My fireplace has small cracks in the bottom of the concrete . Do I have to take out all the concrete and replace it or can I just put some refactory cement over the crackes? I use the fireplace a lot in the winter and go through 2 to 3 metal racks a year thank you for any help you can give.

Most fireplaces do not have a concrete floor just for the reason you state. If you can give up 3" I'd lay in a bed of refractory brick just set without mortar. The refractory cement will give you a repair but not long term.