Home FAQs General Concrete Knowledge What is the formula for mixing concrete?
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What is the formula for mixing concrete?
Q:What is the formula for mixing concrete for a 28x32 foundation I have heard it is a 1 cement 3 sand and 3 aggregate, Is concrete weaker without the aggregate? The foundation will be supporting a 4 foot high slump block wall. I made the forms 16 inches deep and 16 inches wide and will be using rebar reinforcement. The rest of the building will be of wood framing. I do not have to worry about freezing as this is in southern Arizona. The reason I am asking is that I am mixing my own concrete in small batches.
A: The 'rule of thumb' mix design for concrete is 1 part cement, 2 parts sand, and 3 parts gravel or crushed stone. Leaving out the aggregate will definitely weaken your concrete mix. If you want to strengthen your mix, add additional cement without changing the proportion of sand and stone. Remember that the addition of excess water will also weaken your mixture, so don't get carried away with the water hose. It is impossible to address the suitability of this concrete mixture for your intended application without knowing more about your soil conditions and loading, so I would recommend that you contact a local engineering firm for this question.

Finally, you don't mention why you are mixing by hand, but this method usually will not save money compared to ordering from a ready-mix company unless you are in a very remote location ... and hand-mixing is very hard work. The other consideration is the quality of the concrete, and here again the ready-mix option is the best bet, due to much better proportioning of the ingredients and more thorough mixing.

Q:(FOLLOW UP) Thank you very much for the information on mixing concrete. I would certainly use ready-mix but I am at a remote location and a ready-mix truck could not possible get back here.
A: Here is a little more specific information on the amount of water to use in your mix. Nearly all of the properties of any concrete mix are primarily controlled by the water/cement ratio, especially strength and durability. For your hand mixing, you should try not to exceed a w/c ratio of .55, which means 55 lbs of water for every 100 lbs of cement. Since you will most likely be measuring water in gallons, remember that a gallon of water weighs 8.3 lbs. If you can achieve a workable mix with less water, that will be better for the concrete. You should also do some type of curing for your concrete after it is placed in the footing, especially if you are in a hot, dry climate. You can purchase curing compound, which is sprayed on the concrete after your final finishing operation. I would recommend a white-pigmented curing compound, which you should be able to purchase in a 5-gal quantity. The other alternative would be to use burlap or a similar material, and keep the concrete wet. It is important to keep the concrete wet continuously for at least three days if you use this method. (Don't let the burlap dry out before you soak it again)