Settlement under a concrete floor causes the floor to crack and cave in because the ground under the floor no longer provides the concrete with a solid base to sit on. Unfortunately, this type of damage is the first sign of settlement under concrete. Repairs to concrete are not only possible, but also successful.
Use a cold chisel and a heavy masonry hammer to chop out small sunken, damaged areas, chopping out 10 to12 inches past the damage in all directions. Use a heavy sledgehammer to break up larger areas of if the area is too large to chop out by hand; rent a jackhammer if needed, extending 10 to 12 inches past the damage.
Remove all concrete pieces and crushed stone from the space.
Inspect the area under the edges to ensure there is no air space between the concrete and the base. If there is space between the bottom of the concrete and the base, chop out the concrete in that section to avoid further floor failure in the future.
Clean the edges of the concrete by hand with a wire brush or wire brush attached to a drill to remove all loose pieces of concrete from the perimeter.
Fill the base of the hole with dirt to make it level with the existing dirt under the floor.
Tamp the dirt down with a hand tamper or rent a gas-powered tamper to pack the base down firmly.
Add crushed stone to the hole to bring it up to the same height as the crushed stone under the existing floor.
Sweep a piece of wood across the top of the crushed stone to level the surface, knocking down high spots and filling in low spots.
Mix ready-mix concrete in a trough, wheelbarrow or bucket, according to the directions on the package.
Paint the edges of the existing concrete with an epoxy-bonding agent.
Pour wet concrete into the hole up to the surface of the existing concrete immediately after painting the edges with the epoxy-bonding agent.
Use a trowel to smooth the surface of the fresh concrete.
Lay a large plastic tarp over the newly poured concrete and weigh down the edges.
Lightly mist the concrete with water as necessary to allow the new concrete to cure properly. Keep the new concrete damp for one week, misting it as needed.
Wear safety goggles, dust mask, gloves and hearing protection while working with power tools and concrete.