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.