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It
is well known now that both classes of fly ash (viz. Class C &
Class F)
improve the properties of concrete, but several factors and cautions
should be
considered when using fly ashes especially in highway construction,
where fly
ash is heavily used. In a report prepared by the Virginia
Highway and Transportation
Research Council (VHTRC) and summarized by Halstead (1986), several
restraints
relating to the use of fly ash concrete for construction of highways
and other
highway structures were discussed. These restraints include the
following:
- Special
precautions may be necessary to ensure that the proper amount of
entrained air
is present.
- Not
all fly ashes have sufficient pozzolanic activity to provide good
results in
concrete.
- Suitable
fly ashes are not always available near the construction site, and
transportation costs may nullify any cost advantage.
- Mix
proportions might have to be modified for any chance in the fly ash
composition.
Since
the cement-fly ash reaction is influenced by the properties of the
cement, it
is important for the executing agency not only to test and approve each
fly ash
source but also to investigate the properties of the specific fly
ash-cement
combination to be used for each project.
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