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I have an 8-year old concrete slab around my pool. One slab has sunk by about 2 inches on the short side. The slab measures about 6 feet by 3 feet by 4 inches. It rests against the pool structure on the long side. The opposite long side is grass. The two short sides are separated by a 1 inch wood separator from other slabs. There is a thin "cool deck" covering of on the slab.

My question: what is the best choice of action for me to repair this?

Can I raise it to fill underneath it with sand? How? How do I prevent the slab from cracking?

Should I pour concrete on it? It will be tapering from 2 inches to 0 and probably won't stick. Also the cool deck cover gets covered by concrete.

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    I would imagine you could pour self-leveling cement on top of the sunken slab. It will (as it says on the tin) level itself on the surface. You'd probably have to create a dam to prevent it from running off the low edge. You may need to rough up the surface of the existing slab to create a key so they stick together well, or maybe even some holes for it to key into. You'd probably need to rough up the surface as it cures so it's not slick under wet feet. You may want to investigate and fix the cause of the sinking first, though. (Not enough detail/experience to warrant a full answer.) – FreeMan Mar 5 '19 at 16:49
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This slab is small enough that you should be able to easily lift it out and repair the base soil. Unless the concrete was weak initially or in otherwise poor condition it won't break.

  1. From the lawn side, trench along the slab about 6" wide and to the bottom of the concrete. Save the sod by wrapping it in a tarp.
  2. Using heavy steel or wood bars and some fulcrum blocks, begin lifting the concrete. Work in stages and block it up as you go.
  3. Once you're high enough, move the blocks rearward, underneath the slab. Continue maneuvering it until the entire slab is floating above the level of the surrounding slabs.
  4. Lever the raised slab sideways as needed to gain access to the soil below. It's not necessary to move it completely out of the hole area.
  5. Repair your soil base using non-organic sand or gravel. Tamp it well and leave it 1/2" higher than necessary.
  6. Move your slab back into position and remove the blocks. Be careful not to drop it to prevent damage to it and the surrounding slabs.

Allow the slab to settle for a few weeks, ideally through a rain event. If it doesn't rain, flood the area well to allow the soil to compact and settle.

If you find that you've left it too high, a hose worked under the slab from the lawn side will allow you to wash out some fill soil. Work slowly so as to not go too far.

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  • This has the added benefit of seeing what caused the sinking originally - an animal burrow, erosion, bad installation, whatever - and take steps to remedy that. In the case of erosion, for example, you might add a drainage tube to allow water through without washing out more dirt. – ArmanX Mar 6 '19 at 4:17
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You might want to look around your local area for contractors that offer 'mudjacking'. Essentially they pump a grout-like mixture using a powerful hydraulic pump under the slab and bring it level. I think it generally requires cutting a hole in the slab.

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    This works well, but would be costly for a single small repair. The holes are about 1" and the contractor fills them with repair cement. – isherwood Mar 5 '19 at 18:48
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One method to correct this is to jack up the sunken edge to its original height and fill the space between the slab and the ground underneath with polyurethane expanding foam. The details of the process are way beyond an answer here, but in a nutshell:

You dig a trench along the sunken edge so you can get jacks under the slab then slowly, carefully jack it into position. Once it's in position, you drill holes through the slab and squirt the expanding foam into the void below. Once the foam sets you can remove the jacks, backfill the trench you dug, and you're set.

Note - per comments - the spray foam that's readily available for filling gaps and cracks is probably not what you want, they make specialty foam for this purpose.

Note that this doesn't necessarily fix whatever made the slab sink. If it was just settling of the soil under the slab it might not happen again and you're done. If there is water runoff eroding the soil under the slab, it's likely the soil under the slab and foam will erode again. Whatever method you use to fix this - even if you remove and replace the slab - you'll want to remedy any drainage problems first so that your fix holds.

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  • I wouldn't use foam. Your procedure is fine, but foam isn't a reliable support. – isherwood Mar 5 '19 at 13:59
  • @isherwood - have you seen problems with foam? I did the drilling for a friend that used this method but I don't know how it's held up for him. I wouldn't have thought of foam as reliable enough, but I did see there were a lot of DIY videos on YouTube talking about foam. – batsplatsterson Mar 5 '19 at 14:22
  • It compresses and degrades over time, especially where freeze-thaw cycles occur and where there's regular vibration, such as from foot traffic. That said, I'm talking about commonly available cans of urethane. Maybe there's something more dense to be had. – isherwood Mar 5 '19 at 14:29
  • There is ... not sure if something like this is available in small quantities - specialty-products.com/spi_products/eco-rise-3-0 ... I am sure that my friend used something from the helpful hardware place – batsplatsterson Mar 5 '19 at 15:14
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    I used car jacks to lift smaller sections ( 4' X 2.5' ) and pushed in rocks ; it takes time and work but can be done . Scissors jacks can fit into a fairly low space. Crank it up, put in second jack, remove first jack and put rock into the hole under the jack , repeat,repeat,repeat. – blacksmith37 Mar 5 '19 at 16:52
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In the first two minutes of this video you can see a possible way of lifting a slab in place from one side. But it uses a huge C-clamp which I am not sure where you get it from. The largest I have seen locally is an 8-inch type. The large-size variety might be available for rental. The cost depends on the weight you want to lift (a cubic foot of concrete weighs about 150 pounds). Due to heavy weights involved you want to be careful that all components are well-supported, otherwise serious injury can happen.

Here is an alternative approach.

If you have access to both sides this video shows how to do it. Notice they fill underneath with high density foam!

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