Why concrete? Two reasons:
1. Beautiful and incredibly flexible material (surprisingly enough) with so many options and ways to form it.
2. It costs practically nothing and can be bought at nearly any common hardware store.
(You know how much artists love hardware stores, right?)
Of course the actual objects look nothing like the drawings. I made the sketches before I'd ever even mixed my first batch of concrete. (Wishful thinking?) As soon as I started applying my mix to my armature everything changed. But hey, that's half the magic!