concrete-mix design according to NAVFAC
In table 6-3, one of the formulas for 3,000 psi
concrete is 5.80 bags of cement per cubic yard, 233
pounds of sand (per bag of cement), 297 pounds of
coarse aggregate (per bag of cement), and a
water-cement ratio of 6.75 gallons of water to each
bag of cement. These proportions are based on the
assumption that the inert ingredients are in a saturated
surface-dry condition, meaning that they contain all
the water they are capable of absorbing, but no
additional free water over and above this amount.
We need to point out that a saturated surface-dry
condition almost never exists in the field. The
amount of free water in the coarse aggregate is
usually small enough to be ignored, but the ingredient
proportions set forth in the specs must almost always
be adjusted to allow for the existence of free water in
the fine aggregate. Furthermore, since free water in
the fine aggregate increases its measured volume or
weight over that of the sand itself, the specified
volume or weight of sand must be increased to offset
the volume or weight of the water in the sand.
Finally, the number of gallons of water used per sack
of cement must be reduced to allow for the free water
in the sand. The amount of water actually added at
the mixer must be the specified amount per sack, less
the amount of free water that is already in the
ingredients in the mixer.
Except as otherwise specified in the project
specifications, concrete is proportioned by weighing
and must conform to NAVFAC specifications. (See
table 6-3 for normal concrete.)
When tables, such as table 6-3, are not available
for determining quantities of material required for 1
cubic yard of concrete, a rule of thumb, known as rule
41 or 42, may be used for a rough estimation.
According to this rule, it takes either 41 or 42 cubic
feet of the combined dry amounts of cement, sand,
and aggregates to produce 1 cubic yard of mixed
concrete. Rule 41 is used to calculate the quantities of
material for concrete when the size of the coarse
aggregate is not over 1 inch. Rule 42 is used when the
size of the coarse aggregate is not over 2 1/2 inches.
Here is how it works.
As we mentioned earlier, a bag of cement
contains 94 pounds by weight, or about 1 cubic foot
by loose volume. A batch formula is usually based on
the number of bags of cement used in the mixing
For estimating the amount of dry materials
needed to mix 1 cubic yard of concrete, rules 41 and
42 work in the same manner. The decision on which
rule to use depends upon the size of the aggregate.
Lets say your specifications call for a 1:2:4 mix with
2-inch coarse aggregates, which means you use rule
42, First, add 1:2:4, which gives you 7. Then
compute your material requirements as follows:
Adding your total dry materials, 6 + 12 + 24= 42,
so your calculations are correct.
Frequently, you will have to convert volumes in
cubic feet to weights in pounds. In converting,
multiply the required cubic feet of cement by 94 since
1 cubic foot, or 1 standard bag of cement, weighs 94
pounds. When using rule 41 for coarse aggregates,
multiply the quantity of coarse gravel in cubic feet by
105 since the average weight of dry-compacted fine
aggregate or gravel is 105 pounds per cubic feet. By
rule 42, however, multiply the cubic feet of rock
(1-inch-size coarse aggregate) by 100 since the
average dry-compacted weight of this rock is 100
pounds per cubic foot.
A handling-loss factor is added in ordering
materials for jobs.
An additional 5 percent of
materials is added for jobs requiring 200 or more
cubic yards of concrete, and 10 percent is added for
smaller jobs. This loss factor is based on material
estimates after the requirements have been calculated.
Additional loss factors may be added where
conditions indicate the necessity for excessive
handling of materials before batching.
The water-measuring controls on a machine
concrete mixer are described later in this chapter.
Water measurement for hand mixing can be done with
a 14-quart bucket, marked off on the inside in gallons,
half-gallons, and quarter-gallons.
Never add water to the mix without carefully
measuring the water, and always remember that the
amount of water actually placed in the mix varies
according to the amount of free water that is already
in the aggregate. This means that if the aggregate is