Does the tortoises outside enclosure need to be on grass? 

I have seen something saying no concrete but cant find out why? And also read the opposite that it shouldn't be on garden as they can dig out and get lost.

I guess my main question is... if an enclosure with a base (filled with appropriate material) was laid on the concrete would that be ok? 

Thanks for your help 

What size is the garden ?  And the biggest border is ?

Regards,

Tom

Perhaps a concrete tortoise.