I would not use asphalt.
Granite grit is good. Nice and sharp, not rounded. Here is a pic of optimal grit (what my feed store sells):
http://cherrystonegrit.com/grit_sizes.html
size #3 is large fowl
#2 is up to around 7 weeks
#1 is chick grit
If it is just regular gravel, I personally would...