Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 105

more extensively dealt with in Chapter 7. The subject is only briefly
covered here.
Apart from slump, workability affects some or all of mobility, fluidity,
pumpability, compactability, and, negatively, segregation and bleeding.
A factor other than water content is clearly involved and this is best
described as cohesion. Cohesion may be physically evaluated in terms of
resistance to segregation and bleeding but a numerical measure is needed
for use in mix design. Ken Day developed the term MSF (mix suitability
factor). This factor is derived from the overall mix specific surface adjusted
for the content of cementitious material and entrained air, all of which
increase cohesion.
The use of rheometers to measure the yield strength and plastic viscosity
of concrete is taking over from traditional testing and traditional charac-
terisation in the laboratory, but their use rarely extends to the field, and
these are measured parameters rather than something calculable from
gradings and mix proportions. So they are to date a means of establishing
whether the desired concrete properties have been achieved rather than a
means of calculating how to achieve them, although this may change in
future.
MSF is certainly a big advance on characterising mixes only by slump
and a verbal description such as pump, structural, or paving mix. However,
it is not sufficient alone to cope with the 'new' material, self-compacting
or flowing concrete. Even normal pumped concrete needs a measure of
grading continuity and bleed resistance. The latter is a matter of having
sufficient fine material (at least passing a 200 sieve) or using a suitable
chemical admixture such as a VMA.
5.4.2.1 Slump
Although Chapters 8 and 10 use slump as a measure of relative workability,
it is important to realise that this is a matter of convenience and that the
slump test is a very poor measure of the relative workability of different
mixes. One reason for retaining slump as a criterion is that it is so deeply
ingrained in the theory and practice of concrete technology. Another is
that slump in combination with Day's MSF does have a little more validity
as an absolute criterion than slump alone. A third, and probably the most
important, is that it is a useful detector of a change in of water content
between successive deliveries of the same concrete mix within particular
workability ranges.
What is important is not to stop using the slump test, but to realise
and allow for its limitations. For example, a limiting slump value is
often included in a job specification. With few exceptions, this is not
the best way to achieve the specifier's objective. First, there should be
an objective for the specification of anything, rather than it having