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PerpetuallyRepairing 04-07-2008 11:34 AM

Concrete Form for Raised Patio
I want to put a 9 foot by 8 foot patio two feet above grade. I was thinking of digging two trenches on the 8 foot edges, 4 feet below grade, which is below the frost line. To keep the concrete a uniform 10" thick I was going to put bales of hay in the patio area and cover them with plastic. I would not have access to them after the concrete is poured as the trenches would block access. The outside edges would be standard concrete forms. The reason for the trenches is to prevent frost heaves and also make this project consume 6 yards of concrete, which is the minimum for a truck delivery. Will I have issues using hay for the internal support of the concrete? Will I have any issues getting a 1/8" per foot grade on the top for water drainage? Will the concrete hold that grade? This entire surface is being covered by 18" x 18" two inch thick granite tiles. Thanks!

concretemasonry 04-07-2008 01:13 PM

You should pour the frost walls and the slab separately.

The bales of hay will not support the concrete after it cures. - The reason is that the hay will rot and shrink. You the have to have a structural slab capable of spanning the 8' dimension.

You will need steel reinforcement in the concrete. The amount will depend on the slab thickness and concrete strength.

You can finish the slab so the slope on 1/8" per foot works, but your slab will be thinner by about 1" at the low end unless you slope the under side.

Why design based on the size and cost of a truck? - Just trying to get everything you pay for? - Usually it is easier/cheaper to pay the short load charge.

dec-conc-artisan 04-07-2008 02:02 PM

not only what the guy from mn said,,,
but why not place a grout fill'd block wall in your trenches, backfill w/base mtl, & float a 4"/6" slab on top of the walls ?,,, since this is exterior work, how're you anchoring those 70# granite tiles ?,,, eventually water'll loosen & move them as it freezes | thaws | freezes | thaws | freezes,,, while 10" is sufficient to bridge 9' when cured, the 7,800#'ll crush your hay,,, i'd place 5"/6" patio & add a mat of #4 rebar 12" o/c.

PerpetuallyRepairing 04-07-2008 08:03 PM

Yes, I can pay the short load charge & do it in 2 pours. It would allow me to pull the forms from the inside but if the hay works, it would be easier.

I can also do the inside with concrete blocks but if the hay works, it would be easier. The only load the patio is supporting is its own weight. Although I have no experience with this, if the wall thickness is inconsistent, I think? it will be okay.

Yes, #4 rebar 12" O.C. in both directions under the tile. Also a piece of rebar at the base of the footings.

I was going to glue the granite down with latex modified thinset made for showers. The granite is cut about as perfect as can be so I was not going to use grout. Just butt the tiles against each other and use clear silicone caulk.

The sides are going to be used brick. This will be interesting as I have never laid brick before... The granite will overhang the brick by 1.5". I was planning on a 1" gap behind the brick and drainage holes in the bottom row. The brick will rest on a step in the poured concrete.

PerpetuallyRepairing 04-07-2008 08:17 PM

Okay, reread the concrete block post. So the concrete blocks would be load bearing? Yes, that makes sense but it gets complicated as I would have to step the blocks to support the brick.

So I would pour the concrete into the blocks and pour the slab at the same time? I assume I would have to support the outside of the blocks to keep them from moving.

The slab only needs to be 6" thick if I use rebar? If I get the 10" for the same price, why not pour 10"? Thanks

PerpetuallyRepairing 04-07-2008 08:37 PM

Okay, so I am little slow. The grout goes between the blocks. I let that harden and then pour the slab. Does the bottom row of blocks get anything underneath it such as pavers to increase the load bearing area? How do I support the bricks? Jog in the wall? Have something stick out of the wall? Do I need rebar in the block wall? Should I pour concrete into the holes in the blocks since it is free (except for my time)? :(

joasis 04-08-2008 06:33 PM

If you choose to form it, go that way. Do not put hay in the space. I have built similar, and did it in a monolithic pour by filling the raised area with fill sand and plastering it with mortar to prevent a cave in. Picture trenching for the footing, and then placing the fill...then form the 2 foot outside perimeter and brace. Use steel as described above.

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