Dave, you sound like my kind of masochist. After maybe two months of training in aikido a friend told me that my ukemi was ok but it was nothing if I couldn't do it on the sidewalk. I went out immediately and took several breakfalls and was only slightly sore the next day. It may be linked to a lack of brains but I didn't think when I was being thrown and I just let it happen, not worrying about the surface. This training did help because when we got snow the sidewalks would sometimes become like glass and I took a couple of breakfalls later with no pain at all. I still continue with this masochism periodically to keep myself honest but I don't often because I am concerned about the long term effects on my body. I do recommend working up to falling on concrete so that you know you can do it if you need to.