a screed which is "dry enough" i.e. 75% rh will not be dry in terms that it will still have water in it. a cement based screed will have around 3 to 4% free water which is effectively locked within the pore structure of the screed and a calcium sulphate based screed will have around 0.5% free water trapped between the gypsum chrystals.
at this level of moisture it is deemed that the vapour pressure within the screed reltive to the vapour pressure in the atmosphere above it will be in equilibrium so that there is negligible moisture egress from the screed. No screed will offer a uniform moisture reading across its entire surface as the relative densitiy and porosity across its surface will vary meaning that hte rate of moisture egress and the vapour re-uptke rate will vary.
Bear in mind that screeds like most porous materials will accept mosture from the atmosphere as well as allow it to move the other way. If the vapour pressure in the atmosphere increases the screed will absorb moisture. lots of folks say this is a problem with calcium sulphate but in reality ts the same issue for all porous materials. thats why its important to contrl the atmosphere above the screed i.e. temperature and humidity levels.
the action of heating the screed essentially increases the vapour pressure within it so that the moisture is forced out. you must always carry out the moisture testing with the screed in a cold state though. 6 days to get up to 55 C and 4 days at 55 is unlikely to be long enough to dry a traditional screed and you are lucky that you have not turned it to crazy paving although I appreciate hat is the way the heating standard says tocomission the heating.
Our advice in general is to over ride room stats so you are working off the heting manifold mixer stat. Start the heating at its lowest temperature Usually 35C and leave for 3 days before increasing the temp by 5C per day up to the 55C. Leave it there for minimum 3 days (7 ays when force drying) before reversing the process. Once you get back to the minimum temperature switch it off for 48 hours before doing a hygrometer or CM moisture test.
I'd be interested to know the screed type. If the screed is cement based liquid screed there is likley to be a curing agent on the surface which is designed to hold the moisture in the screed so youd get a nice dry result but the moisture is still there and will work its way out slowly. The curing agent should be removed by lightly abrading (sanding) the screed. If its anhydrite screed then the depth will determine the drying time as much as anything and 28 days only just long enough for a 50mm depth in good dring conditions which we have not enjoyed this winter. if its traditional sand cement then 28 days is no where near long enough. you need ot be llooking more like 6 to 10 weeks at 65mm depth.
