Like most secrets, the knack of making lasting repairs boils down to an easily comprehended rule: use the appropriate materials the right way and you will get satisfactory results. Below are listed the basic requirements for achieving a durable patch. These hold true for virtually any repair job.
For assurance that the patch will last: The repair material must be thoroughly bonded to the sound concrete of the cavity; the shrinkage of the patch should be small enough not to jeopardize this bond; the patch and its substrate should be free of cracks; the patch and the old concrete should respond to changes in temperature, moisture and load similarly enough to avoid gross differences in movement; the patch should be sufficiently low in permeability that moisture will not migrate through it to the old concrete underneath; and the patch should be resistant to weathering or traffic when these are factors.
In addition to these requirements, it is sometimes necessary to match the color and texture of the old concrete. If the old concrete has special exposure requirements, such as the ability to resist chemical attack, the patching material should also possess these qualities.