Figure 4-4. Charging mixer with mechanical skip.
Discharging the mixer.
When the mix
is ready for discharge from
move the discharge
into place to receive the concrete from the drum. Concrete that is somewhat dry tends to cling to the top
of the drum and not drop onto the chute in time. Very wet concrete may not carry up high enough on the
drum to drop onto the chute. You can correct these problems by adjusting the mixer speed. Increase the
speed for very wet concrete and decrease the speed for dry concrete.
Mixing time. The mixing time starts when water runs into the dry mixture, which should be during the
first quarter of the mixing period. The minimum mixing time per batch of concrete is 1 minute, unless the
batch exceeds 1 cubic yard. Each additional 1 cubic yard of concrete, or fraction thereof requires an
additional 15 seconds of mixing time.
Cleaning and maintaining the mixer. Clean the mixer daily if it operates continuously or following each
period of use if it operates less than 1 day. The exterior cleaning process goes faster if you coat the
outside of the mixer with form oil before you use it. Knock off all accumulated concrete on the mixer
exterior and wash it down with a hose. Mixer blades that are worn or coated with hardened concrete
provide less efficient mixing action. Replace badly worn blades, and do not allow hardened concrete to
accumulate in the mixer drum. Clean it out whenever you shut it down for more than 1 1/2 hours. To do
this, place a volume of coarse aggregate equal to one-half the mixer capacity in the drum and allow it to
revolve for about 5 minutes. Then discharge the aggregate and flush out the drum with water. Never
strike the discharge chute, drum shell, or skip to remove aggregate or hardened concrete because concrete
adheres more readily to dents and bumps.
Operating the M919 Concrete Mobile Mixer Unit
The concrete mobile mixer unit is a combination material transporter and on-site mixing plant. Table 4-2 on page
4-10 gives its physical characteristics and overall dimensions. The special body is mounted on an M900 truck
chassis. The unit carries enough unmixed material to produce up to 8 cubic yards of fresh concrete. Because the
unit is precisely calibrated, you can produce mixes that meet or exceed both the ACI and American Association of
State Highway and Transportation Officials' (AASHTO) standards for design strength. The unit operates on
either an intermittent or continuous basis, although continuous operation depends on raw material availability at
the site. Certain control settings for the mix operations vary from truck to truck and from site to site.