Hi Anthony, thanks for your response

After some researching I believe I've found the reason for the above described behavior. This is an extract of some info found in the web: "....changing the recovery mode to bulk logged before index rebuild, any extents that are allocated and changed through a minimally-logged operation are marked, and then the next transaction log backup will also read all those extents and include them in the backup" This is the cause of huge log backups files. Unfortunately no enough available free space left in the server and no idea how much the log backup file will need to grow after an index rebuild operation.

We are continue working to find the best schema for our maintenance plan task.

Thanks and regards