A CREED FOR A DRY SCREED
Here, Steven Richford MD of Richfords Fire & Flood offers expert guidance on the importance of screed moisture levels.
Any construction project manager will be able to list numerous issues which cause delays and make completion dates harder to meet. One such issue is where delays are caused due to acceptable moisture levels not being reached in time. If this is not recognised early enough it can be costly to resolve when time is short.
The biggest challenges arise from floor screeds and slabs. Most screeds will have a floorcovering installed and many of these floorcoverings have a stringent specification for moisture conditions that they can tolerate. Floorcoverings can even fail completely if not provided with the right conditions.
If the focus is on the moisture content of screeds then steps must be taken to reduce the risk of last minute problems such as the contractor refusing to lay the floorcovering after he has measured the screed moisture levels.
Firstly consider how you could protect the screed from being re-wetted after it has been laid. Re-wetting typically occurs from exposure to rain, although some site activities could also cause it to occur. A screed that is re-wet after curing can then take twice as long to dry. As you will know most of the water added to a screed becomes incorporated into the material due to the hydrolysis reaction which takes place as it cures. The water that has to be evaporated after this curing phase is the excess moisture not used in the hydrolysis reaction, or what we could call the ‘remnant water’. The problem with subsequent re-wetting is that a cured screed can absorb much more water than the amount of the remnant water. So the drying can take more than twice as long to complete if a screed is re-wetted after curing.
Anhydrite screeds do need further attention after curing when the surface laitance material should be ground off to allow the release of
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water vapour and to permit the proper adhesion of floorcoverings.
Of course, no matter what happens to the screed you need to be able to plan and so you need information on the moisture content during the drying phase. This will provide you with reassurance that deadlines will be met or will help you decide if action is needed to speed the rate of drying.
If a floorcovering has a moisture tolerance value it will be expressed as an Equilibrium Relative Humidity value (ERH). One measuring standard is to test relative humidity (RH) at the surface - achieved by testing within a small box on the surface in equilibrium with the relative humidity of the screed. To measure this you would need a well calibrated digital hygrometer. Acceptable conditions for floorcoverings vary between 75-85%.
Note that ceramic tiles laid with cementitious adhesive can tolerate much higher humidity levels. There is less of a need to get down to these levels when such types of floor- coverings are being used. This means that ceramic tiles can be installed at an earlier stage without any risk. Thus, it is important to remember that there is variation in the relative humidity specified by the suppliers of adhesives and floorcoverings.
Screed materials vary in how quickly the ERH drops as it dries. In particular, it is more difficult to evaluate the moisture levels of a gypsum (anhydrite) screed. This is because, even at very low total moisture by weight content, anhydrite screeds can display very high ERH values at the surface. It is these surface values that affect the chosen floorcovering.
In the case of an anhydrite material, while the moisture content decreases as it dries the moisture content by weight drops down incrementally from 4% to 3% to 2% and then to 1% and yet there is no
significant shift in the relative humidity. The RH can stay stuck at 95-100% and therefore mean a nerve-racking time for the project manager unless they fully understand the material.
In contrast a cement screed will show a much earlier tapering off of the relative humidity. You can see that the anhydrite screed needs to
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