I'm trying to figure out my required materials for a large wall project. I know that an 80lb sack of premix makes about 2/3 ft^3 of concrete. We don't get premix here and have to mix ourselves.

Our bags of portland cement are 40Kg or 88.2 lbs. so if that were premix it would be .735 ft^3. If we did a 123 mix with 2 bags sand and 3 bags gravel (which is pretty strong I believe) that would be 6 bags of material producing 4.41 ft^3 or about 1/6th yd^3 or 1/8 mtr^3.

Is this correct logic or am I missing something here?

 

Close but can be misleading on the strength or the amount of cement needed.

Pre-blended bags are made using controlled aggregate with known properties in order consistently get the proper strength and workability.

Finer or "dirty" or poorly graded aggregate will need more cement for the best strength and use of cement, which is normally the most important cost item.

Coarse aggregate, especially angular particles can give decent strength, but will be harsh, difficult to place and hard to finish if it is for flatwork.

With poor or uncontrolled aggregates, you will need more cement. If you are lucky and have ideal aggregate available, your properties can be very good even with less cement. Excessive cement requires more water and can give more shrinkage cracking of slabs.

Sacks of cement per yard is just a general rule of thumb that can be misleading for many aggregates.

Dick

 

This is mainly for footers and bond beams in a block wall. I know what is good sand, the play sand they sell at HW stores but we don't have that here. I'm told not to use beach sand because A. It looks better on the beach, and B because it is fairly round and doesn't bond well. I'm not sure on the gravel to use. About how big is the right size for good gravel. I'm assuming larger than pea gravel but they sell a lot of crushed rock about 3/4 inch. Would that be good aggregate for footers and possibly slabs? Perhaps there's a site showing various aggregates for various applications?