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Specifying stone

By Giles Heap

Frost cracking of ‘frost-resistant’ paving.
Photographer: Giles Heap

It is important to specify stone correctly for external paving. This piece discusses why labels of frost resistance are not always adequate, and looks at the advantages and limitations of the introduction of CE labelling for stone.

When specifying with stone, it is common practice to refer to the British Standards BS EN 1341:2001, BS EN 1342:2001 and BS EN 1343:2001 which set out the requirements and test methods for natural stone (external) paving, setts and kerbs respectively. One of the tests specified within these documents is that for testing a stone for frost resistance, BS EN 12371:2010, but there is a little problem of which landscape architects should be aware.

It is true that some stones work well in cold weather and some do not, but in the last few years there have been a number of failures of natural stone paving that had passed the test as stated within BS EN 12371:2010. As a result of these failures, some of which were quite high profile, experts have looked a little more closely at the test itself, only to realise that as the original test was designed with cladding in mind, there may be a fundamental flaw in the testing methodology when it is applied specifically to paving products. The principle is simple: stone cladding gets wet, but unlike paving it doesn’t sit in water for any protracted length of time and any water that is soaked up, is much more likely to drain away or evaporate within hours.

Here in the UK our winter temperature can cycle above and below freezing many times over the course of the season, indeed many times each night. Generally when it does, the ground is also pretty wet and probably has been for some time. When stone is effectively sitting in water for weeks at a time (as paving stones do), moisture is able to soak deep into the stone and when the temperature drops below freezing, that moisture will expand as it turns to ice. It is this constant expansion and contraction of moisture within the stone that can lead to the stone failing.

So be warned, just because a stone passed the ‘Frost Test’ doesn’t necessarily mean that it will survive a British winter!

How can you tell which stones will be suitable for paving? You can’t, but if you use a trusted supplier you will have a better chance of not getting caught out.

As a rule of thumb, stones that have a low water absorption (< 3%) tend to cope best, so most granites and some British sandstones and ‘yorkstones’ will be a safe choice. A number of imported sandstones and ‘yorkstones’ will not succeed and many limestones will not be suitable for external use.

CE Marking

In July 2013, the new CE Marking regulations for natural stone come in to force, with the aim of ensuring that what the client orders complies with the appropriate set of European Standards. However there has been some uncertainty as to what this actually means to the supply chain.

From July, all materials (natural stone) supplied within the EU must have a CE Mark attached to the packaging with the relevant European Nomenclature (EN) clearly stated. In the case of external paving for example, that is going to be BSEN 1341:2012 (slabs), BSEN 1342:2012 (setts) and BSEN 1343:2001 (kerbs). BSEN 1341 & 2 were published this year.

On each crate of stone it will be required to show a CE Mark and possibly the certificate stating certain facts about the products contained. For paving, these are likely to be as follows:

A. name or identifying mark of the manufacturer;

B. last two digits of the year in which the marking was affixed;

C. reference to the appropriate Standard and the year of its publication (i.e. EN 1341:2010);

D. description of the product and its intended use:
1) generic name: ‘natural stone slabs’;
2) traditional name, petrological family, typical colour and place of origin;
3) intended use: ‘for external pedestrian and/or vehicular circulation areas’;
4) surface treatment of the stone (if any).
E. performance on the essential characteristics listed:

1. release of dangerous substances: where relevant;
2. breaking strength, dealt with by flexural strength;
3. slipperiness, dealt with by slip resistance for pedestrian areas
4. skid resistance, where required for vehicular areas;
5. durability of breaking strength, of slipperiness and of skid resistance:
i. freeze/thaw resistance, measured as the mean flexural strength (in MPa) after 56 freeze/thaw cycles;
ii. freeze/thaw resistance with de-icing salts;
iii. polished slip or skid resistance declared in accordance with national provision.

A different set of details will be required for kerb or setts.

CE Marking will probably make the use of trade names less common, bringing some transparency to the specifier. However It is important to remember that many stones have two or more different variations with the same ‘name’. Take G654 granite from China for instance. There are many different quarries in the same region that sell G654 granite, but not all G654 is the same. Some is fine-grained, some is coarsegrained. Some has many veins running through it, some has very few and some

G654s are technically just not as good as others.

So it will still be important for the specifier to ensure that their choice of stone is technically competent to do the job required. Once that stone has been specified, however, CE Marking will help to reduce opportunities for the more unscrupulous suppliers to supply cheaper materials under the guise of those specified. Of course this is still open to abuse, but as CE Marking is effectively a guarantee of conformance regarding source, colour, tolerances and technical capability, it will allow an easier avenue for compensation if the goods do not conform.

- BS EN 12371:2010,
- BS EN 1341:2012,
- BS EN 1342:2012,
- BS EN 1343:2001,

Giles Heap is a director of CED Stone.

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