Concrete is a very robust product but will deteriorate over time especially where it is heavily trafficked, is subject to harsh weather conditions (freeze/thaw cycles) or water infiltration. Corrosion of the steel reinforcement (through a process called carbonation) can also lead to concrete failure.
Unsound and failed areas of concrete need to be broken out back to a sound base. If steel reinforcement is present this must be cleaned by shotblasting or other mechanical means and treated with highly alkaline primers.
For vertical and overhead applications a thixotropic lightweight mortar is applied. This incorporates microfibre reinforcement and styrene acrylic technology. Finished with a trowel it matches the profile of the existing surface.
For horizontal flooring applications, a range of products may be suitable depending on the depth and expected loading of the repair. For smaller holes epoxy based mortars are likely to provide the best solution as the epoxy bond works much more effectively than cement, but for larger holes cementitious repair mortars may be more suitable.
For rough concrete a range of cementitious levelling compounds are available to provide a very hardwearing screed but at low thickness (6mm for example).