Any suggestions for repairing a leak in a buried waste pipe?

The leak is under a bathroom slab, under the tub location, between the end of a 6 inch fiber conduit pipe and a 45 degree iron elbow, which is inserted in the fiber pipe. I was wondering if there is a product that could seal this leak from the outside instead of having to break up the slab to repair it. I've read that fiber pipe is pretty brittle.