cement) to fluid (as many as 10 gallons of water per
sack of cement), depending upon the nature of the
grouting job at hand.
When bagged cement is used, the field mix
proportions are usually given in terms of designated
amounts of fine and coarse aggregate per bag (or per
94 pounds) of cement. The amount of material that is
mixed at a time is called a batch. The size of a batch
is usual] y designated by the number of bags of
cement it contains, such as a four-bag batch, a six-bag
batch, and so forth.
The process of weighing out or measuring out the
ingredients for a batch of concrete is called batching.
When mixing is to be done by hand, the size of the
batch depends upon the number of persons available
to turn it with hand tools. When mixing is to be done
by machine, the size of the batch depends upon the
rated capacity of the mixer. The rated capacity of a
mixer is given in terms of cubic feet of mixed
concrete, not of dry ingredients.
On large jobs, the aggregate is weighed out
in an aggregate batching plant (usually shortened
to batch plant), like the one shown in figure
6-5. Whenever possible, a batch plant is located near
to and used in conjunction with a crushing and
screening plant. In a crushing and screening
Figure 6-5.-Aggregate batching plant
plant, stone is crushed into various particle sizes,
which are then screened into separate piles. In a
screening plant, the aggregate in its natural state is
screened by sizes into separate piles.
The batch plant, which is usually portable and can
be taken apart and moved from site to site, is
generally set up adjacent to the pile of screened
aggregate. The plant may include separate hoppers
for several sizes of fine and coarse aggregates, or only
one hopper for fine aggregate and another for coarse
aggregate. It may have one or more divided hoppers,
each containing two or more separate compartments
for different sizes of aggregates.
compartment can be discharged into a weigh box,
which can, in turn, be discharged into a mixer or a
batch truck. When a specific weight of aggregate is
called for, the operator sets the weight on a beam
scale. The operator then opens the discharge chute on
the storage hopper. When the desired weight is
reached in the weigh box, the scale beam rises and
the operator closes the storage hopper discharge
chute. The operator then opens the weigh box
discharge chute, and the aggregate discharges into
the mixer or batch truck. Batch plant aggregate
storage hoppers are usually loaded with clamshell-
The following guidelines apply to the operation of
All personnel working in the batch plant area
should wear hard hats at all times.
While persons are working in conveyor line
areas, the switches and controls should be
secured and tagged so that no one can engage
them until all personnel are clear.
When hoppers are being loaded, personnel
should stay away from the area of falling
The scale operator should be the only person on
the scale platform during batching operations.
Housekeeping of the charging area is
important. Personnel should do everything
possible to keep the area clean and free of
spoiled material or overflow.
Debris in aggregate causes much of the damage
to conveyors. Keep the material clean at all