Civil Engineering Reference
In-Depth Information
Figure 3.3
Sand flow cone apparatus.
It is often a satisfactory arrangement to use a combination of crusher
fines and natural sand. Ken has formed an opinion (rather than definitely
established) that there tends to be more benefit than expected from such a
combination (see Figure 3.3).
Apart from gradings often fitting well together (crusher fines tending to
be deficient in middle sizes and natural sand to have an excess) a small pro-
portion of a fine, rounded, natural sand appears to have a disproportionate
effect on reducing any negative effects of angularity. Also the first 2% or
so by weight of silt in a fine aggregate appears not to be deleterious so that
halving the amount of a silty sand will more than halve the water increasing
effect of its silt.
Air entrainment and crusher fines should be approached with a little more
caution. Trial mixes will very clearly show a significant advantage for air
entrainment. However stone dust inhibits air entrainment and, if its propor-
tion varies, can result in a high variability of air content, which may be unac-
ceptable in practice. Note that fly ash (pfa) gives a similar effect on workability
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