Question
In July of this year, we as desgner/gen hardscape contractor, had a flagstone walkway installed in colored mortar on a year old concrete walkway done by the builder.  This walkway is about 60 ft. longand is lined iwth brick pavers.  My client is insisting the mortar is pulling away from the flagstone and has marked about 8 areas where there is a faint line (could maybe fit in a side of a piece of paper or a thin fingernail) between the stone and the mortar.  I have had 3 expereinced stone masons (other than the installer) look at this walkway and these areas.  All say the walkway and mortar is fine and solid, one suggested total regrouting MIGHT help and another suggested sealing to ease the clients fear of freezing damage.  Also the client says some stones sound hollow if tapped with a rubber hammer yet walkway is solid. We need help to answer this clients concerns.

Answer
Sounds like the wrong mortar was used (maybe type M) or too much portland cement or possibly not enough sand/filler. In any case there is no easy fix. I had this exact same issue on one of my first jobs. The contractor did not add enough sand to the mix. You have two choices, chizzle out all the affected grout and replace it or add a very thick sealant that will fill the larger voids. (rez-seal or something similar). A sealant capable of filling larger cracks will possibly result in a glossy finish on the stone so beware and maybe do a test area first.
Hollow sound means a void in the mortar beneath the stone. If there are just a few, that is typical. You may break these loose and re-set them. If a lot are hollow sounding you may need to redo the entire job. If the project is in an area subject to significant frost cycles these voids are very bad. Also be sure stones are always brushed clean and wet when applied to ensure a god grip and bond. Adding an acrylic modifier to the mortar is also helpful.
Good luck.

Sean J Murphy, LA,ISA, LEED AP
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