I guess what I am asking is that since a lot of these take downs seem to involve your knees slamming into the mat -and again I am new so forgive me if I am wrong- would that kill your knees if you try that on pavement?

You could be hurt by OTHER things on the street, such as by something your opponent does, rather than something you do to yourself.

People have differing opinions regarding the takedowns that are better for the street. Personally, I favor upper-body wrestling takedowns that are more low-risk, in that they don't leave you in bad positions should they fail. Many of them involve the saying of, "'we' land on 'you'" - in that your opponent acts that the landing pad. You lower the risk of being hurt when attempting these. Of course, if you're worried about injuring yourself as a result of your own actions in a street fight, you should probably avoid fighting altogether as that's just the wrong attitude to take (kind of like people worrying about breaking their hands in a street fight). You treat your injuries afterward. In a real fight, you commit everything you have and leave the rest to "God", chance and fate bro.

Now the takedowns that I'm speaking of are mostly from wrestling, but the same applies to virtually any throwing art. There will be a select few that you'll probably gravitate to the most over the course of your training. You will spend a lot of time practicing and training those throws. Over that time, you'll develop a depth of knowledge which within that, will enable you to develop a feel for that which will basically keep you safe. There's no shortcut. Experience matters.