We want to build a dog run in the backyard for our German Shepherd pup. She likes to chew rocks. Horse stall rubber mats? Cement blocks?
Cement is a good option, its very easy to wash if she poops or pees/makes a mess(wash with a hose). It also prevents dogs from digging under the fence. Though instead of using blocks just fill the entire area.
Shelters in my area use cement, really convenient.
As a side note make sure that there is a shaded area so she can get out of the sun if need be :).