Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 35

What matters to the eventual owner of the concrete structure is not the
time aggregate itself but the resulting concrete. Essentially this means that a
technically satisfactory natural or manufactured sand can be defined as one
that enables the production of satisfactory concrete. The required concrete
properties should be fully specified by the purchaser and the sand proper-
ties should be at the discretion of the concrete producer. The same situation
applies to coarse aggregates, but it is easier to justify with fine aggregates
because the effects of a substandard fine aggregate tend to be more imme-
diately experienced, especially in fresh concrete. Such effects may include
retarded set, increased bleeding, excessive air entrainment, poor workabil-
ity, and increased water requirement, the last leading to increased shrink-
age and extra cost.
Seven features of a fine aggregate affect its suitability as a concrete
aggregate:
1. Grading
2. Particle shape and surface texture
3. Clay/silt/dust content
4. Chemical impurities
5. Presence of mechanically weak particles
6. Water absorption
7. Mica content
Any of these, with the possible exception of being porous of low density,
can have such serious effects on concrete as to preclude the use of the aggre-
gate. However, this discussion will concentrate on grading, with comments
on other features. This is partly because the topic's views on the other six
features are not significantly different to those of many others, whereas
the treatment of grading is original and has permitted the use of sands
considered not economically useable by others.
Much of the material in this chapter was presented in a paper titled
“Marginal Sands” presented at an American Concrete Institute (ACI) con-
vention in San Antonio, Texas, in March 1987 (and available on the website).
3.1.2 Grading
Grading is frequently regarded as the main feature of a fine aggregate, and
the feature that often stops a particular sand being exploited. Although not
all gradings are equally suitable for the production of concrete, there is no
one ideal grading. Over the wide range of gradings that may be encountered,
differences can be compensated by adjusting the percentage of fine aggre-
gate in the aggregate combination without any need for additional cement
The basic concept is to use a smaller amount of a finer sand so as to leave
unchanged both the water requirement and the cohesiveness of the mix. In any