above grade patio retaining walls

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Old 04-30-14, 09:14 PM
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above grade patio retaining walls

I want to build a patio not more than 3 feet above grade, my plan essentially is to build a 30'x15'x3' box, back fill it, and cover with a 3" slab of 3000psi 3/8" concrete. The concrete slab will not rest on the walls, it will cover the fill and be level with the top of the walls.
Question: if I build the walls out of cinder block reinforced with rebar and filled with concrete, what size cinder block do I use? 8x8x16, 4x8x16? and if I build the walls of poured concrete, how thick do they need to be?
 
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Old 04-30-14, 09:39 PM
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In AZ, you will not find "cinder block" unless you are willing to pay the price for volcanic cinder block. I assume you are considering regular concrete block that will work well also. No matter what material (block, concrete) you use for the walls make sure you have some reinforcement that can be continued/bent into the patio slab. Then, pour (or place) the slab over the walls.

This will give you an attractive patio that will last. If you are nit-picking the thickness of the walls, that is a false economy, because a 4" wall could cost you more than an 8" wall in the end.

If you pour the wall inside the walls, it will eventually settle and crack along the line of the interior of the retaining wall. Use a 4" or so slab and compact the soil behind the wall.

Dick
 
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Old 04-30-14, 09:47 PM
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I misspoke, I meant concrete block.

if I pour the slab on top of the walls how do I build a form that is tight enough to keep the wet concrete from running down the walls?
 
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Old 05-01-14, 03:45 AM
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why would any conc run down the wall(s) ? no one's forms up that sloppy, do they ?,, IF there's too much wtr in the mix, that's someone's fault,,, good conc doesn't 'run' our of good forms.

as dick sez, 4" slab on TOP of the walls,,, btw, that slab needs jnts otherwise it'll crack all by itself - you don't want that

proper base compaction is extremely important so reinforce your walls to resist lateral compaction stress's,,, rebar bents in the walls but none nec in the slab,,, 3/8" suggests either pump mix OR apron/vest bagg'd conc mix - which is it ?
 
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Old 05-01-14, 09:34 AM
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There will be some loose mortar leakage from the form/block junction, so the tighter the forms, the less leakage. A bead of caulk at any obvious openings before placement will help minimize any bleeding. I would overlap the block wall with 5" or 6" of the forms, by using horizontal 1 x 8 form material. Concrete blocks are usually quite brittle, so you may have to attach the formwork to the walls by drilling and anchoring instead of using a nail gun. Drill the anchors at block cells you've prefilled with concrete to support the vertical rebar. Running an outside horizontal 2 x 4 top rib attached to the forms will help stabilize them, especially if you use a vibrating screed for striking off the slab.
 
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Old 05-01-14, 07:10 PM
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Thanks for the advice, I'll let you know how it turns out.
 
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