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Recommended Concrete Design
Mix
BuildBlock recommends the use of a 3/8" chip or
rock mix 3000 PSI or stronger concrete mix for your walls. Your
ready-mix plant will most likely have a mix design for a 3/8"
chip mix and will refer to it as a pump mix. There are several
alternations you can make to the recipe of concrete to combat
weather occurrences, such as fly ash and air entrainment. Avoid
adding calcium to your mixes as it has a negative impact on the
rebar reinforcement. If pouring in cold weather, use hot water
to formulate the mix.
Below is an
example of a typical 3000 PSI - 3/8" chip mix design:
Entrained Air**** (for
workability): 6% or 1 oz. per bag cement/fly ash
7.
Water Reducer: Encouraged
8.
Slump = 5-1/2" to 6" out of the pump
(Proper slump is very important. Do not use less than
5-1/2" slump concrete.)
Depending on the type of material and individual
gradation, these ratios may have to be adjusted. Consult with
your local ready-mix supplier. The pump may be the controlling
factor (for example, new pump vs. old pump, boom pump vs.
trailer pump, etc.).
To test slump, take
a 5-gallon sample and make five 4" diameter by 8" high
cylinders. Your mix design should yield 3000 PSI at a
designed slump of 5-1/2" to 6" to pour properly. (If not
conveyed properly to your concrete company, they may bring out a
3000 PSI mix with a 4" slump design. If you wet it to pour, your
concrete will not be 3000 PSI strength.)
NOTES
*
The use of Fly Ash
improves the flowability of the concrete and reduces the
amount of Portland cement required. This saves you money
and results in a concrete mix which is more "green" in
terms of LEED points.
**
The maximum aggregate
size for 6" block is 1/2" (3/8" is recommended). The
maximum aggregate size for 8" block is 3/4" (1/2" or
3/8" is recommended.) The larger the aggregate, the more
problems you will have with concrete flow.
***
FM = Fineness Modulus
for sand.
****
6% Entrained Air results
in better flowing concrete.
