So I'm building a concrete gate pier, 18" square by 53" tall. I'm butting it against an existing solid concrete wall. I'm going for more of a spread footing, making the foundation twice the width of the pier, 36" in one direction (so that's a 9" projection each side of the 18" pier).
Some questions:
To extend the foundation towards the adjacent wall which will be butted to the pier, I would have to undercut the wall and remove a section of it's footing. Do I need to do this, or could I rely on the wall (about 2' 6" tall) to prevent any leaning towards it? If I don't, the pier will be sitting on the edge of it's foundation.
I planned to make the foundation 12" deep, should I make it even deeper? E.g. 18"?
What strength mix should I use for the footing? I have seen C25 mentioned on forums?
I don't understand the progression in the strength of mixes
C20 is 1:2:4 cement, sand, stone
C25 is 1:1:2
C30 is 1:2:3
C25 has a higher proportion of cement than C20 and is stronger than it, but a higher proportion of cement than C30 and is weaker? I know the stone/sand ratio is also of significance, but does it follow that less cement and a stronger result can be produced by simply making a C30 mix?
Are these ratios correct?
