Led by Europe, more tests are being devised all the time to try to evaluate stone. In this column Barry Hunt explains the tests and discusses what the results show… and what they don’t. This time he looks at the water absorption test.

This is probably the simplest yet most versatile test available. But that means it is also the one most used out of context.

The test involves dunking a piece of stone in water and see how much it absorbs. How simple is that? The question is: what does the passage of moisture into a stone really tell us. Just as importantly, what doesn’t it tell us?

The test typically involves cubes of stone, but any size and shape can be accommodated. The test specimens are dried to constant mass – which means they can no longer release any moisture. They are then immersed completely in water. The test specimens are left until they become saturated to the point where they can no longer absorb any more moisture, which can take several days. The gain in mass from the added water is divided by the original dry mass of the sample, which provides a fraction that can be converted into a percentage.

In Europe, the standard for the test is EN 13755. In America it is ASTM C97. Both tests are basically the same although there are differences in specimen size and number. For slate and other thinly laminated and cleaved stones typically used in sheet forms, cube specimens might not be used. The standards for testing roofing slate are set out in EN 12326 in Europe and ASTM C121 in America.

In stones with low absorptions the test values tend not to vary much, but as the values increase so does the variation – because features forming the void spaces introduce greater natural variation.

The surface area to volume ratio is also a factor. Water ‘sticks’ to the outside of test specimens, which is sometimes termed the adsorption rather than absorption and may be critical for slates with low absorption.

The results can be applied to all stones to provide a simple measure of quality when comparing similar stones. The test may also be used as an indicator of potential durability, especially to spot variation within a given stone.

The American standards for dimension stone provide a number of maximum limits for different types of stone. In Europe there are no limits. Values are just declared, except for roofing slate, where a limit of 0.6% is required for material classed as A1. Any greater absorption and the slate has to pass a frost cycle test to demonstrate potential durability.

Part 12 of the British Standard for pavements (BS 7533-12:2006) gives maximum limits of 0.40% and 0.25% for igneous materials (such as granite) and 3.5% and 2.5% for sedimentary and other materials (York stone, for example) to be used for surface course materials. The materials achieving these results are described as Class 1 and Class 2, respectively. This recognises that low water absorption is generally a good indicator of potential performance.

For igneous rocks, increases in water absorption often suggest the alteration to more unsound materials due to natural weathering. That can dramatically decrease strength, so it is always best to compare any unexpected higher absorption values with the strength.

There is little that can go wrong with the absorption test because of its simplicity. Only operator error and incorrectly calibrated or insufficiently accurate weighing equipment could lead to potentially serious misleading error. Some porous materials might have air pockets that cannot be reached by the water so that actual absorption would be significantly higher. Such pockets could be one reason why water absorption increases under vacuum.

In real life, if water is unable to reach these pockets they are not really an issue and might even prove to be beneficial to actual performance.

A good water absorption result is a fine start for any material, but there are issues that may affect stone unrelated to this property. However, when combined with a strength result, most other properties can usually be predicted with reasonable accuracy, which is useful if you are trying to compare lots of different stones when only minimal or selective data are available.

Don’t be sucked into large testing regimes until the potential choices have been narrowed down in this way.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646

Email: info@ibis4u.co.uk

The advice offered in answer to readers’ questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought in respect of each specific stone-related issue.