Structurally sound concrete that contains up to 20% fly ash in the total cementitious materials should not present any problems for subsequent bonding of shotcrete. Concrete with fly ash contents up to 30% have been used in recent years without any reported problems with strength and bond. Although concrete mixtures with higher levels of fly ash (up to 55%) have been proposed, we don’t have direct experience with their bonding characteristics. We suspect it would be fine as long as the base concrete develops adequate compressive and tensile strength. This could be confirmed by a simple bond strength test of shotcrete on the concrete substrate in question.
The existing surface needs to be properly prepared, removing all soft or deteriorated material back to sound concrete. For extensive defects in the existing concrete, chipping hammers may be required. For removal of light surface carbonation or laitance, a strong, high-pressure water blast or sand/bead blasting may be adequate. Depending on the thickness of the shotcrete, reinforcing may be required in the overlay. Specific details of the repair are best developed by an engineer experienced in shotcrete repair.