Most of our new mosaics were set in existing walkways. We used a diamond saw and cold chisels to hog out a shallow trough in the concrete. A small angle grinder with a diamond wheel honed down the uneven bottom of each trough. The floppy mosaic panels were set with polymer modified thin set and grouted with sanded grout. We did most the concrete removal and tile setting during 3 weekend sessions. All 22 mosaics (one on each diagonal of each intersection) are in good shape as of 2012. Well, except one sidewalk was replaced and the mosaic was not saved by the permitting agency or the crew. GRRRRR! Oh, well. History repeated itself, constant vigilance and all that. I guess we could make and install that mosaic again.