Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 43

air entrainment can both increase the cohesion of a harsh mix and lubri-
cate a sticky mix since these are virtually positive and negative effects on
the same property of the concrete. Viscosity modifying admixtures (VMA)
have a similar effect to increasing the fines content. The interaction of the
polymer chains can achieve similar cohesion. These admixtures are another
important tool in the use of marginal aggregate gradings.
3.1.3 Particle shape
The third edition of this topic strongly supported the use of natural
sand over manufactured sand. However, the availability of suitable
natural sands is diminishing and concerns regarding the environmen-
tal impact of their use rising. Indeed Europe is poised to outlaw the
use of natural sand. We have seen that a fine sand has a higher water
requirement but, over a wide range, it can simply be used in smaller
proportion to give a normal water requirement. An angular sand, or
especially crusher fines, also has a higher water requirement for a given
grading. However, this does not justify a reduction in its proportion (it
may even justify a small increase, thus further increasing water require-
ment, but this is fine-tuning to a precision more than required by a rela-
tively simple system). There can be an increase in the water requirement
of the mix, which may lead to an additional cost in cement or admix-
ture when an angular fine aggregate is used. Measuring workability will
indicate higher water demand. It is important to differentiate between
true water demand, and admixture demand, as the latter can be a major
influence.
• A coarse grade of crusher fines may be needed to fill the gap between
the top of a fine sand grading and the bottom of the coarse aggregate
grading. This may be essential to provide pumpability or to avoid
segregation where high workability is necessary.
• It should be remembered that a higher water requirement is not purely
an economic disadvantage. It also may result in increased shrinkage
and so may be unacceptable for some purposes even if it is the most
economical way of providing the required strength.
• There may normally be a distinct difference in colour between a
crusher fines mix and a natural sand mix. One or the other may there-
fore be architecturally either preferred or rejected for exposed archi-
tectural concrete.
• There may be a substantial difference one way or the other (depend-
ing on actual gradings) in bleeding characteristics, which may have a
substantial effect on surface appearance (coarse crusher fines being
particularly susceptible to bleeding but fine dust inhibiting it).