When batch operations are conducted at night,
good lighting is a must.
Personnel working in batch plants should use
good eye hygiene. Continual neglect of eye care
can have serious consequences.
LEARNING OBJECTIVE: Upon completing
this section, you should be able to determine
methods and mixing times of concrete.
Concrete is mixed either by hand or machine. No
matter which method is used, you must follow well-
established procedures if you expect finished concrete
of good quality. An oversight in proper concrete
mixing, whether through lack of competence or
inattention to detail, cannot be corrected later.
MIXING BY HAND
A batch to be hand mixed by a couple of
crewmembers should not be much larger than 1 cubic
yard. The equipment required consists of a watertight
metal or wooden platform, two shovels, a metal-lined
measuring box, and a graduated bucket for
measuring the water.
The mixing platform does not need to be made of
expensive materials. It can be an abandoned concrete
slab or concrete parking lot that can be cleaned after
use. A wooden platform having tight joints to prevent
the loss of paste may be used. Whichever surface is
used, you should ensure that it is cleaned prior to use
Lets say your batch consists of two bags of
cement, 5.5 cubic feet of sand, and 6.4 cubic feet of
coarse aggregate. Mix the sand and cement together
first, using the following procedure:
1. Dump 3 cubic feet of sand on the platform
first, spread it out in a layer, and dump a
bag of cement over it.
2. Spread out the cement and dump the rest of
the sand (2.5 cubic feet) over it.
3. Dump the second sack of cement on top of
This use of alternate layers of sand and cement
reduces the amount of shoveling required for
Personnel doing the mixing should face each other
from opposite sides of the pile and work from the
outside to the center. They should turn the mixture
as many times as is necessary to produce a uniform
color throughout. When the cement and sand are
completely mixed, the pile should be leveled off and
the coarse material added and mixed by the same
The pile should next be troughed in the center.
The mixing water, after being carefully measured,
should be poured into the trough. The dry materials
should then be turned into the water, with great care
taken to ensure that none of the water escapes. When
all the water has been absorbed, the mixing should
continue until the mix is of a uniform consistency.
Four complete turnings are usually required.
MIXING BY MACHINE
The size of a concrete mixer is designated by its
rated capacity. As we mentioned earlier, the capacity
is expressed in terms of the volume of mixed concrete,
not of dry ingredients the machine can mix in a
single batch. Rated capacities run from as small as 2
cubic feet to as large as 7 cubic yards (189 cubic feet).
In the Naval Construction Forces (NCFs), the most
commonly used mixer is the self-contained Model 16-
S (figure 6-6) with a capacity of 16 cubic feet (plus a
Figure 6-6.-Model 16-S concrete mixer.