24/02/10

The Slip Resistant Properties of Tread Plate

Introduction

Tread plate comes in many variations and under a range of names: Durbar plate, diamond plate, checker plate, 5-bar plate, all describe this variety of flooring product. Commonly constructed from mild steel, aluminium or stainless steel, these products are used in many different applications and environments from marine and food production to general construction and OEM products.

The wide use of tread plate is due to its hard-wearing nature, excellent drainage due to the non-enclosed nature of the raised tread profile, and its perceived high slip resistance in heavily contaminated conditions. Likely contaminants include water, oil and semi-solid greases or foodstuffs.

Commonly constructed from mild steel, aluminium and stainless steel, tread plate is used in many different applications and environments from marine and food production to general construction and OEM products.

The Effect of the Profile on Slip Resistance

While tread plate offers some benefit of slip resistance over a flat steel plate, it is not as good as you might intuitively imagine. The main issue is that a person walking on the plate is only actually walking on the tips of the raised profile, giving a very small contact area between the sole of the shoe and the plate. This contact area is what creates the essential friction to prevent someone from slipping, and making the area smaller usually has a detrimental affect. The harder the shoe sole the less contact area between floor and sole.

Typically installed as external walkways and stairs, usually forming fire escape routes, wet tread plate can pose a serious slip risk. Corroded surfaces offer increased slip resistance despite the worn macro profile.

Sharp Edges

The sharp edges of the profile will aid slip resistance by “biting” into the soles of the shoes, but this effect varies depending on how soft the rubber soles are. These sharp edges will also wear smooth over time, reducing slip resistance.

Interlocking

You might expect that the raised profile and the cleats on the sole of the shoe would inter-lock and provide significant grip. Sadly, this effect cannot be relied upon due to the varying shape of cleats and their position relative to the floor. It is like trying to insert an unmatched key into a constantly moving lock.

Often specified as 'anti-slip' without ever having its slip resistance measured tread plate is popular in industrial applications for stairs and cat walks. Used internally the profile typically becomes worn without corroding. Industrial contaminants such oil or grease on a smoothed surface significantly reduces slip resistance.

Micro Profile

In many applications, the key to good slip resistance in wet or otherwise contaminated conditions is the micro-profile (or micro-surface roughness) of the flooring material. This is distinct from checker plate, whose main feature is a macro-profile – the raised profile pattern which is clearly visible from a distance.

Micro-profile provides a rough, uniform surface like very fine sandpaper. Generally, the larger the area of this you have, the better the slip resistance, so a completely flat surface will usually provide better slip resistance than a macro-profiled surface like checker plate. The roughness is typically measured in microns, and the more viscous the expected contaminants are, the rougher the micro-profile needs to be to cope with them. For example, water often requires a micro-profile of 20µm while engine oil require 60µm or greater.

Some macro-profiles incorporate a micro-profile, such as the aluminium tread plate shown in Image 4. You can see the rough finish to the whole surface of the plate, but only the areas on the tips of the raised profile are likely to have a significant affect on slip resistance. Like the sharp edges of the raise profile mentioned above, this roughness will wear over time, reducing its effectiveness. This wearing of the micro-profile is accelerated compared to a flat surface without a macro-profile because the pressure applied by a foot (or vehicle’s wheel) is concentrated onto the small area of the profile tips.

A micro-profile aids slip resistance but the macro profile on tread plate typically reduces the contact area between micro-profile and sole. Wear of the micro-profile is also accelerated.

Painting

The effects of both micro-profile and, to a lesser extent, the sharp macro-profile edges are, of course, both rendered moot if the surface is painted. A well maintained (and cleaned) tread plate surface coated in a paint containing anti-slip particulate is however likely to perform considerably better. An effective anti-slip coating such as this is likely to be the most cost effective solution to a slippery tread plate.

Directionality

Most tread plate profile patterns exhibit directionality in their slip resistance properties due to the effect of the sharp edges of the raised profile. Contact with a shoe sole or pendulum test slider will strike more edges in some directions than others. It is not unusual to find up to a 50% reduction in slip resistance between the best and worst of the directions measured in a standard BS 7976 Pendulum test.

Tread plate design has a significant impact on its slip resistant properties. Typically a greater density of small sharp extrusions leads to an increased ability to puncture the hydrodynamic squeeze film created in a wet slip.

Rusty Tread Plate

Curiously, bare mild steel tread plate that has been used outdoors and subject to rain for some time will often show an increased slip resistance compared to when it was new. This is because when the steel is allowed to rust, the surface becomes pitted and forms a micro-profile. This profile constantly renews itself as the surface is worn, although high traffic rates may wear the surface faster than it can suitably corrode. The presence of any oil or grease will both make the surface more slippery and prevent the corrosion beneficial to slip resistance.

Rust forms a 'natural' micro-profile on the surface of aged external tread plate applications increasing slip resistance.

The Effect of Production Tolerances

Due to the very small surface area of the tips of the raised profile, small changes in their dimensions result in large percentage changes in their area, which can mean significant variations in slip resistance. During recent routine testing of some stainless steel tread plate for a major distribution company, Grip Potential staff found that each sheet of tread plate (delivered from the same supplier) inspected had slightly different sized profile tips. On a sample size of just three sheets tested, this resulted in a difference of 12 PTV in water-wet conditions for identical macro-profile configurations. This variability means care should be taken when generating a realistic slip resistance value.

Conclusion

While tread plate offers many benefits it should not be considered inherently slip resistant and the same careful selection process should be applied that would be used for other flooring products to ensure suitability for purpose.

Anthony Roberts is a slip risk consultant for Grip Potential Ltd, providing impartial independent slip risk assessment to a wide range of industries. For help, advice and testing contact Grip Potential.

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Further information

Pendulum testing - Surface roughness - Slips Potential Model - Slip testing methods - HSE slips site - Grip Potential Ltd

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