Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 36

particular case, the optimum fine aggregate percentage is not solely a matter of
its grading. Other factors influencing the ideal percentage include cementitious
content, entrained air content, particle shape and grading of the coarse aggre-
gate, and also the intended use of the concrete. It is assumed that the actual
grading of the fine aggregate will only influence the percentage of it to be used
and have no other influence on concrete properties. Although this is the case
over a wide range, there must be limits to its applicability. It is necessary to be
very clear where the limits are and what happens if they are exceeded.
Chapter 8 includes a thorough examination of the coarse and fine limits
on the usability of a sand and on the selection of the most advantageous
combination of two fine aggregates.
Grading indices
There has always been an attraction in representing a fine aggregate grad-
ing by a single number to describe its performance in concrete. This would
avoid the problem of fine aggregate gradings straying into two different
zones and would permit adjustment of sand percentages on a continuous
scale rather than three large steps.
The original, and perhaps most widely known and used grading index
is the fineness modulus (FM). This is the sum of the cumulative percent-
ages retained on each sieve from 150 micron upward. This index is used in
the ACI mix design system to adjust for sand fineness. However, it is used
to indicate adjustment steps rather than to give continuous adjustment in
a formula.
A problem with using a single value to express something like a grading is
that a single value can represent many variations that affect performance. For
example, the ACI concrete mix design method proportions the sand volume
in a concrete mixture by way of the voids in the coarse aggregate and the sand
FM, with FM representing a grading. However, in the majority of cases,
expressing the performance of a sand by characterising it using FM alone is
meaningless. Many sands will have the same FM and, as you may have expe-
rienced, will perform in different ways in the plastic concrete (see Figure 3.2).
By the same token, the coarse aggregate voids factor is influenced by
several variables, including particle size distribution, particle shape, and
particle surface texture. Because two void contents in coarse aggregates are
the same will not result in the same performance of a concrete mix design
using those two aggregates, nor will the optimal content of each of those
two aggregates be the same. The same applies to particle size distribution
or grading of aggregates, fine or coarse.
The specific surface (SS) is the surface area per unit solid volume (some-
times per unit mass is used). This is difficult to measure directly but may
be estimated from measured or assumed values of specific surface for each
individual sieve fraction in a manner similar to fineness modulus. If dealing