Seems the accretion process allows for limestone (calcium carbonate (The coral polyps that produce it)) to grow at a rapid pace. I suppose that once the rebar is encased in stone, there is little chance of anything seeping. As the article points out, it only grows stronger as time goes on and is somewhat "self healing". I've been following more links and honestly, whomever comes up with the home version of this will most assuredly make their millions off us hobbiest. Coral that grows 10 cm in 10 weeks?? Im in! Though from an outisde perspective, it is very sad in way. The only way to continue to support natural life on this planet will apparently be by putting everything on 'life support' at some point..What happens when we start having to pull the plugs??!!