A Step in the Right Direction
The UK Health & Safety Executive is currently concentrating on the assessment of floor surface slipperiness as part of its ‘Revitalising Health and Safety’ strategy. Dr. Paul Lemon and Dr. Marianne Loo-Morrey, scientists at HSE’s Health & Safety Laboratory, discuss the methods of assessment currently favoured by HSL … and the problems associated with the use of many alternative test methods.
HSE statistics regularly show that around 1 in 3 ‘non-fatal major injuries’ (such as broken bones), and over 1 in 5 ‘over-3-day injuries’ (such as strains and sprains) in workplaces throughout Great Britain involve slips and trips. This equates to one serious slip accident every three minutes, a total of over 35,000 per year.
The Workplace (Health, Safety and Welfare) Regulations (1992) require that ‘floors must not be slippery so as to expose any person to a risk to their safety’. However, these Regulations, along with the Approved Code of Practice (ACoP) that supports them, do not give practical advice regarding the methods of assessment of floor surface slipperiness, nor do they advise on the levels of slipperiness which allow classification of floor surfaces as ‘safe’ or ‘unsafe’.
HSE research (undertaken by HSL) has shown that slips may be the result of a number of factors. A model has been developed, known as the ‘slip potential model’, which considers the influence of several factors on the likelihood of a slip accident. These factors include the characteristics of the floor surface, the contamination (if any) present on the floor, the footwear typically worn by pedestrians using the floor surface, the effectiveness and frequency of the methods used to clean the floor surface, along with appropriate human and environmental factors.
Unfortunately, the assessment of floor surface characteristics, specifically the slipperiness of the floor, is the subject of widespread misunderstanding. As such, no existing single test methodology is accepted as a ‘Standard’ test throughout Europe. HSL, along with the UK Slip Resistance Group and the British Standards Institution, have developed a number of methods for the effective assessment of floor surface slipperiness; these will be discussed here. The potential problems associated with the use of alternative test methodologies will also be considered.
The Assessment of Floor Slipperiness
A number of robust and reliable test methods have been identified / developed which allow direct measurement of the slipperiness of flooring materials. These methods can be divided into those which measure Coefficient of Friction (CoF) and those which measure surface microroughness (specifically the ‘Rz’ parameter). These will be considered in turn.
Coefficient of Friction Measurement
HSL currently utilise two separate test methods for the assessment of floor surface coefficient of friction. Both tests measure the sliding or dynamic friction of floor surfaces via the direct sliding contact between a standardised rubber test pad (made from a simulated shoe sole material) and the floor surface under study. Classification systems have been developed which allow the CoF data produced to be directly converted to a measure of the ‘slip potential’ of the floor surface. Of the two methods, the ‘Ramp Test’ is limited to (but is ideally suited for) laboratory-based assessments, whilst the ‘Pendulum Test’ may be used both for laboratory-based assessments and for the ‘on-site’ assessment of installed flooring materials.
The Pendulum Test
The pendulum test (which is also known as the British Pendulum, the TRRL Pendulum and the Portable Skid Resistance Tester (PSRT)), has been used by HSL / HSE staff for many years during both scientific research and during HSE and Local Authority enforcement action. The method has been relied upon to support a wide range of such enforcement, from the generation of formal technical advice through to the issuing of improvement notices, and, ultimately, to prosecution. The test is described in a current set of British Standards (BS 7976: Parts 1-3, 2002) and forms the basis of the Guidelines on floor surface slipperiness assessment issued by the UK Slip Resistance Group.
Although often used to assess the skid resistance of road surfaces, the pendulum instrument was originally specifically designed to simulate the action of a heel slipping over a floor surface. The greater the friction available, the higher the Pendulum Test Value (PTV) that is generated, and the lower the inherent slip risk. If the pendulum is set up and prepared correctly, the data generated can be extremely reliable and accurate. As a result, the pendulum test has been adopted by HSE as its standard test method for the assessment of floor surface slipperiness.
