Rutledge BA, Chimich DD, Elkin BS, Siegmund GP (2018). The effect of contaminant film thickness on slip resistance. 8th World Congress of Biomechanics: Dublin, Ireland.
Walkway tribometers are devices that measure the slip resistance of flooring surfaces to assess walkway safety and pedestrian slip risk. To be useful, tribometers must be repeatable, reproducible, and accurate. ASTM International standard F2508-16 requires all tribometer models to be validated by the manufacturer, undergo an interlaboratory study to evaluate repeatability and reproducibility, and be calibrated annually . This standard is based on four reference surfaces (RSA-Granite, RSB-Porcelain, RSC-Vinyl Composition Tile, and RSD-Ceramic) that human subject tests have shown have different slip risks when contaminated by a fluid film . Many variables can affect slip resistance, and currently few studies have quantified the effect of individual variables on slip resistance. Our goal was to quantify the effect of contaminant film thickness on the slip resistance measured by two common tribometers.
A Brungraber Mark IIIB and an English XL tribometer were used to measure the slip resistance of the four ASTM reference surfaces. Six measurements were made at each of four film thicknesses between 0.3 and 3.3 mm using two contaminants (distilled water and 0.05% Sodium Lauryl Sulfate (SLS) solution). Film thickness was measured with a Filmetrics F70 Thin-Film Analyzer, a non-contact optical system (accuracy 2 μm, precision 0.2 μm). Test order was blocked by surface and contaminant, but the 24 tests (4 film thicknesses x 6 repetitions) within each block were randomized. For each combination of tribometer/contaminant/surface, slip resistance was linearly regressed against film thickness to determine if the slope was significantly different from zero.
The slip resistance measured by the Mark IIIB decreased significantly with increasing film thickness on RSC and RSD using both contaminants, and on RSB using distilled water (Figure 1). There was no significant effect of film thickness on RSA for either contaminant using the Mark IIIB, and no significant effects for any combination of reference surface and contaminant using the English XL.
The film thickness effects we observed were isolated to the Mark IIIB tribometer on the more slip resistant surfaces. This effect may be due to the larger test foot on the Mark IIIB (75 x 75 mm, grooved) versus the XL (31.5 mm diameter, smooth) and a reduced ability to penetrate and disperse the contaminant to allow contact of the test foot and surface. This effect may diminish on slipperier surfaces because foot/surface contact is limited at all film thicknesses. This study demonstrates the need for tribometer manufacturers to evaluate and potentially specify a film thickness for their tribometer. It also highlights the importance of evaluating the effect of individual variables on slip resistance measurements for individual tribometers.