How do you repair a leaking pipe that is under concrete?

You will have to remove the concrete above and around the leaking pipe. All or some of the pipe may need to be replaced. If your house was new in the early 1980s, the PVC pipe that was used has been found to be generally faulty.

ANS 2 - FIRST - make sure it IS under the concrete. In my experience this hardly ever happens unless the foundation has cracked, or building settled considerably.