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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

Moisture in concrete

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We've already talked about the importance of water in the growth of living wood fibres.

In a sense, concrete has a similar relationship to water - for the chemical reaction to take place that turns cement and aggregate into solid concrete, water is an essential ingredient.

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Concrete forms when aggregate particles are 'glued' together with a cement paste.

The paste is made by mixing cement and water, which sets off a chemical reaction called hydration.

Once the mixture cures, or hardens, the concrete becomes a solid mass.

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A typical concrete mix is about 10-15% cement, 15-20% water and the rest in aggregate, generally a combination of sand and blue metal.

If you've ever mixed your own concrete by hand in a barrow, you've probably used the simple ratio of 4:2:1 - four shovelfuls of blue metal (or coarse aggregate), two shovelfuls of sand (or fine aggregate), and one of cement.

Then you would progressively add water until the mix is fluid enough to place easily in position and trowel the surface to a smooth finish.

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Water and strength

The strength and quality of concrete depends on the proportions of the different ingredients.

If there isn't enough cement paste to fill all the voids between the aggregates, the concrete will be porous and the surface will come out honeycombed and rough.

If there's too much paste, the surface will be easy to trowel, but the concrete will shrink more and crack over time.

It will also be much more expensive, because cement is the dearest component in the mix.

So the trick is to get the balance just right, where the fresh concrete is workable and flows easily into the mould or formwork, but the hardened concrete is durable and strong.

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In general, the highest quality concrete is made by using the least amount of water possible without sacrificing the workability of fresh concrete.

However, this means that fresh concrete will have more water in it than is actually used up in the hydration process.

The excess moisture will remain in tiny pores throughout the concrete and gradually dry over time, until the concrete eventually reaches EMC with the surrounding conditions.

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Admixtures are additives that are put into the mixture. They are used for a range of purposes, depending on the site conditions, prevailing weather or needs of the client. Some of the most commonly used admixtures are:

  • Air entraining, such as special detergents, which improves durability and workability and reduces freezing or thawing problems

  • Superplasticizers, which increase strength by reducing the amount of water needed to keep the concrete fluid and workable

  • Retardants, which slow down the setting time, especially in hot weather, to give the concrete more strength

  • Accellerants, which speed up the setting time and are particularly used in cold weather

  • Mineral admixtures, which improve workability, plasticity and strength

  • Pigments, which add colour to the concrete.

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Learning activity

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For more details on how concrete is made and how to work with it, have a look at the information manual published by Cement Concrete and Aggregates Australia (CCAA) called: Concrete Basics: A guide to concrete practice.

You can download it from the web under the link below:

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Concrete Basics: A guide to concrete practice.