It wasn’t that long ago that uneven concrete slabs were handled either by replacing them with new slabs, or via a repair technique called “mudjacking”. Both these methods had their drawbacks though.
It takes time to dig up an uneven concrete slab and replace it with a new one. This is because freshly poured concrete takes time to dry and that means you can’t use the new slab right away. Also, if the slab became uneven because the soil was weak and unable to support it, the same thing will happen with the new slab.
Today, almost all concrete slab lifting and leveling jobs are handled via polyurethane foam injection. It’s a fast, minimally-invasive procedure that only takes a few hours in most cases, and the foam is ultra lightweight. It won’t add more weight to the soil. Also, as soon as the repair is complete, you can start using the slab again.