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Strictly speaking, the criterion of adequate mixing is
“number of revolutions of the mixer” & not the “time of mixing”.
Since
the manufacturer of the mixer recommends the optimum speed at which the mixer
should run, “number of revolutions of the mixer” has a correlation with the
“time of mixing”. As such, “time of mixing” is used as a criterion for adequate
mixing.
Mixing time affects variability
as well as the strength of the mix.
Shalon’s
tests demonstrated that mixing for less than 1 to 1 ¼ minutes produces an
appreciably more variable concrete. However, if the mixing time is prolonged to
more than 2 minutes no significant improvement in uniformity occurs.
Abrams’
tests demonstrated that average strength of concrete also increases with
increase in mixing time. But rate of increase decreases rapidly beyond about 1
minute and is not significant beyond 2 minutes. Sometimes, slight decrease in
strength has also been observed.
Therefore,
mixing beyond a certain period offers no advantages & becomes a waste of
time & money. The concrete should be mixed for such duration so as to yield
optimum benefit.
It
should be noted that minimum mixing time depends on the type of mixer as well
as on the size of mixture. For example, for usual mixers ACI & ASTM C94-78a
recommends minimum mixing times of 1 min for mixers up to a capacity of 1yd3
(i.e 0.8m3) and 3 ¼ minutes for mixer capacity of 10yd3 (i.e
7.6m3).
For
High speed pan mixers, the mixing time can be as short as 35sec.
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