Mix design - Concrete Mix Design, Quality Control and Specification - page 201

reduction in water requirement for this reason, because the finer particles
can actually displace water from between cement particles, reducing
water requirement in the same way as does fine fly ash, calcium carbon-
ate, and so on.
Apart from the increased range of supplementary cementitious materi-
als, the other major change has been an increased range of more power-
ful water-reducing admixtures, to the extent that any degree of fluidity
can be produced with almost any selected quantity of water. Therefore an
increased MSF (i.e., fines content) no longer has to be paid for in increased
water and cementitious content, although the admixture cost will increase.
Especially where sand/fine aggregate is less expensive than coarse aggre-
gate, sand contents unthinkably high on the old basis can now be used with
no concern that fresh or hardened properties, including strength, perme-
ability, and shrinkage, may be affected.
A new aspect of mix design is attention to the elimination of internal
voids. The initial voids figure can be determined by subtracting the volume
of the coarse and fine aggregates from one cubic metre. This volume then
desirably has to be filled, perhaps with a small excess, by the combined vol-
ume of water, cement, entrained air, admixture, and fine material such as
fly ash, GGBFS, silica fume, rice husk ash, or superfine calcium carbonate.
If there is an inadequate volume of such superfine material, it may be neces-
sary to include a VMA (viscosity modifying admixture) to avoid bleeding
and segregation, especially where high workability or good pumpability
are required.
Since the last edition Day has designed only one mix and it is worth
briefly recounting. Using the University of Texas ICAR program recom-
mendations (incorporating the aforementioned concepts), Day took the
aggregates of an existing normal workability mix and in a site laboratory
increased sand content in small increments until a minimum voids content
was found at a little under 50% sand (bulk density less the weights of
coarse and fine aggregates each divided by their SG gives %voids). He then
reduced the water content from 170 to 150 liters/m3 and added superfine
limestone until the volume of cement plus fly ash plus limestone plus water
equaled the volume of voids in the aggregates. Finally a high-range water
reducer was added to provide very satisfactory, fully self-compacting con-
crete (SCC), concrete of slightly higher strength and lower shrinkage than
the original mix. OK, you can do that, but you will not necessarily have the
most economical solution.
There is now a large variety of cement replacement and other fine materi-
als, and there may be variation in characteristics and quality of the same
material from different suppliers. Other than in broad principle, as set out
by Barry Hudson, or in the detail involved in Francois de Larrard's treatise
(1999), such variations are not amenable to theoretical or numerical mix
design and their relative merits must be established by trial mixes.