measurement in a cold slab

[CCR]

You didn't mention the ambient conditions during the required test period of 72 hours. If the slab is 51 degrees, I would guess the ambient condiitons of the space are not within ASTM F2170-11 standards. The space MUST be placed in operating conditions for 48 hours prior to conductiong the RH test and remain at operating conditions during the 72 hour test period. You need to refer to the document and follow the standards outlined.