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Surface Roughness and Slip Risk Assessments

By Louise G

Republish: EasyPublish™ Published: 05Apr2010 Word count: 740 Viewed: 313 time(s) Get Free Content For Your Site

The surface roughness value, specifically Rz, describes the average vertical distance, in microns, between valleys and peaks across a horizontal sample. The measurement is achieved by use of a surface roughness meter of which there are many commercially available. The meter drags a sprung diamond stylus slowly across the floor surface measuring vertical displacement as it goes. Grip Potential use a Surtronic Duo surface roughness meter available from Taylor Hobson. The micro-roughness of a surface has a significant impact on its slip resistance. This is due to a phenomenon known as hydraulic uplift created by the hydrodynamic squeeze film. When a slip occurs in contaminated conditions the heel slides across the surface, compressing the fluid. If the surface is sufficiently rough the fluid disperses into the valleys and the peaks ensure a good sole/floor contact. If the surface is insufficiently rough the sole is unable to make contact with the floor due to the fluid film and friction is significantly reduced. Sampling and Results As surface roughness measurement gives an indication of the expected behaviour in a wet slip rather than measuring slip resistance directly it should be considered as supporting information rather than a slip test in its own right. The UKSRG offer the following classifications for surface roughness to be used in conjunction with pendulum testing; 20µm Low slip potential HSE issued guidance suggests a sampling area of 'a typical tile size'. Measurements are taken in the direction of, at 45 degrees to and perpendicular to the direction of foot traffic with a total of 10 measurements recorded. Further investigation into surface roughness measurement by the UKSRG has prompted a different sampling recommendation. Measurements should be conducted within an area 75mm x 75mm and taken randomly on surfaces without obvious directionality or along/across a directional surface to demonstrate anisotropism. It is also noted that the significant variability of a floor surface may deem 10 measurements an insufficient sample. In the case of significant variability 20 measurements will provide a more accurate average.

Surface roughness measurements are used widely to give an indication of slip risk on their own or as supporting evidence in pendulum testing. Some of the advantages of this method are that; Roughness measurement will give an indication of slip risk with water based contamination. Roughness meters are generally quick and easy to use and require no training. Commercially available meters will normally cost around £1,000, significantly cheaper than alternative test equipment. Roughness measurement can show significant changes to the surface, highlighting problems with wear, contamination or cleaning. When married to pendulum test data surface roughness can be used to predict PTV's of surfaces wearing over time, allowing an inexpensive monitoring regime.

It should be stressed that surface roughness measurements can be considered a guide only. As a slip resistance test method there are serious shortcomings; Floors of similar Rz reading may have significantly different surface construction and slip resistance characteristics. Independent testing by the HSE has shown that risk classification based on surface roughness measurement is often inaccurate. Generally it is not possible to measure surface roughness of macro profiles.

Surface roughness measurements are most effective when used alongside pendulum slip testing to provide additional information about the floor surface. The HSE have produced the following giving guidance on the roughness required to provide safe levels of grip on floors with contamination of varying viscosity; Rz>20µm - Water based Rz>45µm - Soap solution, milk Rz>60µm - Cooking stock Rz>70µm - Motor oil, olive oil Rz>>70µm - Gear oil, margarine The HSE have produced a Slips Assessment Tool (SAT) based around the Slips Potential Model and surface roughness measurement. The SAT is a freely available computer program designed to be run on a laptop whilst assessing the test site. The SAT enables novices to highlight high risk areas; however its basis in Rz measurement limits its accuracy. The SAT can be downloaded from www.hse.gov.uk/slips Conclusion Surface roughness properties have a significant impact on a floor's contaminated slip resistance. Currently, Rz measurements fail to take into account surface configuration and as such can only produce a limited indication of slip resistance. Measurements are further limited on macro profiled surfaces where surface contact, interlock and deformation of footwear alter slip resistance. When used alongside pendulum testing however, surface roughness can be used to predict PTV's on similar surfaces, allowing fast, cheap and easy long term monitoring of wear, cleaning regime and contamination.

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