Our terrace also has decorative grit [Edelsplitt] beneath the top layer. Whether I wanted the surface layer to be installed crunchily in a bed of grit would be questionable. We have joint crosses and the narrow joints were filled with tiny grit, not sand. However, our surface layer consists of natural stone slabs. In the driveway, however, there are smaller concrete paving stones with sand joints. The joints were not technically necessary; they exist only because permeability was required. The very tiny grit interlocks well and no ants get in. The stones stay firmly in place. The sand in the concrete paving stones tends to moss, the ants carry it away, and I have to do some touch-ups from time to time.