Why am I getting LOG_BACKUP as log reuse wait reason?
There is nothing to be alarmed here. Even if you are taking periodic transaction log backup you will see this Log_Backup under log_reuse_wait_desc because last transaction log was not able to clear any VLF (in other word - VLF was not marked for reuse) due to low activity and not many transactions are being logged. 

Here is the precise explanation 
http://www.sqlskills.com/blogs/paul/why-is-log_reuse_wait_desc-saying-log_backup-after-doing-a-log-backup/