Surface roughness Rz measurement - Slip testing and risk assessment - HSE surface roughness guidelines

Image 1. The Surtronic Duo surface roughness meter is one of many commercially available meters capable of measuring Rz properties.

Image 2. The meter drags a tiny spring loaded diamond tipped stylus horizontally across the surface. The vertical displacement of the stylus is measured, producing an average peak to valley height across a given sample.

Image 3. The meter is supplied with a metal plate of known roughness. The meter is calibrated using this plate prior to measurement of the test surface. Grip Potential's Rz plate holds a UKAS calibration certificate.

Image 4. Surface roughness measurements can be misleading. The two surfaces above have identical average valley to peak heights and so produce an identical Rz reading. In practice the surfaces provide significantly different slip resistance characteristics.

Table 1. The table above is the result of testing conducted by the HSE (document here). The mismatched risk classifications are shown in red. This data highlights the shortcomings of risk assessment based solely on surface roughness data. Where risks have been overestimated unnecessary cost may be incurred in remedying the problem. Worse still, where risks have been underestimated a slip is likely to occur.

Surface roughness (Rz) measurements are a useful addition to pendulum slip tests for the purpose of slip risk assessment. Grip Potential conduct surface roughness (Rz) measurement at the site of pendulum testing, giving additional information about the surface's ability to cope with varying contaminants and forming a basis for long term monitoring.

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, as pictured left.

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.

The mechanics of a contaminated slip are similar to that of an aquaplaning tire. Slick tires offer excellent dry grip but perform very poorly in wet conditions. Grooved tires offer reasonable dry grip and far better performance in wet conditions. The profiled tire displaces water and maintains tire/road contact in a similar way that a profiled floor displaces the fluid film and maintains floor/sole contact.

As surface roughness measurement gives an indication of the expected behavior 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;

Surface Roughness (Rz)	Slip Potential
<10µm	High
10µm-20µm	Moderate
>20µm	Low

The HSE give guidance on the minimum surface roughness levels expected to provide a low risk of slip in various contaminated conditions.

Surface Roughness (Rz)	Contaminant
>20µm	Water based
>45µm	Soap solution, milk
>60µm	Cooking stock
>70µm	Motor oil, olive oil
>>70µm	Gear oil, margarine

In addition to giving an indication of contaminated performance, surface roughness can be used in the ongoing monitoring of floors. Surface roughness values married to pendulum test values can be used to predict the roughness at which the floor is likely to produce a PTV taking it from a low to moderate or moderate to high slip risk classification. This allows long term monitoring of floors without requiring high levels of technical knowledge.

Surface roughness can be a good indication of the effects of wear. Over time the peaks of the floor micro-surface are worn down, reducing Rz and slip resistance. A significant change in surface roughness is a likely indication of a poor cleaning regime. Valleys in the floor micro-surface become clogged with dirt over time, effectively reducing Rz.

In conclusion, surface roughness measurements form a basic indication of slip resistance of a floor. When coupled with pendulum slip testing, surface roughness measurements allow a more thorough assessment and on going monitoring of slip risk. Changes in roughness can highlight potential problems with little technical knowledge.

Measurements taken with a surface roughness meter can be used with the HSE's Slips Assessment Tool as a basic slip risk assessment method. Click here for more information on the SAT.

The Surtronic Duo surface roughness meter can be purchased from Taylor Hobson.

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