There are firms that specialize in difficult repair work using special materials and procedures; and unless you have unusual expertise in this area, you would be well advised to consider employing their services. However, a great many repairs are still done by contractors, owners or their maintenance crews using conventional placing techniques and ordinary mortar, either at a plastic consistency or as a dry pack material. Procedures for this work are fairly well standardized and this article gives methods commonly recommended for this type of repair work.
Naturally, the cause of the deterioration should be determined and corrected before the repair is done. Otherwise there is a strong likelihood that the need for the repair will soon recur. First, remove all unsound concrete. Failing to do this completely means your patch will bond to a surface that is not bonded to the concrete mass. The patch, with a thin layer of old surface bonded to it, will soon lie useless in or near the supposedly restored area. The shape and size of the cavity desired will depend on the type of problem you are confronting.
The finished cavity should be at least 1 inch deep and reasonably uniform in depth, although a moderately rough surface texture is desirable to encourage good bond. If any reinforcing bars have become exposed, the opening must extend at least 1 inch behind them. Edges of the cavity must be sharply defined not featheredged or spalled and, if possible, undercut. The slant of the undercut should be about 1:3. If undercutting cannot be achieved, or when the surface area is great, it has been found helpful to position studs around the perimeter of the patch by firing them into the concrete with a stud gun. The patch must be deep enough to accommodate the stud and also to provide sufficient concrete cover over it to prevent subsequent trouble from corrosion and spalling.