Before breaking out is undertaken, it is necessary to be certain that the reduction in structural section will not overstress the member. Propping is essential, with complete removal of load, if a fully structural repair is required, i.e. in cases where the new concrete or mortar is expected to carry its full share of the load on the repaired member.
The edge of the repair must be at least 5 or 10 mm deep depending on the repair material to be used and the cover to reinforcement. To prevent cracking and debonding, the edges of the repair are usually cut with a concrete saw or disc cutter perpendicular to the surface. It can be beneficial if the edges are undercut slightly so that the repair is keyed into the surface.
It is necessary to break out behind the reinforcement so that it can be cleaned properly and so that the repair material can be easily compacted into the space or flow around the bars. There are various methods of breaking out concrete ranging from a simple hammer and cold chisel for small repairs to electrically or pneumatically powered breakers and water jetting for large areas. Whichever technique is used, care must be taken to avoid damage to the underlying concrete.