I would say using a good washer on drives & patio's you should not really need to use any chem to be honest. If you still need too then most including myself would apply sodium hypochlorite (hypo) afterwards to kill any stubborn algae and/or lichen. Either that I've been known just to use hypo instead and not bother using a washer at all as sometimes I can't be arsed to get the washer set up and start blasting crap everywhere. Not only that I've found some hard surfaces are better treated with a softwash approach rather than using high flow and high pressure blasting.
