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I made a concrete slab to house my concrete fountain. The problem is the concrete fountain base wobbles on top of the slab. The concrete fountain itself came with plastic shims that I put in place. However, the fountain only came with four. I was able to break them at perforated points and use more to try and put more all around the fountain. My concern is that these areas where the shims are will create stress points and the rest of the area underneath could be vulnerable to cracking. I'm including photos to display what I am working with. You cannot see the plastic shims because I tucked them underneath and they're not visible from the camera angle.

Would it be possible to use something like a foam like Great Stuff and inject it underneath the fountain in an attempt to help create a more solid contact between both surfaces and alleviate the stress that will be where the shims are located? It's less than a half inch at it's most uneven point. But given that one of these fountains will weigh about 400lb after it's filled with water I think I should do something to help prevent possible cracks in the base.

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    Would rather build up or grind down the surface to flat. Most foams will not hold any weight, you can punch a hole in them with a finger. Epoxy might work.
    – crip659
    Nov 9, 2021 at 22:01
  • I'm not going to even think of trying to grind down the area in an attempt to make it perfectly level. The slab is 4x4. That's 16 square feet I'd need to try and level. I do not have the skill level to do something like that.
    – Adrien
    Nov 9, 2021 at 22:11
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    There are self levelling floor compounds that should be hard enough. Make a form a bit bigger than the fountains. The foam be great for sealing the bottom of the forms.
    – crip659
    Nov 9, 2021 at 22:19
  • I see. These fountain bases are super heavy and I put it in place already and shimmed and leveled them. I didn't know it was going to be unlevel until I got it in place. I was hoping to "inject" something in between the two surfaces and to leave it as is.
    – Adrien
    Nov 9, 2021 at 22:24
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    You will want something like an epoxy(or something) that will dry hard and solid. Do think a foam would be too soft to give support.
    – crip659
    Nov 9, 2021 at 22:45

4 Answers 4

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Actually, I wouldn't spend too much energy on this matter after what you have done so far. You may simply grout the uneven edges using low slump cementitious material to increase the contact area around the perimeter, and finish up with a sealant by tooling.

If you have serious concerns, you can core a few holes in the fountain, using pressure grout technics to force thin cement mix into the seams between the two concrete surfaces. However, it is not a DIY job as the concerns of equipment and experience. Also, it can be costly.

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https://www.walmart.com/ip/Duct-Tape-Glues-Epoxy-PC-Products-Adhesives-Fillers-PC-Concrete-9-oz-Epoxy-072561/20371160?wmlspartner=wlpa&selectedSellerId=101042527

I have used the product above and it simply works. It self mixes in the tube as you pull on the caulk dispenser. It claims to be 3x stronger than concrete. It has 11,410 psi compressive stregnth.

https://www.pcepoxy.com/products/concrete-repair/pc-concrete/ https://www.pcepoxy.com/wp-content/uploads/2017/07/concretedatacolorback.pdf enter image description here

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Your local big-box DIY store will happily sell you packs of plastic shims. Purchase several more packages and add more shims to help take the weight and disperse it evenly.

Do note, however, that it's possible that 400+ pounds could, over time, slowly deform the plastic and allow your fountain to settle out of level again.

Your other option is to do as crip659 suggested in comments on your question and either grind the floor down flat or build it up to level with a self-leveling compound.

You note in a follow on comment that you don't want to have to move the bases again. Unfortunately, you're probably going to have to move them or live with them being out of level. Your other option may be to hire a company that does "mud jacking" to come inject more concrete underneath the fountain, but I'm not sure they'd be willing or able to move it the less than 1/2" you indicate is the maximum gap. Even if they are willing and able, you might find the "price" of moving the fountain and leveling the foundation yourself to be much more preferable to the price quoted to you by the company.

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Rather than polymer foam I would go with a cement foam.

Take a regular 2:1 mortar mix and add in 0.025% powdered aluminium, mix it up and load it into a piping bag (or woven plastic sack) and squirt it under the base.

The aluminium will cause it to expand slightly so it will relieve pressure from the shims.

0.025% is 1 part per 4000 by weight.

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