Deep rooted weeds in concrete joints

I've finally had enough of having weeds grow between the joints in my side-walk so I'm in the process of trying to seal it. My only issue is there are a good number of weeds that I can't pull/clear because they're real thick and the roots are so deep. I got a weed torch to try and help but I'm not sure I'm doing it right. Any pointers on how I can help get rid of them ?