Mix design - Concrete Mix Design, Quality Control and Specification - page 197

where
SS = Modified specific surface of combined coarse and fine aggregates
EC = Equivalent cement content (see later)
8.1.2 Effect of particle shape
An intrinsic assumption in SS mix proportioning is that a finer sand will
cause less disruption to the packing of the coarse aggregate, permitting a
reduction in sand percentage. It is not necessarily obvious that this reduc-
tion is exactly the same as the reduction needed to maintain the same com-
bined specific surface of the combined aggregates but this seems to work in
practice. A more angular particle shape of the coarse aggregate also causes
an increased requirement for sand, since it increases the percentage voids
in the coarse aggregate to be filled by mortar. An increase of up to 3 in the
appropriate MSF may be needed depending on the degree of angularity
(which has a larger effect than flakiness or elongation). The actual surface
area of both coarse and fine aggregates is obviously increased by a more
angular particle shape at a given grading. However, whereas an increased
fineness of a sand can be fully compensated by reducing its percentage (so
there is no increase in water requirement), this is not so for a more angular
fine aggregate since it does not reduce the interference with coarse aggre-
gate packing, and may even increase it. So the angularity of the fine aggre-
gate is neglected in determining the percentage to be used, but the predicted
water requirement may increase by 5% to 15%.
Specific surface cannot be the only criterion for mix proportioning
because it does not take into account particle shape and provides no assur-
ance of continuity in the grading, which may be needed to avoid segre-
gation and achieve pumpability. This is the aspect better covered by the
void-filling theories, but Day believes he achieves a simpler and more work-
able solution by using crude, semiempirical corrections for these purposes.
8.1.3 Grading continuity
In the past, a great deal of research effort has gone into the search for an
ideal aggregate grading. This has been to some extent pointless because,
even if it exists, such a grading may be impossible or too expensive to attain
with the materials available. One still sees requirements for sand grading to
be within certain limits (particularly in the United States), but the move to
abolish them is gaining momentum.
However, it is undeniable that gaps in an aggregate grading, while they
may make the concrete easier to compact under vibration there is an increase
in the tendency of the concrete to segregate. Resistance to segregation is
vital in higher slump and pumped concrete. Gaps (differences) in excess
of 4% to 5% between the percentage retained on consecutive sieves finer