Mix design - Concrete Mix Design, Quality Control and Specification - page 203

The objective when making these vanilla or normal concretes (15-30
MPa, 50-150 mm slump) is typically to meet some type of compressive
strength requirement; to have an agreed workability; be accepted by the
pump/placing contractor; and not to bleed, segregate, shrink, and so forth.
Other constraints can be placed on the material proportions that are
eventually used in production. These typically the cost of materials and,
increasingly, the reduction in energy or the CO 2 emissions required to make
the concrete.
As mentioned at the beginning of this chapter, in reality the range of the
various material proportions to be used in concrete is relatively narrow.
And there are very few materials to proportion.

8.3 PROPORTIONING AGGREGATES

The aggregate fraction of the concrete mix will have two distinct fractions:
(1) fine aggregates or sand (usually aggregates smaller than 5 mm), and (2)
coarse aggregates (greater than 5 mm). Rather than performance or practi-
cal considerations, the main criteria in selecting these sands and aggregates
are usually to conform to a prescribed grading or particle size distribution
specification. This requirement is usually regardless of whether the aggre-
gate is natural sand and gravel, or a quarried material such as limestone,
granite, or basalt (see Table 8.3). Coarse and fine aggregates will make up
by far the majority of the volume of a concrete (between 70% and 90% of
the total volume). There is a lot of debate on how to proportion coarse and
fine aggregates in a concrete mix, and also within themselves. For example,
should the coarse aggregates be proportioned to get a continuous grad-
ing curve? Or, should they be proportioned to have minimum voids (voids
being the measure of air in a given bulk volume of aggregates)? Should the
sand be proportioned to achieve a target FM (see Chapter 3, Figure 3.2) or

Table 8.3 Influence of aggregate shape characteristics on hardened performance of
concrete

Compressive
strength (MPa)

Flexural
strength (MPa)

Angularity
number Aggregate Flakiness Elongation

Natural
quartzite
gravel

40.7 3.65 8 24 1

Crushed
limestone

44.8 5.48 26 33 7

Crushed
basalt

47.6 5.10 31 42 9

Crushed
flint gravel

37.8 4.55 34 42 9