I think you might be thinking about two different things here. Most concrete counter are full thickness "slabs" that are made off site and then installed like a granite counter. As a DIY project these are far beyond most people's skill set.

Replacing your tile however would be a great DIY project. In the last few years I have done a lot of very large tile installs. These give you the look of a granite slab but at tile prices. ...»

I have did a ton of concrete countertops by myself. It's easier when you have help though, and a shop with all the right tools....... And several years of experience under your belt. Making your own concrete countertops may turn out to be a little more involved than what you think.

I just reread your question and you seriously need to know what you're doing before you start pouring concrete over your existing tile countertop, or I promise it's not going to turn out good. There's too much for me to type that you need to know before going that route.