Looked like thin, cast concrete not metal. They are pretty brittle to large shocks. The grenade is still dumb for that job though, there's not a lot of inertia behind the fragments and the explosive concussion is pretty low, it barely cracked at all. Just pick up something heavy you'd probably get more results using it like a sledgehammer.

Also why the hell didn't he try pulling the flimsy wooden fence away from the metal pole behind it instead of pushing into it?