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Coefficient of Friction

Understanding Coefficient of Friction

Coefficient of friction (COF) is a mathematical term used to describe the effect of dragging one substance (shoe sole material) over another (flooring surface).

This coefficient is a measurement of the relative ability of various surfaces to resist the sliding or slipping of the selected material.

The slip-resistance of ceramic tile in ordinary applications is comparable to that of most hard surface flooring materials and it is significantly better than some.

Unglazed tiles have a greater slip-resistance than glazed tiles and are commonly recommended for areas subjected to high water spillage. irregular or textured surfaces can be confused for slip-resistant surfaces. Because a glazed tile is rough or not glossy, doesn’t necessarily mean it is slip-resistant. Even many irregular or textured unglazed tiles can become slippery when wet, allowing surface hydroplaning.

Many glazed and unglazed tiles can feature abrasive grit on their surface, increasing their slip resistance substantially. These tiles are commonly installed in public areas with direct access to the outdoors. Corundum grit surfaces can introduce an element, when traffic acts upon the surface, which will add to the floors deterioration and can be too slip-resistant, when excessive.

Carborundum grit, which appears to be black reflective speckles into the surface of an unglazed tile, will wear flat and become ineffective.

Please Note: that any tile or other hard surface flooring can become slippery when wet or improperly maintained. Slip resistance varies with the many types of footwear, soiling, and cleaning regimen.

Polished wet surfaces give false slip-resistance readings, due to a suction effect developing between the tile surface and sole materials. Even under dry conditions the testing of polished surfaces is questionable.

Avoid trying to raise the COF by using coatings – they will peel, blister, discolour and mar, creating a difficult floor surface to maintain…aside from providing unsatisfactory slip-resistance.

Slip Resistance

The anti-slip qualities of the tile alone do not make walking safe in risky situations.

Many factors are involved, and all should be taken into consideration at the planning and installation stages:

  • Choice of tile
  • Analysis of environment and the conditions of use
  • Checking for irregularities
  • Slopes
  • Configuration of non-traffic and traffic areas, adjoining floors of different friction coefficients
  • Proper fixing
  • Proper maintenance and cleaning

The United States ANSI method (ASTM C1028) gives, through the use of a force gauge (horizontal dynamometer) pull meter method, the static friction coefficient of the surface.

  • Equal to or greater than .60 — Excellent friction.
  • Equal to or greater than .50 — Adequate.
  • Equal to or less than .40 — Caution necessary.

The USA Occupational Safety & Health Administration (OSHA) currently recommends that the static COF be at least 0.5 for all walkway surfaces under both wet and dry conditions. The Americans Disabilities Act (ADA) recommends the dry static COF of 0.6 on any flat or horizontal surfaces both wet and dry, but 0.8 for any inclined or ramp areas.

Coefficient of Friction (COF) is defined as the friction force opposing sliding motion divided by the force normal to the surface.

People walk differently (most people demand a consistent COF of between 0.25 – 0.3 walking normally) Long fast strides demand a higher friction. Although slippery surfaces or steep slopes provide low COF, we often perceive the potentially dangerous floor, shorten our stride and walk slowly.

Including the presence of contaminants a 0.6 COF should be demanded for safety.

Millions of combinations possible: several shoe materials & design features, floor materials, surface profiles, degrees of wear and cleanliness, type & amount of contamination, walk patterns & angle of contact, body weight, vertical force, rate of weight application, lighting & surface perception, eyesight, inebriation, medication, etc.

It’s important to realize that the COF testing is flawed. The current testing methods, both in the testing laboratory and in the field, are flawed. In addition, there are so many other factors and shoe materials used in real pedestrian traffic conditions that the results are inconclusive at confirming how a surface or tile will provide adequate slip-resistance; especially, once down and subjected to periods of differing wear abrasion and daily maintenance regimens.

The commonly specified and often misunderstood value of 0.60 Static Coefficient of Friction (SCOF), determined by the ASTM C1028 test method, has been superseded by a new method – The Dynamic Coefficient of Friction (DCOF) method, and a new threshold value, all of which can be found in the 2012 edition of ANSI A137.1, the American National Standard Specifications for Ceramic Tile.

The Dynamic Coefficient of Friction (DCOF) method determines DCOF under wet conditions using slightly soapy water, or more specifically, water with 0.05% Sodium Lauryl Sulfate (SLS) in solution. SLS is a wetting agent that allows water to spread in a thin film, similar to that found when a slip occurs or when a floor is being cleaned.

This 2012 version states that tiles suitable for level interior spaces expected to be walked upon when wet shall have a wet DCOF of 0.42 or greater when tested per the procedure in the standard (i.e., per the DCOF methodology).

Safe Solution® Meets and Exceeds these New DCOF Requirements.

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