Shotcrete consists of the same basic ingredients as ordinary concrete; it is also feasible to use most of the same admixtures. Shotcrete differs from conventional concrete in the method of construction and in its superior properties for certain types of work. Shotcrete repairs require no forming, and this generally results in simpler construction. Properly placed shotcrete is a superior product for repairs and thin overlays because of its excellent bonding properties. It is particularly well adapted to vertical and overhead work where conventional repairs are difficult to make, costly, and often short-lived. By using a light back form, shotcrete may also be used to construct new sections in rehabilitation work provided they are relatively thin and lightly reinforced.
Despite their excellent bonding properties, shotcrete repairs cannot develop good bond if the old surface is dirty or unsound. Improper preparatory work is responsible for more failures in shotcrete repair work than any other single factor. All unsound material should be removed if at all possible before applying the shotcrete. Chipping should be continued until a satisfactory thickness of repair (never less than 1 inch) is ensured. The edges of the repair area should be tapered on about a 1:1 slope to prevent entrapment of rebound, instead of using square edges as for conventional repairs.
If no chipping is required, the surface should be sandblasted. Existing reinforcement should be cleaned. New reinforcement, if needed, should be designed and placed to minimize interference with shotcrete placement. The prepared surface should be carefully examined for cleanliness and soundness before shotcreting. The surface should be wetted as needed so that it will not absorb much water from the shotcrete mix. However, the surface should not be saturated at the time of shotcreting; a little suction improves the bond.