How much QuickCrete ?

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This is a very green question. But, I suppose if we already had all of the answers, there would be no need for the group, huh ?
I am planting a large signpost, made out of 6 X 6 pt wood. I plan to dig a hole 30 inches deep, and 30 inches square. Of course, the middle of the hole will be filled with the 6 inch square post, leaving 12 inches on all sides.
I guess the area to be actually filled with concrete then is about 30 inches deep, and 24 inches square, not counting the post.
This area seems about 10 square feet (right?) , which I believe is just over 1/3 of a yard of concrete (which would be 27 sq ft I think).
I want to use 40 lb bags of QuickCrete. Can anyone estimate how many bags I will need ?
--James-- Thanks for any comments and help on this !!
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James Nipper wrote:

on my calculator its 30 in = 2.5 feet, 12 +12+6 = 30 in. 2.5 X 2.5 X 2.5 = 15.625 which is about 12 yrd. of concrete.... a 40 lb. bag of quickcrete should be about 1/2 cu. foot??? check on the bag or their web site... so about 30 bags might just do it.. and that is not including the area that the 6 by 6 in. post will be in..... so it is even less... but who is gonna mix 30 bags of concrete???? and the cost is about $3 a bag.. which is about $90... why not just call up those mini concrete companies that deliver it in a pickup truck with a trailer on the back.. for a 1/2 yard of concrete in my area is about $100.00 you might just save yourself some labor...
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This is up a mountain that no concrete truck is willing to go for a yard of concrete.
I thought it would be 2 ft X 2 ft ( 24 inch square) X 2.5 ft (30 inches deep), and that is how I got 10 square feet. I am not sure how you did it in your math, but I thank you for the reply.
--James--
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jim wrote:

12 yards?
The hole is 30 x30 x30 equals 27,000 cubic inches The post is 6 x 6x 30 equals 1080 cubic inches
Concrete needed is 27,000 - 1080 equals 25,920 cubic inches 1728 cubic inches in a cubic foot so 25920/1728 equals 15 cubic feet.
But why the big hole? 12 inch diameter hole would be plenty. That would take about 3500 cubic inches or 2 cubic feet of concrete. If he needs more strength, then dig the hole a foot deeper.
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12 yards? The entire hole without the post is less than one cubic yard.
Here are my calculations.
The hole without the post is 30" X 30" x 30" = 27,000 cubic inches
The post will occupy 6" X 6" X 30" = 1,080 cubic inches
27,000 cubic inches - 1,080 cubic inches = 25,920 cubic inches
One cubic foot is 12" X 12" X 12" = 1,728 cubic inches
25,920 cubic inches / 1,728 cubic inches = 15 cubic feet
This translates to .55 cubic yards, or just a bit over half a cubic yard of finished concrete required for the job.
If one 40 lb bag of Quikrete yields .3 cubic feet of finished concrete (according to their specs), the OP will need between 40 and 45 bags.
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That is a mighty wide hole for a 6x6 post.

the
to
12
inches
just
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Ok folks, and I do say thanks again for all the help. I have tried to put all comments together, and take the good advice offered.
I think that I will go with a 30 inch deep hole, 18 inches square. This is still overkill I know, but this is a heavy post, and near the top of a mountain that get 80 mph winds at times.
So, about 18 inch sq hole, dug about 30 inches deep. Will put the 6 X 6 beam in the center of the hole.
I come up with about 5 cubic feet of concrete.
40 lb bags create .3 of a cubic ft. So, it would seem that I would need about (16) 40 lb bags.
A full cubic yard weighs nearly 4,000 lbs, so the 5 cubic feet should weigh about 750 lbs. Yes, that should hold up my post and sign in any wind !!
Please alert me if I am way off base on my conclusions, and thanks again to all for the nice input !!
--James--
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Depth will give more support than width.
Why not use a piece of sono tube ?

put
should
any
to
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the
How high is the sign post? 30" square can anchor a huge tower.

None. for that much concrete call a reddi-mix guy and have him pour it in until the hole is filled. Takes about 10 minutes instead of 10 hours to haul and mix 40 or so bags. It is only a half yard so there may be a minimum charge but thee are places that specialize in small drops. Ed
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The post is 8 feet high, and has a cross-arm of 4 feet. It is quite heavy, but maybe I don't need quite that big of a hole ? Remember, these beams are 6 x6 and they are quite heavy. The sign itself will not be very heavy, but it will hang from the horizontal cross beam.
This is in a very rural area, so there are not "small" guys to do it. I was thinking along the lines of 10-15 40 lb bags, and that doesn't seem too bad to me, but would not want to do 30 bags etc etc
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James Nipper wrote:

What kind of ground? Dit the hole with a post hole digger. get a 10 foot post put it in a 4-1/2 foot deep hole (10-12 inches in diameter) and you won't need concrete.
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heavy,
are
but
Take a look at the utilities poles. They are set in the ground with NO concrete. I'd go with about 12" dia. I know one of the dig satellite dishes called for 48" deep and 12" dia and that has a LOT of surface area. I'm not sure where to fins specifics, but I'm sue you will be way over the needed size.
You can also add rocks tot he hole to give mass and have less concrete to mix. Ed
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On Wed, 3 Mar 2004 23:04:07 -0500, "James Nipper"

I'd go for a 12 inch hole 30 inches deep and 5 bags of cement in the bottom of the hole. Then I'd add the removed soil plus sand and rocks to finish filling the hole with lots of tamping at every layer.
Guaranteed to hold that sign straight. PJ
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Go to Quickretes web site and try the calculator.
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http://www.clicktoremodel.com/estimaters.html
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the
inches
A hole 30" square and deep comes out to 15.62 cuft, less the post, subtract .62 cuft. A 40 pound bag is .3 cuft.15/.3P, so 50 bags of quickrete! That is alot of mixing! 2000 lbs. of dry mix! Greg
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the
That's a pretty big hole. Are you sure it needs to be this large?
How tall is this signpost going to be and how large (and heavy) a sign is it going to support? What is it's surface area? Are you concerned about wind knocking the sign over? I trust you've considered all this in determining the size of the post hole.
Here's some info that may be helpful.
http://www.quikrete.com/diy/SettingPosts.html

inches
According to my calculations, you're actually going to need about 1/2 (0.5) a cubic yard of concrete.

Information on how much finished concrete each bag of Quikrete will yield is located here...
http://www.quikrete.com/Spec_Data/concrete_mix.pdf
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James Nipper writes:

Yes. Way more than you could possibly mix. You need a different design.
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Thanks to all of the helpful replies, including the Quickcrete url which is very helpful.
All of you are correct, I was thinking of a much larger hole than I would need. I think that no more than about 18 inches square , and no deeper than 30 inches would be more than enough support for this post and sign.
This is a great group, and thanks again to all of the thoughful replies !!
--James--
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James Nipper wrote:

Think smaller.
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