I have a concrete slab in the breezeway of my house that has several settlement cracks in it.
Can I use self-leveling concrete to repair this?
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You should check the label/product literature for the cement products you intend to use. From what I remember self leveling cement is not recommended for filling cracks deeper than 1/2". If you want to use it then you should patch any deep cracks first. Self leveling cement is also about 10x the cost of a bag of regular cement mix.
If the ground is still settling underneath it's best to just break up the section, compact the ground underneath and then pour in that new section. That's going to be your longest strongest fix.
If you're not sure if it's done settling I would just fill in the cracks and see if they patches crack again.
A lot is going to depend on the size of the cracks as far as the best product to use but for most cracks I like to use a fast setting concrete patching mix. Can fill large gaps and can also be feathered out at the edges. It sets up in less than an hour which I like for walkways. You can also use a regular cement with an acrylic fortifier added to it if you can keep people off the repair for 2-3 days as it will be much cheaper.
With the fast set you need to clean out any loose chunks, wet the concrete, make a thin mix of the cement that you paint on the crack to aid in bonding, then mix the regular thickness mix, trowel in the crack, trowel it smooth then use the paint brush to smooth out the edges. I like to build up the level a little higher and then scrape it off with the trowel after a few minutes.
All the concrete repair products I've seen are polymer modified but not to the same extent that self-leveling cement is. Adding an acrylic fortifier to the mix will give you a stronger bond.
the simple answer is yes you can as long as you don't have a problem raising the floor height an inch or so. Regardless what the packages say, a coat of concrete under an inch thick has a tenancy to shell, crack and become a mess. If you do over pour, be sure to prep the existing concrete so you will get a good bond. The real question is if the settling is done. If you have displacements of over 1/4 to 1/2 inch, I would be looking at fracturing the floor and compact before pouring a new one over the old. Large displacement cracks are a sign of a fairly serious problem that should be addressed.