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What are Coefficients of Expansion and why are they important?

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A coefficient of expansion is a measure of how an object expands when exposed to heat or water. It’s important with tiles because unless you understand the principles and apply preventative measures when installing, tiles will expand and crack or lift causing floor failure.

Tiles expand for two reasons: being heated and absorbing water. These are respectively defined by the coefficient of thermal expansion (CoE) and the coefficient of moisture expansion (CME). The thermal coefficient is the amount of expansion (or contraction) per unit length of the material resulting from a change in temperature. In the tile industry the moisture coefficient is normally expressed as a percentage change in size after immersion in water. In tiles the effect of moisture expansion is around ten times larger than that of the coefficient of thermal expansion.

With tiles moisture expansion is commonly called ‘irreversible’ because once water is absorbed in the microscopic holes in the tile there is really no way for it to escape, especially with glazed tiles which are effectively sealed from above. In addition tiles also experience hydration, the chemical capture of water molecules in the tile body. Moisture expansion in tiles is the primary cause of ‘tenting’; when tiles push together and lift from the floor base. It is also the reason for delayed crazing.

The coefficient of moisture expansion is fundamental to the  design of movement joints, the proper installation of which will prevent tenting and cracking. This is even more critical in areas exposed to moisture such as bathrooms and outdoor spaces.

If the CME is a percentage multiply by 10 to give expansion in mm per metre. This can be used to calculate adequate movement joints. But 10% extra should be added.

Class Tile type             Water absorption          Expansion
B1a Porcelain                      < 0,5%                       5 mm/m
B1b Gres porcelain          0,5 – 3%                      30 mm/m
B1b Ceramic                     0,5 – 3%                      30 mm/m
B2a Ceramic                        3 – 6%                      60 mm/m
B2b Fired clay                     < 10%                     100 mm/m

Note the huge difference between possible expansion of real porcelain to fired clay tiles like Terracotta.

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