You should look into the epoxy concrete, or epoxy granite as a filling product. It is stronger than concrete or steel, dampens vabration much better, and it does not expand so it want change the shape of the tube its being poored in. It also adhears/clues very good so holding it in place is not problem, add a few cross screws if you want to help it have something to hold on, but it really doesnt need that.
The truth is with the this 80/20 many people have built machines and found there was no need to fill them because it performed so good the way it was. But filling it will help and make it that much better. You can even run a threaded rod through the epoxy and put a load on it to make it even stronger. They use this type technic in bridge building to add strenth and control flex. If I understand right the rod runs all the way through the concrete, then it has plates or just nuts to clamp down on it after its cured. You can do some research and find the exact methodes.
Jess |