I also have weed problems. OLD cracked asphalt driveway and a long
one. Surely can't afford to replace it. And I read all the posts
here. NONE of it works! Roundup is the closest to a working product
but not against crabgrass. NOTHING kills that. Tried black plastic on
a small veggie plot. Left it there for SIX MONTHS! Yea right. Well
when I picked it up, the crabgrass that was under it was all yellow.
And in a week it sprung right back to life!!!
Tried vinigar, salt, even some of the strong stuff my husband got while
delivering it to farmers. I dont' want to use any of it. I have a
well and don't want to hurt the environment even if I didn't have a
well. This stuff doesn't just go away. It all goes somewhere! I keep
roundup on hand for the occasional large weed that got past me in
spring. But that's all.

It's not the prettiest way but affordable. I make cement stepping
stones to sell at craft shows. The extra cement poured into the cracks
will STOP the problem. First dig out all the weeds and as much dirt as
you can. Then when I clean the bucket anyway, I make sure I have extra
and dump the extra cement in the cracks. Again not pretty but no
weeds! I have even done this by pouring in the dry cement mix into the
cracks and lightly spraying it with a hose. It works.

Don't know what to tell the poster about gravel. Ever try racking it
on occasion? I have a simple weeding tool for my garden works on the
principal that it slices the young weeds at the base before they get
too big. Works well if you go at it on a regular basis. Deeper gravel
will help too.