Scott, are you tring to save the tube or do you want to stop the water from flowing thru it?
If you want the water to continue to go thru it then I have a cure. In Michigan there is a company that makes plastic liners that go inside of the old galvinized tubes. For example, if you have a 15" tube then you would slide this liner in and put a little concrete around the ends and you would end up with a 12" plastic tube.I can't remember the name of the company but if you call your local Road Comission or someone that deals in drainage alot they could give you the name of a company in your area.

I hope this helps, I have put these in before and they work great.