Plumbing – Copper to PEX connection

You'll need to install a drain assembly. In a bathroom, pop-up drain assemblies are common.

Here's an image of the drain assembly, with all the parts labeled.

_{(source: naturalhandyman.com)}

The tailpiece of the drain assembly, should attach to the trap assembly. If the trap is too low, an extension can be used.

When installing the drain, put a bead of plumber's putty around the drain hole of the sink. Then bed the drain flange in the plumber's putty. From below, install the washer and nut, and thread on the tailpiece. When you tighten it down, the excess plumber's putty will squeeze out. Use you finger, or a rag to remove whatever oozes out.

when conduit or plumbing is covered in concrete it must be insulated from the cements high alkalinity, otherwise the metal will corrode and eventually fail. I don't believe the circulating pump has any detrimental effect on your copper pipe. Although it should be on a thermostat so it only works when needed. What I've noticed alot more in recent years is the quality of copper pipe (most imported from overseas) has become much lower. This and a high pH level could be why the copper is failing. If you are going to replace any damaged pipes it would be wise to re-route the pipes in the concrete to be safe. PEX when installed properly is the better alternative than copper.