Civil Engineering Reference
In-Depth Information
30.0
FM = 3.0
25.0
20.0
15.0
10.0
5.0
0.0
4.75
2.36
1.18
0.600
0.300
0.150
Sieve Sizes (mm)
Figure 3.2
Illustration showing that multiple particle size distributions will have the
same FM.
with perfect spheres, halving the diameter roughly doubles the surface area
per unit weight (1/6 πd
3
vs. πd
2
). This simple assumption gives a reason-
able index for aggregate proportioning. Yet what is really required is a
prediction of the water requirement associated with a given amount of the
fine aggregate, and cohesiveness conferred on the mixture of fine aggregate
and cement paste. In general, greater surface area increases both the water
requirement and the cohesiveness of the mixture. However the effect of the
finer sieve fractions on water requirement is not as great as the surface area
suggests (Day, 1959).
Table 3.1 (Popovics, 1982) sets out 10 factors for the numerical char-
acterisation of individual sieve fractions. Ken Day's modified specific sur-
face has been added to form an 11th column (the origin of Day's values is
explained in Chapter 8). Some of these factors have been used as a basis
for selecting the relative proportions of fine and coarse aggregates, some to
calculate water requirement, and some (including Day's) for both of these
purposes.
Popovics (1992) also sets out 26 formulas, 12 of which were developed
by himself, for the calculation of water requirement. Some of the formulas
are quite complex and tedious to evaluate, but this would be no disadvan-
tage if the formula were included as part of a computer program. However,
only a dedicated research worker could consider the time and effort that
would be involved in examining the relative merits of the 26, or even the
12, formulas over a range of actual mix data.
No doubt each proponent of a system (including Day's) considers his
own system quite simple to use. It is not proposed to examine all the
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