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05-19-2009, 08:47 PM   #1

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## concrete slab

hello all, I am in the midst of building a concrete slab foundation for my new garage. I have dug out the site, put up my boards, and have dug the trench for my footers around ther perimeter, but what I need to know is how to backfill on the outside of the boards so that the cement will not flow out underneath them. There is a wide gap at the back corner where the ground landscape is very low in comparrison to the other side. I also need to know how much cement to order if the pad itself is going to be 6 inches thick, but i have to account for the cement which will fill the trenches around the perimeter.

05-19-2009, 09:32 PM   #2
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Quote:
 Originally Posted by GarageGuy09 hello all, I am in the midst of building a concrete slab foundation for my new garage. I have dug out the site, put up my boards, and have dug the trench for my footers around ther perimeter, but what I need to know is how to backfill on the outside of the boards so that the cement will not flow out underneath them. There is a wide gap at the back corner where the ground landscape is very low in comparrison to the other side. I also need to know how much cement to order if the pad itself is going to be 6 inches thick, but i have to account for the cement which will fill the trenches around the perimeter.
The math is:

Length in feet x width in feet x .5 (for 6" slab), then the answer is divided by 27 to = cubic yards.

If it is a larger slab, figure your whole "trench" (it's a footing) area this same way, right from the top to the bottom. You do it this way on a big slab (especially 6") because it is too easy to underestimate on a larger slab.

You can be more precise and calculate closer on a small slab...

If it is a small slab, figure your "trench" area only from 4" to 6" lower than the top of the slab to the bottom. In other words, don't count in most of the part that the 6" slab will be occupying above the "trench".

Backfilling: Just shovel the dirt up against the outsides of the boards, and let it go down into the gap... Stomp it in down there. Now pack it back into the gap from the inside with your hand or feet. If there is some still inside the form, shave it off neatly with a flat nosed shovel. If there are some cupped holes toward the outside, don't worry about that little bit of under flow... as long as you have a fair amount of dirt tightly banked up there to stop the concrete. The dirt, itself, will act like a form board.
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Willie T

Last edited by Willie T; 05-19-2009 at 10:07 PM.

05-19-2009, 09:45 PM   #3

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## thanks

the slab is going to measure 22feet long by 18 feet wide, with a top depth of 6 inches, not including the trenches ie footings, this works out to 7.3 cubic yards, now how would i calculate the footings?

05-19-2009, 09:54 PM   #4
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Quote:
 Originally Posted by GarageGuy09 the slab is going to measure 22feet long by 18 feet wide, with a top depth of 6 inches, not including the trenches ie footings, this works out to 7.3 cubic yards, now how would i calculate the footings?
Width of the trench is pretty clear to see and measure... probably about 16"? (1.333 feet)

The length will be the running length, all the way around your house, couple hundred feet?

And the depth? What? About 42" (3.5 feet) in the North, and maybe 24" (2 feet) in the South? since this is a big pour, I wouldn't measure short. Take the measurement of the depth all the way from the top of the form board, right down to the dirt at the bottom.

Same formula: L x W x D (all done in "feet") divided by 27.

It is true you could end up with a little extra concrete. But far better a little over than find yourself short. For the chance of some extra left over, go ahead and form for that little garbage can pad you've always wanted... or that pad just outside the shop door. Be prepared not to waste the extra.
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Last edited by Willie T; 05-19-2009 at 10:04 PM.

05-20-2009, 12:00 AM   #5
Concrete & Masonry

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Quote:
 Originally Posted by GarageGuy09 hello all, I am in the midst of building a concrete slab foundation for my new garage. I have dug out the site, put up my boards, and have dug the trench for my footers around ther perimeter, but what I need to know is how to backfill on the outside of the boards so that the cement will not flow out underneath them. There is a wide gap at the back corner where the ground landscape is very low in comparrison to the other side. I also need to know how much cement to order if the pad itself is going to be 6 inches thick, but i have to account for the cement which will fill the trenches around the perimeter.

If there's a large gap left under the boards, you're not done forming yet. Fill that gap in with more lumber to save on concrete.

05-20-2009, 08:27 PM   #6

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## so

so I fill the gaps with more boards, brace it, then backfill over it to meat the top of the forming boards, then once the concrete has cured, dig it out and pull the boards off? Seems like a lot of work for something that has to be taken apart again. Are these steps correct?

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