Why slip resistance continues to be a “moving target.”
March 13th, 2013

Specifying Ceramic Tile, Glass Tile, Stone and Terrazzo for Slip Resistant Surfaces…

 By: Donato Pompo


About twenty years ago, slip resistance was a big selling point to architects because of common slip-fall lawsuits.  Back then most ceramic tiles were not offered with a textured surface. Texture was considered the key characteristic for determining the degree of slip resistance. It became a big selling point, and new tile products were developed with a textured finish.

Soon the industry realized some unexpected tradeoffs with too much texture.  Textured surfaces more readily picked up and showed dirt, requiring more maintenance.  Heavily textured tiles were also more difficult to clean to the extent that in some cases the new tile floor had to be torn out and replaced with a tile that had less texture.  As a result, products were adjusted so they wouldn’t be a maintenance problem, but they would meet slip resistant requirements.

These same concerns apply to glass tile, natural stone, terrazzo and other hard-finish surfaces.  Natural stone has been used extensively in malls, airports and other public areas. Recently terrazzo flooring has been used more due to the increased design options.  Glass tile use has greatly increased in recent years.  In each case, the degree of texture influences the degree of slip resistance and maintenance.

Slip resistance depends on a number of factors: surface texture, whether the surface is clean or contaminated with one or more residuals, whether it is wet or dry or frozen, whether the surface is level or sloped, the type of shoe soles, if any (e.g. bare feet), and the condition of the soles that a person is wearing, whether the person is walking or running; their gait, and whether they are turning or transitioning from one plane to another or from one textured surface to another.  There are other dynamics in terms of the size and weight distribution of the person, whether they have any mobility handicaps, and even their state of mind, making this a complex issue.  Traction engineers say that the walking surface has to be slippery to some extent in order to allow the surface to be a walking surface.

Slip resistant standards-of-care and codes have been a moving target.  Years ago a reasonable Static Coefficient of Friction (SCOF) for a wet or dry surface was considered to be 0.50. This is supported by Underwriters Laboratories UL410 that is based on testing with the James machine per ASTM D-2047 on polished coated flooring surfaces. Then various agencies said the requirement should be 0.60 per the ASTM C1028 SCOF test protocol. Later the Americans with Disability Act (ADA) recommended a 0.60 SCOF on level surfaces and a 0.80 for ramp surfaces, wet or dry, without calling out a test method. (ADA has since removed those recommendations and now states that the surface must be firm, stable and slip resistant throughout its lifetime.)

OSHA’s non-mandatory appendix A of the Walking Working Surfaces notice says a reasonable measure of slip-resistance is a 0.50 static coefficient of friction (COF) when using the English XL test device per ASTM F1679 protocol (this ASTM protocol has now been withdrawn), which OSHA says is based upon studies by the University of Michigan.  All of these recommendations apply to any hard walking surface material. The higher the COF number, the more texture or friction the surface is supposed to have.

Years ago the industry discovered that ASTM C1028 gave misleading readings on highly polished surfaces due to a condition called stiction (an artificially high coefficient of friction value in comparison to the traction the surface provides).  Polished stone and polished porcelain tiles would give SCOF readings of over 0.90 in some cases, which was misleading because they were not more slip resistant, and were slippery under wet conditions.  Over the last twenty years, organizations such as the Ceramic Tile Institute, ADA, and many local city building codes required or recommended that a 0.60 SCOF be met for wet or dry surfaces as tested by the ASTM C1028 protocol. The standard of care was expected to be a minimum of 0.60 SCOF even though the ASTM C1028 did not have a required pass – no pass value.

Manufacturers produced some products that would at least meet the 0.60 SCOF, but, in my opinion, they obviously intentionally made no reference to the tile being slip resistant.  I suspect the liability of slip fall cases was too large and there were too many variables that manufacturers had no control over, so they couldn’t make any claims about slip resistance. They generally continue to take that position today.

