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What is the significance of Mixing time of concrete?

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 1 yd3 (i.e 0.8m3) and 3 ¼ minutes for mixer capacity of 10 yd3 (i.e 7.6m3).

For High speed pan mixers, the mixing time can be as short as 35 sec.