The key properties of a walking surface in respect of its capacity to resist the potential for pedestrian slip is its dynamic coefficient of friction (CoF), which will be influenced by a range of factors. There is no universally agreed method of test for CoF throughout the European Community, however, there are a number of slip assessment tools available using different methods.
Ramp Test
The ramp test uses of a human subject, walking backwards and forwards over contaminated flooring at ever increasing angles of inclination, to measure slip resistance. The resulting angle at which slip occurs classifies the flooring via a rating (DIN 51130, DIN 51097).
Versions utilise oil with the subject wearing safety footwear or, in the case of bare foot testing, a soap solution as the contaminant. Whilst others such as HSL-PS-SOP12 ramp test, also known as the UKSRG Ramp Test (HSE, 2007) use potable water.
Draft methods (prENs) are being discussed for wet (water) tests with both Slider 96 (formally known as Four S slider) calibrated soled footwear and bare foot modes as EN laboratory methods to determine the CoF.
TRRL Pendulum
The pendulum test was originally developed to simulate the action of a slipping foot. The method is based on a swing dummy heel, which sweeps over a specified area of flooring (just making contact with the surface) in a controlled manner. The slipperiness of the flooring has a direct influence on the pendulum value obtained (known as the Slip Resistance Value (SRV) or PSV (Pendulum Slip Value) with high CoF values slowing down the pendulum, reducing the distance it travels after contact with the test floor.
This is one of the preferred methods of test in the UK. It is simple to operate and said to demonstrate good correlation to real life conditions. The method of test is recognised in the UK by the Health and Safety Executive and detailed in the UK Slip Resistance Group Guidelines. The added benefit of full portability is essential in the evaluation of on site slip potential.
One thing to remember is the test is measuring the point when a person is already slipping.
SATRA Friction Tester STM 603
SATRA TM144 utilising the SATRA STM 603 slip rig measures the slip resistance of flooring in the laboratory and, though developed for footwear, is based on biomechanical studies of slip. The test defines a dynamic CoF measurement at a constant speed under a vertical load representative of human body weight. The test slider generally employs Slider 96 rubber mounted at a contact angle of 5°.
The method has the added benefit of adaptation to utilise footwear mounted on a foot form as the slider, allowing end-users the option of specifying footwear, pattern of tread, surface contaminants and other parameters to give realistic performance values under normal service conditions.
This method, which is highly regarded worldwide, provides SATRA's customers with informed knowledge of slip resistance of floorings under a variety of conditions. In fact, SATRA TM144 is the only method that can give reliable indication of slip resistance on profiled floor coverings.
Surface Roughness
Measuring surface roughness is also an option. The benefits of a rough surface is that in wet or oily conditions, any peaks will project through the thin fluid film trapped between the shoe sole and flooring, biting into the soling and reducing the risk of hydroplaning. Roughness is measured using a fine stylus that traverses the surface mapping the microscopic profile. The most commonly quoted parameter for roughness is Rz(DIN) or Rtm - the average of all peak and valley heights in a measured length.
However, as with any measurement it is important to understand the limitations of the measurement system. Surface roughness is measured in microns (1 millionth of a metre), so is only applicable to visibly smooth surfaces. It should also be considered that when measuring surface roughness the hardness of the flooring material should be considered as softer floors will demonstrate better CoF with lower Rtm measurements.