when concrete explodes from heated moisture expanding, its called spalling (sounds like falling but with sp). It happens during fires in buildings constructed with concrete decks. Heats up, explodes, and rains down on the firefighters below it... been there, done that.. sucks...:upset:
welding is a very focused point of heat that is not going to be in the same spot long enough to cause anything more than a small crackle or sizzle. but toss a good size chunk of concrete in a camp fire and look out. you're talking about a sustained temperature needed in the 1500 to 2000 degree range for this to happen, a 30 second torch job is not going to acomplish this.