This is what shows in the hep from Winrar regarding repairing archives.
This command is accessible both in file management and in archive management modes. It considers selected files or an opened archive as corrupt archives and attempts to repair it. To run this command use either Tools menu , "Repair" button on the toolbar or Alt+R keyboard shortcut.
WinRAR is able to repair only RAR and ZIP archives. WinRAR tries to detect the type of the archive automatically using archive data and extension, but user also may set an archive type manually.
The source archive is not changed and the result of the operation is stored into a new archive, which is named fixed.arcname.rar or rebuilt.arcname.rar, where 'arcname' is the original archive name. Chances of a successful repair of RAR archive are significantly higher, if it has a recovery record. Note that a solid archive without a recovery record cannot usually be successfully repaired.
Technical details of archive repairing are provided in the command "r" description ("r" is the command line equivalent of "Repair archive"
