Did you use the 5000 with water and nothing else? Why 5000? Did you work in too cold of an area, like outside or an unheated building?
Too wet of a mix can give you crumbly and lots of air bubbles. Weather too cold? I know that 5000 is for working in cold temps but if a frost or freeze get this before it cures it can crumble.
It's hard to tell from the pictures but these look a little thin to be stepping stones. You have to watch out when using rhubarb leaves or any thick leaf. The veins can be very large and lead to cracking of your stepping stone if the stones aren't thick enough. Billie