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 slip resistance.

Slip resistance is significant for stone that is going to be walked on and with injury lawyers touting their wares all over the place it is essential you get this one right.

I first wrote about slip issues in NSS in 1998 (Vol 33, No 12, pp21-28). The conclusion was that you couldn’t prevent slip accidents, only minimise the risk of them.

The first issue regarding slip testing is which method to use. The pendulum test is preferred in the UK and is possibly now the most widely known and enduring of all the available slip tests. It is incorporated into EN 14231 and, in America where it originated, into ASTM E303.

It sounds simple. It involves determining the degree of retardation to the swing of a pendulum by a surface (see the illustrations). To do this, the apparatus needs to be calibrated to achieve a standard swing so that a standardised spring-loaded rubber slider passes a set distance across the surface being tested. Nominally, the higher the value of retardation – or slip resistance value (SRV) – the greater the slip resistance of the surface.

The pendulum test equipment is conveniently portable and the test provides realistic results under many circumstances, but cannot deal with some real-life situations involving, say, bare feet. There are other tests available to investigate such matters, notably the ramp test.

The pendulum test can also be affected by both the operator and external influences such as contaminants or the breakdown of either the test surface or the rubber on the pendulum. There can be other variables – I have known particles from the stone to stick to the slider and reduce resistance, for example.

Test values can vary dramatically with the texture of the stone and any slip resistance value must come with a clear description of the surface tested – it serves little purpose citing a slip test value carried out on a flame textured finish when the stone is going to be used with a honed finish.

Generally, the coarser the texture the higher the slip resistance, though a finish with a regular or repeated lineation can be slippery parallel to the texture but resistant to slipping across it. Ideally, slip results should be available for a range of finishes in both wet and dry conditions.

In isolation, a slip test value misses the point of how a stone will truly behave in service. A stone with apparently excellent slip resistance might perform no better than ice once it has worn smooth, dirt has accumulated between the crystal grains and it is wet.

For these reasons it might be useful to assess the ability of a stone to resist polishing or become flat, and it could be better to look for in-service performance or to carry out the polished paver test, which will be the subject of a future article.

But for now the recommendation from the Health & Safety Executive is that horizontal surfaces should exhibit pendulum slip test values of 36 or more – and certainly more on a slope.

Well, at least it is somewhere to start. Taking the most pragmatic view, I would choose stones that demonstrate good slip resistance regardless of the finish or surface condition. And remember: as slip resistance decreases, maintenance becomes ever more critical.

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.