Jim,
Beware of a second pouring to thicken a concrete shelter lid for added fallout protection. It is essentially a dead load that causes the lid to get closer to it’s stress limit. Thickness adds strength only when it is part of a single pouring, with all the needed rebar integrated. I would advise consulting an engineer before adding unforeseen load to a concrete span. – Mr. Bravo
JWR Replies: Thanks for mentioning that! I have just added a proviso to my original post.