The procedures for leveling an uneven sidewalk via polyurethane foam injection is generally as follows:
First, dime-sized holes are drilled into the affected slab.
The polyurethane foam is injected under the slab using the holes.
The foam begins to immediately expand and this raises the slab.
Once the repair is complete, the holes are patched with a compound that closely matches the color of the concrete.
Fifteen minutes or so after the repair, you can begin using the slab again.