Strengthening and upgrading structural systems to improve fire performance of an existing building is a common building renovation activity. For upgrade projects, design engineers must deal with structures in which every element carries a share of the existing load.

The tendency of concrete to spall, or break up, in a fire can lead to loss of the insulating cover to the steel and reduction in overall thickness of the member. In some constructions, supplementary reinforcement is necessary to reduce these effects. The overall thickness and cover is determined by the properties of the aggregate used.