samples may be taken from the batch immediately
after depositing it on the subgrade. At least five
samples should be taken at different times, and these
samples should be thoroughly mixed to form the test
When making a slump test, dampen the cone and
place it on a flat, moist, nonabsorbent surface, From
the sample of concrete obtained, immediately fill the
cone in three layers, each approximately one-third the
volume of the cone. In placing each scoop full of
concrete in the cone, move the scoop around the edge
of the cone as the concrete slides from the scoop.
This ensures symmetrical distribution of concrete
within the cone. Each layer is then rodded in with
The strokes should be distributed
uniformly over the cross section of the cone and
penetrate into the underlying layer. The bottom layer
should be rodded throughout its depth.
If the cone becomes overfilled, use a straightedge
to strike off the excess concrete flush with the top.
The cone should be immediately removed from the
concrete by raising it carefully in a vertical direction.
The slump should be measured immediately after
removing the cone.
You determine the slump by
measuring the difference between the height of the
cone and the height of the specimen (figure 6-4). The
slump should be recorded in terms of inches of
subsidence of the specimen during the test.
After completing the slump measurement, gently
tap the side of the mix with the tamping rod. The
behavior of the concrete under this treatment is a
valuable indication of the cohesiveness, workability,
and placability of the mix. In a well-proportioned
mix, tapping only causes it to slump lower. It doesnt
crumble apart or segregate by the dropping of larger
aggregate particles to a lower level in the mix. If the
concrete crumbles apart, it is oversanded. If it
segregates, it is undersanded.
A mix must be workable enough to fill the form
spaces completely, with the assistance of a reasonable
amount of shoveling, spading, and vibrating. Since a
fluid or runny mix does this more readily than a dry
or stiff mix, you can see that workability varies
directly with fluidity.
The workability of a mix is
determined by the slump test. The amount of the
slump, in inches, is the measure of the concretes
workabilitythe more the slump, the higher the
The slump can be controlled by a change in any
one or all of the following: gradation of aggregates,
proportion of aggregates, or moisture content. If the
moisture content is too high, you should add more
cement to maintain the proper water-cement ratio.
The desired degree of workability is attained by
running a series of trial batches, using various
amounts of fine to coarse aggregate, until a batch is
produced that has the desired slump. Once the
amount of increase or decrease in fines required to
produce the desired slump is determined, the
aggregate proportions, not the water proportion,
should be altered in the field mix to conform. If the
water proportion were changed, the water-cement
ratio would be upset.
Never yield to the temptation to add more water
without making the corresponding adjustment in the
cement content. Also, make sure that crewmembers
who are spreading a stiff mix by hand do not ease
their labors by this method without telling you.
As you gain experience, you will discover that
adjustments in workability can be made by making
very minor changes in the amount of fine or coarse
aggregate. Generally, everything else remaining
equal, an increase in the proportion of fines stiffens a
mix, whereas an increase in the proportion of coarse
loosens a mix.
Before you alter the proportions set forth
in a specification, you must find out from
higher authority whether you are allowed to
make any such alterations and, if you are, the
permissible limits beyond which you must not
As previously mentioned, concrete consists of
four essential ingredients: water, cement, sand, and
coarse aggregate. The same mixture without
aggregate is mortar. Mortar, which is used chiefly for
bonding masonry units together, is discussed in a later
Grout refers to a water-cement mixture
called neat-cement grout and to a water-sand-cement
mixture called sand-cement grout. Both mixtures are
used to plug holes or cracks in concrete, to seal joints,
to fill spaces between machinery bedplates and
concrete foundations, and for similar plugging or
sealing purposes. The consistency of grout may range
from stiff (about 4 gallons of water per sack of