Slip Prevention on Polished Natural Stone

Gesa Todt

The improvement of step safety or slip prevention is not a new topic. However, the representatives of chemotechnical treatment of stone and of micro roughening by means of laser started quarrelling during Stone+tec 1999, held in Nurnberg. Just before the fair opened, a rumour was spread that laser treatment was ineffective and that the corresponding test certificates were reports written as a favour rather than being scientific proofs.

During Stone+tec 99 in Nuremberg, however, everybody could get an idea of the problem, as many providers of certain procedures and users met there as well as the Fraunhofer Institute for Material and Jet Engineering (IWS), which initiated the development of the laser method and followed it for a long time.

Two basic statements resulted from the conversations:
Both procedures, i.e. laser micro structuring and chemical treatment have, in principle, the same effect caused differently: micropores in the polished surface. The procedure used for making the pores causes only slight differences in the resulting slip prevention. It depends mainly on the degree of the treatment, which is always a compromise between the optical impression and slip prevention.

The methods which are used to investigate the resulting slip prevention are not really suitable for an adequate evaluation. The German standard test (according to the DIN - German Industrial Standard) with oil comes from industrial safety and cannot be applied to the range of application of polished natural stone. Moreover, stationary and non-stationary methods are based on different ideas and thus cannot be compared.

Micro Structuring - One Aim Reached by Different Methods

With micro structuring the limited resolution of the human eye is taken advantage of. A polished surface is even regarded as shining when it is covered with up to 1200 craters per square centimetre, each having a maximum diameter of 440 micrometers. As the slip danger on a surface decreases with its roughness, this optical tolerance is used to take the slipperiness from surfaces. It is applied around hollows so that surfaces are still easy to clean in spite of their roughness.

The three micro structuring methods mainly differ regarding their technical aspects. With laser structuring holes, with a diameter of approximately 200 micrometers and a depth of 50 to 100 micrometers are "shot". Certain parts of the stone evaporate or burst in the heterogeneous structure of the stone leaving more or less round frayed holes, depending on the material. That means, due to the heterogeneous composition of a stone, sharp edges are generated instead of even, soft hollows. These edges increase the so-called deformation friction of shoe soles gliding over it. Chemical treatment has similar effects but here not mainly round hollows are generated , rather, the "softer" substances in the stone are etched away in the form of lines, which follow the structures of grain limits. If laser and chemical treatment are combined there is not much of a difference, only the etching effect is reached by lower concentrations and in the hollows, where it is desired. Moreover, it is quicker and that is an advantage in the production.

Water displacement in or through micropores is of little importance for slip prevention, as well as the so-called sucker effect, even if all factors act in combination in various ways (which have not yet been finally clarified).

Unrealistic Testing Methods

Unfortunately the testing methods for certifying slip prevention have various shortcomings. In Germany slip prevention on floors is investigated according to various legal requirements, such as the Decree for Working Places, the requirements of the accident insurances as well as the German Industrial Standards (DIN) 51097 and 51130 for testing floor covering. These regulations mainly serve for safety purposes in industry and consider the corresponding conditions. That is why the test with oil is part of the testing conditions, which does not make any sense for the discussed problem, as oil or greasy substances only occur in workshops and production sites, i.e. in places, where nobody ever would use polished natural stone.

Furthermore, the investigation methods partially are very difficult to standardize. For example, for the assignment to the important evaluation groups according to ZH 1/571 and GUV 26.17 the experimental set-up is to place a person on the surface and then to incline it. As soon as the person starts slipping, the angle is measured. It is easy to imagine, that this depends on many factors, such as trembling knees or the person might not stand flatly and so on. The ambient temperature and humidity can also be important, or the storing and installation conditions of the testing objects. New plates are tested under laboratory conditions so that the entire ''walking" factor influences the result, whereas already laid surfaces are only tested regarding their sliding friction by means of a measuring instrument. That means, if an objective comparison of the micro structuring procedures is required, new testing methods must be developed, so that this comparison can also be repeated.

However, there are some experts who generally reject any treatment for slip prevention. As absolute safety is not possible without destroying the polish totally, they think, a good expert should not allow the application of polished natural stone in areas endangered by moisture. Anyway, treated surfaces will only have an increased slip prevention, as long as they are laid and cared for expertly. That means: carefully removing the surplus joint cement after washing, leaving no waxes or detergents containing wax, and frequent cleaning with ample water to remove finest dust from the caverns.

Development of Laser Treatment

In 1992, the IWS started investigations of laser treatment of natural stone within a research project supported by the former BMFT (German Federal Ministry of Research and Technology). The aim was to find out about laser cutting and cleaning of historical valuable surfaces. With this experience, during Stone + tec 1993, the idea came up to use the applied micro structuring for slip prevention on polished stone. Up to this time micro structuring had only be done chemically. Industry and IWS together developed and patented the basic procedure of laser micro structuring. Patentees are Thomas Sievers, who has become manager of Renosa Steintechnik by now, and the Fraunhofer Institute. Since then, Renosa Steintechnik GmbH has offered micro structuring by means of laser under the name ''SafeStep" on an industrial scale (according to Sievers, they have already treated approximately 300,000 square metres of floor plates).

As the research project had been supported with public funds, they were obliged to allocate licenses. That is why there is another company apart from Renosa Steintechnik, called Magna Naturstein, which put laser structuring on the market. Whereas they only apply the modified laser procedure, Sievers, in the meantime, has developed a subsequent and also patented method together with the IWS. Here the stone is treated hydromechanically after laser shooting, so that the edges of the craters are smoothened and the surface does not become dirty that easily.

Independently, in the meantime IWS has also started another project and applied for a patent: mobile laser structuring. With these systems it will be possible to treat surfaces, which have already been laid, thus solving the problem with subsequent washing. However, a possibility to certify a surface after laying is missing especially in this case, as well as in the case of chemotechnical treatment. The current methods of non-stationary slip prevention tests have not been standarized and therefore cannot be classified according to the currently valid R groups.