Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 49

30

C/F 100/0

28

C/F 80/20

26

C/F 65/35

24

C/F 50/50

22

C/F 0/100

20

43 44 45

Percent Voids

46 47

Figure 3.7 Blends of a coarse and a fine sand.

both water demand and specific surface are also related. Figure 3.6 shows
“most likely” relationships based on limited data. Individual instances may
not agree closely with the relationships shown, and the pattern itself can be
expected to change, though perhaps not greatly, should more data become
available.

3.1.4 Clay, silt, or dust content
Day's system does not provide for the incorporation of the effect of mate-
rial finer than a 75 micron (200 mesh) on his “specific surface” (it is
counted the same as material passing the 150 micron [100 mesh] sieve
and retained on the 75 micron sieve). This is for the same reason that
the effect of angular grains is not incorporated, that is, it does affect the
water requirement but it does not justify an offsetting reduction in the
proportion of the fine aggregate. A subsidiary reason is that the increase
is not solely dependent on the weight of such material but also on its
character.
It is arguable whether the 75 micron (200 mesh) sieve is worthwhile
for checking fine aggregates for concrete. Certainly it is important how
much of such material there is in the aggregate, but the percentage by
weight gives only half the story and dry sieving rarely removes all such
material. Some materials, such as the montmorillonite (smectite) clay in
sand extracted in Singapore, can have three times as much effect per
unit of weight as other fines such as fine crusher dust also passing the 75
micron sieve.
The definitive test for this property is undoubtedly the French “Valeur
de Bleu” (Bertrandy, 1982). This test involves titrating wash water from the