collection or diversion of wash water from
At the plant, flush a minimum of 150 to 250 gallons
of water, depending on the size of the mixer, into the
drum. With the flushed water in the drum, rotate the
drum in the mixing direction for a few minutes, then
discharge the flushed water at the maximum drum rpm.
Complete the cleaning of the mixer, particularly around
the discharge end.
Never pound the bottom of the drum to loosen
materials, since this procedure may cause dents and
bumps in which concrete and cement can stick. During
cold weather, the water tank, pump, and lines must be
drained to prevent possible damage from freezing.
The mixing blades inside the drum must be kept
clean and free of built-up concrete. If not cleaned
properly, the blades in the drum will wear down and this
can result in improper mixing. If this occurs, the blades
should be either changed or built up by using hardfacing
If a minor repair is required on a loaded transit
mixer, take the TM to the shop for a quick fix. If the
downtime is going to be for more than an hour, mix in
5 pounds of sugar or concrete retarder to keep the
concrete from setting up inside the truck.
NOTE: A small amount of sugar (5 pounds) acts as
a retarder; however, a large amount will act as an
If a quick fix is not possible, the concrete must be
removed as quickly as possible. Either check for a
hydraulic adapter, which can be hooked up to another
TM to operate the drum to discharge the concrete, or
remove the access hatch from the drum, roll the drum
until the access hatch is facing down, and washout the
concrete mix, if possible.
Like most construction equipment, transit mixers,
when not operated safely, can injure or kill personnel
and damage property. As a TM operator, keep in mind
the following guidelines:
l Transit mixers have a high center of gravity.
Their stability is further decreased by the weight of the
load. Use extreme caution when traveling over uneven
. Always use caution and a signalman when
backing on a jobsite.
l Remember to secure the discharge chute properly
with the chute locked to the right side of the truck for
. Make sure the mixer is stopped before making
l Observe environmental regulations concerning
disposal of waste and wash water from mixers.
. Avoid prolonged skin contact with concrete or
MOBILE CONCRETE MIXER PLANT
The mobile concrete mixer plant, sometimes called
a crete mobile, is a combination material transporter
and mobile concrete mixing plant (fig. 14-3). In the
NCF, the unit is mounted on a trailer that carries
sufficient unmixed materials, such as cement, sand,
coarse aggregates, water, and any chemicals required,
for special mix specifications to the jobsite.
The trailer-mounted crete mobile carries the
cement, sand, and coarse aggregates in divided bins,
mounted on the unit. The cement is carried in a separate
bin, located across the rear of the unit, and the sand and
aggregate are carried on each side of the unit. Water is
carried in a single tank, mounted in front of the
aggregate bins, and is pumped to the mix auger.
Sand and aggregates are proportioned accurately by
weight or volume and dropped simultaneously with a
mixture of cement from the material feed system into
the charging end of the mix auger/conveyor at the rear
of the unit. At this point, a predetermined amount of
water enters the mix auger. This action of the combined
auger and paddle homogenizer mixes the ingredients
and water rapidly, thoroughly, and continuously to
produce a continuous flow of uniformed quality
The mixing action is a continuous process that can
proceed until the aggregate bins are empty. On the
other hand, mixing and delivery may be stopped at any
time and then started again at the will of the operator.
This permits production to be balanced to the
demands of the placing and finishing crews and other
NOTE: When cleaning the crete mobile, you must
cycle the auger to remove all sand, aggregate, and
cement. Excess material in the bin and auger will
harden and freeze the components.