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Concrete newbie here. I am building a concrete pad for an A/C unit in an already excavated area of the yard (new plant bed). The excavated area is around 8" deep. I want to build a 4'x4'x16" concrete block for the air conditioning unit.

Here's a visual :

enter image description here

I understand that these are the steps :

  1. Excavate slab area by 8" and backfill ~6" with gravel.

  2. Level & tamp gravel base.

  3. Build forms and adjust pitch of about 1/4 inch for every 12 feet.

  4. Mix the concrete and pour into the form.

  5. Screed and level out concrete surface

  6. Float & finish surface

  7. Water cure for a week.

I have a few questions :

  • Is this the right approach to building a very thick concrete pad/block ?

  • Is the base for this pad sufficient ?

  • Should I use rebar mesh to reinforce the block ?

I would have considered building this using pre-cast concrete blocks , but my masonry skills are next to nothing and I felt that pouring a block is easier than laying courses of concrete blocks.

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    Would you mind telling why are you building the pad this thick? Also, I'd definitely use some rebar. It's a cheap insurance against cracking... Also, depending on your climate, you might want to make sure it's not connected to house foundation, if the pad's bottom is above the frost line. – haimg Aug 25 '16 at 12:22
  • @haimg , I considered the option of backfilling the excavated area first and then building the block on top of it. But the backfill soil is pretty loose and I felt it wouldnt provide a good base. Wouldn't the pad + base sink when the back-filled soil below it compacts? As a workaround , I am building the concrete block down from the level of the existing compacted soil base. Please correct me if I'm wrong. – anset Aug 25 '16 at 12:36
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    Another plan you might want to consider is a base of gravel and a pre-fab air conditioner mounting pad. Faster and cheaper are the reasons. Plus the pre-fab is ABS plastic and won't crack. – Tyson Aug 25 '16 at 14:11
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    the use of the word block is confusing because blocks are different then a pad. use re-bar. tamping is not as good as compacting but if you do it right it will be ok for a small pad, best is use a plate compactor and compact the gravel in stages. – Alaska Man Aug 25 '16 at 20:21
  • I agree with Tyson,+ most units today are set on a prefab "plastic and foam" pad. These absorbe the vibrations from the unit and some units require dampers if directly on concrete. Most of the pads I poured prior to the pre Fab ones were only 4" thick with wire mesh sitting on crushed rock base. Verify your units requirements because after all that work you may find out you will still need the damper pad and all your work or most was a waste of time and $. FWIW The damper pad is a warrantee requirement on some units. – Ed Beal Jan 25 at 14:35
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I think the better approach is to build a thicker gravel base, and use less concrete for the pad. Backfill with gravel, in layers, compacting between each 4"-6" layer.

I'm not sure you're aware of how much concrete you'll need for your proposed 12" thick pad. See this: http://www.calculator.net/concrete-calculator.html. Your 4'x4'x12" pad will use 27 bags of concrete, nearly a ton, if buying pre-mixed. I think building a thicker base, and then pouring a 4" or 6" concrete pad, with some rebar mesh, is a better/easier approach.

Your plan is solid, but I'd add several things to is:

  1. Definitely use a rebar mesh.
  2. Put a 6mil vapor barrier into the gravel, to prevent it sucking all the moisture out of your concrete pad.
  3. Add a release agent to your form, it will be easier to remove it.
  4. Make sure your pad is not connected to the house foundation, especially if the soil freezes in the winter in your area.
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That's rather an overkill approach, to say the least. Expensive and not getting you much benefit for the expense. Huge solid blocks like that are good for a punch press or artillery piece, they don't benefit an air conditioner at all.

If "not moving" is a prime concern, it's likely too shallow to be below frost line if your profile location of Ohio is accurate. And it's a boatload of concrete that's doing nothing...

A fully functionally equivalent, looks identical, much less costly approach would be to dig nearly the same hole (or one 4'8"x4'8"), without worrying too much about digging out the middle, pour a 12"wide footing around the perimeter, pour a 4'x4' 8" thick wall on top of the footing, fill the middle with gravel/sand, and pour a 4"-8" slab on top. Use steel reinforcing in all of it - concrete without steel is spending 95% of the money for 25% of the end product.

You could also build the wall from concrete block; with steel placed and concrete filling the cores, it would be functionally equivalent and save on forming. Having done both, I can assure you that block is easier in many ways - nothing like having a problem with the forms while pouring to raise your blood pressure.

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