Each piece is a different size and they have different shapes to them, even though they all look the same. The pieces that'll go on the roof of the funnel will be thinner than the ones where the raft will travel on them for structure stability and safety. Some also have holes to let water in and out of the slide. Unfortunatley, I don't know the range in the wieght of one piece but I hope this makes it clearer for you.
They do make spare pieces with water slides just incase if they dent or break. That and they also take extreme care in keeping the pieces from being dented or broken. To answer your question though, they do not put the piece in if there's a dent on it. Even if it's a waterslide, the smallest of a problem can make the biggest of an impact.