The Pendulum coefficient of friction (CoF) test in active service
The PTV results generated by the pendulum can be used to determine the slip potential of the floor surface under study using the following table, reproduced from current HSE Guidance and the UK Slip Resistance Group Guidelines:
|Pendulum Test Value|
|0-24||High Slip Potential|
|25-35||Moderate Slip Potential|
|36+||Low Slip Potential|
It should be noted that the classification regime used above is based on the use of one of two standard test slider materials. The most common, a hard rubber material known as ‘Slider 96’ (also referred to as ‘Four-S’ rubber, or Standard Simulated Shoe Sole) is used in the majority of pedestrian areas. In situations where barefoot pedestrians are commonly encountered, or if particularly rough floorings are to be assessed, an alternative, softer rubber material may be used, known as ‘Slider 55’ (also referred to as ‘TRRL Rubber’). Other test slider materials may be used in specific situations. For example, if a pedestrian slip occurs which involves the use of leather-soled footwear, then an appropriate leather slider may be used in place of the standard rubber materials referred to above. However, in such situations, careful thought should be given to classification of the results generated.
Although ideally suited to the assessment of the slipperiness of level floor surfaces (and those laid to a modest slope of below around 10? to the horizontal), the use of the pendulum for the assessment of specialist areas, such as heavily profiled floorings, open grids or stair treads can be difficult, and should only be attempted by experienced operators. In such circumstances, the use of alternative test methods should be considered.
The HSL ramp test
This test method was developed by HSL around ten years ago in order to allow the generation of CoF information for use during formal research into the causes of pedestrian slips. The method, which is loosely based on a similar test used in two German National Standards (DIN 51130 and DIN 51097) uses two or more test subjects (attached to a full fall-arrest system) who move back and forth over a contaminated flooring surface. The angle of inclination of the flooring surface under study is increased gradually until the test subject slips. The angle at which slipping occurs is used to calculate the CoF available from the contaminated test surface. Testing may be undertaken under a range of floor contaminants, such as water, soap solution, oil or grease, although clean water is routinely used by HSL, applied as a fine mist by crop-spray jets. The CoF data generated relates to the use of the floor surface tested when used on the level.
Ramp-type tests are often used by European flooring manufacturers to assess the slipperiness of flooring materials before sale; the German ramp-based tests mentioned above are often used. However, these Standard tests do not relate to typical flooring use; DIN 51097 involves barefoot operators moving on a test surface contaminated with a strong soap solution, while DIN 51130 requires operators to wear heavily cleated safety boots on a test surface contaminated with viscous motor oil.
The classification systems used by both German Standards have also been the cause of much confusion throughout Europe. For example, the classification system used by DIN 51130 is based on the ‘R-scale’, which runs from R9 to R13. This scale is often misunderstood. Flooring specifiers commonly mistake R9 classification as slip resistant, as it is commonly assumed that the scale runs from R1 to R13, where R1 is the most slippery, and R13 the least slippery. In reality, a flooring classified as R9 is likely to be very slippery when contaminated with water or other wet contaminants.
Surface microroughness measurement
The measurement of surface microroughness, specifically measurement of the ‘Rz’ parameter (which is a measure of the overall roughness of a surface) can give useful information which supplements CoF information generated by the Pendulum or HSL Ramp. Floor surface roughness measurements may be made quickly and simply, and the equipment required is often quite inexpensive. A selection of commercially available surface roughness meters is shown below.
Surface microroughness meters
The levels of Rz floor surface microroughness required to generate satisfactory ‘slip resistance’ in a range of contaminated conditions have been identified by HSL and published in HSE Guidance; these levels are shown in the table below.
|Minimum Rz Surface Roughness (microns)||Contaminant|
|20||Clean water, coffee, soft drinks|
|45||Soap solution, milk|
|70||Motor oil, olive oil|
|>70||Gear oil, margarine|
Unfortunately, commercially available surface roughness meters are unsuitable for use on some common flooring types, such as carpet, particularly rough, or heavily profiled floors. As a result, the results of surface roughness measurements should only be used as a guide, unless considered with other information, such as CoF or the recently developed HSE Slips Assessment Tool (SAT).
