Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 62

Strength capacity of different aggregates and different mixes varies
considerably. Some aggregates can be used to produce concretes of 50 MPa
and more, but 40 MPa is a more likely figure.
Shrinkage tends to be somewhat higher, and a higher cement content is
usually needed for a given strength. These are probably both for the same
reason. Lightweight aggregates will usually have a substantially lower
elastic modulus and will therefore tend to shed more stress into the sur-
rounding mortar.
The lighter kinds of lightweight concrete also use lightweight fines, but
this depends substantially on the type of lightweight fines available. It is
generally quite satisfactory to use any fines produced by a rotary kiln type
of process, although a proportion of sand will probably be needed to give
a suitable grading. However, fines produced by crushing lightweight mate-
rial are often unsatisfactory. Low density is often a matter of air voids
in the aggregate rather than a basic low-density material. As the mate-
rial is crushed finer, more voids are exposed to penetration by the cement
paste. There is a tendency to achieve little benefit in lighter concrete and
a substantial disadvantage by increasing water requirement. Much struc-
tural lightweight concrete uses natural sand as the whole or part of its fine
aggregate. Air entrainment often helps improve rheology as well as reduce
densit y.
Although a slightly higher fines content may be necessary, structural
lightweight concrete is generally amenable to a mix design process similar
to that for normal weight concrete. Sometimes it is better to use volume
batching for the lightweight material. This would apply where moisture
content will vary substantially. However it is generally a matter of using the
different specific gravity (SG) of the material in a similar design process.
The ConAd mixtune process described in the third edition can be used for
structural lightweight concrete. If so used, it is likely to require a “strength
factor” of less than one. The value may be of the order of 0.7 to 0.9 but
there are too many different kinds of such concrete to offer any useful
guide. A trial mix will provide a factor that may prove applicable to a range
of mixes using the same aggregate.
3.4 BLAST-FURNACE SLAG
The blast-furnace slag used as a concrete aggregate is quite different to
the ground-granulated blast-furnace slag (GGBS) as cement. It is the same
material in the molten state but has substantially different properties as a
result of the cooling process. For use as an aggregate, slag must be cooled
slowly to allow attainment of a crystalline state. The material is massive,
requiring crushing in the same manner as a natural rock. It is also vesic-
ular, usually to a sufficient extent to make it lighter, but not very much