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A creed for a dry screed

Steven Richford gives information and guidance on strategies to provide you with more control and predictability, to enable you to better understand key attributes of certain building materials and where issues of moisture measuring are likely to be a problem

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.

This article provides information and guidance on strategies that will provide you with more control and predictability – to enable you to better understand some key attributes of certain building materials and where issues of moisture measuring are likely to be more of a problem.

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.

Walls are less of a problem but are not completely without issues. Masonry walls can usually be decorated with a moisture tolerant permeable paint, which will allow the continued release of water vapour during the first months of building occupation. Floorcoverings on the other hand can fail completely if not provided with the right conditions.

If the focus is on the moisture content of screeds what steps can be taken to reduce the risk of last-minute problems? Last minute problems such as the flooring contractor refusing to lay the floorcovering after he’s measured the screed moisture levels.

First, 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 re-wet screeds.

A screed that is re-wetting after curing can then take twice as long to dry. As you will know most of the water added to a screed material 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, ‘remnant water’ we could call it.
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 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’d 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’s less of a need to get down to these levels when such types of floor-coverings are being used. So that ceramic tiles can be installed at an earlier stage without any risk. Thus, it’s important to remember there’s 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’s 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’s these surface values that affect the chosen floorcovering.

In the case of an anhydrite material, as the moisture content decreases as it dries, you’ll see from Fig 1 that the moisture content by weight drops down from 4% to 3% to 2% and then to 1% and yet there’s 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 s/he fully understands the material. In contrast a cement screed will show a much earlier tapering off of the relative humidity, as Fig 1 shows.

You can see that the anhydrite screed needs to get down to a very low percentage moisture content by weight (around 0.7 to 0.8%) before the relative humidity starts to drop very quickly according to the characteristics displayed by the materials’ adsorption curves.

This means someone measuring the humidity on an anhydrite screed for a long period will feel very insecure and not really know if they’re getting anywhere with the drying process.

They might continually measure 95 to 100% ERH without seeing any drop, yet the screed may be drying very well.

It’s only when you use other measurements (measurements of moisture by weight) that you know things are progressing in the right direction and can predict the desired fall in the relative humidity will happen soon.

Such knowledge allows the constructor to be optimistic that deadlines will be met. It also means that when things are not progressing well, action can be taken to install drying processes, or to consider the application of a surface damp proof sealant. Measuring moisture content by weight can be carried out using a Speedy Meter (AKA Carbide test) or by using some specialist radio wave transmission meters (eg Tramex CME).

It’s clear that being conscious of elevated moisture levels, how to measure them and acting at an early stage, saves significant time and money. If it comes close to your handover date and you’re considering a forced drying regime, then you have probably set yourself quite a challenge.

If you’ve left monitoring late and suddenly decide there is a need to reduce moisture levels, you might have to resort to an emergency strategy. This might involve partially drying the floor and then using a surface vapour barrier (an expensive solution) which then locks the moisture into the building.

This also can reduce the building’s insulation value and the moisture could then, depending on construction method, find its way into the walls.
In conclusion, we recommend that the moisture content of screeds is evaluated early, that laitance Is removed from anhydrite screeds and measures taken to ensure the screed is protected from any further wetting from rain or site activities.

Failure to do this can lead to contractors missing deadlines and incurring post-handover claims for deterioration after occupation because of higher moisture levels.

Richfords Fire & Flood is offering CPD training on these issues from this year.
01858 414278
Steven Richford is chairman, British Damage Management Association (BDMA)

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