Rz Surface Roughness

Surface roughness (Rz) measurements are a useful addition to a pendulum slip test 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.

The Rz roughness parameter

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

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, in common use, this meter is sometimes affectionately referred to as a "Kenny".

Rz roughness and slip resistance

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.

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 can be explained in a greatly simplified manner by comparing them to that of an aquaplaning tyre. Slick tires offer excellent dry grip but perform very poorly in wet conditions. Grooved tires offer adequate dry grip and far better performance in wet conditions. The profiled tire displaces water and maintains tyre/road contact in a similar way that a profiled floor displaces the fluid film and maintains floor/sole contact.

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.

Rz surface roughness results

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;

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

Benefits of Rz roughness measurement

Guidance from the HSE and UKSRG suggests that 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.

Guidance also suggests that 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.

Grip Potential is not of the opinion that Rz measurements are a reliable indicator of wear or contaminated slip resistance performance.

Limitations of Rz roughness measurement

There is an on going debate regarding the Rz parameter between those academically involved in furthering the science of slip resistance testing. Some argue that the Rz parameter has no bearing on slip resistance and is a poor indicator of wear or contamination, others argue that the Rz parameter in conjunction with the HSE's SAT provides a good indicative measure of wet slip risk.

The crucial drawback in Rz surface roughness measurement as a slip test is that the Rz parameter itself fails to take into account the shape of the profile. A rounded peak 20 microns high will produce an identical Rz value to a sharp peak 20 microns high. A surface comprised of sharp peaks will offer better wet slip resistance than a surface comprised of rounded peaks.

The UK Slip Resistance Group Guidelines Version 4 states, "The interpretation of micro-roughness measurements is based on comparison with slip resistance measurements, and so on this basis, it cannot be considered as reliable as Pendulum Test Values. Neither can it be considered an independent test of slip resistance".

Grip Potential have no commercial interest in the Rz Surface Roughness Meter or any other slip test. In our experience of tens of thousands of measurements taken with both the Pendulum and Rz Surface Roughness Meter, both carrying valid UKAS calibration certificates, and both being operated to industry best practice, we have found that Rz Roughness values can provide a very good indication of likely wet PTV on some surfaces. We have found that it is fairly common for surfaces to achieve a low PTV and high Rz value, or conversely, high PTV and low Rz value. For this reason we would have serious reservations about using the Rz Roughness parameter alone to determine whether a floor was safe.

Conclusion

It has been argued, and indeed is promoted by the HSE themselves, that the Rz parameter and SAT provides a good indication of slip risk. We are of the opinion that unless the measurement is reliably accurate it is of little use. There could be considerable expense involved in replacing/treating an erroneously classified 'high slip risk' floor, or possibly severe consequences of exposing pedestrians to an erroneously classified 'low slip risk' floor. A slip risk assessment based on the Rz parameter indicating that a surface probably presents a safe level of grip will have limited value in any legal proceedings following a slip and fall injury.

Further information

Slip Test Services
The HSE Slips Assessment Tool (SAT)
Slip Prevention Tips

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