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So I am pouring some footings for a deck I am building and my current plan is to dig a hole that is 18" wide and 18" deep. I'm then going to suspend a sonotube about 6 inches above the bottom (using a scrap wood frame).

My idea is to pour concrete in through the sonotube and have it splay out at the bottom, but once the concrete fills the bottom of the hole up to the sonotube, that it stop filling the hole.

I'm not sure if that was confusing or not, so I drew a crude picture. The blue line is as high as I want the concrete to go in the hole. The red "X"s should be empty space in the hole and the sonotube in the center should be filled with concrete.

enter image description here

My question: If i take this approach will the concrete fill in as I have drawn in the picture, or will the concrete in the sonotube keep pouring out and fill the hole all the way to the top?

  • What size sonotube are you planning on putting in the 18" hole? – Dano0430 Jul 30 '15 at 18:29
  • Either 10 or 12" – Brh Jul 30 '15 at 18:55
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This is a reasonable concern, but not necessarily a problem. It is not uncommon to have concrete forms the way you describe. The solution depends largely on the consistency of the concrete mix. A very wet mix will run all over the place, a very dry mix will hardly flow out the end at all. This property is known as the "slump" of concrete, measured in inches. Slump value range from 1" (very thick, hard to work with) to 6"+ (very liquid).

If you are mixing the concrete yourself, aim for somewhere in the middle. If the concrete is being delivered, talk to the supplier about your use case and they should help you.

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    I wouldn't worry about it. Your plan is fine. There's a slump test that is done on concrete to make sure it has the proper flow. Even if it's a little thin you'd have to have a really long tube for the pressure in the tube to push up on the pressure outside of the tube. Even then the outside concrete will push down on the concrete in the tube. This ratio will depend on the size of your sonotube. (12" tube will push more concrete out the bottom and up the sides than an 8" tube though that doesn't mean 8" is better check codes first) – Dano0430 Jul 30 '15 at 18:28
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Agree with Henry, as long as you can hold/get the tube stable you'll be fine. Use a nice rolling mix, not too wet and use a length of rebar (or wood in a pinch) and drive it in and out of the concrete to act as a crude vibrating poker (gets rid of air and avoids empty pockets).

I'd probably try a slightly dryer mix until you get into the tube to reduce the 'slump' in the bottom of the hole.

(p.s. been paddling around in concrete for 30 years....;-) )

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I poured three Deck Footers (Stairs) - two 10" tubes, and one 8" tube yesterday in 45 degree weather, and each about 30" in length. I filled the concrete to the top of the tubes. The tubes were each about 12" above the hole bottom, and the holes were between 12-14" in diameter - the concrete went down the tube and filled the bottom approx 12" deep cavity below the tube bottoms, and then rose within and inside each tube, without the concrete squeezing down further, and 'oozing' up the outside of the tubes.
I made the mix with a rental electric mixer from Home Depot - highly recommended, and less than $40 for 4 hrs, and used slightly more than the Sakrete bag instructions of water for Sakrete 50 Lb Fast Setting bags - 2 Gallons for 3 bags (2.5-3.0 Qts per bag, or roughly 2 gallons for 3 bags). WORKED GREAT!!!

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How about pour the concrete around the tube first, letting it set a short time, then pulling the tube up, embedding a couple of pieces of rebar in an X formation across that hole, then lifting the form and pouring the rest? That;s what I would do; if I were doing that.

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