the manufacturer of the mixer recommends the optimum speed at which the
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 time affects
variability as well as the strength of the mix.
tests demonstrated that mixing for
less than 1 to 1 ¼ minutes produces an
appreciably more variable concrete. However, if the mixing time is
more than 2 minutes no significant improvement in uniformity occurs.
tests demonstrated that average
strength of concrete also increases with
increase in mixing time. But rate of increase decreases rapidly beyond
minute and is not significant beyond 2 minutes. Sometimes,
slight decrease in
strength has also been observed.
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.
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
to a capacity of 1 yd3 (i.e 0.8m3) and 3 ¼
minutes for mixer capacity of 10 yd3 (i.e
High speed pan mixers, the mixing time can be as short as 35 sec.