The local tip is very picky about throwing stuff like this away - two small bags of hardcore per day, or it is 'commercial waste' and you have to pay.
Now we are about to build a big shed with a concrete base. It occured to me that we could save all sorts of aggravation if we buried the old fence posts in the concrete base. I would wash them first (power washer) to remove all the dirt and detritus. This would presumably not be as strong as a brand new concrete base, but then again this isn't going to do a massive amount apart from being the floor to the shed. As long as they were an inch below the surface then the surface should be fine.
Any good reasons not to do this? The saving on the bulk of the concrete required for the floor, plus the saving from landfill and repeat trips to the tip all sound good.