We recommend consulting a structural engineer to determine the degree of concrete damage and the extent of repair that must be done. Typically, repair of spalling/hollow concrete involves the removal of crumbling/damaged/loose parts up to sound substrate, cleaning the substrate, treating corroded rebars (if any) with Mapefer 1K, and application of the appropriate repair mortar such as Planitop G40 SP, Mapegrout Thixotropic, or Planitop Smooth & Repair R4.