I would put the tiles in some sand and then pour the concrete over them. That way they would stay in place. I have seen some cool pavers made with terra cotta pieces along with stones, glass and shells. A mold was created with wood (screwed together so it could be taken apart easily) and the pieces were put in the bottom and the concrete poured on top of them. In one magazine, they had a "mat" that looked like a patchwork quilt with pavers decorated like that. Below is a picture of what I'm kind of talking about. This was taken at Grandview, the farmer who lived there created all kinds of mosaic sculptures and even mosaiced the outside walls of the house. This is a "mat" in front of the front door. It looks like it is made with stones and glass cut you could do something similar with some of your broken up tiles.

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