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I think many people find that the mortar is softer and that they won't penetrate through it, vs drilling in the center of a hollow concrete masonry unit(CMU)and penetrating a hollow cell. In some designs of large buildings, the CMU can be high strength, I have used certified 5000 psi CMU with 3000 PSI mortar, fully grouted for structural walls, the mortar was definitely softer than the high strength CMU, you can easily differentiate between the 2 when drilling to install a fastener or using a powder actuated fastner There are a variety of fasteners to consider for masonry applications, if it is a small mirror, a threaded type, with or without an insert or sleeve would work fine, tapcon or similar. There are now a threaded type of concrete bolt, kind of a giant tapcon, but these still don't have the higher strength properties like an expansion type fastener and or other types. If you look through a Hilti catalog, technical information is provided for all fastener types and they do list shear, tension and other strength properties. Just reading the catalog can be a learning experience for anyone new to masonry fasteners. If the CMU are hollow, probably standard strength, a threaded type of fastener, one with an insert or sleeve would be fine, as would one with a toggle, as long as this is not a giant mirror with substantial weight that would be dangerous if it fell. The CMU should not crack or weaken. Do not over tighten the fastener, snug it up tight but don't crank on it until it fails. The CMU has cells and if they are hollow, the depth of the fasteners thread engagement is going to be the same as the wall thickness of the cell. If you were to use an expansion type fastener, like a 1/4" Hilti expansion bolt, you would drill a hole and have to set the expanding part of the fastner within the thickness of the wall of the CMU cell, or gain some depth by using the mortar joint, these won't work if you penetrate through and try to tighten them up but I hve used them on hollow CMU, just have to find the sweet spot and get them to lock in the cell wall. It is possible to pull one of these types through and spall the area around the hole, but I doubt it would crack, unless the CMU is in a deteriorated state, it would just make a hole. There are also epoxy type fastners, drill hole, inject epoxy or insert tubes that will be crushed and activated by the fastener when you place it in the hole. Many times in large buildings, certain areas of CMU walls will be filled with mortar or grout, coordinating with other building components that will need suitable places to fasten.
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