Building a thick concrete pad

Question

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 :

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.

Answer 1

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.

Answer 2

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.