A time is coming when it may be less essential to use standard mixes.
The control system pioneered by Day enables results from many grades to
be combined onto a single control graph. The performance of mixes may be
seen in terms of factors in mix design equations rather than a stand-alone
assessment. The same situation has been encountered in many different
industries (Toffler, 1981). Initially, mass production requires acceptance of