Here's a look at that...this crack IS deep, and the concrete all around it is thin and crumbly. I'm guessing this is related to the shape, in that water probably runs down that seam as a channel, keeping it unusually wet (when you crack off loose material it is wet underneath, green with mildew even). I'm not sure what I'll do yet. This is one of those situations where talking it out with Norbu would be helpful, but one does what one must. I'm leaning towards using the thin set, and seeing how it works. The same area was similarly damaged on the Enlightenment Stupa, and we found the thin set works better here. They key will be how well it stands up to the winter!