As ever, I have read my latest (July) copy of NSS with interest. However, I think that you and your readers may be interested in developments linked to ‘The time of tests’ article.

I think we can all agree that testing is very important and likely only to become more so with the introduction and associated requirements of the Construction Products Regulation and so it is very important that we all are able to share in the most up-to-date information.

In the July issue flexural strength testing and more importantly the results are discussed, particularly in relation to frost testing. The author notes in the penultimate paragraph that “Flexural strength testing can provide considerable insight into the quality of any stone material, which is why it is often used to compare specimens before and after they are subjected to cyclic weathering tests.” This I cannot take issue with, however, the paragraph continues “Most notable is frost testing (EN 12371), where a greater than 20% loss in strength is regarded as a failure.”

Unfortunately this last sentence is quite misleading and soon will prove to be completely incorrect. It is important to put the whole situation in context. Current convention is such that test methods are restricted to providing guidance and information as to how to successfully perform the stipulated test. Guidance on interpretation of test data and performance in relation to specific acceptance criteria is most typically restricted to relevant product standards. In line with this convention EN 12371; Natural stone test methods – Determination of frost resistance makes no mention of any acceptance criteria in relation to measured strength loss in either the old (2001) or current (2010) version.

Performance is tackled in two key product specific standards that currently exist dealing with the major external use of stone where frost testing would be considered important:

• BSEN 1341:2001, Slabs of natural stone for external paving – Requirements and test methods (a document not mentioned in the article)

• BSEN 1469:2004, Natural stone products – Slabs for cladding – Requirements (another document not mentioned in the article).

The current ‘Table 6’ of BSEN 1341 identifies that for a stone to be classed ‘Resistant’ then the change in the measured flexural strength before and after frost testing should be less than or equal to 20% - importantly there is no use of highly emotive terms that could be used out of context like ‘failure’. Similarly, in BSEN 1469 when discussing frost resistance the term ‘failure’ is not used. In BSEN 1469, while frost resistance is assessed (Clause 4.2.10) by reviewing the change in mean flexural strength, it is only accompanied by a note that identifies ‘When the mean value of flexural strength decreases by less than 20%, this should not be considered as significant because of the variability of natural stone’.

Most importantly, however, is the fact that both BSEN 1341 and 1469 are in the process of being revised and updated (2011) versions should be issued either later this year or early next year. The thinking incorporated into the revisions of both documents is such that any reference to performance related to percentage changes in the measured flexural strength has been removed. This is because it has been generally accepted by the experts in Europe (including the UK) that the previously quoted figures were in many instances misleading and that greater care in the interpretation of data sets is required.

Another relevant piece of information if the current thinking in Europe is to be followed is that there is, contrary to the stated opinion of the author, a view that the three-point flexural strength test (EN12372) is of greatest value and is highly likely to become the preferred methodology. Frost testing methodology (EN12371) already requires the EN12372 methodology and for the flexural strength testing requirements of the revised versions of BSEN 1341 and 1469 this will also be the case. As CE marking looms this is a relevant development to note.

I hope that both you and your readers find this update both informative and helpful.

Yours sincerely

David Richardson

Chairman Stone Federation, Technical Committee

 

Response from Barry Hunt, author of the Time of Tests column and a Director of consultants IBIS Ltd

 

It was my mistake that I put in the EN 12371 and not EN 1341 reference as the 20% value. The text got hashed around and I did not spot this after making various editorial changes to shorten the article down to the required size. I am fully aware of all the things David says in the letter, but he is right to point these out. I generally try to write articles to provoke a bit of a response.

Unfortunately, half a page is never enough to put in all the things I would like to say – that would run to about five pages – so any letter that wants to expand on things is welcomed to get more points across.

I would like to include performance criteria for a range of stone types, more diagrams and more photos, but I do not have the luxury of that much space in this column.