Anhydrite screed prep and moisture testing

[Deleted member 18697]

I have an anhydrite screed which I have been testing with a digital hygrometer which measure relative humidity. After further research there seems to be conflicting views on how to test if they are dry enough. I was waiting until it read less than 75% RH. Then I was going to proceed with an acrylic primer and a cement based adhesive which was fixing ditra down and then limestone on top.

I would be interested to hear other views on how they test for moisture in these screeds.

Many thanks

 

[bugs183]

Hi Stone D.
Ajax is your man to contact when it comes to the testing of these screeds. He recommends a hair hydrometer, not a digital one (the kind you may find in a FBall Hood tester).
The hood test appears to be the best non destructive test around.
I've spoken to Protimeter and they are positive that their hood testing equipment is accurate.
The second test is a carbide bomb test which is popular on the continent. This only tests a small chunk of the screed taken from the top and is therefore not considered overly accurate by many experts i've spoken too.
The U.S and Australia favour in situ probes that measure the humidity from within and would seem to be the most sensible. But U/F/H pipes could be damaged during the drilling.
I have tested adhesives and so have others on these screeds, and if i was doing your job i would lay it this way. Of course this is up to you !
Buy the best gypsum friendly adhesive possible which is Creative Impressions:
Flooring systems | Creative Impressions
Test for moisture, when dry to manufacturers spec Sand the floor, prime with GBLC, fix ditra with GBTA. Then fix stone to Ditra as normal.
Despite many manufacturers saying it is fine to use cement based adhesives gysum based would be the best way to go in my opinion.