Re: Concrete Ukemi
I trippped on concrete a couple of months back and did a face-plant. While picking gravel out of my hands I thought, first, "Why didn't I try a forward roll?" and second, "If I'd have done a forward roll while wearing that large, bulky backpack I'd have killed myself."
I didn't skin my knees, though, so perhaps all that ikkyo practice did help.
Very lush grass is actually one of the nicest surfaces around. Firm beach sand isn't bad either, but watch out for loose sand. It eats your momentum and changes the dynamic of the roll a lot--I cranked my neck the first time I tried it, because I lost momentum mid-roll and essentially stood on my head.