It is concrete - what happen last year was the momma duck flew down and quacked at the babies. The first baby jumped and just laid there - then one of the workers that noticed this out the window ran outside. He began catching all the babies as they jumped. The little one that landed on the concrete seemed to come to while he was catching the others.
Another worker got quite a few snapshots of all this - you can find it on snopes.com
Not sure as if he survived - one of the last pictures of the momma with the duckies in the pond has one less baby (maybe just not in the shot - but it was not explained).