The single most common reason repairs fail is poor preparation of the area to be repaired. All unsound concrete must be removed, the reinforcing steel exposed and cleaned, and all dust and debris removed before placing the new material.
1) Patch Geometry: Although the damaged area of concrete may be irregularly shaped, cutting out areas as close to rectangular as possible, or at least with square corners, will result in better-looking repairs. Avoid re-entrant corners. Squared off repair areas will also be easier on the workers and result in more durable repairs. Locate all embedded materials such as reinforcing steel, post-tensioning cables, and electrical conduits.
2) Exposing the Reinforcing Steel: Whether concrete removal is accomplished by mechanical means (impact breakers) or by hydrodemolition, performing a successful repair requires removing all loose or delaminated concrete. This includes removing all concrete above, around, and below any corroded reinforcing steel and any concrete “bruised” by heavy impact hammers. Make sure there is at least 3/4-inch clearance around rebar.
3) Repair Area Edges: Saw-cut the edges of the repair area at a right angle to the surface, but be careful not to cut into the reinforcing steel. Feather-edged repairs will almost always fail. In many cases it may be necessary to start with heavier equipment, then switch to lighter equipment for the final stages of removal to eliminate bruising. Remove up to and slightly into the sound concrete and to where rebar shows no sign of corrosion.
4) Cleaning Reinforcing Steel: Oil-free sandblasting is the best way to clean off all loose mortar and rust from reinforcing steel. Be sure to angle the nozzle to get the abrasive to bounce up and clean the back-side of the bars. Bars that have corroded enough to lose some of the cross-section must be replaced or supplemented with additional steel. Make sure in either case that the lap length between new and existing steel is as specified in ACI 318.
Illustrations: Curt Spannraft