Mix design - Concrete Mix Design, Quality Control and Specification - page 196

It was assumed at the time they were originated that these values would
require subsequent refinement, but in spite of attempts to improve them in
the laboratory, and by their use for production concrete in many countries,
the figures have remained substantially unchanged for 50 years. However,
it is now recognised that superfine aggregate particles can actually displace
water from between cement particles. This can only happen in the presence
of a HRWRA, as otherwise the fine particles will clump together and then
will increase water requirement.
It would be more correct to use surface area per unit solid volume than
per unit weight, but the weight basis was been retained because the actual
numbers were familiar to users of the original SS theory. For the same rea-
son, Day's original modified figures have been doubled so that the overall
combined aggregate SS is of the same order as the original. However, where
there is a large difference between the specific gravity (sg) of coarse and fine
aggregates an adjustment is desirable.
Modification of the basic SS values is not the only adjustment required to
make SS mix design work. Other factors to be taken into account include
the following:
1. The effect of cementitious materials and entrained air
2. The effect of particle shape
3. A requirement for continuity of grading
4. Limitation of fineness and coarseness of sand grading
Before discussing these points, some of the objectives of mix design
should be reviewed. Generally a sandier mix will have a higher degree
of cohesion and be easier to handle and place. However, it will have a
higher water requirement. Traditionally, water/cement (w/c) ratio has been
regarded as the best criterion of quality, so that a sandier mix will require
more cement and so be more expensive. Further investigation has shown
that additional water is more deleterious than less cement at a given w/c
ratio, increasing the desirability of minimising water requirement. So the
objective of mix design is to achieve acceptable fresh concrete properties at
minimum water content. With the advent of self-compacting concrete, the
task becomes even more critical.
8.1.1 Effect of cementitious materials
and entrained air
These materials increase cohesion and so reduce the required SS of the
aggregates. Ken coined the term MSF (Mix Suitability Factor) to represent
the combined effect of all constituents on cohesion. The formula is
MSF = SS + 0.025EC + 0.25(air % - 1) - 7.5