How much concrete do I need?

[orangetrader]

Need to fill a hole 8"x8"x24" deep. There will also be a post 4"x4" occupying this space. How much concrete do I need? Will a 5 pound bag be sufficient?

Which type do I need? Regular or quick drying? Is Quikcrete what I need?

I assume I need to put in some gravel, then pour the concrete in, then insert the 4x4 post and make sure it is level. Now do I pour all the concrete in all at once? or should this be done in several steps? While the post is sitting in the wet concrete, do I need to hold it still until the concrete sets?

Should the post be sitting on top of the gravel then concrete is poured? or should I insert the post, pour in gravel to hold it straight, then pour the concrete in?

Is it better to dig a square hole 8x8 or a round hole 8"R for a post intended for a mailbox?

Thanks,

O

[Greg]

1 60 lb bag will be plenty. Mix it up per instructions (just water), set the post in the hole and pour in the concrete.

[Abe]

An 80# bag ought to do it if you direct-submerge the post. If you use an anchor (recommended) 2 60# bags ought to do it.

Quick drying is weaker than regular drying. Buy the regular drying.

If you're set on submerging the post into the concrete, line the bottom of the hole with 4" of gravel, set the post in, plumb it on all sides, and cross-brace it in place. Then mix up the concrete and pour/shovel it in. Then re-check plumb, and leave to set overnight.

A better way is to fill the hole with concrete and insert and plumb a heavy duty 4X4 post anchor. Then you bolt the post to the anchor. This keeps the post out of the ground and it will last much longer. Also, when it comes time to replace the post, it's just a matter of unbolting the old one, and inserting the new one. With a direct-submerged post, you cannot replace the post once it deteriorates.

If you go the direct-submerged route, your first statement is correct. Place post on top of gravel (then plumb and cross-brace post), then pour concrete, and re-plumb.

8X8 square will be stronger, but for a mailbox is probably unnecessary. For the 8X8 square, you will need 2 60# bags of concrete. With a round hole, you might get away with 1 80# bag.

[Colbyt]

Buy a 60 pound bag of regular concrete mix and throw away what you don't use. Cost $2.

2-3" of gravel in the bottom of the hole would be nice if you have a loamy soil to allow for drainage. If in clay you are wasting your time. Just use the concrete.

Mix it according to the package, plumb the post and no temp support should be needed while it dries.

The quick and easy way is to pour the dry mix in the hole, plumb the post, add water and allow to dry. This make weak concrete which is fine for a mailbox post and much easier to break up when you have to replace it in 10 years.

Colbyt

[davefr]

Don't set posts in concrete. A mailbox post will do quite well in compacted dirt/crushed rock.

Concrete will keep to post moist and cause premature rot. It'll also make replacement a PIA.

Just set the post on a bed of crushed rock and backfill with a blend of dirt and rock. Dome the top and tamp it in well.

[Doug Miller]

Hole is (2/3) x (2/3) x 2 = 8/9 cubic feet. Post is (1/3) x (1/3) x 2 = 2/9 cubic feet. Volume of concrete needed is obviously 6/9 = 2/3 cubic feet. That's about one 80-lb bag.

Either one will work.

If you put the concrete in before the post, you'll never get the post in.

Put some gravel in the bottom. Then put the post in. Brace the post so that it stands up straight and stays that way. Fill the hole about half-way with concrete. Check the post to make sure it's still vertical, and adjust the braces if necessary. Fill the hole the rest of the way. Check the post again.

Gravel alone won't hold it vertical. You need braces.

It doesn't really matter that much, but round holes are much easier to dig. And if all you're supporting on that post is a mailbox, 1) you don't need an

8-inch hole (six is enough), and 2) you don't need concrete either. Just backfill it with dirt, and tamp it down. Do that in stages: dump in about a foot of dirt, tamp it down, add more dirt, tamp it down, etc until the hole is filled.

[ABC]

I agree. I think the reason the fence companies use concrete is that tamping is a lot of work and it is faster and easier to set a post in concrete. They never take out the old ones either. They just cut them off.

[Harry K]

Agreed. Concrete for a mailbox post is egregious overkill.

Harry K

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