The first four birds I had, in a town garden, were Black Rocks. I had them in a little P&T Poultry house, which is still going strong.
The section of garden they were in was half concrete and half soil.
The concrete meant everything didn't turn to a mud bath when it rained and the feeders etc. stayed clean. The hens would choose to be on concrete then, so I assume they don't like muddy feet if given a choice.
The soil I dug over every week, much attended by Eenie, Meenie, Minie and Mo who would get the worms before I even saw them

They liked the soil for scratching about in.
So basically, my advice would be not to dig up all the concrete.
For letting them out into the garden, I bought some mesh panels which I think were sold to make a dog run. Anyway, they were 3ft high and clipped together. Again, I've still got those and they meant I could let the hens into the garden for a much bigger run and scratch about without risking them getting into the flower beds!