Re: Concrete Ukemi
Having to 'do' ukemi on hard surfaces is entirely different.
Firstly the area you move over is random, unlike tatami. 2ndly most 'aikido uklemi' are more about form than about actual 'breakfalling' (as in taking the force through our own body) and to do that we need to take more of it at the start (that is ,through our original contact with the ground - usually a forearm).
In real life rolling sometimes you have to avoid bits and counterbalance with other bits...ukemi is an art not a cant.
I don't know if I have communicated this properly.