Shotcrete is a placement method for concrete. Both wet-mix and dry-mix produce quality in-place concrete when using quality materials and proper equipment and placement techniques. There are no compatibility problems with shooting wet-mix over dry-mix. Basically, it is just shooting shotcrete on top of already placed concrete. For proper bond, the surface of the existing dry-mix must be roughened, cleaned, and brought to a saturated surface-dry moisture condition before shooting the wet-mix lining.