Stand Your Ground

Slip resistance is a challenge. It's ideal to work with a safety footwear provider who understands the tradeoffs and can help advise your organization on the right product for the work environment.

• By Lori Hyllengren
• Sep 01, 2017

When it comes to footwear, the topic of slip resistance is almost as treacherous as the slick floors, wet surfaces, and uneven terrain employees encounter every day. Confusion surrounds the definition of slip resistance, as well as the accepted method to evaluate slip resistant footwear. This puts employers in a difficult position because slips, trips, and falls are among the most reported—and most costly—injuries in many industries.

Navigating the Standards
Slip resistance requirements for footwear do not exist as they do for other protective aspects of safety footwear, such as toe protection or puncture resistance. There are a variety of methods for testing slip resistance, and there are guidelines that manufacturers can choose to follow (or not) during product testing. However, there is no current U.S. regulatory body with enforcement power, such as OSHA, that defines or quantifies the slip resistance of footwear.

The relevant testing standard set forth by ASTM International is ASTM F2913, "Standard Test Method for Measuring the Coefficient of Friction for Evaluation of Slip Performance of Footwear and Test Surfaces/Flooring Using a Whole Shoe Tester" ("Whole Shoe test"). This Whole Shoe test standard is applicable to a variety of footwear materials and test surfaces, including those that are contaminated with slippery substances. As such, it offers a comprehensive picture of the slip resistance of footwear. The following text summarizes the standard:

This test method determines the dynamic coefficient of friction between footwear and floorings under reproducible laboratory conditions for evaluating relative slip performance. The method is applicable to all types of footwear, outsole units, heel top-pieces (top-lifts) and sheet soling materials.

In the Whole Shoe test, the footwear sole and underfoot surface are brought into contact, subjected to a specified vertical force for a short period of static contact, and then moved horizontally relative to one another at a constant speed. The horizontal frictional force is measured after movement starts, and the dynamic coefficient of friction is calculated.

An older slip resistance method employers may know is ASTM F1677, also known as the "Mark II" or “Brungraber” test because it was performed with a slip tester developed by Dr. Robert Brungraber. For many years, this test was an accepted standard for measuring slip resistance of walkway surfaces and evolved into testing footwear. This method was withdrawn by ASTM in 2006 and replaced with ASTM F2913 Whole Shoe test in 2011. The Mark II test is no longer supported by ASTM. Employers accustomed to requesting Mark II test data from safety footwear providers should instead ask for Whole Shoe test data according to F2913.

Exploring a Slippery Issue
Measuring slip resistance for safety footwear can be challenging. Excerpts from the Whole Shoe test documentation illustrate three reasons why:

1. "Surface contaminants include, but are not limited to, water, ice, oil, grease and other chemicals." These contaminants are among the most prevalent, but employers cannot predict changes in slip resistance when the contaminant is food, gravel, cleanser, mud, or construction material.

2. "The test does not account for the risk of tripping due to footwear/ground interlock." This language refers to special-purpose footwear with spikes or metal studs. When footwear is too slip resistant for the task, it can raise additional risk of injury.

3. "The standard does not address all safety concerns." A variety of other factors may affect the risk of slipping, including ambient temperature, lighting, surface irregularities, stairs and handrails, floor mats and runners, and human factors such as age, overall health, inattentive behavior, taking shortcuts, or carrying objects that obstruct your view.

The takeaway? While it's possible to measure the coefficient of friction of a shoe, slipping involves other variables to consider when choosing proper safety footwear for the job. For example, one shoe may provide adequate slip resistance on dry concrete but become slippery on wet steel. Another might offer substantial slip resistance initially, but if the sole is not durable and wears down quickly, the slip resistance may also diminish.

Evaluating Slip Resistance
How can employers make a solid assessment of slip resistance? Start by understanding the tradeoffs.

1. Safety vs. cost. It's unlikely the least expensive safety footwear will meet the slip resistance requirements of the job. The slip resistance of any footwear comes down to the integration of several anti-slip features, all of which may affect purchase price. Shortcuts, sloppy manufacturing techniques, or low-quality materials could compromise any of these features as well as create durability issues.

2. Safety vs. comfort. Safety features have to exist in proper balance with comfort. If safety footwear does not fit properly or is uncomfortable, employees may not wear them consistently or find excuses to avoid wearing them.

3. Safety vs. overdesign. It is possible for safety footwear to be too slip resistant for the job. If the shoe or boot sticks to the work surface, employees run the risk of injuries similar to those experienced by athletes who hurt their knees when their cleats stick in the turf.

It is ideal to work with a safety footwear provider who understands these tradeoffs and can help advise your organization on the right product for the work environment. Here are some general guidelines to consider when selecting the right slip resistance features:

Outsole
Soft rubber compounds offer the greatest slip resistance for environments contaminated with oil and grease.

Tread
Look for outsoles that channel oil and grease away so the outsole can reach the work surface more fully, as well as outsoles with split and solid lugs specifically designed to shed debris. Treads should allow a maximum amount of material to grip the floor. Softer soles and more tightly spaced treads are better suited to fluid contaminants in indoor environments. More widely spaced treads are generally better for handling solid contaminants in outdoor areas. It is vital to avoid clogging the tread. A wider or deeper tread pattern may be necessary if footwear needs to be cleaned often. Over time, watch for worn or flattened soles.

Insoles
Insoles do not play a direct role in slip resistance, but they should offer additional cushioning and impact padding for employees who are on their feet for extended periods or work on hard surfaces.

Uppers
Uppers can be made from various leathers, suede, mesh, and combinations of these. Like the insole, the upper does not directly determine slip resistance, but plays a key role in fit and comfort. A shafted boot (e.g., a 6" or 8" tall boot) offers more ankle stability than a shorter boot (e.g., an oxford).

Midsole
The midsole should provide ample support and stability is are important for the overall structural integrity of footwear.

Durability
Slip resistance performance may change as footwear undergoes normal wear and tear. Employers should prioritize durable footwear that provides the most slip resistance for the longest period of time.

Setting the Standard
Any safety footwear provided can claim slip resistance, precisely because there is no uniform standard or rating for slip resistance. For this reason, Red Wing Shoe Company recommends three simple principles for evaluating slip resistance:

1. Follow the ASTM standard test method. ASTM F2913 provides the most complete approach to evaluating how different footwear materials interact with various work surfaces. All slip resistant footwear should be tested to this standard to establish a common basis for comparison.

2. Understand the details. Whether or not a specific shoe or boot will provide adequate, long-term slip resistance requires a deep understanding of what the job actually involves: the surfaces, contaminants and physical requirements of the role. Look for a provider with experience developing purpose-built safety footwear worn in a variety of industries.

3. Share knowledge. Ensuring slip resistance depends on a strong, open and transparent collaboration between employer and safety footwear provider. The footwear manufacturer should provide detailed information about its products and their performance, while understanding the work environment they are best suited for.

This article originally appeared in the September 2017 issue of Occupational Health & Safety.