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Unit: Inspecting and testing subfloors

LMFFL2004A: Moisture test timber and concrete floors
LMFFL3101A: Inspect sub-floors

Section 2: Moisture in subfloors

Drying concrete to EMC

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In general, the longer the concrete is kept moist, the stronger it will become - because the hydration reaction continues to bond the ingredients together for as long as there is water available.

Although most of the hydration occurs within the first month, it actually continues at a slower rate for years, and the concrete gets progressively stronger throughout that time.

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This graph shows the relative strength of concrete that has been cured for different periods of time.

You can see that if the concrete was allowed to dry out immediately after it was poured, it would only achieve 40% of the strength it could have attained if it had been kept continually moist for 180 days.

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The problem for builders is that although they know the best way to maximise the strength of concrete is to slow down the drying process, they also have a competing priority to finish the construction project in a timely manner.

So good building practice often involves a compromise - the concrete is protected from moisture loss in the critical first few days or weeks, and is then allowed to dry naturally while the work continues.

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The simplest method of protecting concrete is to keep the formwork in place and cover the surface with an impermeable membrane, such as plastic sheeting.

In domestic jobs this is often done for three days.

On industrial projects it can be for one week or more.

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Sometimes a liquid curing compound is sprayed on the surface, which then dries to form a membrane.

These compounds include wax, resin or rubber emulsions.

Admixtures can also be used in the concrete mix itself to retard the drying process.

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Achieving an equilibrium

Like timber, a concrete slab exposed to the air will eventually dry to a relative humidity level that's in equilibrium with the surrounding atmosphere.

The rule of thumb used by many flooring installers is that, under normal conditions, the slab will dry at a rate of 1 mm per day in relation to its thickness.

This means that a 100 mm thick slab will take 100 days to reach EMC, and a 150 mm slab will take 150 days.

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While this drying takes place, there is a moisture gradient in the slab.

In other words, the relative humidity is less at the top of the slab and progressively increases with depth.

As the moisture continues to evaporate from the surface, the heavier concentrations of moisture below are drawn upwards.

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Caution on the rule of thumb

Although the '1 mm per day' guideline is handy to know, in practice there are a lot of variables that can make the actual speed of drying faster or slower. These include:

  • temperature and dryness of the surrounding air - higher temperatures and drier air will allow for a faster absorption of moisture

  • air movement over the slab - the more circulation there is, the easier it will be for the air to carry away evaporated moisture

  • coatings or contaminants on the surface - these will slow down evaporation

  • moisture-resistant membranes on the surface - these will also slow the movement of moisture

  • other forms of protection such as a roof and closed-in walls - these will shield the concrete from exposure to the sun and wind.
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Learning activity

Let's have another look at the bar graph showing the strength of concrete in relation to curing time.

Click on the image at right to see an enlarged view.

How much strength will the concrete have achieved if it is cured for 28 days?

(Note that this figure will be a 'relative' strength compared to concrete that has been cured for 180 days.)

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