50 Yr old concrete slab surface humidity

[Licklieder]

Having acquired a floor hygrometer (foam box with temp / humdity sensor in it & butyl tape) i've started checking the humidity of a ~ 50 year old ground floor slab. Construction i think is 50mm screed above DPC. not sure what's below the DPC, but i guess another 100mm of concrete.

I have only done one measurement so far, in a hall cupboard - reset sensor; sealed box to slab left for 24 hrs then measured for 6hr - and got a relative humidity of 76% Rh @ 18.5C.

This seems quite high to me. I need to do more measurements in the same and other locations, and intend to do a spot calibration check (https://www.vernier.com/til/4337) with water and table salt (NaCl) to confirm the sensor is reading right.

What sort of surface humidity values should i be expecting ?

 

[dazlight]

Below 75. best text after 48 hours then 72 hours.

Also test in the middle of the floor.

50 years old the visqueen sheets were very thin so be best to liquid dpm the floor anyway.

 

[Licklieder]

48 - 72 hrs. wow. ok. this is going to take some time.

I have dug out some of the floor previously for other works and from memory what i assume to be the DPM (Clear plastic sheet) looked surprisingly thin. Looks like surface DPM is what is going to be needed - but i'll see what my extended testing reveals.

Thanks.