mixed in a continuous mixer and the mobile concrete
mixer (crete mobile) where weighing facilities are not
Specifications generally require that materials be
measured in individual batches within the following
percentages of accuracy: cement 1%, aggregate 2%,
water 1%, and air-entraining admixtures 3%.
The equipment used should be capable of
measuring quantities within these tolerances for the
smallest to the largest batch of concrete produced. The
accuracy of the batching equipment should be checked
periodically and adjusted when necessary.
Concrete should be mixed until it is uniform in
appearance and all the ingredients are evenly
distributed. Mixers should not be loaded above their
rated capacities and should be operated at
approximately the speeds for which they were
designed. If the blades of the mixer become worn or
coated with hardened concrete, the mixing action will
be less efficient. Worn blades should be replaced and
the hardened concrete removed periodically,
preferably after each production of concrete.
When a transit mixer (TM) (fig. 3-15) is used for
mixing concrete, 70 to 100 revolutions of the drum at
the rate of rotation designated by the manufacturer as
mixing speed are usually required to produce the
specified uniformity. No more than 100 revolutions at
mixing speed should be used. All revolutions after 100
should be at a rate of rotation, designated by the
manufacturer, as agitating speed. Agitating speed is
usually about 2 to 6 revolutions per minute, and mixing
speed is generally about 6 to 18 revolutions per minute.
Mixing for long periods of time at high speeds, about
1 or more hours, can result in concrete strength loss,
temperature rise, excessive loss of entrained air, and
accelerated slump loss.
Concrete, mixed in a transit mixer, should be
delivered and discharged within 1 1/2 hours or before
the drum has revolved 300 times after the introduction
of water to cement and aggregates or the cement to the
aggregates. Mixers and agitators should always be
operated within the limits of the volume and speed of
rotation designated by the equipment manufacturer.
OVERMIXING CONCRETE. Overmixing
concrete damages the quality of the concrete, tends to
grind the aggregate into smaller pieces, increases the
temperature of the mix, lowers the slump, decreases
air entrainment, and decreases the strength of the
concrete. Also, overmixing puts needless wear on the
drum and blades of the transit mixer.
To select the best mixing speed for a load of
concrete, estimate the travel time to the project (in
minutes) and divide this into the minimum desired
number of revolutions at mixing speed70. The results
will be the best drum speed; for instance, if the haul
Figure 3-15.Transit mixer.