The other day I saw my neighbor using an herbicide to kill the grass that was growing up through the cracks in the driveway concrete. I asked him why he was using that expensive stuff (I think it was Round-Up or something similar).
I advised him to just use vinegar instead. Sorry, I don't recall where I first learned of this tip. I was doing some research on organic lawn care several years ago and ran across it. I believe it said that it was the acidity and Ph that did the killing.
This article on using vinegar as a herbicide says it's the acetic acid that's the active ingredient. But then it goes on to explain that common household vinegar is only 5% acetic acid, and as such, poo-poo's the idea that it's very effective. But shhhhhh, don't tell my driveway; it's been working for me and saving a ton of money!