I do too but Pekka disagrees and I see his point about shrinkage. However upon doing some extra reading it seems that most of the shrinkage comes from adding more water than is needed to make the concrete move better it seems that if the added water is calculated correctly then little shrinkage occurs but the mix is extremely thick. Will have to look into it a bit more.
I like the idea of filling with concrete. Adding mass is one of the simplest things you can do to a small machine to improve it.......
The thing is that concrete shrinks for 400 years. First faster and then slower. Many factors affect on shrink rate and some you have influence and some not.
Worst shape for concrete "filled machine" is a hollow thin steel section that you fill with concrete...some machine manufacturers tried this -70/80:s and failed. Concrete filling will shrink in time and delaminate from the skin...if you weld rebar to avoid that it will pull the skin out of alingnment. There are ways to reduce the shrink rate or compensate it, but it will not work in machine building scale.
You can make a servisable machine if you know what you are doing. Basically you cast more or less square section, no voids in it, you may cast some features to it. You don't need rebar and other "iron" on it, unless you have hanging beam or such bad idea. And then let it cure and age some time - longer the better. Then use this cast as a sandwitch between steel members and have it on compression. You must use epoxy concrete, injection epoxy or grind the concrete surface flat prior mechanically mounting the steel sructure to it.
Epoxy concrete works.
And as Andrew et.al. said sand or such will dampen "ringing" pretty well too, it's cheap, reversible and has all good qualities to home build.