Slips assessment tool (SAT)
HSL (under contract to HSE) have recently produced a CD-RoM based package to allow the simple assessment of floor surface slipperiness. The Slips Assessment Tool (SAT), prompts the user to collect surface microroughness data from the test area using a hand held meter and supplements this with other relevant information, such as floor surface contamination, the regimes used to clean the floor surface (both in terms of their effectiveness and frequency), the footwear types worn in the area, and associated environmental and human factors.
On completion, a slip-risk classification is supplied to the user; this gives an indication as to the potential for a slip. SAT is designed to assist in the decision making process when considering the risk of slipping in a defined area, and can be used iteratively to show the influence of different control measures. However, it should not be relied upon when considering the performance of just the flooring; in this instance the pendulum should be used.
Roller Coaster Tests
HSL have evaluated two roller-coaster style instruments for the assessment of floor slip resistance compared with the pendulum on a wide range of installed floor surfaces, in dry, wet and contaminated conditions. The first instrument was a laboratory prototype; the second was the commercially available SlipAlert test. The results showed reasonable agreement between the tests, provided that Slider 96 was used as the test slider material. Practical experience using the roller coaster type tests showed that the large test area required by such instruments limits their applicability in some situations. Also, as the test slider travels a significant distance over the floor surface, the average slipperiness of the whole test area is measured, rather than the ‘worst case’ slip resistance. On such surfaces, microroughness measurements should be taken to highlight variation in slip resistance within the test area. Furthermore, the use of these test methods on small spills (as is often the case in typical pedestrian areas) may lead to the generation of misleading results.
Laboratory-based assessments have strongly suggested that several tests currently available (particularly those based on ‘sled-type’ principles) can produce misleading data. Information from such tests shows that some smooth floorings appear to be less slippery in wet conditions than when dry; this is clearly at odds with everyday experience. Such tests may give credible results in dry conditions, though it should be stressed that the vast majority of slipping accidents occur in wet contaminated conditions.
The assessment of floor surface slipperiness is a complex matter, and should only be attempted using the correct tools, used in the correct manner. The use of standardised slider materials is strongly recommended, as this allows a degree of correlation between suitable test methods; for example, the pendulum and HSL ramp tests agree well when both used with Slider 96 rubber material. Use of surface roughness information should ideally be accompanied with other salient information, such as CoF data. Alternatively, surface roughness information may be fed into HSE’s ‘Slips Assessment Tool’ software, available from www.hsesat.info. See www.hsl.gov.uk/capabilities/pedestrian.htm for further details.
,em>Dr. Paul Lemon ([email protected]) is a Senior Scientist at the Health & Safety Laboratory; the in-house scientific agency of the Health & Safety Executive. He is a founder member of the HSE Slips and Trips Programme Group, the UK Slip Resistance Group, and BSI Committee B/556 (the coordinating Committee for Pedestrian Slips). He was also co-author of BS 7976, BS 8445 and BS5395, and has produced numerous HSE Guidance Documents regarding pedestrian safety. He is a Chartered Physicist and a Chartered Scientist, and has served as HSE’s Expert Witness during numerous slipperiness investigations.
Dr. Marianne Loo-Morrey ([email protected]) is a Higher Scientist at HSL, and currently specialises in the assessment of pedestrian safety on non-level surfaces such as stairs and inclined walkways. She is currently co-opted to BSI Committee B/208 (Stairs, Ladders and Walkways) and is HSL’s in-house specialist regarding footwear slipperiness.
Published: 01st Jul 2006 in Health and Safety International