If your /dev/sda disk are failing, you should replace it with new one. You server should support hot swapping:

mdadm –manage /dev/md0 –fail /dev/sda1

mdadm –manage /dev/md0 –remove /dev/sda1

this will copy partition table:

sfdisk -d /dev/sdb |sfdisk --force /dev/sda

mdadm -a /dev/md0 /dev/sda1

if you have more partitions, you should repeat that steps with sda2, sda3 and etc.