|
Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > How Much Cement?
Q: How much cement is needed for a small concrete mixture?
I am considering mixing my own concrete for a small
project. How much cement, coarse aggregate and sand are needed to
produce a cubic yard of concrete; how do I calculate the volume
of concrete material required for the project; and when does hand
mixing become impractical compared to ready mixed concrete delivery?
A: The actual cement
required for concrete is affected by the choice of the maximum coarse
aggregate size. As the maximum coarse aggregate size is decreased
the cement content must increase to provide the required quantity
of cement paste to coat all of the aggregate particles. The table
gives some guidance for the proper quantities by volume of the separate
components of a concrete mixture.
Table: Proportions by Volume of Concrete for Small Jobs
| Maximum-size coarse aggregate |
Cement |
Sand (wet) |
Coarse aggregate (wet) |
Water |
| 3/8" |
1 |
2 1/2 |
1 1/2 |
1/2 |
| 1/2" |
1 |
2 1/2 |
2 |
1/2 |
| 3/4" |
1 |
2 1/2 |
2 1/2 |
1/2 |
| 1" |
1 |
2 1/2 |
2 3/4 |
1/2 |
| 1 1/2" |
1 |
2 1/2 |
3 |
1/2 |
These proportions are only a guide and may need adjustments
to obtain a workable mix with locally available aggregates (PCA
1988). Packaged, combined, dry concrete ingredients (ASTM C 387)
are also available.
More information is available in Concrete
for Small Jobs (IS174).
Example: Assume that the proportions
will be made based on one part being a cubic foot (this is convenient
since a 94 lb. bag of cement is 1 cubic foot of bulk material).
For a 3/4" maximum coarse aggregate, the mixture would be 1
part portland cement, 2 1/2 parts sand,
2 1/2 parts coarse aggregate, and 1/2 part water. The sum of the
parts is 6 ½. In general the final volume of concrete produced will
be approximately 2/3 of the sum of all the volumes included in the
mixture. Therefore the approximate volume of this concrete mixture
is 4.3 cubic feet. There are 27 cubic feet in a cubic yard so we
divide 27 by 4.3 giving 6.2 batches for a cubic yard. Plan on making
7 batches to assure that you will have more than enough material
at the start of the job.
In short, use the mixture indicated for the 3/4" aggregate
and use 7 bags of cement, 17.5 cubic feet of sand, and 17.5 cubic
feet of ¾ in. coarse aggregate unless you feel comfortable
recalculating for a different size coarse aggregate.
The volume calculation for concrete is based on length x width x
height to determine the cubic feet of concrete required for the
project. EX: 20’ * 12’ * 0.5’ = 120 cubic feet
(be sure you always convert all of you measurements into feet units).
This quantity is divided by 27 (there are 27 cubic feet in a cubic
yard). 120 / 27 = 4.44 cubic yards. A word of caution, an increase
in thickness of 1/2" would increase the required concrete by
.37 cubic yards. Be sure that you order a little more material than
you need.
Typically it is impractical to use bagged materials or hand mix
concrete if the quantity exceeds one to two cubic yards because
bagged materials and hand mixing require you to handle the materials
several times. A cubic yard of concrete will weigh almost two tons
and handling the material three to four times to transport and mix
the material requires considerable labor. Hand mixing two cubic
yards of concrete is the equivalent of handling 12 to 16 tons of
material. As an alternative ready mixed concrete can be delivered
and simply be unloaded from the truck to its final position using
the chute on the truck or in some instances the concrete is discharged
into wheel barrows simplifying the concrete placement. Most ready
mixed concrete producers have minimum requirements for yardage,
and short loads may include additional charges to offset delivery
costs. Concrete is sold in increments of 1/4 yard so your concrete
order for the preceding example would be 4.5 cubic yards. Again,
be sure to order a little more than you need since charges for short
loads to correct this kind of mistake can be very expensive.
|