My father was a general contractor before he retired. But, for years during my life, and many years before, he was a masoonry contractor. As a boy, I learned - if I recall correctly, that mortar mix was 1 sack of mix, 15 shovels of sand, and about 5 gallons of water, in the mixing machine. I don't recall what it was for concrete, as that was usually delivered by truck, to his work sites. So, I am glad I finally - at the age of 50, am learning about the other mix he used on many of the job sites I worked as a boy.

Anyway, while doing some research for my projected footings, I started learning a few things about concrete that I wanted to share with those of you who may not know.

Concrete strength is rated in grades. C15, C25 and C30, for examples. The grade of the concrete is the concrete compression resistance after it has cured for a bit under a month. 28 days, actually. But, who is counting?

The table below will show the parts of cement, sand, and gravel to add, to make the various mixes above, and to achieve the strengths necessary for a given project.

**Cement : Sand : Gravel : Grade**

**Newtons, per square millimeter**, or

**N/mm**. For the yanks in the classroom, who use the Imperial System of measurements, this would be

^{2}**Pounds per square inch**, or

**PSI**. So, C30 would be 30 N/mm

^{2}, or 3,059,000 Kilograms of force per square meter

**kgf/m**, or 4,351

^{2}**PSI**.