Dry-set (thin-set) mortars are designed primarily to bond to the substrate and the back of the tile. Manufacturers work creatively to develop mortars that are easy to mix and place, along with the ability to provide a tenacious bond. However, they do not make one-sided mortars, meaning they only bond to the tile or substrate. If the mortar bonds only to the substrate, the problem may often be due to site conditions.
Case in point: recently, an inquiry asked why the residential tile was loose and breaking. It was a four-year-old installation of large format plank tile on concrete using the direct bond method as found in the Tile Council of North America (TCNA) Handbook method F113, “On-Ground Concrete with Ceramic Tile.” The tile sounded hollow, the grout joints were cracked and crumbling, and numerous pieces of tile were broken.