<< In other words, was light-weight concrete in use for roof planks in 1959
and 1964? >>
Intrestingly enough "Walker's Building Estimator's Reference Book", 22nd
edition gives a pretty good treatise on lightweight concrete including that
made of lightweight aggregate and the aerated type. They refer to
lightweights with a density as low as 20 PCF. They list Haydite, Perlite,
Pottsco, Pumice, Vermiculite and Waylight as representative products.
I can personally recall seeing the use of light weight precast planks back
in the mid 50s. With so many types and brands available I would think that
a little emperical job site research would be in order.