Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 42

as a sand having an SS of 38.85 (or, say, 40, or 34 to 44); with Zone 2
being, say, 48, or 44 to 52; Zone 3 being 56, or 52 to 60; and Zone 4 being
64, or 60 to 70.
It has been contended that, to a very large extent, only the surface area
and not the detailed grading of a sand is of importance. This is not com-
pletely true in all cases and the following exceptions are noted:
1. The existence of gaps in the grading (i.e., the absence of some sieve
fractions) either between the fine aggregate and the coarse aggregate
or within the fine aggregate grading itself can give rise to:
a. Segregation at medium to high workability
b. Severe bleeding
c. Concrete that will not pump
d. Improved workability under vibration for low slump concrete
2. Sands that are almost single-sized can give rise to poor workability
through particle interference.
3. A proportion of large particles in an otherwise predominantly fine
sand can cause problems through interfering with the packing of the
coarse aggregate.
It is emphasised that these are rare exceptions, not glaring deficiencies in
the general assumption.
Admixtures
The use of admixtures can be of considerable assistance in solving grading
problems. Air entrainment is well known to have the capacity to inhibit
bleeding and to assist in overcoming problems of harshness with very
coarse or very angular fine aggregates. An unusual use for air entrainment
is worth recounting. The mix was specified not to contain any siliceous
aggregates (including natural sand) because it was to be used in the base of
a furnace. This left, as the only available fine aggregate, a crusher dust with
almost 20% passing a 150 μm sieve.
Day's system correctly predicted the proportion of this material that
would make reasonable concrete and correctly predicted its water require-
ment. However, especially since a high minimum cement content was also
specified, the mix was very sticky and difficult to handle from skips, even
though it compacted quite well. These days a superplasticising admixture
and a higher slump would probably be used, but this mix was encountered
before such admixtures were readily available in Australia and in any case
would have represented extra cost since the minimum allowable cement
content already provided excess strength. Instead, an air-entraining agent
was used and did produce a substantial improvement. It is interesting that