29 CFR 1926.417
Controls that are to be deactivated during the course of work on energized or deenergized equipment or circuits must be tagged. Equipment or circuits that are deenergized must be rendered inoperative and must have tags attached at all points where such equipment or circuits can be energized. Tags must be placed to identify plainly the equipment or circuits being worked on. CONCRETE AND MASONRY CONSTRUCTION
29 CFR 1926.702
No employee must be permitted to perform maintenance or repair activity on equipment (such as compressors, mixers, screens, or pumps used for concrete and masonry construction activities) where the inadvertent operation of the equipment could occur and cause injury, unless all potentially hazardous energy sources have been locked out and tagged.