I just found out about this today while reading an article on BuzzFeed. Too bad it's over September 7th. Otherwise I'd drive the 4 hours. It looks so fun.
There are tons more if you Google "The Beach National Building Musuem".

Photos for National Building Museum | Yelp

The Building Museum's "Beach" Will Live On at Dupont Underground | Washingtonian

http://www.buzzfeed.com/gabrielsanch...28#.boVp072DAG 3rd from the bottom of the list.