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all.

I am about to build two concrete slabs to support my two decorative outdoor fountains in our front garden area. I am wondering if I can get the best advice for thickness of the slab, PSI for the concrete itself and any other useful information I can get to make this process go as smoothly as possible. First off, here is what one of my fountains looks like:

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It's the bigger of the two and weighs 310lb before I fill it with water. The reservoir looks to hold about eight gallons. I'm assuming we can just round off that this will be a total weight of approximately 400lb once set in place and filled with water. I also live in the desert and it doesn't get colder than 50 degrees. So I won't have to worry about freezing temps in my area. I think that matters in determining the type of concrete you use?

So here were my thoughts and questions:

I want the slab to be approximately 5ft squared (depth and width) so that there is a bit of a ledge all around the sides of the fountains after I center it on top. Would using 2x6 studs as my form to make this 6" high be overkill or advisable? For my application I'd actually like to make it higher off the ground by using 2x6 studs for my form but this is going to require a lot more materials which I find a bit daunting. But I'm will to do it. Otherwise, for the sake of learning, can 2x4 studs to used to frame a fountain that weighs 400lb?

I plan to use 3/4" minus gravel as a base at 3" compacted with a tamper. But this is for 2x4" stud forms. What should it be if I use 2x6 studs instead?

I plan to use the appropriate wire mesh to place inside the concrete as a way to create reinforcement for added strength. I'm hoping to avoid using rebar because it takes forever to cut with a hacksaw for metal. The mesh is easily cut with my lock cutters so I prefer to use that if I can.

Lastly, what PSI should I use? Is 5000 sufficient or overkill?

Any other important suggestions?

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  • I'm guessing that your fountain base is 2' x 2' square. That's 576 square inches. 400 pounds spread across 576 sq/in = 0.7 PSI. I'm pretty sure that 5000 PSI is tending toward the "overkill" range. If the fountain base is larger than 2x2', then the PSI is even lower.
    – FreeMan
    Nov 2, 2021 at 14:23

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  • 2x6 boards aren't 6" wide. They're 5-1/2", and no, that's not overkill. Much.
  • 400 lbs. isn't really more than two humans on a sidewalk. Sidewalks are often just 4" thick.
  • Unless you're going for minimal thickness, high-compression concrete isn't necessary.
  • What's more critical is a good cure, and maybe some steel reinforcement. Keep the slab moist for at least a few days.
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  • So when you say "high compression" does that refer to the PSI? And if that's the case is 4000 sufficient?
    – Adrien
    Nov 1, 2021 at 21:42
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    Steel is good; 10 gage mesh is common; if you can find some rebar trim at a building site , that is better. But as @ isherwood notes, not that much of a load. I have poured auto parking pad with 2 to 4 inches of concrete poured over broken concrete ( from the removed sidewalk) and it was fine for a car. I used whatever the Redimix truck brought, no special cement.
    – blacksmith37
    Nov 1, 2021 at 23:30
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    @Adrien You have a 400lb fountain. You would develop 4000psi of force on the slab if you stood the entire weight of that fountain on a foot with a contact area smaller than a sugar cube. This is an incredibly light-duty application - you'll be fine. This is like going into a library and worrying about whether or not you'll need hearing protection. It's not even close to being a concern. A 5x5' slab of 4000psi concrete would hold several thousand tons.
    – J...
    Nov 2, 2021 at 9:08
  • @J..., your last statement is a bit deceptive in this context. At that point we're talking about crack resistance and not compression (crush) strength. Such a slab would need to be fully supported by another structure of some sort. On soil it would move and crack. Otherwise, I agree.
    – isherwood
    Nov 2, 2021 at 12:53
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    @isherwood Agreed - I actually was editing in the glaringly obvious caveat at the end (ie: namely, that the ground underneath would fail long before the concrete would under such a load), but hit the 5-min edit limit and decided to just leave it at that. Hard to write an entire comprehensive essay in the span of a few hundred characters... In any case, if OP really is worried about load bearing then it's not the concrete they need to worry about so much as ground prep underneath. In this case, OP will be fine - especially in the desert where there's not even frost to worry about.
    – J...
    Nov 2, 2021 at 13:01

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