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Unit weight of precast roof plank from 1960

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We are investigating an existing building in Massachusetts, built in 1959 and 1964, to which an addition is currently being attached.  It is a one-story building which framing consists of open-web joists at 4' oc supported by steel beams and columns on spread and strip footings.  Exterior walls are CMU infill.

The existing drawings call out <3" PRECAST ROOF PLANK> spanning the 4 ft between joists.  Does anyone know the unit weight of this material?  Breyer's 3rd edition lists <concrete plank> at 6.5 psf per inch, which is 78 pcf.  Regular-weight concrete is 145 pcf.

Is Breyer correct?  Is this the same thing?  In other words, was light-weight concrete in use for roof planks in 1959 and 1964?

By the way, if I use 145 pcf, some of the existing joists and beams are overstressed, which seems unlikely.

Any history lessons from you old-timers will be greatly appreciated.

TIA,

Mark Swingle, PE, SE