The impact of maintenance

One important factor was that property owners were not aware of necessary maintenance routines for tile floors; as a result, floors were not kept as clean as they should have been.  Many believed mopping was all the care required. Tile floors offer benefits regarding maintenance, but they are not self-cleaning.  Regular, appropriate cleaning is necessary to maintain the slip resistant properties. Commercial properties that use maintenance companies in particular tend to have problems cleaning tile floors. They hurry through the process, use mops rather than abrasive pads or brushes, don’t change mop water frequently, and  let the dirty water air dry on the tile surface.  This leaves a dirty residual that builds up over time and lessens the floor’s slip resistant potential.

Ride-along or stand-behind scrub cleaning machines that wet, brush and vacuum as they go work well if used properly. However they have the same contamination problems if water is not changed frequently, cleaners leave a residual film, and floors are not rinsed. In particular, textured tiles need to be brushed clean with a neutral based detergent.  The dirty solution needs to be picked up with a wet-dry vacuum, and the floor needs to be rinsed with clean water to avoid residual contaminates.  Now commercial scrubbers can be used with ionized water solutions (e.g. Tennant’s ec-H2O Technology) in lieu of cleaning chemicals that work very well and are environmentally friendly.

As the tile industry performed more research, more practical and reliable test methods developed that were better indicators of slip resistance.  Much of the research was done by the University of Wuppertal in Germany, which developed what some considered the most reliable test method known as the German Ramp test.  A dynamic coefficient of friction (DCOF) was found to be a better way to measure and specify slip resistance. SCOF is a measurement of frictional resistance when an object is subjected to a force necessary to start it in motion to overcome surface friction.  DCOF is a measurement of the resistance that must be overcome to keep an object in motion, which is already in motion. SCOF is normally higher than the DCOF when a surface is being tested under the same conditions.

You can’t specify a slip-resistance rating without identifying the testing method, test device, surface conditions, and sensor material to be used; thus you can’t compare values obtained through one methodology to those resulting from a different one. A number of different SCOF and DCOF devices are available, some more suitable for laboratory testing and others for field testing. They all produce different results to some degree.

The ANSI A137.1 Ceramic Tile Committee, managed by the secretariat Tile Council of North American (TCNA), started researching and testing various test methods over a number of years.  This year a new method for measuring coefficient of friction, titled the DCOF AcuTest, was passed as part of the updated ANSI A137.1 standard.   It uses an automated, portable device called the BOT 3000 that measures DCOF. According to an article written by Eric Astrachan and Katelyn Simpson of TCNAfor Tile magazine in June 2012, the DCOF AcuTest correlates well with the German Ramp test.

According to the TCNA 2012 Handbook for Ceramic, Glass, and Stone Tile Installations, the 0.42 DCOF AcuTest value is considered an additional measure of safety over the SCOF 0.60 when tested per ASTM C1028. The referenced TCNA article notes that in a study of over  300 tile surfaces, TCNA researchers found “on average” that a 0.60 SCOF per ASTM C1028 measured with de-ionized water generally correlated with a 0.38 DCOF per the AcuTest measured with slightly soapy water. Not all products with a DCOF value over 0.42 are suitable for all applications.  Type of use, traffic, contaminants, maintenance, expected wear, and type of tile are important and must be considered by specifiers. Refer to Section of ANSI A137.1-2012 for details.

There are additional standards for keeping a public floor safe, particularly under wet conditions.  When it rains, signage should be placed at the doorway transitions.  Absorbent floor mats should be placed at doors to provide enough length of matting to allow shoes to dry as they walk, which is normally about 4.6 m (15 feet).  Plastic bags should be provided at doorways for wet umbrellas so they don’t drip on the floor.  Spills on interior floors should be cleaned and dried immediately and warning signage should be placed in the area during the clean up process.  OSHA has additional rules for floors in work places to be maintained in a manner that keeps them safe for employees.


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