Slip Resistance: Understanding the Basics

With more than twenty years of experience in slip resistance, Stone Initiatives offers some useful insight into the various testing methods available.

Article: Jim Mann

In Australia slip resistance is measured according to two different (but similar) standards depending on whether the surface is new or existing. New pedestrian surfaces are tested in accordance with the current version of AS 4586 while existing pedestrian surface are tested in accordance with AS 4663.

The standard for new pedestrian surfaces (AS 4586) provides a method for determining dry slip resistance (using a Floor Friction Tester) and a range of different methods for evaluating slip resistance in a wet condition (i.e. Pendulum, Wet/barefoot Ramp and Oil-wet Ramp). Only one of these methods (Pendulum) can be used on a fixed (installed) surface and this method is documented within AS 4663 along with the method for the Floor Friction Tester which is also able to be used on site.

The various slip resistance test methods are described briefly below along with the pros and cons for each method.

Measurement of Dry Slip Resistance

Determination of dry slip resistance is of particular importance for smooth floors (e.g. polished granite or terrazzo) that are located in areas that would not normally become wet. Such areas may include internal foyers, upper lift lobbies or shops inside shopping centres.

The floor friction tester (known by acronym FFT or the popular marketing name of Tortus) is a portable self-powered mobile unit which measures the dynamic coefficient of friction between the surface and a standard rubber slider. The unit is about the size of an old fashioned VCR and basically includes a load cell, a friction foot fitted with a small rubber slider and a means of recording the coefficient of friction as the unit moves across the floor.

The rubber slider within the FFT is conditioned on Grade P400 wet and dry abrasive paper before being run across the surface to be tested. As the unit moves across the floor a print out is produced which plots the coefficient of friction on chart paper. On conclusion of the test run the unit calculates a mean reading for the test run.

As the FFT unit is electronic, it is a very sensitive unit which can differentiate between small changes in surface finish. The compact nature of the unit makes the FFT unobtrusive and a versatile piece of equipment which can be used in small spaces. The slip resistance of sloping surfaces can be determined by running the FFT up and down the slope and averaging out the results.

The rubber slider used is a small disc only 9mm in diameter which means that the area of the surface tested is quite small and the results achieved may not be representative of the floor if testing locations and directions are not chosen carefully. The slider also has difficulty in dealing with changes in surface profile which means it can become caught up by lippage from uneven tiles.

There are only two classifications for dry slip resistance. The current standard for new pedestrian surfaces (AS 4586:2013) classifies a coefficient of friction of greater than or equal to 0.4 as class ‘D1’ which was previously classified as an ‘F’ in the earlier 2004 version of the standard. A coefficient of less than 0.4 is classified as class ‘D0’ (previously classified as a ‘G’). Although the Standards Australia handbook HB198:2014 does not give a recommendation for ‘dry floors’ a D1 classification can be considered a suitable guideline.

Measurement of Wet Slip Resistance

As mentioned earlier, wet slip resistance can be measured by different methods depending on the surface and location. The Pendulum method could be considered the most versatile and relevant method for most pedestrian situations.

Wet Pendulum Test

The Pendulum method measures the frictional resistance between a rubber slider mounted on the end of a pendulum arm and the test surface. The pendulum consists of an arm which rotates about a spindle attached to a vertical support pillar. At the other end of the arm is a mass (shaped like a foot) fitted with a spring loaded rubber slider. The pendulum is locked into a horizontal position and then released so that it strikes the sample surface over a set distance with a constant velocity and energy. The pendulum continues its arc past the strike path and the height of the forward swing is determined by the energy lost due to friction when in contact with the sample surface. A pointer is pushed along by the pendulum and records the height of the swing. A reading is recorded from the scale and recorded as the British Pendulum Number (BPN) or Skid Resistance Values (SRV).

The standards allow the use of two different types of rubber slider with selection being based on the location and roughness of the surface. A rubber slider known as ‘Slider 96’ (previously known as ‘4S’) is normally used when testing polished or light to moderately textured surfaces. More coarsely textured surfaces may be tested with a ‘as ‘Slider 55’ slider (also known as TRL). The Slider 96 simulates a standard shoe sole and is relatively hard rubber typical of dress shoe soles. The TRL slider is a softer rubber compound originally designed to simulate tyre rubber but can also simulate softer shoe soles or even bare feet.

The contact area of a test site is 76 mm x 126 mm which is similar in size to a small footprint which can more readily be set up to include any variations in texture or directionality within the surface compared to the floor friction tester. As the pendulum swings through an arc of 52cm, the tester itself requires an operating area of at least 1.2 m x 1.0 m. Normally this is not a problem but testing can be difficult in areas such as stairwells.

The rubber slider is firstly conditioned on Grade P400 abrasive paper then further conditioned on 3 micron lapping film and is then set up to come in contact with the test surface over a specified distance. Testing is carried out at five sites with a minimum of five swings at each location or until a relatively consistent reading is achieved. The mean BPN value is calculated and converted into a classification ranging from P5 to P0 for new pedestrian surfaces. Up until mid 2013 the classification ratings ranged from V to Z but this has now changed to the new P ratings within the most recent version of the standard. A significant change between the two standards is the fact the slider is conditioned with the use of lapping film which gives the slider a smoother finish prior to testing. This ‘polishing’ provides more sensitivity to the lower test results. Also the new P version adds in an extra classification than the V to Z version had, as shown in the table below. A P5 classification is considered to represent a very low contribution of the surface to the risk of slipping when wet while a P0 classification represents a very high risk.

The slip resistance recommendations for a range of ‘wet floor locations’ is presented within Table 3B of Standards Australia handbook HB198:2014. An example of these locations and the classification recommendations is given below.

Note 1: note 3 of Notes to Table 3B states:
The minimum classification listed in Table 3B is P1. It is inappropriate for Table 3B to list the lower classification, P0, since there is no lower limit on Classification P0.
Notwithstanding, some smooth and polished floor surfaces, which do not achieve a Classification P1, may be considered to provide a safe walking environment for normal pedestrians walking at a moderate pace, provided the surfaces are kept clean and dry; however, should these surfaces become contaminated by either wet or dry materials, or be used by pedestrians in any other manner, then they may become unsafe. Therefore, the type of maintenance, the in-service inspection of floors, other environmental conditions and use should be taken in to account when selecting such products.

The British Pendulum is considered to correlate well with the slip resistance of most pedestrian surfaces and provides a special insight into the interaction of shoe soles with textured surfaces with smooth undulations such as a flamed and wire brushed (antiqued) stone tile.

Testing by Stone Initiatives has found that although antiqued (i.e. flamed and wire brushed) stone finishes and similar finishes on ceramic tiles can appear to be quite ‘slip resistant’ some of these finishes produce a much lower BPN result than would be expected. The low test result is supported by the investigation of a range of wet slip / fall incidents involving tiles with textured finishes. These incidents predominantly involve pedestrians wearing hard rubber sole shoes walking on textured surfaces with rounded edges. In this situation the hard and flat rubber has a tendency to skid over the top of the surface as shown in the sketch below.

Figure 1: This sketch shows the interaction between a relatively rigid surface (e.g. hard rubber shoe sole) and a surface that has a reduced contact area (e.g. dimpled or undulating surface). The moving surface does not deform significantly to grip the protrusions but instead skids over the top.

The good correlation between laboratory and onsite results appears to confirm the reliability of results for situations involving pedestrians wearing normal street shoes.

The main weakness of the British Pendulum method is simulating the interaction of a pedestrian walking over a highly profiled surface wearing highly profiled soles (e.g. work boots in an industrial work site) or is walking barefoot as occurs around a swimming pool. In both of these situations the sole material is more able to deform to the profile of the pedestrian surface providing improved mechanical grip. When evaluating the slip resistance of surfaces for these locations it is recommended that the surface is tested to the Oil-wet Ramp or Wet Barefoot test as well as the Pendulum test. As the Ramp tests require loose tiles they cannot be carried out on fixed tiles onsite and the British Pendulum test can readily be used to validate the slip resistance of the surface after installation.

Oil-wet Ramp Test

An alternative method for determining wet slip resistance is by the Oil-wet Ramp test. This method uses a test panel approximately 1m × 0.5m in size prepared from the pedestrian surface (e.g. loose tiles) which is installed on the ramp test bed. The test involves laying the test surface on the ramp followed by the application of engine lubricating oil on the test surface. A test walker is required to walk up and down the ramp wearing standard test boots while the incline is gradually increased reducing the friction between the surface and the ‘test walker’. The test is stopped once the test person considers the angle of inclination is unsafe for walking. The test is repeated using a different test person and the mean angle is used to assess the degree of slip resistance. Subjective influences affecting the acceptance angle (e.g. variations between test persons) are limited by the use of calibration boards.

The maximum angle of inclination is corrected after taking into account the results from the standard floor coverings to produce an overall mean acceptance angle which is converted into an ‘R’ rating as shown in the table below along with examples of suitable locations as presented in Table 3B of Standards Australia handbook HB198:2014.

Corrected Mean Overall Acceptance AngleSlip Resistance
Assessment Group
Location Example
6° to 10°R 9Entry and access areas – dry
Over 10° to 19°R10Entry and access areas – wet
Over 19° to 27°R11External ramps slope under 1:14
Walkways, colonnades, pedestrian crossings
Over 27° to 35°R12External ramps steeper than 1:14 inc. driveways, footpaths
Over 35°R13

Note that R12 is considered suitable for most locations. The Ramp method measures the interaction of an actual person walking on the pedestrian surface while wearing footwear with a heavily profiled sole and therefore takes into account the complicated movements performed by a foot while walking.

The properties of the fluid will affect the interaction of a shoe sole with a wet pedestrian surface. The oil-wet ramp test uses lubricating oil which has a higher viscosity than water which reduces coefficient of friction between the shoe sole and the test surface.

Wet Barefoot

The ‘Wet Barefoot’ test is similar to the oil-wet ramp except in this case the bare feet of the test person are soaked in water prior to them moving backwards and forwards over the test surface. During the test, a stream of water is poured over the test pieces at a continuous rate. In the case of absorbent floor coverings, uniform wetting of the top surface shall be ensured by preliminary soaking.

As the angle of inclination is increased until they reach their safe limit of walking. The maximum angle of inclination is used to assess the friction characteristics of the test surface.

The maximum angle of inclination is corrected after taking into account the results from the standard boards to produce an overall mean angle of inclination which is converted into a rating as shown in Table 3B.

Corrected Mean Angle of InclinationSlip Resistance
Assessment Group
Location Example
≥12°AChanging rooms
≥18°BShower rooms
≥24°CSloping pool edges

Test Method Selection

Given the broad range of test methods available deciding how to best evaluate the slip resistance of your pedestrian surface can be confusing. Correct test method selection depends on a range of factors including:
– Is the surface unlikely to become wet? Some locations such as upper level lift lobbies or corridors are unlikely to become wet and therefore evaluation of the dry slip resistance using the Floor Friction Tester may be all that is required. The proposed location should be surveyed for occasional ‘water hazards’ such as wet umbrellas and beverage spills and the risk assessed accordingly.
– Is the surface fixed in place? If so the Ramp test methods cannot be used and the Pendulum test is the only recognised alternative for determining wet slip resistance.
– Does the surface need to be monitored on an ongoing basis after installation? As the Ramp tests cannot be carried out on installed tiles it is recommended that the Pendulum method is used to determine the initial slip resistance and set a base line.
– Is the surface highly profiled ? If the surface is not fixed the Ramp test methods are recommended for these areas as they are better suited to highly profiled surfaces.
– Will the surface mainly be trafficked by bare feet? If the surface is not fixed the Wet Barefoot Ramp method is recommended as it accurately evaluates the unique action of a person walking in bare feet. If the surface is fixed the Pendulum method is recommended using a TRL rubber slider.
– Will the surface mainly be trafficked by dress or hard soled shoes? The Pendulum method is recommended using a 4S rubber slider as this closely replicates a hard soled shoe.

Your Next Step

With more than thirty years of experience in-slip resistance testing, the staff at Stone initiatives can assist you with any queries. Let us take care of your slip-resistance testing requirements. Contact us now. Our NATA-accredited laboratory is well equipped to carry out slip resistance tests anywhere in Australia or the world.

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