i wouldnt train on concrete or tarmac surfaces, both surfaces have high impact on your dogs joints and feet, i would cross a tarmac road or use one on a very short section of a trail but that would be my limit, if you choose to train on hard surfaces that hurt the dog, it will learn to cope with that impact by altering its stride the problem with that is that you probably wont get the best out of your dog and the altered stride may put unnatural pressure on another joint or bone.
nush