Removing rust from concrete

I have rust strains on concrete from where my fire drum (truck brake drums stacked 3 high) has been sitting.

This is something I've never had to do before so I'm assuming a scrubbing brush and chemicals.
Will CLR (I've never used the stuff) do the trick or do I need acid - if I need acid what ratio do I dilute it to?
Is there anything else available other than those two?
Cheers
Paul

This stuff should do the trick.

Just a moment...

I have used the above mentioned product to successfully remove rust stains from concrete driveway

Oxalic Acid it the primary product you are looking for which is the key ingredient in the Diggers product recomended by Jack620