Question
Sean,  I have a Pennsylvania bluestone walkway set in concrete.  During a recent remodel several of the stones have popped off the concrete base.  The concrete itself is still in good condition.  How do I re-attach the stones to the concrete base.  If possible I would prefer not to break out the concrete and re-pour the base and reset the stones.  Please advise.

thanks, Steve

Answer
Anything worth doing is worth doing right... so you should first chisle off the old mortar to expose clean concrete slab. The acid etch it with muriatic acid to ensure a good bond. Also clean off the stone piece so no mortar remains. Scrub it off so all loose material is off. Then mix new mortar and apply to the concrete slab in a thickness equal to slightly more than the existing setting bed which you will expose while chipping away the old mortar. This thickness is the dimensions between the bottom of the set stones and the top of the concrete slab. Probably 1/2 inch to a full inch.
Once you spread the mortar you dampen the stone face and press firmly into the desired location. Then tamp lightly with a rubber mallet onto a block of wood set over a folded hand towel. The block of wood and towel are to keep the stone from cracking. Tamp as needed until the stone settles down into the mortar evenly with surrounding stones. This will or should cause excess wet mortar to squeeze out around the edges. Simply scrape and wipe off the excess and discard.
While it is still wet wipe down and remove excess mortar till the joint matches the exiting. Allow the mortar to set for about an hour and then wipe again. Be sure to keep wiping every hour for about 4 hours until the haze created by fine mortar particles is all gone. It shows up as the water dries. Use a damp cloth to wipe.
Now let set for not less then 48 hours before walking on it.
Repeat as needed for all stones.

Good Luck
Sean J Murphy
www.amenityarchitects.com
www.seanjmurphy.com