The Guns of Longues Sur Mer

[30Three]

When you look a the Mulberry harbour parts that are still in the water at Arrowmanche; it's fair to say that the British concrete is fairly robust as well.
The German submarine pen's however are incredible; they'll probably be standing long after us humans have made ourselves extinct!

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[Flying10uk]

The German submarine pen's however are incredible; they'll probably be standing long after us humans have made ourselves extinct!

My understanding was that they put something else in with the concrete mix, other than the normal cement and gravel. I was hoping of confirmation and of what.