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:
|With Fly Ash||Without Fly Ash|
|1.||Cement Content||5.5 bags (517lbs.)||6.5 bags (611lbs.)|
|2.||Fly Ash* (Class C)||1.5 bags (141 lbs.)||0|
**:ASTM C-33 #8
100% passing 1/2″ screen
85-100% passing 3/8″ screen
|1270 lbs.||1270 lbs.|
|4.||Fine Aggregate: ASTMC C-33
(Sand – FM*** 3.70)
|1620 lbs.||1620 lbs.|
|5.||Water||40 – 46 gallons||40 – 46 gallons|
|6.||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.)
*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.