Mix design - Concrete Mix Design, Quality Control and Specification - page 200

3. Use of fly ash, which has 37% greater volume than an equal weight of
cement (if in an area where fly ash is inexpensive, more might be used
than strictly necessary for strength).
4. Use a proportion of GGBS or of superfine calcium carbonate.
5. Use of air entrainment (as valuable, volume for volume, as cement for
this purpose).
6. If no alternative is less expensive, the use of more cement than neces-
sary on strength grounds would certainly solve the problem since it
both reduces the sand percentage required for a given MSF and pro-
vides more paste to fill the sand voids. However “cement” these days,
as noted earlier, is likely to be a composite material.
Extreme testing of the fine limit has also occurred. In 1956 (Day, 1959)
a case was encountered where the sand percentage calculated by Day's sys-
tem came to 15% (virtually all the sand passed the 300 micron [No. 50
ASTM] sieve). It proved possible to obtain a ¼ inch (7 mm) single-sized
crushed rock and the concrete was made with 10% of this material and
15% of sand (the balance being 75 percent of an almost single sized 20 mm
[3/4 inch] crushed rock).
During the early development of the system (in the early 1950s in England)
sand percentages of 22% to 23% were used, but although the sand was
purchased as “plastering sand” rather than “concreting sand”, this was an
example of the use of a very low “MSF” on earth dry concrete rather than
the use of a very fine sand. It should always be possible to use a propor-
tion of crushed fines (choosing a coarse variety) when the natural sand is
too fine for use alone. However, the particle shape of the crushed fines will
increase water requirement, and therefore increase cement requirement, at
least somewhat.
In selecting all constituent materials for concrete it is particularly impor-
tant to take into account consistency of supply. Any variation in the char-
acteristics of a material is likely to cause variation in the resulting concrete,
unless the concrete producer has been given advance warning and been able
to make a compensating mix change. It is bad enough that the concrete
producer has to maintain skilled staff able to make rapid mix adjustment,
worse still if he has to be able to detect change without advice from his
supplier, but worst of all if change is undetected or inadequately compen-
sated for increasing variability and so requiring a higher mean quality, pre-
sumably at higher cost. Hopefully the adjustment will be timely enough to
avoid failures or penalties.
Day's SS of an aggregate differed from true specific surface because
he recognised that as the particle size of the finer sand/aggregate frac-
tion reduced a stage would be reached where less water was required to
fill the voids in the mass of aggregate than to provide a surface coating
of water. It is now clear that it is not sufficient to recognise the assumed