Wouldn't water get trapped between the new plastic culvert and the existing one? Could you ever get the seal strong enough to prevent that?
hydraulic cement is primarily used for underwater applications..........in Jersey it is what was used for Hoffa's shoes that have kept him from being found
In short with a properly sized plastic culvert and hydro cement i think you could have a really good permanent repair.
I see no reason why you couldnt fill the gap with hydro cement,after thinking about it this is what I'd probably try to do.
Again I'm no expert and there are plenty here,but with what i know this is probably how I'd go about fixing the problem,if it was mine.
