A sunken concrete slab whether a sidewalk, driveway, pool deck or patio, will eventually need a concrete repair or replacement. The decision will often come down to the cost, benefits and capability.
Planning to replace your sunken concrete, yourself? If so, keep in mind that you'll need to not only pour a new slab, including building a form and finishing the new concrete, you'll need to remove the old concrete. This means breaking it up and hauling it away.
Another point to remember is, WHY did the settlement occur in the first place? When replacing concrete, it is important to make sure the soil base is solid. Otherwise, your newly poured concrete may move and settle again in the near future.