I appreciate the input from a building perspective and I'll buy that, sure, I don't know shit about pouring concrete.
Cold war paranoia is the likeliest reason.
I still haven't had any rational reason for these to be for bombs. It's just much more effective to have bombs actually in the structure instead of just hanging on it. Imagine trying to blow a safe. Would you do it just by leaning an explosive on it and wishing for the best? Nah. You'd at least try to attach it to the lock somehow.
And every article I can find on bombing bridges in Finland talks about "charge pits", not "a line of hooks well hang explosives off of". So I just don't buy these being military in any way. Not convinced.