Concrete is a building material made up of sand, gravel, water and Portland cement. It is used to make everything from building foundations to flower pots, but is most commonly associated with sidewalks and driveways. This material is known for its strength, durability and affordability. Though it is easy to work with and maintain, it does require proper installation techniques. Concrete that is installed over loose gravel or dirt, without a proper base, will tend to sink, creating low spots that collect water and are unattractive. Fortunately, it is possible to fix sunken concrete yourself and level out the surface.
Clean the surface using a chemical concrete cleaner that contains phosphoric acid. Spray the cleaner onto the concrete and allow it to soak for several minutes, then spray it clean using a garden hose. This roughs up the surface and provides a good base for the levelling compound you will use.
Blend a concrete repair mixture with water according to the instruction on the bag. Add a concrete liquid bonding compound to help the mixture stick when applied. You can use a wheelbarrow or bucket for mixing.
Apply a very thin coat of the concrete repair mixture to the sunken area using a trowel. This coat should be around 1/4 inch thick, and will act as a "slurry" coat, helping the patch to adhere to the slab.
Wait 4 to 6 hours for the slurry coat to dry before proceeding with Step 5.
Pour the concrete mixture onto the sunken area of the slab. You'll need it to be roughly level with the highest points of the slab.
Use your trowel to spread the concrete mixture across the slab, feathering it out thinly as you transition from low to high areas.
Gently sweep the concrete with a broom to give it a nice broom finish. Follow up with a concrete sealer if desired.
If you're concrete is in bad shape, it may be easiest to hire an outside company to raise the slab and inject sand or gravel underneath. This process, known as slab jacking, is fairly quick, but requires a bit of experience as well as some heavy equipment.