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 compressive strength test.

One of the most useful tests available is the determination of the compressive strength of a stone. It is a relatively straightforward test to perform and it provides values that can be compared not just with other stones but also just about any other material. It is also useful for spotting changes in quality and thus guiding users to better materials.

The test essentially involves a cube of stone being placed between two plates (or platens) parallel to each other and being squeezed until it fails.

The highest load achieved is used in the calculation of the stone’s compressive strength, or its resistance to crushing.

The calculation of the results is simple – it is the failure load (in Newtons, N) divided by the cross-sectional area (square millimetres) of the test specimen. The figure is sometimes given in terms of N/mm2, or megaPascals (MPa).

In Europe, the standard for the test is EN 1926 and in America ASTM C170. Both tests are essentially the same although the European test asks for larger pieces of stone to be tested.

The European test also includes a statistical calculation so that natural variation is taken into account and the results lowered accordingly. This is presented as the “lower expected value” and is the value that should be reported.

The American test requires stones to meet minimum compressive strength limits for the type of stone, whereas in Europe it is only necessary to declare the values.

Results of the test can vary for a number of reasons. The same stone cut in different directions to any bedding, foliation or other natural features may exhibit compressive strength differences.

Such differences could also result from the stone being wetter or dryer, partially saturated or at natural moisture content.

Some variations may result in a decrease in strength to less than half the optimal condition, which might be an important consideration to an engineer or designer where the stone will provide support in a particular orientation within a wet environment.

Always check that any strength testing has been undertaken in different directions and conditions. If results remain consistent this typically suggests a sound material.

The results of a test should not be interpreted in isolation for the purposes of engineering design unless the value is stated as being the lowest achieved under applicable conditions.

Results might also depend on the number of specimens tested, a greater number providing a greater degree of confidence in the results.

One of the major issues with the test is that it underestimates the strength of stones with large crystals or grains, and especially limestones incorporating large shells. To counter this, the test sample minimum dimension should be at least 10 times the average grain size.

Ideally, all test samples should meet this criterion, although it is not always be practicable. Some large grained stones would require huge test specimens that could not be accommodated by standard testing equipment.

Another issue is the fact that a split in a stone oriented either parallel or perpendicular to the loading direction may not affect the result, so it is not a good test for assessing fractures or other similar discontinuities.

There are alternative procedures, such as testing cores. This is typically undertaken with a length to diameter ratio of at least 2:1. The calculation of compressive strength would then be: 4πL/d2.

The different test methods can lead to differences in the results achieved, often due to differences in the loading rates and specimen sizes used. The flatness and squareness of the loaded faces can also considerably affect the results.

Nevertheless, once you understand this test you can gain rapid insight into the quality of any stone and know when and how to question the results you are presented with. Few other tests allow you to do this.

References

BS EN 1926:1999. Natural stone test methods – Determination of compressive strength. Published by the British Standards Institution, London, England.

American Society for Testing Materials (ASTM). Test Designation ASTM C170-94, Standard Test Method for Compressive Strength of Dimension Stone.

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

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.