Mark,
Most of the big shed's "Brick Acids" with be about 18% max, might be ok on the tarmac if it is just run-off from your inside cleaning going on it as it will be diluted down with the washing, but I would still try a small test area, say the size on a 10 pence, drop a spot of the brick acid neat in an out the way area on the tarmac, see what happens, at a guess it will fizz up real quick, just be ready to wash it of very quicky, then see if it has marked it, there not really any other way I can think of to test this. If there is no marking from the neat stuff then at a guess you should be ok with the diluted run-off acid, but again that care.