Recommended Concrete Design Mix

The concrete design mix is very important when pouring an ICF project. Not only is the strength of the concrete important, but also the slump when the concrete arrives on the job site and the style of hose used when pumping.

Recommended Mix

The strength of the concrete should be a minimum of 3,000psi. This ensures the concrete is strong enough to carry the loads of the structure based on the BuildBlock Prescriptive Engineering. Greater strength concrete can be used as needed for specific project considerations or specifications.

The slump of the concrete is a measure of the liquidity of the concrete. The greater the slump, the more wet or liquid the mix is. The greater the liquidity the greater pressure inside the forms. ICFs should be poured at a 5.5″-6″ slump. Be cautious of adding water to the mix onsite as it will weaken the concrete strength. And if too wet can cause the aggregate to fall to the bottom of the wall. 

An ICF wall should be pumped using a 5″ hose with a reducer to 3″. The keeps the flow of concrete steady and if a blockage occurs reduces the force of the concrete. It’s important that the low remain steady to avoid bursts of concrete. A flat hose such as a mud snake can also be used and is common. When ordering concrete make sure to notify the redi-mix company that it is for an ICF project.

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 entrained air . 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
3. Coarse Aggregate
**:
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.)

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 the concrete flow.

***FM = Fineness Modulus for sand.

****6% Entrained Air results in better flowing concrete.

Interested in learning more about building with ICFs? Register and enroll in BuildBlock’s free online ICF installation training course!

BuildBlock Concrete Design Mix July 1, 2013