Mix design - Concrete Mix Design, Quality Control and Specification - page 208

Table 8.4 Typical contribution of aggregate properties to
the performance of concrete (Kaplan)
Property of
concrete

Shape Property of

aggregate texture

Modulus of
elasticity

Flexural 31 26 43

Compressive 22 44 34

1. Size

2. Grading

3. Shape

4. Surface texture

5. Coatings

All these characteristics play a significant role in how a solid will behave
in concrete, both in its fresh and hardened states. Table 8.4 shows the
relative effect of different aggregate properties on flexural and compres-
sive strength of concrete. Each of the characteristics influences a particles
specific surface, but expressing the aggregates specific surface by a simple
voids number is meaningless as that single number (voids) can be arrived at
by many combinations of the individual characteristics.
Because coarse aggregates are being used as inert fillers in concrete, we
suggest that getting a coarse aggregate that has the lowest void content (maxi-
mum packing density) and that has the lowest cost. A good range of packing
density (voids content) for a coarse aggregate is 34% to 39% (tested to ASTM
C1252 the test method). Of course, it is also important to take into account
other properties such as maximum particle size, durability criteria, and any
other constraint that may be specified for that aggregate. The aggregate also
has to be a competent material that is fit for purpose. If an aggregate is already
in commercial use as a concrete aggregate, you can operate under the assump-
tion that the aggregate satisfies these requirements. If in doubt, however, have
a reputable laboratory check the aggregate for suitability.
All blending of aggregates should be done on a volumetric basis unless
the aggregates being blended have the same specific gravity.
To improve the packing density of an individual aggregate will typically
require reprocessing the aggregate by some method, typically screening
or crushing the aggregate through a crusher that can improve the particle
shape. Especially in coarse aggregates, the two major influencers of pack-
ing density are both aggregate-process related. The two properties are
particle shape and particle size distribution. The more equidimensional
(closer to a sphere) a particle is, the higher its potential packing density
will be (the lower the voids content). With particle size distribution (or
grading) having an “even” grading will not result in the highest particle
packing. Actually, a grading or particle size distribution where there are