Figure 7-1.-Transit mixer.
equipment.
If
the
concrete
cannot
be
discharged
immediately,
the
operator
should turn the drum at the minimum
agitating
speed
of
2
revolutions
per
minute. When the transit mixer arrives at
the project having used the minimum amount
of mixing turns, the operator is able, if
necessary,
to
delay
discharging
the
concrete. Delay is limited by the maximum
of 300 rotations allowed.
Remixing Concrete
Concrete begins to stiffen as soon
as
the
cement
and
water
are
mixed.
However,
the
degree
of
stiffening
that
occurs in the first 30 minutes is not
usually a problem; concrete that is kept
agitated generally can be placed within 1
1/2 hours after mixing.
Fresh
concrete
left
to
agitate
in
the mixer drum may be used if upon
remixing
it
becomes
sufficiently
plastic
to be consolidated in the forms. Under
careful supervision a small amount of water
may
be
added
to
remix
the
concrete
provided the following conditions are met:
(1) maximum allowable water-cement ratio
is not exceeded, (2) maximum allowable
slump
is
not
exceeded,
(3)
maximum
allowable mixing and agitating time (or
drum revolutions) are not exceeded, and
(4) concrete is remixed for at least half
the minimum required mixing time or number
of
revolutions.
Adding too much water to make
concrete more fluid should not be allowed
because this lowers the quality of the
concrete. Remixed concrete can be expected
to harden quickly. Subsequently, a cold
joint may develop when concrete is placed
next to or above the remixed concrete.
Mixer Cleaning
After
the
load
of
concrete
is
discharged from the mixer, the operator
should wash off all excess concrete in the
mixer drum and blades, the discharge chute
opening, and the discharge chute before it
has a chance to harden. Spraying 15 to 25
gallons of water into the drum while it is
rotating will clean the inside of the drum
as well as remove all grout which may have
collected
in
the
water
nozzle
during
discharge. A washdown hose is provided on
the mixer to clean areas accessible from
the outside. A clean mixer produces a more
satisfactory
mixing
and
discharge
of
concrete.
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 flush water in the drum, rotate
the drum in the mixing direction for a
few
minutes,
then
discharge
the
flush
water at the maximum drum rpm. Complete
the
cleaning
of
the
outside
of
the
mixer,
particularly
around
the
discharge
end.
7-5
