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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.

Image 1.

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

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.

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.

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.

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 and it is recommended all such standard plates are assessed periodically to ensure the meter is producing valid results. Further information on roughness meters.

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.

Image 4.

Whilst surface roughness does have a direct effect on wet slip resistance the Rz parameter, in our opinion, is a poor indicator. The measurement of average peak to valley height means surfaces of an identical profile height, but featuring significantly differing numbers of peaks/troughs, will produce the same Rz value. These surfaces would offer significantly differing wet slip resistances.

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.

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.

We would never recommend a method of testing to our clients which produced less than reliable results. We would not recommend that any measurement of slip risk is based on Rz values alone given their unreliable correlation with measured PTV's. The BS 7976 Pendulum test is the only slip test method expressly recommended by the HSE. 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 wholly accurate, and reliably so, it is of little use. There would 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 has no value in the inevitable legal proceedings following a slip and fall injury.

Further information on surface roughness meters.

Further information on the BS 7976 Pendulum.

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