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NEWSLETTER |
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A.
Transit Mixed (or "truck-mixed") Concrete
In transit-mixed concrete, also called
truck mixed or dry-batched, all of the raw ingredients
are charged directly in the truck mixer. Most or all
water is usually batched at the plant. The mixer drum is
turned at charging (fast) speed during the loading of the
materials. There are three options for truck mixed
concrete:
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Concrete mixed at the
job site. While travelling to the job site the drum
is turned at agitating speed (slow speed). After
arriving at the job site, the concrete is completely
mixed. The drum is then turned for 70 to 100
revolutions, or about five minutes, at mixing speed.
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Concrete mixed in the
yard. The drum is turned at high speed or 12-15 rpm
for 50 revolutions. This allows a quick check of the
batch. The concrete is then agitated slowly while
driving to the job site.
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Concrete mixed in transit.
The drum is turned at medium speed or about 8 rpm for
70 revolutions while driving to the job site. The drum
is then slowed to agitating speed. (More information on
ready mixed concrete trucks can be found in the
Delivery section.)
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B. Shrink Mixed
Concrete
Concrete
that is partially mixed in a plant mixer and then
discharged into the drum of the truck mixer for
completion of the mixing is called shrink mixed concrete.
Central mixing plants that include a stationary,
plant-mounted mixer are often actually used to shrink
mix, or partially mix the concrete. The amount of mixing
that is needed in the truck mixer varies in these
applications and should be determined via mixer
uniformity tests. Generally, about thirty turns in the
truck drum, or about two minutes at mixing speed, is
sufficient to completely mix shrink-mixed concrete.
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C. Central Mixed Concrete
Central-mixing concrete batch plants include a
stationary, plant-mounted mixer that mixes the
concrete before it is discharged into a truck mixer.
Central-mix plants are sometimes referred to as wet batch
or pre-mix plants. The truck mixer is used primarily as
an agitating haul unit at a central mix operation. Dump
trucks or other non-agitating units are sometimes be used
for low slump and mass concrete pours supplied by central
mix plants. About 20% of the concrete plants in the US
use a central mixer. Principal advantages include:
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Faster production capability than
a transit-mix plant
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Improved concrete quality control
and consistency and
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Reduced wear on the truck mixer
drums.
There are
several types of plant mixers, including:
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