The time of tests : In-service slip resistance prediction
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 in-service slip resistance prediction.
I wrote about slip testing in the June issue, when I mentioned the polished paver value as a way of trying to assess how a stone’s slip resistance might change in service, providing a more realistic idea of a stone’s real slip resistance.
The test for this carried out in the UK is BS 7932, which has slowly developed over the years. A similar European version has been developed and refined into DD ENV 12633. The European test is now called “Method of determination of unpolished and polished slip/skid resistance value”, which does not have much of a ring to it.
In a nutshell, stone samples are tested to determine their slip resistance before and after they are subjected to a standardized regime of polishing, so that the loss in slip resistance can be assessed.
The polishing regime was originally developed to reflect that brought about by vehicular trafficking, so there are concerns that the test does not reflect the effects of pedestrian trafficking. Hopefully current research is looking into this issue.
The polishing regime comprises samples of a fixed size with a fixed loading being placed on a lapping plate rotating at a fixed rate for a given time or total number of revolutions. Corn emery of a specific grade is fed on to the lapping plate at a fixed rate.
The process is then repeated using a finer grade of emery powder to provide the final polish. Three specimens are tested and the results averaged. The test is calibrated using reference samples with known polishing characteristics.
Few presently undertake this test as it is difficult to set up and calibrate, with consistent results not easily achieved.
However, once functioning, it generally seems to work well for siliceous stones, although not too well for carbonate-based stones (mainly limestone and marble).
The coarse emery tends to scratch the surface of softer stones, resulting in striations that increase the apparent slip resistance of the finish that should have polished. It has been suggested that the coarse corn emery phase should be removed from the test when soft stones are tested, or at least some adaptation be made to the test to reduce these effects.
Granular stones generally perform well in the test as grains tend to be plucked out, leaving a rough surface despite the polishing. Thus sandstones typically produce excellent results when subjected to this test. Most igneous rocks with a mixture of different crystals will lose slip resistance, although it remains sufficiently high to be acceptable in use, usually because the different minerals undergo different levels of abrasion and polishing.
The test is very much a physical one. It cannot take into account weathering effects that attack certain minerals, helping to maintain a level of roughness despite polishing through wear.
It looks as if some form of the test will be adopted as a full European Standard – and, potentially, one that is more complicated than the current incarnation, given the various issues.
This will be a major step in the right direction for performance specification of stones to be used for flooring and paving.
On a cautionary note, the test must not be used to prevent marbles and limestones from being used for polished and honed floors where these can be kept dry and their suitability for use as paving and flooring has been proven over millennia.