Hi Fred: A concrete ceiling!? Hmmmmm......don't think I've ever seen one of those in a residential home before....must be heavy (smile)
Your best bet is to rip a 2X4 into a triangular strip (a nailer), big enough to just fit behind the crown molding without hitting the inside of the molding. Then drill 1/8" holes through the nailer and mark the concrete ceiling where the hole lines up. Then with a roto hammer, drill 1/4" holes in the ceiling and install plastic anchors. Do this every 24" or so. Then secure the nailer to the ceiling using drywall screws into the plastic anchors. When you put up the crown molding, nail right into the middle of the molding into the nailer. Cope the corners as discussed above and you should have a securely fastened crown molding.