is 10 minutes, 70 divided by 10 equals 7. With this
drum speed, the load will arrive on the jobsite with
exactly 70 turns at mixing speed with no overmixing
of the concrete mix and no unnecessary wear on the
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
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 compacted 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) is not exceeded,
and (4) concrete is remixed for at least half the
minimum required mixing time or number of
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.
Mobile Concrete Mixer Plant
The trailer-mounted mobile concrete mixer plant
(fig. 3-16) carries 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 accurately
proportioned by weight and simultaneously dropped
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.
The action of this combined auger and paddle
homogenizer mixes the ingredients and water rapidly,
thoroughly, and continuously to produce a continuous
flow of uniformed quality concrete. The material
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 job
Operators assigned to the crete mobile must
thoroughly read and understand the technical manual
before operating the plant.
The following are the mobile concrete mixer plant
cautions and warnings:
Follow all preventive maintenance procedures.
Do not allow any foreign matter in the cement
Do not allow particles larger than 1 1/2 inch in
the aggregate bin.
Do not allow the waterlines and flowmeters to
freeze with water in them.
Do not run the water pump dry.
Do not continue to operate the machine if the
hydraulic oil temperature exceeds 190°F.
Wash out the auger within 20 minutes of the last
Never attempt to operate the unit while in
Never attempt to repair the machine while in
operation (always turn the power source off).
Keep your entire body clear from all moving
Never attempt to walk on top of the aggregate
bin to cross from the cement bin to the water tank
( u s e t h e l a d d e r ) .
Never walk or stand under the auger.
Never climb inside the aggregate bin (use a small
pole to dislodge any aggregate that has bridged).
Never enter the cement bin while in operation
(there are moving parts inside the bin).