I have 3 showers that have the same problem. Below the metal part of the top of the drain there is a "clay" section prior to the trap. All three are crumbling and I was wondering if there is a way to "sleeve" them. I really do not want to have to jackhammer the concrete bases if I don't need to.

How old is your home? I'm pretty sure that is not clay but lets start with the age of the house and get back to me. What is the shower pan floor made of and can you tell what is under that material?

"Below the metal part of the top of the drain there is a "clay" section prior to the trap."
Would you be referring to the putty seal under the lip of the drain?

Concrete bases? Where are they located? Are they built into the slab such as mine is or are we talking about a manufactured base? Regards, Tom

The showers are built into a building slab.

Would it be safe to say the showers are built on top of the slab? Never seen one that was actually part of the slab. The drain exiting the shower pan, hand built or pre-made, an enters the drain. This is the point where Tom and I don't think you have clay. Like Tom said it could be old putty. It could even be some of the mortar/cement used to make a hand made pan.