Thanks for all the suggestions guys. I actually have the sledge head attached to some large pieces of steel that go all the way to the bottom of the form so as to take some of the load off the concrete. I was debating whether to coat all of that with silicone caulk like >this guy did in order to reduce vibration. Now back to the original question, I know that adding weight to the concrete won't be the same as having a heavier anvil, but even if it only helps 1% I'd still like to do it. Can anyone weigh in on how big I can go with the steel "aggregate" without making the concrete weaker? Thanks.