You can clean tarmac without problem, you will find the old tarmac comes up to the colour of the tyre marks. you won't damage it unless it was layed wafer thin and it would have shown up already. The only problem is it shows the stripes if you rush, chewing gum appears because you clean the dirt off the top and the silt is a pain to move on. you can use acids on tarmac but do not use degreaser, solvants or petroleum based products as they break down the tar