Hi, we have cut old tyres using a pocketknife slicing around the side wall just where there is a line on the edge, just below the tread, by using it attached to the hub you can then with a bit of an effort turn it inside out giving a sort of urn effect. We have used this for planters at our gate until the OH donated them to the church for entrance in to the church. But getting side tracked you can them fill the hub with your concrete ballast.
Hope this give you an idea
Tala