What is the difference between PU and Epoxy injection?

The main difference is purpose: PU (Polyurethane) is for *waterproofing*, while Epoxy is for *structural repair*. PU is flexible and expands to stop leaks, allowing for slight movement in the crack without tearing. Epoxy is rigid and has incredibly high bond strength; it is used to 'glue' concrete back together to restore its structural integrity. You would not use epoxy on a live leaking crack (it won't bond), and you wouldn't use PU to structurally restrengthen